Oxygen-containing gases from the rapid pyrolysis of a brown coal: The effects of ion-exchanged calcium

In order to examine the effects of ion-exchanged metals on the thermal decomposition of carboxyl groups in coal, we have quantified the distributions of oxygen-containing product gases from raw, acid-washed, and calcium-exchanged Yallourn brown coals, pyrolysed in a fluidized bed reactor at temperatures of 600 to 1050°C. The major products are CO and CO₂. The minor ones are comprised of oxygenated organic species (acetic acid, ketene, formaldehyde, and methanol), whose summed yields approach that of methane from primary pyrolysis. At temperatures less than 900°C, calcium enhances the yield of CO₂ and decreases the production of CO, formaldehyde, and methanol. It completely suppresses the production of acetic acid. These results are consistent with a mechanism involving decomposition of the calcium carboxylate structure to eliminate CaCO₃, which takes with it some of the organically bound oxygen that would otherwise be able to form oxygen-containing organic gases. The CaCO₃ decomposes in turn and is responsible for the increased yields of CO₂. At higher temperatures (>900°C), decomposition of the organic species and char gasification appear to predominate, leading to high yields of CO.