Pneumatic ash conveying transfers dry ash and sorbent from collection points to storage, conditioning or disposal equipment. Pneumatic conveying offers an enclosed method of moving this fine material without open belts or extensive mechanical conveyors.
In a positive-pressure system, the roots blower is installed upstream of the ash feeding equipment. Clean conveying air passes through the blower before the ash is introduced into the pipeline.
In a vacuum system, the blower operates downstream of a receiver and filter. Suction draws ash from multiple collection hoppers toward a centralized receiving station.
A combined vacuum-pressure installation may collect ash under vacuum and then transfer it under positive pressure to a remote silo.
The roots blower should not handle ash directly. Effective gas-solid separation and filtration are necessary to prevent abrasive particles from entering the rotor chamber.
Important design factors include:
- Ash temperature
- Particle-size distribution
- Bulk density
- Abrasiveness
- Moisture content
- Carbon content
- Conveying distance
- Vertical lift
- Number and geometry of bends
- Feeder leakage
- Receiver-filter pressure drop
The terminal section of a positive-pressure pipeline requires particular attention because gas expansion can produce a higher conveying velocity near the receiving silo. Excessive velocity accelerates elbow wear and increases particle impact.
Ash Silo Aeration and Fluidization
Fine ash can compact, bridge or form stable flow channels inside storage silos. Low-pressure air introduced through aeration pads or fluidizing elements reduces interparticle friction and promotes controlled discharge.
A roots blower is suitable where the aeration system requires a dependable air volume against the combined resistance of the distribution piping, control valves, porous media and material bed.
The total aeration pressure can be represented as:
Total Aeration Pressure
- ΔPpipe = distribution-pipe pressure loss
- ΔPvalve = control-valve and fitting loss
- ΔPpad = resistance of the aeration element
- ΔPmaterial = pressure required to pass air through the material bed
Aeration should be controlled by zones. Supplying excessive air continuously can reduce apparent bulk density, overload silo-vent filters and cause uncontrolled ash discharge.
Conveying air and aeration air may sometimes be supplied by a common blower, but only after evaluating the maximum simultaneous flow and pressure requirements. Separate blowers can provide better control and operational redundancy in critical installations.
FGD Oxidation Air
Wet limestone flue-gas desulfurization systems use oxidation air to convert calcium sulphite compounds into stable gypsum. Air is introduced below the slurry surface through spargers, lances or distribution grids.
A simplified overall reaction is:
Overall Oxidation Reaction
The oxidation-air blower must overcome:
- Pressure caused by slurry submergence
- Sparger and distribution-system resistance
- Air-pipeline losses
- Control-valve losses
- Absorber operating pressure
- Fouling allowance
The static pressure associated with submergence is:
Static Pressure (Submergence)
- ΔPliquid = hydrostatic pressure
- ρslurry = slurry density
- g = gravitational acceleration
- hsubmergence = vertical depth below the liquid surface
Roots blowers can be appropriate for low- and moderate-capacity oxidation-air duties requiring stable flow. Large FGD systems with very high continuous airflow may favour centrifugal or turbo-blower technology after lifecycle-energy evaluation.
The blower package must be protected against slurry backflow, humid gas migration and corrosive condensate. Non-return valves, isolation valves, drainage and correct shutdown sequencing are critical.
Fuel-Gas and Biogas Boosting
Biogas, landfill gas and other low-pressure fuel gases may require boosting before they can pass through cleaning equipment and reach gas engines, burners or combined heat and power units.
A purpose-designed roots-type gas blower may overcome pressure losses through:
- Condensate separators
- Gas coolers
- Hydrogen sulphide removal systems
- Activated-carbon filters
- Siloxane-removal equipment
- Fine gas filters
- Flowmeters and control valves
- Flame arresters
- Fuel-gas pipelines
- Engine inlet gas trains
This is not an ordinary air-blower application. The package may require gas-compatible shaft seals, leakage monitoring, hazardous-area-certified components, controlled venting, suitable elastomers and protection against liquid carry-over.
The blower does not increase the heating value of the fuel. It only delivers the available gas at the required flow and pressure.
Gas flow stated at normal conditions must be converted to the actual blower inlet conditions:
Gas Flow Conversion to Actual Conditions
Condensate removal is essential. A roots blower is designed to handle gas, not liquid slugs. Upstream knock-out vessels, demisters, monitored drains and high-liquid-level shutdowns may be required.
Coal, Biomass and Auxiliary-Fuel Conveying
roots blowers can supply conveying gas for prepared coal, biomass and certain auxiliary fuels where the required pressure is within the practical range of a rotary-lobe machine.
Applicable materials may include:
- Pulverized coal
- Recovered coal dust
- Petroleum-coke fines
- Sawdust and wood flour
- Screened biomass particles
- Pellet fragments
- Agricultural residues
- Prepared refuse-derived fuel
- Dried sludge granules
- Combustible process residues
Coal can be abrasive and explosible. Biomass may be fibrous, compressible and variable in moisture content. Alternative fuels can contain irregular particles, metals or mineral contamination.
The conveying velocity must be high enough to prevent deposition but not unnecessarily high.
For a circular pipeline:
Superficial Gas Velocity
- vg = superficial gas velocity
- Qactual = actual gas volume flow
- D = internal pipe diameter
- π = approximately 3.1416
Excessive velocity can increase:
- Bend wear
- Pellet degradation
- Fines production
- Filter loading
- Energy consumption
- Pipeline noise
There is no universal conveying velocity for coal or biomass. Material testing and representative operating data are preferable to generic velocity values.
A roots blower may supply conveying air around a coal-preparation system, but it should not be confused with the primary-air fan serving a large ball-and-race or pulverizer mill. The primary-air system performs a different process duty involving drying, classification and transport through the mill.
Glossary
Fly ash: Fine particulate ash captured from boiler flue gas.
Bottom ash: Coarser combustion residue removed from the furnace bottom.
Dilute-phase conveying: Solids transport in a gas stream with relatively low solids loading.
Dense-phase conveying: Solids transport at higher material concentration and lower gas velocity.
Rotary feeder: Metering device that introduces solids into a pressurized conveying line.
Receiver filter: Device that separates ash from conveying air at the receiving vessel.
Conveying-line pressure drop: Pressure loss through the pipe route, bends, fittings and solids pickup points.
Silo discharge rate: Mass of ash or other bulk solid discharged from a silo per unit time.