Pneumatic Ash Handling System for CFBC Boilers: How It Handles Bed Ash and Fly Ash

Pneumatic Ash Handling System for CFBC Boiler: The Right Solution for Bed Ash and Fly Ash

CFBC and AFBC boilers produce two distinct ash streams that require careful handling: hot, coarse bed ash collected from the furnace bed and fine fly ash captured by the ESP. Both streams can be highly abrasive and can cause significant wear when handled with exposed mechanical conveying equipment.

A pneumatic ash handling system for a CFBC boiler, particularly one based on dense phase pneumatic conveying, provides a sealed method of transporting both ash streams to storage silos. Compared with conventional open mechanical conveying arrangements, pneumatic conveying reduces exposure of moving equipment to abrasive ash and provides greater flexibility in routing the conveying pipeline.

Bed ash from a fluidized bed boiler is also different from the bottom ash typically associated with a conventional pulverized coal (PF) boiler. It is generally sand like, coarser, hotter when discharged from the bed, and produced continuously. Using equipment designed specifically for conventional bottom ash without accounting for these characteristics can result in excessive wear, poor reliability, and operational problems.

For this reason, CFBC boiler bed ash handling should be selected carefully during the initial system design rather than treated as a conventional bottom ash application.

What Makes CFBC and AFBC Bed Ash Different from Conventional Bottom Ash?

In a circulating fluidized bed combustion (CFBC) or atmospheric fluidized bed combustion (AFBC) boiler, combustion takes place within a fluidized bed containing fuel and bed material such as sand/crushed bricks/other inert bed material. Airflow keeps the particles suspended, while ash is continuously removed from the bed during bed discharge.

The resulting bed ash can combine three demanding characteristics:

• High temperature when discharged from the boiler
• High abrasiveness due to its particle characteristics and mineral content
• Continuous generation (in surges) rather than intermittent discharge

These characteristics can accelerate wear on conventional mechanical conveying equipment.

A dedicated bed ash handling system therefore needs to consider the ash temperature, particle size, conveying rate, abrasiveness, cooling requirements, and required conveying distance.

The objective is not simply to move the bed ash. The system must move it reliably while controlling equipment wear and ensuring that the ash reaches the storage or disposal point safely.

How a Dense Phase Pneumatic Conveying Handles CFBC Bed Ash

A typical arrangement uses a chain conveyor to collect cooled bed ash from the boiler area and transfer it into a surge hopper, located suitably outside the boiler area. The surge hopper provides an intermediate collection point before the ash enters the pneumatic conveying system.

From the surge hopper, a dense phase pneumatic conveying system transports the bed ash through a closed pipeline to the bed ash silo.

In this arrangement, the surge hopper also functions as an important staging point for the bed ash cooler and downstream conveying process. Cooling the ash before pneumatic conveying allows the downstream pipeline, conveying vessels, valves, and associated equipment to operate within their intended temperature range.

A dome valve is installed at the top of each conveying vessel. It seals the conveying vessel before compressed air is introduced, allowing the collected ash to be pushed through the conveying pipeline towards the silo.

Dense phase conveying is particularly suitable for abrasive ash applications because the material can be transported at relatively controlled conveying conditions rather than relying solely on very high material velocities.

Fly Ash from CFBC Boilers Also Requires Efficient Pneumatic Handling

The fly ash side follows a similar principle and is an important part of any fluidized bed boiler ash disposal system.

ESP hoppers on a CFBC boiler collect fine fly ash after the flue gas passes through the electrostatic precipitator. The collected fly ash can then be transferred through a dense phase pneumatic conveying system to a dedicated fly ash silo.

The conveying vessels again use dome valves to isolate the vessels before each conveying cycle.

The overall system arrangement can become more complex when several CFBC boilers share a common fly ash silo. The conveying system must accommodate multiple ash streams while ensuring that conveying cycles are properly sequenced and that one boiler’s operation does not interfere with another boiler’s ash discharge cycle. The fly ash silo top equipment (like bag filters, etc.,) and the silo discharge equipment should be designed to take care of entire load of ash generated in boilers.

CFBC Boiler Ash Handling: Bed Ash vs Fly Ash

Although both streams originate from the same boiler, bed ash and fly ash have different physical characteristics and collection points.

This distinction is important when designing an AFBC or CFBC boiler ash handling system. A single generic conveying arrangement may not be appropriate for both streams.

Real Project Example: 2 × 300 TPH CFBC Boilers at HMEL Bhatinda

A practical example of this type of installation is the ash handling system executed by Macawber Beekay for HPCL Mittal Energy Limited (HMEL) at its Guru Gobind Singh Refinery in Bhatinda, Punjab.

The project covered two 300 TPH CFBC boilers, with the scope including design, engineering, manufacturing, procurement, supply, and installation of the complete ash handling system.

Bed Ash Handling System

For both the boilers, Bed ash from each boiler is discharged continuously onto a chain conveyor (supplied by others) and transferred to a surge hopper.

The surge hopper has two outlets, with each outlet feeding an individual conveying vessel and individual pipeline leading to a common bed ash silo. Out of two conveying vessels, one is working and the other is standby.

This arrangement allows the system to collect and stage the hot, abrasive bed ash before the dense phase pneumatic conveying stage transports it to storage.

The bed ash is discharged to a Bed Ash Silo. The silo is equipped with wet (through ash conditioner) and dry (through unloading spouts) unloading equipment for onward disposal of the bed ash, through trucks.

Fly Ash Handling System

Fly ash from each boiler’s ESP is handled through its own set of conveying vessels.
Each ESP has five fields with two collection hoppers per field, with the collected fly ash conveyed through a series arrangement of conveying vessels, including master and slave vessels, through a common conveying line to a shared fly ash silo.
The shared silo arrangement allows the ash from both boilers to be collected at a common storage location while the control system manages the individual conveying cycles.

Ash Storage and Unloading

Fly ash silo is equipped with multiple unloading outlets.

These include:

• Dry ash unloading spouts for truck loading
• An ash conditioner outlet for moist ash disposal
• A reserve outlet blanked off for future requirements

This configuration provides flexibility for both dry ash handling and conditioned ash disposal.

Automatic PLC-Based Control

The complete system, including separate bed material and limestone conveying equipment for the boiler bunkers, operates through a PLC-based control panel.

The control system can switch between timer-based batch conveying and level-probe-triggered conveying according to operating requirements.

This type of automated sequencing is particularly useful where multiple ash streams and multiple CFBC boilers feed common storage facilities.

Why Dense Phase Pneumatic Conveying Is Suitable for CFBC Ash Handling

The main advantage of dense phase pneumatic conveying in CFBC applications is that it allows abrasive ash to be transported through enclosed pipelines with controlled conveying conditions.

For a well-designed system, the key benefits include:

Reduced exposure to abrasive ash

Ash remains within enclosed conveying pipelines rather than being continuously exposed around mechanical conveyor equipment.

Flexible pipeline routing

Pneumatic pipelines can be routed around plant equipment and structures, providing greater layout flexibility than many conventional mechanical conveying arrangements.

Suitable for long-distance transfer

Dense phase pneumatic conveying can transfer ash from a collection or surge point to a remote storage silo.

Reduced mechanical moving equipment along the conveying route

Once ash enters the pneumatic conveying pipeline, the transfer route does not require a continuous chain or belt conveyor.

Centralised ash storage

Multiple boiler ash streams can be directed toward common storage silos when the system is appropriately designed and controlled.

The exact conveying technology and operating parameters should, however, be selected based on ash characteristics, conveying distance, temperature, throughput, pipeline configuration, and the specific boiler arrangement.

What Should Be Considered When Selecting a CFBC Ash Handling System?

Selecting a pneumatic ash handling system for a CFBC boiler requires more than choosing between mechanical and pneumatic conveying.

Key design considerations include:

Ash Temperature

Bed ash can leave the fluidized bed at a temperature that is unsuitable for direct entry into downstream conveying equipment. An appropriate cooling and staging arrangement, including a bed ash cooler where required, is therefore important.

Ash Characteristics

Particle size, abrasiveness, bulk density, moisture content, and temperature all influence the selection and sizing of the conveying equipment.

Conveying Distance

The distance between the boiler, surge hopper, and ash silo affects the selection of conveying technology, pipeline size, air requirements, and conveying cycle.

Number of Ash Sources

A system serving multiple ESP fields, hoppers, or boilers requires appropriate sequencing and control to prevent competing conveying cycles and uneven loading of common storage equipment.

Silo Capacity and Layout

The number, location, and capacity of bed ash and fly ash silos influence the conveying pipeline arrangement and the number of conveying vessels required.

Automation and Control

A PLC-based control system can coordinate conveying cycles, monitor hopper and silo levels, and manage multiple ash streams automatically.

FAQ: Pneumatic Ash Handling System for CFBC Boilers

Q. What is a pneumatic ash handling system for a CFBC boiler?

A. A pneumatic ash handling system uses air pressure to transport ash through enclosed pipelines from collection points to storage silos. In CFBC applications, dense phase pneumatic conveying can be used for both bed ash and fly ash after the bed ash has been appropriately cooled.

Q. What is the difference between CFBC and AFBC boiler ash handling?

A. Both CFBC and AFBC boilers use fluidized-bed combustion, but their operating arrangements differ. In both applications, ash handling needs to account for the abrasive and potentially hot characteristics of bed ash, while coarse fly ash is collected in ESP in AFBC but fine to very fine fly ash is collected in ESP in case of CFBC boiler. The final system design depends on the specific boiler and plant configuration.

Q. Why is a bed ash cooler required in FBC boiler ash handling?

A. Bed ash is discharged from the fluidized bed at a relatively high temperature. Cooling the ash before it enters downstream pneumatic conveying equipment helps bring the material to a temperature suitable for the conveying pipeline, valves and their associated rubber components, vessels, and associated equipment.

Q. Can mechanical conveyors handle CFBC bed ash?

A. Yes. A mechanical chain conveyor can be used for the short initial section, particularly to move bed ash away from the immediate boiler area and into a surge hopper. Dense phase pneumatic conveying can then be used for the onward transfer to the bed ash silo.

Q. Do CFBC boilers need separate systems for bed ash and fly ash?

A. Typically, bed ash and fly ash are handled as separate ash streams because they differ in particle characteristics, temperature, and collection location. They can, however, use similar dense phase pneumatic conveying principles and common control philosophies.

Q. Can multiple CFBC boilers share the same ash silo?

A. Yes. Multiple CFBC boilers can feed common bed ash or fly ash storage silos respectively when the silo capacity, conveying pipelines, conveying vessels, and control system are designed for the combined ash streams from multiple boilers.
A PLC-based control system can sequence conveying cycles from individual boilers to coordinate the flow of ash into shared storage facilities.

Q. Is dense phase pneumatic conveying suitable for fluidized bed boiler ash disposal?

A. Dense phase pneumatic conveying is most suited to fluidized bed boiler ash disposal because it provides enclosed, controlled transport of abrasive ash from collection points to storage silos. The final selection should be based on the ash properties, required capacity, conveying distance, temperature, and overall plant layout.

Conclusion

A pneumatic ash handling system for a CFBC boiler needs to address the specific challenges created by hot, abrasive, continuously generated bed ash as well as the finer fly ash collected by the ESP.

A typical arrangement combines a chain conveyor and surge hopper for the initial bed ash collection and cooling stage, followed by dense phase pneumatic conveying to transport the bed ash to a bed ash silo. Fly ash from the ESP can similarly be conveyed pneumatically to a dedicated or shared fly ash silo.

The right bed ash handling system therefore depends on the complete plant arrangement rather than a single piece of equipment. Ash temperature, abrasiveness, conveying distance, number of collection points, silo configuration, throughput, and automation requirements all need to be considered during system design.

The 2 × 300 TPH CFBC boiler installation at HMEL Bhatinda demonstrates how bed ash, fly ash, bed material, and limestone conveying can be integrated into an automated ash handling arrangement designed around the requirements of a large industrial power plant.