The primary criterion for biodiesel quality is adherence to the appropriate standard. In The United States, this standard is ASTM D 6751-02 “Standard Specification for Biodiesel Fuel (B100) Blend Stock for Distillate Fuels”. Generally, the fuel quality of biodiesel can be influenced by several factors:
• The quality of the feedstock.
• The fatty acid composition of the parent vegetable oil or animal fat.
• The production process and the other materials used in this process.
• Post-production parameters.
Biodiesel produced from acid oil closely meets the standard specifications. Chemical and fuel properties of biodiesel are distinctly different from those of diesel fuels. As the types of compounds associated with biodiesel are different. Biodiesel is a mixture of FAME compounds which are all of narrow range and typical long carbon chain types of saturated and unsaturated oxygenated hydrocarbon structures, while diesel fuel is a mixture of large range alkyl, substituted napthene and aromatics types of hydrocarbon compounds.
Elemental composition and relative amounts of compounds present in biodiesel and diesel fuel are shown in table 2 and table 3. Due to presence of electronegative element oxygen. Biodiesel is slightly more Polar than diesel fuels as a result viscosity of biodiesel is higher than diesel fuel. Presence of elemental oxygen lowers the heating value of biodiesel when compared to diesel fuels.
Table 4 shows the property values required for a mixture of methyl esters to be considered biodiesel. When these limits are met, the biodiesel can be used in most modern engines without modifications while maintaining the engine’s durability and reliability. Even in low level blends with conventional diesel fuel, the biodiesel blending stock is expected to meet the standard before being blended. While some properties in the standard, such as cetane number and density, reflect the properties of the chemical compounds that make up biodiesel. Other properties provide indications of the quality of the production process. Generally, the parameters given in ASTM D6751 are defined by other ASTM standards.
However, other test methods, such as those developed for the American Oil Chemists’ Society, (AOCS) may also be suitable (or even more appropriate as they were developed for fats and oils and not for petroleum-derived materials addressed in the ASTM standards).
This discussion will focus on the most important issues for assuring product quality for biodiesel as it is related to production as well as some post-production parameters.