Modeling and Performance Evaluation of a Large-Scale SOFC-Based Combined Heat and Power System for Biogas Utilization at a Wastewater Treatment Facility

Biogas has been identified as an attractive fuel for solid oxide fuel cells (SOFCs) due to its high methane content and its renewable status. Current experimental and modeling research efforts in this field have focused mainly on single-cell and small-scale systems performance evaluation. In this paper a large scale biogas source (∼15.5 MW) from a large wastewater treatment facility is considered for integration with an SOFC-based system. Data concerning biogas fuel flow rate and composition have been acquired from a wastewater reclamation facility in Denver and are used as inputs to a steady-state model of an SOFC combined heat and power (CHP) system developed with Aspen Plus. The proposed system concept for this application comprises an advanced SOFC system with anode gas recirculation (AGR) equipped with biogas clean-up and a waste heat recovery system. The system performance is evaluated at near atmospheric pressure with a 725°C nominal operating temperature of the fuel cell stack and system fuel utilization of 80%. The average biogas fuel input has a composition of 60% CH₄, 39% CO₂, and 1% N₂ on a dry molar basis. The SOFC-CHP system employs 80% internal reforming at a steam-to-carbon ratio of 1.2. The system offers a net electrical efficiency of 51.6% LHV and a net CHP efficiency of 87.5% LHV. The influence of the operating parameters on the system efficiency is investigated and discussed. The individual contribution of system components to the total inefficiency of the system is quantified with an exergy analysis. Exergy analysis results indicate that the system could offer a tremendous energy efficiency improvement when compared to biogas-supplied combustion turbines currently installed at the facility which operate with an average net electrical efficiency of 25%-LHV.