Experimental studies of wood particles combustion in an intensive air flow

Wood fuel gasification in the downdraft process is characterised by specific modes. In comparison with traditional modes, these do not have a pyrolysis zone in the fuel bed that precedes the fuel oxidation zone. Fuel is supplied into the oxidising zone without charring, where it reacts with the intensive cold air flow from tuyeres. This study aims to simulate conversion of particles in the gasifier close to the tuyeres. For this purpose the particles are burned in the muffle furnace space and the ceramic channel simulating the presence of other bed particles at a first approximation. Burning of particles in the furnace space allowed determining different mechanisms of their combustion. A two-stage process is observed in the range of tuyere velocities below 20 m s-1 and at its first stage the volatiles escape from the particle and burn. At the tuyere velocities above 125 m s-1 combustion of particles is realised primarily as a single-stage process. The intensive air flow reaches the fuel particle surface and initiates combustion of the surface charcoal layer. In this case the stages of devolatilisation and char residue combustion run concurrently. The combustion velocity of particles in the heated muffle furnace space increases steadily with the air flow rate (in the studied range). The combustion velocity of particles in the ceramic channel increases first and then decreases with the growing air velocity in the tuyere. These regularities are explained by the cooling effect of the air flow on the particles. Presumably, this cooling effect can be observed while gasifying the fuel bed, which leads to decrease in the cold gas efficiency and increase in the tar concentration in the producer gas. At air velocity above 95 m s -1, fragmentation of fuel particles commences. A layer of charcoal formed at an initial stage of burning heats up in the intensive air flow and is separated from the particle surface.