Towards the Development of a RRQL System Part 1: Swirl Pattern Effect on Exhaust Emissions and Chemiluminescence Distribution for NH3-Air Premixed Swirl Flames

Abstract Ammonia (NH3) is a carbon-free energy source and hydrogen carrier, but its fuel-bound nitrogen can lead to significant nitrogen oxides (NOx) emissions. Staged combustion strategies, such as Rich-Quench-Lean (RQL), can achieve low NOx emissions. However, improper design may result in high NOx levels when operating with a rich head end without a sufficient post-flame relaxation time. Previous work has shown that a Rich-Relaxation-Quench-Lean (RRQL) configuration can minimize NOx emissions in the rich head end [1]. This study focuses on how different swirl geometries affect exhaust emissions and flame morphology in rich, premixed NH3-air flames, for the design of a rich relaxation head end of an RRQL combustor. Experiments were conducted using a modular swirl burner, and five swirlers were tested, featuring geometric swirl numbers of Sg = 0.7 and 1.1, with varying numbers of vanes (8, 12, and 16) and vane shapes (curved or straight). The paper presents measured NOx, N2O, and NH3 emissions, and recorded natural flame luminosity and NH2* and OH* chemiluminescence images. The equivalence ratio (ϕ) varied from 1.05 to 1.18 to identify an optimal low NOx and NH3 balance for each swirler. Swirler design affects this optimum by influencing flame length (with shorter flames allowing for more post-flame relaxation time) and flame-wall interactions (influencing NH3 and N2O emissions). For example, Sg 1.1-C8 swirler with a swirl number of Sg = 1.1 and eight curved vanes achieved its optimal NOx-NH3 performance at a leaner ϕ, exhibiting high N2O concentrations, possibly due to heat losses from flame interaction with the wall. Increasing the number of vanes shifted the low NOx-NH3 optimum toward a richer ϕ with lower N2O and NH3 emissions. Consequently, among the tested swirler designs, Sg1.1-S16 is preferred for a RRQL configuration, as it generates a compact flame with minimal wall interactions. The short flame height allows for increased NOx relaxation while minimal wall interactions prevent heat losses and stabilities which minimize NH3 and N2O emissions at a richer sweet spot, potentially enhancing H2 production during the Relaxation phase without increasing overall primary stage NOx emissions, thereby improving system efficiency.