Previous studies have shown that FT fuel can reduce engine emissions, thereby helping to mitigate the impacts of aircraft operation on air quality and the environment, although the detailed chemical composition of aircraft emissions with FT fuel is largely unknown. With the potential for large increases in FT usage, more detailed knowledge of the chemical characteristics and evolution of aircraft emissions with FT fuel is needed to accurately assess possible environmental impacts.

...While aircraft directly emit both particles and gases, the latter constitute the vast majority of emissions (>97%). A significant fraction of the organic emissions are low-volatility vapors. The gas−particle partitioning of semi-volatile organic compounds (SVOCs), those with an effective saturation concentration (C*) between 10⁻¹ and 10² μg m⁻³, changes as the emissions are cooled and diluted in the atmosphere, complicating the definition of primary PM emission factors. The importance of SVOC partitioning therefore requires careful consideration of source sampling conditions, specifically the total concentration of organic aerosol, which is related to the dilution ratio and the total emission rate of a given source.

Intermediate volatility compounds (IVOCs; 103 < C* < 106 μg m⁻³) may also undergo gas−particle conversion at the high concentrations present in many source tests but exist as vapors in the ambient atmosphere. The adsorption of IVOC and SVOC vapors to filter samples can result in sampling artifacts and overestimation of POA [primary organic aerosol] emissions. Once emitted to the atmosphere, IVOCs and other organic gases are photo-oxidized, forming secondary organic aerosol (SOA); IVOCs may be an important class of SOA precursors, especially from gas-turbine engines. Constraining the total contribution of gas-turbine engine emissions to ambient PM therefore requires quantification of three critical factors: (1) the direct emissions of soot, particulate sulfate, and low-volatility organic vapors, (2) the volatility distribution of the low-volatility organic vapors, which controls the gas−particle partitioning of SVOCs and, therefore, the amount of POA, and (3) the susceptibility to oxidation of SVOCs and IVOCs, which will largely govern SOA formation. This paper focuses on the first of these factors.

—Drozd et al.

Properties of fuels used in the study. ACS, Drozd et al. Click to enlarge.

The researchers performed their experiments using a T63 gas-turbine engine—an older engine, primarily used in helicopter applications—installed on a test stand at two engine load conditions: idle and cruise. The T63 is an older engine, primarily used in helicopter applications. T63 emissions are much higher in comparison to other gas-turbine engines, such as the CFM56 engine; primary PM emissions are a factor of 10−20 higher for the T-63 engine, and total hydrocarbon emissions are nearly doubled.

The idle load condition was set with no load on the dynamometer and a fixed fuel flow of 8.5 cm³ s⁻¹. For the cruise load condition, the engine was operated at a constant shaft speed of 6000 rpm and a constant turbine outlet temperature of 693 °C. The new study extends previous research on T63 emissions by characterizing both particle- and gas-phase emissions, with particular focus on their volatility and detailed chemical composition/speciation.

Among the findings:

• Primary particulate matter (PM) and gaseous emissions for the neat FT and blend fuels were reduced relative to emissions when using JP-8 fuel at both idle and cruise loads.

• At idle load, PM mass emissions are reduced by 65% with neat FT fuel and by 50% for the 50:50 blend compared to neat JP-8 fuel—i.e., the JP-8/FT blend decreases emissions beyond the linear average of the emissions for the individual fuels.

• At idle load, FT fuel reduced total hydrocarbon emissions by 20%, while the blend showed no significant change compared to neat JP-8.

• At cruise load, neat FT fuel resulted in an 80% reduction in primary PM emissions and a 30% reduction in total hydrocarbon emissions compared to neat JP-8.

• Decreases in PM emissions at idle load come from lower elemental carbon (EC) and primary organic aerosol (POA), while at cruise load emissions, reductions are driven mainly by EC.

• When using FT fuel, POA emissions appear to be largely engine oil, but emissions with JP-8 fuel have a large fraction of partially oxidized organic material. The differences in POA composition may be due to both the presence of partially oxidized fuel as well as greater EC/soot levels when using JP-8 fuel.

• Switching to FT fuel also reduces both the total hydrocarbon and SO² emissions, which reduces the potential to form secondary PM.

• For this engine, both the SO emissions (from JP-8 fuel) and hydrocarbon emissions greatly exceed the primary PM emissions. Photo-oxidation of these gases produces secondary PM in the atmosphere.

Switching to FT fuel reduces both the SO₂ and SOA precursor emissions. Therefore, increasing FT fuel use, by either blending or substitution, can reduce the contribution of aircraft emissions to levels of both primary and secondary PM.

—Drozd et al.

Resources

• Greg T. Drozd, Marissa A. Miracolo, Albert A. Presto, Eric M. Lipsky, Daniel D. Riemer, Edwin Corporan, and Allen L. Robinson (2012) Particulate Matter and Organic Vapor Emissions from a Helicopter Engine Operating on Petroleum and Fischer–Tropsch Fuels. Energy & Fuels doi: 10.1021/ef300651t