Glossary
Pulp and paper
Carry-over (recovery boiler)
Also known as smelt carry-over, recovery boiler carry-over, carryover.
Carry-over is the entrained molten or partly-molten smelt droplets and ash particles that are lifted from the recovery boiler furnace upward into the convective pass instead of falling to the boiler bottom. Carry-over is the dominant fouling agent on recovery-boiler superheater, generating-bank and economiser tubes.
Why carry-over is so problematic
- Particles arrive on the tubes still partly molten or sticky
- They bond on contact, producing a deposit that resists steam sootblowing
- The deposit composition (sodium sulphate + carbonate + sulphide) is alkali-rich and corrosive
- Build-up accelerates if not actively dislodged early
Cleaning
Sonic horns and infrasonic cleaners on recovery boilers target carry-over deposits before they consolidate. The combination of continuous acoustic action and periodic IK retract sootblowing is what allows modern recovery boilers to extend run-time targets to 12–18 months between chill-and-blow campaigns.
Related terms
Related terms
- Recovery boilerA recovery boiler burns kraft black liquor to generate steam, electrical power and recovered pulping chemicals. Iconic application for sonic horns on superheater cleaning.
- Smelt (recovery boiler)Smelt is the molten sodium carbonate and sulphide mixture that accumulates in the bottom of a kraft recovery boiler. It is dissolved into green liquor and recausticised to pulping reagent.
- Fume (recovery boiler)Fume is the fine sub-micron sodium-sulphate particulate that forms in the upper furnace of a recovery boiler. It deposits on superheater and economiser tubes and is captured by the ESP.
- 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.
- Generating bankThe generating bank is the array of evaporator tubes between the steam and mud drums of a recovery boiler, performing bulk heat absorption from cooling flue gas.
- 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.