2 February 2009 Sub-ppm NH3 sensor for control of de-nitrification process using diode laser spectroscopy
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Diode laser absorption spectroscopy (DLAS) has become an important analytical technique for highly sensitive and specific gas concentration measurements as it is reliable, fast and accurate. In this work a diode laser sensor has been developed to monitor ammonia at sub-ppm in 1510-nm region. The system was designed to control the de-nitrification process with selective catalytic reduction (SCR) abatement system. To ensure that NOx is reacted completely and to avoid secondary pollution resulted from ammonia slip, it is essential to monitor emissions of ammonia for controlling the amount of NH3 injection. NH3 concentration is demanded not to exceed 5ppm, preferably 2-3ppm. In order to provide enough sensitivity, the sensor uses DLAS with wavelength modulation as AC detection of absorption line derivatives, at frequencies where the laser noise is reduced, coupled with coherent electronic detection techniques. Experiment results demonstrate that the sensor with the second-harmonic detection shows in-situ, continuous measurements with low detection limit (60 ppb), fast response (<1s) and long-term stability all of which is difficult to obtain with conventional techniques such as wet chemical analysis, non-dispersive infrared (NDIR). In addition, the influence of water vapor on ammonia measurements due to line broadening effects is investigated. A real-time spectrum analysis algorithm is developed. The systematic measurement error is corrected by means of the line width measurements based on fast pattern correlation analysis of second-harmonic line shape. This offers the advantage of accurate NH3 concentration measurement even though the moisture content is above 40%.
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Haitao Gu, Haitao Gu, Lipeng Liu, Lipeng Liu, Ying Li, Ying Li, Ren Chen, Ren Chen, Luhong Wen, Luhong Wen, Jian Wang, Jian Wang, } "Sub-ppm NH3 sensor for control of de-nitrification process using diode laser spectroscopy", Proc. SPIE 7160, 2008 International Conference on Optical Instruments and Technology: Optoelectronic Measurement Technology and Applications, 71600Q (2 February 2009); doi: 10.1117/12.806973; https://doi.org/10.1117/12.806973

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