Oil or natural gas can run gas turbines. In a gas turbine, fuel burning in a high-pressure combustion chamber produces hot gases that pass directly through the turbine, which spins a generator to produce electricity. The turbine also spins a compressor to provide high-pressure air to the combustion chamber.
This cycle is different from the steam turbine, where the heat of combustion turns water into the steam that pushes the turbine.
Combined-Cycle System
The most efficient way for generators to use gas or oil is in a combined-cycle system, which combines two means of producing electricity. Hot gases from the combustion chamber spin the gas turbine and the generator to make electricity. The system then pipes the still-hot exhaust gases leaving the combustion turbine to a "waste heat" steam boiler where their heat produces steam.
This steam turns a turbine, connected to a second generator, to produce electricity. Condensers convert the steam to water that returns to the boiler to repeat the cycle.
Because steam turbines can be built larger than gas turbines, a combined-cycle system often uses several gas turbines to feed hot exhaust to one steam boiler.