Wide-field imaging of gas-phase K species in biomass combustion using photofragmentation laser absorption imaging

[Display omitted] •A technique for quantitative wide-field imaging of K, KOH and KCl is presented.•Below parts-per-million detection limits are reached at 20 ms temporal resolution.•A spatial resolution of 0.13 mm is achieved in a 4-by-16 mm imaging domain.•K species distributions above coffee husk and wheat straw particles are presented.•Reaction kinetics imaging is demonstrated by mapping the K fragment decay curves. Potassium (K) plays a key role in biomass combustion as its presence in the gas phase may lead to slagging, agglomeration and corrosion. To facilitate feedstock flexibility, a detailed understanding of the K chemistry during solid fuel conversion is needed. Here, we present photofragmentation laser absorption imaging for simultaneous, quantitative wide-field imaging of gaseous atomic K, potassium hydroxide (KOH) and potassium chloride (KCl) using a high-speed camera. Concentration images of the K species, at levels down to below parts-per-million, were acquired in a physical domain of 3.8-by-16.4 mm with a spatial resolution of 0.13 mm and a temporal resolution of 0.5 s after averaging. The technique was applied for imaging of the K species release from solid coffee husk and wheat straw samples combusted in a laboratory flame. Conversion of KCl salt was investigated as a reference. Significant differences in K species concentration and spatial distribution were observed between the fuels and between the devolatilization and char conversion stages. We also demonstrate spatial mapping of the atomic K fragment recombination rates with 1.2 µs time resolution, which constitutes a step towards reaction kinetics imaging.