Evaluating the Thermal Extrusion Behavior of a Coking Coal for Direct Carbon Fiber Production

This study outlines a novel thermal extrusion system and methodologies for evaluating the potential to manufacture carbon fiber directly from thermoplastic coals. It is envisioned that the intermediate product will be further refined by spinning down to commercial fiber sizes and thermal annealing. Commercial melt spinning is used for manufacturing carbon fibers from pitch-based feed materials, and a similar approach for plasticized coal is likely to be a lower risk option. However, the critical aspect of using coal for this purpose is its behavior inside a higher pressure extrusion unit and the need to characterize its rheology. This work has evaluated the thermoplastic development needed for extrusion of a single coking coal in terms of the heating rate and residence time and characterized the extruded fiber product. It was observed that the coal underwent a preliminary softening phase prior to extruding at significant speed. This phase appeared necessary to develop the critical viscosity for extrusion and was affected by the heating rate. The size of the orifice that the coal was extruded through also impacted the point of extrusion, with the smaller 0.5 mm hole requiring lower viscosity to be developed to flow at steady state. Other operating modes were developed to examine the thermoplastic properties of the coal over an extended residence time, and it was found that the coal could be maintained up to 60 min at selected temperatures. The product fiber was larger than the commercial size, appearing slightly larger than the orifice size. Internal porosity and surface roughness were observed as coal-based fiber qualities in need of controlling, along with the mineral content and size.