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Natural Gas Power Plant

July 11, 2020

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 most 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 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) that served to turn a roasting skewer. The heats from the fire would rise and pass through the chimney’s blades, 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

In 1687, Isaac Newton announced his three laws of motion, which would significantly impact future inventions. As the gas turbine engine developed, 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 he planned his design to propel a horseless carriage. 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.

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 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 that worked as an aircraft engine.

In 1942, America had its own gas turbine, which included an axial compressor and an annular combustion chamber, nowadays we still use these designs in turbine engines.

Today, gas turbine engines’ developments continue, and two large gas turbine engines will be massive enough to produce 70000 pounds of thrust each.

That’s good, and that makes us wonder how this gas power plant work to produce electricity; let’s see.

Gas Power Plant Working Principle

The working principle of the gas power plant is very simple as:

The air compresses 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.

The turbine’s shaft connects to an electric generator, so it rotates to produce the electric power.
This thermodynamic process for producing electricity uses the Brayton cycle, which increases the pressure difference across the machine to maximize efficiency.

[This video can help you understand, although we will explain it later.]

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.

Compressor

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.

Regenerator

We use this device to recover the exhaust gases’ heat from heating the compressor’s air output.

As the exhaust passes through the regenerator before releasing it into the atmosphere.

Combustion Chamber

It’s one of the most important components of the plant as the high-pressure air from the compressor enters it and heats up to a high temperature (3000 F), a burner injects oil into the chamber at high pressure to heat the air then we apply this high pressure, high-temperature air to the gas turbine.

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 have applied to the regenerator.

Alternator

We directly join an electric generator (alternator) with the turbine at the same shaft. This generator serves to convert the mechanical energy of the turbine into electrical energy. Then the electric power passes through a transformer to the grid.

Starting Motor

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 serves to drive the compressor, and the starting motor turns off.

Support System

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 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 cools the condenser at constant pressure and goes back to the compressor.

And this air doesn’t exhaust into the atmosphere, so we know 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, a gas turbine, and the difference that there isn’t a cooling chamber.

The air is sucked from the compressor’s atmosphere 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 heats in the combustion chamber at a constant volume, not constant pressure. Still, 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 different components’ plant load and vibration.

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 than reciprocating engines of the same speed, despite the compressor producing an objectable high-frequency noise.
  • The blades require special cooling methods due to 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 pressure that it requires.

It also losses a significant amount of thermal energy through the stack.

You should also know that the gas plant’s efficiency depends on the air temperature 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 increasing the air density, so the compressor works less.

The gas turbine power plant’s efficiency can rise to be more than 61% by using a combined cycle power plant.