A Parametric Study on NOx Emissions From Ammonia Containing Product Gas in Rich Quench Lean Combustion

Abstract Due to climate change, there has been an increasing demand for fuels that can accelerate the transition away from fossil fuels to clean energy. Humidified product gas obtained from gasifying biomass is emerging as a promising candidate to replace natural gas, as it is composed of a gaseous mixture of hydrogen, steam, carbon monoxide and methane. However, the gasification process releases ammonia and other nitrogen bearing compounds into the product gas, resulting in substantial increases in nitric oxides, NOx, in the exhaust. As such, there has been a recent push to understand the underlying chemical kinetics that drive NOx formation in order to optimize gas turbines to mitigate emissions at the source. In this study, a simplified CRN model for a gas turbine Rich-Quench-Lean (RQL) type combustor was developed in Cantera. The following parameters were investigated in this study: equivalence ratio of the primary section (1.1 to 1.6), overall equivalence ratio (0.6 to 1.1), steam dilution (25% and 50% molar dilution), post flame residence time (0 to 75 ms) and recirculation from the post quench region to the primary section (0 and 40% by mass). Additionally, a benchmark CRN representing a Lean Burner (LB) as also developed. Results of the CRN model suggest that, when comparing to LB, a RQL type combustor delivers up to a 75% reduction in emissions. Additionally, it was found that, for both the LB and RQL combustors, an overall lean to stoichiometric equivalence ratio is well suited to reduce emissions in highly humidified fuels, while for moderately humidified fuels it is preferable to operate in an overall slightly rich equivalence ratio. The difference observed is mainly due to the fact that, at high humidification and lean conditions, the temperature is favourable for the conversion of ammonia to nitrogen. While, at moderate humidification and rich conditions, NO reacts with ammonia in the reburn process. Finally, it is suggested that the incorporation of recirculation from the secondary section to the primary section of the RQL burner results in a broader low emission region, due to more favourable conditions for ammonia conversion to nitrogen in the primary section.