A carbon dioxide methanation system and method

Abstract

This invention discloses a carbon dioxide methanation system and method, employing a core architecture of a single-stage adiabatic methanation reactor connected in series with a two-stage temperature-controlled methanation reactor. Carbon dioxide gas is distributed in a specific ratio and introduced into the inlet of each reactor. Hydrogen is mixed with the first batch of carbon dioxide and then enters the adiabatic reactor for a high-temperature reaction. The high-temperature gas exiting the reactor is dedicated to superheating the saturated steam produced as a byproduct of the subsequent temperature-controlled reactor. The mixed reaction gas is then mixed with the second and third batches of carbon dioxide and sequentially enters the two-stage temperature-controlled reactor for a deep methanation reaction. The heat of reaction is removed by a steam drum system via water circulation, producing steam as a byproduct. This invention, through optimized combination of reactor types, segmented injection of feed gas, and directional, stepped utilization of heat, achieves efficient conversion of carbon dioxide into methane product gas without a recirculating gas compressor. It offers advantages such as a short process, low energy consumption, flexible operation, and the production of high-quality steam as a byproduct.

Description

Technical Field

[0001] This invention belongs to the field of synthetic natural gas and chemical technology, specifically relating to a carbon dioxide methanation system and method. Background Technology

[0002] Carbon dioxide methane technology uses hydrogen and carbon dioxide as feedstocks to produce methane products through renewable energy or carbon neutrality technologies. It is a clean, low-carbon energy source, significant for reducing greenhouse gas emissions and improving air quality. Compared to traditional natural gas, green methane production is more environmentally friendly, reducing dependence on fossil fuels and contributing to sustainable development. From the perspective of reactants and pathways, green methane effectively couples two major carbon reduction technologies: new energy and carbon capture. It enables the recycling of carbon dioxide and the storage of green hydrogen. The generated methane can also be stored and transported using existing natural gas pipeline networks and infrastructure, potentially becoming a large-scale, long-term energy storage technology in the era of green electricity.

[0003] Therefore, against the backdrop of rapid development in new energy sources, green methane technology is expected to usher in development opportunities. The key to this technology lies in catalysts for carbon dioxide hydrogenation to methane under mild conditions, and in short-process technology that can be easily coupled with green hydrogen methanation.

[0004] Currently, traditional industrial methanation processes mostly employ adiabatic fixed-bed reactors, which are equipped with multiple adiabatic reactors and heat exchange equipment. Part of the reaction gas is circulated to dilute the carbon concentration of the raw materials and control the temperature rise of the reactor.

[0005] Because each adiabatic reactor has a high outlet temperature, and methanation is an exothermic reaction, from a chemical equilibrium perspective, lower temperatures are more conducive to methane production. Excessively high outlet temperatures mean a lower equilibrium conversion rate, making it impossible to completely convert CO2. The equilibrium conversion rate of a single reactor is limited. The process is lengthy and energy-intensive, typically involving 3, 4, or even 5 adiabatic reactors connected in series. Each reactor is followed by a heat exchanger to cool the gas and separate the water produced in the reaction. Furthermore, the need for circulating gas dilution and cooling, requiring a circulating compressor, results in low operational flexibility. Especially when green hydrogen or carbon dioxide fluctuates, controlling the bed temperature is difficult, easily leading to overheating or carbon buildup. Summary of the Invention

[0006] The purpose of this invention is to solve the above-mentioned technical problems by proposing a carbon dioxide methanation system and process with a short process flow, low energy consumption during device operation, high operational flexibility, and the ability to fully recover heat and by-product superheated steam.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a carbon dioxide methanation system, comprising an adiabatic methanation reactor, a first temperature-controlled methanation reactor, and a second temperature-controlled methanation reactor connected in series. The inlet of the adiabatic methanation reactor is connected to the shell-side outlet of the first inlet / outlet heat exchanger. The shell-side inlet of the first inlet / outlet heat exchanger is connected to the outlet of the first gas mixer. Hydrogen and carbon dioxide are introduced into the inlet of the first gas mixer. The outlet of the adiabatic methanation reactor is connected to the tube-side inlet of the first inlet / outlet heat exchanger. The tube-side outlet of the first inlet / outlet heat exchanger is connected to the inlet of the second gas mixer. Carbon dioxide is also introduced into the inlet of the second gas mixer. The outlet of the second gas mixer is connected to the shell-side inlet of the second inlet / outlet heat exchanger. The tube-side inlet and outlet of the first temperature-controlled methanation reactor are connected to the shell-side outlet and tube-side inlet of the second inlet / outlet heat exchanger, respectively. The tube-side outlet of the second inlet / outlet heat exchanger is connected to the inlet of the third gas mixer. Carbon dioxide is also introduced into the inlet of the third gas mixer. The third gas mixer is also connected to the shell-side inlet of the third inlet / outlet heat exchanger. The tube-side inlet and outlet of the second temperature-controlled methanation reactor are connected to the shell-side outlet and tube-side inlet of the third inlet / outlet heat exchanger, respectively.

[0008] This invention designs a three-stage reactor to replace the traditional 4-5 adiabatic reactors. The feed gas passes through in a single pass, and the reaction process in each stage is relatively controllable. This avoids the drawbacks of traditional processes that use circulating gas for dilution and cooling, and the high temperature rise and low equilibrium conversion rate of a single adiabatic reactor. Furthermore, the design of shell-side feed preheating and tube-side reactant gas heat exchange in the outlet heat exchangers of each stage recovers waste heat from the reactant gas for preheating the feed, reducing external heating energy consumption. Preferably, the system further includes a carbon dioxide desulfurization tank, the outlet of which is connected to the inlet of the first gas mixer, the second gas mixer, and the third gas mixer, respectively.

[0009] Sulfur impurities in CO2 are removed, and the total sulfur is reduced to <20vppb, which avoids sulfur poisoning that leads to deactivation of the methanation catalyst, extends the catalyst's lifespan, and reduces replacement costs.

[0010] Preferably, a steam superheater is provided between the outlet of the adiabatic methanation reactor and the tube-side inlet of the first inlet and outlet heat exchanger; the tube-side inlet of the steam superheater is connected to the outlet of the adiabatic methanation reactor, the tube-side outlet is connected to the tube-side inlet of the first inlet and outlet heat exchanger, and the shell-side outlet of the steam superheater is connected to the boundary pipeline used for transporting medium-pressure superheated steam.

[0011] By using the high-temperature reaction gas (550~680℃) at the outlet of the adiabatic reactor to heat the steam, the saturated steam is superheated into medium-pressure superheated steam, thus solving the problem of heat waste from high-temperature reaction gas in traditional processes.

[0012] Preferably, the system further includes a first steam drum and a second steam drum. The liquid phase outlet and gas phase inlet of the first steam drum are respectively connected to the shell-side inlet and shell-side outlet of the first temperature-controlled methanation reactor. The liquid phase inlet and gas phase outlet of the first steam drum are respectively connected to the boundary pipeline for transporting boiler feedwater and the shell-side inlet of the steam superheater. The liquid phase outlet and gas phase inlet of the second steam drum are respectively connected to the shell-side inlet and shell-side outlet of the second temperature-controlled methanation reactor. The liquid phase inlet of the second steam drum is connected to the boundary pipeline for boiler feedwater, and the gas phase outlet is respectively connected to the boundary pipeline for transporting medium-pressure superheated steam and the boundary pipeline for transporting medium-pressure saturated steam.

[0013] Two additional steam drums are added, each connected to the shell side of a two-stage temperature-controlled methanation reactor. The steam drums are connected to boiler feedwater, producing 3.5~6.5MPaG saturated steam as a byproduct. This solves the problems of temperature rise, easy overheating, and catalyst coking in traditional adiabatic reactors, and effectively recovers the heat of reaction.

[0014] Preferably, a first demineralized water preheater and a first product gas cooler are sequentially connected between the tube-side outlet of the first inlet / outlet heat exchanger and the feed inlet of the second gas mixer; a second demineralized water preheater and a second product gas cooler are sequentially connected between the tube-side outlet of the second inlet / outlet heat exchanger and the feed inlet of the third gas mixer; and the tube-side outlet of the third inlet / outlet heat exchanger is sequentially connected to a third demineralized water preheater and a third product gas cooler.

[0015] The demineralized water preheater utilizes the waste heat of the reaction gas to preheat the demineralized water, reducing energy consumption for subsequent boiler feedwater heating and achieving cascaded heat utilization, further reducing system energy consumption. The product gas cooler cools the reaction gas to 40°C, condensing the water vapor generated in the reaction and preventing high-temperature water from entering the next stage mixer / reactor. This prevents moisture from diluting the feed gas and causing mixing ratio deviations, and also prevents moisture from affecting catalyst activity, ensuring efficient execution of the next stage methanation reaction.

[0016] Preferably, a first product gas separator is connected between the tube-side outlet of the first product gas cooler and the inlet of the second gas mixer, a second product gas separator is connected between the tube-side outlet of the second product gas cooler and the inlet of the third gas mixer, and the tube-side outlet of the third product gas cooler is connected to a third product gas liquid separator.

[0017] The cooled condensate is separated from the reaction gas to prevent moisture from entering the next stage reactor with the feed gas, which would reduce CO2 conversion efficiency. The separated condensate can be sent to the boundary condensate refining system for recycling. The methane product gas, after drying, can be sent to the natural gas pipeline network as SNG, or it can be cryogenically liquefied as LNG.

[0018] This invention also provides a carbon dioxide methanation process, comprising: carbon dioxide, after desulfurization, entering a first gas mixer, a second gas mixer, and a third gas mixer respectively; hydrogen from the boundary pipeline mixing with carbon dioxide in the first gas mixer, then heating to 260-330°C through a first inlet / outlet heat exchanger before entering an adiabatic methanation reactor; after methanation in the adiabatic methanation reactor, the outlet temperature of the mixed reaction gas is 550-680°C; after sequentially passing through a steam superheater, a first inlet / outlet heat exchanger, a first demineralized water preheater, and a first product gas water cooler to a temperature of 40°C; after separation by a first product gas separator; the mixed gas then mixing with carbon dioxide in the second gas mixer, and heating to 230-280°C through a second inlet / outlet heat exchanger before entering a first temperature-controlled methanation reactor. In the alkylation reactor, after the methanation reaction in the first temperature-controlled methanation reactor, the outlet temperature of the mixed reaction gas is 330~380℃. After passing through the second inlet and outlet heat exchanger, the second demineralized water preheater, and the second product gas water cooler to cool down to 40℃, and after being separated by the second product gas separator, the mixed gas is mixed with carbon dioxide in the third gas mixer. After being heated to 230~280℃ by the third inlet and outlet heat exchanger, it enters the second temperature-controlled methanation reactor. After the methanation reaction in the second temperature-controlled methanation reactor, the outlet temperature of the product methane gas is 330~380℃. After passing through the third inlet and outlet heat exchanger, the third demineralized water preheater, and the third product gas water cooler to cool down to 40℃, and after being separated by the third product gas separator, the product methane gas is transported to the boundary pipeline for transporting methane.

[0019] Through a single-stage adiabatic reaction and two-stage temperature-controlled segmented conversion, coupled with precise temperature control, the CO2 volume content in the final methane gas is ensured to be <0.4%, meeting the feedstock requirements of Class I SNG or cryogenic LNG, and solving the problem of incomplete CO2 conversion in traditional processes. It integrates multi-stage heat recovery of high-temperature waste heat, medium-high-temperature waste heat, and medium-low-temperature waste heat, with no significant heat waste throughout the process and energy consumption significantly lower than traditional processes. The feedstock gas passes through in a single pass without the need for a circulating compressor, and the operating conditions can be quickly adjusted. The final product gas can be directly connected to the SNG pipeline network or LNG cryogenic unit.

[0020] Preferably, the feed ratio of desulfurized carbon dioxide gas in the first gas mixer, the second gas mixer, and the third gas mixer is 15-25%, 60-75%, and 10-15%, respectively, totaling 100%.

[0021] The carbon concentration entering the first gas mixer is limited to 15-25% to prevent high-concentration CO2 from entering the adiabatic reactor and causing excessive adiabatic temperature rise, while utilizing the high temperature to form superheated steam; the carbon concentration entering the second gas mixer is limited to 60-75% to concentrate the conversion of most of the CO2 in the temperature-controlled reactor, thereby improving the overall conversion efficiency; the carbon concentration entering the second gas mixer is limited to 10-15% to deeply remove residual CO2 in the second temperature-controlled reactor, ensuring that the outlet CO2 is <0.4%.

[0022] Preferably, the boiler feedwater in the first steam drum enters the shell side of the first temperature-controlled methanation reactor, absorbs the heat released by the methanation reaction in the tube side of the first temperature-controlled methanation reactor, and generates 3.5~6.5 MPaG saturated steam; the boiler feedwater in the second steam drum enters the shell side of the second temperature-controlled methanation reactor, absorbs the heat released by the methanation reaction in the tube side of the second temperature-controlled methanation reactor, and generates 3.5~6.5 MPaG saturated steam.

[0023] By using circulating water in the steam drum to transfer heat, the outlet temperature of the temperature-controlled reactor is stabilized at 330~380℃, strictly avoiding the risk of overheating, protecting the catalyst, and solving the overheating problem of traditional adiabatic reactors. The heat of reaction of the methanation reaction is converted into usable steam heat, with no additional heat loss.

[0024] Preferably, the operating pressure of the system is 2.0~5.0 MPaG, and the total volumetric flow rate ratio of hydrogen to carbon dioxide is 4:1~4.2:1.

[0025] The medium pressure range of 2.0~5.0 MPaG avoids both the slow reaction rate and low conversion rate caused by low pressure, and the high pressure requirements and increased investment costs of high pressure equipment, thus achieving a balance between economy and efficiency. The H2 / CO2 ratio is slightly higher than the theoretical 4:1 stoichiometric ratio. Excess H2 can drive the CO2 conversion equilibrium to the positive side, avoiding the problem of excessive CO2 residue due to insufficient H2, while also avoiding excessive waste of H2 and improving raw material utilization.

[0026] The beneficial effects of this invention are as follows: This invention requires only three methanation reactors to completely convert carbon dioxide, with an outlet carbon dioxide volume content of less than 0.4%, which can be directly used to produce Class I SNG or cryogenic LNG. The process flow is short, the reaction is a single pass, and there is no need for a circulating gas compressor, making start-up and shutdown convenient and highly flexible in operation. It can also more quickly adjust operating conditions to handle gas volume fluctuations. Furthermore, the system employing a single-stage adiabatic methanation reactor followed by a two-stage temperature-controlled methanation reactor utilizes the saturated steam byproduct from the high-temperature superheated temperature-controlled methanation reactor at the outlet of the adiabatic methanation reactor, improving steam quality and steam utilization. From the perspective of overall plant steam balance, this process allows for flexible adjustment of the ratio of superheated steam to saturated steam, optimizing the overall plant energy balance. Attached Figure Description

[0027] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate exemplary embodiments of the invention and, together with their description, serve to explain the invention and do not constitute an undue limitation thereof. In the drawings: Figure 1 The flowchart is provided for a carbon dioxide methanation system in Example 1.

[0028] The labels in the attached diagram are explained as follows: 1. Fine desulfurization tank; 2. First gas mixer; 3. First inlet and outlet heat exchanger; 4. Adiabatic methanation reactor; 5. Steam superheater; 6. First demineralized water preheater; 7. First product gas water cooler; 8. First product gas separator; 9. Second gas mixer; 10. Second inlet and outlet heat exchanger; 11. First temperature-controlled methanation reactor; 11-1. Tube-side inlet of the first temperature-controlled methanation reactor; 11-2. Tube-side outlet of the first temperature-controlled methanation reactor; 11-3. Shell-side inlet of the first temperature-controlled methanation reactor; 11-4. Shell-side outlet of the first temperature-controlled methanation reactor; 12. 13. First steam drum; 14. Second demineralized water preheater; 15. Second product gas water cooler; 16. Second product gas separator; 17. Third gas mixer; 18. Third inlet and outlet heat exchanger; 18-1. Tube-side inlet of the second temperature-controlled methanation reactor; 18-2. Tube-side outlet of the second temperature-controlled methanation reactor; 18-3. Shell-side inlet of the second temperature-controlled methanation reactor; 18-4. Shell-side outlet of the second temperature-controlled methanation reactor; 19. Second steam drum; 20. Third demineralized water preheater; 21. Third product gas water cooler; 22. Third product gas separator. Detailed Implementation

[0029] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0030] Example 1 like Figure 1 As shown, this embodiment provides a carbon dioxide methanation system, including a carbon dioxide desulfurization tank 1, a first gas mixer 2, a first inlet and outlet heat exchanger 3, an adiabatic methanation reactor 4, a steam superheater 5, a first demineralized water preheater 6, a first product gas water cooler 7, a first product gas separator 8, a second gas mixer 9, a second inlet and outlet heat exchanger 10, a first temperature-controlled methanation reactor 11, a first steam drum 12, a second demineralized water preheater 13, a second product gas water cooler 14, a second product gas separator 15, a third gas mixer 16, a third inlet and outlet heat exchanger 17, a second temperature-controlled methanation reactor 18, a second steam drum 19, a third demineralized water preheater 20, a third product gas water cooler 21, and a third product gas separator 22.

[0031] The outlet of the carbon dioxide desulfurization tank 1 is connected to the inlet of the first gas mixer 2, the second gas mixer 9, and the third gas mixer 16, respectively.

[0032] The inlet of the first gas mixer 2 is connected to both hydrogen and carbon dioxide gas. The shell-side inlet of the first inlet / outlet heat exchanger 3 is connected to the outlet of the first gas mixer 2. The inlet is connected to the shell-side outlet of the first inlet / outlet heat exchanger 3. A steam superheater 5 is installed between the outlet of the adiabatic methanation reactor 4 and the tube-side inlet of the first inlet / outlet heat exchanger 3. The tube-side inlet of the steam superheater 5 is connected to the outlet of the adiabatic methanation reactor 4, and its tube-side outlet is connected to the tube-side inlet of the first inlet / outlet heat exchanger 3. The shell-side outlet of the steam superheater 5 is connected to the boundary pipeline used for transporting medium-pressure superheated steam. The tube-side outlet of the first inlet / outlet heat exchanger 3 is connected to the tube-side inlet of the first demineralized water preheater 6. The tube-side outlet of the first demineralized water preheater 6 is connected to the tube-side inlet of the first product gas cooler 7. The tube-side outlet of the first product gas cooler 7 is connected to the first product gas separator 8. Demineralized water is introduced into the shell layer of the first demineralized water preheater 6, and circulating cooling water is introduced into the shell layer of the first product gas cooler 7.

[0033] The outlet of the first product gas separator 8 is connected to the inlet of the second gas mixer 9. The drain of the first product gas separator 8 is used to discharge condensate. The outlet of the second gas mixer 9 is connected to the shell-side inlet of the second inlet-outlet heat exchanger 10. The shell-side outlet of the second inlet-outlet heat exchanger 10 is connected to the tube-side inlet 11-1 of the first temperature-controlled methanation reactor 11. The tube-side outlet 11-2 of the first temperature-controlled methanation reactor 11 is connected to the tube-side inlet of the second inlet-outlet heat exchanger 10. The shell-side inlet 11-3 and shell-side outlet 11-4 of the first temperature-controlled methanation reactor 11 are connected to the liquid phase outlet and gas phase inlet of the first steam drum 12, respectively. The tube-side outlet of the second inlet-outlet heat exchanger 10 is connected to the tube-side inlet of the second demineralized water preheater 13. The tube-side outlet of the second demineralized water preheater 13 is connected to the tube-side inlet of the second product gas cooler 14. The tube-side outlet of the second product gas cooler 14 is connected to the second product gas separator 16. Demineralized water is introduced into the shell of the second demineralized water preheater 13, and circulating cooling water is introduced into the shell of the second product gas cooler 14. The liquid phase inlet and gas phase outlet of the first steam drum 12 are respectively connected to the boundary pipe used for transporting boiler feedwater and the shell inlet of the steam superheater 5.

[0034] The outlet of the second product gas separator 15 is connected to the inlet of the third gas mixer 9. The drain outlet of the second product gas separator 15 is used to discharge condensate. The outlet of the third gas mixer 16 is connected to the shell-side inlet of the third inlet / outlet heat exchanger 17. The shell-side outlet of the third inlet / outlet heat exchanger 17 is connected to the tube-side inlet 18-1 of the second temperature-controlled methanation reactor 18. The tube-side outlet 18-2 of the second temperature-controlled methanation reactor 18 is connected to the tube-side inlet of the third inlet / outlet heat exchanger 17. The shell-side inlet 18-3 and shell-side outlet 18-4 of the second temperature-controlled methanation reactor 18 are connected to the liquid phase outlet and gas phase inlet of the second steam drum 19, respectively. The tube-side outlet of the third inlet / outlet heat exchanger 17 is connected to the tube-side inlet of the third demineralized water preheater 20. The tube-side outlet of the third demineralized water preheater 20 is connected to the tube-side inlet of the third product gas cooler 21. The tube-side outlet of the third product gas cooler 21 is connected to the third product gas separator 22. The shell of the third demineralized water preheater 20 is fed with demineralized water, and the shell of the third product gas cooler 21 is fed with circulating cooling water. The outlet of the third product gas separator 22 is used to transport methane product gas, the drain outlet of the second product gas separator 15 is used to discharge condensate, the liquid phase inlet of the second steam drum 19 is connected to the boundary pipeline for boiler feedwater, and the gas phase outlet is connected to the boundary pipeline for transporting medium-pressure superheated steam and the boundary pipeline for transporting medium-pressure saturated steam, respectively.

[0035] The working principle of the carbon dioxide methanation system provided in this embodiment is as follows: After being refined in the carbon dioxide desulfurization tank 1, the carbon dioxide gas is divided into three batches and enters the first gas mixer 2, the second gas mixer 9, and the third gas mixer 16 respectively. After the carbon dioxide gas and the boundary hydrogen are mixed in the first gas mixer 2, they enter the first inlet and outlet heat exchanger 3 for preheating and then enter the adiabatic methanation reactor 4 for the first methanation reaction. The mixed gas discharged from the adiabatic methanation reactor 4 passes through the tube side of the steam superheater 5 and the tube side of the first inlet and outlet heat exchanger 3. The high-grade reaction heat generated by the first methanation reaction is recovered by the steam superheater 5. Part of it is used to generate medium-pressure superheated steam, and the other part is used to heat the first inlet and outlet heat exchanger 3.

[0036] After the effluent mixed gas from the adiabatic fixed-bed methanation reactor 4 is cooled by the first demineralized water preheater 6 and the first product gas cooler 7, it enters the first product gas separator 8 for gas-liquid separation. The mixed gas then enters the second gas mixer 9 to mix with carbon dioxide gas and enters the second inlet and outlet heat exchanger 10 for preheating. After preheating, it enters the first temperature-controlled methanation reactor 11 for the second methanation reaction. The heat of reaction generated is recovered by the first steam drum 12.

[0037] The mixed gas from the first temperature-controlled methanation reactor 11 enters the tube side of the second inlet / outlet heat exchanger 10. After being cooled by the second demineralized water preheater 13 and the second product gas cooler 14, it enters the second product gas separator 15 for gas-liquid separation. The mixed gas then enters the second product gas separator 16 and mixes with carbon dioxide gas before entering the third inlet / outlet heat exchanger 17 for preheating. Finally, it enters the second temperature-controlled methanation reactor 18 for the third methanation reaction, and the heat generated is recovered by the second steam drum 19. The mixed gas from the second temperature-controlled methanation reactor 18 enters the tube side of the third inlet / outlet heat exchanger 17. After being cooled by the third demineralized water preheater 20 and the third product gas cooler 21, it enters the third product gas separator 22 for gas-liquid separation. Methane product gas is collected from the outlet of the third product gas separator 22.

[0038] Methanation is a strongly exothermic reaction, releasing -165 kJ / mol of heat. From the equilibrium constant perspective, the equilibrium constant K corresponding to 360℃ is 0.11 × 10⁻⁶. 5The equilibrium constant K = 0.761 corresponding to 600℃ shows that low temperature is beneficial to the reaction and can significantly increase the equilibrium conversion rate. The first adiabatic methanation reactor 4 is set up as a small-batch methanation reactor, using the high temperature of the adiabatic methanation outlet to superheat the steam. The purpose of connecting the two temperature-controlled methanation reactors in series is to achieve a high CO2 conversion rate by controlling the reaction within the optimal temperature range. The first temperature-controlled methanation reactor 11 is a large-batch methanation reactor, mainly responsible for reacting most of the CO2 (traditional adiabatic reactors cannot achieve the task of reacting so much CO2 in a single reactor due to the need to avoid overheating at the reactor outlet and the limitation of the equilibrium constant). The second temperature-controlled methanation reactor 18 is a small-batch methanation reactor, mainly responsible for controlling the product indicators.

[0039] Example 2 This embodiment, based on the carbon dioxide methanation system provided in Embodiment 1, takes a SNG production scale of 100 million cubic meters per year as an example to provide a carbon dioxide methanation process flow. The hydrogen is obtained from a water electrolysis hydrogen production unit with a flow rate of 50,600 Nm³. 3 The gas concentration is 12600 Nm³ / h, with a volumetric composition of H₂ ≥ 99.99%, a temperature of 40℃, and a pressure of 2.5 MPaG; carbon dioxide gas comes from a carbon capture device at a flow rate of 12600 Nm³. 3 / h, volume composition: CO2: 99.1%, CO: 0.35%, H2: 0.01%, N2: 0.5%, CH4: 0.04%, temperature 40℃, pressure 2.5MPaG.

[0040] Carbon dioxide gas from the boundary area enters the fine desulfurization tank 1, where total sulfur is removed to less than 20 Vppb, and then divided into three parts with a feed rate of 2200 Nm³. 3 Carbon dioxide gas and hydrogen gas from the boundary area are mixed in the first gas mixer 2 and then enter the first inlet / outlet heat exchanger 3. After being heated to 320°C, it enters the adiabatic methanation reactor 4. The outlet temperature of the adiabatic methanation reactor is 575°C. After heat recovery by the steam superheater 5 and superheating of the 30.4 t / h 4.0 MPaG saturated steam byproduct from the steam drum, the temperature is reduced to 400°C. The mixed gas temperature at the outlet of the steam superheater 5 is 392°C. After heat exchange with the reactor inlet gas in the first inlet / outlet heat exchanger 3, the temperature drops to 119°C. Then, it is cooled to 40°C by the first demineralized water preheater 6 and the first product gas water cooler 7. After the product gas is separated by the first product gas separator 8, the mixed gas phase and part of the feed amount is 8600 Nm³. 3The carbon dioxide gas, at a rate of / h, is mixed in the second gas mixer 9 and then enters the second inlet / outlet heat exchanger 10. After being heated to 250℃, it enters the first temperature-controlled methanation reactor 11. The outlet temperature of the reaction gas from the first temperature-controlled methanation reactor 11 is 350℃. Heat exchange is conducted within the first temperature-controlled methanation reactor 11 via circulating hot water, and 26 t / h of 4.0 MPaG saturated steam is produced as a byproduct in the first steam drum 12. The product gas from the outlet of the first temperature-controlled methanation reactor 11 exchanges heat with the reactor inlet gas in the second inlet / outlet heat exchanger 10, reducing its temperature to 180℃. It then passes through the second demineralized water preheater 13 and the second product gas water cooler 14, further cooling to 40℃. After separation by the second product gas separator 15, the mixed gas phase and the remaining feed rate of 1800 Nm³ are... 3 After being mixed in the third gas mixer 16, the carbon dioxide gas at a rate of / h enters the third inlet and outlet heat exchanger 17, where it is heated to 250°C before entering the second temperature-controlled methanation reactor 18. The outlet temperature of the reaction gas from the second temperature-controlled methanation reactor 18 is 350°C. The second temperature-controlled methanation reactor 18 undergoes heat exchange via circulating hot water, and 4.4 t / h of 4.0 MPaG saturated steam is produced as a byproduct in the second steam drum 19. The product gas from the outlet of the second temperature-controlled methanation reactor 18 passes through the third inlet and outlet heat exchanger 17 and the reactor inlet gas heat exchanger, where its temperature drops to 148°C. It then passes through the third demineralized water preheater 20 and the third product gas water cooler 21, where its temperature is further reduced to 40°C. After being separated by the third product gas separator 22, the gaseous methane product gas is sent out of the boundary area. The volume composition of this gaseous methane product gas is: CH4: 94.32%, H2: 4.7%, N2: 0.48%, H2O: 0.31%, CO2: 0.19%. After drying, the methane product gas can be sent to the natural gas pipeline network as an SNG product.

[0041] Example 3: This embodiment, based on the carbon dioxide methanation system provided in Embodiment 1, takes a SNG production scale of 100 million cubic meters per year as an example to provide a carbon dioxide methanation process flow. By adjusting the total volumetric flow rate ratio of hydrogen to carbon dioxide, the feed ratio of carbon dioxide into different reactors, and controlling the reactor outlet temperature, the CO2 content in the outlet methane product gas can be further reduced, allowing the product gas to be directly sent to the downstream cryogenic separation unit to produce LNG.

[0042] Hydrogen is produced from a water electrolysis unit at a flow rate of 51200 Nm³. 3 The gas concentration is 12600 Nm³ / h, with a volumetric composition of H₂ ≥ 99.99%, a temperature of 40℃, and a pressure of 3.0 MPaG; carbon dioxide gas comes from a carbon capture device at a flow rate of 12600 Nm³. 3 / h, volume composition: CO2: 99.1%, CO: 0.35%, H2: 0.01%, N2: 0.5%, CH4: 0.04%, temperature 40℃, pressure 3.0MPaG.

[0043] Carbon dioxide gas from the boundary area enters the fine desulfurization tank 1, where total sulfur is removed to less than 20 Vppb, and then divided into three parts, 2320 Nm 3 Carbon dioxide gas and hydrogen gas from the boundary area are mixed in the first gas mixer 2 and then enter the first inlet / outlet heat exchanger 3. After being heated to 320°C, it enters the adiabatic methanation reactor 4. The product gas temperature at the outlet of the adiabatic methanation reactor 4 is 585°C. After heat recovery by the steam superheater 5 and superheating of the 30.45 t / h 4.0MPaG saturated steam by-product from the steam drum, the temperature is reduced to 400°C. The gas temperature at the outlet of the steam superheater 5 is 405°C. After heat exchange with the reactor inlet gas in the first inlet / outlet heat exchanger 3, the temperature is reduced to 127°C. Then, it is cooled to 40°C by the first demineralized water preheater 6 and the first product gas water cooler 7. After the product gas is separated by the first product gas separator 8, the gas phase is mixed with a portion of 8800 Nm 3 The carbon dioxide gas produced per hour is mixed in the second gas mixer 9 and then enters the second inlet / outlet heat exchanger 10. After being heated to 250°C, it enters the first temperature-controlled methanation reactor 11. The outlet temperature of the reaction gas from the first temperature-controlled methanation reactor 11 is 350°C. Heat is extracted from the first temperature-controlled methanation reactor 11 through circulating hot water, and 26.7 t / h of 4.0 MPaG saturated steam is produced as a byproduct in the first steam drum 12. The product gas from the outlet of the first temperature-controlled methanation reactor 11 passes through the second inlet / outlet heat exchanger 10 and the reactor inlet gas heat exchanger, where its temperature is reduced to 188°C. It then passes through the second demineralized water preheater 13 and the second product gas water cooler 14, where its temperature is further reduced to 40°C. After being separated by the second product gas separator 15, the gas phase and the remaining 1480 Nm³... 3 After being mixed in the third gas mixer 16, the carbon dioxide gas at a rate of / h enters the third inlet and outlet heat exchanger 17, where it is heated to 250°C before entering the second temperature-controlled methanation reactor 18. The outlet temperature of the reaction gas from the second temperature-controlled methanation reactor is 330°C. Heat exchange is conducted within the second temperature-controlled methanation reactor 18 via circulating hot water, and 3.75 t / h of 4.0 MPaG saturated steam is produced as a byproduct in the second steam drum 19. The product gas from the outlet of the second temperature-controlled methanation reactor 18 exchanges heat with the reactor inlet gas through the third inlet and outlet heat exchanger 17, reducing its temperature to 145°C. It then passes through the third demineralized water preheater 20 and the third product gas water cooler 21, further cooling to 40°C. After separation by the third product gas separator 22, the gaseous methane product gas is sent out of the boundary area. The volume composition of this gaseous methane product gas is: CH4: 91.11%, H2: 8.17%, N2: 0.46%, H2O: 0.26%, CO2: ~25 ppmv. The methane product gas can be sent to downstream cryogenic separation units to produce LNG.

[0044] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Those skilled in the art can readily implement the present invention according to the description and above. However, any modifications, alterations, or variations made by those skilled in the art without departing from the scope of the present invention, based on the disclosed technical content, are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, or variations made to the above embodiments based on the essential technology of the present invention are still within the protection scope of the present invention.

Claims

1. A carbon dioxide methanation system, characterized in that, It includes an adiabatic methanation reactor (4), a first temperature-controlled methanation reactor (11), and a second temperature-controlled methanation reactor (18) connected in series. The inlet of the adiabatic methanation reactor (4) is connected to the shell-side outlet of the first inlet / outlet heat exchanger (3), and the shell-side inlet of the first inlet / outlet heat exchanger (3) is connected to the outlet of the first gas mixer (2). Hydrogen and carbon dioxide are introduced into the inlet of the first gas mixer (2), and the outlet of the adiabatic methanation reactor (4) is connected to the tube-side inlet of the first inlet / outlet heat exchanger (3). The tube-side outlet of the first inlet / outlet heat exchanger (3) is connected to the inlet of the second gas mixer (9), and carbon dioxide is also introduced into the inlet of the second gas mixer (9). The outlet of the second gas mixer (9) is connected to the shell-side inlet of the second inlet / outlet heat exchanger (10). The tube-side inlet (11-1) and tube-side outlet (11-2) of the first temperature-controlled methanation reactor (11) are respectively connected to the shell-side outlet and tube-side inlet of the second inlet and outlet heat exchanger (10). The tube-side outlet of the second inlet and outlet heat exchanger (10) is connected to the feed port of the third gas mixer (16). Carbon dioxide gas is also introduced into the feed port of the third gas mixer (16). The third gas mixer (16) is also connected to the shell-side inlet of the third inlet and outlet heat exchanger (17). The tube-side inlet (18-1) and tube-side outlet (18-2) of the second temperature-controlled methanation reactor (18) are respectively connected to the shell-side outlet and tube-side inlet of the third inlet and outlet heat exchanger (17).

2. The carbon dioxide methanation system according to claim 1, characterized in that, The system also includes a carbon dioxide desulfurization tank (1), the outlet of which is connected to the feed inlets of the first gas mixer (2), the second gas mixer (9), and the third gas mixer (16).

3. The carbon dioxide methanation system according to claim 1, characterized in that, A steam superheater (5) is provided between the outlet of the adiabatic methanation reactor (4) and the tube inlet of the first inlet and outlet heat exchanger (3); the tube inlet of the steam superheater (5) is connected to the outlet of the adiabatic methanation reactor (4), and the tube outlet is connected to the tube inlet of the first inlet and outlet heat exchanger (3).

4. The carbon dioxide methanation system according to claim 1, characterized in that, It also includes a first steam drum (12) and a second steam drum (19). The liquid phase outlet and gas phase inlet of the first steam drum (12) are connected to the shell-side inlet (11-3) and shell-side outlet (11-4) of the first temperature-controlled methanation reactor (11), respectively. The liquid phase inlet and gas phase outlet of the first steam drum (12) are connected to the boundary pipe for transporting boiler feedwater and the shell-side inlet of the steam superheater (5), respectively. The liquid phase outlet and gas phase inlet of the second steam drum (19) are connected to the shell-side inlet (18-3) and shell-side outlet (18-4) of the second temperature-controlled methanation reactor (18), respectively.

5. The carbon dioxide methanation system according to claim 1, characterized in that, The tube-side outlet of the first inlet and outlet heat exchanger (3) is sequentially connected to the feed inlet of the second gas mixer (9) via a first demineralized water preheater (6) and a first product gas cooler (7); the tube-side outlet of the second inlet and outlet heat exchanger (10) is sequentially connected to the feed inlet of the third gas mixer (16) via a second demineralized water preheater (13) and a second product gas cooler (14); the tube-side outlet of the third inlet and outlet heat exchanger (17) is sequentially connected to a third demineralized water preheater (20) and a third product gas cooler (21).

6. The carbon dioxide methanation system according to claim 5, characterized in that, The first product gas cooler (7) is connected to the tube outlet of the first product gas cooler (7) and the feed inlet of the second gas mixer (9) via a first product gas separator (8). The second product gas cooler (14) is connected to the tube outlet of the second product gas cooler (14) and the feed inlet of the third gas mixer (16) via a second product gas separator (15). The tube outlet of the third product gas cooler (21) is connected to the third product gas liquid separator (22).

7. A method for the methanation of carbon dioxide, characterized in that, The system according to any one of claims 1-6 comprises: carbon dioxide gas, after desulfurization, entering a first gas mixer (2), a second gas mixer (9), and a third gas mixer (16); hydrogen gas from the boundary pipeline mixed with carbon dioxide gas in the first gas mixer (2), heated by a first inlet / outlet heat exchanger (3), and then entering an adiabatic methanation reactor (4); after methanation reaction in the adiabatic methanation reactor (4), the gas is then cooled to room temperature by passing sequentially through a steam superheater (5), a first inlet / outlet heat exchanger (3), a first demineralized water preheater (6), and a first product gas water cooler (7); after separation by a first product gas separator (8), the mixed gas is mixed with carbon dioxide gas in the second gas mixer (9), and heated by a second inlet / outlet heat exchanger (10). The gas then enters the first temperature-controlled methanation reactor (11). After the methanation reaction in the first temperature-controlled methanation reactor (11), the gas is then cooled to room temperature by passing through the second inlet and outlet heat exchanger (10), the second demineralized water preheater (13), and the second product gas water cooler (14). After being separated by the second product gas separator (15), the mixed gas and carbon dioxide gas are mixed in the third gas mixer (16). After being heated by the third inlet and outlet heat exchanger (17), the gas enters the second temperature-controlled methanation reactor (18). After the methanation reaction in the second temperature-controlled methanation reactor (18), the gas is then cooled to room temperature by passing through the third inlet and outlet heat exchanger (17), the third demineralized water preheater (20), and the third product gas water cooler (21). After being separated by the third product gas separator (22), the product methane gas is obtained.

8. The carbon dioxide methanation method according to claim 7, characterized in that, The feed ratios of desulfurized carbon dioxide gas in the first gas mixer (2), the second gas mixer (9), and the third gas mixer (16) are 15~25%, 60~75%, and 10~15%, respectively, totaling 100%.

9. The carbon dioxide methanation method according to claim 7, characterized in that, Boiler feedwater in the first steam drum (12) enters the shell side of the first temperature-controlled methanation reactor (11), absorbs the heat released by the methanation reaction in the tube side of the first temperature-controlled methanation reactor (11), and generates 3.5~6.5MPaG saturated steam; boiler feedwater in the second steam drum (19) enters the shell side of the second temperature-controlled methanation reactor (18), absorbs the heat released by the methanation reaction in the tube side of the second temperature-controlled methanation reactor (18), and generates 3.5~6.5MPaG saturated steam.

10. A method for carbon dioxide methanation according to claim 7, characterized in that, The operating pressure of the system is 2.0~5.0 MPaG, and the total volumetric flow rate ratio of hydrogen to carbon dioxide is 4:1~4.2:1.

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