Numerical investigation of air-staged combustion emphasizing char gasification and gas temperature deviation in a large-scale, tangentially fired pulverized-coal boiler

A refined char gasification model, successfully validated in a pilot-scale 20 kW down-fired furnace, is now applied to a numerical investigation of the characteristics of the flow, temperature, and species distribution under various air-staged levels of combustion in a 600 MWe tangentially fired (T-fired) pulverized-coal (PC) boiler. The simulation results with char gasification show that the CO concentration profile in both the primary combustion zone and the reduction zone is much higher than the corresponding case without the gasification model for deep (burnout air rate, fS = 0.42), middle (fS = 0.30), and shallow (fS = 0.17) air-staged cases. Moreover, this result is in accordance with the tests from an industrial pulverized-coal-fired furnace. It can be concluded that the char gasification mechanism should be considered in the numerical simulation of large-scale air-staged T-fired PC boilers. On the basis of a reasonable prediction of combustion characteristics, the gas temperature deviation in the crossover pass was also depicted under conditions of various air-staged levels. The result of the thermal load curve of the final super-heater panels clearly presents a saddle-type distribution for the existing two peak values. These inherent deviations originate from the residual swirling flow at the furnace exit. More specifically, parameters of swirling momentum intensity (δ) in the furnace and heat flow intensity (Ψ) at the entry of the final super-heater were employed to identify the temperature deviation in degrees.