Optimal Design of a Biogas-based Renewable Power Production System

Abstract

This paper presents optimal design for an energy-integrated biogas-fuel cell system for renewable electricity generation. The integrated process consists of two steps. The first step generates hydrogen from biogas via methane steam reforming (SMR), whereas the second step electrochemically converts this hydrogen into electricity using a solid oxide fuel cell (SOFC). These two steps are coupled via material and energy integration. Specifically, various design alternatives like anode and/or cathode gas recycling, biogas upgradation by CO2 removal, external versus direct internal reforming, and auxiliary power production through steam and/or micro gas turbine are explored to improve the overall efficiency and total annualized cost of the system. Specifically, a flowsheet superstructure is developed by incorporating all the available design alternatives. An optimal flowsheet with minimum total annualized cost is extracted from this superstructure using formal optimization techniques to meet the desired power target. Heat exchanger network superstructure is used to incorporate energy integration effectively. The proposed flowsheet and the corresponding optimal operating conditions are explained by analyzing the trade-offs associated with the corresponding design variables in terms of power production, capital expenditure, and utility consumption. For a power target of 300 kW, the proposed optimal energy-integrated process has a total annualized cost of $608,955/y with a net electrical efficiency of 67.1% and corresponds to electricity cost of $0.23/kWh.