Desulfurization and Catalytic Gas Cleaning in Fluidized-Bed Co-gasification of Sewage Sludge–Coal Blends

Energy recovery from digested sewage sludge can be achieved by means of co-gasification with coal in a fluidized-bed system. In this regard, one of the main hurdles in developing a feasible process is the need for gas cleaning, with special emphasis on desulfurization and minimization of the tar content of the product gas. In this work, high-temperature catalytic gas cleaning was investigated by means of two fixed beds placed in series downstream of the gasification system: the first containing dolomite for desulfurization and primary tar cracking and the second containing a nickel-based catalyst for additional gas reforming. The effect of the temperature on the performance of the Ni catalyst bed (800–900 °C) was assessed. The use of dolomite in a secondary bed at 800 °C allowed for a significant reduction in both tar [15–0.21 g/m 3 standard temperature and pressure (STP)] and H 2 S (to less than 0.01%) and an increase in the heating value of the gas [lower heating value (LHV) from 2000 to 2800 kJ/m 3 STP]. The use of the Ni catalyst decreased the tar content of the gas to undetectable levels. The best results were obtained with the Ni-based catalyst at 800 °C, in terms of enhanced LHV (increasing from 2000 to 3300 kJ/m 3 STP), gas production, which increased from around 2.40 to 2.75 m 3 STP/kg on a dry and ash-free basis (daf), and energy requirements for the process. However, some evidence of Ni catalyst deactivation was found when operating under these conditions.