Factors affecting slag formation on the heating surfaces of boilers

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The slagging problem in boilers is a common issue in coal-fired power plants. The so-called \"slagging\" refers to the accumulation of molten ash on the heated surfaces of boilers. Essentially, it is a process in which high-temperature flue gases in the boiler carry unburned coal particles that are in a molten or partially molten state; upon encountering the cooler wall surfaces, these particles cool and solidify, forming deposits. Slag formation in boilers is a very complex process involving numerous factors. It is related not only to the composition and physical and chemical properties of the coal used, but also to the design parameters of the boiler (such as the arrangement of burners, the thermal load in the furnace, the aerodynamic structure inside the furnace, the flue gas temperature at the furnace outlet, the location of the superheaters, the flow velocity and temperature of flue gas in various sections, and the negative pressure in the furnace). Additionally, it is influenced by the operating conditions of the boiler (such as changes in load, the excess air coefficient, the fineness of the coal powder, the control of combustion temperature in the furnace, the method of air distribution, and the control of the aerodynamic field of combustion inside the furnace). Generally speaking, these factors can be divided into two main categories: innate factors, such as the characteristics of the coal being burned and the design parameters of the boiler ； The second are external factors, such as the operating conditions of the boiler. Therefore, when analyzing and solving the slagging problem in boilers, it is necessary to consider these two aspects in order to determine the main factors causing slagging in the boilers. 1. The impact of coal quality characteristics on boiler slagging: A significant deviation between the actual coal quality and the designed coal quality is one of the main reasons for slagging in the furnace. The melting properties of ash are an important indicator to determine whether slagging occurs during combustion, and ashes from different coal types have varying compositions and melting properties. Furthermore, if the ratio of basic to acidic oxides in the ash, that is, the basic-acid ratio, is high, then this type of coal is prone to slag formation. 1.1 Among the four characteristic temperatures of ash fusibility, the softening temperature ST is generally used as a representative value for the ash fusion temperature. Generally, ST is considered to be 1350°C; this is a threshold value. Above 1350°C, boilers are less prone to slagging. The higher the softening temperature ST, the lower the likelihood of slag formation. Conversely, when ST is below 1350°C, the boiler is prone to slag formation; the lower the softening temperature ST, the greater the likelihood of slag formation, and thus the more severe it becomes. Based on the melting temperature of coal ash, it is generally classified into four categories: easily fusible, moderately fusible, difficultly fusible, and non-fusible. The melting temperature ranges are as follows: for easily fusible ash, the ST value is below 1,160°C; for moderately fusible ash, the ST value lies within the range of 1,160°C to 1,350°C ； Refractory ash, with an ST value in the range of 1,350°C to 1,500°C ； No fusible ash, with an ST value higher than 15°C. When examining the fusibility of coal ash, special attention must also be paid to the atmospheric conditions under which the fusibility is measured. Since the iron in coal ash exists in different oxidation states under various atmospheres, in an oxidizing atmosphere it is in the +3 oxidation state, with a melting point of 1,565°C. In a reducing atmosphere, iron is in its metallic state, with a melting point of 1,535°C. In a weakly reducing atmosphere, iron is in the +2 valence state, and its melting point is 1,420°C. 1.2 The sulfur content and ash content in coal: The slagging index of ash depends on the ratio of basic oxides to acidic oxides, as well as the sulfur content in the coal. The lower the ratio of basic oxides to acidic oxides in coal ash, and the lower the sulfur content in the coal, the lower the boiler slagging index value. The ratio of basic to acidic oxides in coal ash remains stable, while the slagging index is determined by the sulfur content in the coal. Therefore, a low sulfur content in coal is very beneficial for preventing slag formation in boilers. If the ash content in coal is too high, its amount in the furnace is large; once sludging occurs, the amount of slag generated is also large, and thus the hazards associated with sludging become greater. At the same time, a high ash content in coal means that its calorific value is low; as a result, the coal powder may not burn completely, leading to incomplete combustion and increased heat losses. Reducing gases are likely to be generated inside the furnace, which lowers the ash melting temperature and contributes to slag formation or exacerbates its severity. Coal powder boilers in power plants should also avoid using high-quality coal with too low an ash content and high calorific value, as this can cause the furnace temperature to rise excessively, resulting in slag formation in the boiler. An important measure to reduce slagging in boilers is to appropriately reduce the load in order to lower the temperature inside the furnace. 2. Influence of boiler operating conditions: The operating conditions of the boiler are also one of the important factors affecting slagging inside the furnace. These primarily include pulverized coal, boiler load, excess air coefficient, furnace temperature level, and flame impingement on the furnace walls, among others. 2.1 Fineness of pulverized coal: If the pulverized coal is too coarse, it is difficult for it to burn completely. This increases the heat loss due to incomplete combustion and reduces the boiler efficiency. Second, an excess of reductive gases may be produced, causing the ash melting temperature to decrease and promoting slag formation. Therefore, stable operation of the coal grinder, uniform coal supply by the coal feeder, and appropriate coal particle size are important measures to prevent slag formation in the boiler. It’s not the case that the finer the coal powder, the better. If the coal powder is too fine, it will increase the energy consumption of the coal mill without improving combustion conditions; this is uneconomical. Power plants must monitor various indicators related to boiler operation in order to determine the optimal fineness of the coal powder, thereby reducing energy consumption while ensuring the safe operation of the boiler. There are three main categories of indicators used to predict the slagging tendency of boilers by taking advantage of various physical and chemical properties of coal ash: ash melting point type, ash viscosity type, and ash composition type. Each type, in turn, includes many indicators. Practice has shown that the prediction accuracy of each individual indicator is limited; however, by combining various types of indicators to assess boiler slagging, the accuracy can be **improved**. The comprehensive evaluation method using the slagging index can be used to determine the degree of slagging tendency for any type of coal. 2.2 The higher the load on the boiler, the greater the thermal load, and as a result the furnace temperature and flue gas temperature increase as well; this leads to more pronounced caking of the coal ash. The boiler load is also affected by the oxygen supply amount and the intensity of the secondary air swirl; if an adequate oxygen supply is not ensured at high loads, the excess air coefficient is low, and in a reducing atmosphere the ash melting point of the coal powder decreases, making slag formation more likely ； Excessive swirl intensity of the secondary air, along with an overly large expansion angle of the primary air, can lead to the formation of edge spurs. When the primary air stream brushes against the side wall water-cooled walls and the slopes of the cold ash hoppers, ash particles collide with the heated surfaces, resulting in slag formation. Similarly, if the swirl intensity is too low, the primary air stream will directly brush against the rear wall water-cooled walls, also causing slag formation. 2.3 Excess air coefficient: The amount of air in the furnace affects the flue gas atmosphere inside it. In a reducing atmosphere, the melting point of ash is 30–50°C lower than in an oxidizing atmosphere. For coal with a high iron content, if there is insufficient excess air in certain areas of the furnace and the coal powder is not mixed evenly with air, a reducing atmosphere may be created. In such an atmosphere, the coal powder cannot be fully oxidized, and the iron oxide present in the ash is reduced to ferrous oxide. Ferrous oxide forms eutectics with substances such as silica, which lowers the melting point to as low as 200°C. Moreover, in a reducing atmosphere, ash particles that are already in a molten state will solidify only when the temperature drops significantly lower than it does in an oxidizing atmosphere. If there is a lack of oxygen in the furnace and the coal ash particles are in a reducing atmosphere, then these particles melt more easily. Moreover, the melted coal ash particles are harder to solidify as they move toward the water wall, which leads to increased slag formation on the water wall. Furthermore, changes in air volume also have a significant impact on the temperature inside the furnace. 2.4 Furnace temperature: The higher the temperature in the burner area of the horizontal furnace, the easier it is for the coal ash to reach a softened and molten state, thereby increasing the likelihood of slag formation. There are also many factors that affect the temperature level in the burner area. For example, the heat load on the furnace cross-section, the wall heat load in the burner area, the calorific value of the fuel, its moisture content, and changes in the boiler load, etc. If the boiler is switched to use a similar type of coal with a higher calorific value, the increased heat release leads to higher temperatures in the burner area, thereby increasing the likelihood of slag formation. The higher the load of the boiler, the more coal powder is fed in, resulting in more heat generation and an increased likelihood of slag formation. The cross-sectional heat load in the furnace burner area and the wall heat load are two important parameters that characterize the furnace temperature level. Therefore, when predicting the slagging tendency of boilers, in addition to considering the influence of coal composition, the influence of also needs to be taken into account. The thermal load per unit area of the furnace burner or the heat load on the wall surfaces is too high; the large amount of heat generated by fuel combustion in the burner area is not absorbed by enough water-cooled wall surfaces, which leads to excessively high local temperatures in that area and results in slag formation there. Additionally, the residence time of the fuel and flue gases inside the furnace is too short, preventing complete combustion of the fuel, which in turn causes the flue gas temperature at the furnace outlet to be too high, leading to slag formation on the heat transfer surfaces at that outlet. 2.5 When the flame sticks to the wall in a furnace equipped with DC burners at the four corners, the coal powder airflow is affected by factors such as airflow stiffness, make-up air conditions, and the impact of airflow from adjacent corners; this results in slag formation. In a furnace equipped with swirl burners, when the swirl intensity is too high, it can cause the flame to move close to the wall ； Or if the swirl intensity of a particular burner is too low and the airflow range is too long, it may cause the airflow to strike the opposite furnace wall or collide with the flame on the opposite side, resulting in slag formation. 3. Influence of boiler design parameters The design parameters of a boiler include the furnace structure, boiler load, aerodynamic structure within the furnace, and furnace negative pressure, among others. 3.1 Furnace structure: In boiler design, a too small furnace volume or cross-sectional area can lead to high volumetric heat loads and heat loads in the burner area, resulting in excessively high furnace temperatures and slag formation on the heating surfaces. During the installation of the burner, its installation angle may not meet the design requirements. Alternatively, defects in the burner design could also lead to slag formation on the cold ash hopper slope and the rear wall water wall. 3.2 Boiler load: The higher the boiler load, the greater the heat load, which in turn raises the furnace temperature and flue gas temperature; as a result, the slagging tendency of coal ash becomes more pronounced. The boiler load is also affected by the oxygen supply amount and the intensity of the secondary air swirl; if an adequate oxygen supply is not ensured at high loads, the excess air coefficient is low, and in a reducing atmosphere the ash melting point of the coal powder decreases, making slag formation more likely ； Excessive swirl intensity of the secondary air, along with an overly large expansion angle of the primary air, can lead to the formation of edge spurs. When the primary air stream brushes against the side wall water-cooled walls and the slopes of the cold ash hoppers, ash particles collide with the heated surfaces, resulting in slag formation. Similarly, if the swirl intensity is too low, the primary air stream will directly brush against the rear wall water-cooled walls, also causing slag formation. 3.3 Aerodynamic structure inside the furnace: Firstly, what has a direct impact on slag formation is the distribution characteristics of the aerodynamic field within the furnace. For example, issues such as flame skewing in the furnace caused by an excessively large overall height-to-width ratio of the DC burner and a large tangent circle diameter, as well as the primary air-powder flow sticking to the wall, can all easily lead to slag formation ； The direction of the exhaust airflow changes after slagging or deformation due to burnout at the burner outlet, disrupting the normal aerodynamic field structure and exacerbating slagging in the high-temperature combustion zone. Furthermore, uneven air distribution in the four-corner fan ducts can also affect the combustion conditions inside the furnace and the atmosphere near the walls, leading to slag formation. 3.4 Furnace negative pressure: If the furnace negative pressure is too high, it indicates that the suction force of the exhaust fan is excessive. At this point, the airflow inside the furnace rises significantly upward, the flame center moves upward, and the smoke temperature at the furnace outlet increases, which can easily lead to slag formation in the superheater. 3.5 Temperature in the burner area: A high temperature level in the burner area, along with high flue gas temperatures at the furnace outlet, facilitates stable ignition, but it can also lead to slag formation ； The temperature level in the burner area is low, which helps to reduce slag formation and the amount of pollutants generated; however, this low temperature level makes it difficult to maintain a stable ignition. Furthermore, if the soot blower is not used for an extended period of time, an increase in ash accumulation on the heated surfaces may lead to slag formation ； When burning mixed coal, the properties of the ash and slag may also change. When coal with strong caking tendency and coal with weak caking tendency are burned together, the caking tendency of the mixture is lower than that of the coal with strong caking tendency. 4. Measures and countermeasures to prevent slagging: Based on the causes of slagging in boilers mentioned above, corresponding measures and countermeasures to prevent slagging are proposed from the perspectives of coal properties, combustion within the boiler, and operational management. The coal type should be selected so that its calorific value is comparable to that of the coal fed into the furnace; the fuel coal supplied by the power plant must meet the requirements specified in the boiler design in order to reduce slag formation inside the furnace. For coal types that may cause slag formation inside the furnace, the severity of this slagging phenomenon can be reduced by mixing in other coals that are less prone to slagging. Additionally, deslagging agents can be added to increase the ash fusion temperature of the coal, thereby transforming the glassy slag at high temperatures into a crystalline form and thus reducing or suppressing slagging. Adjust the combustion conditions inside the furnace; based on the actual burning situation of the boiler, determine an appropriate hypothetical tangent circle diameter to ensure that the installation angle of each burner is consistent with this hypothetical tangent circle. It ensures that the diameter of the combustion annulus, as well as the height and position of the flame center, are appropriate; the ignition distance is suitable; the primary and secondary air mix well; and an adequate supply of oxygen is available, resulting in a uniform temperature distribution across the horizontal cross-section of the furnace and at its outlet, with an average temperature that is not too high. Maintain an appropriate boiler heat load; do not operate it beyond its capacity. It ensures a reasonable distribution of the combustion area and smoke temperature field within the furnace, thereby preventing slag formation due to excessive heat load in certain areas. Maintain a moderate coal powder fineness. Depending on the actual coal type, the coal powder fineness is adjusted to an appropriate range by modifying the separator and the system ventilation volume. Since the separator cannot be adjusted frequently, when the volatiles in coal change, adjusting the primary air flow rate can be used as an auxiliary method to prevent slag formation and maintain stable combustion. In production, the selection of coal powder fineness should take into account various factors such as stable combustion, the risk of slag formation on the furnace walls and heat-exchange surfaces at the furnace outlet, mechanical incomplete combustion losses, and the power consumption associated with coal grinding. The calorific value, ash content, and total sulfur content in coal should not be too high, and the coal powder fed into the furnace should not be too coarse. Strengthen operation and maintenance management. The furnace heat load, temperature distribution in the combustion zone, dynamic conditions of the airflow within the furnace, coal powder fineness, oxygen content in the flue gases, and whether slag formation is occurring inside the furnace can all be determined by monitoring and observing the instruments in operation. Adjustments should be made promptly when abnormal conditions are detected, and any existing slag deposits should be removed immediately; this is an important way to prevent slag formation.