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A Natural Gas Power Plant or Combustion turbine is a type of continuous combustion, internal combustion engine that converts the fuel energy (natural gas, or other liquid fuels) into mechanical energy which turns an electrical generator to produce electricity.
The difference between the gas power plant and the other plants is that it uses high-pressure, high-temperature air to rotate the turbine instead of using high pressure, high-temperature steam.
And this gas plant consists of:
- Upstream rotating gas compressor.
- Downstream turbine.
- Combustion chamber (combustor).
- Turboprop (turbofan): We use it sometimes to increase the efficiency or achieve greater power to mass/ volume ratio.
Gas turbine history:
Gas turbine engines power the majority of aircraft and play a significant part in aviation, which means that this engine has a great history.
Around 150 BC the Egyptian -and mathematician ‘Hero’ developed the first jet engine which was designed to be a toy as known as ‘aeolipile’. It consisted of a boiler; two hollow bent tubes; and a sphere.
The steams coming from the boiler enter through the two hollow tubes that supported the sphere and then this steam exited through the bent tubes on the sphere and made the sphere spin.
And around 1500 AD Leonardo DA Vinci designed a machine (chimney jack) to be used to turn a roasting skewer. The heats from the fire would rise up and pass through the blades in the chimney and these blades would turn a series of gears to turn the skewer.
Then in 1232, the Mongols used rockets (which is credited for its discovery to the Chinese) to create fear among enemy troops, and rockets were also used in the Napoleonic war of 1812.
And in 1629 Giovanni Branca invented an impulse turbine which was a stamping mill. A steam-power turbine generates power, as a nozzle directed the steam onto the turbine wheel which turned a series of gears to operate his mill.
Energy Production Basics
Then in 1687, Isaac Newton announced his three laws of motion which would have a significant impact on future inventions. As the development of the gas turbine engine; he invented a steam wagon in 1687 as he propelled the wagon by generating steam in a boiler on this wagon then this steam directed aft through a nozzle.
In 1791 John Barber patented the first gas turbine engine as his design was planned to propel a horseless carriage; and he used the thermodynamic cycle and the design had a compressor; the combustion chamber; and a turbine.
Natural Gas Turbine In 1791
But in 1872 Dr.F.Stolze designed the first true gas turbine engine and this engine used a multistage turbine section and a flow compressor.
And in 1903 Aegidius Elling of Norway managed to use rotary compressors and turbines to build the first successful gas turbine.
Then in 1918 Dr.Stanford A.Moss developed the general electric turbo-supercharger which used the exhaust gas from piston engines to drive a turbine wheel which in turn drove a centrifugal compressor which used for supercharging.
And in 1930 Frank Whittle submitted a patent application for a gas turbine for jet propulsion, and his engine had a single-stage centrifugal compressor coupled to a single-stage turbine. When Whittle was working on his engine, Hans von Ohain and Max Hahn in 1936 patented a jet propulsion engine which was used as an aircraft engine.
Approximately in 1942 America had its own gas turbine which included an axial compressor and an annular combustion chamber and these designs are still used in turbine engines.
Today, developments in gas turbine engines continue and there are two large gas turbine engines that will be massive enough to produce 70000 pounds of thrust each.
That’s good and that makes us ask how this gas power plant work to produce electricity; let’s see.
Gas power plant working principle
Working principle of the gas power plant is very simple as:
The air is compressed in the compressor then passes through the combustion system which is made up of a ring of fuel injectors that inject a steady stream of fuel into the combustion chambers where we mix the fuel with the air to raise its temperature.
When the air reached enough high-temperature and high-pressure it passes through the gas turbine where it suddenly expands and produces mechanical energy that spins the rotating blades.
And as the shaft of the turbine is connected to an electric generator so it rotates to produce the electric power.
This thermodynamic process for producing electricity uses the Brayton cycle which depends on increasing the pressure difference across the machine to maximize efficiency.
Gas turbine power plant parts and functions
The gas turbine power plant basically consists of:
Air Inlet system
We design it to modify the quality of air under various temperatures; humidity; and contamination situations to make it suitable for use and helps to increase the efficiency of the turbine.
We usually use it with the gas turbine a rotating compressor. It draws the air through a filter to remove the dust.
The rotary blades in the compressor push the air through the stationary blades to raise the pressure of this air; So, we can say that we use the compressor to have a high-pressure air.
We use this device to recover the heat from the exhaust gases to heat the air output from the compressor.
As the exhaust passes through the regenerator before releasing it to the atmosphere.
It’s one of the most important components of the plant as the high-pressure air from the compressor enters it to be heated up to a high temperature (3000 F), a burner injected oil to the chamber at high pressure to heat the air then we apply this high pressure, high-temperature air to the gas turbine.
It’s the heart of the plant. The hot high pressure, high-temperature air passes through the turbine causes the gases on the gas turbine blades to expand and causes rotation of these blades to the intended mechanical work, after that the exhaust gasses are applied to the regenerator.
We directly join an electric generator(alternator) with the turbine at the same shaft. This generator is used to convert the mechanical energy of the turbine into electrical energy. Then the electric power passes through a transformer to the grid.
We use it before starting the plant to start the compressor. It simply works as an initial driving component for the compressor.
We drive this motor with batteries and coupled to the shaft of the gas turbine.
When the turbine rotates some of the produced mechanical power used to drive the compressor and the starting motor is turned off.
It consists of lube oil; cooling water; hydraulic oil; ignition; fuel system; fire protection; and wash water.
Applications of gas turbine power plant:
Gas turbine power plants occupy a large place in today’s use, especially when we require high power, lightweight, and speed; so we can use in instances below:
- In jet population unit in aircraft and ships.
- In automobile in the supercharging system, and in locomotives in the electric generating station.
- As a standby plant for the hydroelectric power plant.
- In airplanes and heavy motor vehicles.
- In petroleum and natural gas industries to drive compressors and pumps.
- As a peak load plant for the smaller power units and in steam plants.
Types of gas turbine engines:
We can classify gas turbines according to the path of the working substance as:
1- Closed cycle gas turbine
We can say that it’s the simplest form of gas turbines as it consists of a compressor; a combustion chamber; a gas turbine and a cooling chamber.
It works on the Brayton cycle; the air is compressed in the compressor then passes to the combustion chamber where we burn fuel to heat the air at constant pressure; then this air is passed over the turbine to rotate it.
After that, the air is cooled in the condenser at constant pressure and resend to the compressor to be used.
And this air doesn’t exhaust into the atmosphere so it’s known as a closed cycle turbine.
2- Open cycle gas turbine
It doesn’t differ from the closed cycle gas turbine as it has a compressor; a combustion chamber; gas turbine and the difference that there isn’t a cooling chamber.
The air is sucked from the atmosphere by the compressor to be compressed then sent to the combustion chamber to be heated. Then this high-temperature and pressured air pass over the turbine to spin it.
And after this process, the compressor sends the hot air to the atmosphere and a new air sucked.
We can also classify the gas turbines according to the process of heat absorption into:
1- Constant pressure heat absorption:
It’s an ideal cycle of a gas turbine as the air is compressed isentropically; then the heat is added at constant pressure and it also works on the Brayton cycle.
2- Constant volume of heat absorption:
It differs from other types as the gas is heated in the combustion chamber at a constant volume, not a constant pressure, but the heat is rejected into the condenser at constant pressure, and this turbine works on the Atkinson cycle.
The consideration when designing a gas turbine power plant:
We should consider some factors when we select a site for a gas power plant such as:
- The site should be near to the load center to reduce the transmission costs and minimize losses as much as possible.
- There should be the availability of land at a cheap rate to keep the capital costs low.
- The fuel should be easily available or transported easily with little cost.
- The plant may produce noise so it’s desirable to be away from the populated area.
- There should also be a high bearing capacity land to withstand the plant load and vibration produced by the different components.
Advantages of gas turbine power plant
We would find many advantages for the gas power plant as:
- The initial cost per unit output, the capital cost, and also the running cost are low.
- The size and construction are smaller and simpler than the steam turbine plant.
- It’s very easy to install a gas plant within short periods.
- Its quick starting and smooth running.
- It is capable of using a range of liquid and gaseous fuels.
- The combustion of water is less compared to other plants as a steam power plant.
- As this plant doesn’t deal with steam we wouldn’t need a boiler or cooling tower.
- The maintenance of the plant is easy and requires low costs.
- It’s also environmentally-friendly as it produces less exhaust gas pollution
Disadvantages of gas turbine power plant
Unfortunately, there are some drawbacks with the gas turbine power plant as:
- The plant wouldn’t start without a pre-compressed, which requires an auxiliary power supply for starting the plant.
- It has fewer vibrations compared with reciprocating engines of the same speed, despite the compressor producing an objectable high-frequency noise.
- The blades require special cooling methods as a result of the severity of operating with high temperature and pressure.
- The overall efficiency is low as we use two-thirds of the output power for driving the compressor.
- Also, there would be several restrictions on the serving conditions of the plant from the high temperature.
Gas turbine power plant efficiency
The simple cycle gas power plant isn’t particularly efficient as the first simple model developed in 1939 worked with only 18% thermal efficiency.
And today it’s about 35%; this efficiency is low because as we said the compressor uses a lot of energy to compress the air to the required pressure.
Also, a significant amount of thermal energy lost through the stack.
You should also know that the efficiency of the gas plant depends on the temperature of the air running through the system.
If the air that enters the compressor is warmer, the compressor efficiency will reduce. And if the air that enters the combustion chamber is cooler the combustion efficiency will reduce. We can solve this problem by cooling the air before it enters the system or by increasing the density of the air; so the compressor works less.
And the efficiency of the gas turbine power plant can rise to be more than 61% by using a combined cycle power plant.