Asymmetric combustion characteristics and NOx emissions of a down-fired 300 MWe utility boiler at different boiler loads

To investigate the aerodynamic field, cold airflow experiments were conducted under different boiler loads in a cold small-scale model of a down-fired pulverized-coal 300 MWe utility boiler. At 300 MWe and 250 MWe loads, a deflected flow field appeared in the lower furnace. In contrast, at a 150 MWe load, a U-shaped flow field appeared in regions near the left- and right-side walls in the lower furnace. Concurrently, the regions near the two wing walls adjacent to the front arch had received deflected upward airflow emanating from the region near the rear wall. Moreover, a symmetric W-shaped flow field appeared in the central regions below the front and rear arches.Industrial-sized experiments on the full-scale furnace were also performed at different loads with measurements taken of gas temperatures in the burner region and near the right-side wall, as well as heat fluxes and gas components in the near-wall region. Asymmetric combustion appeared at 300 MWe and 250 MWe loads, with large differences arising in gas temperatures, gas components, and heat fluxes between zones near the front and rear walls. At 150 MWe load, gas temperatures, gas components and heat fluxes are, in general, symmetrically distributed throughout the furnace. By decreasing the load, differences in gas temperatures, gas components, and heat fluxes near the front and rear walls decrease, as did NOx emissions. Meanwhile, the carbon content in fly ash essentially decreased, yielding an increase in boiler efficiency assisted by a drop in exhaust gas temperature.

► Asymmetric combustion and particularly high NOx emissions are universal in MBEL down-fired boilers. ► The boiler load affects strongly the asymmetric combustion extent and levels of NOx emissions. ► Small-scale airflow and industrial-sized experiments were conducted to determine the impact of boiler load. ► Methods for reduction in asymmetric combustion and NOx emissions in these boilers were recommended.