Carbon Capture and Ocean Storage

Earth Ocean & Space Pty Ltd

eos@atp.com.au

28 July 2006

 

SCOPE: This note addresses issues concerning carbon storage in the ocean, relevant to the Clean Development Mechanism of the Kyoto Protocol.  These issues are leakage, permanence and the choice of project boundaries for a new monitoring methodology for an alkalinity shift for permanent sequestration of carbon in the ocean.

 

Key words: Carbon Capture and Ocean Storage

 

Background

 

The projected rise in the concentration of carbon dioxide in the atmosphere is leading to acidification of the ocean (Doney, 2006) and threatens to bring about rapid climate change.  The increasing concentration of carbon in the ocean in the form of carbonic acid is threatening the ocean productivity on which mankind relies for a significant amount of animal protein (mostly fish).  Carbon Capture and Storage, CCS, is one method of reducing the level of carbon dioxide in the atmosphere and has been reviewed by the IPCC (2005).  CDM[1] was introduced under the Kyoto Protocol as a method of reducing the cost of addressing greenhouse gas while transferring technology to developing countries and contributing to sustainable development.  The lower the cost of mitigation, the more acceptable it will be to society to incur the expenditure of managing anthropogenic climate change.

 

Since the ocean is 70% of the surface of the globe it behoves us to investigate the role it might play in managing the carbon dioxide waste from fossil fuel use.  The ocean already stores order (2milliMolar = 14 g/m3 by a volume of 1.4 x 1018m3) 20,000 Gtonnes of volatile inorganic carbon.

 

The CDM process allows for the introduction of small scale projects and these provide an opportunity to gain experience with innovation at low cost and with diminished risk to the environment. 

 

Carbon Capture and Ocean Storage

 

It is possible to capture carbon from rich (concentrated) sources of carbon dioxide and after NEUTRALISATION, store it in the ocean permanently.  A number of issues of concern with this technology under the Kyoto Convention are discussed below.

 

This new method for carbon capture and ocean storage involves dissolving concentrated CO2 (from flue gas) in sea water and then neutralising the carbonic acid formed with calcium carbonate.  The process stores carbon in the form of bicarbonate.

 

 

 

 

Chemistry

 

In equilibrium, the atmospheric partial pressure of carbon dioxide and of the surface ocean below are equal. An increase of partial pressure in the surface water away from equilibrium, will lead to a flux of carbon dioxide out of the ocean.  The partial pressure of the ocean surface water depends on many factors such as temperature, total alkalinity[2], total carbon and pH.  The aim of alkalinity adjustment of surface waters is to increase the total carbon in the sea water without increasing the partial pressure of carbon dioxide in the water.  In simple terms more inorganic carbon cannot be stored in the ocean without increasing the alkalinity, other things staying equal.

 

Total alkalinity of sea water is a measure of the carbonate and bicarbonate ions with a typical concentration of 2200meq/kg.  Increasing the total alkalinity raises the total inorganic carbon in the water while maintaining the same partial pressure of carbon dioxide.  The most straightforward way of increasing total alkalinity is to dissolve calcium carbonate or calcium oxide to form bicarbonate ions.

 

Kheshgi (1995) and others have suggested changing the total alkalinity of sea water to sequester carbon.  Later Caldeira and Rau (2000) suggested CO2 rich gas streams (such as flue gases) could be passed through a porous bed of limestone which was kept wet by water sprays.  Rau and Caldeira (1999) said that the sequestration costs of one tonne of carbon dioxide might be tens of US dollars per tonne of CO2 avoided.  Golomb et al. (2004) suggested mixing the calcium carbonate with liquid CO2 before injecting the mixture into the ocean.

 

Much earlier Broecker and Takahashi (1977) pointed out that, in a high carbon dioxide world, the calcium carbonate stored in the marine sediments will on a very long time scale change the ocean alkalinity.  Dissolution of the marine calcium carbonate sediments, CaCO3, will decrease partial pressure of CO2 in the water and lead to additional carbon leaving the atmosphere and entering the water.  The current discussion focuses on how to mimic this natural process in order to rapidly store carbon dioxide in the ocean.

 

The waters of the surface ocean are supersaturated with respect to calcium carbonate but precipitation is impeded apparently by naturally occurring ions according to Morse and Mackenzie (1990).  The pH of the surface water is between 8 and 8.5 due to the present level of carbon dioxide in the atmosphere.  In the future a rising level of carbon dioxide in the atmosphere is to be expected (IPCC, 2006) and this will lower the pH of the ocean surface water.  Such a change has significant implications for marine life.

 

There is a discussion of CO2 storage by dissolution of carbonate minerals in the IPCC Special Report on Carbon Dioxide Capture and Storage (2006). For further information on ocean carbon chemistry see Jones and Lu (2003).

 

Carbon Capture and Ocean Storage, CCOS

 

The variation of CCOS with neutralisation discussed in this note involves converting carbon dioxide (molecular weight = 44) to bicarbonate by the reaction

 

CO2 + CaCO3 + H2O= Ca(HCO3)2

 

using sea water.  The rate of dissolution of calcium carbonate depends on the concentration of CO2 that is dissolved in the water.  The dissolution comes about because of the temporary decrease in the pH as CO2 from the flue gases is dissolved in the sea water.  As the calcium carbonate reaction proceeds the pH rises.

 

When the partial pressure of carbon dioxide in sea water is in equilibrium with the atmosphere, increasing the alkalinity by one micro equivalent per kg of water, increases the sequestered carbon by approximately one micro mole per kg.  Thus dissolving 2000 micro mole of calcium carbonate (molecular weight = 100) in a cubic meter of sea water (103L) takes up 88 g of CO2.  In a flowing water system of 1 m3/s this would represent an uptake of 7.6 tonnes/d or 2770 t/y of CO2.  This process would consume 100/44 = 2.3 tonnes of limestone for each tonne of carbon dioxide sequestered in the sea water.  Thus limiting the increase of alkalinity to be twice that of the initial sea water, each cubic meter of sea water per second flowing, is likely to cause a carbon dioxide emission reduction of greater than 2000 t per year.

 

Implementation

 

Figure 1 shows one concept for implementation of the CCOS technology.  A fraction of the flue gas from a coastal power station, with a concentration of carbon dioxide of order 15%, is bubbled through the flowing sea water used for cooling with the aid of a booster fan to overcome the water head. The dissolved carbon dioxide from the bleed flue gas will lower the pH of the sea water flowing past the injection points. The cooling water then flows past calcium carbonate broken into small pieces and retained

Fig 1 Conceptual diagram showing the flue gas bled from the chimney of a power station being bubbled through the cooling water. The calcium carbonate is about to be lowered into the cooling water

 

in a number of porous baskets. Additional energy is required to pump the cooling water against the additional head loss of the baskets.  . The baskets are refilled from trucks bringing the waste carbonate from a nearby quarry.  After the water flows through the baskets the pH will rise as the calcium carbonate goes into solution.  The carbonic acid formed in the first step is neutralised by the limestone.  Chemical additives may be used to impede calcium carbonate precipitation. The cooling water then flows into the sea where it is diluted by the natural diffusion in the coastal ocean.

 

Not all of the carbon dioxide in the flue gas bubbled through the cooling water can be expected to go into solution and some will escape to the atmosphere.  Before the injection of flue gas we can expect a typical alkalinity of 2200 meq/kg.  The cooling water after carbon storage will have, in this example, an alkalinity of order 4,400meq/kg.  Not all the carbon dioxide in solution at this level of alkalinity will be in equilibrium with the atmosphere and so there will be degassing adjustment of the cooling water. By measuring the change of alkalinity by the method in EO&S (2005), only the permanently stored carbon dioxide is counted.  This is the carbon dioxide neutralised by the limestone.

 

An alternative approach is to weigh the calcium carbonate baskets at regular intervals and assume all reduction in weight is due to the dissolving of the limestone. This is considered inferior to the previous method as it allows small chips of limestone to escape the basket and these chips then have an uncertain impact on the determination of stored carbon.

 

Depending on the engineering of the flue gas injection, the surface area of the limestone and the flow speed of the cooling water, a design goal could be scrubbing and storage of 50% of the carbon dioxide in the diverted flue gas. Thus a diversion of 15,000 t CO2 per year would sequester 7,500 t CO2 per year and require an already installed cooling water flow of about 3.75m3/s.

 

Impacts on the Ocean

 

The environmental impacts of discharging the high alkalinity water can be assessed from an examination of the discharge water from desalination plants.  Here the salinity and alkalinity are both about double over that of the adjacent ocean.  The reason for limiting the amount of carbonate dissolved per litre of water is to stay within this experience limit.  Flow rates of discharge water from desalination plants of capacity 200ML per day[3] are about the same as a 200MW power station which is typically of order 1m3/s for each 100MW of generating capacity. Thus if the alkalinity adjustment leads to a discharge with a concentration less than 4,400meq/kg, the desalination experience should be directly relevant.  Of course desalination discharge brine has a higher concentration of sodium chloride than a CCOS discharge. Experience, cited by Einav et al. (2002) teaches us that impacts can be expected to be insignificant.  In desalination plants the exit water is typically diluted to within 3% of the initial concentration within a kilometre of the release point.  Outside a semicircle of 20 km radius centered on the cooling water release point, the excess alkalinity and dissolved inorganic carbon in the coastal waters are expected to be at background levels.

 

Since NOx and SOx are soluble, the flue gas that escapes from the cooling water will have reduced concentrations of these gases. The abundance of calcium carbon will neutralise the acids that are potentially formed by the scrubbing of these gases.

 

CCOS mitigates the change in pH in the surface waters which comes from the invasion of carbon dioxide without neutralisation.  The falling pH due to non neutralised carbon dioxide invasion endangers corals and shell fish.  Marubini and Thake (1999) showed healthier responses in marine organisms as pH was restored to pre-industrial values.  Thus CCOS contributes to the sustainability of the ocean protein supply.

 

Project Boundaries

 

The Clean Development Mechanism differentiates between areas within the project boundary where changes in greenhouse gas emissions are monitored and emissions outside the boundary where the emission is termed leakage.  The CCOS methodology is most likely to be applied to coastal power stations, with some of the CO2 from its flue stacks pumped through cooling water and channelled into the sea.  In such a situation, the project boundary can be the physical boundary that encloses the cooling water pumps, a booster fan for the fraction of flue gas diverted and the cooling water channel, with an extension into the adjacent sea.  The choice of the sea boundary deserves some discussion.  One approach is to make the boundary enclose the whole ocean.  The carbon dioxide crossing the sea surface as a result of CCOS activity is considered as a project emission.

 

Another approach is to place the sea boundary at a 20km radius from the cooling water discharge point as shown in Fig 2.  Now the carbon stored in the cooling water is expected to be swept across the boundary by the ocean currents.  While it may cross the boundary it does not leak into the atmosphere (see next section). The flue gases bled from the stack cross (enter) the boundary and the non soluble gases in the bleed gas leaves the water surface of the channel and crosses the boundary. Energy (electricity) is required to cross the boundary to move the fluids within the boundary. Outside the boundary is the calcium carbonate quarry. Also the manufacture of the small amount of equipment needed is considered external.

 

 

sea

 

X% of flue gas

 

sea

 

Energy

 

pump

 

fan

 

Untreated flue gas

 

Waste gas

 

 

Fig 2 Sea water is pumped against the extra resistance of the limestone baskets while the fraction of the flue gas is forced through the cooling water by a fan. The boundary includes the semicircular portion of the sea shown in blue.

 

Leakage of carbon dioxide

 

There are three possible generators of additional carbon dioxide in the alkalinity adjustment system that occur outside the boundary:

1. Mining calcium carbonate. In the current configuration we expect to use waste calcium carbonate from commercial mining.

2. Transport to the power station.  Truck transport generates about 0.2 kg CO2 per tonne-kilometer. This becomes 4.4 kg of CO2 generated per tonne of CO2 sequestered by CCOS.  We have assumed 10 km.

3. Biological consumption of bicarbonate.  As Donet (2006) points out, the rising concentration of carbon dioxide in the atmosphere is believed to be slowing the growth of marine organisms that utilise calcium carbonate in their structure.  Marubini and Thake (1999) show by experiments that additional alkalinity encourages growth.  The biological use of bicarbonate frees the carbon dioxide stored in the bicarbonate but may be associated with enhanced use of carbon dioxide in the production of organic carbon. While the bicarbonate added to the ocean can be expected to contributing to enhancing the health of the ocean, a little of the stored carbon may returned to the atmosphere.  Since the increase of bicarbonate outside the boundary is small (negligible) and the net carbon uptake response of the marine organisms is uncertain, the amount of bicarbonate lost outside the boundary can be assumed to be unimportant.

 

It is proposed the three terms above be considered negligible (less than 1%) of carbon stored in a simplified system of carbon accounting.

 

Unlike land based geological storage, CCOS provides no opportunity for rapid massive release of the carbon dioxide back to the atmosphere.

 

Permanence

 

Once the carbon dioxide is converted to bicarbonate, there is the possibility of enhanced precipitation of calcium carbonate, either on the sea floor or in the hard shells of marine organisms. Such an action will release the carbon dioxide back to the environment. While the pH of the ocean is falling (due to increasing atmospheric concentration of CO2) additional carbonate ions will go into solution (from coral reefs and the like) and join the bicarbonate introduced by CCOS.  There is no evidence to suggest a significant fraction of the additional alkalinity provided by the present process will be lost.

 

Sequestration

 

From within the project boundary there is

 

1. Extra energy, taken from the power station, to move the cooling water against the extra resistance of the calcium carbonate and the extra pressure head on the flue gases. It is estimated for the pilot plant at 10 kWh for 400kg of CO2 per hour. This energy if drawn from the power station can be converted to carbon dioxide (for gas fired power stations) at 0.6 kg CO2 per kWhr.

 

2. Reduced emission of CO2 to the atmosphere is calculated from the change of carbon dioxide in the cooling water (at equilibrium with the atmosphere). This can be done by measuring the titration alkalinity upsteam of the injection of gas and downsteam of the baskets of calcium carbonate.  Such a measurement counts only the carbon dioxide neutralised and not the carbon dioxide that escapes the cooling water.

 

An alternative is to measure the pH of samples of water from the cooling water channel upstream and downstream of the process and allow both samples to come to equilibrium with the atmosphere. This can be hastened by stirring. Then measurement of pH, temperature and salinity allows the calculation of the change in DIC using a computer program.

 

Monitoring

 

Unlike the more usual concept of measuring the carbon dioxide emissions before and after the project is implemented, the methodology proposed by EO&S (2005) for Carbon Capture and Ocean Storage, involves the direct calculation of the carbon stored in the water.  This maybe easily accomplished with some ocean storage technologies and because the ocean is an interconnected environment, render the concept of a boundary inappropriate.  Measuring the change in emissions it is neither practical nor desirable within the project boundary because of the large surface area of the cooling water channel through which gas flows.  The alternative, direct measurement of alkalinity is both well understood, reliable, precise and economical.

 

Measurement of the amount of carbon dioxide stored in the cooling water can be done easily by determining the alkalinity up steam and downstream of the injection and neutralisation process and using tables of the dissolved inorganic carbon that is in equilibrium with the atmosphere. This involves simple measurement of pH and temperature.

 

Additionality

 

At present power stations do not pass their exhaust gas through the cooling water to reduce carbon dioxide emissions. Operating a CCOS plant would be additional to the current practice and would not be expected to occur without financial incentive such as provided by CDM. Without such carbon capture and storage more carbon dioxide would enter the atmosphere. New CCOS projects will be constructed where previously there was no capture of carbon dioxide.

 

Conclusions

 

Scrubbing carbon dioxide from gas flows of concentrated CO2 and neutralising with calcium carbonate appears to be a desirable method to economically address the problem of rising carbon dioxide in the atmosphere. Without the CDM process to provide a financial return from CCOS, there is an investment barrier to the implementation of CCOS for power stations in developing countries.  The measurement of the dissolved inorganic carbon (in equilibrium with the atmosphere) taken up by the sea water flow can be determined by standard chemical procedures that are detailed in document EO&S (2005).  This is deemed more desirable than attempting to measure the carbon dioxide emissions before (termed baseline) and after CCOS in order to estimate the emissions permanently avoided.

 

The economical storage of carbon will allow carbon rich fossil fuels such as coal, which the world has in abundance, to be used to provide the energy without increasing the carbon dioxide in the atmosphere.  Developing countries need economical energy to improve their standard of living and they need to safeguard the health of the ocean to sustain the marine protein supply.  CCOS has a part to play in these processes.

 

References

 

Broecker, W. S., and T. Takahashi (1977) Neutralization of Fossil Fuel CO2 by Marine Calcium Carbonate, In: The Fate of Fossil Fuel CO2 in the Oceans, (Neil Andersen and A. Malahoff, eds), 213-241.

Caldeira, K and G H Rau (2000) Accelerating carbon dissolution to sequester carbon dioxide in the ocean: Geochemical implications. Geophys. Res. Letters, 27, 225-228.

Doney, S C (2006) The Dangers of Ocean Acidification, Scientific America, 294, 38-45.

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.

Golomb, D, E.Barry, D. Ryan, C. Lawton and P. Swett  (2004) Limestone Particle Stabilized Macro-Emulsion of Liquid and Supercritical Carbon Dioxide in Water for Ocean Sequestration, Environmental Science and Technology, 38, 4445-4450.

IPCC (2005)

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).

Kheshgi, H (1995) Sequestering atmospheric carbon dioxide by increasing ocean alkalinity, Energy, 20, 915-922.

Marubini, F and B Thake (1999) Bicarbonate addition promotes coral growth. Limnology and Oceanography, 44, 716-720.

Morse J W and F T Mackenzie (1990) Geochemistry of Sedimentary Carbonates, Elsevier, Amsterdam, 707pp.

Rau, G H and K Caldeira (1999) Enhancing carbonate dissolution: a means of sequestering waste CO2 as ocean bicarbonate. Energy Conversion & Management, 40, 1803-1813.

 

Acronyms and abbreviations

CCOS Carbon capture and ocean storage

CCS Carbon capture and storage

CDM Clean Development Mechanism

CER Certified emission reduction

CO2 Carbon dioxid

DIC Dissolved inorganic carbon

EB Executive Board

GHG Greenhouse gas

IPCC Intergovernmental Panel on Climate Change

NEUTRALISATION returning the pH to a neutral value

pH measure of the hydrogen ions in a solution

 

Units of measure

h Hour

d Day

y Year

m micro (10-6)

k Kilo (103)

M Mega (106)

G Giga (109)

T Tera (1012)

g Gramme

W Watt

m Metre

J Joule

meq/kg micro equivalent per kilogram

Gt Gigatonne = 1015 g

t tonne



[1] See list acronyms at the end of this document

[2] total alkalinity is defined in the next paragraph.

[3]  Note there are larger plants, eg Shoaiba in Saudi Arabia has a capacity of 400MLD