## 3 Results and Discussion
### Flame Speed and Burning Velocity Measurements  
The measured turbulent spherical flame speeds, ST, for SPFR and SPFT are shown in Figure 6 as a function of the equivalence ratio, \(\O\). These measurements of the mixture reactivity are very similar in their dependency on the equivalence ratio, \(\O\), as for the K\({}_{\text{st}}\) results in Figure 4. However, the two peak ST for the raw and torrefied biomass were very similar at close to 1.0 m/s compared with a significant difference in K\({}_{\text{st}}\) in Figure 4. This difference may be due to the turbulent flame speed, ST being measured in the constant pressure period of the explosion and K\({}_{\text{st}}\) is measured just before the peak pressure. Figure 6 also shows that for rich mixtures, the flame speed remains high as the fuel concentration increases. This is considered to be explained by the mechanism of the coarse biomass flame front with the flame driven by the finer particles in the mixture and the coarse particles gasified behind the flame front. As more fuel is added, the \(\O\) of the fine fraction flame increases and the temperature of this initial combustion increases, which results in more efficient devolatilization of the coarse fraction and the gas volume released in the gasification reactions increases which cause the pressure to remain high even though for gases the pressure would fall for richer mixtures. Figure 6: Turbulent flame speed (ST) vs. Equivalence ratio (\(\O\)) for raw SPF and torrefied SPF wood mixture. Figure 7 compares as a function of the mass mean particle size, d\({}_{50}\), the present peak ST with previous measurements [30; 39; 42; 58; 59] of ST for fine pulverized raw biomass and thermally treated biomass. The previous data was all for biomass sieved to <63 \(\upmu\)m for raw and torrefied biomass (different torrefaction processes) and then analyzed for the size distribution. The d\({}_{50}\) as high as 200 \(\upmu\)m was found compared with 350-450 \(\upmu\)m in the present work where the particles were sieved to <1 mm. Figure 8 presents the same data in terms of the K\({}_{\rm st}\). Figures 7 and 9 show that the present results for coarse-based biomass are consistent with previous results with a prime dependence of the mixture reactivity on the particle size. When compared at the same d\({}_{50}\), the difference in mixture reactivity between the raw and torrefied biomass was small, with the torrefied biomass having a slightly greater dependence on size, mainly as a result of the present results. Figure 8: Peak deflagration Index (Kst, max) vs. average size for 50% fraction (d50) for present samples in comparison to other results of raw (R) and thermally treated samples (T) [30; 39; 42; 58; 59]. Figure 7: Maximum turbulent flame speed (ST, max) vs. average size for 50% fraction (d50) for present samples in comparison to other results of raw (R) and thermally treated samples (T) [30; 39; 42; 58; 59]. The laminar burning velocity, UL, of a dust/air mixture was determined from the measured ST in Figure 9 by dividing by the calibrated turbulence factor for the ISO 1 m\({}^{3}\) using the calibrated turbulence factor and then using Equation (1). The results are shown in Figure 9 and show very low values of UL for the coarse particles in the present work. A very good linear correlation was found between the peak turbulent flame speeds and peak deflagration index of the raw and thermally treated wood samples, as shown in Figure 10. It showed a reduction in the explosibility characteristics with increasing particle size. A similar linear correlation was also observed for the raw and thermally treated wood samples' peak laminar burning velocity and peak deflagration index. Figure 10: Correlation of peak turbulent flame speed (ST, max) with peak deflagration index (Kst) for present samples in comparison to other results of raw (R) and thermally treated samples (T) [30; 39; 42; 58; 59].