Biogas liquefaction system with ice blockage prevention desublimation and decarbonization mechanism and biogas liquefaction method

Abstract

The invention discloses a biogas liquefaction system with an anti-ice-blockage desublimation and decarburization mechanism and a biogas liquefaction method. The biogas liquefaction system comprises a raw material gas pretreatment mechanism, a VPSA nitrogen removal mechanism, a desublimation and decarburization mechanism, a biogas liquefaction mechanism and a cascade refrigeration mechanism, the VPSA nitrogen removal mechanism comprises two nitrogen adsorption towers, and the two nitrogen adsorption towers can alternately perform adsorption and regeneration; the desublimation and decarbonization mechanism comprises two desublimation separators, a cold pipe and a heat pipe are arranged in each desublimation separator, and the two desublimation separators can alternately perform desublimation and carbon dioxide crystallization and sublimation and carbon dioxide removal regeneration. The method has the advantages that the carbon dioxide is effectively separated from the biogas in a crystal form, so that the problem of ice crystal blockage caused by the carbon dioxide, water vapor or impurities at low temperature can be prevented, and the whole decarburization and liquefaction process can be stably and continuously operated for a long time.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of biogas liquefaction. BACKGROUND

[0002] With the wide application of biogas resources in small and medium-sized distributed scenarios, the efficient liquefaction of biogas to prepare bio-liquefied natural gas has become an important direction to improve its energy density and economic value. The removal of high-concentration carbon dioxide in biogas is a key pretreatment step for liquefaction. Although traditional chemical absorption and pressure swing adsorption methods are relatively mature, they have problems such as complex equipment, high energy consumption, and the need for chemical regenerations, which are not suitable for small and medium-sized scenarios. In recent years, low-temperature condensation decarburization technology has attracted attention due to its simple process and the absence of added substances. However, in actual operation, especially for small and medium-sized biogas with large fluctuations in gas quality, the core bottleneck is that carbon dioxide, water vapor, and trace impurities in the condensation process are easily co-crystallized on the low-temperature heat exchange surface, forming complex mixed ice blockages, which seriously damage heat transfer and block flow channels, leading to frequent system shutdown and ice removal, and unable to run stably and continuously. Existing ice blockage prevention methods mainly focus on single-factor control or passive ice removal, and lack of integrated solutions for active prevention and dynamic control of condensation decarburization processes, which restricts the reliability and economy of the technology in the field of small and medium-sized biogas liquefaction. Therefore, it is urgent to develop a biogas liquefaction decarburization system and method with high integration, strong adaptability, and the ability to fundamentally prevent ice blockage condensation.

[0003] Patent application CN202410844323.2 (A supercritical pressure low-temperature carbon capture with pressure liquefaction coupling process for biogas) provides a supercritical pressure low-temperature carbon capture with pressure liquefaction coupling process for biogas, which separates carbon dioxide by condensation under high pressure and low temperature conditions, and provides cold energy by combining a single-stage mixed refrigerant cycle. It has the advantages of high equipment integration and low investment cost. However, this invention and similar low-temperature condensation processes are based on the ideal premise of deep dehydration of the inlet gas and constant gas quality. In actual operation, the risk of "ice blockage" caused by the condensation and co-crystallization of carbon dioxide, water vapor, and trace impurities in the biogas on the low-temperature heat exchange surface is completely ignored. Therefore, this invention focuses on the active prevention and coordinated control of ice blockage in actual scenarios with large fluctuations in biogas quality and high operation and maintenance requirements, aiming to develop a condensation decarburization process that integrates ice blockage prevention mechanisms, can adapt to changes in inlet conditions, and can run stably for a long time, to solve the core obstacle from laboratory methods to engineering applications. SUMMARY

[0004] The technical problem to be solved by the present application is to provide a biogas liquefaction system with an ice blockage prevention condensation decarburization mechanism and a biogas liquefaction method, which can effectively remove carbon dioxide and prevent ice blockage, achieving continuous liquefaction of biogas.

[0005] To solve the above problems, the technical scheme adopted by the present application is: the biogas liquefaction system with anti-icing and blocking of the desorption mechanism, comprising a raw gas pretreatment mechanism, a VPSA nitrogen removal mechanism, a desorption mechanism, a biogas liquefaction mechanism and a cascade refrigeration mechanism; the VPSA nitrogen removal mechanism comprises two nitrogen adsorption towers, which can alternately adsorb and regenerate; the desorption mechanism comprises two desorption separators, each of which is provided with a cold pipe for providing cold energy to condense carbon dioxide in the biogas and a hot pipe for providing heat to sublimate the condensed carbon dioxide, and the two desorption separators can alternately condense carbon dioxide and sublimate carbon dioxide to regenerate; the cascade refrigeration mechanism is used to provide cold energy to the cold pipe in the two desorption separators.

[0006] Further, the aforementioned biogas liquefaction system with anti-icing and blocking of the desorption mechanism, wherein the VPSA nitrogen removal mechanism comprises a first gas buffer tank, a first manifold, a second gas flow regulating valve, a second manifold 16, a third gas flow regulating valve, a first nitrogen adsorption tower, a third manifold, a fourth gas flow regulating valve, a fourth manifold, a second gas buffer tank, a fifth gas flow regulating valve, a fifth manifold, a sixth gas flow regulating valve, a second nitrogen adsorption tower, a sixth manifold, a seventh gas flow regulating valve, an eighth gas flow regulating valve, a seventh manifold, a third gas buffer tank, and a vacuum pump; the outlet of the first gas buffer tank is connected with the inlet of the first manifold, the outlet of the first manifold is connected with the inlets of the second gas flow regulating valve and the fifth gas flow regulating valve, the outlet of the second gas flow regulating valve is connected with the inlet of the second manifold, the outlet of the second manifold is connected with the inlets of the third gas flow regulating valve and the first nitrogen adsorption tower, the outlet of the third gas flow regulating valve is connected with the inlet of the seventh manifold, the outlet of the first nitrogen adsorption tower is connected with the inlet of the third manifold, the outlet of the third manifold is connected with the inlets of the fourth gas flow regulating valve and the eighth gas flow regulating valve, the outlet of the fourth gas flow regulating valve is connected with the inlet of the fourth manifold, the eighth gas flow regulating valve is connected with the sixth manifold and the third manifold, the outlet of the fourth manifold is connected with the inlet of the second gas buffer tank, the outlet of the fifth gas flow regulating valve is connected with the inlet of the fifth manifold, the outlet of the fifth manifold is connected with the inlets of the sixth gas flow regulating valve and the second nitrogen adsorption tower, the outlet of the sixth gas flow regulating valve is connected with the inlet of the seventh manifold, the outlet of the seventh manifold is connected with the inlet of the third gas buffer tank, the outlet of the third gas buffer tank is connected with the vacuum pump, the outlet of the second nitrogen adsorption tower is connected with the inlet of the sixth manifold, the outlet of the sixth manifold is connected with the inlet of the seventh gas flow regulating valve, the outlet of the seventh gas flow regulating valve is connected with the inlet of the fourth manifold, and the outlet of the second gas buffer tank is connected with the biogas liquefaction mechanism.

[0007] Further, the biogas liquefaction system with anti-icing and anti-clogging desublimation and decarbonization mechanism as claimed in the preceding claim, wherein the biogas liquefaction mechanism comprises a first LNG heat exchanger, a first gas expander, a flow divider, a second LNG heat exchanger, a third throttle valve, a second gas expander, a fourth gas compressor, a third gas water cooler, a mixer, a fifth gas compressor, and a fourth gas water cooler; a second hot end outlet of the first LNG heat exchanger is connected with an inlet of the first gas expander, an outlet of the first gas expander is connected with an inlet of the flow divider, outlets of the flow divider are respectively connected with a first hot end inlet of the second LNG heat exchanger and a first cold end inlet of the first LNG heat exchanger, a first hot end outlet of the second LNG heat exchanger is connected with an inlet of the second gas expander, an outlet of the second gas expander is connected with a cold end inlet of the second LNG heat exchanger, a cold end outlet of the second LNG heat exchanger is connected with a second cold end inlet of the first LNG heat exchanger, a second cold end outlet of the first LNG heat exchanger is connected with an inlet of the fourth gas compressor, an outlet of the fourth gas compressor is connected with an inlet of the third gas water cooler, an outlet of the third gas water cooler is connected with an inlet of the mixer, a first cold end outlet of the first LNG heat exchanger is connected with an inlet of the mixer, an outlet of the mixer is connected with an inlet of the fifth gas compressor, an outlet of the fifth gas compressor is connected with an inlet of the fourth gas water cooler, and an outlet of the fourth gas water cooler is connected with a second hot end inlet of the first LNG heat exchanger; an outlet of the second gas buffer tank is connected with the first hot end inlet of the first LNG heat exchanger, a first hot end outlet of the first LNG heat exchanger is connected with an inlet of the desublimation and decarbonization mechanism, and an outlet of the desublimation and decarbonization mechanism is connected with a second hot end inlet of the second LNG heat exchanger, and a second hot end outlet of the second LNG heat exchanger is provided with the third throttle valve.

[0008] Further, the biogas liquefaction system with anti-icing and anti-clogging desublimation and decarbonization mechanism as claimed in the preceding claim, wherein the desublimation and decarbonization mechanism comprises a first three-way switch valve, a first desublimation separator, a second three-way switch valve, a second desublimation separator, a third three-way switch valve, and a fourth three-way switch valve; the first desublimation separator is provided with a first desublimation separator heat pipe and a first desublimation separator cold pipe; the second desublimation separator is provided with a second desublimation separator heat pipe and a second desublimation separator cold pipe; The first hot end outlet of the first LNG heat exchanger is connected with the inlet of the first three-way switch valve, the outlet of the first three-way switch valve is connected with the inlets of the first and second condensation separators respectively, the second separation outlet of the first condensation separator is connected with the inlet of the third three-way switch valve, the second separation outlet of the second condensation separator is connected with the inlet of the third three-way switch valve, the outlet of the third three-way switch valve is connected with the third cold end inlet of the first LNG heat exchanger, the third cold end outlet of the first LNG heat exchanger is connected with the inlet of the fourth three-way switch valve, a carbon dioxide storage tank is further arranged between the third cold end outlet of the first LNG heat exchanger and the inlet of the fourth three-way switch valve, the tenth flow regulating valve is arranged on the carbon dioxide storage tank input pipe, the eleventh flow regulating valve is arranged on the carbon dioxide storage tank output pipe, and the carbon dioxide storage tank input pipe and the carbon dioxide storage tank output pipe are both connected to the pipeline between the third cold end outlet of the first LNG heat exchanger and the inlet of the fourth three-way switch valve; the outlet of the fourth three-way switch valve is connected with the hot pipe inlets of the first and second condensation separators respectively, the first separation outlet of the first condensation separator is connected with the inlet of the second three-way switch valve, the first separation outlet of the second condensation separator is connected with the inlet of the second three-way switch valve, and the outlet of the second three-way switch valve is connected with the second hot end inlet of the second LNG heat exchanger.

[0009] Further, the biogas liquefaction system with the anti-icing condensation decarbonization mechanism, wherein the cascade refrigeration mechanism comprises a fifth three-way switch valve, a sixth three-way switch valve, a ninth gas flow regulating valve, a first cascade heat exchanger, a second gas compressor, a second cascade heat exchanger, a first throttling valve, a third gas compressor, a condenser and a second throttling valve; the outlet of the fifth three-way switch valve is connected with the cold pipe inlets of the first and second condensation separators respectively, the cold pipe outlet of the first condensation separator is connected with the inlet of the sixth three-way switch valve, the cold pipe outlet of the second condensation separator is connected with the inlet of the sixth three-way switch valve, the outlet of the sixth three-way switch valve is connected with the inlet of the ninth gas flow regulating valve, the outlet of the ninth gas flow regulating valve is connected with the hot end inlet of the first cascade heat exchanger, the hot end outlet of the first cascade heat exchanger is connected with the inlet of the second gas compressor, the outlet of the second gas compressor is connected with the hot end inlet of the second cascade heat exchanger, the hot end outlet of the second cascade heat exchanger is connected with the cold end inlet of the first cascade heat exchanger, the cold end outlet of the first cascade heat exchanger is connected with the inlet of the first throttling valve, the outlet of the first throttling valve is connected with the inlet of the fifth three-way switch valve, the cold end outlet of the second cascade heat exchanger is connected with the inlet of the third gas compressor, the outlet of the third gas compressor is connected with the inlet of the condenser, the outlet of the condenser is connected with the inlet of the second throttling valve, and the outlet of the second throttling valve is connected with the cold end inlet of the second cascade heat exchanger.

[0010] Further, the aforementioned biogas liquefaction system with anti-icing and anti-clogging desublimation decarbonization mechanism, wherein the raw gas pretreatment mechanism comprises a first gas flow regulating valve, a filter, a first gas-liquid separator, a first water trap, a sulfur-containing gas pre-absorption tower, a heater, a hydrolysis tower, a second water trap, a first gas water cooler, a sulfur-containing gas fine absorption tower, a first gas compressor, and a second gas water cooler; the outlet of the first gas flow regulating valve is connected to the inlet of the filter, the outlet of the filter is connected to the inlet of the first gas-liquid separator, the first gas-liquid separator is connected to the inlet of the first water trap, the outlet of the first gas-liquid separator is connected to the inlet of the sulfur-containing gas pre-absorption tower, the outlet of the sulfur-containing gas pre-absorption tower is connected to the inlet of the heater, the outlet of the heater is connected to the inlet of the hydrolysis tower, the hydrolysis tower is connected to the inlet of the second water trap, the outlet of the hydrolysis tower is connected to the inlet of the first gas water cooler, the outlet of the first gas water cooler is connected to the inlet of the sulfur-containing gas fine absorption tower, the outlet of the sulfur-containing gas fine absorption tower is connected to the inlet of the first gas compressor, the outlet of the first gas compressor is connected to the inlet of the second gas water cooler, and the outlet of the second gas water cooler is connected to the inlet of the first gas buffer tank.

[0011] A biogas liquefaction method, characterized in that the aforementioned biogas liquefaction system with anti-icing and anti-clogging desublimation decarbonization mechanism comprises the following steps: A pretreatment step: after the biogas is produced by the biogas fermentation device, the first gas flow regulating valve is opened, the biogas enters the filter, the impurities in the biogas are filtered out, and then the biogas enters the first gas-liquid separator for dehydration, the condensed water separated out is discharged from the first water trap, the dehydrated biogas enters the sulfur-containing gas pre-absorption tower to remove most of the sulfides, and then enters the heater for heating, the heated biogas enters the hydrolysis tower, the condensed water is discharged from the first water trap, and after the organic sulfur is hydrolyzed into hydrogen sulfide, the biogas enters the third gas water cooler, the cooled biogas enters the sulfur-containing gas fine absorption tower for further desulfurization, and the biogas after dehydration and desulfurization enters the first gas compressor and the second gas water cooler in sequence to complete the pressurization and cooling of the biogas; A denitrification step: the first nitrogen adsorption tower and the second nitrogen adsorption tower in the VPSA denitrogenation mechanism work alternately in adsorption and regeneration, and for example, the first nitrogen adsorption tower is adsorbed and the second nitrogen adsorption tower is regenerated, the steps are as follows: The biogas after desulfurization, dehydration, pressurization, and cooling enters the first gas buffer tank, the stabilized gas enters the first nitrogen adsorption tower through the first manifold, the second gas flow regulating valve, and the second manifold for denitrification, most of the biogas after the removal of nitrogen enters the second gas buffer tank through the third manifold, the fourth gas flow regulating valve, and the fourth manifold, and the stabilized denitrified biogas enters the first hot end inlet of the first LNG heat exchanger. Open the sixth gas flow regulating valve to connect the second nitrogen adsorption tower to the vacuum pump. The pressure inside the second nitrogen adsorption tower drops rapidly to negative pressure, and the adsorbed nitrogen is desorbed from the internal adsorption material. At this time, a small portion of the biogas after denitrification passes through the third manifold, the eighth gas flow regulating valve, and the sixth manifold to purge the nitrogen desorbed from the second nitrogen adsorption tower from top to bottom. This mixed gas is discharged from the system through the third gas buffer tank and the vacuum pump under the action of suction, thus completing the regeneration of the second nitrogen adsorption tower. Sublimation decarbonization process: The two sublimation separators in the sublimation decarbonization unit alternately perform sublimation crystallization of carbon dioxide and sublimation to remove carbon dioxide regeneration. Taking the first sublimation separator for sublimation crystallization of carbon dioxide and the second sublimation separator for sublimation to remove carbon dioxide regeneration as an example, the steps are as follows: After dehydration, desulfurization, and denitrification, and pressurization, the biogas containing carbon dioxide enters the hot end inlet of the first LNG heat exchanger. After being cooled, it flows out from the hot end outlet of the first LNG heat exchanger, passes through the first three-way switching valve, and enters the separation inlet of the first sublimation separator. When the carbon dioxide gas in the mixed gas passes through the cold pipe, it reaches the critical point of phase change and sublimates into solid carbon dioxide crystals. The biogas gas with carbon dioxide removed is cooled and flows out from the separation outlet of the first sublimation separator, passes through the second three-way switching valve, and enters the hot end inlet of the second LNG heat exchanger. When there are too many carbon dioxide crystals in the first sublimation separator, the process switches to the second sublimation separator to sublimate and crystallize carbon dioxide. The carbon dioxide storage tank pre-stores carbon dioxide gas. The carbon dioxide gas enters the heat pipe inlet of the second sublimation separator through the eleventh flow regulating valve and the fourth three-way switching valve. After releasing heat, the carbon dioxide flows out from the heat pipe outlet of the second sublimation separator. The solid carbon dioxide crystals inside the second sublimation separator are heated and undergo a phase change, sublimating into gas. It flows from the second separation outlet of the second sublimation separator through the third three-way switching valve into the third cold end inlet of the first LNG heat exchanger to release its cooling capacity. Then it flows out from the third cold end outlet of the first LNG heat exchanger. Part of it is stored in the carbon dioxide storage tank through the tenth flow regulating valve until the storage capacity in the carbon dioxide storage tank reaches the set value; part of it enters the heat pipe inlet of the second sublimation separator through the fourth three-way switching valve to provide heat. After releasing heat, it flows out from the heat pipe outlet of the second sublimation separator, and this cycle continues. After carbon dioxide is separated by condensation, the methane-containing biogas gas flows into the second hot end inlet of the second LNG heat exchanger. After releasing all the heat, it flows out from the second hot end outlet of the second LNG heat exchanger. After being depressurized and cooled by the third throttle valve, it is output as liquefied biogas. In the above process, the nitrogen gas sequentially passes through the fifth gas compressor and the fourth gas water cooler, the pressurized and cooled nitrogen gas enters the second hot end inlet of the first LNG heat exchanger, and after being preliminarily cooled, it flows out from the hot end outlet of the first LNG heat exchanger into the first gas expander, the expanded nitrogen gas passes through the flow divider, part of the nitrogen gas flows out from the flow divider into the first hot end inlet of the second LNG heat exchanger, and after being deeply cooled, it flows out from the first hot end outlet of the second LNG heat exchanger into the second gas expander, the expanded low-temperature gas flows into the cold end inlet of the second LNG heat exchanger, and after releasing part of the cold energy, it flows out from the cold end outlet of the second LNG heat exchanger into the second cold end inlet of the first LNG heat exchanger, and after releasing the remaining cold energy, it flows out from the second cold end outlet of the first LNG heat exchanger into the mixer after sequentially passing through the fourth gas compressor and the third gas water cooler, and the other part of the nitrogen gas flows out from the flow divider into the first cold end inlet of the first LNG heat exchanger, and after releasing the cold energy, it flows out from the cold end outlet of the first LNG heat exchanger into the mixer, and after the two parts of nitrogen gas are combined, they flow out of the mixer into the fifth gas compressor, thereby continuously circulating refrigeration. In the above process, the cooling medium R170 flows out from the cold end outlet of the first cascade heat exchanger, is depressurized and cooled by the first throttling valve, flows into the cold pipe inlet of the first desublimation separator through the fifth three-way valve, releases the cold energy, and flows out from the cold pipe outlet of the first desublimation separator, sequentially flows into the hot end inlet of the first cascade heat exchanger through the sixth three-way valve and the ninth gas flow regulating valve, the cooled fluid enters the second gas compressor, the compressed gas flows into the hot end inlet of the second cascade heat exchanger, the cooled fluid flows out from the hot end outlet of the second cascade heat exchanger into the cold end inlet of the first cascade heat exchanger, and after releasing the cold energy, the fluid flows out from the cold end outlet of the first cascade heat exchanger to complete the cycle; the cooling medium R404A flows out from the cold end outlet of the second cascade heat exchanger, sequentially enters the third gas compressor, the condenser and the second throttling valve, and flows into the cold end inlet of the second cascade heat exchanger, thereby continuously circulating refrigeration.

[0012] The advantages of the biogas liquefaction system with an anti-icing desublimation decarbonization mechanism and the biogas liquefaction method are as follows: 1. The desublimation decarbonization mechanism is provided, and two desublimation separators can alternately perform desublimation carbon dioxide crystallization and sublimation carbon dioxide removal regeneration, so that carbon dioxide can be effectively separated from biogas in a crystalline form, which can prevent ice crystal blockage caused by carbon dioxide, water vapor or impurities at low temperature, thereby ensuring that the entire decarbonization and liquefaction process can be long-term, stable and continuous, and overcoming the defects of frequent operation interruption and efficiency reduction in traditional low-temperature separation process.

[0013] II. The biogas liquefaction mechanism is deeply coupled and optimized with the nitrogen expansion refrigeration cycle and the carbon dioxide desublimation decarbonization process. The system realizes efficient cascade utilization of cold energy, significantly improves system energy efficiency, and greatly reduces unit product energy consumption. Moreover, the nitrogen expansion refrigeration cycle has high adaptability to fluctuations in the gas source conditions of raw biogas. Under wide gas conditions, compared with traditional mixed refrigerant refrigeration cycles, the system can maintain high liquefaction rate and stable low energy consumption operation, has strong anti-working condition fluctuation ability, and is particularly suitable for small to medium-sized biogas projects with unstable gas sources.

[0014] III. The raw gas pretreatment mechanism, VPSA denitrogenation gas mechanism, desublimation decarbonization mechanism, biogas liquefaction mechanism, and cascade refrigeration mechanism of the system are fully-process cooperatively optimized. Each module is cooperatively designed and optimized, has high overall integration, compact process flow, and high automation, and is particularly suitable for small and medium-sized application scenarios that are sensitive to land area, operation convenience, and initial investment. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 is a schematic diagram of the principle structure of the biogas liquefaction system with an anti-icing desublimation decarbonization mechanism according to the present application.

[0016] In the figure: 1, the first gas flow regulating valve, 2, the filter, 3, the first steam-water separator, 4, the first steam trap, 5, the sulfur-containing gas pre-absorption tower, 6, the heater, 7, the hydrolysis tower, 8, the second steam trap, 9, the first gas water cooler, 10, the sulfur-containing gas fine absorption tower, 11, the first gas compressor, 12, the second gas water cooler, 13, the first gas buffer tank, 14, the first header, 15, the second gas flow regulating valve, 16, the second header, 17, the third gas flow regulating valve, 18, the first nitrogen adsorption tower, 19, the third header, 20, the fourth gas flow regulating valve, 21, the fourth header, 22, the second gas buffer tank, 23, the fifth gas flow regulating valve, 24, the fifth header, 25, the sixth gas flow regulating valve, 26, the second nitrogen adsorption tower, 27, the sixth header, 28, the seventh gas flow regulating valve, 29, the eighth gas flow regulating valve, 30, the seventh header, 31, the third gas buffer tank, 32, the vacuum pump, 33, the first LNG heat exchanger, 33a, the first hot end inlet of the first LNG heat exchanger, 33b, the first hot end outlet of the first LNG heat exchanger, 33c, the second hot end inlet of the first LNG heat exchanger, 33d, the second hot end outlet of the first LNG heat exchanger, 33e, the third cold end inlet of the first LNG heat exchanger, 33f, the third cold end outlet of the first LNG heat exchanger, 33g, the first cold end inlet of the first LNG heat exchanger, 33h, the first cold end outlet of the first LNG heat exchanger, 33i, the second cold end inlet of the first LNG heat exchanger, 33j, the second cold end outlet of the first LNG heat exchanger, 34, the first three-way switch valve, 35, the first desublimation separator, (35e, 35f), the hot pipe of the first desublimation separator, (35c, 35d), the cold pipe of the first desublimation separator, 36, the second three-way switch valve, 37, the second desublimation separator, (37e, 37f), the hot pipe of the second desublimation separator, (37c,37d) Second condensate separator cold pipe; 38) Third three-way switching valve; 39) Fourth three-way switching valve; 40) Fifth three-way switching valve; 41) Sixth three-way switching valve; 42) Ninth gas flow regulating valve; 43) First cascade heat exchanger; 44) Second gas compressor; 45) Second cascade heat exchanger; 46) First throttle valve; 47) Third gas compressor; 48) Condenser; 49) Second throttle valve; 50) First gas expander; 51) Flow divider; 52) Second LNG heat exchanger; 52a) Second hot end inlet of the second LNG heat exchanger; 52b) Second... 52c, the second hot end outlet of the LNG heat exchanger; 52d, the first hot end inlet of the second LNG heat exchanger; 52e, the first cold end inlet of the second LNG heat exchanger; 52f, the second hot and cold end outlets of the second LNG heat exchanger; 53, the third throttle valve; 54, the second gas expander; 55, the fourth gas compressor; 56, the third gas water cooler; 57, the mixer; 58, the fifth gas compressor; 59, the fourth gas water cooler; 60, the tenth flow control valve; 61, the carbon dioxide storage tank; 62, the eleventh flow control valve. Detailed Implementation

[0017] The present invention will now be described in further detail with reference to the accompanying drawings and preferred embodiments.

[0018] like Figure 1 As shown, the biogas liquefaction system with an anti-icing, sublimation, and decarbonization mechanism includes a feed gas pretreatment unit, a VPSA nitrogen removal unit, a sublimation and decarbonization unit, a biogas liquefaction unit, and a cascade refrigeration unit. The VPSA nitrogen removal unit includes two nitrogen adsorption towers that can alternately adsorb and regenerate. The sublimation and decarbonization unit includes two sublimation separators, each equipped with a cold pipe for sublimating and crystallizing carbon dioxide in the biogas and a hot pipe for sublimating the crystallized carbon dioxide. The two sublimation separators can alternately perform sublimation and crystallization of carbon dioxide and sublimation to remove carbon dioxide and regenerate. The cascade refrigeration unit provides cooling to the cold pipes in the two sublimation separators.

[0019] The raw material gas pre-treatment mechanism comprises a first gas flow regulating valve 1, a filter 2, a first gas-water separator 3, a first water trap 4, a sulfur-containing gas pre-absorption tower 5, a heater 6, a hydrolysis tower 7, a second water trap 8, a first gas water cooler 9, a sulfur-containing gas fine absorption tower 10, a first gas compressor 11, and a second gas water cooler 12. The outlet of the first gas flow regulating valve 1 is connected with the inlet of the filter 2, the outlet of the filter 2 is connected with the inlet of the first gas-water separator 3, the first gas-water separator 3 is provided with the first water trap 4, the outlet of the first gas-water separator 3 is connected with the inlet of the sulfur-containing gas pre-absorption tower 5, the outlet of the sulfur-containing gas pre-absorption tower 5 is connected with the inlet of the heater 6, the outlet of the heater 6 is connected with the inlet of the hydrolysis tower 7, the hydrolysis tower 7 is provided with the second water trap 8, the outlet of the hydrolysis tower 7 is connected with the inlet of the first gas water cooler 9, the outlet of the first gas water cooler 9 is connected with the inlet of the sulfur-containing gas fine absorption tower 10, the outlet of the sulfur-containing gas fine absorption tower 10 is connected with the inlet of the first gas compressor 11, the outlet of the first gas compressor 11 is connected with the inlet of the second gas water cooler 12, and the outlet of the second gas water cooler 12 is connected with the inlet of the first gas buffer tank 13.

[0020] The biogas produced by the fermentation tank enters the filter 2 through the first gas flow regulating valve 1, the filter 2 can prevent dust or solid particles from entering the compressor to cause abrasion and blockage, the gas flows out of the filter 2 and then enters the first gas-water separator 3 in sequence, the condensed water separated out is discharged from the first water trap 4, in order to prevent the sulfur-containing gas in the biogas from affecting the subsequent liquefaction process, the dehydrated gas enters the sulfur-containing gas pre-absorption tower 5 to remove most of the hydrogen sulfide, then enters the heater 6, the heated gas enters the hydrolysis tower 7, the condensed water thereof is discharged from the second water trap 8, after the organic sulfur is hydrolyzed into hydrogen sulfide, the biogas enters the third gas water cooler 9, the cooled biogas enters the sulfur-containing gas fine absorption tower 10 for further desulfurization, and the biogas after desulfurization and dehydration enters the first gas compressor 11 and the second gas water cooler 12 in sequence for pressure boosting and temperature reduction.

[0021] The VPSA denitrogen mechanism comprises a first gas buffer tank 13, a first manifold 14, a second gas flow regulating valve 15, a second manifold 16, a third gas flow regulating valve 17, a first nitrogen adsorption tower 18, a third manifold 19, a fourth gas flow regulating valve 20, a fourth manifold 21, a second gas buffer tank 22, a fifth gas flow regulating valve 23, a fifth manifold 24, a sixth gas flow regulating valve 25, a second nitrogen adsorption tower 26, a sixth manifold 27, a seventh gas flow regulating valve 28, an eighth gas flow regulating valve 29, a seventh manifold 30, a third gas buffer tank 31, and a vacuum pump 32. The outlet of the first gas buffer tank 13 is connected to the inlet of the first manifold 14, the outlet of the first manifold 14 is connected to the inlets of the second gas flow regulating valve 15 and the fifth gas flow regulating valve 23 respectively, the outlet of the second gas flow regulating valve 15 is connected to the inlet of the second manifold 16, the outlet of the second manifold 16 is connected to the inlets of the third gas flow regulating valve 17 and the first nitrogen adsorption tower 18 respectively, the outlet of the third gas flow regulating valve 17 is connected to the inlet of the seventh manifold 30, the outlet of the first nitrogen adsorption tower 18 is connected to the inlet of the third manifold 19, the outlet of the third manifold 19 is connected to the inlets of the fourth gas flow regulating valve 20 and the eighth gas flow regulating valve 29 respectively, the outlet of the fourth gas flow regulating valve 20 is connected to the inlet of the fourth manifold 21, the eighth gas flow regulating valve 29 is connected to the sixth manifold 27 and the third manifold 19 respectively, the outlet of the fourth manifold 21 is connected to the inlet of the second gas buffer tank 22, the outlet of the fifth gas flow regulating valve 23 is connected to the inlet of the fifth manifold 24, the outlet of the fifth manifold 24 is connected to the inlets of the sixth gas flow regulating valve 25 and the second nitrogen adsorption tower 26 respectively, the outlet of the sixth gas flow regulating valve 25 is connected to the inlet of the seventh manifold 30, the outlet of the seventh manifold 30 is connected to the inlet of the third gas buffer tank 31, the outlet of the third gas buffer tank 31 is connected to the vacuum pump 32, the outlet of the second nitrogen adsorption tower 26 is connected to the inlet of the sixth manifold 27, the outlet of the sixth manifold 27 is connected to the inlet of the seventh gas flow regulating valve 28, the outlet of the seventh gas flow regulating valve 28 is connected to the inlet of the fourth manifold 21, and the outlet of the second gas buffer tank 22 is connected to the biogas liquefaction mechanism.

[0022] The VPSA denitrogenation mechanism uses a double-tower pressure swing adsorption system, i.e. two nitrogen adsorption towers are provided, which can alternately perform adsorption and regeneration to achieve continuous nitrogen removal. The first nitrogen adsorption tower 18 and the second nitrogen adsorption tower 26 alternately perform adsorption and regeneration. Take the first nitrogen adsorption tower 18 adsorption and the second nitrogen adsorption tower 26 regeneration as an example for illustration. In order to provide stable feeding conditions for the adsorption tower, the gas flowing out from the second gas water cooler 12 enters the first gas buffer tank 13. The second gas flow regulating valve 15 is in an open state, the fifth gas flow regulating valve 23 is in a closed state, and the gas in the first gas buffer tank 13 flows into the first nitrogen adsorption tower 18 through the first manifold 14 and the second gas flow regulating valve 15. The gas passes through the bed filled with adsorption material from bottom to top, and nitrogen is adsorbed at the bottom and middle of the bed. Methane and carbon dioxide pass through the entire bed to the top of the tower while the impurities are adsorbed. The majority of biogas gas containing methane and carbon dioxide gathered at the top of the first nitrogen adsorption tower 18 enters the second gas buffer tank 22 through the third manifold 19, the fourth gas flow regulating valve 20 and the fourth manifold 21. A small part of the biogas gas enters the second nitrogen adsorption tower 26 from top to bottom through the eighth gas flow regulating valve 29 and the sixth manifold 27 for purging. At the same time, the sixth gas flow regulating valve 25 is opened to connect the second nitrogen adsorption tower 26 with the vacuum pump 32, and the pressure in the second nitrogen adsorption tower 26 rapidly decreases to negative pressure. The adsorbed nitrogen is desorbed from the adsorption material inside, and the desorbed nitrogen in the second nitrogen adsorption tower 26 forms a mixed gas with the purge gas. The mixed gas is discharged from the system through the third gas buffer tank 31 and the vacuum pump 32 under the action of suction force until the regeneration of the second nitrogen adsorption tower 26 is completed. The regenerated second nitrogen adsorption tower 26 is ready for the next adsorption work. When the second gas flow regulating valve 15 is in a closed state and the fifth gas flow regulating valve 23 is in an open state, the first nitrogen adsorption tower 18 is regenerated and the second nitrogen adsorption tower 26 is adsorbed. The adsorption process of the second nitrogen adsorption tower is described above.

[0023] The biogas liquefaction mechanism comprises a first LNG heat exchanger 33, a first gas expander 50, a flow divider 51, a second LNG heat exchanger 52, a third throttle valve 53, a second gas expander 54, a fourth gas compressor 55, a third gas water cooler 56, a mixer 57, a fifth gas compressor 58, and a fourth gas water cooler 59. The second hot end outlet 33d of the first LNG heat exchanger is connected with the inlet of the first gas expander 50. The outlet of the first gas expander 50 is connected with the inlet of the flow divider 51. The outlet of the flow divider 51 is respectively connected with the first hot end inlet 52c of the second LNG heat exchanger and the first cold end inlet 33g of the first LNG heat exchanger. The first hot end outlet 52d of the second LNG heat exchanger is connected with the inlet of the second gas expander 54. The outlet of the second gas expander 54 is connected with the cold end inlet 52e of the second LNG heat exchanger. The cold end outlet 52f of the second LNG heat exchanger is connected with the second cold end inlet 33i of the first LNG heat exchanger. The second cold end outlet 33j of the first LNG heat exchanger is connected with the inlet of the fourth gas compressor 55. The outlet of the fourth gas compressor 55 is connected with the inlet of the third gas water cooler 56. The outlet of the third gas water cooler 56 is connected with the inlet of the mixer 57. The first cold end outlet 33h of the first LNG heat exchanger is connected with the inlet of the mixer 57. The outlet of the mixer 57 is connected with the inlet of the fifth gas compressor 58. The outlet of the fifth gas compressor 58 is connected with the inlet of the fourth gas water cooler 59. The outlet of the fourth gas water cooler 59 is connected with the second hot end inlet 33c of the first LNG heat exchanger. The outlet of the second gas buffer tank 22 is connected with the first hot end inlet 33a of the first LNG heat exchanger. The first hot end outlet 33b of the first LNG heat exchanger is connected with the inlet of the desublimation decarbonization mechanism. The outlet of the desublimation decarbonization mechanism is connected with the second hot end inlet 52a of the second LNG heat exchanger. The second hot end outlet 52b of the second LNG heat exchanger is provided with the third throttle valve 53.

[0024] The nitrogen gas is compressed by the fifth gas compressor 58 and the fourth gas water cooler 59, and the compressed and cooled nitrogen gas enters the second hot end inlet 33c of the first LNG heat exchanger, and after being preliminarily cooled, flows out from the second hot end outlet 33d of the first LNG heat exchanger and enters the first gas expander 50. The expanded nitrogen gas enters the flow divider 51, and a part of the nitrogen gas flows out and enters the first hot end inlet 52c of the second LNG heat exchanger, and after being deeply cooled, flows out from the first hot end outlet 52d of the second LNG heat exchanger and enters the second gas expander 54. The expanded low-temperature gas flows into the cold end inlet 52e of the second LNG heat exchanger, releases part of the cold energy, and then flows out from the cold end outlet 52f of the second LNG heat exchanger, enters the second cold end inlet 33i of the first LNG heat exchanger, releases the remaining cold energy, and then flows out from the second cold end outlet 33j of the first LNG heat exchanger, sequentially passes through the fourth gas compressor 55 and the third gas water cooler 56, and then enters the mixer 57. Another part of the nitrogen gas flows out from the flow divider 51, enters the first cold end inlet 33g of the first LNG heat exchanger, releases the cold energy, and then flows out from the first cold end outlet 33h of the first LNG heat exchanger and enters the mixer 57. The two parts of the nitrogen gas flow out from the mixer 57, are sequentially compressed by the fifth gas compressor 58 and cooled by the fourth gas water cooler 59. The above-mentioned nitrogen gas circulation provides the cold energy for the biogas liquefaction.

[0025] The desublimation decarbonization mechanism comprises a first three-way switch valve 34, a first desublimation separator 35, a second three-way switch valve 36, a second desublimation separator 37, a third three-way switch valve 38, and a fourth three-way switch valve 39. The first hot end outlet 33b of the first LNG heat exchanger is connected with the inlet of the first three-way switch valve 34, the outlet of the first three-way switch valve 34 is respectively connected with the inlet of the first condensation separator 35 and the second condensation separator 37, the second separation outlet 35g of the first condensation separator is connected with the inlet of the third three-way switch valve 38, the second separation outlet 37g of the second condensation separator is connected with the inlet of the third three-way switch valve 38, the outlet of the third three-way switch valve 38 is connected with the third cold end inlet 33e of the first LNG heat exchanger, the third cold end outlet 33f of the first LNG heat exchanger is connected with the inlet of the fourth three-way switch valve 39, and the third cold end outlet 33f of the first LNG heat exchanger is further provided with the carbon dioxide storage tank 61 between the inlet of the fourth three-way switch valve 39, the tenth flow regulating valve 60 is arranged on the carbon dioxide storage tank input pipe, the eleventh flow regulating valve 62 is arranged on the carbon dioxide storage tank output pipe, and the carbon dioxide storage tank input pipe and the carbon dioxide storage tank output pipe are connected to the pipeline between the third cold end outlet 33f of the first LNG heat exchanger and the inlet of the fourth three-way switch valve 39; the outlet of the fourth three-way switch valve 39 is respectively connected with the hot pipe inlet 35e of the first condensation separator and the hot pipe inlet 37e of the second condensation separator, the first separation outlet 35b of the first condensation separator is connected with the inlet of the second three-way switch valve 36, the first separation outlet 37b of the second condensation separator is connected with the inlet of the second three-way switch valve 36, and the outlet of the second three-way switch valve 36 is connected with the second hot end inlet 52a of the second LNG heat exchanger.

[0026] The cascade refrigeration mechanism comprises a fifth three-way switching valve 40, a sixth three-way switching valve 41, a ninth gas flow regulating valve 42, a first cascade heat exchanger 43, a second gas compressor 44, a second cascade heat exchanger 45, a first throttling valve 46, a third gas compressor 47, a condenser 48, and a second throttling valve 49. The outlet of the fifth three-way switching valve 40 is connected to the cold pipe inlet 35c of the first desublimator and the cold pipe inlet 37c of the second desublimator respectively. The cold pipe outlet 35d of the first desublimator is connected to the inlet of the sixth three-way switching valve 41. The cold pipe outlet 37d of the second desublimator is connected to the inlet of the sixth three-way switching valve 41. The outlet of the sixth three-way switching valve 41 is connected to the inlet of the ninth gas flow regulating valve 42. The outlet of the ninth gas flow regulating valve 42 is connected to the hot end inlet 43a of the first cascade heat exchanger 43. The hot end outlet 43b of the first cascade heat exchanger is connected to the inlet of the second gas compressor 44. The outlet of the second gas compressor 44 is connected to the hot end inlet 45a of the second cascade heat exchanger 45. The hot end outlet 45b of the second cascade heat exchanger is connected to the cold end inlet 43c of the first cascade heat exchanger 43. The cold end outlet 43d of the first cascade heat exchanger is connected to the inlet of the first throttling valve 46. The outlet of the first throttling valve 46 is connected to the inlet of the fifth three-way switching valve 40. The cold end outlet 45d of the second cascade heat exchanger is connected to the inlet of the third gas compressor 47. The outlet of the third gas compressor 47 is connected to the inlet of the condenser 48. The outlet of the condenser 48 is connected to the inlet of the second throttling valve 49. The outlet of the second throttling valve 49 is connected to the cold end inlet 45c of the second cascade heat exchanger.

[0027] The desublimation decarburization mechanism is provided with two desublimators. Each desublimator is provided with a cold pipe for providing cold to sublimate carbon dioxide in biogas and a hot pipe for providing heat to sublimate carbon dioxide. The two desublimators can alternately sublimate carbon dioxide and regenerate by sublimating and removing carbon dioxide. By alternately sublimating carbon dioxide and regenerating by sublimating and removing carbon dioxide through the two desublimators, continuous removal of carbon dioxide can be achieved, and ice blockage can be effectively avoided. The alternation of the two desublimators is achieved by switching through the first three-way switching valve 34, the second three-way switching valve 36, the third three-way switching valve 38, and the fourth three-way switching valve 39.

[0028] Taking the first desublimator 35 as an example, carbon dioxide is sublimated and crystallized, and the second desublimator 37 is regenerated by sublimating and removing carbon dioxide.

[0029] The biogas containing methane and carbon dioxide after dehydration, desulfurization and denitrification and being pressurized enters the first hot end inlet 33a of the LNG heat exchanger, is cooled and then flows out from the first hot end outlet 33b of the first LNG heat exchanger, enters the separation inlet 35a of the first condensation separator 35 through the first three-way switch valve 34, and the carbon dioxide gas in the mixed gas reaches the phase change critical point when passing through the first condensation separator cold pipe (35c, 35d), and is condensed into solid carbon dioxide crystals. The biogas after the removal of carbon dioxide is cooled and then flows out from the first separation outlet 35b of the first condensation separator 35, and enters the second hot end inlet 52a of the second LNG heat exchanger through the second three-way switch valve 36.

[0030] The cooling medium R170 flows out from the cold end outlet 43d of the first cascade heat exchanger, is depressurized and cooled by the first throttling valve 46, then flows into the first condensation separator from the cold end inlet 35c through the fifth three-way switch valve 40, flows out from the cold pipe outlet 35d of the first condensation separator after releasing the cooling capacity, and then flows into the hot end inlet 43a of the first cascade heat exchanger through the sixth three-way switch valve 41 and the ninth gas flow regulating valve 42 in sequence. The cooled fluid enters the second gas compressor 44, the compressed gas flows into the hot end inlet 45a of the second cascade heat exchanger, the cooled fluid flows out from the hot end outlet 45b of the second cascade heat exchanger and enters the cold end inlet 43c of the first cascade heat exchanger, and the fluid flows out from the cold end outlet 43d of the first cascade heat exchanger after releasing the cooling capacity to complete the cycle. The cooling medium R404A flows out from the cold end outlet 45d of the second cascade heat exchanger and then enters the third gas compressor 47, the condenser 48 and the second throttling valve 49 in sequence, and then enters the cold end inlet 45c of the second cascade heat exchanger. The cooling medium R170 and the cooling medium R404A continuously circulate to provide cooling capacity for the condensation crystallization of the first condensation separator to remove carbon dioxide. When the second condensation separator is used for condensation crystallization to remove carbon dioxide, the cooling medium R170 and the cooling medium R404A continuously circulate to provide cooling capacity for the condensation crystallization of the second condensation separator to remove carbon dioxide, and the fifth three-way switch valve 40 and the sixth three-way switch valve 41 are switched.

[0031] The second desublimation separator 37 sublimes and removes carbon dioxide for regeneration: The carbon dioxide gas is pre-stored in the carbon dioxide storage tank 61, and the carbon dioxide gas enters the heat pipe inlet 37e of the second desublimation separator 37 through the eleventh flow regulating valve 62 and the fourth three-way switch valve 39. The carbon dioxide releasing heat flows out from the heat pipe outlet 37f of the second desublimation separator 37. The solid carbon dioxide crystals in the second desublimation separator 37 are heated to sublimate into gas, and then flow into the third cold end inlet 33e of the first LNG heat exchanger through the second separation outlet 37g of the second desublimation separator and the third three-way switch valve 38 to release cold. Then, the carbon dioxide flows out from the third cold end outlet 33f of the first LNG heat exchanger. Part of the carbon dioxide is stored in the carbon dioxide storage tank 61 through the tenth flow regulating valve 60 until the storage amount in the carbon dioxide storage tank 61 reaches the set value, and then the tenth flow regulating valve 60 is closed. Part of the carbon dioxide enters the heat pipe inlet 37e of the second desublimation separator through the fourth three-way switch valve 39 to provide heat, and then flows out from the heat pipe outlet 37f of the second desublimation separator after releasing heat. The cycle is repeated until the carbon dioxide crystals in the second desublimation separator 37 are exhausted. The carbon dioxide generated by the desublimation separator is used as a heating medium to sublimate the carbon dioxide crystals, which effectively reduces the equipment cost and the sublimation energy consumption.

[0032] The above-mentioned process of sublimating and removing carbon dioxide for regeneration in the second desublimation separator 37 is based on the premise that the second desublimation separator 37 has already crystallized carbon dioxide by desublimation. When the device is started, one desublimation separator crystallizes carbon dioxide by desublimation, and the other desublimation separator waits. When the carbon dioxide crystals in one desublimation separator are too much, the desublimation is switched to the other desublimation separator to crystallize carbon dioxide by desublimation. The desublimation separator with carbon dioxide crystals is used for sublimation to remove carbon dioxide for regeneration. Then, the crystallization by desublimation and the sublimation for regeneration are alternately performed.

[0033] The carbon dioxide crystals are separated from the biogas, and the biogas gas from which the carbon dioxide is removed flows out from the first separation outlet 35b of the first desublimation separator after being cooled, enters the second hot end inlet 52a of the second LNG heat exchanger through the second three-way switch valve 36, and is discharged from the second hot end outlet 52b of the second LNG heat exchanger after being cooled in the second LNG heat exchanger. The liquefied biogas is formed by throttling through the third throttling valve 53 and then outputted outward.

[0034] A biogas liquefaction method using the above-mentioned biogas liquefaction system with an anti-icing desublimation carbon removal mechanism includes the following steps: Pre-treatment step: After the biogas is produced in the biogas fermentation device, the first gas flow regulating valve 1 is opened, and the biogas enters the filter 2, and after the impurities in the biogas are filtered out, the biogas enters the first gas-water separator 3 to be dehydrated, and the condensed water separated out is discharged from the first water trap valve 4, and the dehydrated biogas enters the sulfur-containing gas pre-absorption tower 5 to be pre-removed of sulfides, and then enters the heater 6 to be heated, and the heated biogas enters the hydrolysis tower 7, and the condensed water is discharged from the first water trap valve 8, and after the organic sulfur is hydrolyzed into hydrogen sulfide, the biogas enters the third gas water cooler 9, and the cooled biogas enters the sulfur-containing gas fine absorption tower 10 to be further desulfurized, and the biogas after being dehydrated and desulfurized enters the first gas compressor 11, the second gas water cooler 12 in sequence to be pressurized and cooled and stored in the first gas buffer tank 13.

[0035] Denitrification step: the first nitrogen adsorption tower 18 and the second nitrogen adsorption tower 26 in the VPSA denitrogenation mechanism perform alternating work of adsorption and regeneration in sequence, taking the first nitrogen adsorption tower 18 adsorption and the second nitrogen adsorption tower 26 regeneration as an example, the steps are as follows.

[0036] The second gas flow regulating valve 15 is in an open state, and the gas in the first gas buffer tank 13 enters the first nitrogen adsorption tower 18 to be denitrified through the first manifold 14, the second gas flow regulating valve 15, and the second manifold 16, and most of the biogas after being denitrified enters the second gas buffer tank 22 through the third manifold 19, the fourth gas flow regulating valve 20, and the fourth manifold 21 in sequence, and the denitrified biogas in the second gas buffer tank 22 enters the first hot end inlet 33a of the LNG heat exchanger.

[0037] The sixth gas flow regulating valve 5 is opened to connect the second nitrogen adsorption tower 26 with the vacuum pump 32, the pressure in the second nitrogen adsorption tower 26 rapidly drops to negative pressure, and the adsorbed nitrogen is desorbed from the internal adsorbent material, at this time, a small part of the biogas after denitrification passes through the third manifold 19, the eighth gas flow regulating valve 29, and the sixth manifold 27 to sweep the nitrogen desorbed from the second nitrogen adsorption tower 26 from top to bottom, and this part of mixed gas is discharged from the system in sequence through the third gas buffer tank 31 and the vacuum pump 32 under the action of suction, and the regeneration of the second nitrogen adsorption tower 26 is completed.

[0038] Desublimation decarbonization step: the two desublimation separators in the desublimation decarbonization mechanism alternately perform desublimation crystallization to remove carbon dioxide and sublimation to remove carbon dioxide for regeneration, taking the first desublimation separator 35 to perform desublimation crystallization of carbon dioxide and the second desublimation separator 37 to perform sublimation to remove carbon dioxide for regeneration as an example, the steps are as follows.

[0039] Sublimation of carbon dioxide: The biogas containing carbon dioxide after dehydration, desulfurization and denitrification and being pressurized enters the first hot end inlet 33a of the LNG heat exchanger, is cooled to flow out from the first hot end outlet 33b of the first LNG heat exchanger, enters the separation inlet 35a of the first desublimation separator through the first three-way switch valve 34, and the carbon dioxide gas in the mixed gas reaches the phase change critical point when passing through the cold pipes (35c, d), and is desublimated into solid carbon dioxide crystals. The biogas gas after removal of carbon dioxide is cooled to flow out from the first separation outlet 35b of the first desublimation separator, and enters the hot end inlet 52a of the second LNG heat exchanger through the second three-way switch valve 36; when the carbon dioxide crystals in the first desublimation separator 35 are too much, the second desublimation separator 37 is switched to desublimation of carbon dioxide.

[0040] Sublimation of carbon dioxide: The biogas containing carbon dioxide after dehydration, desulfurization and denitrification and being pressurized enters the first hot end inlet 33a of the LNG heat exchanger, is cooled to flow out from the first hot end outlet 33b of the first LNG heat exchanger, enters the separation inlet 35a of the first desublimation separator through the first three-way switch valve 34, and the carbon dioxide gas in the mixed gas reaches the phase change critical point when passing through the cold pipes (35c, d), and is desublimated into solid carbon dioxide crystals. The biogas gas after removal of carbon dioxide is cooled to flow out from the first separation outlet 35b of the first desublimation separator, and enters the hot end inlet 52a of the second LNG heat exchanger through the second three-way switch valve 36; when the carbon dioxide crystals in the first desublimation separator 35 are too much, the second desublimation separator 37 is switched to desublimation of carbon dioxide.

[0041] Sublimation of carbon dioxide: The biogas containing carbon dioxide after dehydration, desulfurization and denitrification and being pressurized enters the first hot end inlet 33a of the LNG heat exchanger, is cooled to flow out from the first hot end outlet 33b of the first LNG heat exchanger, enters the separation inlet 35a of the first desublimation separator through the first three-way switch valve 34, and the carbon dioxide gas in the mixed gas reaches the phase change critical point when passing through the cold pipes (35c, d), and is desublimated into solid carbon dioxide crystals. The biogas gas after removal of carbon dioxide is cooled to flow out from the first separation outlet 35b of the first desublimation separator, and enters the hot end inlet 52a of the second LNG heat exchanger through the second three-way switch valve 36; when the carbon dioxide crystals in the first desublimation separator 35 are too much, the second desublimation separator 37 is switched to desublimation of carbon dioxide.

[0042] In the above process, the nitrogen gas is compressed by the fifth gas compressor 58 and the fourth gas water cooler 59, and the compressed and cooled nitrogen gas enters the second hot end inlet 33c of the first LNG heat exchanger. After being preliminarily cooled, the nitrogen gas flows out from the second hot end outlet 33d of the first LNG heat exchanger, enters the first gas expander 50, and is expanded. The expanded nitrogen gas passes through the flow divider 51, and part of the nitrogen gas flows out, enters the first hot end inlet 52c of the second LNG heat exchanger, and is deeply cooled. After being deeply cooled, the nitrogen gas flows out from the first hot end outlet 52d of the second LNG heat exchanger, enters the second gas expander 54, and is expanded. The expanded low-temperature gas flows into the second LNG heat exchanger from the cold end inlet 52e, releases part of the cold energy, and then flows out from the cold end outlet 52f. The low-temperature gas enters the second cold end inlet 33i of the first LNG heat exchanger, releases the remaining cold energy, and then flows out from the second cold end outlet 33j. After sequentially passing through the fourth gas compressor 55 and the third gas water cooler 56, the low-temperature gas enters the mixer 57. The other part of the nitrogen gas flows out from the flow divider 51, enters the first cold end inlet 33g of the first LNG heat exchanger, releases the cold energy, and then flows out from the first cold end outlet 33h. The two parts of the nitrogen gas flow out of the mixer 57, enter the fifth gas compressor 58, and are compressed. The above nitrogen circulation provides cold energy for biogas liquefaction.

[0043] In the above process, the cooling medium R170 flows out from the cold end outlet 43d of the first cascade heat exchanger, is depressurized and cooled by the first throttling valve 46, and then flows into the cold pipe inlet 35c of the first desublimation separator through the fifth three-way valve 40. After releasing the cold energy, the cooling medium R170 flows out from the cold pipe outlet 35d of the first desublimation separator, sequentially flows into the first cascade heat exchanger through the sixth three-way valve 41 and the ninth gas flow regulating valve 42, and enters the hot end inlet 43a of the first cascade heat exchanger. The cooled fluid enters the second gas compressor 44, is compressed, and then flows into the hot end inlet 45a of the second cascade heat exchanger. The cooled fluid flows out from the hot end outlet 45b of the second cascade heat exchanger, enters the cold end inlet 43c of the first cascade heat exchanger, releases the cold energy, and then flows out from the cold end outlet 43d of the first cascade heat exchanger to complete the circulation. The cooling medium R404A flows out from the cold end outlet 45d of the second cascade heat exchanger, sequentially enters the third gas compressor 47, the condenser 48, and the second throttling valve 49, and then flows into the cold end inlet 45c of the second cascade heat exchanger. The cooling medium R170 and the cooling medium R404A continuously circulate to provide cold energy for the first desublimation separator to desublimate and crystallize carbon dioxide. When the second desublimation separator desublimate and crystallize carbon dioxide, the cooling medium R170 and the cooling medium R404A continuously circulate to provide cold energy for the second desublimation separator to desublimate and crystallize carbon dioxide.

[0044] The specific operating parameters of the biogas liquefaction system using nitrogen expansion refrigeration during normal operation are described below: Take the first condensation separator 35 condensation carbon dioxide crystallization, second condensation separator 37 sublimation carbon dioxide regeneration as an example, 35 ℃, 0.1 MPa of biogas raw gas through the first gas flow regulating valve 1, filter 2, first water gas separator 3, first hydrophobic valve 4, sulfur containing gas pre absorption tower 5, heater 6, hydrolysis tower 7, second hydrophobic valve 8, first gas water cooler 9, sulfur containing gas fine absorption tower 10 into the first gas compressor 11 is compressed to 0.49 MPa. Subsequently into the second gas water cooler 12 is cooled to 40 ℃, into VPSA denitrogenation mechanism, after denitrogenation of biogas gas into the first LNG heat exchanger 33 is cooled to-50 ℃, then into the first condensation separator 35, condensation carbon dioxide crystallization is cooled to-68 ℃, biogas gas flows out of the first condensation separator 35, after the second LNG heat exchanger 52 gas is cooled to-152 ℃, then through the third throttle valve 53 after biogas is liquefied output. The solid carbon dioxide crystal is heated by the heat pipe (37e, 37f) in the second condensation separator 37, and is sublimated into gas as a cold source. The gaseous carbon dioxide at-56 ℃ flows into the first LNG heat exchanger 33, and is warmed up to 32 ℃ after releasing the cold energy. The gas is used as a heat source and enters the first condensation separator 35.

[0045] 39 ℃, 2 MPa of nitrogen gas is compressed and cooled to 40 ℃ by the fifth gas compressor 58 and the fourth gas water cooler 59, respectively, to 3 MPa, and then flows into the first LNG heat exchanger 33 and is cooled to-65 ℃. The cooled gas is expanded to-87 ℃ and 2 MPa by the first gas expander 50. Most of the nitrogen gas flows into the second LNG heat exchanger 52 through the flow divider 51 and is cooled to-128 ℃ again. The gas is expanded to-165 ℃ and 0.7 MPa by the second gas expander 54, and then releases cold energy by the second LNG heat exchanger 52 and the first LNG heat exchanger 33, respectively. The nitrogen gas at 37 ℃ and 0.7 MPa is compressed and cooled to 20 ℃ and 2 MPa by the fourth gas compressor 55 and the third gas water cooler 56, respectively, and then flows into the nitrogen circulation through the mixer 57. Another small part of the nitrogen gas flows into the first LNG heat exchanger 33 through the flow divider 51. After releasing the cold energy, the nitrogen gas at 20 ℃ and 2 MPa flows into the mixer 57 again, is compressed by the fifth gas compressor 58, and is cooled by the fourth gas water cooler 59. In this way, the nitrogen gas is continuously circulated and refrigerated.

[0046] To prevent ice blocking caused by methane freezing inside the first desublimation separator 35 and the first desublimation separator 37, it is necessary to ensure that carbon dioxide is solidified before methane is solidified. A set of independent two-stage cascade refrigeration systems is used. The special cascade refrigeration system uses R170 (ethane) as the low-temperature loop refrigerant and R404A as the high-temperature loop refrigerant to provide precise directional cooling for the desublimation separator. The R170 (ethane) fluid at -53°C and 0.2 MPa that releases the cold energy flows out of the first desublimation separator 35, releases cold energy again in the first cascade heat exchanger 43 after passing through the ninth gas flow regulating valve 42, and then the R170 (ethane) fluid at -23°C and 0.2 MPa passes through the second gas compressor 44 for compression. The fluid at 108°C and 1.4 MPa is cooled to -21°C and 1.4 MPa twice in the second cascade heat exchanger 45 and the first cascade heat exchanger 43. After expansion cooling through the first throttling valve 46, the fluid at -70°C and 0.2 MPa enters the first desublimation separator 35 to complete the cycle. The R404A fluid at -5°C and 0.3 MPa that releases the cold energy out of the second cascade heat exchanger 45 is compressed to 58°C and 1.6 MPa by the third gas compressor 47, then condensed to 30°C and 1.6 MPa by the condenser 48, and then expanded and cooled to -31°C and 0.3 MPa by the second throttling valve 49, and then enters the second cascade heat exchanger 45 to complete the cycle.

[0047] As can be seen from the above, the biogas liquefaction system with an anti-ice-blocking desublimation decarbonization mechanism and the biogas liquefaction method provided by the present application have the following advantages: 1. The desublimation decarbonization mechanism is provided, wherein two desublimation separators can alternately perform desublimation crystallization of carbon dioxide and sublimation removal of carbon dioxide for regeneration, so that carbon dioxide can be effectively separated from biogas in a crystalline form, which can prevent ice blocking caused by carbon dioxide, water vapor or impurities at low temperature, thereby ensuring that the entire decarbonization and liquefaction process can be operated long-term, stably and continuously, and overcoming the defects of frequent operation interruption and efficiency reduction in traditional low-temperature separation processes.

[0048] 2. The biogas liquefaction mechanism is deeply coupled and cooperatively optimized with the nitrogen expansion refrigeration cycle and the carbon dioxide desublimation decarbonization process. The system realizes efficient cascade utilization of cold energy, the system energy efficiency is significantly improved, and the unit product energy consumption is greatly reduced. Moreover, the nitrogen expansion refrigeration cycle has high adaptability to fluctuations in the gas source conditions of raw biogas. Under wide inlet gas conditions, compared with traditional mixed refrigerant refrigeration cycles, the system can maintain high liquefaction rate and stable low energy consumption operation, has strong anti-working condition fluctuation ability, and is particularly suitable for small to medium-sized biogas projects with unstable gas sources.

[0049] Thirdly, the raw gas pretreatment mechanism, VPSA denitrogenation mechanism, desublimation decarburization mechanism, biogas liquefaction mechanism and cascade refrigeration mechanism of the system are synergistically optimized. Each module is synergistically designed and optimized, and the overall integration degree is high. The process flow is compact, and automation can be realized. It is particularly suitable for small and medium-sized application scenarios which are sensitive to land area, operation convenience and initial investment.

Claims

1. A biogas liquefaction system with anti-icing and anti-clogging desublimation mechanism, characterized in that: It comprises raw material gas pretreatment mechanism, VPSA denitrogenation mechanism, desublimation decarbonization mechanism, biogas liquefaction mechanism and cascade refrigeration mechanism; the VPSA denitrogenation mechanism comprises two nitrogen adsorption towers, the two nitrogen adsorption towers can alternately carry out adsorption and regeneration; the desublimation decarbonization mechanism comprises two desublimation separators, each desublimation separator is provided with cold pipes for providing cold quantity to desublimate carbon dioxide in biogas and hot pipes for providing heat quantity to sublimate desublimated carbon dioxide, the two desublimation separators can alternately carry out desublimation crystallization of carbon dioxide and sublimation regeneration of carbon dioxide; the cascade refrigeration mechanism is used for providing cold quantity for the cold pipes in the two desublimation separators.

2. The biogas liquefaction system with anti-icing and plugging eutectic decarbonization mechanism according to claim 1, characterized in that: The VPSA denitrogen mechanism comprises a first gas buffer tank (13), a first manifold (14), a second gas flow regulating valve (15), a second manifold (16), a third gas flow regulating valve (17), a first nitrogen adsorption tower (18), a third manifold (19), a fourth gas flow regulating valve (20), a fourth manifold (21), a second gas buffer tank (22), a fifth gas flow regulating valve (23), a fifth manifold (24), a sixth gas flow regulating valve (25), a second nitrogen adsorption tower (26), a sixth manifold (27), a seventh gas flow regulating valve (28), an eighth gas flow regulating valve (29), a seventh manifold (30), a third gas buffer tank (31), and a vacuum pump (32); the outlet of the first gas buffer tank (13) is connected with the inlet of the first manifold (14), the outlet of the first manifold (14) is connected with the inlets of the second gas flow regulating valve (15) and the fifth gas flow regulating valve (23) respectively, the outlet of the second gas flow regulating valve (15) is connected with the inlet of the second manifold (16), the outlet of the second manifold (16) is connected with the inlets of the third gas flow regulating valve (17) and the first nitrogen adsorption tower (18) respectively, the outlet of the third gas flow regulating valve (17) is connected with the inlet of the seventh manifold (30), the outlet of the first nitrogen adsorption tower (18) is connected with the inlet of the third manifold (19), the outlet of the third manifold (19) is connected with the inlets of the fourth gas flow regulating valve (20) and the eighth gas flow regulating valve (29) respectively, the outlet of the fourth gas flow regulating valve (20) is connected with the inlet of the fourth manifold (21), the eighth gas flow regulating valve (29) is connected with the sixth manifold (27) and the third manifold (19) respectively, the outlet of the fourth manifold (21) is connected with the inlet of the second gas buffer tank (22), the outlet of the fifth gas flow regulating valve (23) is connected with the inlet of the fifth manifold (24), the outlet of the fifth manifold (24) is connected with the inlets of the sixth gas flow regulating valve (25) and the second nitrogen adsorption tower (26) respectively, the outlet of the sixth gas flow regulating valve (25) is connected with the inlet of the seventh manifold (30), the outlet of the seventh manifold (30) is connected with the inlet of the third gas buffer tank (31), the outlet of the third gas buffer tank (31) is connected with the vacuum pump (32), the outlet of the second nitrogen adsorption tower (26) is connected with the inlet of the sixth manifold (27), the outlet of the sixth manifold (27) is connected with the inlet of the seventh gas flow regulating valve (28), the outlet of the seventh gas flow regulating valve (28) is connected with the inlet of the fourth manifold (21), and the outlet of the second gas buffer tank (22) is connected with a biogas liquefaction mechanism.

3. The biogas liquefaction system with anti-ice blocking desublimation mechanism according to claim 2, characterized in that: The biogas liquefaction mechanism comprises a first LNG heat exchanger (33), a first gas expander (50), a flow divider (51), a second LNG heat exchanger (52), a third throttle valve (53), a second gas expander (54), a fourth gas compressor (55), a third gas water cooler (56), a mixer (57), a fifth gas compressor (58), and a fourth gas water cooler (59); a second hot end outlet (33d) of the first LNG heat exchanger is connected with an inlet of the first gas expander (50), an outlet of the first gas expander (50) is connected with an inlet of the flow divider (51), outlets of the flow divider (51) are respectively connected with a first hot end inlet (52c) of the second LNG heat exchanger and a first cold end inlet (33g) of the first LNG heat exchanger, a first hot end outlet (52d) of the second LNG heat exchanger is connected with an inlet of the second gas expander (54), an outlet of the second gas expander (54) is connected with a cold end inlet (52e) of the second LNG heat exchanger, a cold end outlet (52f) of the second LNG heat exchanger is connected with a second cold end inlet (33i) of the first LNG heat exchanger, a second cold end outlet (33j) of the first LNG heat exchanger is connected with an inlet of the fourth gas compressor (55), an outlet of the fourth gas compressor (55) is connected with an inlet of the third gas water cooler (56), an outlet of the third gas water cooler (56) is connected with an inlet of the mixer (57), a first cold end outlet (33h) of the first LNG heat exchanger is connected with an inlet of the mixer (57), an outlet of the mixer (57) is connected with an inlet of the fifth gas compressor (58), an outlet of the fifth gas compressor (58) is connected with an inlet of the fourth gas water cooler (59), and an outlet of the fourth gas water cooler (59) is connected with a second hot end inlet (33c) of the first LNG heat exchanger; an outlet of a second gas buffer tank (22) is connected with a first hot end inlet (33a) of the first LNG heat exchanger, a first hot end outlet (33b) of the first LNG heat exchanger is connected with an inlet of a desublimation decarbonization mechanism, an outlet of the desublimation decarbonization mechanism is connected with a second hot end inlet (52a) of the second LNG heat exchanger, and a second hot end outlet (52b) of the second LNG heat exchanger is provided with the third throttle valve (53).

4. The biogas liquefaction system with anti-ice blocking and carbon-depleted desublimation mechanism according to claim 3, characterized in that: The desublimation decarbonization mechanism comprises a first three-way switch valve (34), a first desublimation separator (35), a second three-way switch valve (36), a second desublimation separator (37), a third three-way switch valve (38), and a fourth three-way switch valve (39); the first desublimation separator (35) is provided with a first desublimation separator hot pipe (35e, 35f) and a first desublimation separator cold pipe (35c, 35d); the second desublimation separator (37) is provided with a second desublimation separator hot pipe (37e, 37f) and a second desublimation separator cold pipe (37c, 37d). The first hot end outlet (33b) of the first LNG heat exchanger is connected with the inlet of the first three-way switch valve (34), the outlet of the first three-way switch valve (34) is respectively connected with the inlet of the first condensation separator (35) and the second condensation separator (37), the second separation outlet (35g) of the first condensation separator is connected with the inlet of the third three-way switch valve (38), the second separation outlet (37g) of the second condensation separator is connected with the inlet of the third three-way switch valve (38), the outlet of the third three-way switch valve (38) is connected with the third cold end inlet (33e) of the first LNG heat exchanger, the third cold end outlet (33f) of the first LNG heat exchanger is connected with the inlet of the fourth three-way switch valve (39), a carbon dioxide storage tank (61) is further arranged between the third cold end outlet (33f) of the first LNG heat exchanger and the inlet of the fourth three-way switch valve (39), the tenth flow regulating valve (60) is arranged on the input pipe of the carbon dioxide storage tank, the eleventh flow regulating valve (62) is arranged on the output pipe of the carbon dioxide storage tank, and the input pipe and the output pipe of the carbon dioxide storage tank are connected to the pipeline between the third cold end outlet (33f) of the first LNG heat exchanger and the inlet of the fourth three-way switch valve (39); the outlet of the fourth three-way switch valve (39) is respectively connected with the hot pipe inlet (35e) of the first condensation separator and the hot pipe inlet (37e) of the second condensation separator, the first separation outlet (35b) of the first condensation separator is connected with the inlet of the second three-way switch valve (36), the first separation outlet (37b) of the second condensation separator is connected with the inlet of the second three-way switch valve (36), and the outlet of the second three-way switch valve (36) is connected with the second hot end inlet (52a) of the second LNG heat exchanger.

5. The biogas liquefaction system with anti-ice blocking desublimation mechanism according to claim 4, characterized in that: The cascade refrigeration mechanism comprises a fifth three-way switching valve (40), a sixth three-way switching valve (41), a ninth gas flow regulating valve (42), a first cascade heat exchanger (43), a second gas compressor (44), a second cascade heat exchanger (45), a first throttling valve (46), a third gas compressor (47), a condenser (48), and a second throttling valve (49). The outlet of the fifth three-way switching valve (40) is connected with the cold pipe inlet (35c) of the first condensation separator and the cold pipe inlet (37c) of the second condensation separator respectively. The cold pipe outlet (35d) of the first condensation separator is connected with the inlet of the sixth three-way switching valve (41). The cold pipe outlet (37d) of the second condensation separator is connected with the inlet of the sixth three-way switching valve (41). The outlet of the sixth three-way switching valve (41) is connected with the inlet of the ninth gas flow regulating valve (42). The outlet of the ninth gas flow regulating valve (42) is connected with the hot end inlet of the first cascade heat exchanger (43). The hot end outlet (43b) of the first cascade heat exchanger is connected with the inlet of the second gas compressor (44). The outlet of the second gas compressor (44) is connected with the hot end inlet (45a) of the second cascade heat exchanger. The hot end outlet (45b) of the second cascade heat exchanger is connected with the cold end inlet (43c) of the first cascade heat exchanger. The cold end outlet (43d) of the first cascade heat exchanger is connected with the inlet of the first throttling valve (46). The outlet of the first throttling valve (46) is connected with the inlet of the fifth three-way switching valve (40). The cold end outlet (45d) of the second cascade heat exchanger is connected with the inlet of the third gas compressor (47). The outlet of the third gas compressor (47) is connected with the inlet of the condenser (48). The outlet of the condenser (48) is connected with the inlet of the second throttling valve (49). The outlet of the second throttling valve (49) is connected with the cold end inlet (45c) of the second cascade heat exchanger.

6. The biogas liquefaction system with anti-icing and coagulation mechanism according to any one of claims 1 to 5, characterized in that: The raw material gas pre-treatment mechanism comprises a first gas flow regulating valve (1), a filter (2), a first gas-water separator (3), a first water trap (4), a sulfur-containing gas pre-absorption tower (5), a heater (6), a hydrolysis tower (7), a second water trap (8), a first gas water cooler (9), a sulfur-containing gas fine absorption tower (10), a first gas compressor (11), and a second gas water cooler (12); the outlet of the first gas flow regulating valve (1) is connected with the inlet of the filter (2), the outlet of the filter (2) is connected with the inlet of the first gas-water separator (3), the first gas-water separator (3) is connected with the inlet of the first water trap (4), the outlet of the first gas-water separator (3) is connected with the inlet of the sulfur-containing gas pre-absorption tower (5), the outlet of the sulfur-containing gas pre-absorption tower (5) is connected with the inlet of the heater (6), the outlet of the heater (6) is connected with the inlet of the hydrolysis tower (7), the hydrolysis tower (7) is connected with the inlet of the second water trap (8), the outlet of the hydrolysis tower (7) is connected with the inlet of the first gas water cooler (9), the outlet of the first gas water cooler (9) is connected with the inlet of the sulfur-containing gas fine absorption tower (10), the outlet of the sulfur-containing gas fine absorption tower (10) is connected with the inlet of the first gas compressor (11), the outlet of the first gas compressor (11) is connected with the inlet of the second gas water cooler (12), and the outlet of the second gas water cooler (12) is connected with the inlet of the first gas buffer tank (13).

7. A method of liquefying biogas, characterized by: The biogas liquefaction system with the anti-icing and blocking condensation decarbonization mechanism of claim 6 comprises the following steps: A pre-treatment step: after the biogas is produced by the biogas fermentation device, the first gas flow regulating valve (1) is opened, the biogas enters the filter (2), the impurities in the biogas are filtered out, the filtered biogas enters the first gas-water separator (3) for dehydration, the condensed water separated out is discharged from the first water trap (4), the dehydrated biogas enters the sulfur-containing gas pre-absorption tower (5) to remove most of the sulfides, then enters the heater (6) for heating, the heated biogas enters the hydrolysis tower (7), the condensed water is discharged from the first water trap (8), after the organic sulfur is hydrolyzed into hydrogen sulfide, the biogas enters the third gas water cooler (9), the cooled biogas enters the sulfur-containing gas fine absorption tower (10) for further desulfurization, and the biogas after dehydration and desulfurization enters the first gas compressor (11) and the second gas water cooler (12) in sequence to complete the pressurization and cooling of the biogas; A denitrification step: the first nitrogen adsorption tower (18) and the second nitrogen adsorption tower (26) in the VPSA denitrogenation mechanism work alternately in adsorption and regeneration, for example, the first nitrogen adsorption tower (18) is adsorbed and the second nitrogen adsorption tower (26) is regenerated, and the steps are as follows: The desulfurization and dehydration pressurized and cooled biogas enters the first gas buffer tank (13), and the stabilized gas enters the first nitrogen adsorption tower (18) through the first manifold (14), the second gas flow regulating valve (15) and the second manifold (16) in sequence for denitrification, and most of the biogas after the nitrogen is removed enters the second gas buffer tank (22) through the third manifold (19), the fourth gas flow regulating valve (20) and the fourth manifold (21) in sequence, and the stabilized denitrified biogas enters the first hot end inlet (33a) of the first LNG heat exchanger; The sixth gas flow regulating valve (25) is opened to connect the second nitrogen adsorption tower (26) with the vacuum pump (32), the pressure in the second nitrogen adsorption tower (26) is rapidly reduced to negative pressure, the adsorbed nitrogen is desorbed from the internal adsorption material, at this time, a small part of the biogas after denitrification passes through the third manifold (19), the eighth gas flow regulating valve (29) and the sixth manifold (27) to sweep the desorbed nitrogen in the second nitrogen adsorption tower (26) from top to bottom, and the mixed gas is discharged from the system through the third gas buffer tank (31) and the vacuum pump (32) under the action of suction, and the regeneration of the second nitrogen adsorption tower (26) is completed; The desublimation decarburization step: two desublimation separators in the desublimation decarburization mechanism alternately perform desublimation crystallization of carbon dioxide and sublimation removal of carbon dioxide regeneration, taking the first desublimation separator (35) for desublimation crystallization of carbon dioxide and the second desublimation separator (37) for sublimation removal of carbon dioxide regeneration as an example, the steps are as follows: The biogas containing carbon dioxide after dehydration, desulfurization, denitrification and pressurization enters the hot end inlet (33a) of the first LNG heat exchanger, is cooled to the outlet (33b) of the first LNG heat exchanger, enters the separation inlet (35a) of the first desublimation separator through the first three-way switch valve (34), and the carbon dioxide gas in the mixed gas reaches the phase change critical point when passing through the cold pipe (35c, d), and is desublimated into solid carbon dioxide crystals, and the biogas gas after the carbon dioxide is removed is cooled and flows out from the separation outlet 35b of the first desublimation separator, enters the hot end inlet 52a of the second LNG heat exchanger through the second three-way switch valve (36); when the carbon dioxide crystals in the first desublimation separator (35) are too much, the second desublimation separator (37) is switched to perform desublimation crystallization of carbon dioxide; The carbon dioxide gas is pre-stored in the carbon dioxide storage tank (61), and the carbon dioxide gas enters the heat pipe inlet (37e) of the second desublimation separator through the eleventh flow regulating valve (62) and the fourth three-way switch valve (39). The carbon dioxide releasing heat flows out from the heat pipe outlet (37f) of the second desublimation separator. The solid carbon dioxide crystals in the second desublimation separator (37) are heated to sublimate into gas, and then flow into the cold end inlet (33e) of the first LNG heat exchanger through the second separation outlet (37g) of the second desublimation separator and the third three-way switch valve (38) to release cold. Then, the carbon dioxide flows out from the cold end outlet (33f) of the first LNG heat exchanger. Part of the carbon dioxide is stored in the carbon dioxide storage tank (61) through the tenth flow regulating valve (60) until the storage amount reaches the set value. The other part of the carbon dioxide enters the heat pipe inlet (37e) of the second desublimation separator through the fourth three-way switch valve (39) to provide heat. After releasing the heat, the carbon dioxide flows out from the heat pipe outlet (37f) of the second desublimation separator, and the cycle is repeated. After the carbon dioxide is desublimated, the biogas containing methane flows into the second hot end inlet (52a) of the second LNG heat exchanger, releases heat, and then flows out from the second hot end outlet (52b) of the second LNG heat exchanger. After being depressurized and cooled by the third throttling valve (53), the liquefied biogas is output. In the above process, the nitrogen gas successively passes through the fifth gas compressor (58) and the fourth gas water cooler (59). The pressurized and cooled nitrogen gas enters the second hot end inlet (33c) of the first LNG heat exchanger, is preliminarily cooled, and then flows out from the hot end outlet (33d) of the first LNG heat exchanger and enters the first gas expander (50). The expanded nitrogen gas passes through the flow divider (51), and part of the nitrogen gas flows out from the flow divider (51) and enters the first hot end inlet (52c) of the second LNG heat exchanger. After being deeply cooled, the nitrogen gas flows out from the first hot end outlet (52d) of the second LNG heat exchanger and enters the second gas expander (54). The expanded low-temperature gas flows into the cold end inlet (52e) of the second LNG heat exchanger, releases part of the cold, and then flows out from the cold end outlet (52f) of the second LNG heat exchanger. The low-temperature gas enters the second cold end inlet (33i) of the first LNG heat exchanger, releases the remaining cold, and then flows out from the second cold end outlet (33j) of the first LNG heat exchanger. After successively passing through the fourth gas compressor (55) and the third gas water cooler (56), the low-temperature gas enters the mixer (57). The other part of the nitrogen gas flows out from the flow divider (51), enters the first cold end inlet (33g) of the first LNG heat exchanger, releases the cold, and then flows out from the cold end outlet (33h) of the first LNG heat exchanger and enters the mixer (57). The two parts of the nitrogen gas flow out of the mixer (57) and enter the fifth gas compressor (58), so that the refrigeration cycle is continuously repeated. In the above process, the cooling medium R170 flows out from the cold end outlet (43d) of the first cascade heat exchanger, passes through the first throttling valve (46) to reduce the pressure and temperature, passes through the fifth three-way switching valve (40), flows into the cold tube inlet (35c) of the first desublimation separator, flows out from the cold tube outlet (35d) of the first desublimation separator after releasing the cold energy, sequentially passes through the sixth three-way switching valve (41) and the ninth gas flow regulating valve (42), and flows into the hot end inlet (43a) of the first cascade heat exchanger. The fluid after being cooled enters the second gas compressor (44), the compressed gas flows into the hot end inlet (45a) of the second cascade heat exchanger, the fluid after being cooled flows out from the hot end outlet (45b) of the second cascade heat exchanger and enters the cold end inlet (43c) of the first cascade heat exchanger, and the fluid after releasing the cold energy flows out from the cold end outlet (43d) of the first cascade heat exchanger to complete the cycle. The cooling medium R404A flows out from the cold end outlet (45d) of the second cascade heat exchanger, sequentially enters the third gas compressor (47), the condenser (48) and the second throttling valve (49), and flows into the cold end inlet (45c) of the second cascade heat exchanger, so as to continuously circulate and refrigerate.

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