Physical and Chemical Interactions Between Charcoal and Coal During Coking

This project examined the physical and chemical interactions between biomass-derived charcoal and metallurgical coal during cokemaking, with particular emphasis on coal fluidity, volatile release, coking pressure and coke quality.

Charcoals produced from sugarcane bagasse, jarrah and bamboo were assessed with four coals spanning a range of rank and inertinite content. Controlled pyrolysis of pelletised sugarcane bagasse produced a low-surface-area char (5 m²/g) with minimal mesoporosity. Increasing pyrolysis temperature reduced oxygen content to below 6% on a dry ash-free basis above 600°C and below 4% at 800°C. The 800°C bagasse char behaved approximately as an inert solid in a high-swelling coal, allowing additions of up to 15 wt% without a significant loss of fluidity.

By contrast, high-surface-area jarrah char (286 m²/g) substantially reduced viscosity, consistent with adsorption of coal volatiles and accelerated depletion of liquid-phase material. Bamboo char showed intermediate behaviour, with the 600°C product outperforming the higher-surface-area 800°C product. Char carrying capacity was strongly dependent on coal properties: a higher-rank coal (random vitrinite reflectance 1.18%) accommodated 15 wt% bagasse char, whereas a 5 wt% addition destroyed the fluidity of a lower-rank coal (0.82%). High inertinite content also reduced carrying capacity.

Thermogravimetric analysis and microscopy supported mechanisms involving enhanced volatile release, disruption of bubble growth, and differences in coal wetting and penetration of the char structure. In 8 kg coking tests, jarrah and bamboo chars reduced internal gas pressure, indicating potential for coking-pressure control. At 10 wt% addition, charcoal increased the coke reactivity index by 10.2-12.8 points for most coal-char combinations and reduced coke strength after reaction by 5.3-22.3 points, with the response dependent on coal and char type.

These findings identify low-oxygen, low-surface-area grass-derived charcoal as the most promising option for partial incorporation into coking blends, particularly with higher-rank, high-swelling coals. However, differences between small-scale rheometry and larger-scale coking behaviour require further validation before industrial blend limits can be established.