A method and apparatus for producing high-calorific-value synthetic natural gas by CO2 hydrogenation

Abstract

This invention belongs to the field of CO2 catalytic conversion technology, specifically relating to a method and apparatus for producing high-calorific-value synthetic natural gas (SCNF) via CO2 hydrogenation. The invention provides a method and apparatus for producing SCNF via CO2 hydrogenation, mainly comprising CO2 hydrogenation, product separation, and energy cascade utilization units. Only electrical energy and extremely low heat input are required to maintain the normal operation of the process and apparatus. In extreme cases, only electrical energy input is needed without any heat input to achieve the production of SCNF via CO2 hydrogenation. Simultaneously, the energy efficiency of the entire process is 77%, demonstrating high energy utilization. The carbon dioxide emissions are low, at 0.29 tons per ton of SCNF produced, and the lower heating value of the SCNF produced is 39.3 MJ / Nm³. 3 .

Description

Technical Field

[0001] This invention belongs to the field of CO2 catalytic conversion technology, specifically relating to a method and apparatus for producing high-calorific-value natural gas by hydrogenating CO2. Background Technology

[0002] With the accelerated pace of industrialization, excessive CO2 emissions have triggered a series of severe environmental pollution, climate change, and energy crises. Against this backdrop, developing efficient CO2 catalytic conversion technologies to achieve the recycling of waste carbon resources is particularly important. Currently, CO2 hydrogenation conversion technology has made significant progress, capable of producing a variety of high-value-added chemicals such as synthetic natural gas, methanol, dimethyl ether, and low-carbon olefins. Synthetic natural gas, in particular, offers unique advantages in energy supply, heating, and power generation due to its convenient transport via existing natural gas pipeline networks. However, compared to traditionally extracted natural gas, the main component of synthetic natural gas, methane, has a relatively low calorific value. Current solutions typically require blending with liquefied petroleum gas (LPG) containing ethane, propane, and butane to increase calorific value, but this method still heavily relies on traditional fossil fuels such as oil. To fundamentally solve this problem, the development of novel CO2 hydrogenation technology for producing high-calorific-value synthetic natural gas is urgently needed. Breakthroughs in this technology will help promote the gradual replacement of traditional high-polluting fossil fuels such as oil with high-calorific-value synthetic natural gas, thus providing an innovative solution for the strategic adjustment of my country's energy structure.

[0003] Therefore, developing a new method for producing high-calorific-value synthetic natural gas by CO2 hydrogenation, which combines high energy efficiency and low carbon emissions, would not only improve the quality of existing synthetic natural gas but also significantly promote the sustainable use of carbon resources, which is of great strategic significance for achieving the "dual carbon" goal. Summary of the Invention

[0004] The purpose of this invention is to provide a method and apparatus for producing high-calorific-value synthetic natural gas via CO2 hydrogenation. The method provided by this invention requires only electrical energy and extremely low heat input to maintain the normal operation of the process and apparatus. In extreme cases, only electrical energy input is needed, without requiring any heat input, to achieve the production of high-calorific-value synthetic natural gas via CO2 hydrogenation. Simultaneously, the energy efficiency of the entire process is 77%, demonstrating high energy utilization. The carbon emission is low, with only 0.29 tons of carbon dioxide emitted per ton of high-calorific-value synthetic natural gas produced. The lower heating value of the synthetic natural gas produced is 39.3 MJ / Nm³. 3 .

[0005] This invention provides a method for producing high-calorific-value synthetic natural gas by hydrogenating CO2, comprising the following steps:

[0006] CO2 and H2 are mixed to obtain a mixed gas. The mixed gas is then subjected to a first compression, a first preheating, and a second preheating in sequence. CO2 and H2 are then selectively catalytically converted into high-heat synthetic natural gas and by-products via a hydrogenation reaction on an iron-based catalyst. This reaction releases heat.

[0007] A gas stream containing unreacted substances, byproducts, and main products is obtained through a hydrogenation reaction. The gas stream is then subjected to a first cooling, a first dehydration, decarbonization, and hydrogen recovery. A portion of the recovered feed gas is recycled to the hydrogenation reactor after a second compression. The unrecovered feed gas, byproducts, and main products are then preheated in a third process before entering a methanation reactor. The methanation reaction releases heat, and the unrecovered feed gas and byproducts are further preheated in the third process before being completely converted into methane and water through a methanation reaction on a nickel-based catalyst. The resulting gas stream is then subjected to a second cooling and a second dehydration to obtain the target product, high-calorific-value synthetic natural gas.

[0008] The sensible heat released by the first and second coolers serves as the heat source for the first and third preheaters, respectively; part of the heat released by the hydrogenation reactor serves as the energy source required for the decarbonization and hydrogen recovery process, and the other part is converted into electrical energy for the second compression through a Rankine cycle; part of the heat released by the methanation reactor serves as the heat source for the second preheater, and the other part is converted into electrical energy for the second compression through a Rankine cycle.

[0009] Preferably, the molar ratio of H2 to CO2 in the mixed gas is 1 to 3.

[0010] Preferably, the first preheating temperature is 200-300°C, the second preheating temperature is 300-350°C, and the hydrogenation reaction temperature is 300-350°C, with a pressure of 1-3 MPa.

[0011] Preferably, the final temperature of the first cooling is 10–30°C.

[0012] Preferably, the temperature of the third preheating is 400-500°C; the temperature of the methanation reaction is 400-500°C.

[0013] Preferably, the final temperature of the second cooling is 10–30°C.

[0014] Preferably, the CO2 recovery rate during carbon separation is between 88-93%, and the H2 recovery rate during hydrogen recovery is no higher than 85%.

[0015] This invention also provides an apparatus for the method of producing high-calorific-value synthetic natural gas by CO2 hydrogenation as described in the above technical solution.

[0016] include:

[0017] CO2 hydrogenation unit, product separation unit and energy cascade utilization unit;

[0018] The CO2 hydrogenation unit includes a first compressor, a first preheater, a second preheater, a hydrogenation reactor, a first cooler, and a first flash tank connected in sequence.

[0019] The product separation unit includes a CO2 absorption tower, a solvent regeneration tower, an H2 adsorption tower, an H2 desorption tower, a second compressor, a third preheater, a methanation reactor, a second cooler, and a second flash tank, which are connected in sequence.

[0020] The energy cascade utilization unit is used to recover the sensible heat generated by the first and second coolers, as well as the reaction heat released from the hydrogenation and methanation reactions.

[0021] The sensible heat released by the first and second coolers serves as the heat source for the first and third preheaters, respectively; part of the heat released by the hydrogenation reactor serves as the energy source required by the solvent regeneration tower and the hydrogen adsorption tower, and the other part is converted into electrical energy for the second compressor through a Rankine cycle; part of the heat released by the methanation reactor serves as the heat source for the second preheater, and the other part is converted into electrical energy for the second compressor through a Rankine cycle.

[0022] Beneficial effects

[0023] This invention provides a method and apparatus for producing high-calorific-value synthetic natural gas (SMU) from CO2 via hydrogenation. Through efficient catalytic conversion, CO2 and H2 are selectively converted into SMU, achieving high-value utilization of waste carbon resources and providing a sustainable alternative to traditional natural gas, while simultaneously increasing the lower heating value of the SMU. This invention rationally allocates recoverable waste heat within the system through energy cascade utilization. Part of the waste heat is used for preheating the feedstock gas, significantly reducing external heating requirements and even achieving zero heat input under extreme conditions. The remaining waste heat is converted into electricity using Rankine cycle technology to power the compression stage, thereby significantly reducing overall energy consumption. This invention, through heat-electricity synergistic optimization, not only significantly improves energy utilization efficiency but also effectively reduces CO2 emissions, achieving both technological and environmental benefits. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the embodiments will be briefly described below.

[0025] Figure 1 This is a schematic diagram of the process for producing high-calorific-value natural gas by CO2 hydrogenation in an example; where: H represents heat recovery; S represents material flow; and E represents electrical energy transportation. Detailed Implementation

[0026] This invention provides a method for producing high-calorific-value synthetic natural gas by hydrogenating CO2, comprising the following steps:

[0027] CO2 captured from industrial exhaust gas is mixed with renewable H2 obtained through water electrolysis to obtain a mixed gas. The temperature of the mixed gas is at room temperature, and the pressure is between 0.1 and 0.7 MPa, specifically 0.1 MPa, 0.5 MPa, and 0.7 MPa. Then, a first compression is performed, resulting in a mixed gas pressure of 1 to 3 MPa, specifically 1 MPa, 2 MPa, and 3 MPa. A first preheating and a second preheating are then performed. After the first preheating, the temperature of the mixed gas increases to 200 to 300°C, specifically 200°C, 250°C, and 300°C. After the second preheating, the temperature of the mixed gas increases to 300 to 350°C, specifically 300°C. At 320℃ and 350℃, after pressurization and heating, the mixed gas selectively catalytically converts some CO2 and H2 into the main products methane, ethane, propane, and butane, and the byproduct CO, through a hydrogenation reaction on an iron-based catalyst. The hydrogenation reaction temperature is 300-350℃, specifically 300℃, 320℃, and 350℃, and the pressure is 1-3MPa, specifically 1MPa, 2MPa, and 3MPa. The specific reactions are shown in Equations 1-5, where Equations 1-4 are the dominant reactions that release heat, and Equation 5 is the secondary reaction that absorbs heat. The overall hydrogenation reaction releases heat. The iron-based catalyst can specifically be an Fe-Cu / Al2O3 catalyst.

[0028] CO2 + 4H2 → CH4 + 2H2O (1)

[0029] 2CO2 + 7H2 → C2H6 + 4H2O (2)

[0030] 3CO2 + 10H2 → C3H8 + 6H2O (3)

[0031] 4CO2 + 13H2 → C4H 10 +8H2O (4)

[0032] CO2 + H2 → CO + H2O (5)

[0033] A gas stream containing unreacted CO2 and H2, main products methane, ethane, propane, and butane, and byproducts CO and water, obtained through hydrogenation, undergoes a first cooling process, with the temperature reduced to 10–30°C (specifically 10°C, 20°C, and 30°C). The cooled stream then undergoes a first dehydration process to remove moisture. Subsequently, the gas stream containing unreacted CO2 and H2, main products methane, ethane, propane, and butane, and byproduct CO is subjected to chemical absorption using a solvent to absorb some of the CO2. The CO2 absorbed by the solvent is then separated from the solvent through solvent desorption, and the solvent is recycled. The CO2 recovery rate is between 88% and 93%, specifically 88%, 90%, and 93%. The unabsorbed CO2 and other substances are then subjected to pressure swing adsorption (PSA) to adsorb some of the H2 in the mixture. The adsorbed H2 is then separated from the adsorbent through desorption, and the adsorbent is recycled. The H2 recovery rate is not high. The recovery rate is above 85%, specifically 80%, 82%, and 85%. The recovered H2 and CO2 undergo a second compression and are recycled for hydrogenation. The unrecovered CO2, H2, main products methane, ethane, propane, and butane, as well as the byproduct CO, undergo a third preheating, with the temperature increased to 400–500°C, specifically 400°C, 450°C, and 500°C. Subsequently, CO2, H2, and CO are completely converted into methane and water through a methanation reaction on a nickel-based catalyst. The methanation reaction temperature is 400–500°C, specifically 400°C, 450°C, and 500°C, as shown in Formulas 1 and 6. This reaction releases heat, and the resulting gas stream containing methane, ethane, propane, butane, and water is cooled to 10–30°C through a second cooling process, specifically 10°C, 20°C, and 30°C. The cooled stream is then dehydrated through a second dehydration process to obtain the target product, high-calorific-value synthetic natural gas.

[0034] CO + 3H₂ → CH₄ + H₂O (6)

[0035] The high-temperature stream obtained through the hydrogenation reaction releases a portion of its sensible heat through the first cooling process, which serves as the heat source for the first preheating. The high-temperature stream obtained through the methanation reaction releases a portion of its sensible heat through the second cooling process, which serves as the heat source for the third preheating. A portion of the heat released through the hydrogenation reaction serves as the energy source for solvent regeneration during carbon separation and for H2 adsorption during hydrogen recovery. The remaining heat is converted into electrical energy through a Rankine cycle and used in the second compression process. A portion of the heat released through the methanation reaction serves as the heat source for the second preheating, and the remaining heat is converted into electrical energy through a Rankine cycle and used in the second compression process.

[0036] This invention provides an apparatus for producing high-calorific-value synthetic natural gas by CO2 hydrogenation, mainly comprising a CO2 hydrogenation unit, a product separation unit, and an energy cascade utilization unit. The CO2 hydrogenation unit includes a first compressor, a first preheater, a second preheater, a hydrogenation reactor, a first cooler, and a first flash tank connected in sequence. The product separation unit includes a CO2 absorption tower, a solvent regeneration tower, an H2 adsorption tower, an H2 desorption tower, a second compressor, a third preheater, a methanation reactor, a second cooler, and a second flash tank connected in sequence. The energy cascade utilization unit is used to recover the sensible heat generated by the first and second coolers, as well as the reaction heat released from the hydrogenation and methanation reactions. The sensible heat released by the first and second coolers serves as the heat source for the first and third preheaters, respectively. Part of the heat released by the hydrogenation reactor serves as the energy source for the solvent regeneration tower and the hydrogen adsorption tower, while the other part is converted into electrical energy for the second compressor via a Rankine cycle. Part of the heat released by the methanation reactor serves as the heat source for the second preheater, while the other part is converted into electrical energy for the second compressor via a Rankine cycle.

[0037] To further illustrate the present invention, the method for producing methanol from natural gas provided by the present invention will be described in detail below with reference to the accompanying drawings and embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0038] The calculation methods for energy efficiency and carbon emissions in the embodiments and comparative examples of the present invention are shown in Equations 7 and 8.

[0039]

[0040] Example 1

[0041] A mixture of 100 kmol / h CO2 and 300 kmol / h H2 was prepared at a temperature of 25 °C and a pressure of 0.1 MPa. The mixture was then compressed to 3 MPa, consuming 1.71 GJ / h of energy. The temperature was then raised to 300 °C via a first preheating process, followed by a second preheating to 320 °C. Hydrogenation was then carried out on a Fe-Co / Al2O3 catalyst, selectively converting some of the CO2 and H2 into methane, ethane, propane, and butane as the main products, and carbon monoxide as the byproduct. The hydrogenation reaction was conducted at 320 °C and 3 MPa, releasing 15.61 GJ / h of heat.

[0042] A gas stream containing unreacted CO2 and H2, main products methane, ethane, propane, and butane, and byproducts CO and water, obtained through hydrogenation, undergoes a first cooling process to a temperature of 10°C. The cooled stream then passes through a first dehydration process to remove moisture. Subsequently, the gas stream containing unreacted CO2 and H2, main products methane, ethane, propane, and butane, and byproduct CO is subjected to chemical absorption using a solvent to absorb some of the CO2. The CO2 absorbed by the solvent is then separated from the solvent through solvent desorption, with the solvent recycled. The CO2 recovery rate is 90%. The unabsorbed CO2 and other substances are then subjected to pressure swing adsorption (PSA) to adsorb some of the H2 in the mixture. The adsorbed H2 is then desorbed, achieving H2 and adsorption separation. The adsorbent is separated and recycled, with an H2 recovery rate of 82%. The recovered H2 and CO2 are subjected to a second compression and recycled for hydrogenation reaction, with a compression energy consumption of 0.27 GJ / h. The unrecovered CO2, H2, main products methane, ethane, propane, and butane, as well as the byproduct CO, are subjected to a third preheating, with the temperature raised to 450°C. Subsequently, CO2, H2, and CO are completely converted into methane and water through a methanation reaction on a nickel-based catalyst at a temperature of 450°C. This reaction releases 1.11 GJ / h of heat. The resulting gas stream containing methane, ethane, propane, butane, and water is cooled to 10°C in a second cooling process. The cooled stream is then subjected to a second dehydration process to remove moisture, yielding the target product, high-calorific-value synthetic natural gas.

[0043] Of the sensible heat released by the high-temperature stream obtained through the hydrogenation reaction (5.28 GJ / h) during the first cooling, this heat is used as the heat source for the first preheating. Of the sensible heat released by the high-temperature stream obtained through the methanation reaction (2.04 GJ / h) during the second cooling, this heat is used as the heat source for the third preheating. The heat released by the hydrogenation reaction (11.74 GJ / h) is used as the energy source for solvent regeneration during carbon separation, and 1.78 GJ / h is used as the energy source for H2 adsorption during hydrogen recovery. 2.08 GJ / h is converted into electrical energy (0.17 GJ / h) through a Rankine cycle and used in the second compression process. The heat released by the methanation reaction (0.45 GJ / h) is used as the heat source for the second preheating, and 0.66 GJ / h is converted into electrical energy (0.05 GJ / h) through a Rankine cycle and used in the second compression process. Under the above process configuration and heat exchange scheme, no additional heat input is required inside the system, but it still requires 1.76 GJ / h of electrical energy input, with an energy efficiency of 77%. For every ton of high-calorific-value synthetic natural gas produced, 0.29 tons of carbon dioxide are emitted.

[0044] Comparative Example 1

[0045] The only difference from Example 1 is that the sensible heat released during the cooling process of the high-temperature stream, as well as the heat released during the hydrogenation and methanation reactions, are no longer recovered or converted into electrical energy via the Rankine cycle. For compression purposes, all heat is transferred using cooling water, and all energy-consuming equipment within the system (such as heaters and compressors) is supplied externally. Under the above process configuration and heat exchange scheme, the system requires an input heat of 21.29 GJ / h and an electrical energy of 2.13 GJ / h, with an energy efficiency of 62%. For every ton of high-calorific-value synthetic natural gas produced, 1.2 tons of carbon dioxide are emitted. Through Comparative Example 1 and Example 1, this invention significantly improves energy efficiency and reduces carbon emissions through energy cascade utilization.

[0046] Comparative Example 2

[0047] Compared to Example 1, Comparative Example 2 only considers the CO2 methanation process, where 100 kmol / h of CO2 and 300 kmol / h of H2 are mixed to obtain a mixed gas at a temperature of 25°C and a pressure of 0.1 MPa. The mixed gas is then first compressed to 3 MPa, and after a first preheating, the temperature is raised to 450°C. On a nickel-based catalyst, part of the gas is converted to methane through a methanation reaction. Unreacted CO2, H2, the main product methane, and the byproduct water are then first cooled to 10°C and first dehydrated to obtain a gas stream containing unreacted CO2, H2, and the main product methane. This gas stream is then subjected to chemical absorption. The process uses a solvent to absorb some of the CO2. The absorbed CO2 is then separated from the solvent through solvent desorption, and the solvent is recycled. The CO2 recovery rate is 90%. The unabsorbed CO2 and other substances are partially absorbed by the mixture using pressure swing adsorption (PSA). The adsorbed H2 is separated from the adsorbent through desorption, and the adsorbent is recycled. The H2 recovery rate is 82%. The mixture then undergoes carbon separation and hydrogen recovery processes to separate CO2, H2, and the main product, methane. The recovered CO2 and H2 are recycled and, after a second compression, used in a methanation reaction. The resulting methane is the final product. The lower heating value of each standard cubic meter of synthetic natural gas produced is 32.5 MJ / Nm³. 3 By comparing Example 2 and Example 1, this invention first selectively catalytically converts a portion of CO2 and H2 into methane, ethane, propane, and butane via hydrogenation on an iron-based catalyst. Then, through methanation and recycling, the unreacted CO2 and H2 are further converted into the target product, significantly improving the lower heating value of the product.

[0048] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for producing high-calorific-value synthetic natural gas by hydrogenation of CO2, characterized in that, Includes the following steps: CO2 and H2 are mixed to obtain a mixed gas. This mixed gas undergoes a first compression, a first preheating, and a second preheating process. Then, on an iron-based catalyst, a hydrogenation reaction selectively converts some of the CO2 and H2 into high-calorific-value synthetic natural gas and byproducts. The resulting mixed stream undergoes a first cooling and a first dehydration and gas-liquid separation process to obtain a gas stream and a liquid stream. The resulting gas stream sequentially enters a decarbonization separation tower and a hydrogen recovery tower. The recovered CO2 and H2 are second-compressed and returned to the hydrogenation reactor. Unrecovered CO2, H2, byproduct CO, and the main product light hydrocarbons are third-preheated and then enter a methanation reactor. A methanation reaction converts CO2, H2, and CO into methane and water. The resulting gas stream undergoes a second cooling and a second dehydration and gas-liquid separation process to obtain the main product, high-calorific-value synthetic natural gas. The hydrogenation and methanation reactions release heat. The sensible heat released by the first and second cooling processes serves as the heat source for the first and third preheating processes, respectively; part of the heat released by the hydrogenation reaction serves as the energy source required for decarbonization and hydrogen recovery, and the other part is converted into electrical energy for the compression process through a Rankine cycle; part of the heat released by the methanation reaction serves as the heat source for the second preheating process, and the other part is converted into electrical energy for the compression process through a Rankine cycle.

2. The method according to claim 1, characterized in that, The molar ratio of H2 to CO2 in the mixed gas is 1 to 3.

3. The method according to claim 1, characterized in that, The first preheating temperature is 200-300℃, and the second preheating temperature is 300-350℃; the hydrogenation reaction temperature is 300-350℃, and the pressure is 1-3 MPa; the hydrogenation reaction is carried out under the catalysis of a catalyst.

4. The method according to claim 1, characterized in that, The final temperature of the first cooling is 10–30°C.

5. The method according to claim 1, characterized in that, The third preheating temperature is 400–500°C; the methanation reaction temperature is 400–500°C and the pressure is 1–2 MPa; the methanation reaction is carried out under the catalysis of a catalyst.

6. The method according to claim 1, characterized in that, The final temperature of the second cooling is 10–30°C.

7. The method according to claim 1, characterized in that, CO2 recovery uses chemical absorption, and the carbon separation tower consists of a CO2 absorption tower and a solvent regeneration tower. H2 recovery uses pressure swing adsorption, and the hydrogen recovery tower consists of an H2 adsorption tower and an H2 desorption tower.

8. The method according to claim 1 or 7, characterized in that, The CO2 recovery rate is between 88-93%, and the H2 recovery rate is no higher than 85%, ensuring that there is enough H2 in the subsequent methanation process to convert CO2 and CO into methane.

9. The apparatus used in the method for producing high-calorific-value synthetic natural gas by CO2 hydrogenation according to any one of claims 1 to 8, characterized in that, include: CO2 hydrogenation unit, product separation unit and energy cascade utilization unit; The CO2 hydrogenation unit includes a first compressor, a first preheater, a second preheater, a hydrogenation reactor, a first cooler, and a first flash tank connected in sequence. The product separation unit includes a CO2 absorption tower, a solvent regeneration tower, an H2 adsorption tower, an H2 desorption tower, a second compressor, a third preheater, a methanation reactor, a second cooler, and a second flash tank, which are connected in sequence. The energy cascade utilization unit is used to recover the sensible heat generated by the first and second coolers, as well as the reaction heat released from the hydrogenation and methanation reactions. The sensible heat released by the first and second coolers serves as the heat source for the first and third preheaters, respectively; part of the heat released by the hydrogenation reactor serves as the energy source required by the solvent regeneration tower and the hydrogen adsorption tower, and the other part is converted into electrical energy for the second compressor through a Rankine cycle; part of the heat released by the methanation reactor serves as the heat source for the second preheater, and the other part is converted into electrical energy for the second compressor through a Rankine cycle.

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