CFB Combustor

The modelling of CFB combustors for scale-up has been given attention. Recently, Weiss and Fett (1988), Lee and Hyppanen (1989), Hyppanen et al. (1991), Lou et al. (1991), and Wang (1992) have proposed models to predict CFB combustion. 

Therefore, for coal burning, neglecting radial gas dispersion and radial heterogeneity of solids concentration might be acceptable. Gas-solid flow can thus be simplified to a one-dimensional model with axial distributions only. 

Using the axial voidage profile (Li and Kwauk, 1980 Kwauk et al., 1986), 

For mass transfer outside particles, the transfer coefficient can be calculated by using Equation (7). Inside the particles, the diffusion coefficient can be modified by means of the effective porosity (see Section I.B., or determined by another method such as that of Lou et al., 1991). For bed-to-wall heat transfer, the transfer coefficient can be computed by using relevant correlations presented in Chapter 5. 

When coal is heated, it first pyrolyzes into volatile matter, and the remaining char then burns. Char burning is much slower than that of the volatiles, and the rate controlling process has been described by Rajan and Wen (1980) as follows  

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This correlation has been verified for a wide range of operating conditions and Group A and Group B particles. Measurements of radial solids concentration profiles in a large-scale CFB combustor also confirm the validity of this correlation [Werther, 1993]. 

Westphalen, D., and Glicksman, L. Experimental Verification of Scaling for a Commercial-Size CFB Combustor, in Circulating Fluidized Bed Technology IV" (Amos A. Avidan, ed.), pp. 529-534. Somerset, Pennsylvania (1993). 

There exists both axial and radial gas diffusion in the CFB combustor, the radial gas dispersion coefficient being much smaller than the axial. 

Experimental results for the pilot plant CFB combustor at ICM, Academia Sinica, have been simulated by the preceding axial gas-solids distribution two-phase model. In computation, the parameters of the model were taken from actual operations. A bituminous coal from Datong was used, the approximate and ultimate analyses of which are listed in Table VI. 

Fig. 27. Axial profiles of different gas compositions in CFB combustor (after Lin, 1991).

The combustion efficiency of these low-capacity CFB combustors varies from 93% to 96%, with 20% excess air at the furnace exit. Measurements have shown that CO concentration in the flue gas could be reduced from 1,000-2,000 ppm to 100-200 ppm through the separator, indicating continued and rather efficient combustion in the separator. With an artificial sorbent (see Section V), the S02 removal could reach 83-93% at a Ca/S ratio of 1.4-1.7. The NO, in the exit gas is about 100 mg/Nm3. 

As is the case for bubbling fluidized bed combustors, CFB-combustors are operated with an inert (sand) bed. At start-up the CFB-combustor circulates sand (pp = 2600 kg/m ), which is gradually supplemented and replaced by shale and eombustion ash (Pp - 2500 kg/m ). The bed characteristics should therefore relate to ash, rather than to sand. The average particle size is approximately 300 pm. 

CFB-combustors for coal are operated with sand and ash of average particle size 300 pm (Andersscxi et al. [7]). In view of the small difference of density between both bed materials, transport velocities hardly differ. The biomass combustor will be operated with the same particle size. The transport velocity and operating gas velocity are then ... 

In a combustor the mass of products withdrawn from the reactor Mp, represents only the snail stream of valueless ashes. The characteristics of this stream are h ce less critical in the design of the CFB combustor. 

The most commonly encountered height for industrial CFB combustors is 20 m. For one pass, the fraction of devolatilisation is given in Table S. 

Table 8 illustrates the [M-edicted data for the combustion of biomass, coal and sludge. The particle size of the inert bed, solids circulation rate and gas velocity are the same for the 3 combustors. For the same thermal capacity, the coal and biomass CFB-combustors are comparable in size. The specific combustion load for sludge combustors is tower due to the large amount of water vapour. 

Table 8 Comparison of CFB combustors for biomass, coal and sludge. 

A design strategy for a CFB combustor for biomass was presented and illustrated for a 2SOO kg/hr combustor. The main operating parameters are the solids flux and gas velocity in the riser of the CFB. Isothermal conditions will be guaranteed by circulating inert sand particles of 300 pm at a solids flux of 20 kg/m s. The gas velocity in the riser is determined at 7.9 m/s, exceeding the transport velocity (-6.6 m/s) of the inert sand bed by 20%. About 2% of the diar will be lost in the ash stream. The net combustion capacity of the riser is equal to 4.1 M Wih per m of the riser, comparable with a coal CFB combustor. 

The solids residence time distribution (RTD) in the riser may thus be important in non-catalytic gas-solid reactions, as in a combustor, since this distribution characterizes the degree of solids mixing and provides information about the physical properties of the solid particles in the riser [14]. Moreover, lateral mixing and internal recirculation of solids in a CFB combustor are necessary to maintain uniform temperatures over the entire length of the riser. Hence, lateral and longitudinal mixing is advantageous in a CFB combustor. 

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... 

F-1650°F). The CFB combustor boiler is not bound by the tight restrictions on ash content either and can effectively bum fuels with mineral matter content up to 70% w/w (Figure 22.7). 

Basu, P., Wu, S., and Greenblatt, J., Development of a Simplified Model for Sulphur Absorption in Circulating Fluidized Beds and Experimental Verification in Pilot Scale and Large Commercial CFB Combustors , J. Chem. Eng. Japan, 24 (3), 356(1991). 

Reh L Fluid dynamics of CFB combustors. Circulating fluidized bed technology V, Beijing, 1996, Science Press, 1—15. 

A system in which defluidization can present a particular problem is the combustion of low-rank coals in a circulating fluid bed (CFB) combustor. Some low-rank coals have a high content of sodium and sulphur, and at operating temperatures in the region of 850°C... 

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