WO2015052325A1

WO2015052325A1 - Capture of carbon dioxide

Info

Publication number: WO2015052325A1
Application number: PCT/EP2014/071800
Authority: WO
Inventors: Norbert Schmidbauer; Svein Knudsen
Original assignee: NILU - STIFTELSEN NORSK INSTITUTT FOR LUFTFORSKNING
Current assignee: NILU - STIFTELSEN NORSK INSTITUTT FOR LUFTFORSKNING
Priority date: 2013-10-11
Filing date: 2014-10-10
Publication date: 2015-04-16
Anticipated expiration: 2016-04-11

Abstract

A method of treatment of exhaust gases for carbon dioxide capture comprises contacting a combustion gas mixture including carbon dioxide with a solution of sodium bicarbonate and sodium carbonate in water, such that carbon dioxide reacts with hydroxide ions in the solution to form bicarbonate ions; continuing the contacting until sodium bicarbonate precipitates out of the solution as a solid; removing the solid sodium bicarbonate; and (v) heating the sodium bicarbonate to form sodium carbonate, water and carbon dioxide; wherein during steps (i) to (iv) the sodium bicarbonate concentration in the solution is maintained at 90% or more of saturation levels. Advantageously, the process can be made into a continuous recycling loop with the sodium bicarbonate being broken down by heat into sodium carbonate, water and carbon dioxide, and the sodium carbonate being recycled into the solution. The carbon dioxide may be compressed and stored.

Description

CAPTURE OF CARBON DIOXIDE

The present disclosure relates to a method and apparatus for capture of carbon dioxide (C0₂). For example, capture of C0₂ from combustion gases such as flue gases.

Effective C0₂ capture and sequestration is a significant challenge for coming generations in order to reduce man-made global warming. The use of fossil fuels is expected to persist for some time further, especially as new techniques are developed for more efficient extraction of fuels such as natural gas from previously inaccessible reserves and previously depleted wells. For gaseous fossil fuels carbon dioxide can be present as an impurity when the gas is extracted and therefore it is desirably to 'wash' the gas to remove carbon dioxide (and other impurities) prior to use. Clean energy from fossil fuels requires treatment of the exhaust gases produced after combustion, and this requires effective capture of pollutants including C0₂. This applies to numerous areas, for example electrical power production, heating, industrial process, and combustion engines in vehicles including cars, buses, haulage vehicles, trains, ships and the like.

The use of hydrogen instead of hydrocarbons as a main energy source/carrier in a future society has been widely discussed, but thus far has not been possible to implement on a large scale. The use of methanol as a renewable energy has also been proposed in the context of a Methanol Economy. This involves production of methanol using C0₂ amongst other raw materials. Methanol is in many ways superior to hydrogen, for example in terms of safety and storage. However, a prerequisite for a carbon neutral Methanol Economy is that C0₂ used for the production of methanol is captured from the air or from biomass. C0₂ capture from otherwise polluting sources such as fossil fuel combustion would be an advantage in this context. The hydrogen and the energy used for the production of methanol could in a transition phase also derive from fossil fuels as long as the C0₂ produced in that process is sequestered.

Hence, various drivers result in a need for techniques to capture C0₂ from combustion gases and from other sources. There are several industrial processes developed or under development to capture C0₂ from combustion gases. Various known techniques involve different aqueous solutions of alkanolamines, chilled ammonia or different hydroxide solutions to remove C0₂ by either absorption or chemical reaction. However, releasing C0₂ from these absorbents for storage demands quite high amounts of energy, which makes these C0₂ capture processes expensive. It is well known that C0₂ can be physically absorbed in liquid in accordance with Henry's law. Water can be used to absorb C0₂. There are obvious advantages by using water as absorbent. It is cheap, non-poisonous and does not add new compounds to the purified gas stream. There is little corrosion due the low

temperatures used in the processes. Disadvantages are the relative low absorption load which is achievable - hence requiring a high amount of absorbent compared to other absorbents (like alkanol amines etc.). In industrial scrubbing processes - usually the gas is treated with a very thin absorption film on large surfaces. Co- absorption of other gases like N₂ or 0₂ in water is another disadvantage in industrial scrubber philosophy. In traditional gas purification using scrubbing towers aiming at purification of low C0₂ concentrations in large exhaust quantities, water is not the first choice of absorbent. A method of C0₂ capture using water has been proposed in WO 2010/104402. However, although this technique provides significant advantages since it uses water as a solute it cannot avoid the disadvantages of low absorption load and co-absorption of other gases.

US 7727374 discloses a system using a sodium hydroxide ion solution to capture carbon dioxide, which reacts with the sodium hydroxide to form sodium bicarbonate in solution. This sodium hydroxide ion solution is produced by electrolysis, along with the addition of acid. The sodium bicarbonate is extracted from the water using heated precipitation. This process is energy intensive and the electrolysis process produces potentially hazardous waste products in the form of chlorine gas and also hydrogen gas.

There hence remains a need for a simple and cost efficient method and system for capture of C0₂.

Viewed from a first aspect, the disclosure provides a method of treatment of combustion gases for carbon dioxide capture comprising: (i) providing a solution of sodium bicarbonate and sodium carbonate in water; (ii) contacting the solution with a gas mixture, the gas mixture including carbon dioxide, such that carbon dioxide dissolves in water and reacts with hydroxide ions in the solution to form bicarbonate ions; (iii) continuing step (ii) until sodium bicarbonate precipitates out of the solution as a solid; (iv) removing the solid sodium bicarbonate; and (v) heating the sodium bicarbonate to form sodium carbonate, water and carbon dioxide; wherein during steps (i) to (iv) the sodium bicarbonate concentration in the solution is maintained at 80% or more of saturation levels.

In this way carbon dioxide can be effectively captured from the gas mixture by absorption into solution, removal from the solution by means of precipitation and subsequent removal of the sodium bicarbonate. The dissolution of sodium bicarbonate and sodium carbonate in water forms hydroxide, bicarbonate and carbonate ions. Carbon dioxide dissolved into this solution will form bicarbonate ions. The inventors have made the non-obvious realisation that when this is done in a solution close to saturation of sodium bicarbonate then any further carbon dioxide added to the solution will inevitably result in precipitation of sodium bicarbonate as a solid. This is due to the fact that the solubility of sodium bicarbonate in water, which is 96 g/l at 20 °C, will be considerably less than the solubility of sodium carbonate in water, which is 215 g/l at 20 °C. This difference in solubility means that, with an appropriate amount of sodium carbonate in solution, solid sodium bicarbonate will result when sufficient carbon dioxide is added to the solution. One mole of dissolved carbon dioxide reacting with this sodium carbonate / sodium bicarbonate solution will result in precipitation of two moles of sodium bicarbonate. One mole of sodium carbonate needs to be added to renew the solution. The sodium carbonate may be obtained from the calcination of the two moles of precipitated sodium bicarbonate, resulting in a closed loop of involved chemicals for the preferred embodiments of the invention.

A significant advantage with the proposed method is the effective mixing of gas and liquid to facilitate the absorption of C0₂ in the liquid. If solid bed or scrubbers are used, as in some prior art techniques, the relative slow reaction rate of the C0₂ reaction with the OH^" ions results in large and expensive size equipment and low capture rates. In the proposed method, a long residence time and a high reactant volume compensate the slow reaction. In preferred embodiments the method may advantageously make use of a new contactor where gas is introduced via a mixer- injector that creates large numbers of small bubbles of gas in the solution. An advantage of such a contactor is the ability to work with precipitating systems, where scrubbers have a problem. Introducing the combustion gas as bubbles also enables fresh absorbent to be available at all times and the chemistry in the absorbent chamber can be held at the same conditions. This is not the case for scrubbers, where the conditions are different at different heights in the scrubber. In a preferred embodiment the introduction of the bubbles in the absorbent is done with an aspirating turbine, also known as an aerator, one type being manufactured by Heinrich Frings GmbH & Co. KG under the trademark "Friborator", through a manifold where combustion gas is fed into the manifold and ejected into the absorbent as bubbles or by using ultrasonic devices and the like. The fact that combustion gas is introduced in this way will revitalize processes with slow reaction kinetics, such as: carbonate processes, slow amines and amino acid methods. The reaction forms solids that precipitate from the liquid. This enables the process only to treat the solids and not the reactant liquid. The reactant is preferably recycled as discussed further below. The recycling process involves shifting the reactant between phases from liquid to solid and back to liquid. As noted above, carrying out the reaction in solution makes the whole mass available for reaction and not only the surface. Soiling of surfaces is not a problem since the solids are dissolved and reformed and pollutants in the system will not stick to the surfaces. Other gasses and particles in the gas may be absorbed by the liquid: NO_x, metals and particles will to some extent form solids that will precipitate, but will not be re- dissolved in the carbonate resolver and can therefore be separated from the reactant.

The process provides considerable advantages compared to the processes of WO 2010/104402 and US 7727374. It can capture more carbon dioxide with a lower energy usage than these prior art techniques. Compared to US 7727374 there is no need for energy intensive electrolysis or heated precipitation to remove the sodium bicarbonate. There are no hazardous materials like chlorine gas involved in the process. The equipment required to operate the currently proposed process is also simpler and hence cheaper and easier to operate than the equipment required for the process of US 7727374.

Sodium bicarbonate is safe and easy to handle. It can be easily transported or stored. The calcination process can therefore be centralized for small sources e.g. capture on ships with calcination in harbours. The calcination is done by: (v) heating the sodium bicarbonate to form sodium carbonate, water (generally as water vapour) and carbon dioxide. The carbon dioxide may be captured for storage, sequestration or used as an ingredient in industrial processes such as methanol production.

In a particularly preferred process, the method includes: (vi) capturing and, optionally, drying the carbon dioxide and (vii) recycling the sodium carbonate into the sodium carbonate solution used in (i). The water can also optionally be recycled.

This sequence of steps creates a closed or partially closed loop for the sodium carbonate required in the process, which is hence not consumed but instead can be continuously recycled to a full or partial extent. This means that the carbon dioxide capture method needs only a limited amount of sodium carbonate. Once a solution of given concentration and volume has been formed, and sufficient surplus is allowed to cover the sodium carbonate 'removed' by the precipitation of sodium bicarbonate, then no new sodium carbonate needs to be added. Thus, once the working solution has been established then the method does not require any addition of sodium compounds, such as sodium carbonate or sodium hydroxide to maintain the working solution. Thus, in preferred embodiments the process does not include the addition of such sodium compounds. In fact, the only necessary inputs to the process when it is in steady state are the combustion gases and the heat (or energy) required for thermal decomposition of the sodium bicarbonate. The heat required can be minimised by recovering heat from the system after the reaction, although of course there will always be some losses so some extra heat input is always required. Advantageously, this extra heat can be obtained from the combustion gases.

The sodium carbonate solution can be regenerated by the recycled sodium carbonate. The water can advantageously also be re-used in the sodium carbonate solution, and although this is a lesser advantage since water is more abundant than sodium carbonate it is nonetheless a useful feature, especially when pure water is not readily available, for example on board a ship. This provides considerable additional advantages compared to the process proposed in US 7727374, which involves the use of a solution with sodium hydroxide solution to capture carbon dioxide, but involves continued consumption of raw materials and energy, rather than allowing for an efficient recycling system.

Recycling both the water and sodium carbonate creates an essentially closed loop for the water and sodium carbonate. The proposed process will take in a gas mixture such as an exhaust or flue gas, and will produce carbon dioxide and a new gas mixture with reduced or close to zero carbon dioxide, whilst not requiring the addition of any other raw materials or resource other than the heat required at step (v). The gas mixture with depleted carbon dioxide can be removed for further treatment or vented to atmosphere.

After the removal of solid sodium bicarbonate at step (iv) the sodium bicarbonate may be treated in a drier to remove excess moisture. The water removed from the sodium bicarbonate may optionally be recovered and recycled into the sodium carbonate solution.

After the heating at step (v) the carbon dioxide may be treated in a drier to recover water before the carbon dioxide is processed further. The water thus obtained may optionally be recovered and recycled into the sodium carbonate solution.

The heating at step (v) may be at a temperature of 70 °C to 250 °C. In preferred embodiments the sodium bicarbonate is heated at a temperature in the range 150 °C to 210 °C, for example a temperature close to 160 °C or a temperature close to 200 °C. The products of step (v) will be at an elevated temperature. In a continuous process the method may include heat exchange between some or all processes including heat generation and consumption for example heat exchange between the products of step (v) and the sodium bicarbonate that is to be heated in step (v). This can pre-heat the sodium bicarbonate, which reduces the energy required for step (v). For example heat from the sodium carbonate may be recovered to pre-heat the sodium bicarbonate.

The carbon dioxide obtained from step (v) is preferably compressed for storage. The carbon dioxide can be stored for future use, transported elsewhere, or used locally for sequestration as an ingredient in industrial processes.

The concentration of the sodium bicarbonate solution is controlled so that it is close to saturation. The solution may be maintained at 90% or more of saturation levels, preferably at 95% or more. The concentration may be at about fully saturated for sodium bicarbonate. The process is most effective when the concentration is as close to saturation as possible. Sodium carbonate is preferably continually added to the solution (either as a solid or as a solution) as the reaction with carbon dioxide precipitates out sodium bicarbonate and 'removes' reactants from the solution. In preferred embodiments, this sodium carbonate is recycled from the sodium carbonate produced by the heating in step (v). It will be understood that the solubility of sodium carbonate (and sodium bicarbonate) will vary with temperature. The temperature of the solution may be monitored and the amount of sodium carbonate controlled accordingly in order to maintain a concentration at the chosen saturation level.

To preserve a sufficiently high solubility for sodium carbonate and also a sufficient difference between the two solubilities to enable the reaction to capture carbon dioxide at a high rate then it is preferred for the sodium carbonate solution to be at a temperature of at least 15 °C, more preferably at least 20 °C. The temperature may be up to about 70 °C and preferably it is not higher than 70 °C.

A catalyst may be included in the solution in order to accelerate the chemical reactions. . As is known, in living creatures C0₂ uptake and release is accelerated by carbonic anhydrase. In preferred embodiments a stabilised carbonic anhydrase or functional equivalent thereof is added to the solution to accelerate those reactions.

The flue gas will be effectively mixed with the reactants in the solution. The absorption of gas in liquids is in general described by Henry's law. In preferred embodiments an injector/mixer device is used to introduce the gas mixture into the reactant solution. The injector/mixer preferably generates sufficient gas/liquid surfaces such as large numbers of small gas bubbles in the solution. This maximise the surface area of contact between the gas and the liquid, which maximises the speed of dissolution and hence increases the speed of the carbon dioxide capture process. The mixing can be done with a Friborator or other aerator. Replenishment of the reactant solution can be done within this process. The preferred embodiment will be designed to obtain sufficient resident time for absorption and reactions to take place. The embodiment may need a precipitation zone.

The heating/calcination step (v) may be performed under pressure. This results in the carbon dioxide being produced at an elevated pressure and reduced compression is therefore required to bring the carbon dioxide to the required pressure for storage and/or transport. An elevated pressure can be achieved by carrying out the heating step in a pressure container and using the transformation of the solid sodium bicarbonate into sodium carbonate, water and gaseous carbon dioxide to generate an increased pressure. In some embodiments the whole process can be pressurised and may, for example, operate at the pressure of the system that provides the incoming gas mixture. When an increased pressure is used then the temperature of the heating step can be adjusted accordingly.

Preferably the gas mixture is an exhaust/flue gas, which may be from industrial processes, power plants, or combustion processes, for example from an internal combustion engine.

Where the gas mixture is an exhaust gas then the heating at step (v) preferably uses the heat of the exhaust and/or waste heat from the engine. This further reduces the energy required by the process, since the main energy input at step (v) can make use of what would otherwise be waste heat. The system may also be operated under pressure by using the exhaust gas pressure.

Viewed from a second aspect, the invention provides an apparatus for treatment of exhaust gases for carbon dioxide capture, the apparatus comprising: a reaction vessel containing a solution of sodium bicarbonate and sodium carbonate in water; a gas delivery device for contacting the solution with a combustion gas mixture, the gas mixture including carbon dioxide, such that carbon dioxide will react with the hydroxide ions in the solution and form bicarbonate ions, wherein the gas delivery device is arranged for continuous delivery of gas so that as a result of the reaction sodium bicarbonate precipitates out of the solution as a solid; and a heater for heating the sodium bicarbonate in order to form sodium carbonate, water and carbon dioxide.

The gas delivery device may be a gas injector for bubbling the gas mixture through the solution in the reaction vessel. Very effective devices are aerator products called Friborator produced by Heinrich Frings GmbH & Co. KG of Bonn, Germany. These devices come in various configurations that can be adapted to the containers and volumes that are to be processed. The gas delivery device may optionally mix the solution with the gas by movement of the solution through a mixer- injector, for example a Venturi mixer-injector such as that described in US 5863128. The gas delivery device is preferably connected to a supply for the gas mixture, for example it may be connected to an exhaust system in order to receive a gas mixture in the form of exhaust gases.

The gas mixture that emerges from the solution with depleted carbon dioxide content may be removed from the reaction vessel via an outlet, for example an outlet in a headspace of the reaction vessel. This gas may subsequently be treated further or vented to atmosphere.

The apparatus may include a sediment or slurry extraction device for removing the sodium bicarbonate sediment from the reaction vessel. The sediment extraction device may for example be a conveyor such as a belt conveyor or screw conveyor. The sodium bicarbonate sediment may be treated in a drier or in a filtering device to remove excess moisture. The drier may be arranged to recover water for recycling into the sodium carbonate solution.

Preferably the heater is arranged to heat the sodium bicarbonate to a temperature of 70 °C or above, for example a temperature in the range 70 °C to 250 °C. Waste heat from the combustion gas may be used to provide some or all of the heat for the heater. This reduces energy consumption. The carbon dioxide may be captured in gaseous form and optionally the apparatus includes a compressor for compressing the carbon dioxide. The heater may also act to dry the sodium carbonate if necessary. A gas drier may be included in order to remove moisture from the carbon dioxide.

The calcination process may also be done in a sodium bicarbonate slurry preferably under pressure. This will reduce the energy consumption due to less production of water vapour and a higher concentration of C0₂ in the headspace.

Preferably the apparatus includes a sodium carbonate recycling system for recycling the sodium carbonate into the reactant solution. For example, the recycling system could include a mechanism for transporting the sodium carbonate from the heater to the reaction vessel. The sodium carbonate recycling system may dissolve the sodium carbonate in water, which preferably includes water recovered from the heater and/or driers.

In preferred embodiments the apparatus includes a control device for controlling the concentration in the reaction chamber. The concentration may be controlled such that the sodium bicarbonate concentration in the solution is maintained at 80% or more of saturation levels, optionally at 90% or at 95% of saturation levels. The control device may be connected with the sodium carbonate recycling system so that it can control the rate of recycling of sodium carbonate into the solution in the reaction chamber and thereby control the concentration of the solution.

The flue gas will be effectively mixed with the reactants in the solution. The absorption of gas in liquids is governed by Henry's law. The mixing may be done with a Friborator or other suitable mixer device. The fluid comes from the reactant embodiment. Replenishment of the reactant solution can be done within this process. The embodiment will be designed to obtain sufficient resident time for absorption and reaction to take place. Typically a Friborator will need between 10 and 100 m² to distribute the flue gas as bubbles. For a flue gas from a power plant with a C0₂ concentration of 14%, one ton of C0₂ could be captured per hour, resulting in precipitation of 3.8 tons of sodium bicarbonate. The preferred embodiment may include a precipitation zone.

The heater may include a pressure container so that the heating can be carried out under pressure. In some embodiments the whole apparatus may be pressurised and may, for example, operate at the pressure of the system that provides the incoming gas mixture.

The apparatus may be installed on a vehicle for treating its exhaust gases, and the invention hence extends to a vehicle fitted with the apparatus, for example the apparatus may be installed on a ship.

Where the apparatus is used to treat exhaust gases from an engine, the heater preferably uses heat from the exhaust and/or waste heat from the engine. The heater may hence include a heat exchanger for taking heat from the exhaust and/or from the engine coolant.

It should be noted that the method of the first aspect is considered novel and inventive without step (v) and without reference to the saturation level for the solution. Hence, viewed from a further aspect, the invention provides a method of treatment of combustion gases for carbon dioxide capture comprising: (i) providing a solution of sodium bicarbonate and sodium carbonate in water; (ii) contacting the solution with a combustion gas mixture, the gas mixture including carbon dioxide, such that carbon dioxide reacts with hydroxide ions in the solution to form

bicarbonate ions; (iii) continuing step (ii) until sodium bicarbonate precipitates out of the solution as a solid; and (iv) removing the solid sodium bicarbonate.

In addition, a yet further aspect provides an apparatus for treatment of exhaust gases for carbon dioxide capture, the apparatus comprising: a reaction vessel containing a solution of sodium bicarbonate and sodium carbonate in water; and a gas delivery device for contacting the solution with a combustion gas mixture, the gas mixture including carbon dioxide, such that carbon dioxide will react with the hydroxide ions in the solution and form bicarbonate ions, wherein the gas delivery device is arranged for continuous delivery of gas so that as a result of the reaction sodium bicarbonate precipitates out of the solution as a solid.

In each case the broader aspects above could be combined with preferred or optional features of the first or second aspect.

A preferred embodiment will now be described with reference to the accompanying drawing, which shows a flow diagram of the proposed method.

As seen in Figure 1 , the initial capture of carbon dioxide from a gas mixture, which in this example is a flue gas, occurs in a absorber/reaction chamber. The reaction chamber contains a mixture of sodium bicarbonate (NaHC0₃) and sodium carbonate (Na₂C0₃) in solution. The solution will contain sodium ions, HC0₃ ^" ions, C0₃ ^2" ions and OH^" ions together with physically solved C0₂ in equilibrium with carbonic acid. The HC0₃ ^" ion concentration will be kept close to saturation of sodium bicarbonate. The main reactions will be dissolution of C0₂ in water followed by reaction with OH^" ions. Depending on the pH maintained in the vessel C0₃ ^2" ions are acting as buffer towards the OH^" ion concentrations. Each OH^" ion reacting with Carbonic acid will then be replaced by a new OH^" ion coming from the reaction of C0₃ ^2" ions to HC0₃ ^" ions, pushing HC0₃ ^" concentration over the saturation of sodium bicarbonate in solution. The process can be considered to have four main steps as shown:

1) C0₂ uptake / reaction chamber: In the reaction chamber the HC0₃ ^"

concentration is close to saturation of sodium bicarbonate. C0₂ from the flue gas dissolves into the solution and reacts with OH^" forming HC0₃ ^" which falls out as sediment (sodium bicarbonate). The chamber is continuously fed with solved sodium carbonate at a rate that is equimolar with the C0₂ reaction rate.

2) NaHC0₃ sediment preprocessing: The sediment or slurry is extracted from the reaction chamber and excess water may be removed from the sediment.

3) C0₂ stripper and Na₂C0₃ reclaimer: Controlled heating of the NaHC0₃ will form Na₂C0₃, H₂0 and C0₂. The C0₂ needs to be dried before compression and storage.

4) Carbonate (Na₂C0₃) dissolver: Dissolving sodium carbonate in water for recycling into the reaction chamber. The starting solution is prepared by adding sodium bicarbonate and sodium carbonate to water until the bicarbonate concentration is close to the saturation point of sodium bicarbonate. The sodium carbonate is added in order to increase the number of hydroxyl ions and carbonate ions. Starting with sodium bicarbonate provides additional advantages as well since it allows a better pH control of the solution. A pure sodium carbonate solution is more aggressive to materials due to its high pH value. Once the sodium bicarbonate is in the solution it is much easier to control the pH of the reaction liquid.

By continuously recycling the sodium carbonate from the reclaimer into the reaction chamber at a rate that is equimolar to the C0₂ reaction rate the

concentration in the reaction chamber is controlled, so that it is always close to saturation of sodium bicarbonate. This ensures that carbon dioxide is efficiently reacted and precipitated as sodium bicarbonate. In the sediment preprosessing and the reclaiming process, water can be recycled.

The heating in the stripper and reclaimer at (3) will typically be at a temperature in the range 70 °C to 250 °C, for example at a temperature that is close to 160 °C. Other features of the process and the apparatus that carries out the process can be as described above.

Claims

CLAIMS:

1. A method of treatment of combustion gases for carbon dioxide capture comprising:

(i) providing a solution of sodium carbonate and sodium bicarbonate in water;

(ii) contacting the solution with a combustion gas mixture, the gas mixture including carbon dioxide, such that carbon dioxide dissolves and reacts with hydroxide ions in the solution to form bicarbonate ions;

(iii) continuing step (ii) until sodium bicarbonate precipitates out of the solution as a solid;

(iv) removing the solid sodium bicarbonate; and

(v) heating the sodium bicarbonate to form sodium carbonate, water and carbon dioxide;

wherein during steps (i) to (iv) the sodium bicarbonate concentration in the solution is maintained at 80% or more of saturation level.

2. A method as claimed in claim 1 , further comprising:

(vi) capturing the carbon dioxide formed at step (v); and

(vii) recycling the sodium carbonate formed at step (v) into the sodium bicarbonate and sodium carbonate solution used in step (i).

3. A method as claimed in claim 2, wherein the water formed at step (v) is recycled into the sodium carbonate solution used in step (i).

4. A method as claimed in claim 1 , 2 or 3, wherein sodium carbonate formed at step (v) and the combustion gas mixture containing C0₂ are continually added to the solution as the reaction with carbon dioxide precipitates out sodium bicarbonate; and wherein the addition of sodium carbonate acts to maintain the sodium bicarbonate concentration in the solution at 80% or more of saturation levels.

5. A method as claimed in any preceding claim, wherein the method is carried out continuously.

6. A method as claimed in any preceding claims, wherein the step of contacting the combustion gas with the solution uses a gas mixer-injector that mixes the gas into the liquid in the form of a large number of small bubbles.

7. A method as claimed in any preceding claim, wherein the combustion gas mixture is a C0₂ containing gas stream from one or more of industrial processes, power plants, and combustion processes, for example from an internal combustion engine.

8. A method as claimed in claim 7, wherein heat from the exhaust and/or waste heat from the engine is used in the heating at step (v).

9. A method as claimed in any preceding claim, wherein the solution includes an accelerator, such as carbonic anhydrase, for increasing the speed of the reaction.

10. A method as claimed in any preceding claim, wherein step(s) (i), (ii) and/or (v) are carried out at above ambient pressure.

1 1. A method as claimed in any preceding claim being carried out on a vehicle to treat exhaust gases from an engine of the vehicle.

12. An apparatus for treatment of exhaust gases for carbon dioxide capture, the apparatus comprising:

a reaction vessel containing a solution of sodium bicarbonate and sodium carbonate in water;

a gas delivery device for contacting the solution with a combustion gas mixture, the gas mixture including carbon dioxide, such that carbon dioxide will react with hydroxide ions in the solution to form bicarbonate ions, wherein the gas delivery device is arranged for continuous delivery of gas so that as a result of the reaction sodium bicarbonate precipitates out of the solution as a solid; and

a heater for heating the sodium bicarbonate in order to form sodium carbonate, water and carbon dioxide.

13. An apparatus as claimed in claim 12, comprising a sediment extraction device for removing the sodium bicarbonate sediment from the reaction vessel.

14. An apparatus as claimed in claim 12 or 13, comprising a sodium carbonate recycling system for recycling the sodium carbonate produced by the heater into the sodium bicarbonate and sodium carbonate solution in the reaction vessel.

15. An apparatus as claimed in claim 12, 13 or 14, comprising a control device for controlling the concentration in the reaction vessel so that sodium bicarbonate is close to saturation.

16. An apparatus as claimed in any of claims 12 to 15, wherein the gas delivery device is a mixer-injector for mixing the gas into the liquid in the form of a large number of small bubbles.

17. A vehicle comprising an apparatus as claimed in any of claims 10 to 16, wherein the apparatus receives exhaust gases from an engine of the vehicle as the gas mixture.