WO2021187971A1

WO2021187971A1 - Method of processing gas loaded with carbon dioxide

Info

Publication number: WO2021187971A1
Application number: PCT/MY2021/000002
Authority: WO
Inventors: Jeremy Duncan Stuart Joynson; Edwin Stephen Willis
Original assignee: Jeremy Duncan Stuart Joynson; Edwin Stephen Willis
Priority date: 2020-03-17
Filing date: 2021-03-08
Publication date: 2021-09-23

Abstract

The disclosure relates to a method and a plant for processing off-gas streams from industrial processes that generate carbon dioxide. The processing method has application, although not exclusive application, to off-gas streams from cement production, iron-making and steel-making processes and biological processes. The method includes compressing the gas to produce a pressurised gas and associated undesirable pressurized liquids, separating the pressurized gas from the pressurized liquids, mixing the pressurized gas with pressurized water so that most of the carbon dioxide dissolves into the pressurized water to form a pressurized carbonic acid solution and a stream of compressed residual gas and sequestering the pressurized carbonic acid solution.

Description

METHOD OF PROCESSING GAS LOADED WITH CARBON DIOXIDE

TECHNICAL FIELD

A method and a plant are disclosed for processing gas loaded with carbon dioxide. The disclosure also relates to a method for capturing and sequestering carbon.

The processing method has application to processing off-gas streams from industrial processes that generate carbon dioxide. The processing method has application, although not exclusive application, to off-gas streams from cement production, iron- making and steel-making processes and biological processes.

BACKGROUND

If the sequestering of CO2 is to be achieved at a global scale the processes used must be affordable, ideally at a cost below the costs of carbon credits and carbon taxes. The amount of materials used to deliver the processes must also be minimised to ensure that the world is able to manufacture and install enough of them to capture the requisite amount of CO2 and can do so in the shortest possible time. The atmosphere has a very low concentration of CO2 at around 0.04 mol%. Carbon- rich fuel system exhausts from the likes of electrical power plants contain around 8 to 10 mass % CO2. Cement and steel production processes generate more highly concentrated CO2 exhaust streams, containing between 15% and 25 mass % of the gas. The amount of equipment required to capture CO2 from these high concentration sources compared to the atmosphere is two to three orders of magnitude less, and the costs are correspondingly lower. If the world is to capture and sequester CO2, then it makes more sense focus on the concentrated sources first, rather than using air capture methods. SUMMARY OF THE DISCLOSURE

In a first aspect, there is provided a method of processing gas loaded with carbon dioxide, the method including:

(a) compressing the gas to produce a pressurised gas and associated undesirable pressurized liquids;

(b) separating the pressurized gas from the pressurized liquids; (c) mixing the pressurized gas with pressurized water so that most of the carbon dioxide dissolves into the pressurized water to form a pressurized carbonic acid solution and a stream of compressed residual gas; and

(d) sequestering the pressurized carbonic acid solution.

The method may include cooling the gas prior to compressing step (a).

The method may include removing at least some contaminants from the gas prior to compressing step (a).

The method may include cooling the compressed gas after compressing step (a).

The method may include removing some contaminants from the compressed gas after cooling the compressed gas.

The water may be pressurized to a pressure substantially equal to the pressure of the compressed gas prior to mixing step (c).

The cooling steps may include transferring heat from the gas or the compressed gas to the water in a heat exchanger to form heated water.

The water may be sea water.

The compressed gas may be formed by two or more cycles of cooling, cleaning and compression steps.

The method may involve two cycles and the compressed gas after the first cycle may be at a pressure of 2 to 5 bara (bar absolute) and at a pressure of 10 to 12 bara at the end of the second cycle.

The method may further include sequestering the pressurized carbonic acid solution by injecting it into one or more underground formations containing minerals that form a carbonate or a bicarbonate when contacted with carbonic acid solution. The minerals may comprise any one or more of calcium, magnesium, sodium, and potassium. The one or more underground formations may comprise alkali metal or alkaline earth metals in carbonate or silicate form. For example, it will be appreciated that calcium in underground formations will react to form a dissolved calcium bicarbonate solution.

The one or more underground formations may be offshore and hydraulically connected to the ocean so that the bicarbonate solution flows into the ocean.

The method may include adjusting the pressure of the pressurized carbonic acid solution to a pressure that is substantially equivalent to the water pressure in the one or more underground formations so that carbon dioxide is retained in the pressurized carbonic acid solution when it enters the one or more underground formations.

The underground formations may be 200m to 1000m depth below sea level.

The method may include forming hydraulic connections between the one or more underground formations and the ocean.

The hydraulic connections may be formed by drilling boreholes from a bed of the ocean to the one or more underground formations.

The pressurized carbonic acid solution may be produced onshore and may be supplied to injection wells that are configured to commence onshore and to continue underground into the one or more offshore underground formations.

The method may further include recovering energy stored in the compressed residual gas.

Recovering energy stored in the compressed residual gas may comprise generating electricity by passing the compressed residual gas through an expander.

The expander may be a multi-stage expander and the compressed residual gas may be heated between expander stages.

The method may further include utilizing the generated electricity in any one or more of steps (a) to (c). The method may further comprise storing the compressed residual gas for a period of time and then recovering energy from the stored compressed residual gas.

The compressed residual gas may be further compressed to a pressure in the range of 60 to 300 bara.

The method may include splitting the compressed residual gas into first and second streams and recovering stored energy from the first stream by passing the first stream through an expander to generate electricity and storing the second stream of the compressed residual gas.

The underground formations may comprise limestone or dolomite formations.

The method may include utilizing at least some of the generated electricity to drive the compressing step (a).

In a second aspect, there is provided a method of forming a bicarbonate solution, the method including : (a) compressing a gas loaded with carbon dioxide to produce a compressed gas and undesirable pressurized liquids;

(b) separating the compressed gas from the pressurized liquids; (c) mixing the pressurized gas with pressurized water so that most of the carbon dioxide dissolves into the pressurized water to form a pressurized carbonic acid solution and a stream of compressed off-gas; and

(d) injecting the pressurized carbonic acid solution into one or more carbonate containing formations that are hydraulically connected to the ocean such that the carbonic acid reacts with the one or more underground formations to produce the bicarbonate solution.

The pressurised water may be pressurised sea water.

The one or more carbonate containing formations may be offshore and the pressurized carbonic acid solution may be injected into the carbonate containing formation via injection wells that commence onshore and that terminate in the one or more offshore carbonate containing formations.

In a third aspect, there is provided a carbonic acid sequestration plant, the plant including:

(a) an apparatus that converts gas loaded with carbon dioxide into a carbonic acid solution; and

(b) an injection system that connects the apparatus to one or more underground formations containing minerals that form a carbonate or a bicarbonate when contacted with carbonic acid solution and which underground formation or formations are hydraulically connected to a body of sea water; and wherein the injection system is operable to deliver a carbonic acid solution produced by the apparatus to the one or more underground formations.

The one or more underground formations may be offshore and the injection system may include injection wells that commence onshore and that terminate in the one or more offshore carbonate containing formations.

The plant may be located within 50 km of the one or more underground formations.

The plant may be located within 25 km of the one or more underground formations.

The plant may be located within 10 km of the one or more underground formations.

The one or more underground formations may comprise limestone or dolomite formations.

The gas loaded with carbon dioxide may contain at least 0.04 mol % CO2.

The gas loaded with carbon dioxide may contain at least 1 mass % CO2.

The gas loaded with carbon dioxide may contain at least 2 mass % CO2.

The gas loaded with carbon dioxide may contain at least 5 mass % CO2. The gas loaded with carbon dioxide may contain at least 10 mass % CO2.

The gas loaded with carbon dioxide may contain at least 15 mass % CO2.

The gas loaded with carbon dioxide may contain at least 20 mass % CO2.

The gas loaded with carbon dioxide may contain at least 30 mass % CO2.

The world naturally converts CO2 into stable bicarbonates of calcium (Ca(HC03)2) and magnesium (Mg(HCC>3)2), called weathering of limestone and dolomite. This happens when raindrops absorb CO2 from the atmosphere and then fall on carbonate bearing rocks, such as limestone. The problem is that the process is too slow to capture and reduce the currently steadily rising atmospheric CO2 concentration.

The accelerated weathering of carbonate and silicate minerals (caused by capturing CO2 and converting it into a carbonic acid solution that is contacted with underground formations containing minerals that form a carbonate or a bicarbonate when contacted with carbonic acid solution) according to the first and second aspects disclosed above accelerates the conversion of gaseous CO2 into dissolved HCO3 salts by several orders of magnitude. It does this by capturing CO2 from concentrated sources using pressurised seawater which forms a concentrated acid solution. Compared to CO2 dissolved in rainwater, this process results in CO2 dissolution which is more than 5000 times higher when capturing off-gas at 20 mass % CO2 and dissolving it in water at 11 bara. The level of dissolution increases when the CO2 concentration in the off-gas is higher and the water pressure is higher. For example, when the off-gas contains 35 mass % CO2 and the water is at 20 bara, the dissolution is about 17,000 times higher than rain water. The higher dissolution level results in faster conversion of carbonic acid solution into bicarbonate solution when it comes into contact with carbonate containing formations.

This solution is continuously pumped into carbonate rock formations located below the ocean, where it can react to become a permanently dissolved bicarbonate salt which then gradually percolates into the ocean above. The bicarbonates formed are stable over geological timescales.

Bicarbonate is already a naturally occurring component of the world's oceans. Bicarbonate is a pH buffer that contributes to maintaining the right pH for marine life. The rising amount of CO2 dissolved in the oceans is exceeding the limits of the natural bicarbonate production to control the pH of the oceans and, therefore, the oceans are slowly acidifying. The applicant believes that releasing bicarbonates on a larger scale will slow or reverse the trend in ocean acidification.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the apparatus and method disclosed above will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic flow chart of an embodiment of the method of processing gas loaded with carbon dioxide;

Figure 2 is a more detailed flow chart of the process shown in Figure 1; and

Figure 3 is an energy storage and generation circuit which utilizes a residual gas by product from the process shown in Figures 1 and 2.

DESCRIPTION OF EMBODIMENT

A preferred embodiment of the present invention will now be described in the following text which includes reference numerals that correspond to features illustrated in the accompanying Figures. To maintain clarity of the Figures, however, all reference numerals are not included in each Figure.

Referring firstly to Figure 1, a method of processing off-gas 10 loaded with carbon dioxide is shown schematically as including cooling the off-gas 10 in step 23a, typically by way of a heat exchanger, and then cleaning the off-gas in step 24a to remove condensed water and contaminants. The cooled and cleaned off-gas 10 is then compressed in step 21a by a compressor. The compressing step increases the temperature of the off-gas 10, so it is sent through the same cycle of cooling 23b, cleaning 24b and compressing 21b. The compressed off-gas 10 is then mixed with water at step 42 to form a carbonic acid solution (in which most of the carbon dioxide from the off-gas is dissolved) and compressed nitrogen gas.

The other compressed gases that are not dissolved in water (referred to throughout this specification and claims as "residual gas") are separated from the carbonic acid solution which is then further pressurized at step 51 to a pressure that is high enough to keep the carbon dioxide dissolved in the carbonic acid solution when sequestered into an underground formation 55, which takes the form of a limestone rock formation in this embodiment. The carbonic acid solution is then pumped via injections wells 52 into one or more underground formations 55 that are below sea level and which may be hydraulically connected to a body of sea water, such as open ocean, a bay, a harbour, an inlet or an ocean-bound source of water.

The underground formations 55 may be any suitable formation which will produce bicarbonate solution when contacted with carbonic acid. The one or more underground formations 55 may comprise alkali metal or alkaline earth metals in carbonate or silicate form. The minerals in the underground formation 55 may comprise any one or more of calcium, magnesium, sodium, and potassium. Such formations include limestone, dolomite, basalt, olivine and asbestos.

As described above, the carbonic acid solution reacts with the underground formations 55 to produce a solution that carries bicarbonates. That is, the underground formation 55 will include channels through which the carbonic acid solution flows so that it contacts the minerals of the underground formation 55 to form bicarbonate. The hydraulic connection of the underground formation 55 to the ocean means that the carbonic acid solution can displace the water naturally present in the pores of the underground formation 55 to the ocean above, and react with the underground formation 55 to form bicarbonate solution. The resulting bicarbonate solution may flow into the ocean continuously if the carbonic acid solution is delivered to the underground formation 55 via the injection wells 52 or may flow into the ocean in a batch-wise manner if the supply of carbonic acid solution is delivered to the underground formation 55 intermittently, for example, as a continuous supply being delivered sequentially to different injection wells.

The overall effect is that carbon dioxide is captured from the off-gas 10 and converted to bicarbonate which flows into the oceans and, therefore, helps slow or reverse acidification of the oceans.

The process and a plant for producing the carbonic acid solution are described in more detail below with reference to Figures 2 and 3. The following description is in the context of processing off-gas 10 from a lime manufacturing plant. It will be appreciated, however, that the process is applicable to other gas streams that are loaded with carbon dioxide, such as off-gas streams from electricity generating processes which burn carbon-rich fuels, iron-making processes, steel-making processes and some biological process. The applicant believes that the process is applicable to any gas stream that includes CO2. Lime (CaO) is produced by roasting limestone (CaCCb) above 848°C. The heat is typically generated by burning a carbon-rich fuel inside the roasting kiln creating a combined exhaust gas rich in CO2. Many cement and lime kilns employ heat recovery techniques to reduce fuel costs and use the waste exhaust heat to prewarm the incoming limestone. This can bring the exhaust temperature down to around 200°C, but may be high or lower depending on the plant configuration. The composition of the off-gas 10 at that stage is approximately 20 mol % CO2, approximately 75 mol % N, approximately 5 mol % water and trace components including Argon, NO_*, SOx and dust (possibly including heavy metals). A lime production facility which produces 1 million tonnes/year of lime also produces around 0.78 million tonnes/year of CO2 from the limestone and a similar amount of CO2 from the burning of carbon fuels to achieve the limestone decomposition temperature. The CO2 from both are intermixed in the same off-gas stream.

The AWL process takes this C02-rich exhaust gas stream and cools it further, to around 25°C using seawater (which may be around 15°C, but may be higher or lower depending on where the sea water is sourced from, e.g. the temperature may be higher when the sea water is sourced from tropical regions) in a gas/liquid heat exchanger 23. The sea water is prepared by using pump 32 to pass the salt water initially through a coarse filter 31 and then a fine filer 33 to remove fine particles and optionally add chemicals to kill bacteria in the water.

Water vapour, formed by the fuel combustion process in the kiln, condenses in the heat exchangers, absorbing a small amount of CO2 from the exhaust gas along with much of the NO_x and SO* present. The condensed water, NO_x and SO_x (collectively 25) are separated from the gas 10 downstream in a gas/liquid separator 24. It will be appreciated, however, that other suitable forms of gas/liquid separators may be used in place of a scrubber.

The cooled off-gas 10 is then sent to compressor 21 where it is compressed from around atmospheric pressure up to around 3.5 bara (bar absolute) using a centrifugal compressor which may be electrically driven. It will be appreciated, however, that the compressor may be any other suitable form of compressor, such as reciprocating or axial-type compressor.

The compressed gas requires further compression before it reaches sufficient pressure to be delivered into seabed underground formations 55 and still keep the carbon dioxide dissolved. However, the compression step increases the temperature of the compressed gas so further cooling is required before the next compression step, as shown schematically in Figure 2 and described above. Accordingly, the compressed gas is subjected to a second and possibly a third cycle of cooling, cleaning and compression with a separate heat exchanger 23, scrubber 24 and compressor 21 in series and results in the compressed off-gas 10 being further compressed to around 11 bara. For simplicity, the equipment associated with subsequent processing cycles is not shown in Figure 2, but the equivalent processing steps for a two-cycle process are shown schematically in Figure 1.

Compressing the off-gas 10 to around 11 bara means volume of sea water required for mixing with the compressed off-gas 10 is around the same volume of sea water that is used as coolant in the heat exchangers 23. This avoids the need for additional sea water to be prepared through the filters 31 and 33 in anticipation of mixing with the compressed off-gas 10 at 11 bara. However, the pressure of the compressed off gas 10 is not limited to 11 bara and, instead, may be higher (e.g. 15 or 20 bara).

The higher gas and water pressures are, however, accompanied by higher energy consumption A recycling loop is included to supplement the in-flow of off-gas 10 to the compressor 21 during times when the supply of off-gas 10 from the lime manufacturing plant is low. The recycling loop is a safety measure to reduce surging in centrifugal compressors, so the recycling loop may be omitted if other forms of compressors are adopted instead.

Although shown in Figure 2 as a single stream of off-gas 10 being processed by the heat exchanger 23, scrubber 24 and compressor 21 in series, it will be appreciated that the incoming off-gas 10 may be split into two or more separate streams which is processed in parallel by separate heat exchangers 23, scrubbers 24 and compressors 21. The separated compressed off-gas 10 streams may then be combined to form a single stream which is subjected to further processing. No cooling step is required after the second compression cycle because the compressed off-gas 10 is mixed with the sea water which is cooler than the compressed off-gas 10. The sea water that is mixed with the compressed off-gas 10 is the same sea water that is used in the heat exchanger 23 and which is mixed with additional sea water from the fine filter 33 to achieve the required total flowrate, and pumped up to the same pressure as the compressed off-gas 10 leaving the second or final compression cycle, i.e. 11 bara, by a high pressure pump 41. This pressurised seawater is mixed with the compressed gas 10 using a mixing device which may be an eductor to form acidified sea water, i.e. carbonic acid solution.

The flowrate of the sea water is selected to be high enough to absorb most, if not all, of the CO2 in the gas stream, resulting in a two-phase flow comprising residual gas (mainly nitrogen gas) and acidified seawater containing most of the CO2. These streams are separated in a pressure vessel 43 with the residual gas being taken off as streams via valves 44 and 45 and the acidified seawater being sent to an injection pump 51 to raise the pressure high enough to overcome frictional pressure losses along downstream systems, including the pipes and injection wells and the carbonate containing formation (which may contain limestone, dolomite or other carbonate containing rock).

Valve 44 is an automated valve that may be used to regulate the system outlet operating pressure to ensure that the CO2 is absorbed at the desired pressure. Valve 44 controls the flow to the compressed residual gas to a storage compressor 61 (Figure 3). Valve 45 is an automated valve which may be used to regulate the system outlet operating pressure to ensure that the CO2 is absorbed at the desired pressure when compressor 61 is not operating. Valve 45 controls the flow of the residual gas to a low-pressure stage expander 72 if the compressor 61 is not being used. The CO2 containing seawater (i.e. carbonic acid solution) is pumped up from 11 bara to around 20 bara by the injection pump 51, this pressure depends on the distance to the disposal site, the selected piping sizes, and the depth 54 of the disposal underground formation 55. It flows into injection wells 52 that may commence onshore 53 and penetrate below the ocean into a carbonate bearing disposal reservoir, in the form of offshore, below sea-bed limestone or other carbonate rock formations 55. The operating pressure at all points in the system downstream of the injection pump 51 is maintained above the dissolving pressure to provide a safety margin to ensure that the CO2 always remains in solution.

The injection wells 52 terminate in underground formations 55 that may be hydraulically connected to the ocean above and are deep enough to ensure that the water pressure within the rock is significantly above the vapour pressure of the CO2 in the carbonic acid solution. The minimum top-of-reservoir depth required below the lowest astronomical tide (LAT) to achieve 16 bara is around 160m. Reservoirs in the range of 200m to 1,000m are anticipated to be ideal because they are likely to be naturally hydraulically connected to the ocean through cracks and fissures in the underground formation 55 and any overlying rock formations. Deeper depths may not be naturally hydraulically connected and, may therefore require additional drilling through rock strata to form that hydraulic connection. This is undesirable on account of the additional cost of drilling and controlling the different rock strata pressure regimes.

The carbonic acid solution percolates through the underground formation 55 and converts the carbonate to soluble bicarbonate which is carried through natural or artificial (drilled exit wells) paths 56 into the ocean.

In the embodiment shown in Figure 2, the injection wells 52 are drilled from onshore using readily available land-based oil field drilling rigs and equipment. The wells are drilled and cased following simple oil field designs and completed with injection tubing which may be made using chromium and/or nickel containing steels or other suitable materials to resist corrosion by the strongly acidified carbonic acid solution. Alternatively, the injection wells may be drilled from an off-shore platform and pipe may be laid to connect the on-shore processing facility to the off-shore head of the injection well.

The underground formation 55 being accessed is hydraulically connected to the ocean and therefore there will not be any excess pressure at the surface. These wells will therefore not have any risk of 'blowouts' at the wellhead.

The growth of renewable electricity generated by wind and solar PV is creating ever more variable supplies of electricity whose price varies according to the mismatch in supply and demand - these renewable sources in isolation are not 'dispatchable' and this growth in renewable energy generation is creating growing opportunities for arbitrage in the electricity wholesale market.

During periods of low electricity price, the compressed residual gas stream flowing via valve 44 is sent through a gas/liquid separator, in the form of a scrubber 63, to remove residual liquids and is then compressed by the compressor 61 to at least 60 bara. The compressed residual gas is then cooled in heat exchanger 64, cleaned again in scrubber 65 and then sent via valve 66 (which controls the discharge pressure of the pressurised residual gas) to pressurised residual gas storage vessels 67. Although two storage vessels 67 are shown in Figure 3, it will be appreciated that there may be more vessels depending on their capacity and the processing capacity of the off-gas processing circuit.

When the price of electricity is high, compressed residual gas is drawn from the storage vessels 67 and fed into to a turbo-expander 71 which powers an electrical generator 73 to produce electricity for distribution to customers or for the electrical requirements of the carbon dioxide containing off-gas processing circuit. The residual gas from the storage vessel 67 is sent through a heat exchanger 74 to warm the incoming gas to ensure the inlet is free of excess liquids, ensure that the outlets of the expanders do not become too cold, and to avoid generating too much liquid at the outlet. The outlet of the expander 71 feeds to a second heater (on account of the lower pressure nitrogen being cooled in the expanding process) and is then sent to the expander 72 to capture further energy from the pressurised nitrogen gas. The decompressed residual gas from the expander 72 is sent to a further heater 75 to raise the exhaust residual gas temperature above ambient temperature to ensure it disperses safely in the atmosphere above when vented via a stack 76.

During these high price periods the ongoing supply of nitrogen emanating from the off-gas processing circuit is fed directly to a dedicated turbo-expander 72 that receives the nitrogen at a pressure of 11 bara, instead of that stream going via valve 44 to the nitrogen compressor 61 and storage vessels 67. The expander 72 also powers the generator 73 to produce electricity for distribution to customers or for the electrical requirements of the off-gas processing circuit.

It is anticipated that the process described above offers the following advantages:

• The calcium and magnesium bicarbonates that are produced increase and buffer the pH of the oceans where they are released. Marine organisms that form carbonate shells and corals are negatively affected by acidification, and this process helps restore the required pH to ensure healthier oceans;

• Can be made from relatively standardised, existing industrial equipment to make up any required capacity making it easy to add more capacity;

• Mass producible for low cost, rapid deployment globally at the necessary scale; · The carbonate containing formations avoid the need, cost and CO2 production associated with preparing crushed or rubblized limestone for bicarbonate production;

• Utilising pressurised carbonic acid accelerates the carbonate weathering process and allows higher volumes of CO2 to be captured from a given off-gas stream.

Those skilled in the art of the present invention will appreciate that many variations and modifications may be made to the preferred embodiment without departing from the spirit and scope of the present invention.

For example, although the description above relates to using sea water in heat exchangers, the method may involve using freshwater from any suitable water source, such as a reservoir, river or aquifer.

In another example, the carbonate containing formations may not be hydraulically connected to the ocean. Instead, the carbonic acid solution may be injected into carbonate containing formations to form bicarbonate solution which remains within the formations. In this form, the capture and converted CO2 is sequestered underground where the carbonic acid solution undergoes a further chemical reaction to form a stable bicarbonate. According to this example, the carbonate containing formation may be deeper underground that 1000m. The carbonate containing formation may be up to 12km underground. In the claims which follow, and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word "comprise" and variations such as "comprises" or "comprising" are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the apparatus and method as disclosed herein.

In the foregoing description of preferred embodiments, specific terminology has been resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar technical purpose. Terms such as "front" and "rear", "inner" and "outer", "above", "below", "upper" and "lower" and the like are used as words of convenience to provide reference points and are not to be construed as limiting terms. The terms "vertical" and "horizontal" when used in reference to method and plant throughout the specification, including the claims, refer to orientations relative to the normal operating orientation. Furthermore, invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention. Also, the various embodiments described above may be implemented in conjunction with other embodiments, for example, aspects of one embodiment may be combined with aspects of another embodiment to realize yet other embodiments. Further, each independent feature or component of any given assembly may constitute an additional embodiment.

Claims

1. A method of processing gas loaded with carbon dioxide, the method including: (a) compressing the gas to produce a compressed gas and undesirable compressed liquids;

(b) separating the compressed gas from the compressed liquids;

(c) mixing the compressed gas with compressed water so that most of the carbon dioxide dissolves into the compressed water to form a pressurized carbonic acid solution and a stream of compressed nitrogen gas; and

(d) sequestering the pressurized carbonic acid solution.

2. The method defined in claim 1, wherein the method further includes cooling the gas prior to compressing step (a).

3. The method defined in claim 1 or claim 2, wherein the method further includes removing at least some contaminants from the gas prior to compressing step (a).

4. The method defined in any one of the preceding claims, wherein the method includes cooling the compressed gas after compressing step (a).

5. The method defined in claim 4, wherein the cooling step includes transferring heat from the gas or the compressed gas to the water in a heat exchanger to form heated water.

6. The method defined in claim 4 or claim 5, wherein the method includes removing some contaminants from the compressed gas after cooling the compressed gas.

7. The method defined in any one of the preceding claims, wherein the water may be pressurized to a pressure substantially equal to the pressure of the compressed gas prior to mixing step (c).

8. The method defined in any one of the preceding claims, wherein the water is sea water.

9. The method defined in any one of the preceding claims, wherein the compressed gas is formed by two or more cycles of cooling, cleaning and compression steps.

10. The method defined in claim 9, wherein the method may involve two cycles and the compressed gas after the first cycle is at a pressure of 2 to 5 bara (bar absolute) and at a pressure of 10 to 12 bara at the end of the second cycle.

11. The method defined in any one of the preceding claims, wherein the method further includes sequestering the pressurized carbonic acid solution by injecting it into one or more underground formations to form a dissolved bicarbonate solution.

12. The method defined in any one of the preceding claims, wherein the one or more underground formations are offshore and are hydraulically connected to the ocean so that the bicarbonate solution flows into the ocean.

13. The method defined in claim 12, wherein the method includes adjusting the pressure of the pressurized carbonic acid solution to a pressure that is substantially equivalent to the water pressure in the one or more underground formations so that carbon dioxide is retained in the pressurized carbonic acid solution when it enters the one or more underground formations.

14. The method defined in claim 12 or claim 13, wherein the underground formations are 200m to 1000m depth below sea level.

15. The method defined in any one of claims 12 to 14, wherein the method includes forming a hydraulic connection between the one or more underground formations and the ocean.

16. The method defined in claim 15, wherein the hydraulic connection are formed by drilling boreholes from a bed of the ocean to the one or more underground formations.

17. The method defined in any one of claims 12 to 16, wherein the pressurized carbonic acid solution is produced onshore and is supplied to injection wells that are configured to commence onshore and to continue underground into the one or more offshore underground formations.

18. The method defined in any one of the preceding claims, wherein the method further includes recovering energy stored in the compressed residual gas.

19. The method defined in claim 18, wherein the method further comprises storing the compressed residual gas for a period of time before recovering energy from the stored compressed residual gas.

20. The method defined in claim 18 or claim 19, wherein recovering energy stored in the compressed residual gas comprises generating electricity by passing the compressed residual gas through an expander.

21. The method defined in any one of claims 1 to 17, wherein the method includes splitting the compressed residual gas into first and second streams and recovering stored energy from the first stream by passing the first stream through an expander to generate electricity and storing the second stream of the compressed residual gas.

22. The method defined in claim 20 or claim 21, wherein the method further includes utilizing the generated electricity in any one or more of steps (a) to (c).

23. The method defined in any one of claims 20 to 22, wherein the expander is a multi-stage expander and the compressed residual gas is heated between expander stages.

24. The method defined in any one of claims 18 to 23, wherein the compressed residual gas is prepared for storage by further compression to a pressure in the range of 60 to 300 bara.

25. The method defined in any one of the preceding claims, wherein the underground formations comprise limestone or dolomite formations.

26. A method of forming a bicarbonate solution, the method including:

(a) compressing a gas loaded with carbon dioxide to produce a compressed gas and undesirable compressed liquids;

(b) separating the compressed gas from the compressed liquids;

(d) exposing the pressurized carbonic acid solution into one or more carbonate containing formations such that the carbonic acid reacts with the one or more underground formations to produce the bicarbonate solution.

27. The method defined in claim 26, wherein the pressurised water may be pressurised sea water.

28. The method defined in claim 27 or claim 28, wherein the one or more carbonate containing formations are offshore and the pressurized carbonic acid solution is injected into the carbonate containing formation via injection wells that commence onshore and that terminate in the one or more offshore carbonate containing formations.

29. The method defined in any one of claims 26 to 28, wherein the carbonate containing formations are hydraulically connected to the ocean.

30. The method defined in any one of claims 26 to 29, wherein the carbonate containing formations comprise limestone or dolomite formations.

31. A carbonic acid sequestration plant, the plant including:

(a) an apparatus that converts gas loaded with carbon dioxide into a carbonic acid solution; and (b) an injection system that connects the apparatus to one or more carbonate containing formations that are hydraulically connected to a body of sea water; and wherein the injection system is operable to deliver a carbonic acid solution produced by the apparatus to the one or more carbonate containing formations.

32. The plant defined in claim 31, wherein the one or more carbonate containing formations are offshore and the injection system includes injection wells that commence onshore and that terminate in the one or more offshore carbonate containing formations

33. The plant defined in claim 31 or claim 32, wherein the plant is located within 50 km of the one or more carbonate containing formations.

34. The plant defined in claim 31 or claim 32, wherein the plant is located within 25 km of the one or more carbonate containing formations.

35. The plant defined in claim 31 or claim 32, wherein the plant is located within 10 km of the one or more carbonate containing formations.

36. The plant defined in any one of claims 31 to 35, wherein the carbonate containing formations comprise limestone or dolomite formations.

37. The method defined in any one of claims 1 to 30 or the plant defined in any one of claims 31 to 36, wherein the gas loaded with carbon dioxide contains at least 0.04 mol % CO2.

38. The method defined in any one of claims 1 to 30 or the plant defined in any one of claims 31 to 36, wherein the gas loaded with carbon dioxide contains at least

1 mass % CO2.

39. The method defined in any one of claims 1 to 30 or the plant defined in any one of claims 31 to 36, wherein the gas loaded with carbon dioxide contains at least

2 mass % CO2.

40. The method defined in any one of claims 1 to 30 or the plant defined in any one of claims 31 to 36, wherein the gas loaded with carbon dioxide contains at least 5 mass % CO2.

41. The method defined in any one of claims 1 to 30 or the plant defined in any one of claims 31 to 36, wherein the gas loaded with carbon dioxide contains at least 10 mass % CO2.

42. The method defined in any one of claims 1 to 30 or the plant defined in any one of claims 31 to 36, wherein the gas loaded with carbon dioxide contains at least

15 mass % CO2.

43. The method defined in any one of claims 1 to 30 or the plant defined in any one of claims 31 to 36, wherein the gas loaded with carbon dioxide contains at least 20 mass % CO2.