The effect of coal quality on carbon products produced with alternative thermal processing – extrusion and direct casting technologies

This study has investigated two industrially relevant methods of heating coal with the goal of producing “shaped carbon” products. Thermal extrusion was assessed as a means of producing a continuous carbon material for blast furnace use as was direct casting as a means of producing mould-shaped carbon tiles for use in energy storage. The results indicated that both heating methods produced promising “proto-type” products; suggesting the feasibility of the processes.

The thermal extrusion work package involved an initial scoping study on a laboratory extrusion system and a series of trials on a pilot extrusion unit (15kg/hr nominal feed rate). It was found that the higher fluidity coals above 500ddpm generally could be extruded to various extents on the laboratory system. A number of the coals also produced a residual “thermo-pressed” material that was highly densified.

High fluidity coal was selected for further extrusion trials involving a commercial biochar. Mixtures of 10% and 25% biochar were tested with the coal on the laboratory system then replicated on the pilot unit. It was found that pellets of 20mmOD could be produced on the pilot unit from 100% coal and blends with 10% and 25% biochar. Pellets of +60mmOD were also produced from the coal by itself in a bid to produce commercial lump size coke. Reactivity tests on the carbonised samples suggested that the biochar blends were less reactive than weighted models predicted. From this work, it was theorised that the higher pressures and shearing were potentially forcing coal volatiles into pore spaces of the biochar rending it less reactive.

The direct casting work package involved casting powdered coal into rectangular moulds and carbonising at slow heating rate. It was observed that the lower fluidity coals (below 200ddpm) generally produced an adequate carbon tile. A series of activation trials and reactivity testing on the TGA were undertaken. The activated tiles were then assembled as simple supercapacitor cells and tested for their energy storage potential. It was found that the specific energy storage values varied between 0.6-5.2 Wh/kg with efficiency ranges 29-65%. As proto-type cells, these were considered very reasonable compared with commercial supercapacitors (~5.4 Wh/kg). A material and energy cost analysis for the supercapacitors was conducted based on measured carbon yields and storage values and found to range AU $44-430/kWh (installed capacity) These values appeared to be highly competitive with current utility sized lithium-ion batteries (AU$ 825/kWh). Measured discharge rates in the coal-based supercapacitors also appeared to be superior over shorter commercially relevant time periods (1-2 hours).

Overall, the results indicated that each process was feasible. Further die design is needed and it was suggested that improved product quality could be gained by extruding directly into hotter annealing conditions rather than a cold environment. On direct casting, the coals produced a range of results depending on activation levels. Further development work would optimise specific coal types for maximum energy storage. Carbon tile-based supercapacitor design could be scaled to larger stacks of tiles to enable replicable units to be used a basis of bigger energy storage systems.