Heat transfer within highly porous chars: A review

The review summarizes heat transfer mechanisms for highly porous chars. It provides a brief discussion on global models for thermal conductivity that consider mostly total porosity and a more detailed discussion of models considering details of the microstructure and phonon heat transport within microcrystals. The review includes recent methods for experimental determination of thermal conductivity of single small particles. A description of how thermal conductivity of highly porous chars changes under oxidation, in the range of zero to full burnout, was provided. This review emphasizes the necessity of a detailed description of all aspects that may contribute to the thermal conductivity of highly porous chars, such as macro- and microstructures, microcrystal orientation, microcrystal dimensions and connectivity.