Coal-fired power generation boiler system based on low-temperature air preheater to recover flue gas waste heat

Abstract

The present invention belongs to the technical field of boiler flue gas waste heat recovery. Specifically, it relates to a coal-fired power generation boiler system that uses a low-temperature air preheater to recover flue gas waste heat. The system comprises an air preheater and bypass flue subsystem, a low-temperature air preheater subsystem, a bypass flue feedwater heater, a turbine high-pressure heating system subsystem, and a secondary air heater subsystem. The air preheater and bypass flue subsystem comprises a main flue, a bypass flue, and a boiler air inlet. The low-temperature air preheater subsystem includes a low-temperature air preheater, comprising a primary air low-temperature preheater and a secondary air low-temperature preheater, for preheating the primary and secondary air using the waste heat from the flue gas exhausted from the main flue. The secondary air heater subsystem reheats the secondary air heated by the flue gas in the air preheater and bypass flue subsystem. This system can effectively improve flue gas waste heat recovery and reduce the unit's coal consumption by 4-5g / kWh.

Description

Technical Field

[0001] The present invention belongs to the technical field of boiler flue gas waste heat recovery, and specifically relates to a coal-fired power generation boiler system for recovering flue gas waste heat based on a low-temperature air preheater. Background Art

[0002] Efficient recovery of waste heat from flue gas in coal-fired power generation boilers is an important research direction for improving the power generation efficiency of coal-fired power generation units. The most direct way to recover waste heat from flue gas is to use a low-temperature economizer, which uses condensate to reduce the boiler exhaust temperature from approximately 130°C to approximately 90°C. The heated condensate is then returned to the low-pressure heating system. Since the steam extraction volume of the steam turbine is reduced, the steam turbine work is increased, thereby improving the cycle efficiency of the unit. This method is simple, but the efficiency of waste heat recovery is low, and it can usually only reduce power coal consumption by about 2g / kWh. Of this, 0.5g / kWh is due to the reduction in flue gas resistance after the flue gas temperature is reduced. The coal consumption directly reduced by recovering waste heat usually does not exceed 1.5g / kWh.

[0003] At present, a boiler flue gas waste heat recovery system using partial flue gas bypass has been launched in China (such as Figure 1 (as shown in the figure) and has been widely used in newly built large-scale coal-fired power generation units. This system can recover flue gas waste heat relatively efficiently, typically reducing coal consumption by approximately 3g / kWh. However, this boiler flue gas waste heat recovery system still has the following technical issues:

[0004] 1) When the flue gas temperature and heating surface area at the air preheater inlet remain unchanged, the air temperature at the air preheater outlet is lower than the operating condition without flue gas waste heat recovery. That is, after about 15% of the flue gas is bypassed, the air temperature used for combustion is lower than the original design value, which will lead to a decrease in boiler efficiency and an increase in coal consumption. The solution adopted in the existing technology improves the flue gas waste heat recovery effect by setting up a forced circulation heat pipe system to preheat the secondary air and using technical means such as setting up a flue gas bypass. However, the lower air temperature at the air preheater outlet offsets part of the waste heat recovery effect, thereby affecting the overall efficiency of the flue gas waste heat recovery.

[0005] 2) The forced circulation heat pipe system used in existing technologies only heats the secondary air, while the primary air directly enters the air preheater. The primary air accounts for approximately 25% of the total air volume and is not used for flue gas cooling or waste heat recovery in the low-temperature zone, which also reduces the effectiveness of flue gas waste heat recovery.

[0006] 3) A 2-stage heat exchanger is provided in the bypass flue (i.e. Figure 1The forced circulation heat pipe system cannot raise the air preheater outlet temperature sufficiently. The high-temperature economizer outlet water in the bypass flue is directly used as feed water, which has a high waste heat recovery efficiency. The medium-temperature economizer outlet water, on the other hand, is fed to the deaerator, which has a low waste heat recovery efficiency.

[0007] This field urgently needs to find a new technical solution to solve the above problems. Summary of the Invention

[0008] In order to solve the problems in the above-mentioned background technology, the present invention provides a coal-fired power generation boiler system based on a low-temperature air preheater to recover flue gas waste heat. Through the technical solution in the disclosed embodiment of the present invention, the flue gas waste heat recovery effect can be effectively improved, and the coal consumption of the unit power supply can be reduced by 4~5g / kWh.

[0009] The present invention aims to provide a coal-fired power generation boiler system for recovering flue gas waste heat based on a low-temperature air preheater, comprising: a high-temperature economizer, a denitrification reactor, and a low-temperature economizer. The system comprises: an air preheater and a bypass flue subsystem, a low-temperature air preheater subsystem, a bypass flue feedwater heater and a steam turbine high-pressure heating system subsystem, and a secondary air heater subsystem.

[0010] The air preheater and bypass flue subsystem includes: a main flue for discharging flue gas from the boiler, a bypass flue for discharging flue gas from the boiler, and a boiler air inlet passage for allowing air to enter the boiler. The air entering the boiler air inlet passage includes primary air and secondary air, and the primary air and secondary air are heated by the waste heat of the flue gas discharged from the main flue.

[0011] The low-temperature air preheater subsystem includes: a low-temperature air preheater, which is used to preheat the primary air and secondary air by using the waste heat of the flue gas discharged from the main flue. The preheated primary air and secondary air enter the air preheater and bypass flue subsystem;

[0012] The bypass flue feedwater heater and turbine high-pressure heating system subsystem includes: a boiler feedwater pipeline heated by steam recovery from turbine extraction and flue gas in the bypass flue;

[0013] The secondary air heater subsystem is used to reheat the secondary air heated by the flue gas in the air preheater and the bypass flue subsystem, and the reheated secondary air enters the boiler combustion system;

[0014] The low-temperature air preheater subsystem increases the outlet temperature of the air preheater and bypass flue subsystem from 120~130°C to 10K or higher than the outlet water temperature of the feed water pump. Only a bypass flue feed water heater is set in the bypass flue. The outlet water of the bypass flue feed water heater is directly merged with the feed water from the steam turbine high-pressure heater and enters the low-temperature economizer. The feed water pump is used to supply water to the boiler feed water pipeline, and the steam turbine high-pressure heater and feed water heater are used to heat the feed water.

[0015] Furthermore, the low-temperature air preheater subsystem adopts a low-temperature air preheater method to recover the low-temperature waste heat of the flue gas, and adopts a direct heat exchange method or a partial indirect heat exchange method between the flue gas and the air.

[0016] Furthermore, a heat exchange mode of the low-temperature air preheater subsystem is direct heat exchange between flue gas and air;

[0017] The low-temperature air preheater includes: a primary air low-temperature preheater and a secondary air low-temperature preheater, and the air inlet channel includes a primary air inlet channel and a secondary air inlet channel, and heat is directly exchanged through the primary air low-temperature air preheater and the secondary air low-temperature air preheater;

[0018] A primary air fan, a primary air low-temperature preheater and a primary air preheater are sequentially arranged in the primary air inlet channel. The primary air is delivered by the primary air fan, passes through the primary air low-temperature preheater and the primary air preheater in sequence, and is heated by the flue gas discharged from the main flue.

[0019] A secondary air fan, a secondary air low-temperature preheater and a secondary air preheater are sequentially arranged in the secondary air inlet channel. The secondary air is delivered by the secondary air fan, passes through the secondary air low-temperature preheater and the secondary air preheater in sequence, and is heated by the flue gas discharged from the main flue.

[0020] Furthermore, another heat exchange mode between the flue gas and the air in the low-temperature air preheater subsystem is partial indirect heat exchange;

[0021] Furthermore, another heat exchange mode between the flue gas and the air in the low-temperature air preheater subsystem is partial indirect heat exchange;

[0022] The low-temperature air preheater subsystem includes: a heat medium water heat exchanger, a primary air heater, a secondary air heater, a primary air heat medium water regulating valve, a secondary air heat medium water regulating valve, a heat medium water return regulating valve, a heat medium water circulation pump, a heat medium water circulation pipeline, a primary air heat medium water heat exchange pipeline and a secondary air heat medium water heat pipeline;

[0023] The heat medium water circulation pipeline is provided with the heat medium water circulation pump, the heat medium water heat exchanger and the heat medium water return regulating valve in sequence. After the heat medium water from the condensate system or the steam turbine low-pressure heating system is heated by the heat medium water heat exchanger, a portion of the heat medium water flows back to the steam turbine low-pressure heater through the return regulating valve, and the other portion enters the primary air heat medium water heat exchange pipeline and the secondary air heat medium water heat exchange pipeline;

[0024] The primary air heat medium water heat exchange pipeline is sequentially provided with the primary air heat medium water regulating valve and the primary air heater, and the heat medium water returns to the inlet of the heat medium water circulation pump after heat exchange through the primary air heater;

[0025] The secondary air heat medium water heat exchange pipeline is sequentially provided with the secondary air heat medium water regulating valve and the secondary air heater. The heat medium water returns to the inlet of the heat medium water circulation pump after heat exchange through the secondary air heater.

[0026] The primary air is sent out by the primary fan, passes through the primary air heater, the primary air low-temperature preheater and the primary air preheater in sequence, is heated by the flue gas discharged from the main flue, and then goes to the pulverized coal preparation system;

[0027] The secondary air is delivered by the secondary air fan, passes through the secondary air heater, the secondary air low-temperature preheater and the secondary air preheater in sequence, is heated by the flue gas discharged from the main flue, and then enters the secondary air heater subsystem for further heating before entering the boiler combustion system.

[0028] Furthermore, the secondary air heater subsystem includes: a secondary air heater, the secondary air heater being arranged upstream of the denitration reactor and downstream of the high-temperature economizer;

[0029] The secondary air heated by the air preheater and the bypass flue subsystem enters the secondary air heater subsystem, is further heated by the flue gas at the secondary air heater, and then enters the boiler combustion system.

[0030] Furthermore, under design conditions, the amount of flue gas passing through the main flue accounts for approximately 80%, and the amount of flue gas passing through the bypass flue accounts for approximately 20%;

[0031] A flue gas damper regulating valve is provided in the bypass flue, and the flue gas damper regulating valve is used to distribute the flue gas flow discharged from the main flue and the flue gas flow discharged from the bypass flue.

[0032] Furthermore, in the bypass flue feedwater heater and turbine high-pressure heating system subsystem, the boiler feedwater pipeline includes: a first feedwater pipeline passing through the turbine high-pressure heater and a second feedwater pipeline passing through the feedwater heater in the bypass flue; the bypass flue is provided with a bypass flue feedwater heater;

[0033] A bypass feed water regulating valve and a bypass flue feed water heater are sequentially arranged on the second feed water pipeline. The water supply comes from the feed water pump, and the outlet water is combined with the feed water from the first feed water pipeline of the steam turbine high-pressure heater and then supplied to the low-temperature economizer through the boiler feed water pipeline; the bypass feed water regulating valve controls the amount of water entering the bypass feed water heater, thereby controlling the heat exchange between the flue gas and the feed water in the bypass flue and the outlet water temperature of the bypass flue feed water heater to be the same as the feed water temperature from the first feed water pipeline or higher than the feed water temperature of the first feed water pipeline by within 20K.

[0034] In summary, the coal-fired power generation boiler system based on the low-temperature air preheater to recover flue gas waste heat in the disclosed embodiment of the present invention can bring the following beneficial effects:

[0035] (1) Through the coordination of the two heat recovery systems, air-borne heat recovery and water-borne heat recovery, the waste heat of low-temperature flue gas can be efficiently recovered, and the cycle efficiency of the unit can be improved, reducing the coal consumption of the unit by about 4~5g / kWh;

[0036] (2) A flue gas damper regulating valve is installed in the bypass flue to adjust the flue gas volume passing through the bypass to meet the needs of various operating conditions. During low-load operation, the flue gas damper regulating valve can be adjusted to increase the primary and secondary air temperatures, which is beneficial to stable combustion of the boiler.

[0037] (3) By adopting the partial indirect heat recovery method, the low-temperature air preheater is divided into two parts. The upper part adopts the direct heat exchange method between flue gas and air, and the lower low-temperature end area adopts the forced circulation heat pipe indirect heat exchange method, which avoids the corrosion of the low-temperature air preheater and increases the flexibility of operation adjustment.

[0038] (4) By setting up a secondary air heater, the secondary air can be fully heated here, fully reflecting the effect of flue gas waste heat recovery, while increasing the temperature of the secondary air hot air, which is helpful for boiler combustion organization and also has a positive effect on stable combustion under low load. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0040] Figure 1 This is the overall structural diagram of the boiler flue gas waste heat recovery system in the prior art;

[0041] Figure 2 Schematic diagram of a coal-fired power generation boiler system for recovering flue gas waste heat based on a low-temperature air preheater according to an exemplary embodiment;

[0042] Figure 3 Based on Figure 2 The diagram shows the structure of another coal-fired power generation boiler system based on recovering flue gas waste heat using a low-temperature air preheater.

[0043] 1-high temperature economizer, 2-secondary air heater, 3-denitrification reactor, 4-low temperature economizer, 5-turbine high pressure heater, 6-bypass flue feed water heater, 7-secondary air preheater, 8-primary air preheater, 9-feed water regulating valve, 10-bypass feed water regulating valve, 11-feed water pump, 12-secondary air low temperature preheater, 13-primary air low temperature preheater, 14-secondary fan, 15-primary fan, 16-electrostatic precipitator, 17-induced draft fan, 18-flue gas damper regulating valve, 19-secondary air heater, 20-primary Air heater, 21-primary air heat medium water regulating valve, 22-secondary air heat medium water regulating valve, 23-heat medium water return regulating valve, 24-heat medium water heat exchanger, 25-heat medium water circulation pump, 26-boiler air inlet channel, 27-main flue, 28-bypass flue, 29-boiler feed water pipeline, 30-primary air inlet channel, 31-secondary air inlet channel, 32-heat medium water circulation pipeline, 33-primary air heat medium water heat exchange pipeline, 34-secondary air heat medium water pipe heat pipeline, 35-first water supply pipeline, 36-second water supply pipeline. DETAILED DESCRIPTION

[0044] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0045] The following is combined with Figure 2 and Figure 3 The present invention is described in detail with reference to specific embodiments.

[0046] The present invention provides a coal-fired power generation boiler system that uses a low-temperature air preheater to recover waste heat from flue gas. The system comprises a high-temperature economizer 1, a denitrification reactor 3, and a low-temperature economizer 4. The high-temperature economizer 1, the denitrification reactor 3, and the low-temperature economizer 4 are sequentially arranged at the rear of the boiler. The flue gas discharged from the boiler passes through the high-temperature economizer 1, the denitrification reactor 3, and the low-temperature economizer 4 in sequence. The flue gas flowing out of the low-temperature economizer 4 is split into two parts, one for heating the air entering the boiler, and the other for heating the feed water entering the boiler, replacing part of the steam turbine heat recovery system's extraction steam recovery. A coal-fired power generation unit has two heat recovery systems (a boiler air preheating heat recovery system and a steam turbine extraction steam recovery system). Both systems function to recover waste heat that would otherwise be discharged into the environment (low-temperature heat source) and, after improving its energy quality through a high-temperature heat source (furnace), use it for further work, hence the term "heat recovery." The waste heat from the flue gas is used to heat the combustion air and returns to the furnace with the combustion air, which is defined as air-borne heat recovery. The waste heat from the flue gas is used to heat the working water and then returns to the turbine extraction heat recovery system with the working water, which is defined as water-borne heat recovery. The utilization efficiency of air-borne heat recovery is high, which is the cycle efficiency of the unit; the utilization efficiency of water-borne heat recovery is low. The higher the temperature of the returned working water, the higher the utilization efficiency, but it is always lower than the cycle efficiency of the unit. The utilization efficiency of the waste heat from the flue gas is higher when it is carried by the combustion air, while the utilization efficiency is lower when it is carried by the working water. In other words, the efficiency of air-borne heat recovery is always higher than that of water-borne heat recovery. When designing the unit, the two heat recovery systems must be comprehensively optimized to achieve the highest cycle efficiency of the unit. When the waste heat from the flue gas at the tail of the boiler is fully utilized, it is particularly important to comprehensively optimize the above two heat recovery systems.

[0047] Furthermore, in the disclosed embodiment of the present invention, the coal-fired power generation boiler system based on the low-temperature air preheater to recover the waste heat of flue gas also includes: an air preheater and a bypass flue subsystem, a low-temperature air preheater subsystem, a bypass flue feedwater heater and a steam turbine high-pressure heating system subsystem, and a secondary air heater subsystem.

[0048] Specifically, the air preheater and bypass flue subsystem includes: a main flue 27 for discharging the flue gas in the boiler, a bypass flue 28 for discharging the flue gas in the boiler, and a boiler air inlet channel 26 for allowing air to enter the boiler. The air entering the boiler air inlet channel 26 includes primary air and secondary air, and the primary air and secondary air are heated by the waste heat of the flue gas discharged from the main flue 27.

[0049] For example, the flue gas flowing out of the low-temperature economizer 4 is split into two parts. One part is used in the main flue duct 27 of the air preheater and bypass flue subsystem to heat the primary and secondary air entering the boiler air inlet duct 26. The other part is used in the bypass flue duct 28 to reheat the water in the boiler feedwater pipe 29. Under design conditions, the flue gas volume passes through the air preheater, and the flue gas volume passes through the bypass flue duct, accounting for about 20%.

[0050] The low-temperature air preheater subsystem includes: a low-temperature air preheater, which is used to preheat the primary air and secondary air at low temperatures through the waste heat of the flue gas discharged from the main flue 27, so that the primary air and secondary air after low-temperature preheating enter the air preheater and the bypass flue subsystem, and both the primary air and the secondary air participate in the recovery of low-temperature flue gas waste heat.

[0051] For example, a low-temperature air preheater is used to recover the low-temperature waste heat of the flue gas, and the waste heat of the flue gas at the tail of the boiler is deeply utilized. The low-temperature air preheater is used to effectively increase the flue gas temperature level at the outlet of the air preheater and the bypass flue subsystem, thereby improving the efficiency of the bypass flue feedwater heater and the steam turbine high-pressure heating system subsystem in recovering the waste heat of the flue gas.

[0052] The bypass flue feedwater heater and turbine high-pressure heating system subsystem includes: a boiler feedwater pipeline 29 that recovers heat through steam extraction from the turbine and heat from the flue gas in the bypass flue.

[0053] The secondary air heater subsystem is used to reheat the secondary air in the air preheater and the bypass flue subsystem after being heated by the flue gas, and the reheated secondary air enters the boiler combustion system.

[0054] Specifically, the secondary air heater subsystem includes: a secondary air heater 2, which is arranged upstream of the denitrification reactor 3 and downstream of the high-temperature economizer 1; the secondary air heated by the air preheater and the bypass flue subsystem enters the secondary air heater subsystem, is further heated by the flue gas at the secondary air heater 2, and the heated secondary air enters the boiler combustion system.

[0055] For example, the temperature of the secondary air hot air is increased by the secondary air heater 2, thereby improving the boiler efficiency and reducing coal consumption. The secondary air heater 2 is arranged upstream of the SCR denitrification reactor 3 and downstream of the high-temperature economizer 1. The heat transfer temperature difference between the flue gas and the air is high (up to 80K or more), and the heat transfer efficiency is high, so the heating surface area is small. The secondary air is fully heated in the secondary air heater 2 (such as Figure 2 As shown in the figure, the temperature of the secondary air after heating can reach 352°C), which can fully reflect the effect of flue gas waste heat recovery, while helping the boiler combustion organization and also has a positive effect on stable combustion under low load.

[0056] In summary, in the disclosed embodiment of the present invention, both primary and secondary air are involved in the recovery of low-temperature flue gas waste heat, which can fully reflect the flue gas waste heat recovery and improve the waste heat recovery efficiency; under the condition that the heat transfer temperature difference is acceptable, the flue gas temperature at the inlet of the low-temperature air preheater is increased, such as Figure 2The flue gas temperature at the inlet of the medium and low temperature air preheater is T=210°C, which can increase the temperature level of the bypass flue water heater 6, thereby improving the waste heat recovery efficiency. Figure 1 This is difficult to achieve with a forced circulation heat pipe system (as shown in the figure), requiring a second-stage heater in the bypass duct, which reduces waste heat recovery efficiency. Using direct heat exchange between flue gas and air can easily increase the flue gas temperature at the inlet of the low-temperature air preheater, and thus the outlet temperature of the bypass duct.

[0057] The boiler air intake channel 26 includes a primary air intake channel 30 and a secondary air intake channel 31. The low-temperature air preheater includes: a primary air low-temperature preheater 13 and a secondary air low-temperature preheater 12. The primary air intake channel 30 is sequentially provided with a primary fan 15, a primary air low-temperature preheater 13 and a primary air preheater 8. The primary air is delivered by the primary fan 15, passes through the primary air low-temperature preheater 13 and the primary air preheater 8 in sequence, and is reheated by the heat of the flue gas discharged from the main flue 27. The secondary air intake channel 31 is sequentially provided with a secondary fan 14, a secondary air low-temperature preheater 12 and a secondary air preheater 7. The secondary air is delivered by the secondary fan 14, passes through the secondary air low-temperature preheater 12 and the secondary air preheater 7 in sequence, and is reheated by the heat of the flue gas discharged from the main flue 27.

[0058] One heat exchange method between the flue gas and the air in the low-temperature air preheater subsystem is direct heat exchange through the primary air low-temperature air preheater 13 and the secondary air low-temperature air preheater 12 .

[0059] Cold air (ambient air) from the blower (secondary air fan) 14 and primary air fan 15 first enters a low-temperature air preheater to recover waste heat from the flue gas. The low-temperature air preheater comprises a secondary air low-temperature preheater 12 and a primary air low-temperature preheater 13. The flue gas passes through the low-temperature air preheater, reducing its temperature to 85°C before entering the electrostatic precipitator 16, thereby recovering waste heat from the flue gas.

[0060] Since the efficiency of air-borne heat recovery is always higher than that of water-borne heat recovery, the waste heat of flue gas is used as much as possible to heat the combustion air (including primary air and secondary air), especially to heat the secondary air, so that the waste heat recovery benefit obtained by increasing the temperature of the secondary air can be maximized. The boiler air inlet channel 26 includes a primary air inlet channel 30 and a secondary air inlet channel 31. A primary air preheater 8 is provided in the primary air inlet channel 30 for recovering the primary air by utilizing the waste heat of the flue gas discharged from the main flue 27. A secondary air preheater 7 is provided in the secondary air inlet channel 31 for recovering the secondary air by utilizing the waste heat of the flue gas discharged from the main flue 27. The primary / secondary air heated by the low-temperature air preheater enters the air preheater to continue absorbing the waste heat of the flue gas. The primary air is heated by the primary air preheater 8 and then goes directly to the coal powder preparation system. Figure 2 As shown, the primary air temperature t=310°C ( Figure 2 The temperature values indicated in the table are typical design values for a 1000MW unit. Generally, this temperature level meets the drying requirements for the coal type. The primary air temperature generally does not need to be too high. If the primary air temperature is too high, cold air will need to be added before entering the pulverizer, which can cause irreversible losses.

[0061] In order to prevent the primary air temperature from being too high, the method used in the prior art is to prevent the primary air from participating in the heat exchange process of the low-temperature part. However, this method will reduce the efficiency of the flue gas waste heat recovery. The technical solution in the embodiment disclosed in the present invention adopts a moderate air preheater inlet flue gas temperature to control the primary air temperature at a reasonable level. Figure 2 The flue gas temperature at the inlet of the air preheater is T=340°C, which keeps the primary air temperature at a reasonable level. After the secondary air is heated by the secondary air preheater 7, it enters the secondary air heater 2 for further heating. Figure 2 In the scheme, the secondary air temperature reaches t=352°C, which is beneficial to improving boiler efficiency and organizing boiler combustion.

[0062] For example, the core equipment of the disclosed embodiment of the present invention is a low-temperature air preheater (including a primary air low-temperature air preheater 13 and a secondary air low-temperature air preheater 12). In the specific design, attention should be paid to preventing wear, blockage and corrosion. A mixed flow flat plate chamber air preheater or other equipment with good wear, blockage and corrosion resistance and good heat exchange performance can be used (the mixed flow flat plate chamber air preheater includes: a vent and a smoke vent, the vents are arranged at intervals, an air channel for air circulation is formed inside the vent, the air channel is provided with an air inlet and an air outlet, the smoke vent is arranged on one side of the vent, and the smoke vent forms a smoke channel for smoke circulation, the smoke channel is provided with a smoke inlet and a smoke outlet. It can improve heat exchange efficiency, reduce the volume of the air preheater, prevent dust accumulation and blockage, and reduce wear and corrosion, as shown in the content of patent CN118274340B). The low temperature zone of the low-temperature air preheater is prone to corrosion, so the heating surface in this area should be made of corrosion-resistant materials. In order to ensure the safety of the low-temperature end heating surface of the low-temperature air preheater, a partial indirect heat recovery method can also be used to divide the low-temperature air preheater into two parts. The upper part adopts the direct heat exchange method between flue gas and air, and the lower low-temperature end area adopts the forced circulation heat pipe method (such as Figure 3 (As shown). This approach can avoid corrosion of the low-temperature air preheater and increase operational flexibility. However, the low-temperature region of the heat medium water heat exchanger 24 still presents a corrosion risk. Furthermore, the system is more complex and requires a high investment. The specific trade-offs depend on site conditions and requirements, as well as investor preferences.

[0063] Specifically, another heat exchange mode between the flue gas and the air in the low-temperature air preheater subsystem is partial indirect heat exchange; the low-temperature air preheater subsystem includes: a heat medium water heat exchanger 24, a primary air heater 20, a secondary air heater 19, a primary air heat medium water regulating valve 21, a secondary air heat medium water regulating valve 22, a heat medium water return regulating valve 23, a heat medium water circulation pump 25, a heat medium water circulation pipeline 32, a primary air heat medium water heat exchange pipeline 33 and a secondary air heat medium water heat pipe 34;

[0064] The primary air is delivered by the primary fan 15, passes through the primary air heater 20, the primary air low-temperature preheater 13 and the primary air preheater 8 in sequence, and is reheated by the heat of the flue gas discharged from the main flue 27 before going to the coal powder preparation system; the secondary air is delivered by the secondary fan 14, passes through the secondary air heater 19, the secondary air low-temperature preheater 12 and the secondary air preheater 7 in sequence, and is reheated by the heat of the flue gas discharged from the main flue 27, and then enters the secondary air heater subsystem for heating before entering the boiler combustion system.

[0065] The heat medium water circulation pipeline 32 is provided with the heat medium water circulation pump 25, the heat medium water heat exchanger 24 and the heat medium water return regulating valve 23 in sequence. The heat medium water from the condensate system or the steam turbine low-pressure heating system is heated by the heat medium water heat exchanger 24, and then partially flows back to the steam turbine low-pressure heating system through the return regulating valve 23, while the other part enters the primary air heat medium water heat exchange pipeline 33 and the secondary air heat medium water heat exchange pipeline 34.

[0066] The primary air heat medium water heat exchange pipeline 33 is sequentially provided with the primary air heat medium water regulating valve 21 and the primary air heater 20. The heat medium water is returned to the inlet of the heat medium water circulation pump 25 after heat exchange through the primary air heater 20.

[0067] The secondary air heat medium water heat exchange pipeline 34 is sequentially provided with the secondary air heat medium water regulating valve 22 and the secondary air heater 19 . The heat medium water returns to the inlet of the heat medium water circulation pump 25 after heat exchange through the secondary air heater 19 .

[0068] The air preheater and bypass flue subsystem further includes: a flue gas damper regulating valve 18 , which is arranged in the bypass flue 28 and is used to distribute the flue gas flow discharged from the main flue 27 and the flue gas flow discharged from the bypass flue 28 .

[0069] For example, a flue gas damper regulating valve 18 is installed in the bypass flue 28 to regulate the amount of flue gas passing through the bypass 28 and distribute the flue gas volume passing through the air preheater and the bypass flue to meet the needs of various operating conditions. Simultaneously, a feedwater regulating valve 9 and a bypass water regulating valve 10 are installed in the water supply pipeline to distribute water flow and adjust the outlet water temperature of the bypass flue feedwater heater 6 to keep it consistent with or slightly higher than the feedwater temperature.

[0070] During low-load operation, adjusting the flue gas damper regulating valve 18 can increase the primary and secondary air temperatures, facilitating stable boiler combustion. This also increases the flue gas temperature at the low-temperature air preheater outlet, ensuring the inlet flue gas temperature at the electrostatic precipitator 16 is controlled at 85°C, ensuring the safe operation of the electrostatic precipitator and downstream equipment.

[0071] In addition, the air preheater and bypass flue subsystem also includes: an electrostatic precipitator 16 and an induced draft fan 17; the flue gas discharged from the main flue 27 and the bypass flue 28 passes through the heat medium water heat exchanger 24, the electrostatic precipitator 26 and the induced draft fan 27 in sequence, and then enters the flue gas treatment system; or, the flue gas discharged from the main flue 27 and the bypass flue 28 passes through the electrostatic precipitator 26 and the induced draft fan 17 in sequence, and then enters the flue gas treatment system.

[0072] For example, in Figure 2In the process, the flue gas passes through the low temperature air preheater, the flue gas temperature is reduced to 85 ° C, and then enters the electrostatic precipitator 16, thereby realizing the recovery of flue gas waste heat. Figure 3 The heat exchange mode between the flue gas and the air in the low-temperature air preheater subsystem is a partial indirect heat exchange mode. After the flue gas passes through the low-temperature air preheater, it passes through the heat medium water heat exchanger 24, the electrostatic precipitator 26 and the induced draft fan 27 in sequence and enters the flue gas treatment system.

[0073] In the bypass flue feedwater heater and steam turbine high-pressure heating system subsystem, the boiler feedwater pipeline 29 includes: a first feedwater pipeline 35 for recovering heat through steam extraction from the steam turbine and a second feedwater pipeline 36 for recovering heat through the flue gas in the bypass flue 28; a bypass flue feedwater heater 6 is provided in the bypass flue 28.

[0074] The bypass flue feedwater heater and steam turbine high-pressure heating system subsystem also includes: a feedwater pump 11 and a steam turbine high-pressure heater 5. The output port of the feedwater pump 11 is connected to the steam turbine high-pressure heater 5 through a first feedwater pipeline 35, and is connected to the bypass flue feedwater heater 6 through a second feedwater pipeline 36; the water in the first feedwater pipeline 35 flows out from the output port of the feedwater pump 11, flows through the input port of the steam turbine high-pressure heater 5 and the output port of the steam turbine high-pressure heater 5 in sequence through the first feedwater pipeline 35, and supplies water to the low-temperature economizer 4; the water in the second feedwater pipeline 36 flows out from the output port of the feedwater pump 11, flows through the input port of the bypass flue feedwater heater 6 and the output port of the bypass flue feedwater heater 6 in sequence through the second feedwater pipeline 36, and supplies water to the low-temperature economizer 4.

[0075] The bypass flue feedwater heater and steam turbine high-pressure heating system subsystem also includes: a feedwater regulating valve 9 and a bypass feedwater regulating valve 10; the feedwater regulating valve 9 is arranged on the first feedwater pipeline 35, located between the output port of the feedwater pump 11 and the input port of the steam turbine high-pressure heater 5; the bypass feedwater regulating valve 10 is arranged on the second feedwater pipeline 36, located between the output port of the feedwater pump 11 and the input port of the bypass flue feedwater heater 6.

[0076] For example, it should be noted that, Figure 2 and Figure 3As shown, in order to ensure an appropriate heat transfer temperature difference and maintain thermal balance in the air preheater, a bypass flue 28 is set in the air preheater area. The amount of flue gas flowing through the bypass flue 28 accounts for about 20% of the total flue gas volume. A feedwater heater 6 is provided in the bypass flue. The water supply of the feedwater heater 6 comes from the feedwater pump 11. The outlet water is combined with the feedwater from the steam turbine high-pressure heater 5 and enters the low-temperature economizer 4. A feedwater regulating valve 9 and a bypass feedwater regulating valve 10 are provided to control the amount of water entering the bypass feedwater heater 6, thereby controlling the heat exchange between the flue gas and the feedwater in the bypass flue 28 and the outlet water temperature of the bypass flue feedwater heater 6. The outlet water temperature of the bypass flue feedwater heater 6 is controlled to be the same as or slightly higher than the feedwater temperature from the steam turbine high-pressure heater 5. A slightly higher temperature helps to improve the waste heat recovery effect. The product of the bypass flue ... As long as this equivalent temperature drop is less than the temperature drop of the flue gas waste heat recovery (e.g., from 127°C to 85°C), it indicates that some of the flue gas waste heat is entering the boiler furnace through air heat recovery, becoming air-borne heat recovery. The efficiency of this air-borne heat recovery is much higher than that of water-borne heat recovery, which is the expected effect based on the double heat recovery theory.

[0077] It should also be noted that the higher the temperature of the turbine extraction steam replaced by the water outlet of the bypass flue feedwater heater 6, the higher the efficiency of waste heat recovery. The highest waste heat recovery efficiency is achieved when only a high-pressure heater is installed in the bypass flue and its outlet water is directly used as feedwater.

[0078] The present invention is further described above with the aid of specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the essence and scope of the present invention. Various modifications made to the above embodiments by ordinary technicians in this field after reading this specification are all within the scope of protection of the present invention.

Claims

1. A coal-fired power generation boiler system for recovering flue gas waste heat using a low-temperature air preheater, comprising: High-temperature economizer, denitrification reactor and low-temperature economizer, characterized in that the system includes: air preheater and bypass flue subsystem, low-temperature air preheater subsystem, bypass flue feedwater heater and steam turbine high-pressure heating system subsystem, secondary air heater subsystem; The air preheater and bypass flue subsystem includes: a main flue for discharging flue gas from the boiler, a bypass flue for discharging flue gas from the boiler, and a boiler air inlet passage for allowing air to enter the boiler. The air entering the boiler air inlet passage includes primary air and secondary air, and the primary air and secondary air are heated by the waste heat of the flue gas discharged from the main flue. The low-temperature air preheater subsystem includes: a low-temperature air preheater, which is used to preheat the primary air and secondary air by using the waste heat of the flue gas discharged from the main flue. The preheated primary air and secondary air enter the air preheater and bypass flue subsystem; The bypass flue feedwater heater and turbine high-pressure heating system subsystem includes: a boiler feedwater pipeline heated by steam recovery from turbine extraction and flue gas in the bypass flue; The secondary air heater subsystem is used to reheat the secondary air heated by the flue gas in the air preheater and the bypass flue subsystem, and the reheated secondary air enters the boiler combustion system; The low-temperature air preheater subsystem increases the outlet temperature of the air preheater and bypass flue subsystem from 120-130°C to 10K or higher than the outlet water temperature of the feedwater pump. Only a bypass flue feedwater heater is provided in the bypass flue. The water outlet of the bypass flue feedwater heater merges with the feedwater from the turbine high-pressure heater and directly enters the low-temperature economizer. The feedwater pump is used to supply water to the boiler feedwater pipeline. The turbine high-pressure heater and feedwater heater are used to heat the feedwater. The low-temperature air preheater subsystem adopts a low-temperature air preheater method to recover the low-temperature waste heat of the flue gas, and adopts a direct heat exchange method or a partial indirect heat exchange method between the flue gas and the air; In the bypass flue feedwater heater and turbine high-pressure heating system subsystem, the boiler feedwater pipeline includes: a first feedwater pipeline passing through the turbine high-pressure heater and a second feedwater pipeline passing through the feedwater heater in the bypass flue; the bypass flue is provided with a bypass flue feedwater heater; A bypass feed water regulating valve and a bypass flue feed water heater are sequentially arranged on the second feed water pipeline. The water supply comes from the feed water pump, and the outlet water is combined with the feed water from the first feed water pipeline of the steam turbine high-pressure heater and then supplied to the low-temperature economizer through the boiler feed water pipeline; the bypass feed water regulating valve controls the amount of water entering the bypass flue feed water heater, thereby controlling the heat exchange between the flue gas and the feed water in the bypass flue and the outlet water temperature of the bypass flue feed water heater to be the same as the feed water temperature from the first feed water pipeline or higher than the feed water temperature of the first feed water pipeline by within 20K.

2. The coal-fired power generation boiler system based on recovering flue gas waste heat using a low-temperature air preheater according to claim 1 is characterized in that: One heat exchange mode of the low-temperature air preheater subsystem is direct heat exchange between flue gas and air; The low-temperature air preheater includes: a primary air low-temperature preheater and a secondary air low-temperature preheater, and the air inlet channel includes a primary air inlet channel and a secondary air inlet channel, and heat is directly exchanged through the primary air low-temperature air preheater and the secondary air low-temperature air preheater; A primary air fan, a primary air low-temperature preheater and a primary air preheater are sequentially arranged in the primary air inlet channel. The primary air is delivered by the primary air fan, passes through the primary air low-temperature preheater and the primary air preheater in sequence, and is heated by the flue gas discharged from the main flue. A secondary air fan, a secondary air low-temperature preheater and a secondary air preheater are sequentially arranged in the secondary air inlet channel. The secondary air is delivered by the secondary air fan, passes through the secondary air low-temperature preheater and the secondary air preheater in sequence, and is heated by the flue gas discharged from the main flue.

3. The coal-fired power generation boiler system based on recovering flue gas waste heat using a low-temperature air preheater according to claim 1 is characterized in that: Another heat exchange method between the flue gas and air in the low-temperature air preheater subsystem is partial indirect heat exchange; The low-temperature air preheater subsystem includes: a heat medium water heat exchanger, a primary air heater, a secondary air heater, a primary air heat medium water regulating valve, a secondary air heat medium water regulating valve, a heat medium water return regulating valve, a heat medium water circulation pump, a heat medium water circulation pipeline, a primary air heat medium water heat exchange pipeline and a secondary air heat medium water heat pipeline; The heat medium water circulation pipeline is provided with the heat medium water circulation pump, the heat medium water heat exchanger and the heat medium water return regulating valve in sequence. After the heat medium water from the condensate system or the steam turbine low-pressure heating system is heated by the heat medium water heat exchanger, a portion of the heat medium water flows back to the steam turbine low-pressure heater through the return regulating valve, and the other portion enters the primary air heat medium water heat exchange pipeline and the secondary air heat medium water heat exchange pipeline; The primary air heat medium water heat exchange pipeline is sequentially provided with the primary air heat medium water regulating valve and the primary air heater, and the heat medium water returns to the inlet of the heat medium water circulation pump after heat exchange through the primary air heater; The secondary air heat medium water heat exchange pipeline is sequentially provided with the secondary air heat medium water regulating valve and the secondary air heater. The heat medium water returns to the inlet of the heat medium water circulation pump after heat exchange through the secondary air heater. The primary air is sent out by the primary fan, passes through the primary air heater, the primary air low-temperature preheater and the primary air preheater in sequence, is heated by the flue gas discharged from the main flue, and then goes to the pulverized coal preparation system; The secondary air is delivered by the secondary air fan, passes through the secondary air heater, the secondary air low-temperature preheater and the secondary air preheater in sequence, is heated by the flue gas discharged from the main flue, and then enters the secondary air heater subsystem for further heating before entering the boiler combustion system.

4. The coal-fired power generation boiler system based on recovering flue gas waste heat using a low-temperature air preheater according to claim 1 is characterized in that: The secondary air heater subsystem includes: a secondary air heater, which is arranged upstream of the denitration reactor and downstream of the high-temperature economizer; The secondary air heated by the air preheater and the bypass flue subsystem enters the secondary air heater subsystem, is further heated by the flue gas at the secondary air heater, and then enters the boiler combustion system.

5. The coal-fired power generation boiler system based on recovering flue gas waste heat using a low-temperature air preheater according to claim 1 is characterized in that: Under the design working conditions, the flue gas volume passing through the main flue accounts for about 80%, and the flue gas volume passing through the bypass flue accounts for about 20%; A flue gas damper regulating valve is provided in the bypass flue, and the flue gas damper regulating valve is used to distribute the flue gas flow discharged from the main flue and the flue gas flow discharged from the bypass flue.

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