Combustion Of Forest Fuels Research Articles

Suppression Characteristics of Flaming Combustion and Thermal Decomposition of Forest Fuels

An approach to the mathematical modeling of the suppression of thermal decomposition and flaming combustion of forest fuels (FFs) is proposed which differs from the well-known approaches in that the pyrolyzed FF layer is represented as a heterogeneous structure with high porosity and the thermophysical characteristics of this layer are described using additive mathematical models of thermal conductivity, heat capacity, and density. Experimental studies of the main thermophysical characteristics (thermal conductivity, heat capacity, and thermal diffusivity) of typical FFs were performed. Mathematical modeling of heat- and mass-transfer processes under the above conditions was performed, and the suppression characteristics of the destruction reaction of a typical FF were determined using the values of thermophysical characteristics established in the experiments. The ranges of the integral characteristics of FF flame suppression with varying thermophysical characteristics of FFs within permissible limits were identified.

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 mass-loss 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 mass-loss 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.

Amount of Water Sufficient to Suppress Thermal Decomposition of Forest Fuel

AbstractThis study examines how to stop the pyrolysis of fir needles, birch leaves, aspen twigs and their mixture using the minimum volumes of water. The combustion of forest fuels is suppressed by spraying water on their surface. The temperature of thermal decomposition is monitored throughout the layer of forest fuel by thermocouples. A high-speed camera and optical techniques allow us to study water spraying and its interaction with forest fuels. Finally, the study specifies the ranges of the minimum water volumes and the times of ending of the thermal decomposition of forest fuels. When analyzing the energy balance in the thermally decomposing forest fuel, a mathematical expression is formulated to predict the water volume sufficient to suppress thermal decomposition of forest fuel. This expression takes into account the ratio between the heat energy spent on water evaporation in pores of forest fuel and the heat energy of the reacting layer of forest fuel. The obtained dimensionless factor considers the main parameters of water spraying and the properties of forest fuel. This factor enables us to apply the research findings to forest fuel in various regions of the world.

Emissions of PCDD and PCDF from combustion of forest fuels and sugarcane: A comparison between field measurements and simulations in a laboratory burn facility

Release of PCDD and PCDF from biomass combustion such as forest and agricultural crop fires has been nominated as an important source for these chemicals despite minimal characterisation. Available emission factors that have been experimentally determined in laboratory and field experiments vary by several orders of magnitude from <0.5 μg TEQ (t fuel consumed) −1 to >100 μg TEQ (t fuel consumed) −1. The aim of this study was to evaluate the effect of experimental methods on the emission factor. A portable field sampler was used to measure PCDD/PCDF emissions from forest fires and the same fuel when burnt over a brick hearth to eliminate potential soil effects. A laboratory burn facility was used to sample emissions from the same fuels. There was very good agreement in emission factors to air (EF Air) for forest fuel (Duke Forest, NC) of 0.52 (range: 0.40–0.79), 0.59 (range: 0.18–1.2) and 0.75 (range: 0.27–1.2) μg TEQ WHO2005 (t fuel consumed) −1 for the in-field, over a brick hearth, and burn facility experiments, respectively. Similarly, experiments with sugarcane showed very good agreement with EF Air of 1.1 (range: 0.40–2.2), 1.5 (range: 0.84–2.2) and 1.7 (range: 0.34–4.4) μg TEQ (t fuel consumed) −1 for in-field, over a brick hearth, open field and burn facility experiments respectively. Field sampling and laboratory simulations were in good agreement, and no significant changes in emissions of PCDD/PCDF could be attributed to fuel storage and transport to laboratory test facilities.

Characterization of forest fuels in a Mass Loss Calorimeter by short open-path FTIR spectroscopy

A FTIR spectroradiometer working in a short open-path configuration has been implemented coupled to a Mass Loss Calorimeter to measure in situ and simultaneously the concentrations of the main gaseous carbon-related by-products from the combustion of forest fuels. A proper methodology to retrieve accurate values of CO and CO 2 concentrations has been developed. This methodology includes the determination of the true optical path of the exhaust gases by using an infrared camera imaging at 4.4 μm. Average factors have been calculated to correct the effect of the temperature in the retrieval procedure. The determination of temporal evolution of CO and CO 2 concentrations has allowed the characterization of the combustion dynamics of the samples. These results open the door to further extensions of the method to the measurement and characterization of forest fuels in a field-scale fire.

Experimental study of the emissivity of flames resulting from the combustion of forest fuels

Abstract Heat transfer by radiation is taken into account in most models that predict the propagation of forest fires. This heat transfer mechanism is normally formulated according to the Stefan–Boltzmann law in terms of flame temperature and flame emissivity. This study focused on flame emissivity. Experimental studies carried out to compute the emissivity of the flames generated during the combustion of forest fuels were reviewed, thereby highlighting differences in methodologies and results. Since the results of these studies with regard to the exponential relationship between flame emissivity and flame thickness were not in agreement, two methods based on IR imagery were used in the present study to calculate flame emissivity values. Nine circular fuel beds with a diameter of 0.3–2.5 m were prepared with common Mediterranean species and burned as stationary fires. An exponential correlation between flame emissivity and flame thickness was observed for both methods. According to the results of this study, only flames thicker than 3.2 m would exhibit an emissivity close to that of a blackbody (0.9), and the associated extinction coefficient would be 0.72. A long-term retardant product was used to treat the fuel of two of the nine tests that were carried out and no effect on flame emissivity was observed.

On the Interest of Studying Degradation Gases for Forest Fuel Combustion Modeling

The aim of this work is to determine the influence of the degradation gases on the combustion of forest fuels and whether they have to be taken into account in numerical modeling. A laboratory experimental apparatus was designed to generate laminar, axisymmetric, time-varying and non-premixed flames from crushed forest fuels. The experiments highlight that the mass burning rate of the fuel controls the flame dynamics whereas the combustion kinetics depends on the degradation gases. From the analysis of the degradation gases released by one fuel, different combustion mechanisms and gas mixtures were tested numerically. The reaction rates computed with these mechanisms were examined and the temperature distributions were compared with the experimental data. The predictions obtained with the combustion mechanism including both methane and carbon monoxide did agree with the experiments.

Validating a Simplified Determination of Benzo[a]pyrene in Particulate Matter from Prescribed Forestry Burning

A high performance liquid chromatographic (HPLC) method for determining the concentration of trace amounts of benzo[a]pyrene (B[a]P) in particulate matter from combustion of forest fuels was validated. Particulate matter was prepared for analysis by a small-scale (1 mg), simplified preparation followed by HPLC with fluorescence detection. The chromatographic system had a limit of detection of 2.67 ng/mL, a precision of 6.4 percent relative standard deviation, and a working range of 2.67 to 100 ng/mL. The complete method gave a mean recovery of 100.6 percent across its working range with an 8.3 percent relative standard deviation. With National Bureau of Standards' Standard Reference Material 1649 (urban particulate matter), a mean recovery of 93.2 percent of the certified B[a]P content was obtained with a 9.9 percent relative standard deviation. After storing loaded filters for 4 months under refrigeration, the B[a]P content of the particulate matter from forest fire smoke had not significantly changed.

Simulation of Growth Curves from Periodic Increment Data

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