Compared with other refractory materials, silica bricks used in coke oven masonry have higher load softening temperature, better thermal conductivity, and more stable volume above 900 °C. Its biggest disadvantage is its poor thermal stability at 900 °C, which is mainly caused by the transformation of the silicon dioxide crystal form. According to the information, the damage mechanism of the coke oven can be summarized as the following points:
(1) Thermal damage mechanism
During the working process of the coke oven, opening and closing the furnace door when pushing coke and charging coal causes a sharp change in temperature. There is a temperature gradient on the furnace wall of the carbonization chamber, and the temperature of the carbonization chamber drops rapidly from about 1200 °C to about 700 °C during coal charging. There is a temperature gradient in the vertical direction of the furnace wall of Jinan University Master’s Degree Thesis 5, and the temperature of the upper part is lower than that of the lower part. Therefore, the thermal stress of the upper and lower bricks is different due to the temperature difference, resulting in cracks in the vertical direction. The temperature of the carbonization chamber is lower than that of the combustion chamber, which also causes a temperature gradient in the lateral direction. The tensile stress of the silica bricks on the side of the combustion chamber is greater than the compressive stress of the silica bricks on the side of the carbonization chamber, resulting in fractures in the lateral direction.
(2) Graphite adhesion and furnace wall damage lead to mechanical stress failure mechanism of coke push difficulty
When pushing coke, the extrusion of machinery and coke will generate pressure on the furnace wall, and this pressure will directly cause damage to the furnace wall bricks in the carbonization chamber. During the long-term production process, there will be graphite deposition on the surface of the furnace wall. In order to remove graphite, the method of air firing is usually used. However, the temperature of the wall without graphite deposition changes rapidly, which is easy to cause damage to the silica brick.
(3) Crystal transformation damage mechanism
The coke oven wall is mainly made of silica bricks. There are crystal transformations in the furnace wall silica bricks during the oven and use process. The volume expansion and shrinkage caused by the crystal transformation leads to poor thermal stability, which is also one of the main reasons for the damage of the coke oven wall. The three crystal forms of quartz all have volume changes during the transformation process, among which the volume change is the largest when transforming to quartz, followed by cristobalite, and tridymite is the slowest, and tridymite has better thermal conductivity.
This phenomenon is caused by the frequent changes in the temperature at the furnace door caused by pushing coke and charging coal in the carbonization chamber. In addition, there is a certain amount of moisture in the coal material, which causes the temperature on the surface of the furnace wall of the carbonization chamber to drop sharply from about 1200°C to about 700°C. The change is drastic, resulting in the transformation of the crystal form, so the furnace wall is seriously damaged by thermal shock. The furnace door is most severely affected by the thermal shock of the furnace wall.
