Recovery boiler

A recovery boiler is the kraft pulp mill unit that burns concentrated black liquor to produce steam and recover inorganic pulping chemicals. It is both a boiler and a chemical reactor, which makes its operating risk and maintenance profile different from a conventional power boiler.

Black liquor contains dissolved organic material from wood and inorganic sodium and sulphur chemicals. In the furnace, organic material burns and releases heat. Inorganic material forms molten smelt, mainly sodium carbonate and sodium sulphide, which drains from the furnace floor to the smelt dissolving tank. The recovered chemicals then return to the recausticising plant.

Main operating zones

The lower furnace dries and burns liquor droplets on the char bed. The upper furnace, superheater, generating bank, economiser, and air heater recover heat from the flue gas. Ash in recovery boilers is rich in sodium salts and can be sticky, corrosive, and temperature-sensitive. It deposits on pendant tubes, screen tubes, superheaters, and convection banks.

Stable operation depends on liquor solids, spray pattern, air staging, bed control, furnace temperature, and sootblowing. Low solids or poor combustion can increase carryover and fouling. Overheating and deposit build-up can restrict gas flow, raise tube metal temperatures, and force load reduction.

Safety and failure modes

Recovery boilers have a major smelt-water explosion hazard. Water entering the furnace can react violently with molten smelt. This is why leaks, washing, hydrotests, and emergency shutdown procedures are governed by strict mill and BLRBAC practices.

Other concerns include plugging in superheaters and generating banks, fouled economisers, corrosion under deposits, carryover, plugged liquor guns, smelt spout issues, dissolving tank vent deposits, and ash handling problems. Cleaning must balance deposit removal against tube erosion and thermal stress.

Acoustic cleaning context

Recovery boilers are a classic acoustic cleaning application because many ash deposits are dry and friable when removed early. Sonic horns or infrasonic cleaners can keep superheater lanes, generating banks, economisers, and hoppers cleaner between sootblower cycles. They do not replace required sootblowers or safety procedures, but they can reduce steam sootblowing demand, improve gas flow, and delay the point where deposits become hard plugged masses.

Operating variables

Key operating variables include black-liquor dry solids, liquor temperature, spray gun condition, droplet size, firing load, primary and secondary air distribution, char-bed height, excess oxygen, furnace pressure, and sootblower sequencing. Small shifts can change carryover, reduction efficiency, steam temperature, and the amount of ash reaching the convective pass. Operators read the boiler through bed cameras, smelt spout flow, steam temperature control, gas-side pressure drop, economiser outlet temperature, ESP loading, and the amount of plugging found during inspections.

Maintenance and safety context

Maintenance is dominated by tube integrity, smelt handling, liquor gun reliability, air-port cleaning, sootblower availability, and ash deposit management. Tube leaks are uniquely serious because water and molten smelt can create an explosion hazard. Work practices therefore treat pressure-part leaks, washing, hydrotests, dissolving tank operation, and emergency shutdown as safety-critical activities, not routine boiler housekeeping.

Deposit control has to respect this risk. Steam sootblowers, chill-and-blow procedures, acoustic cleaners, and outage washing all remove ash in different ways and at different levels of intrusion. Acoustic cleaners are attractive where they prevent friable sodium-salt deposits from growing into plugged lanes, because they can operate online and without adding water. They still need correct placement, casing integrity, noise review, and coordination with hopper capacity. If deposits have become hard, molten, or chemically bonded, acoustic energy will not remove the need for controlled outage cleaning.

Measurement and commissioning context

A useful recovery-boiler cleaning review starts with a map of where deposits form at each load and liquor condition. The map should separate lower-furnace carryover, pendant-bank build-up, generating-bank pluggage, economiser ash, air-heater fouling, and hopper flow. Each zone has a different temperature, chemistry, access constraint, and safety consequence. The same ash that is loose in an economiser hopper may be sticky on a superheater tube or corrosive under an insulating layer.

Commissioning acoustic cleaners on a recovery boiler should include baseline pressure drop by bank, sootblower steam use, steam temperature spray demand, hopper evacuation, outlet particulate trend, and inspection photographs after comparable run time. The strongest evidence is not that ash falls during a test fire, but that the boiler holds gas flow, heat transfer, and access condition for longer between outages. Any change must remain subordinate to mill procedures for smelt safety, leak response, confined-space entry, and BLRBAC guidance.

Acoustic equipment should also be reviewed after liquor-solids changes, air-system rebuilds, sootblower programme changes, and major outages. Those events can move the deposit zone or change how ash falls through the boiler. A system that worked well on one firing pattern may need retiming, isolation checks, or extra hopper attention after the mill changes production rate or liquor chemistry.

Cleaning boundaries

Recovery-boiler cleaning has a stricter boundary than many other boiler applications because the lower furnace contains molten smelt and the unit carries a severe smelt-water explosion hazard. Sonic horns, sootblowers, water washing and manual cleaning must be considered in relation to leak detection, drain status, bed condition and mill safety rules. Acoustic cleaning is normally discussed for dry ash deposits in upper furnace, superheater, generating-bank, economiser or downstream gas-path areas, not as a lower-furnace intervention. A credible application case identifies the deposit zone, confirms that loosened ash can leave safely, and shows how the horn sequence interacts with existing sootblowers. The goal is longer stable run time without weakening established recovery-boiler protections.