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Evaluation of Different Pretreatment Processes of Lignocellulosic Biomass for Enhanced Biomethane Production Samer Dahadha, Zeid Amin, Amir Abbas Bazyar Lakeh, and Elsayed Elbeshbishy Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02045 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017
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Evaluation of Different Pretreatment Processes of Lignocellulosic Biomass for Enhanced Biomethane Production
3 4
Samer Dahadha, Zeid Amin, Amir Abbas Bayzar Lakeh, Elsayed Elbeshbishy*
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
Department of Civil Engineering, Faculty of Engineering, Architecture and Science, Ryerson University, 350 Victoria Street, Toronto, Ontario, Canada, M5B 2K3
*
Corresponding author. Elsayed Elbeshbishy, Department of Civil Engineering, Faculty of Engineering, Architecture and Science, Ryerson University, 350 Victoria Street, Toronto, Ontario, Canada, M5B 2K3
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Phone: +1 (416) 979-5000 Ext. 7618
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Fax: +1 (416) 979-5122
37
E-mail address:
[email protected] 38
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ABSTRACT
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Lignocellulosic biomass is the most abundant source of organic materials available, yet it
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remains highly underutilized as a source of renewable energy products. The complex and rigid
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properties of lignocellulosic materials make it uneasily digestible and thus does not offer a
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significant energy yield once digested through anaerobic digestion. Several pretreatment
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methods have been developed over the past years to improve the digestibility of lignocellulosic
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biomass and enhance its energy yield potential. This review paper examines the latest
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technologies and methods used in the pretreatment of lignocellulosic biomass for more efficient
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anaerobic digestion and energy yield in the form of methane gas. Such pretreatment processes
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include mechanical, irradiation, thermal, chemical, biological, and combined pretreatment. A
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comparison between the different types of available pretreatment methods shows that the
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different methods have been successful in achieving an improvement in the methane yield from
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lignocellulosic substrates on a lab-scale. There is a clear variation in the energy requirements,
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reaction times, and methane improvement for each method. However, more research is necessary
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to assess the applicability and feasibility of such methods on full-scale facilities. In addition, the
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optimum choice of a pretreatment process will remain highly dependent on the substrate type and
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economic feasibility.
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1. INTRODUCTION
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The past decade has witnessed a surge in challenges arising from the increased consumption of
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fossil fuels and non-renewable energy sources such as the depletion of natural resources and the
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environmental impacts of the use of fossil fuels that contribute to climate change and global
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warming. It is estimated that more than 84% of the world’s energy demand is supplied through
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non-renewable sources such as fossil fuels, coal and natural gas.1 However, such challenges
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paved a wider way for considering and further researching the use of renewable energy sources
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and managing natural resources in a sustainable manner. A great amount of research has been
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dedicated to the use of renewable organic biomass as a more sustainable alternative to fossil
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fuels and a suitable means of waste reduction.
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Particular interest has been given to lignocellulosic biomasses which are the most abundant
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source of organic matter on the biosphere. Lignocellulosic materials (LCM) are organic materials
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usually found in plant cell walls.2,3 Lignocelluloses are composed of a mixture of three main
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polymers: cellulose, hemicellulose, and lignin that bind together to form the rigid and protective
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layer of the plant cell wall.1,4 It can be collected as a waste material from forest, agricultural,
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industrial, and municipal areas.5 In addition, Lignocellulosic biomass could be grown as an
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energy crop that does not compete with food crops and can be planted in areas not suitable for
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food crops as several ethical concerns have arisen from using food crops such as sugarcane for
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biofuels production.2,6
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Although lignocellulosic biomass is abundant, there are still many challenges hindering it as an
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attractive energy source due to the nature and complexity of its components. Lignocellulosic
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materials are often insoluble in water at low temperatures and are not easily digestible by most
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living organisms including bacteria. This is mainly due to the interaction of the cellulose,
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hemicellulose, and lignin which form a highly resistant and recalcitrant structure.1,2 Recent
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studies have shown that lignocellulosic material makes up about 14 to 44% of raw excess sludge
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produced in different wastewater treatment processes, indicating the difficulty in digesting it
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through microorganisms.4
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Anaerobic digestion (AD) of organic matter has proven to be one of the most cost effective and
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efficient biological processes in treating and converting organic matter to energy in the form of
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electricity, heat and natural gas.7 AD is a natural process that relies on microorganisms to digest
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organic matter in the absence of oxygen.1 AD consists of four main steps as shown in Figure 1.
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Hydrolysis is the first step in which hydrolytic microbes secrete enzymes that work to
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decompose complex organic polymers to soluble monomers. Acidogenic (fermentative) bacteria
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then acts to convert the soluble monomers to a mixture of volatile fatty acids and other products.
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The volatile fatty acids are then converted to acetate, carbon dioxide and hydrogen by acetogenic
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bacteria. The products are then used for methane production through methanogenesis.7
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However, lignocellulosic materials have shown great resistance to anaerobic digestion resulting
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in low energy yield and digestibility level if introduced to AD without any pretreatment.4,7
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Hydrolysis is often believed to be the rate limiting step in the AD of lingocelluloses.1,7
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Therefore, to increase the efficiency of AD in treating LCM and improve energy yield, an
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efficient pretreatment process is required to enhance the digestibility of lignocellulosic biomass
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by microorganisms.
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Figure 1. Anaerobic digestion process flow (Adapted from ref 7). The recalcitrant structure of LCM limits its utilization for methane production. For this reason, a
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pretreatment process is required to make lignocelluloses more amenable to biological
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degradation by changing its form. This may be achieved by disrupting the secondary cell wall of
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raw LCM and thus facilitating succeeding steps in the AD process.8 Several pretreatment
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processes have been developed and researched over the past years which could be categorized as
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mechanical, irradiation, thermal, chemical, biological, and combined pretreatment processes.4
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The pretreatment process is essential when handling LCM through AD for a cost efficient and
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economical conversion. However, this pretreatment step is the most expensive and accounts for
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about 20% of the total energy cost yielded from lignocellulosic biomass.9 Hence, it is important
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to improve the available pretreatment processes and find new, affordable and efficient
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2. LIGNOCELLULOSIC BIOMASS OVERVIEW
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2.1. Composition
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The basic composition of lignocelluloses consists of cellulose, hemicellulose, and lignin, in
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addition to lower contents of other organic and inorganic compounds such as proteins, ash, and
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pectin as shown in Figure 2.5
123 124 125 126
Figure 2. Typical composition of lignocellulosic material (Adapted from ref 10).
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lignocellulosic feedstocks as shown in Table 1. In general, the content of cellulose,
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hemicellulose, and lignin in lignocellulosic materials is about 30-60%, 20-40% and 15-25%,
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respectively.11
The percentage of each of these main components varies among the different types of
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Table 1. Composition of certain lignocellulosic biomass from various sources Lignocellulosic material
Cellulose (%)
Hemicellulose (%)
Lignin (%)
Reference
Corn stover Corn cobs Cotton seed hairs Wheat straw Switchgrass Bagasse Sugarcane Rice straw Giant reed stalk Giant reed leaves Sunflower stalk Barley straw Rye straw Eucalyptus Hardwood stems Softwood stems Nut shells Paper Leaves Newspaper Grasses Solid cattle manure
37.5 45 80-95 38.2, 29-35 31.0 - 45 38.2 25.0 32.0 33.1 20.9 31.0 37.5, 31-34 38.0 38.0-45.0 40.0-55.0 45-50 25-30 85-99 15-20 40-55 25-40 1.6-4.7
22.4 35 5-20 21.2, 26-32 20.0 - 31.0 27.1 17.0 24.0 18.5 17.7 15.6 25.3, 24-29 36.9 12.0-13.0 24.0-40.0 25-35 25-30 0 80-85 25-40 35-50 1.4-3.3
17.6 15 0 23.4, 16-21 12.0 -18.0 20.2 12.0 13 - 18 24.5 25.4 29.2 26.1, 14-15 17.6 25.0-37.0 18-25 25-35 30-40 0-15 0 18-30 10-30 20
1, 12 12 12 1, 13 1, 13 1 1 1, 13 1 1 1 1, 13 1 1 12,13,14 12,13,14 12,13,14 12,13,14 12,13,14 12,13,14 2,12,13,14 12
133 134 135
2.1.1. Cellulose Cellulose is composed of D-glucose subunits linked by β -1,4 glycosidic bonds and is the main
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component of most plant cell walls making it one of the most abundant sources of renewable
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polymers available.5,7 Cellulose is insoluble in water and many organic solvents. However, it can
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be dissolved in water at extremely low or high pH levels as well as other solvents such as ionic
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liquids (ILSs) and N-methylmorphloine N-oxide (NMMO). The insolubility of cellulose is
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believed to be a result of the hydrogen bonds holding the crystalline structure.15,16 The
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characteristics of cellulose make it difficult to be biodegraded or digested by most animals.5,7,17
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2.1.2. Hemicellulose Hemicellulose refers to a family of heteropolymers or polysaccharides that are amorphous and
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differ depending on the source of the material and extraction methods. Hemicellulose is part of
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the supporting materials in plants’ cell walls. The composition of hemicellulose is highly
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variable between different plants and materials. For example, hemicellulose found in hardwood
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is mainly composed of xylans, while the main component of hemicellulose in softwood is
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glucomannans.2,7,14 Hemicellulose requires elevated temperatures to become soluble in water
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with its solubilisation starting at 150 to 180 oC.17
152 153 154
2.1.3. Lignin Lignin is a natural three-dimensional polymer with a phenyl-propane base. After cellulose, lignin
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is the second most abundant source of polymers and provides strong mechanical support to the
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plant cell wall and water impermeability. Lignin acts as a cement linking cellulose and
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hemicellulose to form the rigid structure of the plant cell wall.1,14
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Lignin is insoluble in water and is considered an optically inert material that has been shown to
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only dissolve in water at elevated temperatures (starting at 180 oC) and variant pH levels
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depending on its type. Such properties make lignin the most resistant component to biological
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and chemical degradations of lignocelluloses.7,17
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2.2. Sources of Lignocellulosic Biomass
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The abundant supply of lignocellulosic biomass could be attributed to its high variety of sources.
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Lignocellulosic biomass sources can be divided into two main categories which are waste
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sources and energy crops. Waste sources are those sources where lignocellulosic biomass is
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produced as a by-product and waste due to different natural and human activities such as forestry
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and agricultural residues, in addition to municipal solid wastes. Energy crops are those
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specifically grown as organic feedstock for bioenergy production such as fast-growing trees and
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switchgrass.2,18,19 8 ACS Paragon Plus Environment
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Table 2. Typical sources and examples of lignocellulosic materials Category
Source
Example
Reference
Waste
Forestry
Residues resulting from forest logging, harvesting and other operations. Corn stover, sugarcane bagasse, wheat straw, rice husk, pinewood
2,11,18
Agricultural
Energy crops
2,11,18
Municipal
Paper waste, paper mill sludge
2,11,18
Energy crops
Switchgrass, giant reed and miscanthus
2,11,18
175 176
The wide range of lignocellulosic biomass contributing to massive amounts being produced
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annually ranks it on top of the list for alternative energy sources despite the several challenges
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faced in making it an economically viable source for producing energy products.17 Although
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there is a contradiction in the literature on the exact amount of global biomass belonging to
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lignocellulosic biomass, some have claimed that annual LCM production is estimated at 10 to 50
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billion tons annually,20 while others claimed that it is close to 200 billion tons annually.21 On a
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smaller scale, lignocellulosic biomass production in the United States has been estimated at 1.4
183
billion dry tons annually.2
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2.3. Handling Lignocellulosic Biomass
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Lignocellulosic material can be utilized to produce various energy products and other potential
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products. There are many processes that could be applied to convert lignocelluloses to different
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energy products such as biofuels and biogases. Such processes include anaerobic digestion,
189
fermentation, incineration, pyrolysis, gasification and others.22-24 Figure 3 presents some of the
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potential products that could be produced through different processes using lignocellulosic
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materials as feedstock.
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195 196 197
Figure 3. Potential products obtained from lignocellulosic materials through various processes.
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This paper focuses on the AD process as a means of producing biogas (biomethane and carbon
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dioxide) from lignocellulosic biomass. AD is very common as it relies on the use of
200
microorganisms as an economical energy recovery process. However, as mentioned earlier, the
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nature of lignocellulosic material hinders the ability of such microorganisms to efficiently
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recover energy from lignocelluloses and thus process enhancement in the form of pretreatment is
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necessary.
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3. PRETREATMENT OF LIGNOCELLULOSIC BIOMASS
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Lignocellulosic substrates are organic compounds that hold an enormous potential for AD and
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methane production. However, the chemical and physical composition of lignocelluloses hinders
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the ability of microorganisms to breakdown these organic compounds and release biogas.
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However, with the right pretreatment process, the biodegradation of lignocelluloses can be
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improved to enhance biogas and methane production. 10 ACS Paragon Plus Environment
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Several pretreatment processes have been developed over the past years to improve
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lignocellulosic biomass amenity to microorganism and enzymes, and enhance biogas and
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methane production. Pretreatment methods work in different ways to achieve desired goals.
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However, such methods have been observed to alter some common physical and chemical
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characteristics of lignocellulosic biomass such as reducing lignin and hemicelluloses contents,
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reducing cellulose crystallinity, reducing the degree of polymerisation and increasing accessible
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surface area and porosity.12,25,26
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Available pretreatment processes can be categorized into: mechanical, irradiation, thermal,
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chemical, biological and combined pretreatment. Mechanical, irradiation and thermal
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pretreatment mainly rely on applying physical, radiation and heat energy on the lignocellulosic
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substrates prior to hydrolysis and AD. Biological pretreatment relies on the action of added
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microorganisms, fungi, and enzymes to enhance the biodegradability of lignocelluloses.
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Chemical pretreatment involves treatment with chemicals to enhance the anaerobic digestion
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process. The use of two or more different pretreatment methods is considered as a combined
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pretreatment method.
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However, the choice of a suitable pretreatment method depends on the composition and type of
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the lignocellulosic materials considered as the amount of cellulose, hemicellulose and lignin
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varies from one type to another as shown in Table 1. In addition, a variation in the physical
230
characteristics such as accessible surface area among different LCM might require distinct types
231
of pretreatment processes.7 This section provides a review of the latest and most commonly used
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pretreatment processes of lignocellulosic wastes for enhanced methane production.
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3.1. Mechanical Pretreatment
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Mechanical pretreatment methods are some of the most basic methods that can be used to
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pretreat lignocellulosic substrates. The primary function of such methods is the application of
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physical force to breakdown the particles and reduce the substrate particles size. This directly
238
increases the surface area available for microbial and enzymatic attacks and thus improves the
239
AD process for methane production.7,25 Table 3 illustrates some of the methane improvement
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results obtained from different studies on mechanical pretreatment methods.
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Table 3. Impact of mechanical pretreatment on methane production from lignocellulosic substrates Method
Substrate
Pretreatment conditions
AD Mechanism
CH4 Production
Ref
Hollander beater
Macroalgae spp
10 mins of beating
Batch Thermophilic
+ 53%
27
Grubben deflaker
Ley sillage
Grubben deflaker uses rotor discs, Batch which grind the material as it Mesophilic passes through the narrow openings between the discs, particles grinded to less than 2 mm.
+ 59%
28
Krima disperser
Ley sillage
Krima disperser uses rotor discs, Batch which grind the material as it Mesophilic passes through the narrow openings between the discs, particles grinded to less than 8 mm.
+ 43%
28
Heavy plates
Meadow grass
Two heavy coarse side of mesh Batch grating plates (upper and lower), Mesophilic particles grinded to 5 mm a
+ 25%
29
Extrusion
Reygrass
Two extrusion intensity of 100% and 60% were applied by adjusting the opening at the outlet of the extruder.
Batch Mesophilic
+ 8.5%
30
Extrusion
Festulolium
At the outlet of the extruder, an Batch adjustable plate controlled the size Mesophilic of the outlet opening. This opening could be varied in size between 5 and 40 cm2. The maximum opening was used in this experiment.
+ 62%
31
Laminaria
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Method
Substrate
Pretreatment conditions
AD Mechanism
CH4 Production
Ref
Hydrodynamic cavitation
Wheat straw
2300 rpm for 2 min Dry matter content: 0.5%
Batch Mesophilic
+ 144%
32
Refiner/Steami ng
Japanese cedar chips
Refiner treatment as a physical Batch pretreatment was carried out using Mesophilic disk refiner equipment followed by steaming treatment as a chemical pretreatment where the mixture of wood chips and distilled water was heated and then maintained for 30 min at 170 ◦C.
+ 13 mL CH4/g vs. 0 mL CH4/g control
33
Size reduction by blender
Biofibers: separated from digested manure, maize silage and industrial by-product.
Particles size reduced to 2 mm
Batch Thermophilic
+ 10%
25
Size reduction by knife mill
Wheat straw Barley straw
Particles size reduced to 2 mm Particles size reduced to 5 mm
Batch Mesophilic
+ 83.5% + 54.2%
34
a
The meadow grass was placed between the plates, the upper plate was manually pushed back and forth, using two hand grips, across the bottom plate for four times without applying any downward force on it.
244 245
Table 3 demonstrates the impact of different mechanical pretreatment methods on methane yield
246
from lignocellulosic substrates. The tests differ in the types of mechanical processes used to alter
247
the physical characteristic of LCM (mainly particle size). The mechanical processes used include
248
beating, deflaking, dispersing, extruding, refining, knife milling, and cavitation. In addition, a
249
variety of lignocellulosic substrates have been used. Most studies were conducted in a
250
mesophilic anaerobic batch system, while only a few were conducted under thermophilic
251
conditions.
252
The mechanical pretreatment tests demonstrated an increase in methane production in the range
253
of 8.5 to 145% during the AD process with an average of about 50%. However, the range of
254
improvement is relatively broad with most improvements falling in the range of 40 to 60%.
255
Mechanical pretreatment is quite reliable but does not usually achieve the highest methane
256
improvement results as some of the lignocellulosic molecules require more than a physical
257
process to breakdown.29
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The highest methane yield was achieved when wheat straw was pretreated with hydrodynamic
259
cavitation.32 The hydrodynamic cavitation is relatively a new pretreatment technology for
260
delignification of the lignocellulosic materials. The process works by inducing the cavitation
261
phenomena in the LCM through a motorized device equipped with a stator and rotor. Three
262
substrates to water ratios of 0.5%, 1% and 1.5% wt/wt were treated at rotation speeds of 2300,
263
2500 and 2700 rpm for 2, 4 and 6 minutes. The highest improvement in methane yield of 144%
264
more than that produced from the untreated wheat straw was achieved at speed of 2300 rpm and
265
time of 2 minutes.32 At higher rotation speeds This value is relatively high compared to the other
266
mechanical pretreatment studies which are a lot more consistent. The authors of this work
267
highlighted that the high improvement in methane yield was mainly attributed to the proper
268
optimization of the treatment relative to the substrate used. The hydrostatic cavitation process
269
can be easily adjusted for speed, particle size, and intensity which, after proper research, can be
270
optimized to achieve very efficient results as demonstrated in this study.32
271
On the other hand, the lowest methane yield improvement was reported when mechanical
272
extrusion was applied to Ryegrass. Improvement in methane yield of only 8.5% was reported
273
which is considered relatively low compared to other mechanical pretreatment methods.
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However, the study considered that the results achieved are consistent with those obtained in
275
other studies available in the literature.30 This might imply that some types of grasses are not
276
affected by mechanical extrusion as others. Therefore, the type and composition of a substrate
277
plays an important role in how well a substrate responds to a certain pretreatment method.
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Mechanical pretreatment is an excellent and reliable technique that is easily scalable to any
279
substrate volume size. Mechanical pretreatment tools are usually adjustable which can make it
280
easy to modify and optimize the pretreatment conditions.28 One of the biggest drawbacks of
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mechanical pretreatment is the capital costs for the motorized equipment and the inability to
282
improve the degradability of some of the tougher lignocellulosic substrates.29 Mechanical pre-
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treatment processes are much more effective when combined with other pretreatment methods as
284
reduction in particle size can synergize very well with any of the other techniques.
285 286
3.2. Irradiation Pretreatment
287
The primary function of irradiation pretreatment is the use of radiation energy in the form of
288
microwave, gamma-ray and ultrasound to increase the biodegradability of lignocelluloses.7 The
289
pretreatment methods used involve the loading of the substrate into a containment that endures a
290
specific intensity (in Watts) of radiation for a specific time.35 Irradiation energy mainly exerts a
291
disruptive effect on lignocellulosic structures and increases the accessible surface area available
292
for microbial attacks, and in some cases reduces the polymerization of such structures.7 Table 4
293
demonstrates some of these studies utilizing irradiation pretreatment methods to improve
294
methane production from lignocellulosic substrates.
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Table 4. Impact of irradiation pretreatment on methane production from lignocellulosic substrates Method
Substrate
Pretreatment Conditions
AD Mechanism
CH4 Production
Ref
Microwave
Switchgrass
Closed vessel microwave, 2450 MHz Power range: 400 – 1600 W 260 °C and 33 bars pressure for 90 – 120 min
Batch Mesophilic
No change
36
Microwave
Grass (Pennisetum hybrid)
1180 W for 3 min 2450 MHz Max temp: 260 °C
Batch Mesophilic
- 13.8%
37
Microwave
Agricultural straws (4 types)
200 °C for 15 min Cool to 100 °C then placed in a desiccator for 3h
Batch Mesophilic
- 64 to -1%
38
Microwave
Lignocellulosic fractions from MSW
1-10 minutes at 500 W Intensity
Batch Mesophilic
+ 8.5%
39
Microwave
Cattail
500 W intensity for 14 minutes at 100 °C
Batch Mesophilic
+ 19%
35
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Page 16 of 43
Microwave
Corn stalks
680 W intensity for 24 minutes at 35 °C
Batch Mesophilic
+ 20% a
40
Ultrasonic
Corn cob Vine wood trimming
750 W intensity for 62.5 seconds
Batch Mesophilic
- 4.6% - 60.8%
41
Ultrasonic
Lignocellulosic materials in excess sludge
Ultrasonic cleaner P = 500 W for 2 h
Batch Mesophilic
+ 184%
4
Ultrasonic (Sonolysis)
OMSW
Low-frequency (20 kHz) bench sonicator, from 0 to 750 watts for 60 min 0.4 W/mL ultrasonic density
Continuous Mesophilic
+ 24% a
42
a
increase in biogas
298 299
Table 4 demonstrates the impact of different irradiation pretreatment tests on methane yield from
300
lignocellulosic substrates. Microwave and ultrasonic irradiation methods were used for the tests
301
each having unique pretreatment conditions. In addition, a variety of lignocellulosic substrates
302
have been tested. The studies were conducted in a mesophilic anaerobic batch system, except for
303
a single study that was conducted in a mesophilic continuous AD reactor.
304
The irradiation pretreatment tests demonstrated an impact on methane production from a range
305
of -64 to 184% during the AD process with an average of about 12%. However, the range of
306
improvement is relatively broad with most improvements falling below 20%. This broad range in
307
results, with the majority being very low suggests that this type of pretreatment is not very
308
effective or may require further research to find the optimal pretreatment conditions for different
309
substrates.
310
The highest methane yield was achieved by Hu et al.4 using ultrasonic irradiation pretreatment
311
for lignocellulosic substrate in excess sludge with a power of 500 W for a period of 2 hours. The
312
improvement in methane yield reached as high as 184% more than that produced from the
313
untreated substrate.4 This value is significantly higher relative to other irradiation methods
314
considered as the next best improvement result was 24%. The high improvement in methane
315
yield was mainly attributed to the ability of ultrasonic radiation in deconstructing the lignin 16 ACS Paragon Plus Environment
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structure and thus improving degradation of the LCM.4 In addition, the long pretreatment
317
duration of 2 hours might be an important factor which resulted in this high improvement but
318
which requires a very high energy input.
319
On the other hand, a negative impact on methane yield was presented by Pérez-Rodríguez et al.41
320
using ultrasonic irradiation pretreatment of vine wood trimmings. The pretreatment resulted in a
321
60.8% reduction in methane yield compared to the untreated sample. The results are in
322
contradiction with other studies using ultrasonic pretreatment which showed positive
323
improvements. However, there is a clear difference in the pretreatment period which was very
324
short (62.5 s) for this study. The study suggests that ultrasonic pretreatment in this case lead to
325
an undesired restructuring or repositioning of the lignin in the substrates which formed a shield
326
that prevented anaerobic biodegradation.41
327
Microwave pretreatment seems to be the most common irradiation pretreatment method studied.
328
This method works by applying thermal energy generated from electromagnetic energy to
329
substrates. However, unlike conventional heating methods, microwave heating is selective and
330
targets polar substances such as water molecules as opposed to non-polar substances.35,37 This
331
leads to the intense vibration of water molecules and inhomogeneity heating in substrates leading
332
to the deconstruction of lignocellulose and hemicellulose solubilisation.35 Several studies
333
obtained a negative or zero change in methane yield from microwave pretreated LCM.36-38 while
334
a few studies obtained an improvement of only up to 20% in methane yield.35,39 This indicates
335
that the impact of microwave pretreatment on methane production from LCM is still not very
336
clear and requires further research.
337
Irradiation pretreatment seems to have a potential to improve biogas production from LCM,
338
particularly ultrasonic pretreatment showed promising results. However, at this stage,
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339
improvements in methane production remain relatively low. Many studies indicated that
340
irradiation pre-treatment might be more efficient when combined with other treatment types.41
341
One of the main drawbacks of irradiation pretreatment is its constant energy requirements which
342
may be very high for long pretreatment periods. Moving forward, irradiation treatment research
343
should focus on improving methane yield by optimizing pretreatment conditions.
344
3.3. Thermal Pretreatment
345
In thermal pretreatment, heat is often combined with pressure to improve the biodegradability of
346
lignocellulosic biomass. There are several different thermal pretreatment methods that can be
347
applied. Other than the conventional means of simply using an autoclave or oven to heat up the
348
biomass, steam explosion and liquid hot water pretreatment have been also given great attention.
349
Steam explosion pretreatment works by exposing the biomass to high temperature and pressure
350
for short duration time.
351
pretreatment conditions may involve temperatures as high as 260 oC and pressure that may reach
352
4.5 MPa.7
353
In liquid hot water pretreatment, water is heated and maintained at liquid state through
354
pressure.7,43,44 The water is then allowed to penetrate the lignocellulosic biomass, leading to the
355
removal of some the hemicellulose and lignin and hydrating the cellulose.44 Liquid hot water
356
thus improves cellulose accessibility and improves the hydrolysis and digestion of
357
lignocelluloses.43 However, undesirable phenolic compounds and furan derivatives such as
358
furfural and hydroxymethylfurfal (HMF) can still form as a result of high temperature which can
359
inhibit microorganisms and reduce the content of fermentable sugars.44,45
360 361 362
Table 5. Impact of thermal pretreatment on methane production from lignocellulosic substrates Method
Substrate
This causes the biomass to decompose explosively. Thermal
Pretreatment Conditions
AD Mechanism
CH4 Production
Ref
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Energy & Fuels
Method
Substrate
Pretreatment Conditions
AD Mechanism
CH4 Production
Ref
Hyperthermophilic
Grass (Eleusine indica)
Oil bath at 80 °C for 3 days.
Batch Thermophilic
+ 46%
45
Conventional heating
Grass (Pennisetum hybrid)
Autoclave, water vapor for 30 min
Batch Mesophilic
+ 4.5%
37
Conventional heating
Wheat straw Sugarcane baggase
121 °C for 60 min
Batch Mesophilic
+ 29% + 11%
46
Conventional heating
Barley Straw Wheat Straw Maize stalks Rice straw
120 °C for 30 min
Batch Mesophilic
+ 40.8% + 64.3% No Change No Change
34
Conventional heating
Dewatered pig manure and digested sewage sludge
100 °C for 1 h
Batch
+ 25%
47
Conventional heating
Raw excess sludge
150 °C for 2 h
Batch Mesophilic
+ 223%
4
Steam Explosion
Wheat straw
180 °C for 15 min, 2 Mpa steam
Batch Mesophilic
+ 19.7%
48
Steam Explosion
Common reed (Phragmites australis)
200 °C for 15 min, 3.4 MPa steam
Batch Mesophilic
+ 89%
49
Steam Explosion
Miscanthus lutarioriparius (grass)
198 °C for 10 min, 1.5 MPa steam
Batch Mesophilic
+ 49.8%
50
Steam Explosion
Two-phase olive mill solid waste (OMSW) or alperujo
200 °C for 5 min, 1.57 MPa steam
Batch Mesophilic
+ 60.9%
51
Steam Explosion
Wheat straw
140 °C for 60 min
Batch Mesophilic
+ 3.6%
52
Steam Explosion
Wheat straw
200 °C for 5 min
Batch Mesophilic
+ 27%
53
Steam Explosion
Late harvested hay
175 °C for 10 min
Batch Mesophilic
+ 16%
54
Steam Explosion
Japanese cedar chips
258 °C for 5 min, 4.51 MPa steam
Batch Mesophilic
+180 mL Ch4/g vs. 0 mL CH4/g control
33
Liquid hot water
Giant reed
190 °C for 15 min
Batch Mesophilic
+ 31%
43
Liquid hot water
Paddy straw
200 °C for 15 min
Batch Mesophilic
+ 148%
55
Liquid hot water
Sugarcane press mud
150 °C for 20 min
Batch Mesophilic
+ 63%
56
Liquid hot water
Sunflower oil cake
100 °C for 1- 6 h
Batch Mesophilic
+ 6.5%
44
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Page 20 of 43
363 364
Table 5 presents some of the main lignocellulosic thermal pretreatment methods including
365
conventional heating, steam explosion, and liquid hot water. As shown in Table 5, the
366
improvements in methane production ranged from 3.5% to 223%. However, most of the studies
367
reported an improvement in the range of 10-65% under mesophilic batch AD conditions.
368
An elevated temperature achieved through thermal pretreatment has been observed to improve
369
methane yield among different substrates. The improvement in methane yield has been attributed
370
to several factors including opening up the lignocellulosic structure,46 deconstruction of lignin,
371
decrease in hemicellulose content,56 improved glucose yield43 and improved accessibility and
372
degradability.47
373
Optimum temperature levels varied from about 100-250 oC depending on the type of substrate
374
and method. However, it was observed that further increase in temperature levels drastically
375
reduced methane yield.47-49,56 This may be due to the formation of complex and toxic compounds
376
such as phenolic acids, furfural, HMF and in some cases melanoidins.47,56
377
Similar to other pretreatment methods, different substrates are affected differently by the same
378
thermal pretreatment process. For example, maize stalks and rice straw pretreated using a
379
conventional thermal pretreatment method at a temperature of 120 oC for 30 min did not show
380
any signs of improvement in methane production, as opposed to barley straw and wheat straw
381
which showed a 40% and 64% improvement in methane yield, respectively for the same
382
pretreatment process.34
383
Thermal pretreatment methods rely on heat energy, however, the duration of the pretreatment
384
process is relatively short with many successful methods requiring only up to 15 min of heat
385
exposure. A few studies considered the net electricity balance for thermal pretreatment methods.
386
Menadro et al.34 measured a positive net electricity balance from the conventional pretreatment 20 ACS Paragon Plus Environment
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387
of wheat straw at 120 oC for 30 min, highlighting the feasibility of this pretreatment process.
388
While, Jiang et al.43 detected a lower net energy output after pretreating giant reed using liquid
389
hot water at 200 oC for 15 min. The decrease in energy has been considered a result of loss of dry
390
matter during pretreatment and the high-energy requirement of the pretreatment process.43
391
Therefore, in this case thermal pretreatment is not a feasible pretreatment option.
392 393
3.4. Chemical Pretreatment
394
Chemical pretreatment methods are the most commonly used methods in the literature for the
395
degradation of lignocellulosic substrate.57,58 The primary function of this pretreatment type is the
396
use of chemicals such as acids, bases, and ionic liquids to alter the chemical and physical
397
characteristics of lignocelluloses for improved AD.7 Chemical pretreatments of LCM are often
398
used in order to modify or remove lignin and hemicellulose, as well as reducing cellulose
399
crystallinity.59 The pretreatment conditions for this method involve the loading of the substrate
400
into a solvent that contains a chemical compound for a specific time period and temperature.
401
Table 6 demonstrates the impact of different chemical pretreatment methods on methane yield
402
from lignocellulosic substrates
403 404 405
Table 6. Impact of chemical pretreatment on methane production from lignocellulosic substrates Method
Substrate
Pretreatment Conditions
AD Mechanism
CH4 Production
Ref
Alkali pretreatment
Giant reed
20 g/L NaOH for 24 h at 24 °C
Batch Mesophilic
+ 63%
43
Alkaline Hydrogen Peroxide (AHP) pretreatment
Sida Mithcanthus Sorghum
5% (m/m) H O solution for 24 h Batch at 25 °C Mesophilic
+ 44% + 85% + 83%
60
Alkali pretreatment
Rice Straw
1% NaOH for 3 h at room temp. Batch Mesophilic
+ 34%
61
Alkali pretreatment
Sugarcane bagasse and mud
1% NaOH for 45 mins at 100 °C
+ 81%
62
2
2
Batch Mesophilic
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Page 22 of 43
Method
Substrate
Pretreatment Conditions
AD Mechanism
CH4 Production
Ref
Alkali pretreatment
Softwood spruce Hardwood birch
7% wt NaOH and 5.5% wt thiourea at -15 °C for 16 h
Batch Thermophilic
+600% +56.5%
63
Concentrated phosphoric acid
Pine
85% phosphoric acid at 60 °C for 45 mins and 50% ethanol
Batch Mesophilic
+39%
64
Dilute sulfuric acid
Wheat plant
1% v/v dilute sulfuric acid at 121 °C for 2 h
Batch Mesophilic
+15.5%
65
Sodium carbonate
Rice straw
0.5 M sodium carbonate at 100 °C for 2 h
Batch Mesophilic
+124.6%
66
Fentons oxidation reagent
Sida Hermaphrodita
Treated with the reagent for 2 hours with a PH of 3 and mass ratio of Fe to H O equals 1:25
Batch Mesophilic
+ 75%
67
2+
2
2
Calcium Hydroxide
Grass
7.5% lime loading for 20 h at 10°C
-
+ 37%
68
CaO pretreatment
Manure biofibres
8% CaO for 25 days at 15 °C
Batch Thermophilic
+ 66%
25
Combined Fe and NaOH pretreatment
Maize straw
6% NaOH dosed with Fe with and initial pH of 7 at 20 °C
Batch Mesophilic
+ 57% a
69
Ethanol as organic solvent
Elmwood Pinewood Rice straw
75% ethanol and 1% sulfuric acid at 150 to 180 °C for 0.5 to 1h
Batch Mesophilic
+ 73% + 85% + 32%
70
Ethanol as organic solvent
Sweet sorghum stalks
50% ethanol at 160 °C for 30 min
Batch Mesophilic
+ 270%
71
Isopropanol based organosolv pretreatment
Sun flower stalks
50% isopropanol containing 1% Batch sulfuric acid at 160 °C Mesophilic
+ 124%
72
Ionic liquid (IL) pretreatment
Water hyacinth
IL: 1-N-butyl-3methyimidazolium chloride ([Bmim]Cl) Co-solvent: dimethyl sulfoxide (DMSO) At 120 °C for 120 min
Batch Mesophilic
+ 97.6%a
73
N-methylmorpholineN-oxide Pretreatment
Rice Straw
85% NMMO at 120 °C for 3 h
Batch Mesophilic
+ 82%
74
N-methylmorpholineN-oxide Pretreatment
Barley straw Forest residue
85% NMMO at 90°C for 3.5 h
Batch Thermophilic
+ 100% + 100%
75
N-methylmorpholineN-oxide Pretreatment
Pinewood chips
85% NMMO at 120 °C for 1 to 15 h
Batch Mesophilic
+3.5 - 6.8 folds a
76
N-methylmorpholineN-oxide Pretreatment
Forest Residues
75% NMMO at 120 °C for 15 h Mesophilic
+141%
77
a
increase in biogas
406
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407
Several chemical pretreatment methods are presented in Table 6. The tests differ in the types of
408
chemical reagents used and the pretreatment conditions. The main chemical reagents used
409
include acids, alkalis, ionic liquids and other chemical compounds. In addition, a variety of
410
lignocellulosic substrates have been used. Most of the studies were conducted in a mesophilic
411
anaerobic reactor batch system, while only a few were conducted under thermophilic AD
412
conditions.
413
The chemical pretreatment tests demonstrated an increase in methane production from a range of
414
15.5 to 600%. However, the range of improvement is relatively broad with most improvements
415
falling in the range of 35 to 120% increase in methane yield. This supports the argument that
416
chemical pretreatment shows consistent and reliable results for improving methane production.58
417
The highest methane yield improvements ranged from 270 to 600%. Ostovareh et al.71 achieved
418
a 270% improvement in methane yield using ethanol as an organic solvent to pretreat sweet
419
sorghum stalks. The study assessed different combinations of ethanol and sulfuric acid at varying
420
temperatures. The optimum methane improvement was achieved by using ethanol (50%) at 140
421
o
422
methane yield from softwood spruce pretreated with NaOH and thiourea at -15 oC, while Shafiei
423
et al.76 achieved a 6.8 folds increase in biogas yield from pinewood chips treated using NMMO
424
pretreatment. However, the results of these studies are relatively skewed from other studies
425
considered as most studies reviewed achieved less than 140% improvement in methane yield.
426
The high methane improvement was mainly attributed to lignin and hemicellulose removal,
427
reduced cellulose crystallinity and opening up the substrate structure 71-73,76
428
Mirmohamadsadeghi et al.70 used ethanol as organic solvent to pretreat rice straw achieving only
429
32% improvement in methane yield. The study used 75% ethanol at 150 oC with the addition of
C without the addition of sulfuric acid. Mohsenzadeh et al.63 obtained a 600% improvement in
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Page 24 of 43
430
1% sulfuric acid as a catalyst. Although this study is similar to the study by Ostovareh et al.71 in
431
terms of using ethanol as an organic solvent under high temperatures. Ostovareh et al.71 study
432
used a lower concentration of ethanol (50% as opposed to 75%) and did not use sulfuric acid as a
433
catalyst. In fact, the study found that the addition of acid to the pretreatment resulted in lower
434
methane yield, even though it improved enzymatic hydrolysis. Increasing the ethanol
435
concentration at higher temperatures was found to be less effective in improving the methane
436
yield.71 In addition, dilute sulfuric acid had the lowest impact on methane yield among the
437
reviewed chemical pretreatment methods as demonstrated by Taherdanak et al.65 using dilute
438
sulfuric acid at 121 °C for 2 hours to pretreat wheat plant achieving only 15.5% improvement in
439
total methane yield.
440
Chemical pretreatment is a suitable and convenient method for the treatment of lignocellulosic
441
substrates. Chemical pretreatment methods are widely studied and quickly progressing as proven
442
by the large number of related publications in the literature. One of the biggest drawbacks of
443
chemical pretreatment is the requirement for the constant dosing of a chemical. This procedure is
444
both expensive and environmentally unfriendly.57 Reducing the overall concentration of the
445
solvents used in the pretreatment process, while achieving a larger increase in methane yield,
446
should be the goal for future research in this area of study.
447 448
3.5. Biological Pretreatment
449
Biological pretreatment of lignocellulosic biomass is often regarded as a simple and low capital
450
cost technology to improve biogas yield. This is mainly because biological pretreatment does not
451
require high energy inputs or chemicals addition.7,78 The main concept of biological pretreatment
452
is to improve the biodegradability of lignocelluloses through either the application of fungi,
453
microorganism and enzymes, ensiling the biomass or adding a microaerobic step prior to AD. 24 ACS Paragon Plus Environment
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454
Table 7 presents some of the studies conducted on improving methane production through
455
biological pretreatment of LCM.
456 457 458 459
Table 7. Impact of biological pretreatment on methane production from lignocellulosic substrates Method
Substrate
Biological Source
Pretreatment Conditions
AD Mechanism
CH4 Production
Ref
Fungal
Japanese cedar chips
Cyathus stercoreus
Incubated for 20 days at 37 °C
Batch Mesophilic
+ 43 mL vs. 0 mL control
35
Fungal
Sweet chestnut leaves and hay
Wood-rotting fungi: Auricularia auriculajudae
37 °C for 4-5 weeks in the dark
Batch Mesophilic
+ 15% a
79
Fungal
Sisal leaf decortications residue (SLDR)
CCHT-1 strain obtained from dumps of SLDR and Trichoderma reesei
4 days with CCHT1 followed by 8 days with T. ressi at 28 °C
Batch Mesophilic
+ 30 - 101%
80
Fungal
Albizia Chips
White-rot fungus: Ceriporiopsis subvermispora
Incubation at 28 °C for 48 days
SS-AD Batch Mesophilic
+ 3.7 folds
81
Fungal
Bio-waste (Organic fraction of household waste)
Trichoderma viride
Incubated at 25 °C for 4 days
Batch Thermophilic
+ 400%
82
Microbial consortium
Corn straw
Yeast and cellulolytic bacteria
Microbial agent dose 0.01% (w/w) 20 oC for 15 days
Batch Mesophilic
+ 75.6%
83
Microbial consortium
Pulp and paper mill sludge mixed with rice straw
Microbial consortium (OEM1) originating from spent mushroom substrate
28 oC for 9 days
Batch Mesophilic
+ 40%
84
Microbial consortium
Cotton stalk
Thermophilic microbial consortium (MC1):
50 oC for 8 days
Batch Mesophilic
+ 136%
85
50 oC for 4 days
Batch Mesophilic
+ 105%
86
Clostridium
straminisolvens CSK1, Clostridium sp. FG4b, Pseudoxanthomonas sp. strain M1-3, Brevibacilus sp. M1-5, and Bordetella sp. M1-6 Microbial consortium
Lignocellulose of municipal solid waste
Thermophilic microbial consortium (MC1)
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Page 26 of 43
Method
Substrate
Biological Source
Pretreatment Conditions
AD Mechanism
CH4 Production
Ref
Microbial consortium
Wheat straw
G-Proteobacteria Bacteroidetes B-Proteobacteria Firmicutes (3 types) White-rot fungi Brown-rot fungi
37 oC for 15 days
Batch Mesophilic
+ 80.3%
87
Microbial consortium
Cassava residues
Thermophilic microbial consortium
55 oC for 12 h
Batch Thermophilic
+ 96.6%
78
Microbial consortium
Biofibers: separated from digested manure, maize silage and industrial byproduct.
Compost from garden waste and fungi collected from straw and maize silage stored outdoor for 6 months
27 oC for 0-20 days
Batch Thermophilic
No change
25
Enzymatic
Jose Tall Wheatgrass
Cellulase and hemicellulase from Humicola Insolens Cellulase from Trichodermaa ressi β-glucosidase from Aspergillus niger
50 oC for 7 days
Batch Thermophilic
+ 0 – 31%
88
Enzymatic
Sugar beet pulp
Endoglucanase and xylanase from Trichoderma longibrachiatum Pectinase
50 oC for 24 h pH 6.6
Semicontinuous Mesophilic
+ 19% a
89
Enzymatic
Spent hops
Endoglucanase and xylanase from Trichoderma longibrachiatum Pectinase
50 oC for 24 h pH 6.0
Semicontinuous Mesophilic
+ 13%
89
Enzymatic
Wheat grains
Trizyme (cellulase, αamylase, protease)
37 oC for 24 h pH adjusted at 7
Batch Mesophilic
+ 14%
90
Enzymatic
Corn cob
Enzymatic cocktail (endo-1,3(4)- β glucanase, collateral xylanase, cellobiase, cellulase, and feruloyl esterase activities) Aspergillus enzymatic extract.
30 oC for 7 days
Batch Mesophilic
+ 14%
91
Ensilage
Pineapple peel waste
-
Ensilaged for 6 months
Semicontinuous Mesophilic
+ 55%
92
Ensilage
Giant reed
-
Ensilaged for 60 days at room temperature
Batch Mesophilic
+ 14%
93
Microaeration
Corn straw
-
5 ml O2/g VS at 55
Batch
+ 16.2%
94
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Energy & Fuels
Method
Microaeration a
Substrate
Wheat straw
Biological Source
-
Pretreatment Conditions
AD Mechanism
o
Mesophilic
C
5 ml O2/g VS for 3 days
Batch Thermophilic
CH4 Production
Ref
+ 7.2%
95
increase in biogas
460 461
Biological pretreatment methods highlighted in Table 7 include the use of fungi, microbial
462
consortiums, enzymes, ensilage and micro aeration. Fungal and Microbial pretreatment showed
463
the most promising level of improvement in total methane yield from lignocellulosic matter. As
464
shown in the Table 7, fungal pretreatment showed up to 400% improvement in methane yield,
465
while pretreatment with a microbial consortium increased methane yield by about 135%.
466
Enzymatic pretreatment averaged around 15% improvement, while micro aeration and ensilage
467
delivered an improvement of up to 16% and 55%, respectively. Overall, biological pretreatment
468
showed a high success rate with more than 90% of studies improving methane yield by more
469
than 13%.
470
Fungal pretreatment was observed to improve biodegradability of lignocelluloses and increase
471
the available fatty acids which provided better nutrients and more substrates for microorganism
472
in the AD phase, leading to an improvement in the methane yield.79,80,82 In addition, fungal
473
pretreatment showed low energy consumption compared to other non-biological pretreatments.33
474
Pretreatment using microbial consortiums have shown the ability to degrade lignocelluloses and
475
reduce the content of lignin, cellulose and hemicellulose.84-87 Zhong et al.83 demonstrated that
476
biologically pretreated corn straw produced 75% more methane than untreated samples, in
477
addition to a reduction of 5.8 to 25% in total cellulose, hemicellulose, and lignin contents. In
478
addition, an increase in the concentration of the soluble chemical oxygen demand (sCOD) in the
479
hydrolysates was observed after pretreatment using a microbial consortium which is directly 27 ACS Paragon Plus Environment
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480
related to increasing the biomethane yield.84 Ensiling lignocellulosic biomass also demonstrated
481
a potential in improving methane yield as it converts the carbohydrates to volatile fatty acids and
482
thus enhancing methane production.92,93
483
As shown in Table 7, enzymatic pretreatment was less effective in improving methane yield
484
from lignocellulosic biomass. Zieminski et al.89 detected a reduction in sugar concentration by up
485
to 89% and a decrease in organic content by up to 23%, which resulted in 19% higher methane
486
yield. Hydrolytic enzymes showed a decrease in hydrogen production and a shift toward methane
487
production from enzymatically pretreated wheat grains.90 Romano et al.88 demonstrated that
488
enzymatic pretreatment was ineffective in improving methane yield from anaerobically digested
489
Jose tall wheatgrass. The study suggested that microorganisms were already effective in carrying
490
out digestions without the need for additional enzymes. However, the test was conducted under
491
thermophilic anaerobic digestion conditions which might not be suitable for efficient enzymatic
492
activity.
493
Microaeration or microaerobic pretreatment has also been applied as a biological pretreatment
494
method, where an oxygen-induced aerobic pretreatment step is used to improve the overall AD
495
process.95 Fu et al.94 detected lower crystallinity levels and higher amount of amorphous
496
celluloses in the substrate after thermophilic microaerobic pretreatment of corn straw. This
497
resulted in more digestible cellulose and a 16% higher methane yield after AD. The success of
498
microaerobic pretreatment relies on the portion of oxygen supplied which was set at 5 ml O2/g
499
VSsubstrate in the studies reviewed. Higher amounts of oxygen have been observed to inhibit and
500
reduce methane production.58,95
501
Most biological pretreatment studies reviewed were conducted under mesophilic AD conditions.
502
However, a few experiments were conducted under thermophilic AD conditions. Results from
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503
the latter indicate that biological pretreatment shows less consistent results under thermophilic
504
AD conditions compared to mesophilic conditions. This is because some studies have shown
505
almost no improvement in methane yield, while the rest have shown very high improvements
506
such as 100% and even 400%. This inconsistency implies that more studies should be conducted
507
to assess the impact of biological pretreatment methods under thermophilic AD conditions. In
508
addition, it not very clear how different biological pretreatment methods perform in continuously
509
stirred anaerobic reactors. Although positive results were obtained for semi-continuous AD
510
reactors, assessing the performance of continuous reactors after biological pretreatment is
511
necessary to determine feasibility on large-scale treatment plants.80
512 513
3.6. Combined Pretreatment
514
Combined pretreatment methods are those which involve the application of two or more different
515
pretreatment methods often conducted in series. The main purpose of such pretreatment methods
516
is to gain better results from different methods that would not have been gained if the methods
517
were applied individually. For example, Bruni et al.25 demonstrated the impact of different
518
pretreatment methods on the methane yield of biofibers from digested manure. Of such methods,
519
enzymatic pretreatment showed no effect on the methane yield. On the other hand, chemical
520
pretreatment combined with thermal pretreatment improved the methane yield by 26%.
521
However, combining enzymatic, chemical, and thermal pretreatments led to an improvement of
522
34% in the methane yield. This indicates that enzymatic pretreatment becomes more effective
523
and leads to a positive contribution to the methane yield once combined with other methods.
524
Table 8 demonstrates the impact of some combined pretreatment methods on methane
525
production from lignocellulosic biomass.
526 29 ACS Paragon Plus Environment
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Table 8. Impact of combined pretreatment on methane production from lignocellulosic substrates Method
Substrate
Pretreatment Conditions
Thermo-chemical
Grass silage
Thermo-chemical
Dewatered pig manure and digested sewage sludge
AD Mechanism
CH4 Production
Ref
100 °C and NaOH loading rate of 5% Batch (w/w) for 1.9 - 3.6 h Mesophilic
+ 38.9%
96
5% (W/W) Ca(OH)2 added and allowed to react for 1 h Heating at 70°C for 1 h
+ 72%
47
Batch
Thermal
Heating at 100°C for 1 h
+ 28%
Chemical
5% (W/W) Ca(OH)2 added and allowed to react for 2 h HCL added to neutralize pH after reaction
- 10%
Thermo-chemical (NaOH + H2O2)
Paper tube residuals
220°C for 10 min heating, 15-20 bar 2% NaOH and 2% H2O2
Batch Thermophilic
+ 107%
Thermal
220°C for 10 min heating, 15-20 bar
+ 5%
Thermal – NaOH
190°C for 10 min heating, 15-20 bar 2% NaOH
+ 69%
Thermal - H2O2
190°C for 10 min heating, 15-20 bar 2% H2O2
- 15%
97
Thermo-chemical
Digested manure biofibers
55oC and 6% NaOH. Continuous Shaking incubator (120 rpm) for 24h Thermophilic
+ 26%
98
Thermo-chemical
Rice straw
200°C for 10 min 5% NaOH added to maintain pH
+ 222%
99
Chemical Irradiation chemical
Paddy straw
Chemical Mechanicalchemical – enzymatic
Mechanical
Corn Cob
Batch Mesophilic
3% NaOH for 120 h at 37oC
+ 123.9%
4% sodium carbonate for 48 hours 60 Batch min in the microwave (720W) at 180 Mesophilic °C
+ 54.4%a
4% sodium carbonate for 48 hours
+ 41.5% a
Fast Extrusion for 35 s at room temperature 0.4% NaOH (w/v) Enzyme: Ultraflo® L: Enzymatic cocktail (endo-1,3(4)- β -glucanase, collateral xylanase, cellobiase, cellulase, and feruloyl esterase activities) Fast Extrusion for 35 s at room
Batch Mesophilic
+ 22.3%
100
91
+ 7.7%
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Energy & Fuels
Method
Substrate
Pretreatment Conditions
AD Mechanism
CH4 Production
Ref
temperature Mechanicalchemical
Fast Extrusion for 35 s at room temperature 0.4% NaOH (w/v)
+ 7.7%
Enzymatic
Enzyme: Ultraflo® L
+ 7.0%
Mechanicalenzymatic
Fast Extrusion for 35 s at room temperature Enzyme: Ultraflo® L
+ 13.4%
Thermo-chemicalenzymatic
Thermochemical
Biofibers: separated from digested manure, maize silage and industrial by-product
Steam + NaOH: 160 °C for 15 min Laccase at 37 °C for 20 h
Steam at 160°C for 15 min Catalysts: Sodium hydroxide (NaOH) Phosphoric acid (H3PO4)
Batch Thermophilic
+ 34%
25
+ 26% + 8%
a
increase in biogas
533 534
There are many studies and experiments on combined pretreatment methods available in the
535
literature. Table 8 presents a few of those studies which highlight some of the most common
536
pretreatment combinations. In general, an increase in the methane yield due to the combined
537
pretreatment was in the range of 20-80% with a few exceptions. Most notably thermo-chemical
538
methods are the most popular combined pretreatment processes.
539
Individual pretreatment methods similar to those discussed in earlier sections seem to have a
540
different effect on the methane yield when combined with other methods. Rafique et al.47 studied
541
the impact of thermal, chemical and combined thermo-chemical pretreatments on the methane
542
potential of dewatered pig manure. Thermal pretreatment at 100 oC improved methane yield by
543
up to 28%. On the other hand, chemical pretreatment using 5% calcium hydroxide reduced the
544
substrate methane potential by about 10%. However, the combined thermo-chemical
545
pretreatment at 70 oC with 5% calcium hydroxide showed significantly better results with up to
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546
72% higher methane yield. The results clearly indicate that the performance of thermal and
547
chemical pretreatments significantly improved when combined.
548
A similar study by Teghammar et al.97 examined different thermo-chemical pretreatment
549
combinations to improve the biogas production of paper tube residuals. Thermal, thermo-
550
chemical pretreatment with NaOH and H2O2 were conducted within a temperature range of 190-
551
220
552
pretreatments improved methane yield by 5% and 69% respectively, while thermo-chemical with
553
H2O2 reduced methane yield by -15%. Combining thermal pretreatment with both NaOH and
554
H2O2 under similar conditions and chemical concentrations the methane yield improved by
555
107%. This improvement was attributed to the ability of the combined pretreatment to open up
556
the cellulose crystalline structure and delignify the LCM prior to AD. More interestingly, the
557
achieved improvement level due to combine pretreatment is much higher than that of individual
558
pretreatments as well as higher than the sum of all their achieved improvement levels. Similar to
559
Rafique et al.47 study, a pretreatment that negatively impacted methane yield such as the
560
combined thermo-chemical (H2O2) had a positive impact once combined with other methods.
561
Most studies reviewed performed a comparison between the performance of individual
562
pretreatment methods and combined methods. In most cases combined pretreatment enhanced
563
methane production by more than the sum of enhancements achieved by individual pretreatments
564
or at least higher than the best individual pretreatment. It is also evident that there is sometimes a
565
synergetic effect when combining two or more pretreatment methods as clearly demonstrated by
566
Rafique et al.47 and Teghammar et al.97, where the combined pretreatment results in an enhanced
567
methane production level significantly higher than that achieved by the summation of all
568
individual methods. The causes of this synergetic effect are not clearly examined in the studies.
o
C and 15-20 bars of steam pressure. Thermal and thermo-chemical with NaOH
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569
However, this effect might be explained by the ability of a certain pretreatment to impact some
570
specific parameters of the LCM that would allow the other pretreatment methods to work better.
571
For example, in Rafique et al.47 study, explosive thermal pretreatment was mainly responsible for
572
breaking down the lignocellulosic structure and reducing crystallinity. This change might have
573
exposed chemicals to a greater surface area to penetrate and thus had a stronger effect.
574 575
4. DISCUSSION AND CONCLUSION
576
A comparison between some of the key indicators considered when selecting a pretreatment
577
method is presented in Table 9. The comparison identifies pretreatment conditions mainly
578
temperature and reaction time, methane improvement achieved, and cost considerations as the
579
key factors often considered when choosing a pretreatment method. The table summarizes the
580
average ranges as obtained from literature and presented in Tables 3 to 8, while omitting highly
581
dispersed values that may reflect a less accurate presentation if considered.
582 583
Table 9. Comparison of different pretreatment methods Pretreatment Conditions Pretreament
Temperature (oC)
Reaction time
CH4 Improvement (%)
Cost Consideration(s) a
Mechanical
Room temperature
A few minutes
10 - 60
Electrical energy input
Irradiation
35 – 260
1 -120 min
5 - 24
Electrical energy input
Thermal
100 – 200
5 – 60 min
5 - 65
Thermal/electrical energy input
Chemical
25 - 120
1– 24 hr
30 - 120
Biological
25 – 55
days – weeks
15 - 100
Combined
55 - 200
10 min – 20 hr
10 - 80
Cost of chemicals, thermal energy input and storage Cost of microbes, fungi and enzymes and storage Varies
a
capital cost should be considered for all pretreatment methods
584 585
As shown in Table 9, chemical and biological pretreatment methods are of the most reliable and
586
efficient methods available in terms of improving methane yield from lignocellulosic substrates. 33 ACS Paragon Plus Environment
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Page 34 of 43
587
Mechanical and thermal pretreatment have demonstrated very close performance in improving
588
methane yield, while irradiation pretreatment has the lowest performance among all pretreatment
589
methods.
590
However, improving methane yield alone is not enough. Other factors should be taken into
591
consideration when selecting a suitable pretreatment method to ensure the feasibility of such
592
methods. Pretreatment conditions are essential in terms of energy and time requirements. In
593
addition, the cost of chemical and biological additives is an essential factor.
594
In terms of energy requirements, irradiation, thermal, and combined pretreatment often relies on
595
a high and continuous energy input to achieve desirable temperature or irradiations levels.
596
Chemical pretreatment methods may also require a heat energy input for certain processes that
597
rely on elevated temperatures. Mechanical pretreatment also relies on an electrical power supply
598
to operate machinery and equipment used to reduce the size of feedstock particles. Biological
599
pretreatment is the least energy consuming method as it mainly relies on the action of
600
microorganism, fungi, and bacteria.
601
Reaction time is also essential in particular for large scale plants. Biological pretreatment
602
processes are by far the slowest in terms of the reaction time required which may reach up to a
603
few weeks. Such long reaction times may not be suitable for large scale plants if land space is
604
expensive or restricted. This is because long reaction times require additional storage and land
605
space which can be costly for plants receiving large quantities of lignocellulosic wastes on a
606
continuous basis. However, if biological pretreatment can be combined with in-situ storage, then
607
no additional storage is needed. Chemical and combined pretreatment processes are faster than
608
biological and may require a few minutes or hours or up to a day. Thermal and irradiation
609
pretreatment methods can be considered fast as they may take a few minutes and up to a couple
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Energy & Fuels
610
of hours. However, the longer the reaction time, the higher the energy requirements are. In such
611
cases, a difference in the order of hours can make a significant difference in terms of energy
612
input. Mechanical pretreatment can be considered the fastest and most controllable among the
613
different methods, as the reaction time could reach a few minutes only, depending on the type of
614
machine used in terms of power, efficiency and set-up.
615
The most decisive factor in determining the optimum pretreatment methods is often the
616
economic feasibility of the method in terms of the cost of pretreatment versus the value of added
617
methane yield. The cost of pretreatment includes both capital and operational costs. In most
618
cases, the operation and maintenance cost is most significant and it may include the cost of
619
chemical or biological additives, the operational and maintenance costs of operating equipment
620
used to perform mechanical or heating activities. Most studies reviewed assessed the impact of
621
pretreatment processes on the biogas yield from LCM on a laboratory scale with a few
622
determining the net energy gain/loss obtained after pretreatment.25,30,31 For example Bruni et al.25
623
achieved a net increase of 68kwh (tWW)-1 as a result of extra methane produced from the
624
combination of thermal, chemical and enzymatic pretreatment of biofibers. Hjorth et al.31 also
625
obtained 68% higher net electrical energy surplus due to the additional methane yield obtained
626
by mechanically pretreating different LCM and after subtracting the energy consumed by the
627
mechanical extruder.
628
On a commercial scale, a recent study performed an economic evaluation of commercially
629
available pretreatment methods such as steam explosion and combined thermo-chemical
630
pretreatment of three main lignocellulosic substrates: paper, wood and straw. The study took into
631
consideration investment cost, operational and maintenance costs and the cost of chemicals. The
632
results of the evaluation indicated that pretreatment of wood and straw is not economically
35 ACS Paragon Plus Environment
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633
feasible as the cost of pretreating wood is 50% higher than the value of gas yield, while the cost
634
of pretreating straw is almost equal to the value of the biogas produced. The high cost of raw
635
materials was one of the main reasons for this negative outcome. On the other hand, pretreating
636
paper showed promising results as the value of the biogas produced was found to be 15% higher
637
than the cost of the pretreatment.101
638
Yet, there is still a lack of a comprehensive economic evaluation of the different pretreatment
639
methods. However, considering the studies reviewed, it should be emphasized that a small
640
improvement in methane yield that is economically feasible is better than a high improvement
641
that is not economically feasible. This is particularly important when transforming such methods
642
from lab-scale processes to large scale plants. Most studies in the literature are conducted as lab-
643
scale experiments and do not represent the same output that could be achieved through large
644
scale biogas production facilities.
645
Several pretreatment methods to improve biomethane production from lignocelluloses have been
646
reviewed in this paper. Chemical and biological pretreatment have shown promising outcomes in
647
improving the biomethane yield from lignocellulosic substrates. Chemical pretreatment seems to
648
be superior over the biological pretreatment as it resulted in higher biomethane yield as
649
compared to biological pretreatment. However, biological pretreatment methods such as fungal,
650
microbial and enzymatic pretreatment can be considered more environmentally favorable as they
651
mimic natural processes and often require less energy input. Yet, the cost of biological additives
652
remains as one of the major challenges that hinders its commercial viability. Therefore, working
653
on optimizing the pretreatment conditions for biological additivities and lowering production
654
costs as well as improving the process performance can lead to a drastic and positive change
655
converting the lignocellulosic materials to biomethane in a sustainable manner. In addition, a
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656
combined approach utilizing more than one pretreatment method may boost the performance of
657
individual pretreatments and achieve technical and financial feasibility, however, a further
658
assessment of combined pretreaments is necessary in the future.
659
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Figure 1. Anaerobic digestion process flow (Adapted from ref 7). 125x108mm (300 x 300 DPI)
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Figure 2. Typical composition of lignocellulosic material (Adapted from ref 10). 184x123mm (300 x 300 DPI)
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Figure 3. Potential products obtained from lignocellulosic materials through various processes.
165x82mm (300 x 300 DPI)
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