Anti-wear measures for circulating fluidized bed boilers

With the increasingly strict environmental protection requirements, people's awareness of environmental protection is increasing, the new, efficient, low-pollution clean combustion technology - circulating fluidized bed combustion technology is widely used. The most important feature of circulating fluidized combustion is that the flue gas carrying a large amount of material is separated from gas and solid when it passes through the separator, and the separated gas enters the tail section and flows through each heating surface in turn for heat exchange before being purified and discharged into the atmosphere, while the separated solid material is sent back to the furnace chamber for circulating combustion through a non-mechanical return valve. The high concentration of materials in the process of circulating combustion causes wear and tear of the heating surface and refractory materials, which seriously restricts the long-cycle economic operation of the furnace. Through long time operation practice, some anti-wear measures are summarized in two aspects of active and passive anti-wear, which are hoped to be helpful for engineering practice.

      1 The concept of wear and tear and its hazards

　　Wear: The phenomenon of continuous loss, transfer or residual deformation of the material on the working surface of an object in relative motion.

　　The main hazards of wear: The wear of circulating fluidized bed boiler is mainly in two aspects: the wear of heating surface and the wear of refractory material. In the wear of the heating surface, whether it is the steam side or smoke and air side of the pipe wear, will make the heating pipe thermal stress changes, uneven heating phenomenon, resulting in the heating surface burst pipe, leakage, and even lead to boiler shutdown; refractory wear will lead to refractory layer off, increased air leakage, more serious wear of the heating surface. Whether the heating surface wear or refractory wear will have different degrees of impact on the normal, safe and economic operation of the boiler.

　　2 heating surface wear and anti-wear measures

　　As for the furnace chamber, the relative motion between the gas-solid two-phase flow in the circulating fluidized bed boiler is large. Solid particle mass inside the furnace in the upward movement with the airflow, at the same time, due to the high fluidizing wind speed in the center region inside the furnace, the four walls of fluidizing wind speed is low, solid particles will also slide to the low fluidizing wind speed around the furnace, aggregation, while downward flow along the chamber membrane wall, strong circulation movement in the furnace thus generated. The wear and tear of the furnace wall is closely related to the process of wall reflux of materials inside the furnace chamber, wall reflux of materials in the reflux process, when encountering obstacles, the flow direction of the material particles will change, obstacles will shunt the material to both sides of the obstacle, the pipe on both sides of the obstacle will be oblique scouring, which will cause wear; wall reflux of materials encountering obstacles after the force, rebound, when the particles are rebounded to the membrane wall When the particles are rebounded to the membrane wall, the particles will form oblique scouring and positive impact phenomenon with the tube wall, thus causing wear and tear.

　　Anti-wear measures of furnace chamber in engineering practice: in addition to selecting reasonable flue gas flow rate during design, strictly controlling the particle size of incoming coal and fluidization speed of combustion chamber during operation, active anti-wear measures should be taken in the structure such as letting tube technology, adding anti-wear beams and longitudinal ribs, as well as passive anti-wear measures such as metal spraying and laying refractory materials.

　　2.1 Let the tube technology

　　It is mainly used in the upper part of the dense phase area of the furnace and the combination of the membrane wall. Let the tube is by changing the geometry of the membrane wall, so that the vertical section of wear-resistant refractory material and the upper membrane wall tube to keep straight, so that the vortex generated by the wall reflux material formed in the refractory area, so as to avoid the wear of the membrane wall. A large number of engineering practice shows that the use of let tube structure can be effective anti-wear.

　　2.2 Anti-abrasion beam technology

　　In the height direction of the membrane wall every certain distance with refractory wear-resistant material made of some tabs, the tabs are arranged in the form of horizontal or inclined arrangement, used to reduce the speed and concentration of materials in the process of wall reflux. Anti-abrasive beam technology is simple and convenient to install, and it is mostly used in engineering practice.

　　2.3 Adding longitudinal ribs

　　In order to extend the service life of the vertical arrangement of the heating surface in the furnace (water-cooled screen, superheating screen), the longitudinal ribs are generally set on the vertical arrangement of the heating surface tube. The longitudinal ribs can regulate the flow field of the material returning to the wall, slow down the wear of the heating surface and extend the life of the heating surface.

　　2.4 Metal spraying

　　Metal spraying is a kind of high-strength wear-resistant metal ceramic coating sprayed on the surface of the membrane wall tube and fins, which is a passive anti-wear technical measures. Generally, metal spraying is carried out in the dense phase area of the upper part of the membrane wall 1~2 m, the membrane wall in the front of the coal feed opening, the four corners of the furnace chamber, the top of the furnace chamber and other parts that are easy to wear. After metal spraying, the wear of the membrane wall itself can be slowed down, but as the running time increases, the spraying layer itself will be worn out and wrinkled off, causing unevenness of the membrane wall and increasing wear. At the same time, when replacing the sprayed membrane wall pipe, the sprayed layer at the bevel of the pipe end must be treated cleanly because the Cr and Ni in the metal sprayed layer melt at high temperature. When the membrane wall pipe for welding operations, in the high temperature welding environment, the Cr, Ni in the metal spray layer will melt into the welding fluid, forming brittle cracks, resulting in poor welding quality.

　　2.5 Laying refractory materials

　　Laying a certain thickness of wear-resistant refractory castable on the membrane wall to slow down the wear of the membrane wall is a passive anti-wear technical measure. Refractory material is fixed by welding on the membrane wall tube pins to be fixed. Mostly used in the boiler dense phase area and around the exit of the furnace, the method of construction is simple, anti-wear effect is good, but the thermal conductivity of the castables is small, will make the heat transfer characteristics of the furnace decreased, the thickness of refractory laying in the project is generally 50 ~ 70 mm.

　　Superheater, economizer tube: in the superheater, economizer tube system windward surface and elbow with anti-wear guard tile to protect. In order to prevent the formation of the flue gas corridor, can be inserted in the middle of the pipe row a few horizontal stainless steel iron plate fixed, forced to make the pipe row neatly.

　　Air preheater tube: vertical tube air preheater tube flue gas inlet with anti-wear casing, while laying a certain thickness of wear-resistant castables to prevent wear; lying tube air preheater tube in the windward side with anti-wear tiles to protect.

　　3 Refractory material wear and anti-wear

　　Circulating fluidized bed boiler heating surface to take certain active measures to prevent wear at the same time, but also to take passive anti-wear measures, that is, in the wear-prone parts of the heating surface to lay a certain thickness of refractory materials to slow down the wear of the heating surface. Refractory materials and thermal insulation, sealing the role.

　　Because of the different expansion coefficients of refractory and metal parts, with the cyclic temperature fluctuations in the furnace will appear refractory cracks and spalling phenomenon. At the same time, a large number of solid materials in the furnace on the refractory scouring, will also produce some damage to the refractory. The construction method, maintenance and baking of refractory materials will also affect the service life of refractory materials. Therefore, it is necessary to choose wear-resistant materials reasonably, pay attention to construction quality, and run operation and maintenance reasonably to slow down the wear of refractory materials.

　　4 About anti-wear in operation

　　After the boiler is designed, manufactured, installed, built and baked, the wear of boiler heating surface and castables is closely related to the operation process. Theory and practice show that wear and time, fly ash particle size, flow rate and fly ash concentration have a certain relationship, namely

　　T=Cη2μω3τ. (1)

　　In the formula (1): T is the amount of wear, g/m2; τ is the time, h; η is the fly ash impact rate, with the fly ash concentration, fly ash particle diameter, the viscosity of the flue gas flow, flue gas flow rate and the diameter of the pipe, fly ash particle size and its distribution characteristics with the combustion characteristics of coal, fineness, combustion methods and combustion conditions; μ is the mass concentration of fly ash in the flue gas, g/m3; ω is the flow rate of fly ash particles, can be approximated by taking equal to the flue gas flow rate, m/s; C is the proportionality constant, representing the wear characteristics of fly ash particles, and coal type.

　　It can be seen that in operation, the flue gas flow rate should be reduced, the ash concentration in the flue gas should be reduced, the differential pressure in the furnace should be reduced, and the particle size of the incoming fuel should be reduced in order to reduce the wear of the heating surface and the casting material.

　　4.1 Reduce the flue gas flow rate

　　The air volume required for fuel combustion in circulating fluidized bed boilers is composed of primary and secondary air. The primary air provides oxygen for fuel combustion at the same time, the most important role is to provide fluidization wind for the fluidized bed material; the secondary air is the combustion wind, to provide oxygen for full combustion of fuel. The primary air volume has a significant impact on fluidization quality, circulating material volume, circulating material particle size, bed temperature, and NOX generation. Too low primary air volume will lead to poor fluidization causing high bed temperature and even coking in the furnace chamber and other serious consequences, affecting the safe operation of the boiler; too high primary air volume will lead to an increase in the amount of circulating material, circulating material particle size, flue gas flow rate increases, NOX concentration increases, which will accelerate the heating surface and casting material wear, NOX emission reduction management difficulties increase, affecting the economic operation of the boiler. Therefore, the air distribution should be reasonable in operation.

　　4.2 Reduce ash concentration

　　Circulating ash concentration is a measure of the amount of circulating materials, is an important parameter affecting the heat transfer in the furnace, bed temperature, evaporation, steam temperature, and also has a great impact on wear. High concentration of circulating ash, good heat transfer in the furnace, high bed temperature, strong capacity with load, high steam temperature, low concentration of circulating ash, wear and tear reduction. In operation, the amount of circulating ash should be reasonably controlled.

　　4.3 Reduce the differential pressure of the furnace chamber

　　Differential pressure of the furnace chamber is a measure of the amount of bed material inside the furnace chamber, high differential pressure, more bed material, stable combustion, uniform temperature in all parts of the furnace chamber, less bottom slag combustible; differential pressure is too low may lead to high bed temperature in the lower part of the furnace chamber, bottom slag combustible material, poor economy.

　　4.4 Reduce the size of coal combustion

　　Coarse coal particle size, in order to ensure that the bed fluidization, need to increase the primary air volume, thereby increasing the flue gas flow rate. In addition, in the boiler start and stop process, the temperature rise or cooling rate should be limited to prevent excessive thermal stress.

　　5 Conclusion

　　Circulating fluidized bed boiler own material cycle combustion characteristics, inevitably cause wear and tear on the heating surface and refractory materials, we in the design, manufacture, installation, construction, baking and operation of each link, take reasonable and effective anti-wear measures to slow down the heating surface and castings wear, so as to improve the reliability and economy of boiler operation.