The simple method is to measure quantity of fuel input and the steam energy output. This method is the input output method.
Efficiency %= Output / Input X100
=[Steam Flow kg/s x Steam Enthalpy kj/kg] - [Water Flow kg/s×Water Enthalpy kj/kg] / [Coal Flow kg/s x HHV of Coal kj/kg] ×100
In case of reheat units the reheater inlet and outlet enthalpy also has to be considered.
Enthalpy of steam and water is available in steam tables if we know the pressure and temperature. Pressure and temperatures are available online in all the boilers.
Steam Flow and water Flow also is normally available online. However the accuracies are to be verified before using them.
One can determine the higher heating value by taking a sample of coal as it enters the boiler and analyzing it in the laboratory. These are normally done on a daily basis in most power plants.
Fuel flow is more complicated. Gravimetric feeders used in modern power plants can give the coal flows to a certain degree of accuracy. Otherwise this will have to be computed from volumetric flows and bulk density of the fuels.
This method, although it looks simple on paper, is not the industry preferred method because
- Flow measurements are not accurate nor steady
- Good quality flow instruments are costly.
- Flow measurements always involve a co-efficient, which can very much alter the results.
- Trouble shooting problems for determining the reasons for a lower efficiency is difficult.
However, this method finds use for quick calculation if the flow measurements are reliable and steady.