Numerical Simulation Of Gas-solid Flow In A High Density Circulating-Fluidized-Bed Riser

Circulating fluidized bed are widely used in energy, chemicals and pharmaceutical and other industrial processes for its high efficiency and production capacity, etc.. In recent years, high-density circulating fluidized bed are geting people’s attention. In addition to traditional FCC and maleic anhydride synthesis and other industrial processes, high density circulating fluidized bed has been applied to coal/biomass gasification and chemical looping combustion process. High-density circulating fluidized bed riser has a complex gas-solid flow structure that will impact on heat and mass transfer and chemical reaction process, so it is necessary to enhance the high-density solid flow behavior tube research. In this paper, the numerical simulation methods was used to analyzed solid flow behavior characteristics in the high-density circulating fluidized bed riser.The gas-solid flowing control equation and the closed equation is the basis of numerical calculation, so the control equation and closed equation of momentum equation was first established for calculating the solid flow in the article; the drag force as a major force between gas and solid phase plaies a decisive role in gas-solid flows. Literature and simulations show that the drag force calculated by traditional model is too large. This paper established a segmented drag model (step-section model) and energy minimization multi-scale (EMMS) model, and the models were investigated, the results showed that:under simulated conditions of this article, the segmented drag model has a good simulation results.Combined with the high-density experimental study the model parameters such as mesh size, viscosity model, slip boundary condition, inter-particle collision coefficient and maximum filling factor of particles were investigated. The results show that:when the mesh size was 0.2mm,the viscosity model used turbulence model, boundary slip coefficient was 0.1, inter-particle collision coefficient was 0.9 and the largest particles fill factor was 0.63 the simulation is better.The solid flowing characteristics were simulated at the operating conditions of high density based on the verification of the model. The results show that:the axial solid holdup in the riser was typical exponential distribution; the radial solid holdup in the riser was typical "ring-core" structure. With the increase of circulation the axial solid holdup overall increase, but the bottom of the region affected by the circulation more obvious; in the radial direction the solid holdup has increased, but the significantly increased side was the wall area. With the increase of superficial gas velocity the axial solid holdup overall decrease and the region decreased more obviously was the bottom of the riser,and the operating conditions change a greater impact on the bottom section of the acceleration; radial flow at the side walls state improved its solid holdup larger decline.