Multicyclones are used for removal of coarse ash or dust particles formed during combustion of solid fuel from flue gases. In stratified fuel combustion the bulk ash (90%) remains in the furnace, and smaller fractions are brought with flue gases into the atmosphere. Therefore, flue gases before discharging them to the atmosphere must be cleaned of ash in special devices, such as cyclones, which are used in almost all modern boiler houses run on solid fuel.