The main reason for boiler wear is that after coal and desulfurizer are fed into the furnace, they ignite and burn while undergoing desulfurization reaction. Under the action of the rising flue gas flow, they move to the upper part of the furnace, releasing heat to the water-cooled wall and buried pipes arranged in the furnace. After entering the suspension area, coarse particles deviate from the main air flow under the action of gravity and external forces, and then flow downward along the water-cooled wall, causing wear on the heating surface. How to adopt effective anti-wear devices or use wear-resistant pipes to reduce and prevent wear has become the main way to solve the problem of boiler thermal efficiency and boiler failure. In order to effectively solve the problem of boiler heating surface wear, laser cladding wear-resistant technology is applied to boiler buried pipes, which does not affect the furnace temperature and efficiency of the boiler, and solves the risk of buried pipe wear and explosion, and has achieved good results.

Anyang Chemical Industry Group Co., Ltd. Urea expansion project supporting public engineering thermal power station, the scale is 3 models HG-CPC75-3. 82/450-WMI type fine particle low-speed circulating fluidized bed boiler, which adopts the technology of CPC Company of the United States and is manufactured by Harbin Boiler Factory. The rated evaporation capacity of the boiler is 75 t/h, the steam pressure is 3.82 MPa, and the steam temperature is 450 ℃. The fuel used by this boiler is a mixture of anthracite and boiler fly ash. Under the action of Roots blower, the conveying system collects the bed material from the furnace into the bed material bin or sends it into the furnace from the bed material bin. The height of the furnace bed material is adjusted to achieve the purpose of adjusting the boiler load. The boiler feed water enters the steam drum after being heated by the two-stage economizer, and then enters the front and rear wall lower headers respectively, and is distributed to 170 63.5 × 10. ９２ The internal thread inclined buried pipe (7°) and vertical light pipe water-cooled wall are heated and enter the steam drum through the upper header. The saturated steam is heated by the first-stage superheater, desuperheater, and second-stage superheater to reach qualified steam and is sent out through the steam header. The heating surface (water-cooled wall) layout of this boiler is different from that of other high-speed circulating fluidized bed boilers. The heating part of the boiler is mainly the buried pipe. The position of the buried pipe of the boiler is in the dense phase area of ​​material fluidization, and the temperature is about 900 ℃. When the boiler is running, the material wears the buried pipe seriously. The buried pipe is easy to carbonize and fall off when the boiler bed temperature is running. Once it falls off, the wear of the buried pipe is aggravated, which is easy to cause the boiler to shut down, which has a significant impact on the safe and stable operation and operation cycle of the boiler. According to statistics, accidents caused by leakage due to wear of the heating surface account for about 60% of the accidents of circulating fluidized bed boilers.

1. Comparative analysis of buried pipe wear and anti-wear measures
The buried pipe of the company’s 6# boiler is affected by long-term wear of the bed material, resulting in reduced strength. When the boiler is short of water, the pressure is not released in time, causing the buried pipe to overheat and burst, resulting in risks such as damage to the furnace wall and surrounding pipes. Comparison of several common anti-wear technologies:
① Install rare earth wear-resistant alloy tiles at the bottom of the buried pipe of the boiler to reduce the wear of the buried pipe during boiler operation. However, after adding rare earth wear-resistant alloy tiles, the operation cycle of the boiler is significantly extended, but the buried pipe tiles cause the buried pipe heat absorption rate to decrease, the boiler evaporation rate to decrease, and the average load is reduced by about 15 t/h compared with the boiler design evaporation rate of 75 t/h, which has a certain impact on the economic operation of the boiler and the normal supply of system production steam.
② Welding anti-corrosion fins on the buried tubes, so that the bed material separates before contacting the buried tubes, destroying the directional corrosion of the particles on the buried tubes; however, carbon steel or manganese steel is used as fins, which are not strong and are easily oxidized and peeled at high temperatures.
③ Adding anti-wear beams/grids/guide plates to change the direction of boiler flue gas flow (wall flow), but the refractory and wear-resistant materials form bosses, which are arranged horizontally or inclined in multiple stages at a certain interval along the height direction of the water-cooled wall. The anti-wear bosses significantly reduce the speed and concentration of the furnace wall flow (the wall flow speed can be reduced from 8 m/s to 2 m/s). The installation of anti-wear beams can extend the wear life of water-cooled wall tubes, but it will reduce the heating surface by about 4%, and affect the heat absorption of the water-cooled wall in the boiler by about 5%; at the same time, after the anti-wear beams are installed, the bed temperature will increase under the same load. Considering the heat transfer temperature difference of the water-cooled wall heating surface in the furnace as 520℃ (assuming the bed temperature is 880℃ and the saturated water temperature is 360℃), assuming that the heat absorption before and after the installation is equal, the bed temperature after the anti-wear device is installed is 907℃. If the heat absorption remains unchanged, the bed temperature will increase by about 27℃ after the anti-wear beam is installed, which will affect the stable and standard emission of environmental protection.
④Using anti-wear spraying technology, the coating and the substrate are mechanically combined, and the coating is easy to fall off, especially for boilers burning inferior coal or heavy loads. For severely worn parts, the spraying effect is poor.

By comparing the above anti-wear technologies, although the service life of the buried pipe has been extended to a certain extent, the comprehensive economic effect is not ideal. Combining the above situations, the company adopted the buried pipe laser cladding technology, adding a high-temperature wear-resistant composite material to the surface of the buried pipe, and spraying it by adopting a high-power laser preparation process. Due to the high energy density of the laser beam, more new wear-resistant materials with high melting points can be clad, and the new wear-resistant materials clad on the surface of the buried pipe play a good wear-resistant role in protecting the buried pipe parent material.

2 Principle and characteristics of laser cladding technology
The principle of laser cladding technology is to use high-power laser equipment to evenly dissolve high-temperature wear-resistant composite materials on the surface of the pipe to form a complete cladding layer. The high-power laser preparation process expands the selection space of cladding materials. The company uses a high-temperature wear-resistant ceramic-nickel-chromium alloy composite material with a hardness of 1,200 to 1,500 HV on the surface of the buried pipe. The main components are trichromium carbide (Cr3C2), nickel (Ni), chromium (Cr), titanium diboride (TiB2) and other metal materials. In this material, chromium carbide (Cr3C2) is carbide ceramic with high melting point (1890℃) and high hardness; nickel (Ni) is a hard and flexible silver-white metal, which is widely used in alloys to improve hardness and toughness. It is often used in high-temperature wear-resistant parts such as aircraft engine blades and linings. Chromium (Cr) has high hardness and good high-temperature oxidation resistance, and forms a nickel-chromium alloy with nickel (Ni); titanium diboride (TiB2) has a melting point of 2980℃, high hardness, and an oxidation resistance temperature of up to 1000℃.

The material ratio can be adjusted according to different wear conditions of the boiler to achieve a balance between hardness and toughness. This material does not contain iron-based components, avoiding the disadvantage that iron-based components greatly reduce the hardness and high-temperature oxidation resistance of the material. At the same time, the laser beam energy in the high-power laser preparation process is highly concentrated, which can melt high-melting-point, high-hardness wear-resistant materials that cannot be completed by other processes. It can be completely melted without gasification to form a complete cladding layer.

After data research and comparison, laser cladding technology has the following advantages:

①High hardness. The near-surface hardness of the laser cladding layer of the wear-resistant system is ＞1100 HV, which is much higher than the hardness of thermal spraying (600-800 HV) and surfacing (500-650 HV), and also higher than the maximum hardness of impurities in coal (about 1000 HV). The service life of the laser cladding layer can reach more than 8 times that of the thermal spray coating. The thickness of the buried pipe cladding layer is about 1.8 mm, and the diffusion zone is about 0.9 mm, achieving good metallurgical bonding. Laser cladding realizes the “surface-body integration” of the cladding layer and the substrate through metallurgical bonding. The bonding strength is more than 10 times that of the thermal spray coating, which fundamentally eliminates the problem of local shedding of the wear-resistant and anti-corrosion layer.
② Small heat-affected area. The depth of the heat-affected zone of the substrate by laser cladding is only 0.2 to 0.3 mm, which does not change the original structure and mechanical properties of the substrate, and the deformation of the workpiece is very small.
③ High thermal conductivity. The thermal conductivity is equivalent to that of stainless steel, which is much better than other coatings, and has little effect on the heat exchange efficiency of the equipment.
④ Does not affect the bed temperature and efficiency. It has good thermal conductivity and does not change the original flue gas flow direction of the boiler.

The comparison of various sintering wear-resistant process characteristics is shown in Table 1.

3 Implementation of transformation content

3.1 Technical transformation plan
Due to the wear and thinning of the original buried pipe of the thermal power 6# boiler, the surface carbonization and shedding, and the reduction of pipeline strength, the safe and stable operation of the boiler is affected. In order to completely solve the above problems and restore the performance of the boiler, it was decided to replace the 170 old buried pipes in the furnace and add laser cladding layers to the surfaces of the newly purchased 170 buried pipes.

The wear-resistant cladding layer is prepared by laser cladding process. The cladding layer is a composite material composed of carbide ceramic-NiCr, which has high hardness and low toughness. The laser melted diffusion layer on the surface of the tube is <150 μm. After the laser cladding work is completed, the cladding layer in the area near the weld is completely removed by an electric angle grinder. The processing length is 60 to 70 mm. No bosses should appear on the surface of the boiler tube. The length of the transition zone of the cladding layer thickness is 40 to 50 mm. The thickness of the cladding layer side tube wall near the weld area (60 to 70 mm) is h = h0 – Δ. Δ = 250 ~ 300 μm (i.e., 0.25 ~ 0.3 mm), h0 is the average thickness of the tube wall in the area without cladding layer on the same section. There should be no cladding material near the weld and diffusion zone to avoid the cladding material being diffused into the weld during welding and causing a significant reduction in weld strength. After all cladding is completed, macroscopic inspection, surface flaw detection, ball passing test, etc. are carried out on the buried pipe and elbow to ensure that there are no cracks or scratches on the finished pipe.

The thickness of the laser cladding single layer is 0.6 ~ 0.8 mm. The combination of the laser cladding layer and the boiler tube body is metallurgical bonding, and the change in the parent material thickness must be < 0.3 mm. The hardness of the laser cladding layer is ≥1100 HV. The thermal conductivity of the laser cladding layer should be equivalent to the original base material of the water-cooled wall tube, that is, it does not affect the furnace temperature and efficiency.

3.2 Effect diagram of adding laser cladding layer on the surface of buried pipe and the condition after transformation
After the wear-resistant metal layer is laser clad on the surface of buried pipe, it is in direct contact with the material for heat exchange. The cladding metal layer has high wear resistance and can protect the buried pipe from corrosion and leakage without changing the process flow. The transformation process is to remove the castables and outer guard plates within the furnace maintenance range before dismantling the buried pipe. The buried pipe is removed in the furnace. The cutting part depends on the connecting pipe of the buried pipe. It can be cut inside or outside the furnace. The buried pipe should be removed by mechanical cutting, and then the remaining pipe head should be polished manually or mechanically to clean the original weld. It is required to use a grinder to remove 10 to 15 mm of oil and rust on the pipe mouth and inner and outer walls until the metal luster is clear. Before construction, all buried pipes are macroscopically inspected and ball-passed. When installing the reference buried pipe, check the buried pipe head and the length deviation of the connecting box center from the buried pipe end. After the reference pipe is aligned, use the reference pipe as the standard to install and fix each pipe from one side until all the buried pipes are welded. After the weld passes the self-inspection, the welding quality must be macroscopically inspected and non-destructive testing must be performed in accordance with relevant specifications. After the inspection passes, the water pressure test is performed and the furnace wall castable and the through-wall castable are restored.
After the buried pipe is replaced, the boiler load adjustment elasticity increases, which plays a good role in the stable operation of the boiler. After the company’s 6# boiler buried pipe is laser clad with a high melting point wear-resistant new material, the wear resistance of the cladding layer far exceeds that of other processes, which completely solves the problem of cladding layer shedding.

4 Economic Benefit Analysis
After the laser cladding wear-resistant buried pipe of boiler 6# was replaced, on the one hand, the number of shutdowns caused by buried pipe leakage was reduced by at least one per year, and the cost of each shutdown was about 50,000 yuan. The steam and power electricity loss during the start and stop of the boiler was about 50,000 yuan (excluding power generation loss and chemical production loss caused by reduced steam volume), which can reduce economic losses by 100,000 yuan per year; on the other hand, the heat exchange efficiency of the boiler buried pipe was improved, and the boiler load was increased by about 2 t/h. Under the condition of unchanged cost, the additional steam sales income increased by 160 yuan/t. The annual operation time was calculated based on 200 days. The steam sales income increased by about 1.536 million yuan due to the increase in boiler load, and the total annual economic benefits were about 1.653 million yuan.
Compared with boilers of the same model, under the condition of similar calorific load, the flue gas temperature of boiler 6# after transformation was reduced by more than 10℃ on average, the combustion thermal efficiency was improved by nearly 1%, and the average daily fuel saving was 2 tons/day. Calculated based on the coal price of 650 yuan/ton, the daily economic benefit in reducing flue gas loss and improving thermal efficiency was about 1,300 yuan, and the annual economic benefit was about 260,000 yuan in total.