Combustion experiments with spherical and cubical biochar particles in static and agitated beds

The combustion behaviour of spherical and cubical char particles in static and agitated fuel beds is experimentally examined. Beech wood spheres and cubes (10 mm initial size) were pyrolyzed at 1173.15 K to prepare biochar particles. Bulk samples of these char particles with a constant initial fuel bed height are burned in a batch-operated reactor (BORA) at four primary air flow rates (2.16 to 11.85 g/s), with the air flowing upward through the bed. The bed can be agitated by vertically moving grate elements. Bed mass is measured continuously during conversion using a balance. The results from fixed beds were compared with those from agitated beds. Mass loss rates, flue gas composition (O 2 , CO, CO 2 ), and temperatures were continuously monitored to evaluate conversion rates. The mass loss rate during combustion increases with higher mass flow of air, regardless of whether the bed is static or agitated. For spherical particles, agitation reduces the mass loss rate by 17 % to 26 % compared to the static bed. The effect is less pronounced for cubical particles, where the mass loss rate decreases by only 2 % to 13 % under agitated conditions. The lower mass loss rate observed for cubical particles could be attributed to their reduced mobility compared with spherical particles. It is suggested that the lower reaction rate observed in agitated beds arises from a shift from mass-transfer-limited conversion in static beds to a more kinetically controlled conversion in agitated beds.