Highlighting the role of surface groups in the gasification of carbonized raw materials of different natures with steam and supercritical water at pressures ranging from 1 to 1000 bar

This manuscript describes a kinetic and mechanistic study of the gasification with water of two carbonized materials originating from a phenolic textile fiber and holm oak wood at 540–650 °C and 1–1000 bar. The wide experimental range studied causes the number of hydrogen bonds to strongly vary with the gasifying conditions, which generates important changes in the structure and properties of water. The solvent–solvent and solvent–solute interactions are significantly affected, with pressure favoring the formation of clusters. The concentration of the gas produced and the kinetics and pathways of gasification show a strong dependence on pressure. The study reveals the importance of the oxygen surface groups in gasification. These groups take part in some pathways of the gasification and control the gasification rate and the concentration of the gases. The formation of clusters of water molecules on the oxygen surface groups at high pressures enables a new pathway that accelerates the gasification and guides it toward the direct formation of CO2. Although gasification is accelerated by pressure, increasing amounts of the H2 produced are readsorbed onto the active sites of the solid, disabling them from being gasified (auto-inhibition phenomenon). Consequently, a pressure within this range yields the highest gasification rate.