Cellulosic ethanol is a biofuel made using cellulose, the main component of plant cell walls. Cellulose is contained in all plants and is the earth’s most common organic compound. While it is more difficult to create ethanol from cellulose than it is to make it from starch or sugar-based crops like corn, the sources of cellulose are virtually unlimited. This is a major advantage which allows producers to use resources such as agricultural waste, byproducts of lawn, garden and tree care, and non-food crops that are fast-growing and can be raised on less desirable land.
There are two main types of feedstock, or raw material, used to create cellulosic ethanol: agricultural biomass and forest biomass. Agricultural biomass can include waste materials from food crops such as corn cobs, corn stover, or rice straw. It may also come from crops grown specifically for energy production. Some crops that may be used include switchgrass, sugarcane, and miscanthus. Self-seeding crops that do not have to be re-planted after each growing, which are drought- and weather-resistant, 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 valuable alternative. As with agricultural biomass, certain types of trees may be cultivated specifically as energy crops. Short-rotation tree crops that can be planted, harvested and replanted quickly are the ideal candidates for use in cellulosic ethanol production. Forestry byproducts are another option; wood chips, bark and sawdust can all be used. Another alternative is waste paper pulp.
In order 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. The cellulose is pre-treated to break down some of its rigid structure, both physically and chemically. It is then more accessible for the hydrolysis process, which breaks the molecules down into sugars using enzymes called cellulase enzymes. The sugars are separated from any other materials left over, and 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 to hydrogen, carbon dioxide and carbon monoxide using partial combustion. A microorganism called Clostridium ljungdhalii is then used to ferment these gases and convert them to ethanol and water. The ethanol is then distilled from the water.