SNG (Solidified Natural Gas) Technology For Gas Storage

Natural gas (NG) has been recognized as the cleanest burning fossil fuel and a vital resource to alleviate anthropogenic CO₂ emissions to enable a transition into a carbon-constrained world. There is a necessity to develop safe, reliable, and efficient technology for large-scale NG storage.

Fiery ice (Burning Natural Gas Hydrate). Figure courtesy Praveen Linga.

Solidified natural gas (SNG) technology via clathrate hydrates offers a high NG storage capacity at temperate conditions (atmospheric pressure and moderate temperatures) compared to compressed natural gas (CNG) or adsorbed natural gas (ANG). Though Liquefied Natural Gas (LNG) facilities are devoted to transporting NG, the cryogenic temperature requirement (-162 °C) and the continuous boil-off issues limit its adoptability for long-term storage application. Clathrate hydrates or gas hydrates are crystalline ice-like compounds formed by guest molecules (such as CH₄ gas and THF liquid) and host water molecules at suitable pressure and temperature conditions. Clathrate hydrates classically crystalize in three different structures or geometries, named structure I (sI, cubic), structure II (sII, cubic), and structure H (sH, hexagonal).

SNG is nonexplosive, environmentally compatible, and economical. A major concern for the SNG technology is that the use of sI hydrates requires low storage temperature (-20 °C) for storage, and the stability depends on the anomalous self-preservation effect at this temperature. Methane (sI) hydrates are thermodynamically stable at a temperature of -80 °C at atmospheric pressure.

In order to overcome this issue, it is desirable to move away from sI hydrates. In this direction, mixed methane-tetrahydrofuran (CH₄-THF) hydrates (sII) offer a great promise to shift the thermodynamics to very mild conditions, as the mixed CH₄-THF hydrate is thermodynamically more stable than pure methane hydrate (sI). However, molecular-level understanding of these thermodynamically or kinetically controlled hydrate structures is elusive in the open literature. Thus, through a series of carefully planned experimental work utilizing state-of-the-art analytical techniques like high-pressure differential scanning calorimetry (HP μ-DSC) and an in-situ Raman spectroscopy, we investigate the mixed CH₄-THF hydrate formation (with 5.56 mol% THF, stoichiometric amount) in the presence of a surfactant, sodium dodecyl sulfate (SDS, as kinetic promoter).

Fig.1 Co-occurrence of pure methane (sI) & mixed CH4-THF hydrates (sII) evidenced through DSC thermogram and Raman spectra (Republished with permission from iScience)

Through HP μ-DSC analysis, we found that the presence of SDS in water-THF solution promotes the nucleation and growth of sI hydrate (pure methane hydrates) crystals. In other words, we can say that pure methane (sI) and mixed CH₄– THF hydrates (sII) both co-exist in the presence of SDS (Refer to Fig.1). However, in the absence of SDS, we found that instead of sI hydrates, pure THF (sII) and mixed CH₄– THF hydrates (sII) coincide during hydrate formation. Moreover, if thermodynamics restrict the formation of sI hydrates, the presence of SDS in the water-THF system may enhance the formation of mixed CH₄-THF (sII) hydrate with significantly high methane uptake.

Our findings present a significant enhancement in the methane storage capacity through mixed CH₄-THF hydrates in the presence of SDS, possibly due to (i) the sharing of large cages by methane molecules along with THF, and (ii) improving the methane enclathration in the small cages of mixed CH₄-THF hydrates. In summary, we present a kinetically and thermodynamically controlled encaging of methane molecules in small and large cages of sI and sII hydrates. Our findings offer new insights for the development of an efficient process for large-scale methane storage at temperate conditions in mixed CH₄-THF hydrates (sII) through solidified natural gas technology.