Numerical Simulation On 400t/h Tangentially Coal-fired Boiler Combustion And NO_x Emissions

Contents growing demand in China for electric power generation leads to further increase in the use of fossil fuels.Due to combustion of coals in large power plants significant amount of pollutant gasses,such as nitrogen and sulfur oxides,are released into the atmosphere.This leads to an increasing concern on the environmental protection,and more strict regulations on the limitation of emissions from large combustion plants have been proposed.To meet the stringent pollutant emission limits,researchers focused on the power plant boiler with higher efficiency in energy transformation process,reduction of pollutants emission,operation of facilities in a wide range of loads.The generation of nitrogen oxides and reduction mechanism were reviewed in this paper.The domestic power plant NO_x emission levels were investigated.Measurement of a power plant NO_x emissions level and thermal efficiency were performed.The use of CFD codes for modeling utility boilers is becoming a useful tool to predict the performance of boilers among the scientific and industrial communities.Using the Fluent,CFD software,a three-dimensional simulation of the flow,combustion process and NO_x formation in the furnace of a 400T/h tangentially coal-fired utility boiler was presented in this study.Gas phase is described by time-averaged Eulerian conservation equations for mass,momentum,energy,turbulence kinetic energy and its rate of dissipation.The production of turbulence and its dissipation are modeled with k-ε model.The stoichastic tracking model was applied to analyze the gas-solid flow field.Mixture fraction/probability density function model was employed to describe the chemical reaction and heat transfer process accrued in furnace.A post processor is employed for NO_x simulation and thermal and fuel NO_x is considered in this study.The radiative properties of gas are computed using the P-1 model.The boiler operating conditions considered in this study include full load(100%),90%,80%,70%load.Attentions are focused on the effect of excess air ratio,distribution wind and boiler load on the coal combustion,gas flow,heat transfer and NO_x formation in the boiler.Along with the increasing of the excess air coefficient in the range of 1.0-1.3,the whole temperature level in the boiler reduces a little and the diameter of tangential circle and the area of high temperature reduce gradually.When the boiler load reduced,the whole temperature level in the boiler,the average heat flow density of the wall,the average temperature and velocity as well as the average NO_x mass concentration reduce;Amount the three distribution wind,equal distribution wind leads to an increase in high temperature zone in the boiler.Maximum NO_x emission is observed for the normal distribution wind when the excess air coefficient is kept constant.The prediction helps engineers to optimize the operating conditions,reduce pollutant emissions and also improve the design of new boilers.