Various government regulations restrict the amount of sulfur present in fuels due to sulfur dioxide emissions that are harmful to the environment.  In addition, sulfur can lead to acids forming in an engine’s crankcase which can then attack the lubricant.  In terms of engine wear and formation of deposits, the effect of sulfur depends greatly on operating conditions and varies considerably.  Unlike petroleum-based diesel, biodiesel from vegetable and waste oils are naturally low in sulfur and are therefore an emission reducing fuel.  ASTM D6751 includes two limits for the amount of sulfur allowed in biodiesel, Grade S15 which allows for 0.0015 (% mass or ppm) and Grade S500 which allows for 0.05 (% mass or ppm).

Significance

The referee test method for the determination of sulfur in biodiesel is ASTM D5453, with the use of ultraviolet fluorescence.  This method provides the most accurate readings for biodiesel samples containing extremely low sulfur levels.  In this method, the biodiesel sample is oxidized in a high temperature combustion tube in an oxygen rich atmosphere to produce sulfur dioxide.  The combustion gases are then exposed to UV light, causing the sulfur dioxide to be converted to excited sulfur dioxide.  The fluorescence that is emitted from the excited sulfur dioxide as it returns to a stable state is detected by a photomultiplier tube and the resulting signal is used to measure the amount of sulfur.  Other test methods can be used to measure sulfur in biodiesel samples.  ASTM D2622 measures sulfur content through x-ray fluorescence spectrometry, but can report falsely high results in low sulfur level biodiesel due to the presence of oxygen.