Page 375 - Advances in Eco-Fuels for a Sustainable Environment

330 Advances in Eco-Fuels for a Sustainable Environment

hydrogen from carbon and thus forming hydroperoxide (ROOH). The cycle keeps
propagating when the new free carbon radical reacts with oxygen. Finally, in the ter-
mination step, the free radicals react with each other. The ROOH concentration
remains very low for some initial period of oxidation, but only up to sometime. This
period is called the induction period and is determined by the conditions under which
the biodiesel/fatty acid is stressed as well as its oxidation stability. The ROOH level
rapidly increases (after the induction period), indicating the commencement of oxida-
tion. The ROOH induction period can significantly influence other important proper-
ties of biodiesel. In this peroxidation mechanism, the easily abstracted hydrogens are
usually the ones that are bonded to allylic carbons in the long chain fatty acid, and
nonallylic carbon hydrogens do not participate in this mechanism. The resonance sta-
bilized rearrangement of the pi electron system is one major reason for this observa-
tion. Hydrogens from carbons allylic to two other carbon atoms simultaneously (bis-
allylic carbon atoms) are extremely susceptible to abstraction. This is because the
rearrangement of the mechanism is highly resonance stabilized. These products that
are formed in the initial stages of oxidation are known as primary oxidation products.
Oxidation instability of any fatty acid ester is directly related to the number of
allylic and bis-allylic carbon atoms present. Once the hydroperoxides are formed, they
decompose to form aliphatic alcohols, aldehydes, carboxylic acids, and esters. These
are known as secondary products of oxidation. Increased acidity is primarily due to
oxidation of biodiesel where shorter chain fatty acids are formed.

Factors affecting oxidation
l Metals such as Ni, Sn, Fe, Cu, and brass can significantly decrease the Oxidization Stability
Index (OSI) of biodiesels.
l Long chain fatty acids, regardless of whether saturated or unsaturated, have a significant
influence on the OSI of their corresponding biodiesel. The free acids were found to be
far more unstable than their corresponding methyl ester. The trend for increased stability
for long unsaturated chain fatty acids was found to be linolenic < linoleic < oleic.
In AMC biodiesel, the percentage of the oleic acid is 26.69, the percentage of the lin-
oleic acid is 24.48, and the percentage of the linolenic acid is 15.43%. In Jatropha
biodiesel, the percentage of the oleic acid is 40, the percentage of the linoleic acid
is 36.9, and the percentage of the linolenic acid is 0.2%. In Karanja biodiesel, the per-
centage of the oleic acid is 51.59, the percentage of the linoleic acid is 16.64, and the
percentage of the linolenic acid is 5.7.
Hence the oxidation stability of AMC biodiesel was found to be poor when com-
pared to Jatropha and Karanja.
The oxidation stability of the methyl esters will depend on the alcohol group used in the
l
transesterification process.
On exposure to light, the oxidation stability of biodiesels can be decreased. This process is
l
called photo-oxidation, and in this mechanism, the diatomic oxygen attacks the vinylic car-
bon directly. The energy required to proceed with this unfeasible reaction is provided by the
photons.
l Antioxidants are substances that inhibit oxidation process. There are two types of antioxi-
dants, namely chain breakers and hydro-peroxide decomposers, among which chain breaker
antioxidants are more common. Also, there are two types of antioxidants available: synthetic