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The Circulating Fluidized Bed (CFB) technology offers definitely certain advantages over the conventional Pulverized Coal (PC). As with any system there are limitations also. The key advantages and limitations are discussed in this article.
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The most important advantage of the CFB is the flexibility of firing a wide range of coal from 9500 to 25000 kJ/kg. CFB’s can also burn a limitless number of other types of solid fuels, separately or mixed with coal. The limitations if any are only in the feeding and other mechanical equipment. The output capacity and steam parameters depend on the heating value of the fuel.
CFB boiler can use different types of solid fuels that include low quality coal, lignites, coal blends, bagasse in sugar plants, bark in pulp and paper mills, coal washery rejects in mines, petcoke and other bio mass products. Most of these fuels are seasonal, production or availability dependent. Mixing with coal or using coal when other fuels are not available gives an uninterrupted steam and energy supply.
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Sulphur Dioxide removal.
Firing high Sulphur coal requires the use of a Flue Gas Desulphurisation (FGD) plant to keep emissions within statutory limits. An FGD is a complicated plant that requires high capital investment and operational cost. In a CFB limestone addition in the furnace converts the SO2 to sulphates during the combustion and circulation which goes out with the ash. A CFB boiler costs less than a PC fired boiler with an FGD.
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The combustion temperature in the furnace in a CFB boiler is considerably less than PC boilers resulting in lower NOx emissions. There is no additional requirement of separate NOx capturing devices which reduces capital investment.
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Eliminating the pulverizer and pulverized coal piping reduces maintenance, related outages and equipment redundancy. A part of this saving is offset by higher maintenance due to erosion in pressure parts and furnace area, and maintenance of the refractory.
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Eliminating the pulveriser reduces power but is offset by increased power for higher quantity of primary air. Auxiliary power reduction by eliminating the FGD is a definite gain.
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Unit capacities with CFB technology are limited to 200 MW range. Higher capacities are in development.
Higher steam pressures and temperatures increase the power plant cycle efficiency. CFB technology has yet to achieve the same level of parameters in the new large power plants.