Flue gas waste heat deep recovery system, and method for deep recovery of flue gas waste heat

Abstract

Disclosed in the present invention are a flue gas waste heat deep recovery system, and a method for deep recovery of flue gas waste heat. By means of providing a humidification and cooling device on a flue gas discharge flue to increase the water vapor content of a flue gas or bring same to saturation, the partial or complete conversion of a first heat exchange stage (a cooling section) into a second heat exchange stage (a condensation section) is realized during the heat exchange process between the flue gas and cold water. Furthermore, a condensate water pipeline is provided between a flue gas heat exchanger and the humidification and cooling device, and condensate water is used as spray water of the humidification and cooling device to spray and humidify the flue gas, thereby reducing the need for additional configuration of spray water. The system reduces the area of a flue gas heat exchanger, lightens the equipment weight, lowers the equipment investment, and saves the occupied area and space, thereby facilitating equipment installation and project implementation. The use of the system and method can achieve the technical effect of deep recovery of flue gas waste heat.

Description

A deep flue gas waste heat recovery system and method for deep recovery of flue gas waste heat Technical Field

[0001] This invention relates to the fields of energy conservation, environmental protection, and flue gas heat recovery, specifically to a deep flue gas waste heat recovery system and a method for deep recovery of flue gas waste heat. Background Technology

[0002] A boiler's main working principle is to utilize the heat energy released from fuel combustion or waste heat from industrial production to transfer to water within a container, raising the water to the required temperature or pressure for steam. In the combustion section, fuel combustion continuously releases heat, and the high-temperature flue gas produced transfers this heat to the boiler's heating surfaces through heat transfer, while the flue gas itself gradually decreases in temperature and is finally discharged through the chimney.

[0003] The calorific value of fuel is divided into higher calorific value and lower calorific value. The main difference lies in whether the water in the flue gas, a product of fuel combustion, is in a liquid or gaseous state. The calorific value of fuel is determined when the water in the flue gas exists in a gaseous state, while the calorific value of fuel is determined when the water in the combustion products exists in a liquid state. This distinction is of great significance for energy utilization.

[0004] In order to utilize the high calorific value of fuel, industrial applications typically install flue gas heat exchangers on exhaust pipes, using cold water as the refrigerant to reduce the exhaust gas temperature below the water dew point temperature. This heat exchange process is divided into two stages: the first heat exchange stage is the cooling section, during which no liquid water is precipitated, i.e., there is no phase change, until the temperature reaches the water dew point; the second heat exchange stage is the condensation section, during which the flue gas begins to cool down from the water dew point temperature, and liquid water is precipitated, i.e., there is a phase change. The main problems with the current technical approach are: (1) the heat transfer coefficient of the cooling section is small, requiring a large heat exchange area, resulting in large weight and volume, leading to high cost of the flue gas heat exchanger; (2) the large size and weight of the flue gas heat exchanger also make its installation difficult, increasing the difficulty of project implementation.

[0005] Summary of the Invention

[0006] This invention aims to address the technical problems existing in the prior art. According to a first aspect of the invention, the invention provides a deep waste heat recovery system for flue gas, the system comprising a humidification and cooling device and a flue gas heat exchanger installed on the flue gas duct; the flue gas enters the flue gas heat exchanger after passing through the humidification and cooling device to recover the waste heat of the flue gas; the flue gas is discharged after the waste heat is recovered.

[0007] In this invention, the humidification and cooling device is used to atomize and / or vaporize the spray water, and then spray it into the flue gas, so as to increase the water vapor content in the flue gas or reach saturation, thereby increasing the water dew point temperature of the flue gas.

[0008] This invention does not particularly limit the boiler device that generates flue gas; it is applicable to coal-fired boilers, gas-fired boilers, or oil-fired boilers. In practical applications, it can be a hot water boiler, a steam boiler, a waste incinerator, a gas turbine, a waste heat boiler, or an industrial device.

[0009] This invention does not impose any particular restrictions on the flue gas duct of the boiler. This invention is applicable to flue gas ducts of various types of boilers. The flue gas duct can be a vertical flue gas duct or a horizontal flue gas duct. It can be the original flue gas duct of the boiler or a modified flue gas duct.

[0010] The present invention does not impose any particular limitation on the type of humidification and cooling device. It can be a humidification and cooling device commonly used in the prior art, such as a tubular sprayer, a shower head sprayer, a ring-tube multi-hole sprayer, or a nozzle with liquid atomization function.

[0011] Preferably, the humidification and cooling device is a nozzle, which has the function of atomizing liquid.

[0012] In this invention, the flue gas heat exchanger is a partitioned heat exchanger, such as a plate heat exchanger or a tube heat exchanger.

[0013] Preferably, the flue gas heat exchanger is a plate heat exchanger.

[0014] Plate heat exchangers are characterized by high heat transfer coefficient, compact structure, small footprint, and flexible layout. They are not limited by the structure and layout of boiler flue gas ducts, have strong engineering adaptability, and can well meet the retrofit requirements of various new and old boiler systems.

[0015] Furthermore, the present invention provides a condensate pipeline between the flue gas heat exchanger and the humidification and cooling device. The condensate from the flue gas in the flue gas heat exchanger first enters the condensate tank, and then part or all of the condensate is transported to the humidification and cooling device, reducing the need for spray water.

[0016] Based on the actual water consumption of the humidification and cooling device, some or all of the condensate is pumped to the device via a condensate pump. The condensate is used as the spray water for the humidification and cooling device to humidify the flue gas, reducing the need for additional spray water.

[0017] Optionally, a condensate treatment device is installed at the outlet of the condensate tank, and the treated condensate is transported to the humidification and cooling device.

[0018] In this invention, the number of humidification and cooling devices and flue gas heat exchangers can be adjusted according to actual needs. It can be one humidification and cooling device and flue gas heat exchanger, or multiple humidification and cooling devices and flue gas heat exchangers.

[0019] In this invention, the number of condensate pipes can be set according to actual needs. For example, the condensate pipe includes a humidification and cooling device and a flue gas heat exchanger; or it includes a humidification and cooling device and multiple flue gas heat exchangers; or it includes multiple humidification and cooling devices and multiple flue gas heat exchangers.

[0020] Preferably, after the flue gas is discharged from the boiler, it first enters the humidification and cooling device, and then enters the flue gas heat exchanger.

[0021] According to a second aspect of the present invention, the present invention also relates to a method for deep recovery of flue gas waste heat using the aforementioned flue gas waste heat recovery system. Before the flue gas enters the flue gas heat exchanger, a humidification and / or vaporization spray water is sprayed into the flue gas through a humidification and cooling device installed on the flue gas duct, so as to increase or saturate the water vapor content in the flue gas, thereby raising the water dew point temperature of the flue gas, and then the flue gas enters the flue gas heat exchanger to recover the flue gas waste heat.

[0022] For a description of the deep recovery system for flue gas waste heat, please refer to the first aspect of this invention, and it will not be repeated here.

[0023] Preferably, the water used for spraying in the humidification and cooling device comes from the condensate that condenses in the flue gas.

[0024] Preferably, after the water dew point temperature of the flue gas increases, it enters a plate heat exchanger to recover the waste heat of the flue gas.

[0025] The beneficial effects of this invention are:

[0026] Compared with existing technologies, which suffer from problems such as large heat exchange area, large volume, high cost, large footprint, strong spatial and environmental constraints, and poor engineering adaptability, the flue gas waste heat deep recovery system and method provided by this invention have the following advantages:

[0027] (1) After the flue gas is sprayed and humidified, the water vapor content in the flue gas increases or reaches saturation. As a result, during the heat exchange process between the flue gas and the cold water, the first heat exchange stage (cooling section) is partially or completely transformed into the second heat exchange stage (condensation section). The heat transfer coefficient of the flue gas heat exchanger is significantly improved, the heat exchange area is reduced, the equipment weight is reduced, the volume is reduced, the equipment cost is reduced, and the floor space and installation space are saved, which is beneficial to the equipment installation.

[0028] (2) After the flue gas is sprayed and humidified, the heat transfer coefficient of the flue gas heat exchanger is significantly improved, making it easier to recover the waste heat of the flue gas. The flue gas temperature is lower, the flue gas temperature drop is large, the waste heat recovery is large, the boiler efficiency is improved, and the fuel consumption is reduced. Attached Figure Description

[0029] Figure 1: Schematic diagram of a conventional flue gas waste heat recovery system.

[0030] Figure 2: Schematic diagram of the structure of the flue gas waste heat deep recovery system in Embodiment 1 of the present invention.

[0031] Figure 3: Schematic diagram of the structure of the flue gas waste heat deep recovery system in Embodiment 2 of the present invention.

[0032] Figure 4: Schematic diagram of the flue gas waste heat deep recovery system in Embodiment 3 of the present invention.

[0033] Figure 5: Schematic diagram of the structure of the flue gas waste heat deep recovery system in Embodiment 4 of the present invention.

[0034] Explanation of reference numerals in the attached figures:

[0035] 1. First flue gas heat exchanger; 2. Chimney; 3. Condensate tank; 4. Condensate pump; 5. Humidification and cooling device; 6. Second flue gas heat exchanger.

[0036] a. Flue gas; b. First cooling water; c. Flue gas condensate; d. Second cooling water. Detailed Implementation

[0037] Flue gas-water heat exchange for waste heat recovery involves two heat exchange stages:

[0038] The first heat exchange stage is the cooling section. During the flue gas cooling process, no liquid water is precipitated, that is, there is no phase change, until the temperature reaches the water dew point.

[0039] The second heat exchange stage is the condensation stage, where the flue gas begins to cool down from the water dew point temperature, during which liquid water is precipitated, i.e., a phase change occurs.

[0040] For example, when the flue gas temperature drops from 75°C to 30°C and the flue gas water dew point temperature is 40°C, the first heat exchange stage (cooling stage) is when the flue gas temperature drops from 75°C to the dew point temperature of 40°C, and there is no phase change in the flue gas; in the second heat exchange stage (condensation stage), the flue gas temperature drops from the dew point temperature of 40°C to 30°C, and condensate is released in the flue gas, and the water in the flue gas undergoes a phase change.

[0041] In the first heat exchange stage (cooling stage), the heat exchange between flue gas and water is a phase-change-free process. Due to the small gas phase heat transfer coefficient, the overall heat transfer coefficient in the first heat exchange stage is also very small.

[0042] In the second heat exchange stage (condensation stage), the heat exchange between flue gas and water involves phase change, specifically flue gas condensation and water heat exchange. The heat transfer coefficient of flue gas condensation is much larger than that of gas-phase cooling in the first heat exchange stage (cooling stage). Therefore, the total heat transfer coefficient of the second heat exchange stage (condensation stage) is about ten times larger than that of the first heat exchange stage (cooling stage).

[0043] This invention utilizes a humidification and cooling spray device to humidify and cool the flue gas before it enters the heat exchanger. During this process, the flue gas temperature decreases under the spraying action, while the water vapor content in the flue gas increases or reaches saturation, raising the water dew point temperature of the flue gas. Subsequently, the flue gas enters the heat exchanger for heat exchange.

[0044] The technical concept of this invention differs from conventional understanding in the field. Conventionally, a greater temperature difference between the flue gas entering the heat exchanger and the cooling medium is considered to result in greater heat exchange capacity, or a smaller heat exchanger area. Water spraying for humidification lowers the flue gas inlet temperature, which conventionally leads to the assumption that this reduces the heat exchanger's heat transfer capacity, or that a larger heat exchanger area is needed to recover the same amount of heat. However, this is a misconception.

[0045] This invention uses a humidification and cooling spray device to lower the flue gas temperature while simultaneously increasing or saturating the water vapor in the flue gas, thereby raising the flue gas dew point temperature. The effect is that the first heat exchange stage (cooling section) is partially or completely transformed into the second heat exchange stage (condensation section). Since the heat transfer coefficient of condensation is about ten times greater than that of cooling, and much greater than the decrease in temperature difference caused by the reduction in flue gas temperature, the heat exchange area required to recover the same amount of heat after humidification and cooling is significantly reduced. The heat exchange area of ​​the flue gas heat exchanger using this spray humidification and cooling method is reduced by more than 30% compared to traditional flue gas heat exchangers.

[0046] This invention optimizes the technical route for deep recovery of waste heat from flue gas: before the flue gas enters the heat exchanger, it is first sprayed to humidify and cool it, so that the water vapor content in the flue gas increases or reaches saturation, thereby raising the water dew point temperature of the flue gas. Then, it enters the flue gas heat exchanger for heat exchange, so that the first heat exchange stage (cooling section) is partially or completely transformed into the second heat exchange stage (condensation section), which greatly improves the heat exchange capacity and can significantly reduce the heat exchange area of ​​the heat exchanger while recovering the same amount of heat.

[0047] The system and method of the present invention will now be further described with reference to the accompanying drawings, but the following embodiments do not constitute a limitation of the present invention.

[0048] Figure 1 illustrates a conventional deep waste heat recovery system for flue gas: flue gas a, after being discharged from the boiler, directly enters the first flue gas heat exchanger 1 to exchange heat with the first cold water b. After cooling, the flue gas enters the chimney 2 for discharge. Some conventional recovery systems may also include dust removal devices and water washing devices. For example, a certain washing liquid (water) is used to spray and wash the flue gas generated in industrial production to capture pollutants, reduce dust, etc., dissolving the pollutants and dust in the washing liquid. However, this is fundamentally different from the purpose of the active spray humidification and cooling of this invention.

[0049] Example 1

[0050] Figure 2 shows the flue gas waste heat deep recovery system provided by the present invention, including a humidification and cooling device 5, a first flue gas heat exchanger 1, and a chimney 2, which are sequentially arranged on the flue gas duct.

[0051] Before entering the first flue gas heat exchanger 1, the flue gas is sprayed, humidified, and cooled by the humidification and cooling device 5. During this process, the sprayed water is atomized and / or vaporized before entering the flue gas, or it is atomized and / or vaporized after entering the flue gas, so as to increase the water vapor content in the flue gas or reach saturation. The humidified flue gas then enters the first flue gas heat exchanger 1 to exchange heat with the first cold water b to recover the waste heat of the flue gas; after cooling, the flue gas enters the chimney 2 for discharge.

[0052] After spray humidification, the water vapor content in the flue gas increases or reaches saturation. When the flue gas in the first flue gas heat exchanger 1 exchanges heat with the first cold water b, the first heat exchange stage (cooling section) is partially or completely transformed into the second heat exchange stage (condensation section), and the heat transfer coefficient is improved. Although the heat transfer temperature difference is reduced after spray humidification, the heat transfer coefficient of the second heat exchange stage (condensation section) is about ten times larger than that of the first heat exchange stage (cooling section). The effect of the increased heat transfer coefficient is greater than the effect of the reduced heat transfer temperature difference, and the heat exchange area of ​​the flue gas heat exchanger can be reduced in the end.

[0053] In some implementations, the system is equipped with a dust removal device and a water washing device, and deep recovery of waste heat from the flue gas is carried out after dust removal and water washing. The dust removal device and water washing device can be a tubular sprayer, a shower head sprayer, a ring-tube multi-hole sprayer, a nozzle with liquid atomization function, etc., and their function is to use water to wash away pollutants or dust. However, their function is completely different from the purpose of the humidification and cooling device described in this application. The type, state, and method of spraying the liquid are different, and the effects achieved are also different.

[0054] This invention does not have any particular limitation on the type of boiler that generates flue gas. This invention is applicable to coal-fired boilers or gas-fired boilers, and more specifically, this invention is applicable to hot water boilers, steam boilers, or waste incinerators.

[0055] This invention does not impose any particular restrictions on the flue gas duct of the boiler. This invention is applicable to flue gas ducts of various types of boilers. The flue gas duct can be a vertical flue gas duct or a horizontal flue gas duct. It can be the original flue gas duct of the boiler or a modified flue gas duct.

[0056] Example 2

[0057] Figure 3 illustrates another deep waste heat recovery system for flue gas provided by the present invention. Compared with Embodiment 1, the difference is that a condensate pipeline is provided between the first flue gas heat exchanger 1 and the humidification and cooling device 5. The condensate from the flue gas in the first flue gas heat exchanger 1 first enters the condensate tank 3, and then, according to actual needs, all or part of the condensate is sent to the humidification and cooling device 5.

[0058] In some implementations, a condensate treatment device is installed at the outlet of the condensate tank, and the treated condensate is transported in whole or in part to the humidification and cooling device.

[0059] This system uses a condensate tank to collect condensate from the flue gas, and then uses the condensate to spray and humidify the flue gas. This not only improves the utilization rate of condensate, but also reduces the need for additional spray water devices, achieving the effects of simplifying the system and reducing costs while increasing efficiency.

[0060] Example 3

[0061] Figure 4 illustrates another deep waste heat recovery system for flue gas provided by the present invention. The difference between this system and Embodiment 2 is that a second flue gas heat exchanger 6 is added between the humidification and cooling device 5 and the chimney 2; the first cold water b first enters the second flue gas heat exchanger 6 to exchange heat with the flue gas, and then enters the first flue gas heat exchanger 1 to exchange heat with the flue gas. The condensate from the flue gas in the first and second flue gas heat exchangers 1 and 6 first enters the condensate tank 3, and then, according to actual needs, all or part of the condensate is sent to the humidification and cooling device 5.

[0062] This invention does not limit the number of flue gas heat exchangers; it can be a single flue gas heat exchanger or multiple flue gas heat exchangers. When there are multiple flue gas heat exchangers, their type is a plate heat exchanger and / or a tubular heat exchanger.

[0063] Example 4

[0064] Figure 5 illustrates another deep waste heat recovery system for flue gas provided by the present invention. Compared with Embodiment 3, the difference is that a second cold water d is added; the first cold water b enters the first flue gas heat exchanger 1 to exchange heat with the flue gas, and the second cold water d enters the second flue gas heat exchanger 6 to exchange heat with the flue gas.

[0065] In this invention, there are no restrictions on the type and flow of cold water in the first and second flue gas heat exchangers. The same type of cold water can be used, which first enters the second flue gas heat exchanger for heat exchange and then enters the first flue gas heat exchanger. Alternatively, two different types of cold water can be used, which enter the first and second flue gas heat exchangers respectively.

[0066] Test case

[0067] The deep waste heat recovery system for flue gas in Example 2 was tested. Comparing the differences between the present invention and the conventional process, under the condition of recovering an equal amount of waste heat from the flue gas, the system exhibits the following characteristics: flue gas temperature of 98°C, flue gas water dew point temperature of 41°C, cold water inlet temperature of 22°C, cold water outlet temperature of 34°C, and flue gas temperature reduced to 38°C after waste heat recovery.

[0068] Test 1: Traditional process, without humidification and cooling devices.

[0069] Without humidification and cooling devices, the heat transfer temperature difference is 30℃, and the heat transfer coefficient is 74W / m. 2 At ℃, the required heat exchange area for the flue gas heat exchanger is approximately 2200 m². 2 .

[0070] Test 2: The addition of a humidification and cooling device increased the water vapor content in the flue gas, but it did not reach saturation.

[0071] The temperature of the flue gas before humidification and cooling was 98℃, and the temperature of the flue gas after humidification and cooling was 60℃, while the water dew point temperature of the flue gas increased to 46℃.

[0072] The results show that the heat transfer temperature difference is 16℃, and the heat transfer coefficient is 218W / m. 2 At ℃, the required heat exchange area for the flue gas heat exchanger is approximately 1300 m². 2 .

[0073] Test 3: A humidification and cooling device was added, and the water vapor content in the flue gas reached saturation.

[0074] The temperature of the flue gas before humidification and cooling was 98℃, and the temperature after humidification and cooling was 48℃, which is the water dew point temperature of the flue gas.

[0075] The results show that with a heat transfer temperature difference of 15℃, the heat transfer coefficient is 566W / m. 2 At ℃, the required heat exchange area for the flue gas heat exchanger is approximately 560m². 2 .

[0076] The above tests show that the flue gas waste heat deep recovery system provided by the present invention can effectively improve the recovery and utilization rate and significantly reduce the area of ​​the heat exchange device.

[0077] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0078] The present invention has been described in detail above. Specific examples have been used in the specification to illustrate the principles and implementation methods of the present invention. However, the examples are only used to help understand the present invention and should not be construed as limiting the present invention.

Claims

1. A deep waste heat recovery system for flue gas, comprising: Humidification and cooling devices and flue gas heat exchangers installed on the flue gas duct; After passing through the humidification and cooling device, the flue gas enters the flue gas heat exchanger to recover the waste heat of the flue gas. The flue gas is discharged after waste heat recovery.

2. The flue gas waste heat deep recovery system as described in claim 1, characterized in that, The flue gas heat exchanger is a partition wall heat exchanger; The humidification and cooling device is a tubular sprayer, a shower head sprayer, a ring-tube multi-hole sprayer, or a nozzle.

3. The flue gas waste heat deep recovery system as described in claim 2, characterized in that, The flue gas heat exchanger is a plate heat exchanger; the humidification and cooling device is a nozzle.

4. The flue gas waste heat deep recovery system as described in claim 1, characterized in that, The deep waste heat recovery system for flue gas includes a humidification and cooling device and a flue gas heat exchanger; or includes a humidification and cooling device and multiple flue gas heat exchangers; or includes multiple humidification and cooling devices and multiple flue gas heat exchangers.

5. The flue gas waste heat deep recovery system as described in claim 1, characterized in that, The flue gas is either coal-fired boiler flue gas, gas-fired boiler flue gas, gas turbine flue gas, waste heat boiler flue gas, oil-fired boiler flue gas, waste incinerator flue gas, or industrial equipment flue gas.

6. The flue gas waste heat deep recovery system as described in any one of claims 1-5, characterized in that, A condensate pipeline is installed between the flue gas heat exchanger and the humidification and cooling device. The condensate in the flue gas heat exchanger first enters the condensate tank, and then some or all of the condensate is transported to the humidification and cooling device.

7. The flue gas waste heat deep recovery system as described in claim 6, characterized in that, Includes multiple condensate water pipes.

8. The flue gas waste heat deep recovery system as described in claim 6, characterized in that, A condensate treatment device is installed at the outlet of the condensate tank, and the treated condensate is then transported to the humidification and cooling device.

9. The flue gas waste heat deep recovery system as described in claim 1, characterized in that, The humidification and cooling device is used to atomize and / or vaporize the spray water, and then spray it into the flue gas to increase or saturate the water vapor content in the flue gas, thereby raising the water dew point temperature of the flue gas.

10. The flue gas waste heat deep recovery system as described in claim 1, characterized in that, Before the flue gas enters the flue gas heat exchanger, the water vapor content in the flue gas reaches a saturated state.

11. A method for deep recovery of waste heat from flue gas, characterized in that, The flue gas waste heat recovery system described in claim 1 involves spraying atomized and / or vaporized water into the flue gas before it enters the flue gas heat exchanger via a humidification and cooling device installed on the exhaust flue. This increases the water vapor content in the flue gas or makes it saturated, thereby raising the water dew point temperature of the flue gas. The flue gas then enters the flue gas heat exchanger to recover the waste heat.

12. The method for deep recovery of waste heat from flue gas as described in claim 11, characterized in that, The water used for spraying in the humidification and cooling device comes from the condensate that condenses in the flue gas.

13. The method for deep recovery of waste heat from flue gas as described in claim 11, characterized in that, After the water dew point temperature of the flue gas increases, it enters a plate heat exchanger to recover the waste heat of the flue gas.

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