Hydrothermal gasification
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Bioenergy > Technologies > Gasification > Thermochemical technologies > Hydrothermal gasification
Hydrothermal designates an aqueous system at elevated temperatures and pressures. The pressure is above the saturation pressure at the respective temperature. Thus, a liquid water phase is the predominant phase. If both the temperature and the pressure are above water's critical values (374°C, 22.1 MPa), water is present in its supercritical state (see also Supercritical Water Gasification). According to this definition, Aqueous Phase Reforming is a low temperature and low pressure variant of hydrothermal gasification.
Gasifying biomass and other organic matter in a liquid aqueous phase has many advantages over gas-phase (steam) reforming. By proper choice of the operating conditions, either a methane-rich or a hydrogen-rich product gas can be generated. A big advantage is the efficient reforming of tar precursors, such as phenols, because these molecules are completely soluble in hot pressurized water, which behaves like a non-polar organic solvent.
Hydrothermal gasification is well suited for gasifying wet biomass such as manure, sewage sludge (biosolids), black liquor, DDG&S, and other high-moisture containing biomass and wet organic residues. As opposed to Anaerobic digestion, all organic biomass constituents, including in particular the lignin fraction, can be gasified completely in a hydrothermal process.
The first step in a continuous process is the transformation of the feedstock into a pumpable slurry. This slurry must have an organic dry matter content of at least 10-15 wt% to yield good thermal process efficiencies. These efficiencies depend primarily on the organic matter content of the feed and on the efficiency of the heat recovery. Overall thermal process efficiencies for a continuous process producing synthetic natural gas (SNG) from wood as high as 70% (based on wood's LHV) have been reported. This is much higher than conventional steam gasification followed by gas-phase methanation.
As an emerging technology, hydrothermal gasification is still in an R&D phase. Among other research groups, the Paul Scherrer Institut[1]in Switzerland is developing a catalytic hydrothermal gasification process for the production of synthetic natural gas (SNG).
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