Environmental Engineering Reference
In-Depth Information
other factors also influence the suitability of a crop for bioenergy use. The quality and functionality
of the diesel fuel is heavily influenced by the physical/chemical properties and fatty acid composition
of the vegetable oils. The cetane number, viscosity, density, flashpoint, pour and cloud points and
iodine values are all properties of diesel fuel that are affected by the source vegetable oil (Lang et al.
2001; Ramdhas et al. 2004; Al-Zuhair 2007; Canacki and Sanli 2008). Cetane number, a measure
of fuel ignition quality, increases with increasing FA chain length and decreasing unsaturation/
branching. A cetane value of between 40 and 60 is considered satisfactory for ignition quality.
Viscosity describes a fuel's heaviness. High viscosity interferes with the fuel injection process, and
it increases with increasing carbon chain length of the fatty acids and decreases with the increasing
unsaturation (double bonds). Density indicates the fuel's density or compressibility. An increase
in density improves fuel injection, and density is raised by the chain length/saturation of the fatty
acids. A fuel's flashpoint is the temperature at which fuel starts to burn when in contact with fire.
This is a critical temperature from an engine safety point of view. It is usually affected by the degree
of FA saturation. Pour and cloud points are indicators of a fuel's reaction to low temperatures. They
predict the cold flow quality of the fuel and need to be below the freezing points of the vegetable oils
used. Feed stocks with large amounts of saturated FA have higher cloud and pour points. Finally, the
iodine value is a measure of the number of double bonds in oils and determines fuel stability.
Higher iodine values cause the formation of corrosive acids and deposits (oxidation) in the engines.
Table 18.2 provides the comparison of the above properties in selected vegetable oils including the
rapeseed, whereas the FA compositions of some vegetable oils are compared in Table 18.3.
According to The Canola Council of Canada (http://www.canola-council.org), canola's lowest
level of saturated FA and low iodine values make it one of the best feedstocks for biodiesel, imparting
greater cold flow properties and oxidative stability than those of many other vegetable oils.
18.3.3 E
conomic
i
mplicationS
The economic implications of rapeseed/canola as a bioenergy crop depends on the economic
viability of producing methyl or ethyl esters (biodiesel) using rapeseed as a feedstock. Therefore,
the main thrust of an economic analysis is always to compare the cost of biodiesel production
with the cost of diesel fuel, and such an analysis is highly affected by the fluctuations in market
trends for both feedstock and diesel fuel. For example, the cost of diesel fuel in September
2007 was 733 U.S.$/t whereas the cost of RME (rapeseed derived biodiesel) was up to 3 times
higher (1,060 U.S.$/t), but with the increase in fuel prices (1,017 U.S.$/t) this price gap narrowed
taBle 18.2
Properties of major vegetable oils as related to Biodiesel
oil crop
Iv
vs
cn
FP
cP
PP
Rapeseed
94-120
37.3
37.5
246
-3.9
-31.7
Soybean
117-143
33.1
38.1
254
-3.9
-12.2
Sunflower
110-143
34.4
36.7
274
7.2
-15
Palm
35-61
42
Cotton
90-119
33.7
33.7
234
1.7
-15
peanut
80-106
40
34.6
271
12.8
-6.7
Safflower
126-152
31.6
36.7
246
-3.9
-3.17
Source:
Derived from Canakci, M. and Sanli, H.,
J Indust Microbiol Biotechnol Special
Issue
, pp 1-23, 2008; Han, J.,
Advances in Plant Lipid Research
. Secretariado de
Publicaciones de la Universidad de Sevilla, Sevilla, Spain, pp 665-668, 1998.
IV, iodine value; CN, cetane number; VS, viscosity (mm
2
/s); FP, flash point (°C); CP, cloud
point (°C); PP, pour point (°C).
