Bubbling fluidized-bed boilers 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.