The principal components of most plant materials are commonly described as lignocellulosic biomass. This type of biomass is mainly composed of the compounds, cellulose, hemicellulose, and lignin. Cellulose is a primary component of most plant cell walls and is made up of long chains of the 6-carbon sugar, glucose, that are arranged in bundles (often described as crystalline bundles). The cellulose molecules in the plant cell wall are interconnected by another molecule called hemicellulose. The hemicellulose is primarily composed of the 5-carbon sugar, xylose. Besides cellulose and hemicellulose, another molecule called lignin is also present in significant amounts and provides the structural strength for the plant. Technological developments have recently introduced a variety of processes for extracting and dissolving the cellulose and hemicellulose to produce sugars in a form that can be readily fermented to ethanol. Generally speaking, appropriate pretreatment can liberate the cellulose and hemicellulose from the plant material. Further treatment using chemicals, enzymes, or microorganisms can also be applied to liberate simple sugars from the cellulose and hemicellulose, thus making them available to microorganisms for fermentation to ethanol.
The first step involves cellulose hydrolysis, which is essentially cleaving the chemical bonds in the cellulose to produce glucose.
Once the large molecules are extracted from plant cells, they can be broken down into their component sugars, using enzymes or acids. The sugars can be subsequently converted to ethanol, using appropriately selected microorganisms via fermentation.
Actually, three different reactions have been documented with yields of ethanol ranging from 30 to 50% of the weight of xylose as the starting material (i.e., weight ethanol produced/weight xylose). They are:
3 Xylose => 5 Ethanol + 5 Carbon Dioxide
3 Xylose =>4 Ethanol + 7 Carbon Dioxide
Xylose => 2 Ethanol + Carbon Dioxide + Water
The first reaction yields a maximum of 51% (5 × 46/(3 × 150)), the second 41% (4 × 46/(3 × 150)), and the third 61% (2 × 46/150), respectively. Although the maximum theoretical ethanol yields from these fermentation reactions range between 41 and 61%, the practical yields of ethanol from xylose as starting material are in the range of 3050%.
In the discussion of potential yields of ethanol from various starting materials, two different ranges of efficiencies of hemicellulose-to-xylose conversion and xylose-to-ethanol conversion have been combined to provide an overall conversion efficiency of hemicellulose to ethanol of about 50%. Just as with glucose fermentation, the conversion of carbon dioxide to value-added products would vastly improve the economics of ethanol production, because the yield of carbon dioxide is not only significant in amounts but also inevitable. It must be noted that even though xylose fermentation to ethanol is also mentioned in this chapter, the main focus of discussion for this chapter is on glucose fermentation. As conveyed earlier indirectly, ethanolfrom-corn technology involves glucose fermentation, not xylose fermentation.
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