Fume (recovery boiler)

Fume in recovery-boiler service is the very fine sodium-rich particulate formed from vapours in the upper furnace and convective pass. It is distinct from larger black-liquor carry-over droplets and char particles, although all can contribute to deposit formation.

Formation mechanism

During black-liquor combustion, sodium and sulphur compounds volatilise in the high-temperature lower furnace. As gas cools, vapours condense or react into fine sodium sulphate, sodium carbonate and related salts. The resulting fume particles are small enough to remain entrained through the superheater, generating bank, economiser and into the downstream ESP.

Deposit behaviour

Fume deposits can start as light, powdery layers on tubes but become sticky or sintered depending on temperature and chemistry. In the superheater region, fume interacts with carry-over and sulphur chemistry to form deposits that affect heat transfer and corrosion risk. In cooler regions, fine fume contributes to plugging and can increase ash loading to the ESP.

Operating implications

Recovery-boiler operators manage fume through black-liquor dry solids, liquor firing pattern, air distribution, furnace temperature, char bed stability and sootblowing. High fume loading can indicate combustion imbalance or chemistry that increases sodium volatilisation. It also affects ESP performance because very fine particulate is harder to collect than coarse ash.

Acoustic cleaning relevance

Sonic horns can help keep dry fume deposits from consolidating on convective surfaces and in hoppers, but they do not change the underlying liquor chemistry. Their role is strongest in areas where fine deposits are still friable and where reducing sootblower intensity or extending run length has value.

Diagnostic notes

Distinguishing fume from carry-over matters because the corrective actions differ. Fume points toward vapour formation, condensation and fine particulate collection. Carry-over points toward liquor droplet entrainment, firing practice or bed disturbance. Deposit sampling, particle size and sodium-to-sulphur chemistry can help separate the two. If downstream ESP loading rises without obvious large particles, fume formation or fine carry-over should be considered before blaming the precipitator alone.

Measurement and control context

In industrial language, fume often refers to very fine particles formed from condensed vapour, especially in welding, steelmaking, non-ferrous smelting and high-temperature chemical reactions. It may also be used loosely for visible gas or dust clouds, so plant documents should define whether they mean particulate fume, gas, mist or mixed off-gas. The distinction matters because capture velocity, filter media, corrosion control and exposure limits can differ.

Fume systems are judged by hood capture, duct velocity, fan capacity, temperature, spark risk, particulate loading and worker exposure. Failure modes include fugitive release at the source, duct accretion, baghouse blinding, fan erosion and visible stack emissions. Acoustic cleaning is relevant only after the fume has become dry particulate in ducts, filters or hoppers. It does not capture fugitive fume at the source and cannot replace local exhaust design.

Related terms

• Recovery boiler
• Carry-over
• Superheater
• Electrostatic precipitator