Circulating fluidized bed boiler (CFB Boiler) wear and preventive measures

In recent years, the Circulating Fluidized Bed Boiler (referred to as CFB Boiler) has gained widespread adoption as the primary choice for thermoelectric (thermal power) plants. While it offers distinct advantages, operational wear issues have emerged as a significant concern that can jeopardize the boiler’s sustained performance. Component wear and potential leakage pose threats to the long-term stability of the boiler, thereby impacting the production and operations of thermal power plants. Historically, CFB boilers in Jiangxi Province and its neighboring areas experienced severe wear and tear problems, necessitating nearly annual major overhauls, along with several minor maintenance efforts during regular operations.

To address this challenge, a comprehensive analysis of the structural characteristics and operational performance of the circulating fluidized bed boiler has been undertaken. This analysis has identified key components susceptible to wear and, in conjunction with real-world operational conditions, has paved the way for targeted solutions and preventive measures. Addressing specific wear-related issues entails the implementation of localized control methods, such as controlled spraying, effective management of coal particle ingress into the boiler, and adjustments to operational modes. These measures collectively aim to curtail the wear of solid particles on critical components, notably the water-cooled walls and other susceptible areas. The ultimate goal is to ensure the sustained, stable operation of the CFB boiler, continuously enhancing its reliability, stability, and durability in operation.

Overview of Circulating Fluidized Bed Boilers

1.1 Distinguishing Features of CFB Boilers Compared to Other Boiler Types

When compared to alternative boiler systems such as pulverized coal boilers, Circulating Fluidized Bed (CFB) boilers exhibit several distinctive characteristics:

(1) Gas-Solid Flow State: Within the furnace, CFB boilers exhibit a rapid fluidization state characterized by high gas-solid concentration and significant differences in gas-solid velocities. This dynamic environment often results in pronounced particle clustering and remixing phenomena.

(2) Boiler Structure: CFB boilers typically incorporate a gas-solid separation device to maintain a consistently high gas-solid concentration within the furnace. This setup facilitates a more efficient cyclic re-feeding system within the furnace.

(3) Fuel Versatility: CFB boilers boast a broader range of solid fuel compatibility compared to pulverized coal boilers. They are particularly well-suited for burning fuels with lower calorific values, including mud, gangue, coal sludge, straw, and oil shale.

(4) Fuel Particle Size: CFB boilers commonly burn fuel with a particle size ranging from 0 to 8mm (with some variations for specific boilers, such as 0 to 10mm or 0 to 13mm for certain applications, and even up to 0 to 25mm or 0 to 30mm for lignite combustion).

(5) Temperature Distribution: CFB combustion typically occurs at temperatures ranging from 850 to 950°C, which is lower than the flame temperature of pulverized coal boilers, which can reach as high as 1200 to 1600°C.

(6) Energy Consumption Indicators: CFB plants tend to have slightly higher electricity consumption rates compared to equivalently sized pulverized coal boilers. However, there is ample room for improvement, and operational costs are closely tied to fuel quality. CFB boilers offer the advantage of burning lower-quality and low-calorific-value fuels, which can offset the increased coal consumption with favorable fuel replacement benefits.

(7) Emission Concentrations: Due to its lower combustion temperatures, CFB boilers emit lower concentrations of pollutants such as sulfur dioxide, sulfur trioxide, nitrogen oxides, dust, and mercury compared to pulverized coal boilers. They also demonstrate lower emissions of heavy metals.

(8) Operational Impact Factors: In CFB furnaces, extensive material circulation occurs, reducing the risk of high-temperature corrosion seen in pulverized coal boiler walls. Additionally, issues like coking are typically less prevalent in CFB boilers. One notable advantage of CFB is the absence of significant temperature deviations between the water-cooled wall pipes, which is advantageous for maintaining supercritical parameters and mitigating oxide skin shedding.

(9) Automatic Control System Requirements: The unique coupling of the combustion system’s large thermal inertia with the relatively low thermal inertia of the steam and water system in CFB boilers necessitates more advanced automatic control systems compared to other boiler types.

Operational Characteristics of CFB Boilers

2.1 Low-Temperature Power Combustion

In the process of circulating fluidized bed (CFB) combustion, the primary mechanism involves the high-speed movement of flue gases inside the boiler, accompanied by the entrainment of a substantial quantity of particles. These particles aggregate into clusters, subsequently returning to the mixing and reflux stages of the fluidized combustion reaction. This dynamic interplay encompasses both physical and chemical reactions. Additionally, the majority of high-temperature solid particles outside the furnace are captured and reintroduced into the boiler for secondary combustion. This intricate reaction process serves to maximize the combustion duration within the furnace. Simultaneously, considerations are made for denitrogenation and desulfurization to maintain the boiler’s internal temperature around 950°C. This temperature significantly surpasses that observed in conventional pulverized coal combustion.

The approach described above characterizes low-temperature power combustion, a distinctive feature of CFB boilers when compared to conventional pulverized coal furnaces. Operating below the ash melting point temperature, this method effectively prevents ash melting, thus enhancing operational efficiency. This optimized performance is notable for several reasons:

1. Reduced Slagging and Alkali Metal Precipitation: CFB boilers exhibit minimal slagging, with stronger precipitation of alkali metal substances compared to pulverized coal furnaces. Sensitivity to ash characteristics is notably reduced, necessitating careful control to ensure maximum ash cooling. This approach effectively reduces the generation of nitrogen oxides, allowing for the application of cost-effective and highly efficient desulfurization methods.
2. Efficient Combustion Zones: Within CFBs, combustion reactions are divided into distinct zones, including combustion zones and transition zones, which are carefully controlled. The low temperatures inherent in CFB combustion facilitate the efficient mixing of a multitude of solid particles. Combustion efficiency primarily hinges on the rate of chemical reactions, corresponding to the temperature level during the combustion process. In CFB furnaces, coal combustion occurs at elevated levels, with combustion efficiency often exceeding 98%, provided that other operational parameters remain within prescribed limits.

The distinct advantage of CFB boilers lies in their ability to achieve high combustion efficiency through low-temperature combustion, coupled with effective pollutant control measures, ultimately contributing to enhanced operational performance.

2.2 Fluidization of Solid Materials

In the furnace chamber, separator, and return device of a Circulating Fluidized Bed (CFB) boiler, solid materials, including fuel, cinder residue, ash, desulfurization agents, and inert bed materials, are transported in a high flux, high concentration, and high-velocity manner, forming an external circulation process. This process also encompasses the internal circulation of solid materials within the boiler chamber. Consequently, CFB technology relies primarily on two modes of circulation—internal and external—to facilitate the cyclic movement of materials. The key principle underpinning this technology is to enable the flow of materials and airflow in a highly fluidized state. This is achieved by optimizing the utilization of fluidization, allowing materials to participate in both internal and external circulation within the furnace. This dynamic movement continuously enhances the concentration of material particles in the furnace, ensuring that the combustion process proceeds effectively.

2.3 High-Intensity Transfer Process

The operation of a CFB boiler involves a complex heat, mass, and momentum transfer process. It relies on the vigorous turbulence created by a large volume of solid materials coursing through the boiler chamber. This process is continually adjusted by operators to vary the circulation of materials and their distribution within the furnace. These adjustments accommodate varying degrees of combustion conditions, ensuring a balanced temperature distribution throughout the entire boiler chamber. The CFB boiler’s ability to adapt to changing conditions and maintain uniform temperature profiles is a testament to the high-intensity transfer processes at play within the system.

3. Critical Wear-Prone Areas

During the operation of a Circulating Fluidized Bed (CFB) boiler, the continuous high-velocity cyclic movement of solid fuel particles can lead to wear and tear in several key areas. These wear-prone locations include:

Heat Exchange Surface Components, Particularly the Water-Cooled Wall:

• Junction between the protective belt and the furnace wall.
• Areas near the entrance and exit of the boiler flue.
• The opening of the let-off tube, where eddy currents can be generated.
• Corners of the water-cooled wall.

Additional Vulnerable Areas Include:

• Windward side of pipes, such as superheaters and economizers.
• Pipes passing through the walls.
• Turning points within the boiler structure.

These areas are particularly susceptible to wear and tear due to the relentless movement and abrasion caused by the high-velocity solid fuel particles circulating within the CFB boiler. Addressing wear in these critical locations is essential to ensure the long-term reliability and efficiency of the boiler system.

Wear Prevention Methods and Measures

4.1 Preventive Measures During Design

(1) Water-Cooled Wall Anti-Wear Design:

• Designing the water-cooled wall with a focus on anti-wear considerations. Particular attention should be given to designing the transition zone of the dense phase, where the “wall flow” of material particles occurs at a certain velocity.
• Take into account the impact of wind speed, which can introduce a certain amount of rolling action against the water-cooled wall, causing significant scouring.
• Critical areas to address include the return port, coal drop port, fire lookout, and the design of secondary air outlets.
• The water-cooled wall piping should be designed to allow for material casting around the boiler exit to avoid localized eddy current flushing, which can lead to leakage incidents.
• Consider increasing the height of the protective belt to reduce wear and tear on the pipe wall in these areas.

(2) Superheater and Heat Saver Anti-Wear Design:

• Pay meticulous attention to enhancing the separation efficiency of the separator during the design process.
• This improvement helps reduce wear and tear caused by ash particles in the flue gas on the superheater and economizer.
• After pipeline production, mitigate wear by applying a protective spray coating and, where necessary, utilize anti-friction protective tiles to achieve a cumulative wear-resistant effect.

(3) Air Cap and Slag Discharge Pipe Anti-Friction Design:

• Recognize the proximity of coal drop and slag discharge ports and consider their relative positioning.
• During operation, ensure that coal entering the furnace chamber is mixed thoroughly with the furnace’s hot materials to minimize incomplete combustion and coking issues in the discharge pipe.
• In the design phase, separate these two ports to guarantee that larger particles entering the furnace chamber fully mix with the furnace’s hot materials, reducing instances of incomplete combustion within the boiler.

Implementing these design-driven preventive measures can substantially mitigate wear and tear in critical areas of the CFB boiler, contributing to its overall longevity and operational efficiency.

4.2 Application of Wear-Resistant Coatings

To combat wear and tear, particularly on the water-cooled wall, a wear-resistant coating can be applied to the surface. The coating should possess greater hardness than that of the water-cooled wall. Additionally, the coating should generate a compact, durable, and stable oxide layer at high temperatures to shield the water-cooled walls. During the spraying process, it is crucial to adhere to specific operational procedures. LG8 arc spraying material is typically used for spraying onto the heated metal surface. The coating thickness should be controlled to stay below 1mm. Prior to spraying, appropriate technical methods should be employed to remove any rust from the furnace pipe’s surface.

4.3 Control of Material Particle Size

Managing the size of solid fuel particles is essential for reducing wear and tear. Elevated particle concentrations lead to increased gaps between particles and greater friction and collisions with heating surfaces, resulting in wear. When coal particles collide with the water-cooled wall at furnace temperatures, wear increases, thereby impacting the anti-wear coefficient of the water-cooled wall. Additionally, under specific wind pressures, bed material resistance can influence fluidization state changes and accelerate air cap wear, leading to shorter operational cycles. Thus, it is crucial to appropriately control the size of materials entering the furnace to protect the furnace wall.

4.4 Adjustment of Operating Modes

Effective air volume control involves two primary adjustments:

• Primary Air: This air is primarily distributed beneath the bed material within the fluidized furnace.
• Secondary Air: Secondary air is mainly directed into the lower part of the rarefied zone to provide adequate oxygen supplementation for thorough combustion.

To prevent wear on the water-cooled wall, corresponding measures should be implemented in the air volume control process. This includes optimizing air distribution and strictly regulating the excess air coefficient to achieve comprehensive management goals. By fine-tuning operating modes, wear on critical components can be minimized, promoting the long-term reliability of the CFB boiler.

In conclusion, while wear and tear on the heat exchange surfaces are inherent challenges during the operation of circulating fluidized bed (CFB) boilers, a systematic approach involving scientific analysis and effective measures can significantly mitigate these issues. Through the implementation of the design and operational measures discussed above, and by adhering to comprehensive boiler management principles, we can enhance the wear resistance of CFB boilers during daily operations.

By adopting these strategies, we ensure that CFB boilers can operate efficiently and safely over extended periods, contributing to the long-term success of thermal power plants. This sustained efficiency not only enhances the performance of the CFB boiler but also ensures a consistent improvement in the overall efficiency of the thermal power plant.

[Source] Cao Yangru, Gong Binglin, An analysis of wear and tear of circulating fluidized bed boiler and preventive measures, China Salt 2022[12]:52-54