Mid-infrared absorption spectroscopic sensor for simultaneous and in-situ measurements of ammonia, water and temperature

Abstract

A mid-infrared laser-based absorption sensor is developed for in-situ and simultaneous detection of ammonia, water and temperature for selective catalytic reduction (SCR) exhaust monitoring. The sensor used a quantum cascade laser (QCL) to target the NH3 line at 1103.45 cm−1 and a distributed-feedback (DFB) laser for H2O concentration and temperature by exploiting the line pair at 4029.52 cm−1 and 4029.78 cm−1. The time-division multiplexing (TDM) strategy was implemented to fulfill the H2O and temperature measurement with direct absorption spectroscopy (DAS) and the absolute concentration measurement of NH3 by calibration-free wavelength modulation spectroscopy (CF-WMS). All the experiments were conducted in a test chamber with a controlled temperature range of 296–673 K and pressure of 1 atm. The NH3 sensor was validated to achieve the detection limit of 7 ppb at the integration time of 100 s. The temperature measurement was also verified to show an uncertainty less than 3.5 %. We further performed the multi-parameter (NH3, H2O and temperature) measurement under the static condition and gas flowing condition, respectively. Our sensor successfully captured the time-varying species concentrations and temperature with a time resolution of 2.3 s, which is promising for SCR monitoring in terms of accuracy, sensitivity, and time response.