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Ocean sequestration of carbon dioxide (CO2)
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Ocean sequestration of carbon dioxide (CO2) D. G o l o m b and S. P e n n e l l, University of Massachusetts Lowell, USA Abstract: The deep ocean is a potential storage medium for anthropogenic carbon dioxide (CO2). However, there is strong opposition to ocean storage because of the potential acidification of large volumes of seawater around the discharge point. Furthermore, it is not clear whether international regulations would permit deep ocean discharge of anthropogenic CO2. This chapter reviews various proposals for oceanic CO2 injection designed to alleviate the acidification problem and to discharge the CO2 at sufficient depth so it will not rapidly re-emerge into the surface layer and back into the atmosphere. One proposal is described in greater detail, that of discharging the liquid CO2 in the form of an emulsion in seawater stabilized by pulverized limestone (CaCO3). Key words: ocean storage of carbon dioxide, deep ocean, acidification of seawater, legal constraints, sinking plumes, CO2 emulsion.
11.1
Introduction
This chapter attempts to give an overview of the scientific and technological principles of storing anthropogenic carbon dioxide (CO2) in the deep layers of the ocean. Because there is strong public and political opposition to deep ocean storage of CO2, as well as legal constraints enunciated in the London Convention on Ocean Dumping and the United Nations Convention of the Law of the Sea, deep ocean injection is currently not considered as a viable option for disposing of anthropogenic CO2. In the more distant future, if CO2 concentrations in the atmosphere are still on the rise, other storage and sequestration methods prove to be insufficient, and low- and non-carbon energy sources do not adequately supply the rising energy needs of a burgeoning world population, deep ocean storage may again be considered as an option to reduce atmospheric CO2 concentrations. Let us state at the outset that this chapter concerns storage of CO2 in the deep ocean. We are not addressing here storage proposals in the surface layer of the ocean, such as iron fertilization for enhancing the growth of phytoplankton, or dispersing directly gaseous CO2 or an aqueous solution thereof into the surface layer. We define the deep ocean at a depth of 500 m or deeper. The depth of 500 m corresponds to a hydrostatic pressure of about 5 MPa and a temperature of about 4 °C. At these depths, CO2 remains 304 © Woodhead Publishing Limited, 2010
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in its liquid state. If liquid CO2 were injected at a lesser depth, it would flash into vapor, and the resulting gas bubbles would immediately surge upward, eventually re-emerging into the atmosphere. Why deep ocean storage? The ocean is vast. It covers approximately 70 % of the earth’s surface, its average depth is about 3800 m, and the deeper layers of the ocean are greatly unsaturated in regard to CO2 and its dissolved forms, carbonic and bicarbonic acid, and their salts. A cubic km of deep ocean water contains about 3 ¥ 1010 g carbon (Wilson, 1992). Thus, the total carbon content of the deep ocean (> 500 m) is approximately 5.1 ¥ 1022 g. At present, anthropogenic emissions of carbon equal about 6.8 ¥ 1015 g y–1. If all the emissions were injected into the deep ocean, they would scarcely make a dent in the deep ocean’s carbon content. This is not to say that local effects around the discharge point would be negligible. Caulfield et al. (1997) estimated that the injection of liquid CO2 captured from several large coal-fired power plants would reduce the pH of seawater below 7 in a volume of tens of cubic km. Israelsson et al. (2009), reviewing the latest literature on biological effects of CO2 in the marine environment, concluded that the direct toxicity of dissolved CO2, expressed as pCO2, may be a more important metric than pH. Here pCO2 = [CO2]/K0 in units of pressure, where the square brackets denote concentration in mole/L and K0 is Henry’s law constant, which is a function of the partial pressure of CO2 at the gas/ water interface and temperature. However, Israelsson et al. concluded that discharge scenarios could be engineered to achieve perturbations in the marine environment that approach the natural variability of both ocean pH and pCO2.
11.2
History of carbon dioxide (CO2) deep ocean storage proposals
Marchetti (1977, 1979) proposed first the idea of storing CO2 in the deep ocean. A 1000 km pipeline would carry liquid CO2 from about 10 large power stations to a disposal site, e.g. at the outflow of the Mediterranean Sea into the Atlantic Ocean at the Straits of Gibraltar. There, the outflow carries over one million tonnes of water per second, gently sinking into the deep layers of the Atlantic. Similar, but smaller, thermohaline currents exist in the Red, Weddell and Norwegian Seas. Marchetti considered CO2 separation from the flue gas by a scrubbing process or burning coal in pure oxygen. Mustacchi et al. (1979) proposed the direct (unseparated) flue gas bubbling at a depth of about 240 m. They estimated that absorption of CO 2 in seawater will be complete before the bubbles ascend to the surface. A second option was the release of separated liquefied CO2 at a depth of 160 m. A third option was the release at a depth of only 10 m of a seawater solution of CO2. Because of the scarce solubility of CO2 in water, this would
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require pumping very large amounts of seawater, and very large contact towers. Albanese and Steinberg (1980) and Steinberg and Cheng (1985) proposed to inject the separated, liquid CO2 at 300 m depth. Baes et al. (1980) investigated the behavior of a jet of liquid CO2 released in the deep ocean, as well as dropping dry ice blocks from a barge. Saji et al. (1992) suggested releasing the CO2 in the form of a hydrate (clathrate). The hydrate consists of a molecule of CO2 imbedded in a cage of six water molecules, hence the name clathrate (caged). The solid hydrate is denser than seawater (approximately 1130 kg m–3), hence it would sink deeper from the injection point. Saji et al. suggested forming the hydrate on a platform constructed over the deep ocean. Liquid CO2 would be barged to the platform in a ship, and cold seawater would be pumped from great depths. The two liquids would be mixed under high pressure in order to form the hydrates. Hydrates form at a minimum temperature of 10 °C and minimum pressure of 4.32 MPa. The hydrates would be discharged from a pipe dangling from the platform at about 2000 m. Golomb et al. (1992) hypothesized that CO 2 hydrates will form spontaneously when liquid CO2 is released below 500 m, where the temperature is lower than 10 °C and the hydrostatic pressure is about 5 MPa. Golomb et al. estimated that, because of the higher density of the hydrates, the plume would sink all the way to the ocean bottom. Brewer et al. (2003) demonstrated that CO2 hydrates form spontaneously when liquid CO2 was injected from a submersible vehicle at depths greater than 3000 m. Tsouris et al. (2004) produced CO2 hydrates in a liquid CO2–seawater co-injector carried on a submersible. The hydrates were injected between 1100 and 1300 m. Depending on the liquid CO2–seawater flow rates in the co-injector and on the release depth, the formed CO2 hydrates were either positively or negatively buoyant. Ohsumi (1995) suggested that liquid CO2 be released below 3000 m. At those depths, the density of liquid CO2 is about 1050 kg m–3, which is denser than seawater (about 1028 kg m–3 at 3000 m), hence it would sink all the way to the ocean bottom and form a CO2 ‘lake’ there. Adams et al. (1995) proposed to release the liquid CO2 in a confinement vessel at 500 m or deeper. The confinement vessel is open at the top and bottom. Within the confinement vessel, a large fraction of the CO2 would dissolve in seawater, forming a dense, negatively buoyant solution. The vessel acts like an inverted chimney, allowing lighter seawater to be drawn in through the open top of the vessel, and allowing the denser solution to flow out through the open bottom of the vessel. Because of entrainment of ambient seawater, eventually the descending plume would density-equilibrate with the hydrostatically increasing density of seawater. Ozaki et al. (1999) proposed the release of liquid CO2 from a pipe towed by a moving ship between 1000 and 2500 m. This concept would promote the dispersion and dissolution of the CO2 over a wider volume of seawater.
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Israelsson et al. (2009) propose not only to release the CO2 from a moving ship, but that the injection proceed via a liquid CO2-seawater co-injector as described by Tsouris et al. (2004). In such a manner, a part of the CO2 forms a solid hydrate, further promoting negative buoyancy. It will be a naval engineering challenge to design and build a pressurized, refrigerated tanker with an attachable large diameter 1000–2500 m long pipe and co-injector for the discharge of liquid and hydrated CO2 in the open sea. Furthermore, the cost of such conveyance vessels may be prohibitive. Golomb and Angelopoulos (2001) proposed to release liquid CO2 mixed with a slurry of pulverized limestone (CaCO3) in seawater. The amount of limestone in the slurry would balance stoichiometrically the amount of liquid CO2 in order to achieve complete buffering of the carbonic acid formed after dissolution of the liquid CO2 in seawater. This would require about 2.3 tonnes of pulverized limestone per tonne of CO2, raising the cost of deep ocean storage of CO2 substantially in terms of raw materials, handling and transport. Golomb et al. (2007) proposed the release of a CO2/water emulsion stabilized by very fine limestone (CaCO3) particles. As described below, this method of release will require far less than stoichiometric quantities of CaCO3, and may greatly alleviate the seawater acidification problem.
11.3
Legal constraints of deep ocean storage of carbon dioxide (CO2)
There are two international regulations that may constrain the deep ocean storage of CO2: The London Convention on Ocean Dumping and the United Nations Convention of the Law of the Sea.
11.3.1 London convention on ocean dumping This Convention essentially prohibits all dumping activity in the oceans. A resolution in 1991 formally adopted the Precautionary Principle and outlawed the dumping of all radioactive and industrial waste. The resolution defined industrial waste as ‘generated by manufacturing or processing operations.’ Since then, ongoing discussions have not produced a consensus on whether CO2 should be classified as an industrial waste. The London Convention applies only to ships, aircraft and offshore platforms. Apparently, offshore disposal of CO2 by pipeline would not fall under the purview of the London Convention, but would be governed by national laws. In 1997, the Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) reported that unless two-thirds majority of Contracting Parties amended the Convention, CO2 dumping from ships, specifically dry ice and liquid CO2, violates the London Convention.
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11.3.2 United Nations Convention on the Law of the Sea (UNCLOS) Part XII, Article 194 states: ‘Minimize to the fullest extent the release of toxic, harmful or noxious substances, especially those which are persistent, from land-based sources, from or through the atmosphere, or by dumping.’ Article 210 states: ‘Dumping within the territorial sea and the exclusive zone or onto the continental shelf shall not be carried out without the express prior approval by the State, which has the right to permit, regulate and control such dumping after due consideration of the matter with other States which by reason of their geographical situation may be adversely affected thereby.’ Ironically, the UNCLOS tries to minimize the release of toxic substances onto the ocean from or through the atmosphere. It is estimated that about one-third of the anthropogenic emissions of CO2 are inevitably landing on, and absorbed by, the ocean surface waters (Houghton, 1997). It is evident that both the London and the United Nations conventions need to define (i) whether CO2 is an industrial waste and (ii) whether CO2 is toxic, harmful or noxious. Clearly, special expert committees will have to be convened to decide on these issues.
11.4
Sources of anthropogenic carbon dioxide (CO2) for ocean storage
In 2007, global emissions of CO2 amounted to 26.5 Gt y–1. Of this, 17.7 Gt y–1 came from fossil fuel combustion (IPCC, 2007). CO2 capture will most likely be economic only from central, large CO2 emission sources, such as coal-fired power plants. Oil-fired power plants constitute only about 2–3 % of fossil-fueled power plants and, with the escalating price of oil, their percentage of power plants will be even less. Gas-fired power plants amount to 8–10 % of all fossil-fueled power plants globally, but a modern gas-fired combined cycle power plant emits only about 50 % of CO2 per kWh of electricity generated compared to a coal-fueled plant. Therefore, CO2 capture and storage is considered only worthwhile from large coalfueled power plants. At present, about 25–30 % of global CO2 emissions come from coal-fueled power plants. Because overland transport in pipelines of liquid CO2 is about twice as expensive as transport by pipes laid on the sea bed (Golomb, 1993), most likely only coastal, coal-fueled power plants can be considered for deep ocean storage. For example, in the USA less than 10 % of all coal-fueled power plants are located on the seacoasts (US EIA, 2008). If that percentage is similar worldwide, deep ocean storage from coastal power plants may not amount to much more than 2.5–3 % of global CO2 emissions. Of course, insular countries, like Japan and the UK, may have a larger percentage of
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coastal power plants than inland countries. Also, in the future, new coalfueled power plants with CO2 capture may be specifically located near the coasts in order to avail themselves of deep ocean storage if it were permitted by international conventions.
11.5
Ocean structure
The structure of the ocean and the physical-chemical properties of CO2 are thoroughly intertwined in determining the storage capacity and the storage period of CO2. Of foremost importance are the temperature and density profiles of the ocean. For reasons of transporting large quantities, CO 2 will most likely be injected in its liquid rather than in its gaseous phase. Therefore, we must ensure that liquid CO2 will not immediately flash into positively buoyant gas bubbles. Because liquid CO2 is less dense than seawater, we must ensure that the injected liquid CO2 droplets dissolve in seawater before they ascend to a depth where they would flash into gas. A temperature profile for the North Pacific (45° latitude, 165° longitude) is depicted in Fig. 11.1. The well-mixed surface layer extends to about 100 m depth, where the temperature reaches about 5 °C. Below is the thermocline layer where the temperature gradually decreases to about 2 °C at 2000 m 0 200 400
Depth (m)
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11.1 Sample temperature profile for the North Pacific (45° north latitude, 165° east longitude, 17 Sept 1997) (data from World Ocean Database, 2005).
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depth. Because of the low compressibility of water, the density does not change very much with depth. At mid-latitude, at a depth of 500 m, the density is about 1026 kg m–3, increasing to 1027 kg m–3 at 1000 m, and 1028 kg m–3 at 3000 m (Millero, 2006). At 3000 m, the density of liquid CO2 is greater than seawater (see Fig. 11.2). This is because of the larger compressibility of liquid CO2 compared to water. Liquid CO2 released below 3000 m is negatively buoyant. Figure 11.2 depicts a sample pH profile in the North Pacific. The profile shows a pH of about 8 in the surface layer, declining to 7.4 at about 200 m, and gradually increasing to 7.6 at 1800 m. From an ocean storage point of view, it is important to note that at 500–1000 m depth the pH is approximately 7.5. Dissolving carbonic acid would lower the pH perhaps to the detriment of marine organisms that live in those depths. Caulfield et al. (1997) estimated the acidification of seawater when liquid CO2 is released at depth. The release of CO2 captured from one 500 MW coal-fired power plant (125 kg/s) would render the pH less than 7 in a volume of a few km3. This is the major cause of concern to marine biologists about deep ocean storage of CO2. However, as described below, some proposed injection methods may alleviate that concern.
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11.2 Sample pH profile for the North Pacific (48° north latitude, 165° east longitude, 17 Sept 1997) (data from World Ocean Database, 2005).
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H2O: 4 °C
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11.3 Density–pressure–temperature nomogram of liquid and supercritical CO2 and H2O (adapted from NIST, 2003).
11.6
Properties of carbon dioxide (CO2)
The pressure–temperature–density diagram of CO2 is depicted in Fig. 11.3. In the pressure–temperature interval of 5–10 MPa, 1–10 °C, corresponding to 500–1000 m depth, the density of liquid CO2 is in the range 930–950 kg m–3, far less than seawater, hence positively buoyant. Figure 11.3 also shows the density variation of water with pressure at 4 °C. At 30 MPa, corresponding to a depth of about 3000 m (somewhat outside the frame of Fig. 11.3), the density of water is about 1028 kg m–3. At that depth, liquid CO2 has a density of 1050 kg m–3, therefore liquid CO2 released at that depth becomes negatively buoyant, and would sink all the way to the ocean bottom, forming a ‘CO2 lake’ imbedded on the ocean bottom. This led to several proposals to release CO2 at depths greater than 3000 m, thereby ensuring long storage periods (e.g. Ohsumi et al., 1992). However, it may be very expensive to lay pipes offshore to 3000 m depth, or from a pipe suspended from a platform anchored to the ocean bottom, let alone the transportation cost of pressurized, refrigerated liquid CO2 in a tanker ship. Liquid CO2 is sparsely soluble in water. The solubility increases with pressure and decreases with temperature. In the interval 500–1000 m (5–10 MPa, 3–4 °C), it is about 5–6 % by weight (Stewart and Munjal, 1970). The low density and limited solubility constrain the deep ocean storage of CO2. Liquid CO2 has to be injected through a diffuser or atomizer in order that the bulk liquid is dispersed into tiny droplets. In such a fashion, the droplets
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have a chance to dissolve in seawater by mutual diffusion of one liquid into another before the positively buoyant droplets ascend to a depth where they would flash into vapor, which would occur at about 500 m depth (about 5 MPa). Another issue is the likely formation of CO2 hydrates (clathrates). The hydrates are solid, ice-like particles of density of about 1130 kg m –3 and composition CO2·6H2O (Ohmura and Mori, 1998). A phase diagram of CO2–H2O is shown in Fig. 11.4. It is seen that in the 0–10 °C, 5–10 MPa regime, corresponding to 500–1000 m depth, hydrates are stable. Brewer et al. (2003) have shown that releases of liquid CO2 at those depths do form hydrates. However, hydrates seem to form a film at the CO2–seawater interface. The composite density of the hydrate-coated droplets is less than that of seawater, hence they buoy upward from the release point. In fact, the hydrate film may be a hindrance to ocean storage because it may limit the dissolution rate of CO2 droplets.
11.7
Modeling of carbon dioxide (CO2) release
Herzog et al. (1991) discussed the injection of liquid CO2 at depths between 500 m and 2000 m. The injected CO2 breaks up into droplets whose radius depends on the size of the injection nozzle and the flow rate. At depths above 3000 m, liquid CO2 is less dense than ambient seawater, hence the droplets are positively buoyant. As the droplets rise, CO2 dissolves into seawater. The
2 O(I)–
CO2 (I)
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Hydrate
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I Hydrate–CO2(g) O( 2 (g) –H . O 2 C d O(s)– Hy ate–H 2
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(g) O (I)–CO 2 H 2O(I)–C 2
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Pressure (MPa)
Hydrate–CO2(I)
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0 10 Temperature (°C)
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30
11.4 Phase diagram of the water–CO2 system (adapted from NIST, 2003).
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authors calculated the minimum depth at which a droplet of given radius must be released so that it is completely dissolved by the time it reaches a depth of 500 m, at which point it would flash into vapor. The larger the droplet, the greater the release depth must be. These calculations were carried out using four different assumptions regarding the relation between droplet velocity and mass transfer rate between CO2 drop and seawater. For example, a 1 cm radius droplet when released at 700 m would dissolve completely before it reaches a depth of 500 m. Holder et al. (1995) modified the work of Herzog et al. (1991) to take into account the likely formation of hydrates. They assumed that hydrates form on the surface of a rising CO2 droplet at a rate of 0.04 cm/h. Because solid hydrates are denser than seawater, at some point the droplet becomes negatively buoyant and begins to sink. The authors calculated the minimum depth at which a drop of given radius must be released so it would sink by the time it reaches a depth of 500 m. For example, a 1 cm radius droplet must be released at a depth of about 1400 m, so it will sink before it ascends to 500 m. Wannamaker and Adams (2003) modeled the behavior of negatively buoyant solid hydrate particles. They used a double plume model in which the hydrate particles and entrained seawater form a sinking inner plume surrounded by a rising outer plume of detrained seawater. They calculated maximum plume depth, average intrusion depth and average intrusion changes in dissolved inorganic carbon and pH for given initial particle diameter and CO2 release rates. For example, their model showed that a release at a depth of 800 m with a CO2 release rate of 100 kg/s and particle diameter of 2 cm would result in a maximum plume depth of approximately 2500 m below the release point. Adams et al. (1995) modeled the release of liquid CO2 into a confinement vessel open both at the top and the bottom of the vessel, much like an inverted chimney. Because of the confinement, the dissolving droplets form a dense solution of carbonic acid (H2CO3). The dense acid sinks through the bottom of the confinement vessel. In such a fashion, the confinement vessel could be mounted just below the critical depth of 500 m, where liquid CO 2 would flash into vapor.
11.8
Injection of carbon dioxide, water and pulverized limestone (CO2/H2O/CaCO3) emulsion
In the quest to minimize the acidification of seawater and to ensure that the released CO2 sinks to greater depth from the injection point, the authors developed the concept of emulsifying liquid CO2 in water. As noted before, liquid CO2 is sparingly soluble in water. However, when liquid CO2 is mixed © Woodhead Publishing Limited, 2010
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with water in the presence of very fine hydrophilic particles, an emulsion is formed consisting of tiny CO2 droplets sheathed with the particles. The sheathed droplets are called globules (Pickering, 1907). The globules are dispersed in water (Golomb et al., 2006). Pulverized limestone (CaCO3) is an inexpensive and environmentally benign material that could be used for stabilizing the Pickering emulsion. In laboratory tests, it has been demonstrated that a stable emulsion can be created with a proportion of 33.3 % by volume liquid CO2, 66.7 % by volume artificial seawater (3.5 % NaCl solution) and 0.5 kg pulverized limestone per kg of liquid CO2. The limestone particles range from submicron to a few mm in diameter, with a mean diameter of 2 mm. The emulsion is created in a Kenics-type static mixer. A photo of the emulsion is shown in Fig. 11.5. The globules’ diameters range from 100–200 mm. The emulsion has a gross density of 1087 kg m–3, compared to a density of seawater at 500 m of 1026 kg m–3.
11.8.1 Emulsion release into open ocean For injection of the emulsion into the open ocean, we visualize a system depicted in Fig. 11.6. A floating platform is tethered to the ocean bottom. Liquid CO2 is barged to the platform and stored in a tank. Pulverized limestone is barged to the platform and slurried with seawater pumped from a depth of
11.5 Emulsion consisting of 33.3 % by volume liquid CO2, 66.7 % by volume artificial seawater (3.5 % NaCl solution) and 0.5 kg pulverized limestone per kg of liquid CO2 (adapted from Golomb et al., 2007).
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Release in the open ocean Material handling
CaCO3
CO2
Surface seawater
z = –200 m
Ambient seawater Static mixer z = –500 m
Stalled plume
z = –700 m
Rainout and dissolution z = bottom
11.6 Schematic of open ocean CO2 release (from Golomb et al., 2007).
about 200 m (below the photic zone). The liquid CO2 and limestone slurry are piped to a depth of about 500 m into a static mixer. Before entering the mixer, the limestone slurry is diluted by ambient seawater, which is drawn by aspiration. As noted above, the mix ensuing from the static mixer has a gross density of 1087 kg m–3. The only power requirements for creating the mix are for pumping seawater from a depth of 200 m into the slurry mixer and the mechanical mixing of the slurry. No additional power is required for the undersea static mixer; the hydrostatic pressures of the liquid CO2 and the pulverized limestone slurry provide adequate force for mixing the ingredients in the static mixer. Golomb et al. (2007) modeled the behavior of the released emulsion plume. The emulsion sinks because it is denser than ambient seawater. It is assumed that the descending plume entrains ambient seawater at a rate proportional to the plume velocity. Entrainment of seawater decreases the plume density. The plume descends to a level at which it becomes neutrally buoyant with the density stratified seawater. The authors calculated the depth to which the plume sinks before becoming neutrally buoyant, assuming the emulsion is injected at a depth of 500 m from a pipe of diameter 1 m. The plume length increases as the mass flow rate of injected emulsion increases and as the ambient density stratification and entrainment coefficient decrease. Because the injection would occur below 500 m depth, which is below the average picnocline at mid-latitudes, the density stratification is very weak. The density stratification is expressed by the buoyancy frequency N, defined as
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g d ra –2 s ras dz
where ras is the seawater density at the ocean surface, ra is ambient density, z is depth and g is the gravitational constant. Figure 11.7 shows the modeling results. For example, in a weakly density stratified ocean (N2 = 10–6 s–2) a plume containing the Co2 output of a 1000 MW coal-fired power plant (250 kg/s) released at 500 m would descend 800 m before it density equilibrates with ambient seawater.
11.8.2 Emulsion release along sloping continental shelf When a depth of about 500 m can be reached within 100–200 km from shore, a pipe system laid on the continental slope may be more economic than delivering the ingredients of the emulsion to a floating platform by barges. The system is depicted in Fig. 11.8. Liquid CO2 is stored onshore in a tank. A slurry of pulverized limestone in seawater is prepared onshore. The seawater for the slurry is pumped from about 200 m below the surface. liquid Co2 is pumped from the tank into a pipe, where it flows to a depth 800 E = 0.05; CO2 flux = 250 kg/s E = 0.05; CO2 flux = 125 kg/s
700
E = 0.05; CO2 flux = 62.5 kg/s E = 0.1; CO2 flux = 250 kg/s
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E = 0.1; CO2 flux = 125 kg/s E = 0.1; CO2 flux = 62.5 kg/s
500 400 300 200 100 0 10–6
10–5 N2 (s–2)
10–4
11.7 Dependence of open ocean globulsion plume length on density stratification. A CO2 flux of 250 kg/s corresponds to an output of a 1000 MWel coal-fired power plant. Note that the squares and diamonds overlap (adapted from Golomb et al., 2007).
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Material handling Ca CO 3
CO2
Surface seawater
z = –200 m
Ambient seawater Static mixer
z = –500 m
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z = –1000 m
Rainout z = bottom
11.8 Schematic of CO2 release on sloping seabed (from Golomb et al., 2007).
of about 500 m, then into the static mixer. The limestone slurry is diluted with ambient seawater by aspiration before it flows into the static mixer. The mixer lies on the bottom slope. The proportion of the ingredients is the same as in the open ocean release. In locations where the sea floor drops off rapidly, it may be more economic to release CO2 through a pipe from the shore rather than from a ship or offshore platform. Drange et al. (1993) modeled the flow of a plume consisting of CO2enriched seawater along a sloping seabed. The model incorporates changes in plume temperature, salinity, alkalinity and total inorganic carbon content. Effects of ambient density stratification, entrainment of ambient seawater and friction between the plume and seabed are included in the model. Plume behavior is found to be sensitive to both the ambient density stratification and the friction coefficient. Small friction coefficients lead to higher plume velocities and more rapid entrainment of ambient seawater, leading to more rapid dilution of the plume. On the other hand, larger friction coefficients lead to slower and longer plumes, allowing the CO2 to be carried to greater depths. The authors considered typical ambient conditions in the North Atlantic, the North Pacific, and the Norwegian Sea. Plumes are found to sink to greater depths under the conditions found in the Norwegian Sea than in the North Atlantic or North Pacific. Golomb et al. (2007) modeled the flow along a sloping seabed of an injected emulsion as described in the previous section. Chemical and thermodynamic changes were not considered; only the fluid dynamics of the flow were described. Effects of ambient density stratification, entrainment of ambient seawater and friction between the plume and seabed were included
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in the model. The authors calculated the depth to which the plume sinks before becoming neutrally buoyant, assuming the emulsion is injected at a depth of 500 m from a pipe of 1 m diameter. Plume penetration increases with increasing seabed slope, with increasing friction coefficient (because the plume moves more slowly, entraining less ambient seawater), and with decreasing ambient density stratification. Plume penetration on a sloping seabed is found to be greater than in an open ocean release because the entrainment rate is lower. Less plume surface is exposed to the ambient seawater, and friction with the seabed slows the plume, further decreasing the entrainment rate. Model results are shown in Fig. 11.9. For example, with a buoyancy frequency N2 = 10–6 s–2 and with typical entrainment and friction coefficients, a plume containing the CO2 output of a 1000 MW coal-fired power plant (250 kg/s) released at 500 m along a seabed with 10° slope would descend 2200 m before it density equilibrates with ambient seawater. After the plume comes to rest, the globules, together with excess pulverized limestone, will ‘rain-out’ from the plume on their way to the ocean bottom. Using Stokes’ law for the settling of small particles in a viscous medium, it is estimated that 100 mm radius globules sheathed with a 2 mm thick layer of limestone particles sink at a velocity of approximately 2 ¥ 10–3 m s–1, that is, about 200 m d–1 (Golomb et al., 2007). Eventually, the globules may disintegrate due to wave action and bottom friction. A part of the dissolved carbonic acid may be buffered by the CaCO3 particles.
11.8.3 Economics Raw limestone in chunks can be purchased from several quarries for $5–10/t FOB, and milled to the desired size on-site. The total cost of pulverizing the limestone, mixing it with seawater and co-injecting it with liquid CO2 in a static mixer is estimated at $13/t of limestone. Because only 0.5 t of pulverized limestone is required, the total cost is $6.5 per tonne of liquid CO2 (Golomb et al., 2007). Present reports estimate the cost of capturing and liquefying CO2 at a coal-fired power plant at around $50/t CO2 (IPCC, 2007). Thus, the storage of liquid CO2 in the form of an emulsion would add about 13 % to the capture and liquefaction cost of CO2. The additional cost may be justified on account of increasing the storage period of the released CO2, lowering the transport cost to deeper waters and most importantly, of not acidifying the seawater around the injection point.
11.9
Future trends
Given the public opposition and the legal constraints, it is not expected that in the near future further research and development, let alone actual deep ocean injection of CO2, will be practised.
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11.10 Conclusions The deep ocean would be a logical storage medium for anthropogenic CO2. The ocean occupies about 70 % of the planet’s area; its average depth is 3800 m; and the deep layers of the ocean are greatly undersaturated in regard to its carbonate absorption capacity. Furthermore, the ocean is not isolated from the atmosphere. It is estimated that roughly one third of the yearly anthropogenic emissions of CO2 are absorbed into the ocean. The pH of the surface layer of the ocean may have already decreased by 0.15 units (IPCC, 2007) due to the anthropogenic CO2 emissions over the last century. However, there is strong opposition by marine biologists and environmental groups to deep ocean storage of anthropogenic CO2. The major concern arises from the possibility of acidification of large volumes of seawater due to the formation of carbonic and bicarbonic acid. Furthermore, there are international regulations that prohibit ocean dumping of industrial waste, which may include anthropogenic CO2. Several proposals have been put forward in order to minimize the acidification problem. They include releasing the CO2 through diffusers or atomizers or from a moving ship. In such a fashion, the released CO2 would rapidly disperse over large volumes of seawater, so no concentrated carbonic acid would be created. Also, it has been proposed to release the CO2 in the form of hydrates that are heavier than seawater; hence they would sink to greater depth from the release point. The authors propose to release a CO2–water emulsion stabilized by pulverized limestone (CaCO3). The emulsion is denser than seawater; therefore, it would sink several hundred meters from the release point before the emulsion comes to a rest with the density stratified seawater. The preparation of the emulsion would incur an additional cost to ocean storage, but that cost may be amply compensated by the fact that the emulsion can be released at a relatively shallow depth of 500 m (minimizing transport costs), and that the formed carbonic acid would be partially neutralized by the pulverized limestone used to form the emulsion.
11.11 Sources of further information and advice Most papers on deep ocean storage of CO2 have been published in the Proceedings of the Greenhouse Gas Control Technologies (GHGT) biannual conferences. In GHGT-1, Amsterdam, Netherlands, 1992, 10 papers out of a total of 80 addressed ocean storage. The latest GHGT-9 conference, Washington, DC, 2008, devoted two out of 640 papers to ocean storage. Monographs on ocean storage are available, including Handa and Ohsumi (1995) and Omerod (1997).
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11.12 References Adams E E, Golomb D, Zhang X Y and Herzog H J (1995) ‘Confined release of CO2 into shallow seawater’, in Handa N and Ohsumi T (eds), Direct Ocean Disposal of Carbon Dioxide, Tokyo, Japan, Terrapub, 153–164. Albanese A S and Steinberg M (1980) Environmental Control Technology for Atmospheric Carbon Dioxide, Springfield, VA, National Technical Information Service, US Dept of Commerce. Baes Jr E F, Beall S E, Lee D W and Garland G (1980) ‘The collection, disposal, and storage of carbon dioxide’, in Bach W, Pankrath J and Williams J (eds), Interactions of Energy and Climate, Boston, MA, Reidel. Brewer P G, Peltzer E T, Rehder G and Dunk R (2003) ‘Advances in deep-ocean CO2 sequestration experiments’, in Gale J and Kaya Y (eds), Proceedings of the sixth International Conference on Greenhouse Gas Control Technologies: GHGT6, Oxford, UK Elsevier (Pergamon), 2, 1667–1670. Caulfield J A, Auerbach D I, Adams E E and Herzog H (1997) ‘Near field impacts of reduced pH from ocean CO2 disposal’, in Herzog H (ed.), Carbon Dioxide Removal: Proceedings of the Third International Conference on Carbon Dioxide Removal, Oxford, UK, Elsevier, 343–348. Drange H, Alendal G and Haugan P M (1993) ‘A bottom gravity current model for CO2enriched seawater’, Energy Convers Manage, 34, 1065–1072. Golomb D (1993) ‘Ocean disposal of CO2: feasibility, economics and effects’, Energy Convers Manage, 34, 967–976. Golomb D and Angelopoulos A A (2001) ‘Benign form of CO2 Sequestration in the Ocean’, in Williams D J, Durie R A, McMullan P, Paulson C A J and Smith A Y (eds), Proceedings of the Fifth International Conference on Greenhouse Gas Control Technologies: GHGT5, Collingwood, VIC, CSIRO Publishing, 463–467. Golomb D, Zemba S G, Dacey J W H and Michaels A F (1992) ‘The fate of CO2 sequestered in the deep ocean’, Energy Convers Manage, 33, 675–685. Golomb D, Barry E, Ryan D, Swett P and Duan H (2006) ‘Macroemulsions of liquid and supercritical CO2-in-water and water-in-liquid CO2 stabilized by fine particles’, Ind Eng Chem Res, 45, 2728–2733. Golomb D, Pennell S, Ryan D, Barry E and Swett P (2007) ‘Ocean sequestration of carbon dioxide: modeling the deep ocean release of a dense emulsion of liquid CO2-in-water stabilized by pulverized limestone particles’, Environ Sci Technol, 41, 4698–4704. Handa N and Ohsumi T (1995) Direct Ocean Disposal of Carbon Dioxide, Tokyo, Japan, Terrapub. Herzog H, Golomb D and Zemba S (1991) ‘Feasibility, modeling and economics of sequestering power plant CO2 emissions in the deep ocean’, Environ Prog, 10, 64–74. Holder G D, Cugini A V and Warzinski R P (1995) ‘Modeling clathrate hydrate formation during carbon dioxide injection into the ocean’, Environ Sci Technol, 29, 276–278. Houghton J (1997) Global Warming: The Complete Briefing, Cambridge, UK, Cambridge University Press. IPCC (2007) Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M and Miller HL (eds), Cambridge, UK and New York, Cambridge University Press. Israelsson P H, Chow A C and Adams E E (2009) ‘An updated assessment of the acute
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impacts of ocean carbon sequestration by direct injection’, in Gale J, Herzog H and Braitsch J (eds), Greenhouse Gas Control Technologies 9, Proceedings of the Ninth International Conference on Greenhouse Gas Control Technologies (GHGT9), Energy Procedia, 1, 4929–4936. Marchetti C (1977) ‘On geoengineering and the CO2 problem’, Climate Change, 1, 59–68. Marchetti C (1979) ‘Constructive solutions to the CO2 problem’, in Bach W, Pankrath J and Kellogg W (eds), Man’s Impact on Climate, New York, Elsevier, 299–311. Millero F J (2006) Chemical Oceanography, 3rd edn, Boca Raton, FL, CRC Press. Mustacchi C, Armenante P and Cena V (1979) ‘Carbon dioxide removal from power plant exhausts’, Environ Int, 2, 453–456. NIST (2003) National Institute for Standards and Technology Standard Reference Database Number 69, Washington DC. Ohmura R and Mori Y H (1998) ‘Critical conditions for CO2 hydrate films to rest on submarine CO2 pond surfaces: A mechanistic study’, Environ Sci Technol, 32, 1120–1127. Ohsumi T (1995) ‘Disposal options in view of geochemical cycle of carbon’, in Handa N and Ohsumi T (eds), Direct Ocean Disposal of Carbon Dioxide, Tokyo, Japan, Terrapub, 83–88. Ohsumi T, Nakashiki N, Shitashima K and Hirama K (1992) ‘Density change of water due to dissolution of carbon dioxide and near-field behavior of CO2 from a source on deep-sea floor’, Energy Convers Manage, 33, 685–690. Omerod B (1997) Ocean Storage of CO2: Practical and Experimental Approaches, Cheltenham, UK, IEA Greenhouse Gas R&D Programme. Ozaki M, Ohsumi T and Masuda A (1999) ‘Dilution of released CO2 in the mid ocean depth by moving ship’, in Eliasson B, Riemer P and Wokaun A (eds), Proceedings of the fourth International Conference on Greenhouse Gas Control Technologies: GHGT4, Oxford, UK, Elsevier, 275–280. Pickering S U (1907) ‘Emulsions’, J Chem Soc, 91, 2001–2020. Saji A, Yoshida H, Sakai M, Tanii T, Kamata T and Kitamura H (1992) ‘Fixation of carbon dioxide by clathrate-hydrate’, Energy Convers Manage, 33, 643–651. Steinberg M and Cheng H C (1985) A systems study for the removal, recovery, and disposal of carbon dioxide from fossil fuel power plants in the US, Report BNL-36428, Brookhaven National Laboratory, Upton, NY. Stewart P B and Munjal P (1970) ‘Solubility of carbon dioxide in pure water, synthetic sea water, and synthetic sea water concentrates at –5° to 25° and 10- to 45-atm pressure’, J Chem Eng Data, 15, 67–71. Tsouris C, Brewer P G, Peltzer E, Walz P, Riestenberg D, Liang L and West O R (2004) ‘Hydrate composite particles for ocean carbon sequestration: field verification’, Environ Sci Technol, 38, 2470–2475. US EIA (2008) Annual Energy Outlook 2008, Washington DC, US Energy Information Agency, available at http://www.eia.doe.gov/oiaf/aeo/pdf/0383(2008).pdf (accessed January 2010]. Wannamaker E J and Adams E E (2003) ‘Modeling descending carbon dioxide injections in the ocean’, in Gale J and Kaya Y (eds), Proceedings of the Sixth International Conference on Greenhouse Gas Control Technologies: GHGT6, Oxford, UK, Elsevier, Vol. 1, 753–758. Wilson T R S (1992) ‘The deep ocean disposal of carbon dioxide’, Energy Convers Manage, 33, 627–635.
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World Ocean Database (2005) Select and Search (National Oceanographic Data Service, U.S. National Oceanic and Atmospheric Administration) [Online] (Updated 14 Jan 2009), available at: http://www.nodc.noaa.gov/OC5/SELECT/dbsearch/dbsearch.html (accessed January 2010)).
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Ocean sequestration of carbon dioxide (CO2) D. G o l o m b and S. P e n n e l l, University of Massachusetts Lowell, USA Abstract: The deep ocean is a potential storage medium for anthropogenic carbon dioxide (CO2). However, there is strong opposition to ocean storage because of the potential acidification of large volumes of seawater around the discharge point. Furthermore, it is not clear whether international regulations would permit deep ocean discharge of anthropogenic CO2. This chapter reviews various proposals for oceanic CO2 injection designed to alleviate the acidification problem and to discharge the CO2 at sufficient depth so it will not rapidly re-emerge into the surface layer and back into the atmosphere. One proposal is described in greater detail, that of discharging the liquid CO2 in the form of an emulsion in seawater stabilized by pulverized limestone (CaCO3). Key words: ocean storage of carbon dioxide, deep ocean, acidification of seawater, legal constraints, sinking plumes, CO2 emulsion.
11.1
Introduction
This chapter attempts to give an overview of the scientific and technological principles of storing anthropogenic carbon dioxide (CO2) in the deep layers of the ocean. Because there is strong public and political opposition to deep ocean storage of CO2, as well as legal constraints enunciated in the London Convention on Ocean Dumping and the United Nations Convention of the Law of the Sea, deep ocean injection is currently not considered as a viable option for disposing of anthropogenic CO2. In the more distant future, if CO2 concentrations in the atmosphere are still on the rise, other storage and sequestration methods prove to be insufficient, and low- and non-carbon energy sources do not adequately supply the rising energy needs of a burgeoning world population, deep ocean storage may again be considered as an option to reduce atmospheric CO2 concentrations. Let us state at the outset that this chapter concerns storage of CO2 in the deep ocean. We are not addressing here storage proposals in the surface layer of the ocean, such as iron fertilization for enhancing the growth of phytoplankton, or dispersing directly gaseous CO2 or an aqueous solution thereof into the surface layer. We define the deep ocean at a depth of 500 m or deeper. The depth of 500 m corresponds to a hydrostatic pressure of about 5 MPa and a temperature of about 4 °C. At these depths, CO2 remains 304 © Woodhead Publishing Limited, 2010
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in its liquid state. If liquid CO2 were injected at a lesser depth, it would flash into vapor, and the resulting gas bubbles would immediately surge upward, eventually re-emerging into the atmosphere. Why deep ocean storage? The ocean is vast. It covers approximately 70 % of the earth’s surface, its average depth is about 3800 m, and the deeper layers of the ocean are greatly unsaturated in regard to CO2 and its dissolved forms, carbonic and bicarbonic acid, and their salts. A cubic km of deep ocean water contains about 3 ¥ 1010 g carbon (Wilson, 1992). Thus, the total carbon content of the deep ocean (> 500 m) is approximately 5.1 ¥ 1022 g. At present, anthropogenic emissions of carbon equal about 6.8 ¥ 1015 g y–1. If all the emissions were injected into the deep ocean, they would scarcely make a dent in the deep ocean’s carbon content. This is not to say that local effects around the discharge point would be negligible. Caulfield et al. (1997) estimated that the injection of liquid CO2 captured from several large coal-fired power plants would reduce the pH of seawater below 7 in a volume of tens of cubic km. Israelsson et al. (2009), reviewing the latest literature on biological effects of CO2 in the marine environment, concluded that the direct toxicity of dissolved CO2, expressed as pCO2, may be a more important metric than pH. Here pCO2 = [CO2]/K0 in units of pressure, where the square brackets denote concentration in mole/L and K0 is Henry’s law constant, which is a function of the partial pressure of CO2 at the gas/ water interface and temperature. However, Israelsson et al. concluded that discharge scenarios could be engineered to achieve perturbations in the marine environment that approach the natural variability of both ocean pH and pCO2.
11.2
History of carbon dioxide (CO2) deep ocean storage proposals
Marchetti (1977, 1979) proposed first the idea of storing CO2 in the deep ocean. A 1000 km pipeline would carry liquid CO2 from about 10 large power stations to a disposal site, e.g. at the outflow of the Mediterranean Sea into the Atlantic Ocean at the Straits of Gibraltar. There, the outflow carries over one million tonnes of water per second, gently sinking into the deep layers of the Atlantic. Similar, but smaller, thermohaline currents exist in the Red, Weddell and Norwegian Seas. Marchetti considered CO2 separation from the flue gas by a scrubbing process or burning coal in pure oxygen. Mustacchi et al. (1979) proposed the direct (unseparated) flue gas bubbling at a depth of about 240 m. They estimated that absorption of CO 2 in seawater will be complete before the bubbles ascend to the surface. A second option was the release of separated liquefied CO2 at a depth of 160 m. A third option was the release at a depth of only 10 m of a seawater solution of CO2. Because of the scarce solubility of CO2 in water, this would
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require pumping very large amounts of seawater, and very large contact towers. Albanese and Steinberg (1980) and Steinberg and Cheng (1985) proposed to inject the separated, liquid CO2 at 300 m depth. Baes et al. (1980) investigated the behavior of a jet of liquid CO2 released in the deep ocean, as well as dropping dry ice blocks from a barge. Saji et al. (1992) suggested releasing the CO2 in the form of a hydrate (clathrate). The hydrate consists of a molecule of CO2 imbedded in a cage of six water molecules, hence the name clathrate (caged). The solid hydrate is denser than seawater (approximately 1130 kg m–3), hence it would sink deeper from the injection point. Saji et al. suggested forming the hydrate on a platform constructed over the deep ocean. Liquid CO2 would be barged to the platform in a ship, and cold seawater would be pumped from great depths. The two liquids would be mixed under high pressure in order to form the hydrates. Hydrates form at a minimum temperature of 10 °C and minimum pressure of 4.32 MPa. The hydrates would be discharged from a pipe dangling from the platform at about 2000 m. Golomb et al. (1992) hypothesized that CO 2 hydrates will form spontaneously when liquid CO2 is released below 500 m, where the temperature is lower than 10 °C and the hydrostatic pressure is about 5 MPa. Golomb et al. estimated that, because of the higher density of the hydrates, the plume would sink all the way to the ocean bottom. Brewer et al. (2003) demonstrated that CO2 hydrates form spontaneously when liquid CO2 was injected from a submersible vehicle at depths greater than 3000 m. Tsouris et al. (2004) produced CO2 hydrates in a liquid CO2–seawater co-injector carried on a submersible. The hydrates were injected between 1100 and 1300 m. Depending on the liquid CO2–seawater flow rates in the co-injector and on the release depth, the formed CO2 hydrates were either positively or negatively buoyant. Ohsumi (1995) suggested that liquid CO2 be released below 3000 m. At those depths, the density of liquid CO2 is about 1050 kg m–3, which is denser than seawater (about 1028 kg m–3 at 3000 m), hence it would sink all the way to the ocean bottom and form a CO2 ‘lake’ there. Adams et al. (1995) proposed to release the liquid CO2 in a confinement vessel at 500 m or deeper. The confinement vessel is open at the top and bottom. Within the confinement vessel, a large fraction of the CO2 would dissolve in seawater, forming a dense, negatively buoyant solution. The vessel acts like an inverted chimney, allowing lighter seawater to be drawn in through the open top of the vessel, and allowing the denser solution to flow out through the open bottom of the vessel. Because of entrainment of ambient seawater, eventually the descending plume would density-equilibrate with the hydrostatically increasing density of seawater. Ozaki et al. (1999) proposed the release of liquid CO2 from a pipe towed by a moving ship between 1000 and 2500 m. This concept would promote the dispersion and dissolution of the CO2 over a wider volume of seawater.
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Israelsson et al. (2009) propose not only to release the CO2 from a moving ship, but that the injection proceed via a liquid CO2-seawater co-injector as described by Tsouris et al. (2004). In such a manner, a part of the CO2 forms a solid hydrate, further promoting negative buoyancy. It will be a naval engineering challenge to design and build a pressurized, refrigerated tanker with an attachable large diameter 1000–2500 m long pipe and co-injector for the discharge of liquid and hydrated CO2 in the open sea. Furthermore, the cost of such conveyance vessels may be prohibitive. Golomb and Angelopoulos (2001) proposed to release liquid CO2 mixed with a slurry of pulverized limestone (CaCO3) in seawater. The amount of limestone in the slurry would balance stoichiometrically the amount of liquid CO2 in order to achieve complete buffering of the carbonic acid formed after dissolution of the liquid CO2 in seawater. This would require about 2.3 tonnes of pulverized limestone per tonne of CO2, raising the cost of deep ocean storage of CO2 substantially in terms of raw materials, handling and transport. Golomb et al. (2007) proposed the release of a CO2/water emulsion stabilized by very fine limestone (CaCO3) particles. As described below, this method of release will require far less than stoichiometric quantities of CaCO3, and may greatly alleviate the seawater acidification problem.
11.3
Legal constraints of deep ocean storage of carbon dioxide (CO2)
There are two international regulations that may constrain the deep ocean storage of CO2: The London Convention on Ocean Dumping and the United Nations Convention of the Law of the Sea.
11.3.1 London convention on ocean dumping This Convention essentially prohibits all dumping activity in the oceans. A resolution in 1991 formally adopted the Precautionary Principle and outlawed the dumping of all radioactive and industrial waste. The resolution defined industrial waste as ‘generated by manufacturing or processing operations.’ Since then, ongoing discussions have not produced a consensus on whether CO2 should be classified as an industrial waste. The London Convention applies only to ships, aircraft and offshore platforms. Apparently, offshore disposal of CO2 by pipeline would not fall under the purview of the London Convention, but would be governed by national laws. In 1997, the Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) reported that unless two-thirds majority of Contracting Parties amended the Convention, CO2 dumping from ships, specifically dry ice and liquid CO2, violates the London Convention.
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11.3.2 United Nations Convention on the Law of the Sea (UNCLOS) Part XII, Article 194 states: ‘Minimize to the fullest extent the release of toxic, harmful or noxious substances, especially those which are persistent, from land-based sources, from or through the atmosphere, or by dumping.’ Article 210 states: ‘Dumping within the territorial sea and the exclusive zone or onto the continental shelf shall not be carried out without the express prior approval by the State, which has the right to permit, regulate and control such dumping after due consideration of the matter with other States which by reason of their geographical situation may be adversely affected thereby.’ Ironically, the UNCLOS tries to minimize the release of toxic substances onto the ocean from or through the atmosphere. It is estimated that about one-third of the anthropogenic emissions of CO2 are inevitably landing on, and absorbed by, the ocean surface waters (Houghton, 1997). It is evident that both the London and the United Nations conventions need to define (i) whether CO2 is an industrial waste and (ii) whether CO2 is toxic, harmful or noxious. Clearly, special expert committees will have to be convened to decide on these issues.
11.4
Sources of anthropogenic carbon dioxide (CO2) for ocean storage
In 2007, global emissions of CO2 amounted to 26.5 Gt y–1. Of this, 17.7 Gt y–1 came from fossil fuel combustion (IPCC, 2007). CO2 capture will most likely be economic only from central, large CO2 emission sources, such as coal-fired power plants. Oil-fired power plants constitute only about 2–3 % of fossil-fueled power plants and, with the escalating price of oil, their percentage of power plants will be even less. Gas-fired power plants amount to 8–10 % of all fossil-fueled power plants globally, but a modern gas-fired combined cycle power plant emits only about 50 % of CO2 per kWh of electricity generated compared to a coal-fueled plant. Therefore, CO2 capture and storage is considered only worthwhile from large coalfueled power plants. At present, about 25–30 % of global CO2 emissions come from coal-fueled power plants. Because overland transport in pipelines of liquid CO2 is about twice as expensive as transport by pipes laid on the sea bed (Golomb, 1993), most likely only coastal, coal-fueled power plants can be considered for deep ocean storage. For example, in the USA less than 10 % of all coal-fueled power plants are located on the seacoasts (US EIA, 2008). If that percentage is similar worldwide, deep ocean storage from coastal power plants may not amount to much more than 2.5–3 % of global CO2 emissions. Of course, insular countries, like Japan and the UK, may have a larger percentage of
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coastal power plants than inland countries. Also, in the future, new coalfueled power plants with CO2 capture may be specifically located near the coasts in order to avail themselves of deep ocean storage if it were permitted by international conventions.
11.5
Ocean structure
The structure of the ocean and the physical-chemical properties of CO2 are thoroughly intertwined in determining the storage capacity and the storage period of CO2. Of foremost importance are the temperature and density profiles of the ocean. For reasons of transporting large quantities, CO 2 will most likely be injected in its liquid rather than in its gaseous phase. Therefore, we must ensure that liquid CO2 will not immediately flash into positively buoyant gas bubbles. Because liquid CO2 is less dense than seawater, we must ensure that the injected liquid CO2 droplets dissolve in seawater before they ascend to a depth where they would flash into gas. A temperature profile for the North Pacific (45° latitude, 165° longitude) is depicted in Fig. 11.1. The well-mixed surface layer extends to about 100 m depth, where the temperature reaches about 5 °C. Below is the thermocline layer where the temperature gradually decreases to about 2 °C at 2000 m 0 200 400
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depth. Because of the low compressibility of water, the density does not change very much with depth. At mid-latitude, at a depth of 500 m, the density is about 1026 kg m–3, increasing to 1027 kg m–3 at 1000 m, and 1028 kg m–3 at 3000 m (Millero, 2006). At 3000 m, the density of liquid CO2 is greater than seawater (see Fig. 11.2). This is because of the larger compressibility of liquid CO2 compared to water. Liquid CO2 released below 3000 m is negatively buoyant. Figure 11.2 depicts a sample pH profile in the North Pacific. The profile shows a pH of about 8 in the surface layer, declining to 7.4 at about 200 m, and gradually increasing to 7.6 at 1800 m. From an ocean storage point of view, it is important to note that at 500–1000 m depth the pH is approximately 7.5. Dissolving carbonic acid would lower the pH perhaps to the detriment of marine organisms that live in those depths. Caulfield et al. (1997) estimated the acidification of seawater when liquid CO2 is released at depth. The release of CO2 captured from one 500 MW coal-fired power plant (125 kg/s) would render the pH less than 7 in a volume of a few km3. This is the major cause of concern to marine biologists about deep ocean storage of CO2. However, as described below, some proposed injection methods may alleviate that concern.
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11.6
Properties of carbon dioxide (CO2)
The pressure–temperature–density diagram of CO2 is depicted in Fig. 11.3. In the pressure–temperature interval of 5–10 MPa, 1–10 °C, corresponding to 500–1000 m depth, the density of liquid CO2 is in the range 930–950 kg m–3, far less than seawater, hence positively buoyant. Figure 11.3 also shows the density variation of water with pressure at 4 °C. At 30 MPa, corresponding to a depth of about 3000 m (somewhat outside the frame of Fig. 11.3), the density of water is about 1028 kg m–3. At that depth, liquid CO2 has a density of 1050 kg m–3, therefore liquid CO2 released at that depth becomes negatively buoyant, and would sink all the way to the ocean bottom, forming a ‘CO2 lake’ imbedded on the ocean bottom. This led to several proposals to release CO2 at depths greater than 3000 m, thereby ensuring long storage periods (e.g. Ohsumi et al., 1992). However, it may be very expensive to lay pipes offshore to 3000 m depth, or from a pipe suspended from a platform anchored to the ocean bottom, let alone the transportation cost of pressurized, refrigerated liquid CO2 in a tanker ship. Liquid CO2 is sparsely soluble in water. The solubility increases with pressure and decreases with temperature. In the interval 500–1000 m (5–10 MPa, 3–4 °C), it is about 5–6 % by weight (Stewart and Munjal, 1970). The low density and limited solubility constrain the deep ocean storage of CO2. Liquid CO2 has to be injected through a diffuser or atomizer in order that the bulk liquid is dispersed into tiny droplets. In such a fashion, the droplets
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have a chance to dissolve in seawater by mutual diffusion of one liquid into another before the positively buoyant droplets ascend to a depth where they would flash into vapor, which would occur at about 500 m depth (about 5 MPa). Another issue is the likely formation of CO2 hydrates (clathrates). The hydrates are solid, ice-like particles of density of about 1130 kg m –3 and composition CO2·6H2O (Ohmura and Mori, 1998). A phase diagram of CO2–H2O is shown in Fig. 11.4. It is seen that in the 0–10 °C, 5–10 MPa regime, corresponding to 500–1000 m depth, hydrates are stable. Brewer et al. (2003) have shown that releases of liquid CO2 at those depths do form hydrates. However, hydrates seem to form a film at the CO2–seawater interface. The composite density of the hydrate-coated droplets is less than that of seawater, hence they buoy upward from the release point. In fact, the hydrate film may be a hindrance to ocean storage because it may limit the dissolution rate of CO2 droplets.
11.7
Modeling of carbon dioxide (CO2) release
Herzog et al. (1991) discussed the injection of liquid CO2 at depths between 500 m and 2000 m. The injected CO2 breaks up into droplets whose radius depends on the size of the injection nozzle and the flow rate. At depths above 3000 m, liquid CO2 is less dense than ambient seawater, hence the droplets are positively buoyant. As the droplets rise, CO2 dissolves into seawater. The
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11.4 Phase diagram of the water–CO2 system (adapted from NIST, 2003).
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authors calculated the minimum depth at which a droplet of given radius must be released so that it is completely dissolved by the time it reaches a depth of 500 m, at which point it would flash into vapor. The larger the droplet, the greater the release depth must be. These calculations were carried out using four different assumptions regarding the relation between droplet velocity and mass transfer rate between CO2 drop and seawater. For example, a 1 cm radius droplet when released at 700 m would dissolve completely before it reaches a depth of 500 m. Holder et al. (1995) modified the work of Herzog et al. (1991) to take into account the likely formation of hydrates. They assumed that hydrates form on the surface of a rising CO2 droplet at a rate of 0.04 cm/h. Because solid hydrates are denser than seawater, at some point the droplet becomes negatively buoyant and begins to sink. The authors calculated the minimum depth at which a drop of given radius must be released so it would sink by the time it reaches a depth of 500 m. For example, a 1 cm radius droplet must be released at a depth of about 1400 m, so it will sink before it ascends to 500 m. Wannamaker and Adams (2003) modeled the behavior of negatively buoyant solid hydrate particles. They used a double plume model in which the hydrate particles and entrained seawater form a sinking inner plume surrounded by a rising outer plume of detrained seawater. They calculated maximum plume depth, average intrusion depth and average intrusion changes in dissolved inorganic carbon and pH for given initial particle diameter and CO2 release rates. For example, their model showed that a release at a depth of 800 m with a CO2 release rate of 100 kg/s and particle diameter of 2 cm would result in a maximum plume depth of approximately 2500 m below the release point. Adams et al. (1995) modeled the release of liquid CO2 into a confinement vessel open both at the top and the bottom of the vessel, much like an inverted chimney. Because of the confinement, the dissolving droplets form a dense solution of carbonic acid (H2CO3). The dense acid sinks through the bottom of the confinement vessel. In such a fashion, the confinement vessel could be mounted just below the critical depth of 500 m, where liquid CO 2 would flash into vapor.
11.8
Injection of carbon dioxide, water and pulverized limestone (CO2/H2O/CaCO3) emulsion
In the quest to minimize the acidification of seawater and to ensure that the released CO2 sinks to greater depth from the injection point, the authors developed the concept of emulsifying liquid CO2 in water. As noted before, liquid CO2 is sparingly soluble in water. However, when liquid CO2 is mixed © Woodhead Publishing Limited, 2010
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with water in the presence of very fine hydrophilic particles, an emulsion is formed consisting of tiny CO2 droplets sheathed with the particles. The sheathed droplets are called globules (Pickering, 1907). The globules are dispersed in water (Golomb et al., 2006). Pulverized limestone (CaCO3) is an inexpensive and environmentally benign material that could be used for stabilizing the Pickering emulsion. In laboratory tests, it has been demonstrated that a stable emulsion can be created with a proportion of 33.3 % by volume liquid CO2, 66.7 % by volume artificial seawater (3.5 % NaCl solution) and 0.5 kg pulverized limestone per kg of liquid CO2. The limestone particles range from submicron to a few mm in diameter, with a mean diameter of 2 mm. The emulsion is created in a Kenics-type static mixer. A photo of the emulsion is shown in Fig. 11.5. The globules’ diameters range from 100–200 mm. The emulsion has a gross density of 1087 kg m–3, compared to a density of seawater at 500 m of 1026 kg m–3.
11.8.1 Emulsion release into open ocean For injection of the emulsion into the open ocean, we visualize a system depicted in Fig. 11.6. A floating platform is tethered to the ocean bottom. Liquid CO2 is barged to the platform and stored in a tank. Pulverized limestone is barged to the platform and slurried with seawater pumped from a depth of
11.5 Emulsion consisting of 33.3 % by volume liquid CO2, 66.7 % by volume artificial seawater (3.5 % NaCl solution) and 0.5 kg pulverized limestone per kg of liquid CO2 (adapted from Golomb et al., 2007).
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Release in the open ocean Material handling
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11.6 Schematic of open ocean CO2 release (from Golomb et al., 2007).
about 200 m (below the photic zone). The liquid CO2 and limestone slurry are piped to a depth of about 500 m into a static mixer. Before entering the mixer, the limestone slurry is diluted by ambient seawater, which is drawn by aspiration. As noted above, the mix ensuing from the static mixer has a gross density of 1087 kg m–3. The only power requirements for creating the mix are for pumping seawater from a depth of 200 m into the slurry mixer and the mechanical mixing of the slurry. No additional power is required for the undersea static mixer; the hydrostatic pressures of the liquid CO2 and the pulverized limestone slurry provide adequate force for mixing the ingredients in the static mixer. Golomb et al. (2007) modeled the behavior of the released emulsion plume. The emulsion sinks because it is denser than ambient seawater. It is assumed that the descending plume entrains ambient seawater at a rate proportional to the plume velocity. Entrainment of seawater decreases the plume density. The plume descends to a level at which it becomes neutrally buoyant with the density stratified seawater. The authors calculated the depth to which the plume sinks before becoming neutrally buoyant, assuming the emulsion is injected at a depth of 500 m from a pipe of diameter 1 m. The plume length increases as the mass flow rate of injected emulsion increases and as the ambient density stratification and entrainment coefficient decrease. Because the injection would occur below 500 m depth, which is below the average picnocline at mid-latitudes, the density stratification is very weak. The density stratification is expressed by the buoyancy frequency N, defined as
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g d ra –2 s ras dz
where ras is the seawater density at the ocean surface, ra is ambient density, z is depth and g is the gravitational constant. Figure 11.7 shows the modeling results. For example, in a weakly density stratified ocean (N2 = 10–6 s–2) a plume containing the Co2 output of a 1000 MW coal-fired power plant (250 kg/s) released at 500 m would descend 800 m before it density equilibrates with ambient seawater.
11.8.2 Emulsion release along sloping continental shelf When a depth of about 500 m can be reached within 100–200 km from shore, a pipe system laid on the continental slope may be more economic than delivering the ingredients of the emulsion to a floating platform by barges. The system is depicted in Fig. 11.8. Liquid CO2 is stored onshore in a tank. A slurry of pulverized limestone in seawater is prepared onshore. The seawater for the slurry is pumped from about 200 m below the surface. liquid Co2 is pumped from the tank into a pipe, where it flows to a depth 800 E = 0.05; CO2 flux = 250 kg/s E = 0.05; CO2 flux = 125 kg/s
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11.7 Dependence of open ocean globulsion plume length on density stratification. A CO2 flux of 250 kg/s corresponds to an output of a 1000 MWel coal-fired power plant. Note that the squares and diamonds overlap (adapted from Golomb et al., 2007).
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Material handling Ca CO 3
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11.8 Schematic of CO2 release on sloping seabed (from Golomb et al., 2007).
of about 500 m, then into the static mixer. The limestone slurry is diluted with ambient seawater by aspiration before it flows into the static mixer. The mixer lies on the bottom slope. The proportion of the ingredients is the same as in the open ocean release. In locations where the sea floor drops off rapidly, it may be more economic to release CO2 through a pipe from the shore rather than from a ship or offshore platform. Drange et al. (1993) modeled the flow of a plume consisting of CO2enriched seawater along a sloping seabed. The model incorporates changes in plume temperature, salinity, alkalinity and total inorganic carbon content. Effects of ambient density stratification, entrainment of ambient seawater and friction between the plume and seabed are included in the model. Plume behavior is found to be sensitive to both the ambient density stratification and the friction coefficient. Small friction coefficients lead to higher plume velocities and more rapid entrainment of ambient seawater, leading to more rapid dilution of the plume. On the other hand, larger friction coefficients lead to slower and longer plumes, allowing the CO2 to be carried to greater depths. The authors considered typical ambient conditions in the North Atlantic, the North Pacific, and the Norwegian Sea. Plumes are found to sink to greater depths under the conditions found in the Norwegian Sea than in the North Atlantic or North Pacific. Golomb et al. (2007) modeled the flow along a sloping seabed of an injected emulsion as described in the previous section. Chemical and thermodynamic changes were not considered; only the fluid dynamics of the flow were described. Effects of ambient density stratification, entrainment of ambient seawater and friction between the plume and seabed were included
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in the model. The authors calculated the depth to which the plume sinks before becoming neutrally buoyant, assuming the emulsion is injected at a depth of 500 m from a pipe of 1 m diameter. Plume penetration increases with increasing seabed slope, with increasing friction coefficient (because the plume moves more slowly, entraining less ambient seawater), and with decreasing ambient density stratification. Plume penetration on a sloping seabed is found to be greater than in an open ocean release because the entrainment rate is lower. Less plume surface is exposed to the ambient seawater, and friction with the seabed slows the plume, further decreasing the entrainment rate. Model results are shown in Fig. 11.9. For example, with a buoyancy frequency N2 = 10–6 s–2 and with typical entrainment and friction coefficients, a plume containing the CO2 output of a 1000 MW coal-fired power plant (250 kg/s) released at 500 m along a seabed with 10° slope would descend 2200 m before it density equilibrates with ambient seawater. After the plume comes to rest, the globules, together with excess pulverized limestone, will ‘rain-out’ from the plume on their way to the ocean bottom. Using Stokes’ law for the settling of small particles in a viscous medium, it is estimated that 100 mm radius globules sheathed with a 2 mm thick layer of limestone particles sink at a velocity of approximately 2 ¥ 10–3 m s–1, that is, about 200 m d–1 (Golomb et al., 2007). Eventually, the globules may disintegrate due to wave action and bottom friction. A part of the dissolved carbonic acid may be buffered by the CaCO3 particles.
11.8.3 Economics Raw limestone in chunks can be purchased from several quarries for $5–10/t FOB, and milled to the desired size on-site. The total cost of pulverizing the limestone, mixing it with seawater and co-injecting it with liquid CO2 in a static mixer is estimated at $13/t of limestone. Because only 0.5 t of pulverized limestone is required, the total cost is $6.5 per tonne of liquid CO2 (Golomb et al., 2007). Present reports estimate the cost of capturing and liquefying CO2 at a coal-fired power plant at around $50/t CO2 (IPCC, 2007). Thus, the storage of liquid CO2 in the form of an emulsion would add about 13 % to the capture and liquefaction cost of CO2. The additional cost may be justified on account of increasing the storage period of the released CO2, lowering the transport cost to deeper waters and most importantly, of not acidifying the seawater around the injection point.
11.9
Future trends
Given the public opposition and the legal constraints, it is not expected that in the near future further research and development, let alone actual deep ocean injection of CO2, will be practised.
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11.10 Conclusions The deep ocean would be a logical storage medium for anthropogenic CO2. The ocean occupies about 70 % of the planet’s area; its average depth is 3800 m; and the deep layers of the ocean are greatly undersaturated in regard to its carbonate absorption capacity. Furthermore, the ocean is not isolated from the atmosphere. It is estimated that roughly one third of the yearly anthropogenic emissions of CO2 are absorbed into the ocean. The pH of the surface layer of the ocean may have already decreased by 0.15 units (IPCC, 2007) due to the anthropogenic CO2 emissions over the last century. However, there is strong opposition by marine biologists and environmental groups to deep ocean storage of anthropogenic CO2. The major concern arises from the possibility of acidification of large volumes of seawater due to the formation of carbonic and bicarbonic acid. Furthermore, there are international regulations that prohibit ocean dumping of industrial waste, which may include anthropogenic CO2. Several proposals have been put forward in order to minimize the acidification problem. They include releasing the CO2 through diffusers or atomizers or from a moving ship. In such a fashion, the released CO2 would rapidly disperse over large volumes of seawater, so no concentrated carbonic acid would be created. Also, it has been proposed to release the CO2 in the form of hydrates that are heavier than seawater; hence they would sink to greater depth from the release point. The authors propose to release a CO2–water emulsion stabilized by pulverized limestone (CaCO3). The emulsion is denser than seawater; therefore, it would sink several hundred meters from the release point before the emulsion comes to a rest with the density stratified seawater. The preparation of the emulsion would incur an additional cost to ocean storage, but that cost may be amply compensated by the fact that the emulsion can be released at a relatively shallow depth of 500 m (minimizing transport costs), and that the formed carbonic acid would be partially neutralized by the pulverized limestone used to form the emulsion.
11.11 Sources of further information and advice Most papers on deep ocean storage of CO2 have been published in the Proceedings of the Greenhouse Gas Control Technologies (GHGT) biannual conferences. In GHGT-1, Amsterdam, Netherlands, 1992, 10 papers out of a total of 80 addressed ocean storage. The latest GHGT-9 conference, Washington, DC, 2008, devoted two out of 640 papers to ocean storage. Monographs on ocean storage are available, including Handa and Ohsumi (1995) and Omerod (1997).
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11.12 References Adams E E, Golomb D, Zhang X Y and Herzog H J (1995) ‘Confined release of CO2 into shallow seawater’, in Handa N and Ohsumi T (eds), Direct Ocean Disposal of Carbon Dioxide, Tokyo, Japan, Terrapub, 153–164. Albanese A S and Steinberg M (1980) Environmental Control Technology for Atmospheric Carbon Dioxide, Springfield, VA, National Technical Information Service, US Dept of Commerce. Baes Jr E F, Beall S E, Lee D W and Garland G (1980) ‘The collection, disposal, and storage of carbon dioxide’, in Bach W, Pankrath J and Williams J (eds), Interactions of Energy and Climate, Boston, MA, Reidel. Brewer P G, Peltzer E T, Rehder G and Dunk R (2003) ‘Advances in deep-ocean CO2 sequestration experiments’, in Gale J and Kaya Y (eds), Proceedings of the sixth International Conference on Greenhouse Gas Control Technologies: GHGT6, Oxford, UK Elsevier (Pergamon), 2, 1667–1670. Caulfield J A, Auerbach D I, Adams E E and Herzog H (1997) ‘Near field impacts of reduced pH from ocean CO2 disposal’, in Herzog H (ed.), Carbon Dioxide Removal: Proceedings of the Third International Conference on Carbon Dioxide Removal, Oxford, UK, Elsevier, 343–348. Drange H, Alendal G and Haugan P M (1993) ‘A bottom gravity current model for CO2enriched seawater’, Energy Convers Manage, 34, 1065–1072. Golomb D (1993) ‘Ocean disposal of CO2: feasibility, economics and effects’, Energy Convers Manage, 34, 967–976. Golomb D and Angelopoulos A A (2001) ‘Benign form of CO2 Sequestration in the Ocean’, in Williams D J, Durie R A, McMullan P, Paulson C A J and Smith A Y (eds), Proceedings of the Fifth International Conference on Greenhouse Gas Control Technologies: GHGT5, Collingwood, VIC, CSIRO Publishing, 463–467. Golomb D, Zemba S G, Dacey J W H and Michaels A F (1992) ‘The fate of CO2 sequestered in the deep ocean’, Energy Convers Manage, 33, 675–685. Golomb D, Barry E, Ryan D, Swett P and Duan H (2006) ‘Macroemulsions of liquid and supercritical CO2-in-water and water-in-liquid CO2 stabilized by fine particles’, Ind Eng Chem Res, 45, 2728–2733. Golomb D, Pennell S, Ryan D, Barry E and Swett P (2007) ‘Ocean sequestration of carbon dioxide: modeling the deep ocean release of a dense emulsion of liquid CO2-in-water stabilized by pulverized limestone particles’, Environ Sci Technol, 41, 4698–4704. Handa N and Ohsumi T (1995) Direct Ocean Disposal of Carbon Dioxide, Tokyo, Japan, Terrapub. Herzog H, Golomb D and Zemba S (1991) ‘Feasibility, modeling and economics of sequestering power plant CO2 emissions in the deep ocean’, Environ Prog, 10, 64–74. Holder G D, Cugini A V and Warzinski R P (1995) ‘Modeling clathrate hydrate formation during carbon dioxide injection into the ocean’, Environ Sci Technol, 29, 276–278. Houghton J (1997) Global Warming: The Complete Briefing, Cambridge, UK, Cambridge University Press. IPCC (2007) Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M and Miller HL (eds), Cambridge, UK and New York, Cambridge University Press. Israelsson P H, Chow A C and Adams E E (2009) ‘An updated assessment of the acute
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impacts of ocean carbon sequestration by direct injection’, in Gale J, Herzog H and Braitsch J (eds), Greenhouse Gas Control Technologies 9, Proceedings of the Ninth International Conference on Greenhouse Gas Control Technologies (GHGT9), Energy Procedia, 1, 4929–4936. Marchetti C (1977) ‘On geoengineering and the CO2 problem’, Climate Change, 1, 59–68. Marchetti C (1979) ‘Constructive solutions to the CO2 problem’, in Bach W, Pankrath J and Kellogg W (eds), Man’s Impact on Climate, New York, Elsevier, 299–311. Millero F J (2006) Chemical Oceanography, 3rd edn, Boca Raton, FL, CRC Press. Mustacchi C, Armenante P and Cena V (1979) ‘Carbon dioxide removal from power plant exhausts’, Environ Int, 2, 453–456. NIST (2003) National Institute for Standards and Technology Standard Reference Database Number 69, Washington DC. Ohmura R and Mori Y H (1998) ‘Critical conditions for CO2 hydrate films to rest on submarine CO2 pond surfaces: A mechanistic study’, Environ Sci Technol, 32, 1120–1127. Ohsumi T (1995) ‘Disposal options in view of geochemical cycle of carbon’, in Handa N and Ohsumi T (eds), Direct Ocean Disposal of Carbon Dioxide, Tokyo, Japan, Terrapub, 83–88. Ohsumi T, Nakashiki N, Shitashima K and Hirama K (1992) ‘Density change of water due to dissolution of carbon dioxide and near-field behavior of CO2 from a source on deep-sea floor’, Energy Convers Manage, 33, 685–690. Omerod B (1997) Ocean Storage of CO2: Practical and Experimental Approaches, Cheltenham, UK, IEA Greenhouse Gas R&D Programme. Ozaki M, Ohsumi T and Masuda A (1999) ‘Dilution of released CO2 in the mid ocean depth by moving ship’, in Eliasson B, Riemer P and Wokaun A (eds), Proceedings of the fourth International Conference on Greenhouse Gas Control Technologies: GHGT4, Oxford, UK, Elsevier, 275–280. Pickering S U (1907) ‘Emulsions’, J Chem Soc, 91, 2001–2020. Saji A, Yoshida H, Sakai M, Tanii T, Kamata T and Kitamura H (1992) ‘Fixation of carbon dioxide by clathrate-hydrate’, Energy Convers Manage, 33, 643–651. Steinberg M and Cheng H C (1985) A systems study for the removal, recovery, and disposal of carbon dioxide from fossil fuel power plants in the US, Report BNL-36428, Brookhaven National Laboratory, Upton, NY. Stewart P B and Munjal P (1970) ‘Solubility of carbon dioxide in pure water, synthetic sea water, and synthetic sea water concentrates at –5° to 25° and 10- to 45-atm pressure’, J Chem Eng Data, 15, 67–71. Tsouris C, Brewer P G, Peltzer E, Walz P, Riestenberg D, Liang L and West O R (2004) ‘Hydrate composite particles for ocean carbon sequestration: field verification’, Environ Sci Technol, 38, 2470–2475. US EIA (2008) Annual Energy Outlook 2008, Washington DC, US Energy Information Agency, available at http://www.eia.doe.gov/oiaf/aeo/pdf/0383(2008).pdf (accessed January 2010]. Wannamaker E J and Adams E E (2003) ‘Modeling descending carbon dioxide injections in the ocean’, in Gale J and Kaya Y (eds), Proceedings of the Sixth International Conference on Greenhouse Gas Control Technologies: GHGT6, Oxford, UK, Elsevier, Vol. 1, 753–758. Wilson T R S (1992) ‘The deep ocean disposal of carbon dioxide’, Energy Convers Manage, 33, 627–635.
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World Ocean Database (2005) Select and Search (National Oceanographic Data Service, U.S. National Oceanic and Atmospheric Administration) [Online] (Updated 14 Jan 2009), available at: http://www.nodc.noaa.gov/OC5/SELECT/dbsearch/dbsearch.html (accessed January 2010)).
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