Fluidised Bed Combustion (FBC)

FLUIDISED BED COMBUSTION (FBC)

 Burning of pulverised coal has some problems such as particle size of coal used in pulverized firing is limited to 70-100 microns, the pulverised fuel fired furnances designed to burn a particular cannot be used other type of coal with same efficiency, the generation of high temp. about (1650 C)in the furnace creates number of problems like slag formation on super heater, evaporation of alkali metals in ash and its deposition on heat transfer surfaces, formation of SO2 and NOX in large amount.

Fluidised Bed combustion system can burn any fuel including low grade coals (even containing 70% ash), oil, gas or municipal waste. Improved desulphurisation and low NOX emission are its main characteristics. The fuel and inert material dolomite are fed on a distribution plate and air is supplied from the bottom of distribution plate. The air is supplied at high velocity so that solid feed material remains in suspension condition during burning.

The heat produced is used to heat water flowing through the tube and convert water into

steam. During burning SO2 formed is absorbed by the dolomite and thus prevents its escape

with the exhaust gases. The molten slag is tapped from the top surface of the bed. The bed

temperature is nearly 800-900⁰C which is ideal for sulphur retention addition of limestone or

dolomite to the bed brings down SO2 emission level to about 15% of that in conventional firing

methods.

Various advantages of FBC system are as follows:

1.     FBC system can use any type of low grade fuel including municipal wastes and therefore is a cheaper method of power generation.

2.     It is easier to control the amount of SO2 and NOX, formed during burning. Low emission of SO2 and NOX will help in controlling the undesirable effects of SO2 and NOX. During combustion. SO2 emission is nearly 15% of that in conventional firing methods.

3.     There is a saving of about 10% in operating cost and 15% in the capital cost of the power plant.

4.     The size of coal used has pronounced effect on the operation and performance of FBC system. The particle size preferred is 6 to 13 mm but even 50 mm size coal can also be used in this system.

The major portion of the coal available in India is of low quality, high ash content and low calorific value. The traditional grate fuel firing systems have got limitations and are techno-economically unviable to meet the challenges of future. Fluidized bed combustion has emerged as a viable alternative and has significant advantages over conventional firing system and offers multiple benefits – compact boiler design, fuel flexibility, higher combustion efficiency and reduced emission of noxious pollutants such as SOx and NOx.