Scale-resolved prediction of pyrolysis in a packed bed using the extended discrete element method

The demand for a net reduction of carbon dioxide and for restrictions on energy efficiency make thermal conversion of biomass a very attractive alternative for energy production. Although largely based on experimental investigations, numerical methods have advanced significantly to predict pyrolysis of packed beds, and thus, compensate for a mayor disadvantage of generally non-accessible measurements within packed beds. A detailed resolution of packed bed processes is provided by the innovative numerical approach of the extended discrete element method. Within this approach the solid phase consists of individual particles for which both the dynamic state i.e. position and orientation of each particle in space and time and its thermodynamic state e.g. internal temperature and species distribution is determined. The flow of gas in the void space between the particles is predicted by traditional and well-proven computational fluid dynamics taking into account heat and mass transfer between the particles and the surrounding gas phase. This numerical concept was applied to predict pyrolysis of a packed bed of wood particles in a cylindrical reactor. The predicted results of pyrolysis for both individual particles and integral bed processes agreed well with the experimental data. Thus, an analysis of detailed results helps to uncover the underlying physics of the process, and thus, allows for an improved design and operation conditions.