Field of Invention

[0001] The present invention relates to CO ₂ capturing in a gas turbine based power plant wherein a carbonate looping process has been connected.

Background of Invention

[0002] There is a general aim to capture the CO ₂ in gases generated in gas turbine based power generation system to make the process more environmentally friendly and to reduce the effect of global warming. The captured CO ₂ gas may then be compressed and transported to be stored in a suitable place, for example in deep geological formations o deep ocean masses. There are several technologies known to remove CO ₂ from the flue gas generated such as by absorption, adsorption, membrane separation, and cryogenic separation.

[0003] When generating power in gas turbine based power generation including a combined cycle power plant (CCPP) and a unit for carbon capture and storage (CCS) the most common way for capturing the CO ₂ is by so-called wet chemistry, wherein the CO ₂ gas is captured by adsorption to a chemical or physical solvent in an aqueous solution. The process is a wet chemistry post-combustion capture taking place in a CCPP.

[0004] However, there is an aim to improve the CO ₂ capturing process as the amount of captured CO ₂ is reduced when comparing to the effort put into the system to remove the gas. In a conventional CCPP the concentration of CO2 is normally only about 4 percent by volume. The CO ₂ capture unit for a conventional CCPP will be more costly and energy consuming per kg of captured CO ₂ than the ones for other types of fossil power plants, like coal fired plants, which have a relatively higher CO ₂ concentration. Also, the methods previously used require regeneration of the included components, like the solvent. These regeneration processes are time and energy consuming. Therefore the overall energy production of the system is reduced.

[0005] A process for capturing CO ₂ by a dry process is known from EP

1 199444 A1. The process described herein is also called a carbonate looping process. However, the process described herein is used for capturing CO ₂ generated in a plant for in connection with a coal power plant. Summary of Invention

[0006] According to aspects illustrated herein there is provided a system and a method for C02 capturing by which the energy consumption of the gas turbine based power generation system is reduced.

[0007] According to aspects illustrated herein, there is provided a system for CO2 capturing in a gas turbine based power generation system, comprising

[0008] a combined cycle power plant (CCPP), comprising one or more gas turbines; a carbonate looping unit for CO2 capturing comprising

a first reactor comprising solid material able to capture the CO ₂ present in the flue gas comprising C0 ₂ , such that metal carbonate is formed by carbonation;

a second reactor arranged to release CO2 by decarbonation of metal carbonate at elevated temperature.

[0009] The gas turbine based power generation system comprising the carbonate looping unit for CO2 capturing as herein described further comprise a heat recovery system generator (HRGS) which may be arranged downstream the first reactor 20 to receive and recover heat therefrom.

[0010] According to an embodiment of gas turbine based power generation system may also comprise a heat recovery system generator (HRGS) arranged downstream a gas turbine of the CCPP to receive and recover heat therefrom. The system may also optionally comprise a flue gas recirculation (FGR) arranged downstream the HRGS.

[0011] Also, the system for CO2 capturing may comprise a heat recovery system generator (HRGS) is arranged to receive and recover heat from the first reactor. This is advantageously as reaction taking place in the first reactor is exotermic and shall be kept at a constant temperature between 400 and 800 °C. Therefore additional steam is generated which can be used for various applications for power generation, for example a heat recovery system generator (HRSG). The steam may also be used for heating the O2 gas introduced in the second reactor.

[0012] The system for C0 ₂ capturing may also include means where parts of the CO ₂ stream from the second reactor is recycled, after cooling, to the inlet of the gas turbine power generation system.

[0013] By the recycling of the C0 ₂ stream the O ₂ , if present, may be re-used in the gas turbine power generation system. Another advantage achieved by the recycling of the CO ₂ stream is an increased CO ₂ partial pressure, preferably to an optimal level in the first reactor which is working under atmospheric conditions.

[0014] Also, the system for C0 ₂ capturing according to the invention may comprise a flue gas recirculation (FGR) unit combined with the gas turbine based power generation system by that parts of the flue gas from the gas turbine (GT) are recirculated, optionally after cooling, to the inlet of the gas turbine. An advantage achieved with this embodiment is an increased concentration of C0 ₂ . If the partial pressure of C0 ₂ is increased, lower energy input is required and also,

advantageously, a lower operating costs. Further, if lower volumes of flue gas may be used when operating the gas turbine power plants, also smaller dimensions of the first reactor is required and also reduction of cost for equipment.

[0015] The gas turbine in the gas turbine based power generation system of the invention may be a non-reheated gas turbine. Another option of gas turbine which may be included in the gas turbine power plant system of the invention is a reheat gas turbine.

[0016] According to other aspects illustrated herein, there is provided a method for capturing C0 ₂ in a gas turbine based power generation system, comprising

bringing the flue gas comprising C0 ₂ into contact with

solid material able to capture the C0 ₂ present in the flue gas, such that metal carbonate such as CaC0 ₃ is formed by carbonation; and

releasing the C0 ₂ by decarbonation of the metal carbonate at elevated temperature in the second reactor.

[0017] In an embodiment of the invention method for capturing CO ₂ is wherein the solid material captures the C0 ₂ by adsorbing or absorbing. Further the method comprises a solid material which is in form of powder or small particles. Preferably the solid material is a metal oxide such as calcium oxide (CaO).

[0018] In an embodiment of the invention the method for capturing C0 ₂ includes to bring the C0 ₂ to solid material having a temperature of between 400 and 800 °C, preferably between 600 and 700°C, more preferably 650 °C. The releasing of C0 ₂ takes place at temperature of about 900 °C in the second reactor. Also, the heat obtained from the first reactor may be received and recoved by a heat recovery system generator (HRGS). The reaction taking place in the first reactor is exotermic and shall be kept at a constant temperature between 400 and 800 °C. Therefore additional steam is generated which can be used for various applications for power generation, for example a heat recovery system generator (HRSG).

[0019] It has been shown that to combine the CCPP and the carbonate looping unit the system shows surprisingly good economical advantages as the CCPP generates gases having temperatures of the flue fases as is required for the reaction in the first reactor. Thus, no or at least reduced amount of energy is required to be added to the system for an efficient reaction.

[0020] The method for capturing C0 ₂ may be performed where the flue gas comprising CO2 is brought into contact with solid material at atmospheric pressure. An alternative is a method wherein flue gas comprising CO ₂ is brought into contact with solid material at increased pressure. By using an increased pressure for the step wherein the flue gas comprising CO2 is brought into contact with the solid material for adsorbing or absorbing of the CO2, the partial pressure of C0 ₂ is increased and the volumetric flow is reduced and thereby also the equipment size is reduced.

[0021] When the adsorption or absorption of the C0 ₂ from the flue gas takes place at increased pressure a gas turbine acting with reheat may be used, as well as a non-heated gas turbine system.

[0022] The above described and other features are exemplified by the following figures and detailed description.

Brief description of the drawings

[0023] Referring now to the figures, which are exemplary embodiments, and wherein the like elements are numbered alike:

[0024] Figure 1 is illustrating a gas turbine based power generation system having a C0 ₂ capturing system.

[0025] Figure 2 is illustrating a variant of the gas turbine based power generation system.

[0026] Figure 3 is illustrating a further variant of the gas turbine based power generation system.

Detailed description of preferred embodiments

[0027] The gas turbine based power generation system, wherein the system for capturing CO ₂ as described above may be used, comprises a conventional combined cycle power plant (CCPP), optionally together with one or more heat recovery system generators (HRGS), and optionally together with a system for flue gas recirculation (FRG).

[0028] An embodiment of the gas turbine power generation system of the invention is illustrated in fig 1.

[0029] The gas turbine based power generation system 1 comprises a combined cycle power plant CCPP 2, including a compressor 10 which is feeded with ambient air via duct 1 1. A gas turbine 15 is supplied with the compressor outlet gas 12 and with energy obtained from the combustor 56 feeded with fuel via 13. The outlet gas from the gas turbine 5, comprising C02 is forwarded to the first reactor 20, via duct 14.

[0030] The CO2 is captured in the first reactor 20 by adsorbing or absorbing of the gas to the solid material. The solid material is typically in form of powder or of small particles. Examples of solid material are metal oxides for example calcium oxide, CaO.

[0031] By the form of powder or of small particles it is provided a way to increase the mass transfer rate from gas to solid and by that reduce the reaction time.

[0032] The first reactor 20 is operating in a temperature interval between 400 to 800°C, preferably at a temperature between 600 and 700°C, for example about 650 °C. By the process taking place in the first reactor 20 a metal carbonate is produced. When calcium oxide (CaO) is used, CaCO ₃ is formed in the first reactor.

[0033] The carbonate is then transferred to the second reactor 30 for decarbonation at elevated temperature. Typically, the reaction in the second reactor 30 is taking place at about 900 °C when CaCO3 is decarbonated.

[0034] The second reactor 30 is feeded with O2 via duct 31. The second reactor is also feeded with fuel via duct 32 for keeping the determined high

temperature. The second reactor may be heated by fuels like natural gas, biomass, coal etc. Also this heating process is generating CO ₂ which may be captured. The reaction, thus the decarbonation wherein CO ₂ is produced, taking place in the second reactor produces some solid residues, such as ash mixed with fines of metal oxide, for example fines of CaO, when the fuel used is biomass or coal. When the second reactor is heated with natural gas the solid residues comprises CaO fines only. The solid residues may be further processed and/or reused for industrial application, for example cement industry. Some of the solid material is transferred back via the duct 33 to the first reactor for being reused.

[0035] The reaction taking place in the first reactor 20 is exothermic. Further, the temperature selected is typically kept constant during operation which results in heat generated which may be used in other parts of the gas turbine power plant, for example, in a heat recovery steam generator (HRSG) 25.

[0036] A CO ₂ capturing method using carbonation and decarbonation is further described in EP 1193444 A1.

[0037] The heat may also be used for heating the O2 gas before introduced into the second reactor 30, via duct 31. By that the amount of fuel needed for heating the second reactor is reduced. Also the size of the equipment for generation the O ₂ may be reduced by these means.

[0038] The captured CO ₂ gas obtained in the second reactor 30 is forwarded via duct 34 for cooling, for example to a temperature of near ambient, in a cooling unit 40, the cooling heat can be recuperated into the water steam cycle (WSC) to increase the steam turbine output.

[0039] The cooled CO ₂ gas captured is then forwarded for compression, and further processing, via duct 45.

[0040] The method for capturing C0 ₂ capturing may also include recycling of parts of the C0 ₂ stream from the second reactor, after cooling, to the inlet of the gas turbine power generation system.

[0041] By the recycling of the CO ₂ stream the O ₂ , if present, may be re-used in the gas turbine power generation system.

[0042] Another advantage achieved by the recycling of the C0 ₂ stream is an increased C0 ₂ partial pressure, preferably to an optimal level in the first reactor which is working under atmospheric conditions.

[0043] Optionally part of the flue gases at the exhaust of turbine 15 can be recirculated into the gas turbine air intake duct 11 and mixed with fresh air in a mixing device 50 upstream. The system is named Flue Gas Recurculation (FGR) and comprises a heat recovery steam generator (HRSG) 18 that is placed first downstream the gas turbine 15 to recuperate the heat and a flue gas cooler 17 that cools down the flue gas before mixing into mixer 50. The heat in 18 may be recovered into a water steam cycle (WSC) connected to the HRGS 18 to increase the power of the steam turbine. The water steam cycle (WSC) recuperates the heat I _ ^, u u / u j / from process streams generating steam that is expanded into steam turbine to generate power.

[0044] The CCPP 2 shown in fig. 2 is based on a reheat gas turbine optionally comprising a flue gas recirculation (FGR) downstream the gas turbine 15 with HRSG 18 and flue gas cooler 17 before mixing into device 50.

[0045] In fig. 3 it is shown a gas turbine based power generation system 1 comprising a combined cycle power plant CCPP 2, comprising multiple turbines, a C02 capture unit 3, comprising the first reactor 20 and the second reactor 30, and a heat recovery steam generator (HRSG) 25.

[0046] An extraction of flue gases is made along the expansion of the gas and this extraction then feeds the item 20, the first reactor. In an embodiment of the invention the item 20, the first reactor, is fed with flue gases extracted, not at the end of expansion, but in the intermediate phase of such an extraction by arranging two turbine modules, one with lower pressure, one or more with higher pressure, and extraction takes place between two modules. In this "inter-turbine" arrangement, parts of the flue gas comprising CO ₂ for capturing in the carbonate looping is forwarded from this extraction so that the flue gas is processed in reactor 20 at pressure higher than "atmospheric" one being such flue gas delivered via duct 14. By that, the carbonate looping, i.e. the CO ₂ capture unit, is operating at higher pressure compared to exhaust pressure of last expansion turbine module. Referring to Figure 3 the extraction is made after turbine module 15'.

[0047] The C02 depleted flue gas existing the item 20 is then further expanded in a turbine module 6.

[0048] In figure 3 the combined cycle power plant (CCPP) shows a gas turbine arrangement with several turbines modules, items 15, 15', 15", 16 being turbine modules where mechanical power is generated to drive one or more generators. In figure 3 expansion module 16 is connected to a separate generator 55'.

[0049] A portion of the flue gas leaving the turbine 15' is transferred to the CO ₂ capture unit via the duct 14. The unit is operating at high pressure and is located between the two expansion turbines 15' and 15". In the CO ₂ capturing unit the gas is first entering the first reactor 20 for adsorbing or absorbing of the CO ₂ onto the CaO present in the first reactor to form calcium carbonate (CaCOa). The temperature of the first reactor is kept at about 650 °C. [0050] The formed calcium carbonate, CaC03, is transferred to the second reactor 30 wherein a decarbonation at elevated temperature of about 900 °C is preformed, by that C0 ₂ is released. The CO ₂ is cooled in the cooling unit 40 before it is transferred for further processing, such as compression, via duct 45. Item 40 may also be connected to a water steam cycle or for heat recovering by, for example, thermal integration.

[0051] Some of the CaO formed in the second reactor 30 is transferred back to the first reactor via duct 33 for being reused in the first reactor 20.

[0052] The captured C0 ₂ gas obtained in the second reactor is forwarded via duct 34 for cooling, for example to a temperature of near ambient, in a cooling system 40. The cooled C0 ₂ gas captured is the forwarded for compression, and further processing, via duct 45.

[0053] Optionally part of the flue gases at the exhaust of turbine 15 can be recirculated into the gas turbine air intake duct 1 1 and mixed with fresh air in a mixing device 50 upstream. The system is named Flue Gas Recurculation (FGR) and comprises a heat recovery steam generator (HRSG) 18 that is placed first downstream the gas turbine 15 to recuperate the heat and a flue gas cooler 7 that cools down the flue gas before mixing into mixer 50. The heat in 18 may be recovered into a water steam cycle (WSC) connected to the HRGS 8 to increase the power of the steam turbine.

[0054] The water steam cycle (WSC) recuperates the heat from process streams generating steam that is expanded into steam turbine to generate power: items 40, 25 and 18 recuperate heat from process streams and produce steam to drive steam turbines to increase plant electrical power output. The steam turbine arrangement and connection to generators and shaft arrangement are not part of the description.

[0055] The gas turbine shown in Figure 3 is a reheated gas turbine, the same concept can be applied to a non reheat gas turbine with a single combustor stage (item 56 or 56').

[0056] While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.