A photoacoustic based measurement system for dual detection of NO2 and CO2 in combustion exhaust gases

In this paper a low-cost, low-complexity photoacoustic sensing system for the simultaneous detection of CO ₂ and NO ₂ in exhaust gas is presented. The proposed system is designed as part of a Continuous Emissions Monitoring System for gas turbine emissions. The system exploits the amplification of the photoacoustic 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 NO ₂ 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 with a center wavelength of 405 nm matched on an adsorption peak of NO ₂ . This allows for deriving the NO ₂ concentration measurement directly from the amplitude of the photoacoustic 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 percents. The quantity of CO ₂ is sensed exploiting its effect on the sound speed, and consequently on the resonance frequency of the resonator. To measure the CO ₂ 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 NO ₂ with a resolution lower than 1 ppm whereas CO ₂ resolution is about 0.2%.