Thermodynamic Stability, Spectroscopic Identification, and Gas

Article pubs.acs.org/est

Thermodynamic Stability, Spectroscopic Identification, and Gas Storage Capacity of CO2−CH4−N2 Mixture Gas Hydrates: Implications for Landfill Gas Hydrates Hyeong-Hoon Lee,† Sook-Hyun Ahn,† Byong-Uk Nam,† Byeong-Soo Kim,† Gang-Woo Lee,‡ Donghyun Moon,‡ Hyung Joon Shin,‡ Kyu Won Han,‡ and Ji-Ho Yoon*,† †

Department of Energy and Resources Engineering, Korea Maritime University, Busan 606-791, Korea and R&D Center, Yoosung Inc., Ulsan 689-892, Korea

‡

ABSTRACT: Landfill gas (LFG), which is primarily composed of CH4, CO2, and N2, is produced from the anaerobic digestion of organic materials. To investigate the feasibility of the storage and transportation of LFG via the formation of hydrate, we observed the phase equilibrium behavior of CO2− CH4−N2 mixture hydrates. When the specific molar ratio of CO2/CH4 was 40/55, the equilibrium dissociation pressures were gradually shifted to higher pressures and lower temperatures as the mole fraction of N2 increased. X-ray diffraction revealed that the CO2−CH4−N2 mixture hydrate prepared from the CO2/CH4/N2 (40/55/5) gas mixture formed a structure I clathrate hydrate. A combination of Raman and solid-state 13C NMR measurements provided detailed information regarding the cage occupancy of gas molecules trapped in the hydrate frameworks. The gas storage capacity of LFG hydrates was estimated from the experimental results for the hydrate formations under two-phase equilibrium conditions. We also confirmed that trace amounts of nonmethane organic compounds do not affect the cage occupancy of gas molecules or the thermodynamic stability of LFG hydrates.

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INTRODUCTION Landfill gas (LFG) is produced during the anaerobic decomposition of organic solid wastes. LFG generation begins shortly after waste is dumped in a landfill, and it can last for up to 30 years after the landfill is closed. LFG primarily consists of CH4, CO2, and N2 as well as trace amounts of nonmethane organic compounds (NMOCs). These gases (CH4 and CO2) contribute to global warming as green house gases (GHGs). According to an estimate by the Intergovernmental Panel on Climate Change (IPCC), the emission of methane from solid waste disposal accounts for about 5−20% of the total estimated methane emitted from anthropogenic sources globally (IPCC, 1996). Accordingly, LFG should be collected and treated to reduce the emission of GHGs and thus mitigate their possible impact on global warming. Moreover, in terms of energy recovery, LFG has rather high energy content as about half of its composition is methane. As a result, CH4 collected from large landfills can be used to generate electricity and/or heat. However, for small landfills and landfill power plants at the end of their lifetime, CH4 is collected and burned because it is not economically efficient to use this compound as an energy source. This study has focused on the development of a conceptual design for the use of these abandoned energy sources by collecting CH4 gases from LFG sites. Specifically, this study was conducted to determine if a combined LFG-to© 2012 American Chemical Society

energy project would be feasible if multiple landfills sent their collected LFGs (especially CH4) to a centralized location with the hydration process. Gas hydrates are nonstoichiometric inclusion compounds, which are formed by the physically stable interaction between hydrogen-bonded water molecules and relatively small guest molecules occupied in the cavities. On the basis of the difference in cavity shape and size of the gas hydrates, they have been classified into three distinct structures: I (sI), II (sII), and H (sH). The unit crystalline of the cubic sI consists of two small 12-hedra (512) and six large 14-hedra (51262) cages, whereas the cubic sII clathrate hydrate has sixteen 12-hedra (512) and eight 16-hedra (51264) cages.1 It is also well-known that the hexagonal sH clathrate hydrate is composed of three types of cages − 12-hedra (512), 12-hedra (435663), and 20hedra (51268) − and requires large guest molecules such as adamantane and methylcyclohexane with smaller help gas for cage stability.2,3 Over the past 60 years, there has been considerable interest in research on natural gas hydrates, owing to the plugging of gas Received: Revised: Accepted: Published: 4184

September 26, 2011 February 16, 2012 March 1, 2012 March 1, 2012 dx.doi.org/10.1021/es203389k | Environ. Sci. Technol. 2012, 46, 4184−4190

Environmental Science & Technology

Article

dissociation condition. A more detailed description of the experimental apparatus and procedure is given elsewhere.19,20 Sample Analysis. Once the gas hydrate was formed, we kept the sample with the mixture gas for at least two days. After the reaction was completed, the vessel was removed from the bath and then immediately immersed in liquid nitrogen (LN2) in a stainless steel container. The powdered hydrate samples with a diameter range of 100−250 μm were then used for spectroscopic analysis. A Raman spectrometer with a single monochromator of 1800 grooves/mm grating and a multichannel air-cooled CCD detector was used in this study. The excitation source was an Nd:YAG laser emitting a 532 nm line with a power of 150 mW. The Raman spectroscopic measurements were conducted under LN2 at ambient pressure. The NMR spectra were measured using a Bruker DSX400 NMR spectrometer at a Larmor frequency of 100.6 MHz at the Korea Basic Science Institute (KBSI). Two different methods − 13 C cross-polarization (CP) NMR and 13C high-power decoupling/magic angle spinning (HPDEC/MAS) NMR − were used to confirm the occupancy pattern of CO2 and CH4 molecules trapped in the hydrate cages. For clarity, the enriched 13 CO2 gas was used to obtain high-intensity CO2 signals. The powdered sample was placed in a 4 mm zirconia rotor that was loaded into a variable temperature probe and measured at 240 K. The pulse length of the proton was 4.2 μs and a pulse repetition delay of 3 s was employed when a contact time of 2 ms was used for 13C CP NMR measurement. All 13C NMR spectra were recorded at a Larmor frequency of 100.6 MHz with 2.4 kHz spinning speed for the 13C HPDEC/MAS NMR measurements. A pulse length of 2 μs and pulse repetition delay of 10 s with radio frequency field strengths of 50 kHz corresponding to 90° pulses of 5 μs duration were used. The downfield carbon resonance peak of adamantine, which was assigned as a chemical shift of 38.3 ppm at 300 K, was used as an external chemical shift reference. The crystalline structure of the gas hydrate samples was determined by high-resolution synchrotron XRD with a wavelength of 1.5496(1) Å at beamline 8C2 at the Pohang Accelerator Laboratory (PAL). The diffraction pattern was scanned in the range of 8−149° with a step length of 0.05°. The phase identification was conducted using the CMPR program,21 and the unit cell constant and volume were derived by a fullprofile-fitting method using the EXPGUI package.22

pipelines by hydrate formation between water and gas molecules during natural gas transportation.1,4 An important practical feature of gas hydrate is that vast quantities of methane in the form of the gas hydrate exist in the permafrost zone and subsea sediment.5 From an environmental standpoint, recent studies have focused on the formation, dissociation, and/or guest replacement of gas hydrates for CO2 ocean disposal by in situ hydrate6−8 and the separation of specific components such as SF6, H2S, and HFC from gas mixtures.9−12 One of the unique properties of gas hydrate is that it can store large quantities of gas (e.g., 180 SM3 per M3 of hydrate). Additionally, gas hydrates are characterized by a selfpreservation effect that enables them to remain at atmospheric pressure and a few degrees below the melting point of ice.13,14 Accordingly, extensive studies have been conducted to investigate the feasibility of using gas hydrates as storage and transportation media.15−17 For practical applications of gas hydrate to the storage and transportation process, considerable knowledge regarding thermodynamic stability, structural identification, and cage occupancy of guests of gas hydrates would be of particular importance. However, there is little experimental data available regarding the CO2−CH4−N2 mixture gas, which primarily comprises LFG.18 In this study, we investigated the technical feasibility of storage and transportation of LFG from landfill sites by hydrate formation. The phase equilibrium behavior of the CO2−CH4− N2 mixture hydrate was observed over a wide range of temperatures and pressures. In addition, synchrotron X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and Raman spectroscopy were used to identify the crystal structure and guest distribution in the cages of the gas hydrates. The gas storage capacity of landfill gas hydrate (LFGH) was estimated under several experimental conditions. Furthermore, the effect of NMOCs on the hydrate stability was determined based on the trace amounts of H2S and methanethiol (CH3SH).

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EXPERIMENTAL SECTION Materials and Apparatus. An equilibrium cell with a magnetic driver was used to determine the phase equilibria of the gas hydrates. The equilibrium viewing cell has an internal volume of about 300 cm3 and was made of stainless steel and reinforced sight glasses. This material allows visual observation of the phase transitions between the hydrate and liquid phases at pressures up to 30 MPa. A digital thermometer with a resolution of 0.1 K and a digital pressure gauge with a maximum error of 0.01 MPa were used to measure the temperature and pressure in the cell. The initial concentration of gas mixtures in the cylinders and the final concentration of the gas phase in equilibrium with the hydrate phase were checked by gas chromatography (Younglin, ACME 6100). All gases with a minimum purity of 99+% were supplied by DeokYang Energen Co. Procedure. The phase equilibrium experiment was initiated by charging the equilibrium cell with 70 cm3 of distilled water, and then the cell was pressurized to a desired pressure with the mixture gas. Hydrate nucleation was then induced by cooling the cell to about 5 K below the anticipated hydrate-forming temperature. After the gas hydrates were formed and the pressure was kept constant, the cell temperature was slowly elevated to dissociate the formed gas hydrates. When a minute amount of hydrate crystals remained and the temperature was kept constant at least for 6 h after pressure stabilization, the temperature and pressure were determined as an equilibrium

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THEORETICAL PREDICTION

The difference in the chemical potential of water between the empty hydrate and filled hydrate phases, ΔμwMT − H (= μwMT − μwH), is generally derived from statistical mechanics in the van der Waals and Platteeuw model:23 MT − H = RT Δμw ∑ νmln(1 + m

V

∑ Cmjf ĵ j

) (1)

where νm is the number of cavities of type m per water molecule in the hydrate phase, Cmj is the Langmuir constant of component j on the cavity of type m, and fĵ V is the fugacity of component j in the vapor phase, which is in equilibrium with the hydrate phase. Using the Lennard−Jones−Devonshire cell 4185

dx.doi.org/10.1021/es203389k | Environ. Sci. Technol. 2012, 46, 4184−4190


Article

theory, van der Waals and Platteeuw presented the Langmuir constant as a function of temperature as follows: Cmj =

4π kT

∫0

∞

⎡ −ω(r ) ⎤ 2 exp⎢ ⎥r d r ⎣ kT ⎦

(2)

where k is the Boltzmann’s constant, r is the radial distance from the cavity center, and ω(r) is the spherical-core potential. In our previous study,24,25 we presented a unique expression for the fugacity of water in the filled hydrate phase: ⎡ Δμ0 T ΔhwMT − I + Δhwfus H fŵ = fwL exp⎢ w − dT ⎢⎣ RT T0 RT 2

∫

+

∫0

MT − I P Δυw + Δυwfus

RT

⎤ V ⎥ ̂ ∑ νmln(1 + ∑ Cmjf j )⎥ ⎥⎦ m j

dP − Figure 1. Equilibrium dissociation pressures of CO2−CH4−N2 mixture gas hydrates. Solid lines are calculated using the PSRK model,25 and dotted lines represent the dissociation pressures of pure CO2 and CH4 gas hydrates.

(3)

CH4−N2 mixture hydrate with 30 mol % N2 should be located at higher temperatures and pressures than those of pure CH4 hydrate. These findings indicate that the three guest molecules of CO2, CH4, and N2 compete with each other for optimum occupancy of the hydrate cages resulting in the corresponding equilibrium conditions influenced by the crystal structure and the guest distribution in the cages of the mixture gas hydrates. All of the experimental data obtained in the present study agreed with the results calculated by the PSRK model, as shown in Figure 1. Structural Identification and Spectroscopic Analysis. For structural identification of the mixture gas hydrates, we conducted synchrotron XRD measurement of the hydrate sample formed from the CO2/CH4/N2 (40/55/5) gas mixture. The XRD pattern (Figure 2) indicated that the CO2−CH4−N2

Here, the fugacities of pure water and all components in the vapor phase, f wL and fjV̂ , were calculated using the predictive Soave−Redlich−Kwong (PSRK) group contribution method combined with the UNIFAC model.26,27 It should be noted this approach greatly improves upon the accuracy of the modified Huron−Vidal second-order (MHV2) model, especially for three-guest hydrate systems.25,28 Therefore, the Kihara potential parameters for guest-water interactions of guest molecules determined in the previous study25 were used to predict the phase behavior of the CO2−CH4−N2 mixture hydrates.

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RESULTS AND DISCUSSION Hydrate Phase Equilibria. The mixture gas hydrates were formed using specific compositions of gas mixtures according to the LFG characteristics of a landfill site. The LFG usually consists of two main components, 40−50 mol % CO2 and 50− 60 mol % CH4, diluted with approximately 5 mol % N2 and trace amounts of NMOCs. In some cases, the input stream of landfill power plants contains slightly higher N2 concentrations due to inflow from the air, which depends on the environment of the landfill site and delivery and/or the pretreatment process of the LFG. To make this study more practical and feasible, a specific molar ratio of CO2/CH4 (40/55) was selected to observe the characteristic phase behavior and guest distribution in the cages of gas hydrates. Figure 1 shows the phase equilibrium behavior of the CO2−CH4−N2 mixture gas hydrate at temperatures ranging from 273 to 282 K and pressures of up to 6 MPa. The equilibrium dissociation pressures of the mixture hydrates formed from the CO2/CH4/N2 (40/55/5) gas mixture were similar to those of the binary CO2/CH4 (40/ 60) hydrate, although they shifted slightly with temperature/ pressure changes. At the molar ratio of CO2/CH4 (40/55), the equilibrium dissociation pressures were gradually shifted to higher pressures and lower temperatures as the concentration of N2 increased. The equilibrium dissociation pressures of the CO2−CH4−N2 mixture hydrates with 5−20 mol % of N2 are located between the pure CO2 and CH4 hydrate curves. However, when the N2 concentration increases to 30 mol %, the equilibrium dissociation data of the mixture hydrate crosses the pure CH4 hydrate curve. As a result, above 277 K and 3.8 MPa, the equilibrium dissociation conditions of the CO2−

Figure 2. X-ray diffraction pattern of CO2−CH4−N2 mixture gas hydrate. Asterisks indicate peaks from ice Ih.

mixture hydrate is sI clathrate hydrate, which can be indexed using a cubic unit cell (space group Pm3n) with a unit cell parameter of a = 11.856 Å. Apart from some peaks corresponding to hexagonal ice (Ih), all of the distinctive Bragg peaks of the CO2−CH4−N2 mixture hydrate can be attributed to sI hydrate. Further, the Miller indices assigned at each diffraction angle are similar to those of pure CO2 hydrate29 and binary CO2−N2 hydrate.30,31 4186



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cages, as shown in part b of Figure 3.32,33 On the basis of the Raman results for CO2 and CH4 molecules, the CO2−CH4−N2 mixture hydrate forms a sI clathrate hydrate under equilibrium conditions, which is consistent with the XRD measurements. Therefore, it is clear that the Raman peak at 2324 cm−1 (part c of Figure 3) should be recognized as a characteristic peak corresponding to the symmetric N−N vibrations of N2 molecules encaged in the sI hydrate. It should be noted that the Raman peak position was similar to that of N2 molecules encaged in the sII clathrate hydrate.34 However, there was no indication of double occupancy35 or difference between the small and large cages. It should also be noted that the Raman peak at 2330 cm−1 coincides with that of the N2 free gas corresponding to backscattering of air along the laser path in the LN2 environment.18 To confirm the hydrate structure and cage occupancy of CH4 molecules in the hydrate framework, we also observed the 13C HPDEC/MAS NMR spectra for samples of the CO2−CH4−N2 mixture hydrate. Part a of Figure 4 clearly shows that CH4

Raman spectroscopy was used to better understand the properties of guest molecules trapped in the hydrate lattice. Figure 3 shows the Raman spectra of CO2, CH4, and N2

Figure 4. (a) Solid-state 13C HPDEC/MAS NMR and (b) 13C CP NMR spectra of CO2−CH4−N2 mixture gas hydrate at 240 K.

molecules in the small 512 cages of sI have resonances at −4.3 ppm, whereas the resonance of CH4 molecules in the large 51262 cages of sI occurs at −6.7 ppm. These findings are consistent with the 13C MAS NMR results for the pure CH4 and binary CO2−CH4 mixture hydrates,7,36 as well as the XRD and Raman results. 13C CP NMR (static) measurements were conducted to investigate the dynamics of CO2 molecules in the cages of the sI hydrate framework. Part b of Figure 4 shows the powder pattern of CO2 molecules in the large 51262 cages of sI reflecting the anisotropic motion of CO2 molecules due to the asymmetric shape of the large 51262 cages.37 However, there was no clear evidence of an isotropic line at ∼123 ppm to represent CO2 trapped in the small 512 cages of sI.30,38 These findings imply that CO2 molecules are primarily distributed in

Figure 3. Raman spectra of (a) CO2, (b) CH4, and (c) N2 molecules encaged in the hydrate framework.

molecules encaged in the hydrate framework. For CO2 molecules (part a of Figure 3), the characteristic Raman peaks revealed at 1278 and 1381 cm−1 indicate that the CO2 molecules are encaged over the cages of the sI hydrate framework.32,33 Split peaks are caused by the Fermi resonance effect corresponding to the C−O symmetric stretching (ν1) and O−C−O bending (2ν2) mode of the CO2 molecules. Prominent Raman peaks for the C−H stretching vibration of CH4 molecules in the sI hydrate cages were observed at 2905 cm−1 for the large 51262 cages and 2915 cm−1 for the small 512 4187



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Table 1. Gas Storage Capacity of Landfill Gas Hydrates run no. 1 2 3 4 a

condition

composition of initial gas (%)

composition of hydrate (%)

T (K)

P (MPa)

CO2

CH4

N2

CO2

CH4

N2

268 270 270 270

9 7.7 5.5 6.3

40 40 36.2 26.7

55 55 48 39

5 5 15.8 34.3

47.5 58 54.1 50

50.9 40.1 38 39.7

1.6 1.9 7.7 10.3

gas amount (cm3/g)a 121 136 128 118

Total gas volume (at STP) released from 1 g of hydrate.

the large 51262 cages of sI when they compete with CH4 and N2 molecules to form the CO2−CH4−N2 mixture hydrate. We also found that for the static NMR measurements, a broad peak at −6 ppm could be assigned to CH4 molecules trapped in the cages of sI. This should be distinguished from the peaks for CH4 molecules trapped in the cages of sII and for gaseous CH4 at −5 and −11 ppm, respectively.39 For pure CH4 hydrate, the cage occupancy ratio, θL/θS, is ≈1.26, where θL and θS are the cage occupancies of CH4 molecules in the large and small cages of the sI hydrate, respectively.7,32 This indicates that the small cages are less occupied than the large cages. It is also known that CO2 molecules occupy the large cages of the sI hydrate with a higher occupancy ratio than CH4 molecules, whereas N2 molecules preferentially occupy the small cages.7,30 These characteristics are attributed to the effect of the molecular size of guests and the interaction between hosts and guests on the cage occupancy. Thus, when CH4 molecules compete with CO2 and N2 molecules for the best occupancy, the cage occupancy ratio of CH4 would become lower or higher depending on the composition of gas mixtures. The peak area ratio obtained from the Raman and NMR analyses is known to be an indicator of the cage occupancy ratio of CH4 molecules into the 512 and 51262 cages.33 Cross-examination of the Raman and 13C NMR measurements of the CO2−CH4−N2 mixture hydrates revealed θL/θS (CH4) ≈ 0.78 ± 0.05 when the mixture hydrates were produced from the CO2/CH4/N2 (40/55/5) gas mixture. This indicates that the population of CH4 molecules in the small cages is higher than that in the large cages reflecting the strong effect of CO2 molecules to preferentially occupy the large cages. Gas Storage Capacity of Landfill Gas Hydrate. The capacity of LFGH to store gases was estimated from volumetric gas release measurements. Table 1 presents the composition and amount of gases trapped in the hydrate solids at two-phase (hydrate and vapor) equilibrium conditions. As expected, the concentrations of N2 in the hydrate phase were lower than those in the vapor phase. In contrast, the concentrations of CO2 in the hydrate phase were higher than those in the vapor phase. These differences were caused by the preferential occupation of CO2 molecules in the hydrate cages, especially the large cages, which is in good agreement with the results obtained from the Raman and 13C NMR measurements. It should be noted that the LFGHs can store ∼126 ± 10 cm3 gases (at STP) per gram of hydrate crystals. Figure 5 shows the dissociation pressure and the composition of hydrate under three-phase (hydrate, water, and vapor) equilibrium conditions at 277 K, as calculated using the PSRK model. For the specific molar ratio of CO2/CH4 (40/55) in the vapor phase, the dissociation pressure of the hydrate and the concentration of N2 in the hydrate phase increased as the mole fraction of N2 in the vapor phase increased. However, the molar ratio of CO2/ CH4 in the hydrate phase was almost constant (∼60/40), regardless of the concentration of N2. Another aspect to be

Figure 5. Structural transformation and gas concentration of hydrate phase for the CO2−CH4−N2 mixture gas hydrate with an increasing mole fraction of N2 at 277 K, as determined by the PSRK model.25.

noted is that, at concentrations higher than 80 mol % N2 in the vapor phase, the sII hydrate was found to have a more stable structure than the sI hydrate. Therefore, in this region, the concentration of N2 in the hydrate phase increases drastically, whereas the concentration of CO2 becomes lower than 20 mol %. Effect of NMOCs on Hydrate Stability. As mentioned above, LFGs are primarily composed of CH4, CO2, and N2, with trace amounts of NMOCs. To investigate the effects of NMOCs on the thermodynamic stability of the LFGHs, we measured the equilibrium dissociation pressures of the hydrates prepared from the CO2/CH4 (50/50) gas mixtures including the trace amounts (500 ppm) of H2S and CH3SH. As shown in Figure 6, there was no significant shift in the dissociation phase boundary of the CO2−CH4 mixture hydrates in response to the

Figure 6. Equilibrium dissociation pressures of the CO2−CH4 mixture gas hydrates with a trace amount of nonmethane organic compounds. 4188



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2009T100100988) funded by the Korean Government Ministry of Knowledge Economy. We are also grateful for support provided by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (2008-313-D01285, 2008-0061974, 2010-0007026) and the Korea Science and Engineering Foundation through the Nuclear R&D Program (M2AM062008-03931).

addition of the trace amounts of NMOCs. This implies that the trace amounts of NMOCs do not affect the cage occupancy or the phase equilibrium of the LFGHs. It is important to note that the PSRK model also predicted a very small decrease in dissociation pressures (ΔP ≈ 0.01 MPa) in response to enclathrating H2S molecules into the hydrate cages from a gas mixture including 500 ppm of H2S. In summary, we have studied the feasibility of the storage and transportation of LFG via the formation of hydrate by investigating the phase equilibrium, structure, guest occupation, and gas storage capacity of the gas hydrates formed from the CO2−CH4−N2 gas mixtures. Structural analyses and equilibrium measurements show that the CO2−CH4−N2 mixture hydrates with 5−20 mol % of N2 have the sI structure and the dissociation pressures located between the pure CO2 and CH4 hydrate curves. The gas storage capacity of the CO2−CH4−N2 mixture hydrates is ∼126 v/v, which is comparable to natural gas hydrate (∼180 v/v), which also has potential for gas storage and transportation.40,41 It should also be noted that the concentration of CH4 in the hydrate phase is ∼40 mol %, when the LFGHs are formed from the CO2−CH4−N2 gas mixtures with the specific molar ratio of CO2/CH4 (40/55). This implies that the gases produced from the CO2−CH4−N2 mixture hydrates by regasification may be used directly for power plants to generate electricity and/or heat without further separation process. We also conclude that trace amounts of NMOCs do not affect the cage occupancy of gas molecules or the thermodynamic stability of LFGHs. Overall, the experimental and theoretical results presented here provide insight into the formation and dissociation of LFGHs. The results also provide basic knowledge regarding the interaction between host and guest molecules and the cage occupancy of guest molecules trapped in the hydrate frameworks. Furthermore, the present study provides a useful model for industrial applications that are favorable for gas storage under specific conditions. The most common method in Korea for municipal solid waste is burial in landfills. Currently, among total 218 landfill sites in Korea, only 15 sites have the power generation system to produce electricity and/or heat using the LFGs. At over 200 sites, the LFGs are directly emitted to atmosphere or flared because of their economical infeasibility. To reduce the emission of GHGs, the LFGs should be collected and treated by means of compression, liquefaction, or solidification. On the basis of the technical feasibility of LFGHs suggested in this study, a large scale field-test as well as evaluation of economic feasibility regarding the LFGH process are in progress in our team. This involves optimal size of the hydration and regasification process, transportation of LFGHs, treatment of collected LFGs, and appropriate business model for the best profit.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-51-410-4684, fax: +82-51-403-4680, e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the New & Renewable Energy program by the Korea Institute of Energy Technology E v a l u a t i o n a n d Pl a n n i n g ( K E T E P ) ( G r a n t No . 4189



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Thermodynamic Stability, Spectroscopic Identification, and Gas

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