The diagram above compares the flue gas condensation using a traditional scrubber and a scrubber with combustion air humidification. In this case, the district heating return temperature is so that flue gas can be cooled down up to 55 °C.
The lower curve shows the condensation with only a scrubber, for a heat recovery of 17 % of the boiler power output. The curve above shows the condensation of flue gases after that the combustion air has been humidified up to the dewpoint at 48 °C.
Because of the higher moisture rate of the combustion air feeding the boiler, the dewpoint of the flue gas will be accordingly higher. Thus, heat can be recovered at higher temperature and transferred to district heating, allowing a heat recovery of 27 % of the boiler power output, for a flue gas output temperature below 40 °C.
Humidified combustion air is preheated using the energy at low temperature recovered from the flue gas, allowing increasing the combustion efficiency, even if vaporized water is sent to the boiler.
Combustion air humidification is a process which can be used with almost all kind of fuels. In the case of natural gas, the simplicity and high price of gas make this solution highly profitable.