Graphitization: Microstructural and microtextural transformations of residual char from international coal combustion ash

Natural graphite is a critical raw material and its substitution in some applications will contribute to reduce its supply risk. Coal ash, which is still produced in high amounts, contains a carbonaceous solid residue (char) that may be a substitute of natural graphite. For this purpose, char may be graphitized and used in electrocatalysis reactions, however char properties depend on coal rank, composition and combustion conditions that affect the graphitization process. Research on industrial coal char graphitization is limited, resulting in restricted comprehension of the contributing factors. In order to provide novel insights, industrial residual chars (derived from Poland, Portugal, Romania and South Africa) subjected to high-temperature treatments (carbonization at 1000 °C followed by treatment at 2600 °C under environmental pressure) were examined regarding their microstructural and microtextural transformations. The samples were studied by focussing on its heterogeneity, namely its specific properties and interactions, including elemental composition, optical character, and structure. Furthermore, the evolution of structural order of selected char morphotypes in samples derived from coal of similar rank was assessed using Raman microspectroscopy.

The results highlighted a set of aspects that could have influenced the transformations experienced by the different chars during high temperature treatments (graphitization). It was found that Hydrogen might play a role in the graphitization ability of isotropic chars derived from low rank coal. The prior preferential orientation of the Basic Structural Units (BSU) contributes to achieve a higher graphitization degree, but it can be enhanced or hindered by other factors, e.g., hydrogen content. Raman microspectroscopy showed that char morphotypes from the same group underwent different transformations during graphitization, indicating that morphotypes or sections with unfused optical character may be more prone to graphitization. However, the existence of disordered domains and specific microtextures, such as polyhedral pores identified under TEM, likely hindered further graphitization. Further research on this topic is needed.