Circulating fluidized beds cyclones

Circulating fluidized beds (CFBs) are high velocity fluidized beds operating well above the terminal velocity of all the particles or clusters of particles. A very large cyclone and seal leg return system are needed to recycle sohds in order to maintain a bed inventory. There is a gradual transition from turbulent fluidization to a truly circulating, or fast-fluidized bed, as the gas velocity is increased (Fig. 6), and the exact transition point is rather arbitrary. The sohds are returned to the bed through a conduit called a standpipe. The return of the sohds can be controUed by either a mechanical or a nonmechanical valve. 

The chapter begins by describing how temperature and pressure affect parameters important for bubbling fluidized beds, and then discusses their effect on regime transitions, circulating fluidized beds, and cyclones. 

Cyclonic filters (and closely related designs such as U-tubes) are employed as an initial gas cleanup step in most gasifier systems because they are effective and relatively inexpensive to operate. In circulating fluidized-bed or entrained-bed gasifiers, cyclones are an integral part of the reactor design, providing for separation of the bed material and other particulates from the gas stream. 

A circulating fluidized bed (CFB) photoreactor and installation of the UV light lamps is shown in Fig. 1. The CFB photoreactor composed of a riser (13 mm-i.d. x 1200mm-high, Quartz glass), cyclone, downcomer... 

Bankel, J., and Olsson, E. Measurements of Particle Velocity in a Cyclone Separator Using Laser Doppler Anemometer, in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 630-635. Somerset, Pennsylvania (1993). 

The load characteristic of circulating fluidized bed boilers is closely related to the gas-solid flux through the combustor. Thus, choice of coal size and the proper gas-solid separator deserves attention. Generally, the fraction of particles below 0.1 mm is smaller than 10%, which could be removed only by a cyclone separator, while the major fraction of coarser particles could be easily separated by inertia or gravity (Li and Kwauk, 1981). The diagram of ash balance for a circulating fluidized bed boiler, shown in Fig. 22, can... 

The gasifier is of the atmospheric circulating fluidized bed type, operating at temperatures of 800 - 950 C with the addition of bed material and possible limestone or dolomite. After passing the cyclone, the raw product gas is cooled down in a gas cooler to a temperature of about 200 C in that cooler intermediate temperature steam is produced and slightly superheated. At the temperature of 200 - 250 °C, the gas is dedusted in a bag house filter. The dust-free gas is then washed with water in a scrubbing section, in order to remove mainly the ammonia. After this scrubber, the... 

The dried and crushed wood fuel is pressurised in a lock-hopper system to a level which basically is determined by the pressure ratio of the gas turbine, and is fed by screw feeders into the gasifier a few meters above the bottom. The operating ten erature of the gasifier is 950 - 1000 C and the pressure is approximately 17 bar, The gasifier is of a circulating fluidized bed type and consists of the gasifier itself, cyclone and cyclone return leg. The three parts are totally refractory lined. 

Skrifvars B.-J, et al. (1997) Mechanisms of bed material agglomeration in the cyclone and the return leg of a petroleum coke fired circulating fluidized bed boiler. In Proceedings in I4th International Conference on Fluidized Bed Combustion, Vancouver, Canada, 819-829. 

Fig. 10.5. A schematic representation of a circulating fluidized bed. The CFB loop consists of a riser, gas-solid cyclone separators, standpipe type of downcomer, and a non-mechanical solids flow control device. Reprinted from [82] with permission from...

Fig. 10.9. A typical circulating fluidized bed combustor design [63]. The furnace (riser) is normally operated in the fast fluidization regime. The ash which is entrained from the furnace is separated from the flue gas in the cyclone. Most of the ash particles are sent into the siphon. The siphon is a small bubbling fluidized bed acting as a pressure lock. From the siphon the ash flows back into the riser. Reprinted with permission from Elsevier, copyright Elsevier 2007.

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. 

Circulating fluidized-bed technology uses higher air flows to entrain and move the bed material and recirculate nearly all the bed material with adjacent high-volume, hot cyclone separators. 

In this study, it was concluded that although the operation of both CLC reactor configurations is different, the process efficiency is quite close. So, the process efficiency is not an important factor in the selection of the CLC reactor configuration. The reactor configuration of CLC will be selected based on either the application of efficient cyclones with circulated fluidized beds and the possibility to operate continuously at high pressure or high temperature valves in the packed bed configuration. 

Consider the circulating fluidized bed (CFB) boiler. Figure 1 where typical operating conditions might be 3-5 m/s up to the secondary air injection level and 5-8 m/s above that level at a bed temperature of 1,173 K. Coal with a top size of 6 mm is fed at the bottom of the bed consisting of coal-ash particles of mean diameter 0.3 mm. Entrained particles are collected in a cyclone and recycled to the combustor via a non-mechanical return valve. Part of the heat released from the carbon burning in the bed is extracted by water wall tubes placed in the furnace. The remaining heat leaves the furnace as sensible heat in the flue gas. 

In a circulating fluidized bed, the carbon fines generated due to attrition may be lost through the cyclone. It is essential to consider this in any performance models of high-velocity turbulent or fast fluidized bed. 

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