Non-gray Radiative Property Models For Participating Media And Heat Transfer Characteristics In Oxy-fuel Combustion

Oxy-fuel combustion is one of the most promising technologies for large-scale carbon capture and storage,which has been extensively studied in recent years.The heat transfer process is dominated by radiation in coal-fired boilers,which is a key process for the boiler design.However,the variable initial oxygen concentration and the high concentrations of CO2 and H2 O result in a great change in radiative heat transfer in the furnace under oxy-fuel combustion.In this study,by using the experimental and numerical studies,the radiative heat transfer is systematically investigated in oxy-fuel combustion.Firstly,Mie theory was used to calculate the spectral absorption and scattering efficiencies of the fly ash particles.The concept of the full spectrum k-distribution(FSK)and the weighted sum of the gray gas(WSGG)are introduced.Based on the weighted sum of the gray particle,a non-gray radiative property model(WSGP-SK)of fly ash particles is obtained.By using the LBL model as benchmark solution,the prediction accuracy is evaluated in a one-dimensional plane-parallel slab system.The error of WSGP-SK model and Planck mean method are less than 8% and 13%,respectively.In addition,the prediction accuracy of different WSGG models are compared.The results also show that the WSGG model based on the k-distribution method(WSGG-SK)has the best accuracy.The WSGG-SK model is further combined with the WSGP-SK model to evaluate the prediction accuracy in gas and particle mixture.Secondly,the spectral absorption and scattering efficiencies of char particles during combustion are calculated by Mie theory.The effects of particle diameter and burnout ratio on the radiative property are studied.By using the k-distribution to reorder the spectral absorption and scattering efficiencies,a non-gray radiative property model is developed(WSGP-SK_Burnout).The burnout ratio and particle diameter are two important parameters in this model.As a comparison,a gray model based on Planck mean is obtained.The prediction accuracy is evaluated by LBL model in a one-dimensional plane-parallel slab system.The errors are less than 15% for the WSGP-SK_Burnout model,35% for Planck mean method,and 53% for linearly varying gray model,respectively.Then,numerical simulation methods for oxy-coal combustion are developed,and the influence of variation of combustion atmosphere on the applicability of the submodel is considered.The simulation methods are evaluated by the experiment in IFRF 2.5 MWth furnace with a swirling burner.The effects of different gas and particle radiative property models on the prediction accuracy are compared.The results show that ignoring the effect of burnout ratio(the emissivity of particles is set as 0.9)will over-predict the radiative heat transfer and under-predict the flame temperature.Next,by using the experiment and numerical methods,the oxy-coal combustion characteristics are studied in a 35 MWth large pilot boiler.Stable air combustion and oxy-fuel combustion can occur in a same swirl burner system.The in-furnace temperature,composition and heat transfer are measured,and the numerical simulation methods are further eveluated.Compared to air combustion,the peak temperature decreases in oxy-fuel combustion at initial O2 level of 28 vol%.However,the total heat transfer increases by approximately 9% and 6% for wet and dry recycle oxy-fuel conditions,respectively,relative to air combustion.Finally,the air combustion and oxy-fuel combustion in a 200 MWe tangentially fired boiler are numerically studied,and the effects of flue gas recycle mode and initial oxygen level are investigated.Stable air combustion and oxy-fuel combustion can occur in a same tangentially fired system.In oxy-fuel combustion,although the in-furnace temperature is much lower than that in air combustion,the radiative heat transfer at a specific initial oxygen level can match with that in air combustion.In the thermodynamic calculation of the boiler,the radiative heat transfer in membrane-wall is over-predicted by 5-10% and the heat transfer in front superheater is under-predicted by 4-15%,due to the neglect of the slow CO reaction.Moreover,a compatible configuration strategy for burner streams is studied in this 200 MWe tangentially fired boiler.Although there are significant changes in the volume,velocity,and composition of oxidant under oxy-fuel combustion conditions,the key design criterion is consistent between oxy-fuel combustion and air combustion,in terns of primary stream momentum,momentum ratio of primary stream to secondary stream,and bottom secondary stream momentum.These results provide design guidance to avoid flue gas temperature deviation and high temperature corrosion in tangentially fired oxy-fuel combustion boiler.