## 3 Results and Discussion
### TG/DTG of Fuel Mixtures  
The prepared mixtures contained an equal amount of Chernogorskiy coal and cedar needles. Fractions with a particle size of 100-200 \(\upmu\)m (CB-1) and 300-400 \(\upmu\)m (CB-2) were used. The first stage of heating is expressed by the DTG maximum in the temperature range of 82-87 \({}^{\circ}\)C. When moisture evaporates, mixtures CB-1 and CB-2 lose mass by 3% (Figure 3a). The process of moisture evaporation, just like with individual fuels, is accompanied by the absorption of heat (endothermic effect). Figure 3b illustrates the mixture heating process, which is expressed by three DTG maxima corresponding to the main stages (moisture evaporation, volatile combustion, and coke residue combustion). The second stage of heating corresponds to the beginning of the small molecules decomposition and is accompanied by the release of gaseous volatiles and their further combustion. The second stage is in the temperature range of 195-367 \({}^{\circ}\)C for CB-1 and 195-382 \({}^{\circ}\)C for CB-2. At the same time, the mass of mixtures CB-1 and CB-2 is reduced by 29 and 26%, respectively. The reduction in the mass of mixtures in the second stage is less compared to B-1 and B-2 due to a decrease in the content of volatiles. The temperature at which ignition of the coke residue occurs for CB-1 was 276 \({}^{\circ}\)C, and for CB-2 it was 343 \({}^{\circ}\)C. Mass loss during volatile combustion for CB-1 and CB-2 was 29 and 25%, respectively (Figure 3a). The third stage is carried out in the temperature range of 362-597 \({}^{\circ}\)C for CB-1 and 372-622 \({}^{\circ}\)C for CB-2 and is accompanied by heat release due to the combustion of coke residue from coal and biomass (exothermic effect). During the coke residue combustion, the mass loss for mixtures CB-1 and CB-2 was 62 and 63%, respectively (Figure 3a). The combustion index for CB-1 was \(3.12\times 10^{-8}\) min\({}^{-2\circ}\)C\({}^{-3}\); for CB-2, it was \(1.97\times 10^{-8}\) min\({}^{-2\circ}\)C\({}^{-3}\).  
[PERSON] et al. [32] conducted a study on the co-combustion of coal and pine sawdust. They found that the addition of highly reactive biomass to coal affects the reduction of the following temperature parameters of the combustion process: the temperature at which ignition of the coke residue occurs; the temperature at which the combustion process ends; and the temperature corresponding to the maximum rate of mass loss. The maximum mass loss rate during combustion of coke residue has decreased, thereby affecting the decrease in the combustion index.  
In our study, the addition of biomass to coal improved the combustion characteristics of both dispersed fractions of the mixture. That is, the ignition temperature of coke residue and the temperature at which the combustion process ends are reduced, and the maximum mass loss rate increases compared to the coal combustion process. With an increasing particle size of mixture, as for individual fuels, the combustion of volatiles and coke residue occurs in the region of higher temperatures (Figure 3). With an increase in the particle size of mixtures, a shift in the profiles of the TG and DTG curves to the region of higher temperatures at the stages of combustion of volatiles and coke residue is also noticeable (Figure 3), due to the same reasons as for individual fuels. The shift of the main combustion stages to the region of higher temperatures affects the deterioration of the main combustion characteristics of mixtures. The obtained result is explained by the positive interaction of the studied fuel components with the combustion characteristics. That is, some types of biomass, when burned together with coal, can have a positive effect on only part of the combustion process characteristics.