Electricity Generation from Thermal Power

Electricity Generation from Thermal Power 

In thermal power plants, the thermal energy released from the fuels is converted to electrical energy. The main coal is bituminous coal; natural gas and sewage sludge are utilised as secondary fuels. VERBUND has made the strategic decision to withdraw from electricity production from fossil fuels in the medium term. 

Districting heating for Graz 
The Mellach location offers options for low-emissions heating from the south for the greater Graz region. 

How Does a Thermal Power Plant Work? 
Steam is produced when fuel is burned in the steam generator (boiler). This leaves the steam generator and flows through the guide and rotor vanes of the turbine which drives the generator via a gear transmission.

The generator converts this kinetic electricity into electrical energy and the generated electricity is subsequently fed into the distribution system. The steam that exits the turbine is converted back into water in the condenser unit. This water is then fed back into the steam generator and the water-steam cycle starts again. 

In the case of combined heat and power plants, such as the district heating power plant in Mellach, part of the steam flow is decoupled from the turbine and conducted into heat exchangers which transfer the steam heat to the hot water network.

During this heat transmission process, the extraction steam is condensed and subsequently conducted back into the water-steam cycle. Due to the decoupling of district heating less exhaust steam has to be condensed (reduction equivalent to amount of extraction steam) in the condenser, i.e. less waste heat is released into the environment via the cooling water. 

The product from the flue gas desulphurization system can be used, above all in the construction industry, for the construction of thin diaphragm walls or as a sealing compound. The filtered fly ash is used in the production of cement. 

In the catalytic converter, sulphur dioxide is converted into harmless nitrogen and water vapour and released into the air. The amount of sulphur dioxide, nitrous oxide and dust in the air is measured around the clock in automatic emission monitoring systems. In addition, all of the important weather forming components (wind direction, wind speed, precipitation, temperature) are measured.

What are the Advantages of Generating Electricity in a Combined Gas Cycle Power Plant? 
The most important feature of combined gas cycle power plants is the higher level of efficiency that can be achieved compared to conventional thermal power plants. This advantage results from the intelligent utilisation of existing energy.

Generating electricity – phase 1
The process begins with the intake of combustion air (1). This is compressed in the gas turbine compressor (2) and fed into the combustion chamber of the gas turbine. In the combustion chamber the compressed air is added to the natural gas fuel and this fuel-air mixture is then ignited. The combustion gas (approx. 1,400 °C) that originates from the combustion subsequently flows through the power turbine of the gas turbine (3). The turbine is connected to the compressor and generator via a shaft and therewith generates electricity.

Generating electricity – phase 2
When it leaves the power turbine the combustion gas still has a temperature of approximately 600 °C. It is therefore conducted to the so-called heat recovery generator (5), where it converts liquid water into steam – with a temperature of approximately 560 °C and a pressure of around 135 bar. The steam is now led into a steam turbine (6), which in turn drives the generator (4) and again produces electricity.

"Disposal" of steam
At the combined gas cycle power plant Mellach gas and steam turbines drive the same generator, this is known as a single-shaft arrangement. With the help of the block transformer (7) the electricity produced in the generator is converted to the required voltage.

The steam in the steam turbine is fed into a condenser (8), where it condenses and the energy portion not useable in the steam turbine is dissipated with cooling water. The cooling water (9) can be taken directly from a nearby river.

The water liquefied in the condenser is then fed back into the heat recovery generator thus closing the water-steam cycle.

Clean utilisation of steam
Steam can be taken from the steam turbine to produce district heating (10). This steam is fed into a heat exchanger where it heats the district heating water that transports the district heating to the consumer.

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Florian Seidl Florian Seidl

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Ingun Metelko Ingun Metelko

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