Improved carbon dioxide capture plant and method

Abstract

The plant comprises a first heat recovery heat exchanger having a hot side and a cold side; wherein the hot side is fluidly coupled with a flue gas inlet duct. A second heat recovery heat exchanger is positioned downstream of the first heat recovery heat exchanger and has a hot side fluidly coupled with the hot side of the first heat recovery heat exchanger. The plant further includes a flue gas compressor having a suction side and a delivery side. The suction side is fluidly coupled with the hot side of the second heat recovery heat exchanger. A third heat recovery heat exchanger is further provided, the hot side whereof is fluidly coupled with the delivery side of the flue gas compressor and with a CO2-rich flue gas inlet of an absorber. The cold side of the third heat recovery heat exchanger is fluidly coupled with a CO2-lean flue gas outlet of the absorber and with the cold side of the first heat recovery heat exchanger.

Description

IMPROVED CARBON DIOXIDE CAPTURE PLANT AND METHODDESCRIPTIONTECHNICAL FIELD

[0001] The present disclosure relates to carbon capture systems and methods. Embodiments disclosed herein relate to carbon capture systems using hot potassium carbonate technology.BACKGROUND ART

[0002] Fossil fuels are a major source of chemical energy used for the generation of mechanical power. Fossil fuels are mixed with air and combusted to generate a combustion gas at high pressure and temperature, which expands in a turbine or an expander. The expander or the turbine converts combustion gas enthalpy into mechanical power available on the output shaft of the expander or turbine and used to drive a load, such as a compressor or compressor train, or to rotate an electric generator and convert mechanical power into electric power.

[0003] A maj or concern regarding combustion of fossil fuels relates to the production of carbon dioxide, a greenhouse gas which is considered one of the main contributors of global warming and climate changes.

[0004] To reduce the environmental impact of power generation through combustion of fossil fuels, the option of post combustion capture of carbon dioxide has been investigated. Carbon dioxide capture facilities have been developed, to process flue gas exhausted from gas turbines or other sources of flue gas containing carbon dioxide, and remove carbon dioxide therefrom, prior to discharging the flue gas in the environment.

[0005] 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

[0006] The solvent contacts CCh-rich flue gas in an absorber and causes carbon dioxide to remain trapped in the solvent. CCh-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-lean solvent obtained by removing carbon dioxide therefrom is recirculated towards the absorber.

[0007] Some carbon capture systems, such as in particular those based on hot potassium carbonate technology, require large amount of energy to compress the flue gas upstream of the absorber. This negatively affects the overall efficiency of the system.

[0008] Embodiments disclosed herein are aimed at increasing the overall efficiency of carbon capture systems.SUMMARY

[0009] According to one aspect, disclosed herein is a plant for capturing CO2 from a flue gas, wherein the plant comprises a combination of heat recovery heat exchangers to improve the efficiency of the carbon capture process. In some embodiments, the plant comprises a flue gas inlet duct adapted to receive a flow of incoming flue gas and a first heat recovery heat exchanger having a hot side and a cold side; wherein the hot side is fluidly coupled with the flue gas inlet duct. A second heat recovery heat exchanger is positioned downstream of the first heat recovery heat exchanger and has a hot side and a cold side; wherein the hot side of the second heat recovery heat exchanger is fluidly coupled with the hot side of the first heat recovery heat exchanger. The plant further includes a flue gas compressor having a suction side and a delivery side; wherein the suction side is fluidly coupled with the hot side of the second heat recovery heat exchanger. A third heat recovery heat exchanger is further provided, having a hot side and a cold side. The hot side of the third heat recovery heat exchanger is fluidly coupled with the delivery side of the flue gas compressor and with a CO2-rich flue gas inlet of an absorber. The cold side of the third heat recovery heat exchanger is fluidly coupled with a CCh-lean flue gas outlet of the absorber and with the cold side of the first heat recovery heat exchanger.

[0010] As will become apparent from the following description of embodiments, the provision of a plurality of heat recovery heat exchangers optimizes the recover ofwaste heat from the flue gas and reduces the power needed to run the flue gas compressor, required to feed the flue gas to be processed in the absorber at the required pressure.

[0011] According to another aspect, disclosed herein is a method for capturing CO2 from a flue gas, comprising the following steps: cooling an incoming CCh-rich flue gas in a first heat recovery heat exchanger in heat exchange with a CCh-lean flue gas released by a CO2 absorber; further cooling the CCE-rich flue gas from the first heat recovery heat exchanger in a second heat recovery heat exchanger and recovering useful thermal power therefrom; compressing the CCh-rich flue gas from the second heat recovery heat exchanger to a pressure suitable for CO2 capture in the absorber; and cooling the compressed CCh-rich flue gas in a third heat recovery heat exchanger in heat exchange with the CCh-lean flue gas from the absorber, a cold side of the third heat recovery heat exchanger being upstream of a cold side of the first heat recovery heat exchanger.

[0012] Further features and embodiments of the plant and of the method of the present disclosure are set forth in the dependent claims and described with reference to the enclosed drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Reference is now made briefly to the accompanying drawings, in which:Fig.l illustrates a carbon capture system according to the present disclosure in one embodiment; andFig.2 illustrates a carbon capture system according to the present disclosure in a second embodiment.DETAILED DESCRIPTION

[0014] Fig.l illustrates an embodiment of a power generation system 1 including a gas turbine engine 3 and a carbon capture plant 5, adapted to capture carbon dioxide from the flue gas produced by the gas turbine engine 3. The gas turbine engine 3 can be drivingly coupled to a load 7. In some embodiments, the load 7 can include a driven turbomachinery, for instance a compressor or a compressor train. In otherembodiments, the load 7 can include an electric generator. In the schematic of Fig.1 the load 7 includes an electric generator which is electrically connected to an electric power distribution grid 9.

[0015] The carbon capture plant 3 comprises a flue gas inlet duct 11 adapted to receive a flow of incoming flue gas from the gas turbine engine 3. The flue gas can be at around ambient pressure and at a temperature of 350°C or higher, for instance of 400°C or higher.

[0016] The flue gas inlet duct 11 is fluidly coupled with a first heat recovery heat exchanger 13, which has a hot side and a cold side. The first heat recovery heat exchanger 13 is adapted to recover waste heat from the incoming flue gas and use the recovered waste heat for power generation purposes, as will become apparent from the following description. The hot side has in turn a hot side inlet 13.1 and a hot side outlet 13.2. The hot side inlet 13.1 is fluidly coupled with the flue gas inlet duct 11 and the hot side outlet 13.2 is fluidly coupled with a second heat recovery heat exchanger 15. The cold side of the first heat recovery heat exchanger 13 has a cold side inlet 13.3 and a cold side outlet 13.4, fluidly connected to other components of the plant 3 as will be described below.

[0017] The path of the incoming flue gas extends from the flue gas inlet duct 11, through the hot side of the first heat recovery heat exchanger 13 and reaches through the hot side outlet 13.2 a second heat recovery heat exchanger 15 positioned downstream of the first heat recovery heat exchanger 13 and having a hot side and a cold side. The hot side of the second heat recovery heat exchanger 15 comprises a hot side inlet 15.1 and a hot side outlet 15.2. The hot side inlet 15.1 is fluidly coupled with the hot side outlet 13.2 of the first heat recovery heat exchanger 13. The cold side of the second heat recovery heat exchanger 15 has a cold side inlet 15.3 and a cold side outlet 15.4, which fluidly connect the cold side of the second heat recovery heat exchanger 15 to a utility 16, which is adapted to use thermal power at relatively low temperature.

[0018] The hot side outlet 15.2 of the second heat recovery heat exchanger 15 is fluidly coupled with a suction side 17.1 of a flue gas compressor 17. The delivery side of the flue gas compressor 17 is shown at 17.2. The flue gas compressor 17 is adapted to compress the flue gas at a pressure value sufficient to run the carbon dioxideabsorption process. For example, the flue gas compressor 17 can be designed to deliver compressed flue gas at a pressure between 3 barG and 20 barG, or between 5 barG and 15 barG, or between 5 barG and 8 barG. The temperature of the compressed flue gas delivered by the flue gas compressor 17 can be equal to or higher than 180°C, or equal to or higher than 200 °C.

[0019] This temperature, which is reached due to heat generated in the compression step, can be higher than the temperature actually required by the carbon dioxide absorption process. Therefore, useful thermal power can be removed from the compressed flue gas, for instance through a third heat recovery heat exchanger 19, having a hot side and a cold side. The hot side has a hot side inlet 19.1 and a hot side outlet 19.2. The cold side has a cold side inlet 19.3 and a cold side outlet 19.4. The hot side 19.1 of the third heat recovery heat exchanger 19 is fluidly coupled with the delivery side 17.2 of the flue gas compressor 17 and with a CCh-rich flue gas inlet 21.1 of an absorber 21.

[0020] The absorber 21 can be adapted to absorb carbon dioxide from the incoming CCh-rich flue gas using hot potassium carbonate technology. CCh-lean solvent is delivered at the top of the absorber 21 through a CCh-lean solvent inlet 21.2. In the absorber flue gas flows in an upwards direction in counterflow with the solvent, such that carbon dioxide is removed from the flue gas and absorbed by the downwardly flowing solvent. CCh-rich solvent collects at the bottom of the absorber 21 and is removed from a CCh-rich solvent outlet 21.4, while CCh-lean flue gas is removed from the absorber 21 at the top thereof through a CCh-lean flue gas outlet 21.3. The CCh-rich solvent is regenerated in a regenerator 23. CO2 is released at a CO2 outlet 23.1 at the top of the regenerator 23, while regenerated solvent is removed from a CCh-lean solvent outlet 23.2 at the bottom of the regenerator 23. The CCh-rich solvent inlet of the regenerator is labeled 23.4. Reference number 25 indicates a reboiler of the regenerator 23.

[0021] The cold side of the third heat recovery heat exchanger 19 is fluidly coupled with the CCh-lean flue gas outlet 21.3 of the absorber 21 and with the cold side of the first heat recovery heat exchanger 13. Thus, the CCh-lean flue gas removes heat from the compressed CCh-rich flue gas delivered by the flue gas compressor 17. The cold side outlet 19.4 of the third heat recovery heat exchanger 19 is fluidly coupled with thecold side inlet 13.2 of the first heat recovery heat exchanger 13, such that CCh-lean flue gas is further heated in heat exchange with the incoming CCh-rich flue gas in the first heat recovery heat exchanger 13.

[0022] The cold side outlet 13.4 of the first heat recovery heat exchange 13 is fluidly coupled with a flue gas stack 27, wherethrough CCh-lean flue gas is released in the atmosphere. Since the CCh-lean flue gas flowing thorough the cold side of the first heat exchanger 13 is at a pressure well above ambient pressure, for instance at 3 barG or higher, or at 4 barG or higher, a flue gas expander 29 can be positioned between the cold side outlet 13.4 of the first heat recovery heat exchanger 13 and the stack 27, to convert enthalpy of the CCh-lean flue gas into mechanical power.

[0023] In some embodiments, the flue gas expander 29 can be drivingly coupled with an electric generator, or with another driven machine. In the exemplary embodiment of Fig.1 the flue gas expander 29 is drivingly coupled with the flue gas compressor 17, such that the mechanical power available at the shaft of the flue gas expander 29 can be used directly to drive the flue gas compressor 17. In other embodiments, not shown, the mechanical power generated by the expander can be converted into electric power by an electric generator drivingly coupled to the expander. The electric power thus generated can be supplied to an electric motor or helper drivingly coupled to the flue gas compressor 17.

[0024] As the mechanical power generated by the expander 29 is usually insufficient to drive the flue gas compressor 19, when the expander 29 is drivingly coupled to the flue gas compressor 19 a helper motor, such as a helper electric motor, can be drivingly coupled to the flue gas compressor 17. In Fig.1 a helper motor 31 is shown on a common shaft line 33, drivingly coupling the flue gas expander 29, the helper motor 31 and the flue gas compressor 17. If the expander 29 is drivingly coupled to an electric generator, the electric power generated thereby may be supplemented with additional electric power from an electric power distribution grid, or from electric generator, to supply sufficient electric power to an electric motor which is in turn drivingly coupled to the flue gas compressor 19.

[0025] In both embodiments the mechanical power generated by the expander 29 reduces external power needed to drive the flue gas compressor 19, thus improving theoverall energy efficiency of the system.

[0026] The operation of the system of Fig. 1 described so far is easily understood from the above description and is summarized as follows. Exhaust flue gas from the gas turbine 3, which still contains heat power at a high temperature, around 300-400°C or higher, flows through the first heat recovery heat exchanger 13. High-temperature heat is recovered from the CCh-rich flue gas and absorbed by pressurized CCh-lean flue gas exiting the absorber 21. Further heat, at a lower temperature, is recovered by the second heat recovery heat exchanger 15. Improved heat recovery is thus achieved, at different temperature levels, whereby higher-temperature heat can be advantageously used to generate mechanical power through expansion of the CCh-lean flue gas in the flue gas expander 29. The mechanical energy generated by heat recovery reduces the power required from the helper motor 31 to drive the flue gas compressor 17.

[0027] A further reduction of the power required to drive the flue gas compressor 17 is achieved by removing further heat at a lower temperature from the CCh-rich flue gas through the second heat recovery heat exchanger 15. The volumetric flow rate which must be processed through the flue gas compressor 17 is reduced following additional cooling of the CCh-rich flue gas in the second heat recovery heat exchanger 15. At the same time, a source of valuable heat is made available at the cold side of the second heat recovery heat exchanger 15.

[0028] The cooled CCh-rich flue gas from the second heat recovery heat exchanger is compressed by the flue gas compressor 17 to a pressure sufficient to operate the carbon capture system using hot potassium carbonate solvent.

[0029] A further improvement of the overall efficiency of the system is achieved by removing compression heat from the compressed CCh-rich flue gas through the third heat recovery heat exchanger 19, which increases the amount of total mechanical power available at the shaft of the flue gas expander 29.

[0030] Compressed CCh-rich flue gas at a temperature and pressure suitable for CCh capture using hot potassium carbonate solvent flows through the absorber 21. The resulting compressed, CCh-lean flue gas is heated through the third heat recovery heatexchanger 19 and the first heat recovery heat exchanger 13 in sequence, and finally expanded in the flue gas expander 19 to generate at least a portion of the mechanical power required to compress the CCh-rich flue gas through the flue gas compressor 17.

[0031] A further embodiment of a carbon capture plant according to the present disclosure and a system including a flue gas source and a carbon capture plant are shown in Fig.2. The same reference numbers are used in Figs. 1 and 2 to designate the same or equivalent parts, which are not described again in detail. The plant of Fig.2 differs from the plant of Fig.1 mainly in that the former comprises a flue gas treatment section 35 arranged between the hot side of the second heat recovery heat exchanger 15 and the absorber 21.

[0032] Specifically, in the schematic of Fig.2, the flue gas treatment section 35 is positioned between the second heat recovery heat exchanger 15 and the suction side 17.1 of the flue gas compressor 17. In other embodiments, the flue gas treatment section 35 can be positioned between a first flue gas compressor section and second flue gas compressor section. The flue gas treatment section can be combined with, or operate as an intercooler.

[0033] In some embodiments the flue gas treatment section 35 comprises a direct contact cooler. The system 1 of Fig.2 operates in quite the same way as the plant of Fig.1, except that the CCh-rich flue gas is further treated and cooled by direct contact with a flow of water in the direct contact cooler 35.

[0034] The flue gas treatment section 35, and specifically the direct contact cooler 35, can be beneficial to remove unwanted trace components like sulfur oxides, SOx, from the flue gas. Installation of the direct contact cooler 35 in the disclosed embodiment will predominantly serve the flue gas conditioning by trace component removal rather than actual flue gas cooling. The direct contact cooler 35 may generate a water saturated flue gas stream. This will reduce the amount of water picked-up by the flue gas in the absorber 21. A reduction of the associated risk of precipitation of salts in the absorber solvent, caused by concentrating the solvent, losing the water to the flue gas, is thus obtained. Even though the direct contact cooler is shown at the suction side of the compressor 17 it is clear to those skilled in the art that for a multi-stage compression system the direct contact cooler 35 could be installed also at an intermediatecompression stage.

[0035] 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. A plant for capturing CO2 from a flue gas, the plant comprising: a flue gas inlet duct adapted to receive a flow of incoming flue gas;a first heat recovery heat exchanger having a hot side and a cold side; wherein the hot side is fluidly coupled with the flue gas inlet duct;a second heat recovery heat exchanger positioned downstream of the first heat recovery heat exchanger and having a hot side and a cold side; wherein the hot side of the second heat recovery heat exchanger is fluidly coupled with the hot side of the first heat recovery heat exchanger;a flue gas compressor having a suction side and a delivery side; wherein the suction side is fluidly coupled with the hot side of the second heat recovery heat exchanger;a third heat recovery heat exchanger having a hot side and a cold side; wherein the hot side of the third heat recovery heat exchanger is fluidly coupled with the delivery side of the flue gas compressor and with a CCh-rich flue gas inlet of an absorber; and wherein the cold side of the third heat recovery heat exchanger is fluidly coupled with a CCh-lean flue gas outlet of the absorber and with the cold side of the first heat recovery heat exchanger; andan expander having an inlet side fluidly coupled with the cold side of the first heat recovery heat exchanger and adapted to expand compressed and heated CCh-lean flue gas; wherein the expander is configured to supply power to the flue gas compressor2. The plant of claim 1, wherein the absorber is adapted to use hot potassium carbonate-based solvent.

3. The plant of claim 1 or 2, further comprising a flue gas treatment section arranged between the hot side of the second heat recovery heat exchanger and the absorber.

4. The plant of claim 3, wherein the flue gas treatment section comprises a direct contact cooler.

5. The plant of claim 3 or 4, wherein the flue gas treatment section is positioned between the second heat recovery heat exchanger and the suction side of the flue gas absorber, or between a first stage and a second stage of a multi-stage flue gas compressor.

6. The plant of any preceding claim, wherein the expander is drivingly coupled with the flue gas compressor.

7. The plant of claim 6, further comprising a helper, adapted to supply mechanical power to the flue gas compressor.

8. The plant of any one of claims 1 to 5, wherein the expander is drivingly coupled with an electric generator, and wherein the electric generator is configured to supply electric power to an electric motor drivingly coupled to the flue gas compressor.

9. A power generation system comprising a gas turbine engine having a flue gas discharge fluidly coupled with a carbon capture plant according to any preceding claim.

10. A method for capturing CO2 from a flue gas, comprising the following steps:cooling an incoming CCh-rich flue gas in a first heat recovery heat exchanger in heat exchange with a CCh-lean flue gas released by a CO2 absorber;further cooling the CCh-rich flue gas from the first heat recovery heat exchanger in a second heat recovery heat exchanger and recovering useful thermal power therefrom;compressing the CCh-rich flue gas from the second heat recovery heat exchanger with a flue gas compressor to a pressure suitable for CO2 capture in the absorber;cooling the compressed CCh-rich flue gas in a third heat recovery heat exchanger in heat exchange with the CCh-lean flue gas from the absorber, a cold sideof the third heat recovery heat exchanger being upstream of a cold side of the first heat recovery heat exchanger and fluidly coupled therewith; andexpanding the CCh-lean flue gas exiting the first heat recovery heat exchanger in an expander and converting enthalpy of the expanding CCh-lean flue gas into mechanical power; wherein the expander is configured to supply power to the flue gas compressor.

11. The method of claim 10, further comprising a pretreatment step of the CCh-rich flue gas between the second heat recovery heat exchanger and the flue gas compressor.

12. The method of claim 11, wherein the pretreatment step comprises passing the CCh-rich flue gas through a direct contact cooler.

13. The method of any one of claims 10 to 12, wherein the absorber is adapted to remove CO2 from the CCh-rich flue gas through a potassium carbonate-based solvent.

14. The method of any one of claims 10 to 13, wherein the expander is drivingly coupled to the flue gas compressor.

15. The method of any one of claims 10 to 13, comprising the step of converting mechanical power generated by the expander into electric power and using the electric power to drive the flue gas compressor.

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