Numerical investigation of stable combustion at ultra-low load for a 350 MW wall tangentially fired pulverized-coal boiler: Effect of burner adjustments and methane co-firing

In the context of deep peak regulation, utility boilers are often required to operate at ultra-low loads, significantly deviating from their designed conditions, which can lead to unstable combustion or even furnace flameout. However, their operating characteristics at 20 % or even lower loads remain unclear. This study focuses on a 350 MW wall tangentially fired pulverized-coal boiler, exploring its combustion characteristics and pollutant emission patterns at 20 % ultra-low load. Through numerical simulation analysis of 14 different combustion scenarios, this research investigates the impacts of burner arrangements, burner horizontal swing angles (ranging from 5° to 20°), and methane (CH4) co-firing ratios (ranging from 0 % to 20 %, calculated by heat value) on flue gas temperature and main flue gas components (O2, CO, NOx, etc.). The results indicate that as the boiler load decreases to 20 %, the average flue gas temperature inside the furnace drops by about 200 K, accompanied by widespread flame impingement on the water-cooled walls, leading to decreased combustion stability of the pulverized coal. Operating only the lower two layers of burners can achieve a higher average flue gas temperature (1540 K) and lower NO emissions (294 mg/m3). Furthermore, setting the horizontal swing angle of the burners to 15° can mitigate the impact of flames on the water-cooled walls and elevate the flame center, with the average flue gas temperature reaching 1559 K, making it the recommended burner swing angle setting. As the CH4 co-firing ratio increases from 0 % to 20 %, the average flue gas temperature rises by 50 K, and the emissions of NO and CO2 decrease by 15.3 % and 6.2 %, respectively, while H2O concentration increases by 21.3 %, potentially leading to low-temperature corrosion issues. The recommended CH4 co-firing ratio is 15 %.