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%.