Syngas generation from anaerobically digested sludge via microwave-assisted pyrolysis with activated carbon

Sewage sludge management is a significant environmental and economic challenge for wastewater treatment plants, driving the development of energy conversion methods that recover value while reducing disposal. Microwave-assisted pyrolysis (MWP) is a promising technique because it enables rapid volumetric heating and hydrogen-rich syngas formation. However, most studies use high microwave powers, additional catalysts, severe operating conditions, whereas the comparative performance of dried and dewatered sludge under low-power activated carbon (AC) assisted MWP is not well understood. This study compares dried sludge (DS) and dewatered sludge (DWS) under AC-assisted MWP at microwave powers of 100–500 W and residence times of 10–60 min. Product yields, syngas composition (H₂,CO,CO₂,CH₄), cold gas efficiency (CGE), were analyzed to assess how feed moisture affects gas production and upgrading. At 500 W for 60 min, DWS produced hydrogen-rich syngas containing 34.48 vol% H₂ and 24.04 vol% CO, and CGE 38.6%. Gas yield on a dry feed basis was 1.13 g g −1 for DS and 0.99 g g −1 for DWS. While DS generated more total gas, DWS improved syngas quality and energy recovery, indicating a moisture-related upgrading effect through steam-assisted reforming and secondary gas-phase reactions. Low-power, AC-only MWP can effectively convert anaerobically digested sludge into rich hydrogen syngas.