Multi-phase approach to the degradation of solid materials | Projet Feux | Università di Corsica Pasquale Paoli

Dans cette catégorie : The effect of prescribed burning on Pinus laricio ecosystems - Interdisciplinary tools | Characterisation of smoke | Modelling of the behaviour and impacts of fire | Measurement of the geometric characteristics of fire through vision | Behaviour of wildland fires and comprehensive fire model | Simulation of wildland fires at valley level |

Our group was comprised of three Physical Chemistry Senior Lecturers (31st Section), Dominique Cancellieri, Valérie Leroy-Cancellieri and Eric Leoni.

Our study was both experimental and numerical, attesting to its cross-disciplinary nature. This dual expertise allows us to address operational issues drawing on a better understanding of fire. Our research was driven by the desire to address:

• the concrete requirements of modellers seeking the proper characterisation of source terms.

• current environmental concerns regarding emissions and the spread of pollutants.

Fire spread is generally a multi-phase phenomenon, so to take into account its complexity, we work simultaneously on:

characterisation of the thermal degradation of solid fuels
the reaction mechanics of the gas phase

These studies are both experimental and numerical in nature.

Thermochemical characterisation of the degradation of solid materials

In an effort to better define the thermochemical process which entails degrading a vegetable fuel into a gaseous fuel that feeds the flame, we experiment using differential scanning calorimetry (DSC) and thermogravimetry (TGA). These experiments were conducted on various species representative of Mediterranean plant cover. The combined use of various heat analysis techniques allows us to accurately determine the reactions that come into play throughout the degradation process. We work on a wide temperature range with high heating rates (in terms of applications currently studied in thermal analysis) and in different surrounding atmospheres.

Once the reactions have been clearly identified, drawing on the experimental data allows us to develop kinetic models. This parametric modelling is carried out by means of a method that we created and called the Hybrid Kinetic Method. The application of this method leads to determination of rate constants of each reaction according to an Arrhenius law whose parameters (reaction model, activation energy and pre-exponential factor) are also calculated.

Mass loss thermogram comparing experimental data (symbols) with modelling (line) at different heating rates.

Measuring and modelling gas emissions

Forest fires present major upheavals for natural ecosystems and they contribute to increased levels of the atmospheric gases that lead to climate change. In order to describe the impact of a fire on the environment and specifically on the atmosphere, and to model its emissions, we first need to understand the processes involved in the combustion of plants.

To do this, we tested the oxidation of a pyrolytic gaseous mixture in a jet-stirred reactor. These experiments were carried out at the Institut P’ of Poitiers. The mole fraction data obtained was compared with the Chemkin® code simulation results.

During numerical processing, a so-called "chemical" reduction method was applied so as to propose a framework mechanism of 49 elementary reactions followed by a mechanism reduced to four overall equations.

At the end of this initial work, we wanted to provide a useful chemical kinetics system for turbulent combustion models, while preserving in these models the atmospheric markers required to study atmospheric emissions. Working in partnership with the Molecular and Macroscopic Energetics and Combustion Laboratory - EM2C - we then proposed a global combustion mechanism.

Global combustion mechanism of a pyrolytic gas mixture

Experimental diagnostics and numerical processing of plant degradation at field level

Dependence on the level is the crux of the issue and we need to cross match calculated data against measurements taken under real conditions.

To this end, we had to create tools specifically dedicated to taking measurements in the field. Thus, the loss of mass of plants subjected to fire front is measured using a device, designed by us, which allows for the simultaneous recording of three profiles of loss of mass and temperature.

Differential loss of mass device

We worked with the institutions in charge of firefighting and regional town and country planning (General Councils, National Forestry Offices, SDIS) in order to carry out experimental controlled burns and to test our measurement equipment under real conditions.

The data gathered in the field is then compared to measurements carried out in the laboratory.

In order to control air quality while the fire is in progress and form an estimate of the carbon footprint of these events, it appears to be increasingly necessary to qualify and quantify the gaseous plumes emitted. Always with the aim of making our studies align with real wildland fires from the experimental and numerical point of view, we have purchased a Fourier Transform Infrared analyzer which allows for measurement of the gas plumes emitted in real time and as close as possible to the fireline.

FTIR Analyzer in its real situation and gas measurements taken

Skills - Expertise

During the course of our research, we gained several skills in the analytical and thermal domain:

Calorimetric Analysis (Differential Scanning Calorimetry: DSC)
Thermogravimetric analysis (TGA)
Fourier-transform infrared spectroscopy (FTIR)
Gas Chromatography (GC)
High Performance Liquid Chromatography (HPLC)
Gas-stirred reactor (Perfectly Stirred Reactor PSR).

National Scientific Partnerships

Institut PPrime (Poitiers)
Institut de Combustion, Aérothermique, Réactivité et Environnement (Orléans)
Laboratoire d’Aérologie (Toulouse).
Laboratoire Energétique Moléculaire et Macroscopique, Combustion (Châtenay-Malabry)
Laboratoire de Physico-Chimie des Processus de Combustion et de l'Atmosphère (Lille)
Laboratoire Physico-Chimie pour l’Atmosphère (Dunkirk)

International Scientific Partnerships

Department of Physical and Computational Mechanics, State University of Tomsk
Department of Physical and Computational Mechanics, State University of Tomsk (Russia).
BRE Centre for Fire Safety Engineering, University of Edinburgh, (Great Britain).
Department of Fire Protection Engineering, Worcester Polytechnic Institute (Massachusetts, USA).
Institute of Biometeorology, CNR-IBIMET, (Sassari, Italy)

Science Promotion – Technological transfer

The result of our modelling research has led to the creation of a software tool for the numerical processing of the degradation of solid materials. WinGpyro^® software has specifically been designed to model the mechanisms involved in the thermal degradation of solid fuels.

Page mise à jour le 04/12/2017 par MATTHIEU VAREILLE