Page 268 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

244 8 Pre-combustion Air Emission Control

Fig. 8.4 A simplified
representation of biodiesel
production

B
lnm ¼ A þ ð8:17Þ
T þ C
2
where ν (mm /s) is the kinematic viscosity of the biodiesel at temperature T (K); A,
B, and C are correlation parameters, which can be determined from viscosity
measurements at three or more temperatures.
Nowadays most of the biodiesel fuels are produced through the base-catalyzed
transesterification of oils or fats. In this process, the vegetable oil or animal fat is
combined with alcohol, producing biodiesel and a valuable byproduct glycerin.
Figure 8.4 shows a simplified representation of this reaction, where R is the short
hydrocarbon chain of an alcohol. R can be either methanol or ethanol or isopro-
panol. R 1 ,R 2 , and R 3 are fatty acid chains. For naturally occurring oils or fats, these
fatty acids are largely palmitic, stearic, oleic, linoleic, and linolenic acids.
The exact chemical compositions of biodiesel depend on feedstock, the source of
fatty acids and the alcohol used in production. Since alcohol is relatively constant,
fatty acids are the main factor that determines the chemical composition of bio-
diesel. Most of the fats and oils contain 16 and 18 carbon chains. A large number of
vegetable oils contain predominantly unsaturated acids, whilst animal fats like cattle
tallow contain 60 % saturated acids and mono unsaturated acids as the remainder.

8.4.2.1 Air Emissions from Biodiesel Combustion

Biodiesel is generally considered to be oxygenated, sulfur-free, biodegradable, and
nontoxic.
From an environmental point of view, biodiesel is believed to be biodegradable,
renewable, and reduces CO 2 emission. Since most of the carbon in the fuel was
originally removed from the air by plants, there is very little net increase in carbon
dioxide levels.
The post-combustion exhaust gas from biodiesel combustion is characterized
with low carbon monoxide, low unburned hydrocarbons, and low particulate
emissions from diesel engines; hence it reduces further already low carbon mon-
oxide and unburned hydrocarbons. Although particulate emissions, especially the