Diode laser measurements of trace concentrations of ammonia in an entrained-flow coal reactor

Abstract

We used a single-mode Pb-salt diode laser to quantify in situ the amount of ammonia generated during pyrolysis of pulverized coal in an entrained-flow reactor at 1225 K. The combination of wavelength modulation spectroscopy, a Herriott style multiple pass cell, rapid wavelength scanning (to eliminate noise due to turbulence and vibration) and a novel etalon fringe suppression technique provided a minimum detectable absorbance of 2 × 10⁻⁶ (SNR = 1, 1-Hz bandwidth) corresponding to 0.04-ppm ammonia at 1225 K. This is a 4-order-of-magnitude improvement over CO₂ laser based ammonia detectors and is ~2000 times more sensitive than electrochemical detection methods (for equal integration times).