Effects of ion-exchanged calcium on brown coal tar composition as determined by Fourier transform infrared spectroscopy

In order to investigate the effect of ion-exchanged metals on the yield and composition of coal tar, we have pyrolyzed raw and calcium-exchanged Yallourn brown coal in a fluidized bed reactor at temperatures of 600 to 1000°C and have analyzed the product tars by Fourier transform infrared (FTIR) spectroscopy. At the lower temperatures (600-800°C), tar yields are lower for the calcium form coal than for the raw coal, indicating that the calcium either promotes tar conversion to char or tightens the coal structure and makes it difficult for larger tar molecules to escape. No measurable differences in tar yields are observed at higher temperatures. The yield of aromatic hydrogen in the tars varies only slightly with temperature but shows a marked reduction in the presence of calcium, suggesting that the aromatic components of the tar are those that are affected most by the structural or catalytic influences of the calcium. In contrast, the aliphatic components exhibit little influence from the effects of calcium but prove to be most susceptible to the secondary pyrolytic reactions brought about by an increase in temperature. The yields of hydroxyl, etheric, and carbonyl functionalities in the tars of both coals decrease with increasing pyrolysis temperature as these groups form oxygen-containing gases. These functionalities are lower in the tar from the calcium form coal, partly as a result of the ion-exchange process itself and partly as a consequence of the decomposition mechanism of the calcium carboxylate structure. The presence of calcium also brings about a lower yield of tar with unsaturated hydrocarbon substituents - a result of the lower yields of aromatic and of some unsaturated hydrocarbons from the calcium form coal.