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- Selbstentzündung (5)
- Self-ignition (3)
- Konvektion (2)
- Modellierung (2)
- Numerical modelling (2)
- Brandausbreitung (1)
- Brennbarer Staub (1)
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The current paper presents experimental investigations as well as numerical simulations on the influence of water and humidity on the self-ignition of combustible bulk materials.
It is well known, that bulk materials may undergo self-ignition if stored under specific conditions. In some cases, large amounts of these materials are exposed to a humid surrounding, e.g. dried coal in a moist atmosphere. Due to the effects of condensation and adsorption of water, additional heat is generated and transported into the bulk material. If the pile is stored slightly below its self-ignition temperature, the bulk material can become supercritical and an ignition occurs.
Experiments were carried out for German lignite coal sampled in two different particle size fractions. They showed, that subcritical deposits turned to supercritical behaviour if the relative humidity in the surrounding was suddenly increased or water was poured on the surface of the sample. Besides the experiments, a numerical model was established to describe the effects of self-heating until ignition of the deposit, including the transportation of moisture. Simulations with this model led to satisfying results when compared to the experiments.
Einfluss der Konvektion auf das Selbstentzündungs- und Abbrandverhalten von Schüttgütern und Stäuben
(2005)
Selbstentzündung und Brandausbreitung von brennbaren Schüttgütern und Stäuben werden von mehreren Parametern beeinflusst. Verschiedene Einflussgrößen auf die Selbstentzündungstemperatur, wie Materialeigenschaften, Geometrie sowie Feuchtegehalt des Schüttgutes und seiner Umgebung sind bereits früher untersucht worden. Die vorliegende Arbeit stellt experimentelle Ergebnisse zum Einfluss der Konvektion an den Oberflächen von Schüttgutablagerungen auf Selbstentzündung und Brandausbreitung vor. Die Versuche wurden mit Braunkohlenstaub und Korkmehl durchgeführt. Zusätzlich wurde der Einfluss des Neigungswinkels der Schüttung (bei konstantem Volumen-Oberflächenverhältnis) untersucht. Mit Hilfe eines numerischen Modells wurde außerdem ein in der Literatur beschriebener Großversuch an Kohlehalden simuliert und bewertet. Dabei wurde der Selbstentzündungsvorgang, gefolgt von einer Brandausbreitung, berechnet.
A numerical model is presented which consists of a set of partial differential equations for the transport of heat and mass fractions of eight chemical species to describe the onset of self-ignition and the propagation of smouldering fires in deposits of bulk materials or dust accumulations. The chemical reaction sub-model includes solid fuel decomposition and the combustion of char, carbon monoxide and hydrogen.
The model has been validated against lab-scale self-ignition and smouldering propagation experiments and then applied to predictions of fire scenarios in a lignite coal silo. Predicted reaction temperatures of 550 K and propagation velocities of the smouldering front of about 6 mm/h are in good agreement with experimental values derived from lab-scale experiments.
Experiments were performed to investigate the self-ignition behaviour of accumulations of four different technical dusts at oxygen volume fractions ranging from 1.3 to 21%. For this purpose a laboratory oven used for hot storage testing was modified to allow flushing with the pre-mixed oxygen/nitrogen mixture of the desired composition. It was found that for all sample volumes investigated the self-ignition temperatures were higher the lower was the oxygen volume fraction. In addition, the type of reaction changed obviously, since the apparent activation energy significantly decreased at oxygen volume fractions below 6%. However, it was still possible to observe exothermic effects at oxygen volume fractions as low as 1.3%. A numerical model was established to simulate the process of self-ignition including the coupled heat and mass transfer within the dust accumulation using a finite element solver. The model consists of six balance equations for the heat transfer and the transport of five chemical species. It shows that the model reflects self-ignition in dust accumulations with satisfying accuracy, as long as the input data generated by preceding experiments are reliable.