Ind. Eng. Chem. Res. 2009, 48, 3713–3729
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Methods for Pretreatment of Lignocellulosic Biomass for Efficient Hydrolysis and Biofuel Production Parveen Kumar,†,| Diane M. Barrett,‡ Michael J. Delwiche,§ and Pieter Stroeve*,† Departments of Chemical Engineering and Materials Science, Food Science and Technology, and Biological and Agricultural Engineering, UniVersity of California DaVis, DaVis, California 95616
Biofuels produced from various lignocellulosic materials, such as wood, agricultural, or forest residues, have the potential to be a valuable substitute for, or complement to, gasoline. Many physicochemical structural and compositional factors hinder the hydrolysis of cellulose present in biomass to sugars and other organic compounds that can later be converted to fuels. The goal of pretreatment is to make the cellulose accessible to hydrolysis for conversion to fuels. Various pretreatment techniques change the physical and chemical structure of the lignocellulosic biomass and improve hydrolysis rates. During the past few years a large number of pretreatment methods have been developed, including alkali treatment, ammonia explosion, and others. Many methods have been shown to result in high sugar yields, above 90% of the theoretical yield for lignocellulosic biomasses such as woods, grasses, corn, and so on. In this review, we discuss the various pretreatment process methods and the recent literature that has reported on the use of these technologies for pretreatment of various lignocellulosic biomasses. 1. Introduction Long-term economic and environmental concerns have resulted in a great amount of research in the past couple of decades on renewable sources of liquid fuels to replace fossil fuels. Burning fossil fuels such as coal and oil releases CO2, which is a major cause of global warming.1 With only 4.5% of the world’s population, the United States is responsible for about 25% of global energy consumption and 25% of global CO2 emissions.1 The average price of gasoline in 2005 was $2.56 per gallon, which was $0.67 higher than the average price of gasoline in the previous year.1 Yet, in June 2008, the average price of gasoline in the United States reached $4.10 per gallon.2 Conversion of abundant lignocellulosic biomass to biofuels as transportation fuels presents a viable option for improving energy security and reducing greenhouse emissions.3 Unlike fossil fuels, which come from plants that grew millions of years ago, biofuels are produced from plants grown today. They are cleaner-burning than fossil fuels, and the short cycle of growing plants and burning fuel made from them does not add CO2 to the atmosphere. It has been reported that cellulosic ethanol and ethanol produced from other biomass resources have the potential to cut greenhouse gas emissions by 86%.4 Lignocellulosic materials such as agricultural residues (e.g., wheat straw, sugarcane bagasse, corn stover), forest products (hardwood and softwood), and dedicated crops (switchgrass, salix) are renewable sources of energy. These raw materials are sufficiently abundant and generate very low net greenhouse emissions. Approximately 90% of the dry weight of most plant materials is stored in the form of cellulose, hemicellulose, lignin, and pectin.1 The presence of lignin in lignocelluloses leads to a protective barrier that prevents plant cell destruction by fungi and bacteria for conversion to fuel. For the conversion of biomass to fuel, the cellulose and hemicellulose must be broken * To whom correspondence should be addressed. E-mail: pstroeve@ ucdavis.edu. † Department of Chemical Engineering and Materials Science. | Present address: Department of Chemical Engineering and Materials Science, University of Minnesota Twin Cities, Minneapolis, MN 55455. ‡ Department of Food Science and Technology. § Department of Biological and Agricultural Engineering.
down into their corresponding monomers (sugars), so that microorganisms can utilize them. Three major hydrolysis processes are typically used to produce a variety of sugars suitable for ethanol production: dilute acid, concentrated acid, and enzymatic hydrolysis.5 Hemicellulose can be readily hydrolyzed by dilute acids under moderate conditions, but much more extreme conditions are needed for cellulose hydrolysis. In the dilute-acid process, the reaction is carried out at high temperature and pressure, and because of low yields of glucose from cellulose in the hydrolysis step, the ethanol yield is low. The use of concentrated acid in the hydrolysis process can yield higher quantities of ethanol because of the approximately 100% conversion to glucose from cellulose. The dilute-acid hydrolysis process uses high temperatures (160-230 °C) and pressures (∼10 atm).6 The acid concentration in the dilute-acid hydrolysis process is in the range of 2-5%.5,7 The acid concentration used in the concentrated-acid hydrolysis process is in the range of 10-30%.5 Lower operating temperatures ( 160 °C), continuous-flow process for low solids loadings (weight of substrate/weight of reaction mixture ) 5-10%)78,79 and a low-temperature (T < 160 °C), batch process for high solids loadings (10-40%)76 The most widely used and tested approaches are based on dilute sulfuric
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acid. However, nitric acid,80 hydrochloric acid,81,82 and phosphoric acid81 have also been tested. Numerous plant materia1s have been examined, inc1uding legume byproducts; reed canary grass; corn (husks, cobs, and stover); mixed hardwood (10% map1e and 90% birch); and hardwood bark from aspen, poplar, and sweet gum. Thompson et al.83 used dilute H2SO4 to pretreat mixed hardwood and observed that the crystallinity index, although not a function of pretreatment temperature, still increased as a consequence of pretreatment. The removal of amorphous cellulose fractions, leaving a more crystalline fraction behind, could explain this observation. Recently, acid pretreatment has been used on a wide range of feedstocks ranging from hardwoods to grasses and agricultural residues. Ishizawa et al.84 evaluated whether porosity was one of the factors governing the overall enzymatic digestibility of the cellulose in dilute-acid-pretreated biomass. Corn stover was subjected to dilute H2SO4 pretreatment in a pilot-scale vertical reactor using a fixed residence time of ∼1 min at temperatures ranging from 180 to 200 °C, solid loadings between 25% and 35% (w/w), and acid loadings of 0.03-0.06 g of acid/g of dry biomass. All of the pretreated samples showed higher pore volumes than untreated corn stover. The authors determined that porosity might be a factor for materials with low digestibility but that it is not so much of a factor for lignocellulosic materials with high digestibility. Lu et al.85 carried out pretreatment of corn stover for sulfuric acid concentrations of 2%, 4%, and 6% at 80, 100, and 120 °C. The optimum conditions for corn stover pretreatment were a H2SO4 concentration of 2.0% and a reaction time of 43 min at 120 °C. Up to 77% xylose yield was obtained, whereas the glucose yield was only 8.4%. The corresponding solid phase showed good susceptibility toward enzymatic hydrolysis, leading to solutions containing up to 42.1 g of glucose/100 g of substrate, equivalent to a conversion yield of 70% under the optimum conditions. The production of fermentable sugars from olive-tree biomass by dilute-acid pretreatment and further saccharification of the pretreated solid residues was studied by Cara and co-workers.86 Pretreatment was performed at 0.2%, 0.6%, 1.0%, and 1.4% (w/w) sulfuric acid concentrations, and the temperature was varied in the range of 170-210 °C. Sugar recoveries in both the liquid fraction issued from pretreatment (prehydrolysate) and the water-insoluble solid were taken into consideration. A maximum of 83% of hemicellulosic sugars in the raw material was recovered in the prehydrolysate obtained at 170 °C and 1% H2SO4 concentration, but the enzyme accessibility of the corresponding pretreated solid was not very high. A maximum enzymatic hydrolysis yield of 76.5% was obtained from a pretreated solid at 210 °C and 1.4% acid concentration. Cellulose solubilization was detected, but sugar recovery in the prehydrolysate was the poorest among all of the experiments compared. To take into account the fermentable sugars generated by pretreatment and glucose released by enzymatic hydrolysis, an overall sugar yield was calculated. The maximum value of 36.3 g of sugar/100 g of raw material (75%) was obtained for the pretreatment of olive-tree biomass at 180 °C and 1% H2SO4 concentration. Dilute-acid pretreatment improved the enzymatic hydrolysis process compared to water pretreatment. In a recent article, Yat et al. reported dilute-acid pretreatment of four timber species (aspen, balsam fir, basswood, and red maple) and switchgrass using dilute H2SO4 for 50 g of dry biomass/L under similar conditions.1 Xylose formation and degradation at various reactor temperatures (160-190 °C), sulfuric acid concentrations [0.25-1.0% (w/v)], and particle sizes (28-10/20 mesh) in a glass-lined 1-L well-mixed batch reactor were studied. Reaction
rates for the generation of xylose from hemicellulose and the generation of furfural from xylose were found to depend strongly on both temperature and acid concentration. Maximum yields ranged from 70% (balsam) to 94% (switchgrass) for xylose, from 10.6% to 13.6% for glucose, and from 8.6% to 58.9% for other minor sugars. Xylose degradation varied linearly as a function of acid concentration. Addition of very dilute sulfuric acid (about 0.1% versus the 0.7-3.0% typical for batch dilute-acid technology) in a flowthrough reactor configuration is effective at acid levels lower than 0.1%. Despite achieving excellent hemicellulose sugar yields and highly digestible cellulose with low acid loadings, the equipment configurations and the high ratio of water to solids employed in flow-through systems require significant energy for pretreatment and product recovery. Although dilute-acid pretreatment can significantly improve cellulose hydrolysis, its cost is usually higher than those of physicochemical pretreatment processes such as steam explosion or AFEX. Neutralization of pH is necessary for the downstream enzymatic hydrolysis or fermentation processes. Dilute-acid pretreatment is also known to have a negative influence on the enzymatic hydrolysis of biomass. In a recent article, Selig et al.87 reported the formation of spherical droplets on the surface of residual corn stover following dilute-acid pretreatment at high temperature. They suggested that the droplets formed were composed of lignins and possible lignin-carbohydrate complexes. It was demonstrated that these droplets were produced from corn stover during pretreatment under neutral and acidic pHs at and above 130 °C and that they can deposit onto the surface of residual biomass. The deposition of droplets produced under certain pretreatment conditions (acidic pH, T > 150 °C) and captured on pure cellulose was shown to have a negative effect on the enzymatic saccharification of the substrate. Therefore, it is extremely important to carefully examine the appropriate dilute-acid pretreatment of lignocellulose biomass species. It has been shown that materials that have been subjected to acid hydrolysis can be harder to ferment because of the presence of toxic substances.13 Further, acid pretreatment results in costly materials of construction, high pressures, neutralization and conditioning of hydrolysate prior to biological steps, slow cellulose digestion by enzymes, and nonproductive binding of enzymes to lignin.88 4.3.3. Alkaline Hydrolysis. Some bases can be used for the pretreatment of lignocellulosic materials, and the effect of alkaline pretreatment depends on the lignin content of the materials.19,29 Alkali pretreatment processes utilize lower temperatures and pressures than other pretreatment technologies.9 Alkali pretreatment can be carried out at ambient conditions, but pretreatment times are on the order of hours or days rather than minutes or seconds. Compared with acid processes, alkaline processes cause less sugar degradation, and many of the caustic salts can be recovered and/or regenerated. Sodium, potassium, calcium, and ammonium hydroxides are suitable alkaline pretreatment agents. Of these four, sodium hydroxide has been studied the most.89-92 However, calcium hydroxide (slake lime) has been shown to be an effective pretreatment agent and is the least expensive per kilogram of hydroxide. It is possible to recover calcium from an aqueous reaction system as insoluble calcium carbonate by neutralizing it with inexpensive carbon dioxide; the calcium hydroxide can subsequently be regenerated using established lime kiln technology. The apparatus for the laboratory-scale lime pretreatment of biomass is shown in Figure 7.93 The process of lime pretreatment involves slurrying the lime with water, spraying it onto the biomass material, and storing
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Figure 7. Schematic diagram of the jacketed reactor system for lime pretreatment under nonoxidative (N2 supply) and oxidative (air supply) conditions. (Reproduced from ref 93.)
the material in a pile for a period of hours to weeks. The particle size of the biomass is typically 10 mm or less. Elevated temperatures reduce contact time. The enzymatic hydrolysis of lime-treated biomass is affected by structural features resulting from the treatment.93 These are the extents of acetylation, lignification, and crystallization. Lime pretreatment removes amorphous substances (e.g., lignin and hemicellulose), which increases the crystallinity index. Chang et al.94 reported correlations between enzymatic digestibility and three structural factors: lignin content, crystallinity, and acetyl content. They concluded that (1) extensive delignification is sufficient to obtain high digestibility regardless of acetyl content and crystallinity, (2) delignification and deacetylation remove parallel barriers to enzymatic hydrolysis; and (3) crystallinity significantly affects initial hydrolysis rates but has less of an effect on ultimate sugar yields. These results indicate that an effective lignocellulose treatment process should remove all of the acetyl groups and reduce the lignin content to about 10% in the treated biomass.93 Therefore, alkaline pretreatment can play a significant role in exposing the cellulose to enzyme hydrolysis. Lignin removal increases enzyme effectiveness by eliminating nonproductive adsorption sites and by increasing access to cellulose and hemicellulose. Kim et al.93 pretreated corn stover with excess calcium hydroxide [0.5 g of Ca(OH)2/g of raw biomass] in nonoxidative (in the presence of nitrogen) and oxidative (in the presence of air) conditions at 25, 35, 45, and 55 °C. The enzymatic digestibility of lime-treated corn stover was affected by the change of structural features such as acetylation, lignification, and crystallization resulting from the treatment. Extensive delignification required oxidative treatment and additional consumption of lime [up to 0.17 g of Ca(OH)2/g of biomass]. Deacetylation reached a plateau within 1 week, and there were no significant differences between nonoxidative and oxidative conditions at 55 °C; both conditions removed approximately 90% of the acetyl groups in 1 week at all temperatures studied. Delignification highly depended on temperature and the presence of oxygen. Lignin and hemicellulose
were selectively removed or solubilized, but cellulose was not affected by lime pretreatment at mild temperatures (25-55 °C). The degree of crystallinity increased slightly with delignification (from 43% to 60%) because amorphous components such as lignin and hemicellulose were removed. Lee et al.95 reported that the rate of enzymatic hydrolysis depends on enzyme adsorption and the effectiveness of the adsorbed enzymes, instead of the diffusive mass transfer of enzyme. Lignin removal increases enzyme effectiveness by eliminating nonproductive adsorption sites and by increasing access to cellulose and hemicellulose. Kong et al.96 reported that alkalis remove acetyl groups from hemicellulose (mainly xylan), thereby reducing the steric hindrance of hydrolytic enzymes and greatly enhancing carbohydrate digestibility. They concluded that the sugar yield in enzymatic hydrolysis is directly associated with acetyl group content. Lime has been used to pretreat wheat straw (85 °C for 3 h),97 poplar wood (150 °C for 6 h with 14 atm of oxygen),98 switchgrass (100 °C for 2 h),99 and corn stover (100 °C for 13 h).100 Karr et al.101 showed that pretreatment with slake lime (calcium hydroxide) increased the enzymatic hydrolysis of corn stover by a factor of 9 compared to that of untreated corn stover. The effect of lime pretreatment on the enzymatic hydrolysis of corn stover is shown in Figure 8.101 The optimal pretreatment conditions were determined to be a lime loading 0.075 g of Ca(OH)2/g of dry biomass, a water loading of 5 g of H2O/g of dry biomass, and heating for 4 h at 120 °C. It was suggested that that pretreatment with lime can lead to corn stover polysaccharide conversions approaching 100%. Dilute NaOH treatment of lignocellulosic materials has been found to cause swelling, leading to an increase in internal surface area, a decrease in the degree of polymerization, a decrease in crystallinity, separation of structural linkages between lignin and carbohydrates, and disruption of the lignin structure.29 The digestibility of NaOH-treated hardwood was reported to increase from 14% to 55% with a decrease of lignin content from 24-55% to 20%. However, no effect of dilute NaOH pretreat-
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Figure 8. Sugar yields from enzymatic hydrolysis of corn stover after lime pretreatment at various times and temperatures. Pretreatment conditions: 0.1 g of Ca(OH)2/g of dry biomass and 10 g of H2O/g of dry biomass. Hydrolysis conditions: 5 FPU of cellulose/g of dry biomass for 72 h at 50 °C. (Reproduced from ref 101.)
ment was observed for softwoods with lignin content greater than 26%.24 Dilute NaOH pretreatment was also found to be effective for the hydrolysis of straws with relatively low lignin contents of 10-18%.102 Chosdu et al.103 used a combination of irradiation and 2% NaOH for pretreatment of corn stalk, cassava bark, and peanut husk.103 The glucose yield of corn stalk was 20% in untreated samples compared to 43% after treatment with electron beam irradiation at a dose of 500 kGy and 2% NaOH, but the glucose yields of cassava bark and peanut husk were only 3.5% and 2.5%, respectively. Hu et al.104,105 used radio-frequency- (RF-) based dielectric heating in the NaOH pretreatment of switchgrass to enhance its enzymatic digestibility. Because of the unique features of RF heating (i.e., volumetric heat transfer, deep heat penetration of the samples, etc.), switchgrass could be treated on a large scale, at high solids content, and with a uniform temperature profile. At 20% solids content, RF-assisted alkali pretreatment (at 0.1 g of NaOH/g of biomass loading and 90 °C) resulted in a higher xylose yield than the conventional heating pretreatment. The optimal particle size and alkali loading in the RF pretreatment were determined to be 0.25-0.50 mm and 0.25 g of NaOH/g of biomass, respectively. The switchgrass used in this study had a composition of 33.6% glucan, 19.3% xylan, 21.4% lignin, and 3.9% ash. Glucose and xylose are the major sugars (>90%) in switchgrass, therefore; the yields of these two types of sugars were used to evaluate the efficiency of the pretreatment process. The sugar yield was expressed as grams of sugar released per 100 g (dry weight) of original, untreated biomass (switchgrass). Based on the glucan and xylan contents, the maximum yields for glucose and xylose would be 33.6 × 1.111 ) 37.3 g/100 g of biomass and 19.3 × 1.136 ) 21.9 g/100 g of biomass, respectively. At alkali loadings of 0.20-0.25 g of NaOH/g of biomass, a heating temperature of 90 °C, and a solids content of 20%, the glucose, xylose, and total sugar yields from the combined RF pretreatment and enzymatic hydrolysis were 25.3, 21.2, and 46.5 g/100 g of biomass, respectively. Hu and
co-workers soaked switchgrass in NaOH solutions of different concentrations and then treated the samples by microwave or conventional heating. With alkali loadings of 0.05-0.3 g of alkali/g of biomass, microwave pretreatment resulted in higher sugar yields than conventional heating, with the highest yield (90% of maximum potential sugars) being achieved at an alkali loading of 0.1 g/g. These results suggest that microwave-assisted alkali treatment is an efficient way to improve the enzymatic digestibility of switchgrass. Ammonia has also been used as a pretreatment to remove lignin. Iyer et al. described an ammonia recycled percolation process (temperature ) 170 °C, ammonia concentration ) 2.5-20%, reaction time ) 1 h) for the pretreatment of a corn cobs/stover mixture and switchgrass.106 The efficiency of delignification was 60-80% for corn cobs and 65-85% for switchgrass. 4.3.4. Oxidative Delignification. Lignin biodegradation could be catalyzed by the peroxidase enzyme with the presence of H2O2.107 The pretreatment of cane bagasse with hydrogen peroxide greatly enhanced its susceptibility to enzymatic hydrolysis. About 50% of the lignin and most of the hemicellulose were solubilized by 2% H2O2 at 30 °C within 8 h, and 95% efficiency of glucose production from cellulose was achieved in the subsequent saccharification by cellulase at 45 °C for 24 h.107 Bjerre et al. used wet oxidation and alkaline hydrolysis of wheat straw (20 g of straw/L, 170 °C, 5-10 min) and achieved an 85% conversion yield of cellulose to glucose.102 Wet oxidation combined with base addition readily oxidizes lignin from wheat straw, thus making the polysaccharides more susceptible to enzymatic hydrolysis. Furfural and hydroxymethylfurfural, known inhibitors of microbial growth when other pretreatment systems are applied, were not observed following the wet oxidation treatment. 4.3.5. Organosolv Process. The organosolvation method is a promising pretreatment strategy, and it has attracted much
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Figure 9. Flowchart of the laboratory-scale ethanol organosolv process. (Reproduced from ref 112.)
attention and demonstrated the potential for utilization in lignocellulosic pretreatment.108 In the organosolvation process, an organic or aqueous organic solvent mixture with inorganic acid catalysts (HCl or H2SO4) is used to break the internal lignin and hemicellulose bonds. The solvents commonly used in the process are methanol, ethanol, acetone, ethylene glycol, triethylene glycol, and tetrahydrofurfuryl alcohol.109,110 Organic acids such as oxalic, acetylsalicylic, and salicylic acids can also be used as catalysts in the organosolvation process.111 In essence, the organosolv process involves simultaneous prehydrolysis and delignification of lignocellulosic biomass supported by organic solvents and, usually, dilute aqueous acid solutions. A flowchart for organosolvation pretreatment of biomass is shown in Figure 9.112 A high yield of xylose can usually be obtained with the addition of acid. Pulps with residual lignin ranging from 6.4% to 27.4% (w/w) have been prepared from mixed softwoods using a biorefining technology called the lignol process, which is based on an aqueous ethanol organosolvation extraction.112 This process uses a blend of ethanol and water in the ratio of 50:50 (w/w) at ∼200 °C and 400 psi to extract most of the lignin from wood chips or other lignocellulosic biomass. Lignin is recovered as a fine precipitate by flashing the pulping liquor to atmospheric pressure, followed by rapid dilution with water. Other coproducts such as hemicellulose sugars and furfural are recovered from the water soluble stream. Pan et al.112 found that all pulps were readily hydrolyzed without further delignification and more than 90% of the cellulose in low lignin pulps (
