Tunable diode laser absorptionspectroscopy of atomic potassium in a KOH-seeded flat flame

Abstract: Potassium (K) is the main ash-forming element released from biomass during thermochemical conversion. A better understanding of K chemistry and monitoring of K species is needed to optimize combustion systems. Since K species are highly reactive and prevailing concentrations depend on the conversion conditions, accurate quantification requires in situ measurement techniques. Tunable diode laser absorption spectroscopy with a single-mode distributed feedback laser is used to probe the D1 transition of atomic potassium, K(g), at 769.9 nm. The large current tuning range of the diode laser (5 cm-1) enables monitoringthe wings of the absorption profile. Fitting to the acquired line shape wings is used as astrategy to enhance the dynamic range of the sensor and measure K(g) concentrations even under optically thick condition. A potassium-rich combustion environment is simulated by converting KOH salt in a premixed methane/air flat flame. Quantitative measurements of K(g) are made at 75 positions in the flame. This yields radial K(g) profiles at three different heightsin the plume above the KOH salt and an axial profile at the burner center. The acquired average K(g) concentrations are corrected for effective plume size, i.e. the absorption pathlength determined from the radial profiles. Knowledge of the K(g) distribution in flames can lead to a better understanding of K release and primary reaction kinetics.