Buoyant CO2 Research Articles

Small Scale Field Study of an Ocean CO2 Plume

We have carried out a small-scale (∼20 l) CO2 sequestration experiment off northern California (684 m depth, ∼5°C, background ocean pH ∼7.7) designed as an initial investigation of the effects of physical forcing of the fluid, and the problem of sensing the formation of a low pH plume. The buoyant CO2 was contained in a square frame 1.2 m high, exposing 0.21 m2 to ocean flow. Two pH electrodes attached to the frame recorded the signal; a second frame placed 1.9 m south of the CO2 pool was also equipped with two recording pH electrodes. An additional pH electrode was held in the ROV robotic arm to probe the fluid interface. Local water velocities of up to 40 cm sec−1 were encountered, creating significant eddies within the CO2 box, and forcing wavelets at the fluid interface. This resulted in rapid CO2 dissolution, with all CO2 being depleted in a little more than 2 days. The pH record from the sensor closest (∼10 cm) to the CO2 showed many spikes of low pH water, the extreme value being ∼5.9. The sensor 1 m immediately below this showed no detectable response. The electrodes placed 1.9 m distant from the source also recorded very small perturbations. The results provide important clues for the design of future experiments for CO2 disposal and biogeochemical impact studies. These include the need for dealing with the slow CO2 hydration kinetics, better understanding of the fluid dynamics of the CO2-water interface, and non-point source release designs to provide more constant, controlled local CO2 enrichments within the experimental area.

CO2 hydrate composite for ocean carbon sequestration.

Rapid CO2 hydrate formation was investigated with the objective of producing a negatively buoyant CO2-seawater mixture under high-pressure and low-temperature conditions, simulating direct CO2 injection at intermediate ocean depths of 1.0-1.3 km. A coflow reactor was developed to maximize CO2 hydrate production by injecting water droplets (e.g., approximately 267 microm average diameter) from a capillary tube into liquid CO2. The droplets were injected in the mixing zone of the reactor where CO2 hydrate formed at the surface of the water droplets. The water-encased hydrate particles aggregated in the liquid CO2, producing a paste-like composite containing CO2 hydrate, liquid CO2, and water phases. This composite was extruded into ambient water from the coflow reactor as a coherent cylindrical mass, approximately 6 mm in diameter, which broke into pieces 5-10 cm long. Both modeling and experiments demonstrated that conversion from liquid CO2 to CO2 hydrate increased with water flow rate, ambient pressure, and residence time and decreased with CO2 flow rate. Increased mixing intensity, as expressed by the Reynolds number, enhanced the mass transfer and increased the conversion of liquid CO2 into CO2 hydrate. Using a plume model, we show that hydrate composite particles (for a CO2 loading of 1000 kg/s and 0.25 hydrate conversion) will dissolve and sink through a total depth of 350 m. This suggests significantly better CO2 dispersal and potentially reduced environmental impacts than would be possible by simply discharging positively buoyant liquid CO2 droplets. Further studies are needed to address hydrate conversion efficiency, scale-up criteria, sequestration longevity, and impact on the ocean biota before in-situ production of sinking CO2 hydrate composite can be applied to oceanic CO2 storage and sequestration.

Negatively buoyant CO2‐hydrate composite for ocean carbon sequestration

In this article, we propose a concept for an injector system that produces a negatively buoyant CO 2 material by promoting the conversion of CO 2 to CO 2 hydrate.

Dissolution of buoyant CO 2 drops in a counterflow water tunnel simulating the deep ocean waters

We investigated the dissolution of CO 2 drops in a high pressure counterflow water tunnel simulating ocean conditions down to 2000 m below the ocean surface. In comparison of the behavior of the buoyant CO 2 drops observed in our experiments with that reported previously for motionless CO 2 drops in seawater or water, the following significant differences are noticed: (1) the CO 2 drops in buoyant motion dissolve much faster than the motionless CO 2 drops and (2) hydrate does not always form on the buoyant CO 2 drops, as it was reported in the literature to be an unavoidable phenomenon on the motionless drops. This suggests that buoyant motion influences the physical and chemical behavior of the CO 2 drops in seawater dramatically.

The Impact of CO2 Clathrate Hydrate on Deep Ocean Sequestration of CO2: Experimental Observations and Modeling Results

Abstract: CO2 clathrate hydrate is a crystalline compound that can form under temperature and pressure conditions associated with the injection and storage of CO2 in the deep ocean (below 500 m). At depths being considered for injection of CO2 (between 1,000 and 1,500 m), in the absence of hydrate formation, the buoyant CO2 would simply rise as it dissolved in the seawater. If, however, the hydrate phase forms, it will affect this process. The impact could be positive or negative, depending on how hydrate forms and whether it is associated with undissolved CO2. This paper summarizes experimental and theoretical information relating to formation conditions for the hydrate, the relative density of the hydrate, the formation of a hydrate shell on drops of liquid CO2, and the impact that a hydrate shell has on dissolution of CO2. The future direction of the work is also briefly described.

Hydrate Formation on Surfaces of Buoyant Liquid CO2 Drops in a Counterflow Water Tunnel

An experimental investigation of hydrate formation on surfaces of buoyant liquid CO2 drops was conducted in a counterflow water tunnel simulating conditions in the deep ocean. The major component o...

Dissolution of CO 2 droplets in the ocean

Dissolution of CO 2 droplets in the ocean has been studied. Owing to hydrogen bonding and supersaturation with CO 2, seawater at the surface of the CO 2 droplets becomes highly structured. Since this quasi-crystalline structure is similar to that of hydrate crystal elements, hydrate formation on the droplets will not cause a dramatic decrease in the dissolution rate as reported in the literature. Mass transfer of a buoyant CO 2 droplet in Stokes flow in the ocean is analyzed. Based on the derived mass-transfer coefficients, shrinkage rates for CO 2 droplets without and with a hydrate shell are predicted. Good agreement is observed between predictions and experimental data.