Electricity generated by carbon dioxide gas pressure drop

Abstract

The system comprises an inlet pipe, a pressure control valve, a once-through generator, and an outlet pipe. The inlet pipe is connected to a carbon dioxide pipeline through which carbon dioxide flows. The pressure control valve is installed in the inlet pipe and is configured to reduce the pressure of the carbon dioxide to a specified pressure. The once-through generator comprises a turbine wheel, a rotor, and a stator. The turbine wheel is configured to rotate in response to the expansion of carbon dioxide flowing into the turbine wheel inlet and being discharged from the turbine wheel outlet. The once-through generator is configured to generate electricity as the rotor rotates within the stator. The outlet pipe is configured to receive the expanded carbon dioxide that has passed through the once-through generator and to discharge the carbon dioxide into the carbon dioxide pipeline network.

Description

Technical Field

[0001] [Priority Claim] This application claims the priority of U.S. Patent Application No. 18 / 191,646, filed on March 28, 2023, the entire content of which is incorporated herein by reference.

[0002] This disclosure relates to power generation by gas pressure drop.

Background Art

[0003] Gas can be transported between locations via a pipeline network. An extensive and sophisticated transportation system is required for the efficient and effective movement of such gas from its production area to its consumption area. An example of such gas is natural gas, which is one of the major energy sources for many of our daily needs and activities. Natural gas is an attractive fossil fuel due to its abundance. Natural gas transported through a pipeline moves through the pipeline at high pressure.

[0004] Another example of such gas is carbon dioxide, which is due to the increasing interest in the energy transition from fossil fuels to renewable and sustainable energy in the global effort to reduce carbon emissions. Some examples of decarbonization pathways in the energy transition to renewable energy include improving energy efficiency, producing and / or using low-carbon fuels, and carbon capture and storage.

Summary of the Invention

Problems to be Solved by the Invention

[0005] This disclosure describes a technology relating to power generation using gas expansion operations. Specific embodiments of the subject matter described can be implemented as a system. The system includes an inlet pipe, a pressure control valve, a once-through generator, and an outlet pipe. The inlet pipe is connected to a carbon dioxide pipeline carrying carbon dioxide. A pressure control valve is installed in the inlet pipe. The pressure control valve is configured to reduce the pressure of carbon dioxide to a specified pressure. At the specified pressure, carbon dioxide is in a gaseous state. A once-through generator is coupled to the inlet pipe downstream of the pressure control valve. The once-through generator includes a turbine wheel, a rotor, and a stator. The turbine wheel is configured to receive carbon dioxide from the inlet pipe. The turbine wheel is configured to rotate in response to the expansion of carbon dioxide flowing into the turbine wheel inlet and being discharged from the turbine wheel outlet. A rotor is coupled to the turbine wheel. The rotor is configured to rotate with the turbine wheel. The once-through generator is configured to generate power as the rotor rotates within the stator. An outlet pipe is coupled to the outlet of the turbine wheel. The outlet pipe is connected to a carbon dioxide pipeline network. The outflow pipe is configured to receive the expanded carbon dioxide that has passed through the once-through generator.

[0006] This embodiment and other embodiments may include one or more of the following features: The inlet pipe may include an inlet channel and a first channel. The inlet channel may be configured to receive carbon dioxide from a carbon dioxide pipeline. The first channel may be coupled to the inlet channel to receive at least a first portion of carbon dioxide from the inlet channel. A pressure control valve and a once-through generator may be coupled to the first channel. The inlet pipe may include a second channel. The second channel may be coupled to the inlet channel to receive at least a second portion of carbon dioxide from the inlet channel and provide an alternative channel for carbon dioxide around the first channel. The first and second channels may be coupled downstream of a once-through generator to recombine the first portion of carbon dioxide from the first channel and the second portion of carbon dioxide from the second channel, respectively. The system may include a second pressure control valve. The second pressure control valve may be coupled to a second channel. The second pressure control valve may be configured to adiabatically expand the second portion of carbon dioxide through the second pressure control valve by providing a size-adjustable constriction for the second portion of carbon dioxide flowing through the second channel. The first outlet pressure of the first portion of carbon dioxide discharged from the once-through generator can be made substantially equal to the second outlet pressure of the second portion of carbon dioxide discharged from the second pressure control valve. The system may include a heater. The heater may be coupled to the inflow channel. The heater may be configured to heat the carbon dioxide to a specified temperature upstream of the first and second channels. The carbon dioxide at the specified pressure and temperature may be in a gaseous state. The system may include a control unit. The control unit may be communicatively coupled to the second pressure control valve and the once-through generator. The control unit may be configured to adjust the size of the constriction provided by the second pressure control valve, thereby adjusting the torque applied to the once-through generator so that the first outlet pressure of the first portion of carbon dioxide discharged from the once-through generator is maintained substantially equal to the second outlet pressure of the second portion of carbon dioxide discharged from the second pressure control valve. The once-through generator may include a hermetically sealed enclosure housing a turbine wheel.The rotor and stator can be hermetically sealed in a line in a first flow path so that the first portion of carbon dioxide flows across the turbine wheel and stator. The rotor may include a permanent magnet rotor.

[0007] Specific aspects of the subject matter described can be implemented as methods. An inlet pipe receives carbon dioxide from a carbon dioxide pipeline. The inlet pipe flows the carbon dioxide to the turbine wheel of a once-through generator. Before flowing the carbon dioxide to the turbine wheel of the once-through generator, the pressure of the carbon dioxide is reduced so that the carbon dioxide entering the once-through generator is in a gaseous state. The once-through generator generates electricity in response to the carbon dioxide flowing across the turbine wheel. After generating electricity, the carbon dioxide flows from the once-through generator to an outlet pipe connected to a carbon dioxide pipeline network.

[0008] This embodiment and other embodiments may include one or more of the following features: The inlet pipe may include an inlet flow path. The method may include flowing at least a first portion of carbon dioxide from the inlet flow path into a first flow path in the inlet pipe. A once-through generator may be coupled to the first flow path. The method may include flowing at least a second portion of carbon dioxide from the inlet flow path into a second flow path in the inlet pipe. The second flow path may provide an alternative flow path for carbon dioxide around the first flow path. The method may include rejoining the first portion of carbon dioxide from the first flow path with the second portion of carbon dioxide from the second flow path downstream of the once-through generator. A pressure control valve may be coupled to the second flow path. The pressure control valve may provide a size-adjustable constriction for the second portion of carbon dioxide flowing through the second flow path, allowing the second portion of carbon dioxide to adiabatically expand through the pressure control valve. The first outlet pressure of the first portion of carbon dioxide discharged from the once-through generator may be substantially equal to the second outlet pressure of the second portion of carbon dioxide discharged from the pressure control valve. A heater may be coupled to the inlet flow path. The heater may heat the carbon dioxide to a specified temperature upstream of the first and second flow paths. Carbon dioxide at a specified pressure and temperature may be in a gaseous state. The size of the constriction provided by the pressure control valve can be adjusted so that the torque applied to the once-through generator can be adjusted so that the first outlet pressure of the first portion of carbon dioxide discharged from the once-through generator remains substantially equal to the second outlet pressure of the second portion of carbon dioxide discharged from the pressure control valve. The once-through generator may include a hermetically sealed enclosure. The hermetically sealed enclosure may house a turbine wheel. The rotor and stator may be hermetically sealed in a line in a first flow path so that the first portion of carbon dioxide flows across the turbine wheel and stator.

[0009] Certain aspects of the subject matter described can be implemented as a system. The system includes an inlet pipe, a first pressure control valve, a once-through generator, and a second pressure control valve. The inlet pipe is connected to a carbon dioxide pipeline carrying carbon dioxide. The inlet pipe includes an inlet channel, a first channel, a second channel, and an outlet channel. The inlet channel is configured to receive carbon dioxide (e.g., from a carbon dioxide pipeline). The first channel is coupled to the inlet channel to receive at least a first portion of carbon dioxide from the inlet channel. The second channel is coupled to the inlet channel to receive at least a second portion of carbon dioxide from the inlet channel and provides an alternative channel for carbon dioxide around the first channel. The outlet channel is coupled to the first and second channels to recombine the first portion of carbon dioxide from the first channel and the second portion of carbon dioxide from the second channel, respectively. The first pressure control valve is coupled to the first channel. The first pressure control valve is configured to reduce the pressure of the first portion of carbon dioxide to a specified pressure. At the specified pressure, the first portion of carbon dioxide is in a gaseous state. The once-through generator is coupled to the first channel. The once-through generator includes a turbine wheel, a rotor, and a stator. The turbine wheel is configured to receive a first portion of carbon dioxide from a first flow path. The turbine wheel is configured to rotate in response to the expansion of the first portion of carbon dioxide flowing into the turbine wheel inlet and discharged from the turbine wheel outlet. The rotor is coupled to the turbine wheel. The rotor is configured to rotate with the turbine wheel. The once-through generator is configured to generate electricity as the rotor rotates within the stator. A second pressure control valve is coupled to a second flow path. The second pressure control valve is configured to allow the second portion of carbon dioxide to adiabatically expand through the second pressure control valve by providing a size-adjustable constriction for the second portion of carbon dioxide flowing through the second flow path. The first outlet pressure of the first portion of carbon dioxide discharged from the once-through generator is substantially equal to the second outlet pressure of the second portion of carbon dioxide discharged from the second pressure control valve. The outflow channel is connected to the carbon dioxide pipeline network. The outflow channel is configured to receive the expanded carbon dioxide that has passed through the once-through generator.

[0010] This embodiment and other embodiments may include one or more of the following features: The system may include a heater. The heater may be coupled to an inflow channel. The heater may be configured to heat carbon dioxide to a specified temperature upstream of the first and second channels. The carbon dioxide at the specified pressure and temperature may be in a gaseous state. The system may include a control unit. The control unit may be communicatively coupled to a second pressure control valve and a once-through generator. The control unit may be configured to adjust the size of the constriction provided by the second pressure control valve, thereby adjusting the torque applied to the once-through generator so that the first outlet pressure of the first portion of carbon dioxide discharged from the once-through generator remains substantially equal to the second outlet pressure of the second portion of carbon dioxide discharged from the second pressure control valve.

[0011] Details of one or more implementations of the subject matter of this disclosure are described in the accompanying drawings and description. Other features, aspects and advantages of the subject matter will become apparent from the specification, drawings and claims. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic diagram of an exemplary power generation system including an expansion turbine. [Figure 2] This is a schematic diagram of an exemplary expansion turbine system connected to a pipeline through which carbon dioxide flows. [Figure 3] This is a flowchart illustrating an exemplary method for generating electricity from the pressure drop of carbon dioxide. [Modes for carrying out the invention]

[0013] Natural gas, hydrogen, carbon dioxide, and other process gases are pressurized to facilitate efficient transport in pipelines. To safely deliver the gas through local distribution networks, the process gases are reduced to lower levels (often using pressure regulators). The pressure is then reduced at pressure drop (PLD) stations for delivery to industrial, commercial, and residential end-users. PLD stations achieve the required pressure drop using regulating valves, but this process wastes a considerable amount of energy. An expansion turbine generator can be installed in parallel with the regulating valve to recover the waste energy from the pressure drop and generate electricity. By recovering the lost energy from the carbon dioxide pressure drop application, the expansion turbine can generate electricity while reducing CO2 emissions, increasing overall plant efficiency, offsetting electricity costs, and generating additional revenue.

[0014] Figure 1 is a schematic diagram of the power generation system 100. The power generation system 100 can be added to a PLD station to capture energy from gas expansion from a PLD process. The power generation system 100 includes an expansion turbine 102 in parallel with a pressure control valve 130. The expansion turbine 102 is axially positioned so that it can be mounted coaxially with the piping. The expansion turbine 102 functions as a generator by generating electrical energy from the rotational kinetic energy obtained from the expansion of a process fluid 120 (e.g., carbon dioxide flowing through a carbon dioxide pipeline) through a turbine wheel 104. For example, the rotation of the turbine wheel 104 can be used to rotate a rotor 108 in a stator 110, which then generates power.

[0015] The expansion turbine 102 includes a high-performance, high-speed permanent magnet generator with an integrated radially inflow expansion turbine wheel 104 and low-loss active magnetic bearings (AMBs) 116a, b. The rotor assembly consists of a permanent magnet section with the turbine wheel 104 directly mounted to the rotor hub. The rotor 108 is levitated by the magnetic bearing system, forming a frictionless (or nearly frictionless) interface between the dynamic and static components. The AMBs 116a, b facilitate the lossless (or nearly lossless) rotation of the rotor 108.

[0016] The expansion turbine 102 is designed so that process fluid 120 flows through system 100, the process fluid 120 cooling the generator and eliminating the need for auxiliary cooling equipment. The power electronics 118 of the expansion turbine 102, in some implementations, combines a power converter 206 and a magnetic bearing control unit (MBC) 212 into a single chamber. The power converter 206 enables consistent delivery of the power generated from the expansion turbine 102. For example, the power converter 206 adjusts the frequency and voltage of the generated current to match the local power grid. As another example, the power converter 206 adjusts the frequency and voltage of the generated current to suit use by a power user, such as an electrolytic unit. After expansion, the process fluid 120 is discharged from the expansion turbine 102 along the same axial path for downstream processes.

[0017] The expansion turbine 102 includes a through-flow configuration. This configuration allows the process fluid 120 to flow from the inlet side to the outlet side of the expansion turbine 102. The process fluid 120 flows into the radial gas inlet 154 and enters the turbine wheel 104, and flows out through the axial gas outlet 156 and is discharged from the turbine wheel 104. The process fluid 120 then flows out through the generator and exits the outlet 154, where it rejoins the gas pipeline 170. Generally, the high-pressure process fluid 120 is guided to flow into the expansion turbine 102 through a flow control system 126. The flow control system 126 includes flow or mass control valves and an emergency shut-off valve. The flow control system 126 can be controlled by power electronic equipment 118 or other electrical, mechanical, or electromagnetic signals. For example, in a fault condition, the flow control system 126 can be notified to close or partially close, thereby eliminating or limiting the gas supply to the expansion turbine 102. When the rotor 108 is operating at a constant speed, the torque applied to the rotor 108 is reduced by limiting or removing the gas flow to the expansion turbine 102, and as a result, the amount of current generated by the power converter 206 is reduced. In the example shown in Figure 1, the flow control system 126 can be opened and closed using the signal channel 164 from the power electronics 118. In some implementations, the housing 112 of the expansion turbine is hermetically sealed.

[0018] The process fluid 120 expands by flowing across the turbine wheel 104, resulting in a pressure drop in the process fluid 120. The process fluid 120 is discharged from the expansion turbine 102 at the reduced pressure. The expansion of the process fluid 120 across the turbine wheel 104 rotates the turbine wheel 104, which in turn rotates the rotor 108. The rotation of the rotor 108 within the stator 110 generates power. The expansion turbine 102 achieves the desired pressure drop and captures the energy from the pressure drop to generate power. A pressure control valve 130, such as a conventional pressure regulator, can be installed in parallel with the expansion turbine 102. Excess high-pressure process fluid 120 that is not induced into the expansion turbine 102 can be induced through the pressure control valve 130. For example, the pressure control valve 130 may be configured to provide a constriction of adjustable size for a portion of the process fluid 120 flowing through the pressure control valve 130 to undergo adiabatic expansion through the pressure control valve 130. The pressure of a portion of the process fluid 120 discharged from the pressure control valve 130 is equal to the pressure of a portion of the process fluid 120 discharged from the expansion turbine 102. In this way, the pressure control valve 130 and the flow control system 126 can work together to control the pressure of the process fluid 120 flowing through the expansion turbine, and then control the amount of current generated by the power converter 206.

[0019] In some implementations, the heater 122 can heat the process fluid 120 before it flows into the expansion turbine 102. For example, if the expansion of the process fluid 120 as it passes through the turbine wheel 104 causes its temperature to drop to a point where moisture in the process fluid 120 would freeze at the turbine wheel 104 or another downstream location, the process fluid 120 can be heated by the heater 122 before entering the expansion turbine 102. After heating, the process fluid 120 can then be guided into the expansion turbine 102. Heating the process fluid 120 prevents moisture from freezing as the process fluid 120 expands and its temperature drops. The heater 122 and the flow control system 126 can be configured to cooperate in adjusting the operating conditions of the process fluid 120 entering the expansion turbine 102. For example, the heater 122 and the flow control system 126 are configured to cooperate in adjusting the operating temperature and operating pressure of the process fluid 120 upstream of the expansion turbine 102 so that the process fluid 120 enters the expansion turbine 102 in a gaseous state. For example, if the process fluid 120 is in a liquid or supercritical state, the heater 122 and the flow control system 126 can be configured to cooperate in increasing the operating temperature, decreasing the operating pressure, or both, in order to change the state of the process fluid 120 to a gaseous state before it enters the expansion turbine 102.

[0020] The expansion turbine 102 includes a turbine wheel 104. The turbine wheel 104 is shown as a radial inflow turbine wheel, but other configurations such as an axial flow turbine wheel are also within the scope of this disclosure. In this example, the process fluid 120 is received from an inlet conduit 150 of the housing 112 and enters a radially oriented inlet 154 of the turbine wheel 104. In some embodiments, the process fluid 120 flows through the inlet conduit 150 and is split by a flow divider to a radial inlet 154 that directs the fluid to the radial inflow portion of the turbine wheel 104. After expansion, the process fluid 120 exits the turbine wheel 104 through an axially oriented outlet 156 and flows into an outlet conduit 152 of the housing 112.

[0021] The turbine wheel 104 can be directly fixed to the rotor 108 or an intermediate common shaft, for example, by fasteners, a rigid drive shaft, welding, or other means. For example, the turbine wheel 104 may be received at the end of the rotor 108 and held to the rotor 108 by a shaft. The shaft is screwed into the rotor 108 at one end and holds the turbine wheel 104 at the other end between the end of the rotor 108 and a nut screwed onto the shaft. The turbine wheel 104 and the rotor 108 can be coupled without a gearbox and rotate at the same speed. In other examples, the turbine wheel 104 can be indirectly coupled to the rotor 108, for example, by a gear train, a clutch mechanism, or other means.

[0022] The turbine wheel 104 includes a plurality of turbine wheel blades 106 that extend outward from the hub and interact with the expanding process fluid 120 to rotate the turbine wheel 104. Figure 1 shows the turbine wheel 104 without a shroud, where each of the turbine blades 106 has a substantially radially oriented exposed blade tip that extends between a radial inlet 154 and an axial outlet 156. As will be described in more detail below, the blade tips are substantially sealed against the shroud 114 inside the housing 112. In certain examples, the turbine wheel 104 is a shrouded turbine wheel.

[0023] In a configuration having a shroudless turbine wheel 104, the housing 112 includes an inwardly oriented shroud 114 that is positioned proximate to the turbine wheel blades 106 and that experiences little contact during operation. The proximity of the turbine wheel blades 106 and the shroud 114 substantially seals the passage of the process fluid 120 therebetween as the process fluid 120 flows through the turbine wheel 104. A portion of the process fluid 120 may leak or pass between the turbine wheel blades 106 and the shroud 114, but the amount of leakage is an amount that is immaterial in the operation of the turbine wheel 104. In certain examples, the amount of leakage may be equivalent to other similar shroudless turbine / shroud surface interfaces using conventional tolerances between the turbine wheel blades 106 and the shroud 114. The amount of leakage considered an acceptable leakage amount may be predetermined. The operating parameters of the turbine generator may be optimized to reduce the amount of leakage. In some implementations, the housing 112 is hermetically sealed to prevent the process fluid 120 from leaking from the radial inlet 154 of the turbine wheel 104.

[0024] The shroud 114 may be positioned a specified distance away from the turbine wheel blades 106 and is maintained at a distance away from the turbine wheel blades 106 during operation of the expansion turbine 102 by using a magnetic positioning device that includes an active magnetic bearing and a position sensor.

[0025] The bearings 116a and 116b are arranged to rotatably support the rotor 108 and the turbine wheel 104 with respect to the stator 110 and the shroud 114. The turbine wheel 104 is supported in a cantilevered manner by the bearings 116a, 116b. In some implementations, the turbine wheel 104 may be supported in a non-cantilevered manner and the bearings 116a and 116b may be positioned on the outlet side of the turbine wheel 104. In certain examples, one or more of the bearings 116a or 116b can include ball bearings, needle bearings, magnetic bearings, foil bearings, journal bearings, and the like.

[0026] The bearings 116a and 116b may be a combination of a radial bearing and a thrust bearing that support the rotor 108 in the radial and axial directions. Other configurations can also be used. The bearings 116a and 116b do not have to be of the same type.

[0027] In an implementation where the bearings 116a and 116b are magnetic bearings, the magnetic bearing control unit (MBC) 212 is used to control the magnetic bearings 116a and 116b. The position sensors 117a, 117b can be used to detect the position or position change of the turbine wheel 104 and / or the rotor 108 relative to the housing 112 or other reference points (such as predetermined values). The position sensors 117a, 117b can detect displacements in the axial and / or radial directions. The magnetic bearings 116a and / or 116b can respond to the information from the position sensors 117a, 117b and adjust the detected displacement as needed. The MBC 212 may receive information from the position sensors 117a, 117b, process the information, and provide control signals to the magnetic bearings 116a, 116b. The MBC 212 can communicate with various components of the expansion turbine 102 via the communication channel 162.

[0028] By using the magnetic bearings 116a, 116b and the position sensors 117a, 117b to maintain and / or adjust the position of the turbine wheel blade 106 so that it remains close to the shroud 114, the expansion turbine 102 can be operated without the need for a seal (e.g., without the need for a dynamic seal). By using active magnetic bearings 116a, b in the expansion turbine 102, physical contact between the rotating components and the stationary components is eliminated, and lubrication, the lubrication system, and the seal are also eliminated.

[0029] The expansion turbine 102 may include one or more backup bearings. For example, bearings can be used to rotatably support the turbine wheel 104 during startup and shutdown, or in the event of a power outage affecting the operation of the magnetic bearings 116a and 116b. The backup bearings may include ball bearings, needle bearings, journal bearings, and the like.

[0030] As described above, the expansion turbine 102 is configured to generate power in response to the rotation of the rotor 108. In certain examples, the rotor 108 may include one or more permanent magnets. The stator 110 includes a plurality of conductive coils. Power is generated by the rotation of the magnets in the coils of the stator 110. The rotor 108 and stator 110 can be configured as a synchronous permanent magnet multiphase alternating current (AC) generator. The electrical output 160 may be, for example, a three-phase output. In certain examples, the stator 110 may include a plurality of coils (for example, three or six coils for a three-phase AC output). As the rotor 108 rotates, a voltage is induced in the stator 110. At any given moment, the magnitude of the voltage induced in the stator coils is proportional to the rate at which the magnetic field surrounding the coils is changing over time (i.e., the rate at which the magnetic field is passing through the two sides of the coils). When the rotor 108 is coupled to rotate at the same speed as the turbine wheel 104, the expansion turbine 102 is configured to generate power at that speed. Such an expansion turbine 102 is a so-called "high-speed" turbine generator. For example, the expansion turbine 102 can generate up to 280 kW at a continuous speed of 30,000 rpm. In some implementations, the expansion turbine generates about 350 kW at a higher rotational speed (e.g., about 35,000 rpm). In another example, the expansion turbine 102 can generate about 355 kW at a continuous speed of about 9,500 rpm.

[0031] In some implementations, the design of the turbine wheel 104, rotor 108, and / or stator 110 can be based on desired parameters of the output gas from the expansion turbine 102. For example, the design of the rotor and stator can be based on a desired temperature of the process fluid 120 discharged from the expansion turbine 102.

[0032] The expansion turbine 102 can be coupled to power electronic equipment 118. The power electronic equipment 118 may include a power converter 206 and a magnetic bearing control unit (MBC) 212 (as described above). The power converter 206 may be, for example, a variable speed drive (VSD) or a variable frequency drive.

[0033] The electrical output 160 of the expansion turbine 102 is connected to a power converter 206 which can be programmed to specific power requirements. The power converter 206 may include an insulated-gate bipolar transistor (IGBT) rectifier 208 for converting the variable-frequency high voltage output from the expansion turbine 102 to direct current (DC). The rectifier 208 may be a three-phase rectifier for three-phase AC input current. The inverter 210 then converts the DC from the rectifier 208 to AC for supply to the power grid 140. The inverter 210 can convert the DC to 380VAC to 480VAC at 50 to 60Hz for supply to the power grid 140. The specific output of the power converter 206 depends on the power grid 140 and the application. Other conversion values ​​are also within the scope of this disclosure. The power converter 206 matches its output to the power grid 140 by sampling the power grid voltage and frequency, and then changing the output voltage and frequency of the inverter 210 to match the sampled power grid voltage and frequency.

[0034] In some implementations, the power converter 206 is a bidirectional power converter. In such implementations, the rectifier 208 can receive AC from the power grid 140 and convert the AC to DC. The inverter 210 can then convert the DC from the rectifier 208 back to AC to supply to the generator. In such implementations, power can be supplied from the power grid 140 to the generator to drive the rotation of the rotor 108 and, consequently, the turbine wheel 104, thereby inducing a process gas flow. In short, in implementations where the power converter 206 is a bidirectional power converter, the flow of power can be reversed and used by the generator to induce a process gas flow (in contrast to the process gas that contributes to the expansion work for generating power).

[0035] The expansion turbine 102 is also connected to the MBC 212 in the power electronics 118. The MBC 212 constantly monitors position, current, temperature, and other parameters to ensure that the expansion turbine 102 and the active magnetic bearings 116a and 116b are operating as desired. For example, the MBC 212 is coupled to position sensors 117a and 117b to monitor the radial and axial positions of the turbine wheel 104 and rotor 108. The MBC 212 can control the magnetic bearings 116a and 116b to selectively change their stiffness and damping characteristics as a function of spin velocity. The MBC 212 can also control synchronous cancellation, including automatic balance control, adaptive vibration control, adaptive vibration rejection, and unbalanced force rejection control.

[0036] Figure 2 is a schematic diagram of an exemplary expansion turbine system 200 connected to a pipeline 290 through which a process fluid 120, such as carbon dioxide, flows. The expansion turbine system 200 includes an expansion turbine 102 and power electronics 118 (described above and shown in Figure 1) of system 100 for recovering energy by reducing the pressure of the process fluid 120 flowing through pipeline 290. In certain examples, system 200 is located between interconnected pipelines (such as pipeline 290 and receiving pipeline 292). In certain examples, system 200 is located inside or outside a gas processing area that processes the process fluid 120. The expansion turbine 102 and heater 122 are located in the same physical location. In some embodiments, the expansion turbine 102 and heater 122 are located within 10 meters, 50 meters, 100 meters, or 500 meters of each other.

[0037] In some implementations, the process fluid 120 is primarily carbon dioxide. For example, the process fluid 120 contains at least 96 mole percent (mol%) of carbon dioxide. For example, the carbon dioxide content of the process fluid 120 is in the range of approximately 96 mol% to approximately 99.999 mol%, approximately 97 mol% to approximately 99.999 mol%, approximately 98 mol% to approximately 99.999 mol%, approximately 99 mol% to approximately 99.999 mol%, approximately 99.9 mol% to approximately 99.999 mol%, or approximately 99.99 mol% to approximately 99.999 mol%. The process fluid 120 may contain one or more contaminants. For example, the process fluid 120 may contain small amounts of carbon monoxide, hydrogen, hydrogen sulfide, sulfur content, nitrogen oxide content, oxygen, water, hydrocarbons, glycols, non-condensable gases, or any combination thereof. For example, the process fluid 120 may contain up to approximately 0.4 mol% carbon monoxide (CO). For example, process fluid 120 may contain up to about 0.5 mol% hydrogen (H2). For example, process fluid 120 may contain up to about 20 ppm (parts per million) hydrogen sulfide (H2S). For example, process fluid 120 may contain up to about 35 ppm sulfur (i.e., compounds containing at least one sulfur atom, e.g., hydrogen sulfide). For example, process fluid 120 may contain up to about 10 ppm (by weight) oxygen (O2). For example, process fluid 120 may contain up to about 150 ppm water (H2O). For example, process fluid 120 may contain up to about 4 mol% of one or more hydrocarbons (e.g., methane). For example, process fluid 120 may contain up to about 0.3 gallons / 1 million cubic feet (gal / MMCF) glycol. For example, process fluid 120 may contain up to about 3 mol% non-condensable gas. In some implementations, the process fluid 120 has a maximum dew point of approximately 30 degrees Fahrenheit (°F) at 400 pounds / square inch gauge (psig).

[0038] In certain examples, power generation system 250 is the same as power generation system 100. Referring to Figures 1 and 2, system 250 includes, among other things, the aforementioned expansion turbine 102 in a hermetically sealed enclosure 112, and the electrical output of the generator of the expansion turbine 102 is coupled to power electronics 118, which in certain examples includes a VSD 206 having a damping resistor assembly 203. If a damping resistor assembly 203 is included, the damping resistor assembly 203 is electrically connected to the electrical output of the expansion turbine 102 (e.g., the output of the generator via the VSD 206). The damping resistor assembly 203 may have an impedance adjusted to allow efficient power transfer from the expansion turbine 102 to the damping resistor assembly 203. In some embodiments, a contactor may connect the output current of the expansion turbine 102 to the damping resistor assembly 203 if the VSD 206 is in a fault condition. A contactor is an electrically controlled switch for switching in a power circuit. The contactor can accommodate the three-phase current output from the generator and guide the current to the damping resistor assembly 203. In some embodiments, the contactor is directly connected to the (three-phase) electrical output of the expansion turbine 102. In some embodiments, the damping resistor assembly 203 and / or the contactor are connected to the electrical output of the expansion turbine 102 outside of the power electronics 118, rather than being part of the power electronics. The VSD 206 can supply an energizing signal to the contactor coil so that the contactor connects the electrical output of the expansion turbine to the damping resistor assembly 203. Depending on the implementation choice, the contactor can be a normally closed (NC) contactor or a normally open (NO) contactor.

[0039] The expansion turbine 102 can be configured to handle gaseous states supplied by pipeline 290, for example, a supercritical fluid in a gas and / or a specified amount of liquid. In a particular example, the VSD 206 can be coupled to a cooling system 252 to cool the electronics of the VSD 206 and maintain a temperature below a specified operating temperature. The output of the VSD 206 can be electrically coupled to a load such as the aforementioned power grid to supply power to the power grid, and / or to a load such as a microgrid in the gas treatment area 213 to supply power to equipment used to treat carbon dioxide in the gas treatment area 213, and / or directly electrically coupled to one or more pieces of equipment used to treat carbon dioxide in the gas treatment area 213 to supply power to the equipment. In a particular example, the equipment includes flow sensors, pressure sensors, temperature sensors and level sensors for various pieces of equipment, valve actuators, communication equipment to enable remote communication with sensors, other equipment and control units for valve actuators, separators (e.g., sand separators, liquid separators), heat treatment equipment, field lighting, control trailers, and / or other types of equipment. In certain cases, the electricity generated by the power generation system 250 can be used by other equipment in the gas treatment facility 213 that is not involved in the treatment of carbon dioxide.

[0040] System 200 includes a flow path 211 through which carbon dioxide flows from pipeline 290. Flow path 211 includes flow regulating equipment to adjust the flow to specified conditions selected based on the specifications of pipeline 290 and the characteristics of the expansion turbine 102 of power generation system 250. In Figure 2, the regulating equipment is shown as a heater 122 and a pressure control valve 214, but the regulating equipment may include additional, different or fewer pieces of equipment and types (e.g., separators). Returning to the specific example in Figure 2, the flow in flow path 211 flows to the pressure control valve 214. The pressure control valve 214 can be controlled to reduce the pressure of the gas flow to a specified pressure. Each of the valves herein, whether a control valve or isolation valve or otherwise, can be remotely controlled, for example, via an operator of a remote control panel at injection site 213 or other location or both, and / or autonomously controlled by a control algorithm of a control unit located at gas processing site 213 or other location or both.

[0041] The flow from the pressure control valve 214 is divided into a first downstream flow path 216, which includes the power generation system 250 including the expansion turbine 102, and a second downstream flow path 218 that bypasses the expansion turbine 102. The first downstream flow path 216 and the second downstream flow path 218 rejoin upstream of the receiving pipeline 292. The inlet of the airtight enclosure 112 is airtightly coupled coaxially with the first flow path 216, so that all the fluid in the flow path 216 is directed into the airtight enclosure 112, flows through the enclosure 112, and returns to the remainder of the first flow path 216.

[0042] The second flow path 218 includes a pressure control valve 222 (e.g., pressure control valve 130) configured with a specified pressure drop relative to the operating position correlation. The pressure control valve 222 can be controlled to adjust the pressure in the second flow path 218 downstream of the valve 222, and consequently the pressure upstream of the pressure control valve 222 (as a function of the pressure of the flow coming from the supply pipeline 290) and the pressure in the first flow path 216. The first flow path 216 includes a flow control valve 224 (e.g., flow control valve 126) configured with a specified flow rate relative to the operating position correlation. The flow control valve 224 can be controlled in relation to the pressure control valves 214 and 222 to control the flow rate of the fluid flowing through the first flow path 216, and therefore the flow rate flowing through the expansion turbine 102.

[0043] This configuration places the expansion turbine 102 in parallel with the second channel 218, allowing the size of the expansion turbine 102 to be freely determined in relation to the conditions of the supply pipeline 290 and / or the receiving pipeline 292, as will be described in more detail below. This degree of freedom arises in part from the second channel 218, which allows the flow to selectively bypass the expansion turbine 102 as it flows from the supply pipeline 290 to the receiving pipeline 292. However, in short, since not all of the flow does not need to pass through the expansion turbine 102 as it flows from the supply pipeline 290 to the receiving pipeline 292, the expansion turbine 102 does not need to be sized to accommodate all of the flow. The first channel 216 also includes an emergency shut-off valve 226 upstream of the expansion turbine 102 to quickly shut off the flow to the expansion turbine 102 if necessary. When closed, the entire flow flows through the second channel 218. In particular, the inflow channel 211, the first channel 216, and the second channel 218, although not shown, may further be equipped with sensors for monitoring the flow pressure, temperature, flow rate, and / or other characteristics upstream and / or downstream of each channel and each component (e.g., valves, expansion turbines, and other components in the channels).

[0044] The pressure of carbon dioxide is adjusted to a specified pressure by a pressure control valve 214 in the flow path 211. Next, a pressure control valve 222 in the second flow path 218 controls the pressure to maintain the pressure passing through the first flow path 216 and the expansion turbine 102, and as a result, together with the flow control valve 224, the conditions passing through the expansion turbine 102 are maintained within the specified operating range of the expansion turbine. Excess flow exits the second flow path 218 and is directed to the outflow flow path 220, which is connected to the receiving pipeline 292. The flow through the first flow path 216 flows through the expansion turbine 102, generates electricity, and then rejoins with the flow from the second flow path 218 in the outflow flow path 220 before returning to the receiving pipeline 292.

[0045] The characteristics of the expansion turbine 102 are selected based on several factors, including the expected pressure, temperature, and flow rate that can be delivered via pipeline 290, the amount of time during the service life of pipeline 290 and / or receiving pipeline 292 for which the power generated by the expansion turbine 102 is desired or required, the ambient conditions at the gas treatment area 213, the conditions including the pressure, temperature, and / or flow rate specified (often specified by the pipeline operator) for supply via pipeline 290, the conditions for receiving via receiving pipeline 292, and the amount of power desired or required to be generated at the gas treatment area 213 by the expansion turbine 102. The specified pressure to which the pressure control valve 214 should be controlled is then selected based on several factors, including the pressure, temperature, and flow characteristics of the expansion turbine 102, the amount of power desired or required to be generated, and the pressure, temperature, and / or flow rate specified for supply via pipeline 290 and receiving via receiving pipeline 292. For example, in a particular example, the receiving pipeline 292 is configured to operate at a specified pressure. The expansion turbine 102, which causes a pressure drop when extracting energy from the flow, is configured to work in cooperation with pressure control valves 214, 222 to produce an outlet pressure from the expansion turbine 102 equal to a specified pressure in the receiving pipeline 292. In a particular example, the receiving pipeline 292 also has a specified minimum temperature, for example, a temperature selected to prevent the fluid in the pipeline from freezing. The expansion turbine 102, which causes a temperature drop when extracting energy from the flow, is configured to work in cooperation with a heater 122 and pressure control valves 214, 222 (which also cause a temperature drop) to maintain the outlet temperature of the expansion turbine 102 and the inlet temperature of the interconnecting pipeline 292 above a specified pressure and a specified minimum temperature. The expansion turbine 102 can be optimized to operate at peak efficiency under the pressure, temperature, and flow conditions provided by the pipeline 290, and while operating to generate power, it also further drops and maintains the pressure downstream of the expansion turbine 102 to the specified pressure in the receiving pipeline 292.In a particular example, the expansion turbine 102 is configured to generate a usable amount of electricity until the upstream pressure approaches a specified pressure in the receiving pipeline 292.

[0046] In some implementations, the specified temperature of carbon dioxide flowing through the supply pipeline 290 is in the range of approximately 40°F to approximately 50°F. In some implementations, the specified pressure of carbon dioxide flowing through the supply pipeline 290 is in the range of approximately 1,000 psig to approximately 1,500 psig. In some cases, the specified operating conditions of pipeline 290 may depend at least in part on the flow rate of the process fluid 120 flowing through pipeline 290. For example, supply pipeline 290 is designed for a carbon dioxide flow rate of approximately 95 million standard cubic feet / day (MMSCFD), with a specified temperature of approximately 44°F and a specified pressure of approximately 1,250 psig. For example, supply pipeline 290 is designed for a carbon dioxide flow rate of approximately 134 MMSCFD, with a specified temperature of approximately 46°F and a specified pressure of approximately 1,250 psig. In some implementations, the heater 122 and / or pressure control valves 214, 222 work together to adjust the operating conditions of the process fluid 120 flowing from the supply pipeline 290 so that the process fluid 120 entering the expansion turbine 102 is in a gaseous state. For example, the heater 122 and / or pressure control valves 214, 222 work together to adjust the temperature of the process fluid 120 entering the expansion turbine 102 to be within the range of approximately 140°F to 180°F, approximately 140°F to 170°F, approximately 140°F to 160°F, approximately 150°F to 180°F, approximately 150°F to 170°F, or approximately 150°F to 160°F. For example, the heater 122 and / or pressure control valves 214, 222 work together to adjust the pressure of the process fluid 120 entering the expansion turbine 102 to be within the range of approximately 700 psig to 1,200 psig, approximately 700 psig to 1,100 psig, approximately 700 psig to 1,000 psig, approximately 700 psig to 900 psig, approximately 800 psig to 1,200 psig, approximately 800 psig to 1,100 psig, approximately 800 psig to 1,000 psig, approximately 800 psig to 900 psig, approximately 900 psig to 1,200 psig, approximately 900 psig to 1,100 psig, approximately 900 psig to 1,000 psig, or approximately 850 psig to 950 psig.The specified operating conditions of the receiving pipeline 292 may, in some cases, depend at least in part on the flow rate of the process fluid 120 flowing through the receiving pipeline 292, the operating conditions of the process fluid 120 supplied by the pipeline 290, the flow rate through the expansion turbine 102, the operating characteristics of the expansion turbine 102, or any combination thereof. For example, the receiving pipeline 292 is designed for a specified temperature of approximately 100°F and a specified pressure of approximately 460 psig, with a carbon dioxide flow rate of approximately 95 MMSCFD. For example, the receiving pipeline 292 is designed for a specified temperature of approximately 100°F and a specified pressure of approximately 530 psig, with a carbon dioxide flow rate of approximately 134 MMSCFD.

[0047] As carbon dioxide flows through the expansion turbine 102, it expands, causing the turbine wheel 104 to rotate. The rotation of the turbine wheel 104 causes the rotor 108, which supports several permanent magnets, to rotate. The rotation of the permanent magnets on the rotor 108 induces an electric current through coils or windings on the stator 110, generating electricity. In some embodiments, power electronics 118 are configured to provide a consistent supply of the power generated from the expansion turbine 102 to, for example, a power grid or a user.

[0048] Figure 3 is a flowchart of an exemplary method 300 for generating electricity by carbon dioxide gas expansion. Method 300 can be implemented, for example, by implementing an expansion turbine system 200. In block 302, carbon dioxide (process fluid 120, etc.) from a carbon dioxide pipeline (supply pipeline 290, etc.) is received by an inlet pipe (inlet channel 211, etc.). In block 304, the pressure of the process fluid 120 (carbon dioxide) is reduced. In some implementations, the carbon dioxide is heated (e.g., by a heater 122) before proceeding to block 306. In block 306, the process fluid 120 (carbon dioxide) flows from the inlet channel 211 to the turbine wheel (turbine wheel 104, etc.) of the generator (expansion turbine 102, etc.). By adjusting the operating conditions of the process fluid 120 (e.g., by heater 122 and block 304), the process fluid 120 entering the expansion turbine 102 is ensured to be in a gaseous state. In block 308, electricity is generated by the expansion turbine 102 in response to carbon dioxide flowing across the turbine wheel 104. In block 310, the carbon dioxide flows from the expansion turbine 102 to an outflow pipe (outflow channel 220, etc.) connected to a carbon dioxide pipeline network (receiving pipeline 292, etc.).

[0049] As used in this disclosure, the term “substantially” means the majority or most of the total, such as at least about 50%, 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5%, 99.9%, 99.99%, or at least about 99.999% or more.

[0050] Therefore, this specification and the drawings should be considered illustrative rather than restrictive. Furthermore, the above use of implementations and other exemplary language does not necessarily refer to the same implementation or the same example, but may refer to different distinct implementations as well as potentially identical implementations. The above specification provides detailed descriptions with reference to specific exemplary implementations. However, it will be apparent that various modifications and changes can be made without departing from the broader spirit and scope of this disclosure as set forth in the claims.

Claims

1. An inlet pipe connected to a carbon dioxide pipeline through which carbon dioxide flows, A pressure control valve installed in the inlet pipe and configured to reduce the pressure of the carbon dioxide to a specified pressure, wherein the carbon dioxide at the specified pressure is in a gaseous state, A once-through generator coupled to the inlet pipe downstream of the pressure control valve, A turbine wheel configured to receive carbon dioxide from the inlet pipe, flow into the turbine wheel inlet, and rotate in response to the expansion of the carbon dioxide discharged from the turbine wheel outlet, A rotor configured to be coupled to the turbine wheel and to rotate together with the turbine wheel, stator and, A once-through generator comprising a rotor that rotates within the stator and is configured to generate electricity when the rotor rotates within the stator, An outlet pipe is coupled to the outlet of the turbine wheel, connected to the carbon dioxide pipeline network, and configured to receive the carbon dioxide that has expanded through the once-through generator, A system equipped with these features.

2. The aforementioned inlet pipe is An inflow channel configured to receive carbon dioxide from the carbon dioxide pipeline, A first channel is connected to the inflow channel and receives at least a first portion of the carbon dioxide from the inflow channel, Equipped with, The pressure control valve and the once-through generator are coupled to the first flow path. The system according to claim 1.

3. The inlet pipe is connected to the inlet channel and includes a second channel that receives at least a second portion of the carbon dioxide from the inlet channel and provides an alternative channel for the carbon dioxide around the first channel, The first channel and the second channel are coupled downstream of the once-through generator, and the first portion of the carbon dioxide from the first channel and the second portion of the carbon dioxide from the second channel are recombined. The system according to claim 2.

4. The system includes a second pressure control valve coupled to the second flow path, The second pressure control valve is configured to provide an adjustable constriction for the second portion of the carbon dioxide flowing through the second flow path, and to adiabatically expand the second portion of the carbon dioxide through the second pressure control valve. The first outlet pressure of the first portion of the carbon dioxide discharged from the once-through generator and the second outlet pressure of the second portion of the carbon dioxide discharged from the second pressure control valve are substantially equal. The system according to claim 3.

5. The heater is connected to the aforementioned inflow channel, The heater is configured to heat the carbon dioxide to a specified temperature upstream of the first and second flow paths. At the specified pressure and temperature, the carbon dioxide is in the gaseous state. The system according to any one of claims 2 to 4.

6. The system comprises the second pressure control valve and a control unit that is communicatively coupled to the once-through generator, The control unit, By adjusting the size of the constricted portion provided by the second pressure control valve, The torque applied to the once-through generator is adjusted so that the first outlet pressure of the first portion of the carbon dioxide discharged from the once-through generator and the second outlet pressure of the second portion of the carbon dioxide discharged from the second pressure control valve are maintained to be substantially equal. The system according to claim 4 or claim 5.

7. The once-through generator comprises a hermetically sealed enclosure housing the turbine wheel, The rotor and the stator are hermetically sealed in a line in a first flow path such that the first portion of the carbon dioxide flows across the turbine wheel and the stator. The system according to any one of claims 1 to 6.

8. The system according to any one of claims 1 to 7, wherein the rotor includes a permanent magnet rotor.

9. The inlet pipe receives carbon dioxide from the carbon dioxide pipeline. The inlet pipe allows the carbon dioxide to flow to the turbine wheel of the once-through generator. Before the carbon dioxide is flowed through the turbine wheel of the once-through generator, the pressure of the carbon dioxide is reduced to a specified pressure so that the carbon dioxide entering the once-through generator is in a gaseous state. In response to the carbon dioxide flowing across the turbine wheel, the once-through generator generates electricity. After generating the aforementioned electricity, the carbon dioxide is discharged from the once-through generator into an outlet pipe connected to the carbon dioxide pipeline network. A method that includes doing so.

10. The inlet pipe is equipped with an inlet passage, The method includes flowing at least a first portion of the carbon dioxide from the inflow channel into the first channel of the inflow pipe, The once-through generator is coupled to the first flow path, The method according to claim 9.

11. The method involves allowing at least a second portion of the carbon dioxide to flow from the inflow channel into a second channel of the inflow pipe, wherein the second channel provides an alternative channel for the carbon dioxide around the first channel. Downstream of the once-through generator, the first portion of the carbon dioxide from the first channel is recombined with the second portion of the carbon dioxide from the second channel. The method according to claim 10, which includes the following:

12. The method includes providing an adjustable-size constriction for the second portion of the carbon dioxide flowing through the second flow path by a pressure control valve coupled to the second flow path, and adiabatically expanding the second portion of the carbon dioxide through the pressure control valve, The first outlet pressure of the first portion of the carbon dioxide discharged from the once-through generator and the second outlet pressure of the second portion of the carbon dioxide discharged from the pressure control valve are substantially equal. The method according to claim 11.

13. This includes heating the carbon dioxide to a specified temperature upstream of the first and second channels using a heater connected to the inflow channel, At the specified pressure and temperature, the carbon dioxide is in the gaseous state. The method according to claim 11 or claim 12.

14. By adjusting the size of the constriction provided by the pressure control valve, the torque applied to the once-through generator is adjusted so that the first outlet pressure of the first portion of the carbon dioxide discharged from the once-through generator and the second outlet pressure of the second portion of the carbon dioxide discharged from the pressure control valve are maintained to be substantially equal. The method according to any one of claims 11 to 13, including the action of

15. The once-through generator comprises a hermetically sealed enclosure housing the turbine wheel, The rotor and stator of the once-through generator are hermetically sealed in a line in the first flow path such that the first portion of the carbon dioxide flows across the turbine wheel and the stator. The method according to any one of claims 11 to 14.

16. An inlet pipe connected to a carbon dioxide pipeline that carries carbon dioxide, An inflow channel configured to accept the aforementioned carbon dioxide, A first channel connected to the inflow channel, which receives at least a first portion of the carbon dioxide from the inflow channel, A second channel is coupled to the inflow channel and receives at least a second portion of the carbon dioxide from the inflow channel, and provides an alternative channel for the carbon dioxide around the first channel, An outlet channel connected to the first channel and the second channel, which recombines the first portion of the carbon dioxide from the first channel and the second portion of the carbon dioxide from the second channel, respectively. An inlet pipe, A first pressure control valve is connected to the first flow path and configured to reduce the pressure of the first portion of the carbon dioxide to a specified pressure, wherein the first portion of the carbon dioxide at the specified pressure is in a gaseous state. A once-through generator coupled to the first flow channel, A turbine wheel configured to receive a first portion of carbon dioxide from the first flow path, flow into the inlet of the turbine wheel, and rotate in accordance with the expansion of the first portion of carbon dioxide discharged from the outlet of the turbine wheel, A rotor configured to be coupled to the turbine wheel and to rotate together with the turbine wheel, stator and, The once-through generator is equipped with a rotor that rotates within the stator and is configured to generate electricity when the rotor rotates within the stator, A second pressure control valve is coupled to the second flow path and is configured to provide an adjustable constriction for the second portion of the carbon dioxide flowing through the second flow path, and to adiabatically expand the second portion of the carbon dioxide. A system that includes, The first outlet pressure of the first portion of the carbon dioxide discharged from the once-through generator and the second outlet pressure of the second portion of the carbon dioxide discharged from the second pressure control valve are substantially equal. The outflow channel is connected to a carbon dioxide pipeline network and is configured to receive the carbon dioxide that has expanded through the once-through generator. system.

17. The heater is connected to the aforementioned inflow channel, The heater is configured to heat the carbon dioxide to a specified temperature upstream of the first and second flow paths. At the specified pressure and temperature, the carbon dioxide is in the gaseous state. The system according to claim 16.

18. The system comprises the second pressure control valve and a control unit that is communicatively coupled to the once-through generator, The control unit, By adjusting the size of the constricted portion provided by the second pressure control valve, The torque applied to the once-through generator is adjusted so that the first outlet pressure of the first portion of the carbon dioxide discharged from the once-through generator and the second outlet pressure of the second portion of the carbon dioxide discharged from the second pressure control valve are maintained to be substantially equal. The system according to claim 16 or claim 17.

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