Requirements for Refractory Castables in Various Parts of CFB Circulating Fluidized Bed Boilers

Circulating fluidized bed boilers are currently the most widely used and relatively environmentally friendly waste heat boilers in China. They are primarily used in the power industry, petrochemical industry, waste incineration industry, and other sectors. While their applications vary, their structure is largely the same. During boiler operation, high-temperature gases and solid particles in the gases cause severe wear on the refractory linings in various areas, including the furnace and circulation system. Therefore, the wear protection and thermal insulation requirements of these linings are critical to boiler operation. Taking power plant boilers as an example, the operating environment of various boiler components and the requirements for the refractory castable lining are as follows.

Lower Furnace

The dense phase area in the lower furnace is a highly wear-prone area in circulating fluidized bed boilers. The material within this area is coarse, highly concentrated, and fluidized at high speeds. The combustion atmosphere is somewhat corrosive. The wear-resistant refractory castable used in this area must be wear-resistant, have good thermal conductivity, and exhibit excellent resistance to thermal cracking and thermal shock. Steel fiber wear-resistant refractory castables are recommended.

Separator Inlet Flue Duct

The separator inlet flue duct experiences a change in airflow direction, resulting in fine ash particles and a high concentration. The airflow velocity is generally 4.5-6.0 m/s, and the temperature is 850-950°C. The combustion atmosphere is oxidizing, resulting in severe wear and erosion. The castable must exhibit excellent wear resistance, thermal cracking, and thermal shock resistance. Steel fiber wear-resistant refractory castables are recommended.

Cyclone Separator

The high airflow velocity within the cyclone separator causes centrifugal force, causing coarse particles in the airflow to collide with the wall and separate. This results in severe wear at the separator inlet and on the inner wall of the separator. The operating temperature within the separator is less than 1000°C, and the refractory and wear-resistant layer is relatively thick. The atmosphere is oxidizing. The castable must be lightweight, dense, wear-resistant, and have excellent thermal insulation (for adiabatic separators), as well as good resistance to thermal cracking and thermal shock. High-strength, wear-resistant, and refractory castables or wear-resistant, refractory plastics are recommended.

Return Material Transfer Unit

The return material transfer unit transports ash separated by the separator into the furnace for recirculating combustion. The return material transfer unit features a fluidized bed on one side of the furnace and a moving bed on the other. It is a fluidized, sealed ash conveying device. The fluidization velocity is less than 1 m/s, and the material inside is fine and highly concentrated. The operating temperature is approximately 900°C. Compared to other parts, the return material transfer unit's operating environment is not particularly adverse, but construction is challenging. Since combustion does not occur within the return material transfer unit, the castable must possess excellent thermal insulation, thermal cracking, and thermal shock resistance. High-strength, wear-resistant, and refractory castables are recommended.

Platenose Heating Surfaces

Power plant boiler furnaces feature platenose heating surfaces (water-cooled platen and superheater). The lower elbow of the platen is subject to wear and tear from ash particles. This requires the castable to exhibit excellent wear resistance and thermal conductivity. High-strength, wear-resistant, and refractory castables are recommended.

Precautions for High-Strength Wear-Resistant Castable Construction

The quality of high-strength wear-resistant castable construction directly determines its wear resistance and service life. Key considerations lie in controlling the water-cement ratio, ensuring density, and maintaining proper maintenance to prevent cracking and shedding caused by improper construction. Specific precautions can be divided into four key stages:

Refractory Castables in Various Parts of CFB Circulating Fluidized Bed Boilers

1. Pre-construction Preparation: Clean the base surface to remove oil, dust, and loose debris. Sandblast and roughen as necessary. Install anchors according to design requirements, ensuring uniform spacing (usually 150-200mm). Secure with welds and apply anti-rust paint.

2. Mixing: Use a forced mixer. First, add the dry castable and mix for 2-3 minutes. Then, strictly control the amount of water added according to the instructions (generally a water-cement ratio of 0.10-0.14). Mix until a uniform, lump-free paste forms. The mixing time should be minimal (5-8 minutes total). Avoid initial setting.

3. Casting Process: Use an inserted vibrator (30-50mm diameter) to vibrate the lining layer by layer, with each layer no thicker than 300mm. Vibrate until the slurry is smooth and free of bubbles, avoiding missed or over-vibration. Casting should be continuous, with the interval between each layer no longer than the initial setting time to prevent cold joints.

4. Curing and Baking: Cover with plastic sheeting or wet burlap for 12 hours after casting, maintaining an ambient temperature of 5-35°C for at least 7 days. After curing, bake slowly according to a heating curve to remove moisture and avoid cracking caused by rapid heating. Baking time varies depending on the thickness of the lining and generally takes 3-7 days.

Advantages and Disadvantages of Silicon Carbide Refractory Castables

Rongsheng Refractory Manufacturer, solutions for high temperature industrial furnace lining refractory materials. Advantages and disadvantages of silicon carbide refractory castables. In the field of amorphous refractory materials, silicon carbide refractory castables have always been a product that has attracted much attention. Whether from raw materials to research and development, or from production to use, it is a product that users love and hate. Understanding the advantages, disadvantages and why silicon carbide refractory castables can resist slag erosion can help companies make better choices.

Advantages and disadvantages of silicon carbide refractory castable products

The biggest advantage of silicon carbide refractory castable is that it has high thermal conductivity, low thermal expansion, and does not react with slag. It has been used in kiln parts with severe slag reaction and high-temperature spalling since a long time ago.

Rongsheng Silicon Carbide Refractory Castables

The biggest disadvantage is that the chemical properties are very unstable in certain atmospheres. It is easily corroded in oxidizing gases such as (oxygen, water vapor, carbon monoxide, carbon dioxide), iron oxide, etc., and easily oxidized and decomposed in molten iron and vacuum. Another disadvantage is that silicon carbide refractory castables have poor water solubility. In water-based amorphous refractory materials such as castables, the poor fluidity sometimes causes the poor density of silicon carbide refractory castables. The third disadvantage is the lack of sintering and difficulty in obtaining high strength, but sometimes it is difficult to produce over-sintering and shrinkage, which is a strength in amorphous refractory materials.

Why is silicon carbide refractory castable resistant to slag erosion?

When considering the slag resistance and slag resistance of silicon carbide refractory castable, the following aspects must be considered comprehensively: (1) wettability to slag (contact angle) (2) slag invasion (3) reactivity with slag (4) melting point and viscosity of reaction products. Considering these four aspects, although alumina refractory materials are materials that easily react with slag, as raw materials of refractory materials, they are not materials with poor slag resistance. This is because there are not many low-melting point compounds generated by reaction with slag.

The reason why silicon carbide refractory castable is difficult to be wetted by slag is because of the material of silicon carbide itself. SiC has two crystal forms, α and β, and the crystal structure of β-SiC. Among them, α-SiC has about 120 polymorphs such as 4H, 15R and 6H, among which 6H polymorph is the most widely used in industry. In 6H-SiC, Si and C are stacked alternately in layers, the distance between Si layers or C layers is 2.5Å, and the atomic distance between Si-C is about 1.9Å. There is a certain thermal stability relationship between the various forms of SiC, and the α β crystal forms also transform into each other. When the temperature is below 1600℃, SiC exists in the form of β-SiC. When the temperature is higher than 1600℃, β-SiC slowly transforms into various deformation forms of α-SiC (4H, 15R, 6H, etc.) by recrystallization. For α-β transformation, higher pressure is required, while for β-α transformation, only lower pressure is required. The transformation between various types of silicon carbide does not produce volume effect. SiC is a compound with strong covalent bonds. It still maintains high bonding strength at high temperatures, so SiC has high hardness, large elastic modulus, excellent wear resistance, and will not be corroded by most acid and alkali solutions. For the intrusion of slag and the melting point generated after reaction with slag, when compared with oxides, the slag resistance is significantly better.

When using silicon carbide refractory castables in monolithic refractory materials, the advantages, disadvantages, and prices of use should be considered. Only after the use site is determined can the lining material be used more accurately. However, in the actual use environment of monolithic refractory castables, these favorable and unfavorable environments for silicon carbide are mostly mixed. The evaluation of many environments currently used is often inconsistent with the actual use environment.

What are the Application Industries of Refractory Castables?

The advantages of refractory castables are obvious compared to refractory bricks. The production process of refractory castable is simple, labor-saving and energy-saving, high construction efficiency, good quality, and can be prepared on-site or select materials with excellent performance according to needs. Therefore, the refractory castable is an unshaped refractory with a large amount and a wide range of applications in furnace construction. So, what are the main application industries of refractory castables? The Rongsheng refractory castable manufacturer will explain based on the following refractory lining materials for industrial kilns.

Kinds of Refractory Castables for Furnace Lining

1. Application of castables in cement rotary kiln

In the new dry-process cement rotary kiln, castables (http://www.aluminabricks.com/refractory-castable/) have been widely used. There are many types of refractory castables used in this kiln. According to the different working environments of the kiln, the following types of castables are used. Ordinary refractory castables, ultra-low cement refractory castables, non-cement refractory castables, steel fiber refractory castables, explosion-proof refractory castables, anti-skinning refractory castables, phosphate refractory castables, alkali-resistant castables, heat insulation Castable. In addition, there are new types of castables such as self-flowing refractory castables.

2. Application of castable in glass melting furnace

Due to the requirements of the working environment of the glass melting furnace, the castable used should have the characteristics of good high-temperature resistance, strong resistance to corrosion of molten glass, and low porosity. The main refractory castables used are fused silica castables, corundum castables, lightweight mullite castables, and so on.

3. Application of castables in metallurgical industry

1) Castable for ironmaking system.

In the ironmaking system, there are many refractory castables used in the blast furnace taphole, and ASC refractory castables are generally used. ASC castables are generally low cement or ultra-low cement castables, mainly composed of Al2O3 aggregate, SiC, carbon, cement, and various additives.

2) Castable for hot metal pretreatment.

Refractory castables are mainly used in the application of molten iron ladle with alumina-silicon carbide carbon castables in the furnace bottom, molten pool, and slag line; spray guns are important devices for hot metal pretreatment. Since the middle and late 1980s in our country, spray gun refractories have gradually developed into castable integral casting spray guns, whose materials are mainly Al2O3-SiO2 series. The integral spray gun made of this castable has the characteristics of uniform organization, no joints, good thermal shock resistance, and long life.

3) Castable for electric furnace steelmaking.

The application of refractory castables in electric furnaces is mainly concentrated in the area of the furnace cover. In the late 1990s, the fully water-cooled furnace top technology was widely adopted, and the castable integral prefabrication technology was generally used in the electrode triangle area. On the whole, the integral castable furnace cover may become a development trend of electric furnace cover materials in the future.

4) Castable for refining outside the furnace.

There are four types of castables for refining outside the furnace. High-aluminum castables and aluminum-magnesium castables are used in a small amount in the RH furnace lining. The upper part of the RH vacuum chamber adopts Al2O3-MgO·Al2O3 castable integral lining or gunning integral lining, and integral aluminum-magnesium castable on the outer wall of the RH furnace dip tube. LF furnace castables are also castables for refining outside the furnace. The materials used for the curtain wall of the LF furnace cover are mainly high-aluminum or corundum series castables. High alumina-spinel refractory castables are mostly used to pour large bricks in the impact area of the bottom of the package. The third type is the castable for CAS refining device. The CAS refining device is divided into upper and lower parts. The refractory materials used are all castables. The upper part is mostly low-expansion Al2O3-SiO2 castables, and the lower part is mostly corundum-spinel series castables with fused corundum and MgO superfine powder. In the later stage, in order to extend the service life of the device, magnesia-aluminum gunning materials are mostly used.

To learn more about the application industry of monolithic refractories, and to obtain high-quality refractory castable products, please follow our blog.