Wood chips contain 0.5-2.0% of the dry weight in the form of incombustible minerals which are turned into ash in the combustion process. The ash is handled automatically at all district heating plants.
The manual work in connection with the ash system is limited to ordinary inspections and intervention in case of operations
stoppage. The composition of wood ash means that slagging is not a widespread phenomenon at wood chip-fired heating plants.
The ash drops from the grate onto an ash conveyor or other ash collection system. The sludge from the flue gas condensate contains a large amount of
heavy metal and is collected separately for later disposal.
The ash system may be arranged as a wet or dry ash system. A wet ash system is a dual function system, since it is efficient as a trap hindering false air
entering the boiler at the same time as extinguishing glowing ash. A drawback of the system is the heavy weight ash in the ash container and the corrosion resulting from the wet ash. The emptying of the containers varies with the consumption of wood chips, i.e., from approx. every second week to once every three months.