Numerical Simulation Of Slag Flow And Heat Transfer Process Accompanied With Phase Change And Slag Discharge Behavior In The Entrained-Flow Gasifier

The slag flow characteristics on the wall are not only an important parameter affecting whether the slag can be discharged smoothly from the gasifier but also closely related to the corrosion process of refractory bricks.Taking the entrained-flow gasifier as the research object,this paper established a comprehensive model of slag flow and heat transfer process,which included the flow field and chemical reaction model,the gas-flow shear force model,particle deposition model,slag flow model and slag induced refractory brick corrosion model,etc.Based on the detailed analysis of the slag flow and heat transfer process in gasifier,the thickness distribution of slag layer and the corrosion characteristics of refractory bricks were studied under different operating conditions.The main research contents of this paper are given as follows:(1)Based on the heat and mass transfer characteristics of slag in the unit control volume,a comprehensive model of slag flow and heat transfer process accompanied with phase change was established.It was found that the slag viscosity was approximately expressed as an exponential function with different curvatures,which took the temperature of critical viscosity as the discontinuity point.Therefore,a viscosity modeling method suitable for the modeling study of slag flow was proposed.Based on the physical significance of the phase change of slag flow,the judgment basis of phase state of slag was improved.The critical point of transition from liquid slag to solid slag was taken as the interfacial viscosity of slag reaching 100Pa·s,which greatly improved the calculation accuracy.Based on the turbulent boundary layer theory,the shear force exerted by the gas flow on the wall was expressed as the function of the gasflow density and velocity,and the coupling of the flow field in the furnace and the slag flow process on the wall was realized.Considering the disorder migration of particle deposition on the wall in the furnace,a micro model of particle deposition was constructed by Monte Carlo method.In the modeling process,the adhesion difference caused by the simultaneous distribution of the melting zone and non-melting zone on the surface of semi-viscous particles was considered.Based on the coupling of the above sub-models,the computational efficiency and accuracy of the comprehensive model of slag flow and heat transfer process accompanied with phase change were significantly improved.(2)Based on the comprehensive model of slag flow and heat transfer process,the distribution characteristics of the slag layer in the tapping hole of membrane wall lining and refractory brick lining gasifiers were studied.The results showed that operating temperature was the key control parameter of slag thickness distribution,and the variation trend of the slag layer thickness with the operating temperature under different slag types was obviously different.Compared with glassy slag,the thickness of crystalline slag was more sensitive to the change in operating temperature.The increase in operating temperature would decrease the difference in slag thickness between different types of slags.And when the operating temperature was suddenly reduced,a longer time was required for crystalline slag to reach steady-state than glassy slag,the corresponding steady slag thickness would be thicker.Besides,with the increases in the critical viscosity and the temperature of critical viscosity,the slag thickness increased gradually at the same operating temperature.However,compared with the temperature of critical viscosity,the critical viscosity had a more significant effect on slag thickness.(3)Based on the liquid-solid phase diffusion-reaction mechanism,the slag-induced corrosion model of refractory brick was built.The results showed that the external diffusion model could accurately predict the corrosion rate of refractory bricks.The temperature of gasflow was the key control parameter of slag flow characteristics and refractory brick corrosion rate.With the increase in gas-flow temperature,the brick corrosion rate increased exponentially.Besides,when fluctuation occurred in the turbulent field in the furnace,the brick corrosion process would further deteriorate.When the temperature fluctuation range reached 200K and the fluctuation frequency were 1.0.1.0.01.and 0.001Hz.the brick corrosion rate were 1.30.1.36,1.46,and 1.57mm/100h,respectively.Compared with the temperature fluctuation,the gas velocity fluctuation had less effect on the brick corrosion process.(4)Based on heat transfer analy sis of the slag tapping hole in the gasifier,the slag discharge behavior of the membrane wall lining and refractory brick lining gasifier were studied.The results showed that the uneven deposition of ash particles would lead to the abnormal increase of slag thickness in the local area,which was easy to induce slag plugging.With the transformation of slag type from glassy slag to crystalline slag,the uneven deposition of particles had a more significant effect on the overall thickness of the slag layer.The change amplitudes of heat flux were different for different feeding coals.However,the change trends of the hole effective sectional areas with the heat fluxes were consistent.Therefore,the detection of heat flux could be used as an auxiliary means to judge whether the gasifier was blocked.Besides,the slag blocking mechanism of the refractory brick gasifier was revealed.On one hand,the refractory brick was directly placed on the metal support in the slag tapping hole,which led to excessive heat flux in the local area.On the other hand,for the bottom of the shrinkage area of the hole,the thickness of the working brick was thin,which led to slight thermal resistance.The local low-temperature zone was the main reason for the blockage.Based on the heat transfer analysis in the hole,the operating parameters including the ’sensitive characteristic temperature’ and ’no-solid slag characteristic temperature’ were proposed,in which the temperature range was optimal for different slags.On one hand,slag plugging would not occur due to the fluctuation of the operating temperature within this temperature range.On the other hand,there would be a thin layer of solid slag to protect the refractory brick.