Bioenergy: Volume 2 9783110476217, 9783110475517

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Zhenhong Yuan (Ed.) Bioenergy: Principles and Technologies GREEN Alternative Energy Resources

GREEN Alternative Energy Resources

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Volume 2.2

Bioenergy: Principles and Technologies | Edited by Zhenhong Yuan

Editor Zhenhong Yuan Guangzhou Institute of Energy Conversion Chinese Academy of Sciences Guangzhou 510640 China [email protected] (Z. Yuan)

ISBN 978-3-11-047551-7 e-ISBN (PDF) 978-3-11-047621-7 e-ISBN (EPUB) 978-3-11-047567-8 Set-ISBN 978-3-11-047622-4

Library of Congress Cataloging-in-Publication Data A CIP catalog record for this book has been applied for at the Library of Congress. Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de. © 2018 Science Press Ltd. and Walter de Gruyter GmbH, Beijing/Berlin/Boston Cover image: vencavolrab/iStock Typesetting: PTP-Berlin, Protago-TEX-Production GmbH, Berlin Printing and binding: CPI books GmbH, Leck ♾ Printed on acid-free paper Printed in Germany www.degruyter.com

Contents Yu Zhang, Jingliang Xu, Yanling Jin, Xinshu Zhuang, Hai Zhao, Yunyun Liu, Qiang Yu, Guixiong Zhou, and Mingsong Xiao 1 Biomass ethanol fuel technology | 1 1.1 Characteristics and application of ethanol | 1 1.1.1 Physical and chemical properties of ethanol | 1 1.1.2 Performance comparison of ethanol and gasoline/diesel | 2 1.2 Production theory of bioethanol | 3 1.2.1 The main methods of ethanol production | 3 1.2.1.1 Microbial fermentation | 3 1.2.1.2 Syngas to ethanol | 4 1.2.2 Biochemical process of ethanol fermentation | 5 1.2.2.1 Carbohydrate degradation | 5 1.2.2.2 Glycolysis | 7 1.2.3 Ethanol fermentation microbiology | 8 1.2.3.1 Ethanol fermentation microbiology based on a sugar platform | 9 1.2.3.2 Pentose fermenting microbes | 13 1.2.4 Breeding of ethanol fermentation microorganisms | 13 1.2.4.1 High ethanol tolerant strains | 14 1.2.4.2 High temperature resistant strains | 15 1.2.4.3 Pentose fermentation strains | 16 1.2.4.4 Saccharification function strains | 16 1.2.4.5 Engineered bacteria for consolidated bioprocessing | 17 1.2.4.6 The breeding of syngas fermentation microorganisms | 17 1.3 Ethanol production from starch feedstocks | 18 1.3.1 Crushing | 18 1.3.2 Steaming and gelatinization | 19 1.3.3 Saccharification | 20 1.3.3.1 Separate hydrolysis and fermentation process | 20 1.3.3.2 Simultaneous saccharification and fermentation process | 20 1.3.3.3 Partial simultaneous saccharification and fermentation process | 21 1.3.3.4 Ethanol Fermentation | 21 1.3.3.5 Ethanol extraction and purification | 24 1.3.3.6 Examples of industrial production | 26 1.4 Ethanol production from sugar feedstocks | 26 1.4.1 Technical process of ethanol production from sugar feedstocks | 27 1.4.1.1 Technological requirements for processing molasses feedstock | 27 1.4.1.2 Devices for the dilution of molasses | 29 1.4.1.3 The treatment process of molasses | 30

II | Contents

1.4.1.4 1.4.2 1.4.2.1 1.4.2.2 1.4.3 1.4.3.1 1.4.3.2 1.4.3.3 1.4.3.4 1.4.3.5 1.5 1.5.1 1.5.1.1 1.5.1.2 1.5.1.3 1.5.2 1.5.2.1 1.5.2.2 1.5.2.3 1.5.2.4 1.5.3 1.6 1.6.1 1.6.2 1.6.3 1.6.3.1 1.6.3.2 1.7 1.7.1 1.7.2 1.7.2.1 1.7.2.2 1.7.2.3 1.7.2.4 1.7.2.5 1.7.2.6

Acidification, sterilization, clarification, and addition of nutrient salts to molasses | 30 Ethanol production from sweet sorghum stalk | 33 Solid-state fermentation process | 33 Liquid fermentation process | 35 Bioethanol production cases from sweet sorghum | 36 Continuous solid-state fermentation process | 36 Raw material pretreatment | 36 Yeast activation | 38 Joint blender mixer | 38 Fermentation | 39 Ethanol production from cellulosic materials | 41 Pretreatments | 41 Dilute acid pretreatment | 41 Liquid hot water pretreatment | 44 Steam explosion pretreatment | 45 Enzymatic hydrolysis and ethanol fermentation of cellulosic materials | 45 Types of cellulase and their structures | 45 The hydrolysis mechanism of cellulase | 46 Factors affecting enzymatic hydrolysis | 48 Ethanol fermentation | 49 A case of cellulosic ethanol production | 50 Analysis of the economics of ethanol fuel production | 51 Analysis of the economics of ethanol fuel production using starch raw material | 52 Analysis of the economics of ethanol fuel production using sugar raw material | 57 Analysis of the economics of ethanol fuel production using cellulose raw material | 60 Simultaneous saccharification and co-fermentation | 61 Two-stage dilute acid hydrolysis process | 64 Environmental impacts of fuel ethanol production and control approaches | 67 Sources of pollutants during ethanol production | 67 Waste treatment methods | 68 The production of feed | 69 The production of feed yeast | 69 Anaerobic treatment | 70 Aerobic treatment | 70 Dilution agricultural irrigation method | 70 Concentration | 71

Contents |

1.7.2.7 1.7.2.8 1.7.2.9 1.7.3 1.8 1.8.1 1.8.2 1.8.2.1 1.8.2.2 1.8.3 1.8.3.1 1.8.3.2

Combustion | 71 Comprehensive utilization | 72 Fine Chemicals | 72 Approaches to pollutant control | 73 The general situation of fuel ethanol prepared using biomass both at national and international levels | 77 Research and development of fuel ethanol | 77 Situation of the fuel ethanol industry in China | 79 The development of technologies to produce nongrain starch/sugar ethanol | 80 The development of cellulosic ethanol production technology | 81 Prospects for the fuel ethanol industry in China | 85 Developmental trend | 85 Development objectives | 86

Weiming Yi, Xifeng Zhu, and Wei Qi 2 Technologies of biomass pyrolysis | 87 2.1 Principles and technology of biomass fast pyrolysis | 87 2.1.1 Overview of biomass fast pyrolysis | 87 2.1.2 Characteristics of biomass fast pyrolysis | 90 2.1.2.1 Preparation of biomass powder | 92 2.1.2.2 Temperature control of laminar flow furnace | 92 2.1.2.3 Biomass flash volatilization experiments and data processing | 94 2.1.3 Application example of the biomass pyrolysis technology | 98 2.1.3.1 Pretreatment device for feedstock drying | 100 2.1.3.2 Two-stage screw feed device | 101 2.1.3.3 Fluidized-bed pyrolysis reactor | 102 2.1.3.4 Cyclone separator | 103 2.1.3.5 Condensing system | 104 2.2 Upgrading and applications of bio-oil | 105 2.2.1 Physical and chemical properties of bio-oil | 105 2.2.1.1 Ultimate composition of bio-oil | 105 2.2.1.2 Component analysis of bio-oil | 106 2.2.1.3 Physical and chemical properties of bio-oil | 108 2.2.2 Bio-oil upgrading with addition of hydrogen | 112 2.2.2.1 Sulfided Co-Mo and Ni-Mo catalysts | 113 2.2.2.2 Noble metal catalyst for catalytic hydrogenation | 114 2.2.2.3 Two-stage catalytic hydrogenation | 114 2.2.2.4 On-line catalytic hydrogenation | 115 2.2.2.5 Catalytic hydrogenation of bio-oil in situ | 116 2.2.2.6 Upgrading bio-oil by homogeneous catalysis with metal complexes | 116

III

IV | Contents

2.2.2.7 2.2.3 2.2.3.1 2.2.3.2 2.2.3.3 2.2.3.4 2.2.4 2.2.4.1 2.2.4.2 2.2.4.3 2.2.5 2.2.5.1 2.2.5.2 2.2.5.3 2.2.6 2.2.6.1 2.2.6.2 2.2.6.3 2.2.6.4 2.2.6.5 2.2.6.6 2.2.6.7 2.3 2.3.1 2.3.1.1 2.3.1.2 2.3.1.3 2.3.1.4 2.3.2 2.3.2.1 2.3.2.2 2.3.2.3 2.3.2.4 2.3.3 2.3.3.1 2.3.3.2 2.3.3.3 2.3.4 2.3.4.1 2.3.4.2 2.3.4.3 2.3.4.4

Catalytic hydrogenation of light fraction of bio-oil | 118 Bio-oil upgrading with catalytic cracking | 118 Catalyst type | 119 Catalytic reaction conditions | 120 Catalytic mechanism | 121 Catalyst deactivation mechanism | 123 Bio-oil steam reforming for hydrogen production | 125 Catalyst selection | 126 Preparation of catalysts | 127 Characterization of catalysts | 128 Bio-oil combustion technology | 129 Combustion characteristics of bio-oil | 129 Spray characteristics of bio-oil | 131 Bio-oil atomization combustion system | 134 Demo applications of bio-oil | 137 Design of the fuel supply system | 137 Design of gas supply system | 138 Design of the atomizing nozzle | 139 Design of the bio-oil combustor | 140 Design of flue gas emission system | 141 Design of the test monitoring system | 141 The results of bio-oil spray combustion | 141 Carbonization of biomass | 142 Biomass carbonization technology | 142 Principle and characteristics of carbonization | 142 Carbonization products | 143 Factors affecting the carbonization process | 143 Types of carbonization technology | 144 The properties and applications of biochar | 147 Feedstocks of charcoal | 148 Types of charcoal | 148 Properties of the charcoal | 149 Applications of charcoal | 152 The applications of biomass dry distillation gas | 154 Jiaozuo biomass carbon gas oil cogeneration system | 155 Dalian biomass energy engineering | 155 Liaoning biomass distilled gas production device | 157 Case study of biomass carbonization | 158 Estimated sales (ES) | 164 Unit fixed costs (UFC) | 164 Unit variable costs (UVC) | 165 Unit sales tax and surcharges (USTAS) | 166

Contents |

2.3.4.5

Price of main feedstock materials on breakeven point (POMF on BEP) | 166

Changfeng Yan, Quanguo Zhang, and Shunni Zhu 3 Technologies for biomass-based hydrogen production | 169 3.1 Hydrogen energy | 169 3.1.1 Introduction | 169 3.1.2 Properties of hydrogen | 169 3.1.2.1 Physical properties of hydrogen | 169 3.1.2.2 Chemical properties of hydrogen | 170 3.1.2.3 Characteristics of hydrogen energy | 171 3.1.3 Hydrogen production from biomass | 172 3.1.4 Development and utilization of hydrogen | 173 3.2 Thermochemical routes for hydrogen production from biomass | 174 3.2.1 Introduction | 174 3.2.2 Fast pyrolysis of biomass to hydrogen | 175 3.2.3 Biomass gasification to hydrogen | 176 3.2.3.1 Introduction | 176 3.2.3.2 Biomass gasification reactor | 177 3.2.4 Hydrogen production from biomass gasification in supercritical water | 178 3.2.4.1 Reaction mechanism | 179 3.2.4.2 Reactors for biomass gasification in supercritical water | 181 3.2.5 Hydrogen production from biomass gasification via solid heat carrier | 182 3.2.5.1 Hydrogen production from biomass gasification with chemical looping process of carbonation/calcination cycle | 184 3.2.5.2 Chemical looping hydrogen production from biomass | 187 3.2.6 Hydrogen cleanup and purification | 188 3.2.6.1 Catalytic tar removal for hydrogen purification | 188 3.2.6.2 CO removal for hydrogen purification | 189 3.3 Hydrogen production from biomass derivatives | 194 3.3.1 Hydrogen production from bio-oil | 194 3.3.1.1 Properties of bio-oil | 194 3.3.1.2 Catalysts for hydrogen production via bio-oil steam reforming | 194 3.3.1.3 Bio-oil partial oxidation for hydrogen production | 198 3.3.1.4 Autothermal reforming for hydrogen production | 198 3.3.1.5 Reactors for hydrogen production from bio-oil reforming | 199 3.3.2 Hydrogen production from methanol | 199 3.3.2.1 Mechanism | 199 3.3.2.2 Hydrogen production methods | 200 3.3.2.3 Catalyst for methanol steam reforming | 201

V

VI | Contents

3.3.2.4 3.3.2.5 3.3.2.6 3.3.3 3.3.3.1 3.3.3.2 3.3.3.3 3.3.3.4 3.3.4 3.3.4.1 3.3.4.2 3.3.4.3 3.3.4.4 3.4 3.4.1 3.4.2 3.4.3 3.4.4 3.4.5 3.4.5.1 3.4.5.2 3.4.6

Methanol steam reforming reactor | 202 Demonstration of the system | 203 Numerical simulation | 204 Hydrogen production from ethanol | 205 Technologies of hydrogen production from ethanol | 205 Catalysts | 210 Catalyst support | 211 Catalyst promoter | 212 Hydrogen production from dimethyl ether | 212 DME reforming for hydrogen production | 213 Catalysts for hydrogen production from steam reforming | 214 Reactors and their performance for DME steam reforming | 216 Numerical simulation | 216 Biological hydrogen production | 217 Introduction | 217 Direct biophotolysis | 218 Indirect biophotolysis | 220 Photofermentation | 222 Dark fermentation | 224 Dark fermentative microorganisms | 225 Mechanism of dark fermentation and influencing factors | 227 Biological water-gas shift reaction | 231

Tiejun Wang, Longlong Ma, Yujing Weng, Junlin Tu, Mingyue Ding, Huijuan Xu, and Qi Zhang 4 Biomass synthetic fuel technology | 233 4.1 Catalytic synthesis of liquid fuel with synthesis gas | 233 4.1.1 The basic principle of liquid fuel synthesis from syngas | 235 4.1.2 Methanol synthesis from syngas | 236 4.1.2.1 Methanol synthesis process | 236 4.1.2.2 The reaction mechanism of methanol synthesis | 238 4.1.2.3 Research status of catalysts for methanol synthesis | 240 4.1.2.4 Demonstration and application of biomass gasification methanol synthesis | 241 4.1.3 Catalytic conversion of syngas to dimethyl ether | 243 4.1.3.1 Physical properties | 244 4.1.3.2 Production technologies for DME synthesis via syngas | 245 4.1.4 Catalytic conversion of syngas to mixed alcohols | 249 4.1.4.1 Thermodynamics of mixed alcohols synthesis from syngas | 249 4.1.4.2 Catalysts used for mixed alcohol synthesis | 250 4.1.4.3 Reaction mechanism of mixed alcohol synthesis | 252 4.1.4.4 Typical processes of mixed alcohol synthesis | 254

Contents |

4.2 4.2.1 4.2.1.1 4.2.1.2 4.2.1.3 4.2.2 4.2.3 4.2.3.1 4.2.3.2 4.2.3.3 4.2.4 4.2.4.1 4.2.4.2 4.2.5 4.2.6 4.3 4.3.1 4.3.2 4.3.3

VII

Biofuels synthesis via aqueous phase catalytic conversion of biomass | 258 Mechanism of aqueous phase catalytic conversion of biomass | 260 Hydrogenation of sugar into polyol | 261 Alkane production via aqueous-phase catalytic conversion of biomass | 264 Reactor design for aqueous conversion of sorbitol into alkane | 271 Alkane production by aqueous-phase catalytic conversion of biomass | 272 C8–C15 alkane production via aqueous-phase catalytic conversion of biomass | 275 Aldol condensation of furfural via acetone | 278 Jet fuel production from aldol condensation products | 279 Catalysts applied in aldol condensation of furfural with acetone | 280 Oxygenated liquid fuel production via aqueous-phase catalytic conversion of biomass | 280 Advantages of higher chain alcohols | 280 Synthesis of higher chain alcohols | 281 Diesel synthesis from syngas | 282 Gasoline synthesis via methanol from syngas | 283 Biofuel production via polymerization of low carbon number olefins | 286 Mechanism of polymerization of low carbon number olefins | 286 Gasoline production via polymerization of low carbon number olefins | 288 Jet fuel production via polymerization of low carbon number olefins | 289

Changzhu Li, Wen Luo, Zhihong Xiao, Lingmei Yang, Aihua Zhang, and Pengmei Lv 5 Technologies in vegetable oil and biodiesel | 291 5.1 Vegetable oils fuel | 291 5.1.1 Physicochemical properties of vegetable oils | 291 5.1.1.1 Physical and chemical properties of vegetable oils | 292 5.1.1.2 Characteristics of vegetable oil fuel | 294 5.1.2 The use of vegetable oils as diesel fuel | 296 5.1.2.1 Dilution of oils | 297 5.1.2.2 Microemulsion of oils | 297 5.1.3 Vegetable oil producing technology | 298 5.1.3.1 Basic principle and process of oil leaching method | 298 5.1.3.2 Basic principle of mechanical crushing method and process | 299 5.1.3.3 Oil refining | 299 5.1.4 Examples of rapeseed oil fuel tests | 302

VIII | Contents

5.1.4.1 5.1.4.2 5.1.5 5.1.5.1 5.1.5.2 5.1.5.3 5.2 5.2.1 5.2.1.1 5.2.1.2 5.2.2 5.2.2.1 5.2.2.2 5.2.2.3 5.2.3 5.2.3.1 5.2.3.2 5.2.3.3 5.2.3.4 5.2.4 5.2.4.1 5.2.4.2 5.2.5 5.2.5.1 5.2.5.2 5.2.5.3 5.2.5.4 5.2.6

German-made vegetable oil engine | 302 Contrast test of diesel and rapeseed oil | 302 Example of cottonseed oil-diesel blend fuel test | 303 Heating to reduce the viscosity of cottonseed oil | 303 The ratio of cottonseed oil to diesel | 304 Appraisal of cottonseed oil-diesel blend fuel | 305 Biodiesel technology | 306 Principle of biodiesel production | 306 Esterification | 306 Transesterification | 307 Technologies for biodiesel production | 309 Enzymatic transesterification methods | 309 Chemical methods | 309 Supercritical methanol method | 310 Processing and design for biodiesel production | 311 Process flow | 311 Catalysts | 313 Reactor | 317 Separation and purification of crude biodiesel | 321 Case studies for biodiesel engineering | 323 Processing technological analysis | 323 Economic analysis | 323 Global development of biodiesel | 326 China | 326 United States | 327 European union (EU) | 327 Other countries | 328 Environmental impact | 329

Dong Li, Xiaofeng Liu, Feng Zhen, and Haibin Li 6 Technologies of municipal solid waste treatment | 331 6.1 Characteristics of municipal solid waste | 331 6.1.1 Characteristics of municipal solid waste outside China | 331 6.1.2 Characteristics of domestic MSW | 331 6.1.3 Current situation of collection, transportation, and disposal of MSW in China | 334 6.1.3.1 Quantity of MSW collected and transported increases year by year | 334 6.1.3.2 Large-scale processing: An increase in processing capacity and decrease in the number of facilities | 336 6.1.3.3 Landfill-led, rapid development of incineration and a decrease in the composting ratio | 337

Contents |

6.2 6.2.1 6.2.1.1 6.2.1.2 6.2.1.3 6.2.1.4 6.2.1.5 6.2.2 6.2.2.1 6.2.2.2 6.2.2.3 6.2.3 6.2.3.1 6.2.3.2 6.2.3.3 6.2.4 6.2.4.1 6.2.4.2 6.3 6.3.1 6.3.1.1 6.3.1.2 6.3.1.3 6.3.2 6.3.2.1 6.3.2.2 6.3.3 6.3.3.1 6.3.3.2 6.3.3.3 6.3.4 6.3.4.1 6.3.4.2 6.3.4.3 6.4 6.4.1 6.4.1.1 6.4.1.2 6.4.2 6.4.2.1 6.4.2.2 6.4.3

IX

MSW treatment and application technology | 339 Sanitary landfill | 339 Reaction mechanisms in landfill process | 339 Seepage prevention at a landfill site | 340 Leachate treatment | 342 Gathering and applications of landfill gas | 344 Closure of a landfill site | 345 Incineration for power generation | 345 Waste incineration process | 345 Waste incinerator | 346 Process flow of waste incineration | 351 Aerobic compost | 352 Operating principle | 353 Raw materials and technological parameters | 355 Composting process system | 356 Anaerobic digestion | 358 System composition | 358 Anaerobic digestion of organic waste | 359 Cases of urban domestic waste treatment | 364 Liulitun Waste Sanitary Landfill, Beijing | 364 General introduction to the landfill | 364 Engineering contents of the landfill | 365 Leachate treatment | 367 Likeng waste incineration power plant, Guangzhou | 369 General | 369 Process flow and main equipment | 370 Nangong domestic waste composting plant, Beijing | 372 General | 372 Process description | 372 Cost-benefit analysis | 373 Heishizi anaerobic digestion plant for kitchen waste, Chongqing | 373 General | 373 Process system | 374 Product scheme | 375 Outlook for MSW | 375 Separate collection of MSW | 375 Classification and collection of MSW outside China | 375 Classification of China’s advanced cities and collection of MSW | 377 Mechanical sorting technology of mixed MSW | 379 Actuality of MSW sorting technology | 379 Main equipment for sorting MSW | 379 Mechanical-biological treatment of MSW | 384

X | Contents

Xiaoying Kong, Gaixiu Yang, Ying Li, Dongmei Sun, and Huan Deng 7 Microbial fuel cells | 387 7.1 The basics of microbial fuel cells | 387 7.1.1 The historical development of microbial fuel cells | 387 7.1.2 The technological development of microbial fuel cell | 389 7.1.3 The conductive mechanism of the cell | 391 7.1.3.1 The electrolytic cell | 391 7.1.3.2 Original battery | 391 7.1.4 The working principle of the MFC | 391 7.1.5 The mechanism of electron transfer of microbially produced electricity | 393 7.1.5.1 Redox mediator transfer | 394 7.1.5.2 Direct electron transfer | 394 7.1.5.3 Nanowire transfer | 395 7.2 Microbial fuel cell technology | 395 7.2.1 Electrogenesis microorganism | 395 7.2.1.1 Electrogenesis bacteria | 396 7.2.1.2 Fungi | 398 7.2.1.3 Chlamydomonas reinhardtii | 399 7.2.2 MFC substrate | 399 7.2.2.1 Small organic molecules | 399 7.2.2.2 Alcohols | 400 7.2.2.3 Sugar | 400 7.2.2.4 Organic wastewater | 401 7.2.3 MFC materials | 401 7.2.3.1 Anode materials | 402 7.2.3.2 Cathode Materials | 403 7.2.3.3 Membrane | 404 7.2.4 MFC configurations | 404 7.2.4.1 Two-chambered MFC | 404 7.2.4.2 Single-chambered MFC | 405 7.2.4.3 MFC batteries | 406 7.3 Characterization techniques of MFCs | 406 7.3.1 Electrochemical techniques | 406 7.3.1.1 Cyclic voltammetry (CV) | 407 7.3.1.2 Chronoamperometry (CA) | 409 7.3.1.3 Chronopotentiometry (CP) | 410 7.3.1.4 Polarization curves (PCs) | 410 7.3.1.5 Power curves | 411 7.3.2 Coulombic efficiency | 412 7.3.3 Resistance | 412 7.3.4 Degradation efficiency | 414

Contents |

7.3.5 7.3.6 7.4 7.4.1 7.4.2 7.4.2.1 7.4.2.2 7.4.2.3 7.4.3 7.4.3.1 7.4.3.2 7.4.3.3 7.4.4 7.4.5

Energy efficiency | 414 Other characterization techniques | 415 The application and functional extension of MFCs | 415 Electrogenesis | 416 Pollutants remediation and waste reclamation | 417 Treatment of wastewater | 417 Recovery of solid organic waste | 419 Remediation of polluted soil | 420 Biosensors | 420 BOD Biosensors | 422 Toxicity detection biosensors | 422 Soil pollutant detection | 422 Desalinization | 423 Hydrogen production | 426

References | 429 Index | 455 Also of interest | 459

XI

Yu Zhang, Jingliang Xu*, Yanling Jin, Xinshu Zhuang, Hai Zhao*, Yunyun Liu, Qiang Yu, Guixiong Zhou, and Mingsong Xiao

1 Biomass ethanol fuel technology

1.1 Characteristics and application of ethanol 1.1.1 Physical and chemical properties of ethanol Ethanol, C2 H5 OH, is the second member of the aliphatic alcohol series; and it is a colorless, flammable and transparent liquid with a special flavor (volatile) and slight chemical odor. The density of ethanol is less than that of water, and it is miscible with water in all proportions (in general, it cannot be used as an extraction agent). Ethanol is an important solvent, which is miscible with not only water, but also chloroform, ether, methanol, acetone, and most other organic solvents. When exposed to air, its vapor can be transformed to explosive mixtures. The bonds between the hydrogen and carbon atoms in ethanol are nonpolar covalent bonds. The hydrogen–carbon and carbon–oxygen bonds are polar covalent bonds based on sp3 hybrid orbitals. Ethanol can be seen as an ethane molecule in which Yu Zhang: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China *Corresponding Author: Jingliang Xu: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China; E-mail addresses: [email protected] (J. Xu), Tel. +86-20-37029697 Yanling Jin: Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China Xinshu Zhuang: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China *Corresponding Author: Hai Zhao: Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China; E-mail addresses: [email protected] (H. Zhao), Tel. +86-28-82890725 Yunyun Liu: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China Qiang Yu: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China Guixiong Zhou: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China Mingsong Xiao: Biomass Energy Committee, Chinese Association of Rural Energy Industry, Beijing 100125, China https://doi.org/10.1515/9783110476217-001

2 | 1 Biomass ethanol fuel technology

one hydrogen atom has been substituted by a hydroxyl (–OH) group or as a water molecule in which one hydrogen atom has been replaced by an ethyl group (Fig. 1.1). The carbon–oxygen and hydrogen–oxygen bonds in ethanol can be broken relatively easily.

H

H

H

C

C

H

H

O

H

Fig. 1.1: Schematic illustration of molecular structure of ethanol.

1.1.2 Performance comparison of ethanol and gasoline/diesel The natural physicochemical differences between ethanol and gasoline/diesel are listed in Tab. 1.1 Ethanol is an oxygenate, i.e., ethanol molecules contain oxygen; therefore, it burns easily compared to gasoline and diesel. Oxygen atoms inside ethanol join forces with oxygen molecules in the air to help ethanol burn more completely, which reduces above 30 % carbon monoxide (CO) and hydrocarbon emission. However, compared to gasoline and diesel, ethanol has several disadvantages such as high vaporization heat, high ignition temperature, and low cetane number; therefore, the difficulty related to engine starting is slightly increased. Ethanol has lower heating value than gasoline and diesel; however, its cetane number is higher, thus high oil consumption caused by low heating value could be offset by high compression. Tab. 1.1: Physicochemical indexes comparison between ethanol and gasoline/diesel. Physicochemical indexes

Ethanol

Gasoline

Diesel

Carbon number Molecular weight Oxygen content Density (20 °C) in kg m−3 Boiling point in °C Freezing point in °C Flashing point (closing) in °C Viscosity (20 °C) in mPa S Vaporization heat in kJ kg−1 Low heating value in MJ kg−1 Ignition temperature in °C

2 46 34.73 0.7893 78.4 −117.3 13–14 1.2 0.854 26.778 434

5–12 95–120 0 0.72–0.78 40–210 −60 – −56 −45 – −38 0.28–0.59 0.31–0.34 43.9–44.4 350–468

10–21 180–200 0 0.83–0.86 180 – −370 −35–10 65–88 3.0–8.0 0.25–0.30 42.5–42.8 270–350

1.2 Production theory of bioethanol

| 3

1.2 Production theory of bioethanol Bioethanol is a clean and alternative fuel that produced from a variety of available lignocellulosic feedstocks by various conversion technologies. It can be usually used as a gasoline additive to increase octane number and improve vehicle emissions. Gasoline blended with ethanol burns cleaner than pure gasoline. When it is used as a power fuel, bioethanol can be called fuel ethanol. The use of bioethanol in gasoline and diesel blends can reduce carbon dioxide (CO2 ) emissions for each kilometer driven compared to gasoline, thus mitigating the greenhouse gas effect. Bioethanol is prepared by fermentation mostly from carbohydrates produced in sugar or starch crops and can ease the energy crisis [1]. Its coproducts are also extremely valuable, such as fusel oil that can be used generally as organic solvent, and is an important raw material for the spice industry; stillage with rich nutrients can be used as feed after drying gravity; liquid stillage can be fermented to biogas through anaerobic digestion, and its residue and biogas slurry are capable of being used as organic fertilizer.

1.2.1 The main methods of ethanol production Bioethanol can be produced essentially in the following two ways: chemical conversion and biological fermentation. It is mainly produced by using sugar-based crops (sugar beet, sugarcane, etc.), starch-based crops (corn, wheat, potatoes, etc.), and lignocellulosic biomass (straw, etc.) by hydrolysis and fermentation processes, although it can also be produced by chemical processes involving the reaction of ethylene with steam [1, 2]. Thus, the main sources of ethanol are as follows: – fermentation of sugar derived from grain starches (wheat and corn), sugar beets, or sugar crops using microorganisms; – fermentation of the non-sugar lignocellulose fractions of crops (grasses and trees); and – high temperature catalytic conversion of synthesis gas to liquid fuels by the Fischer–Tropsch process to produce a mixture of alcohols.

1.2.1.1 Microbial fermentation Fermentation is a metabolic process that converts sugar to alcohol and other coproducts using microorganisms such as yeast with its specific metabolic enzymes and through a complex biochemical reaction. Biocatalyst yeast can convert six-carbon (6C, hexoses) sugars into ethanol and CO2 and obtain energy under anaerobic conditions using the Embden–Meyerhof–Parnas (EMP) pathway. Five-carbon (5-C, pentoses) sugars can also be converted to ethanol and other by-products; however, through a more complex metabolic process than glucose [3].

4 | 1 Biomass ethanol fuel technology

Production of ethanol from lignocellulose is more difficult compared to sources such as starch and other food crops. There are several approaches for ethanol production from lignocellulosic feedstocks, and they all need to satisfy the following characteristics [4]: (1) effective degradation of cellulose and hemicellulose to sugars; (2) effective co-fermentation of 6-C and 5-C sugars; (3) advanced technology integration in order to reduce the process energy consumption; (4) lignin removal from raw material to reduce the cellulosic ethanol cost. The two ways of producing ethanol from lignocellulose are: sugar-based platform (biochemical conversion) and syngas platform (thermochemical conversion) [1], as shown in Fig. 1.2. The cellulolysis process involves hydrolysis of pretreated lignocellulosic materials, followed by fermentation and distillation. Gasification transforms the lignocellulosic materials into gaseous CO, CO2 , and hydrogen (H2 ), and these gases can be converted to ethanol by microbial fermentation or chemical catalysis.

1.2.1.2 Syngas to ethanol The thermochemical approach of the gasification process for ethanol production does not rely on chemical decomposition of cellulose chains. Instead of breaking the cellulose into sugar molecules, the carbon in the biomass is converted into synthesis gas including CO, CO2 , and H2 , with partial combustion in the catalytic reactor. These gases then can be converted into ethanol using microorganisms such as the Clostridium ljungdahlii by fermentation or a thermochemical process [5]. Gasification of biomass occurs in the controlled presence of oxygen with only limited combustion at high temperature (750–800 °C), with the products including gaseous mixture of CO, H2 , methane (CH4 ), nitrogen (N2 ), CO2 , and some higher hydrocarbons (25–30 % H2 , 40–65 % CO, 1–20 % CO2 , and 0–7 % CH4 ), also small amount of sulfur and nitrogen compounds. The main components of syngas are CO and H2 , which can be converted to ethanol by Fischer–Tropsch synthesis or microbial fermentation after the removal of the contaminants such as tar and solid particles [6]. In general, the Fischer–Tropsch process is nonspecific and operates at high temperature and high pressures (typically 315 °C, 8.2 Mpa). Its products comprise not only ethanol, but also methanol, butanol, and some high molecular weight alcohols, aldehydes, and even some ketones. However, the biochemical conversion process, with high selectivity, high yield, and low power consumption, leads to higher conversion efficiency, for its irreversible bioreaction can avoid the limitation of thermodynamic equilibrium compared to the chemical catalytic process [7].

1.2 Production theory of bioethanol

| 5

1.2.2 Biochemical process of ethanol fermentation The biochemical process of ethanol production from cellulosic biomass and starch can be summarized into the following stages: Degradation of carbohydrates such as starch, cellulose, hemicellulose, and so forth into monosaccharides. This is followed by the formation of pyruvate from glucose, xylose, and other monoses via glycolysis. Then pyruvate is reduced to ethanol under anaerobic conditions, releasing CO2 . Most of the fermentation strains can metabolize sucrose and other disaccharides to monose and enter directly into the glycolytic process [8, 9]. In ethanol fermentation process with glucose, the intermediates pyruvate may be converted to different end products, such as ethanol, lactic acid, etc. depending on the reaction conditions [9]. The basic chemical equation representing the ethanol fermentation of glucose is shown in Fig. 1.2. OH

O CH2OH H

O H OH H + Yeast

OH OH H OH Glucose

C C

OH CH2 +

O

CH3 2 Pyruvate

2CO2

CH3 2 Ethanol

Fig. 1.2: The basic chemical equation representing ethanol fermentation with glucose.

1.2.2.1 Carbohydrate degradation Hydrolysis is the basic chemical reaction of the digestive process of carbohydrates such as starch, cellulose, and hemicellulose. There are some difficulties encountered in the direct fermentation of these substrates for most ethanol fermentation strains do not have the ability to degrade polysaccharides. Typically, monosaccharides in raw materials are linked together by glycosidic bonds, which should be cleaved by hydrolysis via chemical or biochemical approaches before fermentation.

1.2.2.1.1 Degradation of the starch material The hydrolysis of starch can be carried out enzymatically or through acid-mediated processes, and the final hydrolysate is glucose, the hydrolysis reaction is represented as follows: acids/amylases, H2 O

(C6 H10 O5 )n 󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀→ α-1,4-Oligomeric glucose acids/amylases, H2 O

α-1,4-Oligomeric glucose 󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀→ nC6 H10 O5

(1.1) (1.2)

In the acid treatment, the glycosidic linkages of the starch molecules are broken down by acid catalysis, and large molecules are degraded to small ones. Starch is composed

6 | 1 Biomass ethanol fuel technology

of two polysaccharides: amylose, composed of linear chains of glucose linked by α1,4 bonds, and amylopectin (α-1,4 and α-1,6 bonds) which is the major macromolecular component and responsible for the architecture of the starch granules. The α-1,4 and α1,6 glycosidic linkages have different hydrolysis characteristics under acid hydrolysis. Amylose, with special and complicated structure including compact crystalline shape formed by hydrogen bonding in molecules, is resistant to acid penetration. In contrast, amylopectin has a random, amorphous, and branched structure with little resistance to hydrolysis, and it is more easily hydrolyzed by acids to its monomer components. The two steps in acid hydrolysis of starch are: rapid degradation of amorphous regions and the slow hydrolysis of the crystalline regions in amylose and amylopectin. Amylases are digestive enzymes which hydrolyze glycosidic bonds of starch to glucose. The depolymerization of starch is performed in two steps (liquefaction and saccharification). During the liquefaction process, α-amylases convert starch into dextrins along with small amounts of oligosaccharides. In the saccharification process, dextrins and oligosaccharides are further decomposed into glucose and other monosaccharide molecules [9].

1.2.2.1.2 Lignocellulose degradation Lignocellulosic biomass with complex structure is generally composed of cellulose, hemicellulose, and lignin. Cellulose is a polysaccharide consisting of a linear chain of several hundreds to many thousands of β(1→4) linked D-glucose units. The multiple hydroxyl groups on the glucose from one chain form hydrogen bonds with oxygen atoms on the same or on a neighboring chain, thus holding the chains firmly together side-by-side and forming microfibrils with high tensile strength. Hemicellulose with noncrystalline structure is mainly composed of xylose, arabinose, and mannose, and it is susceptible to be hydrolyzed to monosaccharides. Lignin with a three-dimensional polymer structure is connected based on the phenylpropanoid structural unit through ether and hydrocarbon bonds, and it cannot be hydrolyzed. Lignin plays a protective role in the cellulose periphery, thus preventing the hydrolysis and fermentation efficiency of the substrates [10]. Before hydrolysis, lignocellulose is subjected to pretreatment to reduce the crystallinity of cellulose with the objective of improving the hydrolysis efficiency. Pretreatment can be achieved by physical, chemical, physicochemical, and biological methods. After pretreatment, the structure of the substrate becomes loose, lignin is partially removed, the cellulose and hemicellulose are exposed to the surface, which lead to the increase in the accessible surface area of the cellulose and help in the further degradation of cellulose to monosaccharides. Cellulosic biomass is hydrolyzed mainly by acids or enzymes to produce monosaccharides (mainly glucose and xylose), and the hydrolysis reaction equation is follows: acids/enzymes, H2 O

(C6 H10 O5 )n 󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀→ β-1,4-Oligomeric glucose

(1.3)

1.2 Production theory of bioethanol | 7

acids/enzymes, H2 O

β-1,4-Oligomeric glucose 󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀→ nC6 H10 O5

(1.4)

Xylan in hemicellulose hydrolysis reaction is: acids, H2 O

(C5 H8 O4 )m 󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀→ mC6 H12 O6

(1.5)

Acids treatment can improve the hydrolysis efficiency of substrate; however, it generates acetic acid, furfural, and other fermentation inhibitors. Moreover, the product can be decomposed when the acid strength is greater, which then reduces the sugar yield. Furthermore, the acid hydrolysate also needs detoxification and neutralization [11]. Enzyme hydrolysis using cellulases under mild conditions does not generate inhibitors and the enzymes are very specific for cellulose. Cellulase, mainly derived from fungi such as Trichoderma reesei, is a mixture of at least three different enzymes including endo-glucanase, exo-glucanase, and β-glucosidase. There are three types of reaction catalyzed by cellulases: The first reaction involves the breaking of the noncovalent interactions present in the amorphous structure of cellulose (endo-glucanase), the second is the hydrolysis of chain ends to break the polymer into smaller sugars (exo-glucanase), and third is the hydrolysis of disaccharides and tetrasaccharides into glucose (β-glucosidase). Enzymatic hydrolysis has high requirements on pretreatment, and large amounts of enzymes are needed. Finding the microbials with highly efficient enzyme productivity, improving the hydrolysis efficiency, and reducing the loading of enzyme are the research directions in the enzymatic hydrolysis process [12].

1.2.2.2 Glycolysis Fermentation is a metabolic process and it is a form of microbials’ anaerobic digestion involving the use of specific enzymes that catalyze and oxidize certain organic compounds, such as saccharides, amino acids, or organic acids. Sugars including hexoses and pentoses are the main fermentation substrates. Glycolysis is the metabolic pathway that converts glucose into pyruvate. The most common type of glycolysis is the EMP pathway, which uses a series of reactions for oxidizing glucose to pyruvate and many bacteria employ this pathway in their catabolism. Glycolysis also refers to other pathways, such as the Entner–Doudoroff pathway and various heterofermentative and homofermentative pathways. The discussion here is limited to the EMP pathway [13]. The EMP pathway is the most commonly used series of reactions for the vast majority of organisms under anaerobic conditions. The entire EMP pathway is a sequence of ten reactions involving ten intermediate compounds, and can be separated into two phases: (1) The preparatory phase in which adenosine triphosphate (ATP) is consumed; glucose is converted into two three-carbon sugar phosphates by phosphorylation and isomerization cracking; oxidation–reduction reaction does not occur; and two molecules of intermediate glyceraldehyde-3-phosphate are generated. (2) The payoff phase in which ATP is produced; pyruvate is formed by the enzymes including

8 | 1 Biomass ethanol fuel technology

dehydrogenase, kinase, mutase, enolase, and pyruvate kinase; catalytic conversion of glyceraldehyde-3-phosphate occurs, oxidation–reduction reactions occur which are accompanied by the formation of energy-rich ATP molecules and reduced coenzyme NADH. The product includes two molecules of pyruvate, i.e., one molecule of glucose produces two molecules of pyruvate, ATP, and NADH by ten-step reaction. The overall process of glycolysis is as follows: ATP→ADP

Glucose 󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀→ Glucose-6-phosphate → Fructose-6-phosphate → Glyceraldehyde-3-phosphate

(1.6)

NAD+ →NADH+H+ , ADP→ATP

Glyceraldehyde-3-Phosphate 󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀→ Phosphoenolpyruvic acid ADP→ATP

󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀→ Pyruvic acid C6 H12 O6 +2NAD+ +2ADP+2Pi → 2CH3 COCOOH+2NADH+2H+ +2ATP+2H2 O

(1.7) (1.8)

1.2.3 Ethanol fermentation microbiology Ethanol is produced by the fermentation of starch, sugar, or cellulosic biomass by microbials’ metabolism. Biomass should be degraded to monosaccharides before fermentation because most microbes lack or have low ability to digest polysaccharides. The basic process can be summarized as follows: hydrolysis

fermentation

Biomass 󳨀󳨀󳨀󳨀󳨀󳨀󳨀→ Sugars 󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀→ Ethanol

(1.9)

Starch commonly uses mold fungi as a catalyst to produce amylase; however, lignocellulose requires pretreatment before acid or enzyme hydrolysis due to its complex structure. Two types of microorganisms in the ethanol fermentation industry are: mold fungi which produce hydrolytic enzymes such as amylase or cellulase; the other is ethanol fermentation microorganisms which mainly include yeast, bacteria, and mold. Different strains have different fermentative capacity (including ethanol production, sugar utilization and tolerance, etc.), even for the strains isolated from the same species, the production capacity is also uneven. Ethanol-producing bacteria employed in practical processes should meet the following characteristics according to the requirement of industrial production [14]: (1) Capable of producing high efficiency enzymes with high fermentation ability, for rapid and complete conversion of sugars to ethanol. (2) High speed of proliferation with a high specific growth rate and the ability to reproduce. (3) Resistant to high concentrations of sugar and ethanol, with high fermentation ability in concentrated fermentation broth.

1.2 Production theory of bioethanol | 9

(4) Strong resistance against antibacteria components and organic acids. (5) Well adapted to the complex medium components. (6) Adaptability to mutation due to temperature, acidity, and salinity in the environment. (7) Have DNA stability and low variability.

1.2.3.1 Ethanol fermentation microbiology based on a sugar platform Of the numerous microorganisms existing in nature which are capable of converting glucose into ethanol, such as yeasts, molds and bacteria; only a few strains can be successfully applied to large-scale bioethanol production from lignocellulosic biomass. Currently, the main ethanol production strains in industries are yeast strains. Saccharomyces cerevisiae is regarded as an industrial workhorse for ethanol production, for it can produce ethanol in high yield under a wide range of conditions and low pH requirement using hexose sugars and has high ethanol tolerance (up to 23 %, v/v). Bacteria are easily contaminated and susceptible to inactivation by contamination phage because of mild fermentation conditions with pH greater than 5.0. Phage contamination can often lead to catastrophic fermentation failure. Using the thallus as feed remains controversial [15].

1.2.3.1.1 Yeast Yeasts are eukaryote, which generally exist in a single cell state. Most yeasts reproduce asexually by mitosis, and many do so by an asymmetric division process called budding. They like growing in an environment with high sugar content and acidity, and can ferment sugars; however, the biggest limitation is lack of ability to utilize pentose. Yeasts cells are stubby-shaped, some with false hyphae, their colonies are relatively flat and their surface and edges are rough. Some yeasts are without false hyphae, and the colonies are more uplifted with very rounded edges [16]. The shapes and sizes vary for different yeast strains. The typical morphology is shown in Fig. 1.3.

(a)

(b)

(c)

Fig. 1.3: Yeast morphology: (a) Candida tropicalis; and (b,c) Saccharomyces cerevisiae.

10 | 1 Biomass ethanol fuel technology

Yeast can survive in a wide range of temperatures, normal values range between 28 and 34 °C. The strain vitality declines or stops when the yeasts are under stress by keeping them at the lowest or the highest end of temperature tolerance range (the optimum growth temperature can reach as high as 40 °C or more). Higher temperature in the range 50–60 °C results in its death in five minutes, and in the lower temperature corresponding to 5–10 °C yeast only maintains its slow growth rate. The cells can survive freezing under certain conditions, with viability decreasing over time. Yeast prefers slightly acidic conditions to work the best, and a pH ranging from 3.8 to 5.5 gives the best results. When pH is less than 3.5, its growth is inhibited [17]. Yeast physiology can be either obligate aerobic or facultative anaerobic. In the presence of oxygen, yeast multiplies through a mechanism known as budding, and little ethanol is generated. However, in the absence of oxygen, the cells become weak, fermentative yeasts produce their energy by converting sugars into CO2 and ethanol (alcohol). Therefore, ventilation is required in the early stage of fermentation to make the cells multiply, which lasts till the end of the cells’ logarithmic growth phase, when ethanol anaerobic fermentation proceeds. Not all natural yeasts are appropriate for use at a large scale, the strains applied in production should possess the aforementioned fundamental characteristics of the industrial strains. For molasses fermentation, the cells must also have high resistance to osmotic pressure, acids, temperature, and metals, in particular Cu2+ , and produce less foam. Saccharomyces cerevisiae, Rasse II (German II) yeast, Rasse XII (Germany 12th ) yeast, K yeast, and NanYang V (Saccharomyces 1300) are the most frequently used microorganisms for ethanol production in industrial processes [18].

1.2.3.1.2 Bacteria Ethanol fermentation by bacteria was initiated in the 1970s, and it is still in the research and development stage. Thermophilic anaerobic microorganisms with rapid metabolic rate, short fermentation period, and high ethanol yield are the popular fermentation bacteria. Thermophilic anaerobic bacteria exhibit a distinct advantage over conventional yeasts for bioethanol production in their ability to use a variety of inexpensive biomass feedstocks and their rapid and high efficiency towards pentose hydrolysis, sometimes even as rapid as the hydrolysis of the hexose. The process has high ethanol conversion efficiency, while it generates less thallus [19]. Bacteria are able to use a variety of sugars; however, they contain several coproducts in their fermentation broth, thus their ethanol fermentation yield is relatively low. Most bacteria favor conditions near neutral pH 7, which increases the risk of contamination of the fermentation process. Moreover, some bacteria release toxins or poisons which increase the difficulty of products separation and purification, and enhance the production cost. Bacteria used for industrial ethanol fermentation include Clostridium sphenoides, Zymomonas mobilis, Spirochaeta aurantia, Erwinia amylovora, Leuconostoc

1.2 Production theory of bioethanol | 11

Tab. 1.2: Characteristics of potential organisms in bioethanol fermentation. Species

Characteristics Advantages

Saccharomyces Facultative cerevisiae anaerobic yeast

Rasse II

Facultative anaerobic yeast

Rasse XII

Facultative anaerobic yeast

K Yeast

Facultative anaerobic yeast Facultative anaerobic yeast

NanYang ? (CICC1300)

Saccharomyces Facultative carlsbergensis anaerobic yeast

Kluveromyces marxianus

Thermophilc yeast

Naturally adapted to ethanol fermentation, high alcohol yield (90 %), high tolerance to ethanol and chemical inhibitors, and amenability for genetic modifications Thrives on corn mash and adapted to starch feedstock for ethanol fermentation Forms ascospores more easily than Rasse II and rich in glycogen Reproduces rapidly

Drawbacks

Fermentable substrate

Not able to ferment xylose and arabinose sugars, and not able to survive high temperature of enzyme hydrolysis

Glucose, fructose, galactose, maltose, and xylulose

Not able to ferment lactose Glucose, and low tolerance to high sucrose, and maltose salts Not able to ferment lactose Glucose, and low tolerance to high fructose, sucrose, salts maltose, and galactose —— Glucose, xylose, and starch

Maltose, glucose, sucrose, and raffinose Ferments Not able to ferment lactose Glucose, polysaccharides maltose, sucrose, galactose, and melibiose Able to grow at a high Excess of sugars affect its 8538BGlucose, xylose, etc. alcohol yield, temperature above low ethanol tolerance, and 52 °C, suitable for fermentation of xylose is SSF/CBP system, reduces cooling cost poor and leads mainly to the formation of xylitol and contamination, and ferments a broad spectrum of sugars High fermentation —— Glucose, xylose, efficiency and molasses

High tolerance to ethanol (up to 13 % v/v)

Schizosaccharo Facultative mycespombe anaerobic yeast Candida Microaerophilic Ferments xylose shehatae yeast

Not able to ferment lactose, synanthrin, and melibiose

Low ethanol tolerance, Glucose and low ethanol yield, requires xylose micro-aerophilic conditions, and does not ferment xylose at low pH

12 | 1 Biomass ethanol fuel technology

Tab. 1.2: (continued) Species

Characteristics

Advantages

Drawbacks

Fermentable substrate

Candida utilis

Facultative anaerobic yeast Aerobic fungus

Grows rapidly, high adaptability for medium Ferments xylose

——

Glucose, xylose, molasses, and potato starch Glucose and xylose

Resistance to an extremely high temperature of 70 °C, suitable for SSCombF/CBP system. Able to ferment most sugars and can hydrolyze cellulose

Low tolerance to ethanol

Glucose, xylose, and starch

Low tolerance to ethanol

Glucose, cellobiose, and cellulose

Pachysolen tannophilus

Zygosaccharomyces rouxii

Facultative anaerobic yeast

Ps. Lindneri Zymomonas mobilis

Bacterium Ethanologenic gram-negative bacteria

Pichia stiplis

Facultative anaerobic yeast

Escherichia coli Mesophilic gram-negative bacteria

Thermophilic bacteria: Thermoanaero bacterium saccharolyticum

Clostridium thermocellum

Extreme anaerobic bacteria

Extreme anaerobic bacteria

Low ethanol yield, requires micro-aerophilic conditions, does not ferment xylose at low pH Glucose Resistant to high Respiratory activity osmolarity and pH and salt resistance ability are inhibited in acid Glucose Glucose, Not able to ferment High alcohol yield (97 %), high tolerance xylose, low tolerance fructose, and sucrose to inhibitors, and to ethanol and neutral pH range chemical inhibitors, and amenability for genetic modifications Glucose, xylose, Cannot ferment Best performance mannose, xylose at low pH, towards xylose fermentation, ethanol sensitive to chemical galactose, and cellobiose yield is 82 % and able inhibitors, requires micro-aerophilic to ferment most of conditions to reach cellulosic-material peak performance sugars 8602BGlucose Limited ethanol Ability to use both and xylose tolerance, narrow pentose and hexose temperature and pH sugars and growth range, and amenability for genetic modifications 8601B-produces organic acids

1.2 Production theory of bioethanol

| 13

mesenteroides, Thermoanaerobacter ethanolicus, and thermophilic Bacillus. Ethanol fermentation microorganisms and their characteristics are listed in Tab. 1.2 [20–25].

1.2.3.2 Pentose fermenting microbes A range of research progress has been achieved in pentose fermentation since the 1970s, including the selection of the direct xylose fermenting bacteria, fungi, and yeast, and the construction of xylose-fermenting high-performance engineered bacterial strains. So far, more than one hundred types of microbes are capable of fermenting xylose such as bacteria, neurospora, mucor, rhizopus, and other fungi have already been found. Metabolic engineering of a large number of bacterial strains for production of ethanol from xylose has already been well established [26]. Bacteria with rapid xylose metabolism can not only ferment sugars, but also cellulose and bio-high glycans. Zymomonas mobilis is a natural fermentative bacterium that has been studied for decades due to its remarkable ethanologenic properties, with nearly 97 % of ethanol yield. Thermoanaerobacter ethanolicus can produce ethanol from xylose at a continuously high temperature of 68 °C, with ethanol yield up to 0.44 g/g. Clostridium thermosaccharolyticum is able to metabolize xylose, arabinose, mannose, fructose, glucose, sucrose, and cellobiose at temperatures ranging from 55– 60 °C [27]. The main xylose fermenting fungi are Neurospora crassa and Fusarium oxysporum. These two species are currently used for simultaneous saccharification and fermentation, with properties that can produce cellulases and hemicellulases, and ferment five- and six-carbon sugars. However, their xylose fermentation ability has not yet been studied in depth [28, 29]. There are six types of genera in yeast capable of xylose fermentation, including Pachysolen tannophilus, Candida shehatae, Pichia stipitis, Pichia guilliermondii, Brettanomyces anomalus, and Candida utilis. These natural yeasts for xylose metabolism have a high ethanol production rate. However, they cost high energy with strict production regulatory requirements, and must operate under restrictive conditions. Moreover, the yeasts with low tolerance to inhibitors and high concentration ethanol have a large number of co-products in the hydrolysate, which need detoxification and distillation; thus increasing the production costs. At the same time, metabolism of five-carbon sugars is inhibited by sixcarbon sugars, and there are certain difficulties in their large-scale industrial application. Microorganisms involved in xylose metabolism are listed in Tab. 1.3 [27–32].

1.2.4 Breeding of ethanol fermentation microorganisms The fermentable sugar derived from enzymatic hydrolysis can be converted to ethanol, and this conversion can be effectively completed by yeasts. Moreover, some bacteria

14 | 1 Biomass ethanol fuel technology

Tab. 1.3: The main microorganisms in xylose metabolism. Species Bacterium Clostridium thermocellum Clostridium thermohydrosulfuricum Thermoanaerobacter ethanolicus Clostridium thermosaccharolyticum Zymomonas mobilis Thermoanaerobium brockii Thermoanaerobacter acetoethylicus Fungus Neurospora crassa Fusarium oxysporum Pichia stipitis Yeast Pachysolen tannophilus Candida shehatae Pichia stipitis Pichia guilliermondii Candida utilis

Optimum temperature

Optimum pH

Main fermentation products

60–65 °C

6.5–7.0

65 °C

6.9–7.5

Acetic acid, formic acid, lactic acid, and ethanol Ethanol

60–65 °C

5.5–8.5

55–60 °C

6.0–7.0

25–31 °C 65 °C

3.5–7.5

Ethanol and acetic acid Ethanol, acetic acid, and lactic acid Ethanol, acetic acid, and lactic acid

30–37 °C 25 °C 30 °C

5.0 6.2–6.75 5.0

Ethanol Ethanol Ethanol

28–32 °C 28 °C 25–30 °C 28 °C 28–32 °C

5.0 4.5–5.0 4.6–5.8 5.0 4.5–6.5

Ethanol and xylitol Ethanol and acetic acid Ethanol and xylitol Ethanol and xylitol Ethanol

55–75 °C

Ethanol, acetic acid, and lactic acid Ethanol

such as Zymomonas mobilis can also perform this function. The metabolic properties of the fermentation strain directly determine the final ethanol yield, thus the breeding and construction of strains with high ethanol/temperature tolerance and high ability to utilize pentose and hexoses, are the key points to improve the conversion rate. In addition to these microbes, bioengineering technology has studied breeding and constructing high-performance fermentation strains.

1.2.4.1 High ethanol tolerant strains Ethanol tolerance is one of the important characteristics of ethanol-producing microbials. It can be improved by screening, mutagenesis, protoplast fusion, and gene recombination. These are the most economical and practical methods to directly screen the ethanol tolerant yeasts from natural microorganisms to investigate their utility for industrial exploitation. Yeast has long-evolved stable genetic performance, high yield, and tolerance. In 1990, Ernandes et al. isolated two yeasts, namely Et-2 and Et-4, from crude recycled yeast samples. Ethanol concentrations of 18.4 % (v/v) and 18.5 % (v/v) were obtained when the isolated yeasts were cultivated in medium containing 35 % su-

1.2 Production theory of bioethanol | 15

crose syrup. Argirious isolated Saccharomyces cerevisiae AZA21 strains which could ferment glucose syrup from vineyard soil, and produced 17 % (v/v) ethanol. Not only the fermentation time was short, but also the fermentation performance of this strain was very stable [33, 34]. Protoplast fusion technology is an ideal method to improve alcohol tolerance of yeast at present. The method involves the mixing of protoplasts of two complementary auxotrophic progeny strains and screen fusants. It can achieve gene recombination of genetically different strains of different origin and without any genetic background, thus strains with multiple good traits can be obtained which improve the efficiency of the strain’s breeding. According to the literature, fusion strains developed by protoplast fusion technique could tolerate 11 % ethanol (v/v), and the ethanol production rate of this strain was found to be significantly higher than that of the parent strains [34]. Physical mutation can also improve the performance of the strain. Peng et al. [35] obtained Saccharomyces S132 strain by ultraviolet mutation breeding, which could tolerate 16 % ethanol (v/v) and temperature of 40 °C. When the concentration of ethanol was 20 % in medium, the resistance capacity to ethanol of this strain was about 31 % higher than that of the control. Gera [36] obtained Saccharomyces 355 strain, which could grow in medium with 17 % (v/v) ethanol, by heat shock technology. Genetic engineering is one of the important and most effective technologies to modify complex properties of yeast such as high ethanol and temperature resistance. Hou [37] obtained a mutant strain S3-10 by genetic engineering technology. The strain exhibited substantial improvement in multiple stress tolerance to tolerate high concentrations of ethanol, glucose, and high temperature. Moreover, the ethanol yield was increased by up to 10.96 %. There are also some further methods to improve ethanol tolerance, such as ventilation in early growth, controlling the growth fermentation conditions by adding Aspergillus oryzae or fungal hyphae in a soybean flour medium. Addition of new additives during genetic molasses fermentation, addition of commercial thick mash fermentation factors during starch material gravity fermentation, and immobilization of yeast cells can also improve ethanol tolerance of yeast to different degrees.

1.2.4.2 High temperature resistant strains Breeding of high temperature resistant strains mainly concentrates on high temperature domestication and natural selection. Sipiczki [38] constructed new yeast strains by protoplast fusion technique for the first time in 1977, which provided a new research direction for recombinant and genetic improvement of yeast. Subsequently, scholars devoted extensive research efforts to this field, successfully achieved fusion between different species of yeasts, and constructed some yeasts with application value. Intraspecies fusion was carried out between high alcohol producing Saccha-

16 | 1 Biomass ethanol fuel technology

romyces cerevisiae and high temperature tolerant Saccharomyces cerevisiae, and thus obtained the fused Saccharomyces cerevisiae which could produce alcohol at 40 °C. Sun [39] got stable fusants at 45 °C by screening fusion products. Wuhan Institute of Virology, CAS obtained a group of 40–50 °C resistant yeast by mutagenesis and screening, for which the lethal temperature could reach 80–100 °C (5 min). Ethanol and NaCl tolerance concentrations were found to be 13 and 10 %, respectively. This strain can ferment at 40 °C and its fermentation capacity is better than ordinary yeast [40, 41].

1.2.4.3 Pentose fermentation strains The development of gene recombinant strains through utilizing modern genetic engineering technology is an important approach to obtaining ethanol recombinant bacteria capable of efficiently metabolizing glucose and xylose. Host strains used in the research are mainly limited to Escherichia coli, Pichia stipitis, Zymomonas mobilis, Saccharomyces cerevisiae, and Pachysolen tannophilus. The E. coli KO11 rooting in interrupting succinic acid synthesis pathway of fumaric acid synthase gene ferments almost all the sugar of hemicellulose hydrolysate to produce ethanol; with high ethanol producing capacity and high tolerance to the inhibitor of hydrolysate. Moreover, researchers at Purdue University in the USA carried out the transformation of xylitol dehydrogenase, xylose reductase, and xylulose kinase genes to Saccharomces diastaticus and Saccharomces uvarum forming fusion strains which were able to simultaneously ferment glucose and xylose to ethanol, and improved the fermenting degree and substrate utilization [41].

1.2.4.4 Saccharification function strains The traditional ethanol production process includes saccharification of starch and fiber raw material into fermentable sugar followed by fermentation of sugar by metabolized yeast forming ethanol. A yeast with the ability to degrade polysaccharides and hydrolyze starch and dextrin significantly reduces the complexity of the process. Currently, special yeast strains have been developed by protoplast fusion technology, which are capable of performing the dual functions of saccharification and fermentation for achieving better results. For example, fusion of yeast cultures viz., Saccharomyces cerevisiae and Candida tropicalis as parents, was carried out by single inactivated protoplast fusion technique to obtain the fused strains with high saccharifying enzyme activity and enhanced ethanol production. Moreover, yeast strains for fermenting raffinose, lactose, and other polysaccharides have been developed by direct fusion technology, which provides rich resources of strains for industrial applications.

1.2 Production theory of bioethanol | 17

1.2.4.5 Engineered bacteria for consolidated bioprocessing The consolidated biological process (CBP) is a type of reaction using engineered bacteria to complete the multistep biological response, and it does not include preprocessing, enzyme production, and separation processes. CBP is mediated by the combination of saccharification and fermentation by the microbes in a reaction system, with a high degree of integration. Although the CBP technology can significantly reduce production costs, it places great demands on the engineered microbes that exhibit better microbial fermentation substrate utilization ability and better product configuration compared to traditional microbes, thus reforming the strain’s performance. There are two main types of breeding transformation to obtain microbial strains for CBP. First, the modification of microorganism such as Clostridium themocellum, which produces cellulose enzyme strains for fermentation to produce ethanol at the same time. This modification was carried out by importing the new metabolic gene to saccharify all or most of the fermentation product, aiming to increase the rate of ethanol production, reduce the by-products, and improve ethanol tolerance. Furthermore, transformation of Clostridium thermosaccharolyticum not only leads to the natural fermentation of xylan, biomass derived from sugar, but also prevents the production of organic acid and other by-products. The second type of transformation is the modification of ethanol bacteria capable of breaking down cellulose, such as Saccharomyces cerevisiae, Pichia stipitis, Candida shehatae, Pachysolen tannophilus, Escherichia coli, Klebsiella oxytoca, and Zymomonas mobilis. The strains transformed by the genetic recombination method can secrete a series of exo-glucanases and endoglucanases, could completely or mostly ferment sugars from lignocellulosic materials as the sole carbon source. Taking Saccharomyces cerevisiae as an example, genes of different types of microorganisms coding glycoside hydrolase (cellulase, hemicellulose, and β-D-glycosidase enzymes) and pentose degradation enzyme are transformed into Saccharomyces cerevisiae. The strains can grow in cellulose, hemicellulose, cellobiose, xylose, and arabinose medium. However, import and coexpression of foreign genes could lead to instability of gene expression and an adverse effect on cell growth, thus requiring further research [42].

1.2.4.6 The breeding of syngas fermentation microorganisms At present, there are some shortcomings in the syngas ethanol fermentation strains. The main products of syngas fermentation are ethanol and acetic acid, and most of the time the production of acetic acid is higher than ethanol. Strains required for syngas fermentation can be inhibited by impurities such as tar, which results in low production of ethanol thus making the process nonconducive to industrialization. Screening high-yield ethanol and tar-resistant strains, through genetic engineering and other methods to change or inhibit bacteria acetic acid metabolic pathways, thus making

18 | 1 Biomass ethanol fuel technology

them selective towards the production of ethanol, is the key to syngas microbial breeding research [43].

1.3 Ethanol production from starch feedstocks The raw materials and processes for industrial fuel ethanol production vary by country because of different agricultural production, transportation, and socioeconomic systems. In China, fuel ethanol is mainly produced from starch feedstock. In 2002, at the very beginning of China’s fuel ethanol industry, stale corn and wheat were used as feedstocks for fuel ethanol production. With the stale grain being utilized in producing fuel ethanol and the increasing price of corn, China announced that fuel ethanol must be produced only from nongrain feedstock. Nongrain basic roots and tubers, such as sweet potatoes (Ipomoea batatas), cassava (Manihot esculenta), and Canna edulis Ker., are characterized by high concentrations of fermentable sugars, rich total energy resources, and high availability worldwide. These roots and tubers have already been used as sustainable feedstocks for ethanol fermentation in pilot studies. Ethanol production from Jerusalem artichoke (Helianthus tuberosus L.) and Kudzuvine root (Radix Puerariae Lobatae) has also been carried out at the laboratory scale. Moreover, duckweed (Landoltia punctata) has already attracted extensive attention as a focus for research efforts.

1.3.1 Crushing In industrial production, crushing affects the cooking, saccharification, fermentation, and subsequent filtration of the fermentation mash. The smaller the crushed particles, the greater the surface area of the raw material, thus less steam is consumed during cooking. Furthermore, enzyme activity, heat transfer, and mass transfer are also improved [5]. However, considering the economic aspects, it is not correct to aim for smaller particle sizes at the expense of higher costs, because reaching such a small size requires a significantly larger amount of energy [44–46]. Experimental data show that proper crushing ratios (the maximum diameter of material before crushing to the maximum diameter of material after crushing) are necessary to optimize the process. To reduce energy consumption during crushing, two-stage crushing involving coarse crushing and fine crushing should be used in the grinding process. The crushing ratio for coarse crushing should be between 10:1 and 15:1, while the crushing ratio for fine crushing should be between 30:1 and 40:1. These crushing ratios result in lower energy consumption. Crushing ratios can be adjusted by regulating the aperture of the sieve. General sizes of sieve pores are in the range of 1–3 mm, with the majority falling in the 1.5–2 mm range [47]. Crushed materials that are smaller than the size of the sieve’s pores are separated by the sieve. This prevents

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particles that meet the crushing ratio from staying in the crusher for too long, which would lead to high energy consumption and low efficiency of the crusher. Depending on whether additional water is needed, crushing is usually divided into dry grinding and wet grinding. At present, most Chinese ethanol production enterprises use dry grinding production processes. In the United States, some corn and grain fuel ethanol is produced by wet grinding processes [48].

1.3.2 Steaming and gelatinization After crushing, the particle diameter of raw materials becomes smaller, parts of plant cells and intercellular substances break down, and parts of the starch granules are released; however, most of the starch granules remain within the cells of the raw materials. These starch granules are not easily degraded by amylases due to the protection provided by the plant cell wall [49]. Furthermore, starch is not soluble in water at room temperature. The speed and degree of hydrolysis of starch by amylase is very low when starch is in its insoluble state. Starch is a type of hydrophilic colloid, i.e., water molecules can penetrate into starch granules when there is contact between starch and water. When the water temperature is above 53 °C (the exact gelatinization temperature varies with plant species and molecular weight), the volume of the starch particles expands to 50–100 times the original volume, and the connection between starch granules attenuates, leading to partial collapse of the starch granule and an increase in viscosity of the disintegrated starch granules. This process is referred to as “gelatinization”. Cooking is necessary for gelatinization because this process generally occurs in a certain temperature range. Liquefaction is conducted by using the activity of α-1,4-glucan-4-glucanohydrolase (α-amylase, liquefying enzyme) on gelatinized starch. Starch is composed of amylose and amylopectin. Amylose is composed of glucose connected by α-1,4glycosidic bonds. Amylopectin is composed of glucose connected by α-1,6-glycosidic linkages. The work of α-amylase can be divided into the following two stages: The first stage is faster, and the amylose can be completely hydrolyzed to dextrin, maltose, maltotriose, and maltooligosaccharides. The second stage is very slow; maltotriose and maltooligosaccharides are eventually hydrolyzed to maltose and glucose in this stage. When α-amylase acts on amylopectin, it can hydrolyze α-1,4-glycosidic bonds [50]; however, α-1,6-glycosidic bonds and α-1,4-glycosidic bonds adjacent to α-1,6-glycosidic bonds are not affected. Hydrolysis products are maltose, glucose, and amylodextrin with α-1,6-glycosidic bonds. Different compositions and sizes of amylodextrin are obtained depending on the source of α-amylase. Under the action of α-amylase, branched chains of gelatinized starch are turned into dextrin and oligosaccharides, which results in decreased viscosity of the mash to a certain extent. Therefore, cooking, gelatinization, and liquefaction processes lead to thickening and then thinning of the mash.

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According to cooking temperatures, the cooking process can be divided into high pressure and high temperature (HPHT), and atmospheric pressure processes. In recent years, novel pretreatment processes such as spray liquefaction and no-cooking processes have also gained attention.

1.3.3 Saccharification Depending on the source of glucoamylase, saccharification can be carried out either by directly adding commercial glucoamylase or by introducing glucoamylaseproducing microorganisms along with ethanol-producing microorganisms. However, the latter type of saccharification using microorganisms is usually not good because glucoamylase-producing microorganisms do not tend to tolerate the ethanol produced by ethanol-producing microorganisms. Moreover, glucoamylase-producing microorganisms compete with ethanol-producing microorganisms for substrate, leading to low ethanol conversion efficiency. Therefore, commercial glucoamylase is the main trend in saccharification [51–53]. Saccharification can also be divided into batch saccharification and continuous saccharification depending on the scale of equipment used. Further, based on the integration of hydrolysis and fermentation steps, the following process formats for saccharification can be obtained: separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), and partial simultaneous saccharification and fermentation (partial SSF).

1.3.3.1 Separate hydrolysis and fermentation process In the SHF process, pretreated lignocellulose is hydrolyzed to monosaccharides and subsequently fermented to ethanol in separate vessels. Therefore, the SHF process involves saccharification and fermentation being carried out in different reactors. SHF is universally successful in ethanol production from starch feedstock. Fermentation rates of SHF are low because of inhibition due to high concentrations of glucose at the beginning of fermentation and the limit of post-saccharification at the completion of fermentation. The rate of fermentation depends on the rate of glucose consumption by the yeast as well as the rate of post-saccharification at the completion of fermentation because yeast cannot metabolize starch. In SHF, high concentrations of glucose from saccharification inhibit the activity of glucoamylase because of feedback inhibition, thus limiting the post-saccharification and increasing the fermentation time.

1.3.3.2 Simultaneous saccharification and fermentation process Since the 1970s, some scholars who focused on ethanol production from cellulose proposed SSF to avoid over-accumulation of sugar and end-product inhibition in order to

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enhance the hydrolysis efficiency of cellulase and reduce the investment costs. SSF received extensive attention, and scholars worldwide have devoted extensive efforts to the research on SSF. If SSF is employed in ethanol production from starch feedstock, exclusive saccharification can be omitted and energy consumption can also be reduced. Moreover, this may also lead to a decrease in investments in the facility because saccharification and fermentation could be carried out in the same reactor. Furthermore, glucose would be consumed as soon as it is produced from saccharification, because of simultaneous occurrence of saccharification and fermentation processes [54]. Therefore, production inhibition would be relieved and contamination by other microorganism could be avoided. The principal issue associated with SSF is the discrepancy between saccharification temperature and fermentation temperature. In general, optimum saccharification temperature is above 50 °C; however, optimum growth and fermentation temperature for microorganisms is below 40 °C. To resolve this discrepancy, researchers proposed non-isothermal simultaneous saccharification and fermentation (NSSF). However, some research showed that NSSF could not enhance ethanol yield. Screening heat tolerant yeasts is another method to solve this misalignment.

1.3.3.3 Partial simultaneous saccharification and fermentation process Partial SSF indicates separate hydrolysis and fermentation while adding additional glucoamylase during fermentation. The extent of saccharification plays an important role in fermentation; therefore, index detection after saccharification is necessary. Brix degree, acidity, and reducing sugar concentration are usually selected as standards to determine the quality of saccharified mash. To improve the accuracy of the analysis, glucose and maltose concentration in saccharified mash and activity of glucoamylase are also measured in some plants by high-level testing technology.

1.3.3.4 Ethanol Fermentation At a suitable temperature and pH, hexose is converted to ethanol under anaerobic conditions by inoculated yeasts. This biochemical process mainly consists of two steps. In the first step, hexose is converted to pyruvate. In the second step, pyruvate is converted to acetaldehyde and CO2 in a reaction catalyzed by decarboxylase, and acetaldehyde is further reduced to ethanol. In addition to ethanol, a small quantity of other compounds including glycerol, organic acids (mainly succinate), fusel oil (higher alcohol), aldehydes, and esters are also generated during fermentation. In theory, l mol glucose can be converted to 2 mol ethanol, which indicates that 180 g of glucose can be converted to 92 g of ethanol. Therefore, theoretical yield is 51.1 %. However, actual net yield is not so high. This is attributed to the fact that 2 % of the glucose is consumed for the growth of yeast, and another 2 % is consumed to form glycerol, organic acids,

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and fusel oil. After deducting these losses, only approximately 47 % of the initial glucose is converted to ethanol.

1.3.3.4.1 Fermentation modes In ethanol fermentation, some by-products are produced from the metabolism of yeast, and some are produced from the metabolism of other microorganisms that contaminate the mash. Formation of by-products consumes the feedstock and reduces ethanol yield. Thus, various fermentation processes focus on reducing by-product generation by controlling technological conditions, thereby converting more sugar to ethanol. Depending on the manner of placement of the fermentation mash into the fermenter and the mode of operation, existing fermentation processes can be divided into intermittent fermentation, semicontinuous fermentation, and continuous fermentation. (1) Intermittent fermentation (batch fermentation) A batch fermentation process indicates intermittent fermentation mode using one feeding, one inoculation, and one harvest. The entire process is performed in one fermenter. Thus, fermentation is a discontinuous process divided into batches. Most Chinese ethanol plants employ this technology. Based on different volumes of fermenters and different filling methods of mash, batch fermentation can be divided into one-addition, graded-addition, continuous-addition, and split-main-fermentation-mash types. The advantages of batch fermentation include easy operation and management of feedstocks. The drawbacks of batch fermentation include the low density of yeast at the beginning of fermentation and large quantities of fermentable sugars in mash, which restrain growth rate and fermentation rate. (2) Continuous fermentation (fed-bach fermentation) A continuous fermentation process includes the continuous addition of fresh mash into the fermenter at a fixed rate, and discharge of fermented mash and the final product at the same rate. For sweet potatoes and other starchy raw materials, the key to success of this process is the reduction in the viscosity of fermented mash and the increase in its fluidity to achieve continuous flow in system conduits. Moreover, fermentation is continuous and has prolonged operation; therefore, prevention of bacterial contamination requires special attention. (3) Semicontinuous fermentation Semicontinuous fermentation indicates employing continuous fermentation during the main fermentation stage and batch fermentation at a later stage. This method has some of the advantages of the continuous and batch operations. However, there is a

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high risk of contamination and mutation due to long cultivation periods and periodic handling.

1.3.3.4.2 Novel fermentation technology (1) Thick mash ethanol fermentation In recent years, thick mash ethanol fermentation or very high gravity (VHG) fermentation has been gradually employed to improve ethanol production processes at national and international levels. With continuous improvements in technology, final ethanol concentration from thick mash fermentation of tuber crops has also improved. In the 1980s, fermentation with a final ethanol concentration of 7–9 % (v/v) was considered thick mash fermentation. However, currently fermentation of cassava with final ethanol concentration of 12.5 % (v/v) or more is considered thick mash fermentation. It has been proposed that ethanol fermentation with saccharides in excess of 27 % (w/w) or final ethanol concentrations in excess of 13 % (v/v) can be regarded as thick mash ethanol fermentation. Thick mash ethanol fermentation is a new perspective technology with several advantages over the common fermentation technology used in industries. This technology has several advantages: (1) improved productivity; (2) reduced energy consumption; (3) a water conservation process; (4) reduced wastewater emissions; and (5) reduced loss of distilled ethanol and increased extraction rate. High contents of saccharides and ethanol in fermentation medium have a detrimental effect on yeast cells, which result in the suppression of ethanol fermentation. Therefore, technological improvements and breeding of ethanol resistant yeasts are key to achieving VHG fermentation in industrial production. (2) Vacuum fermentation Vacuum fermentation technology was developed to remove most of the ethanol produced during fermentation and reduce its inhibiting effect on yeast. Ethanol is a volatile compound. If a sufficient degree of vacuum is maintained in the fermentation tank, ethanol starts boiling at the optimal fermentation temperature of yeast (30–32 °C), and thus most of the produced ethanol can be easily removed from the fermentation system to ensure continued fermentation by yeast. The removed ethanol vapor can then be condensed and sent for further processing. However, the disadvantages of vacuum fermentation are its susceptibility to contamination, challenges in removing CO2 , high energy consumption, and strict standards for equipment and cooling. (3) Membrane separation ethanol fermentation Membrane separation ethanol fermentation refers to the removal of highly concentrated ethanol by continuously separating ethanol from the fermentation broth using ethanol permselective membranes. This results in a reduction in product inhibition because the ethanol concentration in the broth remains low. Glucose cannot pass through the ethanol permselective membrane; therefore, it re-

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mains in the fermentation system until the completion of the process. Two strategies are involved in membrane separation. First is pervaporation (PV), and second is membrane distillation (MD). Polytetrafluoroethylene (PTFE), polypropylene (PP), or other microporous hydrophobic membranes are employed in PV. In general, the upstream side of the membrane is at ambient pressure and the downstream side is under vacuum to allow for evaporation of ethanol after permeation through the membrane. In MD, the driving force for mass transfer through the membrane pores is the difference in vapor pressures across the membrane, which depends on the temperature and the composition of solutions in the layers adjacent to the membrane. The permselective membrane is the key for membrane separation technology. Membrane separation technology is not extensively applied in industry because of membrane fouling and high cost of membranes. Moreover, membrane separation is suitable only for liquid fermentation. For tuber crops, solid-liquid separation and liquid fermentation are uneconomical because their low content of protein leads to low added value of separated solid content. Furthermore, when whole tuber crops are used for fermentation, the fiber clogs the membrane. (4) Extractive fermentation Extractive fermentation refers to the extraction of ethanol from fermentation mash by solvent to maintain its low concentrations in order to minimize its inhibition effect on yeast. Extractive fermentation is achieved in the case of ethanol production by coupling both fermentation and liquid-liquid extraction. The main process is as follows: fermentation mash is discharged out of the fermenter and centrifuged, yeast is replaced in the fermenter, and then liquid supernatant is mixed with extraction solvent. Further, solvent with extracted ethanol is separated from the fermentation broth and the fermentation broth is re-placed in the fermenter. Then solvent saturated with ethanol is distilled, and the recovered solvent is used for the next round of extraction. The extraction solvent must have the following characteristics: (1) nontoxic to yeast; (2) a high distribution coefficient for ethanol; (3) higher selectivity for ethanol than water and other fermented products; and (4) inability to form emulsions in fermentation broth. The disadvantage of this method is the high cost of extraction solvents. Notably, similar to membrane separation technologies, extractive fermentation technology is only suitable for liquid fermentation.

1.3.3.5 Ethanol extraction and purification The composition of ethanol fermentation mashes varies due to the utilization of different raw materials, fermentation processes, and production managements. In general, they are multicomponent mixtures that consist of water, ethanol, dry matter, and other fusel oils. Currently, distillation is the only feasible method to separate ethanol from the fermented products derived from the global ethanol industry. Recently, various

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types of energy-efficient distillation procedures and methods involving ethanol recovery without distillation have begun to emerge; however, all of them are still at the laboratory scale or at the amplifying trial phase except for a few energy-efficient distillation processes.

1.3.3.5.1 Distillation Distillation can roughly be divided into single-column distillation, twin-column distillation, triple-column distillation, penta-column distillation, and multi-column distillation. Multi-column distillation can definitely improve the quality of ethanol; however, energy consumption has become a difficult challenge. Therefore, research on energy-saving technologies in the distillation process has become increasingly crucial and received significant emphasis [55]. Overall, distillation is mainly divided into the following two parts: crude distillation and rectification. Crude distillation separates ethanol and other volatile impurities from the mature mash and isolates the crude ethanol. Rectification further removes the volatile impurities, such as alcohols, aldehydes, esters, and acids from the crude ethanol, and increases the ethanol concentration. The impurities can be divided into three levels: head, intermediate, and tail. The head level impurities are more volatile than ethanol. The volatility of the intermediate level impurities is similar to that of ethanol; therefore, their separation from one another is extremely difficult. The tail level impurities, called fusel oils, are less volatile than ethanol. Ethanol product is mainly taken away by vinasse, rectification wastewater, and noncondensable gases; or it is also boiled off by equipment, removed through pipings, and unsealed parts during the distillation process. Loss of ethanol within bounds is determined by the production capacities of the devices and the season. Furthermore, the performance of the devices, the number of distillation columns, the operating conditions, the temperature of the cooling water, and the experience of the operators also impact the loss of ethanol. In general, the loss of ethanol is between 0.8 and 1.2 % in the distillation and rectification devices.

1.3.3.5.2 Dehydration The boiling point of ethanol is 78.3 °C, and that of water is 100 °C. When the fermented products are heated, ethanol is vaporized more quickly than water, due to its low boiling point. As a result, high concentrations of ethanol can be obtained by multistage distillation; however, the volatility of ethanol in an ethanol-water solution decreases with increased ethanol concentration. When the concentration increases to 97.6 (v/v) [95.57 % (w/w)], the system becomes an ethanol-water azeotrope, and conventional distillation methods cannot further increase the ethanol concentration. Therefore, it is impossible to obtain anhydrous ethanol by conventional distillation methods at atmospheric pressure. The fuel ethanol used as biological energy generally refers to ethanol concentrations greater than 99.5 %. Therefore, in order to further increase the concentration of ethanol, special methods of dehydration are used to remove the ex-

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cess water. Currently, the methods for producing fuel ethanol include dehydration by chemical reactions, azeotropic distillation, extractive distillation, adsorption, membrane separation, vacuum distillation, and ion exchange resin method.

1.3.3.6 Examples of industrial production In early April 2008, the National Development and Reform Commission (NDRC) announced that a special planning assessment focusing on biofuel ethanol in key provinces has been completed by the China International Engineering Consulting Corporation. The assessment concluded that using tubers as raw material to produce fuel ethanol is economical. Recently, Henan Tianguan Group Co., Ltd., which produces 300,000 tons of cassava fuel ethanol, passed certification in July 2012. In November 2012, China National Cereals, Oils and Foodstuffs Corporation (COFCO) Biochemical (Anhui) Co., Ltd., which produces 150,000 tons/year cassava ethanol fuel, has also passed certification. Of all the fuel ethanol production projects from starch feedstocks, Guangxi COFCO Biomass Energy Co., Ltd.’s 200,000 tons/year cassava ethanol production line is the most representative. The line was started in October 2006 in Guangxi with the approval of the NDRC and put into production in December 2007. This production line is China’s first set of nongrain fuel ethanol projects and is a significant milestone. All technologies involved in ethanol production from cassava in this plant have selfowned intellectual property rights. A mathematical model of the entire process of fuel ethanol production from cassava has been created. Sand cleaning technology to separate sand from thick mash of cassava, integrated spray liquefaction at high temperatures, low-energy heat exchange, SSF of cassava, thick mash fermentation technology, three-way thermally coupled distillation technology, molecular sieve dehydration coupled with distillation, amplification of the large side-stirred fermenter, high wettability filler, antiblocking type tray design, and manufacturing technology were developed and applied in the plant.

1.4 Ethanol production from sugar feedstocks Ethanol can be produced from sugar feedstocks by bioconversion. About 30,000 kJ of heat energy is released during complete combustion of ethanol per kilogram, thus ethanol is a high-quality liquid fuel. Ethanol fuel has several advantages as follows: It provides clean energy with no emission of sulfur and ash, and it also cuts greenhouse gas emissions. Further, ethanol can directly replace gasoline, diesel, and other petroleum fuels; and can be effectively used as combustion or internal combustion engine fuel for civilian automobiles. In fact, pure ethanol or gasoline blended fuel can be used as vehicle fuel, is most likely to be amenable for industrialization, and

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with the current standards of industrial applications and transportation facilities, is the most promising alternative to petroleum fuels. Ethanol can be produced from sugar feedstocks by bioconversion. Fig. 1.4 shows the feedstocks categories for ethanol production.

Sugar feedstocks

Sugarcane molasses: Residual molasses from sugar refinery in southern China

Sweet sorghum and other sugar plants: sugar utilization in the plants

Beet molasses: a byproduct from beet sugar plants in nothern China

Fig. 1.4: Categories of sugar feedstocks for ethanol production in China.

1.4.1 Technical process of ethanol production from sugar feedstocks 1.4.1.1 Technological requirements for processing molasses feedstock Molasses is a viscous by-product from refining sugarcane or sugar beets into sugar. Owing to the high content of sugars, molasses can be a good feedstock for the largescale production of ethanol just requiring small amounts of yeast. With improvements in the sugar producing industry in China, the yield of molasses has shown a tremendous increase. Moreover, many sugar factories have ethanol plants attached, for the comprehensive utilization of molasses for ethanol production. (1) The presence of many nonsugar components in molasses, in particular, salts and heavy metal ions, could inhibit the breeding of yeast, consequently reducing the production of ethanol. Without the removal of nonsugar components during the sugar clarification step, a decrease in molasses purity causes a drop in the fermentation rate. Thus, the improvement of fermentation technology to produce ethanol from molasses with a high yield under the condition of lower molasses purity is one important subject that undeniably requires much more systematic exploration. (2) After dilution, molasses can be fermented to produce ethanol; however, the content of liquid varies with the different production process flows and flow process conditions. In a double concentration process, dilute sugar liquid with dry matter content of 12–14 % is used to culture yeast, and higher sugar solution concentration of 33–35 % is used for fermentation, based on the ethanol content in mature mash, mixing proportion of yeast mash, and concentration of dilute sugar solution used. In a single concentration process, the concentration of dilute sugar solution is 22–25 %.

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(3) In general, molasses contains a significant amount of bacteria, in particular, acid producing bacteria. Therefore, sterilization or acidification treatment of molasses is required in order to ensure the normal order of ethanol fermentation. (4) Beet molasses is generally alkaline, with pH 7.4, which is not suitable for the growth of yeast and fermentation. Therefore, the pH is adjusted to 4–4.5 by additional treatment with sulfuric acid prior to ethanol fermentation. Sugarcane molasses is generally slightly acidic, just requiring a small amount of sulfuric acid to adjust pH to 4–4.5. (5) Molasses contains 5–12 % of colloidal substances, composed of pectin, caramel, black pigment composition, and so on; therefore a large amount of foam is generated during ethanol fermentation, thereby reducing the rate of utilization of the fermentation tank. Moreover, colloidal substances can get absorbed on the surface of yeast, causing difficulties for the yeast’s metabolism. In particular, caramel and melanin can strongly inhibit ethanol fermentation. Therefore, removal of colloidal substances before fermentation of molasses not only improves the utilization rate and effective capacity of fermentation equipment, but also significantly improves the fermentation rate. (6) A large amount of ash with numerous impurities decreases the purity of molasses, which not only leads to a decrease in the fermentation rate, but also accumulation of dirt in equipment such as fermentation tank, cooler, and distillation tower. Therefore, ash and impurities should be removed before fermentation. (7) Though the content of heavy metal ions in molasses, such as copper ions (Cu2+ ) and lead ions (Pb2+ ) is not very large, they should also be considered as inhibitors because their presence even in trace amounts, such as 5–10 ppm Cu2+ , can suppress the activity of yeast. The experimental results of inhibition of yeast growth by a certain amount of Cu2+ indicate that 5 ppm ions begin to restrain yeast and 10 ppm can totally stop the yeast from growing. Therefore, significant attention should be paid to the content of heavy metal ions in molasses during the process of ethanol fermentation. (8) In production practice, it is very important to breed and domesticate a strong yeast strain that can resist high temperature, high acid concentration, and high sugar concentration to overcome the negative effect of large amounts of nonsugar impurities such as ash, colloids and plenty of bacteria. (9) Nitrogen and phosphorous content in molasses varies according to their types, origins, year, and sugar refining methods. Therefore, it is appropriate to add nutrients during the fermentation process. (10) Metabolic impurities of ethanol fermentation from molasses generate ester aldehyde, fusel oil and other impurities, which significantly influence the quality of ethanol produced. In general, a ventilation process is adopted for yeast culturing. Fermented mash with aldehyde can generate foam that leads to fouling in the distillation process. According to these characteristics, if molasses is used as raw material to produce high purity ethanol or process ethanol distillation,

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a row of aldehyde towers is required between the mash tower and rectification tower in the distillation process to establish a three-tower continuous distillation process using vapor or liquid phase over the tower. Currently, the method for producing ethanol from molasses fermentation is comparatively perfect with high efficiency. Some domestic and foreign advanced technology has achieved continuous dilution, continuous fermentation, and continuous distillation processes for the continuous and automatic production of ethanol.

1.4.1.2 Devices for the dilution of molasses Molasses dilution methods involve intermittent and continuous devices.

1.4.1.2.1 The intermittent dilution device Molasses intermittent dilution is conducted in the dilution tank with agitators. If ethanol plants originally have saccharification equipment, it is always used as a dilution device. In the intermittent dilution device, molasses is first pumped into the upper tank, and then after weighing pulled into the dilution tank, with the simultaneous addition of a certain amount of water. The required concentration of diluted sugar liquor is obtained after mixing well, and then it is filtered for yeast culture and fermentation.

1.4.1.2.2 The continuous dilution device At present, domestic molasses ethanol plants usually employ a continuous dilution method by means of a continuous dilution device. The commonly used types are described as follows: (1) Continuous dilution device with stirrers A continuous molasses diluter with stirrers is a cylindrical tube provided with several baffle plates with holes and a plate along the length. For the better mixing of molasses and water, the holes are located in a staggered arrangement, that is, one hole is in the upper part, and the corresponding one is in the lower part. Thus a turbulent fluid is to obtained during the liquid flowing process, and the baffle hole diameter is calculated to ensure this. The baffle plate is fixed on a pair of horizontal shafts, which can be disassembled for cleaning. The dilution device is often installed with a downward slope to obtain a well-mixed effect, and also to save power. (2) Vertical continuous dilution device The vertical continuous dilution device is a cylindrical tube, based on the continuous change in cross-sectional area, to obtain an even mixture of molasses and water through ensuring the turbulent flow of liquid in the reactor. When approaching continuous dilution, first the molasses is pumped to a high trough, and then it is allowed to flow to the dilution device, with simultaneous addition of hot

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water from another high tank. Ensuring a certain concentration of dilute sugar solution is a key operation in the continuous dilution device, which is dependent on the corresponding manually-regulated valve. The large-scale ethanol production plant employs a linkage pump to adjust the flow of water and molasses.

1.4.1.3 The treatment process of molasses 1.4.1.3.1 The pretreatment of molasses before fermentation Molasses used for ethanol fermentation exhibits the following characteristics: presence of high content of dry materials and sugar, plenty of acid producing bacteria, and large amount of ash and colloidal substances, which inhibit the direct fermentation of yeast without pretreatment. The necessary pretreatment procedures include dilution, acidity, sterilization, clarification, and addition of salts.

1.4.1.3.2 The process requirements for molasses dilution In general, the Brix of molasses is 80–90 Bx, with the sugar content above 50 %. Neither cane molasses nor beet molasses ferment readily until they have been subjected to certain preliminary treatments. Thus, molasses must be diluted with water before it can be fermented. The concentration of diluted molasses changes with different production processes and operation conditions, and the commonly used technical conditions are as follows: (1) single concentration process, the dilute sugar solution concentration is 22–25 %; (2) double concentration process, the yeast diluted sugar solution concentration is 12–14 %; (3) basic dilute sugar concentration 33–35 %.

1.4.1.4 Acidification, sterilization, clarification, and addition of nutrient salts to molasses Molasses contains impurities such as ash and colloidal substances. It is often deficient in nitrogen compounds which are essential to yeast production. Furthermore, molasses contains harmful microorganisms and requires adjustment to achieve suitable acidity. Therefore, operations such as acidification, sterilization, clarification, and addition of salts must be implemented during molasses dilution, which are achieved step by step in an intermittent device. Most molasses ethanol industries in China currently use a hot acid continuous dilution method, in which acidification, sterilization, addition of salts, and clarification are carried out simultaneously.

1.4.1.4.1 Acidification The purpose of acidification by adding acids is to prevent bacterial breeding, accelerate precipitation of ash and colloidal substances in molasses, and simultaneously

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adjust the acidity of the diluted sugar solution to make it suitable for the growth of yeast. Owing to the slightly acidic nature of sugarcane molasses, slight alkalescency of beet molasses and optimal pH corresponding to 4.0–4.5 for yeast fermentation, the technical requirement of adding acid into diluted molasses is necessary. For sugar beet molasses, addition of acid can also remove Ca2+ ions generated in the form of calcium sulfate, thus accelerating the removal of colloid material together with ash from molasses by precipitation. Added acid amounts and methods generally vary with different types of molasses. Sulfuric acid with concentration of 0.2–0.3 % of dilute sugar solution is added to dilute sugarcane molasses, and the amount of acid (specific gravity of 1.86, 66 Be industrial sulfuric acid) is 2–3.5 kg tons−1 molasses or 0.7–l L m−3 fermenting mash. Most beet molasses are alkaline; therefore, more acid is required to bring the pH down to 4–4.5 compared to sugarcane molasses. In single concentration process, the acidity of dilute sugar solution should be 6–7° without adding acid. Organic alkali (-NH2 ) in beet molasses can emit toxic yellowish brown nitrogen dioxide (NO2 ) gas when it reacts with acid. In order to avoid poisoning, the acidification tank must have an exhaust hole, and the acidification section should have good ventilation equipment.

1.4.1.4.2 Sterilization Molasses is always polluted by a large number of microbacteria, roughly including wild yeast and acid producing bacteria such as C. albicans and lactic acid bacteria. Sterilization eliminates thermoresistant bacterial spores. In order to prevent the spoilage of the sugar liquid by bacterium and to ensure the normal conduct of fermentation, in addition to the increase in the acidity of fluid, sterilization is also necessary. There are two sterilization methods. (1) Heating sterilization Addition of heating steam (80–90 °C) for 1 h can achieve sterilization. Besides sterilization, heating the dilute sugar solution also plays the role of clarification. However, consumption of large amounts of heating steam also requires the addition of cooling and clarifying equipment, which are not commonly used in plants. (2) Addition of anticorrosive drug The commonly used preservatives in domestic plants are bleach (200–500 g ton−1 molasses), 40 % formaldehyde (600 mL ton−1 molasses), sodium fluoride (0.01 % of the amount of mash), and sodium pentachlorophenolate (0.004 %). Attention should be paid to the use of five chlorinated phenol sodium in the acidic environment because of its decomposition into five phenol and sodium chloride; therefore, it should be added to molasses which is not subjected to acidification. In recent years, antibacterial substances have also been used for preventing microbial contamination during fermentation. Some molasses ethanol plants adopt

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an antibacterial substance, namely, anti-lactobacillus, which is separated from the mycelium of purple actinomycetes 135/1. When l–20 μg mL−1 (dissolved in 50 % alcohol solution) is added to the fermentation liquid, the activity of lactic acid bacteria is inhibited, with no effect on the mold and yeast growth. Addition of 0.005 % antilactobacillus during molasses fermentation does not affect the quality and yield of ethanol, while perfectly inhibiting the activity of lactobacillus.

1.4.1.4.3 Molasses clarification Numerous colloidal substances, ash, and other suspended matter contained in molasses are harmful for the growth of yeast and ethanol fermentation; therefore, they should be removed to the maximum extent. The following are the typical clarification methods. (1) Acidic precipitation with ventilation This method is also known as cold acid treatment with ventilation. The molasses is diluted with water to about 50 Bx, then 0.2 to 0.3 % concentrated sulfuric acid is added, and finally compressed air is injected for 1 h. After static clarification for 8 h, the supernatant is removed. On the one hand ventilation could discharge harmful gases such as sulfur dioxide (SO2 ) or NO2 , volatile acid, and other volatile substances; on the other hand it may increase the oxygen content of the sugar fluid, thus facilitating the proliferation of yeast due to the increase of the oxygen coefficient. (2) Hot acidic treatment Under high temperature and acidity, both the sterilization of harmful microorganism in molasses and the precipitation of colloidal substances and impurities such as ash are strong. During hot acidic treatment, usually acidic sterilization and clarification are performed simultaneously, and the step dilution method is adopted to dilute the original molasses. During the first step, molasses is diluted to 55–58 Bx by adding warm water at 60 °C, at the same time sulfuric acid is used to adjust the acidity to pH 3.0–3.8, and then the contents are allowed to rest without stirring for 5–6 h. During the second step, the acidified sugar liquid is diluted to the required yeast nutrient solution concentration of 12–14 %, and molasses for fermented mash is also diluted to the required concentration at the same time by a continuous dilution device, then the contents are pulled into the main fermentation tank. (3) Mechanical separation process by using pressure filtration or centrifuge separation

1.4.1.4.4 Addition of nutrient salts A certain amount of nitrogen, phosphorus sources, auxin, and magnesium salt are essential for the growth of yeast. These mineral nutrients are already present in fresh sugarcane juice or sugar beet juice; however, most cannot endure the processing of

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sugar production and molasses treatment. Different methods of sugar production lead to different content of these ingredients in molasses. Dilute sugar solution is often deficient in the nitrogen compounds which are essential to yeast production, which not only affects the growth of yeast, but also the production of ethanol. Therefore, analysis of molasses is highly desirable in order to evaluate the nutrient deficiencies, obtain the extent of the shortfall, and finally add the appropriate nutrients.

1.4.2 Ethanol production from sweet sorghum stalk 1.4.2.1 Solid-state fermentation process 1.4.2.1.1 The characteristics of solid-state fermentation of sweet sorghum stalk Sweet sorghum stalk is considered a cost-effective feedstock for bioethanol production due to its higher drought resistance ability, lower production costs, and higher biomass yield. Solid-state fermentation of sweet sorghum stalk is borrowed from traditional brewing of spirits. Compared to liquid fermentation, it has the following characteristics: – The process requires simple equipment, low investment, short construction period and startup time, and easy operation and spread. – The process is less demanding on the environment, and the fermentation is easy to control. Therefore, it is suitable for spreading to and application in rural areas which are undeveloped economically and technologically. Solid-state fermentation to produce crude ethanol, which is concentrated by distillation, could improve equipment utilization. Furthermore, solid-state fermentation offers numerous advantages such as simple equipment, easy operation, lower energy requirements, production of lesser wastewater, and environmentally friendly; therefore, it does not need any environmental protection facilities. The solid-state fermentation cycle lasts for about 2 to 3 days (this varies with the difference in geography and climatic environment). In summary, solid-state fermentation of sweet sorghum stalk to ethanol is favorable for large-scale application in vast rural areas and less investment leads to better economics.

1.4.2.1.2 Process of ethanol production from solid-state fermentation of sweet sorghum stalk The greatest difficulty encountered during solid-state fermentation of sweet sorghum stalk is the material flow, which leads to difficulties in making the processes of fermentation and distillation automatic and continuous. It has become the bottleneck problem hindering the industrialization of this process.

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The material flow and related parameters during solid-state fermentation of sweet sorghum stalk were intensively studied during the National Science and Technology Support Program, “Key technology development and demonstration about nongain fuel ethanol”. Productive experiment results showed that the entire production system was stable; the conversion efficiency was high; the solid-state fermentation and continuous solid-state distillation equipment could provide the required environmental conditions for ethanol conversion and distillation; and continuous fermentation and distillation were carried out successfully. (1) Process description The process is described in Fig. 1.5. (2) Stalk grinding According to the technical requirements for solid-state fermentation, stalks should be first ground and scrubbed. The length and diameter of the ground stalk should be about 10–30 and 2–3 mm, respectively. The ground stalk must have uniform size, which makes it favorable for further fermentation. (3) Stalk pretreatment A self-developed joint machine is used, in which sterilization, temperature/humidity adjustment, and strain mixing could be carried out simultaneously. The mixing degree between stalk and strain is over 0.7, which could meet the requirement of system capacity. (4) Fermentation The fermentation cycle lasts for about 1.5–2.5 days (60 h), which varies with different regions and seasons. A train fermentation tank is used as our fermentation equipment, and each tank is an independent fermentation unit. The complete fermentation system was constructed by chronologically connecting all the fermentation units. The charged fermentation tank is transferred to the closed fermentation workshop for fermentation, and traveled along the predetermined track road. The travel speed could be set manually, so that the fermentation tank arrives at the chute after the completion of fermentation. After mechanical and manual discharging, the tank is returned to the charging area, and a small fermentation unit is completed. The discharged materials are sent to the solid still by lifting auger. Sweet sorghum planting

Stalk acquisition

Roughing stalk process

Stalk grinding

Steaming

Rectification

Coolingwine

Distillation

Fermentation

Yeast introduction

Ethanol

StorageMarket

Vinasse feed/papermaking materials

Temperature and humidity monitoring

Yeast preparation

Fig. 1.5: Process diagram of solid-state fermentation of sweet sorghum stalk.

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1.4.2.2 Liquid fermentation process There are many processes for ethanol fermentation from molasses, which can be basically divided into two categories, namely, batch and continuous processes. Currently, the continuous process is applied in most plants, where production management is perfect and the instrumentation automation is also good. The batch process is applied in small-scaled plants. Fermentation of the juice of sweet sorghum stalk to produce ethanol has incomparable advantages over that from molasses and starch-based materials. It could save many process links. The juice (the Brix is about 16–20 % Bx) from stalk squeezing can be directly used for fermentation after sterilization, thus avoiding dilution, acidification, clarification, and nutrients addition. Moreover, the juice does not contain impurities, and distillation and sewage treatment are easier compared to those for molasses. Ethanol production using immobilized viable microbial cells in fluidized-bed bioreactors is an advanced technology in today’s world. Brazil is one of the advanced countries extensively promoting industrialization for ethanol production from molasses. The technology provides high yield, advanced process, low production cost, and high comprehensive utilization level, which is worth learning from and adopting. The following data show the technical level of molasses fermentation in Brazil. The fermentation time has changed from 18 h in 1975 to 9 h (6–11 h) in 2004, and the ethanol and yeast concentration in fermented liquor is 9 % (v/v) and 13 % (106 cells mL−1 ), respectively. At present, the technology could not reach the industrialization stage. The liquid fermentation of sweet sorghum in China was developed in the scientific research of “Tenth and Eleventh Five-Year Plan” projects. The yeast immobilization and fluidized-bed technology is based on novel technology in the bioengineering field, i.e., immobilized active proliferation cell technology, where the traditional free yeast is entrapped in a solid carrier. The selected carrier provides great stability to the immobilized yeast, thus resulting in a high yield. Thus production is improved and the operation of an otherwise difficult continuous fermentation process becomes easy. This technology could be applied in industrial ethanol production from various materials including molasses, cassava, sweet potatoes, corn, sorghum, and canna. The technology is based on cell immobilization, and the yeast immobilization methods are studied intensively. The repeated carrier tests solve the technical problems such as low mechanical strength and short life, which are encountered extensively in cell immobilization. The highly acid-tolerant and active yeasts are screened by domestication. A novel ethanol production process suitable for high technology has been developed according to the physiology and biochemical properties of yeast. After traditional ethanol fermentation has been industrially grafted and modified by the novel biotechnology, revenue has increased by 15–20 %. Furthermore, the technology could effectively reduce energy consumption and save much water during ethanol production from sweet sorghum stalk juice.

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In the immobilized yeast and fluidized-bed fermentation technology, live yeast cells are immobilized on a carrier. Thus, there is no yeast loss during the fermentation process and yeast concentration is ensured in the primary fermenter. The immobilized yeast has great advantages over traditional free yeast: (1) fermentation is rapid and the conversion rate is high in a fluidized bed; (2) ethanol with higher concentration is obtained, and there is less residual sugar and consumption; (3) the immobilized yeast can be used repeatedly, which saves yeast investment; (4) ethanol production from sweet sorghum stalk juice involves less consumption of water, and the process is simple; (5) sweet sorghum stalk juice is rich in nutrients and contains few impurities. Therefore, the production cost is low, and distillation and sewage treatment are simple.

1.4.3 Bioethanol production cases from sweet sorghum 1.4.3.1 Continuous solid-state fermentation process The transfer of substrates is significant in continuous solid-state fermentation of sweet sorghum. During transportation, fermentation metabolism in substrates still continues, which requires the conditions in the reactor to remain stable during the entire process, and simultaneously meet the continuous production requirements. This process uses a continuous cell fermentation mode, and each fermentation unit runs on a fixed track. The fermentation period and duration are determined based on the process parameters of the material. Once fermentation is completed, the reaction tank is moved to the discharge port, and then the interior door on the tank is opened to allow for the automatic transfer of the fermented material to the unloading pit. When the material is elevated by an auger to the distillation apparatus, the fermentation process so far is complete. The flow chart of ethanol production is shown in Fig. 1.6, and the basic operating principle of a continuous fermentation bioreactor is displayed in Fig. 1.7. Preparation of ethanol from sweet sorghum belongs to multidisciplinary, knowledge-intensive, high-tech, and highly integrated mutual penetration of emerging industries, with long-chain cross-industry characteristics from variety optimization, scale cultivation, bioengineering technology to waste energy utilization technology. The entire production process is complex with multifactor influences. Therefore, a total quality management system should place great emphasis on maintaining lowpower high-quality production.

1.4.3.2 Raw material pretreatment The storage sugars are found in stems such as sugarcane, sweet sorghum, maize, and sugar maple. The stalks are first carried to crushers where they are torn into small

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Sweet sorghum planting Stalk processing Material mixing

Yeast preparation

Pit entry fermentation

Temperature,humidity mornitoring

Distillation

Worse slag feed

Rectification Bioethanol Fig. 1.6: Ethanol production technology and process from solid-state fermentation of sweet sorghum.

Pulverizer

Blender mixer

Fermenting tank Feeding conveyor belt

Material hoist

Distillation still

Fermentation car Continuous distillation plant

Horizon

Pulverizer

Multifunctional stirring mixer

Rolling wheel

Emptyingchute

Fig. 1.7: Schematic representation of continuous solid-state fermentation equipment.

pieces for the complete transformation and utilization of the sugars. In the milling process, a greater degree of crushing is better because it improves sugar conversion and yeast fermentation. A further degree of crushing is beneficial due to an increase in the external surface area of the stalks and by mixing, the yeast and crushed stems can sufficiently come into contact with each other, which therefore accelerates the fermentation speed. In general, the stems are crushed into fine filaments with length of 2–3 cm and stem diameter of 0.5–1 mm.

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1.4.3.3 Yeast activation Sweet sorghum is one of the many varieties of sorghum grass whose stalks have high sugar content. The fermentation process mainly depends on the conversion of sugars, and it occurs in the presence of yeast or bacteria. There is a direct relationship between the yeast level and fermentation time. Yeast cultivation uses sweet sorghum syrup as a medium. The amount of yeast is about 10–15 % of the weight of sweet sorghum juice, and the juice sugar Brix is about 5–8 %. Yeast activation requires strict sterility, and the yeast cells must remain fresh and vital. During cultivation, yeast storage time, application method, and container disinfection should be rigorously controlled.

1.4.3.4 Joint blender mixer The TTDHL-01 joint mixer is specially designed for solid-state fermentation of sweet sorghum. The mixer has multifunctions involving raw material warming, humidification, yeast adding, and materials mixing. Fig. 1.8 shows the workflow of the joint blender mixer. Raw materials humidifying

Crushed material

Raw material warming

Yeast, etc additives

Steam heating Mixing Fig. 1.8: The workflow of the TTDHL-01 joint blender mixer.

The TTDHL-01 joint blender mixer is a biaxial paired, auger propelled, continuous mixer specially designed for crushed sweet sorghum stalk fiber. Materials are stirred through a lifting lever fixed in an auger blade. This machine can uniformly mix the material, meanwhile achieving substrate warming, humidification, and addition of yeast. The mixer uses a continuous injection mode to feed the materials and yeast in the mixing chamber. Substrate warming is achieved by indirect heat exchange between steam and material. Joint blender mixer exhibits the following characteristics: capable of mixing loose materials, stable and safe operation, easy control, and low noise. It can also reduce

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energy consumption and production costs. Moreover, it improves the working environment. The crushed and tempered stalks are mixed with yeast in certain proportions, the yeast and material weight ratio is about 1–1.5, and both should be stirred sufficiently for proper mixing. Furthermore, acidity and moisture adjustments are required, pH should reach 4.5–5, and material water content should be about 70–75 %. The mixed materials can be directly fed into the fermentation car, and another empty one can be used to replace the filled one. The filled car is pushed into the fermentation chamber for initiation of the fermentation process.

1.4.3.5 Fermentation 1.4.3.5.1 Fermentation equipment Fermentation is the key step in solid-state fermentation. The reactors using sweet sorghum as raw materials are different from wine brewing fermentation tanks using grain as substrates. The main fermentation substrates in sweet sorghum are sugars. Substrates transfer is important in sweet sorghum solid-state fermentation. During transportation, the materials need to complete continuous fermentation metabolism, which not only requires the bioreactors to ensure the stable reaction conditions and unaffected product metabolism, but also meet the continuous production requirements. Each fermentation unit is operated in a fixed orbit and the fermentation period and duration are determined by the material fermentation period. Once fermentation is completed, the reaction tank is moved to the discharge point, and the interior door on the tank is opened to allow for the automatic transfer of the fermented material to the unloading pit. Then the material is elevated through an auger to the distillation apparatus. The capacity of the fermentation tank should not be too large, for too large a volume can result in uneven temperature distribution and long discharge times, which easily lead to ethanol evaporation losses. Therefore, the effective tank volume is defined as 8 m3 , and its structure should meet the solid-state fermentation technology of sweet sorghum and easy operation and management requirements. The fermentation workshop equipment layout should take into account the technical requirements related to crushing and distillation. Based on meeting the fermentation process requirements, the reactors should be simple and convenient in operation, and their layout should be reasonable for the production process.

1.4.3.5.2 Monitoring fermentation temperature The optimum temperature range for yeast culture is 25–28 °C, and a suitable fermentation temperature is in the range 30–32 °C. When the temperature increases above 35 °C, various types of infectious microbes start breeding. Low temperature fermentation is an effective approach to keeping yeast vitality strong, elevating alcohol re-

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sistant ability, and restraining the growth of other bacteria. A slow fermentation rate is obtained by controlling the feed temperature, and the entire fermentation temperature should follow the law of “pre-slow, moderate in middle stage, and later slow down”. Too high a feed temperature accelerates the fermentation rate and yeast breeding, which inevitably results in a rapid increase in the prefermentation temperature, thus creating favorable conditions for the growth of infectious microbes. This stops yeast propagation or leads to its death in advance. In this case, post-fermentation is sluggish thus sugars in the substrate are not completely converted. In general, the fermentation period is 1.5–2.5 d, depending on the regional and seasonal conditions. In the first day of feeding, yeast grows exponentially fast, the next day the cell population maintains a relative balance, and fermentation temperature is gradually increased to 30–32 °C, then the acidity begins to increase slowly, and ethanol concentration increases rapidly. During the 2.5 d, the system temperature tends to be stable. When it decreases slightly, the acidity in the fermentation substrates starts to increase rapidly, and large quantities of yeast begin to die. The immediate distillation process is started when the ethanol formation rate starts declining. During vaporization, decreasing the discharging time can effectively improve the ethanol yield. The fermentation temperature, sugars content in substrates, and ethanol conversion rate in solid-state fermentation of sweet sorghum are shown in Fig. 1.9.

Fermentation temperature °C Ethanol conversion rate (%) Sugars content (%)

Fig. 1.9: The fermentation temperature, sugars content in substrates, and ethanol conversion rate in solid-state fermentation of sweet sorghum.

1.4.3.5.3 pH adjustment pH is significantly important for yeast propagation, in general, pH of yeast cells is about 6. However, the suitable pH range for yeast breeding and fermentation is 4.5–5. When the pH value is less than 4.2, yeast cells can still grow normally, while infectious microbes breeding can be effectively inhibited.

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1.4.3.5.4 Fermentation and air Fermentation is a complex biochemical reaction involving gaseous, liquid, and solid substrates coexisting in a system. During anaerobic fermentation, substrates need hierarchical compactness to exhaust air between the materials. The filled tank should be covered by plastic film and sealed with insulation materials to promote anaerobic respiration of yeast or fermentation in anoxic conditions.

1.5 Ethanol production from cellulosic materials 1.5.1 Pretreatments With the objective of improving microbial polysaccharide utilization, breaking down complex and compact cellulose-hemicellulose-lignin structure is highly critical. Furthermore, reduction in crystallinity and polymerization degree of cellulose and removal of hindering lignin are highly desirable in order to increase the mean surface pore size and material surface area [56]. Currently, pretreatment techniques of lignocellulosic biomass include thermal pretreatment, chemical treatment, and biological treatment. Thermal pretreatment employs hot water or steam to pretreat lignocellulosic biomass, the reaction temperature is generally between 150 and 220 °C and most hemicellulose and some lignin are removed. Steam explosion [57, 58] and liquid hot water [59–61] are two methods studied popularly. Chemical treatments include acid [62–64], alkali [65], and organic solvents [66]. Their processes and characteristics are listed in Tab. 1.4. Some pretreatments are being researched extensively and considered most likely to achieve industrial large-scale application, mainly including dilute acid pretreatment, liquid hot water, and steam explosion.

1.5.1.1 Dilute acid pretreatment Dilute acid pretreatment dates from the 1940s and is the most mature technology at present. Different reaction processes and reactor configurations have significant impacts on the result of dilute acid hydrolysis. The reaction processes generally comprise cocurrent, crossflow, and counterflow dynamics. Cocurrent flow indicates the movement and reaction of biomass and hydrolysate in one direction, counterflow indicates that they contact and react in opposite directions; however, crossflow is irregular relative movement of raw material and hydrolysate. From a purely theoretical point of view, the counterflow reaction is better than the others. Reactors include batch fixedbed, plug-flow, flowthrough, shrinking flowthrough, and screw conveyor reactor. Their characteristics and dilute acid hydrolysis results are listed in Tab. 1.5. We speculate that the research trends of efficient dilute acid hydrolysis for lignocellulosic biomass are multistep treatment, countercurrent process, and dynamic reaction. The multistep treatment can lead to different hydrolysis products under different conditions

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Tab. 1.4: Pretreatment processes of several typical biomasses and their characteristics. Steam Saturated steam (160–290 °C) explosion reacts for a few seconds or minutes, then decompresses instantly at 0.69–4.85 MPa pressure Ammonia 1–2 kg ammonia is required explosion per 1 kg biomass at 90 °C, under 1–1.5 MPa pressure for 30 min, then depressurized CO2 explosion Liquid hot water

Dilute acid hydrolysis

Alkali pretreatment

4 kg CO2 per 1 kg material is required, pressure is 5.62 MPa About 200 °C temperature, pressure greater than 5 MPa, reaction time is 10–60 min, concentration of material is less than 20 % 0.01–5 % H2 SO4 , HCl, HNO3 , or organic acids such as formic acid and maleic acid are utilized, the material concentration is 5–10 %, reaction temperature is 160–250 °C, pressure is less than 5 Mpa 1–5 % NaOH, 24 h, 60 °C; saturated lime, 4 h, 120 °C; 2.5–20 % ammonia water, 1 h, 170 °C

Methanol, ethanol, acetone, ethylene glycol, tetrahydrofurfuryl alcohol, or mixtures, treatment temperature is 185 °C or more for 30–60 min; or 1 % H2 SO4 or HCl is added Biological White rot fungi [70] and pretreat- termites [71], etc. ment

Organic solvent pretreatment

The removal of hemicellulose reaches 80–100 %, 45–65 % of which is xylose recovery, cellulose undergoes certain decomposition and less lignin is removed, enzymatic hydrolysis of residues reaches more than 80 % [67, 68], and there are several sugar degradation by-products No significant removal of hemicellulose [69], a certain decomposition of cellulose and 20–50 % delignification, the enzymatic hydrolysis of residues reaches more than 90 %, with less sugar degradation by-products; however, it requires recovery of ammonia Enzymatic hydrolysis of residues reaches more than 75 % with less sugar degradation by-products 80–100 % removal of hemicellulose [32], about 80 % of which is xylose recovery, cellulose breakdown occurs to some extent and less lignin is removed, enzymatic hydrolysis of residues reaches more than 90 %, with less sugar degradation by-products 100 % removal of hemicellulose, about 80 % of which is xylose recovery, cellulose breaks down intensely [32], less lignin is removed, and the residue hydrolysis is greater than 90 %, with many sugar degradation by-products

Compared to acid hydrolysis, the reactor costs are lower; more than 50 % hemicellulose is removed, 60–75 % of which is xylose recovery, cellulose is converted and about 55 %, lignin is removed, and the enzymatic digestibility is higher than 65 % with less sugar degradation products; however, alkali needs to be recovered Almost all hemicellulose and lignin can be removed and recovered; nonetheless, the cost is high and organic solvent needs recovery

Lignin breakdown occurs by producing lignin oxidase, with low energy consumption, mild reaction conditions and no pollution; its drawbacks include long reaction period, some losses of cellulose and hemicellulose, and the process requires using other pretreatments to achieve the best result

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Tab. 1.5: The process and characteristics for some typical pretreatment reactors of cellulosic biomass. Reactor

Characteristics

Hydrolysis result

Batch fixed-bed reactor

Raw materials and reaction liquid are in nonflow dynamics, products are collected after reaction. Structure is simple; however, products are easily degradable

Total sugar yield is about 40 %

Plug-flow reactor

Raw materials and reaction liquid are in noncontinuous flow state, products are collected at any time, the material residence time can be controlled, and products degradation is reduced

Total sugar yield reaches 55 %

Flowthrough reactor

Raw materials and reaction liquid are in continuous flow state, sugar degradation is decreased; however, due to the high liquid-solid ratio, sugar concentration is low

Xylose recovery is 97 %

Bedshrinking flowthrough reactor

Raw materials and reaction liquid are in continuous flow state, and with materials’ consumption height of solid bed is gradually compressed, material density remains stable, which helps in the reduction of sugar decomposition and improvement of sugar yield

Glucose recovery is 90 %

Screw conveyor reactor

It Combines plug-flow and shrinking flowthrough processes, including horizontal spiral push and vertical countercurrent shrinking sections

The sugar yield of hemicellulose and cellulose can both reach 90 %

avoiding degradation of products. Countercurrent contact is considered as the most efficient reaction. Further, dynamic reaction, such as screw extruder, promotes the reaction well. National Renewable Energy Laboratory (NREL) of the USA has designed and developed a continuous two-step reaction system which combines plug-flow and shrinking percolation processes, including horizontal spiral flat pushing and vertical countercurrent shrinking sections. Hydrolysis of hemicellulose of biomass occurs at the horizontal spiral flat pushing section by 170–185 °C steam, and breakdown of cellulose occurs at the vertical countercurrent shrinking section by less than 0.1 % sulfuric acid at 205–225 °C. After preliminary tests, the sugar yield of hemicellulose and cellulose in yellow poplar wood both can reach 90 %. The equipment requirements are relatively high for traditional sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid; moreover, there is severe sugar degradation in the pretreatment process. Therefore, at present domestic and international research focus has changed from dilute acid (1–10 %) to ultra-low acid (less than 1 %) [72], from traditional strong acids to organic acids such as maleic acid, formic acid, acetic acid, and so on, and also to the addition of metal cocatalyst during pretreatment. Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, has hydrolyzed cellulose with 0.1 % maleic acid at 200 °C, and a cellulose

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conversion rate of 95.7 % was achieved. It has been proposed that the carboxylic acid site of maleic acid is similar to the active site of glycosidase, thus it can effectively simulate enzymatic catalysis with good selectivity. Furthermore, the weak ionization ability of hydrogen ions in maleic acid reduces the probability of an attack to the hydroxyl group in glucose molecules by protons, thus preventing the intramolecular dehydration probability within glucose [73].

1.5.1.2 Liquid hot water pretreatment Liquid hot water pretreatment is an emerging green technology because it enables the increase in cellulose digestibility through sugar extraction without the necessity of adding chemical compounds, thus producing less degradation products. Moreover, due to other advantages attributed to it, such as less inhibitory products, it has become a research hot topic [74, 75]. At present, research mainly focuses on improving the yield and concentration of hemicellulose derived sugars, as well as the hemicellulose hydrolysis mechanism by liquid hot water treatment. Factors affecting hemicellulose hydrolysis during liquid hot water pretreatment include: (1) species of raw materials; (2) types of reaction system; (3) reaction process parameters including reaction temperature, time, pressure, warm-up time, and leachate flow rate; and (4) addition of co-catalyst and other reagents. Guangzhou Institute of Energy Conversion in China developed for the first time the process of step-change flow rate liquid hot water pretreatment and step-change temperature liquid hot water pretreatment for sweet sorghum bagasse and eucalyptus wood chips. Aiming at wood biomass with higher lignification and greater cellulose crystallinity index, they proposed variable temperature liquid hot water pretreatment for enhanced sugar recovery, i.e., first the pretreatment is carried out at 180 °C for 20 min to recover hemicellulose derived sugars, which is followed by further breaking down of cellulose at 200 °C for 20 min to reduce crystallinity degree of cellulose and to remove lignin, finally making cellulose enzymatic digestibility reach 93.3 % and the total yield of sugars (xylose and glucose) reach 96.8 % [70]. Moreover, they also found that the liquid hot water pretreatment of existing forms of hemicellulose, irrespective of the type of raw materials and reaction systems, led to production of xylooligosaccharides whose amount accounted for about 40–60 % of total sugars recovered, which can be explained based on the hydrolysis kinetics of hemicellulose. Hemicellulose hydrolysis in liquid hot water involves one reaction occurring in a series, at first hemicellulose hydrolyzes to oligosaccharides, and then further decomposes to monosaccharides. Under vigorous reaction conditions, they are further degraded to furfural and other by-products. Therefore, owing to relatively mild reaction conditions compared to dilute acid hydrolysis, the degradation rate of oligosaccharides to monosaccharides is slower, showing a large number of oligosaccharides in hydrolysate. Therefore, green characteristics combined with inexpensive lignocellulosic biomass make liquid hot water pretreatment for lignocellulosic materials to prepare functional oligosaccharides become an active research hotspot [76].

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1.5.1.3 Steam explosion pretreatment Steam explosion pretreatment involves the use of steam under high temperature and pressure to treat biomass for a specific time, which leads to softening of cellulosic material. In the steam explosion process, high-pressure steam enters into the core of the raw material, which is followed by the explosive decompression of the biomass in order to release the excessive pressure. This leads to the repeated shear deformation of the raw material under the impact of expanding gases, thus achieving rupturing of the biomass fibers’ rigid structure, chemical separation, mechanical division, and rearrangement of material. This results in better accessibility of the cellulose for enzymatic hydrolysis. In China, COFCO has adapted to continuous steam explosion pretreatment, and built a cellulosic ethanol pilot plant for producing bioethanol from corn stover. It has the capacity of 500 tons/year and has been successfully put to trial. However, the pure water steam explosion method leads to a high loss of hemicellulose, low delignification, high concentrations of toxic by-products, and other shortcomings, which significantly affect the yield of ethanol fermentation. Addition of a small amount of a chemical catalyst in the steam explosion process such as SO2 , CO2 , or NH3 can significantly reduce energy consumption and increase enzymatic digestibility of residues. This is attributed to the fact that under acidic or alkaline conditions, hemicellulose and lignin of lignocellulosic biomass are more easily removed, in particular, under alkaline conditions. For example, the reaction temperature of ammonia explosion pretreatment can be decreased to 90 °C, about 20–50 % lignin is removed, and enzymatic digestibility of cellulose can reach 90 %.

1.5.2 Enzymatic hydrolysis and ethanol fermentation of cellulosic materials 1.5.2.1 Types of cellulase and their structures Cellulase is a type of hydrolysis enzyme that can degrade cellulose to oligomeric glucose, cellobiose, and glucose. Cellulase is divided into cellulase complexes and noncomplex cellulase depending on its different structures. Cellulase complexes are multi-enzyme protein complexes of supramolecular structure linked by multiple subunits. They are mainly found in vinegar Vibrio, Bacteroides, Butyrivibrio, Clostridium, Ruminococcus, and some anaerobic bacteria, also partly found in anaerobic fungus [77, 78]. Cellulase complexes produced by different microorganisms vary in structure; however, they basically consist of four parts: scaffolding protein, conjugate of aggregation protein and anchoring protein, substrate-binding region, and subunit [79, 80]. The scaffolding protein does not have catalytic activity, but includes multiple modules of type I aggregation protein, which can combine with an enzyme subunit having anchored protein. Through conjugation of aggregation protein and anchoring protein a variety of subunits are formed. Conjugation of aggregation protein and anchoring protein occurs in two ways: type I anchoring protein can bind to type I aggregation protein and its main function is to anchor a subunit on scaffolding protein. Type II an-

46 | 1 Biomass ethanol fuel technology

choring protein binds to type II aggregation protein and its main function is to anchor scaffolding protein on the surface of the bacterial cell wall. The main function of the substrate-binding region is to combine with substrate. There are a variety of enzyme subunits, which demonstrate activities of endo-glucanase and exo-glucanase, hemicellulase, chitinase, pectinase, and other polysaccharide degrading enzymes, whose main function is degradation of plant cell walls. Noncomplex cellulase is mainly produced by aerobic filamentous fungi, such as Ascomycetes, Basidiomycetes, and other species [81]. It is a mixture composed of three different enzymes, namely, endoglucanase, exo-glucanase, and β-glucosidase. Endo-glucanase acts on amorphous regions of cellulose and randomly cleaves internal glycosidic linkages of cellulose chains, degrading long-chain cellulose to short-chain. Exo-glucanase acts on crystalline regions of cellulose and cuts from the cellulose chain ends in the cellobiose unit, releasing cellobiose. β-glucosidase acts on glucose oligomer and cellobiose to produce glucose [82]. The structure of most cellulases consists of the following three parts: cellulose-binding domain, catalytic domain, and a linker connecting these two domains. The cellulose-binding domain and the catalytic domain have a plurality of families, which are connected in different combinations to form a variety of cellulases. The cellulose-binding domain mainly functions in the initial reaction stage leading to the binding of cellulase to cellulose fiber and ensuring the efficiency of catalytic enzymatic hydrolysis. Some cellulases cannot be desorbed after binding with cellulose with the aid of a binding domain, but some can be, which depends on the desorption temperature. In addition to playing the role of binding, the cellulose-binding domain can penetrate into the cellulose crystalline regions to promote the crystallization zone to turn loose, and to prevent re-crystallization of loose structure. The main role of the catalytic domain is to act on the β-1,4-glycosidic bonds for cellulose degradation. The linker mainly maintains a certain distance between the binding domain and the catalytic domain. Too short a linker can affect the hydrolytic effect of cellulase [31, 83– 87].

1.5.2.2 The hydrolysis mechanism of cellulase The hydrolysis mechanism of cellulase complexes has not been elucidated; therefore, here we focus on the hydrolysis mechanism of noncomplex cellulase. In 1950, Reese et al. [88] put forward the first hypothesis of the cellulose hydrolysis mechanism, namely, the C1/Cx hypothesis. Based on this hypothesis, Wood and McCrae [89] made a synergy model of endo- and exo-enzymes which was then widely accepted in 1972. Endo-glucanase and exo-glucanase first act on the cellulose surface. Endo-glucanase acts on the β-1,4-glycosidic bonds within a cellulose molecule to produce short-chain cellulose and new chain ends. Exo-glucanase acts only on the glucan chains with polymerization degree greater than 10, and cuts cellulose from the chain ends into units of cellobiose, producing cellobiose and glucose. Under the combined effect of endo-enzyme and exo-enzyme, cellulose is degraded to soluble oligomers of glucose

47

1.5 Ethanol production from cellulosic materials |

whose polymerization degree is less than 7, which are further degraded to glucose under the action of β-glucosidase [90–92]. Cellulase acts on the glycosidic bond to degrade cellulose. There are the following two molecular mechanisms for rupturing glycosidic bond by cellulase: the maintain mechanism and the reversing mechanism as shown in Fig. 1.10. Fig. 1.10(a) shows the maintain mechanism, demonstrating that after binding together of the enzyme and cellulose, two catalytic carboxyl groups which are located on flat sides of the sugar ring with about 5.5 Å distance, through double substitution reaction, split up the glycosidic bond and the conformation of substrate anomeric carbon (C1) remains the same. In general, the procedure is as follows: the first step is glycosylation in which one acid-catalyzed carboxyl group offers a proton to a carbohydrate group, and another catalyzed carboxyl group carries out a nucleophilic attack at the anomeric carbon forming a Glycosyl-Enzyme Intermediate. The second step is deglycosylation, in which the carboxyl group which has provided a proton in the first step captures a proton from a nucleophile in the reaction system, leading to the activation of the nucleophile (a water molecule or a new sugar hydroxyl group) which is involved in the reaction of Glycosyl-Enzyme Intermediate to replace a sugar group, which results in the breaking of a glycoside bond. Fig. 1.10(b) outlines the reversing mechanism, demonstrating that when the enzyme and cellulose are bound, two catalytic carboxyl groups which are located on flat sides of the sugar ring with 6.5–9.5 Å distance, through single nucleophilic substitution reaction, break down glycosidic bonds. The conformation of substrate anomeric carbon (C1) becomes opposite, that is, after the hydrolysis of β-glycosidic bonds, β-conformation of substrate anomeric carbon (C1) turns to α-conformation and vice versa. There are two catalytic carboxyl groups particiAB

O HO

OH OH

OH O

AB

O HO

O O

O–

O

OH

O

AB

OH OH O

O

O

OH OH NU

OH OH

OH O

–

AB

O HO

O

O

OH OH NU

O

OH O

H OH OH O–

OH

H

OH O

OH O

O–

H2O O

OH O–

OH OH NU

(a)

O

H H

OH OH NU

O

HO OH OH

O O

OH OH O– HO

O

HO

HO

AB

O

NU

AB

O

O

O

OH

NU

O

(b)

Fig. 1.10: Two mechanisms of hydrolysis of glycosidic bonds by cellulase (a) the maintain mechanism and (b) the reversing mechanism.

48 | 1 Biomass ethanol fuel technology

pating in the reaction mechanism, one carboxyl group obtains a proton from a water molecule by acid catalysis, and another carboxyl group donates a proton to a glycosyl by acid catalysis, subsequently OH− of a water molecule links with the sugar anomeric carbon from the opposite side of other glycosylation ring plane by a nucleophilic reaction, leading to the glycosidic bond cleavage [93, 94].

1.5.2.3 Factors affecting enzymatic hydrolysis Factors affecting the efficiency of enzymatic hydrolysis can be roughly divided into two categories: one is associated with the substrate, such as lignin, hemicellulose, and crystallinity; the other is related to cellulase, such as cellulase activity and feedback suppression of product.

1.5.2.3.1 Factors related to substrate Lignin and hemicellulose are the first and the most important factors associated with the substrate that affect the efficiency of cellulase hydrolysis. On the one hand as a physical barrier, lignin hinders cellulase access to the cellulose; on the other hand by hydrophobic interactions, effects of ionic bonds, and hydrogen bonding, lignin adsorbs cellulase, leading to invalid combination. Moreover, the actual amount of cellulase that participates in enzyme hydrolysis declines [95–99]. Studies have shown that lignin with more phenolic hydroxyl groups exhibits a stronger inhibitory effect on enzymatic hydrolysis compared to lignin containing non-phenolic hydroxyl groups. Furthermore, hemicellulose acts as a physical barrier, which prevents the contact between cellulase and cellulose, thus inhibiting enzymatic hydrolysis [100, 101]. Characteristics of lignocellulosic material, such as grain size, specific surface area, pore size, crystallinity, and the substrate concentration can also affect enzymatic hydrolysis. It has been reported that compared to micron sized particles, the enzymatic hydrolysis effect of biomass with submicron size is better. The specific surface area is inversely proportional to the particle size, and the smaller the particle size, the greater the surface area. This leads to more favorable binding of cellulase, thus resulting in easy hydrolysis. Large material surface aperture is beneficial to digestion. A reduction in crystallinity degree can improve the initial rate of enzymatic hydrolysis. Higher substrate concentration leads to worse heat and mass transfer. Furthermore, increasing the probability of lignin invalid adsorption to cellulose is not conducive to enzymatic hydrolysis [102–105].

1.5.2.3.2 Factors related to cellulase Cellulase with high enzyme activity can help improve the enzymatic hydrolysis efficiency and reduce production costs. Cellobiose products with high concentrations can combine with the tryptophan residue located near the catalytically active site of exo-glucanase, resulting in steric effects which prevent the contact of enzyme with

1.5 Ethanol production from cellulosic materials |

49

substrate, thus hindering digestion. When the glucose concentration is too high, not only the activity of β-glucosidase is inhibited, but also the activity of cellulase is suppressed. Moreover, xylan, xylooligosaccharides, and xylose all can inhibit enzyme activity of cellulase, and the inhibition effect of xylooligosaccharides on cellulase is stronger than that of xylan and xylose [106–109].

1.5.2.3.3 Measures to improve hydrolysis efficiency of lignocellulose To improve the enzymatic hydrolysis efficiency of lignocellulose, it is essential to reduce or even eliminate the impact of negative factors on enzymatic hydrolysis. Pretreatment can eliminate the adverse effects of lignin, hemicellulose, and the lignocellulosic structure itself on cellulose. Adverse effects of substrate concentration on cellulase can be eliminated by reducing viscosity and heat and mass transfer mode of the reaction system. For example, we can carry out a batch feed process in enzymatic hydrolysis [110, 111], and the process is briefly as follows: First, biomass undergoes enzymatic hydrolysis under low substrate concentration for some time, then adequate substrate is fed for further enzymatic hydrolysis for a period of time. Furthermore, substrate is again fed, and according to the actual situation determines digestion and feeding time, until the final substrate concentration meets requirements, and hydrolysis continues for a while. This process ensures that the viscosity of the reaction system is maintained at a low level, which is conducive to efficient digestion and to obtaining a high concentration of sugar. An adverse effect of product concentration on the cellulase can be eliminated by removing the product from time to time to maintain its concentration, always at a low level. For example, addition of β-glucosidase degrades cellobiose to glucose, which can eliminate the inhibition effect of cellobiose. The inhibition effect of glucose can be eliminated in several ways. (1) By using a membrane reactor: Biomass enzymatic reaction is carried out in a membrane reactor, glucose and other small molecules produced can pass through the membrane and be promptly removed by ultrafiltration; however, cellulase, other macromolecules, and biomass remain trapped in the reactor, where enzymatic hydrolysis is continued, thus eliminating the product inhibitory effect on cellulase to improve the biomass efficiency and sugar concentration [112–115]. (2) Simultaneous saccharification and fermentation: In the same reactor, enzymatic hydrolysis and fermentation are carried out at the same time. The monosaccharides from biomass are consumed in a timely fashion to prevent monosaccharide accumulation, which eliminates the product inhibition effect on cellulase, promotes hydrolysis, and increases ethanol yield [116, 117].

1.5.2.4 Ethanol fermentation Pretreated hydrolysate of lignocellulosic materials contains large amounts of hemicellulose derived sugars, and fermentation inhibitors, such as formic acid, acetic acid,

50 | 1 Biomass ethanol fuel technology

furfural, 5-hydroxymethyl furfural (HMF), and aromatic compounds, which lead to poor fermentation by traditional microbes. In general, the detoxification treatments of hydrolysate have the following three aspects: (1) optimization of the pretreatment process to avoid or reduce the generation of inhibitors; (2) detoxification treatment for hydrolysis products prior to fermentation; and (3) development of highly tolerant bacterial strains to achieve in situ detoxification. Lee et al. [118] used activated carbon for detoxification of liquid hot water-treated hydrolysate of hardwood pieces, and found that 2.5 % activated carbon could remove 42 % formic acid, 14 % acetic acid, 96 % 5-hydroxymethyl furfural, and 93 % furfural in hydrolysate; however, about 8.9 % sugar was lost. Then detoxified hydrolysate was fermented with a geneticallymodified thermophilic anaerobic bacterium MO1442 strain, capable of metabolizing glucose, xylose, and arabinose, and reaching 100 % theoretical ethanol yield. Yang Xiushan from Capital Normal University cultivated a new strain of Pichia stipitis Y7, capable of in situ detoxification of lignocellulosic hydrolysate after dilute acid pretreatment. This resulted in efficient conversion of glucose and xylose in lignocellulosic hydrolysate to ethanol, reaching 93.6 % of maximum ethanol theoretical values [119]. In situ detoxification can simplify the ethanol production process from lignocellulosic materials and reduce the cost of ethanol production, thus exhibiting important theoretical and practical significance for lignocellulosic ethanol production and commercialization.

1.5.3 A case of cellulosic ethanol production Shandong Longlive Bio-technology Co., Ltd. and Shandong University have cooperated and used corn cobs as raw material for ethanol production. They have achieved industrialization of a corn cob biorefinery. In “Technology and Application of Cellulosic Ethanol from Corn Cob Waste”, they used waste from corn cob xylose processing for cellulase and fuel ethanol production, which not only transfers the cost of raw materials and pretreatment to high value-added products, but also can prepare enzymes on the spot for ethanol fermentation. At the same time, the pretreatment stage leads to efficient conversion of some corn cob hemicellulose to xylooligosaccharides, xylitol, and other high value-added products, thus solving the issue related to the low conversion rate of hemicellulose sugars in biomass to ethanol. The remaining lignin is also used in the production of high value chemical products, thus improving the overall economic efficiency of the production process and forming a reasonable industrial structure with diversified products. Eight tons of dry corn cob can produce about 1.5 t ethanol, 1.5 t xylose-related products, more than 1 t lignin, and 1.5 t CO2 . Moreover, waste can be fermented to produce biogas, which truly aids in achieving complete use of corn cob (Fig. 1.11). This new technology not only broke through many technical bottlenecks, but also took an international lead by building a pilot plant with an annual output of 3,000 tons cellulosic ethanol from corn cob, which makes the ethanol

1.6 Analysis of the economics of ethanol fuel production | 51

Crystalline

Corn

Corn Grain

Corn Starch

Modified Starch

Corncob

Biogas power

Residue

Maltose

XOS + Xylitol

Cellulosic Ethanol

Fuel Ethanol

Enzymatic Lignin

Polymeric Material

Corncob Residue

Fig. 1.11: Biorefinery strategies of entire corn plant of Shandong Longlive Co., Ltd.

production cost close to the levels of food ethanol. Recently, the National Development and Reform Commission has formally approved Longlive Company’s project for 50,000 tons cellulosic ethanol. It has become the first cellulosic ethanol sentinel plant and the world’s first industrialized business that realizes cellulosic resources in biorefining. Its products have entered the gasoline sales market due to cooperation with Sinopec.

1.6 Analysis of the economics of ethanol fuel production Three types of raw materials can be used to produce ethanol fuel: starch, sugar, and cellulose. Internationally, the cost of producing ethanol fuel from these three types of raw material consists of raw material expenses, the cost of transporting raw material from producers to processors, the fixed plant investment costs, and the expenses involved in daily operations management. C = CRaw material + CTransport + CFixed plant investment + COperation management

(1.10)

where C is the total cost of producing ethanol, and CRaw material , CTransport , CFixed plant investment , and COperation management are constituents of the total cost. – Raw material costs: Raw material costs are the primary constituent of the cost of producing ethanol and generally account for 60–80 % of the total cost. Raw material costs are related to technological levels and market prices and unrelated to plant size. Over a certain period of time, raw material costs vary within a certain

52 | 1 Biomass ethanol fuel technology

–

–

–

range; enterprises with high technological levels incur low raw material costs due to high utilization of raw materials. Transport costs: Transport costs are related to the concentration of raw material and plant size. More concentration of the raw material and smaller plant size lead to a decrease in transportation costs. In contrast, more decentralized raw material and bigger plant size lead to an increase in the radius for gathering raw material, thus resulting in higher expenses. Fixed plant investment: Fixed plant investment is generally calculated on the basis of annual depreciation. Therefore, fixed plant investment costs are related to technological level and plant size. A higher technological level leads to lower investment per unit of product, thus decreasing the equipment costs. Similarly, bigger plant size results in lower investment per unit of product, which finally leads to lower equipment costs. Operations management costs: Operations management costs are also related to technological levels and plant size. The higher the technological level, the lower the operations management costs. Moreover, the bigger the plant size, the lower the operations management costs.

However, from the perspective of China’s current technological level, fuel and power costs should also be included in the cost of producing ethanol fuel. Furthermore, the prices of ethanol and raw material are controlled by the market and cannot be accurately determined; therefore, the economic analysis is limited to the cost of ethanol fuel, regardless of profits and returns on investment.

1.6.1 Analysis of the economics of ethanol fuel production using starch raw material Starch is the primary raw material used for producing ethanol fuel in China, and corn and sweet potatoes account for 90 %. The cost of producing ethanol fuel can be roughly estimated based on The National-level Criteria for the Main Economic and Technical Indicators in the Alcoholic Beverage Industry published by the Food Authority of the former Ministry of Light Industry in 1987 and the current prices of domestic corn, sweet potatoes, electricity, and coal. A systematic analysis of the economics of ethanol fuel production using starch raw material was carried out by taking corn as an example. The description is as follows: Corn is the primary raw material used for production of ethanol fuel in China. Although the preparation technologies differ, about three tons of corn are required for producing one ton of ethanol, and based on this ratio the following calculations were made. The price of raw material is the major factor affecting the cost of ethanol production, and raw material costs account for 60–80 % of the total cost. The current prices of domestic corn are listed in Tab. 1.6.

1.6 Analysis of the economics of ethanol fuel production | 53

Tab. 1.6: The market prices of the raw material for ethanol (corn) production in June 2014 in China. Region

Province (city)

Price (yuan/ton)

Northeast China

Liaoning Jilin Heilongjiang Beijing Tianjin Hebei Shaanxi Shanghai Fujian Zhejiang Jiangsu Guangdong Guangxi Shaanxi Ningxia Chongqing Sichuan

2,240 2,140 2,060 2,300 2,280 2,187 2,115 2,400 2,450 2,527 2,310 2,440 2,470 2,240 2,300 2,480 2,570 2,343 ± 13

North China

East China

South China Northwest China Southwest China Nationwide

According to The National-Level Criteria for the Main Economic and Technical Indicators in the Alcoholic Beverage Industry published in China in 1987, the consumption of the primary matter required for producing ethanol using corn is roughly summarized in Tab. 1.7. Tab. 1.7: The national criteria for the main economic and technical indicators in the alcohol industry (with corn as the raw material). Item/enterprise grade

First-grade national enterprises

Second-grade national enterprise

Third-grade national enterprise

Material consumption Liquor yield of 95° starch % Water consumption/T Coal consumption (standard coal) kg/T Power consumption KWh/T

National criteria 55 100 650

National criteria 53 120 750

National criteria 52 140 850

220

240

260

* Material consumption does not include comprehensive utilization.

Different regions and different production technologies lead to significant differences in the price of equipment, the degree of manufacturing, the source and price of building material, and the prices of raw material purchase and transportation. In order to

54 | 1 Biomass ethanol fuel technology

facilitate the production cost estimation and economic calculation of ethanol fuel produced from starch raw material, a number of coefficients were assumed, as listed in Tab. 1.8. Tab. 1.8: The coefficients for calculating the cost of producing ethanol from corn. No.

Item

Coefficient

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Total investment (yuan/ton of ethanol) Bank interest ( %) Equipment investment (yuan/ton of ethanol) Depreciation of equipment ( %) Equipment maintenance ( %) Personnel costs (yuan/person/year) Biomass material (yuan/ton) Chemicals (yuan/ton of ethanol) Water charges (yuan/ton of ethanol) Coal (yuan/ton) Electricity (yuan/KWh) Transport costs (yuan/ton of corn) Distillers Dried Grains with Solubles (yuan/ton) Carbon dioxide (yuan/ton) Corn oil (yuan/ton) Fusel (yuan/ton)

5,500 5.8 2,000 5 1 20,000 2,300 50 10 200 0.4 30 500 360 9,000 6,000

Based on the assumptions listed in Tab. 1.8, an annual output of 100,000 tons of ethanol fuel is considered as an example. Moreover, in the case of highly comprehensive utilization, namely, simultaneous recovery of by-products including concentrated protein feed, fusel oil, CO2 , and corn oil, the estimated results of the cost of producing ethanol fuel from corn raw material are listed in Tab. 1.9. Plant size and production technology significantly affect the project investment, investment efficiency, and production costs. In general, plants of different sizes for producing ethanol fuel from starch raw material invest 5,500–7,000 yuan in producing one ton of ethanol. According to the values listed in Tab. 1.9, raw material costs are the primary constituent of the cost of producing ethanol fuel and account for 88.7 % of the total cost when coal is priced at 2,300 yuan per ton. Fig. 1.12 shows the effect of the price of corn on the cost of producing ethanol fuel when other conditions remain unchanged. When the raw material is priced at 1,500 yuan per ton, the cost of producing ethanol is 5,000 yuan per ton. However, if the price of the raw material increases to 1,800 yuan per ton, the cost of producing ethanol increases to about 6,000 yuan per ton, indicating that the market price of the raw material is an important factor affecting the cost of ethanol production. Obviously, the price of raw materials also affects other indicators such as the price of ethanol and profits. According to the

1.6 Analysis of the economics of ethanol fuel production

| 55

Tab. 1.9: Analysis of the cost of producing 100,000 tons of ethanol fuel annually from corn. Item

Amount (ten thousand yuan)

Total investment Fixed costs

Cost of production

Total cost of production By-product recovery

1. Bank interest 2. Depreciation of equipment 3. Equipment maintenance 4. Personnel costs 1. Corn 2. Chemicals 3. Water consumption 4. Coal 5. Electricity 6. Transport costs 1. DDGS 2. Carbon dioxide 3. Corn oil 4. Fusel

Cost of ethanol production (Yuan/ton)

Actual cost per ton

45,000 2,030 1,000 200 2,000 69,000 500 100 1,200 880 900 77,810 4,000 5,600 900 270 6,704

8,500 8,000 7,500 7,000 6,500 6,000 5,500 5,000 4,500 1,600

1,800

2,000

2,200

2,400

Corn price (Yuan/ton) Fig. 1.12: The effect of the price of corn on the cost of producing ethanol fuel.

national criteria for the economic and technical indicators in the ethanol industry, the profit-taxation rate of first-grade ethanol manufacturers should reach above 40 %. Based on this result, the curve for the effect of the price of raw material on the price of ethanol is shown in Fig. 1.13. When the price of the raw material increases from 1,500 to 1,800 yuan per ton, due to the addition of taxes and profits of 40 %, the theoretical price of ethanol increases

56 | 1 Biomass ethanol fuel technology

from 7,000 to 8,400 yuan per ton, far exceeding the market price of absolute ethanol (Fig. 1.14) and significantly higher than the price of ethanol as fuel.

Ethanol price (Yuan/ton)

12,000 11,000 10,000 9,000 8,000 7,000 1,600

1,800

2,000

2,200

2,400

Corn price (Yuan/ton)

Fig. 1.13: The effect of the price of corn on the price of ethanol fuel.

Yuan/ton

The price chart of domestic ethanol from 2012 to 2014

6,400 6,200 6,000 5,800 5,600

Northern Jiangsu Shandong Jilin

5,400

14–6–20

14–4–20

14–2–20

13–12–20

13–10–20

13–8–20

13–6–20

13–4–20

13–2–20

12–12–20

12–10–20

12–8–20

12–6–20

5,200

Fig. 1.14: Changes in the average market price of China’s absolute ethanol from June 2012 to June 2014.

Notably, only when the price of the raw material is less than 1,500 yuan per ton, can ethanol fuel be competitive and profitable. Furthermore, only when the price of raw material is below 1,200 yuan per ton is it possible for the price of ethanol fuel to be equal to the price of gasoline and maintain the profit-taxation rate in accordance with the national criterion. In the United States, the low price of corn, 650 yuan per ton, is one of the major reasons for the widespread application of ethanol fuel produced

1.6 Analysis of the economics of ethanol fuel production

| 57

from corn. Therefore, to promote the production of ethanol fuel from starch raw material, China must provide the manufacturers with very preferential support policies including tax breaks in order to keep enterprises operating at low profits.

1.6.2 Analysis of the economics of ethanol fuel production using sugar raw material The common sugar raw materials used for producing ethanol fuel include sugarcane and sweet sorghum. The production cost of ethanol fuel can be roughly estimated based on The National-level Criteria for the Main Economic and Technical Indicators in the Alcoholic Beverage Industry published by the Food Authority of the former Ministry of Light Industry in 1987 and the current prices of domestic sugarcane, sweet sorghum, electricity, and coal. The economics of producing ethanol fuel from sugar raw material was analyzed by considering sugarcane as an example as summarized in Tab. 1.10. Tab. 1.10: The national criteria for the main economic and technical indicators in the ethanol industry (with sugarcane as the raw material). Item/enterprise grade

First-grade national enterprises

Second-grade national enterprises

Third-grade national enterprises

Matter consumption Liquor yield of sugar % Water consumption T/T Coal consumption (standard coal) kg/T Power consumption KWh/T Profits and taxes per capita ten thousand yuan per person

National criteria 55 60 500 40 2

National criteria 53 80 600 45 1.8

National criteria 51 100 700 55 1.5

* Material consumption does not include comprehensive utilization.

In this case also, the price of the raw material is the primary factor influencing the cost of producing ethanol fuel from sugarcane raw material. Irrespective of the preparation technologies, about 14 tons of sugarcane are required for producing one ton of ethanol, and the following calculations were made on this basis. The price of domestic sugarcane in 2014 was 450 yuan per ton. The technical economics of producing ethanol fuel from sugar raw material was analyzed by considering production of ethanol from sugarcane as an example. In order to facilitate the production cost estimation and economic calculation of ethanol fuel produced from sugar raw material, a number of coefficients were assumed, as listed in Tab. 1.11. Unlike starch raw material, which has to be crushed, cooked, and saccharified, sugar raw material only needs to be squeezed; therefore, the investment per unit of

58 | 1 Biomass ethanol fuel technology

Tab. 1.11: The coefficients for calculating the cost of producing ethanol fuel from sugarcane raw material. No.

Item

Coefficient

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Total investment (yuan/ton of ethanol) Bank interest ( %) Equipment investment (yuan/ton of ethanol) Depreciation of equipment ( %) Equipment maintenance ( %) Personnel costs (yuan/person/year) Raw material costs Chemicals (yuan/ton of ethanol) Water charges (yuan/ton of ethanol) Coal (yuan/ton) Electricity (yuan/KWh) Transport costs (yuan/ton of sugarcane) Bagasse (yuan/ton) (3.22 tons/ton of ethanol) Carbon dioxide (yuan/ton)

3,000 5.8 1,800 5 1 20,000 6,300 60 6 200 0.4 30 200 240

product is low, generally being 3,000 yuan per ton. Based on the assumptions listed in Tab. 1.12, considering the annual output of 100,000 tons of ethanol fuel produced from sugarcane as an example, and in the case of highly comprehensive utilization, namely, simultaneous recovery of by-products including bagasse and CO2 , the estimated costs of producing ethanol fuel from sugarcane are listed in Tab. 1.12. Tab. 1.12: Analysis of the cost of producing 100,000 tons of ethanol fuel annually from sugarcane. Item Total investment Fixed costs

Production costs

Total production costs By-product recovery

Cost per ton

Amount (ten thousand yuan) 1. Bank interest 2. Depreciation of equipment 3. Equipment maintenance 4. Personnel costs 1. Sugarcane 2. Chemicals 3. Water consumption 4. Coal 5. Electricity 6. Transport costs 71,740 1. Carbon dioxide 2. Bagasse Subtotal

45,000 1,740 900 180 2,000 63,000 600 60 1,000 160 2,100 5,600 8,440 14,040 5,770

1.6 Analysis of the economics of ethanol fuel production

| 59

Cost of ethanol production (Yuan/ton)

According to the values listed in Tab. 1.12, the cost of sugarcane raw material is still the primary constituent affecting the cost of ethanol fuel production and accounts for 87.6 % of the total cost when sugarcane is priced at 450 yuan per ton. Fig. 1.15 shows the effect of the price of sugarcane on the cost of ethanol fuel production when other conditions remain unchanged.

7,500 7,000 6,500 6,000 5,500 5,000 300

350

400

450

500

Sugarcane price (Yuan/ton) Fig. 1.15: The effect of the price of sugarcane on the cost of producing ethanol fuel.

When the raw material is priced at 300 yuan per ton, the cost of ethanol production is 4,600 yuan per ton. However, if the price of the raw material increases to 450 yuan per ton, the cost of ethanol production increases to more than 72,000 yuan per ton. According to the national criteria for the technical and economic indicators in the ethanol industry, the profit-taxation rate of first-grade ethanol manufacturers should reach above 40 %. Based on this, the curve for the effect of the price of the raw material on the price of ethanol is shown in Fig. 1.16. Similar to the case of corn ethanol, when the price of the sugarcane raw material increases from 300 to 450 yuan per ton, due to the addition of taxes and profits of 40 %, the price of ethanol increases from 6,400 to 10,080 yuan per ton, far exceeding the market price of absolute ethanol and significantly higher than the price of ethanol as fuel. Only when the price of sugarcane is less than 210 yuan per ton will ethanol fuel be competitive and profitable. Moreover, only when the price of the raw material is below 190 yuan per ton will it be possible for the price of ethanol fuel to be equal to that of gasoline, and then only the profit-tax rate will be maintained, thus reaching the national criterion. Similarly, to promote sugarcane ethanol fuel, China must provide the manufacturers with very preferential support policies including tax breaks in order to keep them operating at low profits. In reality, environmental considerations, energy, and tax policies will determine the extent of fuel ethanol utilization in the

Theoretical price of ethanol (Yuan/ton)

60 | 1 Biomass ethanol fuel technology 12,000 11,000 10,000 9,000 8,000 7,000 6,000 300

350

400

450

500

Sugarcane price (Yuan/ton) Fig. 1.16: Effect of the price of sugarcane on the price of ethanol fuel.

future. Furthermore, development of new sugarcane varieties with high-sugar energy and other sugar energy plants will strongly promote the development of sugar ethanol fuel in China.

1.6.3 Analysis of the economics of ethanol fuel production using cellulose raw material Inexpensive and economical raw material is one of the biggest advantages of producing ethanol fuel from cellulose raw material. As analyzed above, when starch and sugar raw materials are used to produce ethanol fuel, the cost of raw material accounts for over 80 % of the total cost. Therefore, cellulose has a significant advantage in terms of its low cost. However, one of the major barriers to cellulose raw material lies in its pretreatment and enzymolysis. The process cost for pretreatment and the price of the cellulase enzyme used in enzymolysis seriously restrict the production of ethanol fuel from cellulose raw material. Bioethanol production, without doubt, needs an economical approach to address global fuel needs. For the long haul, it is extremely important to understand bioethanol production technologies in terms of their economic viability, environmental feasibility, and empowering employment opportunities before implementing a fuel ethanol policy. In 1995, Qureshi and Manderson [120] made economic calculations for the technologies for producing 146.5 million liters of ethanol annually from four types of biomass raw material (wood, molasses, whey, and cornstarch). The process economics aspects indicated that the cost of producing a liter of ethanol from wood was 0.53 US dollars. In the same year, Von Sivers et al. [121] analyzed the cost economics of the following three technologies for producing ethanol from

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lignocellulose: hydrolysis with concentrated hydrochloric acid, hydrolysis with dilute hydrochloric acid, and enzyme hydrolysis. It was assumed that the plant was located in Sweden and was under continuous operation, waste pine was used as the raw material, and 333 tons of raw material (dry) was processed daily. Through calculations, the cost of producing one liter of ethanol by these three technologies was found to be 4.22, 4.29, and 4.03 Swedish krone (1 US dollar is equivalent to about 9 Swedish krone), respectively. Thus, the production costs for these three technologies can be considered to be virtually equal. In 1999, So and Brown [122] also made economic calculations for the following three technologies for producing ethanol from biomass: hydrolysis with dilute sulfuric acid, SSF process, and rapid pyrolysis/fermentation. The assumed annual output of ethanol was 25 million gallons. The cost of producing one gallon of ethanol using these three technologies was calculated to be 1.35, 1.28, and 1.57 US dollars, respectively. Considering calculation errors, So et al. believed that the production costs for these three technologies were not significantly different.

1.6.3.1 Simultaneous saccharification and co-fermentation In 1999,Wooley et al. performed economic evaluations of simultaneous saccharification and co-fermentation (SSCF) process [7]. Hardwood or corn straw was used as the raw material, and it was assumed that 2,000 tons (dry) of the raw material was processed daily. The entire process was operated for 96 % of the year, and the maintenance period for equipment was slightly greater than two weeks. The basic technological process is shown in Fig. 1.17.

Raw material crushing

Pretreatment with parallel flows of dilute acid

Liquid-solid separation and solid washing

Ion exchange and detoxification through addition of excessive alkali Detoxified sugar solution

Washed solids

Fermenter for enzyme production

SSCF fermenter

Ethanol recovery

Disposal of solid residue

Ethanol recovery

Ethanol recovery

Fig. 1.17: Process flow diagram for producing ethanol using the SSCF process.

The entire process is divided into the following parts in order to estimate the total investment: (1) Treatment and storage of raw material: The designed plant receives 136 loads of raw material, and each load of material weighs 47 tons (wet). The raw material is weighed, transported, cleaned, and stored in the plant for seven days.

62 | 1 Biomass ethanol fuel technology

(2) Pretreatment of raw material: The raw material is treated at 190 °C for 10 min using 0.5 % H2 SO4 , liberating the hemicellulose sugars and other compounds. After the pretreated raw material enters the flash evaporator, furfural, HMF, and some acetic acid generated during pretreatment are removed, following which the raw material enters the filter for liquid-solid separation. After pretreatment, a small portion of the obtained slurry solid products is used for producing cellulase, and the major portion enters the SSCF reactor. The liquid obtained through filtration is treated using ion exchange resin in order to remove 88 % of acetic acid and all sulfuric acid, and then it enters the SSCF reactor. (3) SSCF: SSCF of the hydrolysate slurry is carried out in a series of continuous anaerobic fermentation trains. Three groups of 3,600 m2 agitator stainless steel fermenters are used. Each group consists of six fermenters. The dosage of the enzyme is 15 FPUs/g of cellulose. Transgenic Z. mobilis is used as the seed for fermentation. Two groups of seed incubators are used for scale-up, and each group consists of five incubators. The inoculum size for each scale-up step is equivalent to 10 % of the container volume. The operating conditions for SSCF are as follows: temperature of 30 °C, initial concentration of solids (including soluble and insoluble) of 20 %, and a retention period of seven days. The theoretical yields of conversion from glucose and xylose to ethanol are 92 and 85 %, respectively. (4) Enzyme production: Eleven 1,000 m2 inflatable fermenters are used for intermittent operation. At any time, there are eight fermenters in operation. With T. reesei as the seed, three groups of seed incubators are used for scale-up, and each group consists of three incubators. The inoculum size for each scale-up step is equivalent to 5 % of the incubator volume. The initial concentration of cellulose in the incubators is 4 %. One kg of cellulose or hemicellulose can produce 200 FPUs of cellulase on average. The production rate of the incubators is 75 FPUs per liter per hour. (5) Product recovery and water recycling: Traditional two-tower rectification is first used to obtain azeotropic ethanol, and then vapor-phase molecular sieves are used for dehydration to produce ethanol fuel. A multi-effect evaporator is used to treat the waste liquid at the bottom of the tower. The residual slurry at the bottom of the evaporator is used for burning. (6) Wastewater treatment: The wastewater is first discharged into the neutralizer and then into the anaerobic fermentation system. This process can remove 90 % of the organic matter present in the wastewater and additionally produce medium calorific value gas (mainly including CO2 and CH4 ), which can be used as fuel. The remaining organic matter in the wastewater is removed through aerobic treatment. (7) Burner, boiler, and turbogenerator: A fluidized-bed burner is used. The fuel consists of lignin residue, the medium calorific value gas produced during anaerobic fermentation, and the residual slurry at the bottom of the multi-effect evaporator.

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The boiler produces 10.31 MPa (510 °C) to be used by the turbogenerator. 38 MW of electricity is generated, 32 MW of which are used by the plant, and the rest is sold. (8) Auxiliary equipment: This equipment is used to provide chilled water, cooling water, process water, cleaning fluid, and gas used by the plant, warehouse, and equipment. The plant is expected to be used for twenty years, with a depreciation rate of 10 %. Fixed asset investment totals 234 million US dollars, 143.6 million of which is invested in equipment. The above mentioned parts and processes account for the following proportions: 4 % for the treatment and storage of raw material, 19 % for the pretreatment of raw material, 10 % for SSCF process, 11 % for enzyme production, 10 % for product recovery and water recycling, 8 % for wastewater treatment, 1 % for the storage of products and drugs, 33 % for the burner, boiler, and turbogenerator, and 4 % for auxiliary equipment. The raw material is priced at 27.5 US dollars. The production costs are listed in Tab. 1.13. Tab. 1.13: The expected operating costs for SSCF. Item

Annual operating cost (million US dollars)

Cost per gallon of ethanol (cent)

Biomass Drugs Nutrients Diesel Make-up water Auxiliary drugs Solid waste treatment Electricity charges Fixed costs Total cost

19.31 4.0 3.22 0.48 0.45 0.59 0.61 −3.68 7.50 32.48

37.0 8.0 6.2 0.9 0.9 1.2 1.2 −7.2 13.3 61.5

The electricity generated using this technology is surplus and can be sold; therefore, the electricity charges in Tab. 1.13 have a negative value. The fixed costs listed in Tab. 1.13 cover labor, management, maintenance, insurance, and tax. The annual output of ethanol is 52.2 million gallons (198 million liters). The price of the ethanol produced using this technology is estimated to be 1.44 US dollars per gallon; however, the ethanol produced from corn is priced at 1.2 US dollars per gallon. Nonetheless, improvements in the existing technology, such as conversion of more hemicellulose into sugar during pretreatment, use of more effective cellulase, and use of better fermentative microorganisms, can reduce investment and the price of ethanol. The specific data are listed in Tab. 1.14.

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Tab. 1.14: Changes in the production cost of ethanol after technological improvement.

Production cost per gallon of ethanol Output of ethanol per ton of raw material Annual output of ethanol Expected total investment

Existing technology

Improved technology

1.44 US dollars 68 gallons 52.2 million gallons 234 million US dollars

1.16 US dollars 76 gallons 58.7 million gallons 205 million US dollars

1.6.3.2 Two-stage dilute acid hydrolysis process In 2000, Kadam et al. [123] analyzed the economics of producing ethanol by using the two-stage dilute acid hydrolysis process. Historically, a large portion of the excess agricultural residue produced in California, USA, has been disposed of by open-field burning, which leads to severe air pollution. A related concern was the growing volume of dead trees, underbrush, and small diameter green trees, mainly consisting of cork. In order to reduce air emissions and other negative impacts associated with this disposal method, cork trees were used as the raw material for ethanol production. Therefore, the establishment of this plant was also conducive to environmental protection. However, hydrolysis of cork with acid was advised due to poor heat transfer and cellulase diffusion. From the perspective of raw material sources, 800 tons of raw material (dry) was processed daily. Kadam et al. compared the life cycle analyses of acid and enzymatic hydrolysis and found that the dilute acid process was better than the enzyme process in terms of greenhouse gas potential, natural resource depletion, and acidification potential. The process is shown in Fig. 1.18. Biomass raw material

Acid

Pretretment of raw material CO2

Primary hydrolysis Water vapor

Fermentation Water vapor Distillation

Liquid-solid separation

Liquid-solid Solid separation Liquid

Secondary hydrolysis

Water vapor

Neutralization

Neutralization Lime

Lime Ethanol storage

Discharge of exhaust into of air

Acid

Steam and electricity

Boiler

Ethanol fuel Lignin residue Liquid-solid separation Waste liquid

Ash Wastewater treatment

Methane

Fig. 1.18: Flow diagram for the two-stage dilute acid hydrolysis process.

Waste water

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(1) The pretreatment and primary hydrolysis of raw material: The raw material enters the primary acid inundator at 50 °C after being crushed to 2.5 cm and is soaked in 0.7 % sulfuric acid solution. The raw material from the inundator then enters the primary hydrolysis reactor at 190 °C and is hydrolyzed using 0.7 % sulfuric acid for three minutes, which hydrolyses about 20 % of the cellulose and 80 % of the hemicellulose. The hydrolysate leaving the primary hydrolysis reactor has a solid concentration of about 30 % (including suspended solids and soluble particles). The hydrolysate enters the flash evaporator and stays for about two hours at ~ 130 °C, where most of the oligosaccharides are converted into monosaccharides. Further, the hydrolysate enters the countercurrent slurry scrubber for liquid-solid separation and removal of sugar and other soluble substances from the solids. The water used is three to four times as heavy as the solids. The sugar solution and water obtained through separation are mixed, and then enters the primary pH regulator. The solid flow (a solid concentration of 30 %) from the scrubber is further dehydrated through the spiral presser, resulting in an increase in the solid concentration to 45 %, which then enters the secondary acid inundator. (2) Secondary hydrolysis: The solid raw material that has been soaked in acid enters the secondary hydrolysis reactor at 220 °C and is hydrolyzed using 1.6 % sulfuric acid for three minutes, which leads to the conversion of about 70 % of the remaining cellulose into glucose and the other 30 % into HMF and other by-products. The hydrolysate from the secondary hydrolysis reactor also enters the flash evaporator for pressure and temperature reduction but is no longer washed. The efficiency of two-stage hydrolysis is summarized in Tab. 1.15, where the yields of monosaccharides are based on raw material. Whitewash is added to the primary pH regulator in order to neutralize sulfuric acid and increase the pH to about 5.5, which can settle most of the calcium sulfate. After the settled calcium sulfate is removed through filtration, the filtrate enters the fermenter after cooling to 35 °C. (3) Fermentation: The liquid remains in the fermentation stage for 32 hours, and transgenic seeds can be used which can not only ferment glucose but also xylose. It is assumed that the rates of conversion of glucose, mannose, and galactose into ethanol are 90 %, the conversion rate of xylose is 75 %, and the conversion rate Tab. 1.15: Yields of monosaccharides from two-stage dilute acid hydrolysis.

Glucose Xylose Galactose Mannose Arabinose

Yield of monosaccharides through primary hydrolysis ( %)

Yield of monosaccharides through secondary hydrolysis ( %)

Total yield of monosaccharides ( %)

21 70 79 79 90

34 5 11 3 0

55 75 90 82 90

66 | 1 Biomass ethanol fuel technology

of arabinose is zero. The xylose content in the hydrolysate of cork (about 7 %) is much lower than that in the hydrolysate of hardwood (about 20–25 %); therefore, the fermentation efficiency of xylose has an insignificant effect on the production cost of ethanol. (4) Ethanol recovery and refining: The traditional rectification method is used for fermentation broth to obtain azeotropic ethanol, which is then dehydrated using molecular sieves in order to obtain 99.9 % ethanol. Five percent gasoline is added to the ethanol in order to produce and store denatured alcohol. After centrifugal separation and evaporation of the residue in the rectifying column, 8 % of water can be recycled for use. (5) Waste disposal: Methane, concentrated lignin residue, and solids generated from waste liquid treatment are about 45 % and are sent to the boiler for burning and for generation of steam and electricity. It is assumed that this plant can utilize the electricity of the existing power plant, which in turn can use hydrolyzed residue and methane as fuel. Therefore, the investment is expected to be small. The entire investment is 70.4 million US dollars, 46 million US dollars of which is invested in fixed equipment. The raw material is priced at 27.5 US dollars per ton. The annual output of ethanol is 20 million gallons (76 million liters). The price of the ethanol produced using this process is estimated to be 1.2 US dollars per gallon, with a return on investment of 5 % (Fig. 1.19).

Target price US dollars per gallon

$2.00 Short-term: twenty million gallons per year Mid-term: twenty million gallons per year Long-term: twenty million gallons per year

$1.80 $1.60 $1.40 $1.20 $1.00 $0.80 $0.60 $0.40 $0.20 $0.00

Forest raw material

Agricultural waste

Municipal solid waste

Energy plants

Fig. 1.19: Analysis of the trend in the cost of producing ethanol fuel from fiber using two-stage dilute acid hydrolysis. Note: The plant for generation of electricity and ethanol using biomass; The estimates for short and middle terms include the credit for forest and straw raw material; In the long-term estimation, the cost of ethanol incurred by by-product recovery is assumed to be 0.08 US dollars per gallon (excluding raw material credit)

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1.7 Environmental impacts of fuel ethanol production and control approaches In China, ethanol production is one of the largest sources of organic pollutant emissions and this industry produces the most significant environmental pollution. If it is not controlled, the consequences will be disastrous; therefore, cleaner production of fuel ethanol is imperative.

1.7.1 Sources of pollutants during ethanol production Pollutants generated during the ethanol production process include wastewater, waste gas, waste residue, noise, and odor. Wastewater mainly contains production process water, wash water, and cooling water. Waste gas primarily consists of the exhaust gas of boilers and CO2 . Waste residue mainly comprises vinasse, slag, and waste yeast. Transport vehicles and equipment are responsible for producing noise. Gases and odors mainly arise from secondary steam, drying processes, and wastewater treatment. Pollutants can be characterized as follows: multiprocess-originated pollutants, scattered emissions, and nontoxic and harmless pollutants. Harmless pollutants are mostly recyclable. Though large amounts of water are consumed and wastewater discharge is also high, there are low recycling rates of wastewater. Among the pollution-causing products of ethanol generation, ethanol stillage is the serious source of water pollution. During ethanol production by the fermentation of a variety of biomass feedstocks (including starches, sugars, and cellulose-based raw materials), only the starch, sugar, cellulose, hemicellulose, and other carbohydrates are converted to ethanol by microorganism fermentation. The remaining protein, salt, fat, etc. are not utilized. Therefore, other organics, including nonvolatile by-products, are left in the vinasse, resulting in a large volume of high-concentration organic wastewater, which exhibits a considerable pollution potential. In addition to high-concentration organic wastewater such as stillage, low-concentration organic wastewater including wash water, wastewater from the cooking, saccharification, and fermentation processes, and cooling water from the distillation process is also responsible for environmental pollution. Therefore, substantial increases in ethanol production will also require effective solutions for stillage management. According to the statistics, if China’s fuel ethanol production reaches 10 million tons at the current level of technology, annual Biochemical Oxygen Demand (BOD) emissions will reach approximately 3.5 million tons, and Chemical Oxygen Demand (COD) emissions will reach approximately 7 million tons. This increase would lead to 50 % of China’s total industrial wastewater coming from BOD and 40 % from COD emissions. Ethanol stillage itself has no direct toxicity to humans and animals; however, it represents a high pollution load that contains many organic acids. If such a pollutant is discharged into rivers, lakes, and groundwater systems, it would be decom-

68 | 1 Biomass ethanol fuel technology

posed rapidly by microorganisms, thus resulting in an over-propagation of microorganisms. The over-propagated microorganisms could lead to a severe depletion of oxygen in water, inhibiting the growth of aquatic organisms, and destroying the natural ecology of aquatic systems. This phenomenon would also release many harmful and malodorous gases, which would lead to serious atmospheric pollution. Tabs. 1.16,1.17, and 1.18 list the various raw materials that constitute the ethanol production wastewater pollution load and emissions [5, 124]. Tab. 1.16: Wastewater pollution load and emissions in ethanol production from general starchy raw material. Source of wastewater

Vinasse Remaining water at bottom of rectifying tower Wash water Cooling water

Discharge (t/t ethanol)

pH

13–16 3–4 2–4 50–100

Pollution load CODcr (mg L−1 )

BOD5 (mg L−1 )

BSS (mg L−1 )

4.0–4.5 5.0

5–7 × 104 1,000

2–4 × 104 600

1–4 × 104

7.0 7.0

600–2,000 < 100

500–1,000

CODcr (COD is measured by K2 Cr2 O7 ); BOD5 (BOD within 5 days); SS: Suspended solids

Tab. 1.17: Wastewater pollution load and emissions in ethanol production from molasses. Source of wastewater

Vinasse Remaining water at bottom of rectifying tower Wash water Cooling water

Discharge (t/t ethanol)

pH

14–16 3–4 2–4 50–100

Pollution load CODcr (mg L−1 )

BOD5 (mg L−1 )

4–4.5 5.0

8–11 × 104 1,000

4–7 × 104 600

7.0 7.0

600–2,000 < 100

500–1,000

SS (mg L−1 )

CODcr (COD is measured by K2 Cr2 O7 ); BOD5 (BOD within 5 days); SS: Suspended solids

1.7.2 Waste treatment methods The major waste in ethanol production is ethanol stillage, which contains a very high concentration of organic pollutants, has a high degree of biodegradability, and can be comprehensively processed and utilized. This utilization can occur through a variety of physicochemical and biological technologies, such as the production of feed proteins, biogas, fertilizer, and energy, etc., for the cleaner production of fuel ethanol and better pollution control. However, the compositions of ethanol stillage materials

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Tab. 1.18: Wastewater pollution load and emissions in ethanol production from lignocellulose. Source of wastewater

Vinasse Wastewater from concentration process Remaining water at bottom of rectifying tower Wash water Cooling water

Discharge (t/t ethanol)

pH

Pollution load CODcr (mg/l)

BOD5 (mg/l)

4–4.5 2–3.5

6–8 × 104

3–4 × 104

3–4

5.0

1,000

600

2–4 50–100

7.0 7.0

600–2,000 < 100

500–1,000

15–20 25–40

SS (mg/l)

CODcr (COD is measured by K2 Cr2 O7 ); BOD5 (BOD within 5 days); SS: Suspended solids

derived from starch feedstock, sugar feedstock, and cellulose materials are quite different and must be exploited or treated according to their individual characteristics.

1.7.2.1 The production of feed The most simple and direct method for recycling ethanol stillage is to use it as feed. However, the production of biological feeding protein is generally applied only to the treatment of corn-based ethanol stillage because of its high content of proteins (35.1 %) and fats (14.7 %) [125, 126]. In contrast, nonfood starch materials such as potatoes that are used in the production of fuel ethanol contain low levels of protein and fat and high levels of fiber. This combination makes it difficult to meet the nutritional requirements of feed, and the material can only replace part of the corn-based ethanol stillage and bran feed to be used as filler in other full-price feeds.

1.7.2.2 The production of feed yeast Although the performance of ethanol slag derived from starchy material is not viable in feed applications, the residual sugar and other nutrients present in waste mash can be effectively used to produce unicellular protein yeasts as a valuable livestock and poultry feed, in addition to removing the pollutants present in waste mash. One cubic meter of vinasse can produce 20–22 kg of dry yeast. This dry yeast feed contains 8–10 % moisture and 50–52 % protein and is rich in B vitamins. After solid-liquid separation for feed production, the COD chromium (CODcr) of waste liquid can be decreased by 50 %, thus reducing the production intensity of subsequent treatment. Despite the progress in this technique, it requires further refinement to reduce costs and increase the market share of single-cell protein feed so that factories can afford to produce it.

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1.7.2.3 Anaerobic treatment After solid-liquid separation for feed production, the CODcr of vinasse remains too high to be completely eliminated. Currently, ethanol stillage derived from potatoes is mostly recommended for anaerobic treatment, which has the following advantages: it permits recycling biogas (methane or oxygen) energy while degrading COD, entails lower nutrient requirements, produces less sludge, requires a smaller reactor volume, consumes less energy, and results in lower management costs. In general, CODcr values can be reduced to 10,000–15,000 mg L−1 by dilution with low concentration wastewater, such as distilled water and washing water. Then, it can be used in the production of biogas through anaerobic treatment using anaerobic bacteria, and this process is known as biogas fermentation. The biogas, which contains 40 % CO2 and 60 % CH4 , can be directly used as boiler or power fuel, saving steam coal. After biogas fermentation, the CODcr of the waste liquid can be reduced by more than 85 %, and biogas residues can be used as fertilizers. When the CODcr of wastewater decreases to 1,500–3,000 mg L−1 , it becomes suitable for aerobic aeration treatment.

1.7.2.4 Aerobic treatment Aerobic treatment is, in general, used as a deep treatment (in post-processing by anaerobic treatment) to accelerate the metabolism of aerobic microorganisms in wastewater via artificial oxygen flux, and to thoroughly break down all biodegradable materials into CO2 and other simple molecules. The commonly used aeration device is the microporous aerator, in which the CODcr removal rate can reach up to more than 90 % (150–300 mg L−1 ), eventually meeting the national industrial wastewater discharge standards.

1.7.2.5 Dilution agricultural irrigation method Researchers in India and Australia carried out a two-year field experiment to study the effect of sugarcane irrigation with ethanol waste liquid on its yield and quality. The results revealed that the yield of sugarcane irrigated with 16 t/hm2 waste mash that was diluted 50 times and 75 times increased by 20.6 and 17.1 %, while the yield of commercial sugar increased by 25 and 20.5 %, respectively, compared to the controls without mash irrigation. The cane and sugar yields using the dilution agricultural irrigation method were significantly higher than the controls. It has been demonstrated that the quantitative application of alcohol wastewater to sugarcane fields results in a yield increase. When the method of “waste liquid spraying associated with plastic film mulching” and waste liquid spraying after open ridge cultivation of ratoon cane were used during the seeding period, it was shown that drawn cane molasses alcohol wastewater could promote sugarcane seedling growth and could increase plant height during the prometaphase. Further, application of the appropriate quantity of alcohol wastewater could also lead to an increase in stem

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length, the number of effective stems, the weight of single stems, and overall production without affecting sugar content or juice gravity purity. Wang et al. quantitatively irrigated the sugarcane field with molasses alcohol wastewater during the sugarcane seedling stage; investigated the activities of polyphenoloxydase (PPO), peroxidase (POD), catalase (CAT), and agronomic traits; and eventually concluded that the application of alcohol wastewater could improve the activities of three types of enzymes and also the sugarcane tillering rate [127]. Yu et al. studied the effects of dilute molasses alcohol wastewater for the irrigation of cabbage during different growth periods on yields and qualities, and the results showed that the addition of molasses alcohol wastewater could increase yields and promote quality and nitrogen metabolism [128]. Deng et al. studied the effects of alcohol fermentation wastewater irrigation on the growth and yield of banana, concluding that the use of alcohol fermentation liquid can significantly increase the number of green leaves, fruit number of banana plants, and ear weight per plant, simultaneously promoting early budding and increasing yields [129].

1.7.2.6 Concentration Waste mash concentrated by approximately 75x can be used directly as a commodity. For example, concentrated waste mash from some sugar refineries in Australia, Japan, and Brazil is used with other feeds such as animal feeds, mixed with sugarcane press mud to produce organic and granular fertilizers, and used as a water reducer for concrete. Methods of concentration include conventional clarification evaporation, adsorption by bagasse pith, heap fermentation, and concentration by boiler gas. Concentration can lead to the complete elimination of the emission of pollutants to the environment and total recovery of the useful substances in waste mash; therefore, it is an ideal waste treatment technology. More than ten concentration workshops were built in Guangxi Province, while two concentration workshops were constructed in the Zhongshan sugar refinery and the Kejie sugar refinery in Yunnan Province. The technology and equipment used in this method have been proven to be feasible in production practices. However, owing to ongoing problems with the equipment and processes and their high operating costs, many of the concentration workshops that have been built cannot continue to run. Therefore, this technique requires further improvement to reduce energy consumption and costs.

1.7.2.7 Combustion Use of concentrated waste mash as a fuel to produce steam and/or electricity can reasonably take advantage of the bioenergy of waste mash and recycle potassium in the waste at the same time. Combustion has its merits, such as complete processing, minimal influence of geography and climate, and production scale. Combustion exhibits certain economic benefits and was successfully employed by Nanning Sugar Group

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Pumiao paper mills; Yangsen Alcohol Co., Ltd.; and Guangxi, Guangdong Province Suixi Premium Alcohol Brewing Co. However, the comprehensive utilization of the potassium resource in waste is obviously insufficient in the current concentrated combustion systems. A recovery rate below 50 % leads to low potassium content and low added value. Thus, the current use of combustion is only to sell the burnt waste mash to complex fertilizer plants at the price of 100 yuan/ton for use as a raw material in fertilizers. Moreover, the currently operating combustion systems continue to have issues such as high power consumption, generation of coke in the boiler furnaces, and ash clogging in the flue. Therefore, in consideration of the high input of concentrated combustion technology, its economic benefit is not obvious and should definitely be improved.

1.7.2.8 Comprehensive utilization Lignin-based solid residue is generally used as a fuel, and the incineration process is the most simple and direct method for processing the hydrolyzed residue. However, the anticorrosion requirements for the boiler are high because of the acidic nature of residue. From the perspective of improving economic efficiency, the comprehensive utilization of lignin residue is also possible, for example, using hydrolysis residue as a raw material to produce activated carbon, lignin resins, and other chemical products. The East China University employed rice husks and wood hydrolysis residue, separately or mixed with other biomass, in a liquefaction process, and the results demonstrated that the addition of biomass promoted the pyrolysis of coal. Owing to the low oxygen content and high energy density of lignin, the water and oxygen contents of the resulting pyrolysis oil are low, which facilitates subsequent processing.

1.7.2.9 Fine Chemicals Cellulosic material includes cellulose, hemicellulose, lignin, and extractives (including organic and inorganic substances) groups. To improve the utilization of raw materials and economic benefits, John Ferrell [130] from the USA proposed a scheme to produce fine chemical products from biomass, including the production of fine chemical products from waste fermentation and hydrolysis residue. The hydrolyzed residue was hydrotreated to produce phenol, aromatic compounds, dihydrochloride, paraffins, and other products; and several useful organic acids and alcohols can be separated from waste mash. Most compounds are widely used and have great commercial value, such as sitosterol (a hormone precursor worth more than US $100 per liter) and resin acids and their derivatives (worth US $5–10/L). Moreover, the process of extracting terpenes and resin acids from biomass has reached the level of commercialization.

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1.7.3 Approaches to pollutant control Stillage is the main source of pollution in ethanol production. Pollution load, emissions, utilization, and wastewater treatment are related to the raw material, production technology, equipment, and production scale. Therefore, pollution control of ethanol production technology should be a total process control system. Raw materials, production technology, equipment, comprehensive utilization, waste treatment, production organization, and management should be considered an organic whole and should be optimized effectively. Saving energy, lowering consumption, and reducing pollutants should be emphasized throughout the entire production process to minimize the formation of pollutants, thus promoting sustainable development of the fuel ethanol industry [131, 132]. Pollution control for the ethanol industry is of significant importance to China’s government and the relevant agencies. However, the development time of fuel ethanol in China is short, and the relevant pollution control policy is not perfect. Related policies in the alcohol industry can provide reference to the fuel ethanol industry. Water consumption and pollutant emissions in the alcohol industry have always been high, which not only increases the cost of ethanol production but also the burden on investment in wastewater treatment, energy consumption, and operating costs. To minimize water consumption and pollutant emissions and reduce the scale and investment in wastewater treatment, advanced technology and equipment should be employed, wash water should be reduced, and cooling water should be recycled. For this reason, the Energy Saving and Comprehensive Utilisation Department announced on February 22, 2010, the implementation of cleaner production technologies in the alcohol industry, which also has important guiding significance for China’s fuel ethanol production (Tab. 1.19).

1

Thick mash fermentation technology

Technology

The main content of the technology

Enhances the ratio of material to water to 1:2 and employs SSF as fermentation mode. Approximately 15 % (V/V) ethanol will be obtained at the end of fermentation.

Scope of application

Alcohol industry

The stage Promotion stage

Technology source Independent development

The main problems solved The ratio of material to water is enhanced from 1:2.8 to 1:2; reduction in the quantity of fermentation mash, thus reducing energy consumption in distillation and wastewater discharge; and increased production efficiency.

Tab. 1.19: Cleaner production program for the alcohol industry.

Thick mash fermentation technology can be employed in ethanol production from corn and tuber crops. It can enhance the final ethanol titer to 15 %, eliminate 2 tons of water per ton of ethanol, reduce standard coal use by 0.3 tons, increase production efficiency by 25 %, and eliminate 2 tons of wastewater. Although it has significant environmental and economic benefits, the penetration rate is less than 5 %. Considering a company with an annual output of 100,000 tons as an example, then the annual water saving is 200,000 tons, the standard coal saving is 30,000 tons, the increased production reaches 25,000 tons, and wastewater is reduced by 200,000 tons. Industry-wide promotion (corn feedstock) will lead to annual water savings of 7.86 million tons, standard coal savings of 1.18 million tons, a production increase of 1 million tons, and the elimination of 7.86 million tons of wastewater generation.

Application prospect analysis

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2

Vinasse centrifugation and supernatant recycling

Technology

The main content of the technology

Recycles more than 35 % of supernatant into a mixed raw material.

Scope of application

Alcohol industry

Tab. 1.19: (continued) The stage Promotion stage

Technology source Independent development

The main problems solved Significant reduction in wastewater discharge to zero discharge.

Vinasse centrifugation and supernatant recycling can be employed in ethanol production from corn and other crops. This is an important emission reduction technology in the alcohol industry, with significant environmental benefits. The penetration rate is less than 10 % and can be increased to 70 % after promotion and application. 2 tons of water consumption can be eliminated, 2 tons of wastewater can be prevented, and COD can be decreased by 5 kg. A company with an annual output of 100,000 tons is considered as an example. Then the annual water saving is 200,000 tons, the standard coal saving is 7,500 tons, wastewater can be reduced by 200,000 tons, and COD discharge can be reduced by 500 tons. Industry-wide promotion (corn feedstock 70 % basis) will lead to annual water savings of 5.5 million tons, 204,000 tons of standard coal savings, 1 million tons of increased production, a reduction in generation of wastewater by 5.5 million tons, and a reduction of COD emissions by 13,800 tons.

Application prospect analysis 1.7 Environmental impacts of fuel ethanol production and control approaches |

75

Entire vinasse and supernatant treatment technology

Indirect steam distillation technology

3

4

Technology

The main content of the technology

Supernatant of centrifuged vinasse from corn is submitted to internal circulation (IC). Anaerobic treatment technology of entire vinasse from tuber crops.

During distillation, steam is not in contact with the heated material, thereby reducing the addition of steam condensate into the stillage.

Scope of application

Alcohol industry

Alcohol industry

Tab. 1.19: (continued) The stage Promotion stage

Promotion stage

Technology source Independent development

Independent development

The main problems solved Enhanced role of organic matter degradation and transformation to improve biogas production. BOD removal rate ≥ 90 %; reduction in wastewater emissions, and emissions of pollutants; and saves energy.

Reduction in the amount of stillage; after distillation, approximately 3 tons stillage can be reduced per ton of alcohol.

Indirect steam distillation technology can be employed in all plants to reduce vinasse significantly. 3 tons of wastewater would be eliminated for the production of 1 ton of ethanol. The penetration rate is less than 30 %, and can be increased to 70 % after promotion and application. A company with an annual output of 100,000 tons is considered as an example. The annual water saving would be 300,000 tons. 90 % promotion would eliminate generation of wastewater by 17.7 million tons.

Entire vinasse and supernatant treatment technology can be employed for ethanol production from starchy feedstocks. The penetration rate is less than 10 %. 80 % of ethanol factories can use this technology. 30 % COD (6 kg/t ethanol) can be reduced by this technology. Considering a company with an annual output of 100,000 tons as an example. The annual reduction of COD would be 600 tons. 80 % promotion would prevent 31,500 tons of COD generation.

Application prospect analysis

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1.8 The general situation of fuel ethanol prepared using biomass both at national and international levels 1.8.1 Research and development of fuel ethanol As early as in 1908, the American Ford Company produced cars that could burn both gasoline and pure ethanol. In the period between the two world wars, Europe had 4 million vehicles using a mixture of ethanol and gasoline fuel. During the Second World War, most of the German army’s vehicles were using potato ethanol as fuel. Furthermore, at this time, many other countries, including China, also had numerous cars running on ethanol. After the war, ethanol fuel-based cars vanished with the development of cheap oil in the Middle East, thus gasoline and diesel became the motor fuel for the engine [1] because gasoline became cheap and easily accessible. In recent years, research on fuel ethanol as a main direction and a part of biomass energy has received significant attention. This research is being promoted in several countries and its production scale is also expanding rapidly. World fuel ethanol production was 36.28 million tons in 2005, and the production doubled in 2011, to nearly 70 million tons, with an average annual growth rate of 14 %. Brazil is the first country of the world that uses ethanol as an alternative fuel on a large scale. Brazil is located in the subtropical region, with excellent mild four-season climate for growing crops. Sugarcane is used as the main feedstock for the bioethanol. First, considering national energy security and economic development, the Brazilian government planned to greatly develop fuel ethanol in 1975 because the Oil Crisis of 1973 heavily hit Brazil’s national economy. Second, it can not only promote the development of agriculture and planting industry but also protect peasants’ benefits (Brazil is the world’s largest producer of sugarcane). Furthermore, the strategy is also based on the domestic development of green renewable energy and environmental protection. The production of fuel ethanol, which uses 1 % of the arable land (about 3 million hectares), has achieved 50 % domestic gasoline substitution [2]. The use of fuel ethanol in developed countries can not only reduce their dependence on imported oil, but also stems from environmental protection considerations to a great extent. Ethanol releases far fewer harmful gases and a lower volume of greenhouse gases such as CO2 into the atmosphere than gasoline when it is burned as fuel. Gasoline can be burned more completely and would lead to lower carbon oxide emissions when mixed with 10–15 % of ethanol, so that ethanol has safely replaced methyl tertiary-butyl ether (MTBE) as a gasoline additive. MTBE is poisonous and can pose a potential pollution threat to groundwater; therefore, the US federal government announced the cancellation of the use of MTBE in March 2000. At present, the United States is the world’s largest producer and consumer of fuel ethanol. In the United States, the Energy Independence and Security Act (EISA) of 2007 passed by the US congress envisaged that the total amount of biofuels used in the US is to be increased to almost 36 billion gallons (110 million tons) by 2022. The

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act further specified that 21 billion US gallons of the 2022 total must be derived from non-cornstarch products (e.g., sugar, biodiesel, or cellulose). Fuel ethanol production was 12.73 million tons in 2005, and reached 41.7 million tons in 2011, accounting for nearly 60 % of global production, providing 400,000 jobs, creating a US $42.7 billion gross domestic product (GDP), and reducing oil imports by 485 million barrels (worth US $49.7 billion). The development of corn fuel ethanol has contributed significantly to reduce America’s dependence on foreign oil and has become a very important new industry. On February 21, 2012, US President Barack Obama mentioned in the presidential memorandum that US would develop biofuels and bio-based products in the next 10 years, with the GDP growing by US $80 billion and 600,000 jobs being added [3]. In Europe, fuel ethanol has been used for blending with gasoline fuel in various amounts, and then the blended fuel was first utilized as engine fuel. However, recently, the applications of ethanol are expanding, for example, 47 % ethanol mixed with 53 % isobutylene can form ethyl tertiary-butyl ether (ETBE). In Europe, ethanol production is about 1.75 million tons a year; furthermore, the use of ethanol gasoline is more than 1 million tons. The production cost of fuel ethanol is higher than that of gasoline; therefore, the principle of preferential tax has reduced the ethanol gasoline price and made it equal to the price of gasoline. In 1992, Europe passed a law that involved the harmonization of consumption taxes on all types of mineral oils and each member state could implement a duty-free policy for pilot projects using renewable resource as feedstock; moreover, fuel ethanol has also received tax breaks. In 1993, the European Union proposed that Europe should increase the yield of fuel ethanol, should opt for gasoline blended with 5 % ethanol, and the sales taxes of ethanol made from biomass should be reduced to 10 % of mineral fuel. Driven by preferential policies in taxation, the application of fuel ethanol in Europe was expected to expand from 5.75 % in 2010 to 10 % in 2020 with the capacity to further expand. The following are the main reasons for the development of ethanol fuel: First is the ever increasing prices of oil; second is to reduce dependence on petroleum; third is to increase job opportunities; fourth is to keep capital at home; fifth is that an ethanol gasoline blend can improve engine performance; and finally it prevents environment pollution. In Asia, ethanol fuel is promoted actively in Thailand, and the royal family and the government have shown great interest in the development of ethanol. In conjunction with Ford Motor Company, they have established a research institution and a special committee responsible for a feasibility study and development of fuel ethanol. In November 2000, the Thai government signed a cooperative agreement with the government of Brazil regarding the production of bioethanol and then implemented a tax incentive policy to promote the application of ethanol from December 2000. Since 1983, Japan has put into practice a plan to develop fuel ethanol and it has focused on developing the technology of producing ethanol from unused resource such as agricultural and forest waste products by yeast fermentation.

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1.8.2 Situation of the fuel ethanol industry in China The first main objective of China’s fuel ethanol industry is to exploit old crops. According to the important role and contribution of the ethanol industry to agriculture, energy, and the environment, China launched pilot projects for biofuel ethanol production and finally voted for building four pilot projects for ethanol production from grain in Heilongjiang, Jilin, Henan, and Anhui at the beginning of the Tenth Five-Year Plan. At present, China has already developed first generation fuel ethanol production systems including Henan Tianguan, Jilin Petrochemical Company, Anhui Fengyuan, and COFCO Zhaodong, mostly using corn and wheat as feedstock, producing more than 1.5 million tons a year. The production of motor ethanol gasoline has been done safely and smoothly in the provinces of Heilongjiang, Jilin, Liaoning, Henan, Anhui, and 27 other cities in Hebei, Shandong, Jiangsu and Hubei, making China the third largest producer and user of fuel ethanol in the world after Brazil and the US. Overall, China has developed biomass energy represented by biofuel ethanol for more than ten years now, and the production technology has gradually become mature, safe, accessible, and reliable and has attained advanced international standards. Under the current policy circumstances, it has specific economic aspects. Three strategies (agricultural drive, environmental protection, and energy substitution) put forward in the early period of the pilot projects have had some effect and have achieved great results in terms of social and ecological benefits. At the same time, the pilot programs of ethanol gasoline were creative in terms of organizational leadership, law-based administration, supporting policies, indigenous innovation, and economic improvement. Numerous valuable experiences can be used for reference. In view of food security, China enacted laws and regulations to strictly limit the scale of the grain ethanol industry as early as 2006. It stated very explicitly that “shall not occupy land, shall not consume a large number of grains, and shall not imperil the ecological environment” for the production of biofuel ethanol. The Mid- and Long-term Plan and Program for National Science and Technology Development (2006–2020) and the Medium- and Long-Term Development Plan for Renewable Energy have brought forward the principles of developing nongrain ethanol, i.e., “measures to suit local conditions, nongrain-based” and “focus on supporting basic science and applications technology studies of nongrain biomass fuel”. According to the basic principles and guidelines of nongrain competition with people and non-farmland competition with grain, China began to implement a nongrain substitution strategy in the Chinese Eleventh Five-Year Plan. Key policies include the development of uncultivated land and implementing pilot programs for nongrain fuel ethanol production with starch, sugar, and fiber resources as raw materials, including fuel ethanol projects using cassava in Guangxi, sweet sorghum in Inner Mongolia, and xylose residue in Shandong. By 2011, China produced 1.94 million tons of fuel ethanol (including 320,000 tons of nongrain fuel ethanol), and 17.87 million tons of automobile ethanol gasoline blend accounted for about 23 % of the worldwide petrol consumption of 77.38 million tons [5].

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1.8.2.1 The development of technologies to produce nongrain starch/sugar ethanol At present, two relatively mature models for ethanol production are the “corn ethanol model” in the US and Brazil and the “sugarcane ethanol model” in Brazil. However, neither of these applies to the features of nongrain starch/sugar in China, thus there is no ready-made experience to draw upon. In recent years, projections for the production of biofuels under various policy scenarios showed that China has a certain basis for research on fuel ethanol made from nongrain starch/sugar; however, due to a late start, energy efficiency is low and the key technology has not yet achieved a breakthrough in progress [7, 8, 14]. Currently, the following problems have to be resolved: (1) Lack of an evaluation system for the raw material: Not even a single relative evaluation system is available for nongrain starch/sugar as raw material for fuel ethanol. Nongrain starch/sugar raw materials are still under investigation; therefore, it is extremely difficult to identify which species have high energy output levels, rapid energy output speed, and lower pollutant emissions. The government department responsible has already sufficiently explained the importance of establishing such an evaluation system. Considering sweet potatoes as an example, the economic evaluation index of the sweet potato mainly includes the output rate of starch and fresh potatoes yields, ignoring the fermentable sugar content, energy output level, energy output speed, pollutant emissions, and the impact on biodiversity and soil erosion. However, it is necessary to take these aspects into account to design special nongrain feedstock evaluation systems for fuel ethanol. Selection of the most suitable cultivars from the existing varieties is highly desirable for the effective production of fuel ethanol in order to promote rapid development in the nongrain fuel ethanol industry. As a result, difficult-tostore raw materials have become one of the bottlenecks hindering the production of nongrain fuel ethanol and the development of biomass energy. (2) Balanced production over the course of the year still encounters relative difficulties. The nongrain starch/sugar material exhibits several unfavorable properties such as high water content, perishable, difficult to preserve, and strongly seasonal, which lead to limited supply of raw material during the year thus affecting the balanced production of fuel ethanol. For example, sweet potato, produced extensively in the southwest area, is often used as feedstuff. Less than 10 % is consumed or processed, while more than 30 % (about 40 million tons) decays. This leads to severe economic loss and water pollution, as well as affecting the balanced supply of material for fuel ethanol production. In general, producers of ethanol can only use sweet potato as the raw material for four months from November to March. The rest of the time they have to select other food crops such as corn as raw materials, which results in the shutdown of some small plants. In reaction to this phenomenon, development of a rapid ethanol producing technique is required in order to effectively utilize the raw material in a timely fashion during the harvest season. Moreover, development of an efficient storage method

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is also highly desirable in order to prolong the period. Finally, selection of different nongrain materials and their appropriate combinations is also beneficial to further guarantee the annual balanced production according to different plants grown for raw materials production, distribution area, harvest season, and storage characteristics (3) Comprehensive utilization of raw material still needs further improvement. The protein content of fermenting mash producing ethanol from nongrain starch/sugar is low, so it is not suitable for animal feed. Low resource utilization is one of the basic and common problems that restrict the sustainable development of the ethanol industry. If we can use the non-fermenting part efficiently according to the performance characteristics of nongrain material, it will be good for ensuring clean production of fuel ethanol, reducing pollution, protecting human health, and sustaining a good ecological environment. The above mentioned issues are already the common bottlenecks that restrict the sustainable development of the nongrain starch/sugar fuel ethanol industry and will be the focus of research and development for next 5–10 years.

1.8.2.2 The development of cellulosic ethanol production technology Due to its extensive fiber material resources, its great contribution to agricultural development and to improving the ecological environment and sustainable development, the development of cellulosic ethanol has gained widespread attention and high recognition. As a typical representative of advanced bioenergy, it will usher in a new round of revolution in energy technology and industrial structure, once it achieves its major breakthrough and the commercialization of the technology.

1.8.2.2.1 Technical feasibility analysis Cellulosic ethanol technology has made considerable progress in recent years and it fundamentally involves the technological conditions and development basis necessary for industrial-scale development. Cellulosic ethanol technology includes two main routes, namely, biotransformation and thermochemical conversion for ethanol production. The biotransformation route adopts enzymatic hydrolysis technology, which is highly matured, involves low cost of sewage disposal, and is most widely used. The research is focused on efficient pretreatment processes, production of low cost and high quality cellulase enzymes, and technology for producing ethanol strains through pentose fermentation. Among them, the pretreatment technology involves the development of several main techniques such as neutral and acidic hydrolysis, steam explosion, and ammonia explosion. Furthermore, it has been successfully applied to industrial demonstration plant biorefinery industrialization. The conversion ratio of treated cellulose and hemicellulose can reach above 80 %.

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Hydrothermal and ammonia pretreatment has attracted general attention in the world in recent years due to its environmental friendliness and easy solvent retrieving capability. Two-step pretreatment brings better results than one-step. The researchers at GIEC (Guangzhou Institute of Energy Conversion) use high temperature liquid water prehydrolysis treatment for eucalyptus with two-step temperature variation. After pretreatment, the actual ratio of glucose increased from 78.5 to above 98.4 % and total sugar receipt rate increased from 81.2 to 96.6 %. Pretreatment of bagasse by a twostep process, involving the high temperature liquid water method coupled with the ammonia process, led to an increase in the actual yield of glucose from 77.96 (single high temperature liquid water method) to above 97.99 % and the total sugar receipt rate also increased from 81.30 to 87.85 % [59, 60, 70, 133–135]. The cost of cellulase: Owing to continuous new technological breakthroughs since 2005, the efficiency of enzymolysis has improved significantly and enzyme dosage has also been reduced considerably. The cost of enzymes for producing one ton ethanol has also fallen greatly and in some cases, it has already dropped to 500–700 yuan. However, the detailed technical data has not been reported so far and has not been proved by practical operation. High cost of cellulase enzyme is still one of the major barriers for commercialization of cellulosic ethanol production. Modification of strains for the production of cellulase enzyme, improvement of the level of cellulose enzyme fermentation, and establishment of the production mode of cellulase still require tremendous research efforts on our part [136]. Ethanol fermentation strains: Xylose and glucose co-fermentation has been realized by genetically engineered strains and the conversion rates of xylose have already reached close to that of glucose. Fuel ethanol concentration in the fermented liquor has basically met the requirements of industrial production. However, the strains which have horizontal resistance to fermentation inhibitors produced in the highintensity pretreatment, and can directly and efficiently lead to the co-fermentation of a variety of non-fermented sugars have not yet been confirmed by industrial applications. The strains with enhanced resistance to fermentation inhibitors and the rate of co-fermentation of polysaccharides still are restricting factors [16–19]. The problems of high energy consumption and high wastewater discharge, which become visible only with large-scale equipment and long-term operation, are difficult to find solutions to before large-scale industrial plants exist. Energy consumption is directly reflected in the steam consumption of the pretreatment process and energy consumption also occurs in the distillation of low concentration ethanol. The pretreatment process must reduce the amount of high temperature steam, while providing adequate pretreatment intensity, to ensure effective disruption of the supermolecular structure of lignocellulose for implementation of effective cellulase hydrolysis. As feedstock for distillation, ethanol fermentation liquor must have high concentrations of ethanol (8–10 %, v/v), in order to reduce the energy consumption of distillation columns to the acceptable level of industrial devices operation. The prerequisites to achieve high ethanol concentration involve the use of maximum concentration of lig-

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nocellulose corresponding to minimum use of water. However, in a multiphase transformation system, the increase in solid content results in an increase in viscosity, content of inhibitors, and energy consumption for mixing and transfer which seriously restrains the efficiency of enzymatic hydrolysis and ethanol fermentation. Wastewater discharge occurs mainly during the processes of pretreatment, detoxification, and fermentation. The wastewater emissions of mainstream pretreatment used by industrial demonstration projects are 6–10 times those of lignocellulose. Water-washed, lime alkaline treatment, and other methods during the course of detoxification may be inexorable, and lead to the emission of wastewater rich in complex inhibitors. The wastewater volume generated during the cellulosic ethanol process is several times that produced during the grain ethanol process. Under the current technical level, 25 tons or more wastewater per ton of cellulosic ethanol will be produced. Considering the location of the cellulose ethanol producing factory to be in rural areas abundant in biomass, combined with the small-scale production limited by transportation radius and low maturity level of wastewater treatment systems, the environmental problems caused by wastewater will become progressively more serious and difficult to control. Therefore, meeting the lowest wastewater discharging standards in the R&D phase has very important significance for the normal operation of the cellulosic ethanol process and sustainable development of the city [20–22].

1.8.2.2.2 Analysis of economic feasibility Sustainable feedstock supply, system integration technology, and economic aspects of cellulosic ethanol industrialized production definitely require improvement. The bottleneck of cellulosic ethanol industrialized production involves the following aspects: First, it is extremely difficult to maintain a sustainable supply of feedstock due to the spatial distribution and centralized harvest time of straw. Distribution, area, and characteristics of the non-arable land should be investigated in future research. At the same time, the technology of breed selection, production, harvest, and logistical support have fallen behind the times. Second, technology integration needs optimization and near-perfection. With the development and maturity of key technologies related to pretreatment, enzymes, and xylose fermentation, system integration technology and equipment technology appear to be somewhat lagging and industrialization needs further verification. Third, economic aspects still require considerable improvement with respect to obtaining low cost raw materials and reducing energy consumption and wastewater quantity [44, 45, 137]. (1) Raw material system needs to be established Different to the traditional agricultural production and supply mode, the establishment of a sustainable cellulosic ethanol raw material supply system has its own particularity. Currently, the main obstacle is as follows: seasonal raw material production and continuous production on an industrial scale are a certain contradiction. Special large-scale collection and storage of complete sets of equipment is

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lacking. Raw material purchasing and transporting systems and large-scale applications mode are not mature and need to establish an optimization model through industry. The key technology of high production and low cost should be further developed for resistance of the crop to droughts and salinity. Systematic study of the distribution of saline-alkali soil, sandy, hillside, and abandoned land is still unavailable. Its production potential and type of use need comprehensive study and unified planning. Thus, industrial policy related to raw material production and supply should be further implemented. (2) Integration technology needs optimization At present, although the key technology of cellulosic ethanol has partly achieved a breakthrough, the level of system integration remains to be improved. Development of cellulosic ethanol production technology includes neutral/dilute acid pretreatment, enzymatic hydrolysis fermentation technology; steam explosion pretreatment, enzymatic hydrolysis fermentation technology; acid hydrolysis fermentation technology; acid hydrolysis, fermentation, esterification, hydrogenation technology; ammonia explosive pretreatment, enzyme hydrolysis, fermentation technology, and so on. Most of them have been investigated in pilot plants. However, owing to scatteredness and secrecy of technical achievements, their comparison is extremely difficult. The domestic development of cellulose ethanol technology mainly uses acidic or neutral pretreatment, enzymatic hydrolysis, and hexose or pentose fermentation-hexose fermentation process. Compared to other countries, there are still some gaps in enzyme preparation, fermentation strains, the core equipment, process design, and instrument control. To realize industrialization, the organization of several famous universities or institutes working together is highly desirable. According to different raw materials, device size, and production technology, improvement and optimization of the corresponding system integration technology, in particular, development of the complete technology package is undeniably required. (3) Economics need to be improved A significant reduction in the cost of cellulosic ethanol to a for-profit industrial production level is the lifeblood of the industry. The construction of 75,000 tons of cellulose ethanol projects requires an investment of about US $175 million, demonstrating high one-time investment. The cost to produce per ton of ethanol was about US $1,400 in 2007 and could be controlled at about $1,000. In China, according to pilot plant data, the production cost of ethanol in COFCO and Henan Tianguan is higher than grain ethanol except for in Shandong Longli. Due to the special raw material source and product diversification, its price is competitive. Considering crude oil prices, technological progress, and large-scale production, there exists huge potential in reducing investment and improving the economics. Industry competition will be further enhanced if we add high value-added biobased materials and chemical products.

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1.8.3 Prospects for the fuel ethanol industry in China 1.8.3.1 Developmental trend The generation of cellulosic ethanol is the developmental trend for fuel ethanol. The development of a lignocellulosic biorefinery industry can maximize the benefits of the industrial chain, meet the concept of a circular economy, and achieve sustainable development of a liquid biofuel industry. In order to promote the development of new industries, research on the high value utilization technology of cogeneration products, by-products, and waste produced during the process of cellulosic ethanol production is urgent. For example, using waste liquid and waste residues for biogas, feed, and organic fertilizer production is especially important to make more breakthroughs in cogeneration biochemical products [138]. Cellulosic ethanol has a market demand of millions of tons. However, xylitol, furfural, and wood oligosaccharide mostly have a market capacity of only hundreds of thousands of tons. Therefore, the balance between the main products and co-products should be maintained. The benefits will be reduced if the co-products exceed the capacity of market. The United States government selected 12 types of platform compounds from biomass sources, including xylitol [139, 140]. As a platform compound with five carbons, xylitol is not only used as a food additive, but also as a raw material in the chemical industry. Lignin is a class of complex aromatic polymers that can be extracted as pure compounds and used as additives for chemical materials after transformation of the active groups, such as cement water reducing agent, rubber, polyurethane, and other material modifiers. Its market also needs expansion. Shandong Quanlin group used ammonium pulping black liquor to produce humic acid organic fertilizer and lignin green organic fertilizer. In addition to its nutritional role, lignin can also resist disease and continuous cropping and improve soil aggregate structure and water retention, which are widely welcomed by farmers and have great potential for development. At present, biochemicals account for 5–8 % of total world chemicals and are expected to reach 18 % in 2020 and 50 % in 2050. Production of chemicals as raw materials from nongrain biomass is necessary in the age of post-fossil resources. The industrial production of chemicals can achieve profitability and continuous operation with advanced biorefining technology when using cellulose, hemicellulose, lignin, and other nongrain biomass as raw materials. We must seriously study the mature experience of the oil and chemical industries, break the traditional ideas that only a simple single fuel product can be generated from biomass, make complete use of every type of main components obtained from raw material, and transform them into products according to their characteristics, to achieve complete utilization of the raw materials and maximize the value of products and the efficiency of land use. It is necessary to explore new technology for cogenerating cellulosic ethanol and by-products, in particular, for the production of bioplatform compounds using ethanol as raw materials, such as the technology for the transformation of ethanol into ethylene and its

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derivatives, ethanolamine and other fine chemical products; synthetic resin and adhesive production technology; humic acid fertilizer production technology; catalytic hydrogenation deoxidation and hydrogenation of pyrolysis for fuel additive production technology; mixed fuel technology; functional sugar, furfural, lactic acid production technology; carbon dioxide for preparing polycarbonate technology; and gas fertilizer algae breeding and further processing technology.

1.8.3.2 Development objectives Breakthroughs in technology and equipment for materials, transformation, and conversion can be achieved and this depends on the progress of science and technology, indicating the realization of the “safe and full of excellent” operation. The economic income, comprehensive energy consumption, and pollutant emissions of cellulosic ethanol are expected to be far superior to those of first generation fuel ethanol, and have already achieved 50,000 tons demonstration in 2015. There will be 100,000 tons commercial devices operating in China in 2020, as the construction of the raw material system is becoming nearly perfect [141]. The recent specific objectives (2013–2025): The specific objectives for efficient fuel ethanol production technology include the development of culture dedicated energy crops with genetic modification, expansion of raw material types and cogeneration products, development of new technologies with plant fiber as raw material resources, comprehensive utilization of various components, and production of fine chemicals and fuel, fiber, feed, and chemical raw material at the same time. Further, establishment of large demonstration enterprises to make complete use of the entire areais also significantly important. In 2025, China is expected to establish a new industry with an annual output of 10 million tons of biorefined products using plant fiber and increased industrial output value of 100 billion yuan, simultaneously meeting 15 million tons of oil demand, and providing employment for 300,000 members of the rural population. Furthermore, farmers’ income can increase 15 billion yuan by providing the straw raw materials and 40 million tons of carbon dioxide emissions can be reduced. The long-term development targets (2050): The long-term development targets include the realization of the comprehensive utilization of biomass materials (starch, sugars, cellulose, and lignin) and the diversification of products (fuel, bulk and fine chemicals, medicine, feed, plastic, and others, construction of a large biomass refining industry, partial substitution of nonrenewable mineral resources, and achievement of economically and socially sustainable development based on carbohydrates.

Weiming Yi*, Xifeng Zhu, and Wei Qi

2 Technologies of biomass pyrolysis

2.1 Principles and technology of biomass fast pyrolysis 2.1.1 Overview of biomass fast pyrolysis Biomass pyrolysis, also known as “thermolysis” or “thermocracking”, is a type of thermochemical conversion technology which is considered as one of the most promising technologies for biomass conversion. Biomass pyrolysis is a complex process which involves the rapid thermal decomposition of macromolecular compounds in the absence of oxygen to produce liquids, gases, and char. The yields of the liquids, gases, and char depend on the experimental process and conditions [142]. Lignocellulosic biomass consists mainly of cellulose, hemicellulose, lignin, and extractives. In general, the thermal decomposition temperature of hemicellulose, cellulose, and lignin is in the range of 225–350 °C, 325–375 °C, and 250–500 °C, respectively. The main products from the thermal decomposition of hemicellulose and cellulose are volatiles, which can be further converted into gases and liquid products. The main product from thermal decomposition of lignin is char [142]. Depending on the reaction temperature and heating rate, biomass pyrolysis can be divided into four categories: slow pyrolysis, traditional pyrolysis, fast pyrolysis, and flash pyrolysis. Furthermore, biomass gasification is also considered as super flash pyrolysis. The characteristics and main products of the four biomass pyrolysis processes are compared in Tab. 2.1. The product distribution obtained from different modes of pyrolysis, as shown in Tab. 2.1, indicates that considerable flexibility in pyrolysis process is achievable by changing reaction conditions. Therefore, different modes of pyrolysis need to be selected in order to maximize the target products. According to the desired target products, biomass pyrolysis can be divided into three categories as follows:

*Corresponding Author: Weiming Yi: School of Agricultural Engineering of Food Science, Shangdong University of Technology, Zibo 255000, China; Email: [email protected] Xifeng Zhu: Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei 230027, China Wei Qi: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China https://doi.org/10.1515/9783110476217-002

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Tab. 2.1: Biomass pyrolysis conversion technologies. Type

Temperature °C

Heating rate °Cs−1

Residence time

Main products

Slow pyrolysis

< 400

slow

several days

Char

Conventional pyrolysis

< 500

10–100

5–30 min

Liquid 30 %, gas 35 %, char 35 %

Fast pyrolysis

500–650

103 –104

0.5–5 s

Liquid > 50 %

Flash pyrolysis

> 750

> 104

70 %

(1) Biomass pyrolysis for charcoal production Biomass pyrolysis for the charcoal production process belongs to conventional slow pyrolysis. It has been applied for thousands of years and has been mainly used for the production of charcoal. Lower process temperature and longer vapor residence time favor the production of charcoal (Fig. 2.1). The maximum yield of charcoal can be as high as 30 wt.% of the products but about 50 % of the total energy of the biomass feed. (2) Biomass pyrolysis for fuel gas production If the purpose is to maximize the yield of gaseous products resulting from biomass pyrolysis, flash pyrolysis will be preferred to obtain high yield of fuel gas products. When the process temperature reaches over 750 °C, the yield of fuel gas can be up

Fig. 2.1: Charcoal produced from biomass pyrolysis.

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to 80 %. The main components of this fuel gas are CO, H2 , and small molecular weight hydrocarbons. (3) Fast biomass pyrolysis for liquid production If the purpose were to maximize the yield of liquid products resulting from biomass pyrolysis, fast pyrolysis and/or flash pyrolysis would be preferred to ˙ obtain bio-oil (Fig. 2.2). The yield of bio-oil is typically about 50wt.% of the products, but yields of up to 75 % can be obtained from woody biomass [143]. In the fast pyrolysis process, the process temperature is around 500 °C. The heating rate should be over 103 °Cs−1 . The retention time generally should be less than 1 s, and the pyrolysis products need to be rapidly cooled down at the rate of 102 –103 °Cs−1 . Under these conditions, the condensable volatiles can be rapidly converted into free-flowing liquid, which is referred to as “bio-oil” in the literature.

Fig. 2.2: Bio-oil produced from fast biomass pyrolysis.

Pyrolysis oil typically is a dark brown, free-flowing liquid having a distinctive smoky odor. The following are some main properties of bio-oil: density ~ 1,200 Kg/m3 ; pH ~ 2.0–3.8; moisture content ~ 15 %–30 %; and heating value ~ 14–18.5 MJ/Kg. Biooil can not only be used directly as a fuel oil for combustion in a boiler and/or burner, but also can be further upgraded into high quality liquid fuel with similar features to diesel fuel or gasoline for transportation purposes. In addition, various valuable chemicals can be derived from bio-oil for commercial purposes. Bio-oil is considered a green fuel for the twenty-first century since it contains less sulfur and nitrogen than traditional fossil fuels. The noncondensable small molecule gaseous products (or permanent gases) contain some heating value, which can be used to provide the heat for the pyrolysis process. While the char from the pyrolysis is referred to as “bio-char”, it can be used for soil enrichment or as feedstock for activated carbon production [144].

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2.1.2 Characteristics of biomass fast pyrolysis In recent years, biomass pyrolysis liquefaction has become a popular technology for biomass processing and utilization. The core technique is that the raw material powder is heated at high heating rates (up to 1,000 °Cs−1 or more, also known as flash heating). Under these conditions, intramolecular cleavage will occur, and the molecular weight of biomass will decrease from the original thousands or millions of molecules into small molecular structure of tens and/or hundreds. After immediately cooling the steam pyrolysis and terminating the possible secondary reactions, the resulting liquid products will give bio-oil. The bio-oil yield can reach more than 50 %, and some types of timber can give more than 90 % liquid yield. Bio-oil can either be used as a fuel directly or reprocessed further. It can also be used as a raw material for chemical synthesis of many valuable chemicals. It is important to study the biomass liquefaction mechanism in order to provide a theoretical basis for the liquefaction equipment design, and make the selection of pyrolysis parameters and device structures more systematic. For this purpose, many researchers have investigated the use of a thermogravimetric analyzer (TGA) to determine the thermal volatilization characteristics of different types of biomass. The heating rate of a TGA instrument can reach 100 °C/min, and provide slow heating conditions. Using a thermogravimetric analyzer is an effective method to measure the volatilization characteristics of biomass weight and water-gas gasification. Biomass pyrolysis liquefaction is conducted at very high heating rates (also commonly known as flash heating conditions). The theoretical analysis must also be carried out under such heating conditions. The high temperature laminar flow furnace has been successfully used for studying the rapid thermal evaporation characteristics of coal, and it can also be used to study biomass devolatilization at flash heating conditions. In a laminar flow furnace, the working gas is an inert gas such as nitrogen, argon, etc., which is heated to a certain temperature through the flow straightener into the main rectifying tube. To control the gas flow in a laminar flow state, the Reynolds number of the pipe flow must be less than 2,000. Very fine (120 mesh or more) particles of combustible materials (coal, biomass, etc.) are added to this gas stream instantly to reach the furnace gas temperature. A quench collection device is used to collect charcoal. The ash tracer method is used to determine the content of volatiles and the weight loss of biomass particles. By changing the gas flow rate, the ambient heating temperature, quench collection device, biomass export distance and other parameters, the TGA of combustible materials can be obtained under different heating conditions. In a conventional laminar flow furnace, the working gas is heated through external heating technology. It is difficult to achieve the same temperature of the working gas stream with the furnace wall. The furnace tube has both flow velocity distribution and temperature distribution. If the flow temperature can be made uniform and constant in the laminar furnace, it would make the test more convenient and the re-

2.1 Principles and technology of biomass fast pyrolysis

Vibrating feeder Feed gas Compresed air

Cooling water in

Cooling water out

Plasma-heated argon

Thermocouple 1 Flow straightener Thermocouple 2 Cold finger Thermocouple 3

Thermal insulation

Thermocouple 4 PID controller Thermocouple 5

Thermocouple 6 Collector tube Electric heater

Cooling water in

Cooling water out Suction

Cyclone

Fig. 2.3: Schematic diagram of a plasma-heated laminar flow furnace.

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sults more accurate. To solve this problem, a new kind of laminar furnace system with plasma-based heat source and insulated walls is used to carry out the experiments. The basic structure is shown in Fig. 2.3. Using a plasma heating technique, the argon as working gas could be maintained at a set constant temperature. A control system is used so that the furnace wall and the working gas are at the same temperature, with the temperature control error of ± 3 °C. Performance parameters of the system are: continuously adjustable working temperature of 450–1,100 °C, biomass powder addition rate of 0.5 to 1.5 g/min, quench collector and adjustable charging port (cold finger) distance of 100 mm to 300 mm, and adjustable argon gas flow of 1.0–2.5 m3 /h.

2.1.2.1 Preparation of biomass powder The biomass powder of fineness < 120 mesh is used in laminar flow furnace experiments. The preparation of biomass powder is an important factor for the study. The biomass crushing process has been well researched, in order to achieve the biomass powder with desired characteristics and composition. With the use of mill grinding, the crushed biomass powder is a mixture of a lot of needles, fine particles and sheets. The basic size range of such substances is generally: needle diameter ranges from 0.1 to 0.5 mm, length ranges from 1 to 5 mm; the fine powder can achieve a fineness of 120 mesh or more; and the sheet can be of indefinite size. For corn stalks, the general conclusion is that broken needles are made from the skin portion of the corn stalk, the finely crushed particles are made from the central part of the corn stalk, while the broken sheets are made from the leaves and foreskin. Any of the individual components of needles, powder or flakes are not representative of the overall characteristics of the biomass. Thus, the preparation of fine powder which represents the overall characteristics of biomass is a key issue. For materials such as straw, wood, bark, etc., hand grinding is a suitable preparation method. For example, the specific approach with corn stalks is: first select the appropriate corn stalks; include the roots, central part, leaves, foreskin, etc.; tie them together into a bundle with a diameter of about 5 cm; use a sharp knife to cut the ends; use a high strength sandpaper to hand grind the bundle to give a corn stalk powder. This powder contains the overall composition of corn stalks, with a particle size of 120 mesh or more.

2.1.2.2 Temperature control of laminar flow furnace For laminar flow inside of the furnace reactor tube, its energy equation when it is in a stable state can be derived using equation (2.1): 1 ∂ ∂T 1 ∂T (r )= u ⋅ r ∂r ∂r a ∂x

(2.1)

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In our experimental conditions, the system can be considered in a state of constant wall heat flux when it is stable. At this time, ∂T ∂x = constant, the temperature distribution of each reaction tube is similar to the cross section, and the temperature gradually increases with the flow direction. Boundary conditions are: { { r=0 { { { r = r0

→

∂T ∂r

→

−k ∂T ∂r = q w

=0 (2.2)

The result of equation (2.1) can be solved as follows: T=

u0 ∂T 2 ∂T r + x + T0 4a ∂x ∂x

(2.3)

It can now be written in the form of temperature difference vs. the center: T − Tc =

u0 ∂T 2 r 4a ∂x

(2.4)

In equations (2.3) and (2.4) ∂T ∂x = constant. By applying the boundary conditions, the ∂T relationship between ∂x with the steady heat flux density qw of the furnace wall can be determined. ∂T 2a qw (2.5) = ∂x k ⋅ u0 ⋅ r0 By substituting eq. (2.4) back into eqs. (2.2) and (2.3), we obtain: 2a ⋅ q w qw 2 r + x + T0 (2.6) 2k ⋅ r0 k ⋅ u0 ⋅ r0 qw 2 r (2.7) T − Tc = 2k ⋅ r0 Equation (2.6) describes the relationship between temperature distribution of the air flow with the wall surface heat flux q w and the initial temperature T0 of the gas stream. Equation (2.7) is the temperature difference between the gas temperature of furnace cross section T and the central temperature T c . These two formulas show that changes in the temperature of the gas flow and wall heat flux q w are closely related. For insulation measures, as long as there is a temperature difference between the air flow and the furnace wall, the resulting q w will cause the temperature change along the flow direction and the cross section. Thus, in order to prevent the temperature change caused by the insulation measures, it is necessary to control the temperature of the insulated portion to make it consistent with the set temperature of the air flow. In theory, q w can be made equal to zero, to obtain a uniform and stable temperature of the air. In fact, considering the flow temperature drops caused by heat loss and the presence of a cold finger, by making q w = 0, the flow temperature will be below the set value. Therefore, in actual control, an appropriate temperature difference between the insulation and the wall of the laminar flow furnace is necessary to achieve the desired temperature control. T=

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These analyses represent the temperature control principle for the laminar flow furnace. The following are the specific measures: – Adjusting the power of the plasma heating, considering the cooling air flow by the cold finger, so that the set temperature of the gas stream in the buffer chamber is maintained slightly higher than the set value. – Adjusting the temperature of the thermocouple measurement and control point locations. The control points are fixed to the outer wall surface of the furnace tube to obtain a control input signal. – Monitoring the temperature of the silicon carbide electric heating zone, so that it is close to the temperature of the outer wall surface of the laminar furnace. – Monitoring the gas temperature changes in the laminar furnace tube, to determine the adjustment of control programs.

2.1.2.3 Biomass flash volatilization experiments and data processing For Chinese biomass, we classified the different biomass materials as: Straw class (usually high ash content) – corn stalks, straw, cotton stalks, etc.; Hull class (typically high ash content) – peanut shells, rice husk, etc.; and Wood class (typically low ash content) – pine, coconut shell, etc. Since the biomass ash tracer method was used in our experiments to determine the degree of volatilization, the experiments were in accordance with high ash content and low ash materials treatments. The principle of the ash tracer method to determine the extent of biomass volatiles for one specific type of biomass is to get an approximate analysis of its volatiles, provided the fixed carbon and ash content values are stable. In other words, a certain amount of ash must correspond to a certain weight of biomass. For example, the ash content in one kind of biomass is 8 %. Then, 8 grams of this biomass ash will necessarily correspond to the original 100 g biomass feedstock. Biomass ash in the thermal volatilization process is always present in the carbon residue and remains constant. So the ash in biomass can be used for tracer analysis. The specific methods are: To get the biomass approximate analysis first, the biomass ash content percentage P is obtained. After an experiment under certain conditions, the char ash content P1 is obtained to get an approximate analysis of residual charcoal in the collector. According to the above analysis, the biomass devolatilization percentage of W1 accounting for the biomass feedstocks can be obtained using the below equation: W1 = (1 −

P ) × 100% P1

(2.8)

Corn stalks, wheat straw, rice husks, coconut shells, cotton wood, peanut hulls, and white pine were subjected to volatilizing experiments at flash heating rates. The experimental data obtained were analyzed theoretically, and the chemical kinetics parameters were obtained.

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Biomass devolatilization kinetics follow the Arrhenius law equation. That is: E dW = A ⋅ (W∞ − W) ⋅ exp (− ) dt R⋅T

(2.9)

Where, W is the biomass pyrolysis volatile mass in percentage (of the original mass of the biomass %); W∞ is the final total biomass volatile percentage ( %); t is the volatilization time (s); A is the apparent frequency factor (s−1 ); E is the apparent activation energy (kJ · mol−1 ); R is the gas constant (kJ · mol−1 · K−1 ); and T is the biomass temperature (K). Studies have shown that, at very high heating rate in a laminar flow furnace, the material particles instantaneously reach the heated ambient temperature. Therefore, in equation (2.8), with respect to the fixed heating temperature, only W and t are variables. Then, if we set the following conditions: E ) R⋅T

(2.10)

dW = B ⋅ (W∞ − W) dt

(2.11)

B = A ⋅ exp (− The equation (2.8) simplifies to:

Using the boundary conditions of t = 0, W = 0; equation (2.11) can be solved as: ln (

W∞ )=B⋅t W∞ − W

(2.12)

Assuming that the final volatile content of biomass in percentage W∞ is 80 %, the data in Tabs. 2.2 and 2.3 can processed to give Figs. 2.4, 2.5 and 2.6. It can be seen from these figures that the data show a strong linear relationship. The slope of the line gives the corresponding B values. E B is defined by the equation (2.9): B = A ⋅ exp (− R⋅T ). This equation includes the kinetic parameters. The kinetic parameters are apparent frequency factor and activation energy. The biomass temperature T is the variable. Equation (2.9) can be converted into: E 1 (2.13) ln(B) = − ⋅ + ln(A) R T Tab. 2.2: The experimental data of corn stalks. Temperature 800 K Time (s) W1

0.128 44.73

Temperature 850 K 0.160 51.13

0.192 54.69

0.224 59.66

Temperature 900 K Time (s) W1

0.114 63.07

Time (s) W1

0.121 51.44

0.152 57.20

0.182 64.33

0.212 66.60

0.135 66.48

0.162 71.37

0.189 72.68

Temperature 950 K 0.142 64.73

0.170 68.59

0.199 69.85

Time (s) W1

0.108 65.48

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Tab. 2.3: The volatilization percentage of wheat straw, rice husks and coconut shell at different temperatures and time. Temperature 750 K Time (s) Wheat straw Rice husk Coconut shell

Temperature 800 K

0.137 47.54

0.171 52.14

0.206 56.87

0.240 61.27

19.54 25.12

24.54 31.37

30.00 36.34

33.84 38.25

Time (s) Wheat straw Rice husk Coconut shell

Temperature 850 K Time (s) Wheat straw Rice husk Coconut shell

0.128 57.42

0.160 61.52

0.192 65.19

0.230 68.71

29.60 30.78

33.93 42.98

36.70 45.62

42.30 50.05

0.115 64.09

0.148 69.18

0.172 72.32

0.201 75.95

44.75 51.02

46.58 61.91

50.26 66.19

59.22 69.79

Temperature 900 K

0.121 61.15

0.151 65.22

0.181 70.32

0.217 72.87

31.72 46.70

38.24 55.47

42.38 57.93

48.43 62.62

Time (s) Wheat straw Rice husk Coconut shell

Data of corn stalks 3

y = ln(80/80–W)

2.5 2 1.5 1 0.5 0 0

0.05

800K Line (800K)

0.1 0.15 Heating time t(s) 850K Line (850K)

0.2

900K Line (900K)

0.25 950K Line (950K)

Fig. 2.4: Corn stalk experimental data calculated by equation (2.4) and the fitting straight line.

For one kind of biomass material, let ln (B) and 1/T be the variables, then we can get the corresponding kinetic parameters. Fig. 2.7 shows the data curves. The slope of each curve is the ratio of the activation energy to the gas constant; the intercept is a natural response to the frequency factors. Thus, we get the volatilization characteristic parameters of four kinds of biomass at flash heating conditions. The results are presented in Tab. 2.4.

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Data of wheat straw

3.5

y = ln(80/80–W)

3 2.5 2 1.5 1 0.5 0 0

0.05 900K Line (900K)

0.1

0.15 0.2 Heating time t(s)

0.25

750K Line (750K)

800K Line (800K)

850K Line (850K)

0.3

Fig. 2.5: Wheat straw experimental data calculated by equation (2.4) and the fitting straight line.

Rice husk

Coconut shell

1.6

2.5 2

1.2

y = ln(80/80–W)

y = ln(80/80–W)

1.4

1 0.8 0.6 0.4

1.5 1 0.5

0.2 0

0 0

0.05

0.1

0.15

0.2

0.25

0.3

0

0.05

0.1

t(s) 750K 850K

800K 900K

Line (750K) Line (850K)

0.15

0.2

0.25

0.3

t(s) Line (800K) Line (900K)

(a)

750K 850K

800K 900K

Line (750K) Line (850K)

Line (800K) Line (900K)

(b)

Fig. 2.6: Rice husk and coconut shell experimental data calculated by equation (2.4) and the fitting straight line.

Tab. 2.4: The volatilization characteristic parameters of biomass. Biomass

Rice husk

Coconut shell

Corn stalks

Wheat straw

Apparent frequency factor (s−1 ) Apparent activation energy E/R (K)

949.8 4543.9

6554.5 5822.4

945.5 4011.0

909.0 3730.2

98 | 2 Technologies of biomass pyrolysis Data curves 3 y = – 3730.2x + 6.8123 2.5

y = – 4011x + 6.8517 y = – 5822.4x + 8.7879

ln (B)

2 1.5 1

y = – 454.39x + 6.8563

0.5 0 0

0.0005

0.001

0.0015

1/T Corn stalk Rice husk Coconut shell Wheat straw

Line corn stalk Line rice husk Line coconut shell Line wheat straw

Fig. 2.7: ln (B) and 1/T curves.

The equations for the volatilization characteristics for four kinds of biomass can be written as follows: Rice husk: Coconut shell: Corn stalk:

dW 4543.9 = 949.8 ⋅ (80 − W) ⋅ exp (− ) dt T 5822.4 dW = 6554.5 ⋅ (80 − W) ⋅ exp (− ) dt T 4011 dW = 945.5 ⋅ (80 − W) ⋅ exp (− ) dt T

(2.14) (2.15) (2.16)

Wheat straw:

dW 3730.2 = 909 ⋅ (80 − W) ⋅ exp (− (2.17) ) dt T Figs. 2.8, 2.9, 2.10 and 2.11 show the experimental and calculated data of different feedstock.

2.1.3 Application example of the biomass pyrolysis technology The first biomass fast pyrolysis demonstration plant in China is shown in Fig. 2.12. It was constructed at Yang Miao Town, Anhui province by the University of Science and Technology of China in 2008 by a university-industry cooperation. The annual productivity of the plant is 10,000 tons of bio-oil and 6,000 tons of solid char by-product. Bio-oil is used as fuel in the asphalt or metal smelting furnace, while the solid char is usually used to prepare briquette fuel.

2.1 Principles and technology of biomass fast pyrolysis

|

99

90 80 70 60 %

750K 800K 850K 900K 750K EXP. 800K EXP. 850K EXP. 900K EXP.

50 40 30 20 10 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

t(s) Fig. 2.8: Comparison of the experimental and calculated data of coconut shell volatilization characteristics.

90 80 70 60 %

750K 800K 850K 900K 750K EXP. 800K EXP. 850K EXP. 900K EXP.

50 40 30 20 10 0 0

0.2

0.4

0.6

0.8

1

t(s) Fig. 2.9: Comparison of the experimental and calculated data of corn stalk volatilization characteristics.

90 80 70 60 %

800K 850K 900K 950K 800K EXP. 850K EXP. 900K EXP. 950K EXP.

50 40 30 20 10 0 0

0.2

0.4

0.6

0.8

1

t(s) Fig. 2.10: Comparison of the experimental and calculated data of wheat straw volatilization characteristics.

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90 80 70 60 %

50

750K 800K 850K 900K 900K EXP. 850K EXP. 800K EXP. 750K EXP.

40 30 20 10 0 0

0.2

0.4

0.6

0.8

1

t(s)

Fig. 2.11: The results show that, at flash heating rates, the biomass volatilization kinetics follow the first-order Arrhenius equation.

Fig. 2.12: The demonstration plant constructed at Yang Miao Town in 2008.

The demonstration plant is mainly composed of a pretreatment device for feedstock drying, a two-stage screw feed device, a fluidized-bed pyrolysis reactor, a cyclone separator and a condensing system.

2.1.3.1 Pretreatment device for feedstock drying The water in bio-oil mostly comes from biomass materials. In order to reduce the water contained in bio-oil, it is efficient to dry the biomass materials before pyrolysis. It has been found that drying the biomass materials at an appropriate temperature not only removes the water in the materials, but also changes the surface structure of the materials, which is beneficial to improve the pyrolysis rate and the volatiles yield. The most appropriate temperature range for drying the biomass is 110–160 °C. As shown in Fig. 2.13, the biomass drying pretreatment device is composed of a screw feed device, a pneumatic drier, an induced draft fan, a cyclone separator, and a regulating valve. The screw feed device is used to quantitatively and stably transport

2.1 Principles and technology of biomass fast pyrolysis

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Tail gas drier

Materials after drying (to pyrolysis reactor)

Biomass

Pneumatic drier

Screw feed device

Adjustablespeed motor

Induced draft fan Hot gas from pyrolysis reactor

Fig. 2.13: Pretreatment device for drying biomass.

biomass materials to the pneumatic drier. Biomass materials are dried and pretreated in the pneumatic drier. The induced draft fan sucks the hot tail gas from the pyrolysis reactor, and thus improves the gas pressure. Then the tail gas is transported to the bottom of the pneumatic drier. The screw feed device is used to separate the tail gas and the dried biomass materials. The regulating valve is designed to adjust the flow and temperature of the hot tail gas.

2.1.3.2 Two-stage screw feed device The pyrolysis temperature of biomass is about 500 °C. Due to heat conduction, the temperature of the end connection between screw feed device and reactor is 200– 300 °C. Lignocellulose is amorphous, so it will be softened and thus sticky at a temperature of 100 °C. When the temperature increases to 200–300 °C, it gets into a molten state and its viscosity increases simultaneously. Thus, it is necessary to consider the influence of high temperature on the charging when designing the screw feed device. The axial feed rate in a single-stage screw feed device cannot be too fast because of the low density and high flow friction of biomass. Since the biomass particles will be fully heated in the screw feed, the particles will be softened or even liquefied, and then adhere to the screw blade and inner wall of the spiral tube. This will reduce the feed

102 | 2 Technologies of biomass pyrolysis

rate, or even worse, block the screw feed device. On account of this, we designed a twostage screw feed device, shown in Fig. 2.14 below, which has successfully overcome the disadvantage of the single-stage screw feed device in feedstock transportation.

Barrel for biomass material

Low speed screw feed

Pyrolysis reactor High speed screw feed

Fig. 2.14: Two-stage screw feed device.

As shown in Fig. 2.14, the two screw feed devices are in series. The inlet and outlet of the first screw feed are connected to the outlet of the feedstock barrel and the inlet of the second screw feed, respectively. The outlet of the second screw feed is connected to the inlet of the reactor. The structures of the two screw feeds may be the same, but their operating parameters must be different. The working speed of the first screw feed is low, so it is called the low speed screw feed. It is used to transport the biomass materials quantitatively. The second screw feed has a high working speed, so it is called the high speed screw feed. It is designed to transport the biomass particles from the first screw feed to the pyrolysis reactor rapidly so that the materials are not significantly heated. Therefore, the problems in raw material charging can be solved.

2.1.3.3 Fluidized-bed pyrolysis reactor The pyrolysis process of the fluidized-bed reactor is shown in Fig. 2.15: (1) The biomass particles are charged into the flat and closed-end pyrolysis reactor by the screw feed device. The bottom of the reactor is designed to be tilted, and the outer wall of the reactor is a heating surface. (2) The vibration of the reactor and gravity make the biomass particles slip down to the bottom of the reactor. (3) The biomass particles are heated and then subjected to pyrolysis during the slipping process. The gaseous and solid products leave the pyrolysis reactor from their corresponding outlets. (4) The gas product is purified, condensed, and finally divided into two parts. The condensable part is condensed into liquid product, while the noncondensable part is the

2.1 Principles and technology of biomass fast pyrolysis

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Pyrolysis gas

Fluidized bed pyrolysis reactor

Screw feed device

Carrier gas inlet

Fig. 2.15: Fluidized-bed pyrolysis reactor.

pyrolysis gas. (5) The reactor can be used to gasify, pyrolyze or carbonize biomass according to the vibration frequency of the reactor, the inclination angle of the bottom and the pyrolysis temperature.

2.1.3.4 Cyclone separator As shown in Fig. 2.16, the cyclone separator is composed of a barrel, a cone, a gas inlet, a gas outlet and a dust outlet. The separation space of the cyclone separator is also called the cyclone body, including the barrel and the cone. The section of the gas inlet is usually rectangular, which is at a tangent to the barrel. When the pyrolysis gas with solid char and sand enters into the cyclone separator along the tangential direction, the gas flow will transform from rectilinear motion to curvilinear. Most of the gas moves spirally downward along the inner wall of the barrel towards the cone. During the swirling flow, centrifugal force leads to the separation of pyrolysis char and sand from the pyrolysis gas. The solid particles are thrown towards the inner wall. Once they come into contact with the wall, they will lose radial inertial force and fall

104 | 2 Technologies of biomass pyrolysis

Gas outlet

Gas inlet Cyclone body

Dust outlet

Fig. 2.16: Cyclone separator.

downward along the wall driven by momentum and gravity. Finally, the solid particles are collected in the dust collector. On the other hand, when the purified outer spinning gas moves to the cone, the contraction of the cone forces the gas to gather at the center of the cyclone. Finally, the purified gas is sent to the condensing system through the gas outlet with some uncaptured solid particles.

2.1.3.5 Condensing system As shown in Fig. 2.17, a combined spray and falling film condenser system is used to realize the fast condensation of the high-temperature pyrolysis gas. At first, pyrolysis oil is used as a condensate, atomized and sprayed directly into the high temperature pyrolysis gas. Small droplets of the condensate come in contact with the pyrolysis gas and condense the gas rapidly and efficiently. Therefore, the polymerization and polycondensation reactions are inhibited. Then film condensation is adopted to carry out further condensation. After condensing, the heat passes through the liquid membrane, and is brought out of the condenser by the cooling water on the other side of the condenser pipe. At the same time, low boiling components of the pyrolysis gas are further condensed on the liquid-vapor interface. Thus, the combined spray and falling film condenser can significantly improve the yield and quality of the pyrolysis liquid. When straw is employed as the raw material, the yield of bio-oil collected by the condenser is ≥ 50 %, and its heat value is 16–18 MJ/kg.

2.2 Upgrading and applications of bio-oil |

Atomized cooling nozzle

105

Non-Condensable gas

Spray condenser

Pyrolysis gas Tubular heat exchanger

B Water tower

Falling film condenser

Liquid products

Water pump Oil pump

Fig. 2.17: Combined spray and falling film condenser system.

2.2 Upgrading and applications of bio-oil 2.2.1 Physical and chemical properties of bio-oil 2.2.1.1 Ultimate composition of bio-oil Tab. 2.5 shows the ultimate composition of rice husk and wood chip, and the corresponding bio-oils obtained from them. It can be observed that the organic elements of bio-oil and biomass are roughly equal. The carbon content is 40–50 wt.%, hydrogen content is 6–8 wt.%, oxygen content is 45–55 wt.%, and the total content of these elements is more than 95 %. Bio-oil and biomass both have a low sulfur content, and thus they produce only a small amount of SO2 after combustion. Inorganic elements in the biomass are mainly present in the form of minerals, and most of them are removed as ash by the cyclone separator in the pyrolysis process. However, some amount of ash content is entrapped by the pyrolysis steam and collected in the condensation collection flask with bio-oil. Luckily, the inorganic elements content is significantly low in the biomass raw materials. The metal content of bio-oil obtained from most of the biomass materials is too high to exceed the accommodation limit of the cylinder. The metal ions from the metallic elements have strong corrosion, and can decrease the stability of bio-oil. The oxygen content of bio-oil is high, and it is detrimental to the calorific value of the bio-oil. However, a small amount of oxygen is beneficial to combustion of bio-oil. This may result from the fact that the oxygen in bio-oil is more likely to participate in the oxidation reaction in the combustion process compared to the gaseous oxygen in air.

106 | 2 Technologies of biomass pyrolysis

Tab. 2.5: The ultimate composition of rice husk and wood chip and the corresponding bio-oils obtained from them. Elementary composition

Rice husk

Rice husk bio-oil

Sawdust

Sawdust bio-oil

C H O N S Si K Ca Mg Na Mn Fe Al

38.69 5.4 55.03 0.67 0.21 6.82 0.36 0.16 0.051 0.009 0.033 0.006 0.007

39.92 8.15 51.29 0.61 0.03 0.096 0.0094 0.0056 0.0021 0.0006 0.0014 0.0597 0.0006

45.92 6.18 47.70 0.19 0.01 0.019 0.098 0.190 0.021 0.002

41.71 7.71 50.30 0.29 0.01 0.0064 0.0058 0.0108 0.0014 0.0003

2.2.1.2 Component analysis of bio-oil Bio-oil has a complex composition, and includes a large amount of water, a small amount of solid particles and hundreds of organic compounds of different classes, including acid, alcohol, ketone, aldehyde, phenol, ether, ester, sugar, furan, nitrogen compounds and multifunctional organic matter. Bio-oils obtained from different biomass materials have similar chemical compositions to some extent, but the specific chemical components and contents are affected by many factors. The molecular weights of these compounds vary widely from as low as 18 (water) to as high as 5,000 or more (compounds pyrolyzed from lignin). Many chemical composition analyses have been carried out for bio-oils from various agriculture and forestry biomass raw materials, and more than 400 kinds of compounds have been detected. However, there are some compounds that are present only in certain bio-oils. Only few components in bio-oil such as water and acetic acid have contents as high as 5 wt.%. The contents of the other compounds are usually less than 1 wt.%. Thus, it is difficult to identify all of the compounds of bio-oil with a single analytical measurement. Tab. 2.6 shows some of the components of bio-oil from rice husk detected by GC-MS. It is difficult to comparatively analyze bio-oil components due to the remarkable variations in bio-oil samples. This can be attributed to the different raw materials, pyrolysis processes and analytical measurements. IEA-EU has organized 12 laboratories to comparatively analyze four kinds of bio-oil. Four of them quantitatively measured the oil phase containing organic acids, aldehydes, ketones, alcohols, sugars, and aromatic hydrocarbons of bio-oil. Eighteen kinds of carboxylic acid compounds were found, with acetic acid having the largest content of 2–11 wt. %. Aldehydes, ketones

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Tab. 2.6: Main components of bio-oil from rice husk and their contents detected by GC-MS. No.

Compound

Content (wt.%)

No.

Compound

Content (wt.%)

formaldehyde acetaldehyde ethyl formate glycidol ethyl glycolate

0.3904 1.5112 0.036 0.2431 1.3291

29 30 31 32 33

0.6224 0.7506 0.7811 0.1257 0.2373

6 7 8 9 10

glycolaldehyde hydroxy-acetone ethyl formate formic acid acetic acid

0.8893 0.7121 1.2195 0.8554 4.3616

34 35 36 37 38

11 12 13 14 15 16 17

0.5483 0.0662 0.7399 1.0926 0.9335 0.3904 0.5821

39 40 41 42 43 44 45

18 19 20 21

acetic acid-2-O-butyl ester valeraldehyde 2, 2-dimethoxy ethanol propionic acid furfural 2-alkenyl-butanol 2,5-dimethoxy tetrahydrofuran 1, 3-dihydroxyacetone valeric acid 4-hydroxy butyric acid 5H-furan-2- ketone

xylitol o-methylphenol m-cresol methoxy-6-methyl phenol succinic acid methyl ethyl ester 4-dimethyl phenol 4-ethyl phenol lactose hydroxy-Lauric acid methyl benzene formaldehyde isoeugenol catechol 3-methyl catechol coumaric aldehyde 4-methyl catechol melibiose 3-hydroxy-D-tyrosine

0.2418 0.1845 0.1845 0.0073

46 47 48 49

0.3888 0.0791 0.0916 0.2236

22 23

2, 3-dimethyl cyclohexanol 3-hydroxyl cyclohexanone

0.2452 0.4487

50 51

24 25

4-isopropyl-cyclohexanol allyl acetic acid

0.0095 0.0201

52 53

26 27

ring dulcin 4-methyl-5H-furan-2-ketone

0.2993 0.0154

54 55

28

4-methoxy phenol

0.8657

56

4-ethyl-catechol 3-hydroxy-tridecanoic acid 3-methyl ethyl phenol 4-(3-hydroxyl propenyl)phenol levoglucosan 3-(4-hydroxy-2-methoxybenzene)-acrolein heptose 2-(2-isopropyl benzene)propyl alcohol 2-naphthalene acid 3-methoxy-5-methyl-4-nitrophthalate acid 3-methoxy-2-naphthalene acid

1 2 3 4 5

0.1854 0.3933 0.0889 0.3785 0.9041 0.2372 1.2211 0.1881 0.2467 0.8442 0.3468 0.0259

3.8021 0.0282 0.1161 0.0899 0.0098 0.0024 0.0042

and alcohols found in bio-oil mainly included formaldehyde, acetaldehyde, glyoxal, glycolic aldehyde, furfural, hydroxy acetone, acetone, 1-hydroxy-2-valproic ketene, methanol, ethanol, 2-propyl alcohol, and butyl alcohol among others. Among these, formaldehyde, glycolic aldehyde and hydroxy acetone had the largest content of about 1–10 wt. %. Fifteen kinds of phenols were found, and their total content is around 1– 4 wt. %. Levoglucosan is the important carbohydrate compound in bio-oil, and was detected in three laboratories. In the above-mentioned comparative measurements, there are large differences between the quantitative analysis results of bio-oil in different laboratories.

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2.2.1.3 Physical and chemical properties of bio-oil Bio-oil is a colored liquid, ranging in color from brown to black, and the specific color depends on the raw material, chemical composition and amount of fine carbon particles. Moreover, bio-oil has a unique smell. Freshly prepared bio-oil is homogeneous with a single phase. According to the preparation process and separation effect, it is mixed with various contents of solid particles, namely fine solid char. Some raw materials, such as those with a high alkali metal content, are likely to produce bio-oil which is easy to delaminate. Physical and chemical properties of bio-oil include water content, calorific value, density, viscosity, heterogeneity, ash content and solid particles, ignition and combustion characteristics, corrosion, stability, etc. Tab. 2.7 lists the main physical and chemical properties of bio-oil and several other kinds of liquid fuels. Tab. 2.7: Physical and chemical properties of bio-oil and other liquid fuels. Physical and chemical properties

Bio-oil

Crude oil

Heavy oil

Diesel

Gasoline

Moisture/wt.% Solid/wt.% Ash content/wt.% C/wt.% H/wt/ % O/wt.% N/wt.% S/wt.% Stability Viscosity/10−6 m2 · s−1 Density (15 °C)/kg · m−3 Flash point/°C Heat value/MJ · kg−1 pH

15–30 0.3 85 12.5 1 0.2 >1 Stability 20–200 (80 °C) < 980

0.1 < 0.5 < 0.01 85–86 13–15 – 0.1 0.2–0.5 Stability 3–8 (20 °C) 850

0.025 – – 84–88 12–16 – 0.1 0.08 Stability 0.6–0.7 (40 °C) 700–800

70–100 15–18 2.0–4.0

–10 to 28 41.7 –

< 130 38–40 –

40 to 55 40–46 –

–50 to –40 46 –

2.2.1.3.1 Water content Water is one of the major constituent of bio-oil, and its content is about 15–30 wt.%. The water present in the raw biomass material and the water formed during the pyrolysis process by condensation and polycondensation reactions mix with the pyrolysis gas in the form of steam, then enter the condenser through the pyrolysis system, and finally get collected in the bio-oil. The water in bio-oil cannot be removed by conventional distillation methods, mainly due to the poor thermal stability of bio-oil, as the condensation and polycondensation reactions may result in coking when heated to 80–90 °C. The moisture content of bio-oil can be determined by the Karl Fischer method according to the

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ASTM 203 standard. Bio-oil should be completely dissolved in the selected solvent, so the key to accurate determination of moisture content is selection of the solvent. The typical solvent used for bio-oil is a mixture of chloroform (or dichloromethane) with methanol in the ratio of 3:1. There are several adverse effects of the high water content of bio-oil on its applications: Firstly, high water content reduces the calorific value of bio-oil. Generally, the calorific value of bio-oil is about 15–18 MJ/kg, only two-fifths of that of diesel. In order to meet the requirements of the same total quantity of heat equipment, a larger bio-oil mass flow rate is needed, thus increasing the power consumption of the oil pump. Besides, a larger diameter of nozzle is needed to spray bio-oil. Secondly, too much water causes the water phase and oil phase to separate easily. This is unfavorable for the stability and storage of bio-oil. Thirdly, the evaporation of water will consume a large amount of potential vaporization heat, which will have to be provided by the flame. Therefore, the flame temperature is lowered. Additionally, water vapor will dilute the concentration of combustible volatiles. When bio-oil is used as a fuel in engines, the firing delay time is significantly longer than that of gasoline and diesel. Fourthly, soot emissions increase. This is because the oil burning rate is reduced due to the presence of a large amount of water. As a result, some of the solid particles, CO, CH4 and other intermediates generated during the combustion would not be able to complete burn if the time in the furnace is relatively short. On the other hand, the viscosity of oil will be reduced by water, thereby enhancing the flow properties and the quality of atomization in the combustion pipeline system. Besides, a lower combustion temperature is beneficial to reduce the formation of NOx . Microburst phenomenon of water may occur during the combustion process, thereby enhancing secondary atomization of bio-oil and thus the combustion efficiency. The hydroxyl groups of water may also be effective in inhibiting generation of carbon black and accelerate its oxidation.

2.2.1.3.2 Heat value The heat value of fuel refers to the entire amount of heat released when 1 kg of the fuel is completely burned, and then cooled to the original reference temperature. The heat value can be divided into high heat value (HHV) and low heat value (LHV). The difference between these heat values is due to the vaporization heat of water. The high heat value of bio-oil is usually determined by an oxygen bomb calorimeter. However, the presence of large amounts of water in bio-oil may cause difficulties in ignition. At this point, an ignition wire which will ignite bio-oil easily can be used, such as a cotton wick. Then the high heat value of bio-oil will be obtained by subtracting the ignition wire heat. Since the water in bio-oil is difficult to remove, its dry basis heat value is difficult to measure, and can be obtained through calculations. The dry basis (anhydrous) heat values of bio-oils from different materials and pyrolysis processes are almost the same. Oasmaa et al. [145, 146] have reported that the dry basis heat values of bio-oil

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prepared from wood and straw raw materials were 19–22 and 18–21 MJ/kg, respectively. Although the heat value of conventional bio-oil is two-fifths that of diesel and other fossil fuels (41–43 MJ/kg), the volumetric energy density of bio-oil is about 50 % of that of diesel. This is because the density of bio-oil is generally about 1.1–1.3 g/mL, higher than that of diesel and other fossil fuels (0.8–1.0 g/mL).

2.2.1.3.3 Viscosity and rheological properties Viscosity of bio-oil is a measure of the fluid microscopic molecular friction. It is divided into dynamic viscosity and spot viscosity. It is an important factor that affects fuel transportation and atomization. In order to guarantee the smooth delivery of the fuel, stability of the oil pump and favorable atomizing of the burner nozzles are essential. Thus, there are certain requirements for the fuel viscosity in combustion applications. Kinematic viscosity of bio-oil can be measured by a capillary viscometer. If the solid particles content is high or the phase in bio-oil is separated, then there may be large errors in the capillary viscometer measurements. At this point, a rotary viscometer can be used to measure the dynamic viscosity. Bio-oil viscosity varies with the content of water, raw materials, production process and composition, and the range is 10–1,000 cP (1 Pa ⋅ s = 1, 000 cP) at room temperature. 2.2.1.3.4 Flash point and freeze point Flash point is a safety indicator to prevent the occurrence of fire during applications of the fuel. It is the lowest flash temperature when the mixture of liquid fuel and air is exposed to air after being heated to a certain temperature. Measurements of the flash point include the closed-cup and open-cup methods. The closed-cup method is conducted in a closed container, generally to determine fuels with a low flash point. The flash point measured by closed-cup method is lower than that measured by the open-cup method. The flash point of bio-oil is closely related to its moisture and volatile matter content. Due to the high moisture content of bio-oil, its flash point is usually between 70–100 °C. It is difficult to detect the flash point in this range because of the simultaneous evaporation of small amounts of water in bio-oil. Freezing point is an important quality indicator for the handling and transportation of a fuel. It is used to characterize the low temperature properties, and refers to the maximum temperature at which liquid loses its mobility. Under low temperature conditions, bio-oil loses its mobility, mainly due to the increase in viscosity or the crystallization of the high levels of paraffin wax. The freezing point of bio-oil is usually around −35 to −15 °C.

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2.2.1.3.5 Solid particles and ash content The solid particles in bio-oil are mainly composed of carbon powder and ash (there are also some sand particles and other heat carriers in the fluidized-bed reactor). The solid particles in bio-oil are present in various forms, such as particles adsorbed by ions with opposite charge, particles adsorbed by organic matters and precipitated particles. A certain amount of metal elements such as Na, K, and Ca are also present in the ash content, and the particle size is usually 1–200 μm. The content of solid particles varies with the particle size of raw materials, uniformity, pyrolysis conditions, and the separation and collection of the pyrolysis products. Generally, the separation efficiency for solid particles above 10 μm can reach up to 90 % by cyclone separator, while the separation efficiency for solid particles under 10 μm decreases significantly. Without any further filtration, considerable amounts of solid particles will enter bio-oil, and the highest content can be up to 0.3 wt.%. In addition, the metal elements in the raw materials are concentrated in the ash content after the biomass pyrolysis process. Therefore, the metal content of solid particles in bio-oil is 6–7 times that of the biomass feedstock. These solid particles and metallic elements have some negative effects on the storage stability, pollutants generated during combustion, as well as abrasion and corrosion of engines and other thermal equipment.

2.2.1.3.6 Ignition characteristics Compared with gasoline and diesel, bio-oil has poor ignition characteristics. It cannot spontaneously combust, and external energy has to be introduced. Thus, when biooil is used as fuel in a boiler, an external auxiliary ignition source will be needed to ignite it. When it is used as fuel in an engine, the ignition delay period is significantly longer than that of gasoline and diesel. The poor ignition characteristics of bio-oil are related to the high water content, low amount of hydrocarbons, high oxygen content as well as high volatile matter content. Cetane value is used to characterize the ignition characteristics of bio-oil. It is defined as the percentage of n-hexadecane in the mixture of n-hexadecane and αmethylnaphthalene. It is usually specified that the cetane numbers of n-hexadecane and α-methylnaphthalene are 100 and 0, respectively. Fuels with a low cetane number may cause ‘knocking’ in the engine, which will decrease the economy, power and reliability of the engine. However, diesel with a high cetane number may also result in formation of soot due to localized incomplete combustion.

2.2.1.3.7 Density The density of bio-oil is an important indicator for its storage, transportation and utilization. The density ratio with feedstock indicates the volume energy density of bio-oil. The density of bio-oil is not directly linked to the pyrolysis conditions, but to its water content. Bio-oil with low water content has a higher density, and density of bio-oil is usually 15–30 wt.%.

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2.2.1.3.8 Surface tension Surface tension of bio-oil is an important factor that determines its atomization performance. The surface tension value is usually measured by a surface tension meter. Perez et al. [147] studied the surface tension of bio-oil and found that its value was about 38 mN/m at room temperature (25 °C). When bio-oil was heated to 45–50 °C, the tension was rapidly reduced to 24 mN/m. Tzanetaki et al. [148] measured the surface tension and equilibrium surface tension of bio-oil from hardwood at three different temperature conditions. The results showed that the tension value was approximately 35 mN/m at room temperature while it fell to 30 mN/m at 80 °C. The detailed data are shown in Tab. 2.8. Tab. 2.8: Surface tension of bio-oil as a function of temperature. T/°C

Equilibrium surface tension (mN/m)

Difference (mN/m)

25 50 80

36.29 32.99 30.91

34.66 32.67 30.84

1.63 0.32 0.07

2.2.1.3.9 Toxicity The toxicity of bio-oil is determined according to its chemical composition. Pyrolysis reaction conditions strongly influence bio-oil’s chemical composition. It has been found that with an increase of reaction temperature and gas residence time, the toxicity of bio-oil also increased. This is because more toxic substances such as polycyclic aromatic hydrocarbons and benzene derivatives will be generated in the strict reaction conditions during the pyrolysis process. However, the content of toxic substances in bio-oil is low at the reaction temperature of about 450–550 °C at which the maximum yield of bio-oil is usually obtained. Gratson et al. [149] studied the toxicity of two kinds of bio-oil. The experiments evaluated the effects of bio-oil on the eyes and skin, the effects after inhalation and the Ames test (rapid screening tests for genetic mutations). The results showed that bio-oil caused serious damage when it came into direct contact with the eyes, but did not damage the skin tissue even when it penetrated into the skin. Moreover, the stomach showed signs of damage upon inhaling high concentrations of bio-oil.

2.2.2 Bio-oil upgrading with addition of hydrogen Bio-oil is a kind of microemulsion having a complex composition. It contains a variety of organic compounds, including acids, aldehydes, esters, acetal, hemiacetal, alcohols, alkenes, aromatic hydrocarbon, phenols, sugars, lignin oligomers, and water. Poor characteristics of bio-oil such as high water content, high oxygen content,

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high viscosity, strong corrosive nature, low calorific value, volatility, and thermal instability have limited its application as a fuel in motor vehicles and as an additive in fossil oil. Therefore, as a low-grade liquid fuel, bio-oil should be hydrodeoxygenated before it is used as a substitute for fossil fuel in the internal combustion engine. Hydro-upgrading is the process in which bio-oil is subjected to high pressure between 7–20 MPa in the presence of hydrogen, and involves various reactions including hydrogenation, deoxygenation and reforming via catalysis. The oxygen is removed in the form of water, and liquid fuel with high levels of hydrocarbons can be obtained. Hydrodeoxygenation is widely used in the upgrading of bio-oil due to its high yield, high H/C and good quality of liquid product. Various catalysts were applied to achieve the mild catalytic hydrogenation or deep hydrogenation under different catalytic conditions. The related previous works were classified as follows.

2.2.2.1 Sulfided Co-Mo and Ni-Mo catalysts Piskorz et al. [155] studied the hydrogenation of bio-oil via sulfided Co-Mo catalyst, and found that the content of oxygen was reduced sharply to 0.5 wt.%. Moreover, the content of aromatics in the light hydrocarbon liquid was about 38 wt.%, and the heat value increased dramatically. Churin et al. reported that they hydrotreated bio-oil in a fixed-bed reactor using Co-Mo and Ni-Mo catalysts with the pressure between 5– 12 MPa and temperature between 270–400 °C [150]. The results showed that the mass fraction of hydrocarbons was increased from 10 %–20 % to 70 %–75 %, and the mass fraction of phenols was decreased from 40 % to 18 %. Therefore, hydrogenation is significantly beneficial to reduce the oxygen content in bio-oil. Zhang et al. [151] investigated the indirect hydrogenation of oil phase separated from bio-oil using sulfided Co-Mo-P/γ-Al2 O3 catalyst with tetralin as the solvent. The results showed that the bio-oil yield increased with reaction temperature, but did not show any further significant change when the reaction temperature surpassed a certain value. Gas yield increased steadily as temperature increased, while char yield decreased sharply as temperature was raised up to 360 °C, although above this temperature it increased slightly. Love et al. [152] investigated the fixed-bed pyrolysis of kerogens at high hydrogen pressures (> 10 MPa, hydropyrolysis) in the presence of a sulfided molybdenum catalyst and achieved extremely high yields of dichloromethane-soluble oil (> 65 %) and high total yield of aliphatic hydrocarbons (30 wt.%). Senol et al. [153] studied the elimination of oxygen from carboxylic groups with model compounds, methyl heptanoate and methyl hexanoate, on sulfided NiMo/γAl2 O3 and CoMo/γ-Al2 O3 catalysts. The reaction was carried out in a flow reactor under reaction temperatures of 250 °C, 275 °C, and 300 °C, system pressure of 1.5 MPa and hydrogen flow rate of 2.0 L/h. Catalyst performance and reaction schemes were also investigated. Three paths via which aliphatic methyl esters produced hydrocarbons

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were determined. The first path gave alcohols followed by dehydration to hydrocarbons. De-esterification yielded an alcohol and a carboxylic acid in the second path. Carboxylic acid was further converted to hydrocarbons either directly or with an alcohol intermediate. Decarboxylation of the esters led to hydrocarbons in the third path.

2.2.2.2 Noble metal catalyst for catalytic hydrogenation Although sulfided catalysts displayed excellent catalytic activity for hydrogenation, they had some drawbacks including the formation of environment-unfriendly products and easy deactivation of the catalysts. Therefore, researchers turned their attention to other kinds of catalytic hydrogenation technologies according to the specific characteristics of bio-oil. Sheu et al. [154] investigated the catalytic hydrotreatment of bio-oil in a tricklebed reactor system using Pt/Al2 O3 as the catalyst. The reaction pressures varied from 5 MPa to 10 MPa, and the reaction temperatures ranged from 350 °C to 400 °C. The oxygen was removed in the hydrotreated oil, and the content of hydrogen compounds increased significantly. Yakovlev et al. [156] used Ni-Cu catalysts to catalyze the hydrogenation process and researched the corresponding catalytic hydrogenation system. Wildschut et al. [157] reported a study on the hydrogenation of bio-oil using a variety of noble metal catalysts (Ru/C, Ru/TiO2 , Ru/Al2 O3 , Pt/C and Pd/C). The results showed that when the reactions were carried out at temperature of 250–350 °C and pressures of 10–20 MPa, the Ru/C catalyst was superior to the standard hydrogenation catalysts with respect to oil yield (up to 60 wt.%) and deoxygenation level (up to 90 wt.%). The HHV was about 40 MJ/kg, equivalent to that of diesel. Analyses of the products by 1 H-NMR and 2D GC showed that the upgraded oil had lower contents of organic acids, aldehydes, ketones, and ethers than the feed, whereas the amounts of phenolics, aromatics, and alkanes were considerably higher. The above catalysts are all high-temperature catalysts while the thermal stability of bio-oil is poor. When the temperature exceeds 80 °C, polymerization in bio-oil is quite intense, competing with hydrogenation to increase the viscosity sharply. Some reactants enter the catalyst matrix and get deposited on the catalyst active sites, leading to the rapid deactivation of the catalyst.

2.2.2.3 Two-stage catalytic hydrogenation When bio-oil is subjected to harsh conditions, like that of catalytic hydrogenation in the traditional petrochemical industry, coking and deactivation of catalyst become serious problems due to the high thermal sensitivity of bio-oil. In order to overcome the above-mentioned drawbacks, the two-stage catalytic hydrogenation of bio-oil has been developed. Firstly, catalytic hydrogenation of bio-oil is carried out at a relatively low reaction temperature (< 300 °C, called stabilization stage) to remove some compo-

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nents with poor stability. Subsequently, conventional hydrogenation conditions are used for further deoxygenation (350–400 °C, called hydrogenation stage). The synchronization between the two stages is crucial in bio-oil catalytic hydrogenation. An extremely high temperature would result in the deactivation of catalyst directly. Moreover, the performance of the stabilization stage would not be sufficient, making it difficult for the hydrogenation reaction to proceed. Three catalysts (sulfided CoMo/Al2 O3 , NiMo/Al2 O3 , and Ru/Al2 O3 ) were applied in a fixed-bed reactor to hydro-upgrade the bio-oil pyrolyzed from zelkova tree. The catalytic hydrogenation experiments were conducted in two stages: mild conditions (250 °C) and severe conditions (400 °C). In the first stage, amounts of very reactive compounds (i.e., aldehydes) were reduced, which is helpful to improve the stability of bio-oil. Then the hydrodeoxygenation reaction of less reactive compounds (i.e., phenols and acids) occurred in the second stage. The results showed that the NiMo catalyst gave the highest reduction in oxygen content, i.e., from 20.7 wt.% in the crude bio-oil to 9.42 wt.% in the hydrotreated oil. Considering that the composition of bio-oil is considerably complex, and the hydrogenation activity of different compounds varies, some researchers have extended the two-stage hydrogenation technology. Thus, different heating rates have been applied in the hydrogenation process. Dilcio et al. [158] performed the hydrotreatment of bio-oil using hydrogen pressures between 2.5–10 MPa with different heating rates. The oxygen content of the biooil was decreased by 20 wt.%. When the FeS catalyst was used, the yield of bio-oil increased, whereas the oxygen content decreased by 30 wt.%, and the content of aromatics increased markedly.

2.2.2.4 On-line catalytic hydrogenation The hydrogenation methods mentioned above can decrease the oxygen content of bio-oil and increase its heat value. However, the harsh reaction conditions, coking of bio-oil, complex equipment and high cost in the high pressure hydrogenation process limit its marketable application. In order to reduce the cost and operation difficulty, the mixture of bio-oil vapor and hydrogen is treated with the catalyst, such that the pyrolysis vapor passes through the hydrogenation catalyst layer in hydrogen atmosphere, and then is converted to bio-oil. Thus, the reaction heat can be used again to reduce energy consumption, and also the catalyst life can be prolonged due to the low coverage of solid phase when pyrolysis vapor passes through the catalyst bed. Pindoria et al. [159] conducted the hydropyrolysis of cellulose in a two-stage fixedbed reactor, and the pyrolysis vapor was catalytically upgraded using HZSM-5 as catalyst. The oxygen content of upgraded vapor decreased with increasing pressure and temperature, and the tar yield was also decreased. Hence, the HZSM-5 can promote hydrogenation and deoxygenation; moreover, it can be used as a cracking catalyst. Moreover, TGA evidence indicated that the HZSM-5 catalyst trapped more than 40 %

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of the product from the hydropyrolysis stage. Rocha et al. [158] prepared pyrolytic vapor from pyrolysis of cellulose in a fixed-bed reactor, and then subjected the vapor to a temperature of 520 °C for hydrotreatment with sulfided NiMo catalyst. Increasing the hydrogen pressure from 2.5 to 10 MPa decreased the oxygen content of bio-oil from 31.7 wt.% to 9.8 %. The authors also compared the two-stage catalytic hydrogenation with a traditional test of pyrolytic vapor from cellulose [160]. The traditional test was conducted in hydrogen pressure of 10 MPa using colloidal FeS catalyst. On the other hand, the two-stage test was carried out at the temperature of 400 °C with commercial NiMo/γ-A12 O3 catalyst using hydrogen pressure from 2.5 MPa in the first stage up to 10 MPa in the second stage. In two-stage tests, the oxygen content of the bio-oil decreased by 20 % to 10 wt.%. Moreover, the H/C ratios were higher and O/C ratios smaller for the two-stage bio-oil compared to their traditional counterparts. However, the differences in the O/C ratios between the traditional and two-stage biooil increased with pressure.

2.2.2.5 Catalytic hydrogenation of bio-oil in situ Catalytic hydrogenation can decrease the oxygen content of bio-oil and increase its heat value. However, it requires high-level equipment and high cost, and has safety issues. In order to lower the cost and difficulty of operation, in situ hydrogenation was developed in order to achieve the reduction of unstable compounds in bio-oil. The socalled in situ hydrogenation means that some compounds can produce or catalyze other compounds to produce hydrogen in the reaction system. In situ hydrogenation is drawing increasing attention in recent years for its convenient operation, low cost and good performance. Arceo et al. [161] implemented the reactions of sorbitol and other polyols successfully with formic acid as a hydride donor. The dehydrogenation reactions of isopropanol were achieved by Belier et al. [162] with RuCl3 · xH2 O and its phosphine ligands as catalysts at a low temperature. A new method of in situ hydrogenation for bio-oil was proposed by researchers at the University of Science and Technology of China (USTC). In this method, the hydrogen used for hydrogenation was produced from the catalytic dehydrogenation of formic acid. This method can increase the efficiency of hydrogenation and simultaneously avoid the coking problem during hydrogenation.

2.2.2.6 Upgrading bio-oil by homogeneous catalysis with metal complexes Traditional upgrading of bio-oil using solid catalysts has several disadvantages, including severe catalytic conditions, serious coking of bio-oil, fast deactivation of catalyst and low yields of upgraded oil. Due to these drawbacks, the research and development of highly active catalysts that can be used in mild conditions is the key to efficient bio-oil hydrogenation. Based on the results of the analysis of very heat-sensitive compounds and their reactive characteristics, the method of using homogeneous metal

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complexes for the catalytic upgrading of bio-oil was established by researchers at USTC. The highly reactive aldehydes in bio-oil as well as some phenols were hydrogenated quickly under mild conditions to achieve the hydro-upgrading of bio-oil [163]. In this study, firstly the RuCl2 (PPh3 ) catalyst with high hydrogenation activity was chosen to catalytically hydrogenate several representative aldehydes present in biooil. The results showed that the conversion rates of three aldehydes including vanillin were more than 90 %, even at temperatures less than 100 °C. Subsequently, the ethyl acetate fraction extracted from bio-oil and containing a large number of acids, aldehydes and phenols, was hydrogenated. The RuCl2 (PPh3 ) catalyst showed excellent performance in the hydrogenation of unsaturated side chains of aldehydes, ketones and some phenols. Secondly, the Ru, Rh, Pd, and Ni complexes with PPh3 ligand were used to catalytically hydrogenate the entire bio-oil with temperature below 100 °C and pressure ranging from 2.0 MPa to 3.0 MPa. The effect of various parameters on the hydrogenation reaction and the transformation characteristics of bio-oil composition via different catalysts were also studied. Moreover, catalyst recycling is a major issue with homogeneous catalysts since the catalysts are hard to separate from the reaction mixture. Thus, the homogeneous catalysts RuCl2 (PPh3 )3 and RhCl(PPh3 )3 which showed better effects on bio-oil upgrading, were further supported on mesoporous MCM-41 (Fig. 2.18). The utilization of the supported homogeneous metal complex for hydrogenation of the entire bio-oil was researched under the same mild conditions. Moreover, the recyclability and performance of the catalysts were also investigated in detail. Similar to the neat metal complex catalyst, the supported metal complex also converted the highly reactive aldehyRuCl2(PPh3)3 + H2 RhCl(PPh3)3

R1

OH

OH

O Ru Cl H

H2

H Rh

H

Cl

Cl

R2 R2

O

Ru

Cl R1 = Me, Furyl, Phenyl et al.

Cl

H

Rh R3

R1

H2

RhCl(PPh3)2

R2

Ru

H

R3

R2

O R1

R1

–PPh3

PPh3

R1

RuHCl(PPh3)3 + HCl

(Z or E)

H

(Z or E)

Rh R3

H

Cl

R3

R2 = Phenolic, R3 = H, Me.

Fig. 2.18: Catalyst cycle for the hydrogenation of aldehydes (or phenols) in bio-oil using RuCl2 PPh3 )3 and RhCl(PPh3 )3 .

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des and phenols into hydrogenated products. Moreover, due to the mild conditions, no char or coke formation was found in any of the experiments.

2.2.2.7 Catalytic hydrogenation of light fraction of bio-oil The light fraction of bio-oil is mainly responsible for the high heat-sensitivity of bio-oil. A study on the catalytic hydrogenation of light fraction over Ru/γ-Al2 O3 catalyst was carried out in a pilot fixed-bed reactor in Zhejiang University [164]. The effects of reaction temperature (60–140 °C), hydrogen pressure (atmospheric pressure –3 MPa) and feed rates (1.5–4.5 ml/h) on production distribution were also investigated (Figs. 2.19 and 2.20). The results showed that the unsaturated compounds in light fraction were completely converted into saturated compounds when the temperature was above 120 °C and the content of alcohols increased after hydrogenation.

HO

HO

H2

2-Propen-1-01

Propanol

O +

O Propanol

H2

O

HO

Furfural

(a)

O

Furfuryl alcohol

(b) O OH

H2

OH

H2

O

O

O

HO

1-hydroxy-2-propanone 1.2-propanediol

O

2(5H)-Furanone 2(3H)-Furanone, dilhydro-

(c)

(d)

OH H2 O Phenol, 2-methoxy-4(1-propeny1)

HO

OH O O Phenol, 2-methoxy4-propy 1-

(e)

Guaiacol

OH

HO H2

H2

HO Catechol

Phenol

(f)

Fig. 2.19: The hydrogenation reaction pathway of the major components in the light fraction of biooil.

2.2.3 Bio-oil upgrading with catalytic cracking Catalytic hydrogenation can reduce the oxygen content and improve the quality of biooil. However, due to the complex composition and poor thermal stability of bio-oil, the catalytic hydrogenation effect is poor. At the same time, the need for large equipment and strict operating conditions make the cost of operating very high. Low yields can be obtained and the hydrogen consumption is high (600–1,000 L per kg of bio-oil). In addition, the processing can result in the coking and deactivation of catalyst.

14 12 10 8 6 4 2 0

Relative peak area(%)

Relative peak area(%)

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14 12 10 8 6 4 2 0 Original

Original (a)

60 80 120 140 Temperature (MPa) Acetic acid Furfural 5–Hydroxymethyl furfural Furfuryl alcohol Allyl alcohol 1,2–Propylene glycol 1,4–Butanediol

119

(b)

0.1 1 2 3 Pressure (MPa) Acetic acid Furfural 5–Hydroxymethyl furfural Furfuryl alcohol Allyl alcohol 1,2–Propylene glycol 1,4–Butanediol

Fig. 2.20: The component distribution of alcohols, aldehydes and acids in light fraction and hydrogenated light fraction. (a) The effect of reaction temperature on hydrogenation, (b) the effect of H2 pressure on hydrogenation

In order to avoid these disadvantages, several studies have been carried out on the catalytic cracking process. Bio-oil can be converted into small organic molecules by catalytic cracking, and oxygen can be removed in the form of H2 O, CO2 and CO. Although the yield of catalytically cracked bio-oil is low compared to that of hydrogenated biooil, the catalytic cracking process can be performed under mild conditions and atmospheric pressure without reducing gases. C7.5 H7 O6 7H2

→

5H2 O + 0.5CO2 + C7 H11

C7.5 H7 O6 O2

→

5CO2 + 0.6C6 H11.7

(2.18)

The key to catalytic cracking is the selection of the appropriate catalyst. In earlier research, zeolite catalyst (e.g., HZSM-5) was selected for catalytic cracking of bio-oil, and it showed good deoxidization effects under reaction conditions of atmospheric pressure, reaction temperature of 350–600 °C and WHSV of 2 (mass flowmeter of biooil per unit mass of catalyst, kg · h−1 /kg).

2.2.3.1 Catalyst type Since the early 1990s, zeolite catalysts have been widely used in catalytic deoxidation of bio-oil and biomass pyrolytic gas. Some researchers have investigated the effects of catalyst types on bio-oil refining, and explored several catalysts with excellent activity. Williams et al. [165] studied biomass catalytic cracking with different catalysts (e.g., NaZSM-5, HZSM-5 and Y-type), and found that two kinds of ZSM-5 had simi-

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lar effects on the composition and yield of bio-oil from biomass catalytic pyrolysis. However, Y-type catalyst was found to be highly selective for alkanes with serious coking and low yield, which was different from ZSM-5. Adjaye et al. [166] studied the production of hydrocarbons from biomass catalytic pyrolysis with both HZSM-5 and silica-alumina catalysts. They found that the hydrocarbons yield was 27.9 wt.% with HZSM-5 while it was 27.9 wt.% with silica-alumina catalyst. Interestingly, bio-oil refined with HZSM-5 contained more aromatics and bio-oil, while bio-oil refined with silica-alumina catalyst contained more aliphatic hydrocarbons. Aromatics mainly included toluene, xylene, and trimethylbenzene. Aliphatic hydrocarbons mainly included propylene, hexane, pentane, cyclopentane and cyclopropene. Vitolo et al. [167] studied bio-oil catalytic upgrading with HZSM-5 and H-Y zeolite catalysts. The results showed that, in bio-oil upgraded with HZSM-5, the oil could be easily separated from water, while the upgraded bio-oil with H-Y was a homogeneous system, and the oil phase was uniformly mixed with the water phase. The upgraded bio-oil had a lower oxygen content, a higher heat value and a better combustion performance compared with crude bio-oil. However, catalyst coking was very serious. Researchers tried to shorten the total reaction time of catalytic pyrolysis to prevent coking. However, the oxygen was not removed completely in this case.

2.2.3.2 Catalytic reaction conditions Some researchers have studied the influence of reaction conditions on catalytic performance and optimized the reaction conditions by studying the composition of bio-oil and the catalytic mechanism. Vitolo et al. [167] studied the catalytic upgrading of several bio-oils produced from different feedstocks with H-Y and HZSM-5 with different silicon-aluminum ratios in a fixed-bed catalytic reactor (700 mm × 20 mm). The reaction took about 30 min with stable feed speed (5.9 ml/h) at the temperature of 410–490 °C. The activity of catalysts was evaluated with respect to the yield, hydrocarbons content, coking and so on. It was found that HZSM-5 had good catalytic performance and resulted in the highest yield at 450 °C. The upgraded bio-oil had a lower oxygen content, a higher heat value and a better combustion performance compared with crude bio-oil. However, the catalysts have a lot of problems, including serious coking, rapid loss of activity and short catalyst life. Shortening the total reaction time of catalytic cracking can limit the probability of coking. However, it also results in a significant decrease in the bio-oil yield. Guo et al. [54] mixed bio-oil with an equal mass of tetrahydronaphthalene, and investigated the catalytic cracking of the mixture with different catalysts in a fixed-bed reactor. They found that the yield of bio-oil catalyzed by HZSM-5 was influenced by reaction temperature, particle size of catalyst, WHSV, solvent and so on. The highest yield (44.68 %) could be obtained under the optimized reaction conditions (WHSV of 3.7 h−1 , reaction temperature of 380 °C). After analyzing the products in the upgraded bio-oil, it was found that the content of oxygen-containing compounds, such as acids,

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alcohols, aldehydes, ketones and esters was greatly reduced, while the polycyclic and monocyclic aromatic hydrocarbons content was increased.

2.2.3.3 Catalytic mechanism There are few reports about the catalytic cracking mechanism because bio-oil’s composition is complex, and it is difficult to investigate the catalytic reaction process. Thus, the catalytic mechanism cannot be described in detail at present. Researchers usually study the catalytic mechanism by analyzing the composition changes of biooil and trying to optimize the reaction conditions. Sharma et al. [168] studied the catalytic cracking process with ZSM-5 for three oils, which were crude bio-oil, the residual oil after removal of water and water-soluble components, and the residue after recycling phenols. The reason for this was to remove moisture and phenols which can easily inactivate catalysts. In a micro-fixed-bed reactor, the feedstock was mixed with an equal mass of tetrahydronaphthalene at the reaction temperature of 340–410 °C with the catalyst (Si/Al = 59). The objective of the experiment was to obtain the highest yield of organic compounds, especially aromatics. Based on the mass of sawdust, the highest organic compounds yield of bio-oil from biomass was 14.3 wt.% at 310–380 °C, while organic compounds yield of bio-oil from lignin was 9.1 wt.% and the lowest organic yield of bio-oil from residue oil was 5.6 wt.%. Therefore, the best catalytic upgrading path of bio-oil is to recover phenols and then refine the residue. In order to study the catalytic process of bio-oil with HZSM-5, Adjaye [166] selected nine compounds including propionic acid, methyl acetate, 4-methylcyclohexanol, cyclopentanone, 2-methyl-cyclopentanone, anisole, phenetole, phenol and 2-methoxy4-(2-allyl)-phenol to replace acids, esters, alcohols, ethers and ketones in bio-oil. The study found that the process was extremely complex. The main reactions in the process were pyrolysis and deoxygenation. The secondary reactions included secondary pyrolysis, polymerization, cyclization and olefination. The intermediate reactions included alkylation, condensation, polymerization, isomerization and disproportionation. Adjaye et al. [166] studied the catalytic performance of several catalysts which had some common characteristics. HZSM-5 is a special zeolite catalyst and has a good catalytic activity, which is mainly due to its three-dimensional structure, appropriate pore size, strong acidity and an appropriate Si/Al ratio. To test and verify it, the catalytic performance of HZSM-5 was compared with that of H-mordenite, H-Y, silicalite and silica-alumina. It was found that HZSM-5 had strong acid sites, which were easy to approximate, and had a Z-type crosslinked three-dimensional structure and a high Si/Al ratio. Silicalite is a silicon oxide zeolite catalyst, which has uniform pore size. It is a weak acid and does not contain aluminum oxide. H-mordenite and H-Y are silicate catalysts with large pore size and have Brönsted and Lewis acid sites. Pore structure of H-mordenite is different from that of H-Y, which has larger pore size and allows large molecules such as neopentane to diffuse into the pore tunnels. Silica-

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alumina has an amorphous structure and weak acid sites, which cannot be easily approximated. Above all, HZSM-5 catalyzes bio-oil in two ways: thermal reaction and thermocatalytic reaction. In the thermal reaction, bio-oil is converted into light oil, heavy oil and polymerized char. In the thermocatalytic reaction, the resultant light oil, heavy oil and char are converted into gas, water, coke, tar and organic fractions. The experimental data show that the acidity and pore size of catalysts have an influence on yield, production and products distribution. The researchers provided a possible catalytic mechanism. Bio-oil is decomposed into light oil, heavy oil and char in the first step. And then, these in turn are converted into char, tar, gas, water and the expected organic compounds. Corma et al. [169] studied the catalytic cracking paths and catalytic performance of six catalysts, including FCC1, ECat, Al2 O3 , ZSM-5, USY and SiC, and found three gas production paths, including dehydration, hydrogen generation and hydrogenation. Williams et al. [170] studied the catalytic mechanism of bio-oil cracking in a fluidized-bed reactor with HZSM-5 and investigated the catalyst regeneration. It was found that the catalysts functioned in the following two ways: (1) bio-oil was catalytically cracked into alkanes with zeolites, then aromatized; (2) oxygenated compounds were deoxidized into aromatic compounds. The results of catalyst recycle test showed that the catalytic performance of the regenerated catalyst for cracking bio-oil into aromatic compounds decreased significantly, and the oxygenated compounds content increased gradually as the number of regeneration cycles increased. In the catalytic process, the pore size and active sites of zeolites have a significant effect on catalyst performance. Vitolo et al. [167] studied catalytic cracking of bio-oil in a fixed-bed reactor with different catalysts including HZSM-5 (Si/Al = 50), HZSM-5 (Si/Al = 80) and H-Y (Si/Al = 80) under the conditions: reaction temperature of 410– 490 °C, feeding rate of 5.9 ml/h, and catalyst dosage of 2 g. They found that, in bio-oil upgraded with HZSM-5, the oil could easily be separated from water, while bio-oil upgraded with H-Y was a homogeneous system, and the oil phase was uniformly mixed with the water phase. The upgraded bio-oil had a lower oxygen content, a higher heat value and better combustion performance compared with crude bio-oil. The highest yield (22.1–23.4 %) of upgraded bio-oil was obtained with HZSM-5 (Si/Al = 50). The shape-selective catalytic reaction resulted in some coking. The coke yield of products with HZSM-5 was lower than that of products with H-Y, which is mainly because the pore size of HZSM-5 (Å) is smaller than that of H-Y (Å). Small pore size of catalyst can prevent the coking precursor from diffusing into the catalyst pores. The oxygen removal ability of HZSM-5 (Si/Al = 80) is better than that of HZSM-5 (Si/Al = 50) at low temperature. However, the oxygen removal ability of HZSM-5 (Si/Al = 50) is much better than that of HZSM-5 (Si/Al = 80) at high temperature.

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2.2.3.4 Catalyst deactivation mechanism In the research on catalytic cracking, it was also found that the ZSM-5 catalyst could easily undergo coking and deactivation, and it was difficult to prolong the life of the catalyst by adjusting reaction conditions. Adjaye et al. [166] studied the catalytic pyrolytic gas with HZSM-5 in a micro-fixed-bed reactor to upgrade bio-oil and build a mathematical model based on the experimental data. The model suggested that catalytic cracking of bio-oil was a complex process including some parallel and consecutive reactions. Catalyst coking was a very serious side-reaction. By reducing the reaction temperature and bio-oil gas concentration, the coking could be decreased to some extent. However, the catalytic performance of the catalyst was weakened and oxygen could not be removed. Vitolo et al. [167] studied catalytic cracking and upgrading of bio-oil with HZSM-5 and H-Y catalysts. The results show that shortening the gas residence time can decrease the catalyst coking. However, it can also cause the incomplete removal of oxygen. HZSM-5 is used most frequently in the research on bio-oil catalytic cracking. However, HZSM-5 is easily deactivated, suffers from coking and has a short service life. It is mainly because HZSM-5 is a microporous molecular sieve and has an oval pore structure (0.54–0.56 nm), which is suitable for C10 hydrocarbon molecules to penetrate. Larger molecules that are cracked incompletely easily coagulate on the surface of catalysts and thus generate coking, which will cause the deactivation of the catalyst. Horne et al. [171, 172] focused on the research into ZSM-5 deactivation in catalytic pyrolytic bio-oil, and the results were in agreement with the conclusion in the abovementioned study. The catalytic performance decreased significantly with the increase in reaction time, and the following results were observed. Firstly, the alkenes content decreased while the content of oxygen compounds increased. Secondly, average relative molecular mass increased, which was confirmed by molecular-exclusion chromatography. Gayubo et al. [173] studied the influence of temperature, WHSV and reaction time on the aqueous catalytic upgrading of bio-oil. Aqueous phase of bio-oil was catalyzed with HZSM-5 at 400–500 °C. The product mixture was mainly composed of aromatics, olefins and alkenes, which was similar to the catalytic production of diluted methanol and alcohols. Moreover, the mechanism of the renewable deactivation (coking) and the nonrenewable deactivation (leaching of Al) of the catalyst were similar to that of alcohol. The experimental data of catalyst regeneration show that the main reason for deactivation is the leaching of Al. The acidity of the catalyst acid sites is not easy to change, however, the total acidity decreased significantly due to the dealumination of HZSM-5, which results in the catalyst’s deactivation. The carbonization and coking on the catalyst surface can result in catalyst deactivation when it is used to catalyze cracking of large molecular weight compounds in bio-oil. Therefore, the key point in bio-oil catalytic cracking is to select a good catalyst. Recently, MCM-41, SBA-15 and MSU-15 catalysts have attracted a lot of attention. Compared with H-Y and HZSM-5, these catalysts have larger pore size (2–10 nm), and higher specific surface area (> 500 m2 /g). Because of these properties, these three cata-

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lysts displayed good catalytic performances in catalytic reaction with large molecules, and some achievements in this field have been made. Baoet al. [174] studied the catalytic cracking of sawdust pyrolytic gas with MCM41/SBA-15 zeolite catalyst. It was found that the catalyst could reduce the oxygen content and long-chain compounds proportion in bio-oil. The average molecular weight of the upgraded bio-oil with SBA-15 was similar to that of diesel and gasoline, and the aromatics content of bio-oil upgraded with SBA-15 increased. Triantafyllidis et al. [175] studied the catalytic performance of mesoporous aluminosilicates (MSU-SBEA), and compared the catalytic performance of the catalyst with that of Al-MCM-41 and without catalyst. The results show that bio-oil upgraded with MSU-S has low organic phase yield, high PAH and polymer yield in organic phase, serious coking and few acids, alcohols, carbonyls and phenols. The mesoporous catalysts have obvious advantages over the traditional zeolite catalysts. The catalytic performance of the mesoporous catalysts is much better than that of traditional catalysts. Mesoporous catalyst can also reduce the formation of solid products and efficiently prevent catalyst deactivation. However, the main component of this catalyst is silica, which is the reason why the mesoporous catalyst has less active sites and acidity. Metal ions, acidic groups or acidic oxides were supported on it to improve the acidity and improve its catalytic effect. Katikaneni et al. [176] used three kinds of aluminophosphate molecular sieve catalysts (SAPO-5, SAPO-11 and MgAPO-36) to study the influence of pore size, acidity and temperature on the refined products, by comparing the yield of oil and content change of the hydrocarbon and phenols within the product catalyzed by different catalysts. Iliopoulou et al. [177] used Al/MCM-41 as a catalyst, which has different ratios of Si to Al (30, 50), to catalyze the cracking of bio-oil. They found that the yield of liquid was higher with a larger silica-alumina ratio (weaker in acidity), while the yield of gas and solid was higher with a smaller silica-alumina ratio (stronger in acidity). SBA-15 is a new material with hexagonally ordered pore array structure. Compared with some mesoporous materials such as MCM-41, it has the following advantages: ease of synthesis, larger pore size, larger wall thickness and better hydrothermal stability. Adam J et al. [178] studied the catalytic performances of SBA-15 and MCM-41 in catalytic cracking of bio-oil. They found that the yields of solid compounds with the two catalysts are similar and the carboxyls and carbonyls contents were decreased. However, SBA-15 composed of pure silicon, lacks acid sites and the refined bio-oil yield with SBA-15 was much lower than that with Al/MCM-41. Therefore, some researchers prefer to modify SBA-15 with metal ions (eg. Al3+ and Cu2+ ) and solid acid oxides (eg. ZrO2 and TiO2 ) to enhance its acidity. He et al. [179] synthesized SBA-15 in phosphoric acid, and used a grafting method to load Al onto mesoporous SBA-15. They also used thiophene as a sulfur source to investigate the resistance of catalyst to sulfur poisoning in the isopropyl naphthalene reaction. Results showed that the maximum naphthalene conversion rate of the isopropyl naphthalene was 47 % when the thiophene concentration was less

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than 300 × 106 (mole fraction), which indicates that the catalyst has high catalytic activity. However, when the thiophene concentration was higher than 300 × 106 , the catalytic activity decreased and the conversion rate of naphthalene decreased to 30 % or even less. Zhang et al. [180] synthesized zeolite Y/Al-SBA-15 mesoporous composites and dealuminated Y zeolite using a two-step method and a hydrothermal synthesis method, respectively. All of these catalysts show high hydrothermal stability. They also prepared the cracking catalyst by impregnation method. The comparative evaluation of the performance of heavy oil hydrocracking reactions shows that these hydrocracking catalysts on Y zeolite/mesoporous A1-SBA-15 support display a middle distillate oil yield of 66.21 % and a middle distillate oil selectivity of 84.5 %, which are 5.68 % and 5.7 % higher than the results with dealuminated Y zeolite-supported catalyst, respectively. Zhu et al. [181] used butyl titanate and the products of acetylacetone as the titanium precursors to synthesize TiO2 /SBA-15 molecular sieve via one-step hydrothermal method. The results showed that when the Si/Ti ratio was 50, 25 or 20, the titanium atom replaced the SBA-15 silicon atoms successfully without any damage to the highly ordered hexagonal structure of SBA-15. All of the titanium in the catalyst exists in the tetra-coordinated and highly dispersed state. When Si/Ti was 12.5, the dispersion ratio of titanium decreased and some titanium aggregated and then generated titanium dioxide. When compared with the TiO2 /SBA-15 from two-step synthesis, the TiO2 /SBA-15 from one-step synthesis has a better titanium dispersion rate. With the increase in TiO2 content, the catalyst shows higher activity for the cyclohexene catalytic oxidation. Furthermore, the loading of a solid acid such as SO2− 4 /ZrO2 onto SBA-15 is one of the effective methods to enhance SBA-15 acidity. For example, Li et al. [182] used pure SBA-15 as a carrier to synthesize mesoporous materials such as ZrO2 /SBA-15 with pure silica SBA-15 structure. They used this catalyst for the synthesis of n-butyl citrate, and investigated the effects of the silicon zirconium molar ratio of the catalyst, the amount of catalyst, reaction time, reaction temperature, the molar ratio of acid to alcohol and some other factors. Results showed that the optimal reaction conditions for the n-butyl citrate synthesis were silicon zirconium catalyst molar ratio of 100: 3, n-butanol amount of 2 mol, acid-alcohol molar ratio of 1: 6, catalyst mass loading of 2 %, reaction temperature of 130 °C and reaction time of 5 h. The mesoporous catalyst ZrO2 /SBA-15 presents high stability, so it was a potential catalyst for the synthesis of tributyl citrate.

2.2.4 Bio-oil steam reforming for hydrogen production Hydrogen is an important chemical raw material, which is widely used in oil refining, ammonia synthesis, fine chemicals production and the metallurgical industry. Since hydrogen is a kind of high calorific value clean energy, it will play an important role in the energy system in this century.

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For hydrogen production, numerous technology routes have emerged, such as organics reforming, pyrolysis, photolysis, electrolysis of water and biomass, etc. However, the main generation route used in industry is still steam reforming using light oil, natural gas and coal as feedstock. At present, the technology of hydrogen production from bio-oil is still in the developmental stage. The main method applied is steam reforming, in which the design and selection of catalyst are the key factors.

2.2.4.1 Catalyst selection The selection of the catalyst is the key factor for the bio-oil steam reforming process. The catalytic active components loaded in the catalysts are generally precious and nonprecious metals. Precious metal catalysts usually have the advantages of high activity and anti-coking, but their cost is always very high. Dauenhauer et al. [183] used methanol, ethylene glycol and glycerol as model compounds to investigate the catalytic effects of Pd/γ-Al2 O3 and Rh/γ-Al2 O3 on bio-oil steam reforming. Results showed that the maximum hydrogen yields obtained under optimum conditions were 82 %, 92 % and 79 %, respectively. Vagia et al. [184] used acetic acid, ethylene glycol and acetone as bio-oil phase model compounds to simulate the thermodynamic properties of the hydrogen production process. Rioche et al. [185] supported Pt, Pd and Rh on aluminum oxide and zirconium oxide as catalysts and used them to study the steam reforming of bio-oil and model compounds over the temperature range of 650–950 °C. Basagiannis et al. [186] also investigated this process using new catalysts, in which Ni and Ru were the active components while La2 O3 , Al2 O3 or MgO were the supports. All of these catalysts showed good catalytic activity. However, serious coke formation problems still could not be prevented during the bio-oil steam reforming process. To solve this problem, Iojoiu et al. [187] developed a combined catalytic cracking and catalyst regeneration process. Bio-oil was first introduced into the catalyst bed for steam reforming, after which oxygen was swapped into the catalyst bed for catalyst regeneration. The catalysts used were Pt/Ce-Zr and Rh/Ce-Zr. Results showed that the coke formed on Pt and Rh could be completely removed. However, with increasing recycling steps, the catalyst showed some deactivation. This could be due to the change of Ce-ZrO vector lattice which occurred during the catalyst regeneration process. The non-noble metal catalysts used in bio-oil steam reforming are mainly transition metals (such as Ni, Cu, Fe, and Co, etc.) and metal oxides (such as MgO, La2 O3 , C12 A7 , etc.). Nickel-based catalysts have high activity and economic applicability, and thus they are the optimal catalysts for bio-oil steam reforming at present. Wang et al. [188] used a nickel-based catalyst (UCIG-90C) to conduct bio-oil catalytic steam reforming, and obtained a maximum hydrogen yield of 85 %. Based on these results, Garcia et al. [189] studied the effects of loading different additives on nickel-based catalysts. The Ni and Cr metals were supported on an MgO-La2 O3 /Al2 O3 catalyst which contained Co as addition agent, and this composite catalyst showed the highest cat-

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alytic activity. Garcia et al. [190] also used commercial nickel-based catalysts for biooil steam reforming, at higher reaction temperature (about 800 °C), and obtained a maximum hydrogen yield of 89 %. Considering the serious coking problem which occurs during bio-oil steam reforming, modified Ni/Al2 O3 catalysts were developed to improve the resistance of coke formation. The modification of the catalyst mainly includes two methods. One is the addition of Mg and La additives to improve the adsorption capacity of water, which in turn would increase the gasification rate of coke; another is the addition of Co and Cr additives, with the aim of inhibiting carbon deposition reactions. Experimental results show that Ni-Co/MgO-La2 O3 -Al2 O3 and Ni-Cr/MgO-La2 O3 -Al2 O3 catalysts have the highest coke resistance capacity. However, the catalyst deactivation problem still cannot be solved. Recently, China Key Laboratory of Clean Energy Technology investigated various bio-oil steam reforming catalysts, such as Ni-Al, Ni-Cu-Zn-Al, Ni-Co-Al, Ni-ZSM5, C12A7-Mg and Ni-CNTs, among others. They also studied the relationship between the catalyst structures and bio-oil reforming performance. Furthermore, a catalyst system with high activity, selectivity and stability for bio-oil catalytic reforming was proposed [191–194].

2.2.4.2 Preparation of catalysts There are several commercial preparation methods for bio-oil steam reforming catalysts, including impregnation, ion-exchange method, and so on. Precipitation method: This method ensures the uniformity of the catalyst active components and catalyst performance. The main factor for using the precipitation method is suitable precipitation conditions. The precipitation method usually uses one or several kinds of metal salt solutions as the starting material, with the addition of precipitating agent (such as sodium carbonate, sodium hydroxide, etc.). The entire catalyst preparation process includes precipitation, washing, filtering, drying, molding, baking and activation steps. The precipitation method is the commonly used method in the laboratory and industry for the preparation of complex metal oxide catalysts. Impregnation method: This method involves adding high porosity carriers (such as diatomaceous earth, alumina, activated carbon, etc.) into solutions which contain one or several kinds of metal ion. After the processes of draining, desiccation and calcination, the surface of the carrier is covered by a layer of solid metal oxides or salts. The impregnation method can ensure that the active components are dispersed evenly on the support surface. Therefore, this method is commonly used for the preparation of noble metal catalysts, especially platinum, gold, osmium and iridium. Ion exchange method: Some metal ions such as Na+ in the crystal material can be exchanged with other ions. The ion exchange method usually involves placing the catalyst precursor into solutions which contain other noble metal ions, so that the ions can be exchanged under certain conditions (concentration, temperature and pH).

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This method is commonly used for the production of cracking catalysts, especially the zeolite catalysts.

2.2.4.3 Characterization of catalysts The purpose of catalyst characterization is to study the catalyst’s structure and properties, and to reveal the relationship between catalyst structure and bio-oil reforming performance. In addition, determining the catalytic mechanism is also an important target. The main characterization methods used for bio-oil steam reforming catalysts usually are catalyst component analysis, structural analysis, phase composition analysis, specific surface area (BET) and pore size analysis, catalyst morphology analysis and catalyst dynamic analysis. Component analysis is mainly divided into body phase component analysis and surface composition analysis. Body phase component analysis can be divided into two types: solution method and spectroscopy method. Solution methods usually use a solvent to dissolve the specific substances first, and then analyze the solution via chemical methods. This method is particularly suitable for the analysis of noble metal catalysts such as Pt, Pd, Rh, etc. Spectroscopy methods are suitable for the analysis of most elements. These include atomic absorption spectrometry, X-ray fluorescence spectrometry (XRF), inductively coupled plasma spectroscopy (ICP) and so on. Surface analysis usually uses the energy distribution to study the surface composition, structure and properties. This type of analysis mainly includes X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES), Electron Spectroscopy for Chemical Analysis (ESCA) and so on. Phase structure analysis mainly includes X-ray diffraction (XRD), FTIR (Fourier Transform Infrared Spectroscopy), UV-visible absorption spectroscopy, Nuclear Magnetic Resonance (NMR) and so on. XRD can achieve accurate qualitative analysis of the catalyst composition and phase. It can be used for quantitative analysis via external standard and internal standard methods. FTIR can be used to measure the catalyst composition and the solid catalyst surface acidity, and to investigate the interaction between the carrier and active component, providing information about the catalyst’s surface. The catalyst specific surface area (BET) and pore size distribution are closely related to catalyst performance. Specific surface area is the surface area per unit mass of the catalyst (unit: m2 /g). The specific surface area is usually measured by commercial specific surface area test instruments, based on the dynamic specific surface area analysis. The main catalyst morphology analysis methods are electron microscopy techniques, such as Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and so on. TEM is mainly used for the quantitative analysis of the internal structure of material microstructure and micro-ingredients analysis, while

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SEM is mainly used for the observation of fine material surface morphology, fracture, internal structure and the quantitative analysis of material composition. Apart from these common methods of catalyst characterization, there are two more important techniques for dynamic analysis of catalysts, namely thermal analysis and temperature programmed analysis. The principle of the thermal analysis technique involves studying the material characteristics based on the change of temperature and time. Three commonly used techniques are differential thermal analysis (DTA, compared with the reference sample), differential scanning calorimetry (DSC, measuring the change of energy, rather than the temperature) and thermal gravimetry (TG, temperature programmed, measuring the relationship between the mass and temperature). Thermal analysis techniques can be applied to determine the catalyst composition, activity and coke formation. They can also be used to investigate the interactions between the active ingredient and the carrier of the catalysts. These data can then be used for the selection of appropriate catalysts and for obtaining optimal preparation conditions. Programmed temperature desorption analysis techniques include Temperature Programmed Desorption (TPD), Temperature Programmed Reduction (TPR) and Temperature Programmed Oxidation (TPO). TPD results reflect the binding capacity of the adsorbent to the solid surface, and also give the desorption temperature and dynamic behavior under the surface. TPR is a simple and effective method to characterize the reduction catalyst performance, on which surface the sediment reduction reaction will occur during the temperature-programmed process. TPO is a commonly used method to characterize the performance of the oxidation catalyst.

2.2.5 Bio-oil combustion technology 2.2.5.1 Combustion characteristics of bio-oil The simplest and most direct application of bio-oil is as a fuel in boilers and kilns. Due to the differences in the physical and chemical properties between bio-oil and fossil fuels, it is necessary to modify the thermal equipment and injector according to the combustion properties of bio-oil to make bio-oil burn successfully and ensure that the pollutants discharged meet the emission standards. The elemental compositions and main physical and chemical properties of a biooil sample are presented in Tab. 2.9. Bio-oil was produced by fast pyrolysis from rice husk. The pyrolysis temperature was about 475 °C. The TG-DTG experiments were performed using a thermal analyzer (DTG-60H, Shimadzu Co, Japan). The carrier gas was air. The thermal analyzer heated the sample at different heating rates (10, 20, and 30 °C/min) in order to investigate the effects of heating rate on combustion. The TG-DTG curves are illustrated in Fig. 2.21. As shown in Fig. 2.21, the weight loss of bio-oil can be divided into three stages: the volatilization of light components, the pyrolysis of heavy components, and the

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Tab. 2.9: The elemental compositions and main physical and chemical properties of bio-oil. Density kg/m3 , 40°

Viscosity mm2 /s, 40°

Water content wt. yy%

Surface tension mN/m, 40°

Heat value MJ/kg

1096.84

12.13

27.69

33.27

16.2

Elemental compositions (wt. %) C

H

O*

N

37.48

8.28

53.65

0.59

* Oxygen content is obtained by difference.

0 10°C. min–1 20°C. min–1 30°C. min–1

TG/ %

80 60 40

–5 DTG/ % min–1

100

20

–10 –15 –20 10°C. min–1 20°C. min–1 30°C. min–1

–25 –30

0 0 (a)

200

400 T/° C

600

0

800 (b)

200

400 T/° C

600

800

Fig. 2.21: TG and DTG curves of bio-oil at 10, 20, and 30 °C/min in air atmosphere.

combustion of coke. With the increase in heating rate, the maximum weight loss rates are 13.17 %/min, 22.51 %/min and 31.35 %/min, and the corresponding temperatures are 83.52 °C, 107.48 °C and 115.13 °C, respectively. This is due to the poor heat conduction of bio-oil. The ignition temperature and the burnout temperature of coke increase with the increase in heating rate. The first-order reaction is the most common theory used in previous research to describe the mechanism of weight loss of bio-oil during the volatilization and combustion stages. In order to improve the reliability of analysis and determine the most suitable reaction mechanism, 30 reaction mechanism functions were summarized Moreover, the Achar differential equation and Coats–Redfern integral equation proposed by other researchers have been used in research to calculate the kinetic parameters in the volatilization and combustion stages. The most suitable reaction mechanism function of bio-oil in the volatilization stage is: f(a) = α = (1 − α)2

(2.19)

Corresponding to the second-order reaction model; the weight loss mechanism function for the combustion of coke is: 3 f(a) = (1 − α)2/3 [1 − (1 − α)1/3 ]−1 (2.20) 2 The results calculated using equations (2.19) and (2.20) are presented in Tab. 2.10. From this table, we can find that the activation energies of volatilization and com-

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Tab. 2.10: Fitting results of Achar and Coats–Redfern equations. Heating rate °C/min

T °C

10

Achar equation

Coats–Redfern equation

Ea/kJ · mol−1

ln(A/s-1)

r

Ea/kJ · mol−1

ln(A/s-1)

r

30–177 446–520

73.48 407.01

21.15 27.97

0.9909 0.9917

71.06 533.55

20.12 78.34

0.991 0.9967

20

30–181 456–535

65.83 387.07

17.45 53.78

0.977 0.9933

63.53 431.84

16.47 60.86

0.9926 0.9956

30

30–197 470-550

61.90 340.16

16.26 48.00

0.9710 0.9911

59.60 388.03

15.26 55.63

0.9905 0.9902

bustion of bio-oil decrease with the increase in heating rate, indicating that the easier the fuel evaporates, the better the coke burns.

2.2.5.2 Spray characteristics of bio-oil The experimental apparatus used for measuring bio-oil spray characteristics is shown in Fig. 2.22. The apparatus consists of an air distribution system, fuel supply system, nozzle and light measuring system. Bio-oil was filtered through a sieve with mesh size of 74 μm prior to the spray characterization experiment. The experiment was carried out at room temperature. Bio-oil and air in this experiment were used without preheating. Droplet size distribution was measured by Phase Doppler Anemometry (PDA) (BSA-P60, Dantec-Dynamics A/S, Denmark).

Air flow meter Laser receiving probe High-pressure air bottle

Atomizing nozzle

Laser emission probe Pump

PDA control system

Collecting pail

Fig. 2.22: Schematic diagram of PDA spray experimental apparatus.

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The procedures for the spray characteristics experiment are as follows: (1) Check the junctions of the system to ensure that the pipes are clear and sealed. (2) Use a peristaltic pump to suck bio-oil into the nozzle, adjust the nozzle outlet to make the spray perpendicular to the horizontal plane, and check whether there is any blockage. (3) Adjust the position of the PDA probe so that the laser beam shoots directly at the center of the spray. (4) Open the air cylinder, transport the high pressure gas to the nozzle, mix it with bio-oil and spray the mixture out of the nozzle. (5) After the system has stabilized, open PDA control system and measure data. (6) Clean pipes, nozzles and oil pump after the experiment.

2.2.5.2.1 Droplet size distribution of bio-oil spray The droplet size distribution was measured by Phase Doppler Anemometry. The measurement position was determined as follows: measuring once per 5 mm along 200 mm of the axis direction of the nozzle starting from the nozzle exit; in the selected axial section, measuring once per 2 mm along the radial direction until the area where the light beam is absent. The Sauter Mean Diameter (SMD) distribution of bio-oil spray along the radial direction is shown in Fig. 2.23(a). As can be seen, the radius of spray increases with the increase in axial distance. The calculated spray angle is 37°. The SMD increases with the increase in radial distance at the same axial section. 100

110

90

90 80 70

Axial distence 30mm Axial distence 60mm Axial distence 90mm Axial distence 150mm

60 50 0 (a)

SMD (µm)

SMD (µm)

100

5

10 15 20 Radial distance (mm)

80 70 60

25

0 (b)

50

100 150 Axial distance (mm)

200

Fig. 2.23: SMD distribution of bio-oil spray. (a) SMD distribution of bio-oil spray along the radial direction; (b) SMD distribution of bio-oil spray along the axial direction

The SMD distribution of bio-oil spray along the axial direction is shown in Fig. 2.23(b). As can be seen, the SMD increased first and then decreased with increasing axial distance. From the nozzle exit to the position which is 90 mm away from the exit, the SMD of bio-oil decreases from 100.6 μm to 62.1 μm. On the one hand, due to the pres-

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sure difference between inside and outside, the pneumatic pressure is greater than the surface tension of the liquid. After the liquid particles leave the nozzle, a large number of tiny air bubbles contained inside the particles are expanded, broken, and cause the surrounding liquid film to be broken further, resulting in a decrease in particle size, and this is the secondary breakup. On the other hand, high speed hybrid gas-liquid jet entrains the surrounding atmosphere, and the shear effect also causes the decrease in particle size. The SMD increases with the further increase of the axial distance, which is due to reduction of the secondary breakup effect. At the same time, the disturbance of droplets causes particles to collide with each other, large particles agglomerate with small ones and make the diameter of particles increase.

2.2.5.2.2 Effect of air-liquid mass flow ratio on spray characteristics The air-liquid mass flow ratio (ALR) is an important parameter to determine spray quality. The change in bio-oil SMD with ALR is presented in Fig. 2.24. As can be seen, the SMD decreases with increasing ALR. This is because a higher ALR increases the relative velocity of the two-phase flow and strengthens the shear effect to the liquid. At the same time, the volume occupied by the gas in the nozzle becomes large, the surface occupied by the gas in the nozzle exhaust plane increases, and the liquid film or column becomes thin. In addition, more bubbles will form in the nozzle and the pressure in the bubbles will increase, which will make the droplet more susceptible to break. When ALR is larger than 0.5, the decreasing trend of SMD slows down. A further increase in ALR does not provide much improvement in the atomization. The atomizing air flow should be reduced as far as possible to save energy on the premise that the SMD can meet requirements. 110 105

SMD(μm)

100 95 90 85 80 75 70 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 ALR Fig. 2.24: SMD of bio-oil spray droplets as a function of ALR.

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2.2.5.2.3 Fitting formula of bio-oil spray droplets’ particle diameter The viscosity, surface tension, density and ALR have a significant influence on the average diameter of liquid spray droplets due to the atomization mechanism and the structure of the air atomizing nozzle. Air was used as an atomization medium in this experiment to investigate the effects of ALR. Changes in each variable can be described in terms of a power function. The relationship between SMD and these factors can be described by eq. (2.21) below: c DSMD = Aσ aL ν bL FALR ρ dL m eL

(2.21)

In the equation: σL is the surface tension, νL is the viscosity of liquid, ρA is the viscosity of air, ALR is the air-liquid mass flow ratio, mL is the mass flow of liquid, the superscripts of parameters and A are fitting coefficients. Taking logarithms on both sides of the equation, we can get a linear equation. Then, the data obtained from the experiment can be substituted into the linear equation and the equation can be calculated. Finally, the SMD formula below can be obtained: −0.1634 −0.6981 0.1119 ν0.1769 FALR ρL mL DSMD = 0.4172σ0.2804 L L

(2.22)

Equation (2.22) is obtained based on the experiment. The fitting coefficient is reasonable, and the change in SMD with parameters is also reasonable. The SMD value obtained from the equation is similar to that obtained from the experiment, and the maximum deviation is less than 5 %.

2.2.5.3 Bio-oil atomization combustion system A bio-oil combustion system was designed and developed according to the combustion characteristics of bio-oil and numerical simulation of bio-oil combustion, as shown in Fig. 2.25. The combustion system is composed of four subsystems. They are as follows: (1) Oil supply system. The oil supply system contains an oil tank which is placed 10 m above the ground and pipes. Gravity is used to accelerate the oil. (2) Air-supply system. The atomizing air is provided by an air compressor. (3) Burning system. The burning system contains a nozzle and a 2.5 m long combustor. There are three temperature measuring points (130 cm, 180 cm and 230 cm away from the nozzle) in the combustor. From left to right, the three temperature measuring points are denoted by Thermocouple 1, Thermocouple 2 and Thermocouple 3. (4) Water heating system. A water jacket with 50 kg water inside is mounted at the tail of the combustor to evaluate the heating efficiency of the tail gas. At the start of the combustion process, an external ignition source is used to ignite the bio-oil. Burning combustibles are placed at the entrance of the combustor. When

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1 1 11 12 3

2

4

6 7

8

9 3

4 10

5 2 1. Oil barrels 4. Valve

7. Insulation layer 10. Water jacket

2. Oil barrels 5. Air compressor 8. Combustor 3. Oil pump

6. Nozzle

11. Gas analyser

9. Thermocouples 12. Chimney

Fig. 2.25: Bio-oil combustion system.

the temperature of Thermocouple 1 reaches 250 °C after about 10–15 minutes, Valve 4 and the air compressor are opened and the air flow is adjusted to ensure the fuel gets atomized well. The experiments are conducted with different bio-oil mass flow rates (80, 100, and 120 kg/h) to obtain the change in the temperature determined by the thermocouples with time. The results are shown in Fig. 2.26. As shown in Fig. 2.26, the temperature in the combustor increased rapidly from 0 to 6 minutes. The speed of increase in temperature decreased with time because heat dissipation was enhanced with the increase of temperature. The temperature tended to be stable after 10 minutes because the heat transfer from flame to the combustor and the heat transfer from the combustor to outside were almost equal. The temperature of the combustor increased with the increase of bio-oil mass flow. In addition, an exhaust gas analysis was conducted to investigate the degree of combustion. The results are presented in Tab. 2.11. As shown in Tab. 2.11, when the mass flow of bio-oil increased from 80 kg/h to 120 kg/h, the contents of CO and NO increased only a little, and reached the maximum values of 29 ppm and 94 ppm, respectively. When the mass flow of bio-oil increased

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1200

(°C)

1000 800 600 80kg/h 100kg/h 120kg/h

400 200 0

4

8

(a)

12

16

20

(min)

1200

(°C)

1000 800 600 80kg/h 100kg/h 120kg/h

400 200 0

4

8

(b)

12

16

20

(min) 1200

(°C)

1000 800 600 80kg/h 100kg/h 120kg/h

400 200 0 (c)

4

8

12

16

20

(min)

Fig. 2.26: The temperature of thermocouples as a function of time. (a) Thermocouple 1 (b) Thermocouple 2 (c) Thermocouple 3

to 100 kg/h, the maximum temperature of the combustor was more than 1,200 °C. The maximum temperature measured by Thermocouple 3 was 1,260 °C. The heat utilizing efficiency of the exhaust gas was calculated using the following formula: Tthermocouple − Texhaust gas η= × 100 % (2.23) Tthermocouple

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Tab. 2.11: The composition of the exhaust gas in bio-oil combustion. Mass flow of bio-oil (kg/h)

80 100 120

Temperature of exhaust gas ( °C)

Compositions of exhaust gas

O2 / %

CO2 / %

CO/ppm

NO/ppm

230 237 243

6.48 5.10 3.23

8.9 9.3 10.5

15 26 29

73 88 94

Where η is the heat utilizing efficiency, and Tthermocouple (k) and Texhaust gas (k) are the temperatures of Thermocouple 3 and exhaust gas, respectively. The calculations show that the heat utilizing efficiency of bio-oil combustion is about 91 %.

2.2.6 Demo applications of bio-oil Bio-oil obtained from the fast pyrolysis of biomass can be directly used as a fuel in boilers or kilns, and after refining, it can be used as a vehicle fuel or a feedstock for high value-added chemicals. As a result of the high water content, low heat value, high viscosity and surface tension of bio-oil, its evaporation process is lengthened, its ignition is difficult and the ignition delay time becomes very long when it is used for combustion applications. It is hard to combust bio-oil after pre-evaporation. Thus, bio-oil should be atomized into small droplets by a nozzle to realize its stable combustion.

2.2.6.1 Design of the fuel supply system There are two main parameters in the fuel supply system: the flow and the pressure.

2.2.6.1.1 Fuel supply flow There are two ways to design the fuel supply system, one is to make an improvement based on the existing burning device, and the other approach is to design a new burning device. For the first case, if blend oil is used in the existing burning device, the oil supply should be confirmed by the principle of heat flux equality. The specific statements are listed below: (1) Assuming the burning rate of blend oil is 100 kg/h under a rated condition, the heat value of blend oil is 37 MJ/kg; (2) Assuming the heat value of bio-oil is 16 MJ/kg and the thermal efficiency is equal to the blend oil, then the supply flow of bio-oil can be estimated using the follow-

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ing expression: Q = 100 ⋅ 37/16 ≈ 231 (kg/h). For the second case, the supply flow of bio-oil should be confirmed by the practical conditions. The specific procedures are listed as follows: (1) Assuming the steam boiler produces 1 t/h of steam, then the steam would absorb 2,350 MJ of heat based on the specific and latent heat of water. (2) If the thermal efficiency of the boiler is 80 % and the heat value of bio-oil is 16 MJ/kg, then the rated bio-oil supply flow may be estimated by the following formula: Q = 2350/16/0.8 ≈ 184 (kg/h). 2.2.6.1.2 Fuel supply pressure Based on practical applications, it is known that an excellent spraying effect can be achieved when the oil supply pressure of the spray nozzle is stable and more than 1.0 MPa. The simplest and most reliable way to reach the above-mentioned oil supply pressure is to place the bio-oil storage tank on a higher level. Given that bio-oil density is (1.1–1.3) × 103 kg/m3 , the oil supply pressure will be achieved when the bio-oil storage tank is placed at a height of 10 m based on the hydrostatics analysis.

2.2.6.2 Design of gas supply system The gas supply system is used to supply air compression to the spray nozzle. The key to designing an effective system is to determine the gas/liquid mass flow ratio (ALR). ALR is one of the most important parameters affecting the atomization effect. The Sauter Mean Diameter (SMD) of bio-oil spray droplets decreases with increasing ALR. This is because an increase in ALR causes the relative velocity of the two-phase flow to increase, and thus the shear action of bio-oil is strengthened. At the same time, the volume occupied by the gas in the nozzle becomes larger, the surface occupied by the gas in the nozzle exhaust plane increases, and the liquid film or column becomes thin. When the ALR is larger than 0.5, the decreasing trend of SMD slows down, and a further increase in the ALR has a negligible effect on the atomization. The atomizing air flow should be reduced as far as possible to save energy provided the SMD can meet requirements. Therefore, the following parameters should be considered while designing the gas supply system for bio-oil combustion. (1) The range of the ALR is 0.5–0.7. (2) The range of the air compressor pressure is 0.2–0.4 MPa.

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2.2.6.3 Design of the atomizing nozzle The atomizing process involves breaking up the liquid with definite volume into small droplets. In the atomizing process, external force (fluid pressure and aerodynamic force) competes with the surface tension and the viscosity of the liquid. Generally, the atomizing process includes two steps: (1) initial atomizing, in which the continuous fluid phase is broken into discrete phase; (2) secondary atomizing, in which smaller droplets are formed due to the interaction of droplets with the atomizing medium. There are three factors that affect the fluid atomizing process: the fluid surface tension, the atomizing nozzle and the ALR. Fig. 2.27 shows the surface tension variation with temperature. It can be observed that the surface tension of bio-oil is large, but it decreases with increasing temperature. This is due to the fact that the molecular movement becomes violent with the increase in temperature, which will lead to less molecular interaction and lower surface tension. Hence, it is suggested that a heating unit should be added in the bio-oil spray system to guarantee that the temperature of bio-oil can reach 60–70 °C before it enters the atomizing nozzle, so that the bio-oil’s surface tension and viscosity will be effectively lowered, and thus the atomization effect will be improved.

35

Surface tension (mN/m)

30 25 20 15

F=36.31-0.07273xT R2=0.99045

10 5 0 20

30

40 50 Temperature (°C)

60

70

Fig. 2.27: Bio-oil surface tension as a function of temperature.

The nozzle splits the working medium into droplets due to the speed difference between the atomizing medium and the working medium. In addition to SMD, the atomizing angle and distance are used to evaluate the atomizing nozzle. As bio-oil has a high viscosity and surface tension, a long atomizing distance is needed in order to obtain a good atomizing effect. Hence, atomizing angle is a significant parameter for designing a bio-oil atomizing nozzle.

Atomizing distance

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Atomizing angle

Atomizing radius Fig. 2.28: The atomizing distance and angle of atomizing nozzle.

Therefore, the guidelines for designing a good atomizing nozzle are as follows (Fig. 2.28): (1) Bio-oil SMD should be less than 100 μm. (2) The atomizing angle should be less than 50°, and the atomizing distance should be less than 400 μm.

2.2.6.4 Design of the bio-oil combustor The bio-oil combustor could be cylindrical or rectangular. It is suggested that the biooil combustor should be cylindrical if possible as it would be more favorable to flame propagation and gas flow in the combustion chamber. Considering the long evaporation process, difficult ignition and long ignition delay of bio-oil, the bio-oil combustor should be longer than the diesel combustor at the same thermal power. Air compressor

Flowmeter F1

Flue gas Thermocouples

Nozzle

Combustion chamber Primary air flow control

Bio-oil pump Filter Oilcan Fig. 2.29: The cylindrical atomizing combustor for bio-oil.

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For a cylindrical atomizing combustor for bio-oil (Fig. 2.29), the following design principles are suggested: (1) For a combustor with an oil supply of more than 50 kg/h, the effective length should be longer than 2,500 mm. Also, the effective length should be increased with increasing oil supply. (2) The length-diameter ratio of the cylindrical combustor should be in the range of 0.3–0.5.

2.2.6.5 Design of flue gas emission system The design of the flue gas emission system mainly concerns the funnel, which should be designed referring to the conventional design handbook for chimneys.

2.2.6.6 Design of the test monitoring system There are two important aspects that need to be monitored in the bio-oil combustion process: the first is the combustion temperature, and the second is flue gas emissions. Fig. 2.32 shows a simple bio-oil combustion system, in which three thermocouples are fixed at the side of the combustor to monitor the combustion temperature. From left to right, they are denoted Thermocouple 1, Thermocouple 2 and Thermocouple 3, and their distances are 1.3 m, 1.8 m and 2.3 m away from the atomizing nozzle, respectively. A flue gas analyzer is used to detect the contents of CO and NO. A sample connection is set at two-thirds the height of the funnel. A probe is inserted into the sample connection for the analysis, and the sample connection is then sealed.

2.2.6.7 The results of bio-oil spray combustion Bio-oil was ignited with an external ignition source at the start of the combustion test. External fuel was ignited at the entrance of the combustor. Valve 4 and the air compressor were opened, and the air output was controlled to get a good atomizing effect until the temperature of Thermocouple 1 reached 250 °C. The experiments were conducted with different bio-oil mass flow rates (80, 100, and 120 kg/h) to obtain the change in the temperature determined by the thermocouples with time. The results showed that the temperature in the combustor increased rapidly from 0 to 6 minutes, and the temperature tended to be stable after 10 minutes. Additionally, the temperature of the combustor increased with increasing bio-oil mass flow.

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2.3 Carbonization of biomass 2.3.1 Biomass carbonization technology 2.3.1.1 Principle and characteristics of carbonization Carbonization is also a slow pyrolysis process, in which biomass is subjected to thermal decomposition in an oxygen-free or oxygen-limited environment at a temperature of 400–600 °C. The reaction conditions are tailored to maximize the production of char, wood vinegar and gas as by-products. Similar to combustion, carbonization is the one of the oldest biomass energy conversion techniques. Even today, carbonization is an important method to produce solid fuel in developing countries, while it used to be very common in developed countries until recently. Carbonization can be divided into four stages based on the reaction temperature, pyrolysis velocity and products. The first stage is drying, in which water can be evaporated depending on external heat under 200 °C, and there are almost no chemical reactions. At a temperature of 150–275 °C, biomass begins to undergo thermal decomposition, which is called pre-carbonization. The unstable component, hemicellulose, decomposes into CO2 , CO and acetic acid. Drying and pre-carbonization are called endothermic processes because the heat required for decomposition is provided by an external source. The temperature of the comprehensive carbonization stage is 275– 400 °C, and during this stage, there is vigorous thermal decomposition of biomass. The liquid products include acetic acid, methanol, tar, and so on. For the gas products, the yield of methane and ethylene increases while CO2 content decreases. There is considerable heat produced at the third stage, which is called exothermic reaction. At the last stage, with a temperature of 450–500 °C, the volatile products are removed from the char, forming fine porous structures with more than 80 % carbon, and liquid products are much less. Actually, because of the complex carbonization of biomass, it is difficult to clearly define the demarcation line between the four stages. Thermal decomposition can be influenced by type of biomass materials and reaction equipment, different diameters of biomass materials and location of biomass in the reactor. There are three characteristics for carbonization. The first one is low heating rate. When the heating rate is less than 30 °C/min, the char yield can be increased by 5.6 % compared to fast pyrolysis. The second one is low pyrolytic temperature. A carbonization temperature lower than 500 °C is conducive to forming good char. The third one is long gas residence time. The gas residence time is generally controlled from 15 min to several days according to different materials.

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2.3.1.2 Carbonization products 2.3.1.2.1 Solid products Compared to the feedstock, the content of fixed carbon in char is increased while the contents of O and H are decreased sharply. The content of carbon, an important quality index of char, reflects the degree of carbonization. The yield of char is higher at lower temperature, however, this char has a low content of carbon and high contents of O and H due to incomplete carbonization. Thus, it cannot come up to the required commercial standard. The optimum temperature is 500–600 °C for improving the quality of char.

2.3.1.2.2 Liquid products The liquid products generated by carbonization are allowed to stand to ensure stratification. The content of the upper layer is clear wood vinegar and the bottom layer is the settled wood tar. Wood vinegar is a yellow or red brown liquid with a unique smoky smell. It is very complex. Apart from water, more than 200 components have been detected and identified in wood vinegar. The major components are acids, alcohols, phenols, ketones, aldehydes, alkalis, and so on. Some of them are soluble in water, while others are insoluble. Wood tar is a black and viscous oily liquid, and its components are extremely complex with high oxygen content, including some amount of phenols and a small amount of aliphatic compounds.

2.3.1.2.3 Noncondensable gases The noncondensable gases with high heating value are mainly CO2 , CO, CH4 , C2 H4 and H2 . CO2 and CO are the main gases with low yields at low pyrolytic temperature. As the temperature increases, the yield of gases is also increased, with decreasing content of CO2 and CO, and increasing content of H2 , CH4 and C2 H4 . The heat value of gas at 700 °C is 16 MJ/m3 .

2.3.1.3 Factors affecting the carbonization process Carbonization conditions (temperature, charring time and heating rate) and biomass type and characteristics have strong effects on the production of char. Temperature has the most significant effect on biomass carbonization. With the increase in carbonization temperature, the biochar production rate decreases gradually. But its molecular structure becomes more ordered and the spacing between the molecules increases gradually. This is conducive to expansion of the pore structure and increasing the specific surface area. The higher content of ash at higher temperature pyrolysis can give the biochar higher reactivity. At the same time, higher reaction temperature is beneficial to reduce the content of volatile matter in char, decrease the size of biochar particles and improve the degree of graphitization, so as to improve the density and mechanical strength of char. But when the reaction temperature is

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too high (> 900 °C), excessive ablation can cause serious damage to the pore walls. Thus, the pore structure becomes deformed, which can reduce the specific surface area of biochar. Moreover, there are a large number of functional groups on the char surface, such as carboxyl, carbonyl, phenolic, hydroxyl, ester, and other functional groups with poor thermal stability that disappear gradually with increasing temperature. The char production rate decreases gradually with increasing carbonization time, but the reaction goes on to completed carbonization. The proportions of fixed carbon and ash are high in the char, while the volatile content increases with the increase of carbonization time. A higher heating rate can cause the volatile content to decompose and soften. However, the volatiles do not have enough time to leave the char surface. Therefore, the content of hydrogen and oxygen is higher, which is not conducive to the expansion of the pore structure and increasing the specific surface area. The type of raw material also has a significant effect on the characteristics of char, such as elemental composition, content of ash and volatiles, specific surface area and pore structure of char. Some properties of char carbonized from woody plants are better than those of char from herb plants. For example, the content of C is higher, and the specific surface area is greater with developed pore structure. The high ash content of raw material leads to high ash content of char, which can provide more mineral elements, such as N, P and S, although the specific surface area of char is low. The water in raw material can absorb a certain amount of heat during the pyrolysis process, causing low heating rate and reaction rate. Moreover, water can react with carbon to produce volatiles, which is a disadvantage of carbonization.

2.3.1.4 Types of carbonization technology 2.3.1.4.1 Heap roast The traditional biomass carbonization process is mainly performed in an earth kiln or brick kiln, and the picture of a typical heap roast is shown in Fig. 2.30. Biomass material is placed vertically or horizontally into the kiln, and then the kiln is sealed with clay. An exhaust port is built or an exhaust pipe is installed, and then the ignition is fired. After igniting in the combustion chamber, the fire gradually burns into the carbonization chamber by controlling the air supply from the firing mouth. When the ripping soil changes to white color, the ripping soil is dug out of the smoke hole of the kiln. When the smoke (water vapor) from the mouth changes from white to yellow (including volatiles), the smoke hole is covered and then the smoke vent is opened. The flue gas was released from the smoke vent. When the smoke turns to cyan color, or even invisible, then the kiln is suffocated. All the mouths and vents are closed when suffocating the kiln. After cooling for two days, a door is opened for the carbon to be removed from the side of the kiln, and finally, the char is removed.

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Fig. 2.30: Carbonization kiln (heap roast).

The char is removed after cooling in the furnace inside of the kiln, called the flameout. The char is called “black char”. When the carbonization reaction of the wood is finished in the kiln, the hot char is removed out from the kiln, then the flame is put out with wet sand. This method is called flameout outside of the kiln. When the removed hot char is exposed to air, the external carbon gets oxidized to plaster that is attached to the char. This char is called “white char”, which is harder than black char. The quality and yield of char are strongly related to the operation of the kiln. If the fire in the kiln is not controlled properly and if it is too fierce, then the yield of char is low, and even the materials will be burned out. If the temperature is insufficient, raw carbon will be produced which would affect its use. Temperature control needs to be maintained carefully throughout the process. The rate of char production varies with the kiln process, and generally, the yield of “black char” is 15–20 %, while “white char” is less than 1/4–1/3 of “black char”.

2.3.1.4.2 Furnace combustion method Moving Bed Biomass Carbonization Equipment (MBBCE) The construction of kilns is a labor-intensive project and once built, the kiln cannot be moved. The gaseous and liquid products produced by the carbonization process are released into the environment causing serious pollution. To overcome these problems, the portable carbonization system, MBBCE, has been developed. The MBBCE is made of 2 mm thick stainless steel plate and consists of furnace cover, upper furnace body and lower furnace body. This portable carbonization furnace has many advantages, in-

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cluding compact structure, easy operation, convenient to move, high char production yield and good quality. Moreover, the operation period of the furnace is about 24 h, and the yield of char is 25 to 30 %. The schematic diagram of a BA-I Type portable carbonization furnace designed by Japan’s Agriculture, Forestry and Fisheries Province Forestry Research is shown in Fig. 2.31. Furnace lid Upper furnace Flue Gas valves

Handle

Lighting-up tuyere Lower furnace Air inlet

Liquid collector

Fig. 2.31: Schematic diagram of moving bed biomass carbonization equipment.

With the compression molding fuel rods as an example, the operation process of carbonization is illustrated here. Firstly, the molding fuel is placed properly in the furnace before covering the furnace. Then, the upper and lower furnace body and the furnace cover are joined with clay (or other materials). Then, the kindling is thrown from the ignition mouth. When the temperature of the flue is above 60 °C (or hot to the touch), the ignition is closed. At this time, the smoke (containing water vapor) comes out from the flue. After 3–4 h, the material in the furnace dries out, then the smoke changes from white to yellow (including volatiles). At this point, the vent is turned down gradually. After another 6–8 h, a flame appears from the vent and smoke comes out from the flue, indicating that the molding fuel has achieved carbonization. At this time, the temperature in the upper furnace is up to 600 °C, and in the lower is 450–470 °C. Then, the ventilation pipe is moved away and the hole is plugged with sediment. After another half an hour, the flue pipe is moved and the channel is plugged with sediment, cold stoking. After about 10 h, when the temperature has dropped to about 50–60 °C, the char is removed. The flue gas released into the air contains combustible gas, wood tar and various volatile substances. However, combustible gas can be used as fuel by piping it back into the burning char stove. Wood tar and wood vinegar liquid can be recycled by using a cooling method in the middle of the pipe, which can achieve the efficient use of energy, and decrease environmental pollution.

Continuous Biomass Carbonization Equipment (CBCE) CBCE is designed as the system in which biochar, gas and tar are continuously produced by continuous biomass carbonization. Compared with the kiln furnace and

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Feed hopper Heating furnace Electric motor Feed screw conveyor

M Gas Cooling biochar screw conveyor

M

M Biochar box

Fig. 2.32: Schematic diagram of CBCE with external heating and spiral feeding.

MBBCE, the characteristics of CBCE are high production efficiency, simplicity, ease of control, steady working and high quality product. According to the heating mode, CBCE is divided into two types: internal and external equipment. For the internal heating type, there is no extra heat in carbonization. Heat for drying and carbonization just comes from biomass burning. The main characteristics of internal CBCE are high thermal efficiency, but slow heating, poor precision of temperature control, imperfect combustion and high consumption of biomass. For the external heating type, carbonization depends on heat input from outside. The main characteristics of external CBCE are accurate and convenient temperature control, high energy ratio and good quality of biochar. The most important inadequacy is that heat comes from other sources. According to the movement of materials within the furnace, CBCE is divided into horizontal and vertical types. The external heating and horizontal CBCE shown in Fig. 2.32 is taken as an example. The major components are electric motors, feed screw conveyor, screw reactor, pyrolysis furnace, cooling biochar screw conveyor and biochar box. The feed screw conveyor feeds the raw materials into the spiral carbonization reactor, and the materials react during the conveying process under the electric motor drive. After it arrives at the end of the spiral carbonization reactor, biochar enters the cooling screw conveyor, and then is collected in the box with the release of a small amount of pyrolysis gas. Wood vinegar liquid and tar in the pyrolysis gas are collected with a bottle after separation by condensation. Finally, clean noncondensable gas is released. The high temperature gas can be used for drying raw materials. With continuous feeding, the carbonization process continues, and biochar is produced continuously.

2.3.2 The properties and applications of biochar Charcoal is a dark brown or black porous solid fuel obtained by the incomplete burning of wood or other organics, or by pyrolysis of biomass in the absence of oxygen or limited oxygen. This section presents the properties and applications of the biochar fuel – charcoal.

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2.3.2.1 Feedstocks of charcoal (1) Wood: hardwood and conifers, hardwood: tussah Li, oak wood, oak, Ye Hua and wide variety of hardwood are used as raw materials for charcoal. Conifers: larch, pine, cypress, and fir. (2) Bamboo cutting waste. (3) Shells and nuts, such as coconut shells, peach pit shells, apricot pits, etc. (4) Various crops.

2.3.2.2 Types of charcoal White and black carbon: according to carbonaceous content, charcoal can be divided into white and black carbon. The two types of charcoal are fired in the same way, but they are extinguished in different ways. White charcoal is hard and heavy, very resistant to burning, and its surface appears white, while black charcoal is soft and light, easy to ignite and burn. The methods to easily distinguish between white and black charcoal are shown in Tab. 2.12. Tab. 2.12: The simple method to distinguish between white and black charcoal. Parameter

White carbon

Black carbon

Pyrolytic temperature Extinguishing method

> 1,000° extinguished and cooled by fireworks powder after high-temperatrue refining tan-white appearance high density, hard carbon very heavy Instantaneous firepower is not high, but it can last long High ignition point at 350–520 °C (460 °C) barbecue good almost none about 93 % Cooking, clean water, wash bath, purifying air, blocking electromagnetic waves, health supplies, barbecue around 134 per ml (oak charcoal used as standard) magnetic soon weakly alkaline

400–700° Flameout and cooling in a completely sealed stove

Color Density Weight Firepower Ignition Standard fire use Conductivity Impurities content Carbon content Main applications

anion generating magnetic experiment pH

all black low density, fragile light, easily floats on water Instantaneous high firepower, but poor persistency Flammable, ignites at 250–450 °C (350 °C) used for smelting metal, etc. poor low 65–85 % Deodorant, humidity adjustment, industry, agriculture and animal husbandry no no weakly acidic

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Bamboo charcoal: made from bamboo, used in ancient times, but still widely used. Core charcoal: made from raw material such as shells, fruit pits and stones (such as coconut shells, peach pits), and so on. Straw charcoal: made from straw crops. Other charcoal: made from almost all organic matter, from vegetables to fruit.

2.3.2.3 Properties of the charcoal 2.3.2.3.1 The elemental compositions of charcoal Different types of wood have the same pyrolysis characteristics although their chemical compositions are different. That is to say, charcoal is generated in the various stages of pyrolysis from all kinds of wood, which have the same elements. Charcoal with low ash content is mostly made of carbon and small amounts of other elements such as nitrogen, oxygen, hydrogen, and so on, whose contents are largely unaffected by the species of wood, but mainly depend on the pyrolytic temperature. The elemental compositions of birch charcoal and pine wood charcoal obtained at different temperatures are shown in Tab. 2.13. The elemental contents of white charcoal and black carbon made by the same wood are also different, which are shown in Tab. 2.14. Tab. 2.13: The elemental compositions of birch charcoal and pine wood charcoal obtained at different temperatures Temperature/°C

350 400 450 500 600 700 800 900

Charcoal yield %

Charcoal elements %

Birch charcoal

Pine wood charcoal

Birch charcoal C

H

O+N

C

H

39.5 35.3 31.5 29.3 26.8 24.5 23.1 23.5

40 36 32.5 30 27.3 24.9 23.8 22.6

73.3 77.2 80.9 85.4 90.3 92.3 94.9 96.4

5.2 4.9 4.8 4.3 3.3 2.8 1.8 1.3

21.5 17.9 14.3 10.4 6.4 4 3.3 2.3

73.2 77.5 80.4 88.3 90.2 92.9 94.7 96.2

5.2 4.7 4.2 3.9 3.4 2.9 1.8 1.2

Pine wood charcoal

Tab. 2.14: The elemental compositions of charcoal in %. Charcoal category

C

H

O+N

Ash

White charcoal Black charcoal

90–96 79–94

0.1–2.4 1.0–4.0

2.00–6.57 3.03–9.44

1.04–3.66 0.91–3.80

O+N 21.5 17.8 14.4 9.8 6.4 4.2 3.5 2.6

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2.3.2.3.2 The physical properties of charcoal The physical properties of charcoal including mechanical strength, density, porosity, etc. have important significance for its industrial applications. – Mechanical strength of charcoal: Charcoal usually has a lot of cracks, which come from the process of wood growth, wood pyrolysis and the cooling process. The cracks depend on the structural features of timber, especially the density of wood which is most obvious on the radial portion of the trunk. The cracks are associated with the resin duct of annual rings, eccentricity, various growth defects, etc. Therefore, the following two methods can be adopted to improve the strength of charcoal. Firstly, strong and healthy wood should be chosen for pyrolysis. Secondly, appropriate conditions should be created during the process of wood pyrolysis and charcoal cooling to ensure that the cracks are as few as possible. – The relative density of charcoal: The relative density of charcoal varies with the feedstocks, the carbonization temperature, and the carbon content. A higher relative density of charcoal results from a higher relative density of carbonized feedstocks, higher carbon content in charcoal, and higher carbonization temperature. Generally, oak charcoal has the highest relative density of 1.652; pine charcoal has the second highest relative density of 1.613, while miscellaneous charcoal has relative density of 1.602. – Charcoal porosity: Charcoal is a kind of porous material with pore volume of more than 70 %. Therefore, charcoal has stronger reactivity and absorption ability. The total porosity of charcoal mainly depends on the species of wood as well as on the production method of charcoal, the degree of charcoal crushing, wood parts, age of wood, its growth conditions and so on. Porosity of charcoal is one of the reasons for its good activity. Generally speaking, with increasing porosity, the reactive performance of solid carbon becomes better. For example, the reactivity of charcoal is greater than that of coke, which in turn is more reactive than graphite. The porosity order is: charcoal 65–75 %; coke 37– 57 %; and graphite only 12–35 %. – Calorific value of charcoal: The calorific value of charcoal is about 27,210– 33,490 KJ/Kg, which mainly depends on the carbon content. In addition, the calorific value of charcoal is associated with the category of carbonized material. Carbon content increases with increased pyrolytic temperature, which means that the charcoal’s heating value depends on the carbonization temperature. For example, if the carbonization temperature is in the range of 380–500 °C, the charcoal’s calorific value is 31,380–34,183 KJ/Kg. When the carbonization temperature reaches 600 °C, its calorific value would reach 34,476 KJ/Kg. As an example, the calorific value of compression molding straw fuel after carbonization is less than a quarter of the calorific value of sawdust. For this reason, straw compression molding fuel should not be processed into charcoal.

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–

–

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Reaction ability of charcoal: Charcoal with large porosity has strong reactive ability. One of the most typical reactions of charcoal is oxidation, which can occur at room temperature. The oxidation process of charcoal will always occur at all stages, from the hot stage to the storage stage. The strongest reaction will occur when the charcoal just comes into contact with oxygen, and then gradually decomposes. There are many factors affecting the activity of charcoal, including the tree species used for making the charcoal, age and quality of the wood, the structural characteristics of the charcoal, and so on. Many scholars have studied the relationship between the activity of charcoal and pyrolysis temperature, and the results showed that the activity of charcoal increases with increasing carbonization temperature. Its activity reaches the highest level at 500 °C, and then begins to weaken with further increase in temperature. The moisture of charcoal: Hot charcoal contains 2–4 % water, and the moisture content will increase with the duration of the storage period. If charcoal is stored in air for a long time, even without getting wet from rain and snow, the moisture content can reach up to 10–20 %. If stored for a few years, the moisture content can reach more than 50 %, at which point it becomes easy to break and is not useable for smelting. The degree of moisture absorption depends on the nature of the charcoal surface. The nature of the charcoal surface is affected by pyrolysis method and final pyrolysis temperature. The higher the charcoal surface oxidation, the greater is the amount of water it will absorb. Therefore, charcoal absorbs water more easily with longer storage periods. Spontaneous combustion of charcoal: Spontaneous combustion refers to the sudden and rapid ignition of charcoal: Charcoal adsorbs oxygen from the atmosphere emitting heat of reaction. If the heat is not dissipated quickly, it will make the charcoal’s temperature increase. Rapid oxidation reaction will occur when the temperature rises to a certain degree.

Many factors cause charcoal to undergo spontaneous combustion. For example, the charcoal produced from decayed wood can more easily undergo spontaneous combustion than that produced from general wood. Firstly, charcoal made of decayed wood is more easily crushed into pieces and charcoal powder, which allows for easy accumulation and less loss of heat. Secondly, because decayed wood contains more inorganic impurities, the charcoal made of decayed wood has high ash percentage and ignites easily. In order to prevent spontaneous combustion of charcoal, some basic guidelines should be followed during the manufacturing and storage of charcoal: – the use of decayed wood in the process of carbonization should be limited – during the process of carbonization, small logs of uniform thickness should be used; – high carbonization temperature should be used;

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–

the charcoal powder should be sieved before storage, stacking height should not be too high, charcoal should be stored in low humidity conditions.

2.3.2.4 Applications of charcoal Charcoal has high calorific value, smoke-free burning, high reaction ability and other characteristics, so it is widely used in both domestic and industrial applications. (1) Charcoal can be directly used as domestic fuel, for baking food and heating, and it can be transformed into gas fuel in a gasification furnace. Charcoal has many outstanding advantages, such as low volatility, high calorific value, burning completely and cleanly, which make charcoal a good solid biofuel. Food barbecued with charcoal is popular in every corner of the world, due to its unique flavor. With improvements in living standards and growing tourism, consumption of charcoal is growing continuously. In recent years, due to considerations of protecting forest resources and utilizing agricultural residues, various types of charcoal made of sawdust, straw and rice husk have increasingly taken over the markets, and become the main varieties of fuel charcoal. (2) Used for smelting high quality nonferrous metals and casting iron: Charcoal has a long history of being used for iron smelting. Pig iron smelted with charcoal has advantages of fine grain size, good casting and less defects, because the reduction process of iron oxide with charcoal can be performed at low temperature. The iron produced with charcoal contains less hydrogen, oxygen and impurities, and is suitable for production of high quality steel. In the production of nonferrous metals, charcoal is used as surface cosolvent that can form a protective layer on the surface of molten metal to separate metal from gas medium. This can not only decrease the splatter loss of molten metal, but also decrease the gas saturation of melting material. Charcoal is widely used as surface cosolvent in the production of copper and copper alloys (copper phosphorus alloy, copper silicon alloy), tin alloy, aluminum alloy, manganese alloys, silicon alloys and beryllium bronze alloy, etc. (3) Used in the production of crystal silicon: Crystal silicon is an important material used in the electronics industry and as a new energy material. Crystal silicon used for industrial applications has strict requirements on the content of impurities. In order to ensure the purity of products, high quality reductants should be used. The purity, porosity, reaction ability and dielectric properties of charcoal can meet the above requirements better than other carbon materials, so it is widely used in producing crystal silicon. Production of crystalline silicon requires charcoal which does not have a carbon head, and has low contents of ash and impurities. Charcoal, petroleum coke and quartz are all heated in an electric furnace to produce crystal silicon. When the reaction temperature reaches as high as 2,000 °C, silica vapor from the evaporation of quartz reacts with red hot charcoal. Then, carbon

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(4)

(5)

(6)

(7)

(8)

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monoxide is emitted and gradually reduced to silicon monoxide and silicon. The production of 1 t crystal silicon consumes 1.4 t of charcoal. Charcoal used as the carburizer of mechanical parts can improve the hardness and wear resistance of steel parts: Carburizing treatment is an important heat treatment for steel parts. After carburizing, steel parts have high hardness, wear resistance and toughness in the center, thus greatly improving the mechanical properties of the parts. Carbon compounds used for carburizing treatment are known as carburizing agents, and their main component is charcoal. The carburizing effect of pure charcoal is not good. During the process of carburizing, the production of carbon monoxide will decrease gradually, so a certain amount of contact agent, such as barium carbonate and sodium carbonate, etc. should be added. The charcoal content of carburizing agent is about 80 %. The main process is that the barium carbonate slurry is coated onto the charcoal surface in a blender, and then a rotary drum dryer is used for drying it. Charcoal used for manufacturing carbon disulfide: Carbon disulfide is a toxic liquid which is volatile, colorless, highly refractive, and a good solvent that can dissolve sulfur, phosphorus, raw rubber, and all kinds of oils and resin materials. It can be used for the production of rayon, cellophane, rubber tire cord, as well as for the manufacture of carbon tetrachloride, etc. Charcoal is a good raw material for the production of carbon disulfide and a large amount of charcoal is used in the manufacture of carbon disulfide every year. The method used to produce carbon disulfide in industries is as follows: Firstly, charcoal is dried and calcined at a temperature of 500–600 °C in order to get rid of all the water and reduce the content of volatiles. Then, the reaction temperature is increased to about 800 °C to make sulfur steam through the high temperature charcoal layer. The production of 1 t carbon disulfide consumes 0.5 t of charcoal. Made into graphite: Carbon graphite materials have many advantages over metallic and nonmetallic materials, such as self-lubrication, high temperature resistance, corrosion resistance, high thermal conductivity, low expansibility, and ease of processing. So, these materials are widely used in the machinery manufacturing industry. For example, carbon graphite materials can be used as good solid lubricants and lubricant additives, which is very important for rotating parts that cannot use lubricating oil. Applications in agriculture and forestry: Charcoal with abundant pore structure and good adsorption performance can loosen soil, increase ventilation, retain moisture and promote microbial breeding, all of which can help with soil improvement. Charcoal adsorbs chemical fertilizers and pesticides, then releases them slowly, which can prolong the action time of fertilizers and pesticides. In southern China and Southeast Asia, the easily hardened charcoal powder used in acidic soil significantly improves agricultural production. Used for the production of activated carbon: Charcoal is widely used in the area of chemical, pharmaceutical and environmental protection after it is made into

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activated carbon. For example, it is used in the purification of chemicals, decolorization and purification of sugar, fat deodorization, gas separation, recovery of solvents, purification of water and air, filtering harmful substances and making gas masks, etc.

2.3.3 The applications of biomass dry distillation gas Lignocellulosic biomass such as crop straws, sawdust, weeds, etc. can be distilled into combustible gases (hydrogen, methane, carbon monoxide, etc.), along with the by-products, i.e., solid biochar and liquid products. Biomass distillation is a complex process of chemical reactions, including dehydration, pyrolysis, dehydrogenation, condensation and hydrogenation reactions, which are often cross-reactions. The yields of gas, liquid and solid products vary with material properties, heating rate, reaction temperature and reaction pressure. Low temperature distillation (< 550 °C) always leads to the production of more liquid products, while high temperature distillation (> 600 °C) causes the generation of more gaseous products. Therefore, different products could be obtained by changing and regulating the distillation conditions. In general, the yield of dry distillation gas is about 25–35 %, the yield of biochar is 28–35 %, and the yield of wood vinegar and tar is 35–47 %. Biomass dry distillation gas mainly contains CO, CH4 , Cm Hn , H2 , etc., whose calorific value is relatively high. This mixture of gases can be used as fuel for engines, boilers, furnaces, etc. At the same time, charcoal, wood vinegar and tar could be obtained, which can be used as soil improvement agents and important chemical materials. Therefore, biomass distillation technology is one of the most promising biomass energy utilization technologies. In China, the Jiaozuo Straw Gas Equipment Engineering Co., Ltd, has developed the STQ – I biomass carbon gas oil cogeneration system. The scale of gas supply is 500 households and 2,000 tons of straw, sawdust and other waste are treated per year. The main products are biochar, gas, wood vinegar and tar. Dalian Municipal Design and Research Institute of Environmental Science has built a biomass pyrolysis gas processing plant that is available for 1,000 households. This plant has passed the technical appraisement and acceptance which is organized by the State Science and Technology Commission and State Environmental Protection Administration, supplying gas for 115 households. The University of Science and Technology, Liaoning, has built a continuous distillation system of biomass to produce gas at Liaoyuan Huiyu Energy Co., Ltd, Liaoning Province. The program has reached its tentative targets, and was listed as a national demonstration project by the National Development and Reform Commission. These three typical systems will be described in detail in the following sections.

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2.3.3.1 Jiaozuo biomass carbon gas oil cogeneration system Jiaozuo Straw Gas Equipment Engineering Co., Ltd, has developed the STQ-I biomass carbon gas oil cogeneration system. The scale of gas supply is 500 households and 2,000 t of straw, sawdust and other waste are treated per year. The main products are biochar, gas, wood vinegar and tar. The system contributes to protecting the environment, developing rural cooking gas, and accelerating the pace of rural construction.

2.3.3.1.1 The production process The STQ-I biomass coke-gas-oil cogeneration system consists of raw material preparation, carbonization and purification, storage, delivery and user facilities. Firstly, corn stalks, cotton stalks, rice straw, wood chips and other biomass are crushed into particles with diameters less than 4 mm, then they are dried using the air-drying method. After drying, they are discharged from the cyclone, and finally extruded by screw extrusion as molded rods. Hollow rods with a smooth surface and almost no cracks can be obtained at a suitable temperature. 300 kg of biochar, 300 m3 of gas, 200 kg of wood vinegar, and 50 kg of wood tar are obtained through the distillation of one ton of raw materials. The calorific value of char is more than 29 MJ/kg. The contents of tar and ash in combustible gas are less than 10 mg/m3 , and its calorific value is more than 17 MJ/m3 , which is better than that of city gas. The pH of wood vinegar is 2–4, and the moisture content of wood tar is less than 10 %. These two kinds of by-products are both good chemical raw materials.

2.3.3.1.2 Economic, social and environmental benefits The distillation factory in Lubao village’s high-tech zone, Jiaozuo City, supplies gas for 277 households with a total investment of 1,300,000 RMB. Since it was put into operation, the STQ-I system has been operating effectively and steadily with huge profit. Moreover, the system improves the living environment of farmers, and has become a new economic growth point in this rural area. Farmers use biomass gas which saves money and energy, and this kind of resource is quite clean and convenient. It also reuses large quantities of waste and reduces pollution while accelerating the pace of rural construction.

2.3.3.2 Dalian biomass energy engineering Dalian Municipal Design and Research Institute of Environmental Science has designed a biological energy engineering system which is capable of supplying gas for 1,000 households. The raw materials are mainly agricultural straws. The main equipment includes a rotary mill, dryer, molding equipment, pyrolysis furnace and distillation kettle, in addition to the tar, cooler, separator, alkaline cleaning equipment, roots blower, gas holder, etc.

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2.3.3.2.1 Process (1) The raw materials are crushed by a high-speed rotary mill, then the particles are sieved to obtain a size fraction of 0.5–1.0 cm. The moisture content of raw materials is about 20–30 %. (2) The pulverized raw materials are dried in a dryer until the moisture content is below 10 %. (3) General requirements are for the raw material to be pressed to a size about 0.8– 1.2 g/cm3 . Then the raw materials are molded into the following shape: 50 × 50 × 400 mm hexagonal cylinder; 32 × 32 × 100 mm cuboid; and Φ 8.0 × 12 mm cylinder, etc. (4) Pyrolysis is the key process. The main parameters of temperature, pressure, flow, solid phase and residence time of gas phase should be rigidly controlled during pyrolysis. Generally, the pyrolytic temperature should be controlled at about 600 °C, and pressure should be controlled in the range of 0–10 pa. (5) Condensation. The pyrolytic gas is very acidic with many impurities and with a temperature of 300 °C. Pure gas can be obtained through a series of cooling, purification, and separation processes, at the same time, various by-products are separated. (6) The purified gas should be alkaline in order to remove acetic acid, and to avoid holder and pipeline corrosion.

2.3.3.2.2 Main uses of pyrolytic products (1) Purified gas The main components of purified gas are CO, CH4 , C2 H4 , H2 , etc., and the heat value is around 14.7 MJ/m3 . At present, purified gas is mostly used as cooking fuel. (2) Biochar Biochar is a high quality coal with low ash content, good reaction performance, large specific surface area, and its heat value is about 29 MJ/m3 . Biochar has a very wide range of applications, such as nonferrous metal smelting, foundry industry, improving soil properties, and increasing soil fertility. It can also be made into activated coal used for wastewater treatment, emission control, etc. (3) Wood tar Wood tar is a complex mixture of hydrocarbons, phenols, and acids. Its quality is better than that of coal tar and has the advantages of good softness, ageing resistance, high temperature resistance, etc. Thus, it is a good material for producing waterproof coatings, anticorrosive coatings, marine coatings, hard polyurethane foam and anticoagulants. Wood tar can replace coal tar and has a great market prospects. (4) Wood vinegar Wood vinegar mainly contains vinegar liquid, methanol, acetaldehyde, acetone, ethyl acetate, etc. Wood vinegar obtained by different refining methods can be

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used for drug materials, food additives, dye materials, deodorizing agent, and pesticide materials.

2.3.3.3 Liaoning biomass distilled gas production device University of Science and Technology Liaoning (USTL) has built a biomass continuously-distilled gas production device which uses corn stalks as raw material at Liaoning Liaoyuan Huiyu Energy Co., Ltd. It can produce more than 250 m3 of 3,700 kcal high calorific biomass gas per hour. The program has reached its tentative targets, and was listed as a national demonstration project by the National Development and Reform Commission.

Biomass

Elevator

Steam

Chimney

Hopper

Feeder

Hot exhaust gas Distilled Boiler Furnace

Purification system Flue gas Gas tanks

Softened water

Char box

Burner

Char

Air

User

Fig. 2.33: Process flow diagram of biomass continuous distillation for gas production.

The schematic process of biomass continuous distillation for gas production is shown in Fig. 2.33. The crushed straw is conveyed to a hopper placed on top of the furnace by a bucket elevator. The particles are pressed by double auger feeder, and then conveyed into the distillation furnace continuously. When moving from top to bottom, the temperature increases, and continuous distillation takes place to produce the gas. Crude gas moves upwards through the rising tube and gas header and finally enters into the gas purification system. Crude gas is initially cooled down to 80–90 °C by spraying circulating water at the top of the riser, then it is further cooled down to 30–40 °C by the primary cooler. The primary cooler is connected with a blower, which sends part of the gas back to the furnace as a heating resource, and another part of the gas is sent to the households for fuel. The production capacity of each distillation furnace is 6,000–8,000 m3 /d.

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The distillation furnace is a vertical box-shaped structure, externally heated, and is made of special steel. The distillation furnace is composed of distillation chamber and combustion chamber. Each side of the distillation chamber has a combustion chamber, which is divided into heating and air preheating sections. Two distillation chambers are assigned to each group. The average size of distillation chamber is about 400 mm wide, 2.1 m long, and 4.5 m tall. High temperature exhaust gas flows along the horizontal fire trail in the combustion chamber, continuously heating the distillation chamber. The temperature of exhaust gas decreases gradually, and then the exhaust gas flows out from the combustion chamber. Its temperature drops down to 200–250 °C after the recovery of waste heat. Exhaust gas finally flows out through the chimney. The biochar which is produced from the distillation process falls from the bottom of the distillation chamber into the biochar tank. The biochar tank is divided into three segments: top, middle and lower. Several nozzles are set in the inside of the upper section, for spraying a certain amount of water to cool the biochar initially. A water jacket and sealing device are located on the outside of the middle section. Biochar from the upper section is cooled to about 200 °C by circulating water. The lower section is equipped with a special star ejector and variable speed motors for conveying biochar continuously.

2.3.4 Case study of biomass carbonization Biochar is a product of biomass pyrolysis. Historically, it was used to exploit charcoal as a kind of biofuel. Research on bamboo charcoal in China is in a relatively leading position worldwide. Bamboo charcoal is widely used in air purifiers, cosmetics and additives for textiles. In recent years, soil and crop fertilizer experts found that biochar can also be used as soil amendment agent, fertilizer slow release carrier, etc. So biochar appears in scientific journals and media constantly. The research and production of biochar is rapidly increasing. More and more international research institutions are being set up for this field, such as the International Biochar Initiative (IBI), Biochar Technology Research Centre of Liaoning Province, etc. Biochar is a kind of porous material, with superior air permeability and water permeability, low bulk density, large surface area, and excellent water and air absorption. When biochar is oxidized, it can improve the water absorption of silty loam so that the water-holding capacity of soil is improved. Biochar contains a certain amount of mineral nutrients, which can increase the contents of phosphorus, potassium, calcium, magnesium, nitrogen and other mineral elements in soil effectively and improve the nutrients in poor and sandy soil. As biochar is weakly alkaline and its pH value is higher, it can replace lime to alter acidic soil. Biochar has a higher adsorption capacity, CEC and chemical reactivity. It can also delay the release of nutrients from fertilizer in the soil to improve the utilization rate of fertilizer nutrients as a slow release car-

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rier. Biochar also improves the environment for the microorganisms and provides the conditions for growth and reproduction for many important microbes. Microbial respiration releases CO2 . So it can improve the CO2 concentration near the crops and enhance photosynthesis. During the day, it increases the accumulation of organic matter and inhibits respiration. At night, it reduces the consumption of organic matter. All in all, it achieves the effect of improved crop production. Microbial metabolism can also provide nitrogen nutrients for plant growth, and reduce the amount of nitrogen fertilizer used. After it is added into the soil, biochar can also improve the cation or anion exchange capacity of the soil for its specific adsorption. With its strong adsorption ability, it adsorbs nutrients in the soil which are required for crop growth like a sponge. It can not only prevent the loss of nutrients, but also achieve slow release of nutrients. It has many advantages for the growth of crops [195]. Biochar application in agriculture can improve soil fertility and increase crop yield. In addition, it promotes the sustainable utilization of soil and crop production, and effectively promotes the sustainable development of agriculture. Therefore, the natives of South America used charcoal as the main material for improving the local highly weathered alluvial soil hundreds of years ago. The black earth in the central part of the Amazon basin (Terra Preta de Indio) is still one of the world’s most fertile soils. This phenomenon has attracted the attention of climate scholars. If only the gas capture method is used to capture CO2 emissions from fossil fuel combustion and store CO2 underground or at the bottom of the ocean, it does not provide any noticeable economic benefits. However, if biochar is buried underground as a carbon sink, it is equal to carbon sequestration in the soil. Biochar is a stable carbon fixation carrier which can remain for hundreds of years in the soil. Thus, it helps to reduce carbon dioxide emissions and slow down global warming. It is a technology that can not only improve soil, but can also promote carbon reduction, so it has recently aroused wide interest worldwide. In addition, many studies [196] have found that when a large amount of biochar is applied into the soil, it can result in a better passivation treatment for heavy metals in the soil, such as Cu, Pb, Zn, Cd, etc., due to the strong adsorption ability of biochar. Also, it can hold the residues of pesticides, herbicides, and oil pollutants effectively and reduce the content of pollutants available in the environment. Thus, it also reduces the absorption of pollutants by plants and the stress caused by pollutants on microorganisms. Thereby, it improves microbial activity, and promotes the degradation and deactivation of hazardous substances in the soil. It provides an economical and feasible way for large-scale improvement of contaminated soil. Most of the current global enterprises working on the research and development of biomass pyrolysis are centered on bioenergy. Biochar is only a by-product. There are only a few enterprises concerned with the production of carbon products. However, we believe that with the promotion and application of biochar for slow release fertilizer and soil improvement, and more attention for biological carbon sequestration technology, the high value and diversified utilization of biochar will be recog-

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nized, which would greatly promote the rapid development of the industrialization of biochar. Xin Zhongxing Biomass Comprehensive Utilization Demonstration Base Zaozhuang Xin Zhongxing Industrial Company Ltd. was established in 2005, and it is one of the earliest comprehensive utilization demonstration enterprises for biomass energy in Shandong province. It is specialized on the comprehensive utilization of waste from agriculture and forestry and the research and development of biomass energy. The company has about 50 employees, and consists of Zaozhuang straw briquette factory and Lanshan (Rizhao) sawdust briquette and carbonization. There are three straw briquette production lines, five equipment set-ups for dry distillation/carbonization, and two equipment set-ups for airflow drying. This company can produce about 66,000 tons of straw briquettes, and 13.5 million cubic meters of biomass gas a year. It uses a gas powered generator to directly produce green electricity of about 16,200 thousand kW/h. With an annual output of 13,500 tons of carbon, they can produce additional products such as 2,350 tons of wood tar, and

Fig. 2.34: Biomass carbonization kettle.

Fig. 2.35: Sawdust charcoal products.

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Fig. 2.36: Front and back contrasts of firing sawdust charcoal.

Fig. 2.37: Straw tar.

11,200 tons of wood vinegar at the same time, see Figs. 2.34, 2.35,2.36 and 2.37. The heat content of the charcoal can reach 7,600–7,800 kilocalories. It is mainly used for barbecue, food processing, metallurgy, chemical industry and electric heating. Wood vinegar liquid has antibacterial properties, disease resistance, improves the activity of microorganisms in water and soil, promotes the growth of crops and so on. It can also be used in feed additives, natural plant growth regulators, natural pesticides, synthetic natural cosmetics, natural food additives, deodorants, etc. Wood tar is a kind of complex mixture which is isolated during the biomass pyrolysis process, and it mainly contains hydrocarbons, phenolic compounds and acids. Its quality is better than that of coal tar. It has good softness, ageing resistance, high temperature resistance, etc., and it is a good material for producing waterproof coatings, anticorrosive coatings, marine coatings, rigid polyurethane foam and anticoagulants. It can be used as an alternative to coal tar. In addition, 900 m3 of biomass fuel gas can be obtained by pyrolysis with every ton of carbon produced. The gas heat can reach as high as 3,600 kcal/m3 . It is widely used for domestic cooking and heating purposes, and also provides the heat source for agricultural products processing and industrial production. It is even used in gasification and power generation.

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Liaoning Jinhefu Agricultural Development Ltd. cooperated with the team of Chen Wen-Fu, a member of the Chinese Academy of Engineering and a professor of Shenyang Agricultural University to use agriculture and forestry wastes such as straw as raw material in a retort furnace which has proprietary intellectual property rights, to produce charcoal. Then, this charcoal is made into biochar-based specialty fertilizer, biochar-based soil improvement agent and carbonization bio-coal through a special process. The group’s main products include biochar-based specialty fertilizer, compound fertilizer, organic fertilizer, environmentally-safe retort furnace, wood vinegar liquid, biochar, bio-briquette and so on. The group has two production lines of biochar-based fertilizer with an annual output of 300,000 tons, has one production line for environmentally-safe retort furnaces with annual output of 5,000 sets of furnaces and one set of equipment for biochar and bio-briquette with an annual output of 100,000 tons of bio-briquette, see Fig. 2.38 and Fig. 2.39.

(a)

(b)

Fig. 2.38: Jinhefu biochar-based fertilizer.

Zhejiang Suichang Bamboo Charcoal Ltd. was founded in November 2002. It is a company dedicated to the development and processing of bamboo charcoal which has the functions of air purification, water purification, and health-and-fitness regimen. The company has more than 300 employees, 21,000 m2 of workshops, and 5,000 square meters of building on land with an area of 33 acres. It has become one of the leading enterprises in the bamboo charcoal industry in China which has the largest production scale and the most advanced technology., The company has been developing bamboo charcoal, bamboo vinegar liquid-based products and other related products. Bamboo charcoal products mainly include: nano-bamboo charcoal air purifier, bamboo charcoal soap containing skin toner, bamboo vinegar soap, toothpaste, cups and other

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Fig. 2.39: Environmental protection retort furnace.

household cleaning products, beauty care products, health foods, clean fuels and other product lines, see Fig. 2.40 and Fig. 2.41. Using straw and other agricultural waste as raw materials after crushing, Chai [197] analyzed the economic feasibility of the production of straw biochar and produced biochar through self-heat accumulative pyrolysis equipment. The research results are briefly described below.

Fig. 2.40: Bamboo charcoal toothpaste.

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Fig. 2.41: Bamboo charcoal toothpaste.

The treatment capacity of the biochar self-heat accumulative pyrolysis equipment is 7,200 tons per year and the equipment investment is 180,000 yuan (CNY, similarly hereinafter). The main raw material is straw, and the auxiliary materials are binder and antioxidant whose amounts are 240 tons per year each. The final product is barbecue charcoal. The research uses breakeven analysis to determine the market price of the crop straw.

2.3.4.1 Estimated sales (ES) To produce barbecue charcoal using crop straw, a certain amount of binder and antioxidant need to be added. At the same time, the current market price of barbecue charcoal is 3,500–4,000 yuan per ton. So, it may have market competitiveness if we fix the ex-factory price of barbecue charcoal as 3,000 yuan per ton. ES = (quantity of biochar + dosage of binder + dosage of antioxidant) × price of barbecue charcoal = (2, 400 t + 240 t + 240 t) × 3, 000 yuan/t = 8, 640, 000 yuan.

2.3.4.2 Unit fixed costs (UFC) Fixed costs include administrative expenses (AE), marketing expenses (ME), financial expenses (FE) and fixed assets depreciation (FAD). (1) Administrative expenses AE = 1.5 × Total administrative expenses of administrative staff = 1.5 × 3 × 38, 989 yuan/person = 175, 450.5 yuan.

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(2) Marketing expenses ME = 2 × expenses of sales staff = 2 × 38, 989yuan/person = 77, 978 yuan. The income of sales staff is two times that of workers’ average wage (3) Fixed assets depreciation FAD =

2 × Net value of fixed assets = 2 × 180, 000 ÷ 10 = 36, 000 yuan. 10 years

(4) Financial expenses FE = Total loans × EAIR (actual interest rate) = 180, 000 × 10 % = 18, 000 yuan. Assuming that all equipment used for this production will be paid for by loans, the actual interest rate of 10 % should be used. According to the standards published by the People’s Bank of China, in recent years, the annual interest rate of loans is close to 6.56 %, which is the nominal interest rate rather than the market clearing rate. (5) Fixed costs (FC) and unit fixed costs (UFC) FC = AE + ME + DOFA + FE = 175, 450.5 + 77, 987 + 36, 000 + 18, 000 = 307, 434.5 yuan UFC = FC/Q = 307434.5/2880 = 106.748 yuan

2.3.4.3 Unit variable costs (UVC) Variable costs include the cost of main feedstock (COMF), cost of auxiliary feedstock (COAF), and expenses of production workers (EOPW), calculated separately: 1. COMF = price of main feed × annual production capacity = POMF × 7, 200 = 7, 200 POMF yuan 2.

POMF refers to the price of main feed that is the breakeven price. Cost of auxiliary feedstock COAF = Price of auxiliary feedstock × requirement of auxiliary material = (2, 000 + 8, 000) × 240 tons/year = 2, 400, 000 yuan.

3.

Expenses of production workers (EOPW) EOPW = number of production personnel × average wage = 9 × 38, 989 = 350, 901 yuan. Unit variable cost can be calculated by the below equation: UVC = VC/Q = (EOMF + EOAF + EOPW)/Q = (7, 200 POMF + 2, 400, 000 + 350, 901)/2, 880 = 2.5 POMF + 955.17 yuan.

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2.3.4.4 Unit sales tax and surcharges (USTAS) Sales tax and surcharges include sales tax (ST), city development surcharges (CDS) and education surcharges (ES). Taking the total sales tax and surcharges (STAS) respectively: 3.3 % for the city, 3.24 % for the county and 3.09 % in the countryside by calculation (1) USTAS of urban areas STAS1 = STAS Tax rate × ES = 3.3 % × 8, 640, 000 = 285, 120 yuanUSTAS1 = STAS1/Q = 285, 120/2, 880 = 99 yuan. (2) USTAS of county STAS2 = STAS Tax rate × ES = 3.24 % × 8, 640, 000 = 279, 936 yuanUSTAS2 = STAS2/Q = 279, 936/2, 880 = 97.2 yuan. (3) USTAS rural STAS3 = STAS Tax rate × ES = 3.09 % × 8, 640, 000 = 266, 976 yuanUSTAS3 = STAS3/Q = 266, 976/2, 880 = 92.7 yuan.

2.3.4.5 Price of main feedstock materials on breakeven point (POMF on BEP) The breakeven analysis method is used to calculate the profit and loss balance of production by which we can ensure that the quality of biochar production is sufficient to make a profit. Then, we will use the breakeven analysis method to calculate the main material prices. The calculation process is as follows: Q on BEP =

FC (P − UVC − USTAS)

Q on BEP depending on the breakeven point Through the transformation, we get the below equation: UVC = PUFC − USTAS 2.5 POMF + 955.17 = 3000 − 106.7.99 So, POMF = 735.652 yuan/ton. With the change in the value of USTAS, POMF values will change accordingly as follows: urban areas: POMF = 735.652 yuan/ton; county area: POMF = 736.372 yuan/ ton; rural areas: POMF = 738.172 yuan/ton. We can conclude that biochar projects can be profitable when the price of crop straw is less than 735 yuan/tons which is an attractive price. Biomass raw material undergoes the pyrolysis reaction under limiting oxygen environment in a cracking furnace. At the same time, the biochar as well as the pyrolysis

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gas can yield three more products after cooling and separation, which are bio-oil, wood vinegar liquid and combustible gas. A ton of biomass can produce about 300 kg of biochar, 250 kg of wood vinegar liquid, 50 kg of bio-oil and nearly 700 m3 of combustible gas. Bio-oil can be used as a diesel substitute after refining modification or after mixing with diesel oil emulsion. High value-added chemicals which are difficult to synthesize by conventional chemical methods can also be extracted from bio-oil. Wood vinegar liquid, which is weakly acidic, is rich in organic content and has strong permeability. Thus, it can improve the utilization ratio when it is mixed with foliar fertilizer or pesticide. In this way, we can reduce the use of chemical fertilizers and pesticides and improve the quality of agricultural products. Biochar and livestock feces after mixed fermentation can be made into good slow-release fertilizer if mixed with wood vinegar after drying. Therefore, when using agroforestry wastes to produce biochar, we should not just be concerned about the economics of the main product. The use of appropriate by-products should receive attention too. In this way, we can improve the efficiency of biomass resource and achieve comprehensive economic benefits. However, there are some concerns that, with the development of the industrialization of biomass resources, these companies will destroy the earth’s ecological environment as they plant fast-growing forests or perform deforestation blindly just in pursuit of economic interests which will outweigh the environmental benefits.

Changfeng Yan*, Quanguo Zhang, and Shunni Zhu

3 Technologies for biomass-based hydrogen production

3.1 Hydrogen energy 3.1.1 Introduction The supply of fossil fuels is expected to come to a final end during the coming decades. Unrestrained usage of fossil fuels has led to growing concerns about global warming and increasingly extreme climate events. Consequently, alternative energy sources need to be explored and studied. Among these alternatives, hydrogen is considered as the energy of the future, since it is the most abundant element throughout the universe. It is also widespread in water, methane, ammonia, and various compounds that contain hydrogen on earth. Hydrogen can be produced by a variety of primary energy sources and also via renewable energy or secondary energy extraction. The hydrogen atom has a high mass energy density (122 KJ/g), which is almost three times higher than that of hydrocarbon fuels. Moreover, it is an environmentally friendly energy carrier, which does not release carbon dioxide during combustion. In addition, hydrogen can easily be stored and has wide applications; hence, hydrogen is certainly regarded as a promising alternative energy resource for the twenty-first century, solving environmental pollution problems.

3.1.2 Properties of hydrogen 3.1.2.1 Physical properties of hydrogen The atomic weight of hydrogen is 1.008, and hydrogen is located at the very first place in the chemical periodic table. Hydrogen is a type of colorless, odorless, tasteless, flammable, nontoxic gas, insoluble in water and a highly combustible diatomic gas with the molecular formula H2 under atmospheric temperatures and pressures. Hy*Corresponding Author: Changfeng Yan: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China; E-mail:[email protected] Quanguo Zhang: Henan Agricultural University, Zhengzhou 450002, China Shunni Zhu: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China https://doi.org/10.1515/9783110476217-003

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drogen is the lightest of all gases, approximately one-fifteenth of the weight of air. Hydrogen has three phases: solid, liquid, and gaseous; weight and energy density are both severely improved after liquefaction and solidification. Hydrogen is an excellent cooling, refrigerating, and heating medium since it has the largest specific heat capacity, the highest heat conductivity and the lowest viscosity among all gases. Hydrogen has the lowest boiling point (the only exception is helium), and when hydrogen is cooled to −252.76 °C, it changes into a transparent, odorless liquid, which is neither corrosive, nor particularly reactive. During conversion from the liquid state to the gaseous state, hydrogen expands approximately 840-fold. Its low boiling point and low density results in easy and rapid spilling of liquid hydrogen. Major properties of hydrogen are listed in Tab. 3.1. Tab. 3.1: Physical and chemical properties of hydrogen. Properties

Value

Relative molecular mass Boiling point (0.1013 MPa), K Latent heat of vaporization, kJ/kg Melting point (0.1013 Mpa), K Density (0.1013 MPa, 273 K), kg/m³ Specific heat at constant pressure (273–473 K), kJ/(kg · K) Specific heat at constant volume (273–473 K), kJ/(kg · K) Heat conductivity (0.1013 MPa, 293 K), W/(m · K) Diffusion coefficient (0.1013 MPa, 293 K), cm2 /s Diffusion rate (0.1013 MPa, 293 K), cm/s High calorific value of hydrogen gas, MJ/kg Low calorific value of hydrogen gas, MJ/kg Theory air volume of 1 kg hydrogen combustion, Kg Theory air volume at STP of 1 m3 hydrogen combustion, m3 Hydrogen flame propagation speed (0.1013 MPa, 293 K), m/s

2.0158 20.38 445.6 13.90 0.0899 14.4 10.3 18.97 0.61 2.00 143 121 34.27 2.38 2.65-3.25

3.1.2.2 Chemical properties of hydrogen The chemical property of hydrogen is lively, since hydrogen readily forms covalent compounds with most nonmetallic elements. No single hydrogen atom exists anywhere unlike all the other gaseous elements, and most of the hydrogen on Earth exists in molecular forms, such as water or organic compounds. Molecular hydrogen has a high energy level and strong reducibility. Hydrogen is a flammable and explosive substrate, igniting easily with an ignition energy of only 0.2 MJ and an autoignition temperature of 500 °C. Hydrogen burns in air with a pale blue, almost invisible flame, forms water, and the propagation speed of the flame is up to 2.7 m/s. Hydrogen has the highest heat released by combustion per unit of weight among all existing fuels, and a property that renders it the preferable fuel for the upper stage of multistage rockets.

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When combined with oxygen or air within a wide range of 4–74 % by volume, hydrogen forms an explosive oxy-hydrogen gas. Moreover, it is easy to cause explosions within a range of 18–65 %. At normal temperature, hydrogen is not a very reactive substance, unless it has been activated somehow, for instance by an appropriate catalyzer. At high temperatures hydrogen is highly reactive.

3.1.2.3 Characteristics of hydrogen energy Hydrogen energy is a secondary energy and the energy carrier, which can be converted from primary energy sources. The main characteristics of hydrogen are the following: (1) Hydrogen is the cleanest fuel. Hydrogen is a good-quality fuel with high calorific value, good combustibility, wide flammable range, and fast flame speed. The most prominent advantage of hydrogen is that the end product of hydrogen combustion is water, which can realize zero emission, unlike fossil fuels that form pollutants such as carbon monoxide, carbon dioxide, hydrocarbons, sulfur-containing compounds, and dust particles that harm the environment. (2) Hydrogen is a secondary energy that can be stored at large scale. Hydrogen can be obtained from a variety of primary energy sources (coal, oil, natural gas, nuclear), from renewable energy (solar, biomass, wind etc.), and secondary energy (electricity). The main difference between hydrogen and electricity is that hydrogen can be large-scale stored and transported in gaseous form, as a liquid, or as solid metal hydride. Hydrogen is widely used to meet special requirements. (3) Hydrogen has high energy conversion efficiency. According to the second law of thermodynamics, when chemical energy of fuels is transformed into mechanical energy, there are always cold source losses to some extent. At present, the highest energy conversion efficiency of thermal power plants is only approximately 42 %, while the efficiency of internal combustion engines for gasoline ranges below 30 %. Scientists have been actively searching for an energy conversion method to break the Carnot limit without violating the second law of thermodynamics. The fuel cell is considered to be one of the solutions. Theoretically, a variety of fuel gas can be fed into the fuel cell, and the breakthrough is that hydrogen-to-electricity efficiency is approximately 60–70 %. Therefore, hydrogen energy is considered as one of the highest efficiencies for converting fuel into electricity. (4) Hydrogen is a richly abundant resource. Hydrogen is one of main elements, forming compounds of water and of all organic matter. Consequently, it supplies up to 75 % of the mass of all visible matter in the stars and galaxies, hence, hydrogen is referred to as the ultimate energy for human beings. Water can be constantly recycled during the process of hydrogen production and utilization; therefore, hydrogen and water are inexhaustible resources. Biomass is another enormous and endless source of hydrogen, because biomass is the carrier of hydrogen and is produced naturally via photosynthesis of CO2 and water under the influence of the sun.

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3.1.3 Hydrogen production from biomass The production of hydrogen from biomass and its derivatives is an attractive and promising path, which is of special significance in solving the energy shortage as well as environmental pollution problems. There are two routes for the conversion of renewable biomass into hydrogen-rich gas: (1) thermochemical conversion, and (2) biochemical/biological conversion. Thermochemical conversion involves a series of chemical reactions to release hydrogen. There are four main methods to produce hydrogen-rich gas from biomass-based hydrogen production that use thermochemical conversion processes: (i) fast pyrolysis of biomass, (ii) biomass gasification, (iii) supercritical water gasification (SCWG), and (iv) chemical looping. Fast pyrolysis is a process in which biomass is converted into liquids, gases, and solid products via heating at high temperatures of 600–800 K and 0.1–0.5 MPa in the absence of air. Gasification is one of the easiest and simplest ways to produce hydrogen from biomass, in which biomass is gasified into a combustible gas mixture via partial oxidation of biomass at high temperatures, typically in the range of 800– 900 °C. Hydrogen is then gained after the gas cleaning and gas purification processes. Supercritical water gasification is a process to fully gasify biomass and convert it into hydrogen-rich gases using supercritical water at temperatures and pressures above its critical point (374 °C, 22.1 MPa). This can produce hydrogen with a volume content of above 50 % and without tar or coke. Furthermore, this can directly deal with high moisture content of biomass without a prior drying process. Chemical looping is a process during which biomass is converted into hydrogen-rich gases via steam reforming, and CO2 absorption reactions proceed based on the principle of REDOX reactions, including equilibrium shifts to the direction that favors hydrogen production. Besides these, methods of steam reforming of biomass derivatives such as bio-oil, ethanol, methanol, and dimethyl ether for producing hydrogen have been developed rapidly during recent years, which will be introduced in the next section. Biological hydrogen production includes biophotolysis of water using green algae and blue-green algae (cyanobacteria), photofermentation, dark fermentation, and biological water-gas shift reactions. Biophotolysis is the direct path of biological conversion that uses solar energy and the photosynthetic algal system to convert water into chemical energy. Photofermentation produces hydrogen mainly due to the presence of nitrogenase under oxygen-deficient conditions utilizing light energy and reduced compounds (organic acids). Dark fermentation is a ubiquitous phenomenon under anoxic or anaerobic conditions. Carbohydrates, such as glucose, are preferred carbon sources, which predominantly give rise to acetic and butyric acids in combination with H2 evolution. The biological water-gas shift reaction represents an anaerobic, dark-phase pathway, and combines chemical and biological processes to produce hydrogen. The organism uses the biological WGS reaction as a means to obtain energy, which maintains its metabolic processes and growth.

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However, the yield of H2 from the fermentation process is lower than that of other chemical or electrochemical processes and consequently, biological hydrogen production needs to be further explored.

3.1.4 Development and utilization of hydrogen The development of hydrogen energy is critically beneficial for environmental protection and sustainable development, therefore many countries have introduced clean hydrogen energy as the “fuel of the future”. Hydrogen is a valuable gas and is globally in increasing demand in recent years. Hydrogen can be utilized in many sections. It is not only used in the form of energy, but also widely used as feedstock for the production of chemicals, such as hydrogenation of fats and oils in the food industry, the production of electronic devices, the treatment process of high quality steel, and the desulfurization and reforming of gasoline in refineries. It has been reported that 50 million tons of hydrogen are traded globally per year, with a growth rate of almost 10 % annually. At present, the most important use of hydrogen is ammonia synthesis for agricultural fertilizers, cyclohexane, and methanol, which are intermediates in the production of plastics and pharmaceuticals. Hydrogen also plays a central role in fuel refinement, in hydrocracking and the desulfurising process. A large amount of hydrogen is consumed during the catalytic process from unsaturated fatty acid to solid greases. Hydrogenation is also utilized in the manufacture of organic chemical products. Gaseous hydrogen has several other applications, including welding and the reduction of metallic ores. Since the melting point of hydrogen is only 13.9 K above absolute zero, liquid hydrogen is also of great significance in studies of cryogenics and superconductivity. Hydrogen energy is an ideal energy for transportation, replacing fossil fuels such as coal, oil, and natural gas in future, and it can not only be burned in internal combustion engines, turbines and jet engines as energy, but can also be employed as rocket fuel combined with oxygen or fluorine. As a novel technology of hydrogen utilization, numerous studies have focused on hydrogen fuel cells, since hydrogen is a promising clean fuel for the future with end product of water and without emissions of either CO2 or toxic chemicals theoretically when it is oxidized. Hydrogen fuel cells are widely considered as pollution-free power supplies, which has led to a quick rise in hydrogen fuel demand. Along with the development and commercialization of hydrogen energy utilization, approximately 40 million tons of hydrogen would be required per year to supply fuel cells, while about 100 million fuel cell vehicles will soon come into service. Increased generation of hydrogen is the basis of hydrogen applications, while increased storage and transportation are the key links in hydrogen applications. Currently, increasing attention has also been paid to the technologies of hydrogen fuel cell and some novel technologies as well as equipment need undoubtedly to be developed to improve the application of hydrogen.

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3.2 Thermochemical routes for hydrogen production from biomass 3.2.1 Introduction Hydrogen production from biomass thermochemical processes has already been thought to be economically and technologically attractive by the International Energy Agency and has been demonstrated to be a feasible option that the technology develops from the gasification and pyrolysis of coal. The technology could convert biomass into liquids, gases, and solid products by imposing external influence or changing reaction conditions. The research progress and developing tendency about the thermochemical routes for hydrogen production from biomass will be presented in this section, which includes the reaction mechanism, thermodynamic analysis, as well as catalysts and reactors. Thermodynamic analysis could help to predict the main components of the products and to reveal the effect of process parameters on selectivity and hydrogen yield of the reactions. Steam reforming of biomass to produce hydrogen can be described by the following equations: Cn Hm Ok + (n − k)H2 O → nCO + (n + m/2-k)H2 (3.1) The above reaction is followed by the water-gas shift (WGS) reaction: CO + H2 O → CO2 + H2

(3.2)

Thus, the overall process (complete steam reforming of bio-oil) can be represented as follows: (3.3) Cn Hm Ok + (2n − k)H2 O → nCO2 + (2n + m/2-k)H2 The Gibbs free energies could be expressed by equation (3.4) without vapor pressure so that the direction of a chemical reaction and the composition of the system at equilibrium can be predicted for hydrogen production from biomass. NC−1

G = ∑ ni ⋅ (μ0i + RT (ln yi + ln P)) + nC ⋅ μ0c

(3.4)

i=1

where nc and μ0c represent the mole numbers and chemical potential of carbon, respectively. NC represents the numbers of the components and NE represents the numbers of the elements. When reaching the minimization of Gibbs free energy, two conditions must be met as follows: ni ≥ 0

i = 1, . . . , NC

(3.5)

NC

∑ ni aji = bj j = 1, . . . , NE i=1

(3.6)

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3.2.2 Fast pyrolysis of biomass to hydrogen Fast pyrolysis is a process in which biomass is converted into liquid, solid, and gas products (H2 , CO, CO2 , CH4 , etc.) by heating of biomass to 600–800 K in 0.1–0.5 MPa [198]. Fast pyrolysis of biomass to hydrogen can be expressed as the following: Biomass + heat → H2 + CO + CH4 + tar + carbon + other products

(3.7)

However, more hydrogen could be produced by methane and other hydrocarbons via steam reforming, CH4 + H2 O → 3H2 + CO (3.8) and the water-gas shift reaction eq. (3.2). High purity hydrogen can be produced from these hydrogen-rich gases following the gas purification process such as the pressure swing adsorption. Many types of gaseous, liquid, and solid phases can be found in the products of fast pyrolysis: (1) gaseous products include H2 , CH4 , CO, CO2 , and other gases, depending on the organic nature of the biomass and conditions of pyrolysis; (2) liquid products include tar and oils that remain in liquid form at room temperature such as acetone, acetic acid, etc.; (3) solid products are mainly composed of char and almost pure carbon plus other inert materials. Tab. 3.2 presents the yield patterns of different types of biomass using the pyrolysis process. Tab. 3.2: Product pattern of different biomass [199] Biomass

Char (wt.%)

Gas (wt.%)

Liquids (wt.%)

Beech wood Nutshells Olive husks Grape residues Straw pellets

29.00 36.82 34.13 44.84 34.63

16.03 17.91 19.60 17.44 20.10

55.54 41.66 46.01 35.72 41.50

There are essentially two paths for fast pyrolysis of biomass to hydrogen: the one-stage fast pyrolysis process and the two-stage fast pyrolysis process. The one-stage fast pyrolysis process can directly produce hydrogen-rich gas after biomass fast pyrolysis lasting more than 5 s in a reactor without oxygen. In this process, the temperature affects the product distribution. At lower temperatures (< 250 °C), the main products can be CO2 , CO, H2 O, and coke. For a temperature above 400 °C, it can produce CO2 , CO, H2 O, H2 , CH4 , coke, and tar due to depolymerization, polycondensation, repolymerization, cracking reactions, side chain reactions, and branched chain reactions. When the reaction temperature reaches 700 °C, the secondary reaction of tar cracking can start to produce hydrogen, light hydrocarbon, and coke with enough residence time.

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In the two-stage fast pyrolysis process, the hydrogen-rich gas is produced from the products of the one-stage fast pyrolysis process via tar cracking and steam reforming reactions in a successive reaction. Compared to the one-stage fast pyrolysis process, the two-stage fast pyrolysis process could produce more than a hydrogen volume fraction of 55 % because of the decomposition reaction of long-chain hydrocarbons, such as tar and several large alkanes. Since tar is hard to gasify, some catalysts such as dolomite or Ni-based catalysts under high temperature, and water or oxygen atmosphere are usually used in the two-stage fast pyrolysis process, which can promote the tar cracking reaction at 1,073–1,173 K and 973–1,073 K via dolomite or Ni-based catalysts, respectively. Apart from these, other catalysts such as the Y molecular sieve catalyst, K2 CO3 , NaCO3 , CaCO3 , Al2 O3 , SiO2 , ZrO2 , TiO2 , and Cr2 O3 have also been used in this process. In addition to the types of biomass, temperature, heating rate, residence time, and catalysts are also very important factors in the process of biomass fast pyrolysis to hydrogen. High temperature, rapid heating, and long residence time of the volatile phase is helpful to improve the production of hydrogen during fast pyrolysis, which could be adjusted via different reactors and heat transfer systems. In general, the fluidized-bed reactor is considered an ideal reactor for biomass fast pyrolysis due to its excellent heat transfer performance.

3.2.3 Biomass gasification to hydrogen 3.2.3.1 Introduction Gasification is one of the easiest and simplest ways to produce hydrogen from biomass. This conversion denotes the conversion of biomass into a combustible gas mixture via partial oxidation of biomass at high temperatures that typically range from 800 to 900 °C and then hydrogen gas is obtained after the gas cleaning and gas purification process. Biomass can be completely converted to CO and H2 through an ideal gasification, although some CO2 , water, and other hydrocarbons including methane were produced in practice. The tars produced during fast pyrolysis of biomass can be gasified through gasifying agents. Air, oxygen, and steam are widely used as gasifying agents. (1) Air gasification. Air gasification is the most widely used technology with high efficiency, low investment, and ease of operation; however, it can only produce a low hydrogen volume fraction of approximately 8.14 %. (2) Oxygen gasification. Oxygen gasification has the advantages of ease of operation, being a mature technology, and offering stable operation. However, it requires an O2 supply, which needs a high investment when the pressure swing adsorption or membrane separation processes are utilized to produce oxygen. (3) Steam gasification. Steam gasification results in the conversion of biomass to more H2 and CH4 , and less CO2 and CO, which is helpful for further purification of the products. The reaction temperature in an ideal gasification

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should be more than 700 °C and the steam gasification of coke can also happen at a higher temperature, thus the temperature of steam must reach more than 700 °C to improve the overall efficiency of the gasification process. (4) Air-steam gasification. Both air and steam are used in the air (oxygen)-steam gasification as gasifying agents, resulting in a better performance than air (oxygen) gasification or steam gasification. Furthermore, the reaction heat is supplied via oxidation under air or oxygen atmosphere and does not have to be provided externally. However, a portion of the oxygen, which can come from the steam, is used in the gasification to lessen the oxygen supply and ultimately leads to the generation of more H2 . Tab. 3.3 presents the gas product distribution of biomass utilizing four paths of gasification. Air-steam gasification is the best way to increase the content of H2 in the product gases. This is because steam can react with tar to produce CO and H2 in the biomass pyrolysis process, thus reducing the formation of coke and improving the production of hydrogen. Tab. 3.3: The gas product distribution of biomass via four paths of gasification [200].

H2 O2 N2 CO BCO2 CH4 Cn Hm

Air

Oxygen

Air-steam

Steam

12 2 40 23 18 B3 B2

25 0.5 2.0 30 26 13 4

30 0.5 30 10 20 2 7.5

20 0.3 1.0 27 24 20 8

3.2.3.2 Biomass gasification reactor Three kinds of reactors are used in biomass gasification: the updraft gasifier, the downdraft gasifier, and the circulating fluidized-bed gasifier. The updraft gasifier has the advantages of simple structure and easy operation, and can be used for the gasification of biomass with high moisture content (> 40 %). The utilization of updraft gasifiers in biomass gasification has several disadvantages, such as difficultly continuous feed and high tar content in the product gases. Moreover, updraft gasifiers cannot produce a high content of H2 in the gas product because part of the water in the feedstock is easily carried into rising hot product gases when wet material is fed and drops from the top. High content of H2 in the product gases could be obtained in a downdraft gasifier, which has a complex structure and is difficult to operate. A circulating fluidized-bed gasifier with a continuous cycle of carbon-containing or inert material is an ideal reactor for biomass gasification and comes with the advantages of high fluidization velocity, stable reaction temperature and high heat and mass transfer rate.

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As we have mentioned, pyrolysis and gasification are important processes for obtaining gas fuel from biomass. Valuable hydrogen-rich gas can also be generated by pyrolysis and gasification. In order to produce hydrogen-rich gas, catalytic gasification is one of the promising options for tar removal from the resulting gas. Two main consecutive reactions proceed in the biomass catalytic gasification system. After the biomass is gasified in the fluidized-bed gasifier, the first-stage gasified gas will be further converted into syngas through Ni-based catalysts. In this system, biomass is first fed into a fluidized-bed by the screw feeder, in which gasified gas, tar, and coke would be produced due to gasification. The solid product caused by gasification then reacts with air and steam from the bottom of the reactor and produces gasified gas, which would come into a cyclone separator on the top of the fluidized bed and the coke is separated from it. Subsequently, the gasified gas enters a reactor for tar removal during catalytic gasification. Finally, it is fed into a fixed-bed reactor with Ni-based catalyst for further conversion through catalytic cracking and water-gas shift reaction. Theoretically, all gasification processes mentioned above such as fixed-bed, bubbling, and circulating fluidized-bed can be used to produce hydrogen-rich gas, and these processes listed in Tab. 3.4 have been studied in detail in recent years. The syngas containing CO, CH4 , and C2+ can be supplied to produce hydrogen through appropriately increasing the ratio of steam/biomass to enhance the hydrocarbon steam reforming reaction and water-gas shift reaction if this process is applied into the hydrogen production. Tab. 3.4: Biomass gasification processes to be used for hydrogen production [201]. Organization Reactor

Pressure (bar)

Temp. ( °C)

UBC GIEC Renugas BIOSYN Chemerc SVZ BMI FERCO MTCI FICFB

8.3 0 20 16 0.7,32 25

700–850 800–850 850

Circulating fluidized bed Circulating fluidized bed Bubbling fluidized bed Bubbling fluidized bed Moving bed Twin-bubbling fluidized bed Bubbling fluidized bed Bubbling fluidized bed ICFB

Oxidant

Air Air Air Air 950 Air/oxygen 1,600–1,800 Oxygen 525–600 Steam 850–1,000 Steam/air 600 Steam 850–900 Steam/air

Scale 16–45 kg/h 1,500 kg/h 12 t/d, 100 t/d 10 t/h, 50 kg/h 20 t/d, 300 t/d 15 t/h, 35 t/h 1 kg/h 200 t/h 20 t/h,50 t/h 10–40 kg/h

3.2.4 Hydrogen production from biomass gasification in supercritical water Biomass gasification in supercritical water is a highly efficient method for hydrogen production that has received much attention during recent years. The reaction temperature is approximately 600 °C (Fig. 3.1) for complete gasification of biomass, which

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Pressure Critical pressure (22.1MPa)

is far lower than that of conventional thermochemical cycle reactions. It can simplify the reaction process and is likely to decrease the hydrogen production cost compared to traditional thermochemical cycles.

Supercritical water Critical point

Ice

Liquid water

Subcritical water High pressure superheated steam

Triple point Steam

(0.01°C) (100°C)

Critical temperature (374°C)

Temperature Fig. 3.1: Phase diagramm of water.

By means of the particular characteristics of water at the critical point (374 °C, 22.1 MPa), biomass can be completely gasified and part of the hydrogen can be released from water simultaneously, and thus the volume fraction of hydrogen produced can reach 50 % without side products, such as tar or coke. Wet biomass can be directly gasified without any high-energy consumption for the dehydration process. Many research institutions in the United States, Japan, Germany, Britain, and China have been carrying out a large amount of research on hydrogen production from biomass gasification in supercritical water due to good prospects for future development. Research on reactors, mechanisms, influence of operating parameters, and catalysts are proceeded by Xi’an Jiaotong University, Pacific Northwest National Laboratory, University of Hawaii, Tohoku University, Germany’s Karlsruhe Research Center, and others.

3.2.4.1 Reaction mechanism Lignocellulose includes residues from agriculture and forestry, energy crops, and waste paper, which are the inedible parts of plants consisting of cellulose, hemicellu-

180 | 3 Technologies for biomass-based hydrogen production

lose, and lignin. It was formed inside the complex network of cell walls of plants and a combination between molecules is relying on covalent bonds, bridge, and the van der Waals force. In general, lignocellulose contains 30–60 % cellulose, 20–40 % hemicellulose, and 15–25 % lignin. Biomass gasification is defined as a biomass reaction with a certain amount of oxygen or steam to generate H2 , CO, CO2 , and CH4 under the condition of high temperature (> 700 °C), in which supercritical water (374 °C, 22.1 MPa) is used as medium. The gas medium is the main difference between gasification in supercritical water and traditional thermochemical gasification. The former uses supercritical water and the latter uses air or water steam. The total reaction of biomass gasification converting biomass to hydrogen in supercritical water is shown below: CHx Oy + (2 − y)H2 O → CO2 + (2 − y + x/2)H2

(3.9)

CHx Oy + (1 − y)H2 O → CO + (1 − y + x/2)H2

(3.10)

Cellulose hydrolysis: (C6 H10 O5 )n + nH2 O → nC6 H12 O6

(3.11)

Glucose reforming reaction: C6 H12 O6 → 6CO + 6H2

(3.12)

(C10 H10 O3 )n + nH2 O → nC10 H12 O4 → Phenolics

(3.13)

Hydrolysis of lignin:

Steam reforming reaction: Phenolics + H2 O → CO + CO2 + H2

(3.14)

CO + H2 O → CO2 + H2

(3.15)

CO + 3H2 → CH4 + H2 O

(3.16)

CO2 + 4H2 → CH4 + 2H2 O

(3.17)

CO + 2H2 → CH4 + 0.5O2

(3.18)

Water-gas shift reaction: Methanation reaction of CO:

Methanation reaction of CO2 :

Hydrogenation reaction: Fig. 3.2 shows the process of how biomass (lignocellulose) converts into different gas products in supercritical water. During the initial stage, biomass is degraded into sugar, guaiacols, syringols, and phenols. Cellulose and hemicellulose are hydrolyzed to C5 and C6 sugars; lignin is degraded into phenols such as guaiacols and syringols. During gasification in supercritical water, the degradation products further transform

3.2 Thermochemical routes for hydrogen production from biomass

Biomass (Cellulose, hemicellulose and lignin)

H2 CO

Methanation Catalysts C CH4 H2O

181

Sugars (C5 and C6) Guaiacol, Syringol Phenolics

Hydrolysis

Supercritical water gasification (374–650°C, 22.1–25 MPa)

|

Catalysts A

Catalysts B

Acids Alcohols Phenols Aromatics Aldehydes

Water-gas shift reaction Catalysts D H2 CO2

Fig. 3.2: Typical reaction routes of biomass gasification in supercritical water. Note: Catalysts A (e.g., Ni, Ru, Rh, Pt, Pd, Ni/Al2 O3 , Ni/C, Ru/Al2 O3 , Ru/C, and Ru/TiO2 ); Catalysts B (e.g., Ni, Ru, Pt, and activated carbon); Catalysts C (e.g., Ni, Rh, Ru, Pt, and activated carbon), and Catalysts D (e.g., Ni, Ru, NaOH, KOH, K2 CO3 , and Trona).

into simple compounds including acids (carboxylic acid, succinic acid, acetic acid, etc.), alcohols (coumaric, conifers, mustard, etc.), phenol, aromatic compounds, and aldehyde. H2 , CO, CO2 , and CH4 were generated via water-gas shift, methanation, and hydrogenation reactions under heterogeneous catalysts. Inhibition of reaction (3.16) and promotion of reaction (3.13) will increase hydrogen production from biomass gasification in supercritical water.

3.2.4.2 Reactors for biomass gasification in supercritical water Reactors for biomass gasification in supercritical water can be mainly classified as batch reactors (Fig. 3.3) and continuous reactors (Fig. 3.4). Batch reactors are simple and are able to gasify all biomass materials that can be used for gasification mechanism studies and catalyst screening. The continuous reactor system has been used for extensive research on dynamic characteristics and hydrogen generation characteristics in the gasification process. Made possible by financial support of the United States Department of Energy, a set of demonstration units for hydrogen production from biomass gasification in supercritical water with the capacity of 40 t/d has been established [202].

182 | 3 Technologies for biomass-based hydrogen production

P

T

GC

N2

Fig. 3.3: Schematic of a typical batch (autoclave) reactor for biomass gasification in supercritical water.

Cooler

Back pressure regulator Gas products Biomass-water slurry

Reactor

Liquid products

Pre-heater Pump Fig. 3.4: Schematic of a typical continuous reactor for biomass gasification in supercritical water.

3.2.5 Hydrogen production from biomass gasification via solid heat carrier Hydrogen production from biomass often requires external heat sources (such as steam reforming, pyrolysis, etc.), while the use of air or oxygen as gasification agents (such as autothermal reforming) can introduce other components into the producing gas. Moreover, the use of oxygen as the gasification agent will severely increase the cost of hydrogen production. Therefore, a new hydrogen production process, the solid heat carrier method, has been proposed, where the solid heat carrier is circulated to supply the necessary energy for the endothermic gasification reactions as shown in Fig. 3.5. Biomass is gasified into hydrogen-rich gas in the gasifier with the steam, and

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Biomass

Flue gas

Regenerated catalyst and heat Combustor

Steam Gasifier

Deactivated catalyst and char Air

Product gas

Fig. 3.5: Basic concept of the biomass gasification process via solid heat carrier.

heat carried or chemical reaction heat is released by a solid heat carrier. The char from biomass gasification and solid heat carrier is sent to the combustion chamber. In the combustion chamber, the solid heat carrier is heated by energy released from char combustion, and then returns back to the gasifier. Then another cycle of gasification and sequential combustion starts again. Metal oxide or metal can also be used as the solid heat carrier. A chemical looping agent of carbonation/calcination, such as CaO/CaCO3 , is used as carrier of CO2 and supplies the chemical reaction heat for the endothermic reaction. Metal or metal-oxide looping is used as the carrier of oxygen to produce H2 -rich gas directly, and this is named chemical looping reforming for hydrogen production. This biomass gasification process of the solid heat carrier with catalyst includes the fast pyrolysis reaction of biomass, catalytic reforming reaction of tar and hydrocarbons, steam gasification reaction of semi-coke, heating and regeneration of solid heat carrier, and has the following advantages: (1) Because the combustion and hydrogen production processes are separated in the different devices, the gasification process does not need to contact the air directly, thus avoiding dilution of the hydrogen-rich gas by N2 and CO2 , and decreasing the other contaminants in the resultant gas. (2) The tar and semi-coke catalytic reforming reaction can significantly decrease the tar content in the resulting H2 during this process. (3) The catalyst can be continuously and repeatedly regenerated, thus avoiding the rapid deactivation of the catalyst. The solid heat carrier process was adopted in a European Commission project using twin circulating fluidized beds, one for fluidization and gasification, and the other for combustion to supply heat [203]. The bed materials are circulated in the two beds and the gases in two beds are separated respectively. The material of the bed functioning

184 | 3 Technologies for biomass-based hydrogen production

as solid heat carrier was heated in the combustion zone and sent to the gasification zone, where the heat was used for endothermic gasification reactions. When the temperature of bed materials was dropped, bed materials were sent to the combustion zone to complete the cycle.

3.2.5.1 Hydrogen production from biomass gasification with chemical looping process of carbonation/calcination cycle Traditional thermochemical processes of hydrogen production, such as methane or biomass steam reforming, are strongly endothermic reactions. External heating is needed and CO2 will be released during these processes. In general, hydrogen needs to be purified by pressure swing adsorption or membrane separation while CO2 is produced simultaneously after the reforming reaction, however, it will increase the investment and energy consumption. A type of chemical looping technology for hydrogen production has been widely investigated during recent years. This technology is characterized by low cost, high efficiency of hydrogen production, and internal separation of CO2 , and is of great significance for solving the increasing problem of greenhouse gas emissions. Chemical looping processes of the carbonation/calcination cycle are used in biomass gasification for hydrogen production, and some natural ores such as limestone, dolomite, and olivine can be used as bed material, i.e., solid heat carrier. Typically, CaO/CaCO3 is used as a solid heat carrier and supplies heat carried by CaO and chemical reaction heat of CaO and CO2 , shifting the equilibrium to the direction that favors hydrogen production, for the endothermic reaction to obtain H2 -rich gas as shown in Fig. 3.6. Here, coal or natural gas are usually used as fuels. CO2–rich gas

Hydrogen-richgas

CaO + heat

Heat

Calciner (850~900°C)

Carbonator (600~700°C)

Biomass

CaCO3 + coke Steam/CO2/air

Steam

Fig. 3.6: CaO-based solid heat carrier for hydrogen production.

This system mainly consists of two reactors, one is a gasifier and the other is a calciner. Biomass is gasified into hydrogen-rich gas in the gasifier with the steam and

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heat released by CO2 capture using CaO. CO2 from biomass gasification reacts with CaO and becomes CaCO3 , which promotes the hydrogen production reaction due to CO2 abatement during gasification. CaCO3 is sent to the calciner for calcination and releases high concentration CO2 (CaCO3 → CaO + CO2 ) at high temperature supplied by the external heat resource. CaO, which is carrying the reaction heat returns to the gasifier to be re-used for gasification. CaO-based biomass hydrogen production on the basis of 8 MW biomass double fluidized-bed gasification technology was studied by the Vienna University of Technology. Compared to the technology without using CO2 sorption-enhanced reforming (SER), the H2 content in the product gas using SER increased from 35–40 vol % (dry basis) to 75 vol% (dry basis) [204]. The hydrogen in products existed predominantly in the form of H2 , CH4 , H2 O, and HC using traditional double-bed fluidization, while it existed in the form of H2 or H2 O using SER. Experiments conducted with pine sawdust as feedstock and with calcined olivine as solid heat carrier show that gasification temperature and steam/biomass ratio are the two important factors that affect product distribution. The highest H2 concentration of 53.3 vol % (dry basis) and tar content below 0.7 g/Nm3 was obtained at the steam/biomass ratio of 0.6–0.9. Moreover, higher cellulose and hemicellulose contents of biomass are more conducive to the catalytic gasification process, and H2 production can be improved when the water-gas shift reaction is promoted forward the right way if limestone calcined (CaO) is used as sorbent for CO2 removal [205]. Acharya [206] introduced a system for hydrogen production from biomass with a chemical looping process of carbonation/calcination cycle and analyzed the mass and energy balance as shown in Fig. 3.7. The thermal efficiency reached 87.49 % when CaO was used as the sorbent of CO2 . The concentration of H2 could reach 71 %, while the concentration of CO2 was almost 0 %. A regenerative efficiency of CaO as low as 50 % dropped the efficiency of the whole system to 57 %.

Water Gas output 800˚C Thermal losses 0.92mW

0.18kg/s 2.12MW

CaCO3 800˚C

2.84kg/s 3.36MW

CO2950˚C

4.30kg/s 3.092MW

Carbonator Q=‒4.62MW

Calciner Q=7.65MW

2.41kg/s CaO 950˚C 1.59MW

1kg/s Fuel 30˚C 0.012MW 1.07kg/s 0.644MW

Steam 300˚C

CO230˚C

0.10kg/s Gas output 30˚C 10.52MW 1.89kg/s

CO230˚C

Thermal losses 0.92MW 0.95kg/s 0.02MW

Fig. 3.7: Mass and energy balance for gasification with a chemical looping process of carbonation/calcination cycle.

186 | 3 Technologies for biomass-based hydrogen production

The chemical looping of the carbonation/calcination cycle technology, which uses biomass as raw materials, can capture CO2 in the process of producing high-concentration hydrogen. Pollution emissions of greenhouse gases are significantly reduced in the process of the preparation and utilization of hydrogen due to the effective control of CO2 emission. Therefore, it is regarded as an ideal way of energy conversion and utilization due to the near-zero carbon emission. However, the technology is at present still in the stage of concept design and basic testing, and needs to be further studied. NiO/NiAl2 O4 was used as the catalyst and CaO was chosen as the sorbent of CO2 for glycerin reforming in a moving bed [207] through chemical looping. Under a reaction temperature of 500–600 °C and water/carbon ratio between 1.5 and 3.0, the highest hydrogen purity of more than 90 % could be reached. The majority of glycerol turned into H2 and only a small amount of glycerin provided heat by oxidation. Therefore, the system could run with only a small amount of externally provided heat, and the whole system presented a higher efficiency as shown in Fig. 3.8. C2H8O3+H2O

CO+OH→CO2+1/2H2 H2+CaCO3

Slow ∆H0=−245 kJ/mol

C3H8O3

CO+H2O+CO2

CO2+H2+CaO

Slow

∆H0=−178 kJ/mol

CO2 CaO

∆H0=−41.5 kJ/mol

Ni

Fast C3H8O3+NO/NiAl2O4

∆H0=−603 kJ/mol

C3H8O3 H2O

Ni −O

H2

Nio Fast

Fig. 3.8: Reaction and energy scheme of sorption-enhanced steam reforming of glycerol.

Oxidation reaction: C3 H8 O3 + H2 O + NiO/NiAl2 O4 → CO + CO2 + Ni/NiAl2 O4

(3.19)

Steam reforming reaction: C3 H8 O3 + H2 O → CO + H2

(3.20)

Water/steam transforming reaction: CO + H2 O → CO2 + H2

(3.21)

CaO + CO2 → CaCO3

(3.22)

CO2 capture reaction: Overall reaction: C3 H8 O3 + 2H2 O + NiO/NiAl2 O4 + 3CaO → 6H2 (g) + 3CaCO3 + Ni/NiAl2 O4

(3.23)

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3.2.5.2 Chemical looping hydrogen production from biomass Chemical looping technology is a multistep process during which the particular chemical reaction is carried out through a chemical medium. During the process, the products or by-products of detailed reactions can be used as raw materials or reactants for further reactions. Furthermore, the products can return to their original state after a number of reactions. Such a cycle or chain process completed by several elementary chemical reactions is called chemical looping. The concept of chemical looping reforming (CLR) for hydrogen production was first proposed by Mattisson and Lyngfelton on the basis of chemical looping [208]. This technology is based on the principle of a redox reaction and the appropriate oxygen carrier is selected to cycle between both reactors, for generating hydrogen. Hydrogen is generated via reaction of oxygen carrier and steam, without gas impurity, but with metal, H and O elements produced in the progress. H2 typically needs to be separated from the syngas mixture of H2 and CO2 . Chemical looping reforming technology based on chemical looping reaction, steam metal hydrogen production can simultaneously separate H2 from the syngas mixture during the reaction as shown in Fig. 3.9. Therefore, it can simplify the system and significantly reduce the cost of H2 separation. The system is composed of a reduction reactor and a steam oxidation reactor. Metal oxide is reduced by fuel gas in the reduction reactor, and then the solid heat carrier and the metal or low-valent metal oxide are recycled back to the steam oxidation reactor in which H2 O can react with the oxide metal or lowvalent metal oxide, thus forming H2 and metal oxide. If the reducing gas completely reacts with the metal oxide, the output gases of the reduction reactor only contain CO2 and H2 O, and the output gases of the steam reactor contain H2 and excess H2 O. Therefore, pure H2 and CO2 can be harvested without separating process as long as the condensation of steam remains available. H2

H2O, CO2 MyOx

Oxidation reactor

H2 O

MyOx‒1

Reduction reactor

CO, H2, CnHm...

Fig. 3.9: Hydrogen production using chemical looping technology.

The solid heat carrier and the oxygen carrier should have the following properties: (1) They should completely turn fuel gas into CO2 and H2 O, otherwise the unburned fuel gas needs to be entirely transformed by pure oxygen. (2) With fuel, they should

188 | 3 Technologies for biomass-based hydrogen production

change into low-valent metal oxide or metal element. Moreover, the low-valent metal oxide should have a higher performance of hydrogen production. (3) They should have good stability and low cost. In addition, high temperature and high pressure are necessary conditions to reach industrial application. Raising the temperature can increase the production of hydrogen; however, high temperature is prone to deactivate the carrier due to sintering and damaging activity. High pressure can also promote the carbon to deposit on the surface of the particles, which will in turn affect the purity of H2 . Therefore, the key to this technology is to develop a carrier with high activity, selectivity, wear resistance and anticarbon-deposition performance. At present, the carriers studied include Fe3 O4 /FeO, Fe2 O3 /FeO, ZnO/Zn, CeO2 / Ce2 O3 , NiFe2 O4 /NiFe2 O4−δ , SnO2 /Sn, GeO2 /GeO, WO3 /WO2 and so on. From the point of view of thermodynamic equilibrium, the performances of Ni, Cu, Cd, Co, Mn, Sn, and Fe oxides have been analyzed and Fe2 O3 was considered as the most appropriate carrier in the iron-steam hydrogen production process. In the chemical looping hydrogen production process from syngas, Fe2 O3 in general was considered to be the most active metal. Improving reaction performance and stability of Fe2 O3 in the cycles is the key to industrial application. Therefore, stability and performance of the carrier were investigated and improved via addition of different elements. The results reveal that Al, Cr, Zn, Ga, and V elements improve the performance of Fe2 O3 /FeO evidently and the sintering problem of Fe2 O3 was alleviated during the redox reaction.

3.2.6 Hydrogen cleanup and purification Pyrolysis and gasification technology with air or steam reforming, supercritical water, and the solid heat carriers from biomass mentioned above could maximize H2 production; however, other compounds such as CO, CH4 , CO2 , and tar could be produced with H2 . The hydrogen production process, syngas catalytic conversion process, and hydrogen purification process could determine yield and quality of the hydrogen products. In this section, the hydrogen purification technologies will be reviewed with special emphasis on catalytic tar removal and CO cleanup technologies.

3.2.6.1 Catalytic tar removal for hydrogen purification Tar is considered one of the most problematic parameters in biomass gasification and pyrolysis processes, seriously impeding the wide use of biomass. Typical composition of biomass tars are toluene, naphthalene, monocyclic, and polycyclic aromatic hydrocarbons [209]. In general, tars can be removed either during the gasification process or after gasification. In general, the tar removal rate through the former method is utilized far less than the latter although it is the superior method basically. The latter method can be divided into two categories: the physical method and the chemical method. The physical method includes wet purification, dry removal

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process, and electrostatic tar collecting method. Physical methods have the disadvantages of complicated operation, high cost, poor effect of tar removal, and causing secondary pollution, thus rendering them not conducive to large-scale industrial application. The chemical method includes thermal cracking and catalytic cracking. The far higher temperature of 1,000–1,200 °C for thermal cracking is required to decompose the tar into small gas molecules, which can reduce the tar content and recover tar combustion heat. However, it is not reasonable to reach such a high temperature to achieve thermal cracking from an economic point of view. During or after biomass gasification, a catalyst is added into the catalytic cracking process to lower the activation energy, which can lead to tar decomposition at lower temperature. Catalytic cracking is considered as a promising and effective tar removal method for high purity of hydrogen that can be achieved at much lower temperatures than thermal pyrolysis. The most important factors for the catalytic tar removal method are the components and preparation of catalyst. Ni, clay-supported bimetallic Fe/Ni, NiO/γ-Al2 O3 , olivine, dolomite, zeolite, and ceramic catalysts are used to remove tar from biomass gasified gases. A Ni-based catalyst is considered as one of the most promising catalysts and is widely studied for its high efficiency, low cost and abundant resource availability as shown in Tab. 3.5. To improve catalyst deactivation, a new tar removal method has been proposed [233] based on chemical looping reforming with metallic oxide as the catalyst, oxygen, and heat carrier. It is shown in Fig. 3.10. N2,CO2

Clean gas MexOy Tar refoming reactor

Catalyst regenerator MexOy‒1

Feed gas + Tar

Air

Fig. 3.10: A new tar removal method based on chemical looping reforming.

3.2.6.2 CO removal for hydrogen purification Hydrogen is an important feedstock for fuel cells with high energy efficiency and a constant supply is one of the main goals of hydrogen production. An electrode made of a noble metal such as Pt is very sensitive to CO in a proton exchange membrane fuel cell (PEMFC). The electrode will be poisoned if there is a little CO (> 10 ppm) in the feed gas, so it is of practical significance to completely remove CO from the product gas. There are two methods to remove CO: one is a physical method and the other is a chemical method. The physical method includes pressure swing adsorption and

Toluene

Tar or model compound

–

–

Ni

Ni

Ni/MgO

Ni/MxOy

Nix /Ca12 Al14 O33 Ni

–

ZrO2 , Al2 O3 –

–

Al2 O3

Ni

– 500–900 °C

550–650 °C

500–900 °C

CaOx /MgO1-x

Mn

Steam; 500–800 °C

Ca12 Al14 O33 CaOx /MgO1-x

H2 O/C = 3.4; 500–800 °C Palygorskite CO2 reforming; 650 °C, 750 °C, 800 °C MgO CO2 reforming; 500–800 °C MgO, SiO2 ZrO2 , CO2 reforming; Al2 O3 500–800 °C Ca12 Al14 O33 Ca12 Al14 O33 –

CaO-Al2 O3

Operational condition

Ni/Mayenite, Ni, NiO – NiO/Mayenite, Ni/CaOx /MgO1-x – Ni –

–

–

Ni

Nix /PG

–

Ni, Ce

Active Promo- Carrier compo- ter nent

Ni/Ca-Al-Ce(x)

Sample

Catalyst

Tab. 3.5: Catalysts and performance for catalytic tar removal studied.

The addition of CaO and MgO into the catalysts has little effect on the hydrogen yield [215] Ni and MnOx can largely enhance the catalytic activities and suppress carbon deposition [216] ZrO2 enhances the oxidation activity while Al2 O3 promotes selectivity [217]

Free oxygen inside the structure of catalyst helps to suppress carbon deposition [213] Ni/Ca12 Al14 O33 has better H2 yield than Ni/CaOx /MgO1-x and better CO selectivity than Ni/Ca12 Al14 O33 [214]

MgO is the best carrier among the samples [213]

For x = 0.2, the sample has the best performance. The addition of Nickel can enlarge the specific surface area and inhibit carbon deposition [210]. The order of catalytic activities is PG < Ni2/PG < Ni5/PG ≈ Ni8/PG. When temperature increases or the concentration ratio of CO2 and C7 H8 increases from 0 to 4.5 and the hydrogen yield drops slightly [211] 700 °C is the best temperature to remove tar [212]

Conclusion

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Sample

Catalyst

Biomass gasification products

Fe/CS, CS

Fe, CS

RhNi, Co LaCoO3 /Al2 O3 Ni/Al2 O3 , bitter spar, olivine Ni/CS, Fe/CS Ni, Fe

Ni

Ni/MgO

K

–

– –

– Fe, Ni

Ni, Co

–

–

Rh-LaCoO3 /Al2 O3 has a conversion rate of nearly 100 % [223]

Catalysts were stable within ten hours and lost activity after four cycles [222]

Tar conversion rate is up to 80 % after being treated for 8 hours [219] Catalysts have good performance for tar cracking via suppressing carbon deposition [220] When H2 O/C ratio is low, Co/MgO has high catalytic activity [221]

Bio-ash is a low-cost catalyst with high catalytic activity and long life span for tar cracking [218]

Conclusion

Fe and Ni have catalytic activity while their chemical compounds are inactive [224] The yields of gases decrease with a decrease of Fe loading from 2 wt. y% to Calcinated shell Steam 10 wt.%. Hydrogen production rate was largely increased with the dope of K reforming; 510 °C, 610 °C, [225] 660 °C,

100–800 °C

Steam reforming; below 900 °C Steam reforming, below 900 °C

900 °C 650–850 °C

CO2 , steam; 700–900 °C

Operational condition

Calcinated shell Steam; 650 °C

Al2 O3

MgO

– CaO, MgO, (CaMg)O MgO

–

Active Promo- Carrier compo- ter nent

Co/MgO, Ni/MgO

Naphtha- Bio-ash and lene bitter spar, olivine Olivine –

Tar or model compound

Tab. 3.5: (continued)

3.2 Thermochemical routes for hydrogen production from biomass |

191

– –

Ni

Fe, Ni

Ni, Co

–

–

Ni/PG

–

Ni-Co/Iolite

Bitter spar, Olivine Fe2 O3

–

Rh(0.1– – 1 wt.%) Co Ni, Fe, – Rh

–

Zeolite

Al2 O3

–

–

Zeolite

Fe, Mg, Palygorskite Sn, Ce Palygorskite

Active Promo- Carrier compo- ter nent

Sample

Catalyst

Bio-oil Rhproducts LaCoO3 /Al2 O3

Tar or model compound

Tab. 3.5: (continued)

The tar conversion rate was up to 100 % above 700 °C [231]

After tar removal reaction, Fe2 O3 was converted to Fe3 O4 . The regeneration of Fe2 O3 has little effect on performance of its catalytic tar cracking, while the catalytic activity of water-gas shift goes down after regeneration [230]

Ni/PG catalyst with 6 wt.% of Fe performed best, and its tar conversion rate and H2 production rate is 98.2 % and 56.2 %, respectively [226] Doping of Fe on NI/PG can largely enhance the catalytic activity, and the tar conversion rate and H2 production rate reaches 94.4 % and 57.7 %, respectively [227] Bimetallic catalysts have better performance than any monometallic catalysts. When the ratio of Ni and Co reaches 3:1, the tar conversion rate is 96.4 % [228] The tar conversion rate increases with the temperature increase [229]

Conclusion

At 610 °C, the catalysts doped have high hydrogen yield. Rh is the best metal Steam to improve performance of catalyst [232] reforming; 510 °C, 610 °C, 660 °C

500–700 °C

600 °C

800–900 °C

800 °C

700 °C

700 °C

Operational condition

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membrane separation. Chemical methods include water-gas shift, selective methanation, preferential oxidation, and other techniques.

3.2.6.2.1 Pressure swing adsorption (PSA) The capacity of adsorption and desorption for different gases varies, so different components of gases can be separated via pressure swing adsorption when the state of adsorption and desorption equilibrium changes via pressurization and dropping pressure at different temperatures. Pressure swing adsorption is applied to purify oxygen, nitrogen, ammonia tail gas, etc. and the purity of hydrogen can reach 95–99.9995 %. The equipment needs to meet requirements of the cycling process, where the gas is compressed and decompressed repeatedly, and it is favorable to large-scale production.

3.2.6.2.2 Membrane separation method The permeability of different gases differs; therefore, gas could be separated when there is a pressure difference between both sides of the membrane. Generally, the membrane is made of metal or polymer. Nowadays, Pd or Pd alloy membranes are typically used to remove CO because this is a simple and convenient method. The efficiency will drop since a metal membrane can easily be oxidized via gas oxidation. However, its practical application is limited due to its low mass transfer rate, hydrogen embrittlement, and high cost of preparation and application.

3.2.6.2.3 Preferential oxidation The addition of a small amount of oxygen or air to the hydrogen-rich gas will preferentially oxidize CO into carbon dioxide with the help of a catalyst. Meanwhile, the reaction between hydrogen and oxygen or air should be suppressed as much as possible. Preferential oxidation is regarded as the most effective and economical method to remove CO. Catalysts play the most important role in preferential oxidation. Various catalysts used in the preferential oxidation of CO can be roughly divided into two broad categories according to the active component loaded. One is a noble metal catalyst represented by Pt and Au, while the other one is a non-noble metal oxide catalyst such as Cu and Co. In preferential oxidation, Pt-based catalysts have a stronger selectivity because CO is easier adsorbed than H2 . The reaction condition must be strictly controlled since Pt is sensitive to temperature and the oxygen content which will make the reaction system more complex. Therefore, a lot of studies are focused on improving the activity and tolerance of Pt catalysts. Recent studies have shown that in addition to optimizing the preparation method and carrier, doping noble metals such as Au, Ru, and others, or transition metal oxide components such as Sn, Fe, Co, Ni, and others into Pt cata-

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lysts can also improve the activity of the reaction. An Au nanoparticle (3–5 nm) catalyst shows good catalytic performance and water resistance in a CO oxidation reaction at low temperature. However, the activity of the catalyst will decrease or even deactivate at high temperature, due to size increase of Au nanoparticles agglomerated. This will seriously affect the selectivity of preferential oxidation of CO. A lot of transition metals such as Cu, Co, Zr, and Mn are applied in preferential oxidation. Cu catalysts, non-noble metal catalysts, show an improved reaction effect and selectivity. However, if water and carbon dioxide are present in the feed gas, the activity will be seriously affected. In recent years, Co catalysts have attracted significant attention due to their surface and catalyst properties that are similar to noble metal catalysts such as Pt. They also revealed good catalytic reactivity, especially in an oxidation reaction of CO at low temperature and this catalytic reactivity is equally good as that of noble metals such as Pt. In addition, it has shown good performance of water and carbon dioxide resistance. It also has excellent stability and strong value in practical applications.

3.3 Hydrogen production from biomass derivatives Hydrogen production technologies from biomass derivatives are drawing much attention and are being rapidly developed. Bio-oil is a type of representative biomass derivative, and the technical breakthrough that utilize fast pyrolysis of biomass for bio-oil with convenient collection, storage, and transportation has been a little made. Hydrogen production through catalytic reforming is one of the promising methods of upgrading bio-oil. The technologies for hydrogen production from other biomass derivatives, such as methanol, ethanol, and dimethyl ether, which can also be generated from fossil fuels, lead to significant progress.

3.3.1 Hydrogen production from bio-oil 3.3.1.1 Properties of bio-oil In comparison to petroleum oil, typical bio-oil produced via fast pyrolysis of biomass is a very complex mixture of oxygenated components and water, and the heating value of bio-oil with high density and viscosity is approximately half that of petroleum oil. The physical properties of bio-oil produced via sawdust pyrolysis and fuel oil are presented in Tab. 3.6. The components of bio-oil are unstable due to condensation and polymerization during storage. 3.3.1.2 Catalysts for hydrogen production via bio-oil steam reforming Steam reforming for hydrogen production is a catalytic reaction to reform reactants in a steam atmosphere, thus producing hydrogen-rich gas. The majority of research

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Tab. 3.6: Physical properties of bio-oil produced from sawdust pyrolysis and fuel oil [234]. Analysis

Bio-oil

Heavy fuel oil

Light fuel oil

Water wt.% Water and precipitate vol % Solid wt.% Ash content wt.% Nitrogen content wt.% Sulfur content wt.% Stability Viscosity (40 °C) cSt Density (15 °C) kg/dm3 Flash point °C Flow point °C Low heating value MJ/kg pH Distillation

20–30 – ≤ 0.5 0.01–0.1 ≤ 0.4 ≤ 0.05 unstable 15–35 1.1–1.3 40–110 9–36 13–18 2–3 Not possible

0 ≤ 0.5 – ≤ 0.08 0.4 ≤1 – – ≤ 0.995 ≥ 65 ≥ 15 ≥ 40.6 – Possible

0 ≤ 0.02 – ≤ 0.01 0.02 ≤ 0.001 – 2–4.5 ≤ 0.845 ≥ 60 ≥5 42.6 – Possible

on steam reforming mainly focuses on active ingredients, alkali metal co-catalysts, carbon deposition, reaction temperature, ratio of water and oil, space velocity, and reactors. The mechanisms of acetic acid steam reforming via Pt/ZrO2 catalysts (Fig. 3.11) have been studied, and the result suggests that Pt and ZrO2 can activate HAc and H2 O, respectively [235]. On the surface of Pt particles, HAc will be decomposed to H2 , CO, CO2 , CH4 , and others, and carbon could be observed on the surface of particles. H2 O is adsorbed and activated to the form of hydroxyl groups and the hydroxyl groups then react with CHx species on the Pt via the Pt-ZrO2 boundary layer. H2O

H2, COX, CH4

Acetic acid

H2, COx (CHx) H H O O

Low polymer

CO2, H2O CH3, COCH3

Pt Low polymer ZrO2

Fig. 3.11: Proposed pathway for the steam reforming of HAc involving catalysis of Pt/ZrO2 .

A summary of experimental and simulation research on catalysts for hydrogen production via bio-oil steam reforming is presented in Tab. 3.7.

196 | 3 Technologies for biomass-based hydrogen production

Tab. 3.7: Comparison of different reforming methods [236]. Catalyst

Technology

Conclusion

Feedstock

Ni-La/Al

Fluidized-bed reactor, T = 450–700 °C, S/C = 5.58, inlet ratio = 1.84–2.94 g/min

Acetic acid

Nickel-based catalysts

Fixed-bed reactor, T = 600–900 °C, H2 O/C = 2–8.2, GC1 HSV = 300–500 h−1

Ru/MgO/Al2 O3

Nozzle-bed reactor, T = 800 °C, P = 1 atm, S/C = 7.2

The co-catalyst, La, can effectively enhance the production yield of hydrogen. When inlet rate and temperature reach 1.84 g/min and 650 °C respectively, 0.029 g hydrogen would be produced from acetic acid per g. Potassium can inhibit carbon deposition. The hydrogen production yield of the aqueous fraction of bio-oil reaches 70 %. Furthermore, when the temperature increases beyond 600 °C, the production yields of acetic acid, acetone, and ethylene reforming are all over 90 %. MgO can promote the water-gas shift reaction and enhance the steam absorption on catalysts.

Ni/CeO2 -ZrO2

Fixed-bed reactor, T = 450–800 °C, ratio of water and oil = 4.9, Ni 12 %, Ce 7.5 % Fixed-bed reactor, T = 550–750 °C, S/C: 3, SV = 30,000 h−1

Ni, Rh, or Ir loading CaO-Al2 O3

Ni-Ca-Mg/Al

Fluidized-bed reactor, T = 650 °C, GC1 HSV = 11, 800 h−1

The hydrogen production yield is up to 69.7 %. When T = 800 °C, W/B = 4.9, Ni 12 %, and Ce 7.5 %. This yield is higher than the commercial catalyst Z417. Carbon can be more easily deposited on nickel-based catalysts than on rhodium-based, or iridium-based catalysts. When the addition of nickel reaches 5 wt.%, the hydrogen production yield of Ni/CaO-2Al2 O3 is maximal. Reducing the space velocity and the inlet oxygen concentration can effectively reduce the carbon deposition. Mg-modified catalysts display better performance than Ca-modified catalysts.

Acetic acid, acetone, ethylene and aqueous fraction, aqueous fraction of bio-oil

Acetic acid and aqueous fraction, aqueous fraction of bio-oil Aqueous fraction, aqueous fraction of bio-oil

Acetic acid, acetone

Aqueous fraction, aqueous fraction of bio-oil

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Yan and his team studied the method of hydrogen production from acetic acid and aqueous fraction of bio-oil based on Ni/CeO2 -ZrO2 catalysts [237–239]. When the temperature reaches 650 °C and S/C = 3, LHSV = 2.8 h−1 , the maximum hydrogen production yield and minimum methane production yield of acetic acid steam reforming are 83.4 % and 0.39 %, respectively. Furthermore, the maximum hydrogen production yield of bio-oil stream reforming is 69.7 % at a temperature of 800 °C, W/B = 4.9, mass faction of Ni and Ce of 12 wt.% and of 7.5 wt.%, respectively. Further research on catalytic stability indicates that disproportionation of CO and ketonization reaction lead to continuous carbon depositing on the surface of nickel active sites, which proved to be the main reason for the inactivation of catalysts.

3.3.1.2.1 Noble metal catalysts Noble metal catalysts with good absorption performance promote the formation of active intermediate compounds, which lead to high reaction activities. Domine [240] et al. compared the hydrogen production performance between biooil steam reforming and stepwise dissociation based on Pt- and Rh-based catalysts, respectively, and found that the former catalyst had better performance for hydrogen production during steam reforming reaction compared to the latter, which was attributed to its better promotion of water-gas shift reaction under high values of S/C. The reaction of stepwise dissociation can effectively reduce carbon deposition, which helps to increase the stability of catalysts.

3.3.1.2.2 Rare earth metal and alkali metal modified catalysts Catalyst sintering and carbon deposition can affect the stability and activity of catalysts. Catalysts modified by various rare earth metals and alkali metals can restrain sintering of active composition at high temperatures and inhibit carbon deposition caused by CO disproportionation to some extent. Thus, these catalysts are effective to improve hydrogen production and reduce CO and CH4 production. Medrano et al. [241] synthesized Ca- and Mg-modified Ni/Al2 O3 catalysts and designed an atomization device to reduce carbon deposition in a fixed-bed reactor. The result indicates that Mg-modified Ni/Al2 O3 has better performance in hydrogen production and bio-oil conversion during steam reforming, while the outlet gas via Camodified Ni/Al2 O3 results in higher CO and H2 and lower CO2 concentrations. A further study suggests that decreasing the space velocity has little effect on carbon deposition; however, it reduced the CO and CH4 concentration of the gas. A small amount of oxygen in the reactor can effectively reduce carbon deposition on the surface of catalysts. Remón et al. [242] studied bio-oil steam reforming for hydrogen production based on Co- and Cu-modified Ni/Al-Mg-O catalysts and found that catalysts in a fluidized-bed reactor had higher stability than in a fixed-bed reactor. Cu-modified Ni/Al-Mg-O catalysts have little enhancement in catalytic activity, while Co-modified catalysts have improved performance of stability and carbon deposition control. Beatriz Valle et al.

198 | 3 Technologies for biomass-based hydrogen production

[243] studied steam reforming for hydrogen production from the aqueous fraction of bio-oil based on Ni/α-Al2 O3 and Ni/La2 O3 -α-Al2 O3 . The result indicates that the addition of La2 O3 is prone to reduce the formation of methane and short-chain alkanes and promotes steam adsorption to control carbon deposition. This is due to the formation of oxygen vacancies, which leads to the transfer of an O2− ion through a low valence La3+ on the surface of nickel particles.

3.3.1.3 Bio-oil partial oxidation for hydrogen production The partial oxidation reaction is a type of oxidation reaction between a reactant and an insufficient amount of oxygen as shown in eq. (3.24), which can provide sufficient heat to meet the reaction in high temperature, thus enabling the reaction to reach equilibrium. However, excess oxygen will lead to complete oxidation and to finally form CO2 and H2 O. Cn Hm Ok + O2 → CO + CO2 + H2 (3.24) Agrell et al. [264] synthesized Pd/ZnO catalysts with different particle diameters via microemulsion and impregnation method. The result indicates that the diameter of Pd has little effect on methanol conversion; however, the CO production yield increases with increasing Pd particle diameter, while H2 and CO2 production yield decrease under the same condition. Rabe et al. [244] studied the mechanism of bio-oil partial oxidation in a fixed-bed reactor based on Cu/ZnO/Al2 O3 catalysts. The result suggests two possible mechanisms: one of them is a reaction between the adsorbed oxygen and methanol on the surface of Cu. At 180 °C and the ratio of oxygen and methanol concentration below 0.5, the main by-products are methyl formate and CO. Above 220 °C and at oxygen/methanol ratio above 0.5, the main by-product is formaldehyde. Another mechanism is that Cu is oxidized into Cu2 O by adsorbed O2 , which results in more formaldehyde as by-product.

3.3.1.4 Autothermal reforming for hydrogen production Autothermal reforming for hydrogen production couples partial oxidation and steam reforming reaction, which transfers bio-oil, oxygen, and steam into hydrogen as seen in eq. (3.25). Cn Hm Ok + O2 + H2 O → CO2 + H2 + CO (3.25) Theoretically, an external heat supply is no longer needed for this reaction as it reaches heat equilibrium between exothermal partial oxidation and endothermal steam reforming, releasing heat from oxidation reaction, which is then available for steam reforming. However, its hydrogen production yield is low, which limits further development. Vagia et al. [245] studied thermodynamics of autothermal reforming for hydrogen production from bio-oil and its model compounds. Each kmol of hydrogen production

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requires 0.245 kmol of bio-oil or 0.317 kmol of methane. Furthermore, bio-oil as a raw material for producing hydrogen needs more energy than methane during production. Khila et al. [246] analyzed energy and thermodynamics of ethanol steam reforming, partial oxidation, and autothermal reforming for hydrogen production. The ethanol consumptions of these three methods per kmol of hydrogen production are 0.24 kmol, 0.25 kmol, and 0.23 kmol, respectively.

3.3.1.5 Reactors for hydrogen production from bio-oil reforming The design of reactors is one of the key factors for bio-oil reforming. For thermosensitive bio-oil and the problem of carbon deposition, fixed-bed reactors are only available for laboratory investigation but not for industrial production, while fluidized-bed reactors and microreactors can guarantee continuous and stable operations. Remiro et al. [247] designed two sequential reactors with on-line monitoring for bio-oil steam reforming. The first reactor was used for separating carbon via lignin pyrolysis. The second reactor was a device for steam reforming.

3.3.2 Hydrogen production from methanol 3.3.2.1 Mechanism Methanol (CH3 OH) has a boiling point of 64.7 °C, a flash point of 470 °C, an octane number of 106, and a higher heating value of 22.7 MJ/Kg, which is merely half that of gasoline (47.3 MJ/Kg). Methanol steam reforming via copper-based catalysts for hydrogen production has been wildly investigated. The overall reaction consists of (1) methanol steam reforming, (2) methanol decomposition, and (3) water-gas shift. Methanol steam reforming reaction: CH3 OH + 3H2 O → 6H2 + 2CO2 , ∆H = +49 kJ/mol

(3.26)

Methanol decomposition reaction: CH3 OH → 2H2 + CO, ∆H = +91 kJ/mol

(3.27)

Water-gas shift reaction: CO + H2 O → H2 + CO2 , ∆H = −41 kJ/mol

(3.28)

The methanol steam reforming mechanism over copper-based catalysts has been widely discussed in numerous studies. Several mechanisms have been proposed; however, there is still a big argument about which one is the most accurate. Jiang et al. [248] suggested a mechanism based on the Langmuir–Hinshelwood expression

200 | 3 Technologies for biomass-based hydrogen production

on a single type of active sites. CH2 OH + 2∗ ↔ CH2 O∗ + H∗ CH3 O∗ +∗ ↔ CH2 O∗ + H∗ H2 O+∗ ↔ H2 O∗ H2 + 2∗ ↔ 2H∗ 2CH2 O∗ ↔ CH3 OCHO∗ +∗ CH3 OCHO∗ + H2 O∗ ↔ HCOOH∗ + CH3 OH∗ HCOOH∗ ↔ H2 + CO2

(3.29)

* Represents an adsorption site on the copper surface.

3.3.2.2 Hydrogen production methods Three processes can generate hydrogen from methanol: partial oxidation of methanol (POM), methanol steam reforming (MSR), and autothermal reforming of methanol (ATRM). A comparison among these three processes listed in Tab. 3.8 shows that hydrogen yield is maximal for steam reforming. Steam reforming has been commercialized and provides a small-scale hydrogen supply for many sectors. The partial oxidation process features short start-up time, no auxiliary heating devices, and oxygen supply from the air, which is consequently very convenient as a transportation application. However, higher CO concentration in product gases needs a complicated separated device. The autothermal reforming process avoids catalyst sintering and external heat supply via coupling steam reforming and partial oxidation. Tab. 3.8: Three processes for hydrogen production from methanol [249]. Method

Reaction

Characteristics

Steam reforming Partial oxidation Autothermal reforming

CH3 OH + H2 O → CO2 + 3H2 ΔH298 = 49.4 kJ/mol 2CH3 OH + O2 → 2CO2 + 4H2 ΔH298 = −192.2 kJ/mol CH3 OH + 1/8O2 + 3/4H2 O → CO2 + 11/4H2 ΔH298 = −11.5 kJ/mol

Inherently endothermic, highest hydrogen yield Fast exothermic reaction can lead to catalyst deactivation No additional heating devices, short start-up time, complicated system

Methanol steam reforming typically performs at 250–300 °C, 1–5 MPa, and a mole ratio of the steam and methanol of 1.0–5.0. Methanol steam reforming is one of the most likely hydrogen production methods for on-board fuel cell vehicles. Partial oxidation has the following advantages: exothermic reaction, short startup time, mild reaction conditions, and simple operation. The main shortcoming is that air is used to supply oxygen. Nitrogen in the air results in a hydrogen concentration in the product gases below 50 %, which cannot be tolerated by PEMFC due to slow

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kinetics of oxygen reduction at the cathode caused by dilutive effects of the inert contaminant nitrogen. Ideal autothermal reforming lowers the reaction enthalpy to zero, and the released heat from partial oxidation of methanol can accelerate the rate of the reforming reaction to quickly reach the desired reaction temperature and thus decreases start-up time.

3.3.2.3 Catalyst for methanol steam reforming Catalysts for methanol steam reforming may be divided into three main groups: noble metal, copper-based, and non-copper-based catalysts. Copper-based catalysts have improved low temperature activity for steam reforming compared to noble metals and can produce H2 and CO2 with high selectivity; therefore, they are widely used in the methanol steam reforming process.

3.3.2.3.1 Noble metal catalysts Noble metal catalysts for methanol steam reforming are mainly platinum- or palladium-based catalysts with high catalytic activity, selectivity, and stability. Platinumand palladium-based catalysts are mainly composed of Pt-Pd catalysts, with Al2 O3 , TiO2 , or SiO2 as a carrier, and with rare metals Ce or La as a promoter. The catalytic activity of Ni, Pd, Pt, Rh, and others on methanol steam reforming was studied and the results indicated that Pd/ZnO and Pt/ZnO have the highest catalytic activity which is closely related to the formaldehyde species formed during the reaction [250, 251].

3.3.2.3.2 Copper-based catalyst Copper-based catalysts can be divided into two-component and three-component catalysts, such as Cu-Cr, Cu-Cr-Mn, Cu-Zn-Al, Cu-Zn-Si, Cu-Zr and others. The performance of copper-based catalysts depends on the phase of copper with Cr, Mn, Zn, Fe, Al, Si, Ca, Co, Ni, and others as promoters (Tab. 3.9). Copper-based catalysts with high methanol conversion, good selectivity, and low cost have been developed; however, they easily deactivate due to high temperature sintering, coking, and poisoning, which decreases the lifetime of the catalyst. At present, the deactivation mechanism has been the research topic for hydrogen production from methanol steam reforming.

3.3.2.3.3 Non-copper-based catalysts Non-copper-based catalysts include nickel-based catalysts, Zn-Cr catalysts, and others. Nickel-based catalysts provide the advantages of good stability, wide temperature range, and good resistance to poisoning. During low-temperature methanol steam re-

202 | 3 Technologies for biomass-based hydrogen production

Tab. 3.9: Influence of the different promoters on the performance of Cu-based catalysts for MSR. Catalysts

Temperature ( °C)

Methanol conversion ( %)

Mole fraction of CO

CO selectivity ( %)

Cu/ZnO [252] Cu/ZnO/ZrO2 /Al2 O3 [252] Cu/Zn/Al2 O3 [253](commercial) Cu/ZnO/Al2 O3 [254](commercial) Cu/Cr/Al2 O3 [255] Cu/Zr/Al2 O3 [255] Cu-Mn-O [256] Cu-Ce-O [256] Cu/SiO2 ZnO/Cu/SiO2 [253]

260 260 300 270 300 300 240 240 300 300

75 92 57 59 63 44 99 37 50 75

0.0073 0.0010 – – 0.011 0.0075 – – – –

– – – – – – 3.1 0.8 0.15 0.6

forming, nickel-based catalysts have a lower activity and selectivity, and the reforming gases contain more CO and CH4 compared to copper-based catalysts. The higher CO selectivity is ascribed to the preferential adsorption of CH3 OH instead of CO [257]. With the development of noble and copper-based catalysts, nickel-based catalysts have little been applied for methanol steam reforming.

3.3.2.4 Methanol steam reforming reactor Hydrogen efficiency is not only influenced by catalysts, but also by the structure and size of the reactor, because the reactor design depends on heat transfer, mass transfer, and reaction kinetics.

3.3.2.4.1 Microchannel reactor The microchannel reactor is simple and compact in structure, small in size, light in weigh, large in specific surface area for reaction and in space utilization rate, and low in the amount of catalyst. The boundary layer of flowing fluid is thinner in the microchannel than in a conventional reactor. This thin layer significantly reduces diffusion resistance of components on the catalyst surface and reduces heat transfer resistance and time in the heat exchanger. Heat and mass transfer rate in the microchannel reactor increases by 2–3 orders of magnitude compared to the conventional-scale device. Dynamics analysis reveals that a thin layer of catalyst coated on the surface of the microchannel can improve internal diffusion and thermal conductivity. However, reforming gases need to be purified after the microchannel reactor reaction, and a higher manufacture cost is caused by its complicated structure.

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3.3.2.4.2 Plate reactor The plate reactor has a smaller size, which can reduce the heat transfer resistance. The modular design of the plate can be used for easy assembly of bigger reactors to reach the required scale for hydrogen production. The larger the scale of the reactor, the higher the system energy efficiency will be. Square-shaped configurations are beneficial for on-board hydrogen supply; however, it is difficult for the catalyst to be stably coated on the plate due to the possibility of the catalyst peeling from the plate at high temperature.

3.3.2.4.3 Membrane reactor Reforming gases need to be purified before they are fed into the fuel cell, because reforming gases include many types of by-product gases, such as CO, CO2 , CH4 , and others, except for H2 . A membrane reactor can separate hydrogen from product gases during steam reforming and consequently provide high-purity hydrogen, which in turn avoids CO poisoning of the fuel cell.

3.3.2.4.4 Tubular reactor The tubular reactor has a large ratio of length to diameter and no special internal structure; therefore, it is generally good for a continuous operation process. Simple construction, easy processing and manufacturing, as well as convenient refill and replacement of catalysts in the tubular reactor enables simple and fast catalyst screening and testing. Iulianelli et al. [258] reviewed the studies of reactors for hydrogen production via methanol steam reforming and made a comparison of reactors with different flow-field design as seen in Fig. 3.12.

(a)

(b)

(c)

(d)

Fig. 3.12: Different flow-field designs (a) coiled-serpentine, (b) parallel multichannel, (c) pin-hole, (d) radial.

3.3.2.5 Demonstration of the system The Dalian Institute of Chemical Physics, affiliated with the Chinese Academy of Sciences developed a hydrogen-generating system from methanol steam reforming for

204 | 3 Technologies for biomass-based hydrogen production

fuel cell vehicles. The methanol steam reforming reactor consists of a reforming room, a water-gas shift chamber, a heat exchange chamber, and two combustion chambers (Fig. 3.13) [259]. The reactor for 5 kW FCV achieved a cold start within 7 minutes without any external heat. The system showed a good stability during more than 1,600 h of continuous testing and an efficiency of up to 74 %. Thermocouples

Preheating chamber I

Reforming chamber Water gas shift reaction chamber

Methanol+Air

Methanol+H2O

Combustion chamber

Off gas

Preheating chamber II

Off gas Flow direction of combustion Flow direction of reforming (a)

(b)

Fig. 3.13: Methanol autothermal reforming plate reactor.

Shi et al. [260] investigated how feed rate, preheating temperature, and reaction temperature affect the reaction and reforming gases for hydrogen production from steam reforming of methanol in a fluidized-bed reactor as shown in Fig. 3.14. 3.3.2.6 Numerical simulation Mei et al. [261] developed a novel methanol steam reforming device with micro-pin-fin arrays. A three-dimensional numerical simulation model was utilized to investigate the heat and mass transfer inside the reactor. The rationality of the continuum model used in the microchannel was analyzed and a system of steam reforming numerical simulation equations was presented, which includes the continuity, momentum, energy, and species transport equations. The results show that methanol conversion in the micro-pin-fin reactor is higher than that in a microchannel reactor, and that the velocity and temperature distribution is more uniform in the reactor. Peng et al. [262] used the SIMPLE-C algorithm and the Arrhenius rate model to perform three-dimensional linear numerical simulation of methanol steam reforming to

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Condenser Voltalge regulator

Feed tank Pump Flowmeter

PID-temperature-controller FBR FM fan

Control box

Preheater

Fig. 3.14: The configuration of a fluidized-bed reactor.

investigate the influence of the physical structure of the reactor, inlet gas temperature, and wall temperature on the methanol conversion, CO and CO2 concentration.

3.3.3 Hydrogen production from ethanol From a technological and economical point of view, ethanol production from biomass will possess extensive application prospects. Bioethanol has been used to produce blended gasoline as fuel for internal combustion engines (ICE), which energy efficiency of ICEs with blended gasoline is lower than energy efficiency of 30 % or so with gasoline as fuel. Fuel cell vehicles are powered by hydrogen, which was produced from ethanol reforming, and the resulting energy efficiency of FCV can reach up to 60 %. Hydrogen production from ethanol has the following advantages: (1) the feedstock of ethanol is rich and can be made from renewable energy; (2) ethanol is liquid and is easy to store and transport at normal temperature and pressure; (3) the energy density of ethanol by weight is much higher than that of methanol and hydrogen. High efficient fuel cell vehicle prototypes based on the catalytic hydrogen production from ethanol have been developed.

3.3.3.1 Technologies of hydrogen production from ethanol Hydrogen production from ethanol mainly includes three pathways: steam reforming of ethanol, partial oxidation of ethanol, and autothermal reforming of ethanol. Hydrogen production from ethanol steam reforming is the most commonly used method.

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3.3.3.1.1 Hydrogen from ethanol steam reforming The main reactions during ethanol steam reforming are shown in Tab. 3.10. Tab. 3.10: Reaction equations in steam reforming of ethanol. Eq.-No.

Reaction

Equation

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17)

Steam Reforming Steam Reforming Dehydration Polymerization Ethylene reforming Pyrolysis Dehydrogenation Acetaldehyde carbonyl Acetaldehyde reforming Ketonization Acetone reforming CO methanation CO2 methanation Methane cracking CO disproportionation Water-gas shift Methane steam reforming

C2 H5 OH + 3H2 O → 2CO2 + 6H2 C2 H5 OH + H2 O → 2CO + 4H2 C2 H5 OH → C2 H4 + H2 O C2 H4 → coke C2 H4 + 2H2 O → 2CO + 4H2 C2 H5 OH → CO + CH4 + H2 C2 H5 OH → C2 H4 O + H2 C2 H4 O → CH4 + CO C2 H4 O + 3H2 O → 5H2 + 2CO2 2C2 H5 OH → C3 H6 O + CO + 3H2 C3 H6 O + 5H2 O → 8H2 + 3CO2 CO + 3H2 → CH4 + H2 O CO2 + 4H2 → CH4 + 2H2 O CH4 → 2H2 + C 2CO → CO2 + C CO + H2 O → CO2 + H2 CH4 + H2 O → CO + 3H2

kJ/mol 174.2 256.8 45.3 −52.4 49.0 68.4 −134.2

−205.9 −164.7 74.6 −171.5 −41.1 206.0

High temperatures are not only beneficial to promote the main reaction towards the right, but also to reduce the formation of by-products and to improve hydrogen selectivity. The best reaction temperatures range between 850 and 900 K. The ratio of H2 O to C2 H5 OH is higher, the main reaction more easily proceeds in favor of hydrogen, and carbon deposition is largely alleviated. Meanwhile, the lower the system pressure, the higher the ethanol and water conversion, and the higher the hydrogen yield. Therefore, high temperature, low pressure, and high water to ethanol ratio can improve H2 yield and selectivity with undesirable by-products, CO and CH4 , of which CH4 competes with H2 for hydrogen atoms and reduces H2 selectivity. The main impact factors on hydrogen production from ethanol steam reforming are operating conditions and catalyst types. To improve hydrogen yield, several methods have to be implemented to inhibit the production of by-products and to prevent the formation of carbon deposits, consequently improving the performance of the catalysts. Reforming reaction pathways of ethanol over different metal catalysts [263] are shown in Fig. 3.15 and it is generally accepted that: (1) Ethanol can be dehydrated over the acidic catalysts to form ethylene (Tab. 3.10(3)). Some ethylene undergoes polymerization to generate carbon deposition (Tab. 3.10(4)) which leads to catalyst deactivation, while the remaining ethylene

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quickly undergoes a reforming reaction and generates CO and H2 (Tab. 3.10(5)). CO can be converted to CO2 and H2 via water-gas shift reaction. (2) Ethanol can be dehydrogenated on the basic catalysts to generate acetaldehyde (Tab. 3.10(7)). The produced acetaldehyde will either undergo decarboxylation to produce CH4 and CO, or a condensation reaction (Tab. 3.10(10)) to produce acetone, or a reforming reaction to produce hydrogen (Tab. 3.10(9)). A water-gas shift reaction (Tab. 3.10(16)) and steam reforming of CH4 (eq. (Tab. 3.10(17)) will occur simultaneously. Partial CO under the condition of enriched oxygen can also be directly oxygenated to generate CO2 . (3) Due to the presence of water vapor and hydrogen, the WGS and its reverse reaction have a big influence on the selectivity of hydrogen that might occur among the whole temperature range. (4) Carbon deposition is mainly produced via eqs. 3.10(4), (14), and (15). As previously mentioned, high temperatures (> 773 K) are beneficial for hydrogen production; therefore, hydrogen production from ethanol is typically performed under high temperatures. However, high temperatures are beneficial for CO production and lead to high CO content. To meet the requirements of a fuel cell, it is necessary to employ water-gas shift (WGS), preferential oxidation (PROX), or pressure swing adsorption to reduce CO content, which increases the cost of hydrogen production and reaction system volume, and decreases the thermal efficiency. In recent years, progress on hydrogen production from ethanol at low temperatures (300–400 °C) has been made as ethanol reforming through the appropriate catalyst and reaction conditions can realize near-zero CO content in exhaust gas [264]. However, low temperatures contribute to the generation of CH4 and carbon deposition, which reduces the H2 selectivity and life expectancy of catalyst. Therefore, the development of a low-temperature catalyst is a persistent research focus for hydrogen production from ethanol.

3.3.3.1.2 Hydrogen from partial oxidation of ethanol (POE) Partial oxidation is the redox reaction without sufficient oxygen and causes the carbon hydrogen bonds of ethanol to break up. Partial oxidation is the exothermic reaction that can proceed without external heating; however, high-cost pure oxygen need to be supplied to meet the reaction. The hydrogen content will drop in the product gases from the partial oxidation reaction since air is supplied instead of pure oxygen to oxidize the ethanol which complicates the purification. The main reactions during the partial oxidation process are shown in Tab. 3.11. There is a small amount of by-products such as CH3 CHO, CH3 COOH, and CH3 OCH3 in addition to H2 O and CH4 produced from the partial oxidation of ethanol at low temperatures. At high temperatures, fixed carbon will be produced and severe carbon deposition on the catalyst surface will result in the deactivation of the catalyst.

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H

H

O

C

C

OH H

O

H

C

C

C

H Metal catalyst

Dissociative Adsorption

H

H

H

H

C Decomposition

H2O

Coking Metal catalyst

Metal catalyst

H2O

H

H

H2O Steam reforming

C

H

H

OH

Water-gas-shift

H2, CO

Metal catalyst

Metal catalyst

Dehydrogenation

Decomposition Metal catalyst

Metal catalyst

C

Steam reforming

H

Metal catalyst

Methanation Metal catalyst

H2O

H2O

H2

Decomposition

CH4, C2H4, H2, H2O, CO2 H2O

C Polymerization Metal catalyst

CO H

O

C

C

Water-gas-shift Metal catalyst

Decomposition Metal catalyst

H

H2

Metal catalyst Dehydration Metal catalyst

H

H2, CO2

H

Steam reforming Metal catalyst

H2O

Fig. 3.15: Reaction pathways for hydrogen production from ethanol steam reforming via different metal catalysts.

Haryanto et al. [263] found that POE with different catalysts had different reaction pathways that depended on the interfacial interaction of catalyst and ethanol. In general, two types of the following reaction mechanisms on POE have been proposed: The first one is the dehydrogenation–decomposition–oxidation mechanism. Ethanol adsorbs on the metals Rh and Ni in the form of ethoxy and metal-acetaldehyde intermediates. Then, acetaldehyde will continue to form the acetate phase with oxygen on the surface. The C–C of the intermediates is easy to break up and the intermediates decompose to form CH4 and CO2 . Furthermore, acetaldehyde will also directly crack to generate CH4 and CO. The generated CO can be converted into CO2 and H2 by a water-gas shift reaction, and the CH4 can be converted into carbon oxide

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Tab. 3.11: Reactions of hydrogen production from partial oxidation of ethanol. Eq.-No.

Reaction

Equation

kJ/mol

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)

Combustion Partial oxidation Dehydrogenation Intramolecular dehydration Intermolecular dehydration Thermal decomposition Thermal decomposition Methanation Methanation Ethylene polymerization carbide Methane decomposition Boudouard Water-gas shift

C2 H5 OH + 3/2O2 → 2CO2 + 3H2 C2 H5 OH + 1/2O2 → 2CO + 3H2 C2 H5 OH → C2 H4 O + H2 C2 H5 OH → C2 H4 + H2 O 2C2 H5 OH → C2 H5 OC2 H5 + H2 O C2 H5 OH → CO + CH4 + H2 C2 H5 OH → 1/2CO2 + 3/2CH4 CO + 3H2 → CH4 + H2 O CO2 + 4H2 → CH4 + 2H2 O C2 H4 → polymerization → 2C + 2H2 CH4 → C + 2H2 2CO → CO2 + C CO + H2 O → CO2 + H2

−557.2 54.6 68.4 45.3 −24.0 49.0 −74.9 −205.9 −164.7 −52.4 74.6 −171.5 −41.1

and H2 by a reforming reaction. Of course, some CO will also be directly oxidized into CO2 on the oxygen-enriched surface. The second one is the dehydration–decomposition–oxidation mechanism. Ethanol is dehydrated to produce hydrogen and ethylene. Some ethylene reforms rapidly to form CO and H2 . The CO was oxidized into CO2 . Ethylene can be converted into acetylene by a dehydrogenation reaction and also potentially form carbon deposits via polymerization-carbonization interaction. Fixed carbon can easily become CO in the presence of O2 . 3.3.3.1.3 Autothermal reforming of ethanol Autothermal reforming of ethanol means steam reforming of ethanol under oxidizing atmosphere, where endothermic steam reforming and exothermic partial oxidation both proceed. The whole reaction can reach a heat balance by controlling the temperature, the amount of water and oxygen. Advantages of the process include high efficiency, energy self-sufficiency, and mild reaction conditions. CH3 CH2 OH + 2H2 O + 1/2O2 → 5H2 + 2CO2 CH3 CH2 OH + H2 O + O2 → 4H2 + 2CO2

∆H298 °C = −68.5 kJ/mol

∆H298 °C = −266.3 kJ/mol

(3.30) (3.31)

Thermodynamic analysis of hydrogen production via autothermal reforming of ethanol [265] shows that decomposition reactions of ethanol mainly happen under the condition of low temperature and low water/alcohol ratio, while high temperature and high water/alcohol ratio are beneficial to the ethanol steam reforming reaction. Different temperatures cause different by-products and increasing the amount of oxygen can lower the production of CO and CH4 . Temperatures between 500 and 1300 K,

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pressures between 0.1 and 0.5 MPa, a water/alcohol ratio of 6, and an O2 /C2 H5 OH of 0.9 are ideal preconditions for hydrogen production via autothermal reforming reaction.

3.3.3.2 Catalysts Catalysts for ethanol steam reforming can mainly be divided into oxide catalysts, Ni catalysts, Co catalysts, and noble metal catalysts according to the mainly active components of a catalyst. Ni-based catalysts for ethanol steam reforming have a high activity and good selectivity that is attracting widespread attention. Ni catalysts mainly include Ni/Al2 O3 , Ni/MgO, Ni/La2 O3 , Ni/Y2 O3 , Ni/CeO2 , Ni/CeO2 -ZrO2 , Ni/CeO2 -TiO2 , Ni/ZrO2 -TiO2 , and many others. Nickel can promote breakage of the C–C bond of ethanol, which can reduce the selectivity of acetone, acetic acid, and acetaldehyde by-products, decompose condensation products, and improve the selectivity of hydrogen. Furthermore, Ni also has a high activity at low temperatures and has some activity for methane reforming and water-gas shift reaction. However, nickel also has disadvantages for ethanol steam reforming: (1) Nickel is an excellent methanation catalyst that promotes a methanation reaction between H2 and CO or CO2 during ethanol reforming, which may drop off hydrogen yield and selectivity; (2) Nickel is also likely to accelerate methane and other intermediates to generate carbon deposition, which can lead to catalyst deactivation; (3) At high temperatures, especially in the presence of water vapor, nickel is very prone to sintering, which can result in an activity decline in surface area and permanent catalyst deactivation. Therefore, how to improve the selectivity and carbon deposition resistance of a Ni-based catalyst while dropping the reaction temperature is a major trend for improvement of catalyst. Based on the similarity of methanol and ethanol steam reforming, Cu can be used in ethanol steam reforming for hydrogen production. However, Cu is not an ideal active component due to carbon deposition, more by-products, and poor stability during reactions. Co-based catalysts show high selectivity, but present severe carbon deposition due to high-temperature reaction. Ni-, Cu-, and Co-based catalysts usually need the addition of some second component to improve catalytic properties. Therefore, seeking suitable additives has become a key research direction of the catalyst for hydrogen production from ethanol. For noble metals, Rh, Pt, and Pd can widely be selected as active components, and sequential activity and selectivity for hydrogen production from alcohol is Rh > Pt > Pd > Ru [266]. Currently, noble metals are mainly utilized for hydrogen production from ethanol under low temperatures. In the presence of Pt/Ce0.8 Zr0.2 O2 , Rh-FeCa/Al2 O3 , and Pt/CeO2 catalysts, CO selectivity is below 1.5 % [267], 0 % [268], and 0 % [269] at 400 °C, respectively.

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3.3.3.3 Catalyst support The supports of catalysts play an important role during catalytic reactions. The supports not only affect dispersion of active components, but also improve the activity of the catalyst via interaction with active components. Studies on the influence of different supports on the catalysts show that the sequential selectivity of supports is ZnO ≈ La2 O3 > CeO2 > MgO > γ-Al2 O3 > TiO2 > ZrO2 > silica gel > diatomaceous earth under 650 °C and 101.3 kPa [270] (Tab. 3.12). Tab. 3.12: Performance comparison of hydrogen production from ethanol steam reforming by different catalysts. Catalysts

Surface area Ethanol (m2 /g) conversion ( %)

Ni/ZnO 7.8 Ni/La2 O3 7.2 Ni/CeO2 7.4 Ni/MgO 24.0 Ni/γ-Al2 O3 113.0 Ni/TiO2 94.0 Ni/ZnO 18.4 Ni/mSiO2 · nH2 O 239.0 Ni/SiO2 44.0

≈ 100 ≈ 100 99.9 ≈ 100 ≈ 100 99.7 ≈ 100 99.7 ≈ 100

Hydrogen selectivity ( %)

Product concentration ( %)

89.1 89.3 87.0 82.2 78.2 72.2 65.0 63.2 50.9

5.6 2.0 26.3 7.8 2.0 31.4 0.14 0.87 4.6 2.0 16.1 B0.15 2.7 1.9 18.0 10.0 5.0 23.3 6.9 6.3 10.9 0.81 0.17 1.10 11.7 13.7 25.0 10.5 2.9 26.6 4.10 0.28 1.23 23.3 11.6 14.8

CO

CH4

CO2 C2 H4 C2 H4 O C3 H6 O

Note: the reaction proceeds at temperatures of 650 °C, pressure of 1 atm, and a liquid space velocity of 5 h−1 .

3.3.3.3.1 Al2 O3 support The carbon deposition on the γ-Al2 O3 support is severe. The possible reason is that the incomplete coordination of aluminum atoms on the γ-Al2 O3 surface produces an L acid site, which promotes the ethanol dehydration to conversion into ethylene. Furthermore, the ethylene forms carbon deposition by polymerization. Carbon deposition will not only shorten the life of the catalyst, but also attenuate hydrogen selectivity.

3.3.3.3.2 Rare earth oxide support Rare earth metal oxide is a good ingredient to adjust acid and alkali of the catalyst. Rare earth metal oxide has moderate acidic and alkaline properties that can effectively inhibit carbon deposition, present the ability of rapid oxygen exchange, and help to adjust the acid center of active components. Rare earth metal oxides release lattice oxygen, which promotes CO oxidation, water-gas shift, and steam reforming of CH4 . In the process of ethanol reforming for hydrogen production, catalysts supported on rare earth metal oxides show high selectivity and stability. Experimental results on hydrogen production from ethanol reforming over catalyst Ni/Y2 O3 [271] shows that

212 | 3 Technologies for biomass-based hydrogen production

the conversion rate of ethanol achieved 81.9 % at a low temperature of 250 °C, while the conversion rate of ethanol reached 95.3 % at 320 °C with a selectivity of hydrogen of 53.6 %. However, when SrCeO3 was used as the support, hydrogen selectivity reached a maximum of 87.3 % [272].

3.3.3.4 Catalyst promoter At present, alkali metals, alkaline earth metals, and rare earth oxides as promoter attract the most attention. Alkali metals (Li, Na, and K) are added to weaken catalyst acidity and to reduce carbon deposition. Doped K can neutralize acid on the surface of Al2 O3 support, lower Al2 O3 performance for the dehydration of ethanol, reduce ethylene production and control carbon deposition, which moves the reaction toward gasification [273]. Furthermore, addition of alkali metals will also change the number of d-band holes of Ni and the electronic work function, and improve interface interaction between the metal and the intermediate product. In addition, the alkali metals covered on acidic support have some influence on the physical properties of the catalysts via steric effects. Addition of alkaline earth metals (Ca and Mg) mainly modulates support to become alkaline, which can significantly increase the stability and carbon deposition resistance of the catalyst. A study found that MgO and Al2 O3 supports were prone to produce an MgAl2 O4 spinel structure, which significantly weakened the Al2 O3 performance for dehydration of ethanol and reduced ethylene production. Thus, MgAl2 O4 spinel structure improves carbon deposition resistance and increases stability [274]. The electronic configuration of rare earth elements is unique and can form complexation precursors with multiple ligands. Rare earth elements can form reversible reaction activators that generally have an activity of alcohol dehydrogenation. Rare earth metals are ideal catalytic and assistant catalyst materials. With γ-Al2 O3 as the support, and Ce and Nd oxide as the additive, catalytic activity of catalysts with rare earth metal can be significantly improved.

3.3.4 Hydrogen production from dimethyl ether Dimethyl ether (DME, CH3 OCH3 ) is a type of colorless gas and is easy to condense and gasify with a boiling point of −24.9 °C. Furthermore, it can be dissolved in water and some organic solvents such as ethanol, acetone, and others. Below 0.5 MPa, it can be compressed from gas to liquid. Moreover, DME is a type of ideal energy carrier due to its high hydrogen content and other advantages, such as nontoxicity, easy compression and transportation capabilities, as well as being environment-friendly.

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3.3.4.1 DME reforming for hydrogen production The methods that transfer dimethyl ether to hydrogen include steam reforming, partial oxidation, and autothermal reforming.

3.3.4.1.1 Steam reforming for hydrogen production The reaction of DME steam reforming includes two main steps: Hydrolysis from DME to methanol: CH3 OCH3 + H2 O → 2CH3 OH,

∆H = +37 kJ/mol

(3.32)

Steam reforming from methanol to hydrogen: CH3 OH + 3H2 O → 6H2 + 2CO2 ,

∆H = +49 kJ/mol

(3.33)

∆H = +135 kJ/mol

(3.34)

Total equation: CH3 OCH3 + 3H2 O → 6H2 + 2CO2 ,

The product gases of DME steam reforming contain high-concentration hydrogen, which is fit to supply fuel cells. However, this reaction is endothermal and needs external heating. Rising reaction temperatures contribute to the conversion of DME; however, it also accelerates the rate of side reactions such as the reverse water-gas reaction that lead to an increase in CO concentration, so the product gases are difficult to be directly supplied to proton exchange membrane fuel cells (PEMFC) as hydrogen source. Increasing the ratio of steam and DME can substantially decrease CO concentration; however, it increases energy consumption. Therefore, to reduce energy consumption and to drop off the rate of CO production, catalysts need to be selected for DME reforming at the low reaction temperature.

3.3.4.1.2 Partial oxidation Partial oxidation occurs and produces a hydrogen-rich syngas with CO and hydrogen, when a substoichiometric air-DME mixture is partially combusted. The equation of DME partial oxidation is showed below, CH3 OCH3 + 1/2 O2 → 2CO + 2H2 ,

∆H = −25 kJ/mol

(3.35)

This is an exothermal reaction and does not need an external heating supply. Some “hot spots” can be easily caused in the catalyst layer, which may lead to deactivation due to excessive reacting rate and heat release of DME partial oxidation. Excess nitrogen with oxidizing agents of air leads to a further reduction of the hydrogen.

3.3.4.1.3 Autothermal reforming for hydrogen production Autothermal reforming couples partial oxidation with steam reforming and improves the energy efficiency of hydrogen production except solving the problem of “hot spots” in the layer of catalysts. The technology aims at supplying hydrogen for fuel cell pow-

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ered vehicles. However, some problems still remain, such as the complication of reacting systems, the difficultly of accurate control of oxygen or steam and DME ratio.

3.3.4.2 Catalysts for hydrogen production from steam reforming The required catalysts should be bifunctional because the reaction of DME steam reforming is a two-step consecutive reaction including DME hydrolysis and methanol steam reforming. The components of the catalyst for methanol steam reforming have been introduced in Section 3.3.2. Catalysts used for DME hydrolysis will be introduced in this section. The active ingredients need to be utilized for every step of the consecutive reactions. Based on temperature match for the best activity, both some solid acid catalysts for DME hydrolysis and metal catalysts for DME steam reforming with good low temperature performance of dehydration are screened out in order to effectively execute the synergistic catalytic effect both on active centers of DME hydrolysis and methanol reforming. The bifunctional catalysts with high catalytic activity is prepared based on this rule. In general, this type of bifunctional catalyst contains two types of active ingredients. One of active ingredients is a solid acid catalyst that is conducive to DME hydrolysis. The second ingredient is either a metal or metal oxide catalyst, which promotes methanol reforming for hydrogen production. The reaction of DME hydrolysis is a process of thermodynamic equilibrium limit and is a ratedetermining step of the total reaction [15]. DME can be hydrolyzed with solid acid catalysts such as γ-Al2 O3 , HZSM, ZrO2 , or other molecular sieves. The choice of acid carrier can affect the rate of DME hydrolysis. Faungnawakij et al. [275, 276] studied the DME hydrolysis performance of solid acid catalysts, such as γ-Al2 O3 , H-ZSM5, H-mordenite, and TiO2 and found that H-ZSM5 and H-mordenite with strong acid spots has higher DME conversion as the hydrolytic equilibrium was reached under low temperatures of 200–250 °C as shown in Tab. 3.13. However, when the temperature is higher than 300 °C, methanol yield decreases due to the inactivation of a Tab. 3.13: Influence of different types of solid-acid catalysts on DME hydrolysis [83, 84]. Catalyst

H-mordenite ZSM-5 PBT(Al2 O3 ) ALO8 (Al2 O3 ) P25(TiO2 ) CuFe2 O4 Hydrolysis equilibrium

DME conversion ( %) 200 °C

250 °C

275 °C

300 °C

350 °C

400 °C

17 12 0 0 0 0 15

23 17 4 2 0 0 19

25 21 11 5 0 0 21

30 25 21 17 0 0 23

95 97 31 30 1 1 26

98 98 38 36 5 3 30

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catalyst caused by carbon deposition. The conversion rate through γ-Al2 O3 with some acid amount and intensities exceeds thermodynamic equilibrium conversion rate at 300 °C. Nevertheless, TiO2 and CuFe2 O4 have little effect on the hydrolytic reaction. H-ZSM5, a solid acid catalyst for hydrogen production from DME steam reforming, can be modified to become a bifunctional catalyst via mixing with several copperbased catalysts. Chang-Feng Yan and his team synthesized a series of bifunctional catalysts, CuZnAlX/HZSM-5 (X = Cr, Ce, Zr, Co) using the coprecipitation method [277]. The addition of the co-catalyst, Cr can reduce average pore diameter of the catalyst decrease reduction temperature, and significantly improve the performance at low temperatures. At 250 °C, the DME conversion rate can reach 99 % while hydrogen production rate is up to 95 %. Under calcination temperatures of 400 °C, some CuO particles transfer to the spinel phase, which disperses the CuO particles. Dispersed CuO particles can easily be reduced and can enhance the activity of catalysts. Al2 O3 is mainly used as the carrier for DME steam reforming at high temperatures. Chang-Feng Yan and his team also synthesized a series of bifunctional catalysts, CuX/γ-Al2 O3 (X = Fe, Co, Ni, Cr, Zn) via microwave irradiation [278]. The result indicates that the co-catalyst, Fe plays an important role in the high dispersity of CuO on the surface of catalysts, in the enhancement of catalyst activity, and in the reduction of the CO content in product gases. Moreover, a bifunctional catalyst, Cu/ZnO/Al2 O3 /Cr2 O3 + HZSM, was prepared via coprecipitation coupled with mechanical mixing, and effect of calcination temperature on its physicochemical properties and catalytic performance affected by calcination temperature have been investigated. Faungnawakij et al. [275, 276] studied the CuFe2 O4 coupling with molecular sieve and solid acid catalysts and found that γ-Al2 O3 catalysts with Lewis acid performed well under 300–450 °C. Molecular sieve solid acid catalysts including Brønsted acid have a good performance under 200–275 °C, and short-chain alkenes such as C3 H8 , iC4 H10 , and n-C4 H10 would generate under temperatures above 275 °C. Further studies indicated that DME hydrolysis in different temperature ranges is dominated by different acids. Lewis acids dominate a process of hydrolysis above 275 °C while Brønsted acids dominate the process at temperatures between 200 and 275 °C. Semelsberger et al. [279] studied the activity of several molecular sieve catalysts, including molecular sieve Y (Si/Al = 2.5, 15) and ZSM-5 (Si/Al = 15, 25, 40, 140) with different mole ratios of silicon and aluminum, ZrO2 , γ-Al2 O3 , and nonacidic solid silicon for DME hydrolysis. The results suggest the order of catalytic activities as HZSM-5(15) > HZSM-5(25) > HZSM-5(45) > HZSM-5(140), Y(15) > Y(2.5) ≫ γ-Al2 O3 > ZrO2 > nonacidic solid silicon. Moreover, only ZrO2 failed to enable DME hydrolysis reaching the reaction equilibrium and nonacidic solid silicon has no catalytic activity. The addition of a Brønsted acid is the leading factor for DME hydrolysis and the activity increases with increasing Brønsted acid amount. When the commercial catalyst, Cu/ZnO/Al2 O3 is mixed with HZSM-5, ZrO2 , and γ-Al2 O3 , and the activity of mixture catalyst, Cu/ZnO/Al2 O3 + HZSM-5 was found to be the highest among all, with

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a hydrogen production rate of up to 90 % at 275 °C. Thus, catalysts with ZSM-5 have great potential in low-temperature reforming for hydrogen production.

3.3.4.3 Reactors and their performance for DME steam reforming Yan and his team have designed a series of microreactor systems (Fig. 3.16) for DME steam reforming and CO selective oxidation via CuZnAlCr/HZSM-5 catalysts [280]. The preparation of bifunctional catalysts, the ratio of steam and DME, space velocity, and the reaction temperature that affects the hydrogen production yield were investigated. The hydrogen production yield could reach approximately 90 % and the CO concentration was below 10 ppm, which meets the requirements of proton exchange membrane fuel cells (PEMFC).

Fig. 3.16: A metal foam microreactor for DME steam reforming.

3.3.4.4 Numerical simulation The recent research on DME steam reforming mainly focused on the preparation of catalysts; however, the research related to the mass and heat transfer process and reaction process does not go far enough and numerical simulation has rarely been reported. Yan Chang-Feng et al. [281] developed the kinetic equations of hydrogen production from DME steam reforming, which consist of the kinetic equations of DME hydrolysis, methanol steam reforming, methanol decomposition, and water-gas shift reactions. A steady, two-dimension numerical model was developed, where the catalyst layer was considered as porous medium and reactions proceeded, coupling four processes of fluid flow, mass and heat transfer, and chemical reactions. The kinetic equations were programmed and then loaded into the FLUENT program via User-Defined Function (UDF) and the equations were solved via SIMPLE algorithm and finite volume method. The experimental results and simulation results match well with a maximum error of below 7 % (Fig. 3.17). The numerical simulation result indicates that DME has a higher conversion rate in microreactors compared to fixed-bed reactors. Furthermore, the flow field, temperature field, and reactions distributed are more uniform in the microreactor than in the fixed-bed reactor. Simulation results show that DME reform-

3.4 Biological hydrogen production

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217

DME conversion (%)

100

H2 yield (%)

80 60 40 20 0 220

(a)

Temperature (°C)

240

Fixed bed (5/1, experiment) Micro reactor (5/1, experiment) Experiment by Feng et al. [7] Experiment by Wang et al. [23] Fixed bed (3/1, simulation) Fixed bed (3.5/1, simulation) Fixed bed (5/1, simulation) Micro reactor (5/1, simulation)

260

280

300

Temperature (°C)

(b)

Fixed bed (5/1, experiment) Micro reactor (5/1, experiment) Experiment by Feng et al. [7] Fixed bed (5/1, simulation) Micro reactor (5/1, simulation) Fixed bed (3.5/1, simulation)

Fig. 3.17: Comparison between experimental and simulating results.

ing has been improved when the reaction temperature reaches 270 °C and the ratio of water and DME is 5. Li et al. [282] carried out a numerical simulation of DME steam reforming by developing a steady, two-dimensional, and laminar model to investigate the temperature gradient and hydrogen composition in the fixed-bed reactor. Porous media is used for simulating the catalyst layer and the equations are solved by SIMPLE algorithm and finite volume method. The simulation results matched the experimental results well, indicating that the upstream temperature of the microreactor drops distinctly, while the outlet temperature remains relatively constant. When the diameter and length of the reactor increase, the hydrogen concentration at the outlet decreases.

3.4 Biological hydrogen production 3.4.1 Introduction Biological hydrogen production is a process that hydrogen producing microorganisms use to produce hydrogen either via light energy utilization or a fermentative approach, which is dependent on organic compounds as substrates for biochemical reactions of hydrogen producing bacteria. The phenomenon of biological hydrogen production has been first described more than 100 years ago, and was first published by Lewis in 1966. Practicability and feasibility of biological hydrogen production obtained great

218 | 3 Technologies for biomass-based hydrogen production

attention during the global energy crisis of the 1970s. The public has an in-depth understanding of environmental problems (air pollution, global climate changes, etc.) that have been caused by overutilizing fossil energy until the 1990s. From that period onwards, the public all over the world is paying much more attention to hydrogen production technologies. The simple index of the worldwide biggest abstracts and papers database – Scopus – reveals that the quantity of studies on biohydrogen technology has grown significantly over the past 15 years. Biohydrogen is produced by the metabolism of microorganisms. The reaction is a potentially carbon neutral process that is conducted at lower temperature and ambient pressure, which is therefore less energy intensive than thermochemical and electrochemical processes [283]. In addition, unlike chemical methods, which involve the conversion of nonrenewable fossil fuels into hydrogen, a large amount of renewable carbohydrate-based substrates (such as biomass, urban refuse, organic wastewater, and agricultural and forest residues) can be utilized for biohydrogen production. Hence, biological hydrogen production becomes one of the main developmental directions of hydrogen production since it offers the possibility of generating H2 that is renewable and carbon neutral, which has the dual efficacy of waste resource utilization and energy production. Biohydrogen production can be divided into two biological processes: lightdependent reactions and fermentative biological reactions, such as direct biophotolysis (green algae), indirect biophotolysis (cyanobacteria), dark fermentation (fermentative bacteria), and photofermentation (photosynthetic bacteria). Many attempts have been made to improve the rates and yields of biohydrogen production, and several advantages and disadvantages of each biohydrogen production process and their production characteristics are listed in Tab. 3.14.

3.4.2 Direct biophotolysis Direct biophotolysis is associated with plant-type photosynthesis processes, formerly known as green algae, that utilize light and CO2 as the sole source of energy to split water for hydrogen formation under anaerobic conditions [287]. This process can directly produce hydrogen from water using a microalgae photosynthesis system to convert solar energy into chemical energy in the form of hydrogen, which is sustainable and environmentally friendly to produce clean energy from renewable resources. The reaction is as follows: photosynthesis

2H2 O 󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀󳨀→ 2H2 + O2 solar energy

(3.36)

In the direct biophotolysis of green algae, the photosynthetic organ captures photons and produces activation energy to split water molecules. Subsequently, the negative redox potential reductant is formed, which can be further utilized in a reduction of

Large amount of by-products; low COD removal; reactor to reactor variation

Low efficiency hydrogen production by nitrogenase; low photosynthetic conversion efficiencies; need for inexpensive photobioreactors; large surface areas required

Produces oxygen, and destroys the hydrogen evolving catalyst (hydrogenase); low photosynthetic conversion efficiencies; potentially explosive gas mixtures formed; large surface areas required; need for inexpensive photobioreactors; low volumetric rates of production.

Disadvantages

Metabolic engineering that aims at overcoming metabolic limitations; two-stage systems that have higher yield, and more COD removal

Strain improvement through metabolic engineering; replacement of N2ase with H2ase; creation of antenna mutants

Creation of antenna mutants; immobilization technology; creation of an oxygen resistant hydrogenase; materials science breakthrough

Future prospects

a. Sulfur-deprived green algae and cyanobacteria b. Conversion of total incident light energy to hydrogen at full solar power c. References [284] and [285] d. At low light intensities e. Conversion of substrate (organic acid) to hydrogen, does not account for light energy used f. Four mol of hydrogen per mole of glucose equivalent, theoretically, 12 mol are available. There appears to be an inverse relationship between hydrogen production rates and yields, so the high rate reactors that have high volumetric rates [286] have significantly lower yields

A variety of waste streams can be used; easy operating, and sterility is not necessary; immobilized mixed cultures technology will achieve high production rate

33f

10–15 × 103

Dark fermentation

Many waste materials can be utilized; nearly complete substrate conversion; dark fermentation effluents can also be used

≤ 1d, 80e

12–83c

Photofermentation

Inexhaustible substrate (water); carbon independent pathway; simple products, hydrogen and oxygen

≤ 0.1b

2.5–13a

Biophotolysis

Advantages

Yields ( %)

Production rates (mlsH2 /l/h)

Process

Tab. 3.14: Different biohydrogen production processes.

3.4 Biological hydrogen production |

219

220 | 3 Technologies for biomass-based hydrogen production proton (H+ ) in hydrogenase that then combines with electrons released from the surrounding environment. This process is shown in Fig. 3.18 [288]. H2 Fdox Fdred

H2aes or N2aes

H+

ATP

ADP

PSI

e–

PSII H2O

O2

Fig. 3.18: Direct biophotolysis of green algae. PS I: photosynthetic reaction center I, PS II: photosynthetic reaction center II, Fdox : oxidation state ferredoxin, Fdred : reduce state ferredoxin, e− : electron, H+ : proton, ATP: adenosine triphosphate, ADP: adenosine diphosphate, H2 ase: hydrogenase, and N2 ase: nitrogenase

The process of direct biophotolysis of green algae has a major advantage: even in low light intensities, the efficiency for solar energy utilization remains high, almost 22 % when using hydrogen as an electron donor in the process of fixation of CO2 under anaerobic conditions. However, there is still a big challenge for this method, namely, that the hydrogenase is sensitive to oxygen; therefore, further research is needed to overcome this limitation.

3.4.3 Indirect biophotolysis Indirect biophotolysis is a biological process during which cyanobacteria produce hydrogen from water via their photosynthesis system to convert solar energy into chemical energy in the form of hydrogen. Cyanobacteria are aerobic phototrophic bacteria that have two distinct groups. Most of them produce hydrogen via nitrogenase catalysis, others do so via hydrogenase catalysis. The indirect biophotolysis hydrogen production system contains several stages: (1) biomass cultivation via photosynthesis; (2) biomass concentration; (3) dark anaerobic fermentation by cyanobacteria, producing a small amount of H2 and small molecule organic acids. Here, the reaction principle and effect in this stage are similar to that utilized by fermentative bacteria, theoretically, 4 mol hydrogen and 2 mol acetate are produced from 1 mol glucose; (4) shift

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the by-product of dark fermentation stage into photobioreactor, acetate is thoroughly decomposed into H2 via the light aerobic fermentation stage of cyanobacteria (similar to photosynthetic bacteria) [289]. The reaction of the above step is generally listed as follows: 6H2 O + 6CO2 + light → C6 H12 O6 + 6O2 (3.37) C6 H22 O6 + 2H2 O → 4H2 + 2CH2 COOH + 2CO2

(3.38)

2CH2 COOH + 4H2 O + light → 8H2 + 4CO2

(3.39)

The overall reaction is, 12H2 O + light → 12H2 + 6O2

(3.40)

As shown above, the photosynthesis process cycles through several stages. Fig. 3.19 demonstrates the schematic process of electron flow during photosynthesis [290].

CO2

ATP

ADP + PI

CH2O

Glycolytic Acetyl CoA NAD+

NADP NADPH

NADH

FNR

2H+ Qxygenic photosynthesis

Calvin

PSII

PSI

Fdred

ATP

Fdox

PSI

Fdred

H2aes

2H+ 2e– O2

H+

2H2O

e–

N2aes

e–

H+ H2

BacPS

H+

Q pool

Cyt e– External e– donor e– Cyt BacPS

Reserve e– flow ATP

H+

Fdox N2 NH3

Cyanobacteria system

e– N2aes

ATP

H+

NAD+ NADH Fdox

FeS

Fdred

Anoxygenic Photosynthesis

Fig. 3.19: The schematic process of electron flow in oxygenic and anoxygenic photosynthesis: PI: phosphoric acid, Calvin: Calvin cycle, Acetyl CoA: acetyl coenzyme A, NADP- coenzyme II, NADPHreduced coenzyme II, H2 ase: hydrogenase, N2 ase: nitrogenase, NAD+ : oxidation state nicotinamide adenine dinucleotide, NADH: reduced state nicotinamide adenine dinucleotide, ATP: adenosine triphosphate, ADP: adenosine diphosphate, Fdox : oxidation state ferredoxin, Fdred : reduce state ferredoxin, e− : electron,H+ : proton, PS I: photosynthetic reaction center I, PS II: photosynthetic reaction center II, FNR: flavoprotein-NADP+reductase, BacPS: photosynthetic bacteria center, Q pool: quinone pool, Cyt: cytochrome, FeS: ferrous sulfide protein

222 | 3 Technologies for biomass-based hydrogen production

Carbohydrates are oxidized to produce hydrogen in indirect photolysis, and the separation phase O2 and hydrogen production in cyanobacteria are conducted in different stages or spaces, to overcome the inhibition effect of oxygen on hydrogenase and realize a continuous run (Fig. 3.19).

3.4.4 Photofermentation Photofermentation is a fermentative conversion of organic substrates into hydrogen and CO2 by a diverse group of photosynthetic bacteria (PSB) that utilize light as energy. There are several types of PSB such as Rhodobacter sphaeroides, Rhodopseudomonas palustris, Rhodobacter capsulatus, and Rhodospirillum rubrum. PSB utilize organic compounds as a hydrogen donor and carbon source for photosynthesis; using light as energy, the metabolic type will change with changes in operation conditions. Via small-molecule organic acids (acetate, lactate, and butyrate) as carbon sources and light as energy source, the carbon sources are converted into hydrogen under anaerobic conditions. There are several advantages when utilizing photosynthetic bacteria for biohydrogen production: (1) they can utilize a wide range of substrate for growth and hydrogen production; (2) the substrate conversion efficiency is high; (3) they show great metabolic versatility under different environmental conditions; (4) they can absorb light energy at a wide range of the light spectrum and can withstand high light intensities; (5) there is no oxygen inhibition problem because no oxygen is produced as a by-product. In principle, photofermentation is able to completely convert organic compounds into hydrogen, because hydrogen production is driven by ATP-dependent nitrogenase and ATP is formed via the capture of light energy through photosynthesis. The photofermentation process is shown in Fig. 3.20.

H2 ADP H+

N2ase e– Reserve e– flow

ATP

e–

Photosynthetic bacteria system

Organic acid N2ase-Nitrogenase, e––electron, H+–proton, ATP–Adenosine triphosphate, ADP–Adenosine diphosphate

Fig. 3.20: The process of photofermentation in biohydrogen production.

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223

Light energy H+ Outside membrane

H+ , e– Photosynthetic membrane apparatus ATP Synthase

CO2

H TCA cycle

Hydrogenase

ATP H2

+

H+

Nitrogenase

H2

H2

e– Biosynthesis and growth products Fig. 3.21: The process of photofermentation in biohydrogen production. H2 ase: hydrogenase, N2 ase: nitrogenase, e− : electron, H+ : proton, ATP: adenosine triphosphate, TCA cycle: Tricarboxylic acid cycle

Hydrogen production by photosynthetic bacteria is the same as in algae; both are the result of photosynthesis that is driven by solar energy, but photosynthetic bacteria only have one photosynthesis center (equivalent to the photosynthetic system I in photosynthesis of green algae), and lack the photosynthetic system II water splitting ability. The photosynthetic bacteria use captured solar energy to generate ATP and high energy electrons through energy flow, which then reduce ferredoxin, which in combination with the reduction of ATP, drives the hydrogen protons with nitrogenase. The organism is unable to gain electrons from water, hence organic acids are typically utilized as substrates. Several individual components build the overall production system, and these are conveniently grouped as: (1) enzyme systems, (2) carbon flow – specifically the TCA cycle and (3) the photosynthetic membrane apparatus. These groups are interconnected within the hydrogen production process by virtue of electron exchange, protons, and ATP. The overall scheme is shown in Fig. 3.21. Biohydrogen production and hydrogen consumption in the photosynthetic bacteria process are mediated by nitrogenases and hydrogenases. The basic function of the nitrogenase is to fix the molecular nitrogen into ammonia, which can then be utilized by organisms as nitrogen sources. Nitrogenase can reduce protons along with nitrogen and produce hydrogen as a side-product. The other key enzyme in biohydrogen metabolism is hydrogenase. Hydrogenase plays a distinct role in different conditions; it acts as a hydrogen uptake enzyme when in the presence of hydrogen and an elec-

224 | 3 Technologies for biomass-based hydrogen production

tron acceptor, while using the protons from water as electron acceptors and releasing hydrogen in the presence of an electron donor of low potential. PSB can utilize a wide variety of substrate as carbon and nitrogen sources for growth and hydrogen production. Two criteria are frequently used to evaluate the hydrogen production characteristics: The first is the hydrogen production rate, and the second is substrate conversion efficiency. The hydrogen production can be theoretically expressed through the stoichiometric conversion of the specific substrate according to the following hypothetical reaction: Cx Hy Oz + 2(x − z)H2 O → (y/2 + 2x − z)H2 + xCO2

(3.41)

When hydrogen production rate and substrate conversion efficiency are considered in combination, we find that the organic acids have higher substrate conversion efficiencies than the utilizing sugars. Operation parameters such as pH, temperature, medium composition, and light intensity also affect the growth and hydrogen production of photosynthetic bacteria; hence, optimization of parameters plays an important role to obtain high and stable hydrogen production rates and yield.

3.4.5 Dark fermentation Dark fermentation is also called anaerobic fermentation. Under anaerobic conditions, organic waste is converted by anaerobic heterotrophic bacteria to organic acids for further methane fermentation, and hydrogen is obtained as a by-product during this process. Dark fermentation has many advantages in comparison to photofermentation: (1) Dark fermentation obtains energy mainly via degradation of the organic substrate; therefore biohydrogen production is independent of irradiation, enabling simple implementation under moderate process conditions. (2) The hydrogen production capacity of fermentative hydrogen producing microorganisms is generally higher than that of photosynthetic microorganisms. (3) The growth rate of fermentative hydrogen producing microorganisms is fast, which can quickly provide abundant hydrogen producing microorganisms for fermentation equipment. Additionally, facultative fermentative hydrogen producing bacteria are viable for storage and transportation, rendering fermentation biohydrogen scale production easy to realize that the scale production of fermentation biohydrogen is easy to be realized. (4) The available substrates for dark fermentation are widely present, including glucose, sucrose, xylose, starch, cellulose, hemicellulose, and others. Hydrogen production efficiency via dark fermentation is evidently higher than that via photofermentation, thus reducing the overall cost.

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(5) Due to no light limitation, under the conditions without affecting mass and heat transfers of the process, the reactor of dark fermentation can be designed large enough, thus improving the hydrogen yield in one single unit on the large scale.

3.4.5.1 Dark fermentative microorganisms The type of microorganism is an important factor influencing fermentative biohydrogen. Different species and even different strains have various hydrogen producing abilities on the same organic substrate. The microorganisms used in dark fermentation can either be pure strains or mixed culture [291].

3.4.5.1.1 Pure strains in hydrogen production There are many different types of fermentative bacteria capable of producing hydrogen in natural environment, including strictly anaerobic bacteria, facultative bacteria, and obligate aerobic bacteria. The most widely separated hydrogen producing fermentative bacteria are Clostridium and Enterobacter, e.g., Clostridium butyricum, Clostridium pasteurianum, Clostridium welchii, Enterobacter aerogenes, and Enterobacter cloacae. Tab. 3.15 lists the hydrogen production capacity of several pure strains [292]. Tab. 3.15: Hydrogen production capacity and fermentation parameters of pure strains. Strains

Aerobic type

Substrates

Hydrogen yield

Mode of operation

E. aerogenes Clostridiumsp. E. cloacae II T-BT 08 Klebsielleoxytoca HP1 C. acetobutyricum C. termolacticum Clostridium trybutyricum Ethanoligenens harbinense B4 Enterobacter cloacae DM11 Bacillus coagulans Thermotoga neopolitana

Facultative Anaerobic Facultative Anaerobic

Glucose (20 g/L) Glucose (25 g/L) Glucose (1 %) Glucose (50 mmol/L)

1.09 mol/mol glucose 2.18 mol/mol glucose 2.2 mol/mol glucose 1 mol/mol glucose

Batch Batch Batch Batch

Anaerobic Anaerobic Anaerobic

Glucose Maltose (29 mmol/L) Glucose

2 mol/mol glucose 3 mol/mol glucose 1.79 mol/mol glucose

Batch Batch Continuous

Anaerobic

Glucose

2.20 mol/mol glucose

Batch

Anaerobic

Glucose

3.90 mol/mol glucose

Batch

Anaerobic Anaerobic

Glucose Glucose

2.28 mol/mol glucose 1.84 mol/mol glucose

Batch Batch

226 | 3 Technologies for biomass-based hydrogen production

In the studies on pure strains, anaerobic strains have high hydrogen productivity; however, the fermentation process requires a strict environment and operation. Facultative strains not only have a high capacity for hydrogen production, but also have good adaptation to the environment, enabling convenient operation. Currently, pure strains have been used in theoretical studies, such as the classification of hydrogen producing bacteria, adaptable environment, metabolism, and hydrogen production capacities. Pure strains are hardly used in practical applications.

3.4.5.1.2 Mixed cultures in hydrogen production In fermentative hydrogen production, co-cultures of the same or a different genus of bacteria can establish a rational bacteria community. In this community, synergy of various strains can create a favorable ecological condition, compensating for the defects of pure strains’ response to the complex environment, which can improve hydrogen production efficiency to a maximal extent. So far, mixed cultures contain two types. The first type is a mixture of high efficient hydrogen producing strains; the second type is activated sludge in nature. Tab. 3.16 lists the hydrogen production capacity of mixed bacteria [292]. Tab. 3.16: Hydrogen production capacity and fermentation parameters of mixed bacteria. Strains

Substrate

Hydrogen yield

Mode of operation

C. butyricum + E. aerogenes C. butyricum + E. aerogenes

Starch (2 %) Starch residue of sweet potato Glucose (10 g/L) Food waste Organic solid waste Food waste

2.5 mol/mol glucose 2.4 mol/mol glucose

Continuous Continuous

2.1 mol/mol glucose 2.1 mmol/g COD 1.2 mg/g COD 1.82 mol/mol glucose

Batch Intermittent Continuous Continuous

Various sludge composts Mixed sludge Mixed bacteria Anaerobic sludge

Hydrogen production via mixed culture has many advantages: (1) Mixed culture has a higher capacity of hydrogen production than pure strains, especially mixed high efficient hydrogen producing strains. (2) Mixed culture has no contamination issues, therefore, a sterilization process is not necessary. (3) If anaerobic activated sludge is used, the loss of bacteria in the continuous mode can be avoided by good settling flocks. (4) Simple operation facilitates management, improving the feasibility of industrialization of biohydrogen.

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3.4.5.2 Mechanism of dark fermentation and influencing factors 3.4.5.2.1 Mechanism of dark fermentation Under micro-aerobic or anaerobic conditions, many microorganisms reduce protons to hydrogen to remove excess reducing force produced during primary metabolism. Bacteria initially degrade substrates to supply nutrients for biosynthesis and energy for growth. The electrons produced in this oxidation process should be consumed in order to maintain an intracellular balance. Under aerobic conditions, O2 is reduced to H2 O. Under anaerobic or anoxic conditions, other compounds need to act as an electron acceptor, such as protons, which are then reduced to hydrogen. Common dark fermentation to produce hydrogen has three main pathways: the pyruvate decarboxylation pathway, the formate lysis pathway, and the NADH/NAD+ equilibrium regulation pathway [293]. (1) The pyruvate decarboxylation pathway The pyruvate decarboxylation pathway exists mainly in strictly anaerobic bacteria, such as Clostridium. Fig. 3.22 illustrates the pyruvate decarboxylation pathway in Clostridium using glucose as the substrate. One mol glucose is converted to 2 mol pyruvate, 2 mol ATP, and 2 mol NADH via the glycolytic pathway, then 2 mol NADH are converted to 2 mol reduced ferredoxin (Fdred ), catalyzed by NADHferredoxin oxidoreductase (NFOR); 2 mol pyruvate are converted to 2 mol reduced ferredoxin (Fdred ), and 2 mol acetyl-CoA and 2 mol CO2 are catalyzed by pyruvateferredoxin oxidoreductase (PFOR). The above 4 mol Fdred generate 4 mol H2 catalyzed by hydrogenase, while 2 mol acetyl-CoA generate 2 mol acetate via phosphotransacetylase and acetate kinase in succession. With this, 1 mol glucose generates 2 mol acetate, 2 mol CO2 and 4 mol H2 ; therefore, the maximum theoretical hydrogen productivity in this pathway is 4 mol H2 /mol glucose. Actually, most strictly anaerobic bacteria produce acetate and butyrate as by-products, and some Clostridium under certain conditions also produce ethanol, propionate, even butanol, acetone, and lactate, reducing the final hydrogen productivity below the theoretical value. (2) The formate lysis pathway The formate lysis pathway mainly exists in methylotrophic bacteria, facultative anaerobe, and aerobic bacteria. Fig. 3.23 illustrates the formate lysis pathway in Enterobacterium. The main enzymes involved in this pathway are pyruvate formate lyase (PFL) and formate hydrogen lyase (FHL). FHL is a membranebound multienzyme complex system, including formate dehydrogenase (FDH) and hydrogenase, which are connected by an intermediate electron carrier. Since this is the pyruvate decarboxylation pathway, glucose is converted to pyruvate via a glycolytic pathway followed by decarboxylation. Microorganisms use formate generated by pyruvate decarboxylation to produce CO2 initially catalyzed by FDH; electrons generated during this process are transferred to Fdox to produce Fdred ; Fdred electrons reduce protons to hydrogen via hydrogenase, and Fdox is regener-

228 | 3 Technologies for biomass-based hydrogen production

Glucose 2 ATP 2 ADP Fructose-1,6-diphosphate

4 H2

2 Glyceraldehyde-3-phosphate

NFOR

Hydrogenase

2 Fdred

2 NAD+

2 NAD+

4 ATP

2 NADH

4 ADP

2 Fdox

2 NADH

2 Pyruvate

2 Fdox

2 Lactate

4 NADH 2 CoA

4 NAD+ 2 Propionate

PFOR CO2

2 CO2

2 Fdred

Acetone

Acetoacetyl-CoA

2 Acetyl-CoA

2 NADH 2 NAD+ NADH

Butyryl-CoA

+

NAD

2 NADH

2 NAD+ 2 Acetyl phosphate 2 Aldehyde 2 ADP 2 NADH

Butyraldehyde Butyryl phosphate 2 ADP NADH 2 ATP 2 ATP + NAD Butanol

Butyrate

2 NAD+ 2 Acetate

2 Ethanol

Fig. 3.22: The pyruvate decarboxylation pathway in Clostridium. (NFOR, NADH: ferredoxin oxidoreductase; PFOR, Pyruvate: ferredoxin oxidoreductase)

ated. According to this pathway, the maximum theoretical hydrogen productivity in this pathway is 2 mol H2 /mol glucose. (3) NADH/NAD+ equilibrium regulation pathway Strictly speaking, NADH/NAD+ equilibrium regulation is not a hydrogen production mechanism, as it does not involve any substrate. It is a self-regulatory mechanism of redox equilibrium by microorganisms. During glycolysis, NADH can be re-oxidized to NAD+ through the production of butyrate, propionate, ethanol, or lactate, thus maintaining the equilibrium of NADH and NAD+ . However, when the oxidation of NADH is slower than NADH formation, other regulatory mechanisms

3.4 Biological hydrogen production |

229

Sucrose NADH NAD+

NAD+ NADH Mannitol

Fructose

Mannitol

Glucose 2 ATP

+

Glycerol

NAD

NADH

ATP

ATP ATP

ADP ADP

2 ADP F-1,6-D

Xylose ADP ATP

ADP

Gluconate

2 G-3-P

Galactose

+

2 NAD 2 NADH 4 ATP Low pH

ADP ATP NADH NAD+

4 ATP 2 Pyruvate

2 Lactate 2 NAD+ 2 NADH

Exogenous formate Low pH

PFL 2 Formate

α-Acetolactate

CO2

2 Acetyle-CoA

FDH Fdox

2 CO2 Fdred

FH

L

Hydrogenase 2 NADH 2 NAD+ 2 Acetyl phosphate 2 Aldehyde 2 H2 2 ADP 2 NADH

CO2 3-hydroxy-2-butanone NADH

2 ATP

NAD+ 2,3-butanediol

2 Acetate

2 NAD+ 2 Ethanol

Fig. 3.23: Formate lysis pathway in Enterobacterium. (F-1,6-D, Fructose-1,6-diphosphate; G-3-P, Glyceraldehyde-3-phosphate; PFL, Pyruvate formate lyase; FDH, Formate dehydrogenase; FHL, Formate hydrogen lyase)

would work in order to avoid NADH accumulation. For example, H2 is released by reversible hydrogenase to maintain the equilibrium of NADH and NAD+ (Fig. 3.24). Fdred

H2 Hydrogenase

2e–

NADH-ferredoxin

NAD+

oxidoreductase H+ Fdox

Fig. 3.24: NADH/NAD+ equilibrium regulatory hydrogen production.

NADH

230 | 3 Technologies for biomass-based hydrogen production

3.4.5.2.2 Factors influencing dark fermentation The main factors influencing hydrogen production are temperature, pH, metal ion concentration, and oxidation reduction potential (ORP). (1) Temperature Generally, the temperature range of dark fermentation can be classified into medium temperature (25 to 40 °C), high temperature (40 to 65 °C), extremely high temperature (65 to 80 °C), and ultrahigh temperature (above 80 °C). Most fermentative bacteria produce hydrogen at medium temperature, normally between 30 and 40 °C. Recently, researchers found that fermentative bacteria have a faster metabolism, higher hydrogen yield, and hydrogen production rate at high temperatures. (2) pH value During dark fermentation, the pH value influences the efficiency of fermentation to produce hydrogen through the disturbance of intracellular NADH/NAD+ dynamic equilibrium and bacterial physiology. The pH value can influence hydrogenase activity, cellular oxidation reduction potential, and metabolites. pH can also impact the viability of hydrogen-consuming bacteria, such as propionibacteria and methanobacteria. Still, there are great disagreements about the optimal pH range for dark fermentation, which might be because of diverse substrates, inoculum, or culture conditions. It is generally recognized that the optimal pH range is between 5.0 and 6.0 for dark fermentation. Although the initial pH values might be different, the final pH values after fermentation are generally in the range of 4.0–6.0, which is probably due to quick and numerous production of volatile fatty acids weakening the buffer ability of fermentation broth. It is noteworthy that the decrease of pH would impact the hydrogenase activity, thus inhibiting hydrogen production; therefore, the pH value must be controlled at optimal level during fermentation. (3) Metal ions Metal ions can influence the structure and function of hydrogenase, thus influencing hydrogen production ability of fermentative bacteria. According to the biohydrogen principle and microbial nutrition, ion, nickel, magnesium, and others can promote microbial hydrogen production ability below a certain concentration, while heavy metal ions such as mercury and copper can severely inhibit hydrogenase. (4) Oxidation reduction potential (ORP) In the fermentation system, the control of ORP should be set according to the dominant bacterial community. Normally, aerobic bacteria require an ORP between 300 mV and 400 mV, although aerobic bacteria can also grow above 100 mV ORP. Facultative anaerobic bacteria conduct aerobic respiration above 100 mV ORP, while they switch to anaerobic respiration below 100 mV ORP. Obligate anaerobic bacteria require an ORP between −200 mV and −250 mV, while methanogenic

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bacteria require an ORP between −300 mV and −600 mV. Many aspects can affect ORP: (i) ORP is affected by oxygen partial pressure; high oxygen partial pressure results in high ORP and vice versa. (ii) Microbial oxidation of organic compounds and reducing substances (H2 and H2 S) generated during metabolism can reduce ORP. (iii) pH can influence ORP: In low pH, ORP is high, and vice versa. ORP can be reduced in the fermentation system through addition of reductants, such as vitamin C or H2 S. Contrarily, ORP can be elevated through aeration, thus improving oxygen partial pressure.

3.4.6 Biological water-gas shift reaction The water-gas shift reaction converts carbon monoxide (CO) and water (H2 O) to carbon dioxide (CO2 ) and hydrogen (H2 ). The reaction equation is as follows: CO(g) + H2 O(l) → CO2 (g) + H2 (g),

∆G0 = −20 kJ/mol

The reaction is exothermic, which means the reaction equilibrium shifts to the right at lower temperatures. In contrast, the equilibrium shifts to the left, limiting H2 production at higher temperatures. The current “state-of-the-art” water-gas shift technology is a two-stage catalytic process with high temperature and high pressure. Thermal catalysis can oxidize residual CO, while also inevitably oxidizing H2 . The water-gas shift reaction can be operated at ambient temperature using photosynthetic bacteria, where the reaction equilibrium is more favorable to hydrogen production. The advantages of low operation temperature, rapid reaction rate, and no equilibrium limitation introduce the biological water-gas shift technology as a promising alternative to conventional shift technologies [294]. Certain photoheterotrophic bacteria in the superfamily of Rhodospirillaceae can grow in the dark using CO as the sole carbon source to generate ATP with the concomitant release of H2 as well as CO2 . Such a reaction is mediated by proteins coordinated in an enzymatic pathway, thus capable of taking place at low (ambient) temperature and pressure. Thermodynamics show that this reaction is very favorable for CO oxidation and H2 synthesis, since the reaction equilibrium strongly tends towards the right. The enzyme that binds and oxidizes CO is the carbon monoxide-accepting oxidoreductase (carbon monoxide dehydrogenase = CODH), which is part of a membranebound enzyme complex. Rubrivivaxgelatinosus CBS is a purple non-sulfur bacterium that not only uses CO as a sole carbon and energy source to grow in the dark, converting CO in the atmosphere into near stoichiometric amounts of H2 , but it also assimilates CO into new cell mass if light is available (via CO2 fixation) when CO is the sole carbon source. When both an organic substrate and CO are simultaneously available, R. gelatinosus CBS will utilize both of them, suggesting that the CO-oxidation pathway is fully functional even though a more favorable substrate exists.

232 | 3 Technologies for biomass-based hydrogen production

R. gelatinosus CBS exhibits a doubling time of 7 h in light when CO serves as the sole carbon source. The mass transfer of CO may be increased by a high gas-liquid ratio, and by stirring the culture vigorously. The hydrogenase in this organism is tolerant to O2 , exhibiting a half-life of 21 h when the whole cells were stirred in full air. A specific rate of CO oxidation to H2 production of 0.8 mmol/(min g of cells) was obtained using a low-density culture (final OD660 < 0.2), stirring at high rate (250 rpm), and supplementing with 20 % CO in the gas phase. In addition, many other bacteria can convert CO to H2 , for example, photosynthetic bacteria Rhodobacter sp., Rhodopseudomonas gelatinosa, Rhodospirillum rubrum, and Rhodopseudomonas palustris, as well as the strictly anaerobic bacterium Methanosarcina barkeri, and the chemoheterotrophic bacterium Citrobacter sp.

Tiejun Wang*, Longlong Ma, Yujing Weng, Junlin Tu, Mingyue Ding, Huijuan Xu, and Qi Zhang

4 Biomass synthetic fuel technology

4.1 Catalytic synthesis of liquid fuel with synthesis gas According to relevant research and a German forecast, biomass synthetic liquid fuels (BTL—Biomass to Liquids) will develop rapidly during the next couple of decades (Fig. 4.1). It is estimated that in 2020, biosynthetic fuels will meet 25 % of the fuel needs of Germany. BTL is superior to currently used biodiesel in many ways, and there is a general tendency to replace it. The Volkswagen Forecast reported that secondgeneration biofuels (biosynthetic fuels and biomethane) will replace first-generation biofuels (biodiesel and bioethanol) by 2015. Thirty years later, the forecast further reports that 23 % of the fuel in the world will be produced via biomass, and will be mainly second-generation biofuels – biosynthetic liquid fuels. Second-generation biofuels are one of the most important renewable fuels. This will become the next generation of alternative energy sources.

*Corresponding Author: Tiejun Wang: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China; E-Mail:[email protected]; Tel.:+86 20 8705 7751; Fax: +86 20 8705 7789 Longlong Ma: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China Yujing Weng: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China and University of Chinese Academy of Sciences, Beijing 100049, P. R. China Junlin Tu: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China Mingyue Ding: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China Huijuan Xu: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China Qi Zhang: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China https://doi.org/10.1515/9783110476217-004

234 | 4 Biomass synthetic fuel technology

Biosyngas

Off-gas Electricity (for use in plant) Light product

Biomass

Pre-treatment

Gasification

Gas conditioning

Fischertropsch

CC

Fischer-Tropsch Diesel (ultra-pure high-quality designer fuel)

Fig. 4.1: Catalytic synthesis of liquid fuel with synthesis gas.

In 1923, Fischer–Tropsch (FT) synthesis was discovered, which has since developed the research of synthetic oil or liquid hydrocarbons into a hot spot by providing an unparalleled advantage in the field of alternative energy and chemicals. Successful operation of the Sasol I, II, and III plants laid the industrial foundation of coal-based synthetic oil. Since the 1970s, Mobil has developed a series of new catalysts for FT synthesis of a narrow molecular weight range and a particular type of hydrocarbon products has since opened up a new path. In the mid-1980s, the Shell company developed a new type of cobalt-based catalyst and heavy hydrocarbon conversion catalyst for diesel oil, kerosene, and the by-product of hard wax. In the 1990s, with the evergrowing shortage of oil resources, highly efficient utilization of biomass resources via FT synthesis technology has become increasingly urgent. The FT synthetic technique and industrialization of the Sasol Company has long been in a leading position, using its own proprietary technology to synthesize liquid fuel and other chemical products with synthesis gas. There are other and more mature FT synthesis technologies, such as the Shell SMDS technology, the Exxon/Mobil AGG 21 technology, the Syntroleum technology, and the Tigas technology of the Topsoe Company. Currently, there are two main types of FT synthetic processes: high-temperature and low-temperature FT synthesis. The high-temperature synthesis process can be operated at a lower H2 /CO and at 300–350 °C, using Fe-based catalysts, and the resulting products are mainly gasoline and low carbon olefins. The low-temperature FT synthesis process generally happens at a temperature of 200–240 °C, using an Fe-based or Co-based catalyst. The products are long-chain hydrocarbons such as diesel oil and wax, while by-products are small amounts of olefins and chemicals. Two synthetic process conditions and product distributions are shown in Fig. 4.2. Three main types of FT reactors exist for the industrial application: fixed-bed reactor, fluidized-bed reactor, and slurry-bed reactor. The degree of syngas conversion of the FT reactor is restricted by the type of catalyst, the reactor type, and by technical parameters. When the conversion rate is low, the tail gas cycle reaction, containing

4.1 Catalytic synthesis of liquid fuel with synthesis gas

| 235

Fe-based catalysts fluidized bed 300–350°C 20–30 bar H2/CO 12 < 30 < 10 2.1 90 20 0.1 Hexane extraction Technology maturity 1979, Pilot scale test 1982, 15 kt/a Now, 45 kt/a Feed gas sulfur content, ppm 6.31 598 Investment, billion* 62 Cost, yuan/t* Thermal efficiency, %

IFP

Sygmol

< 300 < 10 56 20–30

< 300 < 10 59 18–23 0.25 85 65–70 0.4 5-35 0.2 0.1 Diethylene glycol Molecular sieve extraction method adsorption 1985, 1 t/d 1984, 7,000 Barrels/a for 4 Months 110–140 10.31 875 49

Octamix < 300 < 10 50 4.3 10.2 −85 0.3 0.1 Molecular sieve adsorption Simulation

< 0.1

69

258 | 4 Biomass synthetic fuel technology

conditions are applied in both the Sygmol and Octamix process with high selectivity to alcohol. However, the water content in the raw product is relatively low, which could greatly decrease the cost of product purification. Compared to the Sygmol process, the aromatic yield in the Octamix process is relatively low, along with high yield of iso-butanol. This has increased its prospect for future development. The main difficulty for the industrialization of the Octamix process is less due to its technology, as it is far more mature than those of the other three. At the same time, the Sygmol process is also a potential synthesis route in future due to the MoS2 catalyst it utilizes, which could sustain high content of sulfur in the syngas to decrease the cost of gas purification and catalyst deactivation.

4.2 Biofuels synthesis via aqueous phase catalytic conversion of biomass Nowadays, global fuel supplies mainly originate from oil, natural gas, and other nonrenewable fossil resources. However, the shortage of fossil energy and environmental pollution of the commercial process cause significant problems. Dual pressures from resources and the environment greatly promote research study on fuels and chemicals from renewable resources. Biomass attracts great attention from governments and scientific researchers all over the world, as it is an enormous resource, environment friendly, and the only available carbon source compared to other renewable resources such as wind, solar, or geothermal energy. The International Energy Agency (IEA) released the “Transport of Biomass Fuel Technology Roadmap”, announcing that biofuels will account for 27 % of the total transportation fuel in 2050, which means that the fuel and chemical industry will change from the current unsustainable “hydrocarbon” era of transition to an era of renewable “carbs”. According to China’s development goals, summarized in the “National Medium- and Long-term Science and Technology Development Plan for 2006–2020”, biological liquid fuel production will reach 1,000 tons in 2020. The traditional method of converting biomass into liquid fuel is mainly via biomass gasification syngas, either through chemical synthesis of dimethyl ether, low carbon alcohol, or Fischer–Tropsch synthesis of gasoline and diesel; or through hydrolytic processing of cellulose and hemicellulose to generate corresponding six carbon sugars and five sugars, followed by fermentation for liquid fuels such as ethanol and butanol. However, the process is low on biomass utilization and high on energy consumption. The second generation of biomass liquid fuel technology research and development is still in its infancy; an unbalanced development as a whole compared to the corn-based fuel ethanol industry and biodiesel industry with waste oil as raw material. In the long run, lignocellulose biomass aqueous catalytic synthesis to liquid fuel can provide a new way of comprehensive biomass utilization, and has a broad development range and great potential [310–312].

4.2 Biofuels synthesis via aqueous phase catalytic conversion of biomass

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259

Biological gasoline has significantly differences to the traditional fossil gasoline: lignocellulose biomass is first hydrolyzed in acidic conditions into the corresponding soluble sugars, then the dehydration and hydrogenation of these sugar species can produce C4–C7 low carbon alkanes and small amounts of olefins, which can be used for automotive engines or gasoline additives. In 2004, Dumesic reported biomassbased sugar derivatives (such as sorbitol) reformed on Pt/Al2 O3 -SiO2 catalysts at 225 °C and 3.96 MPa in the aqueous phase and obtained C1–C6 paraffin. Sorbitol was first dehydrated to closed-loop C6 intermediate species, then hydrogenated to generate the corresponding low carbon alkane with paraffin selectivity reaching 58–89 %. Although the technology of paraffin production is giving priority to n-hexane and positive pentane, which can be used as an additive to motor gasoline, its application is still limited to a certain extent. This is due to the heterogeneous component in the mixture of alkanes being less and thus, cannot provide higher octane numbers, and precious metals Pt catalysts are very expensive. Ma and Wang reported that the bifunctional nickel zeolite catalyst can convert sugar alcohol to the corresponding low carbon alkane in the aqueous phase, and their catalytic activity is similar to that of the Pt catalyst, while yielding approximately 20 % isomerization alkane products. This improves the gasoline octane number of biological alkane compounds. The biological jet fuel synthesis process differs to that of gasoline. Biological jet fuel is a transparent liquid usually composed of straight run, hydrocracking, and hydrotreating components and their necessary additives. The molecular general formula CH3 (CH2 ) nCH3 (n is 8–15), is greater than the biological gasoline biomass. First, hydrolysis converts biomass into soluble sugars, then an aldol condensation reaction is used to control the carbon chain length of the intermediate product, followed by further dehydration/hydrogenation to C7–C15 alkanes, which can be used as additive in jet fuel. In recent years, studies on jet fuel reported the use of C5 and C6 monosaccharide derivatives, with the monosaccharide first dehydrated under acidic condition into 5-HMF and furfural, again by adding acetone to control the aldol condensation reaction with the increase of carbon chain lengths. This generates aviation fuel intermediates, and jet fuel, which is mainly C7–C15 chains and heterogeneous liquid paraffin through hydrogenation—dehydration—heterogeneous processes. In summary, there are many ways for conversion of biomass into liquid fuels and its production routes are shown in Fig. 4.17. As this figure shows, biomass hydrolyzed into C5 and C6 monosaccharides is the primary process, combining aqueous phase catalytic reforming to biological gasoline components; this can also yield jet fuel components by controlling the growth of the carbon chain. These sugar aqueous phase catalytic platforms react in the aqueous phase, and the reaction speed avoids vaporization of raw materials. Product paraffin and water phase automatic separation happen simultaneously, which greatly simplifies and reduces the energy consumption of the system. This further realizes the full utilization of monosaccharides and oligosaccharides in the hydrolysate. This section mainly introduces the class of synthetic biology catalytic synthesis process from lignocellulose biomass and the principle of gasoline

260 | 4 Biomass synthetic fuel technology

II

I

III

Biomass Hydrolysis

Water

Bio-gasoline

Pentose

Lignin+cellulose

Bio-jet fuel

Water Reforming for hydrogen production

Phase separation

H2O+hydrocarbon

Reforming

H2

Phase separation Dehydrationhydrogenation heterogeneous

Hexose+pentose oligose+residues Water soluble intermediate

Residues Steam heating

Aldol condensation Hydrolysate

Polyols

Hydrogenation

HMF, Furfural Dehydration

Fig. 4.17: Scheme for biomass into bio-oil and jet fuel [312].

and jet fuel, as well as summarizing industrial applications and developments of biological liquid fuel synthesis.

4.2.1 Mechanism of aqueous phase catalytic conversion of biomass The aqueous catalytic process for biomass conversion into liquid fuel includes biomass hydrolysis, multivariate alcohol production from sugar hydrogenation, alkane production from multivariate alcohol by catalytic reforming as well as other parts. In 2004, a report by the USA Department of Energy named sugar alcohols such as glycerol and sorbitol as one of the 12 most important types of platform chemicals for “constructing compound molecules” in the development process of biomass. Polyol can further be transformed into fuels and chemicals under elaborate routes. Therefore, researchers have recently carried out a series of studies in this field, especially represented by Dumesic and other studies, that reported sugar alcohols such as sorbitol and glycerol can effectively reform the aqueous phase to hydrogen gas, liquid hydrocarbon fuels, and chemicals under milder conditions. Given the lignocellulose structure and present research achievements, polyol is a reasonable and potential technical route that directly utilizes raw material. These polyol-based aqueous-phase catalytic processes avoid a large amount of energy consumption for distillation, compared to the currently widely used thermal cracking, gasification, and biological fermentation of biomass conversion technologies. This method has obvious advantages regarding green energy and resource use efficiency. Here, mainly the sugar

4.2 Biofuels synthesis via aqueous phase catalytic conversion of biomass

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261

hydrogenation system of multivariate alkane alcohol is introduced as well as the polyhydric alcohol aqueous-phase reforming process.

4.2.1.1 Hydrogenation of sugar into polyol At present, carbohydrate synthesis polyols have two basic pretreatment technology routes: neutralizing acid and ion exchange. The difference lies in using a different method to deal with waste acid in the hydrolysate. The neutralizing acid route generally uses neutralizing agents such as CaCO3 to remove the sulfuric acid roots of raw material. The technological route is as follows: Feedstock hydrolysis neutralization, concentration, decolorization, ion exchange concentration, hydrogenation concentration, crystallization, and separation packing. The technology route is easy, features low acid and alkali consumption, low cost, simple equipment, easy operation, and small initial investment. The disadvantage is a high concentration of CaCO3 due to the enrichment thickening of raw material liquid in the supersaturated state during the process, which can result in part of the CaCO3 precipitating and being deposited on the surface of the evaporator, thus reducing the equipment utilization rate. Ion exchange deacidification eliminates the sulfate via ion exchange and the technical route is as follows: Hydrolysis of the raw materials, followed by decoloring ion exchange, concentrated acid ion exchange, hydrogenation ion exchange, concentration crystallization, and separate packing. Firstly, anion exchange was used to remove the raw material liquid sulfuric acid anion, then cation exchange resin was used, followed by the use of both anion and cation resins to ensure the complete removal of the waste acid in the solution. The disadvantage of this technology is complex: large amounts of resin, too many pieces of equipment, large investment, large acid and alkali consumption, and expensive cost. Ion exchange deacidification still has its irreplaceable advantages, including solving equipment scale faults during the neutralizing acid process, improving the utilization rate of the equipment and service life, reducing the ash content and acid content in the feed solution, improving the quality of the hydrolysate, and also improving the product quality. The current main technological routes include kettle-type intermittent hydrogenation, external loop semicontinuous hydrogenation, and the continuous hydrogenation pipe. Kettle-type intermittent hydrogenation technology is the traditional catalytic process for hydrogenation of glucose for the production of sorbitol. The main technological process is: pumping 50–55 wt.% glucose solution into the hydrogenation reaction kettle, adding the activated Raney Ni catalyst, keeping pH at 6–7, temperature at 120–150 °C, and 3–10 MPa hydrogenation pressure for a period of time until the residual sugar content is lower than the set value, then pumping the product into the sedimentation tank, isolating catalyst, and initiating ion exchange and decolorization to receive 50 wt.% liquid sorbitol. The processes of evaporation and concentration can

262 | 4 Biomass synthetic fuel technology

also be continued to obtain liquid sorbitol 70 % with the yield generally more than 96 %. For the outer loop semicontinuous hydrogenation process, first the 50–53 wt.% glucose solution is pumped into the outer circulating autoclave through the gallery of anion and cation resins, then a certain amount of ruthenium catalyst is added, then hydrogen after nitrogen replacement. It should react under the conditions of 120 °C and 3.0 MPa. After the reaction, the catalyst is separated with a separator and the crude alcohol can produce 50 % sorbitol solution after filtration and ion exchange resin treatment of anion and cation resins. The continuous tubular hydrogenation process first began in 1984 and is continually used abroad, mainly using the method of pipeline, paddles, and fixed-bed reactor. Fixed-bed reactors easily cause local overheating reactions, which results in decreasing catalyst activity, sintering, and side effects; thus, pipeline or paddle reactor are generally used. The technological process is as follows: 50–55 % glucose solution is prepared in a tank, using a multivariate modified Raney Ni catalyst, and tuning the pH of the reaction system to 7–8 via ammonia or sodium bicarbonate. The material is then pumped into the tubular hydrogenation system comprising the hydrogen gas preheater, material preheater, tubular reactor, and liquid spray and gas liquid separator, using a high pressure pump. The temperature of the material preheater is controlled between 110–120 °C. A metal-load-type catalyst is usually adopted for the sugar polyol synthesis reaction. This is an important type of catalyst, and it is widely used in catalytic hydrogenation reactions due to its high activity. It is mainly prepared via different inert carriers with one or several zero-valent VIII elements. (1) Raney-Ni Taking sorbitol as an example, we see that it usually originates from industrial glucose hydrogenation and was first obtained via hydrogenation reduction with a floating catalyst in Japan in 1942. Then, the German Company VEB Deutsches Hychico Werk used fixed-bed reactors to produce sorbitol, which elevated the method of catalytically reducing glucose to sorbitol into the level of industrial application. This process used the Raney Ni catalyst, which controlled the pH value of the solution to 7.0–8.0. The characteristics of the catalyst are easily accessible reactants with relatively low price. However, the shortcomings including weak activity, selectivity, and stability, easy to brake, and drain of nickel and aluminum during the hydrogenation process. As a result, scientists put a lot of effort into the modification of Raney Ni catalysts, mainly adding different additives into the catalyst to improve performance. Generally, adding Fe, Cr, Mo, Co, or Ca fertilizers to the Ni-Al alloy can noticeably improve the performance of these catalysts, and the additive amount generally ranged between 1.0–5.0 wt.%. Fe, Cr, and Mo modified catalysts are widely applied in industrial settings at present. Although the main properties of modified Raney Ni catalysts have obviously been improved, when used as glucose hydrogenation catalysts for sorbitol, they still have the following disadvantages: (1) very strict reaction conditions: higher reaction pressure, higher reaction temperature,

4.2 Biofuels synthesis via aqueous phase catalytic conversion of biomass

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263

and longer reaction time. Generally, the glucose conversion rate can reach more than 98 % with a modified Raney Ni catalyst at 5 MPa reaction system pressure and 120 °C reaction temperature. This requires better compression performance of the hydrogenation system and thus increases the cost of the equipment. However, high reaction temperatures and long reaction time can increase reaction by-products and reduce the selectiveness. Since glucose is not stable, isomerization reactions are easy to happen at high temperatures. Furthermore, reactants easily form coke in the reaction process when temperature is too high, which will lead to catalyst pore blockage and thus, lead to a sharp fall in catalyst activity. The decreased stability of catalysts: glucose aqueous solution is slightly acidic at room temperature. However, the ionization constant and acidity increase with the increasing temperature. This acidity exerts a corrosive effect on the Raney Ni catalyst, draining away a large amount of its active component nickel and auxiliary, and dissolving these components into the reaction system. It also reduces the activity of the catalyst, increasing the product refined processing burden and affecting the quality of the products. Therefore, pH 8–9 for the reaction liquid would lead to the best catalytic activity for the Raney Ni catalyst in glucose hydrogenation. However, the alkaline condition would lead to glucose instability, even at normal temperature. This can also lead to an apparent isomerization reaction of fructose, mannose, and mannitol, thus seriously affecting the glucose hydrogenation selectivity. Consequently, choosing the appropriate pH value prolongs the service life of the catalyst, while decreasing the isomerization reaction as little as possible. Overall, the advantage of low cost makes the Raney Ni catalyst comparable with the precious metal catalyst. Therefore, not restricted by the quality of sorbitol, people are now more inclined to use a cheap modified Raney Ni catalyst. (2) Ru/C Previous studies revealed that metal catalysts Ru, Ni, Rh, and Pd show different reactivity during hydrogenation of glucose, which can be ordered as Ru > Ni> Rh> Pd. Therefore, research on Ru catalysts on various supports including a-Al2 O3 , γ-Al2 O3 , C, and TiO2 , have received increasing attention. Ru catalysts show improved catalytic performance both in glucose conversion and in the selectivity of sorbitol. This may be due to Ru-based catalysts having a characteristic and welldeveloped pore structure, surface area, active center, and less density compared to Ni/Al catalysts. Moreover, Auer et al. [313] systematically studied the effects of different carriers (such as activated carbon, alumina, silicon dioxide, and diatomaceous earth) on the catalytic performance of Ru catalysts, and found that Ru/C has the best activity and features a high stability. Hofer studied the effect of different preparation methods on the performance of Ru/C catalysts. In recent years, China also successively developed numerous approaches to develop Ru/C catalysts. Jianqiang Yu reported a reduction temperature study and an activated carbon treatment study on the performance impact of Ru/C catalyst. Sankui Xu re-

264 | 4 Biomass synthetic fuel technology

ported a study on the effects of organic additives on the performance of the Ru/C catalysts. Moreover, the Dalian Institute of Chemical Physics, Northwest Chemical Industry Research Institute, and Zhejiang University of Technology have also extensively studied glucose hydrogenation over Ru/C catalysts. (3) Amorphous alloy Ru Amorphous hydrogen storage alloy is a new type of functional material developed during recent years. As a potential catalyst, it absorbs and releases hydrogen with an efficient dynamic performance, revealing itself as a transition metal compound with excellent catalytic activity. Therefore, it has received much attention recently. It has been used more commonly in the ammonia synthesis gas conversion and hydrogenation reaction. The main advantage of the catalyst is that it works as an alternative precious metal catalyst. Furthermore, it has the advantage of high yield and conversion with no strict demands on pressure equipment. However, as a result of a series of characteristics of amorphous alloy, the use as catalyst needs to solve two problems: first, it needs to improve the specific surface area of amorphous alloys; second, it needs to stabilize the amorphous structure in the process of catalytic reactions. Since there are still no satisfactory solutions to these two problems, amorphous alloys that can be directly applied to the process of glucose hydrogenation to sorbitol in industrial production have rarely been reported so far.

4.2.1.2 Alkane production via aqueous-phase catalytic conversion of biomass 4.2.1.2.1 Technical route of sugar alcohol into fuels Both biomass and oil raw materials can be converted into fuels and chemicals via a catalytic process. However, they have obvious differences. Sugar alcohols from biomass show a low thermal stability and are naturally hydrophilic due to their large oxygen content. Thus, biomass catalytic conversion has its own distinct characteristics. In recent years, studies found that the oxygen in biomass raw material can be partially or completely removed by a catalytic reaction under an aqueous phase condition. The process could eventually obtain products such as hydrogen, intermediate chemicals, and hydrocarbons, thus becoming an important supplement to fossil energy, and aid in solving the fossil energy crisis to a certain extent. Polyols have become a recent focus for research in their role as the platform compound in the production of fuels and chemicals production using an aqueous catalytic process. Reactions include aqueous reforming of sorbitol to hydrogen, sorbitol dehydration and hydrogenation into hydrocarbon, selective oxidation of glycerol into dihydroxyacetone, glycerin hydrogenolysis into 1-2 propanediol, and 1-3 propanediol, glucose dehydration, condensation, and aqueous reforming to C9–C15 alkanes. Recent publications on these aspects are listed in Tab. 4.7. The design of the aqueous catalytic reaction system utilizes the hydrophilic properties of polyols and has developed into a research hot spot. The unique advantages

4.2 Biofuels synthesis via aqueous phase catalytic conversion of biomass

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265

Tab. 4.7: Catalytic conversion polyols technology [312, 314–321]. Institute

Researcher

Feedstock/product

Catalyst

Major progress

Wisconsins

Huber/2004

sorbitol/alkane

Pt /Si-Al

Davda/2005 Huber/2005

hydrocarbon/H2 , alkane biomass/alkane

Pt, Ni, R, Rd, Ir, Ni-Sn –

Solid acid or inorganic acid promoting the reaction; conversion 98 %, alkane selectivity 78 % Carrier effect > metal effect; four phase reactor Selectivity of produced alkane > 70 %; Aldol condensation

glycol/H2

Sn-Raney Ni

R-NiSn

Barrett/2006 Huber/2006

biomass/long-chain alkane biomass/H2 , alkane

5 wt.% Pd/ MgO-ZrO2 –

Huber/2006

polyol/H2

–

Serrano/2006 Chheda/2007

aldehydes/alcohol carbohydrate alkane

Tanksale/ 2008 Kunkes/2008 West/2009

sorbitol/H2

– complex catalysts bimetallic catalysts – Pt/Si-Al, Pt/Nb acid Pt/Si-Al Pd-Ni-Cu-K/ r-Al2 O3

Shabaker/ 2005

Toledo Queensland

ECUST

Huber/2010 Abranham/ 2006 Beltramini/ 2007

carbohydrate/alkane sorbitol /alkane sorbitol/alkane biomass/H2 water-soluble sugars/H2

FahaiCao/ 2008

glycerol/H2

Pt/Al2 O3

YQ ang/2008

glycol/H2

Pt/Fe-Cr

Optimization of Aldol condensation The mechanism and review of biomass to H2 and alkane Pt> Ni> Ru> > Rh> > Pd bimetallic > one metal Pt-CeOx, Pt-SnOx, PtS HMF selectivity Pt.Pd.bimetallic catalysts Carbohydrate into alkane Pt/NbOPO4 > Pt/Nb3 O5 > Pt/Si-Al; Nb = O Sorbitol into isosorbide Glycerol and glucose reforming into H2 Different monosaccharides had a significant effect on the gas production rate; Reduction of the reactant concentration can improve coking and catalytic activity Optimizing the dosage of catalyst, reaction temperature, reactant concentration and other process parameters Pt/Fe-Cr

266 | 4 Biomass synthetic fuel technology

Tab. 4.7: (continued) Institute

Researcher

Feedstock/product

Catalyst

Major progress

Dalian Institute of Chemical Physics

ZJ Tian/2008

glycerol/H2

Pt> Cu> Ni> CoNi, Co; SAPO11< AC< SiO2 < MgO < Al2 O3 MgO

Technische Universiteit Delft

Claus/2008

glycol/H2

Pt, Ni, Cu, Co/SAPO-11, HUSY, SiO2 , SiO2 -Al2 O3 , MgO Pt/Al2 O3

Yale

XM ang/2009

/glycol/H2

PtCo

Cincinnati

Z Tang/2009

Pt/NaY

Finland

West/2009

methanol, ethanol/H2 Sugar alcohol

Kirilin/2010

Sorbitol conversion

Pt/Al2 O3

PtRe/C

High calcination temperature increases Pt particle size, the H2 gas selectivity was 95 %; Response is sensitive to the catalyst structure; The adsorption of reactant and C-C crack occurred in the heart area Pt-Co bimetallic catalyst with core-shell 0.5 %Pt/NaY> 3 %Pt/rAl2 O3 Sugar and polyols were reviewed to generate a reaction of monomer compounds, including a ketone condensation reaction with added H2 reduction for alcohol, acid, alcohol dehydration reaction, and olefin polymerization The reaction including H2 , deoxidization, carbonyl, dehydration, and inverse aldol condensation; The reaction C - O bond rupture > C - C key fracture; Found the rate of H2 addition order: C-O-C< C = O< C = C

include: (1) the reaction happens in the aqueous phase and does not need gasification of water and carbohydrate, thus avoiding a large amount of energy consumption; (2) nontoxic and incombustible carbohydrates are convenient for storage and processing; (3) the reaction temperature and pressure conditions of the aqueous phase reforming can simultaneously give rise to the water-gas reaction and ensure low CO

4.2 Biofuels synthesis via aqueous phase catalytic conversion of biomass

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content in the produced hydrogen; (4) the reaction pressure generally varies from 3.5 to 5.5 MPa, which is very convenient for Pressure Swing Adsorption (PSA) to remove CO2 ; (5) aqueous phase reforming is a low temperature reaction, thus avoiding decomposition of carbide and other side effects to a great extent. As shown in Fig. 4.18, there are two main methods for biomass sugar polyol conversion into hydrocarbons. The technical routes are described in the following: one is dehydration/hydrogenation of sugar alcohols into alkanes (C1–C6), used for synthesis gas, liquefied petroleum gas, and light naphtha. The product distribution can be controlled by efficient catalyst design with simple technological processes. These are conducive to industrialization. However, the main disadvantage is that the longest carbon chain of the produced alkanes does not exceed six carbons (i.e., hexane) and enlarged volatility, which means that it cannot meet the requirements of oil production. Thus, the researchers advanced the second reaction path, namely aldol condensation to increase the length of the produced alkane carbon chain, and then hydrogenation/dehydration under aqueous catalytic systems to obtain longer carbon chain alkanes of C9–C15. Aquesou-phase Hydrogenation /Dehydration OH

OH

Acid sites OH

OH

O

OH Dehydration

OH

Alkanes C1–C6 H2O

H2 H2O HO OH

OH

OH H2

Aquesou-Phase Reforming

Polyols

OH O OH + 6H2O → 6CO2 + 12H2

HO

Hydro genation

HO HO

Sugars Biomass

Acid Hydrolysis

Acid Dehydration

Aldol Condesation

4-PD/H

Alkanes C9–C13

O OH O O

O HO HO

OH H2 O OH

HO HO

OH O H2

O O

H2O

O

HO

HO

HO aldol crossedcondensation (base catalyst) H2 7 H2O OH

HO O

O OH O OH

Fig. 4.18: Scheme for sugar alcohol sugar alcohol into long-chain alkane [314, 315].

268 | 4 Biomass synthetic fuel technology

The typical reaction follows dehydration of carbohydrates first under acidic conditions, e.g., conversion of glucose and xylose into 5-HMF and furfural, respectively. Since no α-H is present in these types of compounds, they cannot apply self-condensation, and thus need to condensate with other molecules, such as acetone or glycerin to form C9–C15 long-chain organics. As a result of their trace solubility in water, these compounds have to be hydrogenated first to improve solubility in water, and then dehydration/hydrogenation to form C9–C15 paraffin. The advantage of this process for long-chain hydrocarbons is that it is sulfur-free and could be directly added to diesel fuel or as vehicle fuel for the development of alternative P-fuel (amount of ethanol, methyl tetrahydrofuran, and a mixture of pentane). The shortcomings exist at the key step of the C-C growth, namely the aldol condensation reaction needs to introduce alkaline catalysts such as Mg-Al oxides, which are different from the subsequent aqueous phase dehydration/hydrogenation with an acid-metal bifunctional catalyst system. Moreover, they introduce a complicated technological process with low efficiency. In addition, the acetone reactant in the aldol condensation reaction is not derived from biomass, and the long-chain organics that are formed are difficult to dissolve in water, thus organic solutions such as methanol or methylene sulfoxide (DMSO) need to be added.

4.2.1.2.2 Study of catalysts for the transformation of sugar alcohols into transportation fuel Catalysts with loading metals of the family often exhibit good catalytic activity for hydrogenation reactions. Furthermore, they can also be used in the dehydrogenation cyclization of aliphatic hydrocarbon and in the heterogeneous reaction. Therefore, for aqueous reforming of sugar alcohol the metals of the ? family and solid acid tend to be chosen often to obtain bifunctional catalysts. The metal sites could be the active sites for hydrogenation and dehydrogenation, while solid acidic sites could be the active sites for pyrolysis. We investigated the aqueous phase reforming of ethylene glycol under the conditions of 483 K and 22 bar using silicon oxide as supports for loading metals of the ? family, and found that the ethylene glycol conversion rate was in the following order: Pt ~ Ni > Ru > Rh ~ Pd > Ir As shown in Fig. 4.19, although Pt, Ni, and Ru showed a high reaction activity, only Pt and Pd showed higher selectivity for hydrogen formation. This is due to the fact that Pt and Pd metals not only have high catalytic activity to crack C–C bonds, but also promote the water-gas reaction (CO + H2 O = CO2 + H2 ). For silicon support of Rh, Ru, and Ni catalysts, due to their low selectivity for hydrogen generation, more paraffin is usually produced. All of these metals have strong C–O bond cracking activity, and can promote FT synthesis and methanation reaction. In addition, the alloy cat-

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alysts can also be used in polyol conversion reactions due to their specific catalytic activity. Shabaker [322] modified the Raney Ni catalyst with Sn, which significantly improved the stability of the Ni catalyst with a 30 % drop on catalyst activity after 48 h and a hydrogen selectivity of 90 % or more. Kunkes [323] used a Pt-Ru bimetallic catalyst, which improved the content of oxygen containing compounds in the reaction, thus improving the octane number of the liquid mix, leading to conformity to the requirements of biological gasoline product. Pt-Ru/C catalyst with 5.1 wt.% Pt and 4.8 wt.% Re can obtain oxygen compounds with total yield of 48 %, including alcohol and cyclic ether. As the total yield of oxygen compounds increases, the yield of ketones and acid compounds is also increased simultaneously with total yields of approximately 20 %, which necessitates adding refining segments in the subsequent process. This is not conducive to the overall economics of the process. Overall, due to their excellent hydrogenation activity and stability, Pt catalysts have been treated as typical aqueous-phase reforming metal catalysts for reaction mechanism research. However, the Ni catalysts are also a direction for future research due to their low cost and high activity for hydrocarbon production.

100 90 80 70 60 50 40 30 20 10 0 Pt

Pd

% H2 Selectivity % Alkane Selectivity Ni

Ru

% CO2 TOF x 103, min–1 Rh

Fig. 4.19: Performance of catalytic reforming over different metal catalysts[316]. (gray: CO2 TOF × 103 min-1; white: alkane %; blank: H2 %)

The utilization of different carriers can prepare catalysts with different acid strength and pore structures, leading to a change of the metal-acid center ratio, thus affecting reaction activity and product selectivity. Cortright et al. [324] investigated the reaction activity of sorbitol conversion on Pt catalysts, e.g., for mechanical mixtures of 4 wt.% Pt/SiO2 -Al2 O3 (Pt-SiAl), 3 wt.% Pd/SiO2 -Al2 O3 (Pd-SiAl), 3 wt.% Pt/Al2 O3 (Pt-Al), Pt-Al and SiO2 -Al2 O3 (Al2 O3 on SiO2 -Al2 O3 with 25 wt.%). An evaluation of the collected data on these four types of catalysts revealed that the Pt-SiAl catalyst can retain a 92 %

270 | 4 Biomass synthetic fuel technology

conversion rate with a reaction temperature of 498 K after 6 d without passivation. Products of butane, pentane, and n-hexane range from 58 % to 89 % under different reaction conditions. Product alkanes are mainly straight-chain compounds, enriched by a few branched chain isomers (less than 5 %), while paraffin with a C content above that of n-hexane cannot be formed under the reaction conditions. For the Pt-SiAl catalyst, the selectivity of n-hexane increased due to increasing pressure from 25.8 bars to 39.6 bars, which may be the cause for the increase of hydrogen partial pressure that leads to the increasing rate of metal hydrogenation. When sorbitol and external hydrogen are supplied simultaneously, the overall alkane selectivity increased significantly. Adding SiO2 -Al2 O3 or inorganic acid HCl into the Pt-Al catalyst system will shift the produced alkanes to heavy paraffins, and increases hexane selectivity from 29 % to 47 % with a stark change of pH (from 3 to 2). This further illustrates the importance of acid centers for alkane formation, and shows that acid and metal can keep a high catalytic activity without close contact in the aqueous catalytic system at the same time. Many researchers have further explored the reactivity of catalyst carriers and a summary is shown in Tab. 4.8. Pt/H-ZSM-5, Pt/zirconium phosphate, and Pt/SiO2 Al2 O3 have high activity for alkane generation. The activity of Pt/H-ZSM-5 catalyst is much higher and alkane yield can reach 65 %. The selectivity for C5–6 alkanes was above 50 %. However, since the aqueous catalytic reaction must be carried out in a high-temperature hydrothermal environment, the Pt/H-ZSM-5 catalytic activity decreased quickly after 36 h with the alkane yield of less than 10 %. Pt/H-ZSM-5, Pt/zirconium phosphate, and Pt/SiO2 -Al2 O3 show good catalytic activity with alkane yield of about 20 %. The catalysts are stable in the hydrothermal environment after 36 h. The products distribution tends to heavy hydrocarbons. Worth pointing out are acidic catalysts with the metal niobium, such as niobium acid or niobium phosphate with Pt, which also revealed good stability in the aqueous reaction. However, catalytic activity is not high, alkane yield ranges around 10 %, and the paraffin product is mainly light alkanes such as methane and ethane, both of which do not conform to the requirements of biological gasoline products (alkane carbon number > 5). Catalysts in the aqueous catalytic conversion of polyhydric alcohol to alkane must have metal-acid combined centers to meet the requirements of the polyhydric alcohol dehydration/hydrogenation reaction. Catalysts should remain stable and retain high catalytic activity under high temperature hydrothermal conditions. By modulating the metal and acid strength on the carrier, the cracking of C–C bonds as well as the rate of dehydration and hydrogenation reactions can be controlled to obtain a high yield of target product.

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Tab. 4.8: Pt catalyst for sorbitol conversion [312, 314–320]. Cgas : percentage of carbon converted to gas phase; b. Salkane = carbon in alkanes/carbon in gas phase × 100 %; c. Specific alkane selectivity = (carbon in specific alkanes)/(carbon in all alkanes detected) × 100 %; d. Activity at 12 h; e. Activity at 36 h. Catalyst

Pt/SiO2 -Al2 O3 Pt/H-ZSM-5 Pt/HY Pt/WOx/ZrO2 Pt/Mox/ZrO2 Pt/Nb2 O5 Pt/zirconium pHospHate Pt/titanium pHospHate Pt/niobium pHospHate-a Pt/niobium pHospHate-b

Cgas a /%

25.8d 23.6e 86.5d 28.9e 26.9d 12.1e 34.2d 7.5e 7.3d 4.6e 33.4d 33.6e 32.8d 34.2e 47.1d 28.3e 33.7d 15.0e 23.3d 24.3e

Salkaneb / %

45.8 43.2 75.4 22.7 45.4 20.8 50.6 16.0 13.5 25.4 29.3 25.7 52.5 50.6 41.6 26.9 73.6 12.9 43.9 26.1

Specific alkane selectivityc / % C1

C2

C3

C4

C5

C6

13 14 4 12 8 30 4 17 9 3 30 32 4 3 10 17 1 15 12 21

20 19 9 16 14 26 8 26 18 8 28 29 9 8 18 24 3 20 19 30

14 14 14 12 13 14 12 17 15 9 17 18 12 12 21 20 6 16 15 18

17 19 21 14 19 13 20 10 7 7 12 12 20 20 20 16 11 16 22 14

14 13 16 18 22 12 23 20 31 29 8 5 22 23 16 10 24 20 18 10

22 21 36 29 24 6 24 10 19 43 5 4 33 34 16 12 56 12 15 7

Comment: reaction conditions: 518 K, 2.93 MPa, WHSV = 2.91 h−1 , 5 wt.% sorbitol solution, 40 ml min−1 H2 flow.

4.2.1.3 Reactor design for aqueous conversion of sorbitol into alkane The aqueous catalytic conversion of sugar alcohols into alkanes usually results in serious coke formation (about 20 % to 50 % of the reactant into coke) on the catalyst surface. Therefore, researchers have developed the novel reactor for the APR reaction, consisting of four-phase flow: (1) a feed flow of water-soluble organic reactant; (2) a C16 H34 feed flow; (3) an H2 feed flow; (4) a solid bifunctional catalyst (Pt/SiO2 -Al2 O3 ). During the process of the dehydration/hydrogenation reaction, the water-soluble organic reactant is gradually transformed into hydrophobic paraffin products and C16 H34 flow will remove the paraffin alkane from the surface of the catalyst before coke formation. The C16 H34 , hydrogen and CO2 exited from the reactor will be cycled as feed flow. According to the reaction kinetics study, pure water in the four phase reactor will lead to low C16 H34 conversion of 0.007 μmol · min−1 · gcat−1 . The route of sugar polyol conversion to alkanes has an obvious advantage in energy efficiency. Due to the spontaneous separation of the produced alkanes from

272 | 4 Biomass synthetic fuel technology

water, this technology avoids the considerable energy consumption of distillation. Fig. 4.20 shows the energy balance of glucose conversion into alkanes. Glucose and hydrogen conversion to alkanes are exothermic reactions. Heat release is approximately 380 KJ/mol. However, the heat release of n-hexane combustion is approximately 3,900 KJ/mol, which is equivalent to 90 % of the energy in hydrogen and glucose. In other words, about 90 % of the reactant energy is retained in the paraffin product. According to stoichiometric calculations, the overall energy efficiency of producing corn alkanes is approximately 2.1 (paraffin calorific value 13,500 KJ/kg of sugar, divided by the energy required to produce paraffin 6,300 KJ/kg sugar, which is much higher than the overall energy efficiency of corn ethanol 1.1 (defined as the heating value of ethanol, divided by the energy required for the production of ethanol from maize). The energy consumption for the latter is used for distillation of approximately 5,000– 5,500 KJ/L ethanol, and the overall cost of the ethanol conversion is approximately 14,432 KJ/L ethanol, revealing that more than half of the energy used stems from water purification.

H

OH H

O

–380 kJ/mol H2O

H 7H2

HO H

HO

OH OH

H

3.5O2

6O2

–1 700 kJ/mol

–2 600 kJ/mol 7H2O 6CO2 6H2O

Energy

9.5O2 –3 900 kJ/mol

6CO2 7H2O

Fig. 4.20: Energy balance of glucose conversion into hexane [318].

4.2.2 Alkane production by aqueous-phase catalytic conversion of biomass Taking xylitol as an example, the aqueous catalytic reforming reaction with the metalacid bifunctional catalyst is shown in Fig. 4.21. The process may include four key reactions: (1) the hydrogenation reaction on the metal center; (2) C–O bond rupture reaction on the acid center, mainly a dehydration reaction; (3) C–C crack on the metal center, which is mainly a carbonyl reaction; (4) C–C couple reaction on acid center, which is mainly an aldol condensation reaction. In the whole process, there are also some side reactions such as water-gas reaction, FT synthesis, and simulta-

4.2 Biofuels synthesis via aqueous phase catalytic conversion of biomass

Aldo Condensation

OH OH OH

HO OH OH

Hydrogenation OH

O OH

OH

OH OH

OH O

Dehydration Dehydration

OH OH OH

273

C6

Dehydration

Hydrogenation OH OH

OH

OH

OH OH

|

Hydrogenation

OH

Dehydration OH

C5

Hydrogenation Isomerization

OH OH OH O OH

OH

OH

Decarbonylation

C1–C4

O OH

Dehydration

OH OH

F–T Decarbonylation CO

OH

OH

Hydrogenation

CH4, C2H6...C6H14

Systhesis WGS Methanation

CO2 CH4

Fig. 4.21: Xylitol APR conversion route for alkanes over bifunctional catalyst.

neous methanol reaction. Xylitol first occurs via intermolecular dehydration of some dehydration intermediates, namely small molecule organics such as ketones and alcohols. These species can repeat dehydration on the center of the metal-acid to generate C5 alkanes, or crack C–C to generate C1–C4 light hydrocarbons and CO. Approximately 20 % of the normal hydrocarbon can be converted into heterogeneous hydrocarbon. HZSM-5 can promote the isomerization reaction on acid centers, to improve the biological gasoline octane number of hydrocarbons. The experimental results show that the CO2 content in the products is extremely low, meaning that the reaction CO + H2 O → CO2 + H2 is inhibited under the presence of external hydrogen. Heavy hydrocarbons with C chains longer than xylitol (such as the generation of C6 paraffin) may be the C–C key growth reaction on the acid center, such as the aldol condensation reaction. Several studies about polyols APR reaction reported that ketone is contained in the dehydration of intermediates, which can lead to aldol condensation in the center of the acid to growing chain compounds. These compounds were hydrogenated again on the metal center, and pentane is eventually generated after repeated hydrogenation-dehydration steps. In addition, it has been reported that CO can include FT synthesis to generate a C1–C6 paraffin product on the center of the metal. Therefore, in the process of the xylitol APR reaction, the paraffin product is mainly

274 | 4 Biomass synthetic fuel technology

affected by the C–C crack, C–O crack, and hydrogenation reaction rate control. To obtain a high yield of liquid paraffin, we should improve the rate of C–O cracking, and achieve a match of the hydrogenation rate, while C–C cracking must be suppressed, which can be adjusted via acidic catalyst carrier and metal loading. In 2004, the Dumesic group [322] introduced acidic centers to the metal catalyst, which effectively control the sorbitol aqueous conversion to C1–C6 paraffin, mainly pentane and hexane. Paraffin selectivity can reach 58–89 % with a Pt/SiO2 -Al2 O3 catalyst at the temperature range of 498–538 K. Dumesic et al. suspect that the processexternal hydrogen source-free condition can be seen as the coupling of the sorbitol reduction reaction (eq. (4.18)) and the reforming reaction (eq. (4.19)) processes. Sorbitol can also be generated via catalytic synthesis of hydrogen under the same reaction conditions, which provides a hydrogen source for final synthetic paraffin. The conversion of sorbitol into alkanes can be expressed by eq. (4.20), as an exothermic process, generating 90 % of the calorific value of reactants and 30 wt.% of feedstocks: C6 H14 O6 + 6H2 → C6 H14 + 6H2 O (4.18) C6 H14 O6 + 6H2 O → 6CO2 + 13H2

(4.19)

19 36 42 (4.20) C6 H14 O6 → C6 H14 + CO2 + H2 O 13 13 13 The sorbitol conversion reaction is depicted in Fig. 4.22. C–C cracking generates hydrogen via metal catalyst. Dehydration species develop to cyclic compounds (such as isosorbide) or enol species under the catalysis of acid. This is followed by a hydrogenaO

OH

HO OH

OH

OH OH HO OH

OH Dehydration

HO

Acid

OH HO

HO

Reforming WGSR CO2, H2

HO OH OH HO OH HO Hydrogenation H2 OH

Methanation FTS

C-C cleavage C-O cleavage

O

HO C-C cleavage C-O cleavage

OH OH HO Recycle: Reforming Dehydration Hydrogenation

CH4, H2O, C2H6, C3H8

Fig. 4.22: Main scheme for the conversion of sorbitol into C6 alkane [321].

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tion reaction on metal centers into heavy alkanes such as n-hexane, after repeated dehydration and hydrogenation. The generation of light alkanes was caused by C–C cracking and hydrogenation reaction of intermediates. More light alkanes may be the product of catalytic hydrogenation of CO or CO2 under the effect of metal. Huber et al. [325] further studied the reaction intermediate compounds, and their effects on paraffin product distribution. The process of sorbitol into paraffin also forms intermediates such as isosorbide, 1-4 isosorbide ester, 1-2 diol, and n-butyl alcohol with identical catalysts and reaction conditions. Researchers utilized these as the model compounds to explore the intermediate compound reaction mechanisms in the process, and found that the aqueous catalytic dehydration/hydrogenation reaction (aqueous-phase hydrodeoxygenation, APHDO) mainly consists of three basic steps: C–C cracking, C–O cracking, and hydrogenation. C–C cracking occurs at the center of the metal, prioritizes inverse aldol condensation, and utilizes the carbonyl reactions. C–O cracking was controlled via dehydration reaction in the center of the B-acid. The hydrogenation process takes place on the centers of the metal and the hydrogenation rate increases as C–O–C ≪ C=O C=C. The detail about various chemical reactions occurring during sorbitol conversion is depicted in Fig. 4.23. Sorbitol is first dehydrated into 1-4 isosorbide ester, then dehydrated to isosorbide, and isosorbide hydrogenation on the metal center leads to an open loop into 1, 2-diol. Subsequently, the dehydration/hydrogenation process is repeated until complete removal of O, eventually leading to hexane. Sorbitol can lead to inverse aldol condensation reaction of C–C cracking into a C3 alcohol, and the alcohol and the intermediate compounds can also continue with the carbonylation, hydrogenation, and dehydration reaction for C1–C5 alkanes and small molecule alcohols. As shown in Fig. 4.24, selectivity of alkanes is decided by reactions of C–C cleavage and HDO. In addition, polyols can be completely converted into alkanes and water, and carbon dioxide generation is inhibited with the supply of external hydrogen. Production of specific alkanes is expected in future from polyols via catalyst design.

4.2.3 C8–C15 alkane production via aqueous-phase catalytic conversion of biomass Huber et al. [314] first reported that liquid alkanes with the number of carbon atoms ranging from C7 to C15 were selectively produced from biomass-derived carbohydrates via acid-catalyzed dehydration, which was followed by aldol condensation over solid base catalysts, thus forming large organic compounds. These molecules were then converted into alkanes via dehydration/hydrogenation over bifunctional catalysts that contained acid and metal sites in a four-phase reactor, in which the aqueous organic reactant becomes more hydrophobic and a hexadecane alkane stream removes hydrophobic species from the catalyst before they convert further, forming coke.

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OH HO

HO

OH OH OH OH OH

OH HO

O

HO

O HO

HO

O

OH

C6

OH OH OH

HO

OH OH

O

O O

O

OH

OH OH

HO

HO O

OH

O OH HO

OH

C4~ C5

HO

Dehydration

OH

OH HO

OH

Hydrogenation

OH + OH HO

Dehydration and Hydrogenation

OH OH

HO CH3OH

CH4

C1~ C3

Retro-Aldol codensation Dehydrogenation and Decarbonylation

Fig. 4.23: Reactions of alkane synthesis from sorbitol by APHDO.

Production of heavier liquid-phase alkanes from carbohydrates involves a series of reaction steps, starting with acid hydrolysis of polysaccharides such as cellulose, hemicellulose, starch, and inulin to produce monosaccharides such as glucose, fructose, and xylose (Fig. 4.24). Hydrolysis involves the breaking of C–O–C linkages and is typically carried out in the presence of mineral acid catalysts. These carbohydrates can further undergo acid-catalyzed dehydration to form carbonyl-containing furan compounds, such as HMF and furfural. Subsequently, these carbonyl-containing compounds can be coupled through an aldol condensation to produce larger organic molecules (> C6) via C–C bond formation. The reaction is typically carried out in polar solvents, such as water or water-methanol in the presence of solid base catalysts, such as mixed Mg-Al oxides or MgO-ZrO2 at low temperatures.

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HMTHFA 2H2

OH

O

Selective hydrogenation (metal)

Cellulose

Hydrolysis OH O dehydration Acid

OH

O

O

Adlol selfcondensation (base)

OH

OH H2

O

OH

OH

O

O

C12-alkane Dehydration/ hydrogenation (metal-acid)

OH

O

O O

H2O

O

O

Adlol crossedHMF condensation HMF (Base) H2O OH

O

O

OH

4H2

OH

O

C9-alkane

Hydrogenation (metal)

O

OH 7H2

OH

OH 4H2 3H2O

Dehydration/ hydrogenation (metal-acid) OH

O

O

Hemicellulose

Acid

OH 7H2 5H2O C15-alkane Dehydration/ hydrogenation (metal-acid)

Hydrogenation (metal)

Hydrolysis dehydration

OH 8H 6H O 2 2

O O

O

H2O

O

Adlol crossedFurfural condensation Furfural (Base) H 2O

O

O

O

OH

4H2

Hydrogenation (metal) Adlol crossedcondensation (Base) O

7H2

3H2 2H2O

O

Dehydration/ hydrogenation (metal-acid)

OH O

O

Hydrogenation (metal)

C8-alkane

5H2 3H2O C13-alkane Dehydration/ hydrogenation (metal-acid)

Fig. 4.24: Reaction pathways for the conversion of biomass into liquid alkanes.

As indicated in Fig. 4.25, acetone forms an intermediate carbanion species that can cross-condense with HMF in the presence of a base catalyst, thus forming C9 species, which can subsequently react with a second HMF molecule to form a C15 species. These aldol adducts have low solubility in water as a result of their nonpolar structure and thus, precipitate out of the aqueous phase. Subsequently, the C = C and C = O bonds in these aldol adducts are saturated via hydrogenation in the presence of a metal catalyst (Pd), thus increasing their solubility and producing large water-soluble organic compounds. These molecules are then converted into liquid alkanes (C7–C15) via aqueous-phase dehydration/hydrogenation (APD/H) over a bifunctional catalyst (Pt/SiO2 -Al2 O3 ) containing acid and metal sites in a four-phase flow reactor. Another possible route for the production of liquid alkanes is to convert HMF and furfural into 5-hydroxymethyltetrahydrofurfural (HMTHFA) and tetrahydrofurfural (THF2A), respectively. HMTHFA and THF2A can be self-condensed to form C12 and C10 species, respectively, that are subsequently hydrogenated to form water-soluble organic species. Raw materials for industrial production of furfural are mainly cottonseed shells, corncobs, sunflower seed shells, bagasse, rice husks, and broadleaf timber. The composition of some commonly used materials is listed in Tab. 4.9.

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Tab. 4.9: Composition of raw materials for furfural production [321, 326, 327]. Raw materials

Pentosan ( %)

Cellulose ( %)

Lignin ( %)

Corncob Bagasse pith Bagasse Seed coats Straw White birch Aspen Wheat straw Sorghum Reed Sunflower seed shells

36–40 29.07 25.6–29.1 22–25 19–24 23.96 19.5 25.56 22.03 18–25 26–28

32–36 48.05 48.2–55.6 37–48 38–43 59.02 59.0 40.40 48.83 43–58 30–40

17–20 19.07 18–20 29–32 16–21 23.84 20.6 22.34 20.12 21–24 27–29

4.2.3.1 Aldol condensation of furfural via acetone Aldol condensation is an organic reaction in which an enol or an enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone, followed by a dehydration to result in a conjugated enone. The first part of this reaction is an aldol reaction, the second part a dehydration — an elimination reaction (involving removal of a water molecule or an alcohol molecule). Dehydration may be accompanied by decarboxylation when an activated carboxyl group is present. The aldol addition product can be dehydrated via two mechanisms; a strong base, like potassium t-butoxide, potassium hydroxide, or sodium hydride in an enolate mechanism, or in an acid-catalyzed enol mechanism. O H3C C CH3

OH-

+

O H3C C CH2

Âý

O O H3C C CH2 O O

CH CH2 OH

O

CH CH2

¿ì +

O

CHO

O

CH CH2

OH O + C CH3

O C CH3

H2O

O

O

CH CH2

+

H2O

(4.21)

O C CH3

(4.22) O + C CH3

OH-

(4.23) O + CH CH C CH3

H2O

(4.24)

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Total reaction:

CHO

O

+

Furfural

O

FA

Acetone

O CH CH C CH3 +

O

O CH CH C CH3

OH-

O H3C C CH3

CHO

O

OH-

HC

O

HC C CH O

Furfural

FA

(4.25) CH

O

F2A

(4.26) In addition, Fakhfakh et al. [328] found that intermediate F3 A2 existed in the reaction kinetics study of condensation of furfural with acetone. They suggested that F3 A2 was formed by three molecules of furfural and two molecules of acetone, then further decomposed to FA and F2 A. Therefore, a novel reaction path as shown in eq. (4.27) was put forward, which was confirmed in the following studies by Xing et al. [329] O O

O

O + 2

3 F

A

-3H2O

O

O

O

O

1,5,9-tri-2-furylnona-1,8-diene-3,7-dione (F3A 2)

O

O

O

O

O

+ FA

F2A

(4.27)

4.2.3.2 Jet fuel production from aldol condensation products Hydrodeoxygenation technology of aldol condensation products from furfural or HMF and acetone for the production of jet fuels is still new, and limited results have been reported so far. According to the hydrogenation process, this technology can be divided into a one-stage and a two-stage hydrogenation process. (1) One-step hydrogenation process. Aldol condensation products of furfural or HMF and acetone lack a hydrophilic group and are thus difficultly soluble in water. Therefore, it is difficult to achieve continuous production in a fixed-bed reactor in a water phase system for hydrodeoxygenation. Chatterjee et al. [330] reported a supercritical CO2 hydrogenation process, in which an aldol condensation product of furfural or HMF and acetone was transformed to long-chain alkanes via one-step hydrogenation. The process parameters are as follows: T = 80 °C, PCO2 = 14 Mpa, PH2 = 4 Mpa, time = 20 h. They found that Pd/Al-MCM-41 exhibited the best reaction performance, with the conversion and selectivity of alkane above 99.0 %. However, continuous production remains difficult, and the reaction time is longer than other processes. (2) Two-step hydrogenation process.

280 | 4 Biomass synthetic fuel technology

Huber et al. [331] and Chheda et al. [332] studied the condensation reaction of furfural or HMF and acetone in the aqueous phase using Mg-Al oxides. The condensation products are treated by a two-step hydrogenation process. Firstly, a Pd/Al2 O3 catalyst was used at 120 °C for the hydrogenation reaction, promoting water solubility of condensation intermediates. Then, 4 wt.% Pt/SiO2 -Al2 O3 was used for the further hydrogenation deoxidization reaction at 250–265 °C in the four-phase hydrogenation reactor.

4.2.3.3 Catalysts applied in aldol condensation of furfural with acetone The aldol condensation reaction is generally catalyzed via alkali catalysts such as alkali metal hydroxide solution (NaOH, KOH, and ammonia). However, these types of liquid alkali catalyst cause corrosion of equipment, are difficult to recycle, and the process is complex. Therefore, the development of stable and highly active solid base catalysts has been a focus of research. Depending on the presence of hydrogenation functionality, the solid base catalysts can be divided into conventional solid base catalysts and bifunctional solid base catalysts. (1) Conventional solid base catalysts. The aldol condensation reaction is mainly catalyzed by the alkaline center, while acid centers are also necessary to promote the condensation reaction. Composite oxide solid alkali catalysts such as Mg-Al oxides show acid-alkali bifunctional properties when Mg2+ in hydrotalcite is replaced by Al3+ . (2) Bifunctional solid base catalysts. To shorten the process routes, Barrett et al. [333] developed bifunctional solid base catalysts such as 5 wt.% Pd/MgO-ZrO2 , which was used for aldol condensation of furfural or HMF with acetone in the water phase at 353 K and 326 K, respectively. Then, the condensation products were hydrotreated at 393 K and 55 bar in the same reactor, promoting water solubility of intermediates for subsequent hydrodeoxygenation. The process successfully combined the condensation reaction with the hydrogenation reaction in the same reactor at low temperature, which shortens the process to some extent.

4.2.4 Oxygenated liquid fuel production via aqueous-phase catalytic conversion of biomass 4.2.4.1 Advantages of higher chain alcohols Ethanol is an excellent biomass liquid fuel, not only improving mixed fuel octane number, but also reducing the toxic emissions of pollutants. However, fuel ethanol as a substitute fuel has some disadvantages. Firstly, the energy density of ethanol is 30 % lower than that of gasoline. Secondly, ethanol with strong water imbibition leads to mixed fuel easily absorbing moisture from the air. Thirdly, the addition

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of ethanol can increase the vapor pressure of the mixed fuel. Therefore, light hydrocarbon components of gasoline must be extracted before utilization. Fourthly, the existing oil storage and transportation pipelines are not suitable for ethanol fuel. Higher chain alcohols refer to alcohols that contain more than four or five carbon linear or branched chain alcohols, including butanol, isobutanol, isopentanol etc. Higher chain alcohols are considered more suitable than ethanol for gasoline alternatives, since the chemical and physical properties of higher chain alcohols are similar to those of gasoline fuels. First of all, compared to ethanol, higher chain alcohols have longer alkyl chains, weaker polarity, and lower water imbibition than ethanol. Higher chain alcohols can decrease the sensitivity to water when mixed with hydrocarbon fuel. Secondly, higher chain alcohols with lower saturated vapor pressure reduce the risks of high evaporative emissions. For example, the vapor pressure of n-butanol is approximately 11 times lower than that of ethanol. Thirdly, well compatible higher chain alcohols can achieve higher mixing ratios. Without reforming automobile engines, the ethanol mixing ratio is approximately 10 %; however, higher chain alcohols have a higher mixing ratio. Fourthly, the per unit volume energy density of higher chain alcohols is high. For example, the energy density of butanol is similar to that of gasoline, while the energy density of ethanol is only 65 % that of the gasoline. Higher chain alcohols have obvious economic efficiency compared to ethanol. Finally, higher chain alcohols are suitable for pipeline transportation, which can utilize the existing infrastructure, such as storage equipment, pipelines, supply, and distribution system. Sandia Labs in the United States found isopentanol as a fuel for the Homogeneous Charge Compression Ignition (HCCI) engine promising, whether used alone or in combination with gasoline.

4.2.4.2 Synthesis of higher chain alcohols Higher chain alcohols were traditionally synthesized via chemical method or via fermentation. There are numerous types of chemical methods for higher chain alcohol synthesis. However, while most of them are used in the laboratory, industrial production is limited. (1) Oxo-synthesis. With propylene and syngas (carbon monoxide and hydrogen) as raw materials, butyraldehyde is synthesized from hydroformylation via catalysts, which convert to n-butanol and isobutanol through hydrogenation. CH3 CH=CH2 + CO + H2 → CO3 CH2 CH2 CHO + CH3 CH(CH3 )CHO

(4.28)

CH3 CH2 CH2CHO + H2 → HO3 CH2 CH2 OH

(4.29)

CH3 CH(CH3 )CHO + H2 → CH3 CH(CH3 )CH2 OH

(4.30)

282 | 4 Biomass synthetic fuel technology

Several transition metal carbonyl ligand compounds exhibit activities for hydroformylation, while the typical active metals used in butyraldehyde synthesis from propylene are Co and Rh. Co has been widely used for oxo-synthesis from propylene, which is known as high-pressure carbonyl synthesis due to the relatively high pressure (27–28 MPa) required in the process. The conversion of propylene is approximately 95–97 % and butyl aldehyde selectivity is 80–85 % with a Cn /Ciso ratio of 3–4 at the typical reaction conditions (150–165 °C). However, this technology with a long process, requiring big equipment, and high energy consumption has other problems too, such as wear of cobalt particle suspension on the equipment, equipment cleaning, and maintenance workload. In the mid-1970s, the United States UCC, British Davy, and Johnson Matthey first successfully developed Rhodium-phosphine ligand catalysts for hydroformylations. Rhodium has higher catalytic activity than cobalt (almost 100 times). The introduction of triphenylphosphine (TPP), or other organic phosphine ligands in rhodium catalysts decreases the reaction temperature (100–120 °C) and pressure (0.7–2 MPa), therefore this process is also called low-pressure oxo-synthesis. In addition, the Cn/Ciso ratio increases to 17–18. Rh catalysts are the mainstream of the current oxo-synthesis technology. However, rhodium is resource-poor, expensive, and triphenylphosphine is toxic and harmful to humans. Therefore, the development of new non-rhodium catalysts will be a future challenge for oxo-synthesis. (2) Hydrolysis of haloalkanes. Alcohols also can be synthesized from haloalkanes via nucleophilic substitution reaction in sodium hydroxide solution. The nucleophilic reaction is often accompanied by an elimination reaction. If primary alcohols are required, more moderate reagents such as Na2 CO3 or Ag2 O (H2 O) can be added. (3) Aldol condensation. Aldol condensation of acetaldehyde is mainly used for the preparation of n-butanol and hydroxybutanal.

(4.31)

4.2.5 Diesel synthesis from syngas Distillate is defined as the material with a boiling range between 175 and 370 °C, and it roughly corresponds to the C11–C22 aliphatic hydrocarbon cut. Diesel fuel however, is defined as distillate that meets legislated fuel specifications. The refining of Fischer–Tropsch syncrude to on-specification diesel fuel will be dealt with sep-

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arately, since it requires satisfaction of more than just the distillation range of the product. There are two key properties of a Fischer–Tropsch syncrude that determine the ease with which it can be converted into distillate and thus, satisfy the distillation range requirement. These properties are its carbon number distribution and nonparaffin content. The carbon number distribution indicates how much straight run distillate material is present in the syncrude. No further refining of the straight run distillate is required to meet distillation requirements, and this fraction can be directly recovered via separation. It is possible to change the carbon number distribution of the syncrude by manipulating the chain growth probability (R-value) of the Fischer–Tropsch process. LTFT syncrude has a significant benefit over HTFT syncrude in terms of straight run distillate production. LTFT syncrude has a higher straight run distillate production than HTFT syncrude. The nonparaffin content of the C10 and lighter fractions (naphtha, gaseous, and aqueous) is a measure of the synthetic capability of the syncrude. Although it is possible to separate and recover gaseous C2 hydrocarbons, this requires cryogenic separation, which is not considered part of a basic Fischer–Tropsch gas loop design. However, the reactive C3–C10 species can easily be recovered and converted into distillate range material via appropriate technologies, such as alkylation and oligomerization. The distinction between paraffins and nonparaffinic species is made due to the energyintensive nature of the paraffin conversion; it is not easy to convert short-chain paraffins into distillate. The nonparaffinic content of the C3–C10 oil and gaseous fraction increases in the order: Co-LTFT (45 %) < Fe-LTFT (75 %) < Fe-HTFT (> 85 %). This indicates that in respect of the nonparaffin contents of the C3–C10 material, HTFT syncrude has a benefit over LTFT syncrude and iron-based catalysts have an advantage over Co-based catalysts.

4.2.6 Gasoline synthesis via methanol from syngas Syngas can also be converted to methanol, which is the primary method of methanol production and has been practiced converting both natural gas and coal into chemical grade methanol. Methanol is typically produced as a building block for the manufacture of other chemicals and, within limits, has been blended with conventional gasoline. An alternative and commercially proven route for converting syngas to liquid fuels is through the conversion of methanol to conventional gasoline (Fig. 4.25). MTG gasoline meets the requirements for conventional gasoline, is fully compatible with refinery gasoline, and meets the ASTM D4814 Standard for Automotive Spark-Ignition Fuels. Mobil developed a fixed-bed methanol to gasoline (MTG) process in the 1970s

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Gasification

Gasification

MeOH synthesis Fischertropsch synthesis

MTG

Refining Liquid fuels

Coal/ biomass Gasification

H2 Direct liquefaction

2 CH3OH Methanol CH3OH, CH3OCH3 Methanol, Di-Methyl Ether

Refining

CH3OCH3 + H2O Di-Methyl Ether Light Olefins + H2O

Light Olefins

C5+ Olefins

C5+ Olefins

Paraffins Naphthenes Aromatics

Gasoline

Fig. 4.25: MTG reaction paths.

using a proprietary ZSM-5 zeolite catalyst. Mobil commercialized the first gas to gasoline plant in New Zealand in 1985. The New Zealand plant produced 14,500 kB/D of gasoline and was operated by the New Zealand Synthetic Fuels Corporation (New Zealand Synfuels), a joint venture between the government of New Zealand and Mobil, until 1995. Operation of the first coal to gasoline plant via MTG technology began in 2009 in China by the Jincheng Anthracite Mining Group (JAMG). This 2,500 B/D gasoline plant began operations in June of 2009 and successfully demonstrated the coal to gasoline concept. Both the Fischer–Tropsch and MTG routes are able to convert synthesis gas into liquid transportation fuels. However, their respective product slates are very different. The Fischer–Tropsch process typically produces a broad spectrum of straight-chain paraffinic hydrocarbons that can be further refined to produce commercial quality gasoline, jet fuel and diesel. In contrast, MTG selectively converts methanol to one liquid product: ultra-low sulfur, low-benzene regular octane gasoline. In the MTG process, the conversion of methanol to hydrocarbons and water is virtually complete with the product being a mixture of synthesis hydrocarbons and water with a limited amount of C2 gases. The dehydration and synthesis reactions release approximately 1.74 MJ per kg of methanol. This heat release would result in a temperature rise of about 600 °C in an adiabatic reactor system. In the MTG design, the temperature rise from methanol feed to MTG

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reactant is limited to about 100 °C by controlling the volume of the recycled gas. The conversion reactor inlet temperature is controlled via temperature adjustment of the recycle gas via heat exchange with the reactor effluent. Reactor effluent is also used to preheat, vaporize, and superheat the methanol feed to the DME reactor. The reactor effluent is then further cooled to 25–35 °C and passed to a product separator where light gases are disengaged and both liquid hydrocarbon and water are separated. The gas phase (mostly consisting of light hydrocarbons) is returned to the recycle gas compressor. The water phase can be sent to effluent treatment or recycled within the overall complex. The liquid hydrocarbon product (raw gasoline) mainly contains gasoline boiling range material as well as dissolved hydrogen, carbon dioxide, and light hydrocarbons. After separation of raw gasoline from light gases and water, the raw gasoline is processed through a de-ethanizer to remove all remaining C2 gases followed by a stabilizer column that removes C3s and C4s as LPG fractions to control the vapor pressure of the resulting gasoline. At this point, the MTG gasoline is consistent with conventional gasoline with the exception of a concentration of 1,2,4,5-tetramethyl benzene (or durene), that is higher than in typical gasoline. Durene is a compound that crystalizes at moderate temperatures and affects gasoline performance and appearance. A maximum durene content of 2 % has been set to ensure drivability and performance consistent with petroleum-based fuels. To achieve this level of durene content, MTG gasoline is split into a light fraction and a heavy fraction, which concentrates the durene with the other higher aromatics. This heavy fraction is processed in a mild hydrotreater that reduces the durene content primarily through isomerization and demethylation reactions with minimal effect on gasoline yield and octane (Fig. 4.26).

Purge gas C2− MTG reactor system (Multiple)

Light Gasoline

LPG

Blending LPG

Methanol

Treated gasoline Heavy gasoline H2O

Stabilized gasoline Raw gasoline Stabillizer DeEthanizer Splitter

Fig. 4.26: MTG process flow diagram.

HGT reactor

Stabillizer

Finished gasoline

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4.3 Biofuel production via polymerization of low carbon number olefins The polymerization of light olefins to produce higher molecular weight hydrocarbon fuels over acid-type catalysts is a well-known area of chemistry. The products of acidcatalyzed reactions of olefins may include primarily olefins from straight oligomerization or mixtures of olefins, paraffins, cycloalkanes, and aromatics from what has been termed “conjunct” polymerization. The product spectrum is influenced both by reaction conditions and the nature of the catalyst; however, it is generally restricted to the gasoline range (< C12). Oligomerization is mostly applied in the refining context and is an acid-catalyzed, very exothermic reaction. Metal-catalyzed oligomerization can also be performed, but it is better suited for chemical applications. There are many potential applications of oligomerizations in Fischer–Tropsch refining: (1) High-octane olefinic motor gasoline can be produced by oligomerization of a wide range of alkenes. This type of conversion is the application of oligomerization most often found in crude oil refineries. (2) It can produce alkylate-equivalent high-octane paraffinic motor gasoline via selective alkene oligomerization followed by HYD. Butenes are usually the feed material for this type of conversion (also called indirect alkylation) and the alkylate can be produced in two different ways. The first way is to skeletally isomerize butene to produce isobutene, which can then be selectively dimerized to trimethyl pentenes over an acidic resin or SPA catalyst. The second way is to dimerize the 1-butene-rich syncrude directly over SPA at low temperatures to mainly produce trimethyl pentenes. (3) It can produce isoparaffinic kerosene for synthetic jet fuel. Oligomerization naturally introduces branching in the product, which provides the hydrogenated kerosene fraction with very good cold flow properties. (4) By selecting an appropriate oligomerization technology that produces more linear distillate range material, the oligomers can be hydrogenated to yield a high cetane number distillate. (5) Lubricating base oils can be prepared via oligomerization of n-1-alkene-rich fractions. (6) Various chemicals can be produced via oligomerization: among others, the alkene feed for hydroformylation to produce plasticizer, detergent alcohols, and alkenes for benzene alkylation to produce detergents.

4.3.1 Mechanism of polymerization of low carbon number olefins The reaction chemistry of alkene oligomerization cannot be described in generic terms without some reference to the catalyst or process. There is not a single mechanism

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and, even when alkene addition is catalyzed by the same general mechanism, different catalysts may still produce products with very different characteristics. In general, there are four different mechanisms of industrial relevance for alkene oligomerization (Fig. 4.27). Olefinic naphtha recycle C3‒C4 olefins Coal tar naphtha LPG quench Liquid petroleum gas

Olefinic naphtha Olefinic distillate Heavy product Fig. 4.27: Oligomerization mechanisms illustrated by propene dimerization.

(1) The classic Whitmore-type carbocation mechanism. Brønsted acid-catalyzed alkene oligomerization takes place through this mechanism. The first step in this mechanism involves the protonation of an alkene to yield a free carbocation intermediate. This carbocation intermediate is capable of all the side reactions associated with such chemistry. Depending on the length of the carbon chain, double bond isomerization, skeletal isomerization, and cracking via β-scission may take place. These are all monomolecular reactions and are always in competition with the bimolecular addition reaction. Subsequent to the alkene addition reaction, the addition intermediate is still a carbocation, which is still capable of further alkene addition as well as the aforementioned side reactions. The final product is an alkene and it is also capable of further protonation and further reactions. Zeolites and acidic resin catalysts oligomerize alkenes via a carbocation mechanism. (2) Ester-based mechanism. Some acid catalysts form strong formal σ-bonds with the protonated intermediate, and this mechanism involves a polarized acid ester, rather than the equivalent carbocation. Transitions that would otherwise involve a primary carbocation intermediate become possible, when the α-carbon of the alkene is bonded to the acid, since the α-carbon is no longer a primary carbon. The polarized intermediates are weaker electrophiles than carbocations and reactions can be more selective; however, typical acid-catalyzed side reactions

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can still occur. Phosphoric acid-based catalysts oligomerize alkenes via an ester mechanism. (3) Free radical mechanism. The free radical oligomerization of alkenes is initiated via formation of a free radical species that propagates via addition to another alkene. The chain growth is terminated when the radical recombines with another radical, or abstracts a hydrogen atom to initiate a new radical chain. This chemistry is also encountered in free radical polymerization. The process is thermally initiated, and co-feeding compounds with low bond dissociation energy (such as peroxides) can lower the initiation temperature. The side reactions occurring during free radical oligomerization are usually fewer than for acid-catalyzed oligomerization. Double bond isomerization occurs via intramolecular hydrogen transfer (Fig. 4.27). Skeletal isomerization and cracking is not prevalent, except at high temperatures (> 400 °C) where C–C bond scission becomes significant. There is no catalyst involved in free radical oligomerization. (4) Organometallic insertion mechanism. Some metals are capable of forming coordination complexes with alkenes. This chemistry forms the basis for Ziegler– Natta and metallocene polymerization catalysis. Chain growth proceeds by 1,2insertion, and is terminated by β-hydride elimination. The metal most often encountered in general alkene oligomerization is Ni. For ethene oligomerization specifically, a variety of metal complexes is used, which include metals such as Ni, Al, Zr, Ti, and Cr. The ligands on the organometallic catalyst can be designed to catalyze oligomerization in a very specific way, making it useful for very selective catalysis. Homogeneous and heterogeneous nickel-based catalysts can oligomerize alkenes via an insertion mechanism to yield more linear products than during acid catalysis.

4.3.2 Gasoline production via polymerization of low carbon number olefins The oligomerization of light olefins (e.g., ethylene, propylene, and butylene) is an important route for the production of liquid higher olefins, which are valuable feedstocks used in the manufacture of high-octane gasoline, detergents, plasticizers, fatty acids, and other petrochemicals. So far, light olefins used as the basic feedstock for the petrochemical industry are primarily produced through steam cracking and fluid catalytic cracking (FCC) using naphtha and other petroleum products as feedstock. Recently, there has been growing interest in the production of light olefins using renewable biomass as feedstock. As a refining technology, oligomerization was originally developed to convert the gaseous products from crude oil cracking operations into liquid products. Oligomerization units started appearing in second-generation crude oil refineries, and some refineries to this day still have oligomerization units. The main product from these units is high-octane olefinic motor gasoline (Fig. 4.28).

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Olefinic naphtha recycle C3‒C4 olefins Coal tar naphtha LPG quench Liquid petroleum gas

Olefinic naphtha Olefinic distillate Heavy product Fig. 4.28: Flow diagram of synfuel production from low-carbon olefins by oligomerization.

4.3.3 Jet fuel production via polymerization of low carbon number olefins Jet fuel is a type of aviation fuel specifically designed for the use in aircraft that are powered by gas turbine engines. It is colorless to straw-colored in appearance. The most commonly used fuels for commercial aviation are Jet A and Jet A-1, which are produced to a standardized international specification. The only other jet fuel commonly used in civilian turbine engine-powered aviation is Jet B, which is used for its enhanced cold weather performance. Jet fuel is a mixture of a large number of different hydrocarbons. The range of their sizes (molecular weights or carbon numbers) is restricted by the requirements for the product, e.g., the freezing point or smoke point. Kerosene-type jet fuel (including Jet A and Jet A-1) has a carbon number distribution between 8 and 16 (carbon atoms per molecule); wide-cut or naphtha-type jet fuel (including Jet B) between 5 and 15. One of the key components of synthetic jet fuel is isoparaffinic kerosene (IPK). IPK is the hydrogenated kerosene produced from oligomerization of C3–C4 olefins over an SPA catalyst. This product is a good source of highly branched C8–C12 aliphatic hydrocarbons and demonstrates how jet fuel yield can be increased via the conversion of lighter material into kerosene. Naphtha can also be converted into kerosene by technologies such as oligomerization and aromatic alkylation. Oligomerization of propylene in combination with aromatics is ideal for jet fuel production. An added advantage of this technology is that it increases refinery flexibility (Fig. 4.29). The material in the light kerosene range can also be used as a highoctane motor gasoline blending component. When oligomerization of C3–C5 olefins is employed for a refinery design to maximize jet fuel it is easy to meet both the yield and quality requirements for jet fuel. Aliphatic hydrocarbons in the kerosene boiling

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FISCHERTROPSCH

H2 + CO

C1 to C40 HC Liquid

Coal gasification

C3 & C4 Olefins

Naphtha cut

Distillate cut

Coal tar

Polymerization

Hydrogenation

Hydrogenation

Hydrogenation

Fractionation

Fractionation

Naphtha Distillate H’treat

H’crack

Petrol

Diesel

Heavy naphtha #2

Light distillate #2

Fractionation

Iso-Paraffinic kerosene

Petrol

Heavy naphtha #1

Blender

Diesel

Light distillate #1

Sasol synthetic jet fuel

Fig. 4.29: Production schematic for Sasol fully synthetic jet fuel.

range require branching to meet the maximum freezing point (47 °C) specification. Monomethyl branching is usually sufficient to lower the freezing point of the mixture sufficiently for use as jet fuel.

Changzhu Li, Wen Luo, Zhihong Xiao, Lingmei Yang, Aihua Zhang, and Pengmei Lv*

5 Technologies in vegetable oil and biodiesel Vegetable oil can be obtained from wild or cultivated oil-bearing plant juice, leaf, or stem, through crushing, extraction and refining. According to the component, the oil can be edible, some can only be used in industry, and some can be directly used as liquid fuel. Biodiesel is a kind of liquid fuel from a variety of raw materials, including vegetable oil, animal fat, waste food oil and some other oil containing microalgae, that is obtained through a series of processing and production steps. It is a good alternative substitute to fossil oil for its physicochemical properties and combustion performance.

5.1 Vegetable oils fuel 5.1.1 Physicochemical properties of vegetable oils Vegetable oils can be directly used as engine fuel. Rudolf Diesel did not plan to use fossil oil as fuel when he invented the engine in 1895. The engine presented at the World’s Exhibition in Paris in 1990 initially ran on peanut oil. However, several obstacles had to be overcome. For example, vegetable oil has high viscosity, low sixteen cetane number and poor atomization, leading to the formation of carbon deposits on the injector nozzles, the lubrication system is easily polluted, and there are cold start problems and so on. At the beginning, some researchers wanted to reduce the viscosity and the mobility of vegetable oil by direct mixing or microemulsion. However, most problems such as carbon deposition and lubricate oil pollution were too difficult to solve.

Changzhu Li: Hunan Academy of Forestry, Changsha 410004, China Wen Luo: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China Zhihong Xiao: Hunan Academy of Forestry, Changsha 410004, China Lingmei Yang: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China Aihua Zhang: Hunan Academy of Forestry, Changsha 410004, China *Corresponding Author: Pengmei Lv: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China; E-mail: [email protected] (P. Lv), Tel. +86-20-87057760 https://doi.org/10.1515/9783110476217-005

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5.1.1.1 Physical and chemical properties of vegetable oils Vegetable oils are composed of oils and fats. The main sources of vegetable oils are oil crops (rapeseed, peanut, soybean, cotton, sunflower, sesame and castor etc.) and woody oil plants (jatropha, Chinese tallow tree, tung tree, oil palm, wilsoniana, coconut etc.). Algae will also be an important source of vegetable oil in the future.

5.1.1.1.1 Chemical structure of vegetable oils Vegetable oil is a large class of natural organic compounds (hydrocarbon derivatives) and always referred to the oils and fats. And it is, from its chemical concept, defined as mixtures of mixed triglycerides. Its general chemical structural formulas are shown as follows:

Where the R, R󸀠 , R󸀠󸀠 represent the three senior fatty acid side chains. Their lengths may be the same, or not; and the fatty acid may be saturated acid or unsaturated acid. Vegetable oils exist in the plant cell, and their main components are higher fatty acids of even numbers of carbon atoms. There are vegetable oils that are normally liquid at room temperature with unsaturated esters and glycerol carbon bonds, which are called oil, and there are vegetable oils that are solid at room temperature with unsaturated bonds, which are called fat. The unsaturated fatty acid is easily oxidized by air or bacteria. The oils contain conjugated double bonds and are easy to polymerize, and they often form complex polymers.

5.1.1.1.2 Determination and analysis of the index of oil Saponification number Through the catalysis of acid, alkali or enzyme, oil can produce glycerol and fatty acids by hydrolysis. Under alkaline condition, the product of oil by hydrolysis is soap.

(5.1)

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The average molecular weight of different oils varies due to the presence of different fatty acid constituents. For an oil of a fixed weight, the higher the molecular weight, the lower the molecular number. The number of milligrams of potassium hydroxide required to saponify 1 g of fat under the conditions specified is called its saponification value. It is a measure of the average molecular weight (or chain length) of all the fatty acids present. The higher the molecular weight, the lower the saponification value is. If an oil sample and a blank sample are placed in a water bath with enough 0.5 N potassium hydroxide alcohol solution and heated to reflux for 30 minutes, using hydrochloric acid titration and phenolphthalein as indicator we can calculate the saponification number with the following formula: K = (ml2 − ml1 ) × N × 56.108/W Where K is the sample saponification number; ml2 is the amount of potassium hydroxide consumed by the blank test solution of (mL); ml1 is a the solution volume of potassium hydroxide consumed by the sample (mL); N is equivalent concentration of acid; W is the weight of the specimen (g).

Iodine number Oils containing unsaturated fatty acid will yield solid saturated fatty acid esters by catalytic hydrogenation with Ni as catalyst, under the conditions 200°, 1–3 bar pressure. The reaction is unsaturated fatty acid addition reaction. Generally, acetic acid solution of iodine chloride or iodine bromide are used as reagent for iodine number analysis. Iodine number is a measure of total unsaturation within a mixture of fatty materials. It is expressed in grams of iodine which react with 100 g of the respective sample. The iodine number is proportional to unsaturation of oils. Dissolve the oil sample with chloroform, carbon tetrachloride or glacial acetic acid. Add excessive Webster’s reagent or Jones reagent (chlorinated iodine or iodine bromide) to the sample. React 60–90 min in the dark, and then add the proper amount of potassium iodide. Determine the iodine precipitation with sodium thiosulfate titration, with starch as indicator. The same experiment is run with an equal amount of blank sample. The results can be calculated with the following formula. I2 = (ml2 − ml1 ) × N × 0.1269 × 100/W Where I2 is the sample iodine number; ml2 is the consumption of sodium thiosulfate solution volume for blank test (mL); ml1 is the consumption of sodium thiosulfate solution volume for the sample (mL); N is equivalent concentrations of sodium thiosulfate solution; W is the weight of the specimen (g).

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Acid value Acid value is a measure of mineral acids and free fatty acids (FFAs) contained in a fuel sample. It is expressed in mg KOH required to neutralize 1 g of fatty acid methyl esters (FAME). AV = ml × N × 56.108/W Where AV is the acid value of sample; ml is sample consumption of potassium hydroxide solution volume (ML); N is the equivalent concentration of KOH; W is the weight of the specimen (g).

5.1.1.2 Characteristics of vegetable oil fuel Vegetable oil is a complex mixture, whose main component is a mixture of triglycerides, FFA and a variety of small amounts of non-oil substances. As a fuel, it has the following characteristics.

5.1.1.2.1 Heating value The heating values of vegetable oils have been found to increase with increasing length of the fatty acids chain, while they are inversely related to the number of double bonds. Therefore, the heating value of the oil is different with different fatty acid composition. In general, if the total amount of carbon and hydrogen in oil is bigger, its heating value is bigger too. Heating value of vegetable oils is about 87–89 % of the heating value of diesel.

5.1.1.2.2 Cetane number The cetane number of diesel used in high-speed diesel engines is generally 40 to 60. The method to measure cetane number is different for vegetable oil and diesel. In the United States, cetane number is researched using standard fuel injectors. The vegetable oil is heated to around 38 °C and sprayed into the cylinder to combust. The cetane number of vegetable oil is assessed according to the actual conditions. The cetane number of vegetable oils is generally 35 to 42.

5.1.1.2.3 Viscosity Viscosity is one of the main quality indicators of fuel. At a certain temperatures, the viscosity can determine the mobility of the fuel and whether it is difficult to supply oil to the injector. Higher fuel viscosity leads to poor fuel atomization, bad mixture formation, and incomplete combustion. The viscosity of vegetable oil is generally 10–20 times higher than that of diesel. And castor oil’s viscosity is about 100 times diesel’s viscosity. A key problem in using biodiesel as a substitute is to reduce the viscosity of vegetable oil. The viscosity of vegetable oil can be reduced by heating,

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and adding diesel to vegetable oil can also reduce the viscosity of the fuel. The primary method for solving the problem with conventional biodiesel is to change the molecular structure of the oil. The most widely used methods are transesterification and pyrolysis.

5.1.1.2.4 Flash point The flash point of a volatile material is the lowest temperature at which it can vaporize to form an ignitable mixture in air. The flash point of diesel is usually around 60 °C. Vegetable oils are biodegradable and nonvolatile oils, their flash points are about 234– 293°, much higher than that of diesel. Flash point is a measure of the flammability of fuels and thus an important parameter for assessing hazards when the fuel is being transported or stored in the warehouse.

5.1.1.2.5 Freezing point and melting point Vegetable oils are composed of not only a single compound but a variety of different esters. The different compounds have different freezing points and melting points. So It is a gradual process from liquid to solid, and the vegetable oil will freeze completely in a certain temperature range. Similarly, the melting point is within a certain range to being completely melted. The highest temperature when the vegetable oil changes state from liquid to solid or the lowest temperature when the vegetable oil changes state from solid to liquid is called melting point.

5.1.1.2.6 Cloud point The cloud point of a fluid is the temperature at which dissolved solids are no longer completely soluble, precipitating as a second phase giving the fluid a cloudy appearance. Vegetable oil has a higher cloud point than diesel because of the saturated fatty acids contained in it. If vegetable oils are used as fuel at low temperatures, we can make a winterization treatment at 4°–5°, and then filter out most saturated fatty acids. For example, 10 % soybean oil which is composed of saturated fatty acids may be filtered by winterization.

5.1.1.2.7 Phospholipids Vegetable oils contain phospholipids. The phosphate can absorb atmospheric water vapor to form an insoluble gel, which leads to the formation of fouling in the fuel tank and tubing, causing faults. Different vegetable oils contain different amounts of phospholipids, such as sunflower oil has less phosphorous, and soybean oil has higher phosphorous. Using vegetable oil as a fuel we should remove the phospholipids from the crude oil. That is to say we should refine the vegetable oil. The method is to add an amount of water or caustic alkali and then centrifuge the mixture to remove the phospholipids. At the same time we can also remove the FFAs in the oil.

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5.1.1.2.8 Wax The wax in vegetable oil is a mixture consisting of long-chain fatty acid esters and alcohols. It is different from the paraffin in diesel. Paraffin wax is a solid, which can dissolve in the oil. High concentrations of plant wax will render oil turbid, and the wax at room temperature will gradually precipitated. The wax will melt when the oil is heated. We can remove the wax with winterization, cold-based or cold wash by detergent. Tab. 5.1 gives several common vegetable oil fuel characteristics, and also lists the characteristics of US No. 2 diesel value as a control. Tab. 5.1: Fuel characteristics of several vegetable oils. Species

Viscosity (cm2/s)

Cetane number

Castor oil Cotton oil Linseed oil Peanut oil Rapeseed oil Sesame oil Soybean oil Sunflower oil 2# diesel

2.97 0.335 0.272 0.396 0.37 0.365 0.326 0.339 0.027

41.8 34.6 41.8 37.6 40.2 37.9 37.1 47

Heating value (kJ/kg)

Cloud point (°)

Flash point (°)

Freezing point (°)

37,274 39,468 39,307 39,782 39,709 39,349 39,623 39,575 45,345

–

260 234 241 271 246 260 254 274 52

−31.7 −15.0 −15.0 −6.7 −31.7 −9.4 −12.2 −15.0 −33.0

1.7 17.0 12.8 −3.9 −3.9 −3.9 −7.2 −15.0

Density (kg/m3)

914.8 902.6 911.5 913.8 912.8 916.1 840.0

As can be seen from Tab. 5.1, vegetable oils generally have a high viscosity, flash point, and cloud point, but a low cetane number. There are several obstacles to be overcome when vegetable oil is used as a diesel fuel. High viscosity will mean poor combustion in the cylinder, easy coke formation, leading to contaminated lubricating oil and accelerating engine breakdown. High flash point and low cetane number are the cause of poor performance of the engine. High cloud point will make it difficult to start in winter. Vegetable oils can become a good substitute for diesel by modifying them through various technologies (including transesterification and heating) to produce fuels.

5.1.2 The use of vegetable oils as diesel fuel The use of vegetable oils as an alternative renewable fuel competing with petroleum was proposed in the early 1980s. However, the direct use of vegetable oils and/or oil blends is generally considered to be unsatisfactory and impractical for both direct injection and indirect type diesel engines because of the oils’ high viscosities and low volatilities, injector coking and trumpet formation on the injectors, higher level of carbon deposits, oil ring sticking, and thickening and gelling of the engine lubricant

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oil, acid composition (the reactivity of unsaturated hydrocarbon chains), and FFA content [334–337]. Consequently, different methods have been considered to reduce the viscosity of vegetable oils such as dilution, microemulsification, pyrolysis, catalytic cracking, and transesterification. Vegetable oils can be used as fuels for diesel engines, but their viscosities are much higher than that of common diesel fuel and require modifications of the engines [338]. Vegetable oils could only replace a very small fraction of transportation fuel. Different methods of reducing the high viscosity of vegetable oils have been considered: (1) dilution of 25 parts of vegetable oil with 75 parts of diesel fuel; (2) microemulsions with short-chain alcohols such as ethanol or methanol; (3) transesterification with ethanol or methanol, which produces biodiesel; (4) pyrolysis and catalytic cracking, which produces alkanes, cycloalkanes, alkenes, and alkylbenzenes.

5.1.2.1 Dilution of oils Dilution of oils with solvents and microemulsions of vegetable oils lowers the viscosity and mitigates some engine performance problems such as injector coking and carbon deposits, etc. To dilute vegetable oils the addition of 4 % ethanol to the oils increases the brake thermal efficiency, brake torque, and brake power while decreasing brake-specific fuel consumption. Since the boiling point of ethanol is less than that of vegetable oils, it could assist in the development of the combustion process through an unburned blend spray [339]. The viscosity of oil can be lowered by blending with pure ethanol. Twenty-five parts of sunflower oil and 75 parts of diesel were blended as diesel fuel [340]. The viscosity was 4.88 cSt at 313 K, while the maximum specified ASTM value was 4.0 cSt at 313 K. This mixture was not suitable for long-term use in a direct injection engine. Another study was conducted using the dilution technique on the same frying oil [341]. The addition of 4 % ethanol to D2 fuel increases the brake thermal efficiency, brake torque, and brake power while decreasing brake-specific fuel consumption. Since the boiling point of ethanol is less than that of D2 fuel, it could assist the development of the combustion process through an unburned blend spray [342].

5.1.2.2 Microemulsion of oils To reduce the high viscosity of vegetable oils, microemulsions with immiscible liquids such as methanol and ethanol and ionic or nonionic amphiphiles have been studied. The short engine performances of both ionic and nonionic microemulsions of ethanol in soybean oil were nearly as good as that of D2 fuel [343]. To solve the problem of the high viscosity of vegetable oils, microemulsions with solvents such as methanol, ethanol, and 1-butanol have been studied. All microemulsions with butanol, hexanol, and octanol met the maximum viscosity requirement for

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D2 fuel. The 2-octanol was found to be an effective amphiphile in the micellar solubilization of methanol in triolein and soybean oil [334, 344]. An emulsion of 53 % (vol) alkali-refined and winterized sunflower oil, 13.3 % (vol) 190-proof ethanol, and 33.4 % (vol) 1-butanol was prepared. This nonionic emulsion had a viscosity of 6.31 cSt at 313 K, a cetane number of 25, and an ash content of less than 0.01 %. Lower viscosities and better spray patterns (more even) were observed with an increase of 1-butanol. In a 200-h laboratory screening endurance test, no significant deteriorations in performance were observed, but irregular injector needle sticking, heavy carbon deposits, incomplete combustion, and an increase in lubricating oil viscosity were reported [334]. A microemulsion prepared by blending soybean oil, methanol, 2-octanol, and cetane improver in the ratio of 52.7:13.3:33.3:1.0 also passed the 200-h EMA test used the ternary phase equilibrium diagram and the plot of viscosity versus solvent fraction to determine the emulsified fuel formulations [344, 345]. Methanol was often used due to economic advantage over ethanol.

5.1.3 Vegetable oil producing technology Production technology of vegetable oil is already quite mature, and mainly includes the mechanical crushing method and solvent leaching method, etc. At present the solvent leaching method is widely used in the oil industry. The leaching process has extensive adaptability for raw material, and oil yield efficiency is high, the meal is of good quality, and low processing cost. But the leaching process yields raw oil of poor quality, the solvents are generally flammable and explosive, and there is a certain toxicity [346–348].

5.1.3.1 Basic principle and process of oil leaching method The oil leaching method is in accordance with the extraction principle of organic chemical separation processes. Oil and some organic solvents are easily miscible by mutual diffusion and oil can be extracted from the pre-pressed cake using n-hexane as a leaching agent. Oil leaching occurs between solvent and the grain under the condition of relative motion, so in addition to the molecular diffusion process in the oil [349], there are solvent flow and convection diffusion processes. The general technological process of the leaching method is shown in Fig. 5.1. The pre-pressed cake is dipped in the selected solvent, making the oil dissolve in the solvent (mixed with oil), and then the mixed oil and solid residue (wet meal) are separated. The mixing oil is selected according to the boiling point of the solvent and steam gasification is used for separation, thus leaching the raw oil. After condensation, the solvent steam is cooled and recycled [350, 351].

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Leaching solution steam

Raw material Filter press

Leaching embryo

Crude oil

Squeeze machine Evaporation kettle

Crude oil Tank

Exsolution

Cooling water Condenser

Crude oil

Leaching solution Residue Leaching solution recycling

Raw oil tank

Fig. 5.1: Flow diagram for the process of extracting oil via the leaching method.

5.1.3.2 Basic principle of mechanical crushing method and process The crushing method is with the aid of external mechanical forces, to squeeze the oil out of the fruit. Currently, the crushing method is the main method of extracting oil from plants in China. The crushing method is adaptable, has a simple operation process, production equipment maintenance is convenient and production scale is flexible. The product has good quality and light luster, the process is suitable for all kinds of vegetable oil extraction, production is safe, but the disadvantages of the crushing method are a high residual oil content in the press cake, low efficiency, large power consumption, and fast wear and tear [352, 353]. Crushing equipment mainly falls into two categories: a hydraulic press for intermittent production or a spiral press for continuous production. The hydraulic press functions in accordance with the theory of fluid transmitting pressure, squeezing the oil out of the fruit. This machine has a simple structure, convenient operation, low power consumption, produces good quality oil cakes and can process a variety of fuel, is suitable for small quantities and a variety of raw materials. But the hydraulic press process required physical labor, strict process conditions, and has been gradually replaced by continuous squeezing devices. The screw press is widely used in the world’s more advanced continuous oil equipment. Its working principle is based on a continuous forward motion, with a spiral altering the root diameter, so the pressing chamber space shrinks and the pressure risesuntil oil flows out from a crack in the press cage.

5.1.3.3 Oil refining The unrefined product of crushing or leaching is known as raw oil. The main ingredient of raw oil is triglyceride oil, commonly known as neutral oil. In addition, the oil also contains other ingredients collectively referred to as impurities. So, to produce vegetable oil raw oil needs to be refined [354–357]. The general process is shown in Fig. 5.2.

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Raw oil Water

Sedimentation tank

After vacuum pump

Steam

Dehydration kettle Pressure filtation or centrifugation

Solvent Refined oil

Condensate Residue Waste water

Refining pot

Residue pot

Dewaxing Phospholipids

Wax Separator

Fig. 5.2: Vegetable oil refining process flow diagram.

5.1.3.3.1 Removal of impurities suspended in raw oil Raw oil usually contains a certain amount of suspended solids. The screw press method yields raw oil containing the most suspended matter, even as high as 15 % and above. Suspended solids accelerate oil hydrolysis and rancidity, which causes excessive floating in the alkali refining. The centrifuge will frequently be down for cleaning if it is used to separate soap. Thus, it is essential to remove suspended impurities before storage, processing or use. Sedimentation, often called precipitation, is the simplest method in oil refining. It is a method that separates suspended solids and oil by using the different densities of suspended impurities and oil. Therefore, relatively clear oil floats and heavier impurities sink to bottom of the device. Filtration is a solid-liquid separation operation in which the suspension passes through a filter medium (within refineries generally a filter cloth or screen) and the solid particles are retained. Grease factories often use intermittent filters. Raw oil has slow filtration at low temperature and has high viscosity. Increasing the temperature is a simple and effective measure to reduce the viscosity, but the oil may oxidize due to too high a temperature. Thus the filtration temperature of raw oil generally does not exceed 70 °C. Centrifugation is a separation method that separates suspended impurities by using centrifugal force. Among the many types of centrifuge equipment, the horizontal spiral decanter centrifuge has the most chemical applications. Some oil factories obtained better process results with removing suspended impurities from raw oil using centrifugation in recent decades.

5.1.3.3.2 Degumming Degumming is the process of removing rubber impurities from raw oil. The gum in raw oil is mainly phospholipids, thus the process is also called dephosphorization. The value and storage stability of oils was reduced in the presence of gum, such as

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phospholipids, and it also has a series of negative effects in oil refining and processing that eventually reduce the quality of the finished oil. Thus early removal glial becomes imperative. There are many degumming methods, among them hydration and acid degumming in the oil refining process. In hydration degumming, a certain amount of water or dilute solution is added to raw oil under stirring, making the gum impurities condense and separate from the raw oil by swelling because of the hydrophilic quality of phospholipids and other impurities. During the hydration degumming process the condensed substances are mainly phospholipids. In addition, there is phospholipid binding with protein, mucus and micrometal ions. The raw oil is preheated, hydrated by adding water, left to settle, and then the oil residue Is separated in the refining pot. The intermittent hydration process is illustrated in Fig. 5.3. Water

Filter oil

Refining pot

Hydration net oil

Edible oil

Residue pot

Recovery oil

Residue

Fig. 5.3: Intermittent hydration process diagram.

According to the different operating temperatures, intermittent hydration can be divided into high-temperature hydration, room temperature hydration and lowtemperature hydration. In high-temperature hydration the raw oil is preheated to a higher temperature, which is beneficial to improving refining rates. As the names imply, the other two types of intermittent hydration take place at room temperature and low temperature.

5.1.3.3.3 Dewaxing Most vegetable oils contain wax, which mainly comes from the fruit and the seed hull. The wax content of vegetable oil generally is 0.06–5.00 %. Wax will dissolve in oil at 40 °C. Wax is contains long-chain monocarboxylic acids and their alcohol esters. It produces coke in the combustion process of biodiesel. The process of dewaxing can be accomplished by the conventional method or the solvent method. The conventional method works from frozen crystals alone, which are then separated by mechanical methods. Solvent dewaxing uses a solvent which is added to induce crystallization of oil, before the wax and oil are separated and the solvent evaporated.

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5.1.4 Examples of rapeseed oil fuel tests 5.1.4.1 German-made vegetable oil engine In the early 1990s, the German BMW company began producing the P135 series engine which was intended to use vegetable oil as energy source. Several types of engine were manufactured, such as engines of four cylinders and six cylinders. The series of vegetable oil engines adapted the direct injection combustion system which was developed by German Ace Institute for a single-hole injector. The top of the pistons was made of cast iron (combined with the cylinder head into a hyperbolic cylinder combustion chamber), the skirt was aluminum. The combustion system was of high thermal efficiency, low fuel consumption, low noise, good starting performance, and had been transferred abroad. The vegetable oil, which was cold pressed, had been through filtration to remove mucus and was not subjected to any chemical treatment, could be burned directly in the P135 series engines.

5.1.4.2 Contrast test of diesel and rapeseed oil Comparative tests used the 6P135 engine with six cylinders, four stroke, water-cooled, direct injection combustion, rated power 80 kW, speed 1,500 r/min. Fuels were 0# diesel and rapeseed oil. Calorific value of 0 # diesel was 44,950 kJ/kg, the cetane number was 45. Calorific value of rapeseed oil was 38,836 kJ/kg, cetane number was 32.2, the rapeseed oil was without any chemical treatment.

1600

nτ pm n

1500 Tτ °C 400

R

Tτ

200

4 2

R ge 300

0

g/hp. h ge

250 200 150 10 gRTτnNe-

15

20

25

Ne (hp)

Fuel consumption rate Exhaust smoke Exhaust temperature Speed Power (lhp=0. 735 KW)

Fig. 5.4: Load characteristic curves of 6P135 vegetable oil engine.

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The 6P135 type engine powered a 75 kW generator, working in the condition of diesel and rapeseed oil, and engine operation was stable and reliable. Fig. 5.4 shows the performance curves of the two fuels. The results showed that the rapeseed oil burned in the 6P135 engine just needed mucus removed by filtering, but no chemical treatment. It also showed that the rapeseed oil is more environmentally friendly and less harmful than diesel. Furthermore, the 6P135 vegetable oil engine, with a hyperbolic combustion chamber formed of a cast iron piston crown and cylinder head and a direct injection combustion system with single-hole pintle injector, performed well economically, and in terms of power, starting and reliability.

5.1.5 Example of cottonseed oil-diesel blend fuel test 5.1.5.1 Heating to reduce the viscosity of cottonseed oil As previously mentioned, vegetable oil is of high viscosity which makes it not suitable to be used as fuel for an internal combustion engine. Shenyang Agricultural University had done a study to improve the combustion performance of cottonseed oil by raising the temperature and lowering the viscosity. The tests showed that cottonseed oil [358–362] viscosity decreased rapidly with increasing temperature. The viscositytemperature curve shown in Fig. 5.5 illustrates that the viscosity of pure cottonseed oil changes gently in the low temperature range. Viscosity 70

Cottonseed oil Cottonseed oil/diesel 50/50 Cottonseed oil/diesel 30/70 Diesel

60 50 40 30 20 10 0

20

40

60

80

(°C)

Fig. 5.5: Viscosity-temperature characteristics of cottonseed oil and diesel.

This showed that when the diesel engine had used up the cottonseed oil, the temperature only needed to be raised to 60–70 °C , at which the viscosity of the blend oil was close to that of diesel at room temperature. The blend oil can be heated to the required temperature by the circulating water of the diesel engine or the waste heat from the exhaust.

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Circulating water and vent heat were utilized to heat the blend oil to 70 °C in the direct injection diesel engine. The blend oil was a mixture of cottonseed oil and diesel at the ratio of 50:50. Hydraulic engine dynamometer was adopted to measure the dynamic rate with D150. Rotational speed and fuel consumption were measured by TCY-69. The results are shown in Fig. 5.6, the solid line represents the unheated blend oil, the dotted line represents the blend oil heated to 70 °C. From the figure we can see the engine performance was significantly improved when the blend oil was heated to 70 °C, also the degree of incomplete combustion was lower, fuel consumption was reduced, and engine speed improved. Tr (°C) 700 600 500 400 300

Diesel Rapeseed oil Tr

Me (N.m) 500 400 300 200 100 ge (g/kwh) 300 280

ge

260 240 20 30 40 50 60 70 80 Ne (kw) Ne– Power Me– Spindle torque g– Fuel consumption rate Tr– Gas exhaust temperature

Fig. 5.6: Effect of heating on blend oil combustion performance.

5.1.5.2 The ratio of cottonseed oil to diesel Six fuels were obtained by mixing cottonseed oil and diesel fuel in the ratios of 0/100, 20/80, 40/60, 60/40, 80/20, 100/0. Burning tests were carried out on a Pegasus X-195 diesel engine, with D150 load characteristic type hydraulic dynamometer; see Fig. 5.7 for the plotted curve. The diesel engine was rated at 8.8 kW. The figure shows that the consumption of oil increased with increasing cottonseed oil ratio, because the calorific value of cottonseed oil is lower than that of diesel. On the premise of keeping the original power performance of the diesel engine, three aspects should be considered. Firstly, the aspect of replacement quantity of

305

450 450

350 Cottonseed oil Diesel Lineshape

150

100: 80: 60: 40: 20: 0:

0 20 40 60 80 100

400

350

ge (g/kwh)

Tr (°C)

5.1 Vegetable oils fuel |

300

250 2.2

4.4

6.6 Pe (kw)

8.8 9.7

Fig. 5.7: Load curve of diesel engine burning different proportions of cottonseed oil.

diesel: the more cottonseed oil, the better. Secondly, the aspect of price of the mixed oil: the less cottonseed oil, the better. Thirdly, the aspect that cottonseed oil could increase fuel consumption, so excessive fuel consumption should be considered. In the test, when the Pegasus X-195-type diesel engine worked at rated power 8.8 kW, the consumption of pure diesel was 243.85 g/kW · h, the consumption of the mixture of 40 % cottonseed oil and 60 % diesel was only 5.15 % higher. The engine worked on the mixture of 40 % cottonseed oil and 60 % diesel for 400 h, the associated moving parts were not attrited. It was thought that the mixing ratio of 40 % cottonseed oil and 60 % diesel was appropriate.

5.1.5.3 Appraisal of cottonseed oil-diesel blend fuel When fueled with mixed diesel and cottonseed oil after appropriatetreatment, the engine was able to maintain its original dynamic performance. However, when the proportion of cottonseed oil was increased, the consumption of blend oil increased. The best results were obtained with 40 % cottonseed oil and 60 % diesel hybrid. Cottonseed oil viscosity can be significantly reduced by heating or mixing with diesel fuel. When cottonseed oil is heated to 70–80° by a modification of the circulating cooling water and vent heat, or when it is mixed with diesel oil, its viscosity can meet the requirements for a steady state and no phase separation. The cold start performance of mixed fuel is poor. This can be solved by taking the diesel first to start the engine, then during operation burning the mixed fuel (diesel tank should be set to start the engine). Compared to diesel, the mixed fuel has lower cetane, poorer ignition performance, and a longer delay period, so the fuel delivery advance angle should be increased.

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When the ratio of cottonseed oil and diesel was 40:60, the fuel supply advance angle should be adjusted to 20°. Long-term damage may be caused by using oil in diesel engines (especially with a large proportion of vegetable oil), to the injector, piston ring groove, valve and combustion chamber became seriously coked, which can contaminate lubricants for certain, so the coke needs regular treatment. Purification measures for mixed oil also should be designed to prevent filter clogging.

5.2 Biodiesel technology 5.2.1 Principle of biodiesel production Several methods have been developed for biodiesel production and applications: direct use of vegetable oil, microemulsions, thermal cracking (pyrolysis), and transesterification. The biodiesel obtained from microemulsions and thermal cracking methods would lead to incomplete combustion because of a low cetane number. Transesterification is the most common method for biodiesel production because of its simplicity, and it has been widely studied and industrially used to convert vegetable oil into biodiesel.

5.2.1.1 Esterification Esterification is the reaction of FFA molecules present in nonedible oil, fried waste oil, and animal fats with an alcohol in the presence of a catalyst, forming the corresponding esters and water as the by-product. The esterification reaction is a reversible reaction. The reversible reaction is hydrolysis. Usually, the hydrolysis reaction requires a long time to achieve equilibrium. The catalyst is necessary to increase the reaction rate and transesterification reaction yield. The catalyst is very important to shorten the reaction time. Significant amounts of work have been carried out on the homogeneous acid and base catalysis transesterification of vegetable oils. Sulfuric acid and hydrochloric acid are normally used as the acid catalysts, especially when the oil contains a high amount of FFAs and water, because these catalysts are capable of handling the esterification and transesterification of triglycerides simultaneously. In industry, ion-exchange resin is also used as a solid catalyst for the esterification reaction.

(5.2) To accelerate the reaction rate during biodiesel production, not only a catalyst should be used, but also more than a stoichiometric molar ratio of methanol is needed. At

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the same time, the esterification reaction is reversible. To produce more biodiesel, the by-product water should be continuously removed. The steps involved in acid-catalyzed transesterification are shown in Fig. 5.8: (1) The initial protonation of an acid to give an oxonium ion, (2) the oxonium ion and an alcohol undergo an exchange reaction, producing the intermediate, (3) and this in turn can lose a proton to become an ester. Each of the above steps is reversible, but the equilibrium point of the reaction is displaced in the presence of excess large alcohol, thus allowing the esterification to be completed.

(1)

R2

+

O

O C

H2 C O

C

C O H2

C

O CH

OH

O R1 R3

H+

R2

C O- CH

O

H2 C C O R1 C O R C 3 H2 O

+

OH

O (2) R2

C O- CH

C H2 R1 C O C O C R3 H2

O +R OH

C R2

O CH

O

HO H2 C +O H R– C O R1 R3 C O C H2 O O H2 + C OH + C +H C O C R1 OR– R3 H2 O

O HO H + C H2 C O C R R2 O CH C O R2 R1 (3) O CH C O C R3 H2 O R1, R2, and R3: carbon chain of fatty acid ; R: alkyl group of the alcohol O

Fig. 5.8: Mechanism of acid-catalyzed esterification.

5.2.1.2 Transesterification Recently, transesterification has been reported as the most common method for biodiesel production from vegetable or algal oil with alcohol, usually methanol or ethanol, in the presence of an acid or a base catalyst. Transesterification is an equilibrium reaction and the transformation occurs essentially by mixing the reactants. Different types of alcohols such as methanol, ethanol, propanol and butanol can been used in order to produce biodiesel. When methanol is used as reactant, reaction product will be a fatty acid methyl esters mixture (FAME), whereas if ethanol is used as reactant, a fatty acid ethyl esters mixture (FAEE) will be obtained. Nevertheless, methanol is commonly used in biodiesel production due to its low cost and industrial availability. The general scheme of the transesterification reaction for FAME is

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presented in eq. (5.3), where R is a mixture of fatty acid chains

(5.3) Base-catalyzed transesterification has some continuous reversible reactions. The actual reactive species of a base-catalyzed esterification reaction is an alkoxide group (RO− ). The mechanism is shown in Fig. 5.9. Concerning the transesterification with methanol using a solid base catalyst, the abstraction of a proton from methanol by the basic sites to form a methoxide anion is the first step of the reaction. The methoxide anion attacks the carbonyl carbon in a molecule of the triglyceride, leading to the formation of an alkoxycarbonyl intermediate. Then, the alkoxycarbonyl intermediate is divided into two molecules: FAME and the anion of diglyceride. (1)

ROH + B

RO– + BH+ RO

O

H2 C O

C (2) R2

O

O C

CH

O R1

C +

–

OR

R2

O CH

C O C H2

C O C R3 H2 O

C (3)

R2

O CH

H2 C O–

C R2

O CH

R3

H2 C O–

C

C O C H2 O

H2 C

C + BH+

O CH

O + RO

C

R1

C O R3 H2 C O O

O (4) R2

O

O– H2 C C O R1 C O C H2 O

R3 O

RO O

O– H2 C C O R1

R2

OH

+ B

O CH

R3

O C C H2 O

R1, R2, and R3: carbon chain of fatty acid; R: alkyl group of the alcohol Fig. 5.9: Mechanism of heterogeneous base-catalyzed transesterification.

R3

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309

5.2.2 Technologies for biodiesel production 5.2.2.1 Enzymatic transesterification methods The enzymatic method for biodiesel production involves a transesterification reaction in the presence of a lipase enzyme as the catalyst. Enzymatic catalytic transesterification is generally carried out at a moderate temperature with high yields, a slight amount of by-product, easy extraction, and refining. However, the enzymes are expensive and have unstable activity. The reaction rates are significantly lower than those using homogeneous base catalysts. Short-chain alcohols such as methanol and ethanol are toxic to lipases. However, enzymatic catalysis can tolerate FFAs and water without soap formation, thus easily separating biodiesel and glycerol. Lipasemediated reactions are environmentally friendly and utilize most of the waste oils. Therefore, enzymatic methods are important for biodiesel production in the future. The lipases are good catalysts for the transesterification reaction of glycerides and alcohols. Enzymatic methods for biodiesel production have mild reaction conditions, requires less alcohol, and are environmentally friendly. However, when methanol or ethanol is used, the biodiesel yield is only 40–60 %. The selectivity of enzymatic reactions is high, and the enzymes can be immobilized onto support materials. Therefore, lipases on different supports obtained by immobilization or encapsulation are used for transesterification reactions. The lipase-dependent catalysis of biodiesel production has significantly progressed in the USA, Sweden, Brazil, Japan, India, Philippines, Australia, Finland, and other developed countries. The cost of enzymes and their deactivation because of feed impurities are the major obstacles for the commercial feasibility of this process. For example, the price of widely used Novozyme lipase produced in Denmark is about US $ 1,200/kg. Moreover, it is difficult to recover the by-product glycerol.

5.2.2.2 Chemical methods looseness=-1The chemical methods for biodiesel production involve the chemical conversion of vegetable or animal oils using a chemical as the catalyst. The chemical reaction changes the molecular structure of fats, the major component of triglycerides in fatty acids. The low molecular weights of lower alkyl esters of fatty acids significantly improve their liquidity and viscosity, making them suitable to utilize as a diesel engine fuel. The methods for biodiesel production using chemical catalysts comprise the homogeneous catalysis, heterogeneous catalysis, thermal cracking, and supercritical methanol methods. The homogeneous catalysis method is the most common industrial method for producing biodiesel. Homogeneous catalysts provide a higher reaction rate and a lower reaction temperature. However, they have several drawbacks, for example, corrosive nature, difficulty in the separation of the catalyst from

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the reaction products and reagents, difficult to conduct continuous production, and secondary pollution.

5.2.2.3 Supercritical methanol method The supercritical methanol method avoids the use of catalysts, and transesterification is completed in a relatively short time. The reaction mass transfer controls the supercritical reaction rate of oil with alcohol. However, under supercritical conditions, triglycerides solubilize well in methanol. The triglycerides-in-methanol mixture becomes a single phase, thus accelerating the reaction because of their high miscibility. A homogeneous reaction under supercritical conditions can be attributed to two reasons: (1) The solubility of triglycerides increases with the increase in temperature and pressure. (2) The polarity of methanol decreases at a high temperature. The solubility of triglycerides in methanol is less at a low temperature, but significantly high at a relatively high temperature. Liquid methanol is a polar solvent; hydrogen bonding is formed between the oxygen and hydrogen atoms of OH, forming methanol clusters. The degree of H-bonding decreases with an increase in the temperature; thus, the polarity of methanol decreases in the supercritical state. Thus, supercritical methanol is hydrophobic with a lower dielectric constant. Therefore, nonpolar triglycerides dissolve well in supercritical methanol, forming a single phase of oil methanol mixture. This phenomenon at a high temperature and high pressure in the supercritical state probably increases the transesterification of rapeseed oil. The supercritical method is a new process for the preparation of biodiesel. A small dielectric constant of supercritical methanol, hydrophobicity, and triglyceride solubility accelerate the reaction rate. In the supercritical transesterification method, catalysts are not added, and the esterification of oil and methanol is carried out in a supercritical state methanol (TC = 512.4 K, PC = 8.09 MPa). The supercritical state refers to the temperature and pressure of a material above its critical temperature and pressure, where the gas and liquid phases cannot be distinguished. A supercritical fluid has different properties than the corresponding gas and liquid. Its density is close to the density of the liquid, and its viscosity is close to the viscosity of the gas. The thermal conductivity and diffusion coefficient of a supercritical fluid is somewhere between those of the corresponding gas or liquid. In this state, methanol can be used as a reaction medium and also directly participates in the reaction. Its solubility is very high. Therefore, the oil is highly soluble in methanol, and the esterification is carried out in a homogeneous phase, thus significantly accelerating the ester exchange reaction rate and improving the yield.

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5.2.3 Processing and design for biodiesel production 5.2.3.1 Process flow At present, the chemical method is mainly used in biodiesel production at home and abroad. The chemical method for biodiesel production has been well established over the years. The main part of this technology is acid/base-catalyzed esterification and transesterification reactions and their conditions for biodiesel production. Chemical catalysis, enzymatic catalysis, supercritical methanol method, pressure conditions (normal pressure or high pressure), and raw materials including animal fats and waste edible oils and fats have been well studied. The chemical methods of normal pressure continuous transesterification and pressurized continuous biodiesel production technology such as transesterification have been used in large-scale industrial productions in Europe, the United States, and other developed countries.

5.2.3.1.1 Chemical process The chemical process for biodiesel production is shown in Fig. 5.10. There are three steps in the transesterification and esterification processes for biodiesel production. First, the methyl process affords the crude ester, by-product glycerin and black mud residue; second, the obtained crude ester is vaporized to remove excessive methanol and water and refined to obtain high carbon methyl and asphalt; third, the post-processing procedure afforded high carbon methyl esters by stirring at room temperature using a metal salt solution. After the separation of metal soap and dehydration, the purified methyl ester was obtained. The biodiesel was obtained after adding antioxidants and a pour point depressant. Asphalt can use raw materials or feedstock oil instead of heavy oil. Methanol can be used as the raw material after distillation. The crude glycerin formed 80 % of industrial glycerin products using lime and after concentrating under reduced pressure. The main raw materials for the biodiesel used in China are a nonedible oil such as trap grease, a highly acidified oil; animal fats, and soap. A two-step process of esterification and transesterification is usually used in biodiesel production. Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences studied a biodiesel production system consisting of a fixed-bed and downstream plug-flow reactor. In this study, the main components of FFAs and fatty acid glycerides were converted into biodiesel in two steps to obtain high-value glycerin. The two-stage process includes the following: separation and purification of raw material, solid acid pre-esterification, transesterification by plug-flow reactors, separation of glycerol, recovery of methanol, refining of crude biofuel oil (see Section 5.2.3 of this chapter). The homogenization pretreatment technology uses diverse raw materials: First, the mechanical impurities are removed through filtration at a low temperature using a trap grease; then, the oil-water separator is used to remove the moisture. Finally, a biological treatment affords the raw oil. The Fujian Longyan Zuoyue Company in China

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Methanol tank Catalyst Methyl esters Separator Oil tank

Methanol removal Acid

Reactor

Water Acid

M

Glycerol (50%)

Neutralization and washing

Acidulation and separation

Free fatty acids

Dryer Wash water

Methanol removal

Methanol /Water rectification

Methanol storage Finished biodiesel

Crude glycerol (85%) Water Fig. 5.10: Process flow schematic for biodiesel production.

has developed a new technology for the separation and purification of trap grease. The technology can be used to continuously separate water and mechanical impurities. Finally, the moisture content of the raw material was less than 0.5 %. Esterification reaction: This can reduce the FFA content in the feedstock oil in the presence of a solid catalyst in a fixed-bed reactor or a stirred reactor. Transesterification reaction: The raw material separated by centrifugation after the esterification reaction and by-product glycerin undergo transesterification reaction in the presence of a base catalyst for biodiesel production. The biodiesel was purified by washing with water, followed by drying and distilling in vacuo. Crude glycerol was recovered by removing the solid impurities. The refined glycerol can be converted to 1,3-glycerol using a polyol resin and other chemicals. The chemical method has many disadvantages: complex process, superfluous methanol, high energy consumption; dark color, and recovery of methanol. Unsaturated fatty acids decompose at high temperatures, the esterification product is difficult to recycle, and wastewater is generated using a basic catalyst.

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5.2.3.1.2 Enzymatic process The enzymatic process was carried out at 40 °C using immobilized enzyme in a fixedbed reactor. The process is shown in Fig. 5.11. The reactant was a mixture of petroleum ether and oil in a 1:1 volume ratio and 10 % (water/oil volume ratio) of water. Methanol was added in a controlled manner. The reaction was divided into three steps. Some methanol was added in each step. At the end of each level, the by-product glycerol was separated using a spin-liquid separator in-line.

Methanol tank

Mixer

Mixer

Substrate mixing tank Reactor 1

Reactor 2

Reactor 3

Product tank

Material transport pump Termastatic waterbath in 60°C

Separate refine

Cyclone hydraulic separators Water pump

Rude glycerin tank

Fig. 5.11: Enzymatic process for biodiesel production in a fixed-bed reactor.

5.2.3.2 Catalysts The transesterification of oils using catalysts are of two types: homogeneous and heterogeneous. Homogeneous catalysts are more important in an industrial scale because these are more active and thus take less time for transesterification, whereas heterogeneous catalysts are more useful for oils with higher FFA contents.

5.2.3.2.1 Homogeneous catalysts The homogenous catalysts used in biodiesel production are of two types: alkaline and acidic homogeneous catalysts. Alkaline homogeneous catalysts are used for virgin oil with less FFA content. This is because a high FFA content causes saponification, thus forming soap and leading to difficult separation. Acidic homogeneous catalysts are suitable for used cooking oil with a high FFA content. Alkaline homogeneous catalysts include NaOH, CH3 ONa, KOH, and CH3 OK. Base catalysis is inexpensive because it is carried out at a low temperature (40–60 °C), less

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time (30–90 min), and under atmospheric pressure. Moreover, the yield is high, and no intermediate steps are present. However, alkaline homogenous catalysts are highly hygroscopic; they absorb water from air during their storage. They also produce water when dissolved in alcohol, thus decreasing the yield. Some examples of acidic homogeneous catalysts are sulfuric acid, phosphoric acid, hydrochloric acid, and sulfonic acid. Compared to alkaline homogeneous catalysts, they take more time and require a higher temperature for complete conversion. Acid-catalyzed transesterification is not as popular as its counterpart, base-catalyzed transesterification, in commercial applications. This is because the acid-catalyzed reaction is about 4,000 times slower than the base-catalyzed reaction. However, acidcatalyzed transesterification has an important advantage compared to base-catalyzed transesterification. The performance of acid catalysts is not affected by the presence of FFAs in the feedstock. Acid catalysts simultaneously catalyze both esterification and transesterification. Thus, acid catalysts have a great advantage: They directly produce biodiesel from low-cost lipid feedstocks such as used cooking oil and grease with high FFA concentrations (≥ 6 %). Recently, this economic advantage of acid-catalyzed biodiesel production over the base-catalyzed process has been demonstrated using virgin oils, particularly when low-cost feedstocks were used in the former process.

5.2.3.2.2 Heterogeneous catalysts Heterogeneous catalysts are used to overcome various challenges when homogeneous bases and liquid acids are used as the alcoholysis catalysts. The solid acids [363–365] mainly include zeolite materials, heteropolyacids, pure or modified oxides of transition metals such as zirconium and molybdenum, silica and alumina catalysts. These materials contain both Brønsted and Lewis acid sites that determine their activities during the transesterification reactions. The activities of zeolites and heteropolyacids may also be affected by shape selectivity.

Solid acid catalysts Solid acids catalysts (both Lewis-type, such as the mixed and sulfated oxides, and Brønsted-type, such as sulfonic acid-containing materials) have the combined advantages of heterogeneous base catalysts and mineral acids. Therefore, a large number of heterogeneous catalysts have been investigated. Many catalysts showed good catalytic performances. Some of these catalysts include oxides, hydrotalcides, zeolites, and ion-exchange resin. Solid superacids. Solid superacids are stronger than 100 % H2 SO4 acid solid acid. Halogen-type superacids were reported for the first time in 1979. A Japanese scholar reported SO2− 4 /MxOy superacid system [366]. After the treatment of a high-temperature sintered metal oxide with dilute sulfate, the acid strength is greater than 100 % of solid superacid H2 SO4 . SO2− 4 /MxOy-type of solid acid catalyst has been widely investigated

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2− in recent years. Other types of metal oxide solid acids, TiO2 /SO2− 4 [367], ZrO2 /SO4 2− 3+ [368], SO4 /ZrO2 -TiO2 /La [369] have also been widely investigated. They are mainly prepared by the sol-gel method and precipitation impregnation method. This type of solid superacid can be easily prepared and stored, particularly liquid and solid superacids containing halogen. They do not corrode the reaction device, do not pollute the environment, and can be used below 500 °C, thus arousing much interest.

Metal oxides and mixed metal oxides. Solid acid catalysts with metal oxides are important catalysts for esterification and transesterification reactions. At the same time, metal oxides do not leach SO2− 4 ; they show good activity. Because metal oxide catalysts are thermally stable, strong acids, easily recycled, acidic metal oxides have attracted much interest. Jitputti et al. [370] reported that when transesterification was carried out using ZnO as the catalyst under reflux in methanol (300 °C) using a 6:1 molar ratio of methanol to palm oil, a high yield (86.1 %) of FAME was obtained within 1 h. Senso et al. [371] prepared WO3 /ZrO2 (WZ) solid acid catalyst by the wet impregnation method. The dried material was calcined at 500 °C under different atmospheres (H2 , N2 , O2 , and air) for 4 h. The results show that WO3 /ZrO2 catalyst has a higher transesterification activity because of a higher surface area (90.2 m2 /g) and higher acid activity (200 μmol/g) in H2 atmosphere. Hence, further studies are needed to improve the activity of metal oxide catalysts using supported zeolites or mixed different metal oxides. Zeolites. Zeolites are naturally occurring crystalline aluminosilicates interlinked with oxygen atoms. They possess three-dimensional framework structures with molecular pores and channels of uniform sizes and are used in transesterification reactions. Therefore, zeolite catalysts, e.g., ZSM-5, Beta, H-ZSM-5, h-Beta have been widely studied. SBA and MCM families have been modified to achieve higher catalytic activities. The modified catalysts FeSBA-1, FeAlMCM-41, FeMCM-41, and AlMCM showed a higher activity [372]. The zeolites have a higher acidic strength, a higher activity, a stable structure, and no equipment pollution, but the reaction conditions of zeolites in transesterification reactions are harsh, usually a higher temperature and a higher pressure than metal oxide catalysts. Heteropolyacid catalysts. The use of polyoxometalates and heteropolyacids in catalysis is promising. The basic structure of these acids comprises an XO4 central tetrahedron surrounded by an octahedral metal oxygen. The best catalyst is XM12 O40 x-8. X denotes the central atom such as Si4+ or P5+ (e.g., H4 SiW12 O40 and H3 PW12 O40 ). “M” represents the metal ions (usually W6+ or Mo6+ ). Heteropolyacid catalysts [373, 374] contain a central atom (e.g., P, Si, and Fe) and other metal ions (e.g., W, V, Nb, and Ta). Heteropolyacid catalysts not only have a high activity, but also have a high oxidation potential. Keggin heteropoly anion is a heteropolyacid with good activities for esterification and transesterification reactions. At the same time, heteropolyacid cat-

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alysts are green catalyst. Fernandes et al. [375] reported that the H3 PW12 O40 (HPW) heteropolyacid showed a high activity in the oleic acid esterification reaction with methanol. After repeated use of the catalyst for six cycles, the catalytic activity did not vary significantly. However, the leaching and high cost of heteropolyacid catalysts limit their industrial applications. Ion-exchange resin. These catalysts are typically sulfonated crosslinked polystyrene and similar to p-toluenesulfonic acid in acidic strength [376]. Although these catalysts are more expensive than mineral acids, their advantages, such as minimized corrosion and environmental effects due to acid waste streams and ease of product separation, compensate their high costs. The most common catalysts among the ion-exchange family are the nonporous Nafion resins. Three types of cation-exchange resins [377] (NKC-9, 001_7, and D61) have been used as solid acid catalysts to prepare biodiesel from the acidified oils generated from waste frying oils. The results show that the catalytic activity of NKC-9 was higher than that of 001_7 and D61. The maximum conversionusing NKC-9 catalyst was approximately 90 %. Generally, the ion-exchange resins as organic solid acids have a low thermostability and are expensive. Consequently, their industrial applications are limited. Therefore, further studies are needed to maintain industrial biodiesel production with these good catalytic candidates.

Solid base catalyst A solid base catalyst is a solid that donates an electron or accepts a proton. Diverse solid base catalysts such as metal oxides, basic hydrotalcite, basic zeolites, and ionexchange resins have been investigated recently for transesterification reactions. Metal oxides. Many metal oxides for biodiesel production include alkali earth metal oxides and transition-metal oxides The structure of metal oxides has positive metal ions (cations) with Lewis acidity and negative oxygen ions (anions) with Brønsted basicity. Several metal oxide catalysts such as magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO), strontium oxide, titanium oxide, zinc oxide, and mixed oxides catalysts have been studied for transesterification reactions. MgO, CaO, SrO, and BaO metal oxides, which have a good heterogeneous structure, are widely used as alkaline earth metal oxides. To enhance the efficiency of the transesterification reaction, various active compounds, mainly those obtained from alkali metals, have been investigated. Several methods such as doping active compounds on a mesoporous support, coprecipitation with other metals to increase dispersion, and promotion of active compounds with other alkaline metals have been studied. Among these, doping active compounds on a mesoporous support and coprecipitation were effective in improving the activity of compounds. The leaching of metal oxide catalysts is accelerated in the presence of polar substances such as methanol, water, and FFA.

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Catalyst supports. The use of catalyst supports is one of the ways to minimize the mass transfer limitation of heterogeneous catalysts in liquid-phase reactions. Supports provide a higher surface area through the existence of pores where metal particles can be anchored. The catalytic performance of solid catalysts was not only influenced by the active compounds, but also by the activity of the support. Supports such as alumina [378], silica, zinc oxide, and zirconium oxide have been used in biodiesel production. Hydrotalcite/Layered Double Hydroxide (LDH)-derived catalysts. Generally, hydrotalcite or LDH is an anionic and basic clay found in nature. The common Mg6 Al2 (OH)16 CO3 · 4H2 O, hydrotalcite catalyst was synthesized by coprecipitation method. Calcination at a higher temperature decomposes hydrotalcite into active and well-dispersed Mg-Al [379, 380] oxides with a higher surface area possessing hydroxyl groups and strong Lewis basic sites associated with O2− Mn+ acid-base pairs. Basic sites associated with structural hydroxyl groups and strong Lewis basic sites associated with O2− Mn+ acid-base pairs have been developed. The difficulty with LDH is its low basicity, and also that it is sensitive to polar substances such as methanol, water, and FFA.

5.2.3.3 Reactor The reactor is the container used for the reactants oil or fat, methanol, and catalyst. It is classified into two categories by intermittent or continuous properties, namely, batch and continuous kettle-type reactors. Batch kettle-type reactors were used early and are common for biodiesel synthesis. However, because of the unstable quality of the product, high energy consumption, high operating cost, security and environmental issues, batch production is not conducive to industrialization and sustainable development of biodiesel. Continuous production processes play a very important role in reducing the cost of biodiesel production, industrial-scale production, and industrial development. Therefore, the reactor was gradually developed to perform the continuous process. Typical chemical continuous reactors for biodiesel production include stirred kettle-type reactor, tower reactor, tubular reactor, fixed-bed reactor, hydrodynamic cavitation reactor, and microchannel reactor [381].

5.2.3.3.1 Stirred tank reactor In the early transesterification reaction of the industrial process, a batch stirred kettletype reactor with a cooling reflux condenser was used. The alkali catalyst was first dissolved in methanol, and then the mixture was added to the oil. The temperature was increased under stirring. The volume of a large kettle-type reactor for transesterification increased, and the reaction period was between several hours and tens of hours. After the reaction, glycerin and soap were separated by centrifugation, and the

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FAME was purified by distillation under decompression. If KOH or NaOH were previously dissolved in methanol, the reaction time was shortened. The typical commercial production of biodiesel is carried out at a low temperature and normal atmospheric pressure. The reaction temperature did not exceed 60 °C because of the boiling point of methanol. The feedstock oil was treated by drying, neutralizing, removing impurities, decoloring, and other steps to reduce the moisture and free acid to the desired level. The addition of methanol was usually 10–80 % higher than the theoretical value. The added base catalyst was 0.1–1 % of oil. A flow-stirred kettle-type reactor is commonly used in the continuous transesterification process in industry, and the reaction is carried out at normal atmospheric pressure and 60–70 °C. Darnoko et al. [335] investigated the transesterification of palm oil in the CSTR under the following conditions: methanol-to-oil molar ratio 6:1, 60 °C, and KOH as the catalyst. The yields of ethyl ester were 58.8 % and 97.3 % at residence times 40 min and 60 min, respectively. Bradshaw–Meuly biodiesel production was carried out in a closed container of stainless steel 305. The reaction temperature was 80 °C, and 0.1–0.5 % NaOH or KOH was used as the catalyst. Excess methanol (> 60 %) was used, and the reaction time was 1 h. The yield of oil reached up to 98 %.

5.2.3.3.2 Tower reactor To enhance the mass transfer efficiency, achieve a high yield, and simplify the separation and purification of product, Behzadi et al. [382] designed a new type of continuous gas and liquid tower reactor (φ 0.38 m × 2.3 m). The preheated raw oil was dispersed to droplets using a high-pressure nozzle with a diameter of 100–200 μm and sprayed into the reactor from the upper portion, while the methanol phase containing the catalyst was gasified and pumped into the reactor from the bottom. The gas and liquid contacted in a countercurrent. When the reactor temperature was maintained at 70– 90 °C, 5–7 g NaOH was added to 1 L methanol. V (methanol phase) = 17.2 L/h, and V (oil phase) = 10 L/h. The yield of triglyceride reached up to 94–96 %. The process does not require an additional device for separating the methanol, and the reaction time was shortened to a few seconds. The reaction temperature in a continuous gas-liquid tower reactor [383] was higher than the boiling point of methanol. It significantly accelerated the reaction rate, and a higher yield was achieved than a liquid-liquid reaction. However, this reactor has some disadvantages such as a large reactor volume and high investment in equipment.

5.2.3.3.3 Tubular reactor Liu Weiwei et al. [384] designed a plug-flow reactor for the continuous preparation of biodiesel. The optimal conditions were as follows: The mass fraction of the catalyst was 1.2 %, the retention time in the reactor was about 17 min, the bed temperature was 65 °C, and the molar ratio of oil-to-methanol was 1:6. Under these conditions, the mass fraction of methyl ester was 96.33 %. When the crude methyl ester was vacuum

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distilled, the mass fraction of methyl ester increased to 98.62 %. The fuel properties of the product agreed with the standard of German biodiesel DIN V 51606. To obtain the same yield with a continuous stirred tank reactor, the volume was reduced to half of that of the continuous stirred tank reactor, and the temperature was decreased 10 °C [385]. However, the presence of a large slenderness ratio of the plug-flow reactor, high operating requirements, difficulty to achieve a high steady state, and a high investment in equipment and pumping costs hinder the application of this process. To improve the deficiencies of the plug-flow reactor, Harvey et al. [386, 387] designed an oscillatory flow reactor to enhance the mass transfer. The bottom of the reactor chamber had an oscillate piston for changing the degree of mixing of the materials by adjusting the oscillation frequency. Experiments were carried out using two 1.5 m-long tubular oscillatory flow reactors, the amount of catalyst NaOH was 32.4 g/L, n (methanol):n reaction = 1.5:1, and the temperature was 60 °C. When the reaction time was 30 min, the yield of methyl ester reached 99 %, whereas when the reaction time was 40 min, the yield of methyl ester increased to 99.5 %. Although an oscillatory flow reactor significantly reduced the amount of methanol, the requirements of equipment are high. In industrial scale, the flow structure and mixing characteristics of the reactor become very complex; more experiments are needed to prove if it can achieve a high yield.

5.2.3.3.4 Fixed-bed reactor To simplify the purification steps of biodiesel products such as neutralization of the catalyst, washing of the products, and separation and recovery of the catalyst, fixedbed reactors have been widely investigated for the production of biodiesel. Bournay et al. [388] carried out the reaction and glycerol separation in two successive stages to shift the equilibrium of methanolysis. The reaction section included two fixed-bed reactors fed with a catalyst consisting of a mixed oxide of zinc and aluminum. This promotes the transesterification reaction without any loss of catalyst. The yields of the two reactors were 94.1 % and 98.3 %. Glycerin was directly produced with a high purity (at least 98 %) without any salt contaminants. This production process is compatible with high production capacities and simplified operations, high yields, and the absence of waste streams. The high quality of glycerol by-product is also a very important economic parameter. Therefore, it is considered as a “green technology.” In the assessment of HYSYS plant biodiesel production process by West [389], the heterogeneous solid acid technology is one of the best and most inexpensive technologies. Lion Corporation in Japan developed the ES process; the FFAs present in the feedstock were pretreated with methanol by pre-esterification using a specific resin catalyst in a fixed-bed reactor.

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5.2.3.3.5 Hydrodynamic cavitation technique The heterogeneous mixing of oil and alcohol phases is one of the key technical problems that should be resolved in the biodiesel production process, i.e., the effect of mass transfer on reaction is remarkable. The hydrodynamic cavitation technique [390] was developed to overcome the shortcomings of the conventional technologies for biodiesel production. Hydrodynamic cavitation mainly depends on the cavitation of a volatile liquid phase to form droplets in sizes of hundreds of nanometers, thus achieving liquid-liquid two-phase micromixing. Wang et al. [391] applied hydrodynamic cavitation to intensify the transesterification reaction for biodiesel preparation. When hydrodynamic cavitation was used, the reaction time was much shorter than conventional mechanical agitation. Under the conditions of 6:1 methanol/oil molar ratio and 1 equiv KOH of rapeseed oil, the reaction time was shortened from 60 min to 20 min. It can be concluded that the hydrodynamic cavitation technology is more simple, safe, efficient, reliable for scale-up, can be used in a continuous process, and has a good prospect in industrial applications.

5.2.3.3.6 Microchannel reactor The microchannel reactor is a type of small or microreactor where the channel size of process fluid is within hundreds of micrometers. This is manufactured using a precision processing technology. Strengthening of microscale heat and mass transfer significantly improves the physicochemical processes of a reaction mixture. At present, the microreactor system for the production of biodiesel mainly consists of a mixer (mixing the raw materials) and microchannel reactor. The first reported microreactor for the synthesis of biodiesel is a capillary microreactor; finely dispersed reactors have also been used. The microreactor process for biodiesel production is as follows: After preheating in a heat exchanger, oil, alcohol, and a basic catalyst are added to the mixer, and the resulting mixture is then pumped to a microchannel reactor. After the reaction, the mixture is neutralized and washed to afford biodiesel. Sun Juan et al. [392] synthesized biodiesel using a φ 0.25 mm microchannel reactor. A FAME yield of 99.4 % was obtained under 0.01 g KOH/g oil catalyst amount, 6:1 methanol-to-oil mass ratio, and 5.89 min residence time. Wen et al. [393] obtained a higher yield of methyl ester using a Z-shaped microchannel reactor under milder conditions and in a shorter time. The FAME yield was 99.4 % under the following reaction conditions: 0.012 g KOH/g oil catalyst amount, 9:1 methanol-to-oil mass ratio, 56 °C reaction temperature, 28 s residence time.

5.2.3.3.7 Membrane reactor To overcome the challenges in conventional batch and continuous flow biodiesel production, the membrane reactor technology was developed for a continuous transesterification reaction. Membrane reactors have several advantages over conventional reactors. A membrane reactor allows the occurrence of reaction and separation within

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a chamber. A membrane reactor ensures that the reversible reaction rapidly proceeds towards the favorable path by removing the reaction products simultaneously, thus leading to high product yields. Moreover, a membrane reactor can perform a selective separation, resulting in the filtration of only the desired products from the reactants. Recently, the two-phase membrane reactor technology for the simultaneous transesterification and separation of high-quality biodiesel has received much attention. Huang et al. [394] fabricated effective PVA-COPAA/AN-PVA membranes for the esterification of FFAs by separating the water. The methanol consumption decreased by 80 %. Cao et al. [395] studied MeOH recycling in the production of biodiesel using a membrane reactor. In their study, the permeate stream of the membrane reactor was separated at room temperature into a FAME-rich nonpolar phase and a MeOH-GLYrich polar phase. The polar phase was recycled at three recycling ratios: 50 %, 75 %, and 100 % under similar operating conditions. At a 100 % recycling ratio, the FAME concentration ranged from 85.7 wt.% to 92.4 wt.% in the FAME-rich nonpolar phase, whereas the overall molar ratio of MeOH/oil in the reaction system significantly reduced to 10:1. Thus, the reduction in the overall molar ratio of MeOH/oil in the system saved the cost of downstream MeOH recovery from the product. In short, the membrane reactor used for biodiesel production allows flexible feedstock with efficient separation and purification; thus, fewer water washing steps are required. Therefore, membrane reactors are not only considered as time-saving, energy efficient, and costeffective, but also more environmentally friendly.

5.2.3.4 Separation and purification of crude biodiesel 5.2.3.4.1 Separation of crude biodiesel and crude glycerin Once the reaction is completed, two major products are obtained: glycerin and biodiesel. Each contains a substantial amount of methanol that was used in the reaction. The reaction mixture is sometimes neutralized at this step if needed. The glycerin phase is much denser (methyl density of 900 kg/m3 , glycerol density 1,260 kg/m3 ) than the biodiesel phase, and the two phases can be gravity separated with glycerin by simply removing from the bottom of the settling vessel. This sedimentation method is often used in industrial production because of simple equipment, simple operation, and low investment. However, the sedimentation method is very time consuming, thus severely affecting the entire biodiesel production cycle and reducing the amount of processing necessary. In some cases, a centrifuge is used to separate the materials faster.

5.2.3.4.2 Removal of soap and alkali and other impurities from crude biodiesel Untreated biodiesel contains several impurities: free glycerol, soap, metals, methanol, FFAs, catalyst, water, and glycerides. The engine life of a vehicle can be reduced by

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high levels of impurities. Two methods are usually used to purify biodiesel: wet and dry washing. The traditional wet washing method is widely used to remove excess contaminants and leftover production chemicals from biodiesel. However, the introduction of additional water to the process has many disadvantages including increased cost and production time. Dry washing replaces water with an ion-exchange resin or a magnesium silicate powder to neutralize the impurities. Both dry washing methods are used in industrial plants. Because both glycerol and methanol are highly soluble in water, water washing is very effective in removing both the contaminants and until recently was the most common method of purification. It also has the advantage of removing any residual sodium salts and soaps, the latter being a by-product of high FFA feeds, because of their high water solubility. Although it is possible to meet the specifications by water washing, this process has some disadvantages. A highly polluting liquid effluent is generated. Significant product loss can occur due to the retention in the water phase. Furthermore, emulsions formation occurs when cooking oils or other feeds with a high FFA content are used due to soap formation. Two alternative commercial processes are now being promoted, one using ionexchange resin and the other using magnesium silicate (Magnesol®). Both the processes have the advantage of being waterless, thus eliminating many of the problems outlined above but, other than some fairly sketchy advertising material, little is known about their performance. Magnesol adsorbent has to be disposed of and landfilled or used in other applications (compost, potential animal feed additive, and potential fuel). Skelton et al. [396] compared three purification methods, namely, water washing and dry washing using ion-exchange resins and magnesium silicate as a solid adsorbent. It is necessary to remove methanol to avoid the saturation of the adsorbents. Glycerol and soap contents have been removed in all the processes. Not many differences are known on the other tested parameters.

5.2.3.4.3 Removal of excess methanol from biodiesel In the transesterification reaction for biodiesel production, the theoretical molar ratio of methanol/oil is 3:1. An excess amount of methanol with 6:1 molar ratio is usually used in industrial production. Therefore, after the reaction, the excess methanol will be distributed in the methyl ester and glycerin layers. To achieve quality standards for biodiesel and recycle excess methanol, it is necessary to remove the excess methanol from the product. The boiling point of methanol is 64.5 °C, much lower than the boiling point of methyl ester (the average boiling point is > 300 °C) and glycerin (boiling point 290 °C). Therefore, methanol can be removed by ordinary distillation. In industry, typically vacuum distillation is used to reduce the impact of high temperature on the color of biodiesel. High purity methanol may be obtained from recovered methanol using a multistage distillation unit.

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5.2.3.4.4 Biodiesel refining Waste edible oil contains more impurities such as polymers formed during the cooking or collection due to the polymerization at high temperature or decomposition in the presence of water. Therefore, if only treated by a simple process such as the removal of methanol and glycerin and washing, the methyl ester purity of biodiesel products from this waste oil is difficult to achieve 96.5 % purity required by European standards. It should be treated by distillation.

5.2.4 Case studies for biodiesel engineering 5.2.4.1 Processing technological analysis The project scale is 3,300 tons annually. The flow diagram of a two-stage reaction process for biodiesel is shown in Fig. 5.12. Raw oil is treated by the two-step catalytic process, pre-esterification with a solid acid as the catalyst and transesterification in plug-flow reactor. Crude biodiesel is separated from glycerin using a vertical inclined plate separator, pumped to a thin-film evaporator to distil off methanol and water, and then purified using a dry washing device. Refined biodiesel products are obtained after the distillation in a rectifying column. Dry washing device Base, methanol

Film evaporator

Rectifying column

Phase splitter

Glycerine Preesterification reactor

Dewatering tank

Raw material mixing tank

Biodiesel

Prup flow reactor

Fig. 5.12: Flow diagram of two-stage reaction process for biodiesel production.

5.2.4.2 Economic analysis 5.2.4.2.1 Total estimated investment The estimated total investment of this project is about 5 million yuan, consisting of construction investment and the construction period interest on borrowings and liquidity.

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5.2.4.2.2 Sales revenue The main product and by-product are biodiesel (FAME) and glycerol, respectively. The number of production days per year under normal circumstances is 320 days. The sales and income are shown in Tab. 5.2. Tab. 5.2: Gross sales of biodiesel. Project

Yield (t)

Price (yuan/t)

Total price (yuan)

Remarks

Biodiesel Crude glycerin Gross sales

3,300 165

7,100 3,500

23,400,000 570,000 23,970,000

Glycerin concentration is > 80 % Including tax

Remark: The Chinese currency is the yuan

5.2.4.2.3 Cost estimate The cost estimation is shown in Tab. 5.3. Tab. 5.3: Cost estimate for biodiesel. Project

Sum (yuan)

Costs of raw material and power Raw oil Auxiliary materials Fuel and power Wages and welfare Manufacturing costs a Depreciation b Repairs c Other manufacturing costs Management fees, interest, sales charges Total cost

6,158 5,530 24 (solid acid catalysis) + 393 (others) 211 40 234 153 55 26 220 6,652

Description: (1) Various raw materials refer to factory price including tax. (2) Wages and welfare estimates for 30 people. (3) Water: 3 yuan/t, electricity is calculated according to the area-weighted average: 0.76 yuan/kwh (4) Manufacturing costs (a) Fixed asset depreciation period is 10 years, the residual rate is 10 %. (b) Repair costs are in accordance with the 40 % depreciation of fixed assets. (c) Other manufacturing costs are estimated as 20 % of depreciation of fixed assets.

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(5) Management fees, marketing costs (a) Amortization period of intangible assets is 10 years; the deferred assets are amortized over 5 years. (b) The total cost of management and sales is estimated as 4 % of annual sales revenue. (6) Reserve fund is prorated to depreciation and amortization of the fixed assets, intangible assets, and deferred assets. The interest incurred during construction is amortized to the total cost as deferred assets. According to calculation results, the average annual total project cost is 21.95 million yuan/year, and the total cost of biodiesel is 6,652 yuan/ton.

5.2.4.2.4 Revenue and profit The revenue and profit are shown in Tab. 5.4. When the project reached the production target, the average total profit is 1.276 million yuan per year. Deducting 15 % of the total profit for corporation tax, the residual profit, namely, average annual after-tax profit, is 1.084 million yuan. No provision is provided for public reserve funds and public welfare funds. Therefore, the project retained average profit is 1,084,000 yuan per year. Tab. 5.4: Revenue and profit. Project

Sum (yuan)

Total cost Total sales Sales tax and surtax Total profit Income tax Profit after tax

21,950,000 23,970,000 713,000 1,303,000 27,000 1,276,000

5.2.4.2.5 Profitability Analysis (1) ROI (ROI =

1, 084, 000 Net profit ) × 100 % = ( ) × 100% = 21.68% total investment 5, 000, 000

(2) The investment recovery period Payback period =

1 1 = ≈ 4.6 years ROI 12.38%

The above indicators show that the annual 3,300 tons biodiesel project using a solid catalyst is economically feasible. The biodiesel feedstock cost is more than 75 % of

326 | 5 Technologies in vegetable oil and biodiesel

the total costs; therefore, the project risk originates mainly from the competing risks of raw materials. The risk can be reduced by controlling the main raw material cost, ensuring abundant source, and improving product quality and sales price.

5.2.5 Global development of biodiesel 5.2.5.1 China The development of biodiesel started late in China, serving as one of the industries vigorously supported by the government. China released the Renewable Energy Law and the National Standard for Biodiesel (B5) Blend in 2005 and 2010, respectively. From 2005, the Chinese biodiesel industry has developed rapidly. Particularly in the recent years, with the growing problem of energy shortages, the Chinese government, research institutions, and related businesses increased their focus and efforts in the research and development of the large-scale production of biodiesel. Studies were conducted related to the distribution, selection, cultivation, genetic improvement of oil plants, processing technology, and equipment. Now, biodiesel production processes using rapeseed oil, soybean oil, and waste oil as raw materials have been developed, including industrial production trials. In 2013, China’s total capacity of biodiesel production exceeded 3.0 million t/a (about 3.3 billion liters); the output was about 1.0 million t/a (about 1.1 billion liters), indicating that China has not fully utilized the available refining capacity. However, with the implementation of National Standard for Biodiesel (B5) Blend and the expansion of application range, it is estimated that China’s new consumption of biodiesel during 2014–2017 will increase beyond 2.5 million tons. Following the formal implementation of consumption tax on biodiesel imports on January 1, 2014 [397] coupled with other policy support as well as the optimism of enterprises about the biodiesel market, there are still many biodiesel projects proposed and under construction in China. As of the end of May 2014, major biodiesel projects proposed and under construction boast a combined capacity of over 2.7 million t/a. Biodiesel companies include Ningbo Tech-Bank Co., Ltd., Jiangsu Yueda Investment Co., Ltd., and Xinjiang International Industry Co., Ltd., of which Ningbo TechBank, the controlling shareholder of Hunan Jindeyi Feed Oil Co., Ltd. (with a capacity of 80 kt/a), reached an equivalent capacity of 70 kt/a in 2013. Yueda Investment, the controlling shareholder of Jiangsu Kate New Energy Co., Ltd. (with a capacity of 80 kt/a), reached an equivalent capacity of 40 kt/a in 2013. Xinjiang International Industry has invested in the construction of a 60 kt/a biodiesel project in Xinjiang, and currently, a 30 kt/a phase I is under construction.

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5.2.5.2 United States The United States is the pioneer in the research and development of alternative energy. Biodiesel and ethanol are two prominent commercial biofuels under development in the United States. The commercial production of biodiesel started in the early 1990s. In 2006, the biodiesel production capacity was 2.6 million tons, and the actual production was 1.25 million tons. After the Energy and Security Act of 2007 addressing the importance of moving the United States away from oil dependence, the production capacity of biodiesel has increased rapidly. By the end of 2007, US had 171 manufacturers, and the biodiesel production of 450 million gallons had increased by 80 % compared to 2006. In mid-February 2013, the US Senate formally submitted a proposal for the resumption of tax credits for the biodiesel industry. The US production of biodiesel has accounted for 17 % of the global total production in 2013 (up from 14.5 % in 2012). Production increased by one-third over the year to approximately 5.1 billion liters, making the United States again the largest national producer. US output exceeded the Environmental Protection Agency (EPA) target under the Federal Renewable Fuels Standard (RFS), which called for the inclusion of 4.8 billion liters (1.28 billion gallons) in diesel fuel markets in 2013 [398]. Soybean oil is the most prominent feedstock in the US biodiesel industry. The United States also actively explored other ways to produce biodiesel. The US Department of Defense announced at the end of 2008 that the US would invest a US $ 20 million fund in the research and development of microalgae biodiesel by agencies all over the United States, expecting to commercialize algae-based biofuels as a substitute for JP-8 jet fuel.

5.2.5.3 European union (EU) The EU has remained the premier region for biodiesel production in the world, and the total biodiesel production has reached 10.5 billion liters. The EU has been the largest regional biodiesel producer for years, and in 2013, it accounted for 10.5 billion liters of FAME production plus 1.8 billion liters of hydrotreated vegetable oil, mainly in Germany and France. However, its share of the global total production (~ 42 %) has remained constant in the recent years. According to the target in the Kyoto Protocol that CO2 emissions should be reduced by 8 % from 2008 to 2012, the EU attaches great importance to the development and utilization of biomass energy. In Europe, biodiesel enjoys government tax incentives, and the retail price is lower than regular diesel. Biodiesel sales are exempted from VAT. To increase the use of biodiesel, policies requiring the blending of biodiesel into fuels have been implemented, including penalties if those rates are not reached. In France, the goal was to reach 10 % integration, but plans for that stopped in 2010. As an incentive for the EU countries to continue the production of biofuel, tax rebates are available for

328 | 5 Technologies in vegetable oil and biodiesel

specific quotas of biofuel produced. In Germany, the minimum percentage of biodiesel in transportation diesel is set at 4.4 % Germany is Europe’s largest producer of biodiesel, and its total production in 2013 amounted to 3.1 billion liters, approximately one-third of the EU total. As Europe’s second largest renewable energy producer and second largest consumer, France has 32 biodiesel producers with a production capacity of 2.05 million tons (~ 2.27 billion liters), which is added to the common standard petroleum diesel in 5 % biodiesel. B5 (5 % biodiesel, 95 % diesel) is commonly used.

5.2.5.4 Other countries The global production of biodiesel increased gradually since 2005, and it increased over 11 % to 26.3 billion liters in 2013 (Fig. 5.13). In Asia, the production of biodiesel continued to increase rapidly. Thailand, for example, continued its rapid expansion of biofuel production (both ethanol and biodiesel), which increased by ~ 30 % in 2013 (after a 28 % increase in 2012). Its growth is primarily due to the Renewable Energy Development Plan. 30 26,3

25

22,4

20

17,8

18,5

15,5

15 10,5

10 5

22,5

6,5 3,8

0 2005

2006

2007

2008

2009

2010

2011

2012

2013

Fig. 5.13: Global biodiesel production from 2005–2013 (billion liters).

The United States being the largest national producer of biodiesel was followed by Germany and Brazil; both of them increased their biodiesel production by ~ 16 % and 5 %, respectively, to 3.1 billion liters and 2.9 billion liters. Argentina was the fourth largest producer at 2.3 billion liters. However, Argentina’s production declined almost 10 % relative to 2012 as a result of antidumping duties placed by the European Commis-

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329

sion on the imports of US and Argentine biodiesel. France and Indonesia tied for the fifth largest producer, followed by Thailand, Singapore, and Poland with production of about 9–11 billion liters.

5.2.6 Environmental impact The environmental benefits of biodiesel are evaluated by the positive net energy balance (NEB), greenhouse gas (GHG) reduction, and air pollutant reduction by replacing diesel with biodiesel. A gallon of biodiesel yields 93 % more energy than required in its production. Although the life-cycle GHG reduction of biodiesel use varies widely depending on the study, most studies conclude that biodiesel has a positive impact on GHG reduction. The American National Renewable Energy Laboratory (NREL) reported that biodiesel production achieved up to 78 % GHG reduction compared to petroleum diesel. In comparison, Hill et al. [399] calculated a reduction of 41 % GHG emissions when using soybean biodiesel to replace diesel at an energy equivalent amount. Moreover, biodiesel has air quality benefits; according to the American EPA report “A Comprehensive Analysis of Biodiesel Impact on Exhaust Emission”, soybean-based B20 reduced particulate matter (PM) by 10.1 %, hydrocarbons (HC) by 21.2 %, and carbon monoxide (CO) by 11.0 %, whereas it increased nitrogen oxides (NOx) by 2.0 %. Chattopadhyay et al. [400] compared the performance characteristics of a diesel engine with the blends B10 and B20 to that of diesel. The B10 and B20 products proved to be an efficient environmentally friendly bioadditive for diesel engines, the average emissions of CO, CO2 , smoke, and unburned HCs considerably reduced by 33.3 %, 8.4 %, 43.4 %, and 29.4 %, respectively. Moreover, toxic emissions and engine noise pollution also decreased when biodiesel was mixed into diesel fuel. More precisely, the exhaust emissions associated with biodiesel blends depend on the feedstock of biodiesel, how much is blended, and what type of diesel is added.

Dong Li, Xiaofeng Liu, Feng Zhen, and Haibin Li*

6 Technologies of municipal solid waste treatment

6.1 Characteristics of municipal solid waste 6.1.1 Characteristics of municipal solid waste outside China The composition of Municipal Solid Waste (MSW) is significantly different in different countries and regions. In general, MSW in developed countries and regions has a relatively high proportion of plastics and papers, because of differences in living standards and habits. For example, more than 40 % of MSW is plastic and paper in Europe and the United States of America (USA), and even up to 60 % in Japan. In addition, the ratio of metal and glass is also relatively high, accounting for more than 10 % of MSW. The ratio of organic waste, including kitchen waste and yard waste, is relatively low, and is usually no more than 30 %. Therefore, the water content of waste is relatively low, while the heating value is comparatively high, typically more than 8,300 kJ/kg. Figs. 6.1 and 6.2 show the composition of MSW in European countries. For the developing countries in Asia (except Japan and Korea), the common feature of MSW is a large quantity of kitchen waste, together with other organic matter in the waste stream, accounting for more than 40 % of MSW, even up to 70 %. The quantity of paper and plastic is low, usually < 30 %. As kitchen waste and other organic waste have a high moisture content, the overall water content of garbage is usually > 50 %. In general, the heating value becomes very low and is 4,000–4,200 kJ/kg. The composition of MSW in Asian countries is listed in Tab. 6.1.

6.1.2 Characteristics of domestic MSW Since 1996, the Chinese Academy of Science, Guangzhou Institute of Energy has analyzed the characteristics of MSW from more than thirty cities including Guangdong,

Dong Li: Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 510640, China Xiaofeng Liu: Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 510640, China Feng Zhen: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Labo-ratory of New and Renewable Energy *Research and Development, Guangzhou 510640, China *Corresponding Author: Haibin Li: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China; E-mail: [email protected], Tel. +86-20-87057732 https://doi.org/10.1515/9783110476217-006

332 | 6 Technologies of municipal solid waste treatment

Paper 26%

Organics 29%

Mixture 1% Textiles 5% Others 18%

Plastic 9% Glass 7%

Metal 4%

Special waste 1%

Fig. 6.1: Average composition of MSW in European cities (1998 and 1999, 39 European cities). Data sources: Municipal Waste Minimisation and Recycling in European Cities, Association of Cities for Recycling, http://www.acrplus.org/upload/documents/document306.pdf

Others 3.40%

Organics 11.90%

Courtyard waste 13.10% Paper 34.20% Wood 5.10% Rubber, Leather, Textiles, etc. 7.30% Glass 5.20%

Metal 7.60%

Plastic 11.80%

Fig. 6.2: Composition of MSW in the USA (2005). Data sources: MSW in the USA, 2005 Facts and Figs., US EPA.

Fujian, Shandong, Shanxi, Henan, Hebei, Hubei, Jiangsu, Anhui, Yunnan, Sichuan, and other provinces. The result shows that a majority of MSW in China is co-mingled and collected (Tabs. 6.2 and 6.3). The composition of MSW is complex, having high quantities of inorganic substances, kitchen waste, and high water content, but a low calorific value. The composition of MSW in China has significantly changed in recent years. Because of the accelerated urbanization process, the popularity of using gas in cities

6.1 Characteristics of municipal solid waste

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333

Tab. 6.1: Composition of MSW in regions and countries in Asia (2001) ( %). Country/Region

Organic

Paper

Plastic

Glass

Metal

Others

China (mainland) Hong Kong Indonesia Japan Laos Malaysia Myanmar Philippines Singapore Korea Thailand

35.8 37.2 70.2 17 54.3 43.2 80 41.6 44.4 31 8.6

7 21.6 10.9 40 3.3 23.7 4 19.5 28.3 27 14.6

3.8 15.7 8.7 20 7.8 11.2 2 13.8 11.8 6 13.9

2.0 3.9 1.7 10 8.5 3.2 0 2.5 4.1 5 5.1

0.3 3.9 1.8 6 3.8 4.2 0 4.8 4.8 7 3.6

54.3 17.6 6.2 7 22.5 14.5 14 17.9 6.6 23 14.2

Data sources: Institute for Global Environmental Strategies (IGES) (2001). Urban environmental challenge in Asia: current situations and management strategies. Part I: The summary of UE 1st phase project. Urban Environmental Management Project, Hayama, Japan.

Tab. 6.2: Average composition of MSW in more than 30 Provinces (wt.%) (2001–2009). Sand

Glass

Metal

Paper

Plastic

Rubber

Fiber

Garden waste

Kitchen waste

15.4

3.6

0.8

8.1

15.72

0.4

5.5

7.2

43.2

Calorific value of MSW (kJ/kg)

Note: The above data is the arithmetic average. In the case of multiple sampling within a year, the mean value is used.

8000 6000 4000 2000 0 1999-12 2001-04 2002-09 2004-01 2005-05 2006-10 2008-02 2009-07 2010-11 Sampling time

Fig. 6.3: Calorific value of MSW in Chinese cities

and towns has increased substantially. In 2012, the average popularization rate of using gas in cities and towns had reached 93.2 %, which is much higher than 45.4 % in 2000. A majority of urban residents do not use coal directly as their energy supply. Therefore, the proportion of inorganic ash in MSW decreases significantly. In many cities, the inorganic waste (except glass, metal) mainly comes from sand and soil in

334 | 6 Technologies of municipal solid waste treatment

Tab. 6.3: Average composition of MSW in Guangzhou City in recent years.

Percentage

Organics (kitchen waste)

Paper

Plastic

Glass

Metal

Textiles

Other inorganics

49.5 %

10.9 %

21.0 %

3.8 %

2.3 %

6.9 %

5.6 %

street cleaning and a small amount comes from construction and home renovation waste mixed together with domestic waste. For example, in Guangzhou and other large cities, the content of inorganic waste in MSW such as sand and soil has fallen below 10 %, thus the calorific value increased to ≥ 5,800 kJ/kg. Overall, the composition of MSW in China is as follows: kitchen waste and other perishable organic matter, in the range 40 to 60 %; flammable organic matter (plastic, paper, fibers, etc.), in the range 20 to 40 %; inorganics (glass, metal, sand, etc.), ≤ 20 %. The moisture content is usually 40–60 % and the average heating value is ≥ 4,500 kJ/kg.

6.1.3 Current situation of collection, transportation, and disposal of MSW in China With the expansion of city size, population growth, improvements in living standards and changing lifestyles, the amount of MSW collected and transported is increasing gradually year by year. From 2006 to 2012, the average annual growth rate of MSW was 2.4 %. In 2012, the amount of MSW collected and transported (171 million tons) had reached its highest level yet. The harmless waste treatment rate reached 84.8 %, which is a new high in recent years; however, still ~ 15 % of MSW has not been properly addressed. In addition, a majority of rural area uses a simple method such as simple stacking and landfilling for waste handling and treatment. Therefore, harmless MSW treatment still has a room for improvement. Besides that, the number of waste treatment facilities and technology should continuously increase. Overall, the tendency of MSW production and disposal in China has the following three characteristics.

6.1.3.1 Quantity of MSW collected and transported increases year by year As shown in Fig. 6.4, the quantity of MSW collected and transported has gone through four growth stages since 1981. (1) Rapid growth before 1995 in Stage 1, the average annual growth rate of quantity of MSW collected and transported was 10.7 %. (2) Steady growth between 1995 and 2000 in Stage 2, and the annual growth rate was 2.1 % on average. (3) The third stage is between 2000 and 2005. It is a fast-growing stage in which the annual growth rate was 5.4 % on average. (4) In the fourth stage from 2006 to present, the average annual growth rate is ~ 2.4 % between 2006 and 2012. The quantity of MSW collected and transported was > 170 million tons for the first time in 2012.

6.1 Characteristics of municipal solid waste

335

|

180.0

149

148 156

140.0

135

120.0 107

114

110

154

158

137

118

113

108

100.0

152

2010

160.0

2008

Quantity of MSW collected and transported (million tons)

171 157 164

155

100 83

88

80.0 68

76

58

60.0

63

50

40.0

54

38

31

45

31 35

20.0

26

25

Year

500

100.0% 446 409 388

400

356

350

315

300 237

250 201

200

180 156

150 100 50

210

63.4% 60.0% 57.3%

239

225 216 220

80.0%

272

77.9%

79.8%

71.4%

70.0%

66.8%

61.4%

62.0%

58.2%

60.0%

51.7% 54.2%

51.4%

256 258

90.0%

84.8%

50.8%

52.2%

53.0%

0

Harmless treatment rate

450

50.0% 40.0%

19

96 19 97 19 98 19 99 20 00 20 01 20 02 20 03 20 04 20 05 20 06 20 07 20 08 20 09 20 10 20 11 20 12

Harmless treatment capacities (103 ton/d)

Fig. 6.4: Quantity of MSW collected and transported in the past years. Data source: collecting and sorting from annual “China Statistical Yearbook”, “China Urban Construction Statistics Annual Report” [401, 402]

Year Fig. 6.5: Treatment capacities for harmless MSW in China. Data source: collecting and sorting from annual “China Statistical Yearbook”, “China Urban Construction Statistics Annual Report”.

2012

2011

2009

2007

2006

2005

2004

2003

2002

2001

2000

1999

1998

1997

1996

1995

1994

1993

1992

1991

1990

1989

1988

1987

1986

1985

1984

1983

1982

1981

1980

1979

0.0

336 | 6 Technologies of municipal solid waste treatment

6.1.3.2 Large-scale processing: An increase in processing capacity and decrease in the number of facilities The capacity for harmless treatment of MSW has been improving each year since 1996 (Fig. 6.5). By the end of 2012, the capacity of harmless treatment has reached its highest level; the daily MSW processing capacity is > 440,000 tons. At the same time, the percentage of harmless waste has also increased to 84.8 %. Notably, as the current standards for harmless waste differ from those in the past, the harmless treatment rates in the 1990s are actually higher than shown. The national standards for landfill, incineration and other treatment facilities are nowadays close to those in the major developed countries. The increase in the waste disposal capacity also contributed to the phasing out of small garbage disposal facilities. Large-scale waste disposal facilities equipped with advanced pollution control measures have been developed. The number of waste treatment facilities decreased from 741 in 2001 to 701 in 2012, while the processing capacity of a treatment facility increased from 303 tons per day in 2001 to 637 tons per day in 2012. The increase in the treatment scale could centralize the pollution source and facilitates reducing secondary pollution by advance technology (Figs. 6.6 and 6.7). 800 750

741

701

MSW facilities quality

700 677

651

650

628 600

575

567

559

550

509

500 471

460

450 419 400 350 300 2001

2002

2003 2004

2005

2006 2007 2008 2009 Year

2010

2011

2012

Fig. 6.6: Change in the number of MSW facilities in China. Data source: collecting and sorting from annual “China Statistical Yearbook”, “China Urban Construction Statistics Annual Report” [401, 402].

Average daily processing capacity of MSW facilities (ton/d)

6.1 Characteristics of municipal solid waste

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337

1000 Landfill Composting Incineration

900

817

800 697

700

638 593

600

612

677

889

705

582

622

500

464 409

400

766

863

338

493

579

588

475

464

576 549 436

552

498

357 319

313

385

300 216

200

190 181

215

236

252

256

2003

2004

2005

100 0 2001

2002

2006 2007 Year

2008 2009

2010

2011

2012

Fig. 6.7: Average daily processing capacity of MSW facilities in China. Data source: collecting and sorting from annual “China Statistical Yearbook”, “China Urban Construction Statistics Annual Report” [401, 402].

6.1.3.3 Landfill-led, rapid development of incineration and a decrease in the composting ratio As shown in Figs. 6.8 and 6.9, landfilling is still the main waste treatment technology being used in China nowadays; however, in recent years, a gradual decrease in using landfill is observed. The proportion of landfilling decreased from 88.7 % in 2001 to 72.6 % in 2012. Because a considerable area of land is required for landfilling and choosing a suitable site is difficult, a significant reduction in volume of MSW could be achieved by incineration, and thus incineration has become the most preferable option among new waste disposal facilities in many cities in China. From 2001 to 2012, the number of waste-to-energy plants increased from 36 to 138, and the proportion of waste treated by incineration also increased rapidly from 2.2 % to 24.7 %. In the near future, waste-to-energy will be the fastest-growing waste disposal method in China. Composting is an ancient method for the treatment of perishable organic waste. However, as living standards have significantly improved in recent years, the amount of noncompostable components in the waste stream has increased, even the contaminants keep increasing which affect the quality of compost. In contrast, because of low economic value of compost, limited application, seasonal fluctuation in sales as well as low subsidies for running a composting plant, the application of this technology is shrinking. The proportion of waste treated by composting reduced from 9.0 % in 2001 to 2.7 % in 2012.

338 | 6 Technologies of municipal solid waste treatment 600

571

Landfill

528

547

Incineration

540

498

500 Number of facilities

Composting

457

447 407

400

366

356 324

300 200 138

134

100 36

78 45

70 47

61 54

46 67

20 69

66 17

93

74 14

16

104

11

109

21

23

2011

2012

0 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 Year

Fig. 6.8: Number of all types of waste disposal facilities in China. Data source: collecting and sorting from annual “China Statistical Yearbook”, “China Urban Construction Statistics Annual Report” [401, 402].

Percentage of MSW treatment facilities

2.2% 3.7% 8.8% 7.0%

4.9%

5.6%

9.5%

9.0%

9.8% 4.3%

14.5% 15.2% 15.2% 18.0% 18.8% 19.9% 3.7%

2.6%

1.7%

1.6%

1.5%

24.7%

1.4% 2.7%

86.7% 89.3% 84.9% 85.2% 85.2% 81.4% 80.9% 81.7% 79.2% 77.9% 76.9%

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

72.6%

2012

Year Landfill

Composition

Incineration

Fig. 6.9: Percentage share of all types of MSW treatment facilities in China. Data source: collecting and sorting from annual “China Statistical Yearbook”, “China Urban Construction Statistics Annual Report” [401, 402].

6.2 MSW treatment and application technology | 339

6.2 MSW treatment and application technology 6.2.1 Sanitary landfill According to CJJ/T 65-2004 Urban Sanitary Terms issued by the Ministry of Construction of the PRC, landfill refers to the process in which wastes are landfilled and left to natural decomposition; sanitary landfill refers to the process in which various measures including seepage prevention, levelling, compacting and coverage are adopted to treat urban domestic waste and gas and dispose of leachate, flies, and worms.

6.2.1.1 Reaction mechanisms in landfill process 6.2.1.1.1 Biological reactions Aerobic and anaerobic decompositions occur to the organic compounds in the waste in the presence of microorganisms and are the most important reactions in waste heaps. The former takes less time and ultimately generates carbon dioxide and water etc., and the latter needs much more time and generates methane, carbon dioxide, and a small amount of hydrogen sulfide, ammonia, etc.

6.2.1.1.2 Chemical reactions Dissolution/precipitation During landfill, the soluble substance is dissolved to generate highly concentrated organic wastewater (leachate). Certain salts in the leachate precipitate as the pH value changes.

Adsorption/desorption Volatile organic compounds are generated in the gas as well as organic and inorganic compounds in the leachate during landfill and are adsorbed by wastes and soil, and will desorb in some conditions to release contaminants that enter into the gas or leachate again.

Dehalogenation/degradation This refers to dehalogenation, hydrolyzation, and chemical degradation of organic compounds.

Oxidation/reduction Soluble or insoluble salts in the waste affect the solubility of metals or metal salts and cause mutual transformation by oxidation or reduction reaction.

340 | 6 Technologies of municipal solid waste treatment

6.2.1.1.3 Physical reactions Evaporation/vaporization The moisture and the volatile and semi-volatile organic compounds in the waste turn into gas through evaporation/vaporization.

Settlement/suspension Suspended solids and colloidal substance in the leachate will settle or float in liquid phase under the force of gravity or buoyancy.

Diffusion/migration This refers to the gas from the waste moving within and beyond the landfill site and the leachate migrating within the landfill site and entering into the lower layer of the soil cover.

Decay This is a natural reaction in the landfill site as time goes by.

6.2.1.2 Seepage prevention at a landfill site Seepage prevention at the landfill site, including horizontal seepage prevention, vertical seepage prevention and top surface seepage prevention, prevents leachate from penetrating into the groundwater basin and surface water from entering the landfill site. One or a combination of seepage prevention methods can be considered based on the landform and geological conditions of the landfill site. According to the seepage prevention modes and their evolution, the materials used for landfill anti-seepage mainly include the following four.

6.2.1.2.1 Natural clay It is only used in landfill sites where the soil, hydrological, and geologic conditions allow. Specifically, the subgrade of the landfill site is composed of anti-seepage clay, which can restrict the leachate within the landfill site. A layer of compacted soil containing adequate high-cohesive soil and sandy silt is added at the bottom and around the landfill site and must be evenly formed to a thickness of at least 2 m and a minimum seepage coefficient of 1 × 10−7 cm/s [403]. Low cost is the greatest strength of this type of seepage prevention; however, unfortunately, the leachate can get into the soil over a period of time causing groundwater pollution. Therefore, natural clay is not recommended for anti-seepage purposes for the time being.

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6.2.1.2.2 Improved liner This refers to nonconforming mild clay or sandy loam artificially improved to conform to the performance requirements for seepage prevention. The additives used for this purpose include organic and inorganic types. The inorganic additives are less expensive and suitable for developing countries. The improved liner includes two types: clay-bentonite and clay-lime-cement. The seepage coefficient of such improved clay is 1 × 10− cm/s, fully conforming to the seepage requirements. The clay-bentonite liner is used in Anding Waste Sanitary and Beishenshu Waste Sanitary Landfill Sites in Beijing.

6.2.1.2.3 Synthetic membrane Strictly speaking either natural clay or improved liner not only slows the seepage of the leachate, but also fails to isolate the leachate from penetrating underground. Therefore, it is important to use a synthetic membrane with better antiseepage performance. There are about ten types of synthetic membranes developed worldwide, including high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyvinyl chloride (PVC), and chlorinated polyethylene (CPE). These synthetic membranes have an extremely low permeability with a seepage coefficient of ≤ 1 × 10−12 cm/s [404]. With its strong resistance to chemical corrosion, mature manufacturing process, easiness to weld on-site, and rich experience accumulated in practical use, HDPE is widely used in vertical seepage prevention, top surface seepage prevention, foundation seepage prevention in the sewage treatment system and HDPE tubes preparation. The thickness of the HDPE membrane should be no less than 1.5 mm. For example, HDPE membrane is used at Xiaping Waste Sanitary Landfill Site in Shenzhen and Liulitun Waste Sanitary Landfill Site in Beijing.

6.2.1.2.4 Curtain grouting This method is used in vertical seepage prevention and is the only option for landfill sites located in valleys, beaches, and lowlands where the groundwater level is high. In China, The cut-off dam and curtain grouting is set on hard bedrocks for most valleytype landfill sites for most valley-type landfill sites, curtain grouting is performed at the outlet where groundwater accumulates. Pressurized grouting is adapted to fill the cracks in the weathered rocks or the gap in the permeable stratum to intercept the leachate and polluted groundwater within the tank in the front of the curtain so as to stop it from flowing downward and into the neighboring areas. This method is used in the Tianziling Sanitary Landfill Site in Hangzhou. For some special landfill sites such as Yanshan Petrochemical Hazardous Waste Landfill site in Beijing, concrete blocking is carried out in addition to curtain grouting in subsurface faults to prevent the leachate from polluting groundwater. The materials used for seepage prevention are a mixture of slurry, emulsion, and solvents such as (1) water and cement, (2) water, cement, and additive, (3) water, clay, and cement, (4) water, water glass, and

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water-insoluble solid, (5) water, asphalt, emulsion, and quick-setting agent, and (6) hydroquinone, formaldehyde, and catalyst. The design, selection of materials, and construction of seepage prevention for landfill sites are subject to the Technical Code for Sanitary Landfilling of Domestic Sewage (GB 50869-2013). A typical anti-seepage structure is shown in Fig. 6.10.

Leachate drainage layer 30cm Nonwoven geotextile 600g/m2 HDPE membrane 1.5mm

Compacted clay 75 cm Anti-seepage coefficient (1 x 10‒7cm/s)

Base layer Groundwater collection and drinage layer Fig. 6.10: Schematic diagram for typical anti-seepage structure.

6.2.1.3 Leachate treatment The leachate treatment processes developed in China and abroad include local circulation, biological treatment, membrane treatment, advanced oxidation and can be used in combination with supplementary measures such as flocculation, adsorption, stripping, filtration, and flotation. A combination of these treatment technologies instead of only one is utilized in practical applications. Among these treatment processes, biological treatment is normally taken as the main measure, physicochemical treatment is used as pretreatment, and treatment of land is adapted for post-treatment. In order to accelerate the stabilization of a landfill site, a part of the leachate is recycled and circulated locally. For the leachate generated in a mature landfill site, reverse osmosis and advance oxidation are additionally applied.

6.2.1.3.1 Local circulation Local circulation, also called landfill reinjection treatment, refers to leachate injected into the waste landfill site in a controllable way to dispose of the leachate by taking advantage of waste heaps. The merits of this treatment are as follows: (1) increasing

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moisture content of waste heaps to accelerate the degradation of waste, (2) leachate reduction because of reflux and evaporation, (3) effective consolidation of heavy metals as a result of metal sediments formed and chelation of humus, (4) nitrogen removal by denitrification realized by taking advantage of anaerobic environment in the lower part of waste heaps, (5) increasing landfill gas production through anaerobic fermentation of organic matter in the leachate, (6) promoting the stabilization of a landfill site and accelerating heap settlement to make them smaller to extend the service life of the landfill site.

6.2.1.3.2 Biological treatment Biological treatment includes aerobic treatment and anaerobic treatment and is the main process for leachate treatment. For high concentration leachate with a COD content of > 5,000 mg/L, the process of aerobic and anaerobic treatment is recommended and is suitable for young landfill leachate. For leachate with a COD content of < 5,000 mg/L, aerobic treatment is normally used and is suitable for metaphase or mature landfill leachate. Anaerobic treatment is characterized by advantages such as high organic load, strong impact load, less sludge produced, small floor area, low energy consumption, and low operating cost. However, effluent fails comply with relevant indicators, and aerobic treatment is often needed. For high and medium concentration organic wastewaters, a combination of aerobic and anaerobic treatments is normally used.

6.2.1.3.3 Membrane treatment Mature landfill leachate has low biological degradability with BOD5 /COD of < 0.1. Very matured landfill leachate even has a ratio of BOD5 /COD < 0.01, for which biological treatment is minimally effective and membrane treatment should be used. The membrane treatment process includes reverse osmosis (RO) and nanofiltration (NF). As RO removes 100 % of organic molecules in the size range 0.3–10 nm (solute), 98 % of inorganic ions from the leachate except for hydrogen ions and hydroxyl ions, and filters all types of pathogenic microorganisms and viruses, it is becoming the main measure in China for leachate treatment at landfill sites. However, membrane treatment only concentrates pollutants in the leachate and fails to completely remove them from the leachate. Leachate concentrate has higher concentration of liquid waste, with its COD and conductivity 3–4 times that of the original leachate. For this reason, leachate concentrate treatment is definitely unavoidable for membrane treatment. Currently, the treatment method for leachate concentrate includes evaporation, drying, incineration, controlled injection, and solidification.

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6.2.1.3.4 Advanced oxidation Biological treatment is not suitable for leachate that is hard to biodegrade in the later stage of landfill or harmful and toxic to organisms. In such case, advanced oxidation is mainly used. Advanced oxidation is developed on the basis of traditional chemical oxidation using ozone and chlorine, where ozone/UV, hydrogen peroxide/UV, chlorine/UV, ozone/activated carbon, Fenton reagent (hydrogen peroxide/iron salts) are used as the oxidant/catalyst for leachate treatment [405]. The mechanism for advanced oxidation is hydroxyl radical (· OH) formed to trigger the following chain reaction to degrade large organic molecules to small molecules or even carbon dioxide and water. Compared to ordinary chemical oxidation, hydroxyl radical (· OH) can continue to react with intermediate products until carbon dioxide and water are formed, with a rather high TOC removal rate. In addition, advanced oxidation can make organic compounds (such as humus) with large molecules, which are normallyhard to degrade, producing organic acid; for example, small organic molecules can be easily degraded to increase the biodegradability (COD/BOD5 decreased to 6 from 16). Therefore, it can be adopted as the pretreatment for leachate that is hard to biodegrade.

6.2.1.3.5 Treatment with land Membrane treatment, advanced oxidation, or even biological treatment require higher construction and operating costs. Therefore, treatment of land such as stabilized ponds and constructed wetlands is a viable operation in less developed areas. This treatment has a number of advantages such as less investment, stable operation, low operating costs, simple operation, resistance to shock, strong ability to remove toxic and hazardous substances, and one disadvantage of large land occupation.

6.2.1.4 Gathering and applications of landfill gas Vertical gathering and horizontal gathering are two common ways for gathering landfill gas. Vertical gathering has wider applications than horizontal gathering and can be used not only in landfill sites in operation, but also in capped landfill sites and landfill sites with partially complete operating area. A pumping well normally has diameter and depth in the ranges 60–90 cm and 50–90 % of height of a waste heap, respectively, with the lower limit at the leachate level at the bottom of the waste landfill. Horizontal gathering is normally used in an operating landfill site. The gathering unit is composed of perforated pipe or other pipes with different diameters connected to each other. In general, the gas gathering pipes are placed in the lower layer of the landfill site, with the trench for the pipes of 0.6–0.9 in width and 1.2 m depth and nonwoven fabrics lining the walls. The landfill gas can be utilized according to its output and characteristics, economic efficiency, and local conditions while considering the operating and mainte-

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nance costs and convenience after utilization facilities are constructed. It can be used as boiler fuel, domestic or industrial gas, compressed gas or pipe gas after purification and for electricity generation for gas engines, gas turbines, and steam turbines. Although it is similar to biogas in this respect, special attention should be paid to the purification process design, as its components are slightly different from those of biogas. For example, landfill gas normally contains 1–2 % of oxygen, 2–5 % of nitrogen, and some nonmethane hazardous gases, which are negligible or absent in biogas.

6.2.1.5 Closure of a landfill site When the volume of landfill reaches the design value, the landfill site should be closed (closure of landfill site). The final cover system serves for separating waste from the surroundings, it should give the site a good appearance, not allow it to become a place where small animals live or bacteria multiply, be convenient for equipment use and vehicle travelling, provide soil for plants to grow, control spreading of landfill gas, and minimize penetration of surface water to reduce leachate generation. The final cover system includes a vegetation nutrition layer, support soil layer, drainage layer, impermeable layer, and gas discharge layer from top to bottom (Fig. 6.11).

Vegetation layer Soil cover Drainable layer Impermeable layer Gas permeable layer Fig. 6.11: Components of final coverage system.

6.2.2 Incineration for power generation 6.2.2.1 Waste incineration process Waste incineration is normally divided into the stages of drying, thermal decomposition, burning, and burning-out for analysis. Notably, these four stages cannot be clearly differentiated in practical incineration, even though they occur at different times.

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6.2.2.1.1 Drying The drying of domestic waste is a process where the waste is heated in the furnace to remove moisture in order to reduce moisture content. In the case of high moisture content in waste, furnace temperature would decrease significantly and combustion would become difficult, thus necessitating the use of auxiliary fuel to increase the furnace temperature and improve combustion conditions. For modern incinerators used to burn urban domestic wastes, pre-heating primary air is blown into the furnace through the dry section to accelerate the drying, thus avoiding a significant effect of radiant drying on the furnace temperature.

6.2.2.1.2 Thermal decomposition Thermal decomposition refers to the process where organic combustibles in the waste decompose into light combustible gases, fixed carbon, and noncombustible materials. During this process, some decomposed products aggregate to form new reaction products. This process is a heat absorption process in most cases.

6.2.2.1.3 Burning Waste burning is a process where combustible components in the waste oxidize rapidly in the presence of oxygen. Actual waste incineration is rather complicated. After drying and thermal decomposition, a variety of gaseous and solid combustibles is generated and burns at the ignition point in the presence of combustion air. Therefore, waste incineration is a process combining homogeneous (gas) combustion and heterogeneous (gas and solid) combustion. In addition, waste combustion can be divided into complete combustion and incomplete combustion. The final products of the former are CO2 and H2 O, and those of the latter include CO or other organic combustibles in addition to CO2 and H2 O. Incomplete combustion should be avoided as far as possible during waste incineration.

6.2.2.1.4 Burning out Burning out refers to the process where solid combustibles such as fixed carbon left over from waste combustion are finally oxidized into incombustible ash, ending the entire combustion process, and the combustion residues enter the slag discharge system.

6.2.2.2 Waste incinerator There are a variety of waste incinerators used worldwide. Among them, the grate incinerator, fluidized-bed incinerator, and rotary kiln incinerator are representative and have the most widespread applications.

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6.2.2.2.1 Incineration technology of the grate incinerator The mechanism for the grate incinerator is as follows: raw waste is delivered to the grate in the furnace, and then the waste moves backward as the grate moves. Strong flame radiation from the furnace, radiation from the arch, fume radiation, and convective heat transfer dries the waste and makes it catch fire at the front section of the grate. At the middle and back sections of the grate, waste burns completely in the combustion air, while hazardous compounds in the waste effectively decompose and burn in the high-temperature furnace. Grate incineration technology was the first to be developed. Therefore, it is the most mature incineration technology and is widely used in waste incineration plants at present, accounting for more than 80 % of the waste incineration market. It is suitable for central treatment of a large amount of waste, and the heat from incineration can be used for power generation or heat supply. However, the applications of this technology are also limited. For example, sludge containing much moisture and large domestic wastes should not be delivered to the furnace directly for incineration. The temperature of the grate incinerator during waste combustion is normally in the range 800–1,000 °C [406]. The time that waste stays in the furnace can be adjusted according to waste components, heat value, and moisture content. In general, the time should be in the range 1–1.5 s. It should be noted that only the hazardous and toxic fumes from waste incineration stay long enough in the high-temperature area to ensure complete detoxification. Under normal circumstance, fumes remaining in the high-temperature (850 °C) area for over 2 s can meet process requirements for waste incineration. See Fig. 6.12 for a schematic diagram of a mechanical grate incinerator. Some examples of the existing grate incinerators are the reciprocating grate incinerator, overturning grate incinerator, roller grate incinerator, and chain grate incinerator.

6.2.2.2.2 Incineration technology of the fluidized-bed incinerator The mechanism for the circulating fluidized-bed incinerator is as follows: silica sand or furnace slag in a certain granularity range are taken as heat carrier. They are blown up and rolled over by pressurized air fed through an air distributor at the bottom of the furnace. The waste fed into the furnace immediately mixes with the hot heat carrier, resulting in strong gas-solid mixing with uniform temperature. Thus the waste is fully dried and heated, and thus easily catches fire and burns completely. The hightemperature solid particles blown out of the furnace return to the furnace via the separator and feedback unit to achieve material balancing in the furnace and ensure uniform temperature inside the furnace. Large incombustibles (granularity of 10–15 mm) are discharged via a continuous slag discharge unit at the bottom of the bed and slag cooler. Fine particles that are incompletely burned enter the cyclone separator through the top of the combustion chamber. Most of these particles, after being collected by the separator, return to the combustion chamber via the feedback

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Waste feeding hopper

Boiler water-cooled wall

Grate in feeding section Furnace bed hydraulic drum Observation window

Grate in drying section Grate in combustion section

Ignition burner

Grate in burning out section

Movable grate Fixed grate

Bottom ash convey belt Slag cleaner

Fig. 6.12: Schematic diagram of a mechanical grate incinerator.

unit for further combustion. This process repeats itself several times until complete combustion. The strongest merits of this technology are that the waste can be burned completely and hazardous substances are completely destroyed. The incompletely burned particles discharged out of the furnace are normally ~ 1 %. It is also characterized by small amounts of incineration slag and high waste compatibility and is suitable for incineration of sludge containing much moisture. The incinerator has a fixed layer, roll-over layer, and circulating layer according to the air speed and mobility of waste particles. At the fixed layer, the air speed is slow and the waste particles are still, allowing gas to pass through waste particles. At the roll-over layer, the air speed exceeds the flow critical point, resulting in bubbling among particles leading to violent mixing of particles. At the circulating layer, the air speed exceeds the limit, causing fierce mixing and collision of gas and solid particles and the particles fly around. See Fig. 6.13 for the structure of the fluidized bed. The waste is crushed to less than 20 cm and cast into the furnace. The waste feed is mixed with high-temperature bed materials. These fuel particles are dried and heated together, the volatile components are separated and burned, swelled, primarily smashed, burned with coke and secondarily smashed until complete combustion. Incompletely burned combustible gas and light components are brought to the secondary combustion chamber by airflow for further

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Waste feed inlet

Bed material feedback pipe

Combustion chamber

Air distributing pipe Material feedback air

Vibrating sieve Incombustibles discharge Fig. 6.13: Schematic diagram of the structure of a fluidized-bed incinerator.

combustion. Combustion in the secondary combustion chamber normally accounts for 40 %; however, its volume is 4–5 times that of the fluidized layer and its temperature is 100–200 °C higher than that of the fluidized layer below [407]. The ash and solid bed materials are discharged from the furnace bottom and then separated before the bed materials return to the furnace for repeated use. Approximately 70 % of ash is brought out of the furnace in the form of flying ash and then goes into the fume purification and treatment system for separation and further proper disposal [408].

6.2.2.2.3 Incineration technology of the rotary kiln A rotary kiln is a drum, which is obliquely or horizontally placed and turns slowly. Its inner wall is made of refractory bricks or pipes to protect the drum. The rotary kiln used for waste treatment usually has diameter and length in the ranges 4–6 m and 10–20 m, respectively, according to the amount of waste to be incinerated. The ratio of the length/diameter should be 2/1–5/1. See Fig. 6.14 for its structure. When a rotary kiln is used to incinerate waste, the waste is fed through one end of the kiln and brought from the bottom to the upper part of the drum by a heat-resistant shoveling plate attached to the inner wall. The drum turns slowly, and as a result, it falls to the bottom again under the effect of gravity. The rollover of waste allows the waste to fully contact air for easy combustion. When the waste starts to burn, hot fumes and the furnace wall dry and heat the waste until the ignition point. As the

350 | 6 Technologies of municipal solid waste treatment Feeding hopper

Fume outlet Hydraulic discharger

Front seal hopper

Drying section Combustion section Combined ash sieve Igniter

Back seal hopper Fig. 6.14: Schematic diagram of a rotary kiln incinerator. 1. Feeding hopper, 2. hydraulic discharger, 3. fume outlet, 4. front seal hopper, 5. drying section, 6. combustion section, 7. combined ash sieve, 8. back seal hopper, 9. igniter

drum turns, the burning waste moves to the tail section of the drum, where the ash is discharged. The rotary kiln incinerator can incinerate different types of solid and liquid wastes, with the incineration temperature in the range 650–980 °C. The waste is dried, burnt, and burnt out in the kiln before being discharged, with a total stay of several hours. The waste is burnt to ash before reaching the other end of the kiln. The rotation speed is adjustable between 0.75 and 2.50 rpm. Mantis technology is suitable for waste containing higher moisture content and different types of components hard to burn [409]. Changing the rotation speed may change the length of stay of waste in the kiln. Rotation brings about violent mechanic collisions of waste in the hot air inside the kiln to facilitate complete combustion of the waste. The rotary kiln is suitable for various types of wastes, especially industrial wastes. For incineration of domestic wastes, the rotary kiln incinerator is normally installed behind the grate incinerator to improve complete combustion and conform to ash reuse requirements. The rotary kiln has a simple structure, and its maintenance is easy, because the rotating device is outside the kiln. Compared to other incinerators, it consumes less power and has a better incineration capability. As cooling water decreases the combustion temperature, the generation of NOx can be effectively decreased; however, the wastes with less calorific value (1,200 kJ/kg) and much moisture are hard to burn. The high kiln temperature may cause problematical sealing and deformation of the rotary parts. Despite of a number of merits, this technology has several flaws such as limited waste treatment amount, flying ash hard to dispose of, and hard to control the combustion process, making it difficult to be used for power generation. Because of all these reasons, this technology is less applied at present.

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6.2.2.3 Process flow of waste incineration Fig. 6.15 shows a schematic diagram of the process flow of urban domestic waste incineration plants. The primary and secondary combustion air from the combustion air system mixes with the wastes in the pretreatment system and burns the wastes in the waste incinerator. The heat from burning is recycled by the heat recovery boiler and the cooled fumes go to the fume treatment system for treatment before being discharged into the atmosphere via the stack. The flying ash from waste burning is specially treated. The wastewater from the systems is piped to the wastewater treatment system before being discharged to public water sources such as a river or being recycled. The whole incineration process of modern waste incineration plants can be controlled by an automatic control system.

Combustion air system Pre-treatment system

Combined ash disposal system

BFW system

Waste incineration system

Automatic control system

Waste heat utilization system

Fume treatment system

Wastewater treatment system

Fig. 6.15: Block diagram for general process flow of waste incineration plant.

Fig. 6.16 shows the technological process of a waste incineration power plant. Wastes are hoisted into the incinerator to transform the chemical energy of wastes to the heat energy of hot fumes. The heat energy is then absorbed by chemically-treated and thermally deoxidized water in the heat recovery boiler to produce steam, transforming the heat energy of fumes into heat energy of steam. The steam goes to a steam turbine where it swells and acts to drive the rotor of the turbine to rotate at a high speed, transforming the heat energy of steam into mechanical energy. The rotor of the turbine drives the stator of the generator to rotate, transforming the mechanical energy of the stator to power energy. This is a production process where waste, water, and steam cycle (Rankine Cycle) are used as the fuel, active media, and thermal cycle, respectively, producing electricity.

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13 14

5

25

15

2

9

17 22

24

6 16

10 11

1

18 7 4

8

21

19 20

23

12

3 1 - Wastes dumping areas 2 - Crane control room 3 - Leachate storage tank 4 - Waste storage pit 5 - Waste crane 6 - Waste feeder hopper 7 - Slag crane 8 - Slag storage pit 9 - Heat recovery boiler

10 - Combustion chamber 11 - Grate combustion chamber 12 - Slag convey belt 13 - Warm water pool 14 - Turbo-generator 15 - Economizer 16 - Flying ash belt 17 - Bag type dust collector 18 - Central control room

19 - Air preheater 20 - Transformer room 21 - Primary blower 22 - Exhaust heater 23 - Scrubber 24 - Induced draft fan 25 - Stack

Fig. 6.16: Schematic Diagram of systems in an urban waste incineration power plant.

6.2.3 Aerobic compost Organic waste compost is declining as a result of factors such as time consumption, cost, land occupation, odor, weather dependence, relatively poor quality, and undesirable market value. A large amount of nitrogen, phosphate, and potash fertilizers are imported every year. Therefore, we believe that organic waste compost will finally have a promising future with its merits such as soil enrichment, implementation and improvement of waste categorization system domestically, as well as the development of organic agriculture and circulating agriculture.

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6.2.3.1 Operating principle Aerobic composting is a highly efficient biochemical process where organic wastes are completely cured and degraded taking advantage of biological energy. During the aerobic fermentation process, pathogens and weed seeds are decomposed due to consistent high temperature with no anaerobic gases such as methane generated, making the organic wastes harmless.

6.2.3.1.1 Process Aerobic composting is basically divided into the following three stages: (1) Heat production (temperature rise) stage: In the early stage of compost (normally 1–2 d), mesophilic microorganisms in the compost heap proliferate rapidly taking advantage of soluble and degradable organic compounds to release heat, thus constantly increasing the temperature of compost. At this stage, the compost heap temperature increases from room temperature to 50 °C and mesophilic and aerobic microorganism including bacteria, actinomyces, and fungi – with non-spore bacteria and molds as the main types. Among these microorganisms, the bacteria mainly rely on water-soluble monosaccharides; actinomyces, and fungi are specific to decompose cellulose and hemicellulose material. (2) High-temperature stage: When the temperature of the compost heap rises over 50 °C, composting reaches the high-temperature stage. For the waste piled up for fermentation, usually 3 days are required for the compost heap temperature to rise to 55 °C and approximately 7 days to rise to the maximum limit (80 °C). At this time, mesophilic microorganisms are suppressed and gradually replaced by thermophilic microorganisms. The soluble organic compounds, which are left over from the previous stage, and newly formed organic compounds continue to decompose and transform, and simultaneously complex organic compounds such as hemicellulose, cellulose, and protein begin to actively decompose. At temperature 50–55 °C, thermophilic fungi and actinomyces are active. At 60 °C, fungi are almost inactive and only thermophilic actinomyces and bacteria are active. At temperature > 70 °C, most thermophilic organisms are in the dormant state or dead. At this time, the heat production decreases, and the heap temperature automatically decreases. When the heap temperature decreases to < 70 °C, the thermophilic microorganisms in the dormant state become active again to continue decomposing organic compounds hard to decompose. At this stage, the heat increases again, and the heap enters a naturally-regulated and long-lasting high-temperature period. High temperature plays an important role in rapid decomposition of compost, and at this stage humus begins to form inside the heap. The C/N decreases significantly, and the height of the compost heap deceases accordingly. Pathogens in organic wastes are effectively killed as a result of high temperature. According to the sanitary standard for high-temperature composting

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(GB7959) the maximum compost temperature is in the range 50–55 °C or > 55 °C and lasts for 5–7d. (3) Curing (temperature decreasing) stage: At the end of the high-temperature stage, only some organic compounds hard to decompose and newly formed humus are left, at which stage the activity of the microorganisms decreases, heat production decreases, and thus temperature decreases. At this time, mesophilic microorganisms dominate again to further decompose organic compounds and humus grows and stabilizes gradually, at which time the compost enters the curing stage. After the temperature decreases, less oxygen is needed and the gaps in the heap expand, resulting in spreading of oxygen. At this time, only natural ventilation is needed for heap curing.

6.2.3.1.2 Reaction Fig. 6.17 shows the aerobic composting process. This process includes the following steps.

Organic material (C, H, O, N, P, S) + microorganism + oxgyen

Growth of Humus substance + microorganisms (C, H, O, N, P, S) CO2, H2O, NH3, PO43–, SO42–

+

Energy

Emitted to the atmosphere Fig. 6.17: Aerobic composting process.

Oxidation of organic compounds (Ca Hb Nc Od · mH2 O): Ca Hb Nc Od ⋅ mH2 O + O2 → Cf Hg Nh Oi ⋅ nH2 O + H2 O + CO2 + NH3 + Energy

(6.1)

Synthesis of cellular substance (C5 H7 NO2 ): n(Cx Hy Nz ) + NH3 + (nx +

ny nz − − 5x)O2 4 2 → C5 H7 NO2 + (nx − 5)CO2 +

ny − 4 H2 O + Energy 2

(6.2)

Oxidation of cellular substance (C5 H7 NO2 ): C5 H7 NO2 + 5O2 → 5CO2 + 2H2 O + NH3 + Energy

(6.3)

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6.2.3.2 Raw materials and technological parameters 6.2.3.2.1 C/N ratio A C/N ratio of 25 or so is suitable for composting. Too high a ratio will result in slow reproduction of the microorganisms due to the lack of nitrogen source, slow decomposition rate of organic substances and long fermentation time, large loss in organic raw materials, low humification coefficient, and soil nitrogen deficiency of compost products. In contrast, too low a ratio will restrict microorganism growth due to the lack of energy source, slow fermentation temperature rise, and loss of organonitrogen and emission of odor as a result of excess ammonia nitrogen release in the form of ammonia gas. When evaluating the C/N ratio, degradable available carbon and available nitrogen in the compost material such as lignocellulose in timber should be considered. Although the lignocellulosic carbon content is rather high, it is hard for the microorganisms to utilize it after degradation by. Therefore, if such substances account for a large proportion, a rather high C/N ratio should be used.

6.2.3.2.2 Water content Compost with water content in the range 40–60 % is recommended for most raw materials. In general, a crushing test that is not very accurate can be used to test the moisture content of the compost. For example, the raw materials should be wet, with water infiltration instead of lots of drips.

6.2.3.2.3 Volume weight Vent ability as well as volume weight can be improved by humidity control. Where the moisture is identical, the smaller the volume weight is, the faster the temperature will rise during composting.

6.2.3.2.4 Grain size Generally recommended grain size is in the range 1.3–7.6 mm, with the lower limit recommended for ventilation or continuously turning the composting system and the upper limit for static stacking or other static ventilation composting system.

6.2.3.2.5 pH value The pH value does not need to be adjusted during composting. However, when it deviates from the normal compost pH value (5–9), it must be adjusted. When pH is > 4, limestone or alkaline phosphate fertilizer is generally utilized for adjustment; when pH is > 9, ferric chloride or alums can be added for adjustment. Sometimes, compost reverting is also an option.

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6.2.3.2.6 Ventilation volume For composting by batch feeding such as windrow composting or static composting, within the temperature range 45–65 °C, the MVV is 108–136 m3 /(h · t dry matter); at the beginning or in the end of composting, the minimum ventilation volume is in the range 5.68–14.2 m3 /(h · t dry) [9]. Normally, it is difficult for a single raw material to conform to the foresaid composting conditions. Therefore, pretreating or adding conditioners to the raw material is required. For example, wet raw material should be subject to air drying or drying treatment, whereas dry raw material needs to be watered. Swarf, straw, rice hull, boll hull, feces, and other structural or nutritional conditioners can be added to adjust the volume weight and C/N ratio. Even expanding agent such as peanut hull and branches can be added to adjust porosity. Meanwhile, ventilation volume should be adjusted during the fermentation process.

6.2.3.3 Composting process system Aerobic composting processes are generally classified into the following types.

6.2.3.3.1 Windrow composting Fermentation through open-air forced ventilation is typically employed for windrow composting. The mix from the previous treatment processes is stacked on the antiseepage controlled ground, looking like trapezoidal, cross-section, long-strip, windrows. The windrows should be regularly turned with wheeled or tracked turning (casting) equipment so that the materials can be fully exposed to the air, supplying more oxygen to facilitate the aerobic fermentation. Normally, the height, width, and length of windrows are in the range 1–3 m, 2–8 m, and 30–100 m, respectively. The oxygen required for window composting comes from the natural ventilation or forced ventilation because of the rise of hot air in the windrow. Besides, air exchange during turning is also available to a small extent. In the forced ventilation windrow system, oxygen will be forced or induced into the windrow with an air blower. Whatever ventilation method is adopted, the windrows should be opened regularly. Fig. 6.18 shows a windrow aerobic composting site under natural ventilation.

6.2.3.3.2 Static-bed composting For static-bed composting, raw mix should be stacked on the aeration layer, which is made of wood particles, broken straw, or other materials with good permeability. A breather line is provided in the aeration layer and connected to the fan to supply air to the windrow to complete the composting process. Compared to windrow composting, a special ventilation system and fan are available for turning the material in the composting process and forced ventilation, respectively.

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Fig. 6.18: Windrow aerobic composting site under natural ventilation.

6.2.3.3.3 Trough composting For trough composting, several troughs should be provided to the plant, with an air supply pipeline and a drainage pipeline at the trough bottom and track on the top of the trough wall, so that the turning equipment can regularly move. Sun shed fermentation tank as well as rare tunnel fermentation are utilized for trough (pool) aerobic fermentation. Sun shed fermentation tanks use the light permeability and insulation properties of the sun shed to raise the temperature within the tank (Fig. 6.19). A ventilation pipe system is provided at the bottom of the tank to supply the oxygen for aerobic fermentation via forced ventilation.

6.2.3.3.4 Reactor composting Reactor composting is classified into vertical solid flow and horizontal/tilt solid flow. The technology is still in the immature stage, thus limiting the treatment scale. Typical composting reactors include silo composting reactor, tower composting reactor, rotary drum composting reactor, agitation tank composting reactor, round stirred-bed composting reactor, and tunnel kiln composting reactor. Fig. 6.20 shows the tilting aerobic composting fermentation reactor.

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(a)

(b)

Fig. 6.19: Sun shed fermentation tank.

Fig. 6.20: Tilting aerobic composting fermentation reactor.

6.2.4 Anaerobic digestion 6.2.4.1 System composition Fig. 6.21 shows a typical waste anaerobic digestion treatment. Although each company or project may adopt different processes, the whole system composition is identical and is composed of waste receiving and mechanical sorting pretreatment system, anaerobic digestion system, biogas treatment and utilization system, biogas residue composting and biogas slurry treatment system, and site stench treatment system. As Chinese kitchen waste contain lots of fat, the anaerobic digestion treatment system differs to some extent. In contrast, fat anaerobic degradation takes more time than that required for saccharides, starch, and protein, and thus fat cannot be degraded within common anaerobic digestion retention time. In contrast, since the additional value of fat is higher than that of biogas, it can be utilized to produce biodiesel or used as the raw material for the oleo chemical industry. Therefore, the kitchen waste anaerobic digestion system also includes a fat removal subsystem and a fat processing subsystem (e.g., biodiesel production). Fig. 6.22 shows the kitchen waste anaerobic digestion system constructed in Xining by Qinghai Jieshen Environment Energy Industry Co., Ltd.

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Biogas treatment and utilization system

Feeding conveyor Anaerobic digestion system

Weighing Unloading hail

Trommel screen Digestion tank

Trommel screen

Biogas storage

Mixing

Outward transport

Feeding pump

Iron removal Waste receiving and pretreatment system

Reflux pump

Flare

Digestion Compressed biogas storage tank

Biogas compress Belt pressure filter Sewage backflow

Fan Biogas power generation and to grid

Composting workshop

Biofilter Odar disposal system

Digested residue decomposition and fertilizer producing system

Outward transport

Fig. 6.21: Schematic diagram for waste mechanical-biological treatment.

6.2.4.2 Anaerobic digestion of organic waste In an urban waste anaerobic digestion system, mechanical sorting is the key process, whereas the anaerobic digestion process is the core process. The sources and component characteristics of the waste determine the type of anaerobic digestion process. Since the principle, process classification, influencing factors, and engineering composition of anaerobic digestion are discussed in Chapter 6, this section focuses on the introduction of several anaerobic digestion processes for organic wastes. It is worth mentioning that most organic waste engineering technologies have been introduced from abroad. Thus, foreign processes will be discussed here. At present, wet anaerobic digestion and dry fermentation are the main anaerobic digestion processes for organic wastes. Dry fermentation refers to the fermentation when the dry matter in the reactor accounts for > 15 %, and wet anaerobic digestion refers to the fermentation when the dry matter accounts for < 15 % [410]. For the wet process, organic waste should be pretreated to largely remove impurities, whereas for the dry process, pretreatment is not required. Each process can be further classified into single- and two-stage types.

6.2.4.2.1 Single-stage wet anaerobic digestion The single-stage wet anaerobic digestion process is simple, and a Continuous Stirred Tank Reactor (CSTR) serves as the reactor. In this process, materials are fed and discharged at a certain rate, and the retention time is in the range 14–28 days ac-

Fig. 6.22: Schematic diagram for Xining waste mechanical-biological treatment.

Sedimentation basin

Wastewater treatment plant

Biogas slurry and residue treatment system

Pre-sorting unit

Aeration basin

JS-BC

Heavy substances setting and mixing pulping trough

Oil storage tank

B

B

Biogas

Warm water tank

Desulfuriza tion column Air storage tank

Cold water

Digestive fluids retention tank

Belt pressure filter

Conditioning vessel

Digestion tank

1

Heat exchanger

Biogas generation via anaerobi digestion and biogas utilization system

Denydrated blogas residue used as organic fertilizer or transported to landfill silver

Regulating basin

Fat separation unit

Oil-water separation

Soild-liquid separation

Hydrothema I reactor

Pretreatment system

Feeding and emptying bin

JFE multi-funtional crushing separator

Double-shaft crusher

Kitchen wastes

Biodiesel system

Biogas compressor

B

Emergency dischargning tower

Biogas power generation Biogas purification plant

PNG vehicle fule

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cording to the material types and digestion temperature. Typically, it includes the Finnish/Swedish Wassa process, and the German Linde and EcoTec processes. For the Wassa process: the Total Solid (TS) are 10–15 %; mesophilic digestion retention time is 20 days; and high-temperature digestion retention time is 10 days. Organic load for mechanical sorting, representation as Volatile Solid (VS), is 9.7 kgVS/(m3 · d), while that for source sorting is 6 kgVS/(m3 ·d), methane production rate 170–320 m3 /tVS, and the VS removal rate 40–75 % [411].

6.2.4.2.2 Two-stage wet anaerobic digestion For two-stage wet anaerobic digestion, acid and methane are generated through hydrolysis in two reactors. The Dutch Pacques process and German BTA and Biocomp processes are typical. Pacques belongs to the mesophilic processes. It can treat fruit and vegetable wastes and sort 10 % of the organic wastes from the sources of hydrolysis reactor through gas agitation. After the digested substances are dehydrated, the liquid is delivered to an Upflow Anaerobic Sludge Blanket (UASB) for methane generation, while some of the solid will be added into the hydrolysis reactor to serve as inoculum, and the remaining 10 % will be used for composting by the BTA process, requiring mesophilic anaerobic digestion. An attached biomembrane reactor is employed as the methane-producing reactor to ensure sufficient retention time for the microorganisms. To prevent such a biomembrane reactor from being blocked, only the liquid can flow into the methane-producing reactor. Meanwhile, to keep the pH value of the hydrolysis reactor between 6 and 7, the digested liquid in the methaneproducing reactor should be recycled to the hydrolysis reactor. The Biocomp process combines composting and anaerobic fermentation. In this process, the waste will pass through the trommel screen first. Inorganic substances and scrap iron are removed by hand and magnetism, respectively. Coarse wastes are sorted out for composting, with fine ones being sent to the anaerobic fermentation tank. The fine organic substances are crushed and then diluted to obtain a solid content of 10 % to reach Level 1 CSTR mesophilic anaerobic fermentation reactor where they will stay for 14 d. After that, they will go to a Level 2 high temperature (55 °C) upflow anaerobic fermentation reactor, with a Hydraulic Retention Time (HRT) of 14 d.

6.2.4.2.3 Single-stage continuous dry fermentation For this type of fermentation, typical processes include the Belgian Dranco process, Swiss Kompogas and Valorga processes, and German Linde process (Fig. 6.23). In the Dranco process, raw materials go into the reactor from the top, and then are discharged from the bottom. In general, agitation is not necessary for this process. The wastes move vertically in plug flow, and part of the digested substances is sent to fresh waste as inoculum. For this process, the fed solid concentration is 15–40 %, load 10 kg COD/(m3 · d), temperature 50–58 °C, digestion time 15–20 days, and biogas for each ton of wastes 100–200 m3 . For the Kompogas process, the waste moves

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Dranco process Mixture of feed and inoculum

Valorga process

Biogas

Inoculum Discharge of digested residue

Feed Discharge of digested residue

Biogas recirculation

Feed

Kompogas process Inoculum recycle

Feed

Discharge of digested residue

Linde dry fermentation Biogas Waste Feeder Dewatering Solid residue Plug flow reactor

Discharge

Liquid residue

Recycle of liquid residue Fig. 6.23: Schematic diagram for different single-stage continuous dry fermentation systems.

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horizontally in plug flow in the round reactor. The main raw materials include kitchen waste and yard waste, with the TS in the range 30–45 %, a grain size smaller than 40 mm, digestion temperature of 54 °C, and digestion time of 15–18 days. The dither reactor used is unique in design. It is a vertical cylinder, and a vertical plate inside it separates the reactor. Wastes go into the reactor from the feed opening at the bottom. Every 15 min, part of the generated biogas will be injected into the reactor from the bottom through the pipe network under a high pressure. This functions as gas agitation and requires TS in the range 25–35 % and retention time 14–28 d. The Linde dry fermentation process is particularly suitable for the treatment of mixed contaminated wastes. After the waste is sorted, it is delivered to the Mechanical Biological Treatment (MBT) and subjected to anaerobic digestion after mechanical treatment. The digested product will be subjected to aerobic composting after dehydration.

6.2.4.2.4 Two-stage continuous dry fermentation Typical two-stage continuous dry fermentation processes include the Biopercolat process of Germany’s Wehrle Werk AG and the German GICON process, which are similar to the Pacques process, except that the hydrolysis requires a high TS content and slight aeration. A microaerobic hydrolysis reactor together with an attached biomembrane methane-producing reactor can shorten the digestion time to 7 d. Two-stage systems have a higher OLR when compared to a single-stage wet system. The OLRs of the BTA process and the Biopercolat process are 10 kgVS/(m3 · d) and 15 kg VS/(m3 · d), respectively. This is because the attached biomembrane can prolong the microorganism retention time, enhance the tolerance of methanogens to high-concentration ammonia, and improve the biological stability.

6.2.4.2.5 Other dry fermentation processes Other dry fermentation processes include batch and semicontinuous dry fermentation, such as the German Bekon garage process (Fig. 6.24) and 3A process and the Dutch Biocel process. The Biocel process is employed to treat organic wastes sorted from the source. The content of waste in the reactor is 30–40 %, digestion temperature 35–40 °C, and solid retention time more than 40 d to the end of gas generation. For the 3A process, aerobic-anaerobic-aerobic fermentation is completed in the fermentation tank. Early aerobic fermentation sanitizes raw materials; mid-anaerobic fermentation produces biogas; later aerobic fermentation directly generates dry organic fertilizer.

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Biogas boiler Biogas

Biogas Gastight door

Percolation liquid

Heating system

Percolation liquid distribution Waste storage

Forklifts for feed and discharge

Percolation liquid storage tank

Concrete fermenter with intergrated heating system Fig. 6.24: Garage type dry fermentation process.

6.3 Cases of urban domestic waste treatment 6.3.1 Liulitun Waste Sanitary Landfill, Beijing 6.3.1.1 General introduction to the landfill 6.3.1.1.1 General In addition to Asuwei, Anding, and Beishenshu, Liulitun Waste Sanitary Landfill is the new landfill built in downtown Beijing, serving an area of 426 km2 (Haidian District) and a population of 1.5 million. It is located in Yongfengtun Township, northwest of Haidian District, Beijing, at a distance of 12 and 25 km to Haidian Town and Haidian Wuluju RTS, respectively. The landfill has a total area of 46.53 hm2 , in which the front and landfill area cover 12 and 34.53 hm2 area, respectively. The total project investment is CNY 170,632,000 [412].

6.3.1.1.2 Technological parameters (1) Design landfill capacity: 1,500 t/d; design service life: 18 years. (2) Ground area of the landfill area: 34.53 hm2 , of which 14.15 and 20.38 hm2 are for Phases I and II, respectively. (3) Gross capacity of the landfill area: 12,450,000 m3 , where 1.25 million m3 are for covering soil and 11,200,000 m3 for waste landfill volume. (4) Waste landfill site is designed to be composed of underground and above-ground parts, with a design landfill depth of 25 and 20 m for the underground and aboveground part, respectively.

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(5) The volume of the underground landfill site is 5,280,000 m3 , whereas that for the above-ground part is 7,170,000 m3 .

6.3.1.1.3 Technical points (1) Sanitary landfill garage treatment process is used for the design, and the distance from landfill to cell should be covered on the same day. (2) 2 mm-thick HDPE membrane and 700 m-thick clay layer are adopted as leachate anti-seepage layer. (3) Graded broken stones covered with geotechnical cloth are used as groundwater and leachate drainage system. (4) Anaerobic and aerobic activated sludge treatment processes are provided to the leachate. (5) Sewage-sludge gas discharge system is provided. (6) Groundwater monitoring well is provided in the landfill area. (7) Soil-nail sprayed anchor is designed for the side slope of the landfill area.

6.3.1.2 Engineering contents of the landfill 6.3.1.2.1 Bottom The design elevation of the bottom is in the range 17–21 m, and the minimum design gradient is 2 %. In the vertical design of Phase I landfill bottom, the line from the southwest corner to the northeast one is designed at a higher point with an elevation of 21 m, and the bottom slope from this line downwards to the lowest points has an elevation of 17 m in the northwest and southeast corners. In the vertical design of Phase II landfill bottom, the line from the northwest corner to the southeast is designed at a higher point with an elevation of 21 m, and the bottom slope from this line downwards to the lowest points is at an elevation of 17 m in the northeast and southwest corners. For access by landfill vehicles, the parkway is arranged in the north of the landfill area.

6.3.1.2.2 Side slope According to the pithead elevation and bottom elevation, the side slope should be controlled between 1:1.3 and 1:1.5. To ensure the stability of the side slope, anchor sprayed concrete with a thickness in the range 100–155 mm is applied to the side slope of the stable layer. Two landfill platforms (with elevations of 28 and 37 m) will be provided for the side slope.

6.3.1.2.3 Groundwater drainage system The system is composed of landfill area bottom drainage and side slope drainage. The landfill bottom is provided with a 300 mm-thick drainage layer and Drainage Channels A and B, which drain water to both sides from the 21 m watershed at the bottom.

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Drainage Channel A (main channel) is interlinked with Drainage Channel B, with a space of 70 m. Groundwater from the north of 21 m watershed will be delivered to Phase I groundwater pumping station in the northwest after it is collected through Drainage Channel A, whereas that from the south to groundwater pumping station in the southeast in Phase I and southwest in Phase II. As for side slope drainage, PVC grid covered with geotechnical cloth is adopted to collect groundwater to Drainage Channel A at the landfill bottom.

6.3.1.2.4 Impermeable layer Two millimeter-thick HDPE membrane is employed at the landfill bottom and side slope for the purpose of seepage prevention. Under the anti-seepage membrane is a 700 mm-thick clay layer and groundwater drainage system.

6.3.1.2.5 Leachate drainage system The impermeable layer of the bottom is provided with a 300 mm-thick drainage layer and Drainage Channels A, B and C, which drain water to both sides from a 22 m watershed at the bottom. Drainage Channel C is interlinked with Drainage Channel B, and B is interlinked with A (main channel). Leachate from the north of the 22 m watershed will be delivered to Phase I leachate pumping station in the northwest after getting collected through Drainage Channel A, whereas that from the south to the leachate pumping station in the southeast in Phase I and southwest in Phase II. Leachate will be delivered to the leachate treatment area for treatment after collecting at the leachate pumping station through Drainage Channels A, B, and C.

6.3.1.2.6 Waste gas exhaust system ABS pipe with a diameter of 200 mm is used as the waste gas exhaust pipe, covered with 1,000 mm diameter gabion made of 50–120 mm-grain size screen. One hundred drainage pipes are provided in total.

6.3.1.2.7 Guard net around pit For the 26 m depth of the pit, a 1.8 m-height guard net is provided at the eastern, southern, and western sides.

6.3.1.2.8 Parkway in the landfill Parkway is provided for Phases I and II landfills, with a design gradient of less than 5 %.

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6.3.1.2.9 Groundwater and leachate pumping stations Three groundwater pumping stations will be provided, with a design capacity of 1,250 m3 /d per station, and DN 400 reinforced concrete pipe and DN 125 cast-iron pipe are provided as the water inlet and effluent outlet, respectively. A submersible pump is used with a lift of 36 m. Three leachate pumping stations will be provided with a design capacity of 400 m3 /d per station, and DN 400 reinforced concrete pipe and DN 100 as the water inlet and effluent outlet, respectively. A submersible pump is used, with a lift of 48 m. Both groundwater and leachate pumping stations have cast-in-place reinforced concrete structure.

6.3.1.2.10 Landfill process The waste of Haidian District will be transported to the sanitary landfill site for landfill after compressing and packing in Wuluju RTS. Thus, the process includes weighing, landfilling, compacting, and covering on that same day. The waste landfill capacity is 1,500 t/d, and compacted waste will be up to 0.9 t/m3 . Taken the daily landfill elevation as 2 m, the daily waste heap volume is 1 667 m3 , and the waste spreading area is 840 m2 . The waste should be covered from a landfill height 20 m above the ground.

6.3.1.3 Leachate treatment 6.3.1.3.1 Water quality Design leachate treatment capacity is 2,000 m3 /d: 1,000 m3 /d for Phases I and II each. Design inflow water quality: BOD = 1,500 mg/L, COD = 2,000 mg/L, and SS = 600 mg/L. Effluent quality: in accordance with GB16889-97 Standard for Pollution Control on the Landfill Site for Domestic Waste, BOD ≤ 60 mg/L, COD ≤ 100 mg/L, and SS ≤ 50 mg/L. 6.3.1.3.2 Process flow Fig. 6.25 shows the process flow for leachate treatment. Raw leachate

Regulating basin

Anaerobic tank

Oxidation ditch

Secondary sedimentation basin Return and surplus sludge pump well Pumped by sludge storage pool pumper to landfill site

Fig. 6.25: Schematic diagram for leachate treatment process flow.

Pump well effluent

368 | 6 Technologies of municipal solid waste treatment

6.3.1.3.3 Phase I structures Regulating basin The quantity and quality of waste leachate vary after rainfall; the regulating basin will regulate the leachate quantity and quality and supply good water for the subsequent biotreatment of the oxidation ditch as well as stable operating conditions. Ther egulating basin is composed of two parts and will be constructed as projected per the scale of Phases I and II projects. The basin has a reinforced concrete structure, and the net plane dimension of a single basin is as follows: area, 30 m × 15 m; effective depth, 5 m; effective volume, 2,250 m3 ; total effective volume, 4,500 m3 ; and HRT 2.25 d. To prevent sedimentation, two 15 kW underwater blenders will be provided to each regulating basin. In addition, two submersible pumps will be provided to each basin, one for use and one as standby, so as to pump the water to the anaerobic pool distribution well. One electromagnetic flowmeter will be provided to the main effluent pipes of the basin effluent lift pump to measure the effluent of the regulating basin.

Anaerobic pool and oxidation ditch The anaerobic pool and oxidation ditch are constructed together in the reinforced concrete structure as per the scale of Phase I project. The anaerobic pool is divided into two parts, with a net plane dimension, an effective depth, and an HRT of 10.3 m × 2.4 m, 3.5 m, and 4 h for each pool, respectively. To prevent sedimentation, one 1.4 kW underwater blender will be provided to each pool. The oxidation ditch is composed of four corridors, and each has the following specifications: width, 5 m; straight length, 34 m; effective length, 4.5 m; HRT, 96.69 h; MLSS = 4 g/L; and sludge retention time 30 d. Six rotating disc type aerators will be provided in the oxidation ditch, with a rotating disc diameter of 1,400 mm, a single motor power of 18 kW, and rotating disc immersion depth of 500 mm.

Secondary sedimentation basin Two basins will be constructed for the secondary sedimentation as per the scale of Phase I project. A vertical sedimentation basin design (center inlet and peripheral outlet of water) will be adopted. The tank is 7 m in diameter and basin water is 3.1 m in depth. A vent pipe will be provided at the bottom of the secondary sedimentation basin to facilitate dredging and overhauling. A sludge pipe is provided in the middle part. Return sludge will be discharged to the anaerobic pool through the return sludge pump well, and the surplus sludge will be discharged to the sludge storage pool. The effluent from the secondary sedimentation basin will be drained to the water lift pump well.

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Effluent lift pump well The effluent from the secondary sedimentation basin will be collected and pumped to the site gutter. The net plane dimension of the pump well is 3 m × 3 m, maximum effective depth 4 m, and minimum effective depth 0.65 m. In Phase I project, two submersible pumps will be provided in the pump well, one for use and one as standby.

Return sludge pump well The sludge from the secondary sedimentation basin will be delivered to the return sludge pump well. Part of the sludge will be used as the return sludge and will be pumped to the anaerobic pool distribution well and then go to the anaerobic pool after being mixed with the regulating basin effluent. The rest will be used as surplus sludge and will be discharged to the sludge storage pool. In Phase I project, two submersible pumps will be provided in such a pump well, one for use and one as standby. The flow of each submersible pump is 40 m3 /h and a lift of 15 m.

Sludge storage pool Surplus sludge will be drained to the sludge storage pool from the return sludge pump well and then transported to the waste spraying operating area of the landfill site by pumping after staying in such a pool for 1 d. One sludge storage pool is projected in Phase I project, with a net plane dimension of 6 m × 6 m, effective depth of 3 m, and retention time of 1 d. To prevent sedimentation, one 3 kW underwater blender will be provided to the pool.

6.3.2 Likeng waste incineration power plant, Guangzhou 6.3.2.1 General Likeng Waste Incineration Power Plant, located in Longgui Town, Baiyun District, Guangzhou City and adjacent to Likeng Waste Landfill Site, is owned by the Bureau of City Appearance, Environment and Sanitation of Guangzhou Municipality and Guangzhou Construction Investment Development Co., Ltd. The plant covers an area of 101,788 m2 , with a total investment of ~ CNY 720 million [44]. The waste of Liwan District, Yuexiu District, and Fangcun District of Guangzhou will be utilized as the design fuel. The waste is characterized as follows: average QDW 6,179 kJ/kg, varying from 4,610 to 7,827 kJ/kg; average moisture content 53.10 %, varying from 47.27 % to 56.08 %; average stacking density 0.41 t/m3 , varying from 0.38 to 0.42 t/m3 .

370 | 6 Technologies of municipal solid waste treatment

6.3.2.2 Process flow and main equipment 6.3.2.2.1 Incinerator and waste heat boiler The Phase I project of the plant was put into operation in 2006. Two 520 t/d waste incinerators, two 47.46 t/h waste heat boilers, and one 22 MW steam turbine generator unit are provided. The Mitsubishi MHI/Martin waste incineration treatment technique is employed for the incinerator. A Mitsubishi Martin mechanical grate functions as the grate, and incinerator and waste heat boiler are provided in parallel. A horizontal natural circulation boiler with single drum will be adopted, with a rated waste treatment Tab. 6.4: Main parameters of the boiler. SN

Item

Unit

#1

#21)

1 2 3 4 5 6 7

Rated capacity Rated design steam pressure Rated design steam temperature Operating pressure of the steam drum Feed-water temperature Boiler flow rate Design thermal efficiency

T/h MPa °C MPa °C % %

45 6.5 450 6.8 125 25mm Trommel 90 % of organic substances. Residue separation equipment can be added to this machine if needed in real applications. Glass, stones, and other nonferrous materials can be artificially selected on the screen surface material flow for recycling.

Garbage sorting machine of fixed windscreen type The fixed windscreen type of garbage sorting machine is produced by Doppstadt Company, Germany (Fig. 6.33). The design of the windscreen was based on the air com-

6.4 Outlook for MSW

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(b)

Fig. 6.32: EuRec DSK disk sieve garbage sorting machine. (a) Arrangement of disk sieve surface (b) The working disk sieve sorting machine

Fig. 6.33: WS 720 E TAIFUN fixed windscreen garbage sorting machine.

pression process [415, 416], and because the separation of matter does not need a special pipe and can be discharged directly, the total operation is safe. Especially when treating wet material, it can eliminate the problems of blocking jams and overloading tubes. According to different materials, it adjusts the wind direction by the two pieces of machinery baffle. In order to reduce dust, the windscreen realizes air circulation. After unloading light material, the dusty air was sucked back into the machine, and thus it could avoid the need for dust removal equipment. The side door is large, allowing easy access and maintenance. Because the control part is relatively concentrated, the operation of the machine is safe.

Nonferrous metal sorting machine of magnetic column type This device was made by Shinko Electric, Japan (Fig. 6.34) with the following characteristics: simple and compact structure; ensures sufficient sorting by column mo-

384 | 6 Technologies of municipal solid waste treatment

Fig. 6.34: Column type sorting machine for nonferrous metals in garbage.

tor and vibration feeder; high load capacity and durability with high reliability. This model can effectively isolate aluminum, copper, and other nonferrous metals from mixed garbage.

6.4.3 Mechanical-biological treatment of MSW Mechanical-biological treatment means machine sorting MSW to yield fermentable organic matter, which is used for the preparation of biogas through anaerobic digestion, and solid residue, which is used to produce organic fertilizer through aerobic fermentation. Furthermore, various non-fermentable wastes separated out by mechanical sorting can be recycled, used or disposed of. For example, plastic, paper, glass, and metal can be recycled; wood, bamboo, fabric, plastic and other combustible materials can be incinerated for power generation; coal, ash, gravel, bricks, and other heavy mineral can be used for the production of building materials or landfill. Mechanicalbiological treatment is essentially an integrated treatment model as shown in Fig. 6.35. Anaerobic digestion only applies to fermentable organic waste; therefore, the sorting and pretreatment requirements for MSW are high. Because Germany, Austria, Belgium, and other European countries have a relatively perfect waste source separation system and mechanical sorting facilities, mechanical-anaerobic digestion technology was first applied in 1984 and is now widely used to handle food waste, fruit and vegetable garbage, yard waste, and organic waste by mechanical sorting. Municipal waste collection started late in China, and the Ministry of Construction designated Beijing, Shanghai, Guangzhou, Shenzhen, Hangzhou, Xiamen, Nanjing, and Guilin as the MSW collection pilot cities in June 2000. In recent years, based on foreign experience and China’s independent research on the actual situation, mechanical-anaerobic digestion technology has been preliminary applied; however, the technique is mainly used for processing food waste with relatively simple ingredients and only in individual projects for processing organic waste sorting. For example, the Beijing Dong village classified waste plant imported foreign dry anaerobic fermen-

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MSW Toxic waste

Mechanical sorting

Chipboard

Pre-treatment

Biological treatment Aluminum

Heavy material

Organics Water pool

Glass

Metal Recycled water Plastics

Kibbler Organic fertilizer

Biological Generator Electricity treatment

Residual waste Water treatment system

Fig. 6.35: Classic mode of MSW mechanical-biological treatment.

tation technology to process the organic waste (Φ15–60 mm) of middle sorting section at a rate of 300 t/d. Mechanical-biological treatment is an integrated MSW treatment technology for optimum use and with the maturing of China’s waste sorting collection system, it will be a trend in MSW disposal, not only for processing food waste, but also for processing kitchen waste, and organic waste separation at the source.

Xiaoying Kong*, Gaixiu Yang, Ying Li, Dongmei Sun, and Huan Deng

7 Microbial fuel cells

7.1 The basics of microbial fuel cells 7.1.1 The historical development of microbial fuel cells A microbial fuel cell (MFC) is a device that converts chemical energy contained in organic waste into electrical energy using cells or enzymes as catalysts of the system via bioelectrochemical processes. In the early 1880s, several studies were published about the relationship between biological and electric energy. Galvani found that electric current could cause frog leg peristalsis in a test of an electrostatic generator. The discovery not only laid the foundation for the field of neurophysiology, but also established a connection between biology and electricity. In 1839, Grove successfully found the reverse reaction of water electrolysis, namely that the combination of hydrogen and oxygen could produce water, and also produce a current. Although the hydrogen-oxygen fuel cell was studied during these early research conditions, both novel findings were not combined. In 1910, the botanist Professor Potter of Durham University established a microbial half-cell, which generated a current via Escherichia coli. This was the earliest microbial fuel cell. Conen assembled a series of microbial fuel cells to generate a voltage above 35 V in 1931. Then, the implementation of the USA space program promoted the development of microbial fuel cells. As a type of practical technology, people began to build a garbage disposal system to provide electric energy for the space program, and continued the research in both theory and application until the late 1950s and early 1960s.

*Corresponding Author: Xiaoying Kong: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China; E-mail: [email protected], Tel. +86-20-37029696 Gaixiu Yang: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China Ying Li: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences; CAS Key Laboratory of Renewable Energy; Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China Dongmei Sun: School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China Huan Deng: School of Environment, Nanjing Normal University, Nanjing 210023, China https://doi.org/10.1515/9783110476217-007

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Due to the restriction of living cells, scientists started to study cell-free enzyme systems in the late 1960s, and the initial goal was to permanently provide the energy for the implantation of artificial hearts. Wingard discussed the operating conditions and introduced recommendations for improving the performance of the microbial fuel cell in 1982. Then, Aston carried out some research on sensor circuits between electrode and enzyme in 1984. In 1985, Van Dick studied the enzymatic biofuel cell mainly considering the current generated and the characteristics of biosensors. In 1997, Willner studied the biosensor electrode ligase, discussed the way to use monolayer or multilayer enzyme, as well as reconstruction enzymes and coenzymes as electrodes. In 1999, Cosnier reviewed the immobilization technology of biomolecules used as biosensor on the surface of electrodes, and introduced a method to affix the biomolecules on the surfaces of biosensor electrodes via trapping, while also discussing adsorption. In 2000, Armstrong explained the dynamics of electrons, proteins, coenzymes, self-assembled monolayers (SAMs), and membrane surface activators. Kano and Ikeda revisited the basic principles and practice of biocatalysis, respectively. The electrochemical reaction under the action of oxidoreductase was studied. They also discussed the application of biosensors and biological fuel cells. In 2001, several theories and methods were developed in the field of protease fixed electrodes in the study of mesothermal biological fuel cells. In 2003, biological fuel cells were divided into microbial fuel cells and enzymatic fuel cells. At the same time, biological fuel via methods of protein engineering was also reported. In 2004, miniature implantable enzymatic biofuel cells appeared, which are a microsystem that uses glucose and oxygen as substrates to provide electrical energy for implanted devices, via double carbon fibers as cathode and anode. The application of biological fuel cells and biosensors has a bright future. The same year, a team from Pennsylvania State University obtained a great achievement on the study of sewage biological fuel cells. Mitsos compared the characteristics of biological fuel cells and nonbiological fuel cells. Polmore wrote a comment on the research progress and potential application prospects of microbial fuel cells. After 2004, the microbial fuel cell has been becoming the new research focus and has made significant progress. In 2006, the Dutch Reiner and the American Bruce introduced research progress on the microbial fuel cell in Harbin, China; Chinese researchers also introduced the research developments on the coupling-type microbial fuel cell. The same year, the student Nelson from the University of Southern California in America found Geological bacteria. Currently, to further study the theory and technology of microbial fuel cells, many countries have initiated long-term research plans, ensuring the energy security of the future. MFC technology, including biosystem, electrode material, membrane material, and system angle, has made great progress. For example, using microbial fuel cell technology to receive high-value compounds, to wipe out organic and heavy metal pollutants, and to recover soil and water pollutants. Microbial fuel cells could utilize short-chain alcohols and the alkane as fuel material to drive the fuel cell. The microbial fuel cell uses enzymes to catalyze various reactions; therefore, it enlarges the

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material source of the microbial fuel cell, e.g., some systems could use soluble starch as fuel. Using an enzyme or a whole living organism has obvious cost advantages as a biological catalyst, compared to precious metal catalysts. Therefore, large-scale production of biocatalysts will be a research direction of the future.

7.1.2 The technological development of microbial fuel cell The technological development of the microbial fuel cell is shown in Fig. 7.1 and this development passed through several research stages. The first stage: introducing the MFC concept. The second stage: since 2002, MFC research has developed in several directions as follows, and MFC technology has made constant breakthroughs. The first research direction: the selection of electricigen in the MFC, including: (1) the direct research result, e.g., the Geobacter sulfurreducens microbe can be absorbed on the electrode and stay active for a long time, thus oxidizing the organic matter and producing electrons that move from one electrode to another. A phenomenon that could effectively enhance the efficiency of the MFC; (2) the choice of MFC microbes and electron transfer mechanism provided important achievements in research on the microbial community of self-adjusting electron transfer, adsorption, and suspend microbes, i.e., to judge whether the bacteria community has the ability to output high energy and whether it is suitable for power generation; (3) for a summary of the electricity-producing bacteria community and the application of the MFC. The second research direction: improvement of the proton exchange membrane (PEM) design, mainly divided into two thoughts: (1) the development of cheap PEM or the response system without film; (2) the improvement of the efficiency of the proton transfer system. The third research direction: the material improvement of both cathode and anode and the exploitation of the electrode space. The fourth research direction: cross applications of MFC, mainly including wastewater treatment and the application of industrial biotechnology by-products, the electricigen is the most critical restraining factor of the MFC. Throughout the developmental history of the MFC, the developmental road has not been straight. During the early stage, fuel cells used the fermentation products of the microbe as fuel source, such as methane gas extracted from livestock manure. Since the late 1960s, scientists combined the processes of microbial fermentation and power generation into the MFC. In the 1980s, the widespread application of the redox mediator made MFC more feasible as a small power source, promoting its research and development. After 2002, scientists found that bacteria could directly transfer electrons to the solid electron acceptor, and scientists invented an MFC that works without redox mediator. The utilized bacteria directly transferred the electron to the electrode. Micro-

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2002 The electrode reduction microorganisms The bacteria absorbed on the electrode 2003 No media MFC

2004

The electron transfer microbial community

Single room air cathode MFC

Aceta/butyrate MFC

Single room MFC wastewater treatment

The improvement for the cathode nature of the MFC

2005 Continuous tubular single room MFC

2006 Porous anode, reduction electrode

The application of FePc and CoTMPP

The improvement for the cathode structure

The application of the electricity production colony 2007

The application of Graphite fiber anode

The design of the biofilm anode

2008

The application of the tubular cathode The upgrade of the MFC with no film

Fig. 7.1: The technological development of the microbial fuel cell.

bial fuel cells have the advantage of providing a stable power source for a long time; therefore, in some special regions like the bottom of the deep sea it has application potential. However, it has many disadvantages, such as low fuel efficiency, low electron transfer rate, and large amount of side reactions. All of these are introducing constraints on the applications of MFC. Therefore, finding microorganism catalysts with high efficiency will gradually develop into a hot spot in the near future.

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7.1.3 The conductive mechanism of the cell 7.1.3.1 The electrolytic cell The electrolytic tank is the equipment where the electrical energy can be converted into chemical energy. The theory of the electrolytic tank is shown in the Fig. 7.2. Chemical reactions with electron gain and loss in the electrode are called electrode reaction and the general effect of the two electrode reactions is expressed as cell reaction. The conduction process of the electrolyte solution includes the electrode reaction and the directional migration of ions in the electrolyte solution. According to the rules of electrochemistry: the electrode where the oxidizing reaction takes place is called anode, the electrode where the reduction reaction takes place is called cathode. According to the high and low potential, the electrode was divided into a positive pole and a negative pole, with the high potential at the positive pole, and the low potential at the negative electrode. In the electrolytic tank, the positive pole lacks electrons, it will utilize the oxidizing reaction to provide the electron, and is therefore called anode. The negative pole has an electron excess, will utilize the reduction reaction to gain an electron and is therefore called cathode.

7.1.3.2 Original battery The device using a bipolar electrode to produce current is called a galvanic cell or the original battery, which exchanges chemical energy into electrical energy. The original battery is composed of two electrode reactions (Fig. 7.3) [417]. The oxidation reaction loses electrons at the anode, while the reduction reaction receives electrons at the cathode. The electrons flow from the anode to the cathode through an external circuit, which equals the current flowing from anode to cathode through this external circuit. Since the cathode provides electrons to the reduction reaction, it subsequently lacks these electrons and thus maintains a high potential, while the anode receives electrons from the oxidation reaction and retains a low potential. Thus, the cathode is a positive electrode and the anode is a negative electrode.

7.1.4 The working principle of the MFC The partial principle of microbial fuel cell power generation is similar to that of the original battery; the main difference is that the catalyst of the anode chamber consists of microorganisms or enzymes. Under the catalytic action of the microorganisms, the MFC is a device that transforms chemical energy into electrical energy. The body of an MFC generally contains two tanks: an anode chamber and a cathode chamber, which are separated by a proton exchange membrane. The anode chamber is an anaerobic tank and the cathode chamber is an aerobic tank. After the microbes have decomposed the matrix in the anode chamber of the MFC, they release protons and electrons. The

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e

Cathode

I

Anode

Fig. 7.2: The working principle of the electrolytic tank.

e

Anode

I

Cathode

Fig. 7.3: The working principle of the original battery.

electrons reach the cathode through the external circuit, while the protons start from the anode, move through the proton exchange membrane, and eventually reach the cathode. The electrons, protons, and oxygen are combined to produce water at the surface of the cathode. In addition to it being slightly different from the generation and transmission of electron paths, the conventional principle of the MFC and the battery capacity are basically identical. Fig. 7.4 provides a flow chart for glucose as fuel to show the principles of MFC biochemical transformation. Firstly, wastewater that contains glucose is injected into the anode chamber; then, in the catalytic action of anaerobic microorganisms, which decompose glucose to carbon dioxide while producing hydrogen protons (H+ ) and

7.1 The basics of microbial fuel cells

e–

CO2

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393

e–

Resistance

H2O

e–

Anode

Cathode

e–

H+ Subtrate

H+ H+

H+

O2

Proton exchange membrane Fig. 7.4: The working principle of the MFC.

electrons (e− ), is shown in the equation (7.1). The electrons generated via this reaction adhere to the microorganism membrane, and pass through to the anode, then pass to the cathode through the conductor. However, H+ cannot reach the cathode through the cathode electrode; it can only pass through the proton exchange membrane to the surface of the cathode. Under the action of a platinum (Pt) catalyst, which generates hydrogen protons and electrons from the external circuit, or an electron acceptor (such as oxygen), the reaction shown in eq. (7.2) occurs. This mechanism and the processes can be simply summarized as complete glucose conversion to carbon dioxide and water, the process forms a closed loop, releasing chemical energy, and producing a current and an output voltage. The overall redox reaction is shown in equation (7.3). Anodic Reaction: C6 H12 O6 + 6H2 O = 6CO2 + 24H+ + 24e−

(7.1)

Cathodic Reaction: 24H+ + 24e− + 6O2 = 12H2 O

(7.2)

Overall redox reaction:

C6 H12 O6 + 6O2 = 6CO2 + 6H2 O + Energie (−2840 KJ/mol) (7.3)

7.1.5 The mechanism of electron transfer of microbially produced electricity The electrode reaction of the original battery is mainly a redox reaction, which has been studied well in electrochemistry. While the anode chamber microbes of the microbial fuel cell decompose organic matter and convert the organic chemical energy into electrical energy, the main research on MFCs is the conductivity mechanism of microbial electricity production.

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With the gradually deepening research on MFCs, scientists have discovered a series of strains with special metabolic pathways that can produce electrons through their own metabolism. The electrons transfer to the outside of cells either directly or indirectly, which plays a crucial role in fuel cells [418]. The electrons that are transferred to the electrode need a physical delivery system, in order to complete the electronic transfer to the outside of cells. This system can be the soluble electron shuttle body or the membrane-bound electron shuttle complex. So far, three methods of bacterial metabolism have been proposed that transfer the electron to the surface of the electrode: by passing the redox mediator, the cell, and the nanowire.

7.1.5.1 Redox mediator transfer Electrons produced by microbial metabolism need the help of the redox mediator to transfer to the electrode surface, such as Escherichia coli, Proteous vulgarish, Bacillus subtilis, Klebsiella and others. This cell is usually called the indirect MFC. For this system, the bacteria can create reduced metabolic intermediates, under an intermediate condition such as Hg and HgS. Rosenbaum et al. used E. coli K12 to produce hydrogen, and reoxidizing the catalytic electrode protected by polyaniline and immersed in a bioreactor. The current density of this experiment was as high as 1.5 mA/cm2 . Similarly, Straub and Schink [419] used Sulfurospirillum deleyianum to reduce sulfur to sulfide, or intermediates, depending on the oxidizing property of oxidizers. Most of the intermediates available for microbes are toxic and easy to decompose; therefore, seriously hampering the commercialization process of this type of MFC.

7.1.5.2 Direct electron transfer Electrons produced during metabolism can be directly transferred to the surface of the electrode through the cell membrane, which is called direct MFC. This approach is embodied in many bacteria species, such as Shewanella putrefaciens and Pseudomonas aeruginosa. Recent studies have shown that the metabolic intermediate of these microbes usually affects the performance of the MFC, and even obstructs the transfer of extracellular electrons. Silencing the related genes of these metabolic intermediates in Pseudomonas aeruginosa can reduce the current to 1/20 of the original. Oxidized metabolic intermediates produced by one bacterium can also be used by other types of bacteria in the process of electron transfer. Kim [420] used a variety of metabolic inhibitors to establish an electron transport chain of the electrochemically active bacterial population in an MFC and found that the electron transport in an MFC shares an early part of the electron transport chain with aerobic bacteria. Biological oxidation occurring in microbial cells is usually divided into substrate dehydrogenation, hydrogen transfer, and receiving hydrogen (or electrons). The [H] (or electron) produced by the substrate under the action of the dehydrogenase requires transport to the final electron acceptor through a series of electron carriers in a specific

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order (electron transport chain), to get the energy to simultaneously maintain growth. While electrons can be transmitted to the acceptor on the outside of cells during the transfer process of the electricigens, this special electron transfer mode produces microorganisms with electrochemical activity.

7.1.5.3 Nanowire transfer Recently, a structure called nanowire was found in the process of the transfer of extracellular electrons. It is a conductive tissue similar to cilia, and exists systematically in the Geobacter sulfurreducens PCA, Shewanella oneidensis MR-1, Cyanobacterium synechocystis PCC6803, and Pelotomaculum thermopropionicum. It is a fine wire produced by Geobacter, which can be used to transfer electrons [421, 422]. Currently, ironreducing bacteria have been used to research the transfer of microbial extracellular electrons. The complete genome sequences of S. oneidensis MR-1 and G. sulfurreducens have been sequenced and revealed 37 and 100 encoding Cyt C genes, respectively. It has benn found that the two types of outer membrane proteins Cyt C of G. sulfurreducens, namely OmcS and OmcE, participate in the transfer process of electrons to the electrode. The transcription level of OmcS increased 19 fold, and the level increases when the current increases; however, the OmcE can suppress current generation when the electrode acts as electron acceptor of G. sulfurreducens; cytochrome proteins MtrC and OmcA of S. oneidensis MR-1 participate in the nanowire pathway, while nanowires with gene defect strains are not conductive.

7.2 Microbial fuel cell technology The microbial fuel cell is a new power producing technology, which includes microbiology, electrochemistry, and materials science. Although, MFC is still at the basic research stage, a comprehensive understanding of the basic composition and the production performance of MFC is of great significance for breaking through the MFC technology bottleneck. This section mainly introduces the electrogenesis microorganism, substrate for MFC, battery materials, and configuration, which are important factors to determine the development level of the MFC.

7.2.1 Electrogenesis microorganism The electrogenesis microorganism is a type of bacteria with special metabolism. They can produce electrons through their own metabolism, and transfer electrons to the outside of the cell, either directly or indirectly. At the early stage of the study of MFC, scientists had to add mediators (neutral red, violet, and AQDS) to transfer electrons to the electrode; however, the costs of these agents are high, and in addition, some of

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them are toxic. In 1999, the direct electron transferring electrogenesis microorganism, Shewanella IR was isolated, constituting a significant turning point in research, and consequently, scientists began to research direct MFC. Electrogenesis microorganism research mainly focused on the isolation of electrogenesis single bacteria and production of electric microbial community analysis. Previous research has shown that bacteria, fungi, and algae all have direct electricity production functionality.

7.2.1.1 Electrogenesis bacteria The MFC anode chamber is in anaerobic environment; therefore, most electrogenesis microorganisms are anaerobic or facultative anaerobic bacteria. It is important to note that not all of the MFC microbes existing in the anode chamber directly use the substrate for the electric products. Although some microorganisms are not involved in the electron transfer processes on the surface of the electrode, they can provide richer substrates for electrogenesis of bacteria via complex substrate metabolisms. Currently, for electrogenesis bacteria, researchers focus on electricity production mechanisms and production performance of a single bacterium, due to a single species being easier to control, and a single bacterial species is often used in the MFC for material, configuration, and other basic research. The following will introduce several types of typical electrogenesis bacteria.

7.2.1.1.1 Shewanella Shewanella are widely studied. Experimental species mainly include S. putrefactions IR-1, S. oneidensis DSP10, and S. oneidensis MR-1. Kim et al. [420], isolated S. putrefactions IR-1 from paddy soil, which was the first reported directly electron transferring bacteria, transferring electrons to the electrode surface. S. oneidensis DSP10 is the first discovered species under aerobic conditions to produce electricity. Ringeisen et al. [423] and Biffinger et al. [424] successively studied its electricity producing performance under aerobic conditions with the micro-fuel cell (mini-MFC), and found that lactic acid can be oxidized to CO2 , and electricity was produced under aerobic conditions, resulting in a power output density of 500 W/m3 . S. oneidensis DSP10 can also use glucose, fructose, or ascorbic acid (vitamin C) as electron donor to produce electricity. When using fructose as electron donor for electricity production, the highest power density was 350 W/m3 [425]. The characteristics of electricity producing aerobic bacteria shows that these types of microorganism have a good application prospect in the microbial fuel cells of the future, as they can use a wide range of substrate. S. oneidensis can reduce MR-1 Fe3+ and Mn5+ , which is used to investigate bacterial electron transfer mechanisms as model strains. The whole genome sequence of S. oneidensis has been revealed, the mutant strain S. oneidensis MR-1 is used to study the key extracellular enzymes involved in the electronic transfer. The previously named

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researchers showed that S. oneidensis MR-1 have 37 genes that encode Cyt c, which was considered the membrane channel where electrons passed.

7.2.1.1.2 Rhodopseudomonas palustris R. palustris DX-1 is a photosynthetic electrogenesis bacterium, and Xing et al. [426] found that this strain has a high power production capacity and can utilize a wide range of substrates, producing MFC maximum power output densities of up to 2,720 mW/m2 with catalysis. This is higher than the same catalytic plant flora of MFC. Moreover, the bacteria can also use acetic acid, lactic acid, ethanol, pentanoic acid, yeast extract, fumaric acid, glycerin, formic acid, butyric acid, and propionic acid, to produce electricity. They employ the highest power density of acetic acid to produce electricity, as high as 450 mW/m2 [427]. Based on the R. palustris variety of metabolic pathways, the source of a wide range of substrates, relatively high production capacity, and many other advantages may be widely applied for the research of microbial fuel cells.

7.2.1.1.3 Rhodofoferax ferrireducens This bacterium was among the first that was reported to directly use microbial oxidation of glucose to produce electricity, most of the other iron-reducing bacteria electron donors are limited to simple organic acids. With glucose as electron donor, R. ferrireducens has an electronic recovery rate of 81 %. Bacterial microbial fuel cells with this bacterium use electric products quickly, after discharge of complementary substrates can restore the original level of electricity production, charge and discharge can be repeated, resulting in stable battery performance.

7.2.1.1.4 Escherichia coli Strains of Escherichia coli) are common and easy to cultivate. Typically, mediators to E. coli in MFC of medium redox activity are needed, such as neutral red or Mn4+ and a maximum power output of 100 mW/m2 . Zhang et al. [428, 429] reported E. coli with our fabrication: oliK12 does not need a fixator for direct electricity production. In the single-chamber air cathode MFC, E. coli with our fabrication: oli has the advantage of direct glucose production, resulting in a maximum power density of 600 mW/m2 .

7.2.1.1.5 Pseudomonas aeruginosa P. aeruginosa is the earliest reported microorganism producing an electronic shuttle fixator. Rabaey et al. [430] found an isolate of p. aeruginosa metabolism in MFCs that can produce pyocyanin. This strain of electron transfer media, where electrons pass on to the electrode, can be used to enrich the understanding of electron transfer mechanisms in MFC; however, adding pyocyanin and other artificial electron transferring

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agents can act as toxic mediators. Although single bacteria and mixed bacteria have a good result for electricity production, they are not ideal catalysts for MFC.

7.2.1.1.6 Thermincola sp. strain JR Strain JR are thermophilic strains to produce electricity, found by US researchers and cultivated from thermophilic temperature H-separated anodic microbial communities of MFC. MFC was running at high temperatures of 55 °C and with maximum power densities of 37 mW/m2 . In a pure bacteria electricity producing experiment, JR with sodium acetate as carbon source produced a current of 0.42 mA, having a Coulomb efficiency of 91 %. The innovation of this study is to tame the electrogenesis thermophilic microbial communities and isolate a strain of direct electricity production. High temperatures can reduce the amount of dissolved oxygen in the MFC, which is conducive to maintaining anaerobic conditions. In addition, high temperature can also inhibit the growth of pathogens.

7.2.1.1.7 Desulfovibrio desulfuricans D. desulfuricans is a combination of desulfuration with electricity producing strains. Electricity is generated based on the reduction of sulfates’ in situ oxidation of sulfides on the electrode. D. desulfuricans uses lactic acid in the single chamber air cathode of the MFC, reducing the sulfate in situ in an anode oxidation. It transforms sulfate into elemental sulfur, while at the same time turning elemental sulfur and sulfides into more sulfides. A sulfur removal rate of 99 %, and electricity production of a high power density of 0.51 mW/cm2 generates a maximum current density of 2.2 mA/cm2 . Compared to the traditional biological sulfur removal method, using sulfate-reducing bacteria to produce electricity in an MFC, sulfur removal has a great potential for development, due to the electrode occurrence of in situ oxide, reducing sulfides to sulfur in the process of biological treatment, inhibiting the reduction reaction.

7.2.1.2 Fungi Abnormal hansunni yeast has recently been reported for the first time to produce electricity. It does not need to be the middle medium body, but rather can be directly uploaded electronically on electrodes. In the two-chamber MFC experiments with K3 [Fe(CN)6 ] as the cathode catalyst and graphite as the cathode material, Hanomala has examined graphite, carbon felt, and platinum-modified graphite as anode materials. Three types of electrodes were tested with high power density of 0.69 W/m3 , 2.34 W/m3 , and 2.9 W/m3 and the platinum-modified graphite electrode power density revealed the largest and longest maximum open-circuit voltage. Analysis of intermediate metabolites showed that the yeast can also use glucose to produce electricity, utilizing pyruvate for the electricity production mechanism and the Krebs cycle. In

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the process of producing energy and electricity via iron cyanide reductase and lactate dehydrogenase, electricity flows directly to the electrode. Compared to bacteria with the ability to produce electricity, that of yeast is relatively low and further studies are required to improve the production capacity of the battery.

7.2.1.3 Chlamydomonas reinhardtii Rosenbaum used C. reinhardtii to build a solar cell (living solar cell) with a structure similar to that of the MFC within a sealed transparent glass bottle as reaction pool, with built-in polymer coated catalytic electrodes, graphite rod auxiliary electrodes, and Ag/AgCl reference electrode, separated by proton exchange membrane graphite electrode and containing algal reaction liquid. Using 0.2 V as the working electrode potential, and running for 30 h the cell began to generate an electric current that gradually increased to 9 mA, coupling the processes of electricity production and hydrogen production phase. The C. reinhardtii battery produced hydrogen gas and the outer potential utilized in situ oxidation under the action of hydrogen protons, and produced the electronic transfer directly to the electrode. Rosenbaum found that such a hydrogen electric power plant provides a new way of thinking, in addition to its use in MFC, but that further research is required.

7.2.2 MFC substrate The substrate of microbial fuel cells consists of raw materials to produce electricity via anodic microbial reactions. At the same time, providing nutrients for the growth of microorganisms is one of the important factors influencing the generation of MFC electricity. Previous research has shown that small molecule organic acids, alcohol, sugar, protein, and organic wastewater can be used as anodic MFC bottom electric products. Electricity power output and Coulomb efficiency (external transfer of electricity accounting for the complete process of substrate degradation transfer power percentage) is an important index for substrates in MFC production performance evaluation [431]. Different substrates in the MFC affect their performance and power output via MFC configurations, cathode catalyst, external resistance, and other factors such as substrate concentration and the requirement for different materials than the substrate of the MFC. Depending on the complexity of their composition, many substrates are widely used to produce electricity.

7.2.2.1 Small organic molecules Acetic acid, butyric acid, and other organic acids are metabolic products of the anaerobic fermentation process, and are common materials in the anaerobic environment. Due to acetic acid being easier to use by microorganisms, it is often used in basic

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research on the MFC, but other acids are also used in the electrochemical activity of microorganism concentration and anode electricity biological membrane. Acetate for the oxidation of anaerobic microorganisms to methane-producing bacteria (methanogens) and the sulfur bacteria thiobacillus in MFC are commonly used for bacteria acetic acid oxidation to produce electricity for bacillus genus Geobacter). Typically, for the complete oxidation to CO2 , acetate metabolism requires eight electrons to be passed on to the electrode. The reaction is as follows: CH3 COO− + 4H2 O → 2HCO−3 + 9H+ + 8e−

(7.4)

Acetic acid showed a very high efficiency of electronic contribution, and a Coulomb efficiency up to 65–65 %. Several studies have shown that compared to other small molecule organic acids, acetate in MFC can generate more power output and current density. An MFC of medium voltage in a single chamber of 28 mL filled with acetic acid 6,800 mg/L, generates a power density of 506 mw/m2 and a 1,000 mg/L acetate power density for 305 mW/m2 [432].

7.2.2.2 Alcohols Ethanol is a more common substrate and also an important product of anaerobic fermentation. At present, ethanol has been confirmed as the MFC baseline for electrical production, in two chambers with a MFC power density of (488 ± 12) mW/m2 and a Coulomb efficiency of approximately 10 %. In addition, xylitol, dulcitol, arabian sugar alcohols, adonitol, mannitol, and sorbitol can also produce electricity. The electricity production of pentose and hexose alcohols were compared in air cathode MFC. High power densities of approximately 1490 to 2650 mW/m2 can be reached by dulcitol with the highest power density, and mannitol with lowest power output, with Coulomb efficiencies between 13 % and 25 %, including dulcitol and arabinose sugar alcohol.

7.2.2.3 Sugar Sugar has a wide distribution in nature. Monosaccharides, disaccharides, and polysaccharides can be used as the baseline of MFC electricity production. Glucose and galactose, fructose, trehalose, rat lee sugar, mannose, xylose and arabinose, RNA, and other monosaccharides in air cathode MFC electricity production have been compared, which found that these simple sugars can produce electricity, with maximum powers between 1,240 and 2,770 mW/m2 and Coulomb efficiencies between 21 % and 34 %. For mannose, the smallest, yet most powerful glucuronic acid, the trehalose Coulomb efficiency is highest. Continuous addition of glucose to a two-chamber MFC, can obtain a maximum power density of 1,540 mW/m2 and a Coulomb efficiency of 60 %. In addition to simple sugars, sugar and cellulose were confirmed to have the ability to produce electricity with the highest power densities of 170 mW/m2 and 143 mW/m2 , respectively. Natural solid cellulose as raw material, such as straw after

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bacteria or steam explosion pretreatment, can also be used as a substrate for an MFC; the maximum power is 406 mW/m2 .

7.2.2.4 Organic wastewater Industrial and residential sewage is rich in organic matter, such as starches, sugars, fats, and proteins and the metabolites of the organic matter can be used as fuel for an MFC. At present, a study found that residential wastewater, starch wastewater, pig wastewater, beer wastewater, and papermaking wastewater can produce electricity. Electricity could be produced from organic wastewater in an MFC The power output and Coulomb efficiency of organic wastewater is usually less than when pure substances are used as the substrate, which is due to the complicated composition of organic wastewater. Here, some difficult biochemical degradation of the material may take place, or the electrogenetic microorganism has difficulty directly utilizing these materials to produce electricity, which calls for the joint effect of fermentation bacteria. Therefore, when using organic wastewater as substrate for an MFC [433], the power output via fermentation bacteria and electricity bacteria with a high metabolic rate have a mutual influence. Wastewater treatment or industrial organic wastewater discharge must be conducted to reduce the wastewater treatment costs. Current wastewater treatment of the intermediate links usually adopts the system of anaerobic fermentation with biogas or biological gas. However, the application of MFC technology for saving energy and reducing consumption in industrial wastewater treatment also requires a certain amount of time and effort. MFCs at a laboratory scale have a reduced power density of mass, while the configuration design, such as the choice of electrode material, still introduces many problems. The cost of the MFC, its power output, and the substrate utilization efficiency are the bottleneck problems of the technology for practical applications. In summary, substrate types used in MFC technology are very broad, from a variety of simple small molecule acids and alcohols to complex cellulose, protein, and benzene content. Each type of organic wastewater can be used as a fuel supply for an MFC to produce electricity. If cost is not a consideration, small molecule acid and glucose in simple organisms are optimal as substrates for MFCs. From an applications point of view, abundant resources, such as sludge wastewater, would be better MFC substrates.

7.2.3 MFC materials The battery material is the key factor for production performance of microbial fuel cells. Research and development of high-performance low-cost battery materials is

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the key to the technology of MFC application. In the following, materials for the MFC anode, cathode, and membrane are introduced.

7.2.3.1 Anode materials The anode material can have the following impacts on the MFC: (1) it affects microorganisms, the electrode contact distance and the number of electrons transmitted at the microbial electrode, which in turn affect the electron transfer in the MFC system; (2) it affects the anode’s potential, which determines the intracellular redox potential, thus affecting microbial metabolic pathways; (3) it affects electrode resistance, and thus the MFC’s power output. Good anode material facilitates electricity production; therefore, microbial adhesion is strong, it features low resistance, good electrical conductivity, large specific surface area, low biological toxicity, corrosion resistance, low cost, and more. At present, mainly carbon anode materials are used for MFC such as carbon, carbon paper, carbon felt, and carbon-based materials such as graphite brush. On the basis of these carbon materials or nanomaterials, other substances such as metal [435] or conductive polymers can be used to modify the electrode.

7.2.3.1.1 Carbon materials Widely used carbon materials in MFCs include carbon cloth, carbon, carbon foam, glass carbon and carbon paper, etc. Carbon can be paper thin, fragile or brittle; it can be carbon fiber woven cloth, paper, soft, and poured over carbon paper. Foam carbon is similar to carbon cloth but slightly thicker, providing more space for microbial adhesion and electron transfer. Porous glass carbon electrodes provide high electrical conductivity, but are very fragile. The production performance of different carbon materials is influenced by many factors; therefore, it is difficult to compare which materials for improving the production capacity of the battery work better. However, considering the scale-up of the MFC application, costs are an important factor in choosing the right material. Despite the good production performance of carbon cloth, significant costs limit its applicability. Studies have found that the carbon fiber cloth material costs only 1/40 of the carbon cloth and that power density (893 mW/m2 ) is higher than that of carbon cloth material (811 mW/m2 ). Thus, the technology is expected to be applied in future large-scale MFCs.

7.2.3.1.2 Graphite material Graphite can be used in MFC anode materials, usually as graphite rod, graphite plate, graphite pieces of foam, graphite felt, graphite particles, graphite fiber, and graphite brush. Graphite rods, graphite plates, and graphite surfaces with levelled surfaces are often used for the separation and electrogenesis of microorganisms and bacteria under microscopic observation research. Due to the hardness of the graphite rod, the material has no pores; therefore, its unit work area capacity is below that of graphite felt

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and graphite foam. The current density varization has been compared when graphite rods, graphite felt, and graphite is used as electrode change, which found three types of materials for the anode cell output current from big to small order: graphite felt, carbon foam, graphite. The reason is that the different surface area, rather than the material, leads to an increase in the electrode surface and thus, the electronic density transmitted to the anode and the electricity output are increased. In addition to graphite felt, the specific surface area of electrodes can also be increased by choosing graphite particles, graphite, or graphite fiber brushes. Graphite particles are often used to fill in the reactor electrodes. The electrode of the packedbed type must have good contact between particles. The graphite brush is composed of titanium wire and tightly bundled graphite fibers, maximally increasing the specific surface area and electrical conductivity of the anode and the titanium wire electronic collector. Air cathode MFCs use graphite brushes that generate up to 2400 mW/m2 power density.

7.2.3.1.3 Carbon nanomaterials Granular activated carbon, carbon nanotubes, graphene, activated carbon fiber, and many other types of carbon nanomaterials are used in the MFC as anode modifiers. Carbon nanotubes have larger specific surface area, and at the same time have good electrical conductivity. Nanomaterial modified anode-anode activation can effectively reduce internal resistance and is conducive to more microbial adsorption to the electrode, thus improving its performance. Good conductivity of the activated electrode leads to lower loss, thus reducing the internal resistance of the anode. Decreasing the overall internal resistance in the MFC increases the MFC’s electricity output. Carbon nanotubes can be fixed on a sponge surface anode which shows a reduction in the internal resistance of the anode materials, thus enhancing the mechanical properties of carbon nanotubes. Sewage as a substrate has a volume power density as high as 182 w/m3 . The carbon nanotube has many advantages; however, a too-high surface area may glue the material together. Therefore, choosing appropriate materials (such as carbon cloth, stainless steel net, sponge, etc.) is key to obtaining high performance anode material.

7.2.3.2 Cathode Materials Cathode performance is the main factor influencing the performance of the MFC. Cathode chamber materials. But the surface area of the electrode as well as the dissolved oxygen concentration in the solution also affect the power output. The aforementioned carbon paper, carbon fabric and felt, graphite, and graphite particles are commonly used as basic cathode materials besides graphite brushes. The effects are not excellent, but direct use of these materials (especially with oxygen as electron acceptor), can improve highly active catalysts. The catalyst can then reduce the reac-

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tion activation potential of the cathode to accelerate the reaction rate. To date, most research on MFCs used platinum as catalyst, e.g., using the highly active catalyst Pt or Pt-Ru raises the efficiency of the cathode [436]. The high-performing platinum catalyst is relatively expensive, while cheaper catalysts are a developmental trend in cathode materials.

7.2.3.3 Membrane The MFC membrane is mainly used for separating the anode and cathode chambers. The most common membranes are proton exchange membrane (PEM) separation materials, anion exchange membrane, and cation exchange membrane. PEM can effectively transport protons, inhibiting reaction gas permeability at the same time; however, it is expensive. Anion exchange membranes are superior in performance to cation exchange membranes, and the thinner anion exchange membrane can obtain higher output power and Coulomb efficiency. Microfiltration membranes and ultrafiltration membranes have good permeability. However, since the separation of the MFC material is unable to stop the spread of the dissolved oxygen in the substrate to the anode, and the destruction of the microbial anode, it leads to a decrease of Coulomb efficiency. Research on separating materials mainly searches for low price, combined with good performance to promote the large-scale application of MFCs.

7.2.4 MFC configurations To meet the needs of basic research or practical applications, the MFC configuration can be varied. According to the number of reaction chambers, the MFC can be divided into two-chambered and singe-chambered MFCs. According to the presence of a diaphragm, they can further be divided into with and without membrane MFC. Shapes of the MFC are: H MFC, high-class tubular, plate-type, mini-MFC, sediment MFC, and others [437]. This section introduces several typical MFC configurations and their performance.

7.2.4.1 Two-chambered MFC The basic configuration of two-chambered MFCs includes an anode chamber, a cathode chamber, and a proton exchange membrane. H MFC and cube MFC have been compared to the traditional two-chambered MFC. In recent years, some researchers combined the wastewater treatment plant design with many novel modified twochambered MFCs or tablet-type MFC, e.g. to dispose of sewage can produce power densities of mW/m2 (72 + 1). With an upflow MFC, the maximum power density can reach 170 mW/m2 . Furthermore, the folding of the three-dimensional electrode microMFC reaches a high power density of 24 mW/m2 .

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A cubic MFC is the most common configuration, usually utilized for MFC structure parameters. Such microbial anodes need a constant supply of oxygen, iron cyanide, and cathode oxidants such as potassium permanganate and manganese dioxide. The plate-type MFC consists of a bipolar proton exchange membrane pressed together. In the anode chamber, microbes generate electricity with a high concentration on the anode, and only the bipolar proton exchange membrane electrode can reduce the internal resistance, thus increasing the output power. The upflow MFC restores wastewater treatment via a Upflow Anaerobic Sludge Blanket (UASB) reactor and has the advantage of UASB and fungus mixing in the anodic reaction liquid to improve production capacity. The miniature MFC uses the folding of the three-dimensional electrode to increase the surface area (36–610 cm2 ), and anode and cathode are very close, leading to low internal resistance. In addition, due to its small volume, it is expected to be utilized as a sensor for military and homeland security applications and in the medical field.

7.2.4.2 Single-chambered MFC The working principles of two-chambered and single-chambered MFC are similar. The sediment MFC is one of the simplest single-chamber MFCs: sludge and solution are automatically layered to form two phases and are equivalent to two chambers of the bipolar MFC. Pressing cathode and proton membrane together results in “combined” MFCs, while pressing anode, proton membrane, and cathode together, results in a “triad” air cathode MFC, which is the widely used configuration in basic research. The cathode on the cylinder center is arranged with the anode around it in the new single-chamber design for MFCs [438]. The big advantage of a single chamber MFC is that it uses air as the cathode oxidants and thus, there is no need for additional liquid oxidizer. The sediment MFC has no proton exchange membrane, which leads to cost reduction. The hydrogen proton easily reaches the surface of the cathode, while reducing the internal resistance of the battery improves the maximum output power of the battery. Moreover, it simplifies the structure of the battery, the MFC is expected to be applied in polar and underwater conditions, and in bioremediative applications. The air cathode MFC has a low internal resistance, and the further advantages of simple structure, easy operation, suitability for large scale. The cylindrical MFC is equipped with eight internal anodes, an encircling graphite rod is supported by porous plastic pipes around the carbon/platinum cathode. The proton exchange membrane is hot pressed to the cathode; the anode has an increased hanging membrane area, improving the MFC electricity output. The porous cathode replaced the liquid air cathode system so that oxygen in the air can be directly utilized in the electrode reaction. COD ranges from 50–220 mg/L of domestic wastewater as the substrate, and the highest power density of 26 mW/m2 reaches a COD removal rate of up to 80 %.

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7.2.4.3 MFC batteries Due to the low power density generated by a single MFC, it is necessary to combine multiple independent MFCs to form a battery pack for improved power. Researchers have studied air cathode MFCs in series. Each reaction chamber used 0.5 kg of graphite particles as anode, the cathode was outwards exposed to air, and the resulting system produced up to 2,000 mW/m2 (cathode area) of electricity with sodium acetate and can be driven by a small fan. Other results showed that six twochamber MFC in series, using sodium acetate as the substrate and potassium ferricyanide as cathode electrolyte, reached a maximum average power output of 258 w/m3 .

7.3 Characterization techniques of MFCs Recently, MFCs have attracted increasing attention. However, their power output is low and stable performance is poor due to the influence of the cathode, electrolyte, membrane, microorganism of the anode, and cell configuration, among others. To solve the bottleneck problems limiting the development of MFCs, the selection of appropriate characterization techniques and evaluation methods is of particular importance. The comprehensive construction and analysis of MFCs requires knowledge of different scientific and engineering fields, ranging from electrochemistry, microbiology, and materials science and engineering to molecular biology and environmental engineering [439]. Along with the deepening of MFC related research, the evaluation system and the characterization techniques are gradually expanding and improving. In summary, the evaluation system for MFCs consists of electrochemical characteristics, columbic efficiency, energy efficiency, internal resistance as well as the wastewater degradation effect.

7.3.1 Electrochemical techniques Electrochemical/analytical techniques are vitally important in analyzing mechanisms (microbial, physiological, chemical, and electron transport), and so are beneficial to optimizing MFC operation, to ensuring the electron transfer process and to promoting continued innovation. Generally, the electrochemical techniques now being more routinely used in MFC studies have been used in the study of traditional electrochemical systems for many decades (e.g., chemical fuel cells and electrocatalytic systems). However, unlike traditional electrochemical systems, the operation of MFCs is restricted by the requirements for optimal microbial growth and sustainability. Typical conditions include ambient temperature and pressure, electrolytes with a low concentration of ions, and near neutral pH, etc. The following sections briefly introduce electrochemical techniques that can be used to study the complex system of MFCs.

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7.3.1.1 Cyclic voltammetry (CV) The mechanism of the electrode reaction can be determined by a variety of electrochemical methods. Voltammetry is the most common and straightforward technology, which is performed by a three-electrode system containing: a working electrode (WE), a reference electrode (RE), and a counter electrode (CE), as shown in Fig. 7.5.

TES

V A

CE

RE

WE

Fig. 7.5: The principle of a three-electrode system.

In the case where a scan only points in one direction, the method is referred to as linear sweep voltammetry (LSV); if the scan is also continued in the reverse direction and comes back to the start potential, the method is referred to as cyclic voltammetry (CV), which is the method commonly used in MFC anode experiments. It is relatively easy to determine whether a reaction is reversible or not via the CV method. For a fully electro-kinetically reversible couple, the ratio of anodic to cathodic peak currents = 1 and the potential separations between the peak potentials = 59.2 mV/n at room temperature (n is the number of electrons transferred in the electrode reaction). Compared to reversible systems, quasi- or irreversible phenomena will exhibit larger separations between peak potentials and a lower number of peaks. This can be attributed to the increased time necessary for electron transfer in these nonreversible systems. MFC studies employing CV generally use forward and backward voltage sweeps with rates in the range of 1–100 mV/s. The observation of multiple peaks in the CV of bioelectrochemical systems can be attributed to the multistep parallel or consecutive (series) mechanisms, or to the presence of several different redox species. The principles of CV have been denoted as the “spectroscopy of the electrochemist”. They enable revealing wherever conditions are such that a soluble electron acceptor or electron donor is not readily available. In general, the electron acceptor has something to do with biofilms “breathing” [440] minerals, such as iron and manganese oxides, while the latter situation is also associated with biocorrosive

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(a)

3 4 3’ 2’ 1’

2

(b)

5

1

6 4 7 6’ 2’ 1’

2 Electrode

5

1 Electrode

Electrode

2

5

5 4 7

6

8

6’ 8’ Cytosol

1

2’ 1’

(c)

Fig. 7.6: Simplified schematic representation of the major, potentially rate-limiting steps during (a) an electrocatalytic reaction, (b) an enzymatic bioelectrocatalytic reaction, and( c) a microbial bioelectrocatalytic reaction (considering a two-enzyme cascade in the cell). The steps are: mass transport in the bulk solution (1/1󸀠 ), mass transport in the electrolyte diffusion layer (2/2󸀠 ), sorption/desorption processes (3/3󸀠 ), bioelectrochemical reaction (4), electron conduction between the active center and the electrode (5), enzymatic uptake/release (6/6󸀠 ), enzyme turnover (7), and uptake/release of substrate to the cell (8/8󸀠 ) [441].

processes. Microbial biofilms are far more complex than inorganic electrocatalytic moieties, and consequently, the number of steps involved in their bioelectrochemical reactions increases dramatically as shown in Fig. 7.6. In MFC research, CV methods have been applied to (1) study the direct or indirect electron transfer mechanisms between biological membrane and electrode; (2) determine the anodic oxidation potential and the cathodic reduction potential, including the biological or chemical potential (for reversible redox couples, the average peak potentials of the cathode and anode show the reversible potential of the substance relative to the reference electrode); (3) test the performance of the catalyst system. For anode reactions, so far, only microbial extracellular cytochromes have been proven to be active for direct electron transfer mechanisms. However, some CV-based MFC studies show characteristics of interconversion between active-inactive states, i.e., inactivation is observed during the positive potential sweep and reactivation is observed on the return sweep. The shape of these voltammograms is an initial indication that “turnover” enzymes or multienzyme systems might be contributing to the anodic current. CV technology is easy to operate but time consuming. However, in order to guarantee the accuracy of the mechanistic studies, background experiments are mandatory [442]. When the anode chamber is filled with microbial communities, the peak current and peak potential coming from the electrode interface reactions might involve direct and/or indirect electron transfer. For complex systems (especially wastewater), there are too many unknown factors. To better study the factors that affect the MFC’s performance, the following CV testing approaches are recommended (Tab. 7.1). The electron transfer mechanism should be clarified to an extent by comparing CV results (i.e., the peak currents and the peak potentials at identical scan rate using a same-size electrode). To ensure the rigor of the experiment, multi-scans are generally performed, especially in systems with low concentrations of electroactive metabolites.

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Tab. 7.1: Recommended CV testing approaches for complex anode systems. Affecting factors

Electrode

Electrolyte

electrode, electrolyte biofilm substrate suspended cells metabolites

biofilm-less biofilm-coated biofilm-less and biofilm-coated biofilm-less biofilm-less

un-inoculated or abiotic un-inoculated or abiotic un-inoculated or abiotic inoculated inoculated electrolyte with consumed substrate(s)

CV curves are influenced by the following factors: pretreatment of the electrode surface, the rate of electron transfer reactions, the species of the microorganism and their thermodynamic properties, the concentration of electroactive species and their rates of diffusion, and the sweep rate. It is worth noting that many electrode materials used in MFCs cannot produce reversible electrochemical reactions even for the classical 4− reversible redox couple Fe(CN)3− 6 /Fe(CN)6 . The main reasons accounting for this are that the heterogeneous processes of electrode reactions can be significantly affected by the microstructure, roughness and functional groups present on the electrode surface. For example, an electrode will produce different electrode reactions before and after polishing, as well as different CV curves; therefore, the electrode and operating conditions must be appropriate for the accurate determination of kinetic parameters.

7.3.1.2 Chronoamperometry (CA) CA is an electrochemical technique where the potential of anode or cathode is controlled (or stepped) and then kept constant with the resulting currents being monitored as a function of time. Bond et al. [443] found that when the MFC system with Geobacter sulfurreducens as the anode and acetate as the electron donor, with graphite as electrode at an oxidizing potential of +200 mV (vs Ag/AgCl), a direct electron transfer process was indicated for the MFC. Since the biofilm electrode continued, this indicated that the anode potential can not only influence and regulate biofilm activity but also affect the growth rate of microbes. The study compared the performance of three MFCs containing microbial communities that were continuously fed with acetate as substrate with anode potentials poised at 0, −200, and −400 mV (vs Ag/AgCl), respectively, and concluded that the optimal anode was at −200 mV under the conditions tested. Zhao et al. [439] investigated the current for microbially produced sulfide at anode potentials of +200 mV (vs Ag/AgCl) on carbon fiber veil and activated carbon cloth anodes, respectively, and reported that an activated carbon cloth electrode could be more favorable to the formation of sulfide.

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7.3.1.3 Chronopotentiometry (CP) Opposite to CA, CP involves the study of the potential as a function of time at an electrode operating with a constant current. A recent study applied this technology to study the air cathode MFC coated with polytetrafluoroethylene (PTFE). The study tested potential changes of a coated carbon cloth electrode at a constant current, and then drew the potential-current density curve aiming to evaluate the performance of the battery. The results indicated that the four-layer-coated PTFE electrode showed maximum performance.

7.3.1.4 Polarization curves (PCs) A PC represents the voltage as a function of the current (density) and can be used to analyze and describe MFC performance. A voltage stabilizer can be easily introduced into one or more of the MFC chambers so that it is possible to record the individual potentials of anode or cathode (as well as the overall cell voltage of the MFC). This method can be used to analyze the characteristics of both anode and cathode. The polarization curve should be recorded both up and down (i.e., from high to low external resistance) and vice versa. In addition, when a variable resistance was employed to measure the polarization curves, the corresponding output voltage and current can be recorded only when quasi-steady-state conditions have been established. It will take several minutes or even longer to reach a quasi-steady state due to the slow mass transfer when the externals have been changed; the required time depends on the system and the external resistance. However, this condition is only a temporary steady state since over longer times the substrate concentration in the reactor will change due to substrate demand at the anode. Thus, the mass transfer of the reactants and products will have an enormous impact on the output voltage and corresponding current and therefore it is inappropriate to spend too much time on the determination of the quasi-steady state. According to previous publications and the reaction kinetics theory of fuel cells, polarization curves can be divided into three zones: (1) Starting from the OCV at zero current, there is an initial steep decrease of the voltage: in this zone, the activation losses are dominant; (2) the output voltage drops slowly and linearly with the current decline; in this zone, the ohmic losses are dominant; (3) the output voltage decrease with increasing current, and in this zone the concentration losses (mass transport effects) are dominant (solid line, Fig. 7.7) [444]. In MFC research, linear polarization curves are often encountered (dashed line, Fig. 7.7). In reference to the literature, for a linear polarization curve, the value of the internal resistance (Ri ) of the MFC is easily obtained from the polarization curve, as it is equal to the slope. Ri = −∆E/∆I

(7.5)

411

Voltage (V)

7.3 Characterization techniques of MFCs |

Current (mA)

Fig. 7.7: Polarization curve of an MFC.

Power (mW)

7.3.1.5 Power curves Power is an important parameter that can be used to reflect MFC performance; it is equal to the product of output voltage and current. Polarization curves can be obtained from the power curve. The solid line and dashed line of Fig. 7.8 are showing the power curves that correspond to the solid line and dashed line of Fig. 7.8, respectively. As no current flows for open circuit conditions, no power is produced. The power output gradually increases with increasing current and will reach a maximum when the current reaches a certain value. The research of You et al. [434] showed that the maximum power output of an MFC with KMnO4 as electron acceptor reached 0.116 mW at a corresponding current of 0.17 mA.

Current (mA)

Fig. 7.8: Power curve of an MFC.

Beyond this point, the power drops due to increasing ohmic losses and electrode overpotentials to the point where no more power is produced (short circuit conditions). The principle is similar to ordinary fuel cells. In many MFCs, the ohmic resistance plays a dominant role in defining the point of the maximally attainable power, partially due to the low ionic conductivity of the substrate solutions, but usually due to a low degree of optimization in fuel cell de-

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sign. When the proportion of ohmic resistance in the internal resistance is small, the changes of power with the current are shown in Fig. 7.8 (solid line). The curve changes into the dashed line of Fig. 7.8 by increasing the ohmic resistance. Here, ohmic resistance is almost dominant in the internal resistance [437]. When a polarization curve is linear, the slope is equal to the internal resistance. According to electrical knowledge, Pm = E2 /4Ri ,

(7.6)

where E is electromotive force and Ri is the internal resistance of the MFC. When the external resistance is equal to the internal resistance, the maximum power output can be obtained. However, if the polarization curve is nonlinear, the achieving of power output is closely related to the concentration losses.

7.3.2 Coulombic efficiency The Coulombic efficiency εCb , is defined as the ratio of total Coulombs actually transferred to the anode from the substrate, to the maximum possible Coulombs if all removal substrate produced current. The total Coulombs can be obtained by integrating the current over time, and the Coulombic efficiency can be calculated based on the conservation law of charge. If the MFC runs in fed-batch mode, εCb can be calculated from the following formula: t M ∫Ob idt εCb = , (7.7) FbVAn ∆COD where M is the molecular weight of oxygen 32 g/mol, F = 96,485 C/mol the Faraday constant, b = 4 and corresponds to the number of electrons exchanged per mole of oxygen, VAn is the volume of liquid in the anode compartment, and ΔCOD is the change in COD over time tb (g/L). For continuous flow through the system, MFC can achieve its continuous steady operation state when the load resistor (external resistor) remains constant and the current does not change with time. Therefore, calculation of εCb can be further simplified and calculated as follows: εCb =

Mi , FbV∆COD

(7.8)

where q is the volumetric influent or effluent flow rate (L/s), and VCOD is the difference in the influent and effluent COD (g/L).

7.3.3 Resistance In a real MFC system, the process of electron transfer from microorganisms to a final electron acceptor is accompanied by energy loss manifested as internal resistance. The internal resistance can reduce the efficiency by reducing the output voltage [445].

413

Voltage/V

7.3 Characterization techniques of MFCs |

Current/A

Fig. 7.9: Zones of an MFC affected by internal resistance . Zone 1 is mainly affected by activation resistance, zone 2 is mainly affected by ohmic resistance, and zone 3 is mainly affected by mass transfer resistance.

It can be divided into activation resistance, ohmic resistance, and mass transfer resistance (Fig. 7.9). Regardless of whether the oxidation process occurred on the anode surface or on the bacterial surface, it is necessary to spend energy activating the reaction, which is called the activation resistance. The potential losses resulting from the activation are called the activation overpotential, which can be described by the Butler–Volmer equation: i η = a log ( ) , (7.9) i0 where η is the overpotential, a is the correlation coefficient, which was determined by the chemical reaction itself, i is the current density, i0 is the exchange current density, which is the current density when the potential is zero. Exchange current density bears a relation with temperature: increasing the reaction temperature or chemically modifying the electrode surface (such as adding an electron mediator) would improve the bacteria’s ability to transport electrons could reduce the activation overpotential, thus reducing the activation of the internal resistance. Ohm internal resistance is caused by physical resistance of the electrode, solution, and membrane. The ohm internal resistance plays an important role in limiting the improvement of MFC power, especially in high current density areas. The ohm internal resistance can be reduced to a minimum by increasing solution conductivity, choosing electrode materials with good conductivity, or increasing the ion diffusion coefficient of a separation medium. When the anodic oxidation rate is very fast, the rate of compound oxidation is greater than the electrode surface supplementary rate, which will cause mass transfer resistance. This only happens in the case of a higher current density. In general, only when the diffusion process is hindered by thick biofilms or the MFC is operated with a small resistance, does diffusion resistance becomes a problem. It can be seen from equation (7.6) that the power production of MFC is affected by E and Ri , E is the driving force, Ri is capacity resistance. Therefore, measurement of the MFC composition of internal resistance is the precondition for improving MFC power production capacity.

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However, there is little research on the method of MFC internal impedance measurement. There are three methods involved in the measurement of MFC internal impedance published in the literature: current interrupt method, polarization curve method, and AC impedance [445]. The three methods can be divided into two types: one type is transient, including the current interrupt method and AC impedance method, where the AC impedance method has been widely used in the determination of hydrogen fuel cells, and has also been used to determine the ohm internal resistance in an MFC study. A further type is the steady state method, including the ohmic resistance, activation resistance, and mass transfer resistance, when the external resistance is equal to the apparent impedance, MFC power output capacity reached its maximum, while external resistance is also known as the best resistance. Therefore, it is necessary to determine the apparent resistance in order to examine the maximum power supply capacity of MFC.

7.3.4 Degradation efficiency MFC can not only be used to produce electricity, but has also been proposed as a method to treat wastewater, and thus it is important to evaluate the overall performance in terms of BOD, COD, or TOC removal. Other factors may also be important, such as soluble versus particle removal, and nutrient removal. Here, we focus on performance in terms of COD removal as it is a common measure for wastewater treatment efficiency, and COD removal is needed for Coulombic and energy calculations. The COD removal efficiency (COD) can be calculated as the ratio between the removed and influent COD, and the equation can be shown as follows: εCOD =

∆COD COD

(7.10)

This parameter measures how much of the available “fuel” has been converted in the MFC, either into electrical current (via Coulombic efficiency), biomass (via growth yield), or through competitive reactions with alternative electron acceptors (e.g., oxygen, nitrate, and sulfate). As the MFC influent can contain both dissolved and particulate COD, it can be difficult to specify what fraction of the effluent particulate COD was due to biomass produced in the reactor, or untreated COD that was originally in the reactor influent.

7.3.5 Energy efficiency Energy efficiency is defined as the ratio of energy recovery for power production and the theoretical energy release of the MFC. The most important factor for evaluating the performance of an MFC for electricity production compared to a sewage treatment system, is to evaluate the system in terms of energy recovery. The overall energetic

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415

efficiency is calculated as the ratio of power produced by the cell over a time interval tb to the heat of combustion of the organic substrate added in that time frame. For batch-type feeding, the equation has been shown as follows: t

ηMFC =

b ∫ Uidt

maddes ∆H

,

(7.11)

where H is the heat of combustion (J/mol) and madded is the amount (mol) of substrate added. This is usually only calculated for influents with known composition when H is not known for actual wastewaters. In MFCs, energy efficiencies range from 2 % to 50 % or above when easily biodegradable substrates are used. As a basis for comparison, the electric energy efficiency for thermal conversion of methane does not exceed 40 %.

7.3.6 Other characterization techniques In addition, according to different research purposes, there will be some specific parameters, such as the loading rate and volumetric loading rates denoted as Bv (COD/(m3 · d), that are important indexes when examining the use of MFCs for wastewater treatment. Typical values for Bv achieved to date were up to 3 kg COD/(m3 · d), compared to values for high-rate anaerobic digestion of 8–20 kg COD/(m3 · d), or activated sludge processes of 0.5–2 kg COD/(m3 · d). These loading rates can be normalized to the total anode volume for comparison with suspended biomass processes (e.g., activated sludge, anaerobic digestion), and to total anode surface area for comparison with biofilm processes. Based on reported areal short-term peak power productions, the anode surface-specific conversion rates for MFCs are up to 25–35 g COD/(m2 · d), which is higher than typical loading rates for rotating biological contactors (RBCs; 10–20 g COD/(m2 · d)) and comparable to those of high rate aerobic biofilm processes such as moving-bed bioreactors (MBBRs). In addition, there are studies on the power output, electron recovery efficiency (the concentration ratio of electricity conversion of organic matter, and the total organic matter), COD balance, and growth yield [446, 447].

7.4 The application and functional extension of MFCs Generating electricity is the basic function of MFCs, and therefore improving the efficiency of electricity production has become one of the major hot spots of current MFC research. In addition, a variety of electricigens for MFCs can be used to generate electricity, so that sustainable electricity production in different environments (such as a submarine) can be achieved to meet special needs. The valuable applications are as follows: (1) Waste remediation. Technically, MFCs that utilize bioremediation are a feasible and economic strategy, during which no external energy is needed, which has

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the added benefit of generating electricity while recycling pollutants. (2) Biosensors. Due to its sensitivity to environmental changes, an MFC can be developed for monitoring the wastewater BOD value and pollutants’ toxicity. (3) Desalination. During electricity production, a series of reactions happen near the electrode in MFCs, which can motivate desalination. (4) Hydrogen production. With a small amount of external electricity, MFCs can produce a range of energy-related substrate in the cathode, such as hydrogen. In this section, an up-to-date view of the progress over the last five years in MFCs applications and expansion capabilities is reviewed.

7.4.1 Electrogenesis Sediment, soil, organic wastewater, insects, and straw, all contain abundant organic matter, can be decomposed, and used to generate electricity by electrogenic bacteria. Plants and algae can also be used to generate electricity since they secrete organic matter into the environment through photosynthesis. In summary, substances or organisms that are rich in or able to produce organic matter can be used for electricity generation. Among them, sediment or soil contain electrogenic bacteria, which can decompose organic matter and then generate electricity. This is attractive and this type of MFC is named sediment microbial fuel cells (SMFCs). SMFCs are usually operated in a one-chamber configuration. The anode is buried in the sediment and the cathode floats in water, using dissolved oxygen as electron acceptor. The sediment effectively prevents the permeation of oxygen from the water to the anode. So far, the majority of successful power for electronic devices in MFCs originates from marine sediments-based microorganism-generated electricity, mainly attributing to higher electrical conductivity of seawater. Tender et al. [448] successfully drove a small marine monitoring buoy at the marine SMFC. Fu et al. [449] in Ocean University of China has constructed marine sediment MFCs, connected either in series or parallel to increase the overall stack voltage or current for successful application on driving small electronic devices (radio, clock, and calculator) in the Jiaozhou Bay shallows. Zhang et al. [450] used sediment and water of Lake Michigan, USA, to assemble an MFC that drove a wireless temperature sensor in the laboratory. SMFCs have good prospects for various applications. For example, the disappearance of flight MH370: After crashing into the sea, the self-contained power pack of its black box could only maintain the radio signal for a short time. If it had been powered by SMFCs, continuous function could have been expected. In 2009, British scientists and engineers cooperated and invented a series of household appliances driven by MFCs. Those MFCs generate electricity, which can drive clocks, night LED lights, and many other small devices. Ieropoulos and Santoro et al. [451, 452] charged a cell phone using electricity generated from urine by electrogenic bacteria. The electricity is enough for the smart phone to send messages, connect to the internet, and transmit a brief conversation. This technology was expected to be applied in space. Similarly, electrogenesis uti-

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lizing organic wastewater or solid organic waste could not only provide electricity but also decrease BOD through anodic oxidation reactions, implementing a remediation for pollution. Currently, the small amount of electricity obtained through organic wastewater remediation, can only drive small devices and cannot satisfy a realistic demand. How to scale up, to dispose of more BOD and to obtain more electricity is becoming the question considered by researchers. However, in contrast to scaling up, the other research area for MFCs is miniaturization. The volume of miniaturized MFCs could be at μL level, raising the future prospect of applications in medical diagnostics. Mink et al. built an MFC with a volume of only 25 μL and achieved a maximum current density of 1,190 A m-3, which would be sufficient as a power source for a portable diagnostic device driven by saliva or blood glucose. Furthermore, miniaturized biosensors based on MFCs for environmental monitoring are also being developed. The electrogenic mechanism of plant MFCs is as follows: organic matter produced through plant photosynthesis is secreted into the surrounding environment through the roots where the anode is buried nearby and electrogenic bacteria decompose the root exudates, thus generating electricity. The power density could be up to hundreds of mW/m2 . Due to photosynthesis, continuously produced root exudates make it possible to retain the current output from electrogenesis for a long time, thus plant MFCs are devices converting solar power to electricity. Helder et al. [453], researchers from the Netherlands, built a green roof using Spartina anglica, which generated electricity for more than seven months. In the report, the authors compared the electrogenic characteristics of two configurations of MFCs with or without membrane. The results show that the configuration without membrane was cheaper and produced higher currents with a shorter start-up time. However, plant electrogenic properties are difficult to stabilize since they are dramatically affected by temperature, light, and growth status of the plant. Algae can also produce organic matter through photosynthesis and generate electricity through decomposition of organic matter or of themselves. Particularly, marine cyanobacteria possess the ability to be electrogenic and can undergo direct electron transfer at the anode. Algae can also produce oxygen through photosynthesis, which acts as electron acceptor near the cathode.

7.4.2 Pollutants remediation and waste reclamation 7.4.2.1 Treatment of wastewater In wastewater treatment, the COD can be degraded via oxidizing reaction at the anode and the oxidative pollutants can be reduced at the cathode. High valance metal ions and SO42− can be reduced to elements to be recovered. The additional advantage of using MFCs to treat wastewater lies in the treatment process not exhausting energy; on the contrary, it produces a small amount of electricity. Therefore, MFC-based treatments deserve exploration.

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7.4.2.1.1 Removal of COD In the anode chamber of MFCs, the degradation of hard-to-decompose organic compounds, including phenol, polyol, petroleum, and polyaromatic hydrocarbons, can be accelerated. Moreover, wastewater, including urban wastewater, leachate, industrial organic wastewater from breweries, coking plants, and paper mills, as well as agricultural wastewater such as swine wastewater, butchery wastewater, and aquaculture wastewater, with large amounts of COD or BOD can be treated effectively using MFCs. Dye (e.g., azo dye, acid orange 7), chlorinated organic compounds and nitrobenzene can be degraded in the cathode chamber. Wang et al. [454] operated membrane-free upflow MFCs applying 0.5 V voltages and thus removing 98 % nitrobenzene in the cathode chamber. The BOD in wastewater with low toxicity can be completely removed during a few days and a power density of several hundred mW/m3 can be produced in the meantime. The removal rate of BOD in wastewater with high toxicity and hard-todecompose organic compounds can reach 60 %, although the removal process is slow. The upflow air cathode MFC has the best structure over other configurations. MFCs of tubular shape with the anode at the inner circle, cation exchange membrane (CEM) in the middle, and air cathode at the outer circle, are very desirable for the improvement of utilization efficiency of the CEM. Given the fact that MFCs have high removal efficiency of BOD but the power output is relatively low, it is worth considering that two steps for the treatment of wastewater might be useful. The first step is to generate hydrogen through anaerobic fermentation and then treat fermenting liquor using MFCs in the second step. Currently there are methods that can be used to promote the degradation of hard-to-decompose organic pollutants. For example, the addition of glucose and acetate into MFCs can enhance the growth of electrogenic bacteria and achieve co-metabolism, so that the treating efficiency and the electricity production can be improved. Adding NaCl or phosphate buffer solution can increase the conductivity. Electron transfer can be accelerated in the presence of shuttles such as sulfate and azo dyes, which can be found in wastewater. In addition, the incorporation of MFCs with electro-oxidation (microbial fuel cell-electro-oxidation, MFC-EO) could accelerate the degradation of organic compounds, and the generated power can partly be used to compensate for the electrical energy that was exhausted in the electro-oxidation. The bioelectrical Fenton system can promote the degradation of hard-to-decompose organic pollutants in the cathode chambers of MFCs. On the cathode surface, aeration leads to production of H2 O2 which reacts with Fe2+ and forms radicals, effectively degrading organic pollutants.

7.4.2.1.2 Removal of heavy metals The cathode can reduce oxidative chemicals and thus lower the toxicity. Cr(VI) in the 2− form of Cr2 O2− 7 and CrO4 mainly exist in electroplating wastewater and has attracted much attention. In the cathode chamber, Cr(VI) can be reduced to Cr(OH)3 , which is

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much less toxic than Cr(VI). + Cr2 O2− 7 + H + e → Cr(OH)3 ↓

(7.12)

The removal rate of Cr(VI) can reach 100 % during a few days, and a power density of several hundred mW/m2 can be reached. In the cathode chamber, Cu2+ , Ag+ , and Hg2+ can be reduced to elemental Cu, Ag, and Hg, which deposit on the cathode surface where they can be recycled. While treating wastewater with heavy metals, the wastewater with organic pollutants can be treated in the anode chamber (Fig. 7.10). The biofilm can remove heavy metal ions such as Zn2+ and Cd2+ via absorption.

Fig. 7.10: A double-chamber microbial fuel cell for the remediation of organic wastes in the anode chamber and of waste with heavy metal Cr(VI) in the cathode chamber.

7.4.2.1.3 Removal of nonmetal ions 2− Ions of NO−3 , ClO−4 , SO2− 4 , and SeO3 have a strong oxidizing effect, and can be reduced 2− + 2− on the cathode, while S , S2 O3 , SO2− 3 , and NH4 can be oxidized and removed at the 2− anode. The oxidation of S in the anode chamber was oxidized and V5+ is simultaneously reduced in the cathode chamber has been reported. After operating the MFCs for 72 h, the removal rate of S2− and V5+ were 82 % and 26 %, respectively. Ca-P-Mg formed stones and deposited on the cathode during the treatment of wastewater containing 2+ Ca2+ , PO3− 4 , and Mg . This can be attributed to the reaction O2 + H+ + e → OH−

(7.13)

which increased the pH value of the wastewater near the cathode and led to deposition.

7.4.2.2 Recovery of solid organic waste Solid wastes that can be degraded in MFCs and generate power include wheat straw, corn straw, animal carapaces, starch, and livestock and poultry manure. Of these, wheat straw, corn straw and animal carapaces contain many hard-to-decompose materials, such as lignin, cellulose, and chitin. Before being subjected to MFCs, a primary

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decomposition of these materials is achieved via enzymes, bacteria, and acids, which can dramatically increase the decomposing efficiency and the power production.

7.4.2.3 Remediation of polluted soil MFC can be used to remediate polluted soil, although related studies are few. Huang et al. [455] used MFCs to remediate phenol-polluted soil and ten days after MFC operation, 90 % of phenol was removed, while only 13 % of phenol was removed in a nonMFC control. Li et al. [455] used MFCs to remediate oil-polluted soil and found that by running MFCs, the degradation of alkanes and polycyclic aromatic hydrocarbons was accelerated. As the soil pollution problem is getting more and more serious, threatening food security and food safety, soil remediation has been increasingly attractive. MFCs are a green and sustainable remediation technique and are worth exploiting.

7.4.3 Biosensors A large number of studies show that the current or charge output of MFCs positively correlates with the concentration of the metabolizable carbon sources or the quantities of microorganisms in anode chambers (Fig. 7.11). Toxic substances would inhibit the metabolic activities of electrogenic bacteria in the anodic chamber. This could lead to a decrease of the cell current or charge output, which also positively correlates with the concentration of toxic substances. Furthermore, the current would gradually recover with the remediation of pollutants (Fig. 7.11). Therefore, MFCs could be used in biosensors for water quality monitoring. Compared with conventional methods via physiochemical detections, MFC-based biosensors are much superior in real-time, continuously and on-line monitoring. At present, MFC-based biosensors are mainly

Current (mA)

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0.10

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80 60

80

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0.00 0

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Fig. 7.11: BOD biosensors monitoring BOD concentration. The number in the current curve stands for the BOD concentration (mg/L).

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applied in monitoring BOD and pollutants in wastewater. In addition, the microbial quantities and activities can also be detected via MFC. Tront et al. [457] discovered that Geobacter sulfurreducens, a species of electrogenic bacteria, generates lower current under oxygen stress, and the current recovered rapidly after the stress was removed. Thus, MFCs could be used as oxygen detectors. Before testing, the inoculation, enrichment, as well as domestication of electrogenic bacteria need to be conducted in the anode chamber with liquid medium. The wastewater is pumped into the MFC after the power output is stable. BOD concentration could be estimated according to current or charge output, and the toxicities of pollutants and their variations with time would be detected through the reduction rate. The configuration of MFC-based biosensors is usually single-chambered or of dual-chambered type, where anode and cathode chambers are separated by a proton exchange membrane (PEM). The PEM can effectively prevent the interference to anodic reactions from oxygen as electron acceptor at the cathode. To guarantee the stability and re-use of electrogenic bacteria, the anode was assembled on carbon nanotubes, gold nanoparticles, polyaniline, ruthenium dioxide, or modified with redox medium, which could intensify the fixation of electrogenic bacteria and thus enhance the power output. The majority of current MFC-based biosensors are mediator-less. Mediators were added into the anode chamber to promote power output in a few studies. For example, potassium ferricyanide was used as electron mediator for BOD detection with E. coli as electrogenic bacteria. Apart from that, dyes were also used as mediators. These electron transfer mediators are usually toxic to microorganisms, resulting in difficulties for long-time stable operation. For mediator-less MFCs, the electric active microorganisms used so far that could directly transfer electrons to the anode include Shewanella putrefaciens, Rhodofoferax ferrireducens, Geobacter fulfurreducens, Enteroccus gallinarum, Desulfoblbus proprionicus, Aeromonus hydrophilia, and Clostridium butyricum. Mediators-less MFCs possess higher stabilities and are more suitable for biosensors.

Current density (A/m2)

2.5 2+ 2.0 Cu addition

1.5 1.0 0.5 0.0 0

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Fig. 7.12: Toxicity biosensors detecting Cu2+ pollution.

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7.4.3.1 BOD Biosensors BOD biosensors have been applied for detecting the BOD concentration in domestic and industrial wastewater. Within a certain range, the current or charge output of MFCs has a favorable linear correlation with BOD concentrations. The linear ranges were within 300 mg/L. However, there were differences in linear ranges among different studies. The differences are related to differences in test conditions such as pH, temperature, conductivity, BOD composition, electrogenic bacteria, and MFCs. The SPEEK was used as a cation exchange membrane and the linear range increased to 650 mg/L. The reason for this increase was that the material has smaller impedance and can prevent oxygen transferring into the anode chamber more effectively compared to Nafion. Modin and Wilen [458] kept the MFC voltage at 0.5 V with a potentiostat to promote the decomposition of BOD by electrogenic bacteria. They increased the upper limit of the linear range to 1,280 mg/L. Moon et al., Ji et al. [459] and Chang et al. [460] enriched oligotrophic microbes in MFCs to detect BOD samples under 10 mg/L by enhancing the cathodic reactions and reducing the oxygen diffusion. MFC-based BOD biosensors could be continuously operated for a long time with distinct advantages. The longest reported running time of MFCs is approximately five years. The shortcomings lie in the length of time that electrogenic bacteria need to respond to BOD change. Studies show that the response time varies from a few minutes to several hours. In consequence, the shortening of the response time and an improvement of the stability and sensitivity of the biosensors are required.

7.4.3.2 Toxicity detection biosensors Once toxic substances enter into the anode chamber, the MFC current sharply decreases. The response of toxicity biosensors occurs rapidly, which differs from BOD biosensors. Studies have tested series of heavy metals (Cu, Ni, Ag, Pb, Cd, Hg, Cr, and As), organic pollutants (PCBs, herbicide, and sodium dodecyl sulfate) and acid pollutant (HCl), all causing decreases of the current. Within a certain range of concentrations, electrical signals (current or charge output) have linear correlations with the concentrations of toxic pollutants. Kim et al. [461] constructed a toxicity detecting system based on the relationship between pollutant concentration and current decrease. The system possesses high sensitivity and could respond to 0.04 mg/L Cr6+ , 0.03 mg/L Hg, 0.04 mg/L Pb2+ , and 0.04 mg/L benzene. Generally, the existence of a substance could be detected according to the current decrease; however, the species of the substance could not be identified.

7.4.3.3 Soil pollutant detection Most of the MFC-based biosensors were applied for water quality detection, and few studies have so far been conducted on the detection of soil pollutants. Soil has abundant organic matter and various microorganisms, many of which reportedly have the

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ability to be electrogenic. Deng et al. continuously recorded the real-time voltage of soil MFCs for a month and found that the voltage was sensitive to and had a significant correlation with temperature changes. The activities of microorganisms also have a significant correlation with temperature. Thus indicating that the voltage or current that soil generated may imply the soil microbial activity, which could be further applied for soil pollutant detection. The ability to record real-time and continuous running data is the superiority of soil MFCs for on-line monitoring of soil microbial activity or soil pollutant by detecting electrical signals generated by the soil. In addition, electrical signals which could respond to soil contamination would be generated by large quantities of electrogenic microorganisms in soil rather than in water quality detection, during which medium and inoculated electrogenic bacteria are needed. Fig. 7.13 shows the MFCs operated for soil pollutant detection in the author’s laboratory. The soil MFCs have dual-chamber configuration with contaminated soil filled in anode chambers and potassium ferricyanide solution in cathode chambers or single chamber configuration. The detection should be conducted at a constant temperature in the incubator to avoid interference to electrical signals due to temperature fluctuation.

(a)

(b)

Fig. 7.13: MFCs applied to soil pollutant detection. RU: dual-chamber MFCs detecting the toxicity of soil pollutants, L: several dual-chamber MFCs testing the toxicities of different contaminated soil samples, RD: single-chamber MFCs detecting the toxicity of soil pollutants.

7.4.4 Desalinization For a long time, people have been hoping to relieve the demand for fresh water via desalinization. Methods usually applied in desalinization include distillation, reverse osmosis, electrodialysis, ion exchange, freezing crystallization, and a few others. However, relatively high energy has to be input for these methods, and devices are required for high pressure and high temperature, which hinder the large-scale application of desalinization technology. Based on MFCs, Cao et al. [463] from the Tsinghua University invented the microbial desalination cells (MDCs) in 2009, providing a new

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method for seawater desalinization. The configuration and operating principle of the MDCs are demonstrated in Fig. 7.14. MDCs consist of an anode chamber, a desalinization chamber, and a cathode chamber. Anode chamber and desalinization chamber are separated by an anion exchange membrane (AEM); desalinization chamber and cathode chamber are separated by a cation exchange membrane (CEM). Electrogenic bacteria decompose the carbon source in the anode chamber, while the electrons produced travel to the cathode through the external circuit. Protons cannot transfer into the cathode chamber blocked by the AEM. To arrive at charge balance, Cl− in the desalinization chamber transfers through the AEM into the anode chamber. In the + cathode chamber, Fe(CN)3− 6 combines with H from hydrolysis and electrons, gener2− − + ating Fe(CN)4 and HCN. OH attracts Na in the desalinization chamber migrating through CEM into the cathode chamber to maintain the charge balance. Seawater in the desalinization chamber gets desalinated in the process. The desalinization process not only requires no external energy input, but also generates electricity. e–

e– Resistance

AEM e–

CEM e–

CO2 Red H+

Na+ CI– Na+

Carbon substrate Anode

Anode chamber

CI–

Desalination chamber

OX Cathode

Cathode chamber

Fig. 7.14: Schematic diagram of a running MDC. AEM: anion exchange membrane, CEM: cation exchange membrane.

Hence, some improvements was made based on the study above by optimizing the process to achieve reduction of pollution and cost, while increasing efficiency. For example, upflow MDCs were used and O2 as electron acceptor. The tubular anode chamber with an AEM was placed in the tubular desalinization chamber with a CEM, forming a casing structure. Amorphous graphite grains were filled into the anode chamber as anode; an air cathode was adhered outside the CEM (including Pt/C catalyst and carbon cloth). Organic wastewater and seawater flowed into the anode chamber and desalinization chamber from the bottom, respectively. In the anode chamber, electrons produced by the decomposition of organic wastewater arrived at the cathode

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through external wires; on the cathodic surface, H+ from water hydrolysis combined with electrons and O2 generating H2 O. Na+ migrated from the desalinization chamber into the cathode chamber to maintain charge balance with OH− . Thus, it is possible to drive seawater desalinization by the electricity generated from organic wastewater. Similarly, domestic wastewater of high BOD concentration could be used as an analyte to simultaneously realize wastewater treatment and desalinization. Several studies utilize multiple desalinization chambers in series, separated by alternating AEM and CEM. Cations and anions could migrate to neighboring chambers except for the cathode or anode chamber. Thus, the efficiency was improved compared to that with a single desalinization chamber (Fig. 7.15). MDCs could also be combined with reverse osmosis technology, which first uses MDCs to reduce the salt concentration and at a next step, uses reverse osmosis to treat low concentration brine. It is beneficial to reduce the membrane pollution from the reverse osmosis operation and thus extend the life of the membrane. Electricity generated by the MDCs operating and passed onto reverse osmosis could reduce overall power consumption. In addition to desalinization, MDCs could be applied in purification of heavy metal wastewater as well. In the desalinization chamber, heavy metal ions and salt ions in the wastewater migrate to the cathode chamber and the anode chamber respectively, to achieve the objective of heavy metal removal. e–

e– Resistance

AEM e

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–

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+

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–

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Desalinated sea water Fig. 7.15: Schematic diagram of a MDC coupled with multiple desalinization chambers in series. Chamber A and Chamber B for desalinization.

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7.4.5 Hydrogen production The calorific value of hydrogen (143 Gjton−1 ) is higher than that of all the fossil and biofuels currently used. It is approximately 3-fold, 4-fold, and 4.5-fold of that of gasoline, ethanol, and coke, respectively. Its final product of oxidation is water, which will not cause any greenhouse effect, acid rain, or ozone depletion. Therefore, hydrogen is an efficient and clean energy source, and offers a broad range of applications with good prospects. However, the majority of hydrogen is produced by pyrolysisof hydrogencontaining substances, such as methanol or by water electrolysis. The methods above consume energy and cause carbon emissions. Microbial electrolysis cells (MECs) could be used to realize hydrogen production from biomass, which is attracting increasing attention. The operating principle is: electrogenic bacteria grown on the anodic surface in MECs oxide organic matter and generate electrons, H+ and CO2 . H+ migrates to the cathodic surface through CEM while electrons get to the cathode through the external circuit after collection by the anode. At the cathode, H+ combines with electrons and produces hydrogen. The reaction of the MEC anode is the same with that of an MFC; however, at pH 7, the reaction of the MECs cathode is: H + + e → H2

E = −0.414 V

(7.14)

Standard electrode potentials of the organic matter as electron donors are mostly higher than −0.4 V. Therefore, hydrogen cannot be produced spontaneously. For example, when acetic acid (with a standard electrode potential of −0.28 V) is used as electron donor at the anode, theoretically, another potential of 0.13 V needs to be applied. In the actual reactions, due to overpotential, the necessary potential will still be higher than the theoretical value, but lower than that of electrolysis of water (1.8 – 2.0 V). The energy of hydrogen from decomposition of organic matters in MECs can reach 4-fold that of the input electricity. Thus, MECs for hydrogen production will obtain energy yield from biomass. Rozendal et al. [464, 465] found that the cathode had an overpotential of 0.28 V, while the anode had an overpotential of only 0.04 V in the process of hydrogen production. The cathode performance may provide plenty of room for improvement. Many studies seek to find appropriate cathode catalysts to reduce impressed potential and energy input. Xie et al. [466] made defect-rich molybdenum disulfide UNCD ultrathin nanosheets, at which overpotential was only about 120 mV. MECs can be used as a way of waste reutilization. The fuel sources for MECs could be domestic wastewater, farm wastewater, brewery wastewater, and straw. MECs can also be applied to realize desalinization and coupling hydrogen production. The rates of hydrogen production of the MECs above were generally lower than 1 m3 H2 /m3 d. The main reason for the low production rate can be concluded as: the fermentative bacterium in the anode chamber consumed organic matters but did not generate electricity; methanogens consumed H2 ; H+ losses via internal resistance migrating from anode to cathode. Improvements could be made by changing the anodic microbial

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diversity, thus inhibiting the methanogens, improving the MECs configuration and membrane materials, and reducing the internal resistance. MECs also offer a new path for the development of MFCs. A more negative potential of cathode could be achieved by providing auxiliary power from anode and electricity from an external power supply. In addition to hydrogen production, a reaction could be conducted around the cathode synthesizing ethanol, formic acid, acetic acid, and others, using HCO−3 as substrates, which is called microbial electro-synthesizer. Furthermore, it would achieve more efficient reduction of various heavy metal ions, high inorganic compounds, and complex organic pollutants. In the reactions above, the effect of auxiliary power is the superior reduction of the power consumption compared to when only power supply is applied.

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Index Aerobic treatment, 70 Air gasification, 176 Air-steam gasification, 177 Al2 O3 support, 211 Aldol condensation, 265, 278–280, 282 aldol condensation, 259, 266–268, 272, 273, 275, 276, 278–280 Amorphous alloy Ru, 264 Anaerobic Digestion, 358 anaerobic treatment, 70 anode, 402 anode chamber, 404 application, 98 APR reaction, 271, 273 aqueous catalytic reaction system, 264 Aqueous catalytic reforming, 271 Auto-thermal reforming, 209, 213 Autothermal reforming, 198 bifuncational catalysts, 268 Bio-DME project, 248 bio-oil, 194 bio-oil steam reforming, 194 Bio-oil Steam-Reforming, 125 bio-photolysis, 218 biochar, 147 biodesel engineering, 323 biodiesel, 306 Biological gasoline, 259 Biological hydrogen production, 217 Biological jet fuel synthesis process, 259 Biomass aqueous chemical catalytic technological process, 260 Biomass Carbonization, 158 biomass derivatives, 194 biomass dry distillation gas, 154 biomass gasification, 242 Biomass gasification reactor, 177 biomass gasification technology, 241 Biomass pyrolysis, 88 biomass synthetic liquid fuel, 233 Biosensors, 416 C-C growth, 268 C/N ratio, 355 carbohydrate synthesis polyols, 261 carbonation/calcination cycle, 184 https://doi.org/10.1515/9783110476217-009

Carbonization, 142 Catalyst deactivation mechanism, 123 Catalytic Cracking, 118 Catalytic tar removal, 188 cathode, 402 cathode chamber, 404 Cellulase, 45 Cellulosic ethanol, 81 Characteristics, 90 charcoal, 148 charcoal production, 88 Chemical-looping, 184, 187 chemical-looping, 184 Chronoamperometry, 409 Chronopotentiometry, 410 cleanup and purification, 188 CO removal, 189 Combustion Technology, 129 Compost, 352 Composting, 372 Condensing system, 104 Consolidated biological process, 17 Copper-based catalysts, 201 Coulombic efficiency, 412 Cyclic Voltammetry, 407 Cyclone separator, 103 Dark fermentation, 224 dehydration of carbohydrates, 268 dehydration/hydrogenation, 259, 267, 268, 270, 271, 275, 277 Demo Application, 137 Desalination, 416 Dimethyl ether, 212 Direct Electron Transfer, 394 DME, 245 DME reforming, 213 DME synthesis, 246 dry fermentation, 361 Electrogenesis microorganism, 395 environmental pollution, 67 ethanol steam reforming, 210 Fast pyrolysis, 175 fast pyrolysis, 87 Fixed bed reactor, 319

456 | 7 Index

flash volatilization, 94 fluidized bed, 347 Fluidized bed pyrolysis reactor, 102 formate lysis pathway, 227 FT synthesis, 234 FT synthesis process, 234 fuel gas production, 88 Gasification, 176 Glycolysis, 7 High-octane olefinic motor-gasoline, 286 Higher-chain alcohols, 281 HMF, 259, 265, 268, 276, 277, 279, 280 Hydrogen production, 172 Hydrogen production from ethanol, 205 Hydrogen production from methanol, 199 hydrogenation, 114 hydrolysate, 261 Hydrolysis, 5 Incineration, 369 Indirect biophotolysis, 220 isoparaffinic kerosene, 286, 289 Kitchen Wastes, 373 Landfill Gas, 344 Leachate, 342, 366 Liquid Hot Water, 44 membrane, 404 Membrane reactor, 203 metal-acid bifunctional catalyst, 272 methanol steam reforming, 201 methanol synthesis process, 236 micro-channel reactor, 202 microbial desalination cells, 423 Microbial fuel cell, 387 monosaccharides and oligosaccharides, 259 MSW sorting technology, 379 MTG, 283–285 Municipal Solid Waste, 331 Noble metal catalysts, 201 Non copper-based catalysts, 201 numerical simulation, 204, 216 Oxygen gasification, 176

Partial oxidation, 207, 213 partial oxidation, 198 partial simultaneous saccharification and fermentation, 20 pentose fermentation, 13 Photo-fermentation, 222 Physical and chemical properties, 108 plate reactor, 203 polymerization of light olefins, 286 Preparation, 92 Pretreatment, 100 pretreatment, 41 proton exchange membrane, 404 Protoplast fusion, 15 pyruvate decarboxylation pathway, 227 Raney Ni, 261–263, 265, 269 Raney-Ni, 262 Rare earth metal oxide, 211 Reactor composting, 357 rotary kiln, 349 Ru/C, 263, 269 Sanitary Landfill, 339, 364 Seepage Prevention, 340 separate hydrolysis and fermentation, 20 simultaneous saccharification and fermentation, 20 simultaneous saccharificationand co-fermentation, 61 Single-stage, 359 SMFC, 416 solid heat carrier, 182 sorbitol aqueous reforms, 264 Static bed composting, 356 Steam explosion, 45 Steam gasification, 176 steam reforming, 213 sugar alcohol, 259, 260, 264, 268 Sugar alcohols, 264 sugar alcohols, 267, 268, 271 sugar derivatives, 259 sugar platform, 9 supercritical water, 178 Syngas, 249, 283 syngas fermentation, 17 Technologies for Biomass-based Hydrogen Production, 169 Temperature control, 92

7 Index | 457

The applications of charcoal, 152 Thermochemical routes, 174 Trough composting, 357 Tubular reactor, 203 Two Chambers MFC, 404 Two-stage, 361 two-stage dilute acid hydrolysis, 64 Two-stage screw feed device, 101 Upgrading, 105, 112

Vegetable oil fuel, 291 Vegetable oil producing technology, 298

Waste Incinerator, 346 Waste remediation, 415 Wastes Incineration, 345 water-gas shift reaction, 231 wet anaerobic digestion, 359 Windrow composting, 356

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Green Processing and Synthesis. Hessel, Volker (Editor-in-Chief) ISSN 2191-9542, e-ISSN 2191-9550