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The use of biologically derived fuels in diesel engines is as old as the diesel engine itself. Rudolph Diesel reportedly considered vegetable oils as a potential fuel for use in his invention. During the 1970s and 1980s, research was conducted with pure vegetable oils and partially esterified oils, both in their neat form and in blends with petrodiesel. Eventually, biodiesel research focused on alkyl esters due to their superior properties.
Compared to alkyl ester biodiesel, virgin oils tend to have significantly lower cetane and much higher viscosity. Pre-heating of the virgin oil is sometimes employed to lower its viscosity . A chemical modification of high molecular mass triglycerides—where some of the long fatty acid chains are replaced with a short acetyl chain—has also been suggested to lower the viscosity of virgin oils . While unprocessed vegetable oils are attractive for some diesel engine applications , pure or partially esterified oils may lead to a variety of engine problems such as long-term engine deposits, piston ring sticking, fuel injector plugging, or lube oil gelling which can cause engine failure or require more frequent maintenance and shorter engine overhaul intervals. Exhaust from engines fueled with raw vegetable oils can also have more adverse health effects, as suggested by their higher mutagenic activity compared to exhaust from the same engine using diesel fuel . It must be emphasized that unprocessed or partially esterified vegetable oils are generally considered unsuitable as diesel fuels and do not meet the requirements set by existing biodiesel standards and specifications.
Many vegetable oils and animal fats have been suggested, investigated, and—some of them—commercialized as feedstocks for the production of diesel fuel substitutes. The common sources of oil include soybean, rape/canola, palm, sunflower, coconut, camelina, used frying oil, as well as a number on non-edible oils including those from jatropha or kanuga seeds. Methods have also been developed to make diesel fuel substitutes from oils naturally produced by certain species of algae . These oils, made of fatty acid triglycerides, are converted into methyl esters before they are used as diesel fuel.
In the USA, the ASTM (American Society for Testing of Materials) Biodiesel Task Force adopted a definition of biodiesel that limited it to “mono alkyl esters of long chain fatty acids derived from renewable lipid feedstocks, such as vegetable oils and animal fats, for use in compression ignition (diesel) engines” . The mono alkyl ester definition eliminates pure vegetable oils as well as monoglycerides and diglycerides from consideration as biodiesel.
At the time this definition was developed, alkyl esters began to receive significant attention and were the only fuel that was considered to carry the label “biodiesel”. As discussed in the paper on alternative fuels, later developments created pressure to expand the definition to include renewable diesel-like fuels made with other processes and composed of chemically different compounds. For instance, the term biodiesel has also been used by some authors to describe hydrocarbon-based fuels manufactured through refinery hydrogenation of fats or vegetable oils—an increasingly more important type of biofuel. Thus the term “biodiesel” is no longer necessarily limited to alkyl esters. To avoid confusion, hydrocarbon fuels produced by hydrotreating oils at a refinery are now commonly referred to as “renewable diesel” fuels. This became a widely accepted naming convention. In the remainder of this paper, we use the term biodiesel in its conventional scope, limited to mono alkyl ester fuels.
The most common alcohol used in the production of mono alkyl esters is methanol and the more specific label “methyl ester” is often attached to this type of esterified biodiesel. Higher order alcohols such as ethanol and propanol can also be used in the production of mono alkyl esters. These produce other classes of compounds called “ethyl esters” and “propyl esters” respectively.