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In circulating fluidized bed (CFB) boiler systems, the center cylinder and air caps are two core components – one is responsible for “separation,” the other for “air distribution.” Though they perform different functions, together they determine the boiler’s combustion efficiency, operational stability, and equipment service life.
If the CFB boiler is an efficient combustion “boiling factory,” then the air caps are the “air‑distribution matrix” at the bottom – hundreds of air caps feed air uniformly into the furnace, making the coal particles boil and burn like a fluid. The center cylinder, on the other hand, is the “separation core” at the top – it separates unburned materials from the flue gas and returns them to the furnace for continued combustion. One at the bottom, one at the top, one feeding air, one returning solids – together they form the critical closed‑loop material circulation of the CFB boiler.
1. Center Cylinder – The Core Tube of the Cyclone Separator
What Is a Center Cylinder?
The center cylinder is the core component of the cyclone separator in a circulating fluidized bed boiler, located at the top of the separator body. Its function is to perform gas‑solid separation on the high‑temperature flue gas containing large amounts of solid particles: clean flue gas exits through the center cylinder into the rear flue, while the separated solid materials are returned to the furnace via the return device to continue burning.
The center cylinder operates in a harsh environment of high flue‑gas temperature, high gas velocity, and high particle concentration. Its performance directly determines the separation efficiency of the cyclone separator, thereby affecting key indicators such as boiler load capacity, carbon content in fly ash, and wear on the rear heating surface.
Material Selection for the Center Cylinder
Center cylinders are mainly divided into two types based on manufacturing method: rolled‑plate type and cast type:
| Type | Material | Thickness | Advantages | Disadvantages |
|---|---|---|---|---|
| Rolled‑plate type | Cr25Ni20 heat‑resistant stainless steel plate | 8‑12 mm | Lower cost, shorter lead time | Insufficient structural strength, prone to deformation (concave/convex, ovality) |
| Cast type | High‑Cr‑Ni heat‑resistant alloys (e.g., Cr25Ni20MoMnSiNRe, ZG3Cr24Ni7SiN, etc.) | 14‑20 mm | High strength, not easily deformed | Higher cost, complex process |
The trend is that cast center cylinders are gradually becoming mainstream. In terms of material formulation, integral cast materials such as Cr25Ni20MoMnSiNRe significantly improve high‑temperature strength and wear resistance through the addition of molybdenum, manganese, silicon, nitrogen, and rare‑earth elements. ZG3Cr24Ni7SiN forms a stable oxide film at 800‑1000°C through the synergistic effect of chromium, nickel, silicon, and nitrogen, effectively resisting high‑temperature oxidation and hot corrosion.
In highly corrosive environments such as waste incineration, the ZG4Cr25Ni20Si2 cast center cylinder shows significantly better overall performance under 1100°C conditions than traditional 310S rolled products.
Structural Design and Installation of the Center Cylinder
The installation method of the center cylinder directly affects its service life:
- Welded fixed type: Does not account for thermal expansion; load‑bearing parts are prone to fracture; gradually being phased out.
- Bolted hanging type: Expansion stress is not properly resolved, leading to bolt breakage or cylinder compression deformation.
- Free‑hanging type: Allows the cylinder to expand freely in all directions; currently the best installation method.
The free‑hanging center cylinder is equipped with upper, middle, and lower (or upper and lower) reinforcing ribs around its periphery. The upper ribs are load‑bearing parts, resting on heat‑resistant steel supports; the middle and lower ribs strengthen the structure. Concentricity must be ensured during installation. To prevent flue‑gas short‑circuiting, a sealing ring plate is installed at the bottom of the sealing castable on the upper end of the cylinder, and the gaps are filled with rock wool.
In recent years, optimised designs such as offset and variable‑diameter structures, tapered bottom sections, and labyrinth seals have also been widely adopted.
Common Failures and Retrofits of the Center Cylinder
The most common failure mode of the center cylinder is high‑temperature deformation – the cylinder becomes concave, convex, or oval under alternating thermal expansion and contraction. Deformation severely reduces separation efficiency, leading to a series of problems including higher bed temperature, increased carbon content in fly ash, and aggravated wear on the rear flue.
Typical retrofit measures include:
- Upgrading the material: Using higher‑performance heat‑resistant alloys in integral casting.
- Optimising the installation method: Replacing hook‑type mounting with free‑hanging.
- Improving structural design: Using a tapered bottom section with offset design.
- Enhancing sealing: Adopting labyrinth seals.
After implementing the above retrofits on a 330 MW CFB boiler, bed temperature dropped by 30°C, the dilute‑phase differential pressure increased by 500 Pa, and NOx emissions were reduced by 50%.
2. Air Caps – The Key Components of the Air Distributor
What Are Air Caps?
Air caps are the core components of the air distributor in a circulating fluidized bed boiler. Their function is to deliver high‑pressure air from the primary air fan uniformly into the furnace through hundreds of air caps, achieving uniform fluidisation of the bed material.
In simple terms, air caps are the “air distributors” of the CFB boiler – they determine whether air can uniformly penetrate the bed layer, directly affecting fluidisation quality, combustion efficiency, and pollutant formation. Though small, air caps are critical to the boiler’s operation.
Types and Structural Evolution of Air Caps
With the development of CFB boiler technology, air caps have undergone several iterations:
| Type | Characteristics | Current Status |
|---|---|---|
| Small‑hole air caps | Small hole diameter, good uniformity | Wear is common |
| Mushroom‑head type | Mostly used in small and medium boilers | Still in use |
| “Γ”‑shaped / 7‑shaped directional caps | Low resistance | Slag leakage is a prominent issue |
| “Pigtail” type | Suitable for central bottom slag discharge | Specific applications |
| Bell‑type (large diameter) | Labyrinth structure prevents slag leakage, long life | Current mainstream |
Large CFB boilers tend to adopt bell‑type air caps. Features of bell‑type caps include:
- Inner pipe with suitable resistance, uniform air distribution, good adjustability.
- Low air velocity through outer cap holes, reducing inter‑cap wear.
- Outer cap threaded to inner pipe for easy maintenance.
- Not prone to clogging or air leakage during operation.
- Long service life.
New bell‑type air caps use a split design – the air feed pipe is welded to the air distributor plate, and the cap is fitted over the feed pipe. The labyrinth structure effectively prevents material from entering the air plenum.
Material Selection for Air Caps
Air caps operate in a harsh environment of high temperature, high wear, and high corrosion, imposing extremely stringent material requirements. Common materials include:
| Material | Key Characteristics | Applicable Temperature |
|---|---|---|
| 16Cr20Ni14Si2 | High‑temperature resistance, corrosion resistance, wear resistance | High temperature |
| ZGCr25Ni20 (310S) | Good high‑temperature oxidation resistance, but wear resistance is not outstanding | 1000‑1150°C |
| ZG40Cr24Ni9Si2NRE | Rare‑earth heat‑resistant steel, excellent overall performance | 1100‑1200°C |
| High‑Cr cast iron | Carbides distributed as fine rods, excellent high‑temperature wear resistance | High temperature |
| Rare‑earth high‑Cr‑Ni‑N heat‑resistant steel | Fully oxidation‑resistant at 1200°C | ≤1200°C |
| Cr‑Mo alloy steel | Good high‑temperature resistance and oxidation resistance | High temperature |
The core conflict in air‑cap material selection is that it must simultaneously resist high‑temperature oxidation and wear. Traditional heat‑resistant steels (e.g., 310S) have good high‑temperature strength but insufficient wear resistance; high‑chromium cast iron has good wear resistance but is more brittle. Therefore, new materials such as rare‑earth high‑Cr‑Ni‑N heat‑resistant steel are becoming the trend. Some new nickel‑free wear‑resistant alloy air caps show only half the wear of traditional products, extending service life from 5‑6 months to more than 2 years.
Common Failures and Prevention of Air Caps
The most common failure modes for air caps are wear and slag leakage. Wear mainly occurs at the outer cap outlet, top, and inner tube. Once the cap is worn through, it causes internal blockage, poor ventilation, and insufficient cooling, leading to overheating, carbonisation, and hardness reduction, and in severe cases, slagging in the air plenum and bed.
Causes of wear include:
- Substandard material quality.
- Slag leakage into the air plenum causing blockage and burn‑through.
- Excessive primary air velocity.
- Improper cap density and orientation layout.
In addition, poor maintenance and improper installation are also major causes of air‑cap problems.
Preventive measures include: selecting high‑quality wear‑ and heat‑resistant materials, controlling primary air flow, controlling feed fuel particle size distribution, and optimising cap layout.
3. Synergy Between the Center Cylinder and Air Caps
Though located at the top and bottom of the boiler respectively, the center cylinder and air caps together form the complete closed‑loop material circulation of the CFB boiler:
- The air caps at the bottom feed air uniformly into the furnace, fluidising and combusting the fuel.
- The dust‑laden flue gas rises into the cyclone separator.
- The center cylinder at the top separates the solids from the flue gas – clean gas exits, and the solids return to the furnace for further combustion.
A problem with either component will trigger a chain reaction. Worn air caps causing slag leakage lead to uneven air distribution and poor fluidisation; a deformed center cylinder reduces separation efficiency and increases carbon content in fly ash. Therefore, coordinated optimisation of both air caps and the center cylinder has become a common approach in CFB boiler retrofits.
4. Selection and Maintenance Guide
Center Cylinder Selection Points
| Consideration | Recommendation |
|---|---|
| Manufacturing method | Prefer cast type (higher strength, less deformation) |
| Material grade | For high‑temperature conditions, choose high‑performance alloys such as Cr25Ni20MoMnSiNRe; for highly corrosive environments, choose ZG4Cr25Ni20Si2 |
| Installation method | Prefer free‑hanging type (allows free expansion) |
| Structural design | Consider offset, variable‑diameter, tapered, and other optimised designs |
| Sealing structure | Use labyrinth seals to prevent gas short‑circuiting |
Air Cap Selection Points
| Consideration | Recommendation |
|---|---|
| Structural type | For large CFB boilers, prefer bell‑type |
| Material | For high‑temperature, high‑wear conditions, choose rare‑earth high‑Cr‑Ni‑N heat‑resistant steel or high‑Cr cast iron |
| Wall thickness | Top wall thickness ≥20 mm, side wall ≥13 mm |
| Layout density | Optimise density based on plenum dimensions, furnace structure, and fuel characteristics |
| Service life | High‑quality caps should last ≥2 years, with annual replacement <5% |
Maintenance Points
Center cylinder:
- Regularly check for deformation (ovality, concavity/convexity).
- Pay attention to cracks at load‑bearing rib locations.
- Check seal integrity to prevent gas short‑circuiting.
- Avoid separator over‑temperature during operation to slow deformation.
Air caps:
- Regularly clean internal ash and debris.
- Check for wear, blockage, or detachment.
- Monitor for slag leakage into the air plenum.
- Control primary air flow and feed fuel particle size.