The predominant biodiesel feedstock used in the United States is soybean oil. Other vegetable oils, such as corn, cottonseed, canola (rape seed), palm, sunflower, flax, and peanut, also can be used. Animal-derived products such as tallow, choice white grease (lard), poultry fat and yellow grease are also triglycerides and are used as a biodiesel feedstock.
There is a substantial amount of research and development into new biodiesel feedstocks such as jatropha and algae. algae have emerged as one of the most promising sources especially for biodiesel production, for two main reasons (1) The yields of oil from algae are orders of magnitude higher than those for traditional oilseeds, and (2) Algae can grow in places away from the farmlands & forests, thus minimizing the damages caused to the eco- and food chain systems. There is a third interesting reason as well: Algae can be grown in sewages and next to power-plant smokestacks where they digest the pollutants and give us oil!
The third main source of triglycerides is recycled oil and grease, usually from restaurants and animal food processing plants. Although more pre-treatment is required for this feedstock compared to virgin vegetable oils, economically it can be a very attractive feedstock. The use of a recycled product such as used cooking oil is an environmentally friendly process since it solves a waste disposal problem.
The EPA mandate to reduce sulphur in transportation diesel fuel from 500 parts per million (ppm) to 15 ppm causes lubricity problems in diesel fuel injection systems. Adding 1 to 3 percent biodiesel to the petroleum diesel can restore the lubricant qualities of the fuel. Based on approximately 35 billion gallons of diesel fuel used in over-the-road transportation, this addition, using a B2 blend, would require 700 million gallons of biodiesel just for this program. This represents 500 million bushels of soybeans (based on 1.4 gallons/bushel).
Other biodiesel market drivers such as the Energy Policy Act, Renewable Fuel Standard Two (RFS2), energy bill and various environmental programs—for school buses, marinas, ski lifts, etc.—will continue to influence biodiesel consumption. RFS2 targets the use of biofuels to increase from approximately 12 billion gallons in 2009 to 36 billion gallons in 2022. However, the big looming potential for biodiesel is the use of biodiesel blends for transportation fuel. Minnesota has a B5 mandate (5% biodiesel) year around and a B20 mandate (20% biodiesel) in the summer. Other states and regions are considering similar mandates.
All biodiesel feedstocks, including soybean oil, require some amount of pre-conditioning, often chemical refining, before being processed into biodiesel. For example, crude soybean oil must be degummed to remove sources of phosphorous, which degrades the catalysts employed in biodiesel production.
Free fatty acids should also be removed from the feedstock before transesterification, especially if recycled cooking oils, which are high in free fatty acids due to their exposure to high temperature during frying, are used. Free fatty acids are corrosive, and therefore hard on fuel storage facilities, fuel systems and engines. ASTM standards set forth tight tolerances on the allowable levels of free fatty acids in biodiesel.
A common approach for removing free fatty acids from the feedstock is via acid catalyzed esterification in the presence of methanol. This converts the free fatty acids to methyl esters (e.g., biodiesel). Acid esterification equipment requires the use of stainless steel equipment due to the corrosive nature of the process.
Crude oil from a crushing operation also may be pre-treated by caustic refining to remove free fatty acid. The soapstocks generated in this step are then removed by washing the oil with hot water. The oil must be dried to remove the water before transesterification, since water inhibits the biodiesel-producing reaction.