Thread Content
In negative-pressure combustion boilers, if the furnace operates at positive pressure, smoke will escape outward, which not only wastes energy but also endangers the safety of the equipment and operators； Conversely, if the negative pressure in the furnace is too high, a large amount of cold air will leak into the furnace, lowering its temperature and increasing the load on the induced draft fan as well as the heat loss due to flue gas exhaust. Therefore, it is very important to keep the furnace pressure within an appropriate range. There are many factors that can cause fluctuations in furnace pressure; these will be analyzed in detail below. 1. Fault in the boiler desulfurization system; the desulfurization flue gas baffle came loose, resulting in positive pressure in the furnace. Handling: 1) If the automatic adjustment of negative pressure in the furnace chamber is not working properly, the automatic control of the supply and exhaust fans should be disabled, and manual adjustment should be used instead. 2) If, after adjustment, the positive pressure in the furnace still rises rapidly and reaches the protection activation value, the boiler’s flame-out protection should activate; if it does not function properly, an manual MFT should be initiated to prevent the high positive pressure in the furnace from damaging the equipment. 3) If the positive pressure in the furnace does not reach the value that triggers protection actions, one coal grinder should be shut down automatically. At this point, the unit will operate in a mode where it follows the boiler’s operations; as the fuel supply to the boiler decreases, the unit’s load will also decrease accordingly. Depending on the water level in the steam drum and the increase in furnace pressure, oil injectors can be used to shut down one coal grinder every 10 seconds, until a slight negative pressure is achieved in the furnace. During this process, it is important to adjust the primary air pressure to prevent surging in the primary air fan. 4) When reducing the load of a stopped coal mill, pay attention to monitoring the automatic tracking of the drum water level; if the water level changes significantly, the load reduction rate should be slowed down to prevent the activation of the high/low drum water level protection systems. 5) If fan surge occurs during this period, the automatic gradual reduction of the fan’s stator blades should be stopped until the fan surge disappears. 6) During the process of reducing the unit load, the shift supervisor promptly switches the boiler feedwater regulation to bypass control based on the load conditions, so as to maintain the pressure difference before and after it at a level meeting the requirements for attemperation water, thereby preventing any abnormalities in the temperatures of the main steam and reheat steam. 7) Once the negative pressure in the furnace is restored, a comprehensive inspection of the boiler body must be carried out immediately. Special attention should be paid to thoroughly inspecting each oil layer of the boiler and the water seal at the furnace bottom, to prevent ignition caused by high-temperature flue gases. In the event that ignition has already occurred, emergency fire extinguishing measures must be taken promptly, and the fire department must be notified. 2. Cold-start ignition of the boiler caused a sudden increase in furnace pressure to positive values. Preventive measures: 1) Ensure high-volatility coal is used as much as possible in the lower coal mills. 2) The plasma arc is normal. 3) The warm air heater of the plasma coal grinder is operating normally. 4) Ensure that the temperatures of the primary and secondary air at the exit of the air preheater are greater than 150–200 degrees. 5) If no flame detection is detected 180 seconds after the oil-free ignition of the plasma coal grinder, and the combustion condition as observed on-site is poor, the plasma coal grinder should be stopped immediately; an oil gun should then be used for ignition, and the plasma coal grinder can be restarted once the appropriate conditions are met. 6) In the absence of oil ignition, strictly follow the procedures for starting the first mill in the boiler. Only after the furnace temperature reaches a certain level should the pulverizing system be put into operation. 7) Thoroughly purge the furnace before ignition to completely remove any combustible materials from it. 3. Falling coke from the boiler causes fluctuations in the furnace negative pressure. Handling and prevention: 1) Carefully monitor the combustion process; when combustion becomes unstable, inject oil to aid combustion, thereby preventing flame extinction caused by the dropping of large coke pieces. 2) Regularly improve combustion adjustment and enhance soot blowing to reduce the likelihood of coking. 3) Adjust the coal-to-air ratio appropriately, use coal types with stable combustion properties, and minimize coking as much as possible. 4) Make appropriate adjustments to the operating mode (primarily the operating mode of coal mills). The specific principles for coal blending are as follows: when burning coals with a low ash fusion temperature, such coals should be fed to the lower-layer coal mills of each boiler (except for plasma coal mills). To reduce the temperature at the furnace flame center, when shutting down the mills, those in the middle layer should be stopped first; however, it is necessary to ensure stable combustion in the boiler. 5) Inspect for air leaks in the boiler, and address any leaks found promptly. 6) Conduct inspections of the water seal of the slag skimmer during boiler operation to ensure its tightness and stability ； Auxiliary control personnel should pay attention to monitoring the slag level in the slag skimmer, and use this information to assist in assessing the degree of coking in the boiler. 7) If the coking problem in the boiler cannot be resolved, descaling additives can be used if necessary. 4. Boiler fire suppression and RB actions cause fluctuations in negative pressure in the furnace. Handling: Upon boiler flameout, the induced draft fan automatically throttled down, and the furnace negative pressure was maintained within the permissible range. If the furnace pressure reaches -4 kPa, check that the induced draft fan trips; otherwise, manually trip it to prevent damage to the furnace. If the RB operates, check that the intake and exhaust fans automatically adjust to an appropriate range based on the load; otherwise, intervene manually. 5. Failures of the induced draft fan static blades and dampers cause fluctuations in the negative pressure in the furnace. Handling: 1) Adjust the output of the exhaust fan in a timely manner based on the furnace pressure conditions. If the stationary vanes of the exhaust fan get stuck, first limit the load; if moving the stationary vanes does not work, manually operate them on-site to balance the output of the two fans. 2) The actuator for the induced draft fan’s stationary blades came loose and could not be adjusted; thus, the faulty fan was shut down under reduced load. 3) Close the inlet and outlet dampers of the exhaust fan, and immediately reduce the load in accordance with the shutdown of the mill. 6. Ash blockage in the air preheater causes fluctuations in the negative pressure in the furnace. Treatment: 1) Strengthen soot blowing of the air preheater. 2) The load-increasing process slows down the rate of load increase. 3) If it is not possible to apply load, limit the load as necessary to prevent fan surge. 7. Leakage in the four tubes causes fluctuations in the negative pressure inside the furnace. Treatment: Leaks in the four pipes cause fluctuations in the negative pressure inside the furnace; therefore, the unit’s load should be reduced appropriately to maintain the allowable negative pressure level. If it is not possible to maintain this negative pressure, the furnace should be shut down immediately. 8. Fluctuations in the furnace negative pressure are caused by the air preheater and reburning in the tail flue. Treatment: 1) If the flue gas temperature rises, immediate measures such as adjusting combustion and soot blowing of the heated surfaces should be taken to reduce the flue gas temperature. 2) If combustion occurs in the preheater, soot blowing or fire-fighting water should be applied. 3) In the event that re-ignition occurs in the economizer, air preheater, etc. and cannot be controlled, or when the flue gas temperature rises to 250°C, the furnace must be shut down urgently. After shutting down the furnace, stop the intake and exhaust fans; keep the primary fan running, and tightly close all flue and air dampers. It is prohibited to open the fire viewing door and manhole door. 4) If it is detected that the air preheater has stopped operating, isolate it immediately and activate the soot blowing and turning device. If the baffle does not seal properly or the rotor disk fails to rotate, the furnace must be shut down immediately. 5) Confirm that the air preheater and the tail flue are on fire; use soot-blowing steam to fill the flue with steam through the fire-fighting pipeline, thereby isolating air and extinguishing the fire. 9. In winter, leaks and icing in the heating units cause blockages in the air ducts, and changes in the airflow rate of the blower and primary fan lead to variations in furnace pressure. Treatment: Determine whether ice formation and blockage are caused by a leak by checking the air pressure at the inlet and outlet of the heater; if such is the case, stop one half of the fans and isolate the relevant equipment to carry out de-icing procedures. 10. Excessive grinding of memory powder causes a sudden change in furnace pressure when grinding starts. Handling: 1) Appropriately increase the ventilation time before mill startup, and appropriately reduce the ventilation volume after startup. 2) Slightly reduce the furnace negative pressure before starting the mill. 11. Oil injection causes a sudden change in furnace pressure. Precautions: 1) Avoid using multiple oil guns simultaneously. 2) If the atomizing element of the oil gun breaks off, a large amount of fuel enters the furnace. Monitor the fuel volume during oil injection; if too much fuel is supplied, stop using that oil gun immediately. 12. Unstable combustion causes fluctuations in negative pressure in the furnace. Treatment: 1) Add oil to stabilize combustion based on the flame detection. 2) Try to ensure that the burners operate in a concentrated manner, reducing operation in separate layers. 3) The combustion is unstable; increase the load appropriately to enhance combustion. 4) The coal quality is poor; contact the shift supervisor to change the coal. 5) Adjust the secondary air, increase the outlet temperature of the coal grinder, and raise the coal powder concentration by reducing the amount of primary air. 6) If the adjustment is ineffective, shut down the coal mill whose flame detection is faulty. 13. Damage to the water seal at the furnace bottom causes fluctuations in negative pressure inside the furnace chamber. Treatment: 1) Automatically deactivate the exhaust fan, maintaining an allowable slight negative pressure in the furnace to minimize the entry of cold air into it; this prevents combustion from deteriorating and also avoids an increase in the exhaust fan’s output, thus preventing a vicious cycle. Based on the furnace pressure, appropriately reduce the unit load and enhance air flow regulation to prevent the activation of the low air flow protection mechanism; simultaneously, contact the ash removal personnel promptly to restore the water seal at the bottom of the furnace as soon as possible. 2) Pay attention to adjusting the main and reheat steam temperatures to prevent excessive overheating of the main and reheat steam as well as the heating surfaces; otherwise, the load should be reduced promptly. 3) Monitor the flue gas temperature carefully to ensure an appropriate air flow rate; it must not exceed 200°C, as the desulfurization system cannot handle higher temperatures. 4) Do not stay near the furnace for long periods while it is operating under positive pressure; once the furnace pressure returns to negative pressure, check the area around the furnace for any potential fire hazards. 14. Fan surge causes fluctuations in negative pressure in the furnace. Treatment: 1) Check whether the blade and static blade openings of the fan, as well as the current, are normal. 2) Change the fan control from automatic to manual. 3) Reduce the unit load based on the negative pressure condition. 4) Manually reduce the opening of the fan’s static blades to lower its output, thereby bringing the fan back into its normal operating range. 5) Monitor and adjust the parameters of the fan operating on the other side to prevent overcurrent in the fan motor or a sudden increase in negative pressure in the furnace. 15. Fluctuations in secondary and primary air flow cause fluctuations in the negative pressure in the furnace. Prevention and handling: 1) Investigate the reasons for fluctuations in secondary and primary air flow, and take measures to eliminate them. 2) Check whether a certain measurement point of the secondary air box pressure fluctuates; if so, switch the adjustment point to another one. 3) The purge pressures for the secondary air gauge tubes and primary air gauge tubes must be set to the current value to prevent large fluctuations.