阅读说明：本技术 具有低温腐蚀自阻断功能的低温省煤器系统及运行方法 (Low-temperature economizer system with low-temperature corrosion self-blocking function and operation method ) 是由 谢昌亚 司派友 刘双白 于 2021-06-02 设计创作，主要内容包括：本发明提供一种具有低温腐蚀自阻断功能的低温省煤器系统及运行方法,该系统包括低温省煤器、循环泵及再循环调门；低温省煤器的换热管的两端设有第一开口及第二开口,凝结水系统通过管路经由循环泵、低温省煤器逆流进口阀门与第一开口连通；第二开口通过管路经低温省煤器逆流出口阀门与凝结水系统连通；低温省煤器逆流进口阀门与循环泵及第一开口之间的管路上分别设有第一旁支管路和第二旁支管路,第一旁支管路经由低温省煤器顺流进口阀门与第二开口连通,第二旁支管路经由低温省煤器顺流出口阀门与凝结水系统连通；低温省煤器逆流出口阀门、低温省煤器顺流出口阀门与凝结水系统之间的管路上设有第三旁支管路,其经再循环调门与循环泵的入口连通。(The invention provides a low-temperature economizer system with a low-temperature corrosion self-blocking function and an operation method, wherein the system comprises a low-temperature economizer, a circulating pump and a recirculation throttle; a first opening and a second opening are formed in two ends of a heat exchange tube of the low-temperature economizer, and a condensate system is communicated with the first opening through a circulating pump and a low-temperature economizer backflow inlet valve through a pipeline; the second opening is communicated with a condensate system through a pipeline and a low-temperature economizer reverse flow outlet valve; a first bypass pipeline and a second bypass pipeline are respectively arranged on pipelines between the low-temperature economizer countercurrent inlet valve and the circulating pump as well as between the low-temperature economizer countercurrent inlet valve and the first opening, the first bypass pipeline is communicated with the second opening through the low-temperature economizer downstream inlet valve, and the second bypass pipeline is communicated with a condensate system through the low-temperature economizer downstream outlet valve; and a third branch pipeline is arranged on the pipeline between the low-temperature economizer reverse flow outlet valve, the low-temperature economizer forward flow outlet valve and the condensate system and is communicated with the inlet of the circulating pump through a recirculation regulating valve.)

1. A low temperature economizer system having a low temperature corrosion self-blocking function, the system comprising: a low-temperature economizer, a circulating pump and a recirculation throttle;

a first opening and a second opening are formed in two ends of a heat exchange tube of the low-temperature economizer, and a condensate system is communicated with the first opening through a circulating pump and a low-temperature economizer backflow inlet valve through a pipeline; the second opening is communicated with the condensed water system through a low-temperature economizer reverse flow outlet valve through a pipeline so as to enable the condensed water in the low-temperature economizer to perform reverse flow heat exchange with the flue gas discharged by the boiler;

a first bypass pipeline and a second bypass pipeline are respectively arranged on pipelines between the low-temperature economizer countercurrent inlet valve and the circulating pump as well as between the low-temperature economizer countercurrent inlet valve and the first opening, the first bypass pipeline is communicated with the second opening through the low-temperature economizer downstream inlet valve, and the second bypass pipeline is communicated with the condensate system through the low-temperature economizer downstream outlet valve, so that condensate in the low-temperature economizer exchanges heat with flue gas discharged by the boiler in a downstream manner;

and a third branch pipeline is arranged on a pipeline between the low-temperature economizer reverse-flow outlet valve and the condensate system, and is communicated with an inlet of the circulating pump through a recirculation regulating valve.

2. The system of claim 1, wherein a circulating pump inlet valve and a circulating pump outlet valve are respectively arranged on the pipelines connected with the inlet and the outlet of the circulating pump.

3. The system as claimed in claim 2, wherein a circulating pump outlet check valve is further provided on the pipeline between the outlet of the circulating pump and the circulating pump outlet valve.

4. A system according to any one of claims 1 to 3, wherein the lines connected to the inlet and outlet of the recirculation valve are provided with a first recirculation valve and a second recirculation valve, respectively.

5. The system according to any one of claims 1 to 3, wherein the heat exchange tubes of the low-temperature economizer are light tubes, spiral finned tubes or H-type finned tubes.

6. The method of operating a cryogenic economizer system having a cryogenic corrosion self-block function as claimed in any one of claims 1 to 5, the method comprising: regularly switching heat exchange modes between condensed water in a low-temperature economizer in the low-temperature economizer system with the low-temperature corrosion self-blocking function and flue gas discharged by a boiler, wherein the heat exchange modes comprise a forward flow heat exchange mode and a reverse flow heat exchange mode;

wherein, the countercurrent heat exchange mode comprises: opening a low-temperature economizer countercurrent inlet valve and a low-temperature economizer countercurrent outlet valve, closing a low-temperature economizer cocurrent flow inlet valve and a low-temperature economizer cocurrent flow outlet valve, pressurizing condensed water by a circulating pump, flowing through the low-temperature economizer countercurrent inlet valve, entering the low-temperature economizer, and carrying out countercurrent heat exchange on the condensed water and flue gas discharged by a boiler in the low-temperature economizer; after the heated condensed water flows through a counter-flow outlet valve of the low-temperature economizer, one part of the condensed water returns to a condensed water system, and the other part of the condensed water returns to an inlet of a circulating pump through a recirculation valve, is mixed with the low-temperature condensed water of the condensed water system and then enters the low-temperature economizer to perform counter-flow heat exchange so as to ensure that the temperature of the condensed water at the inlet of the low-temperature economizer is higher than the acid dew point temperature of the flue gas;

the forward flow heat exchange mode comprises the following steps: closing a low-temperature economizer counter-flow inlet valve and a low-temperature economizer counter-flow outlet valve, opening a low-temperature economizer downstream inlet valve and a low-temperature economizer downstream outlet valve, pressurizing condensed water by a circulating pump, flowing through the low-temperature economizer downstream inlet valve, entering the low-temperature economizer, and performing downstream heat exchange between the condensed water and flue gas discharged by a boiler in the low-temperature economizer; and after the heated condensed water flows through the downstream outlet valve of the low-temperature economizer, one part of the condensed water returns to a condensed water system, and the other part of the condensed water returns to the inlet of a circulating pump through a recirculation regulating valve, is mixed with the low-temperature condensed water of the condensed water system and then enters the low-temperature economizer to perform downstream heat exchange so as to ensure that the temperature of the condensed water at the inlet of the low-temperature economizer is higher than the acid dew point temperature of the flue gas.

7. The operating method according to claim 6, characterized in that the frequency of the periodic switching is half a month or 1 month.

8. The method of claim 6, wherein a counter-current heat exchange is used in summer and a forward heat exchange is used in winter.

9. The method of operation according to any one of claims 6 to 8, wherein the pressure difference of the condensed water before and after the pressurization is 0.15MPa to 0.2 MPa.

10. The operating method according to any one of claims 6 to 8, wherein the temperature of the flue gas discharged from the boiler is 110-150 ℃;

preferably, the temperature of the low-temperature condensed water is 30-45 ℃;

it is also preferred that the temperature of the condensate obtained after mixing is not less than 80 c, more preferably 80-90 c.

Technical Field

The invention relates to a low-temperature economizer system with a low-temperature corrosion self-blocking function and an operation method, and belongs to the technical field of waste heat utilization of thermal power generating units.

Background

Due to the energy structure of rich coal, poor oil and less gas, the coal-fired power generation still occupies the main position in the power supply of China, the current coal-fired power generation still occupies about 65 percent of the total power generation of China, and the coal consumption of the coal-fired power generation occupies 45 percent of the total coal consumption of China. In the utilization process of coal, a large amount of pollutants such as smoke dust, sulfur dioxide, nitrogen oxides and the like are released into the atmosphere, so that the problem of serious environmental pollution is caused, and energy conservation and emission reduction are imperative.

The heat loss of the exhaust smoke in the utility boiler is the largest item of the total heat loss of the boiler, and is about 4% -8%. Along with the improvement of unit parameters, the exhaust gas temperature of the boiler is further improved, and the heat loss of the exhaust gas is increased by about 1% when the exhaust gas temperature is increased by 12 ℃. The smoke exhaust temperature of the current unit design is generally 120-140 ℃, and the smoke exhaust temperature of the boiler is generally 120-150 ℃ in the actual operation. In order to fully recover the waste heat of the flue gas and reduce the temperature of the discharged flue gas, the low-temperature economizer is widely applied to a plurality of thermal power generating units in recent years. The arrangement form of the low-temperature coal economizer on the flue gas side is roughly divided into three types: the dust remover inlet, dust remover export and draught fan export. The working medium for heating is mainly condensed water, and the arrangement mode can be divided into series connection with the low-pressure heater and parallel connection with the low-pressure heater. According to the flowing direction of the shell side flue gas and the pipe side condensed water, the low-temperature economizer can be divided into countercurrent heat exchange and concurrent heat exchange, wherein a larger heat transfer temperature difference can be obtained in a countercurrent mode, the heat exchange effect is good, but a working medium low-temperature region is superposed with a flue gas low-temperature region, the low-temperature heat exchange section of the economizer is more prone to low-temperature corrosion, a heat exchange pipe can be leaked due to long-time operation, and the low-temperature economizer needs to be isolated and overhauled at the moment, so that the safe and stable operation of a unit is influenced; in a forward flow mode, the heat transfer temperature difference is small, the heat exchange effect is poor, the economy is poor, but the possibility of acid corrosion is reduced due to the high temperature of the wall surface of the working medium outlet; in order to obtain a better heat transfer effect, most of the existing low-temperature economizers adopt a counter-flow arrangement mode.

Research shows that the low-temperature corrosion phenomenon of the low-temperature economizer is mainly related to the acid dew point of the flue gas and the wall temperature of the heat exchange tube, and when the wall temperature of the heat exchange tube is lower than the acid dew point of the flue gas, sulfuric acid vapor in the flue gas is condensed to the wall surface of the heat exchange tube, so that the corrosion phenomenon is generated. Because the heat exchange coefficient of the condensed water for scouring the inner wall of the pipe is two orders of magnitude higher than that of the flue gas for scouring the outer wall of the pipe, the temperature of the wall surface of the heat exchange pipe is mainly related to the temperature of the condensed water in the pipe, and the existing method for preventing low-temperature corrosion mainly improves the temperature of the condensed water at the inlet of the low-temperature economizer. Theoretically, as long as the temperature of the condensed water at the inlet of the low-temperature economizer is higher than the acid dew point of the flue gas, the low-temperature corrosion phenomenon cannot be generated, but because the influence factors of the temperature of the acid dew point of the flue gas are numerous and the calculation formula is diversified, the coal burned by the boiler also changes frequently, the actual acid dew point temperature of the flue gas in operation cannot be accurately obtained, and the temperature of the condensed water at the inlet of the low-temperature economizer cannot be guaranteed to be always higher than the designed acid dew point temperature due to the difference of the operation level and the control mode of a unit, so that the low-temperature heat exchange section of the low-temperature economizer cannot be subjected to sulfuric acid vapor condensation in actual operation. For a single heat exchange mode low-temperature economizer, a pipe section which is easy to condense sulfuric acid vapor is determined, namely a pipe section of a water side inlet, because the temperature of the pipe section wall is the lowest in the whole low-temperature economizer. The difference is that the water side inlet of the low-temperature economizer with countercurrent heat exchange is just the low-temperature area of the flue gas outlet, and the sulfuric acid steam condensation is more easily generated at the section; the flue gas at the water side inlet of the downstream heat exchange low-temperature economizer is not subjected to heat exchange and cooling, the temperature is higher, and the possibility of low-temperature corrosion is lower than that of low-temperature corrosion under the condition that the water temperature at the inlet is certain. Because the low-temperature corrosion is a long-term slow accumulation process, if the environment of some pipe sections is always the worst low-temperature part in the whole low-temperature economizer, the pipe sections can be in the slow corrosion process for a long time, and after long-time operation, the pipe wall of the part becomes thinner gradually, and finally leakage occurs.

Therefore, it is an urgent technical problem in the art to provide a low-temperature economizer system with low-temperature corrosion self-blocking function and an operation method thereof.

Disclosure of Invention

To solve the above-described drawbacks and disadvantages, it is an object of the present invention to provide a low-temperature economizer system having a low-temperature corrosion self-blocking function.

It is still another object of the present invention to provide a method for operating the above-described low-temperature economizer system having a low-temperature corrosion self-blocking function.

In order to achieve the above object, in one aspect, the present invention provides a low-temperature economizer system having a low-temperature corrosion self-blocking function, wherein the low-temperature economizer system having a low-temperature corrosion self-blocking function includes: a low-temperature economizer, a circulating pump and a recirculation throttle;

a first opening and a second opening are formed in two ends of a heat exchange tube of the low-temperature economizer, and a condensate system is communicated with the first opening through a circulating pump and a low-temperature economizer backflow inlet valve through a pipeline; the second opening is communicated with the condensed water system through a low-temperature economizer reverse flow outlet valve through a pipeline so as to enable the condensed water in the low-temperature economizer to perform reverse flow heat exchange with the flue gas discharged by the boiler;

a first bypass pipeline and a second bypass pipeline are respectively arranged on pipelines between the low-temperature economizer countercurrent inlet valve and the circulating pump as well as between the low-temperature economizer countercurrent inlet valve and the first opening, the first bypass pipeline is communicated with the second opening through the low-temperature economizer downstream inlet valve, and the second bypass pipeline is communicated with the condensate system through the low-temperature economizer downstream outlet valve, so that condensate in the low-temperature economizer exchanges heat with flue gas discharged by the boiler in a downstream manner;

and a third branch pipeline is arranged on a pipeline between the low-temperature economizer reverse-flow outlet valve and the condensate system, and is communicated with an inlet of the circulating pump through a recirculation regulating valve.

As a specific embodiment of the above system of the present invention, a circulating pump inlet valve and a circulating pump outlet valve are respectively disposed on the pipelines connected to the inlet and the outlet of the circulating pump.

As a specific embodiment of the above system of the present invention, a pipeline between the outlet of the circulation pump and the outlet valve of the circulation pump is further provided with an outlet check valve of the circulation pump.

In an embodiment of the above system, the first recirculation valve and the second recirculation valve are respectively disposed on the pipelines connected to the inlet and the outlet of the recirculation valve.

As a specific embodiment of the above system of the present invention, the heat exchange tube of the low-temperature economizer is a light tube, a spiral finned tube or an H-type finned tube.

In the system of the invention, the valves such as the low-temperature economizer, the circulating pump, the recirculation throttle, the low-temperature economizer counter-flow inlet valve, the low-temperature economizer counter-flow outlet valve, the low-temperature economizer downstream inlet valve and the low-temperature economizer downstream outlet valve are conventional devices, and the valves such as the low-temperature economizer counter-flow inlet valve, the low-temperature economizer counter-flow outlet valve, the low-temperature economizer downstream inlet valve and the low-temperature economizer downstream outlet valve can be manual valves or electric valves.

In another aspect, the present invention further provides an operating method of the above-mentioned low-temperature economizer system having a low-temperature corrosion self-blocking function, wherein the method includes: regularly switching heat exchange modes between condensed water in a low-temperature economizer in the low-temperature economizer system with the low-temperature corrosion self-blocking function and flue gas discharged by a boiler, wherein the heat exchange modes comprise a forward flow heat exchange mode and a reverse flow heat exchange mode;

wherein, the countercurrent heat exchange mode comprises: opening a low-temperature economizer countercurrent inlet valve and a low-temperature economizer countercurrent outlet valve, closing a low-temperature economizer cocurrent flow inlet valve and a low-temperature economizer cocurrent flow outlet valve, pressurizing condensed water by a circulating pump, flowing through the low-temperature economizer countercurrent inlet valve, entering the low-temperature economizer, and carrying out countercurrent heat exchange on the condensed water and flue gas discharged by a boiler in the low-temperature economizer; after the heated condensed water flows through a counter-flow outlet valve of the low-temperature economizer, one part of the condensed water returns to a condensed water system, and the other part of the condensed water returns to an inlet of a circulating pump through a recirculation valve, is mixed with the low-temperature condensed water of the condensed water system and then enters the low-temperature economizer to perform counter-flow heat exchange so as to ensure that the temperature of the condensed water at the inlet of the low-temperature economizer is higher than the acid dew point temperature of the flue gas;

the forward flow heat exchange mode comprises the following steps: closing a low-temperature economizer counter-flow inlet valve and a low-temperature economizer counter-flow outlet valve, opening a low-temperature economizer downstream inlet valve and a low-temperature economizer downstream outlet valve, pressurizing condensed water by a circulating pump, flowing through the low-temperature economizer downstream inlet valve, entering the low-temperature economizer, and performing downstream heat exchange between the condensed water and flue gas discharged by a boiler in the low-temperature economizer; and after the heated condensed water flows through the downstream outlet valve of the low-temperature economizer, one part of the condensed water returns to a condensed water system, and the other part of the condensed water returns to the inlet of a circulating pump through a recirculation regulating valve, is mixed with the low-temperature condensed water of the condensed water system and then enters the low-temperature economizer to perform downstream heat exchange so as to ensure that the temperature of the condensed water at the inlet of the low-temperature economizer is higher than the acid dew point temperature of the flue gas.

In the operation method, part of the heated condensed water is mixed with the low-temperature condensed water and then enters the low-temperature economizer to exchange heat (concurrent flow or countercurrent flow), so that the temperature of the condensed water entering the low-temperature economizer can be increased, and the temperature of the condensed water at the inlet of the low-temperature economizer is higher than the acid dew point temperature of the flue gas; in addition, the flow of the heated condensate returning to the inlet of the circulating pump can be adjusted through the recirculation regulating valve, so that the mixed condensate temperature can be accurately adjusted, and the condensate temperature at the inlet of the low-temperature economizer is ensured to be at a higher temperature level and higher than the acid dew point temperature of the flue gas.

In a specific embodiment of the above operating method of the present invention, the frequency of the periodic switching is half a month or 1 month.

As a specific embodiment of the above operation method of the present invention, a counter-flow heat exchange mode is adopted in summer, and a forward-flow heat exchange mode is adopted in winter.

As a specific embodiment of the above-mentioned operation method of the present invention, wherein the pressure difference of the condensed water before and after the pressurization is 0.15MPa to 0.2 MPa.

As a specific embodiment of the above-mentioned operation method of the present invention, wherein the temperature of the flue gas discharged from the boiler is 110-150 ℃.

As a specific embodiment of the above operation method of the present invention, wherein the temperature of the low-temperature condensed water is 30 to 45 ℃.

As a specific embodiment of the above-described operation method of the present invention, wherein the temperature of the condensed water obtained after mixing is not lower than 80 ℃, preferably 80 to 90 ℃.

The invention can realize the regular switching of the heat exchange mode between the condensed water in the low-temperature economizer system with the low-temperature corrosion self-blocking function and the flue gas discharged by the boiler, in particular, in summer or when the unit load is higher, namely the flue gas temperature is higher, the low-temperature economizer can adopt a counter-flow heat exchange mode, at the moment, because the flue gas temperature is higher, the heat exchange temperature difference is large, the possibility of low-temperature corrosion is lower, and the counter-flow heat exchange mode has good heat transfer effect, and the unit economy is better; in winter or when the unit load is lower, namely the exhaust gas temperature is lower, low-temperature corrosion is relatively easy to occur, at the moment, the low-temperature economizer can adopt a downstream heat exchange mode, the temperature drop of the exhaust gas is reduced, the overall temperature level of the low-temperature economizer is improved, and the possibility of generating low-temperature corrosion is further reduced.

Most importantly: because the switching of the forward and reverse flow heat exchange modes in the low-temperature economizer in the system provided by the invention is convenient, the influence of the exhaust gas temperature can be not considered in the running process of the unit, the forward and reverse flow heat exchange modes in the low-temperature economizer can be switched regularly, the original condensed water inlet pipe section is changed into the condensed water outlet pipe section after the heat exchange modes are switched, the wall temperature of the pipe section which has the corrosion possibility originally is obviously increased, and the pipe section is separated from the severe low-temperature corrosion environment, so that the corrosion process of the pipe section is automatically blocked, the fixed pipe section is different from the fixed pipe section which is in the low-temperature environment for a long time under the single condition of the heat exchange modes, the low-temperature corrosion process of the pipe section can be suspended under the regularly switched running mode, the corrosion process of the pipe wall is slowed down, the continuous and safe running time of the low-temperature economizer is prolonged, and the long-term stable running of the unit is ensured.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to these drawings without creative efforts.

Fig. 1 is a schematic structural diagram of a low-temperature economizer system with a low-temperature corrosion self-blocking function according to embodiment 1 of the present invention.

The main reference numbers illustrate:

1-low temperature economizer;

2-low temperature economizer counterflow inlet valve;

3-a low-temperature economizer downstream inlet valve;

4-low temperature economizer back flow outlet valve;

5-a downstream outlet valve of the low-temperature economizer;

6-outlet valve of circulating pump;

7-a check valve at the outlet of the circulating pump;

8, a circulating pump;

9-inlet valve of circulation pump;

10 — a first recycle valve;

11-recirculation throttle;

12-second recirculation valve.

Detailed Description

In order to clearly understand the technical features, objects and advantages of the present invention, the following detailed description of the technical solutions of the present invention will be made with reference to the following specific examples, which should not be construed as limiting the implementable scope of the present invention.

Example 1

The embodiment provides a low-temperature economizer system with low-temperature corrosion self-blocking function, the structural schematic diagram of which is shown in fig. 1, and as can be seen from fig. 1, the system comprises: the low-temperature economizer 1, the circulating pump 8 and the recirculation damper 11;

a first opening and a second opening are formed in two ends of a heat exchange tube of the low-temperature economizer 1, and a condensate system is communicated with the first opening through a circulating pump inlet valve 9, a circulating pump 8, a circulating pump outlet check valve 7, a circulating pump outlet valve 6 and a low-temperature economizer counter-flow inlet valve 2 in sequence through pipelines; the second opening is communicated with the condensed water system through a low-temperature economizer reverse flow outlet valve 4 through a pipeline, so that the condensed water in the low-temperature economizer 1 and the flue gas discharged by the boiler perform reverse flow heat exchange;

a first bypass pipeline and a second bypass pipeline are respectively arranged on pipelines between the low-temperature economizer countercurrent inlet valve 2 and the circulating pump 8 as well as between the low-temperature economizer countercurrent inlet valve and the first opening, the first bypass pipeline is communicated with the second opening through a low-temperature economizer downstream inlet valve 3, and the second bypass pipeline is communicated with the condensate system through a low-temperature economizer downstream outlet valve 5, so that condensate in the low-temperature economizer 1 exchanges heat with flue gas discharged by a boiler in a downstream manner;

and a third branch pipeline is arranged on a pipeline between the low-temperature economizer reverse-flow outlet valve 4, the low-temperature economizer forward-flow outlet valve 5 and the condensate system, and the third branch pipeline is communicated with an inlet of the circulating pump 8 through a first recirculation valve 10, a recirculation adjusting valve 11 and a second recirculation valve 12 in sequence.

In this embodiment, the heat exchange tube of the low-temperature economizer is a light tube, a spiral finned tube or an H-shaped finned tube.

Example 2

The present embodiment provides an operation method of the low-temperature economizer system having a low-temperature corrosion self-blocking function as described in embodiment 1, wherein the method includes:

regularly switching heat exchange modes between condensed water in a low-temperature economizer in the low-temperature economizer system with the low-temperature corrosion self-blocking function and flue gas discharged by a boiler, wherein the heat exchange modes comprise a forward flow heat exchange mode and a reverse flow heat exchange mode;

wherein, the countercurrent heat exchange mode comprises: opening a low-temperature economizer countercurrent inlet valve and a low-temperature economizer countercurrent outlet valve, closing a low-temperature economizer cocurrent flow inlet valve and a low-temperature economizer cocurrent flow outlet valve, pressurizing condensed water by a circulating pump, flowing through the low-temperature economizer countercurrent inlet valve, entering the low-temperature economizer, and carrying out countercurrent heat exchange on the condensed water and flue gas discharged by a boiler in the low-temperature economizer; after the heated condensed water flows through a counter-flow outlet valve of the low-temperature economizer, one part of the condensed water returns to a condensed water system, and the other part of the condensed water returns to an inlet of a circulating pump through a recirculation valve, is mixed with the low-temperature condensed water of the condensed water system and then enters the low-temperature economizer to perform counter-flow heat exchange so as to ensure that the temperature of the condensed water at the inlet of the low-temperature economizer is higher than the acid dew point temperature of the flue gas;

the forward flow heat exchange mode comprises the following steps: closing a low-temperature economizer counter-flow inlet valve and a low-temperature economizer counter-flow outlet valve, opening a low-temperature economizer downstream inlet valve and a low-temperature economizer downstream outlet valve, pressurizing condensed water by a circulating pump, flowing through the low-temperature economizer downstream inlet valve, entering the low-temperature economizer, and performing downstream heat exchange between the condensed water and flue gas discharged by a boiler in the low-temperature economizer; and after the heated condensed water flows through the downstream outlet valve of the low-temperature economizer, one part of the condensed water returns to a condensed water system, and the other part of the condensed water returns to the inlet of a circulating pump through a recirculation regulating valve, is mixed with the low-temperature condensed water of the condensed water system and then enters the low-temperature economizer to perform downstream heat exchange so as to ensure that the temperature of the condensed water at the inlet of the low-temperature economizer is higher than the acid dew point temperature of the flue gas.

In this embodiment, the frequency of the periodic switching is half a month or 1 month; in addition, a countercurrent heat exchange mode can be adopted in summer, and a concurrent heat exchange mode can be adopted in winter.

In this example, the pressure difference of the condensed water before and after the pressurization was 0.15MPa to 0.2 MPa.

In this embodiment, the temperature of the flue gas discharged from the boiler is 120 ℃.

In this embodiment, the temperature of the low-temperature condensed water is 40 ℃.

In this embodiment, the temperature of the condensed water obtained after mixing (i.e., the condensed water at the inlet of the low-temperature economizer) is not lower than 80 ℃, and may be, for example, 80 to 90 ℃.

In summary, in the embodiment of the present invention, the heat exchange manner between the condensed water in the low-temperature economizer system having the low-temperature corrosion self-blocking function and the flue gas discharged by the boiler can be periodically switched, specifically, in summer or when the unit load is high, that is, the flue gas temperature is high, the low-temperature economizer can adopt a counter-flow heat exchange manner, at this time, because the flue gas temperature is high, the heat exchange temperature difference is large, the possibility of low-temperature corrosion is low, and the counter-flow heat exchange manner has a good heat transfer effect, and the unit economy is also good; in winter or when the unit load is lower, namely the exhaust gas temperature is lower, low-temperature corrosion is relatively easy to occur, at the moment, the low-temperature economizer can adopt a downstream heat exchange mode, the temperature drop of the exhaust gas is reduced, the overall temperature level of the low-temperature economizer is improved, and the possibility of generating low-temperature corrosion is further reduced.

Most importantly: because the switching of the forward and reverse flow heat exchange modes in the low-temperature economizer in the system provided by the invention is convenient, the influence of the exhaust gas temperature can be not considered in the running process of the unit, the forward and reverse flow heat exchange modes in the low-temperature economizer can be switched regularly, the original condensed water inlet pipe section is changed into the condensed water outlet pipe section after the heat exchange modes are switched, the wall temperature of the pipe section which has the corrosion possibility originally is obviously increased, and the pipe section is separated from the severe low-temperature corrosion environment, so that the corrosion process of the pipe section is automatically blocked, the fixed pipe section is different from the fixed pipe section which is in the low-temperature environment for a long time under the single condition of the heat exchange modes, the low-temperature corrosion process of the pipe section can be suspended under the regularly switched running mode, the corrosion process of the pipe wall is slowed down, the continuous and safe running time of the low-temperature economizer is prolonged, and the long-term stable running of the unit is ensured.

The above description is only exemplary of the invention and should not be taken as limiting the scope of the invention, so that the invention is intended to cover all modifications and equivalents of the embodiments described herein. In addition, the technical features and the technical inventions of the present invention, the technical features and the technical inventions, and the technical inventions can be freely combined and used.