WO2011082448A1

WO2011082448A1 - Sorbent regeneration

Info

Publication number: WO2011082448A1
Application number: PCT/AU2011/000007
Authority: WO
Inventors: Bo Feng
Original assignee: The University Of Queensland
Priority date: 2010-01-08
Filing date: 2011-01-04
Publication date: 2011-07-14

Abstract

A method of CO₂release from a CO₂-rich capture material and capturing CO₂ by contacting a CO₂-containing gas and a CO₂ capture material to produce a CO₂-rich capture material. The CO₂-rich capture material is mixed with a metal oxide; and CO₂ is released by reacting the metal oxide with a fuel in the presence of the CO₂-rich capture material to produce a CO₂-deplete capture material, the metal and gaseous CO₂. The CO₂-deplete capture material may then be recycled to the capture step and used as the CO₂ capture material. An apparatus for the CO₂ capture and CO₂ release is also disclosed.

Description

Sorbent regeneration

Field of the invention

This invention relates to a method and apparatus for capturing C02.

Background of the invention

Systems that capture CO₂ are of interest from at least 2 perspectives: (i) producing CO2 for commercial markets, and (ii) capturing CO2 for storage to avoid C0₂ emission to the atmosphere. The former includes processes where C0₂ is a valuable by-product. The latter relates to the concern over global warming due to the emission of C0₂ into the atmosphere, where electricity and heat generation accounted for 41% of the total C0₂ emissions in 2005. One method to control CO₂ emissions is by a process known as C0₂ capture and storage (CCS).

Under a CCS system, C0₂ is 'captured' before being emitted to the atmosphere, and then stored. The capture step typically also includes, a separation or purification step since the C0₂ is typically initially present as part of a gaseous mixture and in most systems the C0₂ must be in a relatively pure state to enable economic storage. Further, for producing CO₂ for commercial markets, and in most storage systems, the capture step also includes the release of the C0₂. Since the CO₂ capture process is the most expensive component in most CCS systems it is critical to develop low-cost CO2 capture technologies. To capture the CO₂, various strategies have been attempted, including membrane separation, cryogenic fractionation, chemical absorption and adsorption using molecular sieves. Preferably, the C0₂ capture method is able to controllably capture, and preferably also subsequently release, the C0₂. Chemical absorption using monoethanolamine (MEA) is most common, but, due to its historical basis being to produce C0₂ for commercial markets, it is not optimized for C0₂ storage.

The CO₂ capture material (or sorbent) may be used in-situ, that is, at the time of the reaction in which the C0₂ is formed, or at some point in the process after the reaction in which the C0₂ is formed. An example of the former include when coal or natural gas is gasified (reformed) together with the C0₂ capture material, and high purity H2 is produced while C0₂ is captured in the material (this and other techniques are referred to as advanced zero emission power (AZEP) generation technologies). An example of the latter is when a CO2 capture material is used in a process to separate C0₂ from a gas mixture. One application for the latter is use of the CO2 capture material in post- combustion capture of C0₂ in the flue gas of coal-fired power plants, however there are obviously other uses for a C0₂ capture material (such as CO₂ capture from chemical plants, CO₂ capture from natural gas, CO2 separation from air). As mentioned above, it is preferable that following capture of C0₂ by the C0₂ capture material that the CO₂ is then removed from the CO2 capture material. This process is referred to as 'regeneration' of the C0₂ capture material and allows it to be reused. From a commercial perspective, the C0₂ capture material should ideally (a) be sufficiently robust that it can be recycled numerous times without any appreciable loss of performance in terms of its C0₂ capture and liberation abilities (known as reversibility), (b) have a fast reaction rate, and (c) have a high capture capacity. In some applications, for instance when used in-situ, a C0₂ capture material must also be functional at high temperatures. A suitable material for C0₂ capture at high temperature is CaO, which is also cheap and widely available (being the calcined product of CaC0₃ (limestone)).

The capture of C0₂ (or O2) onto a solid reactant, and the subsequent regeneration of that solid reactant for reuse, is referred to as 'chemical looping'.

One type of looping involves the use of a solid reactant to transfer CO₂ from one reactor to another. In this situation, 'chemical looping' refers to the sorption of C0₂ onto a C0₂ capture material and its subsequent regeneration for reuse. When the CO₂ capture material is a CaO-type, the looping is referred to as 'calcium looping'. For example, CO2 at relatively low concentration can be captured from flue gases in one reactor and then transported via the solid reactant (ie the CO₂ capture material) to another reactor in which the solid reactant is regenerated to produce CO₂. In this specification, this type of looping is referred to as ΌΟ2 looping'. A schematic for CO2 looping with heat input met by fuel burning is shown in Figure 1.

In CO2 looping, the capture reaction 12 may be referred to as adsorption or carbonation while the release reaction 11 may be referred to desorption, regeneration or calcination. The capture reaction, which takes place in the capture reactor 12, for a CaO-type C0₂ capture material 13 is (1) CaO + C<¼→ CaC0₃ (-179kJ/mol). The release reaction, which takes place in the release reactor 11 , for a CaO-type CO2 capture material is (2) CaC0₃→ CaO and C0₂ (179 kJ/mol). The capture reaction is exothermic, whereas the release reaction is endothermic.

The release reaction requires energy input (ie in the form of heat). When the chemical looping is for the treatment of gaseous mixtures produced by industrial plants, one option for providing at least some of this heat is to use heat generated by the industrial plant. However, as the release reaction occurs at temperatures greater than typical of many industrial plants, this option will only meet part of the heat demand. Typically, the heat can be provided by burning a fuel 14 in the release reactor 11. However, the oxygen used for the combustion must be free from N₂ in order to avoid dilution of CO2 and issues with NOx. The separation of the air 15 to form O2, 16 (and C0₂) and N2 is expensive and energy intensive.

Another type of looping involves the use of a solid reactant to transfer O2 from one reactor to another. This is often referred to as 'chemical looping combustion' (CLC), and is when a fuel is combusted in one reactor in the presence of an oxidised solid reactant which is then re-oxidised in another reactor. For example, oxygen from the air 1 can be adsorbed onto the solid reactant 5' in one reactor 6 and then transported via the solid reactant 5 to react with fuel 9 in another reactor to produce CO2 and H₂0. That is, the solid reactant is repeatedly oxidised (5) and then reduced (5'). In this specification, this type of looping is referred to as '0₂ looping'.

Typically in 0₂ looping, the solid reactant is a metal oxide. In this case, the reduction reaction, which takes place in a fuel reactor 4, for a CuO solid reactant 5 and a CH₄ fuel, is CH + 4CuO - 4Cu + C0₂ + 2H₂0 (-217kJ/mol). The oxidation reaction, which takes place in an air or oxidising reactor 6, for a CuO solid reactant 5 and a CH₄ fuel, is 2Cu + 02 -> 2CuO (-146 kJ/mol). Both the reduction reaction and the oxidation reaction are exothermic.

A schematic for 0₂ looping with a CuO solid reactant is shown in Figure 2.

0₂ looping is effectively a form of indirect oxy-combustion where fuel and air are never mixed as the solid reactant is used to selectively transport oxygen (from air) to the fuel in the solid phase for combustion. If a suitable solid reactant is used, 0₂ looping produces CO2 and H20 only from the fuel reactor 4 which allows for subsequent condensation to remove H2O (7), and storage of CO2 (8). Thus, combusting the fuel 9 in this manner, to say produce steam for electricity generation, avoids the need for post- combustion C0₂ gas separation steps. 0₂ looping does not reduce the amount of CO2 produced from combustion of an amount of fuel, but does serve to make a more storage-ready form of that amount of C0₂ than if the combustion was done by another method. Because of this, use of 0₂ looping for indirect combustion of fuels is being explored as a means to more economically avoid C0₂ emissions from fuel-fired industrial plants.

However, 0₂ looping is not easily retrofitted to existing fuel-fired industrial plants as it requires a change in the way oxygen is fed to the fuel. As well, 0₂ looping is not relevant for applications where the C0₂ to be processed is not originally produced by the combustion of fuel, for instance, processes such as the production of ammonia. Thus, there exists a need for a CO2 capture system that is more readily applied to existing plants, for instance as a downstream processing step, and can be applied to a broader range of applications. .

Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art. Summary of the invention

The inventors recognised that there is a great deal of synergy between the C<¼ and <¾ looping cycles, and have developed a new system based on this recognition.

In a first aspect of the invention there is provided a method of CO2 release from a CO₂- rich capture material including the steps of

- providing a CCVrich capture material;

- providing a metal oxide; and

- releasing C0₂ by reacting the metal oxide with a fuel, in the presence of the C0₂-rich capture material, to produce a CCVdeplete capture material, the metal and gaseous C0₂.

An advantage of the present invention is that by supplying the oxygen to the fuel in the form of a metal oxide, the need to purify air to obtain oxygen is avoided. The process of purify air is costly, and can reduce the efficiency of a plant by 5-10%.

In preferred embodiments, the C0₂-deplete capture material and/or the metal produced in the release step are recycled. For instance, the C0₂-deplete capture material could be used to capture more C0₂ and/or the metal could be re-oxidised.

One embodiment further includes the step of

- capturing C0₂ by contacting a CCVcontaining gas and a C0₂ capture material to produce a CCVrich capture material. In preferred embodiments, the CO^deplete capture material produced in the release step is recycled. For instance, the C0₂-deplete capture material could be used to capture more C0₂ by returning it to the capture step.

In embodiments where the C0₂-deplete capture material is recycled, the C0₂-deplete capture material (i) may be passed directly from the release step to the capture step or (ii) may first be passed through the oxidising step to which the C02-deplete capture material is inert.

In a preferred embodiment of this invention the metal oxide may be provided in an oxidising step of - reacting a metal in an oxidising environment to produce the metal oxide.

In preferred embodiments, the metal produced in the release step is recycled. For instance, the metal could be re-oxidised by returning it to the oxidising step.

In embodiments where the metal of the release step is recycled, the metal (i) may be passed directly from the release step to the oxidising step or (ii) may first be passed through the capture step to which the metal is inert.

According to a preferred embodiment, the metal oxide is provided by passing the metal and CC<2-deplete capture material of the release step through a capture step to produce a CC>2-rich capture material, and then passing the CCVrich capture material and metal through an oxidising step of reacting a metal in an oxidising environment to produce the metal oxide.

The steps of the above aspect need not be conducted in the order indicated.

Hence, in a further preferred embodiment, which may be considered to be the reverse of the above embodiment, the C0₂-rich capture material is provided by passing the metal and COr-deplete capture material of the release step through an oxidising step to produce a metal oxide, and then passing the metal oxide and COrdeplete capture material through a capture step to provide the CCVrich capture material.

An advantage of some embodiments of the present invention is that C0₂ capture material and metal / metal oxide move from one step to the next together.

In a second aspect of the invention there is provided an apparatus for C0₂ capture and release including: - a capture reactor for receiving and reacting C0₂-deplete capture material and C02-containing gas to form a C0₂-rich capture material;

- an oxidising reactor for receiving and oxidising metal to form a metal oxide; and

- a release reactor in fluid communication with the oxidising reactor for receipt of metal oxide and capture reactor for receipt of the C0₂-rich capture material, the release reactor forming a reaction bed in which the metal oxide and a fuel are reacted to form a CO^-deplete capture material, a metal, and gaseous C0₂.

In one embodiment, the capture reactor is in fluid communication with the oxidising reactor Accordingly, in a further preferred embodiment the release reactor is further in communication with capture reactor for the passage to the capture reactor from the release reactor of the C02-deplefe capture material and metal and the release reactor is further in communication with the oxidising reactor for the passage to the release reactor of C0₂-rich capture material and metal oxide from the capture reactor. The C0₂- deplete capture material and metal are passed to the capture reactor from the release reactor and the C0₂-rich capture material and metal oxide are passed from the oxidising reactor to the release reactor.

The steps of the above aspect need not be conducted in the order indicated.

Hence, in a further preferred embodiment, which may be considered to be the reverse of the above embodiment, the release reactor is further in communication with the oxidising reactor for the passage to the oxidising reactor of C0₂-deplete capture material and metal from the release reactor and the release reactor is further in communication with the capture reactor for the passage to the release reactor of the C0₂-rich capture material and metal oxide from the capture reactor. The CC deplete capture material and metal are passed to the oxidising reactor from the release reactor and the C0₂-rich capture material and metal oxide are passed from the capture reactor to the release reactor. An advantage of some embodiments of the present invention is that. C0₂ capture material and metal / metal oxide move from one reactor to the next together. This simplifies the number and extent of fluid transfer conduits and operational control systems / devices necessary. For instance, separate flow controllers and piping for the C0₂ capture material and metal / metal oxide recycling loops is not needed.

In the above aspects, the CCVcontaining gas may be any gas mixture suitable for contact with the C0₂ capture material and metal. An advantage of the apparatus of this invention is that it can be retro-fitted, or added on, to existing industrial plants. Thus, the C0₂-containing gas may be the exhaust gas from an industrial plant. In some embodiments, it is important that the COr-containing gas does not react with the metal. Preferably, the C0₂-containing gas is either a flue gas (ie also including NOx and SOx) from, for example, pulverised coal or natural gas combustion, or syngas (is a mixture of CO and H2), which can undergo a water-gas shift (ie to C0₂ and H2), from, for example, an integrated coal gasification combined cycle plant. Preferably, in the above aspects, the C0₂ capture material is a CaO-based C0₂ capture material. More preferably, the C0₂ capture material is a CaO-metal oxide C0₂ capture material as described in PCT/AU2009/001465.

The CC*2-deplete capture material is contacted with the C0₂-contain^"mg gas in the capture step / capture reactor to become the CCVrich capture material. That is, the input to the capture step / capture reactor includes a C0₂-containing gas, while the output includes COrrich capture material. The metal / metal oxide may pass through the capture step / capture reactor without being chemically altered. The temperature in the capture step / capture reactor is typically from about 500 °C to about 800 °C, but this of course varies depending on the C0₂ capture material. The skilled person would be aware of appropriate temperatures. For instance, for a CaO-based C0₂ capture material, the temperature in the capture step / capture reactor is typically about 650 °C. Since the loading of C0₂ onto the C0₂ capture material is exothermic, the temperature in the capture step / capture reactor is preferably controlled any heat removed can be reused, preferably to partially heat the release step / release reactor. Preferably, in the above aspects the metal is a transition metal. More preferably, the metal is Cu.

The metal is oxidised to form a metal oxide in the oxidising step / oxidising reactor. The input to the oxidising step / oxidising reactor includes a metal, while the output includes a metal oxide. An oxidant, preferably air, is also an input to the oxidising step / oxidising reactor. The CO2 capture material may pass through the oxidising step / oxidising reactor without being chemically altered. As the oxidation of metal is an exothermic reaction, heat energy will be released in the oxidising step / oxidising reactor. This heat can be controlled though by adjusting the flow rate of the oxidant (eg air). The temperature in the oxidising step / oxidising reactor is typically from about 300 °C to about 750 °C, but this of course varies depending on the metal. The skilled person would be aware of appropriate temperatures. For instance, for Cu, the temperature in the oxidising step / oxidising reactor is typically about 650 °C.

Preferably, in the above aspects the fuel is a hydrocarbon. More preferably, the fuel is a gas. In some preferred embodiments, the fuel is methane gas.

The C0₂ is released from the C0₂-rich capture material to form a C0₂-deplete capture material in the release step / release reactor, where the fuel and metal oxide are reacted to provide the requisite heat energy. Thus, the process converts the CC>2-rich capture material to a C0₂-deplete capture material as the C0₂ is released, in a process that may be considered C0₂ capture material regeneration. The process also converts the metal oxide back to the metal as the oxygen is consumed in the combustion of the fuel, in a process that may be considered metal regeneration. Therefore, the inputs to the release step / release reactor include fuel, metal oxide, and C0₂-rich capture material, while the outputs include CO^eplete capture material, metal, water and gaseous C0₂. The temperature in the release step / release reactor is typically from about 850 °C to about 1050 °C, but this of course varies depending on the C0₂ capture material. The skilled person would be aware of appropriate temperatures. For instance, for a CaO-based C0₂ capture material, the temperature in the release step / release reactor is typically about 900 °C. The exothermic reaction of the fuel with the metal oxide in the release step / release reactor supplies sufficient energy for regeneration of CC>₂-rich capture material to C0₂-deplete capture material. Preferably, the compounds and reactions in the release step / release reactor are such that the reaction is heat neutral or slightly exothermic.

Gaseous C0₂ is the term used to refer to the product CO2 gas, as opposed to the original CC containing gas. That is, the gaseous CO2 that is an output of the release step / release reactor is a purer form of C0₂ than original present in the C0₂-containing gas. It is the gaseous C0₂ that is used in further processing, for instance, for storage to reduce global warming effects or as a purified gas. Depending on the application the C0₂ could contain sulphur and nitrogen species in addition to water. Passing of materials from one step, or reactor, to the next may be achieved by any means known in the art. Essentially, the steps or reactors need only be in fluid communication with each other, in the sense that the particulate and gaseous components of the system can flow there between. For instance, in some embodiments, materials may be passed through pipes. Within the context of the invention, the 'reactors' may not be discrete containers, but are instead regions within the one container or the like. For example, for chemical looping combustion of coal, the C0₂ capture reactor and the C0₂ release reaction can be built as a single reactor but in two sections. The particles will move from the top to the bottom while the gases moving from the bottom to the top. As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps.

Brief description of the drawings

Figure 1 is a schematic illustrating C0₂ looping of the prior art; Figure 2 is a schematic illustrating 0₂ looping of the prior art; Figure 3 is a schematic illustrating embodiments 3A, 3B and 3C of the present invention;

Figure 4 is a schematic illustrating embodiments 4A and 4B of the present invention;

Figure 5 is a schematic view of an embodiment of a CO2 capture apparatus of an embodiment of the invention;

Figure 6 is a graph showing the effect of the stoichiometric ratio and temperature on CH₄ conversion of copper oxide;

Figure 7 is a graph showing the effect of the stoichiometric ratio and water (steam) ratio (H₂0:CH₄) on CH conversion of copper oxide; Figure 8 is a graph showing the effect of the stoichiometric ratio, temperature and steam ratio (H₂0:CH₄) on carbon deposition of copper oxide;

Figure 9 is a graph showing the effect of temperature, pressure and copper oxide to fuel ratio on the calcination reaction of the fuel reactor;

Figure 10 is a graph showing the effect of temperature, pressure, and copper oxide to fuel in the fuel reactor;

Figure 11 is a graph showing the effect of the quantity of copper oxide, pressure on the energy balance of the fuel reactor when copper oxide to fuel ratio equals 2:1 ;

Figure 12 is a graph showing the effect of the quantity of copper oxide, pressure on the energy balance of the fuel reactor when the copper oxide to fuel ratio equals 3:1 ; and Figure 13 is a graph of the effect of the quantity of copper oxide, pressure on the energy balance of the fuel reactor when the copper oxide to fuel ratio equals 4:1. Detailed description of the embodiments

The invention may be considered to be a merge of the CO2 looping and 0₂ looping of the prior art, where the release reactor 1 of the C0₂ looping is one and the same with the fuel reactor 4 of the O2 looping. Effectively, the metal oxide 5 in the oxidising reactor 6 of the O₂ looping acts like the air separation unit 17 of the C0₂ looping system, in that it provides oxygen to the release reactor 1 that is free from impurities.

In a particular preferred embodiment, the C0₂-containing gas is flue gas, the C0₂ capture material is a CaO-based C0₂ capture material that is supported by MgO as described in PCT/AU2009/001465, the metal is Cu, the oxidant is air, and the fuel is methane (CH₄).

The basic reactions for the invention are provided in figures 3a, 3b, and 3C. In figure 3A, the basic reaction in the release reactor R is shown where a fuel and a metal oxide (oxygen containing material) react with a C0₂ rich capture material to produce a C0₂ deplete capture material, metal and C0₂ In figure 3B, expansion of the process of figure 3A provides a capture reactor C for conducting a capture step of contacting a C0₂ containing flue gas and a CaO/MgO C0₂ capture material to produce a CO^rich capture material. Preferably Cu is present during the capture step.

In figure 3C, an oxidising reactor O is in fluid communication with the release reactor R . Metal oxide is passed into release reactor R into which the C0₂-rich capture material from capture reactor C also passes. Fuel to the release reactor R combusts the metal oxide to metal oxide causing the release of C0₂ from the C0₂ rich capture material producing C0₂ deplete material.

Thus, in figures 4A, 4B and 5, different versions of an apparatus for C0₂ capture and release is shown. In the apparatus of figure 4A and 5, a capture reactor C is shown for conducting a capture step of contacting a flue gas (C0₂ containing gas) and a C0₂ capture material (CaO/MgO) to produce a CO rich capture material. Cu (metal) is present during the capture step. An oxidising reactor O is in fluid communication with the capture reactor C. The C0₂- rich capture material and Cu (metal) pass from the capture reactor C to the oxidising reactor O where an oxidising step of reacting the Cu (metal) in air to produce a CuO (metal oxide). A release reactor R is in fluid communication with the oxidising reactor O and the C0₂- rich capture material and CuO (metal oxide) pass from the oxidising reactor O to the release reactor R. In the release reactor, a release step of reacting the CuO (metal oxide) with fuel (methane), in the presence of the C02-rich capture material, to produce a regenerated C0₂-deplete capture material, the Cu and releasing gaseous C0₂ The C0₂-deplete capture material and the Cu (metal) then is passed from the release reactor R to the capture reactor C where the C0₂-deplete capture material reacts with co₂.

Due to the nature of the system, the flow of components between the reactors can equally appropriately occur in the reverse direction. Therefore, an equally preferred form of the invention shown in figure 4B is an apparatus for C0₂ capture and release including a capture reactor C for conducting a capture step of contacting a C0₂ containing gas (flue gas) and a C0₂ capture material (CaO/MgO) to produce a CO^rich capture material, wherein a CuO is present during the capture step.

A release reactor R is provided in fluid communication with the capture reactor C and into which the C0₂-rich capture material and CuO may be passed from the capture reactor, the release reactor R being for conducting a release step of reacting the CuO with methane, in the presence of the C02-rich capture material, to produce a CO2- deplete capture material, Cu and gaseous C0₂

An oxidising reactor O is provided in fluid communication with the oxidising reactor O into which the C0₂-deplete capture material and Cu pass from the release reactor, the oxidising reactor being for conducting an oxidising step of reacting the Cu in air to produce a CuO. The oxidising reactor O is in fluid communication with the capture reactor such that the C0₂-deplete capture material and the CuO from the oxidising reactor O passes to the capture reactor. The general processes occurring within the apparatus have been described above. In this preferred embodiment, the reactions occurring are:

Capture reactor: CaO + C0₂→ CaC0₃ (-179kJ/mol)

Oxidising reactor: 2Cu + 02 -» 2CuO (-146 kJ/mol) Release reactor: CH₄ + 4CuO -» 4Cu + C0₂ + 2H₂0 (-217kJ/mol) or H₂ + CuO -» Cu + H₂0 (-85.8 KJ/mol)

CaC0₃ -→ CaO and C0₂ (179 kJ/mol)

The reaction of the methane (or hydrogen) with the CuO is exothermic, and is adapted to supply sufficient energy for regeneration of C0₂-rich capture material to C0₂-deplete capture material. The amount of energy needed can be determined by the skilled person based on the amount of C0₂ capture material, and this can then be used to determine the amount of methane and CuO needed.

Likewise, the skilled person would understand how to optimise the entire process by adjustment of temperatures, quantities, flow rates and the like. The C0₂ capture reactor and C0₂ release reactor are preferably both fixed bed reactors where solid particles move by their own weight and gases move up naturally due to buoyancy. However compressors may be used to move particles in the transport reactor from the bottom to the top. This may be perhaps the easiest and cheapest way of material transportation. The particle size will depend on the actual reactor configuration. For example, if fixed beds are used the particles can be 1-5 mm. However, if a transport reactor is used the particles need to be smaller.

Figures 6 - 13 are the results of a thermodynamic study conducted to identify the operating conditions for the reaction in reactor R. Figure 6 shows the effect of stoichiometric ratio and temperature on CH conversation of copper oxide. Conditions in the reactor: P = 1 atm and steam ratio (H₂0:CH₄) = 1. The area above the curve is the area in which unburnt CH concentration is less than 1%. Higher temperature and higher stoichiometric ratio can be seen to be beneficial for complete CH₄ conversion. Figure 7 shows the effect of stoichiometric ratio and water steam ratio (H₂0:CH₄) on the C0₂ concentration of copper oxide. Conditions in the reactor: P = 1 atm and T = 900 C. It can be seen that a high C0₂ concentration can be achieved if the stoichiometric ratio is higher than 1.0.

Figure 8 shows the effect of stoichiometric ratio, temperature and steam (H₂0:CH₄) on carbon deposition of copper oxide. The areas above the lines are the areas with zero carbon formation and areas below the line show carbon formation. Condition: P = 1 atm.

Figure 9 shows the effect of temperature, pressure and copper oxide to fuel ratio on the calcination reaction of the fuel reactor. The areas above the lines show full calcination. Lower pressure and higher temperature favor full calcination. Figure 10 shows the effect of temperature, pressure and copper oxide to fuel ratio on complete combustion of fuel in the fuel reactor. The areas above the line show 100% of combustion and areas below the lines show less than 100% of combustion.

Figure 11 shows the effect of the quantity of CuO, pressure on the energy balance of the fuel reactor when copper oxide to fuel ratio equals to 2:1. Conditions: Ti = 900 C. Reactor R has to be operated at a high pressure at a CuO:CH₄=2:1 in order to maintain the reactor conditions as exothermic.

Figure 12 shows the effect of the quantity of CuO, pressure on the energy balance of the fuel reactor when copper oxide to fuel ratio equals to 3:1. Conditions: Ti = 900 C. Reactor R can be operated under exothermic conditions. Figure 13 shows the effect of the quantity of CuO, pressure on the energy balance of the fuel reactor when copper oxide to fuel ratio equals to 4:1. Conditions: Ti = 900 C. Reactor R can be operated under exothermic conditions. It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Claims

1. A method of C02 release from a C02-rich capture material including the steps of

- providing a C02-rich capture material;

- providing a metal oxide; and - releasing C02 by reacting the metal oxide with a fuel in the presence of the C02-rich capture material to produce a C02-deplete capture material, the metal and gaseous C02.

2. A method of C02 capture and release of claim 1 , wherein the step of providing a C02-rich capture material includes the step of - capturing C02 by contacting a C02-containing gas and a C02 capture material to produce a C02-rich capture material.

3. The method of C02 capture and release according to claim 2, wherein the C02- deplete capture material is recycled to the capture step and used as the C02 capture material.

4. The method of C02 capture and release according to claim 2, wherein the step of providing a metal oxide includes an oxidising step of

- reacting a metal in an oxidising environment to produce the metal oxide.

5. The method of C02 capture and release according to claim 4, wherein the C02- deplete capture material is recycled to the capture step via the oxidising step and used as the C02 capture material.

6. The method of C02 capture and release according to claim 4, wherein the metal of the release step is recycled to the oxidising step and re-oxidised.

7. The method of C02 capture and release according to claim 6, wherein the metal is recycled to the oxidising step via the capture step.

8. The method of C02 capture and release according to claim 4, wherein a metal is present in the capture step and the C02-rich capture material and metal is passed through the oxidising step of reacting the metal in an oxidising environment to produce a metal oxide.

9. The method of C02 capture and release according to claim 1 wherein the C02 capture material is CaO.

10. The method of C02 capture and release according to claim 1 wherein the C02 capture material is CaO supported on MgO.

11. The method of C02 capture and release according to claim 1 or 10 wherein the metal is Cu.

12. The method of C02 capture and release according to claim 1 or 10 wherein the fuel is methane.

13. An apparatus for C02 capture and release including:

- a capture reactor for receiving and reacting C02-deplete capture material and C02- containing gas to form a C02-rich capture material;

- a release reactor in fluid communication with the oxidising reactor for receipt of metal oxide and capture reactor for receipt of the C02-rich capture material, the release reactor forming a reaction bed in which the metal oxide and a fuel are reacted to form a C02-deplete capture material, a metal, and gaseous C02.

14. The apparatus for C02 capture and release of claim 13, wherein the capture reactor is in fluid communication with the oxidising reactor.

15. The apparatus for C02 capture and release of claim 14, wherein the release reactor is further in communication with capture reactor for the passage to the capture reactor from the release reactor of the C02-deplete capture material and metal and the release reactor is further in communication with the oxidising reactor for the passage to the release reactor of C02-rich capture material and metal oxide from the capture reactor.

16. The apparatus for C02 capture and release of claim 14, wherein the release reactor is further in communication with the oxidising reactor for the passage to the oxidising reactor of C02-deplete capture material and metal from the release reactor and the release reactor is further in communication with the capture reactor for the passage to the release reactor of the C02-rich capture material and metal oxide from the capture reactor.

17. The apparatus for C02 capture and release according to claim 13 wherein the C02 capture material is CaO.

18. The apparatus for C02 capture and release according to claim 13 wherein the C02 capture material is CaO supported on MgO.

19. The apparatus for C02 capture and release according to claim 13 or 18 wherein the metal is Cu.

20. The apparatus for C02 capture and release according to claim 13 or 18 wherein the fuel is methane.