Boilers
Recovery boiler
Also known as kraft recovery boiler, black-liquor recovery boiler, BLRB.
A recovery boiler (also kraft recovery boiler, black-liquor recovery boiler, or BLRB) is a unique industrial boiler at the centre of every kraft pulp mill. It burns concentrated black liquor — the spent cooking-chemicals stream — to generate steam, electrical power and to recover the sodium and sulphur compounds that re-enter the pulping cycle as smelt. Recovery boilers are large, complex, expensive and irreplaceable to mill operation.
The iconic sonic-horn application
Recovery boilers are the iconic application for sonic horns. Three features combine to make them so:
- Sticky, alkali-rich ash — sodium-sulphate carry-over deposits aggressively on superheater and generating-bank tubes
- Long-run-time targets — mills target 12–18 months between chill-and-blow wash cycles, and every extra week of run time is worth tens of thousands of dollars
- Deep cavities — the superheater bundles are tall and bafflingly inaccessible to short-throw cleaning
Both conventional sonic horns at 60–125 Hz and infrasonic cleaners below 30 Hz are deployed on recovery boilers. Major OEM aftermarket teams (ANDRITZ, Valmet, Babcock & Wilcox Vølund) all integrate acoustic cleaning into their service portfolios.
Other applications inside the recovery island
- ESP hoppers — sodium-rich fly-ash bridging
- Economiser pluggage — salt-cake build-up on tube bundles
- Lime kiln preheater — see lime kiln
- Smelt dissolving tank vent stack — sodium-fume build-up
Safety
Recovery-boiler operations are governed by BLRBAC Recommended Good Practices. Any cleaning intervention — including acoustic — is reviewed against BLRBAC water-side-incident and emergency-shutdown protocols.
Related terms
Related terms
- BoilerA boiler is a vessel that converts fuel chemical energy into steam by heating water. Coal-fired, biomass, oil, gas and recovery boilers all foul; sonic horns clean heat-transfer surfaces.
- 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.
- 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.
- EconomiserAn economiser is the final tube bank in a boiler's convective pass that recovers heat from the flue gas by preheating feedwater. Ash bridging in the economiser is a routine cleaning challenge.
- Chill-and-blowChill-and-blow is the thermal-shock cleaning campaign on a recovery-boiler superheater. The boiler is rapidly cooled to crack deposits; intense sootblowing then dislodges them.
- BLRBACBLRBAC (Black Liquor Recovery Boiler Advisory Committee) publishes Recommended Good Practices governing safe operation of kraft recovery boilers. Mandatory reference for any cleaning-system change.
- 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.
- Infrasonic cleanerAn infrasonic cleaner operates below the audible threshold (typically 12–30 Hz). The very long wavelength penetrates further than a conventional sonic horn and is preferred on recovery boilers and WtE flue paths.