Biofuels - The Oxford Topic of Food, Politics, and Society - page 185

Photosynthetic efficiency (the percentage of visible light energy used for carbon fixa-
tion by plants) for intensively managed plants can average as high as 3%, while most
crops range from 1 to 4% (Moore et al. 1998), but this efficiency only holds true while
the plants are actively growing. Corn in temperate climates such as the US Corn Belt
only grow for 120 days or so, leaving 240 days of no corn productivity, which lowers
photosynthetic efficiency from perhaps 3% during the growing season to about 1% for
the whole year. If not for the fact that almost the entire transport system of the industri-
alized world depends on liquid fuel, no one would consider using cellulosic biomass or
crops such as sugarcane or corn as raw material for making liquid fuel.
Corn Ethanol
Having considered the aggregate biomass resources available for conversion to biofuels,
we now turn to consideration of specific biofuels in use. Each has a particular profile of
ecological and energy costs, and thus different implications for the political and ethical
questions of sustainable development.
In the United States, ethanol constitutes 99% of all biofuels (Farrell et al. 2006). For
capital expenditures, new plant construction costs from $1.05 to $3.00 per gallon of eth-
anol (Shapouri and Gallagher 2005). Fermenting and distilling corn ethanol requires
large amounts of water. The corn is finely ground and approximately 15 liters of water are
added per 2.69 kg of ground corn. After fermentation, to obtain a liter of 95% pure etha-
nol from the 10% ethanol and 90% water mixture, 1 liter of ethanol must be extracted
from the approximately 10 liters of the ethanol/water mixture. To be mixed with gas-
oline, the 95% ethanol must be further processed and more water must be removed,
requiring additional fossil energy inputs to achieve 99.5% pure ethanol (Pimentel et al.
2009). Thus, a total of about 12 liters of wastewater must be removed per liter of ethanol
produced, and this relatively large volume of sewage effluent has to be disposed of, at
an energy, economic, and environmental cost. Manufacture of a liter of 99.5% ethanol
uses 45% more fossil energy than it produces and costs $1.05 per liter ($3.97 per gal-
lon) (Pimentel et al. 2012). The corn feedstock alone requires more than 33% of the total
energy input.
The largest energy inputs in corn ethanol production are for producing the corn feed-
stock plus the steam energy and electricity used in the fermentation/distillation process.
The total energy input to produce a liter of ethanol is 7,438 kcal (Pimentel et al. 2009).
However, a liter of ethanol has an energy value of only 5,130 kcal. Based on a net energy
loss of 2,344 kcal of ethanol produced, 45% more fossil energy is expended than is pro-
duced as ethanol. The total cost, including the energy inputs for the fermentation/distil-
lation process and the apportioned energy costs of the stainless steel tanks and other
industrial materials, is $1,045 per 1,000 liters of ethanol produced (Pimentel et al. 2009).
All differences in ethanol production processes from biomass and genetic manipula-
tion of crop plants to biofuel plants are peripheral to the point that only from 1-4% of