This effort describes laboratory evaluations of six alternative (nonpetroleum) jet fuel candidates derived from coal, natural gas, camelina, and animal fat. Three of the fuels were produced via Fischer-Tropsch (FT) synthesis, while the other three were produced via extensive hydroprocessing. The thermal stability, elastomer swell capability, and combustion emissions of the alternative jet fuels were assessed. In addition, detailed chemical analysis was performed to provide insight into their performance and to infer potential behavior of these fuels if implemented. The fuels were supplied by Sasol, Shell, Rentech, UOP, and Syntroleum Corporation. Chemical analyses show that the alternative fuels were comprised of mostly paraffinic compounds at varying relative concentrations, contained negligible heteroatom species, and were mostly aromatic-free. The six paraffinic fuels demonstrated superior thermal oxidative stability compared to JP-8, and therefore, have increased resistance to carbon formation when heated and can be exposed to higher temperatures when used to cool aircraft systems. Material compatibility tests show that the alternative fuels possess significant seal swelling capability in conditioned nitrile O-rings; however, elastomer swelling was significantly lower than for JP-8, which may likely result in fuel leaks in aircraft systems. Engine tests with the alternative fuels demonstrated no anomalies in engine operation, production of significantly lower nonvolatile particulate matter (soot), and moderately lower unburned hydrocarbons and carbon monoxide emissions compared to baseline JP-8 fuel. Also, no penalty (i.e., increase) in fuel flow requirement for equal engine power output was observed. In general, this study demonstrates that paraffinic fuels derived from different feedstocks and produced via FT synthesis or hydroprocessing can provide fuels with very similar properties to conventional fuels consisting of excellent physical, chemical, and combustion characteristics for use in turbine engines. These types of fuels may be considered as viable drop-in replacement jet fuels if deficiencies such as seal swell, lubricity, and low density can be properly addressed.

The emissions characteristics of two combustion platforms, a T63 turboshaft engine and an atmospheric swirl-stabilized research combustor, fueled with conventional military jet fuel (JP-8), a natural-gas-derived Fischer−Tropsch synthetic jet fuel (also referred herein as synjet or FT), and blends of the two were investigated. Nonvolatile particulate matter (PM) and gaseous emissions were analyzed to assess the impacts of the aromatic- and sulfur-free synjet fuel on the combustion products of the two platforms. The engine was operated at two power settings, and the combustor at several equivalence ratios, to evaluate the emission production over a wide range of combustion temperatures. Conventional aerosol instrumentation was used to quantify particle number (PN), size, and PM mass emissions, while a Fourier Transform Infrared analyzer was used to quantify the gaseous species. Planar laser-induced fluorescence and laser-induced incandescence techniques were employed on the research combustor to study the effects of the FT fuel on the formation and oxidation of particles in the combustor primary zone. Test results show dramatic reductions in particle concentrations and mean size on both combustion platforms with the neat FT and synjet fuel blends relative to operation with JP-8. Reductions of over 90% in PN were observed on both platforms for several operating conditions with neat FT fuel. For the engine, over an 80% reduction in smoke number was observed with neat synjet relative to operation on JP-8. As expected, reductions in sulfur oxide emissions and slight increases in water vapor (measured only in the atmospheric combustor) resulted due to the sulfur-free nature and higher hydrogen-to-carbon ratio of the synthetic fuel. Minor impacts were observed for other gaseous emissions. American Society for Testing and Materials fuel specification tests showed that JP-8/synjet blends up to 50/50% by volume satisfied the JP-8 military fuel requirements and that only the minimum specific gravity requirement was not satisfied at higher synjet concentrations. Impacts of the synjet fuel on the emissions of the atmospheric combustor and the T63 engine, a comparison of emissions between the two platforms, and results of in situ laser-based measurements in the combustor reaction zone are discussed.

We have performed a comprehensive test of the effects of alternative fuels on the trace gas, nonvolatile particulate material (PM), and volatile PM emissions performance of a PW308 aircraft engine. The tests evaluated standard JP-8 jet fuel, a "zero sulfur" and "zero aromatic" synthetic fuel produced from a natural gas feedstock using the Fischer-Tropsch (FT) process, and a 50/50 blend of the FT fuel and JP-8. A Pratt & Whitney PW308 engine was operated under the same thrust and combustion conditions to ensure that the tests captured fuel differences, rather than engine operation differences. Emissions of trace gases, soot particles, and nucleation/growth PM were directly impacted by the sulfur and aromatic content of the fuel. FT fuel combustion greatly reduced SO 2 (>90%), gaseous hydrocarbons (40%), and NO (6-11%) content compared to JP-8 combustion. In general, combustion of the JP-8/FT fuel blend resulted in emissions intermediate to the FT and JP-8 values. FT combustion dramatically reduces soot particle number, mass, and size relative to JP-8, but increases effective soot particle density. In all cases, the drag behavior of the soot particles indicates deviations from spherical shape and effective soot particle densities are consistent with the soot particles being aggregates of primary spherules. As expected, FT combustion plumes support negligible formation of nucleation/growth mode particles (the number of nucleation growth mode particles is <20% the number of soot particles compared to >500% for sulfur containing JP-8). However, particle nucleation/growth for blended fuel combustion is enhanced relative to JP-8, despite the lower sulfur content of the FT/JP-8 fuel blend. A computational model explains the unexpected particle formation result primarily as the effect of much lower soot emissions present in blended fuel combustion exhaust compared to JP-8. Fuel composition, specifically aromatic and sulfur content, affect all aspects of emissions performance and the effect of simultaneously reducing aromatic and sulfur content can lead to surprising behavior.