Four lower rank coals were oxidized in 0.5 N Na2CO3 aqueous solution, into which atmospheric oxygen gas was bubbled. The reaction was carried out at 20-85°C, and, after cooling to ambient temperature, the slurry was acidified (pH 1.3) with a 5 N HCl solution. The coals oxidized at 85 °C for 6-24 h were extracted with methanol/tetrahydrofuran mixtures of 25/75 and 50/50 in volume ratio at 30°C under ultrasonic irradiation. The O2 oxidation dramatically enhanced the extractability of the coals, and extraction yields reached 80-90 wt % daf. Dimethyl sulfoxide and a mixture of benzene and methanol gave similar extraction yields for each oxidized coal. Pyridine showed lower extraction yields in spite of its higher hydrogen-bond-breaking ability. Extractability was, thus, better correlated with the solubility parameter of solvents than with the heat of hydrogen bond complexation. Diffuse reflectance FTIR analysis revealed that the O2 oxidation introduced carboxylic, phenolic, and alcoholic hydroxyls into the coals with a decrease in alkyl groups and aryl-alkyl ethers. This structural change was due to the oxidation of aliphatic carbon-hydrogen bonds to peroxides and subsequent dissociation by acid-catalyzed hydrolysis. The O2 oxidation also converted a portion of the coals to water-soluble acids such as oxalic, formic, acetic, and malonic acids and carbon dioxide. The carbon conversion to CO2 was negligible for the O2 oxidation at 20-50°C and was 5-9 wt % daf for that at 85°C.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)
- Fuel Technology
- Energy Engineering and Power Technology