Bioethanol


Ethanol from Corn

The agricultural production of corn is subsidized in the US. The glucose from the stored starch in this grain can be converted to ethanol through fermentation and the alcohol can be burned in internal combustion engines to supplement or replace fossil fuels. Baker's yeast or Saccharomyces cerevisiae is the microorganism that converts glucose to ethanol and carbon dioxide.

Most corn-ethanol comes from the dry milling process.
    Most of the ethanol plants in the country utilize a dry-milling process. The major steps of dry-milling are outlined below:
  1. Milling: The corn is cleaned and passed through hammer mills which grind it into a fine powder.

  2. Liquefaction: The meal is mixed with water and the enzyme alpha-amylase, and heated to that the starch is liquefied. Sulfuric acid or sodium hydroxide is added to adjust the pH to 7.

    Many types of glucose polymers store energy in the corn kernel. Amylopectin, for example, is a long-chain, branched polymer. The enzyme alpha amylase catalyzes the partial hydrolysis of the polymer to single strand and shorter strand starches.



  3. Saccharification: The enzyme gluco-amylase is added to convert starch to fermentable sugars containing glucose or glucose dimers. The linear and cyclic forms of glucose are below



  4. Fermentation: Yeast is added to the mash to ferment the sugars to ethanol and carbon dioxide. Using a continuous process, the fermenting mash flows through several fermenters until it is fully fermented and leaves the tank. In a batch fermentation process, the mash stays in one fermenter for about 48 hours.

    Fermentation is part of the respiration of the yeast organisms. First, the glucose molecules are cleaved to 3-carbon units. Then each of these is converted to an ethanol molecule and a carbon dioxide molecule.



  5. Distillation: The fermented mash contains about 10% ethanol. The mixture is pumped to the continuous flow, multi-column distillation system. The distillate is about 96% pure. The mixture of this amount of alcohol/water is an azeotrope, that is the mixture distills at a lower temperature than either pure ethanol or pure water.

    The distillation step requires much energy and water for cooling.

  6. Dehydration: The water is removed from the ethanol by molecular sieves (3-4 angstrom zeolites) to produce anhydrous ethanol.

  7. Denaturing: Ethanol that will be used for fuel is then denatured with a small amount (2-5%) of some product, like gasoline, to make it unfit for human consumption.
Last year, 1.8 x 109 bushels of corn were converted to 4.9 x 109 gallons of ethanol fuel in the US. The energy bill passed by the US Senate in June 2007 requires ethanol production to grow to at least 36 billion gallon a year by 2022. (Provisions to increase other renewable fuels were eliminated.)

However, under best practices the energy yield from the ethanol is only about 1.25 times the energy required to produce it. A paper by Tad Patzek details the thermodynamics of the corn-ethanol biofuel cycle.

Intensive farming of corn or other energy crops can have negative environmental effects due to the fertilizers and other agricultural chemicals requuired, increased erosion, increased water demand, and a reduction of high quality land available for the production of food. A new ethanol plant recently approved for Champaign county, IL will withdraw 1.5 x 106 gallons of water per day from the Mahomet Aquifer.

Ethanol from Cellulose

Cellulosic materials contain lignin, hemicellulose, and cellulose and comprise approximately 80 % of biomass. Because these are not in the human food chain, they are typically much cheaper than the glucose-rich agricultural products. However, they are much more difficult to convert into ethanol.

Both cellulose and hemicellulose are much more difficult to hydrolyze into the component sugars. Acid hydrolysis of the cellulosic material with H2SO4 at 237 deg gives about a 50 % yield of component sugars. Another process uses dilute acid followed by enzymatic hydrolysis to form free sugars.

The hemicellulose contains both 5-carbon and 6-carbon sugars and the 5-carbon sugars don't ferment with the same microorganisms that can be used for glucose. Organisms have been genetically engineered to ferment the pentoses into ethanol.

Iogen Corporation in Ottawa, Canada produces just over a million gallons annually of cellulose ethanol from wheat, oat and barley straw in their demonstration facility.




News reports:

  1. Times Topics: News about Biofuels (New York Times)

  2. More Bad News for Ethanol (Wall Street Journal)

  3. How Green Are Biofuels? (Science)



Professor Shapley, University of Illinois, 2011