Environmental Engineering Reference
In-Depth Information
1.1. History and Development
Conventional diesel fuel is the portion of crude oil that is distilled between approximately
200°C (392°F) and 370°C (698°F), higher than the boiling range of gasoline and consistent
with its heavier, oilier composition. Diesel fuel is ignited in a CIE cylinder by the heat of air
under high compression, in contrast to motor gasoline, which is ignited by an electrical spark.
Because of the mode of ignition, a high cetane number (cetane is the hydrocarbon C16H34, or
1 -hexadecane, that ignites very easily under compression and is therefore used as a standard
in determining diesel fuel ignition performance) is required in a good diesel fuel. Two grades
of diesel fuel have been established by the ASTM: Diesel 1 and Diesel 2. Diesel 1 is a
kerosene- type fuel, which is lighter, more volatile, and cleaner than Diesel 2, and is used in
engine applications with more frequent changes in speed and load. Diesel 2 is used in
industrial and heavy mobile service [5, 6]. The term Biodiesel, in turn, is collective describing
fuels comprised of esterified plant oils or animal fats. These biological lipids originate as
mixtures of triglycerides and free fatty acids that are derivatized through transesterification
(also known as alcoholysis) with acid, base, or enzymatic catalysis to form, most commonly,
methyl or ethyl esters (Figure 14) [7, 1, 3].
Before the advent of biodiesel, biological oils were investigated extensively in their
native forms for use in diesel engines. Indeed, Rudolf Diesel himself experimented with
vegetable oils in his engine (Figure 15) over 100 years ago [7]. Unfortunately, however, plant
oils and especially animal fats typically have sufficiently high viscosities, and sufficiently low
cetane numbers, flash points, and combustibility, that they show numerous undesirable
properties in CIEs. Principal among these are carbon deposition on engine parts, gelling and
polymerization during storage and in cool temperatures, and contamination of engine
lubricating oils leading to deterioration of lubricant and ultimately engine performance [8, 7].
As a result, the availability of inexpensive petroleum led quickly to the nearly exclusive
use of diesel fuel, which lacked the undesirable performance properties but brought with it its
own undesirable, if not initially appreciated, environmental consequences. Nevertheless,
experimentation with soybean, canola, and other vegetable oils, as well as beef tallow and
other animal fats, continued for many decades as researchers attempted to discover mixtures
with petroleum fuels that minimized the problems of incomplete combustion posed by the
biolipids. While these measures successfully diminished the rate of engine and lubricating oil
deterioration, most problems persisted to some extent, and at present the use of and
experimentation with unmodified biolipids has effectively ceased [7].
Figure 14. Transesterification of a triglyceride with an alcohol, showing the production of fatty acid
esters and glycerol. Adapted from [1].
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