Material
Fly Ash
Answer in brief
Fly ash behaves like a liquid when aerated and like concrete when it is not.
That switch is what breaks feeders: an ash that floods through a rotary valve in the morning can arch over the same outlet after a weekend of consolidation.
- Control aeration deliberately instead of letting it happen by accident.
- Design the silo outlet for the consolidated state.
- Expect abrasive wear despite the small particle size.
Reviewed October 21, 2026 · Updated October 21, 2026
Handling challenges
Fly ash is the classic example of a powder with two completely different personalities, and the plant usually meets both in the same week.
Aeration and flooding
Freshly conveyed ash retains air for a long time because the particles are fine and spherical.
In that state it flows through valves and feeders uncontrolled, overfills the next vessel and defeats gravimetric feeding.
Deaeration volume above the feeder, a controlled fill sequence and rotary valves with the right clearance class are the usual answers.
Consolidation and ratholing
Left to stand, the same ash consolidates and forms a stable rathole over the outlet, with the stored inventory sitting untouched around it.
- Size outlets from measured flow properties for the time-consolidated condition.
- Prefer mass flow geometry where the silo duty allows it.
- Use aeration pads as a flow aid with care, since over-aeration swings the material back to flooding.
Wear
Fine does not mean gentle: fly ash contains hard, angular particles and erodes bends, valve seats and feeder housings quickly.
Long-radius bends, wear-resistant inserts at impact points and a defined inspection interval are cheaper than an unplanned outage.
Moisture and set
Ash with free lime reacts with water and sets hard, which turns a small leak into a solid block inside a silo or a truck.
Weatherproofing, dry venting air and prompt removal of any wetted material are practical rather than theoretical requirements.
Health and emissions
Respirable fractions include crystalline silica, so dust control at loading, filter maintenance and operator protection are regulated matters rather than housekeeping.
What to establish before buying equipment
- Flow properties in both the aerated and time-consolidated states.
- Free lime content, which predicts set behavior on contact with water.
- Particle hardness and the expected wear rate at design velocity.
- Peak loading rate and the dust control method at the truck or railcar.
Material behavior
- Flowability
- Flooding when aerated, poor when consolidated
- Abrasiveness
- High despite fine particle size
- Cohesiveness
- Moderate; consolidates in storage
- Moisture behavior
- Moisture causes hard set, especially with high free lime
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