Carry-over (recovery boiler)

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.

Where it forms

Carry-over begins in the lower furnace. Black-liquor droplets should dry, pyrolyse and burn while falling to the char bed. If droplets are too fine, too wet, poorly distributed, or caught in a high upward gas stream, they can be carried into the upper furnace before they have completed combustion. Smelt splash from an unstable bed can also create sticky droplets that travel with the flue gas.

The first severe deposition zones are usually the screen tubes, superheater inlet banks and generating-bank entrance. These surfaces see high particle loading and enough temperature for sodium-rich material to remain tacky. Once a small deposit exists, it changes local flow, catches more ash and creates a ledge that grows faster than a clean tube surface.

Operating causes

Carry-over increases with high firing rate, poor liquor gun atomisation, incorrect liquor solids, unbalanced air staging, unstable char bed operation and blocked or misdirected air ports. Furnace draft and gas velocity matter as much as chemistry: a unit pushed above its intended load can lift material that would otherwise fall out.

Operators infer carry-over from flue-gas pressure drop, superheater spray flow, steam temperature control movement, sootblower effectiveness, tube-metal temperature and inspection findings. A rapid rise in draft loss after a firing increase is a stronger sign than a slow seasonal fouling trend.

Risk and maintenance implications

The deposit is not just an insulation layer. Alkali salts can corrode tube metal, and large deposits can bridge between tube banks, fall as heavy slabs, damage tubes or block gas lanes. If deposits become too hard for online sootblowing, the mill may need a chill-and-blow or water wash, both of which reduce recovery-boiler availability.

Acoustic cleaning is useful because it acts before a deposit becomes massive. Low-frequency energy works through the gas space and flexes fresh deposits at their attachment points. It is normally combined with sootblower logic rather than replacing it: horns reduce the rate of build-up, while retractable sootblowers remove deposits that still require direct jet energy.

Field checks

Carry-over is monitored through furnace draft, liquor gun condition, droplet size, bed stability, flue-gas oxygen, superheater pressure drop, sootblower performance and deposit sampling. A change in black-liquor solids, viscosity or spray pattern can change carry-over before operators see a visible furnace problem. Large deposits on pendant banks often point back to atomisation, air distribution or bed behaviour rather than to the sootblowers alone.

The maintenance risk is that carry-over deposits grow from soft sticky material into hard fused masses that bridge tube lanes and overload supports. Heavy deposits can shed unexpectedly during load changes or cleaning, so inspection and cleaning plans must account for falling material and tube impact. Acoustic cleaners are most useful as continuous prevention on early deposits in the screen, superheater and generating-bank approach areas. Once carry-over has sintered into thick masses, plants usually need steam sootblowing, load reduction, chill-and-blow, or an outage cleaning method.

Related terms

• Recovery boiler
• Smelt
• Fume
• Superheater
• Generating bank
• Sonic horn