CFB boiler - circulating fluidised-bed combustion for fuel flexibility

A circulating fluidised-bed (CFB) boiler burns fuel in a turbulent bed of sand, ash and limestone, circulated by an upward-flowing combustion-air stream and recirculated through external cyclone separators. Combustion temperature (~850 deg C) is much lower than in a PC boiler, giving naturally lower NOx and the capability to capture SO2 in the bed by limestone addition.

Fuel flexibility

CFB boilers tolerate a far wider range of fuels than PC boilers:

Coal (anthracite, bituminous, sub-bituminous, lignite)
Petroleum coke
Biomass (wood, agricultural residues, bagasse)
RDF and waste fractions
Mixed and low-grade fuels

This fuel flexibility makes CFB the technology of choice for biomass conversions, waste-fired plants and lignite-rich regions.

Fouling pattern

Cyclone fouling - recirculating bed material accumulates on cyclone walls and downcomers
Backpass fouling - fine ash on economiser, superheater and air-heater tubes
Refractory wear in high-velocity zones

Sonic horns on the backpass surfaces and cyclone walls extend run length between maintenance outages.

Operating mechanisms

The furnace behaves as a solids circulation loop rather than a simple flame box. Primary air fluidises the lower bed; secondary air completes combustion higher in the furnace; entrained solids leave with the gas and are captured by cyclones. The hot return solids stabilise temperature and let low-grade fuels burn more completely than they would in a short-residence pulverised system.

Limestone addition captures sulphur by forming calcium sulphate in the bed. This reduces downstream sulphur dioxide but adds calcium-rich ash that can be abrasive, cohesive or reactive depending on fuel chemistry. Bed temperature must be controlled carefully: too low gives poor burnout and carbon loss; too high risks agglomeration and loss of fluidisation.

Failure modes and maintenance

CFB maintenance focuses on refractory, erosion and solids circulation. Cyclone refractory wear, dipleg pluggage, loop-seal instability, bed agglomeration, tube erosion and ash-cooler failures can all limit load. Convective-pass fouling raises draft loss and reduces heat transfer, while ash-hopper bridging can interrupt discharge and force manual intervention.

Instrumentation includes bed pressure, furnace temperature profile, oxygen, carbon monoxide, cyclone pressure drop, loop-seal pressure and ash-discharge behaviour. A change in any one value is interpreted against the circulation pattern. For example, a rising cyclone pressure drop with lower return-solids flow can indicate build-up rather than higher load.

Acoustic-cleaning fit

Sonic horns do not affect the dense lower furnace, where erosive solids and refractory dominate. Their value is in the cooler, dustier gas paths: convective banks, economisers, air heaters, cyclones, hoppers and ash chutes. Low-frequency cleaning helps keep fine ash mobile, reducing the need for manual poking and supporting stable draft between planned outages.

Field checks

CFB boiler performance is read through bed temperature, cyclone pressure drop, loop seal behaviour, furnace draft, limestone feed, sulphur capture, fuel sizing and ash recycle rate. Because solids circulation is central to heat transfer and combustion, a small restriction in a cyclone dipleg, return leg or external heat exchanger can change the whole furnace balance. Operators watch for unstable bed inventory, temperature spread, rising fan load and reduced load capability.

Maintenance inspections focus on refractory wear, erosion shields, cyclone inlets, loop seals, air nozzles, ash coolers and convective-pass deposits. The main failure modes combine abrasion and fouling: high solids velocity removes metal, while sticky ash or sulphate deposits narrow gas paths. Acoustic cleaning has a stronger role in the convective pass, economiser and air preheater than in the dense circulating bed itself. It is used to keep ash mobile where deposits are dry enough to respond to cyclic pressure waves.

Circulating fluidised-bed boiler