## 3 Results and Discussion
### Ignition of Waste Wood Dust
#### 3.3.3. Monitoring of Waste Wood Dust Ignition by a Smouldering Cigarette  
Smouldering is a heterogeneous combustion where oxygen molecules (a gaseous oxidant) affect the surface of a combustible solid substance--tobacco. The combustion reaction takes place on the surface of the combustible substance [76]. Two thermocouples, as in previous experiments, were used to monitor the surface and bottom temperature of the dust layer. The depicted thermal dependence of wood dust layers in the case of ignition by a smouldering cigarette, shows a significant difference in the oak sample (Figure 6c,f,i). [71]. The smouldering cigarette only affected the surface of the dust (Figure 8), the fire did not burn through the sample layer and the temperature at the bottom of the dust layer increased only slightly. Upon application of the cigarette, a charred layer of dust was formed at the tip of the burning cigarette and it progressed in the direction of the burning cigarette. The figures show that the surface temperature increased, although the smouldering part did not act at a given location for about 100 s. After the cigarette had completely burned out, the charred layer gradually expanded, with strong smoke and smouldering observed in oak dust. Figure 7. Charred surface of dust ignited by an electric coil, documented by camera (Olympus PEN E-PL8 + ED 14-42 II R lens, Olympus Czech Group, s.r.o., Czech Republic). Legend: (**a**) spruce dust, (**b**) beech dust, (**c**) oak dust at 3 rd min. The layer of spruce dust was charred (Figure 8a) only at the area of application of the smouldering tip of the cigarette, followed by light smouldering, without the tendency to expand the charred area. When the beech dust sample was ignited (Figure 8b), the area of the charred layer was larger and the smouldering more intense. At the ignition of the oak dust sample (Figure 8c), the charred surface rapidly spread across the surface with multiple smouldering locations, and the sample burned all the way through, which was also indicated by an increase in temperature at the bottom of the dust layer. The charred areas ignited by a smouldering cigarette (Figure 8 and Table 4) were significantly smaller than those ignited by the electrical coil (Figure 7 and Table 4). A smouldering cigarette poses a fair ignition risk especially for fine dust mixtures (Figure 8c). Summarising the results is difficult. The experiments were carried out on the same hot plate apparatus with the addition of electric coil and smouldering cigarette as ignition sources. Hot plate apparatus is recognised by the academic community as suitable equipment for testing the minimum ignition temperature of a deposited layer of dust [47; 51]. The first experiments analysed the behaviour of the dust layer on the surface of the hot plate. The obtained results are in alignment with the research of other authors [47]. The difference in temperature curves (Figure 7a,d,g) is related to the diversity of wood dust (spruce, oak, beech) and the diversity of the granulometric composition of wood dust (prepared by sawing and grinding). [PERSON] et al. [20] analysed the granulometric composition of various types of wood dust produced by a circular saw and statistically evaluated the influence of wood species and particle size on the granulometric composition of wood dust. Differences in the dust structure of individual tree species were not shown to be statistically significant. [PERSON] et al. [77] confirm this finding. [PERSON] [77] also notes that the species of wood has not shown a clear effect on the course of the granulometric composition of the wood when working with different tools. Differences in the dust structure of individual tree species are shown to be statistically significant among the fractions [45]. This fact is also confirmed in the research of [PERSON] et al. [78]. The results show the influence of the granulometric composition of individual samples on the course of thermal