Carbon crystallization during high-temperature pyrolysis of coals and the enhancement by calcium

Pyrolysis of five coals has been carried out at 1000-1350 degreesC and different heating rates with fixed-bed and free-fall reactors to examine carbon structures in devolatilized chars. The X-ray diffraction measurements show the formation of crystallized carbon with turbostratic structures, depending strongly on coal type and severity of pyrolysis. The proportion of the carbon with low rank coals decreases by demineralization with acid washing but contrarily increases by subsequent addition of Ca2+ ions, irrespective of heating rate, and a small amount of 0.5-1 wt % Ca works efficiently. It is thus likely that Ca2+ ions naturally present as ion-exchanged forms in low rank coals determine dominantly the extent of carbon crystallization at higher temperatures. The Ca added is transformed to fine particles of CaO upon pyrolysis, and a larger amount of CO is formed in the presence of the Ca. A mechanism for the Ca-enhanced carbon crystallization is discussed in terms of solid-solid interactions between CaO particles and amorphous carbon.