Numerical Simulation And Optimization Of MILD Oxy-fuel Combustion Characteristics

MILD oxy-fuel combustion is the next generation of oxy-fuel combustion technology for carbon dioxide capture and sequestration?CCS?with the combined advantages of the first generation oxy-fuel combustion and advanced Moderate and Intense Low-oxygen Dilution?MILD?combustion.It features as increasing flue gas temperature uniformity,enhancing radiation heat transfer,and achieving“near zero”pollutant emission.MILD oxy-fuel combustion process in a 200kW pilot scale furnace at the Royal Institute of Technology was carried out with Computational Fluid Dynamics?CFD?software FLUENT in present paper.GAMBIT software was applied to set up three-dimensional model and generate mesh based on the structure and operation parameters of the furnace.The results showed that intensive recirculation and turbulent mixing of hot flue gas exists in furnace quickly dilute and preheat the fuel and oxygen concentration before ignition.Therefore,the chemical reaction rates and peak flame temperature distinctly decrease.Mean while,Damk?hler?Da?number is much smaller than 1 and Karlovitz?Ka?is greater than 1 in the main reaction zone,which revealed the mechanisms of moderate chemical reactions in low oxygen concentrations of MILD oxy-fuel combustion.Based on the above work,a symmetrical MILD oxy-fuel burner was designed to numerically analyze the influence of different burner parameters on MILD oxy-fuel combustion characteristics.It was found that the flue gas recirculation became weaker as the oxygen jet velocity decreases,and local reactants could not be well preheated and diluted.Consequently,the combustion reaction rates and peak flame temperature significantly increased and combustion mode gradually transfered to conventional flame combustion mode from flameless combustion mode.On the other hand,increasing the distance between oxygen jet and fuel jet could delay the mixing of reactants and decrease the concentration of reactants when the momentum ratio of oxygen jet to fuel jet was small,thus reduce the chemical reaction rates and peak flame temperature.However,the effect of distance between oxygen jet and fuel jet had little impacts on combustion characrteristics when the momentum ratio of oxygen jet to fuel jet is large enough.In addition,the gas radiation characteristics model and pulverized coal ignition model in O₂/CO₂ environment were also improved in this paper.Firstly,the user-defined function?UDF?was used to refine the weighted sum of gray gases model?WSGGM?and reduce the error of applying the original model directly under O₂/CO₂ environment,and the calculation results of gas radiation characteristics were validated with the standard exponential wide band model?EWBM?.Then a refined pulverized coal ignition model was rewritten by UDF considering the default pulverized coal ignition model of the FLUENT software was not able to accurately predict the actual ignition process of pulverized coal.The simulation results of standoff distance were compared with the experimental measurements in an ignition test facility,and the influence of second air temperature and combustion atmonsphere on standoff distance were further discussed.