Multi-scale kinetic model for forest fuel degradation

Abstract

In modelling wildfire behaviour, a good knowledge of mechanisms and kinetic parameters controlling the thermal decomposition of forest fuel is of great importance. Generally, the pyrolysis models are determined from experiments carried out in thermal device (i.e. TGA and DSC). This kind of tools ensures an accurate determination of kinetic parameters in perfectly controlled conditions. But the gap of a larger scale, for their uses in forest fire conditions, is still far. To corroborate the kinetic models in realistic forest fire conditions, a mass loss device specially designed for field scale has been developed. The system includes three load cells with a max load at 400 g and 2.7 Hertz for the frequency acquisition. Each load cell is sit on top of a stainless tube in which the sample is hold. The position and height of the sample in the tube could be adjusted to optimize the interaction with the height of the flame and the sample. To obtain the temperature impacting the sample, a thermocouple K is placed at the end of each tube set to the same frequency acquisition as load cells. The device is integrated in a welded ceramic box that was lined with a 15 mm thick refractory lining. The acquisition system is included into an armored thermal box. For this first campaign of measures, we have selected species which have been previously studied for the kinetic modeling and presenting different typologies: Rockrose, Heather and Pine. One of the main advantages of this prototype is that 3 different species can be submitted, in line, to the same external heating conditions and are simultaneous analyzed. The heating source is a fire spread of wood wool bed. This fuel have been selected for a good repeatability of heating conditions Before each test, a fuel bed with various loads (0.5, 1 and 2 kg/m2) has been uniformly distributed and the prototype has been placed around the end of the bed to ensure the steady state. In these experimental conditions the samples, of intact branches and leaves, are closest to their natural state. Experimental field-scale results have been compared to numerical simulations based on Arrhenius law. The simulations have been performed considering a two-steps mechanism previously obtained by the authors using TGA data. In a general point of view, the simulations have a good agreement with the experimental mass loss rate even if some differences appear (as attempt). For the rockrose, the model do not match accurately in the range of 0.7 < m⁄m0 < 0.9 probably because the mechanisms of initiation and preheating are more complex than a simple Arrhenius equation of order n. Concerning the heather, experiments exhibit an accelerate degradation process which can be explain by the very fine structure of this specie. Conversely, Pine has a different structure which is constituted by a single branch with a diameter of 6 mm. This thickness of sample involves an incomplete degradation. Considering the very important gap between laboratory and field scale, the kinetic scheme gives satisfactory modeling.