December 10, 2014
How is coal converted to electricity?
Coal is first milled to a fine powder, which increases the surface area and allows it to burn more quickly. In these pulverised coal combustion (PCC) systems, the powdered coal is blown into the combustion chamber of a boiler where it is burnt at high temperature (see diagram below). The hot gases and heat energy produced converts water – in tubes lining the boiler – into steam. The high pressure steam is passed into a turbine containing thousands of propeller-like blades. The steam pushes these blades causing the turbine shaft to rotate at high speed. A generator is mounted at one end of the turbine shaft and consists of carefully wound wire coils. Electricity is generated when these are rapidly rotated in a strong magnetic field. After passing through the turbine, the steam is condensed and returned to the boiler to be heated once again.
The electricity generated is transformed into the higher voltages (up to 400,000 volts) used for economic, efficient transmission via power line grids. When it nears the point of consumption, such as our homes, the electricity is transformed down to the safer 100–250 voltage systems used in the domestic market (230V in Poland).
Diagram 1: How a coal-fired power plant works
- Coal is milled and then dried.
- Coal is burnt in the boiler’s combustion chamber (typically, at the rate of 50 kg/s or 250 kg/s for lignite).
- Air is preheated using flue gas and fed into the boiler.
- Heat is recovered from flue gas on its way to the stack.
- High-pressure water is heated and vaporised. Steam is superheated.
- Superheated steam is fed into the turbines.
- Steam expands to the lowest possible pressure. Steam’s thermal energy is converted to mechanical movement of the turbine’s rotor.
- Electricity is generated in the generator attached to the turbine.
- Steam passes into the condenser.
- Steam condenses back into water
- Circulation water, which collects heat from the condensed steam, is cooled in the cooling tower
- Condensed steam is pumped back to the boiler, closing the loop.
PKN ORLEN is constructing a CCGT plant in Włocławek and is going to erect a similar facility in Płock. How does a CCGT plant work?
Combined Cycle Gas Turbine plants utilise both gas and steam turbines within a single power generating unit. The steam turbine and generator operate in a similar fashion as in a coal-fired power plant, which we have discussed above, while the gas turbine captures hot exhaust from gas combustion. The first gas turbines used in power generation were large aircraft engines placed on bearings and attached to generators. In an attempt to improve the energy efficiency of power generating units based on gas turbines, it was decided that the thermal energy of the turbine’s exhaust gases should be utilised by directing the hot exhaust from the gas turbine to the Heat Recovery Steam Generator (HRSG), where the heat is captured by water, turning it into steam, which is then superheated to over 550°C. The HRSG is a boiler without a combustion chamber or flame. The transfer of heat from the exhaust gas to water creates steam, which then enters the steam turbine, where it drives the rotor, condenses and is ultimately returned to the boiler. In a CCGT system, approximately two-thirds of the total power is generated by the gas turbine and one-third by the steam turbine. By capturing the heat from exhaust gases, CCGT power plants have a net efficiency of over 60%. In a typical CCGT power plant, the gas turbine, steam turbine and electric generator work on
a single shaft, which translates into lower investment costs and a smaller footprint area of the plant. This is the configuration used at PKN ORLEN’s CCGT plant in Włocławek. Under other configurations, two gas turbines may be used to drive a single steam turbine. Such units have a high generating capacity, which can currently reach as much as 1 GW. Double-shaft configurations are typically used in plants geared towards generating more process steam or heat, where a non-standard steam turbine, which cannot operate on the same shaft as the gas turbine, is needed.
Diagram 2: A single-shaft Combined Cycle Gas Turbine plant (PKN ORLEN’s plant in Włocławek, with process steam and electricity for use by Anwil)