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 etal. 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.
Highlights
IntroductionMainly alkali chlorides can cause severe problems, e.g. corrosion, slagging and fouling in boilers when combusting biomass fuels[1,2,3] and coals.[4, 5] The release of alkali species from burning biomass and coal has been widely studied using different optical measurement techniques, such as planar laser-induced fluorescence,[6, 7] laser-induced breakdown spectroscopy,[8,9,10,11] excimer laser-induced fragmentation,[12] photofragmentation and atomic absorption spectroscopy,[13] tunable diode laser absorption spectroscopy[14, 15] and spontaneous emission.[16]
To obtain absorption cross-sections of NaCl, NaOH and KOH, the following strategies were adopted: vapors of NaCl, NaOH, KCl and KOH were prepared in the homogenous hot gas environments at known concentration; spectrally resolved absorbance spectra of corresponding species were obtained through UV absorption measurements; the cross-sections of NaCl, NaOH and KOH were derived using the known cross-sections of KCl from Leffler et al.[21] with using the Beer-Lambert law
(b).The same amount of alkali elements should be introduced into the flue gas regardless of the seeding of different alkali species
Summary
Mainly alkali chlorides can cause severe problems, e.g. corrosion, slagging and fouling in boilers when combusting biomass fuels[1,2,3] and coals.[4, 5] The release of alkali species from burning biomass and coal has been widely studied using different optical measurement techniques, such as planar laser-induced fluorescence,[6, 7] laser-induced breakdown spectroscopy,[8,9,10,11] excimer laser-induced fragmentation,[12] photofragmentation and atomic absorption spectroscopy,[13] tunable diode laser absorption spectroscopy[14, 15] and spontaneous emission.[16]. Related to the UV absorption spectra, photolysis cross-section spectra of alkali chlorides have been investigated.[13] Recently, a more reliable measurement of the spectrally resolved UV absorption cross-sections of KCl at temperatures between 973 K and 1073 K was reported by Leffler et al.[21] In their work, a novel quartz calibration cell was developed, which was characterized by a closed design with accurate control of the temperature homogeneity to ensure well-defined and reproducible alkali vapor pressure. The absorption cross-sections were determined for NaCl, KOH and NaOH in the hot flue gas at temperatures ~1400 K Their absolute cross-section values were calibrated based on the results for KCl reported by Leffler et al.[21] In addition, the temperature dependence of the absorption spectra was investigated between 1200 K and 1850 K
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