## 3 Results and Discussion
### Particles Combustion in the Channel
#### 3.2.3 Single-Stage Particle Combustion Mechanism  
The process of particle combustion at a furnace temperature of 800 \({}^{\circ}\)C and air velocity of 125 m s\({}^{-1}\), has common features with the aforementioned smoldering combustion. From the moment the sample is put into the furnace until the fifth second, the sample is heated and charred. Then the individual dots on the side and back faces of the particle begin to smolder and glow. By the 22 nd second, the glow area spreads along the entire length of the faces, and by the 43 rd second, it covers all the side and back faces. The change in the relative mass loss of the sample (\(m\)) is consistent with the propagation of the glow region; the conversion process accelerates with the increase in the smoldering surface of the particle (Figure 12).  
Charred wood fuel samples tested under these conditions and taken out of the furnace at different times consist almost entirely of an unreacted wood core surrounded by a thin layer of char (Figure 12). Combustion is characterized by a small degree of char accumulation on the particle surface and a significant overlapping of the char residue combustion stage with the devolatilization stage. The degree of overlapping of the process stages at a furnace temperature of 800 \({}^{\circ}\)C and air velocity of 125 m s\({}^{-1}\) ranges from 40 to 68%.  
The degree of overlapping of combustion stages can be estimated not only for the overall process running from the beginning of sample input to the time \(t\), but also for  
Figure 12: Characteristics of single-stage combustion of wood particles, T furnace—800 \({}^{\circ}\)C, air velocity—125 m s\({}^{-1}\): \(m\)—mass of the charred particle, \(m_{cure}\)—mass of particle wood core, \(m_{char}\)—mass of the char layer, X—degree of overlapping of stages; \(t_{ign.}\)—average time of particle ignition start, s; \(T_{c}\)—temperature of the sample center, \({}^{\circ}\)C.  
different time intervals. For example, the initial period of sample conversion time can be excluded from the calculation of \(X\). During this period, the charring of the particle occurs, and the smoldering of the charcoal layer begins and evolves. The initial period is transitional and does not fully characterize the course of the main combustion process. If it is excluded, the degree of stage overlapping increases at various time intervals up to 60-95% (Figure 13). A significant overlapping of the conversion stages allows us to characterize this kind of sample combustion process as predominantly single-stage, in which the consumption of most of the char occurs simultaneously with the process of heating and wood pyrolysis.  
In the investigated single-stage process, the flame on the surface of the particle and in the inert channel behind it is visually absent, and smoke, which is gas sol from the tar particles, comes out of the exhaust port of the rig. The cumulative particle conversion process is characterized by a high equivalence ratio of 6.3. The air flow rate far exceeds the needs of the combustion process in the oxidizer. The products of combustion are greatly diluted with cold air, and the intense flow of the latter quickly carries the products out of the furnace. The low temperature and relatively short residence time of the vapor-gas conversion products in the furnace result in a high tar content in the exhaust.  
The equipment that consumes producer gas can have high standards with respect to gas quality. Depending on the technology of gas use, the amount of tar in it should not exceed the level of 0.1-100 mg/m\({}^{3}\)[57; 58; 59]. Fuel gasification conditions with insufficient tar conversion can also take place in downdraft gasifiers operating with intensive air supply to the reactor. The capacity of gasifiers of this type is limited by the 1.4-2.5 MW range with respect to fuel and is associated with the unsatisfactory quality of the gasifier gas obtained by scaling up and boosting the reactor of the unit [60; 61; 62].