## 3 Results and Discussion
### Particles Combustion in the Channel  
#### 3.2.1 Low Temperature Smoldering Combustion  
Smoldering combustion is characteristic of particles in the channel at a furnace temperature of 600 \({}^{\circ}\)C. The combustion of the sample includes the charring and smoldering stages of the char layer of the particle. At low air velocities, smoldering that develops on the surface of the particle contributes to the ignition of volatiles. Further combustion of the particle occurs with the presence of flame on its side and back surface. When the air velocity is increased to 40-80 m s\({}^{-1}\), no flame is formed and the combustion of the sample proceeds completely as a smoldering process. The particle burns from the back to the side and front faces (Figure 9). Significant amounts of smoke are emitted from samples in the absence of a flame. Tar particles that form smoke interact weakly with air oxygen due to low temperature and low concentration of radicals. Changes in a number of particle parameters during smoldering combustion are shown in Figure 10. Particle conversion begins with a slow charring of its surface. It is only by the 27 th second of the sample being in the furnace that the char on one of the particle vertices begins to smolder on the surface of the sample. In the time interval from 0 to 54 s, volatiles are released from the sample, and the consumption of the surface char layer proceeds relatively slowly. The degree of overlapping of the conversion stages (\(X\)) in this time interval proves close to zero, with predominantly heating and charring of the particles taking place. In the time interval between 40 and 60 s, negative values of \(X\) are observed. Such values are due to the fact that the yield of fixed carbon from the charred samples exceeds the yield of carbon from raw wood. A similar phenomenon was noted earlier in [PERSON] et al. and explained by chemisorption of oxygen on the carbon surface [56]. Figure 8: Change in the relative mass of the sample, its wood core and char layer over time (T furnace—800 \({}^{\circ}\)C, air velocity—20 m s\({}^{-1}\), burning takes place in the furnace space). The vertical lines indicate the beginning and end of the cyclic movement of the flame. In the time interval from 54 to 102 s, the process of smoldering of the char layer, which spreads over the surface of the particle, evolves. Smoldering results in char consumption. The overlapping of the char residue combustion stage with the devolatilization stage is 12-19% in this case. The growth rate of the surface char layer mass slows down. By the 102 nd second of the process, the thermocouple readings reach 460 \({}^{\circ}\)C. This level of temperature is sufficient to complete the intensive release of volatiles from the particle. Nevertheless, by the 102 nd second of conversion, the wood core remains in the sample, with a weight of 49% of the initial particle weight. Under the experimental conditions, heat comes to the thermocouple junction from the holder heated in the furnace, despite the protective outer steel cover and the ceramic protection sheath that thermocouple wires run through. After 103-104 s of sample combustion, the char layer smoldering process spreads to all side and back faces of the particle, which further leads to a simultaneous increase in the rate of consumption and formation of the char layer. The mass of this layer peaks at 7% by the 114 th second of sample combustion. It is noteworthy that at the maximum yield  
Figure 10: Changing of parameters of particle smoldering process (T furnace—600 \({}^{\circ}\)C, air velocity—40 m s\({}^{-1}\), combustion in the channel): \(m\)—relative mass of the charred particle, \(m_{cure}\)—mass of particle wood core, \(m_{chur}\)—mass of char layer, \(X\)—degree of overlapping of stages; \(i_{ign.