A major problem encountered with the use of gaseous acid catalysts for the steam pre-treatment is the fact that most of these gases, for example SO2, are regarded as a major air pollutant. Potential SO2 escapes could be toxic, having significant negative impacts on human health and safety, as well as the environmental impacts (i. e. as a precursor to acid rain) associated with such emissions [36]. The use of acid catalysts (especially using high pre-treatment severities) was seen to lead to an increase in the formation of toxic degradation products from the biomass sugars which in turn negatively affect further hydrolysis [33]. Other operating issues associated with the use of acid catalysts such as acid corrosion and the need to implement an extensive downstream effluent treatment processes further limit the use of this method [13].

Hydrothermal carbonization (HTC), also termed wet torrefaction, is a pretreatment process for woody biomass where the biomass is treated with hot compression water at the temperature between 180 and 250 °C for 1-12 h depending on the system design [51, 52]. This is different from steam explosion as the woody biomass is treated at highly pressurized liquid water instead of saturated steam. The reaction chemistry is similar to the steam explosion for which hydrolysis is the preliminary reaction to take place during treatment [52]. HTC is used to carbonize the biomass, making products with higher carbon content. The product characteristics, their relative proportions in the gas/liquid/solid phases, and the process energy requirements depend upon the input material and the process conditions. The advantage of HTC is that it can convert wet input material (MC > 50 % (wb) into carbonaceous solids at relatively high yields without the need for an energy-intensive drying before or during the process. Moreover, some mono-sugars can be recovered from the liquid water, and the dried solid char can be used to produce pellets for energy production. However, there are limited published literatures evaluating the quality of the HTC wood pellet. There is lots of room for future R&D to evaluate the quality of HTC wood pellet.

5.2.2 Pelletization

Pellets quality is controlled by the pelletizing conditions, system design of the pel­let mill, and feedstock parameters. Pelletization conditions include die temperature, pretreatment conditioning, biomass preheating, pressure, feeding speed, and reten­tion time/relaxation time. The design parameters of densiflcation units including types of densiflcation unit, die shape, die specifications, and material to make the die determine the pellet quality.

Similar to DAP, alkaline treatment has been less effective on softwood than for hardwood, herbaceous plants or agricultural residues at the same process conditions because of the generally higher lignin content of wood. Zhu et al. [101] reported that a cold NaOH pretreatment could achieve about 70 % enzymatic hydrolysis Glu yield from spruce when pretreatment was conducted at -15 °C in a 7 % (w/v) NaOH solution with 12 % (w/v) urea for 24 h. However, Mirahmadi et al. [102] obtained only 35.7 % cellulose conversion yield when treated spruce with 7.0 % (w/w) NaOH for 2h at 5 °C. In addition, research revealed that the addition of air/oxygen to the reaction mixture could enhance the cellulose conversion yield and improve the deligniflcation of the biomass, especially highly lignifled materials [85].

The fermentable sugar loss and relatively long time of the pretreatment compared to physical/chemical pretreatment are major challenges in biological pretreatment pro­cess. As discussed earlier, brown-rot fungi are the major consumers of fermentable sugars in the biological pretreatment. Furthermore, biological pretreatment requires

more space and longer time; hence the probability of risk of contamination increases. Consequently, these factors increase the process cost. In order to overcome the above problems and making the process more cost effective and beneficial, a dedicative mi­croorganism must be used in the process, where it could decrease the lignocelluloses recalcitrance with a minimum loss of sugar and a short time for incubation. The effec­tive biological pretreatment process is influenced by many factors, such as (i) strain selection: The strain must have a high affinity to lignin rather than the other part of

lignocelluloses; (ii) high degradation rate of lignin; (iii) simple nitrogen source re­quirement; (iv) simple micronutrient requirements. These factors have already been optimized and implemented by many researchers in their biological pretreatment process for various applications.

In view of reducing the capital cost, incubation time and effective biological pretreatment with minimum fermentable sugar loss, the following approaches can be implemented in near future:

1. Combined biological and chemical/physical treatment may be effective for treatment of lignocelluloses.

2. Using some advance tools like bioinformatic tools, metagenomic tools, and high throughput screening, the process can be implemented effectively. For exam­ple, as discussed earlier, altering the pathway of lignolytic enzyme or removing cellulase/hemicellulase enzymes may provide the alternative solution.

3. Novel strains or novel enzymes can be isolated with the help of metagenomic tools for the better degradation or conversion of lignocelluloses.

4. To inhibit the action of cellulolytic enzyme or to increase the lignolytic enzyme action during the process, a specific enzyme inhibitor or mediator can be used.

Acknowledgments We thank Dr. A. K. Jain, Director, SSS-NIRE, Kapurthala, for his valuable suggestions and encouragement to write this chapter. One of the authors (Nandhagopal N.) gratefully acknowledges the Senior Research Fellowship under ‘Bio-energy Promotion Fellowship’ awarded by the SSS-NIRE, Kapurthala and MNRE, Govt. of India, New Delhi.

Heterogeneous raw materials demonstrate heterogeneous conversion property as mentioned above. Therefore, if only one product is prepared from stalk, other com­ponents would be wasted or even become pollution. What is worse, yield is low if only one fraction is applied and all cost is assumed to one product. On the contrary, stalk could be fractionated into different fractions according to intrinsic characteris­tics, and different fractions could be converted into different products, respectively. With this refining way, the whole stalk is converted into multiple products without pollution. Moreover, the cost of each product is reduced because of cost apportion.

To solve the problems of energy, environment, and poverty, ethanol production from stalk cellulose has been researched for years [1, 23]. However, it has not been industrialized till now. The essential reason is that the yield is low for single product, leading to high cost. Therefore, it would be an effective model to convert stalk into multiple products.

MW has also been applied to the pretreatment of biomas or sludge for methane production as summarized in Table 6.3. Solyom et al. [22] investigated the use of MW as a pretreatment of secondary wastewater sludge for biogas production. They found that the highest solubilization was achieved at an absorbed MW energy of

0. 54 kJ/mL using MW power of 1000 W for 90 g of sludge. Under this condition, an improvement of 7.1 % in methane production was observed compared to the untreated sample. As expected, the methane production could be further increased using a higher absorbed energy but solubilization decreased.

MW pretreatment was also applied to disintegration and digestion of different types of sludges including waste-activated sludge (WAS), primary sludge (PS), combined (PS + WAS), sequencing batch reactor (SBR) sludge, and anaerobically digested biocake [23]. Results showed that MW pretreatment could increase bioavail­ability of sludge components under batch anaerobic digestion and enhance the dewaterability of pretreated sludges after digestion. The degree of solubilization and biodegradation depended on the types of waste sludges. The characteristics of

the sludge may influence final pretreatment outcomes, and general effect of MW on the pretreatment of sludge cannot be concluded.

In the works of Chang et al. [24] on the effects of MW and alkali on pretreatment of sludge, they reported that the synergistic effects of MW and alkali could enhance sludge solubilization, obtaining 46 % COD solubilization. This is equivalent to al­most two-fold increase compared to the combined values obtained with only MW (8.5 %) or alkali pretreatment (18 %) (total = 26.5 %).

The combined MW and alkali method was also investigated on the pretreatment of thickened waste activated sludge (TWAS) to improve thermophilic anaerobic digestion efficiency [25]. The effects of 12 different pretreatment methods were investigated in 28 thermophilic batch reactors by monitoring cumulative methane production. Improvements in methane production in the TWAS were directly related to the MW and alkali pretreatment of the sludge. An improvement in the highest cumulative methane production of about 27 % over the control was obtained.

MW pretreatment was also found to be most effective compared with ultrasonic and chemo-mechanical pretreatments of pulp mill wastewater treatment sludge, in­creasing specific methane yields of WAS samples by 90 % compared to controls after 21 days of mesophilic digestion [26].

Park andAhn [27] investigated optimum MW pretreatment conditions for methane production in anaerobic sludge digestion. They found out that both MW heating rate and final temperature significantly affected solubilization of the sludge and methane production as well, obtaining maximum methane production of 2.02 L/L at optimum heating rate of 9.1 °C/min and final temperature of 90 °C. MW pretreatments also showed to enhance efficiency of anaerobic digestion of food industrial sewage sludge [28]. Due to increased solubility (from 9.7 % to more than 40 %), the specific bio­gas product could be increased three-folds from 220mL/g to more than 600mL/g.

Also, with MW pretreatment, the decomposing bacteria could easily access organic compounds resulting to more efficient digestion.

Jackowiak et al. [29] also made an optimization of MW pretreatment for solu­bilization and anaerobic biodegradability of wheat straw. An improvement of about 28 % on methane production was obtained with MW pretreatment, with the maximum yield obtained at 150 °C.

Nandhagopal Narayanaswamy, Pratibha Dheeran,

Shilpi Verma and Sachin Kumar

Abstract Biological deligniflcation is an attractive approach for pretreatment of lignocellulosic biomass. This approach is very cost effective, low-energy require­ment, environment friendly, low formation of toxic materials such as furfural, hydroxymethylfurfural, etc. Biological approach has been demonstrated using direct microorganism as well as using enzymes extracted from microbes. The microbial treatment includes fungi such as white-rot fungi, brown-rot fungi and soft-rot fungi, and bacteria. Both of brown-rot and soft-rot fungi principally degrade the plant polysaccharides with minimal lignin degradation, while white-rot fungi are capable of complete mineralization of both the lignin and the polysaccharide components. This chapter presents a brief review of the relevant and updated literature on biolog­ical pretreatment of lignocellulosic biomass. Various approaches used by different researchers for biological deligniflcation of lignocellulosic biomass, including mi­crobial and enzymatic approaches, mode of action, effect of biological pretreatment on lignocellulosic biomass, effect of biological pretreatment on enzymatic hydrol­ysis, have been included in this chapter. The chapter also provides a glimpse of the gaps, which need to be studied.

Keywords Lignocelluloses ■ Pretreatment ■ Lignin ■ Biological deligniflcation ■ Fungi ■ Bacteria

1.1 Introduction

In view of environmental and fossil fuel security concern, the future energy economy will probably be based on a broad range of alternative energy resources such as wind, water, sun, nuclear fission as well as biomass. Extensive use of fossil fuels in the

S. Kumar (H) ■ N. Narayanaswamy

Sardar Swaran Singh National Institute of Renewable Energy, Jalandhar-Kapurthala Road, Wadala Kalan, Kapurthala 144601, Punjab, India e-mail: sachin. biotech@gmail. com

P. Dheeran

Biotechnology Area, Indian institute of Petroleum, Dehradun, India S. Verma

Department of Chemical Engineering, Indian Institute of Technology, Roorkee, India

Z. Fang (ed.), Pretreatment Techniques for Biofuels and Biorefineries,

Green Energy and Technology,

DOI 10.1007/978-3-642-32735-3_1, © Springer-Verlag Berlin Heidelberg 2013 last century has greatly depleted the energy reserves. Presently, the petroleum-based fuels—gasoline, aviation turbine fuels, and diesel—all liquid fuels, and compressed natural gas (CNG) are almost excessively used in the transportation sector. The increasing rate of consumption of fossil fuels has raised severe problems includ­ing the issues of depletion of energy resources, increase in fuel prices, and global climate change. The major attraction of the use of renewable energy fuels is the reduction of environmental impacts that are associated with the use of the fossil fuels [1]. Therefore, an imperative technology is required to ward off the appre­hensive problems of meager fossil fuels and its negative impact on environments. Finally, researchers are looking for the economical way to produce alternative fuels and energy preferably from abundantly available biodegradable or eco-friendly and renewable raw materials such as biomass or renewable resources such as sun, wind, water, etc. These resources have a vital role and equal contribution in the energy sector [2].

Among the potential bioenergy resources, lignocellulosic biomass has been iden — tifled as a cheap and effective feed-stock for the production of biofuels such as bioethanol, biobutanol, and biogas. Lignocellulosic biomass is available about 180 million tons per year from agriculture and other sources [3, 4]. Lignocellulose is the most abundant renewable and natural resource, which have a promising role in renewable energy sector and have fetched many researchers toward a new road map to the biofuels production. Biofuels such as ethanol, butanol, hydrogen, biogas, etc. from lignocellulosic biomass and non-food sources have caught worldwide attention. The lignocellulosic biomass has increased its attention because these raw materials do not compete with food crop and is less expensive than conventional feed-stock like sugarcane, corn, etc. In general, lignocellulosic feed-stocks are observed as promising alternative sources because it consist massive amount of carbohydrates

[5] . All lignocellulosic biomass predominantly comprise cellulose, hemicellulose, and lignin, but in a different ratio with respect to distinct biomass [6]. However, lignocellulosic materials are naturally recalcitrant and have more complex structure [3,5]. Lignocellulosic biomass for the production of biofuels includes forest residues such as wood; agricultural residues such as sugarcane bagasse, corn cob, corn stover, wheat, and rice straws; industrial residue such as pulp and paper processing waste; municipal solid wastes; and energy crops such as switch grass [7-11].

Cellulose is the most abundant organic compounds on the earth and this polysac­charide consists of linear chain of several hundred to 10 thousands recurring D-glucose units with molecular formula (C6H10O5) n, linked by в(1 ^ 4) glycosidic bonds. Cellulose is a structural component of a primary cell wall in green plants and algae. Naturally, cellulose can be found in two different forms in the plant materials, consists of parts with crystalline structure and amorphous structure. The crystalline celluloses are well organized, which are tightly bundled and bound together by strong inter chain hydrogen bonds while this is less pronounced in amorphous cellulose.

Hemicelluloses, the second most abundant natural polymer on the earth [3, 12], are the heterogeneous polymers consisting of pentoses (D-xylose, D-arabinose), hexoses (D-glucose, D-mannose, and D-galactose), and sugar acids. Hemicellulose is a connector between cellulose and lignin, and it leads to more rigidity. In hardwood, hemicelluloses are dominantly found as xylan, whereas in softwood glucomannan are most common [3, 12, 13]. Xylans are commonly found as heteropolysaccharide in many plants with backbone chain of 1,4-linked в-D-xylopyranose units. Along with xylose unit, xylan may comprise arabinose, glucornic acid or its 4-O-dimethyl ether, acetic acid, ferulic, andp-coumaric acids. Xylan can be simply extracted in an acid or alkaline environment but in the case of glucomannan requires stronger acid or alkaline environment [3, 12]. Hemicellulose is also an economically important natural polymer as it contains ample amount of pentose sugar, which can be used as a substrate in food, pharmaceutical, and biofuels industries.

Lignin, the third largest available biopolymer in nature [3, 12], is a heterogenous and irregular arrangement of phenylpropanoid polymer that reduces the chemical and enzymatic degradation to maintain the recalcitrant and insoluble properties of lignocellulose. Three phenylpropionic alcohols primarily exist as monomers of lignin

(i) coniferyl alcohol, (ii) coumaryl alcohol, and (iii) sinapyl alcohol. In general, herbaceous plants such as grasses, rice, and wheat straws have the lowest contents of lignin, while in softwoods lignin content is found to be higher. Lignin is the major rate-limiting component in the carbon recycling reaction, as its oxidation rate is naturally very slow [14, 15]. Furthermore, lignin has an important role in conducting water in plant stems and giving physical strength to the plants.

The main routes to produce fuels from biomass (biofuels) include fermentation of sugars to alcohol, gasification and chemical synthesis, and direct liquefaction. The biological process for converting lignocellulose to biofuels requires: (1) delignifica — tion to liberate cellulose and hemicelluloses from the matrix; (2) depolymerization of the carbohydrate polymers to produce free sugars; and (3) fermentation of mixed hexose and pentose sugars [16-19]. All these processes comprise the same main components: hydrolysis of the hemicellulose and the cellulose to monomer sugars, fermentation, and product recovery. The main difference between the process alter­natives is the hydrolysis steps, which can either be accomplished by an acid or by enzymes [20].

Lignocellulosic materials need to be saccharified to produce fermentable sug­ars. This is an intensive process involving a combination of pretreatment and either chemical (acid hydrolysis) or enzymatic hydrolysis [20-22]. In the chemical process, the hydrolysis of sugar polymers in lignocellulosic material is catalyzed by an acid, whereas in the enzymatic process, enzymes are used for hydrolyzing cellulose and hemicellulose to sugar monomers [23-25]. Several factors influence the yields of the monomeric sugars from the lignocellulosic matter and the by-products during hydrolysis. These factors include biomass particle size, liquid-to-solid ratio, type and concentration of acid used, temperature, reaction time, length of the macromolecules, porosity of the biomass, degree of polymerization of cellulose, configuration of the cellulose chain, association of cellulose with other protective polymeric structures within the plant cell wall such as lignin, pectin, hemicellulose, proteins, and mineral elements, etc. [26-28].

Enzymatic hydrolysis offers major advantages over other chemical routes (e. g., acid hydrolysis) such as higher yields, minimal by-product formation, low-energy requirements, mild operating conditions, and low-chemical disposal costs [29]. Hy­drolysis of cellulose to glucose in aqueous media catalyzed by the cellulase enzyme suffers from slow reaction rates due to high crystalline structure of cellulose, degree of polymerization, pore volume, acetyl group bound to hemicellulose, surface area, hydrophobicity, and biomass particle size, which make the penetration of enzymes to the active sites very difficult [30-32]. The enzymatic hydrolysis without pretreat­ment yields sugars which is <20 % of the theoretical quantity, whereas >90 % of the theoretical quantity of sugars are obtained with enzymatic saccharification after pretreatment [33, 34]. Therefore, pretreatment is a necessary and prudent step to break the crystalline structure of the lignocelluloses, the removal of lignin to expose the cellulose and hemicellulose molecules for efficient enzymatic conversion, and saccharification of feed-stock [5, 31, 35-39].

Physical, physico-chemical, chemical, and biological processes have been stud­ied for the pretreatment of lignocellulosic materials [40-42]. Enzymatic hydrolysis of lignocellulosics can be significantly enhanced by physical, chemical, and biolog­ical pretreatments of the lignocellulosic materials to remove and modify the lignin and hemicellulose and to reduce the fiber crystallinity. The physical and chemi­cal pretreatment including grinding, organosolv process involving extraction with hot aqueous ethanol, ozonolysis, acid/alkaline treatment, oxidative delignification, carbon dioxide explosion, hydrogen peroxide, ultrasonic irradiation, ammonia fiber expansion, wet explosion, and acid or SO2-catalyzed steam explosion, ammonia fiber explosion (AFEX) and biological pretreatment have been followed and optimized up to certain levels [5, 31]. The objective of physical pretreatment or mechanical pretreatment is generally used to reduce the particles size, crystallinity, and degree of polymerization; and consequently it leads to increase the surface area for enzyme and/or chemical accessibility. In thermal pretreatment method, various methods have been investigated such as steam explosion/steam pretreatment, liquid hot water, etc.

Chemical pretreatment is another important technique that has been commonly followed by many industries like paper and pulp industries for few decades. This treat­ment is mostly used by the researchers, which includes catalyzed steam explosion, acid/alkali treatment, ammonia fiber/freeze explosion (AFEX), ionic liquid pretreat­ment, organosolv, and pH-controlled hot water treatment. All the above treatments require different chemicals and different operating conditions [3,43].

Biological pretreatment have been studied elaborately by various researchers be­cause this technique is very cheap, less energy consuming process, and the refulgent area of research. In this method, microorganisms or enzymes are used as catalyst in order to modify lignin and to degrade the hemicellulosic content in the biomass. Several white-rot fungi and brown-rot fungi, such as Phanerochaete chrysosporium, Pleurotus ostreatus, Ceriporiopsis subvermispora, Postia placenta, Phanerochaete carnosa, Gloeophyllum trabeum and Trametes versicolor have been studied for pre­treatment of biomass such as wheat and rice straws, corn stover and switch grass [31, 44]. An overview of biological pretreatment and its applications are shown in Fig. 1.1.

All the pretreatment methods, except biological method, require expensive equip­ment that have demand of high energy depending on which the process to be carried out. Furthermore, these techniques often result in effluent and residue that tremen­dously have negative impacts on environments, inhibit the enzymatic reaction and the growth of microorganisms, which suppose to ferment the product of enzymatic sac­charification [5, 31]. Indeed, biological pretreatment method using white-rot fungi has increased its attention because of the following inherent advantages, (i) safe and environmental-friendly method; (ii) low-energy consumption and cost effective; (iii) selective degradation; (iv) in some cases treated biomass directly can be used for

enzymatic conversion or fermentation; (v) increase the cellulose digestibility of many types of forage fiber and agricultural wastes [45].

This pretreatment retains many special features itself, that is why now researchers looked into biological route to achieve desired target. So far, many research papers have already been reported that the biological pretreatment has been tested and estab­lished beyond its level. Organo-solvent (ethanol, methanol, butanol, ethylene glycol, я-butylamine, etc.) also used along with biological treatment to enhance the degra­dation of internal lignin seal, removing hemicelluloses and disturbing crystalline nature of cellulose [46].

Despite all these advantages, however, biological pretreatment is a very slow pro­cess; and moreover some important components (hemicelluloses and cellulose) of biomass are also consumed either by same microorganism or by some foreign in­vaders. Low-saccharification rate (35-40 %) is found when compared with chemical and physical treatment methods [47]. Main objective of this chapter is to discuss various biological pretreatment methods, advantages, disadvantages, and to come out with the key to resolve the barriers in biological treatment.

Lignocellulosic biomass is also a cheap and abundant alternative for microbial biopolymer production, especially for microbial systems with hydrolytic capability via endoglucanases or cellobiose. Otherwise, it is utilized to a limited extend during the fermentation and hence requires pretreatments beforehand. The filamentous fungi

S. rolfsii and other medicinal mushrooms (Basidiomycetes) can naturally metabolize different five carbon sugars like xylose and arabinose and hence are especially well suited for EPS production from lignocellulosic substrates. Some examples include the simultaneous production of schizophyllan and arabinoxylan by Schizophyllum commune strain ATCC 38548 cultures grown on alkaline H2O2-pretreated corn fiber as a sole carbon source [93]. Same strain was also used for schizophyllan production from activated charcoal detoxificated rice hull hydrolysate [94]. Influences of indi­vidual or combined inhibitors as well as the importance of detoxification step in EPS production were systematically investigated in this study. Under SSF conditions, a high temperature tolerant white rot fungus Lentinus squarrosulus MBFBL 201 was reported to degrade cornstalks very fast and up to 5 g/L EPS could be recovered from the fermentation media [95].

On the other hand, there are only very few reports on the bacterial EPS pro­duction using cellulose-rich biomass. Acid hydrolaysates of wood were used for succinoglycan production by Pseudomonas sp. ATCC 31260 cultures. The produced EPS was found to be rheologically comparable with commercially available xan — than [96]. When cultured on acid-hydrolyzed sawdust, Brevundimonas vesicularis LMG P-23615 and Sphingopyxis macrogoltabida LMG 17324 bacterial strains were found to accumulate high amounts of PHA with yields ranging from 64 to 72 % of the dry cell weight [97]. In another study, lignocellulosic fibers with 58-63 % cellulose content were used as a low cost natural complex carbon source for EPS production by Bacillus megaterium RB-05 cells with known cellulase activity. The fibers immersed in production medium were pretreated by autoclaving for 15 min, however, once inoculated with cells, EPS production was found to be driven solely by the bacterial cellulase activity. Moreover, recovery of the EPS from the culture required several steps due to the biofilm formed along the fibers [98]. In another recent study, rice bran was subjected to serial enzymatic treatment using amylase, amyloglucosidase, alcalase, and lipase enzymes and then the hydrolysate was used for the co-production of intacellular and extracellular polymers by nitrogen-fixing Sinorhizobium meliloti MTCC 100 shaking bacterial cultures. Supplementation of the medium with 20 % rice bran hydrolysate resulted in maximum yields of 11.8 and 3.6g/L EPS and PHA, respectively [99].

Pneumatic dryers are gas-solid transport systems with continuous convective heat and mass transfer process [21]. This type of dryers can achieve rapid drying with short residence time (5-10 s) by fully entraining the material with a high velocity gas flow [19, 27]. The high-velocity gas transports the solid particles along the pipeline and mixing takes place between them. The gas stream also serves as a drying medium to supply heat to remove moisture away from the biomass particles. Since pneumatic dryers operate with a low solid content, it is easy to control and allow the materials to be dried to desired equilibrium MC rapidly [13].

8.2.1 Composition of Lignocellulosic Biomass

The term “lignocellulosic biomass” is used when referring to higher plants, such as grasses, SW or HW. Understanding lignocellulosic biomass, particularly its chemi­cal composition, is a prerequisite for developing effective pretreatment technologies to deconstruct its rigid structure, designing enzymes to liberate sugars, particularly cellulase to release glucose (Glu), from recalcitrant cellulose, as well as engineer­ing microorganisms to convert sugars into ethanol and other bio-based chemicals. The main components of the lignocellulosic materials are cellulose, hemicellu — lose, lignin, and a remaining smaller part (extractives and ash). The composition of lignocellulose highly depends on its source. There is a significant variation of the lignin and (hemi)cellulose content of lignocellulosics depending on whether it is derived from hardwood, softwood, or grasses. Table 8.1 summarizes the com­position of lignocellulose encountered in some of the most common sources of biomass.