Quantitative imaging of KOH vapor in combustion environments using 266 nm laser-induced photofragmentation fluorescence

In biomass thermal conversion processes, the release of potassium in high temperature environments crucially influences the operating efficiency and safety. The dominant potassium species can be potassium hydroxide (KOH) and/or potassium chloride (KCl). We report a species-specified quantitative measurement of potassium hydroxide (KOH) in combustion environments using laser-induced photofragmentation fluorescence (LIPF). Ultraviolet (UV) light sources with different wavelengths (193, 213 and 266 nm) were investigated to select a proper source ensuring that the excited potassium atoms in the 42P state could be only generated from the KOH molecules, not another major potassium compound, potassium chloride (KCl), and emit fluorescence at 766/769 nm after the photodissociation. After direct comparison, the fourth-harmonic of Nd:YAG laser at 266 nm was found to be the most proper light source. The fluorescence signal was strongly influenced by temperature as KOH molecules at thermally populated excited vibrational levels were needed to produce excited potassium atoms after the 266 nm photolysis. After the calibration using broadband UV absorption spectroscopy, the detection limit of the LIPF planar imaging system was determined to be about 3 ppm at 1750 K under a harsh condition, where about 80% of the fluorescence was re-absorbed by the potassium atoms present in the background gas. The technique was applied to quantitatively measure KOH concentration in the hot flue gasses provided by potassium carbonate seeded flames with varying equivalence ratios, and it was also used to visualize the distribution of KOH vapor above a piece of burning wood char