1 A substance flow analysis of a combustion- and a gasification- based waste-to-energy units F. Di Gregorio 1 , N. Arena 2 and U. Arena 1, 3 1 Department of Environmental Sciences – Second University of Naples, Caserta, 81100, Italy. fabrizio.digregorio@unina2.it 2 Institute for Combustion Research – National Research Council, Napoli, 80125, Italy 3 The paper aims to provide data for a critical comparison of the two WtE options, by means of a description of the performance of a combustion- and a gasification-based process (in the following indicated as CB-WtE and GB-WtE), applied to the same solid waste and carried out in a moving grate furnace and in a high temperature vertical shaft gasifier with melting system, respectively. To AMRA scarl – Analysis and Monitoring of Environmental Risk, Napoli, 80125, Italy Abstract The paper aims to provide data for a critical comparison between a combustion- and a gasification- based waste-to-energy process carried out in a moving grate furnace and in a high temperature shaft reactor, respectively. Data from several units in operation have been processed by means of the Material and Substance Flow Analysis, which developed a systematic assessment of the flows and stocks of materials and elements within the waste-to-energy unit by connecting the sources, the pathways, and the intermediate and final sinks of each species. The different layers obtained by this analysis highlight, in particular, the patterns of crucial elements, such as carbon, hydrogen, chloride and heavy metals, in the different output streams of the two compared processes. They also allow to quantify the potential reduction of the amount of residues to be sent to final disposal. Keywords waste-to-energy, combustion, gasification, substance flow analysis, landfill INTRODUCTION The environmental and energetic performance of a modern, adequately constructed and operated, waste-to-energy (WtE) unit is today assessed as comparable to that of a medium industry (Rechberger and Schöller, 2006), "with less environmental impact than almost any other source of electricity" (US EPA, 2003). Moreover it is recognized that "increased recovery of waste and diverting waste away from landfill play a key role in tackling the environmental impacts of increasing waste volumes" (EEA, 2008). Nevertheless, fear of pollution still brings WtE plants to the center of emotional public debate, much of it based on perception rather than on objective scientific evidence. This public perception forces the manufacturers of WtE plants to continuously improve the performance of the chemical conversion process and to develop advanced technologies for pollution (especially air pollution) control (Arena et al., 2012a). With specific reference to the chemical conversion process, the whole range of technologies can be grouped into two main categories: combustion based- and gasification based-thermal treatment. The first is a well established technology, with more than 900 plants, having a capacity that ranges from 50 to 1000kt/y: the most common type of combustion-based WtE technology used worldwide is mass burn moving grate incinerator, which is that utilized by 420 of the 450 combustion-based WtE plants in Europe (Van Berlo and Simoes, 2011). The second involves more complex processes and is less proven on a commercial scale, even though about 100 gasification-based WtE plants, having a capacity that ranges from 10 to 250kt/y, are in continuous operation by years, mainly in Japan but also in Korea and Europe. This suggests that gasification could today be proposed as a viable alternative for a WtE treatment, particularly if a dramatic reduction of the amount of residues to be disposed in landfills is required (Arena, 2012).