Carbon dioxide hydrogenation system for synthesizing methane adaptive to hydrogen source fluctuation

Abstract

The application discloses a carbon dioxide hydrogenation system for synthesizing methane, which is suitable for hydrogen source fluctuation, and solves the technical problems of the existing methanation reaction system, such as large equipment investment, large floor area and potential safety hazards caused by the hydrogen storage buffer device for suppressing hydrogen source fluctuation. The system comprises a raw material supply unit, a preheating unit, a reaction unit, a waste heat recovery unit, a cooling and separation unit, a circulation unit and a control unit. The hydrogen feeding pipeline of the raw material supply unit is provided with a flow detection device. The raw material preheater of the preheating unit performs primary preheating on the mixed raw material gas, and an electric heater performs secondary heat supplement. The reaction unit comprises a primary adiabatic reactor and a secondary adiabatic reactor. The waste heat recovery unit comprises a primary steam generator and a secondary steam generator. The total outlet pipeline connected with the gas phase outlet of the water separator of the cooling and separation unit is divided into an external delivery pipeline and a circulation pipeline. The control unit takes fresh hydrogen flow as a feedforward variable to perform collaborative adjustment.

Description

Technical Field

[0001] This invention relates to the field of carbon capture and utilization and renewable energy consumption technology, specifically to a carbon dioxide hydrogenation to methane synthesis system adapted to fluctuations in hydrogen source. Background Technology

[0002] The carbon dioxide hydrogenation to methane technology reacts industrially captured carbon dioxide with hydrogen in the presence of a catalyst to produce methane, thus both absorbing carbon dioxide and producing clean fuel. When the hydrogen is derived from renewable energy sources such as water electrolysis, this technology can also convert intermittent renewable energy sources like wind and solar power into chemical energy that is easy to store and transport, achieving an electro-gas coupled energy conversion.

[0003] Existing methanation reaction systems typically include a feedstock supply unit, a preheating unit, a reaction unit, a waste heat recovery unit, and a cooling and separation unit. The feedstock supply unit mixes carbon dioxide and hydrogen to form a mixed feedstock gas. The preheating unit preheats the mixed feedstock gas using the waste heat from the system's reaction products via a feedstock preheater, and supplements this preheating with an electric heater. The reaction unit includes adiabatic reactors arranged in series, where carbon dioxide and hydrogen react under the action of a catalyst to produce methane and water. The waste heat recovery unit recovers heat from the reaction products and generates steam via a steam generator. The cooling and separation unit cools the reaction products and separates the condensate, obtaining methane-rich gas which is then exported. The feedstock supply unit is equipped with numerous hydrogen storage tanks as a hydrogen storage buffer; hydrogen is first stored in high-pressure hydrogen storage tanks and then output at a stable flow rate.

[0004] Hydrogen storage tanks need to withstand high storage pressures, resulting in large equipment investments, large floor space requirements, and potential safety hazards. Furthermore, the hydrogen storage component increases system complexity and maintenance costs, reducing overall economic efficiency. Summary of the Invention

[0005] The inventors noted that renewable energy power generation has significant volatility and intermittency characteristics. Wind power generation is affected by wind speed and exhibits intraday and seasonal fluctuations, while photovoltaic power generation varies with the intensity of solar radiation. The output of hydrogen produced by water electrolysis also fluctuates. If the carbon dioxide hydrogenation to methane synthesis system itself can be made capable of adapting to fluctuations in hydrogen source, then hydrogen storage buffer devices can be eliminated or simplified, reducing investment and operating costs.

[0006] Therefore, the purpose of this invention is to provide a carbon dioxide hydrogenation to methane synthesis system that adapts to fluctuations in hydrogen source, thereby solving the technical problems of existing methanation reaction systems that rely on hydrogen storage buffer devices to smooth out hydrogen source fluctuations, resulting in large equipment investment, large footprint, and safety hazards.

[0007] This invention provides a carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations, comprising: a raw material supply unit, including a carbon dioxide feed pipeline and a hydrogen feed pipeline, wherein the hydrogen feed pipeline is equipped with a flow detection device; the carbon dioxide feed pipeline and the hydrogen feed pipeline merge to form a main feed pipeline, which is used to transport a mixed raw material gas composed of carbon dioxide, hydrogen, and recirculated gas; and a preheating unit, including a raw material preheater and an electric heater, wherein the raw material preheater has a cold side channel and a hot side channel; the inlet of the cold side channel is connected to the main feed pipeline for introducing the mixed raw material gas, and the outlet of the cold side channel is connected to the inlet of the electric heater for outputting the mixed raw material gas after primary preheating. The inlet of the hot-side channel is used to introduce system reaction products for primary preheating of the mixed feed gas in the cold-side channel. The outlet of the hot-side channel is used to output the system reaction products after heat exchange and cooling. The electric heater is used for secondary reheating of the preheated mixed feed gas, and the outlet of the electric heater is used to output the reheated mixed feed gas. The reaction unit includes a primary adiabatic reactor and a secondary adiabatic reactor. The inlet of the primary adiabatic reactor is connected to the outlet of the electric heater to receive the reheated mixed feed gas. The outlet of the primary adiabatic reactor is used to output the primary reaction product gas. The inlet of the secondary adiabatic reactor is used to receive the primary reaction product gas cooled by the primary steam generator. The outlet of the secondary adiabatic reactor is used to output secondary reaction product gas; the waste heat recovery unit includes a primary steam generator and a secondary steam generator. The material inlet of the primary steam generator is connected to the outlet of the primary adiabatic reactor to receive the primary reaction product gas, and the material outlet of the primary steam generator is connected to the inlet of the secondary adiabatic reactor to output cooled primary reaction product gas. The material inlet of the secondary steam generator is connected to the outlet of the secondary adiabatic reactor to receive the secondary reaction product gas, and the material outlet of the secondary steam generator is connected to the hot-side channel inlet of the raw material preheater to output cooled secondary reaction product gas as the system reaction product; cooling section The separation unit includes a water cooler and a water separator. The inlet of the water cooler is connected to the hot-side channel outlet of the raw material preheater to receive the reaction products after heat exchange and cooling. The outlet of the water cooler is connected to the inlet of the water separator to output the reaction products after deep cooling. The water separator is used to perform gas-liquid separation on the reaction products after deep cooling. The liquid phase outlet of the water separator is used to discharge condensate. The gas phase outlet of the water separator is connected to a main outlet pipeline for outputting methane-rich gas. The main outlet pipeline is divided into an external delivery pipeline and a circulation pipeline. The external delivery pipeline is used to transport methane-rich synthesis gas to the outside and is equipped with a pressure regulating device. The circulation pipeline is used to return a portion of the methane-rich gas as circulating gas.A circulation unit includes a circulation compressor, the inlet of which is connected to the circulation pipeline to receive the circulating gas, and the outlet of which merges with the main feed pipeline to mix the pressurized circulating gas with fresh carbon dioxide and hydrogen. A control unit is connected to the flow detection device, the circulation compressor, and the pressure regulating device. The control unit is configured to use the fresh hydrogen flow rate detected by the flow detection device as a feedforward variable to coordinately regulate the operating frequency of the circulation compressor and the opening of the pressure regulating device, maintaining the gas mass flow rate in the primary and secondary adiabatic reactors within a preset range. When the fresh hydrogen flow rate decreases, the control unit reduces the opening of the pressure regulating device to increase the flow resistance of the external feed pipeline, allowing more methane-rich gas to return through the circulation pipeline, while simultaneously increasing the operating frequency of the circulation compressor to increase the circulating gas flow rate, thereby compensating for the reduction in fresh feed volume. When the fresh hydrogen flow rate increases, the control unit increases the opening of the pressure regulating device and decreases the operating frequency of the circulation compressor to reduce the circulating gas flow rate and increase the external feed ratio.

[0008] As an optimization and / or instantiation of the aforementioned carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations, further: the primary steam generator includes a shell, a heat exchange tube bundle disposed within the shell, and a steam drum communicating with the shell; the heat exchange tube bundle has a tube-side inlet and a tube-side outlet, the tube-side inlet being connected to the outlet of the primary adiabatic reactor for receiving the primary reaction product gas, and the tube-side outlet being connected to the inlet of the secondary adiabatic reactor for outputting the cooled primary reaction product gas; the shell side of the shell is provided with a medium inlet for introducing into the boiler. Boiler feedwater or low-pressure steam is used as the heat exchange medium. After absorbing heat from the first-stage reaction product gas in the tube side, the heat exchange medium enters a boiling state with both vapor and liquid phases coexisting, generating steam. This steam enters the steam drum for steam-water separation. A steam outlet pipeline is located at the top of the steam drum, and a first steam regulating valve is installed on the steam outlet pipeline. Adjusting the opening of the first steam regulating valve controls the steam discharge rate, thereby regulating the steam pressure within the steam drum. The steam pressure within the steam drum determines the saturation temperature of the heat exchange medium in the shell side, which in turn determines the temperature of the first-stage reaction product in the tube side. The heat transfer temperature difference between the gas and the heat exchange medium in the shell side; the control unit is signal-connected to the first steam regulating valve and configured to adjust the opening of the first steam regulating valve according to the inlet temperature detection signal of the secondary adiabatic reactor, so that the inlet temperature of the secondary adiabatic reactor is maintained within the range of 250°C to 280°C; wherein, when the fresh hydrogen flow rate decreases, causing the temperature of the primary reaction product gas to decrease, the control unit increases the opening of the first steam regulating valve to increase the steam discharge, reduce the steam pressure in the steam drum, thereby reducing the saturation temperature of the heat exchange medium, reducing the heat transfer temperature difference, reducing the heat exchange, and maintaining the inlet temperature of the secondary adiabatic reactor within the range of 250°C to 280°C; when the fresh hydrogen flow rate increases, causing the temperature of the primary reaction product gas to increase, the control unit decreases the opening of the first steam regulating valve to reduce the steam discharge, increase the steam pressure in the steam drum, thereby increasing the saturation temperature of the heat exchange medium, maintaining the heat transfer temperature difference, increasing the heat exchange, and maintaining the inlet temperature of the secondary adiabatic reactor within the range of 250°C to 280°C.

[0009] As an optimization and / or instantiation of the aforementioned carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations, further: the structure of the secondary steam generator is the same as that of the primary steam generator, including a shell, a heat exchange tube bundle disposed within the shell, and a steam drum communicating with the shell; the tube-side inlet of the heat exchange tube bundle of the secondary steam generator is connected to the outlet of the secondary adiabatic reactor for receiving the secondary reaction product gas, and the tube-side outlet is connected to the hot-side channel inlet of the feed preheater; a second steam outlet pipeline is provided at the top of the steam drum of the secondary steam generator, and a second steam regulating valve is provided on the second steam outlet pipeline, by adjusting the opening degree of the second steam regulating valve. The steam discharge rate of the secondary steam generator is controlled to regulate the steam pressure in the steam drum and the saturation temperature of the heat exchange medium in the shell side of the secondary steam generator. The steam outlet pipeline of the primary steam generator and the second steam outlet pipeline merge to form a medium-pressure steam main. The medium-pressure steam main is connected to a medium-pressure steam pipeline located between the cold-side channel outlet of the raw material preheater and the inlet of the electric heater, so that the medium-pressure steam generated by the primary steam generator and the secondary steam generator is transported to the mixed raw material gas after primary preheating for supplementary heating. A first pressure detection device and a first flow meter are sequentially installed along the steam flow direction on the steam outlet pipeline of the primary steam generator. The system includes a second pressure detection device and a second flow detection device sequentially installed along the steam flow direction on the second steam outlet pipeline; pressure safety valves are respectively installed on the steam drums of the first-stage steam generator and the second-stage steam generator; the control unit is signal-connected to the second steam regulating valve, the first pressure detection device, the first flow detection device, the second pressure detection device, and the second flow detection device, and is configured to adjust the opening of the second steam regulating valve according to the inlet temperature detection signal of the hot side channel of the raw material preheater; wherein, when the fresh hydrogen flow rate increases and the operating frequency of the circulating compressor decreases, the circulating gas flow rate decreases. When the temperature of the primary and secondary reaction product gases rises, the control unit reduces the opening of the first and second steam regulating valves to increase the steam pressure in each steam drum. At this time, the steam output of the primary and secondary steam generators decreases accordingly, and the amount of supplementary heating steam delivered to the preheated mixed feed gas via the medium-pressure steam main and the medium-pressure steam pipeline also decreases. Simultaneously, the increased flow rate of fresh hydrogen increases the heat released by the reaction, and the heat carried by the reaction products in the hot side channel of the feed preheater increases, thus increasing the primary preheating heat of the mixed feed gas and compensating for the decrease in the amount of supplementary heating steam.When the fresh hydrogen flow rate decreases and the operating frequency of the circulating compressor increases, causing an increase in the circulating gas flow rate, the temperatures of the primary and secondary reaction product gases decrease. The control unit then increases the opening of the first and second steam regulating valves to reduce the steam pressure in each steam drum. At this time, the steam output of the primary and secondary steam generators increases accordingly, and the amount of supplementary heating steam delivered to the preheated mixed feed gas via the medium-pressure steam main and medium-pressure steam pipeline also increases. Simultaneously, because the decrease in the fresh hydrogen flow rate reduces the heat released by the reaction, the heat carried by the reaction products in the hot-side channel of the feed preheater decreases, resulting in a corresponding decrease in the primary preheating heat for the mixed feed gas. The increased supplementary heating steam compensates for the insufficient primary preheating heat.

[0010] As an optimization and / or instantiation of the above-mentioned carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations, further: the ratio of the circulating gas flow rate to the fresh hydrogen flow rate is maintained in the range of 3:1 to 8:1.

[0011] As an optimization and / or instantiation of the above-mentioned carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations, further: the electric heater adopts silicon controlled rectifier (SCR) power regulation control, with a power adjustment range of 10% to 100% of the rated power; the control unit is signal-connected to the electric heater and configured to adjust the power output of the electric heater according to the inlet temperature detection signal of the primary adiabatic reactor, so that the inlet temperature of the primary adiabatic reactor is maintained within the ignition temperature range.

[0012] As an optimization and / or instantiation of the aforementioned carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations, further: the cooling separation unit is also provided with a nitrogen interface, which is connected to the circulation pipeline; the control unit is configured to switch the circulation pipeline to nitrogen circulation mode after receiving a shutdown command, introduce nitrogen through the nitrogen interface, and use the water cooler to reduce the bed temperature of the primary adiabatic reactor and the secondary adiabatic reactor to below 100°C at a set cooling rate.

[0013] As an optimization and / or instantiation of the above-mentioned carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations, further: the set cooling rate is 5℃ / min to 15℃ / min; the control unit is configured to adjust the cooling water flow rate of the water cooler according to the bed temperature detection signals of the primary adiabatic reactor and the secondary adiabatic reactor in nitrogen circulation mode, so that the bed cooling rate is maintained within the set cooling rate range.

[0014] As an optimization and / or instantiation of the aforementioned carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations, further: both the primary steam generator and the secondary steam generator employ three-impulse level control, the three impulses including the steam drum level signal, the steam outlet flow signal, and the feedwater flow signal; the control unit uses the steam drum level signal as the main control signal and the steam outlet flow signal as the feedforward signal. When the steam outlet flow suddenly increases, the control unit increases the feedwater replenishment in advance according to the feedforward signal to offset the false level drop caused by intensified vaporization.

[0015] As an optimization and / or instantiation of the aforementioned carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations, further: the circulating compressor is driven by a variable frequency drive, and the ratio of the circulating gas flow rate to the fresh hydrogen flow rate corresponding to the speed adjustment range is 2:1 to 8:1; when the fresh hydrogen flow rate drops below 50% of the rated flow rate, the control unit increases the operating frequency of the circulating compressor to make the ratio of the circulating gas flow rate to the fresh hydrogen flow rate reach 5:1 or higher; when the fresh hydrogen flow rate drops below 30% of the rated flow rate, the control unit increases the operating frequency of the circulating compressor to make the ratio of the circulating gas flow rate to the fresh hydrogen flow rate reach 7:1 or higher.

[0016] As an optimization and / or instantiation of the aforementioned carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations, further: the hydrogen source connected to the hydrogen feed pipeline is a renewable energy water electrolysis hydrogen production device, wherein the renewable energy includes wind energy, solar energy, or wind-solar hybrid energy; the hydrogen source fluctuations are manifested as follows: during peak renewable energy power generation periods, the fresh hydrogen flow rate reaches 80% to 100% of the rated flow rate; during off-peak renewable energy power generation periods, the fresh hydrogen flow rate drops to 20% to 50% of the rated flow rate.

[0017] The carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations provided by this invention is based on the use of techniques such as setting up a circulation unit and a control unit. It uses the fresh hydrogen flow rate as a feedforward variable to coordinately regulate the operating frequency of the circulation compressor and the opening of the pressure regulating device. When the fresh hydrogen flow rate fluctuates, the circulating gas flow rate is adjusted to compensate for changes in the fresh feed rate, thus maintaining the gas mass flow rate in the primary and secondary adiabatic reactors within a preset range, resulting in significant beneficial effects.

[0018] First, it enables the carbon dioxide hydrogenation to methane synthesis system to adapt to fluctuations in hydrogen source, which can greatly simplify the hydrogen storage buffer device, avoid the equipment investment, land area and safety hazards of high-pressure hydrogen storage tanks, reduce system complexity and operation and maintenance costs, and improve overall economic efficiency.

[0019] Second, by dynamically adjusting the circulating gas flow rate to compensate for fluctuations in the fresh feed rate, the gas mass flow rate in the reactor is kept stable, ensuring the heat and mass transfer conditions of the catalyst bed, avoiding local overheating or reaction shutdown caused by a sudden drop in hydrogen flow rate, as well as bed overheating and catalyst deactivation caused by a sudden increase in hydrogen flow rate, thus extending the catalyst's service life.

[0020] Third, the inlet temperature of the secondary adiabatic reactor is controlled by regulating the steam pressure of the primary steam generator. The heat transfer temperature difference and heat exchange capacity are adjusted by utilizing the correspondence between the steam pressure in the steam drum and the saturation temperature of the heat exchange medium. This enables precise control of the cooling degree of the reaction products, allowing the secondary adiabatic reactor to maintain a suitable inlet temperature even under fluctuating hydrogen source conditions.

[0021] Fourth, the medium-pressure steam generated by the primary and secondary steam generators is used to supplement the heat of the mixed raw material gas after primary preheating, forming a self-balancing mechanism between steam output and raw material preheating requirements. When the flow rate of fresh hydrogen changes, causing changes in the heat released by the reaction, the steam output changes accordingly and automatically adjusts the supplementary heat to ensure the heat balance of the preheating unit.

[0022] Fifth, by controlling the power of the electric heater with a thyristor, the secondary heat replenishment can be adjusted within a wide power range to maintain the inlet temperature of the primary adiabatic reactor within the ignition temperature range, thus ensuring stable ignition of the reaction under fluctuating hydrogen source conditions.

[0023] Sixth, by setting up a nitrogen inlet and a nitrogen circulation mode, the reactor bed temperature can be safely cooled at a set cooling rate using nitrogen circulation and a water cooler during shutdown, preventing the catalyst from being damaged due to excessive cooling and improving the system's operational safety and catalyst protection capabilities.

[0024] Seventh, by using three-impulse water level control in the primary and secondary steam generators, the steam outlet flow signal is used as a feedforward signal to adjust the feedwater supply in advance, offsetting the false liquid level changes caused by load fluctuations and ensuring the stable operation of the steam generator under hydrogen source fluctuation conditions.

[0025] Eighth, the variable frequency drive of the circulating compressor enables wide-range circulation ratio adjustment. When the fresh hydrogen flow rate drops significantly, the circulation ratio is increased accordingly to ensure that the gas mass flow rate in the reactor is not lower than the minimum limit required to maintain normal heat and mass transfer, thus broadening the load adaptability range of the system.

[0026] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Additional aspects and advantages provided by the present invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice. Attached Figure Description

[0027] The accompanying drawings, which form part of this specification, are used to aid in understanding the invention. The contents provided in the drawings and their related descriptions in this specification can be used to explain the invention, but do not constitute an undue limitation of the invention.

[0028] Figure 1 This is a view of the hydrogen feed line section of the feedstock supply unit in a PID diagram (Piping and Instrumentation Diagram) of an embodiment of the carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations of the present invention.

[0029] Figure 2 The PID diagram of an embodiment of the carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations of the present invention shows a view of the preheating unit, the first-stage adiabatic reactor, and the first-stage steam generator.

[0030] Figure 3 This is a PID diagram of an embodiment of the carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations of the present invention, showing a view of the secondary adiabatic reactor and the secondary steam generator.

[0031] Figure 4 This is a view of the cooling separation unit portion in a PID diagram of an embodiment of the carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations of the present invention.

[0032] Figure 5 This is a view of the circulating compressor section in a PID diagram of an embodiment of the carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations of the present invention.

[0033] The following are labeled in the diagram: pressure transmitter 101; flow transmitter 102; temperature sensing element 103; flow regulating valve 104; raw material preheater 201; electric heater 202; medium-pressure steam main 203; primary adiabatic reactor 301; secondary adiabatic reactor 302; primary steam generator 401; secondary steam generator 402; first steam regulating valve 403; second steam regulating valve 404; water cooler 501; water distributor 502; external pipeline 503; circulation pipeline 504; pressure regulating device 505. Detailed Implementation

[0034] The present invention will now be clearly and completely described in conjunction with the accompanying drawings. Those skilled in the art will be able to implement the present invention based on these descriptions. Before describing the present invention in conjunction with the accompanying drawings, it should be particularly noted that:

[0035] The technical solutions and features provided in the various sections, including the following description, can be combined with each other without conflict. Furthermore, where possible, these technical solutions, features, and related combinations can be given specific technical subject matter and protected by relevant patents.

[0036] The embodiments of the present invention described below are generally only some embodiments and not all embodiments. Based on these embodiments, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of patent protection.

[0037] The terms "comprising," "including," "having," and any variations thereof in this specification, the corresponding claims, and related sections are intended to cover non-exclusive inclusion. Other related terms and units can be reasonably interpreted based on the relevant content provided in this specification.

[0038] In addition, the accompanying diagrams in this manual use actual PID diagrams, and the alphanumeric combinations labeled next to each pipe, instrument, and valve in the diagrams (e.g., Figure 1 In the PID diagram, TE11102, FT11102, FV11102, etc., are instrument tag numbers compiled according to instrumentation specifications. They indicate the instrument type and its position in the process flow and are inherent technical information of the drawing itself, which does not constitute any limitation on the present invention. For ease of description, this specification has separately compiled patent reference numerals for referring to corresponding equipment, instruments, or components in the specification and claims. The above two numbering systems are independent and do not conflict with each other; those skilled in the art can clearly distinguish them based on the context.

[0039] like Figures 1 to 5 As shown, this invention provides an embodiment of a carbon dioxide hydrogenation to methane synthesis system adapted to fluctuations in hydrogen source. The system includes a feedstock supply unit, a preheating unit, a reaction unit, a waste heat recovery unit, a cooling and separation unit, a circulation unit, and a control unit.

[0040] The feedstock unit includes carbon dioxide feed lines and hydrogen feed lines. For example... Figure 1As shown, a flow detection device is installed on the hydrogen feed line to monitor the fresh hydrogen flow rate in real time. The carbon dioxide feed line merges with the hydrogen feed line to form a main feed line, which is used to transport a mixed feed gas composed of carbon dioxide, hydrogen, and recycle gas. In this embodiment, the hydrogen source connected to the hydrogen feed line is a renewable energy water electrolysis hydrogen production device, and renewable energy sources include wind power, solar power, or wind-solar hybrid energy. The hydrogen source fluctuations are as follows: during peak renewable energy power generation periods, the fresh hydrogen flow rate reaches 80% to 100% of the rated flow rate; during off-peak renewable energy power generation periods, the fresh hydrogen flow rate drops to 20% to 50% of the rated flow rate.

[0041] See details Figure 1 , Figure 1 This is a schematic diagram of the hydrogen feed pipeline section according to an embodiment of the present invention. A pressure transmitter 101 and a pressure indicator are provided on the hydrogen feed pipeline for detecting and displaying the hydrogen feed pressure. A flow transmitter 102 is also provided on the hydrogen feed pipeline, and the flow transmitter 102 is signal-connected to a flow indicator and controller. The flow indicator and controller detects and indicates the fresh hydrogen flow rate and outputs a flow signal to the control unit. A temperature detection element 103 and a temperature indicator are also provided on the hydrogen feed pipeline for detecting and displaying the hydrogen feed temperature. A flow regulating valve 104 is provided on the hydrogen feed pipeline, and the flow regulating valve 104 is signal-connected to the flow indicator and controller. The valve opening is adjusted according to the flow detection signal to control the hydrogen feed flow rate. The flow regulating valve 104 is a gas-to-close valve, meaning the valve closes when gas is lost to ensure safety. Manual shut-off valves are provided upstream and downstream of the flow regulating valve 104, and a bypass manual shut-off valve is also connected in parallel with the flow regulating valve 104 for shut-off or bypass during maintenance.

[0042] like Figure 2 As shown, the preheating unit includes a raw material preheater 201 and an electric heater 202. The raw material preheater 201 has a cold-side channel and a hot-side channel. The inlet of the cold-side channel is connected to the main feed line for introducing the mixed raw material gas, and the outlet of the cold-side channel is connected to the inlet of the electric heater 202 for outputting the mixed raw material gas after primary preheating. The inlet of the hot-side channel is used to introduce system reaction products to perform primary preheating of the mixed raw material gas in the cold-side channel, and the outlet of the hot-side channel is used to output the system reaction products after heat exchange and cooling. The electric heater 202 is used for secondary supplementary heating of the mixed raw material gas after primary preheating, and the outlet of the electric heater 202 is used to output the mixed raw material gas after secondary supplementary heating. In this embodiment, the electric heater 202 uses thyristor power regulation control, with a power adjustment range of 10% to 100% of the rated power. The control unit is signal-connected to the electric heater 202 and is configured to adjust the power output of the electric heater 202 according to the inlet temperature detection signal of the primary adiabatic reactor 301, so that the inlet temperature of the primary adiabatic reactor 301 is maintained within the ignition temperature range.

[0043] See details Figure 2 , Figure 2 This is a schematic diagram of the preheating unit, the primary adiabatic reactor 301, and the primary steam generator 401 in an embodiment of the present invention. The raw material preheater 201 is a shell-and-tube heat exchanger. Its cold-side inlet receives circulating gas from the outlet of the circulating compressor, carbon dioxide from the carbon dioxide feed line, and hydrogen from the hydrogen feed line via pipelines. These three materials merge to form a mixed raw material gas that enters the raw material preheater 201. The cold-side outlet of the raw material preheater 201 is connected to the inlet of the electric heater 202 via a pipeline. The electric heater 202 is an electrically heated heat exchanger used for secondary reheating of the mixed raw material gas after primary preheating. The hot-side inlet of the raw material preheater 201 receives secondary reaction product gas from the tube-side outlet of the secondary steam generator 402 via a pipeline. The medium-pressure steam main 203 is connected to the pipeline located between the cold-side channel outlet of the raw material preheater 201 and the inlet of the electric heater 202, allowing the medium-pressure steam generated by the primary steam generator 401 and the secondary steam generator 402 to be transported to the preheated mixed raw material gas for supplementary heating. The inlet of the primary adiabatic reactor 301 is connected to the outlet of the electric heater 202 via a pipeline to receive the preheated mixed raw material gas. A temperature sensing element and a temperature indicator are installed on the inlet pipeline of the primary adiabatic reactor 301 to detect the inlet temperature. The outlet of the primary adiabatic reactor 301 is connected to the primary steam generator 401 via a pipeline. The primary adiabatic reactor 301 also has a bed temperature detection point to monitor the catalyst bed temperature. The primary steam generator 401 is a shell-and-tube heat exchanger with a steam drum; its tube-side inlet is connected to the outlet of the primary adiabatic reactor 301 to receive the primary reaction product gas, and its tube-side outlet is connected to the secondary adiabatic reactor 302. The shell side of the first-stage steam generator 401 is equipped with a boiler feedwater inlet and a low-pressure steam inlet for introducing boiler feedwater or low-pressure steam as the heat exchange medium. A steam outlet pipeline is located at the top of the steam drum of the first-stage steam generator 401, and a first steam regulating valve 403 is installed on the steam outlet pipeline. The steam outlet is connected to the medium-pressure steam main 203. A pressure relief valve is installed on the steam drum of the first-stage steam generator 401 for overpressure protection. A level transmitter and a level indicating and regulating instrument are also installed on the steam drum of the first-stage steam generator 401 for steam drum water level control.

[0044] The reaction unit includes a primary adiabatic reactor 301 and a secondary adiabatic reactor 302. The inlet of the primary adiabatic reactor 301 is connected to the outlet of the electric heater 202 to receive the mixed feed gas after secondary heating, and the outlet of the primary adiabatic reactor 301 is used to output the primary reaction product gas. The inlet of the secondary adiabatic reactor 302 is used to receive the primary reaction product gas after cooling by the primary steam generator 401, and the outlet of the secondary adiabatic reactor 302 is used to output the secondary reaction product gas.

[0045] See details Figure 3 , Figure 3 This is a schematic diagram of the secondary adiabatic reactor 302 and the secondary steam generator 402 according to an embodiment of the present invention. The inlet of the secondary adiabatic reactor 302 receives cooled primary reaction product gas from the tube-side outlet of the primary steam generator 401 via a pipeline. A temperature detection element and a temperature indicator are provided on the inlet pipeline of the secondary adiabatic reactor 302 to detect the inlet temperature of the secondary adiabatic reactor 302. The control unit adjusts the opening of the first steam regulating valve 403 according to the inlet temperature detection signal to maintain the inlet temperature of the secondary adiabatic reactor 302 within the range of 250°C to 280°C. Multiple bed temperature detection points are provided on the secondary adiabatic reactor 302 to monitor the catalyst bed temperature distribution. The outlet of the secondary adiabatic reactor 302 is connected to the secondary steam generator 402 via a pipeline for outputting the secondary reaction product gas. The structure of the secondary steam generator 402 is the same as that of the primary steam generator 401, including a shell, a heat exchange tube bundle disposed within the shell, and a steam drum communicating with the shell. The tube-side inlet of the heat exchanger tube bundle of the secondary steam generator 402 is connected to the outlet of the secondary adiabatic reactor 302 to receive the secondary reaction product gas. The tube-side outlet is connected to the hot-side channel inlet of the raw material preheater 201 via a pipeline. The shell side of the secondary steam generator 402 is equipped with a boiler feedwater inlet, a steam generator feedwater inlet, a low-pressure steam inlet, and a blowdown outlet. A second steam outlet pipeline is located at the top of the steam drum of the secondary steam generator 402, and a second steam regulating valve 404 is installed on the second steam outlet pipeline. The second steam regulating valve 404 is a loss-opening valve, and the steam discharge rate of the secondary steam generator 402 is controlled by adjusting the opening degree of the second steam regulating valve 404. The steam outlet pipeline of the primary steam generator 401 and the second steam outlet pipeline merge to form the medium-pressure steam main 203. A pressure relief valve is installed on the steam drum of the secondary steam generator 402 for overpressure protection. The steam drum of the secondary steam generator 402 is also equipped with a level transmitter and a level indicating controller for three-impulse water level control. In the steam drum level control loop, the level transmitter detects the steam drum level signal, and the level indicator and controller uses the steam drum level signal as the main control signal and the steam outlet flow signal as the feedforward signal to adjust the opening of the feedwater regulating valve to control the feedwater flow, thereby achieving three-impulse level control.

[0046] The waste heat recovery unit includes a primary steam generator 401 and a secondary steam generator 402. The material inlet of the primary steam generator 401 is connected to the outlet of the primary adiabatic reactor 301 to receive the primary reaction product gas, and the material outlet of the primary steam generator 401 is connected to the inlet of the secondary adiabatic reactor 302 to output the cooled primary reaction product gas. The material inlet of the secondary steam generator 402 is connected to the outlet of the secondary adiabatic reactor 302 to receive the secondary reaction product gas, and the material outlet of the secondary steam generator 402 is connected to the hot-side channel inlet of the raw material preheater 201 to output the cooled secondary reaction product gas as the system reaction product.

[0047] The primary steam generator 401 includes a shell, a heat exchange tube bundle disposed within the shell, and a steam drum communicating with the shell. The heat exchange tube bundle has a tube-side inlet and a tube-side outlet. The tube-side inlet is connected to the outlet of the primary adiabatic reactor 301 to receive the primary reaction product gas, and the tube-side outlet is connected to the inlet of the secondary adiabatic reactor 302 to output the cooled primary reaction product gas. The shell side of the shell has a medium inlet for introducing boiler feedwater or low-pressure steam as the heat exchange medium. After absorbing heat from the primary reaction product gas in the tube side, the heat exchange medium enters a boiling state with a vapor-liquid two-phase coexistence and generates steam. The steam enters the steam drum for steam-water separation. A steam outlet pipeline is located at the top of the steam drum, and a first steam regulating valve 403 is installed on the steam outlet pipeline. By adjusting the opening of the first steam regulating valve 403, the steam discharge rate is controlled, thereby regulating the steam pressure in the steam drum. The steam pressure in the steam drum determines the saturation temperature of the heat exchange medium in the shell side, which in turn determines the heat transfer temperature difference between the primary reaction product gas in the tube side and the heat exchange medium in the shell side.

[0048] The control unit is signal-connected to the first steam regulating valve 403 and configured to adjust the opening of the first steam regulating valve 403 based on the inlet temperature detection signal of the secondary adiabatic reactor 302, thereby maintaining the inlet temperature of the secondary adiabatic reactor 302 within the range of 250°C to 280°C. When the fresh hydrogen flow rate decreases, causing a decrease in the temperature of the primary reaction product gas, the control unit increases the opening of the first steam regulating valve 403 to increase the steam discharge, reduce the steam pressure in the steam drum, thereby reducing the saturation temperature of the heat exchange medium, decreasing the heat transfer temperature difference, and reducing the amount of heat exchanged, thus maintaining the inlet temperature of the secondary adiabatic reactor 302 within the range of 250°C to 280°C. When the fresh hydrogen flow rate increases, causing an increase in the temperature of the primary reaction product gas, the control unit decreases the opening of the first steam regulating valve 403 to reduce the steam discharge, increase the steam pressure in the steam drum, thereby increasing the saturation temperature of the heat exchange medium, maintaining the heat transfer temperature difference, and increasing the amount of heat exchanged, thus maintaining the inlet temperature of the secondary adiabatic reactor 302 within the range of 250°C to 280°C.

[0049] The secondary steam generator 402 has the same structure as the primary steam generator 401, including a shell, a heat exchange tube bundle disposed within the shell, and a steam drum communicating with the shell. The tube-side inlet of the heat exchange tube bundle of the secondary steam generator 402 is connected to the outlet of the secondary adiabatic reactor 302 to receive the secondary reaction product gas, and the tube-side outlet is connected to the hot-side channel inlet of the raw material preheater 201. A second steam outlet pipeline is provided at the top of the steam drum of the secondary steam generator 402, and a second steam regulating valve 404 is provided on the second steam outlet pipeline. By adjusting the opening degree of the second steam regulating valve 404, the steam discharge rate of the secondary steam generator 402 is controlled, thereby regulating the steam pressure inside the steam drum of the secondary steam generator 402 and the saturation temperature of the heat exchange medium in its shell side.

[0050] The steam outlet pipeline of the first-stage steam generator 401 and the second steam outlet pipeline merge to form a medium-pressure steam main 203. The medium-pressure steam main 203 connects to a medium-pressure steam pipeline located between the cold-side channel outlet of the raw material preheater 201 and the inlet of the electric heater 202, allowing the medium-pressure steam generated by the first-stage steam generator 401 and the second-stage steam generator 402 to be transported to the preheated mixed raw material gas for supplementary heating. A first pressure detection device and a first flow detection device are sequentially installed along the steam flow direction on the steam outlet pipeline of the first-stage steam generator 401, and a second pressure detection device and a second flow detection device are sequentially installed along the steam flow direction on the second steam outlet pipeline. Pressure safety valves are respectively installed on the steam drums of the first-stage steam generator 401 and the second-stage steam generator 402.

[0051] The control unit is signal-connected to the second steam regulating valve 404, the first pressure detection device, the first flow detection device, the second pressure detection device, and the second flow detection device, and is configured to adjust the opening of the second steam regulating valve 404 according to the inlet temperature detection signal of the hot side channel of the raw material preheater 201. When the fresh hydrogen flow rate increases and the operating frequency of the circulating compressor decreases, resulting in a decrease in the circulating gas flow rate, the temperature of the primary and secondary reaction product gases increases. The control unit reduces the opening of the first steam regulating valve 403 and the second steam regulating valve 404 to increase the steam pressure in each steam drum. At this time, the steam output of the primary steam generator 401 and the secondary steam generator 402 decreases accordingly, and the amount of supplementary heating steam delivered to the mixed raw material gas after primary preheating via the medium-pressure steam main 203 and the medium-pressure steam pipeline decreases accordingly. At the same time, due to the increase in the fresh hydrogen flow rate, the heat released by the reaction increases, and the heat carried by the reaction products in the hot side channel of the raw material preheater 201 increases, which correspondingly increases the primary preheating heat of the mixed raw material gas, thereby compensating for the decrease in the amount of supplementary heating steam. When the fresh hydrogen flow rate decreases and the operating frequency of the circulating compressor increases, causing the circulating gas flow rate to increase, the temperature of the primary and secondary reaction product gases decreases. The control unit increases the opening of the first steam regulating valve 403 and the second steam regulating valve 404 to reduce the steam pressure in each steam drum. At this time, the steam output of the primary steam generator 401 and the secondary steam generator 402 increases accordingly. The amount of supplementary heating steam delivered to the preheated mixed feed gas through the medium-pressure steam main 203 and the medium-pressure steam pipeline also increases. At the same time, due to the decrease in the fresh hydrogen flow rate, the heat released by the reaction decreases, and the heat carried by the reaction products in the hot side channel of the feed preheater 201 decreases. The primary preheating heat of the mixed feed gas decreases accordingly, and the increased supplementary heating steam compensates for the insufficient primary preheating heat.

[0052] Both the primary steam generator 401 and the secondary steam generator 402 employ three-impulse level control, with the three impulses including the steam drum level signal, the steam outlet flow signal, and the feedwater flow signal. The control unit uses the steam drum level signal as the main control signal and the steam outlet flow signal as the feedforward signal. When the steam outlet flow suddenly increases, the control unit increases the feedwater supply in advance based on the feedforward signal to offset the false level drop caused by intensified vaporization.

[0053] The cooling and separation unit includes a water cooler 501 and a water separator 502. The inlet of the water cooler 501 is connected to the hot-side channel outlet of the raw material preheater 201 to receive the reaction products after heat exchange and cooling. The outlet of the water cooler 501 is connected to the inlet of the water separator 502 to output the deeply cooled reaction products. The water separator 502 is used for gas-liquid separation of the deeply cooled reaction products. The liquid phase outlet of the water separator 502 is used to discharge condensate, and the gas phase outlet of the water separator 502 is connected to a main outlet pipeline for outputting methane-rich gas. The main outlet pipeline is divided into an external delivery pipeline 503 and a circulation pipeline 504. The external delivery pipeline 503 is used to transport methane-rich synthesis gas to the outside and is equipped with a pressure regulating device 505. The circulation pipeline 504 is used to return a portion of the methane-rich gas as circulating gas.

[0054] See details Figure 4 , Figure 4This is a schematic diagram of the cooling and separation unit in an embodiment of the present invention. Secondary reaction product gas from the tube-side outlet of the secondary steam generator 402 enters the hot-side channel of the raw material preheater 201 for heat exchange. After being cooled by heat exchange, the reaction product enters the water cooler 501. The water cooler 501 includes a first water cooler and a second water cooler arranged in series, both of which are shell-and-tube heat exchangers. The tube-side inlet of the first water cooler is connected to the hot-side channel outlet of the raw material preheater 201, and its tube-side outlet is connected to the tube-side inlet of the second water cooler. The shell-side of the first and second water coolers are respectively provided with a circulating cooling water inlet and a circulating cooling water outlet for deep cooling of the reaction product. The tube-side outlet of the second water cooler is connected to the inlet of the water separator 502 via a pipe. The water separator 502 is a horizontal gas-liquid separation vessel used for gas-liquid separation of the deeply cooled reaction product. The water separator 502 is equipped with a level transmitter and a level indicator regulator for monitoring and controlling the liquid level within the water separator 502. The liquid phase outlet of the water separator 502 is equipped with a level regulating valve, which is connected to a level indicator and controller to adjust the condensate discharge rate based on the liquid level signal. The gas phase outlet of the water separator 502 is connected to a main outlet pipeline for outputting methane-rich gas. The main outlet pipeline is divided into an external delivery pipeline 503 and a circulation pipeline 504. The external delivery pipeline 503 is equipped with a pressure regulating valve, which is connected to a pressure transmitter 101 and a pressure indicator and controller to form a pressure regulating device 505, used to regulate system pressure and control the external delivery flow rate. The pressure regulating valve is a gas-to-close valve, meaning it closes when gas is lost. The circulation pipeline 504 is connected to the inlet of the circulating compressor to return a portion of the methane-rich gas as circulating gas. The circulation pipeline 504 is equipped with a flow transmitter and a flow indicator to detect and display the circulating gas flow rate. The cooling separation unit also has a nitrogen inlet, which is connected to the circulation pipeline 504 via a pipe. The nitrogen interface pipeline is equipped with a shut-off valve and a manual shut-off valve. The shut-off valve is a normally closed valve. During normal operation, the shut-off valve is closed. When the control unit receives the shutdown command, it switches the circulation pipeline 504 to nitrogen circulation mode, opens the shut-off valve to introduce nitrogen through the nitrogen interface, and uses the water cooler 501 to reduce the bed temperature of the primary adiabatic reactor 301 and the secondary adiabatic reactor 302 to below 100°C at a set cooling rate.

[0055] The cooling separation unit is also equipped with a nitrogen inlet, which is connected to the circulation pipeline 504. The control unit is configured to switch the circulation pipeline 504 to nitrogen circulation mode upon receiving a shutdown command, introduce nitrogen through the nitrogen inlet, and use the water cooler 501 to reduce the bed temperature of the primary adiabatic reactor 301 and the secondary adiabatic reactor 302 to below 100°C at a set cooling rate. The set cooling rate is 5°C / min to 15°C / min. In nitrogen circulation mode, the control unit is configured to adjust the cooling water flow rate of the water cooler 501 based on the bed temperature detection signals of the primary adiabatic reactor 301 and the secondary adiabatic reactor 302, so that the bed cooling rate remains within the set cooling rate range.

[0056] The circulation unit includes a circulation compressor. The inlet of the circulation compressor is connected to circulation line 504 to receive circulating gas, and the outlet of the circulation compressor merges with the main feed line to mix the pressurized circulating gas with fresh carbon dioxide and hydrogen. The circulation compressor is driven by a variable frequency drive, and its speed adjustment range corresponds to a circulating gas flow rate to fresh hydrogen flow rate ratio adjustment range of 2:1 to 8:1. When the fresh hydrogen flow rate drops below 50% of the rated flow rate, the control unit increases the operating frequency of the circulation compressor to ensure that the circulating gas flow rate to fresh hydrogen flow rate ratio reaches 5:1 or higher; when the fresh hydrogen flow rate drops below 30% of the rated flow rate, the control unit increases the operating frequency of the circulation compressor to ensure that the circulating gas flow rate to fresh hydrogen flow rate ratio reaches 7:1 or higher. The circulating gas flow rate to fresh hydrogen flow rate ratio is maintained within the range of 3:1 to 8:1.

[0057] See details Figure 5 , Figure 5This is a schematic diagram of the circulating compressor section of an embodiment of the present invention. The circulating compressor inlet receives circulating gas from the gas phase outlet of the water distributor 502 via a pipeline. The circulating compressor outlet outputs pressurized circulating gas to the main feed line via a pipeline, where it merges with fresh raw materials and enters the raw material preheater 201. The circulating compressor is a multi-stage reciprocating compressor, including an inlet buffer tank, a first-stage compressor cylinder, an interstage cooler, a second-stage compressor cylinder, and an outlet buffer tank. The circulating compressor inlet pipeline is equipped with an inlet shut-off valve and a manual shut-off valve. The inlet shut-off valve is used to cut off the flow of circulating gas into the compressor in an emergency. The circulating compressor inlet buffer tank is equipped with a pressure transmitter and a pressure indicator for monitoring the inlet pressure. The circulating compressor inlet pipeline also has a nitrogen inlet and a vent inlet, connected via shut-off valves for system purging and emergency venting. The inlet of the first-stage compressor cylinder of the circulating compressor is equipped with a temperature detection element and a temperature indicator for monitoring the first-stage inlet temperature. An interstage cooler is located between the first-stage and second-stage compressor cylinders of the circulating compressor. The shell side of the interstage cooler has a circulating cooling water inlet and outlet. The recirculating compressor's interstage piping is equipped with pressure transmitters and temperature sensors to monitor interstage pressure and temperature. The outlet of the second-stage compressor cylinder is equipped with a pressure transmitter, temperature sensor, and pressure safety valve to monitor outlet pressure and temperature and provide overpressure protection. The recirculating compressor's outlet piping is equipped with an outlet shut-off valve, a check valve, and a manual shut-off valve. The outlet shut-off valve is used to cut off the compressor outlet in emergencies. The recirculating compressor's outlet piping also has a return line with a return regulating valve connected to a flow indicator controller for surge control or flow regulation. The recirculating compressor also has multiple nitrogen inlets, connected via shut-off valves, for system purging, sealing gas supply, and nitrogen protection in emergencies. The recirculating compressor is driven by a variable frequency drive (VFD). The VFD receives frequency command signals from the control unit and adjusts the operating frequency of the recirculating compressor according to changes in the fresh hydrogen flow rate.

[0058] The control unit is signal-connected to the flow detection device, the circulating compressor, and the pressure regulating device 505. The control unit is configured to use the fresh hydrogen flow rate detected by the flow detection device as a feedforward variable to coordinately regulate the operating frequency of the circulating compressor and the opening of the pressure regulating device 505, maintaining the gas mass flow rate within the primary adiabatic reactor 301 and the secondary adiabatic reactor 302 within a preset range. When the fresh hydrogen flow rate decreases, the control unit reduces the opening of the pressure regulating device 505 to increase the flow resistance of the delivery line 503, allowing more methane-rich gas to return through the circulation line 504. Simultaneously, it increases the operating frequency of the circulating compressor to increase the circulating gas flow rate, thereby compensating for the reduction in fresh feed. When the fresh hydrogen flow rate increases, the control unit increases the opening of the pressure regulating device 505 and decreases the operating frequency of the circulating compressor to reduce the circulating gas flow rate and increase the delivery ratio.

[0059] The working principle of this embodiment is as follows: Carbon dioxide and hydrogen are combined through their respective feed lines and mixed with circulating gas from the circulating compressor to form a mixed feed gas. The mixed feed gas first enters the cold side channel of the feed preheater 201, where it exchanges heat with the system reaction products in the hot side channel to complete the first-stage preheating. Then, it undergoes secondary heating by the electric heater 202 to reach the reaction ignition temperature and enters the first-stage adiabatic reactor 301 to undergo a methanation reaction. The first-stage reaction product gas exits from the outlet of the first-stage adiabatic reactor 301 and enters the tube side of the first-stage steam generator 401, where it exchanges heat with the heat exchange medium in the shell side to cool down before entering the second-stage adiabatic reactor 302 to continue the reaction. The second-stage reaction product gas exits from the outlet of the second-stage adiabatic reactor 302 and enters the tube side of the second-stage steam generator 402 for heat exchange and cooling. Then, as a system reaction product, it enters the hot side channel of the feed preheater 201 to perform the first-stage preheating of the mixed feed gas. After heat exchange and cooling, the reaction products enter the water cooler 501 for deep cooling, and then enter the water separator 502 for gas-liquid separation. The separated condensate is discharged from the system. Part of the methane-rich gas is sent out through the external pipeline 503, and the other part enters the circulating compressor through the circulation pipeline 504 for pressurization and then flows back to the main feed pipeline to mix with fresh raw materials.

[0060] When fluctuations in renewable energy power generation lead to changes in the fresh hydrogen flow rate, the control unit uses the fresh hydrogen flow rate as a feedforward variable to coordinately adjust the operating frequency of the circulating compressor and the opening of the pressure regulating device 505. If the fresh hydrogen flow rate decreases, the control unit reduces the opening of the pressure regulating device 505 to increase the flow resistance of the external delivery pipeline 503, while simultaneously increasing the operating frequency of the circulating compressor to increase the circulating gas flow rate. This increased circulating gas compensates for the reduced fresh feed, maintaining a stable gas mass flow rate within the reactor and ensuring the heat and mass transfer conditions of the catalyst bed. If the fresh hydrogen flow rate increases, the control unit increases the opening of the pressure regulating device 505 and decreases the operating frequency of the circulating compressor, reducing the circulating gas flow rate and increasing the external delivery ratio to prevent excessive gas flow within the reactor.

[0061] Meanwhile, the control unit adjusts the opening of the first steam regulating valve 403 according to the inlet temperature of the secondary adiabatic reactor 302, and regulates the saturation temperature and heat transfer temperature difference of the heat exchange medium by changing the steam pressure in the steam drum of the primary steam generator 401, so as to maintain the inlet temperature of the secondary adiabatic reactor 302 within the range of 250℃ to 280℃. The control unit also adjusts the power output of the electric heater 202 according to the inlet temperature of the primary adiabatic reactor 301, so as to maintain the inlet temperature of the primary adiabatic reactor 301 within the ignition temperature range. The medium-pressure steam generated by the primary steam generator 401 and the secondary steam generator 402 is transported to the mixed raw material gas after primary preheating through the medium-pressure steam main pipe 203 and the medium-pressure steam pipeline for supplementary heating. The steam output varies with the change in the heat released by the reaction, forming a self-balance with the primary preheating heat of the raw material preheater 201, ensuring the heat balance of the preheating unit.

[0062] In the shutdown condition, the control unit switches the circulation pipeline 504 to nitrogen circulation mode, introduces nitrogen through the nitrogen interface, and uses the water cooler 501 to safely reduce the reactor bed temperature to below 100°C at a set cooling rate of 5°C / min to 15°C / min, so as to avoid damage to the catalyst due to excessive cooling.

[0063] The above embodiments of the present invention, by setting up a circulation unit and a control unit, use the fresh hydrogen flow rate as a feedforward variable to coordinately regulate the operating frequency of the circulation compressor and the opening of the pressure regulating device 505. When the fresh hydrogen flow rate fluctuates, the change in the fresh feed amount is compensated by adjusting the circulation gas flow rate, so that the gas mass flow rate in the reactor is maintained within a preset range. Thus, the carbon dioxide hydrogenation to methane synthesis system itself has the ability to adapt to hydrogen source fluctuations, which can greatly simplify the hydrogen storage buffer device, reduce system complexity and operation and maintenance costs, and improve overall economic efficiency.

[0064] It should be noted that the above-mentioned technical solutions of the present invention form a complete system adaptive adjustment strategy, and there is an inherent logical progression relationship between the various technical features.

[0065] First, when fluctuations in the hydrogen source cause changes in the fresh hydrogen flow rate, the control unit coordinates the operation frequency of the circulating compressor and the opening of the pressure regulating device 505 to maintain the gas mass flow rate within the preset range in the primary adiabatic reactor 301 and the secondary adiabatic reactor 302, thereby ensuring the heat and mass transfer conditions of the catalyst bed. However, although the gas mass flow rate is stabilized, changes in the fresh hydrogen flow rate directly lead to changes in the heat of reaction: when the fresh hydrogen flow rate decreases, the amount of hydrogen participating in the reaction per unit time decreases, the heat of reaction decreases accordingly, and the temperatures of the primary and secondary reaction product gases decrease accordingly; when the fresh hydrogen flow rate increases, the heat of reaction increases, and the temperature of the reaction product gas increases accordingly. Fluctuations in the temperature of the reaction product gas will cause the inlet temperature of the secondary adiabatic reactor 302 to deviate from the optimal reaction temperature range, affecting the reaction efficiency and conversion rate of the secondary adiabatic reactor 302.

[0066] To address the problem caused by the temperature fluctuations of the reaction product gas, this invention further incorporates a temperature control strategy based on steam pressure regulation. The control unit adjusts the opening of the first steam regulating valve 403 according to the inlet temperature detection signal of the secondary adiabatic reactor 302. By changing the steam pressure in the steam drum of the primary steam generator 401, the saturation temperature of the heat exchange medium in the shell side is adjusted, thereby regulating the heat transfer temperature difference and heat exchange capacity between the primary reaction product gas in the tube side and the heat exchange medium in the shell side. When the temperature of the primary reaction product gas decreases, the opening of the first steam regulating valve 403 is increased to reduce the steam drum pressure and the saturation temperature of the heat exchange medium, thus reducing the heat transfer temperature difference and the heat exchange capacity. When the temperature of the primary reaction product gas increases, the opening of the first steam regulating valve 403 is decreased to increase the steam drum pressure and the saturation temperature of the heat exchange medium, maintaining the heat transfer temperature difference and increasing the heat exchange capacity. Through these adjustments, the inlet temperature of the secondary adiabatic reactor 302 is maintained within the range of 250°C to 280°C. However, this temperature control strategy brings new problems: the steam discharge rate fluctuates with the opening of the first steam regulating valve 403. When the fresh hydrogen flow rate decreases, the steam discharge rate increases, and when the fresh hydrogen flow rate increases, the steam discharge rate decreases. How to effectively utilize the fluctuating steam becomes a problem that needs to be solved.

[0067] To address the steam utilization issue and achieve overall system heat balance, this invention transmits medium-pressure steam generated by the primary steam generator 401 and the secondary steam generator 402 to the preheated mixed feed gas via the medium-pressure steam main 203 for supplementary heating. This design creates an automatic complementary balance between the supplementary steam heating and the primary preheating of the feed gas preheater 201: when the fresh hydrogen flow rate decreases, the heat released by the reaction decreases, and the heat carried by the reaction products in the hot side channel of the feed gas preheater 201 decreases, resulting in insufficient primary preheating of the mixed feed gas. However, at this time, the opening of the first steam regulating valve 403 and the second steam regulating valve 404 increases, increasing the steam discharge and supplementary heating steam volume, which precisely compensates for the insufficient primary preheating. When the fresh hydrogen flow rate increases, the heat released by the reaction increases, and the primary preheating of the feed gas preheater 201 is sufficient. However, at this time, the opening of the steam regulating valve decreases, reducing the steam discharge and supplementary heating steam volume, thus preventing overheating of the mixed feed gas. Through the above design, a self-balancing relationship is formed between the steam supplementary heat and the primary preheating heat, which can achieve the heat balance of the preheating unit without the need for an additional control loop. At the same time, the steam generated by the steam generator is fully utilized, improving the energy utilization efficiency of the system.

[0068] Thus, a complete system adaptive adjustment strategy was formed, enabling the carbon dioxide hydrogenation to methane synthesis system to maintain stable operation under conditions of large fluctuations in hydrogen source, while achieving self-balance of system heat and efficient energy utilization.

[0069] This invention is particularly applicable to applications that couple renewable energy-based water electrolysis for hydrogen production with carbon dioxide hydrogenation for methane synthesis. When the hydrogen source is a water electrolysis hydrogen production unit driven by wind, solar, or wind-solar hybrid energy, the carbon dioxide hydrogenation for methane synthesis system provided by this invention, which adapts to fluctuations in hydrogen source, can flexibly adjust its operating load according to fluctuations in renewable energy power generation output. It achieves direct coupling between the electrolysis hydrogen production unit and the methanation reaction system without the need for a large-capacity hydrogen storage buffer. Through the technical solution of this invention, intermittent and fluctuating renewable energy electricity is converted into a stable chemical energy carrier—methane. Methane can be directly injected into the natural gas pipeline network for storage and transportation, or used as a clean fuel or chemical feedstock, thereby achieving efficient conversion and cross-temporal transfer of electrical energy to chemical energy. This invention provides key technical support for constructing renewable energy-electricity coupling systems and has significant engineering application value for promoting large-scale renewable energy consumption and driving the low-carbon transformation of the energy system.

[0070] The foregoing has described the relevant content of the present invention. Those skilled in the art will be able to implement the present invention based on these descriptions. All other embodiments obtained by those skilled in the art based on the foregoing content of this specification without inventive effort should fall within the scope of the present invention.

Claims

1. A carbon dioxide hydrogenation to methane synthesis system adapted to fluctuations in hydrogen source, characterized in that: include: The raw material supply unit includes a carbon dioxide feed line and a hydrogen feed line. The hydrogen feed line is equipped with a flow detection device. The carbon dioxide feed line and the hydrogen feed line merge to form a total feed line. The total feed line is used to transport a mixed raw material gas composed of carbon dioxide, hydrogen and recirculated gas. The preheating unit includes a raw material preheater and an electric heater. The raw material preheater has a cold-side channel and a hot-side channel. The inlet of the cold-side channel is connected to the main feed line for introducing the mixed raw material gas. The outlet of the cold-side channel is connected to the inlet of the electric heater for outputting the mixed raw material gas after primary preheating. The inlet of the hot-side channel is used to introduce system reaction products to perform primary preheating on the mixed raw material gas in the cold-side channel. The outlet of the hot-side channel is used to output the system reaction products after heat exchange and cooling. The electric heater is used to perform secondary supplementary heating on the mixed raw material gas after primary preheating. The outlet of the electric heater is used to output the mixed raw material gas after secondary supplementary heating. The reaction unit includes a primary adiabatic reactor and a secondary adiabatic reactor. The inlet of the primary adiabatic reactor is connected to the outlet of the electric heater to receive the mixed raw material gas after secondary heating. The outlet of the primary adiabatic reactor is used to output the primary reaction product gas. The inlet of the secondary adiabatic reactor is used to receive the primary reaction product gas after cooling by the primary steam generator. The outlet of the secondary adiabatic reactor is used to output the secondary reaction product gas. The waste heat recovery unit includes a primary steam generator and a secondary steam generator. The material inlet of the primary steam generator is connected to the outlet of the primary adiabatic reactor to receive the primary reaction product gas. The material outlet of the primary steam generator is connected to the inlet of the secondary adiabatic reactor to output the cooled primary reaction product gas. The material inlet of the secondary steam generator is connected to the outlet of the secondary adiabatic reactor to receive the secondary reaction product gas. The material outlet of the secondary steam generator is connected to the hot-side channel inlet of the raw material preheater to output the cooled secondary reaction product gas as the system reaction product. The cooling and separation unit includes a water cooler and a water separator. The inlet of the water cooler is connected to the hot-side channel outlet of the raw material preheater to receive the reaction products after heat exchange and cooling. The outlet of the water cooler is connected to the inlet of the water separator to output the reaction products after deep cooling. The water separator is used to perform gas-liquid separation on the reaction products after deep cooling. The liquid phase outlet of the water separator is used to discharge condensate. The gas phase outlet of the water separator is connected to a main outlet pipeline for outputting methane-rich gas. The main outlet pipeline is divided into an external delivery pipeline and a circulation pipeline. The external delivery pipeline is used to transport methane-rich synthesis gas to the outside and is equipped with a pressure regulating device. The circulation pipeline is used to return a portion of the methane-rich gas as circulating gas. The circulation unit includes a circulation compressor, the inlet of which is connected to the circulation pipeline to receive the circulation gas, and the outlet of which merges with the main feed pipeline to mix the pressurized circulation gas with fresh carbon dioxide and hydrogen. The control unit is signal-connected to the flow detection device, the circulating compressor, and the pressure regulating device. The control unit is configured to use the fresh hydrogen flow rate detected by the flow detection device as a feedforward variable to coordinately adjust the operating frequency of the circulating compressor and the opening degree of the pressure regulating device, so as to maintain the gas mass flow rate in the primary adiabatic reactor and the secondary adiabatic reactor within a preset range. When the fresh hydrogen flow rate decreases, the control unit reduces the opening of the pressure regulating device to increase the flow resistance of the external pipeline, allowing more methane-rich gas to flow back through the circulation pipeline. At the same time, the operating frequency of the circulation compressor is increased to increase the circulation gas flow rate, thereby compensating for the reduction in the fresh feed amount. When the fresh hydrogen flow rate increases, the control unit increases the opening of the pressure regulating device and reduces the operating frequency of the circulating compressor to reduce the circulating gas flow rate and increase the external delivery ratio. The primary steam generator includes a shell, a heat exchange tube bundle disposed within the shell, and a steam drum communicating with the shell; The heat exchange tube bundle has a tube-side inlet and a tube-side outlet. The tube-side inlet is connected to the outlet of the first-stage adiabatic reactor to receive the first-stage reaction product gas, and the tube-side outlet is connected to the inlet of the second-stage adiabatic reactor to output the cooled first-stage reaction product gas. The shell side of the shell is provided with a medium inlet for introducing boiler feedwater or low-pressure steam as a heat exchange medium. After absorbing the heat of the first-stage reaction product gas in the tube side, the heat exchange medium is in a boiling state with both vapor and liquid phases coexisting and generates steam. The steam enters the steam drum for steam-water separation. The top of the steam drum is provided with a steam outlet pipeline, and the steam outlet pipeline is provided with a first steam regulating valve. By adjusting the opening of the first steam regulating valve, the steam discharge rate is controlled, thereby adjusting the steam pressure in the steam drum. The steam pressure in the steam drum determines the saturation temperature of the heat exchange medium in the shell side, and thus determines the heat transfer temperature difference between the first-stage reaction product gas in the tube side and the heat exchange medium in the shell side. The control unit is signal-connected to the first steam regulating valve and is configured to adjust the opening of the first steam regulating valve according to the inlet temperature detection signal of the secondary adiabatic reactor, so that the inlet temperature of the secondary adiabatic reactor is maintained in the range of 250°C to 280°C. When the fresh hydrogen flow rate decreases, causing the temperature of the primary reaction product gas to drop, the control unit increases the opening of the first steam regulating valve to increase the steam discharge, reduces the steam pressure in the steam drum, thereby reducing the saturation temperature of the heat exchange medium, reducing the heat transfer temperature difference, reducing the heat exchange, and maintaining the inlet temperature of the secondary adiabatic reactor within the range of 250°C to 280°C. When the increase in the fresh hydrogen flow rate causes the temperature of the primary reaction product gas to rise, the control unit reduces the opening of the first steam regulating valve to reduce the steam discharge, increases the steam pressure in the steam drum, thereby increasing the saturation temperature of the heat exchange medium, maintaining the heat transfer temperature difference, increasing the heat exchange capacity, and keeping the inlet temperature of the secondary adiabatic reactor within the range of 250°C to 280°C. The structure of the secondary steam generator is the same as that of the primary steam generator, including a shell, a heat exchange tube bundle disposed in the shell, and a steam drum communicating with the shell; the tube-side inlet of the heat exchange tube bundle of the secondary steam generator is connected to the outlet of the secondary adiabatic reactor for receiving the secondary reaction product gas, and the tube-side outlet is connected to the hot-side channel inlet of the raw material preheater. The top of the steam drum of the secondary steam generator is provided with a second steam outlet pipeline, and a second steam regulating valve is provided on the second steam outlet pipeline. By adjusting the opening of the second steam regulating valve, the steam discharge of the secondary steam generator is controlled, thereby adjusting the steam pressure in the steam drum of the secondary steam generator and the saturation temperature of the heat exchange medium in the shell side. The steam outlet pipeline of the first-stage steam generator and the second steam outlet pipeline merge to form a medium-pressure steam main. The medium-pressure steam main is connected to a medium-pressure steam pipeline located between the cold-side channel outlet of the raw material preheater and the inlet of the electric heater, so that the medium-pressure steam generated by the first-stage steam generator and the second-stage steam generator is transported to the mixed raw material gas after the first-stage preheating for supplementary heating. The first-stage steam generator is provided with a first pressure detection device and a first flow detection device in sequence along the steam flow direction on the steam outlet pipeline, and the second steam outlet pipeline is provided with a second pressure detection device and a second flow detection device in sequence along the steam flow direction. Pressure safety valves are respectively installed on the steam drum of the primary steam generator and the steam drum of the secondary steam generator; The control unit is signal-connected to the second steam regulating valve, the first pressure detection device, the first flow detection device, the second pressure detection device, and the second flow detection device, and is configured to adjust the opening of the second steam regulating valve according to the detection signal of the inlet temperature of the hot side channel of the raw material preheater. When the fresh hydrogen flow rate increases and the operating frequency of the circulating compressor decreases, resulting in a reduction in the circulating gas flow rate, the temperatures of the primary and secondary reaction product gases rise. The control unit then reduces the opening of the first and second steam regulating valves to increase the steam pressure in each steam drum. Consequently, the steam output of the primary and secondary steam generators decreases, and the amount of supplementary heating steam delivered to the preheated mixed feed gas via the medium-pressure steam main and medium-pressure steam pipeline decreases accordingly. Simultaneously, the increased fresh hydrogen flow rate increases the heat released from the reaction, increasing the heat carried by the reaction products in the hot-side channel of the feed preheater, thus increasing the primary preheating heat of the mixed feed gas and compensating for the reduction in supplementary heating steam. When the fresh hydrogen flow rate decreases and the operating frequency of the circulating compressor increases, causing the circulating gas flow rate to increase, the temperature of the primary and secondary reaction product gases decreases. The control unit increases the opening of the first and second steam regulating valves to reduce the steam pressure in each steam drum. At this time, the steam output of the primary and secondary steam generators increases accordingly, and the amount of supplementary heating steam delivered to the preheated mixed feed gas via the medium-pressure steam main and the medium-pressure steam pipeline also increases. Simultaneously, because the decrease in the fresh hydrogen flow rate reduces the heat released by the reaction, the heat carried by the reaction products in the hot side channel of the feed preheater decreases, and the primary preheating heat of the mixed feed gas decreases accordingly. The increased supplementary heating steam compensates for the insufficient primary preheating heat.

2. The carbon dioxide hydrogenation to methane synthesis system adaptable to hydrogen source fluctuations as described in claim 1, characterized in that: The ratio of the circulating gas flow rate to the fresh hydrogen flow rate is maintained in the range of 3:1 to 8:

1.

3. The carbon dioxide hydrogenation to methane synthesis system adaptable to hydrogen source fluctuations as described in claim 1, characterized in that: The electric heater is controlled by a silicon controlled rectifier (SCR), and the power adjustment range is 10% to 100% of the rated power. The control unit is signal-connected to the electric heater and configured to adjust the power output of the electric heater according to the inlet temperature detection signal of the primary adiabatic reactor, so that the inlet temperature of the primary adiabatic reactor is maintained within the ignition temperature range.

4. The carbon dioxide hydrogenation to methane synthesis system adaptable to hydrogen source fluctuations as described in claim 1, characterized in that: The cooling separation unit is also provided with a nitrogen port, which is connected to the circulation pipeline; The control unit is configured to switch the circulation pipeline to nitrogen circulation mode after receiving a shutdown command, introduce nitrogen through the nitrogen interface, and use the water cooler to reduce the bed temperature of the primary adiabatic reactor and the secondary adiabatic reactor to below 100°C at a set cooling rate.

5. The carbon dioxide hydrogenation to methane synthesis system adapted to hydrogen source fluctuations as described in claim 4, characterized in that: The set cooling rate is 5℃ / min to 15℃ / min; The control unit is configured to adjust the cooling water flow rate of the water cooler according to the bed temperature detection signals of the primary adiabatic reactor and the secondary adiabatic reactor in nitrogen circulation mode, so that the bed cooling rate is maintained within the set cooling rate range.

6. The carbon dioxide hydrogenation to methane synthesis system adaptable to hydrogen source fluctuations as described in claim 1, characterized in that: Both the primary steam generator and the secondary steam generator adopt three-impulse water level control, the three impulses including the steam drum liquid level signal, the steam outlet flow signal and the feedwater flow signal; The control unit uses the steam drum liquid level signal as the main control signal and the steam outlet flow rate signal as the feedforward signal. When the steam outlet flow rate suddenly increases, the control unit increases the feedwater replenishment in advance according to the feedforward signal to offset the false liquid level drop caused by the increased vaporization.

7. The carbon dioxide hydrogenation to methane synthesis system adaptable to hydrogen source fluctuations as described in claim 1, characterized in that: The circulating compressor is driven by a variable frequency drive, and the ratio of the circulating gas flow rate to the fresh hydrogen flow rate corresponding to the speed adjustment range is 2:1 to 8:

1. When the fresh hydrogen flow rate drops below 50% of the rated flow rate, the control unit increases the operating frequency of the circulating compressor to make the ratio of the circulating gas flow rate to the fresh hydrogen flow rate reach 5:1 or higher; when the fresh hydrogen flow rate drops below 30% of the rated flow rate, the control unit increases the operating frequency of the circulating compressor to make the ratio of the circulating gas flow rate to the fresh hydrogen flow rate reach 7:1 or higher.

8. The carbon dioxide hydrogenation to methane synthesis system adaptable to hydrogen source fluctuations as described in claim 1, characterized in that: The hydrogen feed pipeline is connected to a renewable energy water electrolysis hydrogen production device, and the renewable energy includes wind energy, solar energy or wind-solar hybrid energy. The fluctuation of the hydrogen source is manifested as follows: during peak periods of renewable energy power generation, the flow rate of fresh hydrogen reaches 80% to 100% of the rated flow rate; during off-peak periods of renewable energy power generation, the flow rate of fresh hydrogen drops to 20% to 50% of the rated flow rate.

Images