Furnace coking is a common combustion anomaly in coal-fired boilers. Due to the heat-absorbing effect of water cooling wall, the flue gas temperature decreases gradually outward along the furnace center. When coal ash particles meet the heating surface, they suddenly freeze and stick to the heating surface, forming coking. The coking of boiler has a great influence on the safe and economic operation of boiler, and the most common one is water wall coking. The coking problem of water wall of boiler can be solved by modification of low nitrogen burner.
In view of China's current energy policy and the status quo of the coal market, in order to reduce the cost of power generation, most of the coal purchased by power generation enterprises are of low quality and inferior quality. Moreover, the coal quality varies in a wide range, and the actual burning coal is far from the designed coal, which makes the coking accidents happen frequently in the furnace.
When the boiler is coking in a large area, if the combustion condition changes, the coke block will fall off from the heating surface. Large area of coke will cause combustion instability, furnace pressure fluctuation, and even furnace fire extinguishing. In addition, under the action of high-temperature flue gas, a series of complex chemical reactions will occur between the coke slag adhering to the water-cooled wall and the pipe wall, forming high-temperature corrosion and increasing the occurrence of pipe explosion accidents of water-cooled wall.
Analysis of the influencing factors of coking
1.1Smoke atmosphere
The flue gas atmosphere, namely the oxidizing and reducing atmosphere in the furnace, is another important factor affecting the coking in the furnace. If the flue gas oxygen content in the furnace outlet is low when the boiler is in operation, the reducing atmosphere in the furnace is strong, the ash melting point will be reduced, and the water wall of the boiler is more prone to coking. This is mainly related to the form of iron in fly ash. Therefore, in reducing atmosphere, the furnace is easier to coking, when the content of iron in fly ash increases, the influence of flue gas atmosphere on ash melting point is more obvious.
1.2 Aerodynamic field in furnace
The water wall coking is easily caused by the unsatisfactory aerodynamic field, the deviation of burner jet Angle from the design value, the deviation of furnace flame center, flame sticking to the wall and the larger diameter of tangential circle. If the tangential diameter of the furnace chamber is larger than a certain value, the coking degree of the water-cooling wall will be intensified with the increase of tangential diameter, which is caused by the large number of semi-molten and molten particles will directly wash the water-cooling wall and adhere to the water-cooling wall. In addition, if the air distribution is not reasonable, coal powder and air gas cannot be well mixed in the furnace, resulting in incomplete combustion of coal powder in the furnace and water cooling wall coking [8-9].
1.3 Temperature field
The temperature field is one of the most important factors affecting the coking in the furnace. The research and analysis show that the degree of coking in the furnace increases exponentially with the increase of temperature in the furnace. When the temperature in the furnace is at a low level, the probability of coal particles softening or melting is very small, and coking is not easy to occur. If the temperature level in the furnace is high, but the temperature near the water cooling wall is relatively low, the coal ash particles have been well cooled before hitting the water cooling wall, and the probability of sticking to the water cooling wall will also decrease due to the decrease of their own temperature.
Analysis of the influence of the modification of low nitrogen burners on coking characteristics
2.1 Aerodynamic field in furnace
After the transformation of the low-nitrogen burner, no organized air leakage in the main combustion area was controlled. The primary air burner was replaced with a horizontal concentration-light separation pulverized coal burner on the opposite mound body, so that the primary air flow carrying coal powder was divided into two streams of air flow into the furnace chamber at a concentration-light ratio of 8:2. Thus, a layer of air film is formed between the primary air and the water cooling wall, forming a typical combustion form of "wind cladding powder", improving the oxidizing atmosphere in the water cooling wall area, so as to realize the purpose of restraining coking in the furnace.
2.2temperature field
The modification of low-nitrogen burner adjusted the air distribution mode and adopted the low-nitrogen combustion technology of air classification. The excess coefficient of air in the main burning area was kept at about 0.85, forming the under-oxygen combustion, making the temperature of the main burning area lower than before the modification, thus reducing the proportion of molten or softened coal ash. "Wind bag powder" form of combustion can be in a form a layer of air film between wind and water wall, making it nearly wall inside certain limits of temperature gradient is relatively flat, causing half melting, molten particles collide with the wall before fully cooling solidification, reduce its own temperature, adherence to the possibility of water-cooled wall decreases. It can be seen that the influence of low-nitrogen burner modification on the temperature field in the furnace is beneficial to prevent the water wall coking of the boiler.
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