Fly ash handling: keeping a fine powder contained

Fly ash is what a boiler makes when it burns pulverised fuel: the mineral fraction of the coal, melted in the flame, quenched into fine glassy spheres, carried up with the flue gas and caught in the precipitator or the baghouse. It is the finest bulk material most power plants handle, and it behaves less like a solid than like a slow, dense fluid. That is the whole problem. A line designed for a powder that sits still will leak, flush and cake on fly ash; one designed for a fluid that occasionally sets like cement will work.

Where does the ash go?

1. Collection — the ash settles in precipitator or filter hoppers, kept warm to stay above the acid dew point of the flue gas and fitted with heaters, vibrators or aeration to keep it moving.
2. Extraction — a rotary airlock or a double-flap valve under each hopper meters the ash out and separates the flue-gas side from the conveying side; the feeders for fine powders in the Flow Finder show the usual options.
3. Transport — pneumatic conveying, pressure or vacuum, is the usual choice; mechanical routes use airslides, screws, drag chains and, over longer distances, enclosed pipe conveyors.
4. Storage — a silo with a fluidised bottom, aeration, a vent filter, and level and pressure instruments.
5. Unloading — dry into sealed tankers through a telescopic loading spout, or through a conditioner that moistens the ash for open trucks and disposal.

What are we handling?

Particle size

Mostly very fine, with a large fraction finer than a typical cement and a coarser tail; it varies with the fuel, the mill and where in the precipitator it was collected, so measure your own.

Bulk density

Indicatively 0.7–1.1 t/m³ loose, lower still when freshly aerated and noticeably higher once settled; use the loose figure for feeder capacity and the settled figure for silo mass.

Fluidisation

Fluidises with very little air and holds that air for a long time, which is why it flows through feeders like water and then refuses to flow at all a day later.

Temperature

Hot from the hoppers — commonly well above 100 °C — hard on seals, rotor clearances and anything rubber.

Abrasiveness

High. Glassy spheres and quartz grains polish and cut pipework bends, airlock rotors and screw flights.

Water

Reacts. Wetted ash cakes, and ash with free lime sets hard; a hopper that takes in rain or condensation becomes a block.

What makes fly ash awkward?

Two habits do most of the damage. The first is flushing: aerated ash runs through any feeder that relies on the material to hold itself back — an open gate, a screw fed from a fluidised hopper — and arrives downstream at many times the intended rate. The second is leakage: a powder this fine passes through gaps that stop everything else, so it emerges at flanges, rotor clearances, shaft seals and the exit of any transfer that is not sealed, and settles on every horizontal surface within reach. Add heat, which opens clearances and hardens seals, and abrasion, which enlarges every gap it passes through, and the line has to be designed for containment first.

Which equipment does which job?

How do you keep it contained?

Containment on an ash line is a chain of small decisions. The pillar note on transfer points, dust and spillage sets out the mechanics for any material; for fly ash the emphasis moves from impact to air. Every volume that receives ash also receives the air that carried it, and that air must leave through a filter rather than through a gap.

• Enclose every transfer — airslide to silo, silo to spout, screw to elevator — with the enclosure vented to a filter, not to the room.
• Vent, do not pressurise — a silo filled pneumatically takes in the whole conveying air flow; the vent filter is sized for it and its differential pressure is monitored.
• Seal the rotating parts — shaft seals with purge air on screws and airlocks, and a maintenance routine for them, because they are the first thing the ash defeats.
• Housekeeping as a design input — surfaces that shed dust, access to clean them, and no ledges where a layer can build up.

What goes wrong in the silo?

A fly ash silo has two opposite failure modes, and the pillar note on bridging, ratholing and mass flow explains both. Freshly filled, aerated ash flushes: it runs through the discharge as a fluid and past any feeder that cannot shut it off. Left to settle, or wetted by moist conveying air, it consolidates and ratholes: a channel empties above the outlet and the rest stays. Aeration pads in the cone, run in sequence rather than continuously, keep the material near the outlet fluid enough to discharge without turning the whole silo into a liquid; a feeder with positive shut-off — an airlock with a vented housing, or a rotary valve with an isolation gate above it — handles the rest.

What to check on an existing ash line

• Airlock clearances — rotor tip to housing, hot and cold; leakage air rate; the state of the shaft seals.
• Hopper heaters and level probes — a cold hopper wall is where the ash first cakes.
• Air quality — dew point of aeration and conveying air at the point of use, in winter.
• Airslide membranes — blinding, tears and the air distribution along the length.
• Silo aeration sequence — what the controls actually do, against what the designer intended.
• Vent filter — differential pressure trend, cleaning cycle, and whether the pressure relief has ever lifted.
• Loading spout — seal condition, level sensor, filter; the truck bay is where most visible dust escapes.
• Deposits — where the grey layer is, and which flange, seal or door it points to.

Most ash lines were built with the plant and modified since, and what they usually need is a survey rather than replacement. Our storage and flow control work covers ash silos, aeration, feeders and unloading, and the energy duties in the Flow Finder list the equipment types involved.