Exhaust emissions point measurements on the secondary stage of an NH3-RRQL system

Abstract

The Rich-Relaxation-Quench-Lean (RRQL) combustion system was recently proposed to increase combustion efficiency in ammonia (NH₃)-fueled gas turbines while reducing nitrogen oxides (NO_x). Success requires precise control of the lean secondary stage, where air oxidizes the remaining fuel from the rich primary stage. However, understanding pollutants distribution across the secondary stage is crucial for designing RRQL systems that minimize NO_x and nitrous oxide (N₂O) emissions. This study experimentally investigates axial and radial distributions of exhaust species across the secondary stage of a lab-scale NH₃-fueled RRQL burner. The burner operated with rich premixed NH₃–air mixtures and employed a secondary air injection geometry of five-4.1 mm diameter holes, selected for its ability to stabilize diffusion-like flames at ϕ_global = 0.85 and premixed-like flames at ϕ_global = 0.60. A fused quartz probe enabled gas sampling at multiple locations upstream within, and downstream from the secondary injection plane. At ϕ_primary = 1.13, results show that NO is formed slightly upstream the air injection plane, whereas NO₂ is rapidly produced downstream and NH₃ and N₂O slip through the walls between the jets. A notable fraction of total NOₓ was found to be NO₂, post-secondary combustion zone, underscoring the need to consider NO₂ explicitly in overall emissions assessments of staged NH₃ combustion systems. Decreasing ϕ_global to 0.60 reduces the overall emissions, highlighting the importance of premixed-like combustion in pollutants mitigation in the secondary stage. Although similar trends were observed at ϕ_primary = 1.15, higher NO_x levels resulted from increased NH₃ availability. However, thermal insulation of the combustor walls effectively reduced NH₃ slip, likely due to enhanced non-oxidative cracking, enabling sub-10 ppm N₂O emissions. These findings emphasize the importance of simultaneous primary and secondary stage tuning, reduction of heat losses, and the promotion of premixed-like combustion through adequate jet momentum for optimal performance of NH₃-fueled RRQL systems.