SUBJECT CODE : ME8792

SUBJECT NAME : Power Plant Engineering

STAFF NAME : Prof.V.Tamil Selvi

Ms.P.Aileen Sonia Dhas

UNIT- I

COAL BASED THERMAL POWER

PLANTS

Rankine cycle - improvisations, Layout of

modern coal power plant, Super Critical

Boilers, FBC Boilers, Turbines, Condensers,

Steam & Heat rate, Subsystems of thermal

power plants – Fuel and ash handling,

Draught system, Feed water treatment.

Binary Cycles and Cogeneration systems

RANKINE CYCLE

• Rankine cycle is a thermodynamic cycle on which thesteam turbine works. The line diagram is shown infig.

Thermal Efficiency of Rankine Cycle

P-V Diagram

T-S Diagram

Improvisations of Rankine Cycle

• Rankine cycle efficiency can be improved by using the following three methods.

1. Reheating

2. Regeneration

3. Combined reheating and regeneration

Reheat Rankine Cycle

• The efficiency of the simple Rankine cycle canbe improved by increasing the pressure andtemperature of steam entering into theturbine and keeping the steam as dry aspossible at the end of turbine.

T-S Diagram

Regenerative Rankine Cycle

• The Condensate coming out of the turbine isheated with the help of steam in a reversiblemanner. This type of heating is known asregenerative heating.

Practical single stage regenerative Rankine cycle

T-S Diagram

Reheat-Regenerative Cycle

T-S Diagram

Layout of modern coal power plant

• A steam power plant converts the chemical energy of the fossil fuels (coal, oil, gas) into mechanical / electrical energy.

• Coal based thermal power plant are meant for base load requirements.

• The following two purposes can be served by a steam power plant.

– To produce electric power.– To produce steam for industrial purposes besides

producing electric power.

• The layout of modern steam power plant comprises of four main circuits namely

– Coal and ash circuit

– Air and gas circuit

– Feed water and steam flow circuit

– Cooling water circuit

Advantages

• They respond rapidly to the load variations withoutdifficulty.

• Can be located very conveniently near to loadcentre.

• Transmission cost is reduced.

• Less space is required compared to hydel powerplants

• Cheaper in production and initial cost compared todiesel power stations.

Disadvantages

• Maintenance and operating cost are high.

• Plant construction time is more.

• Very large quantity of water is required.

• Coal handling is a tedious process.

BOILERS

• A boiler is a closed vessel in which the steam isgenerated from water by applying heat.

• A boiler or steam generator is used where asource of steam is needed.

• The boilers are mainly used in mobile steamengines such as– Steam locomotives

– Portable engines

– Steam powered road vehicles

– Industrial installations

– Power stations

Types of Boilers– Fire tube boiler

• If the hot gas is passed through tubes and the water is circulated around tubes, it is called fire tube boiler.

• Examples: Cochran boiler and Locomotive boiler

– Water tube boiler• If the water is circulated through a large number of tubes and the

hot gases pass around the tubes, it is called water tube boiler. Examples: Babcock and Wilcox boiler

– Low pressure and High pressure boiler• Low pressure boiler: Steam pressure range from 3.5 to 10 bar.

Example: Cochran boiler• High pressure boiler: Steam pressure greater than 25 bar and

temperature of 500oC. • Examples: Babcock and Wilcox boiler

ONCE THROUGH BOILER

• In once through boiler if the water is fed to the boiler, it willbe fully converted into dry or superheated steam withoutany water content present in it.

Advantages• Easy control of steam temperature.• Easy to adopt variable pressure operation.• Starting and cooling down of the boilers is fast.• It is smaller in size and weighs less.

FLUIDIZED BED COMBUSTION BOILERS (FBC BOILERS)

• When a gas is passed through a packed bed of finelydivided solid particles, it experiences a pressuredrop across the bed.

• When the velocity of the gas is increased further, ata stage the particles get suspended in the gasstream and the new packed bed becomes afluidized bed. Burning of a fuel in such as state isknown as fluidized bed combustion.

Advantages of FBC boilers

• Size is small hence capital costs are reduced.

• Responds rapidly to changes In load demand.

• Less pollution.

• Combustion temperature can be controlled accurately.

TYPES OF FBC1. Bubbling Fluidized Bed Boilers (BFB)

2. Circulating Fluidized Bed Boilers (CFB)

TURBINES

CONDENSERS

Functions of condenser

Elements of condenser

Classification of condenser

Surface condenser

Down flow surface condenser

Central flow surface condenser

Low level parallel flow

Low level counter flow

Parallel and counter flow

Ejector condenser

Evaporative condenser

SUBSYSTEMS OF THERMAL POWER PLANTS

a) Fuel and Ash Handling

b) Draught system

c) Feedwater treatment

d) Binary cycles and Cogeneration systems

A)COAL HANDLING

OUT PLANT HANDLING transport to Stations by sea/river, rail, road, ropeways, pipelines.

IN PLANT HANDLING– Coal Preparation

– Transfer of coal and related equipments

• Belt Conveyors

• Screw conveyors

• Bucket elevators

• Grab bucket conveyor

– Covered storage and required equpments

– Inplant handling of coal

– Coal weighig measures.

ASH HANDLING

i. Mechanical Handling System

ii. Hydraulic Ash handling System

iii. Pneumatic Ash handling System

iv. Steam Jet System

i.Mechanical handling System

• In this system, ash cooled by water seal falls on the belt conveyor and is carried out continuously to the bunker.The ash is then removed to the dumping site from the ash bunker with the help of trucks

ii)Hydraulic system

iii.Pneumatic Ash Handling System

In this system , ash from the boiler furnace outlet falls into crusher

The ash is then carried by high velocity air or steam to the point of delivery

Air leaving the ash separator is passed through filter to remove dust etc.,

so that the exhauster handles clean air which will protect the blades of the exhauster.

iv) Steam Jet system

c)DRAUGHT SYSTEM

• The purpose of draught is to supply required quantity of air for combustion and remove the burnt products from the system.

• DraughtNatural DraughtArtificial Draught

Forced Draught Induced Draught

Balanced Draught-Forced draught+InducedDraught

FORCED DRAUGHT

• Draught produced by chimney is due to the temperature difference of hot gases in the chimney and cold air outside the chimney.

Induced Draught

• In this blower is located near the base of chimney instead of grate. The air is sucked in the system by reducing the pressure throughtthe system below atmospheric pressure.

BALANCED DRAUGHT

FEED WATER TREATMENT

Feed water

Binary cycle

• It is one type of combined cycles in which usually two working fluids mercury and water are used to improve the overall thermal efficiency of the power plant.

• For getting best performance of vapour power cycle, the working fluid should have the following characteristics

» High enthalpy of vaporization» High critical temperature with a low corresponding

saturation temperature.» Freezing temperature should be below room temperature.» High condenser temperature

• Low specific heat

• Chemically stable

• Good heat transfer characteristics

• The moisture should not appear during expansion