WO2010142943A2

WO2010142943A2 - Process for reducing carbon dioxide emissions

Info

Publication number: WO2010142943A2
Application number: PCT/GB2010/001111
Authority: WO
Inventors: John Munford; Richard Sanders
Original assignee: University Of Southampton
Priority date: 2009-06-10
Filing date: 2010-06-08
Publication date: 2010-12-16
Also published as: WO2010142943A3; GB0910043D0

Abstract

A process for removing carbon dioxide from seawater by intercepting the cold upwelling ocean water at depth (12) and extracting a proportion of dissolved inorganic carbon before it warms up at the ocean surface (13) emitting carbon dioxide into the atmosphere (15).

Description

Process for Reducing Carbon Dioxide Emissions

Background of the Invention

The present invention is in the field of reducing carbon dioxide emissions. In particular, the invention relates to a process and apparatus for removing dissolved inorganic carbon (DIC) from seawater, a vessel comprising such apparatus and storage of carbon.

It is believed that around 7.2 billion metric tons of carbon are liberated annually from fossil fuel sources to the atmosphere in the form of carbon dioxide. Concerns over global warming and ocean acidification due to increased atmospheric carbon dioxide levels mean that there is some pressure on the International Community to establish targets to reduce the amounts of such anthropogenic carbon emissions. Some data on warming and ocean acidification trends suggest that certain events could positively reinforce global warming and trigger an accelerated warming process with potentially serious consequences for the Earth's environment. In order to reverse atmospheric carbon dioxide accumulation, it is believed that an offset mechanism capable of capturing more than 8 billion metric tons of carbon equivalent per year may be required.

Various carbon capture mechanisms have been deployed on both new and existing power stations to reduce anthropogenic carbon dioxide output, but these may be expensive to install, consume substantial quantities of process energy and be difficult to scale up to the capacity levels sufficient to significantly reduce anthropogenic inputs of carbon dioxide into the atmosphere.

There have also been proposals for so called "air capture" of carbon dioxide by passing air through tower mounted filters and scrubbing out the carbon dioxide. The concentration of carbon dioxide in the air is around 0.04% and such air capture systems have been estimated to be able to capture around 20m tons of carbon dioxide per sq.m per year. While such systems use substantial process energy in their operation, they are not necessarily directly coupled to power plants and may be remotely located from the anthropogenic source. Such systems are often geographically remote from long term storage locations and it can be expensive to transport the carbon to the storage locations.

The oceans are thought to act as both a source and a sink for atmospheric carbon dioxide. It is believed that 38 thousand billion metric tons of dissolved inorganic carbon are stored in the oceans, compared to around 760 billion metric tons of carbon equivalent which are believed to be stored in the atmosphere.

The anthropogenic carbon flux to the environment due to the combustion of fossil fuels is believed to be small compared to natural terrestrial and marine fluxes of billions of metric tons of carbon per year, which appear to balance to leave substantially no net atmospheric accumulation of carbon dioxide. Most of the oceanic carbon dioxide emissions are believed to take place over warm tropical regions, while absorption is believed to occur in the colder oceanic regions .

The net anthropogenic carbon flux to the atmosphere as a consequence of the combustion of fossil fuels is believed to be increasing both atmospheric carbon dioxide levels, at a rate of 4 billion metric tons of carbon equivalent per year, and oceanic dissolved inorganic carbon levels, at around 1.6 billion metric tons per year. It is believed that such increases may be causing oceanic acidification and posing a potential threat to the base of the marine food chain.

The concentration of dissolved inorganic carbon in seawater is a strong inverse function of the seawater temperature, with deep oceans having a concentration of dissolved inorganic carbon in excess of that which would be in equilibrium with the atmosphere. This is due to the cold temperature of the deep oceans, which results in them having high dissolved inorganic carbon levels, and to the dissolution of biological material in the deep oceans. There are regions of the warm tropical ocean where large natural upwelling of cold water from depth occurs and substantial quantities of carbon dioxide are released into the atmosphere.

The present invention seeks to mitigate at least some of the above mentioned problems. Summary of the Invention

According to a first aspect of the invention there is provided a process for removing dissolved inorganic carbon from seawater, the process comprising collecting seawater containing dissolved inorganic carbon in solution, degassing the seawater to release a portion of the dissolved inorganic carbon as gaseous carbon dioxide and collecting the released gaseous carbon dioxide.

The collected carbon dioxide may be passed to a carbon capture apparatus. It will be appreciated that separating and collecting gaseous carbon dioxide from liquid seawater may be more efficient than separating and collecting carbon dioxide from the air.

The degassing may occur by exposing the seawater to an atmosphere having a lower than equilibrium concentration of carbon dioxide. The equilibrium concentration is the atmospheric concentration of carbon dioxide that is in equilibrium with the concentration of the seawater. It will be appreciated that the equilibrium concentration will be affected by factors such as temperature and pH of the seawater.

The seawater may be upwelling ocean water. As such, the seawater may be rich in dissolved inorganic carbon. The seawater may comprise at least 24 ppm dissolved inorganic carbon. Preferably, the seawater comprises at least 36 ppm dissolved inorganic carbon, more preferably, the seawater comprises at least 48 ppm dissolved inorganic carbon. It will be appreciated that it may be more efficient to separate and collect carbon dioxide from seawater with a high dissolved inorganic carbon content.

The released portion may comprise at least 10% of the dissolved inorganic carbon originally present in the seawater. More preferably the released portion comprises at least 20% of the dissolved inorganic carbon originally present in the seawater. Yet more preferably the released portion comprises at least 30% of the dissolved inorganic carbon originally present in the seawater. Still more preferably the released portion comprises at least 40% of the dissolved inorganic carbon originally present in the seawater. Even more preferably the released portion comprises at least 50% of the dissolved inorganic carbon originally present in the seawater.

The process may comprise heating the seawater. The heating may raise the temperature of the seawater by at least 10⁰C. Preferably, the heating may raise the temperature of the seawater by at least 15°C. More preferably, the heating may raise the temperature of the seawater by at least 20⁰C. It will be appreciated that changing the temperature of the seawater changes the solubility of the dissolved inorganic carbon in the seawater. Heating the seawater therefore releases gaseous carbon dioxide from the seawater. The heating may comprise solar heating, which may be direct solar heating or indirect solar heating. The solar heating may raise the temperature of a heat exchange fluid. The heat exchange fluid may raise the temperature of the seawater. Alternatively or additionally, the heating may comprise an ocean heat pump. Another possibility is that the heating comprises nuclear power. The heating may comprise a combination of some or all of the above heating methods .

The process may comprise adding an acid to the seawater. The acid may be added prior to the degassing. The addition of the acid may lower the pH of the seawater by at least 0.1. Preferably the addition of the acid lowers the pH of the seawater by at least 1. More preferably, the addition of the acid lowers the pH of the seawater by at least 2. It will be appreciated that changing the pH of the seawater changes the solubility of the dissolved inorganic carbon in the seawater. Lowering the pH of the seawater may cause more carbon dioxide to be released.

The process may comprise adding an alkali to the seawater. The alkali may be added after the degassing. The alkali may thus be used to neutralise the acid that was added to the seawater prior to separation of the gaseous carbon dioxide. The alkali may restore the pH of the seawater to substantially the pH at which it was collected. The acid and the alkali may be obtained from seawater. The acid and the alkali may be obtained by electrolysis, for example by the Chloralkali process. The process may comprise passing the separated gaseous carbon dioxide to a carbon capture apparatus. The carbon capture apparatus may transform the gaseous carbon dioxide into a form suitable for long term storage. The carbon capture apparatus may comprise electro chemical accelerator carbonate dissolution. The carbon capture apparatus may comprise a Sabatier reaction.

The process may comprise returning the seawater to the sea. It will be appreciated that the seawater returned to the sea contains less dissolved inorganic carbon than the seawater withdrawn from the sea. In this way carbon is removed from the oceans, but is not released into the atmosphere. This anthropogenic interruption in the natural release of carbon dioxide from the oceans may be used to cancel out a portion of anthropogenic emissions from other sources. The seawater may be collected at a greater depth than at the depth it is released. The released seawater may contain a substantially similar amount of dissolved inorganic carbon to seawater at the depth at which it is released; advantageously, the marine ecosystem may thus be substantially unaffected.

The process may be carried out in oceanic regions where natural or artificial upwellings occur. Such upwellings may generate high fluxes of carbon dioxide into the atmosphere. Such regions may, for example, be found in the tropics and particularly along the Equator. The process may comprise transporting the transformed carbon dioxide to a long term storage location. The transformed carbon dioxide may be transported as a liquid or as a solid.

According to a second aspect of the invention, there is provided an apparatus for removing dissolved inorganic carbon from seawater, the apparatus comprising an input for receiving sea water containing dissolved inorganic carbon in solution, a degasser for releasing a portion of the dissolved inorganic carbon as gaseous carbon dioxide and a gas liquid separator for separating the gaseous carbon dioxide from the seawater for passing to a carbon capture apparatus .

The apparatus may comprise a heater for heating the seawater. The heater may comprise means for heating the seawater to release a portion of the carbon dioxide. The heater may comprise a heat exchanger. Alternatively, the heater may comprise a direct heater. The heater may comprise a solar heater. The heater may be an ocean heat pump. The heater may use heat from a power plant, for example, a nuclear reactor, gas turbine, biofuel generator, or Ocean Thermal Energy Conversion (OTEC) plant. The heater may comprise an electric heater. Electricity may, for example be generated using wave power, solar photovoltaic power, wind power, nuclear power, a gas turbine, a biofuel generator or an OTEC plant. The apparatus may comprise a pump. The pump may raise water from below the surface of the sea. The water raised by the pump may be colder than seawater at the surface of the sea. It will be appreciated that colder seawater may be richer in dissolved inorganic carbon.

The apparatus may comprise a reactor for producing an acid. The source may be an electrolytic reactor. The reactor may also produce an alkali. For example the reactor may electrolyse seawater to produce hydrochloric acid and sodium hydroxide.

The apparatus may comprise a mixer for mixing the acid with the seawater. The mixer may be upstream of the gas liquid separator.

The apparatus may comprise a mixer for mixing the alkali with the seawater. The mixer may be downstream of the gas liquid separator.

The apparatus may comprise means for transforming the released carbon dioxide into a form suitable for long term storage. For example, the apparatus may comprise a compressor. The compressor may compress the carbon dioxide into a liquid state. Another possibility is that the apparatus comprises an accelerated carbonate dissolution system.

At least part of the apparatus may be contained in a container. For example, at least part of the apparatus may be contained in a freight container. The freight container may be a so-called 40-foot freight container. The apparatus may be contained in a plurality of freight containers. For example, a first part of the apparatus may be contained in a first freight container. A second part of the apparatus may be contained in a second freight container. The first and second freight containers may be fastened together to form all or part of the apparatus.

The apparatus may be a modular apparatus. The modular apparatus may comprise a multiplicity of modules. The modules may be provided with fastenings suitable for engagement by standard load handling apparatus for freight containers. The modules may be contained within freight containers. Each module may be contained in a freight container.

It will be understood that a freight container comprises corner fastenings suitable for engagement by standard load handling apparatus for freight containers.

The apparatus may be installable in a ship. The apparatus may be installable in a floating rig. The ship or rig may be tetherable to the seabed. Another possibility is that the apparatus is installable in a platform fixed to the seabed. A further possibility is that the apparatus is installable in a semi-submersible or floating unit. The apparatus may be installable in a land based installation. The seawater may be pumped to the installation. The apparatus may be suitable for positioning in the tropics. The apparatus may be suitable for positioning in the regions of the Equator. In that way the apparatus may be provided with access to a consistent upwelling. By providing the apparatus in the region of the Equator, it will be appreciated that the apparatus may reduce natural carbon dioxide emissions to the atmosphere in a region where infra-red radiation from the earth is generally large.

The apparatus may form part of an ocean thermal energy conversion process.

According to a third aspect of the invention there is provided a floating vessel comprising an apparatus as described above. The floating vessel may be a ship. The apparatus may be retro-fitted to the ship. The ship may be purpose built. The floating vessel may be a floating or semi-submersible rig.

According to a fourth aspect of the invention there is provided storage of carbon captured using a process, apparatus or vessel as described above.

It will be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the process of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. Description of the Drawings

Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

Figure 1 is a schematic representation of carbon fluxes; Figure 2 is a schematic representation of emission of carbon dioxide associated with upwelling of cold deep water; Figure 3 is a schematic representation of regions of the earth where natural upwelling of cold water occurs; Figure 4 is a schematic representation of equatorial upwelling; Figure 5 is a schematic representation of an apparatus for removing carbon dioxide from seawater; Figure 6 is a schematic representation of a vessel containing an apparatus for removing carbon dioxide from seawater; and Figure 7 is a schematic representation of a process for removing carbon dioxide from seawater.

Detailed Description

Figure 1 is a schematic representation of carbon fluxes. Carbon in the form of carbon dioxide is exchanged each year between the oceans 2 and the atmosphere 1. Around 92.2 billion metric tons of carbon equivalent is absorbed in this way 8 by the oceans 2 and around 90.6 billion metric tons of carbon equivalent per year naturally emitted in the form of carbon dioxide 7. Carbon in the form of carbon dioxide is also exchanged, in a quantity of about 120 billion metric tons of carbon equivalent per year in each direction 5 and 6, between terrestrial vegetation and soils

3 and the atmosphere 1. Changing land use contributes to this exchange of carbon with a net absorption by the terrestrial vegetation and. soils 3 of 600 million metric tons of carbon equivalent per year. Anthropogenic emissions

4 from fossil fuel combustion and industrial processes result in a release of about 7.2 billion metric tons per year of carbon equivalent in the form of carbon dioxide to the atmosphere 1. Imbalance in the fluxes means the oceans 2 are absorbing 1.6 billion metric tons net of carbon equivalent per year and the atmosphere 1 is absorbing around

5 billion metric tons net of carbon equivalent per year.

Figure 2 is a schematic representation the emission of carbon dioxide associated with upwelling of cold deep water. In cold regions carbon dioxide is absorbed 8 from the atmosphere 1 into the ocean 2. Cold dense water sinks 9 to the bottom of the ocean 2 and circulates 11 towards warmer latitudes. As it circulates 11 the cold water picks up some dissolved inorganic carbon from the sediment 10. In warmer latitudes the cold current of water upwells 12, due to rising sea bed levels, equatorial upwelling or natural or artificial stimulation, and warms 13 as it reaches the surface of the ocean 2. As the water is warmed 13 it liberates part of its stored carbon as carbon dioxide 7 into the atmosphere 1. Part of the stored dissolved inorganic carbon is retained as carbonic acid in solution as the water circulates 14 back towards to colder regions due to the net effect of the earth's rotation, winds, salinity, temperature and tides.

Figure 3 is a map of the world's oceanic regions. In some regions 16 natural upwelling of cold water occurs on a consistent basis. The natural upwelling results in substantial natural carbon dioxide emissions into the atmosphere. The regions 16 and in particular the line along the Equator itself 17 may be particularly suitable for deployment of the present invention.

Figure 4 is a schematic representation of equatorial upwelling. In the region of the Equator 17 a balance between Coriolis and turbulent drag forces and surface wind cause westward-flowing, wind-driven, warm surface currents 19 near the Equator 17 to turn away from the Equator 17, breaching the thermocline 20. The water carried in the surface currents 19 is replaced by cold upwelling water 18 from the deep. The cold upwelling water 18 mixes at the surface and increases in temperature, liberating carbon dioxide into the atmosphere 1.

Figure 5 is a schematic representation of an apparatus for removing dissolved inorganic carbon from seawater in the form of carbon dioxide. An inlet 21 is connected, via a pump 22, heat exchanger 23 and gas trap 38, to a mixer 24. The mixer 24 is upstream of a degasser in the form of a heat exchanger 25, which is upstream of a gas liquid separator 33. The liquid outlet from the gas liquid separator 33 is connected via the input heat exchanger 39 where waste heat is recovered, to a mixer 34. Downstream of the mixer 34 there is an outlet 40. The vapour outlet from the gas liquid separator 33 and the gas trap 38 is connected to a carbon capture apparatus 37, which is connected, via a pump 32, to a storage accumulator 31. The storage accumulator 31 is connected to a sorbent regenerator 29 and an outlet 30. The sorbent regenerator 29 and the heat exchanger 24 are connected to a heat transfer fluid system 41. The heat transfer fluid system 41 comprises a pump 28 and a solar heater 27, which is heated by the sun 26. The heat transfer fluid is water or another suitable heat transfer fluid. An electrolytic reactor 35 is connected to the mixer 23 and the mixer 34. The electrolytic reactor 35 and the pumps 22, 28 and 32 are powered by a power plant 36.

In use, cold water is collected via the inlet 21 and pumped 22, via the preliminary heat exchanger 23, through the mixer 24, to the main heat exchanger 25. Heat exchange fluid is heated in the heater 27 and pumped by the pump 28 to the heat exchanger 25. The temperature of the collected seawater is raised in the heat exchanger 25 and the pH of the collected seawater is lowered by acid added in the mixer 24. The rise in temperature and the drop in pH cause gaseous carbon dioxide to be released from the seawater. The gaseous carbon dioxide is separated from the seawater in the gas liquid separator 33. The seawater leaves the gas liquid separator 33 and passes, via the preliminary heat exchanges 23 where it transfers thermal energy to the incoming seawater, through to the mixer 34. At the mixer 34, it is mixed with an alkali, which returns the pH of the seawater to the level at which it was collected. The seawater is returned to the sea via the outlet 40. The released gaseous carbon dioxide leaves the gas liquid separator 33 and enters the carbon capture apparatus 37 where it is transformed into a form suitable for long term storage. The captured carbon dioxide is pumped, by pump 32, to the storage accumulator 31. Captured carbon can be removed from the storage accumulator 31 via the outlet 30 for transportation to a location for long term storage and the sorbent regenerated in sorbent regenerator 29.

Figure 6 is a schematic representation of an apparatus for removing carbon dioxide from seawater installed in a floating vessel 38. The vessel 38 has a solar heater 27 on the upper deck. The solar heater 27 is warmed by the sun 26. Below decks the heat exchanger 23 and 25, mixers 24 and 34, carbon capture apparatus 37, storage accumulator 31 and sorbent regenerator 29 are arranged. Pumps 28, 32 and 22 are arranged towards the stern of the vessel 38. Electrolyser 35 and power source 36 are also arranged towards the stern of the vessel 38. Extending from the hull of the vessel 38 are inlet pipe 21 and outlet pipe 40. Inlet pipe 21 extends deeper than outlet pipe 40. The vessel 38 may be self-propelled. Power source 36 may be integral with the propulsion source of the vessel 38. At the stern of the vessel 38 there is provided an outlet 30. The outlet 30 is used for offloading captured carbon dioxide for transportation to a storage location.

Figure 7 is a schematic representation in block diagram form of chemical pathways involved in the removal of the dissolved inorganic carbon from solution. Energy 43, including solar power 26, is used to split brine into hydrochloric acid and sodium hydroxide via the Chloralkali process 44. The incoming seawater 21 is filtered and pumped 42 before being passed to the first mixer 46. The hydrochloric acid is mixed with the seawater 46 reducing the pH and moving the dissolved inorganic carbon into the carbon dioxide (aqueous) spectrum. Mechanical and thermal agitation 47 coupled with a mechanism that reduces the partial pressure above the liquid 45 degasses the liquid and extracts the gas in the form of carbon dioxide. The released gaseous carbon dioxide is passed to an absorber 50 for drying and liquefaction before being accumulated 51 ready for permanent storage. Sodium hydroxide is added to the carbon depleted seawater in the second mixer 48 to restore the seawater to its original pH before the seawater is pumped 49 via the outlet 40 where the seawater dissolved inorganic carbon equilibrium is restored. The net effect of this process is to reduce natural carbon dioxide emissions to the atmosphere exactly by the amount of carbon dioxide placed in permanent storage.

Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein.

Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments .

Claims

1. A process for removing carbon dioxide from seawater, the process comprising collecting seawater containing dissolved inorganic carbon in solution, degassing the seawater to release a portion of the dissolved inorganic carbon as gaseous carbon dioxide and collecting the released gaseous carbon dioxide.

2. The process of claim 1, wherein the process comprises heating the seawater.

3. The process of claim 1 or claim 2 wherein the seawater is upwelling ocean water.

4. The process of any preceding claim wherein the seawater comprises at least 24 ppm dissolved inorganic carbon.

5. A process according to claim 4, wherein the released portion comprises at least 10% of the dissolved inorganic carbon.

6. The process of any preceding claim wherein the heating comprises solar heating.

7. The process of any one of claims 1 to 5 wherein the heating comprises an ocean heat pump.

8. A process according to any one of claims 1 to 5 wherein the heating comprises nuclear power.

9. A process according to any preceding claim wherein the process further comprises adding an acid to the seawater prior to the degassing.

10. A process according to claim 9 wherein the process further includes the step of adding an alkali to the seawater after the degassing.

11. A process according to any preceding claim wherein the process further comprises transforming the gaseous carbon dioxide into a form suitable for long term storage.

12. A process according to claim 11 wherein the transforming comprises an electro-chemical accelerator carbonate dissolution.

13. A process according to claim 11 wherein the transforming comprises a Sabatier reaction process.

14. A process according to any preceding claim, wherein the heating raises the temperature of the seawater by at least 15°C.

15. An apparatus for carbon capture, the apparatus comprising an inlet for receiving seawater containing dissolved inorganic carbon in solution, a degasser for releasing a portion of the dissolved inorganic carbon as gaseous carbon dioxide and a gas-liquid separator for separating the gaseous carbon dioxide from the seawater for passing to a carbon capture apparatus.

16. The apparatus of claim 15, the apparatus comprising a heater for heating the seawater.

17. An apparatus according to claim 15 or claim 16 wherein the apparatus comprises a mixer, upstream of the degasser, for mixing an acid with the seawater.

18. An apparatus according to claim 17 wherein the apparatus further comprises a mixer, downstream of the degasser, for mixing an alkali with the seawater.

19. An apparatus according to claim 18 wherein the apparatus comprises a reactor for producing the acid and the alkali from the seawater.

20. An apparatus according to claim 19, wherein the reactor comprises an electrolytic reactor.

21. An apparatus according to any one of claims 15 to 20, wherein at least part of the apparatus is contained in a container.

22. An apparatus according to claim 21, wherein a first part of the apparatus is contained in a first container and a second part of the apparatus is contained in a second container.

23. An apparatus according to claim any one of claims 15 to 20, wherein the apparatus is a modular apparatus comprising a multiplicity of modules.

24. An apparatus according to claim 23, wherein the modules are contained within freight containers.

25. A floating vessel comprising an apparatus according to any one of claims 15 to 24.

26. Storage of carbon captured using the process of any one of claims 1 to 14, or the apparatus of any one of claims 15 to 24.