Operability, Emissions, and Optical Measurements of Combustion of Conventional and Alternative Liquid Fuels

The push for a decarbonized power grid requires a significant reduction in CO2 emissions produced by gas turbines. Several current gas turbines utilize traditional fossil fuels such as kerosene or #2 fuel oil/diesel fuel, and replacing these fuels with sustainably produced fuels is crucial for decarbonizing gas turbine infrastructure. Fuels produced from biomass, such as renewable diesel, bioethanol, and biomethanol have shown substantial reduction in life cycle CO2 emissions when replacing traditional fossil fuels. Work is being conducted to characterize the pollutant emissions created by these fuels, with most existing studies indicating either no change or a decrease in important emissions, such as NOx, CO, and particulate matter when burning sustainable fuels. However, there are comparatively few studies comparing many of these biofuels with each other. This report details the results of experiments conducted to evaluate emissions produced by fossil fuels, synthetic fuels, and alcohol fuels, without changing hardware, and while maintaining a constant flame temperature. To this end, an 8 bar capable, optically accessible combustion test rig was constructed, utilizing aeroderivative combustor hardware. A gas analyzer measured the emissions concentrations in a sample pulled from a water-cooled probe placed in the exhaust of the liner, and filter paper was used to measure the particulate matter produced. OH* chemiluminescence images and Mie scattering images were also captured to make qualitative measurements of the flame shape and spray from a pressure-swirl atomizer in a counter-rotating, dual-passage, radial swirler. Burning the alcohol fuels presented important operating challenges regarding inlet air temperature, the fuel supply system, and blowoff boundaries, which indicate the limitation in using ethanol and methanol as ‘drop-in’ fuels. It was found that the renewable diesel and sustainable aviation fuels had a negligible effect on NOx and CO emissions compared to diesel and Jet A when operating at a consistent set of conditions, while ethanol and methanol showed a decrease in NOx emissions and an increase in CO emissions when compared to the distillate fuels. Particulate matter production was significantly reduced when alcohol fuels were used. The chemiluminescence images indicated that the flame shape did not significantly differ between fossil and synthetic fuels. The alcohol fuels, however, did differ slightly in flame shape. The Mie scattering images showed that the macro spray cone did not differ between fuels. Droplet-scale measurements could not be made due to the limited resolution of the diagnostic relative to the size of the fuel droplets. This paper contributes considerations for operating with different fuels, emissions trends for a range of different distillate and alcohol fuels, physical insight from optical measurements, and opportunities for future work.