Environmentally Sustainable Biofuels – The Case for Biodiesel, Biobutanol and Cellulosic Ethanol - Sustainable Biotechnology: Sources of Renewable Energy - page 56

fermentation of sugars from biomass or by the gasification of cellulosic biomass.
It can be blended in any ratio with gasoline and be used in existing automobiles
without any need for engine or fuel line modifications. It is an attractive substitute
to gasoline because its BTU content is 110,000 BTU's per gallon, which is very
close to the 115,000 BTU per gallon of gasoline, resulting in little change to fuel
economy. The Reid vapor pressure (RVP) of butanol (0.33 psi) is low compared
to ethanol (2 psi) or gasoline (4.5 psi), resulting in lower evaporative emissions.
The octane values and energy density of butanol are also closer to gasoline than is
ethanol. Ethanol is 100% soluble in water whereas the solubility of butanol is 9.1%
at 25 ◦ C [10]; this results in less water absorbed and rust dissolved into the fuel from
tanks and pipelines. An added benefit to the low solubility is reducing the spread
into groundwater in case of a spill.
However, biobutanol is not a perfect fuel and has several disadvantages. Butanol
is more toxic to humans and animals than lower carbon alcohols. The LD 50 oral
consumption for a rat for butanol is 790 mg/kg compared to 7,060 mg/kg for ethanol
[13]. However, it is well known that gasoline contains chemicals such as benzene,
which is toxic and carcinogenic. There have been no definitive tests as to whether
butanol will degrade the materials in an automobile over time, but current evidence
suggests that this is unlikely [10]. Environmental Energy, Inc. tested a 1992 Buick
Park Avenue by driving it 10,000 miles on 100% butanol [33]. No modifications
were done to the car and it passed all emission tests performed in 10 states with
an average increase in gas mileage of 9%. Compared to gasoline, combustion of
butanol reduces the amount of hydrocarbons, carbon monoxide, and smog-creating
compounds that are emitted [33].
Butanol is used as an industrial solvent and the market demand is about 350
million gallons a year worldwide, with the United States accounting for 63%. The
production of butanol via fermentation is the second oldest fermentation process,
next only to ethanol. Since the 1950s however, production of butanol via fermen-
tation has not been an economically viable alternative due to the historic low cost
of petroleum. A new push for renewable alternative fuel sources has been fueled
by the increasing cost of petroleum combined with the generation of more green-
house gases. These two reasons and the development of new technologies form the
underpinnings of the reemergence of the butanol fermentation process.
3.2 Comparison of Processes
The oldest method of butanol production is the acetone-butanol-ethanol (ABE)
bacterial fermentation by Clostridium acetobutylicum , which dates back to Louis
Pasteur in 1861 [13]. The bacterial microorganism, C. acetobutylicum , was first
isolated by Weizmann [13]. In the ABE fermentation process, C. acetobutylicum
produces acetic, butyric, and propionic acids from glucose that can be generated
from various biomass sources. Potential feedstocks include corn, molasses, whey
permeates, or glucose. An enzyme catalyzed reaction of acetoacetyl-CoA transfers