Renewable Energy - Energy and the Environment - page 150

produces a combination of solid, liquid, and gaseous products that are combustible. Fermenta-
tion and distillation of carbohydrates produces ethanol (C 2 H 5 OH), a valuable liquid fuel that is
commonly blended with gasoline for motor vehicle use. Anaerobic digestion produces a gaseous
mixture of CO 2 and CH 4 . The gaseous fuels can be upgraded to more desirable forms, albeit at
some loss of energy.
The processing of biomass to a form that is more readily used as a replacement for fossil
fuel inevitably results in a loss of some of its heating value and generates a production cost of
the conversion. To compete economically with fossil fuels, which have small processing costs,
biomass-derived fuels must utilize low cost forms of primary biomass.
Except for wood, which may be harvested year-round, biomass energy crops, like food crops,
must be stored after harvesting to provide a steady supply of feed stock for the remainder of the
year. The economic cost of storage and maintaining a year's inventory adds to the price differential
between biomass and fossil fuels, which have much smaller inventories.
During the 1970s, when international oil prices skyrocketed, many agricultural-based, biomass-
derived liquid fuel production schemes were investigated. Of particular interest was the energy gain
factor, the ratio of the energy value of the fuel produced (usually ethanol) to the energy consumed
in producing, harvesting, and processing the biomass. Not only must this ratio exceed unity, so that
there is a net output of energy from the biomass conversion process, but it must be sufficiently greater
that other, non-energy costs of production can be covered by the sales revenues generated. 4 Such
requirements could only be met by utilizing a high-energy crop like sugar cane and incorporating
the energy supply of the crop residue into the fuel synthesis process. Currently, these energy and
economic constraints make fuels derived from biomass food crops, such as grains or sugar cane,
noncompetitive with fossil fuels at current world market prices.
As an example, in the United States, corn is currently employed as a feedstock to manufacture
ethanol for use as an additive in motor vehicle fuel. In the fermentation and distillation process that
converts corn starch to ethanol, only 65% of the grain heating value is preserved in the ethanol
output from the process. 5 In addition, fossil fuel is consumed in the production and harvesting of
corn and the production of ethanol. As a consequence, use of ethanol as a vehicle fuel additive
results in only a 50-60% reduction in fossil fuel energy use and a 35-46% reduction in greenhouse
gas emissions, compared to the use of ordinary vehicle fuel. 6 Thus the energy gain factor for

corn-generated ethanol is 2-2.5. 7
The use of wood harvesting residues to generate process heat and electric power in paper mills,
eliminating their use of fossil fuels, is now common in the paper industry. The residues are collected
along with the wood used to feed the pulping operation. While the residues supply some or all of the
mill's energy requirements, there is generally no significant surplus that might otherwise generate
electric power for other purposes. (The same is true for wood harvested for lumber.) In contrast

4 For fossil fuels, the energy consumed in production, processing, and distribution is less than 10 % of the
energy of the end product.
5 Probstein, Ronald F., and R. Edwin Hicks, 1990. Synthetic Fuels. Cambridge: pH Press.
6 Wang, M., C. Saricks, and M. Wu, 1999. J. Air Waste Manage. Assoc. 49, 756-772.
7 The heating value of a bushel of corn is about 400 MJ. At an average price of $3 per bushel, this energy costs
about $8 per million Btu. Average prices of coal and natural gas in the 1990s were about $1.25 and $3 per
million Btu.