This study aims to investigate the thermal stability of biomass piles through a combination of experimental and numerical approaches and explore the impact of particle size and oxygen diffusion. Isothermal basket tests were carried out according to EN-15188:2020 on raw and grinded pellets and sieved dust samples. The progression of the thermal wave inside the baskets was particularly studied by positioning thermocouples at different depths of the pile. The role of oxygen diffusion in the pile was examined by varying the basket size, by modifying the particle size distribution and by partially wrapping the baskets in a protective film. The self-heating behaviour of these piles was also assessed by using the crossing point method. The time evolution of the gases generated was analysed by micro-gas chromatography, especially around the cross-point.
In parallel, a three-dimensional model was developed to simulate the thermal behaviour of a quarter cube. The model includes energy balance, considering conductive, convective and radiative heat transfers and a heat-source term. Mass balances for particle size and each species are also considered through consumption and diffusion terms. Shrinking core models were implemented to represent the consumption of reactants. Moreover, thermogravimetric analyses were performed to identify the various reaction stages and determine the activation energy by using Flynn-Wall-Ozawa, Friedman and Kissinger’s methods.
This study demonstrates the significant influence of bed permeability, especially related to oxygen accessibility, on the thermal stability of storage facilities. Finally, the predictive model developed could be used to explore the efficiency of safety measures and technological solutions (compaction, storage size reduction, bagging...).