NH2 detection in fuel-nitrogen combustion using an Alexandrite laser

A scheme for NH2 detection by means of laser-induced fluorescence (LIF) with excitation in the ultraviolet-blue regime around 385 nm, in particular using the second harmonic of a solid-state Alexandrite laser, is presented. This alternative provides interference-free detection with improved signal compared with previously employed concepts based on Nd:YAG and dye-laser excitation in the visible regime. A detection limit in the range 500-1000 ppm was estimated for planar single-shot LIF imaging on centimeter-scale.  Corresponding author: [email protected] Proceedings of the European Combustion Meeting 2015 Introduction With a large part of energy-production world-wide based on combustion and increased attention to limited resources of fossil fuels, interest in renewable, biomassderived, nitrogen-containing fuels has increased. This makes improved understanding of fuel-nitrogen combustion chemistry highly relevant and insights in these processes for development and verification of kinetic mechanisms require accurate experimental data on the combustion process. Figure 1 shows a simplified schematic layout of the combustion chemistry of a fuel containing hydrocarbon and nitrogen components. The hydrocarbon and nitrogen chemistries are presented in the left and right parts of Fig. 1, respectively. The fuel-nitrogen chemistry contains two paths, involving oxidation of hydrogen cyanide (HCN) and ammonia (NH3) [1]. The amidogen (NH2) radical is a key species in the ammonia oxidation path as shown in the right part of Fig. 1. In addition it is an important component in the thermal De-NOx process [1, 2]. Thus, experimental data on NH2 are very valuable for understanding fuel-nitrogen combustion. Laser-based techniques provide powerful nonintrusive methods with high temporal and spatial resolution for characterization of combustion [3]. Laser-induced fluorescence (LIF) provides specific detection with high sensitivity for many combustionrelevant species. In addition, the technique allows for planar and in some cases also volumetric imaging. Related to fuel-nitrogen chemistry, laser-based methods have been readily developed and applied for detection of diatomic nitrogen radicals such as NH [4] and CN [5]. Investigations have also been carried out monitoring more complex molecules containing three or more atoms such as HCN [6], NH2 [4, 7-12] and NH3 [13]. Measurements of NH2 in flames have been made using absorption spectroscopy, for example in ammonia-oxygen flames at low as well as atmospheric pressure [7, 8]. In addition, sensitive intracavity absorption spectroscopy has been employed for NH2 detection in NH3-doped CH4-air flames [11]. The NH2 radical has a wide absorption spectrum covering the range 390-800 nm for transitions in the à 2 A1 ← X 2 B1 system [14]. The NH2 ground state is bent whereas the excited state is a close to linear configuration and the spectrum consists of multiple bands of the bending vibrational mode. Rotational transitions are assigned by three quantum numbers N, Ka, and Kc written in notation NKaKc. Further details on NH2 spectroscopy and spectroscopic notation are given in ref. [14]. The absorption studies cited above were made at wavelengths around 600 nm probing the (0,9,0) vibrational band. Excitation in this wavelength regime has also been employed for LIF measurements of NH2 in nitrogen-doped hydrocarbon flames at low pressure [4, 12] as well as NH3 flames at atmospheric pressure [10]. In addition NH2 LIF measurements in atmospheric pressure flames have been made using a krypton ion laser at 647 nm [9]. This work presents an alternative approach for NH2 detection by means of LIF with excitation in the ultraviolet-blue regime around 385 nm. In particular this Figure 1. Schematic layout of the chemistry for combustion of a fuel containing hydrocarbon and nitrogen components represented by oxidation pathways of CH4 (left) and HCN/NH3 (right). The NH2 radical is a key species in the NH3 pathway (upper right).