Eric Leoni

Investigation on the Emission of Volatile Organic Compounds from Heated Vegetation and Their Potential to Cause an Accelerating Forest Fire

An experimental study is conducted on the emission of volatile organic compounds (VOCs) emitted by Rosmarinus officinalis plants when exposed to an external radiant flux. The thermal radiation heats the plant and causes the emission of VOCs. The thermal radiation simulates the radiant flux received by vegetation in a forest fire. The results of the experiments are used in a simplified analysis to determine if the emissions of VOCs in an actual forest fire situation could produce a flammable gas mixture and potentially lead to the onset of an accelerating forest fire. The experiments consist of placing a plant in a hermetic enclosure and heating it with a radiant panel. The VOCs produced are collected and analyzed with an automatic thermal desorber coupled with a gas chromatograph/mass spectrometer (ATD-GC/MS). The effects of the fire intensity (radiant panel heat flux) and the fire retardant on the VOCs emission are then investigated. Two thresholds of the VOCs emission are observed. The first is for plant temperatures of around 120°C and appears to be caused by the evaporation of the water in the plant, which carries with it a certain amount of VOCs. The second one is around 175°C, which is due to the vaporization of the major parts of VOCs. The application of a fire retardant increases the emission of VOCs due to the presence of the water (80%) in the fire retardant. However, the use of the retardant results in a lower production of VOCs than using water alone. The measurements are used to estimate the concentration of VOCs potentially produced during the propagation of a specific fire and compared to the flammability limits of α-pinene. It is concluded that the quantities of VOCs emitted by Rosmarinus officinalis shrubs under certain fire conditions are capable of creating an accelerating forest fire.

Fire spread across pine needle fuel beds: characterization of temperature and velocity distributions within the fire plume

The aim of this article is twofold. First, it concerns the improvement of knowledge on the fundamental physical mechanisms that control the propagation of forest fires. To proceed, an experimental apparatus was designed to study, in laboratory conditions, the flame of a fire spreading across a pine needle fuel bed. Characterization of temperature was managed by using a reconstruction method based on a double thermocouple probe technique developed recently. The vertical gas velocity distribution was derived from the previous reconstructed signals by measuring the transit time of a thermal fluctuation between two points of the flow. Second, the experimental data were used for the testing of a physical two-phase model of forest fire behavior in which the decomposition of solid fuel constituting a forest fuel bed as well as the multiple interactions with the gas phase are represented.

Emission of biogenic volatile organic compounds involved in eruptive fire: implications for the safety of firefighters

Forest fires are can be fatal for firefighters owing to the phenomenon of eruptive fire. The hypothesis of this study is that biogenic volatile organic compounds (BVOCs) accumulate in the vicinity of the fire front. One of the factors required for an eruptive fire to take place is that BVOC concentrations must be between their lower flammable limit and upper flammable limit. When this accumulation of BVOCs is exacerbated by specific geographical zones (e.g. small valleys, thalwegs, canyons), the combination of these two factors can lead to situations with a very high flammability potential, representing a considerable risk for firefighters. In France, 16 firefighters have been fatally injured over the last 15 years. This work was carried out on three species of the Mediterranean basin: Pinus laricio Poir., Pinus pinaster Ait. and Cistus monspeliensis L. The maximum BVOCs emitted as a function of temperature (50–200°C) by these species were 147.9, 11.6 and 56.0 g m–3 respectively. The quantities of BOVCs emitted by P. laricio and C. monspeliensis were sufficiently high for eruptive fires to occur.

Measurement of fluctuating temperatures in a continuous flame spreading across a fuel bed using a double thermocouple probe

Abstract Although the modeling of the spread of a forest fire has made considerable progress recently, there remains a lack of reliable measurements of such fluctuating scalar quantities as temperature for the validation of the different existing models. This led us to use double thermocouples to measure fluctuating temperatures in the continuous flame region of a fire plume. An enthalpy balance to model the temperature of a thermocouple-junction immersed in a flame was written. Then a linearized first-order model was derived and a method for identifying the model’s parameters, i.e., the time constant and gain coefficient, was provided. This model was tested with a clean, diffusion flame of ethanol to compensate for the gas temperature. These measurements agree with the model for the temperature of a thermocouple’s hot junction. Finally, the compensated gas temperature for the turbulent flame of a fire spreading across a bed of pine needles was investigated.

Volatile and semi-volatile organic compounds in smoke exposure of firefighters during prescribed burning in the Mediterranean region

Prescribed fires can be used as a forest management tool to reduce the severity of wildfires. Thus, over prolonged and repeated periods, firefighters are exposed to toxic air contaminants. This work consisted in collecting and analysing smoke released by typical Mediterranean vegetation during prescribed burning. Sampling was performed at five active zones on the island of Corsica. Seventy‐nine compounds were identified: volatile organic compounds and semi‐volatile organic compounds, including polycyclic aromatic hydrocarbons. Depending on exposure levels, the toxins present in smoke may cause short‐term or long‐term damage to firefighters’ health. The dangerous compounds emitted, benzene, toluene, ethylbenzene and xylenes, were quantified. Their concentrations varied as a function of the study site. These variations were due to the intrinsic and extrinsic characteristics of the fire site (e.g. plant species, fire intensity and wind). Our results show that benzene concentration is high during prescribed burning, close to the exposure limit value or short‐term exposure limit. Benzene can be considered as a toxicity tracer for prescribed burning because its concentration was above the exposure limit value at all the study sites. The authors suggest that respirators should be used to protect staff during prescribed burning operations.

Thermal degradation of pinus pinaster needles by DSC, part 2: kinetics of exothermic phenomena

Fire behavior prediction models required for controlling wildland fires can be calculated from a mathematical approach, taking the thermal and chemical properties of forest fuels into account. There is a need for a better understanding of the thermal decomposition of forest fuels. Two steps have been observed during the thermal degradation of Pinus pinaster needles under air sweeping. The two corresponding exothermic peaks showed by DSC curves are superimposed. An original and simple method is proposed to separate each peak from the global exotherm then the kinetic analysis is performed using single peak method (1 scanning rate) and isoconversion method (at least 3 different scanning rates).

Thermal Degradation of Pinus pinaster Needles by DSC. Part 1: Dehydration Kinetics

Fire behaviour prediction models, required for controlling wild-land fires, can be calculated from a mathematical approach, taking the thermal and chemical properties of forest fuels into account. To improve and extend applications ofthe model, we need a better understanding ofthe different phenomena involved in the thermal decomposition of forest fuels. Three steps have been observed during thermal degradation of Pinus pinaster needles: dehydration, oxidization of evolved gases and char combustion. Kinetic parameters of the first step were determined from DSC curves using various known methods which enable the determination of activation energy, pre-exponential Arrhenius factor and order of reaction.

The decomposition of hydrogen peroxide : A non-linear dynamic model

All thermal systems are subject to problems of thermal regulation. These can be understood through the use of thermochemical systems, in particular for those in the liquid phase. A dynamic linear model was earlier applied to obtain both the reaction enthalpy and the rate constant at constant temperature for the catalytic decomposition of hydrogen peroxide. This first model did not yield a good fitting between the calculated and experimental data. The hypothesis that the rate constant was independent of temperature was too strong.In the present study, a more elaborate, non-linear model was developed, which takes into account the rate constant variations as a function of temperature (Arrhenius law). This model allowed the activation energy to be determined. The calculated data then successfully fitted the experimental data. The literature indicates that the first-order rate law is not valid for a certain range of concentrations; the present model verified this.The results of dynamic modelling confirm and increase the precision of results obtained in different ways. The developed model is validated through these comparisons.

A GLOBAL KINETIC MODEL FOR THE COMBUSTION OF THE EVOLVED GASES IN WILDLAND FIRES

The analysis of combustion kinetics in the gas-phase is decisive for wild land fire behavior modeling. However, the use of detailed reaction mechanisms, which involves a large number of species and reactions, is impractical due to large computational time requirements. The present work proposes a five-step chemical kinetic mechanism to simulate the gas phase combustion processes taking place in wildland fires. Both experimental data and data from simulations run using the PSR code from the CHEMKIN-II package with a detailed kinetic mechanism (GDF-kin 3.0) have been used to calibrate and evaluate the global model under typical wild land fire conditions in terms of the inlet mixture composition, equivalence ratio, and range of temperatures.

Sampling and Quantitative Analysis of Smoke during a Fire Spreading Through a Mediterranean Scrub

This work consists in sampling and analyzing volatiles and smoke released by a typical Mediterranean vegetation during a fire. On an experimental burning plot, we used two original devices to collect volatiles and smoke. Thanks to air sampling pumps, atmosphere samples were taken, into cartridges filled with an adsorbent and into tedlar bags. The test site was instrumented with different other sensors (thermocouples, fluxmeters, anemometers, IR and visible cameras) in order to get the maximum data [1]. Analyses were performed at the laboratory by gas chromatography one day after the field experiment. Samples were thermally desorbed from the cartridges in the GC column coupled to a MS detector. We aim to characterize the risks related to the toxicity of smoke in actual conditions. Benzene, Toluene and Xylene (BTX) are highly toxic compounds that we propose to quantify in the smoke sampled during the fire. Quantification of such compounds was done with an external calibration using commercial mixtures of BTX.