Green methane synthesis system and process thereof

Abstract

The invention discloses a green methane synthesis system and a process thereof, the system comprises a water electrolysis device, a hydrogen storage tank, a fine desulfurization reactor, a gas holder, a main methanation reactor and a gas-liquid separation tank, a hydrogen outlet of the water electrolysis device is communicated with the hydrogen storage tank, and the fine desulfurization reactor is provided with a carbon source gas inlet; the hydrogen storage tank and the fine desulfurization reactor are both communicated with the gas holder, the gas-liquid separation tank comprises a first gas-liquid separation tank and a second gas-liquid separation tank, the gas holder, the main methanation reactor, the first gas-liquid separation tank and the second gas-liquid separation tank are respectively communicated in one time, and the first gas-liquid separation tank and the second gas-liquid separation tank are both provided with process condensate outlets. And the second gas-liquid separation tank is also provided with a methane outlet. The device has the advantages that by continuously adjusting the components of the mixed gas in real time, the stability of the components of the mixed gas can be realized under the condition that the front-end hydrogen flow changes too fast, so that the methanation reaction efficiency is ensured, and the overall economic benefit can be effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of green methane synthesis, and in particular to a green methane synthesis system and process thereof. Background Art

[0002] In recent years, with the growing global demand for a low-carbon economy and the depletion of fossil fuel resources, there has been an urgent need to combine renewable energy sources such as wind and solar power with non-fossil carbon sources to produce gaseous or liquid fuels. Methane, as the primary component of natural gas, stands out among other fuels due to its high calorific value, clean operation, and low CO2 emissions. Two main types of green methane synthesis processes are currently attracting significant attention. One involves using green electricity generated by wind or solar power to electrolyze water to produce green hydrogen. This hydrogen is then combined with captured green CO2 as feedstock to synthesize methane. This process offers significant CO2 emission reduction advantages and is expected to generate a significant green premium. The other combines captured green hydrogen with biomass gasification gas to produce methane. The future of this process depends on the maturity and economic feasibility of the underlying biomass gasification technology. In summary, both green methane synthesis processes require the addition of green hydrogen. Due to the naturally fluctuating nature of wind and solar resources, the green hydrogen feedstock fluctuates, posing a significant challenge to the stable operation of methane synthesis units.

[0003] Chinese patent CN117776833A discloses a low-energy methane synthesis method and synthesis device, which reduces system energy consumption and alleviates the carbon deposition problem of methane synthesis catalysts by combining adiabatic reaction with controllable heat transfer reaction. Chinese patent CN119158391A discloses a purification system for adjusting the hydrogen-carbon ratio of the raw gas at the methane synthesis inlet. By setting connecting pipelines inside the methane synthesis device and between different series of devices to adjust the gas composition, the risk of carbon deposition of the methane synthesis catalyst is reduced and the service life of the methane synthesis catalyst is extended. Chinese patent CN118480384A discloses a device for producing natural gas from lignite tail gas and its use method, which can produce various qualities of natural gas from lignite tail gas, has low comprehensive power consumption, and improves the utilization value of lignite tail gas.

[0004] None of the existing patents mentions the corresponding mitigation and solution measures when the methane synthesis feed gas is in a state of frequent fluctuations. Summary of the Invention

[0005] The purpose of the present invention is to provide a green methane synthesis system and process. By continuously adjusting the mixed gas components in real time, the mixed gas components can be stabilized when the front-end hydrogen flow rate changes too quickly, thereby ensuring the efficiency of the methanation reaction, reducing the number of monthly shutdowns, and achieving increased production of green methane, thereby effectively improving the overall economic benefits.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions:

[0007] A green methane synthesis system is characterized in that it includes a water electrolysis device, a hydrogen storage tank, a fine desulfurization reactor, a gas holder, a main methanation reactor and a gas-liquid separation tank, the hydrogen outlet of the water electrolysis device is connected to the hydrogen storage tank, the fine desulfurization reactor is provided with a carbon source inlet, the hydrogen storage tank and the fine desulfurization reactor are both connected to the gas holder, the gas-liquid separation tank includes a first gas-liquid separation tank and a second gas-liquid separation tank, the gas holder, the main methanation reactor, the first gas-liquid separation tank and the second gas-liquid separation tank are respectively connected once, the first gas-liquid separation tank and the second gas-liquid separation tank are both provided with a process condensate outlet, and the second gas-liquid separation tank is also provided with a methane outlet.

[0008] Preferably, the carbon source gas inlet of the fine desulfurization reactor is further connected to a first preheater, and the carbon source gas enters the fine desulfurization reactor through the first preheater.

[0009] Preferably, it also includes a second preheater and a first waste boiler, the outlet of the gas cabinet and the outlet of the main methanation reactor are both connected to the inlet of the second preheater, the inlet of the main methanation reactor and the inlet of the first waste boiler are both connected to the outlet of the second preheater, and the outlet of the first waste boiler is connected to the inlet of the first gas-liquid separation tank.

[0010] Preferably, a first hydrogen compressor is provided between the hydrogen storage tank and the gas cabinet, and a second hydrogen compressor is provided between the first gas-liquid separation tank and the second preheater.

[0011] Preferably, 2 to 4 groups of the main methanation reactors are arranged in parallel, and the inlet of each group of the main methanation reactors is individually provided with a shut-off valve, and the main methanation reactors are of an adiabatic type or a temperature-controlled type.

[0012] Preferably, a third preheater, a supplementary methanation reactor, a second waste boiler and a cooler are sequentially connected between the first gas-liquid separation tank and the second gas-liquid separation tank, and the cooler is water-cooled or air-cooled.

[0013] A green methane synthesis process, characterized by comprising the following steps:

[0014] The hydrogen coming out of the water electrolysis device first enters the hydrogen storage tank. The pressure of the hydrogen storage tank is controlled at 0.2-1.5 MPaG, and the total buffer time is controlled at 1-5 hours under normal load. The hydrogen compressor is regulated by PLC according to the pressure of the hydrogen storage tank and enters the gas cabinet;

[0015] Carbon dioxide or biomass gasification gas enters the preheater through the carbon source inlet, is preheated to 150-250°C, and then enters the fine desulfurization reactor. The total sulfur content of the gas leaving the fine desulfurization unit is controlled below 30ppb, and is initially mixed with the hydrogen in the hydrogen storage tank through PLC using a proportional pre-adjustment method. The mixed gas then enters the gas cabinet;

[0016] The gas composition analyzer is used in the gas cabinet to further monitor and adjust the mixed gas composition;

[0017] The mixed gas from the gas cabinet and the outlet gas of the methane reactor are heated in the second preheater to a temperature of 250-350°C before entering the main methanation reactor to synthesize methane.

[0018] The gas from the main methanation reactor is first preheated to a reaction temperature of 250-350°C in a preheater. After heat exchange, the reaction gas enters the first waste boiler for further heat recovery, while also producing medium- and low-pressure steam. The reaction gas then enters the first gas-liquid separator to separate the water produced by the methanation reaction as process condensate.

[0019] The reaction gas separated by condensate is preheated to 250-350°C in a preheater, and then enters the supplementary methanation reactor to further generate methane. The gas after exiting the supplementary methanation reactor passes through the second waste boiler to further recover heat, while producing medium and low-pressure steam as a by-product. The reaction gas then enters the cooler to further cool down to 30-40°C. After exiting the cooler, the reaction gas enters the second gas-liquid separation tank to further separate the water produced by the reaction as process condensate and methane product gas.

[0020] Preferably, the ratio of the gas from the refined desulfurization unit to the hydrogen in the hydrogen storage tank is pre-adjusted by controlling the coefficient M between 3.0 and 3.1. The coefficient M is calculated as follows:

[0021]

[0022] in is the molar fraction of hydrogen in the mixed gas, X CO is the molar fraction of carbon monoxide in the mixed gas, is the molar fraction of the carbon dioxide component in the mixed gas. The control method is to adjust the flow rate of carbon dioxide or biomass gasification gas based on the hydrogen flow rate so that the coefficient M is between 3.0 and 3.1.

[0023] Preferably, the gas cabinet is also equipped with a gas component analyzer to further monitor and secondary adjust the mixed gas components, specifically:

[0024] a. When the coefficient M is less than 2.80, the hydrogen storage capacity is first determined based on the hydrogen storage tank pressure feedback. If the storage tank pressure is greater than 0.3 MPaG, the PLC automatically controls the opening of the automatic valve on the secondary line between the hydrogen storage tank and the gas holder to replenish hydrogen until M is adjusted to the normal range. If the storage pressure is less than 0.3 MPaG, the PLC controls the increase of the flow rate of the second hydrogen compressor to ensure the stability of the bed temperature of the main methane synthesis reactor.

[0025] b. When the coefficient M>3.10, in order to ensure the efficiency of the methanation reaction, the flow rate of carbon dioxide or biomass gasification gas is increased through PLC control instructions until M is adjusted to the normal range.

[0026] In summary, the beneficial effects of the present invention are as follows: by continuously adjusting the mixed gas components in real time, the present invention can achieve stability in the mixed gas components when the front-end hydrogen flow changes too quickly, thereby ensuring the efficiency of the methanation reaction, reducing the number of monthly shutdowns, and achieving increased production of green methane, thereby effectively improving the overall economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the system flow structure of the present invention. DETAILED DESCRIPTION

[0028] The specific embodiments of the present invention will be further described below in conjunction with the accompanying drawings. This embodiment does not constitute a limitation to the present invention.

[0029] like Figure 1 A green methane synthesis system shown includes a water electrolysis device 1, a hydrogen storage tank 2, a fine desulfurization reactor 5, a gas cabinet 6, a main methanation reactor 9 and a gas-liquid separation tank. The hydrogen outlet of the water electrolysis device 1 is connected to the hydrogen storage tank 2. The number of hydrogen storage tanks 2 depends on the storage pressure and the volume of a single tank. The fine desulfurization reactor 5 is provided with a carbon source inlet. The hydrogen storage tank 2 and the fine desulfurization reactor 5 are both connected to the gas cabinet 6. The gas-liquid separation tank includes a first gas-liquid separation tank 11 and a second gas-liquid separation tank 16. The gas cabinet 6, the main methanation reactor 9, the first gas-liquid separation tank 11 and the second gas-liquid separation tank 16 are respectively connected once. The first gas-liquid separation tank 11 and the second gas-liquid separation tank 16 are both provided with a process condensate outlet, and the second gas-liquid separation tank 16 is also provided with a methane outlet.

[0030] The carbon source gas inlet of the fine desulfurization reactor 5 is also connected to the first preheater 4 , and the carbon source gas enters the fine desulfurization reactor 5 through the first preheater 4 .

[0031] It also includes a second preheater 7 and a first waste boiler 10. The outlet of the gas cabinet 6 and the outlet of the main methanation reactor 9 are both connected to the inlet of the second preheater 7, the inlet of the main methanation reactor 9 and the inlet of the first waste boiler 10 are both connected to the outlet of the second preheater 7, and the outlet of the first waste boiler 10 is connected to the inlet of the first gas-liquid separation tank 11.

[0032] A first hydrogen compressor 3 is provided between the hydrogen storage tank 2 and the gas cabinet 6 , and a second hydrogen compressor 8 is provided between the first gas-liquid separation tank 11 and the second preheater 7 .

[0033] The main methanation reactors 9 are arranged in parallel in 2 to 4 groups. Different from the traditional methanation process configuration, in order to adapt to the low-load operating conditions (load <20%) under fluctuating feed, the present invention adopts 2 to 4 groups of main methanation reactors 9 in parallel to flexibly adjust the operating load. The inlet of each group of main methanation reactors 9 is separately provided with a shut-off valve, and the main methanation reactors 9 are adiabatic or temperature-controlled.

[0034] A third preheater 12, a supplementary methanation reactor 13, a second waste boiler 14 and a cooler 15 are sequentially connected between the first gas-liquid separation tank 11 and the second gas-liquid separation tank 16. The cooler 15 is water-cooled or air-cooled.

[0035] A green methane synthesis process comprises the following steps:

[0036] The hydrogen coming out of the water electrolysis device 1 first enters the hydrogen storage tank 2. The pressure of the hydrogen storage tank 2 is controlled at 0.2-1.5 MPaG, and the total buffer time is controlled at 1-5 hours under normal load. The hydrogen compressor is regulated by the PLC according to the pressure of the hydrogen storage tank 2 and enters the gas cabinet 6;

[0037] Carbon dioxide or biomass gasification gas enters the preheater through the carbon source inlet, is preheated to 150-250°C, and then enters the fine desulfurization reactor 5. The total sulfur content of the gas leaving the fine desulfurization unit is controlled below 30ppb, and is initially mixed with the hydrogen in the hydrogen storage tank 2 by PLC using a proportional pre-adjustment method. The mixed gas enters the gas cabinet 6 to further alleviate the fluctuation of the gas flow at the inlet of the main methanation reactor 9. The size of the gas cabinet 6 is designed to provide a buffer time of 1-5 hours under normal load.

[0038] Considering that the rapid change of hydrogen flow at the front end leads to untimely adjustment of carbon dioxide or biomass gasification gas flow, a gas component analyzer is used in gas cabinet 6 to further monitor and adjust the mixed gas composition;

[0039] The mixed gas from the gas cabinet 6 and the methane reactor outlet gas are heat-exchanged in the second preheater 7, and the temperature is raised to 250-350° C., and then enters the main methanation reactor 9 to synthesize methane.

[0040] The gas exiting the main methanation reactor 9 is first preheated in a first preheater 7 to a reaction temperature of 250-350°C. After heat exchange, the reaction gas enters a first waste gas boiler 10 for further heat recovery. This by-product is low- and medium-pressure steam. The steam specifications (temperature and pressure) are determined based on the overall plant steam requirements, typically producing saturated or superheated steam at a grade of 0.5-4.0 MPaG. The reaction gas then enters a first gas-liquid separator 11 to separate the water produced by the methanation reaction as process condensate.

[0041] The reaction gas separated by condensate is preheated to 250-350°C in the second preheater 12, and then enters the supplementary methanation reactor 13 to further generate methane. The gas after exiting the supplementary methanation reactor 13 passes through the second waste boiler 14 to further recover heat, and at the same time produces medium and low-pressure steam as a by-product. The steam specifications (temperature and pressure) are based on the steam demand of the entire plant, and saturated or superheated steam of 0.5-4.0 MPaG grade is usually produced as a by-product. The reaction gas then enters the cooler 15 to further cool it to 30-40°C. After exiting the cooler 15, the reaction gas enters the second gas-liquid separation tank 16 to further separate the water produced by the reaction as process condensate and methane product gas.

[0042] The ratio of the gas leaving the refined desulfurization unit to the hydrogen in the hydrogen storage tank 2 is pre-adjusted by controlling the coefficient M between 3.0 and 3.1. The coefficient M is calculated as follows:

[0043]

[0044] in is the molar fraction of hydrogen in the mixed gas, X CO is the molar fraction of carbon monoxide in the mixed gas, is the molar fraction of the carbon dioxide component in the mixed gas. Taking into account the fluctuation of hydrogen flow rate, while the source and composition of carbon dioxide / biomass gasification gas are relatively stable, this adjustment method is based on the green hydrogen flow rate, and the coefficient M value is between 3.0 and 3.1. That is, when the hydrogen flow rate changes, the carbon dioxide or biomass gasification gas flow rate is controlled by M value and automatically adjusted proportionally to ensure the stability of the component.

[0045] The gas cabinet 6 is also equipped with a gas composition analyzer to further monitor and adjust the mixed gas composition. Specifically:

[0046] a. When the coefficient M is less than 2.80, the adequacy of hydrogen reserves is first determined based on the pressure feedback from hydrogen storage tank 2. If the tank pressure is greater than 0.3 MPaG, the PLC automatically controls the opening of the automatic valve on the secondary line between hydrogen storage tank 2 and gas holder 6 to replenish hydrogen until M is adjusted to the normal range. If the reserve pressure is less than 0.3 MPaG, the PLC controls the increase of the flow rate of the second hydrogen compressor 8 to ensure the stability of the bed temperature of the main methane synthesis reactor.

[0047] b. When the coefficient M>3.10, in order to ensure the efficiency of the methanation reaction, the flow rate of carbon dioxide or biomass gasification gas is increased through PLC control instructions until M is adjusted to the normal range.

[0048] Example 1

[0049] The process flow diagram for a green methane synthesis unit with an annual production capacity of 50,000 tons is shown below. Hydrogen is generated by a wind-powered water electrolysis unit 1, and CO2 is captured from a biomass power plant. Due to the fluctuating nature of front-end wind power generation, the unit's annual equivalent operating time is 5,000 hours. The system is equipped with five 2,000-cubic-meter hydrogen gas tanks and one 100,000-cubic-meter gas holder 6, with an M-value control index of 3.01-3.03. The main reactor is equipped with three adiabatic reactors of uniform size. Relying on coordinated regulation between the hydrogen storage tanks 2, the gas holder 6, and the main methanation reactor 9, the unit operates stably, with a minimum operating load of 10-15% and a maximum operating load of 110-120%. The average number of shutdowns per month has been reduced by three. Based on a minimum three-day recovery time between shutdowns and restarts, the unit's monthly green methane production has increased by 1,350 tons. At a cost of RMB 10,000 per ton of green methane, this translates to an additional RMB 13.5 million in monthly revenue, representing significant economic benefits.

[0050] Example 2

[0051] A green methane synthesis unit with an annual output of 80,000 tons uses biomass gasification to provide the unit's carbon source and some hydrogen. The remaining hydrogen comes from a water electrolysis unit powered by wind power generation. The biomass gasification unit in this project has a stable gas supply, while the front-end wind power generation fluctuates greatly. The annual equivalent operating time of the unit is 6,000 hours. The unit is equipped with three 2,000-cubic-meter hydrogen gas cylinders and one 100,000-cubic-meter gas cabinet. Because the methane product gas is supplied to the downstream cryogenic liquefaction unit, the CO2 content in the methane product gas is required to be less than 50 ppm, and the M value control index is 3.05-3.07. The main reactor is equipped with four temperature-controlled reactors of uniform size. Relying on the coordinated regulation of the hydrogen storage tank 2, the gas holder 6 and the main methanation reactor 9, the device operates stably, with the minimum operating load as low as 10% and the maximum operating load at 110% to 120%. The average number of shutdowns per month has been reduced by 2 times. Based on the recovery time from shutdown to start-up of at least 3 days each time, the monthly production of green methane is increased by 1,440 tons. Calculated at 10,000 yuan per ton of green methane, the monthly income can be increased by 14.4 million yuan, with significant economic benefits.

[0052] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art may make various modifications or equivalent substitutions to the present invention within the essence and protection scope of the present invention, and such modifications or equivalent substitutions should also be deemed to fall within the protection scope of the technical solution of the present invention.

Claims

1. A green methane synthesis system, characterized in that: It includes a water electrolysis device, a hydrogen storage tank, a fine desulfurization reactor, a gas holder, a main methanation reactor and a gas-liquid separation tank. The hydrogen outlet of the water electrolysis device is connected to the hydrogen storage tank. The fine desulfurization reactor is provided with a carbon source inlet. The hydrogen storage tank and the fine desulfurization reactor are both connected to the gas holder. The gas-liquid separation tank includes a first gas-liquid separation tank and a second gas-liquid separation tank. The gas holder, the main methanation reactor, the first gas-liquid separation tank and the second gas-liquid separation tank are respectively connected once. The first gas-liquid separation tank and the second gas-liquid separation tank are both provided with a process condensate outlet, and the second gas-liquid separation tank is also provided with a methane outlet.

2. A green methane synthesis system according to claim 1, characterized in that: The carbon source gas inlet of the fine desulfurization reactor is also connected to a first preheater, and the carbon source gas enters the fine desulfurization reactor through the first preheater.

3. A green methane synthesis system according to claim 1, characterized in that: It also includes a second preheater and a first waste boiler. The outlet of the gas cabinet and the outlet of the main methanation reactor are both connected to the inlet of the second preheater, the inlet of the main methanation reactor and the inlet of the first waste boiler are both connected to the outlet of the second preheater, and the outlet of the first waste boiler is connected to the inlet of the first gas-liquid separation tank.

4. A green methane synthesis system according to claim 3, characterized in that: A first hydrogen compressor is provided between the hydrogen storage tank and the gas cabinet, and a second hydrogen compressor is provided between the first gas-liquid separation tank and the second preheater.

5. A green methane synthesis system according to claim 1, characterized in that: The main methanation reactors are arranged in parallel in 2 to 4 groups, and the inlet of each group of the main methanation reactors is individually provided with a cut-off valve. The main methanation reactors are of adiabatic type or temperature-controlled type.

6. A green methane synthesis system according to claim 1, characterized in that: A third preheater, a supplementary methanation reactor, a second waste boiler and a cooler are sequentially connected between the first gas-liquid separation tank and the second gas-liquid separation tank. The cooler is water-cooled or air-cooled.

7. A green methane synthesis process, characterized by: The steps include: The hydrogen coming out of the water electrolysis device first enters the hydrogen storage tank. The pressure of the hydrogen storage tank is controlled at 0.2-1.5 MPaG, and the total buffer time is controlled at 1-5 hours under normal load. The hydrogen compressor is regulated by PLC according to the pressure of the hydrogen storage tank and enters the gas cabinet; Carbon dioxide or biomass gasification gas enters the preheater through the carbon source inlet, is preheated to 150-250°C, and then enters the fine desulfurization reactor. The total sulfur content of the gas leaving the fine desulfurization unit is controlled below 30ppb, and is initially mixed with the hydrogen in the hydrogen storage tank through PLC using a proportional pre-adjustment method. The mixed gas then enters the gas cabinet; The gas composition analyzer is used in the gas cabinet to further monitor and adjust the mixed gas composition; The mixed gas from the gas cabinet and the outlet gas of the methane reactor are heated in the second preheater to a temperature of 250-350°C before entering the main methanation reactor to synthesize methane. The gas from the main methanation reactor is first preheated to a reaction temperature of 250-350°C in a preheater. After heat exchange, the reaction gas enters the first waste boiler for further heat recovery, while also producing medium- and low-pressure steam. The reaction gas then enters the first gas-liquid separator to separate the water produced by the methanation reaction as process condensate. The reaction gas separated by condensate is preheated to 250-350°C in a preheater, and then enters the supplementary methanation reactor to further generate methane. The gas after exiting the supplementary methanation reactor passes through the second waste boiler to further recover heat, while producing medium and low-pressure steam as a by-product. The reaction gas then enters the cooler to further cool down to 30-40°C. After exiting the cooler, the reaction gas enters the second gas-liquid separation tank to further separate the water produced by the reaction as process condensate and methane product gas.

8. A green methane synthesis process according to claim 7, characterized in that: The ratio of the gas from the refined desulfurization unit to the hydrogen in the hydrogen storage tank is pre-adjusted by controlling the coefficient M between 3.0 and 3.

1. The coefficient M is calculated as follows: in is the molar fraction of hydrogen in the mixed gas, X CO is the molar fraction of carbon monoxide in the mixed gas, is the molar fraction of the carbon dioxide component in the mixed gas. The control method is to adjust the flow rate of carbon dioxide or biomass gasification gas based on the hydrogen flow rate so that the coefficient M is between 3.0 and 3.

1.

9. A green methane synthesis process according to claim 8, characterized in that: The gas cabinet is also equipped with a gas composition analyzer to further monitor and adjust the mixed gas composition. Specifically: a. When the coefficient M is less than 2.80, the hydrogen storage capacity is first determined based on the hydrogen storage tank pressure feedback. If the storage tank pressure is greater than 0.3 MPaG, the PLC automatically controls the opening of the automatic valve on the secondary line between the hydrogen storage tank and the gas holder to replenish hydrogen until M is adjusted to the normal range. If the storage pressure is less than 0.3 MPaG, the PLC controls the increase of the flow rate of the second hydrogen compressor to ensure the stability of the bed temperature of the main methane synthesis reactor. b. When the coefficient M>3.10, in order to ensure the efficiency of the methanation reaction, the flow rate of carbon dioxide or biomass gasification gas is increased through PLC control instructions until M is adjusted to the normal range.

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