Use of wastewater treatment plant biogas for the operation of Solid Oxide Fuel Cells (SOFCs)

Solid Oxide Fuel Cells (SOFCs) perform well on light hydrocarbon fuels, and the use of biogas derived from the anaerobic digestion (AD) of municipal wastewater sludges could provide an opportunity for the CH₄ produced to be used as a renewable fuel. Greenhouse gas (GHG), NO_x, SO_x, and hydrocarbon pollutant emissions would also be reduced. In this study, SOFCs were operated on AD derived biogas. Initially, different H₂ dilutions were tested (N₂, Ar, CO₂) to examine the performance of tubular SOFCs. With inert gases as diluents, a decrease in cell performance was observed, however, the use of CO₂ led to a higher decrease in performance as it promoted the reverse water-gas shift (WGS) reaction, reducing the H₂ partial pressure in the gas mixture. A model was developed to predict system efficiency and GHG emissions. A higher electrical system efficiency was noted for a steam:carbon ratio of 2 compared to 1 due to the increased H₂ partial pressure in the reformate resulting from higher H₂O concentration. Reductions in GHG emissions were estimated at 2400 tonnes CO₂, 60 kg CH₄ and 18 kg N₂O. SOFCs were also tested using a simulated biogas reformate mixture (66.7% H₂, 16.1% CO, 16.5% CO₂, 0.7% N₂, humidified to 2.3 or 20 mol% H₂O). Higher humidification yielded better performance as the WGS reaction produced more H₂ with additional H₂O. It was concluded that AD-derived biogas, when cleaned to remove H₂S, Si compounds, halides and other contaminants, could be reformed to provide a clean, renewable fuel for SOFCs.