Study On The Mixing Characteristics Of Gas-solid In The Ash-agglomerating Fluidized Bed Gasifier

The mixing characteristics of gas-solid in the ash-agglomerating fluidized bed gasifier do not only show the axial and radial movement and the transfer characteristics of gas and rigid granular in the bed, but also play a critical role in recognizing bed heat transfer and mass transfer mechanism, which is quite meaningful for improving the ash-agglomerating fluidized bed gasifier efficiency and carbon conversion efficiency. However, people are still lack of systematic research on the mixing characteristics of gas-solid in bed, which results in constraints in large-scale industrial applications.On the basis of the Eulerian two-fluid models and the kinetic theory of granular flow(KTGF),a numerical simulation for the mixing characteristics of gas-solid, in one company's running ash-agglomerating fluidized bed gasifier, is conducted using the commercial CFD software FLUENT in this paper.In this paper, the mixing characteristics of gas-solid is simulated under these conditions:the granular's diameter is 2mm, the static bed height is separately 4.5m,3.6m,2.7m; the granular's diameter is 1mm, the static bed height is 3.6m.The granular volume concentration distribution under different spouting gas velocity, fluidizing gas velocity and the flowing field distribution, the pressure distribution under some typical operating conditions are showed graphically. The flow structure of the spouting, the internal jet, the jet in fluidized bed with bubbling, the jet in fluidized bed with slugging as well as the plug flow presented in the gasifier is separately obtained under a different ration between the spouting gas velocity and the fluidizing gas velocity. The simulation results show that, in the former three types of flow structure, the granular move upward along the central axis and the moving speed decreases gradually with the increasing height, the granular near the wall move downward and the moving speed increases with the increasing depth. In the spouting structure, the gas stay in bed a short time; in the internal jet structure, the granular volume concentration around the annular area is relatively high; Structure of the jet in fluidized bed with bubbling is consisted of center jet area and the surrounding annular area, and the gas-solid mixture is relatively uniform around the annular area. And this kind of structure can enhance the granular horizontal and vertical mixing, accelerate the movement speed, so it is conducive to fully carry out gasification reaction and enhance the quality and efficiency of coal gasification.In this paper, calculation results are compared and analyzed under different spouting gas velocity and fluidizing gas velocity, and the effect of the above two parameters on granular mixing characteristics in bed are concluded, with the increase of spouting gas velocity, gas-solid flow structures change from internal jet to spouting or the jet in fluidized bed with slugging, in which pressure drop decreases,and inflexion exists during the decreasing process;While, with the increase of fluidizing gas velocity, gas-solid flow structures change from spouting gradually to fluidizing or the jet in fluidized bed with slugging through the jet in fluidized bed with bubbling, and the pressure drop increases,however, the rate of increase is smaller and smaller. The increase of spouting gas velocity and fluidizing gas velocity both can increase the axial velocity, the radial velocity and the average voidage.Based on the above conclusions,the optimal speed corresponds to the different static bed height and different granular diameter, which is suitable for this company's gasifier, under cold running condition is obtained:Provided that the granular diameter is 2mm, fluidizing gas velocity is separately lOm/s,15m/s and 20m/s. The optimal spouting gas velocity is about 30m/s,30m/s and 35m/s if the static bed height is 4.5m; the optimal spouting gas velocity is separately 25m/s,30m/s and 30m/s if the static bed height is 3.6m;the optimal spouting gas velocity is about 20m/s,25m/s and 25m/s if the static bed height is 2.7m. The results show that:there is an optimal spouting gas velocity corresponding to a certain fluidizing gas velocity to make the gasifer present Structure of the jet in fluidized bed with bubbling.This conclusion will not be influenced by the static bed height, however, the optimal spouting gas velocity will decrease while the quality of gas-solid mixture will improve when the static bed height decreases. When the diameter of the granular equals 1mm,the static bed height equals 3.6m, and the fluidizing velocity equals lOm/s, the optimal spouting gas velocity is about 20m/s.When the fluidizing velocity increases to 15m/s, with the increase of spouting gas velocity, the flow structures change from fluidizing bed to the jet in fluidized bed with slugging, and the jet in fluidized bed with bubbling suitable for ash-agglomerating fluidized bed gasifier does not exist. But because of the diameter of the granular is a little smaller, the fluidizing effect is better, gas and solid are mixed more thoroughly compared with effect of larger granular under a certain gas velocity. The above conclusion can supply reference for predetermining the performance of practical device, properly controlling the cold running parameters of ash-agglomerating fluidized bed, eliminating running loss and guiding the hot running of ash-agglomerating fluidized bed.