Modeling Study Of Slag Flow And Heat Transfer Process On The Wall For An Entrained-flow Gasifer

The slag flow and heat transfer characteristics are key factors affecting the safe operating of entrained-flow gasifier.Based on the mass transfer and heat transfer of slag on the wall,the slag flow and heat transfer model and molten slag residence time calculation model were established.And the slag layer characteristics were analyzed with the effect of operating conditions and coal slag physical properties.Moreover,from the purpose of maintaining the safe and stable operation of the gasifier,the slag layer dynamic response and factors that may cause ablation and slagging were studied.Thus the conclusions provide theoretical data support and reasonable suggestions for improving the safety of gasifiers.The main research contents in this paper are given as follows:1.The slag flow and heat transfer model in straight part and cone part were built under steady state conditions in an entrained-flow gasifier.The slag flow and heat transfer basic model was obtained under stable conditions based on the mass transfer and heat transfer of each slag unit in vertical cross section.The slag flow area was shrinking on the cone part of gasifier due to the structure,thus the flow model were different the cone part and straight part.Moreover,according to the assumption that liquid slag was laminar flow,the molten slag residence time calculation model was built by the tracing unit method.2.The slag layer characteristics in straight part were obtained from the slag flow and heat transfer model,which included slag layer thickness,velocity,heat flux density,and residence time distribution.The results shown that the liquid slag thickness increased along the slag flow direction,liquid slag velocity increased from the top and inside to the bottom and outside,but the solid slag thickness and heat flux density were influenced by the gas temperature and slag deposited rate distribution.The heat flux density results calculated from the industrial gasifier steam output were closed to the modeling results,which indicated that the model results are reliable.Moreover,the mean residence time of slag was about 100-500 s,and decreased with the increasing coal ash content,but increased with the increasing critical viscosity.The complex reactor model was used to simulate the slag flow,and the two RTD curve are consistent.In addition,the relationship between the model parameters of RTD and operating conditions was obtained.3.The relationship between the slag characteristics on the cone and cone angles were analyzed based on the slag flow model.The results shown that the slag mean thickness on the top cone were thinned and then thickened with the increasing cone angle,and the SiC surface temperature were increased and then decreased with the increasing cone angle within a narrow range.The slag thickness in a same height on bottom cone increased with the increasing cone angle,but the SiC surface temperature and slag heat flux density decreased with the increasing cone angle.Moreover,the heat transfer rate of cone part and liquid slag mean velocity obviously decreased with the increasing cone angle.The safety of gasifier was considered in this paper,on the one hand,the SiC surface mean temperature obviously influenced by the operating temperature,thus the operating temperature should not be too high in case of refractory lining ablation.On the other hand,the slag thickness at the slag tap hole obviously increased with the decreasing critical temperature interval,therefore,the slagging temperature were suggested to be higher than the critical temperature of coal slag above 50 K to prevent blockage.4.The slag flow and heat transfer dynamic model was built to obtain the slag characteristics dynamic response with the change of operating conditions,and the stiff equations solve method was used to make the results convergent.When the coal ash content transient increased,the slag thickness increased at the beginning and tended to be stable as time increasing,while the slag heat flux density and SiC surface temperature decreased at the beginning and then tended to be stable.The change trend was opposite when the coal ash content transient decreased.When the operating temperature transient increased,the slag thickness decreased at the beginning and tended to be stable,while the slag heat flux density and SiC surface temperature had opposite trend.However,when the operating temperature transient decreased,the slag thickness decreased and the slag heat flux density and SiC surface temperature increased at the beginning,then changed in the opposite direction and tend to be stable as time increasing.The slag behaviors were also periodic change as the operating temperature periodic changed within a specific frequency range,and the frequency of the two is similar.Moreover,the amplitude of slag behaviors increased with the increasing amplitude of the operating temperature,but decreased with the increasing frequency of the operating temperature.And the dimensionless amplitude of each slag behaviors has approximately the same trend with the change of amplitude and frequency of operating temperature.5.The slag flow and heat transfer characterisitcs were analyzed by the effect of slag physical properties.The porous structure was observed inside slag through microscope and the porosity was obtained.Thus the slag in gasifier was a heterogeneous material and the bubble volume fraction was equal to the coal porosity.The effects of bubbles on slag layer characteristics were studied based on the sub-model for the effects of bubbles on slag physical properties.The results shown that once the presence of bubbles was considered,the effective viscosity and thermal conductivity decrease,the liquid slag velocity increased about 22%,and the liquid slag layer thickness decreased about 10%.In addition,the value of solid slag thickness and heat flux density were related to the selection of the effective thermal conductivity model.The heat flux density results calculated from the industrial gasifier steam output were closed to the results from Maxwell-Eucken model,thus the Maxwell-Eucken model was more suitable for this coal.Moreover,the Tcv was used as the transition temperature between the liquid and solid slag layers in most of literatures without considering the rationality of the assumption.Based on the model results in this paper,the smoother the viscosity-temperature profile of slag,the higher the effects of the fluid-solid slag layers interface viscosity on slag layer characteristics.Therefore,the critical viscosity can be approximated regarded as the fluid-solid interface viscosity when the slag type in gasifier is crystalline slag,while interface viscosity can be defined as about 100 Pa s for plastic slag and glassy slag.