Dust explosion experiments. Measurements of explosion indices of graphite dust in hydrogen-containing atmospheres [report]

A. Denkevits
2005
To address the hazard of combined hydrogen/dust explosions in ITER, the maximum overpressures and rates of pressure rise are measured using a standard method of 20-l sphere of hybrid hydrogen/graphite dust mixtures. The hydrogen concentration is varied from 4 to 18 vol. %. The tested graphite dust concentrations range from 25 to 300 g/m 3 . To ignite the mixtures, strong 10 kJ chemical igniters and electric sparks are used. When ignited by the chemical igniters, the H 2 /graphite dust/air
more » ... hite dust/air mixtures produce higher overpressures than the hydrogen/air mixtures. At low dust concentrations two separate phases of the mixture explosion process can be distinguished: an initial phase of igniter/hydrogen explosion followed by a slower dust-explosion phase. At high dust concentrations only one fast phase is observed, in which hydrogen and dust explode like a monofuel. The pressure rise rates in this case are higher that those measured for pure hydrogen/air explosions. Electric spark ignition can induce the combined hydrogen/graphite dust explosions at 10 vol. % of hydrogen and higher. The combined explosions have also two phases, if hydrogen concentration is lower 12 vol. %. At 14 vol. % and higher the hybrid mixtures explode faster than hydrogen alone. Staubexplosionsexperimente Messungen der Explosionskenngrößen von Graphitstaub in Wasserstoff-Luft Mischungen Zusammenfassung Um das Gefahrenpotential von kombinierten Wasserstoff/Staub Explosionen zu untersuchen, werden maximale Überdrücke und Druckanstiegsraten für hybride Wasserstoff/Graphitstaub Mischungen mit der Standardmethode der 20-Liter Kugel gemessen. Der Wasserstoffanteil variiert dabei zwischen 4 und 18 Volumenprozent, die Graphitstaubkonzentrationen zwischen 25 und 300 g/m 3 . Zur Zündung werden 10 kJ starke chemische Zünder sowie elektrische Funken benützt. Bei Zündung mit chemischen Zündern sind die gemessenen Überdrücke höher als für Wasserstoff/Luft Mischungen. Bei niedrigen Staubkonzentrationen kann man dabei zwei Explosionsphasen unterscheiden: eine Anfangsphase mit Zünder/Wasserstoff Explosion, gefolgt von einer langsameren Staubexplosionsphase. Dagegen wird bei hohen Staubkonzentrationen nur eine einzige Phase beobachtet, in der Wasserstoff und Staub wie ein Monobrennstoff explodieren. In diesem letzteren Fall sind die Druckanstiegsraten höher als für reine Wasserstoffexplosionen. Zündung mit elektrischen Funken kann kombinierte Wasserstoff/Graphitstaub Explosionen bei Wasserstoffkonzentrationen von 10 Volumenprozent und höher induzieren. Liegt die Wasserstoffkonzentration unter 12 Volumenprozent, so haben diese vereinten Explosionen ebenfalls zwei Phasen. Bei einem Wasserstoffanteil von 14 Volumenprozent und höher explodieren diese hybriden Mischungen schneller als Wasserstoff allein. Executive summary This work addresses the hazard of combined hydrogen/dust explosions in ITER. Two aspects of the problem are concerned: whether the ITER-relevant dusts mitigate or enhance hydrogen explosions, and the possibility of dust explosion initiation by hydrogen combustion. To investigate these issues, the explosion behaviour of fine graphite dust/hydrogen/air mixtures ignited by either strong chemical igniters or electric sparks is studied using the standard 20-l sphere method. With this aim the existing DUSTEX facility has been modified to allow tests with hydrogen. The explosion indices of four-micron graphite dust were measured in hydrogen-containing atmospheres at 1 bar initial pressure and ambient initial temperature. The dust was tested in the 25-300 g/m 3 concentration range, and the hydrogen content varied from 4 to 18 vol. %. Both strong 10 kJ chemical igniters and electric sparks were used to ignite the mixtures. The maximum overpressures generated during the explosions and maximum rate of pressure rise were measured as functions of hydrogen content and dust concentrations. Adding hydrogen to dust cloud atmospheres makes the 4-μm graphite dust explosible by 10 kJ igniters at any tested concentrations. At low concentrations from 4 to 8 vol. %, hydrogen acts like an additional igniter. The explosion process of hybrid dust/hydrogen/air mixtures is similar to the explosion of dust/air mixture. If the dust concentration is low, fast hydrogen explosion induced by the chemical igniter is followed by slower dust explosion. With increasing concentrations the dust explodes faster, and the two explosions overlap. At medium hydrogen concentrations from 8 to 12 vol. %, some part of the dust explodes together with hydrogen participating in the fast explosion phase. At higher hydrogen concentrations, from 14 to 18 vol. %, most of the dust is involved in the fast phase; dust and hydrogen explode together like a monofuel. At all tested combinations of hydrogen/dust concentrations the maximum overpressures are higher than those produced by hydrogen or dust alone. At low hydrogen concentrations maximum rates of pressure rise for the hybrid mixtures are lower than those measured for the corresponding hydrogen/air mixtures. At higher hydrogen concentrations the maximum rates of pressure rise for the hybrid mixtures are higher than those for the hydrogen/air mixtures. The hydrogen/graphite dust mixtures can be ignited by electric spark at medium and high hydrogen concentrations only. In case of 8 vol. % hydrogen, the electric spark ignites only the hydrogen constituent of the fuel mixtures; the graphite dust is not involved in the explosion process at any tested dust concentrations. In this case the dust acts as a heat-sink agent decreasing the maximum overpressure and rate of pressure rise. At medium hydrogen concentrations (10 to 12 vol. %) both hydrogen-and graphite dust-constituents can be induced to explode by electric spark. Like in case of strong ignition, the hybrid explosion has two phases. Initially the spark ignites the hydrogen which explodes like it does without dust. Then the graphite dust starts to react, if the dust concentration is high enough. The dust Lower Explosion concentration Limit is 75 g/m 3 for 10 and 10.5 vol. % hydrogen. With increasing dust concentrations the dust constituent explodes faster generating higher overpressures. For these hydrogen concentrations there are also the dust Upper Explosion concentration Limits for dust explosion development; these UELs are 150 and 175 g/m 3 for 10 and 10.5 vol. % [H 2 ], respectively. With 11 vol. % hydrogen added, the dust/air mixtures exploded from 50 to 250 g/m 3 . At low hydrogen concentrations the maximum rates of pressure rise in the hybrid mixtures are generally lower that the corresponding values of hydrogen/air mixtures. At hydrogen concentrations from 14 to 18 vol. % there is only one fast phase for the hybrid mixture explosions. The maximum rates of pressure rise are higher than those measured without dust.
doi:10.5445/ir/270061382 fatcat:w3nmoxi63zhrrfsb4e7hsexz5a