Cellulose is one of the most abundant organic compounds, available primarily in the primary and secondary cell walls of green plant matter, as well as in some algae, bacteria and some fungus-like organisms. Cellulose can be converted into biofuels, most commonly cellulosic ethanol, via chemical pre-treatment and fermentation. It is also possible to convert saccarides and starches in to ethanol, however, more cellulose per unit land area is attainable and therefore growing cellulose-rich biomass is deemed a better use of land and has become the common basis for industrial production.
Accumulating large quantities of cellulose-rich biomass feedstock is the first stage in the sequence of events that ultimately leads to the fermentation of ethanol for transportation fuel. For industrial use, the source stock is often wood pulp (about 50% of wood is cellulose) or cotton (up to 90% of cotton by mass is cellulose) but increasingly comes from specialized high-biomass energy crops such as corn, cultivated poplar trees and low-maintenance perennial grasses like switch-grass. Researchers are examining the potential of dedicated energy crops, including certain wood and grass species, in order to offset the extent to which rising demand for ethanol may effect crops pricing.
Once crop has been accumulated, it is pre-treated. Pre-treatment involves soaking the feedstock in concentrated enzymes and acids; pre-treatment can also involve heating the stock. This process disengages unwanted polymers, such as hemicellulose and lignin, from the cellulose base prior to hydrolysis. Under current production standards, pre-treatment is the most expensive step in the production process and is open to refinement through research and development efforts.
Next, the pre-treated base undergoes hydrolysis. The goal of the hydrolysis step is to reduce the cellulose in to its component sugars. The hydrolysis can be performed in two ways: chemically using acids, or enzymatically. Enzymatic hydrolysis is a process similar to that which occurs in the stomachs of ruminant animals. Further enzyme treatment with cellulase enzymes breaks down the cellulose in to double glucose molecules, called cellobiose. Cellobiose is then reduced to single glucose by contact with a second type of cellulase enzyme.
Chemical hydrolysis is the traditional means of breaking down cellulose in to free sugars prior to fermentation. The cellulose is exposed to acids under heat and pressure. When exposed to water the reaction results in free sugars. A significant disadvantage in this process is that the harshness of the hydrolysis results in the production of toxic by products which can be carried through to hinder fermentation.
The final stage in the process is microbial fermentation. Microbes ferment the sugars to ethanol. Baker’s yeast has always been the favoured agent in fermenting ethanol from sugars. Recent advances have led to other micro-organisms, such as Escherina coli, which have been engineered to increase fermentation rates. Fermentation yields a mix of ethanol, water, microbes and some residues. The resultant ethanol is purified through distillation.
As global demand for ethanol rises, increased research and development efforts will be brought to bear on all stages of the ethanol production process.
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