Numerical Simulation On Gasification Process In Fixed Bed Slagging Coal Gasifier

Coal gasification technology is considered as one of the effective ways to utilize coal cleanly and efficiently.Of which,fixed bed coal gasification is widely applied due to the simple structure and low technology costs,especially in China.However,on account of more than half of the coal with high ash fusion temperature(>1400?),slag blockage of fixed bed gasifier occurs frequently and brings adverse impact on the long time and stable running.Recently,a new type of fixed bed slagging gasifier for the utilization of the coal with high ash fusion temperature has been proposed.In order to get optimized operating and design parameters,it's essential to study the gasification process within the gasifier.In this paper,the new type of fixed bed was took as the research object,and a three dimensional Eulerian-Eulerian multiphase CFD model to describe the mass,momentum and heat transfer behavior on gasification process was established.The methods for defining the boundary of high turbulence zone were discussed and a sub-model for the variation of coal particle size was also developed.The effects of following parameters on the shape and size of high turbulence zone,mass flow rate of solid phase at slag tapping,gas and solid temperature,gas composition distribution were investigated,such as the gasification agent charging velocity,the tuyere diameter,the coal particle size,the filling height of bed and oxygen/steam ratio.The results were as follows.Solid phase volume fraction combined with Reynolds number were adopted to define the boundary of high turbulence zone which formed in front of the tuyere.According to the original and final coal particle size,a sub-model for the variation of coal particle size which considered linear change of particle diameter along the gasifier axis was developed.The coal particle size within the gasifier had significant effects on the shape and size of high turbulence zone.The size of high turbulence zone had no obvious changes under the uniform particle size,while increased with the decreasing of particle size when the coal particle size treated as linear variation along with the height.Coal bed filling height had weak influence on the high turbulence zone,the size of high turbulence zone increased slightly with the decreasing of bed filling height.2m of bed filling height was advisable in this paper.The effect of gasification agent velocity on the shape and size of high turbulence zone was no obvious under 1.9mm tuyere diameter.When the tuyere diameter is 4mm,with the gasification agent velocity increased from 150m/s to 200m/s,the high turbulence zone could keep stable shape and the depth of high turbulence zone was 130mm and 160mm respectively.However,when gas velocity increased to 250m/s,the coal bed lost stability and slugging was occurred simultaneously.The mass flow rate of solid phase was increased with the decrease of coal particle size and increased with the increase of gas velocity under 4mm tuyere diameter,especially,when the gas velocity increased to 250m/s,the leakage reached to maximum.The combustion reactions were mainly concentrated in Z=0-0.5m,heat exchange was occurred more than 0.5m,combustion zone accounts for about 1/6 of the whole bed.Oxygen/steam ratio has distinct effects on the gas and solid temperature and the components of the gas phase.With the increase of oxygen/steam ratio,gas and solid temperature increased significantly,and CO content increased while H2 content decreased simultaneously.