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Water column release

Dispersal of liquid CO2 at a depth of 1000 m or deeper is technologically feasible. Since liquid CO2 may be relatively easily transported to appropriate depths, the preferred release mode is thought at this time to be as a liquid or dense gas phase (achieved by compression beyond its critical point, 72.8 bar at 31°C). The pipes that would carry this CO2 to the deep ocean would be similar to the pipes that have been used commercially on land to transport CO2 for use in CO2 enhanced oil recovery projects (Ozaki et al., 1997). Models (Liro et al, 1992, Drange and Haugan, 1992) predict that, with a properly designed diffuser, nearly all the CO2 would dissolve in the ocean within a 100 m of the injection depth. Then, this CO2-rich water would be diluted as it disperses, primarily horizontally along surfaces of constant density.

Water column injection schemes typically envision minimizing local changes to ocean chemistry by producing a

Figure 6.21 Relationship between depth and sea floor area. Flow in ocean bottom boundary layers would need to be taken into account when selecting a site for a CO2 lake. Bottom friction and turbulence can enhance the dissolution rate and vertical transport of dissolved CO2 and lead to a short lifetime for the lake (Section 6.2.1.6). It has been suggested that CO2 lakes would be preferentially sited in relatively restricted depressions or in trenches on sea floor (Ohsumi, 1995).

Figure 6.21 Relationship between depth and sea floor area. Flow in ocean bottom boundary layers would need to be taken into account when selecting a site for a CO2 lake. Bottom friction and turbulence can enhance the dissolution rate and vertical transport of dissolved CO2 and lead to a short lifetime for the lake (Section 6.2.1.6). It has been suggested that CO2 lakes would be preferentially sited in relatively restricted depressions or in trenches on sea floor (Ohsumi, 1995).

relatively dilute initial injection through a series of diffusers or by other means. Dilution would reduce exposure of organisms to very low pH (very high CO2) environments (Section 6.7).

One set of options for releasing CO2 to the ocean involves transporting liquid CO2 from shore to the deep ocean in a pipeline. This would not present any major new problems in design, 'according to petroleum engineers and naval architects speaking at one of the IEA Greenhouse Gas R&D Programme ocean storage workshops' (Ormerod et al., 2002). The oil industry has been making great advances in undersea offshore technology, with projects routinely working at depths greater than 1000 m. The oil and the gas industry already places pipes on the bottom of the sea in depths down to 1600 m, and design studies have shown 3000 m to be technically feasible (Ormerod et al., 2002). The 1 m diameter pipe would have the capacity to transport 70,000 tCO2 day-1, enough for CO2 captured from 3 GW of a coal-fired electric power plant (Ormerod et al., 2002). Liro et al. (1992) proposed injecting liquid CO2 at a depth of about 1000 m from a manifold lying near the ocean bottom to form a rising droplet plume. Nihous et al. (2002) proposed injecting liquid CO2 at a depth of below 3000 m from a manifold lying near the ocean bottom and forming a sinking droplet plume. Engineering work would need to be done to assure that, below 500 m depth, hydrates do not form inside the discharged pipe and nozzles, as this could block pipe or nozzle flow.

CO2 could be transported by tanker for release from a stationary platform (Ozaki et al., 1995) or through a towed pipe (Ozaki et al., 2001). In either case, the design of CO2 tankers would be nearly identical to those that are now used to transport liquid petroleum gas (LPG). Cooling would be used, in order to reduce pressure requirements, with design conditions of -55 degrees C and 6 bar pressure (Ormerod et al., 2002). Producing a dispersed initial concentration would diminish the magnitude of the maximum pH excursion. This would probably involve designing for the size of the initial liquid CO2 droplet and the turbulent mixing behind the towed pipe (Tsushima et al., 2002). Diffusers could be designed so that CO2 droplets would dissolve completely before they reach the liquid-gas phase boundary.

CO2 hydrate is about 15% denser than sea water, so it tends to sink, dissolving into sea water over a broad depth horizon (Wannamaker and Adams, 2002). Kajishima et al. (1997) and Saito et al. (2001) investigated a proposal to create a dense CO2-seawater mixture at a depth of between 500 and 1000 m to form a current sinking along the sloping ocean bottom. Another proposal (Tsouris et al., 2004; West et al., 2003) envisions releasing a sinking CO2-hydrate/seawater slurry at between 1000 and 1500 m depth. This sinking plume would dissolve as it sinks, potentially distributing the CO2 over kilometres of vertical distance, and achieving some fraction of the CO2 retained in deep storage despite the initial release into intermediate waters. The production of a hydrate/seawater slurry has been experimentally demonstrated at sea (Tsouris et al., 2004). Tsouris et al. (2004) have carried out a field experiment at 1000 m ocean depth in which rapid mixing of sea water with CO2 in a capillary nozzle to a neutrally buoyant composite paste takes place. This would enhance ocean retention time compared to that from creation of a buoyant plume. Aya et al. (2004) have shown that a rapidly sinking plume of CO2 can be formed by release of a slurry combining cold liquid and solid CO2 with a hydrate skin. This would effectively transfer ship released CO2 at shallow ocean depth to the deep ocean without the cost of a long pipe. In all of these schemes the fate of the CO2 is to be dissolved into the ocean, with increased depth of dissolution, and thus increased retention.

6.5.3 Production of a CO2 lake

Nakashiki (1997) investigated several different kinds of discharge pipes that could be used from a liquid CO2 tanker to create a CO2 lake on the sea floor. They concluded that a 'floating discharge pipe' might be the best option because it is simpler than the alternatives and less likely to be damaged by wind and waves in storm conditions.

Aya et al. (2003) proposed creating a slurry of liquid CO2 mixed with dry ice and releasing into the ocean at around 200 to 500 m depth. The dry ice is denser that the surrounding sea water and would cause the slurry to sink. An in situ experiment carried out off the coast of California found that a CO2 slurry and dry ice mass with initial diameter about 8.0 cm sank approximately 50 metres within two minutes before the dry ice melted (Aya et al., 2003). The initial size of CO2 slurry and dry ice is a critical factor making it possible to sink more than 3000 m to the sea floor. To meet performance criteria, the dry ice content would be controlled with a system consisting of a main power engine, a compressor, a condenser, and some pipe systems.

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