## 4 Conclusions  
Experimental studies have been conducted for three groups of fuel pellets based on coal processing waste with porous polymer material particles, impregnated with a combustible liquid, and wood sawdust of different particle size. The ignition and combustion of single fuel pellets were researched under radiant heating in an air medium, while varying the temperature from 800 to 1000 \({}^{\circ}\)C. Using a hardware and software system of high-speed video recording, the main characteristics of the processes were established: ignition delay times (2-8 s) and duration of fuel burnout at different heating temperatures (40-90 s). For the fuel compositions under study, the same main stages of the process were distinguished, when single pellets were heated in motionless heated air: inert heating of the fuel; moisture evaporation; thermal decomposition of solid combustible components; 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. A difference was established in the combustion mechanisms of pellets, when adding various components to the fuel mixture composition. This has a direct influence on the induction period duration and combustion time, other conditions being equal. When wood sawdust is distributed evenly throughout the fuel pellet, solid components burn out layer-by-layer. The ignition temperature of wood is much lower than that of dry FC, therefore, wood particles are the first to ignite in the combustion front, and the heat released intensifies the ignition of the coal component. For compositions containing a polymer material particle, impregnated with a combustible liquid, the evaporation of oil or water-oil emulsion in a small area, located inside the dense structure of pressed FC, results in a higher pressure of vapors in a closed space. This leads to the emergence and formation of a developed structure of pores with sizes comparable with the typical pellet size. The combustion front of such pellets has a rather complex structure. The developed structure of pores increases the combustion area and, thus, accelerates the burnout of fuel pellet components. The component composition of fuel pellets also affects the concentration of anthropogenic emissions in flue gases. The concentrations of carbon oxides from the combustion of the fuel compositions under study are similar: CO\({}_{2}\)--17-18%, CO--no more than 400 ppm. NO\({}_{\rm x}\) and SO\({}_{\rm x}\) concentrations in flue gases per a unit of heat, released from the fuel pellet combustion at 1000 \({}^{\circ}\)C, are by 30% and 25% lower for the compositions with 30% of wood sawdust, while being by 30% and 20% higher for compositions with 10% of porous polymer, impregnated with a combustible liquid, as compared with identical characteristics from the combustion of FC without any additional components. The NO\({}_{\rm x}\) and SO\({}_{\rm x}\) concentrations from the combustion of FC pellets are less than 150 and 60 ppm, respectively.  
Conceptualization, P.S.; data curation, K.P.; formal analysis, K.P. and D.K.; funding acquisition, P.S.; investigation, K.P. and D.K.; methodology, T.K.; project administration, P.S.; resources, T.K.; supervision, P.S.; validation, K.P.; visualization, D.K.; writing--original draft, P.S.; writing--review and editing, P.S. All authors have read and agreed to the published version of the manuscript.  
The study was supported by Russian Foundation for Basic Research and the Government of the Tomsk region of the Russian Federation (grant number 18-43-700001).  
The authors declare no conflict of interest.