Numerical simulations of the evolutionary patterns of multi-physical fields during the in-situ pyrolysis of tar-rich coals

Objective The in-situ pyrolysis of tar-rich coals, an emerging technology for coal resource utilization characterized by cleanliness, high efficiency, and safety, is still in its initial development stage. This technology is greatly influenced by the evolutionary patterns of multi-physical fields and critical process parameters.

Methods A numerical simulation model involving multi-physical fields was established by coupling fluid flow, heat transfer, and chemical reactions, and its reliability was verified by comparing the numerical simulation results with the pyrolysis experimental results of cylindrical coal samples. Based on the pilot test conditions, the impacts of permeability, heat-carrier flow rate, and fracture zone height on the heat and mass transfer in the pyrolysis process were investigated using homogeneous and `heterogeneous permeability models.

Results and Conclusions The results indicate that increasing the permeability can facilitate the rapid production of coals. For the homogeneous permeability model, the pyrolysis reaction can be completed in only 38 days under permeability of 1 μm², and impact-induced fracturing is recommended to achieve coal seam permeability of a Darcy level. For the heterogeneous permeability model, increasing the heat-carrier flow rate in the preheating stage can enhance the mass and heat transfer rates. Under a heat-carrier flow rate of 0.12 kg/s, the pyrolysis reaction in the central high-permeability zone was completed in only about 12 days. In the later stage, the reaction and seepage rates decelerated as the pyrolysis of coals in the central high-permeability zone was completed, and increasing the heat-carrier flow rate produced minor impacts on the reaction rate. Therefore, it is recommended that a high heat-carrier flow rate (0.12 kg/s) be adopted in the early production stage to accelerate the pyrolysis in the high-permeability zone and produce tar more quickly and that the heat-carrier flow rate be reduced in the later stage to save costs. Increasing the fracture zone height can enhance the pyrolysis reaction rate. With a fracture zone height of 6 m, the pyrolysis reaction can be completed in about 130 days. Additionally, it is necessary to consider the costs of impact-caused fracturing and pyrolysis time in actual production.