Spectrally Resolved Ultraviolet (UV) Absorption Cross-Sections of Alkali Hydroxides and Chlorides Measured in Hot Flue Gases

Abstract

Spectrally resolved ultraviolet (UV) absorption cross-sections of gas-phase sodium chloride (NaCl), potassium hydroxide (KOH), and sodium hydroxide (NaOH) were measured, for the first time, in hot flue gases at different temperatures. Homogenous gas-phase NaCl, KCl (potassium chloride), NaOH, and KOH at temperatures 1200 K, 1400 K, 1600 K, and 1850 K were prepared in the post-flame zone of laminar flames by seeding nebulized droplets out of aqueous solution of corresponding alkali species. The amount of droplets seeded into the flame was kept constant, so the relative concentration of different alkali species can be derived. The broadband UV absorption cross-section of KCl vapor reported by Leffler et al. was adopted to derive the absorption cross-section curves of NaCl, NaOH, and KOH with the corresponding measured spectrally resolved absorbance spectra. No significant changes in the spectral structures in the absorption cross-sections were found as the temperature varied between 1200 K and 1850 K, except for NaOH at around 320 nm. The difference between the absorption spectral curves of alkali chlorides and hydroxides is significant at wavelengths above 300 nm, which thus can be used to distinguish and obtain the concentrations of alkali chlorides and hydroxides in the broadband UV absorption measurements.

Ultraviolet Absorption Cross Sections of KOH and KCl for Nonintrusive Species-Specific Quantitative Detection in Hot Flue Gases

“…Rowland and Makide 21 estimated the absorption cross sections of NaOH in flames based on the measurement of Daidoji, 22 but the values relied on the calculated amount of NaOH seeded into the flame, which introduced a severe uncertainty. Recently, Weng et al 23 measured the UV absorption spectrum of KOH and KCl in flame environments, and the UV absorption cross sections were evaluated based on the KCl absorption data presented by Leffler et al, 20 which were obtained at 1073 K. In the study of Weng et al, 23 the influence of even higher temperature on the absorption cross-section values was assumed to be negligible.…”

Detection of gaseous species during KCl‐induced high‐temperature corrosion by the means of CPFAAS and CI‐APi‐TOF

“…At the test temperatures of 550°C, 650°C, and 750°C, the calculated p(KCl) were 1.6 × 10 −10 , 3.4 × 10 −9 , and 4.3 × 10 −18 Torr, respectively. The absorption cross‐section of KCl at 355 nm is orders of magnitude lower than that of KOH . However, as the detection system was tuned to be as sensitive as possible by using a small beam diameter, KCl caused interferences when high laser pulse energy was used.…”

“…Differential optical absorption spectroscopy (DOAS) is capable of providing sensitive online monitoring of alkali species . As it, similar to PF‐LIF, relies on alkali molecule absorption in ultraviolet (UV) region of the electromagnetic spectrum, DOAS suffers from spectral overlapping of alkali species’ absorption bands, causing interference in the collected signal . A recently introduced technique called collinear photofragmentation and atomic absorption spectroscopy (CPFAAS) has shown the ability for sensitive molecule specific detection of alkali species, providing sub‐ppm detection limits .…”

TDLAS-based photofragmentation spectroscopy for detection of K and KOH in flames under optically thick conditions

“…Existing spectroscopic techniques to selectively detect KOH(g) in situ include direct UV absorption [5][6][7], laser induced fluorescence [8], and collinear photofragmentation (PF) atomic absorption spectroscopy (CPFAAS) [9,10]. The latter method was developed by Sorvajärvi et al and bears similarities to other implementations of photofragmentation absorption spectroscopy [11,12].…”

“…The temperature was a fixed value obtained from water vapor two-line thermometry at 1398 nm [20]. The absorption cross sections for KOH(g) at the flame temperatures were taken from [6]. The difference in UV pulse energy before and after the KOH(g) sample was indistinguishable from the pulse-to-pulse variations and detector noise, therefore, in this work it was sufficient to measure the pulse energy after the sample volume.…”