Described is a methodology aiming to the removal of harmful ash constituents from the ash of biomass, such as alkali metals, chlorine and sulfur, prior to is thermochemical conversion, in order to minimize/eliminate the ash-related corrosion, deposition and agglomeration problems, as well as the emissions of alkali metals, chlorine and sulfur. This removal is achieved by a combined pre-treatment which includes prepyrolysis of biomass at temperatures varying in the range of 200-300° C. and for a period of 5 min up to 2 h, followed by the leaching of the biomass materials using water with a solid/water mass ratio varying from 33 g/L up to 300 g/L with water temperature varying from 13° up to 55° C. and residence time varying from 5 min up to 24 h.