Lignocellulosic Biomass to Value-Added Products: Fundamental Strategies and Technological Advancements

By Mihir Kumar Purkait and Dibyajyoti Haldar

Lignocellulosic Biomass to Value-Added Products: Fundamental Strategies and Technological Advancements focuses on fundamental and advanced topics surrounding technologies for the conversion process of lignocellulosic biomass. Each and every concept related to the utilization of biomass in the process of conversion is elaborately explained, with importance given to minute details. Advanced level technologies involved in the conversion of biomass into biofuels, like bioethanol and biobutanol, are addressed, along with the process of pyrolysis. Readers of this book will become fully acquainted with the field of lignocellulosic conversion, from its basics to current research accomplishments.

The uniqueness of the book lies in the fact that it covers each and every topic related to biomass and its conversion into value-added products. Technologies involved in the major areas of pretreatment, hydrolysis and fermentation are explained precisely. Additional emphasis is given to the analytical part, especially the established protocols for rapid and accurate quantification of total sugars obtained from lignocellulosic biomass.

• Includes chapters arranged in a flow-through manner
• Discusses mechanistic insights in different phenomena using colorful figures for quick understanding
• Provides the most up-to-date information on all aspects of the conversion of individual components of lignocellulosic biomass

• Language: English
• Publisher: Elsevier Science
• Release date: Jun 17, 2021
• ISBN: 9780128235911

Mihir Kumar Purkait

Dr. Mihir Kumar Purkait is a Professor in the Department of Chemical Engineering at the Indian Institute of Technology Guwahati, Assam, India. His current research activities are focused in four distinct areas viz. i) advanced separation technologies, ii) waste to energy, iii) smart materials for various applications, and iv) process intensification. In each of the area, his goal is to synthesis stimuli responsive materials and to develop a more fundamental understanding of the factors governing the performance of the chemical and biochemical processes. He has more than 20 years of experience in academics and research and published more than 300 papers in different reputed journals (Citation: >16,500, h-index = 75, i-10 index = 193). He has 12 patents and completed 43 sponsored and consultancy projects from various funding agencies.

Book preview

Chapter 1: Introduction to lignocellulosic biomass and its potential

Abstract

In the present context of an energy crisis, the exploitation of lignocellulosic biomass (LCB) has emerged as a potential alterative to petroleum-based products. Primarily, the biomass obtained from woody and nonwoody plants is mainly consisted of cellulose, hemicellulose, and lignin. Due to its global abundance, biomass exhibits the potential to be a renewable feedstock in a variety of different fields including bioenergy generation and the formation of numerous value-added products. This chapter deals with several aspects of lignocellulosic biomass and its potential in the conversion of value-added products. In view of that, an insight on biomass composition with its global availability of varieties is discussed in detail. Further, the applications in the bioenergy sector with the formation of several cellulose- and lignin-derived products are summarized. Readers of this chapter will get to know every potential aspect of LCB and its conversion into value-added products.

Keywords

Lignocellulosic biomass (LCB); Cellulose; Hemicellulose; Lignin; Bioenergy; Value-added products

1.1: Overview

Worldwide, the sustainable conversion of renewable lignocellulosic biomass (LCB) into the production of bioenergy, including gaseous forms with liquid fuels such as bioethanol, biobutanol, and biodiesel are considered as viable substitutes for our dwindling fossil fuel resources (Dai et al., 2019; Dong et al., 2019). Further, the emergence of biomass-derived cellulosic derivatives is of high importance in order to replace petroleum-based synthetic materials (Haldar & Purkait, 2020a). Microcrystalline cellulose (MCC) and nanocrystalline cellulose (NCC) are the most common forms of cellulosic derivatives chemically obtained from LCB (Mondal, 2017; Trache et al., 2016). Similarly, the formation of lignin nanoparticles (LNPs) derived from the lignin fractioning of LCB are the latest discovery, with potential applications in several promising fields including biomedical technology (Tian et al., 2017). Recently, microbial production of enzymatic complexes such as cellulases, hemicellulases, pectinases, ligninases, and other accessory enzymes, such as exoglucanases, β-glucosidases, endoglucanases, and xylanases from biomass is a sustainable alternative to decrease the high cost of synthetic enzymes (Osorio-González, Chaali, & Avalos-Ramírez, 2020).

Consequently, LCB is an inevitable and renewable form of natural resources for sustainable production of a number of value-added products. It has also been observed that the complicated composition of biomass is a serious concern: lignin forms a typical barrier at the outside of carbohydrate layers present in LCB. Therefore, the process of pretreatment is employed for breaking down the outside layer of lignin in order to expose the polymeric cellulose and hemicelluloses and assist subsequent hydrolysis (Haldar & Purkait, 2020b). However, severe pretreatment of biomass results in the formation of unwanted chemicals known as inhibitors.

Enzymatic hydrolysis is aimed at maximizing the conversion of polysaccharides into monosaccharides with no coproduction of inhibitors in the reaction system (Haldar, Sen, & Gayen, 2018). During enzymatic hydrolysis, a rapid saccharification of oligomers is generally observed at the initial phase of the process, following an asymptotic nature with time and manifests a steady process. The steady process of hydrolysis is attributed to the substantial presence of produced monomeric sugars in the reaction mixture, which inhibits the enzymatic reaction and restricts further hydrolysis. Further, the process of enzymatic hydrolysis necessitates a mechanistic and kinetic understanding of the reaction system in order to implement the technology on a large scale.

The quantitative analysis of sugars is a key factor in evaluating the performance of the enzymatic hydrolysis and fermentation process, where sugars are consumed by several microorganisms for biofuel production. Over the years, the anthrone protocol has been exhaustively used as the conventional method for the quantification of sugars. However, the practical application of this method becomes more complicated once the system is a complex mixture of both monomeric and polymeric sugars, which is a common occurrence in most biological systems. Hence, subsequent research in the development of a protocol for an accurate quantification of total sugars present in the hydrolysate obtained from LCB is also important (Haldar, Sen, & Gayen, 2017). The selection of the biomass, effectiveness of pretreatment process, enzymatic hydrolysis, and proper analysis of reaction products are typical factors in the substantial conversion of LCB into biofuel. Additionally, pyrolysis is another promising technology in the sector of bioenergy for converting biomass into renewable biofuels such as bio-oil. In the recent past, with the advent of technological progress, pyrolysis has been well adapted with several renewable energy policies for its possible applications in different sectors of bioenergy (Roy & Dias, 2017). With respect to environmental sustainability, LCB is well explored as an indispensable option for the conversion of numerous value-added products.

This chapter is focused on several aspects of LCB including its composition and potential use. Further, global availability of various types of LCB are summarized with an attempt to explore its diversity. The potential of LCB is discussed for applications in different fields such as bioenergy generation and formation of several cellulose- and hemicelluloses-based value-added products. Readers of this chapter will acquire an indepth knowledge of the fundamentals of lignocellulosic biomass and its potential in several applications.

1.2: Basic structure of lignocellulosic biomass

The term LCB itself suggests that the biomass is made up of cellulose and lignin. It is important to note that plant cells are preferentially considered biomass rather than that from animals. Primarily, LCBs are mostly originated from wood and nonwood resources. Wood materials are further subcategorized into hardwood and softwood biomass, while nonfood crops and waste residues that are left over from other crops are considered nonwood residues. Structurally, LCB is consisted of holocellulose and lignin units. Fig. 1.1 shows the structural organization of cellulose, hemicellulose, and lignin present in LCB. The polymeric cellulose and hemicelluloses are combined together to form holocellulose, where lignin is another polymer made up of several phenolic monomeric units. Cellulose is the major component of the biomass and exists in the form of polysaccharides that contains several hundreds to thousands of units of d-glucose units that are linked together by numerous glycosidic bonds. Hemicelluloses are a branched biopolymer made up of different 5-C pentose and 6-C hexose sugars (Haldar, Sen, & Gayen, 2016).

Fig. 1.1

Fig. 1.1 Basic structural organization of LCB, consisting of cellulose, hemicellulose, and lignin. Reprinted with permission from Raud, M., Kikas, T., Sippula, O., & Shurpali, N. (c. 2019). Potentials and challenges in lignocellulosic biofuel production technology. Renewable and Sustainable Energy Reviews, 111, 44–56. https://doi.org/10.1016/j.rser.2019.05.020.

1.3: Global availability of different types of lignocellulosic biomass

Agricultural and forestry residues are the most abundant form of LCB. Primarily, all kinds of LCB are classified into wood and nonwood biomasses, based on their structural properties and chemical compositions. Woody biomass is physically denser and structurally stronger than nonwood biomass. Further, the wood fibers are often subcategorized into hardwood and softwood biomass. Table 1.1 shows the different varieties of hardwood and softwood LCB. Wood fibers are observed to contain a large amount of lignin with relatively low pentose sugars, manifesting an advantage for bioethanol production using cellulosic fraction of the biomass. However, under acidic conditions, a high pentose content may result in the formation of more decomposed furfurals as an inhibitor during the treatment of LCB, which subsequently limits the rate of fermentation process. It is important to note that worldwide, wood fibers represent 60% of the total supply of biomass (Zhu, Pan, & Zalesny, 2010).

Table 1.1

Reprinted with permission from Haldar, D., Sen, D., & Gayen, K. (c. 2016). A review on the production of fermentable sugars from lignocellulosic biomass through conventional and enzymatic route—a comparison. International Journal of Green Energy, 13(12), 1232–1253. https://doi.org/10.1080/15435075.2016.1181075.

However, wood biomass necessitates a size reduction in order to increase the specific surface area of the residues for more efficient bioethanol production. Further, a relatively high energy input is required for size reduction of wood fibers which accounts to around 200–600 Wh/kg, compared to the requirement of around 50 Wh/kg for nonwood fibers (Zhu et al., 2010). Moreover, due to a high lignin content, wood fibers manifest a strong recalcitrance or physical barrier in any subsequent hydrolysis process.

1.3.1: Nonwood lignocellulosic biomass

Over the years, nonwood fibers have been gaining widespread attention as biomass feedstocks for the production of biofuel, due to their higher availability than wood fibers. Further, compared to rigid and compact wood fibers, nonwood fibers have more flexible structures. Therefore, they are easy to process and require relatively low process energies. Moreover, nonwood fibers are cheaper sources of LCB than wood fibers. The presence low lignin content in nonwood fibers provides an additional benefit during the delignification process. Nonwood LCB is commonly classified into two groups according to its sources: agricultural residues and native plants.

1.3.1.1: Agricultural residues

Agricultural residues are the unutilized biomass that remains in an agricultural field after crops are harvested, or the unused portions after the useful parts of crop is processed into a usable product. In remote areas, agricultural residues are frequently utilized as an animal feed. It is often disposed of by burning or as landfill. However, burning of residues in an open atmosphere severely affects the environmental sustainability by escalating pollution levels. However, such waste biomass of agricultural residues has been well introduced into the application of biofuel production, including bioethanol, owing to their promising compositional contents (Qin et al., 2018). Among agricultural residues, cereal straw, sugarcane bagasse, and oil-palm wastes are mostly explored as feedstock materials for this application.

Cereal straw

Due to the presence of high carbohydrate contents (60%–70% starch), cereal grains are well known as an excellent energy-rich food materials for humans. During the process of cereal grain production, crop straws, the byproduct of the cereal crops or the dry stalks of cereal plants, are generated in large amounts. Straw is collected from the upper-ground part of the cereal plant, after the removal of usable nutrient grain or seeds. Owing to the slow rate of digestibility, cereal straws exhibit a major limitation for animal feed, providing insufficient energy for growth. Therefore, an ample availability of cereal straw has been potentially explored for other advanced applications including bioenergy generation through bioethanol production (Ghosh, Chowdhury, & Bhattacharya, 2017).

Among the different agricultural cereal crops, barley, corn, oat, rice, sorghum, and wheat are mostly explored. According to Tye, Lee, Wan Abdullah, and Leh (2016), an annual production of approximately 141.4, 962.4, 22.9, 473.1, 61.9 and 705.6 million tons of barley, corn, oat, rice, sorghum and wheat, respectively, are reported in the world. Among the agricultural residues, corn crops are reported to have the highest production yield. Compared to corn crops, the production of rice straw is relatively low, as it is not generally left in the field, to prevent erosion. It is reported that the total cereal straws production per year is near to 1580.2 million tons, which is primarily produced in Europe (barley and oat), the United States (corn and sorghum) and China (rice and wheat) (Kim & Dale, 2004).

Cellulose is the main polysaccharide present in LCB that is converted chemically or enzymatically into glucose, with subsequent fermentation into the production of biofuel such as bioethanol, biobutanol, and biodiesel. In general, all cereal straws are composed of approximately 33%–50% cellulose, and exhibit a tremendous potential as a feedstock resource for biofuel production. It is important to note that the percentage of cellulose in all straw fibers is indeed similar to each other, however, the low annual production yield of oats (10.4 million tons) and sorghum straws (12 million tons) has a low total cellulose availability per year, that is, less than 4.0 million tons. Among the cereal straw fibers, rice straw contributes the highest annual total cellulose availability (210.4–309 million tons), owing to its relatively higher annual yield of 657.5 million tons (Tye et al., 2016).

Bagasse

Sugarcane is a type of perennial grass that matures in 12–16 months and is mostly seen in the tropical and subtropical climate zones. Once sugarcane is harvested, the sugar mills, commonly located near to the fields, extract sugars from the cane part and the leftover waste material is known as bagasse. Brazil is the world's largest sugarcane producer, where 50 million tons of sugarcane leads to produce approximately 5 million tons of sugar. Hence, sugarcane exhibits a potential as the substrate materials for bioethanol production with the aspiration of replacing petroleum-based products. The utilization of sugarcane bagasse for bioethanol production not only generates the scope for ethanol production, but also eliminates food/fuel conflict.

In 2018, the annual global production of sugarcane was reported as 1907 million tons, of which Brazil produced 39% and India 20%, followed by China and Thailand contributing 6% individually (https://en.wikipedia.org). Brazil is the greatest and sole producer of sugarcane with 27% of the world's total production. It is also important to note that the yield of bagasse is about 60% for the total amount of sugarcane used (Kim & Dale, 2004). Therefore, about 1144 million tons of waste bagasse is generated annually based on the present scenario of global sugarcane production. Hence, sugarcane bagasse is a feasible and abundantly available biomass resource that shows tremendous potential for bioethanol production.

Oil-palm biomass

The oil-palm tree is an economically important agricultural residue with an average productive lifespan of about 25–30 years. Most importantly, the tree is observed to produce between 8 and 12 fruit bunches. Each of the fruit bunches contains near to 1000–3000 fruitlets in the bunch. Primarily, palm oil is extracted from fruitlet part of the plant. According to the reported data, the global production of palm oil was estimated at 62.6 million tons in 2016. Currently, Indonesia (53.2%) is the major palm-oil producer, while Malaysia ranks second with 32.9% (https://en.wikipedia.org). Additionally, 70% of the processed fresh fruit bunches (FFBs) becomes waste materials after the edible oil is extracted from the biomass (Rupani, Singh, Hakimi, & Esa, 2010). Oil-palm fronds and trunks are generated in large amounts as remainder waste from the biomass, and are left on the plantation field during fruit harvesting. The total waste production generated by FFBs is about 214.5 million tons (or 77% by weight) per annum from biomass. The oil-palm trunk contains a large quantity of sap with a high glucose content, and it is directly fermented for the production of ethanol. Moreover, oil-palm trunks fibers that are usually left in the field as waste primarily contain a relatively high cellulose level of 39.9%. Hence, oil-palm residues is recognized as the most viable and suitable feedstock for its conversion into biofuel because of the high total annual cellulose availability.

1.3.1.2: Native plants

Coastal Bermudagrass, Miscanthus, and Switchgrass are the common native plants always referred to when discussing herbaceous crops. In comparison to others, herbaceous crops are relatively easy to grow, harvest, and process. Primarily, the growth of the grass largely depends on range of geography, climate, and soil type. Several advantages are associated with grasses, such as high yielding biomasses and a low to negligible magnitude of maintenance.

Coastal Bermudagrass

This is an herbaceous perennial crop that is widely grown in the United States during the summer season. Further, this kind of grass biomass bears the capacity to grow on various soil conditions with good annual yield. Outside the United States, the biomass is widely available in the other parts of the globe. Further, the biomass contains a substantial quantity of polymeric carbohydrates in terms of cellulose and hemicelluloses (Xu, Wang, & Cheng, 2011). This biomass exhibits a full potential for value-added production.

Miscanthus grass

Originating from Southeast Asia, currently this grass is also recognized as a promising biomass residue in Asian and European countries. Depending on the area, its growth can vary. The lignocellulosic C4 perennial crop miscanthus and, more particularly, one of its species, Miscanthus giganteus, are of special interest for bioenergy production as they combine high biomass production with a low environmental impact. The composition of biomass varies significantly from one to another grass species. The lignin content of M. giganteus and M. sacchariflorus species are substantially high, while the M. sinensis species is observed to have low lignin content, manifesting in a suitability for thermo-chemical conversions (Arnoult & Brancourt-Hulmel, 2015).

Switchgrass

Switchgrass is another type of perennial native grass of a warm season climate that grows in America and Africa. It can grow up to a height of 3 m and in a variety of soils, with low fertilizer requirements. The production yield varies significantly with the growth conditions and maximum growth rate of 390 kg/ha/day indicates a high change in dry weight over summer time. Nevertheless, its average dry biomass yield of 18 Mg/ha is comparatively higher than other crops such as corn (Giannoulis et al., 2016).

According to the reported literature, the cellulose content of Coastal Bermudagrass (25%) is relatively low compared to Miscanthus and switchgrass, which can attain total annual cellulose availabilities of 84.9–144.0 million tons (Tye et al., 2016). Hence, Miscanthus and switchgrass show a greater potential for utilization and subsequent conversion into value-added products.

1.4: Potential of LCB for bioenergy generation and formation of other value-added products

From an energy and environmental point of view, the biofuels obtained from LCB possess several advantages, including noninterference with the traditional food crops thus maintaining a balance between energy and food security. The recent imbalance in the oil market and regular fluctuation in fuel costs highlights the necessity of biofuel production from sustainable resources such as lignocelluloses. The advantage of using lignocellulosic biofuel is its capability of reducing carbon dioxide emissions to almost zero. This is attributed to the potential of such biomass to absorb carbon dioxide from the atmosphere as they grow. The same byproduct (carbon dioxide) is released into the atmosphere once ethanol is combusted in an automobile engine. Therefore, the phenomenon of renewable energy cycle benefits by reducing the detrimental emission of greenhouse gases (GHG) from the atmosphere.

The efficient bioconversion of renewable lignocellulosic materials into bioenergy and other value-added products are not only receiving immense global attention, but also face several challenges. Primarily, biomass provides near to 10.2% (~ 50.3 exajoule (EJ)/yr) of the annual total primary energy supply (TPES) globally from a wide variety of different LCBs (Menon & Rao, 2012). It is observed that more than 80% of biomass that is used for energy is directly obtained from wood and shrubs. The rest of the remaining feedstocks are mostly derived from agricultural residue including energy crops and from other various commercial ventures with postconsumer waste products. According to the IEA report for the assessment of available residues in 2030 (https://www.iea.org, accessed on 21.04.2021), 10% of global residues contribute 4.1% of the projected transport fuel demand in 2030 and 25% of global residues show potential towards 13–23.3 of EJ. However the conversion process relies on major technological innovations to tackle its economic viability centered on low cost enzymes, feedstocks, and efficient process design. Fig. 1.2 depicts different thermochemical and biochemical routes for the conversion of LCB into value-added products.

Fig. 1.2

Fig. 1.2 Different routes for the conversion of LCB into value-added products. Reprinted with permission from Menon, V., & Rao, M. (c. 2012). Trends in bioconversion of lignocellulose: Biofuels, platform chemicals & biorefinery concept. Progress in Energy and Combustion Science, 38(4), 522–550. https://doi.org/10.1016/j.pecs.2012.02.002.

1.4.1: Potential of biomass for the production of bioethanol

Every year, the limited sources of fossil fuels are exhaustively utilized as the most vulnerable resources to meet the increasing demand for an alternative energy (Carrillo-Nieves et al., 2019; Hosseini Koupaie, Dahadha, Bazyar Lakeh, Azizi, & Elbeshbishy, 2019). As a result, the overall environmental sustainability is hugely affected due to rapid urbanization including global industrialization along with fast exploitation of fossil fuels. Therefore, research regarding the production of green fuels using renewable feedstock such LCB has been a high priority as the biomass does not necessarily compete with conventional food crops. The process for the production of liquid biofuels such as bioethanol and biobutanol from LCB is of low toxicity and cost effective. Further, LCB is well known as the reservoir of carbon in different polymeric forms of cellulose, hemicellulose, and lignin (Chen et al., 2018; Yu, Wu, & Chen, 2018). Therefore, the biomass exhibits every potential for utilization as a potent and sustainable bioresource, once it is transformed economically into value-added products.

The technologies in the bioconversion of liquid fuels involve some major steps including pretreatment, hydrolysis, and fermentation. Pretreatment is aimed at the removal of lignin which is the barrier of the inner core materials of the plant that are made up of cellulose and hemicelluloses. Hydrolysis is conducted using chemicals or enzymes for the depolymerization of polymeric carbohydrates into monomeric sugars. Finally, sugars are utilized as the sole source of carbon by different fermenting microorganisms for the production of liquid biofuels (Haldar et al., 2016; Haldar & Purkait, 2020a). Consequently, the selection of the correct biomass is an important factor for implementing the processes in tandem to produce liquid fuels for a commercial use.

However, plantation crops that are frequently explored for the production of biofuels can create environmental pollution from the farming materials and practices. In addition, the consumption of pesticides and fertilizers are also increased substantially with a higher number of plantations. Furthermore, with the increasing demand for biofuel, some food crops are facing severe challenges in order to contribute to regular food supplies, resulting in fluctuation in food prices. According to the latest report by the United Nations Department of Economic and Social Affairs, from an estimated 7.7 billion people worldwide in 2019, the global population may reach to around 8.5 billion in 2030 with further projections of near to 9.7 billion in 2050 and 10.9 billion in 2100 (United Nations, Department of Economic and Social Affairs, 2019). With an increasing world population, the demand for crop production is rising at an escalated rate. It is estimated that more than a 60% increment in global agricultural production is required by 2050 in order to maintain the balance between population growths and food security in an equal proportions (Ray, Mueller, West, & Foley, 2013). Moreover, the production cost of bioethanol using traditional food crops is substantially high. As a result, the emergence of liquid biofuel production from lignocellulosic biomasses such as nonfood crops residues and food/crop waste is an alternative way to alleviate the conflict between food and fuel