Gerhard Knothe

5.1 Introduction

Biodiesel is an alternative diesel fuel (DF) derived from vegetable oils or animal fats [1, 2]. Transesterification of an oil or fat with a monohy — dric alcohol, in most cases methanol, yields the corresponding mono­alkyl 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 stands for American Society for Testing and Materials) and the European standard EN 14214, which was developed from pre­viously existing standards in individual European countries.

The suitability of any material as fuel, including biodiesel, is influenced by the nature of its major as well as minor components arising from pro­duction or other sources. The nature of these components ultimately deter­mines the fuel and physical properties. Some of the properties included in standards can be traced to the structure of the fatty esters in the biodiesel. Since biodiesel consists of fatty acid esters, not only the structure of the fatty acids but also that of the ester moiety can influence the fuel proper­ties of biodiesel. The transesterification reaction of an oil or fat leads to a biodiesel fuel corresponding in its fatty acid profile with that of the parent oil or fat. Therefore, biodiesel is largely a mixture of fatty esters with each ester component contributing to the properties of the fuel.

Properties of biodiesel that are determined by the structure of its component fatty esters and the nature of its minor components include [9]

ignition quality, cold flow, oxidative stability, viscosity, and lubricity. This chapter discusses the influence of the structure of fatty esters on these properties. Not all of these properties have been included in biodiesel standards, although all of them are essential to proper func­tioning of the fuel in a diesel engine.

Generally, as the least expensive alcohol, methanol has been used to produce biodiesel. 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 the fatty acid chain and the alcohol functionality contribute to the overall properties of a fatty ester. It is worthwhile to consider the properties imparted by other alcohols yield­ing fatty acid alkyl esters (FAAE) that could be used for producing biodiesel. Therefore, both structural moieties will be discussed in this chapter. Table 5.1 lists fuel properties of neat alkyl esters of fatty acids. Besides the fuel properties discussed here, the heat of combustion (HG) of some fatty compounds [3] is included in Table 5. 1, for the sake of under­scoring the suitability of fatty esters as fuel with regard to this property.