Creating Biodiesel from Grease
Oil and grease from restaurants can be recycled into energy. The biodiesel created is one of the most environmentally friendly feedstocks. In fact, when compared to petro-diesel, biodiesel from waste oil results in a significant reduction in greenhouse gases.
Cooking oil, or waste vegetable oil (i.e., used fryer oil), is generally edible oil that has been re-used multiple times in a fryer. This oil is the easiest and cleanest to work with.
The second general category is “trap grease.” Grease traps are installed in kitchen drains to prevent oils and fats from entering sewer pipes. This grease is collected to also make biodiesel.
In biodiesel production, free fatty acids are created when the fats (triglycerides) are split and the free fatty acids are separated from the glycerol molecules. Free fatty acids cause the formation of soap which results in less biodiesel. Therefore, grease with less than 15% free fatty acids are better for the production of biodiesel and called “yellow grease”, while grease with more than 15% free fatty acids are called “brown grease.”
Used Cooking Oil
Used cooking oil is collected by companies that purify the oil and use it for animal feed supplements as well as biodiesel. Used cooking oil is not technically considered “waste.”
“Trap grease” refers only to kitchen waste. This type of grease is considered waste and disposed of at landfills or treatment plants. It can be purified but sold for non-edible applications such as burning fuel.
The difficulties encountered in using trap grease for biodiesel production include contamination with food and trash, foul odor, up to 98% free fatty acid content, water contamination, heavy emulsification, and cold flow issues caused by the grease turning solid at room temperature. Also, the collection system for trap grease is fragmented and disorganized.
However, technology in producing high-quality biodiesel from trap grease has been developed.
Converting Used Oil and Trap Grease to Biodiesel
When converting used kitchen oil and trap grease to biodiesel, the free form acids need to be reduced.
On method is using an acid pre-treatment process which involves adding an acid catalyst and alcohol that reacts. The resulting methanol, water, and acid separates and is removed. If needed, this process is repeated until the free fatty acids are less than 1%. Disadvantages to this method is that the acid may cause damage to the metal tanks used, and the methanol becomes contaminated with water and must be recovered before re-use.
An alternative method is a process called “glycerolysis.” Glycerin is added at 400°F. It reacts with the free form acids to form monoglycerides that are then removed.
This method can be expensive due to the high heat and vacuum required in the process. The glycerin must also be removed.
More methods to reduce the free form acids in used oil and grease are continually being tried and developed.