3.3 Background

Henry Ford test drove his first prototype automobile called the Ford Quadracycle in July 1896 that ran on pure ethanol. He told the New York Times in 1925 that “The fuel of the future is going to come from fruit like that sumach out by the road, or from apples, weeds, sawdust — almost anything” [41]. Ethyl alcohol, or ethanol, is a two carbon, straight chain alcohol that is found in alcoholic beverages. Ethanol is a renewable, biodegradable, clean burning, alternative fuel that is usually produced by the fermentation of carbohydrates from sugar, corn, or fruits [13]. Ethanol has replaced methyl tert-butyl ether (MTBE) as an emissions reducing additive in gaso­line due to concerns of MTBE ground water contamination that arose in late 2005. Ethanol can be used in current automobiles in blends up to 10% (E10) in gasoline without any engine modifications. Higher percentages of ethanol blends (E85 and E100) can be used in Flex Fuel Vehicles (FFVs).

Sugarcane-based ethanol edges out gasoline at an oil equivalent economic price of $40 per barrel [42]. In contrast, US corn-based ethanol has an edge over gasoline when oil price is $60 or higher. “Flex-fuel” vehicles are designed to run on ethanol, gasoline, or a mixture of the two. Ethanol is made through the fermentation of sug­ars, and sugar cane offers particular advantages. The energetic balance in ethanol production shows that for each unit of energy invested, sugar cane based ethanol yields eight times as much energy as corn [43]. Unlike corn-based fuels, sugarcane requires no fossil fuels to process. Cellulosic ethanol, derived from a range of crops, such as switchgrass and crop waste, is more economical than corn ethanol because it requires far less energy to produce. However, the economics of corn or cellu — losic ethanol has been discussed widely in many articles. A central argument is that corn-based ethanol is literally a waste of energy. Detractors say that it takes more energy to grow the corn, process it, and convert it to ethanol than would be saved by using it. According to Pimentel and Pazek [44] “Ethanol production using corn grain required 29% more fossil energy than the ethanol fuel produced.” Wang et al. dispute this and state that it takes 0.74 BTU of fossil fuel to create 1 BTU of ethanol fuel, compared with a ratio of 1.23 BTUs to 1 BTU for gasoline or 66% more than ethanol [45]. The conclusions of Wang et al. have largely been corroborated by Farell et al. [46]. According to them, “current corn ethanol technologies are much less petroleum-intensive than gasoline but have greenhouse gas emissions similar to those of gasoline.” The authors however opined that cellulosic ethanol would be key to large-scale use of ethanol as a fuel. Hammerschlag compared data from ten dif­ferent studies and used a parameter, rE, defined as the total product energy divided by nonrenewable energy input to its manufacture [47]. Thus, rE > 1 indicates that the ethanol has captured some renewable energy. The corn ethanol studies showed rE in the range 0.84 < rE < 1.65, and three of the cellulosic ethanol studies indicated a range of 4.40 < rE < 6.61.

Because ethanol is made from crops that absorb carbon dioxide, it generally helps reduce greenhouse emissions. Although it is carbon neutral and renewable, the GHG impact depends on farming practices, particularly the use of fertilizers. This is specifically true for ethanol made from corn. When ethanol is made from cellulosic sources there is considerable reduction in GHGs [48]. This is because producers of cellulosic ethanol burn lignin to heat the plant sugars whereas most producers of corn ethanol burn fossil fuels to provide the energy for fermentation. Cellulosic ethanol is a renewable, biodegradable, clean burning, alternative fuel. Cellulosic biomass typically contains 40-50% cellulose, 20-30% hemicellulose, and the remainder, 15-30%, is lignin and other components [49]. Cellulose consists of glucose monomers linked by a в-1,4 bond which forms a linear polymer [50]. Hemicellulose is a highly-branched complex polymer that is composed mainly of xylose and other five-carbon sugars [50]. Lignin is a phenyl propane polymer that acts as a binder, which cannot be converted into useful products. The hemicellulose is randomly acetylated and acts as an interface between the cellulose and lignin. The cellulose and hemicellulose can be broken down into simple sugars that are used to produce ethanol, while the lignin can be burned to produce heat, which helps to increase overall efficiency. What makes cellulosic ethanol promising is the diverse, abundant, low cost feedstock that is readily available. There are two main methods for the production of ethanol from biomass; enzymatic saccharification and fermentation, and fermentation by cellulolytic microorganisms.

However, cellulosic ethanol is not without its challenges and drawbacks. Commercial production of cellulosic ethanol currently requires high initial capital costs and involves risk. In 2002, a DOE study determined that for cellulosic ethanol to be competitive, the production cost would need to be $1.07 per gallon or less [51]. One of the most expensive steps in the production of cellulosic ethanol involves the pretreatment of biomass.