Method for discharging liquid carbon dioxide

Abstract

The invention discloses a method for releasing liquid carbon dioxide in the technical field of high-pressure pipeline safety release, which is used for a refrigerating system and comprises the following steps of: reducing the flow rate of carbon dioxide entering a well and releasing the carbon dioxide in the well based on the liquid level of a gas-liquid separation tank and the inlet pressure of a throttling and pressure reducing device; the compression power is reduced along with the reduction of the pressure of the air return port of the combined cooling and heating device; after the device is shut down, the carbon dioxide release rate of the lifting well is reduced, and gas return is stopped; when the pressure of the carbon dioxide outlet of the heat exchanger is reduced to a threshold value, discharging is stopped; when the system pressure is balanced, the carbon dioxide in the well is released. The flow of carbon dioxide entering the well is reduced based on the liquid level of the gas-liquid separation tank and the pressure of the throttling and pressure reducing device, well rising carbon dioxide is released, the compression power of the combined cooling and heating device is coupled with the pressure of an air return port, then releasing is stopped, global releasing is conducted after the system pressure is balanced, and the risks of dry freezing, freezing and valve freezing are eliminated; and the discharge safety and the system stability are improved.

Description

Technical Field

[0001] This invention relates to the field of safe venting of high-pressure pipelines, and specifically to a method for venting high-pressure liquid carbon dioxide over long distances in deep underground environments for transcritical carbon dioxide combined cooling and heating systems. Background Technology

[0002] Transcritical carbon dioxide combined cooling and heating systems have been widely used in industrial heating and cooling, regional energy stations, and cold chain transportation due to their significant advantages such as high efficiency, environmental friendliness, and extremely low refrigerant GWP (Global Warming Potential). During operation, the high-pressure carbon dioxide is in liquid phase (CO2), typically maintained at a high pressure of 3.5–8.5 MPa, and the system has a large stockpile of refrigerant.

[0003] During system operation and maintenance, scenarios such as pipeline repair, modification, and equipment overhaul are unavoidable, necessitating planned venting of high-pressure carbon dioxide within the system. During pressure reduction and venting, the CO2 medium undergoes drastic phase changes, leading to risks such as cryogenic cooling and dry ice blockage. During pipeline pressure reduction and venting, the coke oven effect caused by the phase change of liquid CO2 and gas expansion absorbs a large amount of heat, causing a rapid drop in CO2 temperature. Cryogenic temperatures reduce the toughness of valve and pipeline materials, leading to pipeline failure. Simultaneously, dry ice particles formed in the low-temperature environment gradually deposit inside valves and pipelines, further increasing the risk of freezing blockage.

[0004] Existing technologies for carbon dioxide venting primarily focus on short-distance, low-flow scenarios, failing to adequately consider the phase transition patterns of deep, long-distance, high-flow transcritical combined cooling and heating systems. They also lack a coordinated control mechanism among system modules during venting, making it difficult to effectively suppress the risks of dry ice freezing and valve malfunction. Consequently, they cannot guarantee the safety and stability of venting operations for multi-phase, multi-system high-pressure CO2 systems, thus hindering the large-scale promotion and safe operation and maintenance of transcritical carbon dioxide combined cooling and heating systems.

[0005] Therefore, developing a high-pressure multiphase carbon dioxide release method that is suitable for deep well long-distance, high-pressure, and large-pipe storage conditions of transcritical carbon dioxide combined cooling and heating refrigeration systems, can accurately suppress dry ice freezing and blockage, avoid freezing valve risks, and ensure safe and stable release process has become a key technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to provide a method for releasing liquid carbon dioxide, which solves the problem that existing carbon dioxide release methods are prone to dry ice freezing and valve freezing risks when releasing deep, long-distance, and large-flow liquid carbon dioxide.

[0007] The present invention achieves the above objectives through the following technical solutions: A method for releasing liquid carbon dioxide is provided for use in a refrigeration system. The refrigeration system includes a transcritical carbon dioxide combined cooling and heating device and a gas-liquid separator on the ground, as well as a throttling and pressure-reducing device and a heat exchanger downhole. The combined cooling and heating device separates carbon dioxide through the gas-liquid separator, which allows liquid carbon dioxide to enter the well. The liquid carbon dioxide is then vaporized and lifted to the well after passing through the throttling and pressure-reducing device and the heat exchanger. It is then compressed by the combined cooling and heating device to the heat-using equipment end and then flows back to the combined cooling and heating device. The venting method includes the following steps: Based on the liquid level in the gas-liquid separator and the pressure before the inlet of the throttling and pressure-reducing device, the inflow rate of liquid carbon dioxide into the well is reduced, and the carbon dioxide brought to the well is released based on the inflow rate. As the pressure at the return gas port of the combined cooling and heating system decreases, the compression power of the combined cooling and heating system is reduced. When the pressure at the return gas port drops to the return gas pressure threshold, the gas-liquid separator returns gas to the combined cooling and heating system. After the combined cooling and heating unit's compression power is reduced to shutdown, the rate of carbon dioxide release from the well is reduced, and the gas-liquid separator stops returning gas. When the carbon dioxide outlet pressure of the heat exchanger drops to the lower pressure limit, stop releasing carbon dioxide. When the pressure of the refrigeration system is balanced, the carbon dioxide that has been raised and lowered into the well is released simultaneously until the carbon dioxide is completely emptied.

[0008] As a further optimization of the invention, when the carbon dioxide from the well begins to be released, the flow rate of the heat exchange medium in the heat exchanger is controlled so that the carbon dioxide outlet pressure of the heat exchanger is within the pressure setting range and the temperature is within the temperature setting range.

[0009] As a further optimization of the invention, during the process of reducing the compression power of the combined cooling and heating system, when the return gas pressure of the combined cooling and heating system reaches the preset pressure, the number of compressors in operation is reduced; when the return gas pressure of the combined cooling and heating system reaches the return gas pressure threshold, the gas-liquid separator returns gas to the combined cooling and heating system; if the return gas pressure of the combined cooling and heating system continues to decrease, the number of compressors in operation is further reduced; when the return gas pressure of the combined cooling and heating system does not meet the preset operating parameter threshold, the combined cooling and heating system is shut down.

[0010] As a further optimization of the invention, when the gas-liquid separator returns gas to the combined cooling and heating system, gaseous carbon dioxide is transported to the combined cooling and heating system through the return gas pipeline; liquid carbon dioxide enters the well through the well inlet pipeline and sequentially enters the throttling and pressure reducing device and the heat exchange device, where it is vaporized to form gaseous carbon dioxide, which then enters the return gas port of the combined cooling and heating system through the well lift pipeline.

[0011] As a further optimization of the invention, the injection flow rate is adjusted by setting a liquid injection control valve on the well inlet pipeline, the well-lift carbon dioxide is released by setting a first vent valve on the well lift pipeline, the return gas is controlled by setting a return gas control valve on the return gas pipeline, and the well-entry carbon dioxide is released by setting a second vent valve on the well inlet pipeline.

[0012] As a further optimization of the invention, when the flow rate of the well inlet pipeline is within the set flow rate range, the opening degree of the first relief valve is adjusted to the first preset value. When the combined cooling and heating device is shut down, the opening degree of the first relief valve is reduced to the second preset value. When the pressure of the refrigeration system is balanced, the first relief valve and the second relief valve are opened simultaneously.

[0013] As a further optimization of the invention, when simultaneously releasing carbon dioxide both rising and falling into the well, outside air is introduced into the refrigeration system to replace the residual carbon dioxide within the system.

[0014] As a further optimization of the invention, the combined cooling and heating device outputs carbon dioxide to the heat-using equipment through a high-heat pipeline, and the heat-using equipment returns the carbon dioxide to the combined cooling and heating device through a low-heat pipeline.

[0015] As a further optimization of the invention, pressure sensors and temperature sensors are provided in the high-heat pipeline, low-heat pipeline, return gas port of the combined cooling and heating device, inlet and outlet of the throttling and pressure reducing device, and carbon dioxide outlet of the heat exchanger. A level gauge is also provided on the gas-liquid separator, and a flow monitoring device is provided on the well inlet pipeline.

[0016] As a further optimization of the invention, before the carbon dioxide is released from the well, the pressure of the carbon dioxide entering the well is in the range of 3.5MPa to 8.5MPa, and the temperature is in the range of 0℃ to 20℃; the pressure of the carbon dioxide outlet of the heat exchanger is in the range of 2.8MPa to 3.6MPa, and the temperature is in the range of 15℃ to 29℃.

[0017] The beneficial effects of this invention are as follows: 1) This invention reduces the inflow of carbon dioxide into the well based on the liquid level of the gas-liquid separator and the pressure before the throttling and pressure-reducing device, and initially releases the carbon dioxide from the well based on this flow rate. The compression power of the combined cooling and heating device is coupled with the return gas port pressure, gradually reducing the flow rate until it is shut down. During this process, the return gas control node of the gas phase carbon dioxide obtained from the gas-liquid separation is reasonably adjusted to stop the release and then perform a global release after the system pressure is balanced. The refrigeration system is coordinated and controlled in stages and with multiple parameters linked. The release process is flexible and orderly, effectively controlling the pressure drop gradient and the phase change rate of the medium, avoiding the formation and deposition of dry ice particles, eliminating the risk of dry ice freezing and valve freezing when releasing large-flow liquid carbon dioxide over long distances in deep underground areas, and improving the safety and stability of the release process. 2) By monitoring and controlling the pressure and temperature of the carbon dioxide outlet of the heat exchanger within a set range in real time during the venting process, this invention ensures the stable operation of the downhole heat exchange unit during the venting phase, prevents the risk of secondary phase change or equipment damage caused by drastic fluctuations in outlet parameters, and further guarantees the smooth transition of the venting process. 3) By setting up pressure sensors, temperature sensors, level gauges and flow monitoring devices at multiple key nodes, this invention achieves refined and data-driven monitoring of the entire venting process. Based on multiple preset pressures, lower pressure limits and flow ranges, the adjustment of the injection valve, venting valve, return valve and compression power are adjusted, making the entire venting method logically clear, controllable and highly automated, reducing the possibility of human error. 4) The venting method of the present invention fully covers the entire process from normal system operation to complete shutdown, and then to complete venting after the internal and external pressures are balanced. This method is not only applicable to planned maintenance, but also provides a standardized operating procedure for safe venting in case of system failure or emergency. It enables the combined cooling and heating refrigeration system to have high overall maintainability and safe operation and maintenance level under complex conditions such as deep wells, long distances, and large-scale high-pressure pipes. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the overall structure of the transcritical carbon dioxide combined cooling and heating refrigeration system of the present invention; Figure 2 This is a flowchart of the venting method of the present invention. Detailed Implementation

[0019] The present application will now be described in further detail with reference to the accompanying drawings. It should be noted that the following specific embodiments are only used to further illustrate the present application and should not be construed as limiting the scope of protection of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the present application based on the above application content.

[0020] Example like Figure 1 and 2 As shown, this embodiment provides a method for releasing liquid carbon dioxide. This method is based on releasing transcritical carbon dioxide from a mine heat source through a deep, long-distance, high-pressure liquid carbon dioxide system. It is applied to a coal mine refrigeration project. The transcritical carbon dioxide system includes a transcritical carbon dioxide system and a gas-liquid separator located on the ground, as well as a throttling and pressure-reducing device and a heat exchanger located underground. The heat exchanger is preferably a carbon dioxide-water heat exchanger.

[0021] The transcritical carbon dioxide combined cooling and heating system includes a compressor unit consisting of multiple compressors, a throttle valve, and cooling equipment. A riser pipeline connects the gaseous carbon dioxide outlet of the heat exchanger to the transcritical carbon dioxide combined cooling and heating system. A well inlet pipeline connects the liquid carbon dioxide outlet of the gas-liquid separator to the throttle valve, and a return gas pipeline connects the gaseous carbon dioxide outlet of the gas-liquid separator to the riser pipeline. The return gas pipeline is used to introduce a small amount of gaseous carbon dioxide carried in the liquid carbon dioxide into the compressor of the transcritical carbon dioxide combined cooling and heating system via the riser pipeline. The gas-liquid separator has a gas phase zone and a liquid phase zone, capable of storing both gaseous and liquid carbon dioxide from the refrigeration system.

[0022] Please refer to the following: Figure 1 The transcritical carbon dioxide combined cooling and heating system of the mine uses low-temperature liquid carbon dioxide as the cooling medium. The low-temperature liquid carbon dioxide enters the throttling and pressure reducing device through the inlet pipeline, forming low-temperature and low-pressure liquid carbon dioxide. The low-temperature and low-pressure liquid carbon dioxide exchanges heat with the chilled water, which is the heat medium, in the heat exchanger. The chilled water absorbs heat from the mine heat source underground and transfers the heat to the low-temperature and low-pressure liquid carbon dioxide, causing the low-temperature and low-pressure liquid carbon dioxide to vaporize and form gaseous carbon dioxide.

[0023] Gaseous carbon dioxide enters the transcritical carbon dioxide cogeneration unit on the surface via a riser pipeline. The unit then compresses the gaseous carbon dioxide into high-temperature, high-pressure supercritical carbon dioxide using a compressor, and outputs it to the heat-consuming equipment via a long-distance high-heat pipeline. After heating, it forms sub-high-temperature, high-pressure supercritical carbon dioxide. The heat-consuming equipment then outputs this sub-high-temperature, high-pressure supercritical carbon dioxide to the transcritical carbon dioxide cogeneration unit via a low-heat pipeline. The transcritical carbon dioxide cogeneration unit cools and depressurizes the sub-high-temperature, high-pressure supercritical carbon dioxide using cooling equipment and a throttling device, reverting it to low-temperature liquid carbon dioxide. This achieves the cyclical transformation and circulation of multiphase CO2.

[0024] The transcritical carbon dioxide cogeneration unit is equipped with a gas-liquid separator at the liquid phase carbon dioxide outlet. The gas-liquid separator can input liquid carbon dioxide into the wellhead pipeline. In addition, if a small amount of gaseous carbon dioxide is generated in the low-temperature liquid phase carbon dioxide, the gas-liquid separator can introduce the gaseous carbon dioxide into the wellhead pipeline, so that the carbon dioxide vaporized at the liquid phase carbon dioxide outlet of the transcritical carbon dioxide cogeneration unit can be directly returned to the transcritical carbon dioxide cogeneration unit through the wellhead pipeline.

[0025] A liquid injection control valve V1 is installed on the surface section of the wellhead pipeline, a first vent valve V2 is installed on the surface section of the wellhead pipeline, and a return gas control valve V3 is installed on the return gas pipeline. Both the liquid injection control valve V1 and the first vent valve V2 are located near the liquid outlet of the gas-liquid separator. In addition, a second vent valve V4 is installed on the wellhead pipeline. Pressure and temperature sensors are installed on the high-heat pipeline, low-heat pipeline, the return gas port of the transcritical carbon dioxide combined cooling and heating unit, the inlet and outlet of the throttling and pressure-reducing device, and the carbon dioxide outlet of the heat exchanger. Figure 1 The PI / TI in the diagram indicates that pressure and temperature sensors are installed on the pipeline. Additionally, the gas-liquid separator is equipped with a level gauge LIA. A flow monitoring device is installed on the wellhead pipeline.

[0026] Before carbon dioxide release, the operating parameters of the combined cooling and heating system were as follows: Liquid carbon dioxide downhole flow rate was 4 kg / s to 25 kg / s; pressure at the end of the downhole pipeline ranged from 3.5 MPa to 8.5 MPa; and temperature was controlled within the range of 0℃ to 20℃. Gas phase carbon dioxide pressure in the uphole pipeline ranged from 2.8 MPa to 3.6 MPa; and temperature was controlled within the range of 15℃ to 29℃. The temperature in the high-heat pipeline was controlled within the range of 80℃ to 130℃; and the pressure range was 8.0 MPa to 12 MPa. The temperature in the low-heat pipeline was controlled within the range of 25℃ to 40℃; and the pressure range was 7.5 MPa to 10 MPa.

[0027] Figure 2 This is a flowchart of a method for releasing liquid carbon dioxide. (Refer to...) Figure 2 The specific steps of this venting method are as follows: The liquid level signal of the gas-liquid separator and the pressure signal before the inlet of the throttling and pressure-reducing device in the combined cooling and heating refrigeration system are monitored by a flow monitoring device. Based on the liquid level of the gas-liquid separator and the pressure before the inlet of the throttling and pressure-reducing device, the opening of the liquid injection control valve V1 is reduced. By adjusting the opening of the liquid injection control valve V1, the liquid level of the gas-liquid separator is controlled at the intermediate level, and the pressure before the inlet of the throttling and pressure-reducing device is controlled between 6MPa and 7MPa. The intermediate level is between 20% and 80% of the full level. The opening of the liquid injection control valve V1 is controlled between 4% and 7%. The flow monitoring device acquires flow data from the well inlet pipeline. When the flow in the well inlet pipeline enters the set flow range, the opening of the first relief valve V2 is adjusted to the first preset value. The set flow range is 4 kg / s to 10 kg / s, and the first preset value is between 50% and 60%.

[0028] Gas-phase carbon dioxide is extracted from the wellhead pipeline of the combined cooling and heating system by the compressor to maintain stable system pressure; liquid continues to be injected into the wellhead pipeline.

[0029] Real-time monitoring of temperature and pressure sensor data changes in various pipelines of the combined cooling and heating refrigeration system, and monitoring of pressure and liquid level display data of the gas-liquid separator.

[0030] The flow rate of the heat exchange medium in the heat exchanger is controlled so that the outlet pressure of the gas phase carbon dioxide in the heat exchanger is controlled within the pressure setting range of 2.8MPa to 3.6MPa, and the outlet temperature of the gas phase carbon dioxide in the heat exchanger is controlled within the temperature setting range of 15℃ to 29℃.

[0031] Pressure monitoring is performed on the section of the riser pipeline near the return gas inlet of the transcritical carbon dioxide combined cooling and heating unit to obtain the initial pressure. When the initial pressure drops to the preset pressure, the number of operating compressors is reduced. The preset pressure is set between 3.1 MPa and 3.3 MPa.

[0032] Continue monitoring the pressure of the aforementioned section of the wellhead pipeline to obtain a second pressure. When the second pressure drops to the return gas pressure threshold, open the return gas control valve V3 of the gas-liquid separator. The gas-liquid separator will then return gas to the combined cooling and heating unit, and the gaseous carbon dioxide obtained from the gas-liquid separation will be input into the combined cooling and heating unit along the return gas pipeline. If the second pressure continues to drop, further reduce the number of operating compressors. The return gas pressure threshold is set between 2.8 MPa and 3 MPa. Returning gas when the second pressure drops to the return gas pressure threshold effectively stabilizes the system pressure.

[0033] It should be noted that before releasing carbon dioxide, there is a large amount of high-pressure liquid carbon dioxide in the wellhead pipeline. By reducing the wellhead flow rate, the high-pressure liquid carbon dioxide in the wellhead pipeline is transferred to the gas-liquid separator for storage. The compressor unit is equipped with a return gas pressure threshold. When the second pressure drops to the return gas pressure threshold, return gas is carried out to supplement the return gas pressure. At the same time, the compressor can continue to operate and continue to pump gas from downhole. The liquid carbon dioxide can continue to vaporize and release pressure, while also preventing the pressure in the gas-liquid separator from becoming too high.

[0034] When the second pressure falls below the preset operating parameter threshold, the carbon dioxide combined cooling and heating unit is shut down, and the opening of the first relief valve V2 is controlled to the second preset value. At this time, the return gas control valve V3 of the gas-liquid separator is closed to prevent the gas phase pressure in the gas-liquid separator from failing to decrease further. The pressure difference before the throttling and pressure-reducing device is maintained, and throttling and pressure reduction continue. After the liquid carbon dioxide is vaporized through the carbon dioxide-water heat exchanger, it is released through the well. The released cooling capacity is carried away by the chilled water, making the refrigeration system more stable. The preset operating parameter threshold is set between 2.6MPa and 2.8MPa. The second preset value is set between 25% and 35%.

[0035] Furthermore, when the liquid level in the gas-liquid separator falls below the warning threshold, the control device issues a warning to indicate that the liquid carbon dioxide in the gas-liquid separator is nearing empty. The warning threshold is set between 5% and 15%.

[0036] When the outlet pressure of the gas phase carbon dioxide in the heat exchanger drops to the lower pressure limit, the first relief valve V2 is closed. The lower pressure limit is set between 0.5 MPa and 0.7 MPa.

[0037] Once the combined cooling and heating system reaches pressure equilibrium, simultaneously open the first vent valve V2 and the second vent valve V4 to completely vent the carbon dioxide from the system. This complete venting means that the carbon dioxide concentration within the system matches the concentration in the outside atmosphere. It should be noted that when the combined cooling and heating system pressure reaches atmospheric pressure, it indicates that the carbon dioxide in the system piping has been vented. Additionally, during the release of carbon dioxide from the wellhead and its lowering points, a vacuum pump can be used to evacuate the system, introducing outside air to replace any remaining carbon dioxide. In this process, air can be drawn from one vent valve while air is introduced from the other, thus achieving the goal of carbon dioxide replacement.

[0038] All pipelines in a combined cooling and heating refrigeration system are high-pressure pipelines. To reduce the risk of blockage or freezing of these pipelines, the initial temperature can be appropriately increased during the carbon dioxide release process. This will result in a higher internal temperature and a shorter release time. The specific process is as follows: appropriately reduce the heating power at the heat-using equipment end to increase the initial temperature of the liquid carbon dioxide entering the well, thereby accelerating the release of carbon dioxide.

[0039] Based on the above technical solution, when a combined cooling and heating refrigeration system is shut down, under maintenance, or when components are replaced, a large amount of multiphase carbon dioxide will remain in the pipeline. If it is not released in time, the carbon dioxide in the system will undergo drastic phase changes, leading to risks such as ultra-low temperature cooling and dry ice blockage. This embodiment can actively release carbon dioxide from the pipeline upon receiving a request for release, reducing the carbon dioxide concentration in the pipeline to below a safe threshold, thus providing a safe environment for subsequent maintenance operations.

[0040] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for releasing liquid carbon dioxide, the method being used in a refrigeration system, the refrigeration system comprising a transcritical carbon dioxide combined cooling and heating device and a gas-liquid separator on the ground, and a throttling and pressure-reducing device and a heat exchanger downhole, wherein the combined cooling and heating device separates carbon dioxide through the gas-liquid separator, the gas-liquid separator allows liquid carbon dioxide to enter the well, the liquid carbon dioxide is vaporized and lifted to the well after passing through the throttling and pressure-reducing device and the heat exchanger, and is compressed by the combined cooling and heating device to the heat-using equipment end, and then flows back to the combined cooling and heating device; Its features are, The venting method includes the following steps: Based on the liquid level in the gas-liquid separator and the pressure before the inlet of the throttling and pressure-reducing device, the inflow rate of liquid carbon dioxide into the well is reduced, and the carbon dioxide brought to the well is released based on the inflow rate. As the pressure at the return gas port of the combined cooling and heating system decreases, the compression power of the combined cooling and heating system is reduced. When the pressure at the return gas port drops to the return gas pressure threshold, the gas-liquid separator returns gas to the combined cooling and heating system. After the combined cooling and heating unit's compression power is reduced to shutdown, the rate of carbon dioxide release from the well is reduced, and the gas-liquid separator stops returning gas. When the carbon dioxide outlet pressure of the heat exchanger drops to the lower pressure limit, stop releasing carbon dioxide. When the pressure of the refrigeration system is balanced, the carbon dioxide that has been raised and lowered into the well is released simultaneously until the carbon dioxide is completely emptied.

2. The method for releasing liquid carbon dioxide according to claim 1, characterized in that: When the carbon dioxide from the well begins to be released, the flow rate of the heat exchange medium in the heat exchanger is controlled so that the carbon dioxide outlet pressure of the heat exchanger is within the pressure setting range and the temperature is within the temperature setting range.

3. The method for releasing liquid carbon dioxide according to claim 1, characterized in that: During the process of reducing the compression power of the combined cooling and heating unit, when the return gas pressure of the combined cooling and heating unit reaches the preset pressure, the number of operating compressors is reduced; when the return gas pressure of the combined cooling and heating unit reaches the return gas pressure threshold, the gas-liquid separator returns gas to the combined cooling and heating unit; if the return gas pressure of the combined cooling and heating unit continues to drop, the number of operating compressors is further reduced; when the return gas pressure of the combined cooling and heating unit does not meet the preset operating parameter threshold, the combined cooling and heating unit is shut down.

4. The method for releasing liquid carbon dioxide according to claim 3, characterized in that: When the gas-liquid separator returns gas to the combined cooling and heating unit, it delivers gaseous carbon dioxide to the combined cooling and heating unit through the return gas pipeline; the liquid carbon dioxide enters the well through the well inlet pipeline and enters the throttling and pressure reducing device and the heat exchange device in sequence, where it is vaporized to form gaseous carbon dioxide, and then enters the return gas port of the combined cooling and heating unit through the well lift pipeline.

5. The method for releasing liquid carbon dioxide according to claim 4, characterized in that: The flow rate into the well is regulated by installing a liquid injection control valve on the well inlet pipeline, the carbon dioxide is released from the wellhead by installing a first vent valve on the wellhead lifting pipeline, the return gas is controlled by installing a return gas control valve on the return gas pipeline, and the carbon dioxide is released into the well by installing a second vent valve on the well inlet pipeline.

6. The method for releasing liquid carbon dioxide according to claim 5, characterized in that: When the flow rate in the well inlet pipeline is within the set flow rate range, the opening of the first relief valve is adjusted to the first preset value. When the combined cooling and heating system is shut down, the opening of the first relief valve is lowered to the second preset value. When the pressure of the refrigeration system is balanced, the first relief valve and the second relief valve are opened simultaneously.

7. The method for releasing liquid carbon dioxide according to claim 1, characterized in that: When releasing carbon dioxide both uplifted and downlifted from the well, outside air is introduced into the refrigeration system to replace any remaining carbon dioxide.

8. The method for releasing liquid carbon dioxide according to claim 1, characterized in that: The combined cooling and heating system outputs carbon dioxide to the heat-consuming equipment through a high-heat pipeline, and the heat-consuming equipment returns the carbon dioxide to the combined cooling and heating system through a low-heat pipeline.

9. The method for releasing liquid carbon dioxide according to claim 8, characterized in that: Pressure and temperature sensors are installed in the high-heat pipeline, low-heat pipeline, return gas port of the combined cooling and heating unit, inlet and outlet of the throttling and pressure reducing device, and carbon dioxide outlet of the heat exchanger. A level gauge is also installed on the gas-liquid separator, and a flow monitoring device is installed on the well inlet pipeline.

10. The method for releasing liquid carbon dioxide according to claim 1, characterized in that: Before the carbon dioxide is released from the well, the pressure of the carbon dioxide entering the well is in the range of 3.5MPa to 8.5MPa, and the temperature is in the range of 0℃ to 20℃; the pressure of the carbon dioxide outlet of the heat exchanger is in the range of 2.8MPa to 3.6MPa, and the temperature is in the range of 15℃ to 29℃.

Images