Comparative evaluation of semi-synthetic jet fuels

Abstract

This report compares the properties and characteristics of five blends of individual synthetic paraffinic kerosenes with petroleum-based Jet A, Jet A-1 or JP-8 fuel to make semi-synthetic jet fuels (SSJF). The study was requested by the aviation fuel community to provide technical support for the acceptance of synthetic paraffinic kerosenes (SPK) derived from synthesis gas as blending streams up to 50%(v) in fuel specifications for aviation turbine fuel. The methodology for comparison was to be the properties and characteristics used in the original evaluation of the Sasol semi-synthetic jet fuel (SSJF) which has experienced 9 years of successful service since it was approved for use as commercial jet fuel by DEF STAN 91-91 in 1998. The SPK used by Sasol in the original SSJF was produced by a Fischer-Tropsch (F-T) process using synthesis gas derived from coal. The synthesis gases for the four new candidates were produced from natural gas. The details of the F-T process conditions and the downstream processing differed among the five SPK fuels. Although all five SPK fuels were comprised almost entirely of saturated hydrocarbons, i.e., normal, iso-, and cyclo-paraffins, there were distinct differences in the ratio of the three families and in the distribution of carbon numbers. Despite these differences, when blended at 50%(v) with conventional jet fuels, these five SPK fuels produced semi-synthetic jet fuels that were very similar to each other and had fit-for-purpose properties and characteristics that were very typical of conventional jet fuel. Moreover, all five SPKs met all of the requirements of Table 1 with the exception of density. It is important to realize there are no new chemical compositions involved in SSJF, just a change in the ratios of the aromatics to the saturates, i.e., the paraffin families. It is believed that these five fuels covered a large range of SPK compositions likely to result from F-T catalysis of synthesis gas based on the ratios of the paraffin families and the variation in the range of carbon numbers. It is concluded that semi-synthetic kerosenes produced by blending conventional jet fuels with up to 50%(v) SPK derived from synthesis gas by F-T catalysis and downstream processing and having compositions similar to that described in this report are fit-for-purpose as jet fuel. This conclusion has been validated by nine years of operation on one SSJF and in-depth flight-testing and test experience in ground support systems on another two of the five SSJFs evaluated here. Based on the property data of the five SPKs evaluated, it was possible to develop a composition and performance based definition of SPK derived from synthesis gas through an F-T process that would assure that SSJF with up to 50%(v) such SPK would be fit-for-purpose as jet fuel and certifiable under major fuel specifications. This definition is based on meeting a modification of Table 1 requirements designed to assure that the producer has control over the processes for making SPK and to assure a minimum quality of product, both as an item of commerce and for making SSJF.