District Heat and Power Plant

District Heat and Power Plant

 

a steam power plant that produces not only electricity but also heat, which is distributed to customers in the form of steam and hot water. Practical use of the wasted heat from the engines that turn the electric generators is the distinguishing characteristic of district heat and power plants and in Russian is called teplofikatsiia (district heat supply). The combined production of two forms of energy promotes more economical use of fuel in comparison with separate production of electric energy at condensation electric power plants (in the USSR, in state regional electric power plants) and of thermal energy at local boiler installations. The replacement of local boilers, which use fuel inefficiently and pollute the atmosphere in cities and communities, with a district heat supply system not only promotes a significant saving of fuel but also helps clean air basins and improves health conditions in populated areas.

The initial source of energy at district heat and power plants may be an organic fuel (at steam-turbine and gas-turbine plants) or atomic fuel (at the nuclear plants now planned). As of 1976, district heat and power plants with steam turbines operating on organic fuel were the most common type in operation; this type of plant and the condensation electric power plant are the principal types of thermal steam-turbine power plants. A distinction is made between industrial district heat and power plants, which supply heat to industrial enterprises, and district heating plants, which supply heat and hot water to residential and public buildings. Heat supplied by the latter is transported up to 20–30 km primarily in the form of hot water; heat from industrial plants is supplied over distances up to several kilometers in the form of steam.

The principal equipment of a steam-turbine district heat and power plant consists of the turbine units, which convert the energy of the working fluid (steam) into electrical energy, and the boiler units, which produce the steam for the turbines (see). A turbine unit includes a steam turbine and a synchronous generator. The steam turbines used at such plants are called district heat and power plant turbines. Three types are distinguished: back-pressure turbines, usually 0.7–1.5 meganewtons per m2 (MN/m2), installed at plants that supply steam to industrial enterprises; turbines with condensation and steam extraction at pressures of 0.7–1.5 MN/m2 (for industrial customers) and 0.05–0.25 MN/m2 (for municipal and household customers); and turbines with condensation and steam extraction (for heating) at pressures of 0.05–0.25 MN/m2.

The waste heat of back-pressure turbines can be used in full. However, the electric power developed depends directly on the magnitude of the heating load: if there is no heating load (as occurs, for example, in the summer at plants supplying residential and municipal buildings), the turbines do not produce electric power. Therefore, back-pressure turbines are used only in the presence of a sufficiently even heating load throughout the period of plant operation, that is, primarily at industrial plants.

In turbines with condensation and steam extraction for supplying heat to customers, only steam from extraction is used; the heat of the condensation stream is fed to the cooling water in the condenser and lost. In order to reduce heat losses, such turbines should work most of the time with a minimum of steam being passed to the condenser. The USSR has developed and is producing district heat and power turbines with condensation and steam extraction that have provision for using the heat of condensation. Given a sufficient heating load, such turbines can operate as heating back-pressure turbines. Turbines with condensation and steam extraction are the type most often installed in district heat and power plants because they can operate in all regimes. Their Use allows practically independent regulation of the heating and electric loads; in the particular case where heating loads are reduced or absent, the power plants may operate in a mode producing the required (full or nearly full) electric power.

The electric power capacity of district heat and power turbine units (as distinguished from condensation units) is better selected by the amount of live steam used than by a given scale of capacities. In the USSR, therefore, large district heat and power turbine units are standardized by this parameter. Thus, the R-100 turbine units with back pressure, the PT-135 units with extraction for industrial and comfort heating, and the T-175 units with extraction for comfort heating use the same amount of live steam (approximately 750 tons/hr) but have different electric power capacities—100, 135, and 175 megawatts (MW), respectively. The boiler units that produce steam for such turbines have identical productivity ratings (approximately 800 tons/hr). This standardization makes it possible to use different types of turbine units with the same heating equipment for the boilers and turbines at a single heat and power plant. In the USSR the boiler units used for work at different kinds of thermal steam-turbine power plants are also standardized. Thus, boiler units producing 1,000 tons of steam per hour are used to supply steam for both 300-MW condensation turbines and the 250-MW district heat and power plant turbines, the largest in the world.

The pressures of live steam at district heat and power plants in the USSR are set at ~13–14 MN/m2 (most common) and ~24–25 MN/m2 (at the largest district heating and power units, with 250 MW capacity). Unlike the state regional power plants, the district plants with steam pressures of 13–14 MN/m2 do not have intermediate reheating of the steam because such reheating does not produce such significant technical and economic advantages as it does at regional plants. Power units with 250 MW capacity at district heat and power plants with a heating load use intermediate steam reheating.

The heating load at district plants supplying residential and municipal buildings varies during the year. In order to reduce expenditures on primary power equipment, part of the heat (40–50 percent) during periods of increased load is delivered to customers from peak-load hot-water boilers. The proportion of heat delivered by primary power equipment during maximum load determines the magnitude of the plant’s coefficient of heat supply (usually 0.5–0.6). Peak industrial heating (steam) loads (approximately 10–20 percent of the maximum load) can also be covered in a similar way by low-pressure peak-load steam boilers. Two methods of heat delivery are possible: steam from the turbines may be sent directly to the customers, or heat may be passed through heat exchangers (steam-steam and steam-water exchangers) to the heat-transport medium (steam or water), which is transported to the customers. The choice of methods is largely determined by the plant’s regime.

District heat and power plants use solid, liquid, and gaseous fuels. Because the plants are closer to population centers than state regional power plants, they make more extensive use of ma-zut and gas—more expensive fuels that create less atmospheric pollution in the form of particulate matter. Like state regional power plants, district plants use ash traps to protect the air basin from pollution by particulate matter; stacks up to 200–250 m high are built to disperse particulate matter and oxides of sulfur and nitrogen into the atmosphere. District plants built near heat consumers are usually significantly removed from sources of water supply. Most district plants therefore use water recycling systems with artificial cooling in cooling towers. Direct-flow water supply is rarely provided for district plants.

The gas turbines installed at gas-turbine district heat and power plants are used to drive electric generators. Heat supplied to customers is the heat extracted during the cooling of air compressed by the turbine compressors and the heat of gases spent in the turbine. Steam-gas power plants equipped with steam-turbine and gas-turbine units and nuclear power plants can also operate as district heat and power plants.

District heat and power plants have become most common in the USSR. The first heat lines were laid from plants in Leningrad in 1924 and Moscow in 1928. District plants with capacities of 100–200 MW were planned and built beginning in the 1930’s. By the end of 1940, the capacity of all existing district plants had reached 2 gigawatts (GW), the annual delivery of heat was 108 gi-gajoules (GJ), and the total length of heat supply nets was 650 km. In the mid-1970’s, the total electric capacity of district plants was approximately 60 GW (for a total electric power plant capacity of approximately 220 GW and a total capacity for steam power plants of approximately 180 GW). The annual production of electricity at district heat and power plants reached 330 billion kilowatt-hours, and heat delivery was 4 × 109GJ.

Certain new district plants have capacities of 1.5–1.6 GW with hourly heat delivery up to (1.6–2.0) × 104 GJ; the specific production of electricity for the delivery of 1 GJ of heat is 150–160 kilowatt-hours (kW-hr). The specific expenditure of standard fuel to produce 1 kW-hr of electricity averages 290 g (compared to 370 g at state regional power plants). The minimum average annual specific expenditure of standard fuel at district plants is approximately 200 g/kW-hr, compared to approximately 300 g/kW-hr at the best state regional power plants. This lower specific expenditure of fuel results from the combined production of two forms of energy making use of the heat of the waste steam. District plants in the USSR save up to 25 million tons of standard fuel per year—11 percent of all fuel used for energy production.

The district heat and power plant is the basic production link in the system of centralized heat supply. Construction of district plants is one of the basic areas of concentration in the development of power systems in the USSR and the other socialist countries. District plants have also found limited application in the capitalist countries, primarily in industry.

REFERENCES

Sokolov, E. Ia. Teplofikatsiia i teplovye seti. Moscow, 1975.
Ryzhkin, V. Ia. Teplovye elektricheskie stansii. Moscow, 1976.

V. IA. RYZHKIN

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