Atmospheric fluidised-bed combustion boiler

An AFBC boiler is an atmospheric fluidised-bed combustion boiler. The term covers fluidised-bed boilers that operate near atmospheric pressure, mainly BFB boilers and CFB boilers. It is most often used in contrast to pressurised fluidised-bed combustion, which is uncommon in ordinary industrial power and process-steam service.

In AFBC designs, combustion takes place in a bed of hot inert solids, fuel ash, limestone and fuel particles suspended by upward air flow. The bed gives strong mixing, stable combustion of low-grade fuels and relatively low flame temperature. Limestone can be added for in-bed sulphur capture, and staged air helps reduce NOx.

Where it appears

AFBC units are used for coal, lignite, biomass, sludge, petroleum coke, rejects and mixed industrial fuels. They are common where fuel quality is variable or ash and moisture make pulverised-fuel firing difficult. The furnace, cyclones, loop seals, convective pass, economiser and air heater all see abrasive or sticky ash depending on fuel chemistry.

Fouling and maintenance

Fluidised-bed ash is often coarser and more abrasive than pulverised-coal fly ash, but biomass and waste blends can add alkalis, chlorides and low-melting species. Maintenance concerns include bed agglomeration, tube erosion, cyclone refractory wear, deposit growth in the backpass, blocked drains and air-distributor problems.

Acoustic-cleaning relevance

Acoustic cleaners do not control bed fluidisation. Their role is downstream: keeping convective tube banks, economiser sections, air heaters, hoppers and dust collectors clearer between outages. They are most useful where deposits are dry enough to respond to vibration and where steam sootblowing or manual cleaning causes erosion, water damage or lost availability.

Operating variables

AFBC performance depends on bed temperature, fluidising-air distribution, fuel sizing, sorbent feed, bed inventory, cyclone or multiclone performance and ash withdrawal. Poor air distribution can create local defluidisation, while excessive fines can increase carry-over into the backpass. Operators watch bed pressure drop, oxygen, carbon monoxide, steam temperature, fly-ash carbon, limestone utilisation and the condition of refractory around air nozzles and fuel feed points.

Fouling downstream is shaped by the same variables. High carry-over loads the economiser and air heater; sulphur capture chemistry can change ash stickiness; low-load operation can cool surfaces into corrosion or deposition windows. Acoustic cleaners are therefore specified after the solids balance is understood. They help most when the ash arriving at the target surface is dry and friable, and when hopper removal systems can handle the extra material that is released online.

Field evidence

Cleaning results should be compared with bed inventory and carry-over data. If downstream fouling rises after a fuel-sizing or sorbent change, acoustic-cleaner settings may need adjustment, but the furnace solids balance should be checked first.

Related terms

• BFB boiler
• CFB boiler
• Boiler