The boiler is the first stage in which energy conversion from fuel – coal, oil, gas or any other- to steam takes place. The steam is used further for power production in a turbine, for a process reaction, or for heating another fluid or gas, or simply for heating the air in the house.
What we mean by boiler efficiency is the efficiency of conversion from the chemical energy in the fuel to the heat energy in steam. This is shown as a percentage of the fuel energy. Modern boilers of large capacity used in power plants have an efficiency ranging from 80 to 90 %.
Here we look at simple methods to calculate boiler efficiency. Those interested in an elaborate calculation method should follow ASME PTC 4, which is the performance test code for boilers.
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.
Another method and a more practical approach is to measure the losses and then calculate the efficiency.
Efficiency % = 100 – Losses %.
The big advantage is that the calculation is on unit basis i.e.: for 1 kg of coal. This eliminates any inaccuracies in flow measurements.
Air and gas quantities are determined on theoretical basis and from laboratory analysis of the fuel. This is more accurate than the field flowmeters.
Since each loss is separately calculated it is easy to identify problem areas.
The most important factor that affects the Boiler efficiency is the Gas temperature leaving the boiler.
In a coal fired boiler of a 500 MW power plant if the temperature increases by ten degree centigrade the efficiency reduces by about 0.5 %.
Another main factor is the unburnt coal in the in the ash. One % increase in unburnts is around 0.5 % reduced efficiency.
These two are controllable by the proper maintenance and operational adjustments. The operator has to find out reasons why these two parameters are high and take necessary corrective action.