Environmental Engineering Reference
In-Depth Information
process produces only a small amount of microbial biomass or sludge, mini-
mizing the cost for disposal, and captures 80-90% of the electrons from
biodegradable biomass as CH
4
. Third, because methane is poorly soluble in
water, it naturally evolves into the gas phase and can be captured for energy
generation. Although methanogenesis approximately produces equal molar
ratio of CO
2
and CH
4
, some CO
2
remains in solution as bicarbonate. Therefore
typical effluent gas mixture from a digester is 55-75% methane. After gas
cleaning, the CH
4
can be burned in a piston engine or microturbine to produce
energy. CH
4
can be combusted to yield electrical energy, at
35%VE, and this
makes its ECE
30%.
Amajor drawback for anaerobic digestion is its unit cost of methane, since it
is still higher than fossil fuels [3]. The biogas produced from methanogenesis is
mostly CH
4
and CO
2
, but contains trace levels of H
2
and H
2
S. The removal of
H
2
S to prevent combustion-associated byproducts is expensive and energy
intensive, which results in a decrease in PEE. Furthermore, the disposal of
non-degraded residual solids is costly, which means that improving the biode-
gradability of wastes is important for expanding the usefulness of methanogen-
esis. Improving biodegradability also results in more CH
4
generation, the main
source of economic value. Thus, the net cost of the methane production needs to
be lowered by improving the quality of biogas produced and biodegradability
of the wastes.
1.3.2 Bioethanol
Most naturally occurring bacteria also produce other products, such as acetate
and hydrogen, during glucose fermentation; therefore, the stoichiometric con-
version of one mole of glucose molecule to two moles of ethanol is rarely
observed naturally. Hence, the ethanol production industry uses mainly yeasts
to selectively convert glucose to ethanol [50]. Today, ethanol production with
yeast is limited to a narrow range of substrates, mainly hexose derived from
plants. While metabolic engineering approaches are extending the capacity of
yeasts to utilize pentoses such as xylose by introducing new genes to yeasts [51],
it will be interesting to see how far these approaches will take us.
Figure 1.11 shows an energy balance for producing 1 gallon of ethanol from
corn (based on Shapouri et al. [52]). An input energy of 81 MJ is required to
produce corn, transport corn, convert corn to ethanol, remove the ethanol from
water, and distribute the ethanol. Since the energy value of 1 gallon of ethanol is
89 MJ, the production of ethanol results in a PEE of only 9%. Co-products
from ethanol production, such as corn gluten meal, can have economic value.
Since producing co-products normally requires energy input, some energy
balances consider the benefit of co-product production as ''energy credits.''
Energy credits from co-products add about 18% to the energy captured, mak-
ing PEE at most 22%. Given a CCE = 85% for bioethanol, if the ethanol is
