Characterization of successive time/temperature-resolved liquefaction extract fractions released from coal in a flowing-solvent reactor

Abstract

This paper reports the characterization of successive extract fractions collected during the liquefaction in tetralin of a UK low-rank bituminous coal (Linby), by the use of size exclusion chromatography and u.v. absorption and u.v. fluorescence spectroscopies in a closely related manner. Successive liquefaction product fractions shift progressively to greater molecular masses with increasing extent of reaction and increasing intensity of reaction conditions; parallel increases in relative concentrations of larger aromatic ring systems are observed. These data suggest that material released from coal during earlier stages of the liquefaction process would be easier to hydrocrack than that released during later stages, under more intense reaction conditions. However, quantitative differences in reactivity between product fractions, resulting from such structural changes, remain to be demonstrated. The results also provide evidence that local distribution densities of larger aromatic ring systems might severely affect the relative ease with which coal dissolution (or in pyrolysis, tar precursor formation) takes place at any given intensity of reaction. This is explained in terms of large aromatic ring systems serving as cross-linking points of high coordination number in coal. Increased densities of larger fused aromatic ring systems near edges or external surfaces of potential molecular fragments would render fragment release from the coal mass — i.e. the depolymerization process — progressively more difficult. This picture of coal thermal breakdown would be consistent with existing data showing (1) yields and molecular mass distributions of pyrolysis tars decreasing rapidly with increasing coal rank for high-rank coals, and (2) yields and molecular mass distributions of liquefaction products decreasing rapidly beyond ∼87 wt% carbon content.