Thermodynamic Stability, Spectroscopic Identification, and Gas Storage Capacity of CO₂–CH₄–N₂ Mixture Gas Hydrates: Implications for Landfill Gas Hydrates

Abstract

Landfill gas (LFG), which is primarily composed of CH(4), CO(2), and N(2), 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 CO(2)-CH(4)-N(2) mixture hydrates. When the specific molar ratio of CO(2)/CH(4) was 40/55, the equilibrium dissociation pressures were gradually shifted to higher pressures and lower temperatures as the mole fraction of N(2) increased. X-ray diffraction revealed that the CO(2)-CH(4)-N(2) mixture hydrate prepared from the CO(2)/CH(4)/N(2) (40/55/5) gas mixture formed a structure I clathrate hydrate. A combination of Raman and solid-state (13)C 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.

Phase Equilibrium Data for the Hydrates of Synthesized Ternary CH4/CO2/N2 Biogas Mixtures

“…We also compared the experimental data obtained herein with previously reported literature data for similar systems. , As shown in Figure , our results complemented the hydrate phase equilibrium data of the ternary gas mixtures with CH 4 contents of 49.05–70.05%, and CO 2 contents of 19.98–45.03% under extended pressure and temperature ranges. In addition, it was apparent that hydrate phase equilibrium conditions shifted to higher pressures and lower temperatures upon decreasing the CO 2 content.…”

“…For example, Kakati studied the formation of hydrates in the CH 4 /CO 2 /N 2 gas mixture followed by dissociation in various aqueous electrolyte concentrations, and provided ternary gas hydrate phase equilibrium data for a single given component (i.e., CH 4 /CO 2 /N 2 = 89.89/5/5.11). In addition, Lee investigated the phase equilibrium behavior of landfill gas with lower CH 4 contents (i.e., 41, 46, 52, and 55 mol %) and four different N 2 contents (i.e., 5, 10, 20, and 30 mol %) between 273 and 282 K and at pressures < 6 MPa. The obtained results indicated that the CO 2 /CH 4 /N 2 hydrates have the potential to be utilized in gas storage and transportation applications.…”

Instrumental Methods for Cage Occupancy Estimation of Gas Hydrate

“…Ota et al [70] investigated the CO 2 -CH 4 replacement in CH 4 hydrate with in situ Raman spectroscopy and provided their cage occupancy change during the replacement process. Lee et al [71] measured the Raman spectrum of CH 4 -CO 2 -N 2 mixed gas hydrate, which is the sI hydrate identified by XRD. They calculated the cage occupancy of the gas hydrate synthesized at different initial gas compositions and discussed its application on landfill gas treating.…”

Integrated Process Study on Hydrate-Based Carbon Dioxide Separation from Integrated Gasification Combined Cycle (IGCC) Synthesis Gas in Scaled-Up Equipment

“…3,4 In gas hydrates, the water molecules connect each other by hydrogen bonds and form different cavities; meanwhile, the small gas molecules stably occupy the cavities by van der Waals interaction forces. 5 The basis of gas-hydrate-based CO 2 separation technology is the selective partition of the CO 2 component between the hydrate phase and the gaseous phase.…”