Numerical simulation of an externally heated fixed bed reactor for coal pyrolysis based on porous media

A cutting-edge three-dimensional transient model utilizing porous media has been developed to accurately simulate the pyrolysis process of externally heated coal. This innovative model comprehensively considers the evaporation and condensation of water, as well as the release of volatiles. In addition, it meticulously examines the change in coal seam porosity and the interphase heat transfer between pyrolysis gas and solid coal seam. The model's precision has been appropriately confirmed through validation against the central temperature evolution inside the experimental reactor. Notably, this model systematically illustrates various aspects, including the evolution of coal layer temperature, evaporation and moisture density, changes in coal layer density, porosity distribution, volatile release, and interphase heat transfer. The obtained results reveal that the heat absorption by moisture phase change causes the coal seam temperature to relatively stabilize at around the water boiling point for a period, thus delaying the initiation of the coal pyrolysis reaction. The pyrolysis gas released by center coal seam pyrolysis tends to flow radially toward the heating wall before flowing out of the reactor. Additionally, the change in coal seam porosity increases the heat transfer rate by an average of 1.5 °C/min during the rapid heating stage. The analysis also highlights the significant occurrence of interphase heat transfer throughout the pyrolysis process and elucidates its mechanism in various stages. Ultimately, this work offers essential theoretical guidance for the design, optimization, and scaling of subsequent externally heated fixed-bed coal pyrolysis reactors.