Fuel is often burned to heat air, water, or steam for a direct use such as heating of buildings. Another major use, however, is to generate heat that is then converted into power. An example is the generation of electrical power, as shown in Figure 15.5
Combustion of fuel in the boiler heats steam that is used to drive a steam turbine. The heat in the steam has been converted into mechanical power in the turbine. The turbine drives an electrical generator, creating electrical power. In this case, however, there is a very severe limit on how much of the heat energy can be converted into work, according to the second law of thermodynamics. Under the best practical circumstances, only about 30 BTU of the 80 BTU delivered to the turbine can be converted into work. The remainder (50 BTU) must be rejected in the form of heat. Of course, this heat may still have a use.