Ultraviolet Absorption Cross-Sections of Ammonia at Elevated Temperatures for Nonintrusive Quantitative Detection in Combustion Environments

Ammonia (NH3) is regarded as an important nitrogen oxides (NOx) precursor and also as an effective reductant for NOx removal in energy utilization through combustion, and it has recently become an attractive non-carbon alternative fuel. To have a better understanding of thermochemical properties of NH3, accurate in situ detection of NH3 in high temperature environments is desirable. Ultraviolet (UV) absorption spectroscopy is a feasible technique. To achieve quantitative measurements, spectrally resolved UV absorption cross-sections of NH3 in hot gas environments at different temperatures from 295 K to 590 K were experimentally measured for the first time. Based on the experimental results, vibrational constants of NH3 were determined and used for the calculation of the absorption cross-section of NH3 at high temperatures above 590 K using the PGOPHER software. The investigated UV spectra covered the range of wavelengths from 190 nm to 230 nm, where spectral structures of the (A) over tilde (1)A(2)'' <- (X) over tilde (1) A(1)' transition of NH3 in the umbrella bending mode, v(2), were recognized. The absorption cross-section was found to decrease at higher temperatures. For example, the absorption cross-section peak of the (6, 0) vibrational band of NH3 decreases from similar to 2 x 10(-17) to similar to 0.5 x 10(-17) cm(2)/molecule with the increase of temperature from 295 K to 1570 K. Using the obtained absorption cross-section, in situ nonintrusive quantification of NH3 in different hot gas environments was achieved with a detection limit varying from below 10 parts per million (ppm) to around 200 ppm as temperature increased from 295 K to 1570 K. The quantitative measurement was applied to an experimental investigation of NH3 combustion process. The concentrations of NH3 and nitric oxide (NO) in the post flame zone of NH3-methane (CH4)-air premixed flames at different equivalence ratios were measured.

Automated summary

NH3 absorption cross-sections in hot gas environments were experimentally measured at different temperatures, allowing for quantitative in situ detection of NH3 in various hot gas environments. The absorption cross-section decreases at higher temperatures, enabling nonintrusive quantification of NH3 in different environments with detection limits varying from below 10 ppm to around 200 ppm. The quantitative measurement was successfully applied to an experimental investigation of NH3 combustion.