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Figure 6.15 Estimated volume of pH perturbations at global scale for hypothetical examples in which injection of CO2 into the ocean interior provides 100% or 10% of the mitigation effort needed to move from a logistic emissions curve cumulatively releasing 18,000 GtCO2 (=5000 GtC) to emissions consistent with atmospheric CO2 stabilization at 550 ppm according to the WRE550 pathway (Wigley et al., 1996). The curves show the simulated fraction of ocean volume with a pH reduction greater than the amount shown on the horizontal axis. For the 10% case, in year 2100, injection rates are high and about1% of the ocean volume has significant pH reductions; in year 2300, injection rates are low, but previously injected CO2 has decreased ocean pH by about 0.1 unit below the value produced by a WRE550 atmospheric CO2 pathway in the absence of CO2 release directly to the ocean (Caldeira and Wickett, 2005).

dynamics of the ocean bottom boundary layer and its turbulence characteristics, mechanism of CO2 hydrate dissolution, and properties of CO2 in solution (Haugan and Alendal, 2005). The lifetime of a CO2 lake would be longest in relatively confined environments, such as might be found in some trenches or depressions with restricted flow (Ohgaki and Akano, 1992). Strong flows have been observed in trenches (Nakashiki,

1997). Nevertheless, simulation of CO2 storage in a deep trench (Kobayashi, 2003) indicates that the bottom topography can weaken vertical momentum and mass transfer, slowing the CO2 dissolution rate. In a quiescent environment, transport would be dominated by diffusion. Double-diffusion in the presence of strong stratification may produce long lake lifetimes. In contrast, the flow of sea water across the lake surface would increase mass transfer and dissolution. For example, CO2 lake lifetimes of >10,000 yr for a 50 m thick lake can be estimated from the dissolution rate of 0.44 cm yr-1 for a quiescent, purely diffusive system (Ohsumi, 1997). Fer and Haugan (2003) found that a mean horizontal velocity of 0.05 m s-1 would cause the CO2 lake to dissolve >25 times more rapidly (12 cm yr-1). Furthermore, they found that an ocean bottom storm with a horizontal velocity of 0.20 m s-1 could increase the dissolution rate to 170 cm yr-1.

6.2.2 CO2 storage by dissolution of carbonate minerals

Over thousands of years, increased sea water acidity resulting from CO2 addition will be largely neutralized by the slow natural dissolution of carbonate minerals in sea-floor sediments and on land. This neutralization allows the ocean to absorb more CO2 from the atmosphere with less of a change in ocean pH, carbonate ion concentration, and pCO2 (Archer et al., 1997,

1998). Various approaches have been proposed to accelerate carbonate neutralization, and thereby store CO2 in the oceans by promoting the dissolution of carbonate minerals2. These approaches (e.g., Kheshgi, 1995; Rau and Caldeira, 1999) do not entail initial separate CO2 capture and transport steps. However, no tests of these approaches have yet been performed at sea, so inferences about enhanced ocean CO2 storage, and effects on ocean pH are based on laboratory experiments (Morse and Mackenzie, 1990; Morse and Arvidson, 2002), calculations (Kheshgi, 1995), and models (Caldeira and Rau, 2000).

Carbonate neutralization approaches attempt to promote reaction (5) (in Box 6.1) in which limestone reacts with carbon dioxide and water to form calcium and bicarbonate ions in solution. Accounting for speciation of dissolved inorganic carbon in sea water (Kheshgi, 1995), for each mole of CaCO3 dissolved there would be 0.8 mole of additional CO2 stored in sea water in equilibrium with fixed CO2 partial pressure (i.e., about 2.8 tonnes of limestone per tonne CO2). Adding alkalinity

2 This approach is fundamentally different than the carbonate mineralization approach assessed in Chapter 7. In that approach CO2 is stored by reacting it with non-carbonate minerals to form carbonate minerals. In this approach, carbonate minerals are dissolved in the ocean, thereby increasing ocean alkalinity and increasing ocean storage of CO2. This approach could also make use of non-carbonate minerals, if their dissolution would increase ocean alkalinity.

to the ocean would increase ocean carbon storage, both in the near term and on millennial time scales (Kheshgi, 1995). The duration of increased ocean carbon storage would be limited by eventual CaCO3 sedimentation, or reduced CaCO3 sediment dissolution, which is modelled to occur through natural processes on the time scale of about 6,000 years (Archer et al., 1997, 1998).

Carbonate minerals have been proposed as the primary source of alkalinity for neutralization of CO2 acidity (Kheshgi 1995; Rau and Caldeira, 1999). There have been many experiments and observations related to the kinetics of carbonate mineral dissolution and precipitation, both in fresh water and in sea water (Morse and Mackenzie, 1990; Morse and Arvidson, 2002). Carbonate minerals and other alkaline compounds that dissolve readily in surface sea water (such as Na2CO3), however, have not been found in sufficient quantities to store carbon in the ocean on scales comparable to fossil CO2 emissions (Kheshgi, 1995). Carbonate minerals that are abundant do not dissolve in surface ocean waters. Surface ocean waters are typically oversaturated with respect to carbonate minerals (Broecker and Peng, 1982; Emerson and Archer, 1990; Archer, 1996), but carbonate minerals typically do not precipitate in sea water due to kinetic inhibitions (Morse and Mackenzie, 1990).

To circumvent the problem of oversaturated surface waters, Kheshgi (1995) considered promoting reaction (5) by calcining limestone to form CaO, which is readily soluble. If the energy for the calcining step was provided by a CO2-emission-free source, and the CO2 released from CaCO3 were captured and stored (e.g., in a geologic formation), then this process would store 1.8 mole CO2 per mole CaO introduced into the ocean. If the CO2 from the calcining step were not stored, then a net 0.8 mole CO2 would be stored per mole CaO. However, if coal without CO2 capture were used to provide the energy for calcination, and the CO2 produced in calcining was not captured, only 0.4 mole CO2 would be stored net per mole lime (CaO) to the ocean, assuming existing high-efficiency kilns (Kheshgi, 1995). This approach would increase the ocean sink of CO2, and does not need to be connected to a concentrated CO2 source or require transport to the deep sea. Such a process would, however, need to avoid rapid re-precipitation of CaCO3, a critical issue yet to be addressed.

Rau and Caldeira (1999) proposed extraction of CO2 from flue gas via reaction with crushed limestone and seawater. Exhaust gases from coal-fired power plants typically have 15,000 ppmv of CO2 - over 400 times that of ambient air. A carbonic acid solution formed by contacting sea water with flue gases would accelerate the dissolution of calcite, aragonite, dolomite, limestone, and other carbonate-containing minerals, especially if minerals were crushed to increase reactive surface area. The solution of, for example, Ca2+ and dissolved inorganic carbon (primarily in the form of HCO3) in sea water could then be released back into the ocean, where it would be diluted by additional seawater. Caldeira and Rau (2000) estimate that dilution of one part effluent from a carbonate neutralization reactor with 100 parts ambient sea water would result, after equilibration with the atmosphere, in a 10% increase in the calcite saturation state, which they contend would not induce precipitation. This approach does not rely on deep-sea release, avoiding the need for energy to separate, transport and inject CO2 into the deep ocean. The wastewater generated by this carbonate-neutralization approach has been conjectured to be relatively benign (Rau and Caldeira, 1999). For example, the addition of calcium bicarbonate, the primary constituent of the effluent, has been observed to promote coral growth (Marubini and Thake, 1999). This approach will not remove all the CO2 from a gas stream, because excess CO2 is required to produce a solution that is corrosive to carbonate minerals. If greater CO2 removal were required, this approach could be combined with other techniques of CO2 capture and storage.

Process wastewater could be engineered to contain different ratios of added carbon and calcium, and different ratios of flue gas CO2 to dissolved limestone (Caldeira and Wickett, 2005). Processes involving greater amounts of limestone dissolution per mole CO2 added lead to a greater CO2 fraction being retained. The effluent from a carbonate-dissolution reactor could have the same pH, pCO2, or [CO32 ] as ambient seawater, although processing costs may be reduced by allowing effluent composition to vary from these values (Caldeira and Rau, 2000). Elevation in Ca2+ and bicarbonate content from this approach is anticipated to be small relative to the already existing concentrations in sea water (Caldeira and Rau, 2000), but effects of the new physicochemical equilibria on physiological performance are unknown. Neutralization of carbon acidity by dissolution of carbonate minerals could reduce impacts on marine ecosystems associated with pH and CO32- decline (Section 6.7).

Carbonate neutralization approaches require large amounts of carbonate minerals. Sedimentary carbonates are abundant with estimates of 5 x 1017 tonnes (Berner et al., 1983), roughly 10,000 times greater than the mass of fossil-fuel carbon. Nevertheless, up to about 1.5 mole of carbonate mineral must be dissolved for each mole of anthropogenic CO2 permanently stored in the ocean (Caldeira and Rau, 2000); therefore, the mass of CaCO3 used would be up to 3.5 times the mass of CO2 stored. Worldwide, 3 Gt CaCO3 is mined annually (Kheshgi, 1995). Thus, large-scale deployment of carbonate neutralization approaches would require greatly expanded mining and transport of limestone and attendant environmental impacts. In addition, impurities in dissolved carbonate minerals may cause deleterious effects and have yet to be studied.

6.2.3 Other ocean storage approaches

Solid hydrate. Water reacts with concentrated CO2 to form a solid hydrate (CO26H2O) under typical ocean conditions at quite modest depths (L0ken and Austvik, 1993; Holdren and Baldwin, 2001). Rehder et al. (2004) showed that the hydrate dissolves rapidly into the relatively dilute ocean waters. The density of pure CO2 hydrate is greater than seawater, and this has led to efforts to create a sinking plume of released CO2 in the ocean water column. Pure CO2 hydrate is a hard crystalline solid and thus will not flow through a pipe, and so some form of

CO2 slurry is required for flow assurance (Tsouris et al., 2004).

Water-CaCO3-CO2 emulsion. Mineral carbonate could be used to physically emulsify and entrain CO2 injected in sea water (Swett et al. 2005); a 1:1 CO2:CaCO3 emulsion of CO2 in water could be stabilized by pulverized limestone (CaCO3). The emulsion plume would have a bulk density of 40% greater than that of seawater. Because the emulsion plume is heavier than seawater, the CaCO3coated CO2 slurries may sink all the way to the ocean floor. It has been suggested that the emulsion plume would have a pH that is at least 2 units higher than would a plume of liquid CO2. Carbonate minerals could be mined on land, and then crushed, or fine-grained lime mud could be extracted from the sea floor. These fine-grain carbonate particles could be suspended in sea water upstream from the CO2-rich plume emanating from the direct CO2 injection site. The suspended carbonate minerals could then be transported with the ambient sea water into the plume, where the minerals could dissolve, increasing ocean CO2 storage effectiveness and diminishing the pH impacts of direct injection.

Emplacement in carbonate sediments. Murray et al. (1997) have suggested emplacement of CO2 into carbonate sediments on the sea floor. Insofar as this CO2 remained isolated from the ocean, this could be categorized as a form of geological storage (Chapter 5).

Dry ice torpedoes. CO2 could be released from a ship as dry ice at the ocean surface (Steinberg,1985). One costly method is to produce solid CO2 blocks (Murray et al., 1996). With a density of 1.5 t m-3, these blocks would sink rapidly to the sea floor and could potentially penetrate into the sea floor sediment.

Direct flue-gas injection. Another proposal is to take a power plant flue gas, and pump it directly into the deep ocean without any separation of CO2 from the flue gas, however costs of compression are likely to render this approach infeasible.

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