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CCOS using Neutralisation

Ocean Technology Group

University of Sydney

NSW 2006 Australia

CCOS using Neutralisation 

Interim Report 02/06 

July 2006

 What is CCOS using Neutralization?

Carbon Capture and Ocean Storage is the process of dissolving carbon dioxide in flue gases from power stations in sea water and neutralising the carbonic acid formed by use of limestone. It uses the reaction

 CO2 + CaCO3 + H2O= Ca(HCO3)2 

Two issues that have been examined are reported on here.

 

Can the Saturation of Calcium Carbonate in Sea Water be Increased?

The saturation state of calcium carbonate in sea water, W, can be defined as the mole concentration of [Ca++] ions multiplied by the mole concentration of carbonate ions [CO3=] divided by their product at saturation.

 

The surface water of the ocean is already supersaturated with calcium carbonate.

 

Experiments were performed by the Ocean Technology Group that increased the concentration of dissolved material in sea water.  These experiments show the saturation state of sea water can be raised by a factor of at least four without immediate precipitation of calcium carbonate.  When observed 24 hours later there was still no precipitation. The test sea water was sampled from sea water collected at (34degrees 5 minutes S, 151degrees 16 minutes E) known as the Port Hacking Reference Station. For climatology of water properties at this location see Humphries (1963)

 

Desalination plants discuused in  Einav et al (2002) confirm this in the field.

 

Dissolving Calcium Carbonate with CO2

Procedure

 

500 ml of sea water at room temperature was placed in a 9 cm dia beaker and the pH measured. The sample was stirred for 24 hours before again measuring the pH to ensure the dissolved CO2 was in equilibrium with the atmosphere. The present concentration of CO2 in the air is 375-385 ppmv [http://www.cmdl.noaa.gov/aggi/]. Using the computer program written by E Lewis of Brookhaven National Laboratory, USA, kindly provided by Frank Millero, the Dissolved Inorganic Carbon, DIC, in the sample was calculated at 1736 micro moles per litre. Since the molecular weight of CO2 is 44 this is 76 mg/L.

 

Figure 1 Change in pH at he time after addition of calcium carbonate.

 

Carbon dioxide gas was bubbled thru the sample until the pH was stable at 5.5. Since the total alkalinity did not change with the addition of the carbon dioxide, the dissolved inorganic carbon was now calculated to have a concentration of 6822 micromole/L. Thus 112 mg of carbon dioxide was dissolved in the 500ml of sea water. Note this situation is not in equilibrium with the atmosphere.

 

0.83 grams of granular calcium carbonate was added to the sea water. The surface area, assuming spherical grain and the specific gravity of CaCO3 is 2.83, was calculated. Figure 1 shows the response of the pH as the calcium carbonate neutralised the carbonic acid. As the pH approaches 7 the reaction slows down. Note there is degassing as well as neutralisation occurring in this experiment but that degassing is responsible for only a small change in pH over the duration of the experiments in Fig 1.

 

The experiment was repeated with twice the surface area of CaCO3. The increased area made the rate of change of pH more rapid as expected.

 

Conclusion

Two issues to do with CCOS using neutralisation have been resolved. The supersaturation of sea water can be increased without precipitation of calcium carbonate. Carbon dioxide that has been dissolved in sea water can be neutralised by calcium carbonate at a rate dependent on the surface area of CaCO3. 

 

References

Einav, R, K Harussi and D Perry (2002) The footprint of desalination process on the environment, Desalination, 152, 141-154.EO&S (2005) Anthropogenic Ocean Sequestration by Changing the Alkalinity of Ocean Surface Water (Alkalinity Shift). Earth Ocean & Space Pty Ltd, Sydney.(www.earthoceanspace.com)Jones, Ian S F and Chien Hsing Lu (2003) Engineering Carbon Sequestration in the Ocean Second Annual Conference on Carbon Sequestration, Washington, May, 2003 (http:/hdl.handle.net/2123/986).

Humphries (1963) Australian Journal of Marine and Freshwater Research, 14, 24-36.

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