Some CFB boiler wear-resistant refractory castables will fall off during the construction process. The reasons are as follows:
1. Unreasonable furnace construction process
(I) Improper control of the “ash-water ratio” when mixing the castable
During the construction process, the ash-water ratio should be strictly controlled when preparing the castable. If too much water is added, the internal porosity of the material will be high after the castable is formed, and the material strength will be greatly reduced, especially in low temperature environments, and the natural coagulation time will be prolonged. If too little water is added, the material fluidity is poor, the vibration is not dense, and it is easy to leave pores, caves, etc., and the strength of the castable will also be greatly reduced.
(II) Improper control of mixing time and castable vibration time
When pouring castables, a vibrator should be used for layering. If the mixing time is too short, the material is not mixed evenly; if the vibration time is too long, the material is prone to stratification, fine powder floats on the surface, and aggregate sinks to the bottom, resulting in a decrease in material strength and easy falling. The castable should be used up within 30 minutes after mixing, and it is advisable to pour it to the specified thickness and height at one time. The construction of the castable should be carried out at an ambient temperature of ≥5°. If the temperature is too low, the material is not easy to naturally solidify, or “false solidification” may occur. The castable should generally be poured continuously, and the next layer of castable should be poured before the previous layer of castable is initially solidified. If the construction gap exceeds its initial solidification, it should be treated according to the requirements of the construction joint.
(III) Poor control of demoulding time
The castable cannot be demoulded before it hardens. It should be removed when the strength of the castable can ensure that its edges and corners are not damaged by demoulding. The load-bearing formwork should be demoulded when the castable reaches 70% of its strength. To facilitate demoulding, all mold casting surfaces should be coated with a layer of engine oil before pouring.
(IV) Improper control of the curing time of the castable
After the boiler is built, there must be a sufficient natural drying period to allow most of the moisture in the refractory layer to be separated to avoid the refractory layer cracking and falling due to the inability to discharge a lot of moisture in time during the furnace drying.
(V) Improper control of the furnace heating speed and temperature gradient
2. Subjectively, the importance of furnace drying is not fully understood
(I) Before furnace drying, the furnace lining material must have a certain natural strength
Furnace drying is to remove the free water and crystal water in the furnace lining material that cannot be removed by natural drying, and at the same time, the castable is cured at high temperature to achieve a certain strength. Therefore, before furnace drying, the furnace lining material must have a certain natural strength (satisfactory curing period) before it can be carried out.
(II) Furnace drying principle-“It is better to be long than short, and slow than fast”
The furnace should be dried according to the furnace drying curve formulated in advance, the heating speed should be uniform and stable, the constant temperature time and temperature should be controlled well, and the temperature fluctuation should be maintained within ±20℃ to avoid heating too fast during the furnace drying process. The moisture generated in the material layer cannot be discharged in time, resulting in excessive internal water vapor pressure and breaking through the wear-resistant refractory layer to cause cracks. In addition, the temperature gradient during furnace drying should not be too large to avoid huge thermal stress, which will cause the wear-resistant refractory layer to crack, bulge, and fall off.
(III) Too few exhaust holes are reserved
When the exhaust holes of the outer steel shell of the wear-resistant refractory material layer (such as the return leg and the cone of the cyclone separator) are too few, the water vapor in the refractory layer is blocked from being discharged externally during the furnace drying process, and the refractory layer can only be discharged internally, resulting in bursting and falling, which is also an important factor that cannot be ignored.
3. The design of the refractory and wear-resistant structure is not perfect
(I) The expansion joint design of the refractory and wear-resistant materials is unreasonable
Due to the insufficient number of circumferential and longitudinal expansion designs or other reasons, the volume of the boiler wear-resistant refractory materials expands after heating, squeezes each other, and cracks occur.
1. Considering the casting factors, the expansion joint at the joint between the wear-resistant refractory castable and the refractory brick of the cyclone separator inlet flue is appropriately widened from 5mm to 10mm, and sufficient space is reserved to meet the expansion requirements. The expansion joint is changed to a Z shape to avoid the gap from directly penetrating and forming furnace ash scouring the insulation layer. The expansion joint filling material requires kraft paper layers of refractory fiber felt on both sides to prevent cement from directly penetrating into the filling material, solidifying and occupying the expansion space of the expansion filling material. The planning interval of the circumferential expansion joint was changed from 1500m to 1000mm.
2. For large areas such as the separator outlet flue and separator cone, the casting surface was changed from (2.5m×2.5m) to a small area of square blocks (1.5m×1.5m) for one-time masonry, and expansion joints were set at the same time. The expansion joint width was ≤3mm. The filling material required rigid plywood to prevent deformation during vibration. After the boiler is running, the plywood is burned at high temperature and the space becomes an expansion joint.
3. The cyclone separator cylinder is equipped with brick lining support plates along the height direction to achieve the purpose of layered unloading of the brick wall. By calculating the actual weight of the refractory bricks of the cylinder, the planning of one layer of support plates cannot achieve the purpose of unloading. According to the actual situation on site, the support plates are changed to two layers.
4. The wear-resistant refractory castable support plate of the return feeder riser is changed from 3 layers to 4 layers, and the distance is reduced to 2.5m, so as to achieve layered unloading of castables.
(II) The shape of the gripping nail and the strength of the nail legs need to be improved
1. The shape of the Y-shaped gripping nail in the boiler needs to be improved. The density is not good enough. The optimal angle of the gripping nail legs of the gripping nail material is between 60° and 80°. The heads of the two nail legs are each simmered with a 10mm horizontal section, so as to enhance the gripping force of the gripping nail on the castable.
2. The gripping nails in the boiler are welded on metal plates such as air distribution plates, slag discharge pipes, and air ducts. The expansion coefficient of the gripping nails after heating is much greater than the expansion coefficient of the refractory material. If the gripping 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 material and the refractory material, causing the refractory material to crack and fall off in the end. The surface of the refractory material in contact with the metal should be cleaned of oil and ash stains and evenly coated with asphalt of more than 0.5mm. To ensure the concentration and thickness of asphalt, resolutely put an end to the use of asphalt paint instead of asphalt oil as a nail coating paint.
4. Problems with the quality of wear-resistant refractory castables
Poor quality or poor storage of refractory wear-resistant materials will also cause the material to fall off. The matching and bonding between the aggregate and the matrix of the wear-resistant material have an important impact on the wear resistance of the material: the mismatch between the aggregate and the matrix will cause cracks in the material body after molding; and the poor bonding between the aggregate and the matrix will cause the matrix to be eroded first during scouring, and then the aggregate will be isolated and fall off. It is crucial to ensure the quality of aggregates and matrices and to select suitable binders and additives.
Different material manufacturers have different batching methods, but for users: ultimately, it is necessary to ensure the various performance targets of wear-resistant refractory materials, ensure that the material quality is up to standard, and meet the requirements of the operating conditions and expansion targets of various parts of the boiler.
