Natural gas storage in hydrates in the presence of thermodynamic hydrate promoters: Review and experimental investigation

Natural gas (NG), the cleanest fossil fuel, is playing an increasingly important role in the current energy supply. However, the safe storage and transportation of flammable NG is a long-standing challenge. Furthermore, NG emission has a stronger per molecule greenhouse effect on the environment than CO2. Therefore, efficient and effective methods of NG storage and transportation are needed. Storing NG in the form of gas hydrate offers advantages over common compression or liquefaction methods, but the thermodynamic conditions required for gas hydrate formation hinder the large-scale application of solidified natural gas (SNG) technology. This work presents a review of phase equilibrium conditions of gas hydrates formed by greenhouse gases including CH4, CO2 and NG in the presence of thermodynamic hydrate promoters. This study uses available thermodynamic software to calculate gas hydrate phase equilibrium using different Equations of State (EoS). We include an experimental investigation using a 2 L autoclave reactor to evaluate the effect of mass transfer, the presence of cyclopentane as a thermodynamic promoter, and the level of subcooling on the NG hydrate formation kinetics. The results show that: 1) Tetrahydrofuran and cyclopentane generally have the strongest thermodynamicpromoting effect; 2) Thermodynamic promotion of cyclopentane on NG hydrate is validated experimentally; 3) NG hydrate formation kinetics is greatly influenced by mechanical stirring (mass transfer), cyclopentane as a co-former and its concentration and subcooling; 4) At high subcooling, cyclopentane-promoted systems show a significantly improved gas storage capacity than the baseline sample; and 5) NG hydrate particles have a size distribution of hundreds of microns under current experimental conditions. This study offers new insight into NG hydrate formation thermodynamics and kinetics that has application to SNG technology.