Chemical Kinetic Study of Pressure and Fuel Staging Effects on Nox Emissions in Premixed Methane/hydrogen Flame Combustion

Given the ongoing strict regulation on NOx emissions and additional challenges relating to carbon abatement targets, research into hydrogen – natural gas blends is becoming increasingly relevant in flexible power generation. In this study, a chemical kinetics model has been developed using ANSYS Chemkin-Pro R3 to assess the effect of combustor pressure on NOx formation from the premixed combustion of methane/hydrogen fuel blends with and without staged fuel injection. This study intends to replicate the combustion conditions within commercially available GTs that utilise sequential combustion, which have been shown to exhibit high fuel flexibility, in particular to examine the NOx characteristics of H2 – CH4 blends.  A parametric study of a range of given fuel blends is performed across a range of pressures, up to pressures experienced in F-class GTs (up to 30 bar). Equilibrium and flame speed calculations are used to investigate the reactivity of these fuel blends, with hydrogen addition shown to increase both the laminar flame speed and adiabatic flame temperature of CH₄-H₂ blends.   NO_x emissions are estimated initially using a standalone premixed flame in a chemical reactor network model.  The study is then recreated with an additional fuel injection further downstream, after the pressure drop disc to replicate the turbine stage in a commercial sequential combustion GT. With increasing hydrogen content in the fuel, a greater level of NO_x is observed due to the higher adiabatic flame temperature which promotes the formation of thermal NO_x (Zeldovich mechanism).  At lower pressures, prompt NO_x (Fenimore mechanism) is the prevalent mechanism, yet as the pressure of the combustion chamber increases additional NOx formation pathways (e.g. N₂O and NNH pathways) contribute to overall NO_x formation. Ideal compositions of primary and secondary injected fuel are investigated to reduce NO_x formation under these pressurised, staged combustion conditions, while also comparing predicted NO_x formation rates across different chemical reaction mechanism pathways.  This model will be validated by future experimental work to be undertaken at Cardiff University’s Gas Turbine Research Centre.