Lignocellulosic biomass includes various agricultural residues (straws, hulls, stems,
stalks), deciduous and coniferous woods, municipal solid wastes (MSW, paper,

This chapter is not subject to U. S. copyright. Published 1997 American Chemical Society

cardboard, yard trash, wood products), waste from pulp and paper industry and herbaceous energy crops. The compositions of these materials vary. The major component is cellulose (35-50%), followed by hemicellulose (20-35%) and lignin (10­25%). Proteins, oils and ash make up the remaining fraction of lignocellulosic biomass

(2) . The structure of these materials is very complex and native biomass is resistant to an enzymatic hydrolysis. In the current model of the structure of lignocellulose, cellulose fibers are embedded in a lignin-polysaccharide matrix. Xylan may play a significant role in the structural integrity of cell walls by both covalent and non-covalent associations

(3) . The pretreatment of lignocellulosic biomass is crucial before enzymatic hydrolysis. Various pre-treatment options are available now to fractionate, solubilize, hydrolyze and separate cellulose, hemicellulose and lignin components (4-7). These include steam explosion, dilute acid treatment, concentrated acid treatment, alkaline treatment, treatment with S02, treatment with hydrogen peroxide, ammonia fiber explosion, and organic solvent treatments. In each option, the biomass is treated to reduce its size and open its structure. Pretreatment usually hydrolyzes hemicellulose to its sugars (xylose, L — arabinose, and other sugars) that are water soluble (4). The residue contains cellulose and lignin. The lignin can be extracted with solvents such as ethanol, butanol or formic acid. Alternatively, hydrolysis of cellulose with lignin present makes water — soluble sugars and the residues are lignin plus unreacted materials. The use of S02 as a catalyst during steam pretreatment resulted in the enzymatic accessibility of cellulose and enhanced recovery of the hemicellulose derived sugars (8). Steam pretreatment at 200- 210°C with the addition of 1% SO 2 (w/w) was superior to other forms of pretreatment of willow (9). A glucose yield of 95%, based on the glycan available in the raw material, was achieved. A summary of various pretreatment options is given in Table I.

Recently, supercritical carbon dioxide explosion was found to be very effective for pretreatment of cellulosic materials before enzymatic hydrolysis (10). The sequential steps for production of fuels and chemicals from lignocellulosic biomass involve feedstock preparation, pretreatment, fractionation, enzyme production, hydrolysis, fermentation, product recovery, and waste treatment. The pretreatment of lignocellulosic biomass is an expensive procedure with respect to cost and energy.