Selection of refractory materials for coal injection pipes for cement kilns

Selection of refractory materials for coal injection pipes for cement kilns

The coal injection pipe is an independent fuel delivery system component. After the coal powder and high-pressure air are mixed, they are transported into the rotary kiln for combustion by the coal injection pipe. The coal injection pipe is composed of an internal precision air duct steel pipe, which is exposed to a flame close to 1700 degrees and works in a strong scouring environment of secondary air in the kiln for a long time. Therefore, a layer of castables needs to be poured on the outside of the coal injection pipe to protect the coal injection pipe. The coal injection pipe castable is in direct contact with the flame. Ordinary castables will shrink and crack under high temperature conditions. They will fall off quickly under the scouring of high-pressure wind. The high-temperature performance cannot meet the working conditions of the coal injection pipe, so special castables for coal injection pipes with excellent high-temperature performance must be used. The coal injection pipe castable is mainly produced by the composite of corundum and silicon carbide, which can withstand the scouring of high temperature and high-speed airflow. Under normal circumstances, the service life is more than 6 months.

In the cement production process, the working conditions of the cement kiln coal injection pipe are very harsh, which puts extremely high demands on the selection of its refractory materials. The coal injection pipe not only has to withstand high temperatures, but also faces high-speed scouring of coal powder airflow, rapid temperature changes, and erosion of complex chemical atmosphere in the kiln. Therefore, suitable refractory materials are essential to ensure the stable operation and extend the service life of the coal injection pipe.

Working characteristics of coal injection pipe

The internal temperature of the coal injection pipe of the cement kiln can be as high as 1400℃ – 1600℃, and due to the combustion process of coal powder, the temperature fluctuates frequently and violently. At the same time, the coal powder airflow passes through the coal injection pipe at a high speed, causing strong scouring and wear on the inner lining of the pipe. In addition, the cement kiln contains a large amount of alkaline substances, such as calcium oxide and magnesium oxide, which will form an alkaline atmosphere, chemically corrode the refractory material, and reduce its performance and life.

Performance requirements of refractory materials

High temperature resistance: Refractory materials must be able to withstand the high temperature environment in the coal injection pipe, maintain structural stability under continuous high temperature, and not soften or melt, etc., to ensure the normal operation of the coal injection pipe. Generally, its refractoriness is required to be not less than 1750℃ to meet the high temperature requirements in actual production.

Thermal shock resistance: In view of the rapid temperature change in the coal injection pipe, the refractory material should have good thermal shock resistance stability, and can maintain its integrity and thermal insulation performance without cracking or peeling under frequent thermal shock. For example, after multiple (more than 10) 1100℃ water-cooled thermal shock tests, the strength retention rate of the material should be at a high level (such as more than 70%).

Abrasion resistance: Due to the high-speed scouring of coal powder airflow, the refractory material needs to have excellent wear resistance to reduce the wear of the pipe wall and ensure the wall thickness and structural strength of the coal injection pipe. The wear amount should be controlled at a low value, such as less than 8cm³ (under specific wear test conditions).

Corrosion resistance: It can effectively resist the chemical erosion of alkaline substances and other harmful components in cement kilns, prevent the structure and performance of the material from being destroyed, and thus maintain long-term stable performance.

Common types and characteristics of refractory materials

Corundum refractory materials: Corundum (Al₂O₃) has a high content, high melting point, high strength and good high temperature stability. It has outstanding performance in high temperature resistance and can withstand the high temperature environment in the coal injection pipe. However, its thermal shock resistance is relatively weak, and it is easy to crack when the temperature changes sharply. To improve this performance, an appropriate amount of additives (such as titanium dioxide, etc.) can be added to improve its toughness and thermal shock resistance. It is suitable for areas with high and relatively stable temperatures in the coal injection pipe, such as the rear part of the coal injection pipe near the kiln head.

Silicon carbide refractory materials: With silicon carbide (SiC) as the main component, it has good thermal conductivity, high temperature strength and excellent wear resistance, and can effectively resist the erosion of coal powder airflow. At the same time, silicon carbide refractory materials also have good thermal shock resistance and can adapt to the frequent fluctuations in temperature in the coal injection pipe. However, its anti-oxidation performance is poor, and anti-oxidation measures such as surface coating need to be taken during use. It is often used in the front section of the coal injection pipe, that is, the area where the coal powder airflow velocity is large and the temperature change is more obvious.

Zircon refractory material: The main component is zircon (ZrSiO₄), which has high refractoriness, good thermal shock resistance and certain corrosion resistance, especially good resistance to alkaline corrosion. It can maintain good performance in the complex chemical environment of cement kiln, and is suitable for parts of coal injection pipes that are severely chemically corroded, such as the part where the coal injection pipe is connected to the kiln head.

Principles of refractory selection

Optimize selection according to working conditions: Conduct a detailed analysis of the temperature distribution, airflow velocity, wear degree and chemical corrosion of various parts of the coal injection pipe, select appropriate refractory materials according to the specific working conditions of different parts, and realize the zoning selection of coal injection pipe linings to give full play to the advantages of various refractory materials and improve the overall performance and service life of the coal injection pipe. For example, corundum refractory materials can be used in areas with high and stable temperatures but less wear; silicon carbide refractory materials are preferred in areas with large temperature fluctuations and severe wear; and zircon refractory materials are more suitable for areas that are severely chemically eroded.

Comprehensive consideration of cost performance: When selecting refractory materials, we should not only focus on the initial purchase cost of the materials, but also comprehensively consider factors such as their service life, maintenance costs, and impact on cement kiln production efficiency. Although the purchase price of some high-performance refractory materials is high, if they can significantly extend the service life of the coal injection pipe, reduce the number of repairs and downtime, their comprehensive cost may be lower in the long run. Therefore, it is necessary to conduct a full life cycle cost analysis of different refractory material solutions and select the material combination with the best cost performance.

Ensure material quality and construction performance: Select refractory products with reliable quality and stable performance, whose production process should be mature and quality control should be strict to ensure that the materials can achieve the expected performance indicators in actual use. At the same time, the material should have good construction performance and be convenient for on-site construction operations, such as suitable plasticity, easy masonry molding and good baking performance, etc., to ensure the construction quality of the coal-injection pipe lining, reduce material loss and quality defects during the construction process, and thus ensure the long-term and stable operation of the coal-injection pipe.