Biodiesel—Mono Alkyl Esters

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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 increases engine-out emissions of NOx, while its effect on PM, HC, and CO depends on the engine technology.

What Is Biodiesel

Biodiesel fuels, defined as fatty acid alkyl esters, are the outcome of research that investigated the use of vegetable oils as diesel engine fuels. Compared to virgin oils, alkyl ester biodiesel has much lower viscosity and significantly higher cetane number. 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.

In the United States, 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.

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.

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 petroleum diesel, while B20 is composed of 20% biodiesel and 80% petroleum diesel. Under the same convention, neat biodiesel is termed B100.