Method and system for producing natural gas product from coal with near-zero carbon dioxide emission

Abstract

The invention relates to a method and a system for preparing a natural gas product from coal with near-zero carbon dioxide emission. The method comprises the following steps: S1, enabling purified feed gas to enter a low-temperature methanol washing unit for separation treatment to respectively obtain product gas containing carbon dioxide, waste gas containing hydrogen sulfide and synthesis gas containing hydrogen, carbon monoxide and methane; s2, the product gas and first externally supplied hydrogen are subjected to first mixing treatment, obtained first mixed gas enters a carbon dioxide methanation unit to make contact with a carbon dioxide methanation catalyst for a carbon dioxide methanation reaction, then obtained natural gas crude products are subjected to condensation separation treatment, and natural gas products are obtained; and S3, carrying out second mixing treatment on the synthesis gas and second externally supplied hydrogen, and enabling the obtained second mixed gas to enter a methanol synthesis unit to be in contact with a methanol synthesis catalyst so as to carry out methanol synthesis reaction. The method is simple in process, low in energy consumption and capable of achieving near-zero emission of carbon dioxide.

Description

Technical Field

[0001] This disclosure relates to the fields of energy and chemical technology, and more specifically, to a method and system for producing coal-to-natural gas products with near-zero carbon dioxide emissions. Background Technology

[0002] Current coal-to-oil and coal-to-natural-gas processes, whether the two-step method of synthesizing oil and gas after coal gasification or the one-step method of directly hydrogenating coal to produce oil and gas, suffer from complex process routes and low energy conversion efficiency. For example, the coal utilization rate in coal-to-oil and coal-to-natural-gas processes is only 40% and 50%, respectively, resulting in large amounts of carbon dioxide emissions from coal chemical industry. These large carbon dioxide emissions cause serious environmental problems and are incompatible with a low-carbon economy and climate change.

[0003] In recent years, with the increasing demand for natural gas, advanced technologies and demonstration projects for producing natural gas, such as coal gasification, catalytic gasification, and hydrogenation gasification, have developed rapidly. However, coal utilization in the clean conversion of coal has become a major scientific and technological problem that urgently needs to be solved. Currently, most coal-to-natural-gas technologies involve first gasifying coal under high temperature and pressure to obtain feedstock gas rich in crude syngas, and then producing natural gas products through conversion and methanation processes. This is commonly known as the "two-step coal-to-natural-gas technology" or the indirect coal-to-natural-gas technology. Typical coal gasification technologies include the Lurgi fixed-bed gasifier, with an outlet gas methane content of approximately 10%; typical methanation technologies include the Davy methanation technology and the Haldor Topsoe methanation technology. Because the entire natural gas production process requires coal gasification, dust removal and oil removal, sulfur-resistant conversion, desulfurization and decarbonization before the methane synthesis section, it involves high-temperature, low-temperature, and then high-temperature processes. This results in poor energy utilization efficiency, leading to complex processes, large investments, high energy consumption, and poor project economics.

[0004] Currently, coal-to-natural-gas technologies only utilize carbon monoxide for methanation, releasing the separated carbon dioxide into the atmosphere. Therefore, the overall carbon utilization rate in existing coal-to-natural-gas processes is only 22.7%. In conclusion, effectively addressing the fundamental issue of carbon dioxide emission control while simultaneously improving the economic viability of coal-to-natural-gas remains a pressing practical problem. Summary of the Invention

[0005] The purpose of this disclosure is to provide a method and system for producing coal-to-natural gas products that is simple in process, low in energy consumption, and has near-zero carbon dioxide emissions.

[0006] To achieve the above objectives, the first aspect of this disclosure provides a method for producing near-zero carbon dioxide emissions coal-to-natural gas, the method comprising the following steps: S1. The purified raw material gas is introduced into the low-temperature methanol washing unit for separation and treatment to obtain product gas containing carbon dioxide, waste gas containing hydrogen sulfide, and synthesis gas containing hydrogen, carbon monoxide and methane. S2. The product gas and the first externally supplied hydrogen gas are subjected to a first mixing process, so that the first mixed gas enters the carbon dioxide methanation unit and contacts the carbon dioxide methanation catalyst to carry out the carbon dioxide methanation reaction. Then, the obtained crude natural gas product is condensed and separated to obtain the natural gas product. S3. The synthesis gas and the second externally supplied hydrogen are subjected to a second mixing process, so that the resulting second mixed gas enters the methanol synthesis unit and contacts the methanol synthesis catalyst to carry out the methanol synthesis reaction.

[0007] Optionally, in step S2, the molar ratio of carbon dioxide in the product gas to the first externally supplied hydrogen gas is 1:(4.0-5.5).

[0008] Optionally, in step S2, the conditions for the carbon dioxide methanation reaction include: a temperature of 200-350℃ and a pressure of 0.1-0.5 MPa; the carbon dioxide methanation catalyst is a metal catalyst supported on a first support; wherein the support for the metal catalyst supported on the first support is selected from silica. 、 One or more of cerium dioxide, zirconium dioxide, and titanium dioxide; the active metal of the metal catalyst supported on the first support is selected from one or more of iron, cobalt, nickel, and ruthenium.

[0009] Optionally, in step S3, the molar ratio of carbon monoxide in the synthesis gas to the second externally supplied hydrogen is 1:(0-0.25).

[0010] Optionally, in step S3, the conditions for the methanol synthesis reaction include: a temperature of 200-300℃ and a pressure of 0.5-10 MPa; the methanol synthesis catalyst is a metal catalyst supported on a second support; wherein the support for the metal catalyst supported on the second support is selected from one or more of zinc oxide, alumina, zirconium dioxide, chromium trioxide, and indium oxide; and the active metal of the metal catalyst supported on the second support is selected from one or more of copper, palladium, ruthenium, and platinum.

[0011] Optionally, the method further includes: gasifying the coal raw material in a gasification unit, and then subjecting the resulting crude raw material gas to a dust removal and purification unit for dust removal and purification to obtain purified raw material gas containing hydrogen, carbon monoxide, carbon dioxide, methane, and hydrogen sulfide.

[0012] Optionally, the method further includes performing gas-liquid separation on the mixed crude product obtained from the methanol synthesis reaction, and sending the obtained crude methanol product into a distillation unit for purification.

[0013] Optionally, in steps S2 and S3, the first externally supplied hydrogen and the second externally supplied hydrogen are each independently green hydrogen; optionally, the first externally supplied hydrogen and the second externally supplied hydrogen are each independently derived from water electrolysis, wherein the electrical energy of the water electrolysis comes from renewable energy power generation; preferably, the renewable energy power generation is photovoltaic power generation or wind power generation.

[0014] The second aspect of this disclosure provides a system applicable to the method for producing near-zero carbon dioxide emissions coal-to-natural gas products as described in the first aspect, the system comprising: a low-temperature methanol washing unit, a first gas mixing unit, a second gas mixing unit, a carbon dioxide methanation unit, and a methanol synthesis unit; The low-temperature methanol washing unit includes a purified raw material gas inlet, a product gas outlet, a waste gas outlet, and a synthesis gas outlet; the product gas outlet is connected to the product gas inlet of the first gas mixing unit, and the first mixed gas outlet of the first gas mixing unit is connected to the first mixed gas inlet of the carbon dioxide methanation unit. The syngas outlet is connected to the syngas inlet of the second gas mixing unit, and the second mixed gas outlet of the second gas mixing unit is connected to the second mixed gas inlet of the methanol synthesis unit.

[0015] Optionally, the system further includes a gasification unit, a dust removal and purification unit, a condensation and separation unit, a gas-liquid separation unit, and a distillation unit; the crude feed gas outlet of the gasification unit is connected to the crude feed gas inlet of the dust removal and purification unit, and the purified feed gas outlet of the dust removal and purification unit is connected to the purified feed gas inlet of the low-temperature methanol washing unit; the natural gas crude product inlet of the condensation and separation unit is connected to the natural gas crude product outlet of the carbon dioxide methanation unit; the mixed crude product inlet of the gas-liquid separation unit is connected to the mixed crude product outlet of the methanol synthesis unit, and the methanol crude product outlet of the gas-liquid separation unit is connected to the methanol crude product inlet of the distillation unit; preferably, the system further includes a water electrolysis unit, and the external hydrogen supply outlet of the water electrolysis unit is connected to the first external hydrogen supply inlet of the first gas mixing unit and the second external hydrogen supply inlet of the second gas mixing unit, respectively.

[0016] Through the above technical solution, this disclosure separates carbon dioxide and produces natural gas products through carbon dioxide methanation, enabling the complete recovery and utilization of carbon dioxide and achieving zero carbon dioxide emissions. This disclosure also enables the production of methanol as a byproduct from syngas (which contains hydrogen, carbon monoxide, and methane). Furthermore, the methanation operation temperature in this method is low, significantly reducing energy consumption. This method not only achieves near-zero carbon dioxide emissions and full carbon utilization, but also boasts advantages such as simple process, low energy consumption, and good economic efficiency.

[0017] Other features and advantages of this disclosure will be described in detail in the following detailed description section. Attached Figure Description

[0018] The accompanying drawings are provided to further illustrate the present disclosure and form part of the specification. They are used together with the following detailed description to explain the present disclosure, but do not constitute a limitation thereof. In the drawings: Figure 1 This is a process flow diagram of natural gas preparation according to Embodiment 1 of this disclosure.

[0019] Figure 2 This is a process flow diagram of natural gas preparation according to Comparative Example 1 of this disclosure. Detailed Implementation

[0020] The specific embodiments of this disclosure will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit this disclosure.

[0021] The first aspect of this disclosure provides a method for producing near-zero carbon dioxide emissions coal-to-natural gas, the method comprising the following steps: S1. The purified raw material gas is introduced into the low-temperature methanol washing unit for separation and treatment to obtain product gas containing carbon dioxide, waste gas containing hydrogen sulfide, and synthesis gas containing hydrogen, carbon monoxide and methane. S2. The product gas and the first externally supplied hydrogen gas are subjected to a first mixing process, so that the first mixed gas enters the carbon dioxide methanation unit and contacts the carbon dioxide methanation catalyst to carry out the carbon dioxide methanation reaction. Then, the obtained crude natural gas product is condensed and separated to obtain the natural gas product. S3. The synthesis gas and the second externally supplied hydrogen are subjected to a second mixing process, so that the resulting second mixed gas enters the methanol synthesis unit and contacts the methanol synthesis catalyst to carry out the methanol synthesis reaction.

[0022] In this disclosure, the purified feed gas contains hydrogen, carbon monoxide, carbon dioxide, methane, and hydrogen sulfide. This disclosure separates the carbon dioxide and produces natural gas products through carbon dioxide methanation, enabling complete recovery and utilization of carbon dioxide, achieving near-zero carbon dioxide emissions. This disclosure also enables the production of methanol as a byproduct from syngas (which contains hydrogen, carbon monoxide, and methane). Furthermore, the method of this disclosure eliminates the need for a sulfur-resistant shift conversion process, significantly reducing energy consumption. This method not only achieves near-zero carbon dioxide emissions and full carbon utilization, but also boasts advantages such as simple process, low energy consumption, and good economic efficiency.

[0023] In this disclosure, the condensation separation process can employ condensation separation devices conventionally used in the art, which will not be described in detail here.

[0024] The hydrogen sulfide-containing waste gas obtained by this disclosure can be treated by Claus sulfur recovery and ammonia desulfurization, effectively achieving environmental protection and the recovery and reuse of sulfur resources.

[0025] In one embodiment of this disclosure, in step S2, the molar ratio of carbon dioxide in the product gas to the first externally supplied hydrogen is 1:(4.0-5.5). For example, the molar ratio of carbon dioxide in the product gas to the first externally supplied hydrogen can be 1:4.0, 1:4.5, 1:5, or 1:5.5. To ensure complete conversion of carbon dioxide to methane, the minimum molar ratio of carbon dioxide in the product gas to the first externally supplied hydrogen is 1:4.0.

[0026] In the above embodiments, there may be a small amount of the first externally supplied hydrogen remaining. The remaining first externally supplied hydrogen can be used together with methane as a natural gas product. Alternatively, the remaining first externally supplied hydrogen can be subjected to first externally supplied hydrogen separation treatment as needed. The first externally supplied hydrogen separation treatment can be carried out using methods conventionally used by those skilled in the art, which will not be described in detail here.

[0027] Existing coal-to-natural-gas methanation reactions involve carbon monoxide methanation, a highly exothermic reaction. During reactor operation, the temperature of the catalyst bed can reach over 600°C, requiring significant energy consumption. This disclosure utilizes carbon dioxide methanation to produce natural gas, reducing reaction temperature and energy consumption. In one embodiment, the carbon dioxide methanation reaction conditions include a temperature of 200-350°C and a pressure of 0.1-0.5 MPa. Based on heating 150 mL of reaction gas (carbon dioxide:hydrogen = 1:4), this carbon dioxide methanation process saves approximately 0.32 kWh of electricity compared to conventional coal-to-natural-gas methanation, significantly reducing the energy consumption of the carbon dioxide methanation process.

[0028] The carbon dioxide methanation reaction disclosed herein requires the catalysis of a carbon dioxide methanation catalyst, wherein the carbon dioxide methanation catalyst is a metal catalyst supported on a first support; wherein the support for the metal catalyst supported on the first support is selected from one or more of silicon dioxide, cerium dioxide, zirconium dioxide and titanium dioxide; and the active metal of the metal catalyst supported on the first support is selected from one or more of iron, cobalt, nickel and ruthenium.

[0029] To further improve the utilization of carbon in coal, the method of this disclosure can also utilize syngas to produce methanol as a byproduct. In one embodiment of this disclosure, the molar ratio of carbon monoxide in the syngas to the second externally supplied hydrogen is 1:(0-0.25). For example, the molar ratio of carbon monoxide in the syngas to the second externally supplied hydrogen can be 1:0.1, 1:0.15, 1:0.2, or 1:0.25. In the above embodiments, in the methanol synthesis reaction, the minimum molar ratio of carbon monoxide to hydrogen is 1:2; when there is sufficient hydrogen in the syngas to convert carbon monoxide into methanol, the molar ratio of carbon monoxide in the syngas to the second externally supplied hydrogen can be 1:0.

[0030] In the above embodiments, there may be a small amount of residual second external hydrogen. The residual second external hydrogen needs to be separated. The second external hydrogen separation process can be carried out using conventional gas-liquid separation methods in the art, which will not be described in detail here.

[0031] In one embodiment of this disclosure, the conditions for the methanol synthesis reaction include: a temperature of 200-300°C and a pressure of 0.5-10 MPa. Preferably, the conditions for the methanol synthesis reaction include: a temperature of 230-280°C and a pressure of 5-9 MPa.

[0032] The methanol synthesis reaction disclosed herein requires catalysis by a methanol synthesis catalyst, wherein the methanol synthesis catalyst is a metal catalyst supported on a second support; wherein the support for the metal catalyst supported on the second support is selected from one or more of zinc oxide, alumina, zirconium dioxide, chromium trioxide and indium oxide; and the active metal of the metal catalyst supported on the second support is selected from one or more of copper, palladium, ruthenium and platinum.

[0033] Since the syngas may contain a small amount of methane, the method further includes gas-liquid separation of the mixed crude product obtained from the methanol synthesis reaction. This gas-liquid separation process can yield both methane and crude methanol products, and the separated methane can still be used as a natural gas product. The gas-liquid separation process can employ gas-liquid separation methods conventionally used by those skilled in the art.

[0034] In one embodiment of this disclosure, to further improve the purity of the methanol, the obtained crude methanol product can be sent to a distillation unit for purification. The distillation method used in this disclosure can be one commonly used by those skilled in the art, such as the three-tower distillation method.

[0035] In this disclosure, the first and second externally supplied hydrogen gases can each be independently green hydrogen; the first and second externally supplied hydrogen gases each originate independently from water electrolysis, wherein the electrical energy for the water electrolysis comes from renewable energy generation; preferably, the renewable energy generation is photovoltaic power generation or wind power generation. The hydrogen production via water electrolysis can be alkaline water electrolysis, proton exchange membrane water electrolysis, or anion exchange membrane water electrolysis.

[0036] In this disclosure, green hydrogen prepared by water electrolysis is used. The energy saved in the carbon dioxide methanation reaction process can generate a large amount of hydrogen, which can not only meet the hydrogen requirements of the carbon dioxide methanation reaction, but also provide sufficient hydrogen source for the methanol synthesis reaction, thus further improving the economic efficiency of this disclosure.

[0037] The second aspect of this disclosure provides a system applicable to the method for producing near-zero carbon dioxide emissions coal-to-natural gas products as described in the first aspect, the system comprising: a low-temperature methanol washing unit, a first gas mixing unit, a second gas mixing unit, a carbon dioxide methanation unit, and a methanol synthesis unit; The low-temperature methanol washing unit includes a purified raw material gas outlet, a product gas outlet, a waste gas outlet, and a synthesis gas outlet; the product gas outlet is connected to the product gas inlet of the first gas mixing unit, and the first mixed gas outlet of the first gas mixing unit is connected to the first mixed gas inlet of the carbon dioxide methanation unit. The syngas outlet is connected to the syngas inlet of the second gas mixing unit, and the second mixed gas outlet of the second gas mixing unit is connected to the second mixed gas inlet of the methanol synthesis unit.

[0038] In this disclosure, the carbon dioxide methanation unit includes, sequentially along the flow direction of the first mixed gas, a first mixed gas preheating device and a carbon dioxide methanation reaction device connected in series; wherein, the carbon dioxide methanation reaction device is provided with multiple catalyst beds for loading a metal catalyst supported on a first support. The methanol synthesis unit includes a methanol synthesis reaction device, in which multiple catalyst beds are provided for loading a metal catalyst supported on a second support.

[0039] According to this disclosure, the system further includes a gasification unit, a dust removal and purification unit, a condensation and separation unit, a gas-liquid separation unit, and a distillation unit; The crude raw material gas outlet of the gasification unit is connected to the crude raw material gas inlet of the dust removal and purification unit, and the purified raw material gas outlet of the dust removal and purification unit is connected to the purified raw material gas inlet of the low-temperature methanol washing unit. The crude natural gas inlet of the condensation separation unit is connected to the crude natural gas outlet of the carbon dioxide methanation unit. The mixed crude product inlet of the gas-liquid separation unit is connected to the mixed crude product outlet of the methanol synthesis unit, and the methanol crude product outlet of the gas-liquid separation unit is connected to the methanol crude product inlet of the distillation unit. Preferably, the system further includes a water electrolysis unit, wherein the external hydrogen outlet of the water electrolysis unit is connected to the first external hydrogen inlet of the first gas mixing unit and the second external hydrogen inlet of the second gas mixing unit.

[0040] The present disclosure is further described in detail below through examples.

[0041] Example 1 The system settings used in this embodiment are as follows: Figure 1 As shown, the system includes: a low-temperature methanol washing unit, a first gas mixing unit, a second gas mixing unit, a carbon dioxide methanation unit, and a methanol synthesis unit; The low-temperature methanol washing unit includes a purified feed gas inlet, a product gas outlet, a waste gas outlet, and a synthesis gas outlet; the product gas outlet is connected to the product gas inlet of the first gas mixing unit, and the first mixed gas outlet of the first gas mixing unit is connected to the first mixed gas inlet of the carbon dioxide methanation unit. The syngas outlet is connected to the syngas inlet of the second gas mixing unit, and the second mixed gas outlet of the second gas mixing unit is connected to the second mixed gas inlet of the methanol synthesis unit.

[0042] The system also includes a gasification unit, a dust removal and purification unit, a condensation and separation unit, a gas-liquid separation unit, and a distillation unit; The crude raw material gas outlet of the gasification unit is connected to the crude raw material gas inlet of the dust removal and purification unit, and the purified raw material gas outlet of the dust removal and purification unit is connected to the purified raw material gas inlet of the low-temperature methanol washing unit. The crude natural gas inlet of the condensation separation unit is connected to the crude natural gas outlet of the carbon dioxide methanation unit; The mixed crude product inlet of the gas-liquid separation unit is connected to the mixed crude product outlet of the methanol synthesis unit, and the methanol crude product outlet of the gas-liquid separation unit is connected to the methanol crude product inlet of the distillation unit. The system also includes a water electrolysis unit, whose external hydrogen outlet is connected to the first external hydrogen inlet of the first gas mixing unit and the second external hydrogen inlet of the second gas mixing unit.

[0043] The method for preparing coal-to-natural gas using this system includes the following steps: Two kilograms of Xinjiang coal raw material are gasified in the gasification unit, and the resulting crude raw material gas is then fed into the dust removal and purification unit for dust removal and purification treatment to obtain purified raw material gas containing hydrogen, carbon monoxide, carbon dioxide, methane and hydrogen sulfide.

[0044] The purified raw gas is fed into a low-temperature methanol washing unit for separation and treatment to obtain product gas containing carbon dioxide, waste gas containing hydrogen sulfide, and synthesis gas containing hydrogen, carbon monoxide and methane. The carbon dioxide in the product gas and the first externally supplied hydrogen gas are mixed in a first gas mixing unit at a molar ratio of 1:4. This first mixed gas then enters the carbon dioxide methanation unit and comes into contact with a carbon dioxide methanation catalyst (supported by a composite carrier of silica and cerium dioxide, with nickel as the active metal) to carry out the carbon dioxide methanation reaction. The reaction conditions include a temperature of 300℃ and a pressure of 0.3 MPa. The resulting crude natural gas product is then condensed and separated to obtain the final natural gas product. The synthesis gas is mixed with carbon monoxide and hydrogen at a molar ratio of 1:2 (where the molar ratio of carbon monoxide to the second externally supplied hydrogen is 1:0.08) in a second gas mixing unit. This second mixed gas then enters the methanol synthesis unit and comes into contact with the methanol synthesis catalyst (a composite support of zinc oxide and alumina, with copper as the active metal) to carry out the methanol synthesis reaction. The reaction conditions include a temperature of 250°C and a pressure of 7 MPa. The crude methanol product obtained after gas-liquid separation is sent to a distillation unit for purification using a three-tower distillation method.

[0045] Example 2 The same system and method as in Example 1 are used, except that: The carbon dioxide in the product gas and the first externally supplied hydrogen gas are mixed in a 1:4 molar ratio in a first gas mixing unit. This first mixed gas then enters a carbon dioxide methanation unit where it contacts a carbon dioxide methanation catalyst (supported by a composite carrier of silica and cerium dioxide, with nickel as the active metal) to carry out a carbon dioxide methanation reaction. The reaction conditions include a temperature of 350℃ and a pressure of 0.3 MPa. The resulting crude natural gas product is then condensed and separated to obtain the final natural gas product. The carbon monoxide and hydrogen in the synthesis gas are mixed in a second gas mixing unit at a molar ratio of 1:2 (where the molar ratio of carbon monoxide to the second externally supplied hydrogen is 1:0.08). The resulting second mixed gas enters the methanol synthesis unit and comes into contact with the methanol synthesis catalyst (a composite support of zinc oxide and aluminum oxide, with copper as the active metal) to carry out the methanol synthesis reaction. The reaction conditions include a temperature of 300°C and a pressure of 7 MPa.

[0046] Example 3 The same system and method as in Example 1 are used, except that: The carbon dioxide in the product gas and the first externally supplied hydrogen gas are mixed in a 1:4 molar ratio in a first gas mixing unit. This first mixed gas then enters a carbon dioxide methanation unit where it contacts a carbon dioxide methanation catalyst (supported by titanium dioxide, with ruthenium as the active metal) to carry out the carbon dioxide methanation reaction. The reaction conditions include a temperature of 200℃ and a pressure of 0.3 MPa. The resulting crude natural gas product is then condensed and separated to obtain the final natural gas product. The carbon monoxide and hydrogen in the synthesis gas are mixed in a second gas mixing unit at a molar ratio of 1:2 (where the molar ratio of carbon monoxide to the second externally supplied hydrogen is 1:0.08). The resulting second mixed gas enters the methanol synthesis unit and comes into contact with the methanol synthesis catalyst (a composite support of zinc oxide and aluminum oxide, with copper as the active metal) to carry out the methanol synthesis reaction. The reaction conditions include a temperature of 200°C and a pressure of 10 MPa.

[0047] Example 4 Two kilograms of Ningxia coal raw material are gasified in the gasification unit, and the resulting crude raw material gas is then fed into the dust removal and purification unit for dust removal and purification treatment to obtain purified raw material gas containing hydrogen, carbon monoxide, carbon dioxide, methane and hydrogen sulfide.

[0048] The purified raw gas is fed into a low-temperature methanol washing unit for separation and treatment to obtain product gas containing carbon dioxide, waste gas containing hydrogen sulfide, and synthesis gas containing hydrogen, carbon monoxide and methane. The carbon dioxide and externally supplied hydrogen in the product gas are first mixed in a first gas mixing unit at a molar ratio of 1:4. This first mixed gas then enters the carbon dioxide methanation unit and contacts a carbon dioxide methanation catalyst (a composite support of silica and cerium dioxide, with nickel as the active metal) to carry out the carbon dioxide methanation reaction. The reaction conditions include a temperature of 300℃ and a pressure of 0.3 MPa. The resulting crude natural gas product is then condensed and separated to obtain the final natural gas product. In a second gas mixing unit, carbon monoxide and hydrogen in the synthesis gas are mixed at a molar ratio of 1:2 (where the molar ratio of carbon monoxide to the second externally supplied hydrogen is 1:0.07). This second mixed gas then enters the methanol synthesis unit and contacts the methanol synthesis catalyst (a composite support of zinc oxide and alumina, with copper as the active metal) to carry out the methanol synthesis reaction. The reaction conditions include a temperature of 250°C and a pressure of 7 MPa. The crude methanol product obtained after gas-liquid separation is sent to a distillation unit for purification using a three-tower distillation method.

[0049] Comparative Example 1 The settings of this comparative system are as follows: Figure 2 As shown, the method for preparing coal-to-natural gas using this system includes the following steps: 2 kg of coal raw material is gasified in the gasification unit, and the resulting crude raw material gas is then fed into the dust removal and purification unit for dust removal and purification treatment to obtain purified raw material gas containing hydrogen, carbon monoxide, carbon dioxide, methane and hydrogen sulfide.

[0050] The purified feed gas is fed into a sulfur-resistant shift unit for sulfur-resistant shift processing. The resulting sulfur-resistant shift feed gas is then fed into a low-temperature methanol washing unit for separation to obtain carbon dioxide, hydrogen sulfide, and refined desulfurization feed gas (carbon monoxide + hydrogen). The refined desulfurization feed gas is then fed into a carbon monoxide methanation unit for carbon monoxide methanation treatment, followed by condensation and separation to obtain natural gas products.

[0051] Comparative Example 2 The same system and method as in Example 1 are used, except that: The carbon dioxide methanation reaction conditions include: a temperature of 150℃ and a pressure of 0.05 MPa. The resulting crude natural gas product is then subjected to condensation and separation to obtain the final natural gas product. The methanol synthesis reaction conditions include a temperature of 150℃ and a pressure of 0.2 MPa.

[0052] The results of Examples 1-4 and Comparative Examples 1-2 are shown in Table 1.

[0053] Table 1

[0054] A comparison of Example 1 and Comparative Example 1 shows that Example 1 did not use the sulfur-resistant conversion process, significantly reducing energy consumption. Furthermore, carbon dioxide was not emitted in Example 1; instead, it entered the carbon dioxide methanation process, achieving zero carbon dioxide emissions. The natural gas yield obtained by the method in Example 1 was 3.4 times that of Comparative Example 1, not only increasing natural gas production but also saving coal consumption. Additionally, Example 1 was able to produce methanol as a byproduct.

[0055] A comparison of Examples 1 and 3 shows that, compared with the nickel-based catalyst used in Example 1, the ruthenium-based catalyst used in Example 3 has a higher conversion rate of carbon dioxide at low temperatures. Even at low temperatures, more natural gas can be obtained than in Example 1, and the energy consumption is lower.

[0056] A comparison of Example 1 and Comparative Example 2 shows that the carbon dioxide methanation reaction conditions and methanol synthesis reaction conditions in Comparative Example 2 are not within the scope of this disclosure. The natural gas yield and methanol yield in Example 1 are significantly higher than those in Comparative Example 2, indicating that when the carbon dioxide methanation reaction conditions and methanol synthesis reaction conditions are within the scope of this disclosure, the yield of natural gas and methanol is higher.

[0057] Compared with Comparative Examples 1-2, the carbon utilization rate of Examples 1-4 is significantly higher than that of Comparative Examples 1 and 2, and can achieve near-zero carbon dioxide emissions.

[0058] In summary, the method disclosed herein for preparing natural gas has low energy consumption and can achieve near-zero emissions, thus bringing better economic value.

[0059] The preferred embodiments of this disclosure have been described in detail above with reference to the accompanying drawings. However, this disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of this disclosure, various simple modifications can be made to the technical solutions of this disclosure, and these simple modifications all fall within the protection scope of this disclosure.

[0060] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately.

[0061] Furthermore, various different embodiments of this disclosure can be combined in any way, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content disclosed in this disclosure.

Claims

1. A method for producing natural gas from coal with near-zero carbon dioxide emissions, characterized in that, The method includes the following steps: S1. The purified raw material gas is introduced into the low-temperature methanol washing unit for separation and treatment to obtain product gas containing carbon dioxide, waste gas containing hydrogen sulfide, and synthesis gas containing hydrogen, carbon monoxide and methane. S2. The product gas and the first externally supplied hydrogen gas are subjected to a first mixing process, so that the first mixed gas enters the carbon dioxide methanation unit and contacts the carbon dioxide methanation catalyst to carry out the carbon dioxide methanation reaction. Then, the obtained crude natural gas product is condensed and separated to obtain the natural gas product. S3. The synthesis gas and the second externally supplied hydrogen are subjected to a second mixing process, so that the resulting second mixed gas enters the methanol synthesis unit and contacts the methanol synthesis catalyst to carry out the methanol synthesis reaction.

2. The method according to claim 1, wherein, In step S2, the molar ratio of carbon dioxide in the product gas to the first externally supplied hydrogen gas is 1:(4.0-5.5).

3. The method according to claim 1, wherein, In step S2, the conditions for the carbon dioxide methanation reaction include: a temperature of 200-350°C and a pressure of 0.1-0.5 MPa. The carbon dioxide methanation catalyst is a metal catalyst supported on a first support; wherein the support for the metal catalyst supported on the first support is selected from one or more of silicon dioxide, cerium dioxide, zirconium dioxide and titanium dioxide; and the active metal of the metal catalyst supported on the first support is selected from one or more of iron, cobalt, nickel and ruthenium.

4. The method according to claim 1, wherein, In step S3, the molar ratio of carbon monoxide in the synthesis gas to the second externally supplied hydrogen is 1:(0-0.25).

5. The method according to claim 1, wherein, In step S3, the conditions for the methanol synthesis reaction include: a temperature of 200-300℃ and a pressure of 0.5-10 MPa. The methanol synthesis catalyst is a metal catalyst supported on a second support; wherein the support for the metal catalyst supported on the second support is selected from one or more of zinc oxide, alumina, zirconium dioxide, chromium trioxide and indium oxide; and the active metal of the metal catalyst supported on the second support is selected from one or more of copper, palladium, ruthenium and platinum.

6. The method according to claim 1, wherein, The method further includes: gasifying the coal raw material in a gasification unit, and then subjecting the resulting crude raw material gas to a dust removal and purification unit for dust removal and purification to obtain purified raw material gas containing hydrogen, carbon monoxide, carbon dioxide, methane, and hydrogen sulfide.

7. The method according to claim 1, wherein, The method further includes performing gas-liquid separation on the mixed crude product obtained from the methanol synthesis reaction, and sending the obtained crude methanol product into a distillation unit for purification.

8. The method according to claim 1, wherein, In steps S2 and S3, the first externally supplied hydrogen and the second externally supplied hydrogen are each independently green hydrogen; Optionally, the first and second externally supplied hydrogen gases each originate independently from water electrolysis, wherein the electrical energy for water electrolysis comes from renewable energy generation. Preferably, the renewable energy power generation is photovoltaic power generation or wind power generation.

9. A system applicable to a method for producing near-zero carbon dioxide emissions coal-to-natural gas products according to any one of claims 1-8, characterized in that, The system includes: a low-temperature methanol washing unit, a first gas mixing unit, a second gas mixing unit, a carbon dioxide methanation unit, and a methanol synthesis unit; The low-temperature methanol washing unit includes a purified raw material gas inlet, a product gas outlet, a waste gas outlet, and a synthesis gas outlet; the product gas outlet is connected to the product gas inlet of the first gas mixing unit, and the first mixed gas outlet of the first gas mixing unit is connected to the first mixed gas inlet of the carbon dioxide methanation unit. The syngas outlet is connected to the syngas inlet of the second gas mixing unit, and the second mixed gas outlet of the second gas mixing unit is connected to the second mixed gas inlet of the methanol synthesis unit.

10. The system according to claim 9, wherein, The system also includes a gasification unit, a dust removal and purification unit, a condensation and separation unit, a gas-liquid separation unit, and a distillation unit; The crude raw material gas outlet of the gasification unit is connected to the crude raw material gas inlet of the dust removal and purification unit, and the purified raw material gas outlet of the dust removal and purification unit is connected to the purified raw material gas inlet of the low-temperature methanol washing unit. The crude natural gas inlet of the condensation separation unit is connected to the crude natural gas outlet of the carbon dioxide methanation unit. The mixed crude product inlet of the gas-liquid separation unit is connected to the mixed crude product outlet of the methanol synthesis unit, and the methanol crude product outlet of the gas-liquid separation unit is connected to the methanol crude product inlet of the distillation unit. Preferably, the system further includes a water electrolysis unit, wherein the external hydrogen outlet of the water electrolysis unit is connected to the first external hydrogen inlet of the first gas mixing unit and the second external hydrogen inlet of the second gas mixing unit.

Images