Page 374 - Advances in Eco-Fuels for a Sustainable Environment

Physicochemical fuel properties and tribological behavior of aegle marmelos correa biodiesel 329

As a fuel ages and oxidizes, some of the fuel will polymerize into heavier, more
viscous compounds and some will split into lighter, volatile molecules that may
depend on storage conditions and escape the bulk fluid. More viscous fuel will leave
more carbon residue, but that is not the only factor responsible. If the fuel is oxidized
in a sealed container, these lighter compounds may not be able to escape, and the over-
all viscosity of the fuel mixture will remain largely unchanged. In this case, viscosity
changes should have less of an effect on carbon residue, and degree of unsaturation
will have a dominating effect.
Another main effect that oxidation could have on carbon residue is the break down
of double bonds in the fuel for the creation of hydroperoxides. If these double bonds
are eliminated in the reaction, a fuel could become more saturated. If the degree of
saturation in the fuel increases during oxidation, then carbon residue can be expected
to decrease as the fuel oxidizes. Any increase in viscosity will likely be overshadowed
by an increase in saturation. Increase in saturation during oxidation is only possible
when there is more unsaturation initially.

11.3.3.10 Oxidation stability

In general, the unsaturated components are particularly unstable and are subjected to
deterioration on oxidation, which is always undesirable as it leads to fuel darkening,
reduced induction periods, and formation of complex polymers. Storage of biodiesel
and long use of it in an engine will lead to aging of the biodiesel, forming hydroper-
oxides initially, then subsequently giving rise to acids, aldehydes, and polymers.
Ultimately, the acidity increases as a result of oxidation products which leads to
the formation of corrosion and rusting on the metal surface. The formation of insoluble
substances makes the blockage in nozzle, filter and increases the viscosity in the form
of oil thickening.
The role of antioxidants is to increase the induction period or the period for which
the biodiesel can be safely stored before any undesirable oxidation products appear.
However, this response of biodiesel to any antioxidant additive to achieve an increase
in induction period and ultimately oxidation stability depends on both fuel composi-
tion and the type of antioxidant additive. Hence complete research of antioxidants and
their concentration is important to obtain the best results for a fuel blend and also a
cost-effective solution. Biodiesel contains a mixture of long chain fatty acids, both
saturated and unsaturated. Saturated fatty acids undergo very minimal oxidation
because they are more stable and nonreactive. Hence unsaturation indicates oxidative
instability in biodiesels.

Mechanism of oxidation
The first step involves the formation of carbon-based free radicals by removing a
hydrogen from a carbon atom. The subsequent reaction, in the presence of oxygen,
is extremely fast to form peroxy radicals and this reaction is so fast that it does not
allow other alternatives for the carbon-based free radical to react. Although the peroxy
free radical is not even close to being as reactive as the carbon free radical, it is suf-
ficiently reactive to form another carbon-based free radical by abstracting free