Effects of hydrogen donor additives on the coking properties of high-temperature coal extracts

Abstract

Refcoal is a carbon precursor obtained by alkali-mediated extraction of coal with aprotic solvents such as DMF. Refcoal can be converted into a graphitic material through appropriate heat treatment. Graphitisable materials require the development of an intermediate liquid crystalline mesophase. Thus formation of a mesophase during the carbonisation of Refcoal is essential for obtaining highly graphitisable anisotropic cokes suitable for nuclear graphite applications. Anisotropic carbons are even more important in other commercial and industrial application because of their distinctive properties. The formation of anisotropic carbon depends on the nature of the parent precursor, temperature and carbonisation conditions, especially the molecular mobility during the mesophase stage. High-temperature extraction of coal produces Refcoal that yield cokes with a low level of anisotropy. Good control of the mesophase stage during carbonisation may lead to the development of anisotropic cokes. Hydrogen donor additives increase the molecular mobility in the liquid phase by stabilising the free radicals formed by thermal decomposition of coal. Hydrogen donor additives also increase the temperature range over which fluidity occurs, thereby allowing the formation of large sized mosaic structures. Mittal pitches (CTP) and tetralin were examined for their effectiveness as hydrogen donor additives. Refcoal blends containing 10 to 50% by mass additive were prepared by mixing and carbonisation conducted at temperatures ranging form 400 to 1000 °C. Samples were analysed using thermogravimetric analysis (TGA), diffuse reflectance infrared Fourier transform (DRIFT), optical microscope techniques, scanning electron microscopy (SEM), Raman spectroscopy and X-ray diffraction (XRD). It was found that addition of at least 10% tetralin improves the optical texture of Refcoal cokes and also increases the carbon yield. In both tetralin and pitch addition, 20% additive gave coarse circular anisotropic cokes after carbonisation at 650°C. However the carbon yield was reduced for the Refcoal coke treated with Mittal pitch. In fact the yield roughly decreases with an increase in pitch content. Although the cokes from pitch treated Refcoals showed a low ratio of d/g (where d indicates the degree of disordering and g the degree of ordering), Raman and XRD results indicate little further improvement in crystallinity but SEM showed development of a smooth morphology with increasing pitch content. Previous literature results suggest that this indicates that the material must have passed through a mesophase stage. Surprisingly, the addition of tetralin yielded cokes with high ratio of d/g, but improved the carbon yield and optical anisotropy. Increasing tetralin content in the carbonising system increased the size of anisotropic textures but the SEM micrographs show rough morphology with compounds that appear to inhibit mesophase spheres coalescence.