This work investigated the washing pre-treatment in order to upgrade a low grade biomass (greenhouse crop residue). Characteristics of biomass and biomass ashes before and after water washing were studied and compared. Characterization was carried out using techniques such as elemental analysis, proximate analysis, Inductively Coupled Plasma Optic Emission Spectroscopy (ICP-OES), Fourier Transform Infrared Spectroscopy (FT-IR), Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC).The main aim of washing was to decrease the quantity of ashes present in this kind of biomass since inorganics cause operational problems in thermal processes. Water washing process at 80 degrees C was selected as the best temperature since the higher calorific value (HCV) increased a 7.00%. Moreover, a percentage of 46.16% of ash elimination was achieved at that temperature. Additionally, the quantity of problematic inorganic elements such as Cl, Na, Al, Fe and Si was lowered. Moreover, the ash melting temperature shifted to higher temperatures (1200 degrees C) after the washing. After carrying out the treatment, HCV was enhanced up to 18.07MJ/kg and the characteristics for the use of the material as a fuel were improved. (c) 2019 Elsevier Ltd. All rights reserved.