Biodiesel—Mono Alkyl Esters

W. Addy Majewski, Hannu Jääskeläinen

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Abstract: Biodiesel consisting of mono alkyl esters is one type of a renewable diesel fuel derived from a number of vegetable oils or animal fats. As a renewable fuel, biodiesel has been promoted to reduce petroleum consumption. However, the life cycle analysis for biodiesel remains uncertain. Biodiesel reduces tailpipe emissions of PM, HC, and CO, while increasing NOx.


What Is Biodiesel

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 [1208]. 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 [2273]. While unprocessed vegetable oils are attractive for some diesel engine applications [1388][1389], 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 [1390]. It must be emphasized that unprocessed or partially esterified vegetable oils—unless specifically accommodated for in the engine design—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 [495]. These oils, made of fatty acid triglycerides, are converted into methyl esters before they are used as diesel fuel.

In the USA, the ASTM 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” [148]. 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.

The structure of three example methyl esters representative for biodiesel—those of oleic acid, C17H33COOH, linoleic acid, C17H31COOH, and linolenic acid, C17H29COOH, three acids of one, two, and three unsaturated bonds, respectively—is shown in Figure 1.

[SVG image]
Figure 1. Fatty acids methyl esters

The most common source of biodiesel in the USA is soybeans, while in Europe the main source of biodiesel is rapeseed. Other significant biodiesel resources—estimated to be more abundant than soy—are greases and animal fats [1209]. These feedstocks are also potentially more sustainable than agricultural crops.

Oil from jatropha (Jatropha curcas) nuts was considered as a potential biodiesel feedstock in tropical climates in the early years of the 21 century because jatropha plants tolerate poor, degraded soils and are resistant to pest and disease. Plantations for biodiesel production were started in India and in some African countries [1159]. However, under marginal growing conditions, jatropha plants produce few of the nuts from which the oil is extracted and good yields typically require growing conditions that make it compete with food crops. Additionally, unlike other crops, the residuals from jatropha plants have no commercial value which makes revenue from its cultivation entirely dependent on marketing the nuts. Since interest in jatropha has waned, some hybrid varieties have been developed that make it a more appealing crop [3580][3581].

Palm (Elaeis guineensis) oil is one of the most cost effective feedstocks for the production of biodiesel, as well as renewable diesel. Palm oil has been increasingly produced in Southeast Asia (Malaysia, Indonesia, Thailand) and in South America. From the commercial point of view, palm oil is particularly attractive because its yield per unit area of land is much higher than that of soy, rapeseed or other crops. Demand for palm oil is therefore likely to continue all over the world, in spite of the negative association of palm oil plantations with deforestation and environmental degradation of large areas of tropical forest.

Depending on the feedstock, biodiesel may be referred to by a number of more specific terms and corresponding acronyms, the most common ones being:

In France, biodiesel fuel—mostly rape methyl ester—is also referred to as diester.

Biodiesel is usually blended with petroleum diesel. The blends are often designated by a capital B followed by biodiesel percentage (by volume) in the blend. For example, B5 indicates a blend containing 5% of biodiesel and 95% of conventional petrodiesel, while B20 is composed of 20% biodiesel and 80% petrodiesel. Under the same convention, neat biodiesel is termed B100.