Creating biodiesel from cooking oil waste is a technique for generating diesel engine fuel from leftover grease. It is popular as a way to avoid paying for commercially-sold diesel fuel. Biodiesel from cooking oil requires no special modifications to a diesel engine and can be produced from waste oil found in kitchen grease traps, which is often obtainable for free from restaurants.
Biodiesel, known technically as Fatty Acid Methyl Esters (FAME), is derived from combustible fatty acids known as triglycerides that come from plants or animals. In unprocessed cooking oil, triglycerides consist of three fatty acid molecules attached to a single glycerin molecule. The process of generating biodiesel from cooking oil involves chemically stripping away the glycerin molecules, leaving only the fatty acids. From start to finish, this procedure is called transesterification.
The properties of traditional diesel fuel and biodiesel from cooking oil are very similar, including a high viscosity and flash point. This means it does not clog fuel systems and is comparatively difficult to ignite, making it safer to transport. Advocates say biodiesel also tends to burn more cleanly than fossil fuel diesel, making it a greener option.
Homebrewing of biodiesel from waste grease can save upwards of 75% on the per-unit cost of fuel over prices at the pump. There are initial costs associated with setting up the at-home system to process oil into biodiesel, but the reduced ongoing costs generally make up the deficit in short order. A variety of recipes exist that take advantage of different ingredients that may vary due to availability and preference.
Most experts recommend starting out making very small batches of biodiesel from cooking oil, typically .25 gallons (1 liter) at a time or less. The chemicals needed for the process include methyl or ethyl alcohol and lye, which acts as the catalyst that severs the glycerin molecules. Waste oil must typically be pre-treated before applying these chemicals to remove any water and free fatty acides (FFAs) that may have accumulated, both of which can impede transesterification. Pre-treatment generally involves titration of the oil, which is a test that determines how much lye will be needed to separate out the FFAs.
In very basic terms, appropriate proportions of lye and alcohol are mixed and then added to the supply of oil. A chemical reaction occurs, and after a period of settling the usable biodiesel can be decanted out, washed, dried, and used. Glycerine and FFAs, effectively soap, is left over. Lye and the types of alcohol used in transesterification can be dangerous if mishandled, and it is important to adhere to appropriate safety protocols to ensure the process remains safe.