Marine ammonia fuel engine flue gas purification and waste heat recovery system and purification method

Abstract

The invention relates to the technical field of ship tail gas treatment, and discloses a marine ammonia fuel engine flue gas purification and waste heat recovery system and a purification method.The marine ammonia fuel engine flue gas purification and waste heat recovery system comprises an inlet flue, a reactor and an outlet flue which are sequentially communicated; an SCR denitration catalyst, a first heat exchanger, a second heat exchanger, an ammoxidation catalyst and a laughing gas decomposition catalyst are sequentially arranged in the reactor from bottom to top, a third heat exchanger is arranged at the top of the reactor, the shell pass of the second heat exchanger is communicated with flue gas at the inlet side of the ammoxidation catalyst, and the tube pass of the second heat exchanger is communicated with high-temperature flue gas at the outlet side of the laughing gas decomposition catalyst; and the heating device is used for heating the flue gas in front of the ammoxidation catalyst to 400-410 DEG C. The device can efficiently remove NOx, NHH and NO in sequence, the key reaction temperature is accurately controlled through the unique heat exchange design so as to restrain generation of NO, waste heat of the system is recycled, and integration of energy conservation and environmental protection is achieved.

Description

Technical Field

[0001] This invention relates to the field of marine exhaust gas treatment technology, and in particular to a system and method for the synergistic purification of multiple pollutants and waste heat recovery of flue gas from marine ammonia-fueled engines. Background Technology

[0002] Ammonia, as a zero-carbon fuel, has broad application prospects in marine propulsion. However, ammonia-fueled engines produce nitrogen oxides (NOx) during operation. x The exhaust gas contains various pollutants, including incompletely burned ammonia (NH3) and nitrous oxide (N2O). Among them, N2O is a potent greenhouse gas with a greenhouse effect potential up to 265 times that of CO2, and its emission control is receiving increasing attention from the International Maritime Organization (IMO).

[0003] Currently, selective catalytic reduction (SCR) technology is a mature technology for removing NO from the exhaust of traditional diesel engines. x However, for ammonia-fueled engines, the exhaust gas composition is more complex: it contains high concentrations of both NH3 and NO. x It can also generate N2O under certain conditions. Studies have shown that the concentration of unburned NH3 in the exhaust gas of ammonia-fueled engines is typically significantly higher than that of NO. x This leads to an excess of NH3. Under these conditions, conventional SCR technology suffers from a lack of sufficient NO. x The reaction is difficult to effectively convert excess NH3. If a separate ammonia oxidation catalyst is used to treat NH3, it is easy to generate a large amount of N2O byproducts at lower temperatures (especially below 400℃), causing secondary pollution. In addition, the ammonia oxidation reaction itself is highly exothermic, and directly treating high concentrations of ammonia gas carries the risk of catalyst sintering and deactivation due to high temperatures, while the effective decomposition of N2O also requires a suitable high-temperature environment.

[0004] In the prior art, patent CN120444117A discloses an emission control system for ammonia fuel engines based on catalytic reduction. Through a nested SCR component design, the heat of the main SCR reaction is used to insulate the auxiliary SCR component, improving the system's response speed and thermal management efficiency under varying load conditions. However, this scheme still focuses on the traditional SCR denitrification path, without addressing the synergistic oxidation treatment of excess NH3, and lacks methods for removing the byproduct N2O. Overall, it still falls under the NO... x Targeted treatment technologies have failed to address the complex system of multiple pollutants coexisting in the exhaust gas of ammonia-fueled engines. Patent application CN120402257A discloses a denitrification system and method for ammonia-fueled engine exhaust gas, proposing the recovery of unburned ammonia gas to prepare ammonia water for denitrification, thus achieving the recycling of ammonia resources and reducing the consumption and storage requirements of reducing agents. However, its core remains focused on NO. xThe reduction and removal approach does not establish a synergistic removal pathway for NH3 and N2O. Furthermore, the ammonia preparation, storage, and injection systems increase process complexity and space requirements, posing significant challenges in the limited space of a ship. In addition, this scheme also fails to address N2O emission control and does not integrate effective precise temperature regulation and flow field uniformity mechanisms.

[0005] In summary, although existing technologies have made improvements in the specific aspects mentioned above, they still have the following common limitations: First, they target only a single pollutant and lack specificity for NO. x The problems are: 1) lack of coordinated control of NH3 and N2O; 2) failure to achieve precise control of reaction temperature, making it difficult to suppress N2O generation; and 3) insufficient system integration and internal flow field uniformity, affecting overall treatment energy efficiency and operational stability.

[0006] Therefore, an integrated purification and waste heat recovery system that can comprehensively solve the above problems is still needed. Summary of the Invention

[0007] The purpose of this invention is to provide a multi-pollutant synergistic purification and waste heat recovery system for marine ammonia fuel engine flue gas, which can efficiently remove NO in sequence. x The system precisely controls the critical reaction temperature of NH3 and N2O through a unique heat exchange design to suppress N2O generation and recover waste heat from the system, achieving integrated energy conservation and environmental protection.

[0008] The technical solution adopted in this invention is: A marine ammonia fuel engine flue gas purification and waste heat recovery system includes an inlet flue, a reactor, and an outlet flue connected in sequence. Inside the reactor, from bottom to top, are arranged an SCR denitrification catalyst, a first heat exchanger, a second heat exchanger, an ammonia oxidation catalyst, and a nitrous oxide decomposition catalyst. A third heat exchanger is provided at the top of the reactor. The shell side of the second heat exchanger is connected to the flue gas at the inlet side of the ammonia oxidation catalyst, and its tube side is connected to the high-temperature flue gas at the outlet side of the nitrous oxide decomposition catalyst. This system is used to heat the flue gas before the ammonia oxidation catalyst to 400~410℃. The top of the reactor and the outlet flue are connected by an upper constriction.

[0009] By adopting the above structure, the system achieves denitrification through SCR (first consuming part of NH3 and NO). xThe three-stage catalytic process of "ammonia oxidation (removal of residual NH3) → nitrous oxide decomposition (removal of N2O)" achieves synergistic and deep removal of three characteristic pollutants in the exhaust gas of ammonia-fueled engines, with a high degree of integration. The design cleverly utilizes the reaction heat released during the ammonia oxidation and nitrous oxide decomposition processes to precisely preheat the flue gas entering the ammonia oxidation catalyst to the optimal temperature range of 400-410℃ through heat exchange. This range is chosen based on a thorough consideration of reaction selectivity: although ammonia oxidation catalysts with precious metals as active components are active in the 400-600℃ range, strictly controlling their inlet temperature within a narrow window of 400-410℃ can maximally suppress the formation of N2O byproducts, while achieving an optimal balance between reaction efficiency and energy-saving system operation. The maximum safe operating temperature of the catalyst is 600℃, and the integrated thermal management strategy of this system ensures its long-term stable operation within the above-mentioned optimized range, thus achieving self-balancing operation driven by the system's reaction heat to regulate its own temperature.

[0010] Preferably, the inlet flue and the reactor are connected by a lower constriction, which is equipped with a crown-shaped flow equalization device. A flow equalization plate is provided between the lower constriction and the SCR denitrification catalyst. The crown-shaped flow equalization device has a spherical crown-shaped structure with strip-shaped holes on its surface determined according to flow field simulation. The flow equalization plate is provided with multiple frustum-shaped diffusion holes, wherein the opening ratio of the lower small circle and the upper large circle is 30% to 50%, and the included angle of the generatrix of the frustum is 30° to 45°.

[0011] By adopting the above structure, combined with the crown-shaped flow equalization device, the specially designed flow equalization plate, and the first and second flow equalization pipe groups driven by the circulating fan, the flue gas is ensured to be evenly distributed in the cross section of each catalyst bed in a multi-level and active manner, which improves the catalyst utilization rate and reaction efficiency, effectively avoids pipeline blockage, and extends the catalyst life.

[0012] Preferably, a first flow equalization pipe group and a second flow equalization pipe group are provided above the nitrous oxide decomposition catalyst. The inlet pipe of the second heat exchanger is connected to the first flow equalization pipe group, and the outlet pipe of the second heat exchanger is connected to the inlet of the circulating fan. The outlet pipe of the circulating fan is connected to the second flow equalization pipe group, thereby forming a continuous airflow path. The first flow equalization pipe group and the second flow equalization pipe group have the same structure, consisting of a main pipe and a branch pipe, and uniformly distributed round holes are opened on the side of the main pipe and the branch pipe facing away from the flue gas flow direction.

[0013] Preferably, the inlet pipe of the first heat exchanger is equipped with a regulating valve, which is used to introduce an external heat source to assist in heating the flue gas before it enters the ammonia oxidation catalyst when the heat of the ammonia oxidation reaction is insufficient.

[0014] Preferably, the system also includes multiple monitoring and control components. A regulating valve, a pressure sensor, and a temperature sensor are provided in the inlet pipe of the second heat exchanger. A pressure sensor, a temperature sensor, and a flow meter are provided in the outlet pipe of the circulating fan. Regulating valves, pressure sensors, temperature sensors, and flow meters are provided on both the inlet pipes of the first and third heat exchangers. Pressure sensors, temperature sensors, and flow meters are provided on both the outlet pipes of the first and third heat exchangers. Pressure sensors and temperature sensors are provided below the SCR denitrification catalyst, the ammonia oxidation catalyst, and the nitrous oxide decomposition catalyst, below the third heat exchanger, and on the side wall of the outlet flue. In addition, online monitoring instruments for monitoring flue gas composition are provided in the inlet and outlet flues.

[0015] Preferably, the SCR denitrification catalyst uses vanadium oxide as the active ingredient, and its reaction temperature window is 300~400℃. The ammonia oxidation catalyst has a double-layer coating structure, wherein the inner layer uses noble metal as the active ingredient for efficient catalysis of ammonia oxidation reaction, and the outer layer uses copper-based molecular sieve as the active ingredient for removing NO and NO2 generated during ammonia oxidation. The overall reaction temperature window of this catalyst is 400~600℃. The nitrous oxide decomposition catalyst uses transition metal oxide as the active ingredient, and its reaction temperature window is 400~800℃.

[0016] By adopting the above structure, the waste heat of high-temperature flue gas is recovered through the third heat exchanger to generate steam. At the same time, the heat exchange design embedded in the system also effectively utilizes the heat of reaction, reducing auxiliary energy consumption. The overall energy efficiency of the system is high. The first heat exchanger serves as an auxiliary heat source for backup, which can cope with low ammonia concentration and low nitrous oxide concentration conditions and provide temperature protection for the platinum-based catalyst. The system’s intelligent control and safety monitoring are achieved through a comprehensive sensor network and variable frequency circulating fan, ensuring that the system operates safely, stably and efficiently under varying conditions.

[0017] A preferred purification method for a marine ammonia fuel engine flue gas multi-pollutant synergistic purification and waste heat recovery system includes the following steps: Step 1: The raw exhaust gas enters the reactor through the inlet flue, and after being evenly distributed by the crown-shaped flow equalization device and the flow equalization plate, the unburned NH3 and NO in the exhaust gas are separated. x NO is removed through a chemical reaction under the action of SCR denitrification catalyst. x ; Step 2: After denitrification, the flue gas passes through the first heat exchanger and is heated to 400~410℃ in the second heat exchanger. Then it enters the ammonia oxidation catalyst to remove the remaining NH3. Subsequently, the flue gas continues to pass through the nitrous oxide decomposition catalyst to remove N2O. Step 3: Some of the high-temperature flue gas after the decomposition of nitrous oxide is extracted by the circulating fan through the first flow equalization pipe group, and after releasing heat through the tube side of the second heat exchanger, it is then evenly injected into the front end of the third heat exchanger through the second flow equalization pipe group to condition and equalize the flue gas. Step 4: The purified flue gas is used to recover waste heat through the third heat exchanger to generate steam, and finally discharged through the outlet flue.

[0018] By employing the above method, NO can be removed efficiently in sequence. x The system precisely controls the critical reaction temperature of NH3 and N2O through a unique heat exchange design to suppress N2O generation and recover waste heat from the system, achieving integrated energy conservation and environmental protection.

[0019] Compared with the prior art, the present invention has the following advantages: 1. The device of this invention uses SCR denitrification (first consuming part of NH3 and NO) to remove nitrification. x The three-stage catalytic process of "ammonia oxidation (removal of residual NH3) → nitrous oxide decomposition (removal of N2O)" achieves synergistic and deep removal of three characteristic pollutants in the exhaust gas of ammonia-fueled engines, with a high degree of integration. Utilizing the heat of reaction from ammonia oxidation and nitrous oxide decomposition, the inlet flue gas is preheated through a second heat exchanger, forming a self-balancing thermal management system that ensures the ammonia oxidation reaction takes place within a temperature window of 400~410℃, suppressing the generation of N2O during the ammonia oxidation process.

[0020] 2. The device of the present invention combines a crown-shaped flow equalization device, a specially designed flow equalization plate, and first and second flow equalization pipe groups driven by a circulating fan to ensure that the flue gas is evenly distributed in the cross section of each catalyst bed in a multi-level and active manner, thereby improving the catalyst utilization rate and reaction efficiency and extending the catalyst life.

[0021] 3. The device of the present invention recovers the waste heat of high-temperature clean flue gas to generate steam through the third heat exchanger. At the same time, the heat exchange design embedded in the system also effectively utilizes the heat of reaction, reduces the consumption of auxiliary energy, and the overall energy efficiency of the system is high. The first heat exchanger is used as an auxiliary heat source for backup to deal with low ammonia concentration and low nitrous oxide concentration conditions, and temperature protection is provided for the platinum-based catalyst. Intelligent control and safety monitoring of the system are achieved through a comprehensive sensor network and variable frequency circulating fan. Attached Figure Description

[0022] Figure 1 This is a process flow diagram of the present invention; Figure 2 This is a schematic diagram of the coronal flow equalization device of the present invention; Figure 3 This is a schematic diagram of the flow equalization plate structure of the present invention; Figure 4 This is a cross-sectional view of the diffusion hole of the present invention; Figure 5This is a schematic diagram of the first flow equalization tube assembly of the present invention.

[0023] The components include: 1. Inlet flue; 2. Lower constriction; 3. Crown-shaped flow equalization device; 3-1. Strip-shaped orifice; 4. Reactor; 5. Flow equalization plate; 5-1. Diffuser hole; 6. SCR denitrification catalyst; 7. First heat exchanger; 7-1. Inlet pipe of the first heat exchanger; 7-2. Outlet pipe of the first heat exchanger; 8. Second heat exchanger; 8-1. Inlet pipe of the second heat exchanger; 8-2. Outlet pipe of the second heat exchanger; 9. Ammonia oxidation catalyst; 10. First flow equalization tube assembly; 10- 1. Main pipe; 10-2. Branch pipe; 10-3. Round hole; 11. Circulating fan; 11-1. Circulating fan outlet pipe; 12. Nitrous oxide decomposition catalyst; 13. Third heat exchanger; 13-1. Third heat exchanger inlet pipe; 13-2. Third heat exchanger outlet pipe; 14. Upper constriction; 15. Outlet flue; 16. Regulating valve; 17. Pressure sensor; 18. Temperature sensor; 19. Flow meter; 20. Online monitoring instrument; 21. Second flow equalization pipe assembly. Detailed Implementation

[0024] like Figure 1-5 As shown, the system mainly includes an inlet flue 1, a reactor 2, an outlet flue 15, and a multi-stage catalyst and heat exchanger installed inside the reactor. It also includes a circulating fan, monitoring and control instruments, and corresponding connecting pipes connected to the outside of the reactor.

[0025] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Example

[0026] This embodiment describes the structure and operation of a typical marine ammonia fuel engine flue gas purification system. See appendix. Figure 1 The system mainly includes an inlet flue 1, a reactor 4, and an outlet flue 15. The reactor 4 is a vertical tower. The flue gas enters from the inlet flue 1 and first undergoes preliminary diversion through the crown-shaped flow equalization device 3 in the lower constriction 2. This device is spherical, and the arrangement of the strip holes 3-1 on its surface has been optimized by CFD flow field simulation. Subsequently, the flue gas passes through the flow equalization plate 5, and the frustum-shaped diffusion holes 5-1 on it (the lower small circle opening rate is 35%, the upper large circle opening rate is 45%, and the included angle of the frustum generatrix is ​​35°) further diffuse and equalize the flue gas.

[0027] Uniform flue gas enters the SCR denitrification catalyst 6. At the initial exhaust temperature of 300-350℃, the NO in the flue gas... x The excess NH3 is removed by reaction, and then the flue gas flows upward, passing sequentially through the shell side of the first heat exchanger 7 (which is in standby mode and the regulating valve is closed) and the second heat exchanger 8. In the second heat exchanger 8, the flue gas is heated to 405°C by high-temperature gas (from the outlet of the nitrous oxide decomposition catalyst 12) from its tube side.

[0028] The heated flue gas enters the ammonia oxidation catalyst 9, where the remaining NH3 is efficiently oxidized to N2 and H2O, with very little N2O generated. Subsequently, the flue gas continues to pass through the nitrous oxide decomposition catalyst 12, where the N2O from the original flue gas and any trace amounts of N2O that may remain from the ammonia oxidation process are efficiently decomposed into N2 and O2. A portion of the high-temperature flue gas (approximately 500-550°C) after the reaction is evenly drawn into the tube side of the second heat exchanger 8 by the circulating fan 11 through the first equalization pipe group 10 to release heat. The cooled flue gas, under the suction of the circulating fan 11, is evenly injected into the front end of the third heat exchanger 13 through the second equalization pipe group 21.

[0029] Finally, the purified flue gas (still at a relatively high temperature) enters the third heat exchanger 13 at the top, where it exchanges heat with water to generate saturated steam at 0.5 MPa, which is then sent to the ship's steam system. The cooled clean flue gas enters the outlet flue 15 through the upper constriction 14, and is discharged after being confirmed to meet standards by the online monitoring instrument 20.

[0030] In the system, pressure sensors 17, temperature sensors 18, and flow meters 19 in each heat exchanger pipeline, circulating fan pipeline, and key components transmit data to the control unit to achieve automatic monitoring. Example

[0031] This embodiment is based on Embodiment 1 and focuses on the operating conditions where the engine is running under low load and the concentrations of NH3 and N2O in the original exhaust gas are low.

[0032] Under this condition, the heat released by the ammonia oxidation reaction and the nitrous oxide decomposition reaction is relatively small. The second heat exchanger 8 alone may not be able to heat the flue gas entering the ammonia oxidation catalyst 9 to 400~410℃. At this time, the control system will start the first heat exchanger 7 as an auxiliary heat source.

[0033] The specific operation is as follows: Open the regulating valve 16 on the inlet pipe 7-1 of the first heat exchanger 7 to introduce an external heat source. This heat source preheats the flue gas flowing through its shell side through the first heat exchanger 7, for example, preheating the flue gas from 350°C to 380°C, and then it enters the second heat exchanger 8. In this way, the second heat exchanger 8 can heat the flue gas from 380°C to the target temperature of 405°C by utilizing the limited heat of reaction of ammonia oxidation and nitrous oxide decomposition. This ensures that the ammonia oxidation catalyst 9 always works within the temperature window for efficiently inhibiting N2O generation, ensuring the ammonia removal effect and controlling N2O generation. Example

[0034] This embodiment focuses on illustrating the system's flow field control and safety protection functions.

[0035] Flow field control: The system employs a three-stage flow equalization system. The first stage uses a crown-shaped flow equalization device 3 to break up the intake vortex. The second stage uses frustum-shaped diffuser holes in the flow equalization plate 5 to make the velocity field more uniform as the flue gas diffuses upwards. The third stage consists of a first flow equalization pipe group 10 and a second flow equalization pipe group 21 driven by a variable frequency circulating fan 11. The flow equalization pipe group 10, with its circular holes 10-3 facing away from the opening, can uniformly draw in the flue gas, eliminating any potential radial velocity gradients. The second flow equalization pipe group 21 uniformly sprays the flue gas delivered by the circulating fan, serving as a conditioning and secondary flow equalization mechanism to ensure the final heat exchange and purification effect.

[0036] Security protection: Over-temperature protection: Temperature sensor 18 located below ammonia oxidation catalyst 9 monitors its inlet temperature in real time. If the temperature exceeds 410℃ (possibly due to a sudden increase in ammonia concentration), the control system will reduce the opening of regulating valve 16 or reduce the frequency of circulating fan 11 to reduce the flow of hot flue gas entering the second heat exchanger 8, thereby reducing the heat exchange. At the same time, temperature sensors above and below the ammonia oxidation catalyst 9 bed can monitor the temperature rise. If the temperature at the highest point of the bed approaches the safety limit of 600℃, the system will alarm and take interlocking measures such as reducing the ammonia injection volume of the engine.

[0037] Catalyst protection: Pressure sensors 17 below each catalyst bed are used to monitor differential pressure. An abnormally large increase in differential pressure indicates possible blockage, requiring maintenance. Temperature monitoring of the nitrous oxide decomposition catalyst bed 12 ensures that it operates below a safe temperature (<800℃).

[0038] Closed-loop control: Exit online monitoring instrument provides real-time feedback of NO x The concentrations of NH3 and N2O are recorded and uploaded to the controller, which can fine-tune the frequency of the circulating fan and the opening of the heat exchanger regulating valve to achieve closed-loop control of pollutants and optimization of system energy consumption.

[0039] The system of this invention uses SCR denitrification (which first consumes part of NH3 and NO) to remove nitrification. xThe three-stage catalytic process of "ammonia oxidation (removal of residual NH3) → nitrous oxide decomposition (removal of N2O)" achieves synergistic and deep removal of three characteristic pollutants in the exhaust gas of ammonia-fueled engines, with a high degree of integration. Utilizing the heat of ammonia oxidation and the heat of nitrous oxide decomposition, the inlet flue gas is preheated through a second heat exchanger, forming a self-balancing thermal management system that ensures the ammonia oxidation reaction takes place within a temperature window of 400-410℃, suppressing the generation of N2O during the ammonia oxidation process. Combined with a crown-shaped flow equalization device, a specially designed flow equalization plate, and first and second flow equalization pipe groups driven by a circulating fan, the process achieves comprehensive and deep removal of these three characteristic pollutants in the exhaust gas of ammonia-fueled engines. The system employs a multi-layered, proactive approach to ensure uniform distribution of flue gas across the cross-sections of each catalyst bed, improving catalyst utilization and reaction efficiency, and extending catalyst lifespan. Waste heat from the high-temperature purified flue gas is recovered via a third heat exchanger to generate steam. Simultaneously, the system's embedded heat exchange design effectively utilizes reaction heat, reducing auxiliary energy consumption and resulting in high overall system energy efficiency. The first heat exchanger serves as a backup auxiliary heat source to handle low ammonia and nitrous oxide concentrations; temperature protection is provided for the platinum-based catalyst; and intelligent system control and safety monitoring are achieved through a comprehensive sensor network and variable frequency circulating fan.

[0040] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should be included within the scope of protection defined by the claims of the present invention.

Claims

1. A marine ammonia fuel engine flue gas purification and waste heat recovery system, comprising an inlet flue, a reactor, and an outlet flue connected in sequence, characterized in that: The reactor contains, from bottom to top, an SCR denitrification catalyst, a first heat exchanger, a second heat exchanger, an ammonia oxidation catalyst, and a nitrous oxide decomposition catalyst. A third heat exchanger is located at the top of the reactor. The shell side of the second heat exchanger is connected to the flue gas at the inlet of the ammonia oxidation catalyst, and its tube side is connected to the high-temperature flue gas at the outlet of the nitrous oxide decomposition catalyst. This heats the flue gas before the ammonia oxidation catalyst to 400-410°C. The top of the reactor is connected to the outlet flue via an upper constriction.

2. The marine ammonia fuel engine flue gas purification and waste heat recovery system according to claim 1, characterized in that: The inlet flue is connected to the reactor via a lower constriction. A crown-shaped flow equalization device is installed inside the lower constriction. A flow equalization plate is installed between the lower constriction and the SCR denitrification catalyst. The crown-shaped flow equalization device has a spherical crown structure with strip-shaped holes on its surface determined according to flow field simulation. The flow equalization plate has multiple frustum-shaped diffusion holes, wherein the opening ratio of the lower small circle and the upper large circle is 30% to 50%, and the included angle of the generatrix of the frustum is 30° to 45°.

3. The marine ammonia fuel engine flue gas purification and waste heat recovery system according to claim 1, characterized in that: Above the nitrous oxide decomposition catalyst, there is a first flow equalization pipe group and a second flow equalization pipe group. The inlet pipe of the second heat exchanger is connected to the first flow equalization pipe group, and the outlet pipe of the second heat exchanger is connected to the inlet of the circulating fan. The outlet pipe of the circulating fan is connected to the second flow equalization pipe group, thus forming a continuous airflow path. The first flow equalization pipe group and the second flow equalization pipe group have the same structure, consisting of a main pipe and a branch pipe. On the side of the main pipe and the branch pipe facing away from the flue gas flow direction, there are evenly distributed round holes.

4. The marine ammonia fuel engine flue gas purification and waste heat recovery system according to claim 1, characterized in that: The first heat exchanger is equipped with a regulating valve on its inlet pipe, which is used to introduce an external heat source to assist in heating the flue gas before it enters the ammonia oxidation catalyst when the heat of the ammonia oxidation reaction is insufficient.

5. The marine ammonia fuel engine flue gas purification and waste heat recovery system according to claim 1, characterized in that: The system also includes multiple monitoring and control components. A regulating valve, pressure sensor, and temperature sensor are installed in the inlet pipe of the second heat exchanger. A pressure sensor, temperature sensor, and flow meter are installed in the outlet pipe of the circulating fan. Regulating valves, pressure sensors, temperature sensors, and flow meters are installed on the inlet pipes of both the first and third heat exchangers. Pressure sensors, temperature sensors, and flow meters are installed on the outlet pipes of both the first and third heat exchangers. Pressure sensors and temperature sensors are installed below the SCR denitrification catalyst, ammonia oxidation catalyst, and nitrous oxide decomposition catalyst, below the third heat exchanger, and on the side wall of the outlet flue. In addition, online monitoring instruments for monitoring flue gas composition are installed in the inlet and outlet flues.

6. The marine ammonia fuel engine flue gas purification and waste heat recovery system according to claim 1, characterized in that: The SCR denitrification catalyst uses vanadium oxide as the active ingredient, and its reaction temperature window is 300~400℃. The ammonia oxidation catalyst has a double-layer coating structure, in which the inner layer uses noble metal as the active ingredient for efficient catalysis of ammonia oxidation reaction, and the outer layer uses copper-based molecular sieve as the active ingredient for removing NO and NO2 generated during ammonia oxidation. The overall reaction temperature window of this catalyst is 400~600℃. The nitrous oxide decomposition catalyst uses transition metal oxide as the active ingredient, and its reaction temperature window is 400~800℃.

7. A purification method for a marine ammonia fuel engine flue gas purification and waste heat recovery system, characterized in that: Includes the following steps: Step 1: The raw exhaust gas enters the reactor through the inlet flue, and after being evenly distributed by the crown-shaped flow equalization device and the flow equalization plate, the unburned NH3 and NO in the exhaust gas are separated. x NO is removed through a chemical reaction under the action of SCR denitrification catalyst. x ; Step 2: After denitrification, the flue gas passes through the first heat exchanger and is heated to 400~410℃ in the second heat exchanger. Then it enters the ammonia oxidation catalyst to remove the remaining NH3. Subsequently, the flue gas continues to pass through the nitrous oxide decomposition catalyst to remove N2O. Step 3: Some of the high-temperature flue gas after the decomposition of nitrous oxide is extracted by the circulating fan through the first flow equalization pipe group, and after releasing heat through the tube side of the second heat exchanger, it is then evenly injected into the front end of the third heat exchanger through the second flow equalization pipe group to condition and equalize the flue gas. Step 4: The purified flue gas is used to recover waste heat through the third heat exchanger to generate steam, and finally discharged through the outlet flue.

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