Lignocellulosic biomass

Lignocellulosic biomass [Lin, Yan, and Tanaka, Shuya. " [http://www.springerlink.com/content/fm9l941r87w26048/ Ethanol fermentation from biomass resources: current state and prospects] ." Applied Microbiology and Biotechnology 69.6 (2006): 627-642.] refers to plant biomass that is composed of cellulose, hemicellulose, and lignin. The carbohydrate polymers (cellulose and hemicelluloses) are tightly bound to the lignin, by hydrogen and covalent bonds. Biomass comes in many different types, which may be grouped into four main categories: (1) wood residues (including sawmill and paper mill discards), (2) municipal paper waste, (3) agricultural residues (including corn stover and sugarcane bagasse), and (4) dedicated energy crops (which are mostly composed of fast growing tall, woody grasses).

Application

Fermentation of lignocellulosic biomass to ethanol is an attractive route to energy feedstocks that supplements the depleting stores of fossil fuels. Biomass is a carbon-neutral source of energy, since it comes from dead plants, which means that the combustion of ethanol produced from lignocelluloses will produce no net carbon dioxide in the earth’s atmosphere. Also, biomass is readily available, and the fermentation of lignocelluloses provides an attractive way to dispose of many industrial and agricultural waste products. Finally, lignocellulosic biomass is a very renewable resource. Many of the dedicated energy crops can provide high energy biomass, which may be harvested multiple times each year.

One barrier to the production of ethanol from biomass is that the sugars necessary for fermentation are trapped inside the lignocellulose. Lignocellulose has evolved to resist degradation and to confer hydrolytic stability and structural robustness to the cell walls of the plants. This robustness or "recalcitrance" is attributable to the crosslinking between the polysaccharides (cellulose and hemicellulose) and the lignin via ester and ether linkages. [http://genomicsgtl.energy.gov/biofuels/2005workshop/b2blowres63006.pdf] Ester linkages arise between oxidized sugars, the uronic acids, and the phenols and phenylpropanols functionalities of the lignin. To extract the fermentable sugars, one must first disconnect the celluloses from the lignin, and then acid-hydrolyze the newly freed celluloses to break them down into simple monosaccharides. Another challenge to biomass fermentation is the high percentage of pentoses in the hemicellulose, such as xylose, or wood sugar. Unlike hexoses, like glucose, pentoses are difficult to ferment. The problems presented by the lignin and hemicellulose fractions are the foci of much contemporary research.

References