Thermal power plants
Thermal power plants play a very important role in Slovak power grid. They provide reliable electricity generation both for the baseload as well as for peak load demand within the transmission and distribution grid. The total installed capacity of Slovenské elektrárne's thermal power plants is 486 MW.
Electricity generation in thermal power plants if characterized by the main source of generation being firing of coal. Steam is produced in a boiler, and it drives a turbine connected to an alternator. Heat energy is converted to electric energy within the so-called steam cycle. A thermal plant usually comprises of several separate production units with specific size and power.
A conventional power plant consists of a boiler room, interposed machine room, machine room, electric power output, and auxiliary operations (coal loading, water treatment, water management, back fuel cycle, etc.). The following types of thermal power plants exist:
- condensating, whose main focus is generation of electricity,
- thermal plants whose main focus is combined generation of electricity and heat.
In a conventional condensating type thermal power plant, the electricity generation part is dominated by arrangement in production units. Every production unit of the power plant represents a separate generation entity - a separate power plant. By the method of combustion, solid fuel firing boilers are classified into grate, granulation, fusion, and fluidized-bed type. Boilers firing solid and gas fuels are in addition to the above mentioned boilers.
Every power plant unit may be operated independently. The principle of operation is quite simple. Stockpile coal is taken by a coaling belt into a coal holder located at every boiler. The coal is gradually dried and ground to powder that is subsequently fired in the boiler. Pipe or membrane type evaporators are located in the boiler walls; there, water turns into steam and the steam generated (of a high temperature and pressure) is led to steam cylinder, wherefrom it is led through pre-heaters and postheaters via steam distribution pipes to turbine blades. The turbine is connected to a generator.
Turbine and electric generator comprise a single train - turbogenerator. In the turbogenerator, heat energy is converted into electric energy. Electric energy thus produced is led through a system of transformers and distribution grid to end-consumers. Upon passing the turbine, the steam temperature and pressure get reduced. Having delivered its energy to turbine blades, the steam condensates in heat exchanger - condenser. The steam changes its state and turns into water called condensate. Large quantities of cooling energy are needed for steam to condensate. Surface water from a stream or a reservoir is used for cooling. If there is a plenty of cooling water, flow-through system of cooling is used; circulation system of cooling with water being cooled in cooling towers is used for places with insufficient supply of cooling water.
On their way to the stack, flue gases produced during the firing of coal heat water in economizer, which is a heat exchanger for combustion gas. Cooled stack gases then pass through electrostatic filters where ash is caught, and continue to the stack. To reduce nitrogen and sulfur oxides, desulfurization and denitrification equipment are installed to conventional boilers. For fluidized-bed boilers, desulfurization and denitrification is resolved directly by the boiler technology.