Gas Turbine Power Plant

Because natural gas burns cleanly, it has become a popular fuel to generate electricity. As a result of environmental concerns and technological advances, natural gas power plants have become more appealing than coal or nuclear power plants in some contexts. The main components used to generate power in a gas turbine power plant are a compressor, a combustor, and a gas turbine. Because they can be started up quickly, gas turbines are ideal for meeting peak loading demands.

GE 7FA
GE 7FA gas turbine.

Basic Operations

While gas turbine power plants be operated with an open or closed cycle, open cycle plants are more common. The working fluid in an open cycle plan is atmospheric air, constantly drawn in to the compressor where it is typically compressed up to 18 times atmospheric pressure and then sent to a combustor [46]. In the combustor, natural gas is burned to heat the air and expand it before it reaches the turbine. The exhaust further expands in the turbine, to approximately atmospheric pressure, and moves the turbine blades to create work. The exhaust is then released to the environment.

German Kraftwerk Simmering Gas Turbine
German Kraftwerk simmering gas turbine [25].
In a closed cycle, the working fluid is cycled through the compressor and then heated by an external source before it enters the turbine. Instead of being released to the atmosphere, the exhaust is sent through a heat exchanger that extracts heat from the exhaust before it is returned to the compressor. In both cycles the turbine shaft is connected to a generator that converts the rotational power into electrical power.

A gas turbine cannot 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. For example, the starter system at Faribault Energy Park includes an electrically motorized generator that spins the turbine shaft to the rate of 500 rpm. At that point the combustors are turned on and the turbine begins to spin more rapidly. The generator motor is turned off when the system is spinning at 3200 rpm. The system will continue to speed up until it reaches the normal operating rate of 3600 rpm.

Fuel

Gas power plants can be powered by several different types of fuels, but natural gas is the most suitable, because it is the least expensive and causes less pollution. If natural gas is too expensive or not readily available, however, gas power plants can operate using most liquid fuels. For example No. 2 fuel oil, also known as heating oil, is a common substitute for natural gas. No. 2 fuel oil is a petroleum derivative very similar to diesel fuel. It is used to fuel furnaces, boilers, and gas turbines. Biogas, which is created when organic matter decomposes in the absence of oxygen, is also used to fuel gas turbine power plants. Biogas can be generated at landfill sites and sewage plants, or by decaying agricultural waste. The gas is collected and used in the combustion chamber in place of natural gas.

When choosing fuels, it is often necessary to consider them impact on emissions that that particular fuel will have. While fuel oils such as No. 2 are usable in gas turbines, the increased emissions from burning it may not make it a suitable alternative for long term usage. Some of the other main pollutants that are closely monitored are: Nitrogen oxides (NOX), carbon monoxide (CO), and volatile organic compounds (VOC). Some of the pollutants are simply just byproducts of the combustion reactions, some are due to incomplete combustions, and some are due to specific fuel characteristics. All of these things must be taken into account when choosing a fuel source for each gas turbine application. For example, the low CO2 emissions is one of the reasons why natural gas is the primary fuel type in gas turbines.

Efficiency

Simple cycle natural gas power plants are not particularly efficient. The simple cycle gas power plants in use currently are about 35% efficient for two main reasons. First, the compressor uses a lot of energy to compress the air to the required pressure to run the air through the turbine.

Gas Turbine Piping at TCNJ
Gas turbine piping at TCNJ.
Because up to 2/3 of the turbine output is used to drive the compressor, only 1/3 of the rotational energy remains for conversion into electrical energy. Second, after the combustion gasses pass through and exit the turbine they are still extremely hot (around 900–1200°F), so a significant amount of thermal energy is lost through the stack. In a simple cycle, this energy is wasted when the exhaust is released directly into the atmosphere. However, the energy can be recovered by using the exhaust to preheat the compressed air going into the combustor (which would reduce the fuel requirements), or by heating the air in the building or steam using a heat recovery steam generator.

The efficiency level of a gas turbine power plant depends on the temperature of the air running through the system. The warmer the air is that enters the compressor the less efficient the compressor will be. The cooler the air is that enters the combustion chamber, the less efficient the combustion will be. There are different ways to approach these problems. One way to make the compressor more efficient is by cooling the air before it enters the system. This process, called turbine inlet cooling, increases the density of the air, thus enabling the compressor to do less work with for the same output.

Intercooling can be achieved using a series of compressors. Since air temperature increases as pressure increases, cooling down the air after it travels through the first compressor can increase efficiency levels.

After the air is compressed it travels into the combustion chamber; keeping the air as warm as possible reduces the energy needed for the combustion process. Preheating the air can be achieved without additional energy input by passing it through a heat exchanger, called a regenerator. A regenerator enables the considerable amounts of heat still contained in the exhaust exiting the gas turbine to be transferred from the exhaust to the air that is about to enter the combustor. This process increases the efficiency of the combustor, which then produces hotter exiting air that in turn generates more power from the gas turbine.

Advantages

Natural gas power plant
Natural gas powerplant in Bellingham, WA [20].
There are several advantages to using a gas power plant to generate electrical power as compared to other systems. Gas turbine power plants can be started up and run at full capacity in only 10 to 20 minutes, making them well suited as backup plants for utility companies that require additional electricity immediately. Because they are smaller than coal or nuclear plants, gas power plants can be built faster and at a lower cost. Gas turbine systems also require much less water than steam power plants, and they are easily converted into combined cycle power plants, which are much more efficient.

Disadvantages

Gas turbine power plants have disadvantages as well. The power needed to drive the compressor reduces the net outputs, consuming more fuel to do the same amount of work. The operating temperature in gas turbines is higher than in other power plant systems and can shorten the lifespan of some of the system components. Furthermore, because the thermal energy is wasted when the exhaust is released, the efficiency levels of gas turbine plants are lower than those of other types of power plants.

Components

Components on Engaged that are used in Gas Turbine Power Plants:
Gas Turbine

The time that it takes a gas turbine power plant to reach full load capacity could be as little as 10-20 minutes. It takes hours for a normal coal plant and up to two days for a nuclear plant to reach full capacity.
Video on gas turbines.
Table comparing types of fuel.
How much carbon dioxide is produced when different fuels are burned?
See how using a multi-stage compressor can save on the amount of work needed.
Line drawing of an AGT 1500 shoing how multi-stage compressors and turbines are used.