Fluidized-Bed Combustion System
The fluidized bed combustor (FBC) is designed to operate as a circulating fluidized-bed combustor but can be operated as either a circulating or bubbling fluidized bed combustor. When used as a bubbling FBC (as shown in the figure), a wall is installed between the main bed and sluice.
The combustor includes a 1 foot diameter main bed and freeboard. The freeboard height is approximately 115 inches. The bed velocity is typically maintained at 6-8 ft/s when operated in the bubbling-bed mode and 12-18 ft/s when operating in the circulating fluidized-bed mode. At a fluidization velocity of 12 ft/s (circulating-bed mode), the bulk gas residence time is approximately 1 second in the freeboard.
When operated as a bubbling FBC, a wall separating the sluice from the main bed is extended to prevent the loss of bed material into the sluice during testing. Fluidizing air is supplied by bubble caps located on four standpipes below the bed. A slide gate valve allows for the removal of bottom ash as necessary.
The FBC was cast in approximately 2-foot sections with a layer of insulating brick covered with a layer of hot face refractory made of Plibrico Hymor 3000. Numerous ports are provided in the main bed and freeboard for the measurement of pressures and temperatures.
The unit has two feeders that rest on load cells from which the feed rates of fuels or make-up sand can be monitored. Material from the hoppers combine in a transition hopper before being introduced to the feed tube through a rotary valve. The fuel is then gravity fed into the combustor approximately 1 to 2 feet above the bed. Additional air can be introduced into the feed tube to promote movement of the fuel down the tube and to prevent plugging. Sight glasses on the feed tube and over the main bed allowed viewing of the bed and any slag that might be present in the freeboard.
The FBC has been integrated with a 2 million Btu/h watertube boiler to recover heat from the flue gas. A negative pressure is maintained in the boiler by adjusting the speed of the induced draft fan to avoid gas/particulate leakage.
Flue gases are cooled below 500°F in an economizer prior to passing through a baghouse. The baghouse contains sixteen 5 inches diameter by 96 inches high-temperature filters sewn with felted P84 fibers.
The products of combustion are monitored at the FBC outlet with a complete analytical package consisting of on-line analyzers. The flue gas analyzers used include: a Beckman Industrial Model 755 O2 analyzer, two Beckman Industrial Model 864 infrared analyzers for CO and CO2, a Rosemount Analytical Model 890 SO2 analyzer, and a Thermo Electron Model 10 chemiluminescent NO-NO2-NOx analyzer. The flue gas was also analyzed for total hydrocarbons by a separate California Analytical Instruments Model 300 HFID heated hydrocarbon analyzer. An additional gas sample was collected in a 1 liter Tedlar bag approximately every 30 minutes and analyzed using a Perkin-Elmer Autosystem gas chromatograph fitted with a flame ionization detector and an Alltech Chemipak 018 80/100 column.