## 3 Results and Discussion  
### Experimental Results  
Figure 2 shows typical combustion stages of a composite fuel droplet. Such fuel contains several components with different thermophysical properties and states of matter. Therefore, the combustible components of the mixture are not ignited simultaneously. Slurries, like many other fossil fuels, are characterized by a homogeneous (gas-phase) as well as heterogeneous combustion mechanisms (Figure 2). Adding biomass can significantly affect the duration and intensity of both combustion regimes. In particular, biomass with a high content of volatiles (such as, leaves, sawdust) accelerates the gas-phase ignition and facilitates a larger flame and its long burnout (these parameters also depend on heating conditions). Figure 1: Scheme of experimental setup. Figures 3 and 4 present the experimental research findings for a group of fuel compositions. These include the established ignition delay times, minimum air temperatures sufficient for fuel ignition, maximum combustion temperatures of fuel (\(T_{\rm b}\) max was measured at \(T_{\rm g}\) = 700 \({}^{\circ}\)C), as well as anthropogenic emission concentrations. It can be clearly seen (Figures 3 and 4) that not all the biomass kinds, added to the fuel compositions, produce significant changes in the characteristics under study. This is explained by the component composition of some additives, which is quite close to the coal processing waste being used. The fuel with vegetable (rapeseed) oil features the longest ignition delay times. When it is heated, the vegetable oil components decompose. Unlike petroleum oil, vegetable oil does not produce combustible gas components during decomposition. Thus, extra time is required for the endothermic processes to develop and the coke residue to ignite. For instance, ignition delay time \(\tau_{\rm d}\) of the composition with rapeseed oil differs 1.8-2.0 times from that of the composition with biomass (sunflower hulls) in the heating temperature \(T_{\rm g}\) range from 700 to 900 \({}^{\circ}\)C (Figure. 3a). Varying the kind of plant biomass (sunflower waste, straw, sawdust, leaves) had a marginal effect on the changes of the ignition delay times and ignition temperature (Figure 3), which is explained by similar properties of these components with a major influence of volatiles at the ignition stage. The content of volatiles in the dry ash-free substance was no less than 76% for all the solid biomass additives used in present study. At a relatively low ash and moisture content of the plant biomass, volatiles can rapidly form combustible reaction mixtures with the air. The shortest ignition delay times were typical of coal (Figure 3a), which is due to a small fraction of water (as compared with slurries). Moisture evaporation consumes a lot of energy (specific vaporization heat of water is 2.2 MJ/kg). Compared to that, the thermal decomposition of the organic mass of coal is less energy-consuming. Therefore, a reduction in the fraction of water decreases the inert period. However, the experiments show that dry coal is ignited at higher temperatures, than wet slurries do. This is probably explained by the reactions of interaction of water vapors with carbon [48]. The combustion temperature of a coal particle was much higher than that of slurries (Figure 3b) due to low moisture content and greater combustion heat. Value of \(T_{\rm b}\)max is directly proportional to the combustion conditions and calorific value of the fuel. Thus, an increase in the fuel moisture content reduces the temperature in its combustion zone. However, using high-energy additives (e.g., sawdust) can increase this parameter. When analyzing the anthropogenic emissions, it was established that any plant-based component, added to coal or coal processing waste, substantially reduces sulfur and nitrogen oxide concentrations (Figure 4). The co-combustion of biomass with coal processing waste reduces NO\({}_{\rm x}\) by 25%-62% and SO\({}_{\rm x}\) by 61%-88%.