System and method for carbon capture

Abstract

A duct burner and heat exchanger arrangement system for a carbon capture plant, the system includes an incoming exhaust gas pipe, a fuel gas pipe, and a duct burner connected to a heat exchanger arrangement, wherein a bypasser is arranged to lead a part of the incoming exhaust gas past the duct burner and the first heat exchanger portion to the second heat exchanger portion. A method for CO2 capture including introducing CO2 rich exhaust gas into a duct burner, introducing fuel gas into the duct burner, bypassing the duct burner by leading part of the CO2 rich exhaust gas past the duct burner and the first heat exchanger portion towards the second heat exchanger portion.

Description

SYSTEM AND METHOD FOR CARBON CAPTUREField of invention

[0001] The present invention is related to improvements in CO2 capture. More specifically,the present invention is related improvements to the use of duct burners to increasethe CO2 content, increase the temperature and / or to reduce the oxygenconcentration in an incoming exhaust gas from which CO2 is to be captured.Background

[0002] CO2 capture by using absorbents that can absorb CO2 is well known. The mostcommonly used absorbents for industrial use are absorbents that may beregenerated by releasing CO2 and re-usable absorbents. The most commonly usedand suggested absorbents for industrial large-scale use are aqueous solutions ofamines and aqueous solution of potassium carbonate. The present invention isrelated to aqueous absorbents and will below be exemplified with a pressurized hotpotassium CO2 plant.

[0003] The composition of exhaust gases from gas turbines can vary depending of the gasturbine in question, the fuel composition and the operation control of the gasturbine. However, the main constituents in the exhaust gas from a stationary gasturbine operating on conventional fuel, such as natural gas, are nitrogen (66-72 %volume), oxygen (12-18% volume), carbon dioxide (1-5% volume), and water vapor(1-5 % volume). Additionally, the exhaust gas typically comprises several minorpollutant species that are not important for the present application.

[0004] TW201303142A discloses systems, methods, and apparatus provided for generatingpower in low emission turbine systems and capturing and recovering carbon dioxidefrom the exhaust. In one or more embodiments, the exhaust is cooled, compressed,and separated to yield a carbon dioxide-containing effluent stream and a nitrogen-containing product stream.

[0005] WO2019172772A1 discloses a method and a plant for capturing CO2 from anincoming flue gas. The flue gas can be exhaust gas from coal and gas fired powerplants, cement factories or refineries. The incoming exhaust gas is cooled, mixed withair and compressed, and thereafter introduced into a combustion chamber togetherwith gas and / or liquid fuel. Part of the combustion is achieved by separate burnerswith cooling / combustion air feed with a volume equal to the volume of CO2captured. Said burners will elevate the temperature in the combustion chamberallowing combustion of exhaust gas with low oxygen content. CO2 is captured at highpartial pressure before expansion by the gas turbine to produce power and generatesteam in the heat recovery unit. The gas turbine will operate with high efficiencyclose to design parameters with respect to inlet temperature, pressure and flow.

[0006] US2018243688A1 discloses a method and plant for capturing CO2 from a CO2containing exhaust gas (1), where the exhaust gas is compressed (10) and thereaftercooled (13, 15, 22) before the exhaust gas is introduced into an absorber (30), wherethe exhaust gas is brought in counter-current flow with an aqueous CO2 absorbentsolution (49), to give a lean exhaust gas (31) that is withdrawn from the absorber(30), reheated 22, 13) against incoming compressed exhaust gas, and thereafterexpanded (34) and released into the atmosphere (4), where the aqueous CO2absorbent solution is an aqueous potassium carbonate solution, and that the steamand CO2 withdrawn from the regenerator (40) is cooled in a direct contact cooler (61)by counter-current flow of cooling water (62), to generate a gaseous flow (70) ofcooled CO2 and steam that is withdrawn for compression and drying of the CO2, anda liquid flow (64) of cooling water and condensed steam that is withdrawn andflashed (80), to give a cooled liquid phase (84) that is recycled as cooling water forthe direct contact cooler (61) for the withdrawn CO2 and steam, and a gaseous phase(81) that is compressed (82) and thus heated, and introduced into the regenerator(40) as stripping steam (83).

[0007] In a plant for capturing CO2 from the exhaust gas, the concentration of CO2 andoxygen are often of interest, as the absorption of CO2 is highly dependent on theconcentration of CO2 in the exhaust gas, and as oxygen may be detrimental to someof the absorbents used in commercial plant for CO2 capture. Accordingly, severalsuggestions have been made to increase the concentration of CO2 in an exhaust gasbefore exposing the exhaust gas for the absorbent. One of the options for increasingthe concentration of CO2 in an exhaust gas is the use of a duct burner, in which theincoming exhaust gas is used as an oxygen source for combustion of added naturalgas. The result of such a “post combustion” in a duct burner is that the CO2concentration in the exhaust gas is increased, and the oxygen concentration isdecreased. The present invention relates to an improvement to a duct burner andheat exchanger arrangement to improve the overall efficiency and reduce the heatloss in a plant for carbon capture, preferably a plant for pressurized capture of CO2.

[0008] The term “incoming exhaust gas” or “incoming flue gas” is in this description andclaims used for any incoming gas from which CO2 is to be captured. The incoming fluegas is normally flue gas from industrial processes like flue gas from wasteincineration, plants for combustion of carbonaceous fuels, cement production, etc..The term “lean flue gas” or “lean exhaust gas” is used for flue gas from which thecontent of CO2 is substantially reduced. Normally, the CO2 content in the lean fluegas is reduced by more than 80%, such as more than 90 or even more then 95% ofthe CO2 compared to the CO2 content in the incoming flue gas.

[0009] The skilled person will understand that through the present description, theexpression “about” relating to specific temperatures in the plant as described meansthat the temperature may differ over time, with the incoming exhaust gas, andoperational conditions, with as much as ± 50 deg C, ± 30 deg C, or preferably lessthan ± 25 deg C. ±

[0010] The term “rich absorbent” is herein used to describe absorbent having absorbed CO2,and the term “lean absorbent” is used to describe absorbent from which CO2 isremoved or substantially reduced so that it can be used to absorb CO2.Summary of the invention

[0011] According to one aspect, it is provided a duct burner and heat exchangerarrangement system for a carbon capture plant. The system comprising an incomingexhaust gas pipe for delivering CO2 rich exhaust gas to the duct burner. The systemfurther comprises a fuel gas pipe for delivering fuel gas to the duct burner. The ductburner is connected to a heat exchanger arrangement for cooling of the exhaust gasleaving the duct burner against the lean exhaust gas. The heat exchangerarrangement comprises a first heat exchanger portion and a second heat exchangerportion. The first heat exchanger portion is arranged upstream the second heatexchanger portion, where the first heat exchanger portion is arranged to cool theincoming exhaust gas from the duct burner and to heat the CO2 lean exhaust gas,wherein a bypasser is arranged to lead a part of the incoming exhaust gas past theduct burner and the first heat exchanger portion to the second heat exchangerportion.

[0012] According to another aspect, it is provided a system wherein the fuel gas may benatural gas.

[0013] According to another aspect, it is provided a system wherein the fuel gas may becombusted utilizing oxygen present in the CO2 rich exhaust gas.

[0014] According to another aspect, it is provided a system the first heat exchanger portionmay be a first heat exchanger and the second heat exchanger portion may be asecond heat exchanger. The first and second heat exchangers may be fluidlyconnected via a first connecting line, wherein the bypasser may be a line connectingthe incoming exhaust gas pipe with the first connecting line.

[0015] According to another aspect, it is provided a system wherein the first heat exchangerportion may be a first heat exchanger and the second heat exchanger portion may bea second heat exchanger. The first and second heat exchangers may be fluidlyconnected via a first connecting line, wherein the bypasser may be a line connectingthe incoming exhaust gas pipe with the second heat exchanger.

[0016] According to another aspect, the system may further comprise an enclosuresurrounding the duct burner and the heat exchanger arrangement. The enclosuremay have a first end portion connected to the incoming exhaust gas pipe. The firstend portion may be arranged upstream the duct burner. The bypasser may be achannel formed at least partially by a separation wall extending through the first heatexchanger portion. The bypasser may be connecting the first end portion with thesecond heat exchanger portion.

[0017] According to another aspect, it is provided a system wherein a third heat exchangerportion may be arranged downstream of the second heat exchanger portion. Thethird heat exchanger portion may be a third heat exchanger.

[0018] According to another aspect, it is provided a system a fourth heat exchanger portionmay be arranged between the second heat exchanger portion and the third heatexchanger portion, for cooling the incoming exhaust gas and heat a heat mediumused to heat a reboiler. The fourth heat exchanger portion may be a fourth heatexchanger.

[0019] According to another aspect, it is provided a method for CO2 capture. The methodcomprises introducing CO2 rich exhaust gas into a duct burner. The duct burner isconnected upstream a heat exchanger arrangement. The heat exchangerarrangement is arranged to cool the incoming exhaust gas from the duct burner andto heat the CO2 lean exhaust gas leaving a CO2 absorber. The heat exchangerarrangement comprises a first portion and a second portion. The first portion isupstream the second portion. The method further comprises introducing fuel gasinto the duct burner. The method further comprises bypassing the duct burner byleading part of the CO2 rich exhaust gas past the duct burner and the first heatexchanger portion towards the second heat exchanger portion.

[0020] According to another aspect, it is provided a method wherein the CO2 rich exhaustgas may be pressurized CO2 rich exhaust gas.

[0021] According to another aspect, it is provided a method wherein the fuel gas may benatural gas.

[0022] According to another aspect, it is provided a method wherein the fuel gas may becombusted utilizing oxygen present in the CO2 rich exhaust gas.

[0023] According to one aspect, the present invention relates to a duct burner and heatexchanger arrangement for heating of a pressurized CO2 lean exhaust gas leaving aCO2 absorber in a CO2 capture plant against incoming exhaust gas, the arrangementcomprising an incoming exhaust gas pipe for delivering CO2 rich exhaust gas into aduct burner, a natural gas pipe for delivering natural gas to the duct burner in whichthe natural gas is combusted using oxygen present in the exhaust gas for combustion,the duct burner being connected to one or more heat exchanger(s) or heat exchangesection(s) for cooling of the exhaust gas leaving the duct burner against the leanexhaust gas, where a hot exhaust gas heat exchanger or heat exchange section isarranged to cool the incoming exhaust gas from the duct burner and to heat the CO2lean exhaust gas , wherein a bypass pipe or a bypass channel is arranged to lead apart of the incoming exhaust gas past the duct burner and directly to a medium hotexhaust gas heat exchanger or heat exchange section.

[0024] According to another aspect, the present invention relates to duct burner and heatexchanger arrangement, wherein a cold exhaust gas heat exchanger or heatexchange section is arranged downstream of the medium hot heat exchanger or heatexchanger section, to further cool the incoming exhaust gas and heat the CO2 leanexhaust gas. The term heat exchanger section can be interpreted as a heat exchangerportion of the heat exchanger arrangement.

[0025] According to a another aspect, the present invention relates to a duct burner andheat exchanger arrangement, wherein a reboiler heat exchanger or heat exchangesection is arranged between the medium hot exhaust gas heat exchanger or heatexchange section and the cold exhaust gas heat exchanger or heat exchange section,to cool the incoming exhaust gas and heat a heat medium used to heat a reboiler fora stripper for generation of steam for regeneration of a CO2 absorbent.

[0026] According to a another aspect, the present invention relates to a method forpressurized CO2 capture where an incoming exhaust gas from which CO2 is to becaptured is pressurized and introduced into an absorber where the incoming exhaustgas is caused to flow countercurrent to an aqueous CO2 absorbent, withdrawing leanexhaust gas and the rich absorbent having absorbed CO2 from the absorber,regenerating the withdrawn CO2 rich absorbent by introduction of the absorbent to astripper wherein CO2 is stripped of the aqueous CO2 absorbent by stripping withsteam, withdrawing the stripped off CO2, which is further treated, and regeneratedabsorbent which is returned to the absorber to absorb CO2, where the pressurizedlean exhaust gas withdrawn from the absorber is heated and expanded before beingreleased into the surroundings, characterized in that the lean exhaust gas is heatedby heat exchange against incoming CO2 rich exhaust gas in a duct burner and heatexchanger arrangement according to the above mentioned first aspect of theinvention.Short description of the figures

[0027] The present invention will now be described in detail with reference to the appendeddrawings, whereFigure 1a is an exemplary sketch of the system, including its connection to a plant forCO2capture, figure 1b is an alternative exemplary sketch of the system,figure 2 corresponds to fig 1a and 1b, wherein the duct burner and heat changerarrangement is arranged in a common enclosure or mantle,Figure 3 show the temperature development in the heat exchanger arrangementwithout bypass, andFigure 4 show the temperature development in the heat exchanger arrangementwith bypassDetailed description of the invention

[0028] The present invention relates to improvements related to a duct burner for a plantfor CO2 capture from an exhaust gas including a concentration of oxygen that cansupport combustion of further natural gas. The invention will be further describedbelow exemplified with a high-pressure hot potassium CO2 capture plant.

[0029] With reference to Figures 1a, 1b and 2, a duct burner and heat exchangerarrangement system (2) for a carbon capture plant is illustrated. The system may besuitable for heating of pressurized CO2 rich exhaust gas leaving a CO2 absorber (12) ina CO2 capture plant against incoming exhaust gas. The system (2) comprises anincoming exhaust gas pipe (1) for delivering CO2 rich exhaust gas to the duct burner(4). The system (2) further comprises a fuel gas pipe (3) for delivering fuel gas to theduct burner (4). The fuel gas may be natural gas, but other combustible gases arealso suitable. The fuel gas may be combusted utilizing oxygen present in the CO2 richexhaust gas. Alternatively, an additional oxygen inlet may be provided, if needed. Theduct burner (4) is connected to a heat exchanger arrangement (7,7’,8,8’) for coolingof the exhaust gas leaving the duct burner (4) against the lean exhaust gas. The heatexchanger arrangement (7,7’,8,8’) comprises a first heat exchanger portion (7,7’) anda second heat exchanger portion (8,8’). The first heat exchanger portion (7,7’) isarranged upstream the second heat exchanger portion (8,8’). The first heatexchanger portion (7,7’) is arranged to cool the incoming exhaust gas from the ductburner (4) and to heat the CO2 lean exhaust gas. Wherein a bypasser (6,6’) isarranged to lead a part of the incoming exhaust gas past the duct burner (4) and thefirst heat exchanger portion (7,7’) to the second heat exchanger portion (8,8’).

[0030] The described system outlines a method for CO2 capture comprising introducing CO2rich exhaust gas into a duct burner. The duct burner is connected upstream a heatexchanger arrangement. The heat exchanger arrangement is arranged to cool theincoming exhaust gas from the duct burner and to heat the CO2 lean exhaust gasleaving a CO2 absorber, the heat exchanger arrangement comprising a first portionand a second portion, the first portion being upstream the second portion. Themethod further comprises introducing fuel gas into the duct burner. The methodfurther comprises bypassing the duct burner by leading part of the CO2 rich exhaustgas past the duct burner and the first heat exchanger portion towards the secondheat exchanger portion. The CO2 rich exhaust gas may be pressurized CO2 richexhaust gas. The fuel gas may be natural gas, or other combustible gas. The fuel gasmay be combusted utilizing oxygen present in the CO2 rich exhaust gas.

[0031] With further refence to Figures 1a, 1b, and 2, a third heat exchanger portion (10,10’)may be arranged downstream of the second heat exchanger portion (8,8’). The thirdheat exchanger portion (10,10’) may be suitable for cooling the incoming exhaust gasand heat the lean exhaust gas. A fourth heat exchanger portion (9,9’) may bearranged between the second heat exchanger portion (8,8’) and the third heatexchanger portion (10,10’). The fourth heat exchanger portion (9,9’) is suitable forcooling the incoming exhaust gas and heat a heat medium used to heat a reboiler(32).

[0032] The skilled person will understand that the heat exchanger portions are part of theheat exchange arrangement, and the term may be used to describe individual heatexchanger sections of the heat exchanger arrangement or individual heatexchangers.

[0033] Figure 1 is a flow diagram illustrating one embodiment of the present invention andincludes the basic elements for an exemplary plant for CO2 capture, except anydetails on liquefaction of the captured CO2.

[0034] With further reference to Figure 1, the first heat exchanger portion is a first heatexchanger (7) and the second heat exchanger portion is a second heat exchanger (8),the first and second heat exchangers are fluidly connected via a first connecting line(44). The bypasser (6) is a line, such as a bypasser pipe, connecting the incomingexhaust gas pipe (1) with the first connecting line (44). An alternative solution ispresented in Figure 2, wherein the bypasser (6) a line connecting the incomingexhaust gas pipe (1) with the second heat exchanger (8).

[0035] The flue gas is introduced into a duct burner and heat exchanger arrangementsystem 2 in the present CO2 capture plant through an incoming exhaust gas pipe 1.The flue gas from which CO2 is to be separated from, is normally at a pressure closeto ambient pressure and at a temperature from 450 to 550 deg C. The incomingexhaust gas from the incoming exhaust gas pipe is split in two flows, a duct burnerflow in a duct burner pipe 5 that is led into a duct burner 4 where the incomingexhaust gas is used as a source for oxygen for combustion of fuel gas, such as naturalgas introduced through a fuel gas pipe 3, and a bypass flow that is led through abypasser 6. The skilled person will understand that additional oxygen or air may beintroduced to the duct burner if the rest oxygen in the introduced exhaust gas is notsufficient for a complete combustion of the added natural gas.

[0036] The exhaust gas directed through the duct burner 4 is heated by the combustiontherein to a temperature of about 600 to 650 deg C, is withdrawn from the ductburner 4 and introduced into a hot exhaust gas heat exchanger 7 where the hotincoming exhaust gas from the duct burner 4 is cooled against lean exhaust gas ledthrough a lean exhaust pipe 11’’.

[0037] The thus cooled incoming exhaust gas flow is withdrawn from the hot lean exhaustheat exchanger 7 and introduced, together with the incoming exhaust gas flow in thebypasser 6, into an optional medium hot exhaust gas heat exchanger 8 and the mixedincoming exhaust gas flow is further cooled against lean exhaust gas introduced fromthe lean exhaust gas pipe 11’.

[0038] The hot lean exhaust gas heat exchanger 7 is dimensioned that the incoming exhaustgas from the duct burner 4 is cooled to a temperature substantial equal to thetemperature in the incoming exhaust gas in the flue gas pipe 1 and in the bypass pipe6, i.e. about 450 to 550 deg C. By “substantially equal to it is here meant that thetemperature of the incoming exhaust gas from the bypass pipe 6, is less than 50 degC, such as less than 25 deg C higher or lower than the temperature of the gas fromthe hot exhaust gas heat exchanger before the two gas flows are mixed. The actualtemperature difference is also dependent on optimalisation of the heat exchangers.

[0039] The incoming exhaust gas flow cooled in the medium hot exhaust gas heat exchanger8, is withdrawn therefrom and introduced into a reboiler heat exchanger 9, wherethe incoming exhaust gas is further cooled against a heat medium, such as water,typically at a temperature of about 125 deg C, to produce steam at about 125 deg C,as will be further describes below. The incoming flue gas is cooled from atemperature of typically about 200 deg C to about 140 deg C in the reboiler heatexchanger 9.

[0040] The further cooled incoming exhaust gas is withdrawn from the reboiler heatexchanger 9 and introduced into a cold exhaust gas heat exchanger 10, where the gasis cooled to typically about 120 deg C against pressurized lean exhaust gas introducedfrom a pressurized lean exhaust gas pipe 11 at a temperature of typically about 120deg C.

[0041] The pressurized lean exhaust gas used for heat exchanging in the heat exchangers 7,8 and 10, is received from an absorber 13 as will be further described below andintroduced into the cold heat exhaust gas heat exchanger 10. The pressurized leanexhaust is withdrawn from the cold exhaust gas heat exchanger 10 in a lean exhaustgas pipe 11’ and introduced into the medium hot exhaust gas heat exchanger 8. Thelean exhaust gas heated in the medium hot exhaust gas heat exchanger 8 iswithdrawn via a hot lean exhaust gas pipe 11’’ and introduced into the hot leanexhaust gas heat exchanger 7. The thus heated hot lean exhaust gas is withdrawnfrom hot lean exhaust gas heat exchanger 7 through a lean exhaust gas withdrawalpipe 12 and introduced into an expander 14, where the lean exhaust gas is expandedto ambient pressure and released into the atmosphere through a lean exhaustrelease pipe 15. The expander 14 is arranged at shaft 16 at which also a generator / motor 17 is arranged.

[0042] The incoming exhaust gas being heated in the duct burner 4 and cooled in the heatexchangers 7, 8, 10, is withdrawn from the cold exhaust gas heat exchanger 10 in acooled incoming exhaust gas pipe 18. The exhaust gas withdrawn in pipe 18 typicallyhas a temperature of about 120 deg. C. The gas is further cooled in an exhaust gascooler 19 and is introduced into a series of compressors 20, 20’, 20’’ with intercoolers21’, 21’’ for intercooling of the gas between the compression steps. The compressorsare preferably arranged at the shaft 16, that is also connected to the expander 14and the motor / generator 17. The incoming exhaust gas is typically compressed to apressure from about 800 to 2500 kPa (8-25 bara) in the series of compressors.

[0043] The compressed incoming exhaust gas is withdrawn from the compressor 20’’ in acompressed incoming exhaust gas pipe 22 and introduced into the bottom part of anabsorber 13, where the exhaust gas is caused to flow upwards and countercurrent toa liquid absorbent through a packed absorber section 23, to ensure intimate contactbetween the gas and the absorbent. After passing through the packing 23 in theabsorber 13, the thus cleaned exhaust gas from which a major part of the CO2 isremoved, is collected at the top of the absorber, withdrawn therefrom though thelean absorbent pipe 11, and heated and expanded as described above.

[0044] The absorbent having absorbed CO2, also called “rich absorbent, is collected at thebottom of the absorber, withdrawn through a rich absorbent pipe 24, pumped by arich absorbent pump 25 and introduced into a stripper 26. A rich absorbent controlvalve 27 is preferably arranged to control the flow of rich absorbent into the stripper26.

[0045] The rich absorbent is introduced into at the top of a stripper packed section 28 in thestripper 26 and is caused to flow through the stripper packed section 28countercurrent to steam introduced below the stripper packed section 28 to releaseCO2 absorbed by the liquid absorbent. The absorbent being stripped from CO2 iscollected at the bottom of the stripper 26, is withdrawn through a lean absorbentpipe 29, pumped by a lean absorbent pump 30 and introduced as lean absorbent atthe top of the absorber packing. A lean absorbent control valve 31 is preferablyarranged to control the flow of the lean absorbent into the absorber 13.

[0046] Steam for stripping of the absorbent is generated in a reboiler 32, receiving leanabsorbent collected at the bottom of the stripper 26, through a lean absorbentreboiler pipe 33 generating steam by heating of the lean absorbent in the reboiler 32,and introducing the generated steam into the stripper below the stripper packedsection 28 as stripping gas through a stripper steam pipe 34. The steam is generatedby the heated heat medium received from the reboiler heat exchanger 9 in a reboilerheater pipe 35, normally in the form of steam at about 125 deg C. The steamintroduced into the stripper from reboiler heater pipe is at least partly condensed inthe reboiler to heat and produce steam from the lean absorbent therein. The heatmedium, normally in the form of hot water at about 125 deg, is withdrawn from thereboiler in a heat medium return pipe through steam pipe 34 and returned to thereboiler heat exchanger through a heat medium return pipe 36.

[0047] Steam and CO2 being stripped from the absorbent, leaves the top of the packing andis cooled in one or more cooling sections 37, 38, before being withdrawn though thetop of the stripper through a CO2 pipe 39 for further treatment, such as drying,cooling and liquefaction.

[0048] With reference to figure 2, the duct burner and heat exchanger arrangement system2 may further comprise an enclosure, such as a mantle 2’, surrounding the ductburner 4 and the heat exchanger arrangement. The enclosure has a first end portionconnected to the incoming exhaust gas pipe. The first end portion being arrangedupstream the duct burner. The bypasser 6’ may be a channel formed at least partiallyby a separation wall 41 extending through the first heat exchanger portion 7’, thebypasser connecting the first end portion 40 with the second heat exchanger portion8’.

[0049] Whereas figure 1 is a principle sketch of a plant for capturing CO2 comprising a ductburner and heat exchanger arrangement 2, figure 2 illustrates a preferredembodiment of a duct burner and heat exchanger arrangement 2. Splitting the ductburner and heat exchanger arrangement into separate connected units as illustratedin figure 1a and figure 1b, causes increase of the flow resistance in the arrangementwhich reduces the energy efficiency of the plant. The arrangement illustrated infigure 2 is an arrangement according to the general principles of figures 1a and 1bwhere the duct burner and heat exchange arrangement system 2 is arranged in onesingle physical unit where the duct burner and heat exchangers are arranged in onecommon enclosure, a mantle 2’ where the incoming exhaust gas is allowed to flowfreely through the unit and through heat exchanger sections corresponding the heatexchangers 7, 8, 9, 10 corresponding to all the different heat exchangers illustrated inthe arrangement 2 as illustrated in figures 1a and 1b. The heat exchanger sectionscan be interpreted as heat exchanger portions of the heat exchanger arrangement.

[0050] The incoming exhaust gas is introduced to an inlet chamber 40 at a first end of theenclosure, for example the bottom part of the mantle 2’ through the incomingexhaust pipe 1. A first part of the incoming exhaust gas in the inlet chamber 40 isintroduced into the duct burner 4, in which natural gas is introduced from the naturalgas pipe 3, and the natural gas is combusted using remaining oxygen in the exhaustgas for combustion. After the further combustion in the duct burner 4, thecombustion gases flow upwards in the inner space of the enclosure, and is cooled byheat exchange against lean exhaust gas flowing in a hot exhaust gas heat exchangesection 7’, where the combustion gases are cooled to a temperature correspondingto the temperature of the gases leaving the hot exhaust gas heat exchanger 7 withreference to figure 1. The skilled person will understand that the hot exhaust gasheat exchange section 7’ corresponds to the hot exhaust gas heat exchanger 7 withreference to figures 1a and 1b.

[0051] A second part of the incoming exhaust gas in the inlet chamber 40 is led via a bypasschannel 6’, separated from the hot exhaust heat exchange section 7’ by a separationwall 41. At the top of the hot exhaust gas heat exchange section 7’ the temperatureof the combustion gases flowing upwards from the duct burner 4 are substantiallyequal to the temperature of the incoming exhaust gas in the incoming exhaust pipe 1.The separation wall 41 ends at the same level as the top of the hot exhaust gas heatexchange section 7’, and the exhaust gas in the bypasser, such as a bypass channel,will after reaching the top of the separation wall, be mixed with the gases leaving thehot exhaust gas heat exchange section 7’. The mixed gas flow is further cooled in amedium hot exhaust gas heat exchange section 8’, reboiler heat exchange section 9’and cold exhaust gas heat exchange section 10’, corresponding to the medium hotexhaust gas heat exchanger 8, reboiler heat exchanger 9 and the cold exhaust gasheat exchanger 10, respectively.

[0052] Lean exhaust gas heat exchange pipes 42 receiving lean exhaust gas through the leanexhaust gas pipe 11, are arranged in the inner space of the mantle 2’ for heating ofthe lean exhaust gas in the heat exchange sections 7’, 8’, 10’, whereas reboiler heatexchange pipes 43 are arranged in the reboiler heat exchange section, to heat a heatmedium received through the reboiler heater pipe 35 and returned to the reboiler inthe reboiler steam pipe 34.

[0053] The skilled person will understand that the reboiler heat exchanger 8, and thereboiler heat exchange section 8’, are optional and are not a mandatory part of thepresent invention. However, the reboiler need heat energy for regeneration of theabsorbent, and one convenient source of this heat energy is transfer of heat energyfrom the duct burner and heat exchange arrangement 2. However, any other sourcesof the necessary heat energy may be used for the reboiler without leaving theintention of the present invention.

[0054] As illustrated in the figures, the remaining part of the CO2 capture plants illustrated inboth figures 1a, 1b and 2 are identical.

[0055] The present invention with reference to pressurized CO2 capture, i.e. CO2 capturewhere the gas from which CO2 is to be captured, is pressurized. Normally, the exhaustgas is pressurized to a pressure of about 800 to 2500 kPa to increase the partialpressure of CO2 in the absorber. Duct burners for increasing the concentration ofCO2 in the exhaust gas is well known. The main reason for using duct burners in aplant for capturing CO2 in a pressurized system is to make use of the pressurizedconditions for generation of more power with a rather high thermal efficiency withrather marginal need for extra capital investments. The advantage of using ductburners at this point in the process is that the extra CO2 produced in the duct burnersadd to the CO2 concentration in the flue gas entering the capture plant and therebymakes it easy to capture 100% of the extra CO2 generated by the duct burners.

[0056] Usually, the duct burning is done on the full flow of incoming gas. Since the heatcapacity flow of the incoming flue gas is higher than that of the cleaned gas returningfrom the CO2 removal in the absorber 13, the smallest temperature difference in theheat exchanger will be at the hot end of the heat exchanger. The problem with this isthat the temperature difference in the cold end of the heat exchanger will be ratherlarge and thereby the amount of energy lost to lower temperatures will be big.

[0057] Using a bypass for a part of the exhaust gas as described above, will make it possibleto do the same amount of duct burning to a higher temperature on a heat capacityflow that is smaller than that of the cleaned return gas. This means that the smallesttemperature difference in the heat exchanger will be further down in the heatexchanger at the position where the temperature of the flue gas in the duct burnedpart of the flue gas is the same as the temperature of the bypassed flue gas. Theresult is that the temperature difference in the cold end of the heat exchanger will besmaller and thereby the lost energy smaller. This arrangement makes sure that all theadded energy from the duct burners end up in the gas entering the expander.

[0058] The two graphs illustrated in figures 3 and 4 show the temperature development inthe heat exchangers of two cases one without bypasser and one with bypasser. Inboth of those two cases the duct burning has been adjusted so that the inlet temperature tothe expander is the same, 560 degrees C, and thereby the power delivered is the same. Whatdiffers is the amount of duct burning needed. In the case without the bypass the marginalthermal efficiency of the duct burning is 42,6% and in the case with bypass it is 49,9%,corresponding to approximately 10% increase in efficiency.

[0059] Dependent on the actual design of a plant and the incoming exhaust gas, about 10 to 60%,such as from 20 to 50% or 30 to 40%, of the incoming exhaust gas is passed through thebypass pipe or bypass channel to obtained the effect indicated above.

[0060] The skilled person will understand that oxygen may be added to the duct burner to increasethe oxygen level in exhaust gas introduced into the duct burner, to better support thecombustion therein, and to increase the heat energy of the gas even further to give moreheat to transfer to the outgoing lean exhaust gas to increase the power given by theexpander 14 to give power for the compression and / or to the generator 17.

[0061] The methods and plants described herein for capture of CO2 preferably use an aqueoussolution of carbonate, preferably potassium carbonate, as absorbent, even though theinvention may also be used for other liquid absorbents. Potassium carbonate is anenvironmentally friendly absorbent. Additionally, potassium carbonate as such is nontoxicand do not degrade into poisonous compounds, as is a problem with other suggestedabsorbents.

[0062] The following statements encompass example embodiments of the systems andmethod described herein, and do not define the scope of the invention, which isinstead defined in the appended claims.Statements of invention:1. A duct burner and heat exchanger arrangement for heating of a pressurized CO2 leanexhaust gas leaving a CO2 absorber (12) in a CO2 capture plant against incomingexhaust gas, the arrangement comprising an incoming exhaust gas pipe (1) fordelivering CO2 rich exhaust gas into a duct burner (4), a natural gas pipe (3) fordelivering natural gas to the duct burner (4) in which the natural gas is combustedusing oxygen present in the exhaust gas for combustion, the duct burner (4) beingconnected to one or more heat exchanger(s) or heat exchange section(s) (7, 8, 10) forcooling of the exhaust gas leaving the duct burner (4) against the lean exhaust gas,where a hot exhaust gas heat exchanger (7) or heat exchange section (7’) is arrangedto cool the incoming exhaust gas from the duct burner (4) and to heat the CO2 leanexhaust gas , c h a r a c t e r i z e d i n that a bypass pipe (6) or a bypass channel (6’) isarranged to lead a part of the incoming exhaust gas past the duct burner (4) anddirectly to a medium hot exhaust gas heat exchanger (8) or heat exchange section(8’).2. A duct burner and heat exchanger arrangement according to statement 1, wherein acold exhaust gas heat exchanger (10) or heat exchange section (10’) is arrangeddownstream of the medium hot heat exchanger (8) or heat exchanger section (8’), tofurther cool the incoming exhaust gas and heat the CO2 lean exhaust gas.3. A duct burner and heat exchanger arrangement according to statement 1 or 2,wherein a reboiler heat exchanger (9) or heat exchange section (9’) is arrangedbetween the medium hot exhaust gas heat exchanger (8) or heat exchange section(8’) and the cold exhaust gas heat exchanger (10) or heat exchange section (10’), tocool the incoming exhaust gas and heat a heat medium used to heat a reboiler (32)for a stripper (26) for generation of steam for regeneration of a CO2 absorbent.4. A method for pressurized CO2 capture where an incoming exhaust gas from whichCO2 is to be captured is pressurized and introduced into an absorber where theincoming exhaust gas is caused to flow countercurrent to an aqueous CO2 absorbent,withdrawing lean exhaust gas and the rich absorbent having absorbed CO2 from theabsorber, regenerating the withdrawn CO2 rich absorbent by introduction of theabsorbent to a stripper wherein CO2 is stripped of the aqueous CO2 absorbent bystripping with steam, withdrawing the stripped off CO2, which is further treated, andregenerated absorbent which is returned to the absorber to absorb CO2, where thepressurized lean exhaust gas withdrawn from the absorber is heated and expandedbefore being released into the surroundings, c h a r a c t e r i z e d i n that the leanexhaust gas is heated by heat exchange against incoming CO2 rich exhaust gas in aduct burner and heat exchanger arrangement according to any of the statements 1 to3.A duct burner and heat exchanger arrangement system for a carbon capture plant,the system comprisingan incoming exhaust gas pipe (1) for delivering CO2 rich exhaust gas to the ductburner (4),a fuel gas pipe (3) for delivering fuel gas to the duct burner (4)the duct burner (4) being connected to a heat exchanger arrangement for cooling ofthe exhaust gas leaving the duct burner (4) against the lean exhaust gas, the heatexchanger arrangement comprising a first heat exchanger portion and a second heatexchanger portion, the first heat exchanger portion being arranged upstream thesecond heat exchanger portion, where the first heat exchanger portion is arranged tocool the incoming exhaust gas from the duct burner (4) and to heat the CO2 leanexhaust gas, wherein a bypasser is arranged to lead a part of the incoming exhaustgas past the duct burner (4) and the first heat exchanger portion to the second heatexchanger portion.The system according to statement 5, wherein the fuel gas is natural gas.The system according to statement 5 or statement 6, wherein the fuel gas iscombusted utilizing oxygen present in the CO2 rich exhaust gas.The system according to any one of statements 5 – 7,wherein the first heat exchanger portion is a first heat exchanger and the secondheat exchanger portion is a second heat exchanger, the first and second heatexchangers being fluidly connected via a first connecting line, wherein the bypasser isa line connecting the incoming exhaust gas pipe with the first connecting line.The system according to any one of statements 5 – 7,wherein the first heat exchanger portion is a first heat exchanger and the secondheat exchanger portion is a second heat exchanger, the first and second heatexchangers being fluidly connected via a first connecting line, wherein the bypasser isa line connecting the incoming exhaust gas pipe with the second heat exchanger.The system according to any one of statements 5 – 9, further comprising anenclosure surrounding the duct burner and the heat exchanger arrangement, theenclosure having a first end portion connected to the incoming exhaust gas pipe, thefirst end portion being arranged upstream the duct burnerthe bypasser being a channel formed at least partially by a separation wall extendingthrough the first heat exchanger portion, the bypasser connecting the first endportion with the second heat exchanger portion.The system according to any one of statements 5 – 10, wherein a third heatexchanger portion is arranged downstream of the second heat exchanger portion.The system according to statement 11, wherein the third heat exchanger portion is athird heat exchanger.The system according to statement 11 or statement 12, wherein a fourth heatexchanger portion is arranged between the second heat exchanger portion and thethird heat exchanger portion, for cooling the incoming exhaust gas and heat a heatmedium used to heat a reboiler (32).The system according to statement 13, wherein the fourth heat exchanger portion isa fourth heat exchanger.A method for pressurized CO2 capture where an incoming exhaust gas from whichCO2 is to be captured is pressurized and introduced into an absorber where theincoming exhaust gas is caused to flow countercurrent to an aqueous CO2 absorbent,withdrawing lean exhaust gas and the rich absorbent having absorbed CO2 from theabsorber, regenerating the withdrawn CO2 rich absorbent by introduction of theabsorbent to a stripper wherein CO2 is stripped of the aqueous CO2 absorbent bystripping with steam, withdrawing the stripped off CO2, which is further treated, andregenerated absorbent which is returned to the absorber to absorb CO2, where thepressurized lean exhaust gas withdrawn from the absorber is heated and expandedbefore being released into the surroundings, wherein the lean exhaust gas is heatedby heat exchange against incoming CO2 rich exhaust gas in a duct burner and heatexchanger arrangement system according to any of the statements 5 to 14.A method for CO2 capture, comprising:- introducing CO2 rich exhaust gas into a duct burner, the duct burner beingconnected upstream a heat exchanger arrangement, the heat exchangerarrangement being arranged to cool the incoming exhaust gas from the ductburner and to heat the CO2 lean exhaust gas leaving a CO2 absorber, the heatexchanger arrangement comprising a first portion and a second portion, the firstportion being upstream the second portion;- introducing fuel gas into the duct burner;- bypassing the duct burner by leading part of the CO2 rich exhaust gas past theduct burner and the first heat exchanger portion towards the second heatexchanger portion.The method according to statement 16, wherein the CO2 rich exhaust gas ispressurized CO2 rich exhaust gas.18. The method according to statement 16 or 17, wherein the fuel gas is natural gas.19. The method according to any one of statements 16 – 18, wherein the fuel gas iscombusted utilizing oxygen present in the CO2 rich exhaust gas.

[0063] Any of the above statements can be combined and the skilled person wouldunderstand such examples do not limit the potential embodiments of the presentdisclosure. Although one or more concepts have been explained in relation to theabove embodiment, it is to be understood that many other possible modificationsand variations can be made without departing from the spirit and scope.

[0064] The skilled person will also understand that any use of “or” throughout thestatements of invention or description herein encompasses use of “or”, “and / or”,and “and”. For example, the term "or" within the discourse is construed toencompass both "and" and "and / or" owing to its inherent inclusivity. Within linguisticreasoning, "or" denotes an inclusive disjunction, allowing for the consideration ofscenarios wherein either one condition holds true, the other condition holds true, orboth conditions hold true concurrently. This interpretation inherently incorporatesthe conjunction "and", permitting the acknowledgment of scenarios wherein multipleconditions coexist. Additionally, the term "and / or" explicitly acknowledges thepossibility of either condition being singularly true or both conditions being truesimultaneously, thus aligning with the broader meaning of "or" within the context ofthis disclosure. Consequently, "or" functions as a flexible connector within thestatements of invention, accommodating both exclusive and inclusive interpretationsto suit the nuanced requirements of embodiments described herein.

Claims

Patent claims1. A duct burner and heat exchanger arrangement system (2) for a carbon captureplant, the system (2) comprisingan incoming exhaust gas pipe (1) for delivering CO2 rich exhaust gas to the ductburner (4),a fuel gas pipe (3) for delivering fuel gas to the duct burner (4)the duct burner (4) being connected to a heat exchanger arrangement for cooling ofthe exhaust gas leaving the duct burner (4) against the lean exhaust gas, the heatexchanger arrangement comprising a first heat exchanger portion and a second heatexchanger portion, the first heat exchanger portion being arranged upstream thesecond heat exchanger portion, where the first heat exchanger portion is arranged tocool the incoming exhaust gas from the duct burner (4) and to heat the CO2 leanexhaust gas, wherein a bypasser is arranged to lead a part of the incoming exhaustgas past the duct burner (4) and the first heat exchanger portion to the second heatexchanger portion.

2. The system according to claim 1, wherein the fuel gas is natural gas.

3. The system according to claim 1 or claim 2, wherein the fuel gas is combustedutilizing oxygen present in the CO2 rich exhaust gas.

4. The system according to any one of claims 1 – 3,wherein the first heat exchanger portion is a first heat exchanger and the secondheat exchanger portion is a second heat exchanger, the first and second heatexchangers being fluidly connected via a first connecting line, wherein the bypasser isa line connecting the incoming exhaust gas pipe with the first connecting line.

5. The system according to any one of claims 1 – 3,wherein the first heat exchanger portion is a first heat exchanger and the secondheat exchanger portion is a second heat exchanger, the first and second heatexchangers being fluidly connected via a first connecting line, wherein the bypasser isa line connecting the incoming exhaust gas pipe with the second heat exchanger.

6. The system according to any one of claims 1 – 3, further comprising an enclosuresurrounding the duct burner and the heat exchanger arrangement, the enclosurehaving a first end portion connected to the incoming exhaust gas pipe, the first endportion being arranged upstream the duct burnerthe bypasser being a channel formed at least partially by a separation wall extendingthrough the first heat exchanger portion, the bypasser connecting the first endportion with the second heat exchanger portion.

7. The system according to any one of claims 1 – 6, wherein a third heat exchangerportion is arranged downstream of the second heat exchanger portion.

8. The system according to claim 7, wherein the third heat exchanger portion is a thirdheat exchanger.

9. The system according to claim 7 or claim 8, wherein a fourth heat exchanger portionis arranged between the second heat exchanger portion and the third heat exchangerportion, for cooling the incoming exhaust gas and heat a heat medium used to heat areboiler (32).

10. The system according to claim 9, wherein the fourth heat exchanger portion is afourth heat exchanger.

11. A method for pressurized CO2 capture where an incoming exhaust gas from whichCO2 is to be captured is pressurized and introduced into an absorber where theincoming exhaust gas is caused to flow countercurrent to an aqueous CO2 absorbent,withdrawing lean exhaust gas and the rich absorbent having absorbed CO2 from theabsorber, regenerating the withdrawn CO2 rich absorbent by introduction of theabsorbent to a stripper wherein CO2 is stripped of the aqueous CO2 absorbent bystripping with steam, withdrawing the stripped off CO2, which is further treated, andregenerated absorbent which is returned to the absorber to absorb CO2, where thepressurized lean exhaust gas withdrawn from the absorber is heated and expandedbefore being released into the surroundings, wherein the lean exhaust gas is heatedby heat exchange against incoming CO2 rich exhaust gas in a duct burner and heatexchanger arrangement system according to any of the claims 1 to 10.

12. A method for CO2 capture, comprising:- introducing CO2 rich exhaust gas into a duct burner, the duct burner beingconnected upstream a heat exchanger arrangement, the heat exchangerarrangement being arranged to cool the incoming exhaust gas from the ductburner and to heat the CO2 lean exhaust gas leaving a CO2 absorber, the heatexchanger arrangement comprising a first portion and a second portion, the firstportion being upstream the second portion;- introducing fuel gas into the duct burner;- bypassing the duct burner by leading part of the CO2 rich exhaust gas past theduct burner and the first heat exchanger portion towards the second heatexchanger portion.

13. The method according to claim 12, wherein the CO2 rich exhaust gas is pressurizedCO2 rich exhaust gas.

14. The method according to claim 12 or 13, wherein the fuel gas is natural gas.

15. The method according to any one of claims 12 – 14, wherein the fuel gas iscombusted utilizing oxygen present in the CO2 rich exhaust gas.

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