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

Certain vegetable oils, and in particular used 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). Free fatty acids are generally breakdown products of vegetable oil, and free fatty acids are separated from triglycerides (hence the name “free”). In other words, a triglyceride molecule under prolonged heat exposure will convert to one diglyceride and one free fatty acid, or one monoglyceride and two free fatty acids, or potentially three free fatty acids.

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

However, in the presence of free fatty acids, 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 more base to overcome the free fatty acid reaction and can cause problems with soap formation and product separation after the reaction is complete. In extreme cases, the soaps are mixed with the water from the fuel wash stage to create an emulsion that can greatly prolong or even prevent settling of the fuel bed wash water layer.

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

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