Gas Turbines Explained (Combustion Turbine)

What is a gas turbine?

A gas turbine is a type of internal combustion engine that converts the chemical energy of fuel into mechanical power. It consists of three main components, these are the compressor, combustor, and turbine. The applications of gas turbines are numerous because of their efficiency, cost, and reliability. Gas turbines are also referred to as combustion turbines (they are the same thing). A gas turbine mounted to an aircraft is referred to as a jet engine.

Gas Turbine

Interesting note: Gas turbines are referred to as gas turbines because they operate using hot gases, not because they burn natural gas! It is possible for gas turbines to burn over 30 different types of fuel, but natural gas is the most common.

Basic Gas Turbine Working Principle

The basic process of a gas turbine involves air being drawn in through an air inlet and passing through the compressor where it is pressurised (pressure and temperature increased). The compressed air then passes through the combustor where fuel is injected and ignites with assistance from an ignition system. As heat is released during combustion, this causes the gases to expand rapidly which results in high velocity hot exhaust gases exiting the combustor. The hot gases pass through a turbine, which causes it to rotate. This rotary action produces mechanical work which can be connected to various assemblies such as drive shafts or propeller shafts etc. to perform desired tasks. In addition to providing useful power output, some turbines are also designed for other purposes like powering aircraft engines.

See the next section for a more detailed version concerning how a combustion turbine works.

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How Gas Turbines Work

A gas turbine is an internal combustion engine that uses a continuous stream of gases to produce mechanical power (gas turbines are also known as 'steady flow' combustion engines due to the steady flow of gases they require and produce). It works by compressing air, burning fuel inside of a combustion space (combustor), and then using the hot exhaust gases from the combustion process to rotate a turbine which produces mechanical power (kinetic energy).

The main parts of a gas turbine are the compressor, combustor, and turbine. It is possible to divide a gas turbine roughly into a cold section (air inlet and compressor) and hot section (combustor and turbine). As with other internal combustion engines, gas turbines use the compression, ignition, power, exhaust, process; this process is also referred to as suck, squeeze, bang, blow!

Gas Turbine Parts

Interesting note: Internal combustion engines are also referred to as IC engines.

Compression

Air is drawn into the turbine using a multistage axial fan; this type of fan uses multiple fan blades installed in rows and arranged in series. Each row of fan blades is referred to as a pressure stage because the pressure of the air increases as it passes over each row. The section of the gas turbine housing the multistage fan is referred to as the compressor, because the air pressure increases as it passes through each pressure stage. Aeroderivative gas turbines may have pressure ratios up to 30:1, whilst heavy frame gas turbines have pressure ratios of up to 18:1. It is possible to control how much fuel will be burnt during combustion by regulating the air pressure supplied and consequently how much oxygen is available for combustion.

Due to the large amount of power required to drive the compressor, gas turbines have a typical efficiency of between 30-35% (max approx. 40%) although this can be increased up to 60% if the waste heat is recaptured (combined cycle power plants are a good example of this).

Gas Turbine Efficiency Based Upon Parallel or Series Operation

Ignition

Compressed air from the compressor enters the combustor (combustion chambers) where fuel is injected and mixed with it.  The fuel is ignited by an igniter, which is similar in design to a spark plug. Once ignited, the air/fuel mixture combusts and there is a large increase in temperature. Discharge temperatures from the combustion area vary depending upon design, but temperatures up to 1,600°C (2,900°F) have been achieved.

Interesting note: A combustor is also referred to as a burner, combustion chamber, or flame holder.

Power (Expansion)

The hot gases from combustion are discharged through a stationary nozzle guide vane (NGV) then across rows of turbine blades installed in series. Like with the compressor blades, each turbine blade row is referred to as a ‘stage’. Individual pressure stages are sometimes grouped into a low-pressure stage and high-pressure stage, depending upon how many individual stages are present.

A force is imparted onto each turbine blade as the hot gases pass through the turbine, these combined forces produce torque which act upon a central shaft causing it to rotate. Rotational shaft speeds vary, but some turbines may reach rotational speeds in excess of 20,000 rpm.

Interesting note: As the hot gases pass through the turbine, their pressure and temperature decreases; this part of the turbine is referred to as the expansion section due to this decrease in pressure.

Exhaust

Hot exhaust gases are discharged directly to atmosphere or to another process in order that the heat may be reclaimed. For example, a heat recovery heat generator (HRSG) may be installed directly after a combustion turbine in order to reclaim heat from the exhaust gases, this heat can then be used to generate steam. The amount of heat available is significant, as the discharged exhaust gas temperatures from a typical gas turbine may be up to 550°C (1,002°F).

Heat Recovery Steam Generator (HRSG)

Gas Turbine Designs

Gas turbines may be designed for a specific purpose, such as generating thrust (aircraft engines), or mechanical power for driving machinery like pumps or generators (electrical power generation). Aeroderivative gas turbines are utilised by aircraft due to their high thrust power output, whilst heavy frame gas turbines are used within the electrical power generation industry.

Gas Turbine Used For Power Generation

It's important when constructing and operating gas turbines that all components are designed for endurance; for example, combustors must be able withstand extreme temperatures without damage occurring and blades should be made from heat-resistant material such as titanium so they don't overheat during operation.

Additional Resources

https://en.wikipedia.org/wiki/Gas_turbine

https://www.ge.com/gas-power/resources/education/what-is-a-gas-turbine

https://www.siemens-energy.com/global/en/offerings/power-generation/gas-turbines.html