Under circulating fluidized bed (CFB) combustion conditions, erosion of heat-transfer surfaces by solid particles has long been a key factor limiting economic operation and further development of CFB boilers. Inside a CFB boiler, gas flows, jets, and bubbles exist in multiple directions, velocities, angles, and concentrations. They act on internal surfaces in different ways. Combined with corrosive gases and other media, this creates a complex wear mechanism.
A primary maintenance approach is applying abrasion-resistant refractory linings in critical areas to protect heat-transfer surfaces. Under reducing atmospheres, abrasion-resistant refractories typically offer better wear resistance than steel. In practice, these materials serve as a cost-effective barrier layer to prevent erosion and support safe, reliable boiler operation.
Because CFB boilers operate at high temperature with frequent temperature fluctuations, cyclic thermal shock is common. Meanwhile, large quantities of high-temperature solids move at high speed and continuously scour heat-transfer surfaces. Therefore, abrasion-resistant refractories are required for protection. This article focuses only on abrasion-resistant refractories on the fire side (flue-gas side).
Abrasion-resistant refractory materials can be classified by supply form as shaped materials and unshaped (monolithic) materials, and by function into three groups:
Abrasion-resistant refractories (including bricks, castables, plastics, and mortars)
Refractories (including bricks, castables, and mortars)
Insulating refractories (including insulating bricks, insulating castables, and mortars)
Abrasion-resistant refractories (high-density refractories):
Aluminosilicate bricks (silica bricks, fireclay bricks, high-alumina bricks)
Zircon-containing silicate bricks
Non-oxide bricks (carbon bricks, silicon carbide–carbon bricks)
Magnesia–calcia–chrome bricks and fused-magnesia products (magnesia bricks, magnesia–chrome bricks, chrome bricks, dolomite bricks)
Insulating refractory bricks, insulation bricks, insulation blocks, ceramic fiber products
Common monolithics include castables, ramming mixes, plastics, patching materials, gunning mixes, shotcreting materials, vibration mixes, and refractory coatings/plasters. In form, they may appear as powder, paste, or plastic mass.
Abrasion-resistant refractories are specialized materials designed to resist damage and deformation at high temperature. To reduce erosion from flue gas and fly ash, wear linings are installed inside components subject to severe wear. Correct selection and correct installation are critical—they help ensure long-term performance, reduce lining detachment, and minimize repair frequency.
Chemically, these materials are mainly composed of aluminum and silicon compounds, typically totaling 80%–95%. In CFB boilers, to withstand service conditions, abrasion-resistant refractories should have sufficient:
refractoriness
compressive strength
flexural strength
thermal shock resistance
low permanent linear change
Key physical and chemical indicators include:
Refractoriness is the ability to resist softening or melting at high temperature without external load. It is often expressed as maximum service temperature, defined here as the temperature at which, after calcining for 5 hours, the linear change does not exceed 1.5%.
Bulk density (also called volumetric weight) is the mass per unit volume and reflects material compactness. Unit: kg/m³.
Thermal conductivity is the heat passing through a unit area per unit time under a unit temperature gradient. Unit: W/(m·K). It relates not only to application needs but also directly influences thermal shock resistance.
Thermal shock resistance is the ability to withstand rapid temperature change without cracking, spalling, or failure. It is influenced by thermal expansion, thermal conductivity, microstructure, product shape, and particle size distribution.
This refers to the irreversible length change relative to original length under temperature change, expressed as a percentage. It is an important basis for lining design and expansion-joint design. Excessive linear change increases spalling risk.
Where:
L1: specimen length at room temperature (mm)
L2: specimen length at test temperature tt (mm)
Cold crushing strength (CCS): the maximum pressure per unit area the material can withstand at room temperature before failure, reflecting sintering/bonding and structural integrity.
Where:
CCS: compressive strength (MPa)
F: ultimate load
A: loaded area
In service, linings also face tensile, bending, and shear stresses.
Cold modulus of rupture (flexural strength): the limiting stress at fracture under bending at room temperature (MPa).
Both compressive and flexural strength depend on binder type/dosage and additives, and are also affected by raw material purity, formulation, mixing liquid content, installation method, and curing practice.
Abrasive wear is evaluated by blasting a specified amount of quartz sand at a certain speed onto the material and measuring the mass loss. Unit: g/cm². The abrasion index is a key indicator for abrasion-resistant castables and bricks.
In a CFB boiler, failures typically result from a combination of:
continuous scouring by high-speed, high-temperature solids
temperature fluctuation, thermal shock, and mechanical stress causing cracks and spalling
penetration of alkali metals and other corrosive species leading to degradation
In CFB boilers, major high-wear zones include:
water walls and in-furnace heat-transfer surfaces
cyclone separator and return system
rear pass inlet (tail flue entrance)
ash discharge system
Typical lining locations include:
furnace dense phase zone
lower region beneath in-furnace platen heating surfaces
furnace outlet
separator (cyclone)
standpipe/loop seal area
return valve
separator outlet duct
tail convective pass
Based on wear characteristics, material properties, and installation constraints, linings are commonly designed as single-layer or multi-layer systems.
Single-layer linings are used where thin linings are required (e.g., water walls, platen heating surfaces, water-cooled partitions, double-sided water walls, steam-cooled separators). Abrasion-resistant castables are supported on the flue-gas side using Ø6 or Ø10 cylindrical studs (or Y-type/V-type anchors). Thin linings tolerate rapid thermal shock during boiler start/stop. To improve rigidity and impact resistance, stainless steel fibers are often added to the castable.
Thick linings are typically built in two or three layers. The hot face uses abrasion-resistant/high-temperature materials such as abrasion-resistant bricks or abrasion-resistant plastic/castable. The backing insulation layer reduces heat loss and lowers shell temperature.
During installation of shaped wear materials, loads are transferred in layers through support plates to the outer steel shell. Because different materials have different thermal expansion behavior, expansion joints are critical. Construction joints also function as expansion joints. In large castable areas, staged installation should be used, leaving proper joints between sections and staggering joint locations.
Due to the mismatch in thermal expansion between metal and refractory, before installing abrasion-resistant refractories, the surfaces of metal parts (including studs, support plates, and tubes) should be coated with asphalt/bitumen. This creates a small gap between castable and the heated surface, reducing damage caused by differential expansion during operation.
Vibration (or compaction) is essential for castable installation. Insufficient or improper vibration leads to inadequate or uneven density, resulting in low strength and poor wear protection. In hard-to-reach areas, formwork should be made as small and practical as possible to improve compaction effectiveness.
Because raw minerals contain moisture and additional water is added during mixing, water must be removed through natural drying and controlled heat curing. Abrasion-resistant castables are typically dense, and chemically bound water may be retained within the microstructure, so bake-out requires adequate time.
Heat curing is usually performed using controlled-temperature hot flue gas, while boiler water circulation is maintained to protect heat-transfer surfaces. The recommended heat-up curve should be provided by the material supplier or follow a supplier-approved schedule.
After curing:
all casting holes opened on sealing boxes must be closed and welded;
all temporary steam channels used for curing must be sealed and welded.
To ensure safe unit operation, abrasion-resistant refractories for CFB boilers should offer:
high cold and hot strength
low wear loss
strong resistance to chemical attack/penetration
good high-temperature volume stability
At present, abrasion-resistant refractory castables for circulating fluidized bed boilers are widely produced in accordance with YB/T 4109-2002, which defines three grades: NMJ-1, NMJ-2, and NMJ-3, corresponding to alumina contents of 60%, 65%, and 70% respectively. In real operation, however, performance targets often need to be adjusted according to specific operating conditions and installation locations.
