Biodiesel is one of (and the preferred one) products of the reaction between an oil and an alcohol. Vegetable oils and animal fats consist of triglycerides, or molecules with a 3-carbon backbone (tri-) that can be written chemically as CH2OCOR1-CHOCOR2-CH2OCOR3. Ideally, the oil (virgin vegetable oil, waste vegetable oil, algae oil, or animal fat) reacts with 3 alcohol molecules to form a glycerin molecule, chemically CH2OH-CHOH-CH2OH and 3 molecules of biodiesel (fatty esters) CH3OCOR1, CH3OCOR2, and CH3OCOR3, where R1, R2, and R3 are one of several hydrocarbon chains, referred to fatty acyl groups.

Certain vegetable oils, and in particular waste vegetable oil, consist not only of triglycerides (a 3-carbon backbone with an R1, R2, and R3 group), but may also consist of diglycerides (a 3-carbon backbone with an R1 and R2 group), monoglycerides (a 3-carbon backbone with only an R1 group), and/or free fatty acids (chemically RCOOH). The free fatty acids are generally degradation products of the vegetable oil, with the free fatty acids breaking away from the triglycerides (hence, the name “free”). In other words, a triglyceride molecule under exposure to prolonged heat will become a diglyceride and one free fatty acid, or a monoglyceride and two free fatty acids, or potentially three free fatty acids.

Vegetable oils with as little as 1-2 percent free fatty acid have been observed to cause difficulties with biodiesel production and/or separation. Although some biodiesel reaction systems are designed to convert free fatty acids to biodiesel using an acid-catalyzed esterification reaction, the most common biodiesel systems lack this capability. Most often, a homebrew biodiesel system or first-generation commercial system designed to handle virgin vegetable oils, uses a base-catalyzed reaction to convert the triglycerides to biodiesel and glycerin.

However, with free fatty acids present, a portion (perhaps all if there is enough free fatty acid) of the basic catalyst will react with the free fatty acid to create a soap. This depletes the base used to catalyze the transesterification reaction or requires the addition of a larger amount of base to overcome the free fatty acid reaction and can cause problems with soap formation and the separation of products after the reaction is complete. In extreme cases, the soaps mix with water from the fuel wash stage to create an emulsion that can greatly lengthen or even prevent settling of the wash water layer from the fuel layer.

There are several ways to compensate for the free fatty acids present in the vegetable oil:

  1. Using great care and equipment designed for strong acids, add sulfuric acid or another strong acid with alcohol to the oil assuming a certain level of free fatty acids (some users suggest using 1 miliLiter of acid per liter of oil. Adding excess acid will increase your chemical costs, both in terms of the acid used and the base necessary to neutralize the excess acid prior to the base-catalyzed transesterification reaction;
  2. Test the free fatty acid content, and if it is low enough (less than 1 percent), ignore it and take care with washing to prevent the formation of an emulsion;
  3. Test the free fatty acid content and add the appropriate amount of strong acid to convert the free fatty acids to biodiesel. This approach requires the most skill, but will increase your yield of biodiesel fuel while minimizing your chemical costs and washing/separation problems.