Structure and Extinction Characteristics of Methane/Ammonia/Pulverized Coal Premixed Flames

Co-firing ammonia with pulverized coal offers a promising approach to reducing carbon emissions from coal-fired power plants. However, ammonia, as a fuel, presents challenges for combustion stability when burning compared to traditional fuels like methane. This study focuses on the extinction limits of ammonia-pulverized coal co-firing flames, a critical factor for efficient and safe combustion. A counterflow flame configuration was developed to investigate ammonia/pulverized coal premixed flames. The apparatus, with two opposed nozzles generating a planar flame, enabled the examination of gas-solid reactions. Pulverized coal was fed into the system, with methane representing the volatile matter to create a high-temperature environment for ignition. Stable methane/ammonia/pulverized coal/air premixed flames were successfully established, whose structure was observed under different conditions. The results show that in counterflow premixed flames, assisted by the methane flame, the ammonia/pulverized coal co-firing process begins with the ignition of gaseous components, forming a gaseous flame front, followed by the ignition of pulverized coal downstream in the high-temperature flue gas. The flame structure features in the coal combustion zone, the gaseous flame zone, the preheat zone, and the unburned zone. Increasing the flame stretch rate results in flame extinction, and the extinction limit of gaseous fuel plays a decisive role in the extinction of the overall gas-solid two-phase flame. For high ammonia blending ratios (50%, 75%, by energy) in the ammonia/pulverized coal mixture, the addition of pulverized coal decreases the overall extinction gaseous flame equivalence ratio at relatively high stretch rates (≥ 40 s-1) but increases the extinction gaseous flame equivalence ratio at relatively low stretch rates (< 40 s-1). For low ammonia blending ratios (0%, 25%), the addition of pulverized coal consistently lowers the extinction gaseous flame equivalence ratio across the range of stretch rates covered in the experiments. Additionally, the study examined synergistic effects, finding limited interaction near extinction, implying that extinction of ammonia/pulverized coal combustion closely follows the behavior of gaseous flame extinction in the mixture in weakly-stretched premixed flames. These findings provide insights for enhancing the stability of ammonia/pulverized coal co-firing.