Energy Fuels 2010, 24, 3789–3795 Published on Web 06/17/2010
: DOI:10.1021/ef100315t
Progress in Studies of Natural Gas Storage with Wet Adsorbents Li Zhou,* Jia Liu, Wei Su, Yan Sun, and Yaping Zhou High Pressure Adsorption Laboratory, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Received March 17, 2010. Revised Manuscript Received May 13, 2010
Progress in natural gas storage with wet adsorbents is presented. The storage mechanism switched from adsorption for adsorbed natural gas (ANG) to hydrate formation when wet adsorbents were used. The activated carbon with a pore size of 1.6-3 nm was shown more suitable for the wet storage method, and a larger than 40 wt % gravimetric storage capacity and a deliverable capacity of more than 150 (v/v) were experimentally observed under conditions of 273-283 K and pressure ca. 8 MPa. The stored amount in wet carbon is 2 times higher than that of adsorbed and 1.5 times than that of compressed at the indicated conditions. All of the inherent technical drawbacks of ANG were overcome, and the storage pressure reduced more than half compared to compressed natural gas (CNG). Both investment and energy costs of fuel are expected to reduce, and the safety of the natural gas (NG) vehicle will improve radically.
and adsorbed natural gas (ANG) and NGHs received more attention. However, ANG has never been commercialized because of its technical drawbacks. First, the storage capacity of ANG is low. In comparison to 200 (v/v) of the deliverable gas quantity of compressed natural gas (CNG),4 the most likely chargeable quantity of ANG is 150 (v/v), but less than 140 (v/v) is deliverable6 because a large amount of natural gas cannot be released at atmospheric pressure. Some novel materials other than activated carbon claimed a much larger storage capacity;7 however, it turned out to be a mistake,8 and super-activated carbon is still the best carrier of natural gas for the ANG method with respect to storage capacity and material cost. Second, the C2þ components of natural gas cannot totally be released from activated carbon upon releasing the tank pressure to atmospheric pressure, which leads to a gradual decrease of storage capacity unless all C2þ components were sifted out before storage. Third, the thermal effect on charging/discharging is also a big problem of the ANG technology. The temperature of carbon bed can increase as much as 80 K on fast charging, which decreases the storage capacity to as much as 20%. On the other side, the fast decrease of the bed temperature upon discharging may retard the release of the adsorbed natural gas. Fourth, the activated carbon must be compressed to a bulk density of 0.6-0.7 g cm-3 to enhance the volumetric storage capacity;9 however, it is not an easy job to reach the density in a large-scale production. The surface area and the amount adsorbed per gram of carbon decreased remarkably after making pellets. Therefore, the ANG technology does not seem to have commercial potential. With regard to the NGH method, both kinetics and conversion of the hydrate formation reaction are
1. Introduction A sustainable energy supply and environmental protection are the challenges that the world faces presently. Besides the source of natural gas known today, the natural gas buried in coal seals and contained in black shale was recently exploitable. The former is more than 2 times that buried in natural gas reservoirs,1 and the latter can satisfy the world energy requirement for several hundred years.2 Therefore, natural gas may be a tentative solution for both energy and environmental problems before the commercialization of hydrogen energy. Natural gas is clean and results in much less carbon emission upon combustion. An important usage of natural gas is as an alternative fuel of vehicles, which can relieve the pressure on petroleum supply and emission control; therefore, on-board storage of natural gas received extensive studies. Compression is a simple and available method to enhance the on-board fuel density; however, as high as 20-25 MPa of storage pressure causes problems in investment and energy costs, and carrying a high-pressure gas cylinder on vehicles may often be reluctant for drivers. Therefore, efforts to improve the on-board storage technology of natural gas have never been stopped. The major target of research is to reduce the storage pressure because the reduction of pressure leads to lower cost and better safety. Absorption of natural gas in solvent3 and adsorption on activated carbon4 and natural gas hydrates (NGHs)5 are well-known methods proposed in the literature, *To whom correspondence should be addressed. Telephone and Fax: 86-22-87891466. E-mail:
[email protected] or
[email protected]. (1) Jiang, Q. Report on China Coal Bed Methane Industry for 2010-2015, 2010 (http://www.ocn.com.cn/reports/2006131meiceng.htm). (2) New exploitation method may increase global natural gas supply. Reported in the New York Times, Oct 10, 2009 (http://news.xinhuanet. com/world/2009-10/13/content_12223287.htm). (3) Horstkamp, S. W.; Starling, K. E.; Harwell, J. H.; Mullinson, R. G. High-energy density storage of natural gas in light hydrocarbon solutions. AIChE J. 1997, 43, 1108. (4) Parkyn, A. D.; Quinn, D. F. Natural gas adsorbed on carbon. In Porosity in Carbon; Patrick, J. W, Ed.; Edward Arnold: London, U.K., 1995; pp 292-325. (5) Gudmundsson, J. S.; Børrehaug, A. Natural gas hydrate, an alternative to liquefied natural gas. Pet. Rev. 1996, 50, 232. r 2010 American Chemical Society
(6) Bose, T. K.; Chahine, R.; St-Arnold, J. M. High-density adsorbent and method of producing the same. U.S. Patent 4,999,330, 1991. (7) Ma, S. Q.; Sun, D. F.; Simmons, J. M.; Collier, C. D.; Yuan, D. Q.; Zhou, H. C. Metal-organic framework from an anthracene derivative containing nanoscopic cages exhibiting high methane uptake. J. Am. Chem. Soc. 2008, 130, 1012. (8) Sun, Y.; Liu, C. M.; Su, W.; Zhou, Y. P.; Zhou, L. Principles of methane adsorption and natural gas storage. Adsorption 2009, 15, 133. (9) Tan, Z.; Gubbins, K. E. Selective adsorption of simple mixtures in slit pores, a model of methane-ethane mixture in carbon. J. Phys. Chem. 1990, 94, 6061.
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: DOI:10.1021/ef100315t
Figure 1. Experimental apparatus.
Figure 3. Methane storage capacity in wet carbons: 1, coconut shell carbon BY-1; 2, activated carbon AX-21; 3, coal carbon JX-406; 4, mesoporous carbon molecular sieve CMK-3; 5, multi-walled carbon nanotubes; 6, corncob carbon.
temperature and pressure, and the quantity of gas stored in the wet adsorbent is determined by the equilibrium. The effect of the pressure on the equilibrium at a constant temperature is reflected in isotherms; therefore, isotherms were collected under different conditions to observe the effects that affect the storage capacity in wet adsorbents. These isotherms were measured exactly the same way as for the adsorption isotherms on dry adsorbents, but special attention was paid to prevent the loss of water on evacuating the instrument. A volumetric setup was used in the experiments and is schematically shown in Figure 1. There are two containers of known volume and are connected by tubes via valve C. One container named reference cell with volume Vr is kept at a constant temperature, Tr, which is usually close to room temperature. The value of Vr includes the volume of the tube between the reference cell and valve C. The other container named the sorption cell is where the wet adsorbent locates and an equilibrium temperature, Td, is maintained. The volume of the tube connecting the sorption cell and valve C is divided into two parts: one part of volume Vt is exposed to the room and has the same temperature as the reference cell, and the other part with temperature Td is immersed in the atmosphere of a cryostat and, thus, its volume was added to the sorption cell, Vd. The amount entered in the wet adsorbent was calculated from the p-V-T values before and after opening valve C. Precision and reliability of the measurement rely on how accurately p, V, and T were measured and how reliably the compressibility factor of gases was determined. A pressure transmitter model PAA-23r8465.1-200, manufactured by Keller Druckmesstechnik, Switzerland, was used to measure the pressure. The deviation from linearity was less than 0.05% for the whole range of 20 MPa. The volume of the reference cell was determined by titration, and volume Vd was determined by helium expansion at temperature Tr. The amplitude of temperature variation was (0.1 K for both Tr and Td. The purity of helium and methane used was higher than 99.995%. The condition of most experiments covered the range of 275-283 K and 0-10 MPa. Because each isotherm starts from zero pressure, it is necessary to place the system in a vacuum before measurement. To minimize the vaporization loss of water, the wet sample was cooled to 253 K for every time the sorption cell was evacuated. The temperature was raised to a specified value after evacuation. The water loss during experiments can be ignored because the total weight of the sample was almost unchanged before and after an isotherm measurement.
Figure 2. Effect of the water content on the storage of methane in wet carbon.
not satisfactory if the hydrates were formed in water media, and any catalyst tested thus far did not much change the situation. The storage of natural gas in wet adsorbents is radically different from that in dry adsorbents because the storage mechanism switched from adsorption to hydrate formation. In comparison to NGH, the hydrates are not formed in water media but in spaces of nanometer dimension. An appropriate quantity of water was preadsorbed in the space, and the interaction between the wall surface and molecules of water and gas accelerated the formation process of hydrates under appropriate conditions. As a consequence, the reaction kinetics increased radically and the reaction conversion goes to the end. The formation pressure of hydrates decreases from methane to propane for the same temperature, and the formation pressure of methane hydrates is less than 7 MPa10 for temperatures lower than 283 K. The formation pressure goes up exponentially at higher temperatures; therefore, the storage method on wet adsorbents cannot be used at ambient temperature. However, the storage pressure dropped for more than 1/2 times the CNG, and the storage temperature is about the same as a refrigerator; therefore, the wet storage method (WNG) is technically feasible. A series of studies was carried out with different adsorbents in the High Pressure Adsorption Laboratory, Tianjin University, Tianjin, China, and the optimal storage condition for the wet storage method was allocated. 2. Measurement of Isotherms There is also an equilibrium state for methane or natural gas between the gas phase and the wet adsorbent at a specified (10) Sloan, E. D.; Parrish, W. R. Gas hydrate phase equilibrium. In Natural Gas Hydrates: Properties, Occurrence and Recovery; Cox, J. L., Ed.; Butterworth: Woburn, MA, 1981.
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: DOI:10.1021/ef100315t
Figure 4. TEM photos of CMK-3-1.25 taken along the nanorod direction and perpendicular to it (the nanorods are circular black dots).
succeeding studies on other adsorbents.15-18 The highest amount of methane entered in wet carbon is nearly 32 wt % for the carbon, with Rw = 1.4 at pressure 9.26 MPa. However, too large of a water content leads to the loss of the storage capability, as shown with curve d. Each carbon has an optimal water ratio depending upon pore volume and pore size distribution. Celzard and Mar^eche tried to optimize the water content in four commercial carbons.14 They did not reach an optimal result because their samples did not cover a wide range of pore size, pore size distribution, and pore volume of adsorbents; however, these properties exert a critical effect on the storage capacity. In addition, normal pore size classification is not appropriate for the identification of carbon suitable for the wet storage method. The clathrate side length of type-I hydrate is 1.2 nm,19 and the collision diameter of the methane molecule is 0.381 nm;20 therefore, the slit pore width must be larger than 1.6 nm to allow for at least one layer of clathrates and one methane molecule to transmit. As such, superactivated carbons possessing abundant micropores and suitable for ANG are of no use for the wet storage method because, as shown in Figure 321 with curve 2, the storage capacity of wet carbon is always less than dry carbon. Curves 1 and 3 represent the performance of activated carbons possessing partial micropores and partial mesopores, and the highest storage capacity in the presence of water is larger than that without water. Curves 4 and 5 represent the performance of carbons possessing only mesopores. The storage capacity of wet carbons is always larger than dry carbons, and more than 40 wt % storage capacity was observed on a synthesized mesoporous carbon of orderly structure named CMK-3.15 This capacity is double that on dry activated carbon. Therefore, mesoporous carbons are more suitable for the wet storage method. However, the interaction between the carbon surface and water/methane molecules will become too weak to catalyze the formation of hydrates if the opposite pore surfaces are far apart. It is inferred that 1.6-3 nm might be the optimal pore size for the wet storage usage. The TEM photo of CMK-3-1.25 is shown in Figure 4, from which the pore size distribution is determined as centering at 2.5 nm.15
3. Factors Affecting Storage in Wet Adsorbents The content of water in adsorbents exerts a significant effect on the storage capacity. The water content is defined as “water ratio, Rw”, the ratio of water weight to that of dry adsorbent. Shown in Figure 2 are isotherms collected on a wet carbon, loading different amounts of water at 275 K.11 The amount of methane entered in wet carbon is expressed as the weight percent of methane on a dry carbon basis, that is, grams of methane entered in 100 g of dry activated carbon. This quantity was expressed as v/v in the literature;12-14 however, the denominator value may be ambiguous for both dry and wet carbons or other adsorbents. In fact, the package/bulk density of powder materials may not have a unique value in different laboratories; therefore, we take the gravimetric basis. Curve a is the isotherm collected on dry carbon showing the highest adsorbed amount of 19.4 wt % at pressure 7.46 MPa. Different from the isotherm on dry carbon, the isotherms on wet carbons show an inflection at pressure 4.6 MPa, where a sharp increase in the sorbed amount was observed. The sorbed amount is considerably less than that recorded on dry carbon before the inflection pressure, and the more water added in carbon, the less amount sorbed. However, the isotherm goes up rapidly at the inflection pressure, and the more water added to carbon, the higher the isotherm level reached. The isotherm inflection marks the formation of hydrates because the inflection pressure is close to the formation pressure of hydrate in pure water; however, the inflection pressure was affected by the transportation resistance of methane in the wet carbon, and thus, varied with the pore size and pore size distribution of adsorbents as well as the water content, as shown by the (11) Zhou, L.; Sun, Y.; Zhou, Y. P. Enhancement of the methane storage on activated carbon by pre-adsorbed water. AIChE J. 2002, 48, 2412. (12) Perrin, A.; Celzard, A.; Mar^eche, J. F.; Furdin, G. Methane storage within dry and wet active carbons: A comparative study. Energy Fuels 2003, 17, 1283. (13) Perrin, A.; Celzard, A.; Mar^eche, J. F.; Furdin, G. Improved methane storage capacities by sorption on wet active carbons. Carbon 2004, 42, 1243. (14) Celzard, A.; Mar^eche, J. F. Optimal wetting of active carbons for methane hydrate formation. Fuel 2006, 85, 957. (15) Liu, X. W.; Zhou, L.; Li, J. W.; Sun, Y.; Su, W.; Zhou, Y. P. Methane sorption on ordered mesoporous carbon in the presence of water. Carbon 2006, 44, 1386. (16) Liu, X. W.; Zhou, L.; Chang, H.; Sun, Y.; Zhou, Y. P. Methane sorption on large pore MCM-41 in the presence of water. J. Porous Media 2006, 9, 769. (17) Zhou, L.; Liu, X. W.; Sun, Y.; Li, J. W.; Zhou, Y. P. Methane sorption in ordered mesoporous silica SBA-15 in the presence of water. J. Phys. Chem. B 2005, 109, 22710.
(18) Zhou, L.; Liu, X. W.; Li, J. W.; Sun, Y.; Zhou, Y. P. Sorption/ desorption equilibrium of methane in silica gel with pre-adsorption of water. Colloids Surf., A 2006, 273, 117. (19) Englezos, P. Ind. Eng. Chem. Res. 1993, 32, 1251. (20) Aukett, P. N.; Quirke, N.; Riddiford, S.; Tennison, S. R. Carbon 1992, 30, 913. (21) Zhou, Y. P.; Dai, M.; Zhou, L. Storage of methane on wet activated carbon: Influence of pore size distribution. Carbon 2004, 42, 1855.
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Figure 5. Sorption/desorption isotherms of methane on wet activated carbon made from corncobs.
Figure 7. Effect of the packing density of wet carbon on the release amount at 275 K.
The dynamic behavior of charging and discharging natural gas into/from wet carbons is certainly of big concern from the
point of view of drivers; therefore, charging and discharging experiments were carried out in a setup shown in Figure 8. A pressure sensor and a thermocouple were used to measure the pressure and temperature of the carbon bed, and the signals were sent to a computer via lab cards. The carbon bed was initially evacuated at 243-248 K for 6 h and then put in a water bath of 275 K to obtain a constant temperature. Then, methane was fed to the container to maintain a normal pressure of 101.325 kPa. The quantity of methane charged in the wet carbon was determined by the weight increase measured by an electronic balance. Because the electronic balance was set at zero when the pressure inside the container was atmospheric, the balance outputs only the weight increase of charging. The charging rate was controlled by a needle valve. The quantity of methane discharged was measured using two pails. Pail 10 was full of water, and pail 11 was empty initially. When discharging started, the water in pail 10 was expelled to pail 11 in accordance with the volume of discharged gas, and the water in pail 11 was then weighed. The quantity of gas released from the carbon bed was determined from the weight of water collected in pail 11. The curves shown in Figure 923 recorded the variation in weight and temperature of the carbon bed during a charging process. While the formation kinetics of methane hydrates in wet carbon was not fast, as observed upon measuring isotherms,12 the hydrates were formed in several minutes upon charging, as indicated by the temperature curve. The temperature of the carbon bed was initially maintained at 275 K, with cycling water flowing out of a cryostat. The charging process is initially just expansion of methane from a highpressure cylinder into the container of atmospheric pressure. Because the arriving methane is of room temperature, which is much higher than 275 K, the temperature of the carbon bed rose rapidly to the first peak but soon dropped down because of the cooling effect of the surrounding water bath. The second rise of the bed temperature at 4 min marks the formation of methane hydrates because the heat source for the second temperature rise can only be the formation heat of
(22) Lastoskie, C.; Gubbins, K. E.; Quirke, N. Pore size heterogeneity and the carbon slit pore: A density functional theory model. Langmuir 1993, 9, 2693.
(23) Zhou, Y. P.; Wang, Y. X.; Chen, H. H.; Zhou, L. Methane storage in wet activated carbon: Studies on the charging/discharging process. Carbon 2005, 43, 2007.
Figure 6. Pore size distribution of the corncob activated carbon.
However, this kind of material can hardly be applied in practice because of the high manufacture cost. Nonetheless, a cheaply made activated carbon using corncobs as a precursor reached a storage capacity of 37 wt %, as shown in Figure 5, which is quite close to that of CMK-3. The pore size distribution determined by treating the nitrogen adsorption isotherms at 77 K with the DFT theory22 is shown in Figure 6, which indicates that most pores of the carbon fall in the size range of 1.5-4 nm. The packing density of wet carbon is hopefully the higher the better to increase the volumetric storage density. Unlike dry activated carbon, the package density of wet carbon is easily adjusted because of the plasticity of wet carbon powder. However, as shown in Figure 7, the released amount increases with packing density slowly and there is an upper limit for the density, beyond which the released amount (storage capacity) dropped drastically. The highest density depends upon the charging pressure. The higher the charging pressure, the larger the highest density and the larger the released amount observed. The suitable package density of wet carbons is around 0.7 g/cm3. 4. Dynamic Behavior of Charging/Discharging Wet Carbons
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Figure 8. Schematic apparatus for charging and discharging experiments: 1, methane cylinder; 2, pressure stabilization valve; 3, needle valve; 4, bypass valve; 5, back-pressure valve; 6, wet carbon container; 7, electronic balance; 8, water bath of constant temperature; 9, pressure releasing valve; 10 and 11, water pail; 12, computer; 13, thermostat supplying cycling water.
Figure 10. Variation of the released amount and bed temperature upon discharging.
Figure 9. Variation of the charged amount and bed temperature upon charging (charging pressure, 7 MPa; temperature, 275 K; bulk density, 0.514 g/cm3).
In addition, the cooling atmosphere of the wet carbon bed tempered the release of formation heat. Such a low temperature rise did not affect the storage capacity of wet carbon. Although the bed temperature dropped 16 K upon discharging, a continuous release of the stored gas was observed, as shown in Figure 10. Although the decomposition pressure of methane hydrates was reported to be 2.6 MPa,24 the stored methane released immediately when the pressure of the container began to reduce and the amount of released methane increased with time at an almost constant rate. The total released amount is almost equal to the total quantity charged, which is different from that observed upon discharging a dry carbon bed, where a considerable amount of the adsorbed methane could not be released. The fact that the decreasing temperature did not affect the release of methane indicates that the decomposition of methane hydrates would not stop halfway, as long as pressure continues to decrease. Obviously, this is an advantage of the wet storage method for vehicle application.
hydrates. The bed temperature reached 279 K at about 6 min and then dropped slowly. Because the formation pressure of methane hydrates increases drastically as the temperature rises, the rate of hydrate formation decreased, following the increase of the bed temperature, and the heating effect became weaker than cooling and allowed the the bed temperature to decrease from the apex. The rate of hydrate formation increases again when the temperature decreases. Therefore, the thermal equilibrium between the formation heat and the cooling of the media controlled the formation rate of hydrates. The bed temperature dropped gradually to 275 K, following the completion of hydrate formation. The bed weight increases quickly in the early period of charging and approached the largest portion of saturation amount in 15 min. The equilibrium charge amount was reached in about 40 min. The thermal effect on charging and discharging does not affect the storage capacity or release of the stored gas. The temperature curve shows that the thermal effect on charging wet carbon is very low. The bed temperature increased only 4 K during charging against the temperature rise of 80 K upon charging a dry carbon bed. Because of the high heat capacity of wet carbon, an overly large temperature rise was prevented.
(24) Davidson, D. W.; Desando, M. A.; Gough, S. R.; Handa, Y. P.; Ratcliffe, C. I.; Ripmeester, J. A.; Tse, J. S. A clathrate hydrate of carbon monoxide. Nature 1987, 328, 418.
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Figure 11. Sorption isotherms of methane on dry and wet MCM-41 samples at 275 K (1, dry; 2, wet).16 Figure 13. Sorption isotherms of methane on silica gel with different water contents at 275 K (1, Rw = 0; 2, Rw = 0.61; 3, Rw = 0.7; 4, Rw = 0.8; 5, Rw = 0.93; 6, Rw = 1.05; 7, Rw = 1.17; 8, Rw = 1.3).18
higher than that on the dry sample.16 The MCM-41 sample obtained without using TMB (pore size expander) possesses a much smaller pore size (3.4 nm16); therefore, it may be more suitable for the wet storage method. However, the recorded storage capacity (11.23 wt %) is far less than that observed on activated carbon. SBA-15 looked better than MCM-41, and the highest storage capacity of methane reached 23.2 wt %, with Rw = 2.92, which is 3.75 times higher than the capacity of the dry sample.17 While the highest adsorption amount on the dry sample was reached at about 10 MPa, the highest amount stored in the wet sample was reached at a pressure less than 4 MPa. A prominent feature of the isotherms on wet SBA-15 is that the inflection pressure decreases following the increase of the water content, as shown in Figure 12. The lowest reflection pressure (with Rw = 2.92) is quite close to the formation pressure of methane hydrates in water media. This feature is very attractive for reducing the storage pressure; however, the storage capacity is much lower than that reached by the mesoporous carbon. Silica gel is not suitable for methane storage, no matter dry or wet. The storage capacity is rather low, although higher in the wet state than in the dry state.18 Besides, the pre-adsorbed water may cause hydrolysis of the silicon-hydroxyl groups of silica gel, which again causes changes in the pore size distribution and the abnormal behavior of isotherms.
Figure 12. Sorption isotherms of methane over SBA-15 with different water contents at 275 K (1, Rw = 0; 2, Rw = 1.31; 3, Rw = 1.5; 4, Rw = 2; 5, Rw = 2.69; 6, Rw = 2.92).17
5. Storage in Other Wet Materials In addition to silica gel, the synthesized mesoporous materials of orderly structure MCM-41 and SBA-15 among others are important silicon adsorbents and received extensive studies in the adsorption area. The sorption isotherms of methane on them were also collected in the presence of water and shown in Figures 11-13, respectively. The MCM-41 sample was synthesized using tetraethylorthosilicate as the silica source and cetyltrimethylammonium bromide (CTMA-Br) as the surfactant. Besides, 1,3,5-trimethylbenzene (TMB) was used as the pore size expander, and the molar ratio of TMB over the surfactant was indicated with TMB/surf. The sample used for isotherm measurement was synthesized with TMB/surf = 6, and the water ratio in wet sample was Rw = 2. The highest amount adsorbed on the dry sample (curve 1 in Figure 11) was 4.68 wt % reached at pressure 8.57 MPa, and the highest amount adsorbed on the wet sample was 11.23 wt % at pressure 8.85 MPa (curve 2 in Figure 11), which is 2.4 times
6. Further Comparison to ANG and CNG The advantage of the wet storage method over CNG is obvious if the releasable methane at different charging pressures is compared. The experimentally delivered methane quantity from a wet carbon 23 is drawn against the charging pressure in Figure 14 together with the quantity deliverable by CNG at the same pressure. Apparently, the wet storage method does not have an advantage below 5.5 MPa; however, its deliverable methane is about 1.5 times that of CNG at about 8 MPa. The advantage of the wet storage method over ANG becomes more clear if the storage amount in wet carbon is further analyzed. The total methane maintained in the wet carbon container can be classified into free 3794
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Figure 15. Comparison of the releasable methane fixed on dry and wet carbon.23
Figure 14. Comparison of the releasable methane amount from wet carbon and CNG.23
advantage of wet carbon cannot be shown at lower pressures. However, the releasable quantity from wet carbon is almost double that from dry carbon at about 8 MPa. It seems 8 MPa is the appropriate storage pressure for wet carbons, which is considerably lower than the storage pressure of CNG (2025 MPa).
methane and fixed methane. Assuming that Vt is the total volume of the container, it must be the sum of three parts: Vt ¼ Vc þ Vw þ Vv where Vc is the volume occupied by the carbon skeleton, which can be determined by helium expansion when carbon is dry, Vw is the volume occupied by water, which was calculated from the water ratio and water density (1.0 g/cm3), and Vv is the volume of the void space and was obtained by subtracting Vc and Vw from the known value of Vt. The stored methane in the void space is named free methane. The quantity of free methane per unit mass of dry carbon, nf, can be calculated on the basis of the values of temperature, pressure, and volume Vv via the relation nf = pVv/zRT, where z is the compressibility factor and was determined by, for example, the virial equation. All other parts of the stored methane are thought of as fixed methane, nx, which might be the sum of the adsorbed methane, na, and methane contained in hydrate, nh. Obviously nx ¼ nh þ na ¼ nt - nf
7. Conclusion The following results are concluded on the basis of the aforementioned studies: (1) The storage method on wet carbons provides a storage capacity higher than ANG, and it overcomes all technical drawbacks of the latter. A storage capacity higher than CNG was reported in the literature; however, it was based on static experiments, and the stored amount was based on a not well-defined package density of wet activated carbon. The volumetric capacity obtained in charging/discharging experiments was a little larger than 150 (v/v), which is lower than that of CNG, but the storage pressure reduced more than half; therefore, the fuel cost will reduce remarkably, and the safety of the natural gas vehicles will be improved radically. Nonetheless, the deliverable quantity is 1.5 times higher than that of CNG at the storage pressure of wet carbon. (2) Although hydrates can be formed in different kinds of porous materials in the presence of water, activated carbon with a pore size of 1.6-3 nm is more suitable for the wet storage method and a 37-40 wt % gravimetric capacity was guaranteed. (3) Although quite a long time was taken to reach equilibrium in the static experiments, hydrates were formed quickly in wet carbons upon charging and the charging process may finish essentially in 15 min. The thermal effect does not negatively affect the charging or discharging process.
where nt is the experimentally measured total quantity of methane entered in wet carbon. Figure 14 is converted to Figure 15 by subtracting nf from nt. The result for dry carbon is also shown in Figure 15 for a comparison. The released amount shown in Figure 15 is the released quantity when the storage pressure drops to atmospheric pressure (but not to zero). Therefore, the value of nt is the total amount of releasable methane but not the total quantity stored. Therefore, the value of nx is also the quantity releasable for dry carbon. While adsorption on dry carbon closely approaches saturation at 4 MPa, the amount of fixed methane in wet carbon is almost zero for the same pressure, indicating that methane can neither form hydrate nor be adsorbed. However, the amount of fixed methane in wet carbon increased quickly upon further increasing the charging pressure, and it ultimately reached a much higher level than on dry carbon. The amount of fixed methane released from wet carbon is a linear function of the charging pressure for the range 5-8 MPa. Extrapolating the straight line to abscissa, 4.12 MPa was recognized as the transition pressure of hydrate formation in the wet carbon; therefore, the
Acknowledgment. Financial support of the National Natural Science Foundation of China (Grants 29936100, 20336020, and 90510013) and the Tianjin Municipal Science and Technology Commission (Grant 07JCYBJC00800) is sincerely acknowledged. The corresponding author is also grateful to other graduate students that had worked on the topic of natural gas storage in the lab, although their names do not appear in the paper. 3795
