Ind. Eng. Chem. Res. 2009, 48, 4083–4087
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Fire Extinction Using Carbon Dioxide Hydrate Takashi Hatakeyama, Eisuke Aida, Takeshi Yokomori, Ryo Ohmura,* and Toshihisa Ueda Department of Mechanical Engineering, Keio UniVersity, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
Clathrate hydrates formed with nonflammable gases may be suitable for use as fire extinguishing agents because the dissociation of the hydrates decreases the temperature in the flame base and the nonflammable gases released from the dissociated hydrates prevent the supply of oxygen to the flame base. In the present study, we performed experiments to extinguish pool flames using the hydrate crystals formed with carbon dioxide. From the experimental result, the critical mass of CO2 hydrate to extinguish the flame was found to increase when the size of the pool flame is increased. The critical mass to extinguish the same size flame of ordinary ice and dry ice was also measured. These results showed that the CO2 hydrate can extinguish the pool flame with less water as compared to ordinary ice and less releasing of the CO2 as compared to the dry ice. 1. Introduction The major objective of this Article is to report the results of an experimental study on a novel clathrate-hydrate-based technology, that is, the use of clathrate hydrates formed with nonflammable gases as fire extinguishing agents. Clathrate hydrates are crystalline compounds consisting of hydrogenbonded water molecules forming cages that enclose the guest substance (i.e., species other than water). The amount of a guest gas contained in 1 m3 of a hydrate may be 160 m3 at STP. As hydrates are ice-like crystalline solids, the heat of formation and dissociation of hydrates are typically 300-500 kJ/kg.1 These values of the heat of hydrate dissociation are comparable to or even larger than the heat of fusion of ice, 333 kJ/kg. Thus, if a clathrate hydrate formed with a nonflammable guest gas such as CO2 is dissociated in and/or around a flame base, the temperature would decrease due to the heat of hydrate dissociation. At the same time, the dissociation of the hydrate in the combustion field would prevent the supply of oxygen due to the release of the nonflammable gases from the dissociating hydrate, and the fuel concentration would also decrease. Consequently, clathrate hydrates are considered to be good fire extinguishing agents. When compared to a conventional fire extinguishing method using water spraying, the amount of the hydrate would be much lower than that of water because of the large heat of hydrate dissociation and the release of nonflammable gases. Halon gas is one of the conventional fire extinguishing agents,2 typically used for fires in which water spraying cannot be applied, for example, fire in a subway station. Hydrates may be expected as an environment friendly alternative to the halon gas, because hydrates may be exclusively formed with water and nontoxic substances such as CO2, and such hydrates are less harmful to humans and environment. We may also note the advantage of the dense, solid hydrate as compared to the fluid fire extinguishing agents. An industrial scale method to form pellets, each 20 mm in diameter, has recently been developed.3 We can supply such solid hydrate pellets to the center of fire, even if the fire is large and has a strong buoyancy. As a result, the hydrate is able to reach the flame base and extinguish the fire due to the endothermic reaction of the hydrate dissociation and the release of the nonflammable gases at the flame base. Therefore, we can expect the effective extinction in difficult situations such as large fires in forests or buildings. * To whom correspondence should be addressed. Fax: +81-45-5661495. E-mail:
[email protected].
In the present study, we selected CO2 as the nonflammable gas guest to form a hydrate, considering the relatively low equilibrium pressure, inexpensiveness, and nontoxicity. This study aims to experimentally demonstrate pool flame extinction using a CO2 hydrate and to determine the amount of the hydrate required for the flame extinction. The discussion will provide a physical interpretation of the experimental results. 2. Experimental Section 2.1. Apparatus and Procedure for the Preparation of CO2 Hydrate Crystals. Figure 1 shows a schematic diagram of the experimental setup used for the preparation of the CO2 hydrate crystals. The main part of the setup is a stainless-steel cylinder with an internal volume of 200 cm3 (80 mm in diameter and 40 mm in depth) with a magnetic stirrer. The vessel is immersed in a temperature-controlled bath to maintain the temperature inside the vessel at a prescribed level. The vessel is first charged with 50 g of liquid water and then immersed in the temperature-controlled bath to set the temperature inside at 274 K. Gaseous CO2 was supplied from a highpressure cylinder into the once evacuated vessel to set the pressure inside at 3.1 MPa. This experimental pressure is higher than the equilibrium pressure of the CO2 hydrate at 274 K, 1.3 MPa,4 and is lower than the saturated vapor pressure of CO2 at the same temperature, 3.4 MPa. The CO2 gas was continuously supplied to compensate for the pressure reduction in the vessel due to the CO2 hydrate formation, so that most of the liquid water is converted into the CO2 hydrate. The pressure and the
Figure 1. Schematic of the reactor to form CO2 hydrate.
10.1021/ie8019533 CCC: $40.75 2009 American Chemical Society Published on Web 03/12/2009
4084 Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009
Figure 2. Photograph of CO2 hydrate crystal formed in the present study.
temperature were kept constant for over 24 h after the initial hydrate formation with continuous agitation in the vessel at 400 rpm. During this period of hydrate formation, the line connecting the vessel and CO2 cylinder was intermittently shut. If the pressure reduction is observed after the line was shut, it indicates that the hydrate formation in the vessel still continues. On the other hand, the almost complete conversion of the water in the vessel to hydrate is confirmed if the pressure reduction is not observed. After it was confirmed that the pressure reduction was not observed, the vessel was removed from the bath and then immediately immersed into a liquid-nitrogen pool in a stainless-steel container. We allowed 20 min to decrease the temperature inside the vessel to below 200 K (equilibrium temperature of CO2 hydrate under the pressure of 0.1 MPa is 217.8 K5) and then disassembled the vessel to remove the hydrate crystals. Figure 2 shows a picture of the CO2 hydrate crystals. The water-to-CO2-hydrate conversion ratio was experimentally evaluated to be 76% by dissociating some portion of the CO2 hydrate crystals in liquid water and measuring the volume of the CO2 gas released from the hydrate crystals. In this evaluation, the hydration number of the CO2 hydrate was assumed to be 6.2 on the basis of the single-crystal X-ray diffraction measurement.6 2.2. Procedure and Apparatus for the Flame Extinction Experiments. Figure 3 shows a schematic diagram of the experimental apparatus used for the pool flame extinction experiments. The apparatus consists of the flame to be extinguished, an insulation board above the flame, and the section to supply the hydrate crystals down to the flame. The pool flame is formed over the liquid methanol in a stainless-steel cylindrical container. The initial depth of the methanol was set at 10 mm in all of the experimental runs. Each experimental run was started by packing the CO2 hydrate crystals into the section to supply the hydrate crystals to the flame. The mass of the CO2 hydrate crystals was measured by an electronic balance (A&D, GF3000). The uncertainty of the mass measurements is (0.02 g. By using a pestle, a mortal, and a sieve in the low temperature (below 160 K) nitrogen atmosphere, the hydrate crystals were ground and sieved into fine powders having a diameter less than 1.0 mm prior to each experimental run. This operation was completed within 10 min. Figure 4 shows the picture of the prepared CO2 hydrate powders. The section to supply the hydrate crystals is made of a Teflon cylinder with an inner diameter of 15 mm. The lower opening of the Teflon cylinder is sealed with an iris. The temperature of the hydrate crystals was measured by a T-type thermocouple. The initial temperature of the CO2 hydrate crystals set in the Teflon cylinder was approximately 170 K. When the hydrate temperature increased to 193 K, the CO2 hydrate crystals were
Figure 3. Schematic diagram of the experimental apparatus used for the flame extinction experiments: (a) before and (b) during supply of extinguish agent.
Figure 4. Photograph of CO2 hydrate powder.
supplied to the flame due to gravity by opening the iris (Figure 3b). The supply of the hydrate to the flame was completed within 1 s. The liquid methanol was ignited just after packing the CO2 hydrate into the Teflon cylinder. The supply of CO2 hydrate crystals to the flame was completed within 10 s after the ignition of the liquid methanol. The behavior of the flame was monitored and recorded by a digital video camera (Panasonic, model NVGS300-S). As the experimental parameters, the characteristic size of the pool flame D and the mass of CO2 hydrate supplied to the flame were varied. Here, the characteristic size of the pool flame, D, is defined as the diameter of the stainless steel container, because this diameter determines the flame size and the heat release rate. The experiments were also performed using the CO2 hydrate crystals having 10 mm diameters, solid CO2 (dry ice), and ordinary ice instead of the above-mentioned CO2 hydrate crystals having a diameter less than 1.0 mm for comparison. The other experiments were performed to determine the heat release rate of the pool flame. The stainless-steel container containing the liquid methanol was placed on the electric
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Figure 5. Sequential video graphs of the flame extinction by the supply of CO2 hydrate crystals to the methanol pool flame and the relevant illustration. (a) Flame was extinguished with 1.25 g of the hydrate crystals (D ) 90 mm). (b) Flame was not extinguished with 1.20 g of the hydrate crystals (D ) 90 mm).
balance, and then the methanol was ignited. The decrease in the mass of the liquid methanol was measured. The heat release rate was calculated from the measured rate of the decrease in the mass of the liquid methanol on the basis of the assumption that the decrease in the methanol mass is due to the vaporization of the methanol and the vaporized methanol was completely burnt. 3. Results and Discussion Sequential video images for the flame extinction by the supply of CO2 hydrate crystals to the methanol pool flame and the relevant illustrations are shown in Figure 5. These results were obtained with the following experimental parameters: the diameter of the container containing the liquid methanol was 90 mm, and the mass of the supplied CO2 hydrate crystals having the 1.0 mm diameter was 1.25 or 1.20 g. The supply of 1.25 g of the hydrate crystals resulted in a complete flame extinction (Figure 5a), while the flame was not extinguished by 1.20 g of the hydrate crystals (Figure 5b). When the CO2 hydrate crystals were supplied, all of the crystals passed through the flame and reached to the methanol
pool surface. It was observed that the flame gradually vanished from its center to the periphery after the CO2 hydrate crystals reached the flame, and then the flame was finally completely extinguished. It was visually confirmed that the time required for the flame extinction was shorter when a larger amount of the CO2 hydrate crystals was supplied to the flame. On the basis of the experimental results over the range of the mass of the supplied CO2 hydrate crystals, we determined the critical mass of the CO2 hydrate crystals below which the flame was not completely extinguished. The critical mass is defined as the smallest mass with which the flames were successfully extinguished in the series of the three experimental runs. The quantitative results will be discussed later. When the mass of the supplied CO2 hydrate crystals was less than the critical value, the flame vanished from the center to the periphery of the flame, but the flame at the periphery was not completely extinguished, and then the flame recovered from the periphery to the center. The above-mentioned flame extinction phenomena, that is, the gradual flame vanishing from the center to the periphery, is considered to be the extension of the low temperature area around the dissociating hydrate crystals in the flame base, the
4086 Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 Table 1. Critical Mass of CO2 Hydrate, Dry Ice, and Ice for the Methanol Container of 50 mm Diameter fire extinguishing agent
critical mass (g)
CO2 hydrate dry ice ice
0.3 0.3 9.0
lack of oxygen supply, and the decrease in the methanol vapor concentration that resulted from the release of the CO2 gas from the hydrate crystals. Table 1 compares the experimentally determined critical mass of the CO2 hydrate, dry ice, and ordinary ice (all 1.0 mm diameters) for the 50 mm diameter container. The critical mass of the CO2 hydrate and the dry ice is 30 times less than that of the ice, indicating the high performance of the CO2 hydrate as a fire extinguishing agent similar to that of dry ice. Figure 6 shows the experimental results of the critical mass of the CO2 hydrate and dry ice for a pool flame with a different characteristic size of D. In Figure 6, the heat release rate of the pool flame is also shown. This figure indicates that the critical mass increases with an increase in the characteristic size of the pool flame D, that is, the size of the stainless steel container, which corresponds to an increase in the heat release rate. When D is greater than 60 mm, the critical mass of the dry ice is clearly less than that of the CO2 hydrate. The difference between the critical mass of the dry ice and the CO2 hydrate increases with an increase in the size of the pool flame. The smaller critical mass of the dry ice is ascribed to the larger volume of the CO2 gas released from the dry ice than that from the CO2 hydrate. In fact, the volume of a guest gas released from 1 m3 of the CO2 hydrate is 122 m3 at STP, while that from 1 m3 of dry ice is 800 m3. Figure 7 compares the volumes of the CO2 gases that were released from the critical mass of the CO2 hydrate and the dry ice. The results in Figures 6 and 7 indicate that the critical mass
Figure 6. Relation among the critical mass of CO2 hydrate and dry ice, the size of the pool flame, and the heat release rate of the pool flame.
Table 2. Heat of CO2 Hydrate Dissociation, Water Vaporization, and Sublimation of Dry Ice
CO2 hydrate
[heat of dissociation (277.15 K)] + [heat of water vaporization] ) 566.1 + 1791 ) 2357.1 kJ/kg
dry ice
[heat of sublimation(-194.67 K)] ) 573 kJ/kg7
of the CO2 hydrate is more than that of dry ice, while the volume of CO2 gas from the critical mass of the CO2 hydrate is less than that of dry ice. It suggests that a flame can be extinguished with a lower amount of CO2, if the CO2 hydrate is used as a fire extinguishing agent instead of dry ice. These results indicate that the CO2 hydrate as well as dry ice can prevent the supply of oxygen and decrease the methanol vapor concentration by release of the CO2 gas from the dissociating or sublimating crystals. Table 2 summarizes the heat of hydrate dissociation, vaporization of water, and sublimation of dry ice. It showed that the heat consumption due to the endothermic reaction of the hydrate dissociation and the vaporization of the water generated from the dissociated hydrate is greater than that of the dry ice due to the sublimation by a factor of about 4. This means that the effect to decrease the flame base temperature of the CO2 hydrate is supposed to be superior to that of dry ice. The density of CO2 hydrate is approximately 1110 kg/m3, which is larger than that of liquid methanol (∼790 kg/m3). However, it was observed that the dissociating CO2 hydrate crystals floated at the methanol pool surface. This float of the denser hydrate crystals is considered to be due to the bubbling at the dissociating hydrate surface. These results suggest that the two mechanisms, that is, the release of CO2 gas and the heat absorption due to the hydrate dissociation and the water vaporization, synergistically work to extinguish a flame. In addition, the CO2 hydrate may be considered as a more environmentally friendly and less toxic fire extinguishing agent, because a flame is extinguished with a smaller amount of CO2 when using the CO2 hydrate versus the dry ice. Additional experiments were performed to elucidate the effect of the size of the hydrate to the flame extinction using CO2 hydrate crystals having a size of approximately 10 mm. The critical mass of the CO2 hydrate crystals with a diameter less than 1.0 mm was 1.25 g for D ) 90 mm as shown in Figure 6. A 1.25 g amount of the CO2 hydrate crystals having a 10 mm size was then supplied to the pool flame with D ) 90 mm, but the flame was not extinguished. This result suggests that the decrease in the rate of the hydrate dissociation is due to the increase in the size of the hydrate crystals, and thus the decrease in the total surface area of the supplied CO2 hydrate crystals, resulting in a decrease in the rates of CO2 gas release and heat removal from the flame base. For an effective flame extinction, the CO2 hydrate should be ground into fine particles to such an extent that the hydrate particles can reach the flame base and do not dissociate before they reach the flame base. 4. Conclusions
Figure 7. Volume of CO2 released from the critical mass of CO2 hydrate and dry ice.
Experiments were performed to demonstrate flame extinction using CO2 hydrate as a fire extinguishing agent. The experiments were performed using ordinary ice and dry ice as well to evaluate the performance of the CO2 hydrate as a fire extinguishing agent. The critical mass of the CO2 hydrate required to extinguish a flame was much less than that of ordinary ice, indicating the superiority of CO2 hydrate to the ice. The critical mass of dry ice was less than that of the CO2 hydrate, while
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the amount of CO2 required to extinguish a flame was less when using the CO2 hydrate than dry ice, which may indicate the environmental friendliness of the CO2 hydrate as a fire extinguishing agent. The experiments also revealed that the size of the CO2 hydrate particles had a significant effect on the flame extinction performance. Acknowledgment This research is partly supported by a Grant-in-Aid for Exploratory Science Research (19656059), a Grant-in-Aid for the Global Center of Excellence Program for “Center for Education and Research of Symbiotic, Safe, and Secure System Design” from the Ministry of Education, Culture, Sport, and Technology in Japan, and the Keio Gijuku Academic Development Fund. Literature Cited (1) Fournaison, L.; Delahaye, A.; Chatti, I.; Petitet, J. P. CO2 hydrates in refrigeration processes. Ind. Eng. Chem. Res. 2004, 5, 6521–6526.
(2) Su, J. Z.; Kim, A. K. Suppression of pool fires using halocarbon streaming agent. Fire Technol. 2002, 38, 7–32. (3) Iwasaki, T.; Katoh, Y.; Nagamori, S.; Takahasi, S. Continuous natural gas hydrate pellet production (NGHP) by process development unit (PDU). Proceedings of the 5th International Conference on Gas Hydrates; Trondheim, Norway, June 13-16, 2005; pp 1107-1115. (4) Yasuda, K.; Ohmura, R. Phase equilibrium for clathrate hydrates formed with methane, ethane, propane, or carbon dioxide at temperatures below the freezing point of water. J. Chem. Eng. Data 2008, 53, 2182– 2188. (5) Fallablla, B. J. A study of natural gas hydrate. Dissertation, University of Massachusetts, 1975. (6) Udachin, K. A.; Ratcliffe, C. I.; Ripmeester, J. A. Structure, composition, and thermal expansion of CO2 hydrate from single crystal x-ray diffraction measurements. J. Phys. Chem. B 2001, 105, 4200–4204. (7) Giauque, W. F.; Egan, C. J. Carbon dioxide. The heat capacity and vapor pressure of the solid. the heat of sublimation. thermodynamic and spectroscopic values of the entropy. J. Chem. Phys. 1937, 5, 45–54.
ReceiVed for reView December 18, 2008 ReVised manuscript receiVed February 14, 2009 Accepted February 16, 2009 IE8019533
