Pretreatment of corn stover by aqueous ammonia
Introduction
Corn stover is one of the most promising renewable feedstocks for biological conversion to fuels and chemicals. Pretreatment is an essential element in the bioconversion of lignocellulosic substrates. Ammonia has a number of desirable characteristics as a pretreatment reagent. It is an effective swelling reagent for lignocellulosic materials. It has high selectivity for reactions with lignin over those with carbohydrates. It is one of the most widely used commodity chemicals with about one-fourth the cost of sulfuric acid on molar basis. Its high volatility makes it easy to recover and reuse. It is a non-polluting and non-corrosive chemical. One of the known reactions of aqueous ammonia with lignin is the cleavage of C–O–C bonds in lignin as well as ether and ester bonds in the lignin–carbohydrate complex (LCC). Indications are that ammonia pretreatment selectively reduces the lignin content in biomass. There are many advantages to removing lignin early in the conversion process before it is subjected to biological processing. Lignin is believed to be a major hindrance in enzymatic hydrolysis (Chang and Holtzapple, 2000; Cowling and Kirk, 1976; Dulap et al., 1976; Lee et al., 1995; Mooney et al., 1998; Schwald et al., 1988). Lignin and its derivatives are toxic to microorganisms and inhibit enzymatic hydrolysis. Low-lignin substrates have improved microbial activity and enzyme efficiency, eventually lowering the enzyme requirement.
Complete biomass delignification, however, is difficult because of its location within the deep cell wall (Timell, 1967), hydrophobicity, physical stiffness, strong poly-ring bonds of C–O–C, C–C, and the tendency of recondensation (LCC) during delignifying. A number of factors other than lignin have also been suggested to affect enzymatic hydrolysis, including biomass crystallinity (Caufield and Moore, 1974; Cowling and Kirk, 1976; Fan et al., 1980; Polcin and Bezuch, 1977; Sasaki et al., 1979; Schwald et al., 1988), degree of polymerization (Puri, 1984), particle size (Converse, 1993), surface area (Lee et al., 1995; Burns et al., 1989), and pore size (Grethlein, 1985; Knappert et al., 1980; Mooney et al., 1998).
We have previously investigated various pretreatment processes using a flow-through (percolation) reactor system. Among them is the ammonia recycled percolation (ARP) process which we have studied to pretreat various lignocellulosic biomass feedstocks including hardwood (Yoon et al., 1995), herbaceous biomass (Iyer et al., 1996; Kim and Lee, 1996), and pulp mill sludges (Kim et al., 2000). Process modifications have also been attempted using an additional reagent, i.e. hydrogen peroxide (Kim and Lee, 1996; Kim et al., 2000). The primary purpose of this investigation was to assess the effectiveness of the ARP pretreatment process for corn stover. We were interested in verifying the changes in chemical composition and physical characteristics caused by the pretreatment, and how those factors affected enzymatic digestibility.
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Materials
Corn stover was supplied by the National Renewable Energy Laboratory (NREL, Golden, CO). It was ground and screened. The fraction collected between 9 and 35 mesh was used in all experiments. The initial composition of corn stover was determined to be 40.19% glucan, 21.71% xylan, 2.61% arabinan, 0.29% mannan, 0.68% galactan, 18.53% Klason lignin, 2.30% acid-soluble lignin, 7.08% ash, 2.20% acetyl group, 2.90% protein, and 1.51% unaccounted for. α-Cellulose was purchased from Sigma (cat. no.
Effect of ARP treatment on composition of corn stover
On the basis of our previous investigation and the results of preliminary experiments for this work, 170 °C and 15 wt.% of ammonia concentration was chosen for the ARP treatment of corn stover (Iyer et al., 1996; Yoon et al., 1995). Compositional changes in solid and liquid samples, and their effects on enzymatic hydrolysis are summarized in Table 1 and Fig. 2.
The most significant (p⩽0.05) composition change is in the lignin. The ARP process removed 70–85% of the total lignin of the corn stover
Conclusions
Pretreatment of corn stover by aqueous ammonia was highly effective in enhancing enzymatic digestibility and reducing lignin content. The ARP process removed 70–85% of the total lignin and solubilized 40–60% of hemicellulose, but retained more than 95% of the cellulose. The carbohydrates of corn stover were well preserved during the process with total accountability being above 95% for all pretreatment conditions. Longer ARP treatment resulted in more delignification as well as higher enzymatic
Acknowledgements
This research was conducted as a part of a research project with USDA (IFAFS no. 5-36275, subcontract through Dartmouth College). We gratefully acknowledge the DRIFT tests performed for some of our samples by Professor Bruce Dale and his coworkers of Department of Chemical Engineering, Michigan State University.
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