Method for preventing heating surface over-temperature and NOX emission from exceeding standard in starting and low-load quick response stages of coal-fired boiler

Abstract

The invention discloses a method for preventing overtemperature of a heating surface and exceeding of NOX emission in the starting and low-load quick response stages of a coal-fired boiler. The aim of preventing the NOX emission from exceeding the standard in the starting stage of the coal-fired boiler is achieved by increasing the boiler feed water temperature, increasing the flue gas flow of a front-side flue and starting a coal mill in advance. By the adoption of the method, on the premise that the temperature of the heated surface does not exceed the temperature, the flue gas temperature of the inlet of the denitration system in the starting stage of the unit can be remarkably increased, it is guaranteed that the denitration system is put into operation in time, therefore, NOx emission in the starting stage is prevented from exceeding the standard, and the operation safety performance and the environment-friendly performance of the unit in the starting stage are greatly improved.

Description

Technical Field

[0001] The invention belongs to the technical field of coal-fired power station boilers, and in particular relates to a method for preventing overheating of heating surfaces and excessive NOX emissions during startup and low-load rapid response stages of coal-fired boilers. Background Art

[0002] For the emission control of nitrogen oxides from coal-fired units, the most widely used technology in China is the selective catalytic reduction (SCR) technology. Although the ultra-low emission transformation of the denitrification system can meet environmental protection requirements under normal unit operation, it is impossible to obtain stable NO during the unit startup process and the unit's low-load rapid response process. X Removal efficiency, NO X The emission volume cannot meet the emission standards. This is mainly because the catalyst used in the SCR technology is limited by the flue gas temperature conditions. During the boiler startup process, the amount of coal burned is small, the boiler steam temperature is low, etc., which causes the flue gas temperature at the inlet of the denitrification system to be lower than the normal operating temperature of the system. When the flue gas temperature at the inlet of the denitrification system is low, side reactions will occur on the catalyst surface, and NH3 will react with SO3 and H2O to generate (NH4)2SO4 or NH2HSO4, reducing the reaction between NH3 and NO X The reaction products will be adsorbed on the surface of the catalyst, blocking the channels and micropores of the catalyst, reducing the activity of the catalyst and increasing the NH3 escape. Therefore, how to increase the inlet flue gas temperature of the denitrification system during the boiler startup phase to prevent NO X Excessive emissions is a problem that needs to be addressed urgently. Summary of the invention

[0003] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a method for preventing overheating of the heating surface and excessive NOX emissions during the startup and low-load rapid response stages of a coal-fired boiler.

[0004] In order to achieve the above purpose, the present invention adopts the following technical scheme:

[0005] The present invention provides a method for preventing overheating of the heating surface and excessive NOx emissions during the startup and low-load rapid response stages of a coal-fired boiler, by increasing the boiler feed water temperature, increasing the flue gas flow rate of the front flue, and starting the coal mill in advance to prevent NOx emissions during the startup stage of the coal-fired boiler. X The purpose of exceeding emission standards.

[0006] According to an embodiment of a method for preventing overheating of the heating surface and excessive NOX emissions during startup and low-load rapid response of a coal-fired boiler according to the present invention, a pulverizing system adopts a positive-pressure direct-blowing medium-speed coal mill, the combustion method is front and rear wall counter-combustion, front and rear branch flues are arranged in the tail flue, and a flue gas regulating baffle is used to distribute the flue gas to adjust the outlet steam temperature of the reheater.

[0007] According to an embodiment of a method for preventing overheating of the heating surface and excessive NOx emissions during the startup and low-load rapid response stages of a coal-fired boiler according to the present invention, during the startup process, the existing boiler water in the boiler is discharged and the boiler water is replaced to make the water quality of the boiler water qualified in advance. At the same time, auxiliary steam is used to heat the water in the deaerator to increase the water temperature of the deaerator, thereby increasing the boiler feed water temperature. This increases the metal wall temperature of the economizer, which is equivalent to preheating the boiler as a whole, so that the boiler is in a "hot air hot furnace" environment, thereby increasing the inlet temperature of the denitrification system.

[0008] According to an embodiment of a method for preventing overheating of the heating surface and excessive NOX emissions during the startup and low-load rapid response phase of a coal-fired boiler according to the present invention, during the startup process, the opening of the high and low pressure bypass is increased, and the high pressure bypass diverts part of the main steam flow, reducing the flow of the working medium entering the reheater. At the same time, the opening of the front flue gas regulating damper on the reheater side is increased to maintain the reheater flow to meet the operating requirements, reduce the heat absorption of the working medium, increase the flue gas flow in the front flue, and increase the SCR inlet flue gas temperature to meet the temperature required by the catalyst. At this time, although the flue gas temperature of the rear flue has decreased, the overall flue gas temperature increases with the increase of the flue gas temperature of the front flue, that is, the flue gas temperature of the reheater side.

[0009] The flue gas at the inlet of the SCR system is composed of two parts, namely the flue gas at the outlet of the reheater side in the tail flue of the boiler and the flue gas at the outlet of the superheater side in the tail flue of the boiler. Before the turbine is run, the steam temperature after the high-pressure bypass regulating valve determines the steam temperature at the inlet of the reheater, which is generally higher than 300°C, but at this time, the feed water temperature at the inlet of the economizer is only about 220°C. The flue gas temperature at the outlet of the reheater is about 330°C, which is significantly higher than the flue gas temperature at the outlet of the economizer (about 260°C). Therefore, during the startup of the boiler, the opening of the flue gas regulating damper on the reheater side (front flue) of the tail flue of the boiler is kept at a large level to increase the flue gas flow at the outlet of the front flue. First, the flue gas flow rate through the front flue increases, while the heat exchange rate of the flue gas in the front flue is less, thereby increasing the temperature of the flue gas after mixing in the front and rear flue gases; second, when the flow rate of the reheater steam does not change, the flue gas flow rate through the front flue increases, and the heat exchange rate of this part of the reheated steam does not change much, thereby increasing the flue gas temperature at the front flue outlet. After the flue gas in the front and rear flues is mixed, the overall mixed flue gas temperature is also increased. Therefore, during the unit startup process, in order to put the boiler denitration system into operation early, the opening of the front flue gas regulating damper on the front side of the tail flue, that is, the front side flue gas regulating damper on the reheater side, is gradually opened to 100%, and the rear side flue gas regulating damper is closed. When the rear side flue gas regulating damper is closed to 20%, it is not further closed.

[0010] According to an embodiment of a method for preventing overheating of the heating surface and excessive NOX emissions during the startup and low-load rapid response stages of a coal-fired boiler according to the present invention, during the startup process, the B coal mill is started in advance, and the B coal mill provides fuel for the second-layer burner. The smoke temperature requirement of the unit during the startup process is met by controlling the center position of the furnace flame and cooperating with the damper opening temperature adjustment method. Actual operation shows that a flame center position that is too high or too low will affect the operation of the unit. A flame center that is too high will cause part of the tube wall of the screen superheater to overheat, threatening the safety of the unit; and a flame center position that is too low will cause the smoke temperature entering the denitrification reaction zone to be too low, failing to reach the temperature required for the catalyst reaction, affecting the commissioning of the denitrification system. Through experiments, it is believed that starting the B coal mill meets the height requirement of the flame center.

[0011] The present invention achieves the purpose of preventing NO from escaping during the startup phase of a coal-fired boiler by non-renovation means by increasing the boiler feed water temperature, increasing the flue gas flow rate of the front flue, and starting the coal mill in advance. X The purpose of exceeding emission standards. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a schematic diagram of the system structure of the present invention.

[0013] Among them, 1. B coal mill, 2. second layer burner, 3. reheater, 4. low-temperature superheater, 5. front flue, 6. rear flue, 7. front economizer, 8. rear economizer, 9. front flue gas regulating damper, 10. rear flue gas regulating damper. DETAILED DESCRIPTION

[0014] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. Note that the following embodiments are merely illustrative in nature, and the present invention is not intended to limit its applicable objects or uses, and the present invention is not limited to the following embodiments.

[0015] Example

[0016] A 660MW supercritical power plant coal-fired boiler in a power plant was used as the object. During the startup process,

[0017] (1) The existing boiler water in the boiler is discharged and the water in the boiler is replaced to make the water quality of the boiler water qualified in advance. At the same time, auxiliary steam is used to heat the water in the deaerator to increase the water temperature of the deaerator, thereby increasing the boiler feed water temperature.

[0018] (2) During the unit startup process, increase the opening of the high and low pressure bypasses, and the high pressure bypass will divert part of the main steam flow to reduce the working fluid flow entering the reheater. At the same time, gradually increase the opening of the front flue gas regulating damper 9 on the reheater 3 side to 100%, and close the rear flue gas regulating damper 10. When the rear flue gas regulating damper 10 is closed to 20%, it will not be further closed. While maintaining the reheater flow to meet the operating requirements, reduce the working fluid heat absorption, increase the flue gas flow on the reheater 3 side, and increase the SCR inlet flue gas temperature to meet the temperature required by the catalyst. At this time, although the flue gas temperature of the rear flue has decreased, the overall flue gas temperature increases with the increase of the flue gas temperature of the front flue, that is, the flue gas temperature of the reheater side.

[0019] (3) During the startup process, start the B coal mill 1 in advance, and the B coal mill 1 provides fuel for the second layer burner 2. By controlling the center position of the furnace flame and coordinating the damper opening temperature control method, the smoke temperature requirement of the unit during the startup process can be met.

[0020] In view of the phenomenon that the denitrification system could not be put into operation during the startup process, the method of increasing the feed water temperature, increasing the flue gas flow rate of the front flue, and starting the coal mill in advance was adopted to successfully realize the operation of the denitrification system during the startup process. Before the unit was connected to the grid, the flue gas temperature at the outlet of the A and B sides of the SCR was basically maintained between 300 and 330°C (see Table 1 for specific data), which met the minimum temperature required by the catalyst (300°C), ensuring the operation of the denitrification system during the startup process and preventing NO X Exceeding the standard.

[0021] Table 1 Unit operation data table

[0022]

[0023] The above embodiments are merely examples and do not limit the scope of the present invention. These embodiments can also be implemented in various other ways, and various omissions, substitutions, and changes can be made without departing from the technical concept of the present invention.

Claims

1. A method for preventing overheating of the heating surface and excessive NOX emissions during the startup and low-load rapid response phases of a coal-fired boiler, characterized in that: By increasing the boiler feed water temperature, increasing the flue gas flow rate in the front flue, and starting the coal mill in advance, the NOx in the startup phase of the coal-fired boiler can be prevented. X The purpose of exceeding emission standards.

2. The method for preventing overheating of the heating surface and excessive NOx emissions during startup and low-load rapid response of a coal-fired boiler according to claim 1, characterized in that: During the startup process, the existing boiler water in the boiler is discharged and the water in the boiler is replaced to make the water quality of the boiler water qualified in advance. At the same time, auxiliary steam is used to heat the water in the deaerator to increase the water temperature of the deaerator, thereby increasing the boiler feed water temperature.

3. The method for preventing overheating of the heating surface and excessive NOx emissions during startup and low-load rapid response of a coal-fired boiler according to claim 1, characterized in that: During the startup process, increase the opening of the high and low pressure bypasses, and the high pressure bypass will divert part of the main steam flow to reduce the flow of the working fluid entering the reheater. At the same time, increase the opening of the front flue gas regulating damper on the reheater side to maintain the reheater flow to meet the operating requirements, reduce the heat absorption of the working fluid, and increase the flue gas flow in the front flue. In the low load rapid response stage, increase the opening of the front flue gas regulating damper on the reheater side to maintain the reheater flow to meet the operating requirements, reduce the heat absorption of the working fluid, and increase the flue gas flow in the front flue.

4. The method for preventing overheating of the heating surface and excessive NOx emissions during startup and low-load rapid response of a coal-fired boiler according to claim 1, characterized in that: During the startup process, start the B coal mill in advance to provide fuel for the second-layer burner. By controlling the center position of the furnace flame and adjusting the temperature by the damper opening, the smoke temperature requirement of the unit during the startup process can be met.

5. The method for preventing overheating of the heating surface and excessive NOx emissions during startup and low-load rapid response of a coal-fired boiler according to claim 1, characterized in that: The method is applicable to coal-fired power station boilers with double flues at the tail flue and tower-type coal-fired power station boilers that use split flue temperature regulating dampers to regulate reheat steam.

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