Qualitative Analysis of Biogas by Raman Spectroscopy: Effect of Pressure on Major Components

ABSTRACT This work investigates the pressure‐dependent behavior of Raman spectra of methane, carbon dioxide, nitrogen, and selected minor biogas components in the pressure range up to 100 bar. A clear pressure‐induced red shift was observed for the ν₁ and 2ν 4 bands (≈0.02–0.024 cm −1 ·bar −1 ), in agreement with literature data, while the ν 3 band exhibited a significantly larger shift (0.05 cm −1 ·bar −1 ). In contrast, the ν 2 and 2ν 2 bands were nearly pressure‐insensitive. Pressure‐dependent band broadening was observed mainly for fundamental stretching modes, whereas overtone bands remained largely unaffected. For CO 2 (10–50 bar), pressure sensitivity was confirmed not only for the fundamental and overtone Q‐branches but also for hot bands, which demonstrated measurable red shifts and quadratic broadening behavior. Nitrogen exhibited the smallest pressure shift (0.008 cm −1 ·bar −1 ) and showed band narrowing with increasing pressure. Raman spectra of minor but technically critical components, H 2 S and NH₃, are reported here for the first time in this context. Spectral overlap between H 2 S and methane was identified as a limiting factor for qualitative analysis, whereas ammonia bands were clearly distinguishable. Finally, the applicability of the method was verified by measuring a real biogas sample. The results demonstrate that Raman spectroscopy is a promising tool for high‐pressure qualitative analysis and pressure monitoring in biogas systems.