A typical BFB furnace (Fig. 2) consists of a horizontal air
distributor with an array of bubble caps. This provides the
fluidizing air to the lower furnace bed material. The bubble
caps are closely spaced so that air flow is distributed uniformly
over the furnace plan area. The lower furnace is filled
with sand or other noncombustible material such as crushed
limestone or bed material from prior operation. Air flow is
forced upward through the material, and the bed expands.
The air flow through the bed is very uniform due to a high
number bubble caps and bed pressure drop.
The B&W PGG BFB boiler is an open bottom design.
The open bottom system is characterized by the fluidizing air
bubble caps and pipes mounted on widely spaced distribution
ducts. Stationary bed material fills the hoppers and furnace
bottom up to the level of the bubble caps, above which the
bed material is fluidized by the air flow. The open spacing
is effective in removing larger rocks and debris from the
active bed area as bed material moves down through hoppers.
This design is particularly attractive in biomass and
waste fuel applications containing noncombustible debris.
The typical operating temperature range of a bubbling
bed is 1350 to 1650F. Actual operating bed temperature is
dictated by fuel moisture, ash analysis and alkali content.
Bed temperature is controlled by combustion stoichiometry.
Even at these low combustion temperatures, high convective
and radiative heat transfer from the bed material to the fuel
particles provides sufficient ignition energy to evaporate
moisture, heat the ash, and combust the remaining fuel
without significantly changing the bulk bed temperature