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.