Laser-Induced Photofragmentation Fluorescence Imaging of Alkali Compounds in Flames

Abstract

Laser-induced photofragmentation fluorescence has been investigated for the imaging of alkali compounds in premixed laminar methane-air flames. An ArF excimer laser, providing pulses of wavelength 193 nm, was used to photodissociate KCl, KOH, and NaCl molecules in the post-flame region and fluorescence from the excited atomic alkali fragment was detected. Fluorescence emission spectra showed distinct lines of the alkali atoms allowing for efficient background filtering. Temperature data from Rayleigh scattering measurements together with simulations of potassium chemistry presented in literature allowed for conclusions on the relative contributions of potassium species KOH and KCl to the detected signal. Experimental approaches for separate measurements of these components are discussed. Signal power dependence and calculated fractions of dissociated molecules indicate the saturation of the photolysis process, independent on absorption cross-section, under the experimental conditions. Quantitative KCl concentrations up to 30 parts per million (ppm) were evaluated from the fluorescence data and showed good agreement with results from ultraviolet absorption measurements. Detection limits for KCl photofragmentation fluorescence imaging of 0.5 and 1.0 ppm were determined for averaged and single-shot data, respectively. Moreover, simultaneous imaging of KCl and NaCl was demonstrated using a stereoscope with filters. The results indicate that the photofragmentation method can be employed for detailed studies of alkali chemistry in laboratory flames for validation of chemical kinetic mechanisms crucial for efficient biomass fuel utilization.

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

“…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 213 nm. 11,12 A homogeneous distribution of KOH in the interrogated region was confirmed in this visualization measurement. As shown in Figure 5a, spectrally resolved absorbance A(λ) of different hot flue gases is obtained through the analysis of the collected spectrum using the Beer−Lambert law: 23…”

“…Finally, the UV light is collected to a spectrometer (USB 2000+, Ocean Optics). 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 213 nm. , A homogeneous distribution of KOH in the interrogated region was confirmed in this visualization measurement. As shown in Figure a, spectrally resolved absorbance A (λ) of different hot flue gases is obtained through the analysis of the collected spectrum using the Beer–Lambert law: where I 0 (λ) and I s (λ) are the UV light intensity before and after the seeding of potassium to the hot gas, respectively, N A is the number density of the alkali species in the hot gas, σ A (λ) is the absorption cross-section of alkali species, and L is the absorption path length.…”

“…Optical diagnostics have been widely employed for concentration measurements of potassium species in many related studies. − Laser-induced breakdown spectroscopy (LIBS) was adopted to measure the total amount of potassium in flames, , while excimer laser-induced fragmentation fluorescence (ELIF) was used to detect the total amount of KOH and KCl compounds. , Planar laser-induced fluorescence (PLIF) has been used in measuring K atoms released from a burning biomass pellet . To obtain reliable quantitative results, the application of absorption spectroscopic techniques is essential.…”

Advanced biofuel production via gasification – lessons learned from 200 man‐years of research activity with Chalmers’ research gasifier and the GoBiGas demonstration plant

“…Even measurement after a high temperature conversion of a slip stream to assess the overall carbon amount gives information about the yield of condensable hydrocarbons . For alkali species, spectroscopic methods are under development but are not yet ready for application . Surface ionization detectors for alkali species have been developed by different groups and shown to be applicable for measurements.…”

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

“…Also, alkali species in high‐temperature environments have been investigated using laser methods. Photofragmentation and laser‐induced fluorescence (PF‐LIF) is extensively used for the detection of alkali chlorides and hydroxides . This method allows sensitive detection and imaging of alkali species; however, it is incapable of distinguishing between, for example, KCl and KOH, but relies on equilibrium calculations when concentrations of individual molecules are given.…”