System Design and Optimization Studies of Co-Digestion of Food Waste with Sewage Sludge into Methane Enriched Gas for Heat, Electricity and Activated Char

Recently special attention is paid to make fuel and energy using anaerobic co-digestion of animal wastes, crop and crop residues, municipal solid waste (MSW), as well as municipal sewage sludge. One of the options for improving the biogas yields of anaerobic digestion of solid wastes is co-digestion of multiple substrates. However, this paper compares a conventional co-digestion process for biogas production for CHP with a hybrid pyrobiomethane integrated hydrogasification process for enhanced methane rich gas production. This paper also presents system simulation and analysis of integrated fuel cell based combined heat and power (CHP) systems using the SuperPro bioprocess design software. The innovative CHP system uses biogas as fuel, produced from co-digestion of food waste and sewage sludge, and methane rich gas produced from unconverted sludge, with coproduction of activated char. The methodology encompasses co-digestion, hydro-methanation, syngas production and integration, thermodynamic and electrochemical aspects of a solid oxide fuel cell (SOFC) system, as well as solving the problem of chemical equilibrium in complex systems. The optimized results are compared with other studies and reveals that an integrated two stage biothermal process involving biogasification (anaerobic digestion process) and a thermochemical process optimized using hydro-gasification can produce good quality biofuel for a SOFC. The cost economics of the final selected pyrobiomethane hybrid process for processing 30000 tonnes per annum of dry solids is feasible at an electricity selling cost of 0.3 USD/kWh, providing project benefits such as an IRR of 29.7%, a benefit to cost ratio of 1.38 and a net present value (NPV) of 123 million USD. This innovative scheme not only reduces CO₂ but also improves the economic return and reduces the waste discharge.
Keywords: Fuel Cell; Hydrogen; Anaerobic; Co-digestion; Biomass; Methane; Syngas; heat, electricity; activated, char.