## 2 Materials and Methods
### Physical Properties of Wood Dusts and Sieve Analysis  
As the water content of waste wood dust (determined by a gravimetric method [46]) increases, the resistance of the dust to ignition also increases ([47, 48]. Part of the supplied energy is used to evaporate free water, break bonds and subsequently evaporate bound and chemically bound water. Combustible gases diluted with water vapour have a lower concentration and thus lower flammability. The higher the moisture content of dust, the higher the ignition time [49]. Sieve analysis in accordance with [50] was carried out by an automatic vibratory sieve shaker Retsch AS 200 control and with a set of control stainless steel sieves: diameter of sieve, 200 mm; height, 50 mm; diameter of sieve mesh. \(<\)0.056; 0.056; 0.071; 0.090; 0.150; 0.200; 0.250; 0.500 (mm). Shares of residues on each sieve and bottom were researched using digital laboratory balance Radwag KERN PLT (KERN and SOHN, Balingen, Germany) with accuracy of weighing 0.001 g. Sieving parameters include: amplitude 2 mm-g\({}^{-1}\), with an interval of 10 s, sieving time 20 min. The measurement process was realised by [50]. Determining the Minimum Ignition Temperature According to EN 50281 of Waste Wood Dust Layer Samples from Hot Surfaces  
The minimum ignition temperature was determined on an electrically hot plate (Figure 2) 185 mm in diameter according to EN 50281-2-1: 2002 Methods for determining the minimum ignition temperatures of dust [51]. This standard defines minimum temperature for ignition of a dust layer as a minimum temperature of a hot surface by which ignition of a dust layer can occur with a setting thickness located on that hot surface [47]. According to the procedure EN 50281-2-1:2002 (Method A), the ignition of dust in the layer could occur when [51]:  
1. [label=()]
2. Glowing or flame combustion was observed;
3. Measured temperature of dust have reached 450 \({}^{\circ}\)C;
4. Measured temperature of dust exceeded 250 K (the temperature of the furnace plate). Figure 1: Waste wood dust samples. Legend: (**a**) spruce, (**b**) beech, (**c**) oak. Figure 2: The test equipment establishing minimum ignition temperatures of the dust layer. Legend: (**a**) Hot plate apparatus with full metal ring of beech dust; (**b**) electric coil with a direct current source (TTAD3, JLT Slovakia). In reference to point (b) and (c), the ignition of dust has not occurred if it could be indicated that oxidise reaction does not change into glowing or flame combustion process. It characterises dust according to its ability to ignite in a layer under heat stress from a hot environment or by contact with another hot surface [47]. The hot plate assembly temperature is set and controlled to the desired temperature by a temperature controller. The temperature of the hot plate assembly and the dust layer sample are continuously monitored and recorded on a data logger/data sheet as a function of time to the end of the test. The test is continued until the sample layer changes into glowing or flame combustion process ignites, or reaches a maximum temperature without igniting, and is cooled down. Visual observations are also made during the test. If after 120 min, self-heating is not apparent, the test is terminated [47]. The height of the waste wood dust layer sample placed on the surface of the hot plate was 15 mm. The layer height was chosen to be the highest, which is probably in grinding processes. At the same time, the stated layer height is recommended in the methodology [47] and in the references [43; 49].