The problem of cross-contamination, which was first addressed as far back as 2008, arises as a consequence of the composition of biodiesel and one particular ingredient, FAME, (Fatty Acid Methyl Ester), which can leave a residue on pipe and tank walls as it passes through. When the following product, which may be jet fuel, passes through, these traces are released. At high enough concentrations, the presence of FAME can alter the thermal stability and freezing point of jet fuel, which can subsequently cause engine problems and potential engine flame-out.
"Chemically, FAME biodiesel is a quite different molecule compared to those found in jet fuel and, indeed, diesel, and it does impact on fuel properties," commented Chris Lewis, a former fuels specialist with Rolls-Royce. The maximum FAME contamination of jet fuel had previously been set at 5 parts per million (ppm) but after years of further research and testing, fuel certification body ASTM has increased the limit to 50 ppm.
As it is far cheaper to supply fuel through pipelines than through trucks, both biodiesel and jet fuel providers have been collaborating on a programme testing FAME in jet engines with companies such Boeing, Rolls-Royce and others ever since the issue was first established.
