Gas-Solid Flow and High-Efficiency and Low-NO_x Combustion of a Novel Arch-Fired Boiler with Swirl Burners: Impact of Over-Fire Air

For a 300 MW_e anthracite- and arch-fired boiler with biased-swirling-secondary-air (BSSA) technology, the distribution law of gas/solid two-phase velocity in the furnace is obtained with the help of a 1:10 cold modeling experiment bench, and the combustion and NO_x formation characteristics in the furnace are revealed by numerical calculation. The results show that, with the increase of over-fire air rate, the reflux velocity in the reflux zone under the arch decreases continuously, and when the main air flow from the furnace arch reaches the staged air region from the vent air region, the position of particle flow in the primary air turn upward is advanced, and the utilization rate of the space in the lower furnace will decrease continuously. In the regions of vent air and staged air, when the over-fire air rate increases from 15% to 25%, the maximum vertical velocity decreases by approximately 2 times, and the downward depth of air flow from the furnace arch decreases significantly. When the over-fire air rate is increased from 10% to 25%, the thermal resistance between the primary air of the burner and the high-temperature flue gas in the furnace center increases, the ignition of the primary pulverized coal air flow is advanced, the flue gas temperature in the center of the lower furnace is decreased. As a result, with the increase of over-fire air rate, the combustible content of fly ash increases continuously. Especially, when the over-fire air rate increases from 20% to 25%, the combustible content of fly ash increases significantly. With the increase of the over-fire air rate, the high CO concentration region in the primary combustion zone expands, the reducing atmosphere is strengthened, the formation of NO_x is effectively restrained, and the NO_x emission concentration at the furnace outlet continues to decreases. In particular, when the over-fire air rate increases from 15% to 20%, the NO_x emission concentration decreases greatly. After the swirling secondary air of the arch-fired boiler is biased, the setting of over-fire air rate is recommended to be 20%, and the NO_x emission concentration and combustible content of fly ash are 694 mg/m³(O₂ = 6%) and 7.32%, respectively. Compared with a traditional swirling arch-fired boiler, the combustible content of fly ash and NO_x emission concentration are reduced by 46.8% and 34.1%, respectively.