A Photoacoustic-Based Measurement System for Dual Detection of NO₂ and CO₂ in Combustion Exhaust Gases

In this article, a low-cost, low-complexity photoacoustic (PA) sensing system for the simultaneous detection of CO2 and NO2 in exhaust gas is presented. The proposed system is designed as part of a continuous emissions monitoring system (CEMS) for gas turbine emissions. The system exploits the amplification of the PA signal provided by an acoustic ring resonator, which is characterized by a simple and robust structure and is suitable for in-field measurements. The dual gas detection is obtained by exploiting two measurement principles; the first one, dedicated to the detection of NO2, which is present in the target mixture in the ppm range, is the classical PA effect. In fact, the optical source is a light-emitting diode (LED) with a center wavelength of 405 nm matched on an adsorption peak of NO2. This allows for deriving the NO2 concentration measurement directly from the amplitude of the PA signal. The other mechanism is used to measure the concentration of one of the major components of the exhaust gas, with a concentration in the range of some percentage. The quantity of CO2 is sensed, exploiting its effect on the sound speed, and consequently on the resonance frequency of the resonator. To measure the CO2 concentration, the system automatically tracks the acoustic resonance shift. The detection of the two gases is realized simultaneously by a unique sensor with real-time measurements. A laboratory characterization of the proposed systems showed its feasibility. Experimental results show the possibility to detect NO2 with a resolution lower than 1 ppm, whereas CO2 resolution is about 0.2%.