## 3 Results and Discussion
### Characteristics of Fuel Ignition in a Heated Airflow
Figure 6: Videograms of ignition and combustion of coal particles (composition C-1) during their movement in a heated airflow at \(T_{a}=700\)\({}^{\circ}\)C (\(\Delta t=0.01\) s): (**a**) \(t_{d}=0.099\) s; (**b**) \(t=t_{d}+\Delta t\); (**c**) \(t=t_{d}+2\Delta t\); (**d**) \(t=t_{d}+3\Delta t\). Figure 7: Videograms of ignition and combustion of cedcar needle particles (composition B-1) during their movement in a heated airflow at \(T_{a}=700\)\({}^{\circ}\)C (\(\Delta t=0.01\) s): (**a**) \(t_{d}=0.017\) s; (**b**) \(t=t_{d}+\Delta t\); (**c**) \(t=t_{d}+2\Delta t\); (**d**) \(t=t_{d}+3\Delta t\). The smallest values of \(t_{d}\) are typical for biomass (composition B-1). This result is explained by the lowest moisture content and the highest content of volatiles in its composition (Table 1). After the introduction of fine coal particles into the heated airflow, almost all of the supplied heat from an external source is spent on the endothermic processes of phase transformation (moisture evaporation) and thermal decomposition, accompanied by heat absorption. In addition, coal has a fairly dense structure, mainly with closed pores relative to the external gaseous medium [38]. Therefore, coal (composition C-1), in contrast to biomass, requires more energy (and, accordingly, time at \(T_{a}\) = const) to start heterogeneous combustion. The maximum difference between the ignition delay times of compositions C-1 and B-1 is 0.13 s at \(T_{a}\) = 600 \({}^{\circ}\)C. When biomass particles (100-200 \(\upmu\)m) are added to coal (100-200 \(\upmu\)m), ignition delay times (when \(T_{a}\) changes from 600 to 800 \({}^{\circ}\)C) are reduced by 50-55% compared to coal in the initial state (composition C-1). It should be noted that the smaller the biomass particle size in the fuel mixture, the shorter the ignition delay times. Larger particles (300-400 \(\upmu\)m) require more time for their heating and, as a result, the onset of combustion, compared to smaller particles (100-200 and 200-300 \(\upmu\)m). With a  
Figure 11: Ignition delay times of coal, biomass particles, and their mixtures vs. heated airflow temperature. Figure 10: Videograms of ignition and combustion of coal and cedar needle mixture (composition CB-4) during their movement in a heated airflow at \(T_{a}\) = 700 \({}^{\circ}\)C (\(\Delta t\) = 0.01 s): (**a**) \(t_{d}\) = 0.076 s; (**b**) \(t\) = \(t_{d}\) + \(\Delta t\); (**c**) \(t\) = \(t_{d}\) + \(2\Delta t\); (**d**) \(t\) = \(t_{d}\) + \(3\Delta t\). decrease in the size of biomass particles (from 300-400 to 100-200 \(\upmu\)m) in the fuel mixture, the ignition delay times are reduced by up to 2 times compared to the composition CB-4 (Table 4). Thus, the use of biomass particles (of different sizes) as an additive to widespread coal helps to reduce ignition delay times and increase the completeness of fuel burnup. The Supplementary Materials include Figure S1, which shows the dependence of the ignition delay time of coal, biomass, and their mixtures on the size of fuel particles. The obtained result is consistent with the studies conducted by [PERSON] et al. [39]. In this work, it was noted that biomass ignites at a lower temperature since the content of volatiles in its composition is higher (compared to coal). The ignition delay times of biomass are lower compared to coal throughout the entire range of temperatures studied. In addition, ignition times decrease as the temperature of the furnace increases.