The Behaviour of Export Thermal Coals

The combustion behaviour of inertinite and vitrinite macerals in coal were studied by two methods: Drop Tube Furnace (ACIRL) and Laser Nanoreactor (CSIRO).

 Drop Tube Furnace combustion tests were performed on four coals and on maceral-enriched fractions of those coals.  The tests were performed over short residence times (in the approximate range 0.25 – 0.55 seconds) in order to observe the relative rates of combustion of the different coals components; as well as over longer residence times (approximately 3 seconds) to simulate the time-temperature history in a utility boiler.  The microlitho types of the coal particles and the microstructures of the partly burnt char particles were characterised microscopically.

 The Laser Nanoreactor is a system for measuring the combustion reactivity of individual coal particles, chosen to represent the various maceral types, in air.  The particle of coal is held on a needle attached to a mass analyser and heated rapidly with a laser.  This can measure mass changes to ±2 nanograms (ng) and the reaction kinetics can be determined whilst the particle is burning.  For each coal, one vitrinite and three inertinite macerals, covering a range of reflectances, were chosen and tested.  The individual maceral reaction rates varied by a factor of up to three, and the reaction rates observed were comparable to bulk chemical engineering data in the literature, ie 10^-2 – 10^-3 g/cm²s.

 The results showed that there is no general correlation between the combustion reactivities of vitrinite and inertinite macerals – a case by case study has revealed that in some coals vitrinite macerals are more reactive, whereas in other coals the inertinite macerals are more reactive.

 It is incorrect to assume that inertinite does not fuse (ie become plastic) under pulverised fuel heating conditions, because a very significant proportion of the inertinite particles were observed to do so.  It is also incorrect to equate fusibility with combustion reactivity because, for some of the coals tested, the unfused inertinite particles were more reactive than the fused particles.