Modelling of an indirectly heated fixed bed pyrolysis reactor of wood: Transition from batch to continuous staged gasification

Gasification is today a mature technology and staged processes know an important development. The design of this kind of reactor is still a sensitive work, and numerical simulation, as a flexible and economic tool, has to be developed. The present work concerns the first pyrolysis stage of a staged gasification process and is organised into two parts. The first part deals with the calibration of the effective thermal conductivity of the bed. This is the key parameter in the heat transfer modelling. The calibration is obtained by comparison between experimental and modelling results in batch reactor. The second part is the development of a transient two dimensional model of a continuous indirectly heated fixed bed pyrolysis reactor. The model uses a finite difference formulation to solve mass and energy balances in a cylindrical vessel. The assumptions of local thermal equilibrium, constant particle size and thermally thin particles are considered. The simulations are used to design a pyrolysis reactor in function of the type of biomass, the wall temperature and the direction of gas circulation. The parametric study is presented to highlight the limitations of the external heating regarding the wood conversion rate.