Biodiesel is readily produced from refined vegetable oils; however, many low-value degraded and waste oils contain a high concentration of free fatty acids, which are difficult to convert to fatty add alkyl esters for use as biodiesel. This paper evaluates the performance of an acid-catalyzed bubble column reactor that is highly robust for the esterification of free fatty acids to fatty acid alkyl esters. The bubble column reactor typically operated at 120 degrees C and ambient pressure; methanol bubbling through the reactor reacts with free fatty acids and strips by-product water, which enables high conversions and makes the reactor more robust to water than other reactor designs. This paper shows the effects alcohol, feedstock type and quality, alcohol flow rate, and oil feedstock on the reactor performance. Most reaction conditions produce greater than 98% conversion in less than 2 h, including reactions with alcohols containing 10% water by volume and lipids extracted from trap grease. (C) 2014 Elsevier B.V. All rights reserved.