LNG production system and LNG production method

Abstract

This invention provides an LNG preparation system and an LNG preparation method. The LNG preparation system includes: a mixing and pressurizing unit for receiving and mixing converter gas and coke oven gas to obtain a mixed gas, and pressurizing the mixed gas to obtain feed gas; a purification unit for receiving and purifying the feed gas to obtain purified gas; a methanation unit located downstream of the purification unit for receiving the purified gas and reacting it to generate product gas containing methane; and a liquefaction and separation unit for receiving the product gas and performing cryogenic liquefaction and separation to obtain LNG product, hydrogen-rich tail gas, and nitrogen-rich tail gas; wherein the liquefaction and separation unit is connected to the mixing and pressurizing unit to transport the hydrogen-rich tail gas to the mixing and pressurizing unit for pressurization to become part of the feed gas.

Description

Technical Field

[0001] This invention relates to the field of LNG preparation technology, and in particular to an LNG preparation system and LNG preparation method. Background Technology

[0002] Coke oven gas, a byproduct of the coking industry, is a hydrogen-rich gas. Its main components include hydrogen (54%–60%), methane (23%–27%), carbon monoxide (5%–8%), and trace amounts of carbon dioxide, nitrogen, hydrocarbons, and sulfur-containing impurities. With increasing demand for clean energy and growing pressure to reduce carbon emissions, converting coke oven gas into high-value-added liquefied natural gas (LNG) has become a key technological pathway for the coking industry to achieve resource recycling and energy conservation and emission reduction. However, the efficiency of converting coke oven gas into LNG still needs improvement. Summary of the Invention

[0003] The purpose of this invention is to provide an LNG preparation system and an LNG preparation method to solve the problems of the prior art.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: an LNG preparation system, comprising:

[0005] A mixing and pressurizing device is used to receive and mix converter gas and coke oven gas to obtain mixed gas, and pressurize the mixed gas to obtain raw material gas; A purification device for receiving and purifying the raw gas to obtain purified gas; A methanation unit is located downstream of the purification unit and is used to receive the purified gas and react the purified gas to generate product gas containing methane. A liquefaction and separation unit is used to receive the product gas and perform cryogenic liquefaction on the product gas and separate it to obtain LNG product, hydrogen-rich tail gas and nitrogen-rich tail gas. The liquefaction and separation device is connected to the mixing and pressurizing device to deliver the hydrogen-rich tail gas to the mixing and pressurizing device and pressurize it to become part of the raw material gas.

[0006] In one embodiment, the mixing and pressurizing device includes a mixer, a first pressurizing device, and a second pressurizing device arranged sequentially. The mixer is used to receive and mix the converter gas and the coke oven gas to obtain a mixed gas. The first pressurizing device is used to initially pressurize the mixed gas. The second pressurizing device is connected to the liquefaction and separation device and is used to receive the hydrogen-rich tail gas and the initially pressurized mixed gas and perform secondary pressurization to obtain raw material gas.

[0007] In one embodiment, the methanation apparatus includes a preheater, a regulating valve connected in parallel with the preheater, and a methane reactor located downstream of the preheater and the regulating valve. The preheater is used to preheat the purified gas, and the regulating valve is used to regulate the amount of purified gas entering the methane reactor.

[0008] In one embodiment, the inlet of the methane reactor is equipped with a temperature sensor for detecting temperature. When the real-time temperature at the inlet is less than a preset value, the amount of purified gas is increased by adjusting the regulating valve. When the real-time temperature at the inlet is greater than the preset value, the amount of purified gas is decreased by adjusting the regulating valve. The preset value is 280℃~350℃.

[0009] In one embodiment, the methanation apparatus includes a multi-stage series of methane reactors for a methanation reaction. The methanation apparatus includes a heat recovery device disposed between any two adjacent methane reactors, and a control valve disposed corresponding to the heat recovery device and connected in parallel with the upstream methane reactor. The control valve is used to regulate the amount of gas entering the heat recovery device and the amount of gas delivered to the downstream methane reactor.

[0010] In one embodiment, the methanation apparatus includes a primary methane reactor, a control valve connected in parallel with the primary methane reactor, and a secondary methane reactor located downstream of the primary methane reactor and the control valve. The control valve is capable of regulating the amount of gas entering the primary methane reactor and the amount of gas entering the secondary methane reactor.

[0011] In one embodiment, a first temperature detector is provided in the primary methane reactor to monitor the temperature inside the primary methane reactor. When the temperature inside the primary methane reactor reaches a first preset temperature, the control valve is adjusted to reduce the amount of gas entering the primary methane reactor; the first preset temperature is less than or equal to 600°C. The secondary methane reactor is equipped with a second temperature detector to monitor the temperature inside the reactor. When the temperature inside the secondary methane reactor is lower than a second preset temperature, the control valve is adjusted to increase the amount of gas entering the reactor. The second preset temperature is less than or equal to 350°C.

[0012] In one embodiment, the methanation apparatus includes a primary methane reactor, a first heat recovery unit, and a secondary methane reactor arranged sequentially, as well as a first control valve. The first control valve is connected in parallel with the first heat recovery unit and is used to regulate the amount of gas entering the heat recovery unit.

[0013] In one embodiment, the methanation apparatus includes a second control valve located downstream of the first heat recovery unit, and the second control valve is simultaneously connected to the inlet of the primary methane reactor and the inlet of the secondary methane reactor. The second regulating valve is used to control the amount of gas entering the first-stage methane reactor to be 50% to 65%, and the amount of gas entering the second-stage methane reactor to be 35% to 40%. A heat exchanger is provided between the first heat recovery unit and the second control valve.

[0014] In one embodiment, the methanation apparatus includes a primary methane reactor, a secondary methane reactor, and a tertiary methane reactor arranged sequentially. A second heat recovery unit and a cooling separator are sequentially provided between the secondary methane reactor and the tertiary methane reactor.

[0015] In one embodiment, the LNG preparation system includes a pretreatment device located upstream of the mixing and pressurizing unit. The pretreatment device includes a first pretreatment device for receiving coke oven gas and pretreating the coke oven gas, and a second pretreatment device for receiving converter gas and pretreating the converter gas. Both the first pretreatment device and the second pretreatment device are connected to the mixing and pressurizing unit. The purification device includes desulfurization equipment for desulfurization, deoxygenation equipment for deoxygenation, and decarbonization equipment for removing carbon dioxide. The liquefaction and separation device includes a dryer, a liquefaction unit, and a gas-liquid separator arranged in sequence. The dryer is used to remove moisture, the liquefaction unit is used to liquefy methane gas, and the gas-liquid separator is used to separate the gas and liquid to obtain hydrogen-rich tail gas, nitrogen-rich tail gas, and LNG product.

[0016] This application also provides a method for preparing LNG, including the following steps: The converter gas and coke oven gas are mixed to obtain a mixed gas, which is then pressurized to obtain the raw material gas; The raw material gas is purified to obtain purified gas; The purified gas undergoes a methanation reaction to generate a product gas containing methane. The product gas is liquefied and gas-liquid separation is performed to obtain LNG product, hydrogen-rich tail gas and nitrogen-rich tail gas; The hydrogen-rich tail gas is transported into the mixed gas and pressurized to become part of the raw material gas.

[0017] In one embodiment, in the step of pressurizing the mixed gas, the gas pressure is first increased to 1.0~1.5MPa through a first-stage pressurization, and then increased to 2.5~2.8MPa through a second-stage pressurization. The hydrogen-rich tail gas is mixed with the gas after the first-stage pressurization, and then subjected to the second-stage pressurization.

[0018] In one embodiment, the step of subjecting the purified gas to a methanation reaction to generate a product gas containing methane includes: The purified gas is preheated, and the preheated purified gas is then sent to the methanation reactor. The real-time temperature of the purified gas entering the methanation reactor is detected. When the real-time temperature is greater than a preset value, the amount of purified gas entering the methanation reactor is reduced. When the real-time temperature is less than the preset value, the amount of purified gas entering the methanation reactor is increased. The purified gas enters a multi-stage series methane reactor for methanation to generate a product gas mainly composed of methane, and the heat generated during the reaction process is recovered.

[0019] In one embodiment, the multi-stage methane reactor in series includes a primary methane reactor, a control valve connected in parallel with the primary methane reactor, and a secondary methane reactor located downstream of the primary methane reactor and the control valve. The step of the purified gas entering a multi-stage series-connected methane reactor for methanation includes: When the temperature inside the primary methane reactor reaches a first preset temperature, the amount of gas entering the primary methane reactor is reduced; the first preset temperature is less than or equal to 600°C. When the temperature inside the secondary methane reactor is lower than the second preset temperature, the amount of gas entering the secondary methane reactor is increased; the second preset temperature is less than or equal to 350°C.

[0020] In one embodiment, in the step of mixing converter gas and coke oven gas to obtain a mixed gas: the amount of the converter gas and / or the coke oven gas is adjusted so that the hydrogen-to-carbon ratio in the mixed gas is (2.8~3.2):1.

[0021] As can be seen from the above technical solution, the present invention has at least the following advantages and positive effects: The LNG production system of this invention produces LNG by introducing the abundant carbon source from converter gas into coke oven gas. It precisely adjusts the hydrogen-carbon ratio from the source and uses circulating hydrogen to bring the effective components (H2 and CO / CO2) in the two gases close to the theoretical optimal ratio for methanation reaction. This maximizes the utilization of the effective components in the two gases, significantly improves the methane yield and LNG production per unit gas source, eliminates ineffective hydrogen circulation, greatly reduces system energy consumption, and improves the economics of the entire process. It fundamentally solves the inherent defects of producing LNG from coke oven gas alone, achieving a leap in LNG production and energy efficiency. It also provides a high-value-added utilization path for a large amount of surplus converter gas, and is a key direction for realizing the resource utilization, clean utilization, and value maximization of coal gas in steel enterprises.

[0022] Furthermore, the heat of methanation is efficiently recovered (e.g., to generate steam), and the recycling of hydrogen-rich tail gas reduces the compression work of fresh feed gas and the consumption of system cooling capacity, resulting in a 10% to 15% reduction in overall energy consumption. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the LNG preparation system in this invention.

[0024] Figure 2 This is a schematic diagram of the LNG preparation system in this invention.

[0025] Figure 3 This is a schematic flowchart of the LNG preparation method in this invention.

[0026] The annotations in the attached figures are explained as follows: 800. LNG Preparation System; 1. Mixing and Pressurization Unit; 11. Mixer; 12. First Pressurization Equipment; 13. Second Pressurization Equipment; 2. Purification Unit; 3. Methanation Unit; 31. Preheater; 32. First-Stage Methane Reactor; 33. Second-Stage Methane Reactor; 34. Third-Stage Methane Reactor; 351. Regulating Valve; 352. Control Valve; 36. First Heat Recovery Unit; 371. First Control Valve; 372. Second Control Valve; 38. Heat Exchanger; 391. Second Heat Recovery Unit; 392. Cooling Separator; 4. Liquefaction and Separation Unit; 41. Dryer; 42. Liquefaction Unit; 43. High-Pressure Distillation Column; 44. Low-Pressure Distillation Column; 51. First Pretreatment Equipment; 52. Second Pretreatment Equipment; 6. Controller. Detailed Implementation

[0027] Although the invention can be readily embodied in various forms, only some specific embodiments are shown in the accompanying drawings and will be described in detail in this specification. It is understood that this specification should be regarded as an exemplary illustration of the principles of the invention and is not intended to limit the invention to what is described herein.

[0028] Therefore, a feature pointed out in this specification is used to illustrate one feature of one embodiment of the invention, and does not imply that every embodiment of the invention must have the described feature. Furthermore, it should be noted that this specification describes many features. While certain features may be combined to illustrate possible system designs, these features may also be used in other combinations not explicitly stated. Therefore, unless otherwise stated, the described combinations are not intended to be limiting.

[0029] In the embodiments shown in the accompanying drawings, the directional indications (such as up, down, left, right, front, and back) used to explain the structure and movement of the various elements of the invention are relative rather than absolute. These descriptions are appropriate when these elements are in the positions shown in the drawings. If the descriptions of the positions of these elements change, these directional indications also change accordingly.

[0030] The main components of coke oven gas include hydrogen (54%~60%), methane (23%~27%), carbon monoxide (5%~8%), and small amounts of carbon dioxide, nitrogen, hydrocarbons, and sulfur-containing impurities. The hydrogen content in coke oven gas is far higher than the amount of carbon oxides (CO+CO2) it can consume, resulting in a significant hydrogen surplus. That is, the inherent hydrogen-to-carbon ratio (H / C) of coke oven gas is severely imbalanced, failing to meet the stringent stoichiometric requirements of the methanation reaction (H2 / CO ≈ 3:1), thus limiting LNG production and wasting hydrogen resources.

[0031] Converter gas is rich in CO (50%~60%) and CO2 (15%~20%), with a total carbon oxide content as high as 65%~80%, making it an excellent carbon source. The characteristics of converter gas are perfectly complementary to those of coke oven gas.

[0032] Therefore, this application provides an LNG preparation system 800, which couples coke oven gas produced by coking plants and converter gas produced by steel plants, and performs a methanation reaction of carbon monoxide, carbon dioxide and hydrogen under the regulation of circulating hydrogen and the action of a catalyst to produce methane, and then prepares LNG through liquefaction separation.

[0033] Combination Figure 1 and Figure 2The LNG production system 800 includes a mixing and pressurizing unit 1, a purification unit 2, a methanation unit 3, and a liquefaction and separation unit 4. The mixing and pressurizing unit 1 receives and mixes converter gas and coke oven gas to obtain a mixed gas, which is then pressurized to obtain feed gas. The purification unit 2 receives and purifies the feed gas to obtain purified gas. The methanation unit 3, located downstream of the purification unit 2, receives the purified gas and reacts it to generate product gas containing methane. The liquefaction and separation unit 4 receives the product gas and performs cryogenic liquefaction and separation to obtain LNG product, hydrogen-rich tail gas, and nitrogen-rich tail gas. The liquefaction and separation unit 4 is connected to the mixing and pressurizing unit 1, which then delivers the hydrogen-rich tail gas to the mixing and pressurizing unit 1 for pressurization, making it part of the feed gas.

[0034] By introducing the abundant carbon source from converter gas into coke oven gas to produce LNG, the hydrogen-to-carbon ratio can be precisely adjusted from the source. Circulating hydrogen enrichment ensures that the effective components (H2 and CO / CO2) in both gases approach the theoretical optimal ratio for methanation, maximizing the utilization of these components. This significantly increases methane yield and LNG production per unit gas source, eliminates ineffective hydrogen circulation, drastically reduces system energy consumption, and enhances the overall economic efficiency. It fundamentally solves the inherent defects of producing LNG from coke oven gas alone, achieving a leap in LNG production and energy efficiency. Furthermore, it provides a high-value-added utilization pathway for a large amount of surplus converter gas, representing a key direction for realizing the resource utilization, clean production, and maximum value maximization of coal gas in steel enterprises.

[0035] Specifically, the LNG preparation system 800 includes a pretreatment unit located upstream of the mixing and pressurizing unit 1 to pretreatment coke oven gas and converter gas. The pretreatment unit includes a first pretreatment device 51 for receiving and pretreating coke oven gas and a second pretreatment device 52 for receiving and pretreating converter gas. Both the first pretreatment device 51 and the second pretreatment device 52 are connected to the mixing and pressurizing unit 1 to deliver the pretreated gas to the mixing and pressurizing unit 1.

[0036] The crude coal gas produced from the coke oven contains impurities such as tar, benzene, naphthalene, ammonia, and dust. It needs to undergo preliminary treatment to remove pollutants that could clog pipes and poison subsequent catalysts, thus creating conditions for deep purification. The first pretreatment equipment 51 includes compression and pressure stabilization, electrostatic precipitator for tar and naphthalene removal, washing oil absorption for benzene removal, and water washing or acid washing for ammonia removal.

[0037] The second pretreatment device 52 is used to remove impurities such as dust, tar, benzene, naphthalene, organic sulfur and inorganic sulfur from converter gas.

[0038] The mixing and pressurizing unit 1 is used to receive and mix converter gas and coke oven gas to obtain a mixed gas, which is then pressurized to obtain feed gas. Specifically, the mixing and pressurizing unit 1 receives converter gas and coke oven gas, mixes them to achieve the theoretical ratio, and then performs multi-stage compression to pressurize to 2.5~2.8 MPa to obtain feed gas. The feed gas is in a hydrogen-rich environment with an H / C ratio of approximately 3.1, ensuring that the carbon dioxide concentration in the gas after methanation is below 50 ppm.

[0039] Specifically, the mixing and pressurizing device 1 includes a mixer 11, a first pressurizing device 12, and a second pressurizing device 13 arranged sequentially. The mixer 11 is used to receive and mix converter gas and coke oven gas to obtain a mixed gas. The first pressurizing device 12 is used to initially pressurize the mixed gas. The second pressurizing device 13 is connected to the liquefaction and separation device 4 and is used to receive hydrogen-rich tail gas and the initially pressurized mixed gas and perform secondary pressurization to obtain raw material gas.

[0040] The mixer 11 is connected to both the first pretreatment device 51 and the second pretreatment device 52 to receive pretreated coke oven gas and converter gas. After pretreatment by the first pretreatment device 51 and the second pretreatment device 52, the impurity content in the gas is as follows: tar content ≤ 1 mg / Nm³. 3 Benzene content ≤10mg / Nm 3 Naphthalene content ≤ mg / Nm 3 .

[0041] The mixer 11 mixes the purified coke oven gas and converter gas in a precise ratio, using CO and CO2 in the converter gas to supplement the carbon in the coke oven gas which is high in hydrogen and low in carbon, and adjusts the hydrogen-to-carbon ratio (H / C) of the mixed gas to the ideal reaction range of 2.8:1 to 3.2:1.

[0042] The first pressurizing unit 12 pressurizes the mixed gas to 1.0~1.5 MPa, and the second pressurizing unit 13 pressurizes the mixed gas to 2.5~2.8 MPa. The hydrogen-rich tail gas also enters the second pressurizing unit 13 and is pressurized together with the mixed gas. The recovery and utilization of the hydrogen-rich tail gas increases the H content, improving the overall conversion rate. Furthermore, the use of the recycled gas as a heat carrier helps stabilize the reaction temperature.

[0043] Purification unit 2 is used to receive and purify the raw material gas to obtain purified gas. Purification unit 2 is used to perform deep purification of the raw material gas. The purpose of deep purification is to completely remove trace impurities that are toxic to the subsequent methanation catalyst, which is a core link to ensure long-term stable operation.

[0044] Purification device 2 includes desulfurization equipment for desulfurization, deoxygenation equipment for deoxygenation, and decarbonization equipment for removing carbon dioxide.

[0045] Desulfurization typically employs a combination of wet and dry processes. First, most of the hydrogen sulfide (H2S) is removed using methods such as tannin removal. Then, fine desulfurization technologies such as hydrogenation conversion and zinc oxide absorption are used to reduce the total sulfur (including organic sulfur) concentration to below 0.1 ppm.

[0046] In this embodiment, mature iron-molybdenum and nickel-molybdenum catalysts are used for the hydrogenation reaction. After hydrogenation conversion, the gas undergoes deep desulfurization in a zinc oxide desulfurization tank to ensure that the total sulfur content of the effluent gas is below 0.1 ppm, meeting the stringent requirements of highly active methanation catalysts. Specifically, the desulfurization equipment includes a hydrogenation converter and a zinc oxide desulfurization tank connected in sequence, configured to convert organic sulfur in the mixed gas into hydrogen sulfide and adsorb and remove it, so that the total sulfur content is below 0.1 ppm.

[0047] The presence of approximately 0.5% to 1% oxygen in coal gas can cause catalyst sintering. Catalytic oxidation is typically used to remove oxygen by reacting oxygen with hydrogen to produce water.

[0048] To control the temperature rise of the methanation reaction and optimize the hydrogen-to-carbon ratio, some carbon dioxide (CO2) needs to be removed. Commonly used techniques include MDEA decarbonization or low-temperature methanol washing.

[0049] The methanation unit 3 is located downstream of the purification unit 2 and is used to receive the purified gas and react it to generate product gas containing methane.

[0050] In the purified coke oven gas, hydrogen (H2) reacts with carbon monoxide (CO) and carbon dioxide (CO2) in the presence of a nickel-based catalyst to produce methane (CH4) and water. The equation for the methanation reaction is as follows: CO + 3H2→ CH4+ H2O206 KJ / mol CO2+ 4H2→ CH4+2H2O165 KJ / mol The above reaction is a strongly exothermic reaction, and the heat of reaction needs to be removed, the catalyst protected, and energy recovered in order for the reaction to proceed smoothly.

[0051] Synthetic natural gas (SNG) produced by methanation contains approximately 70% methane, 22% hydrogen, and 8% nitrogen. It needs to be liquefied and separated to obtain liquefied natural gas (LNG). Specifically, the methanation unit 3 includes a preheater 31, a regulating valve 351 connected in parallel with the preheater 31, and a methane reactor located downstream of the preheater 31 and the regulating valve 351. The preheater 31 is used to preheat the purified gas, and the regulating valve 351 is used to regulate the amount of purified gas entering the methane reactor.

[0052] The inlet of the methane reactor is equipped with a temperature sensor to detect the temperature. When the real-time inlet temperature is lower than a preset value, the flow rate of purified gas is increased by adjusting the regulating valve 351. When the real-time inlet temperature is higher than the preset value, the flow rate of purified gas is decreased by adjusting the regulating valve 351. The preset value is 280℃~350℃.

[0053] The methanation unit 3 includes multiple methanation reactors connected in series, which are used for the methanation reaction. The unit also includes a heat recovery device positioned between any two adjacent methanation reactors, and a control valve corresponding to the heat recovery device and connected in parallel with the upstream methanation reactor. The control valve is used to regulate the amount of gas entering the heat recovery device and the amount of gas supplied to the downstream methanation reactor.

[0054] Specifically, the methanation unit 3 includes a primary methane reactor 32, a control valve 352 connected in parallel with the primary methane reactor 32, and a secondary methane reactor 33 located downstream of the primary methane reactor 32 and the control valve 352. The primary methane reactor 32 is located downstream of the preheater 31, and the inlet of the primary methanation reactor constitutes the inlet of the methane reactor.

[0055] Control valve 352 can be used to regulate the amount of gas entering the primary methane reactor 32 and the amount of gas entering the secondary methane reactor 33. Specifically, control valve 352 is installed on a branch pipe, with the outlet end of the branch pipe connected to the upstream of the secondary methane reactor 33 and the inlet end connected to the upstream of the primary methane reactor 32. Thus, the amount of gas entering the secondary methane reactor 33 through the branch pipe can be regulated by adjusting the opening degree of control valve 352.

[0056] A first-stage methane reactor 32 is equipped with a first temperature detector to monitor the temperature inside the reactor. When the temperature inside the first-stage methane reactor 32 reaches a first preset temperature, a control valve 352 is adjusted to reduce the amount of gas entering the reactor. The first preset temperature is less than or equal to 600°C. For example, the first preset temperature can be set to 600°C, or it can be set to 590°C or 580°C, depending on actual needs.

[0057] When gas composition fluctuates, such as an increase in carbon monoxide concentration, the reaction temperature in the primary methane reactor 32 must be controlled to be below 600°C. Therefore, when the first temperature detector detects that the temperature has reached the first preset temperature, the opening of the control valve 352 is adjusted to reduce the amount of gas entering the primary methane reactor 32 and send some of the gas into the secondary methane reactor 33.

[0058] A second temperature detector is pre-installed inside the secondary methane reactor 33 to monitor the temperature inside the reactor. When the temperature inside the secondary methane reactor 33 is lower than a second preset temperature, the control valve 352 is adjusted to increase the gas flow into the secondary methane reactor 33. The second preset temperature is less than or equal to 350°C.

[0059] If the reaction temperature in the secondary methane reactor 33 is too low (e.g., less than 350°C, where the second preset temperature is 350°C), adjust the opening of the control valve 352 to increase the gas flow into the secondary methane reactor 33, increase the secondary methanation reaction load, and adjust the heat balance.

[0060] The methanation unit 3 also includes a tertiary methane reactor 34 located downstream of the secondary methane reactor 33. That is, the methanation unit 3 in this embodiment includes a primary methane reactor 32, a secondary methane reactor 33, and a tertiary methane reactor 34 arranged sequentially.

[0061] The primary methane reactor 32, the secondary methane reactor 33, and the tertiary methane reactor 34 are all adiabatic fixed-bed reactors, and are filled with highly active nickel-based catalysts, which enable CO, CO2 and H2 to undergo a strongly exothermic reaction to efficiently generate CH4.

[0062] The methanation unit 3 also incorporates interstage cooling between reactors to prevent catalyst runaway and deactivation. Simultaneously, high-grade waste heat from the reaction is recovered via a heat recovery unit and a steam generator to produce steam, achieving cascaded energy utilization and reducing the system's overall energy consumption.

[0063] Specifically, the methanation unit 3 includes a primary methane reactor 32, a first heat recovery unit 36, and a secondary methane reactor 33 arranged sequentially, as well as a first regulating valve 371. The first regulating valve 371 is arranged in parallel with the first heat recovery unit 36, and is used to regulate the amount of gas entering the heat recovery unit.

[0064] The gas output from the outlet of the primary methane reactor 32 enters the first heat recovery unit 36 ​​to recover heat, while simultaneously producing steam as a byproduct. Increasing the opening of the first regulating valve 371 reduces the amount of gas entering the heat recovery unit, thereby reducing steam production and transferring the system's reaction heat downstream, thus increasing the temperature of the reaction gas entering the secondary methane reactor 33.

[0065] The methanation unit 3 includes a second regulating valve 372. The second regulating valve 372 is located downstream of the first heat recovery unit 36 ​​and is connected to both the inlet of the primary methane reactor 32 and the inlet of the secondary methane reactor 33. The second regulating valve 372 controls the flow rate of gas, with 50%–65% entering the primary methane reactor 32 and 35%–40% entering the secondary methane reactor 33.

[0066] A heat exchanger 38 is provided between the first heat recovery unit 36 ​​and the second control valve 372.

[0067] A second heat recovery unit 391 and a cooling separator 392 are sequentially provided between the secondary methane reactor 33 and the tertiary methane reactor 34.

[0068] The gas output from the outlet of the secondary methane reactor 33 enters the second heat recovery unit 391 to recover heat and produce by-product steam. After passing through the cooling separator 392, it enters the tertiary methane reactor 34, where the carbon monoxide and carbon dioxide in the mixed gas are fully reacted before being sent to the downstream liquefaction and separation unit 4.

[0069] The product gas after the methanation reaction (mainly composed of CH4, N2, H2O, and unreacted H2) enters the liquefaction and separation unit 4, where it undergoes dehydration and purification before cryogenic liquefaction and dehydrogenation. The gas is cooled to below -160°C via a cryogenic heat exchanger 38, where methane is liquefied, while hydrogen and nitrogen remain gaseous. Finally, the liquid LNG is separated from the hydrogen and nitrogen tail gases in the cryogenic separator, yielding pure LNG product.

[0070] Specifically, the liquefaction and separation device 4 includes a dryer 41, a liquefaction unit 42, and a gas-liquid separator arranged in sequence. The dryer 41 is used to remove moisture, the liquefaction unit 42 is used to liquefy methane gas, and the gas-liquid separator is used to separate the gas and liquid to obtain hydrogen-rich tail gas, nitrogen-rich tail gas, and LNG product.

[0071] Dryer 41 uses a molecular sieve adsorption tower to deeply remove moisture from the product gas and prevent ice formation and equipment blockage during the cryogenic process.

[0072] The liquefier 42 uses a mixed refrigerant and other processes to liquefy methane gas through staged cooling. The liquefier 42 lowers the gas temperature to -160°C.

[0073] The gas-liquid separator performs high-pressure and low-pressure distillation at low temperatures. High-pressure distillation separates hydrogen from the methanation synthesis gas, yielding hydrogen-rich tail gas and liquid-phase methanation synthesis gas. The liquefied methanation synthesis gas is then subjected to low-pressure distillation to produce LNG and nitrogen-rich tail gas.

[0074] In this embodiment, the gas-liquid separator includes a high-pressure distillation column 43 and a low-pressure distillation column 44 arranged sequentially. The top gas of the high-pressure distillation column 43 is hydrogen-rich tail gas, and the bottom gas is methane-rich liquid. After depressurization, the methane-rich liquid enters the low-pressure distillation column 44 and separates LNG product and nitrogen-rich tail gas. The nitrogen-rich tail gas is sent out of the boundary area as the system tail gas outlet.

[0075] After being reheated, the hydrogen-rich tail gas enters the mixing and pressurizing unit 1 to realize the recycling of hydrogen resources and further improve the overall carbon conversion rate and energy efficiency; the nitrogen-rich tail gas is discharged from the system as the outlet for non-condensable vapors such as nitrogen.

[0076] The LNG preparation system 800 includes a controller 6, which is communicatively connected to temperature monitoring equipment and valves.

[0077] Specifically, the controller 6 is communicatively connected to the temperature sensor and the regulating valve 351, and the controller 6 has a preset value. The controller 6 compares the temperature detected by the temperature sensor with the preset value, analyzes and outputs control commands to control the regulating valve 351 to make adjustments.

[0078] The controller 6 is communicatively connected to the first temperature detector and the control valve 352, and the controller 6 has a first preset temperature. The controller 6 analyzes the temperature detected by the first temperature detector and compares it with the first preset temperature, and outputs control commands to control the control valve 352 to adjust.

[0079] The controller 6 is communicatively connected to the second temperature detector and the control valve 352, and the controller 6 has a second preset temperature. The controller 6 analyzes the temperature detected by the second temperature detector and compares it with the second preset temperature, and outputs control commands to control the control valve 352 to adjust.

[0080] The controller 6 is communicatively connected to the first regulating valve 371, and the controller 6 controls the first regulating valve 371.

[0081] The controller 6 is communicatively connected to the second control valve 372, and the controller 6 controls the second control valve 372.

[0082] In the methanation reaction, parameters are preset in controller 6, and the controller 6 receives the monitored temperature. When the monitored temperature deviates from the preset parameters, the opening of the corresponding valve is adjusted according to the preset program in controller 6 to ensure the reaction heat balance of the system.

[0083] The LNG production system 800 of this application precisely supplements carbon to coke oven gas using converter gas, optimizes the balance of reactants, and ultimately achieves energy-saving and increased LNG production, with the following significant beneficial effects: 1. Targeted carbon supplementation for significant production increase: By precisely adjusting the hydrogen-to-carbon ratio (H / C) of coke oven gas through converter gas, the reactants reach or approach the stoichiometric ratio, fundamentally solving the problem of excess hydrogen when coke oven gas is used to produce LNG alone. LNG production can be increased by 16-20%, and the hydrogen conversion rate can be increased from ~60% to over 95%.

[0084] 2. Energy cascade utilization and low system energy consumption: The heat of methanation reaction is efficiently recovered (e.g., to generate steam), and the recycling of hydrogen-rich tail gas reduces the compression work of fresh feed gas and the consumption of system cooling capacity, which can reduce the overall energy consumption by 10% to 15%.

[0085] 3. Resource synergy and green environmental protection: Coke oven gas and converter gas, two types of waste gas from steel plants, are synergistically converted into clean energy LNG, realizing "turning waste into treasure" and meeting the requirements of circular economy and low-carbon development.

[0086] 4. High system integration and stable operation: Through modular design and process optimization such as hydrogen-rich tail gas recirculation, the system's ability to withstand load fluctuations has been enhanced, ensuring long-term stable operation.

[0087] See Figure 3 This application also provides a method for preparing LNG, comprising the following steps: S1. Mix converter gas and coke oven gas to obtain mixed gas, and pressurize the mixed gas to obtain raw material gas.

[0088] Converter gas and coke oven gas are pretreated separately before being mixed. During mixing, the amount of converter gas or coke oven gas is adjusted to achieve a hydrogen-to-carbon ratio of (2.8~3.2):1 in the mixture. The amount of coke oven gas can be fixed while increasing or decreasing the amount of converter gas, or vice versa, or both can be adjusted simultaneously.

[0089] During pressurization, the pressure is increased to 2.5~2.8MPa through multi-stage compression to obtain the feed gas. That is, the pressure of the feed gas is 2.5-2.8MPa.

[0090] The pressurization process specifically includes: firstly, pressurizing the gas pressure to 1.0~1.5MPa through a first-stage pressurization, and then pressurizing the gas pressure to 2.5~2.8MPa through a second-stage pressurization.

[0091] Furthermore, the hydrogen-rich exhaust gas is mixed with the gas that has undergone primary pressurization, and then subjected to secondary pressurization.

[0092] S2. Purify the raw gas to obtain purified gas.

[0093] Specifically, purifying the raw gas includes a deep desulfurization step: the raw gas is subjected to deep desulfurization treatment to reduce its total sulfur content to below 0.1 ppm.

[0094] Purifying the raw gas includes a deoxygenation step, which causes oxygen and hydrogen to react and produce water, thus removing nutrients.

[0095] Purifying the feed gas includes a carbon dioxide removal step to control the hydrogen-to-carbon ratio.

[0096] S3. The purified gas undergoes a methanation reaction to produce product gas containing methane.

[0097] This step specifically includes: S31. Preheat the purified gas, and then send the preheated purified gas to the methanation reactor.

[0098] S32. Detect the real-time temperature of the purified gas entering the methanation reactor. If the real-time temperature is greater than the preset value, reduce the amount of purified gas entering the methanation reactor. If the real-time temperature is less than the preset value, increase the amount of purified gas entering the methanation reactor.

[0099] Specifically, the methanation unit 3 includes a preheater 31, a regulating valve 351 connected in parallel with the preheater 31, and a methane reactor located downstream of the preheater 31 and the regulating valve 351. The preheater 31 is used to preheat the purified gas, and the regulating valve 351 is used to regulate the amount of purified gas entering the methane reactor. A temperature sensor is installed at the inlet of the methane reactor to detect the temperature. When the real-time inlet temperature is lower than a preset value, the regulating valve 351 is adjusted to increase the amount of purified gas; when the real-time inlet temperature is higher than the preset value, the regulating valve 351 is adjusted to decrease the amount of purified gas. The preset value is 280℃~350℃.

[0100] S33. The purified gas enters a multi-stage series methane reactor for methanation reaction to generate product gas mainly composed of methane, and the heat generated in the reaction process is recovered.

[0101] The methane reactor includes multiple methane reactors connected in series. Specifically, the multiple methane reactors connected in series include a primary methane reactor 32, a control valve 352 connected in parallel with the primary methane reactor 32, and a secondary methane reactor 33 located downstream of the primary methane reactor 32 and the control valve 352.

[0102] The steps for the purified gas to undergo methanation in a multi-stage series methane reactor include: When the temperature inside the primary methane reactor 32 reaches the first preset temperature, the amount of gas entering the primary methane reactor 32 is reduced; the first preset temperature is less than or equal to 600°C.

[0103] When the temperature inside the secondary methane reactor 33 is lower than the second preset temperature, the amount of gas entering the secondary methane reactor 33 is increased. The second preset temperature is less than or equal to 350°C.

[0104] Specifically, a first temperature detector is installed inside the first-stage methane reactor 32 to monitor the temperature inside the first-stage methane reactor 32. When the temperature inside the first-stage methane reactor 32 reaches a first preset temperature, the control valve 352 is adjusted to reduce the amount of gas entering the first-stage methane reactor 32.

[0105] A second temperature detector is pre-installed inside the secondary methane reactor 33 to monitor the temperature inside the reactor. When the temperature inside the secondary methane reactor 33 is lower than the second preset temperature, the control valve 352 is adjusted to increase the amount of gas entering the secondary methane reactor 33.

[0106] The multi-stage methane reactor also includes a tertiary methane reactor 34 located downstream of the secondary methane reactor 33.

[0107] A heat recovery device and a control valve are also installed between any two adjacent methane reactors. The amount of gas entering the heat recovery device and the amount of gas delivered to the next stage methane reactor are adjusted by the control device.

[0108] Generally speaking, 50% to 65% of the gas volume is controlled to enter the primary methane reactor 32, and 35% to 40% of the gas volume is controlled to enter the secondary methane reactor 33.

[0109] S4. The liquefied product gas is then separated into LNG product, hydrogen-rich tail gas, and nitrogen-rich tail gas.

[0110] The product gas is successively dried, liquefied, and separated into gas and liquid to obtain LNG product, hydrogen-rich tail gas, and nitrogen-rich tail gas.

[0111] In this process, the hydrogen-rich tail gas is transported into the mixed gas and pressurized to become part of the feed gas.

[0112] For example, with a typical 45000 Nm 3 / h coke oven gas was used as a comparative example for methanation reaction, coupled with 45000 Nm 3 / h coke oven gas and 5000Nm 3 / h converter gas was used as the material for methanation reaction as an example for comparison.

[0113] Table 1 shows the parameter table of the materials in the example, and Table 2 shows the material balance table of the comparative example. Coke oven gas and converter gas are pressurized and mixed with circulating hydrogen-rich gas, and then used as the inlet gas for the first-stage methanation reactor. The reaction process of the third-stage methanation reactor 34 is adjusted by controller 6. The reaction products are cooled, separated, and deeply dehydrated. Deep cryogenic separation yields LNG, hydrogen-rich tail gas, and nitrogen-rich tail gas.

[0114] Table 1. Parameter table of materials in the embodiment

[0115] Table 2 Material Balance Sheet for Conventional Coke Oven Gas Methanation Reaction

[0116] All current shares are calculated on a dry basis.

[0117] A comparison of Tables 1 and 2 shows that, under the same coke oven gas volume, the LNG production increased from 17280 Nm³ / h to 20527 Nm³ / h, representing an 18% increase in LNG production.

[0118] In summary, this LNG preparation system has the following advantages: The LNG production system of this invention produces LNG by introducing the abundant carbon source from converter gas into coke oven gas. It precisely adjusts the hydrogen-carbon ratio from the source and uses circulating hydrogen to bring the effective components (H2 and CO / CO2) in the two gases close to the theoretical optimal ratio for methanation reaction. This maximizes the utilization of the effective components in the two gases, significantly improves the methane yield and LNG production per unit gas source, eliminates ineffective hydrogen circulation, greatly reduces system energy consumption, and improves the economics of the entire process. It fundamentally solves the inherent defects of producing LNG from coke oven gas alone, achieving a leap in LNG production and energy efficiency. It also provides a high-value-added utilization path for a large amount of surplus converter gas, and is a key direction for realizing the resource utilization, clean utilization, and value maximization of coal gas in steel enterprises.

[0119] Furthermore, the heat of methanation is efficiently recovered (e.g., to generate steam), and the recycling of hydrogen-rich tail gas reduces the compression work of fresh feed gas and the consumption of system cooling capacity, resulting in a 10% to 15% reduction in overall energy consumption.

[0120] Although the invention has been described with reference to several typical embodiments, it should be understood that the terminology used is illustrative and exemplary, and not restrictive. Since the invention can be embodied in many forms without departing from the spirit or essence of the invention, it should be understood that the above embodiments are not limited to any of the foregoing details, but should be interpreted broadly within the spirit and scope defined by the appended claims. Therefore, all variations and modifications falling within the scope of the claims or their equivalents should be covered by the appended claims.

Claims

1. An LNG production system, characterized in that, include: A mixing and pressurizing device is used to receive and mix converter gas and coke oven gas to obtain mixed gas, and pressurize the mixed gas to obtain raw material gas; A purification device for receiving and purifying the raw gas to obtain purified gas; A methanation unit is located downstream of the purification unit and is used to receive the purified gas and react the purified gas to generate product gas containing methane. A liquefaction and separation unit is used to receive the product gas and perform cryogenic liquefaction on the product gas and separate it to obtain LNG product, hydrogen-rich tail gas and nitrogen-rich tail gas. The liquefaction and separation device is connected to the mixing and pressurizing device to deliver the hydrogen-rich tail gas to the mixing and pressurizing device and pressurize it to become part of the raw material gas.

2. The LNG preparation system according to claim 1, characterized in that, The mixing and pressurizing device includes a mixer, a first pressurizing device, and a second pressurizing device arranged in sequence. The mixer is used to receive and mix the converter gas and the coke oven gas to obtain a mixed gas. The first pressurizing device is used to initially pressurize the mixed gas. The second pressurizing device is connected to the liquefaction and separation device and is used to receive the hydrogen-rich tail gas and the initially pressurized mixed gas and perform secondary pressurization to obtain raw material gas.

3. The LNG preparation system according to claim 1, characterized in that, The methanation apparatus includes a preheater, a regulating valve connected in parallel with the preheater, and a methane reactor located downstream of the preheater and the regulating valve. The preheater is used to preheat the purified gas, and the regulating valve is used to regulate the amount of purified gas entering the methane reactor.

4. The LNG preparation system according to claim 3, characterized in that, The inlet of the methane reactor is equipped with a temperature sensor to detect the temperature. When the real-time temperature at the inlet is less than a preset value, the amount of purified gas is increased by adjusting the regulating valve. When the real-time temperature at the inlet is greater than the preset value, the amount of purified gas is decreased by adjusting the regulating valve. The preset value is 280℃~350℃.

5. The LNG preparation system according to claim 1, characterized in that, The methanation unit includes multiple methanation reactors connected in series, which are used for the methanation reaction; The methanation apparatus includes a heat recovery device disposed between any two adjacent methane reactors, and a control valve disposed corresponding to the heat recovery device and connected in parallel with the upstream methane reactor. The control valve is used to regulate the amount of gas entering the heat recovery device and the amount of gas delivered to the downstream methane reactor.

6. The LNG preparation system according to claim 5, characterized in that, The methanation apparatus includes a primary methane reactor, a control valve connected in parallel with the primary methane reactor, and a secondary methane reactor located downstream of the primary methane reactor and the control valve. The control valve is used to regulate the amount of gas entering the primary methane reactor and the amount of gas entering the secondary methane reactor.

7. The LNG preparation system according to claim 6, characterized in that, The first-stage methane reactor is equipped with a first temperature detector to monitor the temperature inside the first-stage methane reactor. When the temperature inside the first-stage methane reactor reaches a first preset temperature, the control valve is adjusted to reduce the amount of gas entering the first-stage methane reactor. The first preset temperature is less than or equal to 600℃; The secondary methane reactor is equipped with a second temperature detector to monitor the temperature inside the secondary methane reactor. When the temperature inside the secondary methane reactor is lower than the second preset temperature, the control valve is adjusted to increase the amount of gas entering the secondary methane reactor. The second preset temperature is less than or equal to 350°C.

8. The LNG preparation system according to claim 5, characterized in that, The methanation apparatus includes a primary methane reactor, a first heat recovery unit, and a secondary methane reactor arranged in sequence, as well as a first control valve. The first control valve is connected in parallel with the first heat recovery unit and is used to regulate the amount of gas entering the heat recovery unit.

9. The LNG preparation system according to claim 8, characterized in that, The methanation unit includes a second control valve, which is located downstream of the first heat recovery unit and is connected to both the inlet of the first-stage methane reactor and the inlet of the second-stage methane reactor. The second regulating valve is used to control the amount of gas entering the first-stage methane reactor to be 50% to 65%, and the amount of gas entering the second-stage methane reactor to be 35% to 40%. A heat exchanger is provided between the first heat recovery unit and the second control valve.

10. The LNG preparation system according to claim 5, characterized in that, The methanation unit includes a primary methane reactor, a secondary methane reactor, and a tertiary methane reactor arranged sequentially. A second heat recovery unit and a cooling separator are sequentially provided between the secondary methane reactor and the tertiary methane reactor.

11. The LNG preparation system according to claim 1, characterized in that, The LNG preparation system includes a pretreatment device located upstream of the mixing and pressurizing unit. The pretreatment device includes a first pretreatment device for receiving coke oven gas and pretreating the coke oven gas, and a second pretreatment device for receiving converter gas and pretreating the converter gas. Both the first pretreatment device and the second pretreatment device are connected to the mixing and pressurizing unit. The purification device includes desulfurization equipment for desulfurization, deoxygenation equipment for deoxygenation, and decarbonization equipment for removing carbon dioxide. The liquefaction and separation device includes a dryer, a liquefaction unit, and a gas-liquid separator arranged in sequence. The dryer is used to remove moisture, the liquefaction unit is used to liquefy methane gas, and the gas-liquid separator is used to separate the gas and liquid to obtain hydrogen-rich tail gas, nitrogen-rich tail gas, and LNG product.

12. A method for preparing LNG, characterized in that, Includes the following steps: The converter gas and coke oven gas are mixed to obtain a mixed gas, which is then pressurized to obtain the raw material gas; The raw material gas is purified to obtain purified gas; The purified gas undergoes a methanation reaction to generate a product gas containing methane. The product gas is liquefied and gas-liquid separation is performed to obtain LNG product, hydrogen-rich tail gas and nitrogen-rich tail gas; The hydrogen-rich tail gas is transported into the mixed gas and pressurized to become part of the raw material gas.

13. The LNG preparation method according to claim 12, characterized in that, In the step of pressurizing the mixed gas, the gas pressure is first increased to 1.0~1.5MPa through a first-stage pressurization, and then increased to 2.5~2.8MPa through a second-stage pressurization. The hydrogen-rich tail gas is mixed with the gas after the first-stage pressurization, and then subjected to the second-stage pressurization.

14. The LNG preparation method according to claim 12, characterized in that, The step of subjecting the purified gas to a methanation reaction to generate a product gas containing methane includes: The purified gas is preheated, and the preheated purified gas is then sent to the methanation reactor. The real-time temperature of the purified gas entering the methanation reactor is detected. When the real-time temperature is greater than a preset value, the amount of purified gas entering the methanation reactor is reduced. When the real-time temperature is less than the preset value, the amount of purified gas entering the methanation reactor is increased. The purified gas enters a multi-stage series methane reactor for methanation to generate a product gas mainly composed of methane, and the heat generated during the reaction process is recovered.

15. The LNG preparation method according to claim 14, characterized in that, A multi-stage series methane reactor includes a primary methane reactor, a control valve connected in parallel with the primary methane reactor, and a secondary methane reactor located downstream of the primary methane reactor and the control valve. The step of the purified gas entering a multi-stage series-connected methane reactor for methanation includes: When the temperature inside the primary methane reactor reaches a first preset temperature, the amount of gas entering the primary methane reactor is reduced; the first preset temperature is less than or equal to 600°C. When the temperature inside the secondary methane reactor is lower than the second preset temperature, the amount of gas entering the secondary methane reactor is increased; the second preset temperature is less than or equal to 350°C.

16. The LNG preparation method according to claim 12, characterized in that, In the step of mixing converter gas and coke oven gas to obtain a mixed gas: the amount of converter gas and / or coke oven gas is adjusted so that the hydrogen-to-carbon ratio in the mixed gas is (2.8~3.2):1.

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