A large number of unburned combustible materials are deposited on the heat transfer elements, which is the necessary condition and key to the ignition of the air preheater. The heat transfer elements of air preheater are composed of thin plates, which are closely arranged, with high heating surface per unit volume, low metal temperature at its lower part, and the lowest in low load operation, which is easy to accumulate dust. 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, the condensed oil mist and unburned combustible materials brought in from the furnace accumulate on the heat transfer elements at the lower middle layer and the cold end of the air preheater, which is the fire source.
The main reasons for the accumulation of a large number of oil mist and unburned combustible materials on the air preheater are:
(1) There are many faults in the unit equipment, and the operation and commissioning personnel are inexperienced, resulting in too long commissioning period, too much oil burning and poor combustion;
⑵ Improper oil pressure or atomizing steam pressure, blocked steam holes and oil holes, and poor atomization;
(3) Improper air distribution;
(4) The igniter leaks oil;
5] Long-term low-load operation, more boiler start-stop times;
[6] The soot blower of air preheater failed to be put into operation effectively. During boiler start-up and low load operation, due to the low metal temperature at the cold end, the air preheater is easy to accumulate dust, so soot blowing should be strengthened at this time. However, due to some reasons, the steam pressure and temperature of soot blowing medium are insufficient and the superheat is low at the initial stage of start-up, so the soot blowing effect is not ideal.
fire precautions
Minimize the deposition of unburned combustible substances on the air preheater.
The key is to make the fuel burn completely. When oil is used for ignition and low-load injection, the oil temperature, oil pressure and steam pressure should be appropriate to ensure good oil atomization and correct gas distribution; The oil gun is not blocked and does not leak oil; Combustion is stable and sufficient to minimize unburned combustible substances.
Strengthen the monitoring of air preheater
Before starting the unit, check the air preheater thoroughly. If it is found or judged that there are more unburned combustible materials accumulated in the air preheater, it must never be put into hot operation, and it must be cleaned with water before drying. Ensure that the soot blower can be put into normal operation and the parameters of soot blowing medium are qualified.
In the process of operation, especially after the air preheater is started, hot standby, suddenly stopped and stopped, the terminal temperature of the air preheater (inlet and outlet temperature of flue gas of the air preheater) should be closely monitored. If one or more of the temperatures rises abnormally, it should be analyzed and studied immediately so as to find the fire immediately. For example, when the exhaust temperature exceeds the normal value by about 30℃, it means that there may be a fire. If it continues to rise, it is considered a fire. If the temperature continues to rise, it will definitely catch fire. When the boiler is stopped, the air preheater has stopped rotating, and the inlet smoke temperature remains unchanged or changes little, while the outlet smoke temperature rises slowly first and then sharply, which also indicates that the air preheater is on fire. Operators should monitor the furnace flame and observe the chimney regularly. After the air preheater rotor stops running, observe the driving motor current.
Experience shows that the fire is most likely to occur within a few hours after the boiler is stopped, and at this time, most operators have evacuated from the site, and the left-behind personnel often do not pay attention to the terminal temperature of the air preheater. When a fire is suddenly discovered, it has generally burned more than 1 hour, which increases the loss. Therefore, being familiar with the performance and operation procedure of the air preheater is the premise of monitoring the air preheater.
Install a fire detection system
Infrared fire detection system: several infrared probes are installed at the inlet or outlet of the air side of the air preheater, and the probes move linearly. When the preheater rotates, it takes about 1 0 minute to scan all heat transfer elements1time. If the air preheater catches fire locally, and the infrared ray emitted exceeds the normal level, the system will send out an alarm signal. The advantages of this system are high sensitivity and early fire detection. At this time, a lot of water is put into the fire immediately, and the fire can usually be put out soon.
Effective soot blowing
A large amount of ash accumulation will not only easily cause fire, but also reduce the heat transfer efficiency and increase the resistance of air preheater. Because the cold end of the air preheater is the easiest to accumulate dust, a soot blower is usually installed at the flue gas side of the outlet of the air preheater. In order to prevent fire, blowing must be effective. During normal operation, it is usually blown for 8 hours 1 time; Blow/kloc-0 every 4 hours when starting; Blow before stopping the furnace; During the ignition period, the number of soot blowing should be increased when running at low load for a long time or seriously blocking ash.
At the initial stage of start-up, if the parameters of soot blowing steam cannot meet the requirements, compressed air can be used for purging.
Proper water cleaning
The soot blower of air preheater has limited function. For example, the heat transfer element at the lower part of the middle layer of air preheater is easy to block ash, and the soot blower can't be removed generally. Therefore, it is also important to carry out correct water cleaning. In addition, the scaling of some combustible materials can only be removed by washing with water. General water cleaning cycle is 1~2 times/year. After water cleaning, the air preheater must be dry, otherwise the air preheater will be quickly blocked by corrosion.
6. Increase the fire water system.
On the flue gas side, the primary air side and the secondary air side of the air preheater, annular fire water pipes are installed, and dozens of nozzles are installed on the pipes.
Ignition treatment of air preheater
The fire of the air preheater often originates from the heat transfer elements at the lower part of the middle layer and the cold end. It takes a long time from local minimum area ignition to large area combustion, generally 1~2h, sometimes longer. In order to reduce the loss, judge whether there is a fire as soon as possible. Usually we start with the terminal temperature of the air preheater, and sometimes we observe whether there is fire through the observation hole at the air outlet of the air preheater. If there is a fire, we should 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 operated manually 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. The air preheater keeps rotating, the fire water and multi-nozzle cleaning water pipes are put into fire immediately, and the drainage on the flue is opened at the same time. Experience shows that it is ineffective to extinguish the flame with foam, chemicals or steam, and it is very important to ensure the fire water quantity at this time. After the air preheater catches fire, do not open the air inlet to prevent air from entering the combustion-supporting device. As a last resort, if there is no water input due to the failure of fire water and cleaning water pipes, you can open the manhole and use the fire water of the power plant to extinguish the fire. Pay attention to the use of fire must be thorough, and carefully check, and never leave any future trouble.