biodiesel, fuel made from natural, renewable sources, such as new and used vegetable oils and animal fats, for use in a diesel engine. Biodiesel has physical properties very similar to petroleum-derived diesel fuel, but its emission properties are superior. Using biodiesel in a conventional diesel engine substantially reduces emissions of unburned hydrocarbons, carbon monoxide, sulfates, polycyclic aromatic hydrocarbons, nitrated polycyclic aromtic hydrocarbons, and particulate matter. Diesel blends containing up to 20% biodiesel can be used in nearly all diesel-powered equipment, and higher-level blends and pure biodiesel can be used in many engines with little or no modification. Lower-level blends are compatible with most storage and distribution equipment, but special handling is required for higher-level blends.
Biodiesel is made from oils or fats, which are hydrocarbons. Fresh soybean oil is most commonly used, although biodiesel can be made from mustard seed oil or waste vegetable oil (such as used oil from restaurant deep fryers). These hydrocarbons are filtered and mixed with an alcohol, such as methanol, and a catalyst (sodium hydroxide or potassium hydroxide), resulting in a chemical reaction whose major products are the biodiesel fuel and glycerol.