Numerical studies of solid-solid mixing behaviors in a downer reactor for coal pyrolysis

A downer reactor is ideal for fast pyrolysis during coal gasification as solids holdup distributions and residence time distribution in the reactor are fairly uniform and residence time is short. Because pyrolysis is a rapid reaction, in the downer solid-solid mixing is critical to promote pyrolysis reaction. Thus, solid-solid mixing in the downer was studied through numerical modeling in this study. An Eulerian-Lagrangian method was applied and implemented by combining Computational Fluid Dynamics and Discrete Element Method (CFD-DEM). The heat carrying media of silica sand was modeled to flow from the top of the downer while coal particles were introduced through two lateral nozzles in normal and tangential arrangements. Numerical results showed that tangential arrangement had lower solids holdups while higher particle velocities than normal arrangement. Mixing among binary solid particles was poor at the entrance of downer. The extent of mixing increased rapidly and approached an almost constant value in the downstream regions of the downer. High air velocity and small solid particle size could lead to better mixing for both normal and tangential arrangements. But tangential arrangement had better mixing of binary solid particles than normal arrangement, and when the particle sizes were reduced from 2 to 1 mm, the influence of nozzle arrangement was reduced. (C) 2013 Elsevier B.V. All rights reserved.