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Is ethanol produced only from corn?
When we talk about ethanol, most of us think of ethanol produced from corn. And it is probably true to say that almost all the commercial fuel grade ethanol currently produced in the United States is, indeed, produced from corn. However, ethanol can also be produced from fibrous biomass, such as wood, hay, straw, broiler litter and even garbage. Ethanol produced from these resources is commonly referred to as “bioethanol,” because it is produced from biomass.
Corn ethanol is already playing an important role in the U.S. as a transportation fuel, and especially as a replacement octane-enhancer for MTBE which has been widely banned due to its contamination of ground and surface water. As such, ethanol also provides agriculture with an alternative market for corn, thus alleviating problems related to surpluses and depressed rural economies. However, the potential contribution of bioethanol to meeting national transportation needs and stimulating rural economies in the South is much greater than that of corn ethanol.
Corn Ethanol Technology
The process for producing ethanol from corn involves grinding the corn into a meal that is then mixed with water to form a slurry. Enzymes are added to the slurry to convert the starch to sugar. Ammonia is added to control acidity and to provide nutrition for yeast which is introduced to the slurry after a high temperature cook step, and subsequent transfer to fermenters where the conversion of sugars to ethanol occurs.
After fermentation, the resultant mixture is transferred to distillers where ethanol is separated from the residual. Conventional distillation is used to concentrate the ethanol to about 190 proof and this product is then dehydrated to about 200 proof before it is blended with 5% denaturant for shipment to gasoline terminals or retailers. An important byproduct of this process is corn Distillers Dried Grains with Solubles (DDGS) which is widely used as a feed ingredient, and which plays a significant role in ensuring economic viability of the corn-to-ethanol process.
The main disadvantages of corn ethanol are its localized agricultural benefits, which are restricted to the corn producing regions of the country, and its relative inefficiency. A few years ago it required more energy input to produce a gallon of ethanol from corn than was contained in that gallon of ethanol. This was partly due to the high energy requirement of growing corn, including annual tillage and fertilizer, only part of the plant (the grain) being used, and inefficient conversion in the ethanol production plants. However, recent projections suggest that overall efficiency has improved to 1.25 – 1.65 units of energy out, for every unit of energy input. This has been attributed to increased all round efficiency, including no-till planting of corn, more precise application of fertilizer, and increased conversion efficiency at the ethanol production plants.
Bioethanol Technologies
In contrast to corn grain, which is made up largely of starch that can be easily broken down to sugars, fibrous biomass that is used to make bioethanol has a more complex structure. It is made up mainly of cellulose, hemicellulose and lignin which are interwoven in different proportions in different materials. Wood contains high levels of cellulose (which is the primary compound used for the production of paper) and lignin, while grasses have lower levels of these compounds and higher levels of
hemicellulose.
In general, there are two procedures for processing fibrous biomass into ethanol. The first involves acid or enzyme hydrolysis (use of water to decompose chemical compounds), and fermentation of the subsequent sugars to ethanol. The second involves gasification (burning under starved air or oxygen conditions) and conversion of the resultant synthesis gas, or syngas, to ethanol by means of either a biological or physical catalyst.
The acid hydrolysis and fermentation process was widely researched by TVA in Muscle Shoals, Alabama, and has been commercialized by Arkenol
(http://www.arkenol.com/). Its efficiency is low (about 60 gal ethanol/dry ton of biomass), so economic viability depends on availability of low or negative cost raw material, such as garbage for which a tipping fee might be paid. Production of co-products is often needed too, in order to ensure economic viability.
Enzyme hydrolysis and fermentation is the technology in which the United States Department of Energy has invested most of its ethanol research dollars. The process is on the verge of commercialization by a Canadian company, Iogen,
(www.iogen.ca) that plans to build a 50 million gal/year commercial ethanol plant at a cost of $250 million. The plant will use 800,000 tons of wheat straw per year. However, efficiency seems low (62.5 gal/ton) and capital costs are high: $5.00 per annual gallon of ethanol, whereas this figure for most plants is around $2.00/annual gallon.
Gasification and biological or catalytic conversion of the syngas to ethanol is being developed by several companies. Brienergy
(http://www.brienergy.com/) uses a bacteria to convert the syngas to ethanol, while Phoenix CGI
(http://www.phoenixcgi.com/) uses a physical catalyst to do the same thing. The latter process is one in a family known as Gas to Liquid (GTL) technologies, which have already been widely applied to coal on a commercial basis. In fact, South Africa produces a substantial amount of its gasoline from coal by using this technology.
Typically, conversion efficiencies of the latter technologies are higher than the hydrolysis and fermentation processes; over 100 gal of ethanol/ton of biomass. This is partly because gasification makes more carbon available for conversion to ethanol than does hydrolysis. Phoenix CGI is at the verge of commercialization too, and projects a cost of 40 – 80 cents/gal of ethanol. Clearly, at this price they would be competitive with gasoline, even with no subsidy. So let’s hope they commercialize soon so that the multiple benefits of their technology can be widely realized. |
