Environmental Engineering Reference
In-Depth Information
federal biodiesel excise tax credit that provides one cent credit per percent of biodiesel per
gallon. Under this incentive, taken by the producer and passed on to the consumer, B100
therefore realizes a $1.00 per gallon-cost reduction, and the price of 20 percent biodiesel
combined with 80 percent petroleum diesel (B20) becomes comparable to petroleum diesel
[14]. In other countries, biodiesel production and use is facilitated by higher petroleum prices
(diesel fuel cost is greater by a factor of 1.5-2.5 across Europe [13]), by use of less-expensive
feedstocks, and by a variety of creative pro-biodiesel incentives. In Europe, biodiesel
production has been encouraged by EU farm production programs: much of the biodiesel
expansion in the 1990s occurred as a result of EU policies that allowed farmers to grow crops
for industrial uses, including oilseeds, on set-aside land. Tax benefits from Germany, Austria,
and France have also encouraged biodiesel production and use, allowing biodiesel to find far
greater success in these countries than is possible in the United States with its lack of
government incentives [15]. Rapeseed oil, the predominant feedstock in Europe, is also priced
lower than soybean oil [16].
Nevertheless, the cost of feedstock oils is expected to continue to be a concern in
biodiesel success, promoting investigation of alternative oils. Waste cooking oil, tallow, and
lard are quite inexpensive feedstocks, for example, that are currently used for biodiesel
production in Japan and are promising as well for other areas of Asia that have limited
agricultural land and where vegetable oils are fairly expensive [7, 17].
Another approach is the development of processes that yield valuable coproducts:
glycerol is a clear candidate coproduct in biodiesel production, especially if the
transesterification of plant oils is accomplished enzymatically. Enzyme use eliminates the
need for an alcohol evaporation process, necessary for glycerol recovery from alkali
transesterifications, and also minimizes saponification (soap formation) of glycerol and it
attendant purification difficulties. Because of the favorable commodity market for glycerol,
the cost of biodiesel could be lowered significantly if biodiesel plants incorporated glycerol
recovery facilities [7] or provisions for production of other high-value coproducts, such as
caproic or propionic acids [15].
2.2. Abiotic Processing
2.2.1. Pyrolysis
. Pyrolysis, the use of heat or heat plus a catalyst in the absence of oxygen
to convert one substance into another, has been investigated worldwide for much of the last
century for biofuel production. In this process, triglyceride fatty acids are “cracked” from the
glycerol backbone and further decomposed, yielding a mixture comprised primarily of
alkanes and alkenes with smaller proportions of carboxylic acids and aromatics. While this
approach effectively diminishes the viscosity of the oils and fats, a number of new problems
arise: the process tends to generate a greater proportion of lower molecular-weight products
(gasoline), the equipment required is expensive for modest throughputs, and the removal of
oxygen during the thermal processing also removes the environmental benefits of using an
oxygenated fuel [1, 7].
2.2.2. Microemulsions
. Another approach to minimize vegetable oil viscosity is the
formation of water-oil microemulsions, in which an oil is stably dispersed in a solvent such as
methanol, ethanol, or butanol in 1-150 nanometer micelles by association with ionic or
nonionic amphiphiles. While viscosity has been successfully diminished, and spray
performance has been successfully enhanced by this technique, the problems of incomplete
