Experimental and numerical study of combustion and emission characteristics of NH3/CH4/air premixed swirling flames with air-staging in a model combustor

In the context of low-carbon transition, ammonia (NH3) is considered as a promising zero-carbon fuel owing to its high hydrogen density, well-established storage, and transportation systems. However, its low reactivity and high nitrogen content could induce flame instability and high NO emission problems during combustion, which hinders its large-scale application in industrial furnaces, gas turbines, and engines. The reactivity of NH3 can be effectively boosted by blending it with a more reactive fuel, such as methane (CH4). In addition, it has been demonstrated that burning NH3 at slightly rich equivalence ratios produces low NO emissions, but air-staging is needed to oxidize the unburned fuel in a secondary lean combustion zone. This paper carries out an experimental and numerical study for NH3/CH4 premixed swirling flames in a model combustor with and without air-staging. The main objective is to investigate the potential of using air-staging for controlling NO emissions from NH3/CH4 flames, and to examine the influence of key air-staging parameters, such as staged air ratio (SAR), height of staged-air (H) and number of staged-air nozzle (N), on flame typology as well as NO and CO emissions. Experimental results show that globally lean condition fails to generate satisfactory NO emission for NH3/CH4 mixtures under non-staging mode, and the maximum NO emission is produced by the X-NH3 = 50% mixture which exhibits the largest challenge for NO control. By initiating air-staging for X-NH3 = 50% mixture, the flame topology as well as CO and NO emissions are strongly affected by the key air-staging parameters. In relatively large SAR (> 30%) and small H (< 80 mm) conditions, the staged-air impinging effect should be taken into consideration since it could destroy the locally rich atmosphere in the primary combustion zone, creating lean burning pockets and enhancing NO formation. Increasing N plays an accelerating role on NO generation due to improved mixing homogeneity in the vicinity of staged-air, while this effect can be counteracted by mitigating the staged-air impinging effect via decreasing the staged-air injection momentum. Moreover, an improved NH3/CH4 reaction mechanism based on Okafor's is proposed which shows remarkably high accuracy in predicting NO emission for NH3/CH4 mixtures under both non-staging and air-staging modes. This work provides new insights towards the understanding of NH3/CH4 combustion using air-staging.