Entrained Flow Gasification of Sewage Sludge─Performance Parameters and the Fate of Phosphorus, Potassium, Sulfur, and Heavy Metals

Gasification has the potential to combine energy production and resource recovery in the utilization of sewage sludge. We present an experimental study on sewage sludge gasification in a 90 kW entrained flow reactor, investigating air and O₂ as gasification agent and the influence of CO₂ or KCl on gasification performance. The overall conversion in the entrained flow gasification of sewage sludge ranged from 88 to 95%, and the cold gas efficiency was between 45 and 51% on a dry basis. The lower heating value of the syngas from air-blown gasification was around 3.5 MJ/N m³. We demonstrated the increase in the lower heating value using CO₂ instead of N₂ for equipment cooling. O₂-blown gasification increased the lower heating value to 8.4 MJ/N m³ and reduced the syngas volume flow by approximately 60%. Plant nutrients such as P and K remained in the gasification ash, making it a potential raw material for fertilizer production. However, a significant share of S became gaseous, which needs to be considered when further using the syngas. The entrained flow gasification of sewage sludge enabled the partial volatilization of heavy metals such as Hg, Tl, Cd, Pb, As, Zn, Cu, and Ni. The bottom ash from O₂-blown gasification complied with all limits from the German Fertilizer Ordinance. However, the bottom ash from air-blown gasification exceeded the limits for Ni, Cu, and Cd. In general, only small amounts of Cu and Ni evaporated during gasification. KCl acted as a catalyst in the gasification process and improved the gasification performance for the overall conversion and cold gas efficiency. Based on our results, KCl did not increase the partial volatilization of heavy metals in gasification. Therefore, we advise against using KCl as an additive in entrained flow gasifiers with a water quench.