More than 90% of the fuel ethanol produced in the United States is made from corn. Most analysts agree that the ability to significantly increase ethanol production using corn grain is limited, and that large increases will require the use lignocellulose resources such as agricultural residues, grasses, and wood.
Ethanol is produced by fermenting plant carbohydrates with yeast. The principal carbohydrates contained in lignocellulose resources are the structural carbohydrates. Along with proteins and lignin, they form the complex matrix of plant cell walls that gives plants structural stability and protection from the environment. The inherent stability and chemical complexity of cellulose increases the difficulty of breaking it down into fermentable simple sugars. A number of pretreatment approaches are being explored to overcome this problem, but few have so far been fully commercialized or tested at industrial scales. Scaling pretreatment processes to commercial sizes, and the associated reactor design issues, at present remain a major barrier to the commercial production of fuel ethanol from lignocellulosic biomass.
Apart from the necessity for pretreatment and breakdown of of the carbohydrates in the cell walls to simple sugars, fuel ethanol production from lignocellulose is similar to that from corn. However, the residual solids produced in lignocellulose processes have little value as an animal feed, being high in lignin and low in fiber and protein. Instead they are used to produce the heat, steam, and electricity needed to run the ethanol production facility, with the excess electricity sold to the electrical grid.