A flue gas recirculation system for preventing spontaneous combustion of pulverized coal in the pulverized coal pipeline of a medium-speed coal mill

Abstract

A flue gas recirculation system for preventing spontaneous combustion of accumulated powder in a medium-speed coal mill coal powder pipeline comprises a medium-speed coal mill, a coal-fired boiler, an induced draft fan and a recirculating flue gas pipeline; the coal-fired boiler comprises a burner, a coal powder pipeline is arranged at the outlet of the medium-speed coal mill, the outlet of the coal powder pipeline is connected to the burner via a mixing pipeline, the coal-fired boiler is connected to the inlet of the induced draft fan, and the outlet of the induced draft fan is divided into two paths, one path discharges flue gas, and the other path is connected to the coal powder pipeline via a recirculating flue gas pipeline. The present invention mixes the low-dust and low-oxygen flue gas at the outlet of the induced draft fan of the coal-fired power station boiler with the air-powder airflow at the outlet of the medium-speed coal mill by setting a recirculating flue gas pipeline, and the oxygen content of the medium after mixing is reduced to less than 14%, effectively preventing the spontaneous combustion of coal powder deposited in the coal powder pipeline, and eliminating the conditions for the occurrence of deflagration in the pulverizing system; the flow rate of the mixed medium is increased by 30% to 50%, preventing the deposition of coal powder in the medium. And prevent boiler slagging caused by high flame temperature in the burner area.

Description

Technical Field

[0001] The invention belongs to the field of coal-fired power station boilers, and in particular relates to a flue gas recirculation system for preventing spontaneous combustion of accumulated pulverized coal in a medium-speed coal mill pulverized coal pipeline. Background Art

[0002] The main function of the coal-fired power plant boiler pulverizing system is to prepare qualified coal powder for the boiler and transport the coal powder to the combustion equipment for combustion.

[0003] The three conditions that must be met simultaneously for a pulverizing system to deflagrate are: (1) the presence of a mixture of coal powder and air in a confined space; (2) the concentration of coal powder in the combustible mixture reaches the explosion condition; (3) there is an open flame that can cause deflagration. The open flame that causes deflagration during operation comes from the spontaneous combustion of coal powder deposits in the pulverizing system and the combustion flashback that occurs when the air-powder velocity at the primary air outlet of the burner is lower than the flame propagation velocity. When grinding Indonesian coal and South African coal, it is easy for the pulverizing system to deflagrate due to ignition of accumulated powder in the pulverized coal pipeline. The main reasons for deflagration caused by accumulation of powder in the pulverized coal pipeline of a medium-speed coal mill are as follows: (1) the moisture content of the pulverized coal is too high, resulting in a low outlet temperature of the pulverizer. When the outlet temperature of the pulverizer is close to or lower than the water dew point, the pulverized coal is easily deposited in the pulverized coal pipeline; (2) the air-powder airflow velocity is too low, resulting in pulverized coal deposition; (3) pulverized coal is easily deposited at the clamps and adjustable shrinkage holes on the pulverized coal pipeline; (4) when the pulverizer is shut down, the pulverized coal pipeline is not thoroughly pumped out and the pulverized coal pipe is not purged, resulting in pulverized coal pipeline accumulation. Generally, the volatile matter of dry ash-free basis of imported Indonesian coal is about 48%, and the explosion index of coal is K d It is too large and belongs to the type of coal that is prone to explosion; the total moisture content is about 24%. High moisture content increases the risk of coal powder deposition, and when a medium-speed pulverizer grinds high-moisture coal, the pulverizer outlet temperature is generally 56°C to 62°C, and the coal powder pipeline is prone to condensation and deposition of coal powder, increasing the risk of deflagration.

[0004] Currently, most power plants have taken measures such as extending the purge time during the start-up and shutdown process of the coal mill, controlling the outlet temperature of the coal mill, and increasing the inlet air volume of the coal mill. However, the spontaneous combustion of coal powder accumulated in the coal powder pipeline still occurs from time to time, and the explosion problem of the pulverizing system has not been completely solved. Summary of the invention

[0005] The object of the present invention is to provide a flue gas recirculation system for preventing spontaneous combustion of accumulated pulverized coal in a medium-speed coal mill pulverized coal pipeline.

[0006] To achieve the above object, the present invention adopts the following technical solution:

[0007] A flue gas recirculation system for preventing spontaneous combustion of accumulated pulverized coal in a medium-speed coal mill pulverized coal pipeline, comprising a medium-speed coal mill, a coal-fired boiler, an induced draft fan and a recirculating flue gas pipeline; the coal-fired boiler comprises a burner, a pulverized coal pipeline is arranged at the outlet of the medium-speed coal mill, the outlet of the pulverized coal pipeline is connected to the burner via a mixing pipeline, the coal-fired boiler is connected to an induced draft fan inlet, the outlet of the induced draft fan is divided into two paths, one path discharges flue gas, and the other path is connected to the pulverized coal pipeline via the recirculating flue gas pipeline.

[0008] A further improvement of the present invention is that a recirculation fan is provided on the recirculation flue gas duct.

[0009] A further improvement of the present invention is that the recirculation fan is driven by a 380V motor.

[0010] A further improvement of the present invention is that the recirculation fan is a centrifugal fan, and the blades of the centrifugal fan are anti-wear sprayed blades.

[0011] A further improvement of the present invention is that a dust removal device is provided on the recirculation flue gas duct between the outlet of the induced draft fan and the inlet of the recirculation fan.

[0012] A further improvement of the present invention is that the dust removal device is a metal filter bag.

[0013] A further improvement of the present invention is that an electric regulating door, an electric stop door and an adjustable shrinkage hole are sequentially arranged on the recirculating flue gas pipeline between the outlet of the recirculating fan and the pulverized coal pipeline at the outlet of the medium-speed coal mill.

[0014] A further improvement of the present invention is that the pulverized coal pipeline, the recirculating flue gas pipeline and the mixing pipeline are all made of Q235 steel, and the outsides of the pulverized coal pipeline, the recirculating flue gas pipeline and the mixing pipeline are all covered with insulation materials.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] The present invention mixes the low-dust and low-oxygen flue gas at the outlet of the induced draft fan of the coal-fired power station boiler with the air-powder airflow at the outlet of the medium-speed coal mill by setting a recirculating flue gas pipeline. After mixing, the oxygen content of the medium is reduced to less than 14%, which effectively prevents the spontaneous combustion of coal powder deposited in the coal powder pipeline and eliminates the conditions for the explosion of the pulverizing system; the flow rate of the mixed medium is increased by 30% to 50%, which prevents the deposition of coal powder in the medium. When the medium-speed coal mill is shut down, the flue gas at the outlet of the induced draft fan still enters the burner through the recirculating pipeline, the recirculating fan, and the mixing pipeline, which, on the one hand, cools the burner nozzle to prevent the corresponding burner nozzle from burning and deforming when the medium-speed coal mill is shut down; on the other hand, it can reduce the furnace flame temperature, reduce the NOx generated by combustion, and prevent boiler slagging caused by the high flame temperature in the burner area. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A schematic diagram of a flue gas recirculation system for preventing spontaneous combustion of pulverized coal accumulated in a medium-speed coal mill pipeline according to the present invention;

[0018] Among them, 1 is a medium-speed coal mill, 2 is a coal-fired boiler, 3 is a dust collector, 4 is an induced draft fan, 5 is a recirculation fan, 6 is a pulverized coal pipeline, 7 is a mixing pipeline, 8 is a burner, 9 is a recirculating flue gas pipeline, 10 is an electric regulating door, 11 is an electric stop door, 12 is an adjustable shrinkage hole, and 13 is a metal filter bag. DETAILED DESCRIPTION

[0019] The present invention is further described in detail below with reference to the accompanying drawings.

[0020] See also Figure 1 A flue gas recirculation system for preventing spontaneous combustion of accumulated pulverized coal pipeline of a medium-speed coal mill of the present invention comprises a medium-speed coal mill 1, a coal-fired boiler 2, a dust collector 3, an induced draft fan 4 and a recirculating flue gas pipeline 9. The coal-fired boiler 2 comprises a burner 8, a pulverized coal pipeline 6 is arranged at the outlet of the medium-speed coal mill 1, the outlet of the pulverized coal pipeline 6 is connected to the burner 8 via a mixing pipeline 7, the coal-fired boiler 2 is connected to the inlet of the induced draft fan 4 via the dust collector 3, the outlet of the induced draft fan 4 is divided into two paths, one path discharges flue gas, and the other path is connected to the pulverized coal pipeline 6 via the recirculating flue gas pipeline 9. Specifically, the pulverized coal pipeline 6 of the medium-speed coal mill 1 is connected to the recirculating flue gas pipeline 9, and the flue gas in the recirculating flue gas pipeline 9 is mixed with the pulverized coal and wind in the pulverized coal pipeline 6, and then connected to the burner 8 via the mixing pipeline 7.

[0021] A recirculation fan 5 is provided on the recirculation flue gas duct 9. The recirculation fan 5 is driven by a 380V motor, the recirculation fan 5 is a centrifugal fan, and the blades of the recirculation fan 5 are anti-wear sprayed blades.

[0022] In the present invention, part of the flue gas is extracted from the outlet flue of the induced draft fan 4, and the flue gas flows through the recirculation fan 5 through the recirculation flue gas duct 9. After the pressure is increased by the recirculation fan 5, the flue gas is introduced into the four pulverized coal pipes 6 at the outlet of the medium-speed coal mill 1 through the recirculation flue gas duct 9, so that the flue gas at the outlet of the induced draft fan 4 is mixed with the air-powder airflow and enters the burner 8 through the mixing pipe 7 for combustion.

[0023] A dust removal device is provided on the recirculation flue gas duct 9 from the outlet of the induced draft fan 4 to the inlet of the recirculation fan 5 , and the dust removal device is a metal filter bag 13 .

[0024] In order to meet the explosion-proof requirements of the boiler, an electric regulating door 10, an electric stop door 11 and an adjustable shrinkage hole 12 are sequentially arranged on the recirculating flue gas duct 9 between the outlet of the recirculating fan 5 and the pulverized coal duct 6 at the outlet of the medium-speed coal mill 1.

[0025] The materials of the pulverized coal pipeline 6 , the recirculating flue gas pipeline 9 and the mixing pipeline 7 are all Q235 steel, and the outsides of the pulverized coal pipeline 6 , the recirculating flue gas pipeline 9 and the mixing pipeline 7 are all covered with thermal insulation materials.

[0026] Due to the different unit capacities and coal mill outputs, the temperature of the air-powder airflow in the outlet coal pulverized pipe of the medium-speed coal mill is in the range of 55℃~95℃, the air-powder airflow pressure is 2kPa~3kPa, the oxygen content in the air-powder airflow is 21%, and the air-powder airflow velocity is 18m / s~28m / s. The outlet flue gas temperature of the induced draft fan of the coal-fired power station boiler is 95℃~120℃, the outlet flue gas pressure of the induced draft fan is 0.8kPa~2kPa, and the oxygen content in the flue gas is 4%~6%.

[0027] The flue gas is extracted from the exhaust duct of the induced draft fan and sent to the inlet of the recirculation fan through the recirculation flue gas duct. At this time, the flue gas pressure at the inlet of the recirculation fan is 0.8kPa~2.5kPa, and the dust content in the flue gas is 30mg / m 3 After passing through the metal filter bag arranged on the recirculating flue gas duct, the dust content will be reduced to about 5mg / m 3 The flue gas after dust removal by metal filter bags flows through the recirculation fan, and the flue gas pressure is increased to about 3.5kPa. The pressurized flue gas passes through the recirculation flue gas duct, electric regulating door, electric stop door, and adjustable shrinkage hole. The flue gas pressure is adjusted to be slightly higher than the air-powder airflow pressure, and is introduced into the four pulverized coal pipes at the outlet of the medium-speed pulverizer to mix with the air-powder airflow.

[0028] Since the oxygen content of the flue gas at the outlet of the induced draft fan is 4% to 6%, the oxygen content of the air-powder airflow at the outlet of the medium-speed coal mill is 21%, and the oxygen content of the mixed medium is 12% to 13%, it meets the requirements of DL / T 5203-2005 "Explosion-proof Design Code for Coal and Pulverizing Systems in Thermal Power Plants" that the maximum allowable oxygen content at the outlet of the coal mill designed for inert atmosphere should not exceed 14%. Even if there is coal powder deposited in the medium in the mixing pipeline, the accumulated powder will not spontaneously combust, and the conditions for coal mill explosion will not be met.

[0029] The flue gas is mixed with the air-powder airflow, and the temperature of the mixed medium is 75℃~107℃. Although the temperature of the mixed medium is 12℃~20℃ higher than the temperature of the air-powder airflow, it is beneficial to the initial ignition of the coal powder in the mixed medium entering the burner; considering that the oxygen content of the mixed medium is less than 14%, the mixed medium is an inert atmosphere, so the coal mill will not explode due to the increase in the medium temperature.

[0030] The flue gas is mixed with the air-powder airflow, and the flow rate of the mixed medium will increase by 30% to 50%, reducing the risk of coal powder deposition in the mixed medium. The mixing pipe is connected to the coal powder nozzle of the burner, and the mixed medium enters the furnace for combustion after passing through the burner.

[0031] Furthermore, since the dust content in the exhaust gas from the induced draft fan is 30 mg / m 3 Although the dust content in the flue gas is still about 5mg / m 3 In order to prevent dust in the flue gas from eroding and wearing the recirculation fan blades, the recirculation fan blades are sprayed with anti-wear blades. The recirculation fan inlet flue gas pressure is 0.8kPa~2kPa, the outlet flue gas pressure is 3.5kPa, and the recirculation fan inlet and outlet pressure difference is 1.5kPa~2.7kPa, so the recirculation fan adopts a centrifugal fan, and the recirculation fan motor is a 380V low-voltage motor.

[0032] Furthermore, in order to ensure that the deviation of the flue gas flow rate of the four pulverized coal pipes entering the outlet of the medium-speed coal mill is within ±5%, an adjustable shrinkage hole is provided on the recirculation pipe between the recirculation fan and the pulverized coal pipe to balance the deviation of the flue gas flow rate between the recirculation pipes. In order to ensure that the deviation of the medium flow rate after the flue gas and the air-powder airflow are mixed is within ±5%, an adjustable shrinkage hole is provided on the mixing pipe to balance the flow deviation between the four mixing pipes.

[0033] Furthermore, when the medium-speed coal mill is shut down, the electric stop door on the pulverized coal pipeline is closed, the electric stop door on the recirculation pipeline is fully open, the recirculation fan is still running, and the electric regulating door maintains an appropriate opening. The flue gas enters the furnace through the recirculation pipeline, the mixing pipeline, and the burner nozzle. On the one hand, the temperature of the recirculating flue gas is 95℃~120℃, which will cool the nozzle when flowing through the burner to prevent the corresponding burner nozzle from burning and deformation when the medium-speed coal mill is shut down. On the other hand, the flue gas from the induced draft fan outlet enters the furnace through the recirculation pipeline, the mixing pipeline, and the burner, which can reduce the flame temperature of the furnace and reduce the NOx generated by combustion, while preventing boiler slagging caused by high flame temperature in the burner area.

Claims

1. A flue gas recirculation system for preventing spontaneous combustion of pulverized coal in a medium-speed coal mill pipeline, characterized in that: The invention comprises a medium-speed coal mill (1), a coal-fired boiler (2), an induced draft fan (4) and a recirculating flue gas duct (9); the coal-fired boiler (2) comprises a burner (8); a pulverized coal duct (6) is arranged at the outlet of the medium-speed coal mill (1); the outlet of the pulverized coal duct (6) is connected to the burner (8) via a mixing duct (7); the coal-fired boiler (2) is connected to the inlet of the induced draft fan (4); the outlet of the induced draft fan (4) is divided into two paths, one path discharges flue gas, and the other path is connected to the pulverized coal duct (6) via the recirculating flue gas duct (9); A recirculation fan (5) is arranged on the recirculation flue gas duct (9); the recirculation fan (5) is a centrifugal fan; the recirculation fan blades (5) are anti-wear sprayed blades; An electric regulating door (10), an electric stop door (11) and an adjustable shrinkage hole (12) are sequentially arranged on the recirculating flue gas pipeline (9) between the outlet of the recirculating fan (5) and the pulverized coal pipeline (6) at the outlet of the medium-speed coal mill (1), and the pulverized coal pipeline is provided with an electric stop door (11); When the medium-speed coal mill is shut down, the electric stop door (11) on the pulverized coal pipeline is closed, the electric stop door (11) on the recirculation flue gas pipeline (9) is fully opened, the recirculation fan (5) is running, the electric regulating door (10) maintains an appropriate opening, and the flue gas enters the furnace through the recirculation pipeline, the mixing pipeline, and the burner nozzle.

2. A flue gas recirculation system for preventing spontaneous combustion of pulverized coal in a medium-speed coal mill pipeline according to claim 1, characterized in that: The recirculation fan (5) is driven by a 380V motor.

3. A flue gas recirculation system for preventing spontaneous combustion of pulverized coal in a medium-speed coal mill pipeline according to claim 1, characterized in that: A dust removal device is provided on the recirculating flue gas duct (9) between the outlet of the induced draft fan (4) and the inlet of the recirculating fan (5).

4. A flue gas recirculation system for preventing spontaneous combustion of pulverized coal in a medium-speed coal mill pipeline according to claim 3, characterized in that: The dust removal device is a metal filter bag (13).

5. A flue gas recirculation system for preventing spontaneous combustion of pulverized coal in a medium-speed coal mill pipeline according to claim 1, characterized in that: The pulverized coal pipeline (6), the recirculating flue gas pipeline (9) and the mixing pipeline (7) are all made of Q235 steel, and the outsides of the pulverized coal pipeline (6), the recirculating flue gas pipeline (9) and the mixing pipeline (7) are all provided with thermal insulation materials.

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