Differential pressure power generation system of CNG (compressed natural gas) supply station

Abstract

The invention discloses a differential pressure power generation system of a CNG gas supply station. The differential pressure power generation system comprises the following steps that S1, the expansion pressure of natural gas is reduced; s2, reducing the complexity of the system; s3, using a stable expansion machine; s4, ensuring the stability of the system; a high-pressure gas expansion method is adopted to recycle mechanical energy, a traditional multi-stage pressure regulator pressure reduction method is replaced, energy waste is avoided, meanwhile, in the aspect of utilization of pressure energy with inconstant pressure (gradually reduced), multi-stage expansion machines are adopted to efficiently recycle the pressure energy, the expansion machines at different pressure stages are connected with a main shaft through a one-way coupler, and the pressure energy utilization efficiency is improved. When the high-pressure-level expansion machine does not work, other expansion machines which work normally are not affected, the continuity of the system is guaranteed, when the pressure is reduced and the high-pressure-level expansion machine does not work, the one-way coupler is automatically disconnected with the transmission shaft, the low-pressure-level expansion machine continues to work continuously, and the continuity of system operation is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of pressure difference power generation, and specifically to a CNG gas supply station pressure difference power generation system. Background Art

[0002] Compressed natural gas, abbreviated as CNG, refers to gaseous natural gas compressed to a pressure greater than or equal to 10 MPa and not greater than 25 MPa, which is pressurized natural gas stored in a container in gaseous form.

[0003] CNG is a clean vehicle fuel. For user areas not covered by natural gas pipelines, CNG gas supply stations are a convenient energy supply alternative. CNG is transported to the gas station by special trailer tank trucks, and after unloading gas, heating, pressure regulation, metering, and odorization, it is sent into the urban gas transmission and distribution pipeline or the user's dedicated pipeline.

[0004] Generally, the pressure of the tank truck at the CNG gas supply station is 20 Mpa, and it needs to be reduced to 0.2 - 0.4 Mpa for downstream use. The gas supply station uses a multi-stage pressure regulator to reduce the pressure. During the pressure reduction process, the pressure energy of natural gas is not effectively recovered and utilized, resulting in a certain amount of energy waste.

[0005] In the existing CNG gas supply stations, multi-stage pressure regulators are mostly used for pressure reduction, and the pressure energy is not recovered, resulting in energy waste. The existing technologies are all used in scenarios where the pressure entering the expander is relatively constant, such as in urban natural gas pipelines, usually downstream of the pressure regulating station, where the natural pressure in the pipeline is relatively stable. For a CNG gas supply tank truck, the pressure continuously decreases during the gas supply process. Therefore, the present invention adopts a multi-stage expander to adapt to different pressures to ensure the expansion efficiency. The traditional multi-stage expansion power generation is that one expander drives one generator. To reduce the number of devices and the complexity of the system, a CNG gas supply station pressure difference power generation system is now needed. Summary of the Invention

[0006] The purpose of the present invention is to provide a CNG gas supply station pressure difference power generation system, which adopts a complete CNG pressure energy recovery power generation system to recover the energy during the pressure reduction process, convert it into mechanical energy, drive the generator, and thus convert the pressure energy into electrical energy to solve the technical problems mentioned in the background art.

[0007] To achieve the above purpose, the present invention provides the following technical solution: A CNG gas supply station pressure difference power generation system, including the following steps:

[0008] S1. Reduce the expansion pressure of natural gas: The device uses an expander to expand high-pressure natural gas to reduce the pressure, and at the same time outputs mechanical work, replacing the traditional pressure reduction process of the pressure regulator, effectively recovering the pressure energy and reducing the power consumption of the system;

[0009] S2. Reduce system complexity: During the transportation process, the device arranges multiple expanders coaxially to jointly drive a single generator, reducing the number of devices and thus the system complexity;

[0010] S3. Ensure stable operation of expanders: Between different expanders, one-way couplings are used. When the high-pressure expander is operating normally, it outputs mechanical work to drive the generator. When it cannot operate due to low pressure, the high-pressure expander does not affect the normal operation of the low-pressure stage expander;

[0011] S4. Ensure system stability: During normal operation, the gas paths of the high- and low-pressure stage expanders adopt a series connection process. When the pressure drops and the high-pressure stage expander cannot operate and its outlet cannot discharge gas, the automatic opening and closing valve in the system directly inputs the natural gas in the CNG storage tank into the lower-stage expander to ensure the continuous operation of the system.

[0012] Preferably, in S1, the power generation system will adopt multi-stage expansion power generation. Since the pressure needs to be reduced from 20 Mpa to 0.2 - 0.4 Mpa, the pressure drop is too large. The existing expander technology cannot achieve single-stage expansion pressure reduction. Moreover, when the pressure drop is too large, the theoretical expansion efficiency is relatively low, and the recovered mechanical work becomes less, that is, the power generation will be less.

[0013] Preferably, in S2, through the joint control of multiple expanders, multiple expanders can stably control a single generator for use, thereby reducing the overall operation difficulty of the device, improving the convenience of use of the device, and also greatly reducing the actual operation problems of the device during the natural gas supply process.

[0014] Preferably, when multi-stage expansion is adopted in S3, the shutdown of the high-pressure expander does not affect the operation of the low-pressure expander, that is, it can ensure the continuity of power generation;

[0015] The pressure characteristic of the CNG gas supply station is that as the gas usage time increases, the pressure in the CNG tanker will gradually decrease. And the expander has certain requirements for the inlet pressure. Low pressure will not enable the expander to rotate. When the rotational speed of the high-pressure stage expander decreases or it does not work, it will not affect the operation of the low-pressure stage expander.

[0016] Preferably, when the pressure stage expander does not work in S4, the outlet gas pressure of the expander will not be able to meet the inlet pressure of the lower-stage expander. By using the method of automatic opening and closing valves, the inlet pressure of the lower-stage expander is ensured.

[0017] Preferably, the expander calculation formula is

[0018] W = ΔHi × G

[0019] ΔHi = C P × T 1 × [1 - (P 2 / P 1 ) (K-1) / K

[0020] Among them, W is the output power of the expander, ΔHi is the adiabatic enthalpy drop between the high-pressure station and the low-pressure end of the natural gas, G is the mass flow rate of the natural gas, Cp is the average isobaric specific heat, T1 is the inlet temperature of the natural gas, P1 is the inlet pressure of the natural gas, P2 is the outlet pressure of the natural gas, and K is the adiabatic index of the natural gas.

[0021] Preferably, the comparative calculation divides the expansion process into one stage and multiple stages, and the comparative calculation results (gas flow rate 1 kg / s).

[0022] <![CDATA[Pressure mechanism, Mpa > 20-0.4 Total output power, Kw Output power, Kw 26.2 26.2 <![CDATA[Pressure mechanism, Mpa > 20-10 10-1 1-0.4 Output power, Kw 6.5 18.1 8.4 33 <![CDATA[Pressure mechanism, Mpa > 20-15 15-10 10-5 5-1 1-0.4 Output power, Kw 2.8 3.9 6.5 13.7 8.4 35.3

[0023] Through preliminary calculation and comparison, it can be concluded that the total output power of multi-stage expansion is greater than that of single-stage expansion, that is, multi-stage expansion recovers more energy.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] The present invention uses the high-pressure gas expansion method to recover mechanical energy, replacing the traditional multi-stage pressure regulator step-down method, avoiding energy waste. At the same time, in the utilization of pressure energy with non-constant (gradually decreasing) pressure, a multi-stage expander is used to efficiently recover pressure energy. Moreover, expanders at different pressure levels are connected to the main shaft using one-way couplings. When the high-pressure level expander does not work, it has no impact on other normally working expanders, ensuring the continuity of the system. In addition, multiple expanders drive a generator coaxially, reducing the number of devices and the complexity of the system. The present invention uses multiple expanders to drive a generator coaxially. When the pressure decreases and the high-pressure level expander does not work, the one-way coupling will automatically disconnect from the drive shaft, enabling the low-pressure level expanders to continue working uninterruptedly, ensuring the continuity of system operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the working system of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the drawings in the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0028] ​The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. 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.

[0029] Embodiment 1

[0030] As Figure 1 shown, the present invention provides a CNG supply station differential pressure power generation system, including the following steps:

[0031] S1. Reduce the natural gas expansion pressure: The device uses an expander to expand high-pressure natural gas and reduce the pressure, while outputting mechanical work, replacing the traditional pressure regulator step-down process, effectively recovering the pressure energy, reducing the power consumption of the system. The power generation system will adopt multi-stage expansion power generation. Since the pressure needs to be reduced from 20 Mpa to 0.2 - 0.4 Mpa, the pressure drop is too large, and the existing expander technology cannot achieve single-stage expansion pressure reduction. Moreover, when the pressure drop is too large, the theoretical expansion efficiency is relatively low, and the recovered mechanical work becomes less, that is, the power generation will become less. The overall device adopts multi-stage expansion power generation to better recover the pressure of high-pressure natural gas when reducing the natural gas pressure.

[0032] S2. Reduce the system complexity: The device arranges multiple expanders coaxially during the transportation process, jointly driving a generator, reducing the number of devices and the system complexity. By jointly controlling multiple expanders, multiple expanders can stably control a single generator for use, thereby reducing the overall operation difficulty of the device, improving the convenience of use of the device, and also greatly reducing the actual operation problems of the device during the natural gas supply process.

[0033] S3. Stabilize the use of the expander: Between different expanders, a one-way coupling is adopted. When the high-pressure expander works normally, it outputs mechanical work to drive the generator. When it cannot work due to low pressure, the high-pressure expander does not affect the normal operation of the low-pressure stage expander. When multi-stage expansion is adopted, the shutdown of the high-pressure expander does not affect the operation of the low-pressure expander, that is, it can ensure the continuity of power generation; The pressure characteristic of the CNG supply station is that as the gas usage time increases, the pressure in the CNG tanker will gradually decrease, and the expander has certain requirements for the inlet pressure. Low pressure will not cause the expander to rotate. When the rotational speed of the high-pressure stage expander decreases or it does not work, it will not affect the operation of the low-pressure stage expander.

[0034] S4. Ensure system stability: During normal operation, the expansion engine gas circuits of the high and low pressure stages adopt a series connection process. When the pressure drops and the high pressure stage expansion engine cannot operate and no gas can be discharged from the outlet, the automatic opening and closing valve in the system directly inputs the natural gas in the CNG storage tank into the lower stage expansion engine to ensure the continuous operation of the system. When the expansion engine of a certain pressure stage does not work, the outlet gas pressure of the expansion engine will not be able to meet the inlet pressure of the lower stage expansion engine. By using the automatic opening and closing valve method, the inlet pressure of the lower stage expansion engine is ensured.

[0035] Preferably, the calculation formula of the expansion engine is

[0036] W = ΔHi × G

[0037] ΔHi = C P × T 1 × [1 - (P 2 / P 1 ) (K-1) / K

[0038] Wherein, W is the output power of the expansion engine, ΔHi is the adiabatic enthalpy drop between the high pressure station and the low pressure end of the natural gas, G is the mass flow rate of the natural gas, Cp is the average isobaric specific heat, T1 is the inlet temperature of the natural gas, P1 is the inlet pressure of the natural gas, P2 is the outlet pressure of the natural gas, and K is the adiabatic index of the natural gas.

[0039] Furthermore, in the comparative calculation, the expansion process is divided into 1 stage and multiple stages, and the comparative calculation results (gas flow rate 1 kg / s) are obtained.

[0040] <![CDATA[Pressure mechanism, Mpa > 20-0.4 Total output power, Kw Output power, Kw 26.2 26.2 <![CDATA[Pressure mechanism, Mpa > 20-10 10-1 1-0.4 Output power, Kw 6.5 18.1 8.4 33 <![CDATA[Pressure mechanism, Mpa > 20-15 15-10 10-5 5-1 1-0.4 Output power, Kw 2.8 3.9 6.5 13.7 8.4 35.3

[0041] Through preliminary calculation and comparison, it can be concluded that the total output power of multi-stage expansion is greater than that of single-stage expansion, that is, multi-stage expansion recovers more energy.

[0042] The present invention adopts a complete CNG pressure energy recovery power generation system to recover the energy during the pressure reduction process, convert it into mechanical energy, drive the generator, and thus convert the pressure energy into electrical energy. The present invention uses an expander to expand the high-pressure natural gas to reduce the pressure, and at the same time output mechanical work, replacing the traditional pressure regulator pressure reduction process, effectively recovering the pressure energy and reducing the power consumption of the system. Multiple expanders are arranged coaxially to jointly drive a generator, reducing the number of devices and the complexity of the system. Between different expanders, a one-way coupling is used. When the high-pressure expander works normally, it outputs mechanical work to drive the generator. When it cannot work due to low pressure, the high-pressure expander does not affect the normal operation of the low-pressure stage expander. When working normally, the gas paths of the high-pressure and low-pressure stage expanders adopt a series connection process. When the pressure decreases and the high-pressure stage expander cannot work and the outlet cannot discharge gas, the automatic opening and closing valve in the system directly inputs the natural gas in the CNG storage tank into the lower-stage expander to ensure the continuous operation of the system.

[0043] Different from the traditional multi-stage expansion power generation where one expander drives one generator, to reduce the number of devices and the complexity of the system, the present invention uses multiple expanders to coaxially drive one generator. When the pressure decreases and the high-pressure stage expander does not work, the one-way coupling will automatically disconnect from the drive shaft, allowing the low-pressure stage expander to continue working uninterruptedly and ensuring the continuity of the system operation. While recovering the mechanical energy from the high-pressure natural gas, the present invention also implements cascade utilization of different pressure energies, improving the efficiency of energy recovery. Since the pressure at the natural gas source continuously decreases, when the high-pressure stage expander stops working, it does not affect the continuous operation of the system, improving the stability of the system. Multiple expanders coaxially drive the generator, reducing the number of devices and the complexity of the system.

[0044] Embodiment 2

[0045] A differential pressure power generation system for a CNG gas supply station, as Figure 1 shown, the specific working process includes:

[0046] ① Process description (taking 3-stage expansion as an example)

[0047] The high-pressure natural gas (20 Mpa) released from the CNG tanker first enters the heater. Since after the high-pressure gas passes through the expander, while the pressure decreases, the temperature also drops sharply. If the temperature is too low, liquid droplets will appear, which will damage the impeller of the expander; at the same time, the low temperature will also reduce the strength of metal equipment such as pipelines and valves, creating potential safety hazards. Therefore, before the high-pressure natural gas enters the expander, it needs to be heated to increase the temperature so that the temperature of the expanded gas also increases accordingly.

[0048] Under normal circumstances, the heated high-pressure gas first enters the first-stage expander, then enters the second stage from the outlet of the first-stage expander, and so on until the outlet of the third-stage expander. The pressure reaches the requirements of the pressure regulator and enters the downstream users.

[0049] ② High-pressure working condition (>10 Mpa)

[0050] When the CNG tanker is in the initial unloading stage, the pressure in the tank is relatively high, greater than 10 Mpa. At this time, the high-pressure natural gas enters the first-stage expander through the check valve 1. The natural gas after the first-stage expansion enters the second-stage expander through the check valve 2. In turn, the expanded natural gas enters the third-stage expander through the check valve 3. The three expanders work together to drive the generator to generate electric energy.

[0051] In the high-pressure working condition, under the control of the PLC, the automatic valves 4 and 5 are both in the closed state. The inlet gas of the lower-stage expander can only be the outlet gas of the upper-stage expander.

[0052] ③ Medium-pressure working condition (10 - 1 Mpa)

[0053] As the CNG continues to be unloaded, the pressure of the CNG tanker continuously decreases. When the pressure is lower than the set value of the system, the automatic control valve 4 automatically opens, and the natural gas in the CNG tanker directly enters the second-stage expander through the valve 4. At this time, the automatic valve 5 in front of the inlet of the third-stage expander is in the closed state, and the inlet gas of the third-stage expander still passes through the check valve 3.

[0054] For the first-stage expander, due to the opening of the automatic valve 4, the back pressure after the check valve 2 is relatively high, causing the check valve 2 to be in the automatically closed state. At this time, no natural gas passes through the first-stage expander, that is, there is no energy input, and the expander is in an idling state with the speed continuously decreasing. When the speed of the first-stage expander is lower than the speed of the main shaft, the single universal coupling disconnects the first-stage expander from the main shaft, that is, the first-stage expander does not affect the operation of the entire system at this time.

[0055] ④ Low-pressure working condition (<1 Mpa)

[0056] As the pressure of the CNG tanker continues to decrease and reaches the set value of the automatic valve 5, the valve 5 automatically opens, and at the same time the valve 4 automatically closes. At this time, the check valve 3 cannot be opened due to the large back pressure, so that there is no air flow through the second-stage expander, and the speed gradually decreases. The single universal coupling disconnects the second-stage expander from the main shaft. At this time, only the third-stage expander works to drive the generator to generate electric energy.

[0057] ⑤ Re-pressurization working condition

[0058] When the pressure of the CNG tanker continuously decreases and reaches 0.4 Mpa required by the downstream pressure regulator, a new CNG tanker with full pressure needs to be connected to the system.

[0059] When the inlet pressure is detected to be too high, the automatic valve 5 is automatically closed to protect the expander from damage by excessive pressure. The high-pressure natural gas from the CNG tank truck enters the secondary expander from the automatically opened valve 4, the secondary expander starts, and the natural gas discharged from the outlet of the secondary expander enters the tertiary expander through the one-way valve 3, and the tertiary expander continues to operate normally.

[0060] As the pressure rises rapidly and reaches the set value of automatic valve 4, valve 4 closes, and the high-pressure natural gas from the CNG tank truck enters the first-stage expander through the one-way valve 1. The first-stage expander starts, and the natural gas discharged from the outlet of the first-stage expander enters the second-stage expander through the one-way valve 2. The second-stage expander continues to operate normally.

[0061] The second-stage expander and the first-stage expander start up, and after gradually reaching the rated operating conditions, the mechanical work output by the expander is added to the main shaft through the coupling, and together they drive the generator to generate electrical energy.

[0062] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A pressure difference power generation system for a CNG gas supply station, characterized in that: The following steps are involved: S1. Reduce the expansion pressure of natural gas: The device uses an expander to expand high-pressure natural gas and reduce its pressure, while outputting mechanical work, replacing the traditional pressure regulator pressure reduction process, effectively recovering pressure energy and reducing the power consumption of the system; S2. Reduce system complexity: During the transportation process, the device uses multiple expanders arranged coaxially to jointly drive a generator, which reduces the number of equipment and the complexity of the system; S3. Stable use of expanders: One-way couplings are used between different expanders, so that when the high-pressure expander works normally, it outputs mechanical work to drive the generator. When it cannot work due to low pressure, the high-pressure expander does not affect the normal operation of the low-pressure expander. S4. Ensure system stability: During normal operation, the gas circuits of the high- and low-pressure expanders are connected in series. When the pressure drops, the high-pressure expander cannot work and the outlet cannot discharge gas, the automatic opening and closing valve in the system directly inputs the natural gas in the CNG storage tank into the lower-level expander to ensure continuous operation of the system.

2. A CNG gas supply station pressure difference power generation system according to claim 1, characterized in that: The power generation system in S1 will use multi-stage expansion power generation. Since the pressure needs to be reduced from 20Mpa to 0.2-0.4Mpa, the pressure drop is too large. The existing expander technology cannot achieve single-stage expansion and pressure reduction. Moreover, when the pressure drop is too large, the expansion efficiency is theoretically low, and the recovered mechanical work becomes less, that is, the power generation will be reduced.

3. A CNG gas supply station pressure difference power generation system according to claim 2, characterized in that: In S2, multiple expanders are controlled jointly so that they can stably control a single generator for use, thereby reducing the overall operating difficulty of the device, improving the convenience of using the device, and greatly reducing the actual operating problems of the device as a whole during the natural gas supply process.

4. A CNG gas supply station pressure difference power generation system according to claim 3, characterized in that: When multi-stage expansion is adopted in S3, the deactivation of the high-pressure expander does not affect the operation of the low-pressure expander, thus ensuring the continuity of power generation; The pressure characteristic of the CNG gas supply station is that as the gas usage time increases, the pressure in the CNG tank truck will gradually decrease, while the expander has certain requirements for the inlet pressure. Low pressure will not allow the expander to rotate. When the speed of the high-pressure expander decreases or does not work, it will not affect the operation of the low-pressure expander.

5. A CNG gas supply station pressure difference power generation system according to claim 4, characterized in that: When the pressure-stage expander in S4 is not working, the gas pressure at the expander outlet will not be able to meet the pressure at the inlet of the lower-stage expander. The method of automatically opening and closing valves is used to ensure the inlet pressure of the lower-stage expander.

6. A CNG gas supply station pressure difference power generation system according to claim 5, characterized in that: The expansion machine calculation formula is W = ΔHi ×G ΔHi =C P ×T1×[1-(P2 / P1) (K-1) / K ] Wherein, W is the output power of the expander, ΔHi is the adiabatic enthalpy drop between the natural gas high-pressure station and the low-pressure end, G is the natural gas mass flow rate, Cp is the average isobaric specific heat, T1 is the natural gas inlet temperature, P1 is the natural gas inlet pressure, P2 is the natural gas outlet pressure, and K is the adiabatic index of natural gas.

7. A CNG gas supply station pressure difference power generation system according to claim 6, characterized in that: The comparative calculation divides the expansion process into single-stage and multi-stage, and compares the calculation results (gas flow rate 1kg / s); After preliminary calculation and comparison, it can be concluded that the total power output of multi-stage expansion is greater than that of single-stage expansion, that is, multi-stage expansion recovers more energy.

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