###### Abstract  
Downdraft gasification is a promising process of energy conversion of wood biomass. There are such fuel conversion conditions that differ favorably from conventional conditions. In such conditions, there is no pyrolysis zone in the fuel bed, which precedes the oxidation zone. Fuel is supplied into the oxidizing zone without charring, where it reacts with the intensive cold air flow from tuyeres. The study aims to replicate the conversion of particles in a gasifier close to tuyeres. For this purpose, the individual particles are burned in the muffle furnace space and the quartz channel replicating presence of other bed particles at a first approximation. In the experiment, the furnace temperature was varied, as well as the velocity of air supplied to the particle. Two-stage and single-stage mechanisms of particle combustion were identified. A two-stage process is observed in the range of tuyeres velocities below 20 m s\({}^{-1}\). The two-stage mechanism is characterized by a stage of devolatilization and volatiles combustion, followed by a stage of char residue combustion. The stages are predominantly separate from each other, and their degree of overlapping is low, amounting to 24%. At the tuyere velocities above 125 m s\({}^{-1}\) combustion of particles is realized primarily as a single-stage process. The intensive air flow reaches the fuel particle surface and initiates combustion of the surface char layer. In this case, the stages of devolatilization and char residue combustion run concurrently for the most part. In the single-stage mechanism, the degree of stage overlapping is significantly higher and amounts to 60-95%. For the two-stage combustion mechanism, the effect of cyclic movement of the flame across the particle surface is evident. The number of cycles can reach eight. This effect is due to the change of conversion stages. At air velocity above 95 m s\({}^{-1}\), fragmentation of fuel particles commences. A layer of char formed at an initial stage of burning heats up in the intensive air flow and is separated from the particle surface. The heated walls of the quartz channel contribute to the intensification of particle combustion. This effect is probably due to the swirling of the flame between the wall and the particle surface. biomass; downdraft gasification; single particle; combustion; mechanism; stages overlapping; fragmentation +
Footnote †: journal: Applied Sciences  
the process of biomass gasification. In some cases, it proves more attractive than pyrolysis and combustion technologies from a technical and economic standpoint [8]. For small-scale power generation, biomass downdraft gasifier units are promising. Reactors of the downdraft type are characterized by simplicity of design and use, as well mathematical model allowed the authors to establish a significant overlapping of stages for particles smaller than 15 microns, a slight overlapping for particles ranging from 15 to 65 microns, and staged combustion for particles larger than 65 microns. [PERSON] et al. later refined the [PERSON] and [PERSON] data. Detailed kinetic modeling attests to the overlapping of volatile and coke residue combustion stages even for coal particles 100 microns in size [32]. Fuels with high volatile yields, such as biomass, are characterized by staged combustion, even for relatively small particles of 53-75 microns [33] and 0-100 microns [34]. However, spectral analysis of the radiation of burning biomass particles of 224-250 microns revealed a slight overlapping of the stages of homogeneous and heterogeneous combustion [35]. Large biomass particles burn mostly in separate stages. [PERSON] et al. found that the volatile matter flux from the particle surface is an order of magnitude greater than the flux of oxygen diffusing to the particle surface [36]. The volatile matter flux protects the solid surface of the particle from oxidation by oxygen. [PERSON] et al.