Ignition Characteristics and Interaction of Coal and Ammonia Co-Combustion on a Diffusion-Flamelet-Based Hencken Burner

Co-combustion of coal and ammonia presents an effective strategy to gradually reduce the share of coal-fired power generation and, thereby, achieve CO2 reduction. In this work, the ignition characteristics and interaction of coal and ammonia co-combustion with varying ammonia blending ratios (0–100%) under different oxygen concentrations (5%–30%) were investigated on a diffusion-flamelet-based Hencken burner. The flame morphologies of coal and ammonia co-combustion under different operating conditions were captured through the co-combustion flame images. And the flame radial intensity profiles at the cross sections of the ignition positions exhibited a bimodal distribution at 20% and 30% O2 concentrations with the addition of ammonia, which were not observed at 5% and 10% O2 concentrations or in pure coal combustion scenarios. Correspondingly, the trend in the flame diameter of coal and ammonia co-combustion with the increase of ammonia blending ratio shifted from initially increasing and then decreasing at 5% and 10% O2 concentrations to a monotonically increasing one at 20% and 30% O2 concentrations. Furthermore, the chemiluminescence spectrum of coal and ammonia co-combustion was measured to analyze the evolution of the radicals at different ammonia blending ratios, and the NH* and OH* radical signals were used to correlate the intensity of the gas-phase reaction of ammonia and coal volatile matter combustion. It was found that the interaction between ammonia and coal became more pronounced at higher oxygen concentrations. Notably, the NH*/OH* ratio at the outlet of the central fuel tube was strongly correlated with the ignition delay distance of coal and ammonia co-combustion. Finally, the variation of the coal particle temperatures during the ignition stage was analyzed to elucidate the interaction of the co-combustion process. In most cases, the addition of ammonia led to an increase in the maximum temperature of the coal particles. This effect was primarily influenced by the particle number density (PND) of the pulverized coal and the distribution of the thermal input between the coal and ammonia.