WO2023282735A1

WO2023282735A1 - Method of processing gas loaded with carbon dioxide

Info

Publication number: WO2023282735A1
Application number: PCT/MY2022/000004
Authority: WO
Inventors: Edwin Stephen Willis; Jeremy Duncan Stuart Joynson
Original assignee: Cquestr8 Sdn. Bhd.
Priority date: 2021-07-09
Filing date: 2022-06-22
Publication date: 2023-01-12

Abstract

A method of controlling the pH of a carbon dioxide sequestration stream to be released into a body of water is disclosed. The method comprises combining a gas loaded with carbon dioxide with a first brine stream and carbonate to produce a reaction stream. The method further comprises controlling the pH of the reaction stream by reacting the reaction stream with an hydroxide-rich stream that is produced by electrolysis of a brine. The reaction is controlled to produce the sequestration stream having the predetermined pH by controlling a supply rate of the reaction stream and/or by controlling a supply rate of the hydroxide rich stream.

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 sequestering carbon and increasing total alkalinity of the oceans.

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, ironmaking and steel-making processes, biological processes and direct air capture processes.

BACKGROUND

Carbon dioxide dissolves In raindrops, some of which then fails on or percolates through carbonate bearing rocks, such as limestone and dolomite, in the earth's crust. The water, now containing a very weak carbonic acid solution, slowly reacts with the formations to form stable bicarbonates, such as bicarbonates of calcium (Ca{HC03)2) and magnesium (Mg(HC0₃}₂), which then flow into the oceans. The frequent and very large amounts of rainfall globally together with the long residence times within the rock formations allows this very siow reaction to progress sufficiently to earn/ billions of tonnes of bicarbonates into the oceans. This is referred to genericaily as limestone weathering, a process which adds to the total alkalinity of the oceans.

Bicarbonate is already a naturally occurring component of the world's oceans which contain an estimated 37,000 gigatonnes of carbon as bicarbonate ions. Bicarbonate is a pH buffer that contributes to maintaining a suitable pH for marine life. However, the quantity of bicarbonate being produced naturally is not enough to overcome the negative impact of the recent ongoing natural absorption by the oceans of carbon dioxide directly from the atmosphere and via rivers conveying carbon dioxide-laden rains to the oceans that doesn't flow through limestones or other calcium or magnesium bearing rocks. This heightened carbon dioxide addition is gradually overcoming the ability of the ocean salts to buffer the incoming carbon dioxide and the pH of the oceans is failing, and adversely Impacting the balance of marine life.

There are proposals for accelerated weathering of limestone (AWL) tb strengthen the natural chemical buffer that exists to resist the ongoing

The proposals are based on the simple rection of dissolving carbon dioxide in water and bringing the solution in contact with solid limestone (CaCO₃) to create a calcium bicarbonate solution,, which may be expressed as;

When carbon dioxide is dissolved in water it forms carbonic acid, termed a weak acid, by partially dissociating into hydrogen ions and bicarbonate ions.

One problem with proposals based on this reaction is reaction kinetics. Specifically, the carbonic acid is consumed in the reaction reducing its strength which slows the reaction such that some carbonic acid remains in the bicarbonate-rich product stream. In other words, if the input quantities of carbon dioxide and limestone are stoichio metrically equivalent, then the strength of the carbonic add reduces as the carbon dioxide is consumed. The concentration gradually becomes too low to complete the reaction in any economically sized reactor. The remaining acid, produced by the residual carbon dioxide dissolved in the calcium bicarbonate product stream, lowers the pH below that of the ocean background pH, Despite the presence of increased calcium bicarbonate in the water, the residual carbonic acid is also discharged into the oceans reducing the benefits of the added calcium bicarbonate and can be locally more damaging to species that engage in biogenic calcification (use limestone as the basis for their structures), such as corals, than the ongoing ocean acidification caused by direct dissolving of carbon dioxide into the oceans from the atmosphere.

One option that addresses the problem of residual carbonic acid Involves neutralising the residual acid by adding calcium oxide (also called lime). Calcium oxide is not a naturally occurring mineral. It must be manufactured in an industrial process that typically involves roasting limestone (CaCO₃) above 848°C in a kiln to remove carbon dioxide and leaving behind solid calcium oxide (CaO). This can be converted to calcium hydroxide Ca(OH)₂ by adding water, The other periodic table Group 1 (including sodium, potassium) or Group 2 metals (including magnesium), when in the form of hydroxides, can also be used to raise the pH, but none of them exist in nature and must also he manufactured industrially. These are all known as strong alkalis. The problem with this option is that the carbon dioxide emissions associated with (a) burning carbon -based fossil fuels to roast the limestone and (b) the carbon dioxide evolved from converting limestone to lime off-set all of the benefits of sequestering carbon dioxide by converting it Into a bicarbonate unless there is an additional sequestration route for the evolved carbon dioxide. By way of Illustration, a lime production facility which produces 1 million tonnes/year of lime also produces around 0,78 million tonnes/year of carbon dioxide from the limestone and a similar amount of carbon dioxide from the burning of carbon fuels to achieve the limestone roasting temperature.

An alternative approach to the reaction in Equation 1 accompanied by neutralisation with a strong alkali involves generating an hydroxide rich-stream of liquid and contacting It with carbon dioxide gas or with a carbonic acid solution, for example, the hydroxide rich-stream of liquid can be generated by electrolysis of seawater which lowers the concentration of hydrogen ions by evolving both hydrogen and chlorine gases. The hydroxide rich-stream is richer in hydroxide ions and the pH of the residual water increases. For example, in the case of seawater with pH 8, the resultant electrolysed stream will have a pH noticeably higher than pH 8. This allows for the hydroxide rich-stream to be mixed with carbon dioxide gas or with a carbonic acid solution and for the pH of the mixture to be controlled to be very dose to the background pH of the ocean.

However, in the case of electrolysis of sea water, the process typically generates a combination of hydrogen and chlorine gases at the electrodes. These gases are designated for industrial use and may be combined to form hydrochloric acid, and variously reacted with waste industrial or mining streams that have high pH; or neutralised by reacting with minerals including silicates. For example, asbestos, a dangerous industrial waste Is also a magnesium rich silicate. If hydrochloric acid is added to carbonate minerals, then the reaction releases carbon dioxide, which largely reverses the earlier described benefit of residual carbonic acid neutralisation using hydroxide-rich seawater from the electrolysis process.

A further problem with sequestration processes that purely utilise electrolysis of sea water to achieve the capture and neutralisation of carbon dioxide is that the process requires a large scale process and, therefore, very high electrical Input. Such processes are high cost and, therefore, economically unattractive. Furthermore, the process requires so much electricity that, if using electricity generated from fossil fuel, it is uncertain whether the carbon dioxide sequestered is sufficient to offset the carbon dioxide emitted in generating the power. A variation of the electrolysis option involves raising the pH sufficiently that the solubility limit of calcium carbonate is exceeded, and a solid precipitate is formed. The precipitate is sequestered safely in the ocean and won't have a negative Impact on the pH of the ocean, However, the reaction which forms the precipitate also releases some carbon dioxide. So, in addition to the same problems mentioned above (namely cost, disposal of the hydrogen and chlorine gas by-products apply and how much carbon dioxide is sequestered compared to how much is emitted in generating the required electrical input to this variation)., there is additional carbon dioxide generated which further off-sets the carbon dioxide sequestration benefits of this process.

The above references to processes, options and variations do not constitute an admission that those processes, options and variations form part of the common general knowledge of a person of ordinary skill in the art. Those references are also not intended to limit the application of the scope of the piant arid process disclosed herein.

SUMMARY OF THE DISCLOSURE

In a first aspect, there is provided a method of controlling the pH of a carbon dioxide sequestration stream to be released into a body of water, the method including:

(a) combining a gas loaded with carbon dioxide with a first water stream and a first reactant including carbonate or silicate mineral of one or more alkali or alkaline-earth metals to produce a first reaction stream including alkali metal or alkaline earth metal ions, bicarbonate ions and an excess of unreacted carbon dioxide; and

(b) producing the sequestration stream by reacting the first reaction stream with an hydroxide-rich stream that is produced by electrolysis of a first brine stream; and wherein the reaction in step (b) is controlled to produce the sequestration stream having a predetermined pH by controlling a supply rate of the first reaction stream and/or by controlling a supply rate of the hydroxide rich stream.

The predetermined pH may be in the range of 7.5 to 8.5,

The predetermined pH may be the pH of a body of water into which the sequestration stream Is to be released.

The predetermined pH may be 8,0, The method may include electrolysing the first brine stream to produce the hydroxide-rich stream.

The electrolysis may also produce hydrogen gas and chlorine gas.

The method may include forming the strong add by reacting the hydrogen gas with the chlorine gas to form hydrogen chloride gas.

The method may Include reacting the hydrogen gas with the chlorine gas in a fuel cell to produce the hydrogen chloride gas and to generate electricity.

The method may include controlling the pH of the sequestration stream by mixing a second reaction stream with the first reaction stream,

In one embodiment, the method includes forming the second reaction stream by mixing a second water stream with a strong acid-forming material to produce an acidic brine solution and mixing the acidic brine solution with a second reactant including carbonate or silicate mineral of one or more alkali or alkaline-earth metals to produce the second reaction stream and produce evolved carbon dioxide.

Mixing the acidic brine solution with the second reactant may comprise passing the acidic brine solution through a packed bed of the second reactant.

In another embodiment, the method includes forming the second reaction stream by mixing a second reactant including carbonate or silicate mineral of one or more alkali or alkaline-earth metals with a second water stream and then mixing the mixture of the second water stream and the second reactant with a strong acid-forming material to produce the second reaction stream and if the reactant is a carbonate produce evolved carbon dioxide.

The method may include separating the second reaction stream from the evolved carbon dioxide and include mixing the evolved carbon dioxide with the hydroxide rich stream.

The second reaction stream may be pH neutral or alkaline.

The strong acid forming material may be hydrogen chloride gas. Step (b) may include producing the sequestration stream by reacting the first reaction stream., the second reaction stream and the hydroxide-rich stream and controlling a supply rate of the first reaction stream, the second reaction stream and/or the hydroxide rich stream so the sequestration stream has the predetermined pH.

A portion of the electricity generated by the fuel ceil may be supplied for the electrolysis of the second brine stream as part of the total electrical input requirement to electrolysis.

The method further includes diluting the reaction stream by mixing the first reaction stream with a third water stream.

The hydroxide-rich stream is mixed with the third water stream upstream of where the first reaction stream is mixed with the third water stream.

The evolved carbon dioxide Is mixed with the third water stream upstream of the location where the first reaction stream is mixed with the third wafer stream, The second reaction stream is mixed with the first reaction stream downstream of the location where the first reaction stream is mixed with the third water stream.

The gas loaded with carbon dioxide is conditioned prior to mixing it with the first water stream_:.

The gas loaded with carbon dioxide is conditioned by compression to an elevated pressure.

The gas loaded with carbon dioxide is conditioned by removing contaminants. The gas loaded with carbon dioxide is conditioned by changing the temperature of the gas.

One or more of the first,, second and third water streams and the first brine stream may be pressurized to a pressure substantially equal to the pressure of the compressed gas prior to mixing step.

The first water stream may be sea water or fresh water. The second water stream may he sea water or fresh water.

The third water stream may be sea water or fresh water.

The first brine stream may be sea water.

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 gas loaded with carbon dioxide may contain at least 80 mass % CO₂.

The gas loaded with carbon dioxide may contain at least 90 mass % CO₂.

Aside from incidental impurities,, the gas loaded with carbon dioxide may contain 100 mass % CO₂.

In a second aspect, there is provided a plant for producing a carbon dioxide sequestration stream having a controlled pH, the plant including:

(a) a sea water conditioner which prepares the sea water for subsequent treatment;

(b) a first mixer that is adapted to mix carbon dioxide with a first stream of sea water from the sea water conditioner

(c) a first reactor configured to contact carbonate material with the first stream of sea water; (d) an eiectrolyser adapted to electrolyse a second stream: of sea water from the sea water conditioner to produce chlorine gas, hydrogen gas and a hydroxide- rich water stream; and

(e) a second mixer that is configured to combine the first stream of sea water after passing through the mixer and the first reactor with the hydroxide rich stream and with a third stream of sea water from the conditioner and wherein the second mixer enables the supply of the first stream of sea water, the hydroxide rich stream and the third stream of sea water from the sea water conditioner to be controlled so that the output from the second mixer is a sequestration stream with a controlled pH,

The first mixer may be upstream or downstream of the first, reactor.

The plant may further include a fuel cell that is configured to react the hydrogen gas and the chlorine gas to produce hydrogen chloride gas and to generate electricity.

The fuel ceil is electrically connected to the eiectrolyser to supply generated electricity to the eiectrolyser. The plant further includes;

(a) a third mixer that is adapted to mix a fourth stream of sea water from the sea water conditioner with the hydrogen chloride gas from the fuel cell to produce a hydrochloric acid solution; and

(b) a second reactor that which is configured to contact carbonate material with the fourth stream of sea water; and wherein the second mixer is adapted to receive the fourth stream of sea water after passing through the third mixer and the second reactor.

The third mixer may be upstream or downstream of the second reactor.

The fourth stream of sea water after passing through the third mixer and the second reactor may include evolved carbon dioxide gas and the plant may Include a separator which is configured to separate the fourth stream into an evolved carbon dioxide gas stream and a reaction stream.

The second mixer may comprise a series of sub-mixers and respective sub-mixers are configured to mix the third stream of sea water from the sea wafer conditioner in respective sub-mixers with one of (i) the first stream of sea water after passing through the mixer and the first reactor, {»} the hydroxide rich stream and (»i) the fourth stream of sea water after passing through the third mixer and the second reactor, so that an effluent stream from the last sub-mixer comprises the sequestration stream.

Ordinal references to aspects disclosed above serve to differentiate the aspects from one another only. The ordinal references are not to be interpreted as the order of importance of the aspects.

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 i is a schematic flow chart of an embodiment of the method of controlling the pH of a carbon dioxide sequestration stream; and

Figure 2 is a schematic flow chart of another embodiment of the method of controlling the pH of a carbon dioxide sequestration stream.

DESCRIPTION OF EMBODIMENT

Embodiments of the above disclosed aspects will now be described in the following text which includes reference numerals that correspond to features illustrated in the accompanying Figures.

One embodiment of a method of controlling the pH of a carbon dioxide sequestration stream to be released Into a body of water is shown in Figure 1,

In this embodiment, first second and third streams 1, 6 and 13 of brine are utilised to form a sequestration stream 18. In this embodiment, the brine streams l, 6 and 13 are streams of sea water. The sea water is pumped from the ocean, filtered, and treated in a similar manner to any number of industrial processes that make use of seawater such as seawater used to cool power plants and to feed reverse osmosis membranes. The seawater is then split into the first second and third streams 1, 6 and 13. First and second streams 1 and 6 are smaller than third stream 13. The first stream I is fed by a pump 26 into a mixer 28 into which a carbon dioxide stream 2 is also fed. The carbon dioxide stream 2 mixes with the first stream 1 to produce an acidic carbonic add stream 3. The acidic carbonic add stream 3 is supplied to a packed bed reactor 30, which may be a packed bed reactor, where it is contacted with a reactant supply 4, The reactant supply 4 in this embodiment is a supply of ground limestone, a mineral comprising mainly calcium carbonate. In other embodiments, however, the reactant may be another carbonate, for example, dolomite, a mineral rich in calcium carbonate or magnesium and calcium carbonate, such as limestone or dolomite. Alternatively, the reactant may be a calcium-rich silicate or a magnesium rich-silicate. Examples of suitable silicates include basalt, o!ivlne, talc, and asbestos,

In this embodiment with calcium carbonate as the reactant, the reaction according to

Equation 1 occurs between the acidic carbonic acid stream 3 and the reactant supply 4 to produce a first reaction stream 5 which Is rich in calcium bicarbonate and, owing to the diminishing kinetics of the reaction, a residua! amount of carbonic add (i.e, dissolved carbon dioxide), which makes the first reaction stream 5 slightly acidic.

The packed bed reactor 30 comprises a packed bed of calcium carbonate particles, In an alternative however, the calcium carbonate may be reduced in size further to enable contact with the acidic carbonic acid stream 3 in a fluidised bed reactor. Therefore, the packed bed reactor 30 may be replaced with any suitable reactor that enables the carbonate supply to be reacted with the acidic carbonic acid stream 3. A second brine stream 6 is fed by a pump 22 into an electrolyser 45, The eiectrolyser 30 receives a supply of electricity arid is configured to produce hydrogen gas and chlorine gas at the electrodes. The electrolyser 45 also produces an output stream 7 enriched with hydroxide ions. The hydroxide-rich stream 7, having been depleted of hydrogen and chloride ions, is now rich in hydroxide ions and is therefore more alkaline than the second stream 6.

A third brine stream 13 Is fed by a pump 22 a mixer 34 where it is combined with the hydroxide- rich stream 7 to produce a diluted hydroxide- rich stream 14, The diluted hydroxide-rich stream 14 is combined with the first reaction stream 5 in a mixer 38. The available hydroxide neutralises the residual acid and increases the pH of the sequestration stream 18. The combination of the first reaction stream 5 with the diluted hydroxide-rich stream 14 produces a sequestration stream 18, The feed rates of the first reaction stream 5 and the diluted hydroxide-rich stream 14 determine the pH of the sequestration stream 18. The feed rates can he adjusted to provide the sequestration stream 18 with a predetermined pH. However, the relative flow rates of the first reaction stream 5 and the diluting sea water stream 13 are controlled to ensure that the solubility limit of calcium carbonate is not exceeded. If it is exceeded_,, calcium carbonate may precipitate which would cause a proportion of the newly sequestered carbon dioxide to be evolved. According, the flow rates are controlled so that repredpitation of calcium carbonate mineral and re-evolution of carbon dioxide is avoided. Given that the sequestration stream 18 is Intended to be supplied to a body of water, such as an ocean or another body which feeds into an ocean, the predetermined pH is equal to the pH of the ocean or very slightly higher than the pH of the ocean If there is a need to reverse acidification of the body of water in that region. Accordingly, the sequestration stream 18 may have a predetermined pH that is adjusted to match the pH of the body of water, The body of water may be a body of sea water. In this embodiment, the predetermined pH is the pH of the incoming seawater which forms the first, second and third streams 1, 6 and 13.

This embodiment enables the sequestration stream 18 to have a pH which is consistent with the pH of the ocean into which will be delivered and, therefore, the sequestration stream (on a large-scale and over a !ong period of time) won't affect the overall pH of the ocean. Alternatively, the sequestration stream may have a pH that is slightly higher than of the body of water to counter prior acidification of the body over time. The pH is selected to avoid harming the local biota. Furthermore, the sequestration stream 18 carries the sequestered carbon dioxide in the safe and stable form of calcium bicarbonate solution. in a second embodiment shown in Figure 2, a strong acid is used to dissolve carbonate into a stream as a dissolved alkali and to use the dissolved alkali to neutralise residual acid in the reaction stream. The process of the second embodiment builds upon the process of the first embodiment described above in respect of Figure 1 and, therefore, the same reference numerals are used In Figure 2 and in the following description to describe the same process features.

In this second embodiment, the hydrogen gas and the chlorine gas from the e!ectrolyser 45 are reacted to form hydrogen chloride gas 9, and this reaction may be performed In a fuel ceil 46 to recover electrical energy. Hydrogen chloride gas 9 forms a strong acid in water. The reaction in the fuel cel! 46 generates electricity. In this embodiment, the electricity from the fuel cel! 46 is supplied to the electroiyser 45 as part, of the total electrical input requirement for the electrolysis of the second stream 6. While utilising the electricity generated in the fuel ceil 46 in the electroiyser 45 reduces the total electrical input from external sources to the electroiyser 45, it will be appreciated that it is not essential for the electricity generated in the fuel cell 46 to be supplied to the electroiyser 45, Electroiyser 45 may be supplied with none or only part of the electricity generated In the fuel cell 46,

The second embodiment further includes a fourth brine stream 8 which is fed by a pump 24 to a mixer 34. The hydrogen chloride gas 9 is also supplied to the mixer 34 where it mixes with the fourth brine stream 8. The output from the mixer 34 is a hydrochloric acid solution 10. The hydrochloric acid solution 3,0 is supplied to a further packed bed reactor 32 where it is reacted with a reactant supply 11. The hydrochloric acid solution 10 reacts with the reactant supply 11 to produce a second reaction stream 17. A by-product of this reaction is carbon dioxide gas which is separated from the second reaction stream 17 by a separator 42 to form a separate evolved carbon dioxide gas stream 12, Alternatively, the evolved carbon dioxide may be kept in suspension as a separate gas phase carried along in the second reaction stream 17. As with the first embodiment, the reactant supply 11 is limestone. Accordingly, the second reaction stream 17 comprises dissolved calcium chloride,

However, the reactant supply 11 may be different to the reactant supply 4, For example, the reactant supply 11 may be a carbonate or silicate mineral of an alkali or alkaline-earth metal, for example a calcium silicate-rich mineral. Not having any carbonate present will similarly dissolve in the hydrochloric acid solution 10 to produce a dissolved alkali stream rich in dissolved calcium ions but, as there is no carbonate, there will not be any evolved carbon dioxide gas. In this case twice as much of the incoming carbon dioxide can be sequestered by the calcium ions formed from calcium silicate and less acid is therefore required.

In this embodiment, the reactant supply 11 supplied to the packed bed reactor 32 is from the same source of reactant that forms the reactant supply 4 to the packed bed reactor 30. In other words, the reactant source is split into two streams which form the respective reactant supply 4 which is supplied to the packed bed reactor 30 and the reactant supply 11 which is supplied to the packed bed reactor 32. As with the first embodiment, the packed bed reactor 32 may be replaced with an alternative reactor which is suitable for contacting the hydrochloric acid solution 10 with the reactant supply ii to form the second reaction stream 17,

The sequestration stream 18 is produced by combining the diluted hydroxide-rich stream 14 with the evolved carbon dioxide gas stream 12, the first reaction stream 5 and the second reaction stream 17 and by controlling the feed rates of these streams to arrive at a predetermined pH, More specifically, . the sequestration stream 18 is produced by combining the diluted hydroxide-rich stream 14 with the evolved carbon dioxide gas stream 12 in a mixer 36 to produce a first intermediate stream 15. The intermediate stream 15 is mixed with the first reaction stream 5 in a mixer 38 to produce a second intermediate stream 16, The second intermediate stream 16 is mixed with the second reaction stream 17 in a mixer 40. The output from the mixer 40 is the sequestration stream 18.

The relative flow rates of the evolved carbon dioxide stream 12, the diluted hydroxide-rich stream 14, the first reaction stream 5, and the second reaction stream 17 are controlled to provide the sequestration stream with the predetermined pH. In this embodiment, the predetermined pH is the pH of the incoming seawater which forms the first, second, third and fourth streams 1, 6, 13 and 8. However, given that the sequestration stream is intended to be supplied to a body of water, such as an ocean or another body which feeds into an ocean, the predetermined pH is the pH of the ocean, Accordingly, the sequestration stream 18 may have a predetermined pH that matches the pH or is slightly higher than the pH of the body of water. The body of water may be an ocean, for example the Arctic Ocean, the Atlantic Ocean, the Indian Ocean, the Pacific Ocean or the Southern Ocean, into which it will be delivered. As with the first embodiment, the sequestration stream may have a pH that is slightly higher than of the body of water to counter prior acidification of the body over time, The pH is selected to avoid harm to the local biota.

Furthermore, the relative flow rates of the evolved carbon dioxide stream 12, the diluted hydroxide-rich stream 14, the first reaction stream 5, and the second reaction stream 17 may be adjusted to ensure that the solubility limit of calcium carbonate is not exceeded. If it is exceeded, then calcium carbonate may precipitate causing carbon dioxide to be evolved. Accordingly, the flow rates are controlled so that reprecipitation of calcium carbonate mineral and re-evolution of carbon dioxide Is avoided.

The order In which the evolved carbon dioxide stream 12, the diluted hydroxide·· rich stream 14, the first reaction stream 5, and the second reaction stream 17 are mixed can be other than described above and shown in Figure 2, provided that the flow rates are controlled so that the sequestration stream 18 has the predetermine pH.

While the above description is in the context of limestone being the carbonate, it will be appreciated that other carbonates may be utilised in place of or in conjunction with calcium carbonate. Accordingly, the application of the above embodiments is not limited to calcium carbonate as the carbonate in the process.

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, while the above embodiments describe that the hydroxide- rich stream is produced from a brine comprising sea water, the brine may he any other alternative brine. That Is, the stream 6 to the eiectrolyser, in the first embodiment and in the second embodiment, may be from a source that is separate from the source of the streams 1, 8 and 13 that form the basis of the second reaction stream 17, the first reaction stream 5 and the diluting stream 13. The brine may be a naturally occurring brine. Additionally, the brine is not limited to sodium chloride- based brines, However, the reaction products from electrolysis of the brine ideally should be compatible with the chemistry of the body of water to avoid damaging the environment. For example, the brine may be a sulfur- based brine for bodies of water that are sea water. The brine may be a product of an industrial process. For example, the brine may be a product of a mining process. As another example of an industrial process that produces a brine, the brine may be from a reverse osmosis process, such as a desalination process. It follows that the brine may be a brine stream from a desalination process.

As another example, any one or more of the first and second brine streams 1, 13 in the first embodiment and the fourth brine stream 8 in the second embodiment may be fresh water streams. Accordingly, the term "water stream” used in the claims is taken to mean a brine stream or a fresh water stream. 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

L A method of controlling the pH of a carbon dioxide sequestration stream to be released into a body of water, the method comprising: (a) combining a gas loaded with carbon dioxide with a first water stream and a first reactant including carbonate or silicate mineral of one or more alkali or alkaline-earth metals to produce a first, reaction stream including alkali metal or alkaline earth metal ions, bicarbonate ions and an excess of unreacted carbon dioxide; and (b) producing the sequestration stream by reacting the first reaction stream with an hydroxide-rich stream that is produced by electrolysis of a first brine stream; and wherein the reaction in step (b) is controlled to produce the sequestration stream having a predetermined pH by controlling a supply rate of the first reaction stream and/or by controlling a supply rate of the hydroxide-rich stream.

2. The method defined in claim 1, wherein the predetermined pH is in the range of 7,5 to 8,5.

3, The method defined in claim 1, wherein the predetermine pH is the pH of a body of water into which the sequestration stream is to be released,

4, The method defined in claim 1, wherein the predetermined pH is 8,0,

5 , The method defined in any one of the preceding claims, wherein the method includes electrolysing the first brine stream to produce the hydroxide-rich stream.

6. The method defined in claim 5, wherein the first brine stream is sea water.

7, The method defined in claim 5 or claim 6, wherein the electrolysis aiso produces hydrogen gas and chlorine gas,

8. The method defined in any one of the preceding claims., wherein the method includes controlling the pH of the sequestration stream by mixing a second reaction stream with the first reaction stream.

9. The method defined in claims 8, wherein step (b) may include producing the sequestration stream by reacting the first reaction stream, the second reaction stream and the hydroxide-rich stream and controlling a supply rate of the first reaction stream, the second reaction stream and/or the hydroxide rich stream so the sequestration stream has the predetermine pH.

10. The method defined in claim 9, wherein the method includes forming the second reaction stream by mixing a third water stream with a strong acid-forming material to produce an acidic brine solution and mixing the acidic brine solution with a second reactant including carbonate or silicate mineral of one or more alkali or alkaline-earth metals to produce the second reaction stream and if the reactant is a carbonate produce evolved carbon dioxide.

11. The method defined In claim 10, wherein the method includes mixing the acidic brine solution with the second reactant by contacting the acidic brine solution with the second reactant in a reactor,

12. The method defined in claim 11, wherein the reactor is a packed bed reactor or is a fluidised bed reactor,

13. The method defined in claim 9, wherein the method Includes forming the second reaction stream by mixing a second reactant Including carbonate or silicate mineral of one or more alkali or alkaline-earth metals with a third water stream and then mixing the mixture of the third water stream and the second reactant with a strong acid-forming material to produce the second reaction stream and produce evolved carbon dioxide.

14. The method defined in any one of claims 10 to 13, wherein the method includes separating the second reaction stream from the evolved carbon dioxide and includes mixing the evolved carbon dioxide with the first reaction stream.

15. The method defined In any one of claims 10 to 14, wherein the method includes forming the strong acid-forming materia! by reacting the hydrogen gas with the chlorine gas to form hydrogen chloride gas.

16. The method defined in claim 15, wherein the method includes reacting the hydrogen gas with the chlorine gas in a fuel ceil to produce the hydrogen chloride gas and produce electricity,

17. The method defined in claim 16, wherein the electricity produced in the fuel cell is supplied, at least in part, to electrolysis of the first brine stream as part of the total electrical input requirement to electrolysis,

18. The method defined in any one of the preceding claims, wherein the method further includes diluting the first reaction stream by mixing the first reaction stream with a second water stream.

19. The method defined in claim 18, wherein the hydroxide-rich stream is mixed with the second water stream upstream of where the first reaction stream is mixed with the second water stream.

20. The method defined in claim 19, wherein the evolved carbon dioxide is mixed with the second water stream upstream of the location where the first reaction stream is mixed with the second water stream,

21. The method defined in any claim 19 or claim 20, wherein the second reaction stream is mixed with the first reaction stream downstream of the location where the first reaction stream is mixed with the second water stream.

22. The method defined in any one of the preceding claims, wherein the gas loaded with carbon dioxide is conditioned prior to mixing it with the first water stream.

23. The method defined in claim 22, wherein the gas loaded with carbon dioxide is conditioned by compression to an elevated pressure.

24. The method defined in claim 22, wherein the gas loaded with carbon dioxide is conditioned by removing contaminants.

25. The method defined in claim 22, wherein the gas loaded with carbon dioxide is conditioned by changing the temperature of the gas.

26. The method defined in claim 22, wherein the one or more of the first., second and third water streams and the first brine steam are pressurized to a pressure substantially equal to the pressure of the compressed gas prior to mixing step,

27. The method defined in any one of the preceding claims, wherein the gas loaded with carbon dioxide may contain at least 20 mass % CO₂.

28. The method defined in any one of claims 1 to 26, wherein the gas loaded with carbon dioxide may contain at least 30 mass % CO₂.

29. The method defined in any one of claims 1 to 26, wherein the gas loaded with carbon dioxide may contain at least 80 mass % CO₂.

30. The method defined in any one of claims 1 to 26, wherein the gas loaded with carbon dioxide may contain at least 90 mass % CO₂.

31. The method defined in any one of claims 1 to 26, wherein, aside from incidental impurities, the gas loaded with carbon dioxide contains 100 mass % CO₂.

32. The method defined in any one of the preceding claims, wherein any one or more of the first water stream, the second water stream and the third water stream are sea water or fresh water.

33. A plant for producing a carbon dioxide sequestration stream having a controlled pH, the plant including:

(b) a first mixer that is adapted to mix carbon dioxide with a first stream of sea water from the sea water conditioner;

(c) a first reactor configured to contact carbonate material with the first stream of sea water;

(d) an electrolyser adapted to electrolyse a second stream of sea water from the sea water conditioner to produce chlorine gas, hydrogen gas and an hydroxide- rich water stream; and

(e) a second mixer that is configured to combine the first stream of sea wafer after passing through the mixer and the first reactor with the hydroxide-rich stream and with a third stream of sea water from the conditioner and wherein the second mixer enables the supply of the first stream of sea water, the hydroxide- rich stream and the third stream of sea water from the sea water conditioner to be controlled so that the output from the second mixer is a sequestration stream with a controlled pH,

34. The plant defined in claim 33, wherein the first mixer is upstream or downstream of the first reactor.

35. The plant defined in claim 33 or claim 34 further including a fuel cell that is configured to react the hydrogen gas and the chlorine gas to produce hydrogen chloride gas and to generate electricity.

36. The plant defined in claim 35, wherein the fuel ceil is electrically connected to the electrolyser to supply generated electricity to the elect rolyser.

37. The plant defined in any one of claims 33 to 36, wherein the plant further includes:

38. The plant defined in claim 37, wherein the third mixer is upstream or downstream of the second reactor,

39. The plant defined in claim 37 or claim 38, wherein the fourth stream of sea water after passing through the third mixer and the second reactor includes evolved carbon dioxide gas and the plant Includes a separator which is configured to separate the fourth stream Into an evolved carbon dioxide gas stream and a reaction stream.

40. The plant defined in any one of claims 37 to 39, wherein the second mixer comprises a series of sub-mixers whereby respective sub-mixers are configured to mix the third stream of sea water from the sea water conditioner in respective sub- mixers with one of (i) the first stream of sea water after passing through the mixer and the first reactor, (ii) the hydroxide rich stream, (Hi) the reaction stream and (iv) the evolved carbon dioxide gas stream, so that an effluent stream from a last sub- mixer comprises the sequestration stream,