Addition of kaolin as potassium sorbent in the combustion of wood fuel – Effects on fly ash properties

The combustion of forest residues and other biomass is often hampered by ash sintering, fouling and corrosion caused by potassium salts. Kaolin has been shown to be an efficient absorbent for different species of potassium. This investigation was carried out with the aim to clarify if the leaching of Al or metals bound in aluminium silicate forms is an obstacle to the utilisation of kaolin as fuel additive and if the resulting ash would react differently with water than a normal wood ash. It was shown that kaolin addition decreases the leaching of K at high pH values, but leaching of Al and all trace elements was generally low at all pH (4, 7 and 13) used. Addition of kaolin during the combustion reduced the initial setting time of ash–water mixtures and the presence of kaolin also stabilised the ash structure. From theses results it can be concluded that kaolin addition seems to be a good way to decrease ash related problems during combustion, without deteriorating the properties of the resulting ash or preventing the use of the ash as mineral nutrient in forestry.