An ammonia pyrolysis mixed combustion system and method based on flue gas waste heat and molten salt coupling

Abstract

This invention discloses an ammonia pyrolysis co-firing system and method based on flue gas waste heat and molten salt coupling, belonging to the field of power system technology. The ammonia pyrolysis co-firing system includes a boiler, a heat storage unit, and a pyrolysis heating unit. The heat storage unit utilizes the high-temperature flue gas generated during boiler operation to exchange heat with low-temperature molten salt, obtaining and storing high-temperature molten salt, thus completing the time transfer and storage of thermal energy. The pyrolysis heating unit completes ammonia pyrolysis for hydrogen production, urea pyrolysis for ammonia production, and low-temperature steam heating through three controllable branches, achieving a cascade utilization cycle of "high-medium-low" grade energy. Simultaneously, a central control system monitors operating parameters in real time and sends commands to the molten salt flow controller to regulate the molten salt flow rate of the three controllable branches, thereby adapting to the different boiler operating requirements. This invention utilizes flue gas waste heat as the initial energy source for ammonia and hydrogen production, is easily integrated with existing boiler systems, and solves the problems of low utilization efficiency, high operating costs, and unstable operation of existing hydrogen production systems.

Description

Technical Field

[0001] This invention belongs to the field of power system technology, specifically relating to an ammonia pyrolysis co-firing system and method based on flue gas waste heat and molten salt coupling. Background Technology

[0002] In coal-fired power plants or industrial boilers, flue gas emissions carry a large amount of residual heat, which would result in energy waste if directly discharged. Common methods for utilizing residual heat include preheating air, heating feedwater, or generating electricity with low-grade steam, but there is still room for improvement in utilization efficiency. On the other hand, to reduce carbon emissions from coal-fired boilers, ammonia, as a hydrogen-rich, carbon-free fuel, can be co-combusted with pulverized coal to achieve low-carbon operation. However, large-scale storage and transportation of ammonia pose safety hazards, and direct combustion results in high nitrogen oxide emissions. In existing technologies, ammonia is typically produced through urea pyrolysis or ammonia pyrolysis, but the pyrolysis process requires a continuous and stable high-temperature heat source. Directly using electric heating or fuel heating would increase system energy consumption and operating costs. Furthermore, when boiler load fluctuates, the demand for flue gas residual heat and ammonia pyrolysis often mismatches, leading to low energy utilization efficiency. Therefore, how to efficiently store flue gas residual heat and flexibly utilize it for ammonia production and pyrolysis, achieving an integrated system of "residual heat recovery - heat storage - hydrogen production - hydrogen supply," has become an urgent technical problem to be solved.

[0003] Existing related technologies mainly include direct electric / gas-fired heating for hydrogen production, direct use of flue gas waste heat for urea thermal denitrification, and independent hydrogen production systems based on solar molten salt thermal storage. These solutions either rely on expensive external energy sources and suffer from poor economic efficiency, are limited by real-time flue gas conditions and lack flexible scheduling capabilities, or, while employing thermal storage, depend on intermittent external energy sources, resulting in complex and costly systems. They generally fail to efficiently and economically solve the key problem of converting fluctuating industrial flue gas waste heat into a stable, on-demand supply of hydrogen fuel, and still have significant shortcomings in energy cascade utilization, system flexibility, and overall energy efficiency. Specifically: 1. While direct electric / gas heating hydrogen production can provide a stable supply of hydrogen, it relies on external high-quality energy sources, has high operating costs, and causes carbon emission transfer. 2. Although the direct pyrolysis of urea with flue gas waste heat utilizes waste heat, it cannot achieve real-time air conditioning due to boiler load constraints, and it is only used to produce denitrified ammonia gas rather than fuel hydrogen. 3. Solar molten salt thermal energy storage hydrogen production systems have energy storage capabilities, but they rely on intermittent solar energy and require the construction of a complete set of facilities, resulting in huge investments and making them difficult to integrate with existing boiler systems. Summary of the Invention

[0004] The technical solution of this invention aims to solve the problem of efficient recovery, flexible storage, and on-demand use of waste heat from flue gas in coal-fired boilers or industrial furnaces, as well as the integrated hydrogen production and ammonia pyrolysis hydrogen supply. Its technical essence lies in coupling multiple unit processes, including waste heat recovery from flue gas, molten salt sensible heat storage, urea pyrolysis for ammonia production, ammonia catalytic pyrolysis for hydrogen production, and hydrogen combustion, to construct a comprehensive system with coordinated scheduling.

[0005] The present invention first provides an ammonia pyrolysis co-firing system based on flue gas waste heat and molten salt coupling, including a boiler, a heat storage unit and a pyrolysis heating unit; The thermal storage unit is equipped with low-temperature molten salt. The high-temperature flue gas generated by the boiler operation is transported to the thermal storage unit to exchange heat with the low-temperature molten salt to obtain high-temperature molten salt. The pyrolysis heating unit is equipped with an ammonia pyrolysis heat exchanger, a urea pyrolysis heat exchanger and a steam-molten salt heat exchanger. The ammonia pyrolysis heat exchanger is used for ammonia pyrolysis, the urea pyrolysis heat exchanger is used for pyrolysis of urea solution to generate ammonia, and the steam-molten salt heat exchanger is used for heating steam. The molten salt side outlet of the thermal storage unit is connected to the molten salt side inlet of the ammonia pyrolysis heat exchanger. Molten salt replenishment lines are provided between the molten salt side outlet of the thermal storage unit and the molten salt side inlets of the urea pyrolysis heat exchanger and the steam-molten salt heat exchanger. The molten salt sides of the ammonia pyrolysis heat exchanger, the urea pyrolysis heat exchanger, and the steam-molten salt heat exchanger are connected in series. High-temperature molten salt flows through the ammonia pyrolysis heat exchanger, the urea pyrolysis heat exchanger, and the steam-molten salt heat exchanger in sequence. The gas side outlet of the urea pyrolysis heat exchanger is connected to the gas side inlet of the ammonia pyrolysis heat exchanger. The gas side outlet of the ammonia pyrolysis heat exchanger is connected to the boiler, and the ammonia pyrolysis gas generated by ammonia pyrolysis is transported to the boiler to replace fuel for combustion. Low-temperature steam is introduced into the steam-molten salt heat exchanger. The low-temperature steam exchanges heat with the molten salt flowing through the steam-molten salt heat exchanger to obtain high-temperature steam, which is then transported to the external thermal system for heating. The molten salt after heat exchange returns to the thermal storage unit to continue participating in the thermal storage and pyrolysis heating cycle.

[0006] Preferably, a molten salt control pump is provided before the molten salt side inlet of the ammonia pyrolysis heat exchanger, the urea pyrolysis heat exchanger and the steam-molten salt heat exchanger to control the flow rate of high-temperature molten salt. The molten salt side outlet of the heat storage unit is connected to the ammonia pyrolysis heat exchanger, the urea pyrolysis heat exchanger and the steam-molten salt heat exchanger respectively through the corresponding molten salt control pump.

[0007] Preferably, a burner is provided on the outer side of the bottom of the boiler. An ammonia pyrolysis gas inlet and a pulverized coal and primary air inlet are provided on the outer side of the burner. The ammonia pyrolysis gas enters the furnace through the ammonia pyrolysis gas inlet. At the same time, fuel is added to the furnace of the burner through the pulverized coal and primary air inlets. The ammonia pyrolysis gas and fuel are mixed and burned. A burner outlet jet is formed at the outlet on the inner side of the burner and injected into the boiler.

[0008] Preferably, it also includes a molten salt flow controller, which is signal-connected to three molten salt control pumps to form an actuator for distributing molten salt flow.

[0009] Preferably, it also includes a central control system, which monitors the boiler load command, the heat storage capacity and temperature of the heat storage unit, and the target fuel blending ratio in the boiler in real time, and sends commands to the molten salt flow controller to adjust the flow of the three molten salt control pumps to adapt to different boiler operating requirements.

[0010] The present invention also provides a co-firing method using the aforementioned ammonia pyrolysis co-firing system based on flue gas waste heat and molten salt coupling. First, the high-temperature flue gas generated during boiler operation is transported to the heat storage unit to exchange heat with low-temperature molten salt to obtain high-temperature molten salt, thereby completing the time transfer and storage of thermal energy. When the boiler needs to co-fire ammonia or hydrogen fuel, three molten salt control pumps are started. According to the target fuel co-firing ratio in the boiler, the central control system regulates the flow of the three molten salt control pumps through the molten salt flow controller, and executes the following process: (1) Extract the required amount of high-temperature molten salt from the heat storage unit into the ammonia pyrolysis heat exchanger, and at the same time, introduce the ammonia gas generated in the urea pyrolysis heat exchanger into the ammonia pyrolysis heat exchanger; the high-temperature molten salt releases heat energy, causing the ammonia gas to undergo a pyrolysis reaction in the ammonia pyrolysis heat exchanger; the ammonia pyrolysis gas after the reaction is drawn out from the gas side outlet of the ammonia pyrolysis heat exchanger and transported to the boiler for combustion, and the high-temperature flue gas generated during the combustion process is output from the flue gas outlet of the boiler to the heat storage unit; after the high-temperature molten salt releases heat energy, its temperature decreases to become medium-temperature molten salt and flows into the urea pyrolysis heat exchanger; (2) The required amount of high-temperature molten salt is supplied from the molten salt supply line to the urea pyrolysis heat exchanger as needed, and urea solution is pumped into the urea pyrolysis heat exchanger; the medium-temperature molten salt and the high-temperature molten salt release heat energy, causing the urea solution to pyrolyze and produce a gaseous mixture containing ammonia, and the ammonia is transported to the ammonia pyrolysis heat exchanger as ammonia pyrolysis raw material; after the urea solution is pyrolyzed, the temperature of the medium-temperature molten salt and the high-temperature molten salt decreases to obtain low-temperature molten salt, and the low-temperature molten salt enters the steam-molten salt heat exchanger; (3) The required amount of high-temperature molten salt is supplied from the molten salt supply line to the steam-molten salt heat exchanger as needed. At the same time, low-temperature steam is introduced into the steam-molten salt heat exchanger. The low-temperature molten salt and high-temperature molten salt release heat energy to heat the low-temperature steam, so that the low-temperature steam becomes high-temperature steam and is then transported to the external thermal system for heating. Finally, the molten salt returns to the heat storage unit to continue to participate in the heat storage and pyrolysis heating cycle.

[0011] Preferably, the gas-side outlet of the urea pyrolysis heat exchanger is also connected to the boiler. When only ammonia needs to be co-fired or the demand for ammonia is large, the gas-side outlet of the urea pyrolysis heat exchanger is disconnected from the gas-side inlet of the ammonia pyrolysis heat exchanger, and the gas-side outlet of the urea pyrolysis heat exchanger is connected to the boiler. After the heat storage unit completes the time transfer and storage of thermal energy, the two molten salt control pumps corresponding to the urea pyrolysis heat exchanger and the steam-molten salt heat exchanger are started. According to the target fuel co-firing ratio, the central control system regulates the two molten salt control pumps through the molten salt flow controller. The required amount of high-temperature molten salt is extracted from the molten salt replenishment line and sent to the urea pyrolysis heat exchanger. Urea solution is pumped into the urea pyrolysis heat exchanger. The high-temperature molten salt heats the urea solution to produce ammonia. After the urea solution is pyrolyzed, the ammonia enters the boiler for combustion from the gas side outlet of the urea pyrolysis heat exchanger. The temperature of the high-temperature molten salt decreases to medium-low temperature molten salt and enters the steam-molten salt heat exchanger. Subsequently, low-temperature steam is introduced into the steam-molten salt heat exchanger. The medium-low temperature molten salt heats the low-temperature steam, turning it into high-temperature steam, which is then delivered to the external thermal system for heating.

[0012] Compared with the prior art, the innovative aspects and beneficial effects of this invention are as follows: 1. High-efficiency cascaded energy utilization: By using the high-temperature flue gas generated by the boiler to exchange heat with the low-temperature molten salt, the heat energy is transferred and stored over time. Then, through the central control system, three controllable branches are regulated to realize a complete energy cascaded utilization chain from flue gas waste heat → high-temperature molten salt (heat storage) → ammonia pyrolysis (highest grade requirement) → urea pyrolysis (medium grade requirement) → steam heating (low grade requirement), which greatly improves the overall energy utilization efficiency.

[0013] 2. Low-carbon fuel production at low cost: Using waste heat from flue gas as the initial energy source for ammonia and hydrogen production replaces traditional electric or gas heating, significantly reducing the production cost of zero-carbon fuels (hydrogen / ammonia) and improving the economics of power plant co-firing.

[0014] 3. Enhanced grid flexibility: The molten salt thermal storage system achieves thermoelectric decoupling. During periods of low grid load, the boiler can maintain operation, absorbing waste heat from flue gas and storing it in molten salt; during periods of high grid load or when rapid load changes are required, the stored heat energy is quickly released to produce a hydrogen-nitrogen mixture to aid combustion, thereby improving the boiler's ramp-up rate and peak-shaving capacity.

[0015] 4. Flexible and controllable operation: Through independent molten salt flow control and multi-channel heat exchanger design, the system can flexibly switch between multiple modes such as pure heat storage, ammonia production, and hydrogen production combined with steam production to adapt to the needs of different boiler operating conditions.

[0016] 5. Reduce nitrogen oxide emissions: By first pyrolyzing ammonia into a hydrogen-nitrogen mixture and then burning it, the combustion stability can be improved compared to direct ammonia combustion, and the NOx generation potential is lower. Attached Figure Description

[0017] Figure 1 This is a diagram of an ammonia pyrolysis co-firing system based on flue gas waste heat and molten salt coupling; Figure 2 This is a schematic diagram of an ammonia pyrolysis heat exchanger; Figure 3 This is a schematic diagram of a urea pyrolysis heat exchanger; Figure 4 This is a schematic diagram of a burner.

[0018] In the diagram, 1. Boiler, 2. High-temperature flue gas, 3. Flue gas-molten salt heat exchanger, 4. Low-temperature flue gas, 5. Molten salt pump at the heat exchanger inlet, 6. Low-temperature molten salt tank, 7. High-temperature molten salt tank, 8. Ammonia pyrolysis heat exchanger, 9. Urea pyrolysis heat exchanger, 10. Steam-molten salt heat exchanger, 11. Urea solution tank, 12. First molten salt flow controller, 13. First-stage molten salt control pump, 14. Second-stage molten salt control pump, 15. Third-stage molten salt control pump, 16. Burner, 17. Burner inlet flow control pump, 18. Second molten salt flow controller, 1 9. Low-temperature steam; 20. High-temperature steam; 21. Ammonia pyrolysis heat exchanger inlet; 22. First-stage molten salt inlet; 23. First-stage molten salt outlet; 24. Ammonia pyrolysis heat exchanger outlet; 25. Catalyst; 26. Urea pyrolysis heat exchanger inlet; 27. Urea pyrolysis heat exchanger outlet; 28. Second-stage molten salt inlet; 29. ​​Second-stage molten salt outlet; 30. Ammonia pyrolysis gas inlet; 31. Pulverized coal and primary air inlet; 32. Burner outlet jet. Detailed Implementation

[0019] The present invention will now be described in detail with reference to the accompanying drawings and preferred embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0020] This invention is proposed to solve the problem of efficient storage of flue gas waste heat and flexible use for ammonia preparation and pyrolysis. It realizes the time-varying distribution and grade classification of flue gas waste heat through a molten salt thermal storage system. Without consuming external fuel, the fluctuating flue gas waste heat is converted into hydrogen fuel that can be stably supplied on demand, while reducing the system modification and operation costs.

[0021] One objective of this invention is to provide an ammonia pyrolysis co-firing system based on flue gas waste heat and molten salt coupling. This system can utilize flue gas waste heat and molten salt thermal storage coupling to complete urea-to-ammonia production, ammonia pyrolysis-to-hydrogen production, and flexible heating in a cascade manner. Figure 1The flue gas-molten salt heat exchanger 3 is a shell-and-tube heat exchanger. The flue gas outlet of boiler 1 is connected to the high-temperature side of the flue gas-molten salt heat exchanger 3, allowing the high-temperature flue gas 2 generated by boiler 1 to be transported to the heat exchanger 3. A low-temperature molten salt tank 6 is connected to the molten salt inlet of the flue gas-molten salt heat exchanger 3 via a molten salt pump 5, supplying low-temperature molten salt into the heat exchanger 3. The molten salt outlet of the heat exchanger 3 is connected to a high-temperature molten salt tank 7. After entering the flue gas-molten salt heat exchanger 3, the high-temperature flue gas 2 exchanges heat with the low-temperature molten salt entering from the low-temperature molten salt tank 6, resulting in low-temperature flue gas 4 and high-temperature molten salt. The high-temperature molten salt is stored in the high-temperature molten salt tank 7, while the low-temperature flue gas 4 is discharged from the heat exchanger 3. This circuit converts the dispersed and fluctuating waste heat of the flue gas into concentrated and stable molten salt thermal energy storage.

[0022] The outlet of the high-temperature molten salt tank 7 is divided into three controlled branches, which are respectively connected to the molten salt side inlet of the ammonia pyrolysis heat exchanger 8, the urea pyrolysis heat exchanger 9, and the steam-molten salt heat exchanger 10. All three heat exchangers are shell-and-tube heat exchangers. Specifically: The first branch is connected to the first-stage molten salt inlet 22 on the molten salt side of the ammonia pyrolysis heat exchanger 8 via the first-stage molten salt control pump 13. Its first-stage molten salt outlet 23 on the molten salt side can be connected to the second-stage molten salt inlet 28 on the molten salt side of the urea pyrolysis heat exchanger 9. The ammonia pyrolysis heat exchanger inlet 21 on the gas side of the ammonia pyrolysis heat exchanger 8 is connected to the urea pyrolysis heat exchanger outlet 27 on the gas side of the urea pyrolysis heat exchanger 9. The ammonia pyrolysis heat exchanger outlet 24 on the gas side of the ammonia pyrolysis heat exchanger 8 is connected to the boiler burner 16 via pipelines and the burner inlet flow control pump 17. In this embodiment, when the system is working, ammonia gas from the urea pyrolysis heat exchanger 9 enters the gas channel of the ammonia pyrolysis heat exchanger 8 through the urea pyrolysis heat exchanger outlet 27 and the ammonia pyrolysis heat exchanger inlet 21. Simultaneously, the required amount of high-temperature molten salt is extracted from the high-temperature molten salt tank 7 and enters the molten salt channel in the ammonia pyrolysis heat exchanger 8. The high-temperature molten salt provides a heat source for ammonia pyrolysis. A catalyst 25 is installed in the gas channel of the ammonia pyrolysis heat exchanger 8. Ammonia undergoes a pyrolysis reaction under high temperature and the action of the catalyst to generate ammonia pyrolysis gas (i.e., a high-temperature mixture of nitrogen and hydrogen). After the high-temperature molten salt is cooled, medium-temperature molten salt is obtained. The medium-temperature molten salt enters the urea pyrolysis heat exchanger 9 through the first-stage molten salt outlet 23 and the second-stage molten salt inlet 28. The ammonia pyrolysis gas is transported to the burner 16 through pipelines and the burner inlet flow control pump 17, and injected into the furnace to participate in combustion, replacing part of the pulverized coal fuel combustion and reducing carbon dioxide emissions at the source. The burner 16 is connected to the inside of the boiler 1 on the inside, and has an ammonia pyrolysis gas inlet 30 and a pulverized coal and primary air inlet 31 on the outside. The ammonia pyrolysis gas enters the furnace through the ammonia pyrolysis gas inlet 30, and fuel is added to the furnace through the pulverized coal and primary air inlet 31. The ammonia pyrolysis gas mixes with the fuel and burns, forming a burner outlet jet 32 ​​at the inner outlet of the burner 16 and being injected into the boiler 1. The first branch utilizes high-temperature molten salt for ammonia pyrolysis, achieving high-grade energy utilization.

[0023] The second branch is connected to the second-stage molten salt inlet 28 on the molten salt side of the urea pyrolysis heat exchanger 9 via the second-stage molten salt control pump 14. Its second-stage molten salt outlet 29 on the molten salt side can be connected to the molten salt side inlet of the steam-molten salt heat exchanger 10. The urea pyrolysis heat exchanger inlet 26 on the gas side of the urea pyrolysis heat exchanger 9 is connected to the urea solution tank 11. The urea pyrolysis heat exchanger outlet 27 on the gas side can also be directly connected to the burner 16 via a pipeline and the burner inlet flow control pump 17. A switch valve is provided at the urea pyrolysis heat exchanger outlet 27 to control the connection and disconnection of the pipeline between the urea pyrolysis heat exchanger outlet 27 and the ammonia pyrolysis heat exchanger inlet 21 on the gas side of the ammonia pyrolysis heat exchanger 8, and between the urea pyrolysis heat exchanger outlet 27 and the burner 16. During system operation, the medium-temperature molten salt flowing from the ammonia pyrolysis heat exchanger 8 is introduced into the molten salt channel of the urea pyrolysis heat exchanger 9 through the first-stage molten salt outlet 23 and the second-stage molten salt inlet 28. Simultaneously, the required amount of high-temperature molten salt is extracted from the high-temperature molten salt tank 7 and introduced into the molten salt channel as needed. The urea solution in the urea solution tank 11 is pumped into the solution channel through the urea pyrolysis heat exchanger inlet 26. The medium-temperature and high-temperature molten salts are mixed to provide a heat source for the pyrolysis of the urea solution to produce ammonia, carbon dioxide, and water vapor. After separation, in this embodiment, ammonia is transported to the gas channel of the ammonia pyrolysis heat exchanger 8 through the urea pyrolysis heat exchanger outlet 27 and the ammonia pyrolysis heat exchanger inlet 21 as ammonia pyrolysis feedstock. After the urea solution is pyrolyzed, the temperature of the medium-temperature molten salt is further reduced to obtain low-temperature molten salt, which enters the steam-molten salt heat exchanger 10 through the second molten salt flow controller 18. The second molten salt flow controller 18 controls the flow rate of the low-temperature molten salt into the steam-molten salt heat exchanger 10, thereby controlling the heat exchange rate.

[0024] The third branch is connected to the molten salt side inlet of the steam-molten salt heat exchanger 10 via the third-stage molten salt control pump 15, and its molten salt side outlet is connected back to the low-temperature molten salt tank 6. During system operation, low-temperature molten salt from the urea pyrolysis heat exchanger 9 is introduced into the molten salt channel through the second molten salt flow controller 18 and the molten salt side inlet of the steam-molten salt heat exchanger 10. Simultaneously, the required amount of high-temperature molten salt is extracted from the high-temperature molten salt tank 7 and introduced into the molten salt channel as needed. Low-temperature steam 19 from the boiler system is introduced into the steam channel through the steam side inlet of the steam-molten salt heat exchanger 10. The low-temperature and high-temperature molten salts are mixed to heat the low-temperature steam 19, transforming it into high-temperature steam 20, which is then output from the steam side outlet of the steam-molten salt heat exchanger 10 and transported to an external thermal system for heating. Finally, the molten salt, having released all its effective heat, returns to the low-temperature molten salt tank 6, completing a full "high-medium-low" grade energy cascade utilization cycle.

[0025] The first molten salt flow controller 12 is signal-connected to the first-stage molten salt control pump 13, the second-stage molten salt control pump 14, and the third-stage molten salt control pump 15, forming an actuator for distributing molten salt flow.

[0026] The entire system is centrally controlled and dispatched, receiving signals such as boiler load and fuel demand. The central control system monitors in real time the load command of boiler 1, the heat storage and temperature of the high-temperature molten salt tank 7, and the target fuel blending ratio within the boiler. Based on these parameters, the central control system sends commands to the first molten salt flow controller 12 to dynamically and precisely adjust the flow rates of the first-stage molten salt control pump 13, the second-stage molten salt control pump 14, and the third-stage molten salt control pump 15. For example, when a rapid increase in hydrogen fuel production is needed, the central control system sends a command to the first molten salt flow controller 12 to increase the molten salt flow rate to the ammonia pyrolysis heat exchanger 8 and the urea pyrolysis heat exchanger 9; when boiler 1 has high steam demand or insufficient heat storage, the molten salt flow rate to the ammonia pyrolysis heat exchanger 8 and the urea pyrolysis heat exchanger 9 is reduced, and the flow rate to the steam-molten salt heat exchanger 10 is correspondingly increased.

[0027] This dynamic allocation enables the optimal configuration of thermal energy among hydrogen production, ammonia production, and steam generation, ensuring that the system always responds to actual needs and maintains efficient operation.

[0028] Another objective of this invention is to provide an ammonia thermal co-firing method for an ammonia thermal co-firing system based on the coupling of flue gas waste heat and molten salt, which is achieved through the following technical solution: High-temperature flue gas 2 generated by boiler 1 is introduced into flue gas-molten salt heat exchanger 3. Simultaneously, the inlet molten salt pump 5 is started to pump low-temperature molten salt from the low-temperature molten salt tank 6 into the molten salt side of the flue gas-molten salt heat exchanger 3. High-temperature flue gas 2 and low-temperature molten salt undergo indirect heat exchange within the flue gas-molten salt heat exchanger 3. After the high-temperature flue gas temperature decreases, it is discharged from the system as low-temperature flue gas 4. The molten salt, having absorbed heat, increases in temperature, becoming high-temperature molten salt, which is then transported and stored in the high-temperature molten salt tank 7, completing the time-based transfer and storage of thermal energy.

[0029] When boiler 1 needs to co-fire ammonia or hydrogen fuel, the switch valve at the outlet 27 of the urea pyrolysis heat exchanger is set to connect the pipeline between the outlet 27 of the urea pyrolysis heat exchanger and the inlet 21 of the ammonia pyrolysis heat exchanger on the gas side of the ammonia pyrolysis heat exchanger 8, and disconnect the pipeline between the outlet 27 of the urea pyrolysis heat exchanger and the burner 16. The first-stage molten salt control pump 13, the second-stage molten salt control pump 14, and the third-stage molten salt control pump 15 are started. According to the target fuel co-firing ratio in the boiler, the central control system regulates the flow rates of the first-stage molten salt control pump 13, the second-stage molten salt control pump 14, and the third-stage molten salt control pump 15 through the first molten salt flow controller 12, and executes the following process: (1) The required amount of high-temperature molten salt is extracted from the high-temperature molten salt tank 7 by the first-stage molten salt control pump 13 and sent to the molten salt channel of the ammonia pyrolysis heat exchanger 8. At the same time, the ammonia gas generated in the urea pyrolysis heat exchanger 9 is introduced into the gas channel of the ammonia pyrolysis heat exchanger 8. The high-temperature molten salt provides the required high-grade heat energy for the ammonia pyrolysis to produce a mixture of hydrogen and nitrogen. In the ammonia pyrolysis heat exchanger 8, ammonia gas undergoes a pyrolysis reaction under the action of catalyst 25. The high-temperature ammonia pyrolysis gas after the reaction is drawn out from the outlet 24 of the ammonia pyrolysis heat exchanger and transported to the boiler burner 16 through pipelines and the burner inlet flow control pump 17. Ammonia pyrolysis gas enters the furnace through ammonia pyrolysis gas inlet 30, while fuel is added to the furnace through pulverized coal and primary air inlet 31. The ammonia pyrolysis gas and fuel are mixed and burned, forming a burner outlet jet 32 ​​at the inner outlet of burner 16 and being injected into boiler 1. The high-temperature flue gas 2 generated during combustion is output from the flue gas outlet of boiler 1 to the flue gas-molten salt heat exchanger 3 to continue participating in the heat storage and pyrolysis heating cycle.

[0030] (2) The medium-temperature molten salt, whose temperature has decreased, flows out of the molten salt channel of the ammonia pyrolysis heat exchanger 8 and is introduced into the molten salt channel of the urea pyrolysis heat exchanger 9. At the same time, the required amount of high-temperature molten salt is extracted from the high-temperature molten salt tank 7 by the second-stage molten salt control pump 14 and enters the molten salt channel of the urea pyrolysis heat exchanger 9. The urea solution in the urea solution tank 11 is pumped into the solution channel of the urea pyrolysis heat exchanger 9. The medium-temperature molten salt and the high-temperature molten salt provide the heat source for the pyrolysis of urea solution to ammonia, carbon dioxide and water vapor. After the gaseous mixture generated by the pyrolysis of urea solution is separated, the ammonia is transported to the gas channel of the ammonia pyrolysis heat exchanger 8 as the raw material for ammonia pyrolysis. The temperature of the medium-temperature molten salt and the high-temperature molten salt decreases to obtain low-temperature molten salt, which enters the steam-molten salt heat exchanger 10 through the second molten salt flow controller 18.

[0031] (3) The low-temperature molten salt flowing out of the molten salt channel of the urea pyrolysis heat exchanger 9, with its temperature further reduced, is introduced into the molten salt channel of the steam-molten salt heat exchanger 10. At the same time, the required amount of high-temperature molten salt is extracted from the high-temperature molten salt tank 7 by the third-stage molten salt control pump 15 and enters the molten salt channel of the steam-molten salt heat exchanger 10. The low-temperature steam 19 of the boiler system is introduced into the steam channel of the steam-molten salt heat exchanger 10. The low-temperature molten salt is mixed with the high-temperature molten salt to heat the low-temperature steam 19, turning it into high-temperature steam 20, which is then transported to the external thermal system for heating. Finally, the molten salt that has released all its effective heat is returned to the low-temperature molten salt tank 6 to continue participating in the heat storage and pyrolysis heating cycle.

[0032] When only ammonia needs to be co-fired or the demand for ammonia is large, the switch valve at the outlet 27 of the urea pyrolysis heat exchanger is set to disconnect the pipeline between the outlet 27 of the urea pyrolysis heat exchanger and the inlet 21 of the ammonia pyrolysis heat exchanger on the gas side of the ammonia pyrolysis heat exchanger 8, and connect the pipeline between the outlet 27 of the urea pyrolysis heat exchanger and the burner 16. The second-stage molten salt control pump 14 and the third-stage molten salt control pump 15 are started. According to the target fuel co-firing ratio, the central control system regulates the flow rates of the second-stage molten salt control pump 14 and the third-stage molten salt control pump 15 through the first molten salt flow controller 12, and executes the following process: The required amount of high-temperature molten salt is extracted from the high-temperature molten salt tank 7 by the second-stage molten salt control pump 14. This high-temperature molten salt is then guided to the molten salt channel of the urea pyrolysis heat exchanger 9. Urea solution from the urea solution tank 11 is pumped into the solution channel of the urea pyrolysis heat exchanger 9. The high-temperature molten salt is directly used to heat the urea solution to produce ammonia. After pyrolysis, ammonia is directly introduced into the burner 16 from the urea pyrolysis heat exchanger outlet 27 via the burner inlet flow control pump 17, and then injected into the furnace for combustion. The temperature of the high-temperature molten salt decreases to that of medium- and low-temperature molten salt, which then enters the molten salt channel of the steam-molten salt heat exchanger 10 via the second molten salt flow controller 18. Subsequently, low-temperature steam 19 from the boiler system is introduced into the steam channel of the steam-molten salt heat exchanger 10. The waste heat of the medium- and low-temperature molten salt is used to heat the low-temperature steam 19, transforming it into high-temperature steam 20, which is then delivered to the external heating system for heating. This process bypasses the ammonia pyrolysis step, prioritizing ammonia supply and steam parameters.

[0033] This completed the coupling of flue gas waste heat with molten salt and its flexible application in ammonia preparation and pyrolysis, achieving the goal of energy cascade utilization.

[0034] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the appended claims.

Claims

1. An ammonia pyrolysis co-firing system based on flue gas waste heat and molten salt coupling, characterized in that, Includes boilers, thermal storage units, and pyrolysis heating units; The thermal storage unit is equipped with low-temperature molten salt. The high-temperature flue gas generated by the boiler operation is transported to the thermal storage unit to exchange heat with the low-temperature molten salt to obtain high-temperature molten salt. The pyrolysis heating unit is equipped with an ammonia pyrolysis heat exchanger, a urea pyrolysis heat exchanger and a steam-molten salt heat exchanger. The ammonia pyrolysis heat exchanger is used for ammonia pyrolysis, the urea pyrolysis heat exchanger is used for pyrolysis of urea solution to generate ammonia, and the steam-molten salt heat exchanger is used for heating steam. The molten salt side outlet of the thermal storage unit is connected to the molten salt side inlet of the ammonia pyrolysis heat exchanger. Molten salt replenishment lines are provided between the molten salt side outlet of the thermal storage unit and the molten salt side inlets of the urea pyrolysis heat exchanger and the steam-molten salt heat exchanger. The molten salt sides of the ammonia pyrolysis heat exchanger, the urea pyrolysis heat exchanger, and the steam-molten salt heat exchanger are connected in series. High-temperature molten salt flows through the ammonia pyrolysis heat exchanger, the urea pyrolysis heat exchanger, and the steam-molten salt heat exchanger in sequence. The gas side outlet of the urea pyrolysis heat exchanger is connected to the gas side inlet of the ammonia pyrolysis heat exchanger. The gas side outlet of the ammonia pyrolysis heat exchanger is connected to the boiler, and the ammonia pyrolysis gas generated by ammonia pyrolysis is transported to the boiler to replace fuel for combustion. Low-temperature steam is introduced into the steam-molten salt heat exchanger. The low-temperature steam exchanges heat with the molten salt flowing through the steam-molten salt heat exchanger to obtain high-temperature steam, which is then transported to the external thermal system for heating. The molten salt after heat exchange returns to the thermal storage unit to continue participating in the thermal storage and pyrolysis heating cycle.

2. The ammonia pyrolysis co-firing system according to claim 1, characterized in that, The heat storage unit includes a flue gas-molten salt heat exchanger, a molten salt pump at the heat exchanger inlet, a low-temperature molten salt tank, and a high-temperature molten salt tank. The high-temperature flue gas generated by the boiler is introduced into the flue gas-molten salt heat exchanger, and at the same time, low-temperature molten salt is pumped into the low-temperature molten salt tank through the molten salt pump at the heat exchanger inlet. The high-temperature flue gas and the low-temperature molten salt exchange heat in the flue gas-molten salt heat exchanger. After the temperature of the high-temperature flue gas decreases, it is discharged as low-temperature flue gas. After the low-temperature molten salt absorbs heat, its temperature rises and it becomes high-temperature molten salt, which is then transported and stored in the high-temperature molten salt tank.

3. The ammonia pyrolysis co-firing system according to claim 1, characterized in that, The ammonia pyrolysis heat exchanger, urea pyrolysis heat exchanger, and steam-molten salt heat exchanger are all equipped with molten salt control pumps before the molten salt side inlet to control the flow rate of high-temperature molten salt. The molten salt side outlet of the heat storage unit is connected to the ammonia pyrolysis heat exchanger, urea pyrolysis heat exchanger, and steam-molten salt heat exchanger respectively through the corresponding molten salt control pumps.

4. The ammonia pyrolysis co-firing system according to claim 1, characterized in that, A catalyst is installed in the gas channel of the ammonia pyrolysis heat exchanger. Ammonia undergoes a pyrolysis reaction under the action of the catalyst to generate ammonia pyrolysis gas, which is a mixture of hydrogen and nitrogen.

5. The ammonia pyrolysis co-firing system according to claim 1, characterized in that, The pyrolysis heating unit also includes a urea solution tank, which is connected to the gas-side inlet of the urea pyrolysis heat exchanger and is used to introduce urea solution into the urea pyrolysis heat exchanger.

6. The ammonia pyrolysis co-firing system according to claim 1, characterized in that, A burner is installed on the outer side of the bottom of the boiler. An ammonia pyrolysis gas inlet and a pulverized coal and primary air inlet are installed on the outer side of the burner. The ammonia pyrolysis gas enters the furnace through the ammonia pyrolysis gas inlet. At the same time, fuel is added to the furnace of the burner through the pulverized coal and primary air inlets. The ammonia pyrolysis gas and fuel are mixed and burned. A burner outlet jet is formed at the outlet on the inner side of the burner and injected into the boiler.

7. The ammonia pyrolysis co-firing system according to claim 3, characterized in that, It also includes a molten salt flow controller, which is connected to three molten salt control pumps to form an actuator for distributing molten salt flow.

8. The ammonia pyrolysis co-firing system according to claim 7, characterized in that, It also includes a central control system, which monitors the boiler load command, the heat storage capacity and temperature of the heat storage unit, and the target fuel blending ratio in the boiler in real time, and sends commands to the molten salt flow controller to adjust the flow of the three molten salt control pumps to meet the different operating requirements of the boiler.

9. A co-firing method using the ammonia pyrolysis co-firing system based on flue gas waste heat and molten salt coupling as described in claim 8, characterized in that, First, the high-temperature flue gas generated by the boiler is transported to the heat storage unit to exchange heat with the low-temperature molten salt to obtain high-temperature molten salt, thus completing the time transfer and storage of thermal energy. When the boiler needs to co-fire ammonia or hydrogen fuel, three molten salt control pumps are started. According to the target fuel co-firing ratio in the boiler, the central control system regulates the flow of the three molten salt control pumps through the molten salt flow controller, and executes the following process: (1) Extract the required amount of high-temperature molten salt from the heat storage unit into the ammonia pyrolysis heat exchanger, and at the same time, introduce the ammonia gas generated in the urea pyrolysis heat exchanger into the ammonia pyrolysis heat exchanger; the high-temperature molten salt releases heat energy, causing the ammonia gas to undergo a pyrolysis reaction in the ammonia pyrolysis heat exchanger; the ammonia pyrolysis gas after the reaction is drawn out from the gas side outlet of the ammonia pyrolysis heat exchanger and transported to the boiler for combustion, and the high-temperature flue gas generated during the combustion process is output from the flue gas outlet of the boiler to the heat storage unit; after the high-temperature molten salt releases heat energy, its temperature decreases to become medium-temperature molten salt and flows into the urea pyrolysis heat exchanger; (2) The required amount of high-temperature molten salt is supplied from the molten salt supply line to the urea pyrolysis heat exchanger as needed, and urea solution is pumped into the urea pyrolysis heat exchanger; the medium-temperature molten salt and the high-temperature molten salt release heat energy, causing the urea solution to pyrolyze and produce a gaseous mixture containing ammonia, and the ammonia is transported to the ammonia pyrolysis heat exchanger as ammonia pyrolysis raw material; after the urea solution is pyrolyzed, the temperature of the medium-temperature molten salt and the high-temperature molten salt decreases to obtain low-temperature molten salt, and the low-temperature molten salt enters the steam-molten salt heat exchanger; (3) The required amount of high-temperature molten salt is supplied from the molten salt supply line to the steam-molten salt heat exchanger as needed. At the same time, low-temperature steam is introduced into the steam-molten salt heat exchanger. The low-temperature molten salt and high-temperature molten salt release heat energy to heat the low-temperature steam, so that the low-temperature steam becomes high-temperature steam and is then transported to the external thermal system for heating. Finally, the molten salt returns to the heat storage unit to continue to participate in the heat storage and pyrolysis heating cycle.

10. The method according to claim 9, characterized in that, The gas-side outlet of the urea pyrolysis heat exchanger is also connected to the boiler. When only ammonia needs to be co-fired or the demand for ammonia is large, the gas-side outlet of the urea pyrolysis heat exchanger is disconnected from the gas-side inlet of the ammonia pyrolysis heat exchanger, and the gas-side outlet of the urea pyrolysis heat exchanger is connected to the boiler. After the heat storage unit completes the time transfer and storage of thermal energy, the two molten salt control pumps corresponding to the urea pyrolysis heat exchanger and the steam-molten salt heat exchanger are started. According to the target fuel co-firing ratio, the central control system regulates the flow of the two molten salt control pumps through the molten salt flow controller. The required amount of high-temperature molten salt is extracted from the molten salt replenishment line and sent to the urea pyrolysis heat exchanger. Urea solution is pumped into the urea pyrolysis heat exchanger. The high-temperature molten salt heats the urea solution to produce ammonia. After the urea solution is pyrolyzed, the ammonia enters the boiler for combustion through the gas side outlet of the urea pyrolysis heat exchanger. The temperature of the high-temperature molten salt decreases to that of medium-low temperature molten salt and enters the steam-molten salt heat exchanger. Subsequently, low-temperature steam is introduced into the steam-molten salt heat exchanger. The medium-low temperature molten salt heats the low-temperature steam, turning it into high-temperature steam, which is then delivered to the external thermal system for heating.

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