Assessment of improved banded model for spectral thermal radiation in presence of non-gray particles in fluidized bed combustors

In this study, Bordbar's banded model is coupled with a 3-D radiation model based on method of lines (MOL) solution of discrete ordinates method (DOM) for modeling of radiative heat transfer in freeboard of METU 0.3 MWt ABFBC test rig where a typical Turkish lignite is fired with and without fly ash recycling. The accuracy and the computational efficiency of the model were assessed by benchmarking its predictions against banded SLW model. Furthermore, in order to show the effect of gas composition on predictive accuracy of Bordbar's 10 band model, assessment of the accuracy of Bordbar's 10 band model is also tested by using gas compositions corresponding to oxy-fired combustion. Bordbar's 10 band model requires around 50 times less CPU than that of banded SLW under all conditions. Incident wall heat flux predictions of Bordbar's 10 band model are found to be in reasonable agreement with those of banded SLW and measurements in all test cases. However, Bordbar's 10 band model considerably overestimates source terms in the air-fired test case without recycle. The change in gas composition to oxy-fired combustion conditions and higher particle loads lead to significant improvement in the accuracy of source term predictions. In conclusion, Bordbar's 10 band model can be utilized to reduce CPU requirement of the spectral RTE solution for bubbling fluidized bed combustors with recycle or circulating fluidized bed combustors where radiative heat transfer is dominated by particles.