## 4 Conclusions  
As part of the work, visual observation of combustion and measurement of the temperature of the center of the particles were carried out. The particles were burned in the furnace space, and the furnace temperature and air velocity were varied in the experiments. This study showed the existence of two mechanisms of particle combustion, namely two-stage and single-stage. In the range of tuyere air velocity below 20 m s\({}^{-1}\), the two-stage process occurs. Volatiles are released from the particle and bum at the initial stage of this process. Flame around the particle, with the help of the flow of conversion products, limits the diffusion of oxygen to the char surface of the particles and also chemically binds it during the oxidation of combustible components. Char residue burns at the second stage of the process. At the air velocity above 125 m s\({}^{-1}\) a single-stage mechanism of single particles combustion prevails. Intensive air flow reaches the fuel particle surface and causes burning of a surface layer of char. The stages of devolatilization of fuel and that of char residue combustion occur simultaneously. In single-stage combustion, there is no flame around the particles. At the tuyere air velocity above 60-95 m s\({}^{-1}\) the fragmentation of fuel particles commences. The surface layer of char formed on the particle surface starts intensively reacting with the air oxygen, which may cause its rapid heating and separation from the particle. In this study, combustion conditions were found under which cyclic movement of the flame across the surface of the particle is observed. This process takes about one third of the total combustion time of the particle and consists of 6-8 cycles. Cyclicity might be due to the changing stages of combustion of volatiles and char residue. If there is a flame near the surface of the particle, the release of volatiles slows down over time. This deceleration leads to a flame blowout and intense subsequent combustion of the char layer. This layer heats the particle's wood core and intensifies the pyrolytic processes in it. An increase in the release of volatiles in turn contributes to the return of the flame to the surface of the particle. The cyclic process takes place during the period of change of the devolatilization to the char residue combustion stage. The combustion of particles in the channel at low temperature (600 \({}^{\circ}\)C) proceeds with a prolonged charring stage of the sample and the subsequent spread of smoldering over the char layer. At air velocities up to 40-80 m s\({}^{-1}\), flame formation around the sample is observed. At high air velocities, there is no flame, and the process proceeds with the formation of large amounts of smoke, the particles of which consist mainly of tar. The degree of overlapping of stages proposed in the study provides a quantitative assessment of the combustion conditions and allows us to determine the mechanism of the process. The two-stage process of particle combustion in the channel is characterized by a relatively small degree of stage overlap, amounting to 24%. The observed process is far from the ideal case in which there is complete separation of the stages, and it can be characterized as predominantly two-stage. By contrast, the single-stage process is characterized by a stage overlapping degree of 60-95% for different combustion time intervals. This process is also far from ideal and can be characterized as predominantly single-stage since there is always a small layer of burning char on the surface of the particle. The heated walls of the channel intensify the process of particle combustion. Such an effect is probably due to the swirling of the flame between the particle and the channel. **Funding:** The research was carried out under State Assignment Project (no.