Due to the high ash concentration in the circulating fluidized bed boiler, which is usually several tens of times, several hundred times, or even thousands of times of the pulverized coal furnace, the wear of the circulating fluidized bed boiler is much more serious than other types of boilers. Special attention should be paid to the wear protection of refractory materials. How to solve the wear problem is directly related to the success or failure of the design of the circulating fluidized bed boiler, and also directly affects the availability of the circulating fluidized bed boiler unit.
Concept and Form of Wear and Tear
Due to mechanical action, or chemical or electrical interaction, the phenomenon that the working surface material of the object is continuously lost in relative motion is called wear.
According to different wear mechanisms, wear can be generally classified into adhesive wear, abrasive wear, corrosion wear, contact fatigue wear, erosion wear, fretting wear, and the like. The wear caused by the impact of fluid or solid particles on the surface of the refractory material at a certain speed and angle is called erosion (or impact wear). There are two basic types of erosion, called erosion wear and impact wear, which are completely different in the microscopic morphology of the erosion process.
Scouring wear is a small, or even nearly parallel, impact angle of the particles relative to the solid surface. The velocity of the particles perpendicular to the surface of the solid causes it to break into the object being impacted, and the speed at which the particles are tangent to the surface of the solid causes it to slide along the solid surface, and the effect of the two split-speed synthesis acts as a planing. If the impacted object cannot withstand this effect, it is cut off by a small piece, so that after a large number of repeated actions, the solid surface will wear. Impact wear refers to the impact angle of the particles relative to the solid surface is large, or close to vertical, impacting the solid surface at a certain speed to produce tiny plastic-induced micro-cracks, and gradually deforming plastic under repeated impact of a large number of particles for a long time. The wear of the entire layer of the layer is detached.
According to different wear methods, the wear can be divided into two object wear and three object wear. In the wear of two objects, the solid hits and washes the wall by its own momentum. In the three-object wear, the solid particles moving along the wall surface are impacted by the clusters, and the clusters use the former as the wear medium to wear the heated surface. Although the wear mechanism of circulating fluidized bed boilers is not fully understood, it is predicted that three object wear is the main cause of wear of circulating fluidized bed boilers. Three object wear can occur in three situations: particle enrichment settles at a large density, feed is sufficient to produce a large particle density, and large particle densities can exist in the vicinity of the wear surface within the particle flow tolerance.
Factors Affecting Wear
The wear of coal ash particles in the circulating fluidized bed boiler to the boiler material belongs to the erosion of the particle flow, which has the impact of the particles on the material in the furnace and the scouring of the material by the high concentration ash containing gas flow. The wear of circulating fluidized bed boiler materials depends largely on the size of the particles, the shape of the particles, the impact velocity, the angle of impact, the amount of feed, the strength and hardness of the particles, and the like. In addition, the degree of wear is also related to the material of the impacted surface, and is also affected by fuel characteristics, operating parameters, and the like.
Wearable Parts and Anti-Wear Measures for Circulating Fluidized Bed Boilers
The wearable parts of circulating fluidized bed boilers have heated surface tubes and refractory materials. The wearable metal parts of the circulating fluidized bed boiler are the junction of the refractory material and the water wall, the irregular wall area, the four corners of the water wall, the furnace heating surface, the furnace top heating surface, the cyclone separator, and the tail convection heating surface.
Wear of Transition Zone Between Ignition Zone and Water Wall
The wear in this area occurs at the junction of the furnace and the water wall tube, and the four corners are more severely worn, within a range of 500 mm above the refractory belt. Among them, when there is a step of refractory material in this part, the water wall and fin wear speed is accelerated. There are two reasons for the wear. First, the refractory zone is vortexed locally due to the opposite direction of the solid material flowing down the wall and the solid material flowing upward in the furnace. On the other hand, the solid refractory material flows down the wall of the furnace. A change in the direction of flow at the junction creates a repulsive force that causes erosion of the waterwall tube.
At present, the commonly used methods for erosion protection in this position are structural wear and material wear. Structural anti-wear mainly includes measures such as tube design, anti-wear beam and anti-wear plate design. Allow the tube design to remove eddy and rebound force wear at that location. Methods such as anti-wear beams, wear plates and welded steel bars achieve wear protection by reducing material speed. The material anti-wear mainly adopts methods of replacing wear-resistant materials, anti-wearing tiles and wear-resistant coatings. The preparation methods of wear-resistant coatings include arc spraying, oxyacetylene flame powder spray melting, surfacing welding, thermal diffusion and other measures. .
Wear of the Heated Surface in the Furnace
The heating surface in the furnace mainly has double-sided water-cooled wall, screen superheater and screen reheater. Their wear mechanism is similar to that of the furnace water wall tube, which depends mainly on the specific structure of the heated surface and the flow characteristics of the solid material. . Abrasion-resistant materials are usually applied to the bottom of them, and the rest are subjected to wear-resistant coatings for wear protection purposes.
Wear of the Heating Surface of the Top of the Furnace
The wear of the heated surface of the furnace roof is mainly caused by the gas-solid flow turning away from the top of the furnace, generating centrifugal force and smashing the large-grained material to the top of the furnace. As the capacity of the circulating fluidized bed increases, the furnace height also increases. The problem of the wear of the heated surface of the furnace is not serious, and it can generally be solved by pulling the top of the furnace away from the horizontal flue of the cyclone.
Wear of the Cyclone Separator
Most of the components of the cyclone separator of the circulating fluidized bed boiler (except the central cylinder) are generally coated with refractory materials. Therefore, the wear of the metal components of the cyclone separator is not very serious, and the damage of the central cylinder of the cyclone separator is often caused by thermal deformation. of. The wear of the cyclone mainly occurs in the inlet flue and the upper part of the cylinder, where the direction of the airflow is deflected by 90 degrees, and the disappearance of the annular core zone also causes a large pulsation, which greatly affects the vicinity of the inlet end. Therefore, necessary measures must be taken to prevent the wall of the pipe from failing.
Wear of the Tail Convection Heating Surface
The main parts of the tail convection heating surface occur in the superheater, reheater, first and second rows of tubes of the economizer, and at the outlet of the air preheater. This part of the pipe row often forms a flue gas corridor due to structure, installation or thermal deformation, and the material speed is high, resulting in accelerated wear. It is usually worn by a method of adding a protective plate, a flow equaling plate, and a wear-resistant cover.
