A system for deep cascade utilization of waste heat in the flue gas at the tail of a boiler

Abstract

The present invention provides a system for deep cascade utilization of flue gas at the boiler tail. In this system, the flue gas output from the flue gas outlet of the boiler is divided into three paths. One path of flue gas supplies heat to the feed water and condensate circuits respectively through the feed water and condensate heating unit; another path of flue gas supplies heat to the primary air flue gas air preheater and the secondary air flue gas air preheater respectively through the flue gas chamber of the three-compartment air preheater; the last path of flue gas supplies heat to the primary air tubular air preheater; the boiler forced draft fan sends the primary air through the primary air flue gas air preheater and into the boiler through the primary air chamber of the primary air tubular air preheater or the three-compartment air preheater; the secondary air enters the boiler successively through the primary air flue gas air preheater and the three-compartment air preheater. Beneficial effects: The waste heat of the flue gas at the boiler tail is fully and reasonably utilized, energy cascade utilization is adopted, the heat transfer end difference of each system is reduced, the heat transfer efficiency of each heat exchange system is enhanced, the heat exchange area of the system is reduced, and the flexibility of system operation is also increased.

Description

Technical Field

[0001] The present invention belongs to the technical field of safety and energy conservation of thermal power units, and particularly relates to a system for deep cascade utilization of flue gas at the tail of a boiler. Background Art

[0002] There are many technical routes for the recovery and utilization of flue gas waste heat in domestic power station boilers. Currently, the mature and reliable application methods include: flue gas waste heat utilization of low-temperature economizers, combined flue gas-air heat exchange waste heat utilization of low-temperature economizers and condensate, and cascade utilization of flue gas waste heat. The technology of flue gas waste heat utilization of low-temperature economizers has been developed earlier and is a technical solution widely adopted in China at present. The low-temperature economizer uses the flue gas at the outlet of the air preheater to heat the low-temperature condensate to achieve waste heat recovery. Since the replaced extraction grade is relatively low, the energy-saving amount is limited, generally not exceeding 2 g / kWh. The combined flue gas-air heat exchange waste heat utilization technology of low-temperature economizers and condensate heats the condensate and uses the waste heat to heat the air supply while replacing the original air heater. However, in winter, due to the low ambient temperature, the recovery amount of condensate waste heat is limited. The cascade utilization technology of flue gas waste heat bypasses part of the flue gas at the inlet of the air preheater to heat the feed water, condensate, and air supply, resulting in an increased energy efficiency ratio. The coal-saving amount is generally 3 - 4 g / kWh. However, this transformation will cause a significant decrease in the air supply temperature at the outlet of the air preheater, affecting the economy and safety of unit operation.

[0003] Currently, the mainstream technologies do not consider the utilization of the latent heat of vaporization of saturated wet flue gas after desulfurization, and the energy cascade utilization is not reasonable enough, resulting in a low energy efficiency ratio and limited energy-saving amount, which will affect the safe operation of the system to a certain extent. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a system for deep cascade utilization of flue gas at the tail of a boiler, which is specifically realized by the following technical solutions:

[0005] The system for deep cascade utilization of flue gas at the tail of the boiler includes: a boiler, a denitration system, a feed water and condensate heating unit, a primary air tubular air preheater, a primary air flue gas air heater, a secondary air flue gas air heater, an air heat exchanger, a primary air fan, a secondary air fan, a dust collector, a boiler induced draft fan, a desulfurization unit, and a three-compartment air preheater;

[0006] The flue gas output from the boiler through the denitration system is divided into three paths. One path of flue gas supplies heat to the feed water and condensate circuits through the feed water and condensate heating unit; another path of flue gas heats the primary air and secondary air or only heats the secondary air through the three-compartment air preheater; the last path of flue gas supplies heat to the primary air tubular air preheater; the three paths of flue gas finally converge to heat the air supply at the outlets of the primary air fan and the secondary air fan, and then are discharged through the dust collector, the boiler induced draft fan, and the desulfurization unit by the chimney, and are used to heat the air at the inlets of the primary air fan and the secondary air fan;

[0007] The boiler forced draft fan supplies air to the primary air fan and the secondary air fan respectively through the air heat exchanger to form preheated primary air and secondary air. The primary air enters the boiler through the primary air tubular air preheater or the primary air chamber of the three-chamber air preheater via the primary air flue gas air heater; the secondary air enters the boiler through the primary air flue gas air heater and the three-chamber air preheater in sequence.

[0008] The further design of the boiler tail flue gas deep cascade utilization system lies in that the system further includes a saturated wet flue gas waste heat utilization loop. A saturated wet flue gas bypass fan is provided in the saturated wet flue gas waste heat utilization loop. The air inlet of this loop is the air outlet of the desulfurization unit, and the air outlet of the loop is the chimney. The saturated wet flue gas output from the desulfurization unit supplies heat to the air heat exchanger through the saturated wet flue gas bypass fan. The air heat exchanger is arranged at the air inlets of the boiler primary air fan and secondary air fan to preheat the air passing through the air heat exchanger.

[0009] The further design of the boiler tail flue gas deep cascade utilization system lies in that the air inlets of the primary air chamber and the secondary air chamber of the three-chamber air preheater are connected by a pipeline provided with a connection door. When the connection door is opened, secondary air is injected into the primary air chamber and the secondary air chamber simultaneously, forming a two-chamber structure with the flue gas chamber; when the connection door is closed, the three-chamber air preheater retains its original three-chamber structure.

[0010] The further design of the boiler tail flue gas deep cascade utilization system lies in that the feed water and condensate heating unit includes a feed water heater, a condensate heater, a deaerator, a high-pressure heater, and a low-pressure heater. The condensate flows from the outlet of the condensate pump through the low-pressure heater and the condensate heater, and then sequentially passes through the deaerator, the high-pressure heater, and the feed water heater to complete the feed water supply; the flue gas supplying heat to the feed water and condensate heating unit passes through the feed water heater and the condensate heater in sequence, and then mixes with the flue gas at the outlet of the air preheater and the outlet of the tubular air preheater to heat the air supply.

[0011] The further design of the boiler tail flue gas deep cascade utilization system lies in that there are four low-pressure heaters, and the four low-pressure heaters are connected in series in sequence. The condensate heater is connected in parallel to one of the low-pressure heaters.

[0012] The further design of the boiler tail flue gas deep cascade utilization system lies in that there are three high-pressure heaters, and the three high-pressure heaters are connected in series in sequence. The condensate heater is connected in parallel to the three high-pressure heaters.

[0013] The further design of the boiler tail flue gas deep cascade utilization system lies in that a flue gas regulating baffle is provided on the input side of the flue gas gas path of the feed water and condensate heater and the primary air tubular air preheater.

[0014] The further design of the system for deep cascade utilization of flue gas at the boiler tail is that there is a primary air side inlet door between the primary air flue gas air preheater and the primary air chamber of the three - compartment air preheater. When the primary air side inlet door is closed, the primary air passes through the primary air flue gas air preheater and enters the boiler through the primary air tubular air preheater; when the primary air side inlet door is open, the primary air passes through the primary air flue gas air preheater and enters the boiler through the primary air chamber of the three - compartment air preheater.

[0015] The further design of the system for deep cascade utilization of flue gas at the boiler tail is that both the feed water heater and the condensate heater are multi - pass tubular heat exchangers and are arranged in a counter - current manner.

[0016] The further design of the system for deep cascade utilization of flue gas at the boiler tail is that the primary air tubular air preheater adopts an efficient tubular gas - gas heat exchanger. The flue gas is in the in - tube process, and the air is in the out - of - tube process. The heat exchange tubes of the heat exchanger are arranged with internal and external fins, and the internal fins of the heat exchanger adopt burr - shaped fins.

[0017] The advantages of the present invention are as follows:

[0018] The system for deep cascade utilization of flue gas at the boiler tail of the present invention adds an air preheater flue gas bypass to heat the feed water, condensate and cold primary air, and adds a primary air flue gas air preheater and a secondary air flue gas air preheater, which can reduce the flue gas at the dust collector inlet to about 90 °C. While realizing the waste heat utilization of the unit, it improves the comprehensive cold - end temperature of the original three - compartment air preheater, and at the same time raises the hot air temperature of the boiler, improving the boiler combustion efficiency; by adding a primary air tubular air preheater, the original three - compartment air preheater is indirectly transformed into a "two - compartment" air preheater, while ensuring the hot air temperatures of the primary and secondary air, greatly reducing the air leakage rate of the air preheater; by adding a feed water and condensate heating unit, high - parameter extraction steam is displaced, increasing the power generation of the unit; an air heat exchanger is added, which can utilize the latent heat of vaporization of saturated wet flue gas to heat the supply air, achieving a way of deep waste heat utilization.

[0019] The deep cascade utilization system of the flue gas at the boiler tail of the present invention can make full and reasonable use of the waste heat of the flue gas at the boiler tail, adopt energy cascade utilization, reduce the heat transfer terminal difference of each system, enhance the heat transfer efficiency of each heat exchange system, reduce the heat exchange area of the system, and also increase the operation flexibility of the system. Through the transformation of this system, the power supply coal consumption of the unit can be effectively reduced by about 6 g / kwh under the pure condensing condition. In addition, through the transformation of this system, the operation safety of the unit can be improved, the risk of air preheater blockage and corrosion is reduced. Compared with other transformation methods, the operation temperature of the secondary air at the outlet of the air preheater is ensured or increased, and the boiler combustion efficiency is improved. Through the transformation of this system, the flue gas temperature at the dust removal inlet is reduced to about 90 °C, which improves the dust removal efficiency of the electrostatic precipitator and increases the removal efficiency of SO3 in the tail flue gas. In addition, while utilizing the waste heat of the saturated wet flue gas at the desulfurization outlet, the water collection function is realized, the water consumption of the power plant is reduced, and through the condensation of the saturated wet flue gas, the emissions of pollutants such as filterable particulate matter, soluble salts, and SO3 aerosols in the flue gas can be reduced at the same time. Brief Description of the Drawings

[0020] Figure 1 It is a schematic diagram of the overall structure of the deep cascade utilization system of the flue gas at the boiler tail of the present invention and the flue gas, water flow, and wind direction.

[0021] Among them, 1 - coal-fired power station boiler, 2 - flue gas at the outlet of the denitration system, 3 - bypass flue gas at the denitration outlet, 4 - flue gas after the feed water and condensate heating unit, 5 - flue gas after the primary air duct type air preheater, 6 - flue gas at the outlet of the three-chamber air preheater, 7 - mixed flue gas after the three-chamber air preheater, 8 - flue gas after the air heater for the supply air, 9 - flue gas at the inlet of the dust collector, 10 - saturated wet flue gas after the desulfurization system, 11 - bypass flue gas of the saturated wet flue gas, 12 - flue gas after the air heat exchanger, 13 - finally emitted flue gas, 14 - regulating baffle for the bypass flue gas of the saturated wet flue gas, 15 - bypass fan for the saturated wet flue gas, 16 - air heat exchanger, 17 - dust collector, 18 - boiler induced draft fan, 19 - desulfurization system, 20 - chimney, 21 - air intake of the boiler forced draft fan, 22 - preheated air at the inlet of the boiler forced draft fan, 23 - shut-off door at the inlet of the boiler secondary air fan, 24 - shut-off door at the inlet of the boiler primary air fan, 25 - boiler primary air fan, 26 - boiler secondary air fan, 27 - primary air at the outlet of the boiler primary air fan, 28 - secondary air at the outlet of the boiler secondary air fan, 29 - boiler primary air - flue gas air heater, 30 - boiler secondary air - flue gas air heater, 31 - primary air heated by the air heater, 32 - secondary air heated by the air heater, 33 - inlet door of the primary air side of the three-chamber air preheater, 34 - connection door between the primary and secondary air inlets of the three-chamber air preheater, 35 - adjustment door at the inlet of the primary air duct type air preheater, 36 - primary air chamber of the three-chamber air preheater, 37 - secondary air chamber of the three-chamber air preheater, 38 - flue gas chamber of the three-chamber air preheater, 39 - regulating baffle for the flue gas at the inlet of the dust collector, 40 - regulating baffle for the flue gas at the inlet of the primary air duct type air preheater, 41 - regulating baffle for the flue gas at the inlet of the feed water and condensate heating unit, 42 - primary air duct type air preheater, 43 - hot air at the outlet of the primary air chamber of the three-chamber air preheater, 44 - shut-off door for the hot primary air at the outlet of the primary air chamber of the three-chamber air preheater, 45 - connection door between the primary and secondary air outlets of the three-chamber air preheater, 46 - hot secondary air at the outlet of the three-chamber air preheater, 47 - hot primary air at the outlet of the primary air duct type air preheater, 48 - feed water heater, 49 - condensate heater, 50 - adjustment door for the bypass heating system of the feed water, 51 - feed water at the inlet of the feed water heater, 52 - feed water at the outlet of the feed water heater, 53 - condensate at the inlet of the condensate heater, 54 - adjustment door for the bypass heating system of the condensate, 55 - condensate at the outlet of the condensate heater, 56 - condensate at the outlet of the condensate pump, 57, 58, 59, 60 - low-pressure heaters, 61 - deaerator, 62, 63, 64 - high-pressure heaters, 65 - final feed water, 66 - shut-off door for the mixed flue gas after the three-chamber air preheater. Detailed implementation manners

[0022] The technical solution of the present invention will be further described in combination with specific embodiments and the accompanying drawings.

[0023] As Figure 1, this embodiment provides a system for deep cascade utilization of waste heat in the flue gas at the tail of a coal-fired power plant boiler. This system mainly includes a feed water-condensate heating system, an air preheater system, a flue gas-air heater system, and a wet saturated wet flue gas waste heat utilization system. Each system is independent and can be decoupled from the system for separate operation. At the same time, each system is coupled and used together.

[0024] At present, the flue gas at the outlet of the denitration system of the vast majority of coal-fired power plant boilers (1) all enters the flue gas chamber (38) of the three-compartment air preheater. In this embodiment, a part of the flue gas is by-passed on the original basis and is respectively used to heat the feed water, condensate, and primary cold air. Among them, the by-passed flue gas (3) at the denitration outlet passes through the feed water heater (48) to heat the feed water at the outlet of the deaerator (61) to the temperature corresponding to the final feed water (65). After mixing with the final feed water (65), it is injected into the economizer of the boiler. The water flow of this part is adjusted by the adjustment valve (50) of the feed water by-pass heating system, and the heating flue gas volume of this part is adjusted by the inlet flue gas regulating baffle (41) of the feed water-condensate heating unit. Through the coupled adjustment between the adjustment valve (50) of the feed water by-pass heating system and the inlet flue gas regulating baffle (41) of the feed water-condensate heating unit, it can be ensured that the temperature of the feed water (52) at the outlet of the feed water heater is consistent with the temperature of the final feed water (65). To make full use of the waste heat of this part of the flue gas, this embodiment designs a condensate heating system. The condensate is heated by the flue gas at the outlet of the feed water heater (48). The condensate is taken from the outlet of the low-pressure heater (59) and the condensate is heated by the condensate heater (49). The heated condensate is injected into the inlet of the deaerator (61). The condensate flow of this part is adjusted by the adjustment valve (54) of the condensate by-pass heating system, and the temperature of the flue gas (4) after the feed water-condensate heating unit can be controlled in the range of 120-130°C. Controlling the flue gas discharge temperature in this range mainly has two purposes: First, to ensure that the flue gas discharge temperature is in a relatively high range for heating the air supply. Second, to ensure that the flue gas discharge temperature is in a relatively high range, more than 40°C higher than the acid dew point of the flue gas, which can avoid the problem of low-temperature corrosion at the cold end of the heat exchanger. Both the feed water heater (48) and the condensate heater (49) adopt multi-pass tubular heat exchangers and are arranged in a counter-current manner. It is recommended to use H-type finned tubes, and the material can be 20G. The flue gas (4) after the feed water-condensate heating unit is finally mixed with the flue gas (6) at the outlet of the three-compartment air preheater, and the temperature of the mixed flue gas is 130-140°C.

[0025] Combined Figure 1 , the flue gas (4) after the feed water-condensate heating unit is finally mixed with the flue gas (6) at the outlet of the three-compartment air preheater to become the mixed flue gas (7) after the three-compartment air preheater, and the temperature of the flue gas is 130-140°C. The mixed flue gas adjusts the flue gas volume entering the heater heating system and short-circuiting into the dust collector through the inlet flue gas regulating baffle (39) of the dust collector to flexibly adjust the flue gas discharge temperature and the required heat power of the heater.

[0026] The primary air (27) at the outlet of the boiler primary air fan and the secondary air (28) at the outlet of the boiler secondary air fan are respectively heated by the boiler primary air-gas air heater (29) and the boiler secondary air-gas air heater (30). It is recommended to use a highly efficient heat exchanger with double ribs inside and outside for the air heater, with the flue gas as the in-tube process and the primary and secondary air as the out-of-tube process. Through this heat exchanger, the flue gas temperature of 130 - 140 °C can be reduced to 90 °C, and the temperature of the primary and secondary air can be increased by 40 - 50 °C. The heat transfer amount of this part can be reasonably configured by adjusting the flue gas volume through the flue gas regulating damper (39) at the inlet of the dust collector. It is recommended to use ND steel for this part of the heat exchanger. Through this system, the comprehensive cold-end temperature of the three-compartment air preheater is greatly increased, the risk of cold-end corrosion and blockage of the air preheater is reduced, and under the winter operating conditions, it can replace the steam consumption of the original steam air heater, reduce the extraction volume of the turbine regenerative system, and increase the power generation of the unit.

[0027] Such as Figure 1, in this embodiment, the original three - compartment air preheater is reconfigured. After the denitration system, the flue gas still passes through the flue gas compartment (38) of the three - compartment air preheater. The primary air (31) after being heated by the air heater does not enter the primary air compartment (36) of the three - compartment air preheater, but bypasses into the primary air tubular air preheater (42). The bypass flue gas (3) at the denitration outlet is used to heat the primary air. The hot primary air (47) at the outlet of the heated primary air tubular air preheater is then mixed with the original cold primary air system and enters the coal - pulverizing system. The temperature of this part of the hot primary air is adjusted by the flue gas volume and the primary hot air volume through the flue gas regulating baffle (40) at the inlet of the primary air tubular air preheater and the inlet regulating valve (35) of the primary air tubular air preheater, so as to adjust the temperature of the hot primary air at the outlet of the primary air tubular air preheater and the temperature of the flue gas after the primary air tubular air preheater. Under this condition, it is required that the inlet regulating valve (35) of the primary air tubular air preheater is in the open state, while the inlet door (33) on the primary air side of the three - compartment air preheater is in the closed state, and the primary hot air shut - off door (44) at the outlet of the primary air compartment of the three - compartment air preheater is in the closed state. At the same time, the connection doors (34) between the primary and secondary air inlets of the three - compartment air preheater and the connection doors (45) between the primary and secondary air outlets of the three - compartment air preheater are in the open state. The secondary air after being heated by the air heater enters the primary air compartment (36) and the secondary air compartment (37) of the three - compartment air preheater respectively through the connection doors (34) between the primary and secondary air inlets of the three - compartment air preheater for heat exchange, and the hot secondary air (46) at the outlet of the three - compartment air preheater enters the boiler wind box. Through this design, the original three - compartment air preheater is changed into a two - compartment air preheater, greatly increasing the heat exchange area of the secondary air. The temperature of the hot secondary air (46) at the outlet of the three - compartment air preheater can be further increased or maintained on the original basis, avoiding the problem of the reduction of the secondary air temperature caused by the decrease in the flue gas volume entering the preheater, and at the same time reducing the resistance of the air preheater on the secondary air side and the flue gas side. On the other hand, since the primary air bypass is used to heat the tubular air preheater, its air leakage rate is almost zero, while the air leakage rate of the two - compartment Ljungstrom air preheater can generally be guaranteed to be within 2%, greatly reducing the air leakage rate of the air preheater system, and can reduce the unit's power supply coal consumption by about 0.6 g / kwh. Since the tubular air preheater has problems of being worn or corroded, which will also cause air leakage, in order to increase the stability of the system operation, the structure of the original three - compartment air heat exchanger is retained in this design.The primary air duct type air preheater and the feed water and condensate heating system can be isolated by closing the flue gas regulating damper (40) at the inlet of the primary air duct type air preheater and the flue gas regulating damper (41) at the inlet of the feed water and condensate heating unit. Close the regulating valve (35) at the inlet of the primary air duct type air preheater, open the primary air side inlet door (33) of the three - compartment air preheater, close the connection door between the primary and secondary air inlets of the three - compartment air preheater, open the primary hot air shut - off door (44) at the outlet of the primary air chamber of the three - compartment air preheater, and close the connection door (45) between the primary and secondary air outlets of the three - compartment air preheater. The system will return to the traditional three - compartment layout mode, isolating the newly added system outside the original system. Preferably, the primary air duct type air preheater (42) adopts a high - efficiency tubular gas - gas heat exchanger. The flue gas is in the in - tube process, and the air is in the out - tube process. The heat exchange tubes are arranged with internal and external ribs. The internal ribs are preferably burr - shaped fins, which can prevent corrosion and ash accumulation problems. The heat exchanger material is preferably ND steel.

[0028] This embodiment also provides a heat recovery and utilization loop for the flue gas after desulfurization. A part of the saturated wet flue gas (10) after the desulfurization system bypasses, and the bypass saturated wet flue gas (11) is led out by the bypass saturated wet flue gas fan (15), and preheats the air at the inlets of the primary fan and the forced draft fan through the air heat exchanger (16) to form the pre - heated air (22) at the inlet of the boiler forced draft fan. The flue gas (12) after the air heat exchanger is discharged to the final discharged flue gas (13), and finally discharged to the chimney (20). The flue gas volume of the bypass system can be adjusted by the bypass saturated wet flue gas regulating damper. The air heat exchanger (16) is designed as a tubular gas - gas heat exchanger. The saturated wet flue gas is in the out - tube process, and the air is in the in - tube process. The heat exchanger material is preferably 2205 stainless steel. The heat exchanger is preferably a high - efficiency heat exchanger with high - efficiency fins inside the tube and smooth tubes outside the tube. Through this heat exchanger, under winter conditions, 1 million m 3 / h of cold air at - 10°C can be heated to about 30°C, and 400,000 m 3 / h of saturated wet flue gas at 50°C needs to be bypassed, and the temperature of the saturated wet flue gas is reduced to about 35°C, and about 25 t / h of condensate is condensed out. Under summer conditions, 1 million m 3 / h of air at 20°C can be heated to about 40°C, and 300,000 m 350 °C saturated wet flue gas at / h, the temperature of the saturated wet flue gas is reduced to about 40 °C, and about 18 t / h of condensed water is condensed out at the same time. The system is designed with a shut-off door (23) at the inlet of the boiler secondary fan and a shut-off door (24) at the inlet of the boiler primary fan. When the bypass system cuts off the unit operation, the shut-off door (23) at the inlet of the boiler secondary fan and the shut-off door (24) at the inlet of the boiler primary fan can be closed to cut off the operation of this system. The air inlet of the fan uses the original system. When the bypass system is operating, the original system is cut off from operation, which will not be elaborated here. Through this system, the air temperature at the inlet of the fan can be heated to 30 °C under winter conditions, effectively preventing the cold-end corrosion of the boiler primary air-flue gas air preheater (29) and the boiler secondary air-flue gas air preheater (30). Under summer conditions, the waste heat of the flue gas can be effectively recovered, increasing the heat absorption of the feed water and condensate systems.

[0029] Through the transformation of this system, the waste heat of the flue gas at the tail of the boiler can be fully and reasonably utilized. By adopting cascaded energy utilization, the heat transfer end difference of each system is reduced, the heat transfer efficiency of each heat transfer system is enhanced, the heat transfer area of the system is reduced, and the flexibility of the system operation is also increased. Through the transformation of this system, the power supply coal consumption of the unit can be effectively reduced by about 6 g / kwh under the pure condensing condition. In addition, through the transformation of this system, the operation safety of the unit can be improved, the risk of air preheater blockage and corrosion is reduced. Compared with other transformation methods, the operation temperature of the secondary air at the outlet of the air preheater is guaranteed or increased, and the boiler combustion efficiency is improved. Through the transformation of this system, the temperature of the flue gas at the dust removal inlet is reduced to about 90 °C, improving the dust removal efficiency of the electrostatic precipitator while increasing the removal efficiency of SO3 in the tail flue gas. In addition, while utilizing the waste heat of the saturated wet flue gas at the desulfurization outlet, the water collection function is realized, reducing the water consumption of the power plant. By condensing the saturated wet flue gas, the emissions of pollutants such as filterable particulate matter, soluble salts, and SO3 aerosols in the flue gas can be reduced at the same time.

[0030] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and deformations can be made, and these improvements and deformations should also be regarded as the protection scope of the present invention.

Claims

1. A deep cascade utilization system for flue gas at the boiler tail, characterized in that Comprising: a boiler, a denitration system, a feed water and condensate heating unit, a primary air duct type air preheater, a primary air flue gas air preheater, a secondary air flue gas air preheater, an air heat exchanger, a primary air fan, a secondary air fan, a dust collector, a boiler induced draft fan, a desulfurization unit, and a three - compartment air preheater; The flue gas output by the boiler through the denitration system is divided into three paths. One path of flue gas supplies heat to the feed water and condensate circuits respectively through the feed water and condensate heating unit; another path of flue gas heats the primary air and secondary air or only heats the secondary air through the three - compartment air preheater; the last path of flue gas supplies heat to the primary air duct type air preheater; the three paths of flue gas finally converge to heat the air sent out from the outlets of the primary air fan and the secondary air fan, and then are discharged through the dust collector, the boiler induced draft fan, and the desulfurization unit by the chimney, and are used to heat the air at the inlets of the primary air fan and the secondary air fan; The boiler forced draft fan supplies air to the primary air fan and the secondary air fan respectively through the air heat exchanger to form preheated primary air and secondary air. The primary air enters the boiler through the primary air flue gas air preheater or the primary air chamber of the three - compartment air preheater after passing through the primary air flue gas air preheater. The secondary air enters the boiler successively through the primary air flue gas air preheater and the three - compartment air preheater. The system also includes a saturated wet flue gas waste heat utilization circuit. A saturated wet flue gas bypass fan is provided in the saturated wet flue gas waste heat utilization circuit. The air inlet of this circuit is the air outlet of the desulfurization unit, and the air outlet of the circuit is the chimney. The saturated wet flue gas output by the desulfurization unit supplies heat to the air heat exchanger through the saturated wet flue gas bypass fan. The air heat exchanger is arranged at the inlets of the primary air fan and the secondary air fan of the boiler to preheat the air passing through the air heat exchanger. The inlets of the primary air chamber and the secondary air chamber of the three - compartment air preheater are connected by a pipeline provided with a connection door. When the connection door is opened, secondary air is injected into both the primary air chamber and the secondary air chamber at the same time, forming a two - compartment structure with the flue gas chamber; when the connection door is closed, the three - compartment air preheater retains its original three - compartment structure.

2. The deep cascade utilization system for the flue gas at the tail of the boiler according to claim 1, wherein: The feed water and condensate heating unit includes a feed water heater, a condensate heater, a deaerator, a high - pressure heater, and a low - pressure heater. The condensate, after flowing out from the outlet of the condensate pump, passes through the low - pressure heater and the condensate heater, and then successively passes through the deaerator, the high - pressure heater, and the feed water heater to complete the feed water supply; the flue gas supplying heat to the feed water and condensate heating unit passes through the feed water heater and the condensate heater successively, and then is mixed with the flue gas at the outlet of the air preheater and the outlet of the tubular air preheater to heat the air supply.

3. The boiler tail gas deep cascade utilization system according to claim 2, characterized in that: There are four low - pressure heaters, and the four low - pressure heaters are connected in series in sequence. The condensate heater is connected in parallel to one of the low - pressure heaters.

4. The boiler tail gas deep cascade utilization system according to claim 2, characterized in that: There are three high - pressure heaters, and the three high - pressure heaters are connected in series in sequence. The condensate heater is connected in parallel to the three high - pressure heaters.

5. The boiler tail gas deep cascade utilization system according to claim 1, characterized in that: A flue gas regulating baffle is provided on the input side of the flue gas gas path of the feed water and condensate heater and the primary air duct type air preheater.

6. The boiler tail gas deep cascade utilization system according to claim 1, characterized in that: An inlet door on the primary air side is provided between the primary air flue gas air heater and the primary air chamber of the three - compartment air preheater. When the inlet door on the primary air side is closed, the primary air passes through the primary air tubular air preheater via the primary air flue gas air heater and enters the boiler; when the inlet door on the primary air side is open, the primary air passes through the primary air chamber of the three - compartment air preheater via the primary air flue gas air heater and enters the boiler.

7. The boiler tail gas deep cascade utilization system according to claim 1, wherein: Both the feedwater heater and the condensate heater are multi - pass tubular heat exchangers and are arranged in a counter - current manner.

8. The boiler tail gas deep cascade utilization system according to claim 1, characterized in that: The primary air tubular air preheater adopts a high - efficiency tubular gas - gas heat exchanger. The flue gas is in the in - tube process and the air is in the out - tube process. The heat exchange tubes of the heat exchanger are arranged with internal and external fins, and the internal fins of the heat exchanger adopt burr - shaped fins.

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