Combustion Efficiency
The combustion efficiency of a boiler is defined by the sum of the energy losses contained in the flue gases at the boiler’s outlet. It represents the residual energy that has not been transferred to the water, steam, or dissipated into the environment by radiation.
These energy losses are distributed as follows:
- Losses from the energy of the gas composition molecules (dry)
- Losses from the energy of water molecules originating from air humidity
- Losses from the energy of water molecules produced by the oxidation of hydrogen (H₂) in the fuel
Neglecting losses associated with unburned fuel, the losses due to the oxidation of hydrogen molecules in the fuel constitute the primary source of inefficiency in a boiler. To maximize efficiency, it is crucial to condense the flue gases to recover the energy contained in the water vapor.
Gas Temperature and Combustion Efficiency
The amount of energy contained in gas molecules is directly proportional to their temperature. Therefore, it is possible to graphically represent combustion efficiency as a function of gas temperature for a given operating condition and fuel.
The graph below illustrates this relationship for natural gas combustion with 15% excess air, showing the correlation between gas temperature and combustion efficiency.
Differences between Efficiencies
The combustion efficiency described above is identical to that measured by a combustion analyzer, which is typically used to take measurements in the flue. This analyzer provides the actual efficiency based on specific operating conditions: fuel type, excess air, altitude, etc.
However, this combustion efficiency does not directly correspond to the boiler efficiency, often referred to as “fuel-to-steam efficiency.” To determine it, the losses due to radiation must be subtracted:
Boiler Efficiency (Fuel-to-steam) = Combustion Efficiency – Radiation Losses
Impact of Radiation Losses
Radiation losses remain relatively constant over the operating range of a boiler (in BTU/hr). However, their percentage relative to the total power increases when the boiler operates at low load. It is therefore essential to specify the boiler’s operating regime when calculating “fuel-to-steam” efficiency.
Boiler Efficiency According to the Operating Range
When the boiler’s power output decreases, the gas temperature drops, resulting in an increase in combustion efficiency. Conversely, radiation losses increase proportionally with the reduction in load. Consequently, there is an equilibrium point within the operating range where the overall boiler efficiency is optimal. This equilibrium point is generally around 75% of the nominal power, as demonstrated by the following graph.
