Thermal radiation accounts for the majority of the heat transfer in solid fuel fired combustion systems due to contributions from particle laden combustion gases hence gas and particle radiative properties play an important role on radiative heat exchange in such systems. Therefore, modelling of radiative heat transfer is of considerable importance and necessitates not only accurate but also computationally efficient radiative property estimation methods for both combustion gases and particles.

Under the operating conditions of circulating fluidized bed combustors (CFBC), it has been shown that gas radiation is suppressed by the particles and particle radiation dominates the total heat transfer in the combustor. In fact, particle radiation depends on composition, density, temperature, concentration and size distribution of particles. Particle concentration is usually recognized as the determining factor for both radiative and total heat transfer in the relevant CFBC literature.

Even though much effort has been placed in understanding the effect of particle concentration on radiative heat transfer, evaluations of the accuracy of previously developed models raised

several issues. In one of their recent publications, Ates et al. (2018) have addressed these issues and investigated the influence of particle concentration distribution on radiative heat flux and source term predictions for the first time in a rigorous radiation model covering particle sizes and conditions of interest for CFBCs. Investigations were carried out for a combustion test previously performed in 150 kWt lignite-fired CFBC test rig in METU Chemical Engineering Department [1]. Comparisons reveal that commonly used 0D and 1D representations of particle concentration distributions lead to significant errors and rigorous evaluation of particle concentration distribution is essential for accurate prediction of radiative heat transfer in CFBCs.

C. Ates, N. Selçuk, G. Kulah, Significance of Particle Concentration Distribution on Radiative Heat Transfer in Circulating Fluidized Bed Combustors, International Journal of Heat and Mass Transfer, Vol. 117, pp. 58-70, 2018. (https://doi.org/10.1016/j.ijheatmasstransfer.2017.09.138)

[1] A. Batu, N. Selcuk, G. Kulah, Design assessment of a 150 kWt CFBC Test Unit, Experimental Thermal Fluid Science, Vol. 117, pp.275-281, 2010 (https://doi.org/10.1016/j.expthermflusci.2009.10.006).

Dense bed pictures during the operation of 150 kWt CFBC Test Rig