This dataset comprises a comprehensive time-series record of flue gas composition measurements obtained from a fluidized bed boiler operation, with data points collected at 10-second intervals over a 40-minute period. The measurements encompass a wide range of gaseous components expressed in volumetric percentages (vol-%) and parts per million (ppm), including water vapor (H2O), carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NO, NO2, N2O), ammonia (NH3), hydrogen chloride (HCl), sulfur dioxide (SO2), methane (CH4), ethene (C2H4), hydrogen fluoride (HF), oxygen (O2), and hydrogen bromide (HBr). This detailed composition profile provides valuable insights into the chemical processes occurring within the boiler during combustion and enables assessment of emission characteristics and operational efficiency of the fluidized bed combustion system.