Analysis of the wear-prone parts and wear mechanism of circulating fluidized bed boiler lining

Analysis of the wear-prone parts and wear mechanism of circulating fluidized bed boiler lining

The circulating fluidized bed boiler lining is made of refractory materials. The purpose of setting refractory materials is mainly to prevent the high-temperature oxidation corrosion and wear of metal components by the high-temperature flue gas and materials of the boiler, and it has a heat insulation effect. The damage and wear of materials first occur on the refractory materials, thereby ensuring the service life of the metal structure. This is one of the main measures to ensure the long-term safe operation of the circulating fluidized bed boiler, and it is also one of the main features of the circulating fluidized bed boiler. Refractory materials are of great significance in reducing the effect of metal structures, reducing construction costs, and overhaul and maintenance. The appearance of refractory materials is generally flat or arc structure, which is basically consistent with the running direction of the material, so the wear is generally more uniform. However, due to the coarse aggregate of its composition, about 0.5mm, its surface cannot be quite smooth, so its wear rate is still fast. In some parts, due to the change of the running direction of the flue gas and materials, the material speed increases, causing serious wear, which must be given enough attention.

The main areas where circulating fluidized bed boilers use refractory materials are: combustion chamber, high-temperature separator, external heat exchanger, flue and material return pipeline, etc.

1. Main parts of lining wear

1.1 Combustion chamber

① Wear reasons

One of the main advantages of circulating fluidized bed boilers is that they have strong load regulation performance. In circulating fluidized bed boilers, the combustion chamber temperature reaches 900℃~1000℃ during normal operation. In order to adapt to load changes or peak-shaving needs, load fluctuations often occur, resulting in heat and temperature cycle changes, or start-up or shutdown due to peak-shaving needs; for example, sometimes the temperature change in the combustion chamber can reach 500℃ in a few minutes, and sometimes there may be more than a dozen starts and shutdowns within a week, which will cause thermal shock and thermal stress to the combustion chamber refractory materials, causing damage to the refractory materials.

The furnace part uses a thick lining, which is composed of (75~150)mm dense and wear-resistant castables or plastics covering insulation materials of similar thickness. Usually, the destruction is caused by excessive cracks and extrusion peeling. Shrinkage during drying, thermal shock, and plastic deformation under stress are the main causes of cracks. Stainless steel fibers help reduce cracks, but they cannot completely solve the problem. When the bed material is caught in the cracks, extrusion and peeling will occur when the furnace lining is repeatedly cycled.

② Common lining materials for combustion chambers

Phosphate-bonded mullite-based refractory plastics are durable in the combustion area, mainly due to their volume stability (resistance to thermal expansion and contraction) and good wear resistance. Since phosphate adhesives have good bonding with existing materials, they are often used to repair defective areas. When using, ensure that the repaired area is supported, and at least two pins should be used. Ceramic or cast alloy pins should be used for plastic linings exposed to ultra-high temperature areas. In addition, pins can be used alone to support the insulation layer.

In the dilute phase area located in the upper part of the furnace, silicon carbide tiles are often used to reduce the wear of water-cooled wall tubes. Diamond mortar is used behind the tiles to improve heat transfer to the tubes, and the tiles are generally supported on the tubes with welded supports. In the design of the section where the water-cooled wall tube extends downward to the bottom of the combustion chamber, the lining usually includes a thin, dense, high thermal conductivity, wear-resistant plastic or castable.

Silicon carbide-based castables are usually used on the tubes with welded pins because these castables can be repaired with phosphate-bonded plastic, especially the filler containing silicon carbide has high thermal conductivity. If the bed material or circulating ash contains a high alkali content, phosphate-bonded plastic should be used because calcium aluminate cement is destroyed after being exposed to alkali at high temperatures.

1.2 Cyclone separator inlet and cylinder

① Causes of wear

The top of the furnace and the inlet section of the separator, the intersection of the cyclone cylinder arc surface and the flue plane is the main wear area. At this location, the flue gas rotates, the material direction changes, the speed is high, the particle size is coarse, and the density is large, so the wear is very fast. At the same time, the refractory material in this part is thicker, generally uneven, and the temperature gradient is also uneven. In addition, it is subjected to high temperatures of about 900℃, and occasionally reaches more than 1100℃. Therefore, excessive thermal shock will cause cracks in the lining material and damage the wear-resistant material. In addition, the separator barrel and cone are subjected to very harsh working conditions, including temperature shocks, cyclic changes and wear of 500℃ to 600℃ within a few minutes. For many linings, repeated thermal shocks, temperature cyclic changes, wear and extrusion peeling have jointly led to large-scale damage. When cracks or wear occur, the surface is rougher or has protrusions, and the wear rate will be further accelerated.

② Commonly used refractory materials

From the perspective of refractory material structure, the casting of the cyclone is layered and block casting, and each layer is fixed to the metal structure with pins. There is an expansion gap between each block, and the layers are staggered. In general, dense and wear-resistant materials containing stainless steel fiber wires are used for both the top of the furnace and the inlet of the separator. This material has a satisfactory service life. If there are too many cracks or damage due to thermal shock, temperature fluctuations, etc., fused silica-based castables can be used instead.

For cyclones and cones, super-strong castables are generally used. When crack wear occurs, one of the repair options is to replace the cast thick lining with refractory bricks or refractory prefabricated blocks covered with wear-resistant mullite bricks. It can also be repaired with phosphate-bonded plastics. Another possibility is to use a thin lining with a low thermal expansion coefficient, such as a fused silica castable. However, compared with phosphate-bonded mullite plastics, most fused silicas have poor wear resistance and thus a shorter service life.

1.3 Riser and returner

In actual boiler operation, this part often has problems. Thermal shock, severe wear and cyclic changes lead to repeated damage. The original design was to use thick dense insulation castables, but this material is difficult to construct properly because it is difficult to have enough construction space. The castable insulation layer of the lining is constructed by vibrating casting. Stainless steel fiber wire should be considered in the wear-resistant castable. The lining with insulation bricks or castables as the base and wear-resistant bricks on top is also effective. Phosphate-bonded plastic containing stainless steel fiber is mainly used for the repair of existing boiler linings.

1.4 Wear of expansion joints

There are two important expansion joints (return leg expansion joint and cyclone separator inlet expansion joint), which are set to compensate for the expansion difference. When the expansion exceeds the designed gap or high-temperature materials enter the gap, the refractory material of the expansion joint is damaged by friction or force extrusion. In this way, a large amount of high-temperature materials enter the expansion joint, aggravating the wear and even directly burning metal objects, causing the boiler to fail to operate.

2. Lining wear mechanism

2.1 Damage caused by thermal stress and thermal shock

The first reason for the destruction of refractory materials is that cracks and spalling of refractory materials are caused by temperature cycle fluctuations, thermal shocks and mechanical stress. When the temperature cycle fluctuates, the internal stress is formed due to the different thermal expansion coefficients between the refractory aggregate and the binder, thereby destroying the refractory layer. Temperature cycle fluctuations often cause large cracks and spalling of the refractory lining. Thermal shock caused by rapid temperature changes can cause stress in refractory materials to exceed tensile strength and cause them to peel off. The damage to refractory materials caused by mechanical stress is mainly caused by the different thermal expansion coefficients between refractory materials and metal parts passing through the lining of refractory materials. Therefore, if the appropriate expansion space is not considered during design, it will cause refractory materials to peel off.

2.2 Damage caused by scouring of solid materials

The wear of refractory materials caused by the scouring of materials on refractory materials is the second reason for the damage of refractory materials. The easily worn areas of refractory materials in circulating fluidized bed boilers include corner areas, cyclone separators and solid material return pipelines. In general, the wear of refractory materials increases with the increase of the impact angle; in addition, when designing cyclone separators, flues, etc., the impact angle should be as small as possible.

2.3 Damage caused by changes in the properties of refractory materials

During operation, damage to refractory materials caused by changes in the properties of refractory materials is another reason for the damage of refractory materials. For example, the gradual failure of refractory materials due to the penetration of alkali metals and the deterioration and damage of refractory materials due to carburization are all damage caused by changes in the properties of refractory materials.