Fly ash or, as it is called in some areas, "pulverised fuel ash" (PFA) is a fine-grained material that is carried from the furnace in the flue gases during incineration, and consists of elements with a low vapour temperature. Fly ash is the lightest component of waste generated by the combustion process (roughly 200¼m). Also, it often contains higher levels of dioxins and heavy metals than bottom ash. Pitman,R.M. 2006. Wood ash use in forestry - a review of the environmental impacts. Forestry, 79, 563-588.
Fly ash from combustion of biofuels have also been shown to have interesting technological qualities, and can often be used as a, for example, reinforcement or load-bearing layer. In Finland, about half of the produced fly ash is used in various construction applications. Fly ash is used similarly in Sweden in many projects, including road building after a Finnish example. An example of this is the project carried out by Vattenfall AB and led by Värmeforsk, which purpose is to create a set of guidelines for building with and dimensioning fly ash. Mácsic, J. 2006. Flygaska som förstärkningslager i väg.
The fly ash is gathered using a dust separator. The most common types of dust separators are cyclone, electric filter and dust filter bag Hjalmarsson, A., Bjurström, H. et al. 1999. Handbok för restprodukter från förbränning.. For the most rigorous emission standards that are usually applied to large boilers, electric filter (a.k.a. electrostatic precipitator, ESP) or dust filter (a.k.a. baghouses) type separators are usually used. Cyclones are sometimes used in conditions where the regulations are less stringent. (coalonline.org).