In general terms, electricity is generated from fossil fuels by burning them in the presence of air. This allows the energy contained in the fuel to be released as heat, resulting in very hot gases.
The energy released can then be used in two ways. In internal combustion engines, the hot combustion gases are compressed and then used to directly drive the electric generator and auxiliary equipments [15, 16, 45, 46].
The other method is to use the hot gases to heat water and produce steam at high temperature and pressure. The steam then drives a turbine or generating unit to produce electricity. In this case, combustion is said to be external.
Steam turbines, steam electric or thermal generating units are external combustion systems when they use fossil fuels to generate the steam.
Internal combustion units include stationary gas turbines, also known as combustion turbines, and reciprocating internal combustion engines. These units are generally less than 100 megawatts in size and they are considered to be less efficient than steam turbines.
Combined-cycle units use a gas turbine to generate electricity in a first stage. The hot exhaust gases from the gas turbine are then used to provide all or a portion of the heat for the boiler that produces steam to drive a steam generator turbine for further electricity generation.
Cogeneration units, also known as combined heat and power, use heat for electricity generation and for other thermal applications at the site.