Numerical Investigation Of Gas-Solid Flow Of Dense Transport Bed Based On Euler-Lagrangian Method

The development of the pressurized dense transport bed gasification technology will promote our level of autonomy on large-scale coal gasification technology and equipment,and it is important for the improvement of industrial competitiveness.Dense transport bed has the characteristics of high solids circulation rate and high solids concentration,so it is one of the key problems to realize high solids circulation rate and stable operation under high temperature and high pressure.In addition to the test method on different device scales,the numerical test method based on computational model plays an increasingly important role in technology R&D.To satisfy various flow regimes in the full-loop of dense transport bed,the computational particle fluid dynamics(CPFD)method based on the MP-PIC model was employed to study the gas-solid flow behavior and the control strategy of solids circulation rate in the dense transport bed.Firstly,the constitutive equations and the solution methods of the MP-PIC model were analyzed.Based on the OpenFoam platform,the effects of different solid stress models and the corresponding velocity correction methods on gas-solid flow were compared.It was found that the stress model had certain influence on the particle motion,and the velocity correction method based on the stress gradient had greaterimpact on the simulation results.The use of the Harris model and the Lun model to calculate the particle stress and the absolute velocity correction method can give more reasonable results.Based on the three cold flow test cases of dense transport bed,the gas-solid flow characteristics in the solids returning system composed of riser,return valve and cyclone were numerically analyzed.It was found that the predicted pressure profiles in the standpipe and the solids circulation rates were in good agreement with the experimental data,which implies that the CPFD method can be effectively used in the simulation of the downward moving bed in the standpipe.Through sensitivity analysis of model parameters,it was indicated that the drag model and the close-pack limit had a significant effect on the simulation results.The standpipe aerations at different heights make the pressure gradient form a zigzag distribution,which increases the average pressure gradient of the standipe,thus providing greater power to circulate solids.The aeration can increase the solids circulation rates,but will produce a dramatic change in the pressure gradient at the same time,and the excessive standpipe aeration will result in the instability of the system operation.The gas-solid flow behavior in the riser under two different solids circulation rates was simulated.It was found that the CPFD method could reflect the characteristics of the accelerating flow in the bottom region and the fully developed flow in upper region,and predict well the radial distribution of the solids concentration.The influence of different outlet structures on the solids residence time distribution and dispersion coefficient in the riser was analyzed.It was found that there was a large nonuniformity in the solids residence time distribution under different outlet structures.There was a small difference in the radial dispersion coefficient,and the axial component was much larger than the radial component,which further indicated the plug flow feature in the riser.Coupling the important components of the dense transport bed,a full-loop simulation method was established,and the gas-solid flow behavior in the full-loop has been simulated.It was found that the predicted pressure profiles along the full-loop were in good agreement with the experimental data,and on this basis,the influence of the standpipe aeration was analyzed.In the bottom zone of the riser,the simulation results deviated to some extent from the experimental data,while the claculated solids circulation rates were consistent with the experimental values,and the maximum relative error was about 10%.The boundary conditions for the full-loop simulation are the same as the actual operating conditions,and the validation results show the feasibility of the full-loop simulation method applied to the numerical experiments on the dense transport bed.The effects of operating parameters such as the bottom aeration,the standpipe aeration,the riser gas flowrate,the solids holdup,and the gas distribution in return valve and the structure parameters such as the aeration port number,the return valve type and the device size on the solids circulation behavior were studied using the full-loop simulation method.The increase of the standpipe aeration and the bottom aeration can result in the increase of solids circulation rate in a certain range,but the required standpipe aeration rate is less than that of the bottom aeration to realize the same increment of solids circulation rate.With the increase of solids holdup,the solids circulation rate increases first and then decreases slightly.With the increase of the riser gas flowrate,the pressure drop of the riser decreases,while the solids circulation rate increases slightly.The gas distribution in the inclined section of the return valve has little influence on the solids circulation rate,but more gas should be close to the standpipe to improve the flow stability.The use of excessive aeration ports does not effectively promote the gas-solid flow in the standpipe,but it may cause the premature entry into the unstable state with the increase of the aeration rate.On the other hand,the use of less aeration ports may produce upward bubble and destruct flow stability.For the amplified apparatus,the operating gas velocity in the riser should be increased to prevent the accumulation of particles at the bottom.To improve the gas-solid flow under high pressure,the influence of two aeration schemes,namely the standpipe aeration and the turning aeration,on solids circulation rate was analyzed by numerical simulation.It was found that consistent solids circulation rate could be obtained if the ratio of the aeration rate to the operation pressure was kept the same under different pressures.The interface became blurred with the increase of solids circulation rate under high pressure,which greatly reduced the height of the packed bed.The role of the turning aeration on the increase of solids circulation rate is more significant than that of the standpipe aeration under the same flowrate.A series of operation conditions under different pressures(1 to 20 bar)and temperatures(300 to 1200 K)were designed based on the same fluidization number.The effects of pressure and temperature on gas-solid flow characteristics in the dense transport bed were studied numerically.The results show that the pressure has a more significant effect on the gas-solid flow behavior than the temperature.It was found that the number of particles remained in the cyclone increased significantly with the increase of pressure and temperature.The particles did not move along a obvious spiral descent trajectory any longer,but downward directly along the wall.With the increase of temperature and pressure,the drag forces on the particles in the upper region of the standpipe were significantly increased,while the downward velocities were reduced.Most particles concentrated on the vicinity of the wall and moves downward with a relatively high speed,however,there were a large number of particles in the central area entrained by the ascending gas.These results provide a reference for the design and operation of dense transport bed under high temperature and high pressure.