This study presents a computational fluid dynamics (CFD) analysis of biomass co-firing in an industrial rotary kiln as a pathway toward reducing fossil fuel dependency and supporting decarbonization strategies. A three-dimensional multiphase model was developed in ANSYS Fluent, where biomass was represented as multi-component particles undergoing sequential processes of drying, pyrolysis, and char oxidation. Two operating scenarios were evaluated: (i) a baseline case with natural gas as the sole fuel, and (ii) a co-firing case in which 10% of the thermal input was provided by carbon-neutral biofuels. The results highlight the influence of biomass addition on the thermal field and particle behavior. While a slight reduction in peak flame temperature was observed after biomass injection, the overall temperature distribution and bed heating profiles remained stable and adequate for process requirements. Particle-scale analysis confirmed complete thermal conversion, with moisture release, volatile consumption, and char oxidation leading to full burnout within the residence time. These findings demonstrate the technical feasibility of partial biomass substitution in rotary kilns and emphasize the value of CFD modeling as a predictive tool for evaluating combustion stability, particle conversion, and process efficiency in high-temperature industrial systems.