distributor with bubble caps (Fig. 3). It provides fluidizing
air to the lower furnace bed material. The bubble caps are
closely spaced for uniform air distribution over the furnace
plan area. 50 to 70% percent of the total combustion air
enters the furnace through the bubble caps with the balance
injected through overfire air (OFA) ports.
The upward flow of solids decreases with increased
furnace height. The heavier particles recirculate within the
furnace resulting in decreased local density as a function of furnace height. In the B&W PGG internal recirculation (IR-CFB) design, U-beam collectors located at the furnace
exit collect and recirculate gas-laden solids prior to entering
the convection pass. A significant portion of the remaining
entrained solids are collected by multi-cyclone dust collectors
(MDC) in the backpass. Nearly 100% particle capture
is possible with this arrangement.
Bed particle size distribution and makeup rate are required
for proper fluidization. If the bed material is too
coarse, the bed will be too deep, de-fluidize, and slump. If
the particles are too fine, they become entrained, leave the
furnace, and create overall solids inventory problems. A
stable lower furnace bed and an adequate solids inventory
in the upper furnace dictate the furnace exit gas temperature.
The furnace enclosure and in-furnace heating surfaces
(water-cooled panels or steam-cooled wing walls) define the
furnace temperature profile. Furnace heating surface and a
high solids recirculation rate result in uniform furnace gas
temperatures throughout the furnace. CFB bed temperatures
range from 1500 to 1600F. The lower furnace above the air
distributor is covered by a thin layer of highly conductive
refractory for tube corrosion and erosion protection. All
other furnace enclosures are bare tube construction