Technology and Feedstocks
Biodiesel feedstocks include nearly any vegetable oil or animal fat. Major feedstock categories include:
- Refined vegetable oils (soybean, corn, canola, peanut, olive, etc.). In the United States, soybean oil is the most popular refined vegetable oil because of the balance of biodiesel properties, soil depletion (or enhancement) and soymeal value. Refined vegetable oils are almost entirely triglycerides and have very little water or free fatty acids. The presence of free fatty acids or water in the feedstock significantly complicates the traditional biodiesel process.
- Animal fats (tallow) from rendering operations.
- Recycled fats, oils, and greases. These are primarily used cooking oils collected from the restaurant industry. Once collected, these materials are purified and resold as "yellow grease". Yellow grease is sold as livestock feed supplements, to biodiesel producers, and sometimes as a low grade industrial lubricant.
- Waste oils and greases. These also are collected from the restaurant industry, but currently are rarely recyled into any useful product. Also know as "brown grease" or "trap grease", these materials come from sanitary drainage system traps outside restaurants. Because of the decomposition process, brown grease can be up to 100% free fatty acids. Most brown grease is removed from the traps as part of a routine maintenance operation to prevent city drainage systems from plugging. Brown grease is normally disposed of in landfills.
- Non-food oils. Research is taking place all over the world on methods to produce oils from biological sources that don't compete for food resources. Among the most promising of these are oil-bearing algae and Jatropha curcas.