APPLICATION OF MICROSCOPY TO THE INVESTIGATION OF BROWN-COAL PYROLYSIS

To examine the influence of calcium on the mechanisms of brown coal py rolysis and gasification, the morphology of chars from raw and calcium -exchanged Yallourn brown coal was analysed. The chars were obtained b y slow pyrolysis in a thermo gravimetric analyser and rapid pyrolysis in fluidized bed reactors operating at atmospheric pressure and at 1.1 MPa. They were examined by optical microscopy to determine reflectanc e and the percentage of particles that had become plastic during pyrol ysis, In addition to confirming calcium's inhibiting effect on tar yie ld, the results from the rapid pyrolysis experiments show that in the presence of calcium, char reflectivity decreases, char H/C ratio incre ases, and the proportion of particles going through a plastic stage de creases. Calcium's inhibition of plasticity development is augmented b y high pressure in the fluidized bed reactor. The effects appear to be attributable to the action of carboxylate calcium as a cross-linking agent, leading to the formation of a tighter char structure which trap s the organic material that would otherwise be liberated as tar. The p resence of Ca also increases the H/C ratio of the chars produced by sl ow pyrolysis, but the mechanism of pyrolysis differs, since in slow py rolysis none of the particles showed evidence of plasticity. In slow p yrolysis, calcium's influence on char reflectivity depends on the hold ing temperature, since temperature determines the extents of both coal devolatilization and catalytic transformations. The roles of calcium in these processes and their influence on optical anisotropy and refle ctance are discussed.