Popcorn ash

Popcorn ash is a coarse, porous, low-density form of fly ash associated with incomplete or uneven combustion. The particles are larger and more irregular than normal fly ash, with a brittle, expanded structure that can resemble popped grains. In SCR systems they are a known cause of catalyst pluggage because they are light enough to travel with the gas but large enough to bridge catalyst openings.

Popcorn ash is usually linked to poor coal fineness, burner imbalance, low local oxygen, rapid temperature changes, high unburned carbon, or unstable furnace aerodynamics. It can also appear when fuel quality changes faster than combustion settings are corrected. Because it is porous, it may have a large apparent size but low settling velocity, allowing it to reach equipment that was designed around finer fly ash.

Where it causes trouble

The most visible problem is high-dust SCR catalyst blockage. Popcorn ash can wedge into honeycomb cells or lodge at plate catalyst leading edges. Once a few particles catch, normal fly ash can build around them and form a plug. The result is rising reactor pressure drop, reduced catalyst area, maldistributed gas, and local ammonia slip.

Downstream effects include air heater fouling, duct deposits, hopper loading, and higher particulate carryover. If particles contain unburned carbon, they can also influence mercury control chemistry, loss-on-ignition results, and fly-ash saleability.

Detection and control

Operators may see pressure-drop growth across the SCR, uneven catalyst face inspections, high large-particle ash counts, furnace carbon-in-ash changes, or burner tuning symptoms. Sampling screens, acoustic pyrometry, combustion balance checks, and mill performance data can help separate a furnace source problem from a cleaning system problem.

The primary control is combustion correction: coal fineness, classifier performance, burner balance, air distribution, excess oxygen, and mill maintenance. Mechanical screens or large-particle ash traps may protect the SCR, but they also need inspection and cleaning.

Acoustic cleaning context

Sonic horns can help keep popcorn ash from settling and bridging on catalyst faces or upstream ledges, especially when deposits are dry and recently formed. They cannot solve the combustion fault that creates excessive popcorn ash. If a plant relies on horns while ignoring mill fineness or burner imbalance, the reactor may stay cleaner for longer but the root cause remains.

Where operators find it

Popcorn ash is commonly reported around biomass boilers, recovery boilers, waste-fired units, and other furnaces with variable ash chemistry or carryover. It can lodge in superheater lanes, generating banks, economisers, air heaters, hoppers, and duct turns. Because the particles are large and irregular, they may not behave like fine fly ash in cyclones, ESPs, or fabric filters. A small number of pieces can obstruct a tube gap or hopper throat that would pass ordinary powder.

Inspection notes should describe particle size, hardness, colour, magnetic response if relevant, and whether the pieces are hollow, fused, or friable. These details help connect the deposit to combustion temperature, unburned carbon, bed carryover, mineral matter, or sootblower timing. Acoustic horns can help prevent loose pieces from settling on ledges, but they should be positioned so released material has a clear path to a hopper. If the root cause is molten ash or severe carryover, cleaning hardware only treats the symptom.

Related terms

• Large-particle ash
• Catalyst pluggage
• Selective Catalytic Reduction