In the current study, the influence of pressure and steam on the emission formation in a premixed natural gas flame is investigated at pressures between 1.5 bar and 9 bar.
A premixed, swirl-stabilized combustor is developed that provides a stable flame up to very high steam contents. Combustion tests are conducted at different pressure levels for equivalence ratios from lean blowout to near-stoichiometric conditions and steam-to-air mass ratios from 0% to 25%.
A reactor network is developed to model the combustion process. The simulation results match the measured NOx and CO concentrations very well for all operating conditions. The reactor network is used for a detailed investigation of the influence of steam and pressure on the NOx formation pathways.
In the experiments, adding 20% steam reduces NOx and CO emissions to below 10 ppm at all tested pressures up to near-stoichiometric conditions. Pressure scaling laws are derived: CO changes with a pressure exponent of approximately −0.5 that is not noticeably affected by the steam. For the NOx emissions, the exponent increases with equivalence ratio from 0.1 to 0.65 at dry conditions. At a steam-to-air mass ratio of 20%, the NOx pressure exponent is reduced to −0.1 to +0.25.
The numerical analysis reveals that steam has a strong effect on the combustion chemistry. The reduction in NOx emissions is mainly caused by lower concentrations of atomic oxygen at steam-diluted conditions, constraining the thermal pathway.