Effective thermal conductivity and internal thermal radiation in burning black liquor particles

The effective thermal conductivity of burning black liquor particles is studied. It is one of the most important parameters affecting the rate of particle heating and, consequently, processes controlled by heat transfer rate, such as the devolatilization studied here. A numerical combustion simulation sensitivity analysis, combined with experimental verification, showed that in order to have reasonable agreement between experimental and modeled carbon release rates and swelling during pyrolysis, a significant addition of internal thermal radiation heat transfer must be included in effective thermal conductivity. In terms of the Rosseland mean absorption coefficient a _R in diffusion approximation for radiative thermal conductivity, the value ∼850m^-1 gave the best correlation with experiments. This corresponds to a mean penetration length of 1.2mm, which is very close to the largest experimentally observed pore size. Other available effective thermal conductivity models were also compared. For the 1-mm pores observed experimentally, these models predict values for thermal conductivity that are too small.