GAS TURBINE

Introduction to Gas Turbines


Gas turbines have been used for electricity generation for many years. In the past, their use has been generally limited to generating electricity in periods of peak electricity demand. Gas turbines are ideal for this application as they can be started and stopped quickly enabling them to be brought into service as required to meet energy demand peaks. However, their previously small unit sizes and their low thermal efficiency restricted the opportunities for their wider use for electricity generation.

There are two basic types of gas turbines - aeroderivative and industrial.
As their name suggests, aeroderivative units are aircraft jet engines modified to drive electrical generators. These units have a maximum output of 40 MW. Aeroderivative units can produce full power within three minutes after start up. They are not suitable for base load operation.

Industrial gas turbines range in sizes up to more than 260 MW. Depending on size, start up can take from 10 to 40 minutes to produce full output. Over the last ten years there have been major improvements to the sizes and efficiencies of these gas turbines such that they are now considered an attractive option for base-load electricity generation. Industrial gas turbines have a lower capital cost per kilowatt installed than aeroderivative units and, because of their more robust construction, are suitable for base load operation.

How does a Gas Turbine Work?

Gas turbines use the hot gas produced by burning a fuel to drive a turbine. They are also called combustion turbines or combustion gas turbines.

The main components of a gas turbine are an air compressor, several combustors (also called burners) and a turbine.
The air compressor compresses the inlet air (raises its pressure). Fuel is mixed with the high pressure air in burners and burnt in special chambers called combustors. The hot pressurised gas coming out of the combustors is at very high temperature (up to 1350° C). This gas then passes through a turbine, giving the turbine energy to spin and do work, such as turn a generator to produce electricity. As the turbine is connected to its compressor, the compressor uses some (about 60%) of the turbine's energy.

Because some of its heat and pressure energy has been transferred to the turbine, the gas is cooler and at a lower pressure when it leaves the turbine. It is then either discharged up a chimney (often called a stack) or is directed to a special type of boiler, called a Heat Recovery Steam Generator (HRSG), where most of the remaining heat energy in the gas is used to produce steam.


The attached cross section of a typical large gas turbine and photo of a similar large gas turbine with its top half casing removed, show these major components.

Air Compressor

The air compressors used in gas turbines are made up of several rows of blades (similar to the blades on a household fan). Each row of blades compress and push the air onto the next row of blades. As the air becomes more and more compressed, the sizes of the blades become smaller from row to row. The row of largest blades can be seen at the left end of the compressor in the photo above, with the smallest blades to the right (the direction of air flow is from left to right). Note: A row of blades fixed to the outer casing of the compressor is also located after each row of moving blades.

Filters are used to remove impurities from the inlet air. However, as they can never completely eliminate all impurities, "washing" of the compressor blades must be carried out whenever blade fouling becomes too severe. This washing can be carried out on line (with the gas turbine operating) or when the compressor is stopped. Demineralised water and detergent are commonly used for washing. Erosion of the blades can be caused by hard particles in the air entering the compressor. Inspections for fouling and erosion are usually carried out at defined intervals of operating time.

This type of air compressor can change its capacity (mass of air sucked through the air compressor) only by changing its speed of rotation. However, when the gas turbine is used to generate electricity, the speed of rotation of the generator, gas turbine and air compressor must remain constant (3000 rpm in Australia). The mass of air being compressed therefore remains constant regardless of the amount of air required for combustion of the fuel at partial loads. The energy used to compress this excess air accounts for most of the reduction in efficiency of a gas turbine at partial loads.

Fuel

Gas turbines can operate on a variety of gaseous or liquid fuels, including:

Liquid or gaseous fossil fuel such as crude oil, heavy fuel oil, natural gas, methane, distillate and "jet fuel" (a type of kerosene used in aircraft jet engines);
Gas produced by gasification processes using, for example, coal, municipal waste and biomass; and
Gas produced as a by-product of an industrial process such as oil refining.
When natural gas is used, power output and thermal efficiency of the gas turbines are higher than when using most liquid fuels.
The fuel must be free of chemical impurities and solids as these either stick to the blades of the turbine or damage the components in the turbine that operate at high temperature.
The fuels used in gas turbines power generation plants are often relatively more expensive and in smaller quantities than those required by power generation plants using other fuels (such as coal).

Inlet Air

The air coming into the compressor of a gas turbine must be cleaned of impurities (such as dust and smoke) which could erode or stick to the blades of the compressor or turbine, reducing the power and efficiency of the gas turbine. Dry filters or water baths are usually used to carry out this cleaning.

The power and efficiency ratings of a gas turbine are usually based on the inlet air being at ISO conditions of 15° C and 65% relative humidity. If the inlet air is hotter and drier than ISO conditions, the power of the gas turbine decreases. This effect can be reduced by cooling the air (by equipment similar to air conditioners) or, more usually, by passing the air through an evaporative cooler (the air evaporates droplets of water, thus cooling the air).

The inlet air is usually passed through silencers before it enters the compressor.

Burners and Combustors

The compressed air and fuel is mixed and metered in special equipment called burners. The burners are attached to chambers called combustors. The fuel & air mixture is ignited close to the exit tip of the burners, then allowed to fully burn in the combustors. The temperature of the gas in the combustors and entering the turbine can reach up to 1350° C. Special heat resistant materials (such as ceramics) are used to line the inside walls of the combustors. The area between the combustors and the turbine are also lined.

Water or steam can be injected into the combustors to reduce the concentration of NOx (oxides of nitrogen) in the exhaust gas (by reducing the temperature of the flame). Special burners (usually called "dry low NOx burners") are used to reduce the concentration of NOx in the exhaust gas to less than 25 ppm at full load, without the use of water or steam injection. These dry low NOx burners usually cannot operate effectively below about 60% load. At this point, another type of burner takes over and allows the fuel to be burnt stably down to low loads. These "low load" burners produce significantly higher concentrations of NOx (over 100 ppm). Some burners incorporate both types of burner into the one arrangement (called "hybrid" burners). Note: the values of NOx concentrations and loads depend on the design of the equipment and on the fuel used.
When a gas turbine starts, the combustor quickly heats up. When the gas turbine shuts down, the combustor cools. This rapid heating and cooling produces stresses in the combustor and can cause cracking, particularly in the heat resistant lining material. The combustors must be inspected for cracks after a certain number of starts.

Turbine

The turbine (also called the "power" turbine) consists of several rows of blades (the "moving" blades) that are fastened to the rotating shaft of the turbine. A row of "fixed" blades is located after each row of the "moving" blades. These fixed blades are attached to the casing of the turbine and do not rotate.

As the hot gas from the combustors passes through the moving and fixed blades of the turbine, energy is transferred from the hot gas to the turbine, causing it to rotate. This energy transfer reduces the pressure of the gas and causes the gas to become cooler as it passes through the turbine. The blades of the turbine become larger from row to row to accommodate the expansion of the gas as its pressure reduces. The smallest row of blades can be seen at the left end of the turbine in the photo of the gas turbine with its top half casing removed, with the largest blades to the right (the direction of gas flow is from left to right).

The moving blades in the turbine are subjected to extreme temperature (from the hot gas exiting the combustors) and stress (from the combination of their rotation and the pressure of the hot gas). The efficiency of the gas turbine improves if the hot gas temperature rises. New materials and techniques used to manufacture the turbine blades have resulted in a significant increase in operating temperatures. Currently, turbine blades are made from exotic alloys that retain their strength at the high temperatures experienced in the turbine. Ceramic blades offer the possibility of still higher operating temperatures. However, materials to withstand the higher temperatures are usually more expensive than those that can withstand lower temperatures. The materials for the turbine blades (and other components of the turbine) are therefore selected to give a balance between hot gas temperature (and efficiency) and material selection (and cost). Research into better (and cheaper) materials for these high temperature, high stress duties is ongoing.

Turbine blades can be manufactured with passages inside the blades that allow air to pass through the blades to keep them cool. The compressor section of the gas turbine provides this cooling air. This allows the blades to operate in combustion temperatures that would otherwise be too hot for the material of the blades.

At these high operating temperatures, hard particles and chemical impurities in the air and fuel (even at extremely low levels) can damage the blades of the turbine, thus reducing their effectiveness. The ability of the gas turbine to do work and the efficiency of the gas turbine are consequently reduced. Some of this reduction can be regained by maintenance of the gas turbine. The type and cleanliness of the air and fuel used therefore has a major impact on the amount of maintenance performed on the gas turbine. Various coatings for turbine blades have been developed as another way to minimise this high temperature damage to the blades.

The hot components of the turbine, particularly the blades, are also subject to "creep" failure. Metals at high temperature & high stress gradually change their metallurgical properties and plastically deform ("creeps"). This deformation could result in the moving parts touching the fixed parts with possible catastrophic results. The turbine components most subject to conditions causing creep are regularly inspected and tested.

Exhaust Gases

The temperature of the exhaust gas from the gas turbine is typically in the range of 500°C to 640°C, depending on the design of the gas turbine and the fuel used. The heat energy in this gas can be extracted in a Heat Recovery Steam Generator (HRSG) to produce steam that can be used to produce electricity (Combined Cycle generating plant) or used for process heating.

If the exhaust gas is not passed to a HRSG, it is ducted through a silencer and then discharged up a stack.

The exhaust gas is usually visually clear and free of particles. Refer to "emissions" for information on the chemical compositions of the exhaust gas.

Emissions

The main chemical emissions from a gas turbine are dependent on the type of fuel used. However, some generalisations can be made.

NOx (oxides of nitrogen) can be controlled either by injecting water or steam into the combustors or by using special dry low NOx burners. Further details of these are given in the "burners and combustors" section above.
SOx (oxides of sulphur) are usually not a problem as most fuels used in gas turbines have low sulphur contents.
The concentration of CO2 (carbon dioxide) in the exhaust gas is dependent on the carbon content of the fuel used. The amount of CO2 produced per unit of electrical energy is also highly dependent on the thermal efficiency of the gas turbine.

Power Output

Gas turbine output power values are usually given for ISO conditions of 15° C, 60% relative humidity and an atmospheric pressure equivalent to average sea level conditions. Variations in these conditions during the operation of the gas turbine will result in changes to the power output of the gas turbine as indicated below.

In general, the power output from the gas turbine is influenced by:

1. The energy used by the air compressor - if less energy is used to compress the air, more energy is available at the output shaft;
2. The temperature of the hot gas leaving the combustors - increased temperature generally results in increased power output;
3. The temperature of the exhaust gas - reduced temperature generally results in increased power output;
4. The mass flow through the gas turbine - in general, higher mass flows result in higher power output;
5. The drop in pressure across the inlet air filters, silencers and ducts - a decrease in pressure loss increases power output;
6. The drop in pressure across the exhaust gas silencers, ducts and stack - a decrease in pressure loss increases power output;
7. Increasing the pressure of the air entering or leaving the compressor - an increase in pressure increases power output.

Various methods that have been used to achieve an increase in power output include:

1. Using the exhaust gas to heat the air from the compressor (mainly used in cold weather conditions);
2. Divide the compressor into two parts and cool the air between the two parts;
3. Divide the turbine into two parts and reheat the gas between the two parts by passing the gas through additional burners and combustors located between the two parts;
4. Cooling the inlet air - mainly used in hot weather conditions;
5. Reducing the humidity of the inlet air;
6. Increasing the pressure of the air at the discharge of the air compressor;
7. Inject steam or water into the combustors or turbine;
8. Wash or otherwise clean the fouling from the blades of the air compressor and turbine at regular intervals; and
9. Combinations of the above methods.
However, all these methods increase costs and some decrease the thermal efficiency of the gas turbine. The methods used are therefore a compromise between cost, power and efficiency for each application.

Thermal Efficiency

The thermal efficiency of a gas turbine is the proportion of the energy in the fuel that is converted to mechanical energy in the output shaft. Gas turbine efficiency values are usually given for ISO conditions of 15° C (dry bulb), 60% relative humidity and an atmospheric pressure equivalent to average sea level conditions. Variations in temperatures and relative humidities during the operation of the gas turbine will result in changes to the thermal efficiency of the gas turbine as indicated below.

In general, thermal efficiency is influenced by:

1. The energy used by the air compressor - if less energy is used to compress the air, more energy is available at the output shaft;
2. The temperature of the hot gas leaving the combustors - increased temperature generally results in increased efficiency;
3. The temperature of the exhaust gas - reduced temperature generally results in increased efficiency;
4. The mass flow through the gas turbine - in general, higher mass flows result in higher efficiencies;
5. The drop in pressure across the inlet air filters, silencers and ducts - a decrease in pressure loss increases efficiency;
6. The drop in pressure across the exhaust gas silencers, ducts and stack - a decrease in pressure loss increases efficiency.

Various methods have been used to achieve the above goals:

1. Using the exhaust gas to heat the air from the compressor (mainly used in cold weather conditions);
2. Divide the compressor into two parts and cool the air between the two parts;
3. Divide the turbine into two parts and reheat the gas between the two parts by passing the gas through additional burners and combustors located between the two parts;
4. Cooling the inlet air - mainly used in hot weather conditions;
5. Reducing the humidity of the inlet air;
6. Increasing the pressure of the air at the discharge of the air compressor;
7. Inject steam into the combustors or turbine;
8. Wash or otherwise clean the fouling from the blades of the air compressor and turbine at regular intervals; and
9. Combinations of the above methods.

However, all these methods increase costs and some decrease the amount of power able to be output by the gas turbine. The methods used are therefore a compromise between cost, power and efficiency for each application.

Reliability

The reliability of a gas turbine depends mainly on the design of its components and the selection of materials used in critical components. Operational factors such as the cleanliness of the fuel and inlet air, the way the gas turbine is operated and the quality of the maintenance practices also have an effect of reliability.

New models of gas turbines often have significant changes to critical components in an effort to improve power output, increase thermal efficiency and reduce costs. However, the use of unproven designs and technologies can result in unforseen failures. The manufacturers analyse these failures and improve the component. The reliabilities of the models improve as these types of failures are designed out.

Noise

Gas turbines are very compact and occupy small ground area. Statutory limits on noise levels at site boundaries can be achieved either by increasing the distance from the boundary to the plant or by installing noise abatement equipment on the machines. Silencers are usually fitted in the inlet air and exhaust gas ducts.

The inlet air (blue) enters the compressor at the left. The exhaust gas (red) leaves the turbine at the right. The burners and combustors are located between the compressor and turbine.

The photo shows what such a gas turbine looks like when its top half casing has been removed for inspection or maintenance. The air compressor is on the left and the turbine is on the right. The section that would hold the burners and combustors is between the compressor and the turbine. Note the large bolts that are used to hold the two halves of the casing together.

The photo shows, for a large gas turbine, the cross-section of a typical burner/combustor combination, the arrangement of these combustors and the area between the combustors and the turbine. The heat resistant ceramic tiles used in these hot areas can be clearly seen.


 

 

 


The combustion (gas) turbines being installed in many of today's natural-gas-fueled power plants are complex machines, but they basically involve three main sections:

The compressor which draws air into the engine, pressurizes it, and feeds it to the combustion chamber literally at speeds of hundreds of miles per hour.

The combustion system, typically made up of a ring of fuel injectors that inject a steady stream of fuel (e.g., natural gas) into the combustion chamber where it mixes with the air. The mixture is burned at temperatures of more than 2000 degrees. The combustion produces a high temperature, high pressure gas stream that enters and expands through the turbine section.

The turbine is an intricate array of alternate stationary and rotating aerofoil-section blades. As hot combustion gas expands through the turbine, it spins the rotating blades. The rotating blades perform a dual function: they drive the compressor to draw more pressurized air into the combustion section, and they spin a generator to produce electricity.

Land based gas turbines are of two types: (1) heavy frame engines and (2) aeroderivative engines. Heavy frame engines are characterized by lower compression ratios (typically below 15) and tend to be physically large. Aeroderivative engines are derived from jet engines, as the name implies, and operate at very high compression ratios (typically in excess of 30). Aeroderivative engines tend to be very compact.

One key to a turbine's fuel-to-energy efficiency is the temperature at which it operates. Higher temperatures generally mean higher efficiencies which, in turn, can lead to more economical operation. Gas flowing through a typical power plant turbine can be as hot as 2300 degrees F, but some of the critical metals in the turbine can withstand temperatures only as hot as 1500 to 1700 degrees F. Therefore air from the compressor is used for cooling key turbine components; however, the requirement for cooling the turbine limits the ultimate thermal efficiency.

One of the major breakthroughs achieved in the Department of Energy's advanced turbine program was to break through previous limitations on turbine temperatures using a combination of innovative cooling technologies and advanced materials. The advanced turbines that emerged from the Department's research program were able to boost turbine inlet temperatures to as high as 2600 degrees F - nearly 300 degrees hotter than in previous turbines.

Another way to boost efficiency is to install a recuperator or aste heat boiler onto the turbine's exhaust. A recuperator captures waste heat in the turbine exhaust system to preheat the compressor discharge air before it enters the combustion chamber. A waste heat boiler generates steam by capturing heat from the turbine exhaust. These boilers are also known as heat recovery steam generators (HRSG). High-pressure steam from these boilers can be used to generate additional electric power with steam turbines, a configuration called a combined cycle.

A simple cycle gas turbine can achieve energy conversion efficiencies ranging between 20 and 35 percent. With the higher temperatures achieved in the Energy Department's turbine program, future gas turbine combined cycle plants are likely to achieve efficiencies of 60 percent or more. When waste heat is captured from these systems for heating or industrial purposes, the overall energy cycle efficiency could approach 80 percent.

Turbine Successes - " Breakthrough" Gas Turbines

For years, gas turbine manufacturers faced a barrier that, for all practical purposes, capped power generating efficiencies for turbine-based power generating systems.

The barrier was heat. Above 2300 degrees F, the scorching heat of combustion gases caused metals in the turbine blades and in other internal components to begin degrading. Since higher temperatures are the key to higher efficiencies, this effectively limited the generating efficiency at which a turbine power plant could convert fuel into electricity.

The Department of Energy's Fossil Energy took on the challenge of turbine temperatures in 1992, and nine years later, two of its private sector partners produced "breakthrough" turbine systems that pushed firing temperatures to 2,600 degrees F and permitted combined cycle efficiencies that surpassed the 60 percent mark - the "four-minute mile" of turbine technology.

Moreover, the advanced turbines achieved the higher firing temperatures while actually reducing the amount of nitrogen oxides formed to less than 10 parts per million (NOx is a product of high temperature combustion).

Among the innovations that emerged from the Department's Advanced Turbine Systems program were single-crystal turbine blades and thermal barrier coatings that could withstand the high inlet temperatures, along with new firing techniques to stabilize combustion and minimize nitrogen oxide formation.



The GE H-System Turbine


On February 18, 2000, GE Power Systems unveiled the first gas turbine slated for the U.S. market that would break through the temperature barrier and push efficiencies to unprecedented levels. Using advanced materials and revolutionary new steam-cooling technology, the new turbine is capable of operating at 2600 degrees F.

The H System was the first turbine to surpass the 60 percent efficiency threshold, nearly five percentage points better than the prior best available system, in an industry where improvements are typically measured in tenths of a percent. Using an innovative dry low-NOx combustion system, the turbine achieved nitrogen oxide emission levels of 9 parts-per-million, half the average of the turbines in commercial use.

The unit announced in February 2000 was slated to be one of two 60-hertz turbines that would have powered the 800-megawatt Heritage Station being built in Scriba, New York. The power plant, however, was not built when the anticipated demand for electric power in the region failed to materialize. A 50-hertz version, specially designed for the European power grid, was shipped to Baglan Bay Power Station near Cardiff, South Wales, in December 2000 and began test operations in November 2002.



Siemens Westinghouse W501G Advanced Gas Turbine

In May 2001, the Energy Department's other advanced turbine development partner, Siemens Westinghouse, announced that its advanced W501G turbine had gone into commercial operation at the 360-megawatt, combined cycle Millennium power plant in Charlton, Massachusetts.

In addition, the City of Lakeland, Florida's McIntosh Unit 5, a 249-megawatt simple cycle plant, also went into operation using the advanced turbine at about the same time. The Siemens Westinghouse engine has demonstrated a net efficiency of approximately 58 percent in combined cycle application.

Reference
Web site
:

http://www.energy.qld.gov.au/electricity/infosite/index.htm
http://www.fossil.energy.gov

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