## 3 Results and Discussion
Figure 5 presents the ignition delay times (\(t_{d}\)) vs. burnout times of fuel pellets (\(t_{b}\)) when varying the heated air temperature in a wide range: \(T_{g}\) = 800-1000 \({}^{\circ}\)C. The temperature range limit on the left (\(T_{g}\) = 800 \({}^{\circ}\)C) corresponds to threshold (minimum) ignition conditions. At air temperatures under 800 \({}^{\circ}\)C, the pellets of all the eight fuel compositions (Table 1) did not ignite. Under such conditions, moisture evaporated and solid components thermally decomposed. However, the intensity of these processes and the oxidizer temperature were not enough for the forming gas mixture to ignite. The combustible gases and vapors diffused into the environment, but their concentration around the fuel pellet was decreasing as the fuel components thermally decomposed (and the content of volatiles decreased). The ambient air temperature was not high enough for the solid residue to ignite. Figure 6: Schemes of combustion mechanisms of FC-based fuel pellets with different components: (**a**) fuel composition No. 1: FC; (**b**) fuel compositions No. 2–No. 4: FC + porous polymer material, impregnated with a combustible liquid; (**c**) fuel compositions No. 5–No. 8: FC + sawdust. The mean ignition delay times of pellets of different compositions (with various additives) decrease from 5-6 s to 2.5 s in the air temperature (\(T_{g}\)) range from 800 \({}^{\circ}\)C to 1000 \({}^{\circ}\)C (Figure 5a). Their combustion becomes more intense under such conditions. The burnout times of fuel pellet components decrease by 20-25% (Figure 5b). It was established that the \(t_{d}\) values for the FC-based fuel compositions under study decrease sequentially (other conditions being equal) when the following components are added to them: a polymer material particle, impregnated with oil-water emulsion (based on rapeseed oil); a polymer material particle, impregnated with rapeseed oil; wood sawdust with a particle size of 200-500 \(\upmu\)m; wood sawdust with a particle size of 100-200 \(\upmu\)m; wood sawdust with a particle size of 45-100 \(\upmu\)m; wood sawdust with a particle size of less than 45 \(\upmu\)m; a polymer material particle, impregnated with oil-water emulsion (based on turbine oil). That means that fuel pellets with the latter component (composition No. 4) are characterized by the lowest ignition delay times. The highest values of \(t_{d}\) are typical of pellets, produced from FC without any additional components (composition No. 1). At ambient temperatures 800-850 \({}^{\circ}\)C, the \(t_{d}\) values for the compositions with various components differed by about 70% at the most (Figure 5a). At \(T_{g}\)\(\rightarrow\) 1000 \({}^{\circ}\)C and higher, the intensity of physical and chemical processes is so high that the ignition delay times for all the fuel compositions differ by no more than 5%, thus being less than the random error of \(t_{d}\) measurement (Figure 5a). Under near-threshold ignition conditions, the physical and chemical processes occur less rapidly. Most of the induction period (80-90%) is made up of the thermal preparation stage, characterized largely by heat and mass transfer processes both in the condensed phase and in the gas medium. Therefore, the differences in the ignition characteristics of fuel compositions with different components are more pronounced at \(T_{g}\) = 800 \({}^{\circ}\)C.  
As it was mentioned above, the gas-phase combustion develops most rapidly for FC-based fuel pellets with additives (sawdust and porous polymer material, impregnated with combustible liquid). This result is attributed to a much higher content of volatiles (Tables 1 and 2) in fuel compositions No. 2-No. 8, as compared with FC (composition No. 1) without any additives. Besides, a further analysis will show that compositions No. 2-No. 4, as compared with composition No. 1, are characterized not only by a higher content of volatiles, but also by a significant difference in the combustion mechanism.