3D modeling of ammonia co-firing in a 50 kWth CFB test rig: NO pathways and environmental implications for carbon-free power generation

Ammonia is increasingly regarded as a promising carbon-free fuel for coal-fired power generation, yet its behavior in circulating fluidized bed (CFB) combustion remains insufficiently understood. This study develops a three-dimensional computational particle-fluid dynamics (CPFD) model for a 50 kWth pilot-scale CFB test rig and provides the first validation against experimental data for coal combustion and 20 % ammonia cofiring. The model reproduced pressure profiles, temperature distributions, and flue gas compositions. Based on this validation, simulations were conducted for ammonia cofiring ratios of 0–30 % to assess effects on hydrodynamics, combustion, and emissions. Global CFB hydrodynamics, including pressure drop, temperature, and solids circulation, were largely unchanged up to 30 % cofiring. Increasing ammonia reduced CO2 and SO2 nearly linearly, while increasing H2O. CO2 declined in proportion to NH3 share, reaching about a 26 % reduction at 30 %. NO showed a nonlinear pattern: sharp rise near the dense bed, reduction through selective non-catalytic reduction (SNCR), and rebound at higher ratios. Among 0–30 % cofiring, 20 % NH3 yielded the lowest NO emissions among co-firing cases, about 13 % lower than the 10 % case and about 5 % higher than coal combustion, while still providing about a 17 % reduction in CO2 relative to coal combustion. These results demonstrate stable coal-ammonia cofiring up to 30 % in a CFB and provide guidance for designing low-NOx, carbon-free strategies in future large-scale plants.