Measuring ammonia: Development and application of measurement techniques for the detection of ammonia

Detta är en Master-uppsats från Lunds universitet/Fysiska institutionen

Sammanfattning: 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. This thesis 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.

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