Characterization of Gaseous and Particulate Emissions From a Turboshaft Engine Burning Conventional, Alternative, and Surrogate Fuels

The effect of fuel composition on the operability and gaseous and particulate matter (PM) emissions of an Allison T63-A-700 turboshaft engine operated at four power settings was investigated in this effort. Testing was performed with a specification JP-8, a synthetic paraffinic kerosene, and four two-component surrogate mixtures that comprise compound classes within current and future alternative fuels. Comparable engine operability was observed for all fuels during this study. Major gaseous emissions were only slightly effected, with trends consistent with those expected based on the overall hydrogen content of the fuels. However, minor hydrocarbon and aldehyde emissions were significantly more sensitive to the fuel chemical composition. Linear correlations between speciated hydrocarbon and aldehyde emissions were observed over the full engine operating range for the fuels tested. The corresponding slopes were dependent on the fuel composition, indicating that fuel chemistry affects the selectivity to specific decomposition pathways. Unburned fuel components were observed in the engine exhaust during operation with all fuels, demonstrating that completely unreacted fuel compounds can pass through the high temperature/pressure combustion zone. Nonvolatile PM emissions (soot) were strongly affected by the fuel chemical composition. Paraffinic fuels produced significantly lower PM number and mass emissions relative to aromatic-containing fuels, with the paraffin structure affecting sooting propensity. The observations are consistent with those expected based on simplified soot formation mechanisms, where fuels with direct precursors for polycyclic aromatic hydrocarbon formation have higher PM formation rates. The effect of a specific chemical structure on the relative PM production is important as this would not be evident when comparing sooting tendencies of fuels based on bulk fuel properties. All fuels produced similar single log-normal size distributions of soot, with higher sooting fuels producing larger mean diameter particles. It is hypothesized that the controlling growth and formation mechanisms for PM production are similar for different fuel chemistries in this regime, with composition primarily affecting soot formation rate. This hypothesis was supported by preliminary TEM analyses that showed similar soot microstructures during operation with either conventional JP-8 or alternative fuels. Overall, this study provides additional and improved insight into the effect of fuel chemical composition on complex combustion chemistry and emissions propensity in a gas turbine engine, and can assist with the successful development of predictive modeling tools.