Oxygen-fuel expander system and method including multiple heat exchangers

Abstract

The oxy-fuel expander system (1) includes a combustor (3) and an expander fluidly coupled to the combustor and rotated by the expansion of pressurized hot combustion gases from the combustor. A high-temperature heat exchanger (22) and a low-temperature heat exchanger (21) are positioned to recover heat from the combustion gases and heat an oxidant stream and a stream of combustion gases that are recirculated toward the combustor and expander (5).

Description

[Technical Field]

[0001] FIELD OF THE DISCLOSURE The present disclosure relates to gas expander or gas turbine systems for power generation. In particular, embodiments disclosed herein relate to an oxy-fuel expander system 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 combusted to produce high-pressure, high-temperature combustion gases, which are expanded in a turbine or expander. In this disclosure, the terms turbine and expander are used synonymously, i.e., the term expander also encompasses turbines. The expander converts the enthalpy of the combustion gases into usable mechanical power at the expander output shaft, which is used to drive a load, such as a compressor or compressor train, or to rotate 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-fuel expanders (also called oxy-fuel turbines or oxygen-fuel turbines) have been developed that use an oxidant stream consisting primarily of oxygen (O2) 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 expander is recirculated within the gas expander combustor so that the working fluid supplied to the combustor consists primarily of oxygen and carbon dioxide, but no nitrogen. The resulting flue gas consists primarily 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 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 expander shaft. For example, when the expander or turbine drives a generator connected to the electrical grid, the load applied to the expander may vary depending on the amount of power absorbed by the electrical load powered by the electrical grid. The rotational speed of the expander and generator remains constant. Thus, variations in the load are balanced by adjusting the fuel delivered to the combustor. Similar adjustment requirements may arise when the expander drives a compressor train or other driving machinery. Variations in fuel flow must be compensated for by adjusting the oxidizer flow.

[0007] Efforts have been made to make oxy-fuel expanders and turbines more responsive to load changes, and in this regard there remains a need for improvements in oxy-fuel or oxy-combustion expanders. Summary of the Invention

[0008] According to one aspect, a gas turbine or expander system, particularly an oxy-fuel expander or turbine system, is disclosed herein, comprising: a combustor adapted to combust a fuel and an oxidizer to generate pressurized hot combustion gases; and an expander or turbine fluidly coupled to the combustor and rotated by the expansion of the pressurized hot combustion gases from the combustor. The system further includes a high-temperature heat exchanger fluidly coupled to the expander and a low-temperature heat exchanger fluidly coupled to the high-temperature heat exchanger. The high-temperature side of the high-temperature heat exchanger and the high-temperature side of the low-temperature heat exchanger are, in turn, fluidly coupled and adapted to receive and cool the expanded combustion gases discharged from the expander. An oxidizer supply line is adapted to supply oxidizer to the combustor through the low-temperature side of the high-temperature heat exchanger. In use, the oxidizer flowing through the high-temperature heat exchanger exchanges heat with the combustion gases discharged from the expander. A fuel supply line is adapted to supply fuel to the combustor. A combustion gas compressor is adapted to compress the combustion gases flowing from the low-temperature heat exchanger. Additionally, a recirculation line is provided for recirculating the combustion gas stream from the combustion gas compressor through the low-temperature heat exchanger to the combustor, the recirculation line extending through a recirculation low-temperature side of the low-temperature heat exchanger in heat exchange with the high-temperature side of the low-temperature heat exchanger.

[0009] The first oxidant control valve is located in the oxidant supply line upstream of the high-temperature heat exchanger. As will become apparent from the description of the embodiments, by dividing the heat exchange function, efficient heat recovery from the flue gases discharged by the expander or turbine is achieved in combination with improved agility in controlling the expander, for example in the event of load changes on the expander shaft.

[0010] As noted above, the term "expander" as understood herein encompasses any turbomachine adapted to expand a process gas and convert the enthalpy contained in the process gas into useful mechanical power. Thus, the term expander also encompasses turbines.

[0011] As described in more detail below, the system may further include a condenser, where the combustion gases from the low-temperature heat exchanger are further cooled to condense steam contained therein. The condenser may be part of a water removal device fluidly coupled to the low-temperature heat exchanger and adapted to receive the cooled combustion gases therefrom and remove water from the cooled combustion gases by condensing steam contained in the combustion gases and separating water from carbon dioxide, for example, in a water / gas separator.

[0012] Downstream of the water removal device, a recirculation line branches off from the combustion gas removal line. The first combustion gas stream flows through the recirculation line to a low-temperature heat exchanger, where it is heated by heat exchange with the hot combustion gas from the expander and sent to a combustor as process gas. The second combustion gas stream is discharged through the combustion gas removal line, for example, to a carbon dioxide capture unit.

[0013] According to a further aspect, provided herein is a method of operating an inflator system, the method comprising the steps of: supplying a fuel flow to the combustor through a fuel supply line; providing an oxidant stream to a combustor through a high temperature heat exchanger; combusting fuel and oxidant from a fuel supply line in a combustor to produce a stream of pressurized hot combustion gases; expanding the combustion gases in an expander, thereby generating mechanical power; Discharging the exhaust combustion gas from the expander; flowing the exhaust combustion gas through a high temperature heat exchanger in heat exchange with an oxidant stream; flowing the exhaust combustion gas from the high temperature heat exchanger through a low temperature heat exchanger in heat exchange with a stream of recirculated combustion gas; and adjusting the oxidant flow with a first oxidant control valve (19) located in the oxidant supply line (13) upstream of the high temperature heat exchanger (22).

[0014] Further features and embodiments of the oxygen-fuel turbine or expander system and method according to the present disclosure are described below with reference to the accompanying drawings and are further set forth in the appended claims. [Brief explanation of the drawings]

[0015] Reference will now be made briefly to the accompanying drawings, in which: [Figure 1] 1 is a schematic diagram of an oxygen-fuel expander system according to one embodiment of the present disclosure; [Figure 2] FIG. 2 is a schematic diagram of an oxygen-fuel expander system according to the present disclosure in a second embodiment. [Figure 3] FIG. 10 is a schematic diagram of an oxygen-fuel expander system according to the present disclosure in a third embodiment. [Figure 4] FIG. 10 is a schematic diagram of an oxygen-fuel expander system according to the present disclosure in a fourth embodiment. [Figure 5] 1 is a flowchart summarizing a method according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0016] To improve the response time of an oxy-fuel turbine or expander when the load applied to the expander shaft varies, the volume of oxidizer between the oxidizer control valve and the expander's combustor is reduced by providing a heat exchanger system including a first high-temperature heat exchanger and a second low-temperature heat exchanger. Both heat exchangers are adapted to remove heat from the combustion gas discharged from the expander and transfer the waste heat to the oxidizer and recirculated combustion gas streams. By splitting the heat recovery between the two heat exchangers, the oxidizer control valve can be located closer to the combustor along the oxidizer supply line, minimizing the total oxidizer supply line between the oxidizer control valve and the combustor while maintaining efficient heat recovery. This allows the system to respond faster to load variations, as will become more apparent from the following detailed description of several system embodiments.

[0017] Referring now to the drawings, Figure 1 shows a schematic diagram of an oxy-fuel expander system 1 according to a first embodiment. The oxy-fuel expander system 1 includes a combustor 3 fluidly coupled to an expander or expander 5. The expander 5 generates mechanical power through the expansion of combustion gases generated in the combustor and is drivingly coupled to a load 9 via a shaft 7. The load 9 is rotationally driven by the mechanical power generated by the expander 5. In the exemplary embodiment of Figure 1, the load 9 includes a generator electrically coupled to an electrical grid 11. In other embodiments not shown, the load may include a rotary machine such as a turbomachine, e.g., a compressor or a pump, instead of or in combination with the generator.

[0018] An oxidant stream is supplied to combustor 3 through oxidant supply line 13. The oxidant stream consists primarily of oxygen or a mixture of oxygen and carbon dioxide. The oxidant can be supplied by any oxidant source. In FIG. 1 , the oxidant source includes an air separation unit 15, which separates oxygen or a mixture of oxygen and carbon dioxide from ambient air and removes nitrogen or nitrogen and carbon dioxide therefrom. The oxidant stream from oxidant source 15 can be compressed in oxidant compressor 17 to reach the pressure required to supply oxidant to combustor 3. The oxidant compressor 17 can be driven by a drive device, such as an electric motor 18. The electric motor 18 can be powered by electrical power from electrical grid 11. In other embodiments, not shown, the oxidant compressor 17 can be driven directly by the expander 5. In such cases, the oxidant compressor 17 can be part of a load drivingly coupled to the expander shaft 7.

[0019] In some embodiments, carbon dioxide can be mixed with oxygen in the oxidant supply line or oxidant source 15, so that the oxidant stream contains a reduced amount of oxygen, for example, about 20% oxygen by volume.

[0020] A first oxidant control valve 19 is disposed along the oxidant supply line 13 between the oxidant compressor 17 and the combustor 3. The first oxidant control valve 19 is adapted to control the oxidant flow rate through the oxidant supply line 13 toward the combustor 3. A first oxidant flow meter 20 may be disposed along the oxidant supply line 13 to detect the flow rate of the oxidant through the oxidant supply line 13 to the combustor 3. An orifice 16 or another flow stabilization device that provides a focused pressure drop along the oxidant supply line 13 may be disposed downstream of the first oxidant control valve 19.

[0021] The oxidant stream supplied through oxidant supply line 13 flows through the cold side 211 of a first low-temperature heat exchanger 21 in heat exchange with the combustion gases (flue gases) discharged from the expander 5. The combustion gases discharged from the expander 5 flow through the hot side 212 of the low-temperature heat exchanger 21. Thus, waste heat is recovered from the combustion gases to heat the oxidant before it is supplied to the combustor 3, as described in more detail below.

[0022] The high temperature side 212 of the low temperature heat exchanger 21 has an inlet fluidly coupled to a high temperature side 222 of a second high temperature heat exchanger 22. The high temperature side 222 of the high temperature heat exchanger 22 is fluidly coupled to the outlet of the expander 5 via a combustion gas discharge line 23. The high temperature heat exchanger 22 further includes a low temperature side 221 having an inlet fluidly coupled to the outlet of the low temperature side 211 of the low temperature heat exchanger 21 and an outlet fluidly coupled to the combustor 3. The high temperature heat exchanger 22 is configured to recover additional waste heat from the flue gases discharged by the expander or turbine 5.

[0023] The first oxidant control valve 19 is disposed between the low-temperature heat exchanger 21 and the high-temperature heat exchanger 22 .

[0024] More specifically, the oxidant supply line 13 extends sequentially through the low-temperature side 211 of the low-temperature heat exchanger 21 and the low-temperature side 221 of the high-temperature heat exchanger 22 before connecting to the combustor 3. In this manner, the oxidant from the oxidant source 15 is heated by the waste heat recovered by heat exchange in the low-temperature heat exchanger 21 from the combustion gas (flue gas) from the expander 5, and is further heated to a high temperature by the waste heat recovered from the combustion gas in the high-temperature heat exchanger 22.

[0025] The first oxidant control valve 19 is disposed between the low temperature heat exchanger 21 and the high temperature heat exchanger 22, and the oxidant flowing through the first oxidant control valve 19 is still at a relatively low temperature and amount.

[0026] Because the first oxidant control valve 19 is located downstream of the low-temperature heat exchanger 21 and upstream of the high-temperature heat exchanger 22, the total amount of oxidant contained in the oxidant supply line 13 downstream of the first oxidant control valve 19, between the first oxidant control valve 19 and the combustor 3, is less than in current technology systems where all heat is recovered in one or more waste heat recovery heat exchangers located entirely downstream of the first oxidant control valve 19. This has beneficial effects with respect to expander control during transient conditions, as will be explained in more detail below, without reducing the thermal efficiency of the system.

[0027] The outlet of the hot side 212 of the low-temperature heat exchanger 21 is fluidly coupled to a water removal device 25 adapted to remove water from the cooled combustion gases exiting the hot side 212 of the low-temperature heat exchanger 21.

[0028] 1, the water removal device 25 includes a condenser 251 and a water / gas separator 252. The combustion gas flowing through the condenser 251 is cooled, so that steam contained in the combustion gas condenses and is separated from the gas in the water / gas separator 252 and removed through the water removal line 253.

[0029] Because the oxidant supplied to combustor 3 consists primarily of oxygen or a mixture of oxygen and carbon dioxide, and the flow rates of oxidant and fuel are controlled to maintain stoichiometric combustion in combustor 3, the flue gases exiting expander 5 consist primarily of carbon dioxide and water, the latter of which is removed by water removal device 25. Thus, the cooled combustion gases exiting water / gas separator 252 consist primarily of carbon dioxide.

[0030] 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 the expander 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.

[0031] The first cooled combustion gas stream delivered by the water / gas separator 25 is recirculated to the combustor 3 through a recirculation line 29. The recirculation line 29 extends through the low temperature heat exchanger 21 and the high temperature heat exchanger 22. The recirculation line 29 branches off from a combustion gas removal line 31 that is fluidly coupled to the discharge side of the combustion gas compressor 27.

[0032] In the embodiment of FIG. 1, the combustion gas removal line 31 is fluidly coupled to a carbon dioxide capture unit 33 that processes the second cooled combustion gas stream coming from the water / gas separator 252, for example to store carbon dioxide and avoid its emission into the environment.

[0033] The first cooled combustion gas stream recirculated through recirculation line 29 flows through the recirculation cold side 213 of low-temperature heat exchanger 21 in heat exchange with the hot side 212 to remove waste heat from the hot combustion gases discharged from expander 5 and flowing through the hot side 212 of low-temperature heat exchanger 21. The partially heated recirculated combustion gas stream discharged from the recirculation cold side 213 of low-temperature heat exchanger 21 flows through the recirculation cold side 223 of high-temperature heat exchanger 22 to further remove waste heat from the combustion gases flowing through the hot side 222 of high-temperature heat exchanger 22. Finally, the heated recirculated combustion gas is delivered from the recirculation cold side 223 of high-temperature heat exchanger 22 to combustor 3 where it is mixed with the oxidant delivered through oxidant supply line 13.

[0034] The flow rate of the combustion gas recirculated through the recirculation line 29 can be adjusted by a recirculation combustion gas control valve 35 disposed in the recirculation line 29. A recirculation combustion gas flow meter 37 can be disposed in the recirculation line 29 to detect the flow rate of the combustion gas recirculated toward the combustor 3.

[0035] 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 proportion of oxygen in line 13 and / or line 53 can be reduced to about 20% by volume for ease of handling.

[0036] The oxidizer flow supplied to the combustor 3 through the oxidizer supply line 13 and the recirculated combustion gas supplied to the combustor 3 through the recirculation line 29 are mixed with fuel, e.g., gaseous fuel, supplied to one or more fuel nozzles 37 in the combustor 3 through a fuel supply line 39. A fuel control valve 41 along the fuel supply line 39 is adapted to adjust the fuel flow rate delivered to the combustor 3. A fuel flow meter 43 provided in the fuel supply line 39 can detect the flow rate of the fuel supplied to the combustor 3.

[0037] The mechanical power required to rotate the generator 9 may vary, for example, as a result of fluctuations in the power absorbed by an electrical load (not shown) connected to the electrical grid 11. Since the rotational speed of the expander 5 and the generator 9 is kept constant, load variations must be balanced by adjusting the fuel flow rate accordingly to prevent angular acceleration or deceleration of the expander 5 and the generator 9. The fuel flow rate 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 operatively coupled to the flow meters 20, 37, 43 and the control valves 19, 35, 41.

[0038] As previously mentioned, the oxidant to fluid molar ratio in combustor 3 is controlled to maintain stoichiometric combustion conditions and avoid residual fuel or oxidant in the combustion gases exiting expander 5. To maintain the stoichiometric ratio in combustor 3, changes in fuel flow rate must be accompanied by rapid adjustments in oxidant flow rate.

[0039] The volume between the first oxidant control valve 19 and the combustor 3 includes the cold side 221 of the high-temperature heat exchanger 22, but does not include the cold side 211 of the low-temperature heat exchanger 22. This volume is therefore minimized compared to a system with a single large heat exchanger, thus resulting in a faster response of the expander 5 to load transients.

[0040] To provide even faster, finer adjustments of the oxidant flow rate during load transients, the system 1 of FIG. 1 further includes at least one oxidant bypass line 52 branching off from the oxidant supply line 13 between the low-temperature heat exchanger 21 and the first oxidant control valve 19. The downstream end of the oxidant bypass line 52 is fluidly coupled to the oxidant supply line 13 between the high-temperature heat exchanger 22 and the combustor 3, or to the fuel supply line 39 (see 52A), or directly to the combustor 3 (see 52B), thus bypassing the high-temperature heat exchanger 22. A second oxidant control valve 53 and an optional second oxidant flow meter 55 are disposed in the oxidant bypass line 52 and operatively coupled to the control unit 51. When finer, faster adjustments of the oxidant flow rate are required, the control unit 51 opens or closes the second oxidant control valve 53.

[0041] The second oxidant control valve 53 is positioned close to the combustor 3 so that the volume between the second oxidant control valve 53 and the combustor 3 is minimized. Actuation of the second oxidant control valve 53 provides rapid and accurate adjustment of the auxiliary oxidant flow rate through the oxidant bypass line 52.

[0042] During expander load transients, the total oxidant flow can be adjusted by rapidly increasing or decreasing the oxidant flow rate by opening or closing the second oxidant control valve 53. Slower, coarser regulation of the oxidant flow rate can be obtained by acting on the first oxidant control valve 19.

[0043] In some embodiments, for example, a sudden change in the load applied to the expander 5 requires a rapid adjustment of the fuel flow rate through the fuel supply line 39, controlled by the fuel control valve 41. To maintain a stoichiometric fuel / oxidizer ratio in the combustor 3, the second oxidizer control valve 53 is controlled to obtain a fast modulation of the auxiliary oxidizer flow rate to the combustor 3. Once the load transient ends and the new load value stabilizes, the second oxidizer control valve 53 gradually returns to its pre-load state in synchronization with the adjustment of the first oxidizer control valve 19. For example, if the load applied to the expander 5 changes from a low state to a high state, the second oxidizer control valve 53 is rapidly opened to maintain the stoichiometric combustion ratio. A subsequent slow adjustment returns the second oxidizer control valve 53 to its original state, resulting in a decrease in the auxiliary oxidizer flow rate through the oxidizer bypass line 52, while the first oxidizer control valve 19 gradually opens to balance the decrease in flow rate through the oxidizer bypass line 52.

[0044] Continuing with reference to Figure 1, a second embodiment of the turbine or expander system 1 is shown in Figure 2. The same reference numbers indicate the same or equivalent components as already disclosed in Figure 1 and described above, and will not be described again in detail.

[0045] The primary difference between the embodiment of FIG. 1 and the embodiment of FIG. 2 is that the system 1 of FIG.

[0046] Additionally, the orifice or other flow stabilization device 16 is placed in parallel with the first oxidant flow control valve 19 rather than in series with the first oxidant flow control valve 19 .

[0047] Additionally, in the embodiment of FIG. 2, a shutoff valve 61 or a second oxidant control valve 61 may be positioned along the oxidant supply line 13 downstream of the high temperature heat exchanger 22 .

[0048] In some embodiments, when the second oxidizer control valve 61 is combined with the first oxidizer control valve 19, the first oxidizer control valve 19 can be a pressure-controlled valve that opens and closes based on a pressure signal. The second oxidizer control valve 61 can also be a flow-controlled valve. Because the heat exchange surfaces of the heat exchangers 21 and 22 are located upstream of the second oxidizer control valve 61, the flow-controlled oxidizer control valve 61 can be located adjacent to or near the combustor 3. Therefore, the oxidizer volume between the second oxidizer control valve 61 and the combustor 3 is minimized, thereby enabling faster system response to load changes. When the load changes, the flow-controlled oxidizer control valve 61 opens and closes, possibly following the opening and closing of the fuel control valve 41, to maintain stoichiometric combustion in the combustor 3. The first pressure-controlled oxidizer control valve 19 then provides slower adaptation to new load and oxidizer flow conditions.

[0049] With continued reference to Figures 1 and 2, a further embodiment of the oxy-fuel expander system 1 is shown in Figure 3. The primary difference between the embodiment of Figure 3 and the embodiments of Figures 1 and 2 is that the oxidant stream flows only through the high-temperature heat exchanger, while the recirculated combustion gas flows only through the low-temperature heat exchanger. The combustion gas exiting the expander flows through the high-temperature side of both the high-temperature and low-temperature heat exchangers.

[0050] More specifically, in Figure 3, expander or turbine system 1 includes combustor 3 fluidly coupled to expander or turbine 5. Expander 5 generates mechanical power and is drivingly coupled to load 9 via shaft 7. In the exemplary embodiment of Figure 3, load 9 includes a generator electrically coupled to electrical grid 11. In other embodiments, load 9 can include turbomachinery, such as a compressor or compressor train. In yet other embodiments, the load can include more rotating machinery, for example, a combination of a generator and a turbomachinery, such as a compressor or pump.

[0051] An oxidant stream is supplied to combustor 3 through oxidant supply line 13. The oxidant may be supplied by any oxidant source 15, such as an air separator that separates oxygen or a mixture of oxygen and carbon dioxide from ambient air and removes nitrogen or nitrogen and carbon dioxide therefrom. The oxidant stream from oxidant source 15 may be compressed in oxidant compressor 17 to achieve the pressure required to supply the oxidant to combustor 3. Oxidant compressor 17 may be driven by a drive device, such as electric motor 18. Electric motor 18 may be powered by electricity from electrical grid 11. In other embodiments, not shown, oxidant compressor 17 may be driven directly by expander 5.

[0052] An oxidant control valve 19 is disposed along the oxidant supply line 13 between the oxidant compressor 17 and the combustor 3 and is adapted to control the flow rate of the oxidant through the oxidant supply line 13 toward the combustor 3. An oxidant flow meter 20 may be disposed along the oxidant supply line 13 to detect the flow rate of the oxidant through the oxidant supply line 13 to the combustor 3.

[0053] The oxidant stream supplied through the oxidant supply line 13 passes through the cold side 221 of the high-temperature heat exchanger 221 and exchanges heat with the combustion gas (flue gas) discharged from the expander 5 to recover waste heat. The combustion gas discharged from the expander 5 flows through the hot side 222 of the high-temperature heat exchanger 221.

[0054] The outlet of the high temperature side 222 of the high temperature heat exchanger 22 is fluidly coupled to the high temperature side 212 of the low temperature heat exchanger 212, which is adapted to further recover waste heat from the discharged combustion gases and increase the temperature of the recirculation flow of combustion gases, as described in more detail below.

[0055] An outlet of the hot side 212 of the low-temperature heat exchanger 21 is fluidly coupled to a moisture removal device 25 adapted to remove moisture from the cooled combustion gases exiting the hot side 212 of the low-temperature heat exchanger 21. In the schematic diagram of Figure 3, the water removal device 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. The cooled combustion gases exiting the water / gas separator 252 consist primarily of carbon dioxide.

[0056] 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 the expander 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.

[0057] The first cooled combustion gas stream routed by the water / gas separator 252 is recirculated through the low-temperature heat exchanger 21 and recirculated towards the combustor 3 via a recirculation line 29. The recirculation line 29 branches off from a combustion gas removal line 31. In the embodiment of Figure 3, the combustion gas removal line 31 is fluidly coupled to a carbon dioxide capture unit 33 that processes the second cooled combustion gas stream routed from the water / gas separator 252 to, for example, store carbon dioxide and avoid its emission into the environment.

[0058] The first cooled combustion gas stream recirculated through recirculation line 29 flows through the recirculation cold side 213 of the cold heat exchanger 21 in heat exchange relationship with the hot side 212 to remove waste heat from the hot combustion gas discharged from the expander 5 and is sent to the combustor 3 where it is mixed with the oxidant sent through the oxidant supply line 13.

[0059] 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.

[0060] The oxidizer flow supplied to the combustor 3 through the oxidizer supply line 13 and the recirculated combustion gas supplied to the combustor 3 through the recirculation line 29 are mixed with fuel, e.g., gaseous fuel, supplied to one or more fuel nozzles 37 in the combustor 3 through a fuel supply line 39. 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 along the fuel supply line 39 can detect the flow rate of the fuel supplied to the combustor 3.

[0061] Adjustments to the oxidant flow to balance fluctuations in fuel flow during load transients are made by acting on the oxidant control valve 19. As with the previous embodiment, the volume between the oxidant control valve 19 and the combustor 3 is minimized because the heat recovery function is performed by two separate heat exchangers 21 and 22. The reduced volume between the oxidant control valve 19 and the combustor 3 improves expander control and operability, similar to the previously described embodiment shown in Figures 1 and 2.

[0062] 2, the option of providing two oxidant flow control valves, one upstream and one downstream of the high-temperature heat exchanger 22, is not excluded. In this case, an additional second oxidant control valve can be arranged between the low-temperature side 221 of the high-temperature heat exchanger 22 and the combustor 3. The second oxidant control valve may be a flow control valve, and the oxidant control valve 19 may be a pressure control valve.

[0063] With continued reference to Figures 1, 2, and 3, a further embodiment of an expander system is shown in Figure 4. Like reference numbers refer to similar or equivalent components to those shown in Figures 1, 2, and 3. The system of Figure 4 includes an additional third medium temperature heat exchanger.

[0064] More specifically, the expander system 1 of Figure 4 includes a combustor 3 fluidly coupled to an expander or expander 5. The expander 5 generates mechanical power through the expansion of combustion gases and is drivingly coupled to an electric generator 9 via a shaft 7, which is electrically coupled to an electrical grid 11. As already mentioned above, other loads can be drivingly coupled to the expander 5 in addition to or instead of the generator 9.

[0065] An oxidant stream is supplied to combustor 3 through oxidant supply line 13. The oxidant stream, which may consist primarily of oxygen or a mixture of oxygen and carbon dioxide, may be provided by any oxidant source, such as air separation unit 15.

[0066] The oxidant flow from oxidant source 15 may be compressed in oxidant compressor 17 to achieve the pressure required to supply oxidant to combustor 3. Oxidant compressor 17 may be driven by a drive device, such as electric motor 18. Electric motor 18 may be powered by electrical power from electrical grid 11. In other embodiments, not shown, oxidant compressor 17 may be driven directly by expander 5. In such cases, oxidant compressor 17 may be part of the load drivingly coupled to expander shaft 7.

[0067] An oxidant control valve 19 is disposed along the oxidant supply line 13 between the oxidant compressor 17 and the combustor 3 and is adapted to control the flow rate of the oxidant through the oxidant supply line 13 toward the combustor 3. An oxidant flow meter 20 may be disposed along the oxidant supply line 13 to detect the flow rate of the oxidant through the oxidant supply line 13 to the combustor 3.

[0068] The oxidant stream supplied through the oxidant supply line 13 passes through the cold side 211 of the first low-temperature heat exchanger 21 to exchange heat with the combustion gases (flue gases) discharged from the expander 5 to recover waste heat therefrom. The combustion gases discharged from the expander 5 flow through the hot side 212 of the low-temperature heat exchanger 21.

[0069] The high temperature side 212 of the low temperature heat exchanger 21 is fluidly coupled to the high temperature side 222 of a second high temperature heat exchanger 22, which is disposed upstream of the low temperature heat exchanger 21 relative to the flow direction of the combustion gases. However, unlike the previous embodiment, a third intermediate temperature heat exchanger 24 is provided between the low temperature heat exchanger 21 and the high temperature heat exchanger 22. As will be explained in more detail below, the combustion gas flow sequentially passes through the high temperature side 222 of the high temperature heat exchanger 22, the high temperature side 242 of the intermediate temperature heat exchanger 24, and finally the high temperature side 212 of the low temperature heat exchanger 21.

[0070] The high temperature side 222 of the high temperature heat exchanger 22 is fluidly coupled to the outlet of the expander 5 via the combustion gas discharge line 23. The high temperature heat exchanger 22 further includes a low temperature side 221 having an inlet fluidly coupled to the outlet of the low temperature side 211 of the low temperature heat exchanger 21 and an outlet fluidly coupled to the combustor 3.

[0071] The expanded combustion gases discharged from the expander 5 exchange heat with the oxidant stream flowing through the oxidant supply line 13, and at least a portion of the waste heat contained in the combustion gases discharged from the expander 5 is recovered and used to heat the oxidant stream before it enters the combustor 3.

[0072] Specifically, the oxidant supply line 13 extends sequentially through the low-temperature side 211 of the low-temperature heat exchanger 21 and the low-temperature side 221 of the high-temperature heat exchanger 22 before connecting to the combustor 3. In this manner, the oxidant from the oxidant source 15 is partially heated by the exhaust heat recovered from the combustion gas (flue gas) from the expander 5 through heat exchange in the low-temperature heat exchanger 21, and is further heated to a high temperature by the exhaust heat recovered from the combustion gas in the high-temperature heat exchanger 22.

[0073] The oxidant control valve 19 is positioned between the low temperature heat exchanger 21 and the high temperature heat exchanger 22 so that the oxidant flowing through the oxidant control valve 19 remains relatively cool.

[0074] Furthermore, because the oxidant control valve 19 is located downstream of the low temperature heat exchanger 21 and upstream of the high temperature heat exchanger 22, the total amount of oxidant contained in the oxidant supply line 13 between the oxidant control valve 19 and the combustor 3 downstream of the oxidant control valve 19 is less than if all the heat were recovered in one or more waste heat recovery heat exchangers located completely downstream of the oxidant control valve 19. This has beneficial effects in terms of expander control and operability during transient conditions, especially in terms of response time to load changes.

[0075] An outlet of the hot side 212 of the low-temperature heat exchanger 21 is fluidly coupled to a water removal device 25 adapted to remove water from the cooled combustion gases exiting the hot side 212 of the low-temperature heat exchanger 21. In the schematic diagram of Figure 4, the water removal device 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.

[0076] 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 the expander 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.

[0077] The first cooled combustion gas stream delivered by the water / gas separator 25 is recirculated through a recirculation line 29 towards the low temperature heat exchanger 21, the additional medium temperature heat exchanger 24, and the combustor 3. The recirculation line 29 branches off from a combustion gas removal line 31. In the embodiment of Figure 4, the combustion gas removal line 31 is fluidly coupled to a carbon dioxide capture unit 33, which processes the second cooled combustion gas stream delivered from the water / gas separator 252 to, for example, store carbon dioxide and avoid its emission into the environment.

[0078] The first cooled combustion gas stream recirculated through recirculation line 29 exchanges heat with the high temperature side 212 of low temperature heat exchanger 21 and flows through the recirculation low temperature side 213 of low temperature heat exchanger 21 to remove waste heat from the hot combustion gases discharged from expander 5 and flowing through the high temperature side 212 of low temperature heat exchanger 21. The partially heated recirculated combustion gas stream discharged from the recirculation low temperature side 213 of low temperature heat exchanger 21 flows through the recirculation low temperature side 241 of intermediate temperature heat exchanger 24 to remove further waste heat from the combustion gases flowing through the high temperature side 242 of intermediate temperature heat exchanger 24. Finally, the heated recirculated combustion gas is passed from the recirculation low temperature side 241 of intermediate temperature heat exchanger 24 to combustor 3 where it is mixed with oxidant passed through oxidant supply line 13.

[0079] 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. In Fig. 4, the combustion gas flow meter 37 and the combustion gas control valve 35 are disposed between the low-temperature heat exchanger 21 and the medium-temperature heat exchanger 24. Alternatively, the combustion gas flow meter and the combustion gas control valve can be disposed upstream of the low-temperature heat exchanger 21, as indicated by 37X and 35X.

[0080] The oxidizer flow supplied to the combustor 3 through the oxidizer supply line 13 and the recirculated combustion gas supplied to the combustor 3 through the recirculation line 29 are mixed with fuel, e.g., gaseous fuel, supplied to one or more fuel nozzles 37 in the combustor 3 through a fuel supply line 39. 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 along the fuel supply line 39 can detect the flow rate of the fuel supplied to the combustor 3.

[0081] The combustion gas discharged from the expander 5 has its waste heat efficiently recovered by three heat exchangers 21, 22, and 24. By performing oxidant heating in two consecutively arranged low-temperature heat exchangers 21 and high-temperature heat exchangers 22, the volume between the oxidant control valve 19 and the combustor 3 is minimized, thereby improving the controllability and operability of the expander 5.

[0082] FIG. 5 shows a flowchart summarizing the major steps of a method performed by the above-described embodiment of an oxy-fuel turbine or expander system. Specifically, the method includes the following steps: supplying a fuel stream to combustor 3 through fuel supply line 39 (see step 101); and supplying an oxidant stream to combustor 3 through high-temperature heat exchanger 22 (see step 102). Further steps include combusting the fuel from fuel supply line 39 with the oxidant in combustor 3 to generate a pressurized, hot combustion gas stream (see step 103). In step 104, the combustion gas is expanded in expander or turbine 5, thereby generating mechanical power. Step 105 is configured to discharge exhaust combustion gas from expander 5. The exhaust combustion gas then flows through high-temperature heat exchanger 22 in heat exchange with the oxidant stream (see step 106). Finally, step 107 is configured to flow the exhaust combustion gas from high-temperature heat exchanger 22 through low-temperature heat exchanger 21 in heat exchange with the recirculated combustion gas stream.

[0083] 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. An oxy-fuel expander system (1), comprising: a combustor (3) adapted to combust a fuel and an oxidant to produce pressurized hot combustion gases; an expander (5) fluidly coupled to the combustor (3) and rotated by expansion of the pressurized hot combustion gases from the combustor (3); a high temperature heat exchanger (22) fluidly coupled to the expander (5); a low-temperature heat exchanger (21) fluidly coupled to the high-temperature heat exchanger (22), the high-temperature side (222) of the high-temperature heat exchanger (22) and the high-temperature side (212) of the low-temperature heat exchanger (21) being fluidly coupled in turn and adapted to receive and cool the expanded combustion gases discharged from the expander (5); an oxidant supply line (13) adapted to supply oxidant to the combustor (3) through the cold side (221) of the high-temperature heat exchanger (22), wherein, in use, the oxidant flowing through the high-temperature heat exchanger (22) exchanges heat with combustion gases discharged from the expander (5); a fuel supply line (39) adapted to supply fuel to said combustor (3); a combustion gas compressor (27) adapted to compress the combustion gas flowing from the low-temperature heat exchanger (21); a recirculation line (29) for recirculating a combustion gas stream from the combustion gas compressor (27) through the low-temperature heat exchanger (21) to the combustor (3), the recirculation line (29) extending through a recirculation low-temperature side (213) of the low-temperature heat exchanger (21) in heat exchange with the high-temperature side (212) of the low-temperature heat exchanger (21); a first oxidant control valve (19) in the oxidant supply line (13) upstream of the high temperature heat exchanger (22).

2. The system (1) of claim 1, wherein the oxidant supply line (13) and the recirculation line (29) are isolated from each other and fluidly coupled to the combustor (3).

3. 3. The system (1) of claim 1 or 2, wherein the recirculation line (29) further extends through a recirculation cold side (223) of the high-temperature heat exchanger (22) in heat exchange with the high-temperature side (222) of the high-temperature heat exchanger (22).

4. The system (1) according to one or more of claims 1 to 3, wherein the first oxidant control valve (19) is arranged between the low-temperature heat exchanger (21) and the high-temperature heat exchanger (22), and the oxidant supply line (13) extends sequentially through the low-temperature heat exchanger (21) and the high-temperature heat exchanger (22).

5. 5. The system (1) of claim 4, further comprising an oxidant bypass supply line (52) branching from the oxidant supply line (13) between the low-temperature heat exchanger (21) and the first oxidant control valve (19), bypassing the high-temperature heat exchanger (22), and adapted to supply oxidant to the combustor (3), wherein a second oxidant control valve (53) is disposed in the oxidant bypass line (52).

6. 6. The system (1) of claim 5, wherein the oxidant bypass supply line (52) is further fluidly coupled to one of the oxidant supply line (13) between the high-temperature heat exchanger (22) and the combustor (3), the fuel supply line (39), and the combustor (3).

7. The first oxidant control valve (19) is disposed between the low-temperature heat exchanger (21) and the high-temperature heat exchanger (22), the oxidant supply line (13) extends sequentially through the low-temperature heat exchanger (21) and the high-temperature heat exchanger (22), and the system includes an intermediate-temperature heat exchanger (24) having a high-temperature side (242) disposed sequentially between the high-temperature side (222) of the high-temperature heat exchanger (22) and the high-temperature side (212) of the low-temperature heat exchanger (21), and an intermediate-temperature heat exchanger (24) having a high-temperature side (242) disposed sequentially between the high-temperature side (222) of the high-temperature heat exchanger (22) and the high-temperature side (212) of the low-temperature heat exchanger (21). and a combustion gas discharge line (23) extending sequentially through the high-temperature side (242) of the intermediate-temperature heat exchanger (24) and the high-temperature side (212) of the low-temperature heat exchanger (21), wherein the low-temperature side (241) of the intermediate-temperature heat exchanger (24) is fluidly coupled to the recirculation low-temperature side (213) of the low-temperature heat exchanger (21) and the combustor (3), and the recirculation combustion gas is heated in the intermediate-temperature heat exchanger (24) and the low-temperature heat exchanger (21) by heat exchange with the combustion gas discharged from the expander (5).

8. 1. A method of operating an inflator system, the method comprising the steps of: supplying a fuel flow to the combustor (3) through a fuel supply line (39); supplying an oxidant stream to said combustor (3) through an oxidant supply line (13) extending through a high temperature heat exchanger (22); 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 combustion gases in an expander (5) thereby generating mechanical power; Discharging exhaust combustion gas from the expander (5); passing the exhaust combustion gas through the high temperature heat exchanger (22) in heat exchange with the oxidant stream; passing the exhaust combustion gas from the high temperature heat exchanger (22) through a low temperature heat exchanger (21) in heat exchange with a stream of recirculated combustion gas; and adjusting the oxidant flow with a first oxidant control valve (19) located in the oxidant supply line (13) upstream of the high temperature heat exchanger (22).

9. 9. The method of claim 8, further comprising the step of flowing the oxidant stream through the low-temperature heat exchanger (21) that exchanges heat with the exhaust combustion gas, the low-temperature heat exchanger (21) being located upstream of the high-temperature heat exchanger (22) relative to a flow direction of the oxidant in the oxidant supply line (13).

10. 10. The method according to claim 8 or 9, wherein the first oxidant control valve (19) is arranged in the oxidant supply line (13) between the low temperature heat exchanger (21) and the high temperature heat exchanger (22).

11. 11. The method of claim 8, 9 or 10, further comprising the step of supplying a second oxidant flow towards the combustor (3) through a second oxidant control valve (53) disposed in an oxidant bypass line (52), the oxidant bypass line (52) branching off from the oxidant supply line downstream of the low-temperature heat exchanger (21) and bypassing the high-temperature heat exchanger (22).

12. The method of claim 8, further comprising the step of passing the recirculated combustion gas stream through an intermediate temperature heat exchanger (24) in heat exchange with combustion gas from the high temperature heat exchanger (22).

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