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Bioconversion of cereal straw to bioethanol is becoming an attractive alternative to conventional fuel ethanol production from grains. In this work, the best operational conditions for steam-explosion pretreatment of wheat straw for ethanol production by a simultaneous saccharification and fermentation process were studied, using diluted acid [H_2SO_4 0.9% (w/w)] and water as preimpregnation agents. Acid- or water-impregnated biomass was steam-exploded at different temperatures (160–200°C) and residence times (5, 10, and 20 min). Composition of solid and filtrate obtained after pretreatment, enzymatic digestibility and ethanol production of pretreated wheat straw at different experimental conditions was analyzed. The best pretreatment conditions to obtain high conversion yield to ethanol (approx 80% of theoretical) or cellulose-rich residue after steam-explosion were 190°C and 10 min or 200°C and 5 min, in acid-impregnated straw. However, 180°C for 10 min in acid-impregnated biomass provided the highest ethanol yield referred to raw material (140 L/t wheat straw), and sugars recovery yield in the filtrate (300 g/kg wheat straw).

The transport of catalysts (chemicals and enzymes) within plant biomass is believed to be a major bottleneck during thermochemical pretreatment and enzymatic conversion of lignocellulose. Subjecting biomass to size reduction and mechanical homogenization can reduce catalyst transport limitations; however, such processing adds complexity and cost to the overall process. Using high-resolution light microscopy, we have monitored the transport of an aqueous solution of Direct Blue-I (DB-I) dye through intact corn internodes under a variety of impregnation conditions. DB-I is a hydrophilic anionic dye with affinity for cellulose. This model system has enabled us to visualize likely barriers and mechanisms of catalyst transport in corn stems. Microscopic images were compared with calculated degrees of saturation (i.e., volume fraction of internode void space occupied by dye solution) to correlate impregnation strategies with dye distribution and transport mechanisms. Results show the waxy rind exterior and air trapped within individual cells to be the major barriers to dye transport, whereas the vascular bundles, apoplastic continuum (i.e., the intercellular void space at cell junctions), and fissures formed during the drying process provided the most utilized pathways for transport. Although representing only 20–30% of the internode volume, complete saturation of the apoplast and vascular bundles by fluid allowed dye contact with a majority of the cells in the internode interior.

Seven cellulase preparations from Penicillium and Trichoderma spp. were evaluated for their ability to hydrolyze the cellulose fraction of hardwoods (yellow poplar and red maple) pretreated by organosolv extraction, as well as model cellulosic substrates such as filter paper. There was no significant correlation among hydrolytic performance on pretreated hardwood, based on glucose release, and filter paper activity. However, performance on pretreated hardwood showed significant correlations to the levels of endogenous β-glucosidase and xylanase activities in the cellulase preparation. Accordingly, differences in performance were reduced or eliminated following supplementation with a crude β-glucosidase preparation containing both activities. These results complement a previous investigation using softwoods pretreated by either organosolv extraction or steam explosion. Cellulase preparations that performed best on hardwood also showed superior performance on the softwood substrates.

Barley is an abundant crop in Europe, which makes its straw residues an interesting cellulose source for ethanol production. Steam pretreatment of the straw followed by enzymatic hydrolysis converts the cellulose to fermentable sugars. Prior to pretreatment the material is impregnated with a catalyst, for example, H_2SO_4, to enhance enzymatic digestibility of the pretreated straw. Different impregnation techniques can be applied. In this study, soaking and spraying were investigated and compared at the same pretreatment condition in terms of overall yield of glucose and xylose. The overall yield includes the soluble sugars in the liquid from pretreatment, including soluble oligomers, and monomer sugars obtained in the enzymatic hydrolysis. The yields obtained differed for the impregnation techniques. Acid-soaked barley straw gave the highest overall yield of glucose, regardless of impregnation time (10 or 30 min) or acid concentration (0.2 or 1.0 wt%). For xylose, soaking gave the highest overall yield at 0.2 wt% H_2SO_4. An increase in acid concentration resulted in a decrease in xylose yield for both acid-soaked and acid-sprayed barley straw. Optimization of the pretreatment conditions for acid-sprayed barley straw was performed to obtain yields using spraying that were as high as those with soaking. For acid-sprayed barley straw the optimum pretreatment condition for glucose, 1.0 wt% H_2SO_4 and 220°C for 5 min, gave an overall glucose yield of 92% of theoretical based on the composition of the raw material. Pretreatment with 0.2 wt% H_2SO_4 at 190°C for 5 min resulted in the highest overall xylose yield, 67% of theoretical based on the composition of the raw material.

The purpose of this research was to study the kinetics of liquefaction of crop residues. The liquefaction of corn stover in the presence of ethylene glycol and ethylene carbonate using sulfuric acid as a catalyst was studied. It was found that the liquefaction yield was a function of ratio of solvent to corn stover, temperature, residence time, and amount of catalyst. Liquefaction of corn stover was conducted over a range of conditions encompassing residence times of 0-2.5 h, temperatures of 150–170°C, sulfuric acid concentrations of 2–4% (w/w), and liquefaction reagent/corn stover ratio of 1–3. The liquefaction rate constants for individual sets of conditions were examined using a first-order reaction model. Rate constant increased with the increasing of liquefaction temperature, catalyst content, and liquefaction reagent/corn stover ratio. Reuse of liquefied biomass as liquefying agent was also evaluated. When using recycled liquefied biomass instead of fresh liquefaction reagent, the conversion is reduced. It appeared that 82% of liquefaction yield was achieved after two times of reuse.

This research investigated a novel process to prepare polyester from corn stover through liquefaction and crosslinking processes. First, corn stover was liquefied in organic solvents (90 wt% ethylene glycol and 10 wt% ethylene carbonate) with catalysts at moderate temperature under atmospheric pressure. The effect of liquefaction temperature, biomass content, and type of catalyst, such as H_2SO_4, HCl, H_3PO_4, and ZnCl_2, was evaluated. Higher liquefaction yield was achieved in 2 wt% sulfuric acid, 1/4 (w/w) stover to liquefying reagent ratio; 160°C temperature, in 2 h. The liquefied corn stover was rich in polyols, which can be directly used as feedstock for making polymers without further separation or purification. Second, polyester was made from the liquefied corn stover by crosslinking with multifunctional carboxylic acids and/or cyclic acid anhydrides. The tensile strength of polyester is about 5 MPa and the elongation is around 35%. The polyester is stable in cold water and organic solvents and readily biodegradable as indicated by 82% weight loss when buried in damp soil for 10 mo. The results indicate that this novel polyester could be used for the biodegradable garden mulch film production.

A novel method of producing food-grade xylooligosaccharides from corn stover and corn cobs was investigated. The process starts with pretreatment of feedstock in aqueous ammonia, which results delignified and xylan-rich substrate. The pretreated substrates are subjected to enzymatic hydrolysis of xylan using endoxylanase for production of xylooligosaccharides. The conventional enzyme-based method involves extraction of xylan with a strong alkaline solution to form a liquid intermediate containing soluble xylan. This intermediate is heavily contaminated with various extraneous components. A costly purification step is therefore required before enzymatic hydrolysis. In the present method, xylan is obtained in solid form after pretreatment. Water-washing is all that is required for enzymatic hydrolysis of this material. The complex step of purifying soluble xylan from contaminant is essentially eliminated. Refining of xylooligosaccharides to food-grade is accomplished by charcoal adsorption followed by ethanol elution. Xylanlytic hydrolysis of the pretreated corn stover yielded glucan-rich residue that is easily digestible by cellulase enzyme. The digestibility of the residue reached 86% with enzyme loading of 10 filter paper units/g-glucan. As a feedstock for xylooligosaccharides production, corn cobs are superior to corn stover because of high xylan content and high packing density. The high packing density of corn cobs reduces water input and eventually raises the product concentration.

Alkaline detoxification strongly improves the fermentability of diluteacid hydrolysates in the production of bioethanol from lignocellulose with Saccharomyces cerevisiae . New experiments were performed with NH_4OH and NaOH to define optimal conditions for detoxification and make a comparison with Ca(OH)_2 treatment feasible. As too harsh conditions lead to sugar degradation, the detoxification treatments were evaluated through the balanced ethanol yield, which takes both the ethanol production and the loss of fermentable sugars into account. The optimization treatments were performed as factorial experiments with 3-h duration and varying pH and temperature. Optimal conditions were found roughly in an area around pH 9.0/60°C for NH_4OH treatment and in a narrow area stretching from pH 9.0/80°C to pH 12.0/30°C for NaOH treatment. By optimizing treatment with NH_4OH, NaOH, and Ca(OH)_2, it was possible to find conditions that resulted in a fermentability that was equal or better than that of a reference fermentation of a synthetic sugar solution without inhibitors, regardless of the type of alkali used. The considerable difference in the amount of precipitate generated after treatment with different types of alkali appears critical for industrial implementation.

Following detoxification of the liquid hydrolysate produced in a corn stover pretreatment process, inhibitor levels are seen to increase with the re-addition of solids for the ensuing hydrolysis and fermentation processes. The solids that were separated from the slurry before detoxification of the liquor contain approx 60% (w/w) moisture, and contamination occurs owing to the diffusion of inhibitors from the moisture entrained in the porous structure of the corn stover solids into the bulk fluid. This evidence suggests the need for additional separation and detoxification steps to purge residual inhibitors entrained in the moisture in the solids. An overliming process to remove furans from the hydrolysate failed to reduce total organic acids concentration, so acids were removed by treatment with an activated carbon powder. Smaller carbon doses proved more efficient in removing organic acids in terms of grams of acid removed per gram of carbon powder. Sugar adsorption by the activated carbon powder was minimal.

Computational fluid dynamic simulations are employed to predict flow characteristics in a continuous auger driven reactor designed for the dilute acid pretreatment of biomass. Slurry containing a high concentration of biomass solids exhibits a high viscosity, which poses unique mixing issues within the reactor. The viscosity increases significantly with a small increase in solids concentration and also varies with temperature. A well-mixed slurry is desirable to evenly distribute acid on biomass, prevent buildup on the walls of the reactor, and provides an uniform final product. Simulations provide flow patterns obtained over a wide range of viscosities and pressure distributions, which may affect reaction rates. Results provide a tool for analyzing sources of inconsistencies in product quality and insight into future design and operating parameters.