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About the prevention and treatment of boiler coking
The so-called boiler coking, is in the coal-fired, biomass-fired pellets or oil-fired boilers, local fuel accumulation in the burner nozzle, fuel bed or heating surface, in the case of high temperature and less oxygen, the formation of coking blocks.
A. What is boiler coking
The so-called boiler coking, is in the coal-fired, biomass-fired pellets or oil-fired boilers, local fuel accumulation in the burner nozzle, fuel bed or heating surface, in the case of high temperature and less oxygen, the formation of coking blocks. How is coking formed? Coking is a relatively common problem in boiler operation, in general, with the movement of flue gas slag particles, due to the furnace water-cooled wall heating surface with the flue gas is cooled, if the liquid slag particles in close proximity to the water-cooled wall or furnace wall before the temperature has been reduced and solidified, when attached to the wall of the heating surface, will form a layer of loose ash layer, the operation of the blowing ash can be removed. When the furnace chamber temperature is high, part of the ash particles have reached the molten or semi-molten state, if this part of the ash particles in front of the heated surface has not been cooled enough to reach the solidified state, has a high bonding capacity, it is easy to adhere to the flue gas flushing heated surface or furnace wall, and even reach the melted state, adhering to the molten or semi-molten state of the ash particles and unburned coke so that coking continues to develop.
In the combustion process, the fusible or easily vaporized material contained in the pulverized coal particles volatilize rapidly, into the flue gas in a gaseous state, when the temperature decreases, condensing, or adhering to the flue gas scouring the heated surface or furnace wall. Or it condenses on the surface of fly ash particles and becomes a molten alkali film, which then adheres to the heated surface to form the initial coking layer and becomes the condition for coking development. If the boiler bed temperature is too high, resulting in high slag temperature, reaching its softening point, usually 1040°C, the slag softens and coking is formed. The coking slag cools sharply and too quickly to form hard lumps, which are not easy to break and cause operational problems such as blocking the slag conveyor.
There is a large amount of ash in the fuel, ash at such high temperatures and a serious lack of oxygen in the furnace chamber, most of the ash will melt into a liquid state, or present a softened state. As the surrounding water-cooled wall constantly absorbs heat, so from the center of the burning flame outward around, the temperature is getting lower and lower. The closer to the water-cooled wall, the lower the temperature. As the temperature decreases, the ash is bound to change from a liquid state to a softened state and harden into a solid state. If the ash in a softened state, it will encounter the heating surface, due to the sudden cooling and direct hardening, bonded to the heating surface, so that the formation of coking.
Second, the formation of coking causes
There are many reasons for the formation of coking, the main ones are as follows. Let's analyze them.
1. The influence of fuel
The formation of coking is directly related to the fuel. If the quality of fuel is not good and the ash content is large, coking will be formed easily. On the contrary, it is not easy to form coking.
2. Influence of temperature in the furnace chamber
The higher the temperature in the burner area of the furnace, the easier it is for the ash to reach a softened state or to reach a molten state. This creates a source for coking. The higher the possibility of coking. The higher the temperature in the combustion area, the stronger the gasification of the part of the coal powder that is easy to volatilize. The conditions are created for coking by. Nanning Jinding Boiler Manufacturing Co.
3. The influence of wind-coal ratio
The impact of the fan on coking is mainly the induced, primary fan, the boiler's induced draft fan is used to enter the flue gas produced by the combustion of fuel in the furnace chamber sucked out and sent into the flue. Because the flue gas sucked out by the induced draft fan to transport is high temperature, and with a large amount of ash and impurities. So if the induced draft fan wind is not enough, there will be flue gas in the ash is not sucked out, it will be high temperature into a soft state and then become liquid, so as to create the conditions for coking again.
4. The influence of aerodynamic field characteristics
In the furnace chamber of the boiler. If the nozzle is not installed at the correct angle, and the air distribution will be unreasonable. This will lead to poor aerodynamic conditions in the furnace. This will cause the cutting circle of combustion to be too large, so that the center of combustion is off. High-temperature flue gas will wash the water-cooled wall surface. The slag in the flue gas in the water-cooled wall before it gets solidified, instant hardening surface and the formation of coking.
5. Coal powder concentration and fineness of the impact
The quality of coal powder for coal injection will also lead to coking.
6. Influence of heat load
The heat load of the chamber volume, the heat load of the chamber section, the heat load of the burner area, and the geometry of the chamber all have an effect on the coking of the boiler.
7. The influence of sootblower
Boiler sootblower is not put into use for a long time. The ash accumulation on the heated surface will gradually increase and also be soft and liquid by the high temperature and lack of oxygen, which leads to coking.
8. Coking is related to the melting point of ash
The root cause of coking is the deposition of ash on the heated surface in the melted state. It can be seen that the melting point of ash is the key to coking. The ash melting point temperature and the main components of ash can be used to determine the coking index of coal ash. Usually, the calcium-acid ratio, silicon-aluminum ratio, iron-calcium ratio and silicon value of ash components can be used to determine its coking tendency, and the melting point of ash is related to the chemical composition of ash, the nature of the medium around ash and ash concentration. The chemical composition of ash and the ratio of the content of each component determine the melting point of ash. Ash melting point is lower than the lowest melting point in its mixture. The lower the melting point of ash, the easier it is for the boiler heating surface to coke. Ash melting point is related to the nature of the medium around the ash. When there are CO, H2 and other reducing gases in the flue gas, the ash melting point is reduced by about 200 ℃. This is because the reducing gas can make the ash in the high melting point of Fe2O3 reduced to low melting point of FeO, the difference between the melting temperature of the two 200 ~ 300 ℃. Ash melting point is also related to the concentration of ash in the flue gas. Under the same other conditions, the ash melting point of ash will also change if the ash content in coal is different. This is because the components of the ash in the heating process, the more frequent contact with each other, the more opportunities for chemistry, decomposition, and flux, the greater the possibility of melting point reduction.
Third, the harm of boiler coking
What is the impact of coking on the boiler steam system?
① coking will cause high steam temperature: in the furnace chamber large area coking will make the furnace chamber heat absorption greatly reduced, the furnace outlet smoke temperature is too high, so that the superheater heat transfer reinforcement, resulting in high superheated steam temperature, resulting in superheater tube superheat.
② damage to the water cycle: after the local coking of the furnace, so that the coking part of the water-cooled wall to reduce the heat absorption, circulation flow rate decreases, and in serious cases, the cycle will be stalled and cause water-cooled wall tube burst accident.
③ lower boiler output: water-cooled wall slagging, will make the evaporation volume drop, become a factor limiting the output.
Boiler thermal efficiency decreases: after the heat surface coking, so that heat transfer deterioration exhaust temperature increases, the boiler thermal efficiency decreases; burner outlet coking, resulting in airflow deflection, combustion deterioration, may burn burner; so that the boiler ventilation resistance increases.
Affect boiler output: water-cooled wall coking will make evaporation drop; furnace exit smoke temperature rise, steam exit temperature rise, tube wall temperature rise, as well as the increase in ventilation resistance, may become a factor limiting the output.
Affect the safety of boiler operation: after coking, the superheater at the smoke temperature and steam temperature are increased, which will cause the tube wall overtemperature; coking is usually uneven, and as a result, the superheater thermal deviation will increase, which will adversely affect the safety of water circulation in natural cycle boilers and the thermal deviation of the water-cooled wall in forced cycle boilers; when the coking block falls from the upper part of the furnace, it may break the water-cooled wall tube of the cold ash hopper, causing fire in the furnace or blocking the And coking is easy to form a large number of large coke blocks, large pieces of coke slag falling in the cold slag pile can not be transported away in time so that the boiler is forced to shut down.
Coking will greatly shorten the service life of the boiler and its equipment. The loss of exhaust smoke increases and the thermal efficiency of the boiler decreases. The electricity consumed by the induced draft fan increases.
Fourth, the prevention of boiler coking
Boiler coking causes are multifaceted, to prevent or solve the problem of boiler coking should first identify the causes of coking, from many aspects, to solve. Specific measures to prevent and reduce boiler coking are as follows.
To have the right fineness of coal powder. Coarse coal powder, torch dragging long, coarse powder due to inertia will directly wash the heating surface.
Appropriate increase in primary air speed can reduce the coking near the burner. Increase the primary air speed can delay the ignition of pulverized coal, can make the ignition point farther away from the burner, the flame high temperature zone also pushed to the center of the furnace accordingly, can avoid additional slagging at the nozzle.
A good aerodynamic field in the furnace is a prerequisite for preventing coking.
The concentration of pulverized coal in the four corners and the air distribution of each burner should be as even as possible: the uneven distribution of pulverized coal at the coal spout is bound to cause local oxygen deficiency and uneven load distribution in the furnace, and the coking condition in the furnace deteriorates in the case of insufficient combustion air.
The most basic principle of preventing boiler coking is to eliminate the conditions and environment that produce coking.
V. Analysis of the causes of coking in circulating fluidized bed boilers
The direct cause of coking is that the local or overall temperature of the bed material exceeds the ash melting point or sintering temperature. When the overall temperature of the bed is lower than the ash deformation temperature, the coking caused by local overtemperature is called low-temperature coking. Low-temperature coking often occurs in the bed during start-up and compression fire, and may also occur in the ash hopper of high-temperature cyclones, as well as in external heat exchangers and return mechanisms. To avoid low-temperature coking, a good way is to ensure a good fluidization state and normal moving state of the bed material, so that the temperature is uniform and prevent local overtemperature. During the pressure fire of the boiler, the bed material is in a stationary state, if a small breeze is leaked into it, the combustible material in the hot bed material gets oxygen and will produce combustion. Due to the heat generated by combustion can not be taken away in time, so that the local area bed material overtemperature and coking.
High temperature coking refers to the coking phenomenon formed when the overall temperature level of the bed is high and the fluidization is normal. When the carbon content in the bed material is too high, if the air volume or return material volume is not adjusted in time to control the bed temperature, the bed temperature will rise sharply and exceed the ash melting point, then high temperature coking will occur.
Progressive coking is a form of coking that is difficult to detect in operation. It is slow growing, when the bed temperature and the observed fluidization quality are relatively normal. The main reasons for progressive coking are bad design and installation quality of the air distribution system, coal granularity beyond the design value, improper control of operating parameters, wrongly installed or blocked air cap, etc.
Sixth, the circulating fluidized bed boiler coking preventive measures
Circulating fluidized bed boiler coking once produced, it will grow rapidly, coke growing faster and faster, so the prevention of coking and early detection of coking and to be cleared is the principle of the operating personnel must master.
1. Ensure good fluidization conditions and prevent bed material deposition
Ensure that the fuel preparation system works normally and the coal feed size meets the design requirements.
Strictly control the differential pressure of the material layer and discharge the slag evenly. The use of manual slagging should be timely, to do less and more diligent, the discharge of slag with slag lumps should be reported to the furnace, slagging door should be closed tightly after the end of slagging.
Carefully monitor the temperature difference between the bottom of the bed and the middle of the bed, if the temperature difference exceeds the normal range, indicating that the fluidization is not normal, the lower part of the deposit or slag, at this time, you can open a large wind for a short period of time, blowing scorched blocks, and open the cold slag tube slagging; such as can not be cleared, should immediately stop the furnace maintenance.
Low-load operation, such as bed temperature suddenly dropped, in addition to broken coal, is likely to bed material deposition, then if you increase the amount of coal, but will intensify the deposition, so that the fluidization quality of the boiling bed becomes poor, resulting in local coking. When the bed material deposition is judged, the cold slag discharge pipe should be opened to release slag, and after the bed temperature is normal, it should be properly adjusted to run under higher load.
2. Strictly control the amount of coal feed during ignition
In the process of ignition, the general bed temperature reaches 500 ℃ or more can add a small amount of coal to improve the bed temperature. If adding too much coal, due to incomplete combustion of coal particles, the carbon content of the entire bed material increases, then once the air volume is increased, it will burn violently, the bed temperature rises quickly, and even exceeds the softening temperature of ash, resulting in the whole bed overtemperature coking. When the bed temperature exceeds 1050 ℃, although the coal reduction and wind measures, the bed temperature is still rising, at this time must immediately stop the furnace pressure fire, generally to bed temperature below 800 ℃ and then start.
3. Variable load operation strictly control the bed temperature
Variable load operation, strict control of bed temperature within the allowable range, so that the rising load first add wind after adding coal, reduce the load first reduce coal after reducing the wind, combustion regulation to do "a small number of times" adjustment method, to avoid a large rise and fall in bed temperature.
4. The correct operation of the pressure fire
When the pressure fire first stop coal, and then run a few minutes to stop the fan, during the pressure fire, be sure to tightly close the furnace door, all into the air door and slag door.
5. Carefully adjust the first and second wind
For high-temperature separator, ensure that the oxygen content at any time is not less than 3% ~ 5%, in order to reduce the content of fly ash combustibles, can prevent the separator and the return of material within the institution of secondary combustion and overtemperature. In operation, we should regularly check the situation of the return material and monitor whether the temperature of the bed of the return material is normal. If it exceeds the normal value a lot, it may be the occurrence of secondary combustion. At this time should increase the return air volume, open the ash discharge valve of the return bed to release ash. If the temperature is much lower than the normal value, it means that the recharger is clogged, then the ash discharge valve should be opened to release the ash, and the recharger air volume should be increased. If still can not eliminate the fault, it is necessary to pressure fire maintenance.
6. Boiler start-up period, the return device should be full of ash
During the boiler startup, the return device should be full of ash before putting in, otherwise the wind will backfire. At the beginning of the ignition, do not throw the return wind, to be the bottom of the fine ash in the return device should be full of return wind (generally half an hour after ignition), to ensure that the bed has material, otherwise, the bed temperature will be difficult to control.
7. Install the furnace chamber differential ash device and recharge booster fan
In order to monitor whether the recharge mechanism works normally and avoid coking or blockage, the furnace chamber differential pressure device and recharge booster fan can be installed. Furnace chamber differential pressure refers to the pressure difference between the upper interface of the combustion chamber and the outlet of the furnace chamber, which is a parameter to monitor whether the recharger is working normally. Generally, the differential pressure of the furnace chamber is controlled above 0.5kPa. If the differential pressure of the furnace chamber suddenly decreases during operation, it indicates that the material circulation is aborted and the recharger is blocked. As long as the differential pressure of the furnace chamber is carefully monitored during operation, the coking of the rebate mechanism can be prevented.
In order to improve the return air pressure head and ensure the good fluidization and moving state of the return bed, when the primary air pressure cannot meet the return requirements, it is very meaningful to install the return air booster fan to prevent the return coking.
8. Change the coking characteristics of combustion coal
Do a good job of matching the incoming coal, change the coking characteristics of coal, to prevent coking in circulating fluidized bed boiler has obvious practical significance.
9. Prepare well before starting operation
Before each boiler start-up, the air cap and air chamber should be carefully checked and cleared of debris, and when starting up, a cold fluidization test should be conducted to confirm uniform bed air distribution and good fluidization.
In the fluidized bed boiler operation, good fluidization quality is the key to prevent coking, while the operation should also carefully adjust the amount of coal, air volume, strict control of bed temperature and differential pressure of the material layer and other operating parameters.
Seven, how to deal with coking?
The first is to change the characteristics of coke slag, so that the coke block becomes loose, so that the coke slag is in the form of powder, so as to avoid the harm caused by large pieces falling off. Second is to increase the number of blowing soot according to the change of coal in time to quickly remove the coke on the heating surface of the boiler, to prevent the burst caused by coking tube walls. Again, the furnace is stopped, manual decoking.
Stabbing slag work, is a high risk of boiler operation professional safety work! Sparks flying around, more than 10 meters of steel flat steel burning red curved, accidentally will burn people!
Eight, concluding remarks
Boiler coking is a phenomenon affected by a variety of factors, it will bring serious impact on the safe and continuous production of boilers, but also the subject of many studies, as long as we study with care to exclude the main factors of coking formation, it will greatly reduce the formation of coking, thereby ensuring the safety, stability, economy and continuity of boiler production.
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