Method and system for controlling hydrogen concentration of natural gas in gas pipelines

Abstract

The present invention discloses a method for controlling the hydrogen blending concentration in a gas pipeline, which includes the steps of obtaining the flow rate Q1 of the natural gas input into the gas pipeline, calculating the flow rate Q2 of the blended hydrogen according to the set value x1 of the hydrogen blending concentration, i.e., Q2 = Q1 * x1, calculating the opening value η0 of the first hydrogen flow rate regulating valve according to the flow rate Q2, driving the first hydrogen flow rate regulating valve to the opening value η0, obtaining the measured hydrogen blending concentration x of the gas pipeline, calculating and adjusting the first hydrogen flow rate regulating valve to the opening value η1 according to the difference (x - x1) to maintain the hydrogen flow rate tending to the flow rate Q2. When x < x1, increase the opening η of the first hydrogen flow rate regulating valve; when x > x1, decrease the opening η of the first hydrogen flow rate regulating valve. When the natural gas flow rate Q1 changes, recalculate the flow rate Q2 of the blended hydrogen, i.e., Q2 = Q1 * x1.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas transmission, and particularly relates to a method and a control system for controlling the hydrogen blending concentration of natural gas in a gas pipeline. Background Art

[0002] A hydrogen regulating valve is used to control the hydrogen blending concentration of natural gas in a gas pipeline. The principle is that natural gas is used as the main gas source, and hydrogen is injected into natural gas as the blending gas. Since hydrogen is chemically active, there is a risk of hydrogen corrosion to the existing gas carbon steel pipeline when the concentration is high. At the same time, the downstream cookers also require that the hydrogen component of the hydrogen-blended natural gas be within a certain range, and the Wobbe number of the mixed gas meets the requirements for stable combustion of the gas appliance, without deflagration or flashback.

[0003] To ensure the long-term safe operation of the gas supply system, it is necessary to control the hydrogen concentration within the range allowed by the corresponding gas pipeline (currently required to be 0 - 20%). The commonly used method in the industry is to control the hydrogen blending concentration by the feedback method of the hydrogen regulating valve, that is, generally taking the control signal given by the downstream gas chromatograph analyzer to control the opening degree of the hydrogen regulating valve, so as to ensure that the concentration of hydrogen blended in natural gas is within the set range. Summary of the Invention

[0004] One embodiment of the present invention, a method for controlling the hydrogen blending concentration of natural gas in a gas pipeline, includes the steps of

[0005] Obtaining the flow rate Q1 of the input natural gas in the gas pipeline,

[0006] Calculating the flow rate Q2 of the blended hydrogen according to the set value x1 of the hydrogen blending concentration, Q2 = Q1 * x1,

[0007] Calculating the opening value η0 of the first hydrogen flow regulating valve according to the flow rate Q2,

[0008] Driving the first hydrogen flow regulating valve to the opening value of η0,

[0009] Obtaining the measured hydrogen blending concentration x of the gas pipeline,

[0010] Calculating and adjusting the first hydrogen flow regulating valve to the opening value η1 according to the difference (x - x1) to maintain the hydrogen flow rate tending to the flow rate Q2.

[0011] When x < x1, increasing the opening η of the first hydrogen flow regulating valve,

[0012] When x > x1, decreasing the opening η of the first hydrogen flow regulating valve. Brief Description of the Drawings

[0013] The above and other objects, features and advantages of the exemplary embodiments of the present invention will become readily apparent by reading the following detailed description with reference to the accompanying drawings, in which several embodiments of the present invention are shown by way of example and not limitation, in which:

[0014] Figure 1 Schematic diagram of a natural gas hydrogen concentration control system in a gas pipeline according to one embodiment of the present invention.

[0015] Figure 2 Schematic diagram of a natural gas hydrogen concentration control system in a gas pipeline according to one embodiment of the present invention.

[0016] 10——Natural gas flow regulating valve,

[0017] 20——the first hydrogen flow regulating valve,

[0018] 30——Second hydrogen flow regulating valve,

[0019] 40——Natural gas flow meter,

[0020] 50——Hydrogen flow meter. DETAILED DESCRIPTION

[0021] The main problem with existing control schemes is that, while the control logic is simple, the control efficiency is low. Gas chromatographs provide relatively accurate measurement data, but the sampling cycle is long, typically 3-5 minutes. Controlling hydrogen concentration based on the final detection results from the gas chromatograph results in a long time to reach the control target. Considering the safety requirements for the upper limit of hydrogen concentration, if the downstream flow demand is stable, the final control result is that the valve opening is close to the set point for at least 5-10 minutes. However, if the demand flow is unstable and changes rapidly, it is easy for the hydrogen concentration in natural gas to exceed the standard during certain periods, which can easily lead to unstable combustion and pose certain risks to the pipeline network and downstream gas stoves.

[0022] According to one or more embodiments, a method for controlling the hydrogen concentration of natural gas in a gas pipeline includes the following steps:

[0023] Get the flow rate Q1 of natural gas input from the gas pipeline,

[0024] According to the set value x1 of hydrogen mixing concentration, the flow rate of mixed hydrogen is calculated as Q2=Q1*x1.

[0025] Calculate the opening value η0 of the first hydrogen flow regulating valve according to the flow Q2,

[0026] Drive the first hydrogen flow regulating valve to an opening value of η0,

[0027] Get the measured hydrogen concentration x of the gas pipeline,

[0028] Calculate and adjust the first hydrogen flow regulating valve to an opening value η1 according to the difference (x - x1) to maintain the hydrogen flow tending to the flow rate Q2.

[0029] When x < x1, increase the opening η of the first hydrogen flow regulating valve.

[0030] When x > x1, decrease the opening η of the first hydrogen flow regulating valve.

[0031] When the natural gas flow rate Q1 changes, recalculate the hydrogen-blended flow rate Q2 = Q1 * x1.

[0032] The natural gas hydrogen blending scheme of the embodiments of the present disclosure adopts a follow-up type flow mixing process. Taking one natural gas source as the main gas source, another hydrogen source changes following the change of the main gas source according to a preset volume ratio. Set the gas source with a larger flow rate as the main gas source and the gas source with a smaller flow rate as the follow-up gas source, and control the opening of the regulating valve on the follow-up gas source pipeline to respond quickly to the change of the main gas source in real time.

[0033] As Figure 1 shown, the control method of the present disclosure is to control the opening of the hydrogen regulating valve to keep the hydrogen concentration after natural gas hydrogen blending within the set range x1 at the maximum. The specific control process can be:

[0034] 1. First, obtain the downstream customer demand Q1 through the natural gas flowmeter, calculate the hydrogen-blended flow rate Q2 according to the set value x1 of the blending concentration, and obtain the opening value η0 of the hydrogen regulating valve through the hydrogen valve coefficient calculation in combination with their respective corresponding operating pressures and temperatures, so as to obtain the driving signal of the initial opening position of the main control signal.

[0035] 2. During the opening process, continuously feedback the actual opening η of the valve until the actual opening reaches the calculated opening value η0. At this time, start to compare the concentration data x of the hydrogen-blended natural gas actually measured by the densitometer with the set concentration signal of x1. If it is lower, increase the opening η of the hydrogen valve; if it is higher, decrease the opening η of the hydrogen valve; if they are the same, keep the opening η1 of the hydrogen valve, and the step change opening value η MIN is determined by the minimum step value of the corresponding hydrogen regulating valve. Here, x and x1 can be expressed as percentages.

[0036] 3. If the natural gas flow rate Q1 remains unchanged for a period of time, this process continues to maintain. If the change range of the natural gas flow rate exceeds ±1%, this process restarts until a stable state Q1 is reached.

[0037] Because the servo control is performed using a forward and backward feedback method, under the premise of stable natural gas and hydrogen inlet pressures, the forward feedback calculates the instantaneous flow meter signals of the main gas source (natural gas) and the servo gas source (hydrogen), and outputs the main control signal for the control valve. The control valve first quickly reaches the required final control valve position based on this main control signal, which can quickly respond to changes in flow and shorten response time. The backward feedback measures the density change of the mixed gas before and after blending, and the control system main controller CPU calculates the actual concentration of the blended hydrogen, providing a precision control signal to the control valve in seconds, improving control accuracy. Through the combination of the above feedforward and backward feedback, the overall control efficiency and control accuracy of the hydrogen control valve in the hydrogen blending process are maximized.

[0038] Therefore, the disclosed embodiments improve the control efficiency of the follow-up hydrogen regulating valve in the hydrogen blending process, increase the response speed of the hydrogen blending process, and meet the requirements for safe operation of gas pipelines and downstream stoves, achieving a fast and efficient level. The biggest feature of the control method disclosed in this disclosure is that it combines the advantages of two different control methods. The feedforward signal solves the problem of the response speed of the regulating valve, targeting the main shortcomings of the original control method, such as low valve efficiency and long opening cycle. First, a rough adjustment is performed. The valve opening is determined based on the measured flow rate of natural gas and hydrogen. The actual control signal is given by the station control system, which can significantly improve the response speed of the equipment and improve the control efficiency of the natural gas hydrogen blending equipment station control. The feedback signal solves the problem of control accuracy. The final control result is quickly obtained based on the actual measured physical quantity calculation. Near the equilibrium position, the control upper limit set by the natural gas hydrogen concentration is continuously corrected. While improving the control speed, it can also improve the final control accuracy of the hydrogen blending equipment system and ensure the overall control accuracy of the pipeline gas hydrogen blending system. The biggest benefit of this method is that it shortens the control cycle from the original minute-level control cycle (3-5 minutes) to the second-level control cycle (5-10 seconds), improving control efficiency.

[0039] According to one or more embodiments, a natural gas hydrogen concentration control system for a gas pipeline includes a main controller, a natural gas flow regulating valve, a natural gas flow meter, a first hydrogen flow regulating valve, and a hydrogen-blended hydrogen flow meter that are communicatively connected to the main controller.

[0040] A natural gas flow control valve and a natural gas flow meter are located before the hydrogen mixing point in the natural gas input section of the gas pipeline. A first hydrogen flow control valve and a hydrogen-blended flow meter are also located before the hydrogen mixing point in the hydrogen input section of the gas pipeline. A main controller obtains the natural gas flow rate Q1 of the gas pipeline input from the natural gas flow meter and calculates the hydrogen-blended flow rate Q2 = Q1 * x1 based on the set value x1 of the hydrogen blending concentration. The main controller calculates the opening value η0 of the first hydrogen flow control valve based on the flow rate Q2 and drives the first hydrogen flow control valve to the opening value η0. The main controller obtains the measured hydrogen blending concentration x in the gas pipeline and calculates and adjusts the first hydrogen flow control valve to the opening value η1 based on the difference (x - x1) to maintain the hydrogen flow rate toward the flow rate Q2.

[0041] Furthermore, the flow rate Q1 of the natural gas is obtained by a natural gas flow meter installed in the natural gas pipeline, and the flow rate Q2 = Q1*x1 of the mixed hydrogen is calculated according to the set value x1 of the hydrogen mixing concentration.

[0042] Compare the sampling speeds of the measured hydrogen blending concentration x and the sampling speeds of the natural gas flowmeter flow rate Q1. If the sampling speed of the measured hydrogen blending concentration x is faster, then: calculate and adjust the first hydrogen flow control valve to the opening value η1 based on the difference (x-x1), and then correct the opening of the first hydrogen flow control valve based on the natural gas flowmeter flow rate Q1. If the sampling speed of the natural gas flowmeter flow rate Q1 is faster, then calculate and adjust the first hydrogen flow control valve to the opening value η0 based on the flow rate Q2=Q1*x1, and then correct the opening of the first hydrogen flow control valve based on the measured hydrogen blending concentration x.

[0043] Compare the sampling speed of the measured hydrogen doping concentration x and the sampling speed of the hydrogen doping flowmeter. If the sampling speed of the measured hydrogen doping concentration x is faster, then: calculate and adjust the first hydrogen flow control valve to the opening value η1 based on the difference (x-x1), and then correct the opening of the first hydrogen flow control valve based on the flow rate Q1 of the natural gas flowmeter; if the sampling speed of the hydrogen doping flowmeter is faster, then record the output value of the hydrogen doping flowmeter as the flow rate Q2, adjust the first hydrogen flow control valve to the opening value η1, and then correct the opening of the first hydrogen flow control valve based on the flow rate Q1 of the natural gas flowmeter.

[0044] In the embodiment of the present disclosure, a SAMSON 3241-1DN15 pneumatic control valve can be selected as the hydrogen control valve, and a high-precision SAMSON 3731-3 pneumatic valve positioner can be used to pneumatically control the valve opening. The system provides a matching nitrogen meter, and the control system can accurately control the valve opening and provide real-time feedback on the actual opening value. Figure 1As shown, a second hydrogen pipeline can be used as a backup, with a second hydrogen flow control valve regulating the hydrogen flow. With two hydrogen control valves, one for operation and one for backup, if the primary line fails, the secondary control valve automatically opens and closes the primary control valve, improving the reliability of hydrogen blending control throughout the system.

[0045] Compared with the self-control of the regulating valve with a single control logic, the control method of the embodiment of the present disclosure is relatively complex in terms of program. It has two signal control logics: feedforward and feedback. It is distinguished by the control priority, that is, efficiency is controlled first and accuracy is controlled later. This method is very consistent with the actual needs of pipeline gas hydrogen blending equipment. At the same time, the two signal sources themselves also shorten the signal sampling period as much as possible and change the detection method from the original minute level to the current second level. Only in this way can the purpose of improving the overall control efficiency be truly achieved and the risk of excessive hydrogen concentration caused by rapid changes in downstream flow can be reduced.

[0046] If the control system's main controller CPU calculates the actual hydrogen concentration from the physical density quickly enough, the actual control method can also directly control the actual opening of the hydrogen regulating valve based on this calculated data, supplemented by data verification of the valve opening calculated using metering data. That is, the feedback signal is used for positioning first, and the feedforward data is used for safety review. This can also improve the control speed and safety level of the entire hydrogen blending station control. In other words, the feedforward and feedforward signals can be switched freely, and the control system determines which is the feedforward signal and which is the feedforward signal based on the actual data acquisition speed.

[0047] According to one or more embodiments, a natural gas hydrogen concentration control system for a gas pipeline includes a main controller, a natural gas flow control valve, a natural gas flow meter, a first hydrogen flow control valve, and a hydrogen-doped hydrogen flow meter, all of which are communicatively connected to the main controller. The control system also includes a first gas density meter and a second gas density meter connected to the main controller.

[0048] A first gas density meter is provided at a sampling location in the gas pipeline before natural gas is mixed with hydrogen, and a second gas density meter is provided at a sampling location in the gas pipeline after natural gas is mixed with hydrogen. The main controller obtains the density ρ1 from the first gas density meter and the density ρ2 from the second gas density meter.

[0049] Assume that the density of hydrogen is ρ H , then the main controller calculates the concentration of hydrogen after natural gas is mixed with hydrogen as x, x=(ρ1-ρ2) / (ρ1-ρ H The gas density meter here can be Emerson Micro Motion's gas density meter GDM5AAAC2Z1MZZZ.

[0050] Furthermore, a static mixer is provided at the point where natural gas is mixed with hydrogen in the gas pipeline. The natural gas and hydrogen are fed into the static mixer and mixed within the static mixer to produce a mixed gas. A gas chromatograph is also provided at the sampling point after the natural gas is mixed with hydrogen in the gas pipeline. The output of the gas chromatograph is input into the main controller.

[0051] like Figure 2 As shown, the natural gas hydrogen blending concentration control system for the gas pipeline is designed with two different mixing methods: one with a static mixer and one without a static mixer. This is designed to test the effectiveness of the static mixer. Sampling is performed using two high-precision gas densitometers, one at the front and one at the back, to obtain corresponding blending operation data. The latter gas density also takes into account test positions at different pipeline heights. The sampling tube can be inserted at different depths within the pipeline to compare the differences in test results and verify whether gas stratification occurs after blending. By comparing data from multiple selected points, the reliability of the test method for calculating the hydrogen concentration after blending is verified. The system has a reserved interface at the outlet to accommodate the addition of a traditional gas chromatograph for comparison.

[0052] Taking 1 standard atmospheric pressure and ambient temperature of 20℃ as an example, let the density of natural gas before blending be ρ1 and the density of natural gas after blending be ρ2. According to the density table of hydrogen, the density of hydrogen at 20℃ is 0.082658×10 3 kg / Nm 3 , the molar concentration of hydrogen after natural gas is added with hydrogen is set to x, and the corresponding formula is ρ1(1-x)+0.082658x=ρ2, the density of natural gas decreases after adding hydrogen, ρ 2〈 ρ1

[0053] Then x=(ρ1-ρ2) / (ρ1-0.082658)……Formula (1)

[0054] Considering that the density of hydrogen also changes with the ambient temperature, let the hydrogen density be ρ H

[0055] Then formula (1) is changed to x=(ρ1-ρ2) / (ρ1-ρ H )...Formula (2)

[0056] where ρ H It is planned to use a hydrogen density database, which can automatically match relevant data based on the actual temperature data measured on site, improve measurement accuracy, and reduce calculation errors.

[0057] The embodiments of the present disclosure adopt Indirect measurement The method is to determine the concentration of hydrogen in natural gas pipeline gas. The specific method is to use a high-precision instrument Rapid measurement of gas physical quantitiesor physical property changes, combined with computer technology, through high-performance microprocessor calculation programs to Calculating the hydrogen concentration in pipeline natural gas This shortens the hydrogen concentration measurement cycle and improves the safety of pipeline gas hydrogen blending. Considering that pipeline natural gas is not a stable mixture of components, the composition of natural gas remains relatively stable under normal temperature conditions, with total mass and molar mass conserved, and no chemical reactions occurring, except for the addition of hydrogen before and after hydrogen blending. For example, by directly measuring the density change of hydrogen-blended natural gas and combining it with a local high-performance microprocessor, the actual concentration of hydrogen in the natural gas can be quickly calculated.

[0058] Through the above method, based on the feedback time of the gas density meter itself, the measurement speed of the hydrogen concentration of natural gas can be greatly improved. Theoretically, the feedback time can be achieved in seconds, which improves the automatic control efficiency of the hydrogen blending process, reduces the probability of excessive hydrogen concentration in the natural gas hydrogen blending process, and reduces the safety risk of operating pipelines.

[0059] The method for measuring the hydrogen concentration of natural gas in a gas pipeline according to the embodiment of the present disclosure can quickly provide control efficiency of the hydrogen blending process. The measurement cycle is reduced from no less than 2 minutes (60 seconds) to seconds, providing accurate gas density values, greatly improving the safety and reliability of the gas pipeline hydrogen blending process, and ensuring the feasibility of hydrogen transportation in existing gas pipelines.

[0060] In summary, the beneficial effects of the present disclosure include:

[0061] (1) First, the control system calculates the initial opening of the regulating valve based on the measured flow value, and then drives the hydrogen regulating valve to open quickly and directly to the initial opening position. This method is different from the slow opening method of the conventional regulating valve. The main purpose is to improve the efficiency of control implementation, reduce the actual difference between the actual hydrogen addition amount and the set value during the hydrogen blending process, and improve the utilization rate of hydrogen energy. Then, a more reliable signal such as the actual measured value of the hydrogen concentration of the control system is used to make a high-precision correction to the opening. Significantly improving the control efficiency is one of the innovations of this method.

[0062] (2) The main idea of ​​the control method disclosed in this disclosure is to use feedforward for coarse adjustment and feedback for fine adjustment, and to closely integrate the control cycle by presetting the priority level. After the main control valve is fully opened, it notifies the upper system and automatically switches to the secondary control state. The close integration of the two methods can improve the control speed without reducing the control accuracy. The rapid and deep integration of the two controls can ultimately achieve the purpose of improving the control efficiency of the hydrogen regulating valve.

[0063] (3) The control system disclosed in the present invention is a breakthrough in traditional control methods. It uses two different data signals to adjust a control variable. During this process, the station control system can optimize the signal based on the acquisition speed of the data signal. The signal with a faster acquisition speed is used as the feedforward signal, and the signal with a slower acquisition speed is used as the feedback signal for review or high-precision correction, thereby enabling flexible selection of the signal source.

[0064] (4) The control system of the present disclosure selects the signal source based on the principle of higher control efficiency, that is, the priority is determined by the speed of signal acquisition. In other words, the signal calculated by the flow meter signal acquisition is compared with the station control signal calculated by the density meter. The signal with the faster acquisition speed is used as the main signal for feedforward, and the other is used as the verification signal or correction signal for feedback, thereby better improving the control efficiency and safety level of the hydrogen blending equipment.

[0065] It is worth noting that although the foregoing content has described the spirit and principles of the present invention with reference to several specific embodiments, it should be understood that the present invention is not limited to the disclosed specific embodiments, and the division into various aspects does not mean that the features of these aspects cannot be combined. Such division is merely for the convenience of expression. The present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A method for controlling the concentration of hydrogen blended natural gas in a gas pipeline, characterized in that: The steps include obtaining the flow rate Q1 of the natural gas input into the gas pipeline. Calculate the flow rate Q2 of the hydrogen to be blended as Q2 = Q1 * x1 according to the set value x1 of the hydrogen blending concentration, and calculate the opening value η0 of the first hydrogen flow rate regulating valve according to the flow rate Q2. Drive the first hydrogen flow rate regulating valve to an opening value of η0. Obtain the measured hydrogen blending concentration x of the gas pipeline. Calculate and adjust the first hydrogen flow rate regulating valve to an opening value η1 according to the difference (x - x1) to maintain the hydrogen flow rate tending to the flow rate Q2. Obtain the flow rate Q1 of the natural gas through the natural gas flow meter installed in the natural gas pipeline, and calculate the flow rate Q2 of the hydrogen to be blended as Q2 = Q1 * x1 according to the set value x1 of the hydrogen blending concentration. Compare the sampling speeds of the measured hydrogen blending concentration x and the flow rate Q1 of the natural gas flow meter. If the sampling speed of the measured hydrogen blending concentration x is faster, then: calculate and adjust the first hydrogen flow rate regulating valve to an opening value η1 according to the difference (x - x1), and then correct the opening of the first hydrogen flow rate regulating valve according to the flow rate Q1 of the natural gas flow meter. If the sampling speed of the flow rate Q1 of the natural gas flow meter is faster, then: calculate and adjust the first hydrogen flow rate regulating valve to an opening value η0 according to the flow rate Q2 = Q1 * x1, and then correct the opening of the first hydrogen flow rate regulating valve according to the measured hydrogen blending concentration x.

2. The control method according to claim 1, characterized in that: Compare the sampling speeds of the measured hydrogen blending concentration x and the sampling speed of the hydrogen flow meter for blending. If the sampling speed of the measured hydrogen blending concentration x is faster, then: calculate and adjust the first hydrogen flow rate regulating valve to an opening value η1 according to the difference (x - x1), and then correct the opening of the first hydrogen flow rate regulating valve according to the flow rate Q1 of the natural gas flow meter. If the sampling speed of the hydrogen flow meter for blending is faster, then record the output value of the hydrogen flow meter for blending as the flow rate Q2, adjust the first hydrogen flow rate regulating valve to an opening value η1, and then correct the opening of the first hydrogen flow rate regulating valve according to the flow rate Q1 of the natural gas flow meter.

3. The control method according to claim 1, wherein when x < x1, increase the opening η of the first hydrogen flow rate regulating valve. when x > x1, decrease the opening η of the first hydrogen flow rate regulating valve.

4. The control method according to claim 1, wherein when the natural gas flow rate Q1 changes, recalculate the flow rate Q2 of the hydrogen to be blended as Q2 = Q1 * x1.

5. The control method according to claim 1, characterized in that: The process of obtaining the measured hydrogen blending concentration x of the gas pipeline includes obtaining the density ρ1 of the natural gas before hydrogen blending in the gas pipeline; obtaining the density ρ2 of the natural gas after hydrogen blending in the gas pipeline; Assume that the density of hydrogen is ρ H , the concentration of hydrogen after hydrogen blending in the natural gas is x. x=(ρ1-ρ2) / (ρ1-ρ H )。 6. A natural gas hydrogen concentration control system for a gas pipeline, characterized in that: The control system includes a main controller, and a natural gas flow regulating valve, a natural gas flow meter, a first hydrogen flow regulating valve, and a hydrogen flow meter for blending that are communicatively connected to the main controller. The natural gas flow regulating valve and the natural gas flow meter are arranged at positions before the hydrogen blending point in the natural gas input section of the gas pipeline. The first hydrogen flow regulating valve and the hydrogen flow meter for blending are arranged at positions before the hydrogen blending point in the hydrogen input section of the gas pipeline. The main controller executes the control method according to any one of claims 1 to 4.

7. The control system according to claim 6, characterized in that: The control system also includes a first gas density meter and a second gas density meter connected to the main controller. The first gas density meter is set at the sampling position before the natural gas is mixed with hydrogen in the gas pipeline, and the second gas density meter is set at the sampling position after the natural gas is mixed with hydrogen in the gas pipeline. The main controller obtains density ρ1 from the first gas density meter and density ρ2 from the second gas density meter. Assume that the density of hydrogen is ρ H , Then the main controller calculates the concentration of hydrogen after natural gas is mixed with hydrogen as x, x=(ρ1-ρ2) / (ρ1-ρ H )。 8. The control system according to claim 6, characterized in that: A static mixer is provided at the gas pipeline where natural gas is mixed with hydrogen. The input natural gas and hydrogen are input into the static mixer and mixed in the static mixer to obtain a mixed gas.

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