Pyrolysis of Spherical Wood Particles in a Packed Bed – Comparison between Resolved and Unresolved Discrete Element Method/Computational Fluid Dynamics

Pyrolysis of Spherical Wood Particles in a Packed Bed – Comparison between Resolved and Unresolved Discrete Element Method/Computational Fluid Dynamics

Two discrete element method/computational fluid dynamics coupling approaches are compared: One method resolves the particle shape in the fluid flow, the other one does not. Differences between these two methods with regard to the evolution of particle-based parameters (mass, temperature) are reported. A bulk of thermally thick, pyrolyzing spherical particles is taken for the numerical study.


Abstract

The combined discrete element method/computational fluid dynamics (DEM/CFD) approach allows for the description of reacting granular assemblies passed by a gas flow. Especially for thermally thick particles, the spatial resolution of the solid object volumes, their surfaces, and of the interstitial flow domain controls the quality of results while at the same time driving computational costs. To evaluate the differences between resolved and unresolved DEM/CFD approaches, pyrolysis of a bulk of spherical wood particles enclosed by a cylindrical heating surface and passed by hot nitrogen is numerically examined. The unresolved simulation is based on the averaged volume method (AVM). For the resolved simulation, the so-called blocked-off (BO) method is applied. The results show that the total mass conversion rate of solid particles into gaseous volatiles is faster when employing the resolved BO approach. The better spatial resolution of local flow field and particle surface representation leads to a more detailed prediction of convective and radiative heat transfer to the particles, but is associated with the penalty of a six-fold computing time.