ppe unit 01 combined power plants and comparison v+

Power Plant Engineering - Unit I -

Combined Power plants and their comparisons

Page 1 of 11

The maximum steam temperature in a power cycle exceeds 600*C but

the pulverized coal furnace temperature is about 1300*C. So there is a

lot of energy wasted in the power plant. To increase the efficiency and

reduce the fuel, the combined power cycles are introduced by'

superposing a high temperature power plant as a topping unit to the

steam plant.

The combined plants may be of the following types:

1. Gas turbine-steam turbine power plant

2. Thermionic-steam power plant

3. Thermo electric-steam power plant

4. M.H.D- steam power plant. .

5. Nuclear-steam combined power plant

6. MHD-Gas turbine power plant.

1.Gas turbine-steam turbine power plant - It is the combination of

simple gas turbine power plant and steam power plant. In the gas turbine

unit, the heat from the exhausted gas is recovered by a heat recovery

boiler and the steam is produced by using this heat in the boiler and it

goes to the steam turbine, Here, two generators are used to produce the

power. One generator is connected to the gas turbine unit and the other

generator is connected to the steam turbine unit. The efficiency of this

combined unit is about 45%. '

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Page 2 of 11

2. Thermionic-steam power plant

Principles of thermionic-power generation

Metals have free electrons. A metal electrode which is called an emitter

is heated until it is hot enough to release electrons from its surface. This

electron crosses a small gap and is stored on a cooled metal electrode

called collector.

To reduce the energy loss, the gap is maintained with vacuum space.

The electrons enter the collector and return through an external load to

the emitter by producing electrical power. The thermionic-generator

transforms heat directly into power. The below fig shows a combined

MHD generator with a thermionic generator.





Page 3 of 11

In this, the waste heat from and MHD generator is (about 1900*C) used

to heat 'the thermionic device with the addition of a high temperature

emitters like tungsten or rhenium.

3. Thermo electric-steam power plant

principles of thermoelectric-steam power generation: - It is similar to

a thermocouple with the difference that the thermo- elements are made

up of semi conductors P and N, heat is given to the hot junction and the

heat is removed from the cold junction.





Page 4 of 11

The thermocouple principle is when the junctions of two dissimilar

wires A and B are maintained at two different temperatures, a potential

difference is developed called Seeback effect

Here applying the first law of thermodynamics to the upper plate as

control volume, T1 - T0 will generate a Seeback voltage. Both junctions

are made up of copper elements.

To obtain large outputs, the thermo electric generator can be

incorporated in the fuel elements of nuclear reactors.

The waste heat of gas turbines and diesel engines can also be utilized for

thermo electric power generation. By operating with a higher

temperature range, the over all efficiency would be higher.

4. M.H.D- steam power plant. .

The combined MHD steam power plant is either in an open cycle or in a

closed cycle. If the gas is entering the MHD duct at about 3000*C, it

could be expanded to ambient temperature and the η would have reached

90%. But MHD power output is restricted to temperature fall of 2000*C

because of very low electrical conductivity. To, the MHD exhausted at

about 2000*C is utilized in raising steam to drive turbine and generates

power.

Open Cycle MBD - In open cycle MHD, the products of combustion

with preheated air are seeded with 1 % potassium and it is entered in the

MHD duct at about 2500-3000K where some part of internal energy is

converted to electricity. The exhaust from the MHD duct is used to

preheat the steam and air.

Closed Cycle MI-ID - In closed cycle, helium (or argon) gas is seeded

with cesium and it is heated in a nuclear reactor and then it is passed into

the MHD duct. After that it is passed into the steam generating system.

The duct wall material used in MHD generator is strontium zirconate,

magnesium oxide, and hafnium. The Tungsten or Carbon electrodes are

used.

5. Nuclear-steam combined power plant - 4. Nuclear-steam

Combined Power Plant - In this combined cycle, heated helium gas is

used to run the helium turbine. First, helium gas is compressed in the

compressor to raise its pressure and temperature. Then it is preheated in

the Regenerators. This preheated helium is again heated in high

temperature gas cooled reactor (HTGR). After this, this high temperature





Page 5 of 11

helium gas is expanded in the turbine to produce electricity in the

generator. Both the compression and expansion processes are carried out

isentropically. Then the exhausted helium gas is sent to the regenerator.

Here it is again heated. This helium gas is passed to the feed water to

make steam.

The feed water coming in to the feed is heated by heat released by

helium gas. The steam is collected in steam boiler. Here supplementary

heat supply is given to the steam-water mixture. The super heated steam

is produced due to this supplementary heat. Then this steam is expanded

in steam turbine isentropically, After expansion, the steam is condensed

in steam condenser. The condensed steam will become as water now. It

is passed to the feed pump to raise its pressure. Finally, it is sent to the





Page 6 of 11

feed water heater. Both the cycles are closed cycles, Here, topping cycle

is helium gas turbine cycle and bottoming cycle is Rankine cycle. So,

the overall efficiency this combined cycle is dependent on the individual

efficiency of topping and bottoming cycles.The only disadvantage is, the

thermal efficiency of this cycle is less because of its high temperature

gas cooled reactor.

6. MHD-Gas turbine power plant. - here One MHD and one gas

turbine power plant are connected in series which means heat rejected by

MHD plant is fully utilized to heat the air inside the combustion

chamber in gas turbine power plant. by this, heat action by the cycle is

reduced drastically so that the efficiency of the combined cycle will

increase. The overall efficiency can be off 70 - 72%.

Combined Cycle Gas Turbine Power Plants -

The maximum steam temperature in a power cycle exceeds 600*C, but

the pulverized coal furnace temperature is about 1300*C. So there is a

lot of energy wasted in the power plant. To increase the efficiency and

reduce the fuel, the combined power cycles are introduced by





Page 7 of 11

superposing a high temperature power plant as a topping unit to the low

temperature as a bottoming unit.

The combined plants may be of the following types:

1. Gas turbine-steam turbine plant .

2. MHD-Gas turbine power plant

3. Gas turbine and combined cycle cogeneration plant.

1. Gas turbine-steam turbine plant: - It is the combination of simple

gas turbine power plant and steam power plant, The two cycles of Gas

turbine Steam turbine power plant are connected in series, the topping

cycle on Brayton cycle and the bottoming one operating on Rankine

In the gas turbine unit, the heat from the exhausted gas turbine is

recovered by a heat recovery boiler and the steam is produced by using





Page 8 of 11

this heat in the boiler and it goes to the steam turbine. Here two

generators are used to produce the power.

One generator is connected to the gas turbine unit and the other

generator is connected to the steam turbine unit. So, the overall heat

rejection will be reduced. As a result of this, the overall efficiency will

increase. It is given by η = η1 + η2 - η1η2

Where :η1 and η2 are the thermal efficiencies of the Brayton cycle and

the steam cycle respectively. The achievable overall efficiency η may be

in the range of 40 - 45%.

2. MHD-Gas Turbine Power Plant - here One MHD and one gas

turbine power plant are connected in series which means heat rejected by

MHD plant is fully utilized to heat the air inside the combustion

chamber in gas turbine power plant. by this, heat action by the cycle is

reduced drastically so that the efficiency of the combined cycle will

increase. The overall efficiency can be off 70 - 72%.





Page 9 of 11

3. Gas turbine and combined cycle cogeneration plant - The power

plant which produces both thermal and electrical energy is called a

cogeneration plant. By this, the conversion efficiency of 33% for

electricity and 53% for thermal output are captured. The remaining 14%

goes as waste heat, In usual cycles, the air after expansion in the turbine

is condensed in condenser. The steam is produced by using heat rejected

by air in the condenser.

The device in which steam is produced is known as heat recovery

generator (HRSG). This steam is for further works. The cogeneration

cycles are mainly used in process heating, absorption cooling and space

heating operations.

Advantages of Combined Cycles 1. Low environmental effect.

2. Small amount of water required.

3. Low investment cost.

4. The efficiency of combined cycle plant is more than the open cycle

power plant.

5. When compared with ordinary steam plants, these plants produce less

smoke,

6. simplicity of operation.

7. Great operating flexibility.





Page 10 of 11

8. It gives high ratio of power output to fuel.

Comparison of various Power Plants

1. Thermal power plant Vs Hydro-plant:

Sl.No Thermal power plant Hydro power plant

1 Initial cost is low. Initial cost is high.

2 Located near to load center.. Not like that

3 Transmission losses are less. Transmission losses are high

4 Power production is not

dependent on nature's mercy.

It is only dependent on nature's

mercy.

5 Construction time is less. Initial construction requires long

time.

6 Power generation cost is high Power generation cost is less

7 Air pollution is more. No air pollution.

8 Fuel transportation is difficult. No fuel transportation.

9 Life of the plant is less. Life of the plant is high.

10 Efficiency of the plant is less Efficiency of the plant is. high.

11 Not suitable for peak load

plant.

It is suitable.

2. Steam power plant Vs Nuclear power plant:

Sl.No Steam power plant Nuclear power plant

1 It is not suitable where water. and

coal resources are not available.

No such constraints.

2 Fuel storage space is required. No fuel storage space.

3 Requirement of workmen is very

high..

Can be managed with very

less number of workmen

4 Capital cost is high.

Capital cost is less when

size of plant is increased.

5 No radioactive materials Radioactive wastes.

6 Space requirement is high. Space requirement is less.

7 Steam power plants are affected by

weather conditions.

It will not be affected by

adverse weather conditions

8 It requires large quantity of ' water. It does not require large

quantity of water.

9 Large quantity of fuel is required.

Less quantity of fuel

required.





Page 11 of 11

10 Maintenance cost is less. Maintenance cost is high.

11 Operating cost is high. Operating cost in less. .

12 Steam Power plant efficiency

= 20 to 30%.

Nuclear Power plant

efficiency = 30 to 32%.'

3. Diesel power plant Vs Gas turbine power plant

Sl.No Diesel power plant Gas turbine power plant

1 The efficiency of the diesel

power plant is about 35 to

25%.

The efficiency of the simple gas

turbine power plant is 20 to 42%.

2 Only particular fuel should

be used.

The fuel of different qualities can

be used in this plant.

3 The work output is high. The network output is less.

4 It is not require special

metals.

The unit is operated at high

temperature and pressure so special

metals are required.

5 Cost of the plant is less. Gas turbine cost is high.

6 Limited plant capacity.

Capacity of the plant is higher than

diesel power plant.

7 Not suitable for continuous

over loads.

It can be work on over loads.

8 Lubrication cost is high. lubrication cost is less.

9 No ash-handling problem. It has ash handling.

10 Life of the plant is less.

Life of the plant is high when

compared to diesel power plant.

Selection of Power Plant - The following factors should be considered

for the selection of power plant:

1 . Depending on the load requirement ; 2. Availability of fuel and water

3. Availability of fuel storage facility; 4. Transportation facility.

5. Availability of land ; 6. Environmental conditions

7. Efficiency of the plant ; 8. Capacity of the plant.

9. Distance from the load center., 10. Life of the plant.

l l . Availability of time period for power production.; 12. Cost of the

fuel used; 13. Nature of losses.; 14. Requirements of labours.

15. Depending on the noise of vibration.



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ppe unit 01 combined power plants and comparison v+