In this work, the composition (chemistry and petrography) and the physical (textural) and thermal (pyrolysis and combustion) properties of fly ash derived from the combustion of forest biomass (Eucalyptus globulus bark), used as feedstock in a pulp and paper plant located in Northern Spain, have been investigated. Moreover, this original biomass has also been characterized in order to determine its composition and some of its properties. To concentrate the unburned carbons from the fly ash, the raw fly ash was subjected to two different procedures: dry sieving and agglomeration with used vegetable oil processes. It was concluded that the first procedure that gives rise to fractions with a high content in unburned carbons and good textural properties is the more effective procedure. Accordingly, these fractions were investigated as precursor materials in the preparation of activated carbons. As for the inorganic part of the fly ash, this is mainly of a basic character with a predominance of alkaline and alkaline-earth elements.