## 3 Results and Discussion  
Figures 3 and 4 present the video frames of ignition and combustion of a group of solid composite fuel pellets with various components and without them. It is clear (Figures 3 and 4) that adding a single particle of porous polymer material, impregnated with different combustible liquids, or wood sawdust of different particle size to the fuel pellet composition intensifies the ignition and combustion of these compositions, as compared with pellets, produced from FC without any additional components (composition No. 1). In general, the physical and chemical processes, occurring during the heating of pellets of fuel compositions under study (Table 1), follow almost identical patterns, since their main component is FC. High-speed video recordings made it possible to distinguish the following main stages of the process under consideration, when single pellets are heated in motionless heated air: inert heating of the fuel; evaporation of liquid components (water or oils for compositions No. 2-No. 4); thermal decomposition of solid combustible components (FC, polymer material for compositions No. 2-No. 4, wood for compositions No. 5-No. 8); formation of gas-vapor mixture around the pellet under the conditions of diffusive-convective heat and mass transfer in the oxidizer medium; its ignition on reaching critical conditions and subsequent burnout; heating of the solid residue from the heat of the gas-phase combustion; its ignition and subsequent burnout. The gas-phase combustion develops most rapidly for FC-based fuel pellets with sawdust (Figure 4). This result is attributed to a much higher content of volatiles in wood, as compared with FC (Table 2). Compositions No. 5-No. 8, as compared with composition No. 1, are characterized not only by a larger (by 10-20%) area around the pellet, in which the combustion of the gas mixture occurs, but also a longer (by 20-25%) duration of this process. Additionally, the burnout times of solid components (\(t_{b}\)) differ by 15-20%, which is also conditioned by differences in their ultimate analyses. The content of carbon in wood is almost one third lower than in FC. Therefore, the solid residue of compositions No. 5-No. 8 burns out much faster than that of composition No. 1 (Figure 4). Similar patterns were established when the characteristics of ignition and combustion for fuel compositions No. 2-No. 4 were compared with those for composition No. 1 (Figure 3), even when the concentration of an additional component is only 10%. In this case, the ignition delay times and complete burnout times differ (Figure 5) due to different combustion mechanisms. A detailed analysis of the latter will be carried out below when describing typical schemes (Figure 6) of combustion of fuel pellets with various components. Figure 3: Video frames of ignition and combustion of FC-based fuel pellets with porous polymer material, impregnated with combustible liquid at \(T_{g}=850\)\({}^{\circ}\)C: (**a**) fuel composition No. 1: FC; (**b**) fuel composition No. 2: FC + porous polymer material, impregnated with rapeseed oil; (**c**) fuel composition No. 3: FC + porous polymer material, impregnated with oil-water emulsion (rapeseed oil); (**d**) fuel composition No. 4: FC + porous polymer material, impregnated with oil-water emulsion (turbine oil). Figure 4: Video frames of ignition and combustion of FC-based fuel pellets with sawdust of different particle size at \(T_{g}\) = 850 \({}^{\circ}\)C: (**a**) fuel composition No. 5: FC + sawdust (less than 45 \(\upmu\)m); (**b**) fuel composition No. 6: FC + sawdust (45–100 \(\upmu\)m); (**c**) fuel composition No. 7: FC + sawdust (100–200 \(\upmu\)m); (**d**) fuel composition No. 8: FC + sawdust (200–500 \(\upmu\)m). Figure 5: Ignition delay times (**a**) and complete burnout times (**b**) of fuel pellets vs. the ambient temperature.