A method for producing synthetic natural gas by partition hydrogenation of carbon dioxide-rich synthesis gas

Abstract

The application discloses a method for preparing synthetic natural gas by partition hydrogen supplement methanation of carbon dioxide-rich synthesis gas. The carbon dioxide-rich synthesis gas after desulfurization is sequentially introduced into three-stage methanation zones without water-gas shift and decarburization. In the first stage, carbon monoxide is preferentially converted and bed temperature rise is controlled by using at least two-stage series reactors. In the second stage, at least three-stage series reactors are adopted, waste heat preheated hydrogen is supplemented along the gas flow direction, the H2 / (CO+CO2) at the inlet of each stage is gradually increased, and the water vapor partial pressure is reduced through inter-stage water removal to promote carbon dioxide methanation. In the third stage, residual carbon monoxide and carbon dioxide are deeply methanated. The outlet temperature of each reactor is lower than 650 DEG C, the pressure is 0.1-10.0 MPa, after cooling and gas-liquid separation, the dry basis product gas CH4 volume fraction is not less than 97%.

Description

Technical Field

[0001] This invention belongs to the fields of energy and chemical engineering, coal chemical engineering, biomass energy utilization and carbon dioxide resource utilization. Specifically, it relates to a method for preparing synthetic natural gas from carbon dioxide-containing syngas, which is particularly suitable for the partitioned hydrogen supplementation methanation of carbon dioxide-rich, low hydrogen-to-carbon ratio syngas to prepare synthetic natural gas. Background Technology

[0002] Natural gas is characterized by clean combustion, high calorific value, convenient transportation, and high end-use efficiency, making it an important clean energy source and chemical raw material. my country's energy resource endowment is characterized by abundant coal, scarce oil, and limited natural gas, leading to a continuous increase in natural gas consumption demand. By 2025, China's dependence on imported natural gas is projected to reach 43%. Developing technologies for coal-to-natural gas, biomass-to-natural gas, and the resource-based methane production from carbon-containing gases is of great significance for improving the country's self-sufficiency in natural gas supply, promoting the clean and efficient use of coal, expanding carbon resource utilization pathways, and achieving the "dual carbon" goal. Thermochemical conversion processes of carbon-containing raw materials, such as coal gasification, coal pyrolysis, biomass gasification, biomass pyrolysis, and oil gasification, can produce syngas containing components such as H2, CO, CO2, CH4, and water vapor. After dust removal and desulfurization treatment, this syngas can be converted into synthetic natural gas through methanation. Methanation reactions mainly include carbon monoxide methanation and carbon dioxide methanation. CO + 3H₂ → CH₄ + H₂O Δ H 298K =-206.4 kJ / mol CO2 + 4H2 → CH4 + 2H2O Δ H 298K =-164.9 kJ / mol;

[0003] Among them, carbon monoxide methanation has a faster reaction rate and higher exothermic intensity; carbon dioxide methanation requires more hydrogen, has a relatively lower reaction rate, and is more sensitive to the hydrogen-to-carbon ratio, reaction temperature, and moisture content of the reaction system.

[0004] Currently, the main routes for methanation of syngas include the Lurgi adiabatic fixed-bed methanation method, the Haldor Topsoe adiabatic fixed-bed methanation method, and the Davy adiabatic fixed-bed methanation method. For syngas rich in carbon dioxide and with a low hydrogen-to-carbon ratio, conventional methanation processes are prone to problems such as strong exothermic reactions in the initial carbon monoxide methanation stage leading to localized overheating of the bed, and incomplete conversion of the subsequent carbon dioxide methanation stage due to insufficient hydrogen or high water vapor partial pressure. Therefore, for syngas rich in carbon dioxide and with a low hydrogen-to-carbon ratio, the key to producing high-quality synthetic natural gas lies in how to rationally control the reaction temperature rise and promote the conversion of CO and CO2.

[0005] Currently, methods for producing synthetic natural gas from syngas require a hydrogen-to-carbon ratio of 2.5 to 4.0 in the feed gas. Traditional coal-to-natural gas processes typically require adjusting the hydrogen-to-carbon ratio through water-gas shift, decarbonization, or hydrogen supplementation to meet the requirements of methanation and product gas quality. For example, patent CN107087415A employs a multi-point feeding strategy and a sequentially arranged reactor system, combined with staged circulating gas reflux dilution and controlled de-circulation operation of some reactors, effectively achieving temperature distribution control and energy optimization in the reaction process. This method requires that the hydrogen and carbon oxides in the feed gas basically meet the stoichiometric relationship for the methanation reaction, i.e., a hydrogen to carbon monoxide / carbon dioxide molar ratio of 3.0 to 4.0. Therefore, this method has high requirements for the hydrogen-to-carbon ratio of the feed gas. CN103119137A moves the pressure regulation stage forward and adopts a distributed pressure regulation method, combined with a mixed gas compression unit and multi-stage... The coupled operation of the reactors reduces system energy consumption and optimizes overall process performance. The hydrogen-to-carbon ratio of the feed gas can be adjusted within a wide range, with the molar ratio of hydrogen to carbon monoxide ranging from 0.4 to 5.0, preferably from 2.5 to 4.0, to improve adaptability to changes in the composition of the feed gas. The feed gas is usually dominated by carbon monoxide with a low carbon dioxide content, resulting in limited utilization of carbon dioxide resources. CN104862024A adopts an indirect heat exchange methanation process that combines fluid heat transfer medium with closed-loop heat circulation to achieve decoupling of reaction and heat exchange, multi-stage reaction temperature control, and efficient heat recovery. It controls the hydrogen-to-carbon ratio of the feed gas within the range of 3.0 to 4.0.

[0006] The feed gas in the aforementioned patents all undergoes shift reaction and desulfurization and decarbonization. Addressing the limited CO2 utilization, CN107916150A constructs a waterless methanation process route by directly controlling the H2 / CO ratio during the gasification stage and eliminating the water-gas shift section. This simplifies the process, achieving zero water consumption and low energy consumption in coal-to-natural gas production. However, issues such as insufficient hydrogen in the feed gas and a lack of hydrogen-to-carbon ratio adjustment methods result in a strong dependence on the feed gas composition, low operational stability, and significant process control difficulties. CN108467756A introduces a sulfur-resistant molybdenum-based catalyst... The catalytic system couples sulfur-resistant shift reaction with methanation to achieve direct methanation of high-sulfur, low-H2 / CO feed gas, eliminating the need for traditional water-gas shift and deep desulfurization units, thus significantly simplifying the process and reducing energy consumption. Instead, it relies on a multi-stage methanation system consisting of a circulating reactor and a supplementary reactor to achieve the step-by-step conversion of CO and CO2 and the internal adjustment of the hydrogen-carbon ratio. However, due to the lack of external water-gas shift and decarbonization adjustment methods, the hydrogen-carbon ratio mainly depends on the composition of the feed gas, resulting in weak overall adjustability. Furthermore, operation under high-sulfur and low-H2 / CO ratio conditions places high demands on the stability of the catalytic system and the management of reaction heat.

[0007] Existing syngas methanation or carbon dioxide methanation processes still have shortcomings when handling CO2-rich, low-hydrogen-to-carbon-ratio syngas. On the one hand, current hydrogen replenishment methods mostly involve overall hydrogen replenishment before the reaction or inter-stage hydrogen replenishment in a few stages, making it difficult to control the process in stages according to the conversion characteristics of CO and CO2 at different reaction phases. CO methanation is highly exothermic and fast-paced; if there is excessive hydrogen in the initial stage or improper temperature control, it can easily lead to localized overheating and catalyst deactivation. Conversely, insufficient hydrogen in the later stages can result in incomplete CO2 conversion and a high residual CO2 content in the product gas. On the other hand, a large amount of water is generated during methanation. The presence of water vapor affects the CO2 methanation equilibrium and may exacerbate catalyst hydrothermal aging. If the moisture is not removed in time, it will affect the reaction equilibrium and be detrimental to CO2 methanation. Existing processes do not adequately consider hydrogen distribution, reaction zoning, heat recovery, and inter-stage water removal, making it difficult to simultaneously achieve high CO and CO2 conversion rates, high methane content in the product gas, low process energy consumption, and long-term stable catalyst operation.

[0008] Therefore, there is an urgent need to develop a multi-stage, partitioned hydrogen supplementation methanation process suitable for carbon dioxide-rich syngas, so that CO methanation, CO2 staged hydrogen supplementation methanation, and further methanation of residual CO and / or CO2 can proceed in an orderly manner. By implementing measures such as circulating gas temperature control, interstage heat exchange, interstage water removal, and heat recovery, the hydrogen supply and reaction pathway can be matched, thereby improving the carbon oxide conversion efficiency and the quality of synthetic natural gas products. Summary of the Invention

[0009] The purpose of this invention is to solve the problems of the existing carbon dioxide-rich syngas methanation process, such as the single hydrogen replenishment method, difficulty in matching the hydrogen-carbon ratio to the reaction path, local overheating in the front stage, insufficient CO2 conversion in the rear stage, catalyst deactivation due to high temperature, low reaction efficiency and high energy consumption.

[0010] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing synthetic natural gas by partitioned hydrogen supplementation methanation of carbon dioxide-rich syngas includes the following steps: (1) The raw material gas, circulating gas and water vapor are mixed and fed into the first methanation zone for methanation reaction, and part of the outlet gas of the second methanation reactor is used as circulating gas to control the bed temperature of each methanation reactor in the first methanation zone. The carbon dioxide-rich syngas is derived from one or more of the gases generated by the thermochemical conversion of carbon-containing raw materials, such as coal gasification, coal pyrolysis, biomass gasification, biomass pyrolysis, and oil gasification. The preferred amount of water vapor added is 2% to 10% of the molar flow rate of the raw material gas, used to suppress the carbon deposition reaction during the methanation process. Since the raw material gas is carbon dioxide-rich synthesis gas, the CO2 contained therein, in addition to participating in the subsequent methanation reaction in the first methanation zone, can also dilute the reaction gas, reduce the partial pressure of CO and the temperature rise of the bed; at the same time, CO2 can react with the deposited carbon to undergo a gasification reaction (C + CO2 = 2CO), thereby reducing the risk of catalyst carbon deposition. Therefore, the present invention only needs to add a small amount of water vapor in the first methanation zone to meet the requirements of temperature control and carbon suppression. The raw material gas mixed with water vapor enters the first methanation reactor (1) at a volume ratio of 0.3 to 0.8. The remaining raw material gas is mixed with the outlet gas of the first methanation reactor (1) and sent to the second methanation reactor (2). The outlet gas of the second methanation reactor (2) enters the first gas compressor as the first circulating gas at a volume ratio of 0.2 to 0.8. The remaining outlet gas is used as the first reactor and passed into the second methanation zone. (2) The first reaction gas obtained from the first methanation zone is fed into the second methanation zone. At least two methanation reactors in the second methanation zone are connected to hydrogen supply lines at their inlets to mix with the outlet gas from the previous stage reactor before entering the current stage methanation reactor for the methanation reaction. Hydrogen is added in stages along the gas flow direction to gradually increase the H2 / (CO+CO2) molar ratio at the inlet of each methanation reactor. The H2 / (CO+CO2) molar ratios at the inlet of the third, fourth, and fifth methanation reactors are 1.0~2, 1.5~3.0, and 3.0~5.0, respectively. Preferably, the molar ratios are 1.1~1.7, 1.8~2.5, and 3.2~4.2, respectively. The outlet gas from the fifth-stage methanation reactor is used as the second reaction gas and fed into the third methanation zone.

[0011] The hydrogen is derived from one or more of the following: coal-based hydrogen production, biomass-based hydrogen production, industrial by-product hydrogen production, natural gas-based hydrogen production, and renewable energy-based hydrogen production through water electrolysis.

[0012] In the method, the outlet gas of the fourth methanation reactor (4) in the second methanation zone is fed into the second gas compressor at a volume ratio of 0.2 to 0.8 as the second circulating gas and sent to the third methanation reactor to control the bed temperature of each reactor in the second methanation zone; A dehydration unit is provided between at least one adjacent reactor in the second methanation zone. The dehydration unit is used to remove water generated during methanation, reduce the partial pressure of water vapor, and thus promote the CO2 methanation reaction towards the production of methane. Preferably, a dehydration unit is provided between the fourth methanation reactor (4) and the fifth methanation reactor (5).

[0013] (3) The second reaction gas obtained from the second methanation zone is fed into the third methanation zone for supplementary methanation reaction, so that the residual carbon monoxide and / or carbon dioxide are further converted into methane. The third methanation zone may include one or more methanation reactors arranged in series. The gas after the reaction in the third methanation zone is cooled and separated into gas and liquid to obtain synthetic natural gas product gas.

[0014] In this invention, the volume fraction of carbon dioxide in the carbon dioxide-rich synthesis gas is 5%–40%, the volume fraction of carbon monoxide is 5%–70%, the volume fraction of hydrogen is 5%–50%, and the volume fraction of methane is 0–20%.

[0015] The initial H2 / (CO+CO2) molar ratio of the carbon dioxide-containing synthesis gas is 0.3 to 4.0, preferably 0.4 to 2.5.

[0016] The operating pressure of the process in this invention is 0.1 to 10.0 MPa, preferably 2.0 to 5.0 MPa.

[0017] The preferred inlet temperature for each methanation reactor is 200–300°C.

[0018] The outlet temperature of each methanation reactor is below 650°C.

[0019] Before entering the corresponding reactor inlet, the hydrogen gas added to the second methanation zone is preferably heated / preheated by the corresponding reactor outlet gas or high-temperature gas between adjacent stages, with a temperature of 200-250°C.

[0020] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention divides the methanation reaction into a first methanation zone, a second methanation zone and a third methanation zone; the first methanation zone mainly completes the methanation of carbon monoxide in the feed gas, the second methanation zone mainly completes the methanation of carbon dioxide under the condition of staged hydrogen supplementation, and the third methanation zone further methanates the residual carbon monoxide and carbon dioxide, so that different carbon-containing components are converted according to their reaction characteristics, thereby improving the process adaptability. (2) Since CO2 in the carbon dioxide-rich feed gas has the functions of dilution, temperature control and carbon deposition gasification in the first methanation zone, the present invention can effectively control the bed temperature rise and catalyst carbon deposition under the condition of low water vapor replenishment, thereby reducing the hydrothermal deactivation of the catalyst, reducing steam consumption and preventing the adverse effects of excessive water vapor on the methanation equilibrium. (3) This invention introduces hydrogen in stages along the second methanation zone, thereby gradually increasing the H2 / (CO+CO2) molar ratio at the inlet of each reactor. This avoids local overheating caused by excessive hydrogen in the front stage and improves the problem of insufficient hydrogen supply in the later stage of CO2 methanation. By coupling the staged hydrogen supply with interstage cooling and dehydration, the partial pressure of water vapor in the reaction system can be reduced, the CO2 methanation equilibrium can be shifted towards methane production, and the CO2 conversion rate and methane content in the product gas can be improved. (4) The present invention can effectively control the bed hot spot temperature and improve the system heat utilization efficiency by circulating gas reflux dilution and multi-stage waste heat recovery, thereby extending the catalyst life and reducing the system energy consumption. (5) This invention is particularly applicable to syngas systems with low hydrogen-to-carbon ratio and rich in CO2. While promoting CO2 emission reduction and resource utilization, efficient conversion of renewable energy, and improving the capacity for green electricity consumption, it effectively strengthens the methanation reaction process, significantly reduces the residual CO and CO2 content in the product gas, and improves the quality of synthetic natural gas. Attached Figure Description

[0021] Figure 1 is a process flow diagram of the carbon dioxide-rich syngas partitioned hydrogen supplementation methanation process for producing synthetic natural gas according to the present invention. In the figure, 1-first methanation reactor; 2-second methanation reactor; 3-third methanation reactor; 4-fourth methanation reactor; 5-fifth methanation reactor; 6-sixth methanation reactor; 7-first waste heat boiler; 8-second waste heat boiler; 9-third waste heat boiler; 10-fourth waste heat boiler; 11-fifth waste heat boiler; 12-sixth waste heat boiler; 13-seventh waste heat boiler; 14-eighth waste heat boiler; 15-ninth waste heat boiler; 16-tenth waste heat boiler; 17-eleventh waste heat boiler. 18-First gas-liquid separator; 19-Twelfth waste heat boiler; 20-Thirteenth waste heat boiler; 21-Fourteenth waste heat boiler; 22-Fifteenth waste heat boiler; 23-Second gas-liquid separator; 24-Sixteenth waste heat boiler; 25-Seventeenth waste heat boiler; 26-Third gas-liquid separator; S101-Raw gas; S102-Steam; S103-Hydrogen; S104-First circulating gas; S105-Second circulating gas; S106-First hydrogen replenishment; S107-Second hydrogen replenishment; S108-Third hydrogen replenishment. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and embodiments, so that those skilled in the art can easily understand and master the technical points of the present invention. However, the scope of protection of the present invention is not limited to the following embodiments. Equivalent substitutions or conventional adjustments made by those skilled in the art to the process conditions, equipment layout, catalyst type, heat exchange network, or material distribution ratio of the present invention without departing from the spirit and scope of protection of the present invention shall all fall within the scope of protection of the present invention.

[0023] In the following embodiments, unless otherwise specified, the volume composition of the synthesis gas is a volume fraction, and the gas flow rate is in standard conditions. M is the molar ratio of hydrogen to carbon oxides in the feed gas, i.e., M=n H2 / (n CO +n CO2 (where n represents volume composition).

[0024] Example 1 Annual production of 1 billion Nm 3 Coal-to-natural gas production on a synthetic natural gas scale, with an annual operating time of 8,000 hours.

[0025] The feed gas comes from a pressurized coal gasification unit. After dust removal and desulfurization, it undergoes no water-gas shift or decarbonization treatment and is directly used as the feed gas for the methanation process of this invention. The feed gas has a pressure of 3.5 MPa, a temperature of 25°C, and a volumetric flow rate of 202048 Nm³. 3 / h, total sulfur less than 0.1 ppm, M=0.6883, the specific volume composition of the feed gas is: H2: 36.298%, CH4: 8.731%, CO: 37.249%, CO2: 15.483%, H2O: 1.712%, N2: 0.527%.

[0026] Referring to the claims, in accordance with Figure 1 The process flow shown involves the methanation reaction of the above-mentioned raw material gas.

[0027] 11.7 t / h of water vapor (7.2% of the molar flow rate of the feed gas) was added to the above feed gas to make the volume fraction of water vapor in the mixed feed gas entering the first methanation zone about 8%, which was used to reduce the tendency of catalyst carbon deposition in the first methanation zone and to help control the reaction temperature rise. The mixed feed gas was fed into the first methanation reactor (1) at a volume flow rate ratio of 0.5 and 0.5, and the other stream was mixed with the outlet gas of the first methanation reactor (1) and then entered the second methanation reactor (2). The inlet temperature of the first methanation reactor (1) and the second methanation reactor (2) were controlled at 220℃, and the inlet pressures were 3.4 MPa and 3.5 MPa, respectively. After the methanation reaction, the outlet gas temperature of the first methanation reactor (1) was 532℃ and the pressure was 3.4 MPa. The outlet gas is successively cooled to 300°C by the first waste heat boiler (7) and the second waste heat boiler (8). Then, it is mixed with the second raw material gas entering the second methanation reactor (2) through the pipeline and fed into the second methanation reactor (2) for reaction. The outlet gas temperature of the second methanation reactor (2) is 529°C and the outlet pressure is 3.3 MPa. The outlet gas of the second methanation reactor (2) is cooled to 220°C by the third waste heat boiler (9) and the fourth waste heat boiler (10). It is fed into the first gas compressor (27) at a volume ratio of 0.6 and 0.4 respectively to obtain the first circulating gas and the second methanation zone as the first reaction gas.

[0028] The third methanation reactor (3), the fourth methanation reactor (4), and the fifth methanation reactor (5) form the second methanation zone, and the inlet temperature of each reactor is controlled at 220°C. 273616 Nm³ of preheated material is then used. 3H2 is fed into the third methanation reactor (3), the fourth methanation reactor (4), and the fifth methanation reactor (5) at ratios of 0.37, 0.44, and 0.19 per hour. The first reaction gas from the first methanation zone is mixed with the first supplementary hydrogen and the second circulating gas and fed into the third methanation reactor (3) for methanation reaction. The inlet temperature is 223°C and the inlet pressure is 3.3 MPa. The outlet gas temperature of the third methanation reactor (3) is 471°C and the pressure is 3.2 MPa. The gas is cooled to 220°C by heat recovery from the fifth waste heat boiler (11), the sixth waste heat boiler (12), and the seventh waste heat boiler (13). Then it is mixed with the second supplementary hydrogen as the inlet gas of the fourth methanation reactor (4). The inlet temperature is 218°C and the inlet pressure is 3.2 MPa. The outlet gas temperature of the fourth methanation reactor (4) is 455℃ and the pressure is 3.1MPa. It is then cooled to 80℃ by the eighth waste heat boiler (14), the ninth waste heat boiler (15), the tenth waste heat boiler (16), and the eleventh waste heat boiler (17) to recover heat. It then enters the first gas-liquid separator (18) to separate condensate. After being heated to 220℃ by the ninth waste heat boiler (15), it is divided into second gas compressor (28) and second circulating gas at a volume ratio of 0.6 and 0.4, respectively. It is also used as inlet gas at the inlet of the fifth methanation reactor (5). After the inlet gas is mixed with the third hydrogen supplement, it is fed into the fifth methanation reactor (5) for reaction. The outlet gas temperature of the fifth methanation reactor (5) is 481°C and the pressure is 3.0 MPa. It is then passed through the twelfth waste heat boiler (19), the thirteenth waste heat boiler (20), the fourteenth waste heat boiler (21) and the fifteenth waste heat boiler (22) to recover heat and cool to 70°C. Then it is fed into the second gas-liquid separator (23) to separate the condensate. Finally, it is heated to 220°C by the thirteenth waste heat boiler (20) and then used as the second reaction gas to enter the third methanation zone.

[0029] The third methanation zone includes the sixth methanation reactor (6), which further methanates the second reaction gas from the second methanation zone, wherein the second reaction gas has a temperature of 220°C and a pressure of 3.0 MPa. The outlet gas temperature of the sixth methanation reactor (6) is 296°C. The gas is cooled to 40°C by the sixteenth waste heat boiler (24) and the seventeenth waste heat boiler (25) and then fed into the third gas-liquid separator (26) to separate the condensate, thus obtaining the final synthetic natural gas.

[0030] In the first methanation zone, pressurized cooling water at a temperature of 25℃ and a pressure of 9.8MPa is heated to a saturation temperature of 309℃ by the second waste heat boiler (8), and then superheated by the third waste heat boiler (9) to obtain superheated steam at a temperature of 502℃, a pressure of 9.8MPa, and a flow rate of 36t / h. In the second methanation zone, pressurized cooling water at a temperature of 25℃ and a pressure of 6.5MPa is heated to a saturation temperature of 280℃ by the tenth waste heat boiler (16). The superheated steam is then obtained by superheating it in the sixth waste heat boiler (12) at a temperature of 402℃, a pressure of 6.5MPa and a flow rate of 63t / h. In the third methanation zone, pressurized cooling water at a temperature of 25℃ and a pressure of 3.5MPa is heated to 106℃ by the sixteenth waste heat boiler (24), and then heated by the fourteenth waste heat boiler (21) in the second methanation zone to obtain saturated steam at a temperature of 242℃, a pressure of 3.5MPa and a flow rate of 36t / h.

[0031] The product gas contains 97.44% CH4, <1.0% H2, <1.0% CO2, and has a total CO conversion rate of 99.99%.

[0032] Example 2 Annual production of 1 billion Nm 3 Coal-to-natural gas production on a synthetic natural gas scale, with an annual operating time of 8,000 hours.

[0033] Produced using pressurized coal-water slurry gasification technology, with a pressure of 3.5 MPa, a temperature of 25℃, and a volumetric flow rate of 179,200 Nm³. 3 / h of desulfurized syngas with total sulfur content less than 0.1 ppm, M=n H2 / (n CO +n CO2 =0.4486 (n represents the volume composition), and the specific volume composition of the synthesis gas is: H2: 30.819%, CH4: 0.0301%, CO: 50.390%, CO2: 18.310%, H2O: 0%, N2: 0.450%.

[0034] Referring to the claims, in accordance with Figure 1 The process flow shown involves the methanation reaction of the above-mentioned raw material gas.

[0035] Add 12.6 t / h of water vapor (8.7% of the molar flow rate of the raw gas) to the above raw gas to increase the volume fraction of water vapor in the raw gas to 8%, which can prevent carbon deposition reaction in the first methanation zone. Then, the mixed raw gas is fed into the first methanation reactor (1) and the second methanation reactor (2) in the first methanation zone at a volume ratio of 0.5 and 0.5, respectively. The inlet temperature of the reactors is controlled at 220℃ and the inlet pressure is 3.4 MPa and 3.5 MPa, respectively. After the methanation reaction, the outlet gas temperature of the first methanation reactor (1) is 563°C and the pressure is 3.4 MPa. The heat is recovered by the first waste heat boiler (7) and the second waste heat boiler (8) and cooled to 300°C. Then, it is mixed with the raw material gas entering the second methanation reactor (2) through the pipeline and fed into the second methanation reactor (2) for reaction. The outlet gas temperature of the second methanation reactor (2) is 559°C and the pressure is 3.3 MPa. The heat is recovered by the third waste heat boiler (9) and the fourth waste heat boiler (10) and cooled to 220°C. It is fed into the first gas compressor (27) at a volume ratio of 0.6 and 0.4 respectively to obtain the first circulating gas and the second methanation zone as the first reaction gas.

[0036] The third methanation reactor (3), the fourth methanation reactor (4), and the fifth methanation reactor (5) form the second methanation zone, with the inlet temperature of each reactor controlled at 220°C. 344064 Nm³ preheated to 220°C... 3H2 is fed into the third methanation reactor (3), the fourth methanation reactor (4), and the fifth methanation reactor (5) at ratios of 0.52, 0.31, and 0.17, respectively. The first reaction gas from the first methanation zone is mixed with the first supplementary hydrogen and the second circulating gas and fed into the third methanation reactor (3) for methanation reaction. Its inlet temperature is 222°C and the inlet pressure is 3.3 MPa. The outlet gas temperature of the third methanation reactor (3) is 572°C and the pressure is 3.2 MPa. After recovering heat from the fifth waste heat boiler (11), the sixth waste heat boiler (12), and the seventh waste heat boiler (13), it is cooled to 220°C and then mixed with the second supplementary hydrogen as the inlet gas of the fourth methanation reactor (4). The inlet temperature is 219°C and the inlet pressure is 3.2 MPa. The outlet gas temperature of the fourth methanation reactor (4) is 482℃ and the pressure is 3.1MPa. It is successively passed through the eighth waste heat boiler (14), the ninth waste heat boiler (15), the tenth waste heat boiler (16), and the eleventh waste heat boiler (17) to recover heat and cool to 80℃ before entering the first gas-liquid separator (18) to separate condensate. Then, it is heated to 220℃ by the ninth waste heat boiler (15) and divided into second gas compressor (28) as second circulating gas and fifth methanation reactor (5) as inlet gas in a volume ratio of 0.6 and 0.4. After the gas is mixed with the third hydrogen supplement, it is fed into the fifth methanation reactor (5) for reaction. The outlet gas temperature of the fifth methanation reactor (5) is 507°C and the pressure is 3.0 MPa. It is then passed through the twelfth waste heat boiler (19), the thirteenth waste heat boiler (20), the fourteenth waste heat boiler (21) and the fifteenth waste heat boiler (22) to recover heat and cool to 70°C. Then it is fed into the second gas-liquid separator (23) to separate the condensate. Finally, it is heated to 220°C by the thirteenth waste heat boiler (20) and then used as the second reaction gas to enter the third methanation zone.

[0037] The third methanation zone includes the sixth methanation reactor (6), which further methanates the second reaction gas from the second methanation zone, wherein the second reaction gas has a temperature of 220°C and a pressure of 3.0 MPa. The outlet gas temperature of the sixth methanation reactor (6) is 318°C. The gas is cooled to 40°C by the sixteenth waste heat boiler (24) and the seventeenth waste heat boiler (25) and then fed into the third gas-liquid separator (26) to separate the condensate, thus obtaining the final synthetic natural gas.

[0038] In the first methanation zone, pressurized cooling water at a temperature of 25℃ and a pressure of 9.8MPa is heated to a saturation temperature of 309℃ by the second waste heat boiler (8), and then superheated by the third waste heat boiler (9) to obtain superheated steam at a temperature of 511℃, a pressure of 9.8MPa, and a flow rate of 45t / h. In the second methanation zone, pressurized cooling water at a temperature of 25℃ and a pressure of 6.5MPa is heated to a saturation temperature of 280℃ by the tenth waste heat boiler (16). The superheated steam is then obtained by superheating it in the sixth waste heat boiler (12) at a temperature of 483℃, a pressure of 6.5MPa and a flow rate of 95t / h. In the third methanation zone, pressurized cooling water at a temperature of 25℃ and a pressure of 3.5MPa is heated to 147℃ by the sixteenth waste heat boiler (24), and then heated by the fourteenth waste heat boiler (21) in the second methanation zone to obtain saturated steam at a temperature of 242℃, a pressure of 3.5MPa and a flow rate of 36t / h.

[0039] The product gas contains 97.43% CH4, <1.0% H2, <1.0% CO2, and has a total CO conversion rate of 99.99%.

[0040] Example 3 Produced using pressurized pulverized coal gasification technology, with a pressure of 3.5 MPa, a temperature of 25℃, and a volumetric flow rate of 197,120 Nm³. 3 / h of desulfurized syngas with total sulfur content less than 0.1 ppm, M=n H2 / (n CO +n CO2 =0.3857 (n represents the volume composition), and the specific volume composition of the synthesis gas is: H2: 26.190%, CH4: 0.0301%, CO: 64.360%, CO2: 3.530%, H2O: 5.390%, N2: 0.520%.

[0041] Referring to the claims, in accordance with Figure 1 The process flow shown involves the methanation reaction of the above-mentioned raw material gas.

[0042] Add 4.5 t / h of water vapor (2.8% of the molar flow rate of the raw gas) to the above raw gas to increase the volume fraction of water vapor in the raw gas to 8%, which can prevent carbon deposition reaction in the first methanation zone. Then, the mixed raw gas is fed into the first methanation reactor (1) and the second methanation reactor (2) in the first methanation zone at a volume ratio of 0.5 and 0.5. The inlet temperature of the reactors is controlled at 220℃ and the inlet pressure is 3.4 MPa and 3.5 MPa, respectively. After the methanation reaction, the outlet gas temperature of the first methanation reactor (1) is 588°C and the pressure is 3.4 MPa. The heat is recovered by the first waste heat boiler (7) and the second waste heat boiler (8) and cooled to 300°C. Then, it is mixed with the raw material gas entering the second methanation reactor (2) through the pipeline and fed into the second methanation reactor (2) for reaction. The outlet gas temperature of the second methanation reactor (2) is 579°C and the pressure is 3.3 MPa. The heat is recovered by the third waste heat boiler (9) and the fourth waste heat boiler (10) and cooled to 220°C. It is fed into the first gas compressor (27) at a volume ratio of 0.6 and 0.4 respectively to obtain the first circulating gas and the second methanation zone as the first reaction gas.

[0043] The third methanation reactor (3), the fourth methanation reactor (4), and the fifth methanation reactor (5) form the second methanation zone, with the inlet temperature of each reactor controlled at 220°C. 353920 Nm³ of preheated fuel is then used. 3H2 is fed into the third methanation reactor (3), the fourth methanation reactor (4), and the fifth methanation reactor (5) at ratios of 0.45, 0.38, and 0.17, respectively. The first reaction gas from the first methanation zone is mixed with the first supplementary hydrogen and the second circulating gas and fed into the third methanation reactor (3) for methanation reaction. Its inlet temperature is 224°C and the inlet pressure is 3.3 MPa. The outlet gas temperature of the third methanation reactor (3) is 554°C and the pressure is 3.2 MPa. The gas is cooled to 220°C by recovering heat from the fifth waste heat boiler (11), the sixth waste heat boiler (12), and the seventh waste heat boiler (13). Then it is mixed with the second supplementary hydrogen as the inlet gas of the fourth methanation reactor (4). The inlet temperature is 219°C and the inlet pressure is 3.2 MPa. The outlet gas temperature of the fourth methanation reactor (4) is 465℃ and the pressure is 3.1MPa. It is successively cooled to 80℃ by the eighth waste heat boiler (14), the ninth waste heat boiler (15), the tenth waste heat boiler (16), and the eleventh waste heat boiler (17) to recover heat. It then enters the first gas-liquid separator (18) to separate condensate. After being heated to 220℃ by the ninth waste heat boiler (15), it is divided into second gas compressor (28) as second circulating gas and fifth methanation reactor (5) as inlet gas in a volume ratio of 0.7 and 0.3. After the gas is mixed with the third hydrogen supplement, it is fed into the fifth methanation reactor (5) for reaction. The outlet gas temperature of the fifth methanation reactor (5) is 488℃ and the pressure is 3.0MPa. It is then passed through the twelfth waste heat boiler (19), the thirteenth waste heat boiler (20), the fourteenth waste heat boiler (21) and the fifteenth waste heat boiler (22) to recover heat and cool to 70℃. Then it is fed into the second gas-liquid separator (23) to separate the condensate. Finally, it is heated to 220℃ by the thirteenth waste heat boiler (20) and then used as the second reaction gas to enter the third methanation zone.

[0044] The third methanation zone includes the sixth methanation reactor (6), which further methanates the second reaction gas from the second methanation zone, wherein the second reaction gas has a temperature of 220°C and a pressure of 3.0 MPa. The outlet gas temperature of the sixth methanation reactor (6) is 302°C. The gas is cooled to 40°C by the sixteenth waste heat boiler (24) and the seventeenth waste heat boiler (25) and then fed into the third gas-liquid separator (26) to separate the condensate, thus obtaining the final synthetic natural gas.

[0045] In the first methanation zone, pressurized cooling water at 25°C and 9.8MPa is heated to a saturation temperature of 309°C by the second waste heat boiler (8), and then superheated by the third waste heat boiler (9) to obtain superheated steam at 513°C, 9.8MPa, and 45t / h. In the second methanation zone, pressurized cooling water at 25°C and 6.5MPa is heated to a saturation temperature of 280°C by the tenth waste heat boiler (16), and then... The waste heat boiler (12) is used to superheat the steam to obtain superheated steam with a temperature of 438℃, a pressure of 6.5MPa and a flow rate of 117t / h. In the third methanation zone, pressurized cooling water with a temperature of 25℃ and a pressure of 3.5MPa is heated to 124℃ by the sixteenth waste heat boiler (24), and then heated by the fourteenth waste heat boiler (21) in the second methanation zone to obtain saturated steam with a temperature of 242℃, a pressure of 3.5MPa and a flow rate of 36t / h.

[0046] The product gas contains 97.63% CH4, <1.0% H2, <1.0% CO2, and has a total CO conversion rate of 99.99%.

[0047] Table 1 Summary of main process conditions and results of Examples 1-3 <![CDATA[Feed gas flow rate / Nm 3 ·h -1 > 202048 179200 197120 Raw material gas pressure / MPa 3.5 3.5 3.5 Raw material gas temperature / ℃ 25 25 25 <![CDATA[H2 / %]]> 36.298 30.819 26.19 CO / % 37.249 50.39 64.36 <![CDATA[CO2 / %]]> 15.483 18.31 3.53 <![CDATA[CH4 / %]]> 8.731 0.0301 0.0301 <![CDATA[Raw gas M=nH₂ / (nCO+nCO₂)]]> 0.6883 0.4486 0.3857 <![CDATA[Added steam / t·h -1 > 11.7 12.6 4.5 Percentage of added water vapor in the molar flow rate of the feed gas / % 7.2 8.7 2.8 <![CDATA[Total hydrogen make-up / Nm 3 ·h -1 > 273616 344064 353920 First / Second / Third Hydrogen Supplement Ratio 0.37 / 0.44 / 0.19 0.52 / 0.31 / 0.17 0.45 / 0.38 / 0.17 First circulating gas ratio 0.6 0.6 0.6 Second circulation gas ratio 0.6 0.6 0.7 First methanation reactor outlet temperature / ℃ 532 563 588 Second methanation reactor outlet temperature / ℃ 529 559 579 Third methanation reactor outlet temperature / ℃ 471 572 554 Fourth methanation reactor outlet temperature / ℃ 455 482 465 Fifth methanation reactor outlet temperature / ℃ 481 507 488 The outlet temperature of the sixth methanation reactor is ℃. 296 318 302 <![CDATA[Product gas CH4 / %, dry basis]]> 97.44 97.43 97.63 <![CDATA[Product gas H2 / %, dry basis]]> ＜1.0 ＜1.0 ＜1.0 <![CDATA[Product gas CO2 / %, dry basis]]> ＜1.0 ＜1.0 ＜1.0 Total CO conversion rate / % 99.99 99.99 99.99

Claims

1. A method for preparing synthetic natural gas by partitioned hydrogen supplementation methanation of carbon dioxide-rich syngas, characterized in that, Includes the following steps: (1) First methanation zone: The carbon dioxide-rich syngas after desulfurization is used as raw material gas, mixed with circulating steam and the first circulating gas, and then sent to the first methanation zone to methanate the carbon dioxide-rich syngas as the main component, to obtain the first reaction gas; the first methanation zone includes at least two methanation reactors arranged in series. The raw material gas (S101) is preheated to 200℃~300℃ by the first waste heat boiler (7), and then mixed with steam (S102), and connected to the inlet of the first methanation reactor (1) and the second methanation reactor (2) respectively through pipelines; the inlet pipeline of the first methanation reactor (1) is also connected to the first circulating gas (S104) for diluting the raw material gas, and the first waste heat boiler (7) and the second waste heat boiler (8) are arranged in sequence on the pipeline from the outlet of the first methanation reactor (1) to the inlet of the second methanation reactor (2) for diluting the raw material gas. Heat is recovered from the outlet gas to reduce its temperature to 200℃~300℃; a third waste heat boiler (9) and a fourth waste heat boiler (10) are installed on the outlet pipeline of the second methanation reactor (2) to reduce the outlet gas temperature of the second methanation reactor (2) to 200℃~300℃, and then the outlet gas is divided into two parts through the pipeline. One part is compressed by the first gas compressor (27) and sent to the inlet of the first methanation reactor (1) as the first circulating gas. The remaining gas is used as the first reaction gas and is introduced into the second methanation zone through the pipeline. (2) Second methanation zone: The first reaction gas obtained in step (1) is introduced into the second methanation zone. The second methanation zone includes at least three methanation reactors arranged in series. Hydrogen is added to the inlet of at least two of the methanation reactors in stages, so that the H2 / (CO+CO2) molar ratio of the gas at the inlet of each methanation reactor increases step by step along the gas flow direction, so that the carbon dioxide in the carbon dioxide-rich synthesis gas is mainly methanated to obtain the second reaction gas. At least one cooling dewatering device is set between adjacent methanation reactors in the second methanation zone to remove the water generated by the carbon dioxide-mainly methanation and the water generated by the carbon dioxide-mainly methanation in the first methanation zone, so as to promote the carbon dioxide methanation reaction. Hydrogen (S103) is divided into three streams through pipelines, namely the first hydrogen replenishment (S106), the second hydrogen replenishment (S107), and the third hydrogen replenishment (S108). The first hydrogen replenishment (S106) is preheated to 200℃~300℃ by the fifth waste heat boiler (11), the second hydrogen replenishment (S107) is preheated to 200℃~300℃ by the eighth waste heat boiler (14), and the third hydrogen replenishment (S108) is preheated to 200℃~300℃ by the twelfth waste heat boiler (19). They are then connected to the inlet of the third methanation reactor (3), the inlet of the fourth methanation reactor (4), and the inlet of the fifth methanation reactor (5), respectively.The first reaction gas from the first methanation zone is mixed with the first supplementary hydrogen (S106) and the second circulating gas (S105) and enters the third methanation reactor (3) for reaction. The fifth waste heat boiler (11), the sixth waste heat boiler (12), and the seventh waste heat boiler (13) are sequentially installed on the pipeline from the outlet of the third methanation reactor (3) to the inlet of the fourth methanation reactor (4) to recover the heat of the outlet gas of the third methanation reactor (3) and reduce the outlet gas temperature to 200℃~300℃. The outlet gas with recovered heat is mixed with the second supplementary hydrogen (S107) and enters the fourth methanation reactor (4) for reaction. The outlet pipeline of the fourth methanation reactor (4) is sequentially installed with the eighth waste heat boiler (14), the ninth waste heat boiler (15), the tenth waste heat boiler (16), the eleventh waste heat boiler (17), and the first gas-liquid separator (18) for recovering heat and removing moisture from the outlet gas. After the gas is dehydrated by the separator (18), it is heated to 200°C~300°C by the ninth waste heat boiler (15), and then the outlet gas is divided into two parts by the pipeline. One part is compressed by the second gas compressor (28) and sent to the inlet of the third methanation reactor (3). The remaining gas is mixed with the third hydrogen supplement (S108) by the pipeline and then enters the fifth methanation reactor (5) for reaction. The outlet pipeline of the fifth methanation reactor (5) is equipped with the twelfth waste heat boiler (19), the thirteenth waste heat boiler (20), the fourteenth waste heat boiler (21), the fifteenth waste heat boiler (22) and the second gas-liquid separator (23) for recovering heat and removing moisture from the outlet gas. After the gas is dehydrated by the second gas-liquid separator (23), it is heated to 200°C~300°C by the thirteenth waste heat boiler (20) and sent to the third methanation zone as the second reaction gas. The H2 / (CO+CO2) molar ratio of the inlet gas of each methanation reactor is such that hydrogen is added to the second methanation zone in stages, so that the H2 / (CO+CO2) molar ratio of the inlet gas of the third methanation reactor (3), the fourth methanation reactor (4), and the fifth methanation reactor (5) is 1.0~2.0, 1.5~3.0, and 3.0~5.0 respectively. (3) Third methanation zone: The second reaction gas obtained in step (2) is introduced into the third methanation zone to further convert the residual carbon monoxide and / or carbon dioxide into methane. After cooling and gas-liquid separation, the synthetic natural gas product gas is obtained. (4) First and second circulating gas: A portion of the outlet gas of at least one stage methanation reactor in the first methanation zone is compressed and returned to the inlet of the first methanation zone as the first circulating gas, and mixed with the carbon dioxide-rich synthesis gas before entering the first methanation zone. The proportion of the first circulating gas to the outlet gas volume of the methanation reactor is 0.2 to 0.

8. A portion of the outlet gas from at least one methanation reactor in the second methanation zone is compressed and returned to the inlet of the second methanation zone as a second recirculation gas. It is then mixed with the first reaction gas and enters the second methanation zone. The proportion of the second recirculation gas to the outlet gas volume of this stage methanation reactor is 0.2 to 0.

8. (5) The amount of water vapor added shall not exceed 10% of the molar flow rate of the raw material gas. The raw material gas after preheating and mixing with water vapor shall be divided into the first raw material gas and the second raw material gas. The first raw material gas shall be mixed with the first circulating gas and then enter the first methanation reactor (1). The second raw material gas shall be mixed with the cooled outlet gas of the first methanation reactor and then enter the second methanation reactor (2). The proportion of the first raw material gas to the volume of the mixed raw material gas shall be 0.3 to 0.

8. (6) Wherein, the process operating pressure is 0.1 to 10.0 MPa, and the outlet temperature of each methanation reactor is below 650°C.

2. The process according to claim 1, characterized in that, The first methanation zone includes a first methanation reactor (1) and a second methanation reactor (2), the second methanation zone includes a third methanation reactor (3), a fourth methanation reactor (4) and a fifth methanation reactor (5), and the third methanation zone includes a sixth methanation reactor (6).

3. The process according to claim 1, characterized in that, The carbon dioxide-rich synthesis gas contains 5%–40% carbon dioxide, 5%–70% carbon monoxide, 5%–50% hydrogen, and 0%–20% methane by volume.

4. The process according to claim 1, characterized in that, The carbon dioxide-rich syngas is derived from one or more of the gases generated by the thermochemical conversion of carbon-containing raw materials, such as coal gasification, coal pyrolysis, biomass gasification, biomass pyrolysis, and oil gasification.

5. The process according to claim 1, characterized in that, The hydrogen added to the second methanation zone comes from one or more of the following: coal-based hydrogen production, biomass-based hydrogen production, oil gasification-based hydrogen production, industrial by-product hydrogen, and renewable energy-based water electrolysis-based hydrogen production.

6. The process according to claim 1, characterized in that, The methanation reactor is one or more of the following: an adiabatic fixed-bed methanation reactor, an isothermal fixed-bed methanation reactor, etc.

7. The process according to claim 1, characterized in that, The methanation reactors in each methanation zone can be filled with a general-purpose methanation catalyst or a methanation catalyst suitable for the function of each reaction zone. The catalyst is a methanation catalyst with one or more of Ni, Ru, Rh, and Co as active components, and one or more of Al2O3, SiO2, ZrO2, TiO2, MgAl2O4, or rare earth modified oxides as supports.

8. The process according to claim 1, characterized in that, Each methanation zone may produce saturated steam or superheated steam at a pressure of 0.1-10.0 MPa as a byproduct.

9. The process according to claim 1, characterized in that, The dry-basis synthetic natural gas product gas, after cooling, gas-liquid separation and removal of condensate, shall contain methane volume fraction not less than 97.0%, carbon monoxide volume fraction not more than 0.1%, carbon dioxide volume fraction not more than 1.0%, and hydrogen gas volume fraction not more than 1.0%.

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