System and method for preparing natural gas by using high-carbon-concentration synthesis gas

Abstract

The invention relates to the technical field of coal-based natural gas, in particular to a system and method for preparing natural gas from high-carbon-concentration synthesis gas. The system comprises an isothermal reaction part which comprises a raw material preheater, a reaction gas heat exchanger and at least two isothermal reaction devices, the isothermal reaction devices are arranged in parallel, the raw material preheater is communicated with feed ports of the isothermal reaction devices, and discharge ports of the isothermal reaction devices are communicated with the reaction gas heat exchanger; a gas outlet of the reaction gas heat exchanger is communicated with a feeding hole of the isothermal reaction device; the adiabatic reaction part comprises an adiabatic reactor, a product gas heat exchanger and a product gas separation tank, a feed port of the adiabatic reactor is communicated with a discharge port of the reaction gas heat exchanger, a discharge port of the adiabatic reactor is communicated with a feed port of the product gas heat exchanger, and a discharge port of the product gas heat exchanger is communicated with a feed port of the product gas separation tank; through the system, the energy consumption of methanation can be reduced, the activity and stability of the catalyst can be maintained, and the number of reactors and the circulating gas amount can be reduced.

Description

Technical Field

[0001] The present application relates to the technical field of coal-to-natural gas, and particularly to a system and method for preparing natural gas by using syngas with a high carbon concentration. Background Art

[0002] In the field of coal-to-natural gas, syngas methanation is a very important process. Due to the characteristics of the methanation reaction being highly exothermic, the adiabatic temperature rise for every 1% of CO to form methane is 74°C, and the adiabatic temperature rise for every 1% of CO2 to form methane is 60°C. The specific reactions are as follows:

[0003]

[0004]

[0005] Currently, most of the adiabatic cycle processes for methanation technology adopt the form of series-parallel connection of multiple adiabatic reactors and product gas circulation. Through this process, natural gas meeting the requirements can be produced. However, this process requires a large number of adiabatic reactors, resulting in a large system pressure drop; at the same time, the reactor outlet temperature is high, and multiple high-temperature resistant heat exchange devices are required, leading to high equipment investment.

[0006] Since the methanation reaction is a highly exothermic reaction, thermodynamically, low temperature is beneficial to the formation of methane. When a lower reaction temperature is adopted, the methane content in the product is higher, and the number of reactors can be reduced. Although there is an isothermal methanation process currently, in which the methanation step adopts at least two or more stages of methanation, the first stage of methanation is adiabatic methanation, and the subsequent stages of methanation adopt isothermal methanation, the methanation temperature in the adiabatic stage is still high, energy consumption is high, and it is not conducive to maintaining the activity of the catalyst. Summary of the Invention

[0007] The present application provides a system and method for preparing natural gas by using syngas with a high carbon concentration, so as to solve the technical problems existing in the isothermal methanation process in the prior art, such as high methanation temperature, high energy consumption, and unfavorable maintenance of catalyst activity in the adiabatic stage.

[0008] In a first aspect, the present application provides a system for preparing natural gas by using syngas with a high carbon concentration, the system comprising:

[0009] An isothermal reaction section, the isothermal reaction section comprising a raw material preheater, a reaction gas heat exchanger, and at least two isothermal reaction devices, the isothermal reaction devices being arranged in parallel, the raw material preheater communicating with the feed inlet of the isothermal reaction devices, the discharge outlet of the isothermal reaction devices communicating with the reaction gas heat exchanger, and the gas outlet of the reaction gas heat exchanger communicating with the feed inlet of the isothermal reaction devices;

[0010] An adiabatic reaction section, the adiabatic reaction section includes an adiabatic reactor, a product gas heat exchanger and a product gas separation tank. The feed port of the adiabatic reactor is connected to the discharge port of the reaction gas heat exchanger. The discharge port of the adiabatic reactor is connected to the feed port of the product gas heat exchanger. The discharge port of the product gas heat exchanger is connected to the feed port of the product gas separation tank.

[0011] Optionally, the isothermal reaction device includes an isothermal reactor and an isothermal heat exchanger. The discharge port of the raw material preheater is connected to the feed port of the isothermal reactor. The discharge port of the isothermal reactor is connected to the feed port of the isothermal heat exchanger. The discharge port of the isothermal heat exchanger is connected to the reaction gas heat exchanger.

[0012] Optionally, the isothermal reaction device further includes a steam drum. The liquid outlet end and the liquid inlet end of the steam drum are respectively connected to the side wall of the isothermal reactor. The steam drum is provided with an outlet for high-pressure steam.

[0013] Optionally, the system further includes:

[0014] A circulation section, the circulation section includes a recycle gas heat exchanger, a recycle gas separation tank and a recycle gas preheater. The inlet of the recycle gas heat exchanger is connected to the outlet of the reaction gas heat exchanger. The outlet of the recycle gas heat exchanger is connected to the feed port of the recycle gas separation tank. The outlet of the recycle gas separation tank is connected to the inlet of the recycle gas preheater. The outlet of the recycle gas preheater is connected to the feed port of the isothermal reaction device.

[0015] Optionally, the circulation section further includes a booster pump. The inlet of the booster pump is connected to the recycle gas separation tank. The outlet of the booster pump is connected to the feed port of the recycle gas preheater.

[0016] In a second aspect, the present application provides a method for preparing natural gas using syngas with a high carbon concentration. The method is adapted to the system described in the first aspect. The method includes:

[0017] Exchanging heat for the syngas, and then adding recycle gas to the syngas to obtain a mixed gas;

[0018] Performing a primary methanation reaction on the mixed gas under isothermal conditions to obtain a primary methanation reaction gas;

[0019] Exchanging heat and separating the primary methanation reaction gas to obtain a recycle gas raw material and an intermediate reaction gas respectively;

[0020] Performing a secondary methanation reaction on the intermediate reaction gas in an adiabatic environment, and then cooling down and separating gas and liquid to obtain a product gas;

[0021] Cooling down, separating gas and liquid and compressing and boosting the recycle gas raw material to obtain recycle gas.

[0022] Optionally, the hydrogen-carbon ratio of the syngas is 2.90 to 3.20, and the methane content in the syngas is ≤1%.

[0023] Optionally, the temperature of the primary methanation reaction is 250°C to 450°C; and / or,

[0024] The primary methanation reaction includes performing a primary methanation reaction using a methanation catalyst, and the nickel content of the methane catalyst is 30% to 65%.

[0025] Optionally, the initial temperature of the secondary methanation reaction is 250°C to 300°C, and the end temperature of the secondary methanation reaction is 270°C to 330°C; and / or,

[0026] The secondary methanation reaction includes performing a secondary methanation reaction using the methanation catalyst, and the nickel content of the methane catalyst is 30% to 65%.

[0027] Optionally, the recycle volume ratio of the recycle gas feedstock to the intermediate reaction gas is 0.1 to 0.4.

[0028] The above technical solutions provided by the embodiments of the present application have the following advantages compared with the prior art:

[0029] A system for preparing natural gas using high-carbon concentration syngas provided by an embodiment of the present application, by designing an isothermal reaction section including a raw material preheater, a reaction gas heat exchanger, and an isothermal reaction device, and an adiabatic reaction section including an adiabatic reactor, a product gas heat exchanger, and a product gas separation tank. By using a parallel-set isothermal reaction device, a large amount of methanation reaction can be carried out on the high-carbon concentration syngas under low-temperature conditions, the number of isothermal reactors can be reduced, and the recycle gas separated by the reactor heat exchanger is introduced into the isothermal reaction device for recycling, thereby reducing the consumption of syngas and reducing the degree of methanation reaction, which is beneficial to improving the stability of catalyst activity. Then, through the adiabatic section, a deep methanation reaction is carried out at a lower temperature, which can avoid the use of high-temperature equipment, reduce equipment investment and operating costs, and at the same time maintain catalyst activity. Therefore, through this system, the methanation temperature in the adiabatic stage can be reduced by the parallel isothermal reaction device, thereby reducing the energy consumption of methanation, which is beneficial to maintaining catalyst activity. Introducing recycle gas can improve the stability of catalyst activity, and at the same time reduce the number of reactors and the recycle gas volume to reduce equipment investment and operating costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The drawings herein are incorporated into the specification and form a part of this specification, showing embodiments consistent with the present application, and are used together with the specification to explain the principles of the present application.

[0031] To more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0032] Figure 1 Schematic structural diagram of a system for preparing natural gas using syngas with a high carbon concentration provided by an embodiment of the present application;

[0033] Figure 2 Schematic logical flow diagram of a system for preparing natural gas using syngas with a high carbon concentration provided by an embodiment of the present application;

[0034] Figure 3 Schematic process flow diagram of a method for preparing natural gas using syngas with a high carbon concentration provided by an embodiment of the present application;

[0035] Among them, 1 - isothermal reaction section, 11 - raw material preheater, 12 - reaction gas heat exchanger, 13 - isothermal reaction device, 131 - isothermal reactor, 132 - isothermal heat exchanger, 133 - steam drum, 2 - adiabatic reaction section, 21 - adiabatic reactor, 22 - product gas heat exchanger, 23 - product gas separation tank, 3 - circulation section, 31 - recycle gas heat exchanger, 32 - recycle gas separation tank, 33 - recycle gas preheater, 34 - booster pump, A1 - input water cooling medium, A2 - output water cooling medium, B1 - input water cooling medium, B2 - output water cooling medium. Detailed implementation manners

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some, but not all, of the embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts fall within the scope of protection of the present application.

[0037] Unless otherwise specifically stated, various raw materials, reagents, instruments, and equipment used in the present application can all be obtained through market purchases or can be prepared by existing methods.

[0038] As Figure 1 and Figure 2 shown, an embodiment of the present application provides a system for preparing natural gas using syngas with a high carbon concentration, and the system includes:

[0039] Isothermal reaction section 1, the isothermal reaction section 1 includes a raw material preheater 11, a reaction gas heat exchanger 12, and at least two isothermal reaction devices 13. The isothermal reaction devices 13 are arranged in parallel. The raw material preheater 11 is connected to the feed inlet of the isothermal reaction device 13. The discharge outlet of the isothermal reaction device 13 is connected to the reaction gas heat exchanger 12. The gas outlet of the reaction gas heat exchanger 12 is connected to the feed inlet of the isothermal reaction device 13;

[0040] Adiabatic reaction section 2, the adiabatic reaction section 2 includes an adiabatic reactor 21, a product gas heat exchanger 22, and a product gas separation tank 23. The feed inlet of the adiabatic reactor 21 is connected to the discharge outlet of the reaction gas heat exchanger 12. The discharge outlet of the adiabatic reactor 21 is connected to the feed inlet of the product gas heat exchanger 22. The discharge outlet of the product gas heat exchanger 22 is connected to the feed inlet of the product gas separation tank 23.

[0041] In some alternative embodiments, the isothermal reaction device 13 includes an isothermal reactor 131 and an isothermal heat exchanger 132. The discharge outlet of the raw material preheater 11 is connected to the feed inlet of the isothermal reactor 131. The discharge outlet of the isothermal reactor 131 is connected to the feed inlet of the isothermal heat exchanger 132. The discharge outlet of the isothermal heat exchanger 132 is connected to the reaction gas heat exchanger 12.

[0042] In the embodiments of the present application, by refining the specific composition of the isothermal reaction device 13, the temperature of the syngas after a large amount of methanation reaction can be further reduced by the isothermal heat exchanger 132, so that the temperature of the deep methanation reaction in the subsequent adiabatic stage can be reduced, thereby reducing the energy consumption of methanation, being beneficial to maintaining the catalyst activity, and at the same time reducing the number of reactors and the recycle gas volume to reduce equipment investment and operating costs.

[0043] It should be noted that the isothermal reactor 131 can adopt a water-cooled heat exchange reactor. The reaction temperature is controlled by the water-cooled heat exchange method, and the transition from cooling water to steam is realized. At the same time, the water-cooled medium can be collected by the following steam drum 133 and discharged from the system to obtain high-pressure saturated steam. The high-pressure saturated steam can be processed through a boiler superheater to obtain high-quality superheated steam.

[0044] According to the processing capacity of the system, the pressure of the high-pressure saturated steam can be 9.0 MPa to 9.9 MPa.

[0045] In some alternative embodiments, the isothermal reaction device 13 further includes a steam drum 133. The liquid outlet end and the liquid inlet end of the steam drum 133 are respectively connected to the side wall of the isothermal reactor 131. The steam drum 133 is provided with an outlet for high-pressure steam.

[0046] In the embodiments of the present application, by introducing a steam drum 133 into the isothermal reaction device 13, the water-cooling medium of the isothermal reactor 131 can be connected through the steam drum 133, so that the high-pressure saturated water vapor formed by the cooled water after heat exchange enters the steam drum 133 for collection, and then is discharged out of the overall system through the steam drum 133. Subsequently, high-quality superheated steam can be obtained through boiler overheating treatment.

[0047] In some alternative embodiments, the system further includes:

[0048] A circulation section 3, which includes a recycle gas heat exchanger 31, a recycle gas separation tank 32, and a recycle gas preheater 33. The inlet of the recycle gas heat exchanger 31 is communicated with the outlet of the reaction gas heat exchanger 12. The outlet of the recycle gas heat exchanger 31 is communicated with the inlet of the recycle gas separation tank 32. The outlet of the recycle gas separation tank 32 is communicated with the inlet of the recycle gas preheater 33. The outlet of the recycle gas preheater 33 is communicated with the inlet of the isothermal reaction device 13.

[0049] In the embodiments of the present application, by introducing the circulation section 3 into the system and designing the circulation section 3 including the recycle gas heat exchanger 31, the recycle gas separation tank 32, and the recycle gas preheater 33, a large amount of intermediate reaction gas after the methanation reaction can be used as recycle gas through the circulation section 3, returned to the isothermal reactor 131 and mixed with the syngas. The recycle gas can be used to dilute the syngas and reduce the severity of the reaction, which is beneficial to improving the stability of the catalyst.

[0050] In some alternative embodiments, the circulation section 3 further includes a booster pump 34. The inlet of the booster pump 34 is communicated with the recycle gas separation tank 32, and the outlet of the booster pump 34 is communicated with the inlet of the recycle gas preheater 33.

[0051] In the embodiments of the present application, by introducing the booster pump 34 into the circulation section 3, the recycle gas can be pressurized so that the recycle gas can quickly enter the isothermal reactor 131 and be fully mixed with the syngas, thereby realizing the dilution of the syngas.

[0052] As Figure 3 shown, based on a general inventive concept, the embodiments of the present application provide a method for preparing natural gas using high-carbon-concentration syngas. The method is adapted to the system, and the method includes:

[0053] S1. Exchanging heat for the syngas, and then adding recycle gas to the syngas to obtain a mixed gas;

[0054] S2. Performing a primary methanation reaction on the mixed gas under isothermal conditions to obtain a primary methanation reaction gas;

[0055] S3. Heat exchange and separation are performed on the primary methanation reaction gas to obtain a recycle gas feedstock and an intermediate reaction gas respectively;

[0056] S4. The intermediate reaction gas is subjected to a secondary methanation reaction in an adiabatic environment, and then cooled and subjected to gas-liquid separation to obtain a product gas;

[0057] S5. The recycle gas feedstock is cooled, subjected to gas-liquid separation and compressed to obtain a recycle gas.

[0058] This method is implemented based on the above system. The specific structural composition and connection relationship of the system can refer to the above embodiments. Since this method adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought by the technical solutions of the above embodiments, which will not be elaborated here one by one.

[0059] It should be noted that the syngas can be the purified gas of the dry pulverized coal gasification process or the purified gas of the water coal slurry gasification process, and the methane content in the syngas is extremely low, below 1%. For example, the syngas can be a mixture of carbon monoxide and hydrogen.

[0060] It should be noted that the primary methanation reaction can be a large-scale methanation reaction of the syngas and the recycle gas using a methanation catalyst to convert most of the syngas into methane gas initially, so that a primary methanation reaction gas containing an intermediate reaction gas, unreacted completely recycle gas and syngas can be obtained.

[0061] It should be noted that the separation means directly using a part of the primary methanation reaction gas as the recycle gas and the other part as the intermediate reaction gas through pipeline separation.

[0062] It should be noted that the intermediate reaction gas can be a mixed gas containing completely reacted methane gas, unreacted syngas and unreacted recycle gas.

[0063] It should be noted that the secondary methanation reaction can be a deep methanation reaction of the intermediate reaction gas using a methanation catalyst to further convert the syngas into methane product gas.

[0064] In some alternative embodiments, the hydrogen-carbon ratio of the syngas is 2.90 - 3.20, and the methane content in the syngas ≤ 1%.

[0065] In the embodiments of the present application, controlling the specific hydrogen-carbon ratio of the syngas can clarify the distribution of carbon and hydrogen in the syngas, facilitate the subsequent methanation reaction and obtain methane products.

[0066] The hydrogen-carbon ratio of the syngas can be 2.90, 2.95, 3.00, 3.05, 3.10, 3.15 or 3.20.

[0067] Controlling the specific methane content in the syngas can, on the one hand, reduce the impact of methane on the methanation reaction, and on the other hand, control the hydrogen-carbon ratio in the syngas within the range of 2.90 to 3.20.

[0068] In some alternative embodiments, the temperature of the primary methanation reaction is 250°C to 450°C; and / or,

[0069] The primary methanation reaction includes performing a primary methanation reaction (a large amount of methane catalytic reaction) using a methanation catalyst, and the nickel content of the methane catalyst is 30% to 65%.

[0070] In the embodiments of the present application, controlling the specific temperature of the primary methanation reaction and the specific nickel content of the methane catalyst can enable the primary methanation reaction to proceed sufficiently to obtain a primary methanation reaction gas.

[0071] The temperature of the primary methanation reaction can be 250°C, 260°C, 270°C, 280°C, 290°C, 300°C, 310°C, 320°C, 330°C, 340°C, 350°C, 360°C, 370°C, 380°C, 390°C, 400°C, 410°C, 420°C, 430°C, 440°C or 450°C.

[0072] The nickel content of the methane catalyst can be 30%, 35%, 40%, 45%, 50%, 55%, 60% or 65%.

[0073] In some alternative embodiments, the initial temperature of the secondary methanation reaction is 250°C to 300°C, and the end temperature of the secondary methanation reaction is 270°C to 330°C; and / or,

[0074] The secondary methanation reaction includes performing a secondary methanation reaction using the methanation catalyst, and the nickel content of the methane catalyst is 30% to 65%.

[0075] In the embodiments of the present application, controlling the specific initial temperature and end temperature of the secondary methanation reaction, and the specific nickel content of the methane catalyst can enable the secondary methanation reaction (deep methanation catalytic reaction) to proceed sufficiently to fully convert the intermediate reaction gas into a methane product gas.

[0076] The initial temperature of the secondary methanation reaction can be 250°C, 260°C, 270°C, 280°C, 290°C or 300°C, and the end temperature of the secondary methanation reaction can be 270°C, 280°C, 290°C, 300°C, 310°C, 320°C or 330°C.

[0077] The nickel content of the methane catalyst can be 30%, 35%, 40%, 45%, 50%, 55%, 60% or 65%.

[0078] In some alternative embodiments, the recycle volume ratio of the recycle gas feedstock to the intermediate reaction gas is 0.1 to 0.4.

[0079] In the embodiments of the present application, by controlling the specific recycle volume ratio of the recycle gas feedstock to the intermediate reaction gas, the recycle gas can be used to dilute the intermediate reaction gas, effectively reducing the intensity of the primary methanation reaction and the secondary methanation reaction, which is beneficial to improving the stability of the catalyst.

[0080] The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present application and not to limit the scope of the present application. The experimental methods without specific conditions in the following embodiments are generally determined according to industry standards. If there is no corresponding industry standard, they are carried out according to general international standards, conventional conditions, or the conditions recommended by the manufacturer.

[0081] Example 1

[0082] As Figure 1 and Figure 2 shown, a system for preparing natural gas using syngas with a high carbon concentration, the system includes:

[0083] An isothermal reaction section 1 including a raw material preheater 11, a reaction gas heat exchanger 12 and at least two isothermal reaction devices 13, the isothermal reaction devices 13 are arranged in parallel, the raw material preheater 11 is connected to the feed port of the isothermal reaction device 13, the discharge port of the isothermal reaction device 13 is connected to the reaction gas heat exchanger 12, and the outlet of the reaction gas heat exchanger 12 is connected to the feed port of the isothermal reaction device 13;

[0084] An adiabatic reaction section 2 including an adiabatic reactor 21, a product gas heat exchanger 22 and a product gas separation tank 23, the feed port of the adiabatic reactor 21 is connected to the discharge port of the reaction gas heat exchanger 12, the discharge port of the adiabatic reactor 21 is connected to the feed port of the product gas heat exchanger 22, and the discharge port of the product gas heat exchanger 22 is connected to the feed port of the product gas separation tank 23.

[0085] The isothermal reaction device 13 includes an isothermal reactor 131, an isothermal heat exchanger 132 and a steam drum 133, the discharge port of the raw material preheater 11 is connected to the feed port of the isothermal reactor 131, the discharge port of the isothermal reactor 131 is connected to the feed port of the isothermal heat exchanger 132, the discharge port of the isothermal heat exchanger 132 is connected to the reaction gas heat exchanger 12, and the liquid outlet end and the liquid inlet end of the steam drum 133 are respectively connected to the side wall of the isothermal reactor 131, and the steam drum 133 is provided with an outlet for high-pressure steam.

[0086] The system further includes:

[0087] The circulation part 3 includes a circulating gas heat exchanger 31, a circulating gas separation tank 32 and a circulating gas preheater 33. The air inlet of the circulating gas heat exchanger 31 is connected to the air outlet of the reaction gas heat exchanger 12, the air outlet of the circulating gas heat exchanger 31 is connected to the feed port of the circulating gas separation tank 32, the air outlet of the circulating gas separation tank 32 is connected to the air inlet of the circulating gas preheater 33, and the air outlet of the circulating gas preheater 33 is connected to the feed port of the isothermal reaction device 13.

[0088] The circulation part 3 further includes a booster pump 34 , a feed port of the booster pump 34 is connected to the circulating gas separation tank 32 , and a discharge port of the booster pump 34 is connected to a feed port of the circulating gas preheater 33 .

[0089] Example 2

[0090] Based on the system disclosed in Example 1, the reaction is further carried out, and the specific steps are as follows:

[0091] The pressure of the synthesis gas used is 2.8 MPa, and its composition is: H2: 74.91 mol%, CO: 23.84 mol%, CO2: 0.77 mol%, CH4: 0, N2: 0.48 mol%, H2O: 0.

[0092] The raw gas is mixed with the circulating gas after heat exchange in the raw material preheater 11, and then divided into two streams and enters two parallel isothermal reactors 131 respectively. The reaction temperature is set to 310°C, and a heat-resistant methanogenic catalyst with a nickel content of 35wt% is filled in the reactor. The isothermal reactor 131 adopts a water-cooled heat exchange type, and the water-cooling medium is connected to the steam drum 133, which can produce 9.9MPa high-pressure saturated steam as a by-product;

[0093] The product gas from the isothermal reactor 131 is heat exchanged in the isothermal heat exchanger 132 and the product gas heat exchanger 22 to obtain primary methanation reaction gas. One of the primary methanation reaction gas is used as circulating gas and is cooled by the circulating gas heat exchanger 31, separated into gas and liquid by the circulating gas separation tank 32, compressed and pressurized by the booster pump 34, and then passed through the circulating gas preheater 33 to be fully mixed with the synthesis gas. The circulation ratio of the circulating gas to the synthesis gas is set to 0.35.

[0094] Another stream of the obtained primary methanogenic reaction gas is directly fed into the adiabatic reactor 21 as the intermediate reaction gas for deep methanogenic reaction. The inlet temperature of the adiabatic reactor 21 is 255° C., and the outlet temperature is 310° C. The adiabatic reactor 21 is filled with a heat-resistant methanogenic catalyst having a nickel content of 50 wt %.

[0095] The intermediate reaction gas after the reaction is cooled by the product gas heat exchanger 22, and then undergoes gas-liquid separation in the product gas separation tank 23 and is output as the product gas. The composition of the obtained product gas is as follows: H2: 2.3 mol%, CO: 0, CO2: 0.3 mol%, CH4: 95.3 mol%, N2: 1.9 mol%, H2O: 0.3 mol%. The temperature of this product gas is 40°C and the pressure is 2.5 MPa.

[0096] The obtained product gas meets the requirements of the first-class natural gas in the national standard (GB17820-2018).

[0097] Example 3

[0098] Comparing Example 3 with Example 2, the differences between Example 3 and Example 2 are as follows:

[0099] The hydrogen-carbon ratio of the synthesis gas is 2.90.

[0100] The temperature of the first-stage methanation reaction is 250°C;

[0101] The first-stage methanation reaction includes carrying out the first-stage methanation reaction using a methanation catalyst, and the nickel content of the methane catalyst is 30%.

[0102] The initial temperature of the second-stage methanation reaction is 250°C, and the end temperature of the second-stage methanation reaction is 270°C;

[0103] The second-stage methanation reaction includes carrying out the second-stage methanation reaction using a methanation catalyst, and the nickel content of the methane catalyst is 30%.

[0104] The circulation volume ratio of the recycle gas raw material and the intermediate reaction gas is 0.1.

[0105] The composition of the obtained product gas is as follows: H2: 2.1 mol%, CO: 0.03%, CO2: 0.21 mol%, CH4: 95.56 mol%, N2: 1.8 mol%, H2O: 0.3 mol%.

[0106] Example 4

[0107] Comparing Example 4 with Example 2, the differences between Example 4 and Example 2 are as follows:

[0108] The hydrogen-carbon ratio of the synthesis gas is 3.20.

[0109] The temperature of the first-stage methanation reaction is 450°C;

[0110] The first-stage methanation reaction includes carrying out the first-stage methanation reaction using a methanation catalyst, and the nickel content of the methane catalyst is 65%.

[0111] The initial temperature of the secondary methanation reaction is 300 °C, and the end temperature of the secondary methanation reaction is 330 °C; and / or,

[0112] The secondary methanation reaction includes carrying out the secondary methanation reaction using a methanation catalyst, and the nickel content of the methane catalyst is 65%.

[0113] The circulation volume ratio of the recycle gas raw material to the intermediate reaction gas is 0.4.

[0114] The composition of the obtained product gas is: H2: 2.4 mol%, CO: 0, CO2: 0.25 mol%, CH4: 95.15 mol%,

[0115] N2: 1.9 mol%, H2O: 0.3 mol%.

[0116] In summary, a system for preparing natural gas using high-carbon-concentration syngas provided by an embodiment of the present application uses an isothermal reactor 131 as a large number of methanation reactors, which can make the reaction temperature lower than 450 °C, and the carbon monoxide conversion rate of the syngas in the isothermal reactor 131 is high, which can reduce the number of reactors, and the product gas after the reaction can be recycled in a small amount to reduce the severity of the reaction in the isothermal reactor 131 to improve the catalyst stability; therefore, the system has the advantages of simple process, easy operation and few equipment. At the same time, since the methanation reaction occurs at a lower temperature, high-temperature equipment is not required, so the equipment investment and operation cost are low, and the system is also applicable to the direct methanation process with a high concentration of carbon monoxide content, especially the syngas formed after dry pulverized coal and water slurry gasification, and has a broad market application prospect.

[0117] At the same time, the conversion rate of carbon monoxide of the system is greater than 99.5%, the conversion rate of hydrogen is greater than 99%, and 5516.27 kmol / h of synthetic natural gas can be obtained from a raw material gas volume of 21420 kmol / h, and 399.5 t / h of high-pressure saturated steam at 9.0 MPa to 9.9 MPa can be by-produced.

[0118] And a method for preparing natural gas using high-carbon-concentration syngas provided by an embodiment of the present application can provide a new process scheme for the coal-to-natural gas process, and the produced natural gas meets the requirements of the first-class product of natural gas in the national standard (GB17820-2018).

[0119] The various embodiments of the present application may exist in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be construed as a rigid limitation on the scope of the present application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single values within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within that range, such as 1, 2, 3, 4, 5, and 6, and this applies regardless of the range. Additionally, whenever a numerical range is indicated herein, it means including any cited number (fraction or integer) within the indicated range.

[0120] In the present application, unless otherwise stated, the orientation terms such as "upper" and "lower" specifically refer to the drawing directions in the figures. Additionally, in the description of the specification of the present application, the terms "comprising", "including", etc. mean "including but not limited to". In this text, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this text, "and / or" describes the associated relationship of associated objects and indicates that three relationships can exist. For example, A and / or B can mean: A exists alone, A and B exist simultaneously, and B exists alone. Where A and B can be singular or plural. In this text, "at least one" means one or more, and "a plurality" means two or more. "At least one kind", "at least one of the following items (pieces)" or similar expressions refer to any combination of these items, including any combination of single item (piece) or plural items (pieces). For example, "at least one of a, b, or c", or, "at least one of a, b, and c" can both mean: a, b, c, a - b (i.e., a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively.

[0121] The above description is only the specific implementation manners of the present application, enabling those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but rather to the broadest scope consistent with the principles and novel features claimed herein.

Claims

1. A system for preparing natural gas using syngas with a high carbon concentration, characterized in that, The system includes: An isothermal reaction section (1), the isothermal reaction section (1) includes a raw material preheater (11), a reaction gas heat exchanger (12) and at least two isothermal reaction devices (13), the isothermal reaction devices (13) are arranged in parallel, the raw material preheater (11) communicates with the feed inlet of the isothermal reaction device (13), the discharge outlet of the isothermal reaction device (13) communicates with the reaction gas heat exchanger (12), and the gas outlet of the reaction gas heat exchanger (12) communicates with the feed inlet of the isothermal reaction device (13); An adiabatic reaction section (2), the adiabatic reaction section (2) includes an adiabatic reactor (21), a product gas heat exchanger (22) and a product gas separation tank (23), the feed inlet of the adiabatic reactor (21) communicates with the discharge outlet of the reaction gas heat exchanger (12), the discharge outlet of the adiabatic reactor (21) communicates with the feed inlet of the product gas heat exchanger (22), and the discharge outlet of the product gas heat exchanger (22) communicates with the feed inlet of the product gas separation tank (23).

2. The system according to claim 1, characterized in that, The isothermal reaction device (13) includes an isothermal reactor (131) and an isothermal heat exchanger (132), the discharge outlet of the raw material preheater (11) communicates with the feed inlet of the isothermal reactor (131), the discharge outlet of the isothermal reactor (131) communicates with the feed inlet of the isothermal heat exchanger (132), and the discharge outlet of the isothermal heat exchanger (132) communicates with the reaction gas heat exchanger (12).

3. The system according to claim 2, wherein The isothermal reaction device (13) further includes a steam drum (133), the liquid outlet end and the liquid inlet end of the steam drum (133) communicate with the side wall of the isothermal reactor (131) respectively, and the steam drum (133) is provided with an outlet for high-pressure steam.

4. The system according to claim 1, wherein The system further includes: A circulation section (3), the circulation section (3) includes a circulating gas heat exchanger (31), a circulating gas separation tank (32) and a circulating gas preheater (33), the inlet of the circulating gas heat exchanger (31) communicates with the gas outlet of the reaction gas heat exchanger (12), the outlet of the circulating gas heat exchanger (31) communicates with the feed inlet of the circulating gas separation tank (32), the gas outlet of the circulating gas separation tank (32) communicates with the inlet of the circulating gas preheater (33), and the outlet of the circulating gas preheater (33) communicates with the feed inlet of the isothermal reaction device (13).

5. The system according to claim 4, wherein The circulation section (3) further includes a booster pump (34), the feed inlet of the booster pump (34) communicates with the circulating gas separation tank (32), and the discharge outlet of the booster pump (34) communicates with the feed inlet of the circulating gas preheater (33).

6. A method for preparing natural gas by using syngas with a high carbon concentration, characterized in that, The method is adapted to the system according to any one of claims 1-5, and the method includes: Exchanging heat for the synthesis gas, and then adding circulating gas to the synthesis gas to obtain a mixed gas; Carrying out a primary methanation reaction on the mixed gas under isothermal conditions to obtain a primary methanation reaction gas; Exchanging heat and separating the primary methanation reaction gas to obtain a circulating gas raw material and an intermediate reaction gas respectively; Carrying out a secondary methanation reaction on the intermediate reaction gas in an adiabatic environment, and then carrying out cooling and gas-liquid separation to obtain a product gas; Cool down, separate gas from liquid, and compress and boost the circulating gas raw material to obtain circulating gas.

7. The method according to claim 6, characterized in that, The hydrogen-carbon ratio of the synthesis gas is 2.90 - 3.20, and the methane content in the synthesis gas is ≤ 1%.

8. The method according to claim 6, wherein The temperature of the primary methanation reaction is 250°C - 450°C; and / or, The primary methanation reaction includes carrying out the primary methanation reaction using a methanation catalyst, and the nickel content of the methane catalyst is 30% - 65%.

9. The method according to claim 8, characterized in that The initial temperature of the secondary methanation reaction is 250°C - 300°C, and the end temperature of the secondary methanation reaction is 270°C - 330°C; and / or, The secondary methanation reaction includes carrying out the secondary methanation reaction using the methanation catalyst, and the nickel content of the methane catalyst is 30% - 65%.

10. The method according to claim 6, characterized in that The circulating volume ratio of the circulating gas raw material to the intermediate reaction gas is 0.1 - 0.4.

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