Enzymatic Conversion of Biomass for Fuels Production - American

Chapter 15

Pretreatment of Lignocellulosic Biomass James

D.

McMillan

Downloaded by NORTH CAROLINA STATE UNIV on October 26, 2012 | http://pubs.acs.org Publication Date: October 7, 1994 | doi: 10.1021/bk-1994-0566.ch015

Bioprocessing B r a n c h , Alternative Fuels D i v i s i o n , N a t i o n a l Renewable Energy L a b o r a t o r y , 1617 Cole B o u l e v a r d , Golden, CO 80401-3393

Pretreatments for lignocellulosic materials include mechanical comminution, alkali swelling, acid hydrolysis, steam and other fiber explosion techniques, and exposure to supercritical fluids. These processes act by a variety of mechanisms to render the carbohydrate components of lignocellulosic materials more susceptible to enzymatic hydrolysis and microbial conversion. A variety of methods are effective on representative biomass feedstocks such as agricultural residues, herbaceous crops, and hardwoods. This chapter reviews pretreatment techniques, focussing on the importance of biomass structure and composition in determining pretreatment efficacy and the mechanisms by which different pretreatments act. The chapter concludes by recommending approaches for achieving further improvements in pretreatment technologies.

Production of fuels and chemicals from renewable lignocellulosic materials is accomplished by hydrolyzing polysaccharide components to soluble sugars which can be fermented to desired end products. Some type of pretreatment is generally required to render the carbohydrate fraction susceptible to enzymatic and microbial action because such materials are only partially digestible i n their native form. Comprehensive reviews of pretreatment are provided by Millett et al. (1), Chang et al. (2), L i n et al. (3), Fan et al. (4), and Dale (5). The specific objectives of pretreatment are dictated by the overall objectives of a biomass conversion process. First, pretreatment must be energetically and chemically efficient, i.e economical, for a biomass conversion process to be profitable. Second, pretreatment must promote effective conversion of available carbohydrate to fermentable sugars so that high product yield can be achieved; pretreatment must maximize the formation of sugars or the ability to subsequently form sugars by

0097-6156/94/0566-0292$10.34/0 © 1994 American Chemical Society

In Enzymatic Conversion of Biomass for Fuels Production; Himmel, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


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enzymatic hydrolysis. Hence, degradation or loss of carbohydrate must be avoided. Because it is also desirable to maximize the rate of enzymatic conversion, pretreatment must yield a highly digestible material that is not inhibitory to cell metabolism or extracellular enzyme function. Therefore, it is preferable to avoid the formation of inhibitory products and the need for detoxification or washing; high sugar losses occur i f pretreated material is washed prior to enzymatic hydrolysis. Finally, pretreated materials are most efficiently hydrolyzed using low enzyme loadings, so the potential for nonspecific binding of enzymes to lignin and other fractions of pretreated biomass must be minimized. Historically, pretreatments were investigated as a means to improve the digestibility of lignocellulosic biomass to increase its value as an inexpensive feed or feed supplement for ruminants. The efficacy of using pretreatment to improve lignocellulosic digestibility has been recognized at least since 1919, when Beckmann patented an alkali pretreatment based on soaking in sodium hydroxide ( N a O H ) for improving the in vitro digestibility of straws by ruminants (7). Numerous pretreatment processes have been investigated since then, most based on a combination of mechanical, physical, and chemical processing steps. Pretreatments that have been used or proposed for use in the context of renewable fuels and chemicals production from lignocellulosic materials include alkali swelling, acid hydrolysis, steam and other fiber explosion techniques, as well as exposure to supercritical fluids. This chapter reviews these pretreatment techniques, focussing on their mechanisms of action. Biomass C o m p o s i t i o n a n d Structure There are many different types of lignocellulosic biomass, including agricultural residues, herbaceous crops, deciduous (hardwood) and coniferous (softwood) trees, and municipal solid wastes ( M S W ) . These biomass types exhibit a wide range of susceptibilities to pretreatment and saccharification because of structural and compositional differences. Thus, before discussing specific pretreatment processes, some background on the chemical composition and physical structure of lignocellulosic materials w i l l be provided. Discussion w i l l focus on woody and herbaceous materials. W o o d y and herbaceous biomass species are composed mostly of cellulose, hemicellulose, and lignin, but also contain ash and other so-called extraneous materials. Typical compositions of representative lignocellulosic materials are shown in Figure 1. Cellulose is the main component, followed by hemicellulose and lignin; the paper fraction of M S W is comprised mostly of cellulose. Hardwoods are composed of about 50% cellulose (dry basis), 23% hemicellulose, and 22% lignin. Herbaceous materials and agricultural residues contain a somewhat higher proportion of hemicellulose (30-33%) relative to cellulose (38-45%), and have lower levels of lignin (10-17%). The composition and amount of extraneous components vary widely among the different biomass types. Extraneous components include extractives, fats, oils, protein, and ash. Extractives are compounds that are soluble i n water or organic solvents. Extractive components in woody biomass include terpenes (isoprene alcohols and ketones), resins (fats, fatty acids, alcohols, resin acids, and phytosterols), and phenols (primarily tannins) (4). Nonextractives are mainly inorganic components



—

Paper - 4 6 %

F i g u r e 1.

Lignin -22%

Hemicellulose -23%

Extractives

Composition of cellulosic biomass types. Cellulosic biomass consists of cellulose, hemicellulose and lignin, and some extractives, as shown here for representative examples of agricultural residues (corn cobs), hardwoods, municipal solid wastes, and herbaceous plants.

Herbaceous Energy Crops

Lignin 1 5 % Other 1 0 %

Hemicellulose 3 0 %

Cellulose 4 5 %

Underutilized and Short Rotation Hardwoods

— Other - 2 2 % — Metal - 1 0 % — Food and yard waste - 1 3 % Wood - 2 % P l a s t i c s a n d textiles - 7 %

Municipal Solid Waste

3L

Agricultural Residues

- Lignin 1 7 % -Other 13%

— Cellulose 3 8 %

- Hemicellulose 3 2 %

Cellulose -50%



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such as alkali earth carbonates and oxalates, as well as some non-cell-wall materials like starches, pectins, and proteins. Nonextractive silica crystals are abundant in herbaceous materials, but not in woody species. Cellulose and hemicelluloses, collectively referred to as holocellulose, are high molecular weight polysaccharides. Cellulose is a linear polymer of anhydro D-glucose units connected by |J-l,4-glycosidic bonds. Native cellulose exists as a fibrous crystal with the so-called Cellulose-I lattice structure. However, Cellulose-I is converted to Cellulose-II by physicochemical treatments such as intercrystalline swelling; mercerized or regenerated cellulose has the Cellulose-II crystalline lattice structure (2). Cellulose crystals (Cellulose-I or Cellulose-II) are organized into compacted crystallite microfibrils measuring 35 x 40 A in width and about 500 A in length, having an average degree of polymerization (DP) of about 1000. A widely accepted model of cellulose crystal structure is the folding chain model in which segments of linear cellulose polymer are folded back and forth along the major axis of the fibrillar crystallite to make up individual microfibrils. Hemicelluloses are shorter chain, amorphous polysaccharides of cellulans and polyuronides (6). Cellulans are heteropolymers made up of hexosans (mannan, galactan, and glucan) and pentosans (xylan and arabinan). Polyuronides are similar to cellulans, but contain appreciable quantities of hexuronic acids as well as some methoxyl, acetyl, and free carboxylic groups. X y l a n and glucomannan are the dominant carbohydrate components of hemicellulose. Hardwood hemicelluloses contain mostly xylan, whereas softwood hemicelluloses contain mostly glucomannan. L i g n i n is composed of polymerized phenylpropanoic acids i n a complex threedimensional structure. Individual monomers are held together by ether and carboncarbon bonds (4). L i g n i n forms by a free radical polymerization mechanism and has a random structure. L i g n i n and hemicellulose are believed to form an effective sheath around cellulose fibers which adds structural strength to the biomass matrix. Covalent bonds may exist between hemicellulose and lignin, but this is uncertain. The amount of lignin in softwoods (25%-35%) is appreciably greater than in hardwoods (18%25%) and herbaceous species and agricultural residues (10%-20%) (4,7-11). Figure 2 illustrates the compositional differences between trembling aspen hardwood and white spruce softwood. The distribution of cellulose, hemicellulose, and lignin within a typical wood fiber cell wall is shown in Figure 3. This diagram demonstrates that lignin is the dominant component in the outer portion of the compound middle lamellae, such that a lignin seal effectively exists at the outer periphery of individual wood fibers. The percentage of lignin in the lignocellulosic matrix decreases with increasing distance into the fiber cell wall, with the percentages of lignin in the primary wall and i n the S I layer of the secondary wall much higher than in the S2 and S3 sections of the secondary wall. Another important structural feature is the asymmetry of fibrous and woody biomass materials. Heat and mass transfer studies by Brownell et al. (72), Tillman et al. (73), and others demonstrate that the fibrous structure of lignocellulosic materials strongly favors transport in the axial rather than radial direction. Tillman et al. (73), for example, determined that acid diffusivities are two-to threefold higher in the direction of the fiber axis than in radial or tangential directions.



296

F i g u r e 2.

ENZYMATIC CONVERSION OF BIOMASS FOR FUELS PRODUCTION

Comparison of compositions of hardwood and softwood. Reprinted with permission from ref. 4. (Copyright 1982; Springer-Verlag: N e w Y o r k , NY.)


McMILLAN


Biomass

297


15.

F i g u r e 3.

Distribution of cellulose, hemicellulose, and lignin i n the wood fiber cell wall. Reprinted with permission from ref. 4.



298


Despite gross similarities, however, significant structural differences exist between different biomass types. For example, softwoods and hardwoods have disparate structural features (7,74). Coniferous (soft) woods are composed primarily of tracheid fiber cells, which are elongated, tubelike structures with closed ends that are square in cross section. Earlywood fibers i n softwoods have a pronounced lumen and are thin-walled, whereas latewood fibers have thicker walls and smaller lumens. Deciduous (hard) woods, on the other hand, are composed of vessels and sclerenchyma or librilform fiber cells. Vessels are long, continuous tubes that serve as a conduit for water and have a much larger diameter than the individual fiber cells. Differences i n the structure of coniferous and deciduous woods contribute to the dramatic differences i n susceptibility to various pretreatment techniques that exist between hardwood and softwood species. The presence of vessels in the hardwoods permits greater penetration of heat, chemicals, and enzymes into the biomass matrix, making hardwoods easier to pretreat. F a c t o r s Affecting E n z y m a t i c Digestibility Major factors that have been identified to influence the digestibility or reactivity of lignocellulosic materials are porosity, cellulose fiber crystallinity, lignin content, and hemicellulose content. Prevailing hypotheses regarding these factors are oudined below. Porosity (Accessible Surface A r e a ) . It is generally believed that the initial rate of cellulose hydrolysis is a function of the specific accessible surface area of cellulose, with the rate increasing with increasing coverage of cellulase enzyme (75). Quantifying the rate of enzymatic hydrolysis as a function of accessible surface area has proven to be difficult. M a n y techniques have been used to estimate surface area, including nitrogen adsorption, mercury porosimetry, dye adsorption (16,17) and solute exclusion (15,18,19). Results indicate that accessible surface area increases following pretreatment, although there is disagreement about how much. A n important but still unresolved issue is how lignin affects accessible surface area (see below). C r y s t a l l i n i t y . M a n y researchers believe that the rate of cellulose hydrolysis is determined not by the amount of adsorbed cellulase enzyme but rather by the D P of cellulose or the extent of cellulose crystallinity (also referred to as the ratio of amorphous to crystalline cellulose) (4,20). This hypothesis is supported by the fact that size reduction, which has little effect on accessible surface area because of the high length to width ratio of lignocellulosic fibers, dramatically improves digestibility. This view is disputed by other researchers like Schurz (27) who observe no change in crystallinity during hydrolysis and attribute differences in rate to differences i n the type or quality of surface adsorption. L i g n i n Content. The reactivity of lignocellulosic materials to cellulase enzyme action generally varies i n negative proportion to lignin content. Figure 4 shows data from Stone et al. (75) on the digestibility of softwoods as a function of the amount of lignin removed by sulfite pulping. A large increase i n 24-h saccharification yields occurs with increasing extent of delignification. The lower digestibility of the dried material



F i g u r e 4.

0

20

30

40

50

60

% of total lignin removed by pulping

70

80

Digestibility as a function of percent delignification. Data for black spruce pulped by the sulfite process are interpolated with permission from Figure 5 in Stone et al. (15). (Copyright 1969; American Chemical Society: Washington, D C . )

10


90


300

ENZYMATIC CONVERSION OF BIOMASS FOR FUELS

PRODUCTION

is attributed to reduced pore volume caused by drying. Figure 5 demonstrates that the in vitro digestibility of alkali-pretreated hardwoods increases substantially as lignin content decreases. L i g n i n is hypothesized to interfere with hydrolysis by blocking access to cellulose fibers and irreversibly binding hydrolytic enzymes (2). This is supported by research showing that cellulase enzyme binds to lignaceous materials (22). The location and nature of lignin-carbohydrate bonding may also affect enzymatic digestibility, since materials with similar lignin content such as jute and hay can exhibit markedly different digestibilities (23). It is unclear, however, to what extent the presence of lignin physically restricts or otherwise blocks enzyme binding to lignocellulosic surfaces. Hemicellulose Content. Removal of hemicellulose dramatically improves the digestibility of hardwood species (24) by increasing porosity and thereby the specific surface area accessible to hydrolytic enzymes (18,19). Figure 6 shows enzymatic digestibility as a function of the amount of xylan removed by dilute acid pretreatment of aspen wood. Digestibility increases i n an approximately linear fashion with increasing extent of xylan removed. There is still debate about which factors most influence enzymatic hydrolysis of pretreated lignocellulosic materials. In addition to the factors discussed above, the composition and amount of extraneous components, which vary widely between different biomass types, strongly influence resistance to enzymatic attack and/or susceptibility to thermo-chemi-mechanical pretreatments. Often a small fraction of a pretreated material remains refractory to enzymatic conversion. M a n y researchers, including Fan et al. (4) and L y n d (25), believe that external surface area and reactivity determine the rate and extent of enzymatic hydrolysis. This theory ascribes subtheoretical saccharification yields to highly crystalline regions of cellulose that remain impervious to enzymatic attack, even following pretreatment. Other researchers hypothesize that structural features control the extent to which a lignocellulosic material is accessible to enzymatic attack. Stone et al. (75), Grethlein (26), and Grohmann (27), for example, believe that the porosity of the lignin-hemicellulose sheath controls enzyme accessibility and hydrolysis rates. This view attributes subtheoretical yields to regions of cellulose that remain inaccessible to enzymatic attack. It is difficult to determine which of these hypotheses is correct, especially since the relative importance of enzyme accessibility and cellulose crystallinity vary depending on the lignocellulosic material used and the pretreatment method employed. Examples undoubtedly exist where either accessible surface area or reactivity control the rate and extent of hydrolysis. However, susceptibility to enzymatic hydrolysis is generally affected by both factors. Pretreatment Techniques Methods of pretreating lignocellulosic biomass that w i l l be described i n some detail in the following sections include mechanical comminution, alkali swelling, acid hydrolysis, steam and other fiber explosion techniques, and exposure to supercritical fluids. There are also a variety of pulping techniques that are potential pretreatments, but it is doubtful that pulping processes can be economically used to pretreat



15.

MCMILLAN


60

I

I

301

Biomass

I

I

I

I

o A s p e n (quaking), b a s s w o o d ( A m e r i c a n ) o A s p e n (bigtooth)

50

^

40

>_Maple (silver)

—

—

o Birch (paper) QAsriV (black) \

CO

S.30

o M a p l e (sugar)

O

£

20

o O a k (white) —

Birch o (yellow)

Elm o (American) 10

18

F i g u r e 5.

I

I

19

20

I

I

21 22 Lignin Content, %

>QL

>c< O a k (red)-

I

I

23

24

25

Effect of lignin content on the in vitro digestibility of NaOH-pretreated hardwoods. Adjacent to individual data points are the names of the hardwood species to which the point refers. Adapted with permission from ref. L (Copyright 1976; John W i l e y & Sons: N e w Y o r k , N Y . )



302

F i g u r e 6.

ENZYMATIC CONVERSION O F BIOMASS FOR FUELS PRODUCTION

Digestibility as a function of percent xylan removed. Reprinted with permission ref. 24. (Copyright 1985; John W i l e y & Sons: N e w Y o r k , NY.)


15.

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lignocellulosics i n the context of renewable fuels and chemicals production. Pulping processes, which are generally applied for relatively high-valued products such as paper, generally exhibit relatively low overall carbohydrate yields and are chemical intensive. Large capital investments are typically required to install integrated chemical recovery systems. For these reasons, potential pretreatments based on pulping techniques are not considered here. M e c h a n i c a l C o m m i n u t i o n . A l l pretreatment processes involve an initial mechanical step i n which the biomass is comminuted by a combination of chipping, grinding, and milling. Steam explosion processes use explosive decompression to significantly reduce the particle size of coarsely chipped biomass, whereas other pretreatment processes typically employ a secondary grinding or milling step to further reduce the particle size of chipped biomass. Chipped biomass has a characteristic dimension of 1 c m to 3 c m , in comparison to 0.2 m m to 2.0 m m for milled or ground material. Power requirements for comminution are strongly influenced by particle size and biomass type. Power requirements increase rapidly with decreasing particle size, as shown in Figure 7 for ball milling of municipal solid waste. M o r e than 25% of the total energy of the substrate is required to m i l l this material to particle sizes below 150 pm. Energy requirements for milling coarse biomass chips into fine particles are also strongly influenced by substrate. For example, the specific milling energy required to produce 0.64-cm (0.25-in) or smaller particles is about two-and-a-half-fold higher for aspen wood chips than for corn cobs; for particles 1.27-cm (0.50-in.) or smaller, the difference is greater than eightfold (28). M u c h of the pretreatment research reported in the literature is carried out using 60 mesh (250 p m = 0.25 mm) wood flour as a substrate (e.g., the extensive work of Grethlein et al.). The energy costs for milling biomass particles to such small sizes are likely to be prohibitively high for low-profit-margin, high-volume, industrial-scale pretreatment processes (7). Some researchers have concluded that milling processes, especially vibratory ball milling, increase the reactivity of cellulose, in addition to increasing the external surface area. Millett et al. (7) report that vibratory ball milling breaks down the crystallinity of cellulose to an extent that complete digestion of milled biomass is possible. Fan et al. (4) also conclude that ball milling causes a reduction in cellulose crystallinity, although they observe that the efficacy of this approach is strongly dependent on the type of material being milled. They cite the additional advantage that ball-milled material is often more dense than the native material and can therefore be slurried to a higher solids loading. A l k a l i Swelling. Soaking in alkaline solutions, most notably dilute N a O H , has been used to pretreat lignocellulosic materials. The efficacy of alkali pretreatment is dependent upon lignin content. A s reference to Figure 5 shows, for hardwoods soaking in N a O H shows increasing efficacy as lignin content decreases from 24% to 18%. N o effect of dilute N a O H pretreatment is observed for softwoods in which the lignin content is 26%-35% (7). The efficacy of dilute N a O H pretreatment is somewhat higher for straws than for hardwoods owing, in part, to the lower lignin content of straws.




304


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Biomass

Treatment with alkali causes lignocellulosic materials to swell, and increased swelling leads to higher digestibility. Certain swelling agents such as N a O H , amines, and anhydrous N H cause limited or intercrystalline swelling. N a O H treatment followed by water washing yields a lignocellulosic residue with a polyionic character, since sodium ions remain to act as counter charges to carboxylate ions. This polyionic character promotes swelling relative to N a O H treatment followed by acid washing i n which the sodium ions are displaced by protons. Other chemicals, such as concentrated sulfuric acid ( H S 0 ) or hydrochloric acid (HC1), or high concentrations of cellulose solvents like cupran, cuen, or cadoxen, cause intracrystalline swelling; these chemicals penetrate into the cellulose crystal structure and totally dissolve (or hydrolyze) holocellulose (329). Although they are powerful cellulose solubilization agents, concentrated acids and metal chelate cellulose solvents are toxic, corrosive, and hazardous i n nature. For these reasons, i n addition to high recovery costs, cellulose solvents may not be appropriate for large-scale pretreatment processes (7). The mechanism of alkali pretreatment is postulated to be saponification of intermolecular ester bonds crosslinking xylan hemicelluloses and other polymeric materials, such as lignin or other hemicelluloses (23). Saponification of the uronic ester linkages in 4-O-methyl-D-glucuronic acids pendant along the xylan chain readily occurs i n the presence of alkali. (So does saponification of acetyl groups pendant along the xylan backbone, which has an autocatalytic effect on hemicellulose hydrolysis when it occurs-see below.) The removal of crosslinks apparently permits swelling beyond normal water-swollen dimensions. Pore volume measurements indicate that intraparticle porosity and channel size increase following dilute alkali treatment (23). There is also a phase change i n the cellulose crystal structure (7-4). 3


2

4

Gaseous and aqueous ammonia ( N H ) cause swelling by a similar mechanism (30). A t ambient conditions, longer reaction times are generally required; for comparable reaction times, digestibility is lower following N H treatment than it is following N a O H treatment. In N H alkali treatment, rather than saponification of ester bonds to yield ionized carboxyl groups, ammonolysis of ester bonds occurs to form amides. The decreased swelling characteristics observed following N H pretreatment indicate reduced polyionic character relative to water-washed NaOH-treated materials. However, chemically combined nitrogen (via amide formation) or residual ammonia/ammonium increases overall nitrogen content, and thus may improve the value of NH -treated materials as feeds for ruminants. 3

3

3

3

3

D i l u t e A c i d H y d r o l y s i s . Exposure to concentrated acid and then later to dilute acid was originally used to directly saccharify lignocellulosic materials (29). A b o v e moderate temperatures, however, direct saccharification suffered from l o w yields because of sugar decomposition. Thus, exposure to dilute acid at high temperature has been developed as a pretreatment prior to enzymatic saccharification to improve overall saccharification rates and yields. Whereas older acid-based saccharification processes largely destroyed the predominantly xylan hemicellulosic fraction, more recently developed processes use less severe conditions and achieve high xylan to xylose conversion yields. Achieving high xylan to xylose conversion yields is necessary to achieve favorable overall process economics because xylan accounts for up to a third of total carbohydrate in many lignocellulosic materials (37). N R E L


306

ENZYMATIC CONVERSION O FBIOMASS FOR FUELS PRODUCTION

currently favors dilute acid hydrolysis as the pretreatment process o f choice for a commercial biomass-to-ethanol process (52,33). In N R E L ' s dilute acid process, chipped and/or milled biomass particles of nominal 1-mm size are impregnated with approximately 1% (w/w) H S 0 (liquid basis) and then incubated at 140°-160°C for a period ranging from several minutes to an hour. N R E L researchers have characterized the susceptibility of a variety o f short rotation woody and herbaceous crops and agricultural residues to this dilute acid pretreatment process (9-1124,34-36). High-temperature dilute acid treatment causes hemicellulose to hydrolyze. Hemicellulose removal increases porosity and improves enzymatic digestibility, as shown i n Figure 6. Hemicellulose hydrolysis rates vary with temperature, but for most short rotation woody species and herbaceous crops, complete hemicellulose hydrolysis occurs i n 5-10 min at 160°C, or i n 30-60 m i n at 140°C. M a x i m u m enzymatic digestibility of the cellulosic fraction usually coincides with complete hemicellulose removal. Dilute acid hydrolysis forms the basis of many pretreatment processes. F o r example, steam explosion and autohydrolysis pretreatments are also based on hightemperature dilute acid-catalyzed hydrolysis o f biomass. The dominant factors influencing hydrolysis yields for a variety of pretreatment processes can be understood by examining the kinetics o f dilute acid hydrolysis. The overall goal of dilute acid pretreatment is to achieve high yield while minimizing the breakdown o f sugars into decomposition products. Cellulose hydrolysis, hemicellulose hydrolysis, and sugar degradation reactions can be considered as pseudo first-order processes, as shown by Equation (1). H i g h selectivity pretreatment is achieved by maximizing the ratio o f the rate constants, l ^ / k ^ , where the subscript i refers to rate constants for either cellulose (c) or hemicellulose (h) hydrolysis. This is accomplished by using acid-catalyzed, high-temperature, shortresidence-time processes.


2

glucose cellulose

cellobiose

\2

4

degradation products

oligosaccharides (1)

pentoses hemicellulose

"'hi

hexoses

*A2

degradation products

oligosaccharides Literature and data on the kinetics of dilute acid pretreatment primarily exist for two types o f processes, l o w solids loading (5%-10% [w/w]), high-temperature (T > 160°C), continuous-flow processes (37-42) and higher solids loading (10%-40% [w/w]), lower temperature (T < 160°C), batch processes (24,43-48). The kinetics of high-temperature wood saccharification catalyzed by dilute acid were first extensively investigated by Saeman (43), who demonstrated that cellulose hydrolysis and monomer sugar decomposition follow first-order kinetics. Although Root (44) and Kwarteng (39) subsequently showed that decomposition kinetics become more


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complex as degradation products accumulate, the approximation is usually made that kinetics remain first order. A generalized relationship for sequential hydrolysis and decomposition of both cellulose and hemicellulose is formulated by representing reactants and products on an equivalent sugar monomer basis. Equation (2) shows the general case in which biomass carbohydrate species A (cellulose or xylan) hydrolyzes to B (glucose/cellobiose or xylose), which then decomposes to C (hydroxymethylfurfural from glucose and furfural from xylose).

Expressions for the net rate of formation of components A , B , and C can be integrated to determine the concentrations of A and B as a function of time and initial conditions. Equations (3) and (4) give the concentrations of A and B , respectively, for the case of batch reaction with no heat or mass transfer limitations (43,49). (3)

(4)

In these equations Q represents the concentration of component i at time t and Q denotes the initial concentration. Grethlein (49) showed that the concentration of product B is maximized at time t,opf L

(5)

opt

In Equation 4, K is the selectivity ratio defined by

K

=

(6)

and Q is the lumped initialization and selectivity parameter given by

Enzymatic Conversion of Biomass for Fuels Production - American

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