Key factors influencing the environmental performance of pyrolysis, gasification and incineration Waste-to-Energy technologies

Highlights: • Environmental impacts of representative Waste-to-Energy technologies are analysed. • Critical parameters, processes, operation and modelling conditions are assessed. • The goal is to identify key factors driving the environmental sustainability. • Energy efficiency, NO_x, CO₂, and metals recycling are most critical parameters. • Potential improvements are identified throughout all stages of each WtE technology. -- Abstract: Waste-to-Energy (WtE) has started playing an increasingly important role in the recovery of energy from municipal solid waste (MSW). A number of WtE technologies are being developed. However, selecting a more environmentally sustainable option is difficult due to data limitation and methodological inconsistencies. Using life cycle assessment (LCA) as a tool, this paper aims to identify key factors influencing the potential environmental impacts of four representative WtE technologies, namely the incineration (S1), pyrolysis (S2), gasification (S3), and gasification coupled with ash melting (S4). The systems are constructed using inventory data based on on-site operation of several industrial-scale reference plants. A comprehensive sensitivity analysis is conducted, assessing a range of critical input parameters, processes, operating conditions and modelling assumptions. The results demonstrate that all analysed WtE systems exhibit environmental benefits (i.e. negative environmental impacts) for most of the impacts, while S3 seems to be more optimal due to an intermediate syngas cleaning process, which results in both reduced emissions and increased energy recovery. Parameters driving the environmental impacts are energy recovery efficiency, feedstock variability, NO_x and CO₂ emissions at stack, and recycling of metals. Moreover, the overall ranking of different WtE systems is strongly dependent on operating conditions, such as effectiveness of the air pollution control process, utilization pathway of pyrolysis char, and to a lesser extent, bottom ash management (landfill or recycling). The LCA modelling conditions, such as substituted source of electricity, choice of functional unit and time frame are also shown to significantly affect the quantified environmental performance. Finally, the study highlights the directions, towards which, efforts should be focused throughout all stages of each WtE technology to obtain further improvements.