Direct Liquefaction of 2 South African Bituminous Coals and their Beneficiated Float Fractions

Abstract

The direct liquefaction of South African coal fines and their density separated (float) fractions were carried out under moderate conditions in a laboratory autoclave. The liquefaction temperature ranged between 380 and 420 oC, using tetralin as a solvent and an initial nitrogen gas pressure of 3 MPa. Results from the liquefaction tests showed that the carbon conversion and oil yields were high for the float fractions when compared to the coal fines. Waterberg and Highveld coal float fractions achieved a high carbon conversion of 50.7 wt % daf and 52.7 wt % daf respectively, compared with < 42 wt % daf carbon conversion for the coal fines. The effectiveness of the carbon conversion was correlated with the reactive macerals and the surface area of the individual samples. It was observed that the density separated coal fraction, which has a higher surface area, higher vitrinite content and higher reactive macerals content yielded higher extraction efficiencies. The residues and extracts obtained during the liquefaction tests were characterised using nuclear magnetic resonance spectroscopy, proximate and ultimate analyses. The analytical results indicated that the residue showed a slight decrease in calorific value and aliphatic components, with a lower H/C ratio and a higher ash content. Also, the results showed that using the float fractions of South African coals at a moderate liquefaction temperature could be beneficial in the production of liquid fuel.