Steam Turbine Generation. The process of generating electricity from steam comprises four parts: a heating subsystem (fuel to produce the steam), a steam subsystem (boiler and steam delivery system), a steam turbine, and a condenser (for condensation of used steam). Heat for the system is usually provided by the combustion of coal, natural gas, or oil. The fuel is pumped into the boiler’s furnace. The boilers generate steam in the pressurized vessel in small boilers or in the water-wall tube system in modern utility and industrial boilers. Additional elements within or associated with the boiler, such as the superheater, reheater, economizer and air heaters, improve the boiler’s efficiency.
Wastes from the combustion process include exhaust gases and, when coal or oil is used as the boiler fuel, ash. These wastes are typically controlled to reduce the levels of pollutants exiting the exhaust stack. Bottom ash, another byproduct of combustion, also is discharged from the furnace. High temperature, high pressure steam is generated in the boiler and then enters the steam turbine. At the other end of the steam turbine is the condenser, which is maintained at a low temperature and pressure. Steam rushing from the high-pressure boiler to the low-pressure condenser drives the turbine blades, which powers the electric generator. Steam expands as it works; hence, the turbine is wider at the exit end of the steam. The theoretical thermal efficiency of the unit is dependent on the high pressure and temperature in the boiler and the low temperature and pressure in condenser.
Steam turbines typically have a thermal efficiency of about 35 percent, meaning that 35 percent of the heat of combustion is transformed into electricity. The remaining 65 percent of the heat either goes up the stack (typically 10 percent) or is discharged with the condenser cooling water (typically 55 percent). Low-pressure steam exiting the turbine enters the condenser shell and is condensed on the condenser tubes. The condenser tubes are maintained at an efficient operation by providing a low pressure sink for the exhausted steam. As the steam is cooled to condensate, the condensate is transported by the boiler feedwater system back to the boiler, where it is used again. Being a low-volume incompressible liquid, the condensate water can be efficiently pumped back into the high-pressure boiler.
A constant flow of low-temperature cooling water in the condenser tubes is required to keep the condenser shell (steam side) at proper pressure and to ensure efficient electricity generation. Through the condensing process, the cooling water is warmed. If the cooling system is an open or a once-through system, this warm water is released back to the source water body. In a closed system, the warm water is cooled by re-circulation through cooling towers, lakes, or ponds, where the heat is released into the air through evaporation and/or sensible heat transfer. If a re-circulating cooling system is used, only a small amount of make-up water is required to offset the cooling tower blowdown, which must be discharged periodically to control the build-up of solids. Compared to a once-through system, a re-circulated system uses about one twentieth the water.
There are several types of coal-fired steam generators: 1) Stoker-Fired Furnace, 2) Tangential-Fired Furnace, 3) Horizontal or Wall-Fired Furnace, 4) Arch-Fired Systems, 5) Fluidized-Bed Combusters, and 6) Cyclone-Fired Furnace.