Combustion and emissions of alternative fuels in gas turbines

Renewable biomass derived fuels are of increasing interest for many applications including industrial and aero gas turbines due to the reduction in fossil fuel CO2 and the improvement in energy supply security. The first part of this work investigated the performance of biodiesel as a fuel in low NOx combustors of the type used in industrial gas turbines. This work included comparison with kerosene and co-firing with natural gas and blends of kerosene/biodiesel. In the second phase of this work an aircraft gas turbine APU with diffusion combustion. This investigated the gaseous and particulate emissions using kerosene as a base fuel for comparison with several second generation biofuels, which covered a range of H/C and showed that emissions were correlated with the H/C. The third phase of the work was concerned with renewable or clean coal derived hydrogen combustion using a low NOx flame stabilizer for industrial power generation applications.
For the industrial low NOx combustor work a radial swirler flame stabiliser was used. However, the high boiling point of B100 made operation in a premixed vane passage fuel injection mode impossible as ignition could not be achieved. The pilot fuel injector in the centre was the only fuel injection location that B100 would stabilise a flame, due to the central recirculation of burnt gases. A central 8 hole radially outward fuel injector was used as WME (B100) would not operate with radial vane passage fuel injection that is conventionally used for low NOx radial swirlers with natural gas.
In the aero engine phase of the research, nine alternative fuels were tested and compared to conventional JetA1 fuel at idle and full power. The results showed that all fuels produced similar level of NOx compared to JetA1 and a slight reduction in CO. A remarkable reduction in UHC was observed at all conditions for higher H/C fuels. The results also show that there was a good correlation between fuels H/C ratio and particle concentrations, particle size and distributions characteristics. The hot idle produced ~20% less particles compare to the cold idle. The alternative fuel blends produced fewer particles than JetA1 fuel.
The alternative source of renewable fuels for industrial power generation gas turbines is that of hydrogen derived from renewable or nuclear electricity or from coal or biomass gasification using the water gas shift reaction and CO2 solvent extraction to leave a pure hydrogen fuel. The key problem are in burning hydrogen in gas turbines is that of the increased NOx formation and the increased risk of flashback into the conventional premixing passages used in natural gas low NOx combustors. This work investigated a novel impinging jet configuration that had previously been used successfully with propane and kerosene fuels. It had no premixing so that there could be no flashback. However, the high reactivity of hydrogen did cause a problem with flame stabilization too close to the jet outlets. This was controlled by reducing the proportion of air added to the initial hydrogen jets. NOx emissions lower than alternative designs were demonstrated at simulated high power conditions. This was a practical combustion technique for high hydrogen content fuels with low NOx emissions and no flashback problems.