4.1.1.1 Carbohydrates

Carbohydrates are polysaccharides, that is, polymers of monosaccharides—sugars that are essentially polyhydroxy ketones or aldehydes. The chemical formula of monosaccharides is C_n(H₂O)_m, for example, that of glucose is C₆(H₂O)₆ or C₆H₁₂O₆. The carbonyl functional group in glucose is an aldehyde, whereas that in fructose—another sugar containing six carbon atoms and having the same molecular formula—is a ketone. Both glucose and fructose are hexoses—sugars or monosaccharides containing six carbon atoms. The molecular formula of monosaccharides may indicate a straight chain compound; however, the existence of polar groups in the molecule results in the formation of ring structures—hemiacetals, hemiketals, furanoses (five-member rings), and pyranoses (six-member rings) [4]. Monosaccharides can also oligomerize and polymerize through the formation of glycosidic bonds between the carbon atom of one molecule and the oxygen in the hydroxyl group of the second molecule in a condensation reaction. The nature of bonding and molecules formed shows a great variety depending upon the structural complexity and stereochemistry. The resulting carbohydrates have strikingly different physical and chemical properties, even though the basic units are essentially the same.

Carbohydrates in biomass are classified into four main categories: starch, cellulose, hemicellulose, and pectins [1].

Starch: The base monomer unit of starch is α-D-glucopyranose or D-glucose, which depending upon the type of the glycosidic bonds, forms two different types of polymers—amyloses and amylopectins [5, 6]. Typically, amylose chains are much longer than the amylopectin chains, and amylopectins are highly branched structures having a higher molecular weight than amyloses [7]. The amylopectin content of starches is generally greater than the amylose content [4, 6]. Plants store starches in the tubers, bulbs, seeds, and rhizomes, that is, parts consumed as food items.

Cellulose: The molecular formula for cellulose is represented by (C₆H₁₀O₅)_n. The base monomer unit of cellulose is β-D-glucopyranose. The glycosidic bonds in cellulose are different from those in starch. Cellulose is a homopolymer with a highly linear structure, and the polymer chain length is two to three times that in the amyloses in the starches. The individual polymer chains are highly crosslinked through hydrogen bonds. A high level of intramolecular hydrogen bonding also exists within the individual polymer chains. These polymer chains aggregate to form microfibrils, and then fibrils, and finally cellulose fibers. The crosslinking imparts to the cellulose fiber a highly ordered crystalline structure that makes it insoluble in many solvents and resistant to chemical and biological treatments [6, 8]. Cellulose is located mostly in the plant cell walls functioning to support the plant. Cellulose accounts for 30%–50% of the dry weight of the plant. Figure 4.2 shows the chemical structures of α-D-glucopyranose and β-D-glucopyranose, the base units of starch and cellulose, respectively.

Hemicellulose: Noncellulose and nonpectin polysaccharides present in plant cell walls are termed hemicellulose that has the molecular formula of (C₅H₁₀O₅)m. Unlike starch and cellulose, hemicellulose is a branched heteropolymer with base units consisting mainly of pentoses (five-carbon sugars, e.g., xylose and arabinose) with some hexoses (six-carbon sugars, e.g., glucose, galactose), higher carbon sugars (fructose, rhamnose), and sugar acids (D-galacturonic acid, D-glucuronic acid, etc.). The degree of polymerization is substantially lower in hemicellulose, giving it a more amorphous structure and lower thermal stability as compared to cellulose. Hemicellulose serves as a binding agent and may account for up to 30% of the dry weight of the biomass [4, 6, 7]. The nature of hemicellulose varies greatly in the plants, depending upon the relative amounts of the base units. Figure 4.3 shows the chemical structures of some of the monomer units of hemicellulose.

Pectins: The dominant monomer unit in pectins is galacturonic acid, shown in Figure 4.3. The linear homopolymer homogalacturonan dominates the composition of pectins. More complex heteropolymers consisting of rhamnogalacturonans, xylogalacturonans, and similar units make up the remainder of the pectin. These heteropolymers have crosslinked side chains of various sugars such as L-rhamnose, xylose, arabinose, and so on. [4]. Pectins are involved in the maintenance of the cell wall structure, cell adhesion, and are most abundant in fruits and young plant tissues [1].

4.1.1.2 Lignin

Lignin is a complex heteropolymer that provides mechanical strength to the plants. It is found in the secondary cell walls formed after the cell growth is complete and serves to transport water and nutrients as well as protect the plant from pathogens [9]. Lignin is composed of aromatic phenylpropane monomer units that form highly crosslinked polymers. The structures of the major base units—the monolignols p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol—are shown in Figure 4.4.

Phenylpropanoid radical subunits syringyl (S), guaiacyl (G), and β-hydroxyphenyl (H), derived from the three monolignols sinapyl alcohol, coniferyl alcohol, and p-coumaryl alcohol, respectively, enter into various covalent bonds, forming lignin. The core of softwoods is composed primarily of G units, while the core of hardwoods consists of G and S units. The chemical formula of lignin is (C₁₀H₁₁O_3.5)_m, indicating its significantly higher carbon content as compared to the carbohydrates.

4.1.1.3 Protein

All biomass contains proteins that are essential constituents of enzymes and photosynthetic systems. Proteins serve many functions including governing cell expansion and cell wall transport [1]. Proteins are the source of N content of the biomass, which can vary from <1 wt% to >10 wt% for food crops such as Jatropha, soybean, mustard, olive, some algae, and so on [6, 10]. It should be noted that while the N content of lignocellulosic biomass is relatively low, that of municipal waste and sludges can be quite high.

4.1.1.4 Lipids

Lipids in plants are composed of vegetable oils and fats—high-value products that are consumed by human beings and used for medicinal/cosmetic purposes, animal feed, and so on. These oils and fats consist of saturated and unsaturated fatty acids and triglycerides—esters of fatty acids with glycerol. Figure 4.5 shows the structures of linoleic acid, oleic acid, and palmitic acid, which are the dominant fatty acids in many oilseeds from soybean to sunflower and peanut [4].

Various microalgae (unicellular eukaryotes), such as Chlorella and Scenedesmus species, and cyanobacteria (blue–green algae) such as Spriulina and Nostoc species, also contain up to 20 wt% of oil on a dry basis. These algae also have a very high protein content, as much as 70 wt% on a dry basis, and can be valuable resources for many bioproducts [11].

4.1.1.5 Ash

The inorganics present in the biomass constitute its ash content. The amount of ash remaining after complete incineration of the biomass may range from ~0.5% in hardwoods and softwoods, to as much as 20% in agricultural crops [6]. Silica is a major component of ash, with metals such as Ca, Mg, and K accounting for the rest.

4.1.1.6 Extractives

Extractives are not the structural components of the biomass, but extracellular products secreted or produced by the cells. As these chemicals are extracellular, they can be easily extracted using water or organic solvents. Extractives serve as an energy reserve, metabolic intermediates, or a defense against microbes and other pests [6]. Extractives impart distinct fragrance and color to the lignocellulosic biomass and provide resistance to decay. Extractives may amount to as much as 10% of the weight of the harvested biomass and can be removed from the biomass rather readily by extraction. Some of the extractives, such as turpentine and tall oil, are economically valuable products as well. Figure 4.6 shows some of the extractive compounds.

4.3.1.1 Combustion

Direct combustion of biomass for power generation is the simplest transformation technique and an extension of the traditional biomass burning used for heating application. Biomass, having a heating value of ~20 MJ/kg, can be combusted in dedicated furnaces similar to those in thermal power plants operating with solid fuels to generate electricity. Practically any type of biomass, from energy crops to municipal waste and agricultural and forest residues, can be transformed into power (and heat) using combustion. This technique can also process biomass with the least amount of pretreatment; however, for efficient combustion and ease of fuel handling, the biomass is generally converted into pelletized form. Typical energy efficiencies of dedicated biomass furnaces are ~60% [29].

There are several challenges in biomass in dedicated furnaces: the biomass composition can vary greatly, particularly with respect to the moisture content; depending on the composition, several pollutants such as gaseous acid anhydrides, nonmetal oxides, and halogens (chlorine and hydrogen chloride), in addition to CO and NO_x, may be emitted into the atmosphere, and a significant quantity of ash may be generated. Halogen emissions may be controlled by adding amendments such as alkali and alkaline earth carbonates, or by installing postcombustion equipment. Both of these options impose additional costs on the process. The ash consisting of silica, alkaline chloride and sulfate salts, oxides, and hydroxides presents disposal problems, and its accumulation in the furnace results in lowering the combustion efficiency.

Combustion of biomass in dedicated furnaces may have a niche in microgenerator systems (10–100 kW thermal, ~10 kW electric) for small-scale combined heat and power applications, such as on small farms; for larger systems, co-firing of biomass with coal is a more promising alternative for biomass combustion [30, 31]. Co-firing involves adding biomass to the coal in the coal thermal plant and has several benefits. First, it substitutes coal, the worst fossil fuel for its greenhouse gas emissions/climate change impact, with the carbon-neutral biomass, making it extremely attractive from the environmental point of view. Second, the severity of challenges inherent in the combustion of biomass, as described earlier, is lessened. Depending upon the configuration of the co-firing system, little or no modification may be needed in the existing coal power plants.

Direct co-firing involves mixing the biomass with coal upstream of any size reduction and feed-conditioning operations on the raw coal and feeding the combined mixture into the furnace. Alternately, biomass size reduction and feed-conditioning operations may be conducted separately from those on coal, and the two streams blended together before being fed into the furnace. The third option for direct co-firing involves using dedicated biomass burners in the coal combustion furnace. The first two direct co-firing options do not require any modification of the furnace. The characteristics of biomass determine which alternative will be suitable for direct co-firing. Separating the size reduction and fuel conditioning systems protects the coal handling system from any disturbances in the biomass handling system. Similarly, using distinct burners will help mitigate disturbances in the biomass combustion and increase the stability of the system. Direct co-firing does result in common emissions, including a single solid reside/ash stream. Direct co-firing systems can usually accommodate up to 10% of biomass in the fuel without a major impact on the operation.

Parallel co-firing involves separating the combustions of coal and biomass and then blending the boiler steams for power generation. The biomass is combusted in a dedicated furnace, and the system is faced with the challenges mentioned earlier. However, benefits are realized through upgrading of the low-quality biomass steam and the use of the existing balance of a coal thermal plant. Further, the volume of the troublesome ash stream from the biomass furnace is smaller and can be handled separately from that of the coal furnace. As the biomass combustion takes place in its dedicated furnaces, the coal combustion process is shielded from any disturbances in biomass combustion.

Indirect/gasification co-firing involves subjecting the biomass to gasification (discussed in the next section) in different process equipment and then feeding the generated gas into the coal furnace for combustion. Depending on the biomass composition, a cleanup system may be needed for sequestering problematic constituents, such as chlorine and alkali metals, from the flue gas before it is fed into the coal furnace [31]. As with parallel co-firing, the ash streams are managed separately. The disturbances in biomass gasification are damped to some extent as relatively clean gas stream enters the coal furnace.

4.3.1.2 Gasification

Biomass combustion takes place in excess of air or oxygen. Gasification is an autothermal process conducted at high temperatures with substoichiometric quantities of oxygen to convert the biomass to syngas—a mixture of CO and H₂. Any carbonaceous material can be converted to syngas via gasification, making it possible to apply it to any type of biomass [29, 32]. The process also generates a solid residue, which differs from the ash in the combustion process in that it contains some carbon and hydrogen and is called biochar.

The fundamental reaction in syngas formation is that of reforming carbonaceous material by steam. The reaction is endothermic, and the high temperature (~1000°C) needed for the reaction is obtained through partial oxidation of the material itself. The basic reactions involved in gasification are shown in reactions R4.2 (partial oxidation) and R4.3 (stream reforming).

${\left({\text{CH}}_2\text{O}\right)}_n+0.5n{\text{O}}_2\to n\text{CO}+n{\text{H}}_2\text{O} \left(\mathrm{R4.2}\right)$

$m\text{C}+m{\text{H}}_2\text{O}\to m\text{CO}+m{\text{H}}_2 \left(\mathrm{R4.3}\right)$

The challenges in the transformation of biomass by gasification stem from low heating value, low density, and high moisture content of the biomass. Co-gasification of biomass with coal provides a possible solution for improving the efficiency of gasification, as well as providing a more consistent process by minimizing the variation in feed quality [33]. The overall reaction in the process can be represented by reaction R4.4.

$\begin{array}{l}{\text{CH}}_m{\text{O}}_n\left(\text{Coal}\right)+{\text{CH}}_x{\text{O}}_y{\text{N}}_z{\text{S}}_s\left(\text{Biomass}\right)+{\text{O}}_2\left(21\%\text{of}\text{air}\right)+{\text{H}}_2\text{O}\left(\text{Steam}\right)\to {\text{CH}}_4\\ +\text{ CO}+{\text{CO}}_2+{\text{H}}_2+{\text{H}}_2\text{O}\left(\text{Unreacted steam}\right)+\text{C}\left(\text{Char}\right)+{\text{CH}}_{m’}{\text{O}}_{n’}\left(\text{Tar}\right)+\text{Ash}\end{array} \left(\mathrm{R4.4}\right)$

The syngas formed can be combusted for power generation or synthesis of hydrocarbon fuels using the Fischer–Tropsch process, or methanol and other chemicals.

The chemical composition of biomass gives rise to many contaminants in the syngas. These contaminants include tar (heavy hydrocarbons), particulates; nitrogen, sulfur, and chlorine compounds; and alkali metals [34, 35]. The levels of these contaminants must be reduced to prevent their emissions into the environment as well as adverse impacts on downstream processes and equipment, which may include corrosion, fouling, catalyst deactivation, and so on. Similarly, any ash generated in the gasification process must be managed to prevent its environmental impact. Particulates can be captured using bag filters, cyclones, or electrostatic precipitators. Cooling of the gas can result in the condensation of tars and other volatiles. Acid gases can be scrubbed from the gas using alkaline solutions. Tars can also be subjected to catalytic and thermal cracking to convert them to lighter gases and possibly increase the yield of syngas [36].

Gasification of biomass, particularly with very high moisture content (algae, kelp, sewage, etc.), can be conducted in a supercritical water medium at temperatures ranging from 500°C to 750°C and pressures from 250 to 300 bar, with the critical point of water being 374°C and 220 bar. This process offers several advantages including obviating the need for drying pretreatment and ease of storage. The process itself is characterized by high efficiency, low tar formation, retention of impurities in the aqueous medium, and a cleaner product gas having a higher H₂:CO ratio that is available at higher pressures [28]. Such hydrothermal gasification, that is, gasification in the presence of water, can also be effected under subcritical conditions. Hydrothermal gasification conducted at temperatures ranging from 250°C to 550°C and pressures below the critical pressure requires the use of catalysts—Ni and Ru being the most promising. Operating at temperatures higher than 550°C obviates the need for a catalyst [37, 38].

4.3.1.3 Pyrolysis

Pyrolysis is the thermal decomposition of materials in the absence of oxygen. As seen from Figure 4.13, pyrolysis of biomass takes place at temperatures ranging from 200°C to 800°C, in very low oxygen environments. Pyrolysis can be further classified as mild pyrolysis, slow pyrolysis, and fast pyrolysis, though in some classifications intermediate pyrolysis and gasification are also included as two additional categories [35].

Mild pyrolysis, variously referred to as low-temperature pyrolysis, thermal rectification, or torrefaction, is conducted at temperatures ranging from 200°C to 320°C. Depending upon the temperature in the process, it can be further classified as light (200°C–235°C), mild (235°C–275°C), or severe (275°C–320°C) [36]. Torrefaction results in the breakdown of the structures of lignin, hemicellulose, and cellulose, releasing volatile matter and leaving behind biochar and ash. The reaction in dry torrefaction can be represented by R4.5 [28].

${\mathrm{CH}}_{1.4}{\mathrm{O}}_{0.6}\left(\mathrm{Biomass}\right)\to {\mathrm{C}}_{0.7}{\mathrm{H}}_{1.0}{\mathrm{O}}_{0.4}\left(\mathrm{biochar}\right)+{\mathrm{C}}_{0.3}{\mathrm{H}}_{0.4}{\mathrm{O}}_{0.2}\left(\mathrm{gases}\right) \left(\mathrm{R4.5}\right)$

Approximately 70% of the biomass is converted to biochar, with the remaining 30% getting volatilized.

Torrefaction of biomass containing large quantities of water, such as manure, sewage, and municipal waste, is conducted under wet conditions, and the process is called wet torrefaction or hydrothermal carbonization (HTC). HTC of biomass is conducted by raising the temperature of its aqueous suspension to 180°C–260°C under an autogenous pressure of 20–60 bar [39]. The processing time may range from 5 minutes to 4 hours. HTC proceeds via depolymerization of lignin, hemicellulose, and cellulose into the oligomers of the building blocks and then into derivatives of monomers, as described in Section 4.1. Finally, an aromatic structured solid that has similar carbon content as lignite is formed [39]. The solid product of HTC is termed hydrochar to distinguish it from biochar, the product of dry torrefaction. Hydrochar is generally more energy dense and has lower levels of metallic impurities and ash [36]. The components released from the biomass are contained in the aqueous medium either as dissolved organics or gas.

Biochar and hydrochar, the torrefaction products, can be used for power generation or subjected to gasification for conversion to biofuels. Partial substitution of coal by biochar/hydrochar will have a positive environmental impact due to the reduction in greenhouse gas emissions.

Slow pyrolysis is conducted at temperatures of 300°C–500°C, higher than those employed in mild pyrolysis. The end product of slow pyrolysis is also the same as mild pyrolysis, that is, biochar. In essence, slow pyrolysis is a carbonization process, as shown in R4.6.

${\mathrm{CH}}_{1.4}{\mathrm{O}}_{0.6}\to \mathrm{C}+\mathrm{tar}+0.6{\mathrm{H}}_2\mathrm{O}+0.1{\mathrm{H}}_2+\mathrm{other} \mathrm{gas} \mathrm{products} \left(\mathrm{R4.6}\right)$

Slow pyrolysis is conducted at lower heating rates and temperatures with a longer residence time (5–30 minutes). Fast pyrolysis employs higher temperatures (500°C–800°C), greater heating rates, and shorter residence times (0.5–5 seconds) yielding bio-oil as the main product [28, 34, 40]. The transformation is represented in reaction R4.7.

${\text{CH}}_{1.4}{\text{O}}_{0.6}\to \text{biooil}+\text{tar}+\text{C}+\text{CO}+{\text{H}}_2\text{O}+{\text{H}}_2+\text{other flammable gases} \left(\mathrm{R4.7}\right)$

Nearly 60–75 wt% of the biomass is converted to bio-oil, with 10%–20% converted to char. The remaining fraction is converted to gases, which do not have the opportunity to react with the char due to low residence times employed in the process. Bio-oil formed in the fast pyrolysis is acidic (pH <4), and contains up to 20–25 wt% of water. The organic fraction of bio-oil is a complex mixture of varied chemical compounds, mostly oxygenates (aldehydes, phenols, ketones, and organic acids). These compounds are unstable and corrosive in nature, and the bio-oil needs to be deoxygenated and upgraded for it to be useful [41, 42]. The upgrading and deoxygenated are generally accomplished through catalytic hydrodeoxygenation (HDO), which involves the reaction of bio-oil with hydrogen, supplied externally or generated from the biomass itself through reforming. Oxygen is extracted from the bio-oil constituents and removed as water. Various catalysts are used for HDO including alumina-supported Ni–Mo, Co–Mo, noble metal catalysts (Pt, Ru, and Rh), and zeolite-supported Pt, as well as metal sulfides, nitrides, and phosphides [42, 43]. Catalytic pyrolysis is another alternative to obtain deoxygenated bio-oil products, where the catalyst is either embedded with the biomass during the pyrolysis reaction (in situ) or deoxygenates the pyrolysis vapors prior to their condensation into the bio-oil product.

Bio-oil with low oxygen content can be produced from biomass through hydrothermal liquefaction (HTL), which involves effecting deconstruction of biomass constituents at high temperature and pressure in an aqueous environment. The temperatures and pressures range from 280°C to 380°C and 70 to 300 bar, respectively. The reaction time ranges from 10 to 60 minutes, and a reducing environment may be created through provision of CO and H₂. The processing may be catalyzed using homogeneous alkaline catalysts (e.g., Na/K hydroxides or carbonates) or heterogeneous transition metals (Fe, Co, Ni, etc.). The conversion and product distribution are dependent on several factors including the type of biomass, pressure, temperature, catalyst employed, processing time, and so on. Heating rate also has a significant impact on the product yield—high heating rates result in the formation of gaseous products, whereas lower heating rates lead to char formation. Moderate heating rates are indicated for the formation of liquid products [37]. The liquid products formed in HTL are characterized by higher C content and much lower water contents (5%) as compared to the fast pyrolysis bio-oil [44]. The operating pressures and temperatures for various hydrothermal processing technologies are shown in Figure 4.15.

4.3.1.4 Hydrotreating

Hydrotreating is also essentially a thermochemical process; however, it differs from the other thermochemical processes described earlier, in that it is rarely applied for direct processing of the raw biomass. The feedstock for hydrotreating consists of lipids (triglycerides), which are typically obtained from virgin vegetable oils, used cooking oils, animal fat from the food industry, waste vegetable oils, and so on. Hydrotreating transforms these oils into renewable gasoline and diesel (green gasoline and diesel) or sustainable aviation fuel (SAF). Hydrotreating involves catalytic hydrogenation at high pressures and moderate temperatures using supported metal catalysts [45].

The unsaturated bonds in triglycerides are first saturated through hydrogenation in the process, and this is followed by hydrogenolysis forming fatty acids and liberating propane. The fatty acids undergo HDO yielding a saturated hydrocarbon (alkane) containing the same number of carbons as the fatty acid. Alternately, the fatty acid can also undergo hydrodecarbonylation/decarbonylation (HDC) forming an alkane containing one less carbon atom. Various reactions that constitute hydrotreating are shown in Figure 4.16 [45].

Hydrotreating is typically conducted at temperatures ranging from 300°C to 350°C and pressures of 20 to 70 bar. The initial product of hydrotreating is typically a normal alkane, which will invariably undergo, to some extent, isomerization to iso-alkane, as well as cracking to a lower alkane. The isomerization is generally desired for improved performance as a transportation fuel, whereas cracking has the opposite effect and is not desired. Further, cracking may also cause catalyst deactivation [45]. As mentioned earlier, the catalysts used for HDO are typically supported noble metals, with Pt and Pd reported to be the most effective ones. Metal sulfides are also commonly used due to their cost advantage and superior resistance to impurities in the feedstock. However, they are also more sensitive to the presence of water and susceptible to losing sulfur, which can affect alkane product quality. It can be seen from Figure 4.16 that the hydrotreating process requires large quantities of hydrogen, with requirements ranging from 7 to 16 moles per mole of the triglyceride. Some of this hydrogen can be recovered from the biomass itself; however, an external supply is invariably needed. Non-fossil-derived hydrogen is expensive and has an adverse impact on the economics of the process, while using fossil-derived hydrogen does not help in averting greenhouse gas emissions nor make the resulting fuel truly renewable.

Thermochemical processing of biomass involves either its direct oxidation (complete, as in combustion or partial, as in gasification,) or conversion to reduced forms of carbon (biochar, bio-oil, or hydrocarbons through the syngas route) via pyrolysis for subsequent oxidation. Some fraction of biomass may also be converted to useful bioproducts, particularly from lignin or using syngas as a platform for chemicals, to improve biomass valorization; however, thermochemical transformations primarily involve conversion of biomass energy into other energy forms and carriers.

4.3.2.1 Enzymatic Hydrolysis of Biomass

Polysaccharides in the biomass can be converted into sugar monomers by abiotic acid-catalyzed hydrolysis at temperatures ranging from 100°C to 240°C. Some fraction of the biomass does get hydrolyzed in this manner in the acid pretreatment. However, acid-catalyzed saccharification (sugar formation) may also result in the formation of compounds that can act as inhibitors in the subsequent steps, and hence enzymatic hydrolysis that is conducted at much milder conditions (50°C–60°C with pH 4.8–5.0) is preferable due to higher recovery of sugar with minimal formation of inhibitors [14].

The various enzymes that act on different constituents of the biomass are classified according to the substrate metabolized by them [6]:

• Cellulases are the class of enzymes that hydrolyze cellulose. Specific enzymes within the cellulases perform specific functions.

• Hemicellulases are the enzymes that affect the deconstruction of hemicellulose.

• Amylases are the class of enzymes that metabolize carbohydrates that are present as starch. Human saliva contains amylases that catalyze the digestion of starch in food.

• Pectinases are enzymes that depolymerize pectins.

• Laccases are the ligninolytic enzymes that depolymerize lignin, converting it to phenolic compounds.

• Lipases are the enzymes that catalyze the breakdown of lipids into fatty acids and glycerides.

• Proteases are enzymes that catalyze proteolysis, that is, the breakdown of proteins into peptide chains and amino acids.

Starch in food crops and cellulose/hemicellulose in lignocellulose biomass dominate the carbohydrate content of the biomass and are the most important resources for obtaining monosaccharides for further conversion to biofuels, specifically bioethanol.

As mentioned earlier, starch is hydrolyzed by amylases, which are complex molecules containing hundreds of amino acids. There are several different types of amylases that attack different sites in the starch polymer, cleaving the bonds. Endoamylases or α-amylases randomly attack the polysaccharide chains, breaking the glycosidic bond and generating oligosaccharides of various lengths. Exoamylase (β-amylase), pullanases, and glycosidases are some of the other amylases that target different sites on the polysaccharide chain/oligosaccharide fragments. Finally, γ-amylase is the enzyme whose action results in the formation of glucose.

Biomass energy systems are transitioning away from the first-generation, starch-based system. The second generation of the systems utilizes lignocellulosic biomass, and the important cellulases that mediate the hydrolysis of cellulose are endoglucanase that has a random cleaving action on the glycosidic bonds resulting in the production of cellobiose and cellotriose units and exoglucanase that generates cellobiose units much the same way endo- and exo-amylases depolymerize starch. Finally, β-glucosidase metabolizes the cellobiose units to form glucose [14]. Figure 4.17 shows the formulas of cellobiose and cellotriose units.

While cellulose is a hexose (six-carbon sugar)-based polysaccharide, hemicellulose, the second most abundant carbohydrate in the lignocellulosic biomass, is a pentose (five-carbon sugar)-based polysaccharide. Similar to starch and cellulose hydrolyzing enzymes, hemicellulases consist of several different enzymes that attack specific sites. Xylanases randomly attack the xylan backbone of the hemicellulose, generating xylooligosaccharide chains of varying lengths. The xylooligosaccharide metabolism is mediated by β-D-xylosidases resulting in the formation of pentoses, primarily xylose [6]. Xylose has a limited utility for microbial conversion to ethanol, as it is not readily fermented to ethanol by the common yeast that ferments glucose. Many bacteria and some fungi do have the ability to convert the xylose to ethanol and other chemicals; however, activity of these microorganisms is inevitably suppressed in the presence of glucose [46]. Xylose, and by extension hemicellulose, is more valuable as a raw material for producing furfural, which can function as a commodity chemical for a variety of other chemical products. Furfural can be converted to other useful compounds. Furfuryl alcohol, methyltetrahydrofuran, furoic acid, tetrahydrofurfuryl alcohol, piperidine furoic esters, and tetrahydrofuroic acid are just some of the chemicals that can be produced from furfural [47].

Furfural is obtained from xylose by dehydration mediated by a homogeneous mineral (hydrochloric, sulfuric) or heterogeneous acid (sulfonic acid) catalysts. Furfural can also be obtained from hexoses, the synthesis route consisting of glucose isomerization to fructose, which upon acid-catalyzed triple dehydration yields 5-HMF. Decarboxylaion of HMF results in the formation of furfural. HMF itself is an important intermediate product and building block for secondary chemical products, including C9–C15 alkanes, γ-valerolactone (a green fuel), 2,5-furandicarboxylic acid, levulinic acid, formic acid, and so on. Obviously, many of these products can be utilized as fuels; however, they likely are more useful for conversion to value-added products including polyesters, pharmaceuticals, and many others. The structures of xylose, furfural, and HMF are shown in Figure 4.18.

The optimum conditions for enzymatic processes depend not only upon the type of enzyme but also the microorganism expressing the enzyme. As can be seen from the above discussion, a large number of enzymes act in an interconnected sequence of reactions to bring about the conversion of polysaccharides into monomeric sugars. The optimum pH and temperature for various microbes and enzymes may range from 4 to 8, and 40°C to 70°C, respectively [48]. Typically, the process is conducted at around pH 5 in the temperature range 45°C–50°C to achieve an optimum balance. Separation of various components may allow a multistep process that can take advantage of different optimum conditions for different microorganisms. For example, hemicellulose-degrading thermophilic bacteria Caldanaerobius polysaccharolyticus and Caldicellulosiruptor bescii have their optimum temperatures at 70°C and 80°C, respectively [49]. Separating hemicellulose from the pretreated biomass will permit its hydrolysis to be conducted in a separate step using these bacteria that have faster kinetics at elevated temperatures.

Removal of lignin or delignification of biomass is essential for effective hydrolysis of the polysaccharides, as lignin inhibits the activity of many enzymes that act on cellulose and hemicellulose [6]. Laccases are multicopper oxidases that can oxidize the lignin fragments, converting them to phenolic compounds [50]. Conversion of lignin to these compounds that can then be processed into valuable chemicals is important for improving the economics of the process. However, this is best accomplished by separating lignin and its derivative compounds before subjecting the pretreated biomass to hydrolysis, as many of the compounds, as well as laccases, are inhibitory to enzymes affecting the hydrolysis of polysaccharides.

The lipid content of lignocellulosic biomass is quite low, and so lipase’s action is not generally of consequence in the processing of this biomass. However, lipase’s action is important for biomass that has high lipid content, such as algal biomass or biomass from waste streams from animal processing facilities or food industries. This biomass can be a source of biodiesel, as discussed in a later section.

4.3.2.2 Fermentation of Sugars

Fermentation of sugar is one of the oldest biochemical processes known to humankind and continues to be practiced on a micro- to megascale all over the world, albeit for producing ethanol for human consumption. Bioethanol for energy applications is also produced through the same basic process using the same principles. The chemical reaction involved can be represented by R4.8.

${\text{C}}_6{\text{H}}_{12}{\text{O}}_6\left(\text{glucose}\right)\to 2{\text{C}}_2{\text{H}}_5\text{OH}\left(\text{ethanol}\right)+2{\text{CO}}_2 \left(\mathrm{R4.8}\right)$

As can be seen by the equation, a fraction of glucose (an acetal, i.e., a carbonyl compound) is reduced to an alcohol, while the other fraction is oxidized to carbon dioxide. The fermentation takes place at ~30°C–35°C and is mediated by baker’s yeast—Saccharomyces cerevisiae. S. cerevisiae is the microorganism of choice, though there are other microorganisms that are capable of fermenting sugar, because of its fast growth and high ethanol yield, as well as robustness in response to environmental stress factors, such as ethanol, low pH, and low oxygen [51].

The hydrolysis and fermentation steps can be spatially separated, or more commonly conducted in the same process vessel simultaneously, the technology commonly being referred to as simultaneous saccharification and fermentation (SSF). Saccharification product inhibition in fermentation is the main drawback of the configuration where the two steps are spatially separated. Further, it has increased capital and operating costs due to the use of two processing units. SSF has a low investment cost, and as glucose produced is consumed immediately by the fermenters, product inhibition is averted, leading to higher ethanol yields. However, optimal temperatures for the two steps are different—hydrolysis requiring temperatures of 45°C–50°C, while fermentation is best carried out at 30°C–35°C [14].

Distillation is the preferred mechanism for the recovery of the product ethanol from the reaction mass. However, the ethanol–water mixture forms an azeotrope at 95.6 wt% of ethanol, and extractive distillation techniques using dissolved salts are needed to break the azeotrope and obtain pure, anhydrous ethanol for use as fuel.

4.3.2.3 Anaerobic Digestion

Anaerobic digestion of biomass results in the formation of biogas that consists primarily of methane (approximately two-thirds of the biogas) and carbon dioxide. Other gases such as hydrogen sulfide, ammonia, and hydrogen may also be present. The exact composition of the biogas depends on the biomass fed to the process and the processing conditions [52]. Practically, any type of biomass, including manure, sewage, and food waste, as well as lignocellulosic and algal biomasses, can be subjected to anaerobic digestion for conversion to biogas. The biogas can be used for electricity generation and heat applications. The various microbial processes occurring in the anaerobic digestion of the solution resulting from the hydrolysis of the pretreated biomass are shown in Figure 4.19 [53].

The first stage of the process involves the conversion of sugars and other organics into low-carbon carboxylic acids such as formic, acetic, propionate, butyric, lactic acid, and so on. This conversion, which leads to the formation of acids, is called acidogenesis and is mediated by various species of bacteria including Streptococcus, Lactobacillus, Bacillus, Escherichia coli, and Salmonella.

The acetogenesis stage of the process involves the conversion of these acids into acetic acid and acetates. Syntrophomonas and Syntrophobacter affect the conversion of compounds to acetate, whereas Methanobacterium may first convert the larger compounds to propionic acid before other bacteria process it further to acetic acid.

The final stage of the process is mediated by methanogens—bacteria belonging to the genera Methanosarcina, Methanothrix, Methanobacterium, Methanococcus, and many others—that convert the acetic acid to methane.

The overall reaction can be represented by reaction R4.9.

${\text{C}}_6{\text{H}}_{12}{\text{O}}_6\to 3{\text{CO}}_2+3{\text{CH}}_4 \left(\mathrm{R4.9}\right)$

Part of the carbon present in glucose is oxidized to CO₂, while the other part is reduced to CH₄. Figure 4.16 shows the anaerobic digestion process starting with hydrolyzed biomass. In a general case, when the biomass fed to the process may not be preprocessed, hydrolysis described in Section 4.3.2.1 is the first step of the anaerobic digestion process. The intermediate acetogenesis reactions are shown by reactions R4.10a and R4.10b, while the two methanogenic reactions are shown by R4.11a and R4.11b [27].

$2{\text{CO}}_2+4{\text{H}}_2\to {\text{CH}}_3\text{COOH}+2{\text{H}}_2\text{O} \left(\mathrm{R4.10a}\right)$

${\text{CH}}_3{\text{CH}}_2\text{COOH}+2{\text{H}}_2\text{O}\to {\text{CH}}_3\text{COOH}+3{\text{H}}_2+{\text{CO}}_2 \left(\mathrm{R4.10b}\right)$

${\text{CH}}_3\text{COOH}\to {\text{CH}}_4+{\text{CO}}_2 \left(\mathrm{R4.11a}\right)$

$4{\text{H}}_2+{\text{CO}}_2\to {\text{CH}}_4+2{\text{H}}_2\text{O} \left(\mathrm{R4.11b}\right)$

Approximately 70% of the methane produced is due to the reaction R4.11a, which transforms acetic acid; the bacteria effecting the conversion are called acetotrophs. Reaction R4.11b accounts for the balance of methane generation and is mediated by hydrogenotrophs.

It can be seen that anaerobic digestion is a complicated process that depends on synergistic actions of a large microbial consortia that perform different functions. These microorganisms or consortia that perform a specific step have their own optimum operating conditions of temperature, pH, substrate concentration, and a delicate balance is achieved among them for biogas generation at reasonable efficiency and costs. The process, conducted in batch mode, starts under acidic conditions, and the pH increases slowly as the process enters the latter stages. The optimum pH for methanogens is 6.8–7.2. Depending on the temperature of conversion, the digestion process is termed either mesophilic (operating at temperatures <45°C, usually at 30°C–38°C) or thermophilic (50°C–60°C, up to 70°C). Thermophilic processes, though not necessarily yielding higher yields of methane, are much faster, offering the benefits of higher throughput/lower equipment volume [53].

Algal biomass, particularly macroalgal biomass that has high carbohydrate content, is quite attractive for biogas generation. However, effective pretreatment and improved cost-effective saccharification technologies are needed before it can be economically viable in a stand-alone biogas generation system. Current algal biogas systems are under development and exist as coupled systems with wastewater treatment plants [22, 54].

At a fundamental level, the fermentation to bioethanol and anaerobic digestion to biogas, discussed in Sections 4.2.3.2 and 4.2.3.3, respectively, are both based on glucose as the starting material. The origin of this glucose is in the cellulose in the lignocellulosic biomass, which contains hemicellulose and lignin as well. Hydrolysis of the biomass deconstructs both of these components; however, the presence of degradation products of these two is undesirable in fermentation and anaerobic digestion. Co-fermentation processes containing enzymes that can ferment pentoses as well as hexoses can possibly accommodate the presence of hemicellulose in bioethanol formation; however, such processes are still under development and it is desirable to separate cellulose from the other two components of lignocellulosic biomass before subjecting it to subsequent conversion processes for energy fuels.

It is possible to separate the fractions through appropriate pretreatment technologies. Hydrothermal processing or alkaline extraction methods are effective in solubilizing hemicellulose and removing it from the biomass [55, 56]. The organosolv process, described earlier, can remove lignin and recover it in distinct fractions, improving the economics of the process. Lignin can also be separated from cellulose by catalytic hydrogenolysis using Raney Ni catalyst to depolymerize it into phenolic compounds [57]. Cellulose, stripped off the two (hemicellulose and lignin), can then be processed effectively as described earlier.