The solid end-product from millions of years worth of organic material decomposition is coal. Coal is stored solar energy. Photosynthesis converts sunlight into plant matter, and plants capture that energy through photosynthesis. The energy is then converted to plant matter by animals that eat them.
Over millions of years, the earth’s crust has covered up the accumulated animal and plant matter. This material is slowly being converted into hard, black solids by the earth’s weight. Like other fossil fuels, coal takes many years to produce, but it can be regenerated quickly when used to generate electricity. Coal is not a renewable resource because it is a finite resource and cannot be replenished after being extracted and burned.
Globally, coal plays an important role in electricity generation. 37% of coal-fired power plants currently generate global electricity. The IEA figures show that coal will generate 22% of world electricity by 2040. This will maintain coal’s status as the largest source of electricity globally.
How does coal become electricity?
The power plants that generate electricity using steam coal are also called thermal coal. The first coal is ground to a fine powder. This increases its surface area and makes it more efficient in burning.
- Pulverized coal combustion (PCC), where the coal powder is blown into a boiler’s combustion chamber, is then burned at high temperatures.
- The heat energy and hot gases produced transform water into steam – through tubes lined by the boiler.
- The steam from high pressure is then passed through a turbine with thousands of blades that resemble propellers.
- These blades are pushed by steam, causing the turbine shaft rotation at high speed.
- One end of the turbine shaft is fitted with a generator. It is made up of carefully wound wire coils.
- These are quickly rotated in strong magnetic fields to generate electricity.
- The steam passes through the turbine and is condensed before being returned to the boiler for heating.
- The electricity generated is converted into higher voltages, up to 400,000 volts. This is used for efficient and economic transmission via power line grids.
- It is converted to safer 100-250 volt systems when it reaches the point of consumption, such as in our homes.
New combustion technologies and improvements in PCC power station design are ongoing. These technologies allow for more electricity to be generated from less coal. This results in lower emissions. This is called improving the thermal efficiency at the power station.
The High-Efficiency Low Emission (HELE), Supercritical, Ultrasupercritical, and Advanced Ultrasupercritical plants work the same way as conventional plants but at higher temperatures and pressures.
Due to higher steam parameters, supercritical boilers are more efficient than subcritical coal combustion. Supercritical systems have steam that reaches supercritical temperatures rather than boiling. Supercritical steam cycles with higher temperatures and pressures result in higher efficiency and lower coal consumption.
INTEGRATED GASIFICATION COMBINED CYCLE
- In IGCC systems, coal is not combusted but reacts with oxygen and steam to create syngas. This is primarily hydrogen and carbon monoxide.
- The syngas is then cleaned and boiled in a boiler to make electricity or steam to power a steam engine.
- The steam cycle can also use heat from the gasification or cleaning stages.