Thermal Study of the Coking Behaviour of Thermoplastic Coals

The objectives of this research were to apply laboratory bench-scale experiments to study the coking behaviour of Australian thermoplastic coals and correlate the results with coke tests obtained under large scale research coke oven conditions.  A novel thermoanalytical technique (Computer Aided Thermal Analysis) was applied for thermal evaluation of specific heat, heats of reactions, thermal conductivity and diffusivity of the coals during their pyrolysis. Volatiles liberated during pyrolysis of the coals were evaluated using a micro gas chromatograph, while the tars were condensed at room temperature and characterised with matrix-assisted laser desorption/ionisation mass spectrometer (MALDI). Thermogravimetric analysis was employed to measure volatile matter release with temperature during pyrolysis.

Coke was prepared from the same coal samples under laboratory and under research coke oven conditions. A small coking reactor was designed to prepare coke samples under laboratory bench-scale conditions. The reactor utilised ~80 g of coal. The research coke oven tests were conducted separately by BHP Billiton Newcastle Technology Centre and the coke test results were obtained under defined process conditions.

The initial thermal conductivity and diffusivity measured at 100°C were found to correlate to volatile matter of the coal. When temperatures reached thermoplastic range, the thermal conductivities and diffusivities increased rapidly with the change dependent on the fluidity of the coal.

Laboratory produced cokes exhibited a linear relationship between their strength and coke density. The properties of laboratory produced cokes, however, differed from the corresponding coke properties produced under research coke oven conditions. This is most likely because of differences in secondary reactions for the small and large-scale coking conditions. Secondary tar cracking is believed to be more pronounced in the laboratory reactor. Also, the strength test used to evaluate laboratory coke properties showed large variability for small measurement variations.

The thermal analysis data showed strong correlations with the properties of the coke produced under research coke oven conditions. The specific heat and corresponding heats of reactions of coals in the temperature range when coals undergo thermoplastic transformation were found to correspond well with the coke quality tests. A linear relationship was found between the thermoplastic range correlation (-DHv×Trough/DT)TP and the coke strength after reactivity with CO2 (CSR index) for the cokes. An empirical equation was derived from this relationship which can potentially be applied to predict CSR values using a simple thermoanalytical test.