Hog-fuel boiler - biomass side boiler at pulp and paper mills

A hog-fuel boiler or bark boiler burns coarse wood residues, bark, sawdust, screenings and other biomass to produce steam and power. In pulp and paper mills it usually operates alongside the kraft recovery boiler, handling biomass fuels that are not part of the black-liquor recovery cycle.

Boiler types

Older bark boilers may be grate-fired. Modern larger units are often bubbling fluidised-bed or circulating fluidised-bed boilers because they tolerate variable moisture, ash and particle size better than many grate designs. Fuel handling can include hog mills, conveyors, magnetic separation, storage piles, live-bottom bins and metering screws.

Fuel and ash challenges

Hog fuel is variable. Moisture can swing widely with season and storage. Bark content raises ash, alkali and dirt loading. Forest residues may bring sand, stones, chlorides and potassium. These variables affect combustion stability, bed agglomeration, slagging, superheater fouling and air-heater plugging.

Failure modes

Common problems include wet-fuel derates, fuel-feed plugging, bed agglomeration in fluidised beds, grate clinker, furnace slagging, superheater corrosion, economiser fouling, air-heater deposits and ash-conveying blockages. Because mills need steam continuously, even a side boiler can become production-critical when the recovery boiler is constrained.

Acoustic cleaning relevance

Sonic horns are useful in hog-fuel boilers on convective passes, superheater regions where deposits remain friable, economisers, air heaters and ash hoppers. They are part of a broader cleaning strategy with sootblowers, bed management, fuel-quality control and outage washing.

Operating checks

Good hog-fuel boiler operation depends on fuel-yard discipline as much as boiler hardware. Operators track moisture, oversize material, tramp metal, pile age, combustion stability, bed temperature, oxygen, CO and ash chemistry. Sudden fouling changes often follow a fuel mix change or wet-fuel period. Acoustic cleaning should be reviewed seasonally because winter bark, spring residues and dry sawmill waste can produce different deposits and different cleaning demand.

Operating variables and fouling

Hog-fuel boilers have to tolerate variable moisture, bark, sand, stones, tramp metal and changing particle size. Key variables include fuel preparation, storage time, grate or stoker loading, underfire and overfire air, furnace temperature, oxygen, carbon monoxide, bed depth and ash removal. Wet fuel reduces heat release and can drive unstable combustion; dry fines can increase carry-over and spark risk.

Common failures include fuel-feed plugging, grate clinker, sand erosion, slagging, superheater fouling, economiser plugging, high carbon monoxide and ash-hopper bridging. Operators watch fuel moisture, steam flow, furnace cameras, draught, flue-gas temperature, opacity and ash consistency. Acoustic cleaning is most relevant downstream of the furnace, where bark ash and sand-laden dust collect on tube banks, air heaters or hoppers. It cannot correct poor fuel sizing, frozen fuel, bad air distribution or grate mechanical failures.

Maintenance checks

Routine checks include fuel-bin condition, drag-chain wear, air-zone dampers, grate seals, ash screws, sootblower availability and tube wastage in high-carry-over zones. Because fuel quality changes quickly, operators should compare fouling and emissions trends with recent moisture and fuel-source records before blaming only boiler hardware.

Hog-fuel boiler and bark boiler