Different Types of Bottom Ash Handling Systems Explained

Bottom ash is the coarse fraction of ash that falls to the bottom of a boiler furnace instead of being carried away with the flue gas. In a typical coal-fired boiler, bottom ash accounts for approximately 20% of the total ash generated, while the remaining ash is predominantly fly ash collected from ESP, duct and economiser/air pre-heater hoppers.

Choosing the right bottom ash handling system is critical for reliable ash removal, efficient plant operation, water management and downstream ash disposal or utilisation.

The principal types of bottom ash handling systems used in thermal power plants include:

• Wet bottom ash handling – Jet Pumping System
• Semi-dry bottom ash handling – Submerged Scraper Chain Conveyor (SSCC)
• Dry mechanical bottom ash handling
• Pneumatic bottom ash handling

Each technology uses a different approach to collect, cool and transport bottom ash. The most suitable configuration depends on factors including boiler type, ash characteristics, available space beneath the furnace, water availability, conveying distance, plant layout, the final ash storage or disposal arrangement, and plant ash disposal philosophy.

1. Wet Bottom Ash Handling – Jet Pumping System

A wet bottom ash handling system typically uses a water-impounded hopper installed beneath the furnace to receive and quench hot bottom ash.

In a jet pumping system, ash collected in the bottom ash hopper is periodically discharged through feed gates and clinker crushers. High-pressure motive water supplied to the jet pumps then transports the bottom ash as slurry to dewatering bins, a slurry sump or another downstream disposal system.

This established bottom ash handling method is particularly relevant to pulverised fuel (PF) boilers and plants where the ash management infrastructure is designed around hydraulic conveying and wet disposal.

When Is Wet Bottom Ash Handling Suitable?

Wet bottom ash handling can be suitable where:

• Adequate process water is available.
• Existing slurry handling and disposal infrastructure is already installed.
• The plant’s ash management strategy is based on wet disposal.
• Intermittent bottom ash removal meets the boiler’s operating requirements.

Advantages and Considerations

Wet bottom ash handling provides effective quenching and transportation of high-temperature bottom ash and has a long operating history in coal-fired power plants.

Its principal consideration is the introduction of water into the ash stream. Once the ash is converted into slurry, additional equipment and infrastructure are required for slurry transportation, dewatering, final ash disposal, and water recovery.

2. Semi-Dry Bottom Ash Handling – Submerged Scraper Chain Conveyor (SSCC)

A semi-dry bottom ash handling system using a Submerged Scraper Chain Conveyor (SSCC) provides continuous removal of bottom ash from beneath the furnace while significantly reducing the requirement for hydraulic transportation of ash.

In this arrangement, bottom ash from the furnace is collected in a bottom ash hopper and subsequently discharged into the water bath of the Submerged Scraper Chain Conveyor. The water bath quenches the hot ash and helps maintain an effective seal at the furnace bottom.

The ash is then continuously extracted from the SSCC trough using scraper chains, sprockets and bars. As the material travels through the inclined section of the conveyor, excess water drains from the ash, resulting in dewatered or moist bottom ash at the discharge point.

The dewatered ash can subsequently be passed through a clinker crusher and routed for further handling or disposal.

How Does a Submerged Scraper Chain Conveyor Work?

The typical SSCC bottom ash handling sequence is:

Furnace → Bottom Ash Hopper → SSCC Water Bath → Continuous Scraper Extraction → Dewatering Section → Clinker Crusher → Downstream Handling/Disposal

This arrangement combines water-based ash quenching with mechanical extraction and dewatering, which is why it can be categorised as a semi-dry bottom ash handling solution rather than a conventional wet slurry conveying system.

Advantages of the SSCC System

A Submerged Scraper Chain Conveyor system can provide several advantages:

• Continuous removal of bottom ash from the furnace area.
• Effective quenching of hot bottom ash.
• Mechanical extraction rather than long-distance hydraulic transportation.
• Dewatering of ash before discharge.
• Reduced quantity of water carried forward with the ash compared with conventional slurry conveying.
• Flexibility in selecting the downstream ash handling and disposal route.
• Potential to transport dewatered ash mechanically to a bottom ash storage silo.

The bottom ash hopper can also provide temporary ash storage capacity, allowing maintenance of the SSCC without immediately affecting boiler operation.

Where Is an SSCC Bottom Ash Handling System Suitable?

A semi-dry SSCC arrangement can be particularly relevant for power plants that require continuous bottom ash removal while seeking to reduce their dependence on conventional slurry-based ash transportation.

The final system configuration depends on boiler capacity, furnace arrangement, ash characteristics, available space and the selected downstream ash disposal or utilisation method.

3. Dry Mechanical Bottom Ash Handling System

A dry mechanical bottom ash handling system removes and transports bottom ash mechanically without using water as the primary quenching or conveying medium.

Depending on the boiler and plant configuration, mechanical conveyors are used to collect cooled ash and transport it from the boiler area to an intermediate storage or transfer point.

For CFBC and AFBC boilers, where the discharged material is commonly referred to as bed ash, a drag chain conveyor can be used to collect cooled bed ash and transfer it to a surge hopper.

Advantages of Mechanical Bottom Ash Handling

Mechanical bottom ash handling can offer:

• Reduced dependence on water for ash handling.
• No requirement to transport bottom ash as dilute slurry.
• Controlled mechanical movement of ash.
• Reduced dependence on ash ponds for the bottom ash stream.
• Compatibility with downstream dry conveying and storage arrangements.
• Potentially greater flexibility for subsequent ash utilisation.

Mechanical Conveying for CFBC and AFBC Boilers

For CFBC and AFBC boilers, mechanical conveying can form the first stage of a combined ash handling arrangement.

A drag chain conveyor collects cooled bed ash and transports it out of the immediate boiler area. The ash can then be discharged into a surge hopper before being transferred to a downstream conveying system.

Where the final bed ash silo is located farther away, this mechanical stage can be integrated with dense phase pneumatic conveying.

4. Pneumatic Bottom Ash Handling

Pneumatic bottom ash handling is particularly applicable to CFBC and AFBC boilers, where bed ash may need to be transported over a considerable distance from the boiler to a storage silo.

In one arrangement, cooled bed ash is mechanically collected close to the boiler and transferred to a surge hopper. A dense phase pneumatic conveying system then transports the material through an enclosed pipeline to the bed ash silo.

Alternatively, depending on the boiler and system configuration, bed ash collected from ash coolers can be fed to surge hoppers before entering the dense phase pneumatic conveying system.

Why Use Dense Phase Pneumatic Conveying for Bed Ash?

Dense phase pneumatic conveying provides several advantages for appropriate bed ash applications:

• Enclosed transportation of abrasive ash.
• Flexibility in locating the bed ash silo.
• Efficient transportation over longer distances.
• Reduced requirement for long mechanical conveyor routes.
• Greater flexibility when routing conveying pipelines through an existing plant.
• Reduced interference with movement and other plant equipment at ground level as the conveying ash pipelines travel overhead from boiler to the silo top.

The combination of short-distance mechanical conveying and long-distance dense phase pneumatic conveying can therefore provide an effective solution for CFBC and AFBC bed ash handling.

Comparison of Bottom Ash Handling Systems

Understanding the differences between the major types of bottom ash handling systems helps plant owners and engineers select a technology appropriate to their boiler, water availability and overall ash management strategy.

Wet vs Semi-Dry vs Dry Bottom Ash Handling

The key distinction between wet, semi-dry and dry bottom ash handling systems is how water is used during ash quenching, extraction and transportation.

In a wet jet pumping system, water is used both to quench the ash and as the primary medium for transporting it from the boiler area. The resulting bottom ash is therefore handled as slurry.

In a semi-dry SSCC system, water is used to quench the hot bottom ash within the submerged conveyor. However, the ash is mechanically extracted and dewatered before leaving the conveyor, substantially reducing the amount of water carried forward with the material.

In a dry mechanical system, water is not used as the primary ash conveying medium. The material remains in a dry or cooled condition using atmospheric air and can subsequently be subsequently transported mechanically or pneumatically.

This distinction is important because it affects water consumption, downstream equipment, disposal infrastructure, ash utilisation opportunities and overall plant layout.

Water Use and Environmental Considerations

Water availability is an important consideration when comparing the different types of bottom ash handling systems.
A wet jet pumping system requires water for ash quenching and hydraulic transportation, resulting in a slurry that requires downstream handling and disposal.

A semi-dry Submerged Scraper Chain Conveyor also uses water for quenching but mechanically extracts and dewaters the ash before discharge. This reduces reliance on hydraulic transportation of bottom ash.

Dry mechanical and pneumatic conveying arrangements further reduce dependence on water by transporting the ash without converting it into slurry.

However, water consumption should not be considered in isolation. System selection should also account for:
Boiler design and operating conditions.

• Bottom ash temperature and particle characteristics.
• Available headspace and plant layout.
• Required conveying distance.
• Existing ash handling infrastructure.
• Storage and silo location.
• Capital and lifecycle operating costs.
• Maintenance requirements.
• Environmental considerations.
• Final ash utilisation or disposal strategy.
• Plant location.

How to Select the Right Bottom Ash Handling System

Selecting the appropriate system requires a plant-specific engineering assessment rather than simply choosing between wet and dry technologies.

For a conventional PF boiler, wet jet pumping or a semi-dry SSCC system may be evaluated depending on the required extraction philosophy, available water, plant layout and downstream ash management arrangement.

An SSCC system can be particularly attractive where continuous extraction and dewatered bottom ash are preferred over conventional slurry transportation.

For CFBC and AFBC boilers, mechanical and pneumatic systems can be configured according to the characteristics of the bed ash and the distance between the boiler and bed ash silo.

The selection should ultimately consider the complete ash handling route—from furnace discharge and cooling through extraction, conveying, storage and final utilisation or disposal.

Frequently Asked Questions About Bottom Ash Handling Systems

Q. What are the main types of bottom ash handling systems?

A. The principal types of bottom ash handling systems include wet jet pumping systems, semi-dry Submerged Scraper Chain Conveyor systems, dry mechanical systems and pneumatic conveying systems. The appropriate technology depends on boiler type, ash characteristics, plant layout, water availability and conveying requirements.

Q. What is a Submerged Scraper Chain Conveyor?

A. A Submerged Scraper Chain Conveyor (SSCC) is a bottom ash handling system in which hot ash is discharged into a water bath for quenching and subsequently extracted continuously using scraper chains. The ash is dewatered as it travels along the inclined conveyor section before being discharged for further handling.

Q. Why is SSCC considered a semi-dry bottom ash handling system?

A. An SSCC uses water to quench hot bottom ash but does not rely on water as the primary medium for long-distance ash transportation. Instead, the quenched ash is mechanically extracted and dewatered, producing moist ash rather than the dilute slurry associated with conventional wet jet pumping systems.

Q. What is the difference between a jet pumping system and an SSCC system?

A. A jet pumping system uses high-pressure water to hydraulically transport bottom ash as slurry. An SSCC system quenches the ash in water but then mechanically extracts and dewaters it before downstream handling.

The key difference is therefore the method of ash extraction and transportation: hydraulic slurry conveying versus submerged mechanical extraction and dewatering.

Q. Which bottom ash handling system uses the least water?

A. Dry mechanical and pneumatic systems generally have the lowest water requirement for ash conveying because they do not depend on water as the transportation medium.

Q. Why is pneumatic bottom ash handling used for CFBC boilers?

A. CFBC boilers generate bed ash that may need to be transported from the boiler area to a remotely located silo. Mechanical collection followed by dense phase pneumatic conveying allows the ash to be transported through an enclosed pipeline over longer distances while providing greater flexibility in silo location.

Q. Can a plant convert from wet to semi-dry bottom ash handling?

A. A conversion from wet jet pumping to an SSCC-based system may be technically feasible, but it requires a detailed assessment of the furnace-bottom arrangement, available space, clearances below the boiler columns, structural requirements, ash characteristics and downstream conveying and storage infrastructure.

Conclusion

There is no single bottom ash handling technology suitable for every thermal power plant. Wet, semi-dry, mechanical and pneumatic bottom ash handling systems address different boiler configurations and operating requirements.

A wet jet pumping system offers an established solution where adequate water and slurry disposal infrastructure are available. A semi-dry Submerged Scraper Chain Conveyor system combines water-based quenching with continuous mechanical extraction and dewatering, reducing dependence on slurry transportation.

Mechanical and dense phase pneumatic systems can provide effective solutions where dry handling, longer conveying distances or CFBC/AFBC bed ash applications are involved.

Choosing the right system therefore requires a comprehensive assessment of boiler type, ash characteristics, water availability, plant layout, conveying distance, environmental requirements and the final ash utilisation or disposal strategy.

The objective is not simply to remove ash from beneath the boiler, but to engineer a complete bottom ash handling solution that delivers reliable evacuation, efficient operation, optimised water use and lifecycle performance.