Steam Turbine

American International University-Bangladesh

(AIUB)

Steam Turbine Power Station

1

Introduction

A steam power plant continuously converts the energystored in fossil fuels (coal, oil, natural gas) or fissile fuels(uranium, thorium) into shaft work and ultimately intoelectricity. The working fluid is water which is sometimesin the liquid phase and sometimes in the vapour phaseduring its cycle of operations. In this type of plant, theproducts of combustion do not form the working mediumfor driving the turbine. These are utilized to produce steamwhich is expanded in the turbines.

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Advantages of Steam Power Plants

The advantages of steam power plants are as follows:

Operating costs are low in comparison to the gas turbine power plants.Efficiency is better.Low graded fuel can be used.Capacity of such plants is high.Initial capital cost is low in comparison to the hydro-electric power plants and nuclear power plants.It can be used efficiently as base load power plant.

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Disadvantages of Steam Power Plants

The followings are disadvantages of steam power plants:

Handling and storage of fuels and disposal of ash create a big problem in steam plants.It requires a huge area (about 250 acres of land is needed for a 2000 MW coal fired steam plant).The time required to start a steam plant from idle is high causing a higher standby losses.It is difficult to maintain the cleanliness of the plant mainly because of the fuel and ash handling problem.

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Disadvantages of Steam Power Plants

Electrostatic precipitator (which removes dust particles from gases) is needed to reduce the stack emissions to an acceptable level.In case of coal fired steam plant, the cost of the pulverized plant may be introduced.A large amount of fresh water will be required.Sometimes water treatment plant is used for the purification of water which introduces an extra investment.

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Site Selection of Steam Power Plants

For steam stations, the choice of plant location is governed by the following considerations:

Transmission of energy:Fuel delivery:Water supply:Realty value and taxes:Disposal of ash:Pollution and noise

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Water, Steam and Flue Gas Flow Diagram of a Steam

Power Plant

FLUE

GAS

FLUE

GAS

FURNACE

FLUE

GAS

FEED

WATER

GRATE

AIR

PREHEATER

EC

ON

OM

ISE

R

SUPERHEATER

BOILER

STEAM

TURBINE

REHEATER

CONDENSER

FEED WATER

HEATER

MAKEUP

WATER TANK

COOLING

TOWER OR

RIVER

FUEL

FE

ED

WA

TE

R

AIR FROM

ATMOSPHERE

BLEEDING

OF STEAM

ALTERNATOR

STEAM

CONDENSATE

PUMP

FEED

WATER

FEED WATER

PUMP

CONDENSATE

STEAM

TURBINE

FLUE

GAS

FLUE

GAS

FURNACE

FLUE

GAS

FEED

WATER

GRATE

AIR

PREHEATER

AIR

PREHEATER

EC

ON

OM

ISE

RE

CO

NO

MIS

ER

SUPERHEATER

BOILERBOILER

STEAM

TURBINE

REHEATERREHEATER

CONDENSER

FEED WATER

HEATER

FEED WATER

HEATER

MAKEUP

WATER TANK

MAKEUP

WATER TANK

COOLING

TOWER OR

RIVER

FUEL

FE

ED

WA

TE

R

AIR FROM

ATMOSPHERE

BLEEDING

OF STEAM

ALTERNATOR

STEAM

CONDENSATE

PUMP

FEED

WATER

FEED WATER

PUMP

FEED WATER

PUMP

CONDENSATE

STEAM

TURBINE

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Main and Auxiliary Equipments in a Steam Power

Plant

The main equipments in a thermal (steam) power plant consist of-

Boiler Steam turbine Alternator

To achieve efficient conversion of heat energy into electrical energy, a variety of auxiliary equipments are needed. So, the main and auxiliary equipments in a thermal plant are:

Coal handling plant Pulverizing plant Draft fans Boiler Ash handling plant Turbine Condenser Cooling towers and ponds Feed water heater Economizer Superheater and reheater Air preheater Alternator with exciter Protection and control equipments Instrumentation

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Coal Handling Plant:

A thermal plant burns enormous amount of coal. The function of coal handling plant is automatic feeding of coal to the boiler furnace. A grate (A frame of iron bars to hold a fire) at the bottom of the furnace holds the fuel bed.

Pulverizing Plant:In modern steam power plant, coal is pulverized i.e.ground to dust like size. Pulverization is a means ofexposing large surface area to the action of oxygen andconsequently helping the combustion.

Pulverization: The act of grinding to a powder or dust.

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Coal handling plant in Belchatow

Power Station

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Advantages of using pulverised coal:

The rate of combustion can be controlled and changed quickly to meet the varying load.The percentage of excess air required is low.Automatic combustion control can be used.Preheated air can be used successfully.Low graded coals can be used.The boiler can be started from the cold condition very rapidly.The pulverising equipment is outside the furnace.Therefore, it can be repaired without cooling down the unit.

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Disadvantages of using pulverised coal:

Investment cost of plant is increased.Explosion hazards exist. Therefore, skilled personnel are required.Auxiliary power consumption of the plant is increased.A lot of extra equipments, mills, burners etc. are needed.The lower rows of boiler tubes get deposited with slag. Removal of this slag requires special extra equipment.

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Draft System:

The circulation of air is caused by a difference in pressure known as draft. Thus draft is the differential in pressure between two points i.e. atmosphere and inside the boiler. A differential in draft is needed to cause flow of gases through the boiler setting. A draft may be natural or mechanical.A natural draft is provided by the chimney or stack.In a mechanical draft system, the movement of air is due to the fan. A mechanical draft system may consist of forced draft or induced draft or both.

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Draft System:

GRATE

FURNACE

BOILER

FORCED

DRAFT FAN

EC

ON

OM

ISE

R

AIR

PR

EH

EA

TE

R

PR

EC

IPIT

AT

OR

IND

UC

ED

DR

AF

T

CHIMNEY

GRATE

FURNACE

BOILER

FORCED

DRAFT FAN

EC

ON

OM

ISE

R

AIR

PR

EH

EA

TE

R

PR

EC

IPIT

AT

OR

IND

UC

ED

DR

AF

T

CHIMNEY

Figure: Flue Gas Flow Diagram of a Thermal Power Plant

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Boiler:

A boiler is a closed vessel in which water, under pressure, is converted into steam. It is one of the major components of a thermal power plant. The heat is transferred to the boiler by all the three modes of heat transfer i.e. conduction, convection (The transfer of heat through a fluid (liquid or gas) caused by molecular motion) and radiation (Energy that is radiated or transmitted in the form of rays or waves or particles). Boilers are generally classified as fire tube boiler and water tube boiler.

Superheater:The function of superheater is to remove the last traces of moisture from the saturated steam leaving the boiler tube and also raises the temperature of the steam. The heat of combustion gases from the furnace is used for superheating.

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Reheater:The function of reheater is to superheat the partly expanded steam from the turbine. This ensures that steam remains dry through the last stage of turbine and also increases the thermal efficiency.

Steam Turbine:A steam turbine converts the heat energy of steam into mechanical energy and drives the generator. It uses the principle that the steam when issuing from a small opening attains a high velocity. The attained velocity during expansion depends on the initial and final heat content of the steam. The difference between initial and final heat content represents the heat energy converted into kinetic energy. They are of two types, impulse turbine and reaction turbine.

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Boiler

18

Boiler

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Ash Handling Plant:

Ash is produced due to combustion of coal. Of this, about 25% is furnace bottom ash and remaining 75% is fly ash or pulverised fuel ash (PFA) or dust ash.

A sluicing system is generally used for removal of furnace bottom ash. The ash falls into a dry hopper which is fitted with access doors and high pressure water jet.

A pneumatic (Of or relating to or using air) system is usually employed for disposal of fly ash. In this system, a high velocity air stream causes the ash to flow into separators where the ash is collected in hoppers (Funnel-shaped container that is used to put or keep things in). The dust air is filtered and exhausted to atmosphere through the exhaust fan.

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Condenser:

The condenser does the job of condensing the steam exhausted from the turbine. Thus it helps in maintaining low pressure at the exhaust, thereby permitting expansion of steam in the turbine to a very low pressure. The exhaust steam is condensed and used as feed water for the boiler.

Cooling Towers and Ponds:A condenser needs huge quantity of water to condense the steam. Cooling towers and ponds are required where river is not available near the thermal power plant for cooling.

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Condenser

22

Feed Water Heater:

It is necessary to heat water before feeding it to the boiler. Normally steam is bleeded from the main turbine for heating the feed water. It is necessary for the following reasons:

It improves overall plant efficiency.Removes dissolved oxygen and CO2 from feed water.Thermal stresses due to cold water entering the boiler drum are avoided.Quantity of steam produced by the boiler is increased.

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Feed Water Heater

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Economiser:

Flue gases coming out of theboiler carry lot of heat. Aneconomizer extracts a part of thisheat from the flue gases anduses it for heating feed water. Inan economizer, large numbers ofsmall diameter thin walled tubes areplaced between two headers. Feedwater enters the tubes throughone header and leaves throughother, thus transmits the heat tothe feed water.

FLUE GAS

FLUE GAS

FEED WATER

INLET

FEED WATER

OUTLET

FLUE GAS

FLUE GAS

FEED WATER

INLET

FEED WATER

OUTLET

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Air Preheater:

After the flue gas leave the economiser, some further heat can be extracted from them and used to heat the incoming air for combustion. Cooling of flue gases by 20C raises the plant efficiency by about 1%.

Auxiliary Supply:

The auxiliaries in a steam plant can be divided into two categories: urgent auxiliaries and service auxiliaries. The urgent auxiliaries are those which are associated with the running of a unit and whose loss would cause an immediate reduction of the unit output. The service auxiliaries are those whose loss would not affect the output until after a considerable time.

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Typical properties of selected fuels

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CV as normally expressedContaminants %

Fuel Gross Net Sulphur Water Ash

Steam coal30.6 MJ/kg 29.7 MJ/kg 1.2 10.0 8.0

Wood waste 15.8 MJ/kg 14.4 MJ/kg0.4 15 Trace

Heavy fuel oil 41.2 MJ/litre38.9 MJ/litre 2.0 0.3 0.04

Gas-oil38.3 MJ/litre 36.0 MJ/litre

0.15 0.05 0.01

Natural gas38.0 MJ/cubic

metre

34.2 MJ/cubic

metre - Trace -

Landfill gas20.0 MJ/cubic

metre

18.0 MJ/cubic

metre Trace Trace -

Mine gas21.0 MJ/cubic

metre

18.9 MJ/cubic

metre Trace 5.0 -

Efficiencies in a Steam Power Plant

1 2

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FURNACE

(SOURCE)SINK

EXHAUST

GAS

FUEL AIR

Q1 Q2

P

G

BRAKE OUTPUT

GROSS POWER (MWe)

NET POWER (MWe)

POWER CONSUMED

TO DRIVE

AUXILIARIES

T

P

B C

11 22

3344

FURNACE

(SOURCE)SINK

EXHAUST

GAS

FUEL AIR

Q1 Q2

P

G

BRAKE OUTPUT

GROSS POWER (MWe)

NET POWER (MWe)

POWER CONSUMED

TO DRIVE

AUXILIARIES

T

P

B C

T

P

B C

Figure: A power plant converts energy of fuel into electricity

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Note:

A. 1 kWh (kilowatt-hour) = 3.6 MJ = 860 kcal (kilogram calories)

B. 1 megajoule per second [MJ/s] = 1 MW

C. The SI unit of calorific value is J/kg.


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Automation

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Documents

Steam Turbine