Preventive measures for ignition of the hottest Ju

Preventive measures for the ignition of Junker air preheater

according to statistics, Junker air preheater can be shimmed at the rear of the spring (hereinafter referred to as air preheater) every 6 years of operation × There will be a fire in 106h, that is, one of about 800 air preheaters running every year will catch fire. However, in recent years, several large-scale coal-fired boiler Junggar air preheater fires have occurred in domestic thermal power plants, causing serious losses

1 analysis of ignition reasons

the deposition of a large number of unburned combustibles on the heat transfer elements is a necessary condition for the ignition of the air preheater, and it is also the key. The heat transfer elements of the air preheater are composed of thin plates, which are closely arranged, and the heating surface per unit volume is very high. The metal temperature at the lower part of the air preheater is low, and it is the lowest during low load operation, so it is easy to accumulate ash. In the initial commissioning stage of the boiler, under the conditions of ignition (oil ignition) and long-term low load operation (oil injection), due to incomplete combustion, condensed oil mist and unburned combustibles brought from the furnace accumulate on the lower middle layer and cold end heat transfer elements of the air preheater, which is the root cause of ignition

the main reasons for the accumulation of a large amount of oil mist and unburned combustibles on the air preheater are:

(1) there are many unit equipment failures and inexperienced operation and commissioning personnel, resulting in long commissioning cycle, excessive oil burning and poor combustion

(2) mutual benefit and win-win of oil pressure or atomized steam. The pressure is inappropriate, the steam hole and oil hole are blocked, and the atomization is poor

(3) improper air distribution

(4) the igniter leaks oil

(5) long term low load operation, many boiler starts and stops

(6) the soot blower of air preheater failed to be effectively put into operation. During boiler startup and low load operation, due to the low metal temperature at the cold end, the air preheater is easy to accumulate ash. At this time, soot blowing should be strengthened. However, due to some reasons, the steam pressure and temperature of soot blowing medium at the initial stage of startup are insufficient, the superheat is low, and the soot blowing effect is not ideal

2 fire prevention measures

2.1 try to reduce the deposition of unburned combustibles on the air preheater

the key is to make the fuel burn completely. When using oil for ignition and low load oil injection, the oil temperature, oil pressure and steam pressure should be appropriate, so that the oil atomization is good and the air distribution is correct; The oil gun is not blocked and does not leak. We focus on technological innovation and independent research and development of oil in the field of modified plastics; The combustion is stable and sufficient to minimize unburned combustibles

2.2 strengthen the monitoring of the air preheater

the air preheater should be comprehensively checked before the unit is started. If it is found or judged that there is a large accumulation of unburned combustibles in the air preheater, it must not be put into hot operation. It must be washed with water and then dried. Ensure that the soot blower can be put into operation normally and the soot blowing medium parameters are qualified

during operation, especially after startup and hot standby, as well as after sudden shutdown and boiler shutdown of the air preheater, the endpoint temperature of the air preheater (inlet and outlet temperature of flue gas and air of the air preheater) should be closely monitored. If the temperature at one or more points rises abnormally, it should be analyzed and studied immediately in order to find the fire immediately. For example, when the smoke exhaust temperature exceeds the normal value by about 30 ℃, it indicates that it may catch fire. If it continues to rise, it is considered that it has caught fire. If the temperature continues to rise, it must catch fire. When the boiler is shut down, the air preheater has stopped rotating, the inlet flue gas temperature remains unchanged or changes very little, while the outlet flue gas temperature increases slowly at first, and then rises sharply, which also indicates that the air preheater is on fire. Operators should monitor the furnace flame and regularly observe the chimney. After the rotor of the air preheater stops rotating, observe the current of the drive motor

experience shows that the fire is most likely to occur within a few hours after the boiler is shut down. At this time, most operators have left the site, and the left behind personnel often do not pay attention to the endpoint temperature of the air preheater. When the fire is suddenly found, it has generally burned for more than 1h, increasing the loss. Therefore, familiarity with preheater performance and operating procedures is the premise of monitoring the air preheater

2.3 install fire detection system

infrared fire detection system: install several infrared probes at the air side inlet or outlet of the air preheater. The probes move in a straight line. When the preheater rotates, it takes about 10min to scan all heat transfer elements once. If the air preheater catches fire locally, the infrared ray emitted exceeds the normal level and has developed into an area of 620000 square meters, the system will send an alarm signal. The advantage of this system is that it has high sensitivity and can detect the fire situation early. At this time, a large amount of water is immediately put out, and generally the fire can be put out soon

2.4 effective soot blowing

in addition to easy ignition, a large amount of ash deposition will also reduce the heat transfer efficiency and increase the resistance of the air preheater. Since the cold end of the air preheater is the easiest to accumulate ash, a soot blower is usually installed on the flue gas side of the air preheater outlet. In order to prevent ignition, soot blowing must be effective. Normally, it is blowing once in 8h during normal operation; Blow once every 4H during startup; Soot blowing shall also be carried out before boiler shutdown; During ignition, long-term low load operation or serious ash blockage, the number of soot blowing should be increased

at the initial stage of startup, if the soot blowing steam parameters cannot meet the requirements, compressed air can be used for purging

2.5 correct water cleaning

the role of the soot blower of the air preheater is limited. For example, the heat transfer element at the lower part of the intermediate layer of the air preheater is easy to block the soot, and the soot blower generally cannot be removed. Therefore, proper water cleaning is also very important. In addition, some combustible scaling can only be removed by washing with water. Generally, the water cleaning cycle is 1-2 times/year. After water cleaning, the air preheater must be dried, otherwise the air preheater will soon be corroded and blocked

2.6 add fire water system

circular fire water pipes are installed on the flue gas side, primary air side and secondary air side of the air preheater, and dozens of nozzles are installed on the pipes

3 air preheater ignition treatment

the ignition of air preheater often originates from the lower part of the intermediate layer and the heat transfer elements at the cold end. It takes a long time from local small area ignition to large area combustion, usually 1~2h, sometimes longer. In order to reduce the loss, judge whether it is on fire as soon as possible. Usually start from the end temperature of the air preheater. Sometimes observe whether there is a fire through the observation hole at the air outlet of the air preheater. If there is a fire, close the observation hole immediately. When the fire is serious, there is smoke leakage or obvious radiation, and the chimney emits black smoke

when the air preheater is found to be on fire, the MFT should be manually operated immediately, the induced draft fan should be stopped, the air preheater should be isolated, and the inlet and outlet baffles of the air preheater should be closed. Keep the air preheater rotating, put the fire water and multi nozzle cleaning water pipeline into fire fighting immediately, and open the drain on the smoke and air duct at the same time. Experience shows that the effect of smothering the flame with foam, chemicals or steam is not good, and it is critical to ensure the fire water volume at this time. After the air preheater is on fire, the inlet hole must not be opened to prevent air from entering the combustion supporting. As a last resort, if there is no water input due to the failure of fire water and cleaning water pipelines, the access hole can be opened to put out the fire with fire water from the power plant. Pay attention to putting out the fire thoroughly, and carefully check to avoid future troubles. (Wang renzhigaohua)