The Bioethanol production process

Whether corn/maize, wheat, sugarbeet, sugar cane or woody biomass (lignocellulose) is the feedstock the final stage of ethanol production is fermentation. The difference with these varying feedstocks is how the starch, sugar or cellulose is extracted. Current technology makes extraction from woody biomass uneconomic, but the view is that progress will have been made by 2015 – the advantage of woody biomass being its low cost.

The process for high starch grains (wheat, barley, maize) is as follows, known as dry milling:

1. Milling:

The wheat (or barley or corn) first passes through hammer mills, which grind it into a fine powder called meal.

2. Liquefaction:

The meal is then mixed with water to form a "mash". The enzyme alpha-amylase is added, and heat is applied at this stage to enable liquefaction. High temperature cooking (120-150 degrees Celsius) and a lower temperature holding period (95 degrees Celsius) eliminate bacteria before fermentation.

3. Saccharification:

The mash from the cookers is then cooled and the secondary enzyme (gluco-amylase) is added to convert the liquefied starch to fermentable sugars (dextrose).

4. Fermentation:

Yeast is then added to the mash to ferment the sugars to ethanol and carbon dioxide. Using a continuous process, the fermenting mash is allowed to flow, or cascade, through several fermenters until it is fully fermented and leaves the final tank. In a batch fermentation process, the mash stays in one fermenter for about 48 hours before the distillation process is started.

5. Distillation:

The fermented mash, now called "beer," contains about 10% alcohol, as well as all the non-fermentable solids from the corn and the yeast cells. The mash is then be pumped to the continuous flow, multi-column distillation system where the alcohol is removed from the solids and the water. The alcohol leaves the top of the final column at about 96% strength, and the residue mash, called stillage, is transferred from the base of the column to the co-product processing area.

6. Dehydration:

The alcohol from the top of the column passes through a dehydration system to remove the remaining water. Most ethanol plants use a molecular sieve to capture the last bit of water in the ethanol. The alcohol product at this stage is called anhydrous (pure, without water) ethanol and is approximately 200 proof.

7. Denaturing:

Ethanol that will be used for fuel is denatured with a small amount (2-5%) of some other product, like gasoline, to make it unfit for human consumption.

8. Co-Products:

There are two main co-products created in the production of ethanol: carbon dioxide and distillers grain.

Carbon dioxide is given off in great quantities during fermentation.
Many ethanol plants collect the carbon dioxide, clean it of any residual alcohol, compress it and sell it for beverage carbonation or the flash freezing of meat.

Distiller grains, wet and dried, are high in protein and other nutrients and are a highly valued livestock feed ingredient.

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