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Analysis and Diagnosis of Typical Failures of CDQ Boiler Sheng Junbo (Wugang Coking Company, Wuhan 430082) In the CDQ process, the CDQ boiler is the center of energy conversion and recovery, and is also one of the most important links to ensure the continuous and stable operation of the CDQ device. This article combines the operation conditions of Wuhan Iron and Steel Co., Ltd. No. 1 and 2 dry quenching boilers to analyze the mutual influence between the boiler and other links in the dry coke quenching process, as well as the causes of boiler tube damage, the judgment and treatment methods after boiler tube damage. 1 The impact of boiler tube damage on coke dry quenching The coke processing capacity of Wuhan Iron and Steel No. 1 and No. 2 dry quenching units is 140t/h, the design pressure of steam produced by the boiler is 3.9±0.2 MPa, the steam temperature is designed to be 450±10°C, and the steam generation is designed to be 76t/h. 1.1 Hazards of furnace tube damage. After the boiler furnace tube is damaged, steam and water with strong pressure accumulate in the furnace tube. If it is not discovered in time and the furnace is shut down for treatment, other furnace tubes of the boiler will be washed away and damaged. ； After the furnace tube is damaged, the water vapor will react with the red coke to produce a large amount of hydrogen, which will explode in the boiler and endanger the boiler and personal safety. ； After the furnace tube is damaged, water vapor will enter the bag warehouse along the dust removal pipe, causing damage to the dust removal bag. 1.2 Judgment method for furnace tube damage The causes of boiler tube damage include the furnace tube material, furnace tube installation quality, etc., as well as improper operation during the dry coke quenching production process. Various influencing factors are shown in Table 1. Table 1 Factors affecting boiler tube damage Category Influencing factors In terms of materials, local slag inclusions in the furnace tube, uneven furnace tube walls, cracks in the furnace tube, etc. In terms of installation, there are welding damage on the furnace tube, damage caused by pickling of the furnace tube, damage caused by the water pressure test of the furnace tube, etc. In terms of operation, the boiler feed water quality exceeds the standard, the boiler water quality exceeds the standard, the boiler inlet temperature exceeds the standard, and the boiler loses water. Boiler overpressure, long-term low temperature at the boiler outlet, coke powder erosion of the furnace tube, etc. After the boiler tube is damaged, the dry coke boiler, dry quenching furnace, and gas circulation system will show a series of abnormal phenomena, especially the hydrogen content in the circulating gas will rise sharply, the boiler inlet pressure will increase significantly, and the pre-storage section pressure adjustment bleed pipe will emit steam. It should be noted that when the water pipe of the water supply preheater is damaged and leaks, the water seal tank on the top of the dry quenching furnace is broken and leaks, or the water in the water seal tank enters the dry quenching furnace due to large pressure fluctuations in the pre-storage section of the dry quenching furnace, it will also cause the hydrogen content in the circulating gas to rise sharply. However, the overall phenomenon is obviously different from that of boiler tube damage. Wuhan Iron and Steel Coke No. 1 and Coke No. 2 dry quenching units experienced boiler tube damage in the early stages of construction. After the No. 1 dry quenching unit was in operation for more than two years, the water supply preheater water pipe was damaged, and both were dealt with promptly and effectively. See Table 2 for a comparison of the phenomena that occur after water enters the gas circulation system under several different circumstances. Table 2 Phenomenon after water inflow into the gas circulation system Boiler furnace tube damage Feed water preheater water pipe damage Water seal groove damage Hydrogen concentration in the circulating gas rises sharply√√√ Boiler steam generation significantly decreases The ash discharge valve of the secondary dust collector discharges wet ash √ The pressure in the pre-storage section fluctuates significantly √ √ The outer wall of the boiler peels off or changes color √ The dry quenching furnace inlet temperature is significantly reduced √ The coke discharge temperature is significantly reduced √ The boiler inlet pressure is significantly increased √ 1.3 Treatment after the furnace tube is damaged Once the boiler furnace tube is confirmed to be damaged, the first task is to stop the production of the dry coke quenching device to reduce other losses caused by the furnace tube damage, and then replace the tube or block the tube to deal with the damaged furnace tube. Under normal circumstances, the heat exchange area of the boiler should be designed with a 10% margin. If an individual furnace tube is damaged and it is difficult to replace, the tube can be blocked. But the best choice should be to replace the damaged furnace tubes to ensure the designed heat exchange area of the boiler. The damage to the water supply preheater pipe is the same as the damage to the boiler tube in some aspects. Water vapor will enter the dry quenching furnace with the circulating gas and react with the red coke to produce a large amount of hydrogen. Therefore, timely measures should be taken. However, the damage to the water pipe of the water supply preheater will not cause the suspension of dry quenching. Once it is confirmed that the water pipe of the water supply preheater is damaged, the inlet and outlet valves should be closed, and the bypass pipe should be used to supply water to the deaerator. Then, according to the actual situation, monthly or annual repairs of dry coke quenching should be used to deal with the damaged furnace pipe of the water supply preheater. For the problem of rupture and leakage of the water seal groove on the top of the dry quenching furnace, the production of dry coke can be temporarily stopped, and the cracked part of the water seal groove can be repaired by welding or the water seal groove can be replaced as a whole. Every time a boiler tube is damaged, the cause must be carefully analyzed and corresponding preventive measures must be taken. Generally speaking, furnace tube damage caused by the material of individual furnace tubes or the construction of individual parts mainly occurs in the early stages of CDQ construction or the early stages of operation, and is relatively easy to judge. If the boiler inlet temperature exceeds the standard or fluctuates significantly for a long time due to operational reasons, resulting in damage to the boiler tube, the operating methods must be strictly regulated, and operators who cannot meet the operating skill requirements must be appropriately replaced. If the boiler inlet temperature exceeds the standard for a long time due to the measuring instrument, the furnace tube is damaged. The measuring element must be replaced and strict corrections must be made. Otherwise, continuous large-area damage to the boiler tube will occur. When the quality of the boiler feed water and boiler water exceeds the standard, it will not affect the operation of the boiler in the short term, nor will it affect the steam production of the boiler. However, long-term water quality exceeding the standard will aggravate the scaling and oxygen corrosion of the boiler tubes. Therefore, the adverse effects on the boiler tubes are a slow and gradual process. Moreover, the damage to the boiler tubes is large-scale and irreversible, which should especially attract the attention of dry coke quenching technicians. Boiler feed water and boiler water quality indicators * * The standards vary depending on the design pressure level of the boiler and should be strictly followed. * * Standards manage boiler feed water and boiler water quality. 2 Conclusion (1) The overall operation of Wuhan Iron and Steel Coke No. 1 and No. 2 dry quenching boilers is stable, and their steam pressure, temperature, and productivity all meet the design requirements, meeting the needs of dry quenching production. (2) There should be sufficient understanding of the causes and judgment methods of furnace tube damage in CDQ boilers, and the occurrence of furnace tube damage should be dealt with in a timely manner. (3) The operation of dry coke quenching and the maintenance of measuring instruments should be strengthened, especially the management of boiler feed water and boiler water quality.