Gas Turbine

Gas turbines provide a means of transferring energy from a pressurized fluid to mechanical energy in the form of a rotating shaft. The gas turbine compresses ambient air in a compressor and then channels the compressed air into a combustion chamber. The movement of the products of combustion through the turbine rotates the turbine shaft. Gas turbines have many applications, but are used most often in aircraft and power generation industries.

Solar Turbine at TCNJ
Solar turbine at TCNJ. [132]

Gas Turbines

Gas Turbine Schematic
Gas turbine schematic [1].
Gas turbines are comprised of a compressor, combustor, and turbine. These components work together to produce power or thrust, depending on the application. To begin the cycle, the compressor rotates and draws in ambient air. As it is taken in by the compressor, the air is pressurized, in some cases to 40 times atmospheric pressure [107]. The pressurized air then moves into the combustion chamber where a fuel mixture is ignited, heating the pressurized air and causing it to expand into the turbine. As the heated air expands through the turbine it pushes against the turbine blades which then rotate the turbine shaft. The rotational energy is used to spin a generator and create electricity. Because they are attached to the same shaft, the rotation of the turbine also rotates the compressor, keeping the system operating. Of the power generated by the turbine, 55%-65% is used to drive the compressor and the remainder is used to drive a generator [46]. This ratio of total turbine power to the power that was used to operate the compressor is called the back work ratio.

The College of New Jersey

Solar Turbine at TCNJ
Solar turbine at TCNJ [129].
The College of New Jersey operates a cogeneration plant that uses a dual fuel Solar Taurus 60 Turbine and a 6MW Ideal generator. The Solar Turbine is a 5.2 MW dual fuel gas turbine that burns natural gas or No. 2 low sulfur oil [62]. The gas turbine in the generator converts the chemical energy into mechanical energy. Unlike the internal combustion engine, the gas turbine, operating under the Brayton cycle, is able to simultaneously complete the combustion cycle and the Brayton cycle. Fuel injected into the gas turbine by the injectors is combusted to high temperatures in the combustion chamber. The hot gas expands and does work on the multi-stage turbine (also known as the power turbine), rotating the shaft. The temperatures inside the combustion chamber are too high to be measured by present-day thermal couples. A typical temperature inside the gas turbine is 2000°F [108]. For this reason, temperature in a Solar Turbine is measured at a distance, where the temperature is lower. The lower temperature measurement, called the T5 temperature, is used to estimate the temperature inside the combustion chamber. The turbine shaft rotates at 15,000 rpm to produce energy—70% of the total mechanical energy produced—to drive the turbine’s multistage compressor. Only the remaining 30% of the mechanical energy is converted to electrical energy. The exhaust gas exits the turbine at 1000°F and is directed to the HRSG, which extracts the thermal energy from this high temperature flue gas for the production of steam [62].

Faribault Energy Park

GE 7FA Gas Turbine at FEP
GE 7FA gas turbine at Faribault Energy Park [130].
The GE 7FA gas turbine, the central component of the plant, uses natural gas and diesel fuel to power the plant. The GE 7FA is suited to a wide range of applications including the combined cycle, cogeneration, and the simple cycle. During simple cycle operation, the 7FA has an output of 143 MW with a heat rate of 9360 Btu/kWh. The heat rate is an important measurement of gas turbine efficiency. Essentially it measures how much energy is used to create one kWh of electricity. Logically, the lower the heat rate the better the performance. The turbine rotates up to speeds of 3600 rpm and has a mass flow rate of 952 lb/sec [109]. The gas turbine contains an 18-stage compressor that has a pressure ratio of 15.5:1. The combustor is equipped with the Dry Low NOX system (DLN 2.6) to remove thermal NOX from the combustion process.

The air filtered into the inlet of the GE 7FA gas turbine, is compressed by an 18-stage axial compressor. The air exits the compressor at a pressure and temperature of 196 psig and 728°F, respectively. The compressed air is then mixed with fuel inside 14 cans, which are distributed radially around the turbine. When firing at maximum capacity, the combustion process consumes approximately 2 million standard cubic feet of natural gas per hour or, when switched over to the fuel oil, 14,000 gallons of No. 2 fuel oil per hour. The spark plugs inside the combustor ignite the fuel mixture during startup and then cut out in order to maintain a continuous flame during operation. When the hot combustion gas reaches a temperature of 2443°F, it is mixed with bleed air from the compression stages and sent through the three-stage turbine to run a generator. A significant amount of the compressed air is used to reduce the temperature of the combustion gases before they enter the blades of the turbine. To prevent turbine blade damage and to increase turbine component life, air is directed through openings along the combustor and base of the turbine blades to create a thermal boundary layer of air that prevents the hot gases from damaging the gas turbine components. The air bled from inside of the turbine blade is also the main component that cools the blades.

Power

The power production of a gas turbine is greatly affected by weather conditions. Generally, the colder the inlet temperatures, the more power can be produced. It is for this reason that many gas turbine manufacturers install air cooling systems before the compressor. Hot weather, which prevents the coolers from lowering the air temperature sufficiently for the given application reduces the power output of the gas turbine. Because cold air is denser than hot air, the colder the air, the higher the mass flow rate of air is through the turbine, producing more power.

When the weather is warmer, less steam is produced by a plant that is using its exhaust heat to produce steam to drive a steam turbine. At first glance it might seem that less steam is produced because of the lower exhaust temperatures, but it is actually because there is less hot exhaust gas flowing out the exhaust because the warmer inlet temperatures of the gases have lower density.

Fuel

Gas power plants can be powered by several different types of fuels, but natural gas is the most suitable. This is because it is the least expensive and causes the least pollution. If natural gas is too expensive or not readily available, 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.

How Stuff Works → Building a Gas Turbine

The selection of materials used to build a gas turbine is very important. For example, compressor blades undergo high rotational speeds that result in large mechanical loads. In order to cope with those loads, the materials used to make the blades must have both a high specific strength and a high specific stiffness. Materials with those characteristics include stainless steel, titanium, and nickel alloys. Of the three, titanium is the lightest weight material, but it is expensive and flammable.

With regard to the turbine blades, the first stage rotor blades undergo the most severe loads, so they must be manufactured from a high-quality material.

J85 ge 17a Turbojet Engine
J85 GE 17A turbojet engine [2].
As with the compressor blades, the material needs to have a high specific strength, but it also must have a high thermal mechanical fatigue resistance, sufficient ductility, reliable oxidation and corrosion resistance, and a high creep resistance. Furthermore, because the first stage rotor blades are exposed to scorching temperatures of up to 2500°F, all of the physical properties of the materials must be maintained in extreme conditions.

Thermal barrier coatings are often applied to provide thermal protection to certain parts of a gas turbine to prevent them from melting under conditions of extreme heat and from corrosion and oxidation. Thermal barrier coatings are usually applied in three different layers on top of any metal part: the bottom layer is a metallic bond coat, the second layer is a thermally grown oxide, and the top layer is a ceramic. There are two main ways to apply the coating: diffusion coating and overlay coating. Diffusion coating employs a chemical reaction to bond the coating to the metal. Overlay coating involves spraying the coating on the metal part like spray paint.

Cogeneration Application

One application of the gas turbine is for electricity production in combined heating and power plants (cogeneration). This system differs from the other applications in that waste heat from the exhaust of the turbine is recovered and used to generate steam that is then used to heat facilities near the plant or to drive another turbine to make more electricity.

An important feature to note about gas turbines is that when they generate power, the exit pressure of the turbine is near atmospheric pressure. This is logical since if the pressure exiting the turbine was significantly higher than atmospheric pressure it would indicate that pressure was not used to drive the turbine and that the system was operating at a lower efficiency level.

Aircraft

Many aircraft industries use gas turbines to propel aircraft. When gas turbines are used to propel aircraft the exit pressure of the turbine should be high enough to provide the thrust for the aircraft but not so high that the turbine cannot drive the compressor.

Centrifugal Compressors

Centrifugal compressors, the driven units in most gas turbine compressor trains, are used in small gas turbines. Centrifugal compressors in experimental models have pressure ratios that range from 3:1 to 10:1 [110].

Centrifugal Compressor
Centrifugal compressor [3].
The most prevalent pressure ratio in industry, however, is 3.5:1. The successful operations of many plants depend on the smooth operation of the compressor. In a typical centrifugal compressor, rapidly rotating impeller blades force the fluid through the impeller. The velocity of the fluid is then converted to pressure in the impeller and in the stationary diffusers. A rule of thumb is that half of the velocity head is converted to pressure in the impeller and the other half is converted to pressure in the diffuser [126].

Centrifugal compressors were used in MiG-15 engines [111]. Turboprop engines use centrifugal compressors because they are small (reducing weight and space needs), but most modern aircraft use axial flow compressors because of the higher compression ratios [4].

Axial Flow Compressors

The compressors in most gas turbine applications— especially ones of 5 MW or more— use axial flow compressors. In an axial flow compressor, the flow enters the compressor in an axial direction, in other words, parallel to the axis of rotation. The axial flow compressor compresses the working fluid by first accelerating the fluid and then diffusing it to obtain a pressure increase. The fluid is accelerated by a row of rotating airfoils (blades) called the rotor, and then diffused in a row of stationary blades called the diffuser (the stator). The diffusion in the stator converts the velocity increase gained in the rotor to a pressure increase. Compressors can have many stages, where one stage is considered a rotor followed by a stator [112].

Animation of an axial compressor
Animation of an axial compressor [4].
Compressors contain inlet guide vanes (IGV). IGV are variable pitch blades that are used to ensure that the air enters the first stage rotors at the desired flow angle. Because these vanes are pitch variable they can be controlled and adjusted to changing flow requirements. Because each stage of axial flow compression , raises the pressure slightly, the compressors tend to have very high efficiencies. The use of multiple stages permits overall pressure levels of up to 40:1 in some aerospace applications and 30:1 in industrial applications. The axial flow compressor in most advanced gas turbines is a multistage compressor with between 17 and 22 stages and an exceedingly high pressure ratio.

Industry has always prioritized the life span of a gas turbine over its efficiency levels. As a result of this conservative approach, rugged construction is valued more than high performance with industrial gas turbines. The introduction of new technologies is changing these priorities, however.

Compressor Choke

The compressor choke point, when no more flow can pass through the compressor, is reached when the flow in the compressor reaches Mach 1 at the blade throat. This phenomenon is known in industry as stone walling. The more stages there are, the higher the pressure ratio is and the smaller the operational margin between surge and choke regions of the compressor.

Axial Flow Turbines

Axial flow turbines are the most widely employed type of turbine. Axial flow are mainly used in gas turbines but are used for steam turbines as well. The axial flow turbines are used differently in steam applications than they are in gas turbines. In the axial flow turbine, flow enters and leaves in the axial direction as it does in an axial flow compressor. The challenge during the operation is how to cool the turbines blades. Cooling the turbine blades allows for higher turbine inlet temperatures and thus higher efficiencies. A new method that uses water to cool the turbine blades is currently being developed that employs a number of tubes imbedded inside the turbine blade to channel the water. The water is converted to steam by the time it reaches the blade tip, and is then injected into the flow stream. It has been predicted that this method of cooling could allow turbine inlet temperatures to reach 3000°F, which is a large improvement to the current 2500°F [113].

Combustor

Heat addition to the gas turbine occurs in the combustor. The combustor accepts air from the compressor and delivers it at an elevated temperature to the turbine (ideally with no drop in pressure).

Animation of a Combustor
Diagram of a tubular combustor [5].
There are three types of combustors in use currently: tubular, tubo-annular, and annular. Tubular-style combustors are preferred by many European turbine designers for their simplicity and longevity. Tubo-annular combustors are the most common type used in gas turbines. Most gas turbines in the United States are designed with tubo-annular (or can-annular) combustors, which are easy to maintain maintenance and have superior temperature distribution compared to tubular styles. Annular combustors are used mainly in aircraft gas turbines where frontal area is important (when the magnitude of drag force on an object must be considered).

How do gas turbines work? This article published by Global Gas Turbine News gives an introduction.
Take a look at this video that shows the inside of a gas turbine
Want to better understand the Brayton Cycle? More information can be found here.
An example of a multi-stage turbine from an M1 Abrams tank.
Can combustors are cylindrical combustion chambers. Each can is self-contained and includes its own fuel injector, igniter, liner, and casing.
Read this table for more information on fuel types.
A video showing how jet turbine blades are made.
Want to learn about a greener cogeneration plant? Read this ASME article.
An in-depth diagram of a Pratt & Whitney turbojet engine.
How does a centrifugal compressor compare to a reciprocating compressor? Read this research paper.
A video showing a gas turbine annular combustion chamber.
Want to learn more about combustion chambers? Click here!