Thermodynamic Assessment of Hydrothermal Combustion Assisted Fossil Fuel In-Situ Gasification in the Context of Sustainable Development

•A hydrothermal combustion assisted in-situ gasification system is proposed.•Coupling concentrated solar heat leads to better thermodynamic performance.•The coupling of the system is better with methanol as the auxiliary fuel.•Fossil fuels with high carbon content have higher H2 production.•The carbon emission of the system is 3.9 kg CO2/kg H2, or even lower.