Measuring ammonia -- Development and application of measurement techniques for the detection of ammonia
by
MrOdd Hole(SU, Fysikum)
→
Europe/Stockholm
FA31
FA31
Description
Ammonia (NH3) is an important chemical both as a chemical precursor and in its
own right. It is therefore of practical importance to be able to visualize
ammonia gas especially when ammonia is present in combustion or flue-gas
environments. I will present the results from my thesis which is focused on the
use of laser-induced fluorescence with the stated aim of imaging ammonia in low
concentrations, with the end-goal of applying the technique to evaluate the
flow of ammonia into, and out from, a NOx-reduction catalysis unit. The
fluorescence was induced using a two-photon excitation scheme using a laser
with an output wavelength of 305 nm and detection of the fluorescence at
555-575 nm. In order to accurately evaluate the signal, extensive testing of
the parameters influencing the signal was carried out. Dependencies of
temperature, pressure, concentration, stimulated emission and laser power was
evaluated experimentally. Using the data from the temperature measurements, a
simulated spectrum of the probed transition was fitted, enabling the relevant
molecular constants to be extracted. At low irradiances, corresponding to the
“linear” regime of regular one-photon LIF, the signal shows a quadratic
dependence on laser power. Stimulated emission, or amplified spontaneous
emission, show a fifth order dependence on laser irradiance, but this emission
was not measurable at the levels of irradiance used for imaging. With a
constant concentration, the signal increases slightly with pressure up to 2 bar
and the decrease due to collisional quenching. The signal shows a linear
dependence on concentration within the evaluated concentration region. The
detection limit with the setup used is estimated at 800 ppm, and single-point
measurements could potentially measure sub-ppm concentrations. The technique
was successfully implemented for the imaging of laminar and turbulent flames,
as well as imaging of the gas immediately above a small sample of
ammonia-reducing catylic material.