Palm Trees and Fruits Residues: Recent Advances for Integrated and Sustainable Management 0128239344, 9780128239346

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PALM TREES AND FRUITS RESIDUES Recent Advances for Integrated and Sustainable Management

Edited by

MEJDI JEGUIRIM The Institute of Materials Science of Mulhouse (IS2M), University of Haute Alsace, University of Strasbourg, CNRS, UMR 7361, F-68100 Mulhouse, France

BESMA KHIARI Laboratory of Wastewaters and Environment, Centre of Water Researches and Technologies (CERTE), Technopark Borj Cedria, Touristic road of Soliman, BP 273, 8020, Tunisia

SALAH JELLALI Center for Environmental Studies and Research, Sultan Qaboos University, Al-Khoudh 123, Muscat, Oman

PALM TREES AND FRUITS RESIDUES

Academic Press is an imprint of Elsevier 125 London Wall, London EC2Y 5AS, United Kingdom 525 B Street, Suite 1650, San Diego, CA 92101, United States 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom Copyright © 2023 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. ISBN: 978-0-12-823934-6 For Information on all Academic Press publications visit our website at https://www.elsevier.com/books-and-journals Publisher: Nikki P Levy Acquisitions Editor: Nina Bandeira Editorial Project Manager: Kathrine Esten Production Project Manager: R.Vijay Bharath Cover Designer: Miles Hitchen Typeset by Aptara, New Delhi, India

Contents Contributors Editors biographies Foreword Acknowledgments

1. Identification, Quantification and Characterization of palm-tree and fruit wastes

ix xi xiii xv

1

Mejdi Jeguirim, Besma Khiari and Salah Jellali 1.1 Introduction 1.2 Date palm tree 1.3 Palm oil waste 1.4 Coconut palm trees 1.5 Sustainable valorization of palm wastes Conclusion References

2. Palm trees and fruits residues use for livestock feeding

1 2 10 32 42 45 46

59

Mubarik Mahmood, Kanwal Rafique, Saima, Zafar Hayat, Muhammad Farooq, Muawuz Ijaz, Muhammad Kashif Yar and Zayrah Rafique 2.1 2.2 2.3 2.4

Palm trees and fruits residues use for livestock feeding Introduction and classification of palm trees and fruits Palm trees and fruits residual products and their estimated production Nutrient profile and use of different residual products in feeding of livestock animals Conclusion References

3. Ingredients for food products

59 60 62 66 101 102

115

Nazir Ahmad, Sakhawat Riaz and Anwar Ali 3.1 3.2 3.3 3.4 3.5 3.6 3.7

Introduction Cultivation Wastes Production Impacts of Palm waste on the Environment Palm waste management Waste and byproduct utilization Bioactive ingredients of by byproducts

115 118 119 121 122 125 129 v

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3.8 Sustainability Conclusion References

4. Palm trees and fruits residues’ usage for human health

135 141 142

153

C. Fiore Apuzzo and Marjorie A. Jones 4.1 Introduction 4.2 Date palm (Phoenix dactylifera) 4.3 Coconut palm (Cocos nucifera) 4.4 Oil palm (Elaeis guineensis) 4.5 Sugar palm (Arenga pinnata) 4.6 Areca palm (Areca catechu) 4.7 Açaí palm (Euterpe oleracea) 4.8 Saw palmetto (Serenoa repens) Concluding remarks References

5. Palm wastes for bio-based materials production

153 153 158 162 166 168 172 175 180 180

191

Selsabil El-Ghezal and Besma Khiari 5.1 Introduction 5.2 Bio-based materials production Conclusion References

6. Agricultural applications

191 191 218 218

223

Sarra Hechmi, Rahma Ines Zoghlami, Sonia Mokni-Tlili, Saoussen Benzarti, Mohamed Moussa, Salah Jellali and Helmi Hamdi 6.1 Importance of organic matter in agricultural soils 6.2 Parts and forms of palm residues used as organic amendments 6.3 Impact of palm residues on amended soils Conclusions References

7. Palm wastes valorization for wastewaters treatment

223 224 230 236 236

243

Mansour Issaoui, Meriem Belhachemi, Khaled Mahmoudi, Mahassen Ben Ali, Salah Jellali and Mejdi Jeguirim 7.1 Synthesis and physico-chemical characterization of palm-wastes-derived biochars

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7.2 Synthesis and physico-chemical characterization of palm-wastes-derived activated carbon 7.3 Organic compounds removal by raw and modified palm wastes 7.4 Application of palm-wastes-derived-adsorbents for heavy metals removal from wastewaters (raw + modified biochars/activated carbons) 7.5 Nutrients recovery by palm-wastes-derived materials Conclusions References

8. Palm wastes reuse for gaseous effluent treatment

vii

256 265

279 293 296 298

309

Madona Labaki 8.1 Introduction 8.2 Adsorbents deriving from palm wastes for removal of gaseous pollutants Conclusion References

9. Biofuels production

309 310 348 348

351

Mejdi Jeguirim and Besma Khiari 9.1 Introduction 9.2 Biogas production 9.3 Biodiesel production 9.4 Bioethanol production 9.5 Conclusion References

351 351 368 376 385 386

10. Thermochemical conversion

391

Mejdi Jeguirim and Besma Khiari 10.1 Introduction 10.2 Palm wastes densification and direct combustion 10.3 Conversion into a solid coal-fuel by torrefaction 10.4 Conversion into energy-rich products by pyrolysis 10.5 Conversion into a hydrochar-fuel by hydrothermal carbonization 10.6 Conversion into combustible gas by gasification 10.7 Conclusion References

391 393 409 413 421 425 430 431

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11. The biorefinery concept for the industrial valorization of palm tree and fruit wastes

437

I. Dávila, L. Sillero, I. Egüés, M.M. Antxustegi and J. Labidi 11.1 Introduction 11.2 Added-value compounds obtained from palm tree and fruit wastes 11.3 Antioxidants 11.4 Composites 11.5 Biofuels and bioenergy 11.6 Other compounds from palm and fruits wastes 11.7 Integral valorisation of palm trees and fruits wastes 11.8 Conclusions References Index

437 443 443 447 451 454 456 469 469 479

Foreword We are delighted to write this foreword for this book entitled: “Palm trees and fruits residues: Recent advances for integrated and sustainable management” because we deeply believe in the importance of integrated management of agricultural wastes in environment preservation and resources sustainability. We also believe that end users including scientists as well as stakeholders can boost their knowledge about the most recent findings and practices regarding palm wastes management presented in this book. Over a fruitful career, Editors of this book have intensively worked on the concept of wastes turning into values for a better and more sustainable world. They deeply investigated the use of various solid wastes (agricultural, industrial, urban, etc.) for energy extraction, and the synthesis of new materials to be reused in the domain of agriculture and environment. They specifically worked on the application of the circular economy concept during solid wastes management. One of their most important related works has concerned the management of olive mill solid and liquid wastes that are available in huge amounts in the Mediterranean region. They successfully transformed these harmful wastes, through the use of the pyrolysis and hydrothermal carbonization processes, into valuable biofuels that can be valorized after purification for renewable energy extraction. Moreover, the solid residues of this thermochemical processes: biochars and hydrochars have been effectively used in agriculture as eco-friendly biofertilizer and in environment as a promising adsorbent for various organic and inorganic pollutants. These results were published in outstanding journals and some of them have been relayed in specific press releases in USA, Europe, and Russia. This book concerns the presentation of the most recent advances regarding the sustainable conversion of oil palm, date palm, and coconut palm wastes into added values products. Indeed, even if palm tree cultivation is considered as an important economic outcome for various countries located in several continents: Asia, America, and Africa, it generates huge amounts of biowastes. Nowadays, there is no clear and sustainable applied option for their management. This book reviews and summarizes the most recent scientific works and initiatives related to the quantification, characterization, and valorization of the related generated wastes. Specific attention is paid to the sustainability and circular economy concepts. End users of this book including both scientists and stakeholders responsible for the development of xiii

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Foreword

strategic domains (i.e., energy, agriculture, environment, biotechnology, etc.) will find an interesting matter in this book allowing them to strengthen their knowledge regarding the transformation of the generated palm wastes into added values.Stakeholders in the concerned cultivating countries will find in this book valuable information regarding the valorization of these wastes in agriculture (as animal food and biofertilizers), in energy (as source of renewable biofuels), in environment (as effective adsorbents of pollutants existing in both liquid and gaseous effluents), in biotechnology (for the extraction of bioactive, antioxidant, and antimicrobial substances), and in biomaterial (for the extraction of particleboard, bioplastics, and biodegradable pots). This information is consigned into 10 different chapters written by skilled authors. Whenever they exist, applied real case investigations are summarized and discussed. Moreover, the application of biorefinery concept was developed in a separate chapter. We hope that this book will become a primer for palm wastes management concerned stakeholders, researchers, and teachers, helping them in applying the best and tailored options and in plasticizing the art of critical review and discussions. Book Editors: Dr. Mejdi JEGUIRIM The Institute of Materials Science of Mulhouse (IS2M), University of Haute Alsace, University of Strasbourg, CNRS, UMR 7361, F-68100 Mulhouse, France [email protected] Dr. Besma KHIARI Laboratory of Wastewaters and Environment, Centre of Water Researches and Technologies (CERTE), Technopark Borj Cedria, Touristic road of Soliman, BP 273, 8020, Tunisia [email protected] Salah JELLALI Center for Environmental Studies and Research, Sultan Qaboos University, Al-Khoudh 123, Muscat, Oman [email protected]

Editors biographies

Dr. Mejdi Jeguirim is a Professor at the University of Haute Alsace (France) in the field of energy, process engineering, and kinetics modeling. He dedicated most of his career to the biomass valorization through thermochemical conversion and the chars elaboration for the treatment of aqueous and gaseous effluents. These research topics were performed in the frame of several international collaborations (Tunisia, Germany, Belgium, Algeria, Lebanon, Greece, Cyprus, Spain, Lithuania, Netherland…) and industrial contracts. He acted as PhD advisor for 10 students and he has coauthored more than 150 referred international journal papers in his research field. He is member of the editorial board of international journals (Energy, Energies, Energy for Sustainable Development, and Biofuels) and scientific committee of several international congresses. He is involved as a scientific expert for more than 40 international scientific journals as well as for several national and international research programs. He has received the French National Research Excellence Award for researcher with high level scientific activity for the 2009-2012, 2013-2016, and 2017-2020 periods. Dr. Besma Khiari is a Professor at the University of Carthage (Tunisia) in the field of energy and environment processes, applied mainly to wastes. Recovery from urban, agricultural, and industrial wastes is the main objective of the different papers, books, academic supervisions and industrial projects, she took part in. Her expertise in the area best met the needs and expectations of professionals such as engineering companies, design offices, health services, environmental businesses and units, farmers, etc. Dr. Salah Jellali is serving as senior researcher at the Centre for Environmental Studies & Research, Sultan Qaboos University, Sultanate of Oman. His research interests include wastewaters treatment by low cost materials, nutrients recovery from wastewaters and reuse in agriculture, local water management, and groundwater flow and pollutants transport modeling for a sustainable management. He was involved in various national and international projects regarding the cited above topics.He has coauthored more than 80 referred international journal papers in his research field. He was involved in the management of several international conferences and scientific journal special issues related to wastes/wastewaters sustainable management. He

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Editors biographies

acted as a national and international expert for the evaluation of research projects related to the domain of water/wastes management. He was the holder of the ministerial award of rural engineering diploma in 1994 and the presidential award for the best national supervised PhD thesis in 2018.

Contributors Nazir Ahmad Department of Nutritional Sciences, Government College University Faisalabad, Pakistan Anwar Ali Department of Epidemiology and Health Statistics, Xiangya School of Public Health, Central South University, Yuelu District, Changsha, Hunan, China Mahassen Ben Ali Laboratory of Composite Materials and Clay Minerals, National Center of Research in Materials Sciences (CNRSM), Soliman, Tunisia M.M. Antxustegi Chemical and Environmental Engineering Department. University of the Basque Country UPV/EHU. Plaza Europa, 1, San Sebastian 20018, Spain C. Fiore Apuzzo Department of Chemistry, Illinois State University, Normal, IL, United States of America Meriem Belhachemi Chemistry and environmental sciences laboratory, Tahri Mohamed university, Bechar, 417, Kenadsa road, 08000, Algeria Saoussen Benzarti Lusail University, Lusail City, Doha, Qatar I. Dávila Chemical and Environmental Engineering Department. University of the Basque Country UPV/EHU. Plaza Europa, 1, San Sebastian 20018, Spain I. Egüés Chemical and Environmental Engineering Department. University of the Basque Country UPV/EHU. Plaza Europa, 1, San Sebastian 20018, Spain Selsabil El-Ghezal Textile Research Unit of ISET of Ksar-Hellal, Ksar Hellal, Tunisia Muhammad Farooq University of Veterinary and Animal Sciences (UVAS), Lahore, sub campus, Jhang-Epidemiology and Public Health, Pakistan Zafar Hayat University of Sargodha, Animal Nutrition, Pakistan Helmi Hamdi Food and Water Security Program, Center for Sustainable Development, College of Arts and Sciences, Qatar University, Doha, Qatar Sarra Hechmi Water Research and Technology Center, University of Carthage, Soliman, Tunisia Mansour Issaoui Wastewaters and Environment Laboratory, Water Research and Technologies Center, P.O. Box 273, Soliman 8020, Tunisia Mejdi Jeguirim The Institute of Materials Science of Mulhouse (IS2M), University of Haute Alsace, University of Strasbourg, CNRS, UMR 7361, F-68100 Mulhouse, France ix

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Contributors

Salah Jellali Center for Environmental Studies and Research, Sultan Qaboos University, Al-Khoudh 123, Muscat, Oman Marjorie A. Jones Department of Chemistry, Illinois State University, Normal, IL, United States of America Besma Khiari Laboratory of Wastewaters and Environment, Centre of Water Researches and Technologies (CERTE), Technopark Borj Cedria, Touristic road of Soliman, BP 273, 8020, Tunisia Madona Labaki Lebanese University, Faculty of Sciences, Laboratory of Physical Chemistry of Materials (LCPM/PR2N), Fanar, Jdeidet El Metn, Lebanon J. Labidi Chemical and Environmental Engineering Department. University of the Basque Country UPV/EHU. Plaza Europa, 1, San Sebastian 20018, Spain Muawuz Ijaz University of Veterinary and Animal Sciences (UVAS), Lahore, sub campus, Jhang-Meat Technology, Pakistan Mubarik Mahmood University of Veterinary and Animal Sciences (UVAS), Lahore, Jhang-Pakistan, Animal Nutrition, Jhang, Pakistan Khaled Mahmoudi Laboratory of Composite Materials and Clay Minerals, National Center of Research in Materials Sciences (CNRSM), Soliman, Tunisia Sonia Mokni-Tlili Water Research and Technology Center, University of Carthage, Soliman, Tunisia Mohamed Moussa Arid Regions Institute, University of Gabès, Médenine, Tunisia Kanwal Rafique University of Veterinary and Animal Sciences (UVAS), Lahore, sub campus, Jhang-Poultry Production, Pakistan Zayrah Rafique University of Veterinary and Animal Sciences (UVAS), Lahore, sub campus, Jhang-Visiting faculty, Pakistan Sakhawat Riaz Deparment of Home Economics, Government College University Faisalabad, Pakistan Saima University of Veterinary and Animal Sciences (UVAS), Lahore, Animal Nutrition, Pakistan L. Sillero Chemical and Environmental Engineering Department. University of the Basque Country UPV/EHU. Plaza Europa, 1, San Sebastian 20018, Spain Muhammad Kashif Yar University of Veterinary and Animal Sciences (UVAS), Lahore, sub campus, Jhang-Meat Technology, Pakistan Rahma Ines Zoghlami Arid Regions Institute, University of Gabès, Médenine, Tunisia

Acknowledgments This book is the outcome of a challenging idea that started approximately 2 years ago. At the end of this pleasing experience, Editors are grateful to all the authors involved in the ten chapters of this book for their big efforts and engagement. Complete thanks to Kathrine ESTEN, Editorial Project Manager in Elsevier for her support and effective accompaniment during the last previous months. Special thanks to Elsevier for its renewed confidence and to the entire production and marketing teams.

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CHAPTER ONE

Identification, Quantification and Characterization of palm-tree and fruit wastes Mejdi Jeguirima, Besma Khiarib and Salah Jellalic

a The Institute of Materials Science of Mulhouse (IS2M), University of Haute Alsace, University of Strasbourg, CNRS,UMR 7361, F-68100 Mulhouse, France b Laboratory of Wastewaters and Environment, Centre of Water Researches and Technologies (CERTE), Technopark Borj Cedria, Touristic road of Soliman, BP 273, 8020, Tunisia c Center for Environmental Studies and Research, Sultan Qaboos University, Al-Khoudh 123, Muscat, Oman

1.1 Introduction Huge amounts of agricultural wastes are annually produced in the world. The sustainable management of these wastes has been identified as an urgent defy to be appropriately handled in order to preserve human health and the environment (Bhatt et al., 2021). Turning these agricultural wastes into values and their safe reuse in a context of circular economy has been stressed and applied in several developed countries (Duan et al., 2020; Ong, Kaur, Pensupa, Uisan, & Lin, 2018). Extrapolating a such strategy to the developing countries will allow their economic development as well as the acceleration of the achievement of several Sustainable Development Goals (SDGs) such as SDG2, SDG3, SDG7, SDG13, and SD15, related the “End hunger, achieve food security and improved nutrition and promote sustainable agriculture”, “Ensuring of healthy lives and promotion of wellbeing for all at all ages”, “Ensure access to affordable, reliable, sustainable and modern energy for all”, “Taking urgent actions to combat climate change and its impacts”, and “Protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, and halt and reverse land degradation and halt biodiversity loss (UN, 2019). Palm trees contain various types. The most planted ones are those destined for dates, coconut and oil production. The date palm trees cultivation is mainly located in the Middle East and North Africa (MENA) region (Bastidas-Oyanedel et al., 2016a). The coconut palm trees are originated from the south-east of Asia and north-west and south America (Ojha, Roy, Das, & Dhangadamajhi, 2019). However, the oil palm trees exist Palm Trees and Fruits Residues: Recent Advances for Integrated and Sustainable Management. DOI: https://doi.org/10.1016/B978-0-12-823934-6.00009-5

c 2023 Elsevier Inc. Copyright  All rights reserved.

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Mejdi Jeguirim, Besma Khiari and Salah Jellali

in tropical countries such as Indonesia, Malaysia, and Thailand (MoraVillalobos et al., 2021). During their growth or industrial processing of the fruits, palm trees could generate huge amounts of solid wastes including falls (discarded fruits), stones, shells, flower stalks, panicles and leaves (Bastidas-Oyanedel et al., 2016b; Chia et al., 2020; Mora-Villalobos et al., 2021). These solid wastes could be turned into values and reused in several domains such as human health improvement, animal feeding, water treatment, crops growth enhancement, biomaterials synthesis, and energy recovery (Awad, Zhou, Katsou, Li, & Fan, 2021a; Bastidas-Oyanedel et al., 2016b; Chia et al., 2020; Hassan et al., 2019; Khokhar & Teixeira Da Silva, 2017; Mora-Villalobos et al., 2021). To identify the best technically-feasible, economically-viable, and socially acceptable valorization options of these wastes, it is imperative to get a more precise assessment of: i) their identification versus the geographical localization and fruits processing methods, ii) their annual worldwide produced amounts, and iii) their physical, chemical and energetic properties. This book chapter aims to summarize and analyze the state of the art regarding the cultivation of palm trees around the world and the estimation of the nature, quantities, and properties of the generated palm solid wastes during the fruits growth and their industrial processing. It gives some recommendations about their sustainable reuse in a context of circular economy.

1.2 Date palm tree The date palm is probably the most ancient cultivated tree in the world. The exact date palm origin is considered to be lost in Antiquity. However, it is certain that the date palm was cultivated as early as 4000 B.C. since it was used for the construction of the temple of the moon god near Ur in Southern Iraq – Mesopotamia (Zaid & Arias-Jimenez, 1999). The date palm trees, with more than 5000 varieties, are cultivated frequently in the arid and semi-arid regions. In fact, the date palm cultivated area exist in Asia, Africa, the Americas, and Europe at percentages of 67.4%, 32.0%, 0.6%, and 0.04%, respectively. The date palm trees cover an area of more than 1.381 million hectares (FAO, 2021). Iran has the largest superficies for date palm cultivation with 180,000 ha, followed by Iraq, 125,000 ha. Morocco has 84,500 ha while Saudi Arabia, Algeria and Egypt each have approximately 45,000 ha, Libya has 27,500 ha and Tunisia 22,500 ha (Zaid & Arias-Jimenez, 1999). Number of date palms worldwide is estimated at 105 million. Fig. 1.1 shows the number of palms for the 12 countries having more than 1,000,000

Identification, Quantification and Characterization of palm-tree and fruit wastes

3

Figure 1.1 Number of palms per country (data from (Zaid & Arias-Jimenez, 1999)).

Figure 1.2 Date palm tree.

palms. It is evident that Iraq, Iran and Saudi Arabia palms accounts for about 55% of world’s total palms (Zaid & Arias-Jimenez, 1999). The date palm tree (Phoenix dactylifera) illustrating the different parts, namely date palm leaflets, rachis, trunk, and fruit stalk pruning is presented in Fig. 1.2. The stem of date palm tree is unbranched and is surrounded by old remaining leaves and palms. The trunk of the date palm is covered by several offshoots at the base of the tree. The average economic life of a date palm tree is about 40 to 80 years. However, some of these trees can live up to 150 years. The average height of a mature date palm tree is about 20 to 23 meters.

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Mejdi Jeguirim, Besma Khiari and Salah Jellali

Table 1.1 (no content from ELSA) Average weight of date palm residues (Ahmed, 2016; Haimour & Emeish, 2006; Kumar et al., 2019; Sait et al., 2012; Zaid & Arias-Jimenez, 1999).

Type of Residue Date palm leaflets (DPL) Date palm rachis (DPR) Fruit pruning (FP) Date palm trunk (DPT)

Weight of Residue (kg/tree/year) 9.2 10.8 0.5 60

1.2.1 Date Palm Residues Quantification Following the date fruits cultivation, the tree parts are separated in different residues. An adult date palm tree has approximately 100 to 125 green palms with an annual formation of 10 to 26 new leaves (Zaid & Arias-Jimenez, 1999b) (FAO, 2021; Zaid & Arias-Jimenez, 1999). Each year, some leaves become dry and their removal from the date palm tree is required. The date palm branches are composed of 46% of leaflets and 53.4% rachis The annual production of dry leaves from date palm tree is about 20 kg (Haimour & Emeish, 2006). The mass of a single bunch of fruit pruning with date fruit is about 8 kg (Sait, Hussain, Salema, & Ani, 2012). On a dry basis, fruit pruning without date fruits has a weight of 0.4 to 0.5 kg. The average fruit stalk pruning in a tree is 11 to 15, and the average number of trees in an acre is 80 to 120 (“Date Palm Plant,” n.d.). The approximate mass of the trunk of tree is estimated to be 60 kg per acre since minimum one or two trees expires annually in an acre, and new trees are planted(Kumar et al., 2019). The average weights of different date palm residues are given in Table 1.1. Therefore, using the data of date palm trees and cultivation area, the annual global production of date palm residues should be about 3 million tones. Moreover, generally about 25% of the total produced dates amount (about 2.27 MT/year) have low quality and could finish as solid wastes (BastidasOyanedel et al., 2016b). The date pits or stones constitutes about 10% of these wastes weight (Ahmed, 2016a).

1.2.2 Date Palm Residues Characterization Date palm residues have been characterized in various investigations through the determination of proximate and ultimate analyses along with bulk densities. The obtained results are given in Table 1.2. The analysis findings show that tested samples have high content of volatiles, carbon, hydrogen and oxygen. On the contrary, the relative contents of nitrogen and sulfur are low. In comparison with lignocellulosic biomass, these materials have

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Identification, Quantification and Characterization of palm-tree and fruit wastes

Table 1.2 Proximate and ultimate analysis and bulk density of DPW (El may et al., 2012). Proximate analysis (%), Biomass wet basis Ultimate analysis (%), wet basis Samples Moisture VM

FC

Ash LHV BD ED C

H

O

N

S

DPL DPR DPT DS PF

9.7 8.3 11.5 17.5 16.8

15.2 6 4.2 1.2 2.8

6 5.7 5.8 6.4 5.8

35.2 43 45.5 40.9 40.9

0.63 0.19 0.21 0.73 532 7.5 ppmv       ET(min): 4000 2.5 ppmv > 2312 5 ppmv > 1574 7.5 ppmv

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Figure 8.1 Effect of influent ammonia gas concentration on the adsorption breakthrough curve profile of ammonia gas on to GAC produced from date palm pits. Ammonia gas flow rate: 1.1 L.min−1 , GAC column length = 8 cm, GAC bed diameter = 6.35 mm, Influent ammonia gas concentrations: 2.5, 5, and 7.5 ppmv. From Vohra, M. (2020). International Journal of Environmental Research and Public Health, 17(5), 1519. https://doi.org/10.3390/ijerph17051519.

Fig. 8.1 gives an example of the obtained results. Furthermore, curves of Fig. 8.1 show wide adsorption breakthrough curve that indicates comparatively larger length of the mass transfer zone. Such profiles are different than those obtained with benzene on GAC derived from DPP (Naushad et al., 2019) where sharp breakthrough curves were observed. Indeed, interaction between the basic ammonia and surface acidic groups (Bandosz and Petit, 2009) may explain the wide curve in the case of NH3 . In addition, the increase in BT and ET is almost proportional to the decrease of ammonia concentration. Such an observation is in line with the absence of mass transfer rate limitation, revealing that no diffusional limitation takes place. Therefore, the decrease of BT and ET with increasing ammonia inlet contents is explained by the limited number of adsorption sites on GAC surface, which will be occupied in less time with higher ammonia amount. Adsorption experiments were also conducted with varying gas flow rates (1.1, 1.65, 2.2, and 3.3 L.min−1 ), keeping GAC column length at 8 cm and inlet ammonia concentration at 5 ppmv.It is found that a progressive increase of the total flow rate led to a respective decrease of BT and ET, as follows:         BT(min): 712 1.1L.min−1 > 383 1.65L.min−1 > 272 2.2L.min−1 > 197 3.3L.min−1

        ET(min): 2312 1.1L.min−1 > 1673 1.65L.min−1 > 1315 2.2L.min−1 > 1213 3.3L.min−1

It was found that the mass transfer rate of ammonia is negligible and therefore the above results are linked with the limited amount of surface sites

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able to adsorb ammonia which will be occupied more fastly when higher flow rates are used. Different GAC column lengths were used (4, 6, and 8 cm) with 1.1 L.min−1 total flow rate and 5 ppmv ammonia content. Experiments showed that increasing column length led to a decrease of BT and ET as follows: BT(min): 712(8cm) > 385(6cm) > 197(4cm) ET(min): 2312(8 cm) > 1134(6cm) > 1054(4cm) In absence of mass transfer limitation, the trend above is explained by the higher number of adsorption sites existing on the surface of longer GAC column resulting in a delay of BT and ET. A comparison between GAC derived from DPP and commercial GAC (Filtrasorb 400, Calgon, Moon Township, USA) showed higher ET and BT for DPP-derived GAC. The larger average pore width of DPP-GAC, 23 Å, compared to that of commercial GAC, 3.26 Å, may explain such a result. Indeed, 23 Å is much higher than ammonia molecular size. Since DPP-GAC is mainly porous, it offers to NH3 molecules high available area for adsorption. Infrared spectroscopy evidenced the presence of O–H, C–H, C–O, and S = O groups on the surface of DPP-GAC.Therefore,it is expected an easier uptake of ammonia. Indeed, gaseous ammonia molecule may establish bonds with acidic functional groups and also oxygen functional groups. NH3 could be a hydrogen-bond donor or acceptor and therefore may react with the chemical functional groups present on DPP-GAC surface. Other interaction forces may play a role,such as Van der Waals ones.One mechanism postulated for ammonia interaction is (Bandosz and Petit, 2009):     NH3 gas ↔ NH3 aqueous     NH3 aqueous +H+ ↔ NH4 + aqueous GAC−O− +NH4 + ↔ GAC − O − NH4 DPP-GAC is found to be a promising candidate for ammonia gaseous removal. Guo et al. (2005) prepared carbonaceous adsorbent materials from palm shell (PS) taken from a palm-oil mill in Malaysia (Guo et al., 2005). The raw material was dried and brought to a size of 1–2 mm. Three different treatments processes were then performed:a) treatment with 200 mL H2 SO4

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Figure 8.2 Effects of H2 SO4 concentration on the BET and micropore surface areas of palm-shell activated carbons. From Guo, J., Xu, WS, Chen, Y. L. & Lua, A. C. (2005). Journal of Colloid and Interface Science, 281(2), 285–290. https://doi.org/10.1016/ j.jcis.2004.08.101.

with known concentration (5–40 percent) at ambient temperature during 24 h, followed by solvent evaporation and then an activation under nitrogen flow during 2 h at different temperatures (300 to 700 °C) and finally, after cooling down to ambient, the samples were washed with distilled water, b) pyrolysis under N2 for 2 h at temperatures ranging from 300 to 700 °C to give char samples, c) the char samples were activated by CO2 for 2 h at temperatures 500 to 900 °C. The specific surface area (BET) and the micropore area of the material increased with increasing the amount of H2 SO4 used, to reach a maximum for 30 percent H2 SO4 after which it decreases (Fig. 8.2). Such a result is explained by the inhibiting effect of H2 SO4 on tar and liquid formation as well as on particle shrinkage and volume contraction. Indeed, water vapor released during acid dehydration leads to higher carbon burning and consequently higher gasification and better porosity. However, a high acid content may result in an over gasification leading to a decrease of porosity. In all cases, values of specific surface areas higher than 600 m2 .g−1 are obtained, as shown in Fig. 8.2, such values are promising for gases adsorption. 2000 ppmv NH3 diluted in N2 was used as adsorbing mixture on 20 mg of carbon materials at different temperatures. Ammonia uptake is

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Figure 8.3 Amounts of NH3 adsorbed onto palm-shell activated carbons prepared by thermal and chemical activation at various adsorption temperatures. From Guo, J., Xu, WS, Chen, Y. L. & Lua, A. C. (2005). Journal of Colloid and Interface Science, 281(2), 285–290. https://doi.org/10.1016/j.jcis.2004.08.101.

higher on samples activated by H2 SO4 than those activated by CO2 as shown in Fig. 8.3. Furthermore, it was proven that NH3 is physically adsorbed (Van der Waals forces) on the carbonaceous material prepared by CO2 activation and that the amount of adsorbed ammonia is proportional to the surface area. It is not the case for materials prepared with H2 SO4 where, besides physical adsorption, chemical one is taking place, which is not only proportional to surface area but also is in direct correlation with the amount of oxygen present on the surface of the adsorbent. Indeed, infrared spectroscopy evidenced the presence of different oxygen functional groups: hydroxyl, phenols, carboxylic acids or anhydrides, carbonyl. Strong hydrogen bonding between H atoms of NH3 and O present on the surface, especially OH and C = O, takes place. In another study from the same authors (Guo and Lua, 2002a), the CO2 activation of oil-palm shell (also called endocarp) was performed in two steps: heating during 3 h under nitrogen at 600 °C followed by a cooling down and a subsequent activation by 100 mL.min−1 flow CO2 during 0.5 h at temperatures ranging between 500 and 900 °C. The increase of activation temperature led to an increase in surface area and micropore volume. In this study, the direct linear relation between surface area and NH3 as well as

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Figure 8.4 Amounts of adsorbed NH3 and NO2 gases versus surface area of the activated carbon obtained from oil palm shell. Adsorption conditions: 25 °C and 1000 ppm NH3 or NO2 balanced in nitrogen. From Guo, J. & Lua, A. C. (2002). Journal of Colloid and Interface Science, 251(2), 242–247. https://doi.org/10.1006/jcis.2002.8412.

NO2 uptake was evidenced as shown in Fig. 8.4. The neutral and slightly acidic surface functional groups of the studied activated carbon, evidenced by infrared spectroscopy, are in the origin of the uptake of the basic gas NH3 and the acidic gas NO2 .

8.2.2 Carbon dioxide The increase in atmospheric level of greenhouse gases (GHG) is in the origin of the global warning and climate change. Carbon dioxide (CO2 ) is considered as the most important GHG with largest impact on climate change. CO2 is mainly emitted by fossil fuel burning. Various processes were suggested to reduce CO2 emissions: liquid solvent absorption mainly by amine solutions, cryogenic techniques, membrane separation, solid sorbents, and pressure (and/or temperature) swing adsorption. Among these techniques, the most advantageous one is CO2 adsorption on solid materials since it does not require energy, it is simple, could be used in wide range of pressure and temperature, and is economically and environmentally feasible (Nasri et al., 2014; Rashidi and Yusup, 2015; Shafeeyan et al., 2010). The challenge is to find low-cost effective adsorbents that are easily regenerable and renewable. Carbonaceous adsorbents deriving from biomass are good candidates. Modification of surface of such adsorbents by some chemical

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functions make them more efficient towards CO2 removal (Shafeeyan et al., 2010). Some studies reported CO2 adsorption on carbon materials issued from palm residues, activated physically or chemically. 8.2.2.1 Sorbents activated physically Nasri et al. (2014) used palm kernel shell (PKS) from Malaysia. PKS was washed with water, dried, then sieved into particles with sizes 0.5–1.18 mm, heated under nitrogen flow to 700 °C during 2 h and then cooled down to give the palm kernel char (PKC). PKC was heated under nitrogen till 800 °C and then subjected to CO2 flow at the same temperature and then cooled down under nitrogen to give the palm activated carbon (PAC). PAC showed higher specific surface area and higher total and microporous pore volumes than PKC (25 m2 .g−1 as surface area and 0.0086 cm3 .g−1 as micropore volume for PKC versus 167 m2 .g−1 and 0.0803 cm3 .g−1 respectively for PAC). It is deduced that physical activation by CO2 led to development of surface area and microporous volume which are the reason behind the higher adsorption capacities of PAC compared to PKC (adsorption capacity of 5.60 mmolCO2 .g−1 for PKC at 30 °C and 4 bars versus 7.32 mmolCO2 .g−1 for PAC) (Nasri et al., 2014). Rashidi and Yusup (2015) collected different Malaysian agricultural wastes, palm shell, palm mesocarp fibre, coconut shell, coconut fiber, and rice husk. After drying the samples and sieving to get a size of 0.25–1.00 mm, a treatment under CO2 flow at temperatures between 500 and 900 °C for 15 to 90 min durations was carried out. It was found that the activation temperature and the origin of the materials have significant impact on CO2 adsorption capacities. Indeed, a maximum CO2 adsorption capacity of 1.78 mmol.g−1 was recorded for coconut shell, with size 0.25 mm, activated 45 min under 150 mL.min−1 CO2 at 900 °C (20 °C.min−1 ) (Rashidi and Yusup, 2015). 8.2.2.2 Sorbents activated chemically Ello et al. (2013) prepared microporous carbon materials from African palm shell (PS) by carbonization at two different temperatures (600 °C or 700 °C during 1 h) under N2 atmosphere followed by chemical activation by potassium hydroxide (KOH) with different weight ratios KOH:char, ranging from 1:1 to 5:1. After mixing with KOH, the sample was subjected to a heating at 850 °C during 1 h under N2 , washed with 1 mol.L−1 HCl solution and then with deionized water and dried (Ello et al., 2013).

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KOH increases the surface area and the total pore volume of the materials from 365 m2 .g−1 and 0.16 m3 .g−1 respectively for the non-activated sample to 770–1890 m2 .g−1 and 0.35–0.90 cm3 .g−1 for the activated ones. It was demonstrated that KOH plays a role in developing microporosity, which was found to be essential for CO2 adsorption, and total surface area of the carbonaceous adsorbents. The volume of the ultramicropores (micropores with size below 0.7 nm) Vumi and that of the supermicropores (size between 0.7 and 2 nm) Vsmi were evaluated in this study. Increasing KOH:char ratio up to 3 increased Vumi . For higher ratios, Vumi decreases. However, Vsmi increases with increasing KOH:char ratio and some mesoporosity was also developed. CO2 adsorption capacity increased significantly with KOH activation as can be seen in Fig. 8.5 (3.9 to 4.4 mmol CO2 .g−1 at 25 °C for activated samples versus 1.9 mmol CO2 .g−1 under the same conditions for the non activated one) and depends mainly on Vumi , the samples with highest Vumi presented the highest CO2 adsorption capacities. Pal et al. (2020) also used KOH as activating agent during elaboration of CO2 adsorbents from waste palm trunk (WPT) and mangrove (M) (Pal et al., 2020). The elaboration procedure is similar to that of (Ello et al., 2013) but with a selected size lower than 5 mm, carbonization temperatures of 500 and 600 °C, a weight ratio KOH:char of 6:1 with a subsequent heat at 900 °C. The resulting total surface area and pore volumes were respectively in the range 2848–2927 m2 .g−1 and 2018–87 cm3 .g−1 . Such values are higher than most of the classical carbonaceous adsorbents and are in favor of good adsorbing capacities. The highest CO2 uptake was found for WPT carbonized at 500 °C (value given in Table 8.1). Sahri et al. (2020) activated palm kernell shell (PKS) with KOH:char weight ratio 1:1 at 85 °C, previously brought to a size of 0.65–0.8 mm and carbonized at 700 °C under nitrogen flow (Sahri et al., 2020). The sample was then subjected to a microwave treatment under nitrogen flow during 6 min with a power of 400 W. Higher surface area and micropore volume were obtained compared to the same sample activated physically with CO2 (PAC) (Nasri et al., 2014); about 323 m2 .g−1 for total surface area (versus 167 m2 .g−1 for PAC) and 0.105 cm3 .g−1 for microporous volume (versus 0.0803 cm3 .g−1 for PAC). Yang et al. (2011) used coconut shell as precursor for sorbents to separate CO2 from CH4 in natural gases (Yang et al., 2011). The coconut shells were cut into small blocks with dimensions 3 cm × 3 cm × 3 cm, washed with

318

Figure 8.5 Isotherms of CO2 adsorption on carbonaceous materials issued from African palm shells CTKy, where T represents the carbonization temperature (600 or 700 °C) and y the weight ratio KOH:char, A) at 25 °C, B) at 0 °C. From Ello, AS, De Souza, LKC, Trokourey, A. & Jaroniec, M. (2013). Journal of CO2 Utilization, 2, 35–38. https://doi.org/10.1016/j.jcou.2013.07.003. Madona Labaki

African palm shell

277.2 193.6

1 atm and 0 °C 1 atm and 25 °C

(Ello et al., 2013)

73.0 170.3 322.1 56.1 88.9 248.2 301.4

1 atm and 30 °C (Nasri et al., 2014) 2 atm and 30 °C 4 atm and 30 °C 5 atm and 25 °C (Sahri et al., 2020) 5 atm and 10 °C 25 atm and 25 °C 25 atm and 10 °C

78.3

1 atm and 25 °C

1791

(Rashidi and Yusup, 2015) 50 atm and 25 °C (Pal et al., 2020)

100

1 atm and 25 °C

(Aroua et al., 2008)

49

1 atm and 25 °C

(Khalil et al., 2012) 319

Carbonization at 600 °C followed by activation with KOH at 850 °C, with weight ratio KOH:char = 3 Malaysian palm Heating under nitrogen at 700 °C and kernel shell activation by CO2 flow at 800 °C Size 0.5–1.18 mm Malaysian palm Heating under nitrogen at 700 °C and kernel shell activation by KOH at 85 °C, with weight ratio KOH:char = 1, treatment by microwave Size 0.65–0.8 mm Malaysian coconut Activation by CO2 flow at 900 °C Size 0.25 mm shell Malaysian waste Carbonization at 500 °C followed by palm trunk activation with KOH at 900 °C, with weight ratio KOH:char = 6 Size < 5 mm Malaysian palm Activation by steam followed by shell polyethyleneimine PEI impregnation (0.26 wt% PEI) Size 0.71–0.85 mm Malaysian palm Activation by steam followed by shell monoethanolamine (MEA) impregnation with weight ratio MEA:char = 0.4 Size 0.5 mm

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Table 8.1 CO2 adsorption capacities (mg CO2 .g−1 adsorbent) on different carbon materials issued from palm residues. CO2 adsorption Adsorption capacity (mg Raw material Activation Reference CO2 .g−1 adsorbent) conditions

(continued on next page)

Malaysian palm shell

Malaysian palm shell

Coconut shell

African palm stone

Activation by steam followed by impregnation with 2-amino-2-methyl-1-propanol (AMP) with a ratio 1.27 × 10−4 mol amine.g−1 adsorbent Size 0.51–0.71 mm Activation by steam followed by impregnation with choline hydroxide:urea (1:0.5) with a ratio carbon:solvent 1:2

64

1 atm and 25 °C

(Lee et al., 2013)

37.2 38.3

1 atm and 25 °C (10 percent CO2 concentration) 1 atm and 25 °C (20 percent CO2 concentration) 1 atm and 25 °C 2 atm and 25 °C

(Hussin et al., 2021)

1 atm and 0 °C

(Vargas et al., 2012)

(Yang et al., 2011)

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Activation under nitrogen at 600 °C followed 66 by mixing with phosphoric acid solution, 88 heating at 600 °C then washing with alkali solution Activation with CaCl2 solution (2 percent 254 w/v), with a ratio 2 mL solution/1 g of palm stone, heating under CO2 at 800 °C followed by heating under nitrogen at 600 °C, washing with acid and water. Monolith form.

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Table 8.1 CO2 adsorption capacities (mg CO2 .g−1 adsorbent) on different carbon materials issued from palm residues—cont’d CO2 adsorption Adsorption capacity (mg Raw material Activation Reference CO2 .g−1 adsorbent) conditions

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(A)

(B)

Figure 8.6 Carbonaceous materials from African palm stones: A) granular activated carbon, B) activated carbon monoliths. From Vargas, DP, Giraldo, L. & Moreno-Piraján, J. C. (2012). Journal of Analytical and Applied Pyrolysis, 96, 146–152. https://doi.org/10.1016/ j.jaap.2012.03.016.

deionized water, dried, carbonized at 600 °C under nitrogen flow. Three samples were then prepared, one by calcination under nitrogen at 800 °C (W-AC), the second by impregnation with phosphoric acid solution, heating at 600 °C under nitrogen,cooling down,then washing with alkali solution to neutrality (P-AC), the third by mixing with KOH solution, drying, heating at 800 °C, then cooling down and washing with hydrochloric acid (HCl) to neutrality (K-AC). All the samples presented microporosity with micropore surface area ranging between 1575 and 1922 m2 .g−1 and micropore volumes between 0.53 and 0.68 cm3 .g−1 with good CO2 adsorption capacities. Nevertheless, P-AC adsorbs hardly methane and adsorbs well CO2 and therefore is promising for purifying natural gas. K-AC satisfies also with separation and concentration of greenhouse gases since both CO2 and CH4 adsorb on its surface. Vargas et al. (2012) prepared granular activated carbon (GAC) and monolith one (M), depicted in Fig. 8.6, from African palm stones. The size of GAC is 2–3 mm and the monoliths have an outer diameter of about 1.5 cm, a height of 8 mm and are traversed longitudinally by six parallel channels of 3 mm each. Three chemicals, phosphoric acid (H3 PO4 ), zinc chloride (ZnCl2 ), and calcium chloride (CaCl2 ) were used to activate African palm stone with a ratio 2 mL solution/1 g solid. Different concentrations of H3 PO4 (32 percent, 36 percent, 40 percent, 48 percent w/v), ZnCl2 (32 percent, 36 percent, 40 percent, 48 percent w/v), and CaCl2 (2 percent, 3 percent, 5 percent, 7 percent w/v) solutions were used. For elaborating

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monoliths with CaCl2 , a mixture of H3 PO4 and CaCl2 is used to facilitate compaction of monoliths. Impregnation is realized at 85 °C for a period of 7 h, followed by a temperature increase till 120 °C during 10 h For monolith, compaction process is realized with a pressure of about 300 atm and a temperature of 150 °C. The materials activated with H3 PO4 and ZnCl2 are heated under nitrogen flow at 500 °C. The materials activated with CaCl2 are heated under CO2 flow at 800 °C during 6 h followed by a heating under nitrogen flow at 600 °C to remove excess CO2 from the materials. Subsequently, the materials were washed with hot deionized water (for solids activated with H3 PO4 ) or with HCl solution then with hot water (for the other solids) and dried (Vargas et al., 2012). The activating agent affects the surface area and porous volume of the solid. With ZnCl2 and CaCl2 solids with narrow microporosity and poor development of mesoporosity are obtained whereas with H3 PO4 , solids with greater proportion of mesopores were obtained. Indeed, H3 PO4 led to materials with higher surface area and developed porous structure due to the insertion of the strong dehydrating H3 PO4 , during heat treatment, into the cellulose chains and the consequent replacement of hydrogen bonds between OH groups in the cellulose polymer by phosphate bonds. Removal of phosphate gives rise to materials with developed structure and surface area. ZnCl2 and CaCl2 are also dehydrating agents for cellulose,hemicellulose,and lignin but their effect is lower than that of H3 PO4 because of their lower acidity. ZnCl2 and CaCl2 hydrates are small molecules, with lower size for zinc than for calcium, and when removed by washing, a solid with uniform and narrow porosity will be generated. In contrary, molecules with different sizes are produced with H3 PO4 , such as H4 P2 O5 and H13 P11 O34 that lead after their elimination to materials with heterogeneity in microporosity. It was shown in this study that the increase of H3 PO4 concentration led to an increase of CO2 uptake. In addition, the GAC with lower concentrations of ZnCl2 were more CO2 adsorbing whereas the M samples with the highest ZnCl2 concentrations were more adsorbing. As about CaCl2 , the lowest concentrations of activating agent led to the most adsorbing materials. This study put also into evidence the close relation between CO2 uptake and narrow micropores volume on one hand, and on the other hand, the increase of CO2 uptake with the increase of total basicity and decrease of total acidity of surface functional groups, CO2 being an acidic molecule. Aroua et al. (2008) investigated activated carbon issued from palm shell (PS). A physical activation by steam was firstly performed, followed by a

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sieving to get sizes between 0.71 and 0.85 mm, washing with water, drying, and then impregnation by polyethyleneimine (PEI) solution followed by washing and drying. PEI contains nitrogen. The weight percentages of PEI are 0.06, 0.11, 0.13, 0.26, 0.27, and 0.28 percent. Adsorption of different gases on the as-obtained materials was studied. The sequence of maximum adsorption capacities is as follows:CO2 >> CH4 > O2 > N2 .This order was found to be interesting for CO2 removal and gases separation, especially for separation of CO2 from CH4 in natural gases. PEI increased the adsorption capacity of the activated carbon, whatever the adsorbate is. Adsorption capacity increases with increasing PEI yield up to 0.26 wt%.Above this value, a decrease in adsorption capacity is noted. It seems that PEI contributed to creation of additional meso– and micropores which enhanced adsorption capacity of the material and also PEI created surface chemical functions which favor gases chemisorption. Further increase in PEI loading may result in an excess reduction of pore size which may result in a decrease of gas accessibility. The highest affinity towards CO2 adsorption is explained by the chemical affinity between CO2 (Lewis acid) and nitrogen chemical functions (Lewis base) present on the surface and evidenced by infrared study (Aroua et al., 2008). In another work from the same group (Khalil et al., 2012), impregnation with nitrogen-based compounds, monoethanolamine (MEA) or 2-amino-2-methyl-1-propanol (AMP), is performed with weight ratio MEA or AMP:char of 0.4. The selected size for adsorption experiments is 0.5 mm. Even though a decrease of surface area and a pore blockage are observed when MEA or AMP is present, introduction of nitrogen groups enhanced CO2 uptake due to chemical affinity between CO2 and nitrogen functions. Such a result puts into evidence that not only CO2 physisorption is taking place but also chemisorption. In addition, a better adsorption improvement is obtained with MEA than with AMP. Indeed, nitrogen sites are more accessible in MEA than in AMP due to the molecular structures of these compounds where steric hindrance is more predominant in AMP. It is also shown that MEA is more homogeneously distributed on the surface and the pores of the adsorbent, leading to more accessible nitrogen sites. Furthermore, adsorbents regeneration is tested by using a flow of nitrogen of 60 mL.min−1 during 4 h at room temperature. The non-impregnated bed was completely regenerated, suggesting mainly a physical adsorption of CO2 on its surface. MEA and AMP based adsorbents were not completely regenerated, due to strong chemical bonds between CO2 and the nitrogen groups present on the surface. A further work from the same research group (Lee et al., 2013) dealt with impregnation of three

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types of sterically hindered amines, in the target to get a compromise between CO2 uptake and the ease regeneration of the adsorbent: 2-amino2-methyl-1,3-propanediol (AMPD), 2-amino-2-methyl-1-propanol (AMP), and 2-(methylamino)ethanol (MMEA), to get a theoretical ratio 1 × 10−4 mol amine.g−1 adsorbent (the experimental ones were 1.27 × 10−4 for AMP, 2.22 × 10−4 for AMPD, and 1.76 × 10−4 for MMEA). The selected size for adsorption studies is 0.51–0.71 mm. Here also, CO2 adsorption improvement due to introduction of nitrogen compounds is noted, despite the decrease of surface area.AMP exhibits the best performances,followed by AMPD and then by MMEA. Indeed, MMEA is a secondary amine whereas AMP and AMPD are primary amines, primary amines are more active in CO2 adsorption than secondary ones. Furthermore, AMP has a higher basicity than MMEA and AMPD, in favor of higher affinity towards acidic CO2 molecule. Regeneration is performed with 15 mL.min−1 nitrogen flow during 1 h at 40–50 °C. The regeneration percentages are 70–90 percent, the non-impregnated sample being more regenerated than the impregnated ones. AMP-based sample requires higher regeneration time due to the higher amount of adsorbed CO2 . In a recent study from the same group (Hussin et al., 2021), palm shell was mixed with two solvents, namely choline hydroxide:urea (ChU) and choline hydroxide:glycerol (ChG), with different ratios choline hydroxide:urea or choline hydroxide:glycerol. The ratio carbon:solvent is 1:2. Also, it was shown the role of activating agent in enhancing CO2 adsorption despite the decrease of surface area, due to chemical affinity between CO2 and nitrogen surface groups. The maximum adsorption capacity was found for the sample carbon:ChU, with ratio choline hydroxide:urea of 1:0.5.The same sample showed good regeneration, with nitrogen flow rate of 5 mL.min−1 at 90 °C during 1 h, even after 11 adsorption-desorption cycles.

8.2.2.3 Parameters affecting CO2 adsorption Rashidi and Yusup (2015) performed a statistical analysis using ANOVA to determine the impact of different parameters on CO2 adsorption capacities. They deduced that the main parameter is activation temperature of the biomass waste with a contribution of about 65 percent followed by the origin of the biomass with about 14.5 percent of contribution. Other parameters such as heating rate, flow rate, particle size, residence time of the activating agent flow, and impregnation ratio have no significant impact on CO2 adsorption capacity (Rashidi and Yusup, 2015).

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8.2.2.3.1 Effect of activation temperature

Rashidi and Yusup (2015) investigated the effect of activation temperature, with CO2 as activating agent, on five different types of biomasses and concluded that increasing the activation temperature increases the adsorption capacities. Indeed, a low activation temperature (about 500 °C in their case) is not enough for suitable porosity development. Indeed, at low temperature, the energy for devolatilization process is low and the activation reaction will not take place easily. High activation temperature is in the origin of the development of high porosity, especially microporosity, which is an important factor for gaseous adsorption. However, a very high excess in activation temperature will reduce the microporosity resulting in a lower adsorption capacity (Rashidi and Yusup, 2015). 8.2.2.3.2 Effect of biomass origin

Different biomasses have different physico-chemical characteristics. For example, the highest carbon content in coconut shell (83.01 wt%) is behind its better CO2 adsorption capacities compared to rice husk (53.70 wt%) (Rashidi and Yusup, 2015). Indeed, it is surface carbon that hosts the adsorbate either by physical interactions (Van der Waals) or by chemical ones. Therefore, higher amount of carbon implies higher number of adsorption sites.Conversely,higher ash contents result in lower CO2 adsorptive properties. 8.2.2.3.3 Effect of residence time of the activating agent flow

The variation of residence time of activating agent flow at the temperature of activation affects porosity and surface area development of carbon-based materials. Short time may lead to insufficient devolatilization and pores will not be developed. In contrary, going beyond an optimum residence time may result in widening of microporosity to mesoporosity (and therefore to less adsorption capacities towards CO2 molecule whose size is about 0.33 nm) and in higher amount of ash that may block the pores and therefore decrease the adsorption capacities (Rashidi and Yusup, 2015). In the study of Rashidi and Yusup (2015), different residence times under CO2 flow for activation of different biomasses were investigated (15 to 90 min). It was found that a residence time of 60 min led to the highest adsorption capacities. However, since no significant differences are noted between residence times of 45 and 60 min, the time 45 min was selected to save operating time and cost.

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8.2.2.3.4 Effect of adsorption temperature

The studies that investigated different CO2 adsorption temperatures demonstrated that the amount of adsorbed CO2 increases with the decrease of adsorbing temperature, in the range of temperatures 0–70 °C (Ello et al., 2013;Hussin et al.,2021;Pal et al.,2020;Sahri et al.,2020).Indeed,adsorption is exothermic and is more favored at lower temperatures whereas higher temperatures favor desorption. Fig. 8.7A. illustrates an example of such results. 8.2.2.3.5 Effect of adsorbing gas inlet flow rate

Lee et al. (2013) investigated different inlet flow rates of CO2 (total flow of 10, 20, 30, and 40 mL.min−1 with 30 percent CO2 ) on activated carbon issued from palm shell impregnated with AMP. The breakthrough time (BT), defined as being the time at which 95 percent of inlet gas exits the adsorbent, decreased with increasing flow rates (Lee et al., 2013). Similar result is obtained by Hussin et al. (2021) with palm shell (before and after activation with ChU or ChG) with inlet flow rates that range between 200 and 600 mL.min−1 and CO2 concentration of 10 percent (Hussin et al., 2021). Increasing flow rates decreases CO2 adsorption capacity and the breakthrough times as well (Fig. 8.7B). Indeed, decreasing flow rate increases the residence time, and therefore the gaseous molecule will get sufficient time to diffuse into the pores. 8.2.2.3.6 Effect of initial CO2 concentration

Hussin et al. (2021) modified CO2 concentration between 10 and 20 percent, in a total flow rate of 200 mL.min−1 . An increase in CO2 adsorption capacity with a decrease of the breakthrough time were noted with the increase of CO2 inlet concentration. An example is illustrated in Fig. 8.7C. 8.2.2.4 Adsorption capacities The highest CO2 adsorption capacities found in the above studies are grouped in Table 8.1. It is inferred that a chemical activation is a crucial condition to get good selectivity towards CO2 adsorption. The number of studies where only physical activation is carried out is small. However, the single stage physical activation performed by Rashidi and Yusup (2015) shows promising results (78.3 mg CO2 adsorbed per g of sorbent at 1 atm and 25 °C). A physical activation may save chemical and time cost and is more environmentally friendly.

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(A)

(B)

(C)

Figure 8.7 Effect on CO2 adsorption at atmospheric pressure on palm shell activated with choline hydroxide:urea (1:0.5) with a ratio carbon:solvent 1:2 of A) adsorption temperature (total flow rate = 200 mL.min−1 , initial CO2 concentration = 10 percent) B) total flow rate (reaction conditions: adsorption temperature = 25 °C, initial CO2 concentration = 10 percent) C) inlet CO2 concentration (reaction conditions: adsorption temperature = 25 °C, total flow rate = 200 mL.min−1 ) [C0 = CO2 concentration (%) in the inlet feed gas and Ct = CO2 concentration (%) in the outlet feed gas]. From Hussin, F., Aroua, M. K. & Yusoff, R. (2021). Journal of Environmental Chemical Engineering, 9(4), 105,333. https://doi.org/10.1016/j.jece.2021.105333.

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Table 8.2 Breakthrough-curve characteristic parameters for the coconut shell and palm shell activated carbons, dynamic adsorption of H2 S.

Activated carbon

Breakthrough Exhaustion time (min) time (min)

Adsorption capacity (mg.g−1 )

Palm shell: CO2 activation Coconut shell: H2 O activation Palm shell: KOH activation Palm shell: H2 SO4 activation

245.6 270.7 291.5 304.3

46 53 68 76

316.1 359.4 323.8 334.0

8.2.3 Hydrogen sulfide Hydrogen sulfide (H2 S) is a gas with a known bad smell, rotten egg smell. It is extremely toxic. It causes diseases to human beings such as nausea, loss of appetite, and other health problems. It also affects the catalyst performance in industries. When oxidized to sulfur dioxide, it is in the origin of acid rains. To meet environmental regulations related to H2 S emissions, this gas may be absorbed by amine solution or adsorbed by a solid. Adsorption technique is more economical and environmentally friendly, it avoids solvent wastes and treatments. Using carbonaceous materials which are abundant and cheap is encouraged in this field to contribute in decreasing the cost and increasing the efficiency and the simplicity of the process. Guo et al. (2007) studied H2 S adsorption on activated carbon prepared from oil-palm shell. The raw material dried and brought to a size of 1.0–2.0 mm, was soaked 24 h in 200 mL solution of 40 percent H2 SO4 or 40 percent KOH at room temperature and then dried and heated 2 h under nitrogen flow at 700 °C. After being cooled down, the sample was washed with distilled water. Another set of the same raw material was thermally activated by CO2 during 2 h at 800 °C. A commercial activated carbon prepared by coconut shell activation by steam (H2 O) was also used for comparison. H2 S static (1000 ppm diluted in helium) and dynamic (2000 ppm diluted in helium, 90 mL.min−1 ) adsorption was investigated at 25 °C and 1 bar Desorption with helium flow at adsorption temperature and also at temperature of 200 °C was also carried out to detect the type of adsorption. Table 8.2 presents the breakthrough times and the adsorption capacities in dynamic conditions. Table 8.3 lists the adsorption capacities and the percentage of H2 S desorbed at each temperature, in static conditions. Indeed, chemical activation by H2 SO4 or KOH led to higher breakthrough and exhaustion times than the physically activated samples, revealing greater H2 S adsorption capacities for the chemically activated carbons than for the

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Table 8.3 Desorption properties of the coconut shell and palm shell activated carbons, static adsorption of H2 S. Total amount Percentage of Percentage of adsorbed desorption at desorption at Percentage of Activated carbon (mg.g−1 ) 25 °C (%) 200 °C (%) residue (%)

Palm shell: CO2 activation Coconut shell: H2 O activation Palm shell: KOH activation Palm shell: H2 SO4 activation

131

100

–

–

162

100

–

–

169

79

21

203

83

10

7

Table 8.4 Number of surface groups (meq.g−1 ) obtained from Boehm titration. Activated carbon Carboxyl Lactone Phenol Carbonyl

Palm shell: CO2 activation Palm shell: KOH activation Palm shell: H2 SO4 activation

– – 0.089

– 0.355 0.416

0.120 – 0.208

0.285 1.167 0.604

physically activated ones (deriving from the same precursor or from coconut) . The results for static and dynamic adsorption present the same trends as shown in Table 8.2 and Table 8.3. In addition, all the H2 S adsorbed on the physically activated oil-palm shell was desorbed at 25 °C (Table 8.3), indicating a weak bond between adsorbate and adsorbent, i.e. a physisorption. Conversely, H2 S adsorbed on chemically activated samples was partly desorbed at 25 °C and partly at 200 °C (Table 8.3), indicating both physisorption and chemisorption of H2 S. Furthermore, some H2 S was not desorbed at all from the sample activated by H2 SO4 (Table 8.3), suggesting an irreversible chemisorption where a chemical reaction took place converting H2 S into other components. Infrared spectroscopy showed the existence, on the surface of the KOH activated sample, of alkaline chemical groups such as cyclic ketones and keto-derivatives of pyran that are responsible for chemical adsorption of H2 S. As for the surface of the carbon activated by H2 SO4 , oxygen functional groups were identified,ascribable to phenols,carboxylic acids,carboxylic anhydrides, and carbonyl groups of typical acidic functions (quinone type). The results of Boehm titration presented in Table 8.4 showed the existence of only phenol and carbonyl groups on carbon issued from

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oil-palm shell activated physically, lactonic and carbonyl groups on the carbon treated by KOH, and carboxyl, phenol, lactonic as well as carbonyl groups on the surface of the carbon activated by H2 SO4 . So Boehm titration confirmed infrared results. It is then concluded that H2 S is chemisorbed on activated carbon via hydrogen bonds with the hydroxyl (O–H) and carbonyl (C = O) groups. Besides, H2 S seems to be easily oxidized on some oxygen functional groups into elemental sulfur that remains strongly and irreversibly attached to the adsorbent, hence the existence of some residual H2 S, not desorbed (Table 8.4), on the surface of the carbon activated by H2 SO4 . Indeed, X-ray photoelectron spectroscopy (XPS) evidenced the existence of elemental sulfur on the surface of the corresponding spent sample. It is then concluded that H2 S adsorption may take place via physisorption, reversible chemisorption, and irreversible chemisorption where the adsorbate is oxidized into elemental sulfur. Several years later, Hussaro (2014) investigated H2 S removal from biogas by adsorption on oil-palm shell activated with other activating agents, namely Na2 CO3 and ZnCl2 . Oil-palm shell was dried and then sieved to get a particle size of 1–2 mm. After that, it was heated under nitrogen flow at 600 °C for 1 h. The obtained char was cooled down and impregnated at 80 °C during 10 h with a saturated solution of activating agent using different weight ratios char:chemical agent, 1:1, 1:2, and 1:3. After this step, the temperature was raised to 700 °C and maintained 2 h at this temperature. Finally, the samples were washed with a 0.1 mol.L−1 solution of hydrochloric acid to reach a pH of 6–7 to remove excess chemical reagent and dried. H2 S in biogas, with total flow rate of 15 mL.min−1 , was passed, at 30 °C, through a column with 5 cm as diameter containing 20 g of adsorbent. It was shown that impregnation process led to higher carbon content, with greater contents noted for Na2 CO3 than for ZnCl2 . In addition, the pore volume and the BET surface area increased with the increase of the amount of chemical activating agent, Na2 CO3 leading to higher values than ZnCl2 . In fact, the highest surface area (742 m2 .g−1 ) and pore volume (0.4181 cm3 .g−1 ) were obtained for the adsorbent prepared with the higher amount of Na2 CO3 (1:3). The same material adsorbed the higher total amount of H2 S, 247.33 ppm. Na2 CO3 easily adsorbs water and therefore it is thought that a basic solution film is formed on the adsorbent surface, activating the dissociation of H2 S. High amount of hydrosulfide ion (HS− ) is formed on the surface of the adsorbent activated with Na2 CO3 leading to H2 S oxidation and consequently high sulfur adsorption capacity. Hussaro (2014) concluded that Na2 CO3 is a better activating agent than

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many others, leading to a better development of the total pore volume, the microporosity, and the specific surface area, besides the formation of chemical groups suitable for H2 S adsorption.

8.2.4 Sulfur oxide The acidic gas sulfur oxide (SO2 ) is emitted by combustion processes of fuel and coal and is generated by some chemical industries such as sulfuric acid production factories and metal smelting. SO2 is responsible for acid rains and respiratory diseases. Its elimination is then of great interest for human life and environment. Carbonaceous materials that derive from biomass waste are very interesting alternative to decrease SO2 emissions by adsorption. Such a dry method to remove SO2 offers many advantages in cost and simplicity compared to wet scrubbing which requires higher cost investment as well as higher time consumption for treating solvents and liquid waste. Many articles were published by the group of Lua and Guo on adsorbents that derived from oil-palm stone and oil-palm shell for SO2 adsorption. Guo and Lua (1999) used oil-palm stone with a size of 1–2 mm, impregnated at room temperature with H2 SO4 or KOH solutions, with different concentrations (5, 10, 20, and 30 percent), for 12 to 72 h. After filtration, the sample was carbonized 2 h under nitrogen flow at 600 °C and then activated 1 h with CO2 at 800 °C. A sample not subjected to any impregnation was also carbonized and activated in the same way (physical activation). It was proven that CO2 activation increased total porosity about 60 percent due to reaction between carbon and CO2 that enlarges the pores and creates new ones. Increasing the impregnation time from 12 to 24 h resulted in higher specific surface areas whereas a further increase (above 24 h) did not affect the surface area. Increasing the concentration of H2 SO4 results in higher surface areas and pore volumes, whereas an increase in KOH concentration from 10 to 30 percent results in decrease of surface area and microporosity. Indeed, the impregnating agent reduces the formation of tars and other liquids that may clog up the pores inhibiting the elaboration of carbonaceous porous structures. However, with excess KOH, K2 O and water are formed (2 KOH = K2 O + H2 O) and water may react with carbon leading rather to gasification (H2 O + C = CO + H2 ) than to surface area and microporosity development. Also, Guo and Lua (2003) compared 10 percent and 40 percent KOH concentration of a solution impregnated

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Figure 8.8 Amount of SO2 adsorbed on the activated carbon that derived from oil-palm stone and the specific surface area determined by BET (Brunauer, Emmet, and Teller) method. Adsorption conditions: 2000 ppm SO2 balanced with nitrogen at ambient temperature. From Guo, J. & Lua, A. C. (1999). Microporous and Mesoporous Materials, 32(1–2), 111–117. https://doi.org/10.1016/S1387-1811(99)00,096–7.

on oil-palm shells, using the same procedure, and found the same result for higher KOH concentration. Adsorption of 2000 ppm SO2 (balanced with N2 ) at ambient temperature on the samples physically or chemically activated evidenced that the increase of surface area and microporosity results in an increase of the amount of adsorbed SO2 . Fig. 8.8 shows the linear relationship, with correlation coefficient of 0.99, between the SO2 adsorption capacities and the BET (Brunauer, Emmet, and Teller) surface area. The same trend is also obtained for oil-palm shells activated by CO2 at 500–800 °C during 15–60 min (Guo and Lua, 2002a; A.C. Lua and Guo, 2001a, 2001b). Comparing samples with similar surfaces areas, Fig. 8.8 shows that SO2 adsorption capacity increases in the order: H2 SO4 activation < physical activation < KOH activation. In addition, adsorption of 1000 ppm SO2 or NH3 (for the sake of comparison, NH3 being an alkaline gas) was also conducted at room temperature. Samples with similar specific surface area and microporosity but impregnated with either H2 SO4 or KOH were compared. The sample treated with H2 SO4 showed higher adsorption capacity towards NH3 than the sample treated with KOH. The latter adsorbed higher amount of SO2 and lower amount of NH3 . It is then inferred that not

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only the textural properties play a role but also the surface chemistry of the sample. Indeed, infrared spectroscopy showed that the surface of samples previously impregnated with H2 SO4 contains acidic groups such as phenols and carboxylic acids that favor the adsorption of alkaline gases such as NH3 whereas the surface of the samples impregnated with KOH contains alkaline groups such as cyclic ketones and keto-derivatives of pyran that favor the adsorption of acidic gases such as SO2 . The same results were obtained by the same authors on adsorbents deriving from oil-palm shells with using 30 percent or 40 percent H3 PO4 instead of H2 SO4 as acid agent (Guo and Lua, 2000, 2003; A.C. Lua and Guo, 2001b). In another study, Lua and Guo (2001) treated oil-palm stone 2 h by nitrogen at 600 °C and then activated it physically with CO2 focusing their study on the effect of activation temperature (500 to 900 °C), holding time (10 to 60 min), particle size (1.0–2.0, 2.0–2.8, and 2.8–4.0 mm), on SO2 adsorption capacity (Aik Chong Lua and Guo, 2001). It was observed that increasing the activation temperature and the holding time increases the development of surface area and microporosity due to the reaction between C and CO2 , as previously explained. However, a severe increase in activation temperature and holding time (for example, 900 °C and 60 min) will not be benefic because it will favor volatilization rather than pore development. Hence, the optimal condition to reach a high surface area and well-developed porosity were an activation temperature of 900 °C and a hold time of 30 min. The specific surface area in this case was 1366 m2 .g−1 . In this study, the linear relationship between specific surface area and SO2 adsorption capacity was also demonstrated using different SO2 concentrations (500, 1000, and 2000 ppm) at 25 °C. In addition, the increase of SO2 concentration results in an increase of its adsorbed amount. Similar result is obtained on materials deriving from oil-palm shell (Guo and Lua, 2000; Aik Chong Lua and Guo, 2001). Conversely, an increase of adsorption temperature from 25 to 80 °C, led to a decrease of the adsorption capacities because adsorption is exothermic. Similar result is obtained on materials deriving from oil-shell (Guo and Lua, 2000, 2002a, 2002b; A.C. Lua and Guo, 2000, 2001a; Lua and Guo, 2001c). Fig. 8.9 displays the effect of particle size on SO2 adsorption behavior using 2000 ppm SO2 at 25 °C. It is clear that the increase of particle size did not affect the final adsorption capacity, even though it increases the time required to reach adsorption equilibrium. In fact, higher is the particle size, longer would be the time taken by SO2 molecules to diffuse inside the pores. Similar result is obtained on materials deriving from oil-palm shell. The adsorption capacity reached for the sample

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Figure 8.9 Effect of particle size on the amount of SO2 adsorbed on the activated carbon that derived from oil-palm stone activated physically with CO2 during 30 min at 900 °C. Adsorption conditions: 2000 ppm SO2 balanced with nitrogen at 25 °C. From Lua, A. C. & Guo, J. (2001). Journal of Environmental Engineering, 127(10), 895–901. https://doi.org/10.1061/(ASCE)0733–9372(2001)127:10(895).

activated 30 min at 900 °C under 2000 ppm SO2 at 25 °C is 76 mg.g−1 . This capacity was found to be comparable to that obtained on commercial activated carbons. Guo and Lua (2000) treated oil-palm shell according to the same procedure as in Guo and Lua (1999) with using as impregnating solutions 10 percent KOH or 30 percent H3 PO4 and different sizes of raw materials (0.3–1.0, 1.0–2.0, 2.0–2.8, and 2.8–4.0 mm). These authors put also into evidence that adsorbed SO2 , regardless of adsorption temperature (25, 50, 75, and 100 °C), is of two types: SO2 (I) weakly bonded to carbon surface and easily desorbed at the adsorption temperature, and SO2 (II) strongly bonded to carbon surface and desorbed at temperature higher of 200 °C than the adsorption one. It is then concluded that some SO2 is chemically adsorbed on the carbonaceous material. Even though the total amount of SO2 decreases with increasing adsorption temperature, the proportion of chemisorbed SO2 follows the reverse trend. Higher adsorption temperature is not in favor of physical adsorption but rather of chemical one. In a further study, Guo and Lua (2003) completed their work on oil-palm shell by investigating the effect of bed-length, SO2 superficial velocity, and particle size, under 2000 ppm SO2 at 25 °C and 1 bar in dynamic conditions rather

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Figure 8.10 Plots of breakthrough time versus column length for the fixed bed operating at various SO2 superficial velocities. Adsorption conditions: 2000 ppm SO2 balanced with nitrogen at 25 °C. From Guo, J. & Lua, A. C. (2003). Separation and Purification Technology, 30(3), 265–273. https://doi.org/10.1016/S1383-5866(02)00, 166–1.

than static ones. Increasing the length of the column of the adsorbing material, between 5 and 30 cm, led to an increase of the breakthrough time and the exhaustion time. Besides, higher superficial velocities (from 38.2 to 114.2 cm.min−1 ) result in lower amount of adsorbed SO2 as well as in lower breakthrough times (from 364.5 to 49.0 min) as depicted in Fig. 8.10. Indeed, higher velocity reduces the time contact between SO2 and the adsorbent and thereby the amount of adsorbed SO2 at breakthrough.The same observations were also obtained in another work (Guo and Lua,2002a). As for the particle size effect, Guo and Lua (2003) reported a decrease of breakthrough times with the increase of particle size due to two parameters: the diffusion inside the pores and the diffusion towards the external surface (the surface outside the pores). In fact, both diffusions become slower (slower mass transfer) with the increase of particle size. However, the adsorptive capacity is not affected by the particle size as previously stated (Guo and Lua, 2000; Aik Chong Lua and Guo, 2001). A comparison between dynamic conditions (Guo and Lua, 2002a, 2003) and static ones (Guo and Lua, 2000, 2002a; Aik Chong Lua and Guo, 2001) reveals that the amount of adsorbed SO2 in dynamic fixed-bed (14.55 mg.g−1 ) is much lower than in static conditions (76 mg.g−1 ) where the contact time between adsorptive and adsorbent is high enough to reach equilibrium and saturation. In fact, in

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dynamic conditions, the breakthrough may take place before the adsorption equilibrium is reached due to short contact time. Lua and Guo (2001b) searched for the optimal CO2 activation conditions (activation temperature and hold time) during elaboration of carbonaceous materials from oil-palm shells. The raw material was subjected to the nitrogen flow during 3 h at 600 °C followed by CO2 activation at 500–800 °C during 10–60 min. This work is similar to the one done on oilpalm stone (Guo and Lua, 2002a; Aik Chong Lua and Guo, 2001) and gave the same findings; higher activation temperature and higher hold time led to more developed microporosity and surface areas due to carbon conversion into volatile products. However, there is a compromise, the association of high activation temperature (800 °C) with high holding time (60 min) will be detrimental to textural properties due to so much important volatilization of carbon. The optimal conditions were an activation temperature of 800 °C with a hold time of 30 min. Many years later, the group of Iberahim and Sethupathi investigated also SO2 adsorption on carbonaceous adsorbents issued from palm waste. Sumathi et al. (2009) carbonized oil-palm shell by nitrogen (N2 ) flow (varying between 100 and 500 mL.min−1 ) during different times (30–60 min) and then activated it by carbon dioxide (CO2 ) flow (varying between 100 and 500 mL.min−1 ) at different temperatures (700–1100 °C) and holding times (30–90 min). Experimental and modeling studies were performed to determine the influence of each parameter on the total specific surface area (BET), micropore volume, and total pore volume of the adsorbent. It is to be reminded that BET surface area and micropores are the main factors responsible for good gas adsorption capacities. The adsorption measurements were performed at 100 °C with a 150 mL.min−1 inlet mixture composed of SO2 (2000 ppm),NO (500 ppm),O2 (10 percent), water vapor, and N2 as gas balance. Experimental and modeling results were in good agreement and showed that the main factor that influences the BET surface area and microporosity is the CO2 activation temperature. The second one is the holding time of the same gas. The third one is the CO2 flow.The influence of N2 holding time and flow were less important.Indeed, when the process is stopped at N2 carbonization step, the BET surface areas are small and the SO2 adsorption capacity also. Indeed, as shown by Lua and Guo group (Guo and Lua, 2002a; A.C. Lua and Guo, 2001a; Aik Chong Lua and Guo, 2001), the presence of CO2 is crucial for microporosity development, due to the reaction between CO2 and the carbon of the raw material. It was also proven that increasing the activation temperature, the

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holding time, and the flow of CO2 increases the BET area and develops the microporosity. However, severe conditions, i.e. high activation temperature associated to long holding time or long holding time with high CO2 flow, will lead to the enlargement of micropores and therefore the decrease of BET area and microporosity, due to excess carbon oxidation by CO2 and volatilization of the adsorbent. Iberahim et al. (2018) carried out a similar study on oil-palm mill sludge by performing a carbonization under 300 mL.min−1 nitrogen flow with heating temperatures of 300–800 °C, heating rates of 10–20 °C.min−1 , and holding times of 60–120 min. The effect of these parameters on biochar yield and SO2 adsorption capacity was examined. The inlet testing flow was 300 mL.min−1 of 300 ppm SO2 diluted in nitrogen.Adsorptions were performed at ambient temperature and atmospheric pressure. It is found that all the three parameters influence the biochar yield. The increase of heating temperature and heating rate results in a decrease of biochar yield whereas the increase of holding time gives an increase of this yield. Indeed, high heating temperature causes a gasification of the raw material. In addition, increasing the holding time allows enough time for cracking and repolymerization of the biomass structure.The heating temperature has also the highest impact on SO2 adsorption capacity whereas heating rate and holding time have a much lower impact. The highest adsorption capacity of SO2 was 10.04 mg.g−1 for a carbonization temperature of 400 °C, a holding time of 88 min, and a heating rate of 20 °C.min−1 . Some chemical functional groups were identified on the surface of the adsorbent (IR bands ascribable to aromatic structures) with maximum amount on the sample treated at 400 °C and this amount decreases with increasing treatment temperature. Hence, the adsorption of SO2 is favored by chemical groups rather than physical properties. Adsorption capacity of the studied biochar is lower than that of activated carbons. Iberahim et al. (2019) pursued their work done in (2018) by a further activation of the biochar samples with CO2 at 300–700 °C, with activation time of 30–150 min, and flow rate of 100–500 mL.min−1 . SO2 adsorption test was performed with the same mixture as in (Iberahim et al., 2018) but at 50–200 °C and with the presence of humidity (15 to 60 percent relative humidity). As stated earlier, the activation temperature followed by the activation time were the factors that influence the most the SO2 adsorption capacity. The CO2 flow has an effect but to a less extent. The influence of these parameters on carbon consumption were discussed earlier. An increase of SO2 adsorption capacity with activation temperature is obtained between 300 and 400 °C to reach a maximum at 400–500 °C and to decrease at higher temperatures. The good

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SO2 adsorption capacity is correlated with the presence of C–O groups ascribable to cellulose, hemicellulose, and lignin, whose amount decreases with the increase of activation temperature from 400 to 700 °C leading to a decrease of the amount of adsorbed SO2 . The adsorption capacity increased with the increase of the adsorption temperature from 50 °C to 100 °C, to reach a maximum at 100 °C, and then decreases for higher adsorption temperatures. Introduction of humidity increased SO2 adsorption capacity. However, the optimum humidity content is 15 percent. A higher percentage will decrease the amount of adsorbed SO2 . Indeed, a reaction takes place between SO2 and water leading to sulfuric acid formation and thereby the improvement of SO2 adsorption. Nevertheless, high amount of humidity led to the decrease of partial pressure of SO2 and competition between SO2 and water on the surface sites would be in favor of water. Also, in presence of high humidity, water molecules repel each other and do not reach the surface sites. In addition, high relative humidity may cause water condensation in the pore walls and therefore less surface sites will be available for adsorption. In addition, in their study, Iberahim et al. (2019) regenerated the spent adsorbent by thermal treatment under nitrogen flow during 40 min at 200 °C or 400 °C or by water treatment at 30 °C or 70 °C (which consists of washing the spent sample with water at 30 °C or 70 °C and then heating it at 105 °C). It was found that regeneration with water at 70 °C is more efficient than the same process conducted at 30 °C. However, thermal treatment is more efficient than water one. Better regeneration takes place at treatment temperature of 400 rather than 200 °C. The optimum conditions were found to be 442 °C as CO2 activation temperature with a CO2 flow rate of 397 mL.min−1 and a holding time of 63 min which led to SO2 adsorption capacity of 16.65 mg.g−1 (Iberahim et al., 2019), higher than that of the same raw material not activated by CO2 (Iberahim et al., 2018). In both studies of Iberahim et al. (2018 and 2019), experimental results and modeling ones were in accordance.

8.2.5 Nitrogen oxides Nitrogen oxides (NOX ) such as NO and NO2 are generated by combustion processes in stationary and mobile sources. These gases are dangerous to human health and cause acid rains and depletion in ozone layer. Adsorption of NOx on carbonaceous materials is a simple and low-cost promising way to reduce their emissions.

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Lua and Guo (2000) performed a one-step activation by CO2 of Malaysian oil palm stone, this means that activation by CO2 was directly performed on the material without a previous heat under inert atmosphere. The effect of particle size and different parameters during CO2 activation (flow rate, heating rate, activation temperature, and hold time) on the characteristics of the carbonaceous material were investigated. The stones were crushed and sieved to different particle sizes: lower than 1 mm, 1.0–2.0 mm, 2.0–2.8 mm, 2.8–4.0 mm, and 4.0–4.7 mm. The CO2 flow rates are between 25 and 200 mL.min−1 , the activation temperatures in the range 650 to 950 °C, the heating rates varied between 5 and 20 °C.min−1 , and the hold time between 0.5 and 3 h. Firstly, it was demonstrated that the porosity of the resulting material was higher than that of the same material pyrolyzed (heated without CO2 ) under the same conditions. This puts into evidence that pores development takes place under both heat and reaction between CO2 and carbon material. As for the holding time, it is found that increasing holding time from 0.5 to 3 h, results in a decrease of the carbon yield, due to the loss of volatile matters (predominant at temperatures lower than 750 °C) and to the carbon burn-off by CO2 (that takes place at temperature 750– 950 °C and leads to the development of surface porosity). Furthermore, the range of studied particle size did not affect the surface area or porosity of the resulting material. Increasing heating rates from 5 to 20 °C.min−1 did not affect significantly the textural properties, even though a slight decrease of surface area is noted with increasing heating rates. Conversely, increasing CO2 flow rates led to an increase in surface areas due to higher occurrence of the reaction between CO2 and carbon. However, an excessive increase of flow rate, especially at high temperatures (950 °C) affects badly the materials properties since a decrease of surface area and porosity will take place. In addition, severe reaction between carbon and CO2 may lead to the development of mesopores and macropores at the expense of the micropores. Micropores are more interesting for gaseous pollutants adsorption. Hence, the optimum activation conditions found by Lua and Guo (2000) are a flow rate of CO2 of 100 mL.min−1 with an activation temperature of 850 °C and a hold time of 2 h (heating rise 10 °C.min−1 ). In the latter case, the surface area is 1410 m2 .g−1 and the micropore surface is 942 m2 .g−1 . It was also shown that the amount of inorganic compounds present in the adsorbent does not depend on the activation temperature whereas the organic surface groups are affected by this parameter. The authors found a direct linear

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correlation between NO2 uptake and the surface area of their materials as well as better adsorptions capacities for their optimal material (250 mg.g−1 at 25 °C for inlet concentration of 1000 ppm NO2 balanced in nitrogen, and about 120 mg.g−1 for 500 ppm inlet concentration) than for commercial activated carbon (about 205 mg.g−1 and 95 mg.g−1 respectively in the same conditions). Hence, adsorbents deriving from palm stone are interesting candidates for NO2 removal. CO2 activation of oil palm shell was carried out by the same authors (Guo and Lua, 2002b) in two steps as mentioned earlier [paragraph 8.2.1]. The linear relation between surface area and amount of adsorbed NO2 is depicted in Fig. 8.4. Another group of scientists, Belhachemi et al. (2014) and Belala et al. (2014) prepared carbonaceous materials from Algerian date pits (DPP) for NO2 removal and focused their studies on understanding NO2 adsorption mechanism (Belala et al., 2014; Belhachemi et al., 2014). Belhachemi et al. (2014) prepared two granular activated carbons, one by physical activation by CO2 and the other by chemical activation with ZnCl2 . The DPP was brought to a size of 0.5–1 mm. For physical activation, carbonization was carried out under nitrogen flow at 825 °C followed by an activation by CO2 flow at the same temperature during 4 h (sample CCO2 ). As for the sample chemically activated, the DPP was impregnated by a solution of ZnCl2 (1 g Zn for 1 g of DPP), heated at 550 °C under nitrogen flow during 1.5 h, then cooled down and washed with hydrochloric acid solution and deionized water to reach neutral pH (CZn ). For the sake of comparison, a commercial activated carbon was taken and oxidized by ammonium persulfate (NH4 )2 S2 O8 solution (sample GAC–O), then washed, dried, and heated at 700 °C under nitrogen flow during 1 h to eliminate some oxygen surface groups (sample GAC–O-T). All the prepared activated carbons were microporous, with CCO2 showing some narrow mesopores. A release of NO was detected during NO2 adsorption, revealing that NO2 reduction takes place on activated carbon surface. It was checked that not all NO2 was reduced into NO. A significant part of NO2 was adsorbed and the amount adsorbed (at 25 °C, under 1 atm, with 500 ppmv NO2 in nitrogen as inlet gas) was 129 mg.g−1 for CCO2 and 136 mg.g−1 for GAC–O-T. Furthermore, it was shown by temperatureprogrammed desorption (TPD) that NO2 adsorbs on two different surface sites and desorbs according to two different reactions pathways. In addition, the authors found that the NO2 adsorption is favored by the amount of adsorbed oxygen which is produced by NO2 reduction on carbon surface.

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Contrary to other works (Guo and Lua, 2002b; A.C. Lua and Guo, 2000), these authors found no correlation between specific surface area and the amount of adsorbed NO2 . Nevertheless, they found that the homogeneous microporosity is a key factor for good NO2 adsorption capacities.In addition, the authors investigated the impact of chemical groups on NO2 uptake.They concluded that the presence of stable oxygen groups as well as basic groups (pyrone and chromene) were associated with a higher NO2 adsorption. Indeed, heating carbonaceous materials at higher temperature led to the removal of acidic groups from the surface and to the enhancement of the basic character of carbonaceous materials, the delocalized π electrons of graphene layers playing the role of Lewis base. The authors noticed a decrease of the amount of adsorbed oxygen with the presence of acidic functions and therefore postulated that acidic groups inhibit the reduction of NO2 into NO on carbon surface. Indeed, a crucial step for NO2 adsorption is the reduction of NO2 into NO and surface oxygen. Acidic groups perturbate this reduction step and therefore decrease the amount of adsorbed NO2 . In their second study (Belala et al., 2014), the same group of authors prepared carbon-based adsorbents from DPP by physical activation by CO2 using the same procedure described in their previous study (Belhachemi et al., 2014) but with an activation temperature with CO2 of 850 °C during different holding times (0.5, 1, and 2 h). Indeed, increasing holding time results in higher loss of carbon by reaction with CO2 (A.C. Lua and Guo, 2000). Surface areas as well as total pore volume and micropore volume increase with the increase of holding time from 0.5 to 2 h and this results in an increase of the amount of adsorbed NO2 (from 4.4 for 0.5 h to 106.9 mg.g−1 for 2 h as holding time, at 20 °C and 1 atm with 500 ppm NO2 inlet concentration with N2 as gas balance).It is also noted the presence of some mesoporosity for the sample with the highest holding time. NO is emitted during NO2 adsorption.In addition,N and O species were adsorbed on the solid material. NO emission suggested that O is present on carbon surface under the form of complexes. In addition, experimental results showed that the significant amount of uptaken NO2 may adsorb directly on oxygenated surface groups or on oxygen complexes created upon NO2 decomposition into NO. The latter oxygen species play a more important role in NO2 adsorption than the former ones. NO2 may also adsorb directly on carbon surface. Temperature programmed desorption (TPD) results showed also the formation of NO revealing that adsorbed NO2 reacted with carbon surface, along with the formation of NO2 , CO, and CO2 .

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The authors proposed a series of reactions to explain the mechanisms of NO2 adsorption and desorption where they showed the formation of the different species desorbed (NO2 , NO, CO, and CO2 ). CO emission during desorption is due to decomposition of ethers, phenols, and quinone groups whereas CO2 release is ascribed to carboxylic groups decomposition and also to lactones and anhydrides ones (whose decomposition take place at temperature higher than 400 °C). All these groups are formed by interaction between NO2 and carbon surface. In the same study, the authors introduced 10 percent O2 in the inlet adsorbing mixture and noticed a slight increase in the amount of adsorbed NO2 with a decrease of the extent of NO2 reduction into NO. Indeed, formation of oxygen species on carbon surface due to the reaction with oxygen increased the NO2 uptake. Finally, it was also shown that the increase of adsorption temperature (from 20 °C to 60 °C) results in a decrease of the amount of adsorbed NO2 (from 106.9 to 85 mg.g−1 ) with an increase of the time required to reach equilibrium between adsorption and desorption. All the mentioned studies found NO2 uptake by palm waste derivatives similar or higher than those found for commercial carbonaceous adsorbents or for carbon materials that derive from lignocellulosic biomass.

8.2.6 Volatile organic compounds (VOCs) Volatile organic compounds (VOCs) are common pollutants. They have high vapour pressure at room temperature. Most of VOCs are toxic, some of them are carcinogenic or mutagenic. They may also impact human beings in different ways: irritation of the eyes, nose, and throat; nausea, dizziness, headaches, allergy, respiratory problems, damage to heart, liver, and kidneys as well as to central nervous system. Under the action of sunlight, VOCs react with other pollutants such as nitrogen oxides to produce excess ozone and with other compounds to give photochemical smog. VOCs are emitted by various sources, outdoor and indoor. Their elimination is carried out by many methods. Among these methods, adsorption is widely used. Carbonaceous adsorbents that derive from palm wastes were found promising as shown by many research groups. Kim et al. (2006) used coconut shell to prepare purified activated carbon as follows: the raw material was brought to a size of 30–35 mesh and boiled 5 h in water to lead to activated carbon named AC. AC was impregnated, using conventional excess solvent method, by various acidic and alkaline solutions: HNO3 (NA), H2 SO4 (SA), HCl (HA), H3 PO4 (PA), CH3 COOH

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(AA), KOH (pH), and NaOH (SH). The excess solvent was eliminated by evaporation.0.2 g of dried adsorbent is used for adsorption process.A various set of 10,000–15,000 ppm VOCs diluted in helium with a total flow rate of 40 mL.min−1 was investigated at 80 °C:aromatic model gases BTX (benzene, toluene, o-, m-, p-xylene), alcohols (methanol, ethanol, i-propanol), and methylethylketone (MEK) (Kim et al., 2006). For a 5 wt% of activating agent, the specific surface areas ranged between 636 and 894 m2 .g−1 . It was found that the surface area of AC decreased significantly upon impregnation by alkaline solution which led to pore blocking as shown by electron scanning microscopy. Table 8.5 reports the different amounts of adsorbed VOCs on the samples with 5 wt% of activating agent (Kim et al., 2006). The highest amount of adsorbed VOCs is obtained with H3 PO4 as activating agent, except for the case of o-, m-xylene, and MEK. Relatively large molecules such as the latter ones are less adsorbed on H3 PO4 /AC than the smaller molecules and therefore it is inferred that H3 PO4 not only changed the chemical nature of AC surface but also narrowed the micropores. In addition, the weight percentage of H3 PO4 was varied between 0 and 20 wt% and its effect on the amount of adsorbed VOC was followed. It is observed that the optimum amount is 1 wt% for benzene, toluene, p-xylene, methanol, ethanol, and i-propanol adsorption. Nevertheless, the amount of adsorbed o-xylene, m-xylene, and MEK decreased with increasing the weight percentage of H3 PO4 . Indeed, the amount of adsorbed benzene, p-xylene, and ethanol on 1 wt% H3 PO4 was 1.5 to 2 times greater than that on AC. The BET surface area was maximum for this weight content (1109 m2 .g−1 ). Consequently, the good adsorbing performances of this material were ascribed to a high specific surface area and to suitable chemical modification. 1 weight percent H3 PO4 is then considered a good adsorbent for the removal of the VOCs benzene, toluene, p-xylene, methanol, ethanol, and i-propanol. This adsorbent was chosen to study the influence of the VOC concentration, the inlet flow rate, and the ratio L/D (L = length of the column of adsorbent, D = diameter of the column) on the adsorbing performances. Toluene and MEK were selected for this study. With increasing toluene and MEK concentrations between 5000 and 25,000 ppm, the amount of VOCs adsorbed at equilibrium increased linearly indicating that toluene and MEK are physically adsorbed, may be in multilayer. It was also noticed that shorter time was required to reach adsorbent saturation when the concentration of VOC increased. In addition, when varying the total flow rate between 20 and 100 mL.min−1 , the amount of adsorbed MEK did

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Table 8.5 Amount of adsorbed VOCs (mmol.g−1 ) on activated carbon (AC) and different chemical agents impregnated on AC with 5 wt%. Aromatics Alcohols Ketone Sample

Benzene

Toluene

o-xylene

m-xylene

p-xylene

Methanol

Ethanol

i-propanol

MEK

AC ∗ AA/AC ∗ NA/AC ∗ HA/AC ∗ SA/AC ∗ PA/AC ∗ pH/AC ∗ SH/AC

5.05 5.69 4.72 4.00 5.13 6.41 4.52 3.81

3.60 3.63 3.08 2.25 3.17 4.05 2.87 2.73

4.65 4.52 5.16 3.85 4.40 4.92 4.11 4.04

6.33 6.21 3.91 3.06 3.86 4.11 3.64 4.50

4.21 4.18 3.93 3.49 4.35 5.19 4.31 3.92

5.31 5.35 4.43 3.68 5.11 5.65 4.32 4.67

1.87 1.56 1.91 1.50 1.92 2.18 1.80 2.02

1.59 1.86 1.56 1.31 1.65 1.92 1.36 1.51

4.41 3.50 3.11 3.00 3.52 4.02 3.15 3.12

∗ : AA

= CH3 COOH, NA = HNO3 , HA = HCl, SA = H2 SO4 , PA = H3 PO4 , pH = KOH, and SH = NaOH.

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not vary significantly whereas that of toluene decreased slightly. With the increase of L/D in the range 1 to 15, the amount of adsorbed MEK was not affected whereas that of toluene increased to reach a maximum for L/D = 6 and decreased for higher ratio. Finally, the authors proved that toluene and MEK were physically and chemically adsorbed since some desorption took place at room temperature under helium flow and some other requires a temperature of 300 °C under the same flow. The authors confirmed also the good regeneration of 1 weight percent H3 PO4 /AC and the possibility to reuse it in many cycles adsorption-desorption. Pal et al. (Pal et al., 2017) prepared activated carbon from waste palm trunk (WPT) by carbonization at 600 °C under inert atmosphere followed by a mixing with solid KOH (carbonized biomass/KOH = 1/6) and then an activation at 900 °C under nitrogen flow, a washing by HCl solution till neutral pH and finally a drying. The surface area of the obtained material was 2927 m2 .g−1 , with a total pore volume of 2.51 cm3 .g−1 and a pore diameter of 1.68 nm. The material was evaluated in ethanol removal, ethanol being considered a VOC used as a refrigerant in heat pump systems. Adsorption experiments were conducted at 30, 40, 50, 60, and 70 °C, under various pressures, on 90 mg adsorbent, in dynamic conditions. Adsorbents’ regeneration was carried out at 120 °C during 3 h The adsorbent contains micropores with some mesopores. Adsorption/desorption isotherms of ethanol at 50 °C do not show any hysteresis. This feature is important for the application in heat pump systems. Studies on isosteric heat of adsorption revealed that the adsorbent has heterogeneous surface and therefore various sites of adsorption leading to different gas-solid interaction are present. The adsorbent prepared from WPT showed an uptake of ethanol of 1900 mg.g−1 which is 35 percent higher than that obtained on commercial Maxsorb III. Vohra (2015) treated date palm pits as described in paragraph 8.2.1. The derived GAC was used for benzene adsorption in dynamic conditions at ambient temperature. The column length was varied between 1 and 6 cm. The total flow rate (between 1.1 and 2.2 L.min−1 ) is composed of 5.75 to 23 ppmv benzene diluted in air. Firstly, a comparison was established between the raw date pit and the GAC deriving from it. The GAC was found to be much more efficient (76 mg.g−1 as adsorption capacity for GAC versus 0.33 mg.g−1 for date pit) with much higher BT and ET, mainly due to the presence of mesopores in GAC. Secondly, the influence of benzene concentration was investigated. Decreasing benzene concentration from 23 to 11.5 to 5.75 ppmv showed an increase of the BT from 774 to 1467 to 2311 min respectively and of the ET from 951 to 1152 to 2736 min

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respectively. Indeed, the number of adsorption sites on the GAC surface is fixed and consequently, the decrease of the inlet benzene concentration will obviously result in a higher BT and ET. Besides, the values of obtained BT and ET are interesting not only for benzene concentrations in the range of ppmv but also for real conditions, where the benzene concentration is even lower, in the ppbv range, meaning that GAC will be efficient for a long period of time. Thirdly, the impact of total flow rates on the GAC behaviour was studied. Increasing total flow rates from 1.1 to 1.65 to 2.2 L.min−1 result in a decrease of the BT and ET, even though the benzene adsorption capacity did not change. Indeed, the presence of higher benzene amount in the inlet mixture requires less time to occupy the adsorption sites. In addition, increasing the length of the GAC column results in an increase of BT and ET due to the increase of the number of adsorption sites. Since the breakthrough times are of several hours to several days, the prepared GAC is a promising candidate for applications in real conditions. Besides the good textural properties of the adsorbent (high porosity and surface area), interactions between surface functional groups and benzene molecule were supposed to take place; namely an electron transfer between surface oxygen-groups (O–H, C–O, S = O, P-O) that act as electron donors and the π electrons of the benzene aromatic ring which is an electron acceptor. Few years later, Vohra et al. (2020) elaborated a carbonaceous adsorbent from date palm-tree branches (DPB) by mixing the dried DPB with a solution of 40 percent weight H3 PO4 with a ratio volume of solution (mL) over weight of raw material (mg) equal to 2. The resulting solid was then heated at 700 °C during 1 h, washed with deionized water till neutral pH and dried. Toluene was used as VOC model. 20 ppmv toluene diluted in air was passed through the adsorbent continuously (Vohra, 2020). The prepared material showed a specific surface area of about 800 m2 .g−1 with microporosity and mesoporosity. It was shown that the conditions used to get such a material led to better textural properties than other ones such as an activation temperature lower than 700 °C (500 or 600 °C) or a lower percentage of phosphoric acid (20 percent instead of 40 percent) or a ratio different than 2 (2.4). In this work also, the authors proved that the raw material was much less efficient than the treated one. Also, the effect of toluene concentration (20 - 10 ppmv), total flow rate (2 to 3 L.min−1 ), and column length (between 4 and 6 cm) was studied and the same trends as in the case of benzene were obtained as depicted in Fig. 8.11. The interaction between the aromatic toluene ring and the oxygen surface groups is also supposed to occur. The work was concluded by a modelling study based

(B)

(C)

Palm wastes reuse for gaseous effluent treatment

(A)

Figure 8.11 Effect of inlet toluene concentrations, total flow rates, and column length on the breakhtrough curve profile of toluene adsorption on activated carbon that derive from branches of palm-tree. In all cases, the column diameter was 6.35 mm. A) toluene gas concentration 10 and 20 ppmv, total gas flow rate 2 slpm (standard liter per minute), column length 4 cm. B) toluene gas concentration 10 and 20 ppmv, total gas flow rate 2 slpm (standard liter per minute), column length 6 cm. C) toluene gas concentration 10 ppmv, total gas flow rate 2 and 3 slpm (standard liter per minute), column length 6 cm. From Vohra, M., Al-Suwaiyan, M. & Hussaini, M. (2020). International Journal of Environmental Research and Public Health, 17(24), 9287. https://doi.org/10.3390/ijerph17249287.

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on response surface methodology, where BT and ET were taken as response factors, and the results obtained agreed well with the experimental ones.

Conclusion Carbonaceous materials deriving from palm wastes are promising candidates for removal of gases. On one hand, the use of these inexpensive, renewable, and abundant biomass to prepare activated carbons will avoid costly problems of solid waste treatment while making an economic benefit from such a product.Indeed,the advantages of using palm wastes to elaborate carbonaceous adsorbents are numerous: low-cost, renewability, waste valorization, compatibility with environment, simplicity of operation, tunable pore structure, thermal stability, ease of preparation, and good chemical resistance to alkaline and acidic media. On the other hand, elimination of gases by adsorption is an easy to use and more economic process compared to washing and scrubbing processes. The scientific research works presented in this chapter highlighted the feasibility and good performances of different types of wastes that derive from palm for application in gaseous pollutants uptake. The well-developed surface areas and porosity of the carbonaceous materials issued from these wastes as well as the chemical functional groups on the adsorbent surface are the main reason behind the good adsorption performances. Palm waste carbonaceous adsorbents are good candidates for various types of gases (acidic, basic, neutral, organic, inorganic) adsorption and are very efficient under various conditions. Furthermore,most of the mentioned studies found similar or higher uptake of gases by adsorbents elaborated from different palm wastes than by commercial activated carbons or other common adsorbents. In some studies, the ease of regeneration of such adsorbents was put into evidence. To get more benefit from the process, it is interesting to see how to use the desorbing gases in chemical processes or what to make from spent materials. The combination of adsorbent and suitable cheap catalyst for chemical reaction may be interesting for industry. The elaboration of catalytic carbonaceous supports from palm wastes is also an interesting way to explore.

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Bandosz, T. J., & Petit, C. (2009). Journal of Colloid & Interface Science, 338(2), 329–345. https://doi.org/10.1016/j.jcis.2009.06.039. Belala, Z., Belhachemi, M., & Jeguirim, M. (2014). International Journal of Chemical Reactor Engineering, 12(2), 717–726. https://doi.org/10.1515/ijcre-2014-0043. Belhachemi, M., Jeguirim, M., Limousy, L., & Addoun, F. (2014). Chemical Engineering Journal, 253, 121–129. https://doi.org/10.1016/j.cej.2014.05.004. Ello, A. S., De Souza, L. K. C., Trokourey, A., & Jaroniec, M. (2013). Journal of CO2 Utilization, 2, 35–38. https://doi.org/10.1016/j.jcou.2013.07.003. Guo, J. & Lua, A.C. (1999). Textural and chemical characterisations of activated carbon prepared from oil-palm stone with H2 SO4 and KOH impregnation, Microporous and Mesoporous Materials 32, 111–117. Doi:10.1016/S1387-1811(99)00096-7. Guo, J., & Lua, A. C. (2000). Journal of Chemical Technology and Biotechnology, 75(11), 971–976. https://doi.org/10.1002/1097-4660(200011)75:113.0.CO;2-V. Guo, J., & Lua, A. C. (2002a). Characterization of adsorbent prepared from oil-palm shell by CO2 activation for removal of gaseous pollutants. Materials Letters, 55, 334–339. https://doi.org/10.1016/S0167-577X(02)00388-9. Guo, J., & Lua, A. C. (2002b). Microporous activated carbons prepared from palm shell by thermal activation and their application to sulfur dioxide adsorption. Journal of Colloid & Interface Science, 251, 242–247. https://doi.org/10.1006/jcis.2002.8412. Guo, J., & Lua, A. C. (2003). Separation and Purification Technology, 30(3), 265–273. https://doi.org/10.1016/S1383-5866(02)00166-1. Guo, J., Xu, W. S., Chen, Y. L., & Lua, A. C. (2005). Journal of Colloid & Interface Science, 281(2), 285–290. https://doi.org/10.1016/j.jcis.2004.08.101. Guo, J., Luo, Y., Lua, A.C., Chi, R.-a., Chen, Y.-l., Bao, X.-t., Xiang, S.-x. (2007). Adsorption of hydrogen sulphide (H2 S) by activated carbons derived from oil-palm shell, Carbon 45, 330–336. Doi:10.1016/j.carbon.2006.09.016. Huang, C. C., Li, H. S., & Chen, C. H. (2008).Journal of Hazardous Materials, 159(2–3), 523–527. https://doi.org/10.1016/j.jhazmat.2008.02.051. Hussin, F., Aroua, M. K., & Yusoff, R. (2021). Journal of Environmental Chemical Engineering, 9(4), 105333. https://doi.org/10.1016/j.jece.2021.105333. Iberahim, N., Sethupathi, S., & Bashir, M. J. K. (2018). Environmental Science and Pollution Research, 25(26), 25702–25714. https://doi.org/10.1007/s11356-017-9180-5. Iberahim, N., Sethupathi, S., Goh, C. L., Bashir, M. J. K., & Ahmad, W. (2019). Optimization of activated palm oil sludge biochar preparation for sulphur dioxide adsorption. Journal of Environmental Management, 248, 109302. https://doi.org/10.1016/j.jenvman.2019. 109302. Khalil, S. H., Aroua, M. K., & Daud, W. (2012). Chemical Engineering Journal, 183, 15–20. https://doi.org/10.1016/j.cej.2011.12.011. Kim, K.J., Kang, C.S., You, Y.J., Chung, M.C., Woo, M.W., Jeong, W.J. et al. (2006). In Catalysis Today (Vol. 111, Issues 3–4, pp. 223–228). https://doi.org/10.1016/j.cattod.2005. 10.030. Lee, C. S., Ong, Y. L., Aroua, M. K., & Daud, W. (2013). Chemical Engineering Journal, 219, 558–564. https://doi.org/10.1016/j.cej.2012.10.064. Lua, A. C., & Guo, J. (2001a). Journal of Environmental Engineering, 127(10), 895–901. https://doi.org/10.1061/(ASCE)0733-9372(2001)127:10(895). Lua, A. C., & Guo, J. (2001b). Langmuir, 17(22), 7112–7117. https://doi.org/10.1021/la010290c. Lua, A. C., & Guo, J. (2000). Carbon NY, 38(7), 1089–1097. https://doi.org/10.1016/ S0008-6223(99)00231-6. Lua, A. C., & Guo, J. (2001). Colloids and Surfaces A, 179(1–3), 151–162. https://doi.org/ 10.1016/s0927-7757(00)00651-8. Molina-Sabio, M., Gonalves, M., & Rodríguez-Reinoso, F. (2011). Microporous and Mesoporous Materials, 142(2–3), 577–584. https://doi.org/10.1016/j.micromeso.2010.12.045.

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Non-Print Items Abstract This chapter deals with the elaboration of carbonaceous adsorbents from different types of palm wastes and their applications for gaseous pollutants removal. The work is focused on the way to produce the adsorbing material, the adsorbing capacities, the factors that affect the amount of adsorbed gas as well as the correlation between adsorbing performances and materials properties. It is mainly inferred that textural properties, mainly specific surface area and micropores’ volume, and chemical properties, mainly the type of chemical groups present on the adsorbent surface play a crucial role in gases adsorption. Keywords Palm; Coconut; Biomass; Wastes; Carbon; Adsorbents; Physical activation; Chemical activation; Gas; Adsorption; Pollutants; Regeneration

CHAPTER NINE

Biofuels production Mejdi Jeguirima and Besma Khiarib

a The Institute of Materials Science of Mulhouse (IS2M), University of Haute Alsace, University of Strasbourg, CNRS,UMR 7361, F-68100 Mulhouse, France b Laboratory of Wastewaters and Environment, Centre of Water Researches and Technologies (CERTE), Technopark Borj Cedria, Touristic road of Soliman, BP 273, 8020, Tunisia

9.1 Introduction Many important factors like vulnerable dependence of the global economy on fossil raw materials, political issues related to oil producing countries, emissions of greenhouse gases and climate changes, have incited scientists, politicians and industrialists to explore alternatives of using new energy sources. For sustainable development, a growing consideration is given to the production of green alternative energies that can be produced from the biowaste available from food residues, agricultural residues or food factory residues (Jeguirim et al., 2020, 2019; Khiari et al., 2019). Palm tree wastes and palm fruit residues fit into this category of wastes. Indeed, converting this biomass to alternative energies or biofuels is a positive aspect due to the continuous increase in waste and to the traditional methods of waste disposal such as landfills or incineration which both creates environmental problems. Some of these biofuels are biogas (a mixture of methane and carbon dioxide) generated from the organic fraction in the different anatomical parts of the palm tree; ethanol, produced by the fermentation of the carbohydrate polymers, mainly in the fruits of the palm; and biodiesel, produced usually by the transesterification of their lipids. Therefore, trends in the production and use of biogas, biodiesel and bioethanol from palm tree wastes and palm fruit residues are the scope of this chapter. The possible different pretreatments methods and the recent technologies that can be enhanced are also reviewed.

9.2 Biogas production Biogas production is one among the practical actions that are taking the humanity towards the low carbon society of the future. This biogas production, named also, anaerobic digestion (AD) or methanation, is a c 2023 Elsevier Inc. Palm Trees and Fruits Residues: Recent Advances for Integrated and Copyright  Sustainable Management. All rights reserved. DOI: https://doi.org/10.1016/B978-0-12-823934-6.00010-1

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biochemical process during which complex organic materials are decomposed in the absence of oxygen promoted by the interaction of various microorganisms. The process of anaerobic activity is common to many natural environments including some soils, lake and oceanic basin sediments, ruminants stomachs and peat bogs. In a biogas installation, this process occurs in a sealed vessel called a reactor, which is constructed in various shapes and sizes specific to the site and feedstock conditions. The useful end product is a gases mixture of called biogas, that can either be combusted for heat and energy or be processed further into a renewable natural gas or as transportation fuels. The AD generates also digestate which contains a low amount of biosolids called sludge together with some liquid material. Currently, the raw material used for the anaerobic digester is a homogenous mixture of two or more substrate types. This process is designed as co-digestion and is rather common to most biogas applications (Tamoši¯unas et al., 2022). During anaerobic digestion, organic polymers are broken down with homogenous molecules into methane-rich biogas. Indeed, the usual composition of biogas is 60 to 70% methane, 30 to 40% carbon dioxide and some other gases in small quantities such as 1 to 2% of hydrogen sulphide, less than 1% of nitrogen and of hydrogen, as well as traces of carbon monoxide, ammonia, water vapor and oxygen. Lignocellulosic biomass materials can also be (co)-digested. However, a pre-treatment is usually applied, in order to lower the biomass recalcitrance and thus enhance their digestibility as the microfibrils of celluloses are confined in a matrix of intertwined hemicelluloses and lignin, forming a resistance to effective biological decomposition (Olatunji et al., 2021). Biomass derived from oil, coconut and date palm trees are among the substrates used in biogas plants applied from small-scale to large-scale, including electric power production. The yielded biogas composition and quality rely on the substrates and the digestion process conditions such as the hydraulic retention time (HRT), the temperature, the pH, etc. Even though, the generated gas could be ready for several usages straight from the biogas plant gas holder, it still has to undergo further stages. These latter may include purification in order to remove the inert or low-value constituents, namely H2 S, CO2 and water (through activated carbon filters), scrubbing (by cascading water at very specific pressures and temperatures) and drying (to prevent condensation in winter subzero conditions). The so-obtained biogas can therefore be injected into the natural gas pipeline networks, compressed and used as vehicle fuels, or processed further to generate alternative transportation fuels and/or energy products.

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Figure 9.1 Flow diagram of the anaerobic digestion process.

9.2.1 The biochemical process of anaerobic digestion As previously mentioned, anaerobic digestion includes a series of microbiological processes through which diverse bacteria break down organic matter in an oxygen free environment. The two main end products are biogas and digestate. The biogas is a combustible gas, consisting mostly of methane and carbon dioxide while the digestate is the decomposed left material. During AD, very little heat is generated and the energy, which is chemically bounded in the feedstock, remains mainly in the produced biogas, in form of methane. The biogas production process of is the result of interlinked steps, during which the initial materials are continuously degraded and divided into smaller units. Specific communities of microorganisms are involved in each individual step and carry out the decomposition of the previous steps products successively. The simplified diagram of the methanation process highlights four steps that run parallel in time and space, within a system centralized in a unit called an anaerobic digester, also known as a biogas reactor or a biodigester or a digester tank, as symbiosis of different groups of organisms (Fig. 9.1). These steps are hydrolysis, acidogenesis, acetogenesis and methanogenesis (Dussadee et al., 2016). Hydrolysis is theoretically the first step of anaerobic digestion, during which hydrolytic microorganisms excrete hydrolytic enzymes (exoenzymes)

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that convert polymers into simpler and soluble compounds as follows: lipase

Lipids → f atty acids, glycerol Polysaccharides

cellulase, cellobiase xylanase, amylase

→

monosaccharide

protease

Proteins → amino acids During acidogenesis, the products resulting from the hydrolysis stage are converted by acidogenic (fermentative) bacteria into methanogenic substrates. Simple sugars, fatty acids and amino acids are degraded at a rate of 70% into acetate (C2 H3 O2 ), carbon dioxide (CO2 ), and hydrogen (H2 ) and at a rate of 30% into volatile fatty acids (VFA) and alcohols. The products from the acidogenesis step, which could not be directly converted to methane by methanogenic microorganisms, are converted into methanogenic substrates during acetogenesis. VFA and alcohols are oxidized into methanogenic substrates like C2 H3 O2 , H2 and CO2 . During methanogenesis, these intermediate products are transformed into CH4 and CO2 according to the following mechanisms: Acetic acid

methanogenic bacteria

→

H ydrogen + carbon dioxide

methane + carbon dioxide

methanogenic bacteria

→

methane + water

The acetogenesis and the methanogenesis run usually in parallel, as symbiosis of two microorganisms groups. While around 70% of the formed methane originate from acetate, the remaining 30% are produced from the conversion of hydrogen and dioxide carbon, according to the following Eqs.: The methanogenesis stage is a critical step in the entire AD process, as it might be severely influenced by operational conditions and environmental data. The feedstock composition, the feeding rate, the temperature and the acidity are among the factors that might impact the overall methanogenesis process. The digester overloading, the temperature fluctuations or the large entry of oxygen can result in an early termination of methane production.

9.2.2 Application to palm tree wastes and palm fruit residues Palm tree residues and palm fruit wastes are composed of cellulose and hemicelluloses, making them possible substrates for methane production by anaerobic digestion.

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For instance, Chaikitkaew et al. (2015) have evaluated different types of wastes issued from a palm oil mill for methane production by anaerobic digestion (Chaikitkaew et al., 2015). During their investigations, the palm residues were mixed with inoculum at different ratios I:F based on the volatile solids, ranging from 2:1 to 6:1. The results brought out that among the five ratios tested, the 2:1 ratio yielded the highest cumulative methane production respectively for the empty fruit bunches (EFB) with 2.180 L CH4 per g of volatile solids, the palm press fibers (PPF) with 1.964 mL CH4 and finally for the decanter cakes (DC) with 1.827L CH4. The highest methane yield of 144 mL CH4 /gVS was again obtained from the bunches followed by the fibers (140 mL CH4 /gVS ) and then from the cakes (130 mL CH4 /gVS ) at F/I ratios of 2:1. Methane productions from EFB, PPF and DC by solid state anaerobic digestion (which is an AD system operating at total solid contents of higher than 15%) were 55, 47 and 41 m3 CH4 per ton, respectively. These findings collectively pointed empty fruit bunches as very promising substrates for methane production by solid state anaerobic digestion (Chaikitkaew et al., 2015). Lattieff (2016) have also optimized the methane production process from date palm fruit residues by investigating the effect of solid to distilled water mixing ratio, at different temperatures (mesophilic (37 °C) and thermophilic (55 °C) conditions) and in presence or absence of recycled digestate (Lattieff, 2016). The main findings included that the best mixing ratio for date palm fruit wastes with distilled water was 3 to17 as these proportions help to reach the highest biogas yield of 182 L/kgVS with 63% methane component. Mesophilic temperature was concluded as more suitable to produce methane, which is probably due to the excessive volatile fatty acid formation in the thermophilic temperature conditions. By mixing recycled digestates with substrates at a ratio of 25%, an improvement of biogas production of almost 12% was achieved. A process for the production of biogas through the anaerobic digestion of date palm tree wastes was developed by Al-Juhaimi et al. (2014). The effect of different pretreatments and the operating conditions on the biogas yield were approached. The best results were reached when applying an alkali pretreatment to the substrates, particles size ranging from 2 to 5 mm, a 40 °C digestion temperature, C/N ratio of 30: 1, initial pH of 7 and 10% volatile solid concentration . The generation of flammable gas, where methane accounts for more than half in its composition, started after only one week of operation and continued for approximately eleven weeks. The highest average gas yield obtained was 342.2 L/kgVS . The highest maximum

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Figure 9.2 From White, J. (2011). Biogas generation potential of coconut copra in the anaerobic digestion process. Methane production and pH vs HRT.

and average volumetric biogas production rates obtained were respectively 674.5 and 404.4 L/m3 of digester volume per day (Al-Juhaimi et al., 2014). White et al. (2011) have examined the biogas generation potential of coconut copra as a biomass source by applying anaerobic fermentation processes (White, 2011). The results indicated that coconut copra could be amenable to AD due to its high lipid contents. Unfortunately, the optimal organic loading rate was limited to within a very narrow interval ranging from 3.6 to 6 g VS (i.e. 2.4 to 4 g VS /L of digester) for the batch reactor, the maximum being 420 mL of CH 4 per g of volatile solids reached at an organic load of 3.6 g VS. Substrate overloading (organic loading rates of more than 15 g of volatile solids) resulted in low pH values and negligible CH 4 productions. Higher average methane yields, of 708 mL CH 4 /g VS per day, were also successfully achieved in the continuously stirred reactors (Fig. 9.2). Moreover, increased mixing was observed to have an improved effect on the CH 4 generation. The failure of an accelerated continuous stirred tank reactor start-up procedure reinforced the requirement for a gradual and steady acclimated period during the anaerobic digestion of substrates like coconut copra. Finally, the addition of nitrogen and phosphorus supplements

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Figure 9.3 Open air lagoon digesters of POME.

not only failed to increase the biological activity but also and ultimately resulted in an accumulation of the ammonia to concentration levels that were toxic to the methanogenic microorganisms. The palm oil mill effluents, due to high organic contents, are easily amenable to biodegradation.However,because of silting and short circuiting, many POMEs do not meet discharge standards to water courses. This situation can be significantly improved by introducing enclosed anaerobic digestion systems which reduce the biological oxygen demand (BOD) of the effluent and capture methane, one of the more potent greenhouse gases. In fact, there are two types of biochemical digestion widely practiced in palm oil mills. The first is aerobic digestion in open lagoons (Fig. 9.3) whereas the second is an anaerobic fermentation, closed with high-density polyethylene (HDPE). Biogas production from palm oil mill effluents ranged usually between 20 and 28 m3 of methane per m3 of POME. Generally a volume of 1 m3 biogas is capable of generating 1.8 kWh, which is equivalent to a power generation efficiency of approximately 25% (Lam and Lee, 2011;

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Ohimain and Izah, 2017). The methane energy content can then be recovered, either as a supplementary boiler fuel, or in a biogas engine generator. Nevertheless, the major issue that constrains biogas production is the complexity of the overall production, which increases the cost of the biogas. Hosseini et al. (2015) have introduced hydrogen to biogas issued from palm oil mill effluents. This hydrogen addition has visibly stretched the flame formation and improved the low calorific biogas nature by increasing the hydrogen percentage from 5 to 10 vol.%. The main drawback of this method is that it enhances the nitrogen oxides, because of increased temperature (Hosseini et al., 2015). Besides, effluents from palm oil mills have high lipid contents that might cause problems during AD due to the accumulation of long chain fatty acids. Indeed, these latter are the metabolites of the lipids hydrolysis and, their accumulation would hinder the production of methane gas (Choong et al., 2018; Rasit et al., 2015). To overcome this problem, pretreatments of POME are needed and some methods have been proposed. One can cite de-oiling, sedimentation, pre-hydrolysis, biological and/or inorganic compounds adding. Other pretreatments are also displayed in Fig. 9.4, together with schematic explanations. Palm oil mill effluent de-oiling results in a better methane yield due to lower long chain fatty acids and protein contents, as well as lower portions of bio-fibers (Choong et al., 2018; Fang et al., 2011; Rasit et al., 2015). Nonetheless, another treatment has to be supplemented to the de-oiling in order to manage the extracted lipids.Sedimentation consists of separating the non-digestable suspended solid fractions. Nevertheless, this strategy requires a particular attention to the design of the reactor and its operation conditions otherwise the methane yields would not increase significantly (Ahmed et al., 2015). The effluents pre-hydrolysis is also thought to improve the biogas productions efficiently as the hydrolysis is the rate limiting step in the whole AD process. For example, the palm oil mill effluent ozonation, as a prehydrolysis method, was reported to simultaneously boost considerably the yield of CH4 from POME by a factor of 925% (Chaiprapat and Laklam, 2011) and reduce some toxicants, namely polyphenols and long chain fatty acids. The addition of inorganic additives such as calcium oxide-cement kiln dust, chitosan and red mud-iron for palm oil mill effluent pretreatment aims to improve the retention of the biomass in the system (Ahmad, 2014; Choong et al., 2018). As for the supplementation of microbial consortium as a biological additive, it is known first to assure a better COD removal and

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Figure 9.4 Some pretreatment techniques.

then to enhance the anaerobic fermentation performance thanks to sufficient acclimatization and startup periods (Ahmed et al., 2015). Although the anaerobic digestion is often put forward as the most fundamental process due to the high volumes of carbon contents in the palm mill oil effluents, this same factor could lead to unstable C: N ratio in the AD

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Figure 9.5 Process flow in biogas power plant.

operation, where thereby an optimal C: N ratio should be in the range of 20: 1 to 30: 1 (Okonkwo et al., 2018). In fact, irregularities in this ratio would result in a more significant release of total ammonia nitrogen and/or in a high build-up of volatile fatty acids, which again would inhibit the digestion process (Suksong et al., 2017). In addition, some other factors might also halt the overall reaction and diminish the biogas production efficiency,such as the ambient temperature,the medium pH,the organic loading rate,the hydraulic retention time, etc. Some of these concerns were addressed in the literature. For example, Shakib and Rashid (2019) have discovered experimentally that the optimum pH value of 6.9 with a C: N ratio of 30: 1 and a VSS of 6 g per L reactor and per day could produce 3.8 L of biogas daily (Shakib and Rashid, 2019). Abu Bakar et al. (2016) have stipulated that biogas co-firing in the palm oil mills boilers could potentially cut the particle matter formation by half and save fuel by up to 80 – 90% (Abu Bakar et al., 2016). Among these significant contributions of palm oil mill effluent in biogas production, the most prominent and essential aspects are the economic analyses. A typical biogas plant’s (Fig. 9.5) (Sinaga et al., 2018) overall facility is estimated to be 1 to 1.5 million US dollar, depending on the palm oil mill capacity. The return on investment would be fruitful after completing 2 to 4 operational

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years with revenues from grid-connected electricity generation and reduced diesel consumption (Kheang et al., 2016). Thus, biogas from palm oil mill effluents could be a sustainable solution as it generates considerable incomes in rural and/or remote areas whereby natural gas infrastructures can not reach. Finally, the residual solids and liquids created along with the biogas production are called digestate. This latter goes into a post-digestion reactor and from there further into storage tanks. The digestate is suitable as a fertilizers in agriculture or in landfilling and can also be turned into gardening soils through a process of maturation involving composting for usages such as fertilization of fields. The digestate may also be centrifuged to yield enough process water for the biowastes slurrification at the beginning of the process. This helps reducing the use of clean waters. The centrifuged liquids are rich in nutrients, particularly nitrogen that may further be separated using technologies such as stripping and, used as fertilizers or as nutrient sources in other industrial processes (Ndubuisi-Nnaji et al., 2020).

9.2.3 The ad parameters and operating data The construction and the operation of a biogas unit is a combination of technical and economic considerations.Attaining the maximum biogas yield, by a complete digestion of a given substrate, would require a long retention time of the feedstock inside the digester and a correspondingly large reactor size. To improve the methane production, the selection of one system design (bioreactor size and type) or of an applicable retention time is always based on a compromise between getting the highest possible gas yields and having a justifiable economy for the operating unit. The common types of anaerobic reactors for the palm oil mill effluents digestion are the continuous stirred tank reactor, the fluidized bed reactor, the expanded granular sludge bed reactor, the up-flow anaerobic sludge fixed-film reactor, the up-flow anaerobic sludge blanket reactor, the anaerobic baffled bioreactor, its modified version, the membrane anaerobic system as well as its variant using an ultrasonic assisted membrane (Rajani et al., 2019). The main principles of some of these reactors as well as some of their advantages and drawbacks are seen in Fig. 9.6 and Table 9.1). Almost all of these reactors have exhibited good efficiencies and promising methane production in many laboratory scale researches. Still, the continuous stirred tank reactor is often the bioreactor that is the most adopted for POME digestion at a commercialized scale, thanks to several strong points such as low investments and low operating costs (simple construction,low electricity

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Figure 9.6 Different types or digesters.

requirement, easy cleaning and maintenance, etc.). Besides, it provides more contact between the wastewaters and the biomass. The co-digestion is proved to be one of the most common strategies to improve the overall anaerobic digestion performance and the biogas production. Indeed, codigestion benefits include an increase in the process stability, a balance in the macronutrients (Carbon and Nitrogen) and the micronutrient contents, the promotion of the synergistic effects of microorganisms and the dilution of the inhibitors in the system (Hagos et al., 2017). Among the co-substrates that have been digested with palm oil mill effluents one can cite the oil palm empty fruit bunches. These latter have altered the C/N ratio within the digester (Wu et al., 2010).

Biofilm structure of microorganisms important Nonattached biomass important Biomass thickness control in reactor Recycle necessary Mixing necessary ∗∗ Separation equipment necessary Phasing possible ∗∗∗ Suitable for wastes with suspended organics Run-through of inerts in raw waste Problems with foaming Problems with gas bubbles in reactor High microorganisms/wastewater contact Tolerates hydraulic overloading Tolerates organic overloading Suitable for high concentration of biodegradable toxics Susceptibility to shock-dose toxicants Start-up problems Restart-up easy

Expanded bed

Fluidized bed

Recycled bed

Recycled flocs

Sludge blanket

(+)

(+)

+

+

(+)

°

(+)

(+) ° ° ° ° (+)∗ (+) ° ° (+) ° + + +∗

(+) (+) ° ° ° (+)∗ (+) ° ° (+) + + +∗

° (+) + ° ° + ° (+) + (+) + + (+) (+)

° + + ° ° + ° + + (+) + + (+) (+)

(+) ° + + + + + ° (+) ° + (+) + (+)

+ ° + + + + + ° (+) ° + (+) + +

+ ° ° (+) + (+)∗ (+) (+) + (+) (+) (+) (+) (+)∗

° (+) +

° (+) +

+ + (+)

+ + (+)

+ (+) +

+ (+) +

+ + +

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Table 9.1 Comparison between the various types of reactors. Fixed Moving bed bed

+: yes; (+): partially; °: no/none; ∗ : recycle or mixing mandatory; ∗∗ : 2 reactors with separate acid and gas phases; ∗∗∗ : in excess of the mixing caused by gas bubbles.

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Another important parameter for the biodigester sizing is the hydraulic retention time. This parameter is the mean period during which the substrates are kept inside the reactor tank. The hydraulic retention time is correlated to the reactor volumes and the substrates feeding per time unit. Increasing the organic load reduces the hydraulic retention time. This last must be long enough to ensure that the amounts of microorganisms removed with the digestate are not higher than those of reproduced microorganisms. The duplication rate of anaerobic bacteria is usually 10 days or more. A short retention time provides a good flow rate of substrate but a lower biogas yield. Therefore, it is crucial to adapt the hydraulic retention time to the specific decomposition rate of the feedstock. The efficiency of the anaerobic digestion process is influenced by some other critical parameters. Consequently, it is very important that appropriate conditions are provided to the anaerobic microorganisms. The growth and the activity of these microorganisms are significantly influenced by internal parameters as well as by environmental conditions, including the stirring intensity, the exclusion of oxygen, the pH value, the temperature and its constancy, the nutrients’ supply, the presence and the amount of inhibitors, etc. More particularly, the temperature has an impact on the microorganisms function and therefore on the biogas generation. There are 3 types of temperature processes that may carry out anaerobic digestion: psychrophilic (