Encyclopedia of Renewable Energy. James G. Speight

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      *Ethanol (ethyl alcohol, C₂H₅OH) could also be used.

      Biodiesel is an alternative to crude oil-based fuels. Biodiesel is made from a variety of natural oils, especially rapeseed oil (a close cousin of canola oil) and soybean oil. Rapeseed oil tends to dominate the growing biodiesel industry in Europe, whereas in the United States, biodiesel is made from soybean oil because more soybean oil is produced here than all other sources of fats and oil combined. Palm and Jatropha oil are favored in Asian countries, but there are many other feedstock candidates, including recycled cooking oils, animal fats, and other oilseed crops.

      Biodiesel is a renewable fuel that can be manufactured from algae, vegetable oils, animal fats, or recycled restaurant greases; it can be produced locally in most countries. It is safe, biodegradable, and reduces air pollutants, such as particulates, carbon monoxide, and hydrocarbon derivatives. Blends of 20% biodiesel with 80% crude oil diesel (B20) can generally be used in unmodified diesel engines. Biodiesel can also be used in its pure form (B100), but may require certain engine modifications to avoid maintenance and performance problems.

      Biodiesel is made by chemically reacting vegetable oil or animal fat (or combinations of oils and fats) with alcohol (usually nearly pure methanol or ethanol) and a catalyst (sodium hydroxide). The chemical reaction converts constituents of the vegetable oil into an ester (biodiesel fuel) and glycerol. Since the biodiesel is less dense than the glycerol, it floats on top of the glycerol and may be pumped off, or the glycerol can be drained off the bottom. The fuel can then be filtered and used in heating or lighting applications. It is possible to use the product in diesel engines without further processing, but caution is advised because of the potential for engine damage and it is recommended that impurities (unreacted alcohol and sodium hydroxide) are removed by a washing.

      Biodiesel is currently produced entirely from vegetable oil, waste cooking oil, animal fats, and algae. It is named biodiesel because it is derived from biological product and has similar physical properties as petro diesel. The biodiesel feedstock (oil and fats) primarily consists of triglycerides and have some amount of free fatty acids depending on their origin. The high viscosity and poor combustion properties of vegetable oils (compared to petro diesel) do not allow them to use directly as diesel substitute in engines. The direct use of vegetable oils can give rise to significant carbon deposit, lubricating oil contamination, excessive engine wear, and thus reduced engine durability. The processes such as transesterification, pyrolysis, catalytic cracking, and non-catalytic cracking have evolved to transform the vegetable oils or fats into biodiesel (fatty acid methyl ester) or biofuel.

Biodiesel is composed of long-chain fatty acids with an alcohol attached, often derived from vegetable oils. It is produced through the reaction of a vegetable oil with methyl alcohol or ethyl alcohol in the presence of a catalyst. Animal fats are another potential source. Commonly used catalysts are potassium hydroxide (KOH) or sodium hydroxide (NaOH). The chemical process is called transesterification which produces biodiesel and glycerin. Chemically, biodiesel is called a methyl ester if the alcohol used is methanol. If ethanol is used, it is called an ethyl ester. They are similar, and currently, methyl ester is cheaper due to the lower cost for methanol. Biodiesel can be used in the pure form, or blended in any amount with diesel fuel for use in compression ignition engines.

      Biodiesel is typically produced through the reaction of a vegetable oil or animal fat with methanol or ethanol in the presence of a catalyst to yield glycerin and mono-alkyl esters. If methanol is used as the alcohol for transesterification, the reaction is referred to as methanolysis and methyl esters are produced, whereas ethyl esters are produced when ethanol is used for transesterification (ethanolysis). However, it is important to point out that glycerin is considered a valuable co-product that can be used in pharmaceuticals, cosmetics, and many other applications, depending on its grade and purity. The process can be represented simply as: