Numerical Simulation And Experimental Study Of Molten Slag Layer Formation,Flow And Heat Transfer On Water-Cooled Wall Of Entrained Flow Gasifier

The resource endowment of China determines the main position of coal as a fossil energy source in China's energy consumption.With the requirement of carbon neutral,the clean,efficient and low-carbon utilization of coal will be a difficult task in the energy field for a long time in the future.Dry pulverized coal gasification technology is a clean and efficient coal conversion technology,which has great market prospect in China.Due to the high operating temperature of dry pulverized coal air bed gasification,liquid slag discharge technology is mostly used,and the control of wall slag film is crucial to the design and safe operation of gasifier.Based on this,this paper used a combination of numerical simulation and experimental research to investigate the deposition process of fly ash on the wall,the flow and heat transfer process of the slag layer on the coolant wall in the gasifier.Firstly,to address the problem of decoupling the slag layer simulation and gasifier CFD simulation in existing studies,this paper coupled the slag model to the CFD model by exchanging the wall heat exchange and slag layer surface temperature to achieve an accurate simulation of the wall slag layer flow and heat transfer process.Using a simplified SHELL gasifier as the research object,the differences between the coupled calculation method and the other two non-coupled methods were compared,and the results showed that the solid slag layer thickness was more sensitive to the wall heat flow than the liquid slag layer thickness.The solid slag layer thickness obtained by the coupled method was 39.5 mm and the uncoupled method resulted in 29.8 mm,a difference of 26.5%,which proved the necessity and accuracy of using the coupled method.By changing the temperature of the gasifier,it can be seen that the total thickness of the slag layer thinned and the wall heat transfer rises as the furnace temperature increases.With the decrease of ash deposition,the slag layer became thicker and the heat transfer from the gas phase to the slag surface decreased,but the heat absorbed by the flowing liquid slag rised.When the heat transfer from the water-cooled wall of the gasifier was 2?3 MW and the furnace temperature was 1700K?1800 K,which was 200?300 K higher than the critical viscosity temperature of coal ash,T_cv,the thickness and fluidity of the slag layer on the wall of the furnace were reasonable.Then,this paper simulated the GSP dry pulverized coal gasifier which has a single-nozzle under steady-state conditions using CFD method,and investigated the deposition of ash particles on the wall surface in the furnace combined with the ash deposition model.The results show that the distribution of particle wall deposition is related to the structure of the gasifier and the distribution of the flow and temperature fields inside the furnace,while the temperature is the key parameter to determine the deposition of particles or not.The deposition amount was very little in the area away from the top of the gasifier.In the middle part of the main body,the deposition of particles was more and evenly distributed.At the conical closing at the bottom of the gasifier,ash deposition was intensive here because the downward airflow carried high temperature ash particles.When increasing the oxygen-coal ratio,although the combustion reaction intensified and the temperature rised,but the top low-temperature zone still existed.Therefore,the total deposition of fly ash on the wall of the gasifier didn't increase significantly.When the nozzle spin angle increaseed,the high temperature zone moved up and the lower low temperature zone expanded,and the amount of particle deposition decreased.Based on the study of CFD simulation and ash particle deposition on the wall of the furnace,we continued to couple the slag layer model to achieve accurate simulation of particle deposition on the wall,slag layer heat transfer,and liquid slag flow.The distribution of solid and liquid slag thickness on the wall of the gasifier was relatively uniform in the circumferential direction of the wall,while the distribution in the axial direction was related to the temperature field of the gas phase in the furnace and the amount of particle deposition on the wall.In the main body of the furnace,the thickness of the liquid slag layer first grew gradually,and then the growth slowed down.At the bottom slag tap,the liquid slag layer thickness grew rapidly due to the slowing down of the wall inclination.As the oxygen-coal ratio increased,the internal temperature of the gasifier rised,the slag layer thickness thinned and the slag surface flow rate increased.When the baseline oxygen-coal ratio was increased by4%and the gas temperature was 1748 K,the heat transfer from the water-cooled wall of the furnace chamber could reach 40 kW/m²,which was close to the actual data of a GSP gasifier of43.8 kW/m²,indicating the accuracy of the simulation.Finally,this paper adopted a self-developed droplet flow observation experimental system to directly observe and quantitatively describe the molten slag droplet flow process.The flow characteristics of silicone oil droplets and high temperature molten pure K₂Si₄O₉ droplets on a tilted alumina plate were compared.The effects of temperature,viscosity,and plane tilt angle on droplet flow were investigated.In this chapter,a simplified model for describing the droplet motion on the tilted plate has been developed.An equation for predicting the droplet flow velocity based on the parameters of maximum droplet height,plane tilt angle,viscosity,contact line resistance,and gravity was obtained.The measured values of droplet flow velocity and the measured values of viscosity showed an obvious inverse relationship,and the slope of the fit of the logarithmic relationship between the two was close to-1,which was consistent with the theoretical value.When contact line resistance was not considered,the predicted value of the model was significantly higher than the measured value.Including the contact line resistance makes the prediction closer to the measured value,proving that the droplet contact line resistance is an important factor affecting the droplet flow of molten slag in addition to viscosity.