Numerical Simulation Of A Pilot Dense Transport Bed

Due to the coal-based energy structure in China and the environmental pollution caused by coal utilization,the development of clean and efficient coal utilization technology is an important task at present.Dense transport bed gasification technology adopts dense phase transportation conditions with high solid flux and high solids concentration,which can achieve high conversion rate and gasification intensity,and has broad application prospects.However,the current research is mainly based on small-scale devices,and there are few studies on the overall circulatory system and coal gasification reaction process.The present work is devoted to revealing the gas-solid flow and coal gasification process of a pilot dense phase transport bed gasifier by numerical simulation approach.The diameter of the riser is 0.3 m,and its height is 30 m.And the characteristics of the amplification process from the small-scale device to the pilot plant of dense transport bed are explored.The results can help to grasp the operation of the device and provide reference for experiments.Firstly,the computational particle fluid dynamics?CPFD?approach was employed to establish a simplified full-loop model for the simulation of gas-solid flow in the pilot dense phase transport bed.Using the numerical model,a gas-solid flow test condition was simulated.The computational results of the system pressure distribution and the solid circulation flow rate are in good agreement with the predicted values of the experimental and empirical formulas respectively,which verifies the reliability of the model.Besides,the appropriate parameter settings were determined.The results of gas-solid flow study have shown that the particle distribution characteristics in the riser are consistent from the small-scale device to the pilot plant of dense transport bed.Due to the effects of inlet and outlet,the radial distribution of particle axial velocity and volume fraction presented asymmetry.And compared with fast fluidization,there is no typical core-annulus flow structure obtained.With the solids circulation rate increases or superficial gas velocity decreases,the solids acceleration length increases.Besides,when the initial bed material stack height increases,the standpipe pressure drop and solids circulation rate increase,and the effect is more obvious when the standpipe fluidization number is higher.At the same time,when the initial bed material stack height is 12 m,15 m,and 20 m,the standpipe pressure drop and solids circulation rate increase first with the standpipe fluidization velocity increases,and after the standpipe fluidization number reaches1.71?2.39?3.08 respectively,it tends to be stable.The gas-solid flow model was coupled with chemical reaction model from C3M software to establish a coal gasification model of the pilot dense transport bed,and it has been employed to simulate the gasification test condition.The predicted gas composition out of the gasifier and riser pressure drop result were in good agreement with the experimental data,indicating that the coal gasification model can predict the coal gasification process well.Based on the coal gasification model,the gas composition and the temperature distribution during the gasification process were analyzed firstly,and then the effects of the standpipe aeration and the initial bed material stack height in increasing the solids circulation rate during the coal gasification conditions were explored.Besides,the effects of the solids circulation rate and the position of coal inlet on coal gasification characteristics were studied.As a result,with the solids circulation rate increased from 176.5 kg/?m²·s?to 376 kg/?m²·s?,the CO and H₂ content at the outlet increased and the CO₂ content decreased,the carbon conversion increased by 8%,and the gasification efficiency increased by 9.1%.The position of coal inlet influences the distribution of gas components in the riser,but when the coal inlet is within 2.65 m to 6.10 m above the return valve,it has no effect on the final gasification results.When the height of coal inlet is too low such as 1.70 m above the return valve,the gasification efficiency is slightly reduced by about 5%.