| OCR Text |
Show fluidized-bed air temperature can be reduced to minimize the compressor power requirements while supplying steam to the partial-combustor vessels for fluidized-bed temperature control. Of the total air needed for overall combustion, about 30% is compressed for the partial-combustor vessel. All other air, including the necessary excess air for combustion, is directed to the boiler with a fan to burn the fuel gas produced in the partial-combustor vessels. UDDer Fluidized-Bed Zone -- Calcinator Zone In the upper calcinator fluidized-bed zone, crushed limestone/dolomite is added using a gravity feed system. The feed rate is controlled by a variable-speed rotary feeder. This limestone/dolomite is heated from ambient temperature to about 14500 F from the heat contained in the fuel gas rising from the fluidized beds below. The primary purpose of this fluidized bed is for limestone/dolomite calcination. The fluidization velocity in this zone is maintained at 7 to 10 feet per second depending on the type and size of the sorbent used. The two main reactions occurring in this upper calcination zone with dolomite and/or limestone are as follows: CaC03 + Heat --> CaO + CO2 MgC03 + Heat --> MgO + CO2 With the fuel gas entering this zone from the middle carbonizer zone below at 1550°F, almost complete calcination of the limestone or dolomite is expected. The calcination in this zone reduces the time necessary for sulfation in the middle carbonizer zone below, and therefore raises the sulfur-capture efficiency of the middle zone. The calcium oxide (lime, CaO) and magnesium oxide (MgO) produced in this fluidized bed are transferred into the lower carbonizer fluidized-bed zone via internal fluidized-bed overflow pipes. These overflow pipes have pneumatic motor-operated flap valves at each end that can be controlled to introduce measured amounts of calcium oxide to the middle carbonizer zone below. Through the use of these overflow tubes, the frequency of operation of the flap valves, and limestone/dolomite feed rate, the bed level in the upper zone can be controlled. This upper calcination zone also serves the secondary purpose of further reducing the sulfur in the fuel gas by acting as a scrubber. The 15500 F fuel gas passing through this calcination zone will be expected to lose the remaining sulfur 8 |
