Refractory castables in circulating fluidized bed (CFB) boilers are a key factor determining continuous operating cycles. Their refractoriness, hot strength, shrinkage, casting quality, curing, and dry-out/heating-up procedures directly affect the strength and service life of the furnace wall in the dense-phase/burner zone—therefore impacting the boiler’s run length.
At the same time, the quality of external insulation and protective layers also restricts thermal expansion behavior. This is especially critical in winter when ambient temperature drops below -5°C. Uneven expansion can easily cause stress concentration at pipe weld joints, leading to leakage and forced shutdowns. This reduces operating cycles, increases start-up frequency and cost, affects stable unit operation, and raises operational risk.
Based on issues identified during boiler operation and outage inspections, the main causes of damage include:
Local patch repairs with poor execution
No reserved expansion gaps during installation
Improper external insulation causing large inside–outside temperature gradients and cracking
By analyzing mechanisms, identifying critical zones, and strengthening process control, the goal is to extend lining life and improve long-cycle operation.
Many captive power plant boilers in coal-mining areas operate at relatively low loads. Most are CFB boilers designed for low-grade fuels (for comprehensive utilization), typically with evaporation capacities of 75 t/h and 130 t/h.
Because the fuel quality is poor (low calorific value, high ash), refractory wear castables in the following areas often suffer crazing, abrasion, and spalling:
Slag discharge nozzle region
Furnace “nose” / flame-folding corner
Coal feed inlet
Return feeder (standpipe legs)
Cyclone separator
Wear-resistant castables are therefore a primary limiting factor for long-term operation.
External protection and insulation selection and workmanship influence:
Heat loss (energy efficiency)
Stress at weld joints caused by start-up heating, sudden ambient temperature drops, and thermal gradients—leading to pressure part leakage and shutdowns
With stricter environmental requirements, in-furnace limestone injection desulfurization is widely used. When Ca/S = 1.5–2.0, desulfurization efficiency can reach 85–90%, and limestone utilization is nearly doubled versus conventional fluidized bed boilers. However, higher sorbent feed increases bed material and circulating solids, accelerating wear of furnace wall castables.
Once wear castables degrade, membrane water-wall and outer protective plates become exposed. Severe cases can lead to:
Protective plate overheating, deformation, cracking
Pressure parts thinning from abrasion, resulting in leakage and shutdown
Shortened operating cycles, higher incident rates
Increased safety risk, material consumption, and labor cost
In low-circulation-ratio CFB boilers:
Furnace walls often use a tube-clad lightweight wall structure
Cyclone separators, sloping flues, furnace roof, and tail flues are built with refractory castables, phosphate-bonded bricks, and aluminosilicate insulation materials
The lower membrane water-wall hot zone typically uses high-strength composite refractory castables
Different refractory types should be selected based on local conditions such as:
Fluidization velocity
Flue gas velocity
Heat exposure (hot-face severity)
The goal is to maintain service life and wear resistance while reducing construction and maintenance cost.
Wear-resistant castables are formulated using oxides and binders. In furnace areas, high-flow high-strength castables are common; however, due to differences in formulation and production process, physical properties can vary significantly, including:
Refractoriness
Linear expansion coefficient
Abrasion resistance / strength
These differences directly affect boiler run length.
High-temperature-zone refractories in CFB boilers are mainly high-strength castables. Their typical main components include Al₂O₃ + TiO₂, blended with calcium aluminate cement in certain proportions. Process differences can cause large variations in refractoriness, linear expansion, and wear resistance, impacting service life.
For bauxite clinker (quality parameters), typical targets include:
Al₂O₃ > 60%
SiO₂ < 20%
Fe₂O₃ < 3%
Concrete castables should be dried to remove moisture. For alkaline refractories bonded with phosphates, drying is not allowed; for acid/neutral refractories bonded with phosphates, drying should be carried out promptly at 350–450°C.
(Phosphate solution preparation: use 50% phosphoric acid and industrial aluminum hydroxide at a mass ratio of 7:1.)
Comparing physical properties shows that phosphate-bonded castables combine high strength and hardness with a favorable linear expansion coefficient, meeting service requirements. They can be anchored using 1Cr18Ni9Ti stainless steel anchors, achieving good bonding with the 20G membrane water-wall.
CFB boilers rely on upward-flowing hot gas carrying ash/slag, which is separated by cyclone action and returned via the return feeder. Under normal operation, the highest-temperature areas are typically:
Furnace: 900°C
Furnace outlet: 892°C
Cyclone separator: 852°C
Typical gas velocities:
Furnace: 5 m/s
Furnace outlet: 5 m/s
Cyclone separator: 9.5 m/s
High temperature plus high gas/solids velocity significantly accelerates wear, requiring enhanced wear protection.
Fuel often consists of coal preparation plant by-products such as:
Washery rejects (gangue)
Coal slime
Mixed washed coal
Low calorific value and high ash lead to strong erosive wear on furnace castables and water-wall tubes, especially at:
Dense-phase/burner zone
Furnace outlet horizontal pass
Cyclone separator
Return feeder
Castable damage can expose outer protective plates to direct heat, causing deformation and cracking, as well as flue gas/ash leakage—affecting stable operation and worsening the site environment.
Construction quality is a major determinant of service life. Risks include:
Seasonal construction without adequate measures
Non-compliant installation procedures
Insufficient curing time
Failure to follow the specified heating-up (dry-out) curve
These issues prevent the castable from developing required bonding and abrasion resistance.
Poor control of heating-up/cooling-down rates creates large thermal gradients and stress. During operation, coal feeder issues or major coal quality changes can cause furnace temperature swings—severe cases may reach 150°C fluctuation.
This leads to:
Unstable steam parameters
Frequent air-flow adjustments and disturbed air–fuel ratio
Local spalling/erosion of hot-face castables
Reduced lining service life
Due to structural differences, performance requirements vary widely by zone. For example:
Post-superheater flue zones have lower temperatures and often use refractory brick walls
Small CFB boilers typically have circulation ratios around 14–20
High-wear zones (dense-phase/burner zone, cyclone inlet and vortex duct, return feeder standpipe legs) demand high-performance wear castables
To minimize ash/slag erosion, reduce alternating stress from temperature variation, and prevent microcrack propagation from volume change, these areas should use high-strength, wear-resistant composite castables with:
High hot strength
Good high-temperature stability
Strong abrasion resistance and ash/slag corrosion resistance
Good thermal shock resistance
Operational experience shows these can meet site requirements over long periods.
Additionally, during each outage, applying a uniform coat of wear-resistant protective mortar to worn areas can effectively slow down abrasion and extend service life.
Upgrade fuel blending by implementing a coal blending system to improve calorific value stability.
A practical approach includes:
Coal slime: dilute and mix thoroughly, then pump into the furnace via pressure pipeline from the top feed port
Washery rejects and mixed washed coal: spread and air-dry, then crush and screen into product fuel with particle size 0–13 mm, store separately
Before use: test calorific value of mixed coal and prepared rejects, input results into the control system
The system controls belt scales for proportional blending to achieve more stable calorific value
Better fuel particle size distribution and calorific stability help:
Stabilize boiler load
Reduce coarse-particle abrasion
Reduce combustion disturbance caused by poor fuel quality and load swings
Key controls include:
Incoming material inspection
Ensure materials meet design/contract requirements
Certificates complete
Witness sampling and third-party tests when required
Classified storage and proper protection
Anchor (stud) design improvement for thick castable sections
Upgrade from I-type 150×150 grid to Y-type anchors
Anchor spacing: 200 mm
This improves near-surface support, bonding, and structural strength.
Strict compliance with installation procedures
Apply two coats of asphalt to anchors to form a 1.5 mm isolation layer
Ensure sufficient formwork strength and rigidity
Keep inner form surface flush; seal joints tightly
Cover formwork face with PVC film
Cast in layers and vibrate thoroughly to ensure compaction
Curing and dry-out/heating-up strictly per curve
Follow curing requirements after casting
During dry-out, match initial heating-up strictly to the curve
Avoid too low temperature (moisture not removed) or too high temperature (microcracking)
Control start-up heating-up time to ≥ 6 hours
Heat up gradually from inside to outside so expansion is uniform and thermal stress is minimized
When using charcoal to assist heating-up, strictly control feed rate to prevent rapid temperature rise and damage to pressure parts or castables
During shutdown, reduce coal feed and air supply gradually
After shutdown, open furnace doors only after temperature drops to the required level to prevent localized rapid cooling and thermal shock damage
Increase inspections of air–coal feeding equipment, ensure good condition
Strengthen coal blending control, prevent high moisture fuel from causing bunker bridging and unstable fuel feed, which leads to load fluctuation
To achieve safe, long-cycle operation in CFB boilers firing low-calorific-value fuels, it is essential to:
Select the correct wear castables for different zones
Ensure high construction quality (materials, anchors, casting, curing, dry-out)
Control start-up/shutdown procedures
Stabilize operating parameters and reduce frequent load swings
These measures can significantly extend refractory castable lining life, improve boiler service life, and increase overall economic benefit.
