Elucidation of Mechanisms Involved in Acid Pretreatment and Thermal Extraction during Ashless Coal Production

Abstract

Some low-rank coals, i.e., Wyodak coal (C%, 75.0%), acid-treated in aqueous methoxyethoxy acetic acid (MEAA) and acetic acid (AA), and extracted in polar N-methyl-2-pyrrolidinone (NMP), showed a considerable increase in thermal extraction yield at 360 °C, as the acid concentration increased from 0.01 to 0.1 M. No significant changes were seen with a further increase in acid concentration to 1.0 M. A corresponding decrease occurred in the intensity of FT-IR spectral bands near 1555 and 1400 cm-1, assigned to metal carboxylate groups, as acid concentrations increased from 0.01 to 0.1 M, while bands assigned to carboxyl groups at about 1720 cm-1 increased over the same range. Furthermore, most Mg2+ and Ca2+ ions could be removed from the coals with acids between 0.01 and 0.1 M. Thermogravimetric analyses showed that the acid-treated coal yielded a weight loss similar to that of raw coal. Thermal decomposition of acid-treated coals cannot play a significant role in the increase in extraction yield that is obt...