Planar laser-induced photofragmentation fluorescence for quantitative ammonia imaging in combustion environments

Abstract

Ammonia is regarded as a potential carbon-free alternative fuel. To have a better understanding of its combustion characteristics, development of feasible techniques for ammonia detection in combustion environments is essential. In the present work, planar laser-induced photofragmentation fluorescence (LIPF) was developed for quantitative measurements of ammonia in hot gas flows. A 193 nm ArF excimer laser was used to photo-dissociate ammonia. As a fragment, NH* radicals were generated, and the fluorescence at 336 nm was used for ammonia detection. Quantitative calibration of the LIPF signal was performed in hot flows either from laminar flames or an electric heating-pipe with a known amount of ammonia and temperature. Ultraviolet absorption was used to obtain accurate concentrations of ammonia in the hot flows from laminar flames. The single-shot detection limit of the LIPF technique was estimated to be ∼50 ppm and ∼130 ppm in hot flue gas at 1140 and 1750 K, respectively, and ∼0.2 ppm in a room-temperature nitrogen flow. The technique was applied to detect the slip of ammonia in a premixed laminar ammonia-air flame with a fuel-air equivalence ratio of 1.2. Over 5000 ppm unburned ammonia was detected in the post-flame region and measurements showed good agreement with predictions of a chemical model.