Numerical Study of the Influence of Bed Stoking on Pyrolysis of a Polydisperse Bulk of Spherical Particles Using DEM/CFD

This study numerically investigates the influence of mechanical agitation on the pyrolysis behavior of a polydisperse particle bed. The bed consists of wooden spheres of three different diameters arranged in a cylindrical reactor. Bed agitation is achieved by three vertically oscillating stoking rings, while convective heat is supplied by preheated nitrogen flowing through the bed and radiative heat by a surrounding high-temperature cylindrical reactor wall. A coupled Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD) approach is employed to resolve particle motion, heat transfer to the particles as well as intra-particle heat conduction and pyrolysis. The comparison between stationary and agitated bed configurations reveals that agitation significantly enhances temperature homogenization within the bed, thereby accelerating the overall pyrolysis process and increasing the mass loss rate. The magnitude of this enhancement, however, strongly depends on the initial particle arrangement. Segregation effects arising from particle size differences alter the local temperature fields and, consequently, the effective bulk conversion rate. The results emphasize that, for the design and optimization of industrial-scale grate systems, understanding the interplay between bed motion, segregation, and conversion is essential.