UNIT-I

INTRODUCTION TO POWER PLANTS AND BOILERS

M.NAMBIRAJASSISTANT PROFESSOR

DHANALAKSHMI COLLEGE OF ENGINEERING,

CHENNAI

• Layout of Steam , Hydro , Diesel , MHD, Nuclear and Gas turbine Power Plants

• Combined Power cycles• Comparison and selection• Load duration Curves Steam• Boilers and cycles

– High pressure and – Super Critical Boilers– Fluidized Bed Boilers

CONTENTS

UNIT-IINTRODUCTION TO POWER PLANTS AND

BOILERS • A power plant may be defined as a machine or assembly of

equipment that generates and delivers a flow of mechanical or electrical energy.

• The main equipment for the generation of electric power is generator. When coupling it to a prime mover runs the generator, the electricity is generated.

The major power plants, which are discussed in this Chapter is,1. Steam power plant2. Nuclear power plant3. Hydro electric power plant4. Diesel power plant5. Gas turbine power plant

STEAM POWER PLANT

The heat energy is converted into mechanical energy by the steam turbine and that mechanical energy is used for generating power with the help of generator.

The layout of the steam power plant consists of four main circuits:

1.Coal and ash circuit2.Air and flue gas circuit3.Water and steam circuit4.Cooling water circuit

Advantages:• Power production does not depends on nature of mercy .• Initial investment is low.• The time requirement for construction and

commissioning of thermal power plant require less period of time.

Drawbacks:• As compared to hydro power plant, life and efficiency

are less .• Transportation of fuel is a major problem in this type of

power plant.• It cannot be used as a peak load power plant .• The coal (fuel) needed may be exhausted by gradual

use.

HYDRO POWER PLANT• Hydro electric power was initiated in India in

1987 near Darjeeling.• In Hydro electric Power plants, the potential

energy of water is converted into kinetic energy.

• The potential energy of water is used to run water turbine to which the electric generator is coupled.

• The arrangement of different components used in hydraulic power plant is discussed below.

Layout of a Hydro power plant

Components of Hydro electric power plant:1.Water reservoir- To store the water during rainy

season.2.Dam- To increase the height of water level and

thereby it increases the capacity of reservoir.3.Spillway- It is acting as a safety valve for dam.4.Trash rack- The water is taken from the dam or from

forebay is provided with trash rack.5.Pressure tunnel- It carries water from the reservoir to

surge tank.6.Forebay- It serves as a regulating reservoir. It stores

the water temporarily when the load on the plant reduces.

7.Penstock- A pipeline fixed between the surge tank and prime mover. It is commonly made of reinforced concrete or steel.

8.Surge tank- The surge tank is introduced b/w the dam and powerhouse to keep reducing sudden rise of pressure (water hammer) in the penstock due to sudden backflow of water.

9.Water turbine- It converts kinetic energy of water into mechanical energy to produce electrical energy.

10.Draft tube- It is connected at the outlet of the water turbine.

11.Tailrace- It is a waterway to lead the water discharged from the turbine to the river.

Advantages:• Water is the renewable source of energy. It is neither

consumed nor converted into something else • Variable load does not affect the efficiency in the case of a

hydro plant• Maintenance cost is low• It requires less supervising staff for the operation of the

plantDisadvantages:• Hydro power projects are capital intensive with low of

rate of return• Initial cost of the plant is high• It takes considerably longer time for its installation

compared with thermal power plants

NUCLEAR POWER PLANT• The heat produced due to fission of U and Pu

is used to heat water to generate steam which is used for running turbo generator.

• 1kg of U can produce as much energy as possible by burning 4500tonnes of high-grade coal.

• The two materials most commonly used are uranium-235 and plutonium-239.

Nuclear fission - This process takes place when the nucleus of a heavy atom like uranium or plutonium is split when struck by a neutron.

Main components of Nuclear power plant

1. Fuel- The fuel which is used in the nuclear reactors are uranium-235, plutonium-239, uranium-233.

2. Nuclear Reactor- A nuclear reactor is a device that permits a controlled fission chain reaction. It consists of Reactor core, reflector, shield etc. The various types of reactors used in nuclear power plant is

- Pressurized-Water Reactor (PWR) - Boiling Water Reactor (BWR)

- Heavy Water Reactor (HWR)

3. Steam generator- It is fed with feed water and the feed water is converted into steam by the heat of the hot coolant. The coolant is used to transfer the heat from reactor core to the steam generator.

4. Moderator- It is used to the slow down the neutrons from high velocities without capturing them.

5. Reflector- It is done by surrounding the reactor core by a material called reflector which will send the neutrons back in to core.

6. Control rods- It is used to control the nuclear chain reaction and functions of the nuclear reactor.

7. Coolant pump and feed pump- It is used to maintain the flow of coolant and feed water in the power plant. The ordinary water or heavy water is a common coolant.

8. Shielding- The reactor is a source of intense radioactivity. These radiations are very harmful, shielding is provided to absorb the radioactive rays. A thick concrete shielding and a pressure vessel are provided to prevent the radiations escaped to atmosphere.

Working principle of Nuclear Reactor

Advantages:• Space requirement of a nuclear power plant

is less• There is increased reliability of operation• No ash handlingDrawbacks:• It is not suitable for variable load condition • It requires high initial cost• It requires well trained personnel• Maintenance cost of plant is high.

Diesel power plant• Diesel engine power plant is suitable for small

and medium outputs. • It is commonly used where fuel prices or

reliability of supply favour oil over coal, where water supply is limited, where loads are relatively small, and where electric line service is unavailable or is available at too high rates.

Main components of Diesel power plant

Components of diesel power plant

1. Diesel engine- this is the main component of a diesel power plant. The engines are classified as 2-stroke and 4-stroke engines. Engine is directly coupled with generator for developing power.

2. Air filter and super charger- The air filter is used to remove the dust from the air which is taken by the engine.

The function of the supercharger is to increase the pressure of the air supplied to the engine and thereby the power of the engine is increased.

3. Engine starting system- This includes air compressor and starting air tank. This is used to start the engine in cold conditions by supplying the air.

4. Fuel system- It includes the storage tank, fuel transfer pump, strainers and heaters. The fuel is supplied to the engine according to the load variation.

5. Lubrication system- It includes oil pumps, oil tanks, filters, coolers and pipes. It is used to reduce the friction of moving parts and reduce wear and tear of the engine.

6. Cooling system- The temperature of burning fuel inside the combustion chamber is 1500oC to 2000oC. To maintain the temperature water is circulated around the engine water jackets.

7. Governing system- It is used to regulate the speed of the engine. This is done by varying fuel supply according to the engine load.

8. Exhaust system- After the combustion process, the burned gases are exhausted to the atmosphere through silencer. The exhaust gas has high temperature and so it is used to preheat the oil and air.

Applications:• Quite suitable for mobile power generation• Used as peak load plants in combined with

thermal or hydro plants• Used as stand by plants for emergency service Advantages:• Diesel power plants are cheaper• Plant layout is simple• Location of the plant is near the load centre• It has no stand by losses

GAS TURBINE POWER PLANT

The gas turbine power plant has relatively low cost and can be quickly put into commission.

It requires only a fraction of water used by their steam turbine counter-parts.

It requires less space. This plant is of smaller capacity and is mainly used for peak load service.

The size of gas turbine plants used varies from 10MW to 50MW and the thermal efficiency of about 22% to 25%.

Elements of Gas turbine plantThe simple gas turbine plant consists of1.Compressor2.Intercoolers3.Regenerator4.Combustion Chambers5.Gas turbine6.Reheating unit.

1. Compressor- In gas turbine plant, the axial and centrifugal flow compressors are used. In most of the gas turbine power plant, two compressors are used.

1. Low pressure compressor2. High pressure compressor

2. Intercooler- It is used to reduce the work of the compressor and it is placed b/w HP and LP compressor. The cooling of compressed air in intercooler is generally done by water.

3. Regenerator- It is used to preheat the air which is entering into the CC to reduce the fuel consumption and to increase the efficiency.

4. Combustion chambers- Hot air from regenerator flows to the CC and the fuel like coal, natural gas or kerosene are injected into the CC. After the fuel injection, the combustion takes place.

5. Gas turbine- (1. Open cycle gas turbine)

2. Closed cycle gas turbine

MAGNETO HYDRO DYNAMICS (MHD) GENERATOR

Contents1. Introduction2. Principle Of MHD Power Generation3. Types of MHD SYSTEM4. Open Cycle MHD System5. Closed Cycle MHD System6. Difference between Open Cycle and Closed

Cycle MHD System7. Advantages OF MHD System8. Disadvantages of MHD System9. Applications10. Conclusion

Introduction Magneto Hydro Dynamic (MHD) system is

a non- conventional source of energy which is based upon Faraday’s Law of Electromagnetic Induction, which states that energy is generated due to the movement of an electric conductor inside a magnetic field.

Concept given by Michael Faraday in 1832 for the first time.

Principle Of MHD Power GenerationFaraday’s law of electromagnetic induction : When an

electric conductor moves across a magnetic field, an emf is induced in it, which produces an electric current .

TYPES OF MHD SYSTEM

(1)Open cycle System(2)Closed cycle System (i) Seeded inert gas systems (ii) Liquid metal systems

OPEN CYCLE MHD SYSTEM

CLOSED CYCLE MHD SYSTEM

DIFFERENCE BETWEEN OPEN CYCLE AND CLOSED CYCLE SYSTEM

Open Cycle System Working fluid after generating

electrical energy is discharged to the atmosphere through a stack .

Operation of MHD generator is done directly on combustion products .

Temperature requirement : 2300˚C to 2700˚C.

More developed.

Closed Cycle System Working fluid is recycled to the

heat sources and thus is used again.

Helium or argon(with cesium seeding) is used as the working fluid.

Temperature requirement : about 530˚C.

Less developed.

ADVANTAGES OF MHD SYSTEM Conversion efficiency of about 50% .Less fuel consumption.Large amount of pollution free power generated .Ability to reach full power level as soon as started. Plant size is considerably smaller than

conventional fossil fuel plants .Less overall generation cost.No moving parts, so more reliable .

DISADVANTAGES OF MHD SYSTEM Suffers from reverse flow (short circuits) of

electrons through the conducting fluids around the ends of the magnetic field.

Needs very large magnets and this is a major expense.

High friction and heat transfer losses. High operating temperature. Coal used as fuel poses problem of molten

ash which may short circuit the electrodes. Hence, oil or natural gas are much better fuels for MHDs. Restriction on use of fuel makes the operation more expensive.

APPLICATIONS

Power generation in space craft.

Hypersonic wind tunnel experiments.

Defense application.

SELECTION OF POWER PLANT1. Availability of raw material.2. Nature of land.3. Cost of land.4. Availability of water.5. Transport facilities6. Ash disposal facilities7. Availability of labour8. Size of plant9. Load centre10. Public Problems11. Future Extension

CLASSIFICATION ON THE BASIS OF PRESSURE

Boilers are classified into two basic classes• Low Pressure Boiler: Pressure is below 80bar.• High Pressure Boiler: Pressure is above 80bar.

High Pressure Boilers are further classified in to Sub Critical, Supercritical and Ultra Super Critical

• Sub-Critical (Operated around 19 Mpa):Example: 170 bar, 540˚C, Efficiency: 38 %

• Super-Critical (Operated above 22.1 Mpa):Example: 250 bar, 600/615˚C, Efficiency: 42 %

• Ultra Super-Critical:Example: 300 bar, 615/630˚C, Efficiency: 44 %

MODERN HIGH PRESSURE BOILERSA boiler which generates steam at pressure

greater than 80bar a temperature of about 500˚C, producing more than 250 tones of steam/hr called high pressure boilers.

By using high-pressure boilers, low grade fuels can be burned easily.

High pressure boilers are water tube boilers and uses pulverized coal firing. Examples of these boilers are Lamont, Banson, Loeffler and Velox boilers.

LAMONT BOILER

Components:Steam separator drumWater circulating pump Distributing headerEvaporatorConvection super heater EconomiserAir preheaterCombustion chamber

Advantages:

1. It is forced circulation boiler

2. High working pressure 

Disadvantages:

1. The salt and sediment are deposited on

the inner surface of water.

2. Danger of overheating of tubes

BANSON BOILER

• This is the first drumless boiler.• The entire process takes place in a continuous

tube. This is also called once through boiler.

COMPONENTS:1.Economiser- To preheat the water2.Radiant evaporator-Partly converted into steam3.Convection evaporator-Remaining water is

absorbed from the hot gases by convection.4.Convection super heater-Saturated high-Pr

steam is converted into superheated steam.

LOEFFLER BOILER

COMPONENTSEconomiser-To preheat the waterEvaporator drum- It contains steam and water, 2/3 of

SH steam is used to heat the water in the drum and evaporates it to saturated steam.

Mixing nozzles-It is used to distribute and mix the SH steam throughout the water in the evaporator drum.

Steam circulating pump-To forces the steam from the evaporator drum to the radiant super heater.

Radiant super heater-It is placed in the furnace. Used for SH the saturated steam from the drum.

Convective super heater-Finally heated to the desired temperature of 500˚C.

VELOX BOILER

This boiler makes use of pressurized combustion. This boiler can generate a pressure of about 84kg/cm².COMPONENTS:1.Economiser2.Axial flow compressor3.Water circulating pump4.Convection super heater

Fluidized Bed (FBC) Boiler

An Overview-Fluidized bed combustion has emerged as a

viable alternative and has significant advantages over conventional firing system.

It offers multiple benefits – compact boiler design, fuel flexibility, higher combustion efficiency and reduced emission of noxious pollutants such as SOx and NOx. The fuels burnt in these boilers include coal, washery rejects, rice husk, bagasse & other agricultural wastes. The fluidized bed boilers have a wide capacity range.

Mechanism of Fluidised Bed Combustion When an evenly distributed air or gas is passed

upward through a finely divided bed of solid particles such as sand supported on a fine mesh, the particles are undisturbed at low velocity. As air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream – the bed is called “fluidized”.

With further increase in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation of dense defined bed surface. The bed of solid particles exhibits the properties of a boiling liquid and assumes the appearance of a fluid – “bubbling fluidized bed”.

At higher velocities, bubbles disappear, and particles are blown out of the bed. Therefore, some amounts of particles have to be recirculated to maintain a stable system – “circulating fluidized bed”. Fluidization depends largely on the particle size and the air velocity.

If sand particles in a fluidized state is heated to the ignition temperatures of coal, and coal is injected continuously into the bed, the coal will burn rapidly and bed attains a uniform temperature. The fluidized bed combustion (FBC) takes place at about 840OC to 950OC.

Since this temperature is much below the ash fusion temperature, melting of ash and associated problems are avoided.

The lower combustion temperature is achieved because of high coefficient of heat transfer due to rapid mixing in the fluidized bed and effective extraction of heat from the bed through in-bed heat transfer tubes and walls of the bed. The gas velocity is maintained between minimum fluidisation velocity and particle entrainment velocity. This ensures stable operation of the bed and avoids particle entrainment in the gas stream.

Combustion process requires the three “T”s that is Time, Temperature and Turbulence. In FBC, turbulence is promoted by fluidization. Improved mixing generates evenly distributed heat at lower temperature. Residence time is many times greater than conventional grate firing. Thus an FBC system releases heat more efficiently at lower temperatures.

Fixing, bubbling and fast fluidized beds As the velocity of a gas flowing through a bed of particles increases, a value is reaches when the bed fluidizes and bubbles form as in a boiling liquid. At higher velocities the bubbles disappear; and the solids are rapidly blown out of the bed and must be recycled to maintain a stable system. principle of fluidization

Types of Fluidised Bed Combustion Boilers

There are two basic types of fluidised bed combustion boilers:

1. Pressurised Fluidised Bed Combustion System (PFBC).

2. Circulating (fast) Fluidised Bed Combustion system(CFBC)

Bubbling Bed Boilers In the bubbling bed type boiler, a layer of solid

particles (mostly limestone, sand, ash and calcium sulfate) is contained on a grid near the bottom of the boiler.

This layer is maintained in a turbulent state as low velocity air is forced into the bed from a plenum chamber beneath the grid.

Fuel is added to this bed and combustion takes place. Normally, raw fuel in the bed does not exceed 2% of the total bed inventory.

Velocity of the combustion air is kept at a minimum, yet high enough to maintain turbulence in the bed. Velocity is not high enough to carry significant quantities of solid particles out of the furnace.

This turbulent mixing of air and fuel results in a residence time of up to five seconds. The combination of turbulent mixing and residence time permits bubbling bed boilers to operate at a furnace temperature below 1650°F. At this temperature, the presence of limestone mixed with fuel in the furnace achieves greater than 90% sulfur removal.

Boiler efficiency is the percentage of total energy in the fuel that is used to produce steam. Combustion efficiency is the percentage of complete combustion of carbon in the fuel.

Incomplete combustion results in the formation of carbon monoxide (CO) plus unburned carbon in the solid particles leaving the furnace. In a typical bubbling bed fluidized boiler, combustion efficiency can be as high as 92%. This is a good figure, but is lower than that achieved by pulverized coal or cyclone-fired boilers. In addition, some fuels that are very low in volatile matter cannot be completely burned within the available residence time in bubbling bed-type boilers.

Features of Bubbling bed boiler

•Fluidized bed boiler can operate at near atmospheric or elevated pressure and have these essential features:

• Distribution plate through which air is blown for fluidizing.

• Immersed steam-raising or water heating tubes which extract heat directly from the bed.

• Tubes above the bed which extract heat from hot combustion gas before it enters the flue duct.

Bubbling Bed Boiler-1

Bubbling Bed Boiler-2

1. Pressurized Fluidized Bed Combustion System (PFBC).

Pressurized Fluidized Bed Combustion (PFBC) is a variation of fluid bed technology that is meant for large-scale coal burning applications. In PFBC, the bed vessel is operated at pressure up to 16 atm ( 16 kg/cm2).

The off-gas from the fluidized bed combustor drives the gas turbine. The steam turbine is driven by steam raised in tubes immersed in the fluidized bed. The condensate from the steam turbine is pre-heated using waste heat from gas turbine exhaust and is then taken as feed water for steam generation.

The PFBC system can be used for cogeneration or combined cycle power generation. By combining the gas and steam turbines in this way, electricity is generated more efficiently than in conventional system. The overall conversion efficiency is higher by 5% to 8%.

At elevated pressure, the potential reduction in boiler size is considerable due to increased amount of combustion in pressurized mode and high heat flux through in-bed tubes.

PFBC Boiler for Cogeneration

2. Circulating (fast) Fluidized Bed Combustion system(CFBC)

The need to improve combustion efficiency (which also increases overall boiler efficiency and reduces operating costs) and the desire to burn a much wider range of fuels has led to the development and application of the CFB boiler.

This CFBC technology utilizes the fluidized bed principle in which crushed (6 –12 mm size) fuel and limestone are injected into the furnace or combustor. The particles are suspended in a stream of upwardly flowing air (60-70% of the total air), which enters the bottom of the furnace through air distribution nozzles.

The fluidizing velocity in circulating beds ranges from 3.7 to 9 m/sec. The balance of combustion air is admitted above the bottom of the furnace as secondary air.

The combustion takes place at 840-900oC, and the fine particles (<450 microns) are elutriated out of the furnace with flue gas velocity of 4-6 m/s. The particles are then collected by the solids separators and circulated back into the furnace. Solid recycle is about 50 to 100 kg per kg of fuel burnt.

There are no steam generation tubes immersed in the bed. The circulating bed is designed to move a lot more solids out of the furnace area and to achieve most of the heat transfer outside the combustion zone - convection section, water walls, and at the exit of the riser. Some circulating bed units even have external heat exchanges.

The particles circulation provides efficient heat transfer to the furnace walls and longer residence time for carbon and limestone utilization. Similar to Pulverized Coal (PC) firing, the controlling parameters in the CFB combustion process are temperature, residence time and turbulence.

For large units, the taller furnace characteristics of CFBC boiler offers better space utilization, greater fuel particle and sorbent residence time for efficient combustion and SO2 capture, and easier application of staged combustion techniques for NOx control than AFBC generators. CFBC boilers are said to achieve better calcium to sulphur utilization – 1.5 to 1 vs. 3.2 to 1 for the AFBC boilers, although the furnace temperatures are almost the same.

CFBC boilers are generally claimed to be more economical than AFBC boilers for industrial application requiring more than 75 – 100 T/hr of steam

CFBC requires huge mechanical cyclones to capture and recycle the large amount of bed material, which requires a tall boiler.

A CFBC could be good choice if the following conditions are met. 1. Capacity of boiler is large to medium 2.Sulphur emission and NOx control is important 3.The boiler is required to fire low-grade fuel or fuel with highly fluctuating fuel quality.

Circulating bed boiler (At a Glance)-

At high fluidizing gas velocities in which a fast recycling bed of fine material is superimposed on a bubbling bed of larger particles.

The combustion temperature is controlled by rate of recycling of fine material.

Hot fine material is separated from the flue gas by a cyclone and is partially cooled in a separate low velocity fluidized bed heat exchanger, where the heat is given up to the steam.

The cooler fine material is then recycled to the dense bed.

Advantages of Fluidized Bed Combustion Boilers

1. High Efficiency FBC boilers can burn fuel with a combustion efficiency of over 95% irrespective of ash content. FBC boilers can operate with overall efficiency of 84% (plus or minus 2%). 2. Reduction in Boiler Size High heat transfer rate over a small heat transfer area immersed in the bed result in overall size reduction of the boiler. 3. Fuel Flexibility FBC boilers can be operated efficiently with a variety of fuels. Even fuels like flotation slimes, washer rejects, agro waste can be burnt efficiently. These can be fed either independently or in combination with coal into the same furnace. 4. Ability to Burn Low Grade Fuel FBC boilers would give the rated output even with inferior quality fuel. The boilers can fire coals with ash content as high as 62% and having calorific value as low as 2,500 kcal/kg. Even carbon content of only 1% by weight can sustain the fluidised bed combustion.

5. Ability to Burn Fines Coal containing fines below 6 mm can be burnt efficiently in FBC boiler, which is very difficult to achieve in conventional firing system.

6. Pollution Control SO2 formation can be greatly minimised by addition of limestone or dolomite for high sulphur coals. 3% limestone is required for every 1% sulphur in the coal feed. Low combustion temperature eliminates NOx formation.

7. Low Corrosion and Erosion The corrosion and erosion effects are less due to lower combustion temperature, softness of ash and low particle velocity (of the order of 1 m/sec).

8. Easier Ash Removal – No Clinker Formation Since the temperature of the furnace is in the range of 750 – 900o C in FBC boilers, even coal of low ash fusion temperature can be burnt without clinker formation. Ash removal is easier as the ash flows like liquid from the combustion chamber. Hence less manpower is required for ash handling.

9. Less Excess Air – Higher CO2 in Flue Gas The CO2 in the flue gases will be of the order of 14 – 15% at full load. Hence, the FBC boiler can operate at low excess air - only 20 – 25%.

10. Simple Operation, Quick Start-Up High turbulence of the bed facilitates quick start up and shut down. Full automation of start up and operation using reliable equipment is possible.

11. Fast Response to Load Fluctuations Inherent high thermal storage characteristics can easily absorb fluctuation in fuel feed rates. Response to changing load is comparable to that of oil fired boilers.

12. No Slagging in the Furnace-No Soot Blowing In FBC boilers, volatilisation of alkali components in ash does not take place and the ash is non sticky. This means that there is no slagging or soot blowing.

13 Provisions of Automatic Coal and Ash Handling System Automatic systems for coal and ash handling can be incorporated, making the plant easy to operate comparable to oil or gas fired installation.

14 Provision of Automatic Ignition System Control systems using micro-processors and automatic ignition equipment give excellent control with minimum manual supervision. 15 High Reliability The absence of moving parts in the combustion zone results in a high degree of reliability and low maintenance costs. 16 Reduced Maintenance Routine overhauls are infrequent and high efficiency is maintained for long periods. 17 Quick Responses to Changing Demand A fluidized bed combustor can respond to changing heat demands more easily than stoker fired systems. This makes it very suitable for applications such as thermal fluid heaters, which require rapid responses. 18 High Efficiency of Power Generation By operating the fluidized bed at elevated pressure, it can be used to generate hot pressurized gases to power a gas turbine. This can be combined with a conventional steam turbine to improve the efficiency of electricity generation and give a potential fuel savings of at least 4%.

General Arrangements of FBC Boiler FBC boilers comprise of following systems: i) Fuel feeding system ii) Air Distributor iii) Bed & In-bed heat transfer surface iv) Ash handling system Many of these are common to all types of FBC boilers 1. Fuel Feeding system For feeding fuel, sorbents like limestone or dolomite, usually two methods are followed: under bed pneumatic feeding and over-bed feeding. Under Bed Pneumatic Feeding If the fuel is coal, it is crushed to 1-6 mm size and pneumatically transported from feed hopper to the combustor through a feed pipe piercing the distributor. Based on the capacity of the boiler, the number of feed points is increased, as it is necessary to distribute the fuel into the bed uniformly.

Over-Bed Feeding The crushed coal, 6-10 mm size is conveyed from coal bunker to a

spreader by a screw conveyor. The spreader distributes the coal over the surface of the bed uniformly. This type of fuel feeding system accepts over size fuel also and eliminates transport lines, when compared to under-bed feeding system. 2. Air Distributor

The purpose of the distributor is to introduce the fluidizing air evenly through the bed cross section thereby keeping the solid particles in constant motion, and preventing the formation of defluidization zones within the bed.

The distributor, which forms the furnace floor, is normally constructed from metal plate with a number of perforations in a definite geometric pattern. The perforations may be located in simple nozzles or nozzles with bubble caps, which serve to prevent solid particles from flowing back into the space below the distributor. The distributor plate is protected from high temperature of the furnace by:

i) Refractory Lining

ii) A Static Layer of the Bed Material or

iii) Water Cooled Tubes.

3. Bed & In-Bed Heat Transfer Surface: a) Bed

The bed material can be sand, ash, crushed refractory or limestone, with an average size of about 1 mm. Depending on the bed height these are of two types: shallow bed and deep bed.

At the same fluidizing velocity, the two ends fluidise differently, thus affecting the heat transfer to an immersed heat transfer surfaces. A shallow bed offers a lower bed resistance and hence a lower pressure drop and lower fan power consumption. In the case of deep bed, the pressure drop is more and this increases the effective gas velocity and also the fan power. b) In-Bed Heat Transfer Surface

In a fluidized in-bed heat transfer process, it is necessary to transfer heat between the bed material and an immersed surface, which could be that of a tube bundle, or a coil. The heat exchanger orientation can be horizontal, vertical or inclined. From a pressure drop point of view, a horizontal bundle in a shallow bed is more attractive than a vertical bundle in a deep bed. Also, the heat transfer in the bed depends on number of parameters like (i) bed pressure (ii) bed temperature (iii) superficial gas velocity (iv) particle size (v) Heat exchanger design and (vi) gas distributor plate design.