Study On Flamelet/Progress Variable Model For No Formation In Pulverized Coal Combustion

Lots of nitrogen oxides(NOx)are released during pulverized coal combustion(PCC),which has very harmful effects on human health and ecological environment.However,PCC and NO formation characteristics are very complicated.Therefore,it is necessary to study these processes deeply.With the improvement of computational efficiency and the development of numerical theory,numerical simulation has become an important way to study the process of PCC.The results of the numerical simulation can provide reference and guidance for the development of PCC technology of high efficiency,safe and low pollutant emissions.This paper aims to develop a high-precision PCC model to improve the prediction accuracy of NO formation,and then study the PCC and NO formation in an industrial scale coal boiler.Firstly,A priori and A posterior analyses are conducted to verify the ability of various progress variables in predicting major thermochemical quantities and NO in laminar flow conditions.The complexities of operating conditions increase gradually,which varies from gas phase combustion to realistic PCC.Based on the results,a hybrid flamelet/progress variable(FPV)model for PCC is developed.In this new model,one progress variable consists of major species and is used to construct the flamelet library for the prediction of major thermochemical quantities,the other progress variable consists of both major species and a certain proportion of NO and is used to construct the flamelet library for the prediction of NO.The new model is evaluated under various operating conditions of PCC.The results show that the new model can not only predict the major thermochemical quantities well but also improve the accuracy of NO prediction significantly.The importance of enthalpy and progress variable for NO prediction is discussed The results show that selecting a reasonable progress variable is very important for the prediction of NO variation trend.In addition,considering the effect of enthalpy is beneficial for the prediction of gas temperature,and NO prediction can thus be improved to some extentSecondly,a new method of the flamelet library construction,which can reasonably consider fuel N partitioning between volatiles and char,is developed.The large eddy simulation(LES)of a laboratory-scale turbulent pulverized coal jet flame is conducted.The simulation results are compared with the experimental data and NO prediction results with a post-processing approach The effect of two ways of fuel N partitioning on the prediction of particle dispersion,particle velocity,temperature,major species and NO formation is studied through qualitative and quantitative analyses.The results show that the characteristics of PCC are not changed by different fuel N partitioning ways significantly as a whole.When a more reasonable fuel N partitioning way is considered,the simulation results of NO are closer to experimental data.Compared with the NO post-processing approach,the new method has greater potential for NO prediction Moreover,the combustion and NO formation characteristics of the turbulent pulverized coal jet flame are further discussedFinally,the hybrid FPV model of PCC is applied to an LES of an industrial coal-fired boiler.The reasonable fuel N partitioning between volatiles and char is considered.A typical burning zone of the supercritical CO2 boiler is selected and is further modeled as the research object Combustion states as well as time and space distribution characteristics of coal particles and fluids at different locations are discussed through qualitative and quantitative analyses.The results show that the mixture of primary air and secondary air,the high-temperature wall as well as the adjacent flame can promote the PCC and NO formation.In addition,the effects of wall temperature variation and flue gas recirculation on PCC and NO formation in the typical modeling burning zone are also investigated.The results show that compared with supercritical H2O boiler,higher furnace temperature and NO production can be observed in the supercritical CO2 boiler due to a higher wall temperature;The introduction of flue gas recirculation can effectively reduce the production of NO in the supercritical CO2 boiler.