Mid-infrared fiber-coupled laser absorption sensor for simultaneous NH3 and NO monitoring in flue gases

We report the development of a mid-infrared fiber-coupled laser absorption sensor for in situ and simultaneous detection of NH3 and NO, which is suitable for flue gas monitoring at the SCR (selective catalytic reduction) exhaust. Two quantum cascade lasers (QCLs) are used to detect the optimal absorption lines of NH3 and NO centered at 1103.45 cm-1 and 1929.03 cm-1, respectively. The two QCLs are coupled into a single hollow-core fiber and delivered to an open-path, single-ended optical probe for in situ gas detection. The gas concentration is determined by an improved calibration-free wavelength modulation spectroscopy (CF-WMS) technique with a reduced line model. A series of experiments are conducted to evaluate the sensor performance at varied tem-peratures from 296 to 573 K and the pressure of 1 atm. Such a dual-species sensor realizes a minimum detection limit of 30 ppb at 100 s average time for NO and 14 ppb at 70 s for NH3, respectively. The uncertainty of NH3 and NO detection is determined to be less than 5 %. By evaluating the WMS spectra of other species, the current sensor shows negligible spectral interferences under typical flue gas conditions.