Gas turbine units (GTU) are a single, relatively compact energy complex, in which a power turbine and a generator are paired. The system has become widespread in the so-called small energy. Great for electricity and heat supply of large enterprises, remote settlements and other consumers. As a rule, gas turbines run on liquid fuel or gas.
On the cutting edge of progress
The leading role in increasing the power capacity of power plants is transferred to gas turbine units and their further evolution - combined cycle plants (CCGT). So, in US power plants since the beginning of the 1990s, more than 60% of commissioned and modernized capacities are already made up by gas turbines and combined cycle power plants, and in some countries their share reached 90% in some years.
Simple gas turbines are also being built in large numbers. The gas turbine unit - mobile, economical in operation and easy to repair - turned out to be the optimal solution for covering peak loads. At the turn of the century (1999-2000), the total capacity of gas turbine units reached 120,000 MW. For comparison: in the 80s, the total capacity of systems of this type was 8000-10 000 MW. A significant part of gas turbines (over 60%) was intended for operation as part of large binary combined cycle plants with an average capacity of about 350 MW.
History reference
The theoretical foundations of the use of combined-cycle technologies were studied in sufficient detail in our country in the early 60s. Already at that time it became clear: the general path of development of the power system is connected specifically with combined-cycle technologies. However, their successful implementation required reliable and highly efficient gas turbine plants.
It is the significant progress in gas turbine construction that has determined the modern qualitative leap in heat power engineering. A number of foreign companies have successfully solved the problem of creating effective stationary gas turbines at a time when domestic leading organizations in the conditions of a command economy were promoting the least promising steam turbine technologies (VET).
If in the 60s the efficiency of gas turbine plants was at the level of 24-32%, then in the late 80s the best stationary power gas turbine plants already had an efficiency (with autonomous use) of 36-37%. This made it possible to create CCGTs based on them, the efficiency of which reached 50%. By the beginning of the new century, this figure was 40%, and in combination with combined-cycle plants, it was 60%.
Comparison of steam turbine and combined cycle plants
In combined cycle plants based on gas turbines, the immediate and real prospect was to obtain an efficiency of 65% or more. At the same time, for steam turbine plants (developed in the USSR), only in the case of successful solution of a number of complex scientific problems related to the generation and use of a pair of supercritical parameters, one can hope for an efficiency of not more than 46-49%. Thus, in terms of efficiency, steam-turbine systems hopelessly lose to steam-gas systems.
Steam turbine power plants are also significantly inferior in terms of cost and construction time. In 2005, in the world energy market, the price of 1 kW for combined cycle power plants with a capacity of 200 MW or more was $ 500-600 / kW. For smaller combined cycle power plants, the cost was in the range of $ 600-900 / kW. Powerful gas turbine units correspond to the values of 200-250 $ / kW. With a decrease in unit power, their price rises, but usually does not exceed $ 500 / kW. These values are several times less than the cost of a kilowatt of electricity of steam turbine systems. For example, the price of installed kilowatts of condensing steam turbine power plants ranges from 2000-3000 $ / kW.
Scheme of a gas turbine installation
The installation includes three basic units: a gas turbine, a combustion chamber, and an air compressor. Moreover, all units are placed in a prefabricated single building. The rotors of the compressor and turbines are rigidly connected to each other, relying on bearings.
Around the compressor are combustion chambers (for example, 14 pcs.), Each in its own separate housing. An air inlet is used to enter the air compressor; air leaves the gas turbine through the exhaust pipe. The GTU case is based on powerful supports placed symmetrically on a single frame.
Principle of operation
Most GTU installations use the principle of continuous combustion, or open cycle:
- Initially, the working fluid (air) is pumped at atmospheric pressure with a suitable compressor.
- Further, the air is compressed to a greater pressure and sent to the combustion chamber.
- Fuel is supplied to it, which burns out at constant pressure, providing a constant supply of heat. Due to the combustion of fuel, the temperature of the working fluid increases.
- Further, the working fluid (now it is already a gas, which is a mixture of air and combustion products) enters the gas turbine, where, expanding to atmospheric pressure, it does useful work (it turns the turbine that generates electricity).
- After the turbine, the gases are discharged into the atmosphere, through which the duty cycle closes.
- The difference in operation of the turbine and compressor is perceived by an electric generator located on a common shaft with the turbine and compressor.
Intermittent combustion plants
Unlike the previous design, intermittent combustion plants use two valves 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 chamber volume is closed.
- With the valves closed, the fuel is burned in the chamber, naturally, its combustion occurs at a constant volume. As a result, the pressure of the working fluid is further increased.
- Then the second valve is opened, and the working fluid enters the gas turbine. In this case, the pressure in front of the turbine will gradually decrease. When it approaches atmospheric, the second valve should be closed, and the first open and repeat the sequence of actions.
Gas Turbine Cycles
Turning to the practical implementation of a particular thermodynamic cycle, designers have to deal with many insurmountable technical obstacles. The most typical example: when the humidity of the steam is more than 8-12%, losses in the flow part of the steam turbine increase sharply, dynamic loads increase, erosion occurs. This ultimately leads to the destruction of the flow part of the turbine.
As a result of these restrictions in the energy sector (to get work), only two basic thermodynamic cycles are still widely used : the Rankine cycle and the Brighton cycle. Most power plants are based on a combination of elements of these cycles.
The Rankine cycle is used for working fluids, which during the implementation of the cycle make a phase transition, steam-powered plants operate in this cycle. For working fluids that cannot be condensed in real conditions and which we call gases, the Brighton cycle is used. Gas turbine units and ICE engines operate in this cycle.
Fuel used
The vast majority of gas turbines are designed to run on natural gas. Sometimes liquid fuel is used in low power systems (less often - medium, very rarely - high power). The transition of compact gas turbine systems to the use of solid combustible materials (coal, less often peat and wood) is becoming a new trend. These trends are related 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 plants capable of efficiently operating on solid fuel is actively gaining momentum.
The difference between ICE and GTU
The fundamental difference between internal combustion engines and gas turbine complexes is as follows. In the internal combustion engine, 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 spaced into separate structural units:
- compression is carried out in the compressor;
- fuel combustion, respectively, in a special chamber;
- expansion of combustion products is carried out in a gas turbine.
As a result, gas turbine units and internal combustion engines are not very similar, although they operate on similar thermodynamic cycles.
Output
With the development of small energy, increasing its efficiency, the systems of gas turbines and vocational schools occupy an increasing share in the global energy system. Accordingly, a promising profession is an operator 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 combined-cycle power plant of the new generation in the Russian Federation was the North-Western Thermal Power Plant in St. Petersburg.