New experimental diagnostics in combustion of forest fuels: microscale appreciation for a macroscale approach

Abstract

In modelling the wildfire behaviour, good knowledge of the mechanisms and the kinetic parameters controlling the thermal decomposition of forest fuel is of great importance. The kinetic modelling is based on the massloss rate, which defines the mass-source term of combustible gases that supply the flames and influences the propagation of wildland fires. In this work, we investigated the thermal degradation of three different fuels using a multi-scale approach.Lab-scale experimental diagnostics such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), use of the cone calorimeter (CC) or Fire Propagation Apparatus (FPA) led to valuable results for modelling the thermal degradation of vegetal fuels and allowed several upgrades of pyrolysis models.However, this work remains beyond large-scale conditions of a wildland or forest fire. In an effort to elaborate on the kinetic models under realistic natural fire conditions, a massloss device specifically designed for the field scale has been developed. The paper presents primary results gained using this new device, during large-scale experiments of controlled fires. The mass-loss records obtained on a field scale highlight the influence of the chemical composition and the structure of plants. Indeed, two species with similar chemical and morphological characteristics exhibit similar mass-loss rates, whereas the third presents different thermal behaviour.The experimental data collected at a field scale led to a new insight about thermal degradation processes of natural fuel when compared to the kinetic laws established in TGA. These new results provide a global description of the kinetics of degradation of Mediterranean forest fuels. The results led to a proposed thermal degradation mechanism that has also been validated on a larger scale.

Influence of particle size and density of pelleted samples of forest fuel on thermokinetic characteristics of pyrolysis and oxidation

“…For example, in [25], the optimal range of particle sizes for forest materials (63-125 μm) and the heating rate of 10 °C/min (the heating rate corresponds to the real rates of the FF thermal decomposition) are determined. From the data analysis in [3,[26][27][28][29], it is possible to distinguish the following ranges: 60-150 μm for the size of components and 200-955 kgm -3 for the density of the sample material.…”

“…Since pyrolysis is an integral part of forest fuel (FF) combustion, its study is expedient with the view of development of mathematical models necessary to elaborate the existing and create the new technologies for suppression of flaming combustion and smoldering [1,2] in effective fire control. To date, the analysis of thermokinetic characteristics of plant materials (mainly, different types of wood) is investigated in numerous papers, among which several of the most significant may be distinguished, in particular [3][4][5][6][7][8]. Research [3] proposed the model, used in the analysis of conditions of the Mediterranean forest fire development.…”

“…To date, the analysis of thermokinetic characteristics of plant materials (mainly, different types of wood) is investigated in numerous papers, among which several of the most significant may be distinguished, in particular [3][4][5][6][7][8]. Research [3] proposed the model, used in the analysis of conditions of the Mediterranean forest fire development. The experiments were carried out in laboratory and field conditions.…”

“…Thermokinetic parameters were determined using thermogravimetric analysis in the temperature range from 77 °C to 627 °C. The main achieved result of experimental studies [3] was the creation of a two-stage kinetic model. Similar studies [4,5] were carried out without sacrificing the material structure, i.e.…”

“…It was concluded that the samples with a particle size of less than 0.5 mm were the most reactive. The characteristics of the FF components (such as density and particle size) were proved to influence the rate of physical and chemical processes and play an important role in predicting conditions and characteristics of FF burning [3,12,13]. It was concluded that in this regard, the study of thermokinetic characteristics of components close to forest cover is necessary for mathematical modeling of forest fire propagation.…”