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Wood Ash Database

– Wood for Energy -

Combustion

Unburned fuelHjalmarsson, A., Bjurström, H., Sedendahl, K. Handbok för restprodukter från förbränning. Fjärrvärmeföreningen.

Besides ash, combustion also generates leftover products. These consist mainly of inert material in the fuel, and unburned fuel. During combustion, the leftover products either falls out together with the bottom ash or is carried with the air stream along with the fly ash, where they are separated using a separator such as a cyclone, electric filter or filter bag.

The amount of leftover products varies depending on the fuel type, combustion method and also, to a certain extent, operational factors. For example, measures taken to reduce emissions of nitrogen oxides can lead to a change in quality of the left-over products.

Furthermore, the age of an combustion facility should also be considered, since newer combustion installations often has a better incineration effectivity, and thus lower left-over product content. A table depicting common levels is shown below.

Combustion method Unburned content in Bottom ash, % Unburned content in Fly ash, %
Fluid Bed Combustor (FBC) 2-5 5-30
Roster 5-10 5-50
Spreader stoker 5-10 less than 5-20
Powder burner 5-10 less than 5-20