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%.