Low-carbon emission ammonia synthesis system having adjustable urea, ammonia and carbon imbalance

Abstract

A low-carbon emission ammonia synthesis system having an adjustable urea, ammonia and carbon imbalance, comprising: a natural gas two-stage conversion subsystem, a conversion gas purification subsystem, a fresh gas drying subsystem and an ammonia synthesis and refrigeration subsystem; natural gas raw material is passed through the natural gas two-stage conversion subsystem and the conversion gas purification subsystem, to generate fresh gas and carbon dioxide, and the fresh gas is sent to the fresh gas drying subsystem, to remove moisture by means of a molecular sieve; at the same time, on the basis of the output of carbon dioxide, a portion of the dried fresh gas is used as molecular sieve regeneration gas, and then sent back to the natural gas two-stage conversion subsystem as supplemental fuel gas, the remaining fresh gas is passed through the ammonia synthesis and refrigeration subsystem to produce a liquid ammonia product, and a balance of ammonia and carbon can be achieved in urea co-production. This solves the problem of a urea unit not being able to completely consume an ammonia product produced by an ammonia synthesis unit, reduces CO2 content in first stage furnace flue gas, achieves low carbon emissions, and can also reduce water content in the ammonia synthesis product and improve the purity of the ammonia synthesis product.

Description

A low-carbon emission ammonia synthesis system with adjustable urea-ammonia carbon imbalance Technical Field

[0001] The invention belongs to the field of chemical equipment, and in particular relates to a low-carbon emission synthetic ammonia system capable of adjusting urea-ammonia carbon imbalance. Background Art

[0002] Natural gas is an important chemical raw material and environmentally friendly energy source. It serves as a crucial feedstock in the production of various chemical products. Natural gas resources are particularly abundant in countries and regions such as Africa, the Middle East, Russia, Southeast Asia, and North America. Natural gas production of downstream chemical products is widely adopted internationally due to its low investment, simple process, minimal environmental pollution, and easy operation.

[0003] Currently, the world's mainstream patent holders for producing synthetic ammonia using natural gas as raw material include KBR, Topsoe, and Casali, etc. Each company has its own unique advantages in natural gas conversion and ammonia synthesis, and each patent holder masters one or more natural gas conversion technologies, such as Topsoe's Process for the production of synthesis gas, PCT / EP2021 / 055050 and Casali's Process for the ammonia production PCT / EP2016 / 051658.

[0004] At present, the process route for producing synthetic ammonia from natural gas mainly consists of natural gas conversion, CO shift, CO2 removal, methanation, deep purification, ammonia synthesis, refrigeration, hydrogen and ammonia recovery, and condensate stripping.

[0005] Two-stage reforming technology, namely a combination of a first-stage steam reforming and a second-stage air autothermal reforming, is commonly used in natural gas reforming processes. The heat required for the first-stage reforming is provided by the heat released by the combustion of the fuel gas through the burner, and the heat required for the second-stage reforming reaction is provided by the oxygen in the air introduced into the second-stage furnace and the heat released by the self-ignition of the reformed gas from the first-stage furnace.

[0006] Typically, off-gas from the ammonia synthesis unit is added to the fuel gas used in the first stage to reduce natural gas consumption. Air is typically added to the second stage to ensure an H2 / N2 ratio of 2.6 to 2.8 in the process gas after the shift reaction.

[0007] Two-stage conversion technology, namely high-temperature CO conversion and low-temperature CO conversion, is often used in the CO conversion process to convert CO into H2 as a synthetic ammonia raw gas as much as possible, while converting the CO in the conversion gas into CO2.

[0008] Physical and chemical absorption methods (such as aMDEA solution absorption method) are often used to remove CO2 from the process gas of the conversion process. After desorption, CO2 is transported to a urea device.

[0009] Usually, more than half of the synthetic ammonia production capacity is used to prepare urea products for easy transportation, so the ammonia carbon balance is particularly important in synthetic ammonia plants.

[0010] In conventional natural gas-based ammonia-urea plants, the amount of CO2 produced as a byproduct is often insufficient relative to the amount of NH3 produced, preventing the synthesis of sufficient urea. This often results in overproduction of liquid ammonia, which can lead to excessive liquid ammonia storage when the liquid ammonia is difficult to sell or consume. In natural gas-based ammonia-urea plants, since the primary furnace fuel gas is primarily natural gas, the calorific value of the fuel gas replenished within the ammonia plant is minimal. This results in high levels of CO2 emitted into the atmosphere, making it difficult to reduce CO2 emissions without excessive investment (flue gas CO2 capture technology is typically expensive and requires significant space). In natural gas-based ammonia-urea plants, since the fresh gas contains both water and CO2, an ammonia scrubber or fresh gas is introduced into the ammonia cooler within the synthesis cycle to meet the oxygen content requirements of the synthesis tower inlet. Compared to anhydrous fresh gas, the higher ammonia content in the synthesis tower inlet results in a lower net ammonia value, a larger recycle gas volume, and a relatively large power consumption for the synthesis gas compressor and the size of the synthesis high-pressure circuit.

[0011] Summary of the Invention

[0012] The purpose of the present invention is to provide a low-carbon emission ammonia synthesis system with adjustable urea-ammonia carbon imbalance in response to the defects of the prior art.

[0013] To achieve the above objectives, the present invention adopts the following technical solutions: a low-carbon emission synthetic ammonia system with adjustable urea-ammonia carbon imbalance, comprising: a two-stage natural gas reforming subsystem, a reformed gas purification subsystem, a fresh gas drying subsystem, and an ammonia synthesis and refrigeration subsystem;

[0014] The natural gas raw material generates fresh gas and carbon dioxide after passing through the natural gas two-stage conversion subsystem and the conversion gas purification subsystem. The fresh gas is sent to the fresh gas drying subsystem to remove moisture through the molecular sieve. At the same time, according to the carbon dioxide production, part of the dried fresh gas is used for molecular sieve regeneration gas and then sent back to the natural gas two-stage conversion subsystem as fuel gas supplement. The remaining fresh gas is passed through the ammonia synthesis and refrigeration subsystem to produce liquid ammonia products.

[0015] Furthermore, the proportion of fresh gas used for regeneration gas in the fresh gas drying subsystem accounts for 8% to 10% of the volume of the fresh gas, and the molar ratio of the output of carbon dioxide to the output of liquid ammonia is adjusted to 1:2.

[0016] Furthermore, the fresh gas drying system includes: a fresh gas ammonia cooler, a fresh gas separator, a molecular sieve dryer, a molecular sieve dryer filter and a molecular sieve regeneration heater;

[0017] The fresh air ammonia cooler is used to cool the fresh air and condense water;

[0018] The fresh gas separator is used to separate the moisture condensed in the fresh gas ammonia cooler from the fresh gas;

[0019] The molecular sieve dryer is used to completely remove moisture from the fresh air separated from the condensed water through the molecular sieve;

[0020] Molecular sieve dryer filters are used to filter solid impurities in fresh gas;

[0021] The molecular sieve regeneration heater is used to send part of the dried fresh gas back to the natural gas two-stage conversion subsystem as fuel gas supplement after being used for molecular sieve regeneration.

[0022] Furthermore, the system also includes a hydrogen and ammonia recovery subsystem, which is used to recover hydrogen and liquid ammonia from the purge gas and flash gas of the ammonia synthesis and refrigeration subsystem, and to add the recovered hydrogen to the fresh gas entering the ammonia synthesis and refrigeration subsystem, thereby adjusting the hydrogen-nitrogen volume ratio to 3:1.

[0023] Furthermore, the raw natural gas is hydrodesulfurized before entering the natural gas two-stage conversion subsystem, and the sulfur content is removed to below 0.1 ppmv.

[0024] Furthermore, the natural gas two-stage conversion subsystem includes a first-stage conversion subsystem and a second-stage conversion subsystem. The regenerated gas of the fresh gas drying subsystem and the tail gas of the ammonia synthesis and refrigeration subsystem are recovered and used as fuel gas supplement for the first-stage conversion subsystem.

[0025] Furthermore, the dry basis content of methane at the outlet of the second-stage conversion subsystem is 0.3% to 0.55% mol.

[0026] Furthermore, the conversion gas purification subsystem includes a high-temperature conversion reactor, a low-temperature conversion reactor, a conversion waste heat recovery system, a CO2 removal system and a methanation system. After being processed by the conversion gas purification subsystem, the CO+CO2 content in the fresh gas entering the fresh gas drying subsystem is less than 5ppm.

[0027] Furthermore, the system also includes a process condensate stripping subsystem including a stripping tower and a bottom heat exchanger. The process condensate of the conversion gas purification subsystem is heated in the bottom heat exchanger and then enters the stripping tower, where CO2, NH3, and CH3OH are removed. The steam at the top of the stripping tower is sent to the natural gas two-stage conversion subsystem as raw steam. The steam condensate at the bottom of the tower recovers heat in the bottom heat exchanger, is then cooled by a water cooler, and is sent to the desalted water station for refining treatment.

[0028] Furthermore, the ammonia synthesis and refrigeration subsystem includes an ammonia synthesis tower. The fresh gas from the fresh gas drying subsystem is mixed with the circulating gas in the ammonia synthesis loop and then reheated and sent to the ammonia synthesis tower. The net ammonia value is above 18%.

[0029] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0030] (1) In the present invention, a fresh gas drying process is provided, and a portion of the fresh gas is used for molecular sieve regeneration and then continues to be used as fuel gas, thereby achieving an ammonia-carbon balance during the co-production of urea. This solves the problem in conventional natural gas-based ammonia-urea plants where the urea unit cannot fully consume the ammonia product produced by the ammonia unit.

[0031] (2) In the present invention, due to the provision of a fresh gas drying process, a portion of the fresh gas is used as fuel gas, significantly reducing the amount of raw natural gas used. Furthermore, because the CH4 content in the synthetic fresh gas is very low, and the majority of the combustible gas is H2, CO2 emissions from the flue gas of the first stage furnace are significantly reduced compared to conventional natural gas-to-ammonia plants.

[0032] (3) In the present invention, since a fresh gas drying process is provided to remove almost all moisture and CO2 in the fresh gas, the fresh gas can be directly mixed with the circulating gas in the ammonia synthesis loop and then reheated and sent to the ammonia synthesis tower. Compared with other processes, since the ammonia and inert gas contents in the process gas at the inlet of the synthesis tower are relatively low, the net ammonia value is usually above 18%, which is much higher than the process without fresh gas drying (usually the net ammonia value is 14%) and the process using the ammonia washing method to dry the fresh gas (usually the net ammonia value is 15.5%), which can reduce the power consumption of the synthesis gas compressor and the size of the synthesis high-pressure loop. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] FIG1 is a process flow diagram of a low-carbon emission synthetic ammonia system with adjustable urea-ammonia carbon imbalance according to an embodiment.

[0034] Among them, 1 is the natural gas compression system, 2 is the natural gas hydrodesulfurization system, 3 is the first-stage conversion system, 4 is the second-stage conversion system, 5 is the conversion gas purification system, which mainly includes a two-stage conversion system, a CO2 removal system, and a methanation system, 6 is the fresh gas drying system, 7 is the ammonia synthesis and refrigeration system, 8 is the hydrogen and ammonia recovery system, 9 is the process condensate stripping system, 10 is the air compression system, and 11 is the molecular sieve regeneration gas. DETAILED DESCRIPTION

[0035] In order to enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of this application.

[0036] It should be noted that the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned drawings are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or are inherent to these processes, methods, products or apparatuses.

[0037] The process of the present invention includes a two-stage natural gas conversion system, a two-stage shift system, a CO2 removal system, a methanation system, a fresh gas drying system, an ammonia synthesis and refrigeration system, a hydrogen and ammonia recovery system, a process condensate stripping system, etc.

[0038] Key equipment in the natural gas conversion system includes an air compressor, a hydrogenation reactor, a desulfurization tank, a steam reformer (first stage), an autothermal reformer (second stage), and a waste heat recovery system. A natural gas compressor is also required when the natural gas pressure outside the facility is insufficient.

[0039] The conversion system includes high-temperature conversion reactor, low-temperature conversion reactor, conversion waste heat recovery system and other equipment.

[0040] The CO2 removal system adopts aMDEA decarbonization technology. This technical route has many technical solutions. One typical technical solution includes a CO2 absorption tower, a CO2 desorption tower, a flash tower and ancillary equipment (such as a reboiler, a heat exchanger, a solution pump, etc.).

[0041] The methanation system includes a methanation reactor, a methanation reactor feed heater, a methanation reactor inlet and outlet heat exchangers, a methanation gas water cooler and other equipment.

[0042] The fresh air drying system includes: fresh air ammonia cooler, fresh air separator, molecular sieve dryer, molecular sieve dryer filter, molecular sieve regeneration heater and other equipment.

[0043] The ammonia synthesis and refrigeration system includes the use of low-pressure ammonia synthesis technology. This technical route has many technical solutions, but each solution should include synthesis gas compressor and inter-stage equipment, ammonia synthesis tower, ammonia synthesis tower outlet heat collection system, gas exchanger, water cooler, ammonia cooler, ammonia separator, ammonia compressor and inter-stage equipment, ammonia receiving tank, liquid ammonia pump and other equipment.

[0044] The hydrogen-ammonia recovery system mainly includes ammonia absorption tower, ammonia recovery tower, hydrogen recovery package and other equipment.

[0045] The process condensate stripping system includes stripping tower, bottom heat exchanger and other equipment.

[0046] In this system, natural gas undergoes hydrodesulfurization to reduce the sulfur content in the natural gas to below 0.1ppmv, meeting the sulfur content requirements of downstream catalysts.

[0047] The desulfurized feed gas is mixed with stripping steam from the stripping tower and heated in the convection section of the first stage furnace before being fed into the first stage furnace to participate in the steam reforming reaction. By adjusting the steam and feed gas ratio entering the steam reformer, the steam reformer inlet water-to-carbon ratio is controlled at approximately 3, and the steam reformer outlet temperature is controlled at approximately 750-800°C.

[0048] After being pressurized by the air compressor and heated by the heating coil of the convection section of the first-stage furnace, the air is fed into the second-stage furnace through the burner of the second-stage furnace, where it contacts the conversion gas from the first-stage furnace and spontaneously ignites to release heat, further undergoing the conversion reaction. The outlet temperature of the conversion furnace is controlled to reach approximately 1000°C, and the outlet methane content is 0.3-0.55% mol (dry basis).

[0049] The converted gas after heat recovery is sent to the shift process for shift reaction. The CO content in the process gas at the outlet of the low-temperature shift furnace is less than 0.35% mol (dry basis).

[0050] The low-temperature shift gas is preheated with desalted water after heat recovery and then sent to the CO2 removal process.

[0051] In the CO2 removal process, the process condensate of the process gas after the waste heat recovery process is collected and sent to the process condensate stripping section through a condensate pump.

[0052] In the CO2 removal process, most of the CO2 in the process gas is removed, and the separated CO2 is transported to the urea unit. The CO2 content in the process gas leaving the CO2 removal unit is less than 500ppm (dry basis).

[0053] The process gas from the CO2 removal process undergoes a methanation reaction in a methanation reactor to remove CO and CO2, and the content of CO+CO2 in the process gas leaving the methanation process is less than 5ppm.

[0054] A fresh gas drying process is set up. The process gas from methanation is dried with a molecular sieve to remove moisture. At the same time, based on the CO2 product output, a small portion of the dried process gas is used for molecular sieve regeneration gas and then sent to the upstream furnace as fuel gas. The majority of the fresh gas is sent to downstream processes.

[0055] After the fresh synthetic gas from upstream is mixed with the hydrogen-rich gas from the hydrogen recovery process, the H2 / N2 ratio in the fresh gas is adjusted to 3:1 and sent to the ammonia synthesis and refrigeration circuit to produce liquid ammonia products.

[0056] The waste gases such as the purge gas and flash steam from the synthetic ammonia unit are sent to the hydrogen-ammonia recovery unit to recover hydrogen and liquid ammonia.

[0057] The process condensate of the synthetic ammonia unit is preheated by the condensate heat exchanger at the bottom of the stripping tower and then enters the stripping tower for stripping. The steam at the top of the stripping tower is fed into the conversion process as conversion raw steam. The liquid at the bottom of the stripping tower is sent to the desalted water station for refining treatment after heat recovery and cooling.

[0058] Example

[0059] Referring to FIG1 , the natural gas raw material from outside the boundary enters the inlet buffer tank of the natural gas compressor system (1), undergoes gas-liquid phase separation, and is sent to the natural gas compressor for compression. The compressed natural gas is sent to the convection section of the first stage furnace of the first stage conversion system (3) for heating, and the temperature after heating is about 370-390°C. The first stage furnace fuel gas uses the tail gas of the flash gas and hydrogen and ammonia recovery system (8) of the downstream conversion gas purification system (5) and the molecular sieve regeneration gas (11) of the fresh gas drying system (6) according to the actual situation of the factory. The insufficient part is supplemented by fuel natural gas. The preheated natural gas enters the cobalt-molybdenum hydrogenation reactor of the natural gas hydrodesulfurization system (2), which converts the organic sulfur in the natural gas raw material into inorganic sulfur, mainly H2S.

[0060] The hydrogenated natural gas is sent to the desulfurization tank for desulfurization, and the sulfur content is reduced to below 0.1ppmv. Two desulfurization tanks are set up, which can be operated in parallel or in series, so that when the desulfurizer in one desulfurization tank is replaced, the other desulfurization tank can still be kept online. After the desulfurization, a certain amount of medium-pressure steam is added to the raw gas to make the water / carbon ratio reach 3.0, and then it is sent to the raw gas heating coil of the convection section of the first furnace to be heated to about 520℃ and enter the reaction tube of the radiation section of the first furnace for reaction. The first-stage conversion catalyst is loaded in the furnace tube of the first furnace. The process gas after the reaction is sent to the upper part of the second-stage furnace in the second-stage conversion system (4), and further reacts with the process air compressed from the air compression system (10) and preheated to about 540-550℃ by the process air preheating coil of the convection section of the first furnace in the first-stage conversion system (3) in the second-stage furnace. The methane content of the reaction gas at the outlet of the second-stage furnace is about less than 0.55% mol (dry basis). The process gas at the outlet of the second stage furnace is sent to the waste boiler of the second stage furnace and the steam superheater for process waste heat recovery and then enters the conversion gas purification system (5).

[0061] In the conversion process of the reformed gas purification system (5), the conversion reaction is first carried out in a high-temperature conversion furnace. After heat recovery, the high-temperature conversion gas is sent to a low-temperature conversion furnace for a conversion reaction. The CO content in the process gas at the outlet of the low-temperature conversion furnace is less than 0.35% mol (dry basis). The process gas at the outlet of the low-temperature conversion furnace is cooled step by step in the CO2 removal process, and the CO2 and water therein are separated and then sent to the methanation process. The process gas is first reheated in the inlet and outlet heat exchangers of the methanation reactor, and then sent to the methanation reactor for a methanation reaction to further remove CO and CO2. The CO+CO2 content in the process gas at the methanation process is less than 5ppm. The cooling capacity is recovered by the inlet and outlet heat exchangers of the methanation reactor, and then cooled by the methanation gas water cooler before being sent to the downstream process.

[0062] The CO2 separated in the conversion gas purification system (5) can be transported to downstream devices or processed separately according to demand, and the separated process condensate is collected and pumped to the process condensate stripping system (9) for further processing.

[0063] In the fresh gas drying system (6), the fresh gas from the reformed gas purification system (5) is cooled by the quencher and then enters the molecular sieve inlet separator for separation. The separated synthesis gas enters the molecular sieve for further drying. A portion of the synthesis gas (about 8-10%) is used for molecular sieve regeneration, and the molecular sieve regeneration gas (11) is sent to the first stage furnace fuel gas system as fuel gas, and the rest enters the downstream device.

[0064] After the above steps, the process gas contains only H2, N2, and a small amount of CH4 and Ar. However, the H2 / N2 ratio still does not reach the required ratio (3:1) for ammonia synthesis, so hydrogen needs to be added. The added hydrogen comes from the hydrogen recovery device (8) of the hydrogen and ammonia recovery system. The hydrogen-nitrogen ratio of the process gas after hydrogen addition is 3:1, and it is sent to the ammonia synthesis and refrigeration system (7) for the production of ammonia products.

[0065] In the process condensate stripping system (9), process condensate from the ammonia synthesis unit is heated in a bottom heat exchanger before entering the process condensate stripping tower. Impurities such as CO₂, NH₃, and CH₃OH are removed in the stripping tower. The steam at the top of the stripping tower is sent to the primary conversion system (3) as feed steam. The steam condensate at the bottom of the tower recovers heat in the bottom heat exchanger, is cooled in a water cooler, and then sent to the demineralized water station for refining.

[0066] The above is a further detailed description of the present invention and should not be considered as a limitation on the specific implementation of the present invention. For those skilled in the art, simple deductions or substitutions that do not depart from the concept of the present invention are within the scope of protection of the present invention.

Claims

1. A low-carbon emission ammonia synthesis system with adjustable urea ammonia carbon imbalance, characterized in that include: Natural gas two-stage reforming subsystem, reforming gas purification subsystem, fresh gas drying subsystem and ammonia synthesis and refrigeration subsystem; The natural gas raw material is converted into fresh gas and carbon dioxide after passing through the natural gas two-stage conversion subsystem and the conversion gas purification subsystem. The fresh gas is sent to the fresh gas drying subsystem to remove moisture through the molecular sieve. At the same time, according to the output of carbon dioxide, part of the dried fresh gas is used for molecular sieve regeneration gas and then sent back to the natural gas two-stage conversion subsystem as fuel gas supplement. The remaining fresh gas is passed through the ammonia synthesis and refrigeration subsystem to produce liquid ammonia products.

2. The low-carbon emission synthetic ammonia system with adjustable urea-ammonia-carbon imbalance according to claim 1, characterized in that: The proportion of fresh gas used for regeneration gas in the fresh gas drying subsystem accounts for 8% to 10% of the volume of the fresh gas, and the molar ratio of the output of carbon dioxide to the output of liquid ammonia is adjusted to 1:

2.

3. The low-carbon emission synthetic ammonia system with adjustable urea-ammonia-carbon imbalance according to claim 1, characterized in that: The fresh air drying system comprises: a fresh air ammonia cooler, a fresh air separator, a molecular sieve dryer, a molecular sieve dryer filter and a molecular sieve regeneration heater; The fresh air ammonia cooler is used to cool the fresh air and condense water; The fresh gas separator is used to separate the moisture condensed in the fresh gas ammonia cooler from the fresh gas; Molecular sieve dryer is used to completely remove moisture from fresh air separated from condensed water through molecular sieve; Molecular sieve dryer filters are used to filter solid impurities in fresh gas; The molecular sieve regeneration heater is used to send part of the dried fresh gas back to the natural gas two-stage conversion subsystem as fuel gas supplement after being used for molecular sieve regeneration.

4. The low-carbon emission synthetic ammonia system with adjustable urea-ammonia-carbon imbalance according to claim 1, characterized in that: The system also includes a hydrogen and ammonia recovery subsystem, which is used to recover hydrogen and liquid ammonia from the purge gas and flash gas of the ammonia synthesis and refrigeration subsystem, and to add the recovered hydrogen to the fresh gas entering the ammonia synthesis and refrigeration subsystem, thereby adjusting the hydrogen-nitrogen volume ratio to 3:

1.

5. The low-carbon emission synthetic ammonia system with adjustable urea ammonia carbon imbalance according to claim 1, characterized in that: The raw natural gas is hydrodesulfurized before entering the natural gas two-stage conversion subsystem, and the sulfur content is removed to below 0.1ppmv.

6. The low-carbon emission synthetic ammonia system with adjustable urea-ammonia-carbon imbalance according to claim 1, characterized in that: The two-stage natural gas conversion subsystem comprises a first-stage conversion subsystem and a second-stage conversion subsystem. The regenerated gas of the fresh gas drying subsystem and the tail gas of the ammonia synthesis and refrigeration subsystem are recovered and used as fuel gas supplement for the first-stage conversion subsystem.

7. The low-carbon emission synthetic ammonia system with adjustable urea-ammonia-carbon imbalance according to claim 6, characterized in that: The dry basis content of methane at the outlet of the second-stage conversion subsystem is 0.3% to 0.55% mol.

8. The low-carbon emission synthetic ammonia system with adjustable urea-ammonia-carbon imbalance according to claim 1, characterized in that: The conversion gas purification subsystem includes a high-temperature conversion reactor, a low-temperature conversion reactor, a conversion waste heat recovery system, a CO2 removal system and a methanation system. The CO+CO2 content in the fresh gas entering the fresh gas drying subsystem after being processed by the conversion gas purification subsystem is less than 5ppm.

9. The low-carbon emission synthetic ammonia system with adjustable urea-ammonia-carbon imbalance according to claim 1, characterized in that: The system also includes a process condensate stripping subsystem including a stripping tower and a bottom heat exchanger. The process condensate of the conversion gas purification subsystem is heated in the bottom heat exchanger and then enters the stripping tower, where CO2, NH3, and CH3OH are removed. The steam at the top of the stripping tower is sent to the natural gas two-stage conversion subsystem as raw steam. The steam condensate at the bottom of the tower recovers heat in the bottom heat exchanger, is then cooled by a water cooler, and is sent to a desalted water station for refining treatment.

10. The low-carbon emission synthetic ammonia system with adjustable urea-ammonia-carbon imbalance according to claim 1, characterized in that: The ammonia synthesis and refrigeration subsystem includes an ammonia synthesis tower. The fresh gas from the fresh gas drying subsystem is mixed with the circulating gas in the ammonia synthesis loop and then reheated and sent to the ammonia synthesis tower. The net ammonia value is above 18%.

Images