Chapter 71 Gas Turbine

Gas turbine

• A gas turbine is basically a constant flow engine. The fuel is burned and serves to add heat to compressed air which then expands through the blades of a turbine.

• Use the same impulse and reaction principle as in steam turbine

Gas turbine• Use in aircraft locomotive and stationary power generation

• Only 1/3 the power produced is useful for work, the rest used to keep itself (driving the compressor) running.

• Gas turbines have been used in small sizes to produce power for portable and standby generators, fire pumps, and compressors.

• Large units are used for electricity generation in stationary plants.

The Single open cycle Turbine Unit Without Heat Exchanger

Basic gas turbine

Principle of operations Open cycle

• The simple gas turbine operates by drawing air into a compressor, compressing it and then discharging the air into a combustion chamber.

• Fuel is added and burned, heating the air further.

• This hot air is then expanded through a turbine, providing available power. Air is then exhausted to atmosphere (open cycle).

• Not self starting; must use starting motor to rotate to 20 to 30% before fuel is turned on

Close cycle • The air used to drive the turbine is in a

close cycle. It is separated from the combustion air.

• Exhaust from turbine goes to the intake of the compressor. It absorbs heat from the furnace and expanded in the turbine blade.

Close cycle gas turbine

• The turbine and compressor are on the same shaft so that the compressor obtains its drive from the turbine.

• About two-thirds of this power is used to drive the compressor, but the remainder is available to drive a load or provide thrust.

Efficiency

• The efficiency of such a machine depends primarily upon the temperature to which the air can be raised before entering the turbine.

• The output power, or power rating, of the machine, depends upon the mass of hot gas flowing through the gas turbine in a given time per period.

• A gas turbine output increases when using air of maximum density.

Common terms

• "simple" - it has no heat exchanger or regenerators.

• "open-cycle" - the air used to drive the turbine is drawn from the atmosphere and returned to the atmosphere after use.

• "closed-cycle" would use the same air again and again.

• "single shaft" - is self-explanatory, there is only one shaft. The gas turbine and the compressor are coupled together mechanically on this shaft.

Gas Turbine Characteristics

• Gas turbines have certain characteristics which distinguish them from other types of prime movers, some of these have proved to be advantages and some disadvantages.

High Power to Weight Ratio

• Compared to a steam plant, a gas turbine is much smaller and lighter for the same power. This is a considerable advantage since the building housing a gas turbine can be much smaller and needs only light foundations. Construction cost is lower.

Low Installed Cost

• The capital cost of a gas turbine is lower per watt of output than a steam plant.

Low Maintenance and Operating Cost

• The absence of reciprocating motion minimizes wear of moving parts. Lubricating oil consumption is low. Because of gas turbine simplicity, many auxiliaries required in a steam plant are eliminated.

Minimum Cooling Water

• Gas turbines require a minimum of cooling water and can be operated economically with a closed circulating water system using air cooled heat exchangers. This makes gas turbines ideal for use wherever water is scarce or unobtainable.

Rapid Start-Up and Loading

• A gas turbine can be started almost instantaneously (Power up in 20 minutes). It does not require slow and elaborate preparations as does a steam turbine supplied by a steam generator.

• The gas turbine is ideal for standby duties, such as for fire pumps and emergency electricity generators where instant starting is essential, or where remote operation by telemetry is required.

Disadvantage

• Low Thermal Efficiency• This is the major drawback of the gas

turbine. The thermal efficiency of the simple, open-cycle gas turbine is considerably below that of a steam plant, a diesel engine or a gasoline engine.

• Auxiliary equipment must be fitted, such as regenerators, in order to make the gas turbine competitive.

High Noise Level

• Unless special precautions are taken to baffle the exhaust, the gas turbine is a noisy machine.

Pollution

• NOx – run at high temperature thus produce NOx

• (NOx is produced around 1500 -1600°C)

Typical temperatures which exist in a gas turbine.

• Simple, Open-Cycle, Single-Shaft Gas Turbine

Objective #3 Regeneration• The purpose of the regenerator on the gas turbine is to

improve the cycle efficiency by recovering some of the heat which would otherwise pass to waste with the exhaust gases.

• The regenerator is placed in the air flow after the compressor and before the combustion chamber.

• The compressor works most effectively with cold air. The heat recovered from the exhaust gas reduces the fuel-quantity required to produce the same load.

Regenerative Gas Turbine

Combined Steam - Gas turbine Plants

• Gas turbines can be used in conjunction with steam boilers in various ways.

• Co-generation

• Waste heat boiler

• Combined cycle

Cogeneration• pressurize the boiler furnace with the air leaving the

compressor and then pass the hot gases from the boiler through a gas turbine to drive the compressor and produce mechanical drive for electricity generation.

• In this type of arrangement the gas turbine contributes to the total plant capacity and the overall plant economy is improved.

• The high furnace pressure at which a boiler of this type operates permits a reduction in boiler size.

Cogeneration plant

Gas Turbine Combined with a Steam Plant

• The high energy content of the gas turbine's exhaust may be used in several ways to increase the overall efficiency of a power plant.

• The most common example is a waste heat boiler where the turbine exhaust gases are used to heat water to supply steam for some other purpose.

Combined plant

• Another method uses the exhaust gases, which are at a temperature of about 480ºC and contain considerable excess oxygen, as preheated air for a boiler furnace.

• Fuel may be used in conjunction with the hot air to further burn the remaining excess oxygen to produce steam.

• By employing a combined cycle, the thermal efficiency can be raised to 60% and the design retains several advantages over a straight steam turbine plant.

• A combined cycle plant can be started up and shut down rapidly.

• Several gas turbines can be used with one steam turbine to produce more power, and higher efficiencies can be maintained across a wider range of loads (for example, 25% to 100% range).

Combined cycle advantages• Faster start-up: full load can be obtained

in 30 minutes.

• Lower cooling water requirements, by as much as 67%.

• Lower atmospheric pollution from oxides of nitrogen (NOX).

• Lower cost of investment per kilowatt installed.

Objective 4 Gas turbine operation

• Turbine capacity is depended on the density of air.

• Any change in atmospheric conditions that affect the temperature and pressure of air will affect the performance of the gas turbine.

• The mass of inlet air flowing through the compressor and, after combustion, through the turbine, is highly dependent on altitude and air temperature.

• Plant capacity will be higher in cold weather as the density of the air will increase, allowing more air into the turbine, increasing mass flow.

• Changes in atmospheric pressure will similarly affect turbine output.

• A turbine operating at sea level will have higher density air (more mass) at its inlet than one operating in higher altitude.

Gas turbine starting

• Manual – small plant

• Semi-automatic – operator starts auxiliary and automatic control starts the turbine

Starting

• Automatic – program starting

• engine starts to rotate the system; when the speed is ~30% of max rpm, the combustor will ignite

• when the rpm is 50% of max, started engine shut off

• if the system does not reach operating speed in a certain time, abort start up.

Load control

• Governor senses load-shaft speed to adjust fuel to combustor to control the shaft speed.

• Small unit uses fly ball governor; big units employ more sophisticated governors.

• Over speed trip – same as in steam turbine.

Auxiliary systems

• Gas turbine is not self starting. It requires electric or engine driven starter. Speed has to be 30% of maximum before it can be self-support.

• Lubrication – could use pump driven by the gas turbine shaft. Auxiliary oil supply is needed for cooling the bearings in case of failure.

Blade cleaning

• Blade cleaning system is used to get rid of foreign material deposited on blades. Medium used could be detergent.

• In extreme case, crushed walnut shells mixed with air could be used!

Fire protection• Pre-charged halon or CO2 cylinders are

located in various parts of the system and can discharge automatically.

• halon breaks down into corrosive and toxic byproducts when it is exposed to surface temperatures above 482°C (900°F).

• Water mist could also be used to for protection

Turbine aging

• Loss of 0.5% per 1000 hours of service or 4.4% per year is expected.

Environmental Considerations• The exhaust gases of all engines contain some

constituents that are harmful in the atmosphere. If these constituents are discharged in large enough quantities for a long enough period of time then they will violate Air Quality Standards.

• Generally, gas turbines produce trace amounts of carbon monoxide and sulphur dioxide, but they can be significant generators of oxides of nitrogen (NOX) and waste heat.

• The operator should maintain proper combustion in the turbine and use any equipment required to meet Air Quality Standards.

Environmental Considerations 2

• Noise pollution by the gas turbine can be a problem, but it may be overcome or reduced by acoustic insulation and exhaust silencers.

• Operators should pay special attention to hearing protection regulations when entering buildings housing gas turbines in operation.