Carbon-Free Ammonia Co-firing in Fluidized Bed Combustion: Analysis of Advanced Chemical Modeling and Detailed...

Current proposals to achieve carbon reduction through the co-firing of ammonia and coal in fluidized bed boilers necessitate further research into the combustion characteristics of ammonia under fluidized bed conditions. Ammonia co-firing has primarily been explored using either detailed combustion mechanisms in pulverized coal boilers or coal combustion reactions in fluidized beds. In this study, we performed experiments and detailed mechanism simulations of coal and ammonia co-firing in a bubbling fluidized bed (BFB) combustion apparatus (operating temperature: approximately 1123 K) and conducted experiments at various primary air (PA) and secondary air (SA) ratios. We constructed a 1-D simulation reactor, mimicking a fluidized bed reactor, based on an equivalent reactor network. We also used three chemical reaction models, the Okafor, Nakamura, and modified mechanisms, to simulate NH₃ combustion reactions in the BFB. Under variable PA and NH₃ ratios, NO emissions in both models were primarily controlled by NH₂, O, and HNO radical reactions. Reaction pathway analysis revealed that both mechanisms exhibited similar characteristics for NO formation and reduction. These results indicate that during the initial NH₃ oxidation reaction, HNO production is governed by NH₂ and O radicals, as illustrated by reactions, such as NH₂ + O = HNO + H and NH₂ + NH₂ = N₂H₂ + H₂, which affect NO formation. Thus, the advanced model for NH₂ and HNO reactions can better simulate ammonia oxidation at lower temperatures. This enhancement aids in bridging the gap between experimental results and simulations, particularly under fluidized bed conditions, providing important theoretical support for planned ammonia/coal co-firing in fluidized bed boilers.