Analysis of air-staged combustion of NH3/CH4 mixture with low NOx emission at gas turbine conditions in model combustors

The NOx formation characteristics have been numerically investigated in the NH3/CH4 fired model combustor consisting of perfectly stirred reactors (PSR) and plug flow reactors (PFR) with 84-species Tian mechanism at gas-turbine relevant conditions of 23 atm and 1873 K at the combustor outlet. Specifically, the impacts of NH3 dilution on NOx formation were first studied in a non-staged model combustor consisting of one PSR representing main combustion zone and one PFR representing post-combustion zone over a wide range of the volumetric ratio of NH3 in the NH3/CH4 fuel. It was found that the NOx emission increases sharply with the NH3 addition mainly through the enhanced HNO pathway in the main reaction zone. Recognizing that the NOx formation via the HNO pathway is limited by the availability of oxygen, the effects of fuel/air equivalence ratio on NOx formation were then quantified under different levels of NH3 dilution. It was revealed that at fuel-rich conditions, the NOx formation via HNO pathway can be offset by the reduction via other pathways, such as pathways related to NHi, leading to the overall low NOx emission. Finally, an air-staged model combustion system was proposed, in which the equivalence ratio in the primary stage was chosen to be 1.5 to take advantage of the low NOx emission under fuel-rich conditions. The air split among the two stages was determined by the fixed outlet temperature of 1873 K. A parametric study demonstrated that less than 30 ppm NOx emission can be achieved with simple two-stage combustion even when the volumetric ratio of the NH3 reaches 40%, which is dramatically lower than the one of thousands ppm in the non-staged combustion. The results were further verified with the 47-species Konnov mechanism, demonstrating the great potential of air-staged combustion for low NOx emission in the NH3/CH4 fired combustion.