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Figure 6.11 Simulated CO2 enriched sea water plumes (left panels; indicated by pH) and CO2 droplet plumes (right panels; indicated by kgCO2 m-3) created by injecting 1 cm and 12 cm liquid CO2 droplets (top and bottom panels, respectively) into the ocean from fixed nozzles (elapsed time is 30 min; injection rate is 1.0 kgCO2 s-1; ocean current speed is 5 cm s-1; Alendal and Drange, 2001). By varying droplet size, the plume can be made to sink (top panels) or rise (bottom panels).

releases of small droplets at slow rates produce smaller plumes than release of large droplets at rapid rates. Where CO2 is denser than seawater, larger droplet sizes would allow the CO2 to sink more deeply. CO2 injected at intermediate depths could increase the density of CO2-enriched sea water sufficiently to generate a sinking plume that would carry the CO2 into the deep ocean (Liro et al., 1992; Haugan and Drange, 1992). Apparent coriolis forces would operate on such a plume, turning it towards the right in the Northern Hemisphere and towards the left in the Southern Hemisphere (Alendal et al., 1994). The channelling effects of submarine canyons or other topographic features could help steer dense plumes to greater depth with minimal dilution (Adams et al., 1995).

6.2.1.5 Behaviour of injected CO2 in the far field The far field is defined as the region in which the concentration of added CO2 is low enough such that the resulting density increase does not significantly affect transport, and thus CO2 may be considered a passive tracer in the ocean. Typically, this would apply within a few kilometres of an injection point in midwater, but if CO2 is released at the sea floor and guided along topography, concentration may remain high and influence transport for several tens of kilometres. CO2 is transported by ocean currents and undergoes further mixing and dilution with other water masses (Alendal and Drange, 2001). Most of this mixing and transport occurs along surfaces of nearly constant density, because buoyancy forces inhibit vertical mixing in a stratified fluid. Over time, a release of CO2 becomes increasingly diluted but affects ever greater volumes of water.

The concept of ocean injection from a moving ship towing a trailing pipe was developed in order to minimize the local

Figure 6.12 Simulated plumes (Chen et al., 2005) created by injecting liquid CO2 into the ocean from a fixed pipe (left panel) and a moving ship (right panel) at a rate of 100 kg s-1 (roughly equal to the CO2 from a 500 MWe coal-fired power plant). Left panel: injection at 875 m depth (12 m from the sea floor) with an ocean current speed of 2.3 cm s-1. Right panel: injection at 1340 m depth from a ship moving at a speed of 3 m s-1. Note difference in pH scales; maximum pH perturbations are smaller in the moving ship simulation.

Figure 6.12 Simulated plumes (Chen et al., 2005) created by injecting liquid CO2 into the ocean from a fixed pipe (left panel) and a moving ship (right panel) at a rate of 100 kg s-1 (roughly equal to the CO2 from a 500 MWe coal-fired power plant). Left panel: injection at 875 m depth (12 m from the sea floor) with an ocean current speed of 2.3 cm s-1. Right panel: injection at 1340 m depth from a ship moving at a speed of 3 m s-1. Note difference in pH scales; maximum pH perturbations are smaller in the moving ship simulation.

environmental impacts by accelerating the dissolution and dispersion of injected liquid CO2 (Ozaki, 1997; Minamiura et al., 2004). A moving ship could be used to produce a sea water plume with relatively dilute initial CO2 concentrations (Figures 6.12 and 6.13). In the upper ocean where CO2is less dense than seawater, nozzles engineered to produce mm-scale droplets would generate CO2 plumes that would rise less than 100 m.

Ocean general circulation models have been used to predict changes in ocean chemistry resulting from the dispersion of

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