Cellulosic ethanol is a biofuel made from cellulose found in all plants. It is harder to produce than ethanol from starch or sugar crops, but sources are unlimited. Agricultural and forest biomass can be used, and two processes, cellulolysis and gasification, can be used to create it.
Cellulosic ethanol is a biofuel produced using cellulose, the main component of plant cell walls. Cellulose is contained in all plants and is the most common organic compound on earth. While it is more difficult to create ethanol from cellulose than it is to make it from starch or sugar based crops such as corn, the sources of cellulose are virtually unlimited. This is a major benefit that allows producers to use resources such as agricultural waste, by-products of lawn, garden and tree care, and non-food crops that grow rapidly and can be grown on less desirable land.
There are two main types of feedstock, or feedstock, used to create cellulosic ethanol: agricultural biomass and forest biomass. Agricultural biomass can include waste materials from food crops such as corn cobs, corn stew, or rice straw. It can also come from crops grown specifically for energy production. Some crops that can be used include switchgrass, sugarcane, and miscanthus. Self-seeding crops that do not have to be replanted after each cultivation, which are drought and weather tolerant are usually preferred.
Forest, or woody, biomass also offers a variety of options for ethanol production and has certain processing advantages over agricultural biomass that make it a viable alternative. As with agricultural biomass, some types of trees can be grown specifically as energy crops. Short rotation tree crops that can be planted, harvested and replanted quickly are ideal candidates for use in cellulosic ethanol production. Forest byproducts are another option; wood shavings, bark and sawdust can be used. Another alternative is waste paper pulp.
To create cellulosic ethanol, one of two main processes can be used. The first is cellulolysis, a process that uses hydrolysis to break cellulose into simple sugars. Cellulose is pre-treated to break down some of its rigid structure, both physically and chemically. It is then more accessible for the process of hydrolysis, which breaks down molecules into sugars using enzymes called cellulase enzymes. The sugars are separated from any other leftover material, then fermented using microbes and distilled.
The second process that can be used is gasification. During gasification, the carbon contained in the cellulose is converted into hydrogen, carbon dioxide and carbon monoxide by partial combustion. A microorganism called Clostridium ljungdhalii is then used to ferment these gases and convert them into ethanol and water. The ethanol is then distilled from the water.
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