## 3 Results and Discussion
A rapid evaporation of combustible liquid leads to the emergence and formation of a developed structure of pores, whose sizes are comparable with the typical pellet size. Under such conditions, the vapors and gases with low resistance enter the oxidizing medium from the deep pellet layers through open pores. The concentration of combustible gases in the vicinity of the pellet grows rapidly, and the gas-phase ignition occurs with a short delay (as compared with fuel composition No. 1). The experimental results enable to draw an important conclusion. Including lumber waste (wood sawdust) in the composition of FC-based pellets in the volumes not affecting the physical and mechanical characteristics of pellets leads to significant (up to 70%) reduction in the ignition delay times at relatively low (800-850 \({}^{\circ}\)C) temperatures. Since the emission of nitrogen oxides in combustion products of coal and coal processing waste is minimum, and this temperature range in boiler furnaces of local heat supply is the most preferred, it can be concluded that including wood sawdust in the composition of fuel pellets based on coal processing waste is highly practical. The times of complete burnout of fuel pellets (Figure 5b) are in good agreement with their ignition delay times. It was established that the \(t_{b}\) values for FC-based fuel compositions under study decrease sequentially (other conditions being equal) when the following components are added to them: wood sawdust with a particle size of 200-500, 100-200, 45-100 \(\upmu\)m and less than 45 \(\upmu\)m; a polymer material particle, impregnated with oil-water emulsion (based on rapeseed oil); a polymer material particle, impregnated with rapeseed oil; a polymer material particle, impregnated with oil-water emulsion (based on turbine oil). This means that fuel pellets with the latter component (composition No. 4) are characterized by the lowest burnout times of components. The highest values of \(t_{b}\) are typical of pellets produced from FC without any other components added to them (composition No. 1). As it will be shown below, the obtained result can be explained by the differences in the elemental composition of fuel components and in the combustion mechanisms (the combustion front of fuel pellets with pores has a rather complex structure; the developed structure of pores increases the combustion area, thus, accelerating the burnout of fuel pellet components). Solid fuel pellets with a polymer material particle, impregnated with oil-water emulsion, burn out within 62 s at relatively low ambient temperatures (\(T_{g}\) = 800 \({}^{\circ}\)C), which is 30% lower than \(t_{b}\) of fuel composition No. 1 based on FC without any additional components, and at \(T_{g}\) = 1000 \({}^{\circ}\)C, the difference in the burnout times is 45%. Thus, adding extra components (a polymer particle, impregnated with oil-water emulsion; wood sawdust) to FC intensifies both the ignition and burnout of fuel pellets. As mentioned above, this result is explained by different combustion mechanisms of fuel compositions (Table 1). Figure 6 presents schemes of combustion mechanisms for three groups of FC-based fuel pellets with different components, established from analyzing the frames of high-speed video recording. The differences established in the combustion mechanisms (Figure 6) are primarily attributed to the structure of fuel pellets. The compositions of the first and third groups (No. 1, No. 5-No. 8) have a uniform structure with a rather dense packaging of fine particles of solid components (coal dust and wood sawdust). Thus, when they are heated, the occurring processes are similar to the combustion of large (several millimeters) solid fossil fuel particles. Internal thermal stress leads to the formation of open pores (Figure 6a,c), through which the thermal decomposition products of solid components escape from the deep layers of fuel pellets into the oxidizer medium.