Coupling radiative properties with detailed char conversion kinetics

Abstract

In pulverized solid fuel combustion, radiation is the primary heat transfer mechanism. Especially in the near-burner region with high particle loadings, particle radiation dominates gas radiation. Thus, adequate modeling of particle-radiation interactions in full-scale simulations of combustion chambers may primarily affect the overall accuracy. The radiation behavior depends strongly on the particles? optical properties, characterized by the complex index of refraction (IOR). The IOR depends on particle morphology and material composition which change during pyrolysis and char conversion. For char conversion, the focus of the present paper, the change depends on how the structural transformations take place inside the particle, characterizes by the reaction regimes, boundary layer diffusion limited, pore diffusion limited or kinetically controlled. The formation of an outer ash film layer with progressing burnout can complicate the situation even more.To quantify the effects of the structural changes during the char conversion process on the radiative properties, the output data of the char conversion kinetics (CCK) model (particle size, ash/carbon content, pore fraction) are used to build artificial particles at different conversion degrees and for the different regimes. Finally, the radiation interaction is calculated by applying an extension of the Mie theory for coated particles, and these results are compared to simplified approaches.Simplified approaches were tested to derive the IOR by applying mixing rules based on the available char and ash content with progressing char conversion. The results indicate that only a non-linear mixing rule for the IORs of ash and char lead to appropriate approximations of the radiation behavior of the particle. Furthermore, it is not necessary to model the spatial arrangement of carbon core and ash film for the radiative properties. Modeling a sphere and calculating an effective IOR depending on the carbon, ash, and pore volume fraction results in an average deviation of less than 3% to the exact solution.