Supersonic arc spraying of water wall of circulating fluidized bed boiler

       Boiler water wall supersonic arc spraying

Circulating fluidized bed boiler water-cooled wall supersonic arc spraying 1. Introduction In view of the serious wear and tear of the combustion chamber heating surface of the circulating fluidized bed boilers that have been put into operation in China, and combined with some domestic circulating fluidized bed boiler users to solve the combustion chamber heating surface As an example of the application of wear and tear technology, the thermal power plant of Hunan Xiangtan Power Generation Co., Ltd. decided to carry out supersonic arc spraying on the heating surface area of the water wall combustion chamber of the circulating fluidized bed boiler to increase the service life of the heating surface area that is easy to wear and achieve energy efficiency. Better long-term stable operation. 2. Spraying area of construction site: water wall specification: Φ38.1×7.5 SA-213T2, water wall pitch: 50.8mm, spraying height: boiler elevation 22--33m, spraying width: furnace depth 6-9 meters, spraying Area: about 150 m2×2 furnaces, calculated according to the actual construction area. 3. Wear Mechanism of Heating Surface of Circulating Fluidized Bed Combustion Chamber Circulating fluidized bed combustion technology is a clean coal combustion technology developed rapidly in the past two decades. The circulating fluidized bed boiler has the advantages of strong coal adaptability, high combustion efficiency, low pollutant emission and good load regulation performance. Since there are a large number of bed materials and circulating materials in the furnace, the coal is cyclically burned at low temperature in a fluidized state. By adding an appropriate amount of limestone, staged combustion, and staged air supply, the furnace can be desulfurized and the amount of nitrogen oxides can be suppressed at the same time. However, due to the large amount of fly ash particles in the flue gas, these ash particles scour the water-cooled wall tubes, convection heating surfaces and other parts at high speed, causing the wall surface to be severely worn, causing local serious damage, and even causing a shutdown accident. We know that the primary air flow from the furnace bottom air distribution device must have sufficient speed, strength and rigidity to generate strong disturbances while supporting solid particles. The operation found that when the density of the bed material is 8-10kg/m3, the particles in the bed will aggregate into large particle clusters, and the agglomerated particle clusters will settle at a relatively large relative speed due to the increase in weight and volume and have a side wall effect. , so that the gas-solid two-phase flow in the fluidized bed forms a very dense particle cluster near the wall, which moves obliquely downward and tangentially, and descends to the furnace wall to swirl upward, and the particles frequently collide and rub against each other and the furnace wall. Serious wear on the furnace wall. It is generally believed that the wear of circulating fluidized bed boilers is mainly related to factors such as solid material concentration, flue gas velocity, fuel characteristics, operation adjustment, circulation rate and structural design. Wear Mechanism of Water Wall on Heating Surface of Combustion Chamber Wear of water wall is one of the most serious material-related problems in CFB boilers. In the CFB boiler furnace, the typical hydrodynamic structure is the ring-core structure. In the inner core zone, the particle mass moves upward; in the outer annular zone, the solid particles flow back down along the water wall of the furnace. The thickness of the annular zone gradually decreases from the bottom of the bed to the top. The downflow of solid materials along the water wall is the main reason for the wear of the water wall. The serious wear of the water wall is closely related to the sudden change of the size and direction of the return material. Usually the parts with sudden change of direction are: (1) The turning point of the water-cooled wall guard-burning belt. (2) The butt joint and surface defects of the membrane water-cooled wall tube are poorly welded, with burrs and protrusions. (3) There are raised parts in other parts of the water cooling wall. Therefore, the wear of the water wall in the furnace can be mainly divided into the following two situations: the wear of the tube wall in the turning area of the sanitation zone and the water wall tube, and the wear of the four corners of the furnace and the wall area of the general water wall. 3.1 The wear of the tube wall in the lower part of the furnace and the turning area of the water-cooled wall The mechanism of the wear of the lower part of the furnace and the turning area of the water-cooled wall has the following aspects: First, the solid material flowing down the inner wall of the furnace and the upward flow in the furnace The moving solid material runs in the opposite direction, thus locally generating a vortex flow. The second is due to the change of the flow direction of the solid material flowing down the wall in the junction area, thus causing wear to the water-cooled wall. The wear of the water wall tube wall in the junction area between the water wall and the fire protection zone does not occur evenly around the furnace, but is related to the overall flow of materials in the furnace. Under normal circumstances, the power plant adopts welding fins on the water wall to destroy the solid material flowing downward, so as to achieve the purpose of anti-wear. Practice has proved that the effect is not very ideal, the reason is that the change of the flow direction of the solid material at the junction of the fin and the water cooling wall produces new wear points. 3.2 Wear of the four corners of the furnace and the general water-cooled wall area In many CFB boilers that have been operating, it is found that the wear of the four-corner area of the furnace and the general water-cooled wall is quite serious, so the proportion of furnace shutdown is very high. The worn parts are not only within two meters above the guard belt, but also appear in higher positions. Its characteristic is that the wear position is not fixed. The main reasons are as follows: 3.2.1 The concentration of solid materials flowing down the wall in the corner area is high, and the flow state is easily changed. 3.2.2 The particles collected in the four corners have a greater chance of impact wear on the metal surface than the particles on one side of the water-cooled wall. 3.2.3 Wear due to poor fluidization or localized jets. After the air cap is damaged by wear, a local high-speed jet is generated in the dense phase area, and the bed material particles entrained by the jet will directly scour the heating surface of the water wall at a higher position, resulting in wear. 4. Basic principle and technical characteristics of supersonic arc spraying The melted wire is atomized into fine and uniform particles, which are sprayed onto the surface of the workpiece to form a mechanically anchored, tightly bound coating. It is especially suitable for protective spraying of boiler tube wall due to the following process characteristics. (1) The particles have high flying speed and are well combined with the matrix. (2) High deposition efficiency and dense coating structure. (3) High arc stability. ⑷ There is no thermal deformation of the sprayed substrate. ⑸ After the first spray coating is worn away, it can be sprayed twice. 5. Basic parameters of supersonic arc spraying Particle velocity ≥ 470m/s Coating porosity ≤ 0.9% Substrate temperature ≤ 80℃ 6. Requirements for spray coating and selection of spray materials 6.1 Requirements for spray coating performance The wear mechanism determines that the coating should have the following properties: 6.1.1 It has high wear resistance 6.1.2 It has good ductility 6.1.3 It should have certain corrosion resistance 6.2 Material selection Thermal spraying technology has developed to the present, applying supersonic arc spraying Spraying a layer of wear-resistant coating in the same way can meet the anti-resistance requirements of the workpiece. Due to the technological limitations of various spraying methods, supersonic arc spraying powder core material is the best solution. According to the wear mechanism of CFB boiler, our company adopts supersonic arc spraying TS70HRC superhard wear-resistant spraying wire for construction. After testing various experimental technical indicators, the anti-wear indicators of this material are higher than The domestic similar spraying wire can fully meet the requirements of CFB boiler anti-wear technology. 6.3 Coating composition TS70HRC super-hard wear-resistant spraying wire composition: Cr (25-30%), Ni (10-15%) B (5-6%), nano-ceramic hard phase agglomeration powder core wire . 6.4 Coating properties The coating prepared by supersonic arc spraying of TS70HRC superhard wear-resistant spraying wire (NiCrB-Cr3C2) selected by our company is described as follows: 6.4.1 The specific microstructure is Cr3C2 and its spraying process The mixed phase of the product acts as a hard spot, which is equivalent to the skeleton in the coating structure; NiCrB has good ductility and corrosion resistance, and acts as a carrier of the hard spot. The hard dots are embedded in the carrier and evenly distributed in the carrier. 6.4.2 Wear resistance requirements Cr3C2 and its mixed phase formed in the spraying process are used as hard points, which become the skeleton in the coating structure and are impacted by the erosion particles. It is well known that chromium carbides have quite high hardness and can fully meet the requirements of wear resistance. 6.4.3 Ductility requires that the carbide of chromium has a relatively high hardness, but the thermal expansion coefficient is very different from that of the matrix, which requires the carrier to have good ductility, that is, to have a thermal expansion coefficient similar to that of the matrix. The transition of NiCrB as a carrier plus a primer layer (NiAl) ensures the ductility of the entire coating and does not crack during temperature changes. 6.4.4 Corrosion resistance requires that the coating work under the condition of corrosive medium in the flue gas. NiCrB has good corrosion resistance to ensure that the hard spots will not fail and fall off due to the corrosion of the carrier, which will directly affect the coating. wear resistance. 7. Description of supersonic arc spraying equipment and technology TS500 type supersonic arc spraying equipment consists of power box, wire feeding mechanism system spray gun, air compressor and other equipment. This technology is a new thermal spraying technology developed by our company through the introduction of advanced equipment and technology. It has been listed as the key technology of surface engineering in the 21st century by developed countries such as the United States and Japan. The spraying technology uses two metal wires to be fed into the arc spray gun at the same time to generate arc melting, and the supersonic airflow accelerated by the Laval nozzle atomizes the spray material into particles with fine particle size and uniform distribution, which are sprayed to the workpiece to form a coating. The molten particles and the substrate are mainly combined mechanically, physically and metallurgically, and the bonding strength can reach more than 63.5MPa. Compared with ordinary arc spraying and flame spraying, supersonic arc spraying has higher particle flight speed, stronger bonding strength, lower porosity, high coating uniformity, good compactness, and the sprayed particles reach the surface of the workpiece When the temperature is less than 80 °C, the workpiece will not be deformed, and high-quality coatings can be obtained. 8. Description of sandblasting equipment and technology 8.1 The effect of sandblasting on the surface of the substrate 8.1.1 Cleaning the surface: remove all kinds of pollutants on the surface to be sprayed, especially grease, dirt, oxide scale, rust and corrosion. The cleanliness of the working surface reaches the Sa3.0 level specified in GB8923-88 "Steel Surface Corrosion Grade and Derusting Grade before Coating". 8.1.2 Surface blasting: The combination of thermal spray coating and substrate is mainly mechanical combination, which requires the pretreatment of substrate not only to remove oil and rust, but also to roughen the surface, and the use surface has a certain roughness. The surface roughness reaches Rz60～90μm stipulated in GB11373-89 "General Rules for Surface Pretreatment of Thermal Spray Metal Parts", which will not cause any damage to the thickness of the pipe wall. The purpose of sandblasting treatment: a) Increase the contact area between the spray coating and the substrate, and improve the adhesion and adsorption force of the joint surface. b) Increase the mutual fitting and occlusion of the coating material and the surface of the substrate, and play the role of "anchor hook" to strengthen the adhesion between the coating and the substrate. 8.2 Activated surface sandblasting provides active surface ability for spraying: such as lattice defects and plastic deformation, to generate a certain stress state, which is beneficial to increase the adhesion between the sprayed particles and the surface of the substrate, and to improve the micro-metallurgical bonding ability of the sprayed particles and the substrate. The analysis of the activation effect is as follows: 8.2.1 Sand blasting makes the surface of the workpiece form a certain residual compressive stress after repeated blows of sand particles. Although the stress value is extremely small, the thermal stress of the coating during the spraying process of the loose workpiece is greatly increased. The bonding strength of the coating is favorable, and it can also improve the fatigue strength of the workpiece. 8.2.2 Spraying can remove organic pollutants and oxide layers on the surface of the workpiece, and can increase the plastic deformation of grains on the metal surface and cause lattice defects, so that the surface of the substrate is in a state prone to chemical reactions, which is helpful for spraying particles Physicochemical bond strength to the substrate surface. 8.2.3 Sandblasting instructions: Sandblasting is only to remove the impurities on the surface. The thickness of the sprayed pipe wall is less than or equal to 15μm, which is the thinnest and will not affect the thickness of the entire pipe wall. For the cracking and blowing damage of the tube wall, the sandblasting operation is carried out uniformly, and the thickness loss of the entire surface is consistent. In summary, the sandblasting operation only dries, cleans and activates the surface of the furnace tube and improves the bonding strength of the coating. One of the most effective methods without damaging the wall thickness. Therefore, it is extremely necessary to carry out sandblasting treatment before the workpiece is sprayed. And the workpiece after sandblasting should be sprayed as soon as possible, the shorter the time, the better the surface activation effect and the higher the coating quality. Nine, the basic parameters of spray coating Coating bonding strength ≥ 55Mpa Coating thickness ≥ 0.6 ~ 0.8mm Coating hardness ≥ 65HRC Coating porosity ≤ 0.9% Substrate temperature ≤ 80 ℃ High temperature oxidation resistance: 6.35mg/cm2 (750 ℃ oxidation for 250 hours) Thermal expansion coefficient: 12.5×10-6 Heat transfer coefficient: 34～38w/m.k