## 3 Results and Discussion
### Ignition of Waste Wood Dust  
#### 3.3.1 Monitoring of Hot Surface Ignition  
The set of experiments was preceded by indicative tests to determine the height of the waste wood dust layer on the hot plate apparatus. The first degradation processes were observed near the test ring (Figure 5), which was used to define the height of the dust layer. Our aim was to remove the effect of the heating from the side wall, because a body with high thermal conductivity which is in contact with a hot surface and reaches a layer of deposited dust can cause the dust to heat up. This effect could not be completely avoided, in all cases of heating by a hot plate apparatus, smouldering nests were formed first around the perimeter of the sample (Figure 5). The ignition of a dust layer by a hot surface at a certain temperature depends strongly on the balance between the rate of heat generation in the layer and the rate of heat release to the environment [62; 63]. The essence of the process results from a comparison of the surface temperature of the hot plate with the minimum ignition temperature of deposited dust (Figure 6). Obtaining sufficient heat from the hot surface to heat the deposited dust to a minimum ignition temperature depends on the amount of dust, i.e., the thickness of the deposited layer. The ignition temperature of a given material, therefore, depends on the thickness of the deposited layer [48]. The purpose of this research was to monitor the influence of selected ignition sources on the deposited layer of dust, and therefore, a uniform thickness of 15 mm was chosen. Figure 5: Smouldering nests around the perimeter of the sample caused by the ignition source—hot surface at the 10 th min. Figure 6: Test data showing an ignition for the samples. Legend: (**a**,**d**,**g**) hot plate source, (**b**,**e**,**h**) electric coil, (**c**,**f**) cigarette. The ignition behaviour of the individual waste wood dust samples was very similar. Differences were in the minimum ignition temperature values and in the time intervals between the observed ignition phases. When the dust layer temperature reached about 100 \({}^{\circ}\)C, smouldering nests began to form in each sample. With subsequent temperature increase, thermal dust degradation spread along the perimeter of the deposited layer and towards the centre of the sample (Figure 6). Subsequently, the increasing temperature of waste wood dust was recorded on thermocouple No. X2, until the temperature of the hot surface was exceeded (Figure 6a,d,g). Table 3 shows the measured values of minimum ignition temperatures and the behaviour of the layer during the ignition of waste wood dust. A difference occurs in the time of realisation of these events. Spruce dust reached the hot plate surface temperature in 33 min (Figure 6a) and the 50 \({}^{\circ}\)C difference was not exceeded. [PERSON] [64] confirmed the results. The fine dust fraction (dust particles passing through 90 \(\upmu\)m sieve) had the higher ignition risk compared to the medium (between 90 and 180 \(\upmu\)m) and coarse (between 180 and 420 \(\upmu\)m) dust fractions [64]. [PERSON] et al. [64] investigated the ignition risk of dust from loblolly pine wood by quantifying (including moisture content and grinding screen size effects) the amount of dust in ground wood chips, and the hot surface ignition temperature. In the case of beech waste wood dust, the process was slower. The surface temperature was reached in 37 min (Figure 6d). The surface temperature of oak dust was reached in 25 min (Figure 6g) at 208 \({}^{\circ}\)C. The given minimum ignition temperature of oak is lower due to the particle composition of oak dust [12], where more than 56% of the volume of particles is 0.71 \(\upmu\)m.  
[PERSON] et al. [65] recorded a minimum ignition temperature of beech dust of 330 \({}^{\circ}\)C in a 5 mm layer. Determination of the minimum ignition temperature of dust in the deposited state is described in detail in their paper.