Why is the thermal efficiency of natural gas boilers over 100%?

The calorific value of fuel is divided into high calorific value and low calorific value. High calorific value refers to the heat released by the complete combustion of a unit of fuel under specific conditions, including the heat released by the condensation of water vapor in the flue gas into water.

Low calorific value refers to the heat obtained by deducting the latent heat of vaporization of water vapor condensed into water from the heat released by the complete combustion of unit fuel under specific conditions.

Such as natural gas composition, the calorific value at 101.325 kPa, 0 ℃ is as follow: Qnet,ar=36.36 MJ/m3; Qgr,ar=40.38MJ/m3. Under the condition of low calorific value, assuming that the heat generated by the combustion of natural gas is completely absorbed by the boiler water, the thermal efficiency of the boiler should be 100%. Similarly, under the condition of high calorific value, assuming that the heat generated by natural gas combustion is completely absorbed by boiler water, the theoretical limit value of boiler thermal efficiency is the ratio of high and low calorific value, that is, 40.38/36.36×100%=111.06%.

In the practical application of natural gas-fired boilers, due to the existence of exhaust heat loss q2, chemical incomplete combustion heat loss q3, heat dissipation loss q5, and the water vapor in the flue gas cannot be fully condensed, the actual operation thermal efficiency of natural gas-fired boilers is difficult to achieve its theoretical limit.