1. Causes of boiler coking
1. Coking is related to ash melting point
The fundamental reason of coking is that ash in molten state is deposited on the heating surface. It can be seen that the melting point of ash is the key to coking.
The slagging index of coal ash can be judged by its melting point temperature and main components. Usually, the ratio of calcium to acid, the ratio of silicon to aluminum, the ratio of iron to calcium and the silicon value in the ash composition can be used to judge its coking tendency, and the mass fraction of Na2O can be used to judge its pollution.
The melting point of ash is related to the chemical composition of ash, the properties of media around ash and the ash concentration. The chemical composition of ash and the content ratio of each component determine the melting point of ash. The melting point of ash is lower than the lowest melting point in its mixture. The lower the ash melting point, the easier it is for the boiler heating surface to coke. The melting point of ash is related to the properties of the medium around ash. When reducing gases such as CO and H2 exist in the flue gas, the ash melting point decreases by about 2℃. This is because reducing gas can reduce Fe2O3 with high melting point in ash to FeO with low melting point, and the melting temperature difference between them is 2 ~ 3℃. The melting point of ash is also related to the concentration of ash in flue gas. Under the same other conditions, the ash melting point will change with different ash content in coal. This is because the more frequently the components in ash contact each other during heating, the more opportunities there are for combination, decomposition and fluxing, and the greater the possibility of melting point reduction.
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, and the pulverized coal airflow is deflected and rubbed against the wall, which will often lead to serious coking of the boiler.
3. Coking is related to combustion adjustment.
Unreasonable combustion adjustment, such as too low primary air pressure, too low wind speed, too fine pulverized coal, early ignition, too high secondary wind speed, uneven distribution of air volume at four corners, uneven powder quantity of four-corner burner, etc., will all cause pulverized coal airflow to wipe the wall and cause coking. If the air distribution is unreasonable or the air volume is insufficient and the oxygen content is low during the operation of the boiler, a reducing atmosphere will be generated in the boiler. When the oil is injected and the combustion is stable, the upper oil gun is used, so that the heat load at the upper primary air is concentrated, and the local furnace temperature is high, reaching the ash melting point, resulting in coking of the boiler.
4. Coking is related to air leakage of boiler equipment.
Air leakage in the furnace and pulverizing system increases the air flow into the furnace, reduces the temperature level of the combustion chamber and delays the combustion process. The air leakage at the cold ash hopper will raise the flame center, and the flame will be elongated, which will lead to the increase of smoke temperature at the outlet of the furnace, which will easily lead to coking of the screen. Air leakage of air preheater not only increases the power consumption of induced draft fan, but also causes partial air supply to enter the flue, which is easy to cause lack of air in the furnace.
Second, the hazards of boiler coking:
1. Coking will cause the superheated steam temperature to rise, which will lead to the expansion of superheated steam temperature and reheat steam desuperheating water, and even lead to the explosion of steam-water pipes; Coking will reduce the output of the boiler, which will cause forced shutdown in serious cases; Coking will shorten the service life of boiler equipment; The smoke exhaust loss increases and the boiler efficiency decreases; The power consumption of induced draft fan increases; Because coking is often uneven, slagging on water wall will adversely affect the water circulation safety of natural circulation boiler and the thermal deviation of water wall of forced circulation boiler.
2. Coking is easy to form large pieces of slag, which makes it difficult to transport slag draggers and slag crushers, and sometimes leads to overload tripping. In serious cases, the slag ditch is blocked, and the operation has to be reduced.
3. If the coking is fused into large pieces, it will fall from the upper part due to gravity, resulting in breaking the water wall of the cold ash hopper. Low load will cause unstable combustion or even flameout due to large coke loss.
4. If all the water walls are coked, only stop the furnace for manual coke cleaning.
5. When the big coke of the boiler falls behind the slag catcher, a large amount of water vapor is generated instantly, which destroys the water seal of the slag catcher, and at the same time, a large amount of cold air leaks into the bottom of the boiler, resulting in a serious deterioration of the ignition condition of pulverized coal flame in the burner area (especially in the lower burner area), which makes the negative pressure in the furnace fluctuate violently (beyond the limit) and causes the boiler to extinguish the fire.
3. Measures to prevent boiler coking
1. In terms of operation
1) Choose reasonable operating oxygen.
The operating oxygen content of the boiler is the oxidizing or reducing atmosphere in the boiler, which has a great influence on the coking of the boiler. If the operating oxygen content of the boiler is low and the reducing atmosphere in the boiler is strong, the ash melting point of coal will drop and the boiler will be easy to coke. This is because the ash melting point decreases with the increase of iron content, and the influence of iron on the ash melting point is also related to the gas properties in the furnace. In the oxidizing atmosphere in the furnace, iron may exist in the form of Fe2O3, and its melting point decreases slowly with the increase of iron content. In the reducing atmosphere (insufficient oxygen) in the furnace, Fe2O3 will be reduced to FeO, and the melting point of ash will decrease rapidly, and FeO is most likely to form 2FeO? The ash melting point of SiO2 _ 2 is only 1 65℃.
When the coal quality fluctuates, the operators can't adjust it according to the actual situation, resulting in that the boiler combustion air distribution mode is not in an optimal state, especially the pulverized coal particles in the upper nozzle have poor burnability, and some large pulverized coal has not been burnt out at the outlet of the furnace, resulting in high flue gas temperature at the outlet of the furnace and serious coking, due to the large section of the furnace and small heat load; When the coal quality deteriorates, the adaptability of burnout performance of pulverized coal is not strong.
improve the operating oxygen content of the boiler to avoid reducing atmosphere in the boiler. Strengthen the 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 when necessary. Because the main area of coking is at the outlet of the furnace, it is easy to block the flue and increase the flue gas resistance, and the output of induced draft fan is even more insufficient, so it is necessary to prevent the vicious cycle of coking and reducing atmosphere. When the unit is overhauled, focus on cleaning the air preheater to reduce the resistance of the air flue and improve the output of the fan.
2) Choose a reasonable furnace outlet temperature
to carry out an optimized combustion adjustment test on the boiler, monitor the furnace outlet flue gas temperature (or the tube wall temperature of high-temperature heating surface) on line, and establish an online optimized operation guidance system on the premise of ensuring the main parameters are qualified; By reasonably adjusting the operating throttle opening and operating oxygen of each primary air and secondary air, the main parameters can be ensured to be qualified and the smoke temperature at the furnace outlet is lower than the melting point of coal ash, so as to prevent coking at the furnace outlet; Through on-line monitoring of various operating parameters such as flue gas temperature at furnace outlet, superheated steam temperature, boiler load, combustion oxygen and flue gas temperature, it is also possible to evaluate whether coking will occur at furnace outlet of boiler, so as to prevent coking in furnace chamber of boiler when burning different kinds of coal, and to obtain maximum boiler efficiency.
3) Ensure the good mixing of air and fuel, avoid forming a reducing atmosphere near the water wall, and prevent local serious dust deposition and coking.
when the location, wind speed and air volume of primary and secondary air are not designed reasonably, despite the large total air volume in the furnace, there will still be hot coke and volatilization in local areas, and there will be a local reducing atmosphere. When the oxygen content in the flue gas of pulverized coal boiler is lower than 3%, the CO content will increase sharply due to local hypoxia.
4) Various operation measures are applied to control the temperature level in the furnace.
high temperature in the furnace will vaporize or sublimate some volatile basic oxides in coal (above 14 degrees), and make alkali metal compounds condense on the heating surface (1~11 degrees). The alkali metal directly condenses on the heating surface to form dense and strongly cohesive ash. It can form the conditions to produce low melting point compound sulfate reaction in the initial ash layer, and also enhance the adhesion of the outer layer of ash containing alkaline compounds, and accelerate the development of ash deposition process. Coal ash is in a molten or semi-molten state, and the molten ash will directly stick to the heating surface, resulting in serious coking.
measures: increase the excess air coefficient in operation, increase the uniformity of air distribution, prevent local heat load from being too high and local reducing atmosphere from being generated, adjust the uniformity of air powder distribution in four corners, prevent the primary air flow from directly scouring the wall, and take load reduction operation when necessary.
5) A well-organized aerodynamic field in the furnace is the premise to prevent coking. When the ash collides with the furnace wall, if it remains softened or melted, it is easy to adhere to the furnace wall to form slagging, so the combustion center must be kept moderate to prevent the flame center from deviating and sticking
6) The pulverized coal concentration in the four corners and the air distribution of each burner should be as uniform as possible.
the uneven distribution of pulverized coal quantity at 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 load of the boiler is reduced, and the burner runs without corners or corners, the flame center in the furnace will be skewed. When running, try to level the air volume at four corners to avoid missing corners.
7) there should be appropriate fineness of pulverized coal.
if the pulverized coal is coarse and the torch is prolonged, the coarse powder will directly scour the heating surface due to inertia. Moreover, the combustion temperature of coarse pulverized coal is much higher than that of flue gas, and the melting ratio is high, which is easy to cause coking after flushing the wall. However, too fine pulverized coal will also bring problems. First, the power consumption is high, which will affect the pulverizing output. Second, the flue gas temperature at the furnace outlet will rise, which will easily cause coking.
8) 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.
increasing the primary air speed can also increase the rigidity of the primary air jet, reduce the deflection caused by the static pressure on both sides of the jet, and avoid the coking caused by the direct scouring of the wall by the primary air.
note that the increase of primary wind speed is limited by the ignition conditions of pulverized coal.
9) The temperature field at the furnace outlet should be as uniform as possible.
reduce the residual rotation at the furnace outlet, and the uniform temperature distribution can make the flue gas temperature in the dense convection tube bundle lower than the coking temperature. Application of secondary air backcutting to reduce residual rotation.
1) Mix and burn different kinds of coal.
coal blending can synthesize the ash and coke characteristics of blended coal to some extent. Coal ash with low ash melting point is still deposited on the heating surface, but solid ash with high melting point has a certain scouring effect on the heating surface, which reduces the deposition amount.
11) air distribution.
open the bottom wind at high load. Strengthen the soot blowing of the furnace to prevent the low-load ash falling from causing adverse disturbance to the boiler combustion.
2. Combined chemical decoking in combustion
According to the analysis of the coking situation of boilers in different power plants at home and abroad, experts have come to a unanimous conclusion: no matter how your boiler is designed and carefully adjusted in operation, no coal-fired boiler in the world can completely stop the problem of boiler coking! According to the present situation of scientific and technological development, chemical decoking in combustion should be the only better way at present.