Biodiesel is a liquid fuel made up of fatty acid alkyl esters, fatty acid methyl esters (FAME), or long-chain mono alkyl esters. It is produced from renewable sources such as new and used vegetable oils and animal fats and is a cleaner-burning replacement for petroleum-based diesel fuel. It is nontoxic and biodegradable. Biodiesel has physical properties similar to those of petroleum diesel:

Like petroleum diesel, biodiesel is used to fuel compression-ignition (diesel) engines. Low-level blends of biodiesel with petroleum diesel also provide benefits.

Biodiesel Production

Biodiesel can be produced using a variety of esterification technologies. The oils and fats are filtered and preprocessed to remove water and contaminants. If free fatty acids are present, they can be removed or transformed into biodiesel using special pretreatment technologies. The pretreated oils and fats are then mixed with an alcohol (usually methanol) and a catalyst (usually sodium hydroxide). The oil molecules (triglycerides) are broken apart and reformed into methyl esters and glycerin, which are then separated from each other and purified.

Schematic of biodiesel production path.

Although the process is relatively simple, homemade biodiesel is not recommended. Diesel engines are expensive, and it is not worth risking damage or even minor operational problems from fuel that does not meet rigorous ASTM D6751-07b specifications. Section 3.1 of the Biodiesel Handling and Use Guidelines (PDF 1.5 MB) provides a summary of this standard. Download Adobe Reader. The full standard can be purchased from ASTM International.

Do not use raw vegetable oil in a diesel engine. It will work, but fats and oils (triglycerides) are much more viscous than biodiesel. Even low-level vegetable oil blends can cause long-term engine deposits, ring sticking, lube oil gelling, and other maintenance problems and can reduce engine life.

The U.S. biodiesel industry is small but growing rapidly. Production tripled from 2004 to 2005 and again from 2005 to 2006. Much of the original biodiesel production capacity comes from companies already making products from vegetable oil or animal fat in the detergent industry among others. More than half the biodiesel industry can use any fat or oil feedstock, including recycled cooking grease. The other half of the industry is limited to vegetable oils; soy oil is the most common source in the United States today. The soy industry has been the driving force behind biodiesel commercialization because of excess production capacity, product surpluses, and declining prices. Similar issues apply to the recycled grease and animal fats industry, even though these feedstocks are less expensive than soy oils. A possibly limiting factor for biodiesel industry growth is potential saturation of the market for glycerin.

Enough virgin soy oil, recycled restaurant grease, and other feedstocks are readily available in the United States to provide feedstock for about 1.7 billion gallons of biodiesel per year (under policies designed to encourage biodiesel use). This represents roughly 5% of on-road diesel used in the United States. For more information on this subject, see Biomass Oil Analysis: Research Needs and Recommendations (PDF 1.3 MB). Also see the National Biodiesel Board's Biodiesel Production fact sheet (PDF 35 KB). Download Adobe Reader.