Process for synthesizing natural gas through methanation of CO2-rich coke-oven gas

Abstract

The invention discloses a process for synthesizing natural gas through methanation of CO2-rich coke-oven gas. The process comprises the following steps: compressing coke-oven gas, pre-purifying, desulfurizing, carrying out CO2-rich conversion and three-stage methanation reaction, separating natural gas, extracting H2, and returning to prepare the natural gas meeting the national standard technical specification of the natural gas. According to the process, first-stage hot gas with water vapor circulation is adopted, the outlet temperature of a first-stage reactor and the CO + CO2 content of inlet gas are controlled, most of carbon in coke-oven gas reacts in the first-stage reactor, the second-stage reaction load and the third-stage reaction load are reduced, the catalyst dosage is reduced, and the process is suitable for high-carbon gas components and particularly suitable for CO2-rich environment gas; cO2 can be effectively converted into CO, the carbon utilization rate is increased, carbon emission is reduced, side reactions such as carbon deposition are reduced, the yield of methane is increased, and the catalyst special for methanation is long in service life and can stably run for a long time.

Description

Technical Field

[0001] The invention relates to the technical field of new energy utilization by synthesizing natural gas through methanation of coke oven gas, and in particular to a methanation reaction process for synthesizing natural gas through methanation of CO2-rich coke oven gas. Background Art

[0002] The CO and CO2 in coke oven gas are converted into methane through methanation reaction, and then compressed natural gas or liquefied natural gas is produced through pressure swing adsorption and cryogenic cooling, which has good economic value and social significance.

[0003] In recent years, although my country's natural gas production has increased rapidly and a large number of natural gas pipelines and infrastructure have been built, the demand for natural gas has increased even faster. As the demand for natural gas continues to increase, various technologies for synthesizing artificial natural gas have emerged, such as coal-to-gas. Considering the economy and applicability, coke oven gas synthesis gas has market competitiveness.

[0004] Coke oven gas is a by-product of the coal distillation and coking process. It is a H2-rich gas containing CO and CO2. The composition is generally shown in Table 1.

[0005] Table 1 Main components of coke oven gas Element <![CDATA[H2]]> <![CDATA[CH4]]> CO <![CDATA[CO2]]> Cm <![CDATA[N2]]> <![CDATA[O2]]> Composition V% 50～60 20～30 5～9 2～3 2～4 3～6 0.3～0.8 At the same time, coke oven gas also contains a large amount of impurities such as sulfide, tar, benzene, naphthalene, and ammonia.

[0006] In recent years, coal prices have risen. In order to save production costs, many companies have purchased some low-quality coal as raw materials. The coke oven gas produced contains less hydrogen, and the CO2 and CO contents have increased, especially CO2 has risen to more than 3.5%, which has increased the hydrogen consumption of the methanation reaction. Some equipment even needs to be converted to decarbonization to meet the conditions for producing liquefied natural gas.

[0007] Coke oven gas methanation reaction: The main components of the syngas obtained after coal gasification, conversion and purification are H2, CO, CO2, CH4, H2O and inert gas N2 or Ar. The gas composition varies with the gasification technology. The gas composition of the syngas changes after the methanation reaction, and the CH4 concentration increases significantly. At the same time, a certain amount of carbon deposits are generated on the catalyst bed. The methanation reaction system that needs to be calculated is a multi-component system involving gas-solid two-phases. The reactions that may occur in this reaction system are as follows:

[0008] In the coke oven gas synthetic natural gas technology, the methanation reaction of CO and CO2 is an important step, but there are still many side reactions. Compared with the maturity of coke oven gas purification and subsequent separation technology, although the coke oven gas methanation technology has been industrialized, there are still many problems in the use process, such as large circulating machine gas volume, high energy consumption, catalyst carbon deposition and damage, poor heat resistance and other issues.

[0009] Patent CN101391935A discloses a method for synthesizing methane using coke oven gas, through purification, desulfurization, first-stage methanation reaction, second-stage methanation reaction, third-stage methanation reaction, separation of methane and other main steps, a product gas with a methane concentration of more than 90% is obtained. By adopting the method of the present invention, using coke oven gas as a raw material, a synthetic natural gas with high methane content, low impurity content and high calorific value can be obtained, and the environment is effectively protected and energy is saved. In this method, before the first stage reaction, an appropriate amount of water vapor is added to the raw material coke oven gas, which appropriately suppresses the concentration of the methanation reaction, reduces the heat released during the entire reaction process, is beneficial to the cooling of the gas after the reaction, and prevents the occurrence of carbon deposition reaction that causes the catalyst to be deactivated, which is beneficial to the continuous and normal operation of the entire synthesis process.

[0010] Patent CN200910018047[1].X discloses a process for synthesizing natural gas from coke oven gas by methanation. The process uses a two-stage adiabatic reactor and operates at low pressure. Self-produced water vapor is added to the inlet of the first reactor to help control the methanation conversion rate, thereby achieving the purpose of controlling the reaction temperature and heat discharge, effectively inhibiting the occurrence of carbonization reaction and protecting the methanation catalyst.

[0011] Patent CN200910058611.0 discloses a method for preparing synthetic natural gas by methanation reaction using coke oven gas. The method for producing synthetic natural gas using coke oven gas as raw material is to first pre-treat, purify and desulfurize the coke oven gas before methanation reaction. The method uses a multi-stage methanation reactor to carry out a multi-stage methanation reaction, controls the inlet gas temperature and inlet gas content of each stage of the methanation reactor, and uses part of the methanated gas as return gas to dilute the raw coke oven gas, which can reduce energy consumption, and can effectively control the outlet gas temperature of the methanation reactor, and is also conducive to the selection of reactor materials to reduce costs.

[0012] The first two patents mentioned above do not use circulating gas to dilute the raw coke oven gas, but use water vapor to control the reaction outlet temperature instead. Although water vapor can effectively inhibit carbon deposition, the addition of water vapor will greatly reduce the conversion rate of the methanation reaction, and may even easily lead to the intensification of the transformation side reaction, which has a great impact on the depth of the next-level methanation reaction. The latter patent uses methanated product gas as return gas to dilute the raw coke oven gas. Although it can well control the temperature rise, the circulating gas volume is large and contains a large amount of CH4 gas. After mixing with the raw gas, the methane content increases, which is not conducive to the methanation reaction. At high temperatures, it may lead to cracking and carbon deposition, which ultimately leads to poor stability of synthetic natural gas. Summary of the invention

[0013] The purpose of the present invention is to provide a process for synthesizing natural gas from coke oven gas rich in CO2. The process adopts a hot gas cycle with partial water vapor in a primary methanogenic process, adds a CO2 conversion reactor, reduces the amount of CO2 that consumes a lot of hydrogen, and converts it into CO. The coke oven gas can be made into natural gas that meets the national technical specifications, and the separated hydrogen can be returned for use, while effectively recovering heat energy for comprehensive utilization.

[0014] The present invention is achieved through the following technical solutions: A process for synthesizing natural gas by methanation of CO2-rich coke oven gas, including pre-purification, desulfurization, CO2-rich conversion and three-stage methanation reaction, natural gas separation and H2 extraction and return, the steps are as follows: (1) Pre-purification: The coke oven gas is compressed to a reaction pressure of 0.3-3.0 MPa, and then treated by adsorption or absorption to remove ammonia, cyanide, tar and dust from the coke oven gas to obtain roughly purified coke oven gas; (2) Hydrodesulfurization: The crude purified coke oven gas prepared in step (1) is treated with a hydroconversion process to remove organic sulfur such as C4H4S, COS, CS2, etc.; (3) CO2-rich conversion: The coke oven gas after hydrodesulfurization directly enters the CO2 conversion reactor, which is equipped with a special catalyst for CO2 conversion; (4) Adsorption desulfurization: After CO2 conversion, the coke oven gas enters the dry fine desulfurization to remove inorganic sulfur. After removal, the sulfur content of the coke oven gas is ≤0.1mg / Nm 3 Then enter the ultra-fine desulfurization to remove the total sulfur of coke oven gas to ≤0.03mg / Nm 3 ; (5) Tertiary methanation reaction: The outlet gas after the primary methanation is used as part of the circulating hot gas to return to the inlet of the primary methanation and mix with the fresh gas. The amount of circulating gas is controlled at 570-600°C according to the outlet temperature of the primary methanation reactor, and the reaction pressure is 0.3-3.0 MPa. The mixed gas is preheated to 280-320°C and enters the primary adiabatic reactor of methanation. After the reaction, the primary reaction coke oven gas is obtained; the primary reaction coke oven gas is heat exchanged and cooled to 250-300°C and enters the secondary methanation reactor. The outlet gas of the reaction is heat exchanged and cooled or directly enters the tertiary methanation reactor. The tertiary outlet product gas is finally cooled to obtain crude natural gas. All three reactors are equipped with special high temperature resistant catalysts; (6) Natural gas separation and H2 extraction and return: The crude natural gas obtained in step (5) is treated by a pressure swing adsorption process or a membrane separation process to obtain natural gas-rich gas and hydrogen gas and other gases. The water-rich natural gas is dehydrated by pressure swing adsorption or temperature swing adsorption to obtain natural gas; the separated hydrogen is returned to the inlet of the coke oven gas raw material compressor to be mixed with the crude coke oven gas.

[0015] The CO2 conversion reactor is equipped with a special catalyst for CO2 conversion, the active component of the catalyst is NiS@Ni, the auxiliary agents are sulfide and alkali metal, and the carrier is bulk silicon-zirconium-aluminum composite oxide; the reaction temperature is 350-450°C.

[0016] The process for synthesizing natural gas by methanation of CO2-rich coke oven gas is characterized in that, in the three-stage methanation reaction, the outlet gas after the first-stage methanation is used as circulating hot gas to control the amount of water vapor in the circulating hot gas to adjust the outlet temperature of the first-stage methanation reactor, the circulation ratio is 1.5-2.0, and the temperature of the hot circulating gas is controlled at 150-180°C so that it circulates with part of the water vapor; the first-stage methanation reactor is equipped with a special high-temperature resistant catalyst, the active component of the catalyst is Ni, the additives are Re and alkaline earth metals, and the carrier is a silicon-aluminum composite oxide.

[0017] During the step (3), the CO2 conversion reactor is arranged after hydrodesulfurization or fine desulfurization, and the heat of the desulfurization reaction is used as the heat source at the inlet of the conversion reactor without the need for separate heating. The special catalysts used in the three-stage methanation reactor are all pre-reduced catalysts, which are activated in hydrogen or purified coke oven gas at 280-350°C for 10-16h before use.

[0018] The process provided by the present invention has the following advantages compared with the prior art: The present invention adopts a hot methanation gas circulation with water vapor at 150-180°C at the outlet of the first-stage methanation, with a small circulating gas volume, strong operability and low energy consumption; ② The catalyst used in the methanation reaction in the present invention is a high temperature resistant type, and the usable temperature range is 280-750°C. The methanation catalyst has high temperature resistance and stable activity, and can operate stably at a high temperature of 600°C without activity decay. ③ In the present invention, a CO2 conversion reactor is added to the front section of the methanation, which can convert CO2-rich coke oven gas into CO-rich coke oven gas, reduce the hydrogen consumption in the methanation section, and maximize the utilization of carbon into methane. At the same time, the catalyst in the converter can also adsorb sulfides to reduce the burden of fine desulfurization. ④ The present invention recovers the hydrogen in the gas after separating methane and returns it to the coke oven gas raw material purification inlet section, effectively utilizes hydrogen, minimizes carbon emissions, and maximizes methane production. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The methanogenic process flow chart of the present invention is Explanation of reference numerals: 1 de-oiling tower, 2 purification tower, 3 pre-desulfurization tower, 4 electric furnace, 5 hydrodesulfurization tower, 6 CO2 conversion reactor, 7 fine desulfurization tower, 8 ultra-fine desulfurization tower, 9 primary methanation reactor, 10 secondary methanation reactor, 11 tertiary methanation reactor, 12 separation and purification unit, 13 circulation machine DETAILED DESCRIPTION

[0020] The present invention is further described in detail below in conjunction with specific embodiments. However, it should not be understood that the scope of the above subject matter of the present invention is limited to the following embodiments.

[0021] A process for synthesizing natural gas by methanation of CO2-rich coke oven gas, including pre-purification, desulfurization, CO2-rich conversion and three-stage methanation reaction, natural gas separation and H2 extraction and return, the steps are as follows: The coke oven gas is compressed to a reaction pressure of 0.3-3.0 MPa, and then treated by adsorption or absorption to remove ammonia, cyanide, tar and dust in the coke oven gas to obtain roughly purified coke oven gas; the roughly purified coke oven gas is treated with organic sulfur such as C4H4S, COS, CS2, etc. through a hydrogenation conversion process. The coke oven gas after hydrogenation desulfurization directly enters the CO2 conversion reactor, which is equipped with a special catalyst for CO2 conversion. After the reaction, the coke oven gas enters the fine desulfurization process and uses dry desulfurization to remove inorganic sulfur. After removal, the sulfur content of the coke oven gas is ≤0.1 mg / Nm 3 The coke oven gas after fine desulfurization directly enters the ultra-fine desulfurization tower to remove the total sulfur of the coke oven gas to ≤0.03mg / Nm 3; After the super-fine desulfurization, the coke oven gas enters the primary methanation reactor. The outlet gas after the primary methanation is returned to the primary methanation inlet as the circulating hot gas part and mixed with the fresh gas. The circulation ratio is 1.5-2.0. The amount of circulating gas is controlled at 570-600℃ according to the outlet temperature of the primary methanation reactor, and the reaction pressure is 0.3-3.0MPa. The mixed gas is preheated to 280-320℃ and enters the primary adiabatic reactor of methanation. After the reaction, the primary reaction coke oven gas is obtained; the primary reaction coke oven gas is cooled to 250- The temperature of the reactor is 300℃ and the gas enters the secondary methanation reactor. The outlet gas of the reaction is cooled by heat exchange or directly enters the tertiary methanation reactor. The product gas at the tertiary outlet is finally cooled to obtain crude natural gas. All three reactors are equipped with special high-temperature resistant catalysts. The obtained crude natural gas is treated by pressure swing adsorption process or membrane separation process to obtain rich natural gas and hydrogen gas and other gases. The water-rich natural gas is dehydrated by pressure swing adsorption or temperature swing adsorption to obtain natural gas. The separated hydrogen is returned to the inlet of the coke oven gas raw material compressor and mixed with the crude coke oven gas. Example 1

[0022] like Figure 1 The process of synthesizing natural gas by methanation of coke oven gas is shown in the figure. 3 / h coke oven gas is compressed to a reaction pressure of 0.3MPa, and is treated by adsorption or absorption in a de-oiling tower 1 and a purification tower 2 to remove ammonia, cyanide, tar and dust in the coke oven gas to obtain a roughly purified coke oven gas; the roughly purified coke oven gas is treated by a hydrogenation conversion process in a pre-desulfurization tower 3 and a hydrodesulfurization tower 5 to treat organic sulfur such as C4H4S, COS, CS2, etc. After hydrodesulfurization, the temperature of the coke oven gas rises to 350℃ and directly enters the CO2 conversion reactor 6, which is equipped with a special catalyst for CO2 conversion. After the reaction, the coke oven gas enters the fine desulfurization tower 7, and the inorganic sulfur is removed by dry desulfurization. After removal, the sulfur content of the coke oven gas is ≤0.1mg / Nm 3 The coke oven gas after fine desulfurization directly enters the ultra-fine desulfurization tower 8 to remove the total sulfur of the coke oven gas to ≤0.03mg / Nm 3 After super desulfurization, the coke oven gas is preheated to 320℃ and enters the primary methanation reactor 9 which is equipped with a high temperature resistant special catalyst 8m 3 , reaction pressure 0.3MPa, the outlet gas after the first-stage methanation is partially cooled to 150℃ and returned to the first-stage methanation inlet as circulating hot gas to mix with fresh gas, the circulation ratio is 1.5, and the amount of circulating gas is controlled at 600℃ according to the outlet temperature of the first-stage methanation reactor; the initial reaction coke oven gas obtained after the reaction is cooled to 300℃ through heat exchange and enters the second-stage methanation reactor 10 equipped with a high-temperature resistant special catalyst 2.8m 3 The reaction outlet gas is cooled by heat exchange or directly enters the three-stage methanation reactor 11, which is equipped with a high-temperature resistant special catalyst 1.8m 3The three-stage outlet product gas is finally cooled to obtain crude natural gas. The three reactors are all equipped with special high-temperature resistant catalysts; the obtained crude natural gas enters the separation and purification unit 12 and is treated by pressure swing adsorption process or membrane separation process to obtain rich natural gas and hydrogen gas and other gases. The water-rich natural gas is dehydrated by pressure swing adsorption or temperature swing adsorption to obtain natural gas; the separated hydrogen is returned to the inlet of the coke oven gas raw material compressor and mixed with the crude coke oven gas. Example 2

[0023] like Figure 1 The process of synthesizing natural gas by methanation of coke oven gas is shown in the figure. 3 / h coke oven gas is compressed to a reaction pressure of 1.5MPa, and is treated by adsorption or absorption in a de-oiling tower 1 and a purification tower 2 to remove ammonia, cyanide, tar and dust in the coke oven gas to obtain a roughly purified coke oven gas; the roughly purified coke oven gas is treated by a hydrogenation conversion process in a pre-desulfurization tower 3 and a hydrodesulfurization tower 5 to treat organic sulfur such as C4H4S, COS, CS2, etc. After hydrodesulfurization, the temperature of the coke oven gas rises to 380℃ and directly enters the CO2 conversion reactor 6, which is equipped with a special catalyst for CO2 conversion. After the reaction, the coke oven gas enters the fine desulfurization tower 7, and the inorganic sulfur is removed by dry desulfurization. After removal, the sulfur content of the coke oven gas is ≤0.1mg / Nm 3 The coke oven gas after fine desulfurization directly enters the ultra-fine desulfurization tower 8 to remove the total sulfur of the coke oven gas to ≤0.03mg / Nm 3 After super desulfurization, the coke oven gas is preheated to 280℃ and enters the primary methanation reactor 9, which is equipped with a high temperature resistant special catalyst 8.4m 3 , reaction pressure 1.5MPa, the outlet gas after the first-stage methanation is partially cooled to 170℃ as circulating hot gas and returned to the first-stage methanation inlet to mix with fresh gas, the circulation ratio is 1.8, and the amount of circulating gas is controlled at 570℃ according to the outlet temperature of the first-stage methanation reactor; the initial reaction coke oven gas obtained after the reaction is cooled to 250℃ through heat exchange and enters the second-stage methanation reactor 10 equipped with a high-temperature resistant special catalyst 2.5m 3 The reaction outlet gas is cooled by heat exchange or directly enters the three-stage methanation reactor 11, which is equipped with a high-temperature resistant special catalyst 1.7m 3 The three-stage outlet product gas is finally cooled to obtain crude natural gas. The three reactors are all equipped with special high-temperature resistant catalysts; the obtained crude natural gas enters the separation and purification unit 12 and is treated by pressure swing adsorption process or membrane separation process to obtain rich natural gas and hydrogen gas and other gases. The water-rich natural gas is dehydrated by pressure swing adsorption or temperature swing adsorption to obtain natural gas; the separated hydrogen is returned to the inlet of the coke oven gas raw material compressor and mixed with the crude coke oven gas. Example 3

[0024] like Figure 1 The process of synthesizing natural gas by methanation of coke oven gas is shown in the figure. 3 / h coke oven gas is compressed to a reaction pressure of 3.0MPa, and is treated by adsorption or absorption in a de-oiling tower 1 and a purification tower 2 to remove ammonia, cyanide, tar and dust in the coke oven gas to obtain a roughly purified coke oven gas; the roughly purified coke oven gas is treated by a hydrogenation conversion process in a pre-desulfurization tower 3 and a hydrodesulfurization tower 5 to treat the organic sulfur such as C4H4S, COS, CS2, etc. After hydrodesulfurization, the temperature of the coke oven gas rises to 450℃ and directly enters the CO2 conversion reactor 6, which is equipped with a special catalyst for CO2 conversion. After the reaction, the coke oven gas enters the fine desulfurization tower 7, and the inorganic sulfur is removed by dry desulfurization. After removal, the sulfur content of the coke oven gas is ≤0.1mg / Nm 3 The coke oven gas after fine desulfurization directly enters the ultra-fine desulfurization tower 8 to remove the total sulfur of the coke oven gas to ≤0.03mg / Nm 3 After super desulfurization, the coke oven gas is preheated to 300℃ and enters the primary methanation reactor 9 which is equipped with a high temperature resistant special catalyst 8m 3 , reaction pressure 3.0MPa, the outlet gas after the first-stage methanation is partially cooled to 180℃ as circulating hot gas and returned to the first-stage methanation inlet to mix with fresh gas, the circulation ratio is 2.0, and the amount of circulating gas is controlled at 580℃ according to the outlet temperature of the first-stage methanation reactor; the initial reaction coke oven gas obtained after the reaction is cooled to 270℃ by heat exchange and enters the second-stage methanation reactor 10 equipped with a high-temperature resistant special catalyst 2.6m 3 The reaction outlet gas is cooled by heat exchange or directly enters the three-stage methanation reactor 11 equipped with a high-temperature resistant special catalyst 2m 3 The three-stage outlet product gas is finally cooled to obtain crude natural gas. The three reactors are all equipped with special high-temperature resistant catalysts; the obtained crude natural gas enters the separation and purification unit 12 and is treated by pressure swing adsorption process or membrane separation process to obtain rich natural gas and hydrogen gas and other gases. The water-rich natural gas is dehydrated by pressure swing adsorption or temperature swing adsorption to obtain natural gas; the separated hydrogen is returned to the inlet of the coke oven gas raw material compressor and mixed with the crude coke oven gas. Example 4

[0025] like Figure 1 The process of synthesizing natural gas by methanation of coke oven gas shown in the figure only moves the fine desulfurization tower 7 to the front of the CO2 conversion reactor 6, and the rest remains unchanged. 3 / h coke oven gas is compressed to a reaction pressure of 2.0MPa, and is treated by adsorption or absorption in a de-oiling tower 1 and a purification tower 2 to remove ammonia, cyanide, tar and dust in the coke oven gas to obtain a roughly purified coke oven gas; the roughly purified coke oven gas is treated by a hydrogenation conversion process in a pre-desulfurization tower 3 and a hydrodesulfurization tower 5 to treat organic sulfur such as C4H4S, COS, CS2, etc. After hydrodesulfurization, the temperature of the coke oven gas is raised to 350℃ and directly enters the CO2 conversion reactor 6, which is equipped with a special catalyst for CO2 conversion. After the reaction, the coke oven gas enters the fine desulfurization tower 7, and the inorganic sulfur is removed by dry desulfurization. After removal, the sulfur content of the coke oven gas is ≤0.1mg / Nm3 The coke oven gas after fine desulfurization directly enters the ultra-fine desulfurization tower 8 to remove the total sulfur of the coke oven gas to ≤0.03mg / Nm 3 After super desulfurization, the coke oven gas is preheated to 320℃ and enters the primary methanation reactor 9 which is equipped with a high temperature resistant special catalyst 8m 3 , reaction pressure 2.0MPa, the outlet gas after the first-stage methanation is partially cooled to 160℃ and returned to the first-stage methanation inlet as circulating hot gas to mix with fresh gas, the circulation ratio is 1.5, and the amount of circulating gas is controlled at 600℃ according to the outlet temperature of the first-stage methanation reactor; the initial reaction coke oven gas obtained after the reaction is cooled to 300℃ through heat exchange and enters the second-stage methanation reactor 10 equipped with a high-temperature resistant special catalyst 2.8m 3 The reaction outlet gas is cooled by heat exchange or directly enters the three-stage methanation reactor 11, which is equipped with a high-temperature resistant special catalyst 1.8m 3 The three-stage outlet product gas is finally cooled to obtain crude natural gas. The three reactors are all equipped with special high-temperature resistant catalysts; the obtained crude natural gas enters the separation and purification unit 12 and is treated by pressure swing adsorption process or membrane separation process to obtain rich natural gas and hydrogen gas and other gases. The water-rich natural gas is dehydrated by pressure swing adsorption or temperature swing adsorption to obtain natural gas; the separated hydrogen is returned to the inlet of the coke oven gas raw material compressor and mixed with the crude coke oven gas. Example 5

[0026] like Figure 1 In the process of synthesizing natural gas by methanation of coke oven gas shown in the figure, only the hydrogen finally separated in Example 3 is not returned to the inlet of the coke oven gas raw material compressor, and other process conditions are exactly the same.

[0027] Comparative Example 1 Figure 1 The process of synthesizing natural gas by methanation of coke oven gas shown in the figure is the same except that the CO2 conversion reactor 6 and the separation hydrogen return line are not included. 3 / h coke oven gas is compressed to a reaction pressure of 3.0MPa, and is treated by adsorption or absorption in a de-oiling tower 1 and a purification tower 2 to remove ammonia, cyanide, tar and dust in the coke oven gas to obtain a roughly purified coke oven gas; the roughly purified coke oven gas is treated by a hydrogenation conversion process in a pre-desulfurization tower 3 and a hydrodesulfurization tower 5 to treat organic sulfur such as C4H4S, COS, CS2, etc., and the coke oven gas after hydrodesulfurization enters a fine desulfurization tower 7, where dry desulfurization is used to remove inorganic sulfur. After removal, the sulfur content of the coke oven gas is ≤0.1mg / Nm 3 The coke oven gas after fine desulfurization directly enters the ultra-fine desulfurization tower 8 to remove the total sulfur of the coke oven gas to ≤0.03mg / Nm 3 After super desulfurization, the coke oven gas is preheated to 300℃ and enters the primary methanation reactor 9, which is equipped with 13.1m of commercial catalyst. 3, reaction pressure 3.0MPa, the separated dehydrated gas after the secondary methanation is returned to the primary methanation inlet as part of the circulating gas and mixed with the fresh gas, the circulation ratio is 2.5, and 15% water vapor is added at the same time, and the amount of circulating gas is controlled at 600℃ according to the outlet temperature of the primary methanation reactor; the initial reaction coke oven gas obtained after the reaction is cooled to 270℃ by heat exchange and enters the secondary methanation reactor 10 equipped with 3.75m of commercial catalyst 3 The reaction outlet gas is cooled by heat exchange or directly enters the three-stage methanation reactor 11 equipped with a commercial catalyst 3m 3 The three-stage outlet product gas is finally cooled to obtain crude natural gas. The three reactors are all equipped with special high-temperature resistant catalysts; the obtained crude natural gas enters the separation and purification unit 12 and is treated by pressure swing adsorption process or membrane separation process to obtain rich natural gas and other gases. The water-rich natural gas is dehydrated by pressure swing adsorption or temperature swing adsorption to obtain natural gas. Example 6

[0028] Reactions were carried out according to the processes of Examples 1-5 and Comparative Example 1, respectively. The components of the raw coke oven gas used are shown in Table 2. After the above reactions, the evaluation results are shown in Table 3.

[0029] Table 2 Composition of raw coke oven gas Element <![CDATA[H2]]> <![CDATA[CH4]]> CO <![CDATA[CO2]]> Cm <![CDATA[N2]]> <![CDATA[O2]]> Composition V% 57.65 20.94 13.4 3.76 2.61 1.45 0.19 Table 3 Evaluation results of different process reactions

[0030] From the data in Table 3, it can be seen that in Examples 1-5 with a CO2 conversion step and hydrogen return, the conversion rates of CO and CO2 are also high when less catalyst is loaded, with a CO conversion rate of 100% and a CO2 conversion rate greater than 95%; while in Comparative Example 1, a larger circulation ratio and more catalysts are used to completely convert CO, but there is still a lot of CO2 left, and the CO2 conversion rate is only 64%, which is lower than that of Examples 1-5. The total carbon in the raw gas is not fully utilized. In Example 1-4, there is more residual hydrogen, which is recycled.

[0031] The above-described embodiments are only preferred embodiments for fully illustrating the present invention, and the protection scope of the present invention is not limited thereto. Equivalent substitutions or changes made by those skilled in the art based on the present invention are within the protection scope of the present invention. The protection scope of the present invention shall be subject to the claims.

Claims

1. A process for synthesizing natural gas from CO2-rich coke oven gas by methanation, including pre-purification, desulfurization, CO2-rich conversion and three-stage methanation reaction, natural gas separation and H2 extraction and return, the steps are as follows: (1) Pre-purification: The coke oven gas is compressed to a reaction pressure of 0.3-3.0 MPa, and then treated by adsorption or absorption to remove ammonia, cyanide, tar and dust from the coke oven gas to obtain roughly purified coke oven gas; (2) Hydrodesulfurization: The crude purified coke oven gas prepared in step (1) is treated with a hydroconversion process to remove organic sulfur such as C4H4S, COS, CS2, etc.; (3) CO2-rich conversion: The coke oven gas after hydrodesulfurization directly enters the CO2 conversion reactor, which is equipped with a special catalyst for CO2 conversion; (4) Adsorption desulfurization: After CO2 conversion, the coke oven gas enters the dry fine desulfurization to remove inorganic sulfur. After removal, the sulfur content of the coke oven gas is ≤0.1mg / Nm 3 Then enter the ultra-fine desulfurization to remove the total sulfur of coke oven gas to ≤0.03mg / Nm 3 ; (5) Tertiary methanation reaction: The outlet gas after the primary methanation is used as part of the circulating hot gas to return to the inlet of the primary methanation and mix with the fresh gas. The amount of circulating gas is controlled at 570-600°C according to the outlet temperature of the primary methanation reactor, and the reaction pressure is 0.3-3.0 MPa. The mixed gas is preheated to 280-320°C and enters the primary adiabatic reactor of methanation. After the reaction, the primary reaction coke oven gas is obtained; the primary reaction coke oven gas is heat exchanged and cooled to 250-300°C and enters the secondary methanation reactor. The outlet gas of the reaction is heat exchanged and cooled or directly enters the tertiary methanation reactor. The tertiary outlet product gas is finally cooled to obtain crude natural gas. All three reactors are equipped with special high temperature resistant catalysts; (6) Natural gas separation and H2 extraction and return: The crude natural gas obtained in step (5) is treated by a pressure swing adsorption process or a membrane separation process to obtain natural gas-rich gas and hydrogen gas and other gases. The water-rich natural gas is dehydrated by pressure swing adsorption or temperature swing adsorption to obtain natural gas; the separated hydrogen is returned to the inlet of the coke oven gas raw material compressor to be mixed with the crude coke oven gas.

2. The process for synthesizing natural gas from CO2-rich coke oven gas by methanation according to claim 1, characterized in that: The CO2-rich conversion reactor is equipped with a special catalyst for CO2 conversion, the catalyst active component is NiS@Ni, the auxiliary agents are sulfide and alkali metal, and the carrier is silicon-zirconium-aluminum composite oxide; the reaction temperature is 350-450°C.

3. The process for synthesizing natural gas from CO2-rich coke oven gas by methanation according to claim 1, characterized in that: The three-stage methanation reaction uses the outlet gas after the first-stage methanation as circulating hot gas to control the amount of water vapor in the circulating hot gas to adjust the outlet temperature of the first-stage methanation reactor. The circulation ratio is 1.5-2.0, and the temperature of the hot circulating gas is controlled at 150-180°C so that it circulates with part of the water vapor. The first-stage methanation reactor is equipped with a special high-temperature resistant catalyst, the active component of the catalyst is Ni, the additives are Re and alkaline earth metals, and the carrier is a silicon-aluminum composite oxide.

4. The process for synthesizing natural gas from CO2-rich coke oven gas by methanation according to claim 1, characterized in that: During the step (3), the CO2 conversion reactor is arranged after the hydrodesulfurization or fine desulfurization, and the reaction heat is used as the heat source at the inlet of the conversion reactor without the need for a separate heating device.

5. The process for synthesizing natural gas from CO2-rich coke oven gas by methanation according to claim 1, characterized in that: The special catalysts used in the three-stage methanation reactor are all pre-reduced catalysts, which are activated in hydrogen or purified coke oven gas at 280-350° C. for 10-16 hours before use.

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