###### Abstract  
Potassium (K) is one of the main and most hazardous trace species released to the gas-phase during thermochemical conversion of biomass. Accurate experimental data and models of K release are needed to better understand the chemistry involved. Tunable diode laser absorption spectroscopy (TDLAS) is used for simultaneous real-time in situ measurements of gas-phase atomic K, water (H\({}_{2}\)O) and gas temperature in the vicinity (boundary layer) of biomass particles during combustion in a laboratory single-particle reactor. Atomic K is detected in a wide dynamic range, including optically thick conditions, using direct absorption spectroscopy at the wavelength of 770 nm, while H\({}_{2}\)O and temperature are determined by calibration-free scanned wavelength modulation spectroscopy at 1398 nm. The high accuracy and repeatability of the setup allows to distinguish measurements with varying initial particle mass, laser beam height above the particle and fuel type. Four types of biomass with different ash composition are investigated: softwood, Salix, Miscanthus and wheat straw. For Salix and wheat straw, the K release behaviour is, for the first time, compared to a detailed numerical particle model taking into account the interaction between K/S/Cl composition in the particle ash. A good agreement is achieved between the measured and calculated time-resolved atomic K concentrations for the devolatilization phase of the biomass particles. potassium (K); biomass; combustion; laser spectroscopy; TDLAS; numerical particle model Giopi 2021 11 8887  
potassium chloride (KCI) [8; 9]. Usually, KOH dominates in combustion, due to the high oxygen availability, while under fuel-rich conditions, the atomic K concentration can be of the same order of magnitude as, or even exceed, the levels of KOH and KCl [10]. An efficient way to improve the understanding of fuel conversion is to perform well-controlled studies of volatile release from biomass particles combusted in laboratory reactors [11; 12; 13; 14; 15; 16; 17; 18]. Small-scale single-particle reactors (SPRs) represent the conditions in a typical furnace, with medium temperatures and low heating rates, and allow to distinguish the main particle conversion phases: drying, devolatilization (or pyrolysis), char conversion and ash cooking. Selective and quantitative in situ detection of all three main K species in high-temperature reacting flows requires relatively complex optical techniques [10; 19; 20; 21]. However, gaseous atomic K, here referred to as K(g), provides a good indication of K release, especially during the devolatilization phase, and can be detected in a wide dynamic range and with high time-resolution using tunable diode laser-absorption spectroscopy (TDLAS) [10; 16; 22; 23]. During recent decades, TDLAS, including direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS), has become an established and widely utilized tool to measure gas parameters, such as major species and temperature, in thermochemical processes on various scales [24; 25]. The technique enables fast in situ measurement with robust setups to obtain absolute species concentrations without calibration, which is particularly important for quantitative assessment of the highly reactive K species. To understand and generalise the time-dependent K release behaviour for different biomass sources, a numerical model of the particle and its surrounding is highly valuable [2]. It is a way to develop a sub-model for the K release, which can subsequently be used in simulations of larger reactors with many particles. Experimental data obtained with optical techniques can be employed to validate such models [17], which is important for an increased understanding of the chemistry involved and improvement of the model. The particle model used in this study has been developed over the years [17; 26; 27; 28; 29; 30] and was validated experimentally regarding particle mass loss, H\({}_{2}\)O concentration and gas temperature [17], as well as total K release [27].