Forest residue is the remaining fraction after the outtake of timber, which comprises the tree tops and branches. It may as fuel cause damage to the combustion device through ash slagging and fouling. The objective of this work was to model the ash composition from well-specified samples of a spruce tree: wood, bark, twigs, needles, and shoots. Their ash at 1000 degrees C was modelled using global chemical equilibrium calculations, and laboratory-made ash of the five samples was analyzed by XRD and SEM-EDXA. According to the results, the risk of slagging arises from the spruce foliage: molten alkali silicates from spruce needles and probably molten alkali phosphates from spruce shoots may cause problems in the furnace. Fouling caused by condensing alkali vapours can be produced by all five samples. The amount of alkali vapours in the flue gas was in the same order of magnitude for all five samples, in spite of large differences in their original alkali contents. (c) 2010 Elsevier Ltd. All rights reserved.