Under the combustion conditions of CFB boilers, the wear of solid particles on the heating surface has always been a key issue restricting the economic operation and further development of CFB boilers. There are airflows, jets and bubbles in different directions, speeds, angles and concentrations in CFB boilers. They act on the inner surface in various forms. In addition, the presence of corrosive gases and other media forms a complex wear process. The main method of maintaining the boiler is to lay wear-resistant refractory materials in certain areas to protect the heating surface. Under reducing conditions, wear-resistant refractory materials are more wear-resistant than steel. Wear-resistant materials are a low-cost barrier layer that prevents wear and protects the safe and reliable operation of the boiler.

CFB boilers operate at high temperatures, and the temperature in the furnace changes frequently, causing cyclic thermal shock. At the same time, a large number of high-speed, high-temperature solid particles exist within the furnace, constantly eroding the heating surfaces. Therefore, wear-resistant refractory materials are required for protection. This article only discusses wear-resistant refractory materials on the fire-facing (flue gas-facing) side.

I. Introduction to Wear-Resistant Materials

(I) Types of Wear-Resistant Refractory Materials

Introduction to Wear-Resistant Refractory Materials for Circulating Fluidized Bed Boilers

Wear-resistant refractory materials can be divided into shaped and unshaped materials based on their supply state. Based on their function, they can be divided into three categories: wear-resistant refractory materials (including bricks, castables, plastics, and mortar); refractories (including bricks, castables, and mortar); and refractory insulation materials (including bricks, castables, and mortar).

1. Shaped Materials

1) Wear-Resistant Refractory Materials (High-Density Refractory Materials): Silica-Alumina Bricks (Clay Bricks, High-Alumina Bricks) and Non-Oxide Bricks (Silica-Molybdenum Bricks, Silicon Carbide Bricks).

2) Insulation Materials: Refractory Insulation Bricks, Insulation Bricks, Insulation Blocks, Ceramic Fiber.

2. Amorphous Materials

Amorphous materials include castables, trowels, plastics, patching materials, spray-on materials, injection-type materials, vibrating materials, and plaster coatings. Based on their form, they can be categorized as powder, mud, or clumps.

(II) Properties of Wear-Resistant Refractory Materials

Wear-resistant refractory materials are specialized products that resist damage and deformation at high temperatures. To prevent wear from smoke and fly ash, wear-resistant materials are applied to the interior of some wear-prone components. Proper selection and installation of these materials are crucial to ensure long-term system performance and reduce wear-resistant material shedding and the need for repairs.

Wear-resistant refractory materials are primarily chemically composed of aluminum and silicon compounds, with a total content of 80% to 95%.

In CFB boilers, in order to withstand the impact of the environment, wear-resistant refractory materials must have a certain degree of refractoriness, compressive strength, flexural strength, thermal shock resistance and a sufficiently small linear change rate.

2. Design of wear-resistant materials for CFB boilers

(I) Causes of wear-resistant material damage

In CFB boilers, on the one hand, a large number of high-speed high-temperature solid particles continuously wash away the wear-resistant materials and damage them. On the other hand, the wear-resistant materials are cracked and peeled off due to temperature fluctuations, thermal shock and mechanical stress in the furnace. At the same time, the penetration of alkali metals and the like can easily cause the failure and damage of the wear-resistant materials.

(II) Design of wear-resistant materials

In CFB boilers, the wear-prone areas mainly include the water-cooled wall and the heating surface arranged in the furnace, the separation and return system, the tail flue inlet and the slag discharge system. The main areas where wear-resistant materials are used include the dense phase area of the furnace, the lower area of the screen-type heating surface in the furnace, the furnace outlet, the separator, the material leg, the return valve, the separator outlet flue and the tail convection flue.

Considering the aforementioned wear characteristics, the physical properties of wear-resistant materials, and their construction requirements, single or multiple layers of wear-resistant refractory materials are generally used. Single-layer applications are typically used in thin linings such as water walls, platen heating surfaces, water-cooled partitions, double-sided water walls, and steam-cooled separators. φ6 or φ10 cylindrical pins (or Y-shaped or V-shaped pins) are used to support the wear-resistant refractory castable on the flue gas side of the tube wall. Thin linings are more resistant to the rapid thermal shock of boiler startup and shutdown. Furthermore, stainless steel fibers are added to the castable to enhance its rigidity and impact resistance. Thick linings are constructed in two or three layers. The flue gas side is constructed with wear-resistant and high-temperature-resistant refractory bricks or cast wear-resistant plastic. The inner lining's insulation minimizes heat loss and lowers the shell temperature.

III. Construction and Maintenance of Wear-Resistant and Refractory Materials

(I) Expansion Joint Setup

During the construction process, shaped wear-resistant materials are unloaded layer by layer via support plates, transferring the load to the outer shell steel plate. Due to the difference in linear expansion coefficients, the placement of expansion joints is critical. Construction joints also serve as expansion joints. For areas with excessively large castable areas, construction joints should be constructed alternately, leaving sufficient construction joints between blocks. Construction joints should be staggered.

(II) Treatment of Metal Anchors

Due to the difference in linear expansion coefficients between metal and wear-resistant materials after heating, asphalt must be applied to the metal surfaces (including pins, support plates, and pipes) before the wear-resistant and refractory materials are installed. This asphalt application leaves a certain gap between the castable and the heated surface. This ensures that the differential thermal expansion of the heated surface does not damage the wear-resistant material during operation.

(III) Vibration

Vibration is an essential procedure for castable construction. If the vibration is insufficient or not in place, the density of the wear-resistant castable cannot meet the requirements or is uneven, which can cause the castable strength to be insufficient and difficult to play a wear-resistant role. The templates in difficult-to-construct locations should be as small as possible.

(IV) Thermal curing of wear-resistant refractory materials

Since the baseless mineral raw materials contain a certain amount of water, a certain amount of water is added as a solvent during the preparation of the castable. This water needs to be discharged through natural drying and thermal curing. Because the density of the wear-resistant refractory castable is high and its crystal water is inside its molecular structure, the thermal curing furnace needs sufficient time. Although the thermal curing adopts controllable temperature hot flue gas curing, water should still be added to the boiler heating surface during curing to protect its safety. Usually, the heating curve of thermal curing is provided by the material supplier, or the heating curve approved by the supplier is used. After heat curing, all casting holes in the sealing box must be sealed and welded. All steam passages opened for heat curing should also be sealed and welded.

IV. Conclusion

To ensure safe operation of the unit, wear-resistant refractory materials for CFB boilers must possess the following properties: high room-temperature and hot strength; low wear; excellent corrosion resistance; and good high-temperature volume stability. Currently, wear-resistant refractory castables for circulating fluidized bed boilers generally comply with the YB/T 4109-2002 standard. The corresponding aluminum contents are 60%, 65%, and 70%, respectively. However, in actual application, various physical and chemical parameters may need to be adjusted based on different production conditions and different locations.