Quantum cascade laser-based MIR spectrometer for the determination of CO and $$\hbox {CO}_2$$ CO 2 concentrations and temperature in flames

Abstract

and nondestructive, are an ideal choice for many flame applications. While many important radicals such as OH and CH have strong absorption bands in the UV/Vis and can easily be detected using techniques such as LIF or CRDS, electronic transitions of stable molecules like CO and CO 2 lie in the deep UV. Detection schemes like twophoton LIF [1] are possible, but quantification is prone to errors. For these molecules, rovibrational transitions in the infrared provide an alternative detection method. Due to the availability of cheap tunable diode lasers in the nearinfrared (NIR), this spectral region has already been widely used for combustion diagnostics in the past [2][3][4][5].The strong fundamental bands in the mid-infrared (MIR) have the potential for a more sensitive detection compared to the weaker overtone bands in the NIR. The development of quantitative detection schemes in the MIR was, however, hindered by the lack of laser sources with adequate qualities. Most lasers available required cryogenic cooling and output powers were low [2, 6]. Wondraczek et al. used difference frequency generation (DFG) as an alternative method to generate MIR radiation. They detected CO is around 5 µm even though the output power of their laser system was only 30 nW [7]. In this work, tomography has been applied to visualize CO in a flat flame.In recent years, quantum cascade lasers (QCL) have proven to be reliable light sources in the MIR. Even though the operating principle for this type of laser has already been proposed in 1971 by Kazarinov and Suris [8], the first working quantum cascade laser has been presented more than twenty years later [9]. QCLs provide high output powers (several milliwatts) and narrow-band output with operation at room temperature which simplifies the experimental setup. This enables QCLs to compete with conventional techniques like Fourier transform spectrometer for highresolution molecular spectroscopy in the infrared [10].Abstract An experimental setup for the simultaneous detection of CO and CO 2 and the temperature in lowpressure flames using a pulsed quantum cascade laser at 4.48 μm is presented. This comparatively new type of laser offers good output energies and beam quality in the mid-infrared, where the strong fundamental transitions of many molecules of interest can be accessed. A single-pass absorption setup was sufficient to obtain good accuracy for the stable species investigated here. Due to the high repetition rate of the laser and the speed of the data acquisition, measurement of two-dimensional absorption spectra and subsequent tomographic reconstruction was feasible. As demonstration of this technique, two-dimensional CO and CO 2 concentrations have been measured in two fuel-rich methane flames with different coflow gases (nitrogen and air). The influence of the coflow gas on the flame center concentration profiles was investigated and compared with one-dimensional model simulations.

Infrared absorption spectrometer for the determination of temperature and species profiles in an entrained flow gasifier

“…To compare the flame center temperatures at a distance of 15 mm above the burner surface with temperatures previously measured with CARS [22,23] the measured line-of-sight spectra were Abelinverted using the method of Dasch [24] prior to fitting the absorption spectrum. Details of the procedure are provided in [25,26]. Temperatures agree very well within the measurement uncertainty of 2-3 % for the absorption measurements and 2.5 % for CARS ( fig.…”

Laser Absorption Sensing Systems: Challenges, Modeling, and Design Optimization

“…Figure 2 summarizes some representative results from a number of sensor development and implementation studies in terms of detection limits and bandwidths. It compares species concentration measurements in a variety of combustion systems including laboratory environments [1,[16][17][18][19][20][21][22], shock tube studies [23][24][25][26][27][28], industry processes [29][30][31][32][33], and engines [34][35][36][37][38][39][40][41][42][43][44][45][46]. For the purpose of making a direct comparison between different studies, the reported absolute minimal detection limits for several commonly probed species in combustion, including CO, CO 2 , H 2 O, and C 2 H 2 , are plotted on the same diagram against detection bandwidths.…”

Detection of Formaldehyde in Flames Using UV and MIR Absorption Spectroscopy

“…Model calculation were performed using cantera[32] and the combustion model of Zhao et al [33]. The temperature profile was measured using CO and CO 2 lines at 4.48 µm with a QCL [25]. An error of 5 % is estimated for the temperature.…”

“…In our case, g(ν) can be well described by a Voigt function [24] with the Doppler broadening being the dominant contribution in the low pressure flames investigated here and pressure broadening playing only a minor role. More details of the experimental setup and the data evaluation procedure for the QCL measurements may be found in a recent publication [25].…”