Modelling of self-heating in open-air woodchip storage piles: an ambient humidity-sensitive approach to predict moisture migration and temperature dynamics

The safe storage of renewable biomass in piles poses a significant challenge. Low-temperature oxidation, water evaporation or condensation, and the metabolic processes of microorganisms occur simultaneously within open-air biomass piles, leading to intense self-heating. Drawing on our knowledge of Computational Fluid Dynamics (CFD) simulation, we successfully integrated the three mechanisms of biomass pile heating in the present work. This study examined moisture migration and temperature changes in a woodchip storage pile over a simulated 3-month period. We focused on enhancing the mathematical model of water migration, developing a method sensitive to environmental humidity, and adding a correction factor to match the experimentally measured water migration rate. Based on the above simulation framework, this study investigated the effects of changes in pile height, particle diameter, ambient humidity, and temperature on self-heating. Finally, a preliminary investigation was conducted on the safety margins that can prevent spontaneous ignition of woodchip storage piles under certain extreme weather conditions. The results indicate that during the self-heating process of a woodchip storage pile, the highest temperature inside the pile differs from the experimental record by only 3 to 4°C, and the duration of the elevated temperature differs by about 2 days. When the correction factor φ is 0.5, the moisture migration process within the pile better matches the experimental data. The dry matter loss of the biomass is approximately 11%, slightly higher than the 9% in the experimental record. Reducing the pile height, increasing particle size, and maintaining lower ambient temperature and humidity are all beneficial for safe storage. Finally, a safety margin to prevent woodchip pile spontaneous ignition is proposed: when the pile height is 6 m, the particle size should not be less than 3 cm; when the stack height is 3.5 m, the particle size should not be less than 2 cm. This long-term simulation of biomass storage piles provides significant input for the safety of biomass storage, as it accurately predicts temperature variation and moisture migration.