## 3 Results and Discussion
### Ignition of Spruce Dust Samples by Hot Surface Ignition  
This set of experiments was preceded by indicative tests to determine the height of the spruce wood dust layer on the hot plate apparatus (Figure 6). The saw sample WD_Saw obtained from hot-plate sawing did not ignite. In the above experiments, no 5 mm layer of settled sawdust dust was initiated. Spruce dust samples from briquetting (Figure 7a), suction (Figure 7b) and grinding (Figure 7c) were ignited. The obtained data were different (Table 6).Schemes followed the same formatting. Figure 5: Presentation of the number of fractions of spruce wood dust samples. Legend: (**a**) WD_Saw; (**b**) WD_ExtrD; (**c**) WD_Briq; (**d**) WD_Grin. The dependence of ignition on surface temperature was statistically evaluated by a t-test and a One-Way ANOVA. However, the amount of data obtained did not confirm the significance of the minimum temperature or ignition time on the type of spruce dust sample. The evaluation follows only from the experimentally obtained data (Table 6, Figure 7). From the point of view of the risk of ignition in the settled layer, dust from the grinder is ignited first, then from brighteting and a sample taken from the container. Th ignition temperature range is comparable. The lowest value of the ignition temperature was in the grinder sample and the highest was in the briquettes. From the point of view of the risk of ignition of the settled dust layer, the spruce dust sample emerges from the briquetting as the worst. The sample WD_Briq has a minimum ignition temperature of 360 \({}^{\circ}\)C and a grinding sample of 350 \({}^{\circ}\)C, but the ignition time of the sample WD_Briq is 7 min. The sample taken from the grinding (WD_Grin) ignited after 12 min. It is possible to assume a slower heating of the WD-Grin layer (with the smallest particle composition) and therefore there is a delay of ignition (12 min), but at a lower temperature. [PERSON] and [PERSON] [53] carried out a hot-plate experiment with samples of spruce, beech and oak dusts. The samples (sawdust) were produced using a circular saw. However, \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Parameters**} & \multicolumn{5}{c}{**5 mm of Spuce Dust Layer**} \\ \cline{2-5}  & **WD\_Saw** & **WD\_Briq** & **WD\_ExtD** & **WD\_Grin** \\ \hline Lowest temperature (\({}^{\circ}\)C) & X* & 360 & 360 & 350 \\ Ignition time (min) & X* & 7 & 11 & 12 \\ \hline \hline \end{tabular}  
* X—not determined.  
\end{table} 
Table 6: Presentation of material parameters of spruce dust samples. Figure 7: Determination of the ignition temperature of settled wood dust (**a**) from briquetting; (**b**) removed from the container; (**c**) removed from grinding. spruce samples have the same moistures and a mean sieve analysis value of 250 \(\upmu\)m. Their minimum ignition temperature (300 \({}^{\circ}\)C) was lesser than ours. After ignition, the deposited dust only ends up slowing oxidation reactions, such as decay, heating or low-temperature carbonization. The burning rate of settled dust can range from a slow spread, and can decay to a violent explosion [35]. [PERSON] and [PERSON] [36] determined the minimum temperature of dusts, from different types of pellets, by isothermal thermal stress of combstible dust on an electrically heated metal plate and continuous measurement of the temperature inside the sample. The results of individual measurements of the ignition temperature of settled dusts (according to EN 50281-2-1: 2002 [52]) differ (Table 7), as the determined sample density differs in the mentioned experiments. [PERSON] [68] states the humidity of oak wood dust to be 4.7%, spruce dust to be 4.2% and DTD dust to be 4.2%.In an experiment from [69], the moisture content of beech dust was 1.002%, and that of oak dust was 0.884%. This was reflected in the lower values of the ignition temperature of the samples. The determination of the minimum ignition temperature of dusts in the settled state is described in detail in the paper [69].