High-temperature determination of ammonia by tunable diode laser absorption spectroscopy (TDLAS)

Abstract

Here is described an instrument for ammonia determination utilizing near-infrared laser absorption suitable for high-temperature applications. The laser wavelength is scanned across the ammonia absorption line at 1.512 µm and wavelength-modulation spectroscopy is simultaneously performed to provide robust and accurate measurements. The thermal effects and beam-steering effects are highlighted in the design. An aluminum baffle is employed to alleviate the adverse impact of elevated temperature on the electronic components and evaluated by an infrared thermographic camera. The beam-steering effects were quantitatively assessed through a combination of two-dimensional computational fluid dynamics and ray tracing analysis. Reference standards from 1 to 20 ppm ammonia were used to verify the system performance. The determined ammonia concentrations exhibit good consistency with the reference values, as demonstrated by a coefficient of determination (R2) of 0.999. Furthermore, a detection limit of 0.02 ppm was achieved using an integration time of 60 s. Pprecision of 0.11 ppm was achieved using 1 s time resolution.