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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 obtained with polar solvent. A mechanism is proposed for the processes involved in the acid treatment and thermal extraction of some low-rank coals:  cation-bridging cross-links existing among metal carboxylate groups in the low-rank raw coals are released upon removal of Mg2+ and Ca2+ ions by acid treatment. The resulting carboxyl groups then form new hydrogen bonds among themselves, which can become released when polar NMP solvent is introduced. Thus, both the disruption of cation-bridging cross-links by acid treatment and the release of the hydrogen bonds by NMP are involved in the enhancement of extraction yields upon acid treatment of some low-rank coals.

The effects of acid and hydrothermal pretreatments and the addition of polar compounds on the production of ashless-coal (HyperCoal) from subbituminous coals using cost-effective industrial solvents were investigated. The extraction yield of Wyodak subbituminous coal (C%, 75.0%) using crude methylnaphthalene oil (CMNO) at 360 °C was increased significantly by 19% following acid pretreatment; it was 41.3% for the raw coal and 60.5% for the acid-treated coal. The addition of strongly polar compounds, such as N-methyl-2-pyrrolidinone (NMP), also increased the extraction yields. For Pasir subbituminous coal (C%, 73.0%) the yield increased by 10% from 54.3% for the raw coal to 64.2% when 20% NMP was added to CMNO. The highest extraction yield of 72.2% was obtained for acid-treated Wyodak coal using CMNO with 20% NMP added. The ash content in HyperCoal tended to decrease following acid pretreatment and was less than 200 ppm in some coals. Hydrothermal pretreatment had a negative effect on the thermal extraction at 360 °C, but increased the yield at extraction temperatures below 200 °C.

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