Circulating Fluidized Bed (CFB) boilers have become one of the most widely used clean combustion technologies in power plants, chemical processing, waste-to-energy plants, and industrial steam generation. Their ability to burn various low-grade fuels, maintain stable combustion, and achieve high desulfurization efficiency makes them especially popular in Asia, the Middle East, and Europe. However, CFB boilers operate under extremely demanding conditions—high temperatures, particle erosion, rapid temperature fluctuations, and chemical corrosion. In such environments, the correct selection, installation, and maintenance of refractory materials in CFB boilers are essential for ensuring long service life and stable performance.
This article explains the functions of refractory linings inside CFB boilers, analyzes the types of commonly used refractory materials, discusses application zones, highlights selection criteria, and provides practical guidance for maintenance and troubleshooting.
1. Why Refractory Materials Are Critical in CFB Boilers
Unlike traditional pulverized coal boilers, CFB boilers involve intense solid particle movement. Fuel and bed materials circulate between the furnace and cyclone separator at high speed, often exceeding 15–25 m/s. These conditions create continuous physical and chemical stress. The main functions of refractory materials in CFB boilers include:
1.1 High-temperature resistance
CFB boilers typically operate around 850–950°C. Refractories must withstand long-term exposure without softening, cracking, or structural degradation.
1.2 Erosion and abrasion resistance
Fluidized particles such as sand and ash collide with refractory surfaces at high velocity. Poor erosion resistance results in rapid wear, leading to steel exposure and eventual failure.
1.3 Thermal insulation
Refractories help maintain furnace temperature, improve combustion efficiency, and reduce heat loss through boiler walls.
1.4 Chemical corrosion resistance
CFB fuel varieties—coal, biomass, petroleum coke, sludge—contain sulfur, chlorine, alkali, and other corrosive compounds. Refractories must resist slag attack, alkali corrosion, and reductive gases.
1.5 Structural protection
Refractory layers protect steel shells, anchors, and water-cooled tubes from excessive heat and mechanical damage.
Without reliable refractory linings, a CFB boiler may suffer frequent unplanned shutdowns, reduced efficiency, or in severe cases, safety hazards.
2. Key Zones That Use Refractory Materials in CFB Boilers
Each zone inside the CFB unit faces different mechanical and chemical conditions. Therefore, different types of refractory materials in CFB boilers are used depending on the local stress environment.
2.1 Furnace Bottom (Dense Phase Zone)
Highest particle concentration
Strongest erosion and thermal cycling
Requires dense, abrasion-resistant refractories
Typical materials:
High-alumina castables (70–85% Al₂O₃)
Abrasion-resistant low-cement castables
Mullite-based bricks or castables
2.2 Furnace Sidewalls
Sidewalls use a combination of insulation and working refractory layers. Erosion is moderate, but thermal shock is frequent.
Materials commonly used:
Lightweight insulating castables
Phosphate-bonded high-alumina bricks
Mullite insulation bricks
2.3 Cyclone Separator
Cyclone separators face the most severe erosion due to high-speed circulation of hot solids.
Refractory solutions include:
Dense, ultra-abrasion-resistant castables
Steel fiber–reinforced castables
Silicon carbide (SiC) castables for extreme erosive conditions
2.4 Loop Seal and Seal Pot
Loop seals handle continuous circulation of bed materials and require excellent wear resistance.
Suitable materials:
High-strength alumina castables
SiC castables with strong particle abrasion resistance
2.5 Return Duct and Reheater Zones
These areas experience moderate erosion but strong thermal fluctuations.
Best options:
Thermal shock–resistant castables
Lightweight insulating backing + wear-resistant hot face layer
2.6 Boiler Bed Ash Coolers
Here, refractories must withstand mechanical impact and chemical corrosion from ash.
Common choices:
Corrosion-resistant alumina-silicate castables
SiC bricks or castables for improved heat conductivity
3. Types of Refractory Materials Used in CFB Boilers
The selection of refractory materials in CFB boilers must consider their performance characteristics and cost-effectiveness. The main categories include:
3.1 High-Alumina Castables
Highly popular due to their stable performance, easy installation, and good abrasion resistance.
Advantages:
High strength
Good thermal shock resistance
Suitable for most critical zones
3.2 Low-Cement and Ultra-Low-Cement Castables
These offer improved density, strength, and chemical resistance.
Benefits:
High abrasion resistance
Better bonding and long service life
Lower porosity
Ideal for: cyclone separators, return ducts, furnace bottoms.
3.3 Silicon Carbide (SiC) Castables
Among the best materials for erosion and thermal conductivity.
Strengths include:
Extremely high wear resistance
Resistance to chemical attack
Excellent heat transfer
Often used in: cyclone inlets, loop seals, refractory risers.
3.4 Phosphate-Bonded Refractories
Suitable for fast repairs and zones with high thermal shock.
Advantages:
Quick setting
Good bonding to old refractories
Works well in corners and edges
3.5 Insulating Refractories
Used as the backup layer for energy-saving purposes.
Types include:
Lightweight castables
Mullite insulation bricks
Ceramic fiber boards
3.6 Precast Shapes
Increasingly popular in modern CFB designs.
Benefits:
Factory-controlled quality
Faster installation
Reduced downtime during repairs
Used in: cyclone separators, return ducts, arches, and burner tiles.
4. Key Factors When Selecting Refractory Materials in CFB Boilers
When designing refractory systems for CFB boilers, engineers should evaluate several important factors to ensure long-term durability.
4.1 Operating Temperature
Different zones operate at different temperatures. For example:
Furnace bottom: 900–950°C
Cyclone separator: 850–900°C
Loop seal: 750–900°C
Select refractory grades accordingly.
4.2 Erosion Intensity
Areas with high particle velocity require wear-resistant materials like SiC or steel-fiber castables.
4.3 Chemical Atmosphere
Fuel composition affects refractory choice:
High sulfur or chlorine → corrosion-resistant materials
Biomass with high alkali → alkali-resistant castables
4.4 Installation Method
Consider whether the material will be:
Pumped
Troweled
Gunned
Cast-in-place
Precast
Different materials have different installation requirements.
4.5 Budget and Lifecycle Cost
Higher-grade refractories have greater upfront cost but drastically reduce maintenance and unplanned downtime.
5. Common Failure Modes of Refractory Materials in CFB Boilers
Understanding why refractory linings fail helps improve future selections.
5.1 Erosion Wear
Caused by high-speed ash particle impact; common in cyclones and furnace bottoms.
5.2 Thermal Shock Cracks
Rapid temperature changes during startup/shutdown.
5.3 Alkali Corrosion
Especially relevant for biomass-fired CFB boilers.
5.4 Improper Installation
Poor curing, inadequate anchoring, or incorrect mixing ratios.
5.5 Steel Shell Overheating
Indicates insufficient insulation or refractory detachment.
6. Maintenance and Repair Strategies
To maximize the service life of refractory materials in CFB boilers, plants should adopt a preventive maintenance approach.
6.1 Regular Inspection
Frequency: every 3–6 months
Check for: cracks, spalling, hotspots, anchor exposure.
6.2 Use of Monitoring Tools
Infrared thermal imaging
Endoscopes for internal inspection
6.3 Scheduled Shutdown Repair
Replace damaged areas before widespread failure occurs.
6.4 On-Site Refractory Patching
Phosphate-bonded materials or quick-repair castables.
6.5 Upgrade to Better Materials
Replacing old refractories with SiC or ultra-low-cement castables significantly increases service life.
Conclusion
CFB boilers require highly reliable refractory systems to withstand extreme thermal, mechanical, and chemical stress. Proper selection of refractory materials in CFB boilers, combined with expert installation and preventive maintenance, significantly enhances boiler performance, reduces downtime, and lowers operating costs. As CFB technology continues to evolve, so will refractory engineering—bringing more durable, efficient, and environmentally friendly solutions. Looking for refractory materials for CFB boilers and other refractory materials? Welcome to Get more from Xintai refractory!
