Gritsyna V.P.
In connection with the multiple growth of electricity tariffs in Russia, many enterprises are considering the construction of their own low-capacity power plants. In a number of regions, programs are being developed for the construction of small or mini thermal power plants, in particular, as a replacement for obsolete boiler houses. At a new small CHP plant with a fuel utilization rate of up to 90% with a full use of the body in production and for heating, the cost of electricity received can be significantly lower than the cost of electricity received from the power grid.
When considering projects for the construction of small thermal power plants, power engineers and specialists of enterprises are guided by the indicators achieved in the large power industry. Continuous improvement of gas turbines (GTU) for use in large-scale power generation has made it possible to increase their efficiency up to 36% or more, and the use of a combined steam-gas cycle (CCGT) has increased the electrical efficiency of thermal power plants up to 54% -57%.
However, in the small-scale power industry it is inappropriate to consider the possibility of using complex schemes of combined cycles of CCGT for the production of electricity. In addition, gas turbines, in comparison with gas engines, as drives for electric generators, lose significantly in terms of efficiency and performance characteristics, especially at low powers (less than 10 MW). Since in our country neither gas turbines nor gas piston engines have yet been widely used in small-scale stationary power generation, the choice of a specific technical solution is a significant problem.
This problem is also relevant for large-scale energy, i.e. for power systems. In modern economic conditions, in the absence of funds for the construction of large power plants on obsolete projects, which can already be attributed to domestic project CCGT 325 MW, designed 5 years ago. Energy systems and RAO UES of Russia should pay special attention to the development of small-scale power generation, at whose facilities new technologies can be tested, which will make it possible to begin the revival of domestic turbine-building and machine-building plants and, in the future, switch to large capacities.
In the last decade, large diesel or gas engine thermal power plants with a capacity of 100-200 MW have been built abroad. The electrical efficiency of diesel or gas engine power plants (DTPP) reaches 47%, which exceeds the performance of gas turbines (36%-37%), but is inferior to the performance of CCGTs (51%-57%). CCGT power plants include a large range of equipment: a gas turbine, a waste heat steam boiler, a steam turbine, a condenser, a water treatment system (plus a booster compressor if it is burned natural gas low or medium pressure. Diesel generators can run on heavy fuel, which is 2 times cheaper than gas turbine fuel and can run on low pressure gas without the use of booster compressors. According to S.E.M.T. PIELSTICK , total costs for 15 years for the operation of a diesel power unit with a capacity of 20 MW is 2 times less than for a gas turbine TPP of the same capacity when using liquid fuel by both power plants.
Promising Russian manufacturer diesel power units up to 22 MW is the Bryansk Machine-Building Plant, which offers customers power units with increased efficiency up to 50% for operation both on heavy fuel with a viscosity of up to 700 cSt at 50 C and a sulfur content of up to 5%, and for operation on gaseous fuel.
The option of a large diesel thermal power plant may be preferable to a gas turbine power plant.
In small-scale power generation, with unit capacities of less than 10 MW, the advantages of modern diesel generators are even more pronounced.
Let us consider three variants of thermal power plants with gas turbine plants and gas piston engines.
The fuel utilization factor for the first two options of power plants (with different electrical efficiency) due to heat supply can reach 80% -94%, both in the case of gas turbines and for motor drives.
The profitability of all variants of power plants depends on the reliability and efficiency, first of all, of the "first stage" - the drive of the electric generator.
Enthusiasts for the use of small gas turbines are campaigning for their widespread use, noting the higher power density. For example, in [1] it is reported that Elliot Energy Systems (in 1998-1999) is building a distribution network of 240 distributors in North America providing engineering and service support for the sale of "micro" gas turbines. The power grid ordered a 45 kW turbine to be ready for delivery in August 1998. It also stated that the electrical efficiency of the turbine was as high as 17%, and noted that gas turbines were more reliable than diesel generators.
This statement is exactly the opposite!
If you look at Table. 1. then we will see that in such a wide range from hundreds of kW to tens of MW, the efficiency of the motor drive is 13% -17% higher. The indicated resource of the motor drive of the company "Vyartsilya" means a guaranteed resource until a complete overhaul. The resource of new gas turbines is a calculated resource, confirmed by tests, but not by statistics of work in real operation. According to numerous sources, the resource of gas turbines is 30-60 thousand hours with a decrease with a decrease in power. The resource of diesel engines of foreign production is 40-100 thousand hours or more.
Table 1
Main technical parameters of electric generator drives
G-gas-turbine power plant, D-gas-piston generating plant of Vyartsilya.
D - diesel from the Gazprom catalog
* The minimum value of the required pressure of the fuel gas = 48 ATA!!
Performance characteristics
Electrical efficiency (and power) According to Värtsilä data, when the load is reduced from 100% to 50%, the efficiency of an electric generator driven by a gas engine changes little.
The efficiency of a gas engine practically does not change up to 25 °C.
Power gas turbine evenly falls from -30 °C to +30 °C.
At temperatures above 40 °C, the reduction in gas turbine power (from nominal) is 20%.
Start time gas engine from 0 to 100% load is less than a minute and emergency in 20 seconds. It takes about 9 minutes to start a gas turbine.
Gas supply pressure for a gas turbine it should be 16-20 bar.
The gas pressure in the network for a gas engine can be 4 bar (abs) and even 1.15 bar for a 175 SG engine.
Capital expenditures at a thermal power plant with a capacity of about 1 MW, according to Vartsila specialists, they amount to $1,400/kW for a gas turbine plant and $900/kW for a gas piston power plant.
Combined cycle application at small CHPPs, by installing an additional steam turbine is impractical, since it doubles the number of thermal and mechanical equipment, the area of the turbine hall and the number service personnel with an increase in power only 1.5 times.
With a decrease in the capacity of the CCGT from 325 MW to 22 MW, according to the data of the NPP "Mashproekt" plant (Ukraine, Nikolaev), the front efficiency of the power plant decreases from 51.5% to 43.6%.
The efficiency of a diesel power unit (using gas fuel) with a capacity of 20-10 MW is 43.3%. It should be noted that in the summer, at a CHPP with a diesel unit, hot water supply can be provided from the engine cooling system.
Calculations on the competitiveness of power plants based on gas engines showed that the cost of electricity at small (1-1.5 MW) power plants is approximately 4.5 cents / kWh), and at large 32-40 MW gas-powered plants 3, 8 US cents/kWh
According to a similar calculation method, electricity from a condensing nuclear power plant costs approximately 5.5 US cents/kWh. , and coal IES about 5.9 cents. US/kWh Compared to a coal-fired CPP, a plant with gas engines generates electricity 30% cheaper.
The cost of electricity produced by microturbines, according to other sources, is estimated at between $0.06 and $0.10/kWh
The expected price for a complete 75 kW gas turbine generator (US) is $40,000, which corresponds to the unit cost for larger (more than 1000 kW) power plants. The big advantage of power units with gas turbines is their smaller dimensions, 3 or more times less weight.
It should be noted that the unit cost of Russian-made electric generator sets based on automobile engines with a capacity of 50-150 kW may be several times less than the mentioned turbo blocks (USA), given the serial production of engines and the lower cost of materials.
Here is the opinion of Danish experts who evaluate their experience in the implementation of small power plants.
"Investment in a completed, turnkey natural gas CHP plant with a capacity of 0.5-40 MW is 6.5-4.5 million Danish krone per MW (1 krone was approximately equal to 1 ruble in the summer of 1998) Combined cycle CHP plants below 50 MW will achieve an electrical efficiency of 40-44%.
Operating costs for lubricating oils, maintenance and staffing at CHPs reach 0.02 kroons per 1 kWh produced at gas turbines. At CHP plants with gas engines, operating costs are about 0.06 dat. kroons per 1 kWh. At current electricity prices in Denmark, the high performance of gas engines more than offsets their higher operating costs.
Danish specialists believe that most CHP plants below 10 MW will be equipped with gas engines in the coming years."
conclusions
The above estimates, it would seem, unambiguously show the advantages of a motor drive at low power of power plants.
However, at present, the power of the proposed Russian-made motor drive on natural gas does not exceed the power of 800 kW-1500 kW (RUMO plant, N-Novgorod and Kolomna Machine Plant), and several plants can offer turbo drives of higher power.
Two factories in Russia: plant im. Klimov (St. Petersburg) and Perm Motors are ready to supply complete power units of mini-CHP with waste heat boilers.
In the case of organizing a regional service center issues of maintenance and repair of small turbines of turbines can be solved by replacing the turbine with a backup one in 2-4 hours and its further repair in the factory conditions of the technical center.
The efficiency of gas turbines can currently be increased by 20-30% by applying power injection of steam into a gas turbine (STIG cycle or steam-gas cycle in one turbine). In previous years, this technical solution was tested in full-scale field tests of the Vodolei power plant in Nikolaev (Ukraine) by NPP Mashproekt and PA Zarya, which made it possible to increase the power of the turbine unit from 16 to 25 MW and the efficiency was increased from 32 .8% to 41.8%.
Nothing prevents us from transferring this experience to smaller capacities and thus implementing a CCGT in serial delivery. In this case, the electrical efficiency is comparable to that of diesels, and the specific power increases so much that capital costs can be 50% lower than in a gas engine driven CHP plant, which is very attractive.
This review was carried out in order to show: that when considering options for the construction of power plants in Russia, and even more so the directions for creating a program for the construction of power plants, it is necessary to consider not individual options that may offer design organizations, but a wide range of issues, taking into account the capabilities and interests of domestic and regional equipment manufacturers.
Literature
1. Power Value, Vol.2, No.4, July/August 1998, USA, Ventura, CA.
The Small Turbine Marketplace
Stan Price, Northwest Energy Efficiency Council, Seattle, Washington and Portland, Oregon
2. New directions of energy production in Finland
ASKO VUORINEN, Assoc. tech. Sciences, Vartsila NSD Corporation JSC, "ENERGETIK" -11.1997. page 22
3. District heating. Research and development of technology in Denmark. Ministry of Energy. Energy Administration, 1993
4. DIESEL POWER PLANTS. S.E.M.T. PIELSTICK. POWERTEK 2000 Exhibition Prospectus, March 14-17, 2000
5. Power plants and electrical units recommended for use at the facilities of OAO GAZPROM. CATALOG. Moscow 1999
6. Diesel electrical station. Prospect of OAO "Bryansk Machine-Building Plant". 1999 Exhibition brochure POWERTEK 2000/
7. NK-900E Block-modular thermal power plant. OJSC Samara Scientific and Technical Complex named after V.I. N.D. Kuznetsova. Exhibition brochure POWERTEK 2000
Like a diesel or gasoline engine, a gas turbine is an internal combustion engine with an intake-compression-combustion (expansion)-exhaust duty cycle. But, the basic movement is significantly different. The working body of the gas turbine rotates, and in the piston engine it moves reciprocating.
The working principle of a gas turbine is shown in the figure below. First, the air is compressed by the compressor, then the compressed air is fed into the combustion chamber. Here, the fuel, continuously burning, produces gases with high temperature and pressure. From the combustion chamber, the gas, expanding in the turbine, presses on the blades and rotates the turbine rotor (a shaft with impellers in the form of discs carrying rotor blades), which in turn again rotates the compressor shaft. The remaining energy is removed through the working shaft.
Features of gas turbines
Types of gas turbines by design and purpose
The most basic type of gas turbine is the jet thruster, which is also the simplest in design.
This engine is suitable for aircraft flying at high speed and is used in supersonic aircraft and jet fighters.
This type has a separate turbine behind the turbojet that spins a large fan in front. This fan increases airflow and draft.
This type is quiet and economical at subsonic speeds, which is why gas turbines of this type are used for passenger aircraft engines.
This gas turbine delivers power as torque, with the turbine and compressor sharing a common shaft. Part of the useful power of the turbine goes to the rotation of the compressor shaft, and the rest of the energy is transferred to the working shaft.
This type is used when a constant rotation speed is needed, for example, as a generator drive.
In this type, the second turbine is placed after the gas generator turbine and the rotational force is transferred to it by the jet. This rear turbine is called the power turbine. Since the shafts of the power turbine and compressor are not mechanically connected, the speed of rotation of the working shaft is freely adjustable. Suitable as a mechanical drive with a wide range of rotational speeds.
This type is widely used in propeller-driven aircraft and helicopters, as well as applications such as pump/compressor drives, marine main engines, generator drives, etc.
What is GREEN series gas turbine?
The principle that Kawasaki has followed in the gas turbine business since the development of our first gas turbine in 1972 has allowed us to offer customers ever more advanced equipment, i.e. more energy efficient and environmentally friendly. The ideas embodied in our products have been highly appreciated by the world market and have allowed us to accumulate references for more than 10,000 turbines (at the end of March 2014) as part of standby generators and cogeneration systems.
Kawasaki gas turbines have always been a great success, and we have given them the new name "GREEN Gas Turbines" to show our even greater commitment to this principle.
Power units - drives of electric generators for autonomous small thermal power plants can be diesel, gas piston, microturbine and gas turbine engines.
It is written about the advantages of certain generating plants and technologies a large number of discussion and polemical articles. As a rule, in disputes in the pen, either one or the other often remains in disgrace. Let's try to figure out why.
The determining criteria for choosing power units for the construction of autonomous power plants are the issues of fuel consumption, the level of operating costs, as well as the payback period for power plant equipment.
Important factors in the choice of power units are ease of operation, level Maintenance and repair, as well as the place of repair of power units. These issues are primarily related to the costs and problems that the owner of an autonomous power plant may subsequently have.
In this article, the author does not have a selfish goal to prioritize piston or turbine technologies. The types of power plants of power plants are more correct, it is best to select directly for the project, based on the individual conditions and technical specifications of the customer.
When choosing power equipment for the construction of an autonomous gas-fired CHP plant, it is advisable to consult with independent specialists from engineering companies that are already building turnkey power plants. The engineering company must have completed projects which you can see and visit with a tour. One should also take into account such a factor as the weakness and underdevelopment of the generating equipment market in Russia, where the real sales volumes, in comparison with developed countries, are small and leave much to be desired - this, first of all, is reflected in the volume and quality of offers.
Gas Reciprocating Plants vs. Gas Turbine Engines - Operating Costs
Is it true that the operating costs of a mini-CHP with reciprocating machines are lower than the operating costs of a power plant with gas turbines?
The cost of a major overhaul of a gas piston engine can be 30–350% of the initial cost of the power unit itself, and not of the entire power plant - during the overhaul, the piston group is replaced. Gas reciprocating units can be repaired on site without complex diagnostic equipment once every 7-8 years.
The cost of repairing a gas turbine plant is 30–50% of the initial investment. As you can see, the costs are about the same. Real, honest prices for gas turbine and piston units of comparable power and quality are also similar.
Capital repairs of the gas turbine plant due to its complexity are not carried out on site. The supplier must take away the spent unit and bring a replacement gas turbine unit. The old unit can only be restored to factory conditions.
You should always take into account compliance with the maintenance schedule, the nature of the loads and the operating modes of the power plant, regardless of the type of installed power units.
The question, which is often exaggerated, about the finickyness of the turbine to operating conditions, is associated with outdated information from forty years ago. Then "on the ground", in the drive of power plants, aircraft turbines "removed from the wing" of the aircraft were used. Such turbines with minimal changes adapted to work as the main power units for power plants.
Today, modern autonomous power plants use turbines of industrial, industrial design, designed for continuous operation with various loads.
The lower limit of the minimum electrical load, officially declared by manufacturers for industrial turbines, is 3–5%, but in this mode, fuel consumption increases by 40%. The maximum load of a gas turbine plant, in limited time intervals, can reach 110-120%.
Modern gas-piston installations have phenomenal efficiency, based on a high level of electrical efficiency. The “problems” associated with the operation of gas piston units at low loads are resolved positively even at the design stage. Design must be of high quality.
Compliance with the operating mode recommended by the manufacturer will extend the life of engine parts, thus saving money to the owner of an autonomous power plant. Sometimes, in order to bring the gas-piston machines to the nominal mode at partial loads, one or two electric boilers are included in the project of the thermal scheme of the station, which make it possible to provide the desired 50% load.
For power plants based on gas piston units and gas turbines, it is important to comply with the N + 1 rule - the number of operating units plus one more for the reserve. “N + 1” is a convenient, rational number of installations for the operating personnel. This is due to the fact that for power plants of any types and types it is necessary to carry out routine and repair work.
A company connected to the network can install only one unit and use its own electricity at cost, and during maintenance, be powered by the public electricity network, paying according to the meter. This is cheaper than "+1", but, unfortunately, is not always feasible. This is due, as a rule, to the lack of an electrical network in general, or to the incredible high cost of technical conditions for the connection itself.
Unscrupulous dealers of gas piston units and gas turbines, before selling the equipment to the buyer, as a rule, provide only prospectuses - commercial literature general plan and extremely rarely - accurate information about the total operating costs and produced technical regulations.
On powerful gas piston units, the oil does not need to be changed. At permanent job it is simply produced, not having time to grow old. Oil on such installations is constantly topped up. Such operating modes are provided for by a special design of powerful gas piston engines and are recommended by the manufacturer.
Engine oil waste is 0.25-0.45 grams per kilowatt produced per hour. Loss is always higher when the load is reduced. As a rule, a gas piston engine kit includes a special reservoir for continuous oil topping up, and a mini-laboratory for checking its quality and determining the replacement period.
Accordingly, they must be replaced oil filters or cartridges in them.
Since engine oil does burn out, piston units have a little more high level harmful emissions into the atmosphere than gas turbines. But since the gas burns completely and is one of the cleanest types of fuel, then talking about serious atmospheric pollution is just “stupid checkers”. A couple of old Hungarian Ikarus buses cause much more serious harm to the environment. To meet environmental requirements, when using reciprocating machines, it is necessary to build higher chimneys, taking into account the already existing MPC level in the environment.
Waste oil from gas piston units cannot simply be dumped on the ground - it requires disposal - this is an "expense" for the owners of the power plant. But you can earn money on this - specialized organizations buy used motor oil.
Many of us use engine oil in our piston engines. If the engine is serviceable, properly operated and refueled with normal fuel, then no financial cataclysms associated with its consumption occur.
The same is true at reciprocating power plants: - there is no need to be afraid of engine oil consumption, it will not ruin you, during the normal operation of modern high-quality gas piston installations, the costs for this article are only 2-3 (!) kopecks per 1 kW of generated electricity.
In modern gas turbine installations, oil is used only in the gearbox. Its volume can be considered insignificant. The replacement of gear oil in gas turbines is carried out on average once every 3-5 years, and its topping up is not required.
To carry out the service in full, a beam crane must be included in the set of a powerful gas piston installation. With the help of a beam crane, heavy parts of piston engines are removed. The use of a beam crane requires high ceilings in the machine rooms of the reciprocating power plant. For the repair of gas-piston installations of small and medium power, simpler lifting mechanisms can be dispensed with.
Gas piston power plants upon delivery can be equipped with various repair tools and devices. Its presence implies that even all critical operations can be carried out by qualified personnel on site. Virtually all repair work on gas turbines can be carried out either at the manufacturing plant or with the direct assistance of factory specialists.
Once every 3-4 months, the spark plugs need to be replaced. Replacing candles is only 1-2 (!) kopecks in the cost of 1 kW / h of own electricity.
Piston units, unlike gas turbine units, are liquid-cooled, so the personnel of an autonomous power plant must constantly monitor the level of the coolant and carry out periodic replacement, and if it is water, then it is necessary to carry out its chemical preparation.
The above features of the operation of reciprocating units are absent in gas turbine plants. Gas turbine plants do not use such consumables and components as:
- engine oil,
- spark plug,
- oil filters,
- coolant,
- sets of high voltage wires.
But gas turbines cannot be repaired on the spot, and much more greater expense gas cannot be compared with the operating costs and consumables for reciprocating units.
What to choose? Gas piston or gas turbine installations?
How do the power of power units of power plants and the ambient temperature correlate?
With a significant increase in temperature environment the power of the gas turbine is reduced. But with a decrease in temperature, the electric power of a gas turbine, on the contrary, increases. Electric power parameters, according to existing ISO standards, measured at t +15 °C.
Sometimes an important point is the fact that a gas turbine plant is capable of delivering 1.5 times more free thermal energy than a piston unit of similar power. When using a powerful (from 50 MW) autonomous CHP in public utilities, for example, this can be of decisive importance when choosing the type of power units, especially with a large and uniform consumption of thermal energy.
On the contrary, where heat is not required in large quantities, but an emphasis is needed on the production of electrical energy, it will be more economically feasible to use gas piston plants.
The high temperature at the outlet of gas turbine plants makes it possible to use a steam turbine as part of a power plant. This equipment is in demand if the consumer needs to obtain the maximum amount of electrical energy with the same volume of gas fuel spent, and thus achieve high electrical efficiency - up to 59%. An energy complex of this configuration is more difficult to operate and costs 30-40% more than usual.
Power plants with steam turbines in their structure, as a rule, are designed for a fairly large power - from 50 MW and above.
Let's talk about the most important thing: gas piston units versus gas turbine power units - efficiency
efficiency power plant more than relevant - because it affects fuel consumption. Average specific consumption gas fuel per 1 generated kW / h is much less for a gas piston plant, and at any load mode (although continuous loads of less than 25% are contraindicated for piston engines).
The electrical efficiency of reciprocating machines is 40–44%, and that of gas turbines is 23–33% (in a steam-gas cycle, a turbine is capable of delivering an efficiency of up to 59%).
The steam-gas cycle is used at high power plants - from 50-70 MW.
If you need to manufacture a locomotive, an aircraft or a sea vessel, then one of the determining indicators is the efficiency factor (COP) of the power plant. The heat that is obtained during the operation of the engine of a locomotive, aircraft (or vessel) is not used and is released into the atmosphere.
But we are building not a locomotive, but a power plant, and when choosing the type of power units for an autonomous power plant, the approach is somewhat different - here it is necessary to talk about the completeness of the use of combustible fuel - the fuel utilization factor (FU).
Burning, the fuel does the main work - it rotates the generator of the power plant. The rest of the fuel combustion energy is heat that can and should be used. In this case, the so-called "overall efficiency", or rather, the fuel utilization factor (FUE) of the power plant will be about 80-90%.
If the consumer expects to use the thermal energy of an autonomous power plant in full, which is usually unlikely, then the coefficient of performance (COP) of an autonomous power plant does not have practical value.
When the load is reduced to 50%, the electrical efficiency of the gas turbine decreases.
In addition, turbines require high gas inlet pressure, and for this, compressors (piston) are necessarily installed, and they also increase fuel consumption.
A comparison of gas turbine plants and gas piston engines as part of a mini-CHP shows that the installation of gas turbines is expedient at facilities that have uniform electrical and thermal needs with a power of more than 30-40 MW.
From the foregoing, it follows that the electrical efficiency of power units of various types has a direct projection on fuel consumption.
Gas piston units consume a quarter or even a third less fuel than gas turbine units - this is the main cost item!
Accordingly, with a similar or equal cost of the equipment itself, a cheaper Electric Energy produced in gas turbines. Gas is the main expense item in the operation of an autonomous power plant!
Gas piston units vs. gas turbine engines - inlet gas pressure
Is it always necessary to have a high pressure gas pipeline when using gas turbines?
For all types of modern power units of power plants, the pressure of the supplied gas is of no practical importance, since the gas turbine unit always includes a gas compressor, which is included in the cost of the energy complex.
The compressor provides the required pressure performance of the gaseous fuel. Modern compressors are extremely reliable and low maintenance units. In the world modern technologies For both gas piston engines and gas turbines, it is only important to have the proper amount of gas fuel to ensure the normal operation of an autonomous power plant.
However, one should not forget that the booster compressor also requires considerable energy, Supplies and service. Paradoxically, reciprocating compressors are often used for powerful turbines.
Gas piston engines vs. gas turbine units - dual-fuel installations
It is often written and said that dual-fuel installations can only be piston. Is it true?
This is not true. All well-known gas turbine manufacturers have dual-fuel units in their range. The main feature of the dual-fuel installation is its ability to work both on natural gas and diesel fuel. Due to the use of two types of fuel in a dual-fuel plant, a number of its advantages can be noted compared to mono-fuel plants:
- in the absence of natural gas, the unit automatically switches to diesel fuel;
- during transients, the unit automatically switches to diesel operation.
When entering the operating mode, the reverse process of switching to operation on natural gas and diesel fuel is carried out;
Do not forget about the fact that the first turbines were originally designed to operate on liquid fuel - kerosene.
Dual-fuel installations are still of limited use and are not needed for most autonomous CHP plants - there are simpler engineering solutions for this.
Gas piston installations versus gas turbine ones - the number of starts
What can be the number of starts of gas piston units?
Number of starts: a gas piston engine can start and stop an unlimited number of times, and this does not affect its engine life. But frequent starts-stops of gas piston units, with loss of power own needs, can lead to wear of the most loaded components (turbocharger bearings, valves, etc.).
Due to the sharp changes in thermal stresses that occur in the most critical components and parts of the gas turbine hot duct during quick starts of the unit from a cold state, it is preferable to use a gas turbine plant for continuous, continuous operation.
Gas piston engines of power plants against gas turbine plants - a resource before overhaul
What can be the resource of the installation before the overhaul?
The resource before overhaul is 40,000–60,000 working hours for a gas turbine. With proper operation and timely maintenance of a gas piston engine, this figure is also equal to 40,000–60,000 operating hours. However, there are other situations when overhaul occurs much earlier.
Gas piston units vs. gas turbine engines - capital investments and prices
What capital investments (investments) will be required in the construction of the power plant? What is the cost of building an autonomous power complex on a turnkey basis?
Calculations show that investments (dollar/kW) in the construction of a thermal power plant with gas piston engines are approximately equal to gas turbine plants. Finnish thermal power plant WARTSILA with a capacity of 9 MW will cost the customer approximately 14 million euros. A similar gas turbine thermal power plant based on first-class units will cost $15.3 million.
Gas piston engines against gas turbine plants - ecology
How are environmental requirements met?
It should be noted that gas piston units are inferior to gas turbine units in terms of NO x emissions. Since engine oil burns out, piston units have a slightly higher level of harmful emissions into the atmosphere than gas turbine units.
But this is not critical: the SES asks for the background level according to the MPC at the location of the mini-CHP. After that, the dispersion is calculated so that the “additive” of harmful substances from the mini-CHP added to the background does not lead to exceeding the MPC. Through several iterations, the minimum height of the chimney is selected, at which the requirements of SanPiN are met. The addition from the plant of 16 MW in terms of NO x emissions is not so significant: at a chimney height of 30 m - 0.2 MPC, at 50 m - 0.1 MPC.
The level of harmful emissions from most modern gas turbine plants does not exceed 20-30 ppm, and in some projects this may have a certain value.
Piston installations during operation have vibrations and low-frequency noise. Bringing noise to standard values is possible, appropriate engineering solutions are simply needed. In addition to calculating dispersion when developing a section project documentation"Environmental Protection" an acoustic calculation is made and it is checked whether the selected design solutions and the materials used meet the requirements of SanPiN in terms of noise.
Any equipment emits noise in a certain frequency spectrum. Gas turbine plants did not pass this cup.
Gas piston units vs. gas turbine engines - conclusions
With linear loads and compliance with the N + 1 rule, the use of gas piston engines as the main source of power supply is possible. As part of such a power plant, backup units and tanks are needed to store the second type of fuel - diesel.
In the power range up to 40-50 MW, the use of reciprocating motors at mini-CHPs is considered absolutely justified.
In the case of using gas piston units, the consumer can completely get away from external power supply, but only with a deliberate and balanced approach.
Piston installations can also be used as backup or emergency sources of electricity.
A certain alternative to piston installations is gas microturbines. True, the prices for microturbines “bite” a lot and amount to ~ $ 2500-4000 per 1 kW of installed power!
A comparison of gas turbine plants and gas piston engines as part of a mini-CHP shows that the installation of gas turbines is possible at any facilities that have electrical loads of more than 14-15 MW, but due to the high gas consumption, turbines are recommended for power plants of much larger capacity - 50-70 MW.
For many modern generating plants, 200,000 hours of operation is not a critical value, and subject to the scheduled maintenance schedule and the phased replacement of turbine parts subject to wear: bearings, injectors, various auxiliary equipment (pumps, fans), further operation of the gas turbine plant remains economically feasible. High-quality gas piston units today also successfully overcome 200,000 hours of operation.
This is confirmed by the modern practice of operating gas turbine / gas piston plants around the world.
When choosing the power units of an autonomous power plant, expert advice is needed!
Expert advice and supervision are also necessary in the construction of autonomous power plants. To solve the problem, we need an engineering company with experience and completed projects.
Engineering allows you to competently, unbiasedly and objectively determine the choice of the main and auxiliary equipment for selection optimal configuration- complete set of your future power plant.
Qualified engineering saves significant cash customer, and this is 10-40% of the total cost. Engineering from professionals in the power industry avoids costly mistakes in the design and selection of equipment suppliers.
The principle of operation of gas turbine plants
Fig.1. Scheme of a gas turbine unit with a single-shaft gas turbine engine of a simple cycle
Clean air is supplied to the compressor (1) of the gas turbine power unit. Under high pressure, air from the compressor is sent to the combustion chamber (2), where the main fuel, gas, is also supplied. The mixture ignites. When a gas-air mixture is burned, energy is generated in the form of a stream of hot gases. This flow rushes at high speed to the turbine wheel (3) and rotates it. rotational kinetic energy through the turbine shaft drives the compressor and electric generator (4). From the terminals of the power generator, the generated electricity, usually through a transformer, is sent to the power grid, to energy consumers.
Gas turbines are described by the Brayton thermodynamic cycle. The Brayton/Joule cycle is a thermodynamic cycle that describes the working processes of gas turbine, turbojet and ramjet internal combustion engines, as well as gas turbine external combustion engines with a closed loop of a gaseous (single-phase) working fluid.
The cycle is named after American engineer George Brighton, who invented the reciprocating internal combustion engine that operated on this cycle.
Sometimes this cycle is also called the Joule cycle - in honor of the English physicist James Joule, who established the mechanical equivalent of heat.
Fig.2. P,V diagram Brighton cycle
The ideal Brayton cycle consists of the processes:
- 1-2 Isentropic compression.
- 2-3 Isobaric heat input.
- 3-4 Isentropic expansion.
- 4-1 Isobaric heat removal.
Taking into account the differences between real adiabatic processes of expansion and contraction from isentropic ones, a real Brayton cycle is constructed (1-2p-3-4p-1 on the T-S diagram) (Fig. 3)
Fig.3. T-S diagram Brighton cycle
Ideal (1-2-3-4-1)
Real (1-2p-3-4p-1)
The thermal efficiency of an ideal Brayton cycle is usually expressed by the formula:
- where P = p2 / p1 - the degree of pressure increase in the process of isentropic compression (1-2);
- k - adiabatic index (for air equal to 1.4)
It should be especially noted that this generally accepted way of calculating the cycle efficiency obscures the essence of the ongoing process. The limiting efficiency of the thermodynamic cycle is calculated through the temperature ratio using the Carnot formula:
- where T1 is the refrigerator temperature;
- T2 - heater temperature.
Exactly the same temperature ratio can be expressed in terms of the pressure ratios used in the cycle and the adiabatic index:
Thus, the efficiency of the Brayton cycle depends on the initial and final temperatures of the cycle in exactly the same way as the efficiency of the Carnot cycle. With an infinitesimal heating of the working fluid along the line (2-3), the process can be considered isothermal and completely equivalent to the Carnot cycle. The amount of heating of the working fluid T3 in the isobaric process determines the amount of work related to the amount of the working fluid used in the cycle, but in no way affects the thermal efficiency of the cycle. However, in the practical implementation of the cycle, heating is usually carried out to the highest possible values limited by the heat resistance of the materials used in order to minimize the size of the mechanisms that compress and expand the working fluid.
In practice, friction and turbulence cause:
- Non-adiabatic compression: For a given total pressure ratio, the compressor discharge temperature is higher than ideal.
- Non-adiabatic expansion: although the temperature of the turbine drops to the level necessary for operation, the compressor is not affected, the pressure ratio is higher, as a result, the expansion is not enough to provide useful work.
- Pressure losses in the air intake, combustion chamber and outlet: as a result, the expansion is not sufficient to provide useful work.
As with all cyclic heat engines, the higher the combustion temperature, the higher the efficiency. The limiting factor is the ability of the steel, nickel, ceramic or other materials that make up the engine to withstand heat and pressure. Much of the engineering work is focused on removing heat from parts of the turbine. Most turbines also try to recover heat from exhaust gases that are otherwise wasted.
Recuperators are heat exchangers that transfer heat from exhaust gases to compressed air before combustion. In a combined cycle, heat is transferred to the steam turbine systems. And in combined heat and power (CHP), waste heat is used to produce hot water.
Mechanically, gas turbines can be considerably simpler than reciprocating internal combustion engines. Simple turbines may have one moving part: shaft/compressor/turbine/alternate rotor assembly (see image below), not including the fuel system.
Fig.4. This machine has a single stage radial compressor,
turbine, recuperator, and air bearings.
More complex turbines (those used in modern jet engines) may have multiple shafts (coils), hundreds of turbine blades, moving stator blades, and an extensive system of complex piping, combustion chambers, and heat exchangers.
As a general rule, the smaller the motor, the higher the speed of the shaft(s) required to maintain the maximum linear speed of the blades.
The maximum speed of the turbine blades determines the maximum pressure that can be reached, resulting in maximum power regardless of engine size. Jet engine rotates at a frequency of about 10,000 rpm and a micro-turbine - at a frequency of about 100,000 rpm.
A traditional modern gas turbine plant (GTP) is a combination of an air compressor, a combustion chamber and a gas turbine, as well as auxiliary systems that ensure its operation. The combination of a gas turbine and an electric generator is called a gas turbine unit.
It is necessary to emphasize one important difference between GTU and PTU. The composition of the PTU does not include a boiler, more precisely, the boiler is considered as a separate source of heat; With this consideration, the boiler is a “black box”: feed water enters it with a temperature of $t_(p.v)$, and steam comes out with parameters $p_0$, $t_0$. A steam turbine plant cannot operate without a boiler as a physical object. In a gas turbine, the combustion chamber is its integral element. In this sense, GTU is self-sufficient.
Gas turbine plants are extremely diverse, perhaps even more than steam turbines. Below we will consider the most promising and most used gas turbines of a simple cycle in the power industry.
circuit diagram such a gas turbine is shown in the figure. Air from the atmosphere enters the inlet of an air compressor, which is a rotary turbomachine with a flow path consisting of rotating and fixed gratings. Compressor pressure ratio p b to the pressure in front of him p a is called the compression ratio of an air compressor and is usually denoted as p to (p to = pb/p a). The compressor rotor is driven by a gas turbine. The compressed air flow is fed into one, two or more combustion chambers. In this case, in most cases, the air flow coming from the compressor is divided into two streams. The first flow is sent to the burners, where fuel (gas or liquid fuel) is also supplied. When fuel is burned, high-temperature combustion products are formed. The relatively cold air of the second flow is mixed with them in order to obtain gases (they are usually called working gases) with a temperature acceptable for parts of a gas turbine.
Working gases with pressure r s (r s < p b due to the hydraulic resistance of the combustion chamber) are fed into the flow path of the gas turbine, the principle of operation of which is no different from the principle of operation of the steam turbine (the only difference is that the gas turbine runs on fuel combustion products, and not on steam). In a gas turbine, the working gases expand to almost atmospheric pressure. p d, enter the outlet diffuser 14, and from it - either immediately into the chimney, or previously into any heat exchanger that uses the heat of the gas turbine exhaust gases.
Due to the expansion of gases in the gas turbine, the latter generates power. A very significant part of it (about half) is spent on the compressor drive, and the rest - on the electric generator drive. This is the net power of the gas turbine, which is indicated when it is marked.
To depict gas turbine diagrams, they use conventions, similar to those used for vocational schools.
There can be no simpler gas turbine, since it contains a minimum of necessary components that provide sequential processes of compression, heating and expansion of the working fluid: one compressor, one or more combustion chambers operating under the same conditions, and one gas turbine. Along with simple cycle gas turbines, there are complex cycle gas turbines that may contain several compressors, turbines and combustion chambers. In particular, GT-100-750, built in the USSR in the 70s, belong to this type of gas turbine.
It is made double. High pressure compressor mounted on one shaft KVD and the high-pressure turbine driving it TVD; this shaft has a variable speed. The low pressure turbine is located on the second shaft TND, driving the low pressure compressor KND and electric generator EG; therefore, this shaft has a constant rotational speed of 50 s -1 . Air in the amount of 447 kg / s enters from the atmosphere into KND and is compressed in it to a pressure of approximately 430 kPa (4.3 atm) and then fed into the air cooler IN, where it is cooled with water from 176 to 35 °C. This reduces the work required to compress the air in the high pressure compressor. KVD(compression ratio p k = 6.3). From there, air enters the high pressure combustion chamber. KSVD and combustion products with a temperature of 750 ° C are sent to TVD. From TVD gases containing a significant amount of oxygen enter the low-pressure combustion chamber KSND, in which additional fuel is burned, and from it - into TND. Exhaust gases with a temperature of 390 ° C go either into the chimney or into a heat exchanger to use the heat of the exhaust gases.
GTU is not very economical due to the high temperature of the flue gases. The complication of the circuit makes it possible to increase its efficiency, but at the same time it requires an increase in capital investments and complicates operation.
The figure shows the GTU V94.3 from Siemens. Atmospheric air from the complex air-cleaning device (KVOU) enters the mine 4 , and from it - to the flow part 16 air compressor. Air is compressed in the compressor. The compression ratio in typical compressors is p k = 13-17, and thus the pressure in the gas turbine tract does not exceed 1.3-1.7 MPa (13-17 atm). This is another major difference between a gas turbine and a steam turbine, in which the steam pressure is 10-15 times greater than the gas pressure in the gas turbine. The low pressure of the working medium determines the small thickness of the walls of the housings and the ease of their heating. This is what makes the gas turbine very maneuverable, i.e. capable of quick starts and stops. If it takes from 1 hour to several hours to start a steam turbine, depending on its initial temperature state, then the gas turbine can be put into operation in 10-15 minutes.
When compressed in a compressor, the air heats up. This heating can be estimated by a simple approximate relation:
$$T_a/T_b = \pi_k^(0.25)$$
in which T b And T a- absolute air temperatures behind and before the compressor. If, for example, T a= 300 K, i.e. the ambient temperature is 27 ° C, and p k \u003d 16, then T b= 600 K and, consequently, the air is heated by
$$\Delta t = (600-273)-(300-273) = 300°C.$$
Thus, the air temperature behind the compressor is 300-350 °C. The air between the walls of the flame tube and the body of the combustion chamber moves to the burner, to which the fuel gas is supplied. Since the fuel must enter the combustion chamber, where the pressure is 1.3-1.7 MPa, the gas pressure must be high. To be able to control its flow into the combustion chamber, the gas pressure is approximately twice as high as the pressure in the chamber. If there is such pressure in the supply gas pipeline, then the gas is supplied to the combustion chamber directly from the gas distribution point (GDP). If the gas pressure is insufficient, then a booster gas compressor is installed between the hydraulic fracturing and the chamber.
The fuel gas consumption is only about 1-1.5% of the air flow from the compressor, so the creation of a highly economical booster gas compressor presents certain technical difficulties.
Inside the flame tube 10 high temperature combustion products are formed. After mixing secondary air at the outlet of the combustion chamber, it decreases somewhat, but nevertheless reaches 1350-1400 °C in typical modern gas turbines.
Hot gases from the combustion chamber enter the flow path 7 gas turbine. In it, gases expand to almost atmospheric pressure, since the space behind the gas turbine communicates either with a chimney or with a heat exchanger, the hydraulic resistance of which is small.
When gases expand in a gas turbine, power is generated on its shaft. This power is partially used to drive the air compressor, and its excess is used to drive the rotor 1 generator. One of characteristic features GTP is that the compressor requires about half the power developed by the gas turbine. For example, in a gas turbine unit with a capacity of 180 MW (this is the net power) being created in Russia, the compressor capacity is 196 MW. This is one of the fundamental differences between a gas turbine and a steam turbine: in the latter, the power used to compress the feed water even up to a pressure of 23.5 MPa (240 atm) is only a few percent of the steam turbine power. This is due to the fact that water is a low compressible liquid, and air requires a lot of energy to compress.
In the first, rather rough approximation, the gas temperature behind the turbine can be estimated from a simple relationship similar to:
$$T_c/T_d = \pi_k^(0.25).$$
Therefore, if $\pi_k = 16$, and the temperature in front of the turbine T s\u003d 1400 ° С \u003d 1673 K, then the temperature behind it is approximately, K:
$$T_d=T_c/\pi_k^(0.25) = 1673/16^(0.25) = 836.$$
Thus, the gas temperature downstream of the gas turbine is quite high, and a significant amount of heat obtained from fuel combustion literally goes into the chimney. Therefore, during autonomous operation of a gas turbine, its efficiency is low: for typical gas turbines, it is 35-36%, i.e. significantly less than the efficiency of vocational schools. The matter, however, changes drastically when a heat exchanger is installed on the "tail" of the gas turbine unit (a network heater or a waste heat boiler for a combined cycle).
A diffuser is installed behind the gas turbine - a smoothly expanding channel, during the flow in which the velocity pressure of gases is partially converted into pressure. This makes it possible to have a pressure behind the gas turbine that is less than atmospheric pressure, which increases the efficiency of 1 kg of gases in the turbine and, consequently, increases its power.
Air compressor device. As already mentioned, an air compressor is a turbomachine, to the shaft of which power is supplied from a gas turbine; this power is transferred to the air flowing through the flow path of the compressor, as a result of which the air pressure rises up to the pressure in the combustion chamber.
The figure shows a gas turbine rotor placed in thrust bearings; in the foreground, the compressor rotor and stator elements are clearly visible.
From mine 4 air enters the channels formed by the rotary vanes 2 non-rotating inlet guide vane (VNA). the main task VNA - to inform the flow moving in the axial (or radial-axial) direction of rotational motion. VNA channels do not fundamentally differ from the nozzle channels of a steam turbine: they are confusing (tapering), and the flow in them accelerates, simultaneously acquiring a circumferential velocity component.
In modern gas turbines, the inlet guide vane is made rotatable. The need for a rotary VNA is caused by the desire to prevent a decrease in efficiency when the GTU load is reduced. The point is that the shafts of the compressor and the electric generator have the same rotational speed, equal to the frequency of the network. Therefore, if VNA is not used, then the amount of air supplied by the compressor to the combustion chamber is constant and does not depend on the turbine load. And you can change the power of the gas turbine only by changing the fuel flow into the combustion chamber. Therefore, with a decrease in fuel consumption and a constant amount of air supplied by the compressor, the temperature of the working gases decreases both before and after the gas turbine. This leads to a very significant reduction in the efficiency of the gas turbine. Rotation of the blades with a decrease in load around the axis 1 by 25 - 30° allows to narrow the flow sections of the VNA channels and reduce the air flow into the combustion chamber, maintaining a constant ratio between the air and fuel consumption. The installation of the inlet guide vane makes it possible to maintain the gas temperature in front of the gas turbine and behind it constant in the power range of approximately 100-80%.
The figure shows the VNA blade drive. A rotary lever is attached to the axes of each blade 2 , which through the lever 4 associated with a swivel ring 1 . If necessary, change the air flow ring 1 rotates with the help of rods and an electric motor with a gearbox; while turning all the levers at the same time 2 and, accordingly, the VNA blades 5 .
The air swirling with the help of VNA enters the 1st stage of the air compressor, which consists of two gratings: rotating and stationary. Both gratings, in contrast to turbine gratings, have expanding (diffuser) channels, i.e. inlet air passage area F 1 less than F 2 at the exit.
When air moves in such a channel, its speed decreases ( w 2 < w 1), and the pressure increases ( R 2 > R 1). Unfortunately, to make the diffuser grill economical, i.e. so that the flow rate w 1 to the maximum degree would be converted into pressure, and not into heat, only possible with a small degree of compression R 2 /R 1 (usually 1.2 - 1.3), resulting in a large number compressor stages (14 - 16 with a compressor compression ratio p k \u003d 13 - 16).
The figure shows the air flow in the compressor stage. From the input (fixed) rotary nozzle apparatus, the air exits at a speed c 1 (see the upper speed triangle), having the necessary circumferential twist (a 1< 90°). Если расположенная за ВНА вращающаяся (рабочая) решетка имеет скорость u 1 , then the relative speed of entering it w 1 will be equal to the difference of vectors c 1 and u 1 , and this difference will be greater than c 1 i.e. w 1 > c 1 . When moving in the channel, the air speed decreases to the value w 2 and it comes out at an angle b 2 determined by the inclination of the profiles. However, due to the rotation and supply of energy to the air from the rotor blades, its speed With 2 in absolute motion will be greater than c 1 . The blades of the fixed grid are installed so that the air inlet into the channel is shock-free. Since the channels of this grating are expanding, the velocity in it decreases to the value c" 1 , and the pressure increases from R 1 to R 2. The grid is designed so that c" 1 = c 1, a a "1 = a 1. Therefore, in the second stage and subsequent stages, the compression process will proceed in a similar way. In this case, the height of their gratings will decrease in accordance with the increased air density due to compression.
Sometimes the guide vanes of the first few stages of the compressor are made rotary in the same way as the VNA vanes. This makes it possible to expand the power range of the gas turbine, in which the temperature of the gases in front of the gas turbine and behind it remains unchanged. Accordingly, the economy also increases. The use of several rotary guide vanes allows you to work economically in the range of 100 - 50% of the power.
The last stage of the compressor is arranged in the same way as the previous ones, with the only difference that the task of the last guide vane 1 is not only to increase the pressure, but also to ensure the axial exit of the air flow. Air enters the annular outlet diffuser 23 where the pressure rises to its maximum value. With this pressure, air enters the combustion zone 9 .
Air is taken from the air compressor housing to cool the elements of the gas turbine. To do this, annular chambers are made in its body, communicating with the space behind the corresponding stage. The air from the chambers is removed by pipelines.
In addition, the compressor has so-called anti-surge valves and bypass pipes. 6 , bypassing air from the intermediate stages of the compressor into the outlet diffuser of the gas turbine when it is started and stopped. This eliminates the unstable operation of the compressor at low air flow rates (this phenomenon is called surge), which is expressed in intense vibration of the entire machine.
The creation of highly economical air compressors is an extremely complex task, which, unlike turbines, cannot be solved only by calculation and design. Since the compressor power is approximately equal to the power of the gas turbine, a deterioration in the efficiency of the compressor by 1% leads to a decrease in the efficiency of the entire gas turbine by 2-2.5%. Therefore, the creation of a good compressor is one of the key problems in the creation of gas turbines. Usually compressors are created by modeling (scaling) using a model compressor created by long experimental refinement.
Gas turbine combustion chambers are very diverse. Above is a gas turbine with two external chambers. The figure shows a GTU type 13E with a capacity of 140 MW from ABB with one remote combustion chamber, the device of which is similar to the device of the chamber shown in the figure. The air from the compressor from the annular diffuser enters the space between the chamber body and the flame tube and is then used for gas combustion and for cooling the flame tube.
The main disadvantage of remote combustion chambers is their large dimensions, which are clearly visible from the figure. To the right of the chamber is a gas turbine, to the left - a compressor. Three holes are visible from above in the body for accommodating anti-surge valves and then - the VNA drive. In modern gas turbines, built-in combustion chambers are mainly used: annular and tubular-annular.
The figure shows an integrated annular combustion chamber. The annular space for combustion is formed by the internal 17 and outdoor 11 fiery pipes. From the inside, the pipes are lined with special inserts 13 And 16 having a thermal barrier coating on the side facing the flame; on the opposite side, the inserts are ribbed, which improves their cooling by air entering through the annular gaps between the inserts inside the flame tube. Thus, the temperature of the flame tube is 750-800 °C in the combustion zone. The frontal microflare burner device of the chamber consists of several hundred burners 10 , to which gas is supplied from four collectors 5 -8 . Turning off the collectors in turn, you can change the power of the gas turbine.
The burner device is shown in the figure. From the collector, gas enters through drilling in the stem 3 to the inner cavity of the shoulder blades 6 swirler. The latter is a hollow radial straight blades, forcing the air coming from the combustion chamber to twist and rotate around the axis of the rod. This rotating air vortex receives natural gas from the inner cavity of the swirler blades. 6 through small holes 7 . In this case, a homogeneous fuel-air mixture is formed, which emerges in the form of a swirling jet from the zone 5 . An annular rotating vortex ensures stable combustion of the gas.
The figure shows a tubular-annular combustion chamber GTE-180. Into the annular space 24 between the outlet of the air compressor and the inlet of the gas turbine using perforated cones 3 place 12 flame tubes 10 . The flame tube contains numerous holes with a diameter of 1 mm, arranged in annular rows at a distance of 6 mm between them; distance between rows of holes 23 mm. Through these openings, "cold" air enters from the outside, providing convective-film cooling and the temperature of the flame tube is not higher than 850 °C. A thermal barrier coating 0.4 mm thick is applied to the inner surface of the flame tube.
On the front plate 8 flame tube, a burner device is installed, consisting of a central pilot burner 6 igniting fuel at start-up using a candle 5 , and five main modules, one of which is shown in the figure. The module allows you to burn gas and diesel fuel. Gas through fitting 1 after filter 6 enters the annular fuel gas manifold 5 , and from it into cavities containing small holes (diameter 0.7 mm, step 8 mm). Through these holes, the gas enters the annular space. There are six tangential grooves in the walls of the module 9 , through which the main amount of air supplied for combustion from the air compressor enters. In the tangential slots, the air is twisted and, thus, inside the cavity 8 a rotating vortex is formed, moving towards the outlet of the burner. To the periphery of the vortex through the holes 3 gas enters, mixes with air, and the resulting homogeneous mixture exits the burner, where it ignites and burns. The combustion products enter the nozzle apparatus of the 1st stage of the gas turbine.
The gas turbine is the most complex element of the gas turbine, which is primarily due to the very high temperature of the working gases flowing through its flow path: the gas temperature in front of the turbine of 1350 ° C is currently considered “standard”, and leading companies, primarily General Electric, work on mastering the initial temperature of 1500 °C. Recall that the "standard" initial temperature for steam turbines is 540 °C, and in the future - a temperature of 600-620 °C.
The desire to increase the initial temperature is associated, first of all, with the gain in efficiency that it gives. This is clearly seen from the figure summarizing the achieved level of gas turbine construction: an increase in the initial temperature from 1100 to 1450 °C gives an increase in absolute efficiency from 32 to 40%, i.e. results in fuel savings of 25%. Of course, part of this savings is associated not only with an increase in temperature, but also with the improvement of other elements of the gas turbine, and the initial temperature is still the determining factor.
To ensure long-term operation of a gas turbine, a combination of two means is used. The first means is the use of heat-resistant materials for the most loaded parts that can resist the action of high mechanical loads and temperatures (primarily for nozzle and rotor blades). If steels (i.e., iron-based alloys) with a chromium content of 12-13% are used for steam turbine blades and some other elements, then nickel-based alloys (nimonic) are used for gas turbine blades, which are capable of and the required service life to withstand temperatures of 800-850 °C. Therefore, together with the first, a second means is used - cooling the hottest parts.
Most modern gas turbines are cooled using bleed air from various stages of an air compressor. Gas turbines are already in operation, which use water vapor for cooling, which is a better cooling agent than air. Cooling air after heating in the cooled part is discharged into the flow path of the gas turbine. Such a cooling system is called open. There are closed cooling systems in which the coolant heated in the part is sent to the refrigerator and then returned again to cool the part. Such a system is not only very complicated, but also requires the utilization of heat taken from the refrigerator.
The gas turbine cooling system is the most complex system in a gas turbine, which determines its service life. It ensures not only maintaining the permissible level of working and nozzle blades, but also body elements, disks carrying working blades, locking bearing seals where oil circulates, etc. This system is extremely branched and organized so that each cooled element receives cooling air of the parameters and in the amount necessary to maintain its optimum temperature. Excessive cooling of parts is just as harmful as insufficient, since it leads to increased costs of cooling air, the compression of which in the compressor consumes turbine power. In addition, increased air consumption for cooling leads to a decrease in the temperature of the gases behind the turbine, which has a very significant effect on the operation of the equipment installed behind the gas turbine (for example, a steam turbine unit operating as part of a steam turbine). Finally, the cooling system must provide not only the required temperature level of the parts, but also the uniformity of their heating, which excludes the appearance of dangerous thermal stresses, the cyclic action of which leads to the appearance of cracks.
The figure shows an example of a typical gas turbine cooling circuit. The values of gas temperatures are given in rectangular frames. In front of the nozzle apparatus of the 1st stage 1 it reaches 1350 °C. Behind him, i.e. in front of the working grate of the 1st stage, it is 1130 °C. Even in front of the working blade of the last stage, it is at the level of 600 °C. Gases of this temperature wash the nozzle and working blades, and if they were not cooled, then their temperature would be equal to the temperature of the gases and their service life would be limited to several hours.
To cool the elements of a gas turbine, air is used that is taken from the compressor in that stage where its pressure is slightly higher than the pressure of the working gases in that zone of the gas turbine into which air is supplied. For example, for cooling the nozzle vanes of the 1st stage, cooling air in the amount of 4.5% of the air flow at the compressor inlet is taken from the outlet diffuser of the compressor, and for cooling the nozzle vanes of the last stage and the adjoining section of the housing - from the 5th stage of the compressor. Sometimes, to cool the hottest elements of a gas turbine, the air taken from the compressor outlet diffuser is first sent to an air cooler, where it is cooled (usually with water) to 180–200 °C and then sent for cooling. In this case, less air is required for cooling, but at the same time, the cost of an air cooler appears, the gas turbine becomes more complicated, and part of the heat removed by the cooling water is lost.
A gas turbine usually has 3-4 stages, i.e. 6-8 rims of gratings, and most often the blades of all rims are cooled, except for the working blades of the last stage. Air for cooling the nozzle vanes is supplied inside through their ends and discharged through numerous (600-700 holes with a diameter of 0.5-0.6 mm) holes located in the corresponding areas of the profile. Cooling air is supplied to the working blades through holes made in the ends of the shank.
In order to understand how cooled blades are arranged, it is necessary to consider at least in general terms the technology of their manufacture. Due to the exceptional difficulty of machining nickel alloys, investment casting is mainly used to produce blades. To implement it, first, casting cores are made from ceramic-based materials using a special technology of molding and heat treatment. The casting core is an exact copy of the cavity inside the future blade, into which cooling air will flow and flow in the required direction. The casting core is placed in a mold, the internal cavity of which fully corresponds to the blade to be obtained. The resulting free space between the rod and the wall of the mold is filled with a heated low-melting mass (for example, plastic), which solidifies. The rod, together with the hardening mass enveloping it, repeating the external shape of the blade, is an investment model. It is placed in a mold, to which the nimonic melt is fed. The latter melts the plastic, takes its place, and as a result, a cast blade appears with an internal cavity filled with a rod. The rod is removed by etching with special chemical solutions. The obtained nozzle vanes practically do not require additional machining (except for the manufacture of numerous holes for the exit of cooling air). Working cast blades require processing of the shank with a special abrasive tool.
The technology described briefly is borrowed from aeronautical technology, where the temperatures achieved are much higher than in stationary steam turbines. The difficulty of mastering these technologies is associated with much large sizes blades for stationary gas turbines, which grow in proportion to the gas flow rate, i.e. GTU power.
The use of so-called single-crystal blades, which are made from a single crystal, seems very promising. This is due to the fact that the presence of grain boundaries during a long stay at a high temperature leads to a deterioration in the properties of the metal.
The gas turbine rotor is a unique prefabricated structure. Before assembling individual discs 5 compressor and disc 7 gas turbine are bladed and balanced, end parts are manufactured 1 And 8 , spacer 11 and center pin 6 . Each of the discs has two annular collars, on which hirts (named after the inventor - Hirth) are made - strictly radial teeth of a triangular profile. Adjacent pieces have exactly the same collars with exactly the same hirts. At good quality the manufacture of a hirt connection ensures the absolute centering of adjacent disks (this ensures the radiality of the hirts) and the repeatability of the assembly after disassembly of the rotor.
The rotor is assembled on a special stand, which is an elevator with an annular platform for assembly personnel, inside which assembly is carried out. First, the end part of the rotor is assembled on the thread 1 and tie rod 6 . The rod is placed vertically inside the annular platform and the disk of the 1st stage of the compressor is lowered on top of it with the help of a crane. The centering of the disk and the end part is carried out by hirts. Moving upwards on a special elevator, the installation staff disc by disc [first of the compressor, then the spacer, and then the turbine and the right end 8 ] collects the entire rotor. A nut is screwed onto the right end 9 , and a hydraulic device is installed on the remaining part of the threaded part of the tie rod, squeezing the discs and pulling the tie rod. After drawing the rod, the nut 9 is screwed up to the stop, and the hydraulic device is removed. The stretched rod securely tightens the discs together and turns the rotor into a single rigid structure. The assembled rotor is removed from the assembly stand, and it is ready for installation in the gas turbine.
The main advantage of the gas turbine is its compactness. Indeed, first of all, there is no steam boiler in the gas turbine - a structure that reaches a great height and requires a separate room for installation. This circumstance is connected, first of all, with the high pressure in the combustion chamber (1.2-2 MPa); in the boiler, combustion occurs at atmospheric pressure and, accordingly, the volume of hot gases formed is 12-20 times larger. Further, in a gas turbine, the gas expansion process takes place in a gas turbine consisting of only 3-5 stages, while a steam turbine with the same power consists of 3-4 cylinders containing 25-30 stages. Even taking into account both the combustion chamber and the air compressor, a 150 MW gas turbine has a length of 8-12 m, and the length of a steam turbine of the same power with a three-cylinder design is 1.5 times longer. At the same time, for a steam turbine, in addition to the boiler, it is necessary to provide for the installation of a condenser with circulation and condensate pumps, a regeneration system of 7-9 heaters, feed turbopumps (from one to three), and a deaerator. As a result, the gas turbine unit can be installed on a concrete base at the zero level of the turbine hall, and the STU requires a 9-16 m high frame foundation with the steam turbine placed on the upper foundation slab and auxiliary equipment in the condensation room.
The compactness of the gas turbine allows it to be assembled at the turbine plant, delivered to the engine room by rail or road for installation on a simple foundation. So, in particular, gas turbines with built-in combustion chambers are transported. When transporting gas turbines with remote chambers, the latter are transported separately, but are easily and quickly attached to the compressor-gas turbine module using flanges. The steam turbine is supplied with numerous assemblies and parts, the installation of both itself and numerous auxiliary equipment and connections between them takes several times more time than a gas turbine.
GTU does not require cooling water. As a result, the gas turbine lacks a condenser and an industrial water supply system with a pumping unit and a cooling tower (with circulating water supply). As a result, all this leads to the fact that the cost of 1 kW of installed capacity of a gas turbine power plant is much less. At the same time, the cost of the GTU itself (compressor + combustion chamber + gas turbine), due to its complexity, turns out to be 3-4 times more than the cost of a steam turbine of the same power.
An important advantage of a gas turbine is its high maneuverability, determined by a low pressure level (compared to the pressure in a steam turbine) and, consequently, easy heating and cooling without dangerous thermal stresses and deformations.
However, gas turbines also have significant drawbacks, of which, first of all, it should be noted that they are less economical than those of a steam power plant. The average efficiency of sufficiently good gas turbines is 37-38%, and for steam turbine power units - 42-43%. The ceiling for powerful power gas turbines, as it is currently seen, is an efficiency of 41-42% (and maybe even higher, given the large reserves for increasing the initial temperature). The lower efficiency of the gas turbine is associated with the high temperature of the exhaust gases.
Another disadvantage of gas turbines is the impossibility of using low-grade fuels in them, at least at present. It can only work well on gas or good liquid fuels such as diesel. Steam power units can operate on any fuel, including the poorest quality.
The low initial cost of thermal power plants with gas turbines and at the same time relatively low efficiency and high cost of the fuel used and maneuverability determine the main area for individual use of gas turbines: they should be used in power systems as peak or backup power sources operating several hours a day.
At the same time, the situation changes dramatically when the heat of the gas turbine exhaust gases is used in heating plants or in a combined (steam-and-gas) cycle.
