Life cycle assessment of two emerging sewage sludge-to-energy systems: Evaluating energy and greenhouse gas emissions implications

► Comparative life cycle assessment of two emerging sewage sludge-to-energy systems. ► One system uses anaerobic digestion (AD) followed by fast pyrolysis while the other excludes AD process. ► Both systems achieve net energy gain and greenhouse gas emission credits. ► The system involving AD process is preferable to the system excluding AD process. ► Detailed contribution and sensitivity analyses were also conducted for both systems. A “cradle-to-grave” life cycle assessment was conducted to examine the energy and greenhouse gas (GHG) emission footprints of two emerging sludge-to-energy systems. One system employs a combination of anaerobic digestion (AD) and fast pyrolysis for bioenergy conversion, while the other excludes AD. Each system was divided into five process phases: plant construction, sludge pretreatment, sludge-to-bioenergy conversion, bioenergy utilizations and biochar management. Both systems achieved energy and GHG emission benefits, and the AD-involving system performed better than the AD-excluding system (5.30 vs. 0.63GJ/t sludge in net energy gain and 0.63 vs. 0.47t CO2eq/t sludge in emission credit for base case). Detailed contribution and sensitivity analyses were conducted to identify how and to what degree the different life-cycle phases are responsible for the energy and emission impacts. The energy and emission performances were significantly affected by variations in bioenergy production, energy requirement for sludge drying and end use of bioenergy.