Comparison of full-loop and riser-only simulations for a pilot-scale circulating fluidized bed riser

In this paper, both full-loop and riser-only simulations of a pilot-scale circulating fluidized bed (CFB) system carried out by the open-source code MFIX (Multiphase Flow with Interphase eXchanges) are presented. Detailed comparison between full-loop and riser-only numerical simulations has been conducted with respect to the flow hydrodynamics inside the riser. The mean solids circulation rate is found to be the most critical parameter for both riser-only and full-loop simulations. On one hand, the mean solids circulation rate is needed for specifying the solids inlet boundary condition for the riser-only simulation. On the other hand, a reasonable prediction of the solids circulation rate is a prerequisite for the full-loop simulation to properly predict the flow hydrodynamics inside the riser. To better account for the full-loop dynamics in the riser-only simulation, the transient solids circulation rate measured from an experimental facility is imposed in the simulation in addition to the mean solids circulation rate. Consistent numerical predictions of the flow hydrodynamics inside the riser are obtained by different types of simulations. In-depth discussion on the advantage and disadvantages of each approach is presented and the riser-only simulation with appropriate boundary conditions is shown to be sufficient for investigating the steady CFB riser flow. The full-loop simulation is promising but more challenging as all major components and associated complicated physics have to be included and correctly modeled which require both advanced model capability and high computer power.