A multi-cold-source system and method of coupling a gas-steam boiler with a two-stage heat pump

Abstract

This invention discloses a multi-cold source system and method for a gas-fired steam boiler coupled with a two-stage heat pump. The flue gas from the gas-fired boiler releases heat through two stages of flue gas-water heat exchangers, then enters a parallel first-stage flue gas-air heat exchanger, a third-stage flue gas-water heat exchanger, and a flue gas-circulating water heat exchanger. The flue gas-circulating water heat exchanger provides a heat source for a low-temperature heat pump or a lithium bromide absorption heat pump, or an air-source heat pump can replace the flue gas-circulating water heat exchanger and the low-temperature heat pump. The heat from the flue gas in the parallel structure is distributed, and the released heat from the flue gas heats the air and feedwater. The flue gas then heats the feedwater through the low-temperature heat pump, air-source heat pump, or lithium bromide absorption heat pump. The heated feedwater then absorbs heat through a high-temperature heat pump and the first-stage flue gas-water heat exchanger before entering the gas-fired boiler along with the air. This invention fully absorbs the latent heat of water vapor in the flue gas of a gas-fired steam boiler, reducing the flue gas temperature to below 30°C, raising the feedwater temperature from 20°C to 120-140°C, raising the air temperature from 20°C to 50-90°C, and increasing the boiler efficiency to over 107%.

Description

Technical Field

[0001] This invention belongs to the field of boiler and heat pump technology, specifically relating to a multi-cold source system and method for a gas-fired steam boiler coupled with a two-stage heat pump. Background Technology

[0002] Waste heat recovery from flue gas is one of the effective ways to improve boiler efficiency. The exhaust temperature of gas-fired boilers is generally between 50℃ and 200℃. Water vapor in the flue gas only partially condenses or not at all, leaving a large amount of latent heat of vaporization unutilized. If the water vapor in the flue gas were completely condensed, it could release 8% to 11% of the fuel's lower heating value. The water dew point of gas-fired flue gas is mostly between 55℃ and 58℃, while the feedwater temperature of gas-fired steam boilers is approximately 20℃ to 70℃, and the feedwater flow rate is much lower than that of gas-fired hot water boilers. Even if the feedwater temperature is lower than the water dew point of the flue gas, only a portion of the latent heat of the water vapor can be utilized. Traditional steam boilers achieve an efficiency of only 97% at a return water temperature of 50℃, and 99% at a return water temperature of 20℃. The highest thermal efficiency calculated based on the lower heating value can reach 111%, meaning a large amount of latent heat of water vapor in the exhaust gas of gas-fired steam boilers is not effectively utilized. The latent heat of steam can only be effectively utilized when the feedwater temperature is below the water dew point. However, steam boilers have very little feedwater, and the feedwater heats up rapidly to above the water dew point after entering the boiler. There is an urgent need for a system that can transfer the latent heat of steam in the boiler flue gas to the boiler feedwater or combustion air to improve the efficiency of gas-fired boilers.

[0003] Heat pumps are highly efficient and energy-saving devices that can fully utilize low-grade heat energy, and have received widespread attention in recent years. The working principle of a heat pump is to absorb heat from a low-temperature object and release it to a high-temperature object, consuming only a small amount of work to obtain a large amount of heat supply, effectively utilizing low-grade heat energy that is difficult to utilize. Typically, the coefficient of performance (COP) of a heat pump is 4-5, meaning that a heat pump can absorb heat from a low-temperature heat source and transfer 4 to 5 times the energy it consumes to a high-temperature object. Currently, the main heat pumps on the market can be divided into compression heat pumps and absorption heat pumps based on their refrigeration principles. Heat pumps are now widely used in waste heat recovery from boiler exhaust gas and wastewater discharge from factories. Coupling gas and heat pumps improves boiler efficiency and saves natural gas by replacing gas with electricity. When the COP of a heat pump increases to 3-6, the system can recover its investment in a short period.

[0004] The CN208253927U patent application from Tianjin Gas and Heat Planning and Design Institute Co., Ltd. describes a waste heat recovery system for gas-fired boiler flue gas. This system utilizes a heat pump to effectively absorb waste heat from the flue gas to preheat the feedwater, thus reducing the exhaust gas temperature. However, the system uses a spray tower to lower the flue gas temperature, resulting in a large water consumption and a large system size, requiring consideration of cost and benefit. The CN111156733A patent application from Northeast Electric Power University describes a biomass flue gas waste heat recovery absorption-compression coupled heat pump system. This system couples a compression heat pump unit and a heating absorption heat pump unit to form a heating absorption-compression coupled heat pump module. The biomass flue gas passes through the first, second, and third flue gas heat exchange units, reducing its temperature from 145℃ to below 40℃, while the heating hot water temperature rises from 45℃ to above 70℃. This invention achieves cascade utilization of waste heat from the flue gas, improving the efficiency of the biomass boiler. However, this invention only uses the heating water as a cold source to absorb waste heat from the flue gas, which is clearly insufficient. Currently, most gas-fired boilers coupled with heat pumps are hot water boilers, and there are very few cases of steam boilers coupled with heat pumps. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a multi-cold source system and method for a gas-fired steam boiler coupled with a two-stage heat pump. By utilizing feedwater, air, and a low-temperature heat pump to fully absorb the condensation heat of flue gas, the flue gas temperature is reduced to below 30°C, and the boiler efficiency is increased to over 107%, thus achieving energy saving.

[0006] To achieve the above objectives, the technical solution adopted by this invention is: a multi-cold source method for coupling a gas-fired steam boiler with a two-stage heat pump. The flue gas from the gas-fired boiler releases heat through two stages of flue gas-water heat exchangers, then enters a parallel first-stage flue gas-air heat exchanger, a third-stage flue gas-water heat exchanger, and a flue gas-circulating water heat exchanger for further heat release; or it enters a parallel first-stage flue gas-air heat exchanger, a third-stage flue gas-water heat exchanger, and an air-source heat pump for further heat release; or it enters a parallel first-stage flue gas-air heat exchanger, a third-stage flue gas-water heat exchanger, and a lithium bromide absorption heat pump for further heat release. The heat release in the parallel structure can be regulated. After releasing heat, the flue gas heats the air and feedwater. The flue gas then heats the feedwater through a low-temperature heat pump, an air-source heat pump, or a lithium bromide absorption heat pump, respectively. The heated feedwater then absorbs heat through a high-temperature heat pump and the first-stage flue gas-water heat exchanger. The heated air and feedwater then enter the gas-fired boiler.

[0007] Furthermore, the air heated by the first-stage flue gas-air heat exchanger is heated by the second-stage flue gas-air heat exchanger, and the flow rate of the flue gas used is regulated.

[0008] Furthermore, the air heated by the first-stage flue gas-air heat exchanger is then heated by the second-stage and third-stage flue gas-air heat exchangers in sequence, and the flow rate of the flue gas used is regulated.

[0009] Furthermore, the air heated by the first-stage flue gas-air heat exchanger is heated in a second stage using a second-stage water-air heat exchanger, with the feed water for the second-stage water-air heat exchanger coming from a low-temperature heat pump.

[0010] Furthermore, the air heated by the first-stage flue gas-air heat exchanger is then heated by the second-stage water-air heat exchanger and the third-stage water-air heat exchanger in sequence, with the feed water for the third-stage water-air heat exchanger coming from a high-temperature heat pump.

[0011] Based on the same inventive concept, this invention provides a multi-cold source system for a gas-fired steam boiler coupled with a two-stage heat pump, comprising a gas-fired boiler, a first-stage flue gas-water heat exchanger, and a second-stage flue gas-water heat exchanger connected sequentially along the flue gas flow direction. The flue gas outlet of the second-stage flue gas-water heat exchanger is connected to a first-stage flue gas-air heat exchanger, a third-stage flue gas-water heat exchanger, and a flue gas-circulating water heat exchanger. The flue gas-circulating water heat exchanger is connected to a low-temperature heat pump, or the flue gas outlet of the second-stage flue gas-water heat exchanger is connected to the first-stage flue gas-air heat exchanger and the third-stage flue gas-water heat exchanger. And an air source heat pump; or the flue gas outlet of the second-stage flue gas-water heat exchanger is connected to the first-stage flue gas-air heat exchanger, the third-stage flue gas-water heat exchanger, and a lithium bromide absorption heat pump; the second-stage flue gas-water heat exchanger is connected to a high-temperature heat pump, while the low-temperature and high-temperature heat pumps are compression-type water source heat pumps; the third-stage flue gas-water heat exchanger, the low-temperature heat pump, the high-temperature heat pump, and the water side of the first-stage flue gas-water heat exchanger are connected; the water side outlet of the first-stage flue gas-water heat exchanger is connected to the feed water inlet of the gas-fired boiler; the air side of the first-stage flue gas-air heat exchanger is connected to a ventilator and a gas-fired boiler.

[0012] Furthermore, a second-stage flue gas-air heat exchanger is connected in parallel to the second-stage flue gas-water heat exchanger. The air-side outlet of the first-stage flue gas-air heat exchanger is connected to the gas-fired boiler via the second-stage flue gas-air heat exchanger. Three-way valves are installed at the flue gas side inlet and outlet of the second-stage flue gas-water heat exchanger and the second-stage flue gas-air heat exchanger.

[0013] Furthermore, a third-stage flue gas-air heat exchanger is connected in parallel to the first-stage flue gas-water heat exchanger. The air-side outlet of the first-stage flue gas-air heat exchanger is connected to the gas-fired boiler via the second-stage and third-stage flue gas-air heat exchangers in sequence. Three-way valves are installed at the flue gas-side inlet and outlet of the first-stage flue gas-water heat exchanger and the third-stage flue gas-air heat exchanger.

[0014] Furthermore, the low-temperature heat pump includes a first evaporator, a first throttling valve, a first condenser, and a first compressor; the cold-side outlet of the first evaporator is sequentially connected to the first compressor, the first condenser, and the first throttling valve along the refrigerant flow direction, and the first throttling valve is connected to the cold-side inlet of the first evaporator; the hot side of the first evaporator is connected to the cold side of the flue gas-circulating water heat exchanger, and the water side of the first condenser is connected to the water side of the high-temperature heat pump.

[0015] Furthermore, a second-stage water-air heat exchanger is installed between the water side of the first condenser and the water side of the high-temperature heat pump. The air side of the first-stage flue gas-air heat exchanger is connected to the gas-fired boiler via the air side of the second-stage water-air heat exchanger.

[0016] Furthermore, the high-temperature heat pump includes a second evaporator, a second expansion valve, a second condenser, and a second compressor; the cold-side outlet of the second evaporator is sequentially connected to the second compressor, the second compressor, and the second expansion valve along the refrigerant flow direction; the outlet of the second expansion valve is connected to the cold-side inlet of the second evaporator; the hot-side of the second evaporator is connected to the cold side of the second-stage flue gas-water heat exchanger; the water-side inlet of the second condenser is connected to the water-side outlet of the low-temperature heat pump; and the water-side outlet of the second condenser is connected to the water-side inlet of the first-stage flue gas-water heat exchanger.

[0017] Furthermore, the water-side outlet of the high-temperature heat pump is connected to the water-side inlet of the first-stage flue gas-water heat exchanger via the third-stage water-air heat exchanger, and the air side of the first-stage flue gas-air heat exchanger is connected to the gas-fired boiler via the air sides of the second-stage and third-stage water-air heat exchangers.

[0018] Furthermore, the air source heat pump adopts a compression heat pump, which includes a third evaporator, a third throttling valve, a third condenser, and a third compressor. The cold-side outlet of the third evaporator is connected to the third compressor, the third condenser, and the third throttling valve in sequence. The outlet of the third throttling valve is connected to the cold-side inlet of the third evaporator. The hot-side inlet of the third evaporator is connected to the flue gas outlet of the second-stage flue gas-water heat exchanger. The hot-side outlet of the third evaporator is vented. The water-side outlet of the third condenser is connected to a high-temperature heat pump.

[0019] Furthermore, the lithium bromide absorption heat pump includes a generator, a fourth condenser, a fourth throttling valve, a fourth evaporator, an absorber, a solution valve, a solution pump, and a solution heat exchanger. The working fluid outlet of the generator is connected to the working fluid side of the fourth condenser. The working fluid side outlet of the fourth condenser is connected to the working fluid side of the fourth evaporator through the fourth throttling valve. The working fluid side of the fourth evaporator is connected to the working fluid inlet of the absorber. The working fluid outlet of the absorber is connected to the dilute solution inlet of the solution heat exchanger. The dilute solution outlet of the solution heat exchanger is connected to the working fluid inlet of the generator. A solution pump is installed on the pipeline from the absorber to the solution heat exchanger. The generator is connected to the solution heat exchanger. A solution valve is installed on the pipeline from the solution heat exchanger to the absorber. The hot side of the fourth evaporator is connected to the cold side of the flue gas-circulating water heat exchanger. The water side outlet of the third-stage flue gas-water heat exchanger enters the water side inlet of the absorber. The water side outlet of the absorber is connected to the water side inlet of the fourth condenser. The water side outlet of the fourth condenser is connected to the high-temperature heat pump. The lithium bromide absorption heat pump is driven by high-temperature steam generated by a gas boiler, or by high-temperature water or heat transfer oil.

[0020] Compared with the prior art, the present invention has at least the following beneficial effects:

[0021] This invention provides a multi-cold source system and method for a gas-fired steam boiler coupled with a two-stage heat pump. It utilizes feedwater, air, and a low-temperature heat pump to fully absorb the latent heat of water vapor and a small amount of sensible heat contained in the flue gas. The invention employs a parallel system, with the final-stage flue gas split into three parts, entering a flue gas-water heat exchanger, a flue gas-air heat exchanger, and a flue gas-circulating water heat exchanger respectively. The feedwater is heated in stages through the flue gas, the low-temperature heat pump, and the high-temperature heat pump. By controlling the valve opening, the heat from the flue gas is rationally distributed, thereby controlling the outlet temperatures of the feedwater and air, ensuring the system operates at its optimal state. This reduces the flue gas temperature to below 30°C, raises the feedwater temperature from 20°C to 120-140°C, and the air temperature from 20°C to 50-90°C, increasing the boiler efficiency to over 107%, achieving energy savings.

[0022] Furthermore, this invention enables the cascade utilization of waste heat from flue gas, resulting in cascade heating of feedwater. Depending on the system, air can also be heated in stages, and two different methods, flue gas heating and feedwater heating, can be used.

[0023] Furthermore, a second-stage flue gas-air heat exchanger is installed, where the air is heated twice by passing through the flue gas.

[0024] Furthermore, a second-stage flue gas-air heat exchanger and a third-stage flue gas-air heat exchanger are installed, allowing the air to be heated three times through the flue gas.

[0025] Furthermore, by setting up a second-stage water-air heat exchanger, the air is heated twice through flue gas and feedwater.

[0026] Furthermore, by simultaneously installing a second-stage water-air heat exchanger and a third-stage water-air heat exchanger, the air is heated three times through flue gas and feedwater.

[0027] Furthermore, the heat pump COP is maintained at 3-6, ensuring that the system efficiency is always at its best. At the same time, as much heat is recovered as possible, and electricity is used instead of gas, which can realize the return on investment in a short period of time, resulting in significant economic benefits.

[0028] Furthermore, compression heat pumps and absorption heat pumps can also be driven by renewable energy sources to recover waste heat from flue gas in a zero-carbon manner. Attached Figure Description

[0029] Figure 1a This is a schematic diagram of a multi-cold source system of a gas-fired steam boiler coupled with a two-stage heat pump.

[0030] Figure 1b This is a schematic diagram of a multi-cold source system of a gas-fired steam boiler coupled with a two-stage heat pump.

[0031] Figure 1cThis is a schematic diagram of a multi-cold source system of a gas-fired steam boiler coupled with a two-stage heat pump.

[0032] Figure 2a This is a schematic diagram of a multi-cold source system of a gas-fired steam boiler coupled with a two-stage heat pump.

[0033] Figure 2b This is a schematic diagram of a multi-cold source system of a gas-fired steam boiler coupled with a two-stage heat pump.

[0034] Figure 3 This is a schematic diagram of a multi-cold source system of a gas-fired steam boiler coupled with a two-stage heat pump.

[0035] Figure 4 This is a schematic diagram of a multi-cold source system of a gas-fired steam boiler coupled with a two-stage heat pump.

[0036] In the diagram, 1-Gas-fired steam boiler, 2-First-stage flue gas-water heat exchanger, 3-Second-stage flue gas-water heat exchanger, 4-First four-way valve, 5-First-stage flue gas-air heat exchanger, 6-Third-stage flue gas-water heat exchanger, 7-Second four-way valve, 8-Low-temperature heat pump, 81-First evaporator, 82-First throttling valve, 83-First condenser, 84-First compressor, 9-High-temperature heat pump, 91-Second evaporator, 92-Second throttling valve, 93-Second condenser, 94-Second compressor, 10-Flue gas-circulating water heat exchanger, 11-Fourth three-way valve, 12-Second-stage flue gas-air heat exchanger 13-Third three-way valve, 14-Second three-way valve, 15-Third stage flue gas-air heat exchanger, 16-First three-way valve, 17-Second stage water-air heat exchanger, 18-Third stage water-air heat exchanger, 19-Air source heat pump, 191-Third evaporator, 192-Third throttle valve, 193-Third condenser, 194-Third compressor, 20-Lithium bromide absorption heat pump, 201-Fourth evaporator, 202-Fourth throttle valve, 203-Fourth condenser, 204-Generator, 205-Solution heat exchanger, 206-Solution pump, 207-Solution valve, 208-Absorber. Detailed Implementation

[0037] The present invention will now be described in further detail with reference to the accompanying drawings:

[0038] like Figure 1aAs shown, the system includes a gas-fired steam boiler 1, various stages of flue gas-water heat exchangers, a first-stage flue gas-air heat exchanger 5, a low-temperature heat pump 8, a high-temperature heat pump 9, valves, and connecting pipes. Both the low-temperature heat pump 8 and the high-temperature heat pump 9 are compression-type water source heat pumps. The flue gas outlet of the gas-fired steam boiler 1 is connected to the flue gas inlet of the first-stage flue gas-water heat exchanger 2. The flue gas outlet of the first-stage flue gas-water heat exchanger 2 is connected to the flue gas inlet of the second-stage flue gas-water heat exchanger 3. A first four-way valve 4 is installed outside the flue gas outlet of the second-stage flue gas-water heat exchanger 3, and a second four-way valve 7 is installed outside the flue gas outlet of the third-stage flue gas-water heat exchanger 6. The three outlets of the first four-way valve 4 are respectively connected to the flue gas inlet of the third-stage flue gas-water heat exchanger 6, the flue gas inlet of the first-stage flue gas-air heat exchanger 5, and the flue gas-circulating water heat exchanger 10. The flue gas outlets of the first-stage flue gas-air heat exchanger 5, the third-stage flue gas-water heat exchanger 6, and the flue gas-circulating water heat exchanger 10 are merged through the second four-way valve 7 and discharged into the atmosphere.

[0039] The air undergoes a heating process, specifically as follows: the air-side outlet of the first-stage flue gas-air heat exchanger 5 is connected to the gas-fired steam boiler 1, and the air enters through the air-side inlet of the first-stage flue gas-air heat exchanger 5, exchanges heat with the flue gas, and then flows out through the air-side outlet of the first-stage flue gas-air heat exchanger 5.

[0040] The feedwater is heated in stages by passing through low-temperature flue gas, low-temperature heat pump, high-temperature heat pump and high-temperature flue gas in sequence. The specific process is as follows: the water-side inlet of the third-stage flue gas-water heat exchanger 6 is connected to the external water pipe, the water-side outlet of the third-stage flue gas-water heat exchanger 6 is connected to the water-side inlet of the first condenser 83, the water-side outlet of the first condenser 83 is connected to the water-side inlet of the second condenser 93, and the water-side outlet of the second condenser 93 is connected to the water-side inlet of the first-stage flue gas-water heat exchanger 2.

[0041] Both the low-temperature heat pump 8 and the high-temperature heat pump 9 adopt indirect heat exchange. Taking the low-temperature heat pump 8 as an example, the refrigerant exchanges heat with the circulating cooling water in the first evaporator 81. After absorbing the heat from the circulating cooling water and evaporating, the refrigerant passes through the first compressor 84, the first condenser 83 and the first throttling valve 82 in sequence.

[0042] One implementation of this system is described below: When the feedwater temperature is around 20°C, this system distributes flue gas with a temperature close to the flue gas dew point, allocating 60% of the flue gas to air and feedwater, with air accounting for 10% and feedwater for 50%. At this point, the 20°C feedwater can be heated to 50°C, and the 20°C air can be preheated to 50°C. The remaining 40% of the flue gas waste heat is absorbed by a low-temperature heat pump, providing a temperature rise of approximately 30°C to the 50°C feedwater. The low-temperature heat pump's COP is 3.4 at this point. A high-temperature heat pump absorbs heat from the 130°C flue gas, lowering it to near the flue gas acid dew point, providing a temperature rise of approximately 25°C to the feedwater. The high-temperature heat pump's COP is 5.4 at this point. After two heating cycles, the feedwater temperature reaches 105°C. Finally, after further heating with 230°C flue gas, the flue gas temperature is lowered to 130°C, bringing the final feedwater temperature to approximately 140°C. By absorbing and utilizing the condensation heat of flue gas through air, feedwater, and low-temperature heat pumps, the exhaust temperature can be reduced to 30℃. At this point, the boiler efficiency can reach 107%, and the system can recover its investment in 1-2 years.

[0043] like Figure 1b As shown, in Figure 1a A second-stage flue gas-air heat exchanger 12 can be installed on top of the existing system to achieve secondary heating of the air. Specifically, a third three-way valve 13 is installed at the flue gas outlet of the first-stage flue gas-water heat exchanger 2, and a fourth three-way valve 11 is installed at the flue gas outlet of the second-stage flue gas-water heat exchanger 3. The first outlet of the third three-way valve 13 is connected to the flue gas inlet of the second-stage flue gas-water heat exchanger 3, and the second outlet of the third three-way valve 13 is connected to the flue gas inlet of the second-stage flue gas-air heat exchanger 12. The flue gas outlets of the second-stage flue gas-air heat exchanger 12 and the second-stage flue gas-water heat exchanger 3 merge through the fourth three-way valve 11 and then enter the inlet of the first four-way valve 4. This system requires the distribution of flue gas at 130℃, and 30% of the flue gas can be distributed to the air, at which point the air temperature can be raised to 75℃. The remaining 70% of the flue gas heat is absorbed by a high-temperature heat pump, which can provide a temperature rise of approximately 15℃ for the feedwater. Finally, the flue gas is heated to 230℃, reducing its temperature to 130℃, and the feedwater temperature eventually reaches around 130℃.

[0044] like Figure 1c As shown, in Figure 1bBased on this, a third-stage flue gas-air heat exchanger 15 can be installed to achieve tertiary heating of the air. Specifically, a second three-way valve 14 is installed between the flue gas outlet of the first-stage flue gas-water heat exchanger 2 and the third three-way valve 13. A first three-way valve 16 is installed between the flue gas outlet of the gas-fired steam boiler 1 and the first-stage flue gas-water heat exchanger 2. The first outlet of the first three-way valve 16 is connected to the flue gas inlet of the first-stage flue gas-water heat exchanger 2, and the second outlet of the first three-way valve 16 is connected to the flue gas inlet of the third-stage flue gas-air heat exchanger 15. The flue gas outlet of the third-stage flue gas-air heat exchanger 15 and the flue gas outlet of the first-stage flue gas-water heat exchanger 2 merge through the second three-way valve 14 and then enter the inlet of the third three-way valve 13. This system requires the distribution of 230℃ flue gas, and can distribute 10% of the flue gas to the air, raising the air temperature from 75℃ to 85℃. The remaining 90% of the flue gas heat is used to heat the feedwater, which can eventually reach about 125°C.

[0045] like Figure 2a As shown, because liquids have a stronger heat transfer capacity, no additional heat transfer enhancement treatment is needed on the liquid side. Figure 1b Based on this, the second-stage flue gas-air heat exchanger 12 can be replaced with a second-stage water-air heat exchanger 17. Specifically, the air-side inlet of the second-stage water-air heat exchanger 17 is connected to the air-side outlet of the first-stage flue gas-air heat exchanger 5. The water-side inlet of the second-stage water-air heat exchanger 17 is connected to the water-side outlet of the first condenser 83, and the water-side outlet of the second-stage water-air heat exchanger 17 is connected to the water-side inlet of the second condenser 93. Air is heated twice through flue gas and feedwater. After passing through a low-temperature heat pump, the feedwater is heated to 85°C. Because water has a stronger heat exchange capacity than air, the feedwater only needs to be cooled by 5°C to raise the air temperature by about 15°C. Then, the 80°C feedwater is further heated by a high-temperature heat pump and 230°C high-temperature flue gas, ultimately reaching a temperature of approximately 135°C.

[0046] like Figure 2b As shown, in Figure 2a A third-stage water-air heat exchanger 18 can be installed on this basis. Specifically, the air-side inlet of the third-stage water-air heat exchanger 18 is connected to the air-side outlet of the second-stage water-air heat exchanger 17. The water-side inlet of the third-stage water-air heat exchanger 18 is connected to the water-side outlet of the second condenser 93, and the water-side outlet of the third-stage water-air heat exchanger 18 is connected to the water-side inlet of the first-stage flue gas-water heat exchanger 2. Figure 2a The system indicates that after being heated by the high-temperature heat pump, the feedwater temperature can be raised to 105℃, and the feedwater temperature can also be lowered by 5℃, while the air temperature can be raised from 70℃ to 90℃. The 100℃ feedwater can be further heated to about 130℃ by the 230℃ flue gas.

[0047] like Figure 3As shown, the low-temperature heat pump 8 and the high-temperature heat pump 9 can also be replaced by the air-source heat pump 19. In this case, the flue gas directly exchanges heat with the refrigerant, eliminating the need for a flue gas-circulating water heat exchanger; the remaining processes are the same as... Figure 1a same.

[0048] like Figure 4 As shown, the low-temperature heat pump 8 and high-temperature heat pump 9 can also be replaced by a lithium bromide absorption heat pump 20. The lithium bromide absorption heat pump system includes a generator 204, a fourth condenser 203, a fourth throttling valve 202, a fourth evaporator 201, an absorber 208, a solution valve 207, a solution pump 206, and a solution heat exchanger 205. The working fluid outlet of the generator 204 is connected to the working fluid side of the fourth condenser 203. The working fluid outlet of the fourth condenser 203 is connected to the working fluid side of the fourth evaporator 201 via the fourth throttling valve 202. The working fluid side of the fourth evaporator 201 is connected to the working fluid inlet of the absorber 208. The working fluid outlet of the absorber 208 is connected to the dilute solution inlet of the solution heat exchanger 205, and the dilute solution outlet of the solution heat exchanger 205 is connected to the working fluid inlet of the generator 204. A solution pump 206 is installed on the pipeline from the absorber 208 to the solution heat exchanger 205. The generator 204 is connected to the solution heat exchanger 205, and a solution valve 207 is installed on the pipeline from the solution heat exchanger 205 to the absorber 208. The water-side outlet of the third flue gas-water heat exchanger 6 is connected in sequence to the absorber 208 and the fourth condenser 203. The fourth evaporator 201 also adopts an indirect heat exchange method. The lithium bromide absorption heat pump 20 can be driven by high-temperature steam generated by the gas-fired steam boiler 1, or it can be driven by other high-temperature heat sources (such as high-temperature water or heat transfer oil).

[0049] This invention provides a multi-cold source system and method for a gas-fired steam boiler coupled with a two-stage heat pump. The flue gas discharged from the gas-fired steam boiler 1 releases heat through two stages of flue gas-water heat exchangers, then enters a parallel-connected first-stage flue gas-air heat exchanger 5, a third-stage flue gas-water heat exchanger 6, and a flue gas-circulating water heat exchanger 10 for further heat release; or it enters a parallel-connected first-stage flue gas-air heat exchanger 5, a third-stage flue gas-water heat exchanger 6, and an air source heat pump 19 for further heat release; or it enters a parallel-connected first-stage flue gas-air heat exchanger 5, a third-stage flue gas-water heat exchanger 6, and a lithium bromide absorption heat pump 20 for further heat release. Furthermore, the heat release in the parallel structure can be... The system controls the flow of heat from the flue gas to heat the air and feedwater. The flue gas then heats the feedwater via a low-temperature heat pump 8, an air-source heat pump 19, or a lithium bromide absorption heat pump 20. The heated feedwater then absorbs heat through a high-temperature heat pump 9 and a first-stage flue gas-water heat exchanger 2. The heated air and feedwater then enter the gas-fired steam boiler 1. By using a gas-coupled high-low temperature dual-stage heat pump, the latent heat of water vapor in the flue gas is fully absorbed, reducing the flue gas temperature to below 30°C, raising the feedwater temperature from 20°C to 120-140°C, and raising the air temperature from 20°C to 50-90°C. This increases the efficiency of the gas-fired steam boiler to over 107%, achieving energy savings.

Claims

1. A multi-cold-source method for coupling a gas-fired steam boiler with a two-stage heat pump, characterized in that: The flue gas from the gas-fired boiler (1) releases heat through two stages of flue gas-water heat exchangers, and then enters the parallel first-stage flue gas-air heat exchanger (5), the third-stage flue gas-water heat exchanger (6), and the flue gas-circulating water heat exchanger (10) to release heat. The flue gas-circulating water heat exchanger (10) is connected to a low-temperature heat pump (8), or enters the parallel first-stage flue gas-air heat exchanger (5), the third-stage flue gas-water heat exchanger (6), and the air source heat pump (19) to release heat, or enters the parallel first-stage flue gas-air heat exchanger (5), the third-stage flue gas-water heat exchanger (6), and the lithium bromide absorption heat pump (2) to release heat. 0) Heat is released and distributed in the parallel structure. The second-stage flue gas-water heat exchanger (3) is connected to the high-temperature heat pump (9). After the flue gas releases heat in the first-stage flue gas-air heat exchanger (5), it heats the air. The flue gas heats the feed water in the third-stage flue gas-water heat exchanger (6). The flue gas is then heated again by the low-temperature heat pump (8), the air source heat pump (19), or the lithium bromide absorption heat pump (20). After being reheated, the feed water is then heated by the high-temperature heat pump (9) and the first-stage flue gas-water heat exchanger (2). The heated air and feed water enter the gas boiler (1).

2. The multi-cold-source method for coupling a gas-fired steam boiler with a two-stage heat pump according to claim 1, characterized in that: The second-stage flue gas-water heat exchanger (3) is connected in parallel with the second-stage flue gas-air heat exchanger (12). The air-side outlet of the first-stage flue gas-air heat exchanger (5) is connected to the gas boiler (1) through the second-stage flue gas-air heat exchanger (12). The air heated by the first-stage flue gas-air heat exchanger (5) is heated by the second-stage flue gas-air heat exchanger (12), and the flow rate of the flue gas used is regulated.

3. The multi-cold-source method for coupling a gas-fired steam boiler with a two-stage heat pump according to claim 2, characterized in that: A third-stage flue gas-air heat exchanger (15) is connected in parallel to the first-stage flue gas-water heat exchanger (2); the air-side outlet of the first-stage flue gas-air heat exchanger (5) is connected to the gas boiler (1) via the second-stage flue gas-air heat exchanger (12) and the third-stage flue gas-air heat exchanger (15); the air heated by the first-stage flue gas-air heat exchanger (5) is heated by the second-stage flue gas-air heat exchanger and the third-stage flue gas-air heat exchanger (15) in sequence, and the flow rate of the flue gas used is regulated.

4. The multi-cold-source method for coupling a gas-fired steam boiler with a two-stage heat pump according to claim 1, characterized in that: The air heated by the first-stage flue gas-air heat exchanger (5) is heated by the second-stage water-air heat exchanger (17), and the feed water of the second-stage water-air heat exchanger (17) comes from the low-temperature heat pump (8).

5. The multi-cold-source method for coupling a gas-fired steam boiler with a two-stage heat pump according to claim 4, characterized in that: The air heated by the first-stage flue gas-air heat exchanger (5) is then heated by the second-stage water-air heat exchanger (17) and the third-stage water-air heat exchanger (18). The feed water for the third-stage water-air heat exchanger (18) comes from the high-temperature heat pump (9).

6. A multi-cold source system for a gas-fired steam boiler coupled with a two-stage heat pump, characterized in that: The system includes a gas-fired boiler (1), a first-stage flue gas-water heat exchanger (2), and a second-stage flue gas-water heat exchanger (3) connected sequentially along the flue gas flow direction. The flue gas outlet of the second-stage flue gas-water heat exchanger (3) is connected to the first-stage flue gas-air heat exchanger (5), the third-stage flue gas-water heat exchanger (6), and the flue gas-circulating water heat exchanger (10). The flue gas-circulating water heat exchanger (10) is connected to a low-temperature heat pump (8). The second-stage flue gas-water heat exchanger (3) is connected to a high-temperature heat pump (9). The third-stage flue gas-water heat exchanger (6), the low-temperature heat pump (8), the high-temperature heat pump (9), and the water side of the first-stage flue gas-water heat exchanger (2) are connected. The low-temperature heat pump (8) and the high-temperature heat pump (9) are compression-type water source heat pumps. Alternatively, the flue gas outlet of the second-stage flue gas-water heat exchanger (3) is connected to the first-stage flue gas-air heat exchanger (5), the third-stage flue gas-water heat exchanger (6), and the air source heat pump (19). The second-stage flue gas-water heat exchanger (3) is connected to the high-temperature heat pump (9). The third-stage flue gas-water heat exchanger (6), the high-temperature heat pump (9), and the water side of the first-stage flue gas-water heat exchanger (2) are connected. Alternatively, the flue gas outlet of the second-stage flue gas-water heat exchanger (3) is connected to the first-stage flue gas-air heat exchanger (5), the third-stage flue gas-water heat exchanger (6), and the flue gas-circulating water heat exchanger (10). The flue gas-circulating water heat exchanger (10) is connected to the lithium bromide absorption heat pump (20). The second-stage flue gas-water heat exchanger (3) is connected to the high-temperature heat pump (9). The third-stage flue gas-water heat exchanger (6), the high-temperature heat pump (9), the lithium bromide absorption heat pump (20), and the water side of the first-stage flue gas-water heat exchanger (2) are connected. The water-side outlet of the first-stage flue gas-water heat exchanger (2) is connected to the feed water inlet of the gas boiler (1); the air-side of the first-stage flue gas-air heat exchanger (5) is connected to the fan and the gas boiler (1).

7. The multi-cold source system of a gas-fired steam boiler coupled with a two-stage heat pump according to claim 6, characterized in that: The second-stage flue gas-water heat exchanger (3) is connected in parallel with the second-stage flue gas-air heat exchanger (12). The air-side outlet of the first-stage flue gas-air heat exchanger (5) is connected to the gas boiler (1) via the second-stage flue gas-air heat exchanger (12). Three-way valves are installed at the flue gas side inlet and outlet of the second-stage flue gas-water heat exchanger (3) and the second-stage flue gas-air heat exchanger (12).

8. The multi-cold source system of a gas-fired steam boiler coupled with a two-stage heat pump according to claim 7, characterized in that: The first-stage flue gas-water heat exchanger (2) is connected in parallel with the third-stage flue gas-air heat exchanger (15). The air-side outlet of the first-stage flue gas-air heat exchanger (5) is connected to the gas boiler (1) via the second-stage flue gas-air heat exchanger (12) and the third-stage flue gas-air heat exchanger (15). Three-way valves are installed at the flue gas inlet and outlet of the first-stage flue gas-water heat exchanger (2) and the third-stage flue gas-air heat exchanger (15).

9. The multi-cold source system of a gas-fired steam boiler coupled with a two-stage heat pump according to claim 6, characterized in that: The low-temperature heat pump (8) includes a first evaporator (81), a first throttle valve (82), a first condenser (83), and a first compressor (84); the cold side outlet of the first evaporator (81) is connected to the first compressor (84), the first condenser (83), and the first throttle valve (82) in sequence along the refrigerant flow direction, and the first throttle valve (82) is connected to the cold side inlet of the first evaporator (81); the hot side of the first evaporator (81) is connected to the cold side of the flue gas-circulating water heat exchanger (10), and the water side of the first condenser (83) is connected to the water side of the high-temperature heat pump (9).

10. The multi-cold source system of a gas-fired steam boiler coupled with a two-stage heat pump according to claim 9, characterized in that: A second-stage water-air heat exchanger (17) is installed between the water side of the first condenser (83) and the water side of the high-temperature heat pump (9). The air side of the first-stage flue gas-air heat exchanger (5) is connected to the gas boiler (1) via the air side of the second-stage water-air heat exchanger (17).

11. The multi-cold source system of a gas-fired steam boiler coupled with a two-stage heat pump according to claim 10, characterized in that: The water-side outlet of the high-temperature heat pump (9) is connected to the water-side inlet of the first-stage flue gas-water heat exchanger (2) via the third-stage water-air heat exchanger (18). The air side of the first-stage flue gas-air heat exchanger (5) is connected to the gas boiler (1) via the air side of the second-stage water-air heat exchanger (17) and the third-stage water-air heat exchanger (18).

12. The multi-cold source system of a gas-fired steam boiler coupled with a two-stage heat pump according to claim 6, characterized in that: The high-temperature heat pump (9) includes a second evaporator (91), a second throttle valve (92), a second condenser (93), and a second compressor (94). The cold side outlet of the second evaporator (91) is connected to the second compressor (94), the second condenser (93), and the second throttle valve (92) in sequence along the refrigerant flow direction. The outlet of the second throttle valve (92) is connected to the cold side inlet of the second evaporator (91). The hot side of the second evaporator (91) is connected to the cold side of the second-stage flue gas-water heat exchanger (3). The water side inlet of the second condenser (93) is connected to the water side outlet of the low-temperature heat pump (8). The water side outlet of the second condenser (93) is connected to the water side inlet of the first-stage flue gas-water heat exchanger (2).

13. The multi-cold source system of a gas-fired steam boiler coupled with a two-stage heat pump according to claim 6, characterized in that: The air source heat pump (19) adopts a compression heat pump. The air source heat pump (19) includes a third evaporator (191), a third throttle valve (192), a third condenser (193) and a third compressor (194). The cold side outlet of the third evaporator (191) is connected to the third compressor (194), the third condenser (193) and the third throttle valve (192) in sequence. The outlet of the third throttle valve (192) is connected to the cold side inlet of the third evaporator (191). The hot side inlet of the third evaporator (191) is connected to the flue gas outlet of the second-stage flue gas-water heat exchanger (3). The hot side outlet of the third evaporator (191) is vented. The water side outlet of the third condenser (193) is connected to the high-temperature heat pump (9).

14. The multi-cold source system of a gas-fired steam boiler coupled with a two-stage heat pump according to claim 6, characterized in that: The lithium bromide absorption heat pump (20) includes a generator (204), a fourth condenser (203), a fourth throttle valve (202), a fourth evaporator (201), an absorber (208), a solution valve (207), a solution pump (206), and a solution heat exchanger (205). The working fluid outlet of the generator (204) is connected to the working fluid side of the fourth condenser (203). The working fluid side outlet of the fourth condenser (203) is connected to the working fluid side of the fourth evaporator (201) through the fourth throttle valve (202). The working fluid side of the fourth evaporator (201) is connected to the working fluid inlet of the absorber (208). The working fluid outlet of the absorber (208) is connected to the dilute solution inlet of the solution heat exchanger (205). The dilute solution outlet of the solution heat exchanger (205) is connected to the working fluid outlet of the generator (204). The inlet is connected, and a solution pump (206) is installed on the pipeline from the absorber (208) to the solution heat exchanger (205). The generator (204) is connected to the solution heat exchanger (205), and a solution valve (207) is installed on the pipeline from the solution heat exchanger (205) to the absorber (208). The hot side of the fourth evaporator (201) is connected to the cold side of the flue gas-circulating water heat exchanger (10). The water side outlet of the third-stage flue gas-water heat exchanger (6) enters the water side inlet of the absorber (208). The water side outlet of the absorber (208) is connected to the water side inlet of the fourth condenser (203). The water side outlet of the fourth condenser (203) is connected to the high-temperature heat pump (9). The lithium bromide absorption heat pump (20) is driven by high-temperature steam generated by the gas boiler (1), or by high-temperature water or heat transfer oil.

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