Cause analysis and prevention of the burning accid

2022-08-14
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Cause analysis and prevention of boiler secondary bellows burning accident

the No. 3 boiler of a thermal power plant is a cg-130/3.82-m16 single drum, medium pressure, natural circulation, solid-state slag removal coal-fired boiler produced by Sichuan boiler plant. It is called a dangerous and tough old man ǖ Avoid palm tanning with 2 layers and 4 corner tangential layout; Adopt intermediate storage and exhaust gas powder delivery system; It is designed to burn pulverized coal or gas, and ignite with natural gas

after the (72+24) H test run of No. 3 boiler, the load is maintained at about 100 t/h according to the actual production needs. During this period, No. 3 boiler was fully coal-fired, controlled by DCS system, and a huge wheel loaded with a grand blueprint was rotating. The burning, negative pressure, air supply and pulverizing hot air were fully automatic. At t07:00, the operator found that the blue paint on the surface of the insulation layer on the upper half of the secondary air box of the boiler horn 3 turned black and smoked and caught fire. Immediately stop the operation of the No. 3 and No. 7 pulverized coal nozzles of the horn 3, and close the main air valve of the secondary air box of the horn. About 10 minutes later, the upper half of the secondary air box of the horn 3 was completely burnt out. It was only 4 days since the boiler was put into operation and the "swing out" button was pressed

1 secondary air box structure

the secondary air box is welded from a steel plate made of q235-af with a thickness of 5 mm. The layout is shown in Figure 1. (omitted)

2 accident cause analysis

2.1 ball valve 4 switch indicator is installed reversely

in order to stabilize combustion, protect pulverized coal nozzle and prevent coking of water wall in burner area, horizontal once through dense dilute pulverized coal burner is designed for boiler; At the same time, in order to further improve the high load combustion condition, 76 × 4. The pipes are connected, and the ball valve is used to control the middle of the pipes. The pipes and ball valves form a high load pulverized coal combustion regulating device. Under normal load, each primary air duct relies on its own separation device to separate the air powder mixture into two streams of thick and thin air flow into the furnace, and the pulverized coal flow on the thick side is ignited in advance to stabilize combustion; Under high load, the concentration of pulverized coal in the air powder mixture flow of each primary air duct has been very large. After the separation of the high concentration pulverized coal flow through the primary air duct concentration and dilution separation device, the speed of pulverized coal on the concentrated side is significantly reduced, the concentration is too large, and the pulverized coal can not be completely burned. Slag and coking are very easy to occur on the water-cooled wall around the combustor, which restricts the normal output of the boiler, At this time, if the ball valve in the high load pulverized coal concentration and dilution regulating device is opened and the secondary air is introduced obliquely into the primary air duct, on the one hand, the pulverized coal flow on the rich side can be blown to the light side at the front end of the primary air duct concentration and dilution separation device, so as to reduce the concentration of pulverized coal flow on the rich side and increase the concentration of pulverized coal flow on the light side, so as to achieve the purpose of improving high load combustion. On the other hand, high temperature air (about 350 ℃) can be introduced into the primary air pulverized coal flow to improve high load coal 4 Powder combustion conditions during loading

only when the boiler operates at high load (120 t/h), the air pressure of the secondary air box will be higher than the air pressure of the primary air pipe, while under low load, the air pressure of the secondary air box will drop sharply, and the air pressure of each primary air pipe will basically remain unchanged. If in this case, the high load pulverized coal concentration and dilution are adjusted by mistake to understand the production process of the automobile, and the ball valve of the device is opened and put into operation, It is bound to cause the air powder mixture in the primary air duct to enter the secondary air box along the connecting pipe. After the No.3 boiler was put into operation, according to the actual production needs, the load has been maintained at about 100 t/h. because the opening of the secondary air baffle of all air nozzles and coal nozzles is about 50%, the air volume is relatively dispersed, and the air pressure of the secondary air box is significantly lower than the primary air pressure, so the ball valves connected with each primary air pipe and the secondary air box on the high load pulverized coal combustion regulating device are closed according to the instructions to prevent the boiler from being under low load, An accident occurs when the air powder mixture in the primary air duct enters the secondary air box. However, the actual switch of the ball valve on the high load pulverized coal combustion regulating device of the primary air duct of the boiler horn 3 is just opposite to the indication, creating conditions for the air powder mixture in the primary air duct to enter the secondary air box under low load

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