Synergetic effects of firing gases/coal blends and adopting deep air staging on combustion characteristics

The synergetic effects of firing gases/coal blends and simultaneously adopting deep air staging on combustion characteristic parameters, such as furnace temperature, burnout of coal and NOx emissions, were firstly experimentally studied and numerically simulated in a 300 MWe unit boiler. The improved multiple surface reaction model, which included pore diffusion resistance of gas phase in ash, was proposed in this paper to simulate actual char combustion. The simulated results quantitatively showed that the proposed model solved the problem of seriously inaccurate calculation of unburned carbon in fly ash for the numerical simulation of multifuel combustion. Under the conditions of firing coal/gases blends and adopting air staging, the models of pulverized coal pyrolysis, coke gasification and gas combustion were established because the reduction atmosphere is formed in the primary combustion zone of the furnace. The reduction of NOx by reductive gases produced from co-firing gases, pulverized coal pyrolysis and coke gasification was considered in numerical simulation. All the numerical models adopted in this study were reasonably validated on the basis of comparing the simulation and experimental results. The temperature field, velocity field, flue gas components, coke burnout, NOx formation and reduction characteristics of three conditions (coal-fired condition, adopting deep air staging condition and firing coal/gases blend and simultaneously adopting deep air staging condition) were studied numerically. After firing coal/gases blend and adopting deep air staging, the furnace temperature field was fairly well-distributed; the average velocity of flue gas decreased lower; the reducing atmosphere zone was found to be lengthened by comparing the concentration of oxygen and carbon monoxide; and it could be concluded that there was more time for the combustible matter burning-out and NOx reduction. The temperature field and reducing gases distribution profile along the furnace height are disadvantageous for generating NOx and beneficial to reducing NOx. The results clarified the complex synergistic effect of firing coal/gases blend and adopting deep air staging on combustion characteristics, which could be applied to increase the boiler performance and optimize the co-firing technology.