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Deflagrations of H2–air and CH4–air lean mixtures in a vented multi-compartment environment
Energy 30 (2005) 1439–1451 www.elsevier.com/locate/energy
Deflagrations of H2–air and CH4–air lean mixtures in a vented multi-compartment environment M.N. Carcassi, F. Fineschi Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione, Universita` di Pisa, Via Diotisalvi 2, 56126 Pisa, Italy
Abstract The use of hydrogen as an energy carrier for the future is conditioned by its safety. Hydrogen is commonly and incorrectly perceived as being a more dangerous gas than methane, since the latter is widely used and thus considered to be acceptable. The paper analyses deflagrations of H2/air and CH4/air mixtures at low concentrations (close to the lower flammability limits) and, in particular, focuses on the phenomenology and dangerous aspects of this kind of combustion. The related possible accidents involve closed environments (garages, laboratories, service rooms, internal volumes of buses and cars, etc.) where ignition sources are present. In these cases, combustion probably takes place as soon as the fuel concentration reaches the lower flammability limit. Hydrogen and methane are compared on the basis of (1) their general energetic characteristics, (2) a theoretical examination of the main parameters related to the combustion phenomenon, and (3) the experimental results of the LargeView2 apparatus, where more than 300 deflagration tests were made with both gases in a vented multi-compartment container. # 2004 Elsevier Ltd. All rights reserved.
1. Introduction The use of hydrogen as an energy carrier is conditioned by cost, technical factors related to its production, transport and usage; and of course safety factors. The problems that hydrogen can create for people and equipment are due to its chemical and physical characteristics which are different from other gaseous fuels widely used on the market, such as methane. Unlike other gases, hydrogen can damage the physical and mechanical properties of metals due to embrittlement: hydrogen atoms can dissolve in metal lattice and accumulate in disturbed lattice regions, thus impairing the main mechanical characteristics.
Corresponding author. Fax: +39-050-836-665. E-mail address: fabio.fi[email protected] (F. Fineschi).
0360-5442/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2004.02.012
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Nomenclature At c C CD ev f LFL LHVx m3n ne P t T V W c
throat area (m2) sonic velocity (m/s) volumetric concentration (%) discharge coefficient efficiency of venting in modifying the fuel/air ratio molar fraction lower flammability limit (%) lower heating value of x (kJ/kmolx or kJ=m3n ) normal cubic meter, gas quantity that occupies 1 m3 at 101 325 Pa and 273.15 K total mole number in an environment pressure (kPa) time (s) discharge period (s) volume of a cylinder (m3) molar flow rate (kmol/s) specific heat ratio
Subscripts 0 initial value c cylinder max maximum mix gas mixture s stagnation
If hydrogen is used in large quantities as an energy carrier, it may need to be stored in such places as distribution stations, containers inside cars, etc., with a consequent possible accumulation in closed or poorly ventilated environments. A fuel–air gas mixture can support deflagration when the fuel volumetric concentration is within the flammability range: from 4% to 75% for hydrogen and from 5% to 15% for methane. The perception of hydrogen as being a particularly dangerous gas comes from public fear; people believe such gas to be highly explosive (many cite the vivid images of the Hindenburg ablaze, or the 1986 Challenger catastrophe, or the hydrogen bomb). In reality, these highly publicized explosions have little or no bearing on the level of safety of hydrogen in the car or home.
2. Level of risk of combustible gases The hazard level of a combustible gas is confined to the casual mixture with air to create a flammable mixture, and to the related destructive effects produced by its possible ignition and
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combustion. Such effects are due to a static and/or dynamic increase in pressure, and therefore, this is the first parameter that should be considered in order to evaluate the risk level. Deflagration (subsonic flame propagation) is more probable than detonation (supersonic flame propagation). This means that we can normally neglect the dynamic effect of the pressure transient and conclude that two fuel–air mixtures produce equivalent effects if their deflagrations produce the same maximum pressure on the structures. 3. Combustion in closed environment For deflagrations inside closed containers, the theoretical maximum pressure is equal to PAICC (pressure produced by an adiabatic isochoric complete combustion). This quantity is plotted in Fig. 1 for methane and hydrogen as a function of the volumetric concentration of the combustible gas and of the ‘‘relative concentration’’, which we define, for low concentrations, as: Rel: Conc: ¼
ðfuel=airÞ ðfuel=airÞLFL ðfuel=airÞstoich ðfuel=airÞLFL
(1)
Fig. 1. PAICC as a function of volumetric and relative concentration and lower heating value as a function of volumetric concentration, for hydrogen and methane.
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where (fuel/air) is the molar ratio of fuel and air in the gas mixture, and stoich and LFL point out the stoichiometric and the lower flammability limit conditions, respectively. The values of Rel. Conc. between 0 and 1 describe the overall flammability field for lean mixtures. This quantity may be useful for comparing different gases. For stoichiometric and lower fuel concentrations, the maximum theoretical pressure for methane combustion in a closed environment is always greater than for hydrogen combustion, if their concentrations are equal (Fig. 1). But, if the lower heating values per unit of normal volume of gas mixture are equal (LHVmix), PAICC is almost the same; in Fig. 1, for LHVmix ¼ a, PAICC ¼ A and A0 for hydrogen and methane, respectively. It is not exactly the same only because pressurization is also affected by the thermal capacity of the gas and by the change in the mole number due to combustion. 4. Combustion in vented environments In vented environments (containers with openings), the maximum pressure is (generally) lower than PAICC and, assuming an adiabatic process, depends above all on: . . . . . .
type of gas; concentration of gas; geometry of the container; location and size of the vent; opening pressure of venting; level of turbulence, which affects the burning velocity.
Thus, Pmax is a term of comparison between different gases and is more problematic in vented compartments than in closed containers, where PAICC is only a function of the type of combustible gas and its concentration. In Fineschi [1] the problem was analysed from a theoretical point of view, whereas in this paper it will be discussed mainly on the basis of experimental data. 5. Experimental apparatus LargeView2 The LargeView2 facility (the latest apparatus of the VIEW series) was designed and built to study deflagrations of hydrogen and methane with air, and in particular, to understand flame behaviour in a vented multi-compartment container during ‘‘slow’’ deflagrations. The apparatus is designed to withstand a maximum internal overpressure of 300 kPa. The vessel (Fig. 2) is a square parallelepiped made up of rectangular panels fastened to a steel framework. The inner dimensions of 0:68 0:68 3:2 m lead to a total volume of 1.48 m3. One side and the upper panels are made of high-strength stratified glass (40 mm thick); the other panels are made of enhanced carbon steel. The ends are closed by two steel plates and the right-hand end has a large opening to allow venting during deflagration. To enable sufficiently high overpressures to be reached inside the vessel, four steel rings were welded onto the outer side of the structure. The vessel is divided into two parts by a steel
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Fig. 2. The LargeView2 facility.
diaphragm, with an orifice in the centre that allows the flame to pass from the left-hand chamber (where ignition takes place) to the right-hand one. In the present configuration, the orifice was changed from 0.053 to 0.1 m in diameter and the diaphragm is placed 1.066 m far from the right-hand end, therefore the volume of the second chamber is twice as big as the first. Since the naturally emitted light of the hydrogen flame is in the invisible ultra violet range, to make the hydrogen flame visible to the camera and human eye an aerosol of 7–10 lm drops of a NaCl water solution is added to the hydrogen–air mixture. A typical photo-sequence of a test is shown in Fig. 3. More than 300 tests have been made with the LargeView2 facility. For the purposes of the present paper, principally results from the fourth series of tests have been used. The main parameters that were changed during the tests were: type of gas (hydrogen and methane), gas concentration (5.5, 6.0, 6.3, 6.5, 6.8, 7.0, 7.2% for methane and 8.0, 8.5, 8.8, 9.0, 9.5, 10.0, 10.5, 10.8, 11.0, 11.2% for hydrogen), and orifice diameter (5.2, 7.0, 8.3, 10.0 cm).
6. Experimental results The deflagration transients obtained in this series of tests can be subdivided into three typologies, which correspond to different pressure transients: . Transients in which the ignition jet occurs as soon as the flame has reached the orifice and the hot gases flow into the second room; a typical pressure transient is shown in Fig. 4. . Transients in which the ignition jet takes place a certain time after the flame has reached the orifice (delayed jet); a typical pressure transient is shown in Fig. 5. . Transients in which jet ignition does not occur; a typical pressure transient is shown in Fig. 6.
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Fig. 3. Photo-sequence of test T33.
The physical reason for these different behaviours is due to the complex phenomenon of the jet ignition, which is related to physical and chemical characteristics of the flame (concentration of free radicals) and to the speed and temperature of the gases that flow through the hole [2]. The pressure values in the first room when the flame reaches the orifice, Por, are compared for hydrogen and methane in several experimental conditions: Fig. 7 shows Por versus initial volumetric concentration for different orifice diameters and Figs. 8–10, each for a different value of the orifice diameter, show Por versus the initial relative concentration. These figures seem to give conflicting indications about how dangerous the two gases are, depending on the type of concentration used as the abscissa, but in Fig. 11, where Por is plotted as a function of the lower heating value initially present in the first room before ignition, hydrogen appears to produce higher Por values than those produced by methane for the range of concentrations examined. This is due to the higher burning velocity of hydrogen in the laminar fluid dynamic conditions that were created during the tests. If the first room were closed, PAICC would not be very
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Fig. 4. Test #90: CH2 ¼ 11%, orifice diameter ¼ 100 mm. Pressure transients in the first (P1) and in the second (P2) rooms. Jet ignition occurs as soon as the flame has reached the orifice.
different between the two gases, lower heating values of the gas mixtures being equal (Fig. 1); whereas in a vented container the dynamics of the flame evolution is the crucial element. If the level of turbulence were very high, the burning velocity would actually only depend on turbulence and the difference in pressure between hydrogen and methane would still be negligible even in vented environments. In the tests with hydrogen that we have examined the concentrations were always greater than 8% in volume in order for there to be an isotropic combustion. Hydrogen has three lower limits of flammability: 4% for upward propagation, 6% for horizontal propagation and 8% for isotropic propagation, in terms of volumetric concentration. For other combustible gases, such as methane, the lower flammability limit is only one (5% in volume for methane) and corresponds to an isotropic propagation. The reason for this behaviour has been explained well by Hertzberg [3]: ‘‘The large gap between the hydrogen lower flammability limits, upward and downward, is caused by the fact that the hydrogen molecule has a much higher molecular diffusivity than the oxygen molecule, which causes the selective diffusional enrichment of the flame front in hydrogen, generating cellular flames, whose curvature is further enhanced by buoyancy, which further enriches the flame front in hydrogen’’. In experiments made with the View, LargeView, and LargeView2 facilities, as well as with the FLAME facility of the SANDIA National Laboratories [4], we have always noted that when the concentration of the hydrogen is close to 4% in volume the flame just jumps to the ceiling and then burns slowly, without causing a significant increase in pressure inside a vented room.
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Fig. 5. Test #47: CH2 ¼ 10:5%, orifice diameter ¼ 70 mm. Pressure transients in the first (P1) and in the second (P2) rooms. Jet ignition takes place a certain time after the flame has reached the orifice (delayed jet).
Fig. 6. Test #06: CH2 ¼ 10:0%, orifice diameter ¼ 50 mm. Pressure transients in the first (P1) and in the second (P2) rooms. Jet ignition does not take place.
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Fig. 7. Pressure when the flame arrives at the orifice versus the initial volumetric concentration.
Fig. 8. Pressure when the flame arrives at the orifice (diameter: 5.2 cm) versus the initial relative concentration.
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Fig. 9. Pressure when the flame arrives at the orifice (diameter: 7.0 cm) versus the initial relative concentration.
Fig. 10. Pressure when the flame arrives at the orifice (diameter: 10.0 cm) versus the initial relative concentration.
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Fig. 11. Pressure when the flame arrives at the orifice versus initial global lower heating value of the gas mixture in the first room.
7. Equivalent concentrations of gaseous fuels Both flammability and pressurisation due to deflagration depend on the volumetric concentration, which in turn depends on the type of accident. For the same type of accident, the volumetric concentration may be different for hydrogen and methane, as in, for example, the following accidents: 7.1. The ‘‘kitchen accident’’ Often in the design of a combustible gas feeding system, the primary target is to obtain the desired thermal power, for any kind of combustible gas. In this case, if the normal combustion of the gas accidentally stops, gas might be released in a confined environment: WH2 ðLHVÞH2 ¼ WCH4 ðLHVÞCH4
(2)
The fuel molar fraction would be f ¼ ev ðWfuel t=ne Þ and if, at the same time, the environment volume, pressure and temperature and the venting efficiency were the same for hydrogen as for
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methane, the equivalent volumetric concentration ratio would be CCH4 WCH4 ðLHVÞH2 241826 ¼ ¼ ¼ ¼ 0:3014 802352 CH2 WH2 ðLHVÞCH4
(3)
7.2. The ‘‘store accident’’ For an accidental ‘‘sonic’’ release through a throat, the discharge period, T, of the gas molar contents in an isothermal cylinder would be constant: 1 1 dnc 1 dPs CD At 2 ðcþ1Þ½2ðc1Þ
¼ ¼ ¼ cs (4) T nc dt Ps dt cþ1 V if the thermodynamic coordinates in the cylinder are assumed to be equal to the stagnation values. The moles released into the confined environment where the cylinder is located would be: (5) n ¼ nc0 1 et=T For cylinders with the same volume of hydrogen or methane, at the same temperature and initial pressure, and same throats and discharge coefficients, the discharge period ratio would be: TH2 ¼ 0:3446 TCH4
(6)
The volumetric concentration ratio would be time dependent, but again the equivalent methane concentration would be lower than the hydrogen concentration.
8. Conclusions A comparison of hydrogen and methane, in terms of the maximum pressure that the combustion of the two gases causes in a closed container, shows a greater hazard for methane than for hydrogen, volumetric or relative concentrations being equal. Comparing the behaviour of the two gases in a vented deflagration is more difficult, because not only are the transients complex and affected by many parameters, but also the comparison criteria are questionable. On the basis of experimental tests carried out with the facility LargeView2 and by utilizing, as a reference parameter, the pressure in the first room when the flame reaches the orifice, hydrogen would seem to be slightly more dangerous than methane, since the pressure is greater for hydrogen than for methane for the same relative concentration or the same lower heating value of the fuel–air mixture. This result is hardly surprising since the burning velocity of hydrogen is greater. It is important to note the burning behaviour of hydrogen mixtures below 6% in volume in a vented environment: in this case, a partial combustion occurs, principally at the ceiling of the room, without substantial rises in pressure.
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This means that in a room where a hydrogen release is occurring, especially if vents are located high up, . hydrogen is vented and thus eliminated from the environment better than methane, because of its high lightness and diffusivity; . at LFL, a H2–air mixture has a much lower LHV than a CH4 mixture and its combustion is not complete because of buoyancy; hence, if ignition occurs as soon as flammability is reached, pressurization for hydrogen is lower than for methane. For this reason, in some particular situations, deliberate ignition can even be a method for eliminating hydrogen from the environment and for limiting the fuel concentration and thus deflagration overpressure; . if the vents are designed for combustion at higher gas concentrations, the deflagration effects can be limited for both gases (and in general for all combustible gases). In summary, although hydrogen is more reactive with air than methane, its deflagration can always be mitigated, if not prevented, and detonation can certainly be avoided. The wide experience gained in the fields of aerospace and nuclear engineering, where technical countermeasures have been set up (early ignition by igniters, catalytic recombining, etc.), can be used to increase the safe use of hydrogen.
References [1] Fineschi F, Fiore G. Criteri di pericolosita` per deflagrazioni di gas in ambienti confinati (Dangerousness criteria for gas deflagration in confined environment). In: Carcassi M, Leonardi M, editors. Proceedings on CD-ROM of VGR2k Convegno Nazionale sulla Valutazione e Gestione del Rischio negli Insediamenti Civili e Industriali, Pisa 24–26 Ottobre 2000, Atti del Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione. 2000 DIMNP 027 (00), file 044.pdf. [2] Jordan M, Ardey N, Mayinger F, Carcassi MN. Effects of turbulence and mixing on flame acceleration through highly blocking obstacles. In: Lee JS, editor. Proceedings of the 11th International Heat Transfer Conference, IHTC, August 23–28, vol. 7. Kyongju, Korea: Korean Society of Mechanical Engineers; 1998, p. 295–300. [3] Hertzberg M. Flammability limits and pressure development in hydrogen–air mixtures. In: Berman M, editor. Proceedings of the Workshop on the Impact of Hydrogen on Water Reactor Safety, Albuquerque, NM, USA, NUREG/CR-2017, vol. 3. Sandia National Laboratories; 1981, p. 17–65. [4] Sherman MP, Tiezsen SR, Benedick WB, Fisk JW, Carcassi M. The effect of transverse venting on flame acceleration and transition to detonation in a large channel. Dynamics of Explosions Progress in Astronautics and Aeronomics 1985;106:66–84.
Deflagrations of H2–air and CH4–air lean mixtures in a vented multi-compartment environment M.N. Carcassi, F. Fineschi Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione, Universita` di Pisa, Via Diotisalvi 2, 56126 Pisa, Italy
Abstract The use of hydrogen as an energy carrier for the future is conditioned by its safety. Hydrogen is commonly and incorrectly perceived as being a more dangerous gas than methane, since the latter is widely used and thus considered to be acceptable. The paper analyses deflagrations of H2/air and CH4/air mixtures at low concentrations (close to the lower flammability limits) and, in particular, focuses on the phenomenology and dangerous aspects of this kind of combustion. The related possible accidents involve closed environments (garages, laboratories, service rooms, internal volumes of buses and cars, etc.) where ignition sources are present. In these cases, combustion probably takes place as soon as the fuel concentration reaches the lower flammability limit. Hydrogen and methane are compared on the basis of (1) their general energetic characteristics, (2) a theoretical examination of the main parameters related to the combustion phenomenon, and (3) the experimental results of the LargeView2 apparatus, where more than 300 deflagration tests were made with both gases in a vented multi-compartment container. # 2004 Elsevier Ltd. All rights reserved.
1. Introduction The use of hydrogen as an energy carrier is conditioned by cost, technical factors related to its production, transport and usage; and of course safety factors. The problems that hydrogen can create for people and equipment are due to its chemical and physical characteristics which are different from other gaseous fuels widely used on the market, such as methane. Unlike other gases, hydrogen can damage the physical and mechanical properties of metals due to embrittlement: hydrogen atoms can dissolve in metal lattice and accumulate in disturbed lattice regions, thus impairing the main mechanical characteristics.
Corresponding author. Fax: +39-050-836-665. E-mail address: fabio.fi[email protected] (F. Fineschi).
0360-5442/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2004.02.012
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Nomenclature At c C CD ev f LFL LHVx m3n ne P t T V W c
throat area (m2) sonic velocity (m/s) volumetric concentration (%) discharge coefficient efficiency of venting in modifying the fuel/air ratio molar fraction lower flammability limit (%) lower heating value of x (kJ/kmolx or kJ=m3n ) normal cubic meter, gas quantity that occupies 1 m3 at 101 325 Pa and 273.15 K total mole number in an environment pressure (kPa) time (s) discharge period (s) volume of a cylinder (m3) molar flow rate (kmol/s) specific heat ratio
Subscripts 0 initial value c cylinder max maximum mix gas mixture s stagnation
If hydrogen is used in large quantities as an energy carrier, it may need to be stored in such places as distribution stations, containers inside cars, etc., with a consequent possible accumulation in closed or poorly ventilated environments. A fuel–air gas mixture can support deflagration when the fuel volumetric concentration is within the flammability range: from 4% to 75% for hydrogen and from 5% to 15% for methane. The perception of hydrogen as being a particularly dangerous gas comes from public fear; people believe such gas to be highly explosive (many cite the vivid images of the Hindenburg ablaze, or the 1986 Challenger catastrophe, or the hydrogen bomb). In reality, these highly publicized explosions have little or no bearing on the level of safety of hydrogen in the car or home.
2. Level of risk of combustible gases The hazard level of a combustible gas is confined to the casual mixture with air to create a flammable mixture, and to the related destructive effects produced by its possible ignition and
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combustion. Such effects are due to a static and/or dynamic increase in pressure, and therefore, this is the first parameter that should be considered in order to evaluate the risk level. Deflagration (subsonic flame propagation) is more probable than detonation (supersonic flame propagation). This means that we can normally neglect the dynamic effect of the pressure transient and conclude that two fuel–air mixtures produce equivalent effects if their deflagrations produce the same maximum pressure on the structures. 3. Combustion in closed environment For deflagrations inside closed containers, the theoretical maximum pressure is equal to PAICC (pressure produced by an adiabatic isochoric complete combustion). This quantity is plotted in Fig. 1 for methane and hydrogen as a function of the volumetric concentration of the combustible gas and of the ‘‘relative concentration’’, which we define, for low concentrations, as: Rel: Conc: ¼
ðfuel=airÞ ðfuel=airÞLFL ðfuel=airÞstoich ðfuel=airÞLFL
(1)
Fig. 1. PAICC as a function of volumetric and relative concentration and lower heating value as a function of volumetric concentration, for hydrogen and methane.
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where (fuel/air) is the molar ratio of fuel and air in the gas mixture, and stoich and LFL point out the stoichiometric and the lower flammability limit conditions, respectively. The values of Rel. Conc. between 0 and 1 describe the overall flammability field for lean mixtures. This quantity may be useful for comparing different gases. For stoichiometric and lower fuel concentrations, the maximum theoretical pressure for methane combustion in a closed environment is always greater than for hydrogen combustion, if their concentrations are equal (Fig. 1). But, if the lower heating values per unit of normal volume of gas mixture are equal (LHVmix), PAICC is almost the same; in Fig. 1, for LHVmix ¼ a, PAICC ¼ A and A0 for hydrogen and methane, respectively. It is not exactly the same only because pressurization is also affected by the thermal capacity of the gas and by the change in the mole number due to combustion. 4. Combustion in vented environments In vented environments (containers with openings), the maximum pressure is (generally) lower than PAICC and, assuming an adiabatic process, depends above all on: . . . . . .
type of gas; concentration of gas; geometry of the container; location and size of the vent; opening pressure of venting; level of turbulence, which affects the burning velocity.
Thus, Pmax is a term of comparison between different gases and is more problematic in vented compartments than in closed containers, where PAICC is only a function of the type of combustible gas and its concentration. In Fineschi [1] the problem was analysed from a theoretical point of view, whereas in this paper it will be discussed mainly on the basis of experimental data. 5. Experimental apparatus LargeView2 The LargeView2 facility (the latest apparatus of the VIEW series) was designed and built to study deflagrations of hydrogen and methane with air, and in particular, to understand flame behaviour in a vented multi-compartment container during ‘‘slow’’ deflagrations. The apparatus is designed to withstand a maximum internal overpressure of 300 kPa. The vessel (Fig. 2) is a square parallelepiped made up of rectangular panels fastened to a steel framework. The inner dimensions of 0:68 0:68 3:2 m lead to a total volume of 1.48 m3. One side and the upper panels are made of high-strength stratified glass (40 mm thick); the other panels are made of enhanced carbon steel. The ends are closed by two steel plates and the right-hand end has a large opening to allow venting during deflagration. To enable sufficiently high overpressures to be reached inside the vessel, four steel rings were welded onto the outer side of the structure. The vessel is divided into two parts by a steel
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Fig. 2. The LargeView2 facility.
diaphragm, with an orifice in the centre that allows the flame to pass from the left-hand chamber (where ignition takes place) to the right-hand one. In the present configuration, the orifice was changed from 0.053 to 0.1 m in diameter and the diaphragm is placed 1.066 m far from the right-hand end, therefore the volume of the second chamber is twice as big as the first. Since the naturally emitted light of the hydrogen flame is in the invisible ultra violet range, to make the hydrogen flame visible to the camera and human eye an aerosol of 7–10 lm drops of a NaCl water solution is added to the hydrogen–air mixture. A typical photo-sequence of a test is shown in Fig. 3. More than 300 tests have been made with the LargeView2 facility. For the purposes of the present paper, principally results from the fourth series of tests have been used. The main parameters that were changed during the tests were: type of gas (hydrogen and methane), gas concentration (5.5, 6.0, 6.3, 6.5, 6.8, 7.0, 7.2% for methane and 8.0, 8.5, 8.8, 9.0, 9.5, 10.0, 10.5, 10.8, 11.0, 11.2% for hydrogen), and orifice diameter (5.2, 7.0, 8.3, 10.0 cm).
6. Experimental results The deflagration transients obtained in this series of tests can be subdivided into three typologies, which correspond to different pressure transients: . Transients in which the ignition jet occurs as soon as the flame has reached the orifice and the hot gases flow into the second room; a typical pressure transient is shown in Fig. 4. . Transients in which the ignition jet takes place a certain time after the flame has reached the orifice (delayed jet); a typical pressure transient is shown in Fig. 5. . Transients in which jet ignition does not occur; a typical pressure transient is shown in Fig. 6.
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Fig. 3. Photo-sequence of test T33.
The physical reason for these different behaviours is due to the complex phenomenon of the jet ignition, which is related to physical and chemical characteristics of the flame (concentration of free radicals) and to the speed and temperature of the gases that flow through the hole [2]. The pressure values in the first room when the flame reaches the orifice, Por, are compared for hydrogen and methane in several experimental conditions: Fig. 7 shows Por versus initial volumetric concentration for different orifice diameters and Figs. 8–10, each for a different value of the orifice diameter, show Por versus the initial relative concentration. These figures seem to give conflicting indications about how dangerous the two gases are, depending on the type of concentration used as the abscissa, but in Fig. 11, where Por is plotted as a function of the lower heating value initially present in the first room before ignition, hydrogen appears to produce higher Por values than those produced by methane for the range of concentrations examined. This is due to the higher burning velocity of hydrogen in the laminar fluid dynamic conditions that were created during the tests. If the first room were closed, PAICC would not be very
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Fig. 4. Test #90: CH2 ¼ 11%, orifice diameter ¼ 100 mm. Pressure transients in the first (P1) and in the second (P2) rooms. Jet ignition occurs as soon as the flame has reached the orifice.
different between the two gases, lower heating values of the gas mixtures being equal (Fig. 1); whereas in a vented container the dynamics of the flame evolution is the crucial element. If the level of turbulence were very high, the burning velocity would actually only depend on turbulence and the difference in pressure between hydrogen and methane would still be negligible even in vented environments. In the tests with hydrogen that we have examined the concentrations were always greater than 8% in volume in order for there to be an isotropic combustion. Hydrogen has three lower limits of flammability: 4% for upward propagation, 6% for horizontal propagation and 8% for isotropic propagation, in terms of volumetric concentration. For other combustible gases, such as methane, the lower flammability limit is only one (5% in volume for methane) and corresponds to an isotropic propagation. The reason for this behaviour has been explained well by Hertzberg [3]: ‘‘The large gap between the hydrogen lower flammability limits, upward and downward, is caused by the fact that the hydrogen molecule has a much higher molecular diffusivity than the oxygen molecule, which causes the selective diffusional enrichment of the flame front in hydrogen, generating cellular flames, whose curvature is further enhanced by buoyancy, which further enriches the flame front in hydrogen’’. In experiments made with the View, LargeView, and LargeView2 facilities, as well as with the FLAME facility of the SANDIA National Laboratories [4], we have always noted that when the concentration of the hydrogen is close to 4% in volume the flame just jumps to the ceiling and then burns slowly, without causing a significant increase in pressure inside a vented room.
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Fig. 5. Test #47: CH2 ¼ 10:5%, orifice diameter ¼ 70 mm. Pressure transients in the first (P1) and in the second (P2) rooms. Jet ignition takes place a certain time after the flame has reached the orifice (delayed jet).
Fig. 6. Test #06: CH2 ¼ 10:0%, orifice diameter ¼ 50 mm. Pressure transients in the first (P1) and in the second (P2) rooms. Jet ignition does not take place.
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Fig. 7. Pressure when the flame arrives at the orifice versus the initial volumetric concentration.
Fig. 8. Pressure when the flame arrives at the orifice (diameter: 5.2 cm) versus the initial relative concentration.
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Fig. 9. Pressure when the flame arrives at the orifice (diameter: 7.0 cm) versus the initial relative concentration.
Fig. 10. Pressure when the flame arrives at the orifice (diameter: 10.0 cm) versus the initial relative concentration.
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Fig. 11. Pressure when the flame arrives at the orifice versus initial global lower heating value of the gas mixture in the first room.
7. Equivalent concentrations of gaseous fuels Both flammability and pressurisation due to deflagration depend on the volumetric concentration, which in turn depends on the type of accident. For the same type of accident, the volumetric concentration may be different for hydrogen and methane, as in, for example, the following accidents: 7.1. The ‘‘kitchen accident’’ Often in the design of a combustible gas feeding system, the primary target is to obtain the desired thermal power, for any kind of combustible gas. In this case, if the normal combustion of the gas accidentally stops, gas might be released in a confined environment: WH2 ðLHVÞH2 ¼ WCH4 ðLHVÞCH4
(2)
The fuel molar fraction would be f ¼ ev ðWfuel t=ne Þ and if, at the same time, the environment volume, pressure and temperature and the venting efficiency were the same for hydrogen as for
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methane, the equivalent volumetric concentration ratio would be CCH4 WCH4 ðLHVÞH2 241826 ¼ ¼ ¼ ¼ 0:3014 802352 CH2 WH2 ðLHVÞCH4
(3)
7.2. The ‘‘store accident’’ For an accidental ‘‘sonic’’ release through a throat, the discharge period, T, of the gas molar contents in an isothermal cylinder would be constant: 1 1 dnc 1 dPs CD At 2 ðcþ1Þ½2ðc1Þ
¼ ¼ ¼ cs (4) T nc dt Ps dt cþ1 V if the thermodynamic coordinates in the cylinder are assumed to be equal to the stagnation values. The moles released into the confined environment where the cylinder is located would be: (5) n ¼ nc0 1 et=T For cylinders with the same volume of hydrogen or methane, at the same temperature and initial pressure, and same throats and discharge coefficients, the discharge period ratio would be: TH2 ¼ 0:3446 TCH4
(6)
The volumetric concentration ratio would be time dependent, but again the equivalent methane concentration would be lower than the hydrogen concentration.
8. Conclusions A comparison of hydrogen and methane, in terms of the maximum pressure that the combustion of the two gases causes in a closed container, shows a greater hazard for methane than for hydrogen, volumetric or relative concentrations being equal. Comparing the behaviour of the two gases in a vented deflagration is more difficult, because not only are the transients complex and affected by many parameters, but also the comparison criteria are questionable. On the basis of experimental tests carried out with the facility LargeView2 and by utilizing, as a reference parameter, the pressure in the first room when the flame reaches the orifice, hydrogen would seem to be slightly more dangerous than methane, since the pressure is greater for hydrogen than for methane for the same relative concentration or the same lower heating value of the fuel–air mixture. This result is hardly surprising since the burning velocity of hydrogen is greater. It is important to note the burning behaviour of hydrogen mixtures below 6% in volume in a vented environment: in this case, a partial combustion occurs, principally at the ceiling of the room, without substantial rises in pressure.
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This means that in a room where a hydrogen release is occurring, especially if vents are located high up, . hydrogen is vented and thus eliminated from the environment better than methane, because of its high lightness and diffusivity; . at LFL, a H2–air mixture has a much lower LHV than a CH4 mixture and its combustion is not complete because of buoyancy; hence, if ignition occurs as soon as flammability is reached, pressurization for hydrogen is lower than for methane. For this reason, in some particular situations, deliberate ignition can even be a method for eliminating hydrogen from the environment and for limiting the fuel concentration and thus deflagration overpressure; . if the vents are designed for combustion at higher gas concentrations, the deflagration effects can be limited for both gases (and in general for all combustible gases). In summary, although hydrogen is more reactive with air than methane, its deflagration can always be mitigated, if not prevented, and detonation can certainly be avoided. The wide experience gained in the fields of aerospace and nuclear engineering, where technical countermeasures have been set up (early ignition by igniters, catalytic recombining, etc.), can be used to increase the safe use of hydrogen.
References [1] Fineschi F, Fiore G. Criteri di pericolosita` per deflagrazioni di gas in ambienti confinati (Dangerousness criteria for gas deflagration in confined environment). In: Carcassi M, Leonardi M, editors. Proceedings on CD-ROM of VGR2k Convegno Nazionale sulla Valutazione e Gestione del Rischio negli Insediamenti Civili e Industriali, Pisa 24–26 Ottobre 2000, Atti del Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione. 2000 DIMNP 027 (00), file 044.pdf. [2] Jordan M, Ardey N, Mayinger F, Carcassi MN. Effects of turbulence and mixing on flame acceleration through highly blocking obstacles. In: Lee JS, editor. Proceedings of the 11th International Heat Transfer Conference, IHTC, August 23–28, vol. 7. Kyongju, Korea: Korean Society of Mechanical Engineers; 1998, p. 295–300. [3] Hertzberg M. Flammability limits and pressure development in hydrogen–air mixtures. In: Berman M, editor. Proceedings of the Workshop on the Impact of Hydrogen on Water Reactor Safety, Albuquerque, NM, USA, NUREG/CR-2017, vol. 3. Sandia National Laboratories; 1981, p. 17–65. [4] Sherman MP, Tiezsen SR, Benedick WB, Fisk JW, Carcassi M. The effect of transverse venting on flame acceleration and transition to detonation in a large channel. Dynamics of Explosions Progress in Astronautics and Aeronomics 1985;106:66–84.