This pathway involves the physical and chemical synthesis of products from feedstocks and is primarily associated with the transformation of fresh or used vegetable oils, animal fats, greases, tallow, and other suitable feedstocks into liquid fuels or biodiesel.
Biodiesel from waste fats, oils, and greases (FOG) is produced by the transesterification of triglyceride molecules in lipids into mono-alkyl esters (triglycerides are the main component of vegetable oils and animal fats and are composed of three fatty acids with a glycerol backbone). This is done by reacting an alcohol with the oil in the presence of an alkaline catalyst--sodium or potassium hydroxide (lye)--creating the ester (biodiesel) and glycerol as a coproduct. Methanol is usually used but ethanol is also suitable. Both can be produced from biomass as renewable feedstocks to the process.
Biodiesel production from unused or virgin vegetable oils is an established technique and can yield high quality fuel as long as best practices are followed. It is more difficult or complex to make high quality biodiesel at reasonable yields from waste oils and fats because they contain high concentrations of free fatty acids (FFA). FFA will form soap in the alkaline-catalyzed main triglyceride reaction, reducing yield and making final glycerol removal and washing difficult. Water in the waste oil is also undesirable.
Good quality raw vegetable oil feedstocks for biodiesel have FFA concentrations below 0.1 percent. Restaurant waste oils, rendered animal fats, and “trap grease” (grease trapped from kitchen wastewater before entering sewer lines) have high FFA content. So-called yellow grease has FFA content between about 1.5 percent and 15 percent. Brown grease, with cooked or burned suspended solids and water, may have FFA content as high as 40 percent. Trap grease FFA content can range from 40 percent to 100 percent and contains large amounts of water and solids. After a cleaning and drying step, high FFA feedstocks undergo a pretreatment step in which the FFA are reacted with the alcohol and acid catalyst to form esters and water. The esters, along with the triglycerides, then go through the base catalyzed transesterification step described earlier. Final steps include washing, alcohol and catalyst recovery, and glycerin separation.
The March 2007 Biofuels from Municipal Wastes-Background Discussion Paper provides additional information on biofuels.