Power gas-turbine plants. Gas turbine cycle cycles

Gas turbine plants (GTU) are a single, relatively compact energy complex, in which a power turbine and a generator work together. The system is widely used in the so-called small-scale power engineering. Excellent for electricity and heat supply of large enterprises, remote settlements and other consumers. As a rule, gas turbines operate on liquid fuel or gas.

On the edge of progress

In building up the power capacity of power plants, the main role shifts to gas turbine installations and their further evolution - combined-cycle plants (CCGT). For example, at the US power plants, since the early 1990s, more than 60% of the installed and upgraded facilities are already made by GTU and CCGT, and in some countries in some years their share reached 90%.

In large numbers, simple GTUs are also being built. The gas-turbine plant - mobile, economical in operation and easy to repair - proved to be the optimal solution for covering peak loads. At the turn of the century (1999-2000), the total capacity of gas turbine plants reached 120,000 MW. For comparison: in the 80 years the total capacity of this type of systems was 8000-10 000 MW. A significant part of the GTU (more than 60%) was intended for operation as part of large binary combined-cycle plants with an average capacity of about 350 MW.

Historical reference

The theoretical foundations of the use of combined-cycle technologies were studied in sufficient detail in our country in the early 1960s. Already at that time it became clear: the general way of development of heat power engineering is connected with steam-gas technologies. However, for their successful implementation, reliable and highly efficient gas turbine units were needed.

It is essential progress in gas turbine construction that has determined the modern qualitative leap of heat power engineering. A number of foreign firms have successfully solved the problems of creating efficient stationary gas turbines at the time when domestic leading leading organizations in the conditions of the command economy were engaged in the promotion of the least promising steam-turbine technologies (PTU).

If in the 60s the efficiency of gas turbine plants was at the level of 24-32%, then in the late 1980s the best stationary power gas turbine plants already had an efficiency (in case of autonomous use) of 36-37%. This allowed them to create CCGTs, the efficiency of which reached 50%. By the beginning of the new century, this indicator was equal to 40%, and in combination with gas-steam - and at least 60%.

Comparison of steam-turbine and combined-cycle plants

In steam and gas installations based on GTU, the closest and realistic prospect was to obtain an efficiency factor of 65% or more. At the same time, for steam-turbine plants (developed in the USSR), only if a number of complex scientific problems related to the generation and use of a pair of supercritical parameters are successfully solved can one hope for an efficiency of not more than 46-49%. Thus, steam engines are hopelessly losing steam-gas systems in terms of efficiency.

Significantly inferior steam turbine power plants also in terms of cost and construction time. In 2005, in the world energy market, the price of 1 kW for a CCP with a capacity of 200 MW or more was 500-600 $ / kW. For CCGTs of lower capacities, the cost was in the range of 600-900 $ / kW. Powerful gas-turbine plants correspond to values of 200-250 $ / kW. With a decrease in unit capacity, their price increases, but does not exceed usually $ 500 / kW. These values are several times less than the cost of a kilowatt of electricity for steam turbine systems. For example, the price of an installed kilowatt for condensing steam-turbine power plants varies between 2000-3000 $ / kW.

Scheme of gas turbine plant

The installation includes three basic units: a gas turbine, a combustion chamber and an air compressor. And all units are housed in a prefabricated single body. Compressor rotors and turbines are connected to each other rigidly, relying on bearings.

Around the compressor are placed combustion chambers (for example, 14 pcs.), Each in its own housing. To enter the compressor air is the inlet pipe, from the gas turbine, air flows through the exhaust pipe. The GTU body is based on powerful supports placed symmetrically on a single frame.

Principle of operation

Most GTU installations use the principle of continuous combustion, or an open cycle:

• At first, the working medium (air) is pumped at atmospheric pressure by a suitable compressor.
• Then the air is compressed to a higher pressure and is sent to the combustion chamber.
• It is fed with fuel, which burns at constant pressure, ensuring a constant supply of heat. Thanks to the combustion of fuel, the temperature of the working fluid increases.
• Further, the working fluid (now it is gas, which is a mixture of air and combustion products) enters the gas turbine, where, expanding to atmospheric pressure, it does useful work (turns the turbine producing electricity).
• After the turbine, the gases are discharged into the atmosphere through which the duty cycle closes.
• The difference between the operation of the turbine and the compressor is perceived by an electric generator located on a common shaft with a turbine and a compressor.

Intermittent combustion plants

Unlike the previous design, in the intermittent combustion plants, two valves are used instead of one.

• The compressor pumps air into the combustion chamber through the first valve with the second valve closed.
• When the pressure in the combustion chamber rises, the first valve is closed. As a result, the volume of the chamber turns out to be closed.
• With closed valves in the chamber, fuel is burned, naturally, its combustion occurs at a constant volume. As a result, the pressure of the working fluid is further increased.
• Next, a second valve is opened and the working fluid enters the gas turbine. The pressure in front of the turbine will gradually decrease. When it approaches the atmospheric one, the second valve should be closed, and the first one should open and repeat the sequence of actions.

Gas turbine cycle cycles

Turning to the practical implementation of this or that thermodynamic cycle, designers have to face a lot of insurmountable technical obstacles. The most typical example: at a steam moisture content of more than 8-12%, losses in the flowing part of a steam turbine increase sharply, dynamic loads increase, and erosion occurs. This ultimately leads to the destruction of the flowing part of the turbine.

As a result of these limitations in the energy sector (to get a job), only two basic thermodynamic cycles are still in widespread use : the Rankine cycle and the Brighton cycle. Most power plants are built on a combination of elements of these cycles.

The Rankine cycle is used for working bodies that perform a phase transition during the course of the cycle, steam power plants operate in this cycle. For working bodies that can not be condensed in real conditions and which we call gases, use the Brighton cycle. For this cycle, gas turbine installations and engines of ICE operate.

Used fuel

The overwhelming majority of GTUs are designed to operate on natural gas. Sometimes liquid fuel is used in low-power systems (less often - medium, very rarely - high power). A new trend is the transition of compact gas turbine systems to the use of solid combustible materials (coal, less often peat and wood). These trends are due to the fact that gas is a valuable technological raw material for the chemical industry, where its use is often more cost-effective than in the energy sector. The production of gas turbine units capable of efficiently operating on solid fuels is gaining momentum.

The difference between ICE and GTU

The principal difference between internal combustion engines and gas turbine complexes is as follows. In the ICE, the processes of air compression, fuel combustion and expansion of combustion products occur within the same structural element, called the engine cylinder. In GTU, these processes are separated by separate structural nodes:

• Compression is carried out in the compressor;
• Combustion of fuel, respectively, in a special chamber;
• Expansion of combustion products is carried out in a gas turbine.

As a result, gas turbine installations and ICEs are not very similar, although they work in similar thermodynamic cycles.

Conclusion

With the development of small-scale power engineering, the increase in its efficiency of the GTU and PTU system takes an increasing share in the global energy system. Accordingly, the increasingly promising profession is the machinist of gas turbine plants. Following the Western partners, a number of Russian manufacturers have mastered the production of cost-effective gas turbine-type plants. The first steam-gas power plant of the new generation in the Russian Federation was the North-West thermal power station in St. Petersburg.