A novel design of alkali chloride vapour-generating cell has been developed, which can serve as a calibration cell for quantitative ultraviolet absorption concentration measurements and meticulous spectral investigations of alkali compounds. The calibration cell was designed to provide alkali vapour of well-controlled concentrations and temperatures, and consisted of a sealed quartz cell measuring 0.4 m in length with a temperature-controlled reservoir containing solid alkali salt. The cell was placed in a furnace and the alkali vapours generated from the reservoir have direct access to the measuring chamber. Investigations of potassium chloride (KCl) were made on sublimated vapour at temperatures 650, 700, 750, 780, and 800 °C while the reservoir temperature was kept 50 °C lower to avoid condensation. The cell provides stable KCl vapour pressures, and the furnace provides a homogenous temperature profile along the cell. KCl vapour pressures are well characterised and conform the base for determination of the KCl concentration in the cell. The alkali chloride levels matched the concentration range of the absorption setup and indicated a previously employed calibration method to overestimate KCl concentrations. The KCl absorption cross sections for wavelengths λ=197.6 nm and λ=246.2 nm were calculated to be 3.4 × 10−17 and 2.9 × 10−17 cm2/molecule, respectively. The absorption cross section spectra did not show any structural differences with increasing temperature, which could indicate influence of dimers or significant changes of the population in the KCl vibrational states. The KCl absorption cross sections thus did not show any temperature dependence in the temperature region of 700–800 °C. Moreover, the applicability of the calibration cell for measurement of other alkali chlorides and hydroxides is discussed.

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.

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An understanding of potassium chemistry in energy conversion processes supports the development of complex biomass utilization with high efficiency and low pollutant emissions. Potassium exists mainly as potassium hydroxide (KOH), potassium chloride (KCl), and atomic potassium (K) in combustion and related thermochemical processes. We report, for the first time, the measurement of the ultraviolet (UV) absorption cross sections of KOH and KCl at temperatures between 1300 K and 1800 K, using a newly developed method. Using the spectrally resolved UV absorption cross sections, the concentrations of KOH and KCl were measured simultaneously. In addition, we measured the concentrations of atomic K using tunable diode laser absorption spectroscopy, both at 404.4 and 769.9 nm. The 404.4 nm line was utilized to expand the measurement dynamic range to higher concentrations. A constant amount of KCl was seeded into premixed CH4/air flames with equivalence ratios varied from 0.67 to 1.32, and the concentrations of KOH, KCl, and atomic K in the hot flue gas were measured nonintrusively. The results indicate that these techniques can provide comprehensive data for quantitative understanding of the potassium chemistry in biomass combustion/gasification.

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.

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A laser diagnostic concept for measurement of potassium chloride (KCl) and potentially other alkali compounds in large-scale boilers and furnaces of limited optical access is presented. Single-ended, range-resolved, quantitative detection of KCl is achieved by differential absorption light detection and ranging (DIAL) based on picosecond laser pulses. Picosecond DIAL results have been compared experimentally with line-of-sight measurements using a commercial instrument, the in situ alkali chloride monitor (IACM), utilizing differential optical absorption spectroscopy. For centimeter-scale range resolution and a collection distance of 2.5 m, picosecond DIAL allowed for measurement of KCl concentrations around 130 ppm at 1200 K, in good agreement with values obtained by IACM. The DIAL data indicate a KCl detection limit of around 30 ppm for the present experimental conditions. In addition, a double-pulse DIAL setup has been developed and demonstrated for measurements under dynamic conditions with strong Mie scattering. The picosecond DIAL results are discussed and related to possible implementations of the method for measurements in industrial environments.