Cellulose is a compound found in all plants, and can be digested by neither humans nor yeast. However, if broken down to its component parts, it can be used in fermentation processes to produce ethanol.
There are two different methods of breaking down cellulose - hydrolysis and gasification.
Hydrolysis method uses either an acid or cellulose enzyme. The latter is produced by certain types of bacteria that breaks down cellulose down to beta-glucose. Once the cellulose is broken down to its component parts, production of ethanol through fermentation is possible. In nature, these processes are performed by different organisms: fungi and bacteria that use enzymes (cellulases) to "free" the sugar in cellulose, and other microbes, primarily yeasts, that ferment sugars into alcohol.
The hydrolysis method using acid was developed in the 19th century and requires high heat and temperature for the reaction to take place.
The gasification method, known as synthesis gas fermentation, involves partial combustion of the material producing carbon monoxide gas. Carbon monoxide is then fed into a specialized fermenter, where the gas is turned into ethanol, water and hydrogen gas by bacteria. Although the gasification method involves partial combustion of the raw material, no gases are released into the atmosphere
According to the U.S. Energy Department, combustion of cellulosic ethanol produces less green house gases compared to both sugar-based ethanol and gasoline. Combustion of ethanol made from sugar reduces green house gas emissions by 18 percent to 29 percent compared to gasoline but cellulose ethanol reduces greenhouse gases emissions by up to 85 percent.
Another advantage of cellulosic ethanol is the plentiful supply of the raw material. Unlike sugar and starch, the whole of the plant contains cellulose, therefore the whole plant can be harvested, multiplying the yield per acre. Agricultural waste like straw and grass can be used as sources of cellulose. Because the plants are not grown as food supplies, plants such as switch grass can be grown on land unsuitable for food production.