Supercritical water gasification of biomass: Thermodynamic constraints

In the present work, the supercritical water gasification (SCWG) of biomass is analyzed with a view to outlining the possible thermodynamic constraints that must be taken into account to develop this new process. In particular, issues concerning the formation of solid carbon and the process heat duty are discussed. The analysis is conducted by means of a two-phase non-stoichiometric thermodynamic model, based on Gibbs free energy minimization. Results show that char formation at equilibrium only occurs at high biomass concentrations, with a strong dependence on biomass composition. As regards the process heat duty, SCWG is mostly endothermic when biomass concentration is low, although a very small amount of oxidizing agent is able to make the process exothermic, with only a small loss in the heating value of the syngas produced.

► Supercritical water gasification of biomass: two-phase equilibrium model.
► High biomass concentration and low temperature: char is favored.
► SCWG is mostly endothermic when biomass concentration is low.
► SCWG becomes exothermic when even small amounts of O2 (ER = 0.2) are added.