Environmental Engineering Reference
In-Depth Information
ester content value of biodiesel increases with distillation, as impurities such as sterols, residual
alcohols, partial glycerides, and unseparated glycerin are removed (Prankl et al. 2004; Rilett and
Gagnon 2008).
It is demonstrable that ester content, as a parameter, is similar to the distillation temperature, T90
parameter. However, this is mainly used by the EU and the biodiesel should have an ester content
value equal or higher than 96.5% m/m (EN 14103) to meet the fuel specifications. The United States
limits biodiesel purity with the use of the T90 parameter (see Section 9.2.12). The main concern
for this parameter is the precision of the methodology used, which may be considered unacceptable
given that this method is not suitable for lauric oils such as coconut or palm kernel oil. Currently,
the upgrade or the removal of the test method is being discussed. In the second case, the ester con-
tent values would be replaced by another parameter such as the content of unsaponifiable material
(White Paper 2007).
9.2.17 i
odinE
n
umBEr
, l
inolEnic
a
cid
m
Ethyl
E
StEr
,
and
p
olyunSaturatEd
B
iodiESEl
Iodine value is indicative of total unsaturation within a mixture of fatty material, regardless of
the relative shares of mono-, di-, tri-, and polyunsaturated compounds (White Paper 2007; Rilett
and Gagnon 2008) and only dependent on the origin of the vegetable oil (Mittelbach 1996).
Biodiesel derived from oil with high unsaturation will also have high iodine values (Rilett and
Gagnon 2008). Unsaturated esters blended with engine oil may form high-molecular-weight
compounds that can negatively reduce the lubricating quality and as a consequence damage the
engine (Schäfer et al. 1997).
Fuels with high iodine number (i.e., consisting of many unsaturated esters) tend to polymerize
and form deposits on injector nozzles, piston rings, and piston ring grooves when they are heated
(Kobmehl et al. 1997; Heinrich 1997). Actually, polymerization reactions are directly related to the
number of double bonds and become significant in fatty acid esters containing at least three double
bonds (Prankl et al. 1999, 2004). However, three- or more-fold unsaturated esters are not a very com-
mon component for all seed oils, and as a result various promising seed oils are incorrectly excluded
from the biodiesel market according to some national standards because of their high iodine value
(e.g., soybean oil with an iodine value of 125-140 g I
2
/100 g). Therefore, it is better to limit the
content of linolenic acid methyl esters and polyunsaturated FAMEs rather than the total degree of
unsaturation (iodine value) (Knothe and Dunn 1998; Prankl et al. 2004; White Paper 2007).
European specification limits the iodine number to 120 g of iodine/100 g of biodiesel maximum
(EN ISO 14111), the linolenic acid to 12.0 mg/kg maximum (EN 14103), and polyunsaturated (four
or more double bonds) methyl ester to 1 mg/kg maximum (method under development). Some coun-
tries, such as Brazil and the United States, disagree with this limit because it is very similar to the
oxidation stability limit. Moreover, the iodine number eliminates soybean, sunflower, and other
unsaturated oils from meeting specifications regardless of the use of stability additives or use as a
blend stock rather than as a standalone fuel. EU experts are currently discussing increasing iodine
number combined with other precautions such as reinforced limits on linolenic acid, polyunsatu-
rates, and oxidation stability (White Paper 2007).
9.2.18 d
EnSity
Density is the mass of a unit volume of material at a selected temperature. The density of the
fuel determines the heating value per volume of this fuel given that the other fuel properties are
unchanged (Bacha et al. 2007). Moreover, the density of the fuel directly influences the injection
timing of mechanically controlled injection equipment as well as the volume of injected fuel, both
of which are related to the engine power and, consequently, engine emissions and fuel economy
(WWFC 2006).
