Another way of CO2 ocean storage is as solid or solid hydrate CO2. Under certain temperature and pressure, CO2 can be changed from gas or liquid to solid state. Its solid is approximately 1.5 times greater than seawater and thus tends to sink to the ocean floor. The dissolving rate of the surface is also measured to be at about 0.2 cm/hr that a small quantity of the sample can be completely dissolved before reaching the sea floor[1]. In addition to pure solid co2, CO2 hydrate is another popular method. The formation takes place when the dissolved concentration of liquid CO2 is high enough, usually around 30%, at below average 400m of the sea level. And they are in the form of either external layer around the liquid CO2 droplets or solid mass[2]. Its molecular composition is just CO2 and water, CO2•nH2O (n ≈ 5.75), in a cage-like structure[3]. The resulted density is denser than seawater as well, by approximately 10%. Compared to the liquid CO2, the hydrate form dissolves significantly slowly into seawater and its timescale can be measured to at least hundreds of years[2]. Additionally, the hydrate remains immobile on the seafloor by occupying pore spaces impeding water flow and can be extracted from a pipeline at a shallower depth[4]. The overall molecular stability relies on the temperature and pressure of the environment, and it only dissociate when placed in direct contact with additional heat and water with concentration of cos less than the equilibrium amount[5]. Moreover, the surface mass dissolving speed is similar to that of the solid CO2, 0,2 cm/hr, which allows the samples to either dissolve or sink to the bottom. Although due to its crystalline structure. pure hydrate doesn’t travel through a pipe , its paste-like composite with water can be obtained from extrusion, which has a dissolution rate between CO2 droplets and pure hydrate[3]. Given that 100% efficiency is extremely difficult to achieve in reality, it’s shown in both laboratory and field experiments that CO2 sinking can reach approximately 15-25% reaction efficiency[3]. Any kind of instability is likely to cause dissolution and dispersion during descending process[4].
Mineralization and Deep Sea Sediments
Similar to mineralization process that takes place within the rocks underground, it can also occur under the sea. The rate of dissolution of CO2 from atmosphere to oceanic regions relies upon the circulation period of the ocean and buffering ability of subducting surface water. The forward and reverse reactions between CO2 and solid CaCo3 reduce atmospheric concentration of CO2. Several studies have shown that the consequences of dissolution between CaCo3 and CO2, or CO2 be released back to the atmosphere, could be negligible for at least a few more centuries. Moreover, researches has demonstrated the CO2 retention time could take up to 500 years for marine sequestration below several kilometers, but can be shortened by different injection operations, such as depth, pressure, and temperature. The neutralization happened on the seafloor can be measured on a timescale of 5-6 kyr[6]. Meanwhile, improvements have been found for the dissolution ofCaCo3 when injection happens near o r downstream of the site.
In addition to carbon mineralization, another proposal is to inject liquid carbon into deep ocean at least 3000 m below the level and a few hundred meters below the ocean sediment to generate CO2 hydrate. Two special regions are defined for the research purpose: negative buoyancy zone (NBZ), which is the region between liquid CO2 that’s denser than surrounding water and the one with neutral buoyancy, and hydrate formation zone (HFZ), which is the area with low temperatures but high pressures. Several researches and models have shown that the optical depth of injection needs to take intrinsic permeability and change in liquid CO2 permeability into consideration in order to yield larger storage. Specifically, if the former below HFZ is lower than the latter, then injection site below HFZ shouldn’t be an option. Generally, it’s found that the formation of hydrate decreases liquid CO2 permeability, and injection below HFZ is more energetically favored that the one inside HFZ. Moreover, the results demonstrate that if NBZ is thicker than HFZ in deep ocean, then the injection should happen below HFZ and directly at NBZ. In this case, liquid CO2 would most likely to sink to NBZ and be stored below the buoyancy and hydrate cap. Lastly, the leakage of CO2 is expected to happen either from its further dissolution into pore fluid or molecular diffusion, which both are on a timecard of at least thousands of years[7][8].
- ^ Caldeira, Ken, et al. “IPCC Special Report on Carbon Dioxide Capture and Storage: Ocean Storage.” International Panel on Climate Change, 2005
- ^ a b ROCHELLE, C. (2003). "CO2 HYDRATE AND UNDERGROUND STORAGE" (PDF). published thesis.
- ^ a b c Adams, E. Eric, and Ken Caldeira. “Ocean Storage of CO2.” Elements, vol. 4, Oct. 2008, pp. 319–324., doi:10.2113/gselements.4.5.319.
- ^ a b Capron, Mark (July 26, 2013). "Secure Seafloor Storage CO2 Storage" (PDF). published thesis.
- ^ "Potential for Very Deep Ocean Storage of CO2 Without Ocean Acidification: A Discussion Paper". Energy Procedia. 114: 5417–5429. 2017-07-01. doi:10.1016/j.egypro.2017.03.1686. ISSN 1876-6102.
- ^ Archer, David (June 1998). "Dynamic of Fossil Fuel CO2 Neutralization by Marine CaCo3" (PDF). Global Biogeochemical Cycles.
- ^ "CO2 disposal as hydrate in ocean sediments". Journal of Natural Gas Science and Engineering. 8: 139–149. 2012-09-01. doi:10.1016/j.jngse.2011.10.006. ISSN 1875-5100.
- ^ Zhang, Dongxiao; Teng, Yihua (2018-07-01). "Long-term viability of carbon sequestration in deep-sea sediments". Science Advances. 4 (7): eaao6588. doi:10.1126/sciadv.aao6588. ISSN 2375-2548.