WO2007003954A1

WO2007003954A1 - Carbon dioxide extraction process

Info

Publication number: WO2007003954A1
Application number: PCT/GB2006/002511
Authority: WO
Inventors: Børge Rygh SIVERTSEN; Håkon RUESLÅTTEN
Original assignee: Statoil Asa; Cockbain, Julian
Priority date: 2005-07-06
Filing date: 2006-07-06
Publication date: 2007-01-11
Also published as: GB2428038A; GB0513866D0; GB2428038B; GB2428038A8

Abstract

The invention relates to a process for the extraction of carbon dioxide from a carbon dioxide-containing gas by calcium oxide by formation of calcium carbonate, said process comprising flowing said carbon dioxide- containing gas past or through a gas-porous, calcium oxide-containing ceramic member disposed in a reactor vessel whereby said carbon dioxide-containing gas may permeate said member, and preferably subsequently heating said member and/or exposing said member to a gas having a lower carbon dioxide partial pressure than said carbon dioxide-containing gas whereby to cause calcium carbonate therein to release carbon dioxide, said member defining at least part of the inner wall of a gas flow conduit within said reactor vessel.

Description

Carbon Dioxide Extraction Process

This invention relates to a process for the extraction of carbon dioxide from the gaseous product of methane reformation (generally using natural gas or a natural gas derived material as the starting material for the process) or combustion (e.g. from flue or exhaust gases) , to apparatus therefor and to ceramics and ceramic-containing elements suitable for use therein.

Methane reformation involves the reaction of methane and water at elevated temperature to produce hydrogen and carbon dioxide. The resultant hydrogen may be used for many purposes but it is of particular interest for use in fuel cells, in chemical processes or as a fuel gas. Where the hydrogen produced is used in fuel cells, reformation effectively centralizes the environmental problems of generating energy from methane since the carbon dioxide generated can be captured at the site of the reactor used for the methane reformation.

The gaseous output of a methane reformation reactor is generally referred to as synthesis gas (or syngas) and comprises hydrogen, water, carbon dioxide, some carbon monoxide and some methane. The carbon monoxide may be converted by reaction with water in a shift reactor to produce hydrogen and carbon dioxide and thus the output from the combination of reformer and shift reactors is hydrogen, water, carbon dioxide and some methane and/or carbon monoxide .

It has been proposed in US-B-6667022 to pass steam and the syngas from a reformer through a fluidized bed in a first reactor comprising a fluidized bed of particulate calcium oxide and ferric oxide whereby the calcium oxide reacts with the carbon dioxide to produce calcium carbonate and the carbon monoxide is transformed to carbon dioxide by a combination of a shift reaction and reduction of the ferric oxide. The upper part of the particulate bed is fed, together with the gas output (essentially wet hydrogen) into a second fluidized bed in a second reactor which functions to separate the gas from the solid and perhaps also to some extent as a shift reactor to transform any remaining carbon monoxide to carbon dioxide which can react with remaining calcium oxide. Gas essentially comprising wet hydrogen is drawn out of the top of this reactor while the particulate material from the base of this reactor is fed into a fluidized bed in a third reactor. The bed in this third reactor is fluidized with oxygen and steam which serves to oxidize the ferrous oxide back to ferric oxide and to regenerate calcium oxide from the calcium carbonate as a result of the heat emitted by the oxidation reaction. The gas drawn off from the top of the third reactor is essentially a mixture of water, carbon dioxide, oxygen and some methane. The regenerated particulate mass is drawn off the base of this reactor and recycled into the base of the first reactor.

This procedure, while attractive as a means of separating the hydrogen in syngas from carbon monoxide and carbon dioxide, is undesirably complicated as it requires a particulate mass to be circulated between three separate reactors in which the particulate components undergo cyclical chemical changes which will of course result in their structural integrity, and hence their ability to withstand the physical cycling procedure, being reduced.

We have now found that carbon dioxide removal may be effected particularly advantageously using a static ceramic calcium carbonate/oxide bed which is cycled between conditions which cause carbon dioxide absorption and conditions which cause carbon dioxide release. The bed may be particulate, in which case it will be retained in place along a length of a gas transmitting conduit by a porous retainer, e.g. a porous metal or ceramic inner wall, or it may be a self-supporting ceramic in which case it may be placed within or form some or all of the inner wall of that length of conduit.

Thus viewed from one aspect the invention provides a process for the extraction of carbon dioxide from a carbon dioxide-containing gas (preferably a gas which contains hydrogen and/or methane and optionally which contains water, especially a gas produced in a methane reformation process, in particular syngas) by calcium oxide by formation of calcium carbonate (and preferably for the release of carbon dioxide from calcium carbonate to regenerate calcium oxide) , said process comprising flowing said carbon dioxide-containing gas past or through a gas-porous, ceramic calcium oxide-containing member disposed in a reactor vessel whereby said carbon dioxide-containing gas may permeate said member, and preferably subsequently heating said member and/or exposing said member to a gas having a lower carbon dioxide partial pressure than said carbon dioxide- containing gas whereby to cause calcium carbonate therein to release carbon dioxide, said member defining at least part of the inner wall of a gas flow conduit within said reactor vessel .

Viewed from a further aspect the invention also provides a carbon dioxide separation apparatus comprising a vessel having a gas inlet and a gas outlet and a gas flow conduit linking said inlet and outlet, at least part of an inner wall of said conduit being provided by a gas porous ceramic member containing calcium oxide and/or carbonate, wherein carbon dioxide-containing gas flowing through said conduit permeates said member permitting carbon dioxide in said gas to react with calcium oxide in said member to form calcium carbonate, said apparatus being arranged such that said member may be heated and/or exposed to a gas having a lower carbon dioxide partial pressure than said carbon dioxide- containing gas whereby to cause calcium carbonate therein to release carbon dioxide into said conduit.

Particularly desirably, the apparatus comprises a first such member and a second such member in thermal contact with each other via a gas-impermeable, thermally conducting barrier (e.g. a metal barrier) whereby the first member may in one stage of operation serve to absorb carbon dioxide from a conduit while the second member is functioning to release carbon dioxide into a conduit and in a further stage of operation the second member may serve to absorb carbon dioxide from a conduit while the first member is functioning to release carbon dioxide into a conduit. In this embodiment, the apparatus is preferably provided with a valve which may be operated to direct the carbon dioxide-containing gas alternatively past the first or second member and also desirably a further valve which may be operated to direct an essentially hydrogen-free gas past the other member .

Thus, for example, in one preferred embodiment, the members may be arranged concentrically between inner and outer concentric conduits with an intervening concentric gas impermeable barrier, and preferably within an outer concentric gas-impermeable shell. In an alternative preferred embodiment, two or more parallel conduits are separated by one or more gas impermeable barriers, with the members either forming the walls of the conduits or being disposed on either side of the barrier (s) between adjacent conduits. In this way an array of two or more conduits, e.g. 1x2, 1x4, 2x2, 2x3, 2x4 or 4x4 conduits may simultaneously operate to absorb or release carbon dioxide from or into the gas passing through them.

In a further preferred embodiment, the members are enclosed by a gas-impermeable, thermally conducting barrier and have extending therethrough a gas flow conduit.

In a yet further preferred embodiment the members have extending therethrough a first gas flow conduit from which gas may permeate the member and a second conduit having a gas-impermeable, thermally conducting wall whereby gas therein may not permeate the member .

The conduit through the members may be a single conduit or a plurality of conduits from which gas may permeate the same volumes of the member, i.e. it may provide multiple flow paths so as to increase the efficiency of carbon dioxide uptake or release.

Such conduit arrangements are novel and form a further aspect of the invention. Viewed from this aspect the invention provides a conduit device comprising an outer gas-impermeable wall containing therein a conduit and a gas permeable ceramic member containing calcium oxide or calcium carbonate, whereby gas from said conduit may permeate said member, said conduit device having a gas- impermeable thermally conductive portion (preferably a barrier) wherefrom heat may be transferred to or from said member. If desired, the barrier may be provided by the said outer wall .

The ceramic may also if desired contain a material which may function as a catalyst in one or more stages of methane reformation or water-shift reactions, for example an iron oxide. With the calcium oxide / carbonate present in the ceramic, some or all of the remaining material making up the ceramic is preferably a clay. For this purpose, any clay from which a ceramic can be prepared may be used. The preferred clays and preferred means of manufacturing the ceramic are discussed further below. Generally however the calcium oxide / carbonate content (expressed in terms of the content of the calcium oxide / carbonate when in calcium carbonate form) of the dry ceramic will preferably be from 30 to 90% wt, more preferably 50 to 85%, especially 60 to 85%, particularly 75 to 80% wt. At too high a metal carbonate content, the ceramic becomes friable and difficult to handle; at too low a calcium carbonate content, the amount of carbon dioxide absorbable per unit volume is undesirably low. In general, gas porosity is substantially similar at 50 to 80% wt. calcium carbonate content.

Where the apparatus of the invention is to be used on a continuous basis, it can desirably be provided with an expansion tank or tanks arranged to receive the gas flow to or from the conduit during changeover from CO₂ absorption to CO₂ release in specific lengths of the conduit or alternatively it may be provided with extra lengths of conduit so that each such length is either in CO₂ uptake, CO₂ release or changeover mode. During changeover mode, the gas content of any such length may be driven out by a gas which does not cause any significant CO₂ release or uptake as desired.

It is also preferred that the apparatus be provided with sensors to monitor CO₂ uptake by the members (e.g. weight sensors or CO₂ sensors) in order that cycling between CO₂ uptake and CO₂ release may be achieved most efficiently. Typically such sensors will be functionally connected to a controller, e.g. a computer, which will switch from CO₂ uptake once a predetermined amount of CO₂ has been taken up, e.g. at least 50%, more preferably at least 75%, for example less than 80% of the theoretical maximum determinable in laboratory conditions for the material used. The use of such a sensor is particularly desirable when the members are fresh or are reaching the end of their operating lives . It is not necessary to monitor CO₂ uptake along the whole length of the CO₂ uptake conduit; desirably this is done at a small number of points, e.g. 1 to 6 points, along the length, preferably including at least one between 20 and 80% of the distance along this length.

The apparatus of the invention has four particularly preferred formats. In the first it comprises a methane reformation reactor, a shift reactor and a carbon dioxide absorber in series; in the second it comprises a carbon dioxide absorber, optionally fed by a combustion apparatus (e.g. an engine or a heat and power generator) ; in the third it comprises a combined methane reformation reactor, shift reactor and carbon dioxide absorber (i.e. a fully integrated reactor); and in the fourth it comprises a methane reformation reactor and a combined shift reactor and carbon dioxide absorber (i.e. a partially integrated reactor) .

Where the apparatus comprises a methane reformation reactor, this may for example be a steam reformer or a thermal reformer. Since the process performed within the reformation reactor generates carbon dioxide, the reactor may comprise a gas porous ceramic calcium oxide/carbonate member as described herein which will operate to remove some of the carbon dioxide that is generated. As described, it is preferred that these members be arranged so that they can be switched between absorption and desorption modes so allowing continuous operation of the reactor. Where the apparatus comprises a shift reactor, this may for example be a "low" temperature shift reactor or a "high" temperature shift reactor, or both may be present. As described above, the shift reactor may comprise a gas porous ceramic calcium oxide/carbonate member as described herein which will operate to remove some of the carbon dioxide that is generated and that is present in the gas fed into the shift reactor. Once again, it is preferred that these members be arranged so that they can be switched between absorption and desorption modes so allowing continuous operation of the reactor .

Where the apparatus comprises a separate carbon dioxide absorber, this will preferably comprise a gas porous ceramic calcium oxide/carbonate member as described herein which will operate to remove some of the carbon dioxide that is present in the gas fed into the absorber. Once again, it is preferred that these members be arranged so that they can be switched between absorption and desorption modes so allowing continuous operation of the absorber.

Typically the reactor temperature in a steam reformer for methane (normally natural gas, optionally desulphurized by conventional techniques) is in the range 500 to HOO⁰C, more preferably 700 to 1020⁰C. The inlet gas is a steam/methane mixture and the outlet gas a steam/hydrogen/carbon dioxide/carbon monoxide/methane mixture. However, when this is done, the operating temperature is preferably kept below 1000⁰C, especially below 950⁰C to avoid undue inactivation of the carbon dioxide absorber.

Where the reformation is by thermal reformation (i.e. also involving addition of oxygen) , the operation temperatures are similar, although operation towards the higher ends of the ranges specified may be preferred.

The outlet gas from the reformer is preferably passed through a heat exchanger before passing into a shift reactor in which the carbon monoxide is transformed to carbon dioxide by reaction with steam and optionally also with a reducable inorganic agent, e.g. Fe₂O₃. Typically either co-called low or high temperature shift reactors (or a combination of both) may be used. Low temperature shift reactors typically operate at 200 to 35O⁰C, while high temperature shift reactors typically operate at 350 to 750⁰C. The preferred operational temperature range is generally 400 to 700⁰C. The outlet gas from the shift reactor is essentially wet hydrogen, possibly with some relatively low content of CO and CO₂. Any remaining carbon oxide content may be removed by passing the outlet gas through a methanation or PROX (preferential oxygenation) reactor whereafter heat is removed (and water is condensed out of the hydrogen flow) by passage through a heat exchanger. The hydrogen may then be fed to a fuel cell and subsequently to an afterburner. The exhaust gas from the afterburner may then be fed to a further carbon dioxide absorber, optionally using a calcium oxide containing material according to the invention. Heat from the afterburner and the heat exchangers may be used to bring the inlet gases for the steam reformer to the desired temperature.

If desired, two shift reactors, in series, may be used, the first being a high temperature shift reactor and the second a low temperature shift reactor.

As an alternative to a methanation or PROX reactor, a pressure swing absorber (PSA) may be used to produce a high pressure pure hydrogen stream. The purge gas from the PSA may then be used to produce a second fuel stream. In a further alternative, the output gas from the steam reformer may be passed to a PSA to produce a high pressure hydrogen stream which may be fed to a fuel cell. The purge gas from the PSA may then be fed into a shift reactor and CO₂ absorber lined with a calcium oxide containing material according to the invention. The output from the shift reactor thus provides a second fuel stream.

If desired the shift reactors may not function as carbon dioxide absorbers but instead may be followed by carbon dioxide absorbers in which the gas flow conduit is lined with a calcium oxide containing material according to the invention. The gas temperatures in the absorber will typically be in the range 350 to 975⁰C, preferably 400 to 860⁰C. The absorber may if desired be preceded or followed by a PSA.

At these operating temperatures, the calcium oxide containing material readily serves to absorb carbon dioxide from the gases flowing through the reactor while it undergoes the reformation and shift reactions . By diverting gas flow to alternative conduits through the reactors (or carbon dioxide absorbers), the now calcium carbonate containing material, which will remain at similar temperatures, will then release carbon dioxide into the conduits from which it may be drawn off.

Carbon dioxide release may be triggered by increasing temperature, reducing pressure or reducing carbon dioxide partial pressure, or a combination of these. Reference to Figure 12 of the accompanying drawings will show how, at a given temperature (or carbon dioxide partial pressure) , a change in carbon dioxide partial pressure (or temperature) can move the calcium oxide/ carbonate system from carbon dioxide absorbing to carbon dioxide desorbing or vice versa. Preferably however, any temperature increase to promote carbon dioxide desorption will not be to a temperature above HOO⁰C, especially not to one above 1000⁰C. In general however a temperature increase to a temperature above 860⁰C may be appropriate.

For the carbon dioxide desorption phase, the calcium oxide/carbonate system may be contacted with an inert gas (e.g. nitrogen) so as to reduce the carbon dioxide partial pressure. However this results in an effluent gas which is not essentially only carbon dioxide. It is therefore preferred to use a condensable gas (e.g. steam or less preferably an organic solvent) or carbon dioxide. However, during the desorption phase there is no requirement for a gas flow through the desorbing part of the apparatus: desorption may be effected without gas flow, with the desorbed carbon dioxide subsequently being flushed from the apparatus . In general therefore there are four main options for the desorption phase: flush with steam; reduce pressure, e.g. with a vacuum pump; increase temperature; or flush with carbon dioxide at an elevated temperature.

The retrieved carbon dioxide is preferably compressed or liquefied for transport and disposal e.g. by injection into subterranean formations .

The operating temperature in any portion of the apparatus according to the invention which contains a calcium oxide/carbonate ceramic carbon dioxide absorbing member according to the invention will preferably be below 1100⁰C, more preferably below 1000⁰C so as to avoid inactivation.

The calcium oxide / carbonate containing ceramic material may for example be prepared as follows : clay and water are mixed and allowed to stand so that coarse particles (e.g. mode particle sizes above 60 :m) settle out; the upper layer of water and clay is separated off; coarser calcium carbonate particles (e.g. mode particle size of less than 100 μm, e.g. 2 to 50 :m) are mixed in with the separated clay and water to the desired content) ; the mixture is poured into a mould and dried; the dried material is sintered (e.g. in air, vacuum or CO₂) ; and, optionally, the sintered material is exposed to carbon dioxide (e.g. to facilitate transport and storage) .

Thus viewed from a further aspect the invention provides a process for the production of a ceramic product, said process comprising: mixing clay and water,- adding calcium carbonate particles to the desired content; extruding or moulding the resulting mixture; drying and sintering the extruded or moulded product; and, optionally, exposing the sintered material to carbon dioxide.

The carbon dioxide released on sintering is preferably captured for disposal.

The calcium carbonate used is preferably calcite or dolomite.

The mould used in this process, a process which forms a further aspect of the invention, is preferably a water- absorbent material, e.g. gypsum, so as to prevent deformation during drying.

Sintering is preferably effected at 800 to 1000⁰C, especially preferably 850 to 925°C. Exposure to carbon dioxide is preferably effected at a temperature above 520⁰C, e.g. above 550⁰C, to avoid formation of calcium hydroxide and to expedite the reaction.

The material may be extruded or moulded into blocks which can be cut and/or built up so as to form structures of the desired shape for use according to the invention; alternatively they can be extruded or moulded in the desired shape, e.g. with channels or voids which will function as the gas flow conduits . In this latter case, the mixture may be added to the mould stepwise so as to build up the desired shape gradually.

The ceramic material may also be produced in particulate or pelletized form, for example for use in an embodiment of the invention in which the ceramic is contained within a porous-walled container.

Viewed from a further aspect the invention provides a ceramic containing at least 60% wt. of calcium carbonate or oxide (calculated as the carbonate) .

The clay used may be any clay suitable for ceramic formation. Examples of preferred clays are set out in WO 02/081409, the content of which is incorporated herein by reference. WO 02/081409 describes preparation via a calcium hydroxide stage of building blocks which have a relatively low calcium carbonate content in order to have the strength necessary for the desired end use.

The ceramic of the invention, when in the calcium oxide containing state, may also be used as a carbon dioxide absorber in other circumstances than in methane reformation, e.g. to absorb carbon dioxide from the exhaust gas of a hydrocarbon burner, for example a heat and/or power generator. For such uses, the ceramic is preferably preconditioned by being subjected to at least one, preferably at least two, e.g. 3 to 10, carbon dioxide absorption and release cycles, as in this way performance is improved. The carbon dioxide loaded ceramic may be collected from the fuel burning site for centralized regeneration and subsequent re-use or it may simply be disposed of as landfill or on fields . The carbon dioxide loaded ceramic material may also be used as a building material or as a fertilizer. Ceramic used in methane reformation may similarly be disposed of. Such disposal represents an environmentally friendly means of carbon dioxide disposal .

Such use of the ceramic of the invention forms a further aspect of the present invention.

The reader will appreciate that using calcium oxide/ carbonate ceramics according to the invention allows processes such as methane reformation and shift processes to be effected at higher pressures than is feasible otherwise and that this also allows facile incorporation into the ceramic of catalysts desirable for use in such processes .

Preferred embodiments of the process, apparatus and ceramics of the invention will now be described further with reference to the following non-limiting Examples and the accompanying drawings in which:

Figures 1 to 7 are schematic drawings of integrated apparatus for methane reformation;

Figures 8 to 11 are schematic drawings of ceramic-walled gas flow conduits for use in methane reformation apparatus ; and

Figure 12 is a plot of temperature versus carbon dioxide partial pressure showing the thermodynamic equilibrium between calcium oxide and calcium carbonate.

Example 1

Ceramic Production

Blue clay from Trøndelag was mixed with water. In order to remove the largest fractions of the clay, and any impurities such as stones and sand, sedimentation in water was used to separate out the clay fraction finer than 25 :m. (The mixture of clay and water is allowed to stand so that coarse particles settle out and the upper layer of water and clay is separated off) .

A sample of the mixture of water and clay (<25 :m) was dried in order to find the dry weight of the mixture

(the weight content of the clay) . Calcite was then added to produce samples containing 20 to 50% wt clay

(e.g. 25% wt) and 80 to 50% wt calcite (e.g. 75% wt) on a dry solids basis using the clay/water mixture (which had a 25% wt clay content) . These samples were stirred and dried in a plaster mould. The calcite was bought from Franzefoss KaIk AS (Franzit Micro with specification (average) (6 :m) 98% CaCO₃, 0.8% MgCO₃, 0.4% SiO₂, 0.2% Al₂O₃ and 0.15% Fe₂O₃, <0.1% Na₂O). The samples were sintered in an atmosphere of air, vacuum, or CO₂ in an oven. The sintering temperature was typically 850-1000⁰C, preferably below 950⁰C (lower sintering temperatures gave better ability to absorb CO₂) . Calcination takes place in the temperature range 600-1100⁰C depending on the partial pressure of CO₂. The sintering time was e.g. 2-6 hours (dependent on the temperature, e.g. 2 hours at 1050⁰C) . The oven was heated relatively slowly up and cooled slowly down, e.g. 300°C/h, in order not to break the ceramics. The sintering temperature is preferably 850-925⁰C.

The ceramics are ready to absorb CO₂ after the sintering. However, if the material is to be transported, or stored it might be beneficial to expose the ceramic to CO₂, e.g. at 700⁰C for 10-20 hours. The ceramic is stronger when it has absorbed CO₂ and CaO is transformed into CaCO₃. However, then the material needs to be treated to desorb the CO₂ before it can be used to capture CO₂. This can be done either by changing the partial pressure of CO₂ in the ceramics atmosphere or elevating the temperature (as described before) .

Referring to Figures 1 to 7 , there are shown schematically apparatus arrangements for methane reformation according to the invention. The boxes represent different reactors or vessels for different process steps. A dotted walled box indicates that the reactor or vessel is optional . Dashed lines indicate that the gas flow indicated is optional . HT and LT represent high and low temperature. Q represents energy removal, e.g. by heat exchange. FC represents fuel cell. SOFC represents solid oxide fuel cell. Alt. Fuel indicates that the gas may be used as a combustible fuel.

Referring to Figure 8, there is shown a reactor element 1 having concentric gas flow conduits 2 and 3 separated by two concentric ceramic tubes 4 and 5 which are separated by a gas impermeable, thermally conducting metal barrier 6. While the inner ceramic tube is absorbing CO₂, the outer ceramic tube is desorbing CO₂. When the inner tube has reached its absorption limit, gas flow is switched and the outer ceramic tube is used for CO₂ absorption. Operating at similar temperatures, CO₂ desorption may be triggered for example by lowering the partial pressure of CO₂ in the conduit into which CO₂ is to be released.

Figure 9 shows a reactor element 7 operating on the same principle as that of Figure 8 but with a plurality of parallel gas flow conduits 8, 9, 10, 11, 12, 13 in two ceramic blocks 14 and 15 separated by a barrier 16.

Figure 10 shows two reactor elements 17 and 18 provided with gas flow conduits 19 and 20, "closed" conduits 20 being separated from the ceramic blocks 21 by gas impermeable barriers 22. In this way the gas flow from the reformer may be passed through the closed conduits of element 18 before entering the CO₂ absorber section in which it flows through the "open" conduits 19 of element

17. In element 17 the closed conduits are used for coolant flow. When element 17 has reached its CO₂ absorption limit, gas flow may be switched. In element

18, the closed conduits are used to carry a heated gas so as to raise the temperature of the element and cause CO₂ desorption into the open conduits .

Figure 11 shows reactor element arrays 23 and 24 each comprising four ceramic blocks 25, surrounded by a gas impermeable barrier 26 which serves as a heat exchange surface, and having running through their centres a gas flow conduit 27. Array 23 may be used for CO₂ absorption while array 24 is used for CO₂ desorption and vice versa.

Claims

Claims :

1. A process for the extraction of carbon dioxide from a carbon dioxide-containing gas by calcium oxide by formation of calcium carbonate, said process comprising flowing said carbon dioxide-containing gas past or through a gas-porous, calcium oxide-containing ceramic member disposed in a reactor vessel whereby said carbon dioxide-containing gas may permeate said member, and preferably subsequently heating said member and/or exposing said member to a gas having a lower carbon dioxide partial pressure than said carbon dioxide- containing gas whereby to cause calcium carbonate therein to release carbon dioxide, said member defining at least part of the inner wall of a gas flow conduit within said reactor vessel.

2. A process as claimed in claim 1 wherein said carbon-dioxide-containing gas is synthesis gas.

3. A carbon dioxide separation apparatus comprising a vessel having a gas inlet and a gas outlet and a gas flow conduit linking said inlet and outlet, at least part of an inner wall of said conduit being provided by a gas porous ceramic member containing calcium oxide and/or carbonate, wherein carbon dioxide-containing gas flowing through said conduit permeates said member permitting carbon dioxide in said gas to react with calcium oxide in said member to form calcium carbonate, said apparatus being arranged such that said member may be heated and/or exposed to a gas having a lower carbon dioxide partial pressure than said carbon dioxide- containing gas whereby to cause calcium carbonate therein to release carbon dioxide into said conduit.

4. Apparatus as claimed in claim 3 comprising a first said gas porous ceramic member and a second said gas porous ceramic member, said first and second members being in thermal contact via a gas-impermeable thermally conducting barrier.

5. Apparatus as claimed in claim 4 having a valve operable to direct said carbon dioxide containing gas alternatively past the first and the second said member.

6. Apparatus as claimed in any one of claims 3 to 5 comprising a methane reformation reactor, a shift reactor and a carbon dioxide absorber in series .

7. Apparatus as claimed in any one of claims 3 to 5 comprising a carbon dioxide absorber fed by a combustion apparatus .

8. Apparatus as claimed in any one of claims 3 to 5 comprising a combined methane reformation reactor, shift reactor and carbon dioxide absorber.

9. Apparatus as claimed in any one of claims 3 to 5 comprising a methane reformation reactor and a combined shift reactor and carbon dioxide absorber.

10. A conduit device comprising an outer gas- impermeable wall containing therein a conduit and a gas permeable ceramic member containing calcium oxide or calcium carbonate, whereby gas from said conduit may permeate said member, said conduit device having a gas- impermeable thermally conductive portion wherefrom heat may be transferred to or from said member.

11. A device as claimed in claim 10 having a plurality of said conduits arranged in parallel .

12. A device as claimed in claim 11 wherein at least one said conduit is present on each side of said gas- impermeable thermally conductive portion.

13. A process for the production of a ceramic product, said process comprising: mixing clay and water; adding calcium carbonate particles to the desired content; extruding or moulding the resulting mixture; drying and sintering the extruded or moulded product; and, optionally, exposing the sintered material to carbon dioxide .

14. A ceramic containing at least 60% wt. of calcium carbonate or oxide (calculated as the carbonate) .

15. A ceramic as claimed in claim 14 further containing a catalyst.

16. The use of a ceramic as claimed in claim 14 as a carbon dioxide absorber.

17. The use of a ceramic as claimed in claim 14 for repeated cycles of carbon dioxide absorption and release.

18. Use as claimed in either of claims 16 and 17 as a recyclable carbon dioxide absorber.

19. The use of a ceramic as claimed in claim 14 as a fertilizer or building material following its use as a carbon dioxide absorber.