Chemical transformation of inherent sodium and calcium species during direct liquefaction of two typical lignites rich in alkali and alkaline earth metals

Lignite, a suitable feedstock for direct coal liquefaction (DCL), is usually rich in sodium and calcium species. To better understand chemical transformation of inherent sodium and calcium species during DCL, two lignites rich in sodium and calcium species were liquefied in this work. The results show that both Australia lignite (AU) and Hami lignite (HM) are preferred raw materials for DCL. After DCL, mineral matters were obviously enriched in residues (DCLR). Under all the temperatures, retention ratios of sodium species were lower than those of calcium species. One obvious transformation of calcium species was reflected in changes of organic-bound calcium species (AS-Ca) and hydrochloric acid-insoluble calcium species (HIS-Ca) in HM. During direct liquefaction of HM, the amount of AS-Ca decreased significantly and the amount of HIS-Ca increased sharply. XRD analyses of low temperature ashes of DCLR confirmed that when temperatures reached above 400 °C, kaoline in HM gradually decomposed and came to react with AS-Ca, which resulted in formation of HIS-Ca. Due to the low ash content, transformation of calcium species in AU showed unobvious tendency and only CaCO3 was detected in DCLR.