The Circulating Fluidized Bed—CFB

The results of the calculations verify the general findings on bubbling fluidized beds. 

Various other hydrodynamic models have been proposed recently, using other combinations of assumptions such as 

Changing bubble size with height in the bed Negligible bubble-cloud resistance Negligible cloud-emulsion resistance Nonspherical bubbles 

In all cases the underlying rationale for these hydrodynamic models rests on the observation that beds with identical solids and gas flow rates may develop either large bubbles or small bubbles depending on bed diameter, distributor design, baffle arrangement, etc. thus, bubble size must enter as the primary parameter in the model. A consequence of this argument is that models which do not allow for different bubble sizes at given imposed bed conditions certainly cannot be adequate. 

The power of this class of model should be apparent. For example, even the simplest of these models, the one considered here, gives unexpected predictions (e.g., that most of the gas in the bed may be flowing downward) which are subsequently verified. More important still, this type of model can be tested, it can be shown to be wrong, and it can be rejected, because its one parameter, the bubble size, can be compared with observation. 

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Figure 11 shows the flow structures of the circulating fluidized bed (CFB) for all the preceding three operating modes. The curves in the lower diagram give computation results of af, ec and s, while the operating conditions and flow structures are summarized at the top. 

In the past 10 years, the circulating fluidized bed (CFB) boiler as an efficient and clean combustion technology has rapidly developed and is now widely accepted throughout the world (Kullendorif and Anderson, 1986 Yerushalmi, 1986 Engstrom and Lee, 1991 Xu and Zhang, 1991). Its pronounced characteristics are... 

CFD has also been applied to predict the flow patterns in the circulating fluidized bed (CFB) riser (Tsuo and Gidaspow, 1990). Despite their widespread in-... 

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. 

In its present commercial operations Sasol uses two different types of FT reactors. The multitubular fixed bed (see Figure 1) produces waxes and the circulating fluidized bed (CFB, see Figure 2) produces light olefins and oils. On a cross-sectional area basis the gas throughput as well as the amount converted is much higher for fluidized- than for fixed-bed reactors. 

Unlike conventional pulverized-coal combustor, the circulating fluidized-bed (CFB) combustor is capable of burning fuel with volatile content as low as 8%-9% w/w (e.g., anthracite coke, petroleum, etc., with minimal carbon loss). Fuels with low ash-melting temperature such as wood and biomass have been proved to be feedstocks in CFB combustor due to the low operating temperature of 850°C-900 C... 

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. 

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