Environmental Engineering Reference
In-Depth Information
sugarcane yields by over 10% [2]. The fuel ethanol industry in the USA has grown
rapidly since 2000, with over 95% of the ethanol being blended into gasoline as
an oxygenate (called E10). Current 2008 production is uncertain due to the volatile
economy and sharp commodity fluctuations; however, we project the final volume
to be around 9.6 billion gallons (equal to about 7% of the US gasoline volume). The
majority of the feedstock for US ethanol is corn (maize) grain, with a small amount
4%) being generated from sorghum. Unlike sugarcane, which cannot be stored
and for which the mills must close for several months each year, grains are easily
stored for over a year and can be managed and transported in the existing infras-
tructure. Another advantage of grains is that only the starch is consumed in ethanol
fermentation. The protein and oil are carried through in the distillers grains (DG) and
are available to go back into the livestock feed system. Nevertheless, there will be an
upper limit on the land and farm resources that can be used for grain-based ethanol
before impacting other commodity food markets (e.g. today the amount of grain
exported from the US is about the same as that used for ethanol). Some analysts
suggest that there is an impact today, others project that the maximum amount of
corn that can be used for ethanol production is approximately 25-30% of the annual
corn production [12]. We estimate that the upper limit will depend on how fast the
expected biotechnology-driven yield increase is achieved [11, 17]. For example, we
can calculate the mathematical outcome for various scenarios:
Yield is somehow frozen today at 12 B bushels grain. E10 (oxygenate additive
value) used in all US gasoline would require 15 B gal ethanol
5 B bu grain.
This would require 41% of the current corn harvest. However, 30% of that goes
back into the feed system as DG so the net utilization is 29% of the available corn
Yields are projected to continue to increase due to various new technologies, with
some industry experts projecting 300 bu/acre in 10-15 years: this would generate
24 B bu grain. Again assuming E10 use at 15B gal ethanol
5 B bu grain, this
would result in only 20% of the crop harvest being taken in. Accounting for the
DG return, the net corn grain use would be 14%.
In reality, there are many factors which will impact the final scenario. Irrespective
of the exact scenario, it seems that corn grain can provide for existing market
demands plus enough grain for future oxygenate use (e.g. E10). While this is
an excellent contribution, it does not meet requirements for majority replacement
of gasoline volume. Obviously, to achieve further energy independence and fur-
ther reduce import of foreign oil, additional renewable feedstocks are required to
contribute to the total liquid fuel demand.
3 Potential of Lignocellulosic Biomass
A 2005 USDA and DOE joint report [12] showed that a combination of crops, agri-
cultural residues, trees, forest residues, and bringing conservation reserve land into
production could generate up to 1.3 billion dry tons of biomass each year. Given
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