METHOD FOR CAPTURING CO2 FROM A FLUE GAS FROM A DISTRICT HEATING PLANT

Abstract

A method for capturing CO.sub.2 from a flue gas from a district heating plant fired on a carbonaceous fuel, where the flue gas is compressed and thereafter cooled before the flue gas is introduced into an absorber (16), where the flue gas is brought in countercurrent flow to an aqueous CO.sub.2 absorbent solution introduced into the absorber (16), to give a lean flue gas that is withdrawn from the absorber (16), reheated against incoming compressed flue gas, and thereafter expanded and released into the atmosphere, where the rich absorbent is introduced into a regenerator (30) and stripped to release CO.sub.2, withdrawing the lean absorbent from the regenerator (30) and introduction of the lean absorbent into the absorber (16), where the flow of lean absorbent is split in two, a first flow which is introduced at the top of an absorbent packing (17) in the absorber (16), and a second flow which is cooled against a heat fluid received from the district heating plant an returned thereto, and where the thus cooled absorbent is introduced at the top of a cooler packing (21) at the top of the absorber for cooling and drying of the lean flue gas before being reheated against incoming compressed flue gas, and thereafter expanded and released into the surrounding. and a plant for performing the method are described.

Claims

1-11. (canceled)

12. A method for capturing CO.sub.2 from a flue gas from a district heating plant fired on a carbonaceous fuel, where the flue gas is compressed and thereafter cooled before the flue gas is introduced into an absorber, where the flue gas is brought in countercurrent flow to an aqueous CO.sub.2 absorbent solution introduced into the absorber, to give a lean flue gas that is withdrawn from the absorber, reheated against incoming compressed flue gas, and thereafter expanded and released into the atmosphere, where the rich absorbent having absorbed CO.sub.2 is collected at the bottom of the absorber, withdrawn therefrom and introduced into a regenerator in which the CO.sub.2 rich absorbent is stripped to release CO.sub.2 by countercurrent flow to steam to give a lean absorbent, withdrawing the stripped, or lean absorbent from the bottom of the regenerator and introduction of the lean absorbent into the absorber, and withdrawing of the CO.sub.2 stripped of the absorbent and steam from the regenerator for further treatment and export for deposition of the CO.sub.2, wherein the flow of lean absorbent is split in two, a first flow which is introduced at the top of an absorbent packing in the absorber, and a second flow which is introduced at the top of a cooler packing at the top of the absorber for cooling and drying of the lean flue gas before being reheated against incoming compressed flue gas, and thereafter expanded and released into the surrounding, and wherein the second absorbent flow is cooled against a heat fluid received from the district heating plant and wherein the thus heated heat fluid is led back to the district heating plant.

13. The method of claim 12, wherein CO.sub.2 and steam stripped of the absorbent is cooled by countercurrent flow to an aqueous cooling fluid in a CO.sub.2 cooler arranged at the top of the regenerator before the CO.sub.2 is withdrawn for further treatment, and where the heat transferred from the CO.sub.2 and steam to the aqueous cooling fluid is transferred directly or indirectly to the district heating plant.

14. The method of claim 13, wherein the aqueous cooling fluid used to cool the CO.sub.2 and steam in the CO.sub.2 cooler is water circulated as heat fluid in the district heating plant, where water received from the district heating plant is introduced as aqueous cooling fluid to the top of the CO.sub.2 cooler, and the used and thus heated aqueous cooling fluid collected below the cooler is returned to the district heating plant.

15. The method of claim 13, wherein the aqueous cooling liquid used to cool the CO.sub.2 and steam in the CO.sub.2 cooler circulates in a loop and where the aqueous cooling fluid is cooled in a CO.sub.2 cooler heat exchanger against a heat medium received from and circulated back to the district heating plant to transfer the heat thereto.

16. The method of claim 12, wherein the second flow of lean absorbent which is introduced to the top of the cooler packing at the top of the absorber constitutes from 10 to 60%, such as 20 to 50% of the total flow of lean absorbent introduced to the absorber column.

17. The method of claim 12, wherein the second flow of lean absorbent is cooled to a temperature from 55 to 75 deg. C., such as from 60 to 70 deg. C.

18. The method of claim 12, wherein the incoming flue gas to be compressed is water saturated and has a temperature of 50 to 70? C.

19. A plant for capturing CO.sub.2 from a flue gas from a district heating plant fired on a carbonaceous fuel comprising a flue gas pipe for introduction of the flue gas into a flue gas compressor operated by a driver, a compressed flue gas pipe for introducing the compressed flue gas into a heat exchange unit wherein the compressed flue gas is cooled against a CO.sub.2 lean flue gas, a cooled flue gas pipe for withdrawing the cooled flue gas and introduction thereof into absorber column below packed section, a lean absorbent pipe for introduction of lean absorbent at the top of the packed section, a lean flue gas pipe connected to the top of the absorbent column for leading the lean flue gas from the absorber column to the heat exchange unit to be heated against the compressed flue gas, a heated lean flue gas pipe for leading the heated lean flue gas from the heat exchange unit into a lean flue gas expander, an expanded lean flue gas pipe for releasing the lean flue gas into the surroundings, a rich absorbent pipe for leading rich absorbent collected at the bottom of the absorber and introduction thereof into a desorber column at the top of packed stripper section, steam pipes for introduction of steam into the desorber column below the packed stripper section, a CO.sub.2 withdrawal pipe arranged to withdraw CO.sub.2 from the top of the desorber column for further treatment and disposal, and a lean absorbent pipe for leading lean absorbent collected at the bottom of the desorber column and introduction thereof into the absorber column, wherein a side draw pipe is arranged to withdraw a part of the lean absorbent in the lean absorbent pipe, introduce the withdrawn absorbent into the cooling water heat exchanger for cooling thereof against heat medium received from the district heating plant in a heat medium pipe, where a heat medium return pipe is arranged to return the thus heated heat medium to the district heating plant, and a cooling medium introduction pipe for leading the cooled lean absorbent from the heat exchanger and introduction thereof at the top of a cooler packing at the top of the absorber.

20. The plant of claim 19, wherein a CO.sub.2 cooler cooling water pipe is arranged to introduce cooling water to the top of a CO.sub.2 cooler arranged at the top of the desorber to cool the CO.sub.2 and steam by countercurrent flow of water and the CO.sub.2 and steam before withdrawal through the CO.sub.2 withdrawal pipe, where a CO.sub.2 cooler collector plate is arranged below the CO.sub.2 cooler to collect the used cooling water and a CO.sub.2 cooling water withdrawal pipe is arranged to withdraw used cooling water from the CO.sub.2 cooler collector plate, and means are provided to deliver the heat of the cooling water in the CO.sub.2 cooler cooling water withdrawal pipe to the district heating plant.

21. The plant of claim 20, wherein the means for delivering heat to the district heating plant comprises a CO.sub.2 cooler heat exchanger where a CO.sub.2 cooler incoming heat medium pipe is arranged to deliver a heat medium to the CO.sub.2 cooler heat exchanger from the district heating plant and a CO.sub.2 cooler heat medium return pipe is arranged to the CO.sub.2 cooler heat exchanger to deliver heated heat medium back to the district heating plant, and the CO.sub.2 cooler return pipe is arranged to deliver hot cooling water to the CO.sub.2 cooler heat exchanger and the CO.sub.2 cooler cooling water pipe is arranged to deliver cooling water cooled in the heat exchanger to the top of the CO.sub.2 cooler.

22. The plant of claim 20, wherein the cooling fluid received in the CO.sub.2 cooler incoming heat medium pipe is water, the CO.sub.2 cooler incoming heat medium pipe is connected to the CO.sub.2 cooler cooling water pipe, and the CO.sub.2 cooler heat medium return pipe is connected to the CO.sub.2 cooler cooling water return pipe.

23. The method of claim 12, wherein the second absorbent flow which is introduced at the top of the cooler packing cooling and drying of the lean flue gas has 25-45 K lower temperature as the first flow which is introduced at the top of the absorbent packing.

24. The plant of claim 19, wherein withdrawn lean absorbent is cooled by 25-45 K before introduction at the top of the cooler packing.

25. The plant of claim 19. wherein the driver comprises at least one of a motor and a steam turbine.

Description

SHORT DESCRIPTION OF THE FIGURES

[0027] FIG. 1 is a flow chart of an embodiment of a plant according to the present invention, and

[0028] FIG. 2 is a flow chart of another alternative embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0029] The present invention relates to a method and a plant for CO2 capture which is optimized for capturing CO.sub.2 from an incoming, substantially water saturated, flue gas. A typical source for the flue gas is a plant for district heating or a combined plant for production of electrical power and heat for district heating. In the present description and claims, the expression plant for district heating or district heating plant is used include both a plant for district heating and a combined plant for production of electrical power and heat for district heating.

[0030] The temperature of the incoming flue gas is dependent on the flue gas generating plant, and the heat recovery system of the plant for district heating generating the flue gas and is normally higher than 50? C., such as higher than 55? C., or higher than 58? C., and lower than 70? C., such as lower than 65? C., or lower than 62? C., such as about 60? C. The incoming flue gas is substantially water saturated at said temperatures, and at ambient pressure.

[0031] FIG. 1 is a flow diagram illustrating one embodiment of the present invention. The flue gas is introduced into the present CO.sub.2 capture plant through a flue gas pipe 1 and is optionally introduced into a pre-treatment unit 2 where the flue gas may be pre-treated before introduction into the present plant. The pre-treatment may comprise scrubbing with water for removal of particles and/or cooling for adjustment of the temperature of the incoming gas. However, the pre-treatment unit may often be omitted as the flue gas is scrubbed, water saturated and at the required temperature as mentioned above. The flue gas from the CO.sub.2 generating plant is normally at a pressure close to ambient pressure and is introduced into a flue gas compressor 6 directly from the flue gas pipe or a pre-treated flue gas pipe 5 for leading the flue gas from the pre-treatment unit 2 and is compressed to a pressure of typically 5 to 12 bara.

[0032] The flue gas compressor is operated by an electrical motor, and/or other driver such as a steam turbine, 7 which is arranged on a common shaft 9 together with the flue gas compressor 6, and a CO.sub.2 lean flue gas expander 8, which will be described further below. As the flue gas introduced into the flue gas compressor 6 has a higher temperature than normally used, and is water saturated, the duty of the compressor 6 has to be increased compared to introduction at 30? C.

[0033] The compressed flue gas is led in a compressed flue gas line 10 into a heat exchange unit 11, where the compressed and thus heated flue gas, is cooled by heat exchanging against CO.sub.2 lean flue gas and heating media for a reboiler 57, as will be further described below.

[0034] The illustrated embodiment of the heat exchange unit comprises three heat exchangers. The compressed and hot flue gas is first introduced into a first flue gas heat exchanger 12 against CO.sub.2 lean flue gas. The flue gas leaving the first flue gas heat exchanger 12 is then introduced into a reboiler heat exchanger 13 and then led to a second flue gas heat exchanger 14, where the gas is further cooled against CO.sub.2 lean flue gas.

[0035] The thus cooled compressed flue gas is then led from the heat exchange unit 11 in a cooled flue gas pipe 15 to the lower part of an absorber 16. Typically, the cooled and compressed flue gas introduced into the absorber 16 has a temperature from about 100 to 110? C. In the absorber the flue gas is brought in countercurrent flow to a liquid CO.sub.2 absorbent introduced through a lean absorbent pipe 18 at the top of a packed absorber sections 17 to ascertain intimate contact between the flue gas flowing upwards in the packed section and the CO.sub.2 absorbent. The skilled person will understand that the packed absorber section may comprise several packed sections, typically 2 or 3, arranged at the top of each other. Absorbent having absorbed CO.sub.2, or rich absorbent, is collected at the bottom of the absorber 16, and withdrawn though a rich absorbent pipe 19, and regenerated as will be further described below. Typically, the temperature of the rich absorbent being withdrawn from the absorber 16 is from about 105 to 115? C. as it is heated by the exothermal absorption reaction of CO.sub.2.

[0036] The flue gas leaving the packed absorber section 17, and from which CO.sub.2 have been absorbed, herein called lean flue gas, typically having a temperature of about 95 to 105? C. is led into a cooler packing 21, where the lean flue gas is cooled by contercurrent flow to cooled CO.sub.2 absorbent introduced through a cooled absorbent line 23 to the top of the cooler packing. The cooler packing 21 is normally arranged close to the top of the absorber 16 but may be arranged in a separate unit. The skilled person will understand that the cooler packing maybe arranged in one or more packings, normally 1 to 3.

[0037] The cooled absorbent introduced at the top of the cooler packing is withdrawn from the lean absorbent line 18 in an withdrawal line 18 and is cooled in a lean absorbent cooler 25 against an aqueous cooling fluid from a domestic heating system which is introduced via an incoming heat medium pipe 3, and returned into the domestic heating plant in a heat medium return pipe 3, for transfer of heat energy from the lean flue gas cooler 20 to the domestic heating plant to produce more heat for domestic heating purposes. The cooled CO.sub.2 absorbent is withdrawn from the lean absorbent cooler 25 in a cooled absorbent pipe 23 and is introduced to the top of the cooler packing 21. The cooling of the CO.sub.2 lean flue gas in the cooling section 20 causes part of the humidity in the lean flue gas to condense and thus reduce the water content in the CO.sub.2 lean flue gas. Normally, from about 10 to 60% of the lean absorbent in the absorbent line 18 is withdrawn through the withdrawal line 18 and is cooled in the lean absorbent cooler 25 from typically 95-105? C. to about 60 to 70? C.

[0038] The cooled CO.sub.2 lean flue gas leaving the lean flue gas cooling section at temperature of typically 60 to 75? C., is withdrawn through a CO.sub.2 lean flue gas pipe 26 and first introduced into the heat exchange unit 11 where the gas is first heated to typically 120 to 130? C. in the second flue gas heat exchanger 14 and thereafter introduced into the first flue gas heat exchanger 12 where the CO.sub.2 lean flue gas is heated by heat exchanging against incoming CO.sub.2 rich flue gas. The thus heated CO.sub.2 lean flue gas, typically at a temperature of about 190 to 200? C., is then introduced into the expander 8 where the lean flue gas is expanded to ambient pressure, and thereby cooled to typically about 25 to 35?, such as 30? C., and released into the surroundings via an expanded CO.sub.2 lean flue gas pipe 27, preferably through a not shown stack.

[0039] As mentioned above, CO.sub.2 rich absorbent is withdrawn from the bottom of the absorber in the rich absorbent pipe 19, pumped by a rich absorbent pump 28 and introduced into a desorber column 30. The temperature of the rich absorbent being introduced into the desorber column is typically about 105 to 115? C. A rich absorbent control valve 29 is preferably arranged to control/reduce the pressure of the rich solvent before introduction into the desorber column 30 at the top of serially arranged packed stripper section 31. The skilled person will understand that the packed stripper section may comprise several packed sections, typically 2 or 3, arranged at the top of each other. In the packed stripper section 31 CO.sub.2 is stripped from the liquid absorbent by countercurrent flow of steam introduced from different sources of steam below the packed stripper section 31, as will be described below.

[0040] CO.sub.2 stripped from the absorbent and steam leaving the packed desorber section 31 typically has a temperature of about 100-120? C. flows upwards in the desorber column 30 through a lower desorber collector plate 32 and into a packed recuperation cooler section 33, where the steam and CO.sub.2 are cooled by countercurrent flow to cooling water. The cooling water is introduced at the top of the packed recuperation cooler section 33 at a temperature of about 90-100? C. from a recuperation cooler water pipe 34. Cooling water and water condensed in the packed recuperation cooler section 33 and having a temperature of about 95-105? C., is collected at the lower desorber collector plate 32 and is withdrawn through a recuperation cooler water withdrawal pipe 35, flashed over a recuperation cooler flash valve 36 and introduced into a recuperation cooler flash tank 37.

[0041] Water collected at the bottom of the recuperation cooler flash tank 37 is withdrawn through a recuperation cooling water recycle pipe 38 and pumped in a recuperation cooler water pump 39 into the recuperation cooler water pipe 34 and introduced at the top of the packed recuperation cooler section as cooling water.

[0042] The steam formed by flashing in the recuperation cooler flash tank 37 is withdrawn through a recuperation cooler steam pipe 40 and compressed in a recuperation cooler steam compressor 41 before being introduced into the desorber below the packed stripper section 31 as stripping steam at a temperature of about 140-150? C.

[0043] Steam and CO.sub.2 leaving the top of the packed recuperation cooler section 33 continues upwards in the desorber 30 through a CO.sub.2 cooler collector plate 42 and is further cooled from about 90-100? C. by countercurrent flow against cooling water in a CO.sub.2 cooler packed section 43, to a temperature of about 30? C. The cooling water is introduced at the top of the CO.sub.2 cooler packed section 43 from a CO.sub.2 cooler cooling water pipe 44. The cooling water is collected on the CO.sub.2 cooler collector plate 42 and is withdrawn through a CO.sub.2 cooler water return pipe 45, pumped by a CO.sub.2 cooler cooling water pump 46 into a CO.sub.2 cooler heat exchanger 47 for cooling of the water against a heat medium received from the district heating plant in an incoming heat medium pipe 4. Heat medium heated in the CO.sub.2 cooler heat exchanger is withdrawn through a heat medium return pipe 4 and returned as heated heat medium to the district heating plant. The water cooled in the CO.sub.2 cooler heat exchanger 47 is re-introduced into the CO.sub.2 cooler via the CO.sub.2 cooler cooling water pipe 44.

[0044] The skilled person will understand that the efficiency of steam generation from the flashing of the recuperation cooling water in the recuperation cooler flash tank 37 may be increased by a two-step flashing process where the water phase leaving the flash tank 37 is flashed over a second flash valve into a second flash tank. In such a two-step flashing process the gas phase generated in the second flash tank will be compressed in a second recuperation cooler steam compressor to the same pressure as the gas phase leaving the recuperation cooler steam compressor 41 and introduced into the desorber column 30 as additional stripping steam.

[0045] As mentioned above, the cooling water for cooling of the CO.sub.2 and steam in the CO.sub.2 cooler packed section 43 in the CO.sub.2 cooler heat exchanger 47 is cooled against a cooling medium received from the district heating in a CO.sub.2 cooler incoming heat medium pipe 4 at a temperature of about 50-70? C., such as about 60? C., and returned into the CO.sub.2 generating plant in a CO.sub.2 cooler heat medium return pipe 4 at a temperature of about 80 to 100? C., such as about 90? C., for transfer of heat energy from the CO.sub.2 cooler packed section 43 to the CO.sub.2 generating plant to produce more heat for domestic heating purpose. The skilled person will understand that the temperature of the incoming heat medium in CO.sub.2 cooler heat medium pipe 4 may vary dependent on the district heating plant, and that the indicated temperature is the normal scope of temperatures of cold heat medium from such plants.

[0046] The cooled CO.sub.2 and steam are withdrawn from the top of the desorber 30 in a CO.sub.2 withdrawal pipe 48 and is introduced into a not shown module for drying, compression and cooling of the captured CO.sub.2 before being exported from the plant.

[0047] Lean absorbent, i.e., absorbent being stripped to remove CO.sub.2, is collected at the bottom of the desorber 30 and withdrawn therefrom through a stripped absorbent pipe 49 at a temperature of about 110-120? C. and flashed through a lean flash valve 50 and introduced into a flash tank 51. Liquid collected in the bottom of the flash tank 51 having a temperature of about 95-105? C., is pumped by a pump 54 into the lean absorbent pipe 18. A lean absorbent control valve 55 is preferably arranged to control the flow of lean absorbent leaving the lean absorbent pump 54.

[0048] Steam is withdrawn from the lean flash tank 51 through a lean flash pipe 52, compressed in a lean flash compressor 53 and introduced into the desorber below the packed section 31 via a compressed lean flash pipe 52 at a temperature of about 150 to 200? C. as stripping steam.

[0049] A part of the stripped absorbent collected at the bottom of the desorber 30 is withdrawn through a reboiler pipe 56 and introduced into a reboiler 57 in which the stripped absorbent is heated to generate steam which is introduced as stripping steam into the desorber 30 via a reboiler steam pipe 56. The reboiler 57 receives hot steam for heating from the reboiler heat exchanger 13 in a reboiler heat pipe 58 at a temperature higher than about 128? C. and the steam used for heating in the reboiler is returned to the reboiler heat exchanger 13 in the form of water by a reboiler water return pipe 59 at a temperature of about 126? C. This heat transfer from reboiler heat exchanger 13 to the reboiler 57 can be by means of other heat transfer media than water, such as oil.

[0050] Introduction of water saturated flue gas at a higher temperature than earlier suggested, such as from 50 to 70? C., typically about 60? C. into the flue gas compressor 6 increases the duty of the flue gas compressor 6 compared to the earlier suggested temperature of 30? C. or lower, see e.g., WO2017042163. The increased compressor duty requires input of more electrical power to the motor 7. However, by compressing water saturated flue gas at comparably higher temperature than earlier suggested and allowing condensing of the flue gas at higher temperature and pressure, the energy from condensing may be obtained at higher temperatures. As a result, both the cooling water heat exchanger 25 and the CO.sub.2 cooling water cooler 47 can deliver hot water to other purposes, such as a domestic heating plant, at temperatures above 80? C., preferably about 90? C. Additionally, the temperature in both the absorber 16 and the desorber 30 increases. As a result of the increased temperatures and the resulting increased flow of steam in the absorber and desorber, flashing of the lean absorbent in the lean flash tank 51 and the recuperation cooler flash tank 37, generates more efficiently the volumes of steam which is necessary for stripping. The generated volumes of steam are compressed and returned into the desorber 30 as stripping steam as described above.

[0051] FIG. 2 illustrates an alternative embodiment of the present invention, in which the CO.sub.2 is cooled in direct contact with the heat fluid from the district heating plant in cooler 43. The heat fluid circulating in a district heating system is normally water.

[0052] According tot this embodiment, the CO.sub.2 cooler 43 is cooled by direct contact with water from the district heating Cold water, typically at a temperature of 50-70? C., such as about 60? C., is introduced from the heat medium pipe 4 to the top of the CO.sub.2 cooler 43 and flows counter currently to the CO.sub.2 flowing upwards in the CO.sub.2 cooler 43. The water heated by cooling the CO.sub.2 is collected at the collector plate 42, withdrawn via the cooling water pipe 45, and pumped into the heat medium return line 4 by the cooling water pump 46. The temperature of the water returned in line 4 is typically about 80 to 100? C., such as about 90? C.

[0053] The skilled person will understand that a plant for district heating may comprise an energy storage unit for storage of heat energy in periods where the heat generation is higher than the demand and deliver heat energy when the heat generation is lower than the demand. Heat may be stored as hot water in large well insulated tanks or in salt solutions having a melting point between 70 and 100? C.