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The degradation of biomass is studied in the ranges of 330-410 °C and 30-50 MPa and at 15 min of reaction time. To characterize the chemistry of biomass degradation, key compounds which are intermediates of the biomass degradation are identified and quantified. These key compounds are used to compare the results from earlier studies of model compounds such as, e.g., glucose or cellulose, with biomass degradation in order to identify chemical reaction pathways. Key compounds identified are phenols (phenol and cresols), furfurals, acids (acetic acid, formic acid, lactic acids, and levulinic acid), and aldehydes (acetic aldehyde and formic aldehyde). In addition, sum parameters such as the total organic carbon content and the composition of the formed gas phase are used to describe the biomass degradation. The results are compared with the hydrothermal upgrading process and with the well-known gas-phase gasification processes. The influence of the change of water properties from subcritical to supercritical conditions on the biomass degradation is also discussed. These comparisons show that most of the main reaction pathways detected by the key compounds can be understood by the studies of model compounds. On the other hand, biomass is much more complex because biomass contains a lot of different substances. Especially, the influence of salts is significant and, in addition, rather complex.

This article reviews the work relating to the supercritical water gasifi cation of biomass with a focus on hydrogen production. The high hydrogen yield predicted by thermodynamic calculations and the special properties of near-and supercritical water support the biomass degradation; these were the main reasons why the process of supercritical water gasifi cation was investigated. The main advantage is that biomass, with a natural water content of 80 wt.% or more, can be converted without drying before. The energy required for heating up the relatively high water amount can be recovered by a compact heat exchanger, which is very important for the overall energy balance. The chemistry of biomass degradation is rather complex: from experiments with model compounds, the main reaction pathways and their dependencies on reaction conditions are identifi ed. This knowledge was applied in studies of biomass conversion. Biomass may include proteins and salts, which have a signifi cant infl uence on the gasifi cation: salts increase and proteins decrease the gas yield at comparable reactions conditions. In addition, the heating-up rate and the reactor type used infl uence the results. For the scale-up in view of a technical application, a bench-scale plant is necessary. This plant exists for some years and demonstrates the process feasibility also in the scale of 100 kg/h. Still challenges for a technical application, like corrosion and solid handling, exist.

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