Dependence of biodiesel fuel properties on the structure of fatty acid alkyl esters
Introduction
Biodiesel is an alternative diesel fuel derived from vegetable oils or animal fats [1], [2]. The transesterification of an oil or fat with a monohydric alcohol, in most cases methanol, yields the corresponding mono-alkyl esters, which are defined as biodiesel. The successful introduction and commercialization of biodiesel in many countries around the world has been accompanied by the development of standards to ensure high product quality and user confidence. Some biodiesel standards are ASTM D6751 (ASTM=American Society for Testing and Materials) and the European standard EN 14214, which was developed from previously existing standards in individual European countries.
While the suitability of any material as fuel, including biodiesel, can be influenced by contaminants arising from production or other sources, the nature of the fuel components ultimately determines the fuel properties. Some of the properties included as specifications in standards can be traced to the structure of the fatty esters comprising biodiesel. However, as biodiesel consists of fatty acids esters, not only the structure of the fatty acids but also that of the ester moiety derived from the alcohol can influence the fuel properties of biodiesel. Since the transesterification reaction of an oil or fat leads to a biodiesel fuel corresponding in its fatty acid profiles with that of the parent oil or fat, biodiesel is a mixture of fatty esters with each ester component contributing to the properties of the fuel.
The properties of a biodiesel fuel that are determined by the structure of its component fatty esters include ignition quality, heat of combustion, cold flow, oxidative stability, viscosity and lubricity. The present work discusses the influence of the structure of fatty esters on these properties. Not all of these properties have been included as specifications in biodiesel standards, although all of them are essential to the proper functioning of the fuel.
Section snippets
Experimental
Cetane numbers (CN) of branched and straight-chain esters were determined in the Ignition Quality Tester™ (IQT™) described previously [3]. The IQT™ was calibrated with hexadecane and 2,2,4,4,6,8,8-heptamethylnonane (HMN) as primary reference fuels. CNs obtained with the IQT™ will be referred to as CNIQT in the text. This method has also become an ASTM standard test method (ASTM D6890), in which the CNs are termed derived cetane number (DCN).
For oxidation studies, an oxidative stability
Results and discussion
Generally, methanol has been mostly used to produce biodiesel as it is the least expensive alcohol. Biodiesel in most cases can therefore be termed the fatty acid methyl esters (FAME) of a vegetable oil or animal fat. However, as mentioned above, both moieties, the fatty acid chain and the alcohol functionality, contribute to the overall properties of a fatty ester and it is worthwhile to consider the properties imparted by other alcohols (yielding fatty acid alkyl esters; FAAE) that could be
Summary and conclusions
The above discussion shows that the fuel properties of biodiesel are strongly influenced by the properties of the individual fatty esters in biodiesel. Both moieties, the fatty acid and alcohol, can have considerable influence on fuel properties such as cetane number with relation to combustion and exhaust emissions, cold flow, oxidative stability, viscosity, and lubricity. Generally, cetane number, heat of combustion, melting point, and viscosity of neat fatty compounds increase with
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