"Cellulose ethanol can be produced from a wide variety of cellulose biomass feed stocks including agricultural plant wastes (corn stover, cereal straws, sugarcane bagasse), plant wastes from industrial processes (sawdust, paper pulp) and energy crops grown specifically for fuel production, such as switchgrass. Cellulose biomass is composed of cellulose, hemi cellulose and lignin, with smaller amounts of proteins, lipids (fats, waxes and oils) and ash. Roughly, two-thirds of the dry mass of cellulose materials is present as cellulose and hemi cellulose. Lignin makes up the bulk of the remaining dry mass.
As with grains, processing cellulose biomass aims to extract fermentable sugars from the feedstock.
But the sugars in cellulose and hemi cellulose are locked in complex carbohydrates called polysaccharides (long chains of monosaccharides or simple sugars). Separating these complex polymeric structures into fermentable sugars is essential to the efficient and economic production of cellulose ethanol.
Two processing options are employed to produce fermentable sugars from cellulose biomass. One approach utilizes acid hydrolysis to break down the complex carbohydrates into simple sugars. An alternative method, enzymatic hydrolysis, utilizes pre-treatment processes to first reduce the size of the material to make it more accessible to hydrolysis. Once pre-treated, enzymes are employed to convert the cellulose biomass to fermentable sugars. The final step involves microbial fermentation yielding ethanol and carbon dioxide.
Grain based ethanol utilizes fossil fuels to produce heat during the conversion process, generating substantial greenhouse gas emissions.
In the Cellulose ethanol production, fossil fuels are substituted by biomass. This property changes the emissions calculations.
Cellulose ethanol showed greenhouse gas emission reductions of about 80% [over gasoline], Corn ethanol showed 20 to 30% reductions. Cellulose ethanol's favourable profile stems from using lignin, a biomass by-product of the conversion operation, to fuel the process. Lignin is a renewable fuel with no net greenhouse gas emissions. Greenhouse gases produced by the combustion of biomass are offset by the CO2 absorbed by the biomass as it grows.
Feedstock sources and supplies are another important factor differentiating the two types of ethanol. Agricultural wastes are a largely untapped resource. This low cost feedstock is more abundant and contains greater potential energy than simple starches and sugars. Currently, agricultural residues are plowed back into the soil, composted, burned or disposed in landfills. As an added benefit, collection and sale of crop residues offer farmers a new source of income from existing acreage.
Industrial wastes and municipal solid waste (MSW) can also be used to produce ethanol."
