Mileva N.Zlateva P.Ivanov M.Krumov K.Terziev A.Comarla A.2026-01-202026-01-202026-01-202026-01-202025-10-012673-411710.3390/eng6100257SCOPUS_ID:105020007096https://rlib.uctm.edu/handle/123456789/1908This study explores the combustion behavior of three biomass pellet types—wood (W), sunflower husk (SH), and a mixture of wood and sunflower husks (W/SH)—in a residential hot water boiler. Experiments were carried out under two air supply regimes (40%/60% and 60%/40% primary to secondary air) to measure flue gas concentrations of oxygen (O2), carbon monoxide (CO), and nitrogen oxides (NOx). The results indicate that SH pellets generate the highest emissions (CO: 1095.3 mg/m3, NOx: 679.3 mg/m3), while W pellets achieve the lowest (CO: 0.3 mg/m3, NOx: 194.1 mg/m3). The mixed W/SH pellets produce intermediate values (CO: 148.7 mg/m3, NOx: 201.8 mg/m3). Overall boiler efficiency for all tested fuels ranged from 90.3% to 91.4%. Numerical simulations using ANSYS CFX (2024 R2 (24.2)) were performed to analyze temperature distribution, flue gas composition, and flow fields, showing good agreement with experimental outlet temperature and emission trends. These findings emphasize that both pellet composition and air distribution significantly influence efficiency and emissions, offering guidance for optimizing small-scale biomass boiler operation.enArticle