What is the specific role of supersonic spraying?

       Arc hypersonic spraying is a process in which an electric arc is used as a heat source to melt a wire and atomize it through an air flow, so that the molten particles are sprayed on the surface of the workpiece at high speed to form a coating. This process is used to spray paint on boiler water walls to prevent high temperature corrosion. , the coating has excellent high temperature corrosion resistance and good wear resistance.

 

Key words: supersonic arc spraying; water wall high temperature corrosion resistance; wear resistance

1 Introduction

 

In recent years, many boilers of XX Company have suffered severe high temperature corrosion of the water wall. The minimum wall thickness of the water wall of the most corroded boilers was only 1.3 mm, and the corrosion rate was 2 mm/a. The high temperature corrosion zone of the boiler is basically similar, i.e. the zone downstream of the burner outlet jet, highly close to the centerline of the burner, and the pitting corrosion of the tubes towards the combustion side. is the fastest. After the high temperature corrosion of the water cooling wall occurs, the wall thickness is reduced and the strength is reduced, which is easy to cause the pipeline to burst and leak, and endanger the safe operation of the boiler. Due to high temperature corrosion, the above boilers are all thinned in a short period of time and have to be replaced in a large area during maintenance. This not only consumes materials, but also prolongs the maintenance time, increases the workload, and causes huge economic losses. The high temperature corrosion of the water wall has become a long-term problem that plagues the safe operation of XX company's boilers.

Due to the shortage of coal resources and the difficulty of coal procurement in recent years, XX Company's coal types are more complex and coal blending is more difficult. Company XX's boilers relied on low-quality coal to maintain normal production, resulting in areas of intense corrosion. In a reducing atmosphere, the O2 content in the flue gas in the severely corroded area is almost zero, while the CO content is as high as 10%, and the CO content in some boilers is even close to 30%. The chemical composition of the corroded zone is similar to the morphology of the corroded tube. The locations of high temperature corrosion are all on the four water walls in the burner area. Most of the severely corroded areas are in the middle and downstream of the combustor exit jet. The water wall in the corroded area is usually dark brown on the fire side, with a softer outer layer and a harder inner layer. After peeling off the hard layer, the connection surface between the scale and the water wall tube is peacock blue.

2. Research and measures of high temperature corrosion

2.1 Coal types

 

Coal type is one of the main reasons for high temperature corrosion. Statistical analysis of the coal quality of high-temperature corrosion boilers shows that the sulfur content of coal in most boilers is above 1.2%, and some are even as high as 2%-3%. The high sulfur content makes the coal produce more corrosive substances during the combustion process and accelerates the corrosion of the water wall.

 

2.2 The four-corner tangential combustion method separates the air and dust in the combustion furnace

 

This is a common problem with tangentially fired boilers. After the primary and secondary air jets are discharged from the burner, due to the squeezing effect of the upstream adjacent corner airflow and the influence of the different air replenishment conditions on the left and right sides, the airflow is biased towards the water-cooled wall fire on the back. At this time, the rigid primary air jet is weaker. Will deflect a greater angle than the secondary air, thereby separating the primary and secondary air. The greater the difference between the stiffness of the primary air and the secondary air, the more pronounced this separation phenomenon will be. When part of the primary air jet deviates from the secondary air, the pulverized coal will burn in a state of insufficient oxygen, thereby forming a local reducing atmosphere near the water wall downstream of the jet, which is an important cause of high temperature corrosion.

2.3 Influence of the burner

 

XX company's boiler adopts elbow type horizontal concentration burner, which has the advantages of reducing NOx emission, improving pulverized coal ignition conditions and improving pulverized coal combustion stability. However, for the horizontal coarse and fine burners, the ratio of rich coal powder to air powder is relatively low. If the latter cannot be enhanced, the reducing atmosphere can be enhanced and the possibility of high temperature corrosion can be increased. In order to give full play to the characteristics of the rich-lean combustion technology, the basic method is to try to achieve partial concentration separation in the primary air of the ignition area after the pulverized coal enters the burner. In the primary air flow, relying on turbulent diffusion, the rich pulverized coal and the air in the primary air will be automatically mixed, which is beneficial to combustion and reduces NOx emissions without the problem of difficult mixing in the later stage. This may be the right direction to use the rich-lean combustion technology.

2.4 Influence of working conditions

 

The effect of combustion on the high temperature corrosion of the water wall is mainly due to imperfect combustion conditions, excessive combustion tangent, and poor boiler air distribution, etc., which lead to the strong combustion of pulverized coal near the water wall or the formation of smoke near the water wall due to direct flushing of water. The heavier reducing atmosphere greatly increases the corrosive medium, resulting in high temperature corrosion of the water wall.

 

According to the specific situation of XX company, we have done a lot of work in the prevention and control of high temperature corrosion: ① increase the oxygen content in the boiler flue gas; ② reduce the fineness of pulverized coal; ③ open the large, medium and secondary secondary second furnace. Air volume ④ Strengthen the blending of fuel coal and reduce the sulfur content of coal. For various reasons, the above measures are not very effective in reducing high temperature corrosion of the water wall. Therefore, XX company used the #3 boiler as the test furnace, and carried out arc supersonic spraying on the combustion zone of the XX#3 boiler.