What are the types of Fluidized Bed Combustion ?

 

AFBC Boiler -  Atmospheric Fluidized Bed Combustion Boiler

An atmospheric fluidized bed boiler operates with higher combustor gas velocities that vary only between the minimum fluidization and the entrainment velocities. If higher air velocity is used in a fluidized bed the bed material is entrained and the unburned particles are carried over from the combustor and lost in the gas stream. When the air velocity is lower the bed is slumped causing de-fluidization. Hence to sustain the stable operation of the bed, it must be ensured that air velocity is maintained between fluidization velocity and particle entrainment velocity

Atmospheric fluidized bed boiler is further classified as BFB and CFB boiler.

BFBC Boiler - Bubbling Fluidized Bed Combustion Boiler

The solid fuel feeding to the Bubbling fluidized bed is carried out either with a pneumatic under-bed feed system or an over-bed pneumatic spreader.

The solid fuel feeders are sized to less than 6 mm before conveying to the combustor with the high-velocity air through the airbox and distributor plate. With the over-bed system, the solid fuel is conveyed mechanically to the pneumatic spreaders through the drag chain feeder, which spreads the fuel evenly into the combustor.

In fluidized bed boilers, only a small percent of the ash (or) inert solids are drained through the bottom and the remaining are collected as fly ash. The separation of fly ash from the flue gas is accomplished by ESP/baghouse or mechanical dust collector.

Key characteristics of BFBC are

• Low fluidizing velocity
• Dense bed depth with submerged heat transfer tube bundles.
• A freeboard region with water-cooled membrane walls

CFBC Boiler - Circulating Fluidized Bed Combustion Boiler

A CFBC boiler consists of a combustion chamber, ‘hot loop’ cyclone separator, convection section (back pass), and bedash coolers. Compared to a bubbling fluidized bed, the relatively higher fluidizing velocity adopted in the CFBC boiler causes the dense mixture of solids to move up through the combustion chamber. This phenomenon, the taller combustion chamber of a CFBC boiler and re-circulation of bed material, results in a longer residence time for the fuel particles in the combustion chamber. The long residence time facilitates the complete combustion of the fuel particles.

Primary air is admitted to the combustor through the airbox and nozzles mounted on the bottom grid panel. The primary air is used mainly for fluidization and meets part of the combustion air requirement. Secondary air is admitted at various levels of the combustion chamber for staged combustion, as fuel particles move up. Heat is continuously transferred to the membrane water wall of the combustor maintaining the furnace temperature.

Start-up burners are used for the initial heating up of the bed material. Once the bed temperature reaches above fuel ignition temperature, solid fuel is introduced. The bulk density of the bed varies along with the height of the combustor. Heavier particles stay in the lower region of the combustor and the lighter particles move up and enter the circulation path of the “hot loop”.

The hot loop comprises the combustor, ‘hot loop’ cyclone separator, return leg, and loop seal, back to the combustor. When the particles break down into finer size, they are carried out of the hot loop (circulating path) with the flue gas as fly ash into the back pass of the boiler. The bed ash is transferred through the bed ash cooler for cooling before discharge.