Pyrolysis and spontaneous ignition of wood under transient irradiation: experiments and a-priori predictions

摘要

Wood is a material widely used in the built environment, but its flammability and response to fire are a disadvantage. Therefore, it is essential to have substantial knowledge of the behavior of wood undergoing external heating such as in a fire. The majority of studies in the literature use constant irradiation. Although this assumption simplifies both modelling and experimental endeavors, it is important to assess the behavior of materials under more comprehensive heating scenarios which might challenge the validity of solid-phase ignition criteria developed previously. These criteria are evaluated here for the spontaneous ignition under transient irradiation by combining experimental measurements and a-priori predictions from a model of heat transfer and pyrolysis. We have applied a two-step transient irradiation in the cone calorimeter in the form of a growth curve followed by a threshold of constant irradiation. We used white spruce samples of size 100 x 100 mm thickness of 38 mm measured the temperature at different depths and the mass loss. A one di- mensional model written in the open source code Gpyro is used to predict the pyrolysis behavior. The model has a chemical scheme in which the virgin components of wood (hemicellulose, cellulose, lignin) become active, then decompose in two competing reactions: char and gas, and tar. The kinetic parameters, as well as the thermal properties of the wood and char are taken from the literature, while ρ and moisture content are measured experimentally. A priori predictions of the temperature, made prior to the experiments, show excellent agreement with the measurements, being within the experimental uncertainty range. The mass loss rate (MLR) predictions are qualitatively similar to the measurements, but there is a large uncertainty in the measurements. For a-posteriori simulations, certain parameters are changed after having access to the mea- surements to improve the simulations. We found that the heat of reaction for exothermic reactions has no influence, the reaction order of the char reaction influences the MLR, whereas the reaction order of the tar re- action is important for temperatures and MLR. The in-depth absorption is very important in cone calorimeter experiments. Also, we perform an evaluation of the solid phase ignition criteria. The ignition criteria found in the literature are the critical temperature, the critical mass loss rate, the critical heat flux, and the time-energy squared correlation. We find that neither criteria is a consistent indicator of ignition. These results help un- derstand the spontaneous ignition of wood subjected to transient irradiation and identify strengths and gaps in the topic.