Incinerator bottom ash

Incinerator bottom ash (IBA) is the coarse, mostly non-combustible residue discharged from the bottom of a grate-fired waste-to-energy boiler. It contains glass, ceramics, stones, fused minerals, ferrous and non-ferrous metals, unburned organics and fine ash that falls through or off the grate.

IBA is commonly about 20 to 25% of the incoming municipal-waste mass, although the exact value depends on waste composition, recycling rates, grate burnout, moisture and how much inert material enters the plant. It is distinct from APC residue, which is the much smaller, finer and more hazardous air-pollution-control fraction captured downstream in filters and scrubbers.

Formation and discharge

On a moving grate, waste dries, ignites, burns and leaves mineral residue at the discharge end. The hot ash usually drops into a water-filled extractor or quench system, which cools it, seals the furnace against false air and conveys it away for storage. Poor burnout leaves excess unburned carbon, while excessive clinker formation can create large fused pieces that disturb discharge.

After quenching, the ash is wet, abrasive and heterogeneous. It can bridge in bunkers, overload conveyors, carry tramp metal and release odour or leachate if stored poorly.

Recovery and reuse

IBA is increasingly processed rather than landfilled:

• Ageing or maturation - storage allows hydration, carbonation and leaching behaviour to stabilise.
• Screening and crushing - oversize pieces are separated and mineral fractions are sized.
• Metal recovery - magnets recover ferrous metals; eddy-current separators recover aluminium, copper and other non-ferrous metals.
• Aggregate use - compliant mineral fractions can be used in road sub-base, asphalt, concrete blocks or other regulated applications.
• Landfill or treatment - fractions that fail leaching or quality criteria need disposal or further treatment.

Quality control focuses on particle size, ferrous and non-ferrous recovery, total organic carbon, loss on ignition, soluble salts, heavy metals and leaching performance.

Operational issues

IBA systems fail through extractor jams, chain wear, quench water carryover, clinker blockage, conveyor overload, bunker bridging and poor segregation of wet and dry streams. Because IBA is coarse and wet, it is usually handled by mechanical conveying rather than pneumatic conveying.

Sonic-horn relevance

IBA itself is rarely a sonic-horn target. The ash is wet, heavy and coarse, so acoustic pressure does not move it reliably. Acoustic flow aids are more relevant in associated dry systems: fly-ash hoppers, APC residue silos, lime and activated-carbon dosing hoppers, and dry fines generated during metal-recovery processing.

For Sylio-style acoustic cleaning, the useful distinction is between wet bottom ash and dry airborne residues. The first needs robust mechanical handling; the second can often benefit from non-contact acoustic flow promotion.

Handling and compliance detail

IBA quality depends on waste mix, grate burnout, quench design, residence time and how much metal or glass remains in the ash. Poor burnout leaves high loss on ignition and can create odour, heat release or leachability problems. Excessive quench water can carry fine ash and dissolved salts into downstream treatment. Metal recovery plants track ferrous recovery, non-ferrous recovery, moisture, particle-size distribution and the amount of fine material returned or disposed.

Maintenance teams focus on extractor chains, rams, water seals, conveyors, wear liners, crushers, screens, magnets and eddy-current separators. Blockages are often mechanical rather than acoustic: oversize scrap, tangled wire, clinker lumps, refractory pieces or frozen wet ash. Safety issues include hot pockets, hydrogen generation in wet ash, sharp metal, biological contamination from waste, confined spaces and alkaline wash water.

The distinction between IBA and fly ash is important for cleaning strategy. Bottom ash needs robust mechanical transport and controlled quenching. Dry fly ash and APC residue need dust-tight conveying, hopper reliability and exposure control. Acoustic cleaning belongs mainly on the dry side of that split, where it can prevent bridging before workers are asked to intervene.

Related terms

• Waste-to-energy
• Grate-fired boiler / mass-burn incinerator
• APC residue