The biochemical conversion process involves fermenting the sugar found in biomass and distilling it to ethanol. The pathway varies depending on the type of feedstock used. Corn (as well as sugar cane) has easily accessible sugars in the form of starch. Cellulosic feedstocks must be pretreated before enzymes can break down the cellulose into sugars. The breakdown or hydrolysis of the cellulose is a tricky step in the conversion process.
Cellulosic Ethanol Refineries
Cellulosic ethanol is produced from a wide variety of non-edible plant materials including corn stover, cereal straws, sawdust, paper pulp, and switchgrass. The biomass must be pretreated in order for enzymes to have access to cellulose. The enzymes break down the cellulose into sugars which undergo microbial fermentation. The end product is distilled to ethanol.
Cellulosic ethanol could be produced in every region of the country using locally grown materials, while producing a fuel that creates less greenhouse gases than corn-based ethanol.
New York businesses currently pursuing ventures in the cellulosic ethanol industry:
- Mascoma has built a demonstration cellulosic ethanol pilot refinery in Rome, NY with a 200,000 gallon per year production capacity.
- Sweetwater Energy, Inc., a value-added feedstock provider to ethanol biorefineries
Corn Ethanol Refineries
Currently, the main biofuel used in the United States is ethanol distilled from kernels of corn. There are two methods used to convert the corn kernel into a usable fuel: wet milling and dry milling. The two different processes yield different by-products.
In wet milling, the kernel is steeped before being separated yielding a final product of ethanol, corn oil, corn gluten, and corn starch which may be modified to corn syrup. Generally large, corporate owned ethanol plants use wet milling. Ethanol plants owned by farmer cooperatives generally utilize the dry milling process.
In dry milling, the kernel is first ground into a fine flour before adding water to produce a mash. After fermentation, the mash is separated into ethanol and stillage. The stillage is centrifuged and dried producing distillers dried grains with solubles (DDGS), a high-protein, high-energy livestock feed. The CO2 produced during fermentation is also sold as a valuable by-product.