Biodiesel is a bio-based fuel produced from vegetable oils and animal fats (lipids). It is used as an alternative and/or additive to petroleum-derived diesel fuel. Biodiesel production in the U.S. has been increasing, from an estimated 0.5 million gallons of production in 1999 to an estimated 250 million gallons in 2006.
The primary resource used to produce biodiesel in the U.S. is soybean oil (about 2.6 billion pounds, 13% of domestic production in 2006). However, other virgin vegetable oils (e.g., Canola, castor, peanut, palm, rapeseed, sunflower), along with greases and recycled oils (e.g., tallow, yellow grease, trap grease) can potentially be used as well, depending on their availability and price. Rapeseed oil, for example, is widely used in Europe; castor, in addition to soybean oil, is used in Brazil (de Oliveira, 2005).
Biodiesel is composed of mono-alkyl esters of long chain fatty acids. Through a process called transesterification, biodiesel is made by reacting triglycerides (vegetable oils or fats) with alcohol (methanol) in the presence of a catalyst to produce either fatty acid methyl esters (FAME) or fatty acid ethyl esters (FAEE) and glycerol (Meher, 2006) (figure 1).
Producing biodiesel is a fairly straightforward process. Alcohol and a catalyst (usually sodium hydroxide, NaOH) are first mixed and then added to the vegetable oil. Often, 80% of the alcohol/catalyst mixture is added and allowed to react initially, with the last 20% added later in order to obtain maximum yield (Ma, 1999). The transesterification reaction can be conducted in batch reactors; however, larger plants sometimes use continuous stirred-tank reactors or plug flow reactors. The mechanism by which biodiesel is formed consists of three consecutive reversible reactions. The rates of these reactions vary, depending on production factors such as temperature, the length of time the reaction occurs, the molar ratio of the alcohol to the triglyceride, the type of catalyst used, and amounts of water and free fatty acids present. For biodiesel derived from soybean oil, one pound of oil yields approximately 1 pound (0.1 gallons) of biodiesel and 0.1 pounds of glycerol, and consumes 0.1 pounds of alcohol in its production. Pure (100%) biodiesel is designated B100. Currently, most fleets use blends consisting of 2 to 20% biodiesel and the remainder petroleum-derived diesel (B2 to B20 blends).
In the process of producing biodiesel, glycerol is produced as a co-product. The two products are separated by removing the glycerol from the bottom of a settling vessel, using a centrifuge if needed. Once phase separation has occurred, the two by-products must be cleaned. Alcohol is removed from the biodiesel by flash evaporation (i.e., pumping heated biodiesel into a chamber where the alcohol rapidly evaporates) or by distillation to remove the water. The alcohol is re-used. The biodiesel is usually washed several times to remove excess catalyst and glycerol. Glycerol is neutralized with acid in order to split any residual soap into fatty acids and salts, which can then be separated, leaving about 85% pure glycerol that can be sold to a glycerol refiner (Van Gerpen, 2005).
In 2005/2006, 123 million metric tons of vegetable oils were produced worldwide, with 10.3 million metric tons produced in the U.S. (USDA, 2006). Conversion of the entire U.S. production of vegetable oils would yield approximately 2.9 billion gallons (11 billion liters) of biodiesel. In 2005, U.S. consumption of diesel fuel was 41 billion gallons, 38 billion gallons for transportation uses, and 3 billion gallons for non-transportation uses (DOE, 2005).