Indexed on: 19 Nov '16Published on: 08 Nov '16Published in: Energy & Fuels
Chemical changes of eight Australian coking coal samples from six different basins with various ranks (RvMax from 0.87 to 1.52) and maceral constituents (vitrinite from 48.2 to 76.5%) were dynamically investigated to explore the mechanism of coal coking linking to the synchronized physical and thermal changes. Volatile release during coal pyrolysis was monitored using a novel technique of dynamic elemental thermal analysis (DETA) that is able to differentiate tar and gas evolution in terms of carbon and hydrogen compositions. Condensed coal tars were characterized both chemically and thermally using the DETA and laser desorption ionization–time of flight–mass spectrometry (LDI–TOF–MS) techniques. Coal pyrolysis experiments were conducted at a heating rate of 5 °C/min from room temperature (25 °C) to 1000 °C with a top coal particle size of 212 μm, which kept the same experimental conditions as the physical and thermal measurements. The results indicated that, overall, the volatile evolution rates decreased with the coal rank but increased with the vitrinite content for the tested coals. This chemical observation between different coals is consistent with swelling and exothermic heat, except for medium-rank coal samples C and D from basin III that evolved a similar amount of gas and tar to the comparative rank coal sample E, but showed lower swelling and smaller exothermic heat than expected. A comparison of the hydrogen/carbon ratio (tar H/C) showed that the volatile tars evolved from the coals C, D, and G that originated from basin III contain more molecular hydrogen than that from the comparative rank coals. These volatile tars appeared to be like those from lower rank coals. Condensed tar analysis showed that volatile tars produced from all coals are in a high molecular weight distribution between 200 and 600 Da. Lower and medium-rank coals that gave higher swelling showed an elongated distribution toward a higher molecular weight, while medium-rank coals C and D from basin III and higher rank coals that gave lower swelling did not. The results implied that the molecular weight distribution and the H/C ratio in the volatile tar might affect its utilization for driving thermo-swelling.