Lignocellulosic Biomass to Liquid Biofuels

By Abu Yousuf, Filomena Sannino and Domenico Pirozzi

Lignocellulosic Biomass to Liquid Biofuels explores the existing technologies and most recent developments for the production of second generation liquid biofuels, providing an introduction to lignocellulosic biomass and the processes for its conversion into biofuels. The book demonstrates biorefinery concepts compared with petro refinery, as well as the challenges of second generation biofuels processing. In addition to current pre-treatment techniques and their technical, environmental and economic implications, chapters included also further examine the particularities of conversion processes for bioethanol, biobutanol and biodiesel through chemical, biochemical and combined approaches. Finally, the book looks into concepts and tools for techno-economic and environmental analysis, which include supply chain assessment, by-products, zero-waste techniques and process evaluation and optimization.

Lignocellulosic Biomass to Liquid Biofuels is particularly useful for researchers in the field of liquid biofuels seeking alternative chemical and biochemical pathways or those interested advanced methods to calculate maximum yield for each process and methods to simulate the implications and costs of scaling up. Furthermore, with the introduction provided by this volume, researchers and graduate students entering the field will be able to quickly get up to speed and identify knowledge gaps in existing and upcoming technology the book’s comprehensive overview.

• Examines the state-of-the-art technology for liquid biofuels production from lignocellulosic biomass
• Provides a comprehensive overview of the existing chemical and biochemical processes for second generation biofuel conversion
• Presents tools for the techno-economic and environmental analysis of technologies, as well as for the scale-up simulation of conversion processes

• Language: English
• Publisher: Academic Press
• Release date: Nov 20, 2019
• ISBN: 9780128162804

Abu Yousuf

Dr. Abu Yousuf is a Research Associate in the School of Aerospace and Mechanical Engineering, University of Oklahoma, in the United States. He holds a PhD in Chemical Engineering from the University of Naples Federico II, Italy. His primary research interests include renewable energy, biorefinery, energy storage and waste-to-energy, and he is currently working with protonic ceramic fuel cells at the University of Oklahoma. Dr. Yousuf has published over 60 papers in reputed ISI and Scopus-indexed journals, 19 conference papers and 15 book chapters, and is the Editor of 5 books, as well as serving as an editorial board member for several reputed journals. He received the Dean’s Award from Shahjalal University of Science and Technology, Bangladesh, in 2019 and 2021, for his research, and won the ‘International invention of the year award’ at the British Invention Show, London, UK in 2017, for his work on Hybrid Energy Generation for Highway Lighting Systems.

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Chapter 1

Fundamentals of lignocellulosic biomass

Abu Yousuf¹, Domenico Pirozzi² and Filomena Sannino³,    ¹Department of Chemical Engineering & Polymer Science, Shahjalal University of Science and Technology, Sylhet, Bangladesh,    ²Department of Chemical, Materials and Production Engineering, University of Naples Federico II, Naples, Italy,    ³Department of Agriculture, University of Naples Federico II, Naples, Italy

Abstract

Liquid biofuel is a promising alternative to petroleum-based fossil fuel. Lignocellulosic biomass (LCB) is considered as a potential source of biofuel since it is not food competitive always. Moreover, it has a wide range of sources, such as agriculture residues, forest residues, grass, and energy crops. Though the resources are abundant and readily available all around the world, its contribution in energy sector is not significant. It may be due to the lacking of economically viable and industrially applicable technology. To design and develop sustainable technology, it is urgent to understand the composition, characters, and their refinery process. Therefore this chapter describes the fundamental structure, chemistry, and biorefinery of LCB.

Keywords

Lignocellulosic biomass; chemistry of LCB; biorefinery of LCB

Contents

Outline

1.1 Introduction 1

1.2 Components of lignocellulosic biomass 3

1.2.1 Cellulose 3

1.2.2 Hemicellulose 4

1.2.3 Lignin 4

1.3 Sources of lignocellulosic biomass 6

1.3.1 Annual and perennial energy grasses 6

1.3.2 Woody biomass 6

1.3.3 Nonwoody biomass 6

1.4 Chemistry of lignocellulosic biomass 7

1.5 Biorefinery of lignocellulosic biomass 11

1.6 Conclusion 14

References 14

1.1 Introduction

Lignocellulosic biomass (LCB), also known as lignocellulose, is the most abundant biorenewable material on the earth [1], produced from atmospheric CO2 and water using the sunlight energy through the photosynthesis process. It is a complex matrix, mainly made of polysaccharides, phenolic polymers, and proteins that constitute the essential part of woody cell walls of plants. LCB has a complex spatial structure, in which cellulose (a carbohydrate polymer) is wrapped by the dense structure formed by hemicellulose (another carbohydrate polymer) and lignin (aromatic polymer).

LCB is usually categorized into three types of waste: biomass, virgin biomass, and energy crops. Trees, bushes, and sand grasses are placed into virgin biomass class, whereas agricultural residue, stover, and bagasse are placed in waste biomass class. Energy crops are raw materials used for the production of second-generation biofuels as they offer high biomass productivity.

LCB has a long history as an energy source: for many centuries, wood has been the most widely used raw material to burn fire. During the Industrial Revolution, due to the increase in energy needs, wood was progressively replaced by fossil fuels. However, from the middle of the 20th century, problems rose from pollution and the exhaustion of fossil fuels has increased the demand of biomass for the production of energy [2].

The first biofuels to be developed have been bioethanol, initially obtained from starch and sugars, and biodiesel, obtained from fats and oils. However, the diffusion of these products, so-called first-generation biofuels, has been limited as they cause direct competition between biofuel and food production.

More recently, second-generation biofuels were developed, based on the conversion of LCB components to liquid fuels. Second-generation biofuels allow the utilization of the entire plants, such as woody crops, agricultural residues, or waste, as well as dedicated nonfood energy crops grown on marginal land, thus allowing a dramatic increase of the productivity.

The production of biofuels and energy from LCB is based on two main routes. Biochemical processes are typically carried out with LCB having C/N ratio lower than 30 and humidity at collection higher than 30%. These processes are based on chemical reactions carried out thanks to the exploitation of enzymes, mushrooms, and microorganisms. An alternative is offered by thermochemical processes, used when the LCB available has C/N ratio higher than 30 and humidity content below 30%. In the last years, novel biofuels have been produced from LCB, such as bio-H2, butanol, dimethylfuran, and gamma-valerolactone [3,4].

Some technological barriers still arise in the production of biofuels from LCB, but robust research is going on to overcome those obstacles. One of such problems is that LCB has evolved to resist deprivation and to deliberate hydrolytic stability and structural toughness to the cell walls of the plants. This robustness is attributable to the cross-linking between the polysaccharides and the lignin via ether and ester linkages [5].

Nevertheless, LCB is considered as the most promising raw material for the renewable biofuel production as it is readily available, low cost, and environment friendly.

Before in-depth discussion of processes for the production of biofuels, this chapter is intended to give some fundamentals of LCB. It also incorporates the concept of LCB biorefinery to produce fuels and chemicals.

1.2 Components of lignocellulosic biomass

The main components of LCB are cellulose, hemicellulose, and lignin (Table 1.1). Other minor parts are ash and extractive components.

Table 1.1

Cellulose molecules are arranged in regular bundles, forming crystalline regions, or in random geometry forming amorphous regions. Microfibrils of cellulose polymers are linked by hydrogen and van der Waals bonds and are protected by hemicellulose and lignin.

Carbohydrates, which are the main component of cellulose and hemicellulose, make up for about 70% of the dry weight of lignocellulose biomass and represent the feedstock for nearly all of the most promising bio-based building blocks and chemical intermediates, whatever the conversion technology (biological or thermochemical) adopted. Lignin is another important constituent of lignocellulose biomass making up for about 25% of its weight and is by far the most important natural resource of aromatics, beside a good solid biofuel.

Lignocellulose can play a vital role in the energy sector as different forms of energy products can be obtained from LCB, such as solid (briquettes), liquid (bioethanol and biodiesel), and gas (biogas and bio-H2).

1.2.1 Cellulose

Cellulose is an organic compound composed of polysaccharides, which consists of a straight chain of D-glucose molecules linked through β-(1→4) glycosidic bonds with the formula (C6H10O5)n. It is the main structural constituent of the primary cell wall of green plants, algae, and oomycetes.

Cellulose is the most common and available organic polymer material in the world. Cotton fiber is containing 90% of cellulose content, wood is 40%–50%, and dried hemp is containing approximately 57%. Higher amounts of cellulose are contained in wood pulp and cotton for the industrial use. Cellulose is frequently used to yield paperboard and paper-type materials. A prospective characteristic of cellulose is crystallinity. Cellulose is transformed into amorphous solid when the reactor environment is controlled as 25 MPa pressure and the temperature of water as 320°C. Several environment-friendly biofuels can be derived from conversion of cellulosic materials, such as agricultural residues and energy crops.

1.2.2 Hemicellulose

Hemicellulose is a branched heteropolymer containing approximately 500–3000 sugar units [8]. It consists in different sugar units, with a prevalence of pentose components (xylose and arabinose) together with hexoses (mannose, glucose, galactose, and rhamnose) and acetylated sugars. Hemicellulose cross-links with either cellulose or lignin, strengthening the cell wall (Fig. 1.1). Although hemicelluloses are widely available, their utilization is more difficult in comparison to cellulose, due to its structural diversity, and also because the enzymatic hydrolysis of pentose sugar units is less simple. However, hemicelluloses offer more possibilities for regioselective chemical and enzymatic modifications in comparison to cellulose, due to the variability in sugar constituents, glycosidic linkages, and structure of glycosyl side chains as well as two reactive hydroxyl groups at the xylose repeating unit. In this view a bog effort is being made in research activity.

Figure 1.1 Structure and components of lignocellulose [9].

1.2.3 Lignin

Lignin is the third major component of LCB having polymeric complex structure, which is responsible for some of structural materials in the particular types of tissues of vascular plants and some of algae [10]. It is an inevitable part of plant cell wall, especially in bark and wood. Because of cross-linked phenolic polymers in its structure, it shows rigidity and hard quality. It is mainly amorphous (noncrystalline). Lignin is branched long-chain polymer made up of three types of monomers, such as primarily three-dimensional polymer of 4-propenyl phenol, 4-propenyl-2-methoxy phenol, and 4-propenyl-2.5-dimethoxyl phenol [7].

1.3 Sources of lignocellulosic biomass

1.3.1 Annual and perennial energy grasses

Urbane grasses are well-thought-out as the most prominent sources of cellulosic biomass. The major herbaceous energy crops that play a vital role as the source of LCB are as follows: switchgrass, Miscanthus, canary grass, giant reed, alfalfa (Medicago sativa), and Napier grass (Pennisetum purpureum). Besides circumventing food versus fuel conflict, they are considered to have energetic, economic, and environmental advantages over food crops for ethanol manufacturing [11,12].

1.3.2 Woody biomass

The forest trees, which are fast growing and have short rotation periods, are major source of cellulosic biomass. Though forests are unequally distributed, they play a vital role on reducing the landslide, redundancy of CO2 into our atmosphere, and maintain an equilibrium condition between human being and wild life. Meanwhile, every year approximately 370 million tons of LCB are producing from forest in the United States [13]; other forest-rich countries are Canada, Russian, China, and Brazil. The collected LCB from the abovementioned countries is the half of the amount of whole world. Sources of woody biomass are as follows:

• Natural forest residues

• Forestry wastes: sawdust from sawmills, wood chips, and branches from dead trees

• Tree bark

• Wood shavings

• Sawdust

• Low-grade lumber

• Rejected part from sawmills

• Rejected log from plywood mills

• Rejected trunk/log/branch from pulp mills

1.3.3 Nonwoody biomass

The lower lignin content biomasses are known as nonwoody biomass. They contain comparatively low energy and bulky in size. However, this type of biomass is abundant and cheap. They can be collected from a wide range of sources, such as

• agriculture field wastes (paddy husks, straw, grasses, crop stubble, and trash) and

• agricultural processing wastes (palm oil waste, sugarcane bagasse and animal paunch waste, cotton gin trash, etc.).

1.4 Chemistry of lignocellulosic biomass

Before studying the biorefinery of LCB, it is essential to understand the chemistry of individual biomass because the structure or composition of biomass affects the chemical pretreatment, enzymatic digestibility, and the generation of compounds inhibiting fermentative microorganisms used to produce the final fuel or chemical. Chemical or spectroscopic analysis can determine the percentages of individual sugars, protein, uronic acids, and lignin. For example, during the conversion of corn stover, hardwoods, or rice straw, we are in fact working primarily with the plant’s structural parts, most of which are cell walls. Therefore more knowledge is required about the natural composition and structure of polymers and chemicals in plant tissue.

Cellulose is a six-carbon compound formed by a plant or microbial cell. Glucose is consisted by carbon, hydrogen, and oxygen. Although glucose (C6H12O6) is the smallest unit (monomer) that can be isolated from cellulose degradation, cellobiose (Scheme 1.1) is normally the fundamental building block of cellulose that is nothing but a dimer of anhydride. Each monomer unit of glucose is named as glucan (C6H10O5)n formed by losing one molecule of water, which makes polymer long chain [14].

Scheme 1.1 Molecular chain structure of cellulose.

The cellulose polymer chain is a rectilinear polymer, which has no branch. The number of glucan present in a polymer chain is the measuring unit for the determination of extent of a particular polymeric cellulose usually expressed as (C6H10O5)n, where n is the degree of polymerization (DP). DP of cellulose hangs on the type of plant or microorganism from which it is isolated and also the method of isolation. For some of the native polymers the DP is estimated to be from 2000 to 14,000 glucan units, DP of the wood pulp in the range of 650–1500 units per glucan chain.

Due to the impending of monomer unit of cellulose to formulate three hydrogen bonds with a monomer of neighboring chain, the chains are fit tightly together for the formation of larger units known as microfibrils. The result is a very stable configuration—essentially free of interstitial spaces, making it anhydrous and quite recalcitrant to hydrolysis by acid, base, or enzyme action. In a polymer chain of cellulose, low chain disturbance is occurred in the crystalline region named micelles. Regions without extensive interchain hydrogen bonding are consequently less structured (amorphous). Amorphous parts are less susceptible to hydrolysis comparative to crystalline part. Natural cellulose consists of cellulose fibrils bound together by an amorphous matrix comprising pectin, hemicellulose, and lignin [14].

The composition of hemicellulose depends on the source biomass or species type. Hardwoods, annual plants, and cereal are predominated by xylan hemicellulose, whereas softwoods are predominated by glucomannan hemicellulose [15]. Hemicelluloses are linked with cellulose by inter- and intramolecular hydrogen bonds and with lignin by ester and ether bonds [16]. Hemicellulose also forms covalent associations with lignin, a complex aromatic polymer, whose structure and organization within the cell wall are not completely understood yet. However, chemical properties of several hemicelluloses of LCB s have been discovered as shown in Table 1.2.

Table 1.2