## 3 Results and Discussion
These pores emerge and develop primarily in the near-surface layer of the pellet, and their size is much smaller than that of the pellet itself. Notably, the shape of pellets under such conditions remains almost the same as they burn (Figures 3a and 4). The solid residue burns out in layers. The front of combustion is formed in the near-surface layer of the pellet and moves steadily towards its deep layers. This process is more intense for compositions No. 5-No. 8, containing wood sawdust, than it is for composition No. 1. Sawdust is actually a thermally active additive, evenly distributed throughout the pellet. The ignition temperature of wood is much lower than that of dry FC (Table 2). Thus, in the layer-by-layer combustion of the pellet, wood particles are the first to ignite in the combustion front, and then the heat released intensifies the coal component ignition (Figure 6c). It reduces the ignition delay of coal. The compositions with a polymer material particle, impregnated with a combustible liquid, are characterized by a different mechanism of ignition and combustion intensification. When a fuel pellet is heated, the thermal decomposition of FC is accompanied by the endothermic evaporation of oil or water-oil emulsion. Under such conditions, the energy supplied to the pellet is spent on the phase transformation, which reduces the intensity of the process under study, on the one hand. On the other hand, the rapid evaporation of liquid 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 (Figure 6b), whose sizes are comparable with the typical pellet size. Under such conditions, the fuel pellet shape changes significantly during combustion (Figure 3b-d). As with the other compositions, in the heating of compositions No. 2-No. 4, the vapors and gases with low resistance enter the oxidizing medium from the deep pellet layers through open pores (Figure 6b). However, 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. Similar processes are typical of abnormal combustion modes of composite solid fuels when fuel charges crack or detach from the engine body walls [41]. The experimental research findings for the ignition delay times and duration of burnout of a group of fuel pellets at different heating temperatures provide a foundation for the development of an industrial technology of using such solid composite fuels to generate power by combustion, e.g., in hot-water boiler furnaces. The established temporal characteristics of ignition and combustion can be applied to calculate the thermal modes of boiler furnace operation. Moreover, it was shown that the burnout rate of fuel components may be increased or decreased by varying the component composition of fuel pellets. Thus, the energy output per a unit of time can be varied in the temperature range of 800-900 \({}^{\circ}\)C, typical of hot-water boiler furnaces. In addition to the ignition and combustion characteristics, we also analyzed the concentrations of anthropogenic emissions in flue gases from fuel pellet combustion. The concentrations of carbon oxides from the combustion of compositions No. 1-No. 8 are similar (the differences do not exceed several percent): CO2--17--18%, CO--no more than 400 ppm. These values correspond to identical characteristics of coal dust combustion in a boiler furnace [5]. The concentrations of nitrogen and sulfur oxides from the combustion of fuel pellets with different components differ much more significantly (Figure 7). In Figure 7, the NOx and SOx concentrations are presented per a unit of heat (emissions for identical heating value of fuel samples are compared), released from the combustion of fuel pellets of different component composition.