---
title: "Recovery boiler"
description: "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."
canonical_url: "https://sylio.co/glossary/recovery-boiler"
last_updated: "2026-06-28T02:29:26.258Z"
---

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](/glossary/sonic-horn). Three features combine to make them so:

- **Sticky, alkali-rich ash** — sodium-sulphate carry-over deposits aggressively on [superheater](/glossary/superheater) and [generating-bank](/glossary/generating-bank) tubes
- **Long-run-time targets** — mills target 12–18 months between [chill-and-blow](/glossary/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](/glossary/sonic-horn) at 60–125 Hz and [infrasonic cleaners](/glossary/infrasonic-cleaner) 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](/glossary/lime-kiln)
- **Smelt dissolving tank** vent stack — sodium-fume build-up

## Safety

Recovery-boiler operations are governed by [BLRBAC](/glossary/blrbac) Recommended Good Practices. Any cleaning intervention — including acoustic — is reviewed against BLRBAC water-side-incident and emergency-shutdown protocols.

## Related terms

- [Boiler](/glossary/boiler)
- [Generating bank](/glossary/generating-bank)
- [Superheater](/glossary/superheater)
- [Economiser](/glossary/economiser)
- [Chill-and-blow](/glossary/chill-and-blow)
- [BLRBAC](/glossary/blrbac)
- [Sonic horn](/glossary/sonic-horn)
- [Infrasonic cleaner](/glossary/infrasonic-cleaner)