Modelling of self-heating and parametric study based on CFD in a 3D biomass pile

Large-scale biomass piles in an open-air environment are prone to self-heating, which can induce spontaneous combustion under certain conditions. This poses significant safety risks for the storage and utilization of biomass fuels. Developing a prediction tool for self-heating within stored biomass is a cost-effective strategy to mitigate these risks. This study proposes a robust and reliable modeling tool to predict self-heating in biomass storage piles. A computational fluid dynamics (CFD) model has been developed using Ansys Fluent, incorporating the three primary mechanisms of biomass self-heating (i.e., chemical, physical, and microbial activities). These mechanisms are mathematically formulated and integrated into the CFD model through user-defined functions. CFD simulations are performed for a well-defined biomass storage scenario to validate the model, using experimental data from the literature. The simulation results show good agreement with the measured temporal evolution of temperatures in the pile. Within the initial 30 days, intense microbial activity leads to significant heat generation from metabolic processes, causing a rapid temperature rise within the pile from 29 °C to 62 °C. Subsequently, as the temperature peaks, microbial activities diminish, leading to a decrease in temperature. With no significant contribution from low-temperature oxidation and physical processes, the heat released from the biomass pile is negligible. Based on this investigation, we further explore various cases to examine the factors that affect the self-heating of biomass piles by altering the external environment or the characteristics of the biomass piles and particles themselves. The results indicate that these factors have varying degrees of influence on the biomass pile, whether in the first or second stage of the self-heating process. When the external ambient temperature exceeds 20 °C or the biomass particles are smaller, the high temperature inside the biomass pile is sustained for an extended period, which is highly unfavourable for its safe storage. The accurate simulation of the self-heating process in biomass piles is significant for safe biomass storage. Such virtual testing will not only enhance our understanding of self-heating in biomass piles but also facilitate the formulation of practical guidelines for the secure storage of solid biomass fuels.