Influence of Humidity and Fuel Hydrogen Content on Ultrafine Non-Volatile Particulate Matter Formation in RQL Gas Turbine Technology

To address the known Local Air Quality impacts of ultrafine combustion derived soot, the International Civil Aviation Organisation (ICAO) have recently adopted a non-volatile Particulate Matter (nvPM) regulation in addition to those of NOx, UHC’s and CO for civil aviation gas turbines. Increased water humidity is known to reduce the formation of NOx in flames through localised temperature reduction, however its impact on emitted nvPM is to date not clearly understood.  To address this knowledge gap, nvPM formation mechanisms were assessed empirically at increased water loadings both at atmospheric pressure, in a RQL representative optical combustor fuelled with Jet A and during a full-scale Rolls-Royce aero derivative Gas Turbine fuelled on Diesel.

In line with previous studies it was observed in the RQL combustor rig that increased hydrogen content in the test fuel associated with a 100% Gas-To-Liquid (GTL) derived aviation kerosene with low aromatic content (0.05%) reduced nvPM number concentrations by up to an order of magnitude compared to a baseline Jet A-1 fuel with representative aromatic content (24.24%).  It was also observed for all fuels that an elevated  water loading in the primary combustion zone to 0.05 kg /kg of dry air, representative of maximum global humidity levels, resulted in reductions of both nvPM number and mass concentrations of 40% and 60% respectively.  During the full-scale Rolls-Royce gas turbine study similar trends were observed with 85% reductions in measured nvPM mass witnessed whilst water was injected into the combustor at flow rates 25% higher than the Diesel fuel flow.

The witnessed nvPM reductions are significantly larger than can be explained by water dilution effects, with less impact witnessed in the cases of fuels with higher hydrogen content,  suggesting the reduction may be a chemistry-based phenomenon. To assess potential causes for the observed reduction, preliminary chemical kinetic investigations, were undertaken using CHEMKIN-PRO and suggest that the soot reduction mechanism is potentially via a reduction in PAH formation within the flame zone. However further analysis is required to validate if this mechanism is dominated by in flame OH reduction mechanisms or significantly influenced by other factors associated with water dilution and reduced flame temperatures.