Circulating fluidized beds advantages

Fast or circulating fluidized bed combustion systems have become popular since the late 1970s and now represent the current level of activity in the area. It is clearly recognized that this technology has a stable future in the boiler market. In competition with conventional bubbling beds, the FAB/CFB boiler demonstrates the following advantages ... 

Today the circulating fluidized bed (CFB) has become the dominating design for combustors operated at atmospheric pressure. Pressurized circulating fluidized bed combustors are under development for combined power cycle applications, but so far no clear advantages have been revealed yet. For this reason the existing commercial pressurized fluid bed systems are bubbling beds. 

Recently, Pallares and Johnsson [106] presented an overview of the macroscopic semi-empirical models used for the description of the fluid dynamics of circulating fluidized bed combustion units. They summarized the basic modeling concepts and assumptions made for each model together with the major advantages and drawbacks. In order to make a structured analysis of the processes involved, the CFBC unit is often divided into 6 fluid dynamical zones like the bottom bed, freeboard, exit zone, exit duct, cyclone and downcomer and particle seal, which have been shown to exhibit different fluid dynamical behavior. 

Fast-circulating fluidized bed (transport reactor). The circulating fluidized bed (transport reactor) achieves a number of advantages over the bubbling fluidized bed ... 

A concurrent downward flow circulating fluidized bed, or a downer, is a new alternative flow arrangement for a high-velocity system. A downer reactor system has similar system configurations to a riser reactor system except that both the gas and the solid particles flow downward. Concurrent downward flow of particles and gas reduces the residence time of solid particles because the downward flow is in the same direction as gravity. More uniform radial gas and solids flow than those in a riser can be achieved. The downer leads to more uniform contact time between the gas and solids. With these advantages, downer reactors have been proposed for processes such as fluid catalytic cracking, which requires short contact time and uniform gas and solids residence time distributions. 

Circulating fluidized bed (CFB) risers are particularly advantageous for the following reactor process characteristics ... 

Historieal aeeounts of the development of circulating fluidized beds have been prepared by Reh (1986) and Squires (1994). Advantages of circulating fluidized beds relative to bubbling beds and other types of gas-... 

Circulating fluidized beds share a number of advantages with bubbling beds. These include the following ... 

A fluidized-bed catalytic reactor system developed by C. E. Lummus (323) offers several advantages over fixed-bed systems in temperature control, heat and mass transfer, and continuity of operation. Higher catalyst activity levels and higher ethylene yields (99% compared to 94—96% with fixed-bed systems) are accomplished by continuous circulation of catalyst between reactor and regenerator for carbon bum-off and continuous replacement of catalyst through attrition. 

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