Off-Resonance Photoacoustic Spectroscopy Technique for Multi-Gas Sensing in Biogas Plants

An off-resonance broadband photoacoustic spectroscopy (PAS) technique with a supercontinuum laser (SCL) in the near-infrared range is demonstrated for biogas measurements with different biomass matrices. The PAS sensor system has been calibrated with known concentrations of methane (CH₄), carbon dioxide (CO₂), water vapor (H₂O vapor), and hydrogen sulfide (H₂S). A laboratory-scale bioreactor was set up to monitor CH₄ and CO₂ generation using the SCL-PA sensor system. The obtained results show the suitability of the PAS sensor to be employed in a fully operational large scale biogas plant for online monitoring. The periodic variations in concentration for CH₄, CO₂, and H₂O vapor were monitored in a large scale cattle dung based biogas plant in real-time, and the operating ranges were measured to be around 50-65%, 34-48%, and 0-1%, respectively. The SCL-PA sensor was also employed at two different Sewage Treatment Plants in Chennai, Tamilnadu, India, where along with CH₄ (60-63%), CO₂ (34-38%), and H₂O vapor (0.8-1%), trace concentration levels of H₂S were found to be around 0.04-1%.