Correspondingly, the absorbance peak was truncated and reached a maximum plateau level, which cannot be fitted by a Voigt profile. However, Qu et al. In the work of Qu et al. In the present study, a similar correction process was used.
It is clear that by using the method described by Qu et al. For the It can be seen that the scanning range of 21 GHz cannot cover the entire absorption peak.
For a low concentration situation, similar to the calculation process for the In order to check the entire absorption profile, measurements with different fixed laser wavelengths were conducted, and the absorption values showed good overlap with the fitting curve from the data measured using laser scanning. The insets show the scanning signal as the function of laser frequency with and without potassium absorption.
The introduction of the A deuterium lamp was used as the broadband UV light source and the light was collimated by a parabolic mirror to form a beam 10 mm in diameter through an aperture, and then guided to the hot flue gas region. With five UV-enhanced aluminum mirrors, the absorption path length above the burner was increased to mm. To determine the path length and check the homogeneity over the line of sight, the distribution of KOH in the flue gas was visualized by laser-induced photofragmentation fluorescence, using the fifth harmonic of a pulsed Nd:YAG laser Brilliant B, Quantel at a wavelength of nm.
Some dips in the absorbance curve near and nm indicate the reduction of OH radicals in the hot gas with the seeding of potassium species. The details of the evaluation process will be described in the following section.
In the present study, the UV absorption cross-section of KOH in a combustion atmosphere was determined. In this evaluation, a constant amount of K 2 CO 3 was introduced into one fuel-lean flame and one fuel-rich flame. The potassium chemistry results in the generation of KOH and K atoms as the dominant potassium species in the hot flue gas.
When changing from a fuel-lean flame to a fuel-rich flame, the increase of the concentration of K atoms will be equal to the reduction of KOH. Based on the measured K atom concentrations for the lean and rich cases, the absorption cross sections of KOH were evaluated using the measured absorbance difference of KOH from the UV absorption spectroscopy.
Derived from eq 8 , the evaluation process can be described with the following equation:. A detailed description of the absorption spectrum of KOH is provided by Weng et al. The absorbance of KOH in the lean case nearly doubles the one in the rich case cf. The influence of the temperature difference between these two cases K vs K is negligible, as reported by Weng et al. Note that, by combining eqs 8 and 9 , one can find that the path length L can be cancelled to obtain the cross-section values.
Comparing the two absorption cross-section profiles of KOH at temperatures around and K, it can be seen that the influence of temperature is small.
The average value of the cross sections at temperatures and K was therefore applied in the calculation of the concentration of KOH in the following evaluation. Furthermore, the UV absorption cross-section of KCl in the high-temperature gas was obtained.
In order to generate KCl in the hot flue gas, CHCl 3 was seeded into the premixed flame as the source of chlorine. Especially in the low-temperature case, Flame 9, there were almost no K atoms present. Similar results have been presented by Schofield 38 on sodium chemistry and showed that NaCl became the dominant species as chlorine was available in excess compared with sodium. The absorption was solely from KCl, as it shows the same profile as the ones presented in previous studies on the UV absorption of KCl.
The variation of the cross-section with temperature is shown to be small, as concluded by Leffler et al. In the measurements by Leffler et al. The accuracy of the calculated KCl vapor pressure was directly affected by the accuracy of the salt reservoir temperature.
First, the measurement was made directly on the KCl vapor in the combustion environment, in which the formation of the K 2 Cl 2 dimer was negligible at temperatures over K. Also, the evaluation of the absorption cross-section was based on the measurement of the concentration of atomic K, which had a relatively low uncertainty.
In addition, uncertainty might originate from the inhomogeneity of the potassium distribution in the hot flue gas. Moreover, under the rich condition, the secondary reactions occurred with the ambient air to form a thin layer of flame front, which might change the balance between KOH and K atoms. However, combining eqs 8 and 9 , it can be found that the path length L can be canceled and there was almost no influence on the determination of the absorption cross section.
To mimic different oxidation and reduction environments, hot flue gases were prepared from flames Flames 1—7 with global equivalence ratios that varied from 0. A constant amount of KCl was seeded into the flames via KCl water solution. Note: the concentration in different cases was corrected due to the difference in total flow rate shown in Table 1. It is clear that there is a significant change of the concentration for all the potassium species as the equivalence ratio varies from lean to rich cases.
For the lean cases, the concentrations of different potassium species are almost kept constant. For the lean cases, the concentration of K atoms was measured both by the In the reduction environments, the concentration of K atoms increased to 8 ppm, and continually increased with equivalence ratio.
The well-recognized spectral features make it possible to perform simultaneous KOH and KCl measurement in combustion environments.
Hence, all the major potassium species, i. Using our experimental setup, a constant amount of KCl was seeded into flames with varying equivalence ratios and the concentrations of KOH, KCl, and K were measured. The chemical balance between different potassium species in different environments was accurately determined. Procedure A Here the potassium chloride filter UV1 with a path length of 10 mm is measured against the potassium chloride reference filter UV1H with a path length of 5 mm.
Procedure B Here the potassium chloride filter UV1 with a path length of 10 mm is measured against the reference filter UV12 filled with pure water. Experiences from daily practice show that the values determined in this stray light measuring method are instrument-dependent. This means the wavelength of the peak varies depending on the spectrophotometer performance. It is important for the user to know that the measured maximum absorption in the checked range is decisive for this checking method and that it should be higher than 0.
German English Japanese. Toric optics and mirrors, bikonvex, bikoncave and convexo-concave. The value of A 1 percent, 1 cm at a particular wavelength in a given solvent is a property of the absorbing substance.
Unless otherwise stated, the measurements are carried out with reference to the same solvent or the same mixture of solvents. A spectrophotometer, suitable for measuring in the ultraviolet and visible ranges of the spectrum consists of an optical system capable of producing monochromatic light in the range of nm to nm and a device suitable for measuring the absorbance. The two empty cells used for the solutions under examination and the reference liquid must have the same spectral characteristics.
Where double-beam-recording instruments are used, the solvent cell is placed in the reference beam. Verify the wavelength scale using the absorption maxima of holmium perchlorate solution, the line of hydrogen or deuterium discharge lamp or the lines of a mercury vapor are shown below.
Check the absorbance using suitable filters or a solution of potassium dichromate UV at the wavelengths indicated in Table 1, which gives for each wavelength the exact values and permitted limits of the specific absorbance. For the control of absorbance at nm, nm, nm and nm, dissolve For the control of absorbance at nm, dissolve
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