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A numerical model is presented which consists of a set of partial differential equations for the transport of heat and mass fractions of eight chemical species to describe the onset of self-ignition and the propagation of smouldering fires in deposits of bulk materials or dust accumulations. The chemical reaction sub-model includes solid fuel decomposition and the combustion of char, carbon monoxide and hydrogen.
The model has been validated against lab-scale self-ignition and smouldering propagation experiments and then applied to predictions of fire scenarios in a lignite coal silo. Predicted reaction temperatures of 550 K and propagation velocities of the smouldering front of about 6 mm/h are in good agreement with experimental values derived from lab-scale experiments.
The current paper presents experimental investigations as well as numerical simulations on the influence of water and humidity on the self-ignition of combustible bulk materials.
It is well known, that bulk materials may undergo self-ignition if stored under specific conditions. In some cases, large amounts of these materials are exposed to a humid surrounding, e.g. dried coal in a moist atmosphere. Due to the effects of condensation and adsorption of water, additional heat is generated and transported into the bulk material. If the pile is stored slightly below its self-ignition temperature, the bulk material can become supercritical and an ignition occurs.
Experiments were carried out for German lignite coal sampled in two different particle size fractions. They showed, that subcritical deposits turned to supercritical behaviour if the relative humidity in the surrounding was suddenly increased or water was poured on the surface of the sample. Besides the experiments, a numerical model was established to describe the effects of self-heating until ignition of the deposit, including the transportation of moisture. Simulations with this model led to satisfying results when compared to the experiments.