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] 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 burning fossil fuels relates to 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 power 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 flue gases emitted from gas turbines and remove carbon dioxide from the flue gases before they are discharged to the environment. The cost of carbon dioxide capture facilities is high, both in terms of CAPEX and the energy required to operate the facilities, reducing the overall thermodynamic efficiency of the system. The percentage of carbon dioxide in flue gases 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 an oxidant stream consisting primarily of oxygen (O2) or a mixture of oxygen and carbon dioxide (CO2) instead of air. The oxygen is obtained by separation from ambient air. A portion of the flue gas from the gas turbine is recirculated within the gas turbine combustor so that the working fluid supplied to the combustor consists primarily of oxygen and carbon dioxide and is nitrogen-free. The resulting flue gas consists primarily of water and carbon dioxide. Water is removed from the flue gas by condensation, and the water-free portion of the flue gas that is not recirculated to the combustor can be efficiently processed 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 track variations in the mechanical load applied to the turbine shaft. For example, when a turbine drives a generator connected to an electrical grid, the load applied to the turbine may vary depending on the amount of electrical power absorbed by the electrical load connected to and powered by the grid. The rotational speed of the turbine and generator remains constant. Therefore, variations in load are balanced by adjusting the fuel supplied to the turbine combustor. Similar adjustment requirements may arise when a 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 stoichiometry, and the 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. There remains a need for improved oxyturbines in this regard.

[0009] To alleviate the shortcomings of current technology gas turbines, an oxyfuel gas turbine system is disclosed herein that includes a combustor adapted to combust a fuel and an oxidizer 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 to the combustor through the heat exchanger. 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 arranged to supply fuel to the combustor, and a secondary oxidant supply line is arranged to supply a secondary oxidant flow into the fuel supply line upstream of a fuel control valve arranged along the fuel supply line relative to the direction of fuel flow.

[0013] Thus, when required by system operating conditions, additional amounts of oxidizer can be supplied to the combustor through the fuel supply line. Improved mixing of the oxidizer and fuel is achieved 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 flow rate and oxidizer flow rate of the secondary oxidizer stream increase to produce more power and maintain the required rotational speed of the turbine.

[0016] Because the varying oxidant 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 a rapid response of the turbine system to such transients.

[0017] In embodiments disclosed herein, a secondary oxidizer control valve is provided in the secondary oxidizer supply line and is 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. Because 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, for example, to minimize the oxidizer volume between the secondary oxidizer control valve and the combustor. This is beneficial in terms of system responsiveness 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 load changes on the turbine. The secondary oxidizer control valve may be actuated first, and then actuated in a second step of the oxidizer flow rate adjustment sequence to obtain a more rapid adaptation of the oxidizer flow rate to the load changes.

[0019] According to a further aspect, there is disclosed herein 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 stream to a combustor through a heat exchanger; combusting fuel and oxidant from a fuel supply line in a combustor to produce a stream of hot, pressurized combustion gases; expanding the combustion gases in a turbine, thereby producing mechanical power; Discharging combustion gases from the turbine; passing the exhaust combustion gases through a heat exchanger to exchange heat with the main oxidant stream, thereby cooling the exhaust combustion gases and heating the main 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; and supplying a blend of oxidizer and fuel to the combustor.

[0020] In embodiments disclosed herein, the method further includes adjusting the 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 oxidant flow rate through a secondary oxidant control valve in response to an increase or decrease in load on the turbine; Thereafter, decreasing or increasing the secondary oxidant flow rate through the secondary oxidant control valve while simultaneously increasing or decreasing the main oxidant flow rate through the main oxidant control valve to maintain a substantially constant oxidant 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 explanation 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. [Figure 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. [Figure 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. [Figure 5] FIG. 5 is a flow chart summarizing the steps of a method according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0024] To achieve faster response to turbine load changes in an oxyfuel turbine system and improved flame stability during oxyfuel 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 supplied 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 in the combustor.

[0025] The secondary oxidant supply line may be arranged 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 that is 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] Referring now to the drawings, Figure 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] Turbine 5 generates mechanical power from the expansion of combustion gases and is drivingly connected to load 9 via shaft 7. Load 9 is rotationally driven by the mechanical power produced by turbine 5. In the exemplary embodiment of FIG. 1 , 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 can be supplied by any oxidant source. In some embodiments, carbon dioxide can 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, which 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 stream 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 driven directly by the turbine 5. In such cases, the first oxidant compressor 17 may be part of a load drivingly coupled 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 through the main oxidizer supply line 13 toward the combustor 3.

[0032] The main oxidant stream supplied through main oxidant supply line 13 flows through the cold side 211 of heat exchanger 21 in heat exchange relationship with the combustion gases (flue gases) discharged from turbine 5 to recover heat therefrom, as described in more detail below. The combustion gases discharged from turbine 5 flow through the hot side 212 of 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 the combustion gas exhaust line 23. The expanding combustion gases discharged from the turbine 5 exchange heat with the main oxidant stream flowing through the main oxidant supply line 13, such that low temperature heat contained in the combustion gases discharged from the turbine 5 is at least partially recovered and used to preheat the main oxidant stream before it enters 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 so 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] The oxidant supplied to the combustor consists primarily 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, so that the resulting flue gases exiting the turbine 5 consist primarily of carbon dioxide and water, the latter being removed by the water removal arrangement 25. The cooled flue gases exiting the water / gas separator 252 therefore consist primarily of carbon dioxide.

[0036] The cooled combustion gases from the water / gas separator 252 are 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 from the electrical grid 11, for example.

[0037] The first cooled combustion gas stream provided by the gas / water separator is recirculated via recirculation line 29 towards heat exchanger 21 and combustor 3. Recirculation line 29 branches off from combustion gas removal line 31. In the embodiment of Figure 1, combustion gas removal line 31 is fluidly connected to carbon dioxide capture unit 33, which processes the second cooled combustion gas stream delivered from water / gas separator 252 to, for example, suitably 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 heat exchanger 21 in heat exchange relationship with hot side 212 and receives heat from the hot combustion gases discharged from turbine 5.

[0039] The flow rate of the combustion gas recirculated 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 combustion gas recirculated toward the combustor 3.

[0040] A side stream of carbon dioxide can 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 can be reduced to about 20% by volume for ease of handling.

[0041] In the combustor 3, the main oxidizer flow supplied through the main oxidizer supply line 13 and the recirculated combustion gas supplied through the recirculation line 29 are mixed with fuel, e.g., gaseous fuel, supplied through a fuel supply line 39 to one or preferably multiple 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 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 fuel supplied 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 electrical grid 11. Since the rotational speed of the turbine 5 and generator 9 remains constant, load variations must be balanced by adjusting the fuel flow rate accordingly to prevent angular acceleration or deceleration of the turbine 5 and generator 9. The fuel flow rate must be adjusted as quickly and finely as possible by the fuel control valve 41 under the control of a control unit 51. The control unit 51 is operatively coupled to the flow meters 20, 37, 43 and the control valves 19, 35, 41. The functional coupling is represented diagrammatically by the circled letters (A)-(H).

[0043] As mentioned above, the oxidizer to fuel molar ratio in combustor 3 is controlled to maintain stoichiometric combustion conditions and avoid residual fuel or oxidizer in the combustion gases exiting turbine 5. To maintain the stoichiometric ratio in combustor 3, changes in fuel flow rate should be accompanied by rapid oxidizer 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, causing the oxidizer flow regulation to lag compared to the fuel flow regulation. Thus, power rate fluctuations can cause transient conditions in which the combustor 3 is operated with either too much oxidizer or too much fuel.

[0045] To avoid or reduce these drawbacks, the oxyfuel 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 connected 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, mixing point 54 is shown downstream of fuel flow meter 43. However, in other embodiments, fuel flow meter 43 may be located downstream of mixing point 54. Fuel flow meter 43 detects the flow rate of fuel when secondary oxidizer control valve 57 is closed, and detects the flow rate of the secondary oxidizer and fuel mixture when fuel flow meter 43 is located downstream of mixing point 54.

[0047] The flow rate of the secondary oxidizer stream flowing through the secondary oxidizer supply line 53 may be substantially less than the flow rate of the main oxidizer stream flowing through the main oxidizer supply line 13. Because sufficient heat recovery is ensured by the heat exchanger 21, there is no need to heat the secondary oxidizer stream. 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 so that the volume between the secondary oxidizer control valve 57 and the fuel supply line 39 is minimized. Operation of the secondary oxidizer control valve 57 results in a fast system response 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 load changes 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 flow 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, thereby matching the amount of power produced by the turbine 5 to the turbine load while maintaining stoichiometry 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 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, for example, an additional electrical load is connected to the electrical grid 11 or an already connected load requires more power from the electrical grid 11. The fuel flow rate increases accordingly.

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

[0057] Slow load changes that do not require rapid adjustment of fuel and oxidant flow rates can be accommodated by actuating the main oxidant control valve 19 rather than the secondary oxidant control valve 57 .

[0058] In some embodiments, it may be beneficial to maintain the secondary oxidizer control valve 57 in an intermediate operating state corresponding to a secondary oxidizer flow rate intermediate between zero and the 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 load increases or decreases).

[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 for the secondary oxidizer control valve 57 to be slowly returned to an intermediate operating condition by either slowly decreasing the secondary oxidizer flow rate while simultaneously slowly increasing the main oxidizer flow rate through line 19, or by slowly increasing the secondary oxidizer flow rate while simultaneously slowly decreasing the main oxidizer flow rate. These two transient conditions are illustrated in the diagrams of FIGS. 3 and 4.

[0060] A flowchart summarizing a method of operating the disclosed oxyfuel turbine system is shown in Figure 5. The method includes step 101 of supplying a fuel stream to a combustor (3) through a fuel supply line (39), and step 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 step 103 of combusting the fuel and oxidant from the fuel supply line (39) and the oxidant from the heat exchanger (21) in the combustor (3) to generate a stream of pressurized, hot combustion gases. In step 104, the high-pressure, hot combustion gases from the combustor (3) are expanded in a turbine (5) to generate mechanical power. In step 105, the combustion gases are exhausted from the turbine (5). In next step 106, the exhausted combustion gases flow through a heat exchanger (21) to transfer heat from the exhausted combustion gases 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 fuel and oxidizer (step 109).

[0061] Exemplary embodiments are disclosed above and shown in the accompanying drawings. Those skilled in the art will understand 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 claims that follow.

Claims

1. A gas turbine system (1), comprising: a combustor (3) adapted to combust a fuel and an oxidant to produce pressurized hot combustion gases; 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) fluidly connected to the turbine (5) and adapted to cool expanded combustion gases discharged from the turbine (5); a main oxidizer supply line (13) adapted to supply a main oxidizer stream to the combustor (3) through the heat exchanger (21), wherein in use the main oxidizer stream flowing through the heat exchanger (21) exchanges heat with combustion gases discharged from the turbine (5), resulting in heat being transferred from the combustion gases to the main oxidizer stream and cooling the combustion gases; 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 rate 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); The gas turbine system (1) further comprises a secondary oxidant control valve (57) in the secondary oxidant supply line (53) adapted to adjust secondary oxidant flow through the secondary oxidant supply line (53) during transient conditions of the turbine (5) in response to turbine load changes.

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. 2. The turbine system (1) of claim 1, further comprising: a main oxidizer control valve (19) in the main oxidizer supply line (13); and a control unit (51), wherein the control unit is 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).

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

6. a main oxidant flow meter (20) adapted to detect the main oxidant flow rate in the main oxidant supply line (13); a secondary oxidant flow meter (55) adapted to detect the secondary 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 main oxidant flow meter (20) and the secondary oxidant flow meter (55); The turbine system (1) of claim 5, further comprising:

7. 7. The turbine system of claim 6, 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 main oxidizer flow meter and the secondary oxidizer flow meter.

8. 1. A method for operating a turbine system, the method comprising: supplying a fuel flow to the 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 produce a stream of pressurized hot combustion gases; expanding the pressurized hot combustion gases in a turbine (5) thereby generating mechanical power; Discharging exhaust combustion gases 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 from a secondary oxidizer supply line (53) connected to the fuel supply line (39) and blending the secondary oxidizer stream with the fuel stream upstream of a fuel control valve (41) located in the fuel supply line (39); adjusting a secondary oxidizer flow rate of the secondary oxidizer stream during transient conditions of the turbine (5) in response to turbine load changes by a secondary oxidizer control valve (57) in the secondary oxidizer supply line (53); supplying the oxidizer and fuel blend to the combustor (3); A method comprising:

9. The method of claim 8, wherein the blend of oxidizer and fuel is supplied to a plurality of fuel nozzles (38) of the combustor (3).

10. 10. The method of claim 8 or 9, further comprising adjusting the flow rate of the secondary oxidant stream in response to load variations on the turbine (5).

11. increasing a secondary oxidizer flow rate through a secondary oxidizer control valve (57) in response to an increase in load on the turbine (3) or decreasing a secondary oxidizer flow rate through the secondary oxidizer control valve (57) in response to a decrease in load on the turbine (3); thereafter, decreasing the secondary oxidizer flow rate through the secondary oxidizer control valve (57) while simultaneously increasing the main oxidizer flow rate through the main oxidizer control valve (19) or increasing the secondary oxidizer flow rate through the secondary oxidizer control valve (57) while simultaneously 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); The method of claim 8 further comprising:

12. The method of claim 8 further comprising capturing carbon dioxide from the second cooled combustion gas stream.

13. 9. The method of claim 8, wherein the secondary oxidant stream is supplied through a secondary oxidant supply line (53) that bypasses the heat exchanger (21).

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