Gas turbine

A gas turbine , also called turbine with combustion , is a thermodynamic revolving machine pertaining to the family of the internal combustion engine whose role is to produce mechanical energy (rotation of a tree) starting from the energy contained in a hydrocarbon (fuel, gas…).
Le Turboréacteur is a particular Turbine with gas which uses the principle of the Réaction to propel certain types of fast planes.

Principle of operation

The gas turbine is a thermal Moteur carrying out the various phases of its cycle Thermodynamique in a succession of bodies crossed by a gas driving fluid in continuous flow. It is a basic difference compared to the piston engines which carry out a temporal succession of the phases in the same body (generally a cylinder).

In its simplest form, the gas turbine functions according to the cycle known as of Joule including/understanding successively and schematically:

a adiabatic compression which consumes mechanical energy,
an isobar heating as for a diesel engine ,
an adiabatic pressure drop until the environmental pressure which produces mechanical energy,
an isobar cooling.

The Rendement is the report/ratio of work useful (work of relaxation - work of compression) for the heat provided by the hot source. The theoretical yield believes with the compression ratio and the temperature of Combustion. It is higher than that of the Diesel cycle because its relaxation is not curtailed.

The gas turbine is generally with open cycle and internal combustion. In this case, the phase of cooling is external with the machine and is done by mixture with the atmosphere. The gas turbine can also be with closed cycle and external combustion. The heating and cooling are then ensured by exchanging S of heat. This more complex provision allows the use of particular gases or to work with a low pressure different from the ambient one.

The basic cycle describes higher can be improved by various complementary bodies:

recovery of heat to the exhaust: the gases slackened at exit of turbine cross an exchanger to preheat the compressed air before its admission in the combustion chamber,
cooled compression: compression includes/understands two stages (or more) separated by an exchanger of heat (air-to-air or air/water) cooling the air. The power necessary to compression is some reduced for the benefit of the output.
staged combustion: the relaxation includes/understands two stages (or more) separated by additional reheatings. The provided power is increased from where improvement of the output.

The two last provisions aim at tending towards transformations Isotherme S instead of the adiabats and are justified especially on the machines athigh compression ratio. The three devices can be realized independently or simultaneously. In this case, one finds the cycle known as of Ericsson which as the cycle of Stirling presents a theoretical yield equal to the maximum output of the Cycle of Carnot. This theoretical superiority compared to the cycles Otto and Diesel is however counterbalanced by practical impossibility to carry out the isothermal transformations. In all the cases, these devices are reserved for the stationary installations because of obstruction and of the weight of the exchangers gas/gas.

Principles

The compressing (reference mark C ), made up of a whole of wheels provided with wings, compresses the surrounding air (reference mark. E ), simply filtered, up to 10 to 15 bars, even 30 bars for certain models. Gas (reference mark. G ), or an atomized liquid fuel, is injected into the combustion chamber (reference mark. CH ) where it mixes with the compressed air and ignites. The hot gases slacken while crossing the turbine (reference mark. T ), or the thermal energy of hot gases is transformed into mechanical energy, the aforementioned Turbine consists of one or more wheels also provided with wings and escape by the chimney (reference mark. EC. ) through a diffuser. The rotation movement of the turbine is communicated to the tree has which actuates on the one hand the compressor, on the other hand a load which is not other than an apparatus (machine) receiving (ice) (pump, alternator…) coupled at its right end. For the startup, one uses an engine of launching (reference mark. M ) which plays the part of starter. The adjustment of the power and number of revolutions is possible while acting on the air flow in entry and on the injection of the fuel.

Output

The weak Rendement of the gas turbine (25 to 35%) is due to the fact that the energy provided by fuel is diverted by the compressor or is lost in the form of heat in exhaust fumes. It is possible to improve slightly the output by increasing the Température in the combustion chamber (more than 1200°C) but one runs up against the problem of behavior of materials used for the realization of the part harnesses. It is by recovering the heat of exhaust fumes (heating, production of vapor…) that the total output of the machine can exceed 50%. One then uses the heat of exhaust fumes (more than 500 degrees) to produce vapor in a boiler. Another possibility of increasing the output of the turbine, is to heat gases at exit of the stages of compression (before the combustion chambers) by making them pass in an exchanger located in the flow of exhaust fumes. One is thus able to approach the outputs of a fast semi diesel engine. It is for example the principle of operation of turbine WR21 of Rolls Royce.

The produced vapor is then used in two manners:

the power station with cycle combined where a steam turbine supplements the gas turbine to actuate an alternator, the total output reaches then 55% even 60% in the last power stations being studied.
the Cogénération where the produced vapor is used in another field (paper mill…)

One manufactures gas turbines of powers energy of a few kilowatts to several hundred megawatts.

Pollution

Significant efforts were made by the manufacturers to limit air pollution by the gas turbines, in particular by reducing the nitrogen oxide rejections (NO_x). The use of natural gas allows a weak emission of sulfur dioxides (SO₂) and carbon monoxide (CO). The polluting models not are very especially installed by the developed countries while the gas turbines of less sophisticated design and less low price are preferred by the countries in the process of development.

Applications of the gas turbine

Practical realization

The phase of compression is carried out by compressor of an axial or centrifugal air. The work of compression can be reduced by water spraying to the admission. The compressed air is divided into three flows:

a Stoechiometric food towards the Burner fed in Fuel,
a flow cooling the wall of the combustion chamber and mixed with the products of combustion of the burner,
a flow intended for the cooling of the turbine.

Contrary to the piston engine, the combustion of a gas turbine is continuous and it is thus necessary to limit the temperature by a broad excess of air to maintain the temperature with a value acceptable for materials (up to 1300 °C into nominal with 2000 °C in short point). This is very penalizing for the output which is maximum towards 4500°C. (the same problem exists for the piston engines)

There exist machines using an injection of vapor in the products of combustion in entry of turbine to increase the flow and thus the power of this one. The vapor is produced by a waste heat boiler heated by the exhaust. It is acted in fact of a Cycle combined simplified.

The turbine generally of axial type includes/understands one or more stages of relaxation. Contrary to the steam turbines, they are always reaction turbines. Two great types of gas turbines are to be distinguished:

simple tree: the compressor and the whole of the stages of relaxation are gathered on the same tree also involving the receiving body,
double tree: the compressor is on the same tree as the stages of turbine strictly necessary to its drive, the other stages of turbine being grouped on a second tree interdependent of the actuated machine.

The second more complex provision allows a better operation load partial and variable what is the case of the engines intended for the Propulsion. The turbines with simple tree are adapted to the electric production which is done with constant mode and higher load.

The realization of the turbine and in particular the stage located behind fire pose metallurgical problems related on the high temperature and the centrifugal force being exerted on the mobile bladings. It requires the use of Acier S strongly allied (Cr-Nor-Goes) and an energetic cooling by air of load taken on the compressor. The use of materials Céramiques is being studied to increase the temperature.

Technical limits. Advantages

Although theoretically higher than the diesel engine, the gas turbine presents severe limitations due to the technical constraints of its realization. These principal limits are the following ones:

ratio compression (and thus output) limited by the number of stage of compression necessary,
significant drops of output of the centrifugal compressors to a mode weaker than the nominal mode,
temperature of combustion (and thus output) limited by the mechanical resistance of the turbine.
falls important of the output with partial load in particular for the machines with simple tree.
cost of Machining of the bladings in particular of the turbine.
Inaptitude with the stops and startings frequent and not very progressive.
Cost of maintenance higher than for a diesel engine
Although being studied, the gas turbines cannot burn heavy fuel contrary to the diesel engine. They thus use expensive fuels.

The advantages inherent in this type of machine are the following:

specific and voluminal power very high because of continuous operation,
simplicity connects construction (a rotor in a casing and a burner) and balancing (few vibrations),
Pollution limited in HC and Nox because of excess of air and the limited temperature,
aptitude for the recovery of heat (cogeneration),
longevity moving stationary.
potential aptitude to use fuels varied and from less quality (producer gas, heavy fuel).

The applications of the gas turbines rise directly from their specific advantages. Thus, the high specific power lends itself well to the aeronautical propulsion in particular on the Hélicoptère S. the marine propulsion calls also more and more upon the gas turbines in particular for the ships at high speed. There exist finally examples of application to the railway propulsion but generally limited to North America and military vehicles like tanks (XM-1 Abrams or Leclerc).

On the other hand, the gas turbine is badly adapted to the road vehicles. Indeed, the mode and radial forces are too important and too rapids to be realizable with a correct output. Moreover, the output reaches with difficulty 30% for compact engines and of low power whereas the Diesel current exceed 40%. On the other hand, they could find an renewed interest for the hybrid chains of propulsion in particular on the heavy trucks, where the installation of the exchangers (in particular recuperator on exhaust) is less problematic.

The other great field of application of the gas turbines is the electrical production. Indeed, they are applications to constant mode and relatively constant load for which the output of these machines is the best. The power varies few hundreds of kw with nearly 300 MW. The most powerful machines are in general associated with steam turbines in combined cycles whose total output currently tends towards 60%. In simple cycle, the output is about 30 to 35% even more for the large machines. In the low powers, the output is even lower than 30% but one then makes profitable the aptitude turbines for combustion for the recovery of heat in applications of Cogénération (simultaneous production of electricity and heat).

Turbocompressor

This term has 2 significances:

a compressor (in general, centrifuges) driven by a turbine (in general, with gas),
a turbine pulled by the exhaust fumes which compress air to inject it into the engine (called usually turbo in the automobile field).

The turbo one indicates a turbine actuated by exhaust fumes of a piston engine and of which work is used to compress the air admitted in the engine. This device represents an important improvement of the traditional engine in particular on the following points:

increase in the specific and voluminal power by a power higher than Rolled equal. In order to maximize this effect, it is necessary to cool the compressed air by an exchanger (intercooler),
suppression of the disadvantage of the curtailed relaxation of the Otto cycles and Diesel from where improvement of output. The improvement of the output is very limited on the petrol engines because the risks of spontaneous combustion (rattling) force to appreciably reduce the compression ratio of the engine itself from where a loss of output.

The turbocompressé engine thus combines a piston engine and a gas turbine, both being bound by a common combustion chamber. It makes it possible to reconcile the advantages of the two types of engines while reducing their respective disadvantages, in particular for the Diesel cycles. This explains the current generalization of this technique. The main issue of the turbocompressor is the same one as the other gas turbines, namely management of walk with weak load or with transitory rate. It mainly is solved today by the turbocompressors called “to variable geometry” provided with fixed bladings with variable Incidence.

Propulsion

It is thanks to their specific Power and density Power density high that small turbines are used to motorize the helicopters. trains (Turbotrain), but also of the tanks, ships… are propelled by gas turbines of average power. The Turbojet S and the Turbopropulseur S are gas turbines used in Aéronautique to propel modern and fast Aéronef S.

Engine

Oil industry uses gas turbines to actuate pumps and compressors for the pipelines.

Electrical production

The gas turbine of great power (>1 MW) is especially used to involve a Alternateur and to produce electricity. The infrastructures and civil engineering necessary for a powerplant equipped with gas turbines are reduced, which makes it possible to install in a few months a power station very close to the place of use of electricity (city, factory) or of the source of fuel (port, drilling, refinery…). Harness and alternator are conveyed in the forms of compact and complete modules which it is enough to assemble and to connect to the networks in climates where the outside temperature can go from -40 to +50°C. One of the advantages of the power stations with gas turbine is the time reduced for the implementation, the manager of an electric distribution network can thus easily modulate production capacity to adapt to the variations of consumption.

The installation of a generator with gas turbine can be accompanied by an installation in Cogénération, in order to recover the significant amounts of energy (approximately 65% of the power consumption) contained in exhaust fumes. The principal application of this type consists in injecting these gases, possibly after passage in a tunnel of post-combustion, in a waste heat boiler, with production of warm water or vapor.

Conclusion

The gas turbine contributes on the whole with the current motorizations. Their advantage of lightness imposes the use in aeronautics of it, while in the field of the strong powers (electrical production) they are dissociated by their adaptation to combined cycles or of cogeneration very powerful. The spark-ignition engines them have their power limited to approximately 10 MW for reasons of mass and obstruction.

External bonds

The World Alliance for Decentralized Energy
French Association of the Gas

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