Clean production of power and heat for waste water treatment plant by integrating sewage sludge anaerobic digester and solid oxide fuel cell

A Sewage Sludge Anaerobic Digester (SSAD) system integrated with a Solid Oxide Fuel Cell (SOFC) was modeled and optimized to reach an efficient/cost effective system for producing clean power and heat in Parand wastewater treatment plant (WWTP). The system modeling was performed by the use of 4E analyses (energy, exergy, economic, environmental). Based on 4E modeling and by the use of two objective functions (payback period and exergy efficiency) and seven design variables, SSAD-SOFC system was optimized for shorter payback period and higher exergy efficiency. This multi-objective optimization provided the optimum system operating specifications. From this type of investigation which was not observed in literature for SSAD-SOFC system, the results showed 58747 m3/day of biogas production as well as 7.5 MWel and 5.3 MWth electricity and heating generation, respectively at the optimum point. Furthermore, at the optimum point the integrated system overall energy and exergy efficiency were also 28% and 21%, with payback period of 1.7 years. Moreover, in comparison with a traditional system in which WWTP had to buy electricity and natural gas from the grid and burns fuel for heating requirements, the relative annual benefit of the SSAD-SOFC system increased by 17.9%. Finally, results from multi-objective optimization showed 6.9% improvement in overall energy efficiency and 7.5% enhance-ment in exergy efficiency in comparison with those for single-objective optimization with only payback period as the single objective function. The development of a sustainable and cost-effective procedure for optimum design of WWTP for electricity and heat generation as well as implementing of the above analysis in other integrated systems with similar equipment are implications of this research.