Accident Handling 24: What are the causes of blockage at the slag discharge port of a circulating fluidized bed boiler furnace?

The reasons for the blockage of the slag discharge port in a circulating fluidized bed boiler furnace include: worn-out materials or slag masses that have fallen into the furnace, blocking the slag discharge port； The direction of the slag discharge nozzle at the slag outlet is skewed, preventing the bed slag from being discharged smoothly; or it acts as an obstruction. The slag discharge air pressure is too low to discharge the slag ； The flow rate or pressure of the fluidizing air at the lower part of the cold slag holder is too low, resulting in blockages or coking in the chamber, which in turn hinders the smooth discharge of slag through the slag discharge pipe.

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Common problems and analysis during the operation of circulating fluidized bed boilers: Boiler coking. The coking in circulating fluidized bed boilers is mainly divided into high-temperature coking and low-temperature coking. High-temperature coking: High-temperature coking occurs when the operating temperature is too high, causing the bed material to burn excessively vigorously and the temperature to rise rapidly. When the temperature exceeds the melting point T2 of the ash, high-temperature coking takes place. Low-temperature coking: Low-temperature coking occurs when poor fluidization causes local areas of the material to reach their ignition temperature. At this point, the air flow is sufficient to cause the material to burn rapidly, but it is not enough to ensure thorough mixing and movement of the material; as a result, the temperature in those local areas exceeds the ash melting point T2. If this situation is not addressed promptly, coking will occur. The main causes of high-temperature coking are excessive and rapid coal feeding during startup and normal operation, without timely increase in the volume of primary and secondary air; large fluctuations in coal and air supply, leading to an imbalance between the two; inadequate monitoring or improper adjustments that result in overheating of the bed material; excessive slag discharge that causes the bed layer to become too thin, thus leading to unstable bed temperatures; abnormal operation or blockage of the return devices in the return feeder; incomplete thermal control systems during operation; unreasonable instrument configuration; insufficient monitoring points; and reckless operation by the boiler operator. The main causes of low-temperature coking are insufficient primary air flow and localized failures. When operating at or below the critical fluidization air volume, no routine cold-state critical fluidization tests were conducted before ignition; there were numerous issues in the operation procedures. During operation, no analysis was carried out based on the bed pressure difference, nor was slag removed, which resulted in an overly thick material layer and a bubbly state of fluidization. Localized failures were caused by the detachment of the boiler’s refractory materials, extensive loss of such materials, or the presence of foreign objects inside the furnace, all of which disrupted the operation of the high-temperature return feeder or led to poor fluidization of the boiler bed. Additionally, many air nozzles were damaged or blocked, causing slag to leak into the air chamber and resulting in uneven air distribution. All of these will result in insufficient fluidization of the material, as well as overheating of the bed material and coking. Boilers are prone to low-temperature coking during shutdown. Since the bed material is at rest, if the boiler body and the smoke and air dampers are not airtight, especially if there is air leakage in the dense-phase zone, the combustible materials in the hot bed material receive oxygen and thus burn, releasing heat. Since the heat generated by combustion cannot be removed in time, it leads to local overheating, causing coking of the bed material. During the ignition process, adding too much coal and insufficient air volume for fluidization also cause localized overheating of the bed material, leading to coking. Note: Once coking occurs in a circulating fluidized bed boiler, improper operation can lead to rapid growth of the coke deposits, with the size of these deposits increasing faster and faster. Therefore, preventing coking as well as detecting and addressing it at an early stage are skills that operators must master. Phenomena of coking in circulating fluidized bed boilers: 1) A rapid rise in bed temperature ； 2) The oxygen level indication drops or even reaches 0 ； 3) Primary air current decreases ； 4) Furnace negative pressure increases ； 5) Induced draft fan current decreases ； 6) The bed material does not fluidize; combustion occurs on the surface of the bed ； 7) Difficult to discharge slag; sparks are ejected outward under positive pressure ； 8) When observing the flame, the local or widespread portion of the flame appears white ； Measures to prevent coking: 1. Ensure good fluidization conditions to prevent bed material deposition. 1) Maintain the critical fluidization air volume; before starting the boiler each time, it is necessary to carefully inspect the air nozzles and air chambers and remove any debris. During startup, a cold-state critical fluidization test should be conducted to confirm that the air distribution within the bed is even, fluidization is proper, and the bed surface is level. 2) Ensure the normal operation of the combustion system; the coal particle size must meet the design requirement of 0–20 mm. Continuously monitor the size of coal particles fed into the furnace and strengthen communication regarding coal control. 3) Strictly control the differential pressure of the material layer to ensure uniform slag discharge. Manual slag removal should be carried out promptly, with small amounts removed frequently. If lumps are present in the slag discharged, it should be reported to the furnace operator; after slag discharge is complete, the slag discharge door must be closed tightly. Regularly remove ash from the water-cooled air chamber and the return conveyor air chamber. Ensure that the water-cooled air chamber and the small air chamber of the return feeder are not clogged with ash. 4) Carefully monitor the temperature difference between the bottom and middle parts of the bed; if this difference exceeds the normal range, it indicates abnormal fluidization, with deposits or slag formation in the lower section. In such cases, increase the amount of primary air to boost the fluidization airflow, and open the hot slag pipe to remove the slag ； If it cannot be removed, the furnace should be stopped immediately for maintenance. 5) Strictly control the temperature at the lower part of the high-temperature cyclone and in the return feeder, and adjust the pressure and airflow of the return feeder blower as needed to ensure proper operation of the return feeder. II. Ignition process and adjustment of combustion conditions 1) During the ignition process, when the bed temperature reaches 450–500°C, a small amount of coal can be added (following the principle of \"small amounts multiple times\") to raise the bed temperature. If too much coal is added, incomplete combustion of the coal particles leads to an increase in the carbon content throughout the bed. Once the air supply is increased, vigorous combustion (deflagration) occurs, and the bed temperature rises rapidly, resulting in high-temperature coking of the entire bed. Therefore, when adding coal, it is necessary to strictly control the timing and amount of coal fed in, while paying close attention to changes in oxygen levels and bed temperature. If the bed temperature exceeds 1050°C, and it still continues to rise despite measures such as reducing the amount of coal and increasing the air supply, it is necessary to shut down the furnace immediately. The furnace can usually be restarted only after the bed temperature drops below 800°C. 2) When adjusting the load of the boiler, strictly keep the bed temperature within the allowable range. When increasing the load, add air first and then coal; when reducing the load, reduce coal first and then air. For combustion adjustment, adopt a frequent adjustment approach of \"small amounts in multiple times\" to avoid large fluctuations in the bed temperature. 3) During operation, it is necessary to closely monitor the recirculation of materials. Check whether the temperature of the material recirculation device is normal; if it exceeds the normal range by a significant margin, secondary combustion may have occurred. At this time, the return air volume should be increased to raise the ash solubility and the ash circulation ratio K, thereby improving the efficiency of the boiler. If the furnace pressure difference is too high, above 500 Pa, the temperature of the return conveyor will also exceed normal levels. It is necessary to discharge ash from the return conveyor when required; if the conveyor becomes clogged, the ash discharge valve should be opened to release the ash, while simultaneously increasing the amount of air supplied to the conveyor. If the fault still cannot be resolved, the furnace must be shut down for maintenance. 4) During normal operation, ensure optimal combustion conditions by keeping the oxygen content in the flue gas at the boiler outlet at a level of not less than 3%–5%. Adjust the ratio of primary to secondary air appropriately to maintain good combustion; generally, this ratio is around 6:4. This ensures sufficient mixing of air and coal for complete combustion, thereby reducing the carbon content in the combustible materials in the fly ash. It also prevents secondary combustion in the separator and return mechanism, which could lead to overheating, as well as minimizing mechanical and chemical incomplete combustion. By controlling the bed pressure difference within the normal range of 7–11 kPa based on the fluidization condition, a proper state of boiling and fluidization of the bed material is ensured, resulting in uniform temperature distribution, appropriate air supply, and no deviation in the flame center. III. Correct operations during fire suppression: During fire suppression, reduce the boiler load to its minimum, stop slag discharge, and maintain a high level of fuel in the boiler. Stop feeding coal and reduce secondary air. Maintain the bed temperature between 920 and 950°C; when the bed temperature shows a downward trend, stop the secondary fan, primary fan, exhaust fan, and return material fan. Quickly close the inlets and outlets of each fan, as well as the air and flue ducts, baffles, and gates to prevent air leakage. During the fire suppression period, strengthen the monitoring and analysis of the rate of bed temperature decline. IV. Improving the health status of operating equipment: The condition of operating equipment has a direct impact on the proper operation of fluidized bed boilers. Issues such as the detachment of refractory materials, extensive loss of such materials, or the presence of foreign objects inside the furnace can disrupt the functioning of the high-temperature return conveyor and prevent proper fluidization of the bed material. Additionally, damaged air nozzles, partial blockages in these nozzles, leakage of ash from them, an excessive amount of ash in the air chambers, damaged and deformed air distribution plates that cause air leaks, inaccurate bed temperature readings that are not repaired in a timely manner, incomplete thermal control systems, inappropriate instrument placement, insufficient monitoring points, and careless operation by the boiler operator are all major causes of coking in boilers. We need to take advantage of the boiler maintenance period to conduct a thorough inspection and repair of the refractory materials, air nozzles, thermal equipment, etc., inside the furnace. V. Changing the caking properties of coal: It is necessary to properly mix the coal fed into the furnace (anthracite and bituminous coal). The coal management team should promptly report on the analysis of different coal types during boiler operation, so as to achieve a scientific and even mixture. Changing the caking properties of coal has significant practical value for ensuring efficient operation and preventing coking in circulating fluidized bed boilers. During the operation of a fluidized bed boiler, good fluidization quality is key to preventing coking. It is also necessary to carefully adjust the ratio between coal supply and air volume during operation, while strictly controlling operational parameters such as bed temperature and pressure difference across the material layer. Coking in fluidized bed boilers can be avoided, thereby ensuring the safe operation of the boiler. Wear of the heated surfaces inside the furnace: Circulating fluidized bed boilers and other types of boilers differ significantly in terms of their combustion mechanisms; during normal operation, there is a large amount of material undergoing internal and external circulation within them. In addition, modern large-scale circulating fluidized bed boilers are equipped not only with water-cooled wall tubes inside the furnace, but also with expanded evaporation heating surfaces, screen-type superheaters, and screen-type reheaters in the upper part of the furnace. The expanded evaporation heating surfaces are also known as water-cooled screens or airfoil-shaped water-cooled walls, and they are added as needed to meet the boiler’s evaporation requirements. This makes the wear of the heating surfaces in circulating fluidized bed boilers one of the biggest threats to their normal operation. Statistics show that shutdown incidents caused by wear (of heating surfaces, refractory materials, air nozzles, etc.) account for nearly 50% of all shutdowns. Wear of the heated surfaces inside the furnace is mainly observed at the junction of the water wall tubes and the wear-resistant materials, within 1 to 5 meters above that junction, at the four corners of the furnace, above the return material inlet, at the inlet of the insulated cyclone separator, and at the lower end of the screen-type superheater. ? Hazards of wear on the heating surfaces inside the furnace: 1. Wear on these heating surfaces leads to leaks, and high-pressure steam-water mixtures cause intense erosion, resulting in further leaks in the adjacent water-cooled tubes. Sometimes it becomes difficult to maintain the water level in the drum; the temperature of the fuel bed at the location of the leak drops sharply, creating a large temperature difference between the two sides of the fuel bed, which forces the shutdown of the furnace. 2. It is quite difficult to deal with tube failures in the heating surface, and it takes a long time to organize personnel to clean the bed material and add new material to it ； The caking of moist bed materials prevents re-fluidization, and it can even lead to extensive blockages in the air nozzles; resolving such issues often requires significant additional effort and resources, making it one of the most troublesome problems for power generation companies. 3. Shutdowns due to wear of the heating surfaces inside the furnace result in an end to the continuous operation of the unit. The maintenance period after a shutdown is long: for leaks in the water wall of coal-fired boilers, repairs and restart are usually possible within 3 days, while for leaks in the water wall of circulating fluidized bed boilers, it takes at least a week to carry out repairs and restart; in cases of severe tube failures, the maintenance period can last 10 to 15 days. This reduces the unit’s effective availability rate, thereby compromising its economic viability. The wear mechanism of the water wall: The main causes of wall wear in the dense-phase region of the furnace and in the transition zone between this region and the water wall tubes are shown in the figure below. At the end of the dense-phase region, solid material forms vortex currents locally; the solid material flowing along the wall surface from top to bottom moves in the opposite direction to that of the solid material moving upward inside the furnace, thereby creating vortex currents locally ； Another reason is that the solid material flowing down along the furnace wall causes a change in flow direction at the junction area, thereby increasing the erosion on the upper tube walls of the overfilled region. ? The tube wall wear in the four corner areas of the furnace chamber is generally more severe. This is because the concentration of solid material flowing downward along the inner walls in these corner areas is much higher than that along the water-cooled walls on the four sides, and the flow pattern there is also disrupted. ? Irregular pipe walls include pipes passing through walls, bends at openings in furnace walls, welds on the pipe walls, etc ； There are also some test elements inside the furnace, such as thermocouples. Based on the actual performance in operational settings, even minor geometric irregularities can cause severe localized wear. ? Wear in the local area of the welds on the water wall tubes occurs first at the upper part of these welds. During shutdown inspections, the weld metal along the upper portion of the welds is eroded, resulting in an inclined shape. According to some references, wear also occurs on the tubes located above the welds in some boilers. This wear phenomenon is more severe in the dense phase region of the furnace. For thermocouples used to measure temperature, a sufficient depth of insertion is required to obtain the true temperature inside the furnace; however, such insertion causes significant disturbances to the local flow patterns, which in turn leads to wear on the thermocouple sheath or the adjacent water-cooled wall tubes. ? Wear that occurs easily on the water wall is often first observed at the weld joints of the water wall. This is because the lap welds were installed under the conditions typical of conventional coal-fired boilers, without taking into account the operating characteristics of circulating fluidized bed boilers. The welds during construction have protrusions (as required by the standards), which create conditions for the rapid cutting of solid materials flowing downward along the wall surface. The irregular surface of the welds makes cutting occur easily and worsen over time; under severe conditions, the water-cooled wall can be worn through in just a few hours. ? The reasons for wear on the walls of other irregular areas are similar; primarily, the irregular wall shape alters the flow properties of the material, resulting in greater cutting and erosion by the material that falls along the wall. ? At the furnace outlet, the sudden change in cross-sectional size causes a sudden change in the direction and speed of the airflow; the fine circulating material carried within the airflow then changes its direction of movement, resulting in intense impact and friction at this point. Anti-wear protection for the heated surfaces: Adopting an externally curved tube design in the transition zone where the anti-wear layer ends in the dense-phase area is a very effective method for preventing wear. Additionally, depending on the boiler’s capacity and specific structure, anti-wear coatings are applied to the water wall tubes within a certain range above the end of the anti-wear layer in that area. The transition on the pipe surface at the sprayed area must be smooth; there should be no protrusions. Anti-wear spraying is generally used to treat the transition zones at the locations where the screen-type superheater and reheater tube screens, as well as the lower anti-wear layers of the water-cooled evaporation screen, are arranged within the furnace. ? The anti-wear coating on metal surfaces prevents wear and corrosion for two reasons: First, the hardness of the coating may be greater than that of the substrate ； Second, at high temperatures, the coating forms a dense, hard oxide layer with better chemical stability; moreover, this oxide layer adheres more firmly to its substrate. Spraying techniques and material selection are highly developed in the machinery manufacturing industry. However, spraying techniques specifically for the internal environment of fluidized bed boilers are still not as mature; in some fluidized beds, after anti-wear spraying was applied, the coating began to peel off after just half a year of operation, requiring repairs. It is particularly urgent to find spraying materials and construction methods suitable for the operating conditions inside circulating fluidized bed boilers. ? For the butt welds of the water wall, the interior part of the furnace chamber must be fully ground and polished; there should be no abnormal protrusions to ensure a smooth transition. The openings on the water wall require careful consideration in the design process; efforts should be made to minimize the number of openings and to arrange their locations reasonably ； After the pouring material is applied, ensure it is level with the water wall surface on the fire-facing side. Wear of refractory and wear-resistant materials: Due to the operating characteristics of circulating fluidized bed boilers, a large amount of refractory and wear-resistant materials are used in these boilers. As these materials are in direct contact with the materials circulating within the furnace, wear is an inevitable phenomenon. Areas prone to wear include: (1) the air distribution plates of the water wall ； (2) Surface of the water-cooling walls around the lower part of the combustion chamber ； (3) The lower surface of water-cooling screens, superheater screens, etc. installed in the combustion chamber, as well as the surfaces of the water-cooling walls surrounding their openings through the walls ； (4) Surroundings of the combustor outlet and inner surface of the flue at the smoke outlet ； (5) Entire inner surface of the separator ； (6) Inner surfaces of the material legs and material return devices ； (7) Inner surface of the flue gas duct at the separator outlet ； (8) Inner surface of the inlet to the tail convection flue. ? The normal operating temperature of the castable for circulating fluidized bed boilers is 830°C to 930°C℃ ； Being exposed to a large flow of highly moving particles ； During the unit startup process, due to the rapid temperature rise, a large temperature difference exists between the inside and outside of the refractory material, which can lead to cracking and peeling of the surface layer, thereby increasing wear. Therefore, circulating fluidized bed boilers have high requirements for the properties of refractory and wear-resistant castables, which should exhibit high wear resistance, pressure resistance, and flexural strength ； Good heat shock resistance ； Good thermal shock resistance. ? When choosing wear-resistant and fire-resistant materials, it is necessary to seek information from various sources; opt for companies with good credentials, those that have a long history of stable production and research and development, and which can provide ongoing technical support after the installation of such materials. Uneven bed temperature refers to the condition in normal operation where the temperature differences between various temperature measurement points horizontally within the fluidized bed exceed 80°C. The reasons for uneven bed temperature include: 1. The particles in the bed are too large, the anti-wear lining in the furnace falls off, or large foreign particles enter the furnace and accumulate on the air distribution plate, resulting in uneven fluidization of the bed in certain areas or even no fluidization at all. 2. The resistance of the air distribution plate is too low, resulting in poor air distribution; this leads to inadequate fluidization of the bed material and uneven fluidization of the material across the horizontal direction of the bed layer. 3. The flow field inside the water-cooled wind box is poor; as a result, under high load conditions, the airflow passing through the distributor plate exhibits significant horizontal deviation along the bed layer, leading to uneven fluidization and backmixing of materials within the furnace. 4. The weighing system for the coal feeding amount of the coal feeder is malfunctioning, or there are differences in the quality of the coal supplied to each coal bin, resulting in uneven fuel distribution throughout the furnace and variations in combustion intensity. Consequences of uneven bed temperature: 1. If accumulation occurs due to poor local fluidization of the bed material, it can easily lead to coking within the bed. In severe cases, this will significantly affect boiler operation, and may even necessitate a shutdown of the boiler. 2. Uneven bed temperature can also lead to irregularities in heat transfer within the furnace. In severe cases, it causes uneven hydrodynamics in the water walls. If superheaters or reheaters are installed in the furnace, this may result in localized overheating of certain tubes. All these factors shorten the service life of the boiler’s heating surfaces. Particle size of coal fed into the furnace and drying issues: If the coal particle size is too large, 1. To ensure proper fluidization of the bed material, the primary air volume must be increased. This, in turn, leads to an expansion of the worn area at the lower part of the furnace and accelerates the rate of wear. 2. Excessively large particles can easily cause coking on the bed surface. If the coal particles fed into the furnace are too large, coupled with the presence of impurities, it will damage the air nozzles, resulting in poor fluidization in certain areas. The accumulation of material in those areas leads to an increase in bed temperature, which in turn causes coking. If other factors are added, such as an overly thick material layer or delayed slag removal, coking is more likely to occur. When coal with a low ash melting point is used, 3. Excessively large particles will result in a lower bed temperature, reduced circulation of material inside and outside the furnace, and the boiler cannot operate at full capacity. The coal particles are too small. 1. When the particle size of the coal fed into the furnace is too small, a large amount of coal ash resulting from combustion cannot form a bed with an appropriate particle size and remains in the dense phase region of the furnace, which can easily lead to an insufficient thickness of the bed layer in the boiler. 2. A large amount of ash particles are generated after combustion, increasing the wear on the heated surfaces at the tail section. 3. The bed temperature is too high, which may cause the temperature at the furnace outlet to be much higher than the temperature in the dense phase zone, posing a risk to the safe operation of the system ； The unit cannot reach its desired load level; the boiler’s coal feeding system is not functioning properly. The main reason for this issue is that the coal arriving is moist, with a high content of fine particles. Fine particles in the fuel tend to stick together easily when there is a high amount of moisture in the coal, resulting in blockages in the coal bins and feeders. Continuous caking reduces the effective volume of the coal bin, eventually causing the raw coal inside to form bridges that block the coal discharge opening. Blockages in the coal feeder mainly occur in the coal chute between the outlet of the coal feeder and the furnace; power plants in the southern region often experience blockages in these coal chutes during the rainy season. Secondly, if the belt of the weighing coal feeder shifts out of alignment, the cleaning chain is unable to remove the coal that has accumulated beneath the weighing device in a timely manner, which can also lead to blockages inside the coal feeder ； Furthermore, debris such as woven bags, branches, and steel bars present in the coal can enter the coal feeder, causing issues like tripping of the feeder, jamming, and scratches or damage to the belts. Therefore, regular inspections are necessary during the operation of the coal feeder, so that any issues can be identified and resolved promptly ； Strengthen the management of coal supplied to boilers; establish a reasonable fuel management system at the coal storage area based on actual conditions, to ensure a stable supply of fuel. The coal entering the facility should not be too wet or too fine ； Breakers, sifters, and iron removal equipment should be properly put into use; dry coal sheds or enclosed coal storage silos should be constructed if necessary ； Regularly operate the coal bin loosening machine and vibrator. Problems with slag discharge from the furnace and slag cooler: In a circulating fluidized bed boiler, the bed pressure needs to be maintained within a reasonable range; both too low and too high bed pressures can pose risks to the boiler, and in severe cases it may be necessary to reduce the boiler’s operating load. The means to control bed pressure is to discharge slag at the appropriate time. Reliable disposal of slag from circulating fluidized bed boilers is the foundation for the stable and efficient operation of such boilers, and it also serves as an indicator of controllable operation of CFB boilers. Therefore, selecting an appropriate slag cooler and ensuring its reliable, rational, and stable operation is a prerequisite for the smooth slag discharge from the boiler; it is also essential for the stable operation of the slag transfer system, which is responsible for transporting the slag out of the boiler. Common slag discharge methods and existing problems I. Wind-water combined slag cooler. The slag discharge from the boiler is controlled using a J-valve air control system, while the slag discharge from the cold slag holder is regulated by a rotary feed valve ； The selection chamber and the first cooling chamber use hot air cooling at 180°C via primary air, while the second and third cooling chambers are cooled directly by the cold air coming from the outlet of the primary fan ； Water-cooled tube bundles are arranged in the first cooling chamber and the second cooling chamber as part of the economizer. a. Existing problems: The amount of slag discharged is high, the temperature of the slag discharged from the slag cooler is too high, and the slag cooler often needs to operate intermittently ； Severe internal wear in the slag cooler ； Coking occurs at the selection chamber area of the slag cooler, resulting in its shutdown ； The slag cooler cannot operate continuously; it fails to meet the requirements of the unit’s operation. The boiler’s operation is constrained by the slag cooler, which often results in the boiler operating at reduced capacity or even shutting down. b. Cause analysis: The loss of control over boiler slag discharge, along with the failure to establish a balance between slag inlet and outlet in the cold slag holder, are the main reasons for the aforementioned problems. Since the design of the slag cooler is based on certain material balance principles, when the slag discharge from the boiler gets out of control (that is, when the inflow of slag into the slag cooler becomes uncontrolled), a large amount of hot slag enters the slag cooler rapidly. The cooling air and water available in the slag cooler are not sufficient to cool this hot slag to the desired discharge temperature. As a result, operators increase the volume of cooling air used, which exacerbates wear inside the slag cooler. When it becomes impossible to cool the hot slag, the slag cooler has to be shut down for cooling purposes ； When a large amount of slag is fed into the selection chamber, a large number of unburned coal particles will flow into this chamber along with the slag flow. In cases where the slag layer is thick, this can lead to localized channeling and areas where fluidization does not occur; as a result, combustible materials may reignite and coking may take place within the selection chamber, causing blockages at its slag discharge outlet. c. Measures to be taken: Control the amount of slag discharged from the boiler by regulating the air pressure acting on the boiler’s slag discharge valve, establish a balance between slag inflow and outflow in the cold slag holder, and maintain stable and controllable bed pressure in the selection chamber ； Control the particle size of the coal fed into the boiler to be within the specified range. II. Wind-water combined drum slag cooler: Drum-type slag coolers have been widely used in circulating fluidized bed boilers in China. These slag coolers can handle large variations in the particle size of the slag discharged from the boiler; it is simple to adjust the boiler’s bed pressure, continuous slag discharge is possible, and the slag discharge temperature meets the design requirements. The cooling water for drum slag coolers is usually condensate, which enables partial heat recovery. However, the use of wall-type cooling water causes heat loss from the slag, reducing the boiler’s efficiency. Problems that often occur during the operation of drum slag coolers: 1. If the cooling water for the slag cooler leaks during operation, contact between the high-temperature slag and water can lead to an explosion of the slag cooler; therefore, the slag cooler should be equipped with a protection mechanism against low pressure of the cooling water ； 2. There is a significant relative displacement between the cold slagger inlet and the slag discharge pipe in the boiler furnace; if the design of these two connections is improper, it can lead to slag spilling over, thereby deteriorating the working conditions at the site. Pay special attention here. 3. The movable joints at the inlet and outlet of the cooling water should be regularly checked for leaks; if the leakage is severe, the sealing packing should be tightened promptly or replaced. 4. When the boiler is in an accident condition requiring large amounts of slag discharge, attention should be paid to ensuring that the temperature of the slag at the outlet of the slag cooler, as well as the temperature of the cooling water at its outlet, do not become too high. Excessively high slag discharge temperatures cause the equipment located downstream in the slag removal system to operate in an overheated environment; when such equipment does not have sufficient heat resistance, it can be damaged ； If the temperature of the cooling water is too high, exceeding the saturated temperature corresponding to the pressure in the cooling water system, there is a risk of overpressure explosion in the cooling water system. The slag leakage problem in the furnace air distribution plates may be caused by: concentrated arrangement of return material, excessive return material in certain areas, low resistance of the air distribution plates, and poor quality of the air nozzles. Especially when the resistance of the air distribution plate is low, material leakage is likely to occur as the circulation volume in the furnace increases. When the amount of material in circulation is small, no leakage occurs. It is essential to take full account of the uniform distribution of the return material volume in the design of large CFB boilers, in order to avoid situations where the return material volume in certain areas becomes excessive. At the same time, attention should be paid to whether the structure of the wind cap is reasonable and whether its manufacturing and processing quality meets the standards ； Make sure that the resistance of the air distribution plate is within an appropriate range (usually around 4 kPa). If there is a problem of slag leakage in the air distribution plate, the slag leakage should be cleaned up when the boiler is shut down, and the water-cooled air chamber should be modified to add slag discharge outlets. The ultimate solution to slag leakage is to modify the resistance of the air distribution plate and improve the structure of the air nozzles so that the resistance is high enough. Return material anomaly – The return material system in circulating fluidized bed boilers. J-type return material structures are commonly used for the return material valves in such boilers. The valve body is divided into a descending section and an ascending section; the descending section stores the circulating ash separated by the separator, while the ascending section fluidizes this circulating ash and returns it to the furnace. The return material valve is essentially a small fluidized bed; a certain number of air nozzles are arranged at the bottom of the valve. The number of air nozzles in the descending section and the ascending section differs, so as to ensure that the air volume in the ascending section is more than twice that in the descending section. A wind chamber is still arranged under the wind cap to serve a pressure equalization function. During normal operation, the ascending section of the return valve remains in a slightly fluidized state. When the level of circulating ash exceeds the upper edge of this section, the ash automatically returns to the furnace. The bed material in the descending section, near the air nozzles, remains loosened and continuously moves upward to replenish the ascending section. In this way, the circulation process of the externally circulated ash is completed. ? When there is a shortage of circulating ash in the downcomer, there is also a shortage in the upcomer; as a result, the circulating ash fails to return to the furnace ； When the circulating ash in the descending section is at a high level, if the amount of fluidizing air in the ascending section and that in the descending section is reduced, the circulating ash in the descending section may become compacted and not remain in a loose state; this will also result in a lack of circulating ash to supply the ascending section, thereby stopping the backflow of material. Therefore, during normal operation, it is necessary to ensure that there is sufficient fluidizing air in the rising section and sufficient loosening air in the descending section, while the separator can properly separate the materials. ? Just like the furnace air distribution plates, if there are large foreign objects on the air distribution plate of the return valve, it will also cause abnormalities in the fluidization of this small fluidized bed, preventing proper return of material and resulting in the return valve not functioning properly. ? After the boiler is installed or repaired, a thorough inspection of the air chamber, air distribution plate, and air cap of the return valve should be carried out to ensure that there are no debris in the air chamber, that the holes in the air cap are not blocked, and that there are no foreign objects on the air distribution plate. ? During normal operation, the pressure in the return air main pipe should be closely monitored; both the pressure in this pipe and the volume of return air should increase as conditions improve and decrease as they deteriorate. It is ensured that the flow rate of the fluidizing air in the rising section of the return valve is greater than the flow rate of the loosening air for the return valve. In actual operation, it is necessary to gradually identify the key control points for the return valve that are suitable for this furnace. Reasons for the failure of fire-resistant and wear-resistant castables: 1) The temperature inside the furnace is subject to significant fluctuations over time, resulting in severe thermal shock ； 2) Vibration during operation and mechanical stresses caused by other factors lead to cracks and spalling in the fire-resistant and wear-resistant materials ； 3) Material erosion ； 4) The production, construction, and curing processes of fire-resistant and wear-resistant materials fail to meet quality requirements ；

1. Severe shedding of the wind cap near the taphole, resulting in poor fluidization or coking. 2. There are large pieces of debris near the slag outlet, such as metal sheets, large stones, rebar, bricks, and large chunks of casting material.

The reasons for the blockage of the slag discharge port in a circulating fluidized bed boiler furnace include: worn-out materials or slag masses that have fallen into the furnace, blocking the slag discharge port； The direction of the slag discharge nozzle at the slag outlet is skewed, preventing the bed slag from being discharged smoothly; or it acts as an obstruction. The slag discharge air pressure is too low to discharge the slag ； The flow rate or pressure of the fluidizing air at the lower part of the cold slag holder is too low, resulting in blockages or coking in the chamber, which in turn hinders the smooth discharge of slag through the slag discharge pipe.