## 3 Results and Discussion  
### Initial Fuel Temperature  
Studies of liquid [35] and gel [24] fuel combustion demonstrated that varying the initial fuel temperature (\(T_{0}\)) or ambient temperature (\(T_{g}\)) does not affect the ignition and combustion mechanisms. However, these factors are decisive for the intensity of physical and chemical processes, and, consequently, for the duration of the main stages during the ignition process [33; 35], the integral characteristic of which is the ignition delay time. The ignition delay time (\(t_{d}\)) of the composite fuel consists of several stages of the induction period [24]. The first stage (Figure 4) is the heating of the fuel accompanied by the melting (if \(T_{0}\) is below the crystallization temperature of liquid fuel components), evaporation of liquid components, and release of volatiles from the solid combustible fuel components. This stage ends with the formation of a combustible vapor-gas mixture in the vicinity of a fuel droplet. The second stage is the exothermic reaction of the formed combustible vapor-gas mixture with the oxidizer. The duration of the second stage, as a rule, is no more than 5% of \(t_{d}\)[24]. Therefore, it is advisable to reduce the first stage of the induction period in order to reduce \(t_{d}\) of composite fuels, which is much greater than \(t_{d}\) of dry solid fuels (power-station coals) due to the content of solid and liquid components, including water, in the composition. In practice, to intensify the ignition process, fuel is preheated, which, generally, increases the efficiency of the energy production cycle [36], for example, by minimizing the chemical and mechanical underburning of fuels in the combustion chambers. In this work, the influence of the initial temperature of fuels (\(T_{0}\)) on the combustion characteristics of droplets with varying the temperature of the heating source (\(T_{g}\)) was established. In real industrial boilers under different operating modes, the temperature in the furnace in the flame formation area varies from 700 \({}^{\circ}\)C (at relatively low loads) to 1000 \({}^{\circ}\)C (at relatively high loads). On this basis, our experimental studies of ignition and combustion of composite fuels at different initial fuel temperatures were conducted at temperatures of the heating source \(T_{g}\) = 700-1000 \({}^{\circ}\)C (with 100 \({}^{\circ}\)C increments). Figure 5 presents the frames of the videograms illustrating the ignition moment of the fuel droplets with the initial temperatures of \(T_{0}\) = \(-\)60 \({}^{\circ}\)C and \(T_{0}\) = 60 \({}^{\circ}\)C at the identical temperature of the heating source \(T_{g}\) = 800 \({}^{\circ}\)C for compositions No. 1-No. 4. Figure 4: Typical temperature trend in the droplet of composite fuel during heating based on [33]. Figure 5: Video frames of the ignition moment of composite fuel droplets at different initial fuel temperatures (left—\(T_{0}=-60\) °C, right—\(T_{0}=60\) °C) and at the heating source temperature of \(T_{g}=800\) °C: (**a**) composition No. 1 (40% FC + 15% rapeseed oil + 45% water); (**b**) composition No. 2 (40% FC + 15% turbine oil + 45% water); (**c**) composition No. 3 (40% coal + 15% rapeseed oil + 45% water); (**d**) composition No. 4 (40% coal + 15% turbine oil + 45% water). The video frames (Figure 5) clearly illustrate that the initial temperatures of composite fuels do not affect the mechanism of their ignition similarly to liquid or gel fuels [24; 35]. Fuel compositions, which ignite according to the gas-phase mechanism at \(T_{0}=-60\)\({}^{\circ}\)C, ignite similarly at \(T_{0}=60\)\({}^{\circ}\)C (for example, No. 1 and No. 2).