Fly ash and dry ash pneumatic conveying depend on material properties, route geometry and the receiver’s ability to separate solids from the gas. Large quantities of fine particulate matter are separated from the flue-gas stream by electrostatic precipitators, fabric filters, cyclones and other pollution-control equipment. This ash must then be removed from collection hoppers and transferred to storage silos, conditioning units, recycling facilities or disposal systems.
Pneumatic conveying provides an enclosed method of transporting dry ash without the extensive spillage, dust release and mechanical complexity associated with some conventional conveying systems. roots blowers are particularly suitable for dilute-phase and selected low-to-moderate-pressure ash transfer systems because they deliver a relatively stable volumetric airflow despite normal changes in pipeline and filter resistance.
The blower must nevertheless be selected as part of the complete ash-handling system. Reliable operation depends not only on air capacity but also on ash temperature, particle characteristics, pipeline geometry, feeding method, conveying regime, filter condition and maximum system pressure.
Types of Ash Suitable for Pneumatic Conveying
The term “ash” covers materials with significantly different physical and chemical characteristics. Pneumatic conveying systems in power plants may handle:
- Fly ash collected from electrostatic precipitators
- Fabric-filter or baghouse ash
- Economizer ash
- Air-preheater ash
- Cyclone ash
- Boiler-bank ash
- Biomass combustion ash
- Waste-to-energy combustion residues
- Dry sorbent reaction products
- Fine, cooled and classified bottom-ash fractions
- Unburned-carbon-rich ash
- Mixed ash and pollution-control residues
Fly ash is normally the most suitable material because it consists predominantly of fine particles that can be suspended in a moving gas stream. However, fly ash properties vary considerably with fuel composition, boiler design, combustion temperature, emission-control process and collection location.
Bottom ash should not automatically be treated as fly ash. Coarse bottom ash may contain large, irregular, fused or highly abrasive particles. It may also leave the furnace at a temperature unsuitable for immediate pneumatic transport. Pneumatic conveying becomes practical only where the bottom ash has been adequately cooled, crushed, screened and conditioned into a transportable dry fraction.
Fly Ash Properties Affecting System Design
A conveying system should not be designed from the material name alone. Representative ash samples should be evaluated for:
- Particle-size distribution
- Particle density
- Loose bulk density
- Compacted bulk density
- Moisture content
- Cohesiveness
- Permeability
- Air-retention capability
- Angle of repose
- Abrasiveness
- Friability
- Unburned carbon content
- Chemical composition
- Hygroscopic behavior
- Maximum temperature
- Tendency to cake or form deposits
Fine fly ash may appear easy to convey, but its behavior can change dramatically after storage. Aerated ash may initially have a low bulk density and fluid-like behavior. As entrained air escapes, the material compacts and its bulk density increases. This can alter feeder performance, conveying capacity and pipeline pressure drop.
Moisture presents another major risk. Even a relatively small amount of water can increase cohesion, promote wall deposits and cause ash to accumulate inside feeders, pipelines and filters. Condensation can therefore create conveying problems even when the ash initially enters the system in a dry condition.
The Function of a Roots Blower
A roots blower supplies the air required to accelerate and transport the ash through the pipeline. Two synchronized lobed rotors rotate without contact inside the casing, trapping volumes of gas and transferring them from the inlet to the discharge side.
Because the machine is positive displacement, its inlet volume is primarily related to blower displacement and rotational speed. Internal leakage increases as differential pressure rises, but the delivered volume remains relatively stable across the normal operating range.
This characteristic is valuable in ash systems where pressure resistance changes due to:
- Variable ash feed rate
- Filter-cake accumulation
- Different conveying routes
- Changing numbers of active pickup points
- Ash deposits in pipelines
- Variations in bulk density
- Receiver pressure
- Material remaining in bends or horizontal sections
A roots blower does not directly regulate material throughput. The feeding device determines how much ash enters the pipeline, while the blower provides the gas flow and pressure capability necessary to transport it.
Vacuum Fly Ash Conveying
Vacuum conveying is frequently used to collect ash from multiple hoppers located beneath electrostatic precipitators, economizers or fabric filters. A central Roots exhauster creates sub-atmospheric pressure in the pipeline, drawing ash and conveying air toward a receiving vessel.
The normal sequence is:
Ash enters the pickup point or conveying line.
Atmospheric air is drawn into the pipeline.
The moving air entrains the ash.
The air–ash mixture travels to a central receiver.
Ash is separated from the gas.
A final filter protects the roots blower.
Clean gas passes through the blower and is discharged.
Vacuum operation provides effective containment. If a small leak develops, ambient air moves into the pipeline rather than allowing fly ash to escape into the plant. This is advantageous in areas where dust control and housekeeping are important.
The principal limitation is the pressure differential available from atmospheric pressure. As vacuum increases, gas density falls and actual gas volume expands. The pipeline velocity near the blower can therefore be considerably higher than at the ash pickup point.
The final receiver and protection filter must be designed for the maximum airflow and dust loading. The roots blower should not be allowed to ingest ash. Fine particulate carryover can erode the rotor surfaces, enlarge internal clearances, contaminate bearings and cause rapid mechanical deterioration.
Positive-Pressure Fly Ash Conveying
In a positive-pressure system, the roots blower supplies clean air to the beginning of the conveying pipeline. Ash is introduced downstream of the blower using a pressure-compatible feeder or transport vessel.
Common feeding arrangements include:
- Rotary airlock valves
- Blow-through rotary valves
- Screw feeders
- Ejectors
- Pressure vessels
- Dome-valve transporters
- Blow tanks
The material is conveyed to a silo, conditioning plant or another receiving point. A silo-top filter separates the ash from the conveying air and prevents dust release.
Positive-pressure conveying is well suited to transporting ash from one collection point to one or more destinations. It can also provide a greater usable pressure range than a vacuum system. However, any pipeline leak can release ash into the surrounding environment, making pipe integrity and joint quality important.
The roots blower is installed upstream of material injection and therefore normally handles clean ambient air. This protects the machine from direct ash contact, although the inlet filter must still prevent environmental dust from entering the blower.
Glossary
Ash moisture content: Water mass in ash expressed relative to the ash mass on a stated basis.
Ash receiver: Vessel that separates conveyed ash from transport gas.
Conveying-line pressure loss: Pressure reduction caused by gas flow through pipe, bends and fittings.
Dry-ash handling: Transfer of ash without slurry transport or water conditioning.
Electrostatic precipitator: Collector that removes charged particles from boiler flue gas.
Fly-ash fineness: Particle-size distribution of ash collected from the flue-gas path.
Hopper evacuation: Removal of collected ash from an electrostatic precipitator or filter hopper.
Vacuum pickup: Solids collection arrangement that draws material toward a vacuum receiver.