Numerical Simulation Of Lignite Gasification Process In A Dense Phase Transport Bed

Lignite accounts for about 13%of total coal reserves in China.It is mainly combusted to generate electricity,but causes serious environmental pollution.Gasification can utilize lignite efficiently.The dense phase transport bed lignite gasification technology can not only ensure the efficient gas-solid mixing,heat/mass transfer and chemical reaction processes,but also improve the carbon conversion.In this paper,the lignite gasification process in a dense phase transport bed is studied,and the numerical simulation method is conducted to analyze the flow and reaction behavior in the riser gasifier of a dense phase transport bed.Based on the the Eulerian-Eulerian multiphase framework and lignite gasification kinetics model,a computional fluid dynamics(CFD)model is established to investigate the lignite gasification process in a dense transport bed gasifier.In the CFD model,the standard k-s model and the EMMS drag model are used for gas-phase turbulence flow and gas-solid drag force in the gasifier,respectively.CPD model and MGAS model are applied for gasification reaction process.Simulation results indicate that under the ligniet inlet,the gas velocity slowly increases along the riser height;near the coal inlet and the recycle inlet,the gas velocity increases significantly.From the riser wall to central region,the gas velocity decreases gradually.The velocity of lignite particle increases continuously in the axial direction of the riser reactor.It increases rapidly near the lignite inlet.Above the coal inlet,the lignite velocity does not vary obviously.The velovity of ash particle increases quickly near the return port and at the lignite inlet,however,its increase amplitude decreases above the lignite inlet.The volume fraction of ash particle reaches up to maximum near the lignite inlet,then decreases and keeps almost unchanged along the riser height.Axial/radial concentration distribution of main gas species is investigated numerically.The simualtion results show that the mole fractions of CO and H2 species are lower below the lignite inlet.Above the inlet,however,their mole fractions increase rapidly.Along the height of the riser,CO2 concentration first increases and then decreases.From the riser wall to the center region,the mole fractions of CO gradually decreases.The effects of operating parameters on the coal gasification process are further investigated by the above CFD model.With increasing the oxygen/coal mass ratio,the mole fraction of CO species decreases gradually,the mole fraction of H2 species increases gradually,and the CO2 concentration first increases and then decreases;and carbon conversion and calorific value of syngas also increase.With the increase in steam/coal ratio,the mole fraction of CO species firstly increases and then decreases,the mole fraction of H2 species increases gradually,and the CO2 concentration decreases.When the steam/coal mass ratio is 0.10,the carbon conversion rate reaches maximum.As increasing the operating pressure,the mole fraction of CO species increases continuously,and the mole fraction of H2 species first increases and then decreases.However,CO2 concentration shows opposite tendency.Moreover,the carbon conversion increases gradually,and the calorific value of syngas first increases and then keeps almost unchanged.