A Photoacoustic-Based Measurement System for Dual Detection of NO<sub>2</sub> and CO<sub>2</sub> in Combustion Exhaust Gases
Enza Panzardi, Klaus Stefan Drese, Marco Mugnaini, Lorenzo Parri, Valerio Vignoli, Ada Fort
Abstract
In this paper a low-cost, low-complexity photoacoustic sensing system for the simultaneous detection of CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . This allows for deriving the NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is sensed exploiting its effect on the sound speed, and consequently on the resonance frequency of the resonator. To measure the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> with a resolution lower than 1 ppm whereas CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> resolution is about 0.2%.