gas turbine plant

POWER PLANT ENGINEERING

Gas turbine plant

Suryakanth L G Asst. Prof Mechanical Dept

NMREC, Hyderabad

History of Gas Turbine…The first gas turbine for power generation became operational in 1939 in Switzerland. It was developed by the company Brown Boveri. They were therefore used to provide power at peak loads. After world war II, gas turbine developed rapidly. It therefore soon became the primary choice for many applications.

GAS TURBINE POWER PLANT

Working principle : Air is compressed(squeezed) to high

pressure by a fan-like device called the compressor.

Then fuel and compressed air are mixed in a combustion chamber and ignited.

Hot gases are given off, which spin the turbine wheels.

Most of the turbine’s power runs the compressor. Part of it drives the generator/machinery.

At present, thermal efficiencies of 45% ,max. temp is about 10000C and at present the maximum gas turbine size limitation is about 30 to 40 MW.

Efficiency is 20 to 30% whereas that of steam power plant is 38 to 48%.Major Applications of Gas Turbine1. Aviation(self contained, light weight don’t require cooling)2. Power Generation3. Oil and Gas industry(cheaper supply of fuel and low installation cost)4. Marine propulsion

Gas turbines burn fuels such as oil, nature gas and pulverized( powdered) coal. Instead of using the heat to produce steam, as in steam turbines, gas turbines use

the hot gases directly to turn the turbine blades. Gas turbines have three main parts:i) Air compressor ii) Combustion chamber

iii) Turbine

SIMPLE GAS TURBINE

A simple gas turbine consists of Turbine A compressor mounted on the same shaft A combustor Auxiliaries such as starring device, lubrication pump, fuel system, oil

system etc

ENERGY FLOW DIAGRAM

The air brings in minute amount of energy Compressor adds considerable amount of energy Fuel carries major input to cycle Sum of fuel & compressed air energy leaves combustor to enter turbine In turbine smallest part of entering energy goes to useful output, largest

part leaves in exhaust.

Advantages of Gas turbine power plants.

Compact design, high speed and high power. Storage of fuel requires less area and handling is easy. The cost of maintenance is less. It is simple in construction. There is no need for boiler,

condenser and other accessories as in the case of steam power plants.

Cheaper fuel such as kerosene , paraffin, benzene and powdered coal can be used which are cheaper than petrol and diesel.

Gas turbine plants can be used in water scarcity areas. Less pollution and less water is required Simple lubrication system. The gas turbine can be driven at a very high speed i.e 40000 rpm.

Disadvantages of gas turbine power plant1. 66% of the power developed is used to drive the

compressor. Therefore the gas turbine unit has a low thermal efficiency.

2. The running speed of gas turbine is in the range of (20,000 to 80,000 rpm) and the operating temperature is as high as 1100 – 12600C. For this reason special metals and alloys have to be used for the various parts of the turbine.

3. High frequency noise from the compressor is objectionable.

4. Starting of the plant is not as simple as diesel plants.5. Compare to other power plant, the overall efficiency of

this plant is quite low.6. In summer, oxygen starvation may occur.

GAS TURBINE POWER PLANTS IN INDIA

TYPES OF GAS TURBINE POWER PLANTS

The gas turbine power plants can be classified mainly intotwo categories. These are :open cycle gas turbine power

plant and closed cycle gas turbine power plant.

1. Open Cycle Gas Turbine Power Plant In this type of plant the atmospheric air is charged into the

combustor through a compressor and the exhaust of the turbine also discharge to the atmosphere.

2. Closed Cycle Gas Turbine Power Plant In this type of power plant, the mass of air is constant or

another suitable gas used as working medium, circulates through the cycle over and over again.

OPEN CYCLE GAS TURBINE POWER PLANT AND ITS CHARACTERISTICS

Gas turbines usually operate on an open cycle. Air at ambient conditions is drawn into the compressor, where its temperature and pressure are raised. The high pressure air proceeds into the combustion chamber, where the fuel is burned at constant pressure. The high-temperature gases then enter the turbine where they expand to atmospheric pressure while producing power output.

Some of the output power is used to drive the compressor.

The exhaust gases leaving the turbine are thrown out (not re-circulated), causing the cycle to be classified as an open cycle.

The ideal cycle that the working fluid undergoes in the closed loop is the Brayton cycle. It is made up of four internally reversible processes:

1-2 Isentropic compression;(No change in entropy)

2-3 Constant-pressure heat addition;

3-4 Isentropic expansion;

4-1 Constant-pressure heat rejection.

The T-s diagrams of an ideal Brayton cycle.

P-v and T-s diagrams of Brayton cycle

Entropy The term entropy was first used by Rudolf clausius to state the

second law of thermodynamics. Entropy is the extensive property of the system and its unit of measurement is J/K.

The measure of a system’s thermal energy per unit temperature that is unavailable for doing useful work. Because work is obtained from ordered molecular motion, the amount of entropy is also a measure of the molecular disorder, or randomness of a system.

The various causes of the increase in entropy of the system are

@ Due to external interaction.

@ Internal changes in the system.

dS = δq/T

Where dS is the change in entropy,

δq is the heat added to the system.

METHODS OF IMPROVEMENT OF THERMAL EFFICIENCY OF OPEN CYCLE GAS TURBINE PLANT

1. Regeneration2. Reheating3. Intercooling

REGENERATION

The exhaust gasses from the turbine carry a large quantity of heat with them since their temperature is far above the ambient temperature.

They can be used to heat air coming from the compressor there by reducing the mass of fuel supplied in the combustion chamber.

Gas turbine with regenerator

T-S DIAGRAM FOR THE UNIT

2’-3: heat flow into compressed air (heat exchanger)

3-4: heat taken in from combustion fuel.

6:Temp of exhaust gases.

Regenerative Cycle has more efficiency than simple cycle only at lower pressure ratios.

Above certain pressure ratio limit, the efficiency of cycle drops since in that case regenerator will cool the compressed air instead of heating it.

THERMAL EFFICIENCY V/S PRESSURE RATIO GRAPH

REHEATING

The output of gas turbine can be improved by expanding the gasses in two stages with a reheater between the two.

The H.P. turbine drives the compressor and the LP turbine provides useful power output.

Gas turbine with reheater


1-2’: Compression

2’-3: C.C (heating)

3’-4’: Turbine(Expansion)

4’-5: Reheater(heating)

5-6’: Turbine(Expansion)

Net Work output increases.

Thermal Efficiency Decreases.


INTERCOOLING

A compressor in a gas turbine cycle utilizes the major percentage of power developed by the gas turbine.

The work required by the compressor can be reduced by compressing the air in two stages and incorporation a intercooler between the two.


1-2’: LP compression 2’-3: Intercooling 3-4’: H.P. compression 4’-5: C.C. Combustion

chamber(heating) 5-6’: T(Turbine) –Expansion

The heat supplied when intercooling is used is greater than with no intercooling.


Work Ratio is increased

Thermal efficiency decreases but it increases at high pressure ratio.

CLOSED CYCLE GAS TURBINE POWER PLANT AND ITS CHARACTERISTICS

The compression and expansion processes remain the same, but the combustion process is replaced by a constant-pressure heat addition process from an external source.

The exhaust process is replaced by a constant-pressure heat rejection process to the ambient air.

T-S DIAGRAM FOR CLOSED LOOP CYCLE TURBINE

1-2 : The air is compressed isentropically from lower pressure p1 to the upper pressure p2, temp rising from T1 to T2.

2-3: Heat flows into the system increasing its volume V2 to V3 and temp T2 to T3, pressure remains constant at p2.

Heat received = mCp(T3-T2)

3-4: The air is expanded isentropically from p2 to p1, and temp T3 to T4, No heat flow occurs.

4-5: Heat is rejected from the system as volume decreases from V4 toV1 and temp from T4 to T1, pressure remains constant p1. Heat rejected= mCp(T4-T1)

MERITS AND DEMERITS OF CLOSED LOOP CYCLE TURBINE OVER OPEN LOOP CYCLE TURBINE

Merits: Higher thermal efficiency Reduced size No contamination Improved heat transmission Lesser Fluid friction No loss in working medium Greater output Inexpensive fuel.

Demerits: Complexity Large amount of cooling

water is required. Dependent System Not economical for moving

vehicles as weight /kW developed is high.

Requires the use of very large air heater.

Gas turbine fuels Important properties:It should have better calorific valueSelf lubrication propertyAvailable at low costIt’s volatility in appreciable valueWell Purified

Various types:

Natural gas (for industrial applications)High grade gasoline (for aircraft applications)Kerosene (distillate oils)Blast furnace gas of steel millsResidual oils as residue of crude oilHeavy oils

STARTING OF A GAS TURBINE PLANT The starting of a gas turbine plant is quite simple and

quick. Oil pumps & intercooler water pumps are run up The starting motor is switched on. The fuel line is opened to the combustion chamber

and the charge is ignited. Fuel charge to the reheater is ignited. As both the shaft gain speed, the starting motor is

automatically switched off. Speed and voltage is adjusted Load can be built up on the alternator by governor

gear control.

COMBINATION OF GAS TURBINE CYCLES

Gas Turbine and Steam Power Plants: The combination of gas-turbine-steam cycle aims

at utilizing the heat of exhaust gases from the gas turbine thus, improve the overall plant efficiency.

The popular designs are: 1. Heating feed water with exhaust gases. 2. Employing the gases from a supercharged

boiler to expand in the gas turbine. 3. Employing the gasses as combustion air in the

steam boiler.

HEATING FEED WATER WITH EXHAUST GASES.

The output heat of the gas turbine flue gas is utilized to generate steam by passing it through a heat recovery steam generator (HRSG), so it can be used as input heat to the steam turbine power plant.

This combination of two power generation cycles enhances the efficiency of the plant.

While the electrical efficiency of a simple cycle plant power plant without waste heat utilization typically ranges between 25% and 40%, a CCPP can achieve electrical efficiencies of 60% and more.

Gas turbine gases used for feed water heating.

EMPLOYING THE GASES FROM A SUPERCHARGED BOILER TO EXPAND IN THE GAS TURBINE.

The boiler furnace works under a pressure of about 5 bars and the gases are expanded in the gas turbine, the exhaust being used to heat feed water before being discharged through the stack.

The heat transfer rate is very high as compared to conventional boiler due to high pressure of gases, and smaller size of steam generator is needed.

No need of forced draught fans as the gases in furnace are already under pressure.

Overall improvement in heat rate is 7%.

Supercharged Boiler

EMPLOYING THE GASES AS COMBUSTION AIR IN THE STEAM BOILER.

Exhaust gases are used as preheated air for combustion in the boiler, results in5% improvement in heat rate.

The boiler is fed with supplementary fuel and air, and is made larger than conventional furnace.

Exhaust heat recovery steam gas plant

COMBINED GAS TURBINE AND DIESEL POWER PLANTS

The performance of diesel engine can be improved by combining it with exhaust driven gas turbine.

Three combinations:1. Turbo charging2. Gas Generator3. Compound engine

TURBO CHARGING

This method is known as supercharging.

The exhaust of the diesel engine is expanded in the gas turbine and the work output of the gas turbine is utilized to run a compressor which supplies the pressurized air to the diesel engine to increase its output. Turbo charging

GAS GENERATOR

The compressor which supplies the compressed air to the diesel engine is not driven from gas turbine but from the diesel engine through some suitable device.

The output of diesel engine is consumed in driving the air compressor and gas turbine supplies the power.

Gas generator

COMPOUND ENGINE

The air compressor is driven from both diesel engine and gas turbine through a suitable gearing and power output is taken from the diesel engine shaft.

Compound engine

gas turbine plant

Engineering