Oxyfuel turbine system and oxidizer control method

Abstract

The gas turbine system (1) comprises a combustor (3) adapted to combust a fuel and an oxidizer to generate pressurized hot combustion gases, and a turbine (5) fluidly connected to the combustor (3) and rotated by expansion of the pressurized hot combustion gases from the combustor (3). A heat exchanger (21) is fluidly connected to the turbine (5) and adapted to cool the expanding combustion gases discharged from the turbine (5). A primary oxidizer supply line (13) is adapted to supply oxidizer to the combustor (3) through the heat exchanger (21). The oxidizer flowing through the heat exchanger (21) is in heat exchange relationship with the combustion gases discharged from the turbine (5). A fuel supply line (39) supplies fuel to the combustor (3). A secondary oxidizer supply line (53) is adapted to supply oxidizer to the fuel supply line (39) upstream of a fuel control valve (41). A method of operating the system is also disclosed.

Description

[Technical field]

[0001] OXYFUEL TURBINE SYSTEMS AND METHODS FIELD OF THE DISCLOSURE The present disclosure relates to gas turbine systems for power generation. In particular, embodiments disclosed herein relate to oxyfuel turbine systems, i.e. oxyfuel expander systems, and related methods. [Background technology]

[0002] Fossil fuels are the primary source of chemical energy used to generate mechanical power. Fossil fuels are mixed with air and burned to produce combustion gases at high pressure and temperature, which are expanded in a turbine. The turbine converts the enthalpy of the combustion gases into usable mechanical power on the turbine's output shaft, which is used to drive a load, such as a compressor or compressor train, or to spin a generator to convert the mechanical power into electrical power.

[0003] One of the major concerns about the burning of fossil fuels concerns the production of carbon dioxide, a greenhouse gas that is considered one of the main causes of global warming and climate change.

[0004] In an attempt to reduce the environmental impact of electricity generation from the combustion of fossil fuels, options for post-combustion capture of carbon dioxide have been investigated. Carbon dioxide capture facilities have been developed to treat the flue gas exhausted from gas turbines and remove carbon dioxide from the flue gas before it is discharged to the environment. The cost of carbon dioxide capture facilities is high, both in terms of CAPEX and in terms of the energy required to operate the facility, reducing the overall thermodynamic efficiency of the system. The percentage of carbon dioxide in the flue gas is low. This requires large volumes of flue gas to be processed through the carbon dioxide capture facility, making the capture process particularly inefficient.

[0005] Recently, oxy-combustion turbines, also known as oxy-fuel turbines or oxy-fuel expanders, have been developed, which use a fuel that is primarily oxygen (O ) instead of air. 2 ), or oxygen and carbon dioxide (CO 2The gas turbine uses an oxidant stream consisting of a mixture of oxygen and carbon dioxide. The oxygen is obtained by separation from ambient air. A portion of the flue gas from the gas turbine is recirculated in the gas turbine combustor so that the working fluid supplied to the combustor consists mainly of oxygen and carbon dioxide, and is nitrogen-free. The resulting flue gas consists mainly of water and carbon dioxide. Water is removed from the flue gas by condensation, and the portion of the water-free flue gas that is not recirculated to the combustor can be efficiently treated in a carbon dioxide capture unit.

[0006] The amount of electrical power generated by a power plant or system may need to be finely and quickly adjusted to follow variations in the mechanical load applied to the turbine shaft. For example, when a turbine drives a generator connected to the electrical grid, the load applied to the turbine may vary depending on the amount of electrical power absorbed by the electrical load connected to the electrical grid and powered thereby. The rotational speed of the turbine and the generator remains constant. Thus, variations in the load are balanced by adjusting the fuel supplied to the turbine combustor. Similar adjustment requirements may arise when the turbine drives a compressor train or any other driven machine. Variations in fuel flow rate must be compensated for by adjusting the oxidizer flow rate. Summary of the Invention [Problem to be solved by the invention]

[0007] Oxyfuel turbines are operated at or near stoichiometric ratios, and flow fluctuations required to balance load changes during turbine operation transients can cause flame instabilities.

[0008] Efforts have been made to make oxyfuel turbines more responsive to load changes and to prevent combustion problems resulting from fluctuations in oxidant and fuel flow rates. A need remains for improvements in oxyturbines in this regard.

[0009] To alleviate the shortcomings of current technology gas turbines, an oxy-fuel gas turbine system is disclosed herein that comprises a combustor adapted to combust a fuel and an oxidant to generate pressurized hot combustion gases, and a turbine fluidly connected to the combustor and rotated by expansion of the pressurized hot combustion gases from the combustor.

[0010] The system further comprises a heat exchanger fluidly connected to the turbine and adapted to cool the expanding combustion gases discharged from the turbine. The main oxidizer supply line is adapted to supply a main oxidizer stream through the heat exchanger to the combustor. In use, the main oxidizer stream flowing through the heat exchanger exchanges heat with the combustion gases discharged from the turbine such that heat is recovered from the combustion gases and used to heat the main oxidizer stream and cool the combustion gases.

[0011] The recirculation line is adapted to recirculate the first cooled combustion gas stream through the heat exchanger to the combustor as a working fluid, while the combustion gas removal line is adapted to discharge the second cooled combustion gas stream, for example toward a carbon dioxide capture system.

[0012] A fuel supply line is positioned to supply fuel to the combustor, and a secondary oxidant supply line is positioned to supply a secondary oxidant flow into the fuel supply line upstream of a fuel control valve positioned along the fuel supply line relative to a direction of fuel flow.

[0013] Thus, when required by the operating conditions of the system, an additional amount of oxidizer can be supplied to the combustor through the fuel supply line. Improved mixing of the oxidizer and fuel is obtained by supplying a secondary oxidizer stream to a mixing point located upstream of the fuel control valve. A uniform composition of the oxidizer-fuel blend supplied to the multiple fuel nozzles is ensured, improving combustion conditions within the combustor.

[0014] In some embodiments, the secondary oxidant supply line bypasses the heat exchanger, i.e., does not extend through the heat exchanger.

[0015] In the case of load variations, the flow rate of the secondary oxidizer stream can be rapidly adjusted in response and in combination with fuel flow variations, for example, as the load applied to the turbine shaft increases, the fuel and oxidizer flows of the secondary oxidizer stream are increased to generate more power and maintain the required rotational speed of the turbine.

[0016] Because the varying oxidizer flow rate is pre-mixed with the fuel before entering the combustor, enhanced flame stability is achieved, especially in the event of load changes, combined with rapid response of the turbine system to such transients.

[0017] In the embodiments disclosed herein, a secondary oxidizer control valve is provided in the secondary oxidizer supply line and adapted to regulate the secondary oxidizer flow rate through the secondary oxidizer supply line during turbine transients, i.e., in response to turbine load changes. Since there is no heat exchanger along the secondary oxidizer supply line, the secondary oxidizer control valve can be located close to the fuel supply line, e.g., to minimize the oxidizer volume between the secondary oxidizer control valve and the combustor. This is beneficial in terms of the responsiveness of the system in adapting to variable operating conditions, i.e., load changes.

[0018] The system may further include a main oxidizer control valve in the main oxidizer supply line and a control unit. The control unit may be adapted to selectively open and close the secondary oxidizer control valve and the main oxidizer control valve in response to a load change of the turbine. The secondary oxidizer control valve may be actuated first and the secondary oxidizer control valve may be actuated in a second step of the oxidizer flow rate adjustment sequence to obtain a faster adaptation of the oxidizer flow rate to the load change.

[0019] According to a further aspect, disclosed herein is a method of operating a turbine system, the method comprising the steps of: supplying a fuel flow to a combustor through a fuel supply line; providing a primary oxidant flow through a heat exchanger to a combustor; combusting fuel and oxidant from a fuel supply line in a combustor to generate a flow of hot pressurized combustion gases; expanding the combustion gases in a turbine, thereby generating mechanical power; discharging combustion gases from the turbine; flowing the exhaust combustion gases through a heat exchanger to exchange heat with the primary oxidant stream to cool the exhaust combustion gases and heat the primary oxidant stream; recirculating the first cooled combustion gas stream through a heat exchanger to the combustor as a working fluid and discharging a second cooled combustion gas stream; providing a secondary oxidant stream to the fuel stream and blending the secondary oxidant stream into the fuel stream; providing a blend of oxidizer and fuel to the combustor.

[0020] In embodiments disclosed herein, the method further includes adjusting a flow rate of the secondary oxidant stream in response to load variations on the turbine.

[0021] Specifically, in some embodiments, the method comprises the following steps: increasing or decreasing a secondary oxidizer flow rate through a secondary oxidizer control valve in response to an increase or decrease in load on the turbine; Thereafter, decreasing or increasing the secondary oxidizer flow rate through the secondary oxidizer control valve while simultaneously increasing or decreasing the main oxidizer flow rate through the main oxidizer control valve to maintain a substantially constant oxidizer flow rate to the combustor.

[0022] Further features and embodiments of the systems and methods according to the present disclosure are described below with reference to the accompanying drawings and are set forth in the appended claims. [Brief description of the drawings]

[0023] Reference will now be made briefly to the accompanying drawings, in which: [Figure 1] FIG. 1 is a schematic diagram of an oxyfuel turbine system according to the present disclosure. [Diagram 2] FIG. 2 is a graph showing load, fuel flow rate, primary oxidizer flow rate and secondary oxidizer flow rate over time in the system of FIG. [Diagram 3] FIG. 3 is a graph showing load, fuel flow rate, primary oxidizer flow rate and secondary oxidizer flow rate over time in the system of FIG. [Figure 4] FIG. 4 is a graph showing load, fuel flow rate, primary oxidizer flow rate and secondary oxidizer flow rate over time in the system of FIG. [Diagram 5] FIG. 5 is a flow chart summarizing steps of a method according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] To achieve faster response to turbine load changes in the oxy-fuel turbine system and improved flame stability during oxy-fuel turbine transients, an additional or secondary oxidizer supply line is fluidly connected directly to the fuel supply line. To enhance combustion conditions, a secondary oxidizer flow rate provided to the fuel supply line is introduced into the fuel stream upstream of the fuel control valve. This ensures, for example, that the same fuel-oxidizer blend composition is supplied to all fuel nozzles and burners of the combustor.

[0025] The secondary oxidant supply line may be positioned to bypass the heat recovery heat exchanger.

[0026] A control valve on the secondary oxidizer supply line is operated via a controller to adjust the flow rate of the secondary oxidizer flow added directly to the fuel stream into the combustor in order to quickly adapt the operation of the turbine system to rapid fluctuations in the load applied to the turbine shaft.

[0027] 1 shows a schematic diagram of an oxyfuel turbine system 1 according to an embodiment. The turbine system 1 includes a combustor 3 fluidly coupled to an expander or turbine 5. The rotating turbomachine 5 is referred to herein as the "turbine."

[0028] The turbine 5 generates mechanical power from the expansion of the combustion gases and is drivingly connected to a load 9 via a shaft 7. The load 9 is rotationally driven by the mechanical power produced by the turbine 5. In the exemplary embodiment of FIG. 1, the load 9 includes an electrical generator electrically connected to an electrical grid 11.

[0029] A main oxidant stream is supplied to combustor 3 through main oxidant supply line 13. The main oxidant stream consists primarily of oxygen or a mixture of oxygen and carbon dioxide. The oxidant may be supplied by any oxidant source. In some embodiments, carbon dioxide may be blended with oxygen in oxidant supply line or oxidant source 15 such that the oxidant stream contains a reduced amount of oxygen, for example, about 20% oxygen by volume.

[0030] In FIG. 1 , the oxidant source includes an air separation unit 15 that separates oxygen or a blend of oxygen and carbon dioxide from ambient air and removes nitrogen or nitrogen and carbon dioxide from ambient air. The main oxidant flow from the oxidant source 15 may be compressed in a first oxidant compressor 17 to a pressure required to supply oxidant to the combustor 3. The first oxidant compressor 17 may be driven by a drive device such as an electric motor 18. The electric motor 18 may be powered by electricity from the electrical grid 11. In other embodiments not shown, the first oxidant compressor 17 may be directly driven by the turbine 5. In such a case, the first oxidant compressor 17 may be part of a load drivingly connected to the turbine shaft 7.

[0031] A main oxidizer control valve 19 disposed along the main oxidizer supply line 13 between the first oxidizer compressor 17 and the combustor 3 is adapted to control the first oxidizer flow rate through the main oxidizer supply line 13 toward the combustor 3. A main oxidizer flow meter 20 may be disposed in the main oxidizer supply line 13 to detect the main oxidizer flow rate flowing through the main oxidizer supply line 13 toward the combustor 3.

[0032] The main oxidant stream supplied through the main oxidant supply line 13 flows through the cold side 211 of a heat exchanger 21 in heat exchange relationship with the combustion gases (flue gases) discharged from the turbine 5 to recover heat therefrom, as described in more detail below. The combustion gases discharged from the turbine 5 flow through the hot side 212 of the heat exchanger 21.

[0033] The hot side 212 of the heat exchanger 21 has an inlet fluidly connected to the exhaust of the turbine 5 via a combustion gas exhaust line 23. The expanding combustion gases exhausted from the turbine 5 exchange heat with the main oxidizer stream flowing through the main oxidizer supply line 13 such that low temperature heat contained in the combustion gases exhausted from the turbine 5 is at least partially recovered and used to preheat the main oxidizer stream prior to entering the combustor 3, while the combustion gases are cooled.

[0034] An outlet of the hot side 212 of the heat exchanger 21 is fluidly connected to a water removal arrangement 25 adapted to remove water from the cooled combustion gases discharged from the hot side 212 of the heat exchanger 21. In the schematic diagram of Figure 1, the water removal arrangement 25 comprises a condenser 251 and a water / gas separator 252. The combustion gases flowing through the condenser 251 are cooled such that steam contained in the combustion gases condenses and is separated from the gases in the water / gas separator 252 and removed through a water removal line 253.

[0035] Since the oxidant supplied to the combustor consists mainly of oxygen or a blend of oxygen and carbon dioxide, and the oxidant flow rate and fuel flow rate are controlled to have stoichiometric combustion conditions in the combustor 3, the resulting flue gas exiting the turbine 5 consists mainly of carbon dioxide and water, the latter being removed by the water removal arrangement 25. The cooled flue gas exiting the water / gas separator 252 therefore consists mainly of carbon dioxide.

[0036] The cooled combustion gas from the water / gas separator 252 is compressed by the combustion gas compressor 27 and split into a first cooled combustion gas stream and a second cooled combustion gas stream. The combustion gas compressor 27 may be driven by a turbine 5 or by a drive device 28, such as an electric motor, which may be powered by electricity, for example, from the electrical grid 11.

[0037] The first cooled combustion gas stream provided by the gas / water separator is recirculated via a recirculation line 29 towards the heat exchanger 21 and the combustor 3. The recirculation line 29 branches off from a combustion gas removal line 31. In the embodiment of Figure 1, the combustion gas removal line 31 is fluidly connected to a carbon dioxide capture unit 33, which processes the second cooled combustion gas stream delivered from the water / gas separator 252, for example to appropriately store carbon dioxide and prevent its release into the environment.

[0038] The first cooled combustion gas stream recirculated through recirculation line 29 flows through a second cold side 213 of the heat exchanger 21 in heat exchange relationship with the hot side 212 and receives heat from the hot combustion gases discharged from the turbine 5.

[0039] The flow rate of the recirculated combustion gas through the recirculation line 29 can be adjusted by a recirculated combustion gas control valve 35. A recirculated combustion gas flow meter 37 can be provided along the recirculation line 29 to detect the flow rate of the recirculated combustion gas towards the combustor 3.

[0040] A side stream of carbon dioxide may be branched off from recycle line 29 or from the dehydrated carbon dioxide stream upstream of compressor 27 and added to the oxygen stream from oxidant source 15. As noted above, the percentage of oxygen in line 13 and / or line 53 may be reduced to about 20% by volume for ease of handling.

[0041] In the combustor 3, the main oxidizer flow provided through the main oxidizer supply line 13 and the recirculated combustion gas provided through the recirculation line 29 are mixed with a fuel, e.g., a gaseous fuel, provided through a fuel supply line 39 to a fuel nozzle 38, or preferably a plurality of fuel nozzles 38, in the combustor 3. A fuel control valve 41 along the fuel supply line 39 is adapted to adjust the flow rate of the fuel delivered to the combustor 3. A fuel flow meter 43 may be provided along the fuel supply line 39 to detect the flow rate of the fuel provided to the combustor 3.

[0042] The mechanical power required to rotate the generator 9 may vary as a result of fluctuations in the power absorbed by the electrical load (not shown) connected to the power grid 11. Since the rotational speed of the turbine 5 and the generator 9 remains constant, the load variations must be balanced by adjusting the fuel flow accordingly to prevent angular acceleration or deceleration of the turbine 5 and the generator 9. The fuel flow must be adjusted as quickly and as finely as possible by the fuel control valve 41 under the control of a control unit 51. The control unit 51 is functionally connected to the flow meters 20, 37, 43 and the control valves 19, 35, 41. The functional connections are diagrammatically represented by the letters (A) to (H) in circles.

[0043] As discussed above, the oxidant to fuel molar ratio in combustor 3 is controlled to maintain stoichiometric combustion conditions and avoid residual fuel or oxidant in the combustion gases exhausted from turbine 5. To maintain the stoichiometric ratio in combustor 3, changes in fuel flow rate are accompanied by rapid oxidant flow rate adjustments.

[0044] The volume between the main oxidizer control valve 19 and the combustor 3 includes the cold side 211 of the heat exchanger 21 and is therefore relatively large. The oxidizer flow regulation by the main oxidizer control valve 19 follows variations in the load applied to the turbine 5, resulting in a delayed regulation of the oxidizer flow compared to the fuel flow regulation. Thus, variations in the power rate cause transients in which the combustor 3 is operated with either too much oxidizer or too much fuel.

[0045] To avoid or reduce these drawbacks, the oxy-fuel turbine system 1 includes a secondary additional oxidizer supply line 53, which may be provided with a secondary oxidizer flow meter 55 and a secondary oxidizer control valve 57, both operatively coupled to the control unit 51. The secondary oxidizer supply line 53 is fluidly coupled to the fuel supply line 39 via the secondary oxidizer control valve 57 at a mixing point 54 and adapted to supply a secondary oxidizer flow to the combustor 3 via the fuel supply line 39. The additional oxidizer supply line 53 bypasses the heat exchanger 21.

[0046] In the drawings, the mixing point 54 is shown downstream of the fuel flow meter 43. However, in other embodiments, the fuel flow meter 43 may be located downstream of the mixing point 54. The fuel flow meter 43 detects the flow of fuel when the secondary oxidizer control valve 57 is closed, and detects the flow of the secondary oxidizer and fuel mixture when the fuel flow meter 43 is located downstream of the mixing point 54.

[0047] The flow rate of the secondary oxidizer stream flowing through the secondary oxidizer supply line 53 may be substantially smaller than the flow rate of the main oxidizer stream flowing through the main oxidizer supply line 13. There is no need to heat the secondary oxidizer stream, since sufficient heat recovery is ensured by the heat exchanger 21. Therefore, no heat exchange surface is required between the secondary oxidizer control valve 57 and the mixing point 54. In other words, the secondary oxidizer control valve 57 can be located close to the fuel supply line 39 such that the volume between the secondary oxidizer control valve 57 and the fuel supply line 39 is minimized. When the secondary oxidizer control valve 57 is actuated, it results in a fast response of the system with respect to the total oxidizer flow rate in the combustor 3.

[0048] Furthermore, according to embodiments disclosed herein, the secondary oxidant flow is pre-mixed with the fuel upstream of the combustor 3. This enhances flame stability, especially during load transients, and avoids or reduces the risk of flameout in the region of the combustor 3.

[0049] The secondary oxidant supply line 53 may be fluidly connected to the oxidant source 15 via a second oxidant compressor 59, which may be driven by a drive device 61, such as an electric motor. In other embodiments, not shown, the second oxidant compressor 59 may be driven by the turbine 5.

[0050] In other embodiments, as shown by the dotted line, the secondary oxidant supply line 53 can branch off from the main oxidant supply line 13 downstream of the delivery side of the first oxidant compressor 17. If a higher oxidant pressure is required in the secondary oxidant supply line 53 than in the main oxidant supply line 13, the second oxidant compressor 59 can be omitted or can be placed in series with the first oxidant compressor 17.

[0051] 1, secondary oxidant supply line 53 is fluidly connected to oxidant source 15, although in other embodiments a separate oxidant source, not shown, may be provided. In some embodiments, pure oxygen may be supplied through secondary oxidant supply line 53. In other embodiments, a blend of oxygen and carbon dioxide may be supplied through secondary oxidant supply line 53.

[0052] During transient conditions caused by changes in load on the turbine, the total oxidizer flow can be adjusted by opening or closing the secondary oxidizer control valve 57 to rapidly increase or decrease the oxidizer flow that is added directly to the fuel stream upstream of the fuel nozzles 38 in the combustor 3. Because the volume between the secondary oxidizer control valve 57 and the fuel supply line 39 is small, adjustments to the oxidizer flow can be made just as quickly as adjustments to the fuel flow, so that the amount of power produced by the turbine 5 can be matched to the turbine load while maintaining the stoichiometric ratio in the combustor 3.

[0053] 2 shows the main oxidizer flow rate (OX-main) through the main oxidizer supply line 13 and the secondary oxidizer flow rate (OX-secondary) through the secondary oxidizer supply line 53 over time during a load transient. The load applied to the turbine is represented diagrammatically by the curve L, which also corresponds to the fuel flow rate F. Time t is plotted on the horizontal axis. At time t1, the load applied to the turbine shaft 7 increases, e.g. an additional electrical load is connected to the power grid 11 or an already connected load requires more power from the power grid 11. The fuel flow rate increases accordingly.

[0054] The fuel flow is controlled by the control unit 51 based on a signal detecting the deceleration of the turbine rotation speed. A rapid increase in fuel flow by opening the fuel control valve 41 maintains the required rotation speed of the turbine 5 and generator 9. To match the oxidizer flow to the changed fuel flow, the control unit 51 increases the secondary oxidizer flow by controlled opening of the secondary oxidizer control valve 57 so that the necessary additional heat output is generated as required to maintain the desired rotation speed of the turbine 5 and generator 9.

[0055] At time t2, the load increase ends and the oxidizer and fuel flow increases also stop.

[0056] In the time interval t3-t4, an opposite transient occurs, the load L is reduced and balanced by a reduction in the fuel flow rate controlled by the control unit 51. To follow the rapid reduction in fuel flow rate and to avoid non-stoichiometric combustion in the combustor 3, the secondary oxidizer control valve 57 is closed to rapidly reduce the oxidizer flow rate through the secondary oxidizer supply line 53 and thus adapt the total oxidizer flow rate to the reduced fuel flow rate.

[0057] Slow load changes that do not require rapid adjustments in fuel and oxidizer flow rates can be accommodated by operating the main oxidizer control valve 19 rather than the secondary oxidizer control valve 57 .

[0058] In some embodiments, it may be beneficial to keep the secondary oxidizer control valve 57 in an intermediate operating state corresponding to a secondary oxidizer flow rate intermediate between zero and a maximum secondary oxidizer flow rate (secondary oxidizer control valve 57 fully open). This ensures that the secondary oxidizer control valve 57 can always be operated to accommodate a rapid response to fast load changes (either an increase or decrease in load).

[0059] Therefore, when the secondary oxidizer control valve 57 operates to balance a sudden increase or decrease in fuel turbine load, it may be beneficial to slowly return the secondary oxidizer control valve 57 to an intermediate operating condition, either by slowly decreasing the secondary oxidizer flow rate while slowly increasing the main oxidizer flow rate through line 19, or by slowly increasing the secondary oxidizer flow rate while slowly decreasing the main oxidizer flow rate. These two transient conditions are illustrated in the diagrams of Figures 3 and 4.

[0060] A flow chart summarizing the method of operating the disclosed oxyfuel turbine system is shown in Figure 5. The method includes steps 101 of supplying a fuel stream to a combustor (3) through a fuel supply line (39) and 102 of supplying a primary oxidant stream to the combustor (3) through a heat exchanger (21) and a secondary oxidant stream to the fuel supply line (39) through a secondary oxidant supply line (53). The method further includes combusting (step 103) the fuel and oxidant from the fuel supply line (39) and the oxidant from the heat exchanger (21) in the combustor (3) and generating a stream of pressurized hot combustion gas. In step 104, the high pressure and hot combustion gas from the combustor 3 is expanded in a turbine (5) to generate mechanical power. In step 105, the combustion gas is exhausted from the turbine (5). In a next step 106, the exhausted combustion gas flows through a heat exchanger (21) to transfer heat from the exhausted combustion gas to the primary oxidant stream. In step 107, a load transient on the turbine (5) is detected (step 108) and the oxidizer flow rate and fuel flow rate through the secondary oxidizer supply line (53) are adjusted in response to the load transient. Once the load transient ends and the new load stabilizes, the secondary oxidizer flow rate can be gradually returned to its initial value while the main oxidizer flow rate through the heat exchanger (21) can be varied to maintain the stoichiometric ratio between the fuel and oxidizer (step 109).

[0061] Exemplary embodiments are disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various modifications, omissions, and additions may be made to what is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

Claims

1. A gas turbine system (1), comprising: a combustor (3) adapted to combust a fuel and an oxidant to generate pressurized hot combustion gases; a turbine (5) fluidly connected to the combustor (3) and rotated by expansion of the compressed hot combustion gases from the combustor (3); a heat exchanger (21) fluidly connected to the turbine (5) and adapted to cool the expanded combustion gases discharged from the turbine (5); a main oxidizer supply line (13) adapted to supply a main oxidizer flow to the combustor (3) through the heat exchanger (21), wherein in use, the main oxidizer flow flowing through the heat exchanger (21) exchanges heat with combustion gases discharged from the turbine (5), such that heat is transferred from the combustion gases to the main oxidizer flow and the combustion gases are cooled; a recirculation line (29) adapted to recirculate the first cooled combustion gas stream through said heat exchanger (21) to said combustor (3) as a working fluid; a combustion gas removal line (31) adapted to discharge the second cooled combustion gas stream; a fuel supply line (39) adapted to supply fuel to said combustor (3); a fuel control valve (41) along the fuel supply line (39) adapted to regulate the fuel flow delivered to the combustor (3); a secondary oxidant supply line (53) connected to the fuel supply line (39) at a mixing point (54) upstream of the fuel control valve (41), the secondary oxidant supply line (53) adapted to supply a secondary oxidant flow into the fuel supply line (39).

2. The turbine system (1) of claim 1, wherein the secondary oxidant supply line (53) is arranged to bypass the heat exchanger (21).

3. The turbine system of claim 1 or 2, wherein the combustor comprises a plurality of fuel nozzles (38) downstream of the fuel control valve (41).

4. 4. The turbine system (1) of claim 1, 2 or 3, further comprising a secondary oxidizer control valve (57) in the secondary oxidizer supply line (53) adapted to regulate secondary oxidizer flow through the secondary oxidizer supply line (53) during transient conditions of the turbine (5) in response to turbine load changes.

5. 5. The turbine system (1) of claim 4, further comprising a main oxidizer control valve (19) in the main oxidizer supply line (13) and a control unit (51), the control unit adapted to selectively open and close the secondary oxidizer control valve (57) and the main oxidizer control valve (19) in response to load variations of the turbine (5).

6. The control unit (51) increasing or decreasing secondary oxidant flow through the secondary oxidant control valve (57) in response to an increase or decrease in load on the turbine (3); and thereafter decreasing or increasing the secondary oxidizer flow rate through the secondary oxidizer control valve (57) while simultaneously increasing or decreasing the main oxidizer flow rate through the main oxidizer control valve (19) to maintain an approximately constant oxidizer flow rate to the combustor (3).

7. a main oxidant flow meter (20) adapted to detect the oxidant flow rate in the main oxidant supply line (13); a secondary oxidant flow meter (55) adapted to detect the oxidant flow rate in the secondary oxidant supply line (53); a secondary oxidant control valve (57) adapted to adjust the flow rate of the oxidant in the secondary oxidant supply line (53); a control unit (51) adapted to act on the secondary oxidant control valve (57) based on detection signals from the first oxidant flow meter (20) and the secondary oxidant flow meter (53); The turbine system (1) according to any one of the preceding claims, further comprising:

8. 8. The turbine system of claim 7, further comprising a fuel flow meter adapted to detect the fuel flow rate through the fuel supply line, and wherein the control unit is adapted to act on the secondary oxidizer control valve based on a detection signal from the fuel flow meter in combination with the detection signals from the first oxidizer flow meter and the secondary oxidizer flow meter.

9. 1. A method for operating a turbine system, the method comprising: Supplying a fuel flow to a combustor (3) through a fuel supply line (39); supplying a main oxidant stream to said combustor (3) through a heat exchanger (21); combusting fuel and oxidant from the fuel supply line (39) in the combustor (3) to generate a flow of pressurized hot combustion gases; expanding the combustion gases in a turbine (5) thereby generating mechanical power; Discharging exhaust combustion gas from the turbine (5); passing the exhaust combustion gas through the heat exchanger (21) to exchange heat with the main oxidant stream, thereby cooling the exhaust combustion gas and heating the main oxidant stream; recirculating a first cooled combustion gas stream through said heat exchanger (21) to said combustor (3) as a working fluid and discharging a second cooled combustion gas stream; supplying a secondary oxidizer stream to the fuel stream and blending the secondary oxidizer stream with the fuel stream upstream of a fuel control valve (41) disposed in the fuel supply line (39); supplying said blend of oxidant and fuel to said combustor (3).

10. The method of claim 9, wherein the blend of oxidizer and fuel is delivered to a plurality of fuel nozzles (38) of the combustor (3).

11. 11. The method of claim 9 or 10, further comprising adjusting the flow rate of the secondary oxidant stream in response to load variations on the turbine (5).

12. increasing or decreasing a secondary oxidant flow rate through a secondary oxidant control valve (57) in response to an increase or decrease in load on the turbine (3); thereafter, decreasing or increasing the secondary oxidizer flow rate through the secondary oxidizer control valve (57) while simultaneously increasing or decreasing the main oxidizer flow rate through the main oxidizer control valve (19) to maintain a substantially constant oxidizer flow rate to the combustor (3); 12. The method of claim 9, 10 or 11, further comprising:

13. A method according to any one of claims 9 to 12, further comprising capturing carbon dioxide from the second cooled combustion gas stream.

14. A method according to any one of claims 9 to 13, wherein the secondary oxidant stream is supplied through a secondary oxidant supply line (53) which bypasses the heat exchanger (21).

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