Study on thermoplasticity of coals by dynamic viscoelastic measurement: effect of coal rank and comparison with Gieseler fluidity

Thermoplastic behaviors of 10 coals of various rank were studied by dynamic viscoelastic measurements using a rheometer. Elastic (G′) and viscous (G″) moduli, and tan δ in which δ is a phase delay of a stress to a strain applied to a coal pellet were determined at 300–550°C. The temperatures corresponding to softening, maximum fluidity state, and resolidification of coals could be estimated from a maximum or minimum of G′ and G″ observed at G′ or G″–temperature profiles, and they are in fair agreement with those measured by Gieseler plastometry for the coals used. Tan δ also gave a maximum near maximum fluidity temperatures estimated from G′ and G″, and showed a fairly good correlation with Gieseler maximum fluidity. The two crossover temperatures in G′ and G″ and the temperature range between them could be determined for five high-fluidity coals among the 10 coals used here. The temperature range is narrower than that between softening and resolidification temperatures from Gieseler plastometry, and gives the range of fluid-like state of coals. Maximum fluidity temperatures from G′, G″ and tan δ were found to correlate very well with maximum vitrinite reflectance and C% of the coals, but maximum tan δ shows a maximum at around 1.3 of maximum vitrinite reflectance, similar to Gieseler maximum fluidity.