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.
Steam gasification of the HyperCoals (ash-free coal extracts) with the physical addition of 5.8%-6.0% K 2 CO 3 was conducted at 1023 K on a thermogravimetric apparatus that was equipped with an on-line quadrupole mass spectrometer. The catalytic gasification of the HyperCoals demonstrated a much higher gasification rate than the catalytic gasification of the raw coals. Interactions of K 2 CO 3 with mineral matter in the raw coal formed water-insoluble potassium compounds, such as potassium aluminosilicates, and reduced the catalytic activity, whereas no such negative reactions occurred for the HyperCoals. The steam gasification of the HyperCoals with K 2 CO 3 was also determined to be favorable for the high-yield production of hydrogen. From these experimental results, the catalytic steam gasification of HyperCoal would potentially be a more efficient process for the production of hydrogen in the future.
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