Humidification
Maintaining temperature levels, low enough for heat recovery, is often a problem. At burning plants the solution is to humidify the combustion air.
The humidification example shown in the figure above has been calculated for a wood-burning district heating boiler. The lower curve represents the cooling of the flue gases without humidification of the combustion air. The upper curve represents the cooling results when the burning air is humidified to dew point of 48oC. When the district heating return water temperature is 50oC, the flue gases can be cooled to a temperature of ~55oC. Then in the boiler, a heat recovery of ~17% of the boiler effect is achieved without humidifying the burning air, while heat recovery rises to ~28% when the burning air is humidified. At the same time it can be seen that the flue gas is cooled from 55oC to ~36oC in the upper bed of the condenser. Warmer return water results in an elevated dew point of the combustion air.
A by-product of the humidification is pre-heating of the combustion air with low temperature energy from the flue gases, which increases the boiler efficiency, even if the water vapour, vaporized into the air flow, is carried through the boiler combustion zone.
In practise, the condensing heat recovery, enhanced by humidifying the combustion air, works with all fuels. It is especially profitable in the case of base load boilers using natural gas or comparable fuels, thanks to the simplicity and fuel high price.