Addition of kaolin as a potassium sorbent in the combustion of wood fuel- effects on leaching of metals from the resulting fly ash

Combustion of biomass is a way to decrease the use of fossil fuels and thereby decrease the emissions of for instance the green house gas CO2. Earlier the forest felling residues, i.e. branches, needles and treetops, were left in the forest, but nowadays it is common to use forest felling residues as fuel. Consequently, there is a risk that the supply of nutrients and essential trace elements to the new generation of trees will be limited in the long perspective. Recycling of biomass ashes to the forest soil as fertilizers is common practice in many places. However, the ash spreading may cause some problems, such as a large initial pH increase that the forest soil may be exposed to from the strongly alkaline ash. The alkalinity is mainly caused by the oxides and hydroxides of potassium and calcium present in the ash. Kaolin has been shown to adsorb and react with vaporous alkali metal species from the flue gas and therefore addition of kaolin to bio fuel was tested in a full scale boiler in this project. Since no information about how kaolin addition in the boiler affects the leaching properties of nutrients and trace elements from the ashes this study was undertaken. Fly ash from combustion cases with and without kaolin were investigated in leaching tests in original form and in self-hardened form, i.e. mixed with water and left to react for a number of months. The leaching tests were carried out in order to compare the different ashes and to simulate the possible release of nutrients and trace elements. The ashes were leached according to EN 12457-2 as well as at two constant pH-values, 7 and 4, using a pH-stat titration system. Preliminary results suggest that some of the investigated trace elements (Cd, Cu, Mn, Pb and Zn) only to low content are found in compounds that are soluble at high pH values. Hardening seems to slightly decrease the leaching of these metals at high pH values, whereas addition of kaolin did not affect the release much. The leaching behaviour of trace elements at lower pH values is under evaluation and the results will be included in the presentation. The consumption of acid to achieve neutral and acidic pH values, respectively, during the titrations was highest in the unhardened ashes without kaolin. Both addition of kaolin and hardening of the ashes decreased the acid demand