The hydrothermal gasification of some hard-shell nut residues (hazelnut, walnut and almond shells) was performed in a batch type reactor at temperature and pressure ranges of 300-600 degrees C and 88-405 bar, respectively. The biomass samples were converted into gaseous product (hydrogen, carbon dioxide, methane, carbon monoxide and C-2-C-4 compounds), aqueous product (carboxylic acids, furfurals, phenols, aldehydes and ketones) and solid products after hydrothermal gasification. Hydrogen production was improved by using natural mineral catalysts (Trona, Dolomite and Borax). The activity of selected natural mineral catalysts in hydrothermal gasification can be ordered as being Trona [Na-3(CO3)(HCO3)center dot 2H(2)O] > Borax [Na2B4O7 center dot 10H(2)O] > Dolomite [CaMg(CO3)(2)]. The most effective catalyst was found to be Trona at 600 degrees C leading enhancement in hydrogen yields (mol H-2/kgC in biomass) for hazelnut, walnut and almond shells as 82.4%, 74.1% and 42.4%, respectively. (C) 2014 Elsevier B.V. All rights reserved.