Online detection of trace volatile organic sulfur compounds in a complex biogas mixture with proton-transfer-reaction mass spectrometry

Abstract

Online monitoring of trace contaminants, especially sulfur-containing compounds, is critical in biogas plants to protect downstream process steps, e.g., fuel cells or catalysts. The high biogas purity requirement for fuel cell applications and the presence of a myriad of unknown contaminants at different concentration levels require untargeted measurement techniques with high sensitivity and high dynamic range. Here, we present an online, real-time measurement of trace contaminants in the cleaning stream of a biogas plant with a Vocus proton-transfer-reaction mass spectrometer (Vocus-PTR-MS). We identified more than 300 Volatile Organic Compounds (VOCs) in the raw biogas in the 30–400 m/z range, with total concentrations varying in 54.6–548.3 ppmv on different days. Among these, more than 30 were Volatile Organic Sulfur Compounds (VOSCs), with total concentration in the range of 0.3–9.3 ppmv during measurement days and dimethyl sulfide (C2H6S, DMS) was the most dominant VOSC. Typical biogas tracers were also identified in the ambient air around the biogas plant at low ppbv levels. These results confirm that VOCUS-PTR-MS can be a rapid, scientific online tool of choice to monitor a biogas facility and, hence, shows excellent ability to characterize biogas production.