Causes and solutions of boiler coking

1, coking is related to ash melting point.

The fundamental reason of coking is that molten ash is deposited on the heating surface. It can be seen that the melting point of ash is the key to coking. The scaling and coking tendency of coal ash on high temperature heating surface can be judged by ash melting point temperature and ash main components.

2. Coking is related to the injection angle of the burner.

If the installation angle of the burner is skewed, the burner itself is defective, the tangential circle of the burner is too large, the pulverized coal airflow deflects and rubs against the wall, which often leads to serious coking of the boiler.

3. Coking is related to combustion adjustment.

Unreasonable combustion adjustment, too low primary air pressure, too low wind speed, too fine pulverized coal, too early ignition, too high secondary wind speed, uneven distribution of air volume in four corners, uneven powder quantity in four corner burners, etc. , will all cause coking pulverized coal airflow crashed into the wall.

4. Coking is related to air leakage of boiler equipment.

The air leakage in the furnace and the air leakage in the pulverizing system increase the air flow into the furnace, reduce the temperature level of the combustion chamber and delay the combustion process. The air leakage of the cold ash hopper will raise the center of the flame, and the flame will be lengthened, which will lead to the increase of the smoke temperature at the outlet of the furnace, which is easy to cause screen coking. The air leakage of air preheater not only increases the power consumption of induced draft fan, but also causes part of the supply air to enter the flue, which is easy to cause the lack of air in the furnace.

Solutions to prevent boiler coking;

1, choose a reasonable operating oxygen.

Improve the oxygen content in boiler operation and avoid reducing atmosphere in boiler. Strengthen soot blowing in the furnace, especially in key areas, and increase the number of soot blowing. If the operating oxygen is still low, reduce the load appropriately if necessary. Because the main area of coking is at the furnace outlet, it is easy to block the flue, increase the flue gas resistance, and the output of induced draft fan is even less, so it is necessary to prevent the vicious circle of coking and reducing atmosphere. When the unit is overhauled, it is important to clean the air preheater to reduce the air duct resistance.

Increase the output power of the fan.

2, choose a reasonable furnace outlet temperature.

Through on-line monitoring of various operating parameters, such as furnace outlet flue gas temperature, superheated steam temperature, boiler load, combustion oxygen, furnace flue gas temperature, etc. It can also evaluate whether coking will occur at the outlet of boiler furnace, so as to prevent coking in boiler furnace when burning different kinds of coal and obtain maximum boiler efficiency.

3, the application of various operational measures to control the temperature level in the furnace.

During operation, increase the excess air coefficient, increase the air distribution uniformity, prevent the local reducing atmosphere from being caused by excessive local heat load, adjust the distribution uniformity of air powder in four corners, prevent the primary air flow from directly scouring the wall, and take load reduction operation when necessary.

4, the four corners of pulverized coal concentration and burner air distribution should be as uniform as possible.

The uneven distribution of pulverized coal quantity in pulverized coal nozzle will inevitably lead to local hypoxia and uneven load distribution in the furnace, and the coking situation in the furnace will deteriorate under the condition of insufficient combustion air. When the air distribution of the burner is uneven, or the boiler load is reduced, and the burner can't run or fall off, the flame center in the furnace will be skewed. When running, try to level the air volume at the four corners to avoid missing corners.

5. Properly increasing the primary air speed can reduce the coking near the burner.

Increasing the primary air speed can delay the ignition of pulverized coal, make the ignition point farther away from the burner, and the high temperature area of the flame will move to the center of the furnace accordingly, thus avoiding additional coking at the nozzle. It can also increase the rigidity of the primary air jet, reduce the deflection caused by static pressure on both sides of the jet, and avoid coking caused by direct scouring of the wall by primary air.

6, furnace outlet temperature field should be as uniform as possible.

Reducing the residual rotation and uniform temperature distribution at the furnace outlet can make the flue gas temperature in the dense convection tube bundle lower than the coking temperature. Reducing residual rotation by using secondary air backcutting.

Extended data

Coking will cause great harm to boiler operation and safety, mainly in the following aspects:

(1) After the large coke block of the boiler falls, a large amount of water vapor is generated instantly, resulting in a large amount of cold air leaking into the bottom of the boiler, which seriously worsens the ignition conditions of pulverized coal flame in the burner area (especially in the lower burner area), causing the negative pressure in the boiler to fluctuate violently (exceeding the limit), leading to the boiler extinguishing; If the coke melts into large pieces, it will fall from the top due to gravity. Causes the water wall of the cold ash hopper to break, resulting in load reduction or even boiler shutdown;

(2) When all water walls are coked, only stop the furnace for manual coke cleaning;

(3) Coking in the furnace leads to the increase of superheated steam temperature, superheated steam temperature and reheated steam temperature, and even leads to the explosion of steam-water pipeline; Coking will reduce the output of the boiler, which will cause forced shutdown in serious cases;

(4) Coking will shorten the service life of each boiler; The smoke exhaust loss increases and the boiler efficiency decreases;

(5) The power consumption of induced draft fan increases.

References:

Baidu encyclopedia-boiler coking