Numerical investigation of combustion and recirculation characteristics in a full-time inner combustion burner

Coal-fired boilers with deep peaking capability facilitates the integration of variable renewable energy sources, thereby enhancing the overall energy utilization efficiency. To improve the stability of low-load combustion in coal-fired boilers, this study proposes a primary self-sustained component based on previous research. Numerical simulations evaluated the effects of reverse air and purge air configurations on the recirculation and combustion characteristics of the component. The return flue gas mass flow was used to characterize the internal recirculation structure of the primary self-sustained component. Under benchmark Case A, the return flow rate was the lowest. In Case B, it increased significantly as the purge air was dispersed and injected into the lower part of the component. In both cases, the return flow rate peaked at X/L = 0.472, where X denotes the measurement cross-section position, and L represents the axial distance measured from the maximum diameter cross-section near the inlet side of the self-sustained component to the outlet end face of the component, exhibiting values of 0.255, and 0.311 kg/s, respectively. This suggests that the reverse flow of flue gas was increased in Case B, providing a consistent heat source for the subsequent incoming pulverized coal, thereby supporting self-sustained ignition. To further improve the safety of the component, Case C modified Case B by relocating the reverse air nozzle upward closer to the upper wall of the component. This adjustment significantly reduced the near-wall temperature, effectively lowering the risk of overheating and coking. However, Case C also resulted in the highest reverse flow of flue gas close to the inlet of the component, where the excessive reverse flow of hot flue gas extended toward the inlet piping of a burner. This increased the risk of premature firing of the pulverized coal, compromising the operational safety of the burner. Based on this analysis, the reverse air and purge air configurations in Case B were identified as the optimal engineering solutions.