Generally, the oil burnt in a diesel (compression-ignition) engine is called diesel. If produced from petroleum, it is called petrodiesel, and if produced from biomass, it is called biodiesel. Mineral diesel (or petrodiesel) is made of a large number of saturated and aromatic hydrocarbons. The average chemical formula can be Ci2H23. Petrodiesel (also called fossil diesel) is produced from the fractional distillation of crude oil between 200 °C and 350 °C at atmospheric pressure, resulting in a mixture of carbon chains that typically contain between 8 and 21 carbon atoms per molecule (Collins, 2007).

According to the American Society for Testing and Materials (ASTM), biodiesel (B100) is defined as "a fuel comprised of mono-alkyl (methyl) esters of long chain fatty acids derived from vegetable oils or animal fats, and meeting

the requirements of ASTM D 6751.” Its characteristics are similar to those of petrodiesel, but not identical. Biodiesel, which can be mixed with petrodiesel for burning in diesel engines, has several positive features for use in engines, as listed in the following:

• Petrodiesel contains up to 20% polyaromatic hydrocarbon, while biodiesel contains none, making it safer for storage.

• Biodiesel has a higher flash point, making it safer to handle.

• Being oxygenated, biodiesel is a better lubricant than petrodiesel is and therefore gives longer engine life.

• Its higher oxygen content allows biodiesel to burn more completely. Biodiesel Production from Methanol

Biodiesel is generally produced from vegetable oil and/or from animal fats with major constituents that are triglycerides. It is produced by transesterification of vegetable oil or fat in the presence of a catalyst. Biodiesel carries the name fatty acid methyl (or ethyl) ester, commonly abbreviated as FAME. A popular production method involves mixing waste vegetable oil or fat with the catalyst and methanol (or ethanol) in appropriate proportion. A typical proportion is 87% oil, 1% NaOH catalyst, and 12% alcohol. Both acid and base catalysts can be used, but the base catalyst NaOH is the most common. Because NaOH is not recyclable, a “nongreen” feed is required to produce “green” biodiesel. Efforts are being made to produce recyclable catalysts and thereby make the product pure “green.”

Figure 9.5 shows the reaction for the conversion of triglyceride into bio­diesel (FAME) and its by-product, glycerol. Glycerol cannot be used as a transport fuel, and its disposal is a major issue.

An alternative noncatalytic conversion route for biodiesel is under develop­ment in which transesterification of triglycerides is by supercritical methanol (above 293 °C, 8.1 MPa) without a catalyst (Kusdiana et al., 2006). The metha­nol can be recycled and reused, but the process for this must be carried out at high temperatures and pressures. Efforts are also being made to use woody biomass (ligno-cellulose) instead of fats or oil to produce biodiesel using the supercritical method (Minami and Saka, 2006). The reaction is carried out in

a fixed or fluidized bed. The fluidized bed has the advantage of continuous catalyst regeneration and efficient removal of the heat of reaction.