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.
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 plant 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 . 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.
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.
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 the 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.
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.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 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.
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 on Engaged that are used in Gas Turbine Power Plants:
|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 this page by the EIA.|
|How can using a multi-stage compressor save on the amount of work needed? Click here.|
|Line drawing of an AGT 1500 showing how multi-stage compressors and turbines are used.|