Advances in Bioenergy and Microfluidic Applications 9780128216019, 0128216019

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Advances in Bioenergy and Microfluidic Applications Edited by

Mohammad Reza Rahimpour Reza Kamali Mohammad Amin Makarem Mohammad Karim Dehghan Manshadi

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2021 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-821601-9 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals Publisher: Susan Dennis Acquisitions Editor: Kostas KI Marinakis Editorial Project Manager: Amy Moone Production Project Manager: Debasish Ghosh Cover Designer: Greg Harris Typeset by TNQ Technologies

Contributors Osama Abdelrehim

Department of Mechanical and Manufacturing Engineering, Schulich School of Engineering, University of Calgary, Calgary, Alberta, Canada

Yaser Balegh Department of Chemical Engineering, Shiraz University, Shiraz, Fars, Iran

Parisa Biniaz

Department of Chemical Engineering, Shiraz University, Shiraz, Fars,

Iran

Giuseppina Anna Corrente

Chemistry and Chemical University of Calabria, Cubo 15/D, Via P. Bucci, Rende, CS, Italy

Technologies Dpt.,

Francesco Dalena

Laboratory of Industrial Chemistry and Catalysis, University of Calabria, Via P. Bucci, Rende, CS, Italy

Morteza Esfandyari

Department of Chemical Engineering, University of Bojnord, Bojnord, North Khorasan, Iran

Mohammad Farsi Department of Chemical Engineering, Shiraz University, Shiraz, Fars, Iran

Niloufar Fouladi

Department of Chemical Engineering, Shiraz University, Shiraz,

Fars, Iran

Mohammad Gholami

Department of Mechanical Engineering, Ohio University,

Athens, OH, United States

Ali Hafizi Department of Chemical Engineering, Shiraz University, Shiraz, Fars, Iran Sareh Hamidpour

Department of Chemical Engineering, Shiraz University, Shiraz,

Fars, Iran

Ahmad Hassanzadeh

Department of Processing, Helmholtz-Institute Freiberg for Resource Technology, Helmholtz-Zentrum Dresden-Rossendorf, Freiberg, Germany

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xvi

Contributors

Hamid Reza Hosseini Mechanical Engineering Department, Shiraz University, Shiraz, Fars, Iran

Reza Kamali

Mechanical Engineering Department, Shiraz University, Shiraz, Fars,

Iran

Mohsen Karimi

University of Porto, Porto, Portugal

Danial Khojasteh

Water Research Laboratory, School of Civil and Environmental Engineering, UNSW Sydney, NSW, Australia

Hossein Khorshidian

Mechanical Engineering Department, University of Calgary,

Calgary, Alberta, Canada

Mohammad Amin Makarem

Department of Chemical Engineering, Shiraz

University, Shiraz, Fars, Iran

Mohammad K.D. Manshadi

Mechanical Engineering Department, Shiraz

University, Shiraz, Fars, Iran

Mehdi Mohammadi

Department of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, Alberta, Canada; Biological Science Department, University of Calgary, Calgary, Alberta, Canada

Mohammad Hossein Mohammadi

Department of Animal Science, Isfahan University of Technology, Isfahan, Iran; Department of Mechanical and Manufacturing Engineering, Schulich School of Engineering, University of Calgary, Calgary, Alberta, Canada

Leila Karami Monfared Mechanical Engineering Department, Yazd University, Yazd, Iran

Zohre Moravvej Department of Chemical Engineering, Shiraz University, Shiraz, Fars, Iran

Mozhgan Naseh

Department of Mechanical and Manufacturing Engineering, Schulich School of Engineering, University of Calgary, Calgary, Alberta, Canada

Hamed Nikookar

Mechanical Engineering Department, Shiraz University, Shiraz,

Fars, Iran

Fabrizio Olivito

Department of Chemistry and Chemical Technologies, University of Calabria, Cubo 12C, Lab. LabOrSy, Arcavacata di Rende, Cosenza, Italy

Contributors

xvii

Emilia Paone

Dipartimento di Ingegneria Civile, dell'Energia, dell'Ambiente e dei Materiali (DICEAM), Università degli Studi “Mediterranea” di Reggio Calabria, Reggio Calabria, Italy; Dipartimento di Ingegneria Industriale (DIEF), Università degli Studi di Firenze, Firenze, Italy

Mehdi Piroozmand

Department of Chemical Engineering, Shiraz University,

Shiraz, Fars, Iran

Mohammad Reza Rahimpour

Department of Chemical Engineering, Shiraz

University, Shiraz, Fars, Iran

Alírio E. Rodrigues University of Porto, Porto, Portugal Tayebe Roostaie

Department of Chemical Engineering, Shiraz University, Shiraz,

Fars, Iran

Farideh Salimian

Mechanical Engineering Department, Yazd University, Yazd, Iran

Amir Sanati-Nezhad

Department of Mechanical and Manufacturing Engineering, Schulich School of Engineering, University of Calgary, Calgary, Alberta, Canada

Mohammad Amin Sedghamiz

Department of Chemical Engineering, Shiraz

University, Shiraz, Fars, Iran

Alessandro Senatore

Chemistry and Chemical Technologies Dpt., University of Calabria, Cubo 15/D, Via P. Bucci, Rende, CS, Italy

Nazanin Abrishami Shirazi

Department of Water Engineering, Shiraz University,

Shiraz, Fars, Iran

José A.C. Silva

Institute of Polytechnic of Bragança (IPB), Bragança, Portugal

Ebrahim Soroush

Department of Chemical Engineering, Shiraz University, Shiraz,

Fars, Iran

Shahram Talebi Antonio Tursi

Mechanical Engineering Department, Yazd University, Yazd, Iran

Department of Chemistry and Chemical Technologies, University of Calabria, Arcavacata di Rende, Cosenza, Italy

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Contributors

Ali Behrad Vakylabad

Department of Materials Science, International Center for Science, High Technology & Environmental Sciences, Kerman Graduate University of Advanced Technology, Kerman, Iran

Federica Verteramo

Chemistry and Chemical Technologies Dpt., University of Calabria, Cubo 15/D, Via P. Bucci, Rende, CS, Italy

Gurkan Yesiloz Department of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, Alberta, Canada

Biomass conversion: general information, chemistry, and processes

1

Antonio Tursi1, Fabrizio Olivito2 1

D EP AR T ME NT O F C HE MI S TR Y AN D CH E MI C A L T ECHNO LOGIES, UNIVERSITY OF C ALABRIA, ARCAVACATA DI RENDE, C OSENZA, ITALY; 2 DEPARTME NT OF CHEMISTRY AND CHEMICAL TECHNOLOGIES, UNIVE RS ITY OF CALABRIA, CUBO 12C, LAB. LABORS Y, ARCAVACATA DI R E N DE , C O S E NZ A , I T AL Y

1. Introduction The exploitation of fossil fuels grew up incessantly and now renewable alternatives are getting necessary. Many states allocated funds in search of sustainable alternatives to finite resources [1]. Biomass was proved to be the key of this passage because it has many applications in the field of bioenergy, biofuel, biomaterials, and so on [2,3]. Today biomass conversion is related to a low environmental impact because it is well known the damage that fossil fuels conversion caused in the past years [4]. The growing concern diffused worldwide pushes the institutions of many states to draw up agreements and restrictions on environmental pollution and renewable energy. Biomass plays a central role that is certified in many documents of different countries, such as the “Biomass Action,” planned by the European Commission and the Multi-Year Plan by the US Department of Energy [5]. The main theme of the first manuscript is the reduction of CO2 emissions in agreement with the Kyoto protocol [6] to face up the problems related to global warming. The second document discusses the policies about agriculture and energy, with the focus on the conversion and valorization of renewable resources for fuels, biodegradable plastics, and useful materials production, through research and public interactions. The exploitation of biomass for bioenergy production represents an opportunity for agriculture and farmers, for a more productive and advantageous use of land, that can produce improvement of farmers’ income, containment of the countrysideecity migrations, and conservation of the environment and rural culture. Climate change today is well recognized from all the countries, and it is no longer a hypothesis but an existing reality; for this reason, the scientific community created the International Panel on Climate Change (IPCC), in 1988, intending to limit the production

Advances in Bioenergy and Microfluidic Applications. https://doi.org/10.1016/B978-0-12-821601-9.00001-7 Copyright © 2021 Elsevier Inc. All rights reserved.

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4 Advances in Bioenergy and Microfluidic Applications

of greenhouse gases [7]. These types of gases are today reduced because of the less exploitation of fossil fuels. First- and second-generation biofuels, heat, and electricity are today produced by common practices worldwide [8]. Tons of bioethanol is produced annually from biomass resources, and the demand is still rising. Biomass is considered renewable because, after consumption, feedstocks are available in a short time due to the fast plants growing. In addition, atmospheric CO2 is absorbed from plants for photosynthesis, in which CO2 and water in the presence of sunlight are converted to carbohydrates and oxygen. Plants are the main source of biomass, and their production does not affect the level of carbon dioxide in the environment, but in contrast they are able to reduce it [9]. The only way to produce carbon dioxide from biomass is combustion, but this remains a closed cycle because the produced CO2 is necessary for the plants growing and consequently to accelerate the production of new biomass that is employed to start a new cycle. These steps represent the CO2 cycle in the atmosphere (Fig. 1.1). The energy produced this way is environmentally friendly and fully renewable. Biomass is any materials derived directly or indirectly from photosynthesis. Today, these definitions remain ambiguous because the term is wide and complicated due to the different plant species, cultivation, geographical position, harvest period, and so on. For these reasons, biomass is defined and classified depending on the particular type of application or the relevant legislation. The term biomass brings together an extremely wide and heterogeneous list of natural materials [10]. The most important sources of biomass (shown in Fig. 1.2) are agricultural, forestry and animal residues, algae and aquatic crops, urban solid waste, and all waste produced by human activities if they store an energy potential that can be exploited through conversion processes [7]. In particular, agricultural waste represents the most advantageous biomass category, as they contain a high quantity of convertible lignocellulosic components [11]. Earth has an enormous amount of biomass available. Recent estimates have predicted that the total exploitable biomass is around 5 billion tons, present both on land and in submerged areas.

FIGURE 1.1 Carbon cycle in biomass.

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Biomass conversion: general information, chemistry, and processes

5

FIGURE 1.2 The most important biomass sources.

Estimates translate, from the energy point of view, to a potential production capacity of 33,000 exa-joules (EJ), which corresponds to almost 100 times the annual energy consumption in the world [12]. However, the exploited biomass is equal to 15% of the total available, reaching around 56 million TJ/year (tera-joules per year), corresponding to 1.230 Mtoe/year (million tons of oil equivalent per year). Furthermore, the use of biomass to produce renewable energy has a heterogeneous distribution on the planet. In particular, developed countries produce on average only 11% from biomass compared to the total energy produced, while developing countries reach values of 50% of the total energy requirement [13]. The United States, for example, uses renewable resources, derived from biomass, for only 3% of the country’s energy needs, corresponding to about 3 million TJ/year (about 60 Mtoe/year). Europe gets about 4% of total energy from biomass (about 50 Mtoe/year), while some Nordic countries far exceed the European average, reaching 20% [14,12]. According to the latest estimates (Eurobarometer of solid biomass 2018), recently published by EurObserv’ER, in 2017 the European solid biomass sector produced 100 Mtoe of primary energy, deriving from forest and agricultural waste, and by-products of the paper industry. Italy is the fourth European consumer in terms of primary energy but does not have a virtuous production role. About 1.3 Mtep (million-ton equivalent of petroleum), compared to 9 Mtep of primary energy from solid biomass consumed in 2017, comes from imported biomass. Furthermore, less than 50% (about 1200 TWh (terawatt per hour), compared to over 4000 TWh of electricity generated with solid biomass,

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were produced in cogeneration plants; the rest was produced in plants without recovery, thus wasting about 60% of the primary energy of the biomass as heat dissipated to the atmosphere [13]. In Europe, the countries that use biomass for energy production are Germany (12.4 Mtoe), France (10.8 Mtep), Sweden (9.3 Mtep), Italy (9 Mtep), and Finland (8.6 Mtoe). All the countries mentioned are self-sufficient, with the exception of Italy and Germany, which imports 1.3 and 0.4 Mtep, respectively. As for the direct consumption of biomass for domestic heating, Italy is in third place (6.9 Mtoe, or 76.6% of total consumption), after Germany (9.24 Mtoe) and France (8.65 Mtoe). The most virtuous countries in the use of solid biomass for the generation of electricity, or those where only cogeneration is used, are as follows: Sweden, Poland, Denmark, Latvia, Lithuania, Slovenia, Croatia, and Luxembourg. England is instead the first producer of electricity from biomass, but 100% of its production comes from the combustion of biomass using old converted coal-fired plants [13]. Countries such as Sweden, Denmark, and Finland have enacted laws to eliminate coal-fired electricity generation by 2030. In particular, Sweden has planned to achieve carbon-neutrality for the entire nation by 2045, increasing the exploitation of forest biomass for cogeneration [12]. In connection with all these things, the use of biomass for energy production is, at present, the most desirable form for protecting the environment, human health, and the world economy; for this reason, using them efficiently and sustainably can significantly reduce gas emissions and the greenhouse effect. In fact, the amount of carbon dioxide that results from their transformation for energy production counterbalances that previously absorbed. Despite these considerations, the political commitment to exploit this renewable resource continues to be limited [15].

2. Chemical characterization of biomass Biomass is a general definition that embraces different renewable resources naturally available [16,17]. These feedstocks are generated from plants and as a consequence can be produced also from animals or humans in the form of wastes. This organic matter is the final product of the plant photosynthesis in which the obtained carbohydrates can assemble themselves to build up the structure of the plant body [18]. The chemical composition depends on many factors, for example not only the kind of plant but also the harvest period and the geographical position. Regarding biomass derived from animals manure, it is important for the species of animals due to their different digestion enzymes. Naturally, the same is true for human wastes that are so diverse. The three main components of lignocellulosic materials are cellulose, hemicellulose, and lignin, and their percentage composition can differ due to the previously reported reasons [19]. The percentage of these three constituents can be referred to as an average, where cellulose is the main component with 40%e50% of the dry weight, hemicellulose the

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Biomass conversion: general information, chemistry, and processes

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second with 25%e30%, and lignin generally represents the less abundant with 15%e30% [20]. The chemical reactivity is strongly dependent on this percentage because the structures of these three components differ from one to another. Hemicellulose due to the more disordered structure decomposes at a lower temperature than cellulose. Lignin is the most stable component, consisting of phenyl propane units organized in an amorphous and branched rearrangement. It has a decomposition temperature that can arrive until 500 C [13]. Other components included in biomass are starch, proteins, and inorganic and organic materials. A discussion of the components will follow in the next paragraphs.

2.1

Cellulose

Cellulose is a polymer of b-D-glucopyranoses units linked by b 1e4 glycosidic bonds, and the range is between 5,000 and 10,000 units. These linear chains are stacked together thanks to hydrogen bonds. Cellulose is quite stable due to the huge number of intra- and intermolecular hydrogen bonds and additional Van der Waals forces. It can exist in two forms, crystalline cellulose that possesses a neat structure and amorphous cellulose. The molecular formula is (C6H12O6)n (n indicates the degree of polymerization) and the structure is shown in Fig. 1.3. Cellulose is widely present in nature, in fact, it is the most abundant organic compounds. Cellulose is naturally abundant in the composition of all the plants present in nature, but it is also largely present in useful materials such as cotton and wood with a percentage, respectively, of 90% and 50% [21]. The presence of intramolecular hydrogen bonds and intermolecular hydrogen bonds between different glucose units strongly affects the reactivity and morphology of cellulose.

H OH

H

H H

O

HO

O

H

OH

H H

H H

OH

H

HO

H

OH

O

OH H

H

O

H OH

H

HO O

HO

H H

OH

H H

H

OH

FIGURE 1.3 Cellulose structure.

O n

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The consequence is the stability and rigidity of the structure. In addition, hydrogen bonds can force cellulosic chains to a crystalline arrangement. Crystalline and amorphous cellulose ratio affects the accessibility of this polymer. Cellulose reactivity is related to primary and secondary hydroxyl groups. Crystalline structure is less accessible than amorphous cellulose in terms of the freedom of the two previously mentioned groups. In addition, it is well known that the reactivity of primary hydroxyl groups is higher with respect to the secondary ones because the first mentioned are less hindered than the second [22]. The linear polymeric chains can arrange in a stacked form together and assume a crystalline or amorphous structure (Fig. 1.4), but this entire organic matter is present in the form of fibrils and microfibrils that constitute the supramolecular structure. The regular and highly ordered array of crystalline cellulose was proved by many studies of X-ray diffraction, with a density value of 1.6 g/cm3. In contrast, in the amorphous structure, the distance between the chains and the single molecules is higher and as a consequence the density value is lower (1.5 g/cm3). The ratio crystalline/amorphous structure is variable and can be expressed only as an average value between 30% and 80% [23]. Amorphous cellulose for the previously mentioned reasons is the most accessible part, and chemical or enzymatic attacks occur easily with respect to crystalline cellulose. Cellulose can be totally or partially hydrolyzed. The total hydrolysis of cellulose can be reached generally using harsh conditions such as strong inorganic acid like sulfuric and hydrochloric acid, and high temperatures until to 400e500 C. In this case, single units of D-glucose are produced. Generally, with milder reaction conditions, the hydrolysis of cellulose is partial and the decomposition products are disaccharides (cellobiose) and polysaccharides. The crystalline portion is more hydrophobic than the amorphous part, but it is possible to make this material more hydrophilic (for example the common cotton commercially available) using procedures such as mercerization, ultrasonic, or microwave pretreatments [24]. In the past years, the advancements of research and industry focused on cellulose application and conversion grew up fast. Cellulosic materials represent today one of the most important energy source derived from renewable materials. Cellulose depolymerization or breaking produce important platform molecules such as glucose, furanic

FIGURE 1.4 Amorphous and crystalline areas of cellulose.

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Biomass conversion: general information, chemistry, and processes

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compounds, alcohols, hydrocarbons, carboxylic acids, and others that found many applications in the field of biofuel, manufacture, polymers, pharmaceuticals, and agriculture. Moreover, its functionalization allows to obtain materials with high purifying capacity with respect to different types of pollutants in water [25,26,27,28e30].

2.2

Hemicellulose

Hemicellulose differs from cellulose for two main reasons, the first is the branched rearrangement that differs from the linear rearrangement of cellulosic chains (Fig. 1.5), the second is the heterogeneous polysaccharide character, due to the different types of sugars such as glucose, mannose, xylose, galactose, and arabinose together with 4-Omethyl glucuronic acid and galacturonic acid units. Also, the chemical composition and content of this component is related to the type of plant. Hemicellulose is associated with the surface of cellulose microfibrils. It decomposes in a range of temperatures between 180 and 350 C to give noncondensable gas, coal, and a variety of ketones, aldehydes, acids, and furans. Importantly, hemicellulose differs from cellulose for its hydrophilicity when treated with water. One of the main reasons is the amorphous character that makes the structure more accessible to water molecules. This behavior gives important properties like adhesivity and after dehydration it tends to cement. For the previously mentioned reasons, the main constituents of lignin belonged to these classes: xylans, mannans, galactans, and arabinogalactans, which are introduced briefly in the following paragraphs [31].

2.2.1 Xylans Xylans are polysaccharides that possess for plants the same structural function of cellulose for their cell walls (Fig. 1.5). Differently from cellulose, it is composed of D-xylose instead of D-glucose units, with traces of L-arabinose. The structural array is linear, with the main backbone composed of D-xylose units linked by b-1,4 linkages and branched substituents. These substituents can form ramifications in different ways and different positions, the mains are L-arabinofuranose linked to the 0e3 positions of D-xylose residues and D-glucuronic acid, acetyl esters, or 4-O-methyl-D-glucuronic acid linked to the 0e2 position. Also, other groups can be linked to these units, like ferulate groups. Also, in this case, the composition of this constituent is strongly associated with the origin of the plant, such as the harvest period, the climate, the geographical position, and the botanical origins [32].

2.2.2 Mannans Mannans are another main component of hemicellulose. They are composed of mannan, galactomannan, glucomannan, glucuronic acid mannan, and other compounds (Fig. 1.5). Although mannans are composed of mannose units linked by b-(1 / 4) bonds, glactomannans are composed of galactose units linked by b-(1 / 6) bonds. Glucomannans are composed of glucose and mannose units, in which the mannose hydroxyl group can also

10 Advances in Bioenergy and Microfluidic Applications

FIGURE 1.5 Chemical structures of hemicellulose, xylanes, mannans, and their derivates.Ă

Chapter 1
Biomass conversion: general information, chemistry, and processes

11

be acetylated, linked by b-(1 / 4) bonds, generally with the two components in a ratio 1:3, where the linear chain is sometimes connected with a branched galactose unit. The chemical composition of the cell wall is composed of glucuronic acid. It is composed of mannose units in the main chain, linked by b-(1 / 4) bonds, with the ramification of glucuronic acid substituents bonded by b-(1 / 2) linkages [33].

2.2.3 Galactans Galactans are another type of polysaccharides biopolymers, less abundant respect to the others, but they can be easily found in larch trees as arabinogalactans. These are composed of galactose units connected by 1,3 and 1,6 linkage. Specifically, arabinogalactans are particularly abundant in larch bark (Larix occidentalis), while red seaweeds contain sulfated galactans. Sulfated galactans are composed of linear chains of galactose with interval residues of 3-b-D-galactopyranosyl and 4-a-galactopyranosyl or anhydrogalactose pyranosyl. In addition, galactans can be extracted from algae (agar), larch (3-galactan), alfalfa seeds (a-galactan), and yellow lupin seeds (b-galactan). The structure of galactans can be linear or complex. Generically, they are composed of linear chains of galactoses but some of them can be in a different form. The backbone is commonly a linear chain of b-D-galactopyranose residues linked through positions 1 and 3 and a-D/Lgalactopyranose residues linked through positions 1 and 4 that can be present in an alternate form. The complexity depends on the way that the structure can be ramified with other natural sugars such as xylose, glucose, arabinose, mannose, that can carry methyl ether groups, sulfate hemiester groups, and pyruvic acid in the form of a cyclic chetal [34].

2.3

Lignin

Lignin is the third and the less abundant wood component, with an average of 15%e30% of the dry weight. In nature, it is associated with cellulose, and it possesses the function to protect the plant structure from chemical or enzymatic attacks. The stability is higher than cellulose and hemicellulose; for this reason, the thermal decomposition occurs between 280 and 500 C. This particular stability is related to the structure. Lignin is composed of phenyl propane units, with an amorphous and highly branched structure. The composition of these units is not constant but changes depending on many factors like the type of plant, the botanical origin, and many others. Biomass and lignocellulosic material are commonly lignin deprived before the conversion to energy or chemicals. Lignin is commonly removed by mechanical or chemical methods [35]. Chemical methods make use of strong alkali or acidic agents [36]. An innovative invention in this field is the use of ionic liquids that allow using milder conditions also in terms of times, solvent, and reagents disposal. The content of lignin depends on the plant species and age. For example, the range is close to 25% up to 50% for hardwoods like, for example, ebony [37]. The elemental composition expressed as an average is 61%e65% carbon and 5%e6% hydrogen, and the remaining part is composed of oxygen. From a structural point of view,

12 Advances in Bioenergy and Microfluidic Applications

the phenyl propane units can be connected in different ways. The connection can exist between phenyl and propyl groups both through oxygen or carbonecarbon bonds [38]. Lignin structure derives from a radical oxidative polymerization of three hydroxycinnamyl alcohols that represent the basic monomer of the structure: p-phenyl monomer (type H), guaiacyl monomer (type G), and siringyl monomer (type S), deriving from coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, respectively (Fig. 1.6). In addition, these units can differ among them with respect to the degree of methoxylation. This structural architecture is composed of many polar groups like hydroxyl groups, which can be interconnected through many hydrogen bonds both in the form of intermolecular and intramolecular. These features explain the great rigidity of this polymer and the inability of dissolving in the most common solvents. One of the common chemical treatments to dissolve lignin makes use of strong alkaline solutions [39]. In the case of lignin, there is a direct correlation between molecular weight and the range of the softening temperature. Generally, the average range for the softening point of dry lignin is between 127 and 129 C. In contrast, moisture content, and consequently water content is inversely correlated with a softening range of temperature. The higher water content generally corresponds to a lower range of softening temperature, because water acts as a plasticizer. In terms of structural resistance of plant cell walls, the more lignin quantity the higher the mechanical resistance, the phenomenon of lignin infiltration is called lignification.

2.4

Starch

Starch, together with glycogen and cellulose, is one of the main plant’s components. It is biocompatible, biodegradable, and bioadhesive. Cellulose is produced by photosynthesis, and for plants, it represents the main energetic reserve of carbohydrates. Starch is very widespread in nature, for example, in seeds, fruits, roots, tubers, and in a high percentage in corn, wheat, potatoes, and rice. Starch is a polysaccharide composed of many molecules of glucose connected together by glycosidic bonds. It is a white, odorless, and tasteless powder, commonly in granular form. The molar mass is variable. It is composed of molecules of amylose and amylopectin that are both made up of glucose units. In nature, the quantity expressed as an average is 25%e27% for amylose and 73%e75% for amylopectin. This kind of compounds differ in structure, while amylose consists of a linear chain of glucose units connected by a-(1,4) glycosidic bonds, amylopectin possesses a branched chain of glucose molecules linked linearly with a-(1,4) glycosidic bonds and a-(1,6) bonds at intervals of 24e30 glucose subunits. Starch is a polysaccharide composed of D-glucose units; for this reason, it enters the group of a-glucans. Amylopectin can be obtained in the isolate form, from “waxy” corn starch, while amylose can be isolated by the hydrolysis of amylopectin with pullulanase. Amylopectin differently from amylose is more soluble in water and easier to digest. The difference is due to the presence of more ending points in amylopectin for the enzymatic attack [40]. Amylose structure and properties are discussed in the following paragraph.

Chapter 1
Biomass conversion: general information, chemistry, and processes

FIGURE 1.6 Chemical structures of lignin, amylose, and amylopectine and their derivates.Ă

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2.4.1 Amylose Amylose is a linear polysaccharide composed of D-glucose units connected by a-(1e4) bonds, in the range of 500e20,000 units (Fig. 1.6). The structure adopts a helical conformation. It is divided in the form A, B, and V. The A and B forms present lefthanded helices, with six glucose units per turn, so the only difference between the two forms is the change in the packing of the starch helices. The V form is obtained through cocrystallization processes assisted by iodine, alcohols, fatty acids, and dimethyl sulfoxide. The resistance of amylose to starch is due primarily to the hydrogen bonds between aligned chains that cause a phenomenon of retrogradation, and this leads to viscosity increase and precipitation of amylose particles. Potentiometric titration is a common method for the determination of the percentage of linear components in starch. It is possible thanks to the affinity between amylose and iodine. Another technique for the quantification of amylose percentage is the colorimetric test. Solutions of amylose and iodine result in a deep blue-purple color that can be measured through the absorbance usually at 620e680 nm [41].

2.4.2 Amylopectine Amylopectine is found as the major component in starch granule, and the structure is highly branched and composed by glucose units linked by a(1e4) bonds with branch points linked by a(1e6) connections (Fig. 1.6). Starch granules can present crystalline domains, due to the amylopectin chains that are packed together. In the amorphous region, the main components are free amylose, amylose complexed with lipids, and branch points. The difference between the two regions is not so straightforward. The remarkable difference is the susceptibility of the crystalline region to enzymatic hydrolysis, water penetration, and other processes. Amylopectin varies depending on the type of starch. For example, waxy type contains 100% amylopectin. The most important features of starch such as retrogradation, gel stability, gelatinization, and viscosity are related to the amylose/amylopectin ratio. Even though amylopectin is the major component of the starch granule, a reliable method for the direct analysis of amylopectin still lacks. Combined preparative and analytical size exclusion chromatography with multiple detection and enzymatic debranching and precipitation techniques represent the only method for the analytical study of the microstructural properties of amylopectin. Studies proved that the average molecular weight (MW) of amylopectin is between 107 and 108 Da [42]; in particular, Bule´on et al. [43] estimated that the number of amylopectin chains in a single starch granule is 5.4  107. Amylopectin can hydrolyze, during the process of seed germination, with the break of the a(1 / 4) bonds by the aand b-amylase enzymes and the relative production of dextrins that are successively attacked by other enzymes called dextrinases. This process occurs together to the degradation of amylose and leads to the fragmentation of all starch into smaller units of glucose and maltose [44].

Chapter 1
Biomass conversion: general information, chemistry, and processes

2.5

15

Other minor components in biomass

Biomass also comprises minor components such as waxes, alkaloids, pigments, and terpenes, but also inorganic substances like ash, which differ in the composition depending on the type of raw material. Also, primary metallic elements such as magnesium, phosphorous, aluminum, calcium, sodium, silicon, iron, and potassium belong to this category. Biomass materials can contain different quantities of minor organic components because the number of these species is extremely high. Differently from a composition point of view, these molecules can affect significantly the way by which these materials needed to be processed. The following list encloses the main important:





1%e40% of lipids, 5%e70% of proteins, and up to 5% of nucleic acid in algae [45]. 2%e5% of acetyls in some straws and flax [46]. 1%e5% of uronic acids in pine, eucalyptus, and sorghum grass [47]. Up to 10% of proteins in pine, reeds, spruce, birch, and maize [48,46].

2.6

Fluid matter

Another type of material that can derive from biomass is the fluid matter. It is an aqueous solution that contains different types of cations and anions. The moisture content in biomass materials is generally in the range of 10%e60% or even higher in some species of raw biomass. Crops that grow up fast, for example, have a higher content of water together with many mobile elements such as Na, Ca, K, Mg, N, P, Cl, and S [49].

2.7

Inorganic components

Together with different kinds of crystallized minerals, the inorganic matter is present in a percentage around 7%. Among the most common solid residues, almost all watersoluble, there are chlorides, nitrates, sulfates, oxalates, and both organic and inorganic amorphous materials. During the common biomass pretreatment for drying the materials, ash conditions are usually required and make use of high temperature, up to 750 C, bringing to the evaporation of common elements such as carbon, hydrogen, oxygen, nitrogen, and sulfur in the form of gaseous compounds, differently from the ash residues that contain mineral elements in the form of oxides [50].

3. Biomass classification Biomass is a general definition that includes an extremely high number of materials, and the complexity is referred to the different origins and the different compositions. The type of biomass subdivision can be done on the base of the scope of the analysis. There are two general ways useful to classify biomass:

16 Advances in Bioenergy and Microfluidic Applications

1. Classification based on the type of biomass existing in nature; 2. Classification based on the industrial value such as uses, applications, and commerce; The first is so wide that there are many groups of classification and also different subgroups for each. Natural sources such as crops, trees, and grass are extremely varied from a botanical point of view. Biomass conversion is related to agriculture, food, energy, biofuel, additive, bioplastics, dyes, and many other purposes. This is a multidisciplinary subject, and it is often misunderstood in meaning by the academic or industrial community because incorrect interpretations are very common. In this paragraph, the effort consists of trying to summarize this topic and divide it based on all the sources available in the environment, not only focused on the sources of origin but also on the chemical characteristics that are summarized in Table 1.1. Here is reported a general list: 1. 2. 3. 4. 5.

Wood and woody biomass Herbaceous biomass Aquatic biomass Animal and human biomass wastes Biomass mixtures The different types of biomass are discussed in the next paragraphs.

3.1

Wood and woody biomass

It derives from trees, and this material includes mainly roots, bark, and leaves of woody shrubs. The main source is represented by forest residues, scrap wood like for example sawmill. The main components of woody biomass are carbohydrates and lignin. One of the main applications is the production of H2 by steam gasification using CaO as a CO2 sorbent. Woody biomass is one of the most useful carbonaceous feedstock for H2 production by steam gasification. In addition, it can be converted into energy using classical combustion or other processes [49]. Below is reported a shortlist of the main primary sources from which biomass for energy production derives:

Table 1.1

Biomass classification groups [51,13].

Biomass group

Varieties and species

Wood and woody biomass Herbaceous biomass Aquatic biomass

Coniferous, deciduous, angiospermous, gymnospermous, stems, branches, foliage, chips, pellets, sawdust, and others from various wood species. Grasses, flowers, straws, fruits, shells, seeds, bagasse, food, pulps, cakes, and others. Marine algae, freshwater algae, macroalgae, microalgae, lake weed, water hyacinth, and others. Bones, meat-bone meal, fertilizers, animal and human dung, manures, and others.

Animal and human biomass

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1. 2. 3. 4.

17

Production residues; Residues of nonmerchant timber; Postconsumption wood waste; Urban and agricultural waste.

Woody biomass among the renewable energy source is at the top of the list, for importance, economic value, and availability. At the end of 2010, the energy conversion of woody biomass produced 30 EJ/year, 16 EJ/year used as household fuelwood and 14 EJ/year for industrial uses [52].

3.2

Herbaceous biomass

Herbaceous biomass was defined by European standard 14961-1 as follows: Herbaceous biomass is from plants that have a non-woody stem and which die back at the end of the growing season. It includes grains or seeds crops from food processing industry and their by-products such as cereal straw. Chum et al. [53].

Herbaceous biomass belong to two general categories: agricultural residues and energy crops, each of these can be further divided into other subgroups [13]: 1. Agricultural residues can also derive from forests or food. These by-products can be collected and used for different purposes like animal feed. They are not usually well characterized from a botanical and chemical point of view, so they can be treated just like a generic material. This way is difficult to predict the availability of certain material in a defined region for energy conversion and other bioenergy applications. 2. Energy crops enter the field of bioenergy and can possess many applications, the most are still undiscovered and for this reason, herbaceous biomass has great potential for the future. The continuous demand for fossil fuels is encouraging this sector of renewable feedstocks because it is a green and high-value alternative both for economic point of view and for the environmental point of view.

3.3

Aquatic biomass

This category includes microalgae, macroalgae, and emerging plants [13]: 1. Microalgae are defined as unicellular photosynthetic microorganisms, can be prokaryotic or prokaryotic, usually found in both marine and freshwater, that are able to convert sunlight, water, and carbon dioxide to algal biomass. They are promising biodiesel feedstock that belonged to the third-generation fuels. First-generation fuels are related to ethanol production from agricultural products, in contrast to second-generation biofuels that are not produced from food crops but from

18 Advances in Bioenergy and Microfluidic Applications

lignocellulosic biomass, like, for example, agricultural and forestry wastes. The last category of third-generation biofuels is commonly produced from marine algae, solid wastes, or sewage sludge [54]. The main components are proteins, carbohydrates, and lipids that are stored inside the cells. Microalgae possess many advantages with respect to food crops. These plants are very adaptable to different environments, and the rate of growth is extremely fast. The relationship between microalgae and biodiesel is due to the high content in lipids due to the fact that with a low content of oxygen, and they can be easily converted to hydrocarbons diesel-like compounds. There are also other minor components in the composition, like pigments such as phycobiliproteins, chlorophylls, and carotenoids, which can be used in industries such as pharmaceuticals, food, and cosmetics. Phosphorous and nitrogen are their main nutrients but they can absorb also other substances like pollutants in wastewater. 2. Macroalgae is a general term referred to seaweed or marine algae that are visible to the naked eye. These are also photosynthetic plants that convert sunlight into nutrients. Distribution and composition are highly variable depending on the geographical position. They make part of the structure of many seagrass families. They are important because they can sequester CO2 mitigating climate change and ozone layer hole growing. At the same time, they can furnish renewable feedstocks for food, fuel, pharmaceutical products, or bioenergy. 3. Emerging plants are those that grow partially submerged in marshes and swamps. Algae are a huge category that embraces 55,000 species and over 100,000 strains. They are a high-value form of biomass because they transform sunlight, water, and CO2 into many metabolites and chemicals [55]. Another great advantage of this thirdgeneration biofuel is that they are not competitors of food crops and generally grow up apart fastly and produce a much higher quantity of renewable materials per hectare compared to land crops. Anyway, the growth of these plants is affected by some parameters such as levels of irradiation, CO2 and O2 concentration, temperature, pH, salinity, and nutrients [56].

3.4

Human and animal biomass wastes

This class includes a wide range of mainly animal waste, which can be used as energy sources. The most common sources are bones, meat meal, various fertilizers, animal, and human dung [49] as indicated in Table 1.1. The most important processes to convert biomass wastes in energy are combustion, gasification, and biogas. Combustion is generally used for large industrial applications related to the fuel industry [57]. Gasification apparatus produce a gas that can be burned in a gas or diesel engine to furnish electricity or motive power or burned to provide heat. Biogas is commonly a mixture of methane, carbon dioxide, and hydrogen sulfide that is produced by fermentation of human and animal wastes in the absence of oxygen [58]. Animal wastes represent an important resource of organic matter. This means that these types of

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19

materials can be used for energetic uses and also as fertilizer to enrich the soil for agricultural purposes. A medium-size cattle produces on average from 4 to 6 tons of dung per year. To convert dung wastes into useful energy, one of the main procedures consists of combustion. This type of process possesses many environmental problems due to the release of harmful gases such as carbon monoxide, fine particulates, nitrogen dioxide, and hydrocarbons. Long-term exposure is related to many pathologies such as cancer and respiratory infections [59]. One of the most important green processes to convert animal dung is the production of methane by anaerobic digestion. Anaerobic digestion reduces air and water pollution and inactivates pathogens and parasites. Biogas production today represents a valid alternative to fossil fuels in terms of energy gains and reduction of health and environmental risks.

4. From biomass to bioenergy production Obtaining energy from biomass allows the exploitation of waste from human activities, reducing dependence on fossil sources. Biomass, through the treatment and conversion processes, supplies various products with high energy power. Generally, biomass sources can be reused mainly for three purposes: 1. Direct production of biological fuels (biofuel); 2. Generation of electrical and thermal energy (biopower); 3. The realization of chemical compounds (bioproduct). The choice of biomass conversion technology essentially depends on the characteristics of the starting material, the required renewable final product, the amount of biomass available, and the costs of the process. Generally, lignocellulosic materials, such as forestry, industrial, and agricultural waste represent the excellent raw material for the production of renewable energy. Unfortunately, the structural physicochemical characteristics and their composition hinder the hydrolysis of the lignocellulosic polymers present in the biomass in simpler molecules that can subsequently be converted into fuels and bioproducts. For this reason, pretreatment processes, described in the following paragraphs, are necessary to make lignocellulosic polymers more suitable for hydrolysis and conversion processes.

5. Pretreatment processes of lignocellulosic biomass The aim of the lignocellulosic materials pretreatment process, as previously introduced, is to remove the lignin and hemicellulose covering the cellulose, reduce its crystallinity, increase porosity, improve the formation of sugar units through hydrolysis, and avoid the degradation of biomass in inhibitory by-products for subsequent conversion processes. The pretreatment methods can be divided into different categories such as physical, chemical, physicalechemical, biological, and electrical pretreatment. These

20 Advances in Bioenergy and Microfluidic Applications

pretreatment processes can also be used in combination to improve yields. The most used pretreatment technologies for biomass exploitation are shown in Table 1.2.

6. Conversion technologies Biomass, following the pretreatment processes, is prepared to be converted. The processes of biomass energy conversion can usually be classified as follows: 1. Thermochemical conversion of biomass; 2. Biochemical conversion of biomass; 3. Physicochemical conversion of biomass. Table 1.3 shows the main conversion technologies.

6.1

Thermochemical conversion

Thermochemical processes are used to convert biomass into renewable fuels with a high calorific value compared to other fuels obtained with other conversion processes. The thermochemical conversion of biomass produces a great variety of solid, liquid, and gaseous fuels with considerable advantages from an industrial and environmental point of view.

6.1.1 Combustion Direct combustion is the most convenient and widely used technology to convert biomass into heat. Combustion is an exothermic reaction between fuel and oxygen to form mainly carbon dioxide, water, and heat, as reported in the approximate chemical Eq. (1.1): Table 1.2

Pretreatment process for lignocellulosic biomass exploitation [51,13].

Pretreatment

Type

Mechanical

Milling Ultrasonic Liquid hot water Weak acid Strong acid hydrolysis Alkaline hydrolysis Organosolv Oxidative Room temperature ionic liquids (RTIL) Steam explosion AFEX CO2 Mechanical/alkaline pretreatment Biological hydrolysis

Chemical

Chemical/mechanical

Biological

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Biomass conversion: general information, chemistry, and processes

Table 1.3

21

Biomass conversion technologies [13].

Conversion category

Conversion process

Thermochemical

Combustion Pyrolysis Gasification Liquefaction Anaerobic digestion Fermentation Transesterification

Biochemical Physicochemical

CH1.44O0.66 þ 1.03 O2 / CO2 þ 0.72 H2O þ Heat (20 MJ/kg of biomass)

(1.1)

In general, it is accepted that biomass is a clean and environmentally friendly fuel. It is possible to agree with the fact that it is neutral in terms of CO2 production. The most commonly used biomass burned are wood, dry leaves, hard vegetable shells, rice husk, and dried dung of animals, which are used with the moisture content of less than 45%e50%. Typical efficiencies for direct biomass combustion plants are between 15 and 55 MWe (Mega-Watts electrical), with electrical efficiencies of 25%e30% [51]. The process involves a biomass combustion step in the presence of excess air to produce heat. Depending on the conditions and the combustion properties of the biomass, it is possible to use different types of furnaces, selecting the ideal parameters to guarantee the best efficiency. Technically, biomass is heated to temperatures above 300 C, decomposing into volatile components (CO, H2, CH4, and others) and coal. At 500 C about 85% by weight of the wood substance is converted into combustible gaseous compounds [13]. The direct combustion systems are fed by organic biomass introduced into the combustion chamber. The comburent material is burned with excess air at a temperature of 800e1000 C, to create steam by heating the water contained in a boiler. The steam from the boiler is then conveyed to a steam turbine, which produces electricity through a generator. A basic scheme of the combustion plant is shown in Fig. 1.7. The combustion efficiency depends mainly on the good contact between the oxygen present in the air and the biomass fuel. Furthermore, hot combustion gases are sometimes used directly for drying the initial product or passed through a heat exchanger to produce hot water or steam. The residual material, in the form of coal, is further burned using a forced air inlet to generate more heat, which, in turn, can be used to produce electricity through a Rankine cycle. In addition to the method described earlier, energy production by combustion of biomass can take place using different types of raw materials and types of plants, such as: 1. Biomass-based generators, in which vegetable oils (i.e., jatropha) are used as fuel, replacing diesel in generators, to produce electricity for limited individual needs.

22 Advances in Bioenergy and Microfluidic Applications

FIGURE 1.7 Biomass combustion scheme.

2. Biomass-based cogeneration plants, which allow the production of two forms of energy such as electricity and heat from the same fuel source. Combined heat and power systems significantly increase the overall thermal and electrical conversion efficiency in the range of 80%e90% [60]. 3. Waste-to-energy plants based on municipal solid waste (MSW) in which the heat, developed during the combustion of waste, is recovered to produce steam, in turn, used for the direct production of electricity or as a heat carrier (for example for district heating). Since the raw material used is very diversified and contaminated, these processes require advanced technologies to reduce emissions, increasing plant costs, and bringing MSW to remain a largely untapped energy resource. Despite the high quality and efficiency of the combustion process, this form of fuel also generates by-products and wastes for the environment and humans, divided into ashes, solid particles, and condensable compounds. The strict control of these emissions and their possible effects are the main objectives in the design of environmentally sustainable biomass combustion plants.

6.1.2 Pyrolysis The biomass pyrolysis process is a thermal decomposition through simultaneous reactions of the organic substrate in the absence of oxygen. The products of pyrolysis are biochar, bio-oil, and gases including methane, hydrogen, carbon monoxide, and carbon dioxide. The first step is the pretreatment of the biomass at a temperature between 100 and 120 C and through the use of a shredding section; in fact, the efficiency and nature of the products depend on the size of the particles and the moisture content [61]. The second step of the process is the pyrolysis reaction: the preheated biomass is introduced into the pyrolysis reactor, surrounded by an external air chamber, in which

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23

combustion takes place, which indirectly heats the biomass contained inside the reaction chamber. At this stage, the thermal decomposition of the organic substances contained in the biomass starts at around 350 C and lasts up to 800 C. Following the pyrolytic degradation process, smaller molecules are obtained in the form of gases, condensable vapors, products liquids such as tar and oils, and solid residues richer in carbon content. The combustion process differs from other high temperature processes such as combustion and hydrolysis due to the absence of reactions with oxygen, water, or other reagents [62]. Subsequently, the raw product gases are introduced into a cyclone separator to purify them and remove the coal (in the case of fast pyrolysis). At this point, the purified gases are distilled and condensed with cold water: the components making up the bio-oils (tar, pyroleptic acid, and derivatives) separate and settle on the bottom while the lighter components of the condensed gases (mainly N2, CO, CO2 and, CH4) settle separately on the surface. On the other hand, noncondensable gases are recycled and reintroduced in the initial combustion process [63]. Depending on the thermal conditions and biomass characteristics, pyrolysis will mainly produce biochar at low temperatures (below 450 C) with slow heating rates. When temperatures exceed 800 C with a rapid heating increase, the main products are gases. Alternatively, the main product is bio-oil when the process takes place at an intermediate temperature and with relatively high heating rates [64]. Among all the conversion techniques of biomass conversion, the pyrolysis process offers numerous advantages, including a reduction in emissions and the reuse of all byproducts. The reactors used in the pyrolysis process can have different designs and practical operation (Fig. 1.8). The different types are as follows: 1. 2. 3. 4. 5. 6. 7. 8.

Fixed bed reactor; Fluidized bed reactor; Circulating fluid bed reactors (CFB); Gurgling fluid beds; Ablative reactor; Vacuum pyrolysis reactor; Rotating cone reactor; Auger reactor.

In conclusion, through this thermochemical conversion a combustible gas component (pyrolysis gas or syngas in a quantity equal to 30%e35% by mass of the input biomass) is obtained, a liquid component (bio-oils equal to 55%e60%) and a solid component (coal or char from pyrolysis) in a quantity of 10%e15%. The gas produced is recovered in energy by steam cycle processes, while the produced coal (char) can be used in cement plants, and coal power plants or further treated in a special section of the pyrolysis plant for its energy recovery; the liquid fraction of bio-oil can be used as fuel for energy production using cogenerators [65]. In summary, through a process that does not

24 Advances in Bioenergy and Microfluidic Applications

FIGURE 1.8 Biomass pyrolysis scheme.

require either direct combustion or the presence of oxygen, almost all of the entire biomass can be used for energy production.

6.1.3 Gasification The gasification process allows the conversion of organic biomass into combustible gases and liquids, as well as carbon, ash, and tar particles as reported in Eq. (1.2). Biomass þ O2 þ H2O / CO þ CO2 þ H2 þ CH4 þ N2 þ traces of other species

(1.2)

The produced gas is a mixture, consisting on average between 17% and 24% of carbon monoxide (CO), 10%e12% of hydrogen (H2), 9%e14% of carbon dioxide (CO2), 1%e4% of methane (CH4), and nitrogen 40%e52% (N2) and other species in small quantities [66]. The gasification process involves several phases: 1. Drying process, in which the biomass, before undergoing any form of conversion, is introduced into a pretreatment system and heated to about 100e120 C to remove the moisture from the combustible material, normally included in a range between 10% and 35%. 2. Pyrolysis process, which involves the complete combustion of the biomass in the absence of oxygen below 600 C in a combustion chamber, consisting of the pyrolysis reactor, an ash tube on the bottom to collect the solid part after the process and an inlet tube for supplying steam to the reactor; in this stage, the volatile components contained in the biomass are released as organic compounds, hydrogen, carbon monoxide, tar, and water vapor. Therefore, the biomass is decomposed or separated into solids (coal), liquids (tar and others), and fuel gas mentioned earlier [67]. 3. The oxidation process, which occurs at about 700e1400 C, in which the air is introduced into the combustion chamber after the pyrolysis process, to allow the

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25

complete oxidation of the solid part producing carbon dioxide and heat as reported in Eq. (1.3).

C þ O2 / CO2 þ heat (DH ¼ 394 kJ/mol)

(1.3)

4. Reduction process, which takes place in reductive conditions (in the absence of oxygen) and at even higher temperatures. Gas and coal mixture, coming from the previous phases, is transformed into the final syngas. The reactions that occur form products such as carbon dioxide, hydrogen, and methane, as reported in Eqs. 1.4e1.7. Boudouard reaction: C þ CO2 / 2CO (DH ¼ 172 kJ/mol)

(1.4)

C þ H2O / CO þ H2 (DH ¼ 131 kJ/mol)

(1.5)

Character reform: Water gas shift reaction: CO þ H2O / CO2 þ H2 (DH ¼ 41 kJ/mol)

(1.6)

C þ 2H2 / CH4 (DH ¼ 75 kJ/mol)

(1.7)

Methanation Reactions, reported in Eqs. (1.4) and (1.5), are favored by temperature increases being endothermic reactions, while the processes displayed in Eqs. (1.6) and (1.7) are exothermic and are strongly supported by low temperatures; ultimately, the entire process requires energy as it provides a global endothermic contribution greater than the exothermic one. In general, the produced gas (CO, H2, CO2, CH4, and N2) has a calorific value of between 4 and 6.5 MJ/m3 depending on the type of biomass used and the operating conditions of the process. This calorific value corresponds to 15%e45% of the calorific value of natural gas [13]. In conclusion, gasification technology can be considered a valid alternative process for biomass energy production. Although the disadvantages due to the polluting byproducts generated by the process (such as ammonia, hydrogen sulfide, and hydrochloric acid) are considerable. After the conversion process, the gases are conveyed toward a purification apparatus (normally a cyclotron), to remove any contaminating particles and obtain a clean syngas. After this step, the syngas is ready to be used for electricity production or other uses (Fig. 1.9). This kind of conversion can be performed by two main types of gasifiers: fixed bed gasifiers and fluid bed gasifiers [68].

26 Advances in Bioenergy and Microfluidic Applications

FIGURE 1.9 Biomass gasification scheme.

6.1.4 Liquefaction Liquefaction is a process of thermochemical conversion of organic material into liquid biocrude and coproducts. The process is carried out at moderate temperatures (between 300 and 400 C) and pressures between 10 and 20 MPa, with the addition of hydrogen or CO as a reducing agent. The main objective of these conversion processes is to obtain a hydrocarbon liquid product with atomic ratio H: C w 2 and boiling point in the range 170e280 C. For this type of conversion, two different classes of starting biomass can be used: lignocellulosic biomass (dry raw material) and algal biomass (wet raw material) [69]. The biomass used has high moisture content and can be transformed into an aqueous suspension (in the case of hydrothermal liquefaction, HTL). Macromolecules such as cellulose, hemicellulose, and starch, can easily decompose in hydrothermal water to form glucose that can be fermented to form alcohols or further degraded in water to produce aldehydes and derivatives. While more complex molecules such as lignin decompose equally in hydrothermal water, but the resulting products are very similar to the constitutive molecules of lignin such as sinapyl and coumaryl alcohol. Some advantages of using the HTL process is obtaining coproducts such as feeds with a high water content and the use of many different types of waste materials, such as solid urban waste, food waste, or animal manure, allowing the process of having a dual usefulness: renewable energy production and waste disposal [70]. During the process, different types of catalysts can be used based on the starting material and the type of process [71]. In the hydrothermal liquefaction process, the most common catalysts used are acid and basic catalysts. The catalyst most used is sodium carbonate in the presence of water and CO. The reaction generates sodium formate according to the reaction reported in Eq. (1.8). Na2CO3 þ H2O þ CO / 2HCOONa þ CO2

(1.8)

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27

Sodium formate has the ability to reduce the carbonyl group to alcohol through dehydration reactions (Eq. 1.9). HCO2Na þ C6H10O5 / C6H10H4 þ NaHCO3H2 þ C6H10O5 / C6H10H4 þ H2O

(1.9)

Other catalysts that behave similarly are K2CO3, KOH, NaOH, and other bases. Nickel (Ni) catalysts, on the other hand, are used to obtain the simultaneous decomposition and hydrogenation of the raw material. The main product of this process (up to 70%) is a liquid biocrude (generally a complex mixture of aromatics, aromatic oligomers, and other hydrocarbons), which appears as a dark viscous tar. The other products depend on the process conditions and on the catalyst used: gases such as CO2, CH4, and light hydrocarbons are those commonly obtained and solid phase material that could be used directly as biofuel or fertilizer [13]. Initially, the process involves a phase of pretreatment and grinding of the biomass to facilitate its pumping in the subsequent reaction vessel. Normally, the ground biomass is mixed with oils (up to 20%e30%), later recycled, to form sewage. The resulting suspension is sent to a preheater by a high pressure sewage pump and heated to 330 C to reach the reaction temperature [69]. Subsequently, the heated liquid material is sent to a stirred and pressurized container, where the conversion reaction takes place (Fig. 1.10). The reactions that occur during the liquefaction process are difficult to detect. In general, thermochemical conversion mechanisms take place through a sequence of structural changes and can be represented by the following list of steps: 1. 2. 3. 4.

Solubilization of biomass; Cracking and reduction of polymers; Hydrolysis of cellulose and glucose hemicellulose; Hydrogenation of the alkyl chains of polymers;

FIGURE 1.10 Biomass liquefaction scheme.

28 Advances in Bioenergy and Microfluidic Applications

5. Molecular rearrangements by dehydration and decarboxylation; 6. Hydrogenation of functional groups. In these process conditions, preliminary spontaneous phase separation takes place, which generates a gaseous phase of CO2, solid, biocrude residues, and small traces of aqueous phase. The crude is then sent to a cyclic centrifuge. Unconverted biomass, suspended solid, and hydrogen sulfide are removed from the crude oil. Finally, the refined oil is stored in a tank for subsequent transport and use. While gases and solid residues are stored in other appropriate tanks [72].

6.2

Biochemical conversion

The conversion of organic biomass into energy through biochemical processes is certainly the most well-known and tested energy transformation method, even at the industrial level. Essentially biological conversion methods can be divided into two processes: 1. Anaerobic digestion; 2. Fermentation.

6.2.1 Anaerobic digestion Anaerobic digestion is a biochemical process through which organic substrates are transformed into biogas. The process consists of the degradation and stabilization of the organic material in anaerobic conditions (absence of oxygen) carried out by a set of microorganisms (hydrolytic bacteria, acidogenic bacteria, acetogenic bacteria, and methanogenic archaebacteria), which leads to the production of biogas with good energy content. The biogas obtained from the anaerobic digestion process is a mixture of various gases. On average, its composition is as follows: methane (CH4), 55%e75%, carbon dioxide (CO2), 25%e45%, hydrogen sulfide (H2S) 1%e2%, and traces of NH3 and H2. The biogas is also saturated with water vapor. The calorific value varies between 10 and 27 MJ/Nm3 depending on the methane content [73]. The degradation process takes place in digesters designed to ensure optimal growth conditions for microorganisms (anaerobic environment, mixing, temperature, pH, organic load, and hydraulic retention time). The biogas obtained can have different uses: heat production, electricity and heat production, biomethane production (Fig. 1.11). The digesters are classified according to the feeding system (batch, continuous, semicontinuous), according to the function of the reactors used to make the process happen (single stage, or double stage, digestion and postdigestion, double acidogenic and methanogenic phase), according to the temperature in which the process takes place (psychrophile 10%), which improves combustion; 4. Absence of sulfur; 5. Greater flash point (120 C compared to 70 C), and therefore greater safety in transport and storage; 6. Higher viscosity, which involves small changes in particular to the rubber seals; 7. Greater cetane number, which corresponds to a more rapid ignition of the engine.

7. Global biomass trends Among the various energy resources available, the energy deriving from biomass is probably the most promising for developing countries. Its use can have positive repercussions both from occupational employment and for the protection of the environment, climate changes, and human health. Lignocellulose could be a cheaper raw material than crude oil although its conversion requires large economic investments due to a large amount of energy required by

34 Advances in Bioenergy and Microfluidic Applications

conversion processes. Cost reduction is the main objective of this sector which is constantly looking for more efficient processes and plants, as well as better biomass collection systems. The International Energy Agency [60,83] has estimated that global energy consumption has grown by about 40% in the last two decades, reaching in 2016 the value of about 370 EJ. Among the renewable energy sources available for energy production (equal to 20% of total world consumption), biomass covers about 72%, followed by hydroelectric (around 17%), geothermal (3.4%), and solar and wind energy (around 6%), as shown in Fig. 1.14 [13,12]. The biomass conversion products are mainly solid residues, liquid biofuels, and biogas. Their production has increased enormously over the past few decades. Taking into consideration the final products, derived from biomass, in all cases, there has been a huge increase in global production over the last 15 years: in the case of biofuels, production increased from about 15 to 150 billion liters, while in the case of electricity, heat and, biogas, production is increased around three times [12]. In conclusion, all these data outline a trend toward a renewable energy future.

8. Conclusion and future outlooks Biomass currently has difficulty playing the role of primary importance for obtaining renewable energy. The goal of the scientific community and others is to allow this sector to take on a strategic role, contributing to the balanced improvement of the conditions of the planet. Widespread use of biomass can generate economic, environmental, and occupational benefits. The advantages of using biomass as an alternative source of energy are as follows: 1. Valorization of industrial and agricultural residues and waste; 2. Development of areas alienated from the world economy despite huge quantities of raw materials that can be exploited by replacing traditional crops with energy crops;

FIGURE 1.14 Global renewable energy sources [64].

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Biomass conversion: general information, chemistry, and processes

35

3. Absence of an increase in the concentration of CO2 in the atmosphere; 4. The creation of new employment activities. The greatest advantage is from the environmental point of view since the biomass is considered carbon neutral since the quantity of carbon that it can release is equivalent to the quantity it has absorbed during its life cycle. Ultimately, biomass is the most desirable renewable energy source to tackle environmental, energy, and economic problems as procurement and conversion technologies are considered ideal to minimize the production of waste products and obtain energy clean and sustainable for the future. However, the problems related to this sector derive from the ideal choice of raw materials and the potential conversion path, due to the different economic and environmental performances. Current conversion technologies include a wide variety of processes such as gasification, liquefaction, pyrolysis, transesterification, and so on, to obtain gaseous and liquid fuels with high purity and yields. Many of the processes are suitable for the direct conversion of biomass or to obtain intermediates. Furthermore, the processes currently adopted are sufficiently controllable to allow the production of liquid and gaseous fuels identical to those deriving from fossil raw materials. In conclusion, the technological challenges to increase the yield and reduce the conversion costs of renewable products are the main objectives of the coming decades. Future actions are directed toward the development of new technologies, mainly from the standpoint of conversion techniques, which are easier to use and cheaper, to attract more capital to this sector.

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[9] Tkemaladze GS, Makhashvili KA. Climate changes and photosynthesis. Ann Agrar Sci 2016;14(2): 119e26. [10] McKendry P. Energy production from biomass (part 1): overview of biomass. Bioresour Technol 2002;83(1):37e46. [11] Panwar NL, Kothari R, Tyagi VV. Thermo chemical conversion of biomass e eco friendly energy routes. Renew Sustain Energy Rev 2012;16:1801e16. [12] Waniska RD, Rooney LW, McDonough CM. Sorghum: utilization. In: Wrigley CW, Corke H, Seetharaman K, Faubion J, editors. Encyclopedia of food grains. 2nd ed. San Diego: Academic Press; 2016. p. 116e23. [13] Tursi A. A review on biomass: importance, chemistry, classification, and conversion. Biofuel Res J 2019;22:962e79. [14] Habert G, Bouzidi Y, Chen C, Jullien A. Development of a depletion indicator for natural resources used in concrete. Resour Conserv Recycl 2010;54(6):364e76. [15] Kumar P, Barrett DM, Delwiche MJ, Stroeve P. Methods for pretreatment of lignocellulosic biomass for efficient hydrolysis and biofuel production. Ind Eng Chem Res 2009;48:3713e29. [16] Kumar A, Kumar N, Baredar P, Shukla A. A review on biomass energy resources, potential, conversion and policy in India. Renew Sustain Energy Rev 2015;45:530e9. [17] De Jong W. Biomass composition, properties, and characterization. In: De Jong W, Van Ommen JR, editors. Biomass as a sustainable energy source for the future: fundamentals of conversion processes. Hoboken, NJ: John Wiley & Sons, Inc; 2014. p. 36e68. [18] Beadle CL, Long SP. Photosynthesis - is it limiting to biomass production? Biomass 1985;8(2): 119e68. [19] Ayeni AO, Adeeyo OA, Oresegun OM, Oladimeji TE. Compositional analysis of lignocellulosic materials: evaluation of an economically viable method suitable for woody and non-woody biomass. Am J Eng Res 2015;4(4):14e9. [20] Vassilev SV, Baxter D, Andersen LK, Vassileva CG. An overview of the chemical composition of biomass. Fuel 2010;89(5):913e33. [21] Ummartyotin S, Manuspiya H. A critical review on cellulose: from fundamental to an approach on sensor technology. Renew Sustain Energy Rev 2015;41:402e12. [22] Heinze T. Cellulose: structure and properties. In: Orlando JR, editor. Cellulose chemistry and properties: fibers, nanocelluloses and advanced materials. Springer International Publishing; 2016. [23] Agarwal UP, Ralph SA, Reiner RS, Baez C. New cellulose crystallinity estimation method that differentiates between organized and crystalline phases. Carbohydr Polym 2018;190:262e70. [24] Chang JKW, Duret X, Berberi V, Zahedi-Niaki H, Lavoie JM. Two-step thermochemical cellulose hydrolysis with partial neutralization for glucose production. Front Chem 2018;6:1e11. [25] Arias FEA, Beneduci A, Chidichimo F, Furia E, Straface S. Study of the adsorption of mercury (II) on lignocellulosic materials under static and dynamic conditions. Chemosphere 2017;180:11e23. [26] De Vietro N, Tursi A, Beneduci A, Chidichimo F, Milella A, Chatzisymeon E, Fracassi F, Chidichimo G. Photocatalytic inactivation of Escherichia coli bacteria in water using low pressure plasma deposited TiO2 cellulose fabric. Photochem Photobiol Sci 2019;18(9):2248e58. [27] Liu W, Hou Y, Wu W, Liu Z, Liu Q, Tian S, Marsh KN. Efficient conversion of cellulose to glucose, levulinic acid, and other products in hot water using SO2 as a recoverable catalyst. Ind Eng Chem Res 2012;51(47):15503e8. [28] Tursi A, Beneduci A, Chidichimo F, De Vietro N, Chidichimo G. Remediation of hydrocarbons polluted water by hydrophobic functionalized cellulose. Chemosphere 2018a;201:530e9.

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[49] Vassilev SD, Andersen L, Vassileva C, Morgan T. An overview of the organic and inorganic phase composition of biomass. Fuel 2012;94:1e33. [50] Chen H. Chemical composition and structure of natural lignocellulose. In: Chen H, editor. Biotechnology of lignocellulose, 25e71. Dordrecht: Springer; 2014. [51] Lebaka V. Potential bioresources as future sources of biofuels production: an overview. In: Gupta V, Tuohy MG, editors. Biofuel technology. Berlin: Springer; 2013. p. 223e58. [52] Lauri P, Havlı´k P, Kindermann G, Forsell N, Bo¨ttcher H, Obersteiner M. Woody biomass energy potential in 2050. Energy Pol 2014;66(C):19e31. [53] Chum H, Faaij A, Moreira J, Berndes G, Dhamija P, Dong H, Pingoud K. Bioenergy. In: Edenhofer O, Pichs-Madruga R, Sokona Y, Seyboth K, Matschoss P, Kadner S, Zwickel T, Eickemeier P, Hansen G, Schlomer S, Von Stechow C, editors. IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation. Cambridge, United Kingdom and New York, USA: Cambridge University Press; 2011. p. 209e332. [54] Gnansounou E, Dauriat A. Ethanol fuel from biomass: a review. J Sci Ind Res (India) 2005;64: 809e21. [55] Vassilev SV, Vassileva CG. Composition, properties and challenges of algae biomass for biofuel application: an overview. Fuel 2016;181:1e33. [56] Strezov V. Properties of biomass fuels. In: Strezov V, Evans TJ, editors. Biomass processing technologies. Boca Raton: CRC Press; 2014. p. 104e14. [57] Hupa M, Karlstro¨m O, Vainio E. Biomass combustion technology development e it is all about chemical details. Proc Combust Inst 2017;36:113e34. [58] Kaygusuz K, Avci AC, Toklu E. Energy from biomass-based wastes for sustainable energy development. J Eng Res Appl Sci 2015;4(2):307e16. [59] Yadav A, Gupta R, Garg VK. Organic manure production from cow dung and biogas plant slurry by vermicomposting under field conditions. Int J Recycl Org Waste Agric 2013;2:21. [60] IEA. World energy balances, IEA world energy statistics and balances (database). International Energy Agency; 2018. [61] Jahirul MI, Rasul MG, Chowdhury AA, Ashwath N. Biofuels production through biomass pyrolysis a technological review. Energies 2012;5:4952e5001. [62] Uddin MN, Techato K, Taweekun J, Rahman M, Rasul MG, Mahlia TMI, Ashrafur SM. An overview of recent developments in biomass pyrolysis technologies. Energies 2018;11(11):3115. [63] Stefanidis SD, Kalogiannis KG, Iliopoulou EF. A study of lignocellulosic biomass pyrolysis via the pyrolysis of cellulose, hemicellulose and lignin. J Anal Appl Pyrol 2014;105:143e50. [64] Demirbas A. The influence of temperature on the yields of compounds existing in bio-oils obtained from biomass samples via pyrolysis. Fuel Process Technol 2007;88:591e7. [65] Tang L, Huang H. Plasma pyrolysis of biomass for production of syngas and carbon adsorbent. Energy Fuels 2005;19:1174e8. [66] Farzad S, Mandegari MA, Go¨rgens JF. A critical review on biomass gasification, co-gasification, and their environmental assessments. Biofuel Res J 2016;12:483e95. [67] Asadullah M. Biomass gasification gas cleaning for downstream applications: a comparative critical review. Renew Sustain Energy Rev 2014;40:118e32. [68] Sharma S, Sheth PN. Air-steam biomass gasification: experiments, modeling and simulation. Energy Convers Manag 2016;110:307e18. [69] Zhang S, Yang X, Zhang H, Chu C, Zheng K, Ju M, Liu L. Liquefaction of biomass and upgrading of bio-oil: a review. Molecules 2019;24:2250.

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An overview on pretreatment processes for an effective conversion of lignocellulosic biomass into bioethanol

2

Alessandro Senatore1, Giuseppina Anna Corrente1, Federica Verteramo1, Francesco Dalena2 1

CHEMISTRY AND CHEMICAL TECHNOLOGIES DP T., UNIVERSITY OF C ALABRIA, CUBO 15/D, VIA P . B UCCI, R ENDE, CS, ITALY; 2 L AB O R A T O R Y O F I N DUS T RI AL C HE MI S T R Y AN D CATALYSIS, UNIVERSITY OF CALABRIA, VIA P. BUCCI, R ENDE, C S, ITAL Y

List of acronyms AFEX ammonia fiber explosion CAC cactus FAO Food and Agricultural Organization FGF first-generation feedstock GCS green coconut shell GHG greenhouse gases HMF hydroxymethylfurfuric ILs ionic liquids LCB lignocellulosic Biomass MCF mature coconut fibers MCS mature coconut shell OS organosolvent SGF second-generation feedstock TGF third-generation feedstock WO wet oxidation SE steam explosion

1. Introduction Nowadays, fossil fuels still represent the primary energy supply resources playing a crucial role in the world energy market. Two key points, such as the increasing pollution Advances in Bioenergy and Microfluidic Applications. https://doi.org/10.1016/B978-0-12-821601-9.00002-9 Copyright © 2021 Elsevier Inc. All rights reserved.

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due to the high greenhouse gas emissions and the depletion of the fossil fuel reserves, generate strong instability on the global market. This instability is certainly reflected in fuel prices [1]. For this reason, scientific communities are trying to optimize various production processes to obtain energy from renewable sources (bioenergy). The use of lignocellulosic materials to obtain bioalcohols and bioethanol particularly seems to be one of the most promising solutions. In many countries, federal laws that allow innovation policies for technological development support this research [2]. An example is given by Brazil. Because of the oil crisis in 1973, Brazil promoted the substitution of crude oil with ethanol produced from sugar cane, through the Pro´-Alcol state program [3e5]. Others, such as Canada, follow state programs that allow scientific and innovation policies and favor the production and consumption of biofuels to support energy diversification [2]. Until 2013, bio-ethanol represented approximately 11% of the World’s energy demand and these results are constantly increasing [6,7]. Following renewable sources used for generating bioethanol can be classified largely into sugars and starch (first-generation feedstocks, FGF), lignocellulosic biomass (second-generation feedstocks, SGF) and algae (third-generation feedstocks, TGF) [8]. The last one is currently contemplated only at the research level, in contrast to the other two that are a reality in the industrial field. Sugars/starch and lignocellulosic biomass significantly differ in terms of yield, costs, and from the industrial plant points of view. Moreover, FGF can be further divided into two groups: sugars and starch. Sugar-based feedstocks (such as sugarcane or sugarbeet) are the most easily fermentable substrate (very favorable thermal conditions around to 37 C) [9]; this process was the first one used for the production of ethanol and, it is still cheaper [8]. Starches based feedstock, employed for their high availability, are also characterized by an easy conversion into ethanol. The fermentation process provides that, when the cereals are crushed and milled, the amylase enzyme contained in cereals converts starches into simple sugars. For the production of nonfood ethanol, the enzymatic hydrolysis of amylase is substituted with acid hydrolysis or even a more practical combination of both [9]. It is noteworthy that, although using FGF for the production of bioethanol is very convenient (from an economic and production process point of view), current studies are looking for new promising lignocellulosic materials as an initial substrate [10,11]. This is because, as stated before, FGF is produced from food crops almost exclusively. FAO [12] reported that in the immediate future, the production of bioethanol on large scale from food sources will lead to its serious reduction favoring the big problem of world hunger. In this direction, a great interest grows up on SGF and TGF, since the primary resources used in these processes are the lignocellulosic plants or waste crops. It should be noted that, depending on the starting feedstock, these may contain amounts of cellulose, from 25% to 70%. The data reported in the literature are very encouraging since they report that about 442 billion L of ethanol was produced in 2016 using

Chapter 2  An overview on pretreatment processes

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lignocellulosic biomass specifically (LCB). The initial feedstock can come from the agricultural waste of agri-food products, grass, softwoods, and hardwoods. With the exception of the first two that contains similar concentrations of hemicellulose, cellulose, and lignin [13], the main difference among the other types of feedstock is the relative composition percentage of those three polymers as it can be seen in Ref. [14]. For example, softwoods contain a high quantity of lignin and an elevated amount of mannose in its hemicellulose structure; compared to these, hardwoods contain less lignin and heterogeneous hemicellulose (xylan, mannose, glucose, and rhamnose) [15]. In the United States, the quantity of LCBs utilizable for bioethanol’s production is 1368 Mton, in which 428 Mton comes from agricultural wastes [16]. The fermentation process, in both SGF and TGF, is more complex than that of the first generation, and this is due to the different compositions of different feedstocks. Lignocellulosic-based materials, because of its complex compositions (more specifically 40%e50% of cellulose, 25%e35% of hemicellulose, and 15%e20% of lignin and only a small percentage, 1%e15% of pectin, protein, extractives, and ash), are not easily fermentable without an adequate pretreatment process [17]. Unlike cellulose and hemicellulose that are polysaccharide polymers and easily convertible in fermentation sugars, lignin is a large amorphous cross-linked heteropolymer constituted by three phenyl propionic alcohol building units: coniferyl alcohol (guaiacyl propanol), coumaryl alcohol (p-hydroxyphenylpropanol), and sinapyl alcohol (syringyl alcohol). Alkylearyl, alkylealkyl, and arylearyl bonds, making a threedimensional network structure, link these phenolic monomers together: a network that is linked to the cellulose and hemicellulose structures by very strong covalent bonds. It helps to confer to the lignocellulosic biomass structure characteristics like impermeability, resistance against microbial attack, and oxidative stress [18e20]. Therefore, the pretreatment is fundamental for the fermentation processes in SG and TG substrates. This chapter is focused on various types of pretreatments. They are evaluated according to their applications and the types of technologies (including chemical, physicochemical, and biological procedures). Moreover, an overview of the most promising existing processes will be provided, both in laboratory and industrial scale and on the key factors influencing each type of this technology.

2. Pretreatment The production process, leading to the formation of ethanol from LCB, consists of different steps that may include pretreatment, hydrolysis, fermentation, distillation, and purification. In Fig. 2.1, in a schematic way, an example of the employed industrial process is reported. As said, the pretreatment is necessary to improve the availability of monomeric sugars on the substrate. Normally the conversion of cellulose into monomeric sugars takes place by an enzymatic route through some enzymes that are also responsible for

44 Advances in Bioenergy and Microfluidic Applications

FIGURE 2.1 Schematic representation of industrial bioethanol from biomass conversion process.

the hydrolysis of cellulose called Cellulose [18]. These enzymes convert only the six carbon atoms and sometimes five carbon atoms sugars (such as xylitol also known as “wood sugar” mainly coming from hemicellulose) monomers of cellulose [21,22]. The limiting aspect is that the ethanol produced by the fermentation process is an inhibitor for yeasts/bacteria that carries out the fermentation [23]. So, after fermentation, the ethanol is subtracted from the fermentation broth and recovered by distillation [13,18]. As mentioned, a key point of the production process is represented by the pretreatment step, which is the most expensive one out of the whole process (about 18% of the total cost) [13,24,25]. It is now clear that the amount of lignin changes depending on the starting matrix as well as on the type of LCB, implying also different physicalechemical properties. This implies different delignification processes and not only a single process that can be applied to all substrates [16]. For the above reasons, there are four large classes of pretreatment: physical, chemical, physicochemical, and biological. Fig. 2.2, shows the number of publications related to physical, chemical, and biological pretreatments in the last 20 years. It is evident that the interest of researchers in this type of technology is exponentially increasing. Fig. 2.2 also shows a homogeneous increase in the publications for all the three types of pretreatment. But, it is clear that a great interest mainly regards the chemical one. As previously mentioned, depending on the type of biomass used as feedstock, it is possible to make a macrodistinction between three types of pretreatment: physical, chemical, and biological. There is also a fourth type of pretreatment, the hybrid

Chapter 2  An overview on pretreatment processes

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FIGURE 2.2 Number of publications versus year. Using the keywords “physical pretreatment biomass” in black; “chemical pretreatment biomass” in light gray; “biological pretreatment biomass” in dark gray. Science direct database (www.sciencedirect.com).

chemical/physical one. In Fig. 2.3, a schematic representation of various types of pretreatment is reported with relative examples. Pretreatments are classified according to the following: (1) final yield of the ethanol; (2) maximizing lignin removal and minimizing polysaccharide modification; (3)

FIGURE 2.3 Schematic representation of types of pretreatment processes and some examples.

46 Advances in Bioenergy and Microfluidic Applications

availability of the feedstock; (4) minimization of the amount of undesired toxic byproducts; (5) minimization of GHG (CO, CO2, NOx); (6) reduction of equipment costs; (7) limiting the formation of degradation products that can inhibit the action of fermenting enzymes [15,26,27]. Table 2.1 presents the various types of pretreatment according to the starting feedstock and the advantages/disadvantages for each of them.

2.1

Physical pretreatment

The physical pretreatment can be performed using several techniques. As shown in Table 2.2, among the pretreatment based on physical principles, it is possible to find milling and grinding, autohydrolysis (or uncatalyzed steam explosion), and liquid hot water pressure [38]. These are based on the reduction of plant size to obtain smaller pieces but are characterized by a higher surface area. In this way, sugars will become more accessible for other treatments [19,38]. In fact, it is known that the physical pretreatments increase the yield of the subsequent pretreatments. In addition, they lower the cellulose degree of polymerization and play an important role in the first phase of the breakdown of crystalline and lignin structures [19,39].

2.1.1 Milling mechanical breakage The mechanical reduction of the plant size is fundamental to extract other products. Depending on the type of industrial plant and raw materials, several types of mills and grinds that cut, break, and reduce the size of the raw material at the beginning of the process (i.e., hammer mill, knife mill, wet or dry disk mill, ball mill) can be chosen [39]. Usually, an initial chipping phase reduces the raw material from 10 to 30 mm, while a subsequent milling or grinding process causes a further reduction of 0.2e2 mm [19]. Even if the ideal size is in the range of 4e8 mm, smaller particles can determinate problems in the recovery process [39a]. The energy required to complete this mechanical breakage varies according to the raw material and will be increased for lignocellulosic biomass, which is more difficult to break [22]. Table 2.3 shows an example of the amount of required energy to break different raw materials with a hammer mill that is able to cut until about 3.2 mm.

2.1.2 Uncatalyzed steam explosion Uncatalyzed steam explosion is the main physical pretreatment used for lignocellulosic biomass that has already been brought on the market to hydrolyze hemicellulose (i.e., masonite process) [13]. Due to its low energy requirement (70% less, compared to the conventional mechanical breakage), this pretreatment has been taken into account for possible production of ethanol from lignocellulosic biomass on an industrial level [19,38]. Lignocellulosic biomass is treated with high-pressure steam from

Table 2.1 Some examples of various types of pretreatment according to the starting feedstock (LCB) and the advantages and disadvantages for each of them. Category

Pretreatment method

LCB

Advantages

Disadvantages

References

Physical

Milling

/

Cost-effective solution

[27,28]

Extrusion

/

(1) Difficulty in removing lignin that prevents the access of enzymes to cellulose; (2) high energy consumption (1) Effect is reduced when no chemical is used, mostly effective on herbaceous type biomass; (2) high energy input (1) The use of concentrated acids is dangerous and corrosive; (2) inhibitor formation at a work pH; (3) need for specialized nonmetallic constructions (1) Significant amounts of water required for washing; (2) reduced digestibility in softwoods

Chemical

Rye straw Sunflowers talk Sorghum straw

(1) Low production of by-products Economic solution and chemical-free

Sorghum straw

(1) Short retention time; (2) absence of some inhibitory by-products; (3) reduction of the lignin fraction

Biological

Bamboo culms, Corn stover

(1) Complete degradation of lignin; (2) low energy requirement; (3) chemical-free; (4) working under mild reaction conditions

Microbial treatment

(1) Lower formation of inhibitory; (2) high recovery

[27e29]

[27,28,30]

(1) Use of harsh organic solvents; (2) high capital investment; (3) formation of inhibitors

[27,28,31]

(1) Expensive process, complexity of synthesis and purification; (2) toxicity; (3) low biodegradability; (4) inhibition of enzymatic activity (1) Costly; (2) cellulose is partly degraded Excessive degradation of the chemical and physical properties of cellulose and release of inhibitors (1) High energy input; (2) almost exclusive removal of hemicellulose

[27,28,32]

[27,28] [27,28,33] [27,28,34]

(1) High capital investment; (2) does not significantly [27,28,35] solubilize hemicellulose; (3) need to recycle ammonia after the pretreatment to reduce the cost and protect the environment; (4) much less effective for softwood Slow reaction time [27,28,36,37]

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Oxidation Physicochemical Steam explosion Liquid hotwater pretreatment Ammonia fiber expansion

[27,28]

Chapter 2  An overview on pretreatment processes

(1) Low making of inhibitory compounds, (2) operating at high solids loadings; (3) short time Acid hydrolysis Corn stover, (1) Good hemicellulose and lignin removal; (2) short retention time rice straw, hardwood Alkaline Barley straw, (1) Removal of lignin and a part of the hemicellulose; (2) decrease the degree of hydrolysis sorghum polymerization straw (1) The production of substrates with low Organosolv Wheat residual lignin reduces the unwanted straw, enzymatic adsorption; (2) enables recycling bamboo and reuse Ionic liquid Rice straw (1) Low vapor pressure designer solvent; (2) mild conditions

48 Advances in Bioenergy and Microfluidic Applications

Table 2.2

The most known physical pretreatments. Physical pretreatment

Mechanical Autohydrolysis

Milling, grinding, extrusion Uncatalyzed steam explosion, Liquid hot water pressure Microwave, electron beam, ultrasound

Irradiation

Table 2.3 Energy required by Hammer Mill machines (particles of about 3.2 mm in size). Raw material

Energy required (kWh/ton)

References

Wheat straw Corn stover Wheat straw Corn stover Hardwood

24.6 11.0 21.0 9.6 115.0

[40] [40] [40] [40] [40]

a few seconds until, at least some minutes, in the range 160e270 C that quickly heats the raw material [19,38]. The best performance is observed at low temperatures and longer times during which hemicellulose is hydrolyzed and lignin structure is transformed by depolymerization and repolymerization processes [38]. At the beginning, the degradation of hemicellulose is due to high temperature, with a consequent lowering of the pH due to the release of acetic acid in the reaction environment (there are no external additions of chemical reagents) [13,19,38]. These conditions also outline the limitations of the process that includes depolymerization of a fraction of xylan and the generation of other compounds that can be inhibitory to microorganisms subsequently used in the production of ethanol [19]. At the moment, despite the above aspect, this pretreatment is very useful for agricultural residues and hardwood, but due to its composition, not for softwood [19]. In Table 2.4, uncatalyzed steam explosion method related to aspen chips, olive-tree pruning, wheat straw, and sorghum bagasse as starting feedstock, to experimental conditions, and to the simple sugars obtained in the process is reported.

2.1.3 Liquid hot water pressure An alternative to the steam explosion process is the liquid hot water (LHW) pressure pretreatment, which is already used both with the milling process in the production of ethanol from corn and sugar cane [5, 38]. In this case, to enhance sugar extraction and to avoid the formation of a high concentration of inhibitors, the pressure is used to maintain water in the liquid state in the range of 200e230 C for 15 min [13,18,19,38]. At

Chapter 2  An overview on pretreatment processes

49

Table 2.4 The uncatalyzed steam explosion method related to aspen chips, olive-tree pruning, wheat straw, and sorghum bagasse as starting feedstock, to experimental conditions, and to the simple sugars obtained in the process. Feedstock

T ( C)

Residence time (min)

Yield

References

Aspen chips Olive-tree pruning Wheat straw

205 190e240 190

3e10 / 10

[41] [41] [41]

Sorghum bagasse

190e210

2e8

10.3 g/100 of xylose 7.2 g/100 of ethanol 102% of glucose 96% of xylose 75%e90% of xylose

[41]

the end of pretreatment, the percentage of lignin is about 35%e60%, the cellulose in the range of 4%e22% and all the hemicelluloses are removed from the raw materials [13]. Based on their configuration, there are three types of plants: cocurrent, counter-current, and flow-through (Fig. 2.4). In the cocurrent type, raw materials and water are simultaneously heated and mixed to form a slurry; in the counter-current reactor, lignocellulosic biomass and water flow in the opposite way; while in the flow-through reactor, hot water flows through a stationary phase lignocellulosic [13]. In Table 2.5, the yield and the experimental condition for corn fiber and also grass and wheat straw as starting feedstocks in the LHW pretreatment are reported.

2.2

Chemical pretreatment

The chemical reagents, mainly used to perform the chemical pretreatments, are acids, alkalis, and oxidizing agents [42]. These classes are very different from each other, and

Biomass + Water

CO-CURRENT

COUNTCURRENT

Biomass Water

FLOW THROUGH

Biomass

Water

FIGURE 2.4 Schematic representation of three types of configuration in LHW pressure plant.

50 Advances in Bioenergy and Microfluidic Applications

Table 2.5 The yield and the experimental condition for some types of feedstocks using LHW pretreatment. Experimental conditions 

Feedstocks

T ( C)

Residence time (min)

Yield

Corn fiber Grass (Tifton 85) Wheat straw

160 200e230 170e200

20 Different time conditions 0e40

74% arabinose and 54% xylose 11.0 and 14.7 g/L ethanol Sugar recovery (53% of the content in raw material) and enzymatic hydrolysis (EH) yield (96% of theoretical)

Adapted from Behera S, Arora R, Nandhagopal N, Kumar S. Importance of chemical pretreatment for bioconversion of lignocellulosic biomass. Renew Sustain Energy Rev 2014;36:91e106.

this causes different effects on the lignocellulosic substrates. More effective lignin removal is well performed with alkaline agents, or ozonolysis, peroxide, and wet oxidation pretreatment, instead the hemicelluloses solubilization occurs easily in a weak acid environment, using dilute acid [41].

2.2.1 Acid pretreatment This pretreatment consists of the acids added to the LCB with consequent hydrolysis of the hemicellulose and the precipitation of the lignin [43]. Generally, there are two types of acid pretreatment: dilute and concentrated, and both of them consist of a continually mixing at 130e210 C [44]. These processes have a high sugar yield and have been used with LCB such as grass, hardwood, and softwood [25]. In the “dilute pretreatment,” a very low H2SO4 concentration (0.2%e2.5% w/w) is commonly used; while in the strong one, high concentration of H2SO4 and HCl are used without subsequent enzymatic hydrolysis [45]. These pretreatments are very effective due to the high solubility of hemicellulose and lignin in acid, but there are some drawbacks such as (1) the recovery of the acid that makes it very expensive; (2) the reaction between acids and biomass that induce the decomposition of the hemicellulose with the formation of undesirable by-products, such as hydroxymethylfurfuric (HMF) acid, acetic acid, and furfural acid (on average in a range of concentration of 2.0e3.0 g/L) [46]. These by-products whose concentration tends to be increased proportionally to the operating temperature and concentration of the cited acid (H2SO4 and HCl) tend to inhibit the fermentative microorganisms. Moreover, the use of corrosive agents causes faster deterioration of the plant, thus increasing the maintenance costs [41,42,47]. A prospect of the acid pretreatment method as a function of the LCB nature, of the experimental conditions, and of the simple sugars obtained in the process (glucose from cellulose and xylose and arabinose from hemicellulose) is shown in Table 2.6. It was shown that the maximum yield of simple sugars is obtained for the sugarcane bagasse with the experimental conditions reported in Table 2.6. But, in the reported

Chapter 2  An overview on pretreatment processes

51

Table 2.6 A prospect of the acid pretreatment method as a function of the LCB nature, of the experimental conditions, and of the simple sugars obtained in the process. Experimental conditions Feedstocks Sugarcane bagasse Eucalyptus residue Grass Corn stover

Acid/ concentration 2%e6% H2SO4 0.65%(w/w) H2SO4 1%e10% H3PO4 0.25%(v/v) H2SO4

Yield

T ( C)

Time (min)

Xylose

Arabinose Glucose

References

100e128

0e300

21.6 g/L

n.g.

3 g/L

[41]

157.1

20

13.65 g/L 1.55 g/L

1.65 g/L

[41,48]

150e200

0e15

121

30e180

6.7% (w/w) 16.56% (w/w)

3.36% (w/w)

2.5% (w/w) 6.1% (w/w) [41,49] 1.55% (w/w)

[41,50]

experimental conditions, a higher concentration of by-products, such as 0.5 g/L of furfural and 3.65 g/L of acetic acid, was obtained. Indeed, also with the other substrates, some by-products were obtained. For example, in the case of forest wastes (in particular for Eucalyptus residue) were obtained: 3.10 g/L of acetic acid, 1.23 g/L of furfural, and 0.20 g/L of HMF [41,48]. From an industrial point of view, diluted acid appears to be a more favorable method. This process can be carried out at high temperatures (160e220 C) in a short time; or at a lower temperature (120 C) in a longer time [19,51]. Different types of reactors, such as percolation, plug flow, and shrinking-bed ones, have been applied for the pretreatment of biomass in the industrial scale [17,52].

2.2.2 Alkaline pretreatment The alkaline pretreatment is a highly used pretreatment method to produce bioethanol. This pretreatment is focused on the removal of the lignin and other inhibitors groups (such as acetyl groups, lignin, and several of uronic acid substitutions) from the biomass, particularly on the fermenting enzymes of the polysaccharides [47]. The yield of this type of pretreatment is strictly dependent on the lignin concentration in the initial biomass; the good results were recorded for feedstock with low content of lignin [19]. The reactions that occur during the alkaline biomass pretreatment, reduce both the crystallinity of the cellulose and the degree of its polymerization [18]. This leads to an easier access to the biomass structure for bacteria and fermentation enzymes. This method has been tested on hardwood, softwood, corn stover, switchgrass, and bagasse [43]. The most widely used alkaline agent is sodium hydroxide (NaOH). The limitation of this alkaline is that if it is used in high concentration, it can be a pollutant for water and

52 Advances in Bioenergy and Microfluidic Applications

can inhibit some production processes (such as anaerobic digestion for the production of methane). Other widely used bases are ammonia hydroxide (NH4OH), sodium carbonate (Na2CO3), and lime (Ca(OH)2) [53,54]. The efficiency and the experimental condition for each type of previously cited alkaline are summarized in Table 2.7. In particular, the pretreatment with lime will result in both economic and high productive (that required low energy cost); this is mainly due to the fact that lime is recovered by precipitation reactions with CO2. Adapted from Kim JS, Lee YY, Kim TH. A review on alkaline pretreatment technology for bioconversion of lignocellulosic biomass. Bioresour Technol 2016;199:42e48. A very interesting study of [47] also reported the yield of ethanol produced from some typologies of LCB using alkaline pretreatment. The referred experimental data are cactus (CAC), green coconut shell (GCS), mature coconut shell (MCS), and mature coconut fibers (MCF) treated with alkaline hydrogen peroxide (AlkH2O2) and sodium hydroxide (NaOH). The ethanol yields obtained after the fermentation process (carried out with Zymomonas mobilis, Pichia stipites and Saccharomyces cerevisiae) are equal to 79.27% e84.64% to 85.04%e89.15% for CAC, GCS, MCF and MCS [47]. Among the main advantages of this method, it is possible to recognize: low cost of pretreatment from both the products used point of view and the experimental conditions and also effective removal of lignin and xylan.

2.2.3 Ionic liquids Ionic liquids (ILs) are liquids composed of ions of organic salts (in cation-anion couple) that generically melt below 100 C [56]. The strong electrostatic forces among ions confer very interesting chemicalephysical properties including low vapor pressure (the reason for which they are called “green solvents”), low volatility, and high chemical and electrochemical stability [57].

Table 2.7 bases.

The efficiency and the experimental condition for some types of alkali Experimental condition

Alkaline base

Temperature ( C)

Time

Concentration

NaOH Na2CO3 NH4OH Ca(OH)2

60e180 60e180 30e210 25e130

5e60 min 5e60 min 5e60 min 1 h to 8 weeks

0.5%e10% 1%e30% 5%e30% 0.005e0.15 g Ca(OH)2/1 g of biomass

Process efficiency Hemicellulose dissolution 50% 20%e40% 10%e50% 20%e40%

Delignification 60%e80% 40%e60% 0%e80% 60%e80%

Adapted from Kim JS, Lee YY, Kim TH. A review on alkaline pretreatment technology for bioconversion of lignocellulosic biomass. Bioresour Technol 2016;199:42e48.

Chapter 2  An overview on pretreatment processes

53

As mentioned, ILs are composed of a mixture of cations and anions. Generally, the cations are of organic nature (i.e., imidazolium, pyridinium, aliphatic ammonium, alkylated phosphonium), while the anions may be both inorganic and organic [58]. The cellulose dissolution mechanism is based on the breaking of the hydrogen bonds of the three-dimensional network of cellulose, with the relative formation of hydrogen bonds between glucan and IL [35,59,60]. ILs are used as reagents in biomass pretreatment. Several advantages have been observed with these green solvents: (1) selective removal of hemicellulose and lignin; (2) mild experimental process conditions; (3) recyclability after the process; (4) easy recovery of cellulose; and (5) toxic and odorous free emissions [59e61]. An additive value is the possibility of designing the physical and chemical properties according to the experimental process. For example, Zhang et al. [60] report that the most used salts as solvents for biomass pretreatment are imidazolium and pyrrolidinium salts (Fig. 2.5). Therefore, it is possible to modulate ILs properties by changing the X, Y, and Z functional groups. Table 2.8 shows some examples of imidazolium and pyrrolidinium salts with their relative melting point. These salts are very effective for the degradation of cellulose and lignin. The capacity of the anion that existed in these salts is directly proportional to the acceptance of the hydrogen bond between the IL and the cellulose. In turn, this parameter is proportional to the solubility of the cellulose itself. Experimental data has shown that the cellulose solubility in ILs varies as follows [OAc] > [Cl] > [BF4] > [PF6] [58]. Instead, it is reported [58,62] that hydrogen bonding strength does not affect the solubility of lignin in ILsa lot, but just a minimum of hydrogen bonding basicity was still required to solubilize the lignin. The lignin solubility in IL varies as [MeSO4] > Cl > Br >> [PF6] [58]. The process of pretreatment with ILs seems to be very promising when compared to the dilute acid pretreatment. This method leads to better results such as higher sugar yields, the feedstocks treated results three times more delignificated, and finally significant enhancement of the enzyme hydrolysis kinetics by a factor of 16.7 [61]. The ILs pretreatment process for wheat straw, wood flour, aspen wood, and rice straw feedstock is summarized in Table 2.9.

FIGURE 2.5 Schematic representation of the chemical structure of imidazolium and pyrrolidinium salts.

54 Advances in Bioenergy and Microfluidic Applications

Table 2.8

Imidazolium and pyrrolidinium salts with relative melting point.

Imidazolium salts

Pyrrolidinium salts

X

Y

Z

Nomenclature

Melting point ( C)

eC2H5 eC2H5 eC2H5 eC2H5 eC2H5 eC2H5 eC4H9 eC4H9 eC4H9 eC4H9 eC4H9 eC4H9 eCH3 eCH3 eCH3 eC4H9 eC4H9 eC4H9 eC4H9

eCH3 eCH3 eCH3 eCH3 eCH3 eCH3 eCH3 eCH3 eCH3 eCH3 eCH3 eCH3 eC8H17 eC8H17 eC8H17 eCH3 eCH3 eCH3 eCH3

Cl Br I PF6 CH3COO BF4 Cl Br I PF6 CH3COO BF4 Cl PF6 BF4 Cl Br N(CF3SO2)2 N(CF2SO2)2

[Emin][Cl] [Emin][Br] [Emin][I] [Emin][PF6] [Emin][OAc] [Emin [BF4] [Bmin][Cl] [Bmin][Br] [Bmin][I] [Bmin][PF6] [Bmin][OAc] [Bmin][BF4] [Omin][Cl] [Omin][PF6] [Omin][BF4] [Bmpyr][Cl] [Bmpyr][Br] [Bmpyr][Tf3N] [Bmpyr][Tf2N]

77e79 70e73 80e82 58e62 ˃30 15 70 65e75 58 8 NA 80 12 NA 88 114 214 18 8

Adapted from Zhang Q, Hu J, Lee DJ. Pretreatment of biomass using ionic liquids: research updates. Renew Energy 2017;111:77e84.

Table 2.9 The ILs pretreatment process for wheat straw, wood flour, aspen wood, rice straw feedstocks as a function of the ionic liquid used, of price, of the experimental condition, and the yield of the process. Experimental conditions Feedstock

ILs

Price

Residence T ( C) time

Wheat straw

[Emin][PF6]

820 $/50 g

30

30 min

Wood flour Aspen wood

[Emin][OAc] [Emin][OAc]

1165 $/1 kg 1165 $/1 kg

/ 120

/ 5h

Rice straw

[Emin][OAc]

1165 $/1 kg

120

5h

Yield

References

Yield of reducing sugar reached at 54.8%; ethanol production was 0.43 g/g glucose About 40% of lignin removed 224 g ethanol from 1 kg using S. cerevisiae at 35.0  0.5 C 19.2  2.4 g of ethanol/100 g of straw

[41]

[41] [59] [59]

Chapter 2  An overview on pretreatment processes

55

The main problem derives from the high cost of ILs; in fact, the prices of these salts are much higher (i.e., 1828 $/kg of [Bmin][Cl]) than the common solvents generally [60]. A way to reduce the pretreatment costs is represented by ILs recycling process. Recent research shows that to make this type of pretreatment economically competitive, a process must be put and it includes >97% of recovered IL and >90% of recovered waste heat [63]. Different research groups are working on the reduction of the production costs and this research concerns: (1) experimentation of Hence’s ILs protocols, which are much less expensive than traditional dialkylimidazolium-based salts; (2) trying to operate at even lower temperatures to facilitate the recycling of ILs; (3) a search to integrate IL pretreatment with hydrolysis [45].

2.2.4 Organosolv The organosolv (OS) process involves the extraction of lignin by organic or aqueous solvents. Indeed, this process involves a solvent mixture ratio of 1:1 water/organic solvent [64,65]. Among the organic solvents, the most used are methanol and ethanol (low-boiling alcohols); but other solvents are also commonly used, such as acetone (more expensive than alcohols, but more suitable for delignification process) and glycols or organic acids such as acetic acid (30% v/v) and formic acid (30% v/v) [42]. Generally, low-boiling solvents are used to distill and recycle them after the process. However, their low boiling point requires high reaction pressure (a very expensive procedure related to an industrial plant) [45]. In addition to the economic factors that represent the recycling, the solvents must be removed from the final product for a better yield of the ethanol; in fact, the solvents can inhibit the subsequent enzymatic fermentation. This pretreatment provides the heating of the solvent mixture in the presence of LCB, under mild temperature and neutral pH condition to obtain a relatively pure lignin yield and also cellulose and hemicellulose with almost no change in their structure [42,45]. In fact, the OS process produces three separate fractions: dry lignin, an aqueous hemicellulose solution, and a relatively pure cellulose fraction. As most organic solvents (except organic acids) are too mild to destroy the bonds of the lignocellulosic network, the OS process has been implemented with pretreatment of acidic catalysis using mineral acids (i.e., H2SO4, HCl) and some types of organic acids (i.e., formic, oxalic, acetylsalicylic acid and also salicylic acid) [66]. The acids have the dual purpose of breaking the hemicellulosic bonds (with a consequent higher yield of xylose), improving the delignification process and lowering the operating reaction temperature (100e150 C) [45,67].

56 Advances in Bioenergy and Microfluidic Applications

For example, [68] carrying out tests on an LCB matrix of willow wood and wheat straw found that the maximum enzymatic conversion of cellulose is 87% and 99% for willow and wheat straw, respectively, compared to 74% (willow wood) and 44% (wheat straw) for a noncatalytic process [68]. However, the methodology, compared to other chemical pretreatments, is extremely innovative to easy recovering of the solvent through distillation, to be able to obtain and separate the components of the LCB and to recover the high quality of lignin as a by-product [68,69]. The OS process is chemically and economically valid for the solvent recovery, pH operating conditions, and very mild temperatures; but, it has not any negligible disadvantages: (1) high-pressure operative conditions for solvent recovery process (with a consequent increase in operating costs); (2) high amount of organic solvents that require the use of a safety system because of the high risk of fire or explosion. In Table 2.10, the OS pretreatment process is reported for hybrid popla chips and pinus radiate as starting feedstocks, as a function of the experimental condition and the yield of the processes.

2.2.5 Ozonolysis In the ozonolysis process, ozone is employed to degrade lignin and hemicellulose in many LCBs, such as wheat straw, peanut, pine, cotton, and poplar sawdust [71]. This process has high selectivity with respect to lignin, but it is less suitable for hemicelluloses and cellulose degradation. The main parameters in ozonolysis are the humidity content of the sample, particle size, and ozone concentration in the gas flow. An advantage of which is that the process does not toxic residues for the downstream process and the reaction is carried out at room temperature conditions. Nevertheless, a huge quantity of ozone is necessary and it makes the process expensive [41]. The ozonolysis pretreatment process for pine, eucalyptus, bamboo, and bagasse feedstock is summarized in Table 2.11.

Table 2.10 The OS pretreatment process for hybrid popla chips and pinus radiate as starting feedstocks as a function of the experimental condition and the yield of the process. Feedstock

T ( C)

Residence time (min)

Hybrid popla chips

180

60

Pinus radiate D. Don

195

5

Yield

References

85% of glucose 99.5% ethanol

[69] [70]

Chapter 2  An overview on pretreatment processes

57

Table 2.11 The oxidation pretreatment process for pine, eucalyptus, bamboo, and bagasse feedstock with the yield of simple sugars obtained before and after the pretreatment. Arabinose (%)

Pine Eucalyptus Bamboo Bagasse

Galactose (%)

Xylose (%)

Mannose (%)

Before

After

Before

After

Before

After

Before

After

0.4 _ 0.7 1.2

_ _ _ _

0.3 _ _ _

_ _ _ _

8.0 18.1 19.5 23.1

5.5 10.5 11.9 16.7

6.5 _ _ _

0.8 _ _ _

Adapted from Meng Q, Fu S, Lucia LA. The role of heteropolysaccharides in developing oxidized cellulose nanofibrils. Carbohydr Polym 2016;144:187e195.

2.2.6 Wet oxidation In this pretreatment, the lignocellulosic biomass is treated with water and high-pressure oxygen (8e33 bar) at a temperature above 120 C for effective separation of the cellulosic fractions from lignin and hemicellulose [73]. The main reactions that occur in wet oxidation (WO) pretreatment are oxidative reactions as well as the formation of acids. Several studies [41,74,75] reported in the literature demonstrated the use of the WO pretreatment strategy on different substrates. Some of these, that is, softwood and corn stover feedstocks are summarized in Table 2.12. As reported in Table 2.12, in WO, lignin degradation occurs at high temperature because phenol-like compounds and carbonecarbon bonds are very reactive under WO conditions. Lignin is decomposed to CO2, H2O, and carboxylic acids [73].

2.3

Physical-chemicalpretreatment

The solubilization of lignocelluloses components depends on temperature, pH, and humidity content. In lignocellulosic, materials such as wheat straw and hemicelluloses are the most thermochemically sensitive fraction. Hemicelluloses compounds are dissolved in water at temperatures higher than 150 C and among its components, xylan can be easily extracted [47]. The most useful method of the physicalechemical pretreatment is the CO2 explosion to separate the hemicellulose, which consists in the reduction of polymeric chains, in

Table 2.12 The WO pretreatment process for softwood and corn stover as starting feedstock as a function of the experimental condition and the yield of the process. Feedstock

T ( C)

Residence time (min)

Yield

References

Softwood Corn stover

200 195

10 15

79% of ethanol 83% of ethanol

[74] [75]

58 Advances in Bioenergy and Microfluidic Applications

glucosidic compounds, to more simple and fractionable sugar. Generically, conventional mechanical methods require 70% more energy than conventional physicochemical pretreatments to reach the same amount of sugar reduction. These methods are mainly useful for agricultural residues, but due to the low content of acetyl groups in the hemicellulosic portion, they are less efficacious for softwoods [11]. These treatments, which are often used with the recovery of the chemical solvent, can improve the yield on lignocellulosic raw material [76]. In Table 2.13, some of the common physicochemical treatments designed for different biomass raw materials are shown.

2.3.1 Steam explosion The catalyzed steam explosion (SE) pretreatment of lignocellulosic biomass and other cellulosic raw materials are similar to the previously discussed uncatalyzed steam explosion. As an improvement to this, SE is characterized by adding chemical reagents like SO2, H2SO4, or oxalic acid [38]. Due to its inexpensiveness and to the increase of sugar yield, which can be easily fermented, such as xylose, SO2 is usually preferred among the others [38]. The process with an SO2 concentration in the range of 1% and 6% (w/w) with respect to the substrate, is carried out in the range 160e230 C, for about 5e10 min (all conditions depend on the type of lignocellulosic raw material) [77]. However, this treatment is considered negative for the environment and generates some inhibitors that can be problematic during the fermentation phase [38].

2.3.2 Ammonia fiber explosion Ammonia fiber explosion (AFEX) is an alkali pretreatment that exploits the high value of temperature and pressure on liquid ammonia and lignocellulosic biomass having a ratio of 1:1 or 2:1 (w/w), depending on the raw material [19,77]. The subsequent increase of pressure opens the rigid structure of lignin while increasing the accessibility to sugars and solubilize the hemicellulose structure [38]. The standard conditions used in this method are temperature less than 90 C and pressure of w27 atm for 30 min. These lead to a hydrolysis of about 90% of cellulose and hemicellulose, for low lignin content raw materials (i.e., bermudagrass and bagasse), and 40%e50% for that with a high lignin

Table 2.13 The most common physicochemical pretreatment used for biomasses treatment. Physico-chemical pretreatment Explosion

Irradiation

Steam explosion Ammonia fiber explosion Ammonia recycle percolation Supercritical fluid explosion Microwave with chemicals

Chapter 2  An overview on pretreatment processes

59

content (i.e., newspaper and aspen chips) [19,38]. The main advantage of this technique is linked to a low formation of fermentation inhibitors and no biomass size reduction pretreatment [38,77]. Moreover, the costs and toxicity of ammonia make the recovery process indispensable and make the whole AFEX process too expensive (25e30$/ton) for an industrial application [38,77]. The AFEX pretreatment processes for corn stover, miscanthus giganteus, and switchgrass feedstock are summarized in Table 2.14.

2.3.3 Supercritical fluid explosion/carbon dioxide explosion The use of supercritical fluids such as CO2 is related to the latest generation of technology, and today its equipment is still expensive for large-scale production [81]. CO2, in its supercritical state (usually obtained at a temperature of 31 C and a pressure of about 73 atm), exploits high pressure and low temperature to form a highly compressed gas that is liquid but behaves like gas for its molecule diffusivity [76,82]. When dissolved in water, CO2 produces carbonic acid that is less corrosive than the same acid at standard temperature and pressure conditions. Some hypotheses considered it as responsible for the hydrolysis rate of cellulose and hemicellulose [19,81,82]. Experiments on aspen wood and southern yellow pine (raw materials high in lignin content), using CO2 supercritical (165 C, w205 atm, 30 min), show a high releasing of sugars (84.7% and 27.3%, respectively) with respect to the untreated raw materials (14.5% and 12.8%) [82]. Despite this aspect, such technology has a great potential due to its low temperature with respect to the catalyzed steam explosion and low costs and toxicity with respect to the AFEX process. Anyway, the costs of the plant are still too high to be supported from an industrial point of view [19,82]. The supercritical fluid explosion/carbon dioxide explosion pretreatment processes for hardwood, softwood, cellulose, and sugarcane bagasse feedstock are summarized in Table 2.15. Table 2.14 The AFEX pretreatment process for corn stover, miscanthus giganteus, and switchgrass as starting feedstocks as a function of the experimental condition and the yield of the process. Feedstock

T( C)

Residence time

Yield

Ref.

Corn stover Miscanthus giganteus Corn stover

90 160

5 min 5 min

[78] [79]

Room temperature

10e60 days

Switchgrass

/

5e10 days

98% of glucose 95% of glucan 81% of xylan 100% ̴ of glucan 85% of xylan 77% of ethanol 40%e50% delignification hemicelluloses content decreased by approximately 50%

[79a]

[80]

60 Advances in Bioenergy and Microfluidic Applications

Table 2.15 The supercritical fluid explosion/carbon dioxide explosion pretreatment process for hardwood, softwood, cellulose, and sugarcane bagasse as starting feedstocks in the function of the experimental condition and the yield of the process. Feedstock

T ( C)

Residence time (min)

Yield

References

Hardwood and softwood Cellulose Sugarcane bagasse

112e165

10e60

[83]

50 35

90 e

67.1  3.5 total sugar content 100% of glucose 50% of glucose

2.4

[84] [38]

Biological pretreatment

Biological pretreatment is characterized by high yield and being eco-friendly; this is mainly due to (1) very low energy requirement; (2) no releasing of toxic compounds; and (3) no generation of fermentation inhibitors during the process [85]. Other advantages of this pretreatment are mild conditions and low production costs, but some disadvantages like lower rates of hydrolysis and longer pretreatment times (between 4 and 8 weeks) make this pretreatment still far from the industrial field [16,86]. The biological pretreatment can be of three types: (1) microbial; (2) enzyme; the one with (3) soft-rot fungi. The microbial pretreatment consists of a solid-state fermentation process in which the microorganisms (soil, cow dung, goat dung, etc.) grow on the LCB through degrading the lignin selectively (and in some cases hemicellulose), while the cellulose should remain intact. This method is an indication of obtaining methane by anaerobic digestion particularly [51]. The enzymatic pretreatment involves the use of enzymes with hydrolytic activity, more commonly cellulase and hemicellulases are used to pretreat the lignocellulosic biomass. The effect of these enzymes is to initiate the decay of LCB and facilitate the penetration of hydrolytic enzymes into cellulosic and hemicellulose substrates [47]. Also, this pretreatment is to be very suitable for the production of biogas. The fungal pretreatment involves the use of soft-rot fungi for lignin removal. The main used fungi are white, brown, and soft rot fungi. Brown mainly prefers to attack cellulose, while white and soft fungi are usually preferred, accordingly with their high selectivity, in the degradation of lignin compared to cellulose loss [87]. Lignin degradation performed by white-rot fungi occurs through the strong oxidative action of ligninolytic enzymes that degrade lignin [68], such as laccase, lignin peroxidase, and manganese peroxidase [87]. Various white-rot mushrooms such as Phanerochaete chrysosporium, Ceriporia lacerata, Cyathus stercoletus, Ceriporiopsis subvermispora, Pycnoporus cinnabarinus, and Pleurotus ostreatus were used to test different lignocellulosic materials [68].

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For example, [88] reported that using white-rot fungi of the Ceriporiopsis subvermispora type starting from corn stover as feedstock, a degradation of up to 31.59% of lignin and 94% of cellulose was observed. However, these good results were obtained in 18 days: too long for an industrial application. Good results in terms of efficiency have been obtained also with Phanerochaete chrysosporiu, due to high growth rate and good ability to biodegrade lignin [88a]. Among the three biological pretreatments, the most controllable one appears to be the microbial pretreatment, because of its low risk of contamination with respect to the fungi pretreatment. In fact, in the last type of pretreatment, a lot of care is needed to prevent contamination of a species of fungi with a consequent decrease in the final ethanol yield [47]. Although biological pretreatments have obvious advantages (less expensive in terms of energy, no chemical requirements, slight pretreatment conditions), the very long reaction time strongly limits their industrial application [68]. However, in general, most biological pretreatments are not as promising as chemical pretreatment for the production of biofuels. Due to the high retention time, cost-effectiveness, and high selectivity of microbes, further studies are necessary to overcome the problems of selectivity, cost, conservation time, and effectiveness [47]. Further studies are needed to understand the kinetic parameter to reduce the time.

3. Combined pretreatment As seen in the previous paragraphs, there is no ideal pretreatment, but all of them have their own defects and strengths. Although choosing the pretreatment depends on the substrate, it is possible to improve and overcome some previously cited issues, and it is possible to increase the performance in terms of production of fermentable sugars yield through the combination of two or more pretreatment methods as shown in Table 2.16 [89]. The best results are recorded combining the biological pretreatment (potentially ideal for the production of bioethanol, but commercially unattainable for long process times) with the other methods to reduce significantly the time required for the whole process, to reduce inhibitor formation and consequently improving the efficiency of the process in terms of ethanol production [45]. From Table 2.16, it is clear how the biological pretreatment reaction times are significantly lowered: from 4 to 8 weeks in the biological pretreatment also (see Section 2.4) to 10 and 21 days in the combined acid and steam explosion pretreatment, respectively. It can be noted that the combined alkaline-biological pretreatment does not lead to satisfactory results both in terms of yield (from 425.7 mg/g to 458.7 mg/g) and in terms of reaction times (28 days) [89,90]. From Table 2.16, you can see how very satisfactory results are achieved for the pretreatment combined with acids and steam explosion (in the latter even an increase of 24% in terms of sugar yield). In particular, in the case of combined pretreatment

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Table 2.16 Effects of the combination of acid/alkaline/steam explosion and biological pretreatment to improve sugar yields. Pretreatment combined

Biological pretreatment

Experimental conditions Feedstock Type

Time Concentration/ pression T ( C) (min)

Wheat straw

0.5%e4.5% H2SO4

121

0.25 M NaOH 0.8/1.7 MPa

30

Acid pretreatment

Cornstalks Alkaline pretreatment Cornstalk Steam explosion

e

45e60

30 1

Experimental conditions Strain Fungal isolate RCK-1 Irpex lacteus Phellinus baumii

Sugar yield

Time T ( C) (days)

Untreated

Combined

37

30.27 g/L

40.82 g/L

10

15e60 28

425.7 mg/g 458.7 mg/g

28

130 g/kg

21

313 g/kg

Adapted from Shirkavand E, Baroutian S, Gapes DJ, Young BR. Combination of fungal and physicochemical processes for lignocellulosic biomass pretreatmentea review. Renew Sustain Energy Rev 2016;54:217e234.

with acid, a sharp decrease of the concentration of toxic inhibitory by-products (from 1.31 g/L to 0.63 g/L) is recorded with a relative improvement in terms of environmental impact [34]. Other tests have been performed, through combining enzymatic hydrolysis coupled with ozonolysis method. The removal of 60% of lignin from wheat straw increasing the rate of enzymatic hydrolysis by a factor of 5. After the ozonolysis pretreatment, there is a decrease in lignin content from 29% to 8% and an increase in the enzymatic hydrolysis yield from 0% to 57% [51,91].

4. Conclusion and future trends In conclusion, a general overview of the most currently used pretreatments from both academic and industrial sectors has been given. The cost strictly depends on catalysts, reagents, solvents, and raw materials used; consequently, it varies strictly from the type of the process. There is not an ideal pretreatment, but there are methodologies that vary mainly according to the chemical composition of the starting feedstock, the final sugar yield, and the energy cost of the entire process. In fact, as a goal for the future, the researchers are trying to follow aspects: (1) energy performance; (2) the use of corrosive chemical agents; (3) the development of inhibitors developed during the process; (4) optimizing the cost/efficiency ratio of the process. In our view, the best results can be achieved by using coupled pretreatment trying to take the strengths from multiple methodologies (i.e., a union between ozonolysis and enzymatic pretreatment or steam explosion with biological pretreatment).

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It is clear that this is a very attractive research area, and, in fact, low-cost ethanol production would lead to lowering global energy costs, with the subsequent possibility of making developing countries partially autonomous from an energy point of view.

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Further reading [1] Chandra R, Takeuchi H, Hasegawa T. Methane production from lignocellulosic agricultural crop wastes: a review in context to second generation of biofuel production. Renew Sustain Energy Rev 2012;16(3):1462e76. [2] Chen X, Yu J, Zhang Z, Lu C. Study on structure and thermal stability properties of cellulose fibers from rice straw. Carbohydr Polym 2011;85(1):245e50.

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[3] Gonza´lez-Garcı´a S, Gasol CM, Moreira MT, Gabarrell X, i Pons JR, Feijoo G. Environmental assessment of black locust (Robinia pseudoacacia L.)-based ethanol as potential transport fuel. Int J Life Cycle Assess 2011;16(5):465. [4] Kadimaliev DA, Revin VV, Atykyan NA, Samuilov VD. Effect of wood modification on lignin consumption and synthesis of lignolytic enzymes by the fungus Panus (Lentinus) tigrinus. Appl Biochem Microbiol 2003;39(5):488e92. [5] Kaparaju P, Serrano M, Thomsen AB, Kongjan P, Angelidaki I. Bioethanol, biohydrogen and biogas production from wheat straw in a biorefinery concept. Bioresour Technol 2009;100(9):2562e8. [6] Kim TH, Lee YY. Pretreatment of corn stover by soaking in aqueous ammonia at moderate temperatures. In: Applied biochemistry and biotechnology. Humana Press; 2007. p. 81e92. [7] Manochio C, Andrade BR, Rodriguez RP, Moraes BS. Ethanol from biomass: a comparative overview. Renew Sustain Energy Rev 2017;80:743e55. [8] Nwufo OC, Nwafor OMI, Igbokwe JO. Effects of blends on the physical properties of bioethanol produced from selected Nigerian crops. Int J Ambient Energy 2016;37(1):10e5. [9] Rabemanolontsoa H, Saka S. Various pretreatments of lignocellulosics. Bioresour Technol 2016;199: 83e91. ´ L, Meireles MAA. New starches are the trend for industry applications: a review. Food [10] Santana A Public Health 2014;4(5):229e41.

3

Biofuel purification and upgrading: using novel integrated membrane technology Parisa Biniaz, Tayebe Roostaie, Mohammad Reza Rahimpour DEPARTMENT OF CHEMICAL ENGINEERING, SHIRAZ UNIVERS ITY, SHIRAZ, F AR S, IRAN

Abbreviations AEM anion exchange membranes CEM cation exchange membranes FAME fatty acid methyl esters HTL hydrothermal liquefaction MBR membrane bioreactor MF microfiltration NF nanofiltration PEM polymer electrolyte membrane RO reverse osmosis SS suspended solid UF ultrafiltration

1. Introduction Indiscriminate use of fossil fuels has caused very serious concerns among environmentalists and economists. Therefore, scientists are expected to conduct extensive research into renewable energy sources. Biofuels have attracted a great deal of attention because of their unique characteristic such as being sustainable, clean, biodegradable, and nontoxic. Consequently, biofuels purification and upgrading are an essential step before marketing [1]. The new generation of high-performance membranes has made it possible for researchers to increase biofuel production speed, enhance the efficiency of biofuel separation, improve the quality of bioproducts, and effectively decrease the biofuel production costs [2]. Removing water [3] from biomass or taking contaminants out of sugar streams has been considered as one of the serious challenges in biorefinery Advances in Bioenergy and Microfluidic Applications. https://doi.org/10.1016/B978-0-12-821601-9.00003-0 Copyright © 2021 Elsevier Inc. All rights reserved.

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applications. Furthermore, the separation of carbon dioxide from both aqueous and vapor-phase materials, which accounts for as much as 50% of the costs of converting biomass to useful products, is another major problem that can be fully addressed by membrane technology [4]. Membrane technology brings tremendous engineering and economic benefits in comparison to other separation processes and provides a high purity and quality biofuel. This is mainly because applying these kinds of technologies in bioprocesses dramatically reduces operating costs compared to the conventional methods that use an evaporator for recovering or removing water. Besides, the remarkable properties of the membranes include their resistance to mechanical, chemical, and thermal stress, large surface area per unit volume, high selectivity, and the ability to control the components contacted between two phases that lead to the attraction of researchers’ interest. These features have made membrane technology a prospective candidate for downstream and upstream biofuel production and refining operations. Moreover, the adoption of these ecofriendly, low-cost, and low-power membrane processes can mitigate the pollution caused by traditional upgrading methods [5]. In general, a membrane is regarded as a barrier to separate two phases that could be liquid or gaseous. Through the process, specific compounds are retained in the feed side known as the retentate phase and other compounds of the initial mixture are allowed to cross into the permeate side or stripping phase [6]. For substance transfer from the feed side to the retentate side, a driving force such as pressure gradient, electric gradient, concentration gradient, or the difference in temperature is needed. In other words, the membrane separation processes are classified as concentration, electrical, and pressuredriven membrane processes [7]. Besides that, membranes utilized in biofuel purification and upgrading might be categorized based on their fabricated materials as metal membranes, ceramic membranes, and polymeric membranes [5]. When it comes to membrane properties, membrane flux is the most sensitive parameter that causes changes in the minimum fuel-selling price, so achieving high flux is the key element of choosing an effective membrane. Furthermore, to some extent, biofuel upgrading depends on the molecular size of the components comprising a biofuel. Therefore, the size of the membrane pores is a determining factor in producing high quality and pure biofuels [8]. Typically, membrane technology is applied in various purification and upgrading processes. The most common applications include ultrafiltration of biodiesel, ultrafiltration/nanofiltration of lignin in a solvent-based lignocellulose conversion process, the recovery of amino acids via electrodialysis [9], fast pyrolysis of by-products for recovering essential and valuable constituents such as acetic acid and hydrogen from liquid and gas phases, hydrothermal liquefaction to create bio-oil from algae [10] and removing the solid components from the organic phase [11]. More importantly, membrane processes enhance the productivity of hydrogen in methane steam-reforming operations and pervaporation for alcohol recovery [12].

Chapter 3  Biofuel purification and upgrading 71

This study examines a comprehensive review of the sophisticated membrane technology covering the applications of the membrane in biofuel purification and upgrading. In this context, different purification and upgrading processes with membrane are evaluated in detail. Besides, the characteristics of the membrane and recent studies in the various associated fields are presented and discussed.

2. Membrane features A membrane is a permeable barrier that transmits some particles through while keeping others. This behavior originates from membrane module characterizations, like the size of membrane pore, the shape of pore, membrane surface feature (porosity and charge/ hydrophobicity), and membrane configuration (dimensions and geometry) [6]. Membranes applied in biofuel production and purification would be categorized based on their materials into ceramic (inorganic) membranes, polymeric (organic) membranes, liquid membranes, and metal membranes. Generally, organic membranes can be classified based on their hydrophilicity or corresponding hydrophobicity. Hydrophobic substances are more useful for oil separation. Although hydrophilic membranes are less feasible to being exposed to the fouling phenomenon, created from different biorefinery feeds, they are more susceptible to being deformed because of temperature and pH swings [13]. Considerably, there is a wide diversity of organic membranes in comparison with their inorganic counterparts. This is due to the fact that inorganic membranes mainly focus on alpha-alumina support structures with titanium oxide or zirconium oxide [14]. Fig. 3.1 illustrates a general layout of the separation phenomenon via membrane [15].

3. Membrane types The membrane separation processes are classified into concentration-driven (pervaporation and membrane extraction) [16,17], pressure-driven (microfiltration (MF),

FIGURE 3.1 Schematic diagram of a membrane system [15].

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ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO) [18], electrical-driven (electrodialysis, electrophoresis), and prospective membrane processes (e.g., reverse membrane bioreactor). Fig. 3.2 clearly illustrates an overview of diverse membrane processes and respective biorefinery applications [7,19,20]. In membrane extraction, two immiscible liquids touch each other inside of the membrane or at either surface of the membrane while low concentrated solutes are allowed to transfer across the liquideliquid interface [21]. In pervaporation, an extraphase change takes place as the fluid feed is transferred into the vapor phase at the permeate side of the membrane. The pervaporation process is applied to recover alcohol from fermentation in which a hydrophobic membrane is utilized for separating the low concentrated ethanol from the aqueous solution. Besides that, a hydrophilic pervaporation unit is employed to dehydrate the ethanol [22]. In the pressure-driven membrane process, the solution is separated into permeate and retentate phases by exerting pressure on the solution as a driving force. In other words, the solvent on the feed side of the membrane is transported through the membrane to the permeate side by the pressure difference between two sides [18]. Immersed/submerged and submerged/ external loop are the two common configurations of pressure-driven membrane bioreactors (MBRs). In the submerged system, the membrane module is submerged in the main bioreactor or a separate section, while in the submerged system module is placed

FIGURE 3.2 Schematic diagram of different membrane processes in biorefinery [7].

Chapter 3  Biofuel purification and upgrading 73

in an external chamber. Also, the pressure-driven membranes can be classified according to the membrane pore size and the necessary transmembrane pressure into MF (0.1e5 mm, 1e10 bar), UF (500e100,000 Da, 1e100 nm, 1e10 bar), NF (100e500 Da, 0.5e10 nm, 10e30 bar), and RO (99%. It is also feasible to utilize the RO membrane process after the distillation stage for polishing the evaporator condensate, which might hold a high amount of COD/BOD. Apart from being integrated into the system directly, membrane processes might be applied in the water recycling stage of the bioethanol plant. To illustrate, nanofiltration and RO can be utilized in the pretreatment of the intake water and the membrane bioreactors can be employed in the wastewater treatment step [20]. Optimum process configurations of the bioethanol purification system with an algae process utilizing the distillation-membrane-pervaporation process such as RO membrane pretreatment were investigated by Ref. [19]. The results indicated that RO pretreatment reduces the costs for dilute ethanol feeds and high throughputs where the pretreatment enables the constraint on the size of a distillation column to be met. Furthermore, separation costs analysis showed that the highest proportion of the operating costs was devoted to distillation. Moreover, including the RO pretreatment significantly decreases distillation operating costs and reboiler energy consumption when the purity of ethanol feed is very low. Mahboubi et al. applied an MBR system utilizing integrated permeate channel membrane panels for fermenting pentose and hexose sugars to bioethanol simultaneously [51]. They employed a continuous process for the fermentation of high suspended solid wheat straw hydrolysate and achieved desirable results of coconsuming all glucose. Furthermore, up to 83% of the theoretical ethanol efficiency was obtained with up to 83% of xylose in a continuous fermentation mode.

10. Upgrading biohydrogen Biohydrogen is generated in the biomass cells such as microalgae through photosynthetic metabolism. This superior fuel is used immediately in PEM fuel cells under sulfur starvation or applied in electricity production. Biophotolysis and catabolism are two common methods employed in biohydrogen production [52]. The separation of

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hydrogen from complex biological gas compounds including carbon dioxide, hydrogen sulfide, and water vapor is a challenging task since they lead to a considerable reduction in the required enrichment efficiency. Therefore, it is essential to separate produced hydrogen from various gaseous by-productsdmainly CO2dformed during the fermentation [53]. A vast variety of operating systems such as adsorption, cryogenic distillation, chemical absorption, and membrane separation is applied in biohydrogen upgrading processes. Energy- and cost-effectiveness of membrane technology along with the possibility of integrated production and purification (membranes can be easily attached to the hydrogen-producing bioreactor) make them an efficient candidate for biohydrogen upgrading. Among different membrane-based purification methods, membrane contactors, supported liquid membranes, and gas separation membranes are highly capable for biohydrogen improvement. The combination of membrane bioreactors with gas separation systems seems to be excellent designs for enhancing the feasibility of fermentative hydrogen production [54]. Furthermore, nonporous, polymeric, and ionic liquid-based membranes are considered as potential candidates to be integrated with hydrogen-producing bioreactors as they can be operated approximately under the conditions in which biohydrogen formation takes place [53]. It should be also underlined that process parameters such as temperature, feed flow, retentate ratio, and the composition of the gas are critical factors in the separation of hydrogen and could considerably influence separation efficiency [55]. Bakonyi et al. were designed a gas separation membrane bioreactor (GSMBR) via combining membrane processes with a continuous biohydrogen fermenter [56]. As shown in Fig. 3.7, GSMBR system is an in situ separation process in which the headspace gas is split into two gas streams comprising higher concentrations of either H2 or CO2 named as

FIGURE 3.7 Schematic diagram of biohydrogen purification by purging the membrane bioreactor with internal gas recycling [56].

Chapter 3  Biofuel purification and upgrading 81

retentate and permeate. As a consequence, at least one of the two streams (retentate or permeate) should be recycled to the bottom of the membrane bioreactor to recognize the intended purging of the liquid phase and put the GSMBR to operate. The practicability of this new construction for improving the capacity of hydrogen production was examined by stripping the fermentation liquor with CO2- and H2-enriched gases collected directly from the bioreactor headspace. The results demonstrated that sparging the bioreactor with the CO2-concentrated fraction of the membrane separation unit (consisting of two PDMS modules) could significantly enhance the steady-state H2 productivity. The study also confirmed that stripping the biohydrogen fermenter with its own, self-generated atmosphere after adjusting its composition (to higher CO2-content) can be an effective and efficient method for intensifying dark fermentative H2 evolution. A summary of various operating conditions including temperature and membrane types for purification of different biofuels such as bioethanol, bio-oil, biodiesel, and biogas applying varying upgrading methods such as pyrolysis, fermentation, transesterification, liquefaction, and anaerobic digestion conducted in 2018, is presented in Table 3.1.

11. Conclusion and future trend Excessive consumption of fossil-based fuels has aroused very serious concerns among environmentalists. The unique properties of biofuels such as being sustainable, clean, biodegradable, and nontoxic have made them an acceptable substitute for fossil fuels. Consequently, biofuels purification and upgrading are an essential step before marketing. The new generation of high-performance membranes could increase biofuel production speed, enhance the efficiency of biofuel separation, improve the quality of bioproducts, and effectively decrease the production costs. Moreover, the adoption of these environmentally eco-friendly, low-cost, and low-power membranes can mitigate pollution caused by traditional upgrading methods. The membrane separation processes are classified according to their driving force as concentration-driven, pressure-driven, and electrical-driven membrane processes. Generally, the existence of impurities in different biofuels like biogas, biodiesel, bio-oil, bioethanol, and biohydrogen exerts significant effects on their performance and complicates their handling and storage. This chapter reviews the most advanced membrane technologies covering the membrane applications in various biofuel purification and upgrading. Brief definitions of some methods are described as follows: Upgrading the biogas is used to remove carbon dioxide, hydrogen, nitrogen, and many different trace compounds such as H2O, H2S, and trace compounds (ammonia, siloxanes) and to increase methane content of biogas. Two types of membrane processes applied in biogas upgrading are gas permeation and membrane contactors. Biodiesel is upgraded for removing impurities like tri-, di-, and monoglycerides, soap, catalyst, and traces of alcohol and selectively permeating FAMEs that comprise a typical biodiesel

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Table 3.1 Review articles concerning various operational conditions in biofuels processing using membranes in 2018. Type of biofuel upgrading process Biogas

Biogas

Biodiesel

Biodiesel

Bio-oil

Bioethanol

Highlight

References

 Hollow fiber membranes modified by selective polyimide were investigated to deliver biomethane  Single-stage permeation system was applied and 93.8 vol% of CH4 mostly biomethane (>95 vol% of CH4) was achieved as retentate  Experiments were carried out at 30 C  Critical factors in biogas upgrading efficiency such as the ratio of permeate-to-feed pressure and the splitting factor were investigated in terms of CH4 recovery and the amount of CH4 on the retentate-side  Crystalline nanocellulose/polyvinyl alcohol (CNC/PVA) membranes were used to enhance the quality of biogas by capturing CO2  The effects of CNC concentration and the pH of the casting solution were investigated to optimize CO2/CH4 separation  The effects of casting solution pH and the concentration of the CNC were examined for optimizing the CO2/CH4 separation  Nanocomposite membranes containing 1% CNC displayed the best performance.  Membranes displayed high CO2 permeability and selectivity at higher pH.  Biodiesel purification was conducted with several solvent-resistant composite nanofiltration membranes.  The poly(vinylidenedifluoride) (PVDF) was used as support and poly(dimethylsiloxane) was employed as the coating layer  Biodiesel was produced from the esterification of partially refined soy oil.  Bioethanol (EtOH) and NaOH were used as the catalyst  The proposed membranes indicated remarkable stability for biodiesel permeation.  A high flux recovery ratio of EtOH (about 0.94) was achieved after 20 cycles of application.  An immobilized-cell biofilm photo-bioreactor with polytetrafluoroethylene membrane was applied  The proposed biofilm cultivation process was made to separate gas from liquid  The effects of CO2 concentration on the system performance was examined  The productivities of lipid and biomass were significantly improved  Bio-oils were stabilized and upgraded by an electrochemical system before high pressure and temperature hydrotreating  The cell was used to separate organic acids and decrease protons to hydrogen utilizing three compartments with the electrodes appended to anion and cation exchange membranes configured  The system was operated at 35 C  A recirculation method was used to limit the length of the flow path in the electrolyzer  Although the novel proposed reactor could upgrade bio-oils effectively through the electrochemical route, considerable improvements are essential in total acid number removal and carbonyl conversion  The production of bioethanol from nanofiltration of biomass chemical hydrolysis solutions was investigated in terms of technical and economic approach

[32]

[57]

[58]

[59]

[46]

[60]

Chapter 3  Biofuel purification and upgrading 83

Table 3.1 Review articles concerning various operational conditions in biofuels processing using membranes in 2018.dcont’d Type of biofuel upgrading process

Highlight

References

 82% of total alcohol efficiency was achieved by fermentability of the purified sugar solution  Bioethanol was produced through chemical hydrolysis of biomass with an economically and environmentally sustainable multiple-nanofiltration process

product. Furthermore, biodiesel upgrading methods are categorized according to the nature of the system into equilibrium-based (liquideliquid extraction, distillation, supercritical fluid extraction), affinity-based (ion exchange, adsorption), membrane-based, reaction-based (reactive distillation, membrane bioreactors), and solideliquid separation processes. Oxygen, water, and nitrogen content are removed during bio-oil upgrading. Besides that, heating value, thermal and chemical gas stability increase. Hydrogenation of unsaturated bonds in bio-oil and related surrogate compounds by PEM electrolyzer is an attractive low-temperature technology in this regard. Bioethanol is another biofuel, which is essential to upgrade to eliminate enzymes and starch residues from the cellulosic or glucose biomass formerly the fermentation step. Membrane processes such as microfiltration, ultrafiltration, nanofiltration, and reverse osmosis as well as the emerging membrane processes pervaporation and vapor permeation are highly capable in bioethanol industry. Biohydrogen is also upgraded to eliminate carbon dioxide, hydrogen sulfide, water vapor, etc. Among the different membrane-based purification methods, membrane contactors, supported liquid membranes, and gas separation membranes have shown considerable potential for enriching biohydrogen.

References [1] Samadi S, Karimi K, Behnam S. Simultaneous biosorption and bioethanol production from leadcontaminated media by Mucor indicus. Biofuel Res J 2017;4(1):545e50. [2] Rahimpour MR. 10 - membrane reactors for biodiesel production and processing. In: Basile A, et al., editors. Membrane reactors for energy applications and basic chemical production. Woodhead Publishing; 2015. p. 289e312. [3] Gerardo ML, Oatley-Radcliffe DL, Lovitt RW. Integration of membrane technology in microalgae biorefineries. J Membr Sci 2014;464:86e99. [4] Petersson A, WeLLInGer A. Biogas upgrading technologiesedevelopments and innovations. IEA bioenergy 2009;20:1e19. [5] Figoli A, Cassano A, Basile A. Membrane technologies for biorefining. Woodhead Publishing; 2016. [6] Chapter 2 - Fundamentals. In: Judd S, Judd C, editors. The MBR book. 2nd ed. Oxford: ButterworthHeinemann; 2011. p. 55e207.

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[7] Abels C, Carstensen F, Wessling M. Membrane processes in biorefinery applications. J Membr Sci 2013;444:285e317. [8] Atadashi I, Aroua M, Aziz AA. Biodiesel separation and purification: a review. Renew Energy 2011; 36(2):437e43. [9] Readi OK, et al. On the isolation of single basic amino acids with electrodialysis for the production of biobased chemicals. Ind Eng Chem Res 2012;52(3):1069e78. [10] Galadima A, Muraza O. Hydrothermal liquefaction of algae and bio-oil upgrading into liquid fuels: role of heterogeneous catalysts. Renew Sustain Energy Rev 2018;81:1037e48. [11] Vane LM. A review of pervaporation for product recovery from biomass fermentation processes. J Chem Technol Biotechnol: Int Res Process, Environ Clean Technol 2005;80(6):603e29.  ski W. Biobutanol concentration by pervaporation using supported [12] Rdzanek P, Marszałek J, Kamin ionic liquid membranes. Separ Purif Technol 2018;196:124e31. [13] Ramaswamy S, Huang H-J, Ramarao BV. Separation and purification technologies in biorefineries. John Wiley & Sons; 2013. [14] Gomes MCS, Arroyo PA, Pereira NC. Influence of acidified water addition on the biodiesel and glycerol separation through membrane technology. J Membr Sci 2013;431:28e36. [15] Abdullah N, et al. Chapter 2 - membranes and membrane processes: fundamentals. In: Basile A, Mozia S, Molinari R, editors. Current trends and future developments on (bio-) membranes. Elsevier; 2018. p. 45e70. [16] Grzenia DL, et al. Conditioning biomass hydrolysates by membrane extraction. J Membr Sci 2012; 415e416:75e84. [17] Iryani D, et al. Lampung natural zeolite filled cellulose acetate membrane for pervaporation of ethanol-water mixtures. In: IOP conference series: earth and environmental science. IOP Publishing; 2018. [18] Van der Bruggen B, et al. A review of pressure-driven membrane processes in wastewater treatment and drinking water production. Environ Prog 2003;22(1):46e56. [19] Kanchanalai P, et al. Cost and energy savings using an optimal design of reverse osmosis membrane pretreatment for dilute bioethanol purification. Ind Eng Chem Res 2013;52(32):11132e41. [20] Lipnizki F. Membrane process opportunities and challenges in the bioethanol industry. Desalination 2010;250(3):1067e9. [21] Berrios J, Pyle DL, Aroca G. Gibberellic acid extraction from aqueous solutions and fermentation broths by using emulsion liquid membranes. J Membr Sci 2010;348(1):91e8. [22] Liu Q, et al. Mixed-matrix hollow fiber composite membranes comprising of PEBA and MOF for pervaporation separation of ethanol/water mixtures. Separation and Purification Technology; 2018. [23] Cui ZF, Jiang Y, Field RW. Chapter 1 - fundamentals of pressure-driven membrane separation processes. In: Cui ZF, Muralidhara HS, editors. Membrane technology. Oxford: ButterworthHeinemann; 2010. p. 1e18. [24] Huang C, et al. Application of electrodialysis to the production of organic acids: state-of-the-art and recent developments. J Membr Sci 2007;288(1):1e12. [25] Mahboubi A, et al. Reverse membrane bioreactor: introduction to a new technology for biofuel production. Biotechnol Adv 2016;34(5):954e75. [26] Meng F, et al. Fouling in membrane bioreactors: an updated review. Water Res 2017;114:151e80. [27] Ullah Khan I, et al. Biogas as a renewable energy fuel e a review of biogas upgrading, utilisation and storage. Energy Convers Manag 2017;150:277e94. [28] Ka´ra´szova´ M, Sedla´kova´ Z, Iza´k P. Gas permeation processes in biogas upgrading: a short review. Chem Pap 2015;69(10):1277e83.

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[29] Pimia¨ T, et al. Organic waste streams in energy and biofuel production. 2014. [30] Molino A, et al. Biogas upgrading via membrane process: modelling of pilot plant scale and the end uses for the grid injection. Fuel 2013;107:585e92. [31] Scholz M, Melin T, Wessling M. Transforming biogas into biomethane using membrane technology. Renew Sustain Energy Rev 2013;17:199e212. [32] Nemesto´thy N, et al. Evaluation of a membrane permeation system for biogas upgrading using model and real gaseous mixtures: the effect of operating conditions on separation behaviour, methane recovery and process stability. J Clean Prod 2018;185:44e51. [33] Brunetti A, et al. Membrane technologies for CO2 separation. J Membr Sci 2010;359(1):115e25. [34] Veljkovi c VB, et al. Biodiesel production from corn oil: a review. Renew Sustain Energy Rev 2018;91: 531e48. [35] Demirbas A. Biodiesel. Springer; 2008. [36] Bateni H, Saraeian A, Able C. A comprehensive review on biodiesel purification and upgrading. Biofuel Res J 2017;4(3):668e90. [37] Jaber R, et al. Biodiesel wash-water reuse using microfiltration: toward zero-discharge strategy for cleaner and economized biodiesel production. Biofuel Res J 2015;2(1):148e51. [38] Atadashi IM, et al. Membrane biodiesel production and refining technology: a critical review. Renew Sustain Energy Rev 2011;15(9):5051e62. [39] Giorno F, Mazzei R, Giorno L. Purification of triacylglycerols for biodiesel production from Nannochloropsis microalgae by membrane technology. Bioresour Technol 2013;140:172e8. [40] Yi L, et al. In situ upgrading of bio-oil via CaO catalyst derived from organic precursors. Proceedings of the Combustion Institute; 2018. [41] Xiu S, Shahbazi A. Bio-oil production and upgrading research: a review. Renew Sustain Energy Rev 2012;16(7):4406e14. [42] Isahak WNRW, et al. A review on bio-oil production from biomass by using pyrolysis method. Renew Sustain Energy Rev 2012;16(8):5910e23. [43] Valle B, et al. Integration of thermal treatment and catalytic transformation for upgrading biomass pyrolysis oil. Int J Chem React Eng 2007;5(1). [44] Sorunmu Y, et al. Life cycle assessment of alternative pyrolysis-based transport fuels: implications of upgrading technology, scale and hydrogen requirement. ACS Sustainable Chemistry & Engineering; 2018. [45] Pintauro P, et al. Electrochemical hydrogenation of soybean oil with hydrogen gas. Ind Eng Chem Res 2005;44(16):6188e95. [46] Lister TE, et al. Low-temperature electrochemical upgrading of bio-oils using polymer electrolyte membranes. Energy Fuel 2018;32(5):5944e50. [47] Saber M, Nakhshiniev B, Yoshikawa K. A review of production and upgrading of algal bio-oil. Renew Sustain Energy Rev 2016;58:918e30. [48] Teella A, Huber GW, Ford DM. Separation of acetic acid from the aqueous fraction of fast pyrolysis bio-oils using nanofiltration and reverse osmosis membranes. J Membr Sci 2011;378(1):495e502. [49] Hahn-Ha¨gerdal B, et al. Bio-ethanol e the fuel of tomorrow from the residues of today. Trends Biotechnol 2006;24(12):549e56. [50] Saı¨dane-Bchir F, et al. 3rd generation bioethanol production from microalgae isolated from slaughterhouse wastewater. Waste Biomass Valorizat 2016;7(5):1041e6.

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[51] Mahboubi A, et al. Continuous bioethanol fermentation from wheat straw hydrolysate with high suspended solid content using an immersed flat sheet membrane bioreactor. Bioresour Technol 2017;241:296e308. [52] Rahimpour MR, Biniaz P, Makarem MA. 14 - integration of microalgae into an existing biofuel industry. In: Dalena F, Basile A, Rossi C, editors. Bioenergy systems for the future. Woodhead Publishing; 2017. p. 481e519. [53] Bakonyi P, Nemesto´thy N, Be´lafi-Bako´ K. Biohydrogen purification by membranes: an overview on the operational conditions affecting the performance of non-porous, polymeric and ionic liquid based gas separation membranes. Int J Hydrogen Energy 2013;38(23):9673e87. [54] Ramı´rez-Morales JE, et al. Evaluation of two gas membrane modules for fermentative hydrogen separation. Int J Hydrogen Energy 2013;38(32):14042e52. [55] Bakonyi P, et al. Biohydrogen purification using a commercial polyimide membrane module: studying the effects of some process variables. Int J Hydrogen Energy 2013;38(35):15092e9. [56] Bakonyi P, et al. A novel gas separation integrated membrane bioreactor to evaluate the impact of self-generated biogas recycling on continuous hydrogen fermentation. Appl Energy 2017;190: 813e23. [57] Jahan Z, et al. Cellulose nanocrystal/PVA nanocomposite membranes for CO2/CH4 separation at high pressure. J Membr Sci 2018;554:275e81. [58] Torres JJ, et al. Biodiesel purification using polymeric nanofiltration composite membranes highly resistant to harsh conditions. Chem Eng Technol 2018;41(2):253e60. [59] Zhang L, et al. Effect of carbon dioxide on biomass and lipid production of Chlorella pyrenoidosa in a membrane bioreactor with gas-liquid separation. Algal Res 2018;31:70e6. [60] Kuo Y-T, et al. Technical and Economic approach of bioethanol production from nanofiltration of biomass chemical hydrolysis solutions. Appl Energy 2018;215:426e36.

4

Chemical looping conversion of biomass and biomass-derived feedstocks

Mehdi Piroozmand1, Yaser Balegh1, Ali Hafizi1, Morteza Esfandyari2 DEPARTMENT OF CHEMICAL ENGINEERING, SHIRAZ UNIVERS ITY, SHIRAZ, F AR S, IRAN; DE PARTMENT OF CHEMICAL ENGINEERING, UNI VERSITY OF BOJNORD, BOJNORD, NORTH K HOR AS AN, IRAN 1

2

1. Introduction Sustainable energy production and consumption result in negligible harmful influence on the environment. Renewable energy sources have the benefit of reducing greenhouse gases (GHG), economic efficiency, and consistent energy source [1]. Biomass is one of the most important sustainable energy sources. The estimations show that more than 1:4
108 tons of biomass such as wood chips of forest harvesting and processing and agriculture sector are available per year. Converting only 1% of this huge biomass into fuel with the overall efficiency of 50% can solely produce more than the world energy consumption per year [2]. Biomass and biomass-derived feedstocks could be applied for the production of heat, electricity, and hydrogen. Hydrogen is widely used as an important chemical feedstock in modern industries for producing various chemical and petrochemical products such as methanol and ammonia [3]. Furthermore, H2 is known as an environmentally benign energy source and could be applied as a clean fuel for the next years [3,4]. Numerous technologies are applied for hydrogen production, but during the recent decades, more than 96% of hydrogen was produced based on nonrenewable sources. The reforming of natural gas is the most known technology for commercial hydrogen production [5]. Accordingly, to meet the fossil fuel consumption and eliminating the CO2 emission, novel sustainable technologies resulting from renewable sources of hydrogen and energy must be developed. The chemical looping (CL) process is a recently proposed technology for the production of hydrogen, synthesis gas (H2 and CO), power, and energy from different hydrocarbon sources. Chemical looping reforming (CLR) and chemical looping steam reforming (CLSR) are novel technologies for converting hydrocarbons to syngas with a Advances in Bioenergy and Microfluidic Applications. https://doi.org/10.1016/B978-0-12-821601-9.00004-2 Copyright © 2021 Elsevier Inc. All rights reserved.

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FIGURE 4.1 Conceptual scheme of chemical looping reforming.

suitable H2/CO ratio using a solid oxygen carrier [6]. The CLR process conventionally includes two different reactors called fuel reactor (FR) and air reactor (AR). In the fuel reactor, the hydrocarbons as fuel are burnt with the oxygen carrier, while the OC (MexOy) is reduced to MexOys. After that, the reduced OC is oxidized with oxidants such as air and returns to its oxidation states in the oxidation reactor (air reactor) as demonstrated in Fig. 4.1 [5,7]. The principal reactions that occur in the CLR process are as follows: Fuel reactor  m H2 sCn Hm þ Mex Oy /Mex Oysn þ ðs þ nÞCO þ S þ 2

(4.1)

CO þ H2 O4H2 þ CO2

(4.2)

Air reactor Mex Oysn þ

sþn O2 /Mex Oy 2

(4.3)

The selection of appropriate oxygen carrier is essential for the development of a chemical looping process. The oxygen carrier should have high chemical and mechanical stability, adequate durability, and reactivity through cyclic chemical looping process. Also, it should have a low tendency to agglomeration, fragmentation, coke deposition, and particle attrition [8]. The conversion of biomass and biomass-derived feedstocks using the chemical looping processes has a promising potential for the production of hydrogen, syngas, and power. Fig. 4.2 represents the various recent technologies of biomass and biomassderived feedstocks into valuable products.

Chapter 4  Chemical looping conversion of biomass

89

FIGURE 4.2 Short configuration of biomass conversion as a renewable fuel for power and hydrogen generation.

According to the above consideration, the objective of this research is to investigate the issues, challenges, and advances associated with the process and catalysis along with the type and properties of biomass and biomass-derived feeds and the chemical looping transformation to pure H2 and syngas. The conversion of biomass and biomass-derived feeds in environmentally benign procedures mentioned herein might be highlighted and is expected to prove advantageous for students, scientists, and technology developers.

2. Basic principles and concepts 2.1

Chemical looping technology/process

Chemical looping technology has attracted much attention in the recent 2 decades. The chemical looping technologies could be applied in many industries and different processes and are being developed for each unit day by day. The chemical looping process could be applied in petrochemical plants such as methanol, ammonia, dimethyl ether, and hydrogen. It is also applicable in industrial installations to produce electricity and heat. There are various reasons why this technology is being developed. During

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recent decades, the replacement of fossil fuels with renewable fuels and energy sources is mentioned, which has neither produced lower pollution nor caused fewer operating difficulties in the process. These processes themselves have been designed and developed from the integration of several primary systems. Chemical looping technology is one of the most important primary units. The most common component in any chemical cycle is a metal oxide, which is expected to continue circulating in this loop. Different processes have been developed based on feed phase and composition, reactor type, oxygen carrier, and final product. The most important proposed processes are investigated in the following.

2.1.1 Chemical looping steam reforming technology/process Many different technologies have been introduced for the chemical looping process. All investigations have been reviewed under different process conditions. Ordinary systems such as chemical looping steam reforming have also been applied to biogas with the aim of producing hydrogen [9]. Fig. 4.3C shows the CL-SR process that has been combined with other processes over time, resulting in a new process or technology.

2.1.2 Chemical looping combustion technology/process Chemical looping combustion (CLC) is another primary process of chemical looping technology that is used for the production of heat and CO2. The most important difference of CLC and CL-SR is the ratio of OC to fuel that results in the combustion of

FIGURE 4.3 Schematic representation of biomass-based chemical looping principal systems/technologies: (A) CLG process modest diagram, (B) Simplified scheme of chemical looping combustion, (C) Schematic diagram of a simple CL-SR process.

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fuel in the CLC process. The CLC process can address gaseous, liquid, or solid materials as primary fuels. As shown in Fig. 4.3B for CLC of gaseous fuels, the oxygen carrier reacts directly with the fuel, for example, biogas, etc. The CLC process has no limitation on the choice of inlet fuel. It uses all conventional gaseous, liquid, and solid fuels, including natural gas, bio-oil, coal, biomass, and refinery gases [10]. The CLC process can proceed with even solid fuels such as biomass and coal. Due to the combustion in the Fuel reactor (FR) of this system and the high heat generation, some research works have used autothermal reactors (coupled reactors) as the heat reactor, which introduced new technology in the CLC system [11]. A fast fluidized-bed riser with high velocity is commonly applied for autothermal reforming, while a lowvelocity bubbling fluidized-bed riser is proposed for fuel reactors in the CLC process [11]. It is more efficient than similar processes in terms of energy and hydrogen production because it uses the membranes system to control the amount of produced hydrogen. It means that steam reforming and chemical looping combustion are proceeding in the proposed technology and an autothermal reactor as one process to become chemical looping combustion-steam reforming (CLC-SR).

2.1.3 Chemical looping gasification technology/process The chemical looping gasification (CLG) process is another foundation of chemical looping technology. As shown in Fig. 4.3A, this process is schematically very similar to the CLC and CL-SR processes, but the differences in the type of reaction between the metal oxide and the feed of both reactors have altered the outcome of the process.

2.1.4 Biomass-based chemical looping principal technology/process To the coproduction of hydrogen and electricity, the following basic processes should be applied. As Fig. 4.4 shows, these biomass-based subprocesses can be combined to achieve a new system for biomass conversion. As indicated in this figure, these subprocesses include biomass-based chemical looping combustion (BCLC), biomassbased chemical looping gasification (BCLG), biomass-based coproduction chemical looping process (BCCLP), biomass-based calcium looping combustion (BCaLC), biomass-based calcium looping gasification, and (f ) sorption enhanced BCLG.

2.2

Chemical looping materials

Two reasons have been described as obstructions on the commercialization of chemical looping processes. The first is the lack of adequate performance of CL oxygen carrier materials, including their reactivity, oxygen capacity, recyclability, and stability against attrition, and the second is related to the reactor design and solid circulatory systems [13]. Therefore, to overcome the insufficiencies of the materials for commercial implementation, the development of such materials seems to be essential. The other concern is the environmental effect; therefore, besides the effect of the CLR process to produce H2 as environmentally friendly fuel, using fewer toxic materials and attempts to capture the CO2 is desirable. The looping materials are commonly composed of four parts including the OCs used for storage and transfer of oxygen, promoter to stabilize or promote OC metals, support, and finally the sorbents as CO2 capturing materials.

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FIGURE 4.4 Schematic representation of biomass-based chemical looping systems: (A) Biomass-based chemical looping combustion (BCLC), (B) Biomass-based chemical looping gasification (BCLG), (C) Biomass-based coproduction chemical looping process (BCCLP), (D) Biomass-based calcium looping combustion (BCaLC), (E) Biomass-based calcium looping gasification and (F) Sorption enhanced BCLG [12].

2.2.1 Oxygen carriers Chemical looping includes two reaction steps: first, the oxidation of carriers with air, CO2, or steam in the regeneration reactor; second, the transfer of oxidized substance to the fuel reactor where the reduction step is performed by the feedstock fuel, resulted in the oxidation of the fuel at high temperatures. Finally, the reduced oxygen carrier returned to the regeneration reactor to complete the cycle. As indicated previously, the most important properties of oxygen carriers are adequate oxygen transfer capacity, high redox-active property, high resistance to attrition and agglomeration and coke formation, low cost, and less environmentally destructive effects [10]. Transition metal oxides such as Ni, Fe, Mn, Co, and Cu are most commonly used for chemical looping purposes.

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2.2.1.1 Ni-based oxygen carriers Ni-based oxygen carriers have shown remarkable reactivity and good catalytic performance, especially for gaseous fuels. In the CLR process, the highest selectivity toward H2 could be achieved with Ni-based oxygen carriers. In addition, their promising catalytic properties make them be outstanding OCs [10,14]. Despite the excellent activity of Ni-based OCs in the reforming process of light hydrocarbons, the widespread utilization and commercialization of Ni-based OCs can be affected due to its high cost, sulfursensitive nature, and environmental effects [12]. Little contents of sulfur, especially for biogas feedstock, can poison Ni-based OC causing destructive operational effects. Despite these well-known problems, many efforts have been made to enhance NiO-supported materials. There are several methods currently being applied to reach this purpose such as optimizing the appropriate supporting materials with enhanced structural and textural properties [15], addition of suitable promoters-additives [16,17], and optimization of the synthesis method [18,19]. 2.2.1.2 Fe-based oxygen carriers The low cost of iron oxide with high mechanical and thermal stability along with its environmentally friendly nature was recommended as a prominent oxygen carrier. Fe-based OCs have a low tendency to poisoning by coke or sulfur [20]. One of the most important requirements of Fe-based OCs for the utilization in CL processes is improving its structural properties [21]. The most important oxidation states of iron are Fe3O4, Fe2O3, FeO, and Fe. Considering the thermodynamic limitation, the desired reduction for total oxidation of hydrocarbons such as methane is Fe2O3 to Fe3O4, while FeO to Fe is more adequate for partial oxidation [22]. The thermodynamic analysis of the CLC process represents that the complete combustion of fuel is only accessible with partially reduction of Fe2O3 to Fe3O4 [23]. On the other hand, agglomeration of Fe-based oxygen carriers and weak reactivity toward gaseous fuels resulted from Fe3O4 are the main drawbacks of this OC [24]. Fe-based oxygen carriers supported on Al2O3 showed better stability toward sintering and the higher oxygen capacity compared to Mn, Co, and Cu using alumina as support [5]. In general, without considering the support or promoting agents used in oxygen carrier structure, Fe-based OCs have relatively low reactivity and oxygen transfer capacity, while low costs, high resistance toward sintering, and resistance to coke and sulfur made it a promising oxygen carrier [12]. 2.2.1.3 Cu-based oxygen carriers Low cost besides significant oxygen transfer capacity, high reactivity, and low toxicity made copper a proper choice as an oxygen carrier. CuO is the oxidized form of copper, and its reduction gives Cu2O or Cu. The relatively low melting point of Cu accelerates the agglomeration tendency of its particles in high-temperature reactions. Therefore, adjustment of the reactor temperature below 800 C is recommended [12]. The other disadvantage of Cu is the weak mechanical strength [25]. Moreover, it is necessary to utilize proper supports or promoters to increase the resistance of Cu particles to avoid agglomeration.

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2.2.1.4 Mn-based oxygen carriers Manganese can be another prominent candidate for the selection of oxygen carrier metal owing to relatively low cost and toxicity along with relatively higher theoretical oxygen transfer capacity. The Mn oxidation states are MnO2 and Mn2O3. The former begins to decompose at almost 500 C under air conditions and the latter is the single state existing at high temperatures, and only its transform to MnO is practical for high-temperature reactions [26]. Using Al2O3, MgAl2O4, SiO2, and TiO2 as support of Mn can restrict its reactivity, nonetheless, utilizing ZrO2 or bentonite as support resulted in good performance and recyclability [12]. 2.2.1.5 Co-based oxygen carriers Similar to nickel, cobalt suffers from toxicity and high cost. Multiple oxidation states of cobalt are Co3O4, CoO, and Co. The transform of CoO to Co is most commonly used in the reduction step, considering the thermodynamical instability of Co3O4 at high temperatures. The interaction of CoO with supports like Al2O3, MgO, and TiO2 can form phases like CoAl2O4, Mg0.4Co0.6O, and CoTiO3 and significantly reduces its reactivity [27]. Co supported on yttria-stabilized zirconia showed good reactivity and high stability against coke formation in the CLC process [28]. The comparison of transition state metals as an oxygen carrier is indicated in Table 4.1. 2.2.1.6 Perovskite Perovskite-type oxides are one of the most promising structures widely investigated for chemical looping processes in recent years due to their high thermal strength and redox activity. The formula of the structure is ABO3, where A mostly represents lanthanum and Table 4.1 [12].

Comparison of utilized prominent transition metal-based oxygen carriers

Metal

Advantages

Drawbacks

Ni

High reactivity

Fe

Low cost High mechanical and thermal stability

High cost Toxic Sulfur poisoning Low oxygen transfer capacity Relatively low reactivity

Cu Co Mn

Resistance to coke or sulfide/sulfate formation Environmentally friendly High reactivity and oxygen transfer capacity Low cost and toxicity High reactivity and oxygen transfer capacity Low cost and toxicity High theoretical oxygen transfer capacity

Low melting point High agglomeration tendency High cost Toxic Low reactivity Low oxygen transfer capacity Sulfur poisoning

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B is an active transition metal [27]. Substitution of lanthanide ion with alkaline earth metals like Sr ion creates an electronic imbalance, which could increase the valence state of active B metal cations or form oxygen lattice vacancies [25]. The mobile lattice oxygen can take part in partial oxidation of hydrocarbons in the reforming process to form H2 and CO [29]. Using La, Nd, Eu as A part for AFeO3 oxides has been studied for syngas production in a fixed-bed reactor. The best conversion of methane to syngas and stability in consecutive cycles are achieved by LaFeO3 oxide [30]. Perovskite oxides showed acceptable reactivity in CLR whether incorporated by active nonnoble metals or used as support materials [31].

2.2.2 Sorbents One of the major challenges in the sorption-enhanced CLR process is the decline of CO2 sorption capacity during sequential operations, and it is mainly affected by sintering, resulting in agglomeration and changing the pore sizes [21]. Calcium oxide is the most investigated sorbent for sorption enhanced reforming processes and is widely utilized as high-temperature in-situ CO2 capture materials. CaO possesses a high tendency to CO2 capture with a high sorption capacity of 17.8 CO2 mol/kg at elevated temperatures [32]. However, the decline in CO2 capture capacity according to sintering is the most important drawback of it. The promotion of Ca-based sorbents with structural promoters such as Zr, Al, Si, and Ce was investigated to improve the lifecycle of these materials [21,33e35].

2.2.3 Supports The oxygen carriers used in the chemical looping processes are synthesized with different methods including mechanical mixing, coprecipitation, solegel, and impregnation. The OC with low metal loading and high dispersion is commonly synthesized using the impregnation method and requires appropriate support to improve the mechanical strength and dispersion of metals to hinder agglomeration and attrition. Support materials should provide high surface area and accessibility of active metal oxides resulting in higher reactivity of oxygen carriers [21]. The most prominent supports that have been investigated recently are oxides with high thermal strength such as Al2O3, SiO2, ZrO2, and TiO2 [36]. 2.2.3.1 Al2O3 High mechanical and thermal strength makes the Al2O3 as one of the best oxygen carrier supports for CL processes [21]. Using alumina as support truly enhanced the stability of oxygen carriers in sequential redox cycles. Nonetheless, the acidic nature of alumina can enhance the activity of active metals, while it highlights the carbon deposition on the OC structure [37]. Additionally, the high tendency of alumina for the formation of undesired spinels such as FeAl2O4, NiAl2O4, and CuAl2O4 can affect the oxygen transferring capacity and the reactivity of OCs [38]. The crystalline nature of alumina can affect the formation of these undesired phases, and some modifications or thermal treatments

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could minimize this problem. It has been reported in many types of research that the addition of some promoters such as Mg could improve alumina stability by forming MgAl2O4 to avoid the formation of other undesired phases with the active materials. 2.2.3.2 SiO2 Improvement of oxygen mobility and reducibility of OC could be performed by means of porous structures. Similar to alumina, silica has high mechanical and thermal strength in addition to the high surface area that makes it another candidate for the selection of OC support [39]. It was reported that one of the most promising supports for a variety of catalytic species is mesoporous silica-based materials, which generally offer well-ordered pore structure and high surface area [40]. However, lack of catalytic active sites in the silica mesoporous materials limits their application. 2.2.3.3 ZrO2 Zirconia is another support material with high thermal stability with an inhibiting effect on carbon deposition [35,41]. ZrO2 was used to modify NieCaO catalyst for steam reforming of methane, and improvement of stability and surface area of the catalyst was observed. Results from the comparison of ZrO2, TiO2, SiO2, Al2O3, and NiAl2O4 as support for Ni-based OCs in the CLR process indicate that NiO/ZrO2 exhibited the best conversion of methane and high stability in 20 redox cycles [42]. However, higher costs and lower surface area compared to alumina are the most important drawbacks of ZrO2 support.

2.2.4 Promoters Promoter materials are mostly applied to avoid the coke deposition and improvement of catalyst activity and textural properties. Adding promoting agents can improve the performance of oxygen carriers by hindering the formation of metal spinels. Deactivation of OCs caused by coke deposition is one of the most important drawbacks of CL processes. This problem can be diminished by adding a small amount of alkali metals that can effectively reduce carbon deposition on the catalyst surface [43]. The most important promoters investigated for different oxygen carriers are Ca, Mg, Ce, Y, La, Ba, Zr, Zn, W, Al, etc. 2.2.4.1 CeO2 High oxygen storage capacity along with redox properties made cerium oxide a good choice for the improvement of the OC. Combining CeO2 with another metal oxide can improve the reaction requisites. The redox feature of cerium oxide can improve the mobility of surface oxygen, resulting in lower coke formation on active sites [44]. Incorporation of cerium to Ni Supported on SBA-15 and SBA-16 showed remarkable improvement in the dispersion of Ni particles and OCs lifetime at high temperatures [44,45]. Using FeeCe metal oxides improved H2 yield and the stability of Fe2O3 oxygen carrier. The addition of CeO2 had promoting effects on the kinetics of reduction by promoting the transformation of Fe3O4 to FeO at lower temperatures [46].

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CeO2 also revealed the promotional effect on the thermal stability and surface characteristics of synthesized sorbents resulted in better CO2 capture performance. Using a small portion of Ce to CaO sorbent improved the activity and structural properties of sorbent, concluding that the addition of Ce shortened the diffusion barrier of CO2 within the CaO sorbent structure [39]. 2.2.4.2 Y2O3 Yttrium-based materials have been recently applied as the support and promoter for the catalytic reforming process. The attained results revealed that yttrium oxide has excellent potential for chemical looping reforming process [47,48]. The surface oxygen mobility of Y2O3 plays a key role in promoting catalytic activity [49]. The addition of Y to Pd/Al2O3 catalyst can suppress the coke formation due to the diffusion and reaction of formed oxygen with carbon deposition [48]. In addition, the presence of yttrium in the framework of support can effectively enhance the oxygen vacancies on the surface of supports and promoted the oxygen species mobility [50]. 2.2.4.3 Mg The addition of magnesium could efficiently enhance the activity and stability of oxygen carriers by adjustment of the support structure and pore size. On the other side, the nature of the Mg promoter could control the carbon deposition in CL processes due to the alkaline nature of it [38]. Introducing excess MgO in the structure of aluminasupported OC could inhibit the formation of FeeAl spinel and form MgAl2O4 resulted in higher oxygen transfer capacity using Fe2O3/MgAl2O4 OC [51]. In comparison with Fe2O3/Al2O3, the Mg-promoted Fe2O3/MgAl2O4 showed higher reactivity and lower coke formation at elevated temperatures [21]. 2.2.4.4 Other promoters The addition of calcium in the OC structure can positively affect the OC resistance to carbon deposition. The improved activity gained by 15 wt.%Fe-5 wt.%Ca/g-Al2O3 OC for methane steam reforming showed 100% conversion and high hydrogen selectivity at a relatively low temperature of 700 C [52]. Utilizing Ca for Ni/a-Al2O3 improved the dispersion and stability of nickel particles [53]. A combination of Zn with iron oxide showed improvement for the kinetic of the iron oxide resulted in decreasing the reaction temperature and increasing the reactivity for ZnFe2O4 OC [54].

3. Chemical looping biomass and biomass-derived conversion 3.1

Processes of chemical looping biomass conversion

Biofuel feedstock as a renewable energy source is also examined in different processes for standalone hydrogen and power generation. These processes have some advantages and disadvantages in their descriptions. In this section, almost all processes of chemical looping biomass conversion have been reviewed.

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3.1.1 Chemical looping systems/technologies concepts for biomass conversion 3.1.1.1 Chemical looping hydrogen system/technology The chemical looping hydrogen (CLH) process of renewable heavy fuels such as a heavy fraction of bio-oil for the production of pure hydrogen and elimination of CO2 has the potential to be introduced as a sustainable and low-cost process [55,56]. This process is designed to upgrade the same process of metal oxide reduction to be applied for new fuels or power generation systems. The heavy fraction of bio-oil that is produced from fast pyrolysis of biomass is an energy carrier in this process [57]. The underlying concepts in the CLH process are the same as those in the steam-iron process integrated with the CLC process. The schematic diagram of CLH of bio-oil is demonstrated in Fig. 4.5. The conventional CLH process involved a steam reactor, fuel reactor, and an air reactor. This process is formed by the transfer of OC particles between these two continuous reactors to produce pure CO2 in FR and high-purity H2 in SR. Practically in the FR reactor, the oxygen carrier particles are reduced with the hydrocarbon fuel to a lower oxidation state. 3.1.1.2 Coke-oven gas chemical looping hydrogen generation system/technology A process based on coke-oven gas (COG) as the feedstock of the CLH process has very high energy efficiency (73.6%) with full CO2 capture efficiency was developed for hydrogen and power generation [58]. Coke-oven gas is a fuel gas having a medium calorific value that is produced during the manufacture of metallurgical coke by heating bituminous coal to temperatures of 900e1000 C. The normal composition of coke gas is hydrogen (w51%), methane (w34%), carbon monoxide (w10%), ethylene (w5%), and may include some other gaseous fuel. Fig. 4.6 represents the process flow diagram of this process. However, COG direct chemical looping hydrogen generation (CG-CLH) includes other processes such as pressure swing adsorption for producing hydrogen with high purity and power generation with a gas turbine. The use of other bio-based fuels could be applied to develop this process. 3.1.1.3 Biomass direct chemical looping system/technology Another system/technology is biomass direct chemical looping that is applied for biomass conversion to both hydrogen and power. The biomass direct chemical looping

FIGURE 4.5 Simplified diagram of CLH process [57].

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FIGURE 4.6 Simplified sketch of CG-CLH process flow diagram [58].

(BDCL) process also eliminates carbon dioxide in addition to the production of energy from biomass [59]. As shown in the process flow diagram of the system indicated in Fig. 4.7, this process has three main reactors including combustor, oxidizer, and reducer. The biomass feedstock is injected into the two-stage moving-bed reactor and steam enters the movingbed oxidizer reactor. The coal direct chemical looping applied to the chemical looping conversion of coal is the head start of the BDCL [61]. One of the most important advantages of the BDCL process for CO2 capture is that it does not require an energy-intensive step. It is because the produced carbon dioxide is separated from the gas stream through other side reactions in the process, so the net result is CO2 negative process. It is calculated by ASPEN plus simulation that the BDCL process yields energy conversion efficiency of above 38% on the higher heating value, in addition to power generation with 99% elimination in CO2 release [60]. In addition to the previous figure, the process is shown in Fig. 4.8 could be used for the production of heat in the combustor of prereduced materials. The proposed process eliminates the oxidizing section to only generate electricity, while the released CO2 is a side product of this reactor.

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FIGURE 4.7 The BDCL process scheme for hydrogen production [60].

FIGURE 4.8 BDCL process scheme for power generation [60].

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3.1.1.4 Chemical looping solid oxide fuel cell system/technology Chemical looping solid oxide fuel cell (CLSOFC) is a process innovation of producing hydrogen and power with renewable energy sources. The block flow diagram (BFD) of this process indicated in Fig. 4.9 represents the generation of power and hydrogen simultaneously from biomass [62]. In this system, two main processes are used in parallel to reach high efficiency. A combination of chemical looping hydrogen process with chemical looping gasification innovates a new system that not only generates power in the power cycle but also generates power with solid oxide fuel cells alongside hydrogen production. Fig. 4.10 represents more detail about this process in the process flow diagram (PFD) scheme. This process revealed a net power efficiency of 55.8% based on biomass lower heating value (LHV) [62]. 3.1.1.5 Biomass gasification dual chemical looping system/technology Another available process system/technology of chemical looping of biomass conversion is biomass gasification with dual chemical looping (BGDCL). The conceptual design of this process is shown in Fig. 4.11 for simultaneous production of hydrogen and power. In fact, this process consists of four main subprocesses: gasification, Fe-CLC, chemical looping air separation (CLAS), and the power generation section [63]. The CLAS process was designed for the production of pure O2 individually [64]. But, in this process, the obligation is to inject crude gas of biomass into the CLC fuel reactor section. The biomass feedstock enters the CLAS and gasification section for combustor and dehydrator/gasifier, respectively. Then produced gas in gasification section inject to

FIGURE 4.9 BFD of CLSOFC process [62].

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FIGURE 4.10 Process flow diagram of biomass conversion to power and hydrogen in CLSOFC process [62].

Fe-CLC for heat generation. Heat recovery steam generator provides the needed heat from reactors hot flue-gas. Therefore, the generated steam injects to high pressure, medium pressure, and Low pressure for power generation. The thermodynamic performance evaluation of this process revealed that exergy and energy efficiencies were 58.1% and 66.23%, respectively. In addition, this process produces hydrogen with very high purity (99.9%) and almost complete CO2 capture (99.9%) because of the Fe-CLC system coupled with this process. 3.1.1.6 Sorption-enhanced chemical looping reforming system/technology Sorption-enhanced chemical looping reforming (SE-CLR) is a process that includes combined sorbent oxygen carrier material as a multifunctional catalyst. The innovation of this process is only the type of catalyst that producing hydrogen with conventional CLR process simultaneous with CO2 capturing. As indicated in Fig. 4.12, this process has three main sections: reforming reactor, calcination reactor, and air reactor. The oxygen

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FIGURE 4.11 Block flow diagram of BGDCL process [63].

FIGURE 4.12 Simplified schematic of SE-CLR process [21].

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carrier and sorbent are combined and formed a uniform structure, and each one is activated in its process [21,33,34]. The main advantage of this process is the production of hydrogen with high purity and further reduction of carbon dioxide emission into the atmosphere [21]. As a case study, SE-CLR of acetic acid as biofuel results 99.99% hydrogen purity at 450 C and 1 bar operating conditions [65]. 3.1.1.7 Chemical looping water splitting system/technology Chemical looping water splitting (CLWS) of biofuel is another proposed process based on the possibility of oxidizing the metal oxide by means of water. For this purpose, biofuel is injected into the fuel reactor and reacts with the oxygen carrier to produce heat and CO2 (in CLC mode of fuel reactor). The reduced metal is oxidized with steam partially and produces hydrogen, simultaneously. Then, the oxidation is completed with the oxidation in air reactor under the stream of hot air [66]. Schematic of chemical looping water splitting is shown as Fig. 4.13. As one of the major problems with bio-based fuels is the deactivation of catalysts and/or especially oxygen carriers, it is vital to use moving-bed reactor or riser with a mixer in the subprocess. 3.1.1.8 In situ gasification-chemical looping combustion system/technology In situ gasification-chemical looping combustion (iG-CLC) is a proper subset of the CLC process [67]. Fig. 4.14A represents the reactions pathway of this process. For the iG-CLC process, CO2 and/or steam are/is injected into the fuel reactor. The supplied CO2 and/or steam, which is injected to the reactor as a fluidizing/gasifying agent, reacted with the char gasification product (H2/CO2) and released volatiles. In this process, the main limiting step is the slow gasification rate of the generated char.

FIGURE 4.13 CLWS process diagram [66].

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FIGURE 4.14 The difference between the (A) iG-CLC and (B) CLOU process from a microscopic perspective.

3.1.1.9 Chemical looping oxygen uncoupling system/technology The limitation stated in iG-CLC is resolved in the chemical looping oxygen uncoupling (CLOU) process, as the lattice oxygen of the oxygen carrier can react directly with the produced char. In general, the CLOU process offers advantages over biomass iG-CLC, especially with no tar compounds at FR and the low solid inventories needed to reach high carbon capture efficiencies. Because of the disappearing cost of steam generation energy, it is more efficient to use CO2 instead of steam [67]. Fig. 4.14B shows a microscopic perspective of reactions in the CLOU process. 3.1.1.10

Pressurized chemical looping combustion combined cycle system/technology The pressurized chemical looping combustion with combined cycle (PCLC-CC) is generally shown in Fig. 4.15. This system includes two pressurized fluidized-bed, mixer-riser and offers an air reactor. Also, this system has operated in a high level of temperature and pressure condition, so the flue gas can lead to a gas turbine [68]. There are currently no studies on biofuel feed but this system was investigated on coal feeder and solid fuels. This system could potentially achieve much higher efficiency with biofuel as feed. 3.1.1.11 Chemical looping zero-emission coal system/technology This system is based on coal/biomass cohydrogasification and CLH technologies. Chemical looping zero-emission coal (CL-ZEC) with wheat straw as feedstock was investigated to have exergy and total energy efficiencies of near 41.2% and 43.6%,

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FIGURE 4.15 Process flow diagram of PCLC-CC power generation and CO2 capturing [68].

respectively. Compared with the BDCL system, this amount of efficiency is higher but the BGDCL process energy efficiency is higher than these two processes. Also, this system has potentials to capture 99.1% of the carbon inside the biomass feedstock [69]. Fig. 4.16 represents a block flow diagram of the CL-ZEC system. Also, this figure shows the SOFC system as a subsystem in this process for power generation. 3.1.1.12

Biomass and coal cofueled gasification chemical looping combustion for combined cooling, heating, and power generation system/technology Biomass and coal cofuel gasification chemical looping combustion with CO2 capture for combined cooling, heating, and power generation (BCCLC-CCHP) schematic is sketched in Fig. 4.17. This detailed configuration represents a process with a combination of CLC, CLG, and trigeneration systems (heating, cooling, and power). The main obligation of gasification and heat recovery is to convert biomass and coal into syngas, which includes a thermochemical reaction to recover excess heat from syngas. The benefit of the CLC system is that it helps to increase efficiency by reducing GHG emissions, and it is responsible for the capturing of CO2. Heating, cooling, and power can be produced

FIGURE 4.16 BFD of CL-ZEC system for power generation [69].

FIGURE 4.17 Detailed process flow diagram of BCCLC-CCHP system/technology [70].

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simultaneously, which is led by flue gas (exhaust gas) from the last step (CLC). The results of this work were investigated in two winter and summer seasons, with the total energy efficiency of 57.46% and 60.16% in winter and summer, respectively. Also, when the amount of biomass in the feedstock is increased from 10% to 50%, the GHG emission rate drops sharply. It was reported to be 121.66% and 123.38% in winter and summer, respectively [70]. Similarly, chemical looping processes have evolved over time and have been studied in various aspects. However, the main purpose of designing new processes is improving the performance along with the reduction in investments and operating costs. The innovations of the extended chemical looping processes from several specific aspects have been examined, such as the overall process systems/technologies, reactor type change, feed phase (solid, liquid, gas), carbon dioxide emissions, and overall performance. Table 4.2 summarized the major chemical looping process/technology for biomass conversion.

Table 4.2

Chemical looping process system/technology for biomass conversion.

System/Technology

Principal technology/ Fuel/ process feed

Chemical looping hydrogen (CLH)

CLC-CLG

Bio-oil/ biofuel

Biomass direct chemical looping (BDCL)

CLC CLG

Biomass

Chemical looping solid oxide fuel cell (CLSOFC)

CLG CLC SOFC Fe-CLC CLG CLAS CLSR CLC Calciner

Biomass

Biomass gasification with dual chemical looping (BGDCL) Sorption enhanced chemical looping reforming (SE-CLR)

Biomass

Biomass/ Bio-oil

Chemical looping with oxygen uncoupling (CLOU) CLC CLG In situ gasification-chemical looping combustion CLC (iG-CLC)

Biomass/ coal

Chemical looping water splitting (CLWS)

Biofuels

CLC

Biomass

Major remarks High-temperature condition Full CO2 capturing High energy efficiency High conversion efficiency Low cost for CO2 capturing High energy efficiency with zero CO2 emission Very high purity H2 and CO2 capture The use of a specific catalytic structure High purity of H2 This process only for solid fuels Very cheap and has available oxygen carrier.

Main purpose References H2-power [58,71]

H2-power [59,60]

H2-power [62]

H2-power [63]

H2

[72]

Power

[67]

CO2-heat [67]

H2

[66]

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Table 4.2 Chemical looping process system/technology for biomass conversion.dcont’d

System/Technology

Principal technology/ Fuel/ process feed

Pressurized chemical looping combustion combined cycle (PCLC-CC)

CLC

Chemical looping zero-emission coal (CL-ZEC)

CLC SOFC CLSR Calciner

Biomass and coal cofueled gasification chemical looping combustion for combined cooling, heating, and power generation (BCCLC-CCHP)

CLC

Major remarks

Main purpose References

Zero CO2 emission Power High-pressure Coal/ level condition solid High-temperature fuel condition Biomass/ Two-step power Power coal generation High efficiency in addition to zero CO2 emission Coupling with SOFC system Pure N2 as side product Biomass/ Cofueled process Power coal Very low power generation High CO2 capturing

[68,73]

[69]

[70]

3.1.2 Main products in chemical looping biomass conversion Each process has different products and outputs depending on the applied technology, the input feed, and the catalyst. The chemical looping process is used to generate heat, carbon dioxide, electricity, hydrogen, etc. In this section, the two main products of biomass-based process/technology, electricity and hydrogen, are investigated. Fig. 4.18 summarized the path between some processes and technologies of chemical looping. 3.1.2.1 The standalone generation of power in chemical looping biomass conversion Two main pathways for power generation are thermoelectrical and thermomechanical power generation. Most CLC-based processes fall into the thermomechanical category due to the conversion of the generated heat to power [74]. At first, the process applies the biomass gasification with pyrolysis. Biochar, tar, and biogas (H2, CO, CO2, CH4, CnH2n), which were produced in the last step, are used in the main combustor. As shown in Fig. 4.19, high-pressure exhaust gas (steam and CO2) drives turbines and turns power to electricity. This process operates at 740e920 C and atmospheric pressure. Without considering whether reactor type and catalysis structure, a fundamental component of such process configuration, are exergy/(net)energy efficiency and economic evaluation of that process. As tabulated in Table 4.2, all the processes that lead to the generation of electricity are applied to their core process. This does not mean that any process that has a

FIGURE 4.18 Classification of chemical looping process with products.

FIGURE 4.19 Simplified process flow diagram of CLC-combine cycle process for power generation [75].

Chapter 4  Chemical looping conversion of biomass 111

FIGURE 4.20 Percentage of absorbed carbon dioxide in the CLC with electricity generation processes.

CLC system in its process necessarily has the capability to generate electricity, but it does mean that the core process of CLC is capable of generating electricity. Therefore, the processes based on CLC are more suitable for generating electricity. As mentioned earlier, the main reason is that in the CLC air reactor section, the bulk temperature reaches about 600e1200 C, in which a completely exothermic reaction improves this ability. The processes are also varied in terms of the feed phase. The bio-based inlet feed in the three phases of solid, liquid, and gas fuels can be used in any process, and as mentioned, the CLC process is an integral part of the energy production system. The important thing is that the fuel used in this process is almost used as gas or liquid: gaseous fuel such as biogas, syngas from biogasification and volatiles from torrefaction [66]. Also, liquids such as bioethanol and glycerin are most commonly investigated recently. Fig. 4.20 shows the net amount of captured CO2 in the power generation process. All these processes are developed for power generation as the main product in addition to no CO2 emission. As indicated in Figs. 4.21 and 4.22, most of the processes have about 100% CO2 capture. The amount of produced energy (MJ) per unit time (s) for various processes are reported in Fig. 4.21. It should be mentioned that the amount of produced electricity depends entirely on the unit capacity, so it is possible to change with the unit capacity. However, the electricity production efficiency of the system/technology is more appropriate for investigating the system performance. Fig. 4.22 shows the efficiency of the as mentioned systems. Even though the CLH process has less unit capacity than the BGDCL, it is clearly evident that it has a higher energy and exergy efficiency than other processes. 3.1.2.2

The standalone generation of hydrogen in chemical looping biomass conversion Advancement in chemical looping biomass conversion for hydrogen generation from biooil, in addition to very low CO2 release, has been studied in several proposed processes.

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FIGURE 4.21 Total power capacity of mentioned process.

FIGURE 4.22 Simultaneous investigation of energy and exergy efficiency of different processes for standalone power generation.

Decomposition, steam reforming, sorption enhanced steam reforming, aqueous phase reforming, and auto thermal steam reforming are some of these underlying concepts for hydrogen production from bio-oil and biomass [76]. The decomposition of bio-oil is just like the decomposition of light hydrocarbons. The main difference between these two is

Chapter 4  Chemical looping conversion of biomass 113

the product, where the decomposition of bio-oil has CO2 as a side product, unlike methane. The temperature of the decomposition reaction was reported in the range of 600e800 C in the presence of nickel oxide as an oxygen carrier [77]. Steam reforming of biofuel for hydrogen production has attracted the attention of many researchers. Steam reforming of biofuel has a higher yield of hydrogen than decomposition of the same biofuel. The operating conditions of such a process are not as high as the similar conventional processes. The temperature is near 400 C that led the process to the top of the pyramid of the same kind from the energy and exergy efficiency point of view. The reaction set of this section is completely endothermic [78]. Aqueous phase reforming of biomass and biomass-derived feedstocks is another hydrogen generation process. Biomass derivatives such as ethylene glycol, sugar, and methanol were examined in this system. By collision of CO and H2O as water gas shift reaction, CO2 and hydrogen are produced simultaneously. This process was also investigated under 200 C and 2.5e3.0 MPa operation conditions [79].

3.1.3 Type of reactor in chemical looping biomass conversion In chemical looping technology, the type of reactor is one of the most important parameters affecting the process. As the reactions, catalysts, targets, and processes are divided into different categories, the reactors also have different types. The most common applied rectors include fixed-bed and fluidized-bed reactors. But the other reactors, such as membrane reactors, noncontinuous reactors, and even parallel fixed reactors are applied for this purpose. A fluidized reactor is one of the reactor types, which was used in chemical looping processes. Followed by the initiation of the flow of fuels such as methane, syngas, and/or solid carbonaceous fuels, steam and/or recycled CO2 are used as fluidizing gases in the reactor. The regeneration in the oxidization reactor could be performed with fluidizing air, which oxidizes the reduced metal oxide as indicated in Fig. 4.23. The catalysts were simultaneously fluidized with the feedstock of the reactor and participate in reaction along the length of the reactor. Fig. 4.24 represents a simplified sketch of this kind of reactor. A fixed-bed reactor is widely used in experimental studies due to a very simple design with successively changing the gas stream to make the conditions of redox cycles [52]. Another kind of reactor applied in the conversion of biomass and biomass-derived feeds is a noncontinuous fluidized-bed reactor [82].

3.2

Catalysts for chemical looping biomass conversion

Proper selection and design of oxygen carrier materials are crucial for improving the production yield of desired products (hydrogen, syngas) in CL reforming/gasification of biomass and biomass-derived feeds. Further, the affection of feedstock nature and the unfavorable existent of fractions on the OC lifetime have to be declined as much as possible. Therefore, extent research works laid on testing oxygen carriers with various sources and structures for the conversion of real or simulated biocomponents into hydrogen-rich gas. Nickel, iron, cobalt, manganese, and copper oxides are the most

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FIGURE 4.23 Chemical looping process for pure CO2 generation with fluidized-bed reactor [80].

FIGURE 4.24 Schematic of fluidized-bed reactor [81].

investigated metals in CL conversion of biomass due to their prominent performance with CLR of fossil fuels. On the other hand, the total elimination of carbon deposition is unavoidable for biomass conversions, and the utilization of less acidic support along with the improvement of catalysts to resist coke deposition is of interest. Three

Chapter 4  Chemical looping conversion of biomass 115

suggestions have been offered to efficiently modify the catalysts, including the introduction of a small amount of another noble or nonnoble metal to the catalyst decreasing the size of active metals utilizing adequate support to reduce sintering [83].

3.2.1 Development of looping materials for solid biomass feedstock The objective of numerous research works in recent years is the direct use of plantderived solid biomass for H2 and syngas production to avoid the costs of using multiple steps to purify the biofuel derived from biomass prior to use them for the production of H2 and CO. Biomass has higher effects of tar, low melting-point ash and fuel-derived metals on looping materials [12]. Various agricultural wastes such as rice straw, corn stalk, peanut shell, and wheat straw tested for syngas production using Fe2O3/Al2O3 as OC showed promising gasification efficiency at a reaction temperature of 850 C and steam to biomass (S/B) ratio of 2.8, proposing Fe-based OC suitable for syngas production [84]. The Al2O3 supported iron oxide revealed better performance alongside satisfactory lattice oxygen activity in comparison with Fe2O3 supported on TiO2, SiO2, and ZrO2 [85]. It worth mentioning that injected steam is needful for biomass gasification to increase the interaction between solidesolid phases of fuel and oxygen carriers and increase the reactivity. As for the gasification of rice straw, introducing CaO in Fe2O3 OC, both with mixing or impregnation method resulted in an improvement in syngas production due to the promoting effect of CaO in reactivity and sintering resistance of OC [86]. However, the recyclability test for impregnated Ca2Fe2O5 showed the formation of CaSiO2 and Fe2O3 due to the segregation of FeeCa structure by Si from biomass ash [86]. Therefore, using better support for Fe-based OC is needed to avoid this phenomenon. Mg, another alkali earth metal used to promote the OC for biomass CLOU as CuO/MgAl2O4, enhanced the gasification activity, stability, and oxygen transfer capacity of CuO OC [87]. The CL gasification of rice husk as a biomass source was investigated recently in a 25 kWth reactor using CaO-decorated NiO/Al2O3 and natural hematite as OC at 750 and 860 C optimal temperatures, respectively, resulted in higher than 60% carbon conversion efficiency [88]. Wood sawdust is the most researched solid source for biomass CL gasification. Bimetallic Fe2O3eNiO/Al2O3 was used as OC for CLG of pine sawdust in 10 kWth interconnected fluidized-bed reactors. The results demonstrated the catalytic effect of Ni on the improvement of gasification efficiency, reaction stability, and sintering resistance at 850 C compared with Fe2O3/Al2O3 [89]. Several studies showed that the application of Cu-based OC in CLR/CLG of pine sawdust using a fluidized-bed reactor at 800 C can increase the carbon conversion efficiency and reduce the tar content but also with a decrement of gasification efficiency and gas yield during the redox cycles [90,91]. Relatively low gasification efficiency and LHV using copper ore as OC are the most important drawbacks of CuO oxygen carrier while the copper ore has a higher activity for carbon conversion compared with hematite and silica sand [92].

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It was mentioned by the researchers that the production of high H2/CO ratio syngas was achieved in an inexpensive and adequate method using hematite in this process with a temperature of 820 C and S/B ratio of 1.5 [93]. Pine sawdust steam gasification in a decoupled dual-loop system using olivine rock mineral as OC demonstrated a high H2 concentration of 40.8% and a low tar content of 14.1 g/Nm3 [94]. Char is the solid product of the biomass pyrolysis and like coal have the potential to be used for reforming processes to generate syngas. Several investigations were performed on biomass char using fixed-bed reactors, recently. It was remarked that spinel-type NiFe2O4 formed in NiO-modified iron ore can improve the reactivity of the OC for biomass char CLG [95]. Experiment with NiFe2O4 as an oxygen carrier at 850 C showed that the synergistic effect between Fe and Ni caused better performance of spinel structure in char gasification compared with mechanically mixed Fe2O3eNiO and Fe2O3 OCs [96]. In addition, it was concluded that NiFe2O4 recyclability was relatively stable, but a downturn in reactivity was observed within 20 cycles due to the sintering of the particles, formation of Fe2O3 phase, and also ash deposition on the surface of active sites [96]. Containment of ashes with low melting-point is one of the matters for oxygen carriers of biomass conversion at high temperatures, particularly for sewage sludge, which can negatively affect accumulation and defluidization of OC [12]. High dispersed Fe0.99Ni0.6Al1.1O4 synthesized by coprecipitation method as OC for CLG of biomass char derived from pine sawdust resulted in maximum mass loss rate and weight loss of 2.46% wt/min and 35.59% along with 36.65% carbon conversion efficiency at 850 C [97]. The addition of steam to the reaction mixture can significantly improve the interaction between the solid char feedstock and OC and higher gas yield could be attained, simultaneously [98]. Several perovskites CaMnO3, CaFeO3, and CaMn1xFexO3 (x ¼ 0.2 and 0.4) synthesized by the solegel method were examined for syngas production from CLG of rice husk char at the optimized temperature of 800 C and S/F mass ratio of 7.2 [99]. The highest H2/CO ratio was gained by using CaMn0.6Fe0.4O3 as an oxygen carrier described into three separate stages: (1) oxygen uncoupling, (2) catalytic gasification, and (3) partial decomposition. The possibility in the production of syngas from microalgae biomass CLG was tested at 850 C with steam input and utilizing Sn-substituted brownmillerite oxide (Ca2Fe2O5) as oxygen carrier yielded in 48.7%, 29.5%, and 11.6% of H2, CO, and CO2 relative percentages, respectively [100].

3.2.2 Development of looping materials for liquid biomass-derived feedstock This subset represents recent research in the advancement of CL materials for the conversion of bio-alcohols, tar, and bio-oil derived from biomass pyrolysis. High oxygen content and low heating value limited their utilization as fuels. Therefore, continuous research recently focused on upgrading bio-oil or if possible converting to a more valuable fuel such as aromatic hydrocarbons, hydrogen, and syngas. Several feeds including acetic acid, ethanol, furfurals, and glycerol were investigated as model bio-oil compounds. Acetic acid as a model compound showed promising results for the SE-CLR process in a fixed-bed reactor for hydrogen production using NieCo and dolomite as OC

Chapter 4  Chemical looping conversion of biomass 117

and sorbent, respectively. The hydrogen with high purity of more than 99% was produced in different documents [101]. The perovskite structure of LaNixFe1xO3 with reasonable coke resistance was successfully tested in CLSR of acetic acid using a fixed-bed reactor at 600 C and S/C mole ratio of 3:1 [56]. Although the same composition was used for other CL transition metals (Cu, Co, and Mn), the activity order of syngas, which was LaNi0.8Fe0.2O3 > LaNi0.8Co0.2O3 > LaNiO3 > LaNi0.8Mn0.2O3 > LaNi0.8Cu0.2O3, confirmed the advantage of NieFe mixed OC [102]. The performance of four model biocompounds (ethanol, acetone, furfural, and glucose) as the fuel of the CLSR fixed-bed reactor has been investigated at a temperature of 650 C with commercial NiO/a-Al2O3 oxygen carrier. It was mentioned that agglomeration of OC due to coking was the major problem for the CLSR of glucose, and to suppress this drawback, higher S/R ratio and temperature above 650 C are suggested. In addition, the SR of furfural was limited due to the high thermal stability of furfural or its derivatives [103]. Bioethanol is a product of starch-rich biomass fermentation, and its potential for hydrogen production was investigated with several Ni-based OCs supported on different structures. The superior activity and stability of Ni/SBA-15 and Ni/montmorillonite oxygen carriers resulted from the CLSR of ethanol [104]. In addition, promoting Ni/ SBA-15 by CeO2 enhanced the dispersion of Ni, improved the metal-support interaction, and increased the coke resistance [105]. Glycerol as the other by-product of biodiesel possesses the high potential to be used for hydrogen production using steam reforming. Hydrogen production from glycerol in the sorption-enhanced CLR system was tested in two moving-bed reactors with the simultaneous flow of the OC and sorbent at temperatures within 500e600 C using NiO/ NiAl2O4 and CaO as OC and sorbent, respectively. The results indicated that NiO cannot be totally reduced to Ni in the direct reaction with glycerol but the hydrogen purity of above 90% was achievable [106]. Furthermore, Ni supported on Al-MCM-41 with high surface area was tested for the same reaction, with the addition of Ce as a promoter. At the temperature of 650 C, glycerol conversion and hydrogen selectivity above 80% and 90%, respectively, aside with high recyclability were achieved by CeNi/Al-MCM-41, showing the important role of Ce promoting effect and ordered mesoporous structure of MCM-41 [107]. Waste vegetable cooking oils are a potential source for hydrogen production and owing to their difficulties for diesel production, have tested in CLSR and sorptionenhanced CLSR processes to produce hydrogen within the temperature range of 600e700 C utilizing a fixed-bed reactor [108]. The latter process was performed in the presence of dolomite as a sorbent and commercially available oxygen carrier (18 wt.% Ni/Al2O3). The SE-CLSR process indicated that the performance of Ni-based OC was almost constant within six cycles with the hydrogen purity of 98% [109]. The pine oil and palm empty fruit brunches bio-oils are the other biomass-derived bio-oils applied for hydrogen production using 18 wt.% Ni/Al2O3 in the CLSR process resulted in the maximum conversion of 97% and 89% and H2 yield efficiency of 60% and 80% for the above bio-oils, respectively [110].

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In addition to nickel-based oxygen carries, the iron-based structures were investigated in the conversion of liquid bio-based feedstocks. The Fe2O3/MgAl2O4 as OC was developed and tested in the CL process of bio-oil feedstock and attained 96% hydrogen yield at 950 C [111]. As a natural ore, ilmenite (FeTiO3) showed promising performance at the first run of reaction to produce a hydrogen concentration of 92% with 85% carbon conversion at a temperature of 950 C using bio-oil derived from cotton stalks pyrolysis as feedstock [57]. Three different feedstock represents different phases of biomass pyrolysis products namely char, pyrolysis gas, and toluene as tar components were selected to recognize the roles of different ferrites to produce hydrogen-rich syngas in a fixed-bed reactor at 850 C [112]. Although all four spinel-type structures of MFe2O4 (M ¼ Cu, Ba, Ni, and Co) showed good oxygen transfer capability affected by their synergistic effect, hydrogen generated by BaFe2O4 was 26.72% higher than that of CoFe2O4 and 13.79% more than NiFe2O4 and CuFe2O4, offering BaFe2O4 as best the choice for the generation of hydrogen-rich syngas. Tar defined as a condensable product of biomass gasification and its contamination in the syngas can make fouling and blocking problems in pipelines and equipment [113]. Therefore, tar removal or conversion is worthwhile more attention in biomass conversion. Benzene was examined as a tar model component in CL dry reforming with Fe- and Ni-based silicon carbide as OC resulting in the benzene conversion of 96% utilizing a fluidized-bed reactor at 730 C with H2 and CO yields of 7.2% and 17% [114]. Toluene is another compound that its conversion was investigated as a biomass tar model. A combination of Ni and Fe oxides prepared by solegel and mechanical mixing methods at 850 C utilizing a fixed-bed reactor [115]. The CuOeNiO/Al2O3, CaFe2O4, and NiFe2O4 oxygen carriers were examined for CLR of tar model and revealed high toluene conversion [55,116].

3.2.3 Development of chemical looping materials for gaseous biomass-derived feedstock While using biomass or biomass-derived components as chemical looping feedstock, considering all the characteristics needed for oxygen carriers of chemical looping processes, improvement of some characteristics are more highlighted because of the nature and impurities of biomass. In some cases (e.g., sulfur), impurities even in a slight amount of ppm can reduce the activity and lifetime of the catalyst [117]. Particularly for biogas, contamination of sulfur and siloxane impurities can result in poisoning the reforming catalysts and proper design of resistant catalysts is required [118]. A comparison of different methods to synthesize NiFe2O4 spinel and its impact on hydrogen production has been studied using biomass pyrolysis model gas in a fixed-bed reactor at 850 C [119]. Accordingly, the characterization results demonstrated the superiority of solegel-prepared NiFe2O4 in hydrogen production and the grade of lattice oxygen recovery in comparison with other synthesis methods (solid-state, coprecipitation, and hydrothermal) and it was mentioned that a combination of NiFe2O4 and SiO2 showed significantly higher stability within 20 cycles compared to individual NiFe2O4

Chapter 4  Chemical looping conversion of biomass 119

particles [119]. The conversion of sweetened biogas to hydrogen was examined by three Fe-based oxygen carriers alongside the 10% NiO/Al2O3. The process was a combination of dry reforming and steam iron process utilizing a fixed-bed reactor including a coke combustion step leading to a continuous output of great purity hydrogen (50 ppm COx) within 13 successive cycles [120]. The same process was performed using cobalt ferrite and modified hematite beside NiAl2O4 concluding that the results were comparable to methane dry reforming [121]. Iron ore modified with Ni, Cu, and Ce oxides was applied as an oxygen carrier in CL hydrogen production from simulated biogas. The reaction was performed at 700 C to produce hydrogen with a purity of above 99% for all the modified iron ores. In addition, NiO-iron ore showed the best hydrogen yield [122].

4. Comparative analysis Chemical looping technology is a flexible process with the potential to be used in a wide range of industrial processes. The early existence of this technology has not been very pronounced, but the global development of this technology is in progress over the past and recent decades. According to the statement, CL technology is in the process to be globalized industrially due to its flexibility. In addition, global warming and the need for the elimination of GHG emission makes this technology more attractive. Fortunately, chemical looping technology has many capabilities such as the removal of carbon dioxide alongside the production of the main products such as power and hydrogen. Different processes have been reported for the conversion of fossil fuels such as methane, ethane, propane, methanol, natural gas, and many other fuels, but renewable sources greatly increase the efficiency and limit the net release of carbon dioxide. However, these developed processes that utilize biomass and biomass-derived feeds with high operating efficiency are still in progress (Table 4.3). In addition to the process, a variety of looping materials have been investigated to find the proper method for the conversion of biomass sources to produce renewable energy in efficient procedures. Ni and Fe-based oxygen carriers were the most tested materials for different biosources owing to their great and appropriate performance in reforming processes of conventional fuels.

4.1

Technical feasibility

It has been found that the conversion of gaseous biofuels in the CLC system is able to produce high values of net energy. But the CLC process alone has low efficiency. Combining this process with others with high conversion efficiency is very convenient due to the environmental benign nature of it. For example, the BCCLC-CCHP process that results in power generation integrated with the CLC process is a process with about 123% CO2 uptake. In addition, a process such as the CLH produces power in addition to hydrogen. This process operated in the range of 900e1000 C, which is too high. However, taking a look at the efficiency of this process (indicated in Fig. 4.24), we would realize the benefits of this process according to the high energy efficiency.

Overview of oxygen carriers used for chemical looping conversion of biomass and biomass-derived feeds. Synthesis method

Woody biomass

Iron-titanium composite metal oxide

Walnut shell

CuO/MgAl2O4

Rice straw Corn stalk Peanut shell Wheat straw

60% Fe2O3/Al2O3

Rice straw

NiOeCaO/Al2O3

Rice straw

Fe2O3 (OC) CaO (sorbent)

Rice straw

Fe2O3/CaO

Rice husk

Natural hematite

Rice husk

Natural hematite

Pine sawdust

Natural iron ore

 syngas H2/CO: 2 1000 Fluidized dry syngas purity>65%s bed combustor Moving bed reducer e Fixed-bed 700e950  H2 volume fraction: 700 C: 18% 950 C: 24.4%  CO molar fraction: 700 C: 48.8% 950 C: 52.9%  Similar performances for all feedstocks Impregnation e 850  CO content 19.2%e23.1%  H2 content 36.5%e41.1%  Carbon conversion 72.3%e82.2%  Gas yield 0.78e1.04 L/g  25 kWth prototype Mechanical mixing Fluidized750  Carbon conversion efficiency max. of 60.28% bed  Syngas yield increased by adding CaO  Syngas yield: 1.367 m3/kg e Fixed-bed 800  Carbon conversion efficiency: 89.28%  Gasification efficiency: 88.81%  63.2 v% H2 content Impregnation Fixed-bed 800  23.07% H2 yield (mmol/g biomass)  25 kWth prototype e Fluidized860  Increment of carbon conversion efficiency from 53.36% to 89.22% bed with the gasification temperature of 800e900 C  Increment of carbon conversion efficiency with the gasification temper850 e Fluidizedature of 750e900 C bed  Cold gas efficiency in FR: 77.21% e Fluidized820  H2 yield in SR: 0.279 Nm3/kg bed

Pine sawdust

Copper Ore Hematite Silica sand Natural hematite

Pine sawdust

Reactor type

T ( C)a

Remarks

e

Ref. [123]

[87]

[84]

[124] [125] [86] [88] [126] [93]

e

Fluidizedbed

800

 Larger weight loss and rate with natural copper ore  Increment of gas yield and carbon conversion using copper ore

[92]

e

Fluidizedbed

840

 Max. gas yield: 1.06 Nm3/kg  Max. gasification efficiency: 83.31%

[127]

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Oxygen carrier sorbent

Feed

120

Table 4.3

Natural olivine

Pine sawdust

a. 60% CuO/ Cu: Solegel 40%CuAl2O4 Fe: Freeze b. 60% Fe2O3/ granulation 40%Al2O3, c. Silica sand d. Refined copper ore

Pine sawdust

Fe2O3eNiO/Al2O3

Pine sawdust

CuOeFe2O3

e

Fluidizedbed

Fluidizedbed

Mechanical mixing Fluidizedand impregnation bed Solegel Fluidizedbed

Microalgae Ca1.4Sr0.6Fe2O5 Heavy metal Natural hematite hyperaccumulators

Solegel e

Rice husk char

800 (fuel  H2 concentration: 40.8 vol% 3 reactor)  Tar content: 14.1 g/Nm 850 (reformer)  Carbon conversion: 800 a. 95.6% b. 81.7% c. 62.3% d. 83.2%  Gasification efficiency: a. 30.8% b. 2. 60% c. 59.1% d. 26.6%  Gasification efficiency: 70.48% 850  Carbon conversion: 93.95% 800

Solegel

Biomass char

CaMnO3 CaFeO3 CaMn1xFexO3 (x ¼ 0.2 and 0.4) Fe0.99Ni0.6Al1.1O4

Fixed-bed 850 Fixed-bed 900 (Microwaveassisted) Fixed-bed 800

Coprecipitation

Fixed-bed

850

Biomass char

NiFe2O4

Soleegel

Fixed-bed

850

Biomass char

NiO-modified iron Impregnation Ore (NiFe2O4) coupled with ultrasonic treatment

Fixed-bed

     

Carbon conversion%: 94.6 Tar yield (g/kg): 7.5 Gas yield (Nm3/kg): 1.16 H2, CO and CO2 relative fractions: 48.7%, 29.5%, and 11.6%. Hematite OC favors H2 and CO production Decay of each heavy metal in char by increment of steam flow

[94]

[90]

[89] [91] [100] [128]

 H2/CO ratio order: [99]  CaMn0.6Fe0.4O3>CaMn0.8Fe0.2O3>CaFeO3>CaMnO3 at 750 C  The best operation conditions for CaMn0.6Fe0.4O3: 800 C and S/C mass ratio of 7.2       

Maximum weight loss 35.59% Largest mass loss rate: 2.46%wt/min Carbon conversion efficiency: 36.65% Max. carbon conversion: 88.12% Max. synthesis gas yield: 2.58 L/g char Carbon conversion: 55.56% Enhancement of OC reactivity by formation of spinel-type NiFe2O4

[97] [96] [95]

Continued

Chapter 4  Chemical looping conversion of biomass 121

Pine sawdust

Oxygen carrier sorbent

Synthesis method

Reactor type

Biomass char

Natural iron ore

e

Fixed-bed

Wheat straw

Fe2O3/support Impregnation (Al2O3, TiO2, SiO2, and ZrO2) 18%Ni/Al2O3 e Dolomite (sorbent)

Waste cooking oil

Fixed-bed

T ( C)a

Remarks

 Carbon conversion and H2 relative content%: Without input steam: Al2O3: 11.1%, 48.8% Fe2O3: 35.7%, 20.0% with input steam: Al2O3: 37.6%, 63.1% Fe2O3: 64.1%, 56.8% 750e850  The highest H2 yield, H2/CO ratio, gas yield, and carbon conversion with 60%Fe2O3/Al2O3 850

 100% dolomite carbonation and 98% purity hydrogen at the beginning, decreased to around 56% with a purity of 95% respectively in the subsequent cycles.  Fuel and steam conversion was higher with the sorbent  Reactivity: NiFe2O4 >NiO þ Fe2O3 > Fe2O3 > NiO >> Al2O3  NiFe2O4 toluene conversion and H2 yield: 96.83% and 0.91 L/g.

Ref. [98]

[85]

Fixed-bed

600

Al2O3 Fe2O3 NiO NiO þ Fe2O3 NiFe2O4 NiFe2O4

Solegel (NiFe2O4) Fixed-bed Mechanically mixed (NiO þ Fe2O3)

850

Solegel

Fixed-bed

Toluene

CuOeNiO/Al2O3 CaFe2O4

Hydrothermal Solegel

Fixed-bed

Toluene

Impregnation NiO/dolomite CuO/dolomite Fe2O3/dolomite Impregnation Fe, Sr-doped La2Zr2O7 supported on ZrO2

Fixed-bed

 Toluene conversion: 96.83% [116]  H2 yield: 0.91 m3/kg [55] 600e700  Fairly high conversion rates of toluene for both oxygen carriers at the beginning and then increment of toluene conversion to about 100% in (CuO the further stage eNiO/  High stability of CaFe2O4 with high products of CO and H2 during a 3 Al2O3) cycles 800e900 (CaFe2O4)  Highest syngas purity for NiO/dolomite 900 [129]  Suitability of Fe2O3/dolomite and NiO/dolomite for syngas production  CuO/dolomite tended to generate CO2

Toluene (biomass tar model compound)

Toluene

Tar

Fluidizedbed

[108]

[115]

850

750e850  Improvement of benzene to syngas conversion by about 50% with decrement of Fe loading from Fe/La ¼ 1.25 to 0.25 at T ¼ 800 C  Stable performance over 3 cycles

[130]

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Feed

122

Table 4.3 Overview of oxygen carriers used for chemical looping conversion of biomass and biomass-derived feeds.dcont’d

Heavy fraction of bio-oil (derived from the fast pyrolysis of cotton stalks) Glycerol Glycerol Glycerol

e

Fixed-bed

NiAl2O4

Coprecipitation

Fixed-bed

NiO/NiAl2O4 CaO (sorbent) Ni/Al-MCM-41 CeNi/Al-MCM-41 Ni@Al-MCM-41

Coprecipitation Direct-synthesis Impregnation (Ni@Al-MCM-41)

950

   Moving-bed 500e600  600

Fixed-bed

650

Ethanol Acetone Furfural Glucose

18%NiO/a-Al2O3

Fixed-bed

650

Ethanol

1. Coprecipitation Fixed-bed 1. Ni/Al2O3 2. Ni/ 2. Ultrasoundmontmorillonite assisted cation exchange 3. Ni/Al-MCM-41 4. Ni/SBA-15 impregnation 3. Postsynthesis 4. Hydrothermalsurfactantassisted isovolumetric impregnation 18%Ni/Al2O3 e Fixed-bed

650

Biomass pyrolysis oils (pine oil and palm empty fruit bunches oil)

e

 Considerable improvement of carbon conversion in the inert gasification in the presence of ilmenite  Decrement of reducibility and H2 production within cycles due to the physical demolition in ilmenite structure

600

H2 concentration: 90% H2 yield efficiency: 80.6% Glycerol conversion: 99% Hydrogen purity of 80% with S/C ¼ 1 and above 90% with S/C ¼ 1.5 e3.0

[57]

[106] [131]

 Glycerol conversion above 80% and selectivity up to 75% and H2 [107] selectivity: Above 90%  Decrement order of glycerol conversion and H2 selectivity: CeNi/AlMCM-41, Ni/Al-MCM- 41, Ni@Al-MCM-41.  H2 yield efficiency (%): [103] Acetic acid: 61.2% Ethanol: 68.9% Acetone: 76.1% Furfural: 71.8% glucose: 76.7%  Decrement order of bio-compound conversion: Ethanol z acetone > glucose > furfural > acetic acid  Redox performance order: Ni/SBA-15, Ni/Al-MCM-41, Ni/MMT, and Ni/ [104] Al2O3  Higher initial activity for Ni/SBA-15 and Ni/AlMCM-41  Lower stability of Ni/Al-MCM-41 than Ni/SBA-15 due to the structure collapse

 Fuel conversion of 97% for pine oil and 89% for EFB oil  Approximate H2 yield efficiency: 60% for pine oil and 80% for EFB oil

[132]

Continued

Chapter 4  Chemical looping conversion of biomass 123

Natural ilmenite

T ( C)a

Remarks

Ref.

Biomass pyrolysis oil 20%NiO/dolomite Impregnation

Fixed bed

900

[110]

Biomass pyrolysis oil Benzene (as gasification tar model) Acetic acid (model compound of biooil) Acetic acid

Fixed-bed Fluidizedbed

950 730

 Cold gas efficiency: 80.76% (slight decrement to 76.26% after 10 cycles)  Syngas yield: 1.318 Nm3/kg  Syngas purity: 92.06%  Weak cyclic at 700 C owning to agglomeration and sintering  H2 purity: 96% with S/O ¼ 1.5 and 84% without steam  NiFe/SiC: benzene conversion of 96%, H2 yield:7.2%, CO yield:17%  H2/CO: NiFe/SiC > Fe/SiC  Formation of excessive carbon deposits and biphenyls

Fixed-bed

525

 H2 yield above 90% and H2 purity above 99.2 vol% [101]  Pd considerably promoted the reduction of NieCo oxides to metallic Ni eCo during the reforming stage

Solegel

Fixed-bed

650

 Conversion and H2 selectivity above 90% and 60% for LNFe [102]  Activity order of H2-rich syngas: LaNi0.8Fe0.2O3>LaNi0.8Co0.2O3>LaNiO3>LaNi0.8Mn0.2O3>LaNi0.8Cu0.2O3

Solegel

Fixed-bed

600

e

Fixed-bed

Acetic acid

Fe2O3/MgAl2O4 Fe, Ni/silicon carbide

Synthesis method

Solegel Impregnation

Pd/NieCo Coprecipitation Dolomite (sorbent) Impregnation LaNi0.8M0.2O3 (M ¼ Fe, co, Cu, and Mn) LaNixFe1xO3 (0 Cu > Fe > Mn [10]. In the intention to obviate the economic matters affected by the synthesis of OC materials and the loss of synthesized OCs after sequential cycles in addition to environmental concerns, several natural ores rich of metal oxides (e.g., hematite, olivine, ilmenite) have been investigated as the OCs for CL processes and exhibited almost good performance, opening a new highway in the synthesis of inexpensive looping materials.

Chapter 4  Chemical looping conversion of biomass 129

5. Concluding challenges and outlooks 5.1

Challenges

The most controversial problems with biofuel chemical looping systems are the deactivation of oxygen carriers with fuel and fuel impurities, process efficiency, economical aspects, and H2 purity. The oxygen carriers tested for CL processes of bio-based fuels have not been developed sufficiently to be proper for large scales that require a long lifetime of stable reactivity, taking into account that the effect of poisoning and catalyst deactivation for such fuels could be much higher than conventional fossil-based fuels due to the impurities and complex components existent in the nonpurified bio-based feedstock. Designing an inexpensive bifunctional catalyst that can afford high and stable reactivity upon numerous sequential redox cycles could be the future aim of researchers.

5.2

Outlooks

With the development of chemical looping processes over the last 2 decades, it is clear that future use of bio-based chemical fuels in all industries would be expected. Although, as mentioned there are problems, but with innovations in processes, reactors, oxygen carriers, and even new bio-based fuels, these problems would be resolved over time and become one of the most important possible processes according to the source of energy and environmental aspects. In view of current human problems such as fossil fuel affecting global warming, it is strongly recommended to use a clean fuel source for use in power generation rather than fossil energy consumption. Chemical looping technology is at the core of this topic and is the bridge between the ingredients and the end product. Without a doubt, using bio-based fuels for chemical looping technology is competitive due to four main reasons: 1. Using renewable fuels helps to directly decrease the consumption of fossil fuel. 2. Using bio-based fuels for power, hydrogen, and coproduction with clean fuel. That hydrogen and syngas could use as a secondary clean fuel or industrial feedstock such as methanol and ammonia plant. 3. Reduce too much CO2 emission with a CLC-type system and become a friendly environmental process. 4. In general, fossil fuels are more expensive than biofuels, which reduce the economic efficiency of the system.

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5

Production of biogas, bio-oil, and biocoal from biomass Morteza Esfandyari1, Ali Hafizi2, Mehdi Piroozmand2 1

DE PARTMENT OF CHEMICAL ENGINEERING, UNI VERSITY OF BOJNORD, BOJNORD, NORTH KHORASAN, IRAN; 2 DEPARTME NT OF CHEMICAL ENGINE ERING, SHIRAZ UNIVERSITY, SHIRAZ, F ARS, IRAN

1. Introduction Global efforts to generate renewable and sustainable energy have been steadily increasing over the past few decades, and The world’s overuse of fossil fuels and the consequent emissions of greenhouse gases (GHGs) have caused a great deal of damage to humanity, such as environmental degradation, disease spread, climate change, and global warming. Biomass, wind, solar and hydroelectric power plants, etc. are renewable energy sources that ensure the continuity of cheaper, safer, and cleaner energy supply and consumption and according to forecasts, renewable energy consumption will reach 129 million Btu by 2040 [1]. Rising demand for fossil fuels and the imminent depletion of these resources, and the subsequent forecast of rising global energy prices due to limited fossil fuel resources, have plunged the world into an energy crisis. Therefore, the importance and necessity of changing the current system of energy production and consumption and replacing it with renewable energy sources to meet the world’s energy needs in the future are undeniable. Biomass is one of the imperative sources of renewable energy that can be converted to fuel and chemicals in a variety of ways, and can be replaced by fossil fuels by improving its quality [2,3]. Biomass as one of the important sources of renewable energy includes all materials in nature that were living latterly, made from living organisms or their waste. The origin of fossil fuels is biomass, but fossil fuels origin is biomass that was alive in the far past and was created under certain pressure and temperature conditions tens of millions of years ago. Biomass is based on organic matter and is formed of a mixture of molecules including carbon, hydrogen, usually oxygen and mostly nitrogen, and a few of other atoms like alkali, alkali earth metals, and metals. Biomass turns carbon dioxide and solar energy in the photosynthesis process into carbohydrates and turns the carbohydrates into chemical energy. Burning of biomass is a process in which the carbon dioxide saved Advances in Bioenergy and Microfluidic Applications. https://doi.org/10.1016/B978-0-12-821601-9.00005-4 Copyright © 2021 Elsevier Inc. All rights reserved.

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during the photosynthesis, gets freed by burning [4]. Therefore, a neutral carbonic cycle starts in the environment; this cycle has been shown in Fig. 5.1. Some important sources of biomass are as follows:
Forestry wastes like wood, pieces of wood and sawdust
Agricultural products and their accompanied wastes like corn, rice, sugar cane, types of fruits, oily plants and their wastes such as rice bran and straw
Factory wastes from various industries including textile, alcohol making, wood and paper, wastewater treatment, and food and juicing industries
Urban solid swage wastes and animal excreta including houses, offices, shops and restaurants garbage, bulky house wastes, garden wastes, and greenhouses wastes
Special wastes such as industrial trashes, construction wastes, rusty rubbers, and radioactive and infected hospital wastes. Using the biomass as a resource of energy is interesting not only for economic purposes but also for economic and environmental reasons. It is also known as a cause for accelerating the sustainable development. The systems that turn biomass into utilizable energy can be used in small scale as a module and in medium and large scales [5]. Agricultural and forestry industries are one of the main biomass resources, which can provide great opportunities for the economic expansion of villages and remote areas. The emission rate of pollution from biomass combustion usually is less than fossil fuels. Furthermore, commercial use and utilizing of biomass can eliminate or reduce the problems accompanied by the destruction of wastes in other industries such as forestry, wood manufacturing, food processing, and especially municipal solid wastes of urban centers [5,6]. Notable features of this energy source include its ability to be supplied in three forms: gas, liquid, and solid, as well as extensive storage capacity, pollution elimination, and

FIGURE 5.1 Neutral carbonic cycle in the process of turning biomass into biofuel.

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applications such as power plants, transportation, combined heat and power (CHP), and heat manufacture [7]. All biomasses are basically made up of three basic polymers, including cellulose, hemicellulose, and lignin, which can be used for power plant operation, heat supply, cooking, transportation, etc. Fig. 5.2 shows technologies that can be used for manufacturing various fuels from various biomass. Solid biomass can be used as a potential primary substance for making various fuels (bio-oils, gases, characters) by using thermal technologies such as pyrolysis, liquefaction, and degassing, while wet biomass (organic wastes, fertilizers, etc.) is transformable to renewable fuels through chemical processes such as fermentation and anaerobic digestion.

2. Biomass technologies Today, many technologies have been developed or are being developed for various sources of biomass and its various applications. Different biomass technologies are in different stages of development and introduction to the market, from laboratory development and pilot scale to fully commercialized. These technologies are divided into two general categories: thermochemical and biochemical processes.

2.1

Thermochemical processes

These processes include combustion (direct and cofiring), pyrolysis, high-temperature gasification, low-temperature gasification, carbonization, and liquefaction.

FIGURE 5.2 Biomass production technologies to different products.

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Direct combustion In this technology, solid biomass resources such as forestry, agricultural, food, and municipal waste are burned directly in certain boilers and the resulting heat is used to generate electricity or heat. The most important technologies for generating electricity in this group are waste incinerators.
Combustion of biomass in modern boilers (cofiring) In this technology, various biomass resources are combined with fossil fuels and directly burned in the fluidized bed, cyclone, etc. This technology has been highly implemented by the United States as “clean coal program” and Europe as combustion with other biomass sources, coal, and fossil fuels.
Pyrolysis Pyrolysis is the reaction of biomass resources at high temperatures without the presence of air, which leads to their decomposition. Final products of pyrolysis are in different forms like solid (coal), liquid (oxygenated oils), and gas (methane, carbon monoxide, carbon dioxide). Biomass can be converted into biogas, bio-oil, and biocoal during pyrolysis.
Low-temperature gasification The basis of this technology is heat decomposition. In this technology, while heating biomass resources and in the presence of very low air, methane gases, carbon dioxide and carbon monoxide, and hydrogen are produced.
High-temperature gasification The basis of this technology is the decomposition with high-temperature ionized gas. In this technology, methane, carbon dioxide, carbon monoxide, and hydrogen are produced by heating biomass resources to a very high temperature of 3500e20,000 C and in the presence of a very low portion of air.
Carbonization This technology is one of the oldest technologies, and its final products are charcoal, electricity, and heat.
Liquefaction In this technology, biomass resources are placed at high temperatures and pressures, and the final product is a liquid with a relatively high thermal value [8].

2.2

Biochemical processes

This group includes some biotechnological processes such as anaerobic fermentation (digestion), aerobic fermentation, and alcoholic fermentation.

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Anaerobic digestion The basis of this process is the decomposition of biomass resources by bacteria in the absence of air, in which methane and by-products of medium thermal value (biogas) are produced.
Aerobic digestion In this method, certain bacteria perform aerobic decomposition. The output product includes heat, carbon dioxide, and a small amount of biogas.
Alcoholic fermentation This technology is used to produce renewable fuels. The final products are bioethanol, biodiesel, and different kinds of oils.

3. Biogas For a half-century, using methane from biological fermentation was raised in European municipal wastewater treatment plants. Nevertheless, the mutual use of biogas has been considered since the second world war and, specifically, in the last 2 decades its usage gained more interest due to lack of energy and also rising energy prices in fuel importing countries, biogas has received superior consideration [9,10]. Today’s world needs to pay much intention to manufacture and using biogas, and most of the developed countries have implanted big plans referred to this area, in countries like China and India. Biogas is being used in a considerable amount [10]. Biogas or swamp gas is a flammable mixture that is formed of fermentation of organic substances in a specified temperature and pH range in an anaerobic condition by the microbes. The outcome gas is odorless, colorless, and flammable that has a blue flame while burning. Based on the raw material, the composition of biogas breakups and changes the process conditions. The biogas extracted from landfills mostly contains between 40% and 58% of methane and biogas received from digestive house wastes may contain more than 70% methane. Generally, fundamental compositions of biogas and their volume fraction are methane (50e80%), carbon dioxide (20e50%), nitrogen (1e4%), hydrogen (0e1%), hydrogen sulfide (50e5000 ppm), ammonia (0e300 ppm), and oxygen (0e2%). The composition of biogas depends on the materials used to manufacture the gas. The volume and structural form of gas producer entirely depend on the amount of heat and time of material staying in the fermentation tank. The combustion temperature is about 700 C (combustion temperature of gasoline is 350 C, and oil and propane combustion temperature is 500 C), and the temperature resulted from the flame is 870 C. Biogas is flammable like other gas fuels and it mixes with air at a ratio of 1e20 and it has a high ignition speed [11,12]. Table 5.1 shows the characteristics of biogas and natural gas.

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Table 5.1

Compositions of biogas and natural gas [13e16]. Biogas

Methane (%) Carbon dioxide (%) Nitrogen (%) Oxygen (%) Hydrogen (%) Water (%) Hydrogen sulfide Specific gravity Ignition temperature ( C) Inflammable limits (%)

3.1

50e75 25e45 0e3 0e2 0e1 2e7 Up to 10,000 ppm 0.78e0.1.1 650e750 7.5e21

Natural gas 87e98 0e1 0e6 0e1 0e0.02 0e2 0e5 0.56e0.65 650e750 5e15

Biogas manufacture

The decomposition of organic materials or biomass by anaerobic bacteria in proper condition and environment (in the absence of oxygen) is called anaerobic fermentation. This process is done in three steps: hydrolysis, acidification, and methane generation. The materials that can be used in this process are livestock and aviculture wastes and generally animal wastes, green vegetable materials (agricultural, industrial products, trees, and green spaces), solid wastes and organic excreta (urban and rural waste wasters, urban and rural garbage), solid materials, and food processing plant wastes (composting plants, canning plants, dairy, pasta, sauce, etc.) [1,12]. By placing the above-mentioned materials in a proper condition meaning temperature, pH, carbonenitrogen ratio, the density of the materials, the stop time of materials, stirring and daily load in a reactor, a little biogas is made including a mixture of methane, carbon dioxide, nitrogen, hydrogen, oxygen, hydrogen sulfur, which we can use as an energy resource. Also, the output sludge of the reactor (peptic), due to the increase of foodstuff (P, K, and N) and the reduction of their contamination directly or after drying, it can be used in farms as fertilizer and in fish ponds as fish feed as well as animal feed [17]. A general categorization of the share of any of the biogas manufacturing sources in the world is as shown in Fig. 5.3. As it can be seen, solid agricultural wastes and animal excreta are the most choice resources of biomass, among others [18]. Biogas technology is acceptable from an economic point of view and based on a natural process, this gas is made without any cost or expense, but controlling, utilizing, and optimization of this gas guarantees of spending money. Biogas can be produced in three ways: biogas plants, landfills, and anaerobic wastewater treatment. The use of biogas, in addition to sanitizing the environment and preparing rich fertilizers and producing fuel gas, is of great economic importance. Manufacturing electricity by burning this gas is far more economical than directly burning this gas. The high price of fossil fuels, an increase in energy demands, environmental pollution, etc. has made bioenergy gets paid high attention. By constructing and expanding biogas power plants in addition to supplying some of the energy needed

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FIGURE 5.3 Categorizing biogas manufacturing resources [2,6].

in our country, it can also help the crisis of urban garbage and decreasing of environmental pollution, which has considerable effects on economics and socials. Using biomass resources is affordable in large capacities and long working hours, and also granting loans with a small profit can be very useful in this field. By utilizing a biogas power plant while collecting and controlling environmental pollution and helping the social hygiene and health of the community, it can provide some of the heat and electrical needs [18,19].

3.2

Affecting factors on the amount of manufactured biogas

Fermentation reactions in the biogas apparatus include a complex of chemical and biological activities of two groups of acidifying and methane generating bacteria in the fermentation tank, which their growth, living, and the amount of the biogas they make depend on the condition of fermentation. Thermal degree, humidity, pH, carbone nitrogen ratio, materials concentration, and inertia time are affective on the biogas apparatus’s function [12,19].

3.3

Biogas manufacturing apparatus

The structure of this device includes two entrance and output pools and one fermentation tank, which usually end to a gas tank at the top segment. Recently, biogas devices with different forms and dimensions have been tested and put into operation in accordance with different climatic conditions. Biogas manufacturing systems, which are built on the ground or usually underground, have three main parts: (A) Entrance pool and canal (B) Digestive tank (C) Output pool and canal

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Generally, the organic materials are mixed with water with the same ratio to get thinner; then, these materials enter the fermentation tank by a pipe. In this tank, anaerobic chemical interactions are performed by a set of bacteria, operations, and production of methane gas, and the resulting gas is collected in the upper part of the tank, which is called gas storage, and is taken out of the system for consumption. Materials and the remains of the fermentation process, after the gas extraction, exit through the output canal, and new organic materials enter through the entry canal. The fermented remains after gas extraction are a very nutritious and highgrade fertilizer for use in agriculture. Usually, the biogas facilities are made by materials such as brick, cement, and metal. The first step of constructing a biogas system is to build a tank; before the construction, there shall be a precise study on the soil condition and groundwater level. For making sure of tanks stability, we should choose a place in which the soil is impenetrable and hard and the underground water reservoir is bottomless or has a deficient volume [20,21]. For calculating the volume of the output gas in biogas devices first, we shall obtain the chemical formula of excessive materials or the entry wastewater to the reactor. Excessive materials have the typical formula of [12,22].

      3 3 3 Ca Hb Oc Nd þ 4A  B  2C þ D / 4A  B  2C  D CH4 þ 4A  B þ 2C þ D CO2 þDNH3 4 8 8 (5.1)

3.4

Biogas, manufacture and utilizing

Biogas production reactors can be classified based on the percentage of water in the raw materials; also, the biogas manufacturing process can be done in one step or two steps. Multiple commercial technologies for manufacturing biogas have been presented based on the long-range of raw materials, which can be classified into three categories from the density of solid material point of view: wet processes, half dry processes, and dry processes. Biogas manufacturing from a type of entrance point of view gets categorized into single digestion and codigestion. In the process of codigestion, two or more different raw materials enter the process. Experience has shown that the yield of biogas (gas volume based on the entry material unit) improves a codigestion process [18,20]. Process temperature, pH, the concentration of volatile acids, being alkaline, organic loading rate, and inertia time are the boldest processing affective conditions in the manufacturing of biogas [23]. The diagram of the digestion process for biogas production has been shown in Fig. 5.4. Utilizing biogas is possible in the frame of few guidelines:






Manufacturing Manufacturing Manufacturing Manufacturing Manufacturing

thermal energy energy fuel heat and electricity at the same time combination of electricity-heat-cool

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FIGURE 5.4 Diagram of the biogas production process.

For utilizing biogas, sometimes it is needed gas to be refined and improve the quality. Type of utilizing determines the measurement, refining range, and the improvement of biogas. For cooking and heating applications in rural areas, biogas refining is often not required and only the drip trap needs to be installed in the biogas pipeline. Because biogas is saturated with moisture and liquid mixtures are used on the inner walls of pipes and equipment. They condense on biogas and may cause failures. In the industrial biogas utilization also in the electricity manufacturing or heat and steam manufacturing or manufacturing both at the same time, it is needed that the biogas gets refined of damaging and corrosive compounds to under the threshold that the device producers determine. The most important ones of these unwanted compounds are hydrogen sulfide, ammonia, organic halogenated compounds, and organic silica compounds (a.k.a., siloxanes) [23]. Biogas can be a substitute for natural gas, get injected into the gas delivery network, or after being compressed be provided as bio compressed fuel gas in the gas station [5,8]. A perspective of biogas utilizing process steps has been shown in Fig. 5.5. In manufacturing electricity from biogas, the chemical energy hidden in biogas turns into thermal energy and then into mechanical energy and finally into electrical energy or the chemical energy directly turns into electrical energy. Combining electricity and heat from biogas is done in a variety of ways. However, the foundation of all them is recycling the output heat of thermal energy transforming process or turning the chemical energy of biogas into electrical energy; also, by using the absorption chillers cycle with the help of materials such as lithium bromide, it can provide chill from the heat resulted by biogas [19,23].

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FIGURE 5.5 Process diagrams in biogas utilization.

4. Bio-oil Biomass can be used to produce bio-oil, a promising alternative energy source for limited crude oil. Biomass can be used for manufacturing bio-oil, as a hopeful source of energy to substitute crude oil. Generally, there are two processes for turning biomass into bio-oil: flash pyrolysis and thermal fluidizing. The costs for producing bio-oil from biomass are rather high based on the current technologies, and the main challenges are a low quality and low applicability of bio-oil. The considerable research tries have been made for producing bio-oil from biomass. Scientific and technical has been studied in the way of improving the function and quality of bio-oil-based renewing research of bio-oil. In addition to the bio-oil produced by biomass properties, it expresses the functions and how to produce bio-oil [24,25].

4.1

History

Since the shortage of oil resources with the global energy crisis in the 1970s, considerable attention has been paid to the development of alternative fuels. Renewable biomass resources can be converted to fuel and are a logical option to replace crude oil [26]. Unlike fossil fuel, biomass exits the carbon out of the atmosphere and returns it as burned. This keeps a closed carbon cycle and without a gross increase of CO2 in the atmosphere. All of the biomass is produced by the green plants that turn sunlight into herbal material through photosynthesis. The potential of biomass fuels includes wood, wood products with short rotation, agriculture wastes, herbal products with short rotation, animal wastes, and many more materials. Biomass can turn into different

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energies by different technical processes based on the properties of the materials and the wanted energy kind. As a result, a wide range of conversion schemes has been expanded. Among different technologies of conversion, the thermal conversion of biomass provides a proper way of producing liquid fuels. Liquid products, known as bio-oils, are considered as the hopeful substitute for oily fuels for producing electricity, heat, or extracting valuable chemical material [7,24,26].

4.2

Bio-oil properties

Bio-oils are usually a dark brown liquid, and it has a special smoky odor. The physical properties of bio-oils are described in various articles. The physical property of bio-oil, which is made of different biomasses, varies a lot from crude oil. Bio-oil is a complex combination of hundreds of organic compounds, which usually include acids, alcohols, aldehydes, esters, ketones, phenols, and lignin-derived oligomers. Some of these compounds have a direct connection with unwanted binds suitable for bio-oil. In comparison with crude oil, bio-oils have many unwanted properties such as the big amounts of water, high viscosity, high ash contains, high amount of oxygen (low amount of heating), and high corrosion (acidity) [27,28]. Bio-oil has the volumes energy up to 10 times bigger than biomass; therefore, it is more proper for transportation because the costs of biogas transportation in comparison with biomass are going to be very lower. Therefore, one of the problems of using biomass in the energy section can be solved by flash pyrolysis [28]. Table 5.2 shows the comparison of mixture and properties of biomass and crude oil.

Table 5.2 Comparison of characterization and composition of Bio-oil and Crude oil [29,30]. Moisture content (wt%) pH Density(kg/m3) C (wt%) H (wt%) O (wt%) N (wt%) Solids (wt%) LHV (MJ/kg) Viscosity (cSt at 40 C) Flash point Pour point S (wt%) Distillation residue (wt%)

Bio-oil

Crude oil

14e30 2e4 1.1e1.3 32e60 5.0e8.6 35e60 0e0.2 0.2e1 13e18 10e1000 60e100 (40)e3 0e0.5 1

0e10 e 0.78e0.98 80e90 10e15 0e2 0.3 0e1 38e47 7e15 70e85 (22)e(10) 0e6 Up to 50

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Bio-oil can be converted into a variety of products and may therefore be a good option in a biomass plant. It can be converted to gasoline such as partial dehydrogenation or gas synthesis by steam modification [29,31].

4.3

Methods of producing bio-oil

Biomass can be turned into bio-oil by two main methods: flash pyrolysis and hydrothermal fluidization (HTL). Flash pyrolysis includes a fast breakup of organic compositions in the absence of oxygen for producing liquids, gasses, and chars. HTL includes the reaction of biomass in water in high temperature and pressure with or without a catalyst in which case in the presence of catalyst it needs slightly dry biomass but for the catalyst-less reaction it needs a very humid condition. Therefore, from the source point of view, the aquatic source is ideal for the biomass. Over the past decade, significant efforts have been made to develop bio-oil production; unlike the flash pyrolysis, the changes in the HTL field are at the first steps of expansion. There is just limited information accessible on the HTL subject of study including process expansion, mechanism study, and the functions of bio-oil [29]. Flash pyrolysis is a compression method in which density, mass, and energy increase by using the raw biomass at the medium temperature (300e600 C) with a high amount of heating (103e104 K/s) and in the time of short residence (1e2 s). Flash pyrolysis is one of the exciting technologies for manufacturing liquid fuel (biooil) from biomass, which has excellent potential for being used in the transportation industry. The flash pyrolysis process product has resulted in three phases of liquid (water þ bitumen þ organic compositions), solid (coal), and gas (H2, CO, CO2, CH4). The liquid product of flash pyrolysis is called bio-oil, which is a dark brown liquid, and is made of highly oxygenated compounds. Its thermal value is between 14 and 18 MJ/kg. Bio-oil is formed by breaking and fast and concurrent decomposition of cellulose, hemicellulose, and lignin available in the biomass with a fast increase of temperature and then fast cooling down of the interface products. The function and the composition of the pyrolysis products (bio-oil, gas, and biocoal) depend on the type of the material and pyrolysis condition. Long inertia time of at the low rate of heating (slow pyrolysis) mostly produces biocoal. Low inertia time and high rate of heating (flash pyrolysis) produce oil. Bio-oil and biocoal are used as alternative fuels for fossil fuels and raw materials to produce chemicals. Many parameters, including the biomass particles, nitrogen flow (fluidizing gas), feeding rate, biomass type, and temperature affect the function and composition of bio-oil made by biomass flash pyrolysis [31]. In the flash pyrolysis process during fast heating, first biomass particles break up and turn into coal in less than 1 s (10e15% of weight percentage) and pyrolysis steams, which contain stable gasses (carbon dioxide, methane, carbon monoxide) and condensable liquids. The condensable part of pyrolysis steams includes an organic part (bio-oil), humidity, and water manufactured during the decomposition. In fact, their pyrolysis

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vapors are the result of the thermal decomposition of biomass components. The redecomposition of these steams to elements with smaller molecular weight (gas) is called secondary break up. In the third phase of the flash pyrolysis process, bio-oil condensed during the saving in a long time, get under the polymerizing reaction that produces water and carbon dioxide by sedimentation. Although the thermal degree is high or second phase time (middle part in Fig. 5.6) gets longer, the second break reaction improves and it produces more gas [24,31]. The development of pyrolysis flash research and the hydrothermal fluid fluorescence fluoride for liquid production has been significantly developed since the late 1970s. Many reactors and GHG production processes have been tested and developed in a way that nowadays flash pyrolysis is an acceptable path and practical for renewable liquid fuels, chemical materials, and derived products. From the 1990s, many research organizations have successfully constructed fast GHG-producing factories [25]. Bridgewater and Peacocke companies have seriously studied the flash pyrolysis key properties and the liquid products and presented the main reaction systems and expanded process during the last 20 years [32,33]. In comparison with heavy fuel oil, bio-oils have these unwanted properties to burn [25,32]:







Functioning programs: High amount of water High viscosity High ash containing High oxygen (low heating) High corrosion (acidity)

FIGURE 5.6 Flash pyrolysis reaction path schematics.

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4.4

Bio-oil advantages

Bio-oil has many advantages on fossil fuels as a clean fuel. Bio-oils have natural GHG/ CO2. Therefore, they can make CO2 credits. GHGs such as SOx would not be manufactured, because herbal biomasses contain an insignificant amount of sulfur. Therefore, the bio-oil will not be taxed by SOx. Bio-oil fuels produce more than 50% less GHGs than gas oil in the wind turbines. Bio-oil is renewable and can be produced in countries that have high amounts of organic wastes. Therefore, bio-oils are cleaner and cause less pollution. However, bio-oil potential for directly replacing oil fuels considering high viscosity, high amount of water and ash, low thermal value, unstability, and high corrosion is limited. In conclusion, for obtaining a liquid product that can be used as liquid fuel or chemical materials in different programs, it requires more study on bio-oils. Bio-oils are obtained by the fast decomposition of biomass and can replace liquid fossil fuels because this material is updated by catalytic dehydrogenating, catalytic break, or steam correction. Bio-oil also can be a source of valuable chemical materials by distillation or extraction. Until now, more than 400 composites have been known in the bio-oil obtained by the fast decomposition of biomass. Expansion concentration on this field has been easily divided on refunding products of the whole bio-oil or from major deficit [32].

4.5

Utilizing bio-oils

As a renewable liquid fuel, bio-oil can easily be stored and transport. This matter is being used in many static functions such as steam boilers, furnaces, engines, and turbines for producing electricity as a substitute for fuel oil or diesel. On the other hand, crude oil can serve as a primary substance for producing glues, phenol-formaldehyde resins, wooden flavors, etc. The followings are a few laboratory uses of bio-oils: (1) combustion fuels in the steam boilers/torches/furnaces for producing heat, (2) combustion in diesel engines/turbines for producing electricity, (3) producing anhydrosugars such as levoglucosan, which has potential for producing medicines, surfactants, and biodegradable polymers, (4) as liquid and wooden smoke flavors, (5) producing chemical compounds and resins (for instance, agriculture chemical compounds, fertilizers, acids, and publishing controllers), and (7) glue production such as bioasphalt glue [31,32,34]. Bio-oils are obtained by the fast decomposition of biomass and can replace liquid fossil fuels because they are updated by catalytic dehydrogenating, catalytic break, or steam correction. Bio-oil also can be a source of valuable chemical materials through the distillation or extraction processes. Until now, more than 400 composites have been known in the bio-oil obtained by the fast decomposition of biomass. Expansion concentration on this field has been easily divided on refunding products of the whole biooil or from major deficit [31,32].

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5. Biocoal In the fast-developing world, biocoal is the biggest helper to humans’ CO2 manufacturing in the atmosphere. The environmental effects of biocoal have been expanded through many areas of the world, from air and water pollution and waste management to increase earth usage [22]. In the face of GHG emissions, industries are looking for a new way to control and manage their environmental effect and reducing it. In this part, we concentrate on biomass as a new process for replacing fossil coals in the industry [35]. Biocoal is a natural carbon fuel that can replace fossil coal in the raw industrial processes [36]. This matter is produced in the biogreen GHG manufacturing process and carbonizing raw biomass in the controlled temperature and residence time. Thermal conversion of biomass, which happens in the absence of oxygen, allows the volatile organic compounds and cellulose part to be put out of the primary compounds and results in a uniform biological fuel with similar properties to fossil coals [36,37]. Biocoal is a product in which biomass turned into biocoal by heat, while countries are trying to decrease their dependency on coal and reduce GHGs, and are also looking for renewable energy sources. Thanks to biocoal volume reduction up to 75% and increase of its density there are options for transporting them in the longer paths. Transforming biomass into biocoal has many advantages including higher energetic value, reducing the humidity content, clean fuel, and easy execution [36,37]. Biocoal is ideal for decentralization, local use of biomass, and highly wet biomass, such as sludge from water treatment plants, garbage obtained by food industries from distillation or manufacturing factories, and also agriculture wastes [8]. Also, using biocoal in coal plants has caused cement manufacturers to pay attention to this matter, where now a considerable amount of energy is being used in the form of coal and coke as a fuel resource [38]. Biocoal is used for the following reasons: (1) more concentrated energy, (2) easy breakability, and (3) more hydrophobic than primary substance like coal. Based on the difference resulted by this fact is that artificial biomass is made of biomass by using the hydrothermal carbonization (HTC) or pyrolysis, and in comparison with fossil coal it provides the following advantages [36,39]. Biocoal is a very uniform fuel, which is made of a very heterogeneous biomass [37]:








A neutral CO2 source of energy 75% of biomass turns into coal High calorie Manufacturing coal with the determined calorie value High burning yield with low NOx publishing rate Very low amounts of toxic, sulfur and heavy metals Lower volume for saving in comparison to biomass

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Unlike the biomass, it will not be exposed to destruction
Lower transportation costs in comparison with biomass
It fixes the cocking biomass is the steam boilers.

5.1

Process of producing biocoal

Preparing the coal process by pyrolysis has been shown in Fig. 5.7. Renewable biomass (for instance, rice husk, sawdust, corn stem, etc.) for the first time is decomposed and manufactured rapidly at 500 C in an anaerobic atmosphere and manufactured for producing bio-oil and bioflux. Then, bio-oil has been divided under the atmospheric pressure and room temperature up to 240 C to liquid chemical substances, and biocoal can be obtained. Production of biocoal only needs the basic procedures of chemical engineering and there is no need for equipment or utilizing complex and costly refineries, which can be increased easily [40,41]. The second process of manufacturing biocoal is HTC, which is shown in Fig. 5.8. Basically, any biomass can turn into biocoal in few hours, by feeding water as a secondary product. Humid biomass in a reactor under high pressure of 16e40 bar and temperature of 200e300 C gets composed and heated. Biocoal is made after 5e12 h. HTC technology can be used for both dry and humid biomasses. The final product of the HTC method is a liquid that contains particles of biocoal. The remained humidity can be easily removed by mechanical methods [37,38]. Table 5.3 shows the comparison between physical and compositions of bio-oil and biocoal, and Table 5.4 also shows the characteristics between coal and biocoal. Biocoal can replace convectional fuels such as (1) diesel, (2) kerosene, (3) furnace oil, (4) firewood, (5) coal, and (6) lignite [35,36].

FIGURE 5.7 Schematic illustration of biocoal preparation from biomass.

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FIGURE 5.8 Biocoal production process using the HTC process.

Table 5.3 Comparison of characterization and composition of bio-oil and biocoal [36]. (%)

Bio-oil

Biocoal

C H N O

34.75 8.04 1.28 55.93

64.82 5.88 1.11 27.42

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Table 5.4 Comparison of characterization and composition of biocoal and coal [42e45]. Biocoal Moisture content (wt%) Density (kg/m3) Energy density (GJ/tonne) Volatiles (%db) C (wt%) H (wt%) O (wt%) N (wt%) Heating value (MJ/kg) S (wt%) Ash (wt%) Fixed carbon (%db)

1e5 750e850 24e26 10e12 30e50 7e10 40e60 0.2e2 20e30 0e0.005 20e30 40e50

Coal 6e32 640e920 23e27 15e30 45e60 4e6 20e25 0e1 20e25 0e1 5e45 35e55

6. Environmental impact and future of biomass and bioenergy 6.1

Environmental impact biogas preparation

Biogas, which is a clean fuel with the potential for replacing fossil fuels, has many benefits in different processes. However, the amount of emitted GHGs and the environmental impact of producing this fuel are extremely important. Biogas production technology as a renewable energy source is highly investigated and optimized to prevent the use of fossil fuels. However, the important point is that each process has its disadvantages and should be avoided. Table 5.5 represented releasing gases through operating direct biogas combustion. Biogas has the potential to significantly reduce GHG emissions. Direct emission of NOx must be considered in the combustion of biogas. Because the amount of this pollutant reported in some case study was at a high level. Therefore, it should be attention for two critical GHGs including nitrogen oxides and methane.

6.2

Environmental impact of bio-oil

Production of bio-oil requires a high level of energy due to biomass drying and size reduction. Recently, a technology was developed to achieve bio-oil at a lower temperature of approximately 200e350 C; namely torrefaction. This innovation decreased the temperature and energy consumption in addition to increasing the quality of produced bio-oil. As expected, reducing energy consumption would also reduce GHG emissions. Reports indicate that this process used as a size reducer caused mitigation of energy and also reduced pollutants. It was represented on the basis of life cycle assessment. This method promises to use two-stage torrefaction to produce bio-oil as an environmentally friendly method [1].

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Table 5.5 The composition of GHG emitted in the combustion of a model biogas. Pollutant Carbon monoxide (CO) Sulfur dioxide (SO2) Nitrogen oxides (NOx) Nonmethane volatile organic compounds (NMVOC) Formaldehyde (CH2O)

6.3

Emission factor (gGJL1) 310 256 25 202 540 10 21.15 8.7 14

References [46] [47] [46] [46] [47] [46] [47] [46] [47]

Environmental impact of biocoal

The biocoal production process was analyzed with two different operational aspects: (A) fast pyrolysis for bio-oil production and (B) fast pyrolysis in addition to atmospheric distillation for bio-coal synthesis. The biochar produced from A/B can also be analyzed in two different methods including the application of biochar as fuel or to be used as soil amendment. It has been reported that when using biochar as fuel produced from pyrolysis for biooil, the net amount of emitted GHG was approximately 450 kg (a negative output), whereas for soil amendment from pyrolysis for bio-oil, the net amount of GHG is around 906 kg (positive output) for the production of the same energy value. Of course, it should be noted that the energy efficiency is greater in the first system. From an economic and energy standpoint, both classes A and B are positive. Generally, for preventing climate change, using biochar as soil amendment produced with both procedures (A and B) is promising.

6.4

Futures of biomass and bioenergy (an outlook)

Nowadays, more than 80% of the world’s energy consumption comes from nonrenewable sources. This is extremely dangerous for the future of human society because fossil fuels have limited sources, and then the amount of carbon dioxide and pollutants from this type of fuel will severely damage human society. Global warming, ozone depletion, polar ice melt, etc. are the consequences of this event. One important method to prevent it is to replace clean fuels and renewable energies, such as solar energy, wind energy, and hydro-energy, as well as energy carrier materials such as hydrogen that can significantly replace fossil fuels. Fig. 5.9 demonstrates the prediction for the various energy sources for the next 30 years. For example, coal as energy source is predicted to be reduced from about 40% in 2020 to 16% and 6% of the overall world energy consumption in 2030 and

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FIGURE 5.9 The prediction of the various energy sources for the next 30 years.

2050, respectively. On the other hand, the renewable sources such as wind, solar, and biomass sources are predicted to be increased from about 19% in 2020 up to 68% in 2050. In addition, as shown in Fig. 5.10, the future of energy consumption is led to renewable sources during the next 30 years. Recently, for each process designed by industries, the environmental impacts of the proposed process along with energy and exergy analyses are required for better investigations. This rule does not exclude clean fuels such as the production processes of bioenergy sources. The economic factor also plays an important role in the future of biomass and biomass-derived bioenergy. The factor scores are numerous for basic

FIGURE 5.10 Future of biomass-based energy consumption [48].

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sectors including combined heat and power industry, civic services-subregion heating, and utilities-co-firing. With more extensive analysis of biomass resource requirements, innovative use of biomass, feasibility, and progress toward practical implementation would be developed. Also, other prospects for this technology are global activity toward the generation of bioenergy (both process and chemical) business income in addition to the benefits of GHG emissions and cost-effective technology.

7. Conclusion and future trends Nowadays, there is an urgent need to the solution of renewable and stable energy for facing the intense challenges of humanity such as population increase, decline of fossil and other nonrenewable resources, environmental health getting worse, environmental balance, and industrial demand in a worldwide scale, which makes heavy demands about energy sources. In line with steady expansion, the stable solutions must reveal economic topics, social and environmental and provide today’s energy needs without satisfying the capabilities of the future generation in providing energy for themselves. Being renewable and giving more credit to the environment means that biotechnology is upgraded by the governments and scientific institutes, and an important solution because replacing fossil fuels with biofuels can reduce the carbon dioxide publish rate, reduce the GHG effect, reduce oil dependency, improve energy security, etc., and save the nonrenewable resources [49,50]. The automatic fermentation of organic wastes causes millions of tons of carbon dioxide and methane to enter the atmosphere, which causes greenhouse effects, increase in temperature, and intense weather changes. The studies show that the earth’s average temperature has raised 0.3e0.6 grades of centigrade in the last century. By designing and manufacturing anaerobic digestives and pointing different wastes to it can produce proper energy while it prevents the entrance of above-mentioned gasses to the atmosphere and the negative effects of GHGs like temperature, increase of seas water surface, and precession of them [50,51]. Research and strategic implementation of biogas, bio-oil, and biocoal in developed countries has improved environmental, economic, social relations, agriculture, energy, and health. However, many of the developed countries have not succeeded in reaching the goal of accessing to energy and lowering the environmental crisis and there are many challenges which prevent the development of biogas, bio-oil and biocoal technologies in developing countries to make sure of the successful implementation of such technologies; therefore, these problems shall be solved [51,52]. Based on the reports in 2018, the volume of the biogas was about 59 billion cubic meters (equal to 39 billion cubic meter of methane), which the European community produces about half of this amount. Therefore, in developing countries, the biogas production was not satisfying. Developing countries in comparison with developed

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countries have a long way ahead for manufacturing biogas. Implementation of biogas is different between places, forms, structure costs, and usage patterns, and these changes depend on the expansion states of the country. In developing countries, the biogas manufacturing challenges include technical and substructures, economic challenges, politics and political challenges, the low prices of fuel in some countries, and lack of cooperating spirit between the people [51,52]. Making a stable technology is still an essential challenge for the implementation of biogas in developing countries in comparison to their developed match. The developing countries are rich in biomass but because of insufficient foundations that can include required technologies, they cannot use the resources properly. In addition to this, the amount of technical understanding of operators provides the effectiveness of the implementation of biogas. Lack of fund is a great challenge for continuing the implementation of biogas in developing countries. At the moment, manufacturing a biodigester in household scale with a capacity of 50 kg per day is estimated about 1500 dollars [51e53]. Considering the additional costs, including storage tanks, operating costs, treatment, biogas compressors, and maintenance, the cost is high for low-income people. Most of the developing countries do not have a trustable system for collecting and organizing garbage. Most of the time, most of the collected wastes get burned or left in the environment and also most of the countries do not have a specified policy for decreasing environmental pollution or using bioenergy. One of the other existing challenges for manufacturing biogas is the noncommitment of the government by using new methods [51,52]. Another challenge is corruption that causes an increase in investment and operational costs for the implementation of biogas and from this way it lowers the investment yield. The effective implementation of biogas technology in developing countries as national programs and innovative strategies causes economic growth, environment safety, and national safety [51e53]. Generally, expanded installation and correct function of biogas manufacturing systems can make many advantages for users and a more expanded society. The advantages are energy stability, saving resources, and guarding the environment. On the other hand, using fossil fuels for a long time has not been considered due to limited sources and nonrenewable nature. The biogas resulted from various biological sources can reduce the heavy dependency on these nature threatening sources and resolve the worries about the energy insecurity due to its renewable, functional, and abundant properties. On the other hand, the credibility of biogas is its use in energetic point of view (for instance a typical amount for electrical yield is 33% while for the thermal yield is 45%) and it also is environment friendly due to publishing low amount of dangerous pollution like volatile organic compositions (VOC) [51,52]. Biomass with high cellulose and amounts of hemicellulose mostly cause bio-oil and syngas, while biomass with higher lignin produces char [54]. Bio-oil is a general production for replacing fuels that depend on oil. However, research and manufacturing in large scales are still limited [50]. Most of the studies on the bio-oil production field are in the laboratory scales and for increasing bio-oil quality its properties must be compared with crude oil. Bio-oil is an environmentally friendly

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fuel in comparison with fossil fuels because of the CO2 release in insignificant during the usage of bio-oil [4]. In addition to this, bio-oil produces 50% less NOx; therefore, it is clean fuel oil and less polluter than fossil fuel. Bio-oil contains phenolic compositions that can be used in medical industries and also a large amount of asphalt in it shows its potential application to the asphalt industry [40,54]. Considering the potentials bio-oil has some problems that can be because of high humidity, it has less heat than petroleum products; other bugs include high viscosity, corrosion behavior, high solid content, high amount of water, chemical unstability, and high ash content. Therefore, its direct application is not possible due to these defects. For fixing these bugs, this fuel must be upgraded for its quality to get better for it so it can be a good replacement for the usual fuels. The conversion rate of biomass to bio-oil varries from 24% to 74% of weight fragments, and this conversion rate depends on the type of biomass, temperature, reactor type, humidity, etc.; the rate of conversion can be obtained by choosing the optimum conditions. Considering that the water content of the oil has a maximum percentage of 26% of weight fragments but according to essays the water in bio-oil has between 15% and 130% of weight fragments. In conclusion, one of the bio-oil problems is the existence of a high amount of water, which cannot be used as fuel directly. Because of such a high water content, it requires more steps of separation compared to the ordinary oil, the refining costs of bio-oil is most probably more than rival sources of biomass and oil [30,55]. To ensure the economic rivalry between fuels, bio-oil shall be prepared from cheap sugar sources; for instance, a price sample of molasses is 300e400 $/t. Based on the essay’s price of producing bio-oil is in the range of 98e860 $/t. So the cost of producing bio-oil is below 100 $/t [30]. Until recently the commercial production of biocoal for industrial purposes was started only in a few countries such as Netherland, Belgium, France, and the United States, but currently because of the biocoal advantages it has expanded to many countries [56]. Biocoal has many advantages on ordinary coal that helps the financial savings of the companies who use the biocoal companies. As a fuel obtained from biomass, biocoal is fully exempt from the carbon taxes and it can be qualified for compensating carbon. Biocoal also contains less sulfur and nitrogen than coal and coke, which means while it is burning it releases less amount of pollution. This means less scrubbing costs and emission penalties [36,56]. Biocoal is the main goal for supplying biocarbon fuels for energy and industrial heat producers, which are looking for reducing the carbon tax burden, improving the production of GHG properties, and accessing the carbon markets. Meanwhile, the plants that use coal as fuel can use biocoal as fuel by doing some small changes. The reduction of gross publication of carbon dioxide and taxes caused by carbon are motives for replacing coals with biocoals. In the conclusion of using biocoal as a substitute, it can reduce 14% of equivalent coal that can cause a reduction of coal demands [36,56].

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Nowadays, the energy crisis has led most of the countries and experts to use bioenergy and renewable resources. In industrial and developed countries, by assigning lots of funds and utilizing extent equipment, extensive research and studies have been performed. From the results, it can be obtained that in nature there are various kinds of energy, which can be recycled by recognizing and utilizing propel technologies and methods that one of the most important ones is biogas, bio-oil, and biocoal. Renewable energy sources as a result of biogas, bio-oil, and biocoal are the least expensive options in boosting electricity access, reducing air pollution, and cutting worldwide carbon dioxide emissions. Biomass can be utilized to produce biogas, bio-oil, and biocoal, a promising alternative energy source for the limited fossil fuels. Biomass sources contain organic compounds with large-scale molecules that break down into simple molecules during digestion processes (buried in the ground, inside special containers, or abandoned in nature), and the end result of this process is biogas. Flash pyrolysis processes are so far the only commercially practiced technology for production of bio-oil or biocoal from biomass. HTL with an appropriate solvent (water or organics) is suitable for high moisture content biomass.

References [1] Maghanaki MM, et al. Potential of biogas production in Iran. Renew Sustain Energy Rev 2013;28: 702e14. [2] Biller P, Ross A. Production of biofuels via hydrothermal conversion. In: Handbook of biofuels production. Elsevier; 2016. p. 509e47. [3] Jafari D, Esfandyari M. Optimization of temperature and molar flow ratios of triglyceride/alcohol in biodiesel production in a batch reactor. Biofuels 2020;11(3):261e7. [4] Ciolkosz D, Wallace R. A review of torrefaction for bioenergy feedstock production. Biofuels Bioprod Biorefining 2011;5(3):317e29. [5] Agbor VB, et al. Biomass pretreatment: fundamentals toward application. Biotechnol Adv 2011; 29(6):675e85. [6] Molino A, Chianese S, Musmarra D. Biomass gasification technology: the state of the art overview. J Energy Chem 2016;25(1):10e25. [7] Mun˜oz R, et al. A review on the state-of-the-art of physical/chemical and biological technologies for biogas upgrading. Rev Environ Sci Biotechnol 2015;14(4):727e59. [8] Zheng Y, et al. Anaerobic fermentation technology increases biomass energy use efficiency in crop residue utilization and biogas production. Renew Sustain Energy Rev 2012;16(7):4588e96. [9] Khalil M, et al. Waste to energy technology: the potential of sustainable biogas production from animal waste in Indonesia. Renew Sustain Energy Rev 2019;105:323e31. [10] Bharathiraja B, et al. Biogas productionea review on composition, fuel properties, feed stock and principles of anaerobic digestion. Renew Sustain Energy Rev 2018;90(C):570e82. [11] Farquhar GJ, Rovers F. Gas production during refuse decomposition. Water Air Soil Pollut 1973;2(4): 483e95. [12] Horva´th IS, et al. Recent updates on biogas production-a review. Biofuel Res J 2016;10:394e402.

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[13] Sun Q, et al. Selection of appropriate biogas upgrading technology-a review of biogas cleaning, upgrading and utilisation. Renew Sustain Energy Rev 2015;51:521e32. [14] Valais M, Boisserpe P, Gadon J-L. The world gas industry. Technip; 1982. [15] Kidnay AJ, Parrish WR, McCartney DG. Fundamentals of natural gas processing. CRC press; 2019. [16] Aregbe AG. Natural gas flaringdalternative solutions. World J Eng Technol 2017;5(01):139. [17] Van der Stelt M, et al. Biomass upgrading by torrefaction for the production of biofuels: a review. Biomass Bioenergy 2011;35(9):3748e62. [18] Ohimain EI, Izah SC. A review of biogas production from palm oil mill effluents using different configurations of bioreactors. Renew Sustain Energy Rev 2017;70:242e53. [19] Dada O, Mbohwa C. Biogas upgrade to biomethane from landfill wastes: a review. 2017. [20] Mao C, et al. Review on research achievements of biogas from anaerobic digestion. Renew Sustain Energy Rev 2015;45:540e55. [21] Tauseef S, Abbasi T, Abbasi S. Energy recovery from wastewaters with high-rate anaerobic digesters. Renew Sustain Energy Rev 2013;19:704e41. [22] Smith AM, Ross AB. Production of bio-coal, bio-methane and fertilizer from seaweed via hydrothermal carbonisation. Algal Res 2016;16:1e11. [23] Deublein D, Steinhauser A. Biogas from waste and renewable resources: an introduction. John Wiley & Sons; 2011. [24] Mortensen PM, et al. A review of catalytic upgrading of bio-oil to engine fuels. Appl Catal Gen 2011; 407(1e2):1e19. [25] Mohan D, Pittman Jr CU, Steele PH. Pyrolysis of wood/biomass for bio-oil: a critical review. Energy Fuels 2006;20(3):848e89. [26] Demirbas A. Current technologies for the thermo-conversion of biomass into fuels and chemicals. Energy Sources 2004;26(8):715e30. [27] Chiaramonti D, et al. Review and experimental study on pyrolysis and hydrothermal liquefaction of microalgae for biofuel production. Appl Energy 2017;185:963e72. [28] Tzanetis KF, Posada JA, Ramirez A. Analysis of biomass hydrothermal liquefaction and biocrude-oil upgrading for renewable jet fuel production: the impact of reaction conditions on production costs and GHG emissions performance. Renew Energy 2017;113:1388e98. [29] Trane R, et al. Catalytic steam reforming of bio-oil. Int J Hydrogen Energy 2012;37(8):6447e72. [30] Pinheiro Pires AP, et al. Challenges and opportunities for bio-oil refining: a review. Energy Fuels 2019;33(6):4683e720. [31] Gholizadeh M, Hu X, Liu Q. A mini review of the specialties of the bio-oils produced from pyrolysis of 20 different biomasses. Renew Sustain Energy Rev 2019;114:109313. [32] Czernik S, Bridgwater A. Overview of applications of biomass fast pyrolysis oil. Energy Fuels 2004; 18(2):590e8. [33] Peacocke G, Bridgwater A. Ablative fast pyrolysis of biomass for liquids: results and analyses. In: Bridgwater AV, Hogan EH, editors. Bio-oil production and utilisation; 1996. p. 35e48. [34] Mahinpey N, et al. Analysis of bio-oil, biogas, and biochar from pressurized pyrolysis of wheat straw using a tubular reactor. Energy Fuels 2009;23(5):2736e42. [35] Asadullah M, et al. Optimization of palm kernel shell torrefaction to produce energy densified biocoal. Energy Convers Manag 2014;88:1086e93. [36] Cheng B-H, et al. Bio-coal: a renewable and massively producible fuel from lignocellulosic biomass. Sci Adv 2020;6(1). eaay0748.

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[37] Cruz DC. Production of bio-coal and activated carbon from biomass. 2012. [38] Smith AM, et al. The potential for production of high quality bio-coal from early harvested Miscanthus by hydrothermal carbonisation. Fuel 2018;220:546e57. [39] Yadav V, Baruah B, Khare P. Comparative study of thermal properties of bio-coal from aromatic spent with low rank sub-bituminous coals. Bioresour Technol 2013;137:376e85. [40] Rasul M, Jahirul MI. Recent developments in biomass pyrolysis for bio-fuel production: its potential for commercial applications. Central Queensland University, Centre for Plant and Water Science, Faculty of Sciences, Engineering and Health; 2012. [41] Smith AM, Ekpo U, Ross AB. The influence of pH on the combustion properties of bio-coal following hydrothermal treatment of swine manure. Energies 2020;13(2):331. [42] Dhanapalan S, Annamalai K, Daripa P. Turbulent combustion modeling of coal: biomass blends in a swirl burner I-preliminary results. In: Energy technology conference and exhibition. ASME; 1997. [43] Demirbas A. Combustion characteristics of different biomass fuels. Prog Energy Combust Sci 2004; 30(2):219e30. [44] Ikelle I, et al. Study on the combustion properties of bio-coal briquette blends of cassava stalk. ChemSearch J 2017;8(2):29e34. [45] Udomchoke T, et al. Performance evaluation of sorption enhanced chemical-looping reforming for hydrogen production from biomass with modification of catalyst and sorbent regeneration. Chem Eng J 2016;303:338e47. [46] Nielsen M, et al. Danish emission inventories for stationary combustion plants. NERI Technical Report No 795; 2010. [47] Kristensen PG, et al. Emission factors for gas fired CHP units< 25 MW. In: IGRC; November 2004. [48] Reid WV, Ali MK, Field CB. The future of bioenergy. Glob Change Biol 2019. [49] Kowalewski A, Jan D. Emissions and properties of bio-oil and natural gas Co-combustion in a pilot stabilised swirl burner. 2015. [50] Isahak WNRW, et al. A review on bio-oil production from biomass by using pyrolysis method. Renew Sustain Energy Rev 2012;16(8):5910e23. [51] Patinvoh RJ, Taherzadeh MJ. Challenges of biogas implementation in developing countries. Curr Opin Environ Sci Health 2019. [52] Chen C, et al. Challenges in biogas production from anaerobic membrane bioreactors. Renew Energy 2016;98:120e34. [53] Mota N, et al. Biogas as a source of renewable syngas production: advances and challenges. Biofuels 2011;2(3):325e43. [54] Baloch HA, et al. Recent advances in production and upgrading of bio-oil from biomass: a critical overview. J Environ Chem Eng 2018;6(4):5101e18. [55] Xiu S, Shahbazi A. Bio-oil production and upgrading research: a review. Renew Sustain Energy Rev 2012;16(7):4406e14. [56] Wang L, et al. Bio-coal market study: macro and micro-environment of the bio-coal business in Finland. Biomass Bioenergy 2014;63:198e209.

Production of biodiesel from biomass

6

Emilia Paone1, 2, Antonio Tursi3 DIP A RT I ME N TO DI I N GE G N ER I A C I VI L E , D E L L ' ENE RG I A , D E L L ' A MB I E NT E E DE I M AT ER I A L I (DICEAM), UNIVERSITÀ DEGLI STUDI “MEDITERRANEA” D I R E GG I O C AL AB R I A , R E G G I O CALABRIA, ITALY; 2 DIPARTIMENTO DI INGEGNERIA I NDUSTRIALE (DIEF), UNIVERSITÀ D EGLI STUDI DI FIRENZE, FIRENZ E, ITALY; 3 DEPARTME NT OF CHEMISTRY AND CHEMICAL TECHNOLOGIES, UNIVERSITY OF C ALABRIA, ARCAVACATA DI RENDE, COSENZ A, ITAL Y 1

List of abbreviations AEOE Association for Environmental and Outdoor Education ASE Accelerated solvent extraction ASTM American Society for Testing and Materials International B100 Pure Biodiesel B2 2% Biodiesel B20 20% Biodiesel B5 5% Biodiesel COP21 21st yearly session of the Conference of the Parties FAAE Fatty acid alkyl esters FAE Fatty acid ester FAME Fatty acid methyl ester FFA Free fatty acid GHG Greenhouse gas MAE Microwave-assisted extraction OPEC Organization of the Petroleum Exporting Countries SFE Supercritical fluid extraction USP United States Pharmacopeia

1. Introduction Petroleum (also called crude oil) was the main source of energy at man’s disposal from a long time ago. Petrol oil and its derivatives can be found everywhere in our daily activity, as a component of the most common things, including road asphalts, domestic detergents, Advances in Bioenergy and Microfluidic Applications. https://doi.org/10.1016/B978-0-12-821601-9.00006-6 Copyright © 2021 Elsevier Inc. All rights reserved.

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films, records, clothes, cars, and so on. The downside stands on its causality with international wars and geopolitical issues [1e5]. Although it is often renamed “black gold” for its importance and preciousness, it creates further drawbacks, among which, one of the most important is certainly pollution. Indeed, everyone is aware of the emission produced by the combustion of fossil fuels, which contain CO2, CO, nitrogen and sulfur oxides, volatile organic compounds, and particulates, that are significantly harmful to human health. They are also the main cause of environmental problems, such as the greenhouse effect (due to nitrogen and carbon oxides), water contamination [6], and acid rain (in the case of sulfur oxides and nitric oxides) [7]. Moreover, in recent years, the increase in the price of the raw oil and the awareness of environmental problems has brought community and authorities to call for alternative energy sources, above all biodegradable and renewable (Fig. 6.1). In fact, several alternative technologies go toward sustainability, including the use of solar, wind, geothermal energies, and biomass use. However, biodiesel seems to stand out in this exclusive club, mainly due to its economical advantage. Vegetable oil, despite it is a good fuel (thanks to its calorific value equal to about 90% of that of diesel, mainly used in transport vehicles), it did not provide a satisfactory performance, due to its low volatility and high viscosity, which lets carbon deposits to arise on the injection system, along with the deterioration of the piston seals after combustion.

FIGURE 6.1 Increase in price of crude oil from 1990 to 2017.

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Since then and for decades, efforts were addressed to obtain a good fuel from the natural substances and different methodologies, to lower the viscosity of vegetable oils or animal origin, were found out for diesel engines [47,50]. Among all, the most known and most used methodology is definitely the transesterification reaction of triglycerides with alcohols, also called alcoholysis. It can significantly reduce the viscosity of the starting oil and improve the atomization of the fuel, and consequently its combustion characteristics. The transesterification reaction is generally catalyzed by hydroxides and alkoxides of alkali metals, and the main products are the alkyl esters of fatty acids and glycerol (the latter obtained as a by-product) with the obtained mixture being an excellent fuel commonly known as biodiesel. There are many advantages deriving from the use of biodiesel, which includes first of all the low environmental impact. It behaves as a renewable source of energy, is a biodegradable product, can be easily and rapidly dispersed, absorbed, and eliminated by soil bacteria, and rapidly degrades in water. Biodiesel is safer to handle and store because it has a higher flash point compared to the diesel of mineral origin. Moreover, it does not contain harmful metals such as lead, cadmium, vanadium, as well as sulfur, and last but not least it does not contribute to the formation of acid rain. Furthermore, the use of biodiesel helps to reduce the greenhouse effect because the amount of carbon dioxide released during the combustion is exactly the same that is absorbed by plants during their growth [8e10].

2. History of biodiesel The history of biodiesel began in Paris in 1898, during the World Exhibition, when Rudolph Diesel proposed the use of vegetable oils as fuel for engines, as an alternative to the expensive oil and coal, demonstrating the feasible use of it in engines, designed at the purpose and supplied with peanut oil. Actually, the engineer Rudolf Diesel was the first one to use substances of natural origin as possible sources of energy, at the beginning of the last century, using a mixture of vegetable oils to fuel the direct injection engine, designed by himself [11]. However, the realization of this idea is quite recent, given that from some decades ago the availability and affordability of oil became a relevant concern for the fuel supplier, because of its fast decrease. Until that period, the search for renewable energy sources was not yet seriously persecuted [12]. As a result, this concern arises from the energy crisis of the 1970s, more precisely in 1973, when the Western world was shaken by a serious crisis, which put an end to the economic boom started immediately after the Second World War. The crisis mainly derived from the involvement of western developing countries in the so-called Kippur War, between Israel and EgypteSyria, which mainly caused the energy crisis as a result of the rise in prices of crude oil and derived products. In this context, the OPEC Arab

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countries supported Egypt and Syria through the reduction of exports and, in some cases, even embargoes against the pro-Israel countries. Western Europe was the most hit by the price hike in oil and governments were forced to launch and impose measures to reduce energy consumption and avoid waste. Therefore, these events promoted an alternative energy source race to compensate for this energy shortage [13]. The downside deriving from the use of pure vegetable oils or animal fats, as fuels, stands on the high viscosity and the tendency to polymerize. Indeed, the prolonged use of these oils in engines causes the formation of carbon deposits, for example, in the injectors, and the thickening and gelling of the lubricating oil. Since then, polyunsaturated fatty acids can be subjected to polymerization and formation of gums due to oxidation during the storage phase or too high temperatures and combustion pressures. The rubber products were not completely burned during the process, so they encourage the formation of the above-mentioned carbon deposits and gelling. These problems are currently solved through the use of a reaction known as transesterification, where the triglyceride molecule reacts with short-chain alcohol in the presence of a catalyst. This reaction is the main way to produce biodiesel at an industrial level. Since biodiesel can be produced using different types of vegetable oils and various types of alcohol, the characteristics of the final product can vary significantly. Generally, however, the most used alcohols are methanol and ethanol, and only short-chain alcohols. They react with the fatty acids of vegetable oils, oleic, stearic, palmitic, lauric acid, etc., thereby giving the corresponding esters. Some of the properties of the deriving ester depend on the length of the chain and on the degree of saturation of the chain itself and, therefore, on the type of fatty acids used as raw material. On the other hand, alcohol affects the reactive phase more than the final properties. Biodiesel can be used in pure form (B100) or it can be mixed in different proportions with the refinery fuel (such mixtures are generally composed of 20%, 5%, and 2% of biodiesel, and are therefore called B20, B5, and B2, respectively). Unlike the B100, these mixtures can be used in a diesel engine without mechanical modification, making the product much more attractive from a commercial point of view. Several standards specify the minimum quality requirements that must be met for a biofuel to be called biodiesel with the US ASTM D6751 standard and the European standard EN 14214: 2001 is the most adopted. Biodiesel is a biofuel, obtained from renewable sources, such as vegetable oils or animal fats, and more recently, from used fried oil and waste resulting from oil refining [14]. It has a transparent and amber color, it is a natural liquid, nontoxic as it does not contain harmful molecules such as benzene or other aromatic molecules that can damage human health. At the same time, it is renewable because it is derived from the cultivation of diffused oil plants or through the reuse of exhausted oil. It is biodegradable because unlike common fuels, it can be decomposed by living organisms (bacteria and fungi) within a few days. Furthermore, biodiesel is easily obtainable from biomass, thereby respecting the criterion of sustainability and economy [15]. Biofuels used in the transport sector (called biofuels) derive (for the most part) from the biomass and can be both liquid and gaseous.

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Being able to be produced from different varieties of biological raw materials, biofuels are renewable, sustainable, and biodegradable; they do not emit fossil CO2, encourage green industry and agriculture, and can be used in normal engines without the need of mechanical modification (depending on the type of biofuel, they are usually used, mixing with classic fossil fuels). From a chemical and biochemical point of view, biodiesel is characterized by a linear carbon chain, with oxygen atoms at the extremities that are more easily attacked by bacteria, that better degrade oils and fats, compared to diesel. It shows poor content of oxygen and derives from a complex mixture of long-chain alkanes (up to C20 with branched chains), aliphatic and polycyclic aromatic hydrocarbons, including benzene. Furthermore, gas emissions are less polluting [16] than those produced by conventional fuels because:
biodiesel does not contain sulfur: the emissions of sulfates and sulfur oxides, which represent most of the acid rain composition, are practically absent compared to those deriving from the refinery diesel;
the oxygen content is 10%e11% (compared to 2%e3%, by weight, of gas oil) and there is no aromatic compound;
the combustion of biodiesel reduces the emissions of particulate and carbon monoxide into the atmosphere [17]. However, the use of biodiesel causes an increase of nitrogen derivatives, Nox emissions, which are slightly higher than those produced in the conventional fuel combustion, consequent to the presence of nitrates in crops deriving fertilizers. Several studies have been carried out to evaluate the biodegradability of biodiesel. Rapid degradability of biological diesel oil in the soil and aqueous systems has been proved to be comparable to that of glucose and much higher than the conventional diesel. In addition, many researchers have recognized that the biodiesel toxicity is significantly lower than the refinery diesel [15,17e20]. On the other hand, biodiesel production depends directly on the cultivation of oleaginous plants that thrives with a relatively small amount of water. This means that they can grow in semidesert soils, thereby avoiding future desertification, while, at the same time, they provide a very economical raw material to produce biodiesel. Furthermore, biodiesel can be produced from recycled waste materials, for instance from grease or fried oil disposed of restaurants or industries with a considerable benefit for the environment since the waste does not need to be treated. As a result, it can reduce significantly negative environmental impact. At the same time, the consumption of biofuels could also lead to an important reduction in the importation of petroleum in many developing countries and a potential source of export in the international market. The most common raw materials used for the production of biodiesel are rapeseed, sunflower, and soya as the first generation materials and Brassica Carniata, Babassu oil, and fry waste as the second-generation materials. Animal oils and fats can also be exploited. Considering the second generation of biodiesel, the aim is to have a raw

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material unusable for food. In this way, it is guaranteed that the production of biofuels does not affect the amount of foods for human use. The important features of this fuel are biodegradability, nontoxicity, and the low content of pollutants in emissions, the reason why it can be considered a form of renewable energy. The ASTM (American Society for Testing and Material Standard) defines biodiesel as a monoalkyl ester of a long-chain fatty acid, derived from renewable fluids as animal fats or vegetable oils, which are used in injection and compression engines. Nowadays, the first and the second generations of biofuels account for about 99% of the global biofuels production; however, in the near future, a relevant contribution from third (advanced algae-based biodiesel) and fourth (photobiological and solar fuels and electrofuels) is expected [21,22].

3. Biodiesel market The biofuels, derived from biomass and used in the past and then supplanted by fossil fuels, due to lower production costs, have recently been “dusted off” just when, during the energy crisis in the 1970s, Western countries were hit by the OPEC countries, and the governments themselves promoted their production. In this regard, since 1997 with the Kyoto Protocol [23], several international conferences have been held, with raising the awareness on environmental issues, in particular regarding the consequences of global warming [24]. The objective was to put the majority of states together to reach some targets and reduce the greenhouse gas emissions assuming as a baseline, the emission values relative to the year 1990 by mutual agreement in different percentages for each State. Reductions were supposed to be 5% on average and regard the greenhouse gases called Green House Gasses (GHGs), such as CO2, CH4, N2O, HFC, PFC, and SF6. The majority of European states have issued decrees and directives to try to limit emissions, often dividing the various sectoral reductions and proposing incentives for the production of energy from renewable sources. However, the Kyoto Protocol, as well as the subsequent international conferences on the environment subject achieved only a few results. This occurred because the most polluting countries, including those in the developing world, refused to ratify the agreements in most cases. In December 2015, the 21st Conference of the Parties (COP21) (United Nations) was held in Paris and the 195 participating countries reached a new global agreement on climate change with an action plan to limit global warming to below 2 C. However, European states were already prepared to accept COP21 issues, because, in 2010, the EU promoted a strategy, for the member states, to set important targets for 2020. In fact, this strategy, known as “Europe 2020” (Fig. 6.2), established ambitious goals to be achieved:  to reduce GHGs by 20%  to reduce energy consumption by 20%, thereby increasing the energy efficiency  to meet 20% of European energy needs with renewable sources

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FIGURE 6.2 European program 20-20-20.

After this declaration of intent, the Climate-Energy Package, which establishes the methods for translating the objectives into practice in 2020 through new European legislative instruments, was approved. Furthermore, since the renewable energy issue is at the heart of the Energy Union’s priorities, the Renewable Energy Directive1 was and will be a central element of the Energy Union policy and a fundamental guide of the provision of clean energy for all European citizens, to make EU the world leader in the renewable energy sector and, at the same time, to contribute to the five dimensions of the Energy Union. Firstly, renewable energy plays an important role in energy security. It is estimated that its contribution to the 2005 fossil fuel imports’ saving was 16 billion EUR and it is expected to rise to 58 billion EUR in 2030 [25]. Secondly, thanks to the rapid drop in prices due to technological progress, in particular in the electricity sector, renewable energy can also be gradually more integrated into the market. The recast of the renewable energy directive for the post-2020 period together with the proposals on the market structure, which is part of the Clean Energy Package for all Europeans, will make it even easier for the renewable energy to participate, on equal terms, with other energy sources. Thirdly, the renewable energy program goes along with energy efficiency. In the electricity sector, the transition from unrenewable fossil fuels to renewable sources could reduce the primary energy consumption. In the construction sector, solutions based on renewable energy can improve the energy performance of buildings in a cost-effective way. Fourthly, renewable energy is an essential driver for decarbonizing the Union’s energy system. In 2015, renewable energies supported the reduction in gross GHG emissions by a share equal to Italy’s emissions. Last but not least, renewable energies play a major role in making the EU a world leader in innovation. With 30% of patents in the renewable energy sector, the EU has been a pioneer in this field and is committed to prioritize research and innovation to advance the energy transition. However, the benefits of renewable energy go much further, as they are a source of economic growth and employment for European citizens, also contributing to reduce air pollution and to help the developing countries to access clean energy at acceptable prices.

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Heating and cooling remain the main sectors in terms of the absolute diffusion of renewable energy. However, the highest market share and the highest growth are in the electricity sector, where the share of energy from renewable sources grew by 1.4 percentage points per year between 2004 and 2014. The share of energy, from renewable sources, in the heating and cooling sector grew by 0.8 percentage points a year over the same period of time, while the transport sector recorded the worst slowdown, with an average growth per year of 0.5 percentage points. The solid biomass provides the largest contribution (82%) for the production of heat from renewable sources (72 Mtoe). In 2015, the use of renewable waste amounted to 3.4 Mtoe. Although in 2004 it was negligible (0.7 Mtoe), in 2015 the share of biogas in the heating and cooling sector exceeded the expected values and reached 3.2 Mtoe [25]. The electricity sector recorded the fastest growth in the share of renewable sources and currently accounts for 28.3% of the total electricity production. In 2015, the sector that mostly contributed to the production of electricity from renewable sources was hydropower. The most significant contribution in terms of growth comes from wind power on land. The development of solar photovoltaics has been irregular, with a peak of growth in 2011 and 2012, and lower growth rates in the next years. Overall, the variable renewable sources represent 12% of the gross electricity production in the EU. The transport sector continues to record slower growth in the use of renewable energy, equal to 0.5 percentage points on average per year, from 2005 to 2014 and a marked slowdown after 2011. Its share of renewable energy was 5.9% in 2014 (estimated at only 6.0% in 2015) compared to a specific target, for the sector, of 10% in 2020. This slow progress is due to various issues, including regulatory uncertainty and a delay in the use of advanced biofuels. Biodiesel is the main fuel used for transportation in the EU, representing 79% of the total use of biofuels in 2015. The main consumers of biodiesel are France, Germany, and Italy. Moreover, in the last few decades, biomass has become an important starting material for different sectors belonging to the chemical industry, thanks to its versatile properties and chemicalephysical characteristics. Currently, an important role is played by lignocellulosic materials in various sectors including environmental protection, such as the purification of water contaminated by pollutants dangerous to human health [6,27,28].

4. Feedstock classification and pretreatment The potential raw materials for the production of biodiesel can be divided into four main categories: edible vegetable oil, nonedible oils, waste or recycled oils, animal fats, and, finally algae [29,30]. Edible oil resources such as soy, palm oil, sunflower, safflower, canola, coconut, peanuts, and rapeseed represent the first generation biodiesel raw materials because they are considered the first materials to be used for the production of biodiesel. Huge

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plantations of these crops have been grown up in many countries of the world such as the United States, Germany, India, and Malaysia. Currently, more than 95% of the world’s biodiesel is produced from edible oils. Such as colza (84%), sunflower oil (13%), palm oil (1%), soya oil, and others (2%) [29,31]. The fatty acids composition of edible oils coming from different plant seeds is different but in all cases, the dominant fatty acids are oleic acid (C18:1), linoleic acid (C18:2), palmitic acid (C16:0), and stearic acid (C18:0). Generally, the fatty acid compositions of a given plant remain unchanged during the conversion of the raw materials into the fuel through transesterification, therefore the composition greatly affects the quality of the produced biodiesel. However, the continuous large-scale use of edible vegetable oils for biodiesel production raised many concerns like the food crisis and major environmental issues, including the deforestation and the conversion of many agricultural lands available into oil crops [22,38]. Therefore, the first generation biodiesel increased the skepticism toward the exploitation of this mode of production during the time. The main concerns were carbon impacts and budgets. These factors limited the biofuels production, bringing up the “versus food” debate or even more whether it was more useful to produce biodiesel raw fuel materials than food [30,31,36]. Another problem was the price of vegetable oils that has increased dramatically over the last few decades, affecting the economic viability of the biodiesel industry [30,31,39]. Therefore, the current use of edible vegetable oils, as a raw material for biodiesel production, is considered ineffective and it is necessary to target the research to find cheaper resources and processes [31,32]. Nonedible vegetable oils known as the second-generation raw materials, are the most potential substitutes of traditional edible (first generation) crops for the production of biodiesel, allowing to overcome some problems encountered with oils deriving from edible sources. In recent years, these oils have attracted a lot of attention thanks to their high oil content, their great availability, and ease of cultivation (in fact these crops can also be made on land unsuitable for agriculture). The main nonedible vegetable oils, most studied for the production of biodiesel, are jatropha seed oil, karanja oil, jojoba oil, linseed oil, oil of cotton seed [30,35,36], among others (Table 6.1). In addition, the fatty acid composition is also an important feature of biodiesel raw materials as it determines the efficiency of the biodiesel production process. The composition and distribution of fatty acids in nonedible oils are generally aliphatic compounds with a carboxylic group at the end of a straight chain [31]. Recent studies have shown that biodiesel with a high level of methyl oleate (monounsaturated fatty acid) could have excellent characteristics in terms of ignition quality, fuel stability, and low temperature flow properties [30]. Furthermore, it has been reported that the cetane number, combustion heat, melting point, and viscosity of clean fatty compounds are increased with increasing chain length and are decreased with increasing unsaturation [40] of the molecule of methyl esters of

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Table 6.1

Main feedstocks for biodiesel production.

Edible plant oils

Nonedible plant oils

Waste oils and animal fats

Other sources

Sorghum Soybean Coconut Barley Groundnut Palm and palm kernel Pumpkin seed Canola Corn Peanut Safflower Wheat Rapeseed Sunflower

Coffee ground Cotton seed Tobacco seed Croton megalocarpus Passion seed Karanja oil Linseed oil Jojoba oil Rubber seed tree Moringa Jatropha curcas

Pork land Chicken fat Beef tallow Fish oil Poultry fat Waste fish

Tall oil Cyanobacteria Bacteria Cooking oil Fungi Microalgae Soapstocks

Based on source: Avhad M, Marchetti J. A review on recent advancement in catalytic materials for biodiesel production. Renew Sustain Energy Rev 2015;50:696e718.https://doi.org/10.1016/j.rser.2015.05.038; Shahid EM, Jamal Y. Production of biodiesel: a technical review. Renew Sustain Energy Rev 2011;15:4732e45. https://doi.org/10.1016/j.rser.2011.07.079; Kibazohi O, Sangwan R. Vegetable oil production potential from Jatropha curcas, Croton megalocarpus, Aleurites moluccana, Moringa oleifera and Pachira glabra: assessment of renewable energy resources for bio-energy production in Africa. Biomass Bioenergy 2011;35:1352e56. https://doi.org/10.1016/j. biombioe.2010.12.048; Supamathanon N, Wittayakun J, Prayoonpokarach S. Properties of Jatropha seed oil from Northeastern Thailand and its transesterification catalyzed by potassium supported on NaY zeolite. J Ind Eng Chem 2011;17:182e5. https://doi.org/10. 1016/j.jiec.2011.02.004; Thiruvengadaravi K, Nandagopal J, Baskaralingam P, Sathya Selva Bala V, Sivanesan S. Acid-catalyzed esterification of karanja (Pongamia pinnata) oil with high free fatty acids for biodiesel production. Fuel 2012;98:1e4. https://doi.org/10.1016/j. fuel.2012.02.047; Dalena F, Senatore A, Tursi A, Basile A. Bioenergy production from second- and third-generation feedstocks. Bioenergy Syst Future 2017;599. https://doi.org/10.1016/B978-0-08-101031-0.00017-X.

fatty acids (FAME). Therefore, the composition of structural fatty acids will affect the physicochemical properties of biodiesel such as cetane number, cold flow properties, combustion heat, and viscosity [30,38]. In recent years, the used cooking oil has been considered as a possible raw material for the production of biodiesel due to its low cost, and given that the deriving biofuel has been suitable to comply with the requirements specified by European standards for biodiesel (EN) and by ASTM standards. However, the residue obtained after using cooking oil, generally discarded without further applications, is highly unclean. These types of oils are mainly composed of the high content of free fatty acids that, after filtration and purification processes, could be used for the production of biodiesel [22,30,41]. Animal fats such as chicken fat, lard, and yellow fat are also considered as secondgeneration raw materials. Animal fat waste is readily available at a low cost. However, it may not be enough in quantity to meet the global energy demand, in couple with the higher complexity for the process of transesterification for some types of fats, due to the presence of a high amount of saturated fatty acids [42].

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The amounts of edible and nonedible oil crops of used animal fats and cooking oils are limited, so they are unlikely to meet the worldwide demand for the biodiesel production. In recent years, a strong interest has been developed toward the production of biodiesel from microalgae (considered belonging to the third-generation biofuels). The use of microalgae for biodiesel production has several advantages such as [30,38] a biomass productivity much higher than terrestrial plants, high content (up to 20%e50% of triacylglycerol), chance to grow the biomass without exploiting agricultural land, and the need for simple sunlight and economic nutrients for their growth [43]. The first step for oil extraction is the seed preparation. This includes taking out the outer layers of the fruit and the drying, to make the grains or seeds available, in addition to reducing the moisture content [30]. The seeds or the separated grains are sieved, cleaned, and stored at room temperature [29]. The main products deriving from the oil extraction processes are the crude oil and some by-products such as seeds or kernel cakes. Seed cakes can be used as fertilizers for the soil enrichment [30], feed for poultry, fish, and pigs, while some oil cakes have applications in fermentation and biotechnological processes [44]. The oil extraction is the next step after preparation, and it is one of the most important steps in this process. Three main conventional methods have been identified for oil extraction: 1. mechanical extraction, 2. chemical or solvent extraction, 3. enzymatic extraction. Recently, new methods of oil extraction, namely accelerated solvent extraction, supercritical fluid extraction, and microwave-assisted extraction, are also used (either individually or in combination with the conventional ones) to overcome some disadvantages of conventional methods [29e31]. It has been observed that mechanical pressing and solvent extraction are the most used methods for oil extraction. The oil extraction through the use of mechanical presses is the most conventional technique. It allows to use different types of presses: manual press or a motorized screw press. It has been found that the engine-driven screw press can extract 70%e80% of the available oil while the piston presses only reach the 60%e65% [30]. This gap is due to the fact that seeds can be subjected to numerous extraction cycles through automated presses. At the end of the extraction process, using mechanical presses, the oil needs a further filtration and dehumidification treatment to produce a more refined raw material [45]. Solvent extraction is a process in which the oil is removed from a solid by a liquid solvent and it is also known as leaching. The chemical extraction, using the n-hexane method, leads to the maximum yield, making it the most used solvent. Although the costs of oil extraction by a mechanical press are lower, the extraction efficiency does not reach the yields obtainable by extraction of oil with a solvent [30,34].

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The aqueous enzyme oil extraction method is a promising technique for the extraction of oil from plant materials. In this method, enzymes are used to extract oil from crushed seeds [36]. The extraction of aqueous enzyme oil can also be used in combination with other oil extraction methods. For example [46] used a combination of ultrasound and aqueous enzyme oil extraction to obtain oil from J. curcas seeds. The yield of this combination was 74% of seed oil, compared to the 20% of oil extracted with the single extraction of aqueous oil. Another advantage of this combined test was the reduction of the processing time from 18 to 6 h. The enzymatic extraction of oil results in having important advantages from the environmental point of view, as it does not produce volatile organic compounds. In contrast, the most relevant downside is represented by the long process times [30,46].

5. Biodiesel production processes The most common and efficient method to produce biodiesel is the Transesterification Reaction [47], in which vegetable fats, including triglycerides, diglycerides, and monoglycerides (all characterized by the presence of ester groups) react with an alcohol, usually methyl and sometimes ethyl alcohol, to form the corresponding alkyl esters. Currently, several production processes are used for biodiesel production through the transesterification reaction and, as shown in Table 6.2 with a brief report on the catalysis Table 6.2 Summarize of catalysis production processes used for biodiesel production by the transesterification reaction. Reaction conditions Origin

Catalyst

Temperature [ C]

Molar ratio [methanol to oil]

Catalyst Reaction amount [wt%] time [h]

Yield/Conversion [Y/C %]

Homogeneous base catalyst Canola, soybean Soybean Used frying Waste cooking Sunflower Soybean

NaOH

25

6:1

0.5

0.33

Y ¼ 98

KOH NaOH KOH

40 60 70

6:1 7:1

1.5e2.2 1.1 1

0.25 0.33 1

Y ¼ 99.4 Y ¼ 88.8 Y ¼ 98.2

CaO MgO

252 130

41:1 55:1

3 5

0.1 7

Y ¼ 100 Y ¼ 60

95

20:1

4

20

C > 90

65 65

30:1 12:1

1 5

69 4

C ¼ 90 Y ¼ 94.7

Heterogeneous acid catalyst Waste H2SO4 cooking Sunflowers H2SO4 Soybean Enzymatic catalyst

Based on source: Thanh LT, Okitsu K, Van Boi L, Maeda Y. Catalytic technologies for biodiesel fuel production and utilization of glycerol: a review. Catalysts 2012;2(1):191e222. https://doi.org/10.3390/catal2010191.

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production processes used. It is worth to underline that, in transesterification reaction, glycerol is obtained as a secondary product that is generally sold to pharmaceutical and cosmetic industries after the refining. The transesterification reaction (Scheme 6.1) is relatively simple and can be represented as follows: Methanol is the most used alcohol for producing biodiesel, thanks to its low cost and high reactivity, in couple with its polarity and its ability to dissolve quickly in basic solutions. In addition, thanks to its small molecule, it reacts with triglycerides very fast [49]. Ethanol is used for the production of biofuels, in particular areas, such as South America, where it is more profitable than methanol. When it is used, ethyl-esters are produced. Furthermore, the use of long-chain or branched alcohols, with up to eight carbon atoms, such as propanol, isopropanol, butanol, isobutanol, or pentanol affords low yields of triglycerides, and it is also disfavoured by their high market price [50]. The chemical composition of biodiesel varies according to the starting substrates [51], and it is generally composed of a variable percentage mixture of fatty acids, which are first etherified and then transformed into methyl esters. Fatty acids, that actually compose the biodiesel, are most commonly found in biodiesel blends: palmitic acid (C16:0), oleic acid (C18:1), stearic acid (C18:0), linoleic acid (C18:2), and linolenic acid (C18:3) [52]. It has also been demonstrated that the composition of fatty acids has a huge impact on the performance of biodiesel [53]. Indeed, the length of the chain, the degree of unsaturation, and the branching change affect the cetane number, the melting point, the oxidation stability, the kinematic viscosity, and the combustion heat. These properties are crucial and necessary to satisfy the standards of biodiesel such as EN 142142 and ASTM D6751. The main physicochemical characteristics of various biodiesels obtained from different process compared with those of petroleum diesel are reported in Table 6.3. The transesterification reaction starts when the reactants (triglycerides and alcohol) are mixed together, and usually the use of an acid or base catalyst is necessary to make the reaction rate acceptable. Reaction products have a much lower viscosity than those of the starting vegetable oil and of the by-product. Glycerol, in particular, contains most of the alkali necessary to perform the reaction.

SCHEME 6.1 Transesterification process for biodiesel production.

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Table 6.3 Main physico/chemical characteristics of various biodiesels obtained from different process compared with those of petroleum diesel. Biodiesel Properties

Palm

Jatroba

Avg. chain length Avg. unsutured Carbon wt% Hydrogen wt% Oxygen wt% Specific gravity Cetane no. Flash point,  C Viscosity

17.20 0.62 76.2 12.6 11.2 0.873 61.9 163 4.61

18.30 1.15 76.2 12.6 11.2 0.876 55.7 152 4.75

Sunflowers 17.80 1.59 76.2 12.6 11.2 0.878 51.1 175 4.42

Soy 17.90 1.50 76.2 12.6 11.2 0.882 51.3 159 4.26

Petroleum diesel 11, 14 1.95 86.8 13.2 0 0.85 40e45 52e82 3

Based on source: Hoekman SK, Broch A, Robbins C, Ceniceros E, Natarajan M. Review of biodiesel composition, properties, and specifications. Renew Sustain Energy Rev 2012;16(1):143e69. https://doi.org/10.1016/j.rser.2011.07.143.

The process takes place in three different phases in which every single step is reversible, and diglycerides and monoglycerides are intermediate products. The transesterification reaction is normally carried out using basic catalysts since they enable a much higher reaction rate than the acid ones. In addition, the reaction temperature is maintained at 50e60 C. Generally, the basic inorganic catalysts for producing biodiesel are alkali metal hydroxides and potassium carbonates [54] and alkaloids of alkali metals (such as methoxide and sodium ethoxide) are the active species formed in the reaction medium. The overall reaction proceeds in about 30 min using a low concentration of catalysts. However, the main downside of performing the process is the difficulty keeping to get the reaction environment completely anhydrous. Furthermore, an accurate control of the entire industrial process is strictly necessary. Although hydroxides are much less reactive than alkoxides, they have a much lower cost and the interest in their use stems from the evidence that, by increasing their concentration, a quite high conversion is easily observed. On the other hand, hydroxides have a strong affinity with short-chain alcohols, generating water molecules as by-products. The presence of water in the reaction medium is an extremely negative factor for the entire biodiesel production process, as it causes the ester hydrolysis, which can cause the formation of undesired intermediates, generally lowering the speed and the efficiency of the overall reaction [55]. All these factors decrease the production of biodiesel and make the subsequent phases of the process purification and washing, more difficult. Conversely, by using carbonates, as catalysts, in sufficient concentration, affords the formation of bicarbonates and then water, to make sure that the purification process is considerably simplified compared to the previous case.

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In addition to the basic catalysis, the transesterification reaction can also be carried out using acid-type catalysts (e.g., hydrogen sulfide) [56]. The fundamental problem, in this case, is the low reaction rate and, consequently, the reaction temperature needs to increase at about 100 C, a significantly higher value than that used with basic catalysts. Again formation of water is an undesired reaction because it involves the production of carboxylic acids through reducing the formation of biodiesel in the global process. In principle, if processes are carried out with heterogeneous catalysis, these problems are completely eliminated.

6. Transesterification of vegetable oil using homogeneous catalyst Conventionally, the transesterification reaction is carried out using basic homogeneous catalysts, and among the homogeneous alkaline transesterification catalysts, sodium hydroxide or sodium methoxide is most widely used in the industrial field (potassium analogs are equivalent) [57]. These catalysts are dissolved in methanol feed or are injected directly, if available in liquid form (sodium methoxide solution). The catalyst feed can be adjusted to meet the required conversion of the product, but generally, an excess is used. The optimum catalyst corresponds to a concentration range from 0.5% to 1.0% by weight based on the vegetable oil feed. The presence of free fatty acids in the reaction mixture can be partly reduced by adding a further excess of catalyst. However, a higher concentration of catalyst leads to greater solubility of methyl ester in the glycerol phase, producing an increase in settling time and requiring a modification of the glycerol purification step. The conditions of the homogeneous reaction promote continuous natural decantation of glycerol, from the reacting mixture, allowing displacement of the reaction equilibrium. Instead, water acts as an inhibitor of the transesterification reaction, causing the catalyst to lose activity. Therefore, it must be removed from the system to avoid its accumulation, and this occurs through a dedicated operation or directly in the final glycerol. The crude product (ester) is purified by a washing step, which removes the last traces of catalyst (the final Na and K content must be less than 5 ppm to satisfy EN14214 rules) [58]. In some cases, a third acidified water reactor can be used to refine the product (FAE, fatty acid esters) through the removal of the last traces of bases and glycerol. Every decanting and neutralization effluents are sent to the glycerol purification section for further treatment. The residual content of methanol and water in the product is removed by evaporation and recycled to the feed. When sodium hydroxide is used as catalyst, side reactions usually occur creating sodium soaps. These reactions are also observed when sodium methoxide is used, with the presence of water traces. Sodium soaps are soluble in the glycerol phase and are

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recovered as fatty acids after acid neutralization and decantation. In this case, glycerol is obtained as an aqueous solution also containing sodium chloride. Depending on the process, the final purity of glycerol is about 80%. The loss of esters can be around 1% of the FAE production. The FAE yields can range from 98.5 to 99.5 (% of weight), depending on the feed quality and the type of the catalyst used. Commercial industrial processes use both batch and continuous processes. In the ESTERFIP batch process (IFP/Axens license), the transesterification reaction takes place in a single stirred reactor. The reaction is carried out in a batch, minimizing the necessary equipment and guaranteeing a constant control of the conversion of the oil into FAME. The final treatment of products (FAME and glycerol) are carried out continuously. Plants of this type started to be built from 1992 for a total of 1.2 million tons/year of capacity. The continuous processes require the use of two or three reactors in series. After each phase of catalytic reactions, glycerol is removed by gravity or centrifugation. The key point is the washing of the final methyl esters, with the aim of eliminating glycerol, water, methanol, and catalysts. The washing is carried out, in some cases, using a counter-current washing column with water. The basic salts coming from both glycerol and washing water are neutralized using a strong acid compound. For both the transesterification processes (continuous and batch), around 4 kg of saline by-products is presented in the glycerol phase for each ton of processed oil. The treatment of this amount of salt is left to the end users of glycerol. Glycerol (containing salts) and free fatty acids are considered waste that leaves the homogeneous phase. The processes, which are carried out in homogeneous phase, can be considered satisfactory for several aspects:
very active catalyst;
good thermal efficiency, low energy consumption;
capacity (10,000e250,000 tonnes/year). The product is mixed up to 30% by weight with diesel fuel for different uses, such as in city buses, while the introduction into normal diesel is made on the regular basis of 5%. The main difficulties are the handling of strong acids and basic chemicals and the production of impure glycerol (containing the salts resulting from the neutralization of the soluble catalyst with a mineral acid), in large quantities compared to the market needs. The main impurities of the glycerol by-product are water, salts (NaCl, Na2SO4, KCl) (which depending on the catalyst and the acid used for neutralization), and organic compounds such as esters and soaps. The possible uses of such crude glycerol are limited. If it is used as fuel in the ovens, it can produce technical difficulties related to its high flash point and the presence of salts. It can moreover be incorporated into animal feeds.

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Raw glycerol can be refined to obtain USP grade, allowing a possible use in pharmaceutical, cosmetic, and food products. The crude glycerol often has a dark brown color and needs to be treated with multiple distillations and bleaching techniques to achieve USP compliance. The purification systems are rather complex and expensive, both in terms of investment and operating costs, especially due to the energy needed for this purification and above all if distillation is necessary. Furthermore, the produced waste (mainly sodium salts polluted with organic compounds) must be disposed of. The massive construction of homogeneous FAE plants normally produces a strong increase in this type of waste, which requires a specific solution. The achievement of a wide diffusion of green fuels, using this process with a high environmental impact, does not seem to be the most practicable solution.

7. Transesterification of vegetable oil using heterogeneous catalyst The classic path, using homogeneous catalysts, involves a considerable amount of these systems, with the serious drawback related to the complex purification of the obtained glycerol. An easy way to avoid this problem is exploiting a heterogeneous catalyst. The process was developed by IFP and is marketed by Axens under the trade name Esterfip-H. The first industrial unit was launched in 2006, in Se`te, in the south of France. In the ensuing continuous process, the transesterification reaction is promoted by a heterogeneous catalyst made by mixed oxides, which do not consume or significantly lose activity over time. The use of a solid catalyst completely eliminates any other catalyst and does not need the use of a neutralization phase and water washing. Thus, the overall water content during al transesterification processes is significantly lower. The residual fatty acids entering the process are esterified into FAME, thereby increasing the overall yield of the process. However, the esterification of fatty acids produces water as a byproduct, thus the fatty acid content entering the process must be adequately monitored to limit the presence of H2O. The desired conversion, required to produce a specific FAE, is achieved with two successive phases of glycerol reaction and separation. The catalytic section includes two fixed bed reactors. The former is fed with vegetable oil and methanol in a given ratio. The excess of methanol is removed by partial evaporation after each reactor. Esters and glycerol are then separated into a simple gravity decanter and, since transesterification is an equilibrium reaction, the intermedium disposal of glycerol allows to achieve very high conversions, close to the thermodynamic limits. Glycerol outputs are collected, and the residual methanol is removed by evaporation. To obtain FAE values in

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compliance with European specifications, even the last traces of methanol and glycerol need to be removed. The purification section includes the elimination of methanol by vacuum vaporization, followed by an adsorber to remove soluble glycerol. The adsorption step uses neutral ion exchange resins that act as traps for glycerol, and allows the elimination of the washing phase, containing water, previously used in homogeneous processes. Glycerol purity is usually higher than 98%, with an average water content of 0.5% by weight, instead of 13.5% typical of standard raw glycerol. The reactors are fixed-bed plug flow, chosen for their ability to achieve a better conversion level than agitated reactors. The reactor design ensures adequate dampening of the catalyst throughout the height of the bed. The operating conditions were chosen to let the reaction occur in a single step and avoid glycerol separation, which would limit the reaction due to one of the external diffusion limitations. Two beds are used to ensure the correct distribution of liquid in the reactor from the bottom to the top. No mechanical stirring system is used in the reactor or before since the fluid turbulence maintains a continuous mixing. This new heterogeneous catalysis process offers the following main advantages:
high performance in FAE, since there is no loss of ester due to the formation of soaps (yield ¼ 100%);
crude glycerol free of salts, with very high purity (>98%), therefore a direct use for chemical applications is allowed;
only the oil fed and the methanol are consumed;
There is no waste as by-products. However, the reaction is carried out at a much higher temperature and pressure than in a homogeneous catalytic process, thanks to the lower activity of the solid catalyst compared to the liquid one.

8. Other processes of biodiesel production Different techniques and technologies for the production of biodiesel are actually known, unlike the catalytic transesterification of vegetable oils. However, the most frequently used method and the better one remains the transesterification reaction, mainly due to its high versatility and applicability. Among the various possible biodiesel alternative production currently applied, we can certainly include the following:

8.1

The thermal cracking

This process can also be defined as the transformation of one substance into another by means of heat in the absence of oxygen or air. The process involves the breaking of some chemical bonds for producing smaller molecules [59] and can be performed in the

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presence of a catalyst. This explains the name of catalytic cracking. The substances that can be subjected to this type of process are vegetable oils, animal fats, natural fatty acids, and methyl esters [15]. For example, thermal cracking of the soybean oil can be accomplished and a standard ASTM distillation is performed. About 60% of the weight of the obtained product are alkanes or alkynes, while about 10%e15% of which are carboxylic acids [59]. As far as the catalytic cracking is concerned, all catalysts typical of oil cracking can be used. An example is SiO2/Al2O3 which produces a mixture of low molecular weight compounds that can undergo a purification treatment to obtain a biofuel at 450 C [59]. However, the obtained product has a high quantity of aromatic compounds, thus it seems more to gasoline than to diesel fuel. In addition, thermal and catalytic cracking for biodiesel production is not widely used due to the complexity of the plant and the high costs associated with it [60].

8.2

The enzymatic catalysis

The enzymatic catalysis attempts to solve problems related to some production processes, such as the purification of glycerol (very contaminated by acid or basic catalysts) and overcame the need to use purified vegetable oils or animal fats [61]. The enzymatic process uses lipase, as a biological catalyst, as it can also exploit and consume residual oils from raw materials, without producing contaminated byproducts. However, the process has several limitations, so that the transesterification with biological catalysts is not currently an economically viable route. Indeed, there is no doubt that many experiments need to be carried out to find the optimal conditions for the reaction, taking place (temperature, pH, microorganism strain to generate the enzyme, solvent, etc.) with the aim of making a competitive process compared to the chemical industry [15]. In addition, lipase is a much more efficient enzyme for transesterification when it is used with long-chain alcohols, rather than with methanol or ethanol, and therefore the enzymatic process requires the involvement of an organic solvent. The main drawback of this technology is the high cost of the biological catalyst (lipase). However, studies are being carried out with the aim of increasing the operational stability of the enzyme (recycling it in a discontinuous process or increasing the use in a continuous process), thereby decreasing the incidence of the catalyst in the overall cost of the process. Nevertheless, the reaction times are higher with biological catalysts than with the basic ones, and it reduces the overall efficiency of the process.

8.3

The FischereTropsch

The FischereTropsch technology allows the production of biodiesel and exploits the use of biomass synthesis gas. The process is very flexible and can be made using different raw materials, thus including vegetable oils and animal fats.

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The process can be divided into two phases: the first includes the production of biomass synthesis gas, by gasification in a fluidized bed or in a laminar flow reactor. Products of biomass combustion are gases (such as hydrogen, carbon dioxide, and methane), tar, and a carbonaceous solid residue. The gases are then separated and a high concentration of H2 þ CO is obtained; in the second phase (the true FischereTropsch process), the synthesis gas is converted into long-chain liquid hydrocarbons using a cobalt iron catalyst [15]. In this way, a residue is transformed into a product with high added value, not derived from the oil refining.

8.4

The supercritical methanol

This process is an alternative method and was proposed for the first time by Refs. [15,55]; it considers the difficulties that arise in the transesterification process when there is a low quality of raw materials. Mostly occurring problems concern the formation of undesired intermediates, the difficulty of separation, and the purification of biodiesel and glycerol. The use of supercritical alcohol (normally methanol) partially solves these problems because a single phase is formed when the reaction is completed. The method avoids the use of catalysts of any type, saves energy, and reduces the reaction time and the direct separation of reaction products. In fact, it takes only 240 s to produce the same amount of obtained methyl ester, unlike the conventional transesterification process, which takes 1e8 h [55]. The reaction with supercritical methanol is carried out in a cylindrical stainless steel reactor (autoclave) at a temperature of 520K and at a pressure of 45 MPa. The disadvantages of the process are the high temperature and the necessary pressure to perform the reaction and a large amount of needed methanol (molar ratio equal to 42:1) [55].

9. Biodiesel byproduct: challenges and opportunities in glycerol valorization The main problem in the transesterification process is a large amount of “crude” glycerol obtained as a byproduct (for every 100 kg of biodiesel obtained, 10 kg of glycerol is formed). Crude glycerol has a total content that ranges from 40% to 90% and is generally refined to increase its grade up to 99%. There are wide applications of glycerol and three different grades are presented in the actual market: (1) “technical” to the chemical industry; (2) “USP” (United States Pharmacopeia), using this molecule for food or pharmaceuticals purposes; and (3) “Kosher”, suitable for the production of kosher food [62]. Today, global glycerol production is about 3 million tons per year and its worldwide market is continuously growing and will exceed 3 billion euros by 2022 [26] (Fig. 6.3).

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FIGURE 6.3 Glycerol market size (Kilo Tons) from 2011 to 2022 in Europe.

Direct uses of glycerol range from personal care and pharmaceuticals to foods and beverage. However, due to the huge amount of biodiesel produced per year, very large quantities of glycerol are actually poured into the market to create a big imbalance between supply and demand. In the past, the direct use of crude glycerol for energy production has been proposed. However, its real application has been strongly limited since acrolein is produced during the combustion process. Surely, glycerol-based polymers are now increasingly attracting attention due to the high possibility to obtain several compositions and architectures [63e66]. There are, actually, various catalytic options for the conversion of glycerol into addedvalue products [67e70] including carboxylation, etherification, oxidation, hydrogenolysis, and aqueous phase reforming. Coupling glycerol with CO2 (another industrial waste), the carboxylation process occurs and leads to glycerol carbonate as the main product [71]. The glycerol carbonate is useable in the pharmaceutical sector, polymers production, cosmetics, etc. Actually, its applications are extended to many industrial productions as solvent, surfactant, lubricant, varnishes, or as precursor for coating materials. A further way to follow to add value to glycerol is the etherification that favors the production of polyoxygenated compounds with a low degree of polymerization (e.g., polyglycerols) which, thanks to their antidetonation properties, can be used as fuel additives [72,73]. Hydrogen is the most important product from several reforming processes (aqueous phase reforming, steam reforming, autothermal reforming, dry reforming, etc.) [74e76]. In modern society, H2 has a huge amount of application including electricity production (either a fuel cell or a gas turbine), petroleum refining, treating metals and food processing.

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Molecular hydrogen [68,77,78] or H-donor molecules [79e82] can be used to transform glycerol into diols including 1,2-propanediol (1,2-PDO), 1,3-propanediol (1,3-PDO), and ethylene glycol (EG). 1,2-Propanediol (1,2-PDO) has wide applications as moisturizer, flavoring agent, and as a building block for food additive and cosmetic products. Being a monomer for the preparation of polyesters, polyethers, and polyurethanes, 1,3-propanediol (1,3-PDO) is one of the most valuable molecules in industrial chemistry. EG is widely used as an automotive antifreeze, as a building block for the preparation of 1,4-dioxane, diethylene glycol, other ethylene glycol, and as comonomer in polyethylene terephthalate (PET) production. The alcoholic groups of glycerol can be selectively oxidized and/or all together to commercial high chemical value [83,84]. The oxidation of all hydroxyl groups yields mesoxalic acid, while the selective oxidation of primary and secondary hydroxyl groups leads, respectively, to glyceric and tartronic acid or dihydroxyacetone.

10. Conclusions Biodiesel is a bioderived fuel that can be used for automotive and heating, whether pure or mixed with fossil fuel. It is a natural product, nontoxic (it does not contain danger molecules), renewable given that it is obtained from the cultivation of oilseed plants grown in marginal or abandoned lands and biodegradable because living organisms can easily decompose it. The primary advantage of biodiesel, in comparison with analogous petrochemical fuels, stands on its essentially carbon-neutral nature with a significant saving of CO2 emission. From a chemical point of view, biodiesel is a mixture of methyl esters of carboxylic acids (FAME, fatty acids methyl esters) obtained from the transesterification reaction of vegetable oils with simple short-chain aliphatic alcohols (generally methanol or ethanol). Biodiesel is almost free of sulfur (SO2 4 mg/L (providing by mechanical aeration)
Recirculating operation systems were applied under batch and SBR pilot-scale
Temperature: 30 C
Feed flow rate: 6 mL/min
Airflow: 0.5 L/h
Tubular packed bed reactor was packed with: 1. Alginate beads: Average density of 1.254 g/cm 2. Polyurethane foam cubes: Average density of 0.0488 g/cm

Fixed bed reactor


Anaerobic codigestion; applying batch condition and
continuous fixed-bed reactor
for slaughterhouse wastewater (SHWW) and grease treatment


Fixed bed reactor

Aerobic fixed bed reactor upflow sludge blanket filtration (FUSBF)





References


Continuous operation [23] mode was employed in the reactor
The initial COD and TOC contents were reduced
The concentration of phenolic was reduced by 90% or more
The PO34 _ P is the total phosphate-phosphorus
The NHþ 4 _N is ammonium-nitrogen Temperature: 37 C.
High yield of 0.6 L/g COD [72] was achieved for biogas pH: 7.2 A polyurethane foam cube under the batch system comprising a 75% of was utilized as random SHWW and 25% hydrosupporting substance in lyzed grease the midst of the reactor
The optimum state for the (specific surface of polyefficiency of digester: 25% urethane: 200 m2/m3) of hydrolyzed grease A cycle pump was used to load the effluent continuously into the FBR Maximum air compressor
In the FUSBF system, at [73] output: 90 L/min phenol concentration of 20% of the volume of 312 mg/L the average COD aerobic reactor was occuand phenol removal was pied by a media fabricated 92.82% and 97.52%, of dense polyethylene respectively Continued

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Table 12.4 Review articles concerning various reactors for wastewater treatment (2015e18).dcont’d Reactor

Method of treatments

Operating conditions




SG-SBR HG-SBR

Aerobic SBR were used to estimate the biodegradation performance of phenol








SBR

Anaerobic treatment of domestic wastewater to remove nitrogen





Granular anaerobic membrane bioreactor (SG-AnMBR)

Anaerobic SG-MBR evaluation for municipal wastewater treatment and biogas production











Description

References

1. Surface area of support material 400 m2/m3 2. Porosity: 56% 3. Mean diameter: 14 mm 4. Density: 97.0e92.0 g/cm3 The sludge that the clarifier of FUSBF produce immediately turned back into the anoxic area Temperature:25 C pH: Neutral The operation modes for
Average phenol removal: [74] fill:react:settle:draw: 80% for both systems
HGSBR minimized the 1:20:2:1 The aeration flow rate: inhibitory effect of phenol 60 mL/min towards the biochemical activities of the microorTemperature: Room temperature ganisms compared with SG-SBR
The coexistence of biosludge and biofilm increased the efficiencies of HG-SBR due to high concentration of biomass Temperature: 18 C
The removal efficiencies: [75] 1% wt. methanol supplied Total nitrogen: 84% solufrom peristaltic pump was ble COD: 77% added for the denitrifica-
SBR in a single reactor at tion step low temperature is an appropriate process for the removal of organic and nitrogen pollutant both together from low COD content domestic wastewater Temperature: 20 C
The SG-AnMBR exhibited [47] efficient nutrient and Anaerobic sludge with organics removal, and mixed liquor suspended methane yield at solids (MLSS) of 20.50  1.53 g/L in the re- 156.3  5.8 mL CH4 (STP)/ action zone was used g CODremoved Membrane characteristics:
The SG-AnMBR also 1. Polyvinylidene hollow showed lower fouling rate fiber with 50.7% reduction in 2. Pore size: 0.22 mm total filtration resistance 3. Surface area: 0.06 m2 than those of the Filtration rate: 5.3 L/m2h

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Table 12.4 Review articles concerning various reactors for wastewater treatment (2015e18).dcont’d Reactor

Method of treatments

Operating conditions

Description

References


Hydraulic retention time: conventional granular 12 h AnMBR
Upflow velocity: 3.2 m/h applying internal recirculation
Very high treatment effi- [55] Membrane Aerobic process was tested
Temperature:24 C and ciencies were obtained 29 C bioreactor in submerged MBR for (suspended solids (SS), (MBR) treatment textile wastewater
60 L of textile industry 100%; COD, 98%; color, sludge was used 100%; biochemical oxygen
The starting mixed lliquor demand (BOD5), 96%). suspended solids (MLSS) concentration: 5.22 g/L
Aeration rate: 1e2 m3 air/h.
The MBR was fed through a peristaltic pump synchronized to permeate suction
The reactor was operated at a preset flow rate that was controlled by adjusting the rotation speed of the peristaltic pumps. [76] Several types of research Fluidized bed Aerobic inverse fluidized bed
pH: 3.5e10.5  revealed that AIFBBRs own reactor biofilm reactors (AIFBBRs) for
Temperature: 18e60 C superior organic removal industrial wastewater capacity and higher treatment(review) operational stability
Temperature: 37 C Fluidized bed Anaerobic COD and volatile suspended [77]
Organic loading rate: reactor solids (VSS) removal for 3 18 kg COD/m d primary sludge showed 62%
Hydraulic retention time: and 63%, respectively 2.2 days

Besides the above-mentioned advances, several other developments such as dualpurpose microalgaeebacteria-based systems are currently occurring in the wastewater treatment industry.

10. Dual-purpose algaeebased systems Interesting progress such as microalgae-based systems has been made to increase energy and resource recovery from wastewater by producing valuable biofuels from WWTPs [78]. These dual-purpose algaeebacteria-based systems remarkably contribute to a substantial saving in the overall biomass production costs and could enhance the prosperity of biodiesel manufacture from algae, as reported by recent life cycle analysis researches. Some microalgal species (algae with 1e50 mm in diameter) very efficiently

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degrade organic contaminants, and also they have been successfully utilized for treating effluents generated from the digestion of biogas anaerobic, pulp/paper mills, olives and swine manure processing, sand, and other refineries. In other words, microalgae can enhance the purifying process of domestic and industrial wastewater via adsorbing and accumulating heavy metals and organic nutrients. They change the adsorbed species to attractive raw substances for biofuels production [79]. The most common production systems of algae are open ponds (e.g., high rate algal ponds, also known as HRAPs or raceway ponds) [10], closed reactors [17] (e.g., tubular, vertical, horizontal, helical reactors), and immobilized cultures (e.g., matrix-immobilized systems and biofilms) [80]. Rawat et al. discussed available technology regarding wastewater treatment applying HRAPs and microalgal biomass production techniques employing wastewater streams [81]. The biomass harvesting techniques and lipid extraction, as well as the processes of biodiesel and biomethane are investigated in detail. Generally, microalgae treatments are used in municipal wastewater management plants as tertiary/advanced treatment units. First, wastewater enters a screening chamber where large solids separate from wastewater, followed by a grit chamber to get rid of the grit, and a primary clarifier to eliminate most of the particles at the bottom. The sludge from the settling tank is transferred to a digester plant for further treatment and solid disposal. The residual wastewater is then sent to an aeration basin followed by a secondary clarifier. Afterward, wastewater is guided to a sedimentation tank or treatment ponds/lagoons [82]. Approximately clear H2O from the sedimentation tanks is directed to tertiary/advanced treatment units including biological processes for microalgae harvesting and biofuel production as shown in Fig. 12.10 [20,21,78].

FIGURE 12.10 Schematic diagram of WWTP with dual-purpose algae-based systems as a tertiary/advanced treatment [78,82].

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11. Economic analysis The cost-effectiveness of wastewater treatment is a very important issue and affects the method selection. When it comes to the cost of the adsorbents produced based on sludge, it mainly depends on various factors such as the sludge local availability, conditions of treatment, type of required processing, the adsorbent lifetime, and recycling method [32]. Wang, Xu [83] studied the transportation cost, chemicals, electrical energy, and other processing. The low cost and comparable capacity for adsorption of the sludge-based adsorbent in comparison to the standard activated carbon make it interesting for wastewater treatment. Moreover, it offers considerable advantages by eliminating the regeneration cost of adsorbent and the waste sludge disposal in the landfill. Besides, the reuse of the sludge in preparation of the adsorbent can enhance the landfill lifespan [84]. Furthermore, the high commercial potential and cost-effectiveness of algae-based technologies have made them a reliable and attractive option for decreasing energy consumption, reducing costs of fertilizer and freshwater, and greenhouse gas emissions in private companies and wastewater treatment industries [80]. Abinandan et al. [85] critically discussed the advantages and disadvantages of microalgal cultivation methods, harvesting techniques, and biofilm technology recently applied to effectively remove pollutants in terms of economic analysis. El Moussaoui et al. [35] estimated cost reduction in cultivation facilities by applying the nutrients from wastewater for algae growth. This analysis considers pipe transport cost from the wastewater facility to the algae ponds. The results illustrated that reducing the expense of nutrient can support transporting wastewater distance up to 10 miles for a 1000-acre-pond facility, with potential adjustments for different operating assumptions. Xin et al. [86] conducted a detailed techno-economic analysis to investigate the technology and economic possibility of the integrated biofuel production system with wastewater treatment that applies algae. The results showed that selling price of the bio-oil ($1.85/ gallon) is closely equivalent to the 5-year averaged crude oil price and the payback period is superior in comparison to other methods. However, several technological, trade and policy barriers impress the economic feasibility of biological treatments. However, there are various potential business opportunities if these alternative methods become an integral part of wastewater treatment industries. Also, several constructive suggestions can be offered to overcome the economic and technological limitations of microalgae-based systems, such as scaling up the laboratory results, establishing pilot plants, and intensifying the studies on integrating systems, particularly within availability of nutrient, the toxic compound effects, and population dynamics in mixed cultures. Moreover, polluted rivers can be used as a source of nutrients for microalgae cultivation that demonstrates the efficiency and cost-effectiveness of various alternative biological technologies on a large scale. Consequently, economic and technical feasibility of process scaling-up and optimization should be accomplished to identify the extremely profitable processes [87].

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12. Conclusion and future trends Applying biological systems provide certain advantages over other conventional methods in wastewater treatments. This chapter presented the applications of biomass in wastewater treatments. There are a large number of different biological technologies utilized for domestic and municipal wastewater treatment including aerobic and anaerobic processes. Aerobic wastewater treatment is an energy-efficient and costeffective biological process in which organic compounds oxide to low-energy form by microorganisms in the presence of oxygen, while anaerobic wastewater treatment is the biological treatment of wastewater in the absence of elemental oxygen or air. Typically, aerobic processes are not preferred due to high energy consumption for aeration, restrictions in transfer rates of liquid-phase oxygen, and high sludge output. Consequently, combining anaerobic and aerobic processes in a single bioreactor (integrated anaerobic-aerobic bioreactors) for high strength industrial wastewater treatment is highly desired from an operational and economical viewpoint. This is mainly due to offering the benefits of anaerobic digestion (biogas production) associated with the benefits of aerobic digestion (better COD and volatile suspended solid removal) and meets strict constraints with respect to space, odors, and minimal sludge production. The design of appropriate reactor configurations can enhance the increasing of natural biological processes rate. Detailed knowledge of microbial populations and the relationships between them s to providing the appropriate environmental conditions for the maximization of microbial growth and pollutant degradation. The membrane bioreactor, sequencing batch reactor, fixed bed reactors, and fluidized bed reactors are the most common reactors in this regard. Despite all mentioned excellent features of biological processes utilized for wastewater treatments there are still considerable challenges that need more studies. The cost-effectiveness of the selected membrane and the utilized technology in membrane photobioreactor operation is of great interest. Solar energy is an economical alternative in microalgae cultivation particularly in scaled up MBR for further implementation. Various microalgae strains may be explored in MBR to investigate the amount of nutrients removal from wastewater. For further work, microalgae could be pretreated to enhance biomass production [88]. Although the tremendous benefits that have made FBR as an appropriate option for wastewater treatments, it will require further research to resolve some of the current literature gaps. Process simulation and optimization of wastewater treatment using FBR are challenging according to extravariables involved in FBR efficiency. However, more analysis for process enhancement and future large-scale implementations are required. In this regard, it is especially essential to optimize the solid carriers utilized in FBR because iron crystallization and removal depend on the form of carriers, particle size, and packing [70]. The Biofilm reactor seems extremely practical owing to resistance to harmful materials, high biomass, long production time, fast recovery, etc. Eventually, it is important to review scale-up or process models to provide reliable data for industrial applications [89]. For the future of waste peels based

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biosorbents, there are already some trends that require yet more focus, including biosorbents modification through changing the active sites to enhance biosorption capacity, avoid secondary pollution, and improve pollutants absorption, evaluation of biosorbents efficiency using various multicomponent pollutants, regeneration investigations, and technological effort to scale up [14].

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[60] Hou J, et al. Effects of CeO2 nanoparticles on biological nitrogen removal in a sequencing batch biofilm reactor and mechanism of toxicity. Bioresour Technol 2015;191:73e8. [61] Miao L, et al. Advanced nitrogen removal from landfill leachate using real-time controlled threestage sequence batch reactor (SBR) system. Bioresour Technol 2014;159:258e65. [62] Santos SCR, Boaventura RAR. Treatment of a simulated textile wastewater in a sequencing batch reactor (SBR) with addition of a low-cost adsorbent. J Hazard Mater 2015;291:74e82. [63] Kermanshahi pour A, Karamanev D, Margaritis A. Biodegradation of petroleum hydrocarbons in an immobilized cell airlift bioreactor. Water Res 2005;39(15):3704e14. [64] Rodrıguez Couto S, et al. Stainless steel sponge: a novel carrier for the immobilisation of the whiterot fungus Trametes hirsuta for decolourization of textile dyes. Bioresour Technol 2004;95(1):67e72. [65] Rajeshwari K, et al. State-of-the-art of anaerobic digestion technology for industrial wastewater treatment. Renew Sustain Energy Rev 2000;4(2):135e56. [66] de Aquino S, Fuess LT, Pires EC. Media arrangement impacts cell growth in anaerobic fixed-bed reactors treating sugarcane vinasse: structured vs. randomic biomass immobilization. Bioresour Technol 2017;235:219e28. [67] Guerrero RBS, Zaiat M. Wastewater post-treatment for simultaneous ammonium removal and elemental sulfur recovery using a novel horizontal mixed aerobic-anoxic fixed-bed reactor configuration. J Environ Manag 2018;215:358e65. [68] Tisa F, Abdul Raman AA, Wan Daud WMA. Applicability of fluidized bed reactor in recalcitrant compound degradation through advanced oxidation processes: a review. J Environ Manag 2014;146: 260e75. [69] Deng Z, et al. Design of anaerobic fluidized bed bioreactor e dyeing effluents. Chem Eng Sci 2016; 139:273e84. [70] Bello MM, Abdul Raman AA, Purushothaman M. Applications of fluidized bed reactors in wastewater treatment e a review of the major design and operational parameters. J Clean Prod 2017;141: 1492e514. [71] Tatoulis TI, et al. A hybrid system comprising an aerobic biological process and electrochemical oxidation for the treatment of black table olive processing wastewaters. Int Biodeterior Biodegrad 2016;109:104e12. [72] Affes M, et al. Effect of bacterial lipase on anaerobic co-digestion of slaughterhouse wastewater and grease in batch condition and continuous fixed-bed reactor. Lipids Health Dis 2017;16(1):195. [73] Ghannadzadeh M-J, et al. Biodegradation of phenol in synthetic wastewater using a fixed bed reactor with up flow sludge blanket filtration (FUSBF). Glob J Health Sci 2015;7(7):120. [74] Yusoff N, et al. Evaluation of biodegradation process: comparative study between suspended and hybrid microorganism growth system in sequencing batch reactor (SBR) for removal of phenol. Biochem Eng J 2016;115:14e22. [75] Pelaz L, et al. Sequencing batch reactor process for the removal of nitrogen from anaerobically treated domestic wastewater. Water Sci Technol 2018;77(6):1581e90. [76] Swain AK, et al. Industrial wastewater treatment by aerobic inverse fluidized bed biofilm reactors (AIFBBRs): a review. J Water Process Eng 2018;23:61e74. [77] Wang Z, et al. Anaerobic fluidized bed digestion of primary and thickened waste activated sludges. Chem Eng J 2016;284:620e9. [78] Arias DM, et al. Integrating microalgae tertiary treatment into activated sludge systems for energy and nutrients recovery from wastewater. Bioresour Technol 2018;247:513e9. [79] Rahimpour MR, Biniaz P, Makarem MA. 14-Integration of microalgae into an existing biofuel industry. In: Bioenergy systems for the future. Woodhead Publishing; 2017. p. 481e519.

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[80] Christenson L, Sims R. Production and harvesting of microalgae for wastewater treatment, biofuels, and bioproducts. Biotechnol Adv 2011;29(6):686e702. [81] Rawat I, et al. Dual role of microalgae: phycoremediation of domestic wastewater and biomass production for sustainable biofuels production. Appl Energy 2011;88(10):3411e24. [82] Nelson MJ, Nakhla G, Zhu J. Fluidized-bed bioreactor applications for biological wastewater treatment: a review of research and developments. Engineering 2017;3(3):330e42. [83] Wang XJ, et al. Adsorption of copper(II) onto sewage sludge-derived materials via microwave irradiation. J Hazard Mater 2011;192(3):1226e33. [84] Djati Utomo H, et al. Thermally processed sewage sludge for methylene blue uptake. Int Biodeterior Biodegrad 2013;85:460e5. [85] Abinandan S, et al. Nutrient removal and biomass production: advances in microalgal biotechnology for wastewater treatment. Crit Rev Biotechnol 2018;38(8):1244e60. [86] Xin C, et al. Waste-to-biofuel integrated system and its comprehensive techno-economic assessment in wastewater treatment plants. Bioresour Technol 2018;250:523e31. [87] Olguı´n EJ. Dual purpose microalgaeebacteria-based systems that treat wastewater and produce biodiesel and chemical products within a biorefinery. Biotechnol Adv 2012;30(5):1031e46. [88] Yu KL, et al. Microalgae from wastewater treatment to biocharefeedstock preparation and conversion technologies. Energy Convers Manag 2017;150:1e13. [89] Ercan D, Demirci A. Current and future trends for biofilm reactors for fermentation processes. Crit Rev Biotechnol 2015;35(1):1e14.

13

Microfluidic devices and their bioprocess applications

Leila Karami Monfared1, Farideh Salimian1, Shahram Talebi1, Hossein Khorshidian2, Mehdi Mohammadi2, 3 1

MECHANICAL ENGINEERING DEPAR TMENT, YAZD UNIVERSITY, YAZD, IRAN; 2 DEPART ME NT OF MECHANICAL AND MANUFACTUR ING E NGINEERING, UNIVERSITY OF C ALGARY, CALGARY, ALBERTA, CANADA; 3 BIOLOGICAL SCIENCE DEPARTMENT, UNIVERSITY OF CALGARY, CALGARY, ALBERTA, CANADA

Chapter points •

Significant advantages of the microfluidic device

•

Introduce several microfluidic devices

•

The most applications of several microfluidic devices in bioprocess

Acronym list BAY bacteria, algae, and yeast Id inner diameter

1. Introduction to microfluidic device Microfluidic devices are classified into two major groups: passive and active tools [1]. In the passive type, no external force is utilized for driving liquid inside the channel like self-capillary device [2,3], while an external force pushes the fluid in the active type, that is, electroosmotic pump [4,5], electro-kinetic [6], acoustic wave and piezoelectric [7], and centrifugal force [8e13]. Furthermore, a large number of microfluidic devices have been developed, for example, micromixer [14e18], micro-bioprocessor [19], micropump [4,20], microvalve [8,9,21], microfilter and separator [22e24], and microdroplet [25,26] for different applications, for example, point of care products [5,27,28], biomedical assay [29,30], energy and oil and gas [31,32], and nanobiosensing [33]. Normally, the photolithography technique has been used for the SU-8 mold generation and soft lithography method for polydimethylsiloxane PDMS channel fabrication [34]. Also, the multilamination method of poly(methyl methacrylate) has been employed for multicomplex Advances in Bioenergy and Microfluidic Applications. https://doi.org/10.1016/B978-0-12-821601-9.00013-3 Copyright © 2021 Elsevier Inc. All rights reserved.

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multilayer microfluidic chip fabrication [8,9]. Furthermore, different surface modification methods have been developed for a large variety of microfluidic applications [32]. Micromixer has been utilized to enhance the contact surface and reduce the diffusion length of mixing samples. For instance, amorphous advection generation has a significant effect on mixing methods, which can be generated by stirring the flow, and it plays an essential role at low Reynolds numbers due to splitting, stretching, folding, and breaking up of the species streams [35]. Micro-bioprocessor systems have been widely studied for continuous production on a small scale that can significantly enhance the mass and heat transfer due to their high surface to volume ratio. As a result, molecular emission through the interface can be less resistant to a two-phase reaction [36]. Controlling parameters in a system plays an important role in system operation [37e39]. Micropumps and microvalves are applied as active microfluidic devices so precisely controlling the fluids flow. Micropumps have also been utilized for delivering fluids, as well as maintaining a constant level or varying flow rate. The microvalves have also been employed to shut off or modify the flow [40], for example, microvalves in centrifugal microfluidic systems [10,41]. The microfilter and microseparator have been applied for filtering and collecting the cell or cell debris from a solution [22,42]. The microsensors control the concentration of fluid flow and measure various parameters in a bioprocessor. For instance, we developed a noncontact microwave sensor for measuring and real-time monitoring of cell and bacteria concentration inside the microfluidic channel [43]. The microdroplet system generates droplets and provides analysis under different conditions [44,45]. Despite various applications in several areas, here we concentrate mainly on the applications of microfluidic devices including micropump, micro-bioprocessor, micromixer, microdroplet in biofuel, and biomass processes.

2. Biomass Energy resources have different forms and can be observed everywhere. The potential of energy is found in fossil fuels, sunlight, forests, water, wind, and even in geological activities [46,47]. The average world population growth rate is about 1.2% per year, resulting in increased energy demand. Therefore, the world is moving toward the use of alternative energy resources that are safer for human life, environmentally friendly, accessible, cheap, and degradable like the renewable and organic waste materials that remain from natural resources such as plants and animals. These materials can be applied in various types, including geothermal energy, wind energy, hydropower, tidal energy, ocean energy, solar energy, and bioenergy. Bioenergy can be easily adapted to provide heat, electricity for industrial and residential testing, and a source to produce transport fuel. The concepts of using biofuels instead of fossil fuels have been widely accepted in recent years due to their less pollution and more cost-effectiveness than fossil fuels. Biomass is the only renewable source of carbon that is widely distributed in

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the Earth’s regions and can be converted into bioenergy [48]. Also, the use of bioenergy reduces national dependence on imported energy sources and brings energy security to the countries.

2.1

Generation of biomass feedstock

The first generation of feedstock for biomass is starch and olive oil since the starch-based biomass contains significant amounts of carbohydrates, it has been the primary source of bioethanol in the last decade [49]. The second-generation feedstock significantly contains lignocelluloses-based biomass, grasses, and waste streams; also, it is the primary feedstock for bioenergy production. The source of the third generation is mainly from algae (rapeseed), and they are present in freshwater and seawater [50].

3. Production technology of bioenergy Various methods have been developed based on thermochemical technology that is utilized for lead bio-energy production such as biomass combustion to produce heat and electricity, pyrolysis, and hydrothermal lubricants to generate liquid biofuels [51]. Also, microalgae and microorganisms have been utilized as a great feedstock for biodiesel production as shown in Table 13.1. The microfluidic devices have been utilized widely in all steps, including research study and process stages of bioenergy application.

4. Microfluidic device application in biofuel 4.1

Micro-bioprocessors

Microbial cells are susceptible to environmental changes, any instability that affects the growth and composition of their products during the process of converting biofuels. Bioreactors are utilized to control physical and chemical parameters (environmental composition, temperature, pH, storage time, mass and the rate of heat transfer, etc.) to increase the growth of microorganisms and products [65e67]. Also, bioreactors support different types of active biological environments, for example, a photobioreactor, which is integrated with a light source (natural sunlight or artificial light) [68]. The cultivation of microbial biomass is one of the key roles of the bioreactor to produce biofuels, which is generally determined by the concentration of microbial biomass and the production capacity of individual biofuels. Microbial electrochemical systems convert microbial metabolism into electrochemical bioenergy that adds energy stored in degradable materials to high energy chemical compounds such as hydrogen and methane. The H-type bioprocessor is a dual conventional mechanism widely used in microbial electrolysis cells (MEC) experiments. This type of bioprocessor is a high internal resistance that mostly limits the functionality of the MEC due to a considerable distance

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Table 13.1

Different compounds as a feedstock [52e64].

Biofuel

Kinds of fuel

Biodiesel

Fatty acid

Animal fats Vegetable oils Nonedible oils

Biogas

Methane

Biohydrocarbons

n-Alkanes

Olive pomace Cow manure Pig slurry Maize silage food residues Glucose and organic acids Heterotrophic fermentation Branched-chain fatty acids Head-to-head condensation

Alkenes

Bioalcohols

Methanol Ethanol

Propanol Butanol

Resource

Methanol, isobutene Glycerol Enzymatically hydrolyzed Starch Glycerol Starch-rich wastes Glucose Glycerol starch-rich wastes, 2-Keto isovalerate

Manufacturing process Microemulsion Pyrolysis, transesterification, and dilution Anaerobic digestion

Acid catalysis Fermentation

Anaerobic fermentation Anaerobic fermentation

Microorganisms sources Yeast, Algae Bacteria (E. coli) Bacterial, archaeal, and fungal community

Bacteria (Vibrio furnissii S. cerevisiae, E. coli) Bacteria (Micrococcus sp., S. cerevisiae, E. coli, S. globisporus) Methanogenic bacteria Yeast (Saccharomyces Cerevisiae), bacteria (E. coli) Bacteria (E. coli) Bacteria (Escherichia coli, Pichia pastoris, Clostridium Acetobutylicum)

between the anode and the cathode, as well as a small size of the isolation of the membrane. There are several ways to increase H-type MEC hydrogen production, such as increasing the membrane size relative to the desired electrode surface, using a highlevel electrode, and reducing the distance between the anode and the cathode. Some micro-bioprocessor applications have been integrated with a micromixer that is referred to in the micromixer section. There is another configuration of the bio-micro-bioprocessor that is used to analyze microorganisms and analyze automatic densities. BAYbioreactor is capable of cultivating microorganisms in separate droplets without utilizing mixers or pumps, and the schematic view of the BAY micro-bioreactor is shown in Fig. 13.1 [69]. Another type of micro-bioprocessor is microcapillary bioprocessors that are mostly utilized for chemical reactions that could regulate mass transfer and heat in reaction. The microcapillary bioprocessor that is utilized to the synthesis of CdSe nanocrystals in the microfluidic chip is shown in Fig. 13.2A [70]. Kralj et al. [71] developed a microfluidic device for mixing and separating several organic-aqueous and fluorous-aqueous liquideliquid systems, see Fig. 13.2B. Nagasawa et al. [72] developed dual-pipe micro-bioprocessor for the synthesis of inorganic nanoparticles as shown in Fig. 13.3. Micro-bioprocessors have different shapes

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FIGURE 13.1 Schematic of BAY micro-bioreactor [69].

and structures designed to mix and complete the reaction. By mass transferring the reactive triglyceride from the oil phase to the methanol/oil interface, the methanolysis reaction rate is limited, and the kinetics are controlled at the beginning of the reaction. Also, in this reaction, the size of the droplet strongly affects the methyl function. Therefore, micro-bioprocessors are used in the reaction of Tran’s oxidation using a high ratio of volume/surface, rapid transfer distance, rapid degradation, thermal efficiency, and mass transfer. A micro-bioprocessor promotes the size reduction of the droplet due to the increase of the specific spatial region, which increases the mass transfer ratio of total methyl esters and ultimately accelerates the triglyceride response.

4.2

Micromixer

Micromixers are developed to mix different fluids and phases, and the KM micromixer has been utilized as an efficient micromixer to blend two incompatible fluids [73]. Elkady MF et al [74]. used KM micromixer comprising two inputs for two different reactive fluids, mixing plates, and output plates containing 14 microchannels for liquid flows. These liquids (preheated oil, and methanol) are pumped to the mixing plate, see Fig. 13.4 [74].

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FIGURE 13.2 (A) Schematic view of a capillary-type micro-bioprocessor for the synthesis of CdSe nanocrystals [70]. and (B) chip-type multiprocess micro-bioprocessor for extraction, mixing, and phase separation [71].

FIGURE 13.3 Schematic view of (A) micro-bioprocessor design and (B) internal flow in the micro-bioprocessor [72].

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FIGURE 13.4 Schematic view of the KM mixer [74].

Guoqing Guan [75] used the micro-bioprocessor and micromixer to transfer sunflower oil to the biodiesel that was used from the microtube bioprocessor and T-type mixer. A scheme for the developed experimental setup is shown in Fig. 13.5 [75].

4.3

Droplet microfluidic

Droplet microfluidics, as another microfluidic method, has been utilized in many areas [6,76e79]. The main feature of the droplet-based microfluidic device is a continuous fluid flow that can be divided into the microdroplet and monodisperses aqueous

FIGURE 13.5 Microreaction system for biodiesel production. (1) Vegetable oil; (2) methanol/KOH; (3) acetic acid; (4) T-type mixer or micromixer; (5) thermostat oil bath; (6) microtube; (7) collection bottle [75].

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droplets (10e200 mm diameter) or nanodroplet. Besides manipulation and analysis, which are the most common applications of microdroplet device, monodisperses are performed by microdroplet platform through different processes [49]. There are two types of microdroplet: digital droplet-based microfluidics and continuous-flow droplet-based microfluidics [80]. Microdroplet has excellent features that make it a unique platform. For instance, ability to compartmentalization, quantitative measurements in droplet scale, manipulation and analyzing in single cell-level, high throughput and fast reactions for various droplet generators, compartmentalizing reagents, screening (monitoring) droplets, and sorting droplets [25,26,81]. According to the essential characteristics and applications of the microdroplet gives an ability to manipulate the droplets and emulsions in a single cell-level so that each droplet is an individual experiment. The droplet microfluidic-based platform has a variety of screening applications containing drug detection, synthesis of biomolecules, recognition, enzymatic activity investigation and biofuel, and bioenergy [82]. Diverse operations of microdroplets are presented in Fig. 13.6 [84]. Various techniques have been applied for analyzing and screening droplets such as laser-induced fluorescence, UV-visible spectrophotometry, mass-spectrometry, liquid-chromatography, or surface enhanced Raman spectroscopy [84]. An essential advantage of the microdroplet platform and cell compartmentalization is the high throughput screening in both the cell and its surrounding microenvironment. In the biofuel area, to understand and access the performance of microalgae and cyanobacteria strains in the production of bioethanol, screening and detection of the bioethanol production processes are required. Taking this

FIGURE 13.6 Different operations of the microdroplet platform. (A) Droplet generation by two streams of fluid like oil and water, (B) droplet incubation for further analysis, (C) droplet reinjection, (D) split droplets to smaller droplets, (E) injecting pico-liter volumes of samples to the preformed droplet, and (F) sorting droplets by using lasers and electrodes [83].

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into account, the application of microdroplet in the biofuel generation involves screening and monitoring important traits of microalgae. The general principles of microfluidic screening technology are as follows (Fig. 13.6): first, individual cells flow past a sensitive tracer at high frequency, and then the specifications of each cell are immediately measured and registered [83]. Pan et al. [85] used a droplet microfluidic platform, including two distinct parts, for encapsulation and culturing of microalgae to monitor the growth of individual microalgal from three different green microalgae species in a stable environment for a long time. Another useful substance that can be the right choice as a biofuel is photosynthetic cyanobacteria. Qu et al. [86] presented droplet electroporation on the microfluidic chip to enhance the efficiency of transformation microalga strain with the cell wall and wall-less. Dewan et al. [87] developed an immobilized droplet array in a microfluidic chip to investigate the growth kinetics of single-cell microalgae. Lee et al. [88] applied a droplet microfluidic platform that used microcapsules containing alginate hydrogel to screen and accelerate the selection of superior species microalgae. Sjostrom et al. [89] used high-throughput droplet microfluidics to screen industrial enzyme production hosts. In comparison with traditional methods, the reduction of consuming reagents and the increase in the process efficiency was significant. Despite the high potential of biofuel due to the low growth rate, monitoring of mutant libraries via the traditional device is not possible. Hammar et al. [90] applied multiple microfluidic devices to encapsulate droplets and subsequently evaluated and sorted L-lactate-producing strains of the cyanobacterium also used a high productivity droplet microfluidics platform to measure L-lactate production. Abalde et al. [91] utilized a microdroplet-based platform to identify microalgae that produces more ethanol. Various applications of the microdroplet platform are shown in Fig. 13.7, the first step

FIGURE 13.7 Various applications of the microdroplet platforms [92].

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is droplet production for the cell encapsulation, the second one is pico-injection, and lastly, after many hours, droplets are prepared to be analyzed via the fluorescence of resorufin in microdroplets [92]. Damodaran et al. [92] applied a millifluidic droplet platform (about 650 mm droplet diameter) to analyze populations from a single algal cell, and then sorting and accumulating drops of interest. Sung et al. [93] applied a high-throughput droplet-based photobioreactor in a PDMS-based microfluidic device that integrated micropillar arrays instead of the traditional and bulk system for the production of microdroplet and screening the microalgal growth kinetics in various cultures conditions including four CO2 concentrations, five light intensity, and four microalgal species. Kim et al. [94] presented a droplet microfluidic platform for microalgae screening to investigate the growth rate of microalgal species. As shown in Fig. 13.8, the first step was the droplet

FIGURE 13.8 Schematic of the droplet-based microfluidics for microalgae monitoring including three sections: culturing section, on-chip staining section, and analysis section [94].

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generation for culture and monitoring of the growth of microalgal in the culture chamber, then was Nile red oil production in the on-chip staining region, and finally analyzing the region. The most important feature of this device is the integration of all steps in one chain. As previously discussed, a vital parameter in the selection of the best strain is the determination of high productivity microalgae strain. The detection of ethanol in growth is critical to recognize the type of strains. In many cases, the fluorescent tracer is used for cell detection in sorting and screening of microdroplet platform. Best et al. [95] presented a novel and labeled free method such that available chlorophyll fluorescence in microalgae can be used as an index to measure the difference between various species in sorting and screening process. Kim et al. [96] applied a high-throughput droplet-based microfluidic to identify and analyze desired specification strains, including rapid growth and higher lipid amount. To address this goal, 200,000 different collection microalgae growth was monitored, and finally, eight species with higher growth rate and more lipid content were determined. Sung et al. [97] developed a microdroplet system that benefits from a magnetic field for speed sorting and separation of various concentrations of microalgae for the selection of fast-growing strains.

4.4

Micropump

The hydrodynamic flow in microscale is different from the macroscale due to the low Reynolds number, and the viscous force is significant compared to the inertial force, so the viscous force is dominant in microscale. As a result, diffusion plays a vital role in a high precision transmission of liquid flow in microchannels. On the one hand, a high ratio of surface-area-to-volume and remarkable surface tension is also a critical subject in microfluidics that leads to enhancing hydrodynamic drag. Pumps as a key component to drive the fluid flow in such devices also need to be present on a mini scale. The diversity of designs of micropumping systems can tackle the above issues. The main advantages of micropumps are the reduced size and volume, portability, low power consumption, low cost, and well-integrated potential with other microfluidic devices [98]. Various factors are considered to select an appropriate micropump such as differential pressure, flow rate, and pump size [98]. Micropumps can be classified based on two facets: (1) actuation principle and (2) flow rectification method [99]. Based on the actuation principle, micropumps are classified into two groups: mechanical/displacement micropumps and nonmechanical/dynamic micropumps [100]. In the mechanical/displacement micropumps, the rotational or oscillatory movement of mechanical parts is used for the transmission of working fluid in each pumping cycle. In contrast, the nonmechanical/dynamic micropumps exploit the fluid properties to drive fluid by converting another form of energy into the kinetic energy [101]. In many works, mechanical and nonmechanical micropumps have several subcategories based on their characteristic: some elements are used in mechanical/displacement micropump such as a diaphragm, piezoelectric, thermopneumatic, electrostatic,

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bimetallic while in nonmechanical micropump, magnetohydrodynamics, electrohydrodynamic, electrowetting, that is, capillary pump [2], electroosmotic pump [4,5], electrochemical, centrifugal microfluidic, and ultrasonic, which have been developed for many different applications, including biomass processing, drug delivery [102] and biomedical assays, microfluidic analyses, cell culture, and others [103]. Here, we highlight several micropump applications in biofuel. Ajilo et al. [104] presented a micro-bioprocessor system to produce biodiesel of vegetable oil (algae oil). They suggested a micropump can be utilized to control the flow rate and supply enough pressure to drive reactants into and out of micro-bioprocessor. Oh et al. [105] developed a microfluidic device with a channel to culture cell base on in vitro to analyze in physical stress-induced behavior of yeasts and microalgae in various physical environments. This device was linked to a micropump to supply driven force flow and controlled the constant or pulsed pressure in the system. The range of that pressure was from 0 to 10 psi, and a fluorescence microscope was used for screening the cell behavior under culture condition as well. Lopresto et al. [106] designed a new model to decline solvent consumption and to improve the sustainability in microalgae processes. Their setup consisted of different steps, and one of the main steps was an organic solvent compatible with gear micropump (GC series) to control fluid flow, and they were able to access excellent results under various conditions. As discussed earlier, following the reduction in the use of fossil fuel, several kinds of research are conducted. One of the few contexts is the production of methane from biomass like palm kernel shell. Shahbaz et al. [107] recently have reported a response surface methodology to survey the influence of operating parameters and to recognize the optimized parameters. They have used a micropump to supply adequate pressure and to drive fluid flow from a heater to the steam generator (Fig. 13.9).

FIGURE 13.9 Schematic of setup [107].

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The thermobalance bioprocessor is used for steam gasification from biomass to produce biofuel. Sun et al. [108] applied this to analyze the effect of divers operating parameters on steam gasification. In this setup, a micropump is utilized to control the flow rate. Hydrogen gas, as another fuel source, can be obtained from biomass via gasification process. This fuel has a high potential to replace fossil fuel. Wu et al. [109] followed an experimental method to investigate the effect of temperature on the biomass process as a vital factor to enrich hydrogen gas from biomass. In their setup, a micropump was applied to control the flow rate. A micropump was used in the biofuel cell development of electrochemical biosensors [110]. Converting biomass to gaseous fuel via gasification utilizing a thermal method can be another source of energy. In Fig. 13.10, a schematic of steam gasification is shown. A micropump is applied to deliver water to a superheater, and a microvacuum-pump is also used to facilitate transforming gaseous production from thermogravimetric analysis [111].

5. Conclusion and future trends The significant advantages of a microfluidic device include miniaturization that leads to the smaller volume of sample and less fabrication cost, also, the quick sample to answer result, automation and portability of the platform, and high sensitivity. Microfluidic products have been widely accepted and utilized in a large domain of applications, for example, (bio) analytical chemistry, biomedical testing, point of care products, and bioenergy due to the global energy problem utilizing microfluidic for biofuel applications is emerging and in high demand. This chapter is provided insight into the microfluidic application for bioprocess application and the most critical applications of several microfluidic devices reviewed, such as micro-bioprocessor, micromixer, droplet microfluidic, and micropump.

FIGURE 13.10 Schematic of the routing system in steam gasification [92].

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Micro-bioprocessors and their applications in bioenergy production

Mohammad K.D. Manshadi1, Mohammad Hossein Mohammadi2, 3, Mozhgan Naseh3, Reza Kamali1, Amir Sanati-Nezhad3 MECHANICA L ENGINEERING D EPARTMENT, SHIRAZ UNIVERS ITY, SHIRAZ, F AR S, IRAN; DEPARTME NT OF ANIMA L SCIENCE, ISFAHAN UNIVERSITY O F T ECHNOLOGY, I SFAHAN, I R A N ; 3 DEPARTMENT OF MECHANICAL AND MANUFACTUR ING E NGINEERING, SCHUL ICH SCHOOL OF ENGINE ERING, UNIVERS ITY OF CALGARY, CALGARY, AL BE RTA, CANADA 1

2

Acronym list LOC Lab on a chip CO Carbon monoxide Rh Rhodium CeO2 Cerium dioxide Al2O3 Aluminum oxide C2H5OH Ethanol H2O Water CO2 Carbon dioxide H2 Hydrogen AgNiCZ 0.1%Age10%Ni/CeZrO2 Ni Nickel CeZrO2 ceriaezirconia mixed oxide

1. Introduction The chemical reaction is a process in which initial materials (reactants) interact to produce new substances. These new chemicals have different properties from the reactants. Chemical reactions can take place in nature without any control, or they can be managed precisely in laboratories using chemical bioprocessors. However, by

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controlling any phenomena in nature, their energy can be harvested [1e3]. Therefore, bioprocessors have been introduced to control chemical reactions. Chemical bioprocessors are enclosures where reactions proceed. The most generalized bioprocessors are bath bioprocessors and continuous bioprocessors. Bath bioprocessors are the most employed systems to carry out chemical reactions under which reactants are mixed together in a bath (e.g., a tank) where chemical reactions occur. On the other hand, reactants in continuous bioprocessors continuously flow into the bioprocessor while the products leave the channels at the outlet. Although bath bioprocessors have been widely employed in laboratories, continuous bioprocessors have shown better controllability over mixing reactants or product properties and have expanded their applications in different industries [4]. In some chemical reactions, especially biochemical ones, the initial amounts of reactants are limited and the use of conventional bioprocessors is not economically efficient [4]. However, lab on a chip microfluidics addresses this challenge [4,5]. Microfluidics refers to the physics of fluids and fluidic devices at the micrometer scale. They have a wide variety of applications, including biological analysis, biochemical detection, inkjet printing, blood analysis, drug delivery, and protein analysis [6e25]. Lab on a chip integrated microfluidic facilitates with sophisticated and complex analysis in biological and biomedical fields [26e28]. These chips consist of micromixers, micropumps, and micro-bioprocessors that each can individually complete a particular task, and they are interconnected to each other by microchannels (modular construction as is shown in Fig. 14.1A and B) [16,29,30]. For instance, micromixers are applied to achieve the mixing of different samples. Rapid mixing is one of the strong characteristics of suitable micromixers [31,32]. Micropumps transfer fluids from reservoirs to other components via microchannels, while microvalves control fluidic flows.

2. Micro-bioprocessors Micro-bioprocessors have been employed to proceed with chemical reactions in microspace [33]. They have attracted the researchers’ attention due to their high potential for manipulation of chemical reactions. They also facilitate chemical synthesis because of their capability of proceeding a reaction in micrometer size. Such physical dimensions lead to an increase in heat and mass transfer among reactants due to high surface to volume ratio. In addition, their configurations result in a higher mixing of reactants comparing conventional bioprocessors and fast responding of fluids flow through microchannels. These characteristics result in enhancing the efficiency of chemical processes [34,35]. One of the most promising fields that require high efficiency in the reaction process is the conversion of biomass to fuels. This field is receiving the attention of researchers and companies due to its potential to be an alternative renewable source for fossil fuels [36]. Biofuels are used as a substitution of producing fossil fuels in large scales to address the

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FIGURE 14.1 (A) Schematic model of one type of microreactor employing for the performance of two consecutive reactions [29]. (B) Schematic of the di-assemble model of microreactor using for the synthesis of a vitamin precursor [29].

ever-increasing energy problems. There is a wide variety of methods to generate biofuels. Recently, micro-bioprocessors have been employed to produce biofuels from biomass [33]. One of the significant advantages of micro-bioprocessors is their scaling-up property where a series of micro-bioprocessors are assembled to increase the production

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capacity. In contrast, those systems need further improvement to enhance the production rate in the larger bioprocessors. In addition, the micro-bioprocessor systems in comparison to conventional bioprocessors work continuously even when one of the bioprocessors fails [4]. Among various energy sources, fuel cells are considered as one of the potential alternatives to batteries for portable devices [36]. Micro-bioprocessors have the advantage to be used in the fields that chemical reactions play a significant role. In the fuel cells field, micro-bioprocessors are used to enhance the efficiency of producing hydrogen feed for fuel cells and also to control required reactions that result in the desired output in fuel processing, such as oxidation of carbon monoxide [36]. The rest of this chapter is dedicated to the application of micro-bioprocessors. The first section is devoted to biodiesel and micro-bioprocessors, and the second section focuses on the fuel cell and the ways that micro-bioprocessors can be adopted in this field to improve efficiency.

3. Biofuels as an alternative to fossil fuels There is a need to have an alternative energy source for fossil fuels due to their impact on the environment that has recently shown its effect on climate change as a threat to human life. Biomass is a renewable energy source that can be an alternative to fossil fuels due to its presence in nature and the ability of nature to absorb its emissions [36]. Pyrolysis is the step to produce liquid oil and gasification to produce syngas as one of the most convenient ways to produce bioenergy. In the gasification route, microbioprocessors are applicable due to their characteristics to enhance power generation.

3.1

Micro-bioprocessors and biofuel production

Micro-bioprocessors can be adapted to convert biomass to energy in an efficient way. Due to their modular construction, numbering-up is cost-efficient and can be easily achieved (Fig. 14.2 [29]). In addition, their high surface to volume ratio characteristics enhances heat and mass transformation and therefore improves the gasification process. More importantly, high efficiency in mass transfer addresses transport problems existing in conventional bioprocessors such as slurry bioprocessors. In the fuel synthesis step, the most common types of biofuels implemented are bioethanol and biodiesel. A large number of micro-bioprocessors have been introduced to generate these biofuels. In this chapter, the micro-bioprocessor applications for these two types of biofuels are described.

3.2

Biodiesel and bioethanol production

Biodiesels are created from monoalkyl esters of long-chain fatty acids obtained from lipid feedstocks of various sources like vegetable oil, nonedible oils, animal fat, and waste

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(A) Scale-up

Complex and cost intesive increase in plot size

(B)

Numbering-up Simple and inexpensive replication

FIGURE 14.2 Schematic of the scale-up strategy for (A) conventional reactors and (B) microreactor [37].

cooking oils [38]. Among different techniques used for producing biodiesels, such as microemulsion, direct/blends, and pyrolysis, transesterification is the most common technique [38]. In this process, lipid feedstocks containing a confined amount of fatty acids (e.g., seed oils) become involved in three reversible processes: triglycerides to diglycerides, diglycerides to monoglycerides, and monoglycerides to glycerol conversions. The disadvantages of such conventional methods are the dependency on reactant and transesterification conditions, the high amount of alcohol consumption that leads to high process costs, and the alternative catalysts drawbacks like decreasing the reaction rate and the yield of biodiesel and increasing initial exerting energy [38,39]. Researchers have employed different types of micro-bioprocessors for the biodiesel production process via transesterification. These investigations have demonstrated the high potential of these kinds of bioprocessors to achieve higher conversion in less time in comparing with conventional bioprocessors. Micro-bioprocessors are capable of producing biodiesels 10e100 times faster than conventional bioprocessors due to their key features listed above [40]. These micro-bioprocessors, including membrane, microtube, microstructure, and oscillatory flow micro-bioprocessors have been proposed to provide appropriate mixing and reaction [39]. Sun et al. [41] proposed a capillary micro-bioprocessor for biodiesel synthesis. They considered various parameters in their study, including methanol/oil molar ratio, residence time, catalyst concentration, reaction temperature, and capillary dimensions.

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Their results demonstrated that the residence time decreased significantly in the microbioprocessor, but it should be adjusted to avoid the saponification of biodiesel with the catalyst. In addition, increasing the catalyst concentration to a certain limit enhanced biodiesel yield. Such a trend was also observed for methanol/oil molar ratio cases. Moreover, the smaller channel size resulted in a higher biodiesel yield due to the larger specific surface area and intensified mass transfer. In another investigation, researchers studied the effect of various cosolvents in biodiesel production from sunflower oil in a microtube bioprocessor. It was proved that cosolvent promoted the transesterification rate and all of the sunflower oil were altered to biodiesel. The results also showed that cosolvent/methanol molar ratio, methanol/oil molar ratio, and catalyst concentration also affected the conversion rate [42]. A corrugated plate’s heat exchanger microbioprocessor was also proposed for biodiesel production. This micro-bioprocessor proved its high efficiency in transesterification of soybean oil with methanol. The results showed that increasing the flow rate largely intensified soybean oil conversion to biodiesel due to the promoting mixing between the corrugated plates [43]. It was also proven that micro-bioprocessors with zigzag microchannels once optimized enhanced alkali-catalyzed biodiesel synthesis. The results demonstrated that smaller zigzag microchannels led to a higher efficiency of biodiesel production comparing with conventional stirred bioprocessors [44]. Flow patterns during the conversion of waste cooking oil in a microtube bioprocessor were also considered in another investigation [45]. Oleic acid and/or water were added to pure sunflower oil in order to find the influences of free fatty acids and water on the flow pattern and fatty acid methyl ester. It was determined that free fatty acids could not generate glycerol or methanol droplets at the microtube inlet due to the formation of soap and changing the interface properties. On the other hand, free fatty acids produced a uniform outflow that decreased the soap and water drawbacks on fatty acid methyl ester yield [45]. Machsun et al. [46] proposed, for the first time, membrane micro-bioprocessors for biofuel production. They converted triolein to methyl oleate by substrate solution passing through the biocatalytic membrane and addressed the challenge of methanol miscibility in oil. Transesterification of soybean oil in a tube was also studied by researchers and the effects of tube dimensions, temperature, pressure, oil/ethanol molar ratio, and cosolvent on the production yield were studied [47]. They found that smaller tubed had higher yields than larger ones due to their larger specific surface area. Furthermore, increasing the temperature, pressure, and oil/ethanol molar ratio resulted in promoting soybean oil conversion. They also reported that the employed cosolvent had no significant effect on biodiesel production [47]. This investigation was also considered by others and almost the same approach was implemented under the same reaction conditions. However, the results reported that adding cosolvent increased the biodiesel yields [48]. Furthermore, a microfluidic platform with a micromixer integrated into a delay loop was proposed in a recent investigation and the prototype was employed in transesterification of cottonseed oil and methanol [49]. Fatty acid methyl ester production was investigated, and the effect of pressure drop and flow patterns under different experimental conditions at high

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temperatures and flow rates were studied. The results demonstrated that this system was capable of producing biodiesel in a short time under high flow rate and low-pressure drop condition. Kalu et al. [50] demonstrated that slit microchannels had the potential to be utilized as micro-bioprocessors for biofuel production. They converted soybean oil into biodiesel in a slit microchannel bioprocessor and observed that the increase in channel depth leads to a higher production due to better mixing of reactants. Micro-bioprocessor channel prototypes were also investigated by some other researchers. Researchers [51] studied three microchannel prototypes in which various internal geometries were employed to increase reactant mixing. Their results proved that tesla shaped microchannels had higher biodiesel yields due to a better reactant mixing during conversion. In another investigation, different micro-bioprocessors for producing biodiesel from soybean oil were considered [52]. The results proved that mass transfer was the main factor of controlling the conversion procedure and the increase in the mass transfer enhanced the production rate. It was also proven that continuous microbioprocessors had higher efficiency than conventional batch bioprocessors [52]. In addition, an increase in the efficiency of a microtube bioprocessor was also demonstrated by employing a wire coil where this coil utilized within a microtube increased the mass transfer [53]. However, it was also reported that the pressure drop increases when the coil was located in the reacting flow [53]. A novel micro-bioprocessor was also proposed in which mixing was enhanced in a micromixer by circular obstructions. Different structures of T-channel, T-channel with circular obstructions and T-channel with alternate circular posts were employed and it was demonstrated that the latter prototype had a higher conversion rate of Jatropha curcas oil to biodiesel fuel [54]. The scaling-up characteristics of a micro-bioprocessor were also studied [55]. A bunch of laminae microchannels were assembled together and showed that the system had the potential of producing biodiesel with a rate of 2.47 L/min and at a capacity of over 1.2 million liters of fuel per year. Rahimi et al. [56] investigated biodiesel production in a T-shaped micro-bioprocessor and investigated the effect of methanol/oil molar ratio, concentration of catalyst, residence time, and temperature on biodiesel production rate. It was shown that the reaction time is more important than mixing time in microbioprocessors. Yamsub et al. [57] converted pork lard to biodiesel in a microbioprocessor. They proposed a quadratic model for the biodiesel production rate regarding several vital parameters in their investigation. Their results demonstrated that the increase in temperature decreased biodiesel production as a consequence of the change inside reactions and methanol phase. Moreover, the reaction was time inversely proportional to the biodiesel fraction. Furthermore, high pressure and droplet aggregations in the system affected the production rate inversely. Aghel et al. [58] studied biodiesel production from soybean oil with the focus on characterizing the effect of catalyst concentration, methanol/oil volume ratio, and reaction time using response surface methodology in a micro-bioprocessor. The main contribution of this study was in employing demineralized water plant sedimentation as the heterogeneous catalyst. Chueluecha et al. [59] demonstrated the capability of a

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packed microchannel bioprocessor in producing biodiesel fuel with high quality and performance. They integrated a T-shape micromixer to a rectangular microchannel filled with catalyst powder to experimentally evaluate the effects of reaction time, temperature, and methanol/oil molar ratio. The packed micro-bioprocessor had superior productivity performance in comparison with other prosed bioprocessors. Santana et al. [60] employed a micro-bioprocessor for sunflower oil conversion to biodiesel fuel and compared their results by a batch bioprocessor. They proved that the efficiency of the micro-bioprocessor was higher than the batch bioprocessor for transesterification of vegetable oils with alcohol. Yao et al. [61] studied biodiesel production with peracetic acid in a micro-bioprocessor. They considered the effect of different parameters such as micromixer configuration, temperature, catalyst concentration, reaction time, and flow rate, and finally proposed the optimal condition for maximizing the production rate. Pontes et al. [62] investigated biodiesel production in a rectangular micro-bioprocessor using theoretical analysis. They considered reaction, convection, and diffusion to model soybean oil conversion to biodiesel fuel. Temperature, reaction time, bioprocessor dimensions, and flow rate were the verified parameters in their study. They reported that the ideal conditions for maximal biodiesel production were proportional to higher residence time, higher temperature, and microchannel geometry with a lower height.

4. Micro-bioprocessors and fuel cells Fuel cells can be used in portable devices due to their capabilities to produce energy in a portable fashion. Nowadays, there is a growing demand to find an alternative for thestate-of-art lithium batteries to fulfill the energy needed for portable devices such as

FIGURE 14.3 Energy of conventional hydrocarbon fuels using for internal combustion engine with the representative battery types. Reproduced picture Karagiannidis S. Introduction. Catalytic microreactors for portable power generation, Springer theses, 2011. p. 15.

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cell phones and laptops. Fuel cells have a much higher energy density (about an order of magnitude) than current lithium portable power generation technologies Fig. 14.3 [63]. Micro-bioprocessors have the potential to generate energy by producing hydrogen from fuel [64,65]. The generated hydrogen is fed into a fuel cell to produce power. In order to generate hydrogen from fuel, several steps should be taken as is shown in Fig. 14.4, which are done efficiently using micro-bioprocessors [29]. One of the most essential parts of hydrogen feeding to a fuel cell is to clean up carbon monoxide (CO), which can ruin the electrodes. By using micro-bioprocessors, we are able to clean the carbon monoxide through a preferential oxidation process with high efficiency due to higher heat and mass transfer rates. In addition, micro-bioprocessors, due to their size, are the most suitable devices to be used in conjunction with fuel cells. The reaction of preferential oxidation of CO is shown below: CO þ O2 / CO2

(14.1)

H2 þ O2 / H2O

(14.2)

Another obstacle for the efficient usage of fuel cells as a new energy source is in finding a proper intermittent as the carrier of the energy. Bioethanol promises its high capability as an ideal carrier and can be employed in fuel cells to produce electricity with high efficiency. In conventional methods, hydrocarbons such as natural gas have been employed to produce hydrogen. However, the process of hydrogen production from these fossil fuels leads to emitting a tremendous amount of greenhouse gases (e.g., CO2). In addition, such processes for secondary energy production are not sustainable. Therefore, a vast variety of investigations have been performed to explore an alternative method for hydrogen production. Among different substitutions, ethanol has attracted attention due to its exceptional characteristics such as high content of hydrogen, accessibility, safe storage, and nontoxicity. In addition, today, ethanol is produced from a wide range of renewable sources of plants and different types of wastes [66e68]. In this

FIGURE 14.4 Process steps in the microreactor [29].

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chapter, hydrogen production from ethanol in micro-bioprocessors is categorized into steam reforming and autothermal reforming and discussed below.

4.1

Ethanol steam reforming process

For producing hydrogen, this process has received significant attention because the hydrogen yield through steam reforming is reasonably high. The mechanism of this procedure is basically the reaction of high tempered steam with the ethanol. The reaction in the ideal pathway is defined as: C2H5OH þ 3H2O / 2CO2 þ 6H2

(14.3)

There are several studies on hydrogen generation from ethanol using microbioprocessors. Herein, an overview of some of these investigations is presented. Wang et al. [69] proposed a system of 14 parallel ceramic microchannels for steam reforming of Ethanol into hydrogen. They selected ceramic due to its high heat resistance, hardness, and corrosion resistivity. They also analyzed this system at temperatures between 500 and 700 C and investigated ethanol conversion, hydrogen selectivity, and product stream composition to evaluate the efficiency of the system. For the first time, Llorca et al. [70] employed macroporous silicon with millions of parallel microchannels for ethanol conversion. They proved that the high efficiency of this system and suggested that this configuration had the high potential in energy supply ¨ ru¨cu¨ et al. [71] characterized the capability of a for portable electronic devices. O bimetallic catalyst for ethanol steam reforming purposes. They considered the effects of water/ethanol ratio and temperature on ethanol conversion and hydrogen formation. Casanovas et al. [72] studied the effect of micro-bioprocessor dimensions, and their results demonstrated that decreasing channel size resulted in enhancing the inner surface area with respect to bioprocessor volume and in consequence increasing in hydrogen production. Anzola et al. [73] analyzed a micro-bioprocessor with the focus on heat transfer and channel size effects on hydrogen yields in concurrent and countercurrent systems. The results demonstrated that the ethanol conversion was highly dependent on the heat transfer rate where higher hydrogen yields were achieved in the concurrent system with smaller channel dimensions. Cai et al. [74] studied catalyst deposition on a stainless steel microchannel wall and proved that this system had higher stability along with long-term runs. Moreover, the efficacy of hydrogen production increased comparing to fixed-bed bioprocessors. Dominguez et al. [75] investigated a catalytic plate micro-bioprocessor (with stainless steel substrate) for ethanol steam reforming since the catalytic plate required no activation treatment. They proved a complete ethanol conversion with a low level of carbon monoxide in the production. Peela and Kunzru [76] studied rhodium-based catalysts for ethanol steam reforming in microchannels where their system was capable of hydrogen production with no diffusional effects in the catalyst layer in comparison with conventional packed bed bioprocessors.

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Peela et al. [77] studied the performance of an ethanol steam reformer microbioprocessor where Rh/CeO2/Al2O3 catalysts were deposited over metallic microchannels. They observed that adding CeO2 to 2Rh/Al2O3 improved the activity and selectivity of the catalyst. However, adding Ni to 2Rh/20CeO2/Al2O3 reduced the activity of the catalysts considerably. Peela and Kunzru [78] proposed a kinetic model for ethanol steam reformer using 2% Rh/20% CeO2/Al2O3 catalysts in a micro-bioprocessor. They explained the process by four main chemical reactions and a kinetic model based on Langmuir Hinshelwood kinetics that can define the conversions of these four reactions. Bruschi et al. [79] studied a parallel plate bioprocessor in which the heat transfer occurs between a flue gas stream in concurrent and countercurrent flows. Their results showed that in low-temperature ethanol and water inlet conditions, the bioprocessor with countercurrent flows had higher yields. In contrast to the ethanol and water inlet preheating situation, the bioprocessor with concurrent flows had a better operation. In addition, their results proved that wall thickness also affected ethanol conversion where thicker walls resulted in a lower conversion. Bruschi et al. [80] introduced an ethanol steam reformer micro-bioprocessor in which the required heat for the ethanol conversion was prepared by ethanol combustion in the air in concurrent contiguous microchannels. Their results proved that preheating of the flow rates resulted in hydrogen yield enhancement. Dehkordi et al. [81] investigated the high efficiency of fixed-bed micro-bioprocessors due to their low price, easy application, and simple structure. They considered conical, cylindrical, and inverted conical configurations with equal volumes in their study to find the effect of changing bioprocessor diameter in the longitudinal direction on its efficiency. Their results demonstrated the higher efficiency of the conical bioprocessor among the case studied. Tripodi et al. [82] studied the effects of temperature, catalyst, and water/ethanol ratio on steam reforming of ethanol in a fuel cell performance. Although the catalyst determined the ethanol conversion, the results of their study proved that the temperature and flow rate also profoundly affected the ethanol conversion rate in the micro-bioprocessor.

4.2

Autothermal ethanol reforming process

The autothermal ethanol reforming process is the process in which the ethanol is oxidized with oxygen and generate steam to achieve a neutral reaction. Although lower hydrogen is produced through this procedure, its neutral thermodynamic features have attracted attention. Casanovas et al. [83], for the first time, introduced micro-bioprocessor application for the autothermal ethanol reforming process. They designed a microchannel that ethanol steam reforming reaction occurs on one side, and ethanol oxidation occurs on the other side where they demonstrated the performance efficiency of 71%. Dantas et al. [84] investigated the effects of some additives such as platinum, palladium, silver, and iron on the efficiency of a microbioprocessor that employed Ni/CeZrO2 for partial oxidation in an autothermal

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ethanol reforming process. Their results demonstrated that AgNiCZ as a catalyst had a better methane conversion and better stability during the reaction. Chen et al. [85] proposed autothermal ethanol reformer bioprocessor in which a flow equalizer with various geometries was used for disturbing the penetration of reactant gases. Their results showed that a hemisphere jet flow splitter resulted in a better reaction. Divins et al. [86] proposed a system of microchannels for steam and oxidative steam reforming. Their results showed a high improvement in hydrogen production rate and operating at considerably reduced residence times.

5. Conclusions and future trends Membrane, microtube, microstructure, and oscillatory flow micro-bioprocessors have been proposed for biodiesel production, where most of them were microtube type. Various parameters such as methanol/oil molar ratio, residence time, catalyst concentration, reaction temperature, and micro-bioprocessor dimensions were investigated in different bioprocessor types. Generally, decreasing micro-bioprocessor dimensions enhances the biodiesel production rate and decreases reaction time. However, methanol/oil molar ratio and catalyst concentration should be optimized to enhance the biodiesel production rate. Most of the presented platforms were simple, while a vast number of microchannel networks have been suggested for micromixers that are also capable of using micro-bioprocessors for biodiesel production. Therefore, it is highly recommended that researchers study the capability of micromixer platforms for biodiesel production. Micro-bioprocessors were also employed for fuel cell applications. In this context, researchers mostly studied the effects of catalyst and parallelized microchannels to achieve higher hydrogen formation. Steam reforming and autothermal reforming of hydrogen production from ethanol were the most applicable methods. The proposed micro-bioprocessors in this area should be capable of working at high temperatures. Therefore, there are limitations on designing various methods, but employing parallelized microchannels using the steam reforming method for hydrogen formation is shown to be an efficient procedure for fuel cell applications of micro-bioprocessors. According to the literature, there are many types of micromixers for biomedical applications that have the potential to be employed in biomass reactions. Therefore, it is suggested that various high performance proposed micromixers in the literature considered for such processes.

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[68] Ni M, Leung DY, Leung MK. A review on reforming bio-ethanol for hydrogen production. Int J Hydrogen Energy 2007;32(15):3238e47. [69] Wang J, et al. Fabrication of ceramic microcomponents and microreactor for the steam reforming of ethanol. Microsyst Technol 2008;14(9e11):1245e9. [70] Llorca J, et al. First use of macroporous silicon loaded with catalyst film for a chemical reaction: a microreformer for producing hydrogen from ethanol steam reforming. J Catal 2008;255(2):228e33. ¨ ru ¨ cu ¨ E, et al. Ethanol steam reforming for hydrogen production over bimetallic PteNi/Al2O3. Catal [71] O Lett 2008;120(3e4):198e203. [72] Casanovas A, et al. Catalytic walls and micro-devices for generating hydrogen by low temperature steam reforming of ethanol. Catal Today 2009;143(1e2):32e7. [73] Anzola AM, et al. Heat supply and hydrogen yield in an ethanol microreformer. Ind Eng Chem Res 2010;50(5):2698e705. [74] Cai W, et al. Hydrogen production from ethanol steam reforming in a micro-channel reactor. Int J Hydrogen Energy 2010;35(3):1152e9. [75] Domı´nguez M, et al. Ethanol steam reforming over cobalt talc in a plate microreactor. Chem Eng J 2011;176:280e5. [76] Peela NR, Kunzru D. Oxidative steam reforming of ethanol over Rh based catalysts in a microchannel reactor. Int J Hydrogen Energy 2011;36(5):3384e96. [77] Peela NR, Mubayi A, Kunzru D. Steam reforming of ethanol over Rh/CeO2/Al2O3 catalysts in a microchannel reactor. Chem Eng J 2011;167(2e3):578e87. [78] Peela NR, Kunzru D. Steam reforming of ethanol in a microchannel reactor: kinetic study and reactor simulation. Ind Eng Chem Res 2011;50(23):12881e94. [79] Bruschi YM, et al. Theoretical study of the ethanol steam reforming in a parallel channel reactor. Int J Hydrogen Energy 2012;37(19):14887e94. [80] Bruschi YM, et al. Coupling exothermic and endothermic reactions in an ethanol microreformer for H2 production. Chem Eng J 2016;294:97e104. [81] Dehkordi TK, Hormozi F, Jahangiri M. Using conical reactor to improve efficiency of ethanol steam reforming. Int J Hydrogen Energy 2016;41(38):17084e92. [82] Tripodi A, et al. Process simulation of hydrogen production by steam reforming of diluted bioethanol solutions: effect of operating parameters on electrical and thermal cogeneration by using fuel cells. Int J Hydrogen Energy 2017;42(37):23776e83. [83] Casanovas A, et al. Autothermal generation of hydrogen from ethanol in a microreactor. Int J Hydrogen Energy 2008;33(7):1827e33. [84] Dantas SC, et al. Effect of different promoters on Ni/CeZrO2 catalyst for autothermal reforming and partial oxidation of methane. Chem Eng J 2010;156(2):380e7. [85] Chen H, et al. Effect of inlet flow distributor for reagent equalization on autothermal reforming of ethanol in a microreformer. Ind Eng Chem Res 2012;51(30):10132e9. [86] Divins NJ, et al. Bio-ethanol steam reforming and autothermal reforming in 3-mm channels coated with RhPd/CeO2 for hydrogen generation. Chem Eng Process Process Intensif 2013;64:31e7.

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An overview on micropumps, micromixers, and their applications in bioprocess Hamid Reza Hosseini1, Hamed Nikookar1, Gurkan Yesiloz2, Mozhgan Naseh2, Mehdi Mohammadi2, 3 MECHANICA L ENGINEERING D EPARTMENT, SHIRAZ UNIVERS ITY, SHIRAZ, F AR S, IRAN; DE PARTMENT OF MECHANICA L A ND MANUFAC TURING ENGINEERING, UNIVERSITY OF C A L G A RY , C A L G A RY , A L B E RT A , C A NA D A ; 3 BIOLOGICAL SCIENCE DEPARTMENT, UNIVERSITY OF CA LGARY, CA LGARY, AL BE RTA, CANADA 1

2

Chapter point •

Review different types of microfluidic devices

•

The applications in bioprocesses and biofuel production

Acronym list AC Alternative current DC Direct current EDL Electrical double layer EHD Electrohydrodynamics EM Electromagnetic EO Electroosmotic EW Electrowetting FAME Fatty acid methyl ester GOX Glucose oxidase LIF Laser-induced fluorescence technique LoaD Lab-on-a-disc MEMS Micro-electro-mechanical systems MHD Magnetohydrodynamic MPO Myeloperoxidase PDMS Polydimethylsiloxane PZT Piezoelectric leadezirconateetitanate SAR Separation and recombine mTAS Micro total analysis systems

Advances in Bioenergy and Microfluidic Applications. https://doi.org/10.1016/B978-0-12-821601-9.00015-7 Copyright © 2021 Elsevier Inc. All rights reserved.

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1. Introduction A bioprocess can change alive cells and their components into particular chemicals, such as biofuels [1e3], polymers [4e6], and food and feed additives [7,8]. These days, scientists are highly interested in producing chemicals in a sustainable and permanent manner due to the world population growth and reduction of nonrenewable energy resources such as coal and oil. Bioprocessing is a new method that uses microbial cells for chemicals synthesis and can be replaced by traditional chemical production. The chemicals produced by biobased methods have “cleaner” components in comparison with the chemicals that are fossil fuel-based. Thus, bioprocessing decreases greenhouse gas emissions and has a favorable effect on the reduction of global warming [9]. Bioprocess engineering attempts to design, manufacture, and develop products (e.g., agricultural products, food, pharmaceuticals, nutraceuticals, chemicals, and polymers) from biological components and also treat wastewater. Bioprocess engineering employs different fields of science from mathematics and biology to mechanical engineering, electrical engineering, and industrial design for the production of chemicals from living cells or their components. A bioprocess may include various procedures such as designing bioreactors, studying fermentors, and separation and counting of cells [10] and different biotechnological processes. Biotechnological processes are essential for large-scale production of biological products in industrial applications to optimize the quantity and quality of products. One parameter that seriously affects the production process efficiency is population heterogeneity in the cells [11]. The main reason of population heterogeneities is the change in the environmental conditions. Population heterogeneity leads to serious negative effects during chemical production. It increases during scale-up and prevents from transfer to industrial dimensions [12]. The microbial growth rate, product yields, and overall productivities are the main characters of measuring microbial bioprocesses to measure the performance. As previously stated, the problem of heterogeneous cells is quite challenging in the bioprocess. This issue has been addressed by several microfluidic studies [13], through the improvements achieved in different subjects like single-cell studies [14].

2. Various types of bioprocess A bioprocess consists of three main steps: upstream, cell, and downstream bioprocesses. The upstream process includes a series of processes from first cell separation and cultivation, to cell banking and culture expansion of the cells to make viable cell batch collection [15]. In fact, in this part, microbes/cells are grown and prepared for cell processes. Cell bioprocess is a precise transformation of cells and their components into chemicals. Cell bioprocess relates downstream and upstream bioprocesses. In downstream bioprocess, the living cells from the upstream step are processed to the final requirements. In this part, microfluidics devices can be used for studying the design advantages of the process and combining them with integrated sensor technology [16].

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3. Microfluidics bioprocess A significant challenge in bioprocessing is improving the in-depth realization of the processes for efficient implementation. This requires obtaining data related to the production scale with minimal labor and cost [16]. As a number of bioprocess variables are possible such as different genotypes, media compositions, and operating situations, it is mandatory to scale-down and automate the process design to lower the costs and manual handling of materials during the process [17]. The only prohibitive factor in the production scale is the cost [18]. In other words, miniaturized systems are necessary as an alternative to microscale production tools to minimize the cost and time needed in process development [19]. Controlling a system plays a significant role in device operation [20e22]. Microfluidics devices with their precise control of the microenvironment can improve the quality of the data and also offer increased throughput. In addition, by combining them with sensing technology, these tools can be a suitable choice for process development. For instance, Narang et al. [23,24] integrated a microfluidic channel and contactless microwave sensor for measuring bacteria growth in a microchannel. Also, the electrochemical nanobiosensor is integrated with microchannel for cancer detection [25]. Overall, microfluidic devices have attracted much attention for various applications [26e30]. To monitor process variables such as oxygen, pH, glucose, and temperature in microfluidic devices, different sensors and techniques have been implemented including optical sensors, electrical sensors, spectroscopy [31], image analysis [32], and microwave sensors [23,24]. A crucial part of bioprocessing is microfluidic cultivation for which main principles, and some properties are shown in Fig. 15.1. Different methods have been suggested for single-cell cultivation experiments. In a number of methods, single-cell trap, mother machine, and monolayer growth chamber, the cells are continuously supplied with nutrients (left). Also, droplets and microbatches are proposed methods for single-cell batch mode [33]. Microfluidics tools have many significant advantages, including parallelization and integration. Device parallelization proposes more throughput, and integration with analytical devices offers more related information per experiment [34]. These privileges make microfluidics tools ideal for examining cell responses in different conditions, for instance, by exerting small disturbance to the cellular microenvironment [35]. Microsystems are capable of precise control, monitoring, and manipulation of small amounts of models, even at scales below nanolitre. Also, microsystems can proceed and analyze cells with high accuracy at single-cell resolution and can manage all these procedures in an entirely parallel model to produce high-throughput constructions. There are two major methods to limit growing cells for tillage and perception: droplets and structures [36]. Droplet microfluidics platforms have attracted the attention to be implemented for various applications [37e43]. They are also excellent candidates

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FIGURE 15.1 Review of microfluidic cultivation methods and their properties.

for receiving and processing millions of individual cells and molecules [44]. It is broadly used for single-cell parallel processes and has been implemented in plenty of applications including primary biological [45] and clinical research [46e49]. Micromixers and micropumps are two essential components of microfluidic devices for reagent mixing and droplet manipulation. In the following part of this chapter, different types of micromixers and micropumps are reviewed, and their performance in bioprocess development is discussed.

4. Micromixers The main goal of mixing in a microfluidic device is to perform a rapid and complete mixing of several reagents in microscale tools. One important factor in a complete mixture is rapid sample preparation, which is necessary for numerous chemical and biological applications such as detection of biological/chemical agents in microscale [50], lab-on-a-chip devices [51,52], micro-bioprocessors [53], DNA hybridization [54], drug delivery [55e58], and so on. Due to the physics of the flow in microscale, which induces low Reynolds numbers, the mixing process leads to a laminar flow regime in microfluidic systems that cause molecular diffusion to be the dominant mixing mechanism. When there is no turbulence, it is more challenging to improve the mixing efficiency only by diffusion. As a result, many micromixers have been proposed to overcome this challenge, which could be classified into passive and active types [59].

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Passive micromixers

Passive micromixers run with no external energy source, and the process of mixing only includes diffusion or chaotic advection. To make suitable mixing between two fluids, the method can be employed in a specific channel design, which produces vorticities in the stream to improve the contact space and time for the two species. Passive micromixers are suggested in various types like lamination, 3D serpentine construction, zigzag, twisted channel, embedded barrier, and surface chemistry.

4.1.1 Lamination-based micromixers Generally, in microfluidics, Reynolds numbers vary between 2 and 100. Therefore, species can be mixed by the inertia effects due to the structure of the design. Different geometries have been studied in the micromixers. Hsieh et al. [60] investigated the influence of angle (a ¼ þ30 degrees, 60 degrees, 90 degrees, and 120 degrees) in Y-type micromixers. To examine mixing efficiency by visualizing and quantifying concentration and velocity field, a microfluorescence laser and particle image velocimetry was utilized. Regarding the mixing performance and mixing length, an optimum value of a was obtained. Also, the Joule heating effect on the mixing performance was explored. Chen et al. [61] studied the mixing efficiency and pressure drop for two micromixers, known as stacking E-shape and folding E-shape micromixers. The results showed that the mixing efficiency and pressure drop increase when the Reynolds number increases. Also, folding E-shape micromixer had better mixing performance compared with stacking E-shape micromixer (SESM) when Re  30. A T-type micromixer with oxygen and nitrogen gases was studied by Huang et al. [62]. Mixing and velocity fields were reported for the two gases in the micromixer. According to the results, the quality of mixing at Re ¼ 596.1 was about 30% more with respect to Re ¼ 376.2. Wang et al. [63] studied the liquid mixing quality in the T-type micromixer by m-LIF technique (micro laser-induced fluorescence). It was found that when Reynolds number is low, the mixing between in the mixture of water and ethanol is significantly better than the mixing of water alone because of the concentration difference between the two liquid flows that cause a suitable driving force for mixing at the interface. Mengeaud et al. [64] integrated a zigzag microchannel with a “Y” inlet junction (Fig. 15.2). They study the effects of dimensions on the mixing index. The results show

FIGURE 15.2 The integration of Y-type and zigzag configuration in a passive micromixer.

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that when Reynolds remains constant, the mixing index enhanced from 65% to 83.8% when geometry ratio s/w was varied from 1 to 8.

4.1.2 Intersecting channels Effectiveness of intersecting channels in improving the mixing efficiency in microfluidic mixers was shown by separating, rearranging, and incorporating the component of flows in the microchannel in a number of studies [65e68]. Tran-Minh et al. [65] designed a planar micromixer with ellipse-liked microcolumns that operated in a laminar flow regime with great mixing efficiency. They studied both typical Newtonian and non-Newtonian models of blood viscosity. Power law and Carreau models were used as non-Newtonian models [69]. The models were utilized to investigate the wall shear stress. Wasim et al. [66] designed a 3D serpentine micromixer to achieve a high mixing quality in a short length and low Reynolds numbers. The mixing index shows the efficiency of mixing and is specified as the mass fraction difference between two fluids in a plane. This micromixer demonstrates mixing index more than 0.87 for different Reynolds numbers in a 1500 mm short length channel. Chung et al. [67] proposed a rhombic microchannel with flat angles and observed that a mixing index of more than 95% occurred at Reynolds numbers of more than 180 because of the Dean vortices and improved flow recirculation effect. Chen and Shie [68] designed a laminar micromixer that was parallel with staggered bent channels to enhance species mixing by using Dean vortex produced in the curved tapered channels. It was favorably observed that the diffusion distance for two fluids was decreased because of the staggered nature of the structure, and also the mixing strength was increased.

4.1.3 3D structures To date, many 3D serpentine microchannels were proposed and built to study the chaotic mixing effect [70e73]. Chen et al. [72] tried to optimize the layout of the obstacles in a 3D T-shaped micromixer. The results of the numerical study revealed that the direction of stream velocity varied steadily because of obstacles blocking, which caused the chaotic convection and improved species mixing efficiently. Experimental studies were also applied to determine the effects of some important parameters on mixing performance. Yang et al. [70] proposed a 3D spiral micromixer consisted of two helix channels overlapped in the perpendicular direction to produce Dean vortices. The experimental results showed that the erect channel connecting the two helical channels played an essential role in enhancing satisfied mixing. In addition, it was concluded that a mixing index of 90% could be obtained when the mass flow rate and the shape of the erect channel were optimized. Nimafar et al. [73] proposed a passive microchannel based on the separation and recombine model with an H-shaped micromixer. It was indicated experimentally that a mixing index of about 98% can be obtained at Re ¼ 0.083. Li et al. [74] designed a 3D chaotic micromixer, including an alternative arrangement of V-type channels and triangular chambers. Also, numerical simulations were performed

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in the study to assess the mixing efficiencies of the device for different Reynolds numbers, varying from Re ¼ 0.001e150, and also for two fluids with kinematic viscosities of 0.00,097 and 0.186. The results indicated that for the latter fluid, a mixing efficiency of higher than 90% could be achieved in a Reynolds number of Re ¼ 0.01. Some other types of passive micromixers have been proposed in the literature including embedded barriers [75,76], staggered herringbone structures [77,78], twisted channels [79,80], and tesla structures [81,82], which were discussed in more detail in Ref. [83,84].

4.2

Active micromixers

Active micromixers, on the other hand, use an external force to make disturbances during the mixing process, unlike the passive mixing process, in which the microchannel design is generally essential to enhance the contact area or diffusion time of different samples [85]. Active mixing increases the mixing process by an external force exerted on the species for decreasing the diffusion layer thickness or producing cross-stream chaotic motion. Usually, active mixing modes are executed by combining a mechanical converter within the microfluidics tool using fabrication techniques on the microscale. The mixing efficiency is enhanced by disturbing the stream using different external energy forms. Active micromixers regarding the applying external force can be categorized into the following types: acoustic/ultrasonic, electrokinetic, dielectrophoretic, thermal actuation electrohydrodynamic force, magnetohydrodynamic (MHD) flow, electrokinetic instability, and thermocapillary effect.

4.2.1 Acoustic actuation Lee et al. [86] proposed a new magnetic-based droplet microfluidic tool combined with acoustic stimulation for the improvement of mixing efficiency. The fluctuation amplitudes of acoustically fluctuating droplets with two volumes (6 and 8 mL) in various frequencies were specified by high-speed images. The result indicated the magnetic droplets had high mixing efficiency in this microfluidic platform. Kishor et al. [87] studied the characterization and properties of surface acoustic waves within a recyclable microfluidic platform. Three different parameters studied in this research were SAW frequency, input voltage, and the thickness of the coupling layer. Zeggari et al. [88] studied the mixing performance by acoustic vibrations. These low-frequency vibrations were produced by a lump piezoelectric leadezirconateetitanate (PZT).

4.2.2 Electrohydrodynamic force actuation Electrohydrodynamics (EHD) is an active actuation in which the flow is enforced by an electric field. This specific type of micromixer in microfluidics generally developed in either physical science or in engineering. Jalaal [89] proposed a new model for the mixing of two soluble dielectric liquids by the DC electric field. In this study, EHD forces were applied to enhance the interface instability and flow circulations in the channel.

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The obtained results helped to introduce a novel EHD mixer that contained only static parts and was easy to combine with related machines during fabrication. Yu et al. [90] designed a capillary-based system as a micromixer, which was combined with a component to perform an active electrothermal mixing. When an active mixing element was used, the time of detection was remarkably decreased from 3.5 h to about 30 min in a part of the study, which detected myeloperoxidase. Vaidyanathan et al. [91] reviewed the application of electrohydrodynamics in micromixers, followed by a lot of new developments in the electrohydrodynamics. They specifically focused on the application of surface shear forces for the transportation of biological molecules or cells on electrodes.

4.2.3 Magnetohydrodynamic force actuation Kusumi et al. [92] designed a new arrangement containing a large number of hemispherical protrusions to produce several vortices in their wakes. It this study, regarding the heat flux propagation on the bottom wall of the channel and heat transport performance, an optimum stream-wise pitch length was found for the thermal mixing improvement. Xiao et al. [93] simulated a novel MHD micromixer by assuming pumping capability. They concluded that the proposed device has high efficient concurrent mixing and pumping, especially for electrolytes. Chen et al. [94] applied a uniform magnetic field and numerically studied the mixing efficiency of a novel MHD mixer for two different electrolyte solutions. According to the results, the high mixing efficiency of 0.999 was achieved. Wang et al. [95] proposed a micromixer that is affected by an alternating magnetic field to actuate magnetic particles within the fluid (Fig. 15.3). The results show that the best efficiency occurs at high operating frequency and narrow microchannels. In addition to the mentioned techniques, there are some other types of actuation in active micromixers including microwave thermocapillary microfluidic mixer [96] electrokinetics [97], electroosmosis [98,99], electrophoresis [100,101], magnetic [102,103], and thermal [104] actuation.

FIGURE 15.3 Schematic diagram of microscale MHD micromixer.

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Bioprocess applications

4.3.1 Sample concentration Concentration gradients of diffusible fluids are crucial in different applications of micromixers especially in biological samples [105]. However, a few methods were presented in biology, for producing and preserving gradients when the flow rate is completely low in laminar flow. Jiang et al. [106] designed microdevice with improved diffusive mixing using a disordered advection effect produced by a construction positioned in the ceiling of the channel. They experimentally demonstrated that the microfluidic device exactly controlled both the combination and the structure of the collected gradient. Yeh et al. [105] designed a polydimethylsiloxane concentration gradient micromixer to define the appropriate mechanism for protein concentration in the proposed device. The concentration efficiency of this microfluidic was measured and fluorescence intensity was characterized with respect to two specific concentrations in the mixture (50 and 100 mM). Kim et al. [107] proposed a microcell arrangement for an integral drug study. The microdevice combined 64 different specified cell culture chambers. The cell culture arrangement was utilized to determine and improve different combinations of drug concentration for prostate cancer cells.

4.3.2 Chemical synthesis Microfluidic chips have many advantages in chemical synthesis and found its way from the laboratory to industrial applications. This is mainly due that microfluidics enhances the efficiency and selectivity of the combination procedure by fast mixing and increased thermal management. The performance of the mixing process can be improved simply by controlling the diffusion length or amplifying the convection effects within the stream mixing zones. Wang et al. [108] designed a microfluidic chemical synthesis device that included a nanoliter mixer, a disordered mixer, a micromultiplexer, and an array with 32-microvessel combined and fabricated on a single microfluidic chip. It was indicated that the mentioned device was capable of up to 32 tick reactions in a parallel manner. In another study [109], the mentioned group fabricated a second-generation device that could perform 1024 in situ instant reactions in parallel by a bovine carbonic anhydrous. In comparison to the prior device, the second-generation kind decreased the time needed to provide a single reaction from 1 min to 17 s. In addition, the use of microfluidic purification and mass spectrometry with several reactions screening improved the hit sensibility of the tool and also decreased the consumption of enzyme and reactant.

4.3.3 Extraction and purification processes Microscale tools are also used for solvent extraction during a process identified as liquidliquid extraction. The process of extraction includes two steps: dispersion and phase dissociation. Mass transport through the phase boundary is the general parameter that describes the extraction process. As a result, to enhance the extraction efficiency, micromixers should be highly efficient [110e113].

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Lin et al. [114] proposed a combined microfluidic device to analyze fast DNA digestion and electrophoresis (CE) on a microscale. The chip constituted two gel-filled chambers to enrich and purify DNA. An electrophoresis T-shaped micromixer is shown in Fig. 15.4 for DNA/limitation enzyme mixing. It consists of a serpentine microchannel for the DNA reaction part and a CE channel for the CE detection part, see Fig. 15.4. Lee and Voldman [115] studied and compared the efficiency of three different micromixers: a single groove, a herringbone, and a staggered herringbone micromixer in improving the particle trapping performance of dielectrophoresis (DEP) microconcentrators. Ufer et al. [116] studied the potential of T-type micromixers to produce hydrogeneration of m-nitrotoluene. Also, Kinahan et al. [117] utilized an automated centrifugal microfluidic platform for mixing and silica bead-based nucleic acid extraction.

4.3.4 Biofuel production A critical application of micromixers is biofuel production. Different types of micromixers have been used in biofuel production. Elkady et al. [118] produced biodiesel from redundant vegetable oil by using KM micromixer, bioprocessors. In this study, some crucial parameters like the molar ratio of alcohol to the oil concentration of catalyst, and the flow rates of fluids have been optimized. Results show that the maximum value of biodiesel production is 97% when the molar ratio of methanol to oil is 12 and the flow rate is 60 mL/min. In another work, soybean oil has been transformed into fatty acid methyl ester by using four-way micromixers [119]. The reaction parameters like residence time, reaction

FIGURE 15.4 Active T-type micromixer for DNA restriction and digestion.

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temperature, methanol to oil volume ratio, and methanol flow rate have been optimized. The maximum efficiency of transformation has been reported 97.6% by using optimum parameters. Also, different configurations of micromixers have been studied in the synthesis of biodiesel by Santana et al. [120]. In this study, mixing and reaction of ethanol and oil in the micromixer have been investigated numerically. Three types of micromixers have been used: T-type, cross-type, and double T-type micromixers. Results show that all three types have an appropriate mixing index when the Reynolds number is low, while the cross-type has the maximum mixing efficiency. In another research, biodiesel synthesis has been investigated numerically and experimentally by Ref. [121]. In this study, two micromixers were evaluated: T-type and MSE (micromixer with the static element). Results show that MSE has the highest performance and oil conversion index. There are many other works that study the possibility of biodiesel synthesis by using different micromixers [122e124].

5. Micropumps The application of microfluidic devices with numerous functions has been improved during the past decades due to the progression in micro-electro-mechanical systems [125]. Micropumps are a special type of microfluidic device that provides the required energy for driving a fluid through the channels and is one of the most necessary parts in each microfluidic chips. Micropumps have been developed significantly during this decade. They have numerous powerful characteristics, like the small size and lightweight, suitable portability, high range of flow rate, low power consumption, low price, and a high potential for combination with other microfluidic tools. Micropumps are an essential part of transporting small volumes of species and reagents needed for different applications [126]. Micropumps have been implemented in various applications, such as drug delivery and biomedical systems [126e128], microfluidic analysis [129], cell culturing and separation [69,128,130e134], and other purposes. Micropumps are usually produced with the aid of micro-electro-mechanical systems (MEMS) technology using biocompatible materials like silicon, glass, or different kinds of polymers, including polymethylmethacrylate, polydimethylsiloxane, and SU-8 photoresist [27,135]. The essential features of mechanical micropumps are moving mechanical parts, including check valves and pumping diaphragms. In contrast, nonmechanical micropumps provide the required energy without any mechanical moving parts. Nonmechanical micropumps perform the fluid actuation using capillary force [131], pneumatic valving [136] “event-triggered” dissolvable film (DF) valves [117] hydrodynamic, electroosmosis [137], or electrowetting activation while mechanical micropumps use a physical actuator to achieve pumping. Also, different methods have been developed to change the surface property of the microfluidic channels for different applications such as hydrophilic surface for micropump [138].

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5.1

Mechanical micropumps

Several mechanical micropumps types have been developed, such as the following: Piezoelectric micropump comprises a membrane and two inactive nonreturn valve. The pumping membrane includes a chamber with a diaphragm. A PZT disk, which is attached to the diaphragm, actuates it, produces a volume, and generates the pressure needed for suction and discharge flows periodically. Feng et al. [139] proposed a resonant micropump to achieve a 205-mL per minute flow rate and 23 kPa pressure. Also, they designed a highly efficient PZT based micropump to achieve1660 mL/min [140]. Thermal actuation micropumps are generally based on thermopneumatic pumps [141], formed by memory alloy [142] or thermally tensile polymer [143] mechanisms. In these actuators, two parameters are important: volume variation of the chamber and phase variation of the fluid. In this case, a peripheral variation of the chamber volume generates a pressure variation in the cavity, which leads to the actuation of the corresponding flexible diaphragm. Electrostatic based actuation pump works based on the Coulomb absorption of two oppositely charged bodies [144]. A parallel plate structure is generally used for electrostatic actuation. The electrostatic force in the micropump is calculated as follows: F¼

1 ε0 2

 2 V A d

(15.1)

Here, V is the actuation voltage, A is the area of the plates, and d is the gap distance between the plates, and ε0 is vacuum permittivity.

5.2

Nonmechanical micropumps

Nonmechanical pumps are other types of micropumps that transform different kinds of nonmechanical energy into kinetic momentum. In these types of micropumps fluid is driven only by a different configuration of microchannels. Magnetohydrodynamic micropump electrically conducts the wide range liquids that are exposed to electric and magnetic fields [145] and operate in DC [146] and AC electric fields [147]. This type of pump includes different components such as a microchannel with two walls as electric field sources, generated by electrodes, and two other walls as magnetic field sources, generated by opposite polarity permanent magnets. Electrohydrodynamic micropump derives liquid due to the electrostatic forces. The fluid flow occurs due to the interaction of the charges and the applied electric field [43,148]. Electroosmosis micropump is based on electrokinetic actuation. Electrokinetics is categorized as Refs. [127,128] electrophoresis [149], and DEP [10,69]. When DC or AC electric field applied in microchannel forms an electric double layer over the microchannel surface that generates flow [69].

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Evaporation-type micropumps [150] manipulate fluid flow by another method, which performs controlled evaporation of the fluid between a membrane within a gas space involving a sorption agent. Centrifugal microfluidic pump: the centrifugal force can be harnessed for pumping, mixing, metering, aliquoting, and liquid handling in centrifugal microfluidic technology [117,151e153]. The three main forces for lab-on-a-disc platform are Centrifugal forces fu ¼ rru2

(15.2)

the Euler force fE ¼ rr $

du dt

(15.3)

and the Coriolis force fC ¼ 2ruv

(15.4)

where u ¼ 2pv is angular velocity on a rotating platform, r is the fluid density, r is the distance from the center of the disc, and v is the speed of flow.

5.3

Biofuel production

Today, micromechanical systems have been studied a lot for biofuel production [154e157]. One necessary part of these studies is micropumps that are investigated by many researchers. Ishida et al. [154] fabricated a micropump by microfabrication processes and a glucose biofuel cell, which is inserted into a chamber of the micropump. The electricity is generated in this device by manipulating a glucose aqueous solution onto two electrodes in the fluid chamber. The maximum produced power was reported at 0.076 mw. The diaphragm pressure varies between 0 and 30 kPa, and the pumping frequency is 3 Hz. Micropumps are usually generated by microelectromechanical systems (MEMS), and used to transport fuels in such systems with biofuel production. Liu et al. [155] studied polyimide diaphragm micropump by MEMS. The flow rate was reported 260 and 310 mL=min with diaphragms of 2 and 5.4 mm thick. In another work, Fukushi et al. [157] studied a biofuel cell in a microchannel that mimicked blood flow from the human body. In this study, the maximum output power is equal to 0.45 mw when the voltage is 0.5 V.

6. Conclusion and future trend In the present study, we first review different types of micromixers including active and passive techniques, followed by the applications in bioprocesses such as sample concentration, chemical synthesis, extraction and purification processes, and biofuel production. Micromixers are widely studied due to the numerous advantages they have in various fields including mechanical, chemical, biological, control, medical, and

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environmental applications. From 2000 to 2018, more than 2300 papers have been published in different categories related to micromixers like biological analysis, chemical synthesis, purification, droplet, and so on. The fraction of papers corresponding to each group published between 2004 and 2017 are shown in the following circular chart of Fig. 15.5 [158]. Recent improvements in MEMS techniques lead to the fabrication of artificial biochips for a wide range of applications. Micromixers have specific properties like short operation time, low cost, enhanced portability, and also a more straightforward combination with different planar bio-medical chips. Furthermore, micromixers can also be used in other structures like lab-on-a-chip (LOC) and micro total analysis systems ðmTASÞ. Passive micromixers are usually utilized in LOC and mTAS because they have no moving parts and need no energy input compared to active micromixers. As a result, they can be fabricated and combined with other parts in LOCs more easily. While notable advances have been achieved in micromixers design and development in the past decade, more studies are required in this field, particularly in active micromixers, to be integrated more easily with different parts in a microfluidic chip. Secondly, we review the advances in several mechanical (piezoelectric, electrostatic, electroactive, thermal, and electromagnetic actuated micropumps) and nonmechanical (magnetohydrodynamic, electrohydrodynamic, electro-osmotic, bubble-type, evaporation-type, electrowetting, and electrochemical) micropumps during the past decade. Despite notable achievements in micropumps technology, more research needs to be conducted in this field especially for biofuel applications. Another concept that also can be studied is the integration of micromixers and micropumps with other devices in a microfluidic chip.

FIGURE 15.5 The percentage of publications in micromixers according to their applications.

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[141] Hamid NA, Majlis BY, Yunas J, Syafeeza A, Wong YC, Ibrahim M. A stack bonded thermopneumatic micro-pump utilizing polyimide based actuator membrane for biomedical applications. Microsyst Technol 2017;23(9):4037e43. [142] Robertson JM, Rodriguez R, Holmes Jr L, Mather P, Wetzel E. Thermally driven microfluidic pumping via reversible shape memory polymers. Smart Mater Struct 2016;25(8):085043. [143] Tripathi D, Sharma A, Be´g OA. Electrothermal transport of nanofluids via peristaltic pumping in a finite micro-channel: effects of Joule heating and Helmholtz-Smoluchowski velocity. Int J Heat Mass Tran 2017;111:138e49. [144] Hsieh H-C, Kim H. A miniature closed-loop gas chromatography system. Lab Chip 2016;16(6): 1002e12. [145] Sheikholeslami M, Ellahi R. Three dimensional mesoscopic simulation of magnetic field effect on natural convection of nanofluid. Int J Heat Mass Tran 2015;89:799e808. [146] Kim CT, Lee J. Design, fabrication, and testing of a DC MHD micropump fabricated on photosensitive glass. Chem Eng Sci 2014;117:210e6. [147] Si D, Jian Y. Electromagnetohydrodynamic (EMHD) micropump of Jeffrey fluids through two parallel microchannels with corrugated walls. J Phys D Appl Phys 2015;48(8):085501. [148] Sato T, et al. Electrohydrodynamic conduction pump with asymmetrical electrode structures in the microchannels. Chem Lett 2017;46(7):950e2. ´ . Recent [149] Ramos-Paya´n M, Ocan˜a-Gonzalez JA, Ferna´ndez-Torres RM, Llobera A, Bello-Lo´pez MA trends in capillary electrophoresis for complex samples analysis: a review. Electrophoresis 2018; 39(1):111e25. [150] Chang FL, Hung YM. Dielectric liquid pumping flow in optimally operated micro heat pipes. Int J Heat Mass Tran 2017;108:257e70. [151] Mohammadi M, Kinahan DJ, Ducre´e J. Lumped-element modeling for rapid design and simulation of digital centrifugal microfluidic systems. In: Lab-on-a-Chip fabrication and application; 2016. [152] Maguire I, O’Kennedy R, Ducre´e J, Regan F. A review of centrifugal microfluidics in environmental monitoring. Anal Methods 2018;10(13):1497e515. [153] Tang M, Wang G, Kong S-K, Ho H-P. A review of biomedical centrifugal microfluidic platforms. Micromachines 2016;7(2). [154] Ishida T, Fukushi Y, Nishioka Y. A glucose biofuel cell equipped in the fluid reservoir of a polyimide diaphragm micropump. J Photopolym Sci Technol 2015;28(2):175e80. [155] Liu Y, Komatsuzaki H, Imai S, Nishioka Y. Planar diffuser/nozzle micropumps with extremely thin polyimide diaphragms. Sensor Actuator A Phys 2011;169(2):259e65. [156] Ishida T, Fukushi Y, Nishioka Y. Integration of glucose biofuel cells with a MEMS diaphragm micropump. Meet Abstr 2015;1. 49-49: The Electrochemical Society. [157] Fukushi Y, Koide S, Ikoma R, Akatsuka W, Tsujimura S, Nishioka Y. Fabrication and characterization of glucose fuel cells with a microchannel fabricated on flexible polyimide film. J Photopolym Sci Technol 2013;26(3):303e8. [158] Lee C-Y, Fu L-M. Recent advances and applications of micromixers. Sensor Actuator B Chem April 15, 2018;259:677e702.

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Droplet-based microfluidic platforms and an overview with a focus on application in biofuel generation Mohammad K.D. Manshadi1, Danial Khojasteh2, Osama Abdelrehim3, Mohammad Gholami4, Amir Sanati-Nezhad3 1 MECHANICA L ENGINEERING D EPARTMENT, SHIRAZ UNIVERS ITY, SHIRAZ, F AR S, IRAN; WATER R ESEARC H LABORATORY, SCHOOL OF CIVIL AND ENVIRONMENTAL ENGINEERING, UN SW SYDNEY, N SW, AUSTRALIA; 3 DEPARTMENT OF ME CHANICAL AND MANUFACTURING ENGINEERING, SCHULICH SCHOOL OF ENGINEERING, UNIVERSITY OF C ALGARY, CALGARY, ALBERTA, CANADA; 4 DEPARTME NT OF MECHANICAL ENGINE ERING, OHIO UN IVERSITY, AT HENS, OH, UNITED STATES 2

Acronym list AC Alternating current CTCs Circulating tumor cells DC Direct current GMP Good manufacturing practice PDMS Polydimethylsiloxane RF Resorufin SAW Surface acoustic wave TiO2 Titanium dioxide

1. Introduction Microfluidics has been widely used by chemical, biological, and medical research due to the unique advantages and capabilities in scientific and industrial applications [1e14]. One of its most significant applications is single-cell analysis. As the size of microfluidic platforms matches with the size of biological cells, microfluidic platforms are highly applicable to biofuel processes via facilitate characterization of biochemical and biophysical properties of single cells [15]. These devices have been employed for collecting, Advances in Bioenergy and Microfluidic Applications. https://doi.org/10.1016/B978-0-12-821601-9.00016-9 Copyright © 2021 Elsevier Inc. All rights reserved.

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analyzing, culturing, and developmental characterization of cells. However, microfluidic devices used for high-throughput analysis or sorting of single cells such as microalgae (the most utilized cells in biofuel production) are mainly limited to droplet-based microfluidic platforms but with a wide range of applications [16e20]. In droplet microfluidic systems, a continuous fluid stream segregates another fluid stream into discrete monodisperse droplets with small volumes. These microdroplets are ideal microenvironments for biological applications and processes [21,22]. Droplet microfluidic devices provide the context to encapsulate single cells in continuous, uniformly sized droplets that are carried by a continuous fluid. Such shielding results in the accumulation of cell secretions in the droplet that can be employed for detecting growth events, cell lysates, and testing of high-throughput cell efficacy [23]. The significant feature of droplet-based microfluidics is in the production of droplets with a controllable size at a high frequency. Various types of controlling techniques have been proposed for different engineering applications [24e26]. For droplet generation, two major techniques of passive and active have been employed for manipulation of droplet generation. In passive techniques, droplet generation is manipulated by channel geometry or variation of the fluid streams by volume or pressure at channel inlets. In the active methods, droplet formation is controlled by external power sources, for example, electric [27], magnetic [28], and acoustic sources [29]. The droplet generation mechanism is based on surface-induced instabilities in various device prototypes. These instabilities are controlled to achieve the desired droplet frequency, droplet size uniformity, or variability. In some methods, surfactants are added to the main fluid stream to improve the monodisperse of the produced droplets and prevent their coalescence. Instabilities in passive devices result in droplet formation in four modes; squeezing, dripping, jetting, tip streaming, and tip multibreaking. In squeezing mood, droplet break up occurs mainly due to the pressure difference around the droplet during separation. However, in other droplet generation modes, surface tension forces that are inclined to minimize interfacial energy are the main factors in droplet separation [30]. These modes are described below. One of the essential parameters in droplet generation is the geometry of microchannels. Each microdroplet formation platform has its own characteristics. Although there are many types of microfluidic devices for droplet formation, these devices have been categorized into three main classes of cross-flow (Fig. 16.1A), co-flow (Fig. 16.1B), and flow-focusing (Fig. 16.1C) devices based on the breaking mechanisms [31]. The rest of this chapter is organized as follows: in Section 2, different droplet formation modes are explained in more detail. Section 3 is devoted to the droplet formation regimes occurring in different devices. Then, droplet manipulation and control using the passive and active methods are presented. Thereafter, droplet microfluidic platforms for bioenergy applications are described, and finally remarkable points and challenges are explained.

2. Droplet formation regimes Under the introduction of a dispersed phase into a continuous phase, an interface is created between the phases. This interface should be deformed enough to produce a

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FIGURE 16.1 The main droplet-based microfluidics platforms. (A) Cross-flow, (B) co-flow, and (C) flow-focusing.

segregated region. Thereby, an increase in interfacial energy results in a higher deformation at the interface, leading to the breakup of the dispersed phase into droplets. Three main modes for droplet separation consist of squeezing, dripping, and jetting. These regimes have been observed in most droplet microfluidic platforms. In the squeezing and dripping regimes, the generated droplets are monodispersed but the droplet size in the former regime is larger than the latter one. In the jetting regime, smaller droplets are produced in polydisperse form. The occurrence of each regime depends on fluid properties and flow characteristics. Various dimensionless numbers have defined these regimes. Capillary number, the ratio of viscous stress to capillary pressure, is one of the essential dimensionless numbers for describing droplet generation regimes [32]. In the following subsection, these separation modes are explained in more detail.

2.1

Squeezing

The interfacial stresses in a microchannel are intensified due to the confinement of the fluids within microscale channel walls. However, in the squeezing mode of droplet generation, the pressure gradient around the droplet during their generation is the primary reason for its separation in low capillary numbers. In this situation, when the dispersed phase introduces into the continuous phase, the pressure gradient increases gradually until the channel is blocked. Then, the continuous phase pushes the dispersed phase until it concurs the pressure inside the droplet and then break-up happens. In this mode of droplet generation, the segregated dispersed phase is confined by the channel wall, and it has a plug-like shape with a larger size than the channel dimension [33]. Researchers have proven that the dimensions of the produced droplets depend on the channel size, and some relations were proposed defining the droplet size as a function of the channel dimensions [34]. In the case of a large capillary number in microchannels, the size of generated droplets depends highly on this number. In this situation, the dispersed phase partially blocks the continuous phase, the breakup point moves toward the downstream, and the

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FIGURE 16.2 Squeezing regime of droplet generation in a co-flow device.

viscous force affects droplet separation. Therefore, as you can see in Fig. 16.2, the squeezing mode is transmitted to the dripping regime [33].

2.2

Dripping

In the dripping mode of droplet generation, the viscous shear force is larger than that of the squeezing regime that leads to segregating almost spherical smaller droplets. Many equations have been proposed to define the size of droplets in this regime and many trials have been made for correlating the droplets sizes as a function of the capillary number, channel dimensions, and flow rates ratio [32]. The dripping regime of droplet generation in a co-flow device is shown in Fig. 16.3.

FIGURE 16.3 Dripping regime of droplet generation in a co-flow device.

2.3

Jetting

Jetting mode occurs when the flow rate of both phases is higher than the dripping regime, but the dispersed phase has a lower flow rate than the continuous one. Thereby, a jet is created from the dispersed phase and droplets separate at the end of the jet due to the surface instabilities. This mode produces polydisperse, smaller droplets compared to the dripping and squeezing regimes [32]. Jetting regime of droplet generation in a coflow device is shown in Fig. 16.4.

FIGURE 16.4 Jetting regime of droplet generation in a co-flow device.

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3. Device fabrications Microfluidic fabrication techniques are almost the same for different applications. However, regarding the microdroplet generation, the wettability of the microchannels plays a significant role in the stability of the process. On the other hand, most of the applications of microdroplet generation platforms (e.g., cell manipulation) are in organic material. Therefore, the biocompatibility of the materials used for microchannel fabrication is also another important feature. There are many fabrication methods such as micromechanical cutting, wet and dry etching, photolithography and soft lithography, embossing and imprinting, and injection molding with various materials such as polymers and glass, semiconductors, ceramics and composites, and metallic substrates [8,35]. Although most of the polymeric-based methods provide great conditions, for example, chemical stability, geometric precision, and durability, their fabrication is usually costly and time-consuming. Soft lithographic techniques are widely used for microchannel construction purposes due to their unique characteristics needed for droplets generation [36]. Among different materials, polydimethylsiloxane (PDMS), a polymeric organosilicon compound, is the material of choice for microchannel fabrication due to easy fabrication and biocompatibility [37]. PDMS-based microchannels fabricated by soft-lithography methods have been widely used for different applications, for example, point of care testing [5,7], cell and bacteria separation [38,39], filtration [40], drug testing [41e43], pumping and valving [3,44], sensing [1], and tissue engineering [45,46]. However, PDMS microchannels sometimes do not have enough wetting properties for continuous microdroplet generation. Different methods have been used to address this challenge. For instance, the continuous phase is injected with a higher concentration of a surfactant to achieve the desired wetting properties [47]. Besides, coating techniques have been employed to adjust wettability of the microchannels. Solegel methods that produce solid materials from small molecules have been used to vary the wettability where the chemical compatibility has been improved by a thin glass coating [48]. Several other methods such as acrylic acid coatings [49], plasma polymerization [50], or deposition of polyelectrolytes [51] as well as alteration of channel dimensions such as rounding rectangular cross-sections [52] have been employed to adjust wettability and chemical compatibility. Due to the swelling challenge of PDMS for some solvents, photo-curable polymers have been alternatively used for droplet generation where alkanes or toluene are used as continuous phase fluids.

4. Passive methods Passive methods have been widely used to design microfluidics platforms for droplet generation due to fast fabrication at low cost. The passive devices that are proposed for droplet generation are explained in different subsections.

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4.1

Droplet formation in cross-flow devices

One of the most implemented droplet generation microfluidic platforms is the crossflow devices where dispersed phase and continuous phase confront each other at an angle of a T-junction (Fig. 16.5A) or Y-junction (Fig. 16.5C). Among these devices, Tjunctions are the most used ones where the dispersed phase and continuous phase meet each other at an angle of 90 degrees. This platform was introduced by Thorsen et al. [53] for the first time. They demonstrated the capability of this configuration to generate water droplets in oil with a determined production rate. Thereafter, this geometry was considered for droplet generation in many applications due to the simplicity and low cost with high controllability of droplet formation [54]. Many experimental and theoretical studies have been carried out to characterize the effect of various physical parameters on the device performance, such as viscosity and flow rate ratio, channel size, surface tension, wettability, inflow angle, and surfactants [31]. Besides, many other works have been performed to increase droplet formation efficiency by various methods, such as integrating active methods with T-junction geometry [55]. In these methods, an external energy source is employed to exert force on the fluids. Such forces manipulate droplet formation in the microchannel. This type of droplet manipulation is described in the section of active droplet generation methods. T-junctions have been widely used for droplet generation purposes. However, some other cross-flow devices have also been proposed for producing droplets. Many investigations have demonstrated that a little deviation in the T-junction from 90 degrees to an arbitrary angle affects the droplet formation process and the separated droplet/ bubble (Fig. 16.5B) [56]. On the other hand, one type of T-junctions is also provided in which two perpendicular inlets are designed faced to each other. The droplet producing mechanism in this design is almost the same as the T-junction [57].

FIGURE 16.5 Cross-flow droplet-based microfluidic devices. (A) T-junction, (B) T-junction with a with a deviation in the inlet channel, (C) Y-junction, and (D) T-junction with two perpendicular inlets.

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Moreover, Y-junction devices with different angles among the inlets are introduced as cross-flow devices for droplet generation [58,59]. Another well-known cross-flow droplet generation device contains two perpendicular inlets for the dispersed phase connected to a microchannel with the flowing continuous phase. This device is highly applicable in situations where two different types of emulsion droplets with the same dimensions are needed [60]. They have also been used for mixing two types of fluids in droplets. In some other types of this device, the inlets do not exactly confront each other and are designed to be located with a distance from each other. Such platforms are also used for cellular characterization [61].

4.2

Droplet formation in co-flow devices

In co-flow configurations, two concentric parallel fluids flow through a microchannel with a dispersed flow as the inner phase and a continuous flow as the outer phase. Various aspects of this parallel flow system have been studied. For instance, Mitropoulos et al. [62] employed these devices for the synthesis of silk fibroin micro/submicron spheres. Hong et al. [63] considered the effects of flow rate on droplet formation. They found that a transition condition existed in which droplet volume changes from being independent to a strongly dependent on the flow rate. They explained this phenomenon by a correlation between strain rate and droplet size. Zhu et al. [64] examined the vibration effects on perturbation generation through a co-flow device and their influences on droplet generation. They proved that such perturbation had the potential in altering the droplet generation regime from dripping to jetting or vice versa. Wu et al. [65] examined various droplet generation regimes in their experimental work where they proposed two types of dripping-to-jetting transition regimes and a wavy regime in which no droplet was generated. Cramer et al. [66] distinguished two different droplet formations of dripping and jetting in their experimental investigation. They studied dripping regime aspects more precisely by considering the effects of flow rate, fluid viscosities, and interfacial tension on droplet size and separation mechanisms. Wu et al. [67] also investigated different droplet regimes in a co-flow device. They demonstrated that interfacial tension had the primary role in droplet separation in the dripping regime, and that viscous force plays a key influence in droplet formation in the jetting regime. Moon et al. [68] explored the effect of flow rate on the size of the generated water droplet in oil. They proved that in the dripping regime, the generated droplets had almost uniform droplet sizes. The increase in the flow rate of the continuous phase resulted in a transition from dripping to the jetting regime. In the latter regime, the produced droplets had variable sizes. In other words, they had multimodal size distributions. Ren et al. [69] presented the mechanism underlying the shifting of the droplet generation regime from dripping to jetting in Newtonian and non-Newtonian fluid flows. Utada et al. [70] provided a better understanding of the transition between dripping and jetting regime. They concluded that their work could be a basis for analyzing such transition in nonNewtonian fluids. Suryo and Basaran [71] studied the dynamics of droplet formation

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in a co-flow device under a creeping flow condition. Sauret and Shum [72] investigated the shift from dripping regime to jetting regime numerically and demonstrated that initial perturbation in the inlet velocity of the dispersed phase led to a faster droplet break-up and controlled producing rate.

4.3

Droplet formation in flow-focusing devices

Flow-focusing devices have almost the same function as the co-flow one, while the dispersed and continuous phases are focused on one single microchannel, followed by entering a contraction where the droplets are generated. These devices were first produced by Anna et al. [73]. Flow-focusing devices have a more complex configuration with respect to other mechanisms of droplet generation, thereby the fabrication and operation costs are higher compared to T-junction droplet generation systems. However, droplet generation controllability is preferable due to the large flexibility in producing droplets with a wide range of sizes. Therefore, many investigations have been focused on the flow-focusing devices. For instance, Anna et al. [73] proposed flow-focusing devices for generating droplets under two different dripping and jetting regimes in which the droplet size is comparable to the orifice width, explained by the diagrams showing the variation of droplet size respect to flow rate at these two regimes. Mulligan and Rothstein [74] presented a platform in which six flow-focusing channels were used together with production rates of hundreds of milliliters droplets per hour. Their prototype had one inlet for the dispersed phase and one inlet for the continuous phase, which was divided into six flow-focusing streams. It was indicated that the droplet size in these devices could be controlled by adjusting flow resistance in the outlet channels. They suggested that such a manipulation system occurred by altering the exit length of the outlets. Ward et al. [75] studied chemical reaction effects on droplet formation in a flow-focusing device. They observed that the droplet shape did not change in the absence of a chemical reaction. However, when some reactants were incorporated into the fluid streams, the droplet configuration changed due to the change in an interfacial chemical reaction. Ong et al. [76] demonstrated a flow-focusing device for water droplet generation in oil. Their device included a circular orifice integrated with a microchannel where they illustrated the immense manipulating potential of such configuration both experimentally and numerically. Gupta et al. [77] explored the effect of orifice dimensions on droplet formation in a co-flow device numerically. It was indicated that the droplet formation mechanism in this device relied on the squeezing process. Besides, they demonstrated that the droplet size increased by a rise in width and orifice distance from the inlet. However, they found a critical value for the orifice length in which the droplet formation became independent from the orifice length. Berthier et al. [78] implemented a flow-focusing device for the encapsulation of biologic objects like proteins and cells. They proved that the viscosity of the liquid has minimal effect on the ability of the system in stable droplet generation. They showed that a high viscous liquid had enough potential to produce droplets. The main consequences of the increase in viscosity were

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shown in reducing the droplet size and production rate. Nie et al. [79] studied droplet generation for the emulsification of liquids in a flow-focusing device. They showed that droplet generation regimes (dripping and jetting) were highly dependent on fluid properties and flow rates. Also, they proved that fluid viscosity also played an influential role in droplet generation where an increase in fluid viscosity generated larger droplets with lower polydispersity. Anna and Mayer [80] examined the capability of surfactantmediated tip streaming to adjust droplet size in a flow-focusing device. They observed different droplet breakup regimes by changing the surfactant concentration and flow rates. Seo et al. [81] studied droplet generation in this type of droplet generation device with a focus on the effect of surface energy. They employed T-junction and flowfocusing devices to produce multiple populations of droplets within a wide range of sizes and proved that surface energy was an effective parameter on droplet generation. Yobas et al. [82] proposed a novel flow-focusing device in which the orifice had a cuspshape edge. This edge led to the great controllability of the device for droplet generation due to applying the maximized stress around the dispersed flow. Their results also demonstrated the sensitivity of their device to a steady rate and frequency of manipulation for manipulating droplet sizes. Hashimoto et al. [83] investigated influential parameters on droplet generation in flow-focusing devices, such as geometry, fluid properties, flow characteristics, and compressibility of the dispersed phase. They showed that there were significant differences between bubble generation and droplet generation, which were mainly due to fluid compressibility.

5. Active methods Besides passive methods, in a vast variety of microfluidics devices, external energy sources have been employed to control droplet generation and increase the efficiency of droplet formation in droplet-based microfluidic devices. The main external energy sources that have been usually used for this purpose are electrical, magnetic, acoustic, and thermal energy sources.

5.1

Electrical manipulation

Aqueous liquids have been widely used in microfluidic systems. The bipolar, leaky dielectric characteristics of these fluids make them useful to be manipulated by AC/DC external electric field [10,84]. Aqueous liquids have also been applied extensively in droplet-based microfluidic devices. For instance, Link et al. [85] employed a DC electric field to control droplet volume. They showed that the increase in electric field strength decreased the droplet size. On the other hand, Kamali et al. [27] studied the effect of an external electric source on leaky dielectric droplets. Their results showed that the electric field influences were highly dependent on the liquids characteristics and electric field enhancement, resulted in changing the droplet volume regarding the electrical properties of dispersed and continuous phases. Xi et al. [86] investigated droplet deformation in

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a T-junction device under the AC electric field. They demonstrated that the increase in electric field strength and frequency of the applied voltage notably affected the deformation of the droplets. Shojaeian and Hardt [87] compared the Newtonian and nonNewtonian droplet generation in an external electric field. They demonstrated that the electric field had a higher influence on Newtonian water droplets than non-Newtonian liquids and decreased the droplet size by a factor of 2.

5.2

Magnetic manipulation

Some liquids such as ferrofluids or biological cells exhibit a response to the external magnetic field [88,89]. An external magnetic source (e.g., permanent magnets) has been employed to control their motion through microfluidic devices for various demands like micropumping and micromixing. Tan et al. [90] proved the capability of an external magnetic field in controlling droplet formation by employing a permanent magnet to steer droplets containing ferrofluid in a T-junction device. They observed that when the magnet was placed after the junction, as a consequence of the applied magnetic field, droplets were separated with a delay but with a larger size. In contrast, when the magnet was located before the junction, the magnetic force pushed the droplet and separated them sooner; therefore, smaller droplets were produced. Liu et al. [91] examined the effects of the magnetic field on droplets generated in a flow-focusing device. Their results demonstrated that increasing the magnetic field strength resulted in a rise in droplet size. Moreover, the flow rate was shown to be effective in the sensitivity of droplets to the magnetic field. Liu et al. [92] also presented a numerical investigation for studying the effect of the magnetic field on ferrofluids droplet generation by making a balance among magnetic force, interfacial tension force, and viscous stresses. They proved that the increase in magnetic field strength and magnetic susceptibility elevates the droplet size.

5.3

Acoustic manipulation

Efficient microfluidic devices should demonstrate fast performance, high integration capability on a chip, compatibility with biological/chemical samples, flexibility with respect to applications and useable fluids, and good manufacturing practice conformal operation. Surface acoustic wave (SAW)-based active microfluidic chip devices operating at MHz to GHz frequencies can provide all these attributes and are compatible with many different types of microfluidic approaches incorporating electric, magnetic, thermal fields as well as optical techniques. SAW-based microfluidic devices have been exploited for a variety of different microfluidic tasks, including microparticle and cell handling, concentration or sorting in acoustic tweezers, droplet manipulation, and aerosol generation to serve different objectives such as separating circulating tumor cells and delivery of pulmonary drugs [93]. Various SAW-based microfluidic platforms have been introduced to manipulate very small droplets, called, microdroplets [94]. For instance, SAWs were used for splitting and merging of droplets to produce the desired

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microdroplet volume. Schmid and Franke [95] applied SAW in a cross-flow junction to control the volume of separated microdroplets. Sesen et al. [96] utilized SAW to control microdroplet movement in a Y-junction and split it at the branch of the Y-junction with the desired volume. Jung et al. [97] exploited SAW to split-microdroplets during their movement in a microchannel with any desired volume. Dong et al. [98] also divided a single microdroplet into different volumes on a substrate using a controlled SAW. In addition to droplet splitting, in some organic reactions or chemical synthesis, two different microdroplets need to be merged [99]. The main advantage of this method is the lack of restriction on the initial distance among microdroplets.

5.4

Thermal manipulation

It is undeniable that most of fluid properties change with heat transfer. Increasing the temperature particularly affects the performance of various microfluidic devices. Nguyen et al. [100] studied the effect of heating on droplets generation in a flow-focusing device. They utilized the dependency of viscosities and interfacial tension on temperature to control droplet formation in the device. Their results demonstrated that increasing the temperature using microheater caused a rising in droplet volume as a result of controlling the viscosity and interfacial tension. Tan et al. [101] studied the effect of heat transfer on the formation of TiO2-water nanofluid droplets in a flowfocusing device. Their results showed that enhancing the heat transfer increased the droplet size. Yap et al. [102] studied the control of droplets formed in a bifurcation using a microheater. Heat was applied to one of the bifurcation branches that decreased the fluidic resistance of that branch. Thereby, adjusting the temperature resulted in either controlling the formation of drops in the branch or even the entire motion of the droplet toward the desired branch. The operating temperatures used in all these studies were compatible with the culture of biological samples.

6. Droplet-based microfluidic platforms for bioenergy applications One of the recent droplet microfluidic applications is in the microbiology field of research. The significant characteristics of microorganisms are their incredible diversity and their ability to grow to high densities in a short time, making them great potential candidates for large-scale industrial processes such as biofuel production. Enzymes provide the condition for chemical reactions to take place with lower activation energy. Therefore, they are widely used in a vast variety of industries, for example, food, bioenergy, and farming. In the past, enzymes have been obtained from natural resources, but nowadays, technology achievements make the enzyme generation possible. Enzyme generation expands the range of applications in biofuel production. Microalgae cells are the potential enzymes producers for biofuel production. Microalgae are microscopic organisms that grow in aqueous suspensions and are photosynthesis to

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feed themselves [103]. The main advantage of these cells for biofuel production are as follows: (1) no need to tap into the global food supply chain, (2) higher energy density due to their simple cellular structure, and (3) absorbing carbon dioxide to mitigate global warming [104]. Therefore, they can produce a massive quality of renewable biofuel feedstocks and high-value chemicals. Many researchers have investigated different aspects of these cells in biofuel processes [105]. Droplet-based microfluidic platforms have the potential to be used for microalgae cell manipulation as they demonstrated their exceptional capability in cell screening, culturing, immobilization, and sorting, as detailed below.

6.1

Cell culturing and screening

Cell culturing and screening play a significant role in biochemical processes and assist in determining the active components of microalgae production and helps to design it to follow the most efficient reaction pathway to ensure that the pathway is economically viable for biofuel production [104]. Droplet microfluidic platforms provide the opportunity to manipulate cells in libraries of mono-dispersed liquid droplet-reactors, making it possible to apply different operations like titration of an additional downstream reagent on each droplet [106]. These platforms are also capable of encapsulating single cells efficiently. In these devices, single cells are confined in a small volume of liquid, and their secreted metabolites are examined effectively [15]. Therefore, various types of these platforms have been investigated for cultivating and screening of cells in droplets [23,105e110]. Lee et al. [111] proposed a microdroplet-based microfluidic platform to encapsulate a single microalga from three species of green microalgae. Kim et al. [112] presented a droplet-based lab-on-a-chip platform for screening microalgal growth and oil production. They employed this device to encapsulate and culture single microalgal cells into droplets and monitor their growth where they performed on-chip oil staining and analysis. Two droplets, one containing microalgal cells and the other containing a neutral lipid fluorescent staining dye, were merged to allow the microalgal oil bodies to be stained and quantified inside droplets. Droplets were washed up after the staining process to remove the background signal resulted from the carrier oil stained by neutral lipid fluorescent staining dye. Abalde-Cela et al. [113] employed a droplet-based platform for ethanol detection. In their process, cells were encapsulated in microdroplets in a cross-flow device followed by using an enzymatic assay that converted ethanol into resorufin (RF), a highly fluorescent compound, to perform re-injection, pico-injection, and mixing processes. Finally, ethanol was detected using the fluorescence of RF in microdroplets. Such detection approach provided an integrated platform for high-throughput quantification of ethanol. Skhiri et al. [114] presented a biofuel cell in which ethanol was used as the fuel and oxygen as an oxidant. They employed a droplet-based microfluidic device to improve enzyme efficacy in their system where the efficiency of these cells depends significantly on the performance of the enzyme, which is shown in Fig. 16.6.

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FIGURE 16.6 Schematic illustration of the proposed platform by Skhiri et al. [114].

Besides screening, cell culturing is also another attractive application of microdroplet platforms. Although cell culturing and screening have been performed simultaneously in droplet-based lab-on-a-chip, some investigators focused on the culturing capability of these devices. For instance, Sung et al. [115] employed a droplet-based microfluidic platform for the photoautotrophic culture of microalgae and rapid measurement of their growth. The main feature of their device was integrated micropillar arrays used to encapsulate cells and facilitate microdroplet capturing for more accurate monitoring of microalgae growth. Pan et al. [116] also developed a simple device that integrated cell encapsulation along with a microfluidic chamber for cell culturing. This platform was capable of monitoring individual cells’ growth in extended periods up to 10 days. In addition, they utilized this device for culturing and monitoring three different green microalgae species as well as changing the diameter of droplets, the initial number of cells per droplet, and different growth conditions, to demonstrate the generality of their proposed method. Devices that Sung et al. and Pan et al. used are shown in Fig. 16.7.

6.2

Sorting

Owing to the fact that there are various types of microalgal species with different growth rates and biomass productivities, the sorting of these cells has become a challenge. Several techniques have been proposed to separate microdroplets from different cell densities, but a few were successfully employed specifically for biofuel applications [117]. For instance, Joon Sung et al. [117] developed a magnetic field to efficiently separate microdroplets encapsulating different numbers of microalgal cells. The main advantage of their method was the ability in separating fast-growing microalgal strains, significantly applicable to convert carbon dioxide to biomass and high-value commercial products. Best et al. [118] also presented a method based on chlorophyll fluorescence detection and sorting of picoliter microdroplets containing different species of cyanobacteria. Fig. 16.8 shows a schematic of microfluidics that Best et al. [118] used.

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FIGURE 16.7 Schematic of cell encapsulation devices presented by (A) Sung et al. [115] and (B) Pan et al. [116].

FIGURE 16.8 Schematic of the microfluidic platform employed by Best et al. [118].

7. Conclusions and future trends The primary role of droplets for biofuel production is in providing appropriate environmental conditions for the successful growth and screening of microalgae. Microalgae cells are potential enzyme producer for chemical reactions and one of the most promising feedstocks for biofuel production. The droplet-based devices for microalgae cell

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manipulation are highly capable of encapsulating cells for culturing, screening, and sorting. With respect to the proposed investigations in this area, cross-flow devices (e.g., T-junction, and Y-junction) are considered as the best platforms to be employed for this purpose. However, various active methods can also be integrated to cross-flow configurations to enhance the efficiency. Therefore, it is highly recommended that scholars harvest the potentials of active methods in this area to promote a powerful biofuel production useful as an alternative future of energy source.

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Application of biomass ash for brick manufacturing Niloufar Fouladi, Sareh Hamidpour, Mohammad Amin Sedghamiz, Mohammad Reza Rahimpour DEPARTMENT OF CHEMICAL ENGINEERING, SHIRAZ UNIVERS ITY, SHIRAZ, F AR S, IRAN

Acronyms AR acoustic response ASTM American Society for Testing and Material(International Standard) BA biomass ash BBA biomass bottom ash BC Before Christ BD bulk density cm3 cubic centimeter FLS firing shrinkage g/cm3 density unit HHV high heating value ISSA incinerated sewage sludge ash kg kilogram kJ kilojoules KWh Kilowatt hours LS linear shrinkage mm millimeter MWe megawatt electrical OPC ordinary Portland cement RH relative humidity SOx sulfur oxide TC thermal conductivity USEPA United States Environmental Protection Agency vol.% volume ratio WL weight loss wt.% weight percentage
C degree celsius mm micrometer Advances in Bioenergy and Microfluidic Applications. https://doi.org/10.1016/B978-0-12-821601-9.00017-0 Copyright © 2021 Elsevier Inc. All rights reserved.

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1. Introduction Biomass energy is now replacing fossil fuels since it reduces the greenhouse gas emission, and also due to its great energy diversification; moreover, it leads to maintenance of activities in rural areas [1,2]. According to the directive 2009/28/EC of the European Parliament, biomass is defined as follows: “The biodegradable fraction of products, waste and residues of biological activities from agriculture (including vegetal and animal substances), forestry and related industries, including fishing and aquaculture, as well as the biodegradable fraction of industrial and municipal waste” [1]. The variation of chemical and phase between natural and human-induced biomass categories and subcategories is notable. This variety is a consequence of the broad diversity of biomass sources, including plant, animal, and fabricated products. Furthermore, semibiomass (biomass that is polluted like sewage sludge, municipal solid wastes, and other industries organic wastes) has an even more complex composition in comparison with natural biomass, due to the inclusion of different nonbiomass substances during the manufacturing process [3]. The biomass is considered as a clean energy resource with no overall production of CO2, meaning that the amount of CO2 generated during biomass combustion is almost equal to the amount taken from the atmosphere during the growth of biomass [4]. Biomass energy utilization is growing worldwide. It is estimated that by 2050, about 33%e50% of the world’s current primary energy consumption will be met by biomass. However, due to recent increases in production, waste products from biomass combustion cause serious environmental and economic problems [3,5]. The significant growth of wide-scale natural biomass combustion and its cocombustion with semibiomass leads to confronting an enormous amount of remaining biomass ash (BA), which is one of the important aspects of biofuel developments in the very near future. The main challenges are how to tackle this increasing quantity of ash and investigating the probable environmental risks regarding BA [3]. This large quantity of incineration produced ash is conventionally directly discharged into landfills, an unacceptable solution both environmentally and financially. Hence, there is a need for finding an economic method of reuse and recycling of the biomass waste residues into the new vendible product [6]. From an ecological point of view, the order of ash management from the worst to the best approved method is as follows: landfill, disposal other than a landfill, disposal with energy conversion, recovery, recycling, product reuse, and limiting ash generation [7,8]. The utilization of wastes in brick production has been the subject of many researchers due to the environmental production aspect and sustainable development. A broad range of waste materials have been studied in different methods, including fly ash, paper production residue, cotton wastes, petroleum effluent treatment plant sludge, cigarette pulp, rice husk ash, waste tea, and wood sawdust [1,9]. In this review, the focus is specifically on biomass wastes utilization in brick production.

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2. Bricks Brick is an ancient construction material that has been utilized in the Mesopotamia region since the third millennium BC [10]. The word “brick” comprises a great number of products made by mixing clay, preparing and shaping it, then slowly drying, and eventually firing it in an oven. Increasing the temperature alters mineralogical and textural properties, which are the outcome of the significant disequilibrium of a system that is somehow similar to a small case of high-temperature metamorphosis. The firing temperature and the mineralogical composition of feedstock affect the porosity of the brick. Overall, the higher the firing temperature, the more vitreous the bricks are and they would experience the most changes in size and porosity [11]. Retaining of humidity for a long time inside the construction substances proceeds the physicochemical decay procedure. Regarding the porous nature of the bricks, they are at the risk of decaying (for instance when it rains). Hence, a good material for brick production is the one that absorbs water gradually but dries rapidly. Relying on the age of the brick and production condition and procedure, the porosity values of bricks can diversify in a wide range (15%e40%). The most porous bricks are the ones in historical buildings [11]. The addition of external materials to improve the quality of the final brick product is nothing new. The addition of body fuels, defined as the burnable materials, combined with the brick raw material, then firing through brick combustion, dates back to when Egyptians utilized straw for manufacturing of the brick. Due to the organic content of biomass, its combustion ashes are recognized as low calorific value body fuel [12]. Another thing to point out is that it is essential to recognize the difference between “adding” material to the clay body (like sawdust or paper sludge), and “substituting” (namely sewage sludge or ash), which is alternating a portion of the initial clay body. The distinguish between adding or substituting for a foreign body fuel is not always a simple work. Improving the intrinsic features of the initial clay raw material, without replacing of clay, is known as an addition. In contrast, substitution is decreasing the volume of the clay for altering it with a substituting material, which can also improve the clay body [12]. The basics of the production of brick have not been changed for a long time. Raw materials are shaped into the small bricks that can be held in one hand and the mortar can be easily picked up by the other hand. Foreign body fuels can be added at the initial step or in the later steps of the brick making process [12]. Ordinary bricks are a product of high-temperature kiln firing of clay or OPC concrete. The clay extraction process is extensively energy consuming, remains negative effects on the landscape, and causes an enormous amount of waste formation. Firing in high temperatures also causes the emission of a significant amount of greenhouse gases. Clay bricks are regularly produced from clay, sand, and a small fraction of proper organic substances such as sawdust that combusts during the sintering stage and provides the required porosity for the bricks. Normally, some inert substance, like crushed used brick or coal ash, is added to the

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mixture to control the quality of the final product [13]. A regular clay brick has an average energy intensity of 2.0 kWh. It also liberates about 0.41 kg of carbon dioxide (CO2) per brick. It should be pointed out that the distribution of clay across the world is not even, and there is a shortage of clay in many places. To preserve the clay resources and the environment, some countries like China have legislated new laws to constrict the usage of fired clay bricks [9]. Adding the industrial and municipal wastes to clay bricks usually improves the quality of the final product. These positive influences involve the whole process of brick making (molding, drying, and heating) and also have impacts on the final products, including shrinkage, porosity, and mechanical strength. Furthermore, different types of wastes have various calorific values that provide a part of the energy requirement of brick production [14]. Fired clay bricks, one of the oldest construction material and the greatest known type of brick is the most popular material for forming an enclosure, because of its low cost, low maintenance expenses, physical features, high mechanical resistance, long lifetime, and also, its equilibrium hygroscopic moisture [15]. Table 17.1, summarizes the features of an appropriate brick.

Table 17.1

Characteristics of a good brick [12,16].

Terms

Features

General terms

Low shrinkage Low swelling Constant firing color Low firing temperature High mechanical strength Proper thermal isolation behavior Quartz, feldspar, and amphibole influence the sintering and might cause unfavorable colors. Form pores in the final product. If the organics contain a high concentration of sulfur, this might have effects on color, the fumes, and the kiln atmosphere. Pyrite and marcasite liberate SOx on firing, generating large pores in brick that result in a reduction of compressive strength of the final product. Calcite, dolomite, and other carbonate minerals, like ankerite and siderite, if well dispersed, enhance releasing of low-temperature carbonization gases related to the generation of channels, in the worse cases (higher quantities), it reduces the compressive and flexural strength of the finished product. To prevent firing interactions that improve discoloring of the finished product, a low alkaline earth content is appropriate. Goethite and hematite, for instance, containing iron oxide (the main colorant responsible), with an optimized range of 5%e12% (for proper constant color), are the most important source of the red brick color.

Associated minerals Organics Sulfur Carbonate minerals

Alkalis Metal oxides and hydroxides

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The quality of the brick is a function of its raw material and its compositions, production process, method of firing, firing temperature, and duration. Sintering increases the brick endurance via bonding the clay ingredient. This bonding is obtained as a result of the heat effect [15].

2.1

Traditional brick making

Initially, water is added to the clay to obtain plasticity, which simplifies the process of shaping. Then, the brick has to be dried and while drying, it is hardened due to the presence of clay minerals. This hardening procedure protects the brick from possible damages during transportation to the kiln. The next stage is firing. During this stage, as the temperature is increased, the minerals melt into the liquid phase. This molten phase comes into contact with other minerals that remain fixed during the firing stage, and during cooling, acts like a glue that makes the whole matrix of combined minerals and rock residue stick together. This process provides the strength and durability of the bricks [17].

3. Necessity of the addition of wastes to the bricks The reduction of waste is not the exclusive reason for adding them to the clay matrix. By raising the local temperature, during the firing stage, waste might reduce energy consumption. Through self-combustion, the wastes would add their higher heating value (HHV) to the system, hence less energy is required for firing. The addition of waste might also result in reducing the water demand for increasing the plasticity of the mixture. Moreover, based on the nature of the waste added, some features of the finished product clay might be modified [18]. In the case of ash amendment to the conventional ceramics, the important advantages are the following: Recycling of ash that its manufacture is rising with establishing new power and electricity plants, decreasing the cost of raw material, production of a more porous material, reduction of the product density, and improving the insulating ability of the final product. The addition amount of ash is determined by certain construction and environmental standards. The finished product must satisfy the requirement of specific standards [6]. Organic residues including sawdust, paper sludge, coal, coke are pore making materials used as an additive to the bricks to increase the porosity of the brick. These organic pore makers are preferentially used due to their low cost and as mentioned before, low combustion energy requirements. The higher the porosity of the brick, the lower its thermal conductivity would be [19]. For better understanding the effect of biomass ash addition to the brick, the following section is allocated to definition and discussion of ash.

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4. Biomass ash Although ash has been the subject of many research works, there is still inadequate understanding regarding this subject. This lack of perception might be attributed to the complicated nature of this term. Therefore, it is essential to recognize and distinguish between the terms like ash yield and ash content, also mineral and inorganic content of biomass (which are not the same, they include ingredients with various composition, magnitude, and also origin) [3,20,21]. As mentioned previously biomass ash (BA) is referred to the solid waste that remains from the combustion (complete or partial) of organic components of biomass. Therefore, biomass is the residuals of inorganic matter associated with or without a quantity of uncombusted organic substance. This combustion might occur in laboratory or industrial circumstances [3]. Biomass ash is made up of minerals that biomass has absorbed or just incorporated during biomass collecting and uncombusted organic substances. The chemical and physical characteristics of these minerals specify the potential reuse of BA. The utilized biomass combustion technology affects the quality and the amount of obtained ash. The recognition of ash properties would facilitate its further implementation [5]. Recently, by increasing the number of biomass power stations producing heat and electricity, the amount of produced biomass ash is developing as well. The utilization area of biomass ash is a function of its properties like morphology, chemical, and mineralogical compositions and leaching manner pattern [8]. The basic categorization criteria of ashes distinguish between: 1. Type of ash: fly ash and bottom ash 2. Incinerator/gasifier type: fluidized bed gasification ash, fluidized bed combustion ash, grate stokers, and entrained flow gasification ash [8]. Regardless of the nature of biomass combusted, two types of ashes are generated: fly ash and bottom ash. Fly ash is caught using electrostatic precipitators or sleeve filters embedded before the exit of the gas and fine species to the atmosphere. Bottom ash is accumulated at the bottom of the boiler. The quantity of the collected fly ash and bottom ash relies on the boiler type, for instance, the amount of bottom ash is more than fly ash in the powder boilers. In contrast, fluidized bed boilers generate more fly ash. The magnitude of fly ash and bottom ash is almost the same in the grate boilers [4]. Reusing biomass ash in building substances is in line with the recommendations of the European Directive on waste 2008/98/CE. Regarding this rule, fly ash and bottom ash should only be reused in concrete, cement, and brick manufacturing or utilized as a filling substance [5].

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Composition of ash

The phases and minerals are BA, regarding their origin can be categorized as follows: 1. Primary: the natural minerals and phases formed before and during plant growth, and after the plant has died. They have experienced no phase transform during combustion. 2. Secondary: technogenic phases and minerals formed during the combustion process. New phases are produced by solid, liquid, and gas reactions among preexisting and newly formed compounds. 3. Tertiary: new natural minerals or minerals created during transport and storage of combustion residues, originated by weathering. The composition of BA determines its features, quality, and further usage. It also specifies the upcoming environmental challenges or advantages regarding the usage of any fuel and its residues as well [3,21]. It is a function of many parameters involving biomass origin, containing biomass type, growth conditions, type of soil, fertilizer used, time of harvest, harvesting technology, transportation and storage, treatment, and others [22]. Moreover, the composition of BA depends on biomass combustion that includes precombustion processes, combustion techniques, collecting, separation, and cleansing facilities. Finally, transportation and storage method of BA has an impact on its composition [3]. The composition of phase and mineral of BA involves the following: 1. Inorganic substance consists of crystalline, semicrystalline(mineral), and amorphous ingredients. 2. Organic substance containing tar and organic minerals 3. Some fluid matter comprising moisture and gas and gaseliquid containments involved with both inorganic and organic substances [3]. Approximately 229 minerals and phases have been found in BAs, which is more than coal ashes (188 detected). The minerals recognized in BAs (in descending order of abundance) include the following: silicates > oxides and hydroxides > sulfates (plus sulfides, sulfosalts, sulfites, and thiosulfates) > phosphates > carbonates > chlorides (plus chlorites and chlorates) > nitrates. The observed phases in BAs (in decreasing number of recognition) involve the following: (1) mostly quartz, calcite, sylvite, arcanite, anhydride, char, glass, lime, periclase, and hematite and (2) subordinately portlandite, cristobalite, hydroxylapatite, larnite, albite, Ca phosphates, fairchildite, K carbonate, K feldspars, halite and KeCa silicates. The abundance of chemical components in BA in descending order are O > Ca > K > Si > Mg > Al > Fe > P > Na > S > Mn > Ti, plus some Cl, C, H, N and trace components. Natural biomass ashes are usually enriched in Mn > K > P > Cl > Ca > Na > Mg and depleted in Al > Ti > Fe > Si > S compared to

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coal ashes. Trace elements such as B > Au > Cd > (Cr, Mn) > Ag > Zn > (Be, Cu, Se) > Ni > Rb in various BAs might contain higher amounts (2e24 times) than the related worldwide mean values for coal ashes [3]. Dondi et al. provided a table of the chemical composition of different fly ash used for brick production. Although the compositions are significantly different and diversified, the most common chemical elements in his table are SiO2, Al2O3, Fe2O3, and CaO [14].

4.2

Fly ash

As it is pointed out in the previous sections, the addition of fly ash to the clay brick feed results in enhanced molding and drying phases. Adding of fly ash reduces the plasticity of the matter and promotes drying, preventing the formation of cracks and fissures [14,23]. Furthermore, by the addition of fly ash, the liberation of the dangerous pollutant from the firing kiln will be reduced, low thermal conductivity and low weight of the fly ash-based bricks lead to lower both constructional and heating/cooling costs [24,25]. On the other hand, there might be a small reduction in the dry strength of the clay matter, which does not change the features of the final product significantly. In the cases that fraction of fly ash exceeds the clay raw material an extreme reduction in the dry shrinkage (from 5% to 1%), as well as deteriorated mechanical properties, would be observed [14]. Cultrone et al. reported the following results as impacts of adding fly ash. These results are related to their specific experiments, but are in good agreement with other observations: 1. As the color of the brick must be considered in restoration fields, the proportion of fly ash added should not exceed the amount that color tests specify (10 wt.% of the initial substances used for their work). 2. Bricks do not experience a significant change in their texture. The only difference is the existence of fly ash particles, which are spheres with a diameter range of 0.1e10 mm. These particles are dispersed in the matrix of clay and relying on the combustion temperature and the nature of raw material (carbonate content); they have higher or smaller vitrification ability. 3. The addition of fly ash would not improve the hydric features but results in the production of a lighter brick. Basically, adding fly ash results in a lower density of bricks. 4. Due to the lower density of the produced bricks, ultrasound velocities are pretty lower in such bricks. 5. When exposed to the cycles of salt crystallization, the bricks made with fly ash, are more resistant than ordinary bricks. This modification is regarding the decline in the surface area of the bricks, in other words, reducing the volume of the smallest pores, which are responsible for the most damage to the brick by crystallization of soluble salt [11].

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Their results indicated that the addition of fly ash to the brick materials can modify the properties of brick. Although, facing bricks are limited to use less than 10 wt.% of fly ash, a higher amount can be implemented in the construction of new buildings. Before that, a petro-physical specification study must be done [11].

4.3

Bottom ash

In a number of developed countries including Germany, Japan, Denmark, and the Netherlands, bottom ash is extensively applied in road construction, concrete, soil conditioning, and production of sound-insulating walls. The recycling pace in these countries is in the range of 70%e90% and is possible to reach approximately 100% [5,26e28]. The recycling and reuse of biomass bottom ash (BBA) in urban applications has been investigated extensively. Its application to agricultural or forests soils has also been proposed [2,5,29].

5. Wastes categorization Dondi et al. categorized the wastes that are potential to use in brick production into four leading categories according to their main impacts:    

Fuel waste Fly ash Fluxing wastes Plasticity reducing and plastifying wastes

Fuel wastes encompass waste from a broad variety of industrial manufacturing units, and their mutual property is a high organic and carbonaceous material content that gives the wastes a high calorific value. Fluxing wastes are expressed mainly by sludge, these wastes have a high content of the heavy metal. The last category has silicate in its structure that has an impact on the plastifying ability of the brick. Adding this waste results in an increase or reduction in the plastifying tendency of the brick material [14]. Industrial wastes are described as potential energy, which is taken in the brick combusting phase, that is supplied from high carbonaceous, and organic content materials exist in such wastes. This division of wastes involves residues from municipal waste treatment plants, textile and tanning industry wastes, coal processing wastes, and the wastes from paper and wood processes [14]. A number of these wastes and their properties are mentioned in Table 17.2. Later, Viera et al. modified the categories defined by Dondi, making them more general. This modification allowed a broader range of waste to be reviewed. Apart from fuel and fluxing wastes, a category of property affecting wastes replaced the originally presented fly-ash and plasticity reducing/plastifying waste categories [30].

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Table 17.2

Addition of different types of wastes to the brick body [14].

Type of waste

Waste addition to the body Calorific value (mass %) (kJ/kg)

Paper industry sludge

2e10

8,400

Sawdust Wool wash water treatment sludge Tanning industry wastes

4e5 10

7,000e19,000 18,900e29,400

10e40

84,000

Coconut pith Rice hull

10e30 10e40

12,000 18,677

Others 20 mass% of dry weight is organic content Up to 18% fuel saving 15% energy saving 20 mass% organic out of the dry amount for wool sludge Problematic due to containing pollutant(most important is chromium) 20 mass% addition leads to 17% fuel saving Consists almost exclusively of silica (98%) and volatile component

6. Constituent of brick Clays utilized in brick manufacturing units include SiO2 content of around 50%e60%. Although SiO2 improves the porosity of the brick, it also enhances the risk of crack formation in the cooling phase. The next most abundant chemical component is Al2O3 that modifies mechanical resistance of the brick via transforming into mullite during combustion. The Al2O3 content is commonly between 10% and 20%. Two other important compounds should be introduced: Fe2O3 and CaO. The existence of Fe2O3 would make the trouble of efflorescence when the clay has long been homogenized. In the case of insufficient oxygen content during combustion, the presence of Fe2O3 might lead to the so-called “black core” phenomena. Hence, there is a limit content amount for Fe2O3, which is around 10%. In the process of combustion, CaCO3 decomposes, generating CO2 and CaO. CO2 exits through the gas stream, and CaO would increase mechanical strength via binding with SiO2. Nevertheless, if free CaO does not combine with SiO2, it would absorb water and causes expansion of the brick, which may lead to the formation of cracks [18].

6.1

Salt

The existence of water-soluble salt in the feedstock of brick making procedure is a substantial problem for brick producing factories. This soluble salts can cause a production fault referred to as “drier scum” in the brick manufacturing. This is a white surface deposit, developed as a result of the migration of dissolved salts to the surface of newly shaped bricks during the drying phase. They form a white film that is apparent after further firing. This scum has a negative aesthetic effect and lowers the market value of the brick. The most common solution for this challenge is adding chemicals namely

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barium carbonate and barium chloride, which immobilizes the salt and prevents their migration. However, constantly adding such antiscumming agents would significantly increase the cost of production [16,31].

7. Biomass origin As it was mentioned before, organic residues are added to the clay matrix because of their high pore-forming ability and also their high heating value (HHV) that lead to saving energy in the combusting process. It should be noted that in lots of cases, pretreatments are essential before adding the waste into clay raw material because of nonuniform particle size distribution. There might be some possible challenges due to organic matter addition, in the homogenizing step or firing stage [18]. Lots of research has been done on the utilization of biomass wastes in fired brick production. A summarized list of some of these works is presented in Table 17.3. In the following sections, the most investigated biomass wastes are reviewed:

7.1

Sewage sludge

Incinerated sewage sludge ash (ISSA) is introduced as an odorless, pathogen-free, freeflowing powder with low bulk density (0.65 tonnes=m3 ). Single particles of ISSA are

Table 17.3

List of research done on adding biomass wastes to the fired brick.

Type of waste

Amount of waste added wt.%

Country

References

Olive pomace ash

Up to 25 5e10 Up to 20 10e20 Up to 20 Up to 10 Up to 60 30 5 Up to 40 Up to 50 20e30 Up to 30 Up to 23.8 Up to 60 10e40 20

Spain Spain Spain Spain Brazil Brazil Brazil Spain England Singapore GermanyeJapan China Brazil Spain Spain Malaysia China

[32] [8] [13] [1] [33] [34] [35] [36] [31] [37] [38] [39] [40] [41] [42] [43] [44]

Sugarcane bagasse ash

Sewage sludge ash

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composed of an accumulation of slightly sintered and partially glassy-phase substances, resulting in a porous structure, which makes it capable of absorbing a relatively large quantity of water (almost twice its dry weight) [31]. To achieve the confidence of the brick manufacturing market for ISSA, it is critical to ensure that the quality of the produced brick is not negatively influenced by the nature of ISSA or the performance of incineration operation [16]. Due to various resources of such wastes, and diverse processing operations executed on them, the composition of this type of wastes is broadly different. A mutual feature among these wastes is the high content of organic material, ranging from 30% to 60%. Utilizing ISSA in the brick manufacturing industry has the following advantages: 1. It has no negative influence on the wire-cutdextrusion shaping technique. 2. By increasing the strength of the dry phase, prevent it from possible damage during the transportation of unfired bricks. 3. By modifying the verification property, the same final features would be achieved at a lower firing temperature. 4. No negative impact on the product color or its texture. 5. Free-lime constitution of ISSA might help controlling the emission of fluoride from the kiln [31]. Eventually as mentioned earlier, it has significant positive environmental impacts including saving raw material and preventing the disposal of possibly polluting matters [14,45,46].

7.2

Olive pomace ash

The average oil content of olive is approximately 20% by weight. The remaining 80% of the fruit along with the extra added water forms olive pomace that contains a small fraction of remained oil. This residual oil is usually extracted using a solvent (such as hexane). After drying and processing the olive pomace, another solid waste is produced, which is called dry olive cake (orujillo in Spanish). This solid cake can be utilized as a fuel [8]. Due to its high calorific value, orujillo is conventionally sold as a fuel supply for small boiling equipment and furnaces. The significant production amount of orujillo, its HHV (about 18,800 kJ/kg), and its simple production procedure, resulted in considering orujillo as a medium-scale power plants (12e20 MWe) fuel [47]. However, a huge quantity of ash (around 4%e8% of the combusted orujillo) is produced as the final waste. The conventional disposal of these wastes is landfilling in the sites near the power plants; though as it is mentioned before, it is not an environmentally accepted way of disposing the wastes. Various studies in this field have proposed that this ash is a proper candidate for adding into construction materials [8]. The organic/ carbonaceous content of orujillo gasification ash, during the sintering stage, leads to the formation of some voids in the clay matrix. Besides, due to the considerable carbon

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content of this ash, energy consumption in the firing stage is reduced [13,48]. Here again, the composition of olive pomace fly ash is affected by the incinerator operating conditions and the composition of olive pomace [8,49].

7.3

Sugarcane bagasse ash

Combusting of bagasse in boilers of sugarcane/alcohol manufacturing factories remains a solid residue which is called sugarcane bagasse ash (SCBA). The sugarcane stems are cut and crushed for obtaining their juice. Bagasse is the remaining fibrous residue. Bagasse is combusted in boilers to supply the required steam for power plants and factories. Approximately the combustion of 250 kg of bagasse results in the production of about 6 kg of ash [35]. The sugarcane industry produces a huge quantity of ash, which is still developing and the produced amount of ash is supposed to constantly improve. Traditionally, SCBA was utilized as soil fertilizer, which is not the most environmentally attractive way of SCBA disposal. Due to new environmental legislation, it is preferential to develop recycling technology [33]. Utilizing SCBA as a feedstock for clay brick manufacturing appears to be an efficient recycling method for disposal of this waste. Beside improving waste disposal management, it also has a positive influence on natural resource preservation and environmental protection [33]. Even though, ceramic industry is proven to be a promising method for the removal of solid residue, information regarding reusing SCBA in clay ceramics is really limited. This lack of knowledge might be attributed to the fact that the main suppliers of sugar cane in the world are developing countries [33]. SCBA is an inexpensive substance containing a high amount of crystalline silica (SiO2) that has plasticity reducing effect. The results of leaching and dissolving tests indicated that SCBA is classified as an inert waste material. The addition of SCBA to the clay raw material would result in an increase in the water absorption and also it would decrease the mechanical strength of the clay. The addition of SCBA concentrations, higher than 10 wt.% is not recommended due to significant adverse effect on the mechanical strength. Like other biomass wastes, the chemical composition of different studied SCBA, is broadly various, mainly because of the soil and the growth condition [35].

7.4

River sediments

Each year huge quantity (several million tonnes) of dredged sediments are formed across the world [50]. The dredging process is essential to preserve natural maritime and river activities. These sediments normally have high water content and significantly poor mechanical strength. Furthermore, they might contain large quantities of organic and inorganic contaminations, particularly the sediments taken from toxic industrial places [51]. Dredged sediments cannot be utilized in construction applications without any treatment. Therefore, handling this huge volume of waste is a serious concern.

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Traditional methods of disposal of these wastes, that is, disposal in the ocean or landfill disposal has been limited due to new environmental legislation. Hence, finding new methods for reuse of these wastes is of great importance [19]. Dredged sediments, like all other biomass wastes, have different chemical and mineral composition, owing to the different origin and the generating conditions of the waste. Xu et al., studied the river sediment of Qinhuai River, China. They reported that due to its moderate organic content (8.6 wt.%), it can be used as pore former in the brick manufacturing procedure. This seems to be a potentially effective way of recycling such wastes [52]. The most serious environmental drawbacks of using urban river sediments as an additive to the clay bricks is the leaching of heavy metal. Therefore, leaching tests are obligatory for this category of biomass wastes [19].

8. Effects of biomass waste addition to the bricks raw material Advantages of using biomass wastes in fired clay brick might be summarized as follows: 1. 2. 3. 4.

A notable reduction in CO2 emission Preservation of natural resources, that is, clay raw material (and reduction of costs) Protection of environment by reduction of landfill disposal High organic contents of these wastes generate pores during sintering, leading to the production of a porous brick with low density and high thermal/sound insulation ability.

Energy-saving is due to high carbonaceous content and hence, the high heating value of wastes (and reduction of energy costs) [2,6,13]. However, the application of wastes has some drawbacks. As discussed in previous sections, the increase of water absorption and reduction of mechanical strength of finally produced brick limit the addition of biomass ash. The added quantity should be optimized in a way that satisfies the specified brick standard [6].

9. Brick production: process and methods The procedure for preparing brick samples significantly affects the characteristics of the produced brick. Hence, the exact procedure should be obviously explained by the researchers. In other words, obtaining a similar product with the same properties on the industrial scale is not possible [18]. Manufacturing methodologies for the brick samples often have the following steps:  Pretreatment and preconditioning (if needed)  Crushing, drying, sieving, powdering, etc.  Mixing with water (usually 10%e30%)

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 Shaping the bricks by pressing, extrusion, or molding. (The size of the reported brick samples in the literature are different.)  Drying at room temperature for 24 h or more, then in an oven of approximately 100
C for (2e48 h, till constant weight)  Firing in an electric furnace or an industrial tunnel kiln at the temperature range of 850e1100
C. Then, cooling at room temperature or in the oven [18]. The pretreatment stage often includes milling, washing, and drying; or milling, washing, drying, and micronizing [8]. Preconditioning of wastes might include some of the following items, for example, crushing, powdering, sieving, dissolving into water, grounding, drying, and homogenizing [18]. Mixing water is added for obtaining the desired plasticity. After the shaping stage, this water should be eliminated. The method of brick shaping has effect on the mixing water demand and consequently, the energy requirement for removal of this moisture in the drying stage. In industrial operations, the drying process includes multiple stages of tunnel drying. Moisture content and temperature should be determined in each drying stage. It is essential to dry the clay gradually because the volume and porosity of the clay alter while drying. Hence, these uncontrolled circumstances might cause the deforming of the brick, the formation of cracks, and/or efflorescence as a result of the presence of soluble salt. To prevent these troubles, in the brick manufacturing factories, the drying phase begins from high relative humidity RH and low temperature. The process ends at RH around 5%, and the final temperature of this stage is about 100
C [18]. In the utilization of ashes to the brick raw material, it should be considered that free calcium ions of clay are present for a pozzolanic reaction with biomass ash. The reaction is slow and definitely speeds up by temperature. In the shaping stage by the extrusion method, this can make trouble: plasticity loss and hardening lead to denoting of the extruded strand. It is essential to prevent or postpone this. Adding ashes might be helpful, as it is used for polystyrene, for instance, exactly before the extrusion. Another solution is hydrophobizing the ash by a proper agent. Two appropriate agents regularly used in brick manufacturing are the following: waste glycerine supplied from biodiesel production or waste antifreeze coming from the car industry [12]. The thermal heating stage (firing the brick at the temperatures between 800 and 1000
C) leads to the destruction of organic residue and fixing of inorganic material and metals by the inclusion of oxides of elemental component into ceramic-like substance [53]. Firing is the last stage in brick production. The final brick property is highly influenced by temperature and time of exposure [54]. In this step, two basic considerations should be taken: the firing process and then the cooling zone. Modern brick manufacturing factories utilized tunnel kiln. Bricks are located on wagons and passed through multiple stages of progressive heating and then, progressive cooling. The time of exposure is specified by the speed of the wagon [18].

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9.1

Different methods of brick making

Based on the method of production, three categorizations are defined as firing, cementing, and geopolymerization. 1. The firing method is almost identical to the conventional method of brick production. Hence, applying this method does not require principal changes in the customary brick production equipment. On the other hand, firing pollutants in the waste substances might be liberated and lead to the generation of new contamination. Besides, this method is extremely energy-consuming and causes the emission of a great amount of greenhouse gases. Thus, the firing method is not in accordance with new environmental regulations. 2. Brick manufacturing through the method of cementing does not require firing but depends on the cementing from waste material [9]. The cementing method is relied on hydration reactions to create particularly CeSeH and CeAeSeH bonds that provide strength. The cementing substance can be either the waste itself or other added cementing compounds, like OPC and lime. Due to the high energy requirement and a great quantity of greenhouse emission for the production of cementing materials, namely OPC and lime, this method also has the same disadvantages of the firing method. In the case of self-cementing of wastes, high content of calcium is required (like class C fly ash). To enhance the kinetics of the reaction, the process of curing is carried out in an autoclave, under pressurized steam at the temperature of 125e200
C. This process condition raises the production costs [9]. 3. The method of geo-polymerization is based on the recent geo-polymerization technique that is not similar to regular cementing technology used in OPC concrete. While ordinary cementing relies on the existence of CeSeH and CeAeSeH compounds for creating matrix and strengthening, this new method is based on poly-condensation of silica and alumina as precursors and also high alkali fraction to achieve the construction strength. Manufacturing of bricks using geopolymerization method requires much lower energy and liberates far less quantities of greenhouse gases compared to firing and cementing bricks. Moreover, the resulted geopolymer bricks, have desirable physical and chemical characteristics. Usage of alkali solution induces additional costs to the system. Moreover, curing at moderate temperature conditions (which is required in some cases) brings other extra costs. However, geopolymerization is introduced as a “green” production method, and there are not many exact assessments on the environmental effects of this method [9,55,56].

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9.2

423

Brick analysis

After preparation of brick samples, they must go through a series of tests to identify physical properties (weight loss on ignition, firing shrinkage (linear shrinkage), water absorption, bulk(apparent) density, and water suction) and mechanical properties (compressive strength) of the bricks comparing with a certain standard to determine the quality of the brick [6,53]. Constant monitoring of the quality and composition of the waste utilized in brick production is critical, to be informed of any irregularity in a composition that might lead to negative effects on the quality of the final product. Long-term manufacturing tests are crucial for determining the final differences in the composition and the effect of these changes on the quality of product or manufacturing procedure [12]. Some necessary information for examining the potential waste substance for addition to the clay body for brick production is given in the following: Oxides and heavy metals including: aluminum oxide, sulfur trioxide, antimony oxide, barium oxides, boron trioxide, calcium oxide, iron oxide, phosphorus pentoxide, manganese oxide, lead oxide, magnesium oxide, potassium oxide, silica, sodium oxide, titanium dioxide, zinc oxide, zirconium oxide, arsenic, cadmium, chromium, mercury, nickel, copper, selenium, lead, zinc, and vanadium. Other characteristics such as humidity (H2O%), acidity (pH), not combusted (%), conductivity, dry substance, chloride (%), sulfur (%), fineness (percentage clay minerals if any), plasticity, firing temperature, mechanical characteristic at various firing temperatures. Finally, the mineralogy, which is defined as kaolinite (%), smectite (%), illite (%), muscovite (%), mica (%), fluor (%), and chloride (%) [12].

9.2.1 Water absorption rate of the bricks The rate of adsorption of water, that identifies as the proportion of the weight of moisture trapped in the pores to the weight of the cured sample is an efficient index of brick quality. In fact, water absorption indicates the volume fraction of open pores. The lower the water permeates the brick, the more durable it would be. The addition of biomass wastes usually results in increasing in absorbed water [33,53].

9.2.2 Weight loss on ignition of bricks This term is referred to as the reduction of weight of a monolith while sintering. This weight loss is attributed to porosity improvement and also densification, and has an impact on the compressive strength of the cured specimen. The weight loss ignition for an ordinary clay brick is approximately 15% [53].

9.2.3 Shrinkage of bricks Another quality control index for bricks is firing shrinkage. This index examines the amount of brick shrinkage after sintering steps and facilitates comparing it with standards requirements. For example, in some regions, it should not exceed 8% [53].

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9.2.4 Bulk density of bricks In the sintering process, a large number of open and blocked pores are created. The minimum density is related to the maximum volume of blocked pores in the specimen. Densification is a pore-filling process that occurs during the liquid-phase flow and by pore shrinkage [57]. The bulk density of a normal clay brick is around 1.8e2.0 g/cm3. Usually, it is observed that by increasing the firing temperature, the bulk density of the samples increases [53].

9.2.5 Compressive strength of bricks The most important engineering quality index for construction substances is compressive strength. This index should meet the specified standard requirements [53].

9.2.6 Leaching tests Waste materials contain various kinds of contamination, toxic material, and heavy metals. To utilize wastes in the brick manufacturing process, it is essential to make sure that these contaminants are fixed and immobilized within the bricks. Especially, in the case of utilizing river sediments in brick production, leaching of heavy metals is the most important environmental concern [9,19]. In the leaching test, the crushed fired brick samples (Smaller than 10 mm) are subjected to a solid waste extraction process. The concentration of heavy metals (i.e., As, Cd, Cr, Cu, Hg, Ni, Pb, Zn, etc.) and other contaminants in the leachates are then specified. Leaching analyses then would be conducted according to USEPA, ASTM, and other standards to ensure that the leached components fulfill the standard requirements [9,19].

9.2.7 Color Color is a critical aesthetic property for construction material like face brick and the one used in fac¸ade design. The results indicate that the sintering temperature greatly influences the produced brick color. The higher the temperature, the darker the red color of the brick [8]. First, the maximum quantity of waste (here, fly ash) that does not considerably alter the color of brick is determined by using colorimetric methods; then other properties of the samples, durability, and petrophysical features will be estimated [11].

9.2.8 Thermal conductivity Thermal conductivity is another substantial feature of fired clay masonry units. When bricks are modified by an appropriate process, they can have a considerably lower thermal conductivity that results in lower energy loss through the walls of the building. This lower conductivity is attributed to the formation of pores within the bricks. The addition of organic and inorganic pore-forming waste additives before sintering leads to having lower thermal conductivity [15,58]. Mineralogical composition, nature of the pores and textural properties of brick, firing temperature, etc. highly affect the thermal

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conductivity of brick. Usually, the fired bending strength of bricks is also proportional to the thermal conductivity of the brick: the higher the thermal conductivity, the higher the bending strength [8]. Although, insulation is of great importance in the building, which provides indoor comfort, there are not many studies focusing on this area, that is, thermal conductivity or acoustic response of fired brick clays produced by additives [18].

10. Commercialization From a commercial perspective, providing and considering some other performance index such as color, efflorescence, and rugosity are required for transferring the research into an industrial scale. Producer point of view, it is important to know how biomass waste additives have an impact on other manufacturing parameters. Hence, papers should specify linear shrinkage, bulk density, and weight losses [18]. Although a broad range of biomass wastes have been investigated for the production of brick, there are limited industrial cases of manufacturing bricks from waste material. The probable causes of this limitation might be the following: the methods of brick manufacturing, contamination of the waste utilized, lack of appropriate standards, slow agreement of industry and public with waste substance-based brick. Additional research and development are required for large-scale production and utilization of such bricks. These research works should be on the technological, economic, and ecological aspects, as well as development in standardization, modification on government policy, and public awareness regarding waste recycling and reuse [9]. However, other possible reasons according to limited reuse of biomass ash in industrial brick manufacturing may be related to the following: the efficiency of biomass combustion, problems due to supply, delivering and treatment (drying, grinding, etc.) of adequate quantity of biomass, cost, and technological restrictions [5]. Moedinger pointed out some problems during the industrial examination of fired brick made by the addition of biomass ash: the ashes are not delivered in a proper acceptable way that can be easily approved by the brickyard with no need to significant modifications in the present facilities of the unit. They are mostly delivered in big pockets. If they were delivered using trucks, a silo could be established in the unit for storing them. The other challenge that he pointed out is the seasonality of ashes. Brickyards do not have the capacity for storage bunker to tackle this problem. Once the ash producer found a solution for these problems, brickyards can become a perfect consumer of these wastes [12].

11. Conclusion and future aspects As it is obvious from the various studies related to biomass ash-based bricks, the physicochemical compositional variations and mineral phases differences among fly ash and the clay matrix, lead to a difference in firing properties, efficiency, and mechanism of fly ash-clay bricks and the conventional bricks [13].

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The addition of fly ash has some impact on the characteristic of the final brick, the most important ones are the following: 1. 2. 3. 4.

A considerable increase in mixing water required for the extrusion. Medium drying linear shrinkage. Decrease in the compressive strength A significant increase in the water absorption relating to a greater porosity of fly ash-based bricks. 5. A notable decrease in bulk density. 6. A remarkable reduction in the dry and fired bending strengths of the samples. 7. Firing linear shrinkage increases with the temperature [13,18]. Many researchers have studied the procedure and effects of the addition of biomass wastes/ash to the bricks. Although the compositions of the clays were the same, due to differences between the experimental brick manufacturing process with the real industrial process, one cannot assure the same effects on the industrial scale: Factories utilize multiple successive drying and sintering stages, under controlled circumstances. The manufacturing operations include progressively rising and reducing both relative humidity and temperature. Only a few researchers have conducted their production following these procedures [18]. Despite this fact, the results of various researches, allow us to be optimistic regarding the use of biomass ash to produce bricks. The addition of biomass wastes for brick production is an environmentally benign process, leading to reduction of greenhouse gas emission, reduction of the energy consumption of brick factories, preservation of natural resources (i.e., clay material resources), reduction in the cost of raw material, decreasing the challenges due to landfill disposal and so on. Moreover, the produced brick has higher insulating abilities, lower density, and probably lower prices. However, still further studies are required. Studies with a more similar procedure to industrial manufacturing studies to examine some properties that have been rarely reported such as thermal conductivity and acoustic response. The next step regarding this subject is convincing both industries and public, to consider this type of bricks as an acceptable building material. This is accomplished through developing policies to raise public awareness regarding recycling necessity and its advantages [9,13,18,59].

References [1] Eliche-Quesada D, Leite-Costa J. Use of bottom ash from olive pomace combustion in the production of eco-friendly fired clay bricks. Waste Manag 2016;48:323e33 (New York, N.Y.). [2] Berra M, Mangialardi T, Paolini AE. Reuse of woody biomass fly ash in cement-based materials. Construct Build Mater 2015;76:286e96. [3] Vassilev SV, Baxter D, Andersen LK, Vassileva CG. An overview of the composition and application of biomass ash. Fuel 2013;105:19e39.

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[4] Berra M, Casa Gd, Dell’Orso M, Galeotti L, Mangialardi T, Paolini AE, et al. Reuse of woody biomass fly ash in cement-based materials: leaching tests. In: Insam H, Knapp BA, editors. Recycling of biomass ashes. Berlin, Heidelberg: Springer Berlin Heidelberg; 2011. p. 133e46. [5] Cabrera M, Galvin AP, Agrela F, Carvajal MD, Ayuso J. Characterisation and technical feasibility of using biomass bottom ash for civil infrastructures. Construct Build Mater 2014;58:234e44. [6] Pe´rez-Villarejo L, Eliche-Quesada D, Iglesias-Godino FJ, Martı´nez-Garcı´a C, Corpas-Iglesias FA. Recycling of ash from biomass incinerator in clay matrix to produce ceramic bricks. J Environ Manag 2012;95(Suppl. l):S349e54. [7] Pels JR, de Nie DS, Kiel JH. Utilization of ashes from biomass combustion and gasification. In: 14th European biomass conference & exhibition; 2005. p. 17e21. [8] La Casa JAd, Castro E. Recycling of washed olive pomace ash for fired clay brick manufacturing. Construct Build Mater 2014;61:320e6. [9] Zhang L. Production of bricks from waste materials e a review. Construct Build Mater 2013;47: 643e55. [10] Warren J. Conservation of brick. 1999. [11] Cultrone G, Sebastia´n E. Fly ash addition in clayey materials to improve the quality of solid bricks. Construct Build Mater 2009;23:1178e84. [12] Moedinger F. The use of biomass combustion ashes in brick making. In: Insam H, Knapp BA, editors. Recycling of biomass ashes. Berlin, Heidelberg: Springer Berlin Heidelberg; 2011. p. 121e32. [13] Ferna´ndez-Pereira C, La Casa JAd, Go´mez-Barea A, Arroyo F, Leiva C, Luna Y. Application of biomass gasification fly ash for brick manufacturing. Fuel 2011;90:220e32. [14] Dondi M, Marsigli M, Fabbri B. Recycling of industrial and urban wastes in brick production- a review. Tile Brick Int 1997;13:218e25. [15] Gencel O. Characteristics of fired clay bricks with pumice additive. Energy Build 2015;102:217e24. [16] Anderson M, Skerratt RG. Variability study of incinerated sewage sludge ash in relation to future use in ceramic brick manufacture. Br Ceram Trans 2003;102:109e13. [17] Yang Y, Cho YL. Plasma discharge in liquid: water treatment and applications. Boca Raton: Taylor & Francis; 2012. [18] Mun˜oz Velasco P, Morales Ortı´z MP, Mendı´vil Giro´ MA, Mun˜oz Velasco L. Fired clay bricks manufactured by adding wastes as sustainable construction material e a review. Construct Build Mater 2014;63:97e107. [19] Xu Y, Yan C, Xu B, Ruan X, Wei Z. The use of urban river sediments as a primary raw material in the production of highly insulating brick. Ceram Int 2014;40:8833e40. [20] Vassilev SV, Baxter D, Andersen LK, Vassileva CG. An overview of the composition and application of biomass ash. Part 1. Phaseemineral and chemical composition and classification. Fuel 2013;105: 40e76. [21] Vassilev SV, Baxter D, Andersen LK, Vassileva CG, Morgan TJ. An overview of the organic and inorganic phase composition of biomass. Fuel 2012;94:1e33. [22] Vassilev SV, Baxter D, Andersen LK, Vassileva CG. An overview of the chemical composition of biomass. Fuel 2010;89:913e33. [23] Anderson M, Jackson G. The beneficiation of power station coal ash and its use in heavy-clay ceramics. Trans J Br Ceram Soc 1983;82:50e5. [24] Cicek T, Tanrıverdi M. Lime based steam autoclaved fly ash bricks. Construct Build Mater 2007;21: 1295e300. [25] Baykal G, Do¨ven AG. Utilization of fly ash by pelletization process; theory, application areas and research results. Resour Conserv Recycl 2000;30:59e77.

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[26] Nishida K, Nagayoshi Y, Ota H, Nagasawa H. Melting and stone production using MSW incinerated ash. Waste Manag 2001;21:443e9. [27] Hjelmar O, Holm J, Crillesen K. Utilisation of MSWI bottom ash as sub-base in road construction: first results from a large-scale test site. J Hazard Mater 2007;139:471e80. [28] Astrup T. Pretreatment and utilization of waste incineration bottom ashes: Danish experiences. Waste Manag 2007;27:1452e7. [29] Po¨ykio¨ R, Ro¨nkko¨ma¨ki H, Nurmesniemi H, Pera¨ma¨ki P, Popov K, Va¨lima¨ki I, et al. Chemical and physical properties of cyclone fly ash from the grate-fired boiler incinerating forest residues at a small municipal district heating plant (6 MW). J Hazard Mater 2009;162:1059e64. [30] Vieira C, Monteiro S. Incorporation of solid wastes in red ceramics: an updated review. Materia 2009;14:881e905. [31] Anderson M. Encouraging prospects for recycling incinerated sewage sludge ash (ISSA) into claybased building products. J Chem Technol Biotechnol 2002;77:352e60. ´ , Eliche-Quesada D, Corpas-Iglesias FA, Lo´pez-Galindo A. [32] La Rubia-Garcı´a MD, Yebra-Rodrı´guez A Assessment of olive mill solid residue (pomace) as an additive in lightweight brick production. Construct Build Mater 2012;36:495e500. [33] Faria KCP, Gurgel RF, Holanda JNF. Recycling of sugarcane bagasse ash waste in the production of clay bricks. J Environ Manag 2012;101:7e12. [34] Teixeira SR, De Souza AE, de Almeida Santos GT, Vilche Pena AF, Miguel AG. Sugarcane bagasse ash as a potential quartz replacement in red ceramic. J Am Ceram Soc 2008;91:1883e7. [35] Souza AE, Teixeira SR, Santos GTA, Costa FB, Longo E. Reuse of sugarcane bagasse ash (SCBA) to produce ceramic materials. J Environ Manag 2011;92:2774e80. ˜ ez LM. Sugar-cane bagasse ash (SCBA): [36] Paya´ J, Monzo´ J, Borrachero MV, Dı´az-Pinzo´n L, Ordo´n studies on its properties for reusing in concrete production. J Chem Technol Biotechnol 2002;77: 321e5. [37] Tay J-H, Show K-Y. Utilization of municipal wastewater sludge as building and construction materials. Resour Conserv Recycl 1992;6:191e204. [38] Wiebusch B, Ozaki M, Watanabe H, Seyfried CF. Assessment of leaching tests on construction material made of incinerator ash (sewage sludge): investigations in Japan and Germany. Water Sci Technol 1998;38:195. [39] Qi Y, Yue Q, Han S, Yue M, Gao B, Yu H, et al. Preparation and mechanism of ultra-lightweight ceramics produced from sewage sludge. J Hazard Mater 2010;176:76e84. [40] Teixeira S, Santos G, Souza A, Alessio P, Souza S, Souza N. The effect of incorporation of a Brazilian water treatment plant sludge on the properties of ceramic materials. Appl Clay Sci 2011;53:561e5. [41] Devant M, Cusido´ J, Soriano C. Custom formulation of red ceramics with clay, sewage sludge and forest waste. Appl Clay Sci 2011;53:669e75. [42] Cusido´ JA, Cremades LV. Environmental effects of using clay bricks produced with sewage sludge: leachability and toxicity studies. Waste Manag 2012;32:1202e8. [43] Liew AG, Idris A, Wong CH, Samad AA, Noor MJM, Baki AM. Incorporation of sewage sludge in clay brick and its characterization. Waste Manag Res 2004;22:226e33. [44] Weng C-H, Lin D-F, Chiang P-C. Utilization of sludge as brick materials. Adv Environ Res 2003;7: 679e85. [45] Churchill W. Aspects of sewage sludge utilisation and its impact on brickmaking. Br Ceram Trans 1994;93:161e4. [46] Slim JA, Wakefield RW. The utilisation of sewage sludge in the manufacture of clay bricks. Water S. A. 1991;17(3):197e202.

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[47] Go´mez-Barea A, Arjona R, Ollero P. Pilot-plant gasification of olive stone: a technical assessment. Energy Fuels 2005;19:598e605. [48] Lingling X, Wei G, Tao W, Nanru Y. Study on fired bricks with replacing clay by fly ash in high volume ratio. Construct Build Mater 2005;19:243e7. [49] Nogales R, Delgado G, Quirantes M, Romero M, Romero E, Molina-Alcaide E. Characterization of olive waste ashes as fertilizers. In: Recycling of biomass ashes. Springer; 2011. p. 57e68. [50] Dubois V, Abriak NE, Zentar R, Ballivy G. The use of marine sediments as a pavement base material. Waste Manag 2009;29:774e82. [51] Baruzzo D, Minichelli D, Bruckner S, Fedrizzi L, Bachiorrini A, Maschio S. Possible production of ceramic tiles from marine dredging spoils alone and mixed with other waste materials. J Hazard Mater 2006;134:202e10. [52] Xu Y, Zhou Y, Ma W, Wang S. A highly sensitive and efficient Fe3O4@ SiO2 nanoparticles chemosensor for Cu2þ removal. Integr Ferroelectr 2013;147:110e4. [53] Lin KL. Feasibility study of using brick made from municipal solid waste incinerator fly ash slag. J Hazard Mater 2006;137:1810e6. [54] Pinto S, Almeida M, Correia A, Labrincha J, Ferreira V, Rosenbom K. Study on the environmental impact of lightweight aggregates production incorporating cellulose industrial residues. In: Proceedings of international RILEM conference on the “use of recycled materials in buildings and structures. Barcelona (Spain): RILEM publications SARL; 2004. p. 771e7. [55] Habert G, De Lacaillerie JDE, Roussel N. An environmental evaluation of geopolymer based concrete production: reviewing current research trends. J Clean Prod 2011;19:1229e38. [56] Pacheco-Torgal F, Abdollahnejad Z, Camo˜es A, Jamshidi M, Ding Y. Durability of alkali-activated binders: a clear advantage over Portland cement or an unproven issue? Construct Build Mater 2012;30:400e5. [57] Nowok JW, Benson SA, Jones ML, Kalmanovitch DP. Sintering behaviour and strength development in various coal ashes. Fuel 1990;69:1020e8. [58] Rimpel E, Rehme F. Development of extruded high-thermal insulating bricks. ZI Int 2001;54:36e41. [59] Monteiro SN, Vieira CMF. On the production of fired clay bricks from waste materials: a critical update. Construct Build Mater 2014;68:599e610.

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Biomass technologies industrialization and environmental challenges Sareh Hamidpour, Niloufar Fouladi, Mohammad Amin Sedghamiz, Mohammad Reza Rahimpour DEPARTMENT OF CHEMICAL ENGINEERING, SHIRAZ UNIVERS ITY, SHIRAZ, F AR S, IRAN

1. Introduction Limited fossil resources and the unavailability of these supplies have led to the policy of energy investors and planners’ structural studies, changing energy carriers, and moving toward cleaner fuels are at the top of their list. One of these options is to use energy carriers from biomass resources. Many countries in the world consider the decent usage of biomass supplies. The application of biomass as a source of power is not only concerning economic reasons (where the fuel to it is not simply and cheaply available), but it is also attractive due to its hygienic and environmental development. Energy production technologies convert biowaste into energy products. Energy production is presented in the form of five major categories: direct combustion, combustion, gasification, anaerobic digestion, and fermentation. Technologies of energy production from biomass resources are classified into two main groups, namely chemical heating and biological techniques. Choosing biomass technology is influenced by three factors: accessibility to raw materials, final use, and cost. The extensive and conducive business application needs new technology development and deployment to enable the masses to compete with traditional energy sources.

2. Technology overview There are numerous conversion methods for producing biomass energy carriers. Fig. 18.1 shows the significant conversion pathways used or under progression for heat, electricity, and transport fuel production. Furthermore, conversion technologies for the generation of power and heat will be examined, followed by technologies available or developed for the production of carrier fuels (fermentation, gasification, and liquefaction), distinguishing current and potential availability. Advances in Bioenergy and Microfluidic Applications. https://doi.org/10.1016/B978-0-12-821601-9.00018-2 Copyright © 2021 Elsevier Inc. All rights reserved.

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FIGURE 18.1 The basic processes of thermal energy conversion.

2.1

Thermochemical conversion

In this section, the thermochemical conversion of biomass is described.

2.1.1 Combustion Biomass combustion is the most well-known biomass conversion technology used at the household and industrial level since ancient times. In the last decades, however, advanced technologies for the combustion of biomass have been developed as entirely automated pellet boilers, cofired, and effective combination of heat and power generation (CHP) for a wide range of biomass supplies. Wood, when available nearby, is a moderately cheap source of energy for stoves and boilers. Otherwise, transportation expenses would typically be restrictive. A significant benefit of wood is that its net carbon dioxide emissions are very low, about 8 kg per MWh of useful energy, compared to 240 kg/MWh for natural gas and 490 kg/MWh for oil. The biggest downside to residential boilers and stoves is that, as seen in Table 18.1, using wood results in far more emissions than using oil or natural gas. A study of the large-scale use of wood pellets for heating in Sweden using community boilers found that it is crucial to monitor pollutant dispersion to ensure that their concentration at critical locations is not too high. As shown in Table 18.1, both boiler design and wood nature have a significant influence on pollutant emissions [1]. Pellets made of sawdust that can be added to wood shavings have clear fuel features and help ensure more normal combustion [1]. Therefore, consistent wood properties are essential for mitigating pollution. The European Standardization Committee shall issue technical requirements for woody feedstocks (and other feedstocks with biomass) [2]. However, the classification is kept versatile because of the extensive variety of woody feedstocks [3]. The combustor

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Table 18.1

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Emissions from residential boilers (in mg/MJ of produced energy).

Product

Old wood boiler

New pellet boiler

Oil boiler

CO CH4 Other volatile organic carbon (VOC) Polycyclic aromatic hydrocarbons (PAH) Particles

5000 700 400 15 100

500 0.5 2 2 15

5 0.5 0.5 0.01 10

structure also has significant impacts on the emissions of contaminants [4]. Heat transferred from the combustion chamber dries the pellets, as wet pellets will significantly increase emissions of contaminants, and the feeder for the screw can be controlled to feed the pellets as required. An additional problem with residential and community heaters and boilers is that they do not respond well to fluctuating thermal demand, resulting in far higher emissions of pollutants. Therefore, it is necessary to have some heat storage to smooth out fluctuations in heat charges [5]. 2.1.1.1 Industrial applications One of the residential boiler issues is that they are not provided with particle separators capable of recuperating a significant fraction of the particulates, which are mainly submicron particles [6]. Industrial boilers, on the other hand, afford much more efficient recovery of particulate matter. Nevertheless, one big pollution issue with industrial boilers is the release of dioxins found both in the particulate and gas phases. Emissions of dioxins are much higher when wood trash is burned than when fresh wood is burned [7]. Emissions of dioxins are much higher when wood trash is burned than when fresh wood is burned [7]. This has inference for the combustion of wood from construction and destruction sites. The formation of dioxins can be prevented or reduced by the addition of chemical inhibitors and suitable combustor design [7]. Fouling is a severe operational problem with industrial wood burners, resulting in low wood ash sintering level. Decreasing the temperature of the combustor negatively affects thermodynamic efficiency and increases the emission of most pollutants. Ash from most woods will sinter at around 1000
C, with bark ash sintering at 850
C [8]. Other than woody feedstocks, some industrial boilers used biomass fuels. Combusting agricultural residues usually increase pollution, corrosion, and fouling. Attribute these issues to high levels of nitrogen, chlorine, and sulfur in straws, cereals, grasses, grains, and residues of fruit relative to wood, resulting in higher nitrogen oxide, hydrogen chloride, dioxins, and sulfur oxides emissions. The increased fouling observed arising from its ash’s lower sintering temperature when burning agricultural biomass. Also, the more under sintering temperature of the ash from most farming residues creates problems when cofired with coal [2]. Biomass applied to coal increases fouling and the chance of slagging [9]. This

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has been observed with straw, sewage sludge, and meat and bone meal [10]. To reduce the risk of slagging, the temperature of the combustor must be reduced to about 800
C, which reduces thermodynamic efficiency [3]. The higher chlorine content of agricultural biomass also causes problems with corrosion [10e12]. Although wood ash generally has a higher sintering temperature, when cofired wood and coal, wood alkali combines with coal sulfur to increase fouling and risk of slagging. 2.1.1.2 Partial combustion The conversion of biomass into gas is highly endothermic. There are two ways to supply the required heat of reaction: by carrying heat from an external burner where byproducts such as charcoal are burned, or by direct injection of oxygen to the combustion reactions within the gasifier. Steam can also be inserted via the water gas shift reaction to regulate the temperature and increase the hydrogen value of the gas. Injecting a mixture of pure oxygen and steam gives a gas with a heating value of 10e18 MJ/Nm3, while natural gas has a heating value of 33e42 MJ/Nm3 [13]. As pure oxygen is expensive, the air is used, but the added nitrogen significantly reduces the product gas heating value to 4e7 MJ/Nm3 [13]. When air is used, the product gas typically contains more than 50 vol.% of nitrogen and about 10 vol.% of carbon dioxide, which means that less than 40 vol.% is fuel gases, primarily carbon monoxide and hydrogen. The cost of oxygen is prohibitive for electric generation from biomass, and the air is used. The low heating value gas can then be burned to produce steam, directly combusted in a turbine, or sent to a modified diesel generator for smaller-scale applications. Even though turbines or engines are more effective than steam generation systems, their safety and performance are compromised even by deficient concentrations of tars and particulate matter in the gas. There are two main classes of gasifiers: fixed beds and fluidized beds. The poor radial transfer of heat in fixed beds restricts their usage to small gasifiers because with larger gasifiers, maintaining a sufficiently high temperature for gasification anywhere in the reactor will result in hot spots where slagging may occur, particularly with feedstocks of biomass that are more sensitive to slag than coal. Fluidized bed gasifiers have a uniform temperature and substantially reduce the slagging chance [12].

2.1.2 Gasification Gasification provides the benefit of generating a homogeneous, intermediate fuel for the secondary conversion of solid inhomogeneous fuel. Gasification of biomass is an endothermal conversion operation that transforms a solid fuel into a burnable gas. The oxidizing agent acts as a small source of oxygen, air, steam, or a combination. Product gas comprises CO, CO2, H2, CH4, bit concentrations of other hydrocarbons, for example, ethene, ethane, and nonhydrocarbons of water, nitrogen, and other pollutants, that is, char, ash, tars, higher hydrocarbons, alkali, ammonia, acids, etc. [14]. Gasification of biomass comprises a convoluted set of reactions, as presented in Fig. 18.2, a great recognition of the fundamental reactions of biomass gasification is

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FIGURE 18.2 Basic biomass gasification chemistry [16].

prime to the other steps of gasification set up. In a normal proceeding method, the subsequent steps regularly occur, which include drying, pyrolysis, char, and tar gasification. A part of biomass is converted toward condensible compounds during the pyrolysis. Hereafter, the gasifier has a reaction series, as well as a homogeneous reaction of gas-phase and a heterogeneous char gasifying reaction of gasesolid. Partial and full combustion, in addition to wateregas shift and hydrogasification, are performed on the char compound, which includes the addition of hydrogen into carbon to generate energy with more eminent hydrogen into carbon (H/C) degree. Table 18.2 summarizes the reactions of gasification. Before the pyrolysis, the biomass is preheated at 100e200
C. In the pyrolysis step, a portion of the carbon is eliminated without increasing the hydrogen content. It occurs in the range of 200e700
C at relatively low temperatures without using a gasifier. A part of biomass is converted toward condensible compounds during the pyrolysis. Hereafter, the gasifier has a reaction series, as well as a homogeneous reaction of gas-phase and a heterogeneous char gasifying reaction of gasesolid. Partial and full combustion, in addition to wateregas shift and hydrogasification, are performed on the char compound, which includes the addition of hydrogen into carbon to generate energy with more eminent H/C degree. Volatiles go through oxidation, steam reforming, and cracking during gas-phase gasification reactions. The wateregas moving reaction is of prominent significance, as

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Table 18.2

Primary chemical reactions for gasification of biomass [15]. DH298 ; KJ=mol

Reaction Pyrolysis yields

Biomass ƒ! char þ tar þ H2 O þ light gas ðCO þH2 þCO2 þCH4 þC2 þ.Þ

DH298 < 0

Char combustion yields

C þ 0:5 O2 ƒ! CO yields C þ O2 ƒ! CO2

111 394

Char gasification yields

C þ CO2 ƒ! 2CO yields C þ H2 O ƒ! CO þ H2 yields C þ 2H2 ƒ! CH4

172 131 75

Homogeneous volatile oxidation yields

CO þ 0:5O2 ƒ! CO2 yields H2 þ 0:5O2 ƒ! H2 O yields CH4 þ 2O2 ƒ! CO2 þ 2H2 O yields CO þ H2 O ƒ! CO2 þ H2 yields CO þ 3H2 ƒ! CH4 þ H2 O Tar reactions n yields m Cn Hm þ O2 ƒ! nCO þ H2 2 2  m yields þn H2 Cn Hm þ nH2 O ƒ! nCO þ 2 m yields  m H4 ƒ! nCO þ n  C Cn Hm þ 4 4

254 242 283 41 88

DH298 < 0

yields

Cn Hm þ ð2n mÞH2 ƒ! nCH4

it performs an essential task in hydrogen generation [15]. The methanation reaction always continues without any catalyst. Above all, the product gas from gasification can be seen as a mixture consisting mostly of hydrogen, carbon dioxide, carbon monoxide, water vapor, and methane. 2.1.2.1 Fixed bed gasifier Fixed bed gasifiers are classified in different types on the basis of entering the mechanism of the gasifying compounds into the reactor. Gasifiers are categorized into the updraft, downdraft, cross draft, and stage gasifier. The gasifiers of downdraft type are Imbert and open core form. Different sources of compounds for gasifying, for example, air, steam, and O2, could be used in gasifiers. The exhaust gas might be used in thermal or engine applications. Operating conditions, biomass type, and gasifier conditions might affect the produced gas composition and contamination level [17e19]. Gasifiers of updraft type are proper for gasifying biomass with considerable ash (about 15%) and moisture value (about 50%) and for producing gas with eminent tar value (50e100 g/Nm3). The biomass

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air gasification with high temperature (830
C) in gasifiers of updraft type enhances the lower thermal amount of the product gas and decreases the tar value. The gasifiers of updraft type were applied as the gasifying media for bark, wood chunks, chips and pellets, hay, maize cobs, refuse-derived fuel, and pellets of waste with air and O2. Imbert downstream gasifiers are proper for fuel from biomass with ash and moisture contents of fewer than 5% and 20%, sequentially. Changes in Imbert downdraft gasifier grate and hopper configuration have been proposed for nonwoody biomass gasifying, which include coir dust, stalks of cotton, straw of wheat, shells of hazelnut, residues of leather, sludge, etc. Gasifiers of downdraft type give less tar content from the producer gas (1e2 g/Nm3) relative to gasifiers from the updraft. Gasifiers of throatless downdraft type were generated in Imbert downdraft gasifiers to defeat the bridging and channeling problems. The throatless gasifiers were effectively used to gasify husk of rice, chips of wood, bagasse, the pulp of sugarcane, shells of coconut, etc. developing insulation of gasifier, producer gas recirculation, and different air dispersal have been proclaimed to improve the efficiency of the gasifiers of throatless type and the value of tar to 50e250 mg/Nm3. The biomass pyrolysis and gasification occur in detached chambers in two-stage gasifiers directing in low-tar (15e50 mg/Nm3) product gas. 2.1.2.2 Updraft gasifier It is the gasifier utilized most generally concerning biomass and materials of solid waste. A primary gasifier of the updraft type is described in Fig. 18.3. Within a specific upstream gasifier, the biomass is fed downward, and also the content passes down over the drying, pyrolysis, reduction, and oxidization of the char region. Air is inserted into the bottom of the gasifier through a grate that has a bed for the feedstock and char. The syngas being generated flows upward, egressing at the top of the gasifier. Properties of updraft gasifiers are as follows:       

Up to about 150 tons/day of feed rates The feed is preheated and dried before the stage of pyrolysis and gasification Practical for humidity content (around 50%) of feedstock and waste The source of heat to power the gasification reactions is partial carbon oxidation. Performs at 1450e2000
F, with syngas leaving at 500e1400
F Tar with high quantity in syngas High efficiency

2.1.2.3 Downdraft gasifier The cocurrent flow of the air and the syngas with feedstock occur in the downdraft gasifier. Downdraft gasifiers are planned primarily to reduce tar products. The volatile gases produced within the pyrolysis region are oxidized partly in the oxidation area, wherever the air is inserted. The oxidation of volatiles produces the heat required for the reactions of gasification. During this high-temperature region, usually, 1500e2200
F, the tars are thermally cracked and partly oxidized. This applies a filtration impact to assist in scrubbing syngas before exiting the gasifier. The typical conversion of tar is more than

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FIGURE 18.3 Schematic of updraft gasifier.

99%. The hot char within the region of reduction reduces CO2 and H2O in the syngas flow to CO and H2, which are the first species of syngas. The temperature of exit syngas is usually 100e1500
F. Downdraft gasifiers get similar overall limitations as the updraft gasifiers on feedstock properties (Fig. 18.4). The biomass requires a relatively uniform distribution of particle size with scant fines, to preserve the physical features of the bed and decrease channeling. To sustain the tar cracking high temperature, the feedstock humidity content must be less than about 20%. To avoid slagging, the biomass must have a low value of ash and high temperatures of ash fusion. Properties of downdraft gasifiers are:     

Feed rate capacity of 65 tons/day Low humidity requirement ( 10e200
C/s, residence time ¼ 0.5e10 s). In the pyrolysis process of flash type, 103e104
C/s heating with the residence time of the biomass lower than 0.5 s is applied [28,29]. Medium temperature (around 350e500
C) and low residence time tend to turn biomass to liquid outputs, whereas low residence time and low temperature essentially convert it to charcoal. The parameters for the generation processes of liquid and charcoal are reviewed in Table 18.3 [30]. There are a wide variety of reactions in pyrolysis, such as dehydration, depolymerization, isomerization, aromatization, decarboxylation, and charring [31,32]. The reactions can be classified as following [31]. Primary reactions: The primary reactions contain the creation of char, depolymerization, and cracking (fragmentation) [33]. (a) Charring: Char is a polycyclic aromatic carbon formed by combining or condensing C6H6 (benzene) rings through pyrolysis [34]. (b) Depolymerization: entails breaking the bond attachments between the monomers. Through pyrolysis, depolymerization is a dominant reaction path leading to the volatiles and gases creation [35,36]. (c) Fragmentation: the unit monomers and polymer’s covalent bond is fragmented. Species, including a short chain and other noncondensable

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FIGURE 18.7 Primary pyrolysis products and applications.

Table 18.3 The yields for product distribution of slow and rapid pyrolysis (on the dry wood basis) [17]. Type

Condition

Solid

Liq (wt%)

Gas(wt%)

Slow Fast

w400
C, residence time of vapor / day w400
C, residence time of vapor w1s

35 12

30 75

35 13

gases, are generated through biomass fragmentation [37,38]. Secondary reactions: The Primary species produced through primary reactions may not be steady and may yet experience secondary reactions, including reactions of cracking or recombination. Further, the produced char can catalyze secondary reactions. Lighter compounds are formed by cracking the primary molecules, while recombination leads to the heavier compounds or sediments formation on the surface of the char [37,39]. Reactions of different process situations: Different forms of the process may have a significant impact on the network of reaction. For example, a lower heating rate (100
C/s) results in more volatile species. Besides, the temperature of the reaction has a vital impact on product distribution. Depolymerization reactions at temperatures between 250 and 500
C are very high. The highest bio-oil yield is obtained from 450 to 550
C, while more reactions of fragmentation occur above 550
C, which results in more gaseous products [26]. Consequently, conditions of the process in biomass pyrolysis can have a significant impact on the reactions and

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mechanisms of pyrolysis product formation. It can be discovered from all the studies carried out so far that the secondary reactions are the governing reactions that can create outputs with particular characteristics [40]. Pyrolysis is desirable since solid biomass and waste can easily be transformed into liquid products, which are difficult and expensive to manage. Such liquids, as liquid or slurry, have benefits in production and marketing in terms of transportation, storage, combustion, retrofitting, and flexibility [17].

2.2

Biochemical conversion

In biochemical processing, bacteria or enzymes break down the biomass molecules into smaller molecules. This method is slower than the thermochemical transformation; however, it needs low external energy. The following are the three main biochemical transfer pathways:  Digestion (anaerobic and aerobic)  Fermentation [41].

2.2.1 Digestion In this section, different types of digestion, for example, anaerobic, aerobic, is described. 2.2.1.1 Anaerobic digestion Anaerobic digestion (AD) is a cycle where microorganisms break down plant and animal materials (biomass) in the absence of oxygen. The AD cycle starts when a sealed tank or digester has biomass within it. Natural microorganisms consume biomass, which releases gas riched in methane (biogas) that can be utilized to produce renewable heat and power; this helps to decrease the usage of fossil fuel and to minimize greenhouse gas (GHG). The residual substance (digestate) is nutrient-rich, and it can be utilized as a fertilizer (Fig. 18.8). Many feedstock sources are ideal for AD, containing food waste, slurry, and manure, besides crop and crop residue. Nevertheless, woody biomass cannot be applied in AD because microorganisms cannot break down lignin, a compound that enhances the strength of wood. Anaerobic digestion consists of a variety of reactions, often biochemical. A simplified scheme of reactions is shown in Fig. 18.9. The initial stage of the process includes the collapse of organic particulate material in carbohydrates, lipids, and proteins that are then extracellular enzymatically toward short-chained carbohydrates, long-chain fatty acids, and amino acids. These hydrolytic enzymes, either exhibit in the liquid bulk or joined to particulates, are secreted by microorganisms [42]. Consequently, acid microorganisms turn these soluble species into alcohols and/or organic acids, which are then transformed by acetoclastic methanogens into acetate and, eventually, CH4 and CO2.

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FIGURE 18.8 Schematic overview of the aerobic digestion method.

Particulates

Lipids

Long chain fatty Acids

Amino acids

Hydrogen

Alcohols

Carbone Dioxide

Acetate

Methane

FIGURE 18.9 Anaerobic digestion process overview [45].

Methanogenesis Microbial

Hydrogen

Alcohols

Acidogenesis Microbial

Carbon Dioxide

Short chained sugars

Glycerine

Acidogenesis Microbial

Volatile Fatty Acids (VFA)

Carbohydrate

Disintegration Hydrolysis Enzymatic

Proteins

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CO2 and H2 are further aggregates to generate CH4 by hydrogenotrophic methanogens. The last process accounts for about 30% of the total production of methane [42,43]. The eventual composition of biogas relies on some parameters: (1) the carbon oxidation status in the substrate, (2) the residence time, which is usually associated emphatically with the methane, (3) the design of the reactor, and (4) the temperature affecting total kinetics and gaseous solubility [44]. A critical factor in the process of digestion is dissolved hydrogen: high concentrations prohibit the phases of acidogenesis/acetogenesis whereas it is a required element of hydrogenotrophic methanogenesis; the entire network of reaction is extremely more intricate and involves, for example, sulfur and nitrate reduction and methanol and format oxidation, siloxane degradation, and lactic acid formation [46]. Furthermore, equilibriums of acidebase and mass transfer of the vaporeliquid phase should also be considered, as well as microbial growth and decay [47]. 2.2.1.2 Aerobic digestion Aerobic digestion is likewise a biochemical degradation of waste, even when there is oxygen. It applies various kinds of microorganisms that entree oxygen of the air, generating CO2, energy, and a solid digestive state [41].

2.2.2 Fermentation Through fermentation, a portion of the biomass is turned into sugars using acid or enzymes. With the help of yeasts, the sugar is then turned to ethanol or other substances. The lignin is not transformed and is leftover both for burning or chemical transformation. In comparison to the digestion of anaerobic, the fermentation output is liquid. Starch and sugar-basis feed fermentation in ethanol is entirely economic. However, with cellulosic biomass, this is not the same due to the cost and complexity of cracking the substances in fermentable sugars. Like wood, lignocellulosic feedstock needs pretreatment with hydrolysis to crack down the cellulose and hemicellulose toward simple sugars required for fermentation by the yeast and bacteria [41].

2.3

Liquefaction

Biomass liquefaction is an essential technology for transforming biomass into precious biofuel. Well-known biomass liquefaction technologies are indirect and direct liquefaction. The FischereTropsch method of indirect liquefaction involves the application of biomass syngas as the raw material for the development of liquid fuel, involving methyl alcohol, ethyl alcohol, and dimethyl ether. The direct liquefaction of biomass as regards the transformation of biomass to bio-oil and the fermentation of hydrolysis and thermodynamic liquefaction are the leading technologies. The process of thermodynamic liquefaction can be classified into pyrolysis of fast type and hydrothermal liquefaction [48].

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2.3.1 Indirect liquefaction Indirect liquefaction is an encouraging technology, split into two steps. The first step is a cycle of thermo-chemical gasification [49,50]. During this process, following the raw material has reacted with air or steam, the syngas is formed. Throughout the method, once the raw material reacts with air or steams, the syngas is generated. Within the syngas, the essential species are CO, CO2, H2, and H2O. The following step is the substantial cycle of (FeT) [51]. The mixture will be added in the FeT process to make a number of chemicals, as well as methyl alcohol, dimethyl ether, and ethyl alcohol, whereas the higher alcohols obtained from the biomass syngas is little studied.

2.3.2 Direct liquefaction Direct liquefaction could be categorized into hydrolysis-fermentation liquefaction and thermodynamic liquefaction. These two types of direct liquefaction are described in this section. 2.3.2.1 Hydrolysis-fermentation liquefaction Ethyl alcohol has drawn a great deal of consideration over the last few decades as a likely option to fossil fuels [52]. The biomass fermentation is currently the leading modern technology for the production of ethyl alcohol, the main raw materials of which are glucose and sucrose [30]. After the biomass is delivered through the manufacturing plant, it will be processed in the house to avoid fermentation and pollution of the bacteria. The raw material could then be pretreated to make it available for extraction more readily. During the process of fermentation, Hydrolysate, yeasts, nutrients, and other ingredients should be supplemented. The fermentation is generally conducted at 25e30
C, and it will last 6e72 h for the appropriate reaction time. The parameters depend primarily on the hydrolysate components, type, density, or yeast activity. Recycling yeasts are used for improving fermentation activity and productivity. A mixture may be obtained after distillation, also referred to as “hydrous” or “hydrated” ethyl alcohol (95% alcohol, 4% water). To produce “anhydrous” ethyl alcohol (99.6% alcohol, 0.4% water), the hydrated ethyl alcohol must then be dehydrated. 2.3.2.2 Thermodynamic liquefaction Typically there are two methods for biomass thermodynamic liquefaction based on conditions of the process: pyrolysis liquefaction and hydrothermal liquefaction [53,54]. This may be categorized in pyrolysis of slow, fast, and flash type in liquefaction with pyrolysis [55]. Bio-oil, also called pyrolysis oil or pyrolysis oil, can be achieved from any of the mentioned methods. Bio-oil is a highly complex substance that compromises hundreds of organic materials like alkanes, aromatic hydrocarbons, derivatives of phenols, ketones, esters, ethers, sugars, amines, and alcohols [56,57]. Additionally, the molar ratio between H and C is greater than 1.5 in bio-oil. The pyrolysis bio-oils might be directly burnt in boilers or adjusted to generate beneficial fuels and chemicals products applying the subsequent techniques: [58], emulsification [59], esterification/alcoholysis [60], supercritical fluids [61], hydrotreating [62], catalytic cracking [63], and steam reforming [64].

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447

Physical conversion

2.4.1 Densification The biomass with low mass density is costly and ineffective to develop and use. Biomass can be densified to tackle this issue; this is typically done with some sort of extrusion and substantially enhancing the bulk density of biomass. Briquetting is a process of densification that basically develops carriage and storage treatment characteristics. Therefore biomass, when applied near the source, is most economically possible. It creates a homogeneous output that has a greater density of energy than the primary raw material. Prior studies have been performed on the method of processing biomass content, extracted from a collection of shells and fibers of oil palm, clusters of empty fruit, dried leaves, husks of rice, and biocoal wood waste. The method involves pyrolysis that comprises applying pressure to loose particles of biomass in a form that is heated concurrently, in the nonexistence, or with a minimum amount of O2 to turn them toward a condensed and agglomerated shape of the desirable biocoal range of product [65]. Several biomass components may be applied, and improvements made by applying pressure, temperature, and interval timing to the physical and chemical specifications of the biocoal may be achieved. The biomass can be combined with other kinds of materials or additives used to enhance efficiency. A certain amount of fixed carbon, ash content, and volatile matter may be provided for the biocoals. The method given concerning each kind of biomass had its carbonization temperature of carbonization, and pressure of pyrolysis timed to provide the necessary physical character.

2.4.2 Torrefaction The process of torrefaction may be defined as a moderate sort of pyrolysis at temperatures between 200 and 320
C. Both the water stored in the biomass and the volatiles of superfluous are extracted during the process. Biomass loses something like 20% about its weight. Also, over 10% about its value of heating, outwardly at whatever alter its size, subsequently reducing its density of energy. The aim is to achieve an extremely better fuel property for combustion and gasification utilization to enable simple pelletizing or briquetting of the materials [65].

3. Environmental challenges Although biomass is desirable as a low-sulfur renewable fuel, the use of biomass by way of a resource for energy is not out of possible effects on the environment. Contest for the arable land needed for the production of food and fiber is the biggest problem regarding the generation of biomass. Soil disruption, loss of nutrients, and poor quality of water are further possible environmental consequences of the processing of biomass feedstock and the use of the energy of farm and forest residues. The extent of these effects is extremely site-dependent and requires regional assessment. Biochemical processes to

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turn biomass substances into fuel create air contaminants, solid waste, and wastewater, which can negatively affect the environment. Thermochemical transformation of biomass into fuels generates air contaminants that include particulates, CO, H2S, and polycyclic organic matterdadditional environmental concerns resulting in the wastewater and solid waste production and handling. Through adopting proper provision and maintenance practices, using precise environmental management technologies, and using any side-products generated, the environmental impacts from the generation and transformation of biomass can be minimized.

3.1

Feedstock

The extent of the environmental effect of the feedstock depends on the size, the management methods used, and the natural characteristics of the site of production [66].

3.2

Residue

Farm residues are the pieces of plants, including stalks, bay, stalks, and leaves, that left after harvesting. Residues from the farm, when remaining on land, take part in the prevention of erosion and nutrient preservation [67]. Extensive residue elimination can extend wind and water depletion and can enhance fertilizer usage. Water and air grade, land constancy, and site productivity may be littered with residue elimination. Quality of water is endangered by raised deposition, enhanced usage of herbicides and the following drainage, and enhanced progress of minerals into surface waters. Wind depletion and herbicide usage are increasing dust and aerosols in the atmosphere, sequentially. By rising erosion and eventual degradation of topsoil, nutrients, and organic element, removal of residues impresses land stability and productivity. Current biomass production and consumption practice having resulted in forest destruction and deforestation, loss of biodiversity, soil depletion, ozone pollution from GHG emissions during wood combustion (with its effects on climate change), and indoor air pollution are resulting in hazards and depletion of nutrients from cattle dung combustion and crop residues.

3.3

Deforestation and land degradation

Deforestation directing to soil erosion, flood threats, desertification due to clearance of forest and woods for farming and livestock, and so on are the underlying issues of environmentalists at the macro-level. At a microlevel, the problems vary from undesirable forest soils for agricultural purposes, health problems caused by the smoke generated by fuelwood burning, loss of soil fertility because of the utilization of agricultural residues, etc. Even a transition to nonwood biomass fuels creates direct competition with animals that depend on crop residue and fodder.

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449

Loss of soil nutrients

Agricultural residues represent a vital supply of energy in rural areas of developing countries once left on fields, enriches the fertility of the soil. The employment of agricultural residues for energy will, therefore, be a problem if it reduces the fertility of the soil. The burning of fuelwood and other biomass fuels results in CO2 emissions, with about 50% of wood being carbon. If fuelwood comes from sustainable extraction methods, then its combustion does not result in net C emissions. Nonetheless, evaluating what percentage of fuelwood usage is from nonsustainable sources is difficult. At a global level, fuelwood combustion accounts for about 2.8% of CO2 emissions [68]. The burning of fuelwood and agro-residues contributes to the emission of incomplete combustion products, in addition to CO2 emissions. Such products are even more powerful carbon-emitted GHGs per gram than CO2 [69].

4. Conclusion and future trends Biomass can be transformed into many valuable energy patterns using the various processes (conversion technologies) mentioned in this chapter. Bioenergy is the expression applied to express the energy that comes from the feedstocks of biomass. To transform raw biomass into useful energy, different processing steps are required using four significant methods: thermochemical, biochemical, liquefaction, and physiochemical. Biochemical conversion involves two key process options: anaerobic digestion and fermentation. The three essential process options discussed here for the thermochemical conversion routes are pyrolysis, gasification, and combustion. Physiochemical conversion mainly composes densification where the bulk density is increased. Bioenergy contains solid, liquid, or gaseous fuels that can be derived from the available technologies. Liquid fuels are widely employed in carrying but can further be utilized in motionless engines. Solid fuels are combusted straight to recover heat, power, or CHP. Gaseous fuels apply to the complete range of end-use applications. Many parameters influence the selection of the transformation process, involving the type, size, and properties of the feedstock of biomass, the necessities for end-use, and the environmental management, economics, area, and parameters related to specifications of the project. The way in which biomass conversion technologies are applied and managed may impact the ecosystem that may result from their implementation. The environmental complexity of biomass production and utilization systems can require technological modifications to maximize yields while minimizing environmental effects. The use of mitigative conversion systems techniques can decrease solids, liquids, and gaseous contaminants.

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´ lvarez-Galva´n M, Loricera C, Fierro J. Upgrading of bio-liquids on [62] Nava R, Pawelec B, Castan˜o P, A different mesoporous silica-supported como catalysts. Appl Catal B Environ 2009;92(1e2):154e67. [63] Guo X, Zheng Y, Zhang B, Chen J. Analysis of coke precursor on catalyst and study on regeneration of catalyst in upgrading of bio-oil. Biomass Bioenergy 2009;33(10):1469e73. [64] Vagia EC, Lemonidou AA. Hydrogen production via steam reforming of bio-oil components over calcium aluminate supported nickel and noble metal catalysts. Appl Catal Gen 2008;351(1):111e21. [65] Ani FN. Utilization of bioresources as fuels and energy generation. In: Electric renewable energy systems. Academic Press; 2016. p. 140e55. [66] Trimble JL, Van Hook RI, Folger AG. Biomass for energy: the environmental issues. Biomass 1984; 6(1e2):3e13. [67] Pimentel D, Fried C, Olson L, Schmidt S, Wagner-Johnson K, Westman A, Whelan A, Foglia K, Poole P, Klein T. Environmental and social costs of biomass energy. Bioscience 1984;34(2):89e94. [68] Ahuja DR. Research needs for improving biofuel burning cookstove technologies: incorporating environmental concerns. In: Natural resources forum. Wiley Online Library; 1990. [69] Change C. In: Houghton JT, Callander BA, Varney SK, editors. The supplementary report to the ipcc scientific assessment. Report of the Intergovernment Panel on Climate Change. Cambridge Univ. Press; 1992. p. 1992.

Index Note: ‘Page numbers followed by “f ” indicate figures and “t” indicate tables.’ A Acid pretreatment, 50e51 Acidogenesis, 244 Acoustic actuation, 371 Acoustic manipulation, 396e397 Activated carbons (AC), 264e269 biomass-derived activated carbon, 265e269 Activated sludge systems, 304e305 Active methods, 395e397. See also Passive methods acoustic manipulation, 396e397 electrical manipulation, 395e396 magnetic manipulation, 396 thermal manipulation, 397 Active micromixers, 371e372. See also Passive micromixers acoustic actuation, 371 EHD force actuation, 371e372 MHD force actuation, 372 Adsorbent, 256e257 Aerated stabilization basins (ASBs), 304e305 Aerobic digestion, 143, 445 Aerobic wastewater treatment, 309 Aerobic water treatment, 309e310, 310f Ageratum conyzoides leaf powder (ACLP), 305e308 Agricultural residues, 449 Air reactor (AR), 88 Alcoholic fermentation, 143 Algae, 18, 308 “Algae-to-biofuels” technologies, 209e210 Alkaline pretreatment, 51e52 Aluminium oxide (Al2O3), 95e96 Ammonia fiber explosion (AFEX), 58e59 Amorphous cellulose, 8

Amylopectine, 14 Amylose, 14 Anaerobic digestion (AD), 28e30, 143, 242e243, 443e445 Anaerobic water treatment, 310e312 Animal biomass wastes, 18e19 Anion exchange membranes (AEM), 72e73, 277e278 Anodes, 285e286 Aquatic biomass, 17e18 Aqueous enzyme oil extraction method, 176 Autothermal ethanol reforming process, 359e360 B BASF process, 231 Bath bioprocessors, 350 Bio-electrochemical system (BES), 311e312 Bio-oil, 148e152 advantages, 152 environmental impact of, 156 history, 148e149 methods of producing, 150e151 properties, 149e150 upgrading, 77e78 utilizing, 152 Biochar, 308 Biochemical conversion, 28e32, 443e445 anaerobic digestion, 28e30 digestion, 443e445 fermentation, 30e32, 445 Biochemical oxygen demand (BOD), 284 Biochemical processes, 142e143 Biocoal, 153e154 environmental impact of, 157 process of producing, 154

455

456

Index

Biodiesel, 167 byproduct, 184e186 feedstock classification and pretreatment, 172e176 history, 167e170 market, 170e172 other processes of biodiesel production, 182e184 production, 176e179, 352e356 purification, 76e77 Bioenergy, 330e331 from biomass to bioenergy production, 19 droplet-based microfluidic platforms for, 397e399 environmental impact and future of, 156e159 futures of biomass and, 157e159 production technology of, 331 Bioethanol, 3e4, 117 production, 352e356 purification of, 79 Biofuel(s), 69 as alternative to fossil fuels, 352e356 biodiesel and bioethanol production, 352e356 micro-bioprocessors and biofuel production, 352 benefits and drawbacks of using membrane technology in biofuel purification and upgrading, 73e74 improvement with metabolic engineered microalga, 217e221 economical extraction of microbial final product, 221 enhancing harvesting, 219e220 metabolic engineering for biofuels production, 217 from microalgae extracted lipid, 215 with ultrasound, 215e217 microfluidic device application in, 331e341 production, 374e375, 377

Biogas, 74e75, 143e147 affecting factors on amount of manufactured biogas, 145 environmental impact biogas preparation, 156 manufacture, 144e145 manufacture and utilizing, 146e147 manufacturing apparatus, 145e146 upgrading, 74e76 Biohydrogen production, 197 upgrading, 79e81 Biological pretreatment, 60e61 Biological processing of biomass, 242e245 Biomass, 3e4, 15e19, 87, 139e143, 209e210, 330e331, 352 aquatic, 17e18 bio-oil, 148e152 biochemical conversion, 443e445 biochemical processes, 142e143 biocoal, 153e154 biogas, 143e147 from biomass to bioenergy production, 19 biomass-based chemical looping principal technology/process, 91 biomass-based cogeneration plants, 22 biomass-based generators, 21 biomass-derived activated carbon, 265e269 biomass samples, 267e269 cellulose, 7e9 chemical characterization of, 6e15 chemical looping, 97e119 composition, 233e234 conversion technologies, 3e4, 20e33 energy, 408 environmental challenges, 447e449 deforestation and land degradation, 448 feedstock, 448 loss of soil nutrients, 449 residue, 448 environmental impact and future of biomass and bioenergy, 156e159

Index

environmental impact biogas preparation, 156 environmental impact of bio-oil, 156 environmental impact of biocoal, 157 fluid matter, 15 generation of biomass feedstock, 331, 332t global biomass trends, 33e34 hemicellulose, 9e11 herbaceous, 17 human and animal biomass wastes, 18e19 hydrogen from biomass feedstock, 195e197 inorganic components, 15 lignin, 11e12 liquefaction, 445e446 to methanol technology, 233e246 olive pomace ash, 418e419 origin, 417e420 other minor components in, 15 physical conversion, 447 pretreatment processes of lignocellulosic biomass, 19e20 production technologies to different products, 141f river sediments, 419e420 SCBA, 419 sewage sludge, 417e418 starch, 12e14 technologies, 431e447 thermochemical conversion, 432e443 thermochemical processes, 141e142 utilized in water and wastewater treatment, 305e308, 306te307t wood and woody, 16e17 Biomass and coal cofuel gasification chemical looping combustion with CO2 capture for combined cooling, heating, and power generation (BCCLC-CCHP), 106e108 Biomass ash (BA), 408, 412e415 biomass origin, 417e420 bottom ash, 415 brick production, 420e425 bricks, 409e411 commercialization, 425

457

composition of ash, 413e414 constituent of brick, 416e417 effects of biomass waste addition to bricks raw material, 420 fly ash, 414e415 necessity of addition of wastes to bricks, 411 wastes categorization, 415, 416t Biomass direct chemical looping system/ technology (BDCL system/ technology), 98e99 Biomass gasification dual chemical looping system/technology (BGDCL system/ technology), 101e102 Biomass-based calcium looping combustion (BCaLC), 91 Biomass-based chemical looping combustion (BCLC), 91 Biomass-based coproduction chemical looping process (BCCLP), 91 Biomass-derived conversion catalysts for CL biomass conversion, 113e119 chemical looping biomass and, 97e119 CL systems/technologies concepts for biomass conversion, 98e108 BCCLC-CCHP, 106e108 BGDCL system/technology, 101e102 biomass direct CL system/technology, 98e99 CL-ZEC system/technology, 105e106 CLH process, 98 CLOU system/technology, 105 CLSOFC system/technology, 101 CLWS system/technology, 104 coke-oven gas CLH generation system/ technology, 98 iG-CLC system/technology, 104 PCLC-CC system/technology, 105 SE-CLR system/technology, 102e104 main products in CL biomass conversion, 109e113 standalone generation of hydrogen in CL biomass conversion, 111e113

458

Index

Biomass-derived conversion (Continued ) standalone generation of power in CL biomass conversion, 109e111 processes of CL biomass conversion, 97e113 type of reactor in CL biomass conversion, 113 Biomethanol biomass to methanol technology, 233e246 challenges on industrialization, 246e247 methanol production routes, 232e233 Bioprocess(ing), 366 biofuel production, 374e375 chemical synthesis, 373 extraction and purification processes, 373e374 microfluidics bioprocess, 367e368 micromixers, 368e375 micropumps, 375e377 sample concentration, 373 Bioremediation, 276 Biosorption, 308e309 Bottom ash, 411, 415 Brick(s), 409e411 analysis, 423e425 bulk density of bricks, 424 color, 424 compressive strength of bricks, 424 leaching tests, 424 shrinkage of bricks, 423 thermal conductivity, 424e425 water absorption rate of bricks, 423 weight loss on ignition of bricks, 423 characteristics of good brick, 410t constituent of, 416e417 salt, 416e417 different methods of brick making, 422 necessity of addition of wastes to, 411 production, 420e425 traditional brick making, 411 Briquetting process, 447 Bulk density of bricks, 424

C Capillary micro-bioprocessor for biodiesel synthesis, 353e354 Carbo-V process, 246e247 Carbon atoms sugars, 43e44 Carbon dioxide (CO2) capture technologies, 255e259 CO2(g) sequestration, 276 Carbon dioxide capture and storage (CCS), 256 Carbon monoxide (Co) Co-based oxygen carriers, 94 droplet formation in Co-flow devices, 393e394 Carbon Recycling International, 232 Carbonization, 142 Catalysts for CL biomass conversion, 113e119 CL materials development for gaseous biomass-derived feedstock, 118e119 looping materials development for liquid biomass-derived feedstock, 116e118 solid biomass feedstock, 115e116 Cathodes, 286e287 Cation exchange membranes (CEM), 72e73 Cell bioprocess, 366 culturing and screening, 398e399 disruption, 214 wall disruption, 215e217 Cellulose, 6e9 Centrifugal forces, 377 Centrifugal microfluidic pump, 377 Ceria (CeO2), 96e97 Char, 116 Charring, 441e442 Chemical bioprocessors, 350 Chemical looping combustion technology/ process (CLC technology/process), 90e91 Chemical looping gasification technology/ process (CLG technology/process), 91 Chemical looping hydrogen process (CLH process), 98

Index

Chemical looping oxygen uncoupling system/technology (CLOU system/technology), 105 Chemical looping process (CL process), 87e91, 88f biomass-based chemical looping principal technology/process, 91 challenges, 129 chemical looping biomass and biomass-derived conversion, 97e119 CL-SR technology/process, 90 CLC technology/process, 90e91 CLG technology/process, 91 comparative analysis, 119e128 economic feasibility, 128 technical feasibility, 119e128 materials, 91e97 oxygen carriers, 92e95 promoters, 95e96 sorbents, 95 supports, 95e96 oxygen carriers, 120te125t Chemical looping reforming (CLR), 87e88 Chemical looping solid oxide fuel cell system/technology (CLSOFC system/ technology), 101 Chemical looping steam reforming (CLSR), 87e88, 90 Chemical looping water splitting system/ technology (CLWS system/ technology), 104 Chemical looping zero-emission coal system/technology (CL-ZEC system/ technology), 105e106 Chemical oxygen demand (COD), 284 Chemical pretreatment, 49e57 acid pretreatment, 50e51 alkaline pretreatment, 51e52 ILs, 52e55 OS process, 55e56 ozonolysis, 56 wet oxidation, 57 Chemical reaction process, 349e350 Chemical sorption, 259e260 Chlorella vulgaris, 279e280

459

Chlorofluorocarbons (CFCs), 255e256 Clay extraction process, 409e410 Coke-oven gas CLH generation system/ technology, 98 Color, 424 Combined heat and power (CHP), 140e141 Combined pretreatment, 61e62 Combustion, 18e22, 432e434 of biomass in modern boilers, 142 industrial applications, 433e434 partial, 434 Commercialization, 425 Compressive strength of bricks, 423e424 21st Conference of the Parties (COP21), 170 Coniferyl alcohol, 43 Continuous bioprocessors, 350 Conventional fossil fuels, 209e210 Conversion technologies, 20e33 biochemical conversion, 28e32 physicochemical conversion, 32e33 thermochemical conversion, 20e28 Copper-based oxygen carriers (Cu-based oxygen carriers), 93 Coriolis force, 377 Coumaryl alcohol, 43 Cross-flow devices, droplet formation in, 392e393 Cross-type micromixers, 375 Crude glycerol, 184 Crude oil. See Petroleum Cultivation, 211 D Deforestation, 448 Dehydration, 213 Densification, 447 Depolymerization, 441e442 Digestion, 443e445 aerobic, 445 anaerobic, 443e445 Direct combustion, 142 Direct liquefaction, 446 hydrolysis-fermentation liquefaction, 446 thermodynamic liquefaction, 446 Domestic wastewater, 303

460

Index

Double T-type micromixers, 375 Downdraft gasifier, 437e438 Downflow FBR, 315e316 Dripping, 390 Droplet formation regimes, 388e390 dripping, 390 jetting, 390 squeezing, 389e390 Droplet generation mechanism, 388 Droplet microfluidics, 335e339 active methods, 395e397 device fabrications, 391 devices, 387e388 droplet formation regimes, 388e390 passive methods, 391e395 platforms, 367e368 for bioenergy applications, 397e399 Drying process, 213 Dual-pipe micro-bioprocessor, 332e333 Dual-purpose algaeebased systems, 319e320 E Economic analysis, 321 Economical extraction of microbial final product, 221 Edible oil resources, 172e173 Electrical manipulation, 395e396 Electroactive bacteria (EAB), 283 Electrogenic microorganisms, 285 Electrohydrodynamics (EHD), 371e372. See also Magnetohydrodynamics (MHD) force actuation, 371e372 micropump, 376 Electroosmosis micropump, 376 Electrostatic based actuation pump, 376 Electrostatic force, 376 Energy production technologies, 431 resources, 330e331 Energy Information Administration, 255e256 Environmental impact biogas preparation, 156

Enzymatic catalysis, 183 Ethanol, 177 steam reforming process, 358e359 Ethyl alcohol, 446 Euler force, 377 Evaporation-type micropumps, 377 Extraction process, 373e374 F Feedstock, 3e4, 6, 448 classification and pretreatment, 172e176 Fenton’s process, 304 Fermentation, 30e32, 445 Fired clay bricks, 410, 420 First-generation feedstocks (FGF), 42 FischereTropsch technology, 183e184, 445 Fixed bed gasifier, 436e437 reactors, 314e315 Flash pyrolysis, 150 Flow-focusing devices, droplet formation in, 394e395 Fluid matter, 15 Fluidized bed gasifiers, 439e441 Fluidized bed reactor (FBR), 315e319 Fluxing wastes, 415 Fly ash, 411, 414e415 Fossil fuels, 41e42, 69, 139, 209e210 exploitation, 3 Fouling, 433e434 Fragmentation, 441e442 Freeze-drying process, 213 Freshwater scarcity, 303 Fuel cells based on biomass MFC, 276e288 microbial culture, 288e289 scale-up and developments in technology, 290e292 micro-bioprocessors and, 356e360 Fuel reactor (FR), 88 Fuel wastes, 415

Index

G Galactans, 11 Gas separation membrane bioreactor (GSMBR), 80e81 Gaseous biomass-derived feedstock, CL materials development for, 118e119 Gasification, 24e25, 434e441. See also Liquefaction of biomass, 200e202, 236e238 downdraft gasifier, 437e438 fixed bed gasifier, 436e437 fluidized bed gasifiers, 439e441 stage gasifier, 438e439 updraft gasifier, 437 Gasifiers, 25 Gene transformation, 217 eukaryotic microalgae with, 218te219t Global biomass trends, 33e34 Global warming, 255e256 Glycerol, 76, 117 valorization, 184e186 Graphene oxide (GO), 285 Greenhouse gas emission (GHG emission), 87, 139, 170, 193, 443 Guaiacyl propanol, 43 H H-type bioprocessor, 331e332 Harvesting, 212e213 Hemicellulose, 9e11 galactans, 11 mannans, 9e11 xylans, 9 Herbaceous biomass, 17 Heterogeneous catalyst, 181e182 High rate algal ponds (HRAPs), 319e320 High-temperature gasification, 142 Homogeneous catalyst, 179e181 Human biomass wastes, 18e19 Hydrogen, 87, 185 application, 194e195 from biomass feedstock, 195e197 in CL biomass conversion, 111e113 different methods of hydrogen production, 197

461

feedstock for hydrogen production, 195 formation, 360 gas, 341 production via thermochemical processes, 198e202 as sustainable and clean energy carrier, 194 Hydrolysis-fermentation liquefaction, 446 Hydrothermal carbonization (HTC), 153 Hydrothermal liquefaction (HTL), 77, 150 I ICI process, 231 In situ gasification-chemical looping combustion system/technology (iG-CLC system/technology), 104 Incinerated sewage sludge ash (ISSA), 417e418 Incineration, 408 Indirect liquefaction, 446. See also Direct liquefaction Industrial wastewater, 303 Inorganic components, 15 Integrated membrane technology benefits and drawbacks of using membrane technology in biofuel purification and upgrading, 73e74 biodiesel purification, 76e77 membrane features, 71 fouling, 74 types, 71e73 operational conditions in biofuels processing, 82te83t purification of bioethanol, 79 upgrading bio-oil, 77e78 biogas, 74e76 biohydrogen, 79e81 International Panel on Climate Change (IPCC), 3e4 Intersecting channels, 370 Ionic liquids (ILs), 52e55 Iron-based oxygen carriers (Fe-based oxygen carriers), 93

462

Index

J Jetting, 390 K Kippur War, 167e168 Kyoto protocol, 3e4 L Lamination-based micromixers, 369e370 Land degradation, 448 Leaching tests, 424 Lignin, 11e12 Lignocellulose, 33e34 Lignocellulosic biomass, pretreatment processes of, 19e20 Lipid extraction, 214 from microalgae by ultrasound, 215e217 procedure from microalgae biomass, 211e214 cell disruption, 214 cultivation, 211 dehydration, 213 harvesting, 212e213 lipid extraction, 214 Liquefaction, 26e28, 142, 445e446. See also Gasification of biomass, 200 direct, 446 indirect, 446 Liquid biomass-derived feedstock, looping materials development for, 116e118 Liquid hot water pressure (LHW pressure), 48e49 Low-temperature gasification, 142 M Macroalgae, 18 Magnesium (Mg), 97 Magnetic manipulation, 396 Magnetohydrodynamics (MHD), 371. See also Electrohydrodynamics (EHD) force actuation, 372 micropump, 376

Manganese-based oxygen carriers (Mnbased oxygen carriers), 94 Mannans, 9e11 Mechanical micropumps, 376 Membrane bioreactors (MBRs), 72e73, 310e313 Membrane fouling, 74 Membrane technology, 70 Metabolic engineering for biofuels production, 217 eukaryotic microalgae with gene transformation, 218te219t Metal-organic frameworks (MOF), 263 Methanogenesis, 244 Methanol, 177, 231 biomass to methanol technology, 233e246 production routes, 232e233 Micro-bioprocessors, 331e333, 350e352 biofuels as alternative to fossil fuels, 352e356 and fuel cells, 356e360 systems, 329e330 Micro-electro-mechanical systems (MEMS), 375, 377 Microalgae, 17e18, 210e211, 308 biofuel production from microalgae extracted lipid, 215 biofuel production from microalgae with ultrasound, 215e217 biofuel production improvement with metabolic engineered microalga, 217e221 lipid extraction procedure from microalgae biomass, 211e214 potential for biofuel production, 210e211 Microalgal film photobioreactors, 211 Microbial culture, 288e289 pH, 288 substrate, 289 temperature, 288 Microbial electrolysis cells (MEC), 331e332 Microbial fuel cells (MFC), 276e288, 291t anodes, 285e286 architecture, 279e282 cathodes, 286e287

Index

chemical aspects of, 278e279 microbial aspects, 283e288 separators/membranes, 287e288 Microcapillary bioprocessors, 332 Microchannel, 350 laminae, 355 parallelized, 360 slit, 354e355 tesla shaped, 355 zigzag, 354 Microdroplet system, 329e330 Microfiltration (MF), 71e72 Microfluidic(s), 350, 387e388 bioprocess, 367e368 chips, 373 device, 329e330 application in biofuel, 331e341 biomass, 330e331 production technology of bioenergy, 331 Micromixers, 329e330, 333e335, 368e375 active, 371e372 passive, 369e371 Micropumps, 329e330, 339e341, 375e377 biofuel production, 377 mechanical, 376 nonmechanical, 376e377 Microscale tools, 373 Microsensors, 329e330 Microwave drying, 213 Millifluidic droplet platform, 338e339 Milling mechanical breakage, 46 Municipal solid wastes (MSWs), 22, 268e269 N Nafion 117, 286 Nanofiltration (NF), 71e72 Nickel-based oxygen carriers (Ni-based oxygen carriers), 93 Nonmechanical micropumps, 376e377 Nonrenewable energy resources, 366 O Oil extraction, 175 Olive pomace ash, 418e419 Organic residues, 411 Organosolv process (OS process), 55e56

463

Oxygen carriers, 92e95 Co-based oxygen carriers, 94 Cu-based oxygen carriers, 93 Fe-based oxygen carriers, 93 Mn-based oxygen carriers, 94 Ni-based oxygen carriers, 93 perovskite, 94e95 Ozonolysis, 56 P p-hydroxyphenylpropanol, 43 Packed-bed reactors (PBRs), 314e315 Palm kernel shell (PKS), 268 Partial combustion, 434 Passive methods, 391e395. See also Active methods droplet formation in co-flow devices, 393e394 droplet formation in cross-flow devices, 392e393 droplet formation in flow-focusing devices, 394e395 Passive micromixers, 369e371. See also Active micromixers 3D structures, 370e371 intersecting channels, 370 lamination-based micromixers, 369e370 Perovskite, 94e95 Petrol oil, 165e166 Petroleum, 165 Photolithography technique, 329e330 Physical adsorption, 260e269 activated carbons, 264e269 MOF, 263 PSA, 261 TSA, 261e262 zeolites, 262e263 Physical conversion, 447 densification, 447 torrefaction, 447 Physical pretreatment, 46e49 liquid hot water pressure, 48e49 milling mechanical breakage, 46 uncatalyzed steam explosion, 46e48 Physical-chemical pretreatment, 57e59 AFEX, 58e59 steam explosion, 58

464

Index

Physical-chemical pretreatment (Continued ) supercritical fluid explosion/carbon dioxide explosion, 59 Physicochemical conversion, 32e33 transesterification, 32e33 Piezoelectric micropump, 376 Plasma processing of biomass, 245e246 Polydimethylsiloxane (PDMS), 391 Polyethylene terephthalate (PET), 186 Polymer electrolyte membrane (PEM), 77e78 Population heterogeneity, 366 Pressure swing adsorption (PSA), 261 Pressurized chemical looping combustion with combined cycle system/ technology (PCLC-CC system/ technology), 105 Pretreatment biological, 60e61 chemical, 49e57 combined, 61e62 feedstock classification and, 172e176 of lignocellulosic biomass, 19e20 physical, 46e49 physical-chemical, 57e59 process, 43e61 Pro´-Alcol state program, 42 Production rate, 351e352 Promoters, 95e96 CeO2, 96e97 Mg, 97 other promoters, 97 Y2O3, 97 1,2-propanediol (1, 2-PDO), 186 Proton exchange membrane (PEM), 276 Purification of bioethanol, 79 process, 373e374 Pyrolysis, 22e24, 142, 352, 441e443 of biomass, 198e200, 238e239 R Rain-water, 303 Renewable energy, 171

Renewable sources of energy, 209e210 Residue, 448 Reticulated vitreous carbon (RVC), 285 Reverse osmosis (RO), 71e72 River sediments, 419e420 S Salt, 416e417 Second-generation feedstocks (SGF), 42 Separators/membranes, 287e288 Sequencing batch reactor (SBR), 313 Sewage sludge, 408e409, 417e418 Shrinkage of bricks, 423 Silicon dioxide (SiO2), 96 Sinapyl alcohol, 43 Soil nutrients, loss of, 449 Solid biomass feedstock, looping materials development for, 115e116 Solid retention time (SRT), 313 Solid wastes, 265 Solvent extraction, 175 Sorbents, 95 Sorption enhanced BCLG, 91 Sorption-enhanced chemical looping reforming system/technology (SE-CLR system/technology), 102e104 Sorting, 399 Squeezing, 389e390 Stacking E-shape micromixer (SESM), 369 Stage gasifier, 438e439 Starch, 12e14, 42 amylopectine, 14 amylose, 14 Steam explosion, 58 Steam reforming method, 360 Sugarcane bagasse ash (SCBA), 419 Sugars, 42 Supercritical fluid explosion/carbon dioxide explosion, 59 Supercritical methanol, 184 Surface acoustic wave (SAW), 396e397 Sustainable energy, 87 Synthesis gas, 87e88 Syringyl alcohol, 43

Index

T T-type micromixers, 375 Temperature swing adsorption (TSA), 261e262 Tetraethylenepentamine (TEPA), 268 Thermal actuation micropumps, 376 Thermal conductivity, 424e425 Thermal cracking, 182e183 Thermal manipulation, 397 Thermal technologies, 141 Thermobalance bioprocessor, 341 Thermochemical conversion, 20e28, 432e443 of biomass, 234e242 biomass gasification, 236e238 biomass pyrolysis, 238e239 combustion, 20e22, 432e434 gasification, 24e25, 434e441 liquefaction, 26e28 process development, 239e242 pyrolysis, 22e24, 441e443 Thermochemical processes, 141e142 gasification of biomass, 200e202 hydrogen production via, 198e202 liquefaction of biomass, 200 pyrolysis of biomass, 198e200 Thermodynamic liquefaction, 446 Third-generation feedstocks (TGF), 42 3D structures, 370e371 3-phase FBR, 315e316 Torrefaction, 447 Total Acid Number (TAN), 78 Total organic carbon removal, 284 Traditional brick making, 411 Transesterification, 32e33 reaction, 167, 176e177 of soybean oil, 354e355 of vegetable oil using heterogeneous catalyst, 181e182 using homogeneous catalyst, 179e181 Triacylglycerols (TAGs), 209e210 2-phase FBR, 315e316

465

U Ultrafiltration (UF), 71e72 Ultrasound biofuel production from microalgae with, 215e217 lipid extraction from microalgae by ultrasound, 215e217 Uncatalyzed steam explosion, 46e48 United Nation Environment Program (UNEP), 265 Updraft gasifier, 437 Upflow FBR, 315e316 V Vegetable oil, 166 transesterification using heterogeneous catalyst, 181e182 using homogeneous catalyst, 179e181 W Waste-to-energy plants, 22 Wastes categorization, 415, 416t Wastewater treatment aerobic water treatment, 309e310 anaerobic water treatment, 310e312 biomass utilized in water and, 305e308, 306te307t dual-purpose algaeebased systems, 319e320 economic analysis, 321 fixed bed reactors, 314e315 fluidized bed reactors, 315e319 membrane bioreactor, 312e313 operating parameters, 308e309 SBR, 313 Wastewater Treatment Plants (WWTP), 309e310 Water, 303 absorption, 419e420, 423 rate of bricks, 423 biomass utilized in, 305e308, 306te307t Weight loss on ignition of bricks, 423 Wet oxidation, 57

466

Index

Wetland roof (WR), 308 Wood sawdust, 115 and woody biomass, 16e17 Wood ashes (WAs), 268 X Xylans, 9

Y Yttrium oxide (Y2O3), 97 Z Zeolites, 262e263 Zigzag microchannel, 369e370 Zirconia (ZrO2), 96