Mumbai University > Mechanical Engineering > Sem 7 > Power Plant Engineering

Marks : 5M

Year: Dec 2015

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 furnaces designed to burn a particular cannot be used other type of coal with same efficiency, the generation of high temp. about (1650 deg 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. Fig. 4.41 shows basic principle of Fluidised bed combustion (FBC) system. 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 800-900’C 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.

The amount of NOX is produced is also reduced because of low temperature of bed and low excess air as compared to pulverised fuel firing. The inert material should be resistant to heat and disintegration and should have similar density as that of coal. Limestone, or dolomite, fused alumina, sintered ash are commonly used as inert materials.

Various advantages of FBC system are as follows:

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

(ii) 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.

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

(iv) 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.

TYPES OF FBC SYSTEMS

FBC systems are of following types :

(i) Atmospheric FBC system :

(a) Over feed system

(b) Under feed system.

In this system the pressure inside the bed is atmospheric. Fig. 4.42 shows commercial circulation FBC system. The solid fuel is made to enter the furnace from the side of walls. The Low Velocity (LV), Medium Velocity (MV) and High Velocity (HV) air is supplied at different points along the sloping surface of the distribution ash are collected from the ash port. The burning is efficient because of high lateral turbulence.

(ii) Pressurised FBC system. In this system pressurized air is used for fluidisation and combustion. This system: the following advantages: (a) High burning rates. (b) Improved desulphurization and low NO, emission. (c) Considerable reduction in cost.