Organic fraction of municipal solid waste treatment plant optimization case study: Circular economy sustainable transition towards a high efficiency Solid Oxide Fuel Cell stack system

The purpose of this study aims to be the energy optimization of a A2A Energiefuture project for an organic fraction of municipal solid waste treatment plant located in San Filippo del Mela. The plant is designed to dispose of an amount of about 75’000 tons per year of organic waste. By means of an anaerobic digestion process, and thus in the absence of oxygen, the fermentation of the organic fraction generates biogas and compost. The former, main focus of this study, once treated and filtered to remove impurities, can be injected, in accordance with the ongoing incentives, into the national gas distribution network as biomethane since it is characterized by similar parameters. The plant's energy demands, both thermal and electrical, are satisfied through withdrawal from the grid in the base configuration. Thermal energy required to maintain the operating temperature of the digester and to preheat the air for the biocells is provided by two similar boilers of 780 kW each fired by natural gas. Electricity supply is provided by a MV electrical substation. An optimized solution has been analysed in which natural gas-fired boilers give way to fuel cell also fed by natural gas. The latter electrochemical device can not only cover both the thermal and electrical needs of the plant but can also generate an electricity surplus that can be injected into the grid or stored in a battery energy system. Moreover a "stand-alone island" configuration was also analysed in which the fuel cell stack is fed by a share of biomethane produced by the treatment plant. The latest configuration, beyond reducing net carbon dioxide emissions to almost zero, provides the significant opportunity to not depend on natural gas market price fluctuations. A model has been created on Aspen Plus® that breaks down the electrochemical process through all the reactions that occur within each fuel cell component. By means of the model, it was possible to run the simulation by which the operating values of the fuel cell under the steady-state conditions were obtained. These parameters were extracted and used in a calculator data sheet to achieve the economic analysis of the new plant design.