Ethanol Production
Feedstock for bioethanol production
Instead of chemical synthesis, fermentation of simple sugars by yeast is currently used extensively to produce ethanol from renewable sugar-containing biomass. The feedstock for bioethanol production is categorized as:
- First generation feedstock which are sugar and starch crops. The major sugar crops are sugar cane, sugar beet and their molasses, i.e. by-product from sugar production and sweet sorghum while the major starch crops are corn and cassava.
- Second generation feedstock which are lignocellulosic materials derived from agricultural residues or agro-industrial waste such as sugar cane bagasse, corn cob or stover, wheat and rice straw, empty fruit bunches of oil palm.
The key producers of bioethanol in the world are USA by using corn, accounting for 61% of total global production, followed by Brazil using sugar cane (26% of total global production). In Europe, sugar beet is mainly used as the feedstock, with some other starch feedstock including corn, wheat, rye and barley. In China, corn, wheat and cassava are the main feedstock for bioethanol production.
World bioethanol production (billion litres) in 2012
Uses of bioethanol
Bioethanol is an alternative liquid fuel for automotive transport, or for producing alcohol. It can be used either in a form of pure hydrous (95%) usually referred to as E100 or anhydrous ethanol (99.5%). Pure hydrous ethanol can be used directly, but only with modified engine or flexible –fuel vehicles (FFVs). Except for Brazil, other countries use primarily anhydrous ethanol in the form of a blend with gasoline, namely gasohol. There are several common blends of anhydrous ethanol and gasoline used around the world and classified by an international nomenclature of E followed by a figure to indicate the proportion of bioethanol content (in % by volume). For example, E10 is a blend of 90% gasoline and 10% of anhydrous ethanol. Currently, different blends are used including E5, E10, E20, E25 and E85. Blends having bioethanol with or more than 20% must be applied with modified automotive or FFVs which can limit its consumption.
Ethanol production
When ethanol is produced by yeast fermentation of sugar feedstock such as sugar cane, molasses, sugar beet and sweet sorghum, yeast can directly consume simple sugars and convert them biologically to ethanol. However, starch and cellulose feedstock are a polymer of glucose and cannot directly be utilized by yeast. They have to be converted or depolymerized to glucose prior to yeast fermentation. Depolymerization or hydrolysis of starch is much simpler and more cost effective than that of cellulosic materials and can be achieved by acid or enzyme or a combination of both.
Conversion (by weight) of glucose to ethanol by yeast fermentation