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What should I do if a coking accident occurs in a circulating fluidized bed boiler? How to prevent it?
Dec 18, 2018

The characteristics of the circulating fluidized bed boiler have the advantages of particularly good fuel adaptability, high combustion efficiency, low gas pollutant emission, large load regulation range, and comprehensive utilization of ash and slag. With the rapid development of circulating fluidized bed boilers, it has rich experience in design, manufacture, installation, commissioning and operation, but it also exposes some problems. The coking accident is the most common problem in the operation of circulating fluidized bed boilers. This article gives a detailed explanation on how to prevent coking accidents in circulating fluidized bed boilers.

1. Mechanism of coking accident in circulating fluidized bed boiler

Circulating fluidized bed coking accidents are the most common accidents in boiler unit operation, and occur frequently during normal operation, ignition start and ignition start. In the dense phase zone of the circulating fluidized bed furnace, the combustion temperature exceeds the ash melting point and causes coking due to the destruction of the flow chemical condition. Combustion coking occurs not only in the furnace but also in the cyclone and the return. In the event of a coking accident, it is necessary to stop the furnace treatment, which has a great impact on the whole process, safety and economy.

In the operation of the circulating fluidized bed boiler, the ash change in the fuel is divided into three stages: deformation temperature, softening temperature and melting temperature. For example, the melting temperature of the coal ash produced by the Yunnan origin used by our company is about 1200 °C. Left and right, and the temperature control range of the two circulating fluidized bed boilers in our company is 850-950 °C. If the controlled operating temperature is lower than the ash melting point temperature of 300-400 °C, maintaining good fluidization and normal flow, it will not Will cause bed material coking accident.

2. The hazard of coking in circulating fluidized bed boiler

2.1 Fluidization is not smooth

When the air volume of the circulating fluidized bed boiler is improperly adjusted, when the fluidization is uneven or the temperature of the combustion chamber exceeds the ash melting point temperature for a certain period of time, local coking in the combustion chamber may occur, causing clogging of the hood and uneven distribution of the air distribution plate. A larger area of coking causes the boiler to run poorly and shut down.

2.2 slag discharge pipe blockage

When the working condition of the circulating fluidized bed boiler is improperly adjusted, the coal slag falls into the slag discharge pipe when the carbon content in the fuel is not completely burned. Due to the negative pressure, the coal slag is burned and agglomerated in the slag discharge pipe, which may cause The slag discharge pipe is blocked, the slag discharge is not smooth, and the furnace is stopped.

2.3 The blockage of the return material causes the furnace to coke

Because the refractory material in the feeder is collapsed or the return air is too small, a large amount of return ash is accumulated in the feeder and cannot enter the furnace. At this time, the temperature in the furnace is increased, but the vapor pressure of the steam will follow. Decrease, such as by adding coal to maintain the load will accelerate the rise of the bed temperature, eventually leading to high temperature coking.

3. Classification and phenomenon of coking

The coking of circulating fluidized bed boilers is divided into low temperature coking and high temperature coking.

3.1 Low temperature coking phenomenon

The performance of low temperature coking is that the temperature in the dense phase shows a large deviation, the temperature of the individual measuring points is extremely low, the negative pressure of the furnace outlet is fluctuating, the steam load is reduced, and the slag discharging of the cold slag machine is not smooth.

3.2 High temperature coking phenomenon

The bed temperature rises sharply, reaches or exceeds the ash melting point temperature of coal, the bed pressure is abnormal, the resistance of the air distribution plate increases, the oxygen volume fluctuates greatly, accompanied by the deflagration phenomenon, the current of the primary fan and the induced draft fan is relatively reduced, and the load carrying capacity is reduced. The combustion proceeds on the surface of the layer, and a clear flame is observed from the observation hole and appears white, and slag discharge is difficult.

Analysis of the cause of coking

4.1 Causes of low temperature coking

Low-temperature coking often occurs during the normal start-up or fire-starting process of the boiler. When the circulating fluidized bed boiler is ignited or raised, the bed temperature is only 400-600 °C, but the fuel is released due to poor local fluidization or clogging of the hood. The heat can not be carried away with the flue gas or absorbed by the heated surface, causing local accumulation of heat, so that the local temperature of the bed temperature is higher than the ash melting point temperature, so that the ash slag melts and agglomerates, and the fluidization condition in this region becomes worse, the coke The surface temperature of the block is low, causing heat in the surrounding area to be absorbed by it, and the agglomeration range is enlarged. In addition, it may also be due to fluidization experiments that are not well done, improper operation during operation, and the operating air volume is lower than the critical fluidized air volume, causing coking due to improper fluidization.

4.2 Causes of high temperature coking

High-temperature coking refers to coking that occurs when the temperature of the hearth is high overall under normal fluidization conditions. High temperature coking may occur during ignition temperature rise and normal operation. When the ignition temperature rises, such as the bed temperature rises to 950 ° C, the high temperature coking accident may occur due to failure to properly input the return material. The high temperature coking is due to the higher temperature in the furnace. The temperature of the ash melting point causes the ash in the hearth to melt and agglomerate. And if the wind and coal are not properly operated during operation, the furnace will easily cause a coking accident.

5. Coking accident handling

5.1 Low temperature coking treatment

Once the furnace personnel judge the low temperature coking, take decisive measures immediately. For our large circulating fluidized bed boiler, we can only increase the fluidized air volume, so that the slag block can be fluidized at high speed in the furnace. Because the low temperature coking slag is loose, there are It is possible to use the wind to break up the slag during the collision of the bed material, and then discharge it from the cold slag machine. If the slag block blocks the cold slag machine, the auxiliary furnace should continuously slag the slag until it is normally slag.

5.2 Treatment of high temperature coking

Once the stove personnel judges that they may be coking, they should stop the coal supply immediately, and increase the air volume greatly, reduce the bed temperature, and then operate normally after the temperature is restored. If it is judged that the furnace has been cokeed, the furnace can increase the fan according to the maximum rated current. The damper blows out the furnace and then stops the slag to reduce the damage to the furnace due to coking and shorten the labor intensity of slag.

6. Summary of the case of coking accident in circulating fluidized bed

6.1 Causes of coking accident

(1) A circulating fluidized bed boiler has controlled the operation of the low bed during the operation. Because the slag discharge rate of the cold slag machine is not controlled, the local fluidized air volume of the material layer is excessively blown through the bottom material, and the surrounding air is blown through. Unevenness causes the bottom material to accumulate, thereby affecting the overall flow chemical condition leading to low temperature coking.

(2) Blockage of return line is also one of the main reasons for over-temperature coking in circulating fluidized bed. During the operation of a circulating fluidized bed, due to the accumulation of return ash in the return line, the return line of the return fan is blocked and crushed. The returning fan is tripped, resulting in a fluidized bed temperature rising to the ash melting point temperature and high temperature coking.

6.2 Measures for coking accidents

Summarize the above two coking accidents, strengthen the technical parameters during the operation of the circulating fluidized bed boiler, strictly control the thickness of the bed, control the differential pressure of the material layer between 6.0 and 7.0 Kpa, and control the cold slag machine according to the thickness of the material layer. The slagging rate ensures that the layer is not blown through to ensure a good fluidization state.

Strictly check the internal condition of the returning device before opening the furnace to ensure that there is no dust accumulation in the returning device, and there is no gap between the returning air distribution plate and the returning material ash discharge pipe to prevent the returning ash from falling into the returning air chamber and blocking the return. Feed air line.

7. Coking prevention measures

Coking is a common failure of circulating fluidized bed boilers. It directly affects the safe and economic operation of the boiler. In order to avoid the economic and safety losses caused by coking, the furnace personnel should sum up experience in operation and strictly keep in mind the operating procedures. , take precautionary measures:

(1) Before entering the primer for each ignition, carefully check the hood for obstruction and clean up the debris.

(2) Make a proper experiment of the air distribution plate every time you open the furnace.

(3) Strictly check the quality of coal:

a. dry basis fixed carbon Fcd ≧ 50%;

b. Ash softening temperature ST ≧ 1200 ° C;

c. particle size ≦ 13mm;

d. Dry basis ashless base volatiles Qnet, v, ad ≧ 14-20%.

(4) Before the ignition, the bottom material should be carefully checked to determine whether the carbon content of the primer exceeds the standard. If the standard exceeds the standard, the primer should be replaced immediately to prevent coking.

(5) The returning fan is not started at the initial stage of ignition. When the returning material is full and the temperature of the returning device reaches 900 °C, the returning fan is started to ensure that there is enough returning ash to enter the furnace and control the bed temperature.

(6) When igniting, carefully check whether the oil gun atomization is good, do the oil gun atomization experiment, if there is a bad atomization, it is necessary to clean the gun head. If the ignition fails due to poor atomization, use compressed air to purge. If there is too much oil leakage, the primer should be replaced to prevent coking.

(7) During the ignition temperature rise, the bed temperature can reach 550 °C, and a small amount of coal can be injected into the furnace. It should be gradual, and the coal should not be too fast and too much. If the coal in the furnace is too much, the carbon content in the bottom of the furnace is too high. When the air volume is increased, the bed temperature rises sharply, and the ash temperature will exceed the ash melting point temperature, causing coking in the furnace.

(8) During low-load operation, if the bed temperature drops sharply, it is likely to be bed material accumulation except for coal breakage. If it is judged that the bed material accumulation should speed up the speed of the cold slag machine, the differential pressure of the material layer is controlled to be normal. Within the range of bed pressure, it is necessary to add wind and then add coal when adding load.

(9) When the circulating fluidized bed boiler is under fire, first stop the returning fan, the secondary fan, and then stop the coal feeder, and then quickly close the damper of the fan and the induced draft fan separately, and isolate the air from the furnace. The secondary combustion of the inner layer of the furnace prevents coking.

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