Revealing the formation mechanism of high-density particle band in a cyclone pyrolyzer via extended kinetic theory of granular flow

In the cyclone pyrolyzer, the formation of high-density particle band (HDPB) is conducive to constructing a localized high solids holdup region, achieving rapid heat transfer and efficient pyrolysis of low-rank coal. However, fundamental understanding of the HDPB formation mechanism remains limited. The kinetic theory of granular flow (KTGF), which reveals the relationship between the microscopic inter-particle interactions and macroscopic flow characteristics, is employed to analyze the formation mechanism of the HDPB in this work. In addition, based on the spatial superposition assumption, a mesoscale solid phase stress model is constructed by extending KTGF that considers the heterogeneous structure to close the two-fluid model (TFM). Meanwhile, a visualized cyclone pyrolysis experimental system is established, and the experimental results obtained using high-speed particle image velocimetry (HPIV) techniques show good agreement with the model predictions. It is indicated that the positive granular pressure gradient induced by airflow between the low solids holdup region and the high solids holdup region drive particles to migrate toward the HDPB, promoting the formation of HDPB. By analyzing the energy dissipation mechanisms in the cyclone pyrolyzer, it is found that the formation of HDPB can significantly reduce hydrodynamic and collisional energy dissipation. The gas-solid flow behavior exhibits distinct characteristics on both sides of the HDPB, forming the windward and leeward regions. These results are expected to deepen the understanding of formation and energy dissipation mechanism of HDPB in the cyclone pyrolyzer.