Evaluation of Different Pretreatment Processes of Lignocellulosic

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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

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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

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E-mail address: [email protected]

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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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techniques. 5 ACS Paragon Plus Environment

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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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random, and have highly branched structures. There are variable structures of hemicellulose that 7 ACS Paragon Plus Environment

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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

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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

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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,

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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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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

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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

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characteristics such as accessible surface area among different LCM might require distinct types

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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

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increases the surface area available for microbial and enzymatic attacks and thus improves the

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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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Energy & Fuels

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

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from lignocellulosic substrates. The tests differ in the types of mechanical processes used to alter

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the physical characteristic of LCM (mainly particle size). The mechanical processes used include

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beating, deflaking, dispersing, extruding, refining, knife milling, and cavitation. In addition, a

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variety of lignocellulosic substrates have been used. Most studies were conducted in a

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mesophilic anaerobic batch system, while only a few were conducted under thermophilic

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conditions.

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The mechanical pretreatment tests demonstrated an increase in methane production in the range

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of 8.5 to 145% during the AD process with an average of about 50%. However, the range of

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improvement is relatively broad with most improvements falling in the range of 40 to 60%.

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Mechanical pretreatment is quite reliable but does not usually achieve the highest methane

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improvement results as some of the lignocellulosic molecules require more than a physical

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process to breakdown.29


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The highest methane yield was achieved when wheat straw was pretreated with hydrodynamic

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cavitation.32 The hydrodynamic cavitation is relatively a new pretreatment technology for

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delignification of the lignocellulosic materials. The process works by inducing the cavitation

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phenomena in the LCM through a motorized device equipped with a stator and rotor. Three

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substrates to water ratios of 0.5%, 1% and 1.5% wt/wt were treated at rotation speeds of 2300,

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2500 and 2700 rpm for 2, 4 and 6 minutes. The highest improvement in methane yield of 144%

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more than that produced from the untreated wheat straw was achieved at speed of 2300 rpm and

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time of 2 minutes.32 At higher rotation speeds This value is relatively high compared to the other

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mechanical pretreatment studies which are a lot more consistent. The authors of this work

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highlighted that the high improvement in methane yield was mainly attributed to the proper

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optimization of the treatment relative to the substrate used. The hydrostatic cavitation process

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can be easily adjusted for speed, particle size, and intensity which, after proper research, can be

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optimized to achieve very efficient results as demonstrated in this study.32

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On the other hand, the lowest methane yield improvement was reported when mechanical

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extrusion was applied to Ryegrass. Improvement in methane yield of only 8.5% was reported

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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

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other studies available in the literature.30 This might imply that some types of grasses are not

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affected by mechanical extrusion as others. Therefore, the type and composition of a substrate

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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

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substrate volume size. Mechanical pretreatment tools are usually adjustable which can make it

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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

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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

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reduction in particle size can synergize very well with any of the other techniques.

285 286

3.2. Irradiation Pretreatment

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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

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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

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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


Energy & Fuels

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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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316

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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Page 18 of 43

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


AD Mechanism

CH4 Production

Ref


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Energy & Fuels

Method

Substrate


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


Wheat straw Sugarcane baggase

121 °C for 60 min

Batch Mesophilic

+ 29% + 11%

46


Barley Straw Wheat Straw Maize stalks Rice straw

120 °C for 30 min

Batch Mesophilic

+ 40.8% + 64.3% No Change No Change

34


Dewatered pig manure and digested sewage sludge

100 °C for 1 h

Batch

+ 25%

47


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)


Batch Mesophilic

+ 49.8%

50

Steam Explosion

Two-phase olive mill solid waste (OMSW) or alperujo


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



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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Energy & Fuels

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


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


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


Barley straw Forest residue

85% NMMO at 90°C for 3.5 h

Batch Thermophilic

+ 100% + 100%

75


Pinewood chips

85% NMMO at 120 °C for 1 to 15 h

Batch Mesophilic

+3.5 - 6.8 folds a

76


Forest Residues

75% NMMO at 120 °C for 15 h Mesophilic

+141%

77

a

increase in biogas

406


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Energy & Fuels

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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Energy & Fuels

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


AD Mechanism

CH4 Production

Ref

Fungal


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


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


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)


Energy & Fuels

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Page 26 of 43

Method

Substrate

Biological Source


AD Mechanism

CH4 Production

Ref


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


Cassava residues

Thermophilic microbial consortium

55 oC for 12 h

Batch Thermophilic

+ 96.6%

78


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


+ 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


+ 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


Page 27 of 43

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Energy & Fuels

Method

Microaeration a

Substrate

Wheat straw

Biological Source

-


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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Page 28 of 43

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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527 528 529 530 531 532

Page 30 of 43

Table 8. Impact of combined pretreatment on methane production from lignocellulosic substrates Method

Substrate


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%


Page 31 of 43

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Energy & Fuels

Method

Substrate


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


Energy & Fuels

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Page 32 of 43

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


Page 33 of 43

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Energy & Fuels

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

Energy & Fuels

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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


Page 35 of 43

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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


Energy & Fuels

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Page 36 of 43

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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Energy & Fuels

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)


Evaluation of Different Pretreatment Processes of Lignocellulosic

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