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Fusion Engineering and Design 85 (2010) 1059–1063
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Hydrogen/dust explosion hazard in ITER: Effect of nitrogen dilution on explosion behavior of hydrogen/tungsten dust/air mixtures A. Denkevits ∗ Forschungszentrum Karlsruhe GmbH, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
a r t i c l e Keywords: ITER safety Hybrid explosions
i n f o
a b s t r a c t The work is aimed at supporting inert-gas dilution method proposed to mitigate hydrogen/dust explosion hazard in ITER in case of severe accidents. A standard method of 20-l sphere is used to study the effect of nitrogen dilution on the explosion behavior of 0.5-m tungsten dust dispersed in hydrogen-containing air atmospheres. The oxygen content varied from normal 21 to 10 vol.%. The hydrogen concentration was varied from 7 to 18 vol.%; the tungsten dust cloud density of 2 kg/m3 was chosen to test as the optimal, i.e. the most dangerous dust concentration. The tested mixtures were formed in a spherical combustion chamber of 20-l volume at normal initial conditions and ignited at its center by a weak electric spark. In general, the tested dust/hydrogen mixtures explode more dangerously than hydrogen alone: they can generate higher explosion overpressures and explode faster than the corresponding hydrogen/air mixtures without dust. For all the tested mixtures the nitrogen dilution reduces both the explosion overpressure and pressure rise rate; however, its influence is more pronounced on how fast combined explosions proceed. In case of 7 vol.% H2 , the explosion overpressure decreases from 4.5 to 3.5 bar at O2 decrease from 21 to 10 vol.%, while the pressure rise rate drops from 400 to 60 bar/s. In case of 18 vol.% of H2 , the corresponding values are 6.2–2.6 bar explosion overpressure and 1370–170 bar/s pressure rise rate. An extrapolation of the obtained results to lower oxygen concentrations gives the value of limiting oxygen concentration, at which the combined explosions are to be suppressed, about 8–9 vol.%. © 2010 Elsevier B.V. All rights reserved.
1. Introduction One of the safety problems considered in the design of ITER is the explosion hazard of the dusts which are produced during normal ITER operation as a result of plasma–wall interaction and accumulated in its vacuum vessel. The dusts of concern are fine graphite, tungsten and beryllium as these are the materials of ITER plasma facing components. In the course of a severe accident with air ingress the dust can be mobilized and form an explosible dust–air cloud. Being ignited, such clouds can explode generating pressure loads dangerous for the ITER construction elements integrity [1]. To quantify the ITER explosion hazard, explosibility of fine graphite and tungsten dusts has been tested using a standard method of 20-l sphere [2]. Explosion overpressure, pressure rise rate, ignition energy, and limiting explosion concentrations have been measured for different dust concentrations and dust particle sizes [3]. It was shown that the dusts can explode generating pressure loads about 5–7 bar; however, the ignition energy necessary
∗ Tel.: +49 7247 828140; fax: +49 7247 824777. E-mail addresses: [email protected], [email protected]. 0920-3796/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2010.01.009
to explode the dusts was several kJ which is rather high for typical accident scenarios in ITER. However, another ignition source seems more realistic. LOCA or LOVA scenarios involve hydrogen presenting in the accidental atmospheres [1]. Hydrogen can be easily ignited by a weak ignition source like an electric spark, and the energy delivered as a result of the hydrogen combustion might be enough to ignite the dust. Such a possibility was investigated in [4]. The explosibility of hydrogen–graphite dust mixtures was studied by use of 20-l method. Fine 4-m graphite dust was dispersed in air containing hydrogen at 8–18 vol.%. The mixtures were ignited by a weak electric spark. The hybrid mixtures exploded, in general, faster and generating higher explosion overpressures than the corresponding hydrogen–air mixtures. The aim of the presented work is to support inertization methods of explosion hazard mitigation in ITER by diluting accidental atmospheres with an inert gas. If the oxygen content is below some level (so called ‘Limiting Oxygen Concentration’), any explosions of flammable substance, mobilized dust or hydrogen, are suppressed. Even being over the threshold, reduced oxygen content can result in significant reduction of the explosion severity [5]. The standard method of 20-l sphere has been used to study the explosibility of <0.5 m tungsten dust dispersed in hydrogen-containing air atmospheres. The mixtures are ignited by a weak electric spark. The
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Fig. 1. Explosion overpressures (a) and pressure rise rates (b) of 0.5-m tungsten dust/hydrogen/air mixtures versus dust cloud concentration.
influence of nitrogen dilution of the pre-explosion atmospheres on the explosion indices is studied. 2. Experimental The tests are performed using a standard method of 20-l sphere [6]. In this method a dust–air cloud is formed in a spherical combustion chamber and ignited at the sphere center. The explosibility of the dust is characterized by some parameters extracted from the pressure–time curve which is recorded during the dust explosion. To create a cloud, a dust sample is injected into the sphere from a vessel of 0.6 l volume connected with the combustion chamber of 20 l volume by a dust outlet valve. Prior to test, the container is pressurized with a compressed air to 21 bar. When the dust outlet valve is open, the dust sample is injected into the chamber with a portion of compressed air. Taking into account this portion, the sphere is pre-evacuated to 0.4 bar to get 1 bar pressure after the completion of the dispersion process. The dispersion lasts about 90 ms and produces well-mixed dust cloud in highly turbulent atmosphere. The turbulence starts to decay 10 ms later [7], and at this moment the mixture inside the sphere is ignited. Normally strong chemical igniters are used to initiate dust cloud explosions (standard value for ignition energy is 10 kJ). In presence of hydrogen it is possible to explode the dust via hydrogen explosion initiated by a weak electric spark [4]. In described tests the mixtures were ignited this way. The spark was discharged through 2 stainless-steel electrodes installed at the sphere center. The spark voltage was about 5 kV, the inter-electrode distance was 2 mm; the spark duration was 40 ms. To dilute the pre-explosion atmosphere with nitrogen, compressed air and compressed nitrogen were mixed in the 0.6-l vessel to get the desired oxygen content. This mixture was used to disperse dust. Because of air presenting in the vessel when the dust was placed there, the minimum initial oxygen content that could be provided in the pre-explosion atmospheres was about 10 vol.%. Hydrogen was added directly to the combustion chamber. First the chamber was evacuated to less than 1 mbar. Then a desired amount of hydrogen was let inside it. After that the ambient air was let into the chamber till the pressure reached 400 mbar. The final test mixture was formed after a dust had been injected inside the chamber with a portion of nitrogen-diluted compressed air from the dust vessel. All the tests were performed at 1 bar initial pressure and room temperature. The values of hydrogen content reported below are its volumetric fractions of the gaseous constituents and calculated as [H2 ]/([H2 ] + [O2 ] + [N2 ]). The values of oxygen content
are oxygen volumetric fraction of N2 -diluted air and calculated as [O2 ] = [O2 ]/([O2 ] + [N2 ]). The facility was equipped with a Pfeiffer quadrupole massspectrometer. It was used to monitor qualitatively the content of the combustion products and trace the changes of the product concentrations. The tested tungsten dust was purchased at the market. The sizes of the dust particles range between 0.2 and 0.55 m, most of the particles are about 0.3 m. 3. Results Following the standard [2], explosibility of a dust is characterized by so called ‘explosion indices’, namely, explosion overpressures and pressure rise rate generated in the course of the dust explosion in a closed vessel. These parameters are derived from the pressure–time curve recorded during a dust explosion; the derivation takes into account the heat losses at the chamber wall and influence of the igniter. Statistical deviation of maximum overpressures is less than 10% and of pressure rise rates is less than 30% at dP/dt <190 bar/s and than 20% if dP/dt <370 bar/s [8]. The presented study starts with the mixtures of the tungsten dust and hydrogen in non-diluted air. The hydrogen concentration was changed from 7 to 18 vol.%; the dust concentration varied from 100 g/m3 (about four times lower than the lower explosion concentration limit) to 7500 g/m3 (about six times higher than stoichiometric concentration of tungsten oxidation reaction). The results are presented in Fig. 1 where (a) the explosion overpressures and (b) explosion pressure rise rates are plotted versus the dust concentrations. The results obtained for four hydrogen concentrations are shown. At each hydrogen concentration the peak explosion overpressure rises with dust concentration, reaches its maximum and then decreases slightly. At the rising part of the curves the fuel concentrations are low; the mixtures have enough oxygen to burn out all the fuel, so here the fuel concentration is limiting the energy release and, hence, the peak pressure. With more fuel the oxygen content in the products goes down, and reaches zero. Here the peak pressure is maximum. This fuel concentration is called ‘optimal’. Note that optimal concentration differs from the corresponding stoichiometric concentration if dust combustion is concerned. The reason is that it is a rare dust explosion at which all the dust burns out completely; usually there is an unburnt residue after the combustion process is completed [5]. Normally the optimal concentration is two to four times higher than the stoichiometric. Further increase in fuel concentration results in decreasing
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Fig. 2. Explosion overpressures (a) and pressure rise rates (b) of 0.5-m tungsten dust/hydrogen/air mixtures (solid squares) and pure hydrogen/air mixtures (open circles) diluted with nitrogen versus initial oxygen content. Hydrogen concentration is 7 vol.%.
Fig. 3. Explosion overpressures (a) and pressure rise rates (b) of 0.5-m tungsten dust/hydrogen/air mixtures (solid squares) and pure hydrogen/air mixtures (open circles) diluted with nitrogen versus initial oxygen content. Hydrogen concentration is 10 vol.%.
explosion peak pressure because the excessive dust acts now as a heat sink reducing the product temperature and hence the peak pressure. At each hydrogen concentration the input from tungsten dust to the hybrid explosion severity is prominent. The most substantial is
at 7% H2 . Here the dust input to pressure loads is two times higher than that of hydrogen: 1.6 bar from H2 against 4.6 bar from H2 + W. The pressure rise rate of the hybrid explosion is 300 bar/s while the pure hydrogen explosion gives 40 bar/s. Actually the hybrid H2 + W explosion with 7% H2 can be considered as a pure dust explosion
Fig. 4. Explosion overpressures (a) and pressure rise rates (b) of 0.5-m tungsten dust/hydrogen/air mixtures (solid squares) and pure hydrogen/air mixtures (open circles) diluted with nitrogen versus initial oxygen content. Hydrogen concentration is 14 vol.%.
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Fig. 5. Explosion overpressures (a) and pressure rise rates (b) of 0.5-m tungsten dust/hydrogen/air mixtures (solid squares) and pure hydrogen/air mixtures (open circles) diluted with nitrogen versus initial oxygen content. Hydrogen concentration is 18 vol.%.
initiated by a weak hydrogen explosion triggered by a weak electric spark. With higher hydrogen concentrations the input from dust decreases because the oxygen amount available for the tungsten dust oxidation decreases. Nevertheless, even at the maximum tested hydrogen concentration of 18% the dust input is still noticeable: pressure peak rises from 5.1 bar without dust to 6.2 bar with dust and pressure rise rate from 1090 to 1350 bar/s, respectively. Mass-spectrometry of the combustion products showed no oxygen starting from the dust concentration of 2000 g/m3 in the test series with hydrogen concentration varied from 7 to 14%. At [H2 ] = 16% there was no oxygen already at 1000 g/m3 and at [H2 ] = 18%, at 500 g/m3 . Some hydrogen remained in the combustion products even in the test series with 10% H2 and higher. The higher the dust concentration, the more hydrogen is found in the combustion products. In the test with 18% H2 and 6000 g/m3 of dust the amount of unburnt hydrogen was estimated as approximately half of the initial hydrogen content. The test results on studying the influence of initial oxygen content on the explosion behavior of the tested mixtures are presented in Figs. 2–5 where the explosion overpressures and pressure rise rates are plotted versus initial oxygen concentration for four hydrogen concentrations. For reference, the results obtained with pure hydrogen–air mixtures diluted with nitrogen are also given. Hybrid mixtures are represented by solid squares, pure hydrogen ones—by open circles. All the tests have been performed at the same dust concentration of 2000 g/m3 . This is the concentration at which most of the hybrid mixtures produce maximum explosion overpressure and which is most close to the corresponding optimal concentrations. In general, nitrogen dilution reduces both explosion overpressure and pressure rise rate of the hybrid mixture. In the test series with 7% H2 the explosion overpressure remains constant down to [O2 ]init = 16.5%; there is some oxygen remaining in the combustion products. At 15.2% [O2 ]init the oxygen is fully burnt out during the hybrid mixture explosion, and starting from this point the explosion overpressure decreases slightly, from 4.4 to 3.6 bar at 10% [O2 ]init . The pressure rise rate decreases monotonically starting from the beginning of the dilution; its decrease is more prominent than that of the overpressure: from 400 bar/s at normal air to 50 at 10% [O2 ]init . It is still a bit higher than the corresponding value obtained in the test with pure oxygen. In the test series with 10% H2 the decrease in the explosion overpressure is more distinct, from 5.5 bar at normal air to 3.8 bar at 10% [O2 ]init . The pressure rise rate of the hybrid mixtures also decreases with initial oxygen content, but not so sharp as in the previous test
series, from 770 to 240 bar/s. However, for each tested dilution the severity of the hybrid mixture explosions is higher than that of the pure hydrogen. In this test series there is no resting oxygen in the combustion products; i.e. even in the test at normal air the mixture is oxygenlimited. Starting from 15.2 vol.% [O2 ]init down, hydrogen presents in the combustion products. Its content increases with decreasing initial oxygen concentration. Both the explosion overpressure and pressure rise decreases due to nitrogen dilution are deeper in the test series with 14% H2 : from 5.9 to 3.8 bar and from 1150 to 310 bar/s in the tested range. Notice that at 10% [O2 ]init the explosion indices of a hybrid mixture are lower than those of the corresponding pure hydrogen–air mixture. There is also no oxygen in the combustion products in any test of this series. Starting from 17. vol.% [O2 ]init down, hydrogen presents in the combustion products. The mitigation effect of the nitrogen dilution is most pronounced in the tests with 18% H2 . The explosion pressure drops from 6.2 to 2.6 bar and the pressure rise rate from 1370 to 170 bar/s at normal air and 10% initial oxygen, respectively. Both the explosion overpressure and pressure rise rate generated by the hybrid mixture are lower than those from the explosion of the pure hydrogen–air mixture below 14% of initial oxygen. In all the tests of this series hydrogen is found in the combustion products. In the test with [O2 ]init = 10% more than a half of the initial hydrogen does not react. Notice that in this test some amount of oxygen remains in the combustion products which means that both hydrogen and dust explosions are partly suppressed. 4. Discussion and conclusions Following the method of 20-l sphere, severity of dust explosion is characterized by explosion overpressure and pressure rise rate. Maximum explosion overpressure indicates how much chemical energy can be delivered in the course of the fuel oxidation. It shows the maximum value of pressure loads which can be expected in slow combustion regimes where flame velocity is much below sonic speed. Real pressure loads are normally lower because of heat losses. More dangerous are regimes of fast combustion, where the flame velocity is comparable to the speed of sound. In this case the flame propagation generates fast pressure waves which intensify the combustion and, hence, result in much higher pressure loads. In the worst case a fast deflagration regime can transit to detonation where pressure peaks are an order of magnitude higher. The potential of the fuel to deflagrate fast is characterized in this method by
A. Denkevits / Fusion Engineering and Design 85 (2010) 1059–1063
Fig. 6. Evaluation of limiting oxygen concentrations of 0.5-m tungsten dust/hydrogen/air mixtures by extrapolating their explosion pressure rise rates to zero values. The straight lines are linear extrapolations of the dependences [O2 ]init (dP/dt). The free terms in the extrapolation equations are extrapolated values of LOC at dP/dt = 0.
the maximum rate of pressure rise (it is this parameter which is used to classify a dust as explosive). The test results show that the tungsten dust/hydrogen mixtures explode more dangerously than hydrogen alone. The hybrid mixtures generate higher explosion overpressures and explode faster than the corresponding mixtures of pure hydrogen with the same concentration. The maximum of the peak pressures obtained at each hydrogen concentration shifts from 4000 g/m3 at [H2 ] = 7% to 1000 g/m3 at [H2 ] = 18% (see Fig. 1a). The fact that the higher the hydrogen concentration, the less dust is needed to complement to the optimal fuel concentration, can indicate that hydrogen oxidation dominates the dust oxidation. Such a case was observed in similar tests with graphite dust [4]. The graphite dust–hydrogen–air mixtures exploded in two steps at low hydrogen concentrations (10–12%): first hydrogen exploded fast and after that the graphite dust started to react much slower. It was clearly seen from the shape of the pressure–time curves which had two distinguished rising parts. At higher hydrogen concentrations of 16% the C–H2 –air mixtures exploded in one step because the energy released at the earlier stage of hydrogen explosion, when not all the hydrogen is burnt out, was enough to ignite the dust. However, the pressure–time curves recorded in these tests with W–H2 –air show that all the mixtures explode in one step. The tested tungsten dust starts to react when not all the hydrogen is burnt out even at 7% hydrogen. It means that fine tungsten dusts are more sensitive and can be easier ignited by hydrogen explosion than fine graphite dusts. Nitrogen dilution of the pre-explosion atmospheres mitigates the explosion severity of the tested mixtures of hydrogen (with concentration from 7 to 18%), 0.5-m tungsten dust (cloud con-
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centration 2000 g/m3 ) and air. Both explosion overpressure and pressure rise rates are reduced. The air dilution to 10% oxygen results in 1.3–1.6 times reduction in explosion overpressure and 4–8 times reduction in pressure rise rate. Less pronounced effect of the dilution on the explosion pressure is due to the fact that even at [O2 ]init = 10% it is still enough oxygen to burn out most part of the fuel. However, the pressure rise rates drop drastically. It can indicate a corresponding decrease in the flame propagation velocities, and this effect is very important: at low flame velocity the fast deflagration regimes are practically excluded. Then, the pressure rise rate in confined geometry is proportional to the flame propagation speed in free volumes. Decrease in the flame propagation velocity can result in the flame propagation quenching in larger volumes. To estimate the necessary dilution, one can extrapolate the dependence of pressure rise rate on the initial oxygen content to lower concentrations. Fig. 6 presents linear extrapolations of the reciprocal dependences of pressure rise rates on oxygen content. The legend comprises the extrapolation equations, where the free terms are the oxygen concentrations at which the pressure rise rates are zero. The estimated limiting oxygen concentrations range between 6 and 9%. However, this estimation could also be too conservative, while even if the flame propagation velocity is far from zero, the flame can quench at some larger distance. To investigate this possibility, it is necessary to measure the flame propagation velocities as function of initial oxygen concentration. Such results would also be of use in modeling the explosion scenarios in the inert atmospheres. Acknowledgments This work, supported by the European Communities under the contract of Association between EURATOM and Forschungszentrum Karlsruhe, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] Generic Site Safety Report, Vol. III, Source terms, ITER Garching JWS, 2001. [2] VDI-Richtlinien, Staubbrände und Staubexplosionen Gefahren-BeurteilungSchutzmaßnahmen, Beuth Verlag GmbH, Berlin, 1990. [3] A. Denkevits, S. Dorofeev, Dust explosion hazard in ITER: explosion indices of fine graphite and tungsten dusts and their mixtures, Fusion Engineering and Design 75–79 (2005) 1135–1139. [4] A. Denkevits, Explosibility of hydrogen–graphite dust hybrid mixtures, Journal of Loss Prevention in the Process Industries 20 (2007) 698–707. [5] R.K. Eckhoff, Dust Explosions in the Process Industries, Elsevier Science, Amsterdam, 2003. [6] R. Siwek, Development of a 20 l Laboratory Apparatus and its Application for the Investigation of Combustible Dust, Ciba-Geigy AG, Basel, Switzerland, 1985. [7] A.E. Dahoe, R.S. Cant, M.J. Pegg, B. Scarlett, On the transient flow in the 20-liter explosion sphere, Journal of Loss Prevention in the Process Industries 14 (2001) 475–487. [8] R. Siwek, Operating Instructions 20-l Apparatus, Adolf Kühner AG, Switzerland, http://www.kuhner.com/DOCUMENT/KSEP E.pdf.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Hydrogen/dust explosion hazard in ITER: Effect of nitrogen dilution on explosion behavior of hydrogen/tungsten dust/air mixtures A. Denkevits ∗ Forschungszentrum Karlsruhe GmbH, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
a r t i c l e Keywords: ITER safety Hybrid explosions
i n f o
a b s t r a c t The work is aimed at supporting inert-gas dilution method proposed to mitigate hydrogen/dust explosion hazard in ITER in case of severe accidents. A standard method of 20-l sphere is used to study the effect of nitrogen dilution on the explosion behavior of 0.5-m tungsten dust dispersed in hydrogen-containing air atmospheres. The oxygen content varied from normal 21 to 10 vol.%. The hydrogen concentration was varied from 7 to 18 vol.%; the tungsten dust cloud density of 2 kg/m3 was chosen to test as the optimal, i.e. the most dangerous dust concentration. The tested mixtures were formed in a spherical combustion chamber of 20-l volume at normal initial conditions and ignited at its center by a weak electric spark. In general, the tested dust/hydrogen mixtures explode more dangerously than hydrogen alone: they can generate higher explosion overpressures and explode faster than the corresponding hydrogen/air mixtures without dust. For all the tested mixtures the nitrogen dilution reduces both the explosion overpressure and pressure rise rate; however, its influence is more pronounced on how fast combined explosions proceed. In case of 7 vol.% H2 , the explosion overpressure decreases from 4.5 to 3.5 bar at O2 decrease from 21 to 10 vol.%, while the pressure rise rate drops from 400 to 60 bar/s. In case of 18 vol.% of H2 , the corresponding values are 6.2–2.6 bar explosion overpressure and 1370–170 bar/s pressure rise rate. An extrapolation of the obtained results to lower oxygen concentrations gives the value of limiting oxygen concentration, at which the combined explosions are to be suppressed, about 8–9 vol.%. © 2010 Elsevier B.V. All rights reserved.
1. Introduction One of the safety problems considered in the design of ITER is the explosion hazard of the dusts which are produced during normal ITER operation as a result of plasma–wall interaction and accumulated in its vacuum vessel. The dusts of concern are fine graphite, tungsten and beryllium as these are the materials of ITER plasma facing components. In the course of a severe accident with air ingress the dust can be mobilized and form an explosible dust–air cloud. Being ignited, such clouds can explode generating pressure loads dangerous for the ITER construction elements integrity [1]. To quantify the ITER explosion hazard, explosibility of fine graphite and tungsten dusts has been tested using a standard method of 20-l sphere [2]. Explosion overpressure, pressure rise rate, ignition energy, and limiting explosion concentrations have been measured for different dust concentrations and dust particle sizes [3]. It was shown that the dusts can explode generating pressure loads about 5–7 bar; however, the ignition energy necessary
∗ Tel.: +49 7247 828140; fax: +49 7247 824777. E-mail addresses: [email protected], [email protected]. 0920-3796/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2010.01.009
to explode the dusts was several kJ which is rather high for typical accident scenarios in ITER. However, another ignition source seems more realistic. LOCA or LOVA scenarios involve hydrogen presenting in the accidental atmospheres [1]. Hydrogen can be easily ignited by a weak ignition source like an electric spark, and the energy delivered as a result of the hydrogen combustion might be enough to ignite the dust. Such a possibility was investigated in [4]. The explosibility of hydrogen–graphite dust mixtures was studied by use of 20-l method. Fine 4-m graphite dust was dispersed in air containing hydrogen at 8–18 vol.%. The mixtures were ignited by a weak electric spark. The hybrid mixtures exploded, in general, faster and generating higher explosion overpressures than the corresponding hydrogen–air mixtures. The aim of the presented work is to support inertization methods of explosion hazard mitigation in ITER by diluting accidental atmospheres with an inert gas. If the oxygen content is below some level (so called ‘Limiting Oxygen Concentration’), any explosions of flammable substance, mobilized dust or hydrogen, are suppressed. Even being over the threshold, reduced oxygen content can result in significant reduction of the explosion severity [5]. The standard method of 20-l sphere has been used to study the explosibility of <0.5 m tungsten dust dispersed in hydrogen-containing air atmospheres. The mixtures are ignited by a weak electric spark. The
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Fig. 1. Explosion overpressures (a) and pressure rise rates (b) of 0.5-m tungsten dust/hydrogen/air mixtures versus dust cloud concentration.
influence of nitrogen dilution of the pre-explosion atmospheres on the explosion indices is studied. 2. Experimental The tests are performed using a standard method of 20-l sphere [6]. In this method a dust–air cloud is formed in a spherical combustion chamber and ignited at the sphere center. The explosibility of the dust is characterized by some parameters extracted from the pressure–time curve which is recorded during the dust explosion. To create a cloud, a dust sample is injected into the sphere from a vessel of 0.6 l volume connected with the combustion chamber of 20 l volume by a dust outlet valve. Prior to test, the container is pressurized with a compressed air to 21 bar. When the dust outlet valve is open, the dust sample is injected into the chamber with a portion of compressed air. Taking into account this portion, the sphere is pre-evacuated to 0.4 bar to get 1 bar pressure after the completion of the dispersion process. The dispersion lasts about 90 ms and produces well-mixed dust cloud in highly turbulent atmosphere. The turbulence starts to decay 10 ms later [7], and at this moment the mixture inside the sphere is ignited. Normally strong chemical igniters are used to initiate dust cloud explosions (standard value for ignition energy is 10 kJ). In presence of hydrogen it is possible to explode the dust via hydrogen explosion initiated by a weak electric spark [4]. In described tests the mixtures were ignited this way. The spark was discharged through 2 stainless-steel electrodes installed at the sphere center. The spark voltage was about 5 kV, the inter-electrode distance was 2 mm; the spark duration was 40 ms. To dilute the pre-explosion atmosphere with nitrogen, compressed air and compressed nitrogen were mixed in the 0.6-l vessel to get the desired oxygen content. This mixture was used to disperse dust. Because of air presenting in the vessel when the dust was placed there, the minimum initial oxygen content that could be provided in the pre-explosion atmospheres was about 10 vol.%. Hydrogen was added directly to the combustion chamber. First the chamber was evacuated to less than 1 mbar. Then a desired amount of hydrogen was let inside it. After that the ambient air was let into the chamber till the pressure reached 400 mbar. The final test mixture was formed after a dust had been injected inside the chamber with a portion of nitrogen-diluted compressed air from the dust vessel. All the tests were performed at 1 bar initial pressure and room temperature. The values of hydrogen content reported below are its volumetric fractions of the gaseous constituents and calculated as [H2 ]/([H2 ] + [O2 ] + [N2 ]). The values of oxygen content
are oxygen volumetric fraction of N2 -diluted air and calculated as [O2 ] = [O2 ]/([O2 ] + [N2 ]). The facility was equipped with a Pfeiffer quadrupole massspectrometer. It was used to monitor qualitatively the content of the combustion products and trace the changes of the product concentrations. The tested tungsten dust was purchased at the market. The sizes of the dust particles range between 0.2 and 0.55 m, most of the particles are about 0.3 m. 3. Results Following the standard [2], explosibility of a dust is characterized by so called ‘explosion indices’, namely, explosion overpressures and pressure rise rate generated in the course of the dust explosion in a closed vessel. These parameters are derived from the pressure–time curve recorded during a dust explosion; the derivation takes into account the heat losses at the chamber wall and influence of the igniter. Statistical deviation of maximum overpressures is less than 10% and of pressure rise rates is less than 30% at dP/dt <190 bar/s and than 20% if dP/dt <370 bar/s [8]. The presented study starts with the mixtures of the tungsten dust and hydrogen in non-diluted air. The hydrogen concentration was changed from 7 to 18 vol.%; the dust concentration varied from 100 g/m3 (about four times lower than the lower explosion concentration limit) to 7500 g/m3 (about six times higher than stoichiometric concentration of tungsten oxidation reaction). The results are presented in Fig. 1 where (a) the explosion overpressures and (b) explosion pressure rise rates are plotted versus the dust concentrations. The results obtained for four hydrogen concentrations are shown. At each hydrogen concentration the peak explosion overpressure rises with dust concentration, reaches its maximum and then decreases slightly. At the rising part of the curves the fuel concentrations are low; the mixtures have enough oxygen to burn out all the fuel, so here the fuel concentration is limiting the energy release and, hence, the peak pressure. With more fuel the oxygen content in the products goes down, and reaches zero. Here the peak pressure is maximum. This fuel concentration is called ‘optimal’. Note that optimal concentration differs from the corresponding stoichiometric concentration if dust combustion is concerned. The reason is that it is a rare dust explosion at which all the dust burns out completely; usually there is an unburnt residue after the combustion process is completed [5]. Normally the optimal concentration is two to four times higher than the stoichiometric. Further increase in fuel concentration results in decreasing
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Fig. 2. Explosion overpressures (a) and pressure rise rates (b) of 0.5-m tungsten dust/hydrogen/air mixtures (solid squares) and pure hydrogen/air mixtures (open circles) diluted with nitrogen versus initial oxygen content. Hydrogen concentration is 7 vol.%.
Fig. 3. Explosion overpressures (a) and pressure rise rates (b) of 0.5-m tungsten dust/hydrogen/air mixtures (solid squares) and pure hydrogen/air mixtures (open circles) diluted with nitrogen versus initial oxygen content. Hydrogen concentration is 10 vol.%.
explosion peak pressure because the excessive dust acts now as a heat sink reducing the product temperature and hence the peak pressure. At each hydrogen concentration the input from tungsten dust to the hybrid explosion severity is prominent. The most substantial is
at 7% H2 . Here the dust input to pressure loads is two times higher than that of hydrogen: 1.6 bar from H2 against 4.6 bar from H2 + W. The pressure rise rate of the hybrid explosion is 300 bar/s while the pure hydrogen explosion gives 40 bar/s. Actually the hybrid H2 + W explosion with 7% H2 can be considered as a pure dust explosion
Fig. 4. Explosion overpressures (a) and pressure rise rates (b) of 0.5-m tungsten dust/hydrogen/air mixtures (solid squares) and pure hydrogen/air mixtures (open circles) diluted with nitrogen versus initial oxygen content. Hydrogen concentration is 14 vol.%.
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Fig. 5. Explosion overpressures (a) and pressure rise rates (b) of 0.5-m tungsten dust/hydrogen/air mixtures (solid squares) and pure hydrogen/air mixtures (open circles) diluted with nitrogen versus initial oxygen content. Hydrogen concentration is 18 vol.%.
initiated by a weak hydrogen explosion triggered by a weak electric spark. With higher hydrogen concentrations the input from dust decreases because the oxygen amount available for the tungsten dust oxidation decreases. Nevertheless, even at the maximum tested hydrogen concentration of 18% the dust input is still noticeable: pressure peak rises from 5.1 bar without dust to 6.2 bar with dust and pressure rise rate from 1090 to 1350 bar/s, respectively. Mass-spectrometry of the combustion products showed no oxygen starting from the dust concentration of 2000 g/m3 in the test series with hydrogen concentration varied from 7 to 14%. At [H2 ] = 16% there was no oxygen already at 1000 g/m3 and at [H2 ] = 18%, at 500 g/m3 . Some hydrogen remained in the combustion products even in the test series with 10% H2 and higher. The higher the dust concentration, the more hydrogen is found in the combustion products. In the test with 18% H2 and 6000 g/m3 of dust the amount of unburnt hydrogen was estimated as approximately half of the initial hydrogen content. The test results on studying the influence of initial oxygen content on the explosion behavior of the tested mixtures are presented in Figs. 2–5 where the explosion overpressures and pressure rise rates are plotted versus initial oxygen concentration for four hydrogen concentrations. For reference, the results obtained with pure hydrogen–air mixtures diluted with nitrogen are also given. Hybrid mixtures are represented by solid squares, pure hydrogen ones—by open circles. All the tests have been performed at the same dust concentration of 2000 g/m3 . This is the concentration at which most of the hybrid mixtures produce maximum explosion overpressure and which is most close to the corresponding optimal concentrations. In general, nitrogen dilution reduces both explosion overpressure and pressure rise rate of the hybrid mixture. In the test series with 7% H2 the explosion overpressure remains constant down to [O2 ]init = 16.5%; there is some oxygen remaining in the combustion products. At 15.2% [O2 ]init the oxygen is fully burnt out during the hybrid mixture explosion, and starting from this point the explosion overpressure decreases slightly, from 4.4 to 3.6 bar at 10% [O2 ]init . The pressure rise rate decreases monotonically starting from the beginning of the dilution; its decrease is more prominent than that of the overpressure: from 400 bar/s at normal air to 50 at 10% [O2 ]init . It is still a bit higher than the corresponding value obtained in the test with pure oxygen. In the test series with 10% H2 the decrease in the explosion overpressure is more distinct, from 5.5 bar at normal air to 3.8 bar at 10% [O2 ]init . The pressure rise rate of the hybrid mixtures also decreases with initial oxygen content, but not so sharp as in the previous test
series, from 770 to 240 bar/s. However, for each tested dilution the severity of the hybrid mixture explosions is higher than that of the pure hydrogen. In this test series there is no resting oxygen in the combustion products; i.e. even in the test at normal air the mixture is oxygenlimited. Starting from 15.2 vol.% [O2 ]init down, hydrogen presents in the combustion products. Its content increases with decreasing initial oxygen concentration. Both the explosion overpressure and pressure rise decreases due to nitrogen dilution are deeper in the test series with 14% H2 : from 5.9 to 3.8 bar and from 1150 to 310 bar/s in the tested range. Notice that at 10% [O2 ]init the explosion indices of a hybrid mixture are lower than those of the corresponding pure hydrogen–air mixture. There is also no oxygen in the combustion products in any test of this series. Starting from 17. vol.% [O2 ]init down, hydrogen presents in the combustion products. The mitigation effect of the nitrogen dilution is most pronounced in the tests with 18% H2 . The explosion pressure drops from 6.2 to 2.6 bar and the pressure rise rate from 1370 to 170 bar/s at normal air and 10% initial oxygen, respectively. Both the explosion overpressure and pressure rise rate generated by the hybrid mixture are lower than those from the explosion of the pure hydrogen–air mixture below 14% of initial oxygen. In all the tests of this series hydrogen is found in the combustion products. In the test with [O2 ]init = 10% more than a half of the initial hydrogen does not react. Notice that in this test some amount of oxygen remains in the combustion products which means that both hydrogen and dust explosions are partly suppressed. 4. Discussion and conclusions Following the method of 20-l sphere, severity of dust explosion is characterized by explosion overpressure and pressure rise rate. Maximum explosion overpressure indicates how much chemical energy can be delivered in the course of the fuel oxidation. It shows the maximum value of pressure loads which can be expected in slow combustion regimes where flame velocity is much below sonic speed. Real pressure loads are normally lower because of heat losses. More dangerous are regimes of fast combustion, where the flame velocity is comparable to the speed of sound. In this case the flame propagation generates fast pressure waves which intensify the combustion and, hence, result in much higher pressure loads. In the worst case a fast deflagration regime can transit to detonation where pressure peaks are an order of magnitude higher. The potential of the fuel to deflagrate fast is characterized in this method by
A. Denkevits / Fusion Engineering and Design 85 (2010) 1059–1063
Fig. 6. Evaluation of limiting oxygen concentrations of 0.5-m tungsten dust/hydrogen/air mixtures by extrapolating their explosion pressure rise rates to zero values. The straight lines are linear extrapolations of the dependences [O2 ]init (dP/dt). The free terms in the extrapolation equations are extrapolated values of LOC at dP/dt = 0.
the maximum rate of pressure rise (it is this parameter which is used to classify a dust as explosive). The test results show that the tungsten dust/hydrogen mixtures explode more dangerously than hydrogen alone. The hybrid mixtures generate higher explosion overpressures and explode faster than the corresponding mixtures of pure hydrogen with the same concentration. The maximum of the peak pressures obtained at each hydrogen concentration shifts from 4000 g/m3 at [H2 ] = 7% to 1000 g/m3 at [H2 ] = 18% (see Fig. 1a). The fact that the higher the hydrogen concentration, the less dust is needed to complement to the optimal fuel concentration, can indicate that hydrogen oxidation dominates the dust oxidation. Such a case was observed in similar tests with graphite dust [4]. The graphite dust–hydrogen–air mixtures exploded in two steps at low hydrogen concentrations (10–12%): first hydrogen exploded fast and after that the graphite dust started to react much slower. It was clearly seen from the shape of the pressure–time curves which had two distinguished rising parts. At higher hydrogen concentrations of 16% the C–H2 –air mixtures exploded in one step because the energy released at the earlier stage of hydrogen explosion, when not all the hydrogen is burnt out, was enough to ignite the dust. However, the pressure–time curves recorded in these tests with W–H2 –air show that all the mixtures explode in one step. The tested tungsten dust starts to react when not all the hydrogen is burnt out even at 7% hydrogen. It means that fine tungsten dusts are more sensitive and can be easier ignited by hydrogen explosion than fine graphite dusts. Nitrogen dilution of the pre-explosion atmospheres mitigates the explosion severity of the tested mixtures of hydrogen (with concentration from 7 to 18%), 0.5-m tungsten dust (cloud con-
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centration 2000 g/m3 ) and air. Both explosion overpressure and pressure rise rates are reduced. The air dilution to 10% oxygen results in 1.3–1.6 times reduction in explosion overpressure and 4–8 times reduction in pressure rise rate. Less pronounced effect of the dilution on the explosion pressure is due to the fact that even at [O2 ]init = 10% it is still enough oxygen to burn out most part of the fuel. However, the pressure rise rates drop drastically. It can indicate a corresponding decrease in the flame propagation velocities, and this effect is very important: at low flame velocity the fast deflagration regimes are practically excluded. Then, the pressure rise rate in confined geometry is proportional to the flame propagation speed in free volumes. Decrease in the flame propagation velocity can result in the flame propagation quenching in larger volumes. To estimate the necessary dilution, one can extrapolate the dependence of pressure rise rate on the initial oxygen content to lower concentrations. Fig. 6 presents linear extrapolations of the reciprocal dependences of pressure rise rates on oxygen content. The legend comprises the extrapolation equations, where the free terms are the oxygen concentrations at which the pressure rise rates are zero. The estimated limiting oxygen concentrations range between 6 and 9%. However, this estimation could also be too conservative, while even if the flame propagation velocity is far from zero, the flame can quench at some larger distance. To investigate this possibility, it is necessary to measure the flame propagation velocities as function of initial oxygen concentration. Such results would also be of use in modeling the explosion scenarios in the inert atmospheres. Acknowledgments This work, supported by the European Communities under the contract of Association between EURATOM and Forschungszentrum Karlsruhe, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] Generic Site Safety Report, Vol. III, Source terms, ITER Garching JWS, 2001. [2] VDI-Richtlinien, Staubbrände und Staubexplosionen Gefahren-BeurteilungSchutzmaßnahmen, Beuth Verlag GmbH, Berlin, 1990. [3] A. Denkevits, S. Dorofeev, Dust explosion hazard in ITER: explosion indices of fine graphite and tungsten dusts and their mixtures, Fusion Engineering and Design 75–79 (2005) 1135–1139. [4] A. Denkevits, Explosibility of hydrogen–graphite dust hybrid mixtures, Journal of Loss Prevention in the Process Industries 20 (2007) 698–707. [5] R.K. Eckhoff, Dust Explosions in the Process Industries, Elsevier Science, Amsterdam, 2003. [6] R. Siwek, Development of a 20 l Laboratory Apparatus and its Application for the Investigation of Combustible Dust, Ciba-Geigy AG, Basel, Switzerland, 1985. [7] A.E. Dahoe, R.S. Cant, M.J. Pegg, B. Scarlett, On the transient flow in the 20-liter explosion sphere, Journal of Loss Prevention in the Process Industries 14 (2001) 475–487. [8] R. Siwek, Operating Instructions 20-l Apparatus, Adolf Kühner AG, Switzerland, http://www.kuhner.com/DOCUMENT/KSEP E.pdf.