Natural gas expansion differential pressure power generation system

Abstract

The present application discloses a natural gas expansion differential pressure power generation system, which includes an automatic monitoring and control subsystem and an expansion differential pressure power generation subsystem. Among them, the expansion differential pressure power generation subsystem has four power generation modes, and the product combinations produced under different power generation modes are different. The product combination includes at least one of electric energy, liquefied natural gas, and compressed natural gas. The automatic monitoring and control subsystem can determine the power generation mode that meets the operating conditions and has the highest operating efficiency according to the operating parameters of the upstream natural gas, and automatically set the expansion differential pressure power generation subsystem to this power generation mode. This power generation system is applicable to the situation where the operating parameters such as the gas volume, pressure, and temperature of the upstream natural gas fluctuate greatly, can ensure that the temperature of the natural gas at the pipeline outlet will not drop below the natural gas water dew point, and realizes the maximum recovery and utilization of the pressure energy of the natural gas.

Description

Technical Field

[0001] This application relates to the technical field of natural gas transportation, and particularly relates to a natural gas expansion differential pressure power generation system. Background Art

[0002] With the rapid development of the natural gas industry, China has accelerated the construction of natural gas pipeline networks. However, the gas sources are far from the main gas-consuming cities. Therefore, high-pressure long-distance pipelines are mostly used for natural gas transportation in China. Before natural gas enters the urban pipeline network system from high-pressure pipelines, pressure regulation is required. If the pressure energy of the pipeline network is not utilized during the pressure regulation process, a large amount of energy will be wasted in vain.

[0003] Therefore, the expansion differential pressure power generation technology is usually used to convert the pressure energy in the process of natural gas collection and transportation in gas fields into electric energy. The principle is to use an expander to replace the pressure regulating valve to reduce the pressure of high-pressure natural gas so that it meets the requirements of the transportation pressure, and use the mechanical energy generated by the free expansion of high-pressure natural gas itself to drive the generator to generate electricity.

[0004] However, with the increase of the gas field exploitation years, the operating parameters such as gas production, pressure, and temperature will surely fluctuate greatly. Continuing to use the current expansion differential pressure power generation system with a single power generation form may cause the temperature of the natural gas at the pipeline outlet to drop below the natural gas water dew point, resulting in the precipitation of water in the natural gas and even icing, leading to serious safety accidents such as ice blockage, and causing the inability to recover the pressure energy of natural gas to the maximum extent, resulting in energy waste. Summary of the Invention

[0005] In view of the above problems, the present invention provides a natural gas expansion differential pressure power generation system to make full use of the pressure energy of natural gas.

[0006] The present application specifically adopts the following technical solutions:

[0007] A natural gas expansion differential pressure power generation system, the system includes an automatic monitoring and control subsystem and an expansion differential pressure power generation subsystem;

[0008] During operation, the expansion differential pressure power generation subsystem is in one of the states of the first power generation mode, the second power generation mode, the third power generation mode, and the fourth power generation mode. Among them, different power generation modes have their corresponding operating conditions, and at the same time, the product combinations produced under different power generation modes are different. Each product combination includes at least one of electric energy, liquefied natural gas, and compressed natural gas;

[0009] The automatic monitoring and control subsystem is used to: obtain the operating parameter information of the upstream natural gas, determine the power generation mode that meets the operating conditions according to the operating parameter information, and set the expansion differential pressure power generation subsystem to the corresponding power generation mode. If there are multiple power generation modes that meet the operating conditions, the operating benefits of each power generation system in the multiple power generation systems are determined according to the operating parameter information, and the expansion differential pressure power generation subsystem is set to the power generation mode with the highest operating benefit.

[0010] Optionally, the expansion differential pressure power generation subsystem includes a first expander, a second expander, a compressor, a first heat exchanger, a second heat exchanger, a first generator, a second generator, an intake manifold, a first flow branch pipe, a second flow branch pipe, a third flow branch pipe, a first outlet manifold, a second outlet manifold, a first gas collecting pipe, a second gas collecting pipe, a third gas collecting pipe, and a gearbox;

[0011] The intake manifold is used to receive and transport the upstream natural gas;

[0012] The intake ends of the first flow branch pipe, the second flow branch pipe, and the third flow branch pipe are respectively connected to the intake manifold;

[0013] The first expander is connected in the second flow branch pipe;

[0014] The compressor is connected in the third flow branch pipe;

[0015] The first expander, the gearbox, and the compressor are coaxially connected in sequence through a main rotating shaft, and the output shaft of the gearbox is connected to the first generator;

[0016] The first intake port of the first heat exchanger is connected to the outlet end of the second flow branch pipe, the second intake port is connected to the outlet end of the first flow branch pipe, the first outlet port is connected to the first intake port of the second heat exchanger through the first outlet manifold, and the second outlet port is connected to the intake port of the second expander through the first gas collecting pipe;

[0017] The outlet end of the second expander is connected to the first gas storage tank through the second gas collecting pipe, and the rotating shaft of the second expander is connected to the second generator;

[0018] The second intake port of the second heat exchanger is connected to the outlet end of the third flow branch pipe, the first outlet port is connected to the second gas storage tank through the third gas collecting pipe, and the second outlet port is connected to the intake end of the second outlet manifold;

[0019] The intake ends of the first flow branch pipe, the second flow branch pipe, and the third flow branch pipe are respectively provided with a first valve, a second valve, and a third valve;

[0020] The opening and closing states of the first valve, the second valve, and the third valve are controlled by the automatic monitoring and control subsystem.

[0021] Optionally, when the first valve is in the open state and the second valve and the third valve are in the closed state, the expansion differential pressure power generation subsystem is in the first power generation mode, and the product combination produced in the first power generation mode is electric energy;

[0022] When the first valve and the second valve are in the open state and the third valve is in the closed state, the expansion differential pressure power generation subsystem is in the second power generation mode, and the product combination produced in the second power generation mode is electric energy and liquefied natural gas;

[0023] When the second valve and the third valve are in the open state and the first valve is in the closed state, the expansion differential pressure power generation subsystem is in the third power generation mode, and the product combination produced in the third power generation mode is electric energy and compressed natural gas;

[0024] When the first valve, the second valve, and the third valve are in the open state, the expansion differential pressure power generation subsystem is in the fourth power generation mode, and the product combination produced in the fourth power generation mode is electric energy, liquefied natural gas, and compressed natural gas.

[0025] Optionally, the operating condition of the first power generation mode is that the outlet temperature of the first expander is more than 5°C higher than the natural gas water dew point;

[0026] The operating condition of the second power generation mode is that the cold source outlet temperature of the first heat exchanger is more than 5°C higher than the natural gas water dew point, and the outlet temperature of the second expander is lower than the liquefaction temperature of natural gas under the current pressure;

[0027] The operating condition of the third power generation mode is that the cold source outlet temperature of the second heat exchanger is more than 5°C higher than the natural gas water dew point, and the power generation amount of the first expander is greater than the power consumption of the compressor;

[0028] The operating condition of the fourth power generation mode is that the cold source outlet temperature of the second heat exchanger is more than 5°C higher than the natural gas water dew point, and the outlet temperature of the second expander is lower than the liquefaction temperature of natural gas under the current pressure, and at the same time the power generation amount of the first expander is greater than the power consumption of the compressor.

[0029] Optionally, the automatic monitoring and control subsystem includes: a total pressure gauge, a first temperature sensor, a total flowmeter, a PLC control device, and a computer;

[0030] The total pressure gauge, the first temperature sensor, and the total flowmeter are arranged at the air inlet end of the intake manifold, and are all electrically connected to the computer, and are respectively used for measuring the pressure value, the temperature value, and the flow value of the upstream natural gas. The operating parameter information of the upstream natural gas includes the pressure value, the temperature value, and the flow value;

[0031] The computer is used to obtain the operating parameter information of the upstream natural gas, determine the power generation system that meets the operating conditions and has the highest operating efficiency according to the operating parameter information, and send control information to the PLC control device. The control information is used to instruct the PLC control device to set the expansion differential pressure power generation subsystem to the corresponding power generation mode;

[0032] The first valve, the second valve, and the third valve are all electrically connected to the PLC control device;

[0033] The PLC control device is electrically connected to the computer, and is used to receive the control information sent by the computer, and control the on-off states of the first valve, the second valve, and the third valve according to the control information, so as to set the expansion differential pressure power generation subsystem to different power generation modes.

[0034] Optionally, the automatic monitoring and control subsystem further includes a circuit protection device and a storage battery. The storage battery is used to supply power to the automatic monitoring and control subsystem, and the circuit protection device is used to protect the automatic monitoring and control subsystem.

[0035] Optionally, a dryer is further provided at the air inlet end of the intake manifold.

[0036] Optionally, a first flowmeter, a second flowmeter, and a third flowmeter are respectively provided on the first flow branch pipe, the second flow branch pipe, and the third flow branch pipe.

[0037] Optionally, check valves are provided on the first flow branch pipe, the second flow branch pipe, the third flow branch pipe, the first air outlet manifold, the second air outlet manifold, the first gas collecting pipe, the second gas collecting pipe, and the third gas collecting pipe. The check valve is used to prevent the natural gas from flowing back.

[0038] Optionally, pressure gauges are provided on the second flow branch pipe, the third flow branch pipe, and the first gas collecting pipe.

[0039] The beneficial effects of the embodiments of the present invention are at least as follows:

[0040] The natural gas expansion differential pressure power generation system provided by the present invention includes an automatic monitoring and control subsystem and an expansion differential pressure power generation subsystem. Among them, the expansion differential pressure power generation subsystem has four power generation modes with different product combinations, and the automatic monitoring and control subsystem can set the expansion differential pressure power generation subsystem to the power generation mode that meets the operating conditions and has the highest operating efficiency according to the operating parameter information of the upstream natural gas. This power generation system is applicable to the situation where the operating parameters such as the gas volume, pressure, and temperature of the upstream natural gas fluctuate greatly, and can ensure that the temperature of the natural gas at the pipeline outlet will not drop below the natural gas water dew point, realizing the maximum recovery and utilization of the pressure energy of the natural gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0042] Figure 1 It is a schematic structural diagram of the natural gas expansion differential pressure power generation system provided by the embodiment of the present invention;

[0043] Figure 2 It is a schematic diagram of the expansion differential pressure power generation subsystem in the first power generation mode provided by the embodiment of the present invention;

[0044] Figure 3 It is a schematic diagram of the expansion differential pressure power generation subsystem in the second power generation mode provided by the embodiment of the present invention;

[0045] Figure 4 It is a schematic diagram of the expansion differential pressure power generation subsystem in the third power generation mode provided by the embodiment of the present invention;

[0046] Figure 5 It is a schematic diagram of the expansion differential pressure power generation subsystem in the fourth power generation mode provided by the embodiment of the present invention.

[0047] Among them, the reference numerals respectively represent:

[0048] 1 - First expander; 2 - Second expander; 3 - Compressor; 4 - First heat exchanger; 5 - Second heat exchanger; 6 - First generator; 7 - Second generator; 8 - Inlet manifold; 9 - First flow branch pipe; 10 - Second flow branch pipe; 11 - Third flow branch pipe; 12 - First outlet manifold; 13 - Second outlet manifold; 14 - First gas collector pipe; 15 - Second gas collector pipe; 16 - Third gas collector pipe; 17 - Main rotating shaft; 18 - Gearbox; 19 - First gas storage tank; 20 - Second gas storage tank; 21 - First valve; 22 - Second valve; 23 - Third valve; 24 - Fourth valve; 25 - Dryer; 26 - Total pressure gauge; 27 - First temperature sensor; 28 - Total flowmeter; 29 - PLC control device; 30 - Computer; 31 - Circuit protection device; 32 - Battery; 33 - First flowmeter; 34 - Second flowmeter; 35 - Third flowmeter; 36 - Second temperature sensor; 37 - Third temperature sensor. Detailed implementation manners

[0049] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0050] The embodiment of the present application provides a natural gas expansion differential pressure power generation system, which includes an automatic monitoring and control subsystem and an expansion differential pressure power generation subsystem;

[0051] During operation, the expansion differential pressure power generation subsystem is in one state among the first power generation mode, the second power generation mode, the third power generation mode and the fourth power generation mode. Different power generation modes have their respective corresponding operating conditions. At the same time, the product combinations produced under different power generation modes are different, and each product combination includes at least one of electric energy, liquefied natural gas and compressed natural gas.

[0052] The automatic monitoring and control subsystem is used to obtain the operation parameter information of the upstream natural gas, determine the power generation mode that meets the operating conditions according to the operation parameter information, and set the expansion differential pressure power generation subsystem to the corresponding power generation mode. If there are multiple power generation modes that meet the operating conditions, the operating benefits of each power generation system among the multiple power generation systems are determined according to the operation parameter information, and the expansion differential pressure power generation subsystem is set to the power generation mode with the highest operating benefit.

[0053] The natural gas expansion differential pressure power generation system provided by this application has multiple power generation modes, and the product combinations produced under different power generation modes are different. The product combination includes at least one of electric energy, liquefied natural gas, and compressed natural gas. The automatic monitoring and control subsystem can determine the power generation mode that meets the operating conditions and has the highest operating efficiency according to the operating parameters of the upstream natural gas, and automatically set it to this power generation mode. This power generation system is applicable to situations where the operating parameters such as the gas volume, pressure, and temperature of the upstream natural gas fluctuate greatly, so as to ensure that the temperature of the natural gas at the pipeline outlet will not drop below the natural gas water dew point, and realize the maximum recovery and utilization of the pressure energy of the natural gas.

[0054] Optionally, as Figure 1 shown, the expansion differential pressure power generation subsystem includes a first expander 1, a second expander 2, a compressor 3, a first heat exchanger 4, a second heat exchanger 5, a first generator 6, a second generator 7, an intake manifold 8, a first flow branch pipe 9, a second flow branch pipe 10, a third flow branch pipe 11, a first outlet manifold 12, a second outlet manifold 13, a first gas collecting pipe 14, a second gas collecting pipe 15, a third gas collecting pipe 16, and a gearbox 18.

[0055] The role of the expander is to use the adiabatic expansion of the gas in the expander to do work externally and consume the internal energy of the gas itself, so that the pressure and temperature of the gas are greatly reduced, so as to achieve the purpose of refrigeration and cooling. In this field, the expanders used for natural gas expansion power generation mainly have two types: screw expanders and turbine expanders. In the embodiments of this application, the first expander 1 and the second expander 2 are preferably turbine expanders.

[0056] A heat exchanger is a device that transfers part of the heat of a hot fluid (liquid or gas) to a cold fluid (liquid or gas). When in use, let two fluids with a temperature difference pass through different pipes of the heat exchanger. Due to the temperature difference, based on the law of heat balance - the heat of a high-temperature object is transferred to a low-temperature object, heat exchange will occur between the two fluids. The temperature of the originally higher-temperature fluid will decrease, and the temperature of the originally lower-temperature fluid will increase.

[0057] Generally, a heat exchanger has two sets of inlet and outlet ports, one set is the cold source inlet and outlet ports, and the other set is the heat source inlet and outlet ports, that is, two inlet ports and two outlet ports. Common types of heat exchangers include floating head heat exchangers, fixed tube sheet heat exchangers, U-tube sheet heat exchangers, plate heat exchangers, etc. In the embodiments of this application, the types of the first heat exchanger 4 and the second heat exchanger 5 can be determined according to the actual situation. In the embodiments of this application, both the first heat exchanger 4 and the second heat exchanger 5 have two inlet ports and two outlet ports. Among them, the first inlet port and the first outlet port are one set, and the second inlet port and the second outlet port are one set.

[0058] A compressor is a fluid machine that raises low-pressure gas to high-pressure gas. It sucks in low-temperature and low-pressure gas from the suction pipe, compresses it by driving a piston through the operation of an electric motor, and then discharges high-temperature and high-pressure gas to the exhaust pipe. Common types of compressors include piston compressors, screw compressors, centrifugal compressors, linear compressors, etc. In the embodiments of this application, the type of compressor 3 can be determined according to the actual situation.

[0059] Among them, the intake manifold 8 is used to receive and transport the upstream natural gas.

[0060] The intake ends of the first flow branch pipe 9, the second flow branch pipe 10, and the third flow branch pipe 11 are respectively connected to the said intake manifold 8. That is, the upstream natural gas flowing into the intake manifold 8 can flow into the three flow branch pipes.

[0061] The first expander 1 is connected in the second flow branch pipe 10.

[0062] The compressor 3 is connected in the third flow branch pipe 11.

[0063] The first expander 1, the transmission 18, and the compressor 3 are coaxially connected in sequence through the main rotating shaft 17, and the output shaft of the transmission 18 is connected to the first generator 6.

[0064] The natural gas enters the first expander 1 and expands freely. During the expansion process, its internal energy decreases, and at the same time, it outputs work externally, driving the compressor 3 to operate to compress the natural gas and driving the generator 6 to generate electricity through the main rotating shaft 17. At the same time, setting the transmission 18 can conveniently adjust the rotational speed of the rotor shaft of the generator 6 to adapt to different situations.

[0065] The first intake port of the first heat exchanger 4 is connected to the outlet end of the said second flow branch pipe 10, the second intake port is connected to the outlet end of the first flow branch pipe 9, the first outlet port is connected to the first intake port of the said second heat exchanger 5 through the first outlet manifold 12, and the second outlet port is connected to the intake port of the second expander 2 through the first collector pipe 14.

[0066] The outlet port of the second expander 2 is connected to the first gas storage tank 19 through the second collector pipe 15, and the rotating shaft of the second expander 2 is connected to the second generator 7.

[0067] The first gas storage tank 19 is provided to collect and store the corresponding gas products generated after switching to the corresponding power generation mode.

[0068] The second intake port of the second heat exchanger 5 is connected to the outlet end of the third flow branch pipe 11, the first outlet port is connected to the second gas storage tank 20 through the third collector pipe 16, and the second outlet port is connected to the intake end of the second outlet manifold 13.

[0069] The second gas storage tank 20 is provided to collect and store the corresponding gas products generated after being set to the corresponding power generation mode.

[0070] The intake ends of the first flow branch pipe 9, the second flow branch pipe 10, and the third flow branch pipe 11 are respectively provided with a first valve 21, a second valve 22, and a third valve 23. The opening and closing states of the first valve 21, the second valve 22, and the third valve 23 are controlled by the automatic monitoring and control subsystem.

[0071] By setting the above valves and controlling the opening and closing states of the valves, it is possible to control whether natural gas flows in the three flow branch pipes, thereby setting the expansion pressure difference power generation subsystem to different power generation modes. At the same time, since the opening and closing states of these valves are all controlled by the automatic monitoring and control subsystem, the expansion pressure difference power generation subsystem can be automatically set to the power generation mode that meets the operating conditions and has the highest operating efficiency.

[0072] Optionally, when the second valve 22 is in the open state and the first valve 21 and the third valve 23 are in the closed state, the expansion pressure difference power generation subsystem is in the first power generation mode. In this power generation mode, the product combination produced by the expansion pressure difference power generation subsystem is electric energy. The power generation principle and products of the first power generation mode will be described below, where the components with darker colors are the components corresponding to this power generation mode: Figure 2 The power generation principle and products of the first power generation mode will be described, where the components with darker colors are the components corresponding to this power generation mode:

[0073] As Figure 2 shown, when the first valve 21 is in the open state and the second valve 22 and the third valve 23 are in the closed state, there is only one flow path for the natural gas in the intake manifold 8, that is, it can only enter the second flow branch pipe 10. When the natural gas enters the first expander 1, the gas expands freely. During the expansion process, its internal energy decreases, and at the same time, it does work externally. Then, the first expander 1 operates and drives the first generator 6 to generate electricity through the main rotating shaft 17. The gearbox 18 can conveniently adjust the rotational speed of the rotor shaft of the first generator 6 to adapt to different situations.

[0074] The natural gas expanded by the first expander 1 flows through the first outlet manifold 12 and the second outlet manifold 13 in sequence, and finally delivers qualified natural gas to the downstream.

[0075] It should be noted that the components of the first power generation mode do not include the compressor 3. In order to prevent the first expander 1 from driving the first generator 6 and also driving the compressor 3 to operate, in the embodiment of the present application, a coupling can be provided between the main rotating shaft 17 and the compressor 3 ( Figure 2 not shown in the figure). When the expansion pressure difference power generation subsystem is set to the first power generation mode, the coupling will be overloaded and automatically disengaged, so that the main rotating shaft 17 cannot transmit mechanical energy to the compressor 3.

[0076] It should also be noted that although the natural gas expanded by the first expander 1 flows through the first heat exchanger 4 after flowing into the first outlet manifold 12, since only this stream of natural gas flows into the first heat exchanger 4, this stream of natural gas will not undergo heat exchange in the first heat exchanger 4. That is to say, in the first power generation mode, the first heat exchanger 4 only serves as a gas flow channel and will not change the temperature of the natural gas.

[0077] When the first valve 21 and the second valve 22 are in the open state and the third valve 23 is in the closed state, the expansion pressure difference power generation subsystem is in the second power generation mode. In this power generation mode, the product combination produced by the expansion pressure difference power generation subsystem is electric energy and LNG (liquefied natural gas). The following will combine Figure 3 to illustrate the power generation principle and products of the second power generation mode, where the components with darker colors are the corresponding components of this power generation mode:

[0078] As Figure 3 shown, when the first valve 21 and the second valve 22 are in the open state and the third valve 23 is in the closed state, the natural gas flowing into the intake manifold 8 is divided into two streams. The first stream of natural gas flows into the first flow branch 9, and the second stream of natural gas flows into the second flow branch 10.

[0079] After the second stream of natural gas enters the first expander 1, the gas expands freely. During the expansion process, its internal energy decreases, and at the same time, it outputs work externally. Then the first expander 2 operates, driving the first generator 6 to generate electricity and produce electric energy.

[0080] The second stream of natural gas after being expanded by the first expander 1 enters the first heat exchanger 4 as a cold source through the first intake port of the first heat exchanger 4. At the same time, the first stream of natural gas flowing into the first flow branch 9 enters the first heat exchanger 4 as a heat source through the second intake port of the first heat exchanger 4. The two streams of natural gas with a temperature difference undergo heat exchange in the first heat exchanger 4. After heat exchange, the temperature of the first stream of natural gas decreases, and the temperature of the second stream of natural gas increases.

[0081] The first stream of natural gas after heat exchange flows into the first gas collector 14 from the second outlet port of the first heat exchanger 4 and freely expands in the second expander 2. During the expansion process, its internal energy decreases, and at the same time, it outputs work externally. Then the second expander 2 operates, thereby driving the second generator 14 to generate electricity and produce electric energy. The temperature of the first stream of natural gas after expansion decreases and is lower than the natural gas liquefaction temperature, so it is all liquefied to generate LNG (liquefied natural gas), and finally is collected and stored through the first gas storage tank 19.

[0082] The second stream of natural gas after heat exchange flows into the first gas outlet header 12 of the first heat exchanger 4, and the natural gas is transported downstream through the second gas outlet header 13.

[0083] It should be noted that the components of the second power generation mode do not include the compressor 3. To prevent the first expander 1 from driving the first generator 6 and also driving the compressor 3 during operation, in the embodiment of the present application, a coupling can be provided between the main rotating shaft 17 and the compressor 3 ( Figure 3 not shown in the figure). When the expansion differential power generation subsystem is set to the second power generation mode, the coupling will be overloaded and automatically disengaged, so that the main rotating shaft 17 cannot transfer mechanical energy to the compressor 3.

[0084] It should also be noted that the components of the second power generation mode do not include the second heat exchanger 5. Although the second stream of natural gas after heat exchange will flow through the second heat exchanger 5, since there is only the second stream of natural gas and no other streams of natural gas enter the second heat exchanger 5, therefore, no heat exchange will occur to the second stream of natural gas in the second heat exchanger 5. That is to say, in the second power generation mode, the second heat exchanger 5 only serves as a natural gas transmission pipeline and will not change the temperature of the natural gas.

[0085] When the second valve 22 and the third valve 23 are in the open state and the first valve 21 is in the closed state, the expansion differential power generation subsystem is in the third power generation mode. In this power generation mode, the product combination produced by the expansion differential power generation subsystem is electric energy and CNG (compressed natural gas). The following will combine Figure 4 to illustrate the power generation principle and products of the third power generation mode, where the components with darker colors are the corresponding components of this power generation mode:

[0086] As Figure 4 shown, when the second valve 22 and the third valve 23 are in the open state and the first valve 21 is in the closed state, the natural gas flowing into the intake header 8 is divided into two streams. The first stream of natural gas flows into the second flow branch pipe 10, and the second stream of natural gas flows into the third flow branch pipe 11.

[0087] After the first stream of natural gas enters the first expander 1, the gas expands freely. During the expansion process, its internal energy decreases, and at the same time, it outputs work externally. Then the first expander 1 operates to drive the first generator 6 to generate electricity and produce electric energy. At the same time, the main rotating shaft 17 will also transfer mechanical energy to the compressor 3, and then the compressor 3 compresses the second stream of natural gas entering the third flow branch pipe 11 to increase the pressure of the natural gas.

[0088] The second stream of compressed natural gas serves as a heat source and enters the second heat exchanger 5 through the second gas inlet of the second heat exchanger 5. The first stream of expanded natural gas serves as a cold source and enters the second heat exchanger 5 through the first gas inlet of the second heat exchanger 5. The two streams of natural gas with a temperature difference undergo heat exchange within the second heat exchanger 5.

[0089] After heat exchange, the temperature of the second stream of natural gas decreases to form CNG (compressed natural gas), which flows into the second gas collection tank 20 through the second gas outlet of the second heat exchanger 5 for collection and storage.

[0090] After heat exchange, the temperature of the first stream of natural gas increases and flows into the second gas outlet manifold 13 through the first gas outlet of the second heat exchanger 5 for downstream transportation.

[0091] It should be noted that the components of the third power generation mode include the compressor 3. When the expansion differential power generation subsystem is in the third power generation mode, the coupling will not be overloaded and disengaged. Thus, when the first expander 1 operates, the compressor 3 can be driven to operate through the main rotating shaft 17 to compress natural gas to produce CNG (compressed natural gas).

[0092] Meanwhile, it should also be noted that the components of the third power generation mode do not include the first heat exchanger 4. Although the first stream of expanded natural gas flows through the first heat exchanger 4, since there is only the first stream of natural gas and no other streams of natural gas enter the first heat exchanger 4, heat exchange will not occur in the first heat exchanger 4. That is to say, in the third power generation mode, the first heat exchanger 4 only serves as a natural gas transportation pipeline and will not change the temperature of the natural gas.

[0093] When the first valve 21, the second valve 22, and the third valve 23 are in the open state, the expansion differential power generation subsystem is in the fourth power generation mode. In this power generation mode, the product combination produced by the expansion differential power generation subsystem is electric energy, liquefied natural gas, and compressed natural gas. The following will combine Figure 5 to illustrate the power generation principle and products of the fourth power generation mode, where the components with darker colors are the corresponding components of this power generation mode:

[0094] As Figure 5 shown, when the first valve 21, the second valve 22, and the third valve 23 are in the open state, the natural gas flowing into the intake manifold 8 is divided into three streams. The first stream of natural gas flows into the first flow branch pipe 9, the second stream of natural gas flows into the second flow branch pipe 10, and the third stream of natural gas flows into the second flow branch pipe 11.

[0095] After the second stream of natural gas flowing into the second moving branch pipe 10 enters the first expander 1, the gas expands freely. During the expansion process, its internal energy decreases, and at the same time, it outputs work externally. As a result, the first expander 2 operates, driving the first generator 6 to generate electricity and produce electrical energy.

[0096] The second stream of natural gas after expanding through the first expander 1 enters the first heat exchanger 4 through the first air inlet of the first heat exchanger 4 as a cold source. At the same time, the first stream of natural gas entering the first flow branch pipe 9 enters the first heat exchanger 4 through the second air inlet of the first heat exchanger 4 as a heat source. The two streams of natural gas with a temperature difference exchange heat in the first heat exchanger 4. After heat exchange, the temperature of the first stream of natural gas decreases, and the temperature of the second stream of natural gas increases.

[0097] The first stream of natural gas after heat exchange in the first heat exchanger 4 flows into the first gas collector 14 from the second air outlet of the first heat exchanger 4 and freely expands in the second expander 2. During the expansion process, its internal energy decreases, and at the same time, it outputs work externally. As a result, the second expander 2 operates, thereby driving the second generator 14 to generate electricity and produce electrical energy. The temperature of the first stream of natural gas after expansion decreases and is lower than the natural gas liquefaction temperature, so it is all liquefied to generate LNG (liquefied natural gas), and finally is collected and stored through the first gas storage tank 19.

[0098] At the same time, when the second stream of natural gas freely expands in the first expander 1, the main rotating shaft 17 will also transfer mechanical energy to the compressor 3. As a result, the compressor 3 compresses the second stream of natural gas to increase the pressure of the natural gas.

[0099] The compressed second stream of natural gas enters the second heat exchanger 5 through the second air inlet of the second heat exchanger 5 as a heat source. The first stream of natural gas after heat exchange in the first heat exchanger 4 enters the second heat exchanger 5 through the first air inlet of the second heat exchanger 5 as a cold source. The two streams of natural gas with a temperature difference exchange heat in the second heat exchanger 5. After heat exchange, the temperature of the second stream of natural gas decreases, and the temperature of the first stream of natural gas increases, forming CNG (compressed natural gas).

[0100] The first stream of natural gas after heat exchange in the second heat exchanger 5 flows out from the first air outlet of the second heat exchanger 5 and enters the second gas storage tank 20 through the third gas collector 16 for storage.

[0101] The second stream of natural gas after heat exchange in the second heat exchanger 5 flows into the second air outlet manifold 13 through the second air outlet of the second heat exchanger 5 and is transported downstream.

[0102] It should be noted that the components of the fourth power generation mode include the compressor 3. When the expansion differential power generation subsystem is in the fourth power generation mode, the coupling will not be overloaded and disengaged. Therefore, when the first expander 1 operates, the compressor 3 can be driven to operate through the main rotating shaft 17, thereby compressing natural gas to produce CNG (compressed natural gas).

[0103] Optionally, as Figure 1 shown, the automatic monitoring and control subsystem includes: a master pressure gauge 26, a first temperature sensor 27, a master flowmeter 28, a PLC control device 29, and a computer 30.

[0104] Among them, the master pressure gauge 26, the first temperature sensor 27, and the master flowmeter 28 are arranged at the inlet end of the inlet manifold 8 and are all electrically connected to the computer 30, and are respectively used to measure the pressure value, temperature value, and flow value of the upstream natural gas. The above-mentioned pressure value, temperature value, and flow value constitute the operating parameter information of the upstream natural gas. Furthermore, according to these operating parameter information, the power generation mode that meets the operating conditions can be determined.

[0105] Optionally, the operating condition of the first power generation mode is that the outlet temperature of the first expander 1 is more than 5°C higher than the natural gas water dew point, so as to ensure that the outlet temperature of the system is not too low in this power generation mode and prevent safety accidents such as icing.

[0106] The operating condition of the second power generation mode is that the cold source outlet temperature of the first heat exchanger 4 is more than 5°C higher than the natural gas water dew point, so as to ensure that the outlet temperature of the system is not too low in this power generation mode and prevent safety accidents such as icing. Moreover, the outlet temperature of the second expander 2 should also be lower than the liquefaction temperature of natural gas under the current pressure, so as to ensure that the temperature of natural gas is reduced to the liquefaction temperature to obtain LNG (liquefied natural gas).

[0107] The operating condition of the third power generation mode is that the cold source outlet temperature of the second heat exchanger 5 is more than 5°C higher than the natural gas water dew point, so as to ensure that the outlet temperature of the system is not too low in this power generation mode and prevent safety accidents such as icing. Moreover, the power generation of the first expander 1 should be greater than the power consumption of the compressor 3, so as to ensure that the power generation system generates sufficient electric energy, which can not only provide the electric energy required for the compressor to produce CNG (compressed natural gas), but also generate additional electric energy.

[0108] The operating conditions for the fourth power generation mode are as follows: the cold source outlet temperature of the second heat exchanger 5 is more than 5 °C higher than the natural gas water dew point, so as to ensure that the outlet temperature of the system is not too low under this power generation mode and prevent safety accidents such as icing. Moreover, the outlet temperature of the second expander 2 also needs to be lower than the liquefaction temperature of natural gas under the current pressure, so as to ensure that the temperature of natural gas drops to the liquefaction temperature to obtain LNG (liquefied natural gas). At the same time, the power generation of the first expander 1 also needs to be greater than the power consumption of the compressor 3, so as to ensure that the power generation system generates sufficient electric energy, which can not only provide the electric energy required for the compressor to produce CNG (compressed natural gas), but also generate additional electric energy.

[0109] That is to say, the operating conditions of each power generation mode should ensure that under the current natural operating parameters, the outlet temperature of the system is not too low to prevent safety accidents such as icing, and each power generation mode can produce corresponding products.

[0110] It should be noted that the water dew point of natural gas and the liquefaction temperature of natural gas can be directly obtained when natural gas is transported from upstream.

[0111] The computer 30 is used to obtain the operating parameter information of upstream natural gas, determine the power generation mode that meets the operating conditions and has the highest economic benefit according to the operating parameter information, and send control information to the PLC control device 29. The control information is used to instruct the PLC control device to set the expansion differential power generation subsystem to the corresponding power generation mode.

[0112] The PLC control device 29 is electrically connected to the computer 30 and is used to receive the control information sent by the computer 30, so as to set the expansion differential power generation subsystem to the corresponding power generation mode.

[0113] Optionally, the first valve 21, the second valve 22 and the third valve 23 are all electrically connected to the PLC control device 29. The PLC control device 29 controls the opening and closing states of the first valve 21, the second valve 22 and the third valve 23 according to the control information sent by the computer, so as to set the expansion differential power generation subsystem to the corresponding power generation mode.

[0114] Optionally, the automatic monitoring and control subsystem further includes a circuit protection device 31 and a storage battery 32. When a circuit fault occurs, the circuit protection device 31 can make the automatic monitoring and control subsystem in an open circuit state to play a protective role. The storage battery 32 can supply power to the automatic monitoring and control subsystem, thus solving the problem that power supply cannot be provided due to a long distance from the power grid.

[0115] Optionally, a dryer 25 can also be provided at the intake end of the intake manifold 8, so as to dry the natural gas from upstream to ensure the accuracy of the measured temperature, flow rate and pressure of natural gas subsequently.

[0116] Optionally, a first flowmeter 33, a second flowmeter 34, and a third flowmeter 35 may be respectively provided on the first flow branch pipe 9, the second flow branch pipe 10, and the third flow branch pipe 11, so that the flow rate of natural gas in each flow branch pipe can be conveniently known to better control the entire power generation system.

[0117] Optionally, check valves may also be provided on the first flow branch pipe 9, the second flow branch pipe 10, the third flow branch pipe 11, the first gas outlet manifold 12, the second gas outlet manifold 13, the first gas collecting pipe 14, the second gas collecting pipe 15, and the third gas collecting pipe 16. The check valves are used to prevent the backflow of natural gas.

[0118] Optionally, pressure gauges may also be provided on the second flow branch pipe 10, the third flow branch pipe 11, and the first gas collecting pipe 14 to conveniently monitor the natural gas pressure values in each branch pipe.

[0119] Optionally, as Figure 1 shown, the gas outlet end of the second gas outlet manifold 13 is connected to the gas outlet end of the intake manifold 8, that is, the natural gas in the second gas outlet manifold 13 can be transported to the downstream through the intake manifold 8.

[0120] Optionally, as Figure 1 shown, a fourth valve 24 may be provided at the gas outlet end of the intake manifold 8. When the fourth valve 24 is in the closed state, the incoming natural gas in the intake manifold 8 can flow toward the flow branch pipe. When the fourth valve 24 is in the open state and the other valves are all in the closed state, the natural gas can smoothly pass through the intake manifold 8, so as to obtain the upstream operation parameter information of the natural gas and at the same time prevent the entire system from being under pressure.

[0121] In order to prevent accidents such as icing due to the temperature of the natural gas delivered to the downstream being lower than the natural gas water dew point at the pipeline outlet, it is necessary to pay attention to the temperature of the natural gas at the outlet of the intake manifold 8. Therefore, as Figure 1 shown, a second temperature sensor 36 may be provided on the pipeline after the convergence of the second gas outlet manifold 13 and the intake manifold 8, so that the temperature of the natural gas at the outlet of the intake manifold 8 can be conveniently monitored.

[0122] Similarly, since the second gas collecting pipe 15 is mainly used to collect LNG (liquefied natural gas), it is necessary to pay attention to the temperature of the natural gas at the outlet of the second gas collecting pipe 15. Therefore, as Figure 1 shown, a third temperature sensor 37 may be provided on the second gas collecting pipe 15, so that the temperature of the natural gas at the outlet of the second gas collecting pipe 15 can be conveniently monitored.

[0123] The working method steps of the natural gas expansion differential pressure power generation system provided by the present application will be described below:

[0124] Step 1: Start all devices within the entire system, including pressure gauges, temperature sensors, flow meters, computers, and PLC control devices, etc.

[0125] Step 2: The upstream natural gas inlet enters the first inlet manifold, and then passes through the main pressure gauge, the first temperature sensor, and the main flow meter on the first inlet manifold, so that the total flow rate, pressure, and temperature of the upstream natural gas inlet can be measured, and the above parameter data is transmitted into the computer.

[0126] It should be noted that when measuring the operating parameters of the upstream natural gas, in order to prevent pipeline overpressure accidents caused by valve closure, the PLC control device only controls the opening of the fourth valve on the first inlet manifold, that is, when measuring the operating parameters of the upstream natural gas, except for the fourth valve, other valves are set to the closed state, so that the natural gas inlet is temporarily transported from the first inlet manifold, so as to measure the pressure, temperature, and flow meter of the natural gas.

[0127] Step 3: The computer determines the currently eligible power generation mode based on the obtained total flow rate, pressure, and temperature data of the upstream natural gas inlet, as well as the operating conditions corresponding to each of the four power generation modes.

[0128] Specifically, the embodiments of the present application use the following principle to determine whether the four power generation modes meet their respective operating conditions:

[0129] Calculate the mechanical work generated by the expander or consumed by the compressor through Equation (1).

[0130]

[0131] In the formula, W is the mechanical power generated by the expander or the mechanical work consumed by the compressor, kW·h; k is the isentropic index of natural gas, which is taken as 1.33 here; P1 is the pressure at the inlet of the expander or compressor, kW, and its value can be measured through the main pressure gauge on the inlet manifold; P2 is the pressure at the outlet of the expander or compressor, kW; V1 is the volume flow rate at the inlet of the expander or compressor, m 3 / d, the values of P2 and V1 are set accordingly according to the specific situation after the upstream natural gas enters the power generation system, that is, in different power generation modes, the gas flow rate entering each flow branch pipe, as well as the pressure at the outlet of the expander or compressor, are artificially set.

[0132] Furthermore, the enthalpy difference between the inlet and outlet of the expander or compressor can be obtained based on the mechanical work generated by the expander or consumed by the compressor obtained above.

[0133]

[0134] Wherein, ΔH is the enthalpy difference between the inlet and outlet of the expander or compressor, in kJ / kg; Q is the volume flow rate of the natural gas entering the expander or compressor, in m 3 / d; ρ is the density of the natural gas entering the expander or compressor, in kg / m 3 .

[0135] Based on the above enthalpy difference, the temperature difference between the inlet and outlet of the expander or compressor can be calculated.

[0136] ΔT = T2 - T1 = c × ΔH (3)

[0137] Wherein, ΔT is the temperature difference between the inlet and outlet of the expander or compressor, in K; T1 is the inlet temperature of the expander or compressor, in K; T2 is the outlet temperature of the expander or compressor, in K; c is the specific heat capacity at constant pressure of the natural gas, in kJ / (kg·K).

[0138] Furthermore, the temperature at the outlet of the expander or compressor can be calculated as follows:

[0139] T2 = T1 - ΔT (4)

[0140] The calculation method for the outlet temperature of the heat exchanger is as follows:

[0141] c h × m h × (T 2,h - T 1,h ) = c c × m c × (T 2,c - T 1,c ) (5)

[0142] Wherein, c h is the specific heat capacity at constant pressure of the heat source, in kJ / (kg·K); c c is the specific heat capacity at constant pressure of the cold source, in kJ / (kg·K); m h is the mass of the heat source, in kg; m c is the mass of the cold source, in kg; T 1,h is the inlet temperature of the heat source, in K; T 2,h is the outlet temperature of the heat source, in K; T 1,c is the inlet temperature of the cold source, in K; T 2,c is the outlet temperature of the cold source, in K. Among them, the values of c h and c c can be directly obtained when the natural gas is transported from the upstream.

[0143] For the first power generation mode, it is mainly judged whether it meets the operating conditions according to formulas (1), (2) and (3), where T1 is the temperature of the upstream incoming gas.

[0144] For the second power generation mode, it is mainly determined whether the operating conditions are met according to Equation (5).

[0145] Since there are two conditions to be considered, it is necessary to first assume that one of the conditions is met, and then determine whether the other condition is met under the premise that this condition holds. Here, it is assumed that the cold source outlet temperature of the first heat exchanger is (natural gas water dew point + 5) °C, that is, the cold source outlet temperature of the first heat exchanger is a known quantity.

[0146] For Equation (5), since the gas volume flow rate in each flow branch is artificially set, that is, the gas volume flow rate in each flow branch is known, m h (heat source mass) and m c (cold source mass) can be calculated.

[0147] T 1,h is the inlet temperature of the heat source, that is, the temperature of the upstream natural gas incoming gas, which can be measured by the first temperature sensor on the intake manifold.

[0148] T 1,c is the inlet temperature of the cold source, that is, the outlet temperature of the first expander, and this temperature can be calculated according to Equations (1), (2) and (3).

[0149] T 2,c is the outlet temperature of the cold source, that is, the cold source outlet temperature of the first heat exchanger, and here it is assumed that its value is (natural gas water dew point + 5) °C.

[0150] T 2,h is the outlet temperature of the heat source, that is, the heat source outlet temperature of the first heat exchanger. The outlet temperature T 2,h of the heat source of the first heat exchanger can be calculated using Equation (5).

[0151] The heat source outlet temperature value of the first heat exchanger is used as the inlet temperature of the second expander. Furthermore, according to Equations (1), (2) and (3), the outlet temperature of the second expander can be obtained. If this outlet temperature is lower than the liquefaction temperature of natural gas, it means that under the current operating parameters of natural gas, it can not only ensure that the outlet temperature of the system is not too low, but also produce liquefied natural gas, meeting the operating conditions of the second power generation mode. Otherwise, the outlet temperature of the second expander is not lower than the liquefaction temperature of natural gas, not meeting the operating conditions of the second power generation mode.

[0152] For the third power generation mode, it is necessary to determine whether the operating conditions are met according to Equations (1), (2) and (3).

[0153] Since there are two conditions to be considered, it is necessary to first assume that one of the conditions is met, and then determine whether the other condition is satisfied under the premise that this condition holds. Here, it is assumed that the outlet temperature of the first expander is (natural gas water dew point + 5) °C, that is, the outlet temperature of the first expander is a known quantity.

[0154] Using equations (1), (2) and (3), the mechanical work generated by the first expander can be calculated, and then the power generation of the first expander can be calculated:

[0155] W fd = W × η1 (6)

[0156] In the formula, W fd is the power generation of the first expander, kW·h; η1 is the mechanical efficiency of the first expander, %.

[0157] Using equation (1), the mechanical work consumed by the compressor can be calculated, and then the power consumption of the compressor can be calculated:

[0158] W hd = W × η2 (7)

[0159] In the formula, W hd is the power consumption of the compressor, kW·h; η2 is the mechanical efficiency of the compressor, %.

[0160] Compare the power generation W fd of the first expander and the power consumption W hd of the compressor. If W fd is greater than W hd , it means that under the condition that the outlet temperature of the first expander is 5 °C higher than the natural gas water dew point, the power generation of the first expander is greater than the power consumption of the compressor to produce CNG (compressed natural gas), which meets the operating conditions of the third power generation mode. Otherwise, if W fd is less than W hd , it does not meet the operating conditions of the third power generation mode.

[0161] For the fourth power generation mode, it is mainly judged whether it meets the operating conditions according to equations (1)-(7).

[0162] There are three conditions to be considered in this power generation mode. Therefore, it is necessary to first assume that one of the conditions holds, and then judge whether the other two conditions hold under this condition. Since the fourth power generation mode is equivalent to the combination of the above three power generation modes, its judgment method is similar to the above three modes, and the specific method can refer to the above, which will not be elaborated here.

[0163] Based on the above calculation principle, the computer determines the power generation mode that currently meets the operating conditions. If only one power generation mode meets the operating conditions, the PLC control device directly sets the expansion differential pressure power generation subsystem to this power generation mode; if multiple power generation modes meet the operating conditions, the computer calculates the operating benefits of the expansion differential pressure power generation subsystem under each power generation mode according to the operating parameters of natural gas and in combination with the electricity price of local industrial electricity, the profit unit price of LNG (liquefied natural gas), and CNG (compressed natural gas). Thus, the PLC control device sets the expansion differential pressure power generation subsystem to the power generation mode with the highest operating benefit.

[0164] The principle of calculating the operating benefits of the expansion differential pressure power generation subsystem under each power generation mode is described below:

[0165] Comparing the economic benefits of multiple power generation modes is to compare the profits brought by the products under each power generation mode.

[0166] The power generation amount W of the expander fd or the power consumption W of the compressor hd The calculation method refers to the above formulas (6) and (7).

[0167] The total profit of the system's daily power generation is:

[0168] M1 = (W fd - W hd ) × c1

[0169] In the formula, M1 is the total profit of the system's daily power generation, yuan / d; W fd is the daily power generation amount of the expander in the system, kW·h; M hd is the daily power consumption of the compressor in the system, kW·h; c1 is the local industrial electricity unit price, yuan / degree (1 degree = 1kW·h).

[0170] The total profit of the system's daily production of LNG (liquefied natural gas) is:

[0171] M2 = Q1 × ρ × c2 / 1000

[0172] In the formula, M2 is the total profit of the system's daily production of LNG (liquefied natural gas), yuan / d; Q1 is the volume flow rate of natural gas in the first flow branch, m 3 / d; ρ is the density of natural gas, kg / m 3 ; c2 is the local industrial LNG profit unit price, yuan / t.

[0173] Among them, the value of Q1 can be measured by the first flowmeter set on the first flow branch.

[0174] The total profit of the system's daily production of CNG (compressed natural gas) is:

[0175] M3 = Q3 × c3

[0176] Wherein, M3 is the total daily profit of the CNG products produced by the system, in yuan / d; Q3 is the natural gas volume flow rate in the third flow branch, in m 3 / d; ρ is the natural gas density, in kg / m 3 ; c3 is the unit price of local industrial CNG products, in yuan / t.

[0177] Among them, the value of Q3 can be measured by a third flowmeter installed on the third flow branch.

[0178] The total daily profit of the system:

[0179] M = M1 + M2 + M3

[0180] Wherein, M is the total daily profit of the system, in yuan / d.

[0181] For the first power generation mode, its product is only electric energy. Therefore, when calculating the total daily profit of the system, only the power generation amount of the first expander needs to be considered.

[0182] For the second power generation mode, its products are electric energy and liquefied natural gas. When calculating the total daily profit of the system, the power generation amount of the expanders in the system and the production amount of liquefied natural gas need to be considered. And since in this power generation mode, the expanders in the system include the first expander and the second expander, the total power generation amount of the two expanders should be calculated.

[0183] For the third power generation mode, its products are electric energy and compressed natural gas. When calculating the total daily profit of the system, the power generation amount of the first expander, the power consumption of the compressor, and the production amount of compressed natural gas need to be considered.

[0184] For the fourth power generation mode, its products are electric energy, liquefied natural gas and compressed natural gas. When calculating the total daily profit of the system, the power generation amount of the expanders in the system (i.e., the total power generation amount of the first expander and the second expander), the power consumption of the compressor, the production amount of liquefied natural gas, and the production amount of compressed natural gas need to be considered.

[0185] Step 4: The computer feeds back the judgment result to the PLC control device, and the PLC control device controls the opening and closing of the corresponding valves to set the expansion differential power generation subsystem to the corresponding power generation mode.

[0186] The present invention provides a natural gas expansion differential pressure power generation system, which includes an automatic monitoring and control subsystem and an expansion differential pressure power generation subsystem. Among them, the expansion differential pressure power generation subsystem has four power generation modes, and the product combinations produced under different power generation modes are different. The product combination includes at least one of electric energy, liquefied natural gas, and compressed natural gas. The automatic monitoring and control subsystem can determine the power generation mode that meets the operating conditions and has the highest operating efficiency according to the operating parameters of the upstream natural gas, and automatically set the expansion differential pressure power generation subsystem to this power generation mode. This power generation system is applicable to the situation where the operating parameters such as the gas volume, pressure, and temperature of the upstream natural gas fluctuate greatly, can ensure that the temperature of the natural gas at the pipeline outlet will not drop below the natural gas water dew point, and realizes the maximum recovery and utilization of the pressure energy of the natural gas.

[0187] Embodiment

[0188] To make the content of the present invention more obvious and understandable, the following uses a specific embodiment to elaborate on the operation method of the natural gas expansion differential pressure power generation system provided by this application.

[0189] It is known that the density of the upstream natural gas is 122.451 kg / m 3 , and the constant pressure specific heat capacity is 2.51 kJ / (kg·K).

[0190] Step 1: Start all the equipment and machines of the system.

[0191] Step 2: The upstream natural gas enters the entire system. First, it passes through a natural gas dryer to remove the moisture in the gas, and then through equipment such as a total pressure gauge, a total flow meter, and a first temperature sensor for measurement, and a series of parameters are obtained: the gas volume is 100×104 m 3 / d, the inlet pressure of the expander and compressor is 20 MPa, the outlet pressure of the expander is 3 MPa, the outlet pressure of the compressor is 22 MPa, the inlet temperature of the expander and compressor is 30 °C, etc., and the data is transmitted to the computer.

[0192] Step 3: The computer first calculates the outlet temperature of the expander, the outlet temperature of the compressor, the outlet temperature of the heat exchanger, etc. according to the data in Step 2, and then determines the power generation mode that meets the operating conditions. The following details the specific calculation method and judgment basis.

[0193] (1) The first power generation mode: When in this power generation mode, the calculated outlet temperature of the entire system, that is, the outlet temperature of the first expander, is -73.6 °C, which fails to meet the condition of being 5 °C above the natural gas water dew point (≥10 °C). The specific parameters are shown in Table 1. Therefore, this power generation mode cannot be adopted.

[0194] Table 1

[0195] Name Value Mechanical work W 750.67 kW·h Enthalpy difference ΔH -41.27 kJ / kg Temperature difference ΔT -103.6℃ System outlet temperature -73.6℃

[0196] (2) Second power generation mode: When in this power generation mode, set the outlet temperature of the entire system, i.e., the cold source outlet temperature of the first heat exchanger, to 11°C, so that it meets the condition of being 5°C (≥10°C) above the natural gas water dew point. After calculation, the outlet temperature of the second expander is -93.77°C, which meets the condition of being lower than the liquefaction temperature of natural gas (-80°C) at the current pressure of 3 MPa. The specific parameters are shown in Table 2. That is to say, it meets the operating conditions of the second power generation mode, so this power generation mode can be adopted.

[0197] Table 2

[0198] Name Value <![CDATA[Power generation of the first generator W1]]> 377.67 kW·h <![CDATA[Power generation of the second generator W2]]> 378.85 kW·h Total mechanical work W 756.52 kW·h Enthalpy difference ΔH -79.73 kJ / kg Temperature difference ΔT -105.13℃ Outlet temperature of the second expander -93.77℃ System outlet temperature 11℃

[0199] (3) Third power generation mode: When in this power generation mode, set the outlet temperature of the entire system, i.e., the cold source outlet temperature of the second heat exchanger, to 11°C, so that it meets the condition of being 5°C (≥10°C) above the natural gas water dew point. Through calculation, the power generation of the first expander is 566.495 kW·h, which is greater than the power consumption of the compressor (51.88 kW·h). The specific parameters are shown in Table 3. That is to say, it meets the operating conditions of the third power generation mode, so this power generation mode can be adopted.

[0200] Table 3

[0201] Name Value <![CDATA[Power generation of the first generator W1]]> 566.495 kW·h <![CDATA[Compressor power consumption W2]]> 51.88 kW·h Total mechanical work W 514.62 kW·h Enthalpy difference ΔH -119.23 kJ / kg Temperature difference ΔT -105.44℃ System outlet temperature 11℃

[0202] (4) Fourth power generation mode: When in this power generation mode, set the outlet temperature of the entire system, i.e., the cold source outlet temperature of the second heat exchanger, to 11°C, so that it meets the condition of being 5°C (≥10°C) above the natural gas water dew point. Through calculation, the outlet temperature of the second expander is -95.54°C, which is lower than the liquefaction temperature of natural gas (-80°C) at the current pressure of 3 MPa, meeting the condition. At the same time, through calculation, the power generation of the first expander is 377.67 kW·h, which is greater than the power consumption of the compressor (38.91 kW·h). The specific parameters are shown in Table 4. That is to say, it meets the operating conditions of the fourth power generation mode, so this power generation mode can be adopted.

[0203] Table 4

[0204] Name Value <![CDATA[Power generation W1 of the first generator]]> 377.67 kW·h <![CDATA[Power generation of the second generator W2]]> 120.82 kW·h <![CDATA[Compressor power consumption W3]]> 38.91 kW·h Total mechanical work W 459.58 kW·h Enthalpy difference ΔH -50.86 kJ / kg Temperature difference ΔT -47.5℃ Outlet temperature of the second expander -95.54℃ System outlet temperature 11℃

[0205] Thus, three power generation modes that meet the operating conditions, namely the second, third, and fourth power generation modes, are determined. Furthermore, combined with the local industrial electricity price of 0.55 yuan / kWh (1 kWh = 1 kW·h), the profit unit price of LNG (liquefied natural gas) of 100 yuan / t, and the profit unit price of CNG (compressed natural gas) of 1 yuan / m 3 , calculate the total profit of the system under the three power generation modes respectively. The calculation results of the second, third, and fourth power generation modes are shown in Tables 5, 6, and 7 respectively.

[0206] Table 5

[0207] Product name Output Profit (yuan / day) Total electric energy 756.52 kW·h 9986.064 LNG 73.946t 7394.6 Total profit — 17380.664

[0208] Table 6

[0209] Product name Output Profit (yuan / day) Total electric energy 514.615 kW·h 6792.918 CNG <![CDATA[875.954m 3 > 875.954 Total profit — 7668.854

[0210] Table 7

[0211] Product name Output Profit (yuan / day) Total electric energy 459.58 kW·h 6066.456 LNG 122.341t 12234.1 CNG <![CDATA[1218.08m 3 > 1218.08 Total profit — 19518.702

[0212] After comparison, under the fourth power generation mode, the total profit of the system is the highest.

[0213] Step 4: The PLC control device controls the opening and closing of the corresponding valves, and sets the expansion differential pressure power generation subsystem to the fourth power generation mode. The upstream natural gas incoming gas is divided into three streams and enters the expansion differential pressure power generation subsystem.

[0214] The natural gas entering the first flow branch pipe is named natural gas I, and the gas flow rate is controlled to be 30×10 4 m 3 / d. This natural gas enters the first heat exchanger 1 and acts as a heat source.

[0215] The natural gas entering the second flow branch pipe is named natural gas II, and the gas flow rate is controlled to be 40×10 4 m 3 / d. This natural gas enters the first expander and expands freely. The first expander operates to drive the compressor to operate through the main rotating shaft, compress the natural gas to produce CNG (compressed natural gas), and at the same time drive the first generator to generate electricity and produce electric energy. After expansion, the pressure of natural gas II drops to 3 MPa and the temperature drops to -37 °C, and it enters the first heat exchanger and acts as a cold source.

[0216] Natural gas I and natural gas II exchange heat in the first heat exchanger.

[0217] After heat exchange, the temperature of natural gas I drops to -12.9 °C and the pressure remains 20 MPa. It enters the second expander for secondary expansion. The second expander operates to drive the second generator to generate electricity and produce electric energy. After expansion, the pressure of natural gas I drops to 3 MPa and the temperature drops to -91.5 °C, which is lower than the natural gas liquefaction temperature of -80 °C, and LNG (liquefied natural gas) is liquefied and finally collected and stored by the first storage tank.

[0218] After heat exchange, the temperature of natural gas II rises to -10.9 °C and enters the second heat exchanger as a cold source.

[0219] The natural gas entering the third flow branch pipe is named natural gas III, and the gas flow rate is controlled to be 30×10 4 m 3 / d, this natural gas enters the compressor for pressurization. The temperature of the compressed natural gas III rises to 76.4 °C and the pressure rises to 22 MPa. Then it enters the second heat exchanger as a heat source.

[0220] The two streams of natural gas, natural gas II and natural gas III, exchange heat in the second heat exchanger. After heat exchange, the temperature of natural gas III drops to 55.2 °C, obtaining CNG (compressed natural gas), which is finally collected and stored in the second storage tank. The temperature of the natural gas II after heat exchange rises to 13 °C and the pressure remains 3 MPa, meeting the pipeline transportation temperature requirement (≥10 °C), and finally is transported to the downstream through the second outlet header.

[0221] In this application, the terms "first", "second", "third", and "fourth" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance. The term "plural" means two or more, unless otherwise clearly defined.

[0222] After considering the specification and practicing the present application disclosed herein, those skilled in the art will readily conceive of other embodiments of the present application. The present application is intended to cover any variations, uses, or adaptations of the present application, which follow the general principles of the present application and include the common general knowledge or conventional technical means in the technical field not disclosed in the present application. The specification and examples are only regarded as exemplary.

[0223] It should be understood that the present application is not limited to the exact structures described above and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present application is only limited by the appended claims.

Claims

1. A natural gas expansion differential pressure power generation system, characterized in that, The system includes an automatic monitoring and control subsystem and an expansion differential pressure power generation subsystem; During operation, the expansion differential pressure power generation subsystem is in one of the first power generation mode, the second power generation mode, the third power generation mode, and the fourth power generation mode. At the same time, the product combinations produced under different power generation modes are different, and each of the product combinations includes at least one of electric energy, liquefied natural gas, and compressed natural gas; The automatic monitoring and control subsystem is used to: obtain the operation parameter information of the upstream natural gas, determine the power generation mode that meets the operation conditions according to the operation parameter information, and set the expansion differential pressure power generation subsystem to the corresponding power generation mode. If there are multiple power generation modes that meet the operation conditions, the operation benefits of each power generation system in the multiple power generation systems are determined according to the operation parameter information, and the expansion differential pressure power generation subsystem is set to the power generation mode with the highest operation benefit; The expansion differential pressure power generation subsystem includes a first expander (1), a second expander (2), a compressor (3), a first heat exchanger (4), and a second heat exchanger (5); The operation condition of the first power generation mode is: the outlet temperature of the first expander (1) is more than 5°C higher than the natural gas water dew point; The operation condition of the second power generation mode is: the cold source outlet temperature of the first heat exchanger (4) is more than 5°C higher than the natural gas water dew point, and the outlet temperature of the second expander (2) is lower than the liquefaction temperature of the natural gas under the current pressure; The operation condition of the third power generation mode is: the cold source outlet temperature of the second heat exchanger (5) is more than 5°C higher than the natural gas water dew point, and the power generation of the first expander (1) is greater than the power consumption of the compressor (3); The operation condition of the fourth power generation mode is: the cold source outlet temperature of the second heat exchanger (5) is more than 5°C higher than the natural gas water dew point, the outlet temperature of the second expander (2) is lower than the liquefaction temperature of the natural gas under the current pressure, and at the same time, the power generation of the first expander (1) is greater than the power consumption of the compressor (3); 2. The natural gas expansion differential pressure power generation system according to claim 1, wherein The expansion differential pressure power generation subsystem further includes a first generator (6), a second generator (7), an intake manifold (8), a first flow branch pipe (9), a second flow branch pipe (10), a third flow branch pipe (11), a first outlet manifold (12), a second outlet manifold (13), a first gas collecting pipe (14), a second gas collecting pipe (15), a third gas collecting pipe (16), and a gearbox (18); The intake manifold (8) is used to receive and transport the upstream natural gas; The intake ends of the first flow branch pipe (9), the second flow branch pipe (10), and the third flow branch pipe (11) are respectively connected to the intake manifold (8); The first expander (1) is connected in the second flow branch pipe (10); The compressor (3) is connected in the third flow branch pipe (11); The first expander (1), the gearbox (18), and the compressor (3) are coaxially connected in sequence through a main rotating shaft (17), and the output shaft of the gearbox (18) is connected to the first generator (6); The first air inlet of the first heat exchanger (4) is connected to the outlet end of the second flow branch pipe (10), the second air inlet is connected to the outlet end of the first flow branch pipe (9), the first air outlet is connected to the first air inlet of the second heat exchanger (5) through a first air outlet manifold (12), and the second air outlet is connected to the air inlet of the second expander (2) through a first gas collecting pipe (14); The outlet of the second expander (2) is connected to the first gas storage tank (19) through a second gas collecting pipe (15), and the rotating shaft of the second expander (2) is connected to the second generator (7); The second air inlet of the second heat exchanger (5) is connected to the outlet end of the third flow branch pipe (11), the first air outlet is connected to the second gas storage tank (20) through a third gas collecting pipe (16), and the second air outlet is connected to the inlet end of a second air outlet manifold (13); The inlet ends of the first flow branch pipe (9), the second flow branch pipe (10), and the third flow branch pipe (11) are respectively provided with a first valve (21), a second valve (22), and a third valve (23); The opening and closing states of the first valve (21), the second valve (22), and the third valve (23) are controlled by the automatic monitoring control subsystem.

3. The natural gas expansion differential pressure power generation system according to claim 2, wherein When the second valve (22) is in the open state and the first valve (21) and the third valve (23) are in the closed state, the expansion differential pressure power generation subsystem is in the first power generation mode, and the product combination produced in the first power generation mode is electric energy; When the first valve (21) and the second valve (22) are in the open state and the third valve (23) is in the closed state, the expansion differential pressure power generation subsystem is in the second power generation mode, and the product combination produced in the second power generation mode is electric energy and liquefied natural gas; When the second valve (22) and the third valve (23) are in the open state and the first valve (21) is in the closed state, the expansion differential pressure power generation subsystem is in the third power generation mode, and the product combination produced in the third power generation mode is electric energy and compressed natural gas; When the first valve (21), the second valve (22), and the third valve (23) are in the open state, the expansion differential pressure power generation subsystem is in the fourth power generation mode, and the product combination produced in the fourth power generation mode is electric energy, liquefied natural gas, and compressed natural gas.

4. The natural gas expansion differential pressure power generation system according to claim 2, wherein, The automatic monitoring control subsystem includes: a total pressure gauge (26), a first temperature sensor (27), a total flowmeter (28), a PLC control device (29), and a computer (30); The total pressure gauge (26), the first temperature sensor (27), and the total flowmeter (28) are arranged at the inlet end of the intake manifold (8), and are all electrically connected to the computer (30), and are respectively used for measuring the pressure value, temperature value, and flow value of the upstream natural gas. The operating parameter information of the upstream natural gas includes the pressure value, the temperature value, and the flow value; The computer (30) is used to obtain the operating parameter information of the upstream natural gas, determine a power generation system that meets the operating conditions and has the highest operating efficiency according to the operating parameter information, and send control information to the PLC control device (29). The control information is used to instruct the PLC control device (29) to set the expansion differential pressure power generation subsystem to the corresponding power generation mode; The first valve (21), the second valve (22), and the third valve (23) are all electrically connected to the PLC control device (29); The PLC control device (29) is electrically connected to the computer (30), and is used to receive the control information sent by the computer (30), and control the opening and closing states of the first valve (21), the second valve (22), and the third valve (23) according to the control information, so as to set the expansion differential pressure power generation subsystem to different power generation modes.

5. The natural gas expansion differential pressure power generation system according to claim 4, characterized in that, The automatic monitoring and control subsystem further includes a circuit protection device (31) and a storage battery (32). The storage battery (32) is used to supply power to the automatic monitoring and control subsystem, and the circuit protection device (31) is used to protect the automatic monitoring and control subsystem.

6. The natural gas expansion differential pressure power generation system according to claim 2, wherein, A dryer (25) is further provided at the inlet end of the intake manifold (8).

7. The natural gas expansion differential pressure power generation system according to claim 2, wherein, A first flowmeter (33), a second flowmeter (34), and a third flowmeter (35) are respectively provided on the first flow branch pipe (9), the second flow branch pipe (10), and the third flow branch pipe (11).

8. The natural gas expansion differential pressure power generation system according to claim 2, characterized in that, Check valves are provided on the first flow branch pipe (9), the second flow branch pipe (10), the third flow branch pipe (11), the first outlet manifold (12), the second outlet manifold (13), the first gas collecting pipe (14), the second gas collecting pipe (15), and the third gas collecting pipe (16). The check valves are used to prevent the natural gas from flowing back.

9. The natural gas expansion differential pressure power generation system according to claim 2, wherein Pressure gauges are provided on the second flow branch pipe (10), the third flow branch pipe (11), and the first gas collecting pipe (14).

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