Coke oven construction, oven baking and start-up (1)

Section 1 Furnace Building Materials

Modern coke ovens are mainly built with refractory materials. For example, a 42-hole 58-Ⅱ type coke oven with an annual output of 300,000 tons of metallurgical coke requires a total weight of about 6,600 tons of refractory materials for the furnace body. Therefore, the performance of refractory materials is closely related to the production capacity and service life of the coke oven.

All materials that can resist high temperatures and physical and chemical effects under high temperatures are collectively referred to as refractory materials.

In addition to refractory materials, the materials used to build coke ovens also include insulation materials and ordinary building materials, which are described as follows:

I. Properties of refractory materials

The performance of refractory materials is usually measured by the following indicators:

(1) Porosity. There are many pores of different sizes and shapes in refractory products. Porosity is the percentage of the total volume of pores to the total volume of refractory products. It indicates the density of the refractory material. The porosity usually refers to the open porosity excluding closed pores (pores that are not connected to the atmosphere).

(2) Bulk density and true density. Bulk density refers to the mass of brick per m3 including all pores. True density refers to the ratio of the mass of refractory material per unit volume excluding pores to water. Different quartz crystals have different true specific gravity. Therefore, measuring the true density of silica bricks can understand the firing conditions.

(3) Thermal expansion. Refractory products generally expand after being heated. This property of the material is called thermal expansion. It can be expressed by linear expansion coefficient or volume expansion percentage.

(4) Thermal conductivity. The thermal conductivity of refractory products depends on its phase composition and organizational structure (referring to the size and distribution of porosity and the distribution of crystal phase and glass liquid phase). It is expressed by thermal conductivity "λ", and its legal unit is: kJ/m·h℃ (or W/m·K). The thermal conductivity of most refractory products increases with increasing temperature (such as silica bricks, clay bricks, etc.), while some products are the opposite (such as magnesium bricks and silicon carbide bricks).

(5) Refractoriness is a measure of the ability of refractory products to resist melting at high temperatures. It is done by placing a pyramidal sample of specified size together with some standard samples and heating them at a certain rate. These standard samples are made of kaolin, alumina and quartz in different proportions and each has a known refractoriness. When the temperature is raised to the point where the sample to be tested and a standard cone sample soften and bend at the same time, that is, when the apex of the pyramid bends and contacts the base, the refractoriness of the sample to be tested is equal to that of the standard cone. Therefore, refractoriness is the temperature at which the sample product softens and bends. The refractoriness of refractory materials is generally not more than 1580℃.

(6) High temperature refractoriness under load (also called load softening point) is an indication of the resistance of refractory materials under the simultaneous action of high temperature and load. It is indicated by heating a cylindrical sample of a certain size at a certain pressure (19.6×104Pa) at a certain heating rate to cause a certain amount of deformation in the sample. According to the amount of deformation, it is divided into the starting deformation temperature (deformation accounts for 0.6%), the temperature when the deformation accounts for 4%, and the final deformation temperature (4%). The load deformation curve of clay bricks is relatively flat, and the difference between the starting deformation temperature and the final deformation temperature can reach 200~250℃, while silica bricks are destroyed immediately after reaching the starting deformation temperature, and the difference between the starting deformation temperature and the final deformation temperature is only 10~15℃. Due to the heavy weight of the refractory materials in the coke oven masonry, coupled with the load of mechanical equipment, the high temperature load deformation temperature of the refractory materials is very important to the life of the furnace body. Among them, the main one that is meaningful to production is the starting deformation temperature, that is, the load softening point.

(7) High temperature volume stability. When refractory materials are used for a long time at high temperature, the phase composition will continue to change, resulting in recrystallization and further sintering. Therefore, the volume of refractory products will change. Due to the different chemical compositions of various products, some shrink and some expand, and this change is irreversible, which is called residual shrinkage and residual expansion. The value is expressed by the volume change percentage (%) of the product when it is heated to 1200~1500℃ (depending on the type of refractory product), kept warm for 2h, and cooled to room temperature.

(8) Temperature shock resistance. It is the performance of refractory products to resist temperature shock without damage. The sample is heated to 850±10℃, cooled in running cold water, and repeated until the sample is damaged and the mass of the detached part accounts for 20% of the original sample mass. At this time, the number of rapid cooling and heating it has undergone is used as an indicator of the product's resistance to rapid cooling and heating.

The thermal stability of products is closely related to their thermal expansion. If the linear expansion coefficient of a product is large, the uneven temperature distribution inside the product will cause different degrees of expansion, which will generate greater pressure and reduce the thermal stability of the product. In addition, the more complex the shape of the product and the larger the size, the worse its thermal stability. The above measurements show that different refractory products vary greatly. For example, the worst resistance of silica bricks is only 1-2 times, ordinary clay bricks are 10-20 times, and coarse-grained clay bricks can reach 25~100 times.