RECIRCULATING FLUIDIZED BEDS WITH A DRAFT TUBE

0 RECIRCULATING FLUIDIZED BEDS WITH A DRAFT TUBE 

This concept was first called a recirculating fluidized bed by Yang and Kearins (1974). Several other names have also been used to describe the same concept the fluid-lift solids recirculator (Buchanan and Wilson, 1965), the spouted fluid bed with a draft tube (Y ang and Keaims, 1983 Hadzismajlovic et al., 1992), the internally circulating fluidized bed (Milne et al., 1992 Lee and Kim, 1992) or simply a circulating fluidized 

The important design parameters for a recirculating fluidized bed with a draft tube were identified by Yang and Keaims (1978a) as the gas bypassing characteristics of the distributor plate, the area ratio between the downcomer and the draft tube, the diameter ratio between the draft tube and the draft tube gas supply, the distance between the distributor plate and the draft tube inlet, and the area ratio of the draft tube gas supply and the concentric solids feeder. The design and operation of a recirculating fluidized bed with a draft tube are discussed below. 

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Most of the applications for the recirculating fluidized bed with a draft tube operate the draft tube as a dilute phase pneumatic transport tube. Hence we will discussed this system in more details. 

Downcomer and Draft Tube Pressure Drop. Typical experimental pressure drops across the downcomer, AP, 4, and the draft tube, AP2 3, show that they are essentially similar. Successful design of a recirculating fluidized bed with a draft tube requires development of mathematical models for both downcomer and draft tube. 

Design Procedure for a Recirculating Fluidized Bed with a Draft Tube... 

Figure 4 Recirculating fluidized bed with draft tube operated as a pneumatic transport tube.

A central draft tube is used primarily for recirculating solids, and the dense, dry char collects in the fluidized bed at the top of the draft tube and is withdrawn at this point. Dolomite or calcium oxide (sorbent) is added to the fluidized bed to absorb the sulfur present as hydrogen sulfide in the fuel gas and spent sorbent is withdrawn from the bottom of the reactor and regenerated. The heat for devolatilization is supplied primarily by the high-temperature fuel gas produced in the gasifier combustor. After separation of fines and ash, product gas is cooled and scrubbed with water for final purification. 

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