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 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.
Fig. Fluidized bed combustion
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.
If a coal-fired fludized bed combustor is operated at elevated pressure, the products of combustion can be expanded through a gas turbine to produce electricity. The products of combustion have to be sufficiently clean for a gas turbine to accept without excessive erosion, corrosion or fouling of the turbine. Pressurized fluidized bed combustion (PFBC) has been under extensive research and development, not only because of its potential as a coal fired gas turbine but also when used in conjunction with a steam plant, a significantly higher efficiency of electricity generation is possible than that from either a gas turbine or steam turbine plant alone. The thermodynamic aspects of combined cycle power generation.
The first PFBC plants with an electrical power of about 80 MW were almost simultaneously installed by ABB Carbon at Vartan (2 units) in Sweden, Escsrton in Spain and TIDD in the USA. Their details are as follows.
Operational success of these demonstration units gave an increased confidence in this technology. The Vartan PFBC plant in Stockholm is a Cogeneration unit, which consists of two modules, each containing one boiler with gas turbine and associated fuel and ash system. The fuel is a mixture of coal, sorbent and water supplied as a paste by means of six piston pumps. Coal is crushed to 0-6 mm size and the sorbent (dolomite) to 0-3 mm size, before they are mixed in water to form the paste.
The flow diagram of the TIDD plant at Ohio, USA, where the PFBC operates at 12 bar increasing the gas density and reducing the bed area for a given heat release. To further improve the overall cycle efficiency, there is a growing interest in combining PFBC with partial or mild gasification of coal to take advantage of a higher gas turbine inlet temperature. This is known as the second generation PFBC. Or the Topping Cycle. Mild gasification is a devolatilization process designed to produce a series of alternate fuels by decomposing coal into simpler components at relatively mild temperatures (5400C-6500C) and pressure (1-2 bar). The coal heated by limited air gets pyrolyzed to yields carbon char (20-40% of feed), a complex of hydrocarbon liquids and hydrocarbon gas. The char is combusted in a conventional fluidized bed boiler to raise steam and hence electricity. The gaseous and liquid hydrocarbon products products are cleaned and then combusted in a gas turbine, also producing electricity. The plant is similar to the British Coal Topping Cycle and the CFBC topping cycle. The significant advantage of partial gasification over total gasification is that devolatilization and gasification of coal are achieved relatively easily, whereas
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