What are the main causes of wear-resistant refractory castables for CFB boilers falling apart?
2025-08-13 09:10:22
Some wear-resistant refractory castables for CFB boilers may fall apart during construction. The reasons are as follows:
I. Improper furnace construction techniques
(I) Improper control of the "ash-water ratio" during castable mixing
During the construction process, the ash-water ratio must be strictly controlled when preparing the castable. If too much water is added, the castable will have a high internal porosity after molding, significantly reducing its strength. This will also increase the natural setting time, especially in low-temperature environments. If too little water is added, the material will have poor fluidity and will not be compacted during shaking, which can easily lead to pores and holes, significantly reducing the castable's strength.
(II) Improper control of mixing and castable vibration time
The castable should be vibrated in layers using a vibrator during pouring. Too short a mixing time will result in uneven mixing of the material; too long a vibration time will cause the material to separate, with fines floating on the surface and aggregate sinking to the bottom. This will result in a decrease in strength and susceptibility to falling. The castable should be used within 30 minutes of mixing and should be poured to the specified thickness and height in one go. Castable construction should be carried out at an ambient temperature of 5°C or higher. Temperatures too low will prevent the material from setting naturally, or may result in "false setting." Castables should generally be poured continuously, with the next layer of castable poured before the previous layer begins to set. If the construction gap exceeds the initial setting time, it should be treated according to construction joint requirements.
(3) Poor demolding timing
The castable cannot be demolded before it has hardened. Demolding should only be performed when the castable's strength ensures that its edges and corners will not be damaged during demolding. Load-bearing formwork should not be removed until the castable reaches 70% of its strength. To facilitate demolding, all mold casting surfaces should be coated with a layer of motor oil before pouring.
(4) Improper control of castable curing time
After the boiler is built, a sufficient natural drying period is required to allow most of the moisture in the refractory layer to dissipate. This prevents cracking and falling of the refractory layer due to inadequate moisture removal during furnace drying.
(5) Improper control of the furnace heating rate and temperature gradient
II. Insufficient subjective understanding of the importance of furnace drying
(1) The lining material must have a certain degree of natural strength before furnace drying.
Furnace drying removes free water and crystallized water in the lining material that cannot be removed by natural drying, while simultaneously curing the castable at high temperature to achieve a certain strength. Therefore, the lining material must have a certain degree of natural strength (a satisfactory curing period) before furnace drying can proceed.
(2) Furnace Drying Principles - "Long, Not Short, Slow, Not Fast"
Furnace drying should be performed according to a pre-defined drying curve. The heating rate should be uniform and steady. The constant temperature time and temperature should be carefully controlled, and the temperature fluctuation should be maintained within ±20°C. Avoid excessively rapid heating during the drying process, which can prevent moisture generated in the material layer from being discharged in time. This can lead to excessive internal water vapor pressure, which can break through the wear-resistant refractory layer and cause cracks.
In addition, the temperature gradient during drying should not be too large to avoid excessive thermal stress, which can cause cracking, bulging, or shedding of the wear-resistant refractory layer.
(3) Insufficient Ventilation Holes
If the outer steel shell of the wear-resistant refractory layer (such as the return leg or cyclone separator cone) has too few vents, the evacuation of moisture from the refractory layer during the drying process is hindered. This can cause the refractory layer to burst and shear due to the water vapor being forced to escape inwards, which is another important factor that cannot be ignored.
III. Inadequate Design of Refractory and Wear-Resistant Structures
(1) Irrational Expansion Joint Design of Refractory and Wear-Resistant Materials
Inadequate circumferential and longitudinal expansion design or other factors caused the boiler's wear-resistant refractory materials to expand upon heating, compressing each other and causing cracks.
1. The expansion joint at the junction of the wear-resistant refractory castable and refractory bricks in the cyclone separator inlet flue was appropriately widened from 5mm to 10mm to account for casting requirements, leaving sufficient clearance to accommodate expansion. The expansion joint was changed to a Z-shaped pattern to prevent direct penetration and ash erosion of the insulation layer. The expansion joint filling material required kraft paper layers of refractory fiber felt on both sides to prevent cement from directly penetrating the filling material, solidifying, and occupying the expansion space of the expansion filler. The spacing of the circumferential expansion joints was changed from 1500mm to 1000mm.
2. For large areas such as the separator outlet flue and separator cone, the casting surface was changed from a 2.5m x 2.5m block to a smaller block (1.5m x 1.5m) constructed in a single step. Expansion joints were also provided, with a width of ≤3mm. The filling material required was rigid plywood to prevent deformation during vibration. After the boiler was operational, the plywood burned at high temperatures, and the space became an expansion joint.
3. Brick-lined support pallets were installed along the height of the cyclone separator cylinder to achieve layered unloading of the brick wall. Based on the actual weight of the cylinder's refractory bricks, a single-layer support pallet was insufficient. Based on the actual site conditions, the support pallets were changed to a two-layer arrangement.
4. The support pallets for the wear-resistant refractory castable in the return feeder riser were changed from three layers to four, with the spacing reduced to 2.5m to achieve layered unloading of the castable.
(II) Need to Improve the Shape and Leg Strength of the Catch Pins
1. The shape of the Y-shaped catch pins within the boiler needs to be improved. The density is insufficient. The optimal angle between the gripper legs of the gripper material should be between 60° and 80°. Each leg should have a 10mm horizontal section at the head to enhance the gripper's grip on the castable.
2. The gripper nails in the boiler are welded to metal plates such as the air distribution plate, slag discharge pipe, and air duct. The expansion coefficient of the gripper nails after heating is much greater than that of the refractory material. If the gripper nails are not pre-treated with asphalt and directly contact the refractory castable, a network of micro-cracks will inevitably form on the contact surface between the metal and the refractory, eventually leading to cracking and shedding of the refractory material. The refractory surface contacting the metal should be cleaned of oil and dust and evenly coated with asphalt at least 0.5mm thick. Ensure the asphalt concentration and thickness, and strictly avoid using asphalt paint instead of asphalt oil as a gripper coating.
4. Quality Issues with Wear-Resistant Refractory Castables
Poor quality or poor storage of refractory wear-resistant materials can also cause material shedding. The compatibility and bonding between the aggregate and matrix of wear-resistant materials significantly impacts the material's wear resistance. A mismatch between the aggregate and matrix can lead to cracks in the formed material. Poor bonding between the aggregate and matrix can lead to erosion of the matrix during erosion, which in turn isolates the aggregate and causes it to fall. Ensuring the quality of both aggregate and matrix, and selecting the appropriate binders and additives, is crucial.
Different material manufacturers use different batching methods, but ultimately, users must ensure that the performance targets of wear-resistant refractory materials are met, ensuring that the material quality meets the operating conditions and expansion targets of various boiler components.