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

- Anaerobic digestion-derived biogas was studied for Solid Oxide Fuel Cell operations.
- Testing was performed with humidified dilute hydrogen fuel and various gas diluents.
- Successful biogas reformate tests were achieved with up to 20 mol% humidification.
- UniSIM™ simulation predicted Solid Oxide Fuel Cell system performance using biogas.
- Biogas could be employed as a renewable Solid Oxide Fuel Cell fuel alternative.

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), NOx, SOx, 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.