Hydrogen and chemical energy storage in gas hydrate at mild conditions
Graphical abstract
In this work additives which contain chemical energy such as 1,1-dichloro-1-fluoroethane (HCFC-141 b) and methane + tert-butyl alcohol (tBA) lower operation condition of hydrate-based hydrogen storage from tough zone to the mild (easy) zone, meanwhile with high energy storage capacity.
Introduction
Gas hydrates is clathrate compound formed by water (host molecule) and gas (guest molecule) under high pressure and low temperature. Gas hydrates reservoir is a promising energy resource, exploration and gas production of it has been studied [1,2]. Meanwhile gas hydrate is a good energy material, hydrated-based technology has been applied on gas storage [3] and hydrogen purification from syngas [4,5]. Hydrogen storage in clathrate hydrates is a promising hydrogen storage technology with large scale industrial application prospects [[6], [7], [8], [9]]. For further improving hydrogen storage properties, there are three generations of hydrate-based hydrogen storage technology have been developed. The first generations was using hydrogen hydrate (structure II) as hydrogen storage material. Mao et al. [10,11]reported hydrogen storage capacity could reach 5.26 wt% when hydrogen formation at 180–220 MPa and 300 K, which is higher than 2020 DOE's target (4.5 wt%).
The second generation was using energy-free liquid or solid additives in hydrate-based hydrogen storage, aiming to lower hydrate phase equilibrium conditions. Weissman et al. [12] studied 5.56 mol% tetrahydrofuran (THF)+ hydrogen + water hydrate forming at 274 K, 6.89 MPa and 274 K, 11 MPa, hydrogen storage capacity was lower than 1 wt% and hydrogen only occupied 512 cage of the gas hydrate. Liu et al. [13] using Ab initio molecular dynamic simulation studied cage occupation of H2 and THF in H2+THF hydrate at 140 K, and theoretical hydrogen storage could reach 1.6 wt% and 3.8 wt% by calculation. Di Profio et al. [14] using THF reverse micelles to enhance the kinetics of the formation process and assist clathrate formation, hydrogen capacity could reach 0.5 wt% within 28min. Kaur et al. [15] examines the host-guest interactions in H2-THF mix hydrate by computational study which proves H2 molecules can be occupied in the hexakaidecahedral cages along with THF as well as single occupied in small cages. Trueba et al.[[16,17]] studied tetrabutylammonium bromide (TBAB) and tetrabutylammonium fluoride (TBAF) thermodynamically promote hydrate-based hydrogen storage, hydrogen capacity are lower than 0.1 wt%.
The third generation was using flammable gas molecules as promoters in hydrate-based hydrogen storage, aiming to increase totally energy storage density. Veluswamy et al. [18,19] studied propane promoting hydrate-based hydrogen storage and tested hydrate phase equilibrium of hydrogen-propane binary hydrate. Matsumoto et al. [20] using in-situ Raman spectra studied methane promoting hydrate-based hydrogen storage, maximum hydrogen storage capacity achieved 0.31 wt% at 263 K and 70 MPa. Zhang et al. [21] studied growth of H2 + CH4 binary hydrates via molecular dynamic simulation, results shown that the solubility and diffusivity of guest molecules especially CH4 dominate the growth process.
However, hydrogen storage capacity of the second and third generations hydrate-based hydrogen storage technology still quite low in ambient temperature and low pressure (lower than 25 MPa). Aiming to lower hydrate formation condition and increase hydrogen energy storage capacity simultaneously, this paper proposes a novel approach that using materials containing chemical hydrogen energy as hydrate formation promoters. In this work hydrogen hydrate phase equilibrium in addition of methane + tert-butyl alcohol (tBA), trichloromethane (CHCl3) and 1,1-dichloro-1-fluoroethane (HCFC-141 b) were tested, and enclathration mechanism of hydrogen + methane + tBA were studied via Raman spectroscopy. This study will provide guidance on hydrate-chemical hybrid hydrogen storage technology, and leads to the next generation of hybrid hydrate-based hydrogen technology in the future.
Section snippets
Material and methods
Experimental apparatus of hydrate phase equilibrium were detailed described in previous work [[22], [23], [24], [25]]. The key parts of this apparatus is a 300 mL stirred high pressure vessel with a cold bath to control temperature. Two thermocouples monitored temperature of gas and liquid phase inside the vessel. A pressure sensor measured pressure of the apparatus. All data were recorded by Agilent 34970 A data logger. Experimental method of hydrate phase equilibrium is like previous work [22,
Results and discussion
- (1)
Enclathration of tert-butyl alcohol in (75 mol%)H2/CH4 hydrate
5.6 mol% tBA (aq) represents all 51264 cages were occupied by tBA molecule while forming sII gas hydrate, and 2.8 mol% tBA (aq) represents half of 51264 cages were occupied by tBA molecule while forming sII gas hydrate. Hydrate phase equilibrium data of (75 mol%)H2/CH4+water and (75 mol%)H2/CH4+tBA (aq) are demonstrated in Table 1 and Fig. 1.
Fig. 1 and Table 1 shown that comparing to hydrate phase equilibrium line of (75 mol%)H2/CH4
Conclusions
In this paper enclathration of methane + tert-butyl alcohol, 1,1-dichloro-1-fluoroethane and trichloromethane in hydrogen hydrates were studied. Methane + tert-butyl alcohol (tBA) as a chemical hydrogen storage could significantly thermodynamically enhanced hydrate-based hydrogen storage. 5.6 mol% tBA and 2.8 mol% tBA shifted hydrate phase equilibrium line of (75 mol%) H2/CH4 about 4 K and 5 K towards higher temperature, respectively. Raman spectra shows the thermodynamic promotion mechanism
Acknowledgements
This work was funded by the National Key Research and Development Program (2016YFC0304006, 2017YFC0307302 and 2017YFC0307303), National Natural Science Foundation of China (21736005, 51576069 and 51876069) and. Raman spectra studies were conducted with the assist of Miss Dongmei Wang and Mr. Ke Zhang from ThermoFisher China Co. . The authors sincerely appreciate their contributions to this work.
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