AFBC is atmospheric fluidized bed combustion, where the furnace pressure is atmospheric pressure. The flue gas from the furnace goes through a cyclone and is let out to the atmosphere.
As for CFBC, circulating fluidized bed combustion, the furnace is pressurized and the furnace gas is recirculated to capture the unburnt carbon, to increase the thermal efficiency of the boiler.

When we comes to the difference between AFBC boiler and CFBC boiler, we can compare them from the following aspects:

1. Velocity

AFBC= 1.2 m/sec to 3.7 m/sec
CFBC= 3.7 m/sec to 9 m/sec

2. Heat Transfer Surface

AFBC= Heat is extracted within the the bed
CFBC= Heat transfer occur outside of combustion zone i-e Convection section, water walls and at the exit of the riser.

3. Size of Coal

AFBC: 6-10 mm
CFBC: 6- 12 mm

4. Combustion Air Supply

AFBC: 3 - 5 psig
CFBC: 1.5 - 2 psig

After comparing, we can know that CFBC boiler is more competitive than AFBC boiler and CFBC technology evolves from AFBC to overcome some of the problems of AFBC and it has the following advantages:

1. Better Space utilization for large Industrial units.
2. Efficient combustion.
3. Better SO2 capture.
4. Better Techniques for Nox Control.
5. Better temperature Control.
6. More economical for industrial application requiring steam demand more than 100 tons/hr.
7. Better for low grade Fuel with Low calorific value and high ash content.
8. Reduced Erosion of heat transfer surface in combustion chamber as surface is parallel to the flow contrary to AFBC.

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