Waste-to-energy and biomass
Low-melt sticky ash
Also known as sticky ash, low-melting ash, alkali-rich sticky ash.
Low-melt sticky ash is the universal headache of biomass and waste-to-energy boiler operation. It forms when ash particles rich in alkali metals (K, Na) and chlorides soften at typical convective-pass gas temperatures (700–900 °C) and bond to cooler tube surfaces on contact.
Why it defeats steam sootblowers
A steam jet from an IK retract sootblower is highly effective on dry, friable ash but largely ineffective on a deposit that has bonded as a continuous sticky film. The steam removes only the loose surface layer; the bonded under-layer remains and continues to grow.
Why sonic horns help
Sonic horns work before the deposit consolidates. Continuous low-amplitude vibration during the early sticky phase prevents the deposit from forming a bonded interface with the tube. The ash remains friable enough to be released by sootblowers or by the next horn pulse, rather than building up into a self-reinforcing sticky mass.
Where it dominates
- Recovery boilers — see carry-over
- Straw and high-alkali biomass
- WtE boilers, especially with high-RDF feed
- Petcoke firing in some configurations
Related terms
Related terms
- Alkali metals in ashAlkali metals (Na, K) in biomass and waste-fuel ash form low-melting compounds that bond to boiler tubes as sticky deposits and poison SCR catalysts.
- Chloride-induced corrosionChloride-induced corrosion is the accelerated tube-wall thinning caused by chlorine-rich deposits on WtE and biomass boilers. The dominant tube-failure mechanism in WtE.
- Waste-to-energyWtE plants burn municipal solid waste, RDF, SRF and biomass to generate steam and electricity. Sticky chloride-rich ash defeats conventional cleaning; sonic horns are the dominant fit.
- SuperheaterA superheater is a tube bank that raises steam temperature beyond the saturation point using flue-gas heat. Sticky alkali ash and slag deposits are the dominant fouling concerns.
- Sonic hornA sonic horn is a pneumatically-driven low-frequency sound emitter (typically 60–400 Hz at 140–180 dB SPL) used to dislodge particulate fouling from boilers, ESPs, baghouses and process vessels.