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The operating temperature of the circulating fluidized bed boiler is high, and the temperature in the furnace changes frequently, resulting in thermal shock. At the same time, there are many high-temperature solid particles in the furnace, which continuously erode the heating surface, so it is necessary to lay wear-resistant refractory materials for maintenance. Types and characteristics of wear-resistant refractory materials for circulating fluidized bed boilers:
Properties of wear-resistant refractory materials
Wear-resistant refractory material is a special product that is not easy to be damaged and deformed under high temperatures. In order to avoid damage from smoke and fly ash, wear-resistant materials are laid inside some vulnerable parts. The correct selection and installation of this wear-resistant material is particularly important. It guarantees the long-term characteristics of the system, reducing the frequency of shedding of wear-resistant materials and maintenance.
The chemical composition of wear-resistant refractory materials is mainly composed of aluminum and silicon compounds, accounting for 80%-95% of the total content.
In order to withstand the environmental hazards of circulating fluidized bed boilers, wear-resistant refractory materials must have certain refractoriness, compressive strength, flexural strength, thermal shock resistance, and a sufficiently small linear change rate. The main physical and chemical indicators of wear-resistant refractory materials are as follows:
A. Refractory materials
Refractoriness refers to the possibility of wear-resistant refractory materials resisting melting at high temperatures without external force. The refractoriness is usually expressed by the highest service temperature, that is, the temperature at which the linear change rate of the material after calcination for 5 hours does not exceed 1.5%.
B. Bulk density
Bulk density, also known as bulk density, refers to the mass per unit volume of wear-resistant refractory materials, which can reflect the density of refractory materials, and the unit is kg/m3.
C. Heat transfer coefficient
The heat transfer coefficient refers to the heat of the wear-resistant refractory material per unit vertical area per unit time under the unit temperature gradient, w/(M.K). The heat transfer coefficient of refractory materials is not only related to their use but also a key factor that directly endangers the thermal shock stability of handicrafts.
D. Thermal shock stability
Thermal shock resistance refers to the potential of wear-resistant refractory products to resist large temperature changes without damage, also known as thermal shock resistance, temperature change resistance, and rapid cooling and heating resistance. During the use of refractory materials, they are often harmed by sharp changes in working temperature, which will cause the materials to crack, fall off or even collapse. Factors affecting thermal shock stability include thermal deformation rate, heat transfer coefficient, material structure, product shape, and particle composition.
E. Electronic circuit change rate
The linear rate of change refers to the ratio of the irreversible variable of the length change of the wear-resistant refractory material to the original length at unit temperature, expressed in percentage, also known as the coefficient of linear expansion. It is one of the bases for the overall design of refractory materials and the layout of expansion joints.
In the formula: L1 is the length of the sample at room temperature, mm; L2 is the length of the sample heated to the experimental temperature T, mm.
F. Constant temperature compressive strength and flexural strength
Compressive strength generally refers to the compressive strength at room temperature, which is the ultimate pressure that a wear-resistant refractory material can withstand per unit area at room temperature. If this value is exceeded, the material will be destroyed. The calcined, molten state and characteristics related to the structure of the refractory are the key performances of its compressive strength. It is a common item for testing wear-resistant refractory materials, also known as cold compression strength. Calculation method of compressive strength:
In the formula: CCS is the compressive strength, the unit is MPa; F is the ultimate pressure that the material can bear; a is the stress area of the material.
In the application of wear-resistant refractory materials, in addition to bearing compressive stress, it also bears tensile stress, bending stress, and shear stress. Bending strength generally refers to the bending strength at room temperature, which refers to the ultimate stress when the specimen is subjected to a bending load at room temperature, the unit is megapascal (MPa).
The compressive strength and flexural strength depend on the type and amount of flux and admixture and are also affected by the purity, proportion, total amount of mixed liquid, construction method, and maintenance method of raw materials.
G. Wear index
Spray one pound of quartz sand on the wear-resistant material at a certain speed. The amount of wear-resistant material is called the wear-resistant index, and the unit is g/cm2. The comprehensive wear resistance index is a key index to measure the wear resistance of castables and bricks.
In order to meet the safe operation of the unit, the wear-resistant and refractory materials of circulating fluidized bed boilers should have the following characteristics: high constant temperature and thermal strength; low wear rate; high-quality corrosion resistance; excellent high-temperature volume stability.
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