Thermal Steam-Turbine Power Plant

a fossil-fuel-fired steam plant in which a steam turbine is used to drive the electric generators. The basic purpose of a thermal steam-turbine power plant, as with any power plant, is to produce electrical energy. In the USSR, large plants that supply only electricity to customers are called state regional power plants. They are equipped with steam turbines that exhibit high coefficients of expansion and condensation of the steam in condensers cooled by circulating water. Plants that supply customers with both electricity and thermal energy from the turbine’s waste steam are called district heat and power plants. The first thermal steam-turbine power plants appeared in the early 20th century when steam and diesel engines, which had been used in power plants to drive electric generators, were supplanted by steam turbines, which deliver very even speed and produce current of constant frequency. Progress in turbine design has led to an increase in the capacity of steam turbines, that is, the electrical capacity of the generator connected to the turbine, from a few megawatts (MW) in the first plants to hundreds of megawatts; steam turbines of more than 1 gigawatt have been developed and are now in operation.

Steam turbines are usually connected directly to generators without intermediate transmission, forming a steam turbine unit that is compact, reliable, and highly efficient. The turbine unit can be almost completely automated and thus controlled from a central control station.

The steam required for the steam turbine is produced in a steam generator. The use of steam at high pressure and temperature increases the specific work of the steam and reduces the expenditure of steam, heat, and fuel, that is, it increases the plant’s efficiency. At large steam turbine plants in the USSR, steam is delivered to the turbines at pressures of ~13–14 or ~24–25 meganewtons per m² (MN/m²) and temperatures of approximately 540°–560°C; in other countries, a pressure of ~16 MN/m² is also used. The output of steam generators at state regional power plants reaches 1,600–4,600 tons per hr for turbine units of 500–1,380 MW capacity; outputs at district heat and power plants with turbine units of 100–250 MW capacity reach 500–1,000 tons per hr. Modern thermal steam-turbine power plants operate on a thermodynamic cycle based on the Rankine cycle. The required steam pressure is ensured by pumping an appropriate amount of water into the steam generator to be converted into steam. The required steam temperature is attained by reheating in the steam generator’s reheater. At the same time, intermediate reheating of the steam takes place. Steam from the intermediate stage of the turbine is fed to the boiler to be reheated again and is then sent to the next stage of the turbine. The turbine unit, steam generator, auxiliary equipment, and steam and water pipes make up the plant’s energy block.

Feed water for the steam generators is taken from the condensate of waste steam from the turbine, which has been heated by steam from the turbine’s regenerative extraction points. The upper limit of the number of stages of regenerative water heating is seven to nine and depends on the number of regenerative extraction points. One of the heating stages is often used for deaeration, that is, removal of gases, such as oxygen, dissolved in the water.

The feed and condensation pumps, regenerative heaters, and deaerators are classified as auxiliary equipment for the turbine installation. Auxiliary equipment for a steam generator operating on solid fuel includes dust pretreatment equipment and ash traps, forced-draft fans that feed air into the furnace chamber of the steam generator, and exhaust fans that suck out the combustion products. Flue gases are discharged into the atmosphere through smokestacks 150–360 m high. In steam generators operating on gasmazut fuel and having excess pressure in the combustion chamber and gas ducts, blowers with increased pressure are used instead of forced-draft fans; exhaust fans are not needed in this case.

General auxiliary installations and structures at a thermal steam-turbine power plant include the installations and structures for industrial water supply and the fuel and ash systems. The basic purpose of industrial water supply is to provide the turbine units with the water required to cool the waste steam; condensation electric power plants use more than 30 m³/sec of water for a turbine of approximately 1,000-MW capacity. The source of water may be a river, lake, or sea. The water supply is usually recycled through cooling ponds (at condensation electric power plants) or cooling towers (primarily at district heat and power plants). Straight-through water supply is sometimes provided, in which the cooling water makes a single pass through the turbine condensers.

The fuel system of a thermal steam-turbine power plant operating on solid fuel, usually coal, includes unloading devices, a system of belt conveyors that deliver fuel to the bins of the steam generators, a fuel storage area with the necessary mechanisms and transport devices, and pulverizing equipment. Solid or liquid slag from the combustion chambers is removed by water through flushing channels, and the slag-water mixture is then pumped into ash dumps by centrifugal pumps. Fly ash caught in the ash traps is removed by water or air. Where mazut is used as fuel, the fuel system includes mazut tanks, pumps, heaters, and piping.

The main building of a thermal steam-turbine power plant, in which the energy blocks are housed, the auxiliary production installations and structures, the switchgear, and the laboratories, workshops, and storehouses are all located on the plant grounds—an area of 30–70 hectares. Sites for condensation electric power plants are selected outside of cities, as close as possible to the source of water and the fuel supply. District heat and power plants are located close to the heat users.

Like any electric power plant, thermal steam-turbine power plants must have high reliability, flexibility, and economy. Equipment reliability should be sufficient to allow the plant to develop at any moment power equal to the electric load (which changes over time) and to ensure the necessary quality of electrical energy in the power system. The reliability of plant equipment and power units is judged by the plant’s ability to provide the required water supply conditions and purity of steam, condensate, and water in the plant’s steam-water lines; it is characterized by the plant’s operational readiness, that is, the length of time spent by the power unit at work relative to the time spent in a state of readiness for work (in reserve). The value of this coefficient for a power unit is determined by the corresponding indexes of the turbine unit and the steam generator and falls in the range 0.85–0.90. Flexibility ensures rapid changing of the plant’s power output to conform with changes in load. The economic efficiency of a plant is characterized by the calculated unit costs required to produce 1 kilowatt-hour of electricity. Calculated unit costs are determined by the capital investments during the plant’s construction and the annual production costs from the moment the equipment is put into operation, including the cost of fuel, wages paid to personnel, and depreciation deductions; in the USSR it is approximately 1 kopek per kilowatt-hour for thermal steam-turbine power plants. Other important economic indexes are the specific amount of capital investment (the cost of one kilowatt of installed capacity depends on the type of plant and other factors and is 100–200 rubles), the specific number of personnel (the staff ratio is 0.5–1.0 persons per MW), and the specific consumption of standard fuel (~340 g per kilowatt-hour). One of the significant requirements of thermal steam-turbine power plants is that they preserve a clean environment (the air and water basins) while producing electric and thermal energy.

Modern thermal steam-turbine power plants are highly automated enterprises. Automatic regulation of all basic processes functions not only during normal equipment operations but also when the power units are started up. The automatic control systems of large plants use computers. In the USSR, computer technology and logic devices are used for power units rated at 200–300 MW or more.