## 2 Experimental Investigation
### Experimental Setup for Fuel Combustion  
The characteristics of the ignition process of both individual fuels and mixtures were studied in an experimental setup (Figure 2) according to a tested method [16].  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Fuels**} & \multicolumn{4}{c}{**Proximate Analysis**} & \multicolumn{4}{c}{**Ultimate Analysis**} \\ \cline{2-9}  & \(\mathbf{W^{a}}\) & \(\mathbf{A^{d}}\) & \(\mathbf{V^{daf}}\) & \(\mathbf{HHV}\) & \(\mathbf{C^{daf}}\) & \(\mathbf{H^{daf}}\) & \(\mathbf{N^{daf}}\) & \(\mathbf{S^{daf}}\) & \(\mathbf{O^{daf}}\) \\ \cline{2-9}  & & \% & & \(\mathbf{M}\)J kg\({}^{-1}\) & & & \% & & \\ \hline C & 4.1 & 11.8 & 44.0 & 31.00 & 77.5 & 5.2 & 2.1 & 0.6 & 14.6 \\ \hline B & 3.7 & 3.4 & 79.5 & 23.42 & 56.3 & 6.7 & 0.3 & 0.1 & 36.6 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Proximate and ultimate analysis of tested samples (air-dry basis).  
Figure 2: Appearance of the experimental setup: 1—air fan; 2—air heater; 3—muffle furnace with quartz tube; 4—air cooler; 5—exhaust ventilation; 6—heated air recirculation system; 7—mechanism for introducing finely dispersed fuel particles into the heated airflow; 8—thermocouples; 9—multipoint recorder; 10—control panel; 11—power supply board; 12—exhaust ventilation control unit.  
Air fan 1 ROBUST (LEISTER, Wuppertal, Germany) together with air heater 2 LEISTER LE 5000 HT (LEISTER, Wuppertal, Germany) pumps heated air into a quartz tube. Muffle furnace 3 General Therm RT 1000.1100 SP (Nevaterm, Saint Petersburg, Russia) maintains uniform temperature distribution along the quartz tube. The airflow temperature was recorded by four thermocouples 8. The thermocouple readings located at the outlet pipe of air heater 2 were used as feedback when regulating the temperature of the heated airflow. The readings of the other three thermocouples were recorded by a multipoint recorder, 9 RMT-59 (Elemer, Moscow, Russia). The air cooler 4 is designed to reduce the temperature of the airflow and remove flue gases from the atmosphere using exhaust ventilation 5. The heated air recirculation system 6 serves to remove heat from the air cooler and increase the efficiency of the experimental setup by supplying heated air to the air fan 1 inlet. The main technical characteristics of the used equipment are presented in [16].  
A series of 5-10 experiments were carried out for coal, biomass, and mixtures based on these components at constant temperatures (\(T_{a}\)) of the heated airflow. Using an automated mechanism 7, a fuel portion of about 5 mg was introduced into the airflow through a ceramic channel. The processes occurring during the movement of finely dispersed solid particles inside a quartz tube were recorded by a high-speed color video camera, the Phantom V411 (Vision Research, New Jersey, USA), complete with a wide-angle lens, the Distagon 1.4/35 ZF2.2 T* (Carl Zeiss, Gottingen, Germany). Automated processing of video recordings to determine \(t_{d}\) was performed using the standard software Phantom Camera Control 3.0 (Vision Research, New Jersey, USA) according to a tested method [16]. The systematic and random errors in determining \(t_{d}\), due to the video recording speed and the scatter of experimental data, did not exceed 0.5% and 15%, respectively.