(photo: Exxon Mobil)
Fri 20 Feb 2015 – The increased global use of biodiesel in ground transport has proved a headache for jet fuel suppliers and aero engine manufacturers as the two fuels are often transported in the same multi-product pipeline and distribution systems, so leading to cross-contamination. Biodiesel is made up of a bio-component called FAME (Fatty Acid Methyl Ester), traces of which can adhere to pipe and tank walls as the biodiesel passes through and then released to the following product, which may be jet fuel. At high enough concentrations, FAME can impact the thermal stability and freezing point of jet fuel, which could result in engine operability problems and possible engine flame-out. Up till now, the maximum FAME contamination of jet fuel was set at 5 parts per million (ppm) but after a number of years of research and testing by fuel and engine experts, fuel certification body ASTM has raised the limit to 50 ppm. Biodiesel is not to be confused with green diesel, which is currently undergoing an ASTM process to allow its use as an approved jet fuel.
FAME is produced from vegetable oils, animal fats or waste cooking oils by transesterification, in which a glyceride reacts with an alcohol in the presence of a catalyst to form a mixture of fatty acids esters and an alcohol.
“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. FAME also has very variable composition, can have carryover of some process by-products and frankly is not made to ‘aerospace’ standards,” explained Chris Lewis, a former fuels specialist with Rolls-Royce with long experience in the development and certification of aviation biofuels, and now an independent consultant.
The problem of cross-contamination was first addressed as far back as 2008 when jet fuel supplied to Birmingham Airport in the UK was tested and found to contain levels of FAME above the 5ppm level, in fact samples of 20ppm were measured. As a result, a number of tanks at the local supply terminal were quarantined and jet fuel supplies had to be sourced elsewhere.
Given that it is considerably cheaper to supply fuel through pipelines than through trucks, both biodiesel and jet fuel providers have been keen to find common ground, and a testing programme on FAME in jet engines has been ongoing since, involving fuel companies, Boeing, Rolls-Royce and others. The experts studied whether the level of allowable FAME in jet fuel could be increased without compromising safety nor adversely affected aircraft operation.
According to Lewis, the programme, led by the Energy Institute, was “very rigorous” and took a long time to complete but had all the key stakeholders involved.
“The jet fuel specification keeps the aviation industry safe while adapting to the expanded presence of biofuels,” said ASTM member David Abdallah of Exxon Mobil Research and Engineering. “In fact, no discernible negative impact on jet fuel product quality was observed with up to 400 ppm of biodiesel.”
Lewis said the target clearance had been 100 ppm but as a prudent move the industry had gone for 50 ppm but with a possible review up to 100 ppm in two years’ time if there are no problems reported, with an emergency allowance of 100 ppm in place if required.
Currently undergoing the ASTM approvals procedures to have it certified as a jet fuel, green diesel, also known as renewable diesel, has a different chemical structure to biodiesel. “It is actually synthetic hydrocarbons and very pure – it has the same molecules as jet fuel – so it slots in nicely and, at the right levels, does not impact on critical properties,” said Lewis. The currently approved synthetics from ASTM-approved Fischer-Tropsch (FT) and hydroprocessed esters and fatty acids (HEFA) fuels are similar to green diesel, he said.
“However, green diesel has a higher freeze point temperature than the already approved FT/ HEFA products that can be blended into jet because some of the processing needed to lower the freeze point – called isomerisation – has been left out. This does make green diesel more environmentally friendly to produce but because of the freeze point issue you can’t blend it as high as 50% with conventional jet fuel.
“In summary, green diesel is a good product, the right molecules, can be blended to a lower level without degrading jet fuel performance and is more efficient to produce.”
ASTM – FAME announcement
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