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[en] Gasification in supercritical water can be assisted with heterogeneous catalysts. Effective salt separation upstream of the catalyst is important to avoid poisoning of the catalyst and to recover nutrients. Recovery of phosphorus and nitrogen as well as gasification of a significant portion of the organic carbon were demonstrated on the pilot plant scale. A Ru/C catalyst was applied to catalyze the formation of CH4, which was the desired primary gasification product. On top of the catalyst, a bed of ZnO was used as sulfur adsorbent to protect the catalyst from deactivation. As feedstock for the process, glycerol, ethanol, and digestate sludge were studied. The results confirm the activity of the catalyst under the applied conditions. At a reaction temperature of 420 °C and a pressure of 280 bar, a gas composition close to thermodynamic equilibrium was achieved. Salt separation performed at 470 °C was effective, but the separation efficiency was less for potassium than for phosphorus. Fifty-six percent of the ash contained in digestate sludge was separated and recovered. Sulfur partly escaped the salt separation system and reached the reactor. The ZnO layer trapped most of this remaining sulfur. The remaining sulfur contamination was low enough not to poison the Ru/C catalyst completely. In total, 326 kg of glycerol, 334 kg of digestate sludge, and 167 kg of ethanol were gasified without any operational issues.
[en] Biogas plants, increasing in number, produce a stream of fermentation residue with high organic content, providing an energy source which is by now mostly unused. We tested this biomass as a potential feedstock for catalytic gasification in supercritical water (T ≥ 374 °C, p ≥ 22 MPa) for methane production using a batch reactor system. The coke formation tendency during the heat-up phase was evaluated as well as the cleavage of biomass-bound sulfur with respect to its removal from the process as a salt. We found that sulfur is not sufficiently released from the biomass during heating up to a temperature of 410 °C. Addition of alkali salts improved the liquefaction of fermentation residues with a low content of minerals, probably by buffering the pH. We found a deactivation of the carbon-supported ruthenium catalyst at low catalyst-to-biomass loadings, which we attribute to sulfur poisoning and fouling in accordance with the composition of the fermentation residue. A temperature of 400 °C was found to maximize the methane yield. A residence time dependent biomass to catalyst ratio of 0.45 g g−1 h−1 was found to result in nearly full conversion with the Ru/C catalyst. A Ru/ZrO2 catalyst, tested under similar conditions, was less active. -- Highlights: ► Fermentation residue of a biogas plant could be successfully liquefied with a low rate of coke formation. ► Liquefaction resulted in an incomplete removal of biomass-bound sulfur. ► Low catalyst loadings result in incomplete conversion, implicating catalyst deactivation. ► At 400 °C the observed conversion to methane was highest. ► A residence time dependent biomass to catalyst ratio of 0.45 g g−1 h−1 was determined to yield nearly complete conversion