A freeboard model of an overall FBC system model where radiation between particle and gas is accounted for through Stefan-Boltzmann law was modified by incorporation of a radiation model based on method of lines solution of discrete ordinates method. The predictive accuracy of the coupled code was tested by comparing its predictions with measurements taken on a refractory-lined, hybrid, pilot scale FBC as well as with predictions of unmodified system model. Comparisons show good agreement particularly for the test with recycle of fine particles. Significance of using a rigorous radiation model was illustrated by validating its predictions against measurements available on an industrial scale fluidized bed boiler. It was concluded that coupling a rigorous radiation model with an overall FBC system model is essential for accurate prediction of the behaviour of industrial boilers characterized by low surface to volume ratio and relatively colder water walls.