## 3 Results and Discussion
### Deflagration Index  
The Deflagration Index, \(\text{K}_{\text{st}}\), as a function of the Equivalence Ratio, \(\mathcal{O}\), is shown in Figure 4. The peak \(\text{K}_{\text{st}}\) was measured to be 27 and 36 bar\(\cdot\)m/s for raw and torrefied wood mixture, respectively. The peak \(\text{K}_{\text{st}}\) for the torrefied wood mixture occurred at a nominal \(\mathcal{O}\) of 6.8 (Fines \(\mathcal{O}\) - 1). For the raw wood mixture, the maximum \(\text{K}_{\text{st}}\) occurred at \(\mathcal{O}\) = 9.6 (Fines \(\mathcal{O}\) - 1.6). Figure 5 shows that the torrefied sample had higher Kst at all \(\mathcal{O}\) and was much more reactive (1/3 rd more) at the most reactive concentration. For \(\mathcal{O}>6\), based on the nominal concentration, there were some fluctuations in K\({}_{\text{st}}\) due to the excess mass of fuel affecting the flame propagation. For higher fuel concentration, the additional mass of fuel absorbs the heat from the burnt gases, and this reduces K\({}_{\text{st}}\) slowly as fuel mass is increased. Biomass with coarse particle size, whether raw or torrefied, showed their most reactive concentration at overall \(\mathcal{O}\), giving the \(\mathcal{O}\) for the fines near stoichiometric. This indicates that the flame propagated in the fines.  
A mechanism for coarse biomass powders to burn in a propagating flame is proposed to explain these results, which is an extension of that used to explain why about half of the initial dust does not burn in the explosion [59; 60]. The action of the wind, induced by the expanding spherical flame, on particles ahead of the flame with a variable size distribution is to blow the smallest particles close to the gas velocity, with the larger particles lagging due to drag effects. The flame front is then driven by the finer particles, and the larger particles lag and are heated to ignition by the hot burnt gases from the flame front. The  
Figure 4: Deflagration Index (K\({}_{\text{st}}\)) vs. Equivalence ratio (\(\mathcal{O}\)) for raw SPF (SPFR) and torrefied SPF (SPFT) wood mixture.  
Figure 5: Normalized maximum explosion pressure (Pmax/Pi) vs. Equivalence ratio (\(\mathcal{O}\)) for raw SPF and torrefied SPF wood mixture.  
mixture has to be very rich (nominal concentration) for the finer particles ahead of the flame to burn close to \(\O=1\), as \(<\)20% of the total mass of particles was in the size fraction that will burn easily, as shown in Table 2. A flammable mixture of 20% fine particles with \(\O_{\text{MEC}}\) of 0.2 needs \(\O_{\text{fines}}\) of at least 1.0 for the overall mixture to burn. This is close to the results in Figure 5 for SPF raw and torrefied samples. With this mechanism, the larger particles are gasified in the rich mixture of the hot burnt gases from the flame burning in the finer fraction. This releases CO and H\({}_{2}\), which have insufficient oxygen to burn, but the volume release keeps the explosion pressure high for rich mixtures.  
The maximum explosion pressure, P\({}_{\text{max}}\), to the initial pressure, P\({}_{\text{i}}\), is shown in Figure 5 as a function of the nominal equivalence ratio, \(\O\). This shows that the normalized peak explosion pressure rise was 7.4 bar for SPFT and 7.3 bar for SPFR, which shows that the two materials burnt with a similar flame temperature. These are large pressure rises indicating that a high proportion of the coarse mixture had burnt as well as the fines burning. In spite of the low reactivity of these mixtures, as shown by their low K\({}_{\text{st}}\), the overpressure was high and would destroy any process plant enclosure used in processing this material. These pressure rises were lower than those of purely fine particles of biomass, whereas, for similar biomass composition, P\({}_{\text{max}}\)/P\({}_{\text{i}}\) was about 8.5 bar [58; 59].