Page 278 - Biosystems Engineering
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256 Cha pte r Ei g h t
(the main contaminants are water and free fatty acids) have influ-
enced transesterification reaction.
As per a survey of the literature, 10,18 one drawback of biodiesel is
that it is more prone to oxidation than petroleum-based diesel fuel. In
its advanced stages, this oxidation can cause the fuel to become acidic
and to form insoluble gums and sediments that can plug the fuel fil-
ter. It is observed that long-term use of pure vegetable oil and its
blend with diesel in diesel engine may result in carbon deposits in the
combustion chamber, coking and trumpet formation on the injectors
to such an extent that fuel atomization becomes difficult, oil ring
sticking, thickening, and gelling formation. Lubricating oil contami-
nation showed high piston, liner, and bearing wear, indicating that
pure vegetable oils are acceptable only for short-term use. However,
engine durability is an issue during extensive use of pure vegetable
oil blends because of carbon deposits and fueling system problems of
critical engine components resulting in premature engine failure.
18
Some investigators have suggested that a fuel additive or a fuel
blend with less vegetable oil is needed for engine durability. Pure
vegetable oil is needed for filtering and chemical treatment to reduce
viscosity and to improve combustion and flow properties. In recent
years, researchers and automobile industries have been trying to
design various parts of engine components so that pure vegetable
oils can be used 100 percent without any unwanted difficulties.
Elsbett AG from Germany has designed a piston/combustion cham-
ber in a compression ignition engine to give excellent running using
pure vegetable oils. Pure vegetable oils are used mainly in large sta-
tionary engines. Large stationary engines run on lower grades of fuel
oil, including thick crude oil full of impurities. The injector is desig-
nated to spray thick oils efficiently. The slow speed and size of these
engines give more time for the fuel to burn completely.
19
Goering et al. suggested that vegetable oils are too viscous for
prolonged use in direct-injected diesel engines that resulted in poor
fuel atomization and inefficient mixing with air, contributing to incom-
plete combustion. Due to different chemical and physical properties of
vegetable oils, vegetable oils accumulate and remain as charred depos-
its when they contact engine cylinder walls. The problem of charring
and deposits of particles on the injector and cylinder wall can be par-
tially overcome by transesterification of the oils. The process of trans-
esterification removes glycerol from the triglycerides and replaces it
with radicals from the alcohol used for the conversion process. 20
8.1.1 Transesterification of Vegetable Oils
Gummy materials that are present in vegetable oils inhibit the rate of
reaction.
Degumming is an economical chemical process of acid treatment by
which the gum of the vegetable oil is removed to improve the viscosity
18
and cetane number (CN) of vegetable oil up to a certain limit. The