A natural gas boiler flue gas deoxidation and waste heat recovery system

Abstract

The present invention relates to the technical field of boiler energy conservation and environmental protection, and discloses a natural gas boiler flue gas deoxidation and waste heat recovery system, which is composed of a gas boiler body, a burner, an economizer, a condenser, a flue gas waste heat recovery unit, a heat exchange layer, a water quality regulating device, a plate heat exchanger, a full-heat air preheater, an air superheater, a heat pump unit, a heat pump condenser, a heat pump evaporator, a blower, a recirculation flue, a waste hot water overflow pipe and corresponding pipelines. The present invention reduces the oxygen content of the wet air by heating and humidifying the combustion-supporting air, while reducing the gas consumption, thereby reducing the amount of NOx generated during the combustion process; by adding water vapor to improve the combustion conditions of the burner, slightly reducing the furnace temperature, it can promote the reaction of methane and OH ‑ The reaction between them improves the combustion efficiency; thus, the latent heat in the flue gas of the gas boiler can be completely recovered, which greatly improves the boiler efficiency and achieves the purpose of reducing pollutant emissions and eliminating smoke plume.

Description

Technical Field

[0001] The invention relates to the technical field of boiler energy conservation and environmental protection, in particular to a natural gas boiler flue gas deoxidation and waste heat recovery system. Background Art

[0002] Driven by the dual carbon goals, all parts of the country are vigorously implementing energy structure reforms. In the short term, decoalization means replacing coal with gas and using gas for heating and power generation. The proportion of natural gas boilers will increase significantly; at the same time, the transformation of natural gas boiler waste heat recovery systems or the construction of new boilers will become a hot spot. Natural gas is mainly composed of methane (85%) and a small amount of ethane (9%), propane (3%), nitrogen (2%) and butane (1%), and a very small amount of hydrogen sulfide; after the natural gas is completely burned, the exhaust gas contains a large amount of CO2 and water vapor.

[0003] Since the flue gas of natural gas boilers contains a large amount of water vapor, there is a large amount of latent heat of vaporization, which accounts for about 12% of the low calorific value of natural gas; the generation of SO2 / SO3 in the flue gas is extremely low, and there is no need to worry about the acid dew point. Under the existing technology, the heat in the water vapor is usually condensed and recovered by a condenser or an economizer. The principle is to use water below the flue gas dew point as the heat exchange medium close to the heat exchange surface area, and the water vapor in the flue gas condenses, while realizing the release of the sensible heat of the flue gas and the release of the latent heat of condensation of water vapor; another way to recover waste heat is to install a spray heat exchange tower, obtain spray waste hot water through direct contact between spray water and flue gas, and then recover the waste heat of the spray waste hot water through a heat pump for heating of heat network return water or boiler feed water.

[0004] Heat pumps generally use gas-driven absorption heat pumps, which are expensive to manufacture, have high operating costs, and a poor return on investment. The heat exchange method of the condenser is inter-wall heat exchange, which is less effective than the water-to-water direct contact heat exchange that occurs simultaneously in the spray heat exchange tower. Summary of the invention

[0005] The object of the present invention is to provide a natural gas boiler flue gas deoxidation and waste heat recovery system to solve the problems raised in the above background technology.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A natural gas boiler flue gas whitening and waste heat recovery system, which consists of a gas boiler body, a burner, an economizer, a condenser, a flue gas waste heat recovery unit, a heat exchange layer, a water quality adjustment device, a plate heat exchanger, a full heat air preheater, an air superheater, a heat pump unit, a heat pump condenser, a heat pump evaporator, a blower, a recirculation flue, a waste hot water overflow pipe and corresponding pipelines;

[0008] The tail flue of the gas boiler body is equipped with an economizer and a condenser from left to right, a recirculation flue is connected between the economizer and the condenser, one end of the recirculation flue is communicated with a blower, a burner is installed inside the gas boiler body, a blower is installed at the input end of the burner, a heat exchange layer is installed inside the flue gas waste heat recovery unit and the full heat air preheater, the heat exchange layer inside the flue gas waste heat recovery unit is divided into two sections, cooling water is provided in the full heat air preheater and the flue gas waste heat recovery unit, the gas boiler body is connected to the condenser and the flue gas waste heat recovery unit through a bypass valve, and a heat pump condenser and a heat pump evaporator are installed inside the heat pump unit from top to bottom.

[0009] A working method of a natural gas boiler flue gas deoxidation and waste heat recovery system, comprising the following steps:

[0010] S1. The external combustion air is drawn into the full-heat air preheater through the blower. At the same time, the full-heat air preheater heats and humidifies the combustion air through the flue gas carrying heat discharged from the tail flue of the boiler;

[0011] S2, the combustion air is heated and humidified and then heated by an air superheater to obtain a temperature rise of 5 to 8°C, and its relative humidity is reduced to below 80%;

[0012] S3, the combustion air heated by the air superheater is drawn into the burner through the blower to mix with the natural gas for combustion;

[0013] S4. Part of the flue gas generated by combustion in step S3 is returned to the burner through the recirculation flue for combustion. Since the condenser is bypassed by the bypass valve, the other part of the flue gas directly enters the cooling water inside the flue gas waste heat recovery unit. The cooling water absorbs heat and heats up to form primary waste water. The primary waste water passes through the water quality regulating device to remove trace soluble salts and SO2 in the flue gas.

[0014] S5. The first-stage waste water after removing trace soluble salts and SO2 enters the plate heat exchanger again, and exchanges heat with the external heat network return water through the plate heat exchanger. After heat exchange, part of the low-temperature first-stage waste water flows back to the lower heat exchange layer inside the flue gas waste heat recovery unit for heat exchange again, and the other part of the low-temperature first-stage waste water enters the full-heat air preheater for heat exchange to obtain the second-stage waste water with a lower temperature. The excess first-stage waste water can be discharged through the waste water overflow pipe;

[0015] S6. The secondary waste water in the full-heat air preheater then enters the heat pump evaporator inside the heat pump unit to absorb heat, and finally produces low-temperature water of about 16°C. The low-temperature water is sent to the upper heat exchange layer of the flue gas waste heat recovery unit for heat exchange, and finally the flue gas is reduced to below 20°C; and the heat absorbed by the heat pump unit from the secondary waste water is released by the heat pump condenser inside the heat pump unit, and then absorbed by the external heat network return water, and enters the gas boiler body together with the heat network return water after heat exchange through the plate heat exchanger for heating.

[0016] As a further solution of the present invention: the heat source of the air superheater can be water supplied by a heat network, and the heat exchange method of the air superheater can be partition heat exchange.

[0017] As a further solution of the present invention: the heat exchange layer in the flue gas waste heat recovery unit and the full-heat air preheater can use an atomizing nozzle or a structured packing. When an atomizing nozzle is used, a demister must be used in conjunction.

[0018] As a further solution of the present invention: the volume expansion and water content of the combustion air after heating and humidification in step S1 do not exceed the maximum limit of the burner.

[0019] As a further solution of the present invention: in the step S4, the water quality regulating device adopts alkali neutralization or multi-layer filter membrane to filter the trace soluble salts and SO2 in the flue gas.

[0020] As a further solution of the present invention: in the step S1, the full-heat air preheater heats and humidifies the incoming combustion air by air-water countercurrent heat exchange; in the step S4, the flue gas waste heat recovery unit uses air-water direct heat exchange to exchange heat.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The present invention reduces the oxygen content of the wet air by heating and humidifying the combustion-supporting air, and at the same time reduces the fuel gas consumption, thereby reducing the amount of NOx generated during the combustion process; the combustion conditions of the burner are improved by adding water vapor, the furnace temperature is slightly reduced, and the methane and OH groups are promoted. - The reaction between the exhaust gas and the exhaust gas improves the combustion efficiency; after the waste water is exchanged with the heat network and preheated with the combustion-supporting air, the heat pump further absorbs the heat and reduces the temperature to about 16°C, and finally exchanges heat with the flue gas to reduce the flue gas temperature to below 20°C; thus, the latent heat in the flue gas of the gas boiler can be completely recovered, greatly improving the boiler efficiency, while achieving the reduction of pollutant emissions and the elimination of smoke plume. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The schematic diagram is a structural diagram of a natural gas boiler flue gas deoxidation and waste heat recovery system.

[0024] In the figure: 1. Gas boiler body; 2. Burner; 3. Economizer; 4. Condenser; 5. Flue gas waste heat recovery unit; 6. Heat exchange layer; 7. Water quality adjustment device; 8. Plate heat exchanger; 9. Full heat air preheater; 10. Air superheater; 11. Heat pump unit; 12. Heat pump condenser; 13. Heat pump evaporator; 14. Blower; 15. Recirculation flue; 16. Waste hot water overflow pipe. DETAILED DESCRIPTION

[0025] See also Figure 1 In the embodiment of the present invention, a natural gas boiler flue gas deoxidation and waste heat recovery system is composed of a gas boiler body 1, a burner 2, an economizer 3, a condenser 4, a flue gas waste heat recovery unit 5, a heat exchange layer 6, a water quality adjustment device 7, a plate heat exchanger 8, a full heat air preheater 9, an air superheater 10, a heat pump unit 11, a heat pump condenser 12, a heat pump evaporator 13, a blower 14, a recirculation flue 15, a waste hot water overflow pipe 16 and corresponding pipelines;

[0026] The tail flue of the gas boiler body 1 is installed with an economizer 3 and a condenser 4 from left to right, a recirculation flue 15 is connected between the economizer 3 and the condenser 4, one end of the recirculation flue 15 is connected to the blower 14, a burner 2 is installed inside the gas boiler body 1, and a blower 14 is installed at the input end of the burner 2. A heat exchange layer 6 is installed inside the flue gas waste heat recovery unit 5 and the full heat air preheater 9. The heat exchange layer 6 inside the flue gas waste heat recovery unit 5 is divided into an upper and lower section. Cooling water is provided in the full heat air preheater 9 and the flue gas waste heat recovery unit 5. The gas boiler body 1 is connected to the condenser 4 and the flue gas waste heat recovery unit 5 through a bypass valve. A heat pump condenser 12 and a heat pump evaporator 13 are installed inside the heat pump unit 11 from top to bottom.

[0027] A working method of a natural gas boiler flue gas deoxidation and waste heat recovery system, comprising the following steps:

[0028] S1. The external combustion air is drawn into the full-heat air preheater 9 through the blower 14. At the same time, the full-heat air preheater 9 heats and humidifies the combustion air through the flue gas carrying heat discharged from the tail flue of the boiler. After the combustion air is humidified, its oxygen content decreases, and the amount of recirculated flue gas is reduced accordingly, so that the amount of flue gas flowing through the flue gas waste heat recovery unit 5 is increased, and the recoverable flue gas waste heat is increased, while reducing the fuel gas consumption and the generation of NOx. The added water vapor improves the combustion condition of the burner 2, slightly reduces the furnace temperature (offsetting the heating and combustion-supporting effect of the combustion air, and finally the exhaust gas temperature rises slightly), the radiation capacity is enhanced, and the water vapor participates in the oxidation process of methane, thereby improving the combustion efficiency;

[0029] S2, the combustion air is heated and humidified and then heated by the air superheater 10 to obtain a temperature rise of 5 to 8°C, and its relative humidity is reduced to below 80%, thereby ensuring that no condensation occurs inside the blower 14;

[0030] S3, the combustion air heated by the air superheater 10 is drawn into the burner 2 through the blower 14 to mix with the natural gas for combustion;

[0031] S4, in step S3, part of the flue gas generated by combustion returns to the burner 2 through the recirculation flue 15 for combustion. Since the condenser 4 is bypassed by the bypass valve, the other part of the flue gas directly enters the cooling water inside the flue gas waste heat recovery unit 5. The cooling water absorbs heat and heats up to form primary waste hot water. The primary waste hot water then passes through the water quality regulating device 7 to remove trace soluble salts and SO2 in the flue gas.

[0032] S5, the first-stage waste water after removing trace soluble salts and SO2 enters the plate heat exchanger 8 again, and exchanges heat with the external heat network return water by entering the plate heat exchanger 8. After the heat exchange, part of the low-temperature first-stage waste water flows back to the lower heat exchange layer 6 inside the flue gas waste heat recovery unit 5 for heat exchange again, and the other part of the low-temperature first-stage waste water enters the full-heat air preheater 9 for heat exchange to obtain the second-stage waste water at a lower temperature. The excess first-stage waste water can be discharged through the waste water overflow pipe 16;

[0033] S6, the secondary waste water in the full-heat air preheater 9 enters the heat pump evaporator 13 inside the heat pump unit 11 to absorb heat, and finally produces low-temperature water of about 16°C. The low-temperature water is sent to the upper heat exchange layer 6 of the flue gas waste heat recovery unit 5 for heat exchange, and finally the flue gas is cooled to below 20°C; and the heat absorbed by the heat pump unit 11 from the secondary waste water is released by the heat pump condenser 12 inside the heat pump unit 11, and then absorbed by the external heat network return water, and enters the gas boiler body 1 together with the heat network return water after heat exchange by the plate heat exchanger 8 for heating.

[0034] Preferably, the heat source of the air superheater 10 can be water supplied by a heat network, and the heat exchange method of the air superheater 10 can be partition heat exchange.

[0035] Preferably, the heat exchange layer 6 in the flue gas waste heat recovery unit 5 and the full-heat air preheater 9 can adopt an atomizing nozzle or a structured packing. When an atomizing nozzle is adopted, a demister is required.

[0036] Preferably, the volume expansion and water content of the combustion-supporting air after heating and humidification in step S1 do not exceed the maximum limit of the burner 2, so as to prevent affecting the normal combustion of the gas boiler body 1.

[0037] Preferably, in step S4, the water quality regulating device 7 filters trace soluble salts and SO2 in the flue gas by adding alkali for neutralization or by using a multi-layer filter membrane.

[0038] Preferably, in step S1, the full-heat air preheater 9 heats and humidifies the incoming combustion air by air-water countercurrent heat exchange; in step S4, the flue gas waste heat recovery unit 5 uses air-water direct heat exchange to exchange heat, so that the thermal effect is better.

[0039] In order to better illustrate the technical effect of the present invention, the following examples are described:

[0040] Take a 70MW gas hot water boiler of a factory as an example, and it is equipped with economizer, condenser, air preheater, heat network return water temperature is 40~45℃, water supply temperature is 65~90℃; condenser outlet flue gas temperature is 49.7℃, air preheater outlet flue gas temperature is 44℃, flue gas flow rate is 1.1×10 5 Nm³ / h, boiler load is 68.89MW, condenser load is 5.16MW, totaling 74.05MW;

[0041] The original air preheater is completely replaced by the full-heat air preheater 9, the combustion air is heated and humidified to 38°C, and the air is heated to 45°C by the air superheater 10 and sent to the burner 2 for combustion with natural gas; the heat source of the air superheater 10 is taken from the water supply of the heating network, and the temperature is reduced to 48°C after heat exchange and sent to the return water of the heating network; the proportion of flue gas recirculation 15 is reduced from 20% to 17%, and the exhaust volume after the furnace is increased by 3%; at this time, the outlet temperature of the condenser 4 rises to 54.5°C, the flue gas is in a wet saturated state, and enters the flue gas waste heat recovery unit 5 to obtain 51°C waste hot water, which is adjusted by water quality. The device 7 adjusts the water quality to be weakly alkaline, and the water is cooled to 47°C after heat exchange with the return water of the heat network at 45°C through the plate heat exchanger 8. Then, a part of the water enters the lower heat exchange layer 6 of the flue gas waste heat recovery unit 5 for heat exchange, and the other part enters the full-heat air preheater 9 for heat exchange and cooling to 42°C, and then enters the heat pump evaporator 13 of the heat pump unit 11 to produce low-temperature water of 16°C, and then sent to the upper heat exchange layer 6 of the flue gas waste heat recovery unit 5 to reduce the flue gas temperature to below 20°C, achieving a large-scale deoxidation, and the excess waste hot water is discharged to the condensate water tank through the overflow pipe 16 for other use;

[0042] In this embodiment, the test results show that: the boiler load is increased to 70.14MW, the condenser load is increased to 6.46MW, the heat network return water heated by waste hot water is 2.2MW, and the heat pump recovers secondary waste hot water of 1.8MW, totaling 80.6MW, and the waste heat recovery capacity of a single boiler is increased by 6.55MW under the rated load;

[0043] The original gas hot water boiler condenser is susceptible to corrosion, which increases the operating cost. The present invention can completely replace the original condenser and air preheater, and can be implemented in conjunction with the plant construction, which can effectively reduce the initial investment and improve the thermal efficiency.

[0044] What is described above is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A natural gas boiler flue gas deoxidation and waste heat recovery system, characterized in that: The invention comprises a gas boiler body (1), a burner (2), an economizer (3), a condenser (4), a flue gas waste heat recovery unit (5), a heat exchange layer (6), a water quality adjustment device (7), a plate heat exchanger (8), a full heat air preheater (9), an air superheater (10), a heat pump unit (11), a heat pump condenser (12), a heat pump evaporator (13), a blower (14), a recirculation flue (15), a waste heat water overflow pipe (16) and corresponding pipelines; The tail flue of the gas boiler body (1) is provided with an economizer (3) and a condenser (4) in sequence from left to right; a recirculation flue (15) is connected between the economizer (3) and the condenser (4); one end of the recirculation flue (15) is respectively connected to a blower (14) and an air superheater (10); a burner (2) is installed inside the gas boiler body (1); a blower (14) is installed at the input end of the burner (2); a heat exchange layer (6) is installed inside the flue gas waste heat recovery unit (5) and the full heat air preheater (9); the heat exchange layer (6) inside the flue gas waste heat recovery unit (5) The heat pump unit (11) is divided into two sections, the full-heat air preheater (9) and the flue gas waste heat recovery unit (5) are both provided with cooling water, the economizer (3) and the condenser (4) are connected with a bypass valve, and the condenser (4) is bypassed through the bypass valve; the outlet end of the condenser (4) is connected to the flue gas waste heat recovery unit (5), a part of the flue gas passing through the economizer (3) enters the blower (14) through the recirculation flue (15), and a part of the flue gas flows into the blower (14) through the bypass valve, and then enters the flue gas waste heat recovery unit (5) for waste heat recovery; the heat pump unit (11) is provided with a heat pump condenser (12) and a heat pump evaporator (13) in sequence from top to bottom; An air superheater (10) is provided at one end of the full-heat air preheater (9); external combustion-supporting air is heated and humidified by the full-heat air preheater (9) before entering the air superheater (10) for heating; and then drawn into the burner (2) by a blower (14); the plate heat exchanger (8) is connected to the gas boiler body (1), the flue gas waste heat recovery unit (5), the full-heat air preheater (9), the air superheater (10), the waste hot water overflow pipe (16) and the heat network return water through pipelines; the flue gas waste heat recovery unit (5) and the plate heat exchanger ( 8), a water quality regulating device (7) is installed between the flue gas waste heat recovery unit (5), and water from the flue gas waste heat recovery unit (5) enters the plate heat exchanger (8) for heat exchange through the water quality regulating device (7); the outlet pipe of the full heat air preheater (9) is connected to the inlet of the heat pump evaporator (13) of the heat pump unit (11); the outlet of the heat pump evaporator (13) is connected to the flue gas waste heat recovery unit (5) through a pipeline; the inlet of the heat pump condenser (12) is connected to the heat network return water and the air superheater (10) through a pipeline; and the outlet of the heat pump condenser (12) is connected to the gas boiler body (1) through a pipeline.

2. A natural gas boiler flue gas deoxidation and waste heat recovery system according to claim 1, characterized in that: The heat source of the air superheater (10) is water supplied by a heat network, and the heat exchange method of the air superheater (10) is wall-type heat exchange.

3. A natural gas boiler flue gas deoxidation and waste heat recovery system according to claim 1, characterized in that: The heat exchange layer (6) in the flue gas waste heat recovery unit (5) and the full heat air preheater (9) adopts an atomizing nozzle or a structured packing. When an atomizing nozzle is adopted, a demister is required.

4. A method for implementing the natural gas boiler flue gas deoxidation and waste heat recovery system of claim 1, characterized in that: The following steps are involved: S1, external combustion-supporting air is drawn into the full-heat air preheater (9) by means of a blower (14), and at the same time, the full-heat air preheater (9) heats and humidifies the combustion-supporting air through the flue gas carrying heat discharged from the tail flue of the boiler; S2, the combustion air is heated and humidified and then heated by an air superheater (10) to obtain a temperature rise of 5 to 8°C, and its relative humidity is reduced to below 80%; S3, the combustion air heated by the air superheater (10) is drawn into the burner (2) through the blower (14) to be mixed with the natural gas and burned; Part of the flue gas generated by the combustion in steps S4 and S3 passes through the condenser (4) and then returns to the burner (2) through the recirculation flue (15) for combustion; the condenser (4) is bypassed by a bypass valve, so that the other part of the flue gas directly enters the cooling water inside the flue gas waste heat recovery unit (5), and the cooling water absorbs heat and rises in temperature to form primary waste hot water. The primary waste hot water then passes through the water quality adjustment device (7) to remove trace soluble salts and SO2 in the flue gas; S5, the first-stage waste water from which trace soluble salts and SO2 are removed enters the plate heat exchanger (8) again, and exchanges heat with the external heat network return water through the plate heat exchanger (8). After the heat exchange, part of the low-temperature first-stage waste water flows back to the lower heat exchange layer (6) inside the flue gas waste heat recovery unit (5) for further heat exchange, and the other part of the low-temperature first-stage waste water enters the full-heat air preheater (9) for heat exchange to obtain second-stage waste water with a lower temperature, and the excess first-stage waste water is discharged through the waste water overflow pipe (16); S6, the secondary waste hot water in the full-heat air preheater (9) then enters the heat pump evaporator (13) inside the heat pump unit (11) to absorb heat, and finally produces low-temperature water of 16°C. The low-temperature water is sent to the upper heat exchange layer (6) of the flue gas waste heat recovery unit (5) for heat exchange, and finally the flue gas is cooled to below 20°C. The heat absorbed from the secondary waste hot water by the heat pump unit (11) is released by the heat pump condenser (12) inside the heat pump unit (11), and then absorbed by the external heat network return water, and enters the gas boiler body (1) together with the heat network return water after heat exchange through the plate heat exchanger (8) to be heated.

5. A natural gas boiler flue gas deoxidation and waste heat recovery system according to claim 4, characterized in that: The volume expansion and water content of the combustion-supporting air after heating and humidification in step S1 do not exceed the maximum limit of the burner (2).

6. A natural gas boiler flue gas deoxidation and waste heat recovery system according to claim 4, characterized in that: In the step S4, the water quality regulating device (7) filters trace soluble salts and SO2 in the flue gas by adding alkali for neutralization or by using a multi-layer filter membrane.

7. A natural gas boiler flue gas deoxidation and waste heat recovery system according to claim 4, characterized in that: In step S1, the full-heat air preheater (9) heats and humidifies the incoming combustion air by means of air-water countercurrent heat exchange; and in step S4, the flue gas waste heat recovery unit (5) performs heat exchange by means of air-water direct heat exchange.

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