Comparative life-cycle energy and environmental analysis of sewage sludge and biomass co-pyrolysis for biofuel and biochar production

Abstract

To address the gaps in current research on sewage sludge (SS) pyrolysis, this study examined the feasibility of the environmental and energy demands of co-pyrolysis as an SS treatment process. The analyses included a cradle-to-gate life cycle assessment, carbon emission and credit assessment, energy recovery calculations, and sensitivity analysis. Sawdust (SD) and wheat straw (WS) were chosen as co-feeds for the co-pyrolysis scenarios; four mixing ratios (20, 40, 60, and 80 wt%) were examined for each co-feed. The functional unit was 1000 kg of bio-oil. Mixing the co-feed biomass with SS reduced the overall environmental burden by 58–83 %; WS/SS co-pyrolysis performed better than SD/SS in this respect. All examined scenarios reduced the global warming potential (GWP) and achieved a net positive energy balance; SD/SS co-pyrolysis was more desirable owing to high energy recovery. The additional gas turbine operation generated a process energy demand of 23–224 %. GWP and terrestrial ecotoxicity potential are indicators that are significantly affected by co-feed material transportation. Sensitivity analysis revealed that the transportation of feedstock and pyrolysis products should be prioritised to limit the environmental burden of the process. Co-pyrolysis is an eco-friendly SS treatment method, and its economic viability should be further investigated.