Photoacoustic monitoring of CO₂ in biogas matrix using a quantum cascade laser

We have developed an on-line sensor for biogas monitoring using mid-infrared quantum cascade laser (QCL) and photoacoustic (PA) detection. This sensor concept takes advantage of the inherent selectivity provided by mid-ER laser spectroscopy and the small gas volume required in photoacoustic spectroscopy allowing rapid response. Using a distributed feedback QCL emitting at 4.3 μm, the sensor concept as been successfully tested for carbon dioxide measurement under harsh conditions during biogas treatment in a pilot plant. In this application a reliable and rapid gas sensor is required for process control to assure a continuous CO₂ concentration below 2%. The obtained results compared favorably with a state of the art NDIR analyzer used as a reference with the distinct advantage of faster response times. Due to the generic nature of the developed PA-QCL sensor concept selective on-line measurement of other important analytes in biogas, such as NH₃ and H₂S is possible as well.