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Positive pressure combustion in boilers not only severely deteriorates the working environment but also causes significant damage to the boiler’s combustion equipment, often leading to malfunctions and shutdowns that disrupt normal production and daily activities. At present, negative pressure combustion is commonly used in industrial boilers in our country, with the negative pressure level generally kept within 2–3 mmH2O columns (a resistance value of 2 mmH2O columns, or 19.8 Pa, is usually adopted for calculations). However, during actual operation, many industrial boilers suffer from positive pressure combustion to varying degrees; in mild cases, ash and smoke are emitted from the furnace, contaminating the workshop environment, while in severe cases, the equipment can be damaged and operators may suffer burns. The 2 SHL20-13-AII type boilers put into use by our unit in 1990 often experienced positive pressure combustion during operation, and this phenomenon was particularly severe when the quality of coal was poor. When encountering positive-pressure combustion in a coal-fired boiler, it can be inspected and addressed from the following aspects. 1. In terms of operation, this leads to a positive pressure inside the furnace. During combustion, if the amount of flue gas expelled from the furnace equals the amount of flue gas generated by combustion, then there is an equilibrium of substances inside the furnace, and the pressure within it remains relatively constant. If the smoke exhaust volume is less than the amount of smoke generated by the fuel, it will inevitably cause a positive pressure inside the furnace. When the heat load increases, the air volume of the exhaust fan should be increased first, that is, the damper should be opened wider, and then the amount of coal burned and the air supply volume should be increased ； Conversely, when the load decreases, the amount of coal burned and the blast volume should be reduced first, followed by the exhaust volume. 2 Improper maintenance of the equipment, or equipment damage, can lead to positive pressure combustion inside the furnace. 1) Clogging and wear of the air preheater tubes are the main causes of positive pressure combustion in boilers. Once the pipes become blocked, the area available for smoke flow decreases and the resistance increases; when the number of blocked pipes exceeds 5% of the total number of pipes, positive-pressure combustion becomes inevitable. When the tubes of the air preheater wear out and cause air leakage, it creates a direct short circuit between the forced air and the exhaust air; with positive pressure on one side and negative pressure on the other, this results in a significant amount of exhaust air being diverted uselessly. By comparing the flue gas pressure changes at the inlet and outlet of the air preheater (by checking the records), it is possible to determine whether there is blockage or wear. Therefore, during shutdown for maintenance, it is essential to clear all blocked pipes ； If there is air leakage in the middle section of individual pipes, the ends of those pipes can be sealed off; however, the number of sealed pipes should not exceed 5% of the total number of pipes. If this proportion exceeds 1/3 of a group, the entire group must be replaced ； The tube ends suffer the most wear (at the flue gas inlet); installing protective sleeves on the tube ends can prevent such wear and also facilitates maintenance. 2) Ash accumulation in the economizer can also cause positive-pressure combustion in the boiler; the ash reduces the flow area of the flue gases, thereby increasing the resistance. Boiler economizers are generally equipped with purging and soot cleaning facilities. Regular purging and soot cleaning are effective measures to prevent ash accumulation in the economizers, and this should be done at least once a week. 3) For positive-pressure combustion that occurs after switching to wet dust removal methods (such as marble dust collectors, integrated dust removal and desulfurization equipment, etc.), it is necessary to first consider whether the flue gas contains water. The method is to compare whether the current of the exhaust fan is significantly higher at the same opening degree of the control valve ； Has the vibration of the exhaust fan increased? ； Is the impeller clogged with dust? ； When the impeller gets covered with ash, its dynamic balance is disrupted, which causes vibration in the exhaust fan and an increase in motor current; this leads to turbulent airflow and a reduction in the exhaust volume. In such cases, which usually occur within the equipment’s warranty period, it is necessary to contact the equipment manufacturer promptly to address the issue of incomplete separation of air and water. Secondly, relevant materials should be consulted or actual measurements taken to verify whether the resulting local resistance is much higher than before the change. 4) For older-type cyclone dust collectors, if the quality of the smoke deteriorates, the pressure loss increases and positive-pressure combustion occurs, it is likely that dust accumulates at the lower part of the outer cylinder of the cyclone collector, causing disruptions in the internal airflow and drawing the dust into the upward-flowing air. When the inner and outer cylinders of the dust collector are worn through by smoke and dust, or when the air-locking device is not airtight, although the pressure loss decreases, the flue gas bypasses its normal path; this not only reduces the dust removal efficiency but can also lead to positive-pressure combustion in the boiler. During maintenance and repair of the equipment, it is necessary not only to thoroughly clean off dust but also to check whether all sealing points such as flanges, dust discharge devices, and air-locking devices are airtight and free from leaks. Those with severe wear should be repaired and replaced promptly. 5) The quality of the smoke is poor, which prevents the furnace temperature from rising; as a result, the temperature of the smoke at the furnace outlet is also low, leading to an increase in the density of the smoke. The exhaust fan is designed to operate at an exhaust temperature of 180–200°C and a pressure of 1 standard atmosphere. When the exhaust temperature is lower than the designed value, the density of the smoke increases, causing the fan to operate under conditions beyond its design capacity ； At the same time, to meet external loads, it is necessary to increase the coal supply, which in turn increases the flue gas output. If the margin considered during fan design and selection is small, it becomes more difficult to establish a negative pressure in the furnace. Due to the positive-pressure combustion caused by coal quality, installing a stratified combustion coal feeding device can improve the furnace’s adaptability to coal. The moisture content of the coal fed into the furnace should also be controlled, as large amounts of water vapor increase the volume of flue gas generated in the furnace. The moisture content of coal generally should not exceed 8% to 12%; in case of rain or snow, coal from the dry coal storage area should be used. 6) Excessive dust accumulation at the bottom of the chimney, as well as failure to seal the inspection holes and dust cleaning holes at the back of the furnace in a timely manner, can also lead to increased resistance and short-circuiting of the exhaust airflow; these aspects should be carefully checked before starting the furnace. It should also be noted that efforts should be made to avoid running several boilers with positive-pressure combustion, or boilers with both positive- and negative-pressure combustion, simultaneously, as this can worsen the positive-pressure combustion. 7) In the event of a sudden positive pressure in the furnace for unknown reasons, it is necessary to first check whether the water wall and the heating surfaces of the economizer are damaged, in order to prevent the situation from worsening. 3. Positive pressure combustion inside the furnace is caused by issues related to design selection and installation. If a newly installed boiler continues to burn under positive pressure even after thorough inspection and adjustment during its 72-hour trial run, it indicates that the air volume generated by the exhaust fan is not sufficient to carry away the smoke produced by combustion; in such cases, either the exhaust fan needs to be replaced or the boiler’s output capacity has to be reduced. At this point, the reasons should be sought mainly in terms of design selection and installation. In our facility, the two SHL20-13-AⅡ type boilers used a Y4－73NO11D exhaust fan during design and installation; this fan was paired with a 75 kw motor, provided an air flow rate of 66,500 m3/h, and had a total pressure of 2,255 Pa. When the exhaust fan’s control valve was opened to its maximum position and the blower’s control valve was only opened to half its capacity, combustion occurred under positive pressure. Later, the fan was replaced with a NO12D model with a power output of 90 kw, an air flow rate of 78,200 m3/h (an increase of 17%), and a total pressure of 2,783 Pa (an increase of 23%); this allowed for combustion under negative pressure with ease. There are several reasons for this situation: 1) When selecting the fan, the impact of the fan’s own total pressure deviation ΔH was not taken into account; when ΔH is positive, the air volume delivered by the exhaust fan increases, while when it is negative, the air volume decreases. 2) The actual resistance of the pipeline differs significantly from the calculated value, resulting in a substantial reduction in the fan’s airflow. As can be seen from the general pipe flow equation H = KQ², when the actual value of K is less than the calculated value of K, the flow rate increases; whereas when the actual value of K is greater than the calculated value of K, the flow rate decreases. When selecting exhaust fans, experience is relied on instead of calculations, ignoring the differences in furnace structure among boiler manufacturers, as well as variations such as the length of the duct from the air preheater outlet to the chimney inlet, the number of bends, the methods used for dust and sulfur removal, and the cross-sectional area of the ducts when the installation location is limited. If these differences cause the actual K value to increase significantly, the air volume delivered by the exhaust fan will decrease greatly. This not only consumes the reserve factors for air volume and pressure considered during the selection of the exhaust fan but also results in an insufficient air volume. 3) As special equipment, boilers require that some installation units be thorough and responsible regarding the items that must be inspected and the pressure-bearing components, but they tend to be careless with the auxiliary systems. If the elbow is not manufactured in accordance with standards, or if the radius of curvature is too small, or if guide vanes that should be installed are not added ； The inner wall of the air duct is uneven ； The flanges are not installed parallel to each other ； Sealing with asbestos rope instead of using other materials can easily lead to air leakage or an increase in resistance (friction loss and local resistance). 4) Due to objective constraints, the actual resistance loss in the flue ducts is often higher than the designed value. Additionally, when multiple boilers in a boiler room share one flue duct and chimney for exhaust, each exhaust fan is effectively pushing gas into a pressurized space, which undoubtedly increases the resistance of the flue duct system as well. As time goes by and the equipment ages, along with wear and tear on the fans and air leaks in the ductwork, these factors inevitably lead to an increase in duct resistance. It is therefore advantageous to provide an appropriate margin in terms of airflow and pressure for the exhaust fans during design, which facilitates adjustments in future use. In practice, both the exhaust and intake control valves of the boiler are set at around 85%, which enables easy adjustment for stable operation at rated load; at this point, the negative pressure gauge can be maintained at 20 Pa, which is ideal for future operations. It should also be noted that a higher negative pressure in the furnace is not necessarily better; excessive negative pressure leads to increased air leakage into the furnace, thereby raising the heat losses associated with smoke exhaust ； Secondly, the power consumption of the fans increases ； 3. Fly ash exacerbates wear on the heat transfer surfaces along the flow path ； These four factors lead to incomplete combustion of coal, **reducing the thermal efficiency of combustion in the furnace. Therefore, the negative pressure should also not exceed 50 Pa. It can be seen that there are various reasons for positive pressure combustion in the furnace. To ensure the safe and economical operation of the boiler and avoid such combustion, it is necessary to take action in the following areas: improving the level of operational skills ； Strengthen the maintenance, repair, and upkeep of boilers and auxiliary equipment ； During design, it is necessary to appropriately increase the air volume and total pressure head reserve of the exhaust fan ； Try to use and design coal with a quality similar to that of the coal being used.