## 3 Results and Discussion
### Characteristics of Fuel Ignition in a Heated Airflow  
When conducting experimental studies, finely dispersed particles of coal (chernogorskiy coal) and biomass (cedar needles) of various dispersions were used as the initial fuel components (Table 4). Figures 6-10 show typical videograms of high-speed video recording of the ignition and combustion processes of coal and biomass particles, as well as their mixtures. The videograms show the dynamics of the combustion process of finely dispersed fuel particles in a heated airflow at Ta = 700 \({}^{\circ}\)C with a step \(\Delta\)t = 0.01 c from the moment of fuel ignition. The resulting videograms show differences in the distances traveled by combustible particles from the beginning of their heating to the ignition moment and the intensity of combustion under identical initial conditions. The results demonstrate (Figure 6) that coal not only ignites longer than other compositions (larger ignition delay times td), but also has the lowest burnout efficiency. This is clearly visible in Figure 6. The main part of the bright dots (flammable coal particles) is located on the right side of the figure. Cedar needles, on the contrary, are characterized by the shortest ignition delay times (Figure 7). Figure 7 clearly shows that all the points are on the left side of the figure. When biomass with a particle size of 100-200 \(\upmu\)m is added to coal, in the video frames of their combustion (Figure 8), all points are located in the left and middle parts of the figure, which characterizes shorter ignition delay times of the fuel mixture. With an increase in the size of biomass particles to 200-300 \(\upmu\)m in the frames of the videogram of their combustion (Figure 9), the points are located in the middle part of the figure, which illustrates the increase in ignition delay times. An increase in the size of biomass particles to 300-400 \(\upmu\)m leads to a shift of points to the right side of the figure (Figure 10). This indicates that the ignition and combustion characteristics of biomass particles are approaching those of coal. Figure 11 shows the dependence of the ignition delay times of coal and biomass particles, as well as their mixtures, on the oxidizer temperature in the range of 500-800 \({}^{\circ}\)C when finely dispersed fuel particles move in a heated airflow at a speed of \(V_{a}\) = 5 m/s. Approximation curves are drawn through points characterizing the average values of \(t_{d}\) established in a series of 5-10 experiments under identical initial conditions. The higher the temperature in the muffle furnace (\(T_{a}\)), the lower the value of \(t_{d}\), since at relatively high ambient temperatures (\(T_{a}\)\(\rightarrow\)800 \({}^{\circ}\)C), the processes of heating, moisture evaporation, and volatile release in fuel particles are most active. At relatively low heating temperatures (\(T_{a}\) = 500 \({}^{\circ}\)C), these processes are less intense. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{3}{*}{**Fuel**} & \multicolumn{4}{c}{**Fuel Component Concentration, wt\%**} \\ \cline{2-5}  & **Coal** & \multicolumn{2}{c}{**Biomass**} \\ \cline{2-5}  & **Particle Size** & **Particle Size** & **Particle Size** & **Particle Size** \\  & **100–200 \(\upmu\)m** & **100–200 \(\upmu\)m** & 200–300 \(\upmu\)m & 300–400 \(\upmu\)m \\ \hline C-1 & 100 & – & – & – \\ \hline B-1 & – & 100 & – & – \\ \hline CB-1 & 50 & 50 & – & – \\ \hline CB-3 & 50 & – & 50 & – \\ \hline CB-4 & 50 & – & – & 50 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Fuel composition. Figure 8: Videograms of ignition and combustion of coal and cedcar needle mixture (composition CB-1) during their movement in a heated airflow at \(T_{a}=700\)\({}^{\circ}\)C (\(\Delta t=0.01\) s): (**a**) \(t_{d}=0.047\) s; (**b**) \(t=t_{d}+\Delta t\); (**c**) \(t=t_{d}+2\Delta t\); (**d**) \(t=t_{d}+3\Delta t\).