Production of Synthetic Fuels and Chemicals from Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 397

operating compression-ignition engines. In most cases, engine modifications
may not be needed for substitution of mineral oil with bio-oil-derived fuels.
Ethanol is produced mainly from food crops, but, less commonly, it can also
be produced from nonfood lignocellulosic biomass.

11.5.1.1 Ethanol from Food Sources
Ethanol (C 2 H 6 O) is presently produced primarily from glucose obtained
from grains (e.g., corn and maize), sugar (sugarcane), and energy crops using
the fermentation-based biochemical process. A typical process based on corn
is shown in Figure 11.11 . The process may comprise the following major
steps:

a. Milling: Corn is ground to a fine powder called cornmeal.
b. Liquefying: A large amount of water is added to make the cornmeal into
a solution.
c. Hydrolysis: Enzymes are added to the solution to break large carbohy-
drate molecules into shorter glucose molecules.
d. Fermentation: The glucose mixture is taken to the fermentation batch
reactor, where yeast is added. The yeast converts the glucose into ethanol
and carbon dioxide as represented by the equation:

Fermentation

C 6 H 12 O 6 ð glucose Þ 1 yeast ! 2C 2 H 6 O ð ethanol Þ 1 2CO 2 (11.20)

e. Distillation: The product of fermentation contains a large amount of
water and some solids, so the water is removed through distillation.
Distillation purifies ethanol to about 95 96%. The solids are pumped

out and discarded as a protein-rich stock, which may be used only for
animal feed.
f. Dehydration: The ethanol produced is good enough for car engines. So,
countries like Brazil use it directly, but in some countries, further purifi-
cation is needed if it has to be blended with mineral gasoline for ordinary
cars. In this stage, a molecular sieve is used for dehydration. Small beads
with pores large enough for water but not for ethanol absorb the water.

A large amount of energy is consumed in the distillation, dehydration,
and other steps in this process. By one estimate, for the production of 1 liter
of purified ethanol, about 12,350 kJ of energy is needed for processing, espe-
cially for dehydration. An additional 7440 kJ/l of energy consumed in har-
vesting the corn is required (Wang and Pantini, 2000). Although a liter of
ethanol releases 21,200 kJ of energy when burnt, the farming and processing
of the corn consume about 19,790 kJ of energy. The net energy production is
therefore a meager 1410 kJ (21,200

19,790) per liter of ethanol.
Major criticism of this process is that it uses a valuable food source—
indeed, a staple food in many countries. The search for an alternative is
therefore ongoing.