## 3 Results and Discussion
### Comparison to Numerical Particle Model--Wheat Straw and Salix  
The numerical particle model and the potassium release sub-model presented in Section 2.4 were used to simulate the particle conversion and K release behaviour for the given experimental conditions for two of the fuels. A straw-based biomass (wheat straw) and a woody-type biomass (Salix) were chosen for the comparison. The particle properties, ultimate and ash analysis of the samples presented in Table 1, were used as input parameters for the numerical model. According to reactions (R2), (R3) and (R10), the fractions of organic and inorganic potassium and sulphur should also be provided to the model. Ideally, these data are obtained through ash analysis. In this study, however, since the data were not available, the values were chosen based on data in the literature. It was assumed that 80% of the potassium is in the inorganic form and 50% of sulphur is in the free ion (S\({}^{2-}\)) form. These values were kept constant for both wheat straw and Salix. The numerical simulation resulted in time-dependent data for volatile release from the fuel during conversion and a corresponding 2D representation of the gas phase boundary layer around the particle. The 2D concentration data was averaged along the line-of-sight at given HAPs to obtain concentration values that can be compared to the TDLAS results. Figure 4 presents a comparison between TDLAS-measured (3 replicates) and model-predicted average H\({}_{2}\)O concentration, gas temperature and average K(g) concentration for wheat straw (Figure 4a-c) and Salix (Figure 4d-f) at a HAP of 4 mm. In addition to H\({}_{2}\)O, panels (a) and (d) also display the predicted normalized particle mass during conversion. In Figure 4b,e, the experimental gas temperature at HAP 4 mm is compared to the predicted temperature at HAP 2 and HAP 4 mm, as well as the predicted temperatures at the particle centre and surface. Overall, a similar behaviour is observed for the two fuels. The minor differences stem from the higher ash content of the wheat straw samples and the differences in the particle densities, as presented in Table 1. As seen in both experiments and simulation, the devolatilization process was shorter for the wheat straw particle (-40 s) than for Salix (-50 s). The present particle model was already validated concerning H\({}_{2}\)O concentration, gas temperature and mass loss in a previous study using a similar experimental setup [17]. Additionally, in this work, measured and simulated H\({}_{2}\)O concentrations are close. As concluded in the study by [PERSON] et al. [17], the discrepancy in gas temperature at a given HAP can be explained by the fact that a global reaction mechanism was used and that the TDLAS data provide an average (and H\({}_{2}\)O-weighted) temperature across the flame (and hot flame front). Figure 4b,e shows that the modelled temperature is considerably lower at HAP 2 mm than at HAP 4 mm, such that the average between these two would correspond rather well to the measured temperature. In general, the experimental results represent an axial average due to the finite size of the laser beam (diameter 1.2 mm). An excellent agreement is found between the measured and simulated K(g) concentration profiles of both wheat straw and Salix (Figure 4c,f). During devolatilization, a significantly lower concentration is correctly predicted for Salix. In the char combustion phase, the measured K(g) is around a factor of 10 lower than the model prediction, presumably because the actual combustion conditions during that phase were different from those assumed in the simulation. Figure 5 shows a more detailed comparison between model-predicted and experimental K(g) concentrations at HAP 2, 4 and 8 mm, focusing on the devolatilization. A good agreement is achieved, where the model predictions are well within the experimental range. In general, K(g) decreases with increasing HAP.