Effects of Blending Ratio on Combustion and NO Emission Characteristics during Co-Firing of Semi-Char and Lignite in a 350 kW Pulverized Coal-Fired Furnace

The influence of the blending ratio of pyrolyzed semi-char (SC) on the ignition, NO emission and burnout characteristics of lignite co-fired with SC was investigated in a 350 kW fuel-rich/lean combustion furnace. The flame temperature and concentrations of gaseous species including O₂, CO, and NO, were measured in detail. The results indicated that the ignition characteristics of the blended fuel worsened with increasing SC blending ratio, such as an elongated ignition standoff distance. The addition of SC to lignite delayed the appearance of a stable flame boundary, and the stable combustion zone moved down, but the final combustion stability was gradually strengthened in the later combustion stage. NO emission concentration at the primary combustion zone (PCZ) outlet was the lowest at 472.6 mg/m³@6% O₂ when the SC blending ratio was 25%. The combustion zone and reducing zone areas in PCZ were defined to evaluate the NO reduction characteristics, and quantitative analysis using a multiple linear regression model showed that heterogeneous reduction was more important than homogeneous reduction in lowering NO emissions. The Raman spectrum of the char sample indicated that the addition of lignite promoted the formation of small aromatic rings in the early ignition stage, corresponding to a higher char reactivity. The burnout ratio of pure lignite was maximal and was decreased by increasing the SC blending ratio. Synthetically, considering the ignition standoff distance, NO emission, and burnout ratio, the optimum SC blending ratio was estimated to be 25%.