Biodiesel is a form of diesel fuel derived from plants or animals and consisting of long-chain fatty acid esters. It is typically made by chemically reacting lipids such as animal fat, soybean oil, or some other vegetable oil with an alcohol, producing a methyl, ethyl or propyl ester by the process of transesterification.

Unlike the vegetable and waste oils used to fuel converted diesel engines, biodiesel is a drop-in biofuel, meaning it is compatible with existing diesel engines and distribution infrastructure. However, it is usually blended with petrodiesel (typically to less than 10%) since most engines cannot run on pure biodiesel without modification. Biodiesel blends can also be used as heating oil.

Blends of biodiesel and conventional hydrocarbon-based diesel are most commonly distributed for use in the retail diesel fuel marketplace. Much of the world uses a system known as the "B" factor to state the amount of biodiesel in any fuel mix.

■ 100% Biodiesel is referred to as B100

■ 20% Biodiesel 80% petrodiesel is labeled B20

■ 10% Biodiesel 90% petrodiesel is labeled B10

■ 7% Biodiesel 93% petrodiesel is labeled B7

■ 5% Biodiesel 95% petrodiesel is labeled B5

■ 2% Biodiesel 98% petrodiesel is labeled B2

Blends of 20% Biodiesel and lower can be used in diesel equipment with no, or only minor modifications, although certain manufacturers do not extend warranty coverage if equipment is damaged by these blends. The B6 to B20 blends are covered by the ASTM D7467 specification. Biodiesel can also be used in its pure form (B100) but may require certain engine modifications to avoid maintenance and performance problems. Blending B100 with petroleum diesel may be accomplished by:

■ Mixing in tanks at the manufacturing point prior to delivery to the tanker truck.

■ Splash mixing in the tanker truck (adding specific percentages of biodiesel and petroleum diesel)

■ In-line mixing, two components arrive at the tanker truck simultaneously.

■ Metered pump mixing, petroleum diesel and biodiesel meters are set to X total volume.

The color of biodiesel ranges from clear to golden to dark brown, depending on the production method and the feedstock used to make the fuel. This also changes the resulting fuel properties. In general, biodiesel is slightly miscible with water, has a high boiling point and low vapor pressure. The flash point of biodiesel can exceed 130 'C (266 'F) significantly higher than that of petroleum diesel which may be as low as 52 'C (126 'F) Biodiesel has a density around -0.88 g/cm3 higher than petrodiesel -0.85 g/cm3.