Studies on the coal gasification process in supercritical water (SCW) were carried out with the ReaxFF molecular dynamics (MD) method, in which the Wiser model of the coal molecule was adopted. The results show that hydrogen production increases with increase of temperature and water-coal mass ratio. It is also found that the coal molecule breaks into small fragments before it reacts with water molecules. The detailed chemical reactions and pathways of hydrogen generation during the gasification process are disclosed. H ions are found to be the main source of hydrogen generation, and C-H-O compounds or radicals are the most essential reactants throughout the reactions producing H-2 and H ions. OH ions can significantly accelerate the oxidization of organic fragments to produce C-H-O compounds and radicals, which explains how catalysts of alkali salts such as NaOH and KOH improve hydrogen production. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.