Converter maintenance

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Accident handling and discussion of conversion furnaces. Keywords: conversion furnace, temperature measurement points, damage due to high temperatures, repair measures. The 100,000-ton-per-year methanol production plant of Tengzhou Shenglong Coking Co., Ltd., which uses coke oven gas as raw material, came online in September 2006. The pure oxygen conversion furnace is a key device in the process of producing methanol from coke oven gas; it consists of a fire-resistant insulation layer, inner and outer walls with water jackets, as well as filling materials. The working medium inside consists of a mixture of coke oven gas and steam fed from the top of the converter, as well as a mixture of oxygen and steam. Upon entering the converter, the coke oven gas and oxygen undergo a partial oxidation and steam conversion reaction; the resulting products are then sent to the waste heat boiler from the bottom. The upper part of the converter consists of the mixture gas inlet and the high-temperature combustion zone; the middle part is the packed catalyst zone; and the lower part is the squirrel-cage collector. Its structure is shown in Figure 1. With a furnace temperature of 900–1650°C and a pressure of 1.9–2.5 MPa, it is difficult to handle accidents and take preventive measures for high-temperature, pressurized equipment such as conversion furnaces. 1 Accident Handling 1.1 Accident 1: On September 13, 2006, not long after the methanol production facility began operating, the outer sleeve of the temperature sensor located in the upper layer of the converter was damaged, rendering that temperature sensor unusable. The material of this sleeve is 0Cr25Ni20; it can withstand temperatures up to 1200°C, with an oxidation resistance up to 900°C. The cause of the burnout was the recent production of methanol, lack of operational experience, and excessively high furnace temperature. The temperature measurement point of the catalyst in the upper section of the converter is an important parameter for controlling the combustion temperature (which can also be regarded as the reaction temperature). Extending the service life of the protective sleeve is a key issue for ensuring the stable and safe operation of the converter. 1.1.1 Treatment method: (1) The original temperature measurement probe consisted of a fixed sleeve that was welded to the converter shell; after the part of this sleeve that extended into the furnace interior was damaged, in order to restore the use of this temperature measurement point as quickly as possible, the damaged sleeve was left unchanged, and an additional movable sleeve was inserted inside it, as shown in Figure 2. The material of the inserted movable sleeve has been changed to GH3039; this material can withstand temperatures up to 1300°C, exhibits good oxidation resistance at temperatures below 1000°C, and has good permanent strength as well as resistance to thermal and fatigue stress. The original sleeve had dimensions of Φ50×10, while the inserted movable sleeve has dimensions of Φ25×5. The movable sleeve is easy to remove for inspection, and it is also easy to replace if the external movable sleeve gets damaged. (2) The length to which the temperature measurement thermocouple and the external sleeve extend inward from the furnace inner wall affects the service life of these components; the shorter their extension inward, the lower the likelihood of them being damaged. With the approval of the process engineers, the length of the sleeves and thermocouples inserted into the furnace was reduced; the protrusion length of the temperature sensing elements vertically into the inner wall of the furnace was decreased from 200 mm to 150 mm. Even after this reduction, the readings obtained from the temperature sensing elements still accurately reflected the temperature inside the furnace. (3) Regarding the requirements for thermocouples, in addition to a shorter length, changes have also been made to the material and outer diameter of the coating surrounding the thermocouple itself. The original coating material was corundum; although corundum coatings can resist high temperatures and oxidation, they cannot withstand impacts, are vulnerable to thermal shock, and tend to break easily. Once the external coating is damaged, the corundum coating is quickly destroyed as well, rendering the thermocouple unusable. The temperature measurement point now uses a thermocouple with a GH3039 coating; the diameter of the coating surrounding the thermocouple itself has been reduced to 12 mm. If the externally mounted ø25 sleeve is damaged, the ø12 sleeve of the thermocouple itself can still be used. 1.1.2 Treatment Effect: The service life of the temperature measurement points in the converter, after being repaired through this treatment, is **extended**; in practical operation, this service life can reach around 5 months. However, the temperature measurement points for the catalysts located in the upper part of the converter still suffer from the problem of the sleeves bending. 1.2 Incident 2: On December 8, 2007, an accident occurred in which the furnace chamber of the converter was damaged. The details of the incident are as follows: the temperature reading at the water jacket of the converter reached its maximum value; the pressure of the coke oven gas entering the converter dropped from 2.45 MPa to 2.2 MPa, while the flow rate of the coke oven gas increased from 24,500 Nm3/h to 34,000 Nm3/h. It was initially determined that the converter had suffered burn-through, and emergency shutdown was carried out for inspection and repair. Upon parking the furnace, it was found that the corundum distributors in the converter were in a melted state in multiple areas, and the inner parts of these distributors were severely damaged. The corundum bricks and lightweight casting layers on the outside of the lining bricks had collapsed, resulting in fire spreading from the furnace chamber to the main housing, which caused varying degrees of damage to the inner walls of the main housing. The burned areas are shown in Figure 3: a hole has formed through position W1, the inner walls at positions W2 and W3 have turned black; at position W4, the inner wall’s middle layer has hardened while the surrounding area has softened due to excessive temperature. The outer wall of the water jacket, which is directly opposite the hole at W1, has bulged and deformed as a result of the impact from high-temperature, pressurized gas ； Additionally, the catalyst filler is severely crushed, with many small particles. 1.2.1 Treatment methods: (1) The microstructure and properties of the steel in the holes where the main shell of the conversion furnace was penetrated, as well as in other areas damaged by overheating, had changed. To restore the shell to properties similar to those of the original material, the following repair methods were employed: 1) The blackened areas at W2 and W3 were removed by using carbon arc gouging to eliminate the black oxide layer, and an angle grinder was used to polish the area until a metallic luster reappeared. The grinding area and its surrounding region are normalized. After normalization, TIG welding is used for the root pass, followed by shielded metal arc welding to restore the thickness of the steel plate to its original level. Stress-relief annealing is carried out after welding. 100% UT testing and 100% MT testing are conducted within the heat-treated area, and the results meet the requirements of grade I as specified in JB/T4730-2005. 2) Normalizing treatment was applied to the burned-through areas on the inner wall of the main shell of the W1 converter and the areas surrounding them. 100% UT testing and 100% MT testing were carried out within the heat-treated area, and the results met the requirements of Grade I as specified in JB/T4730-2005. After normalizing, the burned-through area at W1 was polished, and then repair welding was carried out on the inner side of the shell. Steel plates of material 16MnR with a yield strength of δ=14 were used as lining plates; TIG welding was used for the root pass, while shielded metal arc welding was used for the subsequent layers. The surface was welded back to the original thickness of the steel plate, followed by stress-relief annealing. A 100% RT inspection was conducted, and the results met the requirements of grade II as specified in JB/T4730-2005. 3) Normalizing treatment is applied to the hard tissue at W4 and the surrounding soft tissues; 100% UT testing and 100% MT testing are carried out within the heat treatment area, and the results meet the requirements of Grade I as specified in JB/T4730-2005. 4) After cutting away the bulged area on the outer wall of the water jacket, a new steel plate was installed. Since this steel plate was made of Q235C material, and such material is difficult to obtain, a steel plate with an equal thickness of δ=16 and made of 20g material was used as a substitute. The cut area was welded back together, followed by stress-relief annealing. The welds were subjected to 100% UT testing and 100% MT testing, and the results met the requirements of grade I as specified in JB/T4730-2005. (2) The burned corundum distributor results in uneven mixing of coke oven gas and oxygen, while the large-scale peeling and cracking of the lining material on the upper inner wall of the furnace cause cross-ignition of the furnace’s inner wall. Therefore, all the damaged lightweight castables, refractory bricks, corundum distributors, etc. above the combustion zone were replaced and recast. The heavy castables below the catalyst are basically intact; to reduce maintenance time, no further treatment is required. (3) The original catalyst was severely fragmented, containing many small particles, which created high resistance to the gas flow and affected both the gas flow rate and the removal of heat generated during combustion. Furthermore, considering that converter units have little time for major repairs, in order to extend their operational cycle, the catalyst is replaced during the repair period; currently, catalyst fillers with higher hardness and slightly larger particle size are being used. (4) Since the hard cast material in the middle and lower sections of the conversion furnace had not been replaced, after the repair of the furnace shell walls and the casting in the upper part of the furnace was completed, catalyst filler was installed. The height of this catalyst filler was increased by 300 mm compared to before. After the catalyst filler was in place, furnace drying and catalyst heating were carried out simultaneously, which **reduced the time required for maintenance work** as compared to carrying out furnace drying alone. 1.2.2 Treatment effects: The repaired conversion furnace is now operating normally, with a significant improvement in its performance compared to before the accident. Although the height of the catalyst filler has increased, the airflow within the furnace remains unobstructed, and the heat generated by the combustion reactions can be removed promptly. As a result, the average temperature in the combustion zone has decreased compared to before the repair; the temperature of the upper layer of catalysts has also dropped from around 1030°C to around 940°C. This helps to extend the service life of both the conversion furnace and the thermocouples. Another major effect is that the conversion efficiency increased after the reformer was overhauled, setting new records for methanol production on a shift basis as well as on a daily basis. 2 Discussion: The production process for manufacturing methanol from coke oven gas is now largely mature, and the problems that arise during this process are being gradually identified and resolved. Multiple manufacturers of conversion furnaces in methanol production plants using coke oven gas face problems similar to those encountered by our plant; some have even experienced explosion accidents. In order to reduce the occurrence of converter failures, prevent converter accidents, identify and resolve problems in a timely manner, and ensure the normal and safe operation of the converter, the existing issues with the converter are discussed based on actual production conditions as follows: (1) Regarding the problem of excessive temperature within the furnace that causes damage to the temperature sensing sleeve for the catalysts located in the upper part of the converter, replacing the material of this temperature sensing sleeve is considered as a first step. In a converter where coke oven gas and oxygen burn unevenly, the temperature in certain areas of the combustion zone can occasionally exceed 1650°C for short periods of time. The temperature at the temperature measurement points located in the upper part of the converter can also reach around 1300°C for brief periods. The material used for the original casing is susceptible to being damaged by heat, so choosing a material with better oxidation resistance and higher heat tolerance is key to solving this problem. The 3YC52 material exhibits excellent oxidation resistance; it can be used at temperatures as high as 1200–1300°C over the long term, and up to 1350°C in the short term. This material is suitable for use in the environments of temperature measurement points for catalysts located in the upper sections of conversion furnaces, making it an appropriate alternative material for sleeves that could be considered for trial use. Furthermore, if it is possible to further reduce the length by which the thermocouple and its sleeve extend vertically inward into the inner wall of the furnace while still meeting the requirements of the production process, and if the wall thickness of the sleeve is increased, this can also help to reduce the occurrence of damage to the temperature measurement points. (2) Excessively high temperatures inside the conversion furnace, uneven mixing of coke oven gas and oxygen leading to uneven combustion, as well as rapid fluctuations in temperature, are the direct causes of various faults in the conversion furnace. Controlling the temperature of the conversion furnace properly is key to resolving these problems. Solution: 1) If the oxygen distributor can mix coke oven gas with oxygen evenly, it is possible to avoid uneven burning in the combustion zone and prevent excessively high local temperatures. The damage to the corundum distributor affected the uniformity of mixing between coke oven gas and oxygen; therefore, adopting a type of oxygen distributor that is resistant to damage or using a metal-centered burner is an issue that needs to be addressed in the future. 2) Controlling the amount of oxygen supplied is an important factor in controlling the temperature of the conversion furnace. It is necessary to strengthen operational management, follow the operating procedures strictly, carefully control the amount of oxygen supplied, so as to keep the furnace temperature from rising too high and avoid large fluctuations in temperature, ensuring that the temperature inside the furnace remains within the range suitable for safe and normal production. (3) The quality of the lining material used in the converter furnace is an important factor affecting its service life. By increasing the number of fixing claws during the lining process, using a properly selected lining material, and ensuring that the lining is filled thoroughly to maintain good quality, cracks and detachment of the lining material can be reduced, thereby preventing cross-ignition within the lining layer of the furnace. (4) To prevent accidents, it is also important to make full use of the role of temperature control instruments in production, so as to identify problems promptly and quickly control and resolve faults. (5) Preventive inspection and maintenance are important means of eliminating faults. 1) Regularly inspect each temperature measurement point. When the production process is stopped, thoroughly examine the sleeves and thermocouples at each measurement point; address any issues found promptly to ensure the proper functioning of all these points. 2) During shutdowns of the production process or during regular maintenance shutdowns, after the process personnel have ensured that the process is in proper condition, they enter the upper part of the furnace for inspection. If any issues such as detachment, cracks, or other problems with the lining material or refractory bricks on the inner wall of the furnace are detected, they are repaired promptly. 3 Conclusion Practice has shown that the repair methods adopted after the accident were correct; the repair results were satisfactory, with low costs, short repair times, and high practicality. These methods ensured safe and stable operation of the production processes, yielding significant economic benefits, and are therefore worth promoting within the same industry. However, issues such as the control of combustion temperature in the converter, the control of the construction quality of the lining materials, and the material and structural design of the temperature measurement probes still need further investigation and resolution.