Catalytic gasification of digestate sludge in supercritical water on the pilot plant scale

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.