The flow behaviors in a series of novel gas-solids co-current downflow conical fluidized beds (conical downers; 7 m in height, 0.14 m inlet diameter, 0.02, 0.04, 0.07 and 0.14 m outlet diameters), which are expected to realize a high-density solids holdup along a downer-type pyrolyzer to strengthen the heat transfer efficiency, were systematically investigated by means of a numerical approach. A 2D, transient, Eulerian–Eulerian two fluid model based on the kinetic theory of granular flow with a k-ε turbulence model for gas phase was adopted to simulate the hydrodynamics of the conical downer and explore possibilities of achieving a high density solids holdup in the bed. In order to avoid the different effects of different boundary coefficients on the simulation results, a boundary condition which can realize the adjustment without changing the related parameters was used here. The results demonstrated that the conical downer was beneficial to the enhancement of solids holdup. This was more obvious at Ug = 1 m/s in this study. According to the analysis of the axial distributions of pressure and velocity, the unique hydrodynamics of three axial acceleration regions were characterized, which was different from traditional downer. In addition, it was found that the length of the third acceleration region increased with the increase in gas velocity but decrease in outlet diameter of the conical downer. Also, the conical downer was able to save a large amount of carrier gas and reduce the percentage of carrier gas in the follow-up mixture gas. These results are expected to be a guidance for the design of an efficient downer-type pyrolyzer in a triple-bed combined circulating fluidized bed gasifier.
Keywords: Triple-bed combined circulating fluidized bed, Conical downer, Flow behavior, High density holdup, Numerical simulation,