Performance analysis of solid oxide fuel cell system fed by biogas

Abstract

The research focuses on the simulation of solid oxide fuel cell (SOFC) systems fuelled by biogas. The study is divided into four parts: the investigation of carbon formation in SOFC system, the selection of suitable reforming agent for SOFC system, the technical and economic analyses of different configurations of SOFC systems, and the study on the operation viability of SOFC stack fed by different feedstocks. The study on carbon formation in SOFC system indicates that the addition of supplementary reforming agent such as steam and air simultaneously with biogas to SOFC system is required to alleviate the carbon formation. The use of oxygen-ion conducting electrolyte in SOFC stack is preferred since steam produced in anode section can inhibit the carbon formation. For the topic of reforming agent selection, steam is found to be the most suitable reforming agent for SOFC system relative to air, and combined steam and air. SOFC system fed by steam offers extremely higher power density compared to that fed by the other reforming agents. Three major configurations of SOFC system are proposed in this research; i.e., SOFC equipped with palladium membrane reactor (PMR-SOFC), SOFC equipped with CO2 separator (M-SOFC/A-SOFC) and SOFC equipped with both palladium membrane reactor and CO₂ separator (SE-PMR-SOFC/SER-PMR-SOFC). For PMR-SOFC, it can offer higher power density compared with conventional SOFC system (CON-SOFC); however, it is not a good choice in the economic viewpoint. SOFC equipped with CO₂ separator is divided into two configurations; SOFC equipped with CO₂-selective membrane (M-SOFC) and SOFC equipped with CaO-CO₂ acceptor (A-SOFC). M-SOFC is not the interesting configuration due to the loss of CH4 in permeation section of membrane module. A-SOFC can become attractive candidate for SOFC system since it offers higher performance and potential benefit compared to CON-SOFC. SE-PMR-SOFC can offer superior power density to CON-SOFC. The recycle of retentate gas of PMR (SER-PMR-SOFC) can improve the performance of SE-PMR-SOFC; however, both SE-PMR-SOFC and SER-PMR-SOFC are not the interesting alternative in economic viewpoint. Nevertheless, with this configuration, all CO₂ generated in this system can be captured. Operating viability of SOFC stack fuelled by different feedstocks is finally investigated employing 1-D analysis. Pure-H₂ feed offers the highest performance for SOFC among the other feeds. Extremely high excess air is required for SOFC fed by biogas to become operation viable. Moreover, its power density is much lower than those of SOFCs fed by the other feeds. Methane-reformed feed offers higher power density than biogas-reformed feed since H₂ concentration of the former one is higher.