Numerical Simulation Of The Effect Of Water Vapor On MILD Combustion Of Pulverized Coal

MILD combustion has the characteristics of low reaction rate,uniform heat flow distribution and low peak temperature.Oxy-MILD combustion technology combined MILD combustion with oxy-fuel combustion also has the advantage of low cost capture CO2.Compared with N2,steam has the characteristics of high specific heat capacity,strong radiation capacity,and gasification reaction with carbon,which can reduce the reaction rate of coal and furnace temperature.So,it is theoretically feasible to use steam to promote the the realization of pulverized coal MILD combustion.In this paper,numerical simulations are carried out from the two aspects of water vapor concentration and water vapor velocity,which focus on the influence of water vapor on air-MILD combustion and oxy-MILD combustion of pulverized coal.The results of this study have important scientific significance for further revealing the combustion mechanism of pulverized coal MILD combustion.First of all,based on the IFRF experimental furance,the conventional air combustion and air-MILD combustion of pulverized coal are numerically calculated.The significant difference in the two combustion modes is analyzed from the cold and hot states in the furnace.It is found that air-MILD combustion has the characteristics of large recirculation area,strong entrainment,uniform temperature distribution,high gasification ratio and low NO emission.Then,this paper studies the law of the influence on MILD combustion of water vapor concentration by simulation.We discuss the effect of steam concentration on combustion under different oxygen concentrations in the atmosphere of O2/N2 and O2/CO2.The flow field,temperature distribution,carbon reactivity and NO emission at different oxygen and water vapor concentrations are compared,while the effects of different atmosphere on MILD combustion are also compared.The results show that in O2/N2,increasing water vapor concentrations makes the furnace temperature more uniform,the proportion of carbon gasification increased,the thermal NO and fuel NO reduced;in O₂/CO₂,increasing water vapor concentration makes radial temperature of the main combustion zone increase,the peak temperature rise,reduction zone increase and fuel NO emission reduce.In these two atmospheres,increasing the oxygen concentration increases the gasification reaction ratio of carbon and reduces the fuel NO emission.When the water vapor concentration is30%,the temperature distribution of 28%O₂ in O₂/CO₂ atmosphere.is similar to the one of28%O₂ in O₂/CO₂ atmosphere.Finally,this paper studies the law of the influence on MILD combustion of water vapor velocity by simulation.The influence of water vapor velocity on the combustion in O₂/N₂ and O₂/CO₂ was discussed by changing the the size of the steam nozzle.The flow field,temperature distribution,carbon reactivity and NO emission at different water vapor velocity are analyzed,while the effects of different atmosphere on MILD combustion are also explored.It was found that in O₂/N₂ and O₂/CO₂,improving the water vapor flow rate makes the mixing point of the primary and secondary air in advance,the flue gas entrainment enhanced,the temperature distribution more uniform,the proportion of carbon oxidation reaction increased,the area of the low oxygen concentration area reduced and the fuel NO emission increased.However,in O2/N2,the inhibition of thermal NO is dominant at small water vapor velocity while the promotion of fuel NO is dominant at the large flow rate.Therefore,the sum of the thermal NO and the fuel NO decreases first and then increases as the water vapor flow rate increases.