FLUE GAS CIRCULATION SYSTEM FOR IMPROVING COMBUSTION CHARACTERISTICS AND THERMAL CHARACTERISTICS OF PULVERIZED COAL FIRED BOILER

Abstract

Disclosed in the present application is a flue gas circulation system for improving combustion characteristics and thermal characteristics of a pulverized coal fired boiler. The flue gas circulation system comprises a plurality of circulation flue gas leading-out ports provided in a hearth outlet flue of a boiler, wherein the circulation flue gas leading-out ports are connected to a circulation flue gas leading-out main pipe by means of a circulation flue gas leading-out pipeline; the circulation flue gas leading-out main pipe is connected to a circulation flue gas leading-in main pipe; the circulation flue gas leading-in main pipe is connected to a circulation flue gas leading-in pipeline; and the circulation flue gas leading-in pipeline is connected to a hearth. Flue gas in a flue is introduced into the hearth through the circulation flue gas leading-out ports, thereby achieving flue gas circulation; and at the same time, the problems such as clinkering or overtemperature of a heating surface of a pulverized coal fired boiler of a power station, high-temperature corrosion of a water-cooled wall, burning loss of a combustor, incapability of reaching a design value of steam temperature, and high concentration of nitrogen oxides at an outlet of the hearth can be solved or ameliorated.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Chinese Patent Application No. 202210417474.0 filed to the China Patent Office on Apr. 20, 2022 and entitled “FLUE GAS CIRCULATION SYSTEM FOR IMPROVING COMBUSTION CHARACTERISTICS AND THERMAL CHARACTERISTICS OF PULVERIZED COAL FIRED BOILER”, the entire content of which is incorporated herein by reference.

TECHNICAL FIELD

This application belongs to the technical field of pulverized coal-fired boilers for power plants, and specifically relates to a flue gas circulation system that can significantly improve the combustion characteristics (including nitrogen oxide emissions and clinkering property) and thermal characteristics of pulverized coal-fired boilers.

BACKGROUND

Large-scale coal-fired thermal power generating units are still the main power generating units in China. Pulverized coal-fired boiler is the core main equipment of thermal power generating units, thus the operation safety and reliability of pulverized coal-fired boilers have an important impact on the entire power generating unit and even the power grid. However, in the context of new situations of thermal power generating units such as deep peak load regulating, flexible operation, ultra-clean emission transformation, and actual quality of coal for firing deviating from the design value, the problems of pulverized coal-fired boilers of high-parameter and large-capacity power plant, such as clinkering and overtemperature on the heating surface, high-temperature corrosion of the water-cooled wall, burner burning loss, steam temperature not reaching the design value, high concentration of nitrogen oxides at the outlet of the hearth, and ash blocking in the air preheater, are widespread. The above problems are related to the inherent combustion and thermal characteristics of coal-fired boilers themselves. Factors such as pollutant control requirements, flexible operating conditions, and deterioration of coal quality under the new situation have also made the above problems more prominent.

Under high load operating conditions of the boiler, the high flame temperature of the hearth exacerbates the problems of clinkering on the hearth's water-cooled wall, high temperature corrosion, and overtemperature of the tube wall of water-cooled wall. The above problems are more prominent when the boiler burns low ash melting point and high sulfur coal, or uses separate over-fire air low-nitrogen combustion technology. When the boiler burns high alkali and low ash melting point coal (such as Zhundong coal, etc.), the risk of slag-hanging on the high-temperature heating surface of the boiler, especially on the heating surface of the high-temperature reheater, as well as the risk of blockage of the circulating flue greatly increase. In low-to-medium load conditions of the boiler, in order to ensure steam parameters, the operating oxygen amount is relatively high, and the excess air coefficient is usually greater than 1.2, however, the phenomenon of low steam parameters is still relatively common. As the operating oxygen amount increases, the proportion of separate over-fire air amount decreases, as a result, anoxic combustion or even low-oxygen combustion cannot occur in the burner area, which significantly weakens the effect of the separate over-fire air technology and increases the nitrogen oxide concentration at the outlet of the hearth with the reduction of the boiler load. At the same time, the excess air increases the exhaust heat loss of the boiler and reduces the thermal efficiency of the boiler. Secondly, the small number of burners in operation under medium and low loads causes uneven distribution of local heat loads, resulting in relatively large deviations in exhaust gas temperature at the outlet of the hearth, which in turn causes local overtemperature of tube walls of some high-temperature heating surface.

Pulverized coal-fired boilers that use separate over-fire air technology commonly suffer from burning loss problems of swirl burners. Related studies believe that the burning loss of swirl burners mainly occurs when they are in standby. The main reason is that the separate over-fire air shunts a relatively large amount of secondary air, resulting in insufficient pressure in the secondary air box, relatively small amount of the cooling air of the internal and external secondary air of the backup burner, as well as relatively small amount of the cooling air of the central air.

The causes of the above problems are interrelated and mutually restrictive. It is difficult to solve or alleviate the above problems at the same time due to the current flue gas and air system configuration and environmental protection emission requirements of pulverized coal-fired boilers in power plants. It is necessary to improve the flue gas and air systems of pulverized coal-fired boilers.

SUMMARY OF THE INVENTION

In view of the problems in the prior art, the purpose of the present application is to propose a flue gas circulation system for improving the combustion characteristics and thermal characteristics of pulverized coal-fired boilers.

In order to achieve the above purpose, the technical solutions adopted in the present application are as follows.

A flue gas circulation system for improving the combustion characteristics and thermal characteristics of a pulverized coal-fired boiler, wherein the flue gas circulation system comprises a plurality of circulation flue gas leading-out ports provided on a hearth outlet flue of the boiler, the circulation flue gas leading-out ports are connected to a circulation flue gas leading-out main pipe via circulation flue gas leading-out pipelines, the circulation flue gas leading-out main pipe is connected to a circulation flue gas leading-in main pipe, the circulation flue gas leading-in main pipe is connected to circulation flue gas leading-in pipelines, and the circulation flue gas leading-in pipelines are connected to the hearth.

A further improvement of the present application is that the circulation flue gas leading-out main pipe is connected to the inlet of the boiler flue gas circulation fan, and the outlet of the boiler flue gas circulation fan is connected to the circulation flue gas leading-in main pipe.

A further improvement of the present application is that the circulation flue gas leading-out ports comprise one or more of a first circulation flue gas leading-out port, a second circulation flue gas leading-out port, a third circulation flue gas leading-out port, a fourth circulation flue gas leading-out port and a fifth circulation flue gas leading-out port.

A further improvement of the present application is that the hearth outlet flue of the boiler is provided with a platen superheater, a steering flue gas chamber, an air preheater and an induced draft fan;

• • the first flue gas leading-out port is connected to the induced draft fan outlet flue, the second flue gas leading-out port is connected to the induced draft fan inlet, the third flue gas leading-out port is connected to the air preheater outlet flue, the fourth flue gas leading-out port is connected to an economizer outlet flue, and the fifth flue gas leading-out port is connected to the steering flue gas chamber.

A further improvement of the present application is that a plurality of leading-in ports are provided at the bottom of the hearth, and the leading-in ports comprise one or more of a first circulation flue gas leading-in port provided at the bottom of the hearth, a second circulation flue gas leading-in port provided in a burner area, and a third circulation flue gas leading-in port provided at the bottom of the platen superheater.

A further improvement of the present application is that a leading-out flue gas damper is provided on the circulation flue gas leading-out main pipe.

A further improvement of the present application is that a leading-in flue gas damper and a flue gas flow rate measuring device are provided on the circulation flue gas leading-in main pipe.

A further improvement of the present application is that a flue gas temperature measuring device and a flue gas pressure measuring device are provided on the circulation flue gas leading-in main pipe.

Compared with the prior art, the present application has the following beneficial effects.

Based on the flue gas system of the original pulverized coal-fired boiler, the present application proposed the following solution, a plurality of circulation flue gas leading-out ports are provided on a hearth outlet flue of the boilers, the circulation flue gas leading-out ports are connected to a circulation flue gas leading-out main pipe via a circulation flue gas leading-out pipeline, the circulation flue gas leading-out main pipe is connected to a circulation flue gas leading-in main pipe, the circulation flue gas leading-in main pipe is connected to a circulation flue gas leading-in pipeline, and the circulation flue gas leading-in pipeline is connected to the hearth. Flue gas in a flue is introduced into the hearth through the circulation flue gas leading-out ports, thereby achieving flue gas circulation; and at the same time, the problems, such as clinkering or overtemperature of the heating surface of pulverized coal fired boilers of power plants, high-temperature corrosion of the water-cooled walls, burning loss of combustors, incapability of reaching design value of steam temperature, relatively high concentration of nitrogen oxides at an outlet of the hearth, etc., can be solved or ameliorated.

Furthermore, the introduction of circulation flue gas from the bottom of the platen superheater can significantly improve the temperature distribution of the flue gas at the outlet of the hearth and solve the problems of overtemperature and clinkering on the high-temperature heating surface of the boilers.

Furthermore, the introduction of circulation flue gas from the central air duct of the swirl burner can significantly improve the cooling capacity of the burner, improve the temperature distribution in the nozzle outlet area of the burner, and solve the problems of clinkering and burning loss at the nozzle outlet of the swirl burner.

Furthermore, the introduction of the circulation flue gas from the burner area can significantly reduce the generation of initial nitrogen oxides and solve the problems of high-temperature corrosion of water-cooled walls and burning loss of swirl burners caused by relatively high over-fire air rate.

Furthermore, the introduction of the circulation flue gas from the bottom of the hearth can significantly improve the flue gas flow and heat transfer characteristics, increase the flexibility of steam temperature adjustment of boilers, and solve the problem of steam temperature not reaching the design value or excessive desuperheating water flow.

Furthermore, the present application adds a flue gas circulation system based on the original flue gas and air system of the pulverized coal-fired boiler. In practical applications, flue gas circulation flues, corresponding adjustment flue gas dampers, boiler flue gas circulation fans, etc. can be arranged only between different parts of the hearth (bottom, burner area, bottom of platen superheater, etc.) and flue (flues of parts such as induced draft fan outlet, induced draft fan inlet, air preheater outlet, economizer outlet, steering flue gas chamber, etc.), thus, the system is simple and easy to implement. During operation, the flue gas damper can be adjusted to restore to the mode of the original flue gas system without the risk of application failure. As for different problems such as relatively high temperature of the outlet of the hearth, clinkering on the water-cooled wall of the hearth and nozzle outlet of the burner, burning loss of the burner, relatively high nitrogen oxide concentration, steam temperature parameters not reaching standard, etc., they can be solved by selecting different path of flue gas circulation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the system of the present application.

The numbers in the figure are: 1-pulverized coal burner, 2-over-fire air nozzle outlet, 3-platen superheater, 4-convection heating surface, 5-economizer, 6-denitrification SCR reactor, 7-air preheater, 8-dust collector, 9-induced draft fan, 10-boiler flue gas circulation fan, 11-flue gas damper, 12-circulation flue gas leading-out main pipe, 13-circulation flue gas leading-in main pipe, 14-first circulation flue gas leading-out port, 15-second circulation flue gas leading-out port, 16-third circulation flue gas leading-out port, 17-fourth circulation flue gas leading-out port, 18-fifth circulation flue gas leading-out port, 19-first circulation flue gas leading-in port, 20-second circulation flue gas leading-in port, 21-third circulation flue gas leading-in port.

DETAILED DESCRIPTION

The present application will be described in further detail below in conjunction with the drawings.

Referring to FIG. 1, a flue gas circulation system that can improve the combustion characteristics and thermal characteristics of pulverized coal-fired boilers comprises a plurality of circulation flue gas leading-out ports (comprising first circulation flue gas leading-out port 14, second circulation flue gas leading-out port 15, third circulation flue gas leading-out port 16, fourth circulation flue gas leading-out port 17 and fifth circulation flue gas leading-out port 18) provided on a boiler outlet flue of the boilers, circulation flue gas leading-in ports (comprising first circulation flue gas leading-in port 19, second circulation flue gas leading-in port 20 and third circulation flue gas leading-in port 21), flue gas damper 11, circulation flue gas leading-out pipeline, flue gas flow rate measuring device flue gas temperature measuring device flue gas pressure measuring device circulation flue gas leading-out main pipe 12, boiler flue gas circulation fan 10, circulation flue gas leading-in main pipe 13, circulation flue gas leading-in pipeline, hearth, pulverized coal burner 1, over-fire air nozzle outlet 2, platen superheater 3, convection heating surface 4, economizer 5, denitrification SCR reactor 6, air preheater 7, dust collector 8 and induced draft fan 9.

Specifically, the circulation flue gas leading-out port is connected to circulation flue gas leading-out main pipe 12 via a circulation flue gas leading-out pipeline, circulation flue gas leading-out main pipe 12 is connected to circulation flue gas leading-in main pipe 13, circulation flue gas leading-in main pipe 13 is connected to a circulation flue gas leading-in pipeline, and the circulation flue gas leading-in pipeline is connected to the hearth.

The flue gas generated after the pulverized coal is burned in the boiler hearth flows along the hearth, horizontal flue and tail flue, and passes through the water-cooled wall, platen superheater 3, convection heating surface 4, steering flue gas chamber, economizer 5, denitrification SCR reactor 6, air preheater 7, dust collector 8, and induced draft fan 9 in sequence, and then discharges into the desulfurization device and chimney.

In the original pulverized coal-fired boiler, the steering flue gas chamber, economizer outlet, air preheater outlet, dust collector outlet, five induced draft fan outlets or part of the flue gas leading-out outlets are set up along the flue gas flow path, and different flue gas leading-out outlets can be selected according to needs to achieve the target flow rate and temperature of circulation flue gas. Specifically, first circulation flue gas leading-out port 14 is connected to the induced draft fan outlet flue; second circulation flue gas leading-out port 15 is connected to the induced draft fan inlet, and third circulation flue gas leading-out port 16 is connected to the air preheater outlet flue, fourth circulation flue gas leading-out port 17 is connected to the economizer outlet flue, and fifth circulation flue gas leading-out port 18 is connected to the steering flue gas chamber.

There are three flue gas leading-in ports arranged on the hearth of the original pulverized coal-fired boiler, i.e., first circulation flue gas leading-in port 19 provided at the bottom of the hearth, second circulation flue gas leading-in port 20 provided in the burner area, and third circulation flue gas leading-in port 21 provided at the bottom of the platen superheater, and one or more leading-in ports can be selected to achieve different purposes. Specifically, first circulation flue gas leading-in port 19 is connected to the bottom of the hearth, second circulation flue gas leading-in port 20 is connected to the hearth burner area, and third circulation flue gas leading-in port 21 is connected to the hearth of the bottom of the platen superheater.

Each circulation flue gas leading-in pipeline is connected to circulation flue gas leading-in main pipe 13. Each circulation flue gas leading-out pipeline is provided with leading-out flue gas damper 11, flue gas temperature measuring device flue gas pressure measuring device and flue gas flow rate measuring device Each circulation flue gas leading-out pipeline is merged into the circulation flue gas leading-out main pipe 12, and circulation flue gas leading-out main pipe 12 is provided with a leading-out flue gas damper 11.

Circulation flue gas leading-out main pipe 12 is connected to the inlet of boiler flue gas circulation fan 10, and the outlet of the boiler flue gas circulation fan 10 is connected to circulation flue gas leading-in main pipe 13. The flue gas is pressurized by boiler flue gas circulation fan 10 and sent to the hearth. The energy required for extracting and introducing the circulation flue gas is provided by boiler flue gas circulation fan 10, and its pressure head and flow rate are determined by calculation.

The circulation flue gas leading-in main pipe 13 is provided with leading-in flue gas damper 11, flue gas temperature measuring device flue gas pressure measuring device and flue gas flow rate measuring device

The total flow rate of circulation flue gas can be adjusted as needed, and can be selected within the range of 5% to 20% of the exhaust flue gas flow rate.

The flue gas from the outlet of the air preheater and the outlet of the economizer is extracted to the central air duct of the swirl burner, and the circulation flue gas is used as the cooling medium of the burner.

The flue gas is extracted from different positions of the flue of the existing pulverized coal-fired boiler (flue in parts such as the outlet of the induced draft fan, the inlet of the induced draft fan, the outlet of the air preheater, the outlet of the economizer, steering flue gas chamber and others), pressurized through boiler flue gas circulation fan 10, and sent to the hearth to form a flue gas circulation system. The position where the flue gas is fed into the hearth is first circulation flue gas leading-in port 19 at the bottom of the hearth, second circulation flue gas leading-in port 20 in the burner area, or third circulation flue gas leading-in port 21 at the bottom of the platen superheater.

EXAMPLE 1

For problems such as overtemperature and clinkering on the tube wall of the high-temperature heating surface caused by the high temperature at the outlet of the hearth, flue gas can be extracted from the flue of the positions such as the outlet of the induced draft fan (first circulation flue gas leading-out port 14), the inlet of the induced draft fan (second circulation flue gas leading-out port 15), the outlet of the air preheater (third circulation flue gas leading-out port 16), the outlet of the economizer (fourth circulation flue gas leading-out port 17) and others, fed into the hearth near the bottom area of the platen superheater (third circulation flue gas leading-in port 21) to improve the temperature distribution of the flue gas at the outlet of the hearth. Specifically, the flue gas from the outlet of the economizer (fourth circulation flue gas leading-out port 17) can be led to the bottom of the platen superheater (third cycle flue gas leading-in port 21). When the circulation flue gas flow rate is 10% of the exhaust flue gas flow rate, the temperature of the flue gas at the outlet of the hearth is reduced by about 100° C., which can significantly improve the problems of overtemperature and clinkering on the high-temperature heating surface.

EXAMPLE 2

For problems such as clinkering on the water-cooled wall of the hearth and nozzle outlet of the burner, as well as burning loss of the burner, flue gas can be extracted from third circulation flue gas leading-out port 16 at the outlet of the air preheater, fourth circulation flue gas leading-out port 17 at the outlet of the economizer, fifth circulation flue gas leading-out port 18 of the steering flue gas chamber and other positions, and fed into the hearth from second circulation flue gas leading-in port 20 in the burner area, thereby reducing the flame temperature of hearth. Specifically, the flue gas from the outlet of the air preheater (third circulation flue gas leading-out port 16) and the outlet of the economizer (fourth circulation flue gas leading-out port 17) can be extracted to the central air duct of the swirl burner, and the circulation flue gas is used as the cooling medium of the burner, which can greatly improve the reliability of the backup laminar swirl burner.

EXAMPLE 3

For reducing the concentration of nitrogen oxides, medium-temperature and high-temperature flue gas can be extracted from fourth circulation flue gas leading-out port 17 of the outlet of the economizer, fifth circulation flue gas leading-out port 18 of the steering flue gas chamber, and other positions, and sent into the hearth from second circulation flue gas leading-in port 20 in the burner area, that is, the primary air, secondary air, and over-fire air nozzle outlets, thereby suppressing the generation of initial nitrogen oxides by reducing the temperature and oxygen amount in the combustion zone, and achieving the purpose of reducing nitrogen oxide emissions. Specifically, the flue gas from fourth circulation flue gas leading-out port 17 at the outlet of the economizer and fifth circulation flue gas leading-out port 18 of the steering flue gas chamber can be extracted to the primary air, secondary air, and over-fire air nozzle outlets of the burner, so as to use the circulation flue gas to replace the air that not involved in combustion in this area to reduce the over-fire air rate. When the amount of circulation flue gas introduced reaches 10% of the total flue gas, the nitrogen oxide concentration can be reduced by about 100 mg/m³. At this time, the introduction of circulation flue gas can also improve the wall-adherent atmosphere of the water-cooled wall, alleviate the high-temperature corrosion and coking problems of the water-cooled wall, reduce the power consumption of the induced draft fan, and reduce the reducing agent consumption amount of the denitrification SCR system.

EXAMPLE 4

For the problem that the steam temperature parameters are not up to standard, flue gas can be extracted from positions such as first circulation flue gas leading-out port 14 on the induced draft fan outlet flue, second circulation flue gas leading-out port 15 on the induced draft fan inlet flue, third circulation flue gas leading-out port 16 on the air preheater outlet flue, fourth circulation flue gas leading-out port 17 on the economizer outlet flue, and fifth circulation flue gas leading-out port 18 of the steering flue gas chamber, etc., and fed into the hearth from first circulation flue gas leading-in port 19 at the bottom of the hearth, thereby increasing the flue gas flow rate and the heat absorption of convection heating surface 4, and improving the boiler steam temperature regulation characteristics. Specifically, the flue gas from the outlet of the induced draft fan (first circulation flue gas leading-out port 14) and the outlet of the economizer (fourth circulation flue gas leading-out port 17) can be extracted to first circulation flue gas leading-in port 19 at the bottom of the hearth (upper part of the turning angle of the cooled ash hopper) to change the heat absorption ratio of the radiation heating surface and convection heating surface 4, and improve the adjustment flexibility of the main steam temperature and reheat steam temperature.

Claims

1. A flue gas circulation system for improving the combustion characteristics and thermal characteristics of a pulverized coal-fired boiler, wherein the flue gas circulation system comprises a plurality of circulation flue gas leading-out ports provided on an outlet flue of a hearth of the boiler, the circulation flue gas leading-out ports are connected to circulation flue gas leading-out main pipe (12) via circulation flue gas leading-out pipelines, circulation flue gas leading-out main pipe (12) is connected to circulation flue gas leading-in main pipe (13), circulation flue gas leading-in main pipe (13) is connected to circulation flue gas leading-in pipelines, and the circulation flue gas leading-in pipelines are connected to the hearth.

2. The flue gas circulation system for improving the combustion characteristics and thermal characteristics of a pulverized coal-fired boiler according to claim 1, wherein circulation flue gas leading-out main pipe (12) is connected to the inlet of boiler flue gas circulation fan (10), and the outlet of boiler flue gas circulation fan (10) is connected to circulation flue gas leading-in main pipe (13).

3. The flue gas circulation system for improving the combustion characteristics and thermal characteristics of a pulverized coal-fired boiler according to claim 1, wherein the circulation flue gas leading-out ports comprise one or more of first circulation flue gas leading-out port (14), second circulation flue gas leading-out port (15), third circulation flue gas leading-out port (16), fourth circulation flue gas leading-out port (17) and fifth circulation flue gas leading-out port (18).

4. The flue gas circulation system for improving the combustion characteristics and thermal characteristics of a pulverized coal-fired boiler according to claim 3, wherein the outlet flue of the hearth of the boiler is provided with a platen superheater, a steering flue gas chamber, air preheater (7) and induced draft fan (9);

first flue gas leading-out port (14) is connected to an outlet flue of the induced draft fan, second flue gas leading-out port (15) is connected to an inlet of the induced draft fan, third flue gas leading-out port (16) is connected to an outlet flue of the air preheater, fourth flue gas leading-out port (17) is connected to an outlet flue of an economizer, and fifth flue gas leading-out port (18) is connected to the steering flue gas chamber.

5. The flue gas circulation system for improving the combustion characteristics and thermal characteristics of a pulverized coal-fired boiler according to claim 1, wherein a plurality of leading-in ports are provided at the bottom of the hearth, and the leading-in ports comprise one or more of first circulation flue gas leading-in port (19) provided at the bottom of the hearth, second circulation flue gas leading-in port (20) provided in a burner area, and third circulation flue gas leading-in port (21) provided at the bottom of the platen superheater.

6. The flue gas circulation system for improving the combustion characteristics and thermal characteristics of a pulverized coal-fired boiler according to claim 1, wherein leading-out flue gas damper (11) is provided on circulation flue gas leading-out main pipe (12).

7. The flue gas circulation system for improving the combustion characteristics and thermal characteristics of a pulverized coal-fired boiler according to claim 1, wherein leading-in flue gas damper (11) and a flue gas flow rate measuring device are provided on circulation flue gas leading-in main pipe (13).

8. The flue gas circulation system for improving the combustion characteristics and thermal characteristics of a pulverized coal-fired boiler according to claim 1, wherein a flue gas temperature measuring device and a flue gas pressure measuring device are provided on circulation flue gas leading-in main pipe (13).

9. The flue gas circulation system for improving the combustion characteristics and thermal characteristics of a pulverized coal-fired boiler according to claim 7, wherein a flue gas temperature measuring device and a flue gas pressure measuring device are provided on circulation flue gas leading-in main pipe (13).