Gate-to-Gate Life Cycle Assessment of Solidified Natural Gas Production via Clathrate Hydrate Formation: Environmental Trade-Offs of Thermodynamic Promoter Selection

Solidified natural gas (SNG) is a potential alternative to liquefied and compressed natural gas for natural gas storage, but its environmental performance has not been quantified. This study presents the first life cycle assessment of SNG production via clathrate hydrate formation, comparing unpromoted methane hydrate (sI), methane–tetrahydrofuran (THF) hydrate (sII), and methane–1,3-dioxolane hydrate (sII). A gate-to-gate assessment was performed using ReCiPe 2016 (H) with a functional unit of 1 MJ of natural gas stored as methane hydrate. Since unconverted methane is recycled, all configurations consume the same net quantity of methane, and the higher conversion of the promoted systems (42.0% versus 10.0%) instead reduces electricity demand from 0.116 to approximately 0.030 kW h MJ–1. The unpromoted process had the lowest impact in all 18 midpoint categories, including the lowest global warming potential (0.070 kg CO2-eq MJ–1) and end point single score (1.83 mPt). The global warming potentials of the 1,3-dioxolane- and THF-promoted processes were 3.0 and 4.7 times higher, with promoter production contributing 83% and 89%, respectively. Paired Monte Carlo analysis confirmed these rankings, with 1,3-dioxolane outperforming THF in 87–100% of iterations depending on the category. The ranking was unchanged under a 20-year characterization horizon and would be reversed only if more than 2.5% of the unconverted methane recycle stream were lost, or if the production burden of 1,3-dioxolane were 81% lower than that of its 1,4-dioxane proxy. Thermodynamic promotion therefore increases rather than redistributes environmental burdens, and the stoichiometric promoter makeup, not hydrate formation, is the principal environmental barrier to promoted SNG.