Sludge-to-energy approaches based on pathways that couple pyrolysis with anaerobic digestion (thermal hydrolysis pre/post-treatment): Energy efficiency assessment and pyrolysis kinetics analysis

Abstract

In this study, the apparent energy efficiencies (AEEs) of four sludge-to-energy pathways (raw sludge (RS)→pyrolysis; RS→anaerobic digestion (AD)→pyrolysis; RS→thermal hydrolysis process (THP)→AD→pyrolysis; and RS→AD→THP→pyrolysis) were analyzed and the yields of the pyrolysis products were examined. Compared with simple RS pyrolysis, the other three pathways yielded significantly more biochar and pyrolysis gas (py-gas), but the tar yield was reduced. The pathways that combined AD, THP, and pyrolysis achieved higher energy efficiencies than the pyrolysis of RS. The RS→AD→THP→ pyrolysis pathway had the highest AEE (91.3%). Thermogravimetric analysis and the Coats-Redfern model were used to explore the pyrolysis kinetic characteristics of the different sludge-to-energy pathways. The results showed that AD and THP reduced the activation energy of sludge pyrolysis. The activation energy of the pyrolysis of sludge treated by AD after being treated with THP was the lowest of the treated sludge samples (15.30 kJ/mol and 19.04 kJ/mol at 180.04°C–337.50 °C and 337.50°C–550.03 °C, respectively).