Power generation system and method

Abstract

The power generation system comprises: a compressor; a combustor section; a turbine section; an exhaust gas recirculation path adapted to establish a fluid connection between the turbine discharge and the suction side of the compressor section. The power generation system further includes an exhaust discharge stack and a carbon dioxide discharge. The power generation system further includes a flow adjusting arrangement, adapted to modulate: an air flow to the compressor section; an oxidant flow from an air separation unit towards the combustor section; and an exhaust gas flow recirculating through the exhaust gas recirculation path to the suction side of the compressor section.

Description

POWER GENERATION SYSTEM AND METHODDESCRIPTIONTECHNICAL FIELD

[0001] Disclosed herein are power generation systems and methods of operating these systems.BACKGROUND ART

[0002] A large amount of mechanical and electric power is still generated through thermodynamic cycles which involve the combustion of fossil fuels. Specifically, gas turbine engines are used to drive large rotary equipment such as compressors and electric generators, for the conversion of mechanical power into electric power.

[0003] Combustion of fossil fuels, including natural gas, generates carbon dioxide (CO2) and other polluting gaseous species, such as nitrogen oxides and sulfur oxides. The release of CO2 into the atmosphere has negative effects on the climate, including being a large contributor to global warming.

[0004] Many efforts are being made to reduce the release of carbon dioxide generated by anthropic activities into the atmosphere. Carbon capture systems (shortly CCS) are being developed, which are aimed at removing carbon dioxide from flue gas produced by thermodynamic cycles used for power generation. Because gas turbines operate with a large excess of air, turbine exhaust gas contains a relatively low concentration of carbon dioxide, in the range of 3-4%.

[0005] Such a low percentage of carbon dioxide makes carbon capture processes complex and expensive. The complexity and expense stem, in part, from a necessity to process large flow rates of flue gas. Carbon capture systems also require considerable power, plant and management costs, and the power required for carbon dioxide capture reduces the overall efficiency of the thermodynamic cycles for power generation.

[0006] Therefore, power generation plants have been developed, which use exhaust gas recirculation, aimed at increasing the concentration of carbon dioxide in the exhaust gas. A technology used for this purpose provides for partial recirculation of the exhaust gas from the turbine to the air compressor of the gas turbine engine. The compressor processes a fluid which consists partly of air and partly of previously cooled exhaust gas, which contains a percentage of carbon dioxide. The result is that the carbon dioxide concentration in the exhaust gas increases from 3-4% to about 8-9%, for example. The higher concentration of carbon dioxide in the gas that is discharged from the thermodynamic cycle allows for easier removal of carbon dioxide and reduces the cost of the carbon capture.

[0007] Recently developed oxy-fuel combustion cycles are aimed at addressing the same issue of improving carbon capture and reducing the release of carbon dioxide in the environment. Oxy-combustion technology has been identified as one of the most promising technologies for carbon dioxide capture and sequestration / storage. In oxy- fuel combustion cycles, nitrogen is removed from the process gas and a high-pressure mixture consisting mainly or almost exclusively of carbon dioxide and oxygen is mixed with a fuel, typically natural gas (CT ), and burned. The resulting hot and pressurized combustion gas contains carbon dioxide and water and is expanded in an expander to generate power. The exhaust flue gas is cooled and water is removed therefrom by condensation. Most of the resulting dry exhaust combustion gas, containing almost only carbon dioxide, is recycled and a small fraction is removed to compensate for the addition of oxygen and fuel in the cycle. The high concentration of carbon dioxide in the flow removed from the cycle facilitates carbon capture and sequestration.

[0008] Current oxy-fuel combustion cycles are efficient and environmentally-con- scious, but are complex and technically challenging in view of the high pressure and temperature of the compressed combustion gas.

[0009] An efficient oxy-fuel combustion cycle at low pressure would be therefore welcomed in the art.SUMMARY

[0010] According to a first aspect, disclosed herein is a power generation system including a gas turbine engine comprising a compressor section comprising a suction side adapted to receive exhaust gas and air, specifically ambient air sucked from the environment, a combustor section fluidly coupled to a delivery side of the compressor section and configured to generate a flow of hot and compressed combustion gas (flue gas); and a turbine section having a turbine intake fluidly coupled with the combustor section and a turbine discharge. The system further includes an exhaust gas recirculation path adapted to establish a fluid connection between the turbine discharge and the suction side of the compressor section. The power generation system also includes an exhaust discharge stack adapted to release exhaust gas, i.e., flue gas from the gas turbine engine at least under certain operating conditions. A carbon dioxide discharge is fluidly coupled with the exhaust gas recirculation path to remove carbon dioxide from the power generation system in some operating conditions. An air separation unit of the power generation system is adapted to provide an oxidant flow to the combustor section. The power generation system includes a flow adjusting arrangement, adapted to control: an air flow, i.e. an ambient air flow, to the suction side of the compressor section; an oxidant flow from the air separation unit towards the combustor section; and an exhaust gas flow recirculating through the exhaust gas recirculation path to the suction side of the compressor section; an exhaust gas flow released through the exhaust discharge stack; and a carbon dioxide discharge flow removed from the power generation system; so as to gradually switch the power generation system from a gasturbine operating mode in open cycle, to an oxy-fuel-combustion operating mode in a semi-closed cycle.

[0011] In embodiments disclosed herein, the flow adjusting arrangement is adapted to switch the power generation system from a first operation mode, wherein the gas turbine engine operates in a 100% air, open-cycle condition, in which air is fully taken from the environment, to a semi-closed operating condition, wherein the gas turbine engine operates in an oxy -fuel combustion mode.

[0012] In embodiments disclosed herein, the flow adjusting arrangement comprises: an air flow adjuster at the air inlet; an oxidant flow adjuster between the air separationunit and the combustor section; an exhaust discharge flow adjuster at the exhaust discharge stack; and a carbon-dioxide flow adjuster at the carbon dioxide discharge.

[0013] In some embodiments, the flow adjusting arrangement further comprises a recycling flow adjuster in the exhaust gas recirculation path, between the turbine discharge and the suction side of the compressor section.

[0014] Each flow adjuster disclosed herein can include a device adapted to adjust, i.e., regulate, modulate or control the flow, and consequently the flow rate of a respective fluid stream. The adjustment can be achieved for instance through a flow restriction member, adapted to modify the cross section or the geometry of the respective duct and therefore partialize the fluid flow therethrough and regulate the fluid flow rate. The flow adjuster can for instance modulate the flow rate from 100% to zero by acting on the geometry or on the cross section of the respective flow duct.

[0015] According to a further aspect, disclosed herein is a method for operating a power generation system as outlined above.

[0016] According to embodiments disclosed herein, the method comprises a starting step, wherein the power generation system starts operation in an open-cycle, gas-turbine operating mode with 100% air. A gas turbine engine of the power generation system is operated according to a standard Brayton cycle. Ambient air is sucked by a compressor of the gas turbine engine, mixed with fuel and ignited in a combustor section of the gas turbine engine, to generate high-pressure, hot combustion gas. The combustion gas is expanded in a turbine section of the gas turbine engine to generate power driving the compressor section and useful power available on an output shaft of the gas turbine engine. Exhausted flue gas is released in the environment. The rotation speed and fire temperature of the gas turbine engine increase until a base load condition is reached. Once the base load condition is reached the flow of exhaust gas released from the gas turbine engine through an exhaust discharge stack is reduced gradually. Simultaneously, a gradually increasing flow of exhaust gas is recycled to an intake of the gas turbine engine through an exhaust gas recirculation path arranged between an outlet of the gas turbine engine and the intake of the gas turbine engine. The air flow toward the intake of the gas turbine engine is gradually reduced. Responsive to an oxygen content in the gas turbine engine being below a predetermined thresholdneeded for combustion, the operation of an air separation unit of the power generation system is started to deliver an oxidant flow to the gas turbine engine and maintain the oxygen content in the gas turbine engine at a predetermined value. In some embodiments, the air separation unit can be started in advance, and oxygen can be stored in a storage unit, for instance. The reduction of the flow of exhaust gas discharged through the exhaust discharge stack and the reduction of the air flow towards the intake of the gas turbine engine continue in combination with a continued increase of the flow of exhaust gas recycled towards the gas turbine engine, until the gas turbine engine operates in an oxyfuel-combustion, semi-closed cycle operating mode.

[0017] Further features of the method and of the system according to the present disclosure are described below and outlined in the attached claims.

[0018] As a result of the above steps, the compressor section processes a gas flow which gradually changes from a flow of air to a flow consisting mainly of carbon dioxide or carbon dioxide and oxygen.BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Reference is now made briefly to the accompanying drawings, in which: Fig. l is a schematic of a system according to the present disclosure; and Fig. 2 is a flowchart which summarizes a method according to the present disclosure for operating the system.DETAILED DESCRIPTION

[0020] Disclosed herein is a novel system for operating a power generation system including a gas turbine engine. The system comprises an exhaust gas recycling arrangement, an air separation unit and flow adjustment devices, adapted to handle a transition of the operation mode from an open-cycle, gas-turbine operating mode (Brayton cycle) to a semi-closed oxy-fuel combustion cycle. When the system is started, the gas turbine is fed with air and fuel and generates exhaust gas. A gradually increasing amount of exhaust gas is recycled towards the suction side of the compressor of the gas turbine engine and the amount of exhaust gas released in the environment is correspondingly reduced. The gas turbine engine therefore processes a gas which contains an increasing percentage of carbon dioxide from the recycled exhaust gas anda decreasing amount of environment air. As the carbon dioxide concentration in the circuit increases and the fresh air flowrate decreases, the air separation unit starts feeding an oxidant to the circuit. At the end of the transient period, the fresh air flowrate delivered to the compressor section becomes zero and no exhaust gas is discharged from the circuit. A flow of carbon dioxide is removed from the circuit to compensate for the carbon dioxide generated by the combustion of oxidizer and fuel fed to the combustor of the gas turbine engine and the power generation system operates according to an oxy-combustion cycle with a semi-closed circuit.

[0021] Fig. 1 illustrates a power generation system 1 in one embodiment. The power generation system 1 comprises a gas turbine engine 3 forming part of a top thermodynamic cycle 5. The top thermodynamic cycle 5 converts thermal power into mechanical power and finally into electric power. In some embodiments, the power generation system 1 further includes a bottom thermodynamic cycle 7, which converts waste heat from the top thermodynamic cycle 5 into mechanical power and final into electric power.

[0022] The gas turbine engine 3 includes an intake 3A and an outlet 3B, between which there are arranged: a compressor section 11, a combustor section 13 and a turbine section 15. In some embodiments, the compressor section 11 can include one compressor. In other embodiments, the compressor section 11 can include two or more sequentially arranged compressors. In the embodiment of Fig. 1 the compressor section 11 includes a first compressor and a second compressor arranged in sequence. More specifically, the compressor section 11 includes a low-pressure compressor 11.1 and a high-pressure compressor 11.2. The low-pressure compressor 11.1 includes a suction side 11.11 and a delivery side 11.12. The high-pressure compressor 11.2 includes a suction side 11.21, which is fluidly coupled with the delivery side 11.12 of the low-pressure compressor 11.1, and a delivery side 11.22, which is fluidly coupled with the combustor section 13.

[0023] The compressor section 11 can include an intercooler 11.3. In this case, the delivery side 11.12 of the low-pressure compressor 11.1 can be fluidly coupled with the inlet of the hot side of the intercooler 11.3 and the outlet of the intercooler 11.3 can be fluidly coupled with the suction side 11.21 of the high-pressure compressor 11.2.A coolant fluid flows through the cold side of the intercooler 11.3 to remove heat generated by compression from the partially compressed process gas flowing from the first compressor 11.1 to the second compressor 11.2.

[0024] In some embodiments the first, low-pressure compressor 11.1 can include an axial compressor and the second, high-pressure compressor 11.2 can include a radial compressor, in particular a centrifugal compressor.

[0025] The turbine section 15 can include one or more turbine wheels. In the exemplary embodiment of Fig. 1 the turbine section 15 includes a high-pressure turbine 15.1 and a low-pressure turbine 15.2. A shaft arrangement 17 drivingly connects the turbine section 15 to the compressor section 11, such that power generated by the turbine section is used to drive the compressor section 11. The shaft arrangement 17 can include a single shaft or two or more coaxial shafts. For instance, the high-pressure turbine15.1 can be drivingly coupled through a first shaft to the high-pressure compressor11.2 and the low-pressure turbine 15.2 can be drivingly coupled through a second shaft, coaxial with the first shaft, to the low-pressure compressor 11.1. An output shaft drivingly couples the low-pressure turbine 15.2 to a load, for instance an electric generator 17. The electric generator 17 is electrically coupled to an electric power distribution grid 19.

[0026] In other embodiments a different load can be connected to the output shaft of the gas turbine engine 3. For instance, the gas turbine engine 3 can be used to drive a turbomachine, such as a compressor or a compressor train.

[0027] The structure of the gas turbine engine 3 described above and shown in Fig. 1 is by way of example only. Those skilled in the art of gas turbine engines will understand that different gas turbine layouts can be provided, for instance gas turbine engines including more than two co-axial shafts, and including more than two compressors and two turbines in sequence.

[0028] For instance, in some embodiments, a high-pressure turbine wheel can be drivingly coupled to the high-pressure compressor 11.2 and an intermediate pressure turbine wheel can be drivingly coupled to the low-pressure compressor 11.1. A low- pressure turbine wheel can be provided with an independent shaft which is coupled tothe load. This allows the various turbomachine sections to rotate at different rotary speeds. A single-shaft, heavy duty gas turbine engine can also be used, including a single shaft which drivingly couples one or more compressors to one or more turbine wheels and to the load.

[0029] The turbine section 15 comprises a turbine intake 15.3, fluidly coupled with the combustor section 13, and a turbine discharge 15.4 at the outlet 3B of the gas turbine engine 3.

[0030] The power generation system 1 further comprises an exhaust gas recirculation path 21 having an inlet end 21.1 fluidly coupled with the turbine discharge 15.4 and an outlet end 21.2, fluidly coupled with a compressor inlet section 11.4.

[0031] In some embodiments, an exhaust gas treatment skid 23 can be positioned along the exhaust gas recirculation path 21.

[0032] The exhaust gas recirculation path 21 is fluidly coupled with an exhaust discharge stack 22, wherethrough exhaust gas is discharged from the top thermodynamic cycle in certain operating conditions of the power generation system 1, as will be described later on.

[0033] In some embodiments, the power generation system 1 can include a regenerator 25. The exhaust gas recirculation path 21 can extend through the hot side of the regenerator 25. A compressed gas duct 11.5 leading from the delivery side 11.22 of the second compressor 11.2 to the combustor section 13 extends through a cold side of the regenerator 25. The arrangement is such that heat from exhaust gas flowing from the turbine section 15 is recovered in the regenerator 25 and used to pre-heat the flow of compressed gas from the compressor section 11.

[0034] As mentioned above, in the embodiment of Fig. 1 the power generation system 1 includes a bottom thermodynamic cycle 7. Low-temperature waste heat is transferred from the top thermodynamic cycle 5 to the bottom thermodynamic cycle 7 in a waste heat recovery heat exchanger 29. The exhaust gas recycling path 21 extends through the high-temperature side of the waste heat recovery heat exchanger 29, in heat exchange relationship with a process fluid flowing in the bottom thermodynamic cycle 7, circulating through the low-temperature of the waste heat recovery heatexchanger 29.

[0035] The bottom thermodynamic cycle 7 can be any cycle adapted to convert heat at relatively low temperature into mechanical power. As understood herein a “relatively low temperature” is temperature equal to or lower than the lower temperature of the top thermodynamic cycle. By way of non-limiting example, the bottom thermodynamic cycle is a closed cycle. In some exemplary but not limiting embodiments, the bottom thermodynamic cycle can be a Rankine cycle, such as an organic Rankine cycle (ORC), rather than a steam (water vapor) Rankine cycle.

[0036] In the schematic of Fig. 1 the bottom thermodynamic cycle 7 is represented as including a heater and evaporator 7.1, in the cold side of the waste heat recovery heat exchanger 29, wherein pressurized working fluid, such as an organic fluid, is heated and vaporized. Pressurized and vaporized working fluid expands in a turbine or expander 7.2. Exhaust working fluid is condensed in a condenser 7.3 and pumped by a pump 7.4 back into the waste heat recovery heat exchanger to be vaporized.

[0037] In other embodiments, not shown, a heat transfer loop can be provided between the waste heat recovery heat exchanger 29 and the bottom thermodynamic cycle. In this case, a heat transfer circuit is interposed between the waste heat recovery heat exchanger and the bottom thermodynamic cycle. A heat transfer fluid circulates in the heat transfer circuit. The heat transfer fluid heats up by flowing through the cold side of the waste heat recovery heat exchanger and transfers heat to the working fluid of the low temperature cycle in the evaporator.

[0038] The expander 7.2 is drivingly coupled to a load, for instance an electric generator 31, which converts mechanical power generated by the expander 7.2 into electric power. The electric generator 31 can be electrically coupled to the electric power distribution grid 19.

[0039] In other embodiments the bottom thermodynamic cycle 7 can be omitted. In yet further embodiments, waste heat from the exhaust gas recirculation path can be used differently than to generate further mechanical or electric power. For instance, the power generation system 1 can be a co-generation system used to generate me- chanical / electric power through the top thermodynamic cycle 5 and thermal powerfrom the waste heat recovery heat exchanger, which thermal power can be used as such, for heating purposes or in other processes.

[0040] The exhaust gas recirculation path 21 further includes a cooler 21.3 between the waste heat recovery heat exchanger 29 and the outlet end 21.2 of the gas recirculation path 21. Between the cooler 21.3 and the outlet end 21.2 of the exhaust gas recirculation path 21 a water-gas separator 21.4 can be arranged, wherein water condensed in the cooler 21.3 can be removed from the gaseous flow recirculating in the exhaust gas recirculation path 21. Water can be removed at the bottom of the water- gas separator 21.4 (line 21.5) and dried exhaust gas is delivered to the outlet end 21.2 of the exhaust gas recirculation path 21.

[0041] The power generation system 1 further includes an air separation unit 33, adapted to separate oxygen from ambient air. The air separation unit 33 separates oxygen from air and delivers oxygen, or more generically an oxidant including oxygen, to the top thermodynamic cycle 5 under certain operating conditions, as will be described in more detail below. In the schematic of Fig. 1 the air separation unit 33 is fluidly coupled through an oxidant supply line 35 directly to the combustor section 13. In other embodiments, the output of the air separation unit 33 can be fluidly coupled in a different position, e.g. upstream of the combustor section 13. For instance, oxidant from the air separation unit 33 can be supplied upstream of the regenerator 25, or upstream of one or both compressors 11.1 and 11.2.

[0042] The air separation unit 33 can be configured to supply a stream of substantially pure oxygen in the flow path of the working fluid of the top thermodynamic cycle. In other embodiments, the air separation unit 33 can be configured to supply an oxidant including a blend of oxygen and carbon dioxide, for instance. The carbon dioxide can be delivered from the exhaust gas recirculation path 21, for instance after cooling in cooler 21.3 and water removal in water-gas separator 21.4.

[0043] The power generation system 1 further includes a flow adjusting arrangement adapted to modulate the gaseous flows in the various sections of the power generation system 1, such as to gradually switch the operating mode of the power generation system from a gas-turbine operating mode in open cycle, to an oxy-fuel-combustion operating mode in a semi -closed cycle.

[0044] In the embodiment of Fig. 1, the flow adjusting arrangement includes an air flow adjuster 41, arranged to control and modulate the air flow in an air intake 42 at the inlet of the compressor section 11. As will be explained in more detail below, the air flow adjuster 41 is adapted to modulate the air intake from the compressor section 11.

[0045] The flow adjusting arrangement can further include an oxidant flow adjuster 43 along the oxidant supply line 35. The oxidant flow adjuster 43 is adapted to modulate an oxidant flow from the air separation unit 33 to supply a flow of oxidant at the required flowrate depending upon operating conditions of the power generation system 1, as will be explained in detail below.

[0046] In embodiments, an exhaust discharge flow adjuster 45 is further provided between the exhaust gas recirculation path 21 and the exhaust discharge stack 22. The exhaust discharge flow adjuster is adapted to control and modulate an exhaust gas flowrate through the exhaust discharge stack 22 under certain operating conditions as will be described below.

[0047] In some embodiments, the flow adjusting arrangement further includes a car- bon-dioxide flow adjuster 47 at a carbon dioxide discharge 49, which can be fluidly coupled with the exhaust gas recirculation path 21, downstream of the water-gas separator 21.4. The carbon-dioxide flow adjuster 47 is adapted to control and modulate a carbon dioxide flowrate discharged from the top thermodynamic cycle under certain conditions as described in more detail below. The carbon dioxide discharge 49 can be fluidly coupled with a carbon capture system 51, adapted to capture carbon dioxide exiting the top thermodynamic cycle 7.

[0048] The flow adjusting arrangement can further comprise a recycling flow adjuster 53 arranged along the exhaust gas recirculation path 21, for instance between the water-gas separator 21.4 and the outlet end 21.2 of the exhaust gas recirculation path 21. The recycling flow adjuster 53 is adapted to control and modulate a flowrate of exhaust gas recirculated through the exhaust gas recirculation path 21 towards the compressor inlet section 11.4.

[0049] The power generation system 1 is adapted to operate in a 100% air, gas-turbine operating mode, wherein the power generation system 1 performs a standard open Brayton cycle, and in an oxyfuel-combustion operating mode according to a semi-closed cycle. The power generation system 1 is moreover configured to gradually shift from the open cycle operating mode (standard Brayton cycle) to the semi-closed- cycle operating mode (oxyfuel-combustion cycle) without interruption.

[0050] The process for shifting from the open-cycle operating mode to the semiclosed cycle operating mode is as follows.

[0051] At start-up the power generation system 1 is set to operate according to a standard Brayton open cycle with 100% air. With 10’0% air means that the gas which expands in the turbine section 15 is generated by ignition of an air-fuel mixture, with no recycling of flue gas. The exhaust discharge flow adjuster 45 and the air flow adjuster 41 are fully open. The oxidant flow adjuster 43 is closed. The carbon-dioxide flow adjuster 47 and the recycling flow adjuster 53 are fully closed. The gas turbine engine 3 is started and gradually brought at nominal condition (base load). The compressor section 11 sucks ambient air and delivers compressed air to the combustor section 13. Fuel, for instance methane (CH4), is fed to the combustor section 13 from a source of fuel 55 through a fuel line 57. The fuel flowrate is adjusted via a fuel control valve 59.

[0052] This preliminary step ends when the gas turbine engine 3 reaches the base load condition. The base load condition can be determined by the rotary speed or by the firing temperature of the gas turbine engine. Depending upon the gas turbine engine, the base load can be achieved when the design operating speed is achieved, or when the design firing temperature is achieved. The firing temperature or the rotary speed can be detected by standard sensors and transducers.

[0053] Once the base load has been achieved and a gradual switch, i.e., a transition, to the oxy-fuel combustion cycle is desired, an intermediate operating mode with a gradual increase of exhaust gas recirculation is started. This is achieved by gradually closing the exhaust discharge flow adjuster 45, such that the flowrate of exhaust gas which is released in the atmosphere is gradually reduced. At the same time, the air flow adjuster 41 is gradually closed and the recycling flow adjuster 53 is gradually opened. An increasing flowrate of flue gas, i.e., exhaust combustion gas is recycledfrom the outlet 3B of the gas turbine engine 3 towards the intake 3 A of the gas turbine engine.

[0054] As understood herein, the term “gradual” or “gradually” means that the transition from one operating condition to another is performed as a function of time in a finite time interval, rather than abruptly. Gradual opening or closing of flow adjusters and consequent gradual variation (increase or decrease) of the respective controlled flow rate can be a linear variation occurring in a time interval from tO to tl, wherein tO is the time instant at which the gradual variation starts from a first flow rate value and tl is the time instant at which the gradual variation ends having reached a second flow rate value. As will be described in more detail below, a gradual increase of the flowrate usually starts from zero flow rate and ends at a stationary state value of the flow rate which can depend upon the operating conditions of the power generation system, e.g., from the power required by a load drivingly coupled to the output shaft of the gas turbine engine 3. While a linear transition from the starting value to the final value is possible, this is not the only feasible trend. The time interval tO-tl can be selected based upon needs, for instance also in view of the load applied to the gas turbine engine.

[0055] Because of the gradual increase of the flowrate of recycled exhaust gas, the compressor section 11 processes a blend of air and recycled exhaust gas, containing an increasing percentage of exhaust gas and a decreasing percentage of fresh air.

[0056] Because of the increasing percentage of recycled exhaust gas and reduction of the air at the intake 3 A of the gas turbine engine 3, the percentage of carbon dioxide in the flow processed by the gas turbine engine 3 increases gradually.

[0057] The content of carbon dioxide and of oxygen in the blend of air and recycled exhaust gas processed through the compressor section 11 is detected, i.e., monitored.

[0058] Any suitable device can be used to directly or indirectly detect the percentage of carbon dioxide and oxygen in the flow of gas processed by the gas turbine engine 3.

[0059] In some embodiments, the percentage of carbon dioxide and oxygen in the gas processed by the gas turbine engine 3 can be detected by suitable carbon dioxideand oxygen detectors arranged, for instance, at the intake 3 A of the gas turbine engine 3. In some embodiments, the carbon dioxide and oxygen percentage can be calculated based on the air flowrate and fuel flowrate. The air flowrate at the intake 3 A and the fuel flowrate delivered to the combustor section 13 can be detected with suitable transducers. For instance, a flowmeter can detect the fuel flowrate. Pressure and temperature transducers at the intake 3 A of the gas turbine engine 3 ca be used to calculate the air flowrate.

[0060] The purpose of monitoring, i.e., detecting the carbon dioxide and oxygen content in the gas processed through the gas turbine engine 3 is two-fold. On the one hand the change in composition of the gas mixture processed by the compressor section 11 may require a change in rotary speed of the compressors, to accommodate the operating point of the compressor section 11 and maintain a constant value of the corrected speed, while the speed of sound in the gaseous blend decreases as the carbon dioxide percentage increases. On the other hand, once the oxygen content in the working fluid drops below a percentage needed for correct operation of the gas turbine engine, the oxidant flow adjuster is gradually opened to supply additional oxygen to the working fluid, such that the flowrate of oxygen delivered to the combustor section 13 is always at the correct value, with respect to the fuel flowrate, to maintain stable combustion in the combustor section 13 and reduce or avoid non-combusted fuel from being discharged at the outlet 3B of the gas turbine engine. The oxygen flowrate required depends upon the amount of fuel delivered to the combustor section 13 and in turn depends upon the load applied to the gas turbine engine 3.

[0061] At the end of this transitional operating mode, the airflow adjuster 41 and the exhaust discharge flow adjuster 45 will be fully closed. The recycling flow adjuster 53 will be fully open. Since oxygen and fuel are introduced into the cycle, the cycle cannot be fully closed. To compensate for the addition of chemical species (O2; CH4), the carbon-dioxide flow adjuster 47 is opened to allow a corresponding amount of carbon dioxide, produced by the combustion process, to escape the circuit. The carbon dioxide removed through the carbon-dioxide flow adjuster 47 can be captured in the carbon capture system 51. The flow processed through the carbon capture system 51 is small, as it consists substantially of carbon dioxide. The power required by the carbon capture system 51 is minimized, and the overall efficiency of the power generation system 1is consequently optimized.

[0062] The top thermodynamic cycle 5 is now operating according to an oxy-com- bustion, semi-closed cycle. Fuel and oxygen are fed into the cycle and a corresponding amount of water (from water-gas separator 21.4) and carbon dioxide (through carbondioxide flow adjuster 47 and the carbon-dioxide discharge 49) generated through combustion are removed from the cycle.

[0063] The transition from an open-cycle operating mode to a semi-closed cycle operating mode described above is summarized in the flowchart of Fig. 2 as follows: block 101 represents the step starting the gas turbine with 100% air according to an open Brayton cycle, once the base load condition is achieved (block 102) the exhaust gas recirculation is started by gradually closing the exhaust discharge flow adjuster, gradually closing the air flow adjuster, and gradually opening the recycling flow adjuster. This step is shown in block 103. The CO2 a content in air and recycled exhaust gas mixture entering the gas turbine engine 3 is detected as mentioned above, see block 104, to adapt the rotary speed of the compressor to the variation of the carbon dioxide content within the process gas. Oxygen content is also detected such that when the oxygen content drops below the amount needed for combustion, operation of air separation unit is started and oxygen is supplied therefrom to the gas turbine engine 3, acting upon oxidant flow adjuster, as shown in block 105. The operating point of gas turbine compressor section as a function of CO2 concentration is controlled and modified by modifying the rotary speed thereof as shown in block 106. When the exhaust discharge flow adjuster and the air flow adjuster are fully closed, and the recycling flow adjuster is fully opened, the carbon dioxide flow adjuster opens and excess carbon dioxide produced by combustion is removed from the semi-closed cycle, as shown in block 107.

[0064] The flow adjusters and valves described above can be controlled by a control unit 60 which is functionally connected to the flow adjusters and valves as pictorially shown by dotted lines in Fig. 1. More specifically, the control unit 60 can be functionally connected to the air flow adjuster 41 to reduce the air flowrate from full air flowrate to zero. Moreover, the control unit 60 can be functionally coupled to the exhaust discharge flow adjuster 45 and to the recycling flow adjuster 53, to control thereduction of the flowrate of exhaust gas released through the stack 22 and the simultaneous increase of the flowrate of exhaust gas recycled from the gas turbine engine outlet 3B to the gas turbine intake 3 A through the water-gas separator 21.4. Through a functional connection of the control unit 60 with the carbon-dioxide flow adjuster 47 the latter can be opened to control the flowrate of carbon dioxide removed from the circuit when the power generation system 1 is operating in the semi-closed, oxy-fuel combustion mode. The oxidant flow adjuster 43 is controlled by the control unit 60 to provide supplemental oxidant to the combustor section 13 when the content of oxygen in the process fluid is insufficient. In Fig. 1 the oxygen and carbon dioxide content are detected through a sensor block 64 arranged by way of example at the intake 3 A of the gas turbine engine 3 and delivered to the control unit 60, which is functionally connected to said sensor block 64. The control unit 60 can be further functionally coupled to the fuel control valve 59 to control the fuel flowrate.

[0065] Additionally, the control unit 60 can be functionally coupled, e.g., to a rotary speed sensor 66 adapted to detect the rotary speed of the gas turbine engine 3, and / or of one or more of the shafts thereof, as well as to a temperature sensor 68 adapted to detect the firing temperature of the gas turbine engine 3, such that reaching of the base load can be detected through by the control unit 60.

[0066] In some embodiments the power generation system may include devices or arrangements adapted to start the system in an oxy-fuel combustion mode directly, without a preliminary operating phase in a gas turbine mode. In some embodiments, these arrangements may be used to speed up the transition or switching process from the gas turbine operation mode to the oxy-fuel operation mode.

[0067] Such arrangements or devices may include a carbon dioxide tank or a carbon dioxide source adapted to be fluidly coupled with the suction side of the compressor section 11. In Fig. 1 reference 61 represents a generic carbon dioxide source. In Fig. 1 the carbon dioxide source 61 is pictorially represented as a tank, which can be placed in fluid communication with the exhaust gas recycle path 21 and / or with the suction side 11.4 of the compressor through a valve 63. In other embodiments, the carbon dioxide source can be a separate process or facility, wherefrom a carbon dioxide flow can be available. The valve 61 can be functionally connected to the control unit 60.

[0068] The carbon dioxide source can be used for instance if one or more of the flow adjusters described above, which are required for performing the gradual transition from a gas-turbine operating mode to an oxy-combustion operating mode, is unavailable. The carbon dioxide source 61 can also be used to accelerate the transition from the gas-turbine operating mode to the oxy-fuel combustion mode. This can be done by supplementing carbon dioxide to the compressor section 11 thus increasing the carbon dioxide concentration in the exhaust gas recirculation path 21 at a faster rate.

[0069] In some embodiments, to achieve a smoother operation of the power generation system, one or more dampers can be provided at one or more points along the exhaust gas recirculation path 21. This can be particularly useful downstream of intersections between ducts, where different gaseous flows intersect and merge into one another. For instance, and by way of non-limiting example, a damper 65 can be provided downstream of the intersection between the exhaust gas recirculation path 21 and the air intake 42.

[0070] 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

POWER GENERATION SYSTEM AND METHODCLAIMS1. A power generation system, comprising: a compressor section comprising a suction side adapted to receive exhaust gas and air; a combustor section fluidly coupled to a delivery side of the compressor section and configured to generate a flow of hot and compressed flue gas; a turbine section having a turbine intake fluidly coupled with the combustor section and a turbine discharge; an exhaust gas recirculation path adapted to establish a fluid connection between the turbine discharge and the suction side of the compressor section; an exhaust discharge stack; a carbon dioxide discharge, fluidly coupled with the exhaust gas recirculation path; an air separation unit, adapted to provide an oxidant flow to the combustor section; and a flow adjusting arrangement, adapted to control: an air flow to the suction side of the compressor section; an oxidant flow from the air separation unit towards the combustor section; and an exhaust gas flow recirculating through the exhaust gas recirculation path to the suction side of the compressor section; an exhaust gas flow released through the exhaust discharge stack; and a carbon dioxide discharge flow removed from the power generation system; the flow adjustment arrangement being configured to gradually switch the power generation system from a gas-turbine operating mode in open cycle, to an oxy -fuel-combustion operating mode in a semi-closed cycle.

2. The system of claim 1 , wherein the flow adjusting arrangement comprises: an air flow adjuster at the air inlet; an oxidant flow adjuster at an outlet of the air separation unit;an exhaust discharge flow adjuster at the exhaust discharge stack; and a carbon-dioxide flow adjuster at the carbon dioxide discharge.

3. The system of claim 2, wherein the air flow adjuster is configured to control and modulate an air flow in an air intake at the inlet of the compressor section and to gradually reduce the air flow to the compressor section when the power generation system gradually switches from the gas-turbine operating mode to the oxy-fuelcombustion operating mode; wherein the oxidant flow adjuster is configured to gradually open to supply additional oxygen to a working fluid circulating in the system while the air flow adjuster gradually reduces the air flow to the compressor section.

4. The system of claim 2 or 3, wherein the exhaust discharge flow adjuster is configured to gradually close, to reduce the flowrate of exhaust gas released in the atmosphere while the air flow adjuster gradually reduces the air flow to the compressor section and the oxidant flow adjuster gradually opens; and wherein the carbon-dioxide flow adjuster is adapted to discharge carbon dioxide when the system is operating in the oxy-fuel combustion mode.

5. The system of claim 2 or 3 or 4, wherein the flow adjusting arrangement further comprises a recycling flow adjuster in the exhaust gas recirculation path, between the turbine discharge and the suction side of the compressor section.

6. The system of claim 5, wherein the recycling flow adjuster is configured to gradually open while the air flow adjuster is gradually closed to switch the system from the gas turbine operating mode in open cycle, to the oxy-fuel combustion operating mode in semi-closed cycle.

7. The system of any one of the preceding claims, wherein the exhaust gas recirculation path comprises an exhaust gas cooler.

8. The system of claim 7, when dependent on claim 4, wherein the recycling flow adjuster is positioned downstream of the exhaust gas cooler with respect to the direction of flow of the exhaust gas in the exhaust gas recirculation path.

9. The system of any one of the preceding claims, wherein the exhaust gas recirculation path comprises an exhaust gas treatment skid adapted to treat the exhaust gas recirculating in the exhaust gas recirculation path.

10. The system of any one of the preceding claims, wherein the compressor section is drivingly coupled to the turbine section and driven into rotation thereby.

11. The system of any one of the preceding claims, wherein compressor section comprises: a first compressor having a suction side and a delivery side; and a second compressor having a suction side and a delivery side; wherein the first compressor and the second compressor are arranged in series.

12. The system of claim 11, wherein the compressor section further comprises an intercooler fluidly coupled with the delivery side of the first compressor and with the suction side of the second compressor.

13. The system of claim 11 or 12, wherein the first compressor is an axial compressor and the second compressor is a radial compressor, in particular a centrifugal compressor.

14. The system of any one of the preceding claims, further comprising a regenerator adapted to transfer heat from exhaust gas, circulating in the exhaust gas recirculation path, to compressed gas delivered by the compressor section, upstream of the combustor section.

15. The system of any one of the preceding claims, further comprising a bottom cycle, adapted to convert waste heat contained in exhaust gas recirculating in the exhaust gas recirculation path into mechanical power; wherein a waste heat recovery heat exchanger is arranged along the exhaust gas recirculation path between the turbine section and the compressor section and is adapted to transfer waste heat from the exhaust gas recirculating in the exhaust gas recirculation path to the bottom cycle.

16. The system of claim 15, wherein the bottom cycle is an organicRankine cycle.

17. The system of claim 15 or 16, when depending upon claim 11, wherein the regenerator is positioned upstream of the waste heat recovery heat exchanger with respect to a direction of flow of the exhaust gas in the exhaust gas recirculation path.

18. The system of any one of the preceding claims, further comprising a carbon capture system fluidly coupled to the carbon dioxide discharge.

19. The system of any one of the preceding claims, further comprising at least one damper in the exhaust gas recirculation path.

20. The system of any one of the preceding claims, further comprising a carbon dioxide source, adapted to be fluidly coupled to the system to provide carbon dioxide to the compressor section.

21. A method of operating a power generation system, the method comprising the following steps: starting operation of a power generation system in an open-cycle, gas-turbine operating mode with 100% air; determining that a base load condition is reached; gradually reducing a flow of exhaust gas discharged from a gas turbine engine through an exhaust discharge stack; gradually increasing a flow of exhaust gas recycled to an intake of the gas turbine engine through an exhaust gas recirculation path arranged between an outlet of the gas turbine engine and the intake of the gas turbine engine; and gradually reducing an air flow toward the intake of the gas turbine engine; responsive to an oxygen content in the gas turbine engine being below a predetermined threshold needed for combustion, starting delivery of an oxidant flow to the gas turbine engine; and further reducing the flow of exhaust gas discharged through the exhaustdischarge stack; continuing reducing the air flow towards the intake of the gas turbine engine; and continuing increasing the flow of exhaust gas recycled towards the gas turbine engine, until the gas turbine engine operates in an oxyfuel-combustion, semiclosed cycle operating mode.

22. The method of claim 21, wherein the step of starting delivery of an oxidant flow to the gas turbine engine includes the step of starting operation of an air separation unit of the power generation system to generate said oxidant flow.

23. The method of claim 21 or 22, wherein the step gradually reducing the flow of exhaust gas discharge through the exhaust discharge stack includes the step of gradually closing an exhaust discharge flow adjuster positioned between the outlet of the gas turbine engine and the exhaust discharge stack.

24. The method of claim 21, 22 or 23, wherein the step of gradually increasing the flow of exhaust gas recycled toward the intake of the gas turbine engine includes the step of gradually opening a recycling flow adjuster adapted to connect the exhaust gas recirculation path to the intake of the gas turbine engine.

25. The method of any one of claims 21 to 24, wherein the step of gradually reducing the air flow toward the intake of the gas turbine engine includes the step of gradually closing an air flow adjuster between an air inlet and the intake of the gas turbine engine.

26. The method of any one of claims 21 to 25, wherein the step of delivering an oxidant flow to the gas turbine engine from the air separation unit comprises the step of controlling an oxidant flow adjuster to maintain the oxygen content in a combustor section of the gas turbine engine at a predetermined amount.

27. The method of any one of claims 21 to 26, further comprising the step of opening a carbon dioxide flow adjuster to remove carbon dioxide excess from the power generation system.

28. The method of claim 27, wherein the step of opening the carbondioxide flow adjuster and removing carbon dioxide excess from the power generation system starts when the flowrate of exhaust gas discharged through the exhaust gas discharge stack is approximately zero.

29. The method of any one of claims 21 to 28, further comprising the following steps: controlling a content of carbon dioxide in a gas flow at the suction side of the intake of the gas turbine engine; adjusting a rotary speed of the compressor section responsive to the carbon dioxide content, to maintain a substantially constant corrected speed in the compressor section.

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