The Effect of Particle Size and Heating Rate on the Transformation of Sulphur during Pyrolysis of a South Australian Low-rank Coal

Particle size and heat transfer effects on organic and inorganic sulphur transformations during pyrolysis are investigated using temperature‐programmed pyrolysis and fixed‐bed pyrolysis (700‐900°C) of pulverised Bowmans coal, and fluidised bed pyrolysis at 800°C of 6, 8 and 10mm diameter cylindrical coal pellets. Results are interpreted using a heat transfer model [8]. The pyrolysis experiments reveal that during the initial devolatilisation stage, organic and sulphate sulphur decomposition occurs at a rate directly proportional to heating rate and inversely proportional to particle size. Towards the end of the devolatilisation stage, the transportation of volatiles out of the coal decreases so that sulphate undergoes solid‐state transformations to form organic sulphur in the char. The reincorporation process is accelerated by higher reaction temperatures or heating rates which allows greater decomposition of sulphate and longer periods of slow devolatilisation to support the solid‐state reactions. Smaller particles in the fluidised bed reveal a similar effect due to faster internal heating rates. Larger particle sizes also facilitate organic sulphur increase due to slow internal heating rates and hence slow and extended devolatilisation and sulphate reincorporation.