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Abstract: The cement plant of Taochong Mining Company, under MaSteel Group Holdings Limited, was originally a kiln equipped with a five-stage cyclone preheater of 600 t/d, Φ3.2M*52M, and it came online in 1999. After extensive investigations and thorough comparative analyses, the company decided in June 2004 to technically upgrade the kiln system to a new type of dry pre-decomposition kiln with a capacity of 1200 t/d. During the trial production period from January 8 to January 25, 2005, there were multiple incidents in which the air inlets of the tertiary air ducts at the cone section of the decomposer, as well as the area where these air inlets met the decomposer, became red-hot or even damaged, resulting in the shutdown of the furnace. During this time, we took temporary measures such as applying castable linings to the damaged areas, cutting and repairing those areas, changing the type of castable used, and adjusting the process parameters; however, these measures had a significant negative impact on the production testing process. This article mainly discusses the causes of wall burnout in decomposers, as well as the treatment and preventive measures. 1 Comparison of the main equipment before and after technical upgrades 2 Analysis of the causes of burnout on the furnace walls 2.1 The wind direction entering the furnace is too downwardward and the wind speed is too high. Our factory’s tertiary air valves are Φ1400 high-temperature electric butterfly valves; these valves are located very close to the furnace, with a distance of about 1.5 meters at the farthest point and around 20 centimeters at the nearest point. The butterfly valves rotate in a counterclockwise direction when opened. During normal operation, the opening degree of these valves is between 55% and 60%, which causes the wind entering the furnace to blow at an angled downward direction. As a result, the wind speed inside the furnace increases by nearly half. The fast-moving air carries a large number of clinker particles, which act like grinding wheels and erode the tertiary air valves as well as the conical part of the furnace, quickly wearing through the cast materials. Without the protection provided by these cast materials, the furnace shell suffers from burnout quite rapidly. On January 15, 2005, the temperature at the junction of the bottom of the tertiary air duct inlet and the decomposer reached 450°C; on January 18, 2005, that junction was burned through. The fact that the conical part of the decomposer turned red on January 22, 2005, further confirmed that the wind direction entering the decomposer was too downward and that the high wind speed was the main cause of the decomposer being burned through. 2.2 Poor dispersion of raw materials in the C4 decomposition furnace: The HCFC1200 system is designed with one air inlet, two coal inlets, one material inlet, and one exhaust outlet. The high-temperature gases at the end of the kiln are injected into the decomposition furnace from its bottom in a jet-like manner, while the tertiary air enters the furnace tangentially. Inside the decomposition furnace and the gooseneck tube, the gas, coal, and material mix further and burn, resulting in 90%–95% decomposition of the raw calcium carbonate. The exhaust gas enters the C5 cylinder, and the materials separated in the C5 cylinder are fed into a kiln for calcination. During the trial production phase, material collapse was observed at the neck area of the decomposition furnace. Upon stopping production for inspection, it was found that about 1/3 of the spreading device located in the feed pipe leading to the furnace was damaged; as a result, the raw material did not get properly dispersed upon entering the decomposition furnace, with some of it falling straight down and failing to provide protection for the furnace walls, thereby accelerating their wear and damage. 2.3 Regarding the quality of the casting materials We initially suspected that the quality of these casting materials was the issue. During the temporary repairs of the decomposition furnace, GT—13NL high-strength alkali-resistant casting materials, corundum-based casting materials, and HN15D steel fiber casting materials were used; however, their service life was very short, lasting only 2–5 days. Therefore, we believe that the quality of the casting materials themselves is not the main cause of the problems in the decomposition furnace. 3. Measures taken and preventive actions 3.1 Reapplying casting materials The damaged areas in the conical part of the decomposition furnace, as well as the area where the third-air duct inlet meets the decomposition furnace, were repaired by reapplying casting materials there. The method involves knocking off the severely damaged castable in that area (using as a criterion that the palladium pins should not be exposed; if they are exposed, the area is knocked off and refilled), removing the 100-mm-thick calcium silicate board from that location, and filling it entirely with castable. The castable used is HN15D steel fiber castable, with a pouring thickness of 250 mm. 3.2 Adjusting the direction of the tertiary air Bricks are stacked inside the tertiary air duct, about six meters away from the tertiary air valve in the direction toward the kiln head; the height of this brick stack is 0.8 meters. During normal operation, the tertiary air valve remains open at around 90% of its capacity. The purpose of using refractory bricks in the stack is to control the volume of the tertiary air flow. Keeping the tertiary air valve open at about 90% during normal operation ensures that the direction of the tertiary air remains unchanged. 3.3 Controlling the temperature in the middle section of the decomposer The temperature range for the middle section of the decomposer was adjusted from 900–950°C to 850–900°C. The author believes that different cement plants use different raw materials and have varying process systems; therefore, the control range should be determined based on specific circumstances, provided that the decomposition rate upon entry into the kiln is met. Controlling the temperature too high not only increases coal consumption but also deteriorates the working conditions in the decomposition furnace. Based on the technological characteristics of our plant, by maintaining the temperature in the middle section of the decomposition furnace at 850–900°C, the decomposition rate of the raw material fed into the kiln can be kept at around 92%. 4 Conclusion 4.1 The use of refractory bricks in the stack reduces the range of adjustment for the tertiary air valve; therefore, it is necessary to adjust the opening degree of the exhaust fan appropriately during operation, especially when starting up and heating up, in order to control the amount of air used. If the conditions permit, it is advisable to replace the third-air valve from a butterfly valve to a gate valve and move its location 3–5 meters further toward the furnace entrance; this may yield better results in adjusting the air flow direction into the decomposer. 4.2 In cases where the kiln is shut down for an extended period, such as for brick replacement, the spreading device for C4 in the feeding pipe of the furnace should be reactivated. 4.3 Since the refractory material was refilled in the decomposer on February 3, 2005, no burning damage has occurred in our plant’s decomposer again.