The gasification of biomass in supercritical water is a promising technology for hydrogen production and the paper reports a thermodynamic analysis, based on minimization of Gibbs free energy, of the gasification with supercritical water of different biomass and agro-food residues: almond shells, digestate from wastewater treatment, algae and manure sludge. Numerical simulations were performed in order to assess the effect of temperature, pressure and biomass-to-water ratio on gas-phase yield and composition.A partial energy integration was also discussed, by considering the energy recovery from a turbine expansion of the gas-phase stream leaving the gasifier. The proposed thermodynamic approach allows predicting not only gasification efficiency of gasifier but also energy balance on the entire gasification process. Results showed that the dry substrates (almond shells and algae more than digestate and sludge) tend to form more carbon monoxide. Besides, data comparison revealed that the produced hydrogen comes from biomass and water for high process temperature, while when temperature decreases, the thermodynamic path tends to promote water formation from the hydrogen of the dry biomass.

The gasification of some selected components of food wastes using H₂O₂ as the oxidant and in the presence of NaOH has been investigated under subcritical water conditions. Hydrogen production was enhanced when both NaOH and H₂O₂ were used compared to when either NaOH or H₂O₂ alone was used or in their absence. Results indicated that the H₂O₂ acted to partially oxidize the samples while NaOH significantly increased hydrogen gas yields by promoting the water-gas shift reaction with subsequent CO₂ capture. In the presence of NaOH, the main components were Na₂CO₃, CH₃COONa and CH₃COONa·3H₂O. Char and tar production were suppressed in the presence of NaOH.

Co-digestion of press water from organic municipal wastes and of homogenized food residues with defibered kitchen wastes (food waste) as the main substrate was examined to improve biogas production. Although the biowaste digester was operated already at high organic loading (OLR) of 12.3kgCODm⁻³ d⁻¹ during the week, addition of co-substrates not only increased biogas production rates but also improved total biogas production. By feeding the two co-substrates up to 20kgCODm⁻³ d⁻¹ gas production followed the increasing OLR linearly. When the OLR was further increased with food waste, not more gas than for 20kgCODm⁻³ d⁻¹ OLR was obtained, indicating the maximum metabolic capabilities of the microbes. During weekends (no biowaste available) food waste could substitute for biowaste to maintain biogas production. Addition of press water or food waste to biowaste co-digestion resulted in more buffer capacity, allowing very high loadings without pH control.