3D CFD modelling of self-heating and self-ignition with parametric investigations in solid fuel stockpiles

• A valid and reliable model developed for predicting coal pile self-heating. • The airflow inside the coal pile is laminar, and the convection is dominant. • There are three distinct stages of self-heating of the coal pile. • Lower coal pile height, larger particles, and a low-temperature environment are conducive to solid fuels. Open-air stockpiles of solid fuels, such as coal and biomass, are susceptible to self-heating and spontaneous combustion due to low-temperature oxidation, moisture migration, and, in the case of biomass, biological processes. Computational fluid dynamics (CFD) combined with experimental data plays a crucial role in predicting and preventing such hazardous occurrences. Compared to conventional 1D and 2D models, 3D modelling provides distinct advantages by faithfully replicating the intricate geometry of the stockpile and accurately accounting for realistic ambient flow conditions, resulting in more reliable simulation results. In this study, a 3D CFD model is developed to numerically replicate the self-heating and self-ignition process within a coal pile as previously reported in the literature. The CFD model, developed using Ansys Fluent, incorporates essential considerations such as low-temperature coal oxidation and water evaporation, and implements various source terms reflective of the chemical and physical processes within the coal pile through User Defined Functions (UDFs). Since the thermal conductivity and mass diffusivity in a coal pile may be in the same order as the local turbulent conductivity and diffusivity, it is crucial to assess whether the flow within the pile is turbulent or laminar and accommodate the correct flow regime within the pile. Our simulation results reveal three distinct stages of self-heating within the coal pile, which are consistent with the observations reported in the literature. Notably, the upper-middle zone inside the coal pile exhibits high temperatures and relatively low oxygen content, primarily limited by the internal air velocity. Key parameters, such as coal pile height, moisture content, wind speed, and heat loss at the pile bottom, are found to significantly impact the self-heating process. The developed CFD model lays the groundwork for a comprehensive and systematic parametric study, from which applicable guidelines for preventing and mitigating fire accidents in coal piles are suggested. This model will be extended to biomass stockpiles by incorporating biological reactions as well as addressing the differences between low-temperature biomass and coal oxidation.