Due to its clean burning nature, natural gas has become a popular fuel to generate electricity. Advances in technology and environmental concerns have made natural gas power plants more appealing than coal or nuclear power plants in some situations. The main components used to generate power in a gas turbine power plant are a compressor, combustor and a gas turbine. Gas turbines have rapid start up times which means they are ideal for meeting peak loading demands. As of 2009, 23% of the total U.S. electric generation was produced using gas turbines.
Gas turbine power plants can operate using an open or closed cycle. Open cycle gas power plants are most common. An open cycle is one where atmospheric air is constantly drawn in as the working fluid. Air is brought into the compressor where it is compressed to approximately ten times atmospheric pressure and sent to a combustor. This is where the natural gas is burned to add heat to the air so it will expand before reaching the turbine. The exhaust is then allowed to expand through the turbine where it expands to approximately atmospheric pressure, which is what turns the turbine blades to create work. The exhaust is then released to the environment. In a closed cycle, the working fluid is cycled through the compressor then receives a heat input from an external source before entering the turbine. Instead of being released to the atmosphere, the exhaust is sent through a heat exchanger where it rejects heat before returning to the compressor. In each case the turbine shaft is connected to a generator which changes the rotational power into electrical power.
A gas turbine cannot just be turned on like a gasoline or diesel engine. Since the compressor is driven by the shaft of the turbine, an outside source is required to start the system. An example of a starter system includes at Faribault Energy Park the generator is electrically motorized to spin the turbine shaft until it is spinning at 500rpm. At this point in time the combustors are turned on and the turbine starts spinning faster. The generator motor is shut off when the system is spinning at 3200rpm. The system will continue to speed up until it reaches the normal operating rate of 3600rpm.
Gas power plants can run on a variety of different fuels, but natural gas is most suitable. If natural gas is too expensive, or not readily available, gas power plants can run off of most liquid fuels. No. 2 fuel oil is a common replacement for natural gas in the system. No. 2 fuel oil, also known as heating oil, is a petroleum derivative that is very similar to diesel fuel. It is used as a fuel for furnaces, boilers, and gas turbines. Biogas is also being used as a fuel for gas turbine power plants. Biogas is generated when organic matter decomposes in the absence of oxygen. It can be created at a landfill site, sewage plants, and by decaying agricultural waste. The gas is collected and used in the combustion chamber in place of natural gas.
Natural gas power plants are not known for their efficiency. The current simple cycle gas power plants have efficiencies of around 35%. There are two main reasons for this. First reason is the compressor requires a lot of energy to compress the air to the necessary pressure. Up to 2/3 of the turbine output is used to drive the compressor. This only leaves 1/3 of the rotational energy to be converted into electrical energy. The second reason is because the exhaust leaving the turbine is still incredibly hot, so a lot of thermal energy is lost through the stack. After the combustion gasses pass through the turbine, the temperature is still around 900-1200°F. In a simple cycle, this extra energy is wasted by releasing the exhaust directly into the atmosphere. There are several ways to recover this energy. The exhaust can be used to preheat the compressed air going into the combustor (which would reduce the fuel needed), heat the air in the building or steam using a heat recovery steam generator.
Gas turbine power plant efficiency has a lot to do with the temperature of the air running through the system. The warmer the air is that enters the compressor, the less efficient that compressor is going to be. The cooler the air is that enters the combustion chamber, the less efficient the combustion is going to be. There are different ways to approach these problems. Turbine inlet cooling is one way to make the compressor more efficient. Before the air enters the system it is cooled so that it become more dense, therefore the compressor does less work to get the same results.
When multiple compressors are being used in series, intercooling can be applied. Since air increases in temperature as the pressure is increased it would be beneficial to cool the air back down after going through the first compressor, this is when intercooling can be applied to increase efficiency.
After the compression is done the air travels into the combustion chamber. At this point it is most beneficial to have the air as warm as possible so that the combustion process has less work to do. Preheating the air can be done without any extra energy input by passing it through a heat exchanger called a regenerator. Exhaust coming from a gas turbine is still incredibly hot, and a regenerator allows that heat to be transferred from the exhaust to the air that is about to enter the combustor. This will allow the combustor to be more efficient and produce higher temperatures of exiting air which will in turn create more power from the gas turbine.
When comparing the different options for electrical generation, there are several advantages to building a gas power plant. Gas power plants have a quick startup time to full load, it can take as little as 10-20 minutes to start up the power plant and get it to run at full capacity. This makes gas turbine power plants well suited for backup plants. If the utility company needs more electricity immediately a gas turbine power plant would be called. Gas power plants are smaller in size than coal or nuclear plants which allows them to be built in less time and for less money. Steam power plants require large amounts of water to be used, which is not necessary in a gas turbine system. Gas turbine power plants are easily converted to combined cycle power plants, which increases the efficiency greatly.
There are also disadvantages to gas turbine power plants. They can have lower net outputs due to the power needed to drive the compressor. This would mean more fuel is necessary to produce the same amount of work. Due to the high temperature required to operate the turbine, some parts in the system may have a shorter lifespan than the parts needed for other power plants. The efficiency is also lower than some other types of power plants because of the thermal energy that is being wasted when the exhaust is released.