Influence of steam addition and elevated ambient conditions on NO_x reduction in a staged premixed swirling NH₃/H₂ flame

Abstract

There is growing interest in the application of renewably-generated NH₃ to support future energy requirements, however combustor designs and strategies require considerable development to reduce NO_x emissions in particular. A turbulent swirl burner was used to experimentally and numerically appraise potential pathways for operational NO_x reduction with a premixed NH₃/H₂/air flame. Reactants were supplied at elevated temperature with parametric changes made to pressure and humidity. Favourable agreement was demonstrated between exhaust gas measurements and chemical kinetic simulations with a reactor network model, showing NO_x emissions to be sensitive to operational equivalence ratio, increasing by several orders of magnitude across the experimental range. The lowest NO_x concentrations were achieved at the richest conditions, accompanied by high unburned fuel fractions in the product stream. An increase in combustor pressure remarkably reduced exhaust NO_x concentrations primarily due to enhanced NH₂ formation, and subsequent NO consumption in the post-flame zone. Reactant humidification was explored in detail for the first time with this fuel, and shown to reduce NO_x production limiting thermal pathways with the extended Zel'dovich mechanism. NO consumption in the post-flame zone was also enhanced through an increase in OH-produced NH₂, and together with pressure, resulted in elevated exhaust NH₃ concentrations. Whilst this effect was comparatively small, it meant that leaner humidified operation could be employed to reduce unburned fuel fractions without a NO_x penalty. Emissions performance was further improved by the application of staged combustion, with secondary airflow used to improve fuel burnout. Humidity and pressure were optimised in the staged configuration to achieve operation with sampled respective NO_x and NH₃ exhaust fractions of 32 and 50 ppmvd (15%O₂), at a globally lean equivalence ratio. There is considerable scope for further system optimisation through improved mixing of secondary air and increased ambient pressure.