Alternative aviation fuels are evaluated for lean-flame NOx formation, lean-flame blowout (LFBO), and rich-flame soot threshold. The alternative aviation fuels evaluated are: hydro-processed renewable jet (HRJ) made from camelina and tallow, Fischer-Tropsch (FT) jet made from coal and natural gas, and alcohol-based jet. Chromatographic analysis (from another lab) shows that these fuels are composed mainly of iso-alkanes. These fuels are burned as received and with 10–20% aromatic added. For comparison, petroleum-based jet fuels are burned: JP5, JP8, Jet-A, and an aromatic blend. Additionally, we burn several pure fuels: n-hexane, cyclo-hexane, n-octane, iso-octane, n-dodecane, 1-3-5 tri-methyl benzene (TMB), and toluene. The NOx and blowout experiments are performed in a jet-stirred reactor (JSR), and the soot threshold experiments are performed on a Meker burner. The fuels are burned pre-vaporized, premixed, and preheated.
NOx is measured by probe sampling and chemiluminescent analysis at a fixed temperature for each fuel. Soot tendency is measured as the fuel-air equivalence ratio (phi) for the first appearance of yellow tips in the flame. This is termed the soot threshold. LFBO is examined by measuring the JSR temperature immediately prior to blowout.
Relatively small changes in NOx, LFBO, and soot threshold are measured for the petroleum-based and alternative jet fuels examined. HRJ-tallow is the cleanest burning jet fuel tested with respect to NOx and soot measurements. It exhibits the lowest NOx and the highest soot threshold among alternative fuels tested.
A possible correlation between decreasing NOx and increasing soot threshold is explored. The impact of adding aromatic content to the alternative jet fuels is also examined. With 20% aromatic content added to the alternative jet fuels, the soot threshold decreases, though the HRJ-tallow with 20% aromatic added continues to maintain a higher soot threshold than petroleum-based jet fuels. The addition of 20% aromatic does not show a clear trend in NOx. Over the four alternative jet fuels tested, the 20% aromatic addition causes a maximum change in the NOx emission of ± 13% (relative); the average change is close to zero.
