## 3 Results and Discussion
### Efficiency Indicators of Combustion of Slurry Fuels with Biomass  
The absolute and standardized values of fuel evaluation criteria, obtained in the experimental research and theoretical analysis, are presented in Table 5. It can be concluded that all the slurry fuels are inferior to coal in terms of _energy performance parameters_: ignition delay times, calorific value, and maximum combustion temperature. This result is explained by the fact that slurries contain quite a lot of moisture. Its evaporation prolongs the ignition, reduces energy performance characteristics of the fuel, thus, affecting the maximum combustion temperature. In terms of heat of combustion and maximum combustion temperature, the composition with rapeseed oil is closest to coal. The heat of combustion of this composition is 42.1 MJ/kg. However, the ignition delay times of this composition are, on the contrary, the longest. The cetane number (a measure of the fuel ignition value) of rapeseed oil is 37.6 [58], which is quite a low indicator of ignition. A standard fuel is characterized by a cetane number from 48 to 51, whereas a superior quality fuel--from 51 to 55. At a cetane number lower than 40, the fuel ignition delay time rises sharply. Therefore, the ignition delay of the composition with rapeseed oil is greater than that of the composition with dry biomass. However, due to almost zero ash content of oil, the composition with rapeseed oil performed the best in this criterion, since agricultural and wood biomass can contain quite a lot of ash (Table 2) and additives, affecting not only the ignition, but also slagging indicators. The energy performance indicators for the compositions with dry biomass (Figure 3, Table 5) are in good agreement. This result can be attributed to a low fraction of biomass in slurries and similar characteristics of the materials under consideration. The _environmental criteria_ for slurry fuels are much higher than those for coals within the whole temperature range (Figure 4). That gives slurries a major advantage. A reduction in emission concentrations is explained by the influence of a number of factors. The main ones are a lower fraction of sulfur and nitrogen in the fuel, lower combustion temperature, interaction of sulfur and nitrogen oxides with water vapors and with the components of biomass pyrolysis and gasification, resulting in the formation of salts in ash and a decrease in NOx and NOx concentrations. It is difficult to single out one particular dry biomass additive. For instance, at 700 \({}^{\circ}\)C and 800 \({}^{\circ}\)C, straw and leaves proved to be the best additives, whereas at 900 \({}^{\circ}\)C, using 10% of sunflower cake had a major effect. As mentioned above, this is explained by chemical reactions developing between biomass components and the gases under study [10]. The main conclusion is that this research focuses on the variation ranges of anthropogenic emission concentrations, when biomass is added to fuel slurry compositions. They show great prospects of using biomass for an environmentally friendly recovery of coal processing waste to produce energy. The _economic criteria_, including cost of components and economic indicators of preparation, storage, and combustion, were better for slurries, too. The cost of the slurries under study was derived from the market prices of the components [59; 60; 61]. Using coal processing waste to produce slurries decreases the fuel cost up to sevenfold. Adding biomass can increase or decrease the cost of the resulting slurry, depending on the type of biomass, the degree of its involvement and use in other sectors, accumulated volumes, and location. In this study, it was considered the most typical and widespread biomass (often waste), therefore, the final cost of the slurry did not increase much. Fuel with rapeseed oil is an exception, since the latter is more expensive than coal. At present, rapeseed oil is actively used in the food and energy industries to produce biodiesel. Thus, its extensive use increases the cost of slurries.