Quantitative and Sensitive Mid-Infrared Frequency Modulation Detection of HCN behind Shock Waves

Abstract

Despite its key role for the study and modeling of nitrogen chemistry and NOx formation in combustion processes, HCN has only rarely been detected under high-temperature conditions. Here, we demonstrate quantitative detection of HCN behind incident and reflected shock waves using a novel sensitive single-tone mid-infrared frequency modulation (mid-IR-FM) detection scheme. The temperature-dependent pressure broadening of the P(26) line in the fundamental CH stretch vibration band was investigated in the temperature range 670K≤T≤1460K, yielding a pressure broadening coefficient for argon of 2γAr296K=(0.093±0.007)cm−1atm−1 and a temperature exponent of nAr=0.67±0.07. The sensitivity of the detection scheme was characterized by means of an Allan analysis, showing that HCN detection on the ppm mixing ratio level is possible at typical shock wave conditions. In order to demonstrate the capability of mid-IR-FM spectroscopy for future high-temperature reaction kinetic studies, we also report the first successful measurement of a reactive HCN decay profile induced by its reaction with oxygen atoms.

Mid-infrared Frequency Modulation Detection of HCN and Its Reaction with O Atoms behind Shock Waves

“…Voigt line profiles for the conversion of fractional absorption values to HCN mole fractions were calculated from line strengths and pressure-broadening parameters for the P(26) transition taken from the HITRAN database (line strengths), our previous study on HCN detection (Ar parameters), and a study by Rinsland et al . (N 2 parameters).…”

“…FM detection of HCN was performed on the P(26) transition in the ν 1 CH stretch band at 3228.049 cm –1 . As explained in our previous work in more detail, the selection of this transition combines a relatively high absorption cross section over the studied temperature range with spectral separation. Unlike other absorption lines in the same branch, it is well separated from hot band transitions that could otherwise cause interference .…”

“…Voigt line profiles for the conversion of fractional absorption values to HCN mole fractions were calculated from line strengths and pressure-broadening parameters for the P(26) transition taken from the HITRAN database 30 (line strengths), our previous study on HCN detection 24 (Ar parameters), and a study by Rinsland et al 59 (N 2 parameters). At 1700 K and 1.2 bar, the Voigt line width in pure argon is Δν Voigt FWHM = 1.27 GHz and the pressure broadening of the bath gas accounts for ∼80% of the total line width.…”

“…In a preceding study on HCN detection behind shock waves, we have established a new detection scheme in the ν 1 CH stretch vibration band using single-tone mid-infrared frequency modulation spectroscopy (MIR-FMS). 24 Therein, the temperature-dependent pressure broadening of the P(26) rovibrational line has been investigated under shock wave conditions and it was shown that MIR-FM detection of HCN on the ppm mixing ratio level and with μs time resolution is possible. A first kinetic HCN profile had been included as well.…”

Quantification of Elevated Hydrogen Cyanide (HCN) Concentration Typical in a Residential Fire Environment Using Mid-IR Tunable Diode Laser

“…These measurement methods utilize HCN absorption lines in the near infrared region of the electromagnetic spectrum. [24][25][26][27] With the increased availability of tunable diode lasers in the mid-infrared (mid-IR) region, it has become possible to utilize the much stronger fundamental vibration bands of HCN in the mid-IR region. Absorption lines from the fundamental C-H stretching band have been used to quantify HCN using various mid-IR spectroscopy techniques such as mid-IR polarization spectroscopy in atmospheric pressure flames 28 and mid-IR degenerate four-wave mixing in gasified biomass pellets.…”