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290 IPCC Special Report on Carbon dioxide Capture and Storage 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 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). 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). 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. 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 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 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). 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.PDF Image | CARBON DIOXIDE CAPTURE AND STORAGE
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