Research on Wear-Resistant Application and Engineering Challenges of Ceramic Impellers in Boiler Induced Draft Fans

As the demands for equipment stability and service life are increasingly higher in the thermal power generation industry, traditional metal impellers of boiler induced draft fans undergo severe wear due to long-term exposure to high-temperature dust-laden flue gas, leading to frequent maintenance and high maintenance costs. Ceramic materials possessing excellent hardness, wear resistance, and corrosion resistance have great potential in the anti-wear modification of induced draft fan impellers. In this paper, boiler induced draft fans are considered the study object, and the mechanism of wear resistance, typical material performance, connection method, and practical application example of ceramic blades are systemically analyzed. The feasibility and engineering issues of their application under high-abrasion environments are assessed, and optimization design is put forward.

Introduction

Boiler induced draft fans are key smoke exhaust machinery in thermal power generating systems. Their impellers operate for an extended duration in flue gas with high-temperature particles, and wear is particularly serious in those power stations that burn coal with high ash content. Traditional metal materials such as stainless steel and high-chromium cast iron have a certain strength and toughness, but in the presence of severe abrasion and high-temperature scouring, they wear out quickly and have a short service life. To extend the equipment cycle and reduce maintenance expenses, the use of wear-resistant ceramic materials has been the trend technologically.

Wear Mechanism of Induced Draft Fan Impellers and Traditional Protection Technologies

Boiler induced draft fans are usually placed in the boiler tail section. Their primary functions are to manage the negative pressure of the furnace, expel high-temperature flue gas, and provide smooth discharge of combustion products, which are key fluid transportation equipment in the thermal system. In long-term operation, since some coal has high ash content, it contains a large amount of rigid mineral particles, such as high-hardness minerals quartz and corundum. In addition, the dust removal equipment is not efficient and has no ability to effectively intercept fine particles, leading to an increase in dust concentration in the flue gas. These high-speed particles produce a constant abrasion of the surface of the impeller blade by centrifugal force, with special focus on the blade inlet region and the windward side of the blade tips, forming a typical triangular erosion area. In severe cases, it will lead to substrate thinning, weld cracking, and even perforation failure, which will finally make the impeller lose its dynamic balance, (intensify) vibration, unstable operation, and even forced shutdown accidents.

To delay or reduce such erosion damage, traditional anti-wear measures are mainly in the following technical directions:

• Surface surfacing reinforcement:Nickel-based alloys, high-chromium alloys, and other wear-resistant alloy layers are deposited by surfacing in the high-wear zone. The technology and cost of this method are low, and it can be used for on-site rapid repair, but because the heat-affected zone of the surfacing layer is prone to defects such as cracks and spalling, the anti-wear cycle is short, and it is usually suitable for light and medium wear conditions.
• Thermal spraying/spray welding technology: Tungsten carbide, nickel-based alloy, and other wear-resistant coatings are sprayed onto the surface of the blade via plasma spraying or oxy-acetylene flame spray welding. The coating has high hardness and good density, and the theoretical wear resistance is good. But there are risks such as poor adhesion, peeling of the coating caused by thermal deformation, and stress concentration. The process requires strict equipment cleanliness and process control.
• Ceramic sheet pasting protection: New high-performance anti-wear technology increasingly popularized in recent years. Engineering ceramic sheets such as alumina and silicon carbide are pasted onto the impeller surface wear zone with high-strength adhesives to form a scouring-resistant composite layer. This technology can significantly improve the service life of the impeller with excellent wear resistance and corrosion resistance and is particularly well adapted to bad working conditions with high dust and high temperature. It is currently an important development direction for equipment protection of large thermal power units.

Analysis of Advantageous Properties of Ceramic Materials in Impellers

Comparative Analysis of Material Properties

The excellence of ceramic materials in structural protection is derived from their physical and chemical properties. Engineering ceramics like alumina (Al₂O₃), silicon carbide (SiC), and silicon nitride (Si₃N₄) have a dense microstructure and covalent bond bonding, which allows them to have overall properties much exceeding metal materials for numerous extreme service environments:

Engineering Application Performance and Practical Performance

Alumina ceramic sheets produced by cold isostatic pressing + high-temperature sintering process have Rockwell hardness HRA88 or above and a specific gravity of about 3.7g/cm³, which is far lower than that of traditional metal materials (e.g., carbon steel about 7.85g/cm³). The weight of each square meter of ceramic protective coating is less than 5.5kg, which has small influence on the quality of the impeller and requires slight dynamic balance correction. According to long-term operation feedback, the wear-resistant life of this type of ceramic protective layer is more than 5 times that of high-chromium cast iron, and it can effectively inhibit failure modes such as grooves, pits, and perforations caused by particle scouring.

Analysis of Ceramic Sheet Pasting Technology and Connection Reliability

Performance Requirements and Evaluation of Adhesives

The adhesion of ceramic materials and metal substrates is the most significant bond that determines the performance of anti-wear structures. With the high temperature, high flow rate, and high-frequency vibration conditions of the normal operating condition of induced draft fans, the adhesives selected should have the following most significant performances:

• High-temperature resistance: It does not degrade after long-term service in an environment of 150℃, and its shear strength is still over 20MPa;
• Aging resistance: Deterioration of the bonding performance after long-term high-temperature oxidation and water vapor erosion should be controlled within 10%;
• High toughness and flexibility:  It has the ability to absorb thermal stress caused by the difference of thermal expansion coefficients between ceramics and metals;
• High bonding strength: Under the high-speed rotation of the impeller, the centrifugal force that a 10×10mm ceramic sheet bears is about 0.65kgf, while the bonding strength may reach 300kgf, with a safety factor of more than 450 times, which is highly reliable.

Assembly and Construction Technical Specifications

Ceramic impeller protection and repair operations are typically carried out in line with the following steps:

• Substrate repair:Restore the original profile of the wear area by laser cladding or cold welding so that the quality of the surface meets the requirements of bonding;
• Ceramic sheet pasting: Install standard ceramic sheets in areas with high wear (e.g., inlet segment and blade tips), and U-shaped ceramic blocks can be selected in key stress areas to form a windward shield structure;
• Dynamic balance counterweight: After repair, the dynamic balance check and adjustment of the entire impeller are required to ensure the stability of the high-speed operation of the equipment.

Analysis of Industrial Application Cases

Application Background

State Energy Chengde Power Plant SAF27.5-17-2 boiler induced draft fan has been running with high-ash coal for an extended period, and particle scouring is a significant problem. The nickel-based tungsten carbide thermal spraying original blade showed serious grooves and perforations after only 11 months of operation.

Technical Improvement

Cold-pressed alumina ceramic sheets 1.5mm thick were applied in the blade inlet and high-scouring areas and fixed with special adhesives.

Economic and Engineering Benefit Evaluation

• The repair cost of a single impeller is about 5,000 yuan;
• The average service life is extended to 2–3 years, which is more than 2 times higher than the original spraying process;
• The annual spare parts and maintenance costs are reduced by more than 30%;
• After 18 months of actual operation, the inspection shows that the ceramic layer has almost no obvious (falling off) and wear.

Figures 4 and 5 show the wear comparison between conventional spraying and ceramic pasting technologies in service, intuitively conveying the durability advantage of ceramic protection.

Application Limitations and Engineering Challenges

Although ceramic materials have significant advantages in the field of wear resistance, their engineering applications still have some limitations:

• High risk of brittle fracture: Ceramic materials have low fracture toughness and will fracture into pieces under impact or abrupt temperature change;
• Complex manufacturing and assembly: Traditional welding connection is not able to be used, and mechanical embedding or high-performance adhesive technology must be relied upon;
• High comprehensive cost: The production cost of ceramic sheet, processing, and construction supporting processes are all more expensive than metal materials;
• Size and structure limitations: It is difficult to integrally use ceramic materials for large-size complex blades. Now, structures are primarily used to compromise the performance and manufacturability.

Development Trends and Technical Optimization Suggestions

To achieve the further development of wide applications of ceramic materials in impeller equipment, the key technical directions to be focused on are:

• Ceramic-metal composite structure design: Enhance the overall toughness and impact resistance of ceramic components through metal skeletons;
• Additive manufacturing (3D printing) ceramic technology: Realize the integrated forming of complex streamlined structures and improve functional integration;
• Research and development of high-performance structural adhesives: Achieve breakthroughs in temperature resistance, bonding strength, and flexibility to ensure long-term service;
• Bionic structure design: Learn from the multi-level protection structures in nature (such as fish scales and shells) to improve the fracture tolerance and impact resistance of ceramic structures.

Engineering design guidelines: In the application of ceramic protection design, key operating parameters such as flue gas temperature, particle size and loading, and start-stop frequency of fans need to be comprehensively considered. The form and layout of ceramics must be optimized by the integration of CFD simulation and wear prediction models, and the design hypotheses must be revised progressively with measured information in the field.

Conclusion

The application of ceramic material in engineering provides a subversive protection idea for high-wear parts such as boiler induced draft fans. Supported by the support of reasonable structure arrangement and joining technology, the ceramic sheet pasting anti-wear technology is of superior overall performance and economic benefit as well, and is particularly suitable for the (life extension) modification demand of medium-sized impeller blades. Although it remains weak in brittleness, cost, and production processes, with the further development of composite material technology, structural bionic design, and intelligent manufacturing technology, ceramic-based materials ought to play a central role in a wider range and more complex environment and be one of the important technical trends for future impeller anti-wear engineering.