A TDLAS-based photofragmentation method for spatially resolved measurement of KOH and KCl as well as its application in biomass combustion processes

Abstract

The release of gas-phase potassium species, mainly KOH, KCl, and K atoms, from burning biomass fuels can introduce severe problems to boilers, such as fouling, slagging, and corrosion. In the present work, an optical technique combining laser-induced photofragmentation and tunable diode laser absorption spectroscopy is developed for simultaneously measuring the concentration of KOH, KCl and K atoms with a high temporal and spatial resolution. Two laser sheets with a thickness of about 1 mm at wavelengths of 266 and 355 nm, respectively, were adopted to photodissociate KOH and KCl molecules into K atoms. A continuous wave laser at 766 nm generated by a tunable diode laser passed perpendicularly through the laser sheet to detect the K atom. The measured fragmentation-induced K-atom absorbance was correlated to KOH and KCl concertation through a calibration process in a homogenous combustion environment where the concentrations of KOH and KCl were monitored by UV absorption spectroscopy. The calibration curves were verified to be independent of temperature. A typical spatial resolution of 1 mm3 was realized where the value depended on the overlap volume of the UV laser sheet and the 766 nm laser beam. Finally, this technique was applied to measure the release behavior of KOH, KCl and K atoms from burning wood and straw pellets.