Biomass gasification in supercritical water

The use of biomass is at the heart of the debate over energy resources in the 21st century. It is one of the major issues at stake. The term “biomass” encompasses a wide range of meanings, from high-quality biomass intended for food, such as grains, to biomass comparable to waste, such as beet pulp or sewage sludge from wastewater treatment plants. Particularly in the case of wet biomass, an interesting utilization process is supercritical water gasification. This process eliminates the need for a drying step and, under appropriate pressure and temperature conditions, produces a so-called “low-carbon” energy gas that may contain hydrogen, methane, carbon monoxide, and/or light hydrocarbons. The interest in this process therefore stems from the broader challenge of accessing non-fossil energy sources as well as the issue of greenhouse gases, as the use of biomass fits into a short carbon cycle, thereby creating a virtuous circular approach. Supercritical water gasification is particularly suited to very wet biomass (over 70% moisture content), which therefore does not need to be dried beforehand. Reaction temperatures are relatively low (maximum of 700 °C), compared to conventional or dry gasification processes (typically 900 °C). This limits the production of polluting gases, such as dioxins or NO _x . Similarly, the aqueous solvation medium helps limit the formation of solids and tars. The target gases are primarily hydrogen, but also a mixture of hydrogen and carbon monoxide (a mixture for Fischer-Tropsch synthesis) or methane production. The influence of key operating conditions on the nature and conversion yields will be detailed in this article (pressure, temperature, initial biomass concentration, and the presence or absence of catalysts). The most significant laboratory-scale pilot plants (up to 100 kg/h ⁻¹ ) will be presented, along with the first industrial developments set to begin operation in 2025.