High-temperature ammonia detection using heterodyne phase-sensitive dispersion spectroscopy at 9.06 μm

Abstract

Quantitative, sensitive and in situ detection of ammonia (NH3) is important for industrial process monitoring and emission characterization. We demonstrate high-temperature NH3 detection using heterodyne phase-sensitive dispersion spectroscopy (HPSDS) with a quantum cascade laser (QCL) emitting at 9.06 μm. The injection current of the QCL was modulated at 360 MHz to generate a three-tone laser beam and to maximize the dispersion signal of NH3 at the target temperature of 573 K and pressure of 100 Torr. The laser beam was directed into an optical probe, which can be inserted into the flue gas for in situ monitoring, to obtain an effective path length of 95 cm. The developed NH3 dispersion sensor shows an excellent linear response over the concentration range of 5–100 ppm (parts per million) and a minimum detection limit (MDL) of 0.69 ppm. The MDL of the current NH3 dispersion sensor can be further improved to 70 ppb (parts per billion) at a longer measurement time of 50 s. Compared to absorption spectroscopy, dispersion spectroscopy merits the advantage of immunity to laser power fluctuations caused by unwanted disturbance. The developed sensor is promising for NH3 slip monitoring in the selective catalytic reduction (SCR) used in power plants.