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.
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                   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
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               and cetane number (CN) of vegetable oil up to a certain limit.  The
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