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.
Highlights
Hydrogen cyanide (HCN) is formed by gas phase combustion reactions and during devolatilization of fuels containing organic nitrogen, e.g., biomass residues [1,2]
We report the first Frequency modulation (FM) detection of HCN behind shock waves
Selecting the high-J P(26) line represents a compromise between a high linestrength at high temperatures and spectroscopic isolation to ensure that the recorded FM signal originates only from the absorption line of interest
Summary
Hydrogen cyanide (HCN) is formed by gas phase combustion reactions and during devolatilization of fuels containing organic nitrogen, e.g., biomass residues [1,2]. In-situ detection of HCN in the mid-IR range was realized with polarization spectroscopy in flames at atmospheric pressure by Sun et al [18] and with degenerate four-wave mixing spectroscopy during straw gasification by Hot et al [19], respectively. Both groups have chosen the P(20) rovibrational line in the ν1 band for their measurements. We established single-tone frequency modulation spectroscopy in the mid-IR [27] aiming at sensitive detection of various species on their fundamental rovibrational transitions in shock tube experiments. Future studies on this and other bimolecular reactions of HCN as well as on the important isomerization step of HCN to hydrogen isocyanide (HNC) [28] will benefit from the availability of a sensitive HCN detection scheme as outlined in this work
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