Solvent-Free Liquefaction of Brown and Subbituminous Coals Using NiMo Sulfide Catalyst Supported on Carbon Nanoparticles

Abstract

Single- and two-stage liquefaction processes of Yallourn (YLC), South Banko (SBC), and Tanitoharum (THC) coals were performed in an autoclave of 50-mL capacity at variable solvent (tetralin)/coal ratios from 0 to 1.5 under the reaction hydrogen pressure of 15 MPa, using NiMo sulfide supported on carbon nanoparticles or commercial NiMo/alumina and synthetic pyrite catalysts. Although the oil yield decreased very much with all the catalysts by reducing the amount of solvent to zero, the NiMo/carbon catalyst gave much higher oil yields of 52 and 64%, respectively, in the single-stage (450 °C, 60 min) and two-stage (360 °C, 60 min; 450 °C, 60 min) liquefaction under the solvent-free conditions compared with the yield of around 40% by the other two catalysts in both single- and two-stage liquefaction under the same conditions. The oil yield depended on the coal species under the solvent-free conditions, being in the order of SBC > YLC > THC regardless of the reaction conditions. SBC provided the highest oil yields of 60 and 68% in the single- and two-stage liquefaction, respectively, reflecting its higher reactivity and lower gas yield. THC gave the lowest oil yields among the coals examined, although the oil yield reached 60% by the two-stage liquefaction even under the solvent-free conditions. The oil produced with NiMo/carbon catalyst carried lighter fractions in the boiling range 100-300 °C than those with the other catalysts regardless of the reaction conditions and coal species. Such excellent performance of the NiMo/carbon catalyst reflects its higher hydrogenation activity as well as the high dispersion on the coal surface at the initial stage of coal liquefaction, suppressing the retrogressive reactions. It is confirmed that the major portion of solid coal was solubilized during the heating and the initial stage to work as the self-producing solvent under the solvent-free reaction conditions. The design of coal liquefaction with the least use of solvent is discussed for the higher productivity.