## 3 Results and Discussion
### _Furnace Space Particle Combustion_
#### 3.1.3 Cyclic Movement of the Flame across the Particle Surface  
Combustion of particles in the furnace space at air velocity of 20 m s\({}^{-1}\) and furnace temperature of 800 \({}^{\circ}\)C is accompanied by an interesting effect. This effect consists in the  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**T Furnace, \({}^{\circ}\)C**} & \multicolumn{6}{c}{**Air Flow, m s\({}^{-1}\)**} \\ \cline{2-7}  & **20** & **40** & **50** & **60** & **95** & **125** \\ \hline
800 & 0 & 0 & 0 & 0 & 100 & 80 \\
1000 & 0 & 0 & 0 & 50 & 100 & 100 \\
1200 & 0 & 0 & 0 & 0 & 100 & 100 \\ \hline \end{tabular}
\end{table}
Table 1: Fragmentation event rate (%) as a function of tuyere air velocity and furnace temperature.  
Figure 6: Particle fragmentation and preceding stages. Time elapsed after sample placing into the furnace is given under the photos. On the right of the photos a part of a type A holder is seen. (**a**) 4.3 s, (**b**) 4.5 s, (**c**) 5.0 s, (**d**) 5.4 s.  
cyclic movement of the flame from the side face of the particle to its back face, and back to the side face (Figure 7).  
After the ignition of the particle, its surface is surrounded by flame, except for the front face. This facet faces the tuyere and is perpendicular to the axis of the air jet. The first movement of the flame is observed at 20.5 s from the start of the sample placement into the furnace. The air flow blows the flame off the side face, to which the flame returns after a certain time (21.1 s) and the cycle repeats again. The number of cycles varies from sample to sample and ranges from 6 to 8. The duration of the entire cyclic process takes about one third of the total combustion time of the particle (Table 2).  
After the first movement of the flame, there is a decrease in the combustion rate of the particle. This can be seen from the change in the slope of the mass loss curve in the time interval from 18 to 24 s (Figure 8). It is worth assuming that such a decrease in the combustion rate is due to the movement of the flame away from the side face of the particle. The flame is the source of heat and charged particles needed to sustain the conversion process. In the absence of a flame, the char surface becomes exposed to oxidation by air, begins to react with it, and incandesces. Incandescing of the surface leads to heating up of the inner layers of the particle and intensification of their pyrolysis process with the release of volatiles to the surface. The mass loss rate of the particle increases again, and the volatile matter flame returns to side faces.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Magnitude** \\ \hline Total number of cycles & 6–8 \\ Cycle time, s & 2.2–5.2 \\ Total cyclic process time, s & 18.5 \\ The same, relative to the total combustion time of the particles, \% & 33 \\ T center, \({}^{\circ}\)C: & \\ - at the beginning of the cyclic process & 260 \\ - at the end of the cyclic process & 470 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Characteristics of the cyclic process.  
Figure 7: One of the cycles of changing the position of the flame during combustion of the particle (T furnace—800 \({}^{\circ}\)C, air velocity—20 m s\({}^{-1}\), type B holder without steel cover).  
It should be noted that the middle of the time range of the cyclic process coincides with the maximum amount of char on the particle surface (Figure 8). It is likely that the cyclic movement of the flame is related to the change of combustion stages. The beginning of this process is due to the weakening of the sample devolatilization, and its end is marked by the completion of the devolatilization stage.