By humidifying, the energy content of combustion air is increased prior to the air entering in to the boiler. This additional energy can then be retrieved at the condensing stage. Humidifying is especially beneficial if:

• The temperature of returning district heat water is relatively high
• The water content of fuel is low
• In addition, the humidifying can be used to adjust boiler temperature instead of using returning flue gas circulation.

The water for humidifying is directed from the scrubber condensing water cycle to the top of the humidifier and is evenly distributed across the humidifier cross section. The combustion air is taken in from the bottom of the humidifier, and is directed up through a bed of filler elements. As the water and the combustion air are in a good contact in the filler element bed, the moisture and heat is effectively transferred to the combustion air. The combustion air water content is increased and the temperature of air is also raised. This energy can then be recovered at the scrubber heat recovery phase.

After humidifier, the combustion air is directed through droplet separation after which the air is heated prior to directing the air through the combustion air duct and in to the boiler.