GB2544802A

GB2544802A - A combustion method

Info

Publication number: GB2544802A
Application number: GB1521018.0A
Authority: GB
Inventors: Ingvar åsen Knut; Solgaard Andersen Henrik
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2015-11-27
Filing date: 2015-11-27
Publication date: 2017-05-31
Anticipated expiration: 2035-11-27
Also published as: GB2544802B; GB201521018D0

Abstract

A method comprises combusting a fuel with an oxidant in a combustion process, and desorbing oxygen (O2) adsorbed in an adsorbent material utilising heat produced by the combustion process. The desorbed oxygen may be provided to the combustion process as oxidant and the utilised heat is preferably heat from flue gas produced by the combustion process. The method has particular relevance in the context of flue gas recirculation wherein flue gas is recirculated to the combustion process. Preferably the flue gas is produced in a combustion chamber 30 by combusting fuel such as natural gas or coal with desorbed oxygen 16 received from an oxygen desorber unit 36 prior to being input to the combustion process to provide an increase in carbon dioxide (CO2) in the flue gas thereby improving the ability to separate and capture carbon dioxide.

Description

A Combustion Method

The present invention is concerned with the problem of increasing the C02 concentration in a flue gas from a combustion process (e.g. a power and/or heat generation plant) so as to facilitate improved C02 separation and capture from the flue gas, particularly in the context of a flue gas recirculation (FGR) process. In particular, the present invention is concerned with efficient methods of obtaining and providing oxygen to the combustion process to thereby increase C02 concentration.

The capture of C02 from the flue gas of combustion processes such as power generation processes is an important task in the bid to reduce global C02 emissions. Typically, in a post-combustion carbon capture system, the flue gas from the combustion process (e.g. from a coal-fired or combined cycle gas turbine process) is processed through a C02 capture technology such as amine absorption or membrane separation, in order to remove the C02 from the flue gas. This C02 is then typically transported for sequestration underground. Amine absorption is discussed for example in Rochelle, G. T., Amine Scrubbing for C02 Capture, Science 2009, 325, 1652-1654 and membrane separation processes are known for example from US 7964020.

However, such C02 capture technologies are driven by the partial pressure of the C02 and consequently the lower the concentration of C02 in the flue gas, the more energy intensive the separation and capture process.

This can be a particular issue with combined cycle gas turbine power plants that have a lower flue gas C02 concentration than for example coal fired plants. A gas turbine plant has to operate with a large excess of air so as to keep the temperature at a level that the turbine blades can withstand, and as a result the C02 in the exhaust is typically only 3.5-4.2 vol% compared with 12-14 vol% from a coal fired plant.

Flue gas recirculation (FGR, also known as exhaust gas recirculation EGR, and also sometimes referred to as flue gas recycling) is one technology that has been developed to help address this problem (as discussed in “Selective Exhaust Gas Recycle with Membranes for C02 Capture from Natural Gas Combined Cycle Power Plants”, Merkel T. etal, Ind. Eng. Chem. Res., 2013, 52 (3), pp 1150-1159). By recirculating (recycling) part of the flue gas back to the combustion chamber of the combustion process to act as oxidant in addition to air, the C02 concentration in the flue gas is increased. Consequently, the energy required to separate and capture that C02 is reduced, and thus the overall efficiency of the power plant is improved. (It should be noted here that the terms “flue” and “exhaust” are used here interchangeably to mean the same thing, i.e. the gas that results from a combustion process, regardless of what that process is).

However, the use of recycled (recirculated) flue gas as oxidant in the combustion chamber also lowers the oxygen content in the combustion chamber (as the recycled flue gas has a lower 02 content of around 0.5-5vol% than the normal oxidant, air). In a combined cycle gas turbine plant the oxygen content in the combustion chamber if FGR is not used is around 13 vol%, but when recycling fluegas the oxygen content can decrease to as low as 4-5 vol%. For a boiler plant, oxygen can decrease down to 0.5 vol%.

This means that less oxygen is available to react with the fuel to generate C02, thus limiting the concentration of C02 achievable in the flue gas.

Furthermore, reduced 02 in the combustion chamber can cause reduced combustion stability and lower flame temperatures that lead to increased carbon monoxide production.

As a result, not more than 40-50 % (typically for Natural Gas Combined Cycle Power Plants ) of the flue gas can be recycled in an FGR process, with a maximum C02 content in the flue gas being in the region of 8-9 vol% for a gas turbine, and 13-15 vol% for a coal-fired boiler. In boiler applications less than 10 vol %, e.g. just a few % of the flue gas may be recycled to control NOx output. “Selective” FGR is also known in the art, which, instead of recirculating (recycling) the flue gas, selectively recycles C02 from the flue gas by first separating it from the flue gas using e.g. a membrane separator. For example, the combustion air stream can be used as a sweep gas in a membrane separator to selectively pick up C02 from the flue gas. The higher content of C02 thus in the combustion chamber results in a higher concentration of C02in the flue gas. However, this still suffers from the same problems caused by reduced oxygen, as discussed above.

The document “Flue Gas Recirculation in Gas Turbine: Investigation of Combustion Reactivity and NOx Emission”, Guethe F. etal, Proceedings of ASME Turbo Expo 2009, June 8-12 2009 Orlando, p179-191 recognises that the maximum amount of flue gas that can be recirculated is dependent on the lowest technically acceptable 02 concentration in the combustor. However no solutions are proposed. “Impacts of exhaust gas recirculation (EGR) on the natural gas combined cycle integrated with chemical absorption C02 capture technology”, Li H. et al, Energy Procedia 4 (2011) 1411-1418, also recognises that EGR reduces the 02 concentration in the exhaust gas, which can have negative effects on combustion stability. It proposes that this could be addressed by injecting additional oxygen into the combustor, however it notes that the additional energy consumption required to produce the 02 would have a significant impact on the efficiency of the process.

According to a first aspect, the present invention provides a method comprising: combusting a fuel with an oxidant in a combustion process; desorbing oxygen adsorbed in an adsorbent material utilising heat produced by the combustion process; and providing the desorbed oxygen to the combustion process as oxidant.

The invention is most useful in the context of flue gas recirculation (FGR) and carbon capture, wherein as discussed above in relation to the prior art the reduced oxygen in the combustion process limits the concentration of C02 achievable in the flue gas and thus the proportion of C02 that can be separated and captured. Thus, in one embodiment, the method of the invention further includes the step of recirculating (recycling) flue gas to the combustion process. In fact, the invention may be seen as a method of flue gas recirculation, comprising: combusting a fuel with an oxidant in a combustion process, recirculating flue gas to the combustion process, desorbing oxygen adsorbed in an adsorbent material utilising heat produced by the combustion process; and providing the desorbed oxygen to the combustion process as oxidant. Preferably, the flue gas is enriched with desorbed oxygen prior to recirculating the flue gas to the combustion process.

As mentioned above, flue gas recirculation (FGR, also known as exhaust gas recirculation EGR) is sometimes referred to as flue gas recycling, and references herein to recycling flue gas mean the same thing as recirculating flue gas, and vice versa.

The present inventors have thus recognised that an oxygen source can be obtained by utilising the heat of the combustion process itself to drive the desorption of oxygen adsorbed in an adsorbent material. The adsorbed oxygen will preferably have been adsorbed from air. Thus, the heat of the combustion process itself is used to facilitate the separation of oxygen from air via desorption. Consequently, the majority (if not all) of the energy required to separate oxygen is provided by the combustion process itself, without the need for any substantial external energy source. As a result this is far more energy efficient than e.g. providing an oxygen source in other ways such as using cryogenic separation. Consequently oxygen can be provided to the combustion process without the significant energy penalty warned of in the prior art.

The oxygen content available to react with the fuel is thereby increased in the combustion process, resulting in a greater concentration of C02 in the flue gas. This consequently enables a greater proportion of C02 to be captured in a downstream carbon dioxide separation and capture process.

It will be understood that a combustion process as referred to herein will typically utilise a combustion chamber in which fuel is combusted with an oxidant. Useful work will be generated by means of e.g. a boiler or gas turbine, together with flue gas.

Whilst it will be understood that there are many possible ways to implement the utilisation of heat produced by the combustion process, preferably, the heat produced by the combustion process that is utilised to desorb oxygen is “waste” heat, in the sense that it is heat not otherwise used to produce useful work. Most preferably, the utilised heat is heat from flue gas produced by the combustion process. It should again be noted here that the terms “flue” and “exhaust” are used here interchangeably to mean the same thing, i.e. the gas that results from a combustion process, regardless of what that process is. Thus, flue gas may be that from a boiler process (utilising any fuel), a combined cycle gas turbine process, a fired heater process, or indeed any other suitable combustion process.

Furthermore, the term “flue gas” is intended to mean both flue gas that directly comes from a combustion chamber, and flue gas that has been processed in some way (e.g. heated or cooled; has carbon dioxide removed).

In one embodiment of the invention the desorbed oxygen is provided to the combustion process by enriching flue gas with desorbed oxygen to form oxygen enriched recycled flue gas, and recycling (recirculating) this to the combustion process. In this embodiment the flue gas may directly contact the adsorbent material so as to directly heat the adsorbent material, thereby desorbing oxygen into the flue gas so as to enrich the flue gas with oxygen. Thus the flue gas is acting as a purge gas in the desorption process. This is particularly preferred since it is essentially a “one-step process”, i.e. oxygen is desorbed directly into the gas that is heating it and driving the desorption.

In an alternative embodiment, the flue gas indirectly heats the adsorbent material so as to desorb oxygen. Preferably, the flue gas heats a heating medium by heat exchange, and this heating medium heats the adsorbent material to thereby desorb oxygen. Oxygen desorbed in this way can be mixed with the flue gas downstream of the oxygen desorption process. Alternatively, it may be directly input to the combustion process or it may be mixed with a different process stream (other than flue gas) which is being input to the combustion process. However it is done, in the invention the desorbed oxygen is provided to the combustion process as oxidant.

Although in the preferred embodiment described above the flue gas is enriched with oxygen prior to being recycled to the combustion process, in fact, any gas stream to be recycled to the combustion process can be enriched with desorbed oxygen. For example, any C02 containing gas stream from a downstream C02 capture plant can be enriched with oxygen. In a capture plant using cryogenic C02 purification in combination with membranes, the C02 containing purge gas could be enriched with oxygen and recycled.

In one embodiment, the method of the invention further comprises a downstream carbon dioxide separation process for separating carbon dioxide from the flue gas of the combustion process, the method further comprising the step of enriching any carbon dioxide containing gas stream generated in the separation process with desorbed oxygen and recycling this to the combustion process. For example this may be used in a “selective FGR” process. As discussed previously, selective FGR selectively recycles C02 from the flue gas by first separating it from the flue gas using e.g. a membrane separator. This separated C02 may be enriched with desorbed oxygen before being recycled to the combustion process. This would normally be in addition to recycling flue gas itself, which may or may not additionally be enriched with oxygen.

In one embodiment, recycled flue gas is mixed with any C02 containing gases recycled from a C02 capture plant before being recycled to the combustion process. The flue gas and/or the C02 containing gases may be enriched with oxygen before being mixed, or they may be mixed and then enriched with oxygen.

Thus in one embodiment the method of the invention further comprises a downstream carbon dioxide separation process for separating carbon dioxide from the flue gas of the combustion process, the method further comprising mixing any carbon dioxide containing gas stream generated in the separation process with flue gas, before or after enriching the flue gas with desorbed oxygen.

In an embodiment in which the flue gas is enriched with desorbed oxygen, the oxygen content of this flue gas may be 7 to 21 vol %, preferably 9 to 12 vol %.

Preferably, desorbed oxygen is used as oxidant in the combustion process in combination with air. For example, flue gas enriched with desorbed oxygen may be recycled to the combustion process to act as oxidant in combination with air.

If oxygen enriched recycled flue gas were to be used on its own, its oxygen content of typically 9-12% might not be enough to provide stable combustion, dependent on application. However by also using air as oxidant, the higher oxygen content of air (21%) in combination with oxygen enriched flue gas enables more stable combustion. The weight fraction of oxygen as oxidant in the combustion process provided by the air is preferably less than 25% of the total oxygen, most preferably less than 10%, depending on application. In one embodiment the fraction of oxygen provided by the fresh air is preferably between 10-25 vol% of the total amount of oxygen.

Conversely, the weight fraction of oxygen as oxidant in the combustion process provided by the desorbed oxygen is preferably greater than 75% of the total oxygen, most preferably greater than 90%.

Preferably, the invention enables the C02 content in the flue gas of the combustion process to be above 20 vol % for a natural gas fired gas turbine application; above 25 vol % for a natural gas fired boiler application; and above 30 vol% for a coal-fired boiler. This represents a significant improvement on the C02 content of prior art FGR processes, which for a gas turbine is in the region of 7-8%; for a coal-fired boiler is in the region of 13-15% and for a natural gas fired boiler in the region of 8-10%.

The obtainable C02 concentration in the flue gas will depend to an extent on the amount of desorbed oxygen that is provided to the combustion process. This depends for example on the oxygen selectivity of the adsorbent material and thus the oxygen available to be desorbed. An ideal adsorbent material is one that has a high adsorbent capacity for oxygen and a low adsorbent capacity for carbon dioxide at temperatures below 500°C. Preferably, the adsorbent material comprises any oxide, functional material or nanostructured material such as carbon nanotubes, with the capability to adsorb and desorb oxygen at a temperature below 500°C.

One particularly preferred material is Yo sTbo sBaCO^y.

When the oxygen enriched flue gas is recycled to the combustion process, the C02 in the flue gas essentially replaces nitrogen as an inert component in the combustion process, because the flue gas has more C02 and less nitrogen than the normal oxidant, air. This should beneficially depress the generation of NOx. C02 is also a better temperature moderator than nitrogen and will likely keep NOx formation at a lower level (NOx formation can be reduced by reducing the combustion temperature, see e.g. http://www3.epa.gov/ttncatd/dir1/fnoxdoc.pdf).

Preferably, the oxygen desorption process operates at a temperature below 500°C, to e.g. minimise heat recovery costs and avoid the use of expensive exotic materials in the heat exchangers. At temperatures below 500°C materials such as SS304 or SS315 stainless steel can be used, however above 500-500°C, softening or loss of strength occurs for these materials and more expensive alternatives would be required. Carbon steel which is cheaper can be used if the temperature is below about 400°C, see e.g. https://www.metabunk.org/attachments/174413923-28247782-carbon-steel-handbook-pdf.4544/. The temperature of the flue gas may be adjusted dependent on the adsorbent material used. For example, the temperature of the flue gas may be adjusted utilising a heat recovery process after combustion.

Preferably, an oxygen adsorption process adsorbs oxygen into the adsorbent material, which is subsequently desorbed in the desorption process utilising the heat of the flue gas. The oxygen adsorption and desorption processes may utilise e.g. any fluidised beds, or fixed beds, as known in the art.

In the embodiment where flue gas directly contacts the adsorbent material in order to heat the adsorbent material and desorb oxygen directly into the flue gas, carbon dioxide present in the flue gas may be undesirably adsorbed by the adsorbent material. In the case of a fixed bed process, when, subsequent to the oxygen desorption step, air is flushed through the bed so that further oxygen is adsorbed, the carbon dioxide may be flushed out with this air thus undesirably emitting to the atmosphere. To solve this problem, in one embodiment in which fixed beds are used, the method further includes an adsorber bed cleaning step. Preferably, after a step of desorbing oxygen by direct contact with flue gas optionally mixed with another C02 containing gas stream,, the method further comprises flushing out the fixed bed using an air stream, wherein the air stream is subsequently used in the combustion process as oxidant in combination with desorbed oxygen. Most preferably, the air stream flushes out carbon dioxide that has been adsorbed by the adsorbent material during the oxygen desorption step. Thus, the carbon dioxide flushed out is input to the combustion process, and can ultimately be recovered from the flue gas.

The fuel used for the combustion process can be any fuel containing carbon, such as natural gas or coal. The combustion process itself is generally for the purpose of power or heat generation, thus preferably, the method of the invention is a method of generating power and/or heat. The combustion process may typically be a boiler process (e.g. a coal-fired or gas-fired steam boiler), a gas turbine process (e.g. a combined cycle gas turbine utilising natural gas) or a fired heater which are used in many different processes e.g. within refineries (crude and vacuum heaters) or within gas conversion plants (cracking, reforming etc.)The combustion process will generally utilise a combustion chamber in which the combustion takes place.

Despite the enrichment of the recycled flue gas with oxygen, and the use of air in combination with the enriched flue gas as oxidant in the combustor, the lower overall oxygen content than air may carry a risk of unstable combustion. The present inventors have developed a solution to this problem in which two different oxidants are used, by providing at least one primary and a plurality of secondary burners. Flue gas produced by the combustion process is enriched with the desorbed oxygen to form oxygen enriched recycled flue gas. The oxygen enriched recycled flue gas is provided, optionally in combination with air, to the plurality of secondary burners as oxidant, whilst air is provided to the primary burner as oxidant. This enables a higher flame temperature to be achieved in the primary burner than that possible in the secondary burners as well as securing more complete combustion (at least in the primary burner). Furthermore, it keeps the temperature initially higher in the secondary burners which helps stabilise the combustion in the secondary burners. The flue gas from the primary burner is then mixed with flue gas from the secondary burners before being output as flue gas of the combustion process, to then be recycled.

This solution is particularly relevant to boiler applications, although can also be used with fired heaters and gas turbines (although in the latter separate compression of air will likely be required).

As an alternative to the use of one primary and a plurality of secondary burners, a single burner could be used with one nozzle for primary and a plurality of nozzles for secondary fuel. For large boilers, three or more of such burners having both primary and secondary fuel nozzles could be used.

During start-up of the combustion process, air is preferably provided to both the primary and secondary burners (or nozzles) in order to set up stable combustion. Subsequently, the air supplied to the secondary burners (or nozzles) can gradually be replaced with the oxygen enriched recycled flue gas. Thus, the secondary burners (or nozzles) should be able to operate both using air and oxygen enriched recycled flue gas.

This need to provide stable combustion during start-up of the combustion process is also more generally applicable, i.e. not only limited to the primary/secondary burner (nozzle) concept. Thus, preferably, prior to providing desorbed oxygen to the combustion process, the combustion process utilises only air as oxidant during the start-up phase. This also addresses the issue that at the commencement of the combustion process, there could not yet be any desorbed oxygen to use as oxidant, as the process is yet to produce any heat.

Most preferably, the method of the invention further comprises a postcombustion C02 capture process as known in the art, such as amine adsorption or membrane separation. Energy consumption will be reduced, C02 capture rate increased and equipment cost reduced in comparison with a normal FGR process without oxygen enrichment, due to the higher concentration of C02 in the flue gas. Thus, a greater amount of C02 can be captured more cost efficiently.

The invention also provides a method of increasing the carbon dioxide concentration in the flue gas of a combustion process that utilises flue gas recirculation (FGR), comprising carrying out the steps of the first aspect of the invention.

In yet another aspect the invention provides a method of flue gas recycling, comprising: combusting fuel with an oxidant in a combustion process, thereby producing flue gas; recycling part of the flue gas to an oxygen desorption process; utilising heat of the flue gas to desorb oxygen adsorbed in an adsorbent material thereby releasing oxygen, wherein the recycled part of the flue gas is enriched with this oxygen; and recycling the oxygen enriched flue gas to the combustion process.

In addition to the above described aspects, the present invention also provides a combustion system. Thus, according to a further aspect, the present invention provides a combustion system comprising: a combustion chamber; and an oxygen desorber unit; wherein the combustion chamber is arranged to combust fuel with an oxidant thereby producing flue gas; wherein the oxygen desorber unit is arranged to receive part of the flue gas, the oxygen desorber unit being further arranged to utilise heat from the flue gas to desorb oxygen from an adsorbent material; wherein the system comprises means to enrich the part of the flue gas received by the oxygen desorber unit with oxygen to form oxygen enriched recycled flue gas; and wherein the combustion chamber is arranged to receive the oxygen enriched recycled flue gas as oxidant for the combustion process in combination with air.

According to a further aspect, the present invention provides a method of enriching flue gas from a combustion process with oxygen, comprising: utilising the heat of the flue gas in an oxygen desorption process to desorb oxygen adsorbed in an adsorbent material; and enriching the flue gas with this desorbed oxygen.

The numerous optional and preferred features described above in relation to the method of the first aspect are also applicable to the other aspects of the invention described above.

Where “increasing the carbon dioxide content” is referred to in this specification, it is meant increasing the carbon dioxide content beyond that which it would have been if the recycled flue gas had not been enriched with oxygen. In the prior art wherein recycled flue gas is not enriched with oxygen, a typical CO2 content in the flue gas is around 7-8% for a gas turbine and 9% for a boiler, whereas in the invention it can be more than 15%, preferably more than 25%.

Preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 illustrates a boiler-based system operating according to a first embodiment of the present invention;

Figure 2 illustrates an alternative oxygen enrichment process C that may be used in the boiler-based system of the first embodiment;

Figure 3 illustrates an adsorber bed cleaning step that may be implemented in the alternative oxygen enrichment process of Figure 2;

Figure 4 illustrates a combined cycle gas turbine (CCGT) system operating according to a second embodiment of the present invention;

Figure 5 illustrates a membrane method that can be used in the carbon capture process B of the systems of Figures 1 and 4;

Figure 6 illustrates a combustion chamber comprising primary and secondary burners; and

Figure 7 illustrates a cross-section through the combustion chamber of Figure 6.

The illustrated embodiments relate to a method in which utilised heat is heat from flue gas, and in which flue gas is enriched with desorbed oxygen and recirculated to the combustion process.

The boiler-based system of Figure 1 has been divided into three components for ease of understanding: combustion and heat recovery unit A, C02 capture unit B and oxygen enrichment process C.

In the combustion and heat recovery unit A, air 1, fuel 2 (natural gas, coal or any fuel containing carbon) and oxygen enriched recycled flue gas 16 are fed to combustion chamber 30 of a steam boiler. The fuel 2 (e.g. coal or gas) is combusted utilising the oxygen enriched recycled flue gas 16 as oxidant in addition to air 1, generating H20 and hot flue gas 3 comprising C02. Generated heat is used to evaporate pre-heated boiler feed water 51 to produce steam 52, in order to control the combustion temperature.

The hot flue gas 3 is fed to fluegas heat recovery unit 31 in which it is used to superheat the steam 52 (generated in the combustion chamber 30) and pre-heat boiler feed water 51. Superheated steam 53 enters unit 32 that comprises either steam turbines or a heat transfer unit (using the steam as a heat source for any purpose), with condensed water 50 circulating back to the fluegas heat recovery unit 31.

One part of flue gas 3 is thereby cooled to below 200°C, preferably below 100°C thus generating flue gas stream 5 that is fed to C02 capture unit B. A second part of fluegas stream 3 is cooled to 500°C or below depending on the required operating temperature in the oxygen enrichment process C and is recycled as fluegas stream 9 and fed to oxygen enrichment process C.

In the C02 capture unit B, part-cooled flue gas 5 is further cooled using cooling water in cooler 33 to form cooled flue gas 6. The C02 concentration in cooled flue gas 6 is generally above 15 vol% and preferably above 25 vol% depending on the carbon content in fuel 2 and the C02 content in oxygen enriched recycled flue gas 16.

The cooled flue gas 6 is input to post-combustion C02 capture process 34 which separates C02 from the flue gas 6 to form C02 rich gas stream 7 and C02 depleted flue gas 8. Any suitable post-combustion C02 capture process may be used, e.g. C02 absorption using a solvent (e.g. amines), a C02 membrane process, a cryogenic process, a C02 adsorption process etc. An example membrane method is described later with reference to Figure 5. The temperature to which unit 33 further cools the part-cooled flue gas 5 may depend on the C02 capture process used.

Typically, 80 to 90% of the C02 is removed by process 34.

The oxygen enrichment process C utilises adsorption and includes adsorption unit 35 and desorption unit 36. Ambient air 12 is compressed (fan not shown) and preheated (in this case to about 200 to 300°C, the desired temperature will depend on the adsorption material used) in heat exchanger 39 by heat exchange with oxygen depleted air stream 14. This compressed, heated air 13 is fed to adsorber unit 35 countercurrent to the adsorbent material in the unit 35. The adsorbent material may be hotter than the air (depending on the cooling in cooler 38) and may heat up the air to e.g. 350°C or above. Unit 35 is a fluidised or moving bed unit. It produces oxygen depleted air 14 and oxygen enriched adsorbent material stream 20.

The oxygen depleted air 14 is collected and filtered at the top of the unit 35 and is cooled by heat exchange with ambient air 12 as discussed above, producing cooled oxygen depleted air 15 that is discharged to the environment.

The oxygen enriched adsorbent material stream 20 is preheated by unit 37 to produce heated oxygen enriched adsorbent material stream 21, that is fed to the top of desorber unit 36 where it flows downwards countercurrent to the recycled flue gas stream 9. Oxygen lean adsorbent material stream 22 is cooled in cooler 38 to form cooled adsorbent material stream 23 which is transported back to adsorber unit 35.

The recycled flue gas stream 9 containing 0.5 to 5% oxygen is fed to the bottom of desorber unit 36. The temperature of recycled flue gas stream 9 has been adjusted in heat recovery unit 31 to a temperature sufficient for recovery of adsorbed oxygen in desorber unit (bed) 36. The required temperature depends on the material used in unit 36 to desorb oxygen. Suitable materials include Yo.sTbo.sBaCCUOy (described in “Materials Research Bulletin 47 (2012) 518-520”) or similar, metal-organic framework type of materials: http://www.ncnr.nist.aov/Droarams/CHRNS/Ddf/SURF/2014 Gnewuch.pdf), or any material that can adsorb/desorb oxygen from air below about 400 to 450°C whilst keeping the temperature at a level that will allow use of low temperature construction materials (e.g. carbon steel). In the embodiment of Figure 1,

Yo.5Tbo.5BaC0407 is utilised as the adsorbent material that circulates between the adsorber unit 35 and desorber unit 36, with oxygen being adsorbed at about 350 to 390°C and desorbed at about 430 to 450°C. The temperature of cooled flue gas 9 is in this case at least 450°C.

In desorber unit 36 the heated oxygen enriched adsorbent material stream 21 flows downstream countercurrent to the flue gas stream 9. The heat of the flue gas stream 9 facilitates the desorption of oxygen from the heated oxygen enriched adsorbent material stream 21 into the recycled flue gas 9. This forms oxygen enriched flue gas 10 comprising at least 5-10% and preferably 10-15% oxygen which may be heated or cooled in heat exchanger 40 as necessary, becoming oxygen enriched flue gas 16 which is fed to the combustion chamber 30 in combustion and heat recovery unit A. A fan (not shown) should be installed upstream and/or downstream of desorber unit 36 in order to encourage the flow of cooled flue gas 9 and/or oxygen enriched flue gas 10.

An alternative version of desorber unit 36 is also envisaged in which, instead of direct contact between the adsorbent material and the recycled fluegas, heat transfer tubes are installed within the unit to heat up the adsorbent material in order to release the oxygen. This oxygen can then be mixed with the recycled fluegas upstream of the desorber. This would be particularly suitable if the material used would be degraded if in direct contact with either C02 from the fluegas or water vapour.

An alternative oxygen enrichment process C that can be used in the embodiment of Figure 1 is illustrated in Figure 2. Instead of the continuous fluidised bed process of Figure 1, two or more fixed beds 35A, 35B containing the adsorbent are used.

As with Figure 1, ambient air 12 is compressed (fan not shown) and preheated (in this case to about 250 to 280°C) in heat exchanger 39 by heat exchange with oxygen depleted air stream 14. This compressed, heated air 13 is fed to adsorber unit 35A containing a fixed bed with an appropriate oxygen adsorbent. Oxygen is adsorbed into unit 35A and oxygen depleted air 14 is fed back to heat exchanger 39 and cooled before being discharged to the environment.

Once oxygen has been adsorbed by the unit 35A to its maximum level, the air valve 13a is closed to stop the flow of air to this unit. Air valve 13b is then opened and heated air is instead fed to adsorber unit 35B containing the same type of oxygen adsorbent material as used in unit 35A so as to adsorb oxygen into unit 35B. Oxygen depleted air is fed back to heat exchanger 39 and cooled before being discharged.

Whilst this step of feeding heated air to adsorber unit 35B is carried out, recycled flue gas valve 9b is opened (9a is closed) so that recycled flue gas 9 is fed to unit 35A, which heats up the adsorbent such that oxygen is stripped off and enriches the recycled flue gas. This forms oxygen enriched flue gas 10 which may be heated or cooled in heat exchanger 40 as necessary, becoming oxygen enriched flue gas 16 which is fed to the combustion chamber 30 in combustion and heat recovery unit A. After a certain time the oxygen content in stream 10 will become too low to be used as oxidant in combustion chamber 30 because most of the available oxygen has been desorbed from unit 35A. At this point, recycled flue gas stream 9 is rerouted to unit 35B by closing valve 9b and opening valve 9a, whereas with unit 35A, the recycled flue gas heats up the adsorbent such that oxygen is stripped off and enriches the recycled flue gas to form oxygen enriched flue gas 10.

Whilst recycled flue gas stream 9 is routed to unit 35B, air valve 13b is closed and air valve 13a is opened so that compressed, heated air 13 is once again fed to adsorber unit 35A in order that further oxygen can be adsorbed in 35A - thus the described sequence is repeated in an ongoing cycle so that oxygen is adsorbed/desorbed in units 35A / 35B in a continuous process. In other words whilst oxygen is being adsorbed into unit 35A, oxygen is being desorbed into recycled flue gas stream 9 in unit 35B, and then whilst oxygen is being desorbed into recycled flue gas stream in unit 35A, oxygen is adsorbed into unit 35B.

More than two adsorber units may be used to provide a more continuous process.

An alternative version of units 35A, 35B is also envisaged in which, instead of direct contact between the adsorbent material and the recycled flue gas, heat transfer tubes are installed within the units to heat up the adsorbent material in order to release the oxygen. This oxygen can then be mixed with the recycled flue gas upstream of the units.

Referring now to Figure 3, this illustrates an adsorber bed cleaning step that can be used with a fixed bed oxygen enrichment process e.g. that of Figure 2, except at least three beds (units 35) will be required rather than the two (35A, 35B) illustrated in Figure 2, as discussed further below. C02 may be adsorbed by the fixed beds of units 35A, 35B during the desorption step (i.e. when the recycled flue gas 9 strips off adsorbed oxygen) which may then be flushed out by the air stream 13 during a subsequent adsorption phase, thus causing this C02 to be emitted to the environment. This is clearly undesirable.

To address this problem, the C02 adsorbed by the fixed beds can be flushed out using air stream 1 before air stream 1 is fed to combustion chamber 30. This step may last for a few seconds before air valve 13a (for unit 35A) or air valve 13b (for unit 35B) is opened to introduce heated air 13 to the adsorber unit for a further adsorption phase. Thus, the C02 that has been flushed out will ultimately be recovered in the C02 capture unit B following the combustion process.

In order to provide a continuous cycle, if a cleaning step is to be implemented then at least three beds (units 35) are required so that at any one time oxygen is being adsorbed in one bed, oxygen is being desorbed into the flue gas stream 9 in a second bed, and cleaning is being carried out in a third bed. The air valve and recycled flue gas valve arrangement shown in Figure 2 can be extended to allow this to happen.

Since the beds (units) 35 are quite hot, the air stream 1 will be heated during the cleaning step which is an advantage in the combustion chamber since less fuel will be required. The recovered C02 in the air stream 1 will also be quite hot and thus add further energy to the combustion process.

Figure 4 illustrates a combined cycle gas turbine (CCGT) system operating according to a second embodiment of the present invention. As with Figure 1, the system has been divided into three components for ease of understanding: combustion and heat recovery unit A, C02 capture unit B and oxygen enrichment process C. C02 capture unit B is the same as that of the embodiment of Figure 1 and thus the same description applies. The oxygen enrichment process C is also the same as that of Figure 1, except that the heat exchanger 40 that regulates the temperature of the oxygen enriched flue gas 10 to form oxygen enriched flue gas 16 is part of the combustion and heat recovery unit A as discussed further below. The alternative oxygen enrichment process of Figure 2 could instead be used in Figure 4, just as with Figure 1.

The combustion and heat recovery unit A of Figure 4 is a combined cycle gas turbine (CCGT) system as opposed to the boiler-based system of Figure 1. It comprises a gas turbine including a compressor 42, a combustor 30 and a turbine (expander) 43. Air stream 1 and oxygen enriched recycled flue gas 17 are compressed by compressor 42 to form stream 18, and are input to the combustor 30 together with fuel 2. The mixture is burned and hot exhaust gases 19 expand across the turbine 43 to generate power and hot flue gas stream 3.

Hot flue gas stream 3 enters flue gas heat recovery unit 31 where the hot flue gas 3 heats and boils water thus generating steam 53 (preferably superheated steam). Steam 53 enters unit 32 that comprises either a steam turbine or a heat transfer unit (using the steam as a heat source for any purpose), with condensed water 50 circulating back to the heat recovery unit 31. Available heat in oxygen enriched flue gas stream 10 is also used to generate steam or preheat boiler feed water in heat exchanger 40. Stream 54 can be either boiler feed water or water at the boiling point. Stream 55 can be steam or hot water or a mixture of both.

One part of flue gas 3 is cooled to below 200°C, preferably below 100°C, generating flue gas stream 5 that is fed to C02 capture unit B. A second part of fluegas stream 3 is cooled to 500°C or below depending on the operating temperature in the oxygen enrichment process C, generating recycled fluegas stream 9 which is fed to oxygen enrichment process C as in the embodiment of Figure 1.

In contrast to the embodiment of Figure 1, the oxygen enriched flue gas 10 from the oxygen enrichment process C is cooled to close to ambient temperature by first generating steam or by heating boiler feed water in heat exchanger 40 and then by cooling with e.g. cooling water in unit 41 to a temperature preferably below 40°C, to form cooled oxygen enriched flue gas 17. It is this cooled gas that enters the compressor 42 with air stream 1. Unit 40 is integrated with the heat recovery section 31 to maximise steam production as described.

Figure 5 illustrates a membrane method 34 that can be used in the carbon capture process B of the process of Figures 1 and 4. Cooled flue gas 6 is fed to C02 membrane separation unit 60, wherein 75 to 90% of the C02 is removed from the gas stream to create C02 rich stream 61 at below atmospheric pressure on the permeate side by means of vacuum pump62 that sucks C02 out of the membrane,and producing a C02 depleted flue gas 8 that is emitted to the atmosphere (alternatively if cooled flue gas 6 is being compressed then C02 depleted flue gas 8 may be reheated and fed to an expander that is connected to the compressor). C02 rich stream 63 downstream of the vacuum pump 62 has the same composition as stream 31 but has different pressure and temperature (atmospheric pressure or higher). C02 rich stream 63 is purified further in secondary membrane separation unit 64, thereby producing a further concentrated C02 gas stream 7 that is taken away for sequestration (a vacuum compressor or vacuum pump may also be required here, not illustrated), and further C02 depleted flue gas 8b that is emitted to the atmosphere or is partly recycled to the combustion process to increase the C02 recovery. This C02 depleted flue gas 8b may be enriched with oxygen using the above described desorption method prior being recirculated to the combustion chamber.

Figure 6 illustrates a combustion chamber 70 comprising primary and secondary burners, that may be used in the boiler based system of Figure 1 instead of the standard combustion chamber 30. It is particularly useful if the oxygen concentration in the oxygen enriched recycled flue gas is below 12-15% and thus may cause unstable combustion. It comprises boiler water feed 51, a standard central primary burner 73 for fresh air 1 and fuel 2, and secondary burners 74 surrounding the primary burner 73. Secondary burners 74 receive oxygen enriched flue gas 16 as oxidant, together, at least at start-up, with air. This can also be seen in Figure 7, which illustrates a cross-section through the central part of combustion chamber 70.

Typically (for a boiler, e.g. coal or gas fired), 10-15% of the total oxygen used in the combustion will come from fresh air 1 added through the primary burner 73. The secondary burners 74 use more lean oxidant, the oxygen enriched recycled flue gas 16 with an oxygen content of around 9-12%. The primary burner 73 supplied with air provides a sufficiently high flame temperature to maintain stable combustion and thus steam production throughout the combustion process. The heat generated will enable the secondary burners 74 to perform better since the central primary burner will have a higher flame temperature (due to the higher 02 content) that will support ignition for the surrounding burners and keep the surrounding flames somewhat hotter due to the received heat flux.

Typically, during start-up of the power plant, both the primary 73 and secondary 74 burners will actually be supplied with fresh air 1. Once the power plant is running and maximum steam production is achieved, then the air supplied to the secondary burners 74 can gradually be replaced with the oxygen enriched recycled flue gas 16. If this is done gradually over some time then the use of the oxygen enriched flue gas will have no negative impact on the combustion process and thus steam production, but will have a positive impact on the C02 concentration and thus C02 capture.

Claims

1. A method comprising: combusting a fuel with an oxidant in a combustion process; desorbing oxygen adsorbed in an adsorbent material utilising heat produced by the combustion process; and providing the desorbed oxygen to the combustion process as oxidant.

2. A method as claimed in claim 1, wherein the utilised heat is heat from flue gas produced by the combustion process.

3. A method as claimed in claim 1 or 2, wherein flue gas produced by the combustion process is enriched with the desorbed oxygen and recirculated to the combustion process as oxygen enriched recirculated flue gas.

4. A method as claimed in claim 3, wherein flue gas directly contacts the adsorbent material so as to directly heat the adsorbent material, thereby desorbing oxygen into the flue gas so as to enrich the flue gas with oxygen.

5. A method as claimed in claim 1, 2, or 3, wherein flue gas indirectly heats the adsorbent material so as to desorb oxygen.

6. A method as claimed in claim 5, wherein the flue gas heats a heating medium by heat exchange, and this heating medium heats the adsorbent material to thereby desorb oxygen.

7. A method as claimed in claim 5 or 6, further comprising mixing the desorbed oxygen with the flue gas downstream of the oxygen desorption process.

8. A method as claimed in any of claims 3 to 7, wherein the oxygen content of the oxygen enriched recirculated flue gas is 7 to 21 vol %, more preferably 7 to 15%, most preferably 9 to 12 vol%.

9. A method as claimed in any preceding claim, further comprising a downstream carbon dioxide separation process for separating carbon dioxide from the flue gas of the combustion process, the method further comprising the step of enriching any carbon dioxide containing gas stream generated in the separation process with desorbed oxygen and recirculating this to the combustion process.

10. A method as claimed in any of claims 3 to 9, further comprising a downstream carbon dioxide separation process for separating carbon dioxide from the flue gas of the combustion process, the method further comprising mixing any carbon dioxide containing gas stream generated in the separation process with flue gas, before or after enriching the flue gas with desorbed oxygen.

11. A method as claimed in any preceding claim, wherein the carbon dioxide content in the flue gas from the combustion process when desorbed oxygen is used as oxidant in combination with air is above 20 vol % for a natural gas fired gas turbine combustion process; above 25 vol % for a natural gas fired boiler combustion process; and above 30 vol % for a coal-fired boiler combustion process.

12. A method as claimed in any preceding claim, wherein desorbed oxygen is used as oxidant in the combustion process in combination with air.

13. A method as claimed in claim 12, wherein the weight fraction of oxygen as oxidant in the combustion process provided by the air is less than 25% of the total oxygen, preferably less than 10%.

14. A method as claimed in claim 12 or 13, wherein the weight fraction of oxygen as oxidant in the combustion process provided by the desorbed oxygen is greater than 75% of the total oxygen, preferably greater than 90%.

15. A method as claimed in any preceding claim wherein the oxygen desorption process operates at a temperature below 550°C, preferably below 500°C.

16. A method as claimed in any of claims 2 to 15, wherein the temperature of the flue gas used to desorb oxygen is adjusted dependent on the adsorbent material used.

17. A method as claimed in claim 16, wherein the temperature of the flue gas is adjusted utilising a heat recovery process after combustion.

18. A method as claimed in any preceding claim, wherein the adsorbent material comprises any solid oxide material, any other functional material or nanostructured material such as carbon nanotubes, with the capability to adsorb and desorb oxygen at a temperature below 500°C.

19. A method as claimed in any preceding claim, wherein the adsorbent material is YosTbosBaCCUOy.

20. A method as claimed in any preceding claim further comprising an oxygen adsorption process for adsorbing oxygen into the adsorbent material, which is subsequently desorbed in the desorption process utilising the heat produced by the combustion process.

21. A method as claimed in claim 20 wherein the oxygen adsorption and desorption processes utilise any fluidised bed system, moving bed system or fixed beds.

22. A method as claimed in any preceding claim, wherein the fuel for the combustion process is any fuel containing carbon such as natural gas or coal.

23. A method as claimed in any preceding claim, wherein the combustion process comprises a steam boiler process, a combined cycle gas turbine process, or a direct fired heater process.

24. A method as claimed in any preceding claim, wherein the combustion process comprises a steam boiler process, and wherein the steam boiler has at least one primary and a plurality of secondary burners; wherein flue gas produced by the combustion process is enriched with the desorbed oxygen to form oxygen enriched recycled flue gas; wherein the oxygen enriched recycled flue gas is provided, optionally in combination with air, to the plurality of secondary burners as oxidant; wherein air is provided to the primary burner as oxidant so as to provide a higher flame temperature than that achieved in the secondary burners; and wherein the flue gas from the primary burner is mixed with flue gas from the secondary burners before being output as flue gas of the combustion process.

25. A method as claimed in any preceding claim, further comprising a postcombustion C02 separation and capture process such as amine adsorption, membrane separation or a cryogenic process.

26. A method as claimed in any preceding claim, wherein the adsorbent material is contained within a fixed bed; and wherein after a step of desorbing oxygen by direct contact with flue gas optionally mixed with another C02 containing gas stream, the method further comprises flushing out the fixed bed using an air stream, wherein the air stream is subsequently used in the combustion process as oxidant in combination with desorbed oxygen.

27. A method as claimed in claim 26, wherein the air stream flushes out carbon dioxide that has been adsorbed by the adsorbent material during the oxygen desorption step.

28. A method of flue gas recirculation, comprising: combusting fuel with an oxidant in a combustion process, thereby producing flue gas; recirculating part of the flue gas to an oxygen desorption process; utilising heat of the flue gas to desorb oxygen adsorbed in an adsorbent material thereby releasing oxygen, wherein the recirculated part of the flue gas is enriched with this oxygen; and recirculating the oxygen enriched flue gas to the combustion process.

29. A combustion system comprising: a combustion chamber; and an oxygen desorber unit; wherein the combustion chamber is arranged to combust fuel with an oxidant thereby producing flue gas; wherein the oxygen desorber unit is arranged to utilise heat produced by the combustion chamber to desorb oxygen from an adsorbent material; wherein the combustion chamber is arranged to receive desorbed oxygen as oxidant for the combustion process.

30. A method of generating power and/or heat, comprising: combusting a fuel with an oxidant in a combustion process; and desorbing oxygen adsorbed in an adsorbent material utilising heat produced by the combustion process.

31. A method or combustion system as substantially hereinbefore described with reference to the accompanying drawings.