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On the pressure dependence of flammability limits of CH2=CFCF3, CH2F2 and methane
Fire Safety Journal 46 (2011) 289–293
Contents lists available at ScienceDirect
Fire Safety Journal journal homepage: www.elsevier.com/locate/firesaf
On the pressure dependence of flammability limits of CH2 ¼ CFCF3, CH2F2 and methane Shigeo Kondo n, Akifumi Takahashi, Kenji Takizawa, Kazuaki Tokuhashi National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan
a r t i c l e i n f o
abstract
Article history: Received 7 September 2010 Received in revised form 18 February 2011 Accepted 23 March 2011 Available online 16 April 2011
Flammability limits of CH2 ¼ CFCF3 (HFO-1234 yf), CH2F2 (HFC-32), and methane were measured at pressures from ambient to 2500 kPa in a 5 l stainless-steel spherical vessel. For HFO-1234 yf, as the pressure rises from ambient, the lower flammability limit is shifted downward and the upper limit is shifted upward. The changes to the lower flammability limits are, in general, small compared to the upper flammability limits. Both the lower and upper flammability limits of this compound can be approximated by simple logarithmic functions of pressure. For HFC-32, the behavior of lower flammability limit is similar to that for HFO-1234 yf, but the behavior for the upper limit is rather complicated. As the pressure is increased, it begins to rise upward gradually. Then, as the pressure becomes larger than about 1000 kPa it begins to rise upward rapidly, and then the change becomes moderate again. This must be due to a change of combustion reaction mechanism below 1000 kPa and above 1500 kPa in the upper flammability limit region for this compound. On the other hand, both the flammability limits of methane change almost linearly with pressure, at least in the pressure region considered. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Flammability limits Pressure dependence HFO-1234 yf HFC-32 Methane
1. Introduction Recently, various fluorinated compounds have been synthesized and used mainly as refrigerants. Refrigerant compounds are generally used at ambient pressure or a little higher than ambient pressure. Thus, data for flammability limits of these compounds under pressurized conditions may be useful. With regard to experimental flammability limits, they are more or less dependent on the apparatus and conditions under which they are measured [1–4]. Indeed, the flammability limits are dependent on the experimental temperature and pressure as well. Some data of the pressure dependence of flammability limits are reported in the literature (e.g. [1]). It is known that the flammable ranges of hydrocarbon compounds, in general, become wider as the pressure is raised: the lower limit becomes lower and the upper limit becomes higher. As for the flammability limits of natural gas, Zabetakis [5] has pointed out that both the flammability limits, obtained by Jones et al. [6], can be expressed as simple logarithmic functions of pressure. On the other hand, Vanderstraeten et al. [7] have reported that the pressure dependence of the flammability limits of methane is expressed by a simple power series of pressure. However, since the main component of natural gas is methane, it is hard to accept these two results at the same time. It is desirable to clarify the reason for the
n
Corresponding author. Tel./fax: þ 81 29 861 4770. E-mail address: [email protected] (S. Kondo).
0379-7112/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.firesaf.2011.03.005
apparent inconsistency between the two results. On the other hand, it is of particular interest to know whether the flammability limits of such compounds as HFCs and HFOs behave similarly to natural gas under pressurized conditions. Previously, the authors have published a paper describing the relationship between adiabatic flame temperature and pressure in the lower flammability limit region [8]. The result is that the following equation holds between them:
p p0
a
¼
T T0
b
E E exp RT RT0
ð1Þ
Here, a is a constant and takes a value between 1 and 3 depending on the order of combustion reaction, b is a constant and takes a value between 2 and 2 depending on the flame quenching mechanism at the flame propagation limit, T and T0 are the adiabatic flame temperatures at pressures p and p0, respectively, E is the activation energy of combustion, and R is the gas constant. In the lower flammability limit region, the adiabatic flame temperature T can be considered to be proportional to the flammability limit concentration L. Namely, L/T¼L0/T0 ¼const. Thus, we may be able to rewrite Eq. (1) as
p p0
a ¼
b L cE cE exp L0 RL RL0
ð2Þ
This equation suggests that the lower flammability limit becomes a logarithmic function of pressure. The finding by Zabetakis [5], as mentioned above, is a good example for this. It has also been recently
290
S. Kondo et al. / Fire Safety Journal 46 (2011) 289–293
established that the geometric mean of upper and lower flammability limits is very stable during temperature change [9]. If this also applies to the pressure change, it is possible that the pressure behavior of upper flammability limits follows a logarithmic function as well. In this paper, we report the result of measurements and analysis of the pressure dependence of upper and lower flammability limits for HFO-1234 yf, HFC-32, and methane.
2. Experiments The measurement of flammability limits was done in a 5 l spherical stainless-steel explosion vessel. This vessel is equipped with a fan for gas mixing. The ignition of sample gas–air mixtures was made by fusing tungsten wire, 0.3 mm in diameter and 10 mm in length, stretched between a couple of electrodes. Fusing was made by direct current from two car batteries connected in series, giving 24 V. The electric energy consumed by the tungsten wire by the time of fusion was about 20 J, measured by recording electric current and voltage. However, the actual ignition ability of this ignition source was less than that of 0.4 s AC spark used in ASHRAE method [10], as observed for the measurement of HFO-1234 yf at ambient and at 200 kPa pressures. The 0.4 s AC spark corresponds to about 10 J of energy [11]. The electrodes were positioned about one-third of the distance from the bottom to the top of the vessel ceiling. This is atypical as it is true that a central ignition point is generally used for flammability experiments in spherical vessels. However, since the burning velocity is slow in the flammability limit regions, flames tend to go up to the ceiling after ignition. Therefore, the space above the ignition area is important for these flammability limit experiments. The ignition source should be positioned somewhere between the center and the bottom of the vessel. We followed the setting of standard ASTM procedure [10,12]. A schematic diagram of experimental setup is shown in Fig. 1. All experiments were carried out at room temperature. The flammability limits were determined by assessing the maximum pressure rise after ignition. In this case, the criterion for determining flammability limits is of particular importance. The present criterion has been determined referring
to the one proposed by ASHRAE [10,12]. According to the ASHRAE criterion, the ignition experiment is done in a 12 L spherical flask, and the gas mixture prepared in the flask is determined to be flammable if the flame moves upward and outward from the point of ignition, to reach an arc of the vessel wall subtending an angle larger than 901 as measured from the ignition point. The volume inside the cone of 901 and under the vessel wall amounts to about 30% of the total vessel volume. Now, provided that the volume is completely filled with burnt gas whose temperature is 1500 K for example, the pressure rise in the vessel may become about 30% of the initial value. Considering this, we have introduced a criterion that the mixture is flammable if the maximum pressure rise exceeds 20% of the initial sample pressure. For the measurements, gas mixtures were directly prepared in the explosion vessel by the partial pressure method. Before introduction of gases, the vessel was evacuated to 0.1 Torr (1 Torr¼133.32 Pa). The fuel gas was introduced into the vessel first followed by dry air. Three kinds of MKS Baratrons, 100, 1000, and 20000 Torr heads, were used as necessary for the sample pressure measurements. Gas mixtures were prepared in the vessel, stirred with a fan for 15 min, then left to settle for 1 min before ignition. During and after ignition, the pressure profile was measured using a strain gage and the data were recorded via an amplifier. It is known that the partial pressure method does not necessarily give the correct concentration under high pressures. This is because the gases do not obey the ideal gas law under such conditions. Therefore, it is necessary to investigate the relationship between partial pressures and true concentrations under high pressures. For this purpose, we have analyzed the gas mixtures using IR absorption intensity measurements in order to obtain the relationship between partial pressures and true concentrations for the three compounds mixed with air. The result is that for any of the three compounds considered, the deviations between partial pressures and true concentrations are much less than 1% in relative value even for a mixture pressure of 2500 kPa. Based on this result, the present experiments were all prepared using the partial pressure method. The flammability limits of methane, HFC-32, and HFO-1234 yf were measured at pressures from ambient to 2500 kPa for the lower limits and from ambient to 2000 kPa for the upper limits.
Gas
Air
Pressure-gauge
Amplifier
Strain-gauge
90°
Oscilloscope
Electrode
Electrode Fan Explosion-vessel
Soda-lime Pump
Fig. 1. Experimental setup for measuring flammability limits under high pressures.
S. Kondo et al. / Fire Safety Journal 46 (2011) 289–293
Sample gases were purchased from chemical companies. Purities of fuel gases were all 99% or better. Air was extra-pure and dry. All sample materials were used as supplied without further purification.
3. Results and discussion For methane in particular, there are several studies on the pressure dependence of flammability limits reported in the literature [1,5,7,13]. There are also similar studies of the pressure effect on the flammability limits of natural gas whose main components are methane and ethane [6]. The variation of flammability limits of natural gas with pressure are expected to behave very similarly to those of methane. In general, the lower limit goes down to lower concentrations and the upper limit goes up to higher concentrations when the pressure is increased. The variation is rapid at first and then becomes gradual afterward. On the whole, the change of upper flammability limits is more significant than the lower limit. Zabetakis [5] has pointed out that the change of both the flammability limits of natural gas due to pressure can be expressed by a simple logarithmic function of pressure. 3.1. HFO-1234 yf Fig. 2 shows the result of measurement for HFO-1234 yf, where the observed values are given by black and white circles. As will be explained below, the solid line in this figure shows the calculated values. For this compound, fusing tungsten wire 0.3 mm in diameter and 10 mm long does not provide enough energy for ignition at ambient pressure, thus we could not obtain the values for the flammability limits at the ambient pressure. The black circles in Fig. 2 are the values obtained using a 12 l spherical glass flask with a 0.4 s AC spark ignition. In this figure, the point for the lower flammability limit at 200 kPa looks too high, which may be due to insufficient ignition energy as well. At 500 kPa or higher, the present ignition system seems to give enough energy. As suggested in the preceding section, variation of the flammability limits due to pressure may follow an appropriate logarithmic function of pressure. Zabetakis [5] has found that both the lower and upper flammability data for natural gas can be explained appropriately by the following type of equation: p L ¼ L0 þ k ln ð3Þ p0 Here, L and L0 are the flammability limit values at pressures p and p0, respectively, and k is a constant. In this study, this equation was
applied to explain the lower flammability data of HFO-1234 yf to obtain the following equation: p L ¼ 5:350:55 ln ð4Þ 500 Here, L is in partial pressure % and p in kPa. Similarly, for the upper flammability limit data the following equation was obtained: p U ¼ 16:5 þ 3:2 ln ð5Þ 500 The solid lines for the upper and lower flammability limits in Fig. 2 were obtained using these equations. 3.2. HFC-32 For measurement of HFC-32, fusing tungsten wire 0.3 mm in diameter and 10 mm long gives enough energy for ignition at ambient pressure. The result of measurement for this compound is shown in Fig. 3. For the lower flammability limit of this compound, the behavior against pressure is quite similar to that of HFO-1234 yf. Actually, it has been found that the data can be represented well by the following equation: p L ¼ 11:701:3 ln ð6Þ 500 The solid line for the lower flammability limit in Fig. 3 was obtained using this equation. On the other hand, the behavior of upper flammability limit has been found to be very unusual. From ambient pressure to about 1000 kPa, the behavior is moderate and looks like a mirror image of the lower limit. Then, as the pressure goes up further to exceed 1000 kPa, the upper limit begins to increase very rapidly. The behavior of the upper flammability limit in the whole pressure range from ambient to 2000 kPa cannot be explained by a single logarithmic function. Rather, it may be that the behavior below 1000 kPa is explained by one logarithmic function, and that above 1500 kPa by another. This result strongly suggests that the combustion reaction mechanisms are different in the pressure region below 1000 kPa and in the region above 1500 kPa. The solid line representing the upper flammability limit in Fig. 3 is simply a spline curve connecting the observed values shown by white circles. CH2F2 is a stable compound at ambient temperature and pressure, and the major reaction of this compound may be oxidation reaction in the upper flammability region as well. However, one thing worth noting is that this compound can release a certain amount of heat when it is decomposed as follows: CH2 F2 ¼ C þ 2HF; DH ¼ 95:3 kJ
ð7Þ
50 Flammability limit (%)
25 Flammability limit (%)
291
20 15 10 5
40 30 20 10 0
0 0
500
1000
1500 2000 Pressure (kPa)
2500
Fig. 2. Effect of pressure on the flammability limits of CH2 ¼ CFCF3.
3000
0
500
1500 1000 Pressure (kPa)
2000
Fig. 3. Effect of pressure on the flammability limits of CH2F2.
2500
292
S. Kondo et al. / Fire Safety Journal 46 (2011) 289–293
It is very probable that this reaction plays a role in making the peculiar pressure behavior of upper flammability limit observed for this compound. In this molecule, there are equal numbers of H and F atoms, and all of the H and F atoms are attached to the same carbon atom. We believe that this fact makes its self-decomposition easier to produce the characteristic pressure behavior of upper flammability limit. Further detailed investigation is needed to clarify the reaction mechanism occurring in flames in the upper flammability limit region of this compound. 3.3. Methane The results of measurements for methane are shown in Fig. 4. The behavior of the flammability limits of this compound is a little different from the observations for HFO-1234 yf and HFC-32. Although the variation of lower flammability limit is small and not far out of the uncertainty level (0.05–0.1 vol%), it seems to show tendency to go up slightly at the first stage of pressure increase and then begins to go down to lower concentrations. This tendency is not conspicuous at all but somewhat resembles that of hydrogen flammability limits [1]. At any rate, it appears that the lower flammability limit of methane may be considered almost constant within the pressure range of 100–2500 kPa. The solid line for the lower flammability limit in Fig. 3 was calculated using the following equation: L ¼ 5:09 þ0:000065p
ð8Þ
Regarding the upper flammability limit of methane, the results suggest that the observed values can appropriately be explained by a linear function of pressure. The solid line for the upper flammability limit in Fig. 4 was obtained using the following equation: U ¼ 15:27 þ0:00596p
ð9Þ
As mentioned in Section 1, the pressure behavior of the flammability limits of methane has already been investigated by several authors [1,5,7,13]. Zabetakis [5] pointed out that both the flammability limits of natural gas, which were composed of 85% methane and 15% ethane, can be fitted to simple logarithmic functions of pressure like Eq. (3). On the other hand, Vanderstraeten et al. [7] found that the pressure dependence of upper flammability limits of methane can be expressed by a simple power series of pressure rather than a logarithmic function. As described in the above, we have found that both the upper and lower flammability limits show almost linear dependence on the pressure. In connection with these studies, the experimental work by Berl and Werner [13] is noteworthy. This was done for a pressure range from ambient to as high as 40000 kPa. According to their results, both the upper and lower flammability limits of methane move to
Flammability limit (%)
30 25 20 15 10 5 0 0
500
1000
1500 2000 Pressure (kPa)
2500
Fig. 4. Effect of pressure on the flammability limits of methane.
3000
narrow the flammable range in the initial stages of pressure increase: the lower limit goes to higher concentrations and the upper limit goes to lower concentrations as the pressure is increased. Then, as the pressure becomes about 2000 kPa or higher, both the flammability limits begin to extend the flammable range. This resembles the well known pressure behavior of hydrogen flammability limits [1]. As described in reference [8], the pressure behavior of flammability limits is dependent on the reaction and/or quenching mechanisms. If the limit flame temperature becomes high when the pressure is increased, it indicates that the flame quenching mechanism is determined by a competition between radical increasing and decreasing reactions. And the reversal of pressure behavior at around 2000 kPa must be due to changes of reaction and/or quenching mechanisms. In the vicinity of the pressure behavior reversal region, the flammability limit values are sensitive to the apparatus and experimental conditions employed for the measurement. In fact, the pressure behaviors of hydrogen flammability limits in that region, which have been reported so far by different authors, are quite unstable [1]. Coming back to the present study, the measurements were done in a pressure range from ambient to 2500 kPa for the lower limit and from ambient to 2000 kPa for the upper limit. This means that almost all our measurements were done within that particular region of pressure reversal. The measurement by Vanderstraeten et al. [7] was for the pressure range from ambient to 5500 kPa, and so at least one-third of their data was taken within this region. On the other hand, the measurement of natural gas by Jones et al.[6] was done for the pressure range from ambient to 3000 psi ( ¼20700 kPa), and therefore the major part of their measurements were done well outside this region. As was apparent in the case of hydrogen [1], it is very possible that the pressure behavior of flammability limits of methane in the range from the ambient to around 2000 kPa is sensitive to the apparatus and the conditions employed for the measurements. In addition to this, the difference of pressure range studied may be responsible for the divergence of pressure behaviors of methane flammability limits for various measurements.
4. Conclusion The flammability limits were measured under high pressures for HFO-1234 yf, HFC-32, and methane. The lower limits were measured for a pressure range from ambient to 2500 kPa, and the upper limits were measured from ambient to 2000 kPa. In many cases, it seems that the pressure dependence of upper and lower flammability limits can be fitted to logarithmic functions of pressure. In this sense, the behavior of flammability limits of HFO-1234 yf is a typical one. As for HFC-32, the lower limit can also be fitted to a logarithmic function. However, the upper flammability limit of HFC-32 is very extraordinary. The pressure behavior seems to be composed of two different stages: one is for a pressure range from ambient to approximately 1000 kPa and the other for pressures higher than 1500 kPa. This may be due to change of combustion reaction mechanism. It could be that the main reaction mechanism changes from combustion to thermal decomposition in the pressure region from about 1000 to 1500 kPa. Interestingly, the pressure behavior of the simple compound of methane is a little complicated. The lower limit seems to show a slight increase at the initial stage of pressure increase and then shows a tendency to turn to lower concentrations. This behavior somewhat resembles the behavior of hydrogen gas [1]. It is possible that similar reactions and/or quenching mechanisms exist for the two cases. On the other hand, the behavior of upper limit of methane is quite different from what was expected from the literature data. The results
S. Kondo et al. / Fire Safety Journal 46 (2011) 289–293
for the pressure behavior of flammability limits for methane show an almost linear dependence on the pressure. This is neither consistent with that by Vanderstraeten et al. [7] nor with that by Berl and Werner [13]. Including the report by Zabetakis [5] on flammability limits of natural gas, different measurements were found to give quite different results. The inconsistency of pressure behavior of methane among different studies seems to be related to sensitivity of reaction mechanism and/or quenching mechanism to the change of pressure in the region from ambient to about 2000 kPa. In spite of the exceptions encountered in the present study, it may be said that, in general, the pressure dependence of flammability limits tends to follow a simple logarithmic function. References [1] H.F. Coward, G.W. Jones, Limits of flammability of gases and vapors. US Bureau of Mines Bulletin 503, 1952. [2] A. Takahashi, Y. Urano, K. Tokuhashi, H. Nagai, M. Kaise, S. Kondo, Fusing ignition of various metal wires for explosion limits measurement of methane/ air mixture, J. Loss Prevention Process Ind 11 (1998) 353–360. [3] S. Kondo, Y. Urano, A. Takahashi, K. Tokuhashi, Reinvestigation of flammability limits measurement of methane by the conventional vessel method with AC discharge ignition, Combust. Sci. Technol. 145 (1999) 1–15.
293
[4] A. Takahashi, Y. Urano, K. Tokuhashi, S. Kondo, Effect of vessel size and shape on experimental flammability limits of gases, J. Hazard. Mater. A105 (2003) 27–37. [5] M.G. Zabetakis, Flammability characteristics of combustible gases and vapors. US Bureau of Mines Bulletin 627, 1965. [6] G.W. Jones, R.E. Kennedy, I. Spolan, Effect of high pressures on the flammability of natural gas–air–nitrogen mixtures. U.S. Bureau of Mines Report of Investigation No. 4557, Washington D.C., 1949. [7] B. Vanderstraeten, D. Tuerlinckx, J. Berghmans, S. Vliegen, E. Van’t Oost, B. Smit, Experimental study of the pressure and temperature dependence on the upper flammability limit of methane/air mixtures, J. Hazard. Mater. 56 (1997) 237–246. [8] S. Kondo, S. Horiguchi, Y. Urano, K. Tokuhashi, Pressure dependence of the limit flame temperatures for premixed gas explosions, J. Jpn. Soc. Saf. Eng. 29 (1990) 241–250 in Japanese. [9] S. Kondo, K. Takizawa, A. Takahashi, K. Tokuhashi, On the temperature dependence of flammability limits of gases, J. Hazard. Mater. 187 (2011) 585–590. [10] American Society of Heating, Refrigerating and Air-conditioning Engineers. Number designation and safety classification of refrigerants. ANSI/ASHRAE Standard 34-2007, ASHRAE, Atlanta GA, 2007. [11] S. Kondo, K. Takizawa, A. Takahashi, K. Tokuhashi, Extended Le Chatelier’s formula and nitrogen dilution effect on the flammability limits, Fire Saf. J. 41 (2006) 406–417. [12] American Society of Testing and Materials. Standard test method for concentration limits of flammability of chemicals. Designation: E681-04, Philadelphia, 2004. [13] E. Berl, G. Werner, Z. Angew. Chem. 40 (1927) 245.
Contents lists available at ScienceDirect
Fire Safety Journal journal homepage: www.elsevier.com/locate/firesaf
On the pressure dependence of flammability limits of CH2 ¼ CFCF3, CH2F2 and methane Shigeo Kondo n, Akifumi Takahashi, Kenji Takizawa, Kazuaki Tokuhashi National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan
a r t i c l e i n f o
abstract
Article history: Received 7 September 2010 Received in revised form 18 February 2011 Accepted 23 March 2011 Available online 16 April 2011
Flammability limits of CH2 ¼ CFCF3 (HFO-1234 yf), CH2F2 (HFC-32), and methane were measured at pressures from ambient to 2500 kPa in a 5 l stainless-steel spherical vessel. For HFO-1234 yf, as the pressure rises from ambient, the lower flammability limit is shifted downward and the upper limit is shifted upward. The changes to the lower flammability limits are, in general, small compared to the upper flammability limits. Both the lower and upper flammability limits of this compound can be approximated by simple logarithmic functions of pressure. For HFC-32, the behavior of lower flammability limit is similar to that for HFO-1234 yf, but the behavior for the upper limit is rather complicated. As the pressure is increased, it begins to rise upward gradually. Then, as the pressure becomes larger than about 1000 kPa it begins to rise upward rapidly, and then the change becomes moderate again. This must be due to a change of combustion reaction mechanism below 1000 kPa and above 1500 kPa in the upper flammability limit region for this compound. On the other hand, both the flammability limits of methane change almost linearly with pressure, at least in the pressure region considered. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Flammability limits Pressure dependence HFO-1234 yf HFC-32 Methane
1. Introduction Recently, various fluorinated compounds have been synthesized and used mainly as refrigerants. Refrigerant compounds are generally used at ambient pressure or a little higher than ambient pressure. Thus, data for flammability limits of these compounds under pressurized conditions may be useful. With regard to experimental flammability limits, they are more or less dependent on the apparatus and conditions under which they are measured [1–4]. Indeed, the flammability limits are dependent on the experimental temperature and pressure as well. Some data of the pressure dependence of flammability limits are reported in the literature (e.g. [1]). It is known that the flammable ranges of hydrocarbon compounds, in general, become wider as the pressure is raised: the lower limit becomes lower and the upper limit becomes higher. As for the flammability limits of natural gas, Zabetakis [5] has pointed out that both the flammability limits, obtained by Jones et al. [6], can be expressed as simple logarithmic functions of pressure. On the other hand, Vanderstraeten et al. [7] have reported that the pressure dependence of the flammability limits of methane is expressed by a simple power series of pressure. However, since the main component of natural gas is methane, it is hard to accept these two results at the same time. It is desirable to clarify the reason for the
n
Corresponding author. Tel./fax: þ 81 29 861 4770. E-mail address: [email protected] (S. Kondo).
0379-7112/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.firesaf.2011.03.005
apparent inconsistency between the two results. On the other hand, it is of particular interest to know whether the flammability limits of such compounds as HFCs and HFOs behave similarly to natural gas under pressurized conditions. Previously, the authors have published a paper describing the relationship between adiabatic flame temperature and pressure in the lower flammability limit region [8]. The result is that the following equation holds between them:
p p0
a
¼
T T0
b
E E exp RT RT0
ð1Þ
Here, a is a constant and takes a value between 1 and 3 depending on the order of combustion reaction, b is a constant and takes a value between 2 and 2 depending on the flame quenching mechanism at the flame propagation limit, T and T0 are the adiabatic flame temperatures at pressures p and p0, respectively, E is the activation energy of combustion, and R is the gas constant. In the lower flammability limit region, the adiabatic flame temperature T can be considered to be proportional to the flammability limit concentration L. Namely, L/T¼L0/T0 ¼const. Thus, we may be able to rewrite Eq. (1) as
p p0
a ¼
b L cE cE exp L0 RL RL0
ð2Þ
This equation suggests that the lower flammability limit becomes a logarithmic function of pressure. The finding by Zabetakis [5], as mentioned above, is a good example for this. It has also been recently
290
S. Kondo et al. / Fire Safety Journal 46 (2011) 289–293
established that the geometric mean of upper and lower flammability limits is very stable during temperature change [9]. If this also applies to the pressure change, it is possible that the pressure behavior of upper flammability limits follows a logarithmic function as well. In this paper, we report the result of measurements and analysis of the pressure dependence of upper and lower flammability limits for HFO-1234 yf, HFC-32, and methane.
2. Experiments The measurement of flammability limits was done in a 5 l spherical stainless-steel explosion vessel. This vessel is equipped with a fan for gas mixing. The ignition of sample gas–air mixtures was made by fusing tungsten wire, 0.3 mm in diameter and 10 mm in length, stretched between a couple of electrodes. Fusing was made by direct current from two car batteries connected in series, giving 24 V. The electric energy consumed by the tungsten wire by the time of fusion was about 20 J, measured by recording electric current and voltage. However, the actual ignition ability of this ignition source was less than that of 0.4 s AC spark used in ASHRAE method [10], as observed for the measurement of HFO-1234 yf at ambient and at 200 kPa pressures. The 0.4 s AC spark corresponds to about 10 J of energy [11]. The electrodes were positioned about one-third of the distance from the bottom to the top of the vessel ceiling. This is atypical as it is true that a central ignition point is generally used for flammability experiments in spherical vessels. However, since the burning velocity is slow in the flammability limit regions, flames tend to go up to the ceiling after ignition. Therefore, the space above the ignition area is important for these flammability limit experiments. The ignition source should be positioned somewhere between the center and the bottom of the vessel. We followed the setting of standard ASTM procedure [10,12]. A schematic diagram of experimental setup is shown in Fig. 1. All experiments were carried out at room temperature. The flammability limits were determined by assessing the maximum pressure rise after ignition. In this case, the criterion for determining flammability limits is of particular importance. The present criterion has been determined referring
to the one proposed by ASHRAE [10,12]. According to the ASHRAE criterion, the ignition experiment is done in a 12 L spherical flask, and the gas mixture prepared in the flask is determined to be flammable if the flame moves upward and outward from the point of ignition, to reach an arc of the vessel wall subtending an angle larger than 901 as measured from the ignition point. The volume inside the cone of 901 and under the vessel wall amounts to about 30% of the total vessel volume. Now, provided that the volume is completely filled with burnt gas whose temperature is 1500 K for example, the pressure rise in the vessel may become about 30% of the initial value. Considering this, we have introduced a criterion that the mixture is flammable if the maximum pressure rise exceeds 20% of the initial sample pressure. For the measurements, gas mixtures were directly prepared in the explosion vessel by the partial pressure method. Before introduction of gases, the vessel was evacuated to 0.1 Torr (1 Torr¼133.32 Pa). The fuel gas was introduced into the vessel first followed by dry air. Three kinds of MKS Baratrons, 100, 1000, and 20000 Torr heads, were used as necessary for the sample pressure measurements. Gas mixtures were prepared in the vessel, stirred with a fan for 15 min, then left to settle for 1 min before ignition. During and after ignition, the pressure profile was measured using a strain gage and the data were recorded via an amplifier. It is known that the partial pressure method does not necessarily give the correct concentration under high pressures. This is because the gases do not obey the ideal gas law under such conditions. Therefore, it is necessary to investigate the relationship between partial pressures and true concentrations under high pressures. For this purpose, we have analyzed the gas mixtures using IR absorption intensity measurements in order to obtain the relationship between partial pressures and true concentrations for the three compounds mixed with air. The result is that for any of the three compounds considered, the deviations between partial pressures and true concentrations are much less than 1% in relative value even for a mixture pressure of 2500 kPa. Based on this result, the present experiments were all prepared using the partial pressure method. The flammability limits of methane, HFC-32, and HFO-1234 yf were measured at pressures from ambient to 2500 kPa for the lower limits and from ambient to 2000 kPa for the upper limits.
Gas
Air
Pressure-gauge
Amplifier
Strain-gauge
90°
Oscilloscope
Electrode
Electrode Fan Explosion-vessel
Soda-lime Pump
Fig. 1. Experimental setup for measuring flammability limits under high pressures.
S. Kondo et al. / Fire Safety Journal 46 (2011) 289–293
Sample gases were purchased from chemical companies. Purities of fuel gases were all 99% or better. Air was extra-pure and dry. All sample materials were used as supplied without further purification.
3. Results and discussion For methane in particular, there are several studies on the pressure dependence of flammability limits reported in the literature [1,5,7,13]. There are also similar studies of the pressure effect on the flammability limits of natural gas whose main components are methane and ethane [6]. The variation of flammability limits of natural gas with pressure are expected to behave very similarly to those of methane. In general, the lower limit goes down to lower concentrations and the upper limit goes up to higher concentrations when the pressure is increased. The variation is rapid at first and then becomes gradual afterward. On the whole, the change of upper flammability limits is more significant than the lower limit. Zabetakis [5] has pointed out that the change of both the flammability limits of natural gas due to pressure can be expressed by a simple logarithmic function of pressure. 3.1. HFO-1234 yf Fig. 2 shows the result of measurement for HFO-1234 yf, where the observed values are given by black and white circles. As will be explained below, the solid line in this figure shows the calculated values. For this compound, fusing tungsten wire 0.3 mm in diameter and 10 mm long does not provide enough energy for ignition at ambient pressure, thus we could not obtain the values for the flammability limits at the ambient pressure. The black circles in Fig. 2 are the values obtained using a 12 l spherical glass flask with a 0.4 s AC spark ignition. In this figure, the point for the lower flammability limit at 200 kPa looks too high, which may be due to insufficient ignition energy as well. At 500 kPa or higher, the present ignition system seems to give enough energy. As suggested in the preceding section, variation of the flammability limits due to pressure may follow an appropriate logarithmic function of pressure. Zabetakis [5] has found that both the lower and upper flammability data for natural gas can be explained appropriately by the following type of equation: p L ¼ L0 þ k ln ð3Þ p0 Here, L and L0 are the flammability limit values at pressures p and p0, respectively, and k is a constant. In this study, this equation was
applied to explain the lower flammability data of HFO-1234 yf to obtain the following equation: p L ¼ 5:350:55 ln ð4Þ 500 Here, L is in partial pressure % and p in kPa. Similarly, for the upper flammability limit data the following equation was obtained: p U ¼ 16:5 þ 3:2 ln ð5Þ 500 The solid lines for the upper and lower flammability limits in Fig. 2 were obtained using these equations. 3.2. HFC-32 For measurement of HFC-32, fusing tungsten wire 0.3 mm in diameter and 10 mm long gives enough energy for ignition at ambient pressure. The result of measurement for this compound is shown in Fig. 3. For the lower flammability limit of this compound, the behavior against pressure is quite similar to that of HFO-1234 yf. Actually, it has been found that the data can be represented well by the following equation: p L ¼ 11:701:3 ln ð6Þ 500 The solid line for the lower flammability limit in Fig. 3 was obtained using this equation. On the other hand, the behavior of upper flammability limit has been found to be very unusual. From ambient pressure to about 1000 kPa, the behavior is moderate and looks like a mirror image of the lower limit. Then, as the pressure goes up further to exceed 1000 kPa, the upper limit begins to increase very rapidly. The behavior of the upper flammability limit in the whole pressure range from ambient to 2000 kPa cannot be explained by a single logarithmic function. Rather, it may be that the behavior below 1000 kPa is explained by one logarithmic function, and that above 1500 kPa by another. This result strongly suggests that the combustion reaction mechanisms are different in the pressure region below 1000 kPa and in the region above 1500 kPa. The solid line representing the upper flammability limit in Fig. 3 is simply a spline curve connecting the observed values shown by white circles. CH2F2 is a stable compound at ambient temperature and pressure, and the major reaction of this compound may be oxidation reaction in the upper flammability region as well. However, one thing worth noting is that this compound can release a certain amount of heat when it is decomposed as follows: CH2 F2 ¼ C þ 2HF; DH ¼ 95:3 kJ
ð7Þ
50 Flammability limit (%)
25 Flammability limit (%)
291
20 15 10 5
40 30 20 10 0
0 0
500
1000
1500 2000 Pressure (kPa)
2500
Fig. 2. Effect of pressure on the flammability limits of CH2 ¼ CFCF3.
3000
0
500
1500 1000 Pressure (kPa)
2000
Fig. 3. Effect of pressure on the flammability limits of CH2F2.
2500
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It is very probable that this reaction plays a role in making the peculiar pressure behavior of upper flammability limit observed for this compound. In this molecule, there are equal numbers of H and F atoms, and all of the H and F atoms are attached to the same carbon atom. We believe that this fact makes its self-decomposition easier to produce the characteristic pressure behavior of upper flammability limit. Further detailed investigation is needed to clarify the reaction mechanism occurring in flames in the upper flammability limit region of this compound. 3.3. Methane The results of measurements for methane are shown in Fig. 4. The behavior of the flammability limits of this compound is a little different from the observations for HFO-1234 yf and HFC-32. Although the variation of lower flammability limit is small and not far out of the uncertainty level (0.05–0.1 vol%), it seems to show tendency to go up slightly at the first stage of pressure increase and then begins to go down to lower concentrations. This tendency is not conspicuous at all but somewhat resembles that of hydrogen flammability limits [1]. At any rate, it appears that the lower flammability limit of methane may be considered almost constant within the pressure range of 100–2500 kPa. The solid line for the lower flammability limit in Fig. 3 was calculated using the following equation: L ¼ 5:09 þ0:000065p
ð8Þ
Regarding the upper flammability limit of methane, the results suggest that the observed values can appropriately be explained by a linear function of pressure. The solid line for the upper flammability limit in Fig. 4 was obtained using the following equation: U ¼ 15:27 þ0:00596p
ð9Þ
As mentioned in Section 1, the pressure behavior of the flammability limits of methane has already been investigated by several authors [1,5,7,13]. Zabetakis [5] pointed out that both the flammability limits of natural gas, which were composed of 85% methane and 15% ethane, can be fitted to simple logarithmic functions of pressure like Eq. (3). On the other hand, Vanderstraeten et al. [7] found that the pressure dependence of upper flammability limits of methane can be expressed by a simple power series of pressure rather than a logarithmic function. As described in the above, we have found that both the upper and lower flammability limits show almost linear dependence on the pressure. In connection with these studies, the experimental work by Berl and Werner [13] is noteworthy. This was done for a pressure range from ambient to as high as 40000 kPa. According to their results, both the upper and lower flammability limits of methane move to
Flammability limit (%)
30 25 20 15 10 5 0 0
500
1000
1500 2000 Pressure (kPa)
2500
Fig. 4. Effect of pressure on the flammability limits of methane.
3000
narrow the flammable range in the initial stages of pressure increase: the lower limit goes to higher concentrations and the upper limit goes to lower concentrations as the pressure is increased. Then, as the pressure becomes about 2000 kPa or higher, both the flammability limits begin to extend the flammable range. This resembles the well known pressure behavior of hydrogen flammability limits [1]. As described in reference [8], the pressure behavior of flammability limits is dependent on the reaction and/or quenching mechanisms. If the limit flame temperature becomes high when the pressure is increased, it indicates that the flame quenching mechanism is determined by a competition between radical increasing and decreasing reactions. And the reversal of pressure behavior at around 2000 kPa must be due to changes of reaction and/or quenching mechanisms. In the vicinity of the pressure behavior reversal region, the flammability limit values are sensitive to the apparatus and experimental conditions employed for the measurement. In fact, the pressure behaviors of hydrogen flammability limits in that region, which have been reported so far by different authors, are quite unstable [1]. Coming back to the present study, the measurements were done in a pressure range from ambient to 2500 kPa for the lower limit and from ambient to 2000 kPa for the upper limit. This means that almost all our measurements were done within that particular region of pressure reversal. The measurement by Vanderstraeten et al. [7] was for the pressure range from ambient to 5500 kPa, and so at least one-third of their data was taken within this region. On the other hand, the measurement of natural gas by Jones et al.[6] was done for the pressure range from ambient to 3000 psi ( ¼20700 kPa), and therefore the major part of their measurements were done well outside this region. As was apparent in the case of hydrogen [1], it is very possible that the pressure behavior of flammability limits of methane in the range from the ambient to around 2000 kPa is sensitive to the apparatus and the conditions employed for the measurements. In addition to this, the difference of pressure range studied may be responsible for the divergence of pressure behaviors of methane flammability limits for various measurements.
4. Conclusion The flammability limits were measured under high pressures for HFO-1234 yf, HFC-32, and methane. The lower limits were measured for a pressure range from ambient to 2500 kPa, and the upper limits were measured from ambient to 2000 kPa. In many cases, it seems that the pressure dependence of upper and lower flammability limits can be fitted to logarithmic functions of pressure. In this sense, the behavior of flammability limits of HFO-1234 yf is a typical one. As for HFC-32, the lower limit can also be fitted to a logarithmic function. However, the upper flammability limit of HFC-32 is very extraordinary. The pressure behavior seems to be composed of two different stages: one is for a pressure range from ambient to approximately 1000 kPa and the other for pressures higher than 1500 kPa. This may be due to change of combustion reaction mechanism. It could be that the main reaction mechanism changes from combustion to thermal decomposition in the pressure region from about 1000 to 1500 kPa. Interestingly, the pressure behavior of the simple compound of methane is a little complicated. The lower limit seems to show a slight increase at the initial stage of pressure increase and then shows a tendency to turn to lower concentrations. This behavior somewhat resembles the behavior of hydrogen gas [1]. It is possible that similar reactions and/or quenching mechanisms exist for the two cases. On the other hand, the behavior of upper limit of methane is quite different from what was expected from the literature data. The results
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for the pressure behavior of flammability limits for methane show an almost linear dependence on the pressure. This is neither consistent with that by Vanderstraeten et al. [7] nor with that by Berl and Werner [13]. Including the report by Zabetakis [5] on flammability limits of natural gas, different measurements were found to give quite different results. The inconsistency of pressure behavior of methane among different studies seems to be related to sensitivity of reaction mechanism and/or quenching mechanism to the change of pressure in the region from ambient to about 2000 kPa. In spite of the exceptions encountered in the present study, it may be said that, in general, the pressure dependence of flammability limits tends to follow a simple logarithmic function. References [1] H.F. Coward, G.W. Jones, Limits of flammability of gases and vapors. US Bureau of Mines Bulletin 503, 1952. [2] A. Takahashi, Y. Urano, K. Tokuhashi, H. Nagai, M. Kaise, S. Kondo, Fusing ignition of various metal wires for explosion limits measurement of methane/ air mixture, J. Loss Prevention Process Ind 11 (1998) 353–360. [3] S. Kondo, Y. Urano, A. Takahashi, K. Tokuhashi, Reinvestigation of flammability limits measurement of methane by the conventional vessel method with AC discharge ignition, Combust. Sci. Technol. 145 (1999) 1–15.
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[4] A. Takahashi, Y. Urano, K. Tokuhashi, S. Kondo, Effect of vessel size and shape on experimental flammability limits of gases, J. Hazard. Mater. A105 (2003) 27–37. [5] M.G. Zabetakis, Flammability characteristics of combustible gases and vapors. US Bureau of Mines Bulletin 627, 1965. [6] G.W. Jones, R.E. Kennedy, I. Spolan, Effect of high pressures on the flammability of natural gas–air–nitrogen mixtures. U.S. Bureau of Mines Report of Investigation No. 4557, Washington D.C., 1949. [7] B. Vanderstraeten, D. Tuerlinckx, J. Berghmans, S. Vliegen, E. Van’t Oost, B. Smit, Experimental study of the pressure and temperature dependence on the upper flammability limit of methane/air mixtures, J. Hazard. Mater. 56 (1997) 237–246. [8] S. Kondo, S. Horiguchi, Y. Urano, K. Tokuhashi, Pressure dependence of the limit flame temperatures for premixed gas explosions, J. Jpn. Soc. Saf. Eng. 29 (1990) 241–250 in Japanese. [9] S. Kondo, K. Takizawa, A. Takahashi, K. Tokuhashi, On the temperature dependence of flammability limits of gases, J. Hazard. Mater. 187 (2011) 585–590. [10] American Society of Heating, Refrigerating and Air-conditioning Engineers. Number designation and safety classification of refrigerants. ANSI/ASHRAE Standard 34-2007, ASHRAE, Atlanta GA, 2007. [11] S. Kondo, K. Takizawa, A. Takahashi, K. Tokuhashi, Extended Le Chatelier’s formula and nitrogen dilution effect on the flammability limits, Fire Saf. J. 41 (2006) 406–417. [12] American Society of Testing and Materials. Standard test method for concentration limits of flammability of chemicals. Designation: E681-04, Philadelphia, 2004. [13] E. Berl, G. Werner, Z. Angew. Chem. 40 (1927) 245.