Char Contribution as Heterogeneous Reaction To NO Reduction During Rice Husk Reburning in a Drop Tube Furnace

Abstract

Biomass reburning has emerged as a promising method for reducing nitrogen oxide (NO) emissions. However, a comprehensive understanding of reburning reactions using biomass remains limited. In particular, the char contribution to NO reduction requires further clarification, especially when using rice husk. This study investigates the contribution of rice husk char to NO reduction during the reburning process in a drop-tube furnace. The effects of reaction temperature, NO concentration, and equivalence ratio on the contribution of the homogeneous and heterogeneous reactions were analyzed. Results showed that increasing the reburning temperature enhanced the heterogeneous reaction by promoting CO release from the char surface, thereby increasing the char’s contribution to the overall reburning reaction. As the initial NO concentration increased, NO reduction increased for both rice husk and its char; however, due to the limited amount of fuel for reburning reactions, the rate of increase in NO reduction decreased. The homogeneous reaction decreased above a certain equivalence ratio because oxygen limitation reduced the conversion of intermediate species into N₂. In contrast, the char reaction increased and eventually reached saturation with an increasing equivalence ratio, further enhancing char contribution. These findings provide valuable insights for optimizing biomass reburning systems, particularly by clarifying the distinct role and operational dependencies of heterogeneous char-NO reactions.