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Experimental and simulation studies on the influence of carbon monoxide on explosion characteristics of methane
Journal of Loss Prevention in the Process Industries 36 (2015) 45e53
Contents lists available at ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Experimental and simulation studies on the influence of carbon monoxide on explosion characteristics of methane Jun Deng a, b, Fangming Cheng a, b, *, Yu Song a, Zhenmin Luo a, b, Yutao Zhang a, b a
School of Energy Engineering, Xi'an University of Science and Technology, Xi'an, Shaanxi 710054, PR China Key Western China Laboratory for Coal Exploitation Development and Safety for Ministry of Education, Xi'an University of Science and Technology, Xi'an, Shaanxi 710054, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 August 2014 Received in revised form 9 April 2015 Accepted 1 May 2015 Available online 2 May 2015
In underground coal mining, methane explosions often can cause tremendous disasters. In the meantime, carbon monoxide (CO), generated during the process of coal oxidation, may appear in the air. Therefore, the explosion characteristics of the mixture of CH4 and CO must be investigated to prevent gas explosion accidents in coal mines. We conducted experiments by using a 20-L nearly spherical gas explosion testing device. The software FLACS was used to simulate the explosion of the mixture of CH4 and CO at various mixing concentrations, and the simulation results corresponded to experimental results. With the increase of CO concentration, both upper and lower explosive limits of CH4 decreased. On the whole, the explosion characteristic parameters of CH4 and the mixture are similar. When CH4 concentration was below the stoichiometric concentration, the addition of CO could promote the intensity of gas explosion; oppositely, excessive CO would inhibit the gas explosion reaction. The inhibitory effects become more significant as the concentration of CH4 increases. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Coal mining Methane explosions Carbon monoxide Explosive limits Explosion characteristic parameters
1. Introduction In China, gas explosions are always the greatest threat in underground coal mines because of complicated geological conditions and other objective factors. In recent years, gas explosions have occurred frequently and brought great casualties and economic losses. In underground coal mines, methane is the primary explosive gas. Other gases, such as carbon monoxide (CO), ethane, or hydrogen, produced from the spontaneous combustion of coal, can also affect the explosion characteristics of methane (Li and Si, 2010; Peng et al., 2011; Wang et al., 2011; Huang et al., 2012). Previous studies mainly focused more on the explosion characteristics of methane-air than the multiple combustible gases existing in coal mines. Hu et al. (2002) studied the explosive limits of a mixture of H2, CH4 and CO as well as the influence of the container. Experimental results showed that their explosive limits were affected by
* Corresponding author. School of Energy Engineering, Xi'an University of Science and Technology, Xi'an, Shaanxi 710054, PR China. E-mail address: [email protected] (F. Cheng). http://dx.doi.org/10.1016/j.jlp.2015.05.002 0950-4230/© 2015 Elsevier Ltd. All rights reserved.
various factors. Chen and Zhang (2009) experimentally studied the explosive limits and the critical oxygen concentration of multiple combustible gases. Li et al. (2008, 2012) conducted experiments on explosion characteristics of the mixture of CH4, H2, coal dust, and air. Yetter et al. (1991), Bougrine et al. (2011), Di Sarli et al. (2012), Di Sarli and Di Benedetto (2013) and Salzano et al. (2012) studied the explosion characteristics of CH4eH2 in air under different conditions. Di Sarli et al. (2014) studied the explosion behavior of mixtures with composition representative of wood chip-derived syngas (CO/H2/CH4/CO2/N2 mixtures). The above researches show that the behavior of a fuel blend is strongly non-linear and, as such, it cannot be extrapolated from the behavior of the pure components. CO in coal mines is generated from the oxidation of coal. The content of CO increases exponentially as the temperature of coal rises. The existence of CO may increase the risk and consequence of gas explosion, which necessitates understanding the effect of CO on the methane-air explosion (Jia et al., 2013). In this paper, experiments and simulation were utilized to study the interaction of CO and CH4 in terms of explosion characteristics.
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J. Deng et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 45e53
Fig. 1. Schematic of the experimental system.
2. Experimental and methods 2.1. Experimental system The experimental system, illustrated in Fig. 1, consists of explosion reactor, gas mixing system, ignition system, and measurement system. (1) Explosion reactor: It is a 20-L nearly spherical tank with internal diameter and height 30 and 35 cm, respectively; there are inlet and outlet pipes, sensors, ignition electrode, powder nozzle, and some mounting interfaces in this instrument. The maximum pressure capacity is 2 MPa. (2) Gas mixing system: The structure includes vacuum pump, air compressor, precise digital pressure gauge (range: 0e101.3 kPa; precision: 0.01 kPa), methane cylinder, and carbon monoxide cylinder. The compound gas is mixed based on partial pressure law. (3) Ignition system: The ignition source is a chemical propellant located in the center of the explosion reactor. The ignition is controlled automatically by the computer, and the ignition energy is ca. 1 J. (4) Measurement system: It mainly consists of pressure sensor, controller, and computer. The data can be collected simultaneously when the system is ignited. The response time and the maximum collecting time are 1 and 500 ms, respectively.
the explosive limit was estimated to be from 6.52 to 17.42%. The limit should range from 5.20 to 14.70% according to the stoichiometric concentration method (Wang and Bi, 2000; Parra et al., 2004; Xu and Xu, 2005; Zhang et al., 2011; Liu et al., 2013). To be on the safe side, an explosive limit of 5.2e17.42% was used as the theoretical range of CH4 explosive limit. The explosive limits of the mixtures of CO/CH4 with various concentrations were tested, and the results are shown in Fig. 2. We observed that when the CO concentration increased, the lower explosive limit of mixture decreased, and the lower explosive limit decreased 0.5% with increase of 1% CO concentration. Similarly, when CO concentration increased 0.5%, the upper explosive limit of the mixture went down 0.5e1%. In summary, with increasing CO concentration, both the upper and the lower explosive limits decreased. When the CO concentration was increased from 0 to 3.0%, the upper limit was decreased by nearly 2.5%, while the lower limit dropped by about 1.5%. Therefore, the addition of CO increased the explosibility of lowconcentration methane. 3.2. Explosion characteristics of the CH4/CO mixture Five groups of gas mixtures with CO concentrations of 0.0, 0.5, 1, 2, and 3%, respectively, were used for the explosion tests. Explosion
2.2. Testing conditions and operating process The ambient temperature and humidity were 17e27 C and 50e90%, respectively. The pressure before explosion was standard atmospheric. The test was carried out in the airtight and volumeconstant tank. The occurrence of explosion can be considered when the pressure increases beyond 7% following the definition of ASTM (American Society for Testing and Materials). 3. Experimental results and analysis 3.1. Influence of CO on CH4 explosive limits According to the experimental results, the explosive limit of CH4 was 5.35 and 17.35% by nearly spherical tank test. According to the number of oxygen atoms required for complete CH4 combustion,
Fig. 2. The influence of CO on the CH4 explosive limit.
J. Deng et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 45e53
Fig. 3. Characteristics of the mixture explosion pressure.
parameters, such as the maximum explosion pressure and the time to reach the maximum pressure (TRMP), were obtained under different conditions. The test results are in Fig. 3. As can be seen from Fig. 3(a), the trends of the maximum explosion pressures are similar for the five groups of mixtures with various CO concentrations. Each of them increased to a certain value and decreased thereafter. The maximum explosion pressure was changed with various CO concentrations that are illustrated in Table 1. With increasing CO concentration, the maximum explosion pressure of the mixture was rising. However, the corresponding CH4 concentration decreased. When the CH4 concentration was less than 11%, the explosion pressures of all mixtures were larger than pure CH4. However, it showed opposite results when CH4 concentrations exceeded 11%. From Fig. 3(b), TRMPs for all the groups went down first and then up after the CH4 concentrations exceeded certain values. The shortest TRMPs for different CO and CH4 concentrations are listed in Table 2. When CH4 concentration is below its stoichiometric concentration, increase of CO can enhance the rate of gas explosive reaction. With the increase of CO concentration, the TRMPs became less. When CH4 concentration exceeded its stoichiometric concentration, the addition of CO would inhibit the gas explosion reaction.
Fig. 4. Simulated geometric model of explosion vessels.
Table 1 Concentrations of CO and CH4 when reaching to maximum explosion pressures. CO concentration/% CH4 concentration/% Maximum explosion pressure/MPa 0 0.5 1.0 2.0 3.0
11 10 9 9 9
0.783 0.809 0.836 0.859 0.880
Table 2 Shortest TRMPs for different CO and CH4 concentrations. CO concentration/%
CH4 concentration/%
TRMP/ms
0 0.5 1.0 2.0 3.0
11 10 9 9 9
85 78 75 70 59
Fig. 5. Grid division of simulated model.
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J. Deng et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 45e53
Fig. 6. Simulation results of maximum explosion pressure and TRMP vs. CO and CH4 concentrations.
Both the maximum explosion pressure and the shortest TRMP occurred at the same CH4 and CO concentrations. When CH4 concentration was below its stoichiometric concentration, the maximum explosion pressure rose and the TRMP became shorter with the increase of the CO concentration. Accordingly, the addition of CO could intensify the gas explosion. When CH4 concentration was above its stoichiometric one, the excessive CO would inhibit the gas explosion reaction. 4. Simulation results and analysis using FLACS FLACS (Flame Acceleration Simulator), developed by Norway
CMR research, is the computational fluid dynamics tool to simulate the ventilation, gas diffusion, vapor cloud explosion, and shock waves. FLACS was used to simulate the explosion parameters of mixed gases in this study. 4.1. Establishment of the geometric model The geometry of the explosion vessel is plotted in Fig. 4. A pressure sensor was mounted on the side wall and an ignition source was located in the sealed container. The meshes of the entire explosion reactor are shown in Fig. 5. There are 20 meshes at each coordinate.
Fig. 7. Profiles of explosion parameters.
J. Deng et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 45e53
4.2. Simulation results and analysis The same mixtures as in the experiments were used to conduct the explosion simulation; the results are plotted in Fig. 6. Simulation results indicated that the maximum explosion pressures for all the groups increased at the beginning and decreased after reaching the maximums. In contrast, the TRMPs were just the opposite. They all dropped at first and went up after certain values. The explosion pressures of all the five groups reached the maximum when the CH4 concentration was 10%, at which the TRMPs reached the minimums. In addition, when the CH4 concentration was below 10%, with the increase of CO concentration, the explosion pressure rose and the TRMPs alleviated. However, when the CH4 concentration exceeded 10%, as the CH4 concentration increased, the excessive CO acted as an inhibitor. The maximum explosion pressure was lessened and the TRMP was gradually increased. The simulation results were consistent with experimental results.
49
Mixtures of CH4 of 10% concentration were used for the following simulations. Explosion parameters including pressure, temperature, velocity of the explosive gases, and the concentrations of the explosion products at the wall of the explosion reactor are simulated and plotted in Fig. 7. Fig. 7(a) and (b) indicate that the maximum explosion of 10% CH4 in the air was 0.798 MPa with the maximum temperature up to 2496 K. Before reaching the maximum pressures, the addition of CO made the explosion pressures higher than that of pure CH4 mixed with air. But after the maximum pressures, the explosion pressures became slightly lower than mixtures without CO. From Fig. 7(c), affected by the refluxing swirl, multiple peaks are present for the velocity of the explosive gases (Luo et al., 2013). The addition of CO could promote the explosion velocity. For instance, the velocity changed from 3.42 to 3.92 m/s as the CO concentration was increased from 0 to 3.0%. Fig. 7(d) suggests that the concentration of explosion products changed from 27.7 to 30.1% when the CO concentration was increased from 0 to 3.0%.
Fig. 8. Contours of the explosion parameters (t ¼ 16 ms).
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J. Deng et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 45e53
The trend of TRMP was in accordance with that of concentrations of the explosion products. Both the explosion pressure and the concentrations of the explosion products reached the maximum for a complete explosion reaction. Contours of the pressure, temperature, concentration and velocity fields for the normal section (X ¼ 0) at different times are diagramed in Figs. 8e11. From the pressure distributions at a specific time, the pressures were almost uniform in the explosion reactor due to the limited space. At the early stage, there were few changes among the pressures for different groups. With the increase of the CO concentration, the pressure became larger. The explosion pressure reached its maximum value at a CO concentration of 2.0%. The pressure went down a little when the CO concentration was
3.0%. As can be seen from the temperature distribution, the temperature was higher near to the ignition source and gradually dropped until the leading edge of the combustion wave. Generally, the temperature rose as the CO concentration increased from 0 to 2.0%. The temperature went down a little at a CO concentration of 3.0%. The addition of CO could accelerate the reaction rate. Contours of the concentration of explosion products indicated that the explosion spread in the form of a spherical wave. Because the horizontal width of the reactor was larger than its height, the explosion products reached the horizontal wall first, then the top and bottom. The reactor was filled with explosion products at about 53 ms, when the reaction was complete and the pressure reached
Fig. 9. Contours of the explosion parameters (t ¼ 32 ms).
J. Deng et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 45e53
its maximum. In addition, few differences were observed in terms of the distributions of the concentration for explosion products. According to the velocity distribution of the explosive gases, the burnt gaseous products expanded and propelled the unburned gases. The explosion wave was reflected after reaching the wall. Swirls formed due to the interference of the waves. The velocity of the explosive gases reached its peak value at 42 ms for CH4/air mixture and at 39e40 ms after CO was introduced into the mixture. 5. Conclusions A 20-L near-spherical explosion testing apparatus and FLACS simulation software were used to elucidate the characteristics of explosion for mixture of CH4/CO. The following conclusions were drawn.
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(1) The explosive limits will change after adding CO to CH4. With the increase of CO concentration, the upper and lower explosive limits decrease. Before reaching the stoichiometric explosion concentration of CH4, the addition of CO promotes the explosion reaction of the mixture. (2) The simulation results showed that the stoichiometric concentration of explosion of CH4 is 10%. The highest explosion pressure was 0.798 MPa with the maximum temperature of 2496 K. The explosion pressure and temperature of mixture of CH4/air after adding CO increased until reaching their maximum values and went down a little afterwards. (3) The simulation results of FLACS were corresponding to the experimental results. This study provides a further understanding of the explosion characteristics for the mixture of combustible gases.
Fig. 10. Contours of the explosion parameters (t ¼ 42 ms).
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J. Deng et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 45e53
Fig. 11. Contours of the explosion parameters (t ¼ 53 ms).
Acknowledgments This work was supported byNational Natural Science Foundation of China (Grant No. 51304155, 51134019), the Ministry of Education Funded Project of Innovation Team (Grant No. IRT0856) and the Doctoral Program in Xi'an University of Science and Technology (Grant No. 2013QDJ039), Shaanxi Provincial Science and Technology Innovation Project 13115 (Grant No. 2010ZDGC-14). References ASTM E681-01, Standard Test Method for Concentration Limit of Flammability of Chemical (Vapors and Gases), USA. Bougrine, S., Richard, S., Nicolle, A., 2011. Numerical study of laminar flame properties of diluted methane-hydrogen-air flames at high pressure and temperature using detailed chemistry. Int. J. Hydrog. Energy 18, 12035e12047.
Chen, X., Zhang, E., 2009. Experimental study on the methane mixed combustive gas explosive characteristic. Saf. Coal Mines 10, 7e10. Di Sarli, V., Di Benedetto, A., 2013. Effects of non-equidiffusion on unsteady propagation of hydrogen-enriched methane/air premixed flames. Int. J. Hydrog. Energy 38, 7510e7518. Di Sarli, V., Di Benedetto, A., Long, E.J., Hargrave, G.K., 2012. Time-resolved particle image velocimetry of dynamic interactions between hydrogen-enriched methane/air premixed flames and toroidal vortex structures. Int. J. Hydrog. Energy 37, 16201e16213. Di Sarli, V., Cammarota, F., Salzano, E., 2014. Explosion parameters of wood chipderived syngas in air. J. Loss Prev. Process Ind. 32, 399e403. Hu, Y., Zhou, B., Yang, Y., 2002. The limit and container factors of H2, CH4 and CO multiple explosive mixed gas. Sci. China 1, 35e39. Huang, Z., Si, R., Zhang, Y., 2012. Numerical simulation of the influence of initial temperature on gas explosion characteristics. Saf. Coal Mines 5, 5e7. Jia, B., Wang, X., Wen, H., 2013. Simulation study on reaction kinetics of CO inhibiting gas explosion in roadway of coal mine. Blasting 1, 35e40. Li, Q., Lin, B., Dai, H., 2012. Explosion characteristics of H2/CH4/air and CH4/coal dust/air mixtures. Powder Technol. 229, 222e228.
J. Deng et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 45e53 Li, R., Si, R., 2010. Effect of gas concentration on the explosion pressure and pressure rise rate. J. Xi'an Univ. Sci. Technol. 1, 29e33. Li, Z., Lin, B., Zhang, L., 2008. Effects of hydrogen production on gas explosion. J. China Univ. Min. Technol. 2, 147e151. Liu, Y., Wang, C., Liu, F., 2013. Numerical simulation of the effect of low CO concentration on CH4 explosion in closed space. Saf. Coal Mines 4, 9e13. Luo, Z., Zhang, Q., Wang, H., 2013. Numerical simulation of has explosion in confined space with FLACS. J. China Coal Soc. 8, 1381e1387. Parra, T., Castro, F., Mendez, C., 2004. Extinction of premixed methaneeair flames by water mist. Fire Saf. J. 7, 581e600. Peng, F., Zhang, L., Gao, S., 2011. Impact on the characteristics of methane explosion by special mine gases. Saf. Coal Mines 4, 9e13. Salzano, E., Cammarota, F., Di Benedetto, A., Di Sarli, V., 2012. Explosion behavior of
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hydrogen-methane/air mixtures. J. Loss Prev. Process Ind. 3, 443e447. Wang, H., Deng, J., Ge, L., 2011. Influence of initial pressure on explosion characteristics of flammable gases in coal mine. J. China Coal Soc. 3, 423e428. Wang, S., Bi, M., 2000. The calculation of industrial multivariate mixed gas explosion limit. Chem. Equip. Tech. 6, 28e30. Xu, M., Xu, J., 2005. Explosion limits and calculated methods of combustible gas. J. Xi'an Univ. Sci. Technol. 2, 139e142. Yetter, R.A., Dryer, F.L., Rabitz, H.A., 1991. Comprehensive reaction mechanism for carbon monoxide/hydrogen/oxygen kinetics. Combust. Sci. Technol. 9, 97e128. Zhang, G., Tong, M., Ren, Z., 2011. Experimental studies on effect of alkane gases on lower explosion limits of methane. J. Kunming Univ. Sci. Technol. Nat. Sci. Ed. 2, 004.
Contents lists available at ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Experimental and simulation studies on the influence of carbon monoxide on explosion characteristics of methane Jun Deng a, b, Fangming Cheng a, b, *, Yu Song a, Zhenmin Luo a, b, Yutao Zhang a, b a
School of Energy Engineering, Xi'an University of Science and Technology, Xi'an, Shaanxi 710054, PR China Key Western China Laboratory for Coal Exploitation Development and Safety for Ministry of Education, Xi'an University of Science and Technology, Xi'an, Shaanxi 710054, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 August 2014 Received in revised form 9 April 2015 Accepted 1 May 2015 Available online 2 May 2015
In underground coal mining, methane explosions often can cause tremendous disasters. In the meantime, carbon monoxide (CO), generated during the process of coal oxidation, may appear in the air. Therefore, the explosion characteristics of the mixture of CH4 and CO must be investigated to prevent gas explosion accidents in coal mines. We conducted experiments by using a 20-L nearly spherical gas explosion testing device. The software FLACS was used to simulate the explosion of the mixture of CH4 and CO at various mixing concentrations, and the simulation results corresponded to experimental results. With the increase of CO concentration, both upper and lower explosive limits of CH4 decreased. On the whole, the explosion characteristic parameters of CH4 and the mixture are similar. When CH4 concentration was below the stoichiometric concentration, the addition of CO could promote the intensity of gas explosion; oppositely, excessive CO would inhibit the gas explosion reaction. The inhibitory effects become more significant as the concentration of CH4 increases. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Coal mining Methane explosions Carbon monoxide Explosive limits Explosion characteristic parameters
1. Introduction In China, gas explosions are always the greatest threat in underground coal mines because of complicated geological conditions and other objective factors. In recent years, gas explosions have occurred frequently and brought great casualties and economic losses. In underground coal mines, methane is the primary explosive gas. Other gases, such as carbon monoxide (CO), ethane, or hydrogen, produced from the spontaneous combustion of coal, can also affect the explosion characteristics of methane (Li and Si, 2010; Peng et al., 2011; Wang et al., 2011; Huang et al., 2012). Previous studies mainly focused more on the explosion characteristics of methane-air than the multiple combustible gases existing in coal mines. Hu et al. (2002) studied the explosive limits of a mixture of H2, CH4 and CO as well as the influence of the container. Experimental results showed that their explosive limits were affected by
* Corresponding author. School of Energy Engineering, Xi'an University of Science and Technology, Xi'an, Shaanxi 710054, PR China. E-mail address: [email protected] (F. Cheng). http://dx.doi.org/10.1016/j.jlp.2015.05.002 0950-4230/© 2015 Elsevier Ltd. All rights reserved.
various factors. Chen and Zhang (2009) experimentally studied the explosive limits and the critical oxygen concentration of multiple combustible gases. Li et al. (2008, 2012) conducted experiments on explosion characteristics of the mixture of CH4, H2, coal dust, and air. Yetter et al. (1991), Bougrine et al. (2011), Di Sarli et al. (2012), Di Sarli and Di Benedetto (2013) and Salzano et al. (2012) studied the explosion characteristics of CH4eH2 in air under different conditions. Di Sarli et al. (2014) studied the explosion behavior of mixtures with composition representative of wood chip-derived syngas (CO/H2/CH4/CO2/N2 mixtures). The above researches show that the behavior of a fuel blend is strongly non-linear and, as such, it cannot be extrapolated from the behavior of the pure components. CO in coal mines is generated from the oxidation of coal. The content of CO increases exponentially as the temperature of coal rises. The existence of CO may increase the risk and consequence of gas explosion, which necessitates understanding the effect of CO on the methane-air explosion (Jia et al., 2013). In this paper, experiments and simulation were utilized to study the interaction of CO and CH4 in terms of explosion characteristics.
46
J. Deng et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 45e53
Fig. 1. Schematic of the experimental system.
2. Experimental and methods 2.1. Experimental system The experimental system, illustrated in Fig. 1, consists of explosion reactor, gas mixing system, ignition system, and measurement system. (1) Explosion reactor: It is a 20-L nearly spherical tank with internal diameter and height 30 and 35 cm, respectively; there are inlet and outlet pipes, sensors, ignition electrode, powder nozzle, and some mounting interfaces in this instrument. The maximum pressure capacity is 2 MPa. (2) Gas mixing system: The structure includes vacuum pump, air compressor, precise digital pressure gauge (range: 0e101.3 kPa; precision: 0.01 kPa), methane cylinder, and carbon monoxide cylinder. The compound gas is mixed based on partial pressure law. (3) Ignition system: The ignition source is a chemical propellant located in the center of the explosion reactor. The ignition is controlled automatically by the computer, and the ignition energy is ca. 1 J. (4) Measurement system: It mainly consists of pressure sensor, controller, and computer. The data can be collected simultaneously when the system is ignited. The response time and the maximum collecting time are 1 and 500 ms, respectively.
the explosive limit was estimated to be from 6.52 to 17.42%. The limit should range from 5.20 to 14.70% according to the stoichiometric concentration method (Wang and Bi, 2000; Parra et al., 2004; Xu and Xu, 2005; Zhang et al., 2011; Liu et al., 2013). To be on the safe side, an explosive limit of 5.2e17.42% was used as the theoretical range of CH4 explosive limit. The explosive limits of the mixtures of CO/CH4 with various concentrations were tested, and the results are shown in Fig. 2. We observed that when the CO concentration increased, the lower explosive limit of mixture decreased, and the lower explosive limit decreased 0.5% with increase of 1% CO concentration. Similarly, when CO concentration increased 0.5%, the upper explosive limit of the mixture went down 0.5e1%. In summary, with increasing CO concentration, both the upper and the lower explosive limits decreased. When the CO concentration was increased from 0 to 3.0%, the upper limit was decreased by nearly 2.5%, while the lower limit dropped by about 1.5%. Therefore, the addition of CO increased the explosibility of lowconcentration methane. 3.2. Explosion characteristics of the CH4/CO mixture Five groups of gas mixtures with CO concentrations of 0.0, 0.5, 1, 2, and 3%, respectively, were used for the explosion tests. Explosion
2.2. Testing conditions and operating process The ambient temperature and humidity were 17e27 C and 50e90%, respectively. The pressure before explosion was standard atmospheric. The test was carried out in the airtight and volumeconstant tank. The occurrence of explosion can be considered when the pressure increases beyond 7% following the definition of ASTM (American Society for Testing and Materials). 3. Experimental results and analysis 3.1. Influence of CO on CH4 explosive limits According to the experimental results, the explosive limit of CH4 was 5.35 and 17.35% by nearly spherical tank test. According to the number of oxygen atoms required for complete CH4 combustion,
Fig. 2. The influence of CO on the CH4 explosive limit.
J. Deng et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 45e53
Fig. 3. Characteristics of the mixture explosion pressure.
parameters, such as the maximum explosion pressure and the time to reach the maximum pressure (TRMP), were obtained under different conditions. The test results are in Fig. 3. As can be seen from Fig. 3(a), the trends of the maximum explosion pressures are similar for the five groups of mixtures with various CO concentrations. Each of them increased to a certain value and decreased thereafter. The maximum explosion pressure was changed with various CO concentrations that are illustrated in Table 1. With increasing CO concentration, the maximum explosion pressure of the mixture was rising. However, the corresponding CH4 concentration decreased. When the CH4 concentration was less than 11%, the explosion pressures of all mixtures were larger than pure CH4. However, it showed opposite results when CH4 concentrations exceeded 11%. From Fig. 3(b), TRMPs for all the groups went down first and then up after the CH4 concentrations exceeded certain values. The shortest TRMPs for different CO and CH4 concentrations are listed in Table 2. When CH4 concentration is below its stoichiometric concentration, increase of CO can enhance the rate of gas explosive reaction. With the increase of CO concentration, the TRMPs became less. When CH4 concentration exceeded its stoichiometric concentration, the addition of CO would inhibit the gas explosion reaction.
Fig. 4. Simulated geometric model of explosion vessels.
Table 1 Concentrations of CO and CH4 when reaching to maximum explosion pressures. CO concentration/% CH4 concentration/% Maximum explosion pressure/MPa 0 0.5 1.0 2.0 3.0
11 10 9 9 9
0.783 0.809 0.836 0.859 0.880
Table 2 Shortest TRMPs for different CO and CH4 concentrations. CO concentration/%
CH4 concentration/%
TRMP/ms
0 0.5 1.0 2.0 3.0
11 10 9 9 9
85 78 75 70 59
Fig. 5. Grid division of simulated model.
47
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J. Deng et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 45e53
Fig. 6. Simulation results of maximum explosion pressure and TRMP vs. CO and CH4 concentrations.
Both the maximum explosion pressure and the shortest TRMP occurred at the same CH4 and CO concentrations. When CH4 concentration was below its stoichiometric concentration, the maximum explosion pressure rose and the TRMP became shorter with the increase of the CO concentration. Accordingly, the addition of CO could intensify the gas explosion. When CH4 concentration was above its stoichiometric one, the excessive CO would inhibit the gas explosion reaction. 4. Simulation results and analysis using FLACS FLACS (Flame Acceleration Simulator), developed by Norway
CMR research, is the computational fluid dynamics tool to simulate the ventilation, gas diffusion, vapor cloud explosion, and shock waves. FLACS was used to simulate the explosion parameters of mixed gases in this study. 4.1. Establishment of the geometric model The geometry of the explosion vessel is plotted in Fig. 4. A pressure sensor was mounted on the side wall and an ignition source was located in the sealed container. The meshes of the entire explosion reactor are shown in Fig. 5. There are 20 meshes at each coordinate.
Fig. 7. Profiles of explosion parameters.
J. Deng et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 45e53
4.2. Simulation results and analysis The same mixtures as in the experiments were used to conduct the explosion simulation; the results are plotted in Fig. 6. Simulation results indicated that the maximum explosion pressures for all the groups increased at the beginning and decreased after reaching the maximums. In contrast, the TRMPs were just the opposite. They all dropped at first and went up after certain values. The explosion pressures of all the five groups reached the maximum when the CH4 concentration was 10%, at which the TRMPs reached the minimums. In addition, when the CH4 concentration was below 10%, with the increase of CO concentration, the explosion pressure rose and the TRMPs alleviated. However, when the CH4 concentration exceeded 10%, as the CH4 concentration increased, the excessive CO acted as an inhibitor. The maximum explosion pressure was lessened and the TRMP was gradually increased. The simulation results were consistent with experimental results.
49
Mixtures of CH4 of 10% concentration were used for the following simulations. Explosion parameters including pressure, temperature, velocity of the explosive gases, and the concentrations of the explosion products at the wall of the explosion reactor are simulated and plotted in Fig. 7. Fig. 7(a) and (b) indicate that the maximum explosion of 10% CH4 in the air was 0.798 MPa with the maximum temperature up to 2496 K. Before reaching the maximum pressures, the addition of CO made the explosion pressures higher than that of pure CH4 mixed with air. But after the maximum pressures, the explosion pressures became slightly lower than mixtures without CO. From Fig. 7(c), affected by the refluxing swirl, multiple peaks are present for the velocity of the explosive gases (Luo et al., 2013). The addition of CO could promote the explosion velocity. For instance, the velocity changed from 3.42 to 3.92 m/s as the CO concentration was increased from 0 to 3.0%. Fig. 7(d) suggests that the concentration of explosion products changed from 27.7 to 30.1% when the CO concentration was increased from 0 to 3.0%.
Fig. 8. Contours of the explosion parameters (t ¼ 16 ms).
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J. Deng et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 45e53
The trend of TRMP was in accordance with that of concentrations of the explosion products. Both the explosion pressure and the concentrations of the explosion products reached the maximum for a complete explosion reaction. Contours of the pressure, temperature, concentration and velocity fields for the normal section (X ¼ 0) at different times are diagramed in Figs. 8e11. From the pressure distributions at a specific time, the pressures were almost uniform in the explosion reactor due to the limited space. At the early stage, there were few changes among the pressures for different groups. With the increase of the CO concentration, the pressure became larger. The explosion pressure reached its maximum value at a CO concentration of 2.0%. The pressure went down a little when the CO concentration was
3.0%. As can be seen from the temperature distribution, the temperature was higher near to the ignition source and gradually dropped until the leading edge of the combustion wave. Generally, the temperature rose as the CO concentration increased from 0 to 2.0%. The temperature went down a little at a CO concentration of 3.0%. The addition of CO could accelerate the reaction rate. Contours of the concentration of explosion products indicated that the explosion spread in the form of a spherical wave. Because the horizontal width of the reactor was larger than its height, the explosion products reached the horizontal wall first, then the top and bottom. The reactor was filled with explosion products at about 53 ms, when the reaction was complete and the pressure reached
Fig. 9. Contours of the explosion parameters (t ¼ 32 ms).
J. Deng et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 45e53
its maximum. In addition, few differences were observed in terms of the distributions of the concentration for explosion products. According to the velocity distribution of the explosive gases, the burnt gaseous products expanded and propelled the unburned gases. The explosion wave was reflected after reaching the wall. Swirls formed due to the interference of the waves. The velocity of the explosive gases reached its peak value at 42 ms for CH4/air mixture and at 39e40 ms after CO was introduced into the mixture. 5. Conclusions A 20-L near-spherical explosion testing apparatus and FLACS simulation software were used to elucidate the characteristics of explosion for mixture of CH4/CO. The following conclusions were drawn.
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(1) The explosive limits will change after adding CO to CH4. With the increase of CO concentration, the upper and lower explosive limits decrease. Before reaching the stoichiometric explosion concentration of CH4, the addition of CO promotes the explosion reaction of the mixture. (2) The simulation results showed that the stoichiometric concentration of explosion of CH4 is 10%. The highest explosion pressure was 0.798 MPa with the maximum temperature of 2496 K. The explosion pressure and temperature of mixture of CH4/air after adding CO increased until reaching their maximum values and went down a little afterwards. (3) The simulation results of FLACS were corresponding to the experimental results. This study provides a further understanding of the explosion characteristics for the mixture of combustible gases.
Fig. 10. Contours of the explosion parameters (t ¼ 42 ms).
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J. Deng et al. / Journal of Loss Prevention in the Process Industries 36 (2015) 45e53
Fig. 11. Contours of the explosion parameters (t ¼ 53 ms).
Acknowledgments This work was supported byNational Natural Science Foundation of China (Grant No. 51304155, 51134019), the Ministry of Education Funded Project of Innovation Team (Grant No. IRT0856) and the Doctoral Program in Xi'an University of Science and Technology (Grant No. 2013QDJ039), Shaanxi Provincial Science and Technology Innovation Project 13115 (Grant No. 2010ZDGC-14). References ASTM E681-01, Standard Test Method for Concentration Limit of Flammability of Chemical (Vapors and Gases), USA. Bougrine, S., Richard, S., Nicolle, A., 2011. Numerical study of laminar flame properties of diluted methane-hydrogen-air flames at high pressure and temperature using detailed chemistry. Int. J. Hydrog. Energy 18, 12035e12047.
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