Pulverised-coal boiler

A pulverised-coal boiler, often shortened to PC boiler, burns coal after mills grind it to a fine powder and primary air carries it to burners. The goal is rapid, near-complete combustion in suspension so the furnace can produce high steam output from a compact firing system.

Coal enters pulverisers, is dried and classified, then travels through coal pipes to burner elevations. Secondary and overfire air control flame shape, burnout, furnace temperature, and NOx formation. The furnace waterwalls absorb radiant heat, while superheaters, reheaters, economisers, and air heaters recover heat from the flue gas before particulate and gas-cleaning systems.

Ash and deposition mechanisms

PC boilers create both bottom ash and fly ash. Mineral matter in the coal melts, softens, fragments, or vaporises depending on ash chemistry and local temperature. Sticky ash can form slag on furnace walls and platen surfaces. Cooler downstream tube banks see fouling from ash, sulphates, ammonium salts, and fine particulate.

The same boiler can change behaviour when coal source, mill fineness, excess air, burner tilt, load, or sootblowing pattern changes. Low-sulphur coal can raise fly-ash resistivity and challenge the ESP. High alkali or chlorine can increase sticky deposits. Poor combustion can increase unburned carbon, popcorn ash, and catalyst pluggage.

Design and maintenance implications

Cleaning strategy is divided by temperature and deposit character. Water cannons and long retract sootblowers address hard furnace slag. Steam sootblowers and acoustic cleaners are used in convective passes where deposits are dry or lightly bonded. ESPs, SCR reactors, air heaters, and hoppers require their own ash-management strategy.

Operators watch heat-rate drift, furnace exit gas temperature, superheater and reheater spray flows, tube metal temperatures, draft loss, opacity, and hopper levels. A rise in draft loss or spray demand can mean deposit growth is shifting heat absorption away from the intended section. Aggressive sootblowing can remove deposits but also causes erosion, thermal shock, and tube wear.

Acoustic cleaning context

Sonic horns are most useful where PC boiler ash remains friable: economiser cavities, SCR catalyst faces, air heater approaches, ESP internals, and hoppers. They provide frequent low-energy cleaning rather than periodic impact. They are not a deslagging tool for molten furnace deposits, but they can reduce the downstream fouling and re-entrainment that follows poor ash control.

Operating variables and maintenance focus

Important operating variables include coal grind, burner tilt, excess air, furnace exit gas temperature, sootblower sequencing, air heater leakage, mill balance, and boiler load ramp rate. Small changes can move ash from a dry powder to a sticky or molten deposit. Operators therefore compare steam temperatures, spray flows, gas-side pressure drop, oxygen trim, furnace cameras, and tube metal temperatures before deciding whether a fouling problem is fuel related, combustion related, or cleaning related.

Maintenance checks normally focus on burner condition, pulveriser performance, furnace slag patterns, sootblower travel, lance alignment, wall blower coverage, tube wastage, air heater baskets, expansion joints, and ash hopper evacuation. Acoustic cleaning is considered when deposits are dry enough to be loosened without adding steam or water and when tube erosion from frequent sootblowing is becoming a concern. It should be coordinated with sootblowing logic so ash released from upper banks does not overload downstream hoppers or collectors.

Measurement context

A PC boiler cleaning review should compare gas-side pressure drop, steam temperature spray flow, sootblower operation, furnace observation, ash analysis, and downstream collector loading. A cleaner convective pass may shift ash to an economiser hopper, air heater, or ESP, so the whole gas path should be checked after any cleaning change.

Acoustic cleaning is best justified when it reduces a measurable constraint: fan power, sootblower steam use, manual cleaning, tube wastage from blowing, or forced derates. It should be commissioned with load and fuel noted, because coal quality and mill performance can change deposit behaviour faster than the cleaning system can correct it.

Related terms

• Boiler
• CFB boiler
• Waterwall
• Superheater
• Electrostatic precipitator
• Sonic horn