Ammonia-based carbon capture system and method

Abstract

The carbon capture system (1) comprises an absorber (9) and a regenerator(15). The system further includes a water wash column (13), fluidly coupled with the absorber, adapted to receive CO2-lean flue gas from the absorber and remove residual ammonia from the CO2-lean flue gas. An ammonia stripper (37) is adapted to receive wash water, containing ammonia, from the water wash column and remove ammonia therefrom. A stripper overhead condenser (47) is fluidly coupled with the ammonia stripper and adapted to receive water vapor from the ammonia stripper and return condensed water (61) to the ammonia stripper. The stripper overhead condenser comprises a cold side, wherein a stream of liquid solvent from the regenerator circulates in heat exchange with the condensing water vapor. Also disclosed is a method for condensing water and knocking down non-condensables in from an ammonia stripper of an ammonia-based carbon capture system.

Description

AMMONIA-BASED CARBON CAPTURE SYSTEM AND METHODDESCRIPTIONTECHNICAL FIELD

[0001] The present disclosure relates generally to carbon capture plants and methods, adapted to remove carbon dioxide from a flue gas containing moisture.BACKGROUND ART

[0002] Carbon dioxide (CO2) is a greenhouse gas which is believed to be one of the major contributors to global warming and climate changes. Carbon dioxide is generated by several industrial processes, where thermal power is generated by combustion of fossil fuels, such as natural gas and oil.

[0003] Carbon capture and storage (in short CCS) is a technology effective to make deep cuts in greenhouse gas emissions. Post combustion capture (in short PCC) is a process which uses an aqueous absorption solution (herein referred to also as solvent) containing compounds such as ammonia, potassium salts and the like, to capture carbon dioxide from flue gas generated by combustion of fossil fuel

[0004] The solvent contacts CCL-rich flue gas in an absorber and causes carbon dioxide to remain trapped in the solvent. CCL-rich solvent is then transferred to a regenerator, where carbon dioxide is removed and delivered to a carbon dioxide recovery line for further processing, while the CCh-lcan solvent obtained by removing carbon dioxide therefrom is recirculated towards the absorber. The CO2-lean flue gas is released in the environment.

[0005] Several carbon capture systems use ammonia-based solvents to capture carbon dioxide from the flue gas.

[0006] It is common practice to pass the CCL-lean flue gas exiting from the absorber through a water wash column before releasing it into the atmosphere, to remove any residues of the chemical substances contained in the solvent from the flue gas. This is done to prevent the release of potentially polluting chemical substances into the environment, and to avoid the consumption of such substances. In the water wash column,the CCh-lean flue gas flow moves counter-currently to a flow of water sprayed into the flue gas, so that ammonia or other substances from the solvent, which are carried in the flue gas, are trapped in the water and recovered, preventing them from being carried into the stack by the flue gas flow.

[0007] A wash water stream is often used also to remove residual ammonia from the carbon dioxide which separates from the solvent in the regenerator.

[0008] To recover the ammonia from the various wash water, the carbon capture plant typically includes an ammonia stripper. In the ammonia stripper, the wash water containing ammonia is heated and vaporized, then condensed in an overhead condenser and returned to the stripper, while the vent gas, mainly consisting of gaseous ammonia and other non-condensables, such as carbon dioxide and nitrogen, which separate from the condensed water, is reintroduced into the absorber.

[0009] An exemplary embodiment of a process gas treatment system including an ammonia stripper is disclosed in US2012 / 0063975. The top of the ammonia stripper column is coupled with a liquid-gas separator via a cooler, which removes heat from the gaseous flow exiting from the ammonia stripper. This results in condensation of the water contained in the gaseous flow exiting the ammonia stripper. Condensed water is separated from non-condensables in a liquid-gas separator and recycled. The non- condensables are removed from the top of the liquid-gas separator and fed through a further non-condensables removal section in contact with solvent from an absorber. .

[0010] The vent gas from the ammonia stripper is to a high degree prone for salt sublimation. The chemistry therefore requires costly materials of construction. Additionally extensive interconnecting piping is required, which is prone to plugging due to salt formation.

[0011] Improvements aimed at alleviating the above-mentioned problems and improve the efficiency of the ammonia recovering in the stripper would be welcomed in the art.SUMMARY

[0012] According to an aspect, disclosed herein is an ammonia-based carbon capture system comprising an absorber adapted to circulate a solvent in contact with flue gasand remove carbon dioxide from the flue gas, and a regenerator adapted to receive CCh-rich solvent from the absorber, remove carbon dioxide from the solvent, and return CCh-lean solvent to the absorber. The system further includes a water wash column, fluidly coupled with the absorber, adapted to receive CCh-lean flue gas from the absorber and remove residual ammonia from the CCh-lean flue gas. An ammonia stripper is adapted to receive wash water, containing ammonia, from the water wash column and remove ammonia therefrom. A stripper overhead condenser is fluidly coupled with the ammonia stripper and adapted to receive water vapor from the ammonia stripper and return condensed water to the ammonia stripper. The stripper overhead condenser comprises a cold side fluidly coupled with the regenerator and adapted to circulate a stream of liquid solvent from the regenerator in heat exchange with the condensing water vapor circulating in a hot side of the stripper overhead condenser, and returning heated solvent to the regenerator. Condensing heat removed from the water vapor delivered at the top of the ammonia stripper is thus recovered and used to reduce the amount of thermal energy required by the reboiler of the regenerator.

[0013] The stripper overhead condenser is further combined with a non-condensa- bles removal section, adapted to remove non-condensables carried in the water vapor from the ammonia stripper. Condensed water is removed from the remaining non-con- densables, which flow from the overhead condenser towards and through the non-con- densables removal section.

[0014] The non-condensables removal section can be fluidly coupled with the absorber and can be adapted to receive a stream of CCh-rich solvent from the absorber. In the non-condensables removal section the stream of CCh-rich solvent absorbs the non-condensables. Non-condensables may include carbon dioxide and ammonia, which are thus returned in the solvent.

[0015] The non-condensables removal section can be fluidly coupled with the regenerator to transfer a stream of CCh-rich solvent incorporating the non-condensables to the regenerator.

[0016] According to another aspect, disclosed herein is a method of condensing water and knocking down non-condensables from an ammonia stripper in an ammonia- based carbon capture system. The method comprises the following steps:collecting a gaseous flow containing water vapor and non-condensables on top of an ammonia stripper of the ammonia-based carbon capture system; flowing the gaseous flow through a stripper overhead condenser and condensing water by heat exchange against a flow of a solvent from a regenerator of the carbon capture system; separating condensed water from the non-condensables in a water separating structure; and returning the solvent from the overhead condenser to the regenerator.

[0017] Further features and embodiments of the system and method according to the present disclosure are described below and set forth in the appended claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Reference is now made briefly to the accompanying drawings, in which:Fig.l illustrates a schematic of an ammonia-based carbon capture system based; andFig.2 illustrates an enlargement of a combined overhead condenser and non- condensables removal section, combined to the regenerator of the carbon capture plant.DETAILED DESCRIPTION

[0019] In the following description, reference is specifically made to a carbon capture plant based on the so-called mixed salt-process (MSP). Nevertheless, novel features disclosed herein can be used with advantage also in other ammonia-based carbon capture systems and plants, like the chilled ammonia process.

[0020] The general configuration of the MSP carbon capture plant is familiar to experts in the field and will therefore only be briefly outlined here to provide sufficient context for comprehending the innovative aspects revealed herein, as well as their operational mechanisms and associated benefits.

[0021] Referring to Fig.l, the carbon capture plant is labeled 1 as a whole. The plant 1 includes a flue gas inlet duct 3, wherethrough flue gas from a combustion process, is delivered to the carbon capture plant 1. The plant 1 further includes a direct contact cooler 5 and a direct contact heater 7. In the direct contact cooler 5 the flue gas is cooled to a temperature suitable for processing of the flue gas in an absorber 9.

[0022] After undergoing treatment in the absorber 9, the flue gas, depleted of CO2 (i.e., flue gas from which carbon dioxide has been partially or almost entirely removed through the reaction with a CCh-lcan solvent in the absorber 9), flows through the direct-contact heater 7 to be reheated to approximately ambient temperature before being discharged into the environment, typically through a stack 11.

[0023] Between the absorber 9 and the direct contact cooler 7 a water wash column 13 is arranged, wherethrough the flue gas flows in counter- flow with water. Ammonia carried from the absorber 9 by the processed flue gas is dissolved in water, prevented from being released in the atmosphere and recovered to be re-used in the absorber.

[0024] The absorber 9 is fluidly coupled thorough lines 16 and 18 to a regenerator 15, where CCh-rich solvent from the absorber 9 is regenerated; carbon dioxide is released from the solvent and removed through a CO2 removal line 19. The regenerator 15 includes a reboiler 14, which feeds thermal energy to the regenerator 15 to support the regeneration reaction.

[0025] CCh-lean solvent is returned through lines 23 and 25 to the absorber 9. The CO2 removal line 19 is fluidly coupled with a CO2 wash column 27, where the CO2 stream from the regenerator 15 is washed in counter- flow with wash water to remove residual ammonia drawn by the carbon dioxide, before the carbon dioxide is finally removed along line 29 from the plant 1 and transported to compression system, a liquefaction system or the like.

[0026] The bottom of the water wash column 13 is fluidly coupled through a line 31 with a water recovery line 33, which is in turn also fluidly coupled with the bottom of the CO2 wash column 27. The recovered water fed through the water recovery line 33, which can be loaded with ammonia, is passed through an ammonia stripper 37.

[0027] Clean water, wherefrom residual ammonia and carbon dioxide have been removed in the ammonia stripper 37, is collected at the bottom of the ammonia stripper 37. From the bottom of the ammonia stripper 37, clean water is delivered through a line 41 to the top of the CCh-wash column 27 and through a line 43 to the top of the water wash column 13.

[0028] In the ammonia stripper 37, water containing ammonia and carbon dioxide isheated in the bottom of the ammonia stripper 37 through a heater 38 and vaporized. Water vapor, gaseous ammonia and gaseous carbon dioxide flow upwardly in through the ammonia stripper 37 and the gaseous stream is collected in a line 45 and delivered to a composite treatment column 47, which features a stripper overhead condenser and a non-condensables removal section, which will be described in more detail below, reference being made to Fig.2.

[0029] The gaseous stream containing water vapor, ammonia and non-condensables, including carbon dioxide, which leaves the ammonia stripper 37 at the top thereof and delivered through the line 45 to the column 47, must be treated in the column 47 such that water is condensed, separately recovered and recycled towards the ammonia stripper 37, while the non-condensables are fed to the regenerator 15.

[0030] With continued reference to Fig. 1, Fig. 2 depicts a schematic vertical section of column 47. This column comprises a stripper overhead condenser 47.1 and a non- condensables removal section 47.2. In the embodiment of Fig.2, the overhead condenser 47.1 is positioned above the non-condensables removal section 47.2. A gaseous stream containing water vapor and non-condensables flows into the column 47 from the top and flows therethrough in a downward direction as indicated by arrow f47. The overhead condenser 47.1 includes a heat exchanger with a hot side, wherethrough gaseous species from the line 45 flow in heat exchange with a cold side 49. C Ch-lean solvent from the regenerator 15 flows through the cold side 49 of the heat exchanger and is then returned to the regenerator 15. Reference numbers 51 and 53 indicate a solvent inlet and a solvent outlet of the overhead condenser 47.1, respectively.

[0031] The temperature of the solvent entering the cold side of the overhead condenser 47.1 is lower than the condensing temperature of the water flowing through the hot side of the overhead condenser 47.1. Thus, the solvent flowing in the cold side 49 of the overhead condenser 47.1 removes condensation heat of the water flowing through the hot side of the overhead condenser 47.1. Thus, water condenses in the overhead condenser 47.1 and solvent returns through the solvent outlet line 53 at a temperature higher than the inlet temperature and eventually even partially vaporized.

[0032] The overhead condenser is separated from the non-condensables removal section by water separating structure generically shown at 57. The water separatingstructure can comprise a chimney tray arrangement 59.

[0033] While flowing downwards (arrow f47) the condensed water collects in the chimney tray 59 (see W) and is removed therefrom through a water recovery line 61. The water recovery line 61 is fluidly coupled with the top of the ammonia stripper 37, such that liquid water, wherefrom non-condensables, including ammonia and carbon dioxide, have been removed is returned into the ammonia stripper 37. Condensed water transport may be realized by a pump (not shown).

[0034] Non-condensables flow in a downward direction through the chimney tray 59 into the non-condensables removal section 47.2, where spraying nozzles 63 are placed. The spraying nozzles 63 are fluidly coupled with the absorber 9 through the line 18.

[0035] As schematically shown in Fig.l , the inlet end of the line 18 is positioned at an intermediate position along the absorber 9, where a chimney tray 67, or other liquid collecting structure, collects solvent, which is partly loaded with carbon dioxide. A pump 67 pumps the CCh-rich solvent from the absorber 9 through line 18 toward the regenerator 15.

[0036] Before entering the regenerator 15, the CCh-rich solvent is sprayed into the downwardly flowing stream of non-condensables in the non-condensables removing section 47.2 of column 47.

[0037] Non-condensables are absorbed and dissolved by the CCh-rich solvent before the solvent is finally delivered to the regenerator 15 through a line 71. The physical conditions of the C Ch-rich solvent from the absorber 9 are such that the solvent sprayed in the non-condensables removal section 47.2 has still sufficient capacity to absorb the non-condensables which flow from the top of the ammonia stripper 37, and which mainly include carbon dioxide, ammonia, which are chemically absorbed in the solvent, and possibly nitrogen or oxygen which can be physically dissolved in the solvent.

[0038] The carbon dioxide is then removed in the regenerator 15 and the ammonia is recycled from the regenerator towards the absorber 9.

[0039] The column 47 thus includes an integrated equipment that combines the stripper overhead condenser 47.1 and the non-condensables removal section 47.2. for vent gas disposal. The vent gases (non-condensables) are treated with rich solvent from theammonia-based carbon capture process to knock down the vent gases competely. This results in avoidance of any further vapor handling. Use of a spray system (spray nozzles 63) allow to permanently remove any forming salts from surfaces of the equipment.

[0040] Using CCh-rich solvent from the absorber 9 is beneficial, since the pressure thereof matches well the operating conditions in the overhead condenser, eliminating the need for an additional pump.

[0041] The overhead condenser 47.1 can be advantageously integrated into the regenerator system.

[0042] The temperature level of the overhead condenser 47.1 allows to use the condensation heat removed by the solvent flowing through the cold side of the overhead condenser 47.1 and returned to the regenerator 15 as a side reboiler of the regenerator 15. This is particularly beneficial from an energy efficiency point of view, since the condensation heat recovered in the overhead condenser 47.1 reduces the thermal energy needed from the reboiler 14 at the bottom of the regenerator 15.

[0043] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

Claims

CLAIMS1. An ammonia-based carbon capture system comprising: an absorber adapted to circulate a solvent in contact with flue gas and remove carbon dioxide from the flue gas; a regenerator adapted to receive CCh-rich solvent from the absorber, remove carbon dioxide from the solvent, and return CCh-lcan solvent to the absorber; a water wash column, fluidly coupled with the absorber, adapted to receive CCh-lean flue gas from the absorber and remove residual ammonia from the CO2- lean flue gas; an ammonia stripper adapted to receive wash water, containing ammonia, from the water wash column and remove ammonia therefrom; a stripper overhead condenser fluidly coupled with the ammonia stripper and adapted to receive water vapor from the ammonia stripper and return condensed water to the ammonia stripper; wherein: the stripper overhead condenser comprises a cold side fluidly coupled with the regenerator and adapted to circulate a stream of liquid solvent from the regenerator in heat exchange with the condensing water vapor circulating in a hot side of the stripper overhead condenser, and returning heated solvent to the regenerator; and wherein the stripper overhead condenser is combined with a non-condensables removal section, adapted to remove non-condensables carried in the water vapor from the ammonia stripper.

2. The ammonia-based carbon capture system of claim 1, wherein the non-condensables removal section is fluidly coupled with the absorber and is adapted to receive a stream of CCh-rich solvent from the absorber; such that in the non-condensables removal section the stream of CCh-rich solvent contacts and possibly absorbs the non-condensables.

3. The ammonia-based carbon capture system of claim 2, wherein the non-condensables removal section is fluidly coupled with the regenerator to transfer a stream of CCh-rich solvent incorporating the non-condensables to the regenerator.

4. The ammonia-based carbon capture system of claim 2 or 3, wherein the non-condensables removal section comprises at least one spraying nozzle adaptedto spray CCh-rich solvent from the absorber into a stream of non-condensables.

5. The ammonia-based carbon capture system of any one of the preceding claims, wherein the overhead condenser is arranged on top of the non-condensables removal section in a composite treatment column, such that a gaseous stream containing water vapor and non-condensables from the ammonia stripper flows in a downward direction in the composite treatment column through the hot side of the overhead condenser and through the non-condensables removal section.

6. The ammonia-based carbon capture system of claim 5, comprising a water separating structure, adapted to separate condensed water from non-condensables which flow in a downstream direction from the overhead condenser to the non- condensables removal section; wherein the water separating structure is arranged in the composite treatment column in an intermediate position between the overhead condenser and the non-condensables removal section; and wherein the water separating structure is fluidly coupled with a water recirculating duct fluidly connecting the water separating structure with the ammonia stripper.

7. The ammonia-based carbon capture system of claim 6, wherein the water separating structure comprises a chimney tray structure.

8. The ammonia-based carbon capture system of claim 6 or 7, wherein the non-condensables removal section comprises spraying nozzles adapted to spray a CCh-rich solvent from the absorber in the stream of non-condensables flowing downwardly from the overhead condenser through the non-condensables removal section.

9. A method of condensing water and knocking down non-condensables from an ammonia stripper in an ammonia-based carbon capture system, comprising the following steps: collecting a gaseous flow containing water vapor and non-condensables on top of an ammonia stripper of the ammonia-based carbon capture system; flowing the gaseous flow through a hot side of a stripper overhead condenser and condensing water by heat exchange against a flow of a solvent from a regenerator of the carbon capture system flowing through a cold side of the stripper overhead condenser; separating condensed water from the non-condensables in a waterseparating structure; and returning the solvent from the cold side of the overhead condenser to the regenerator.

10. The method of claim 9, further comprising the step of contacting the non-condensables, wherefrom condensed water has been removed, with ammonia- based solvent and absorbing or dissolving the non-condensables in the ammonia-based solvent.

11. The method of claim 10, wherein the step of contacting the non-condensables with ammonia-based solvent includes the step of spraying ammonia-based solvent into a stream of non-condensables.

12. The method of claim 10 or 11, further comprising the step of feeding the ammonia-based solvent, in which the non-condensables have been absorbed or dissolved, to the regenerator.

13. The method of claim 10 or 11 or 12, further comprising the step of feeding CCh-rich ammonia-based solvent from an absorber of the ammonia-based carbon capture system to a non-condensables removal section, in which the non-condensables are contacted with the CCh-rich ammonia-based solvent to absorb or dissolve the non-condensables in the CCh-rich ammonia-based solvent and feeding the CO2- rich ammonia-based solvent, in which the non-condensables have been absorbed or dissolved, to the regenerator.

14. The method of claim 13, wherein the overhead condenser and the non-condensables removal section are positioned in a composite treatment column; wherein the overhead condenser is arranged above the non-condensables removal section; wherein the water separating structure is positioned in an intermediate position between the overhead condenser and the non-condensables separating structure; and wherein the method further comprises the step of flowing the gaseous flow in a downward direction sequentially through the hot side of the stripper overhead condenser, the water separating structure and the non-condensables separating structure.

15. The method of claim 13 or 14, when depending at least upon at least claim 11 , wherein the step of spraying ammonia-based solvent into the stream of non-condensables includes spraying said ammonia-based solvent through spraying nozzles arranged in the non-condensables removal section.

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