Guide: Due to the poor working environment of coal-fired boilers and the long working duration, many problems are inevitable. Therefore, in the operation of coal-fired boilers, a comprehensive analysis should be carried out based on the actual situation, and the most reasonable treatment measures should be adopted for the problems that occur, so as to comprehensively improve the effective use of the boiler and maximize its effectiveness.
Due to the poor working environment of coal-fired boilers and the long working duration, many problems are inevitable. Therefore, in the operation of coal-fired boilers, a comprehensive analysis should be carried out based on the actual situation, and the most reasonable treatment measures should be adopted for the problems that occur, so as to comprehensively improve the effective use of the boiler and maximize its effectiveness.
Some common problems in the operation of coal-fired boilers:
1. Low-temperature corrosion generally occurs in places where the flue gas temperature is relatively high, such as the heating surface of the boiler tail, such as the air preheater economizer, steel pipes, dust collectors and induced draft fans. There are many factors that affect low-temperature corrosion. The more prominent ones are the sulfur content of the fuel, the coefficient of excess air, the quality of the boiler combustion conditions, etc.; low-temperature corrosion will cause equipment wear and leakage, and have a greater impact on the safe operation of the boiler. . Therefore, effective measures should be taken to prevent low-temperature corrosion during operation. Common measures mainly include using additives, increasing the wall temperature of the heating surface, low-oxygen combustion, and adopting new anti-corrosion materials. From the operating point of view, the use of low-oxygen combustion control methods, reasonable control of the oxygen content in the flue gas, hot air recirculation in winter, strengthening of soot blowing on the heating surface of the tail or use of new materials, etc., can play a certain role in alleviating low-temperature corrosion .
2. If the thermal protection device fails, it will cause the unit to trip. Although this expands the application range of the protection device to a certain extent, it is also the main reason why the new unit is prone to failure in the early stage of operation. Therefore, when equipment is installed, it is necessary to apply relevant control measures to ensure the matching between equipment installation, design and performance, which also plays an important role in shortening the running-in period. In particular, it is necessary to strengthen analysis and research infrastructure coordination and arrangements, but it should be noted that inadvertent protection devices and failure trends should be avoided as much as possible.
If the heating surface of the boiler water wall is damaged, hydrochloric acid will damage the condensate system of the water wall pipe in a low pH environment, and then under the condition of scale concentration, hydrogen atoms will directly enter the water wall pipe and directly reduce The intergranular strength of steel and methane formed by bonding and metal carbides. After hydrogen damage occurs, although the thickness of the water wall pipe will not be significantly thinned in the early stage, the problem of "open window" rupture will occur. Since there is no universal ultrasonic inspection technology, this will cause the metal position to become more and more fragile, which greatly increases the difficulty of troubleshooting.
3. In the process of thermal fatigue crack prevention, you can start with temperature control. Regarding the cracks in the recirculation pipe holes of coal burners, relevant departments have made clear regulations. It is necessary to start with water supply pipes and circulating pipes. While reducing the temperature of the pipes, strengthen protection of pipe sleeves, sockets, feed pipes, heating pipes and other pipes. The effective management of the joints avoids the phenomenon of alternating cold and heat to the greatest extent, which causes the insulation fatigue problem of the wall body and the steam drum. For the safety valve, the temperature difference caused by the long time required for the condensate in the inlet pipe to cool down, the pipe flow control, small pipes and other measures can be adopted to avoid the reverse flow of steam or high temperature as a whole to heat the joint again. Steam, and avoid directly spraying water on the head wall to cool down. By using these measures reasonably during the operation of coal-fired boilers, thermal fatigue damage can be effectively avoided.
4. With the gradual increase of steam parameters and water quality, water cycle accidents in small and medium-sized boilers occur from time to time, and with the gradual improvement of the automation control level, the current accidents such as fires and full water explosions in coal-fired boilers can also be effectively controlled. In addition, power plants have gradually begun to apply computer control, fully enclosed structure, supercritical parameters, and subcritical suspensions, which can also solve related problems to a certain extent. By understanding the current operating failures of coal-fired boilers in power plants in China, the following analysis is made according to their frequency distribution and severity: As a kind of thermal fatigue, low fatigue damage generally refers to the local thermal stress of the pressure element due to thermal expansion and the change of boiler parameters Or start-stop operation may cause fatigue damage. If the stress changes significantly, the local stress may reach the yield limit, which will cause low-cycle fatigue failure, and several alternating cycles will occur. In addition, when the boiler is turned on or off, the back wall pressure of the boiler will alternate, and low-cycle fatigue damage may also occur. However, from the current point of view, the probability of this failure is generally not high. As far as the failure principle is concerned, as long as there is a temperature difference between the boiler components, or the expansion coefficients of the connecting parts are different, and there are different expansion dead zones, a certain thermal stress will be formed. The key is the magnitude between the local yield and the thermal stress. difference. If only part of the pipe wall is heated during operation, there will be a temperature difference between the pipe walls, there will be a certain difference in the relative position of the pipe row in the operating state, or the pipe wall disturbance is not enough, etc., the head will be hot. Low-cycle fatigue damage occurs under the influence of stress.
5. After the coal-fired boiler of the power plant is put into operation, the brittleness of the pipe wall will gradually decrease, and then the failure problem will occur. Therefore, the relevant personnel need to check the pipe regularly and strengthen the macro or multi-phase corrosion test to determine whether hydrogen occurs on the pipe wall. damage. If it is determined that it is hydrogen damage during the inspection, the problematic tube wall should be replaced in time. If the problem is not serious, chemical cleaning can be applied to prevent the tube wall strength and thickness from continuing to decrease.
6. By analyzing the reasons for the failure of coal-fired boilers, it can be found that if you want to avoid boiler failures during operation, you need to start with all aspects of commissioning, installation, manufacturing, and design, and implement full process control. The power plant also needs to arrange special personnel to regularly inspect and maintain coal-fired boilers, conduct serious rectification of equipment, and conduct inspections of coal-fired boiler equipment in strict accordance with relevant regulations and requirements to fundamentally improve the safety and reliability of equipment. In the process of high-temperature flue gas heating, if the pipe wall is not properly cooled, its working temperature will not reach the design value within the specified time, resulting in overheating. If the overheating time is short, the strength of the material will be reduced, causing problems such as water circulation boiling, water circulation destruction, water shortage, pipe blockage, etc., and obvious shrinkage or expansion deformation will occur. Generally, the rupture of the pipe wall will have leaf-like edge characteristics. Among them, only when the temperature exceeds the phase transformation temperature Ac3, the ferrite of the austenitic steel will be transformed, and the tube wall will not be significantly thinned. If the overheating time is long, the microstructure and creep temperature of the tube wall will be changed, and the austenitic steel, graphite, molybdenum steel, carbon steel and other substances such as pearlite spheroidization or precipitation of carbides will appear, destroying the strength of metal crystals , Among them, in the high temperature creep rupture, there is generally no obvious thinning of the pipe wall thickness, but a thick lip problem.