Numerical Simulation Of Two-stage Dry Feed Entrained Flow Gasification

Coal is the dominant primary energy source in China,and gasification is an important way for its efficient and clean utilization.Compared with the traditional entrained-flow gasification technology,the two-stage dry feed pressurized gasification technology exhibits improved carbon conversion and higher cold gas efficiency,and has been applied industrially.In this thesis,the two-stage dry feed pressurized gasifier in the Huaneng Tianjin IGCC Demonstration Power Plant is studied by CFD method.The flow and gasification behavior,the effect of operating conditions and the coupling relationship between the two stages have thus been investigated.Based on the ANSYS FLUENT commercial software platform,a flow,heat transfer and gasification reaction model is established for the two-stage dry feed pressurized gasifier.In the model,the Eulerian-Lagrangian method and P1 model are used to describe gas-solid flow and radiative heat transfer in the gasifier,respectively.The realizable k-? model is applied for the gas-phase turbulence flow.Furthermore,the grid independence is examined.It is found that as the number of grid is greater than 324 000,the effect of grid number on the simulation results could be neglected.Gas velocity is higher in the lower part of the gasifier,which benefits for gas-solid mixing and the gasification reaction.In the upper part,the gas velocity is relatively lower,which prolongs the residence time of coal particles in flue gas and promotes the carbon conversion.The concentration of O2 and H2O around nozzles is highest in the gasifier.With increasing the gasifier height,the concentrations of CO and H2 increase while the concentration of CO2 decreases.As the gasifier height is greater than 20 m,the contents of these species do not vary obviously.From nozzles to center in the lower part of gasifier,the concentrations of CO and H2 gradually increase while the concentration of CO2 first increases and then decreases.From nozzles to center in the upper part of gasifier,the concentrations of CO and CO2 gradually decrease while the concentration of H2 first increases and then decreases.The effects of operating conditions,such as gasification pressure,coal particle size,oxygen/coal ratio,water/coal ratio,first/second stages feed ratio and gasification load,on the gasification performance are further investigated.It is found that that pressure has little impact on the gasification performance.Gasification efficiency decreases with increasing the coal particle size,but increases with increasing the mass ratio of water to coal.The gasification efficiency and effective gas yield reach the maximum when the coal feed ratio of second stage is about 0.20 and the oxygen/coal ratio is in the range of 0.75-0.85.The gasification performance is optimal when the gasification load is about 80%.The above researches could provide necessary information for the optimization and design of the two-stage dry feed pressurized gasifier.