Method and system for carbon dioxide capture

Abstract

Disclosed is a method and system to recover heat from hot flue gas and use said heat to regenerate a used CO2 capturing fluid. The system comprises, in downstream order, a quenching section, an absorption section, a regeneration section, and a reboiling section. The invention presents the addition of a heat pump configured to recover heat from the quenching section and provide heat to the reboiling section.

Description

[0001] Title: METHOD AND SYSTEM FOR CARBON DIOXIDE CAPTURE

[0002] Field of the Invention

[0003] The invention is in the field of capturing carbon dioxide from flue gas. Particularly, the invention pertains to the energy balance in capturing carbon dioxide by a suitable fluid, with regeneration of said fluid.

[0004] Background of the Invention

[0005] Today it is well-established that emissions of carbon dioxide (CO2) from flue gas, such as emissions resulting from the combustion of fossil fuel, as in power plants, fired heaters or cement production plants, present a major challenge to climate development. Accordingly, the capture of CO2 is pivotal to at least reducing effects such as global warming.

[0006] Various technologies for CO2 capture from flue gas exist, the most important of which being chemical absorption by means of a suitable CO2 - capturing liquid. Such liquids most typically are aqueous amine solutions, a prominent example being a 30% mono ethanol amine (MEA) solution.

[0007] In such processes, the flue gas is initially subjected to quenching, i.e., essentially humidifying and cooling it by direct contact with a quenching medium in a dedicated column. Typically, the quenching medium can be the condensed water from the flue gas, which is recovered from the bottom of the quenching column and it is cooled down in a dedicated heat exchanger, before being recirculated to the top. The resulting quenched flue gas is then subjected to CO2 absorption by allowing it to be contacted, generally in counter-current flow, with the CO2 -capturing liquid. This results in treated flue gas, i.e., flue gas from which CO2 has been at least partially removed (sometimes indicated as “sweetened gas”), and used CO2- capturing liquid, i.e., a CC -rich liquid, generally a CO2-rich amine solution.

[0008] An essential part of the chemical absorption method, involves the regeneration of the used CC -capturing liquid. This usually comprises a stripping process, typically conducted in a regenerator column, resulting in removing CO2 from the liquid. Thereby CO2 is obtained, which can be worked-up into a CO2 product, and a regenerated CC -capturing liquid. The latter liquid is subjected to reboiling, typically using low pressure steam from an external steam header as heating medium, thereby obtaining vapor and a residual liquid. The vapor is sent to the bottom of the regenerator column and used as stripping medium. The residual liquid is regenerated CO2-capturing liquid, i.e., generally a CC -lean amine solution. The regenerated CC -capturing liquid can thereupon be recirculated and reused in capturing CO2.

[0009] In the art, it is recognized that a drawback of the aforementioned CO2 capturing method resides in the additional energy needed for providing heat to accomplish the regeneration of the CO2 -capturing liquid. Also, the conventional set-up of a carbon-capture facility operating on the basis of the chemical absorption method, in fact generates additional CO2 emissions due to the production of steam required to provide heat to the reboiler. Since the overall system is normally designed to reach a net CO2 emission reduction target these additional emissions associated with steam production must be captured as well, therefore a substantial overdesign of such carbon capture facility is often required. This adversely affects the capital investments (Capex) and / or the operational costs (Opex) of the CC -capturing facility.

[0010] It is thus a desire in the art to provide a solution to improve energy efficiency in processes and systems for CC -capture by chemical absorption, possibly with better balanced Capex and Opex. Summary of the Invention

[0011] In order to better address one or more of the above-mentioned desires, the invention provides, in one aspect, a method for capturing carbon dioxide from flue gas, the method comprising contacting the flue gas with a quenching medium so as to obtain cooled flue gas and heated quenching medium, subjecting the cooled flue gas to treatment with a CO2-capturing liquid so as to provide treated flue gas and used CC -capturing liquid, and subjecting the used CC -capturing liquid to stripping, thereby obtaining a CO2-comprising gas stream and a stripped CC -capturing liquid; subjecting said stripped CC -capturing liquid to a reboiling step, thereby generating water vapor and regenerated CO2-capturing liquid, wherein said water vapor is applied as a stripping medium for the used CC -capturing liquid; the process comprising recovering heat from the heated quenching medium by means of a heat pump, and allowing the heat pump to provide said heat to the reboiling step by direct heat exchange with the CO2 capturing liquid to be reboiled.

[0012] In another aspect, the invention presents a system for capturing carbon dioxide from flue gas, comprising a quenching section configured to subject flue gas to contact with a quenching medium, said quenching section comprising an inlet for hot flue gas, an outlet for quenched flue gas, an inlet for the quenching medium, and an outlet for used quenching medium, the outlet for used quenching medium being connected with the inlet for the quenching medium via a recirculation circuit, said circuit being configured to cool the used quenching medium upstream of the inlet for the quenching medium; an absorption section configured to contact the quenched flue gas with a CO2-capturing liquid, said absorption section comprising an inlet for quenched flue gas, said inlet being in fluid communication with the outlet for quenched flue gas of the quenching section, an inlet for a CO2-capturing liquid, an outlet for used CC -capturing liquid, and an outlet for a CO2- containing gas stream; a regeneration section configured to subject used CO2-capturing liquid to stripping with a stripping medium, said regeneration section comprising an inlet for used CC -capturing liquid in fluid communication with the outlet for used CC -capturing liquid of the absorption section, an inlet for said stripping medium, an outlet for stripped CC -capturing liquid; and an outlet for vapor; a reboiling section comprising an inlet for stripped CC -capturing liquid, an outlet for regenerated CO2- capturing liquid and an outlet for vapors, said outlet for vapors being in fluid communication with an inlet for stripping medium of the regeneration section; wherein the system comprises a heat pump configured to recover heat from the used quenching medium, and to provide heat to the reboiling section, said heat pump being connected such as to have direct input heatexchange with the used quenching medium and direct output heat-exchange with the CO2 capturing liquid to be reboiled.

[0013] In yet another aspect, the invention presents a method for modifying a pre-existing system for capturing carbon dioxide from flue gas, said system comprising a quenching section configured to subject flue gas to contact with a quenching medium, said quenching section comprising an inlet hot flue gas, an outlet for quenched flue gas, an inlet for the quenching medium, and an outlet for used quenching medium, the outlet for used quenching medium being connected with the inlet for the quenching medium via a recirculation circuit, said circuit being configured to cool the used quenching medium upstream of the inlet for the quenching medium; an absorption section configured to contact the quenched flue gas with a CC -capturing liquid, said absorption section comprising an inlet for quenched flue gas, said inlet being in fluid communication with the outlet for quenched flue gas of the quenching section, an inlet for a CC>2-capturing liquid, an outlet for used CC -capturing liquid, and an outlet for a CO2- depleted gas stream; a regeneration section configured to subject used CO2- capturing liquid to stripping with a stripping medium, said regeneration section comprising an inlet for used CC -capturing liquid in fluid communication with the outlet for used CO2-capturing liquid of the absorption section, an inlet for said stripping medium, an outlet for stripped CC>2-capturing liquid, and an outlet for CO2 vapor; a reboiling section comprising an inlet for stripped CC -capturing liquid, an outlet for regenerated CC -capturing liquid and an outlet for vapors, said outlet for vapors being in fluid communication with an inlet for stripping medium of the regeneration section; wherein the method comprises adding to the system a heat pump configured to recover heat from the used quenching medium, and to provide heat to the reboiling section, said heat pump being connected such as to have direct input heat-exchange with the used quenching medium and direct output heat-exchange with the CO2 capturing liquid to be reboiled.

[0014] In a still further aspect, the invention presents the use of a heat pump to recover and re-apply heat from a heated quenching medium, said medium being applied in quenching flue gas prior to carbon capture, the carbon capture comprising subjecting the flue gas to chemical absorption with a CC>2-capturing liquid, subjecting the resulting used CC -capturing liquid to stripping, and subjecting the resulting stripped CO2-capturing liquid to a reboiling step, wherein re-applying said heat is in said reboiling step by direct heat-exchange with the CC -capturing liquid to be reboiled.

[0015] Brief description of the drawings

[0016] FIG. 1 is a schematic drawing reflecting a conventional process and system for capturing CO2 from flue gas by chemical absorption, not in accordance with the invention.

[0017] FIG.2 is a schematic drawing reflecting the conventional system as depicted in FIG.l, with a heat pump added and connected in accordance with the invention. FIG.3 is a schematic drawing indicating the operation of a heat pump.

[0018] FIG.4 is a schematic drawing indicating an embodiment of the heat pump implementation of the invention, in conjunction with a vapor compression unit.

[0019] Detailed description of the invention

[0020] In a broad sense, the invention is based on the judicious insight to apply heat that is available as a result of quenching flue gas prior to treatment with a CO2-capturing liquid, to satisfy at least part of the heat required in regenerating the CO2 capturing liquid. In terms of a CO2- capturing facility, this effectively results in an integration of the quenching section and the regeneration section of such facility. Particularly, in accomplishing said judicious integration, the inventors have realized to apply a heat pump in such a manner that a direct heat exchange can take place between the hot circuit of the heat pump and the CO2 capturing liquid to be reboiled.

[0021] The process of the invention can be applied to any gas stream from which CO2 is to be removed by treatment with a CO2-capturing liquid, in which said is cooled down, more particularly to a temperature aa generally applied for CO2 removal. These gas streams are generally referred to as “flue gas”, and typically are combustion gases from a powerplant, or exhaust gases from steam generators used in various industrial processes.

[0022] The technology to remove CO2 from flue gas by means of chemical absorption by a CO2-capturing liquid has been well-established. A variety of such CO2-capturing liquid exists, all of which serving to present a suitable base that is capable of directly or indirectly binding carbon dioxide. Generally, this involves organic bases, typically amines. Alternatively, the CO2-capturing liquid can be an inorganic solvent, such as potassium carbonate, particularly the “Hot Potassium Carbonate Process” (HPC) with which the skilled person is familiar. Suitable amines include diethanolamine (DEA), monoethanolamine (MEA), methyldiethanolamine (MDEA), diisopropanolamine (DIP A), and aminoethoxyethanol (Diglycolamine) (DGA). The skilled person is familiar with suitable CC - capturing liquids (also known as CC -absorbing liquids).

[0023] Generally, the capture of CO2 by an amine, will result in the corresponding salt, in dissociated ionic form, viz. dissolved in water. As a result, the flue gas is depleted in CO2, whilst the CO2-capturing liquid, i.e., when used, becomes rich in CO2, generally in the form of carbonate or, more typically, bicarbonate. It will be understood that the used CO2-capturing liquid is not waste, but can be treated in order to obtain and rework the captured CO2 as a product, and to regenerate the CO2-capturing liquid, particularly for recirculation and re-use in the same CO2-capturing process.

[0024] The regeneration of used CO2-capturing liquid, i.e., notably used aqueous solutions of organic amines, is well-established. Typically, as also applied in the present invention, this involves stripping with water vapor, whereby customarily the stripping process is intrinsically linked with reboiling the stripped liquid. The skilled person is familiar with the equipment applied for such stripping and reboiling. Typically, the stripping occurs in a column or tower provided with trays (also known as a plate column), or in a packed column, and generally in such a way as to allow counter-current flow of the liquid to be stripped and the stripping medium. Typically, the stripping medium is the vaporized CO2-capturing liquid and the heat medium is steam which exchange heat inside a dedicated reboiling exchanger. The stripping process effectively is steam stripping.

[0025] The stripper will generally have a gas outlet connected to an overhead condenser, in which the water fraction of the gas leaving the stripper will condense, and will generally be returned to the top of the stripper as a reflux. Non-condensing substances particularly CO2, will be allowed to leave the condenser. The skilled person will understand that, over time, residual amine can build up in the circulating water to the stripper column.

[0026] Generally, prior to being subjected to regeneration, the used CO2- capturing liquid, i.e., the CC -rich amine solution, is desirably heated, generally from 60°C to 100°C, preferably from 80°C to 100°C. In an interesting embodiment this accomplished by heat exchange with the regenerated CC -capturing liquid, as obtained from the reboiling step. Herein various type of heat exchangers can be used, such as a plate heat exchanger, as shell-and tube heat exchanger having a single tube or a tube bundle. Typically, in the event of using a shell-and-tube heat exchanger, the hot regenerated CC -capturing liquid will be at the shell side thereof.

[0027] Prior to treatment with the CC -capturing liquid, it is necessary to subject the flue gas to quenching. Typically, hot flue gas will enter the system for carbon capture at 110°C-150°C and is cooled, as a result of quenching, generally from 25°C to 55°C, preferably from 30°C to 45°C, by direct contact with a quenching medium, typically an aqueous quenching medium, generally circulation water supplied from the top of the quenching section. As a result, the water is heated. Typically, the quench causes the water vapor present in the flue gas to condensate and this condensate is used as quenching medium after cooling it in a dedicated heat exchanger. Typically, such heat exchanger uses cooling water as cooling medium. In accordance with the invention, the heat transferred to the quenching medium is applied to the aforementioned reboiling step, i.e., in the generation of the vapor to be used as a stripping agent in regenerating the CO2-capturing liquid.

[0028] To this end, the process of the invention comprises recovering heat from the heated quenching medium, and providing said heat to the reboiling step. This can be accomplished in a system according to the invention, which comprises a heat pump.

[0029] The system of the invention, for capturing carbon dioxide from flue gas, comprises a quenching section for quenching flue gas, an absorption section for absorbing carbon dioxide by a CO2-capturing liquid, and a regeneration section for regenerating used CC -capturing liquid. By judiciously presenting a heat pump, the invention provides a heat-managing connection between the quenching section and the regeneration section. Particularly, the invention hereby does not require producing steam to achieve reboiling of the CC -capturing liquid. Specifically, to this end a heat pump is installed and connected such as to have direct input heat-exchange with the used quenching medium and direct output heat-exchange with the CO2 capturing liquid to be reboiled. By direct input heat-exchange, reference is made to the fact that the used quenching medium provides heat, by direct heat-exchange, to a cold service fluid circulating in the heat pump. The direct output heat-exchange from the heat pump refers to the direct heat exchange from a hot service fluid, resulting from a refrigerating cycle in the heat-pump, in which a service fluid transfers thermal energy from a cold source to a hot source, to the CO2 capturing liquid to be reboiled.

[0030] The quenching section comprises an inlet for hot flue gas, an outlet for quenched flue gas, an inlet for a quenching medium, and an outlet for used quenching medium.

[0031] Quenching serves to condition the gas stream, particularly by generating an atmosphere having a 100% relative humidity (RH), and thereby cooling it. Quenching involves adding a quenching medium (i.e. generally an aqueous quenching liquid, preferably water) to the gas, e.g. by mixing water with the gas. This is generally done by one or more quenchers, i.e. devices that serve to introduce water into the gas stream. This introduction will generally be done in such a way that the water is well- dispersed into the gas, e.g. having water droplets present throughout the gas.

[0032] Typically, in the field of carbon capture, quenchers are packed towers in which the quenching medium and the hot flue gas are contacted counter-currently (i.e. the hot flue gas is fed at the bottom of the quencher, whereas the quenching medium is fed at the top of the quencher through a liquid distributor). Alternatively, quenching can be done by spraying a quenching liquid co-currently with the gas stream, and / or to provide a quenching chamber wherein the gas and the quenching liquid are subjected to a residence time that is sufficiently long for the gas to be conditioned at, or at least close to 100%RH.

[0033] The system of the invention is provided with a circuit allowing the recirculation of the quenching medium. This recirculation circuit effectively connects the outlet for used quenching medium from the quenching section with an inlet for the quenching medium into the quenching section. It will be understood that the used quenching medium will be heated up as a result of its directly cooling the hot flue gas. Accordingly, said circuit is configured to cool the used quenching medium upstream of the aforementioned inlet for the quenching medium. Conventionally, this cooling is accomplished by means of cooling water in a dedicated heat exchanger, while in accordance with the invention, this cooling is accomplished by means of a heat pump. It will be understood that the benefits of the present process are optimally obtained in the event that the cooling duty is entirely transferred by means of a heat pump to the reboiling step.

[0034] A heat pump, which may be a single heat pump unit or a heat pump system, can be understood to function on the basis of a cold fluid, particularly a non-aqueous fluid, to which heat can be transferred, a refrigerating cycle with a service fluid to transfer thermal energy from a cold source to a hot source, and a hot fluid from which heat can be obtained. Typically, service fluids are standard hydrocarbon-based refrigerants like propane or butane.

[0035] Accordingly, the used quenching medium is allowed, by direct heat exchange, to transfer heat to the cold service fluid side of the heat pump. The heated cold service fluid will thereupon be allowed to vaporize in an evaporating section of the heat pump, and the resulting vapor is subjected to compression in a compression section of the heat pump. The compressed service fluid is the subjected to cooling by direct heat exchange with a condensation fluid in a condensation section of the heat pump. Upon such cooling condensation, the compressed service fluid transfers heat to said condensation fluid. In accordance with the invention, the stripped CO2- capturing liquid serves as the condensation fluid. This heat exchange thus takes place at a downstream end of the regeneration section, upstream of the reboiling section. Effectively, thereby the condensation heat obtained from the compressed service fluid, serves to generate water vapor applied as a stripping medium in the regeneration section.

[0036] In accordance with the customary operation of a heat pump, the resulting liquid service fluid is subjected to expansion with its temperature dropping back to the level required for its functioning as a cold service fluid, and recirculated to heat exchange with the used quenching medium. The expansion with further cooling is a customary feature of refrigerants such as used in a heat pump. When the liquid passes through a suitable expansion valve, typically a lamination valve, its pressure is reduced, resulting in evaporation of the refrigerant, whereby the necessary heat of evaporation is taken from the refrigerant itself, thus causing its temperature to drop, resulting in a low-temperature, low-pressure mix of liquid and vapor.

[0037] The absorption section comprises an inlet for quenched flue gas, obtained from the quenching section. Generally, the absorption section comprises a column or tower, with said inlet, which is in fluid communication with an outlet for quenched flue gas from the quenching section, being positioned at or near the bottom of such column or tower. This allows the treatment of the quenched flue gas to be conducted in countercurrent flow with the CC -capturing liquid, said liquid flowing down, and the gas to be treated flowing up. Accordingly, the absorption section has an inlet for the CC -capturing liquid which is preferably positioned at or near the top of the absorption section. Conversely, the absorption section has an outlet for used CC -capturing liquid which thus will preferably be positioned at or near the bottom of the absorption section, and an outlet for the CC>2-depleted gas stream resulting from the treatment, preferably positioned at or near the top of the absorption section. In an interesting embodiment, the absorption section comprises one or more heat exchangers, that use a cooling medium, preferably water, for reducing the temperature profile of the absorption column. The resulting used, i.e. heated, cooling medium can subsequently be used as a heat source for the heat pump.

[0038] The regeneration section is configured to subject used CO2- capturing liquid obtained from the absorption section to stripping with water vapor obtained from the reboiling section. To this end, the regeneration section is provided with the corresponding inlets for said water vapor, positioned at a lower end of the stripping section, and for the used CC -capturing liquid in fluid, positioned at a higher end, such as an upper end, of the regeneration section. The regeneration section has an outlet for stripped CC>2-capturing liquid which is in fluid communication with a corresponding inlet into the reboiling section. At a point between said outlet and said inlet, direct thermal communication is provided with the heat pump. Particularly, the invention allows that a direct heat exchange can take place between the hot compressed liquid section of the heat pump and the CO2 capturing liquid to be reboiled.

[0039] The reboiler, as is customary, comprises a flash drum or other expansion vessel allowing vaporization with gas from liquid separation to occur. It will be understood that separation will involve positioning outlets for gas and liquid such as to allow the gas (i.e. the vapor obtained from vaporizing the stripped CC -capturing liquid) to flow up, and the residual liquid (i.e., the regenerated CC -capturing liquid) to flow down.

[0040] In an interesting embodiment, allowing to increase the energy efficiency of the CO2 capture process, a vapor compression unit is added to receive the vapor from the reboiling section. E.g., the stripped CO2- capturing liquid (i.e., CO2-lean amine solvent) as obtained from the bottom of the regeneration section is sent to the regeneration section, preferably to a regenerator flash drum, where the amine solution is vaporized by means of the heat provided by the heat pump. The heat pump heats the lean solvent to around 80-100°C, i.e., lower than the temperature of typically 110-130°C preferably reached in absence of the vapor compression unit. The vaporized amine solution is then compressed before entering the regeneration section as a stripping medium.

[0041] As an advantage of the introduction of the vapor compression unit, the overall efficiency of the system is improved. Since the heat pump operates at a lower temperature lift, thereby requiring less power, and thus presenting a higher coefficient of performance. It will be understood that the addition of the vapor compression unit entails additional costs, e.g., for investment and maintenance. Accordingly, depending on the scale of the system and the investments required, e.g., when modernizing a pre-existing system, it can also be advantageous to dispense with a vapor compression unit, and elect maximum heap pump capacity.

[0042] In connection with the regeneration section, one or more auxiliary units can be provided, as the skilled person knows, in order to enhance the efficiency and durability of the process, particularly with respect to the regeneration of the CO2-capturing liquid. These units may include filter units, energy saving systems, and reclaiming systems. With respect to reclaiming, the invention allows obtaining a further advantage in conjunction with a thermal reclaiming process. To this end, the system as substantially described hereinbefore preferably comprises a reclaiming section. Said reclaiming section is in fluid communication with the reboiling section and is configured to subject CO2- capturing liquid obtained from the reboiling section to a thermal reclaiming process, thereby separating the CC -capturing liquid from contaminations contained therein.

[0043] With respect to reclaiming, the CC>2-capturing liquid, notably in the event of such liquid being an amine solution, over time can accumulate impurities and stable salts in the regeneration section. These impurities can include various contaminants like corrosion products, hydrocarbons, and solid particles. It is generally desired to remove such impurities, in order to optimize the performance of the CO2-capturing liquid, i.e., notably of the amine solution, and to timely prevent fouling and corrosion in the system. To this end a reclaiming process aims at removing these impurities and restoring the amine solution to a better acceptable level of purity for reuse.

[0044] Reclaiming processes and applicable systems do not require elucidation to the skilled person. The reclaiming process typically takes place in a dedicated piece of equipment, often referred to as a reclaiming column, vessel or heat exchanger. This equipment is specifically designed for the removal of impurities from the amine solution. Typically, a side stream is taken from the main process and sent to the reclaiming process.

[0045] Reclaiming comprises converting the amine salts to sodium salts and boiling the amine away from the resulting salt solution. Accordingly, the reclaiming process requires heat to raise the temperature of the amine solution. The elevated temperature also helps to enhance the dissolution of impurities in the solution, and in vaporizing volatile impurities or components with low boiling points, facilitating their removal from the solution. The mixture of amine and impurities may then pass through a filtration system or allowed to settle in a separate vessel. This helps to physically separate the impurities from the amine solution.

[0046] The treated amine solution, now free of impurities, is collected and can be returned to the absorption section for reuse in the CO2 removal process. The separated impurities, often in the form of a slurry or concentrated solution, are typically sent for further treatment, disposal, or regeneration depending on the specific impurities involved.

[0047] Typically, the heating medium used for reclaiming is steam. In the present invention, excess recovered heat from the heat pump can advantageously be used as a heat source for reclaiming.

[0048] In another aspect, the invention provides a method for modifying a pre-existing system for capturing carbon dioxide from flue gas. This method will normally start from a system comprising quenching, absorption, regeneration, and reboiling sections as described above, but which does not have a heat pump as specified. The method of the invention then comprises adding to the system a heat pump configured to recover heat from the used quenching medium, and to provide heat to the reboiling section. It is also conceivable that the pre-existing system comprises a heat pump, but directed at recovering heat from an external source. In that event the present invention presents an alternative method of modernizing the preexisting plant. Provided that the already present heat pump has a suitable capacity, the method of the invention then comprises changing he connections of the heat pump such that the heat pump becomes configured to recover heat from the used quenching medium, and to provide heat to the reboiling section.

[0049] In sum, a method and system are disclosed to recover heat from hot flue gas and use said heat to regenerate the used CO2 capturing fluid. The system to which the invention is applied comprises, in downstream order, a quenching section, an absorption section, a regeneration section, and a reboiling section. The invention presents the addition of a heat pump configured to recover heat from the quenching section and provide heat to the reboiling section.

[0050] The invention is further illustrated with reference to the drawings. It will be understood that the drawings do not limit the invention. E.g., the invention is not limited to the specific types of equipment and specific plant systems as shown. The figure shows schematically equipment parts and process streams relating to embodiments of the invention.

[0051] FIG. 1 is a schematic drawing reflecting a conventional process and system for capturing CO2 from flue gas by chemical absorption, not in accordance with the invention. The figure shows a system comprising a quenching section (1) which is in fluid communication with an absorption section (2), and a regeneration section (3) which is in fluid communication with the absorption section (2). Flue gas (a) is subjected to quenching in the quenching section, resulting in quenched flue gas (b) and heated used quenching liquid (not shown), generally resulting in wasted heat (Al). The quenched flue gas (b) is sent to the absorption section (2), in which it is treated with an amine solution so as to result in treated flue gas (c) and a CO2-rich solution (d). The latter solution is sent to the regeneration section (3), in which it is subjected to solvent regeneration with heat input (A2), resulting in a CO2- product (e) that is sent to battery limits, and a CO2-lean solution (f) that is returned to the absorption section (2).

[0052] FIG.2 is a schematic drawing reflecting the conventional system as depicted in FIG.l, with the same sections (1) to (3), and the same streams (a) to (f) as discussed with reference to FIG.l. In accordance with the invention, a heat pump (4) is added. This heat pump is thermally connected such that it recovers heat from the used quenching liquid (Al) and delivers heat (A3) to the regeneration section (3). It will be understood that the heat pump is connected to a power source (not shown) providing power (P) to operate the heat pump. FIG.3 is a schematic drawing indicating the operation of a heat pump. The figure depicts the circuit of the service fluid in the heat pump. Via a heat exchanger, the cold service fluid receives heat from a used flue gas quenching liquid (warm circulating water, thereby generating cold circulating water); the resulting heated service fluid is allowed to vaporize in an evaporator; the resulting vapor is subjected to compression in a compressor. The compressed service fluid is subjected to cooling by direct heat exchange with a condensation fluid in a condenser. Thereby the compressed service fluid transfers heat to the condensation fluid. In accordance with the invention, the stripped CC -capturing liquid (lean solvent) serves as the condensation fluid. The resulting liquid service fluid is passed through a lamination valve, and thereby brought back to its original condition as a cold service fluid.

[0053] FIG.4 is a schematic drawing indicating an embodiment of the heat pump implementation of the invention, in conjunction with a vapor compression unit. Herein 002-lean amine solvent (f) as obtained from the bottom of a regenerator is sent to a regenerator flash drum, where the amine solution is vaporized by means of the heat (A3) provided by a heat pump unit. The resulting vaporized amine solution is then compressed in a vapor compression unit before entering the regenerator as a stripping medium. Part of the vapor is sent as a heat source to a reclaiming system.

Claims

Claims1. A method for capturing carbon dioxide from flue gas, the method comprising contacting the flue gas with a quenching medium so as to obtain cooled flue gas and heated quenching medium, subjecting the cooled flue gas to treatment with a CO2-capturing liquid so as to provide treated flue gas and used CC -capturing liquid, and subjecting the used CC -capturing liquid to stripping, thereby obtaining a CO2-comprising gas stream and a stripped CO2-capturing liquid; subjecting said stripped CC -capturing liquid to a reboiling step, thereby generating water vapor and regenerated CO2-capturing liquid, wherein said water vapor is applied as a stripping medium for the used CC -capturing liquid; the process comprising recovering heat from the heated quenching medium by means of a heat pump, and allowing the heat pump to provide said heat to the reboiling step by direct heat exchange with the CO2 capturing liquid to be reboiled.

2. A method according to claim 1, wherein the quenching medium is water.

3. A method according to claim 1 or 2, wherein the CO2-capturing liquid is an aqueous solution of an amine.

4. A method according to claim 3, wherein excess recovered heat from the heat pump is used as a heat source for thermally reclaiming amine.

5. A system for capturing carbon dioxide from flue gas, comprising a quenching section configured to subject flue gas to contact with a quenching medium, said quenching section comprising an inlet for hot flue gas, an outlet for quenched flue gas, an inlet for the quenching medium, and an outlet for used quenching medium, the outlet for used quenching medium being connected with the inlet for the quenching medium via a recirculation circuit, said circuit being configured to cool the used quenching medium upstream of the inlet for the quenching medium; an absorption section configured to contact the quenched flue gas with a CO2-capturing liquid,said absorption section comprising an inlet for quenched flue gas, said inlet being in fluid communication with the outlet for quenched flue gas of the quenching section, an inlet for a CO2-capturing liquid, an outlet for used CO2-capturing liquid, and an outlet for a CC -depleted gas stream; a regeneration section configured to subject used CO2-capturing liquid to stripping with a stripping medium, said regeneration section comprising an inlet for used CC -capturing liquid in fluid communication with the outlet for used CC -capturing liquid of the absorption section, an inlet for said stripping medium, an outlet for stripped CO2-capturing liquid; and an outlet for vapor; a reboiling section comprising an inlet for stripped CO2- capturing liquid, an outlet for regenerated CC -capturing liquid and an outlet for vapors, said outlet for vapors being in fluid communication with an inlet for stripping medium of the regeneration section; wherein the system comprises a heat pump configured to recover heat from the used quenching medium, and to provide heat to the reboiling section, said heat pump being connected such as to have direct input heat-exchange with the used quenching medium and direct output heat-exchange with the CO2 capturing liquid to be reboiled6. A system according to claim 5, wherein the absorption section includes one or more heat exchangers configured to use a cooling medium for reducing the temperature profile of the absorption column, wherein the system is preferably configured to allow the used cooling medium to serve as a heat source for the heat pump.

7. A system according to claim 5 or 6, further comprising a reclaiming section, said reclaiming section being in fluid communication with the reboiling section and being configured to subject CC -capturing liquid obtained from the reboiling section to a thermal reclaiming process, thereby separating the CC -capturing liquid from contaminations contained therein.

8. A system according to claim 7 wherein the heat pump is further configured to provide heat to the reclaiming section.

9. A system according to any one of the claims 5 to 8, wherein the reboiling section comprises a flash vessel configured to subject the stripped CO2-capturing liquid to expansion.

10. A system according to any one of the claims 5 to 9, wherein the reboiling section comprises a vapor compression unit positioned such that vapor obtained from vaporizing the stripped CC -capturing liquid is allowed to be compressed resulting in compressed vapor, and having an outlet for compressed vapor in fluid communication with the inlet for stripping medium of the regeneration section.

11. A method for modifying a pre-existing system for capturing carbon dioxide from flue gas, said system comprising a quenching section configured to subject flue gas to contact with a quenching medium, said quenching section comprising an inlet hot flue gas, an outlet for quenched flue gas, an inlet for the quenching medium, and an outlet for used quenching medium, the outlet for used quenching medium being connected with the inlet for the quenching medium via a recirculation circuit, said circuit being configured to cool the used quenching medium upstream of the inlet for the quenching medium; an absorption section configured to contact the quenched flue gas with a CC -capturing liquid, said absorption section comprising an inlet for quenched flue gas, said inlet being in fluid communication with the outlet for quenched flue gas of the quenching section, an inlet for a CC -capturing liquid, an outlet for used CO2- capturing liquid, and an outlet for a CC -depleted gas stream; a regeneration section configured to subject used CO2-capturing liquid to stripping with a stripping medium, said regeneration section comprising an inlet for used CC -capturing liquid in fluid communication with the outlet for used CC -capturing liquid of the absorption section, an inlet for said stripping medium, an outlet for stripped CO2-capturing liquid, and anoutlet for CO2 vapor; a reboiling section comprising an inlet for stripped CO2-capturing liquid, an outlet for regenerated CC -capturing liquid and an outlet for vapors, said outlet for vapors being in fluid communication with an inlet for stripping medium of the regeneration section; wherein the method comprises adding to the system a heat pump configured to recover heat from the used quenching medium, and to provide heat to the reboiling section, said heat pump being connected such as to have direct input heatexchange with the used quenching medium and direct output heat-exchange with the CO2 capturing liquid to be reboiled.

12. A method according to claim 11, comprising adding to the system a heat pump configured to recover heat from the used quenching medium, and to provide heat to the reboiling section.

13. The use of a heat pump to recover and re-apply heat from a heated quenching medium, said medium being applied in quenching flue gas prior to carbon capture, the carbon capture comprising subjecting the flue gas to chemical absorption with a CCh-capturing liquid, subjecting the resulting used CC>2-capturing liquid to stripping, and subjecting the resulting stripped CO2-capturing liquid to a reboiling step, wherein re-applying said heat is in said reboiling step by direct heat-exchange with the CC -capturing liquid to be reboiled.

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