Article pubs.acs.org/EF
Effect of Direct Coal Liquefaction Conditions on Coal Liquid Quality Moshfiqur Rahman, Toluwanise Adesanwo, Rajender Gupta, and Arno de Klerk* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada S Supporting Information *
ABSTRACT: Solvent extraction of coal was investigated with a focus on the quality of the coal liquids rather than coal conversion. The aim was to determine how the hydrogen/carbon ratio and other quality measures were influenced by liquefaction conditions. Liquefaction was performed using Canadian Bienfait lignite in the temperature range of 350−450 °C, 4 MPa H2, solvent/coal ratio of 2:1, and residence times up to 30 min at liquefaction temperature. An industrial hydrotreated coal liquid was used as the solvent. The hydrogen/carbon ratio of the coal liquids decreased with an increase in coal conversion, so that coal liquid quality decreased with an increase in the maximum liquefaction temperature. Selective extraction of hydrogen-rich material during the initial stages of liquefaction could be explained in terms of the low solubility parameter of the solvent, the weaker association of less polar molecules, and the limited extent of hydrogen transfer between phases. At longer residence times, especially at higher temperature, the coal liquids became heavier (>550 °C boiling material) and more aromatic and had a higher density and refractive index. These changes were partly due to increased coal conversion and partly due to increased time for hydrogen transfer, cracking, and recombination reactions to take place. It was further found that the nitrogen content of the coal liquids increased with increasing temperature and residence time. Some industrial implications of the changes in coal liquid quality on process development for coal liquefaction were discussed.
1. INTRODUCTION Direct coal liquefaction (DCL) investigations are mostly approached from a coal conversion perspective, where the liquid yield is the dominant metric. An analogous situation can be found in the industrial development of DCL technology for coal−liquid conversion in the 1970−1980s, where increasing the liquid yield was emphasized.1 The liquefaction yield passes through a maximum, which is a function of the temperature and time,2 with best results usually obtained in the temperature range of 400−450 °C with an appropriate residence time for the coal used as feed material. The liquid yield can be further improved through hydrogenation and hydrogen transfer.3 It makes sense to maximize the liquid yield during DCL, because the coal liquefaction residue remaining after coal liquid recovery is considered less valuable than the raw coal used as feed material to the process. However, the maximum liquefaction yield and maximum profitability may not coincide at the same operating point. Apart from the obvious operating cost versus liquid yield trade-off, the quality of the coal liquid should be considered. It was shown previously that it is difficult to refine highly aromatic coal liquids to on-specification fuels.4 The optimum operating point for DCL is not necessarily the best operating point for the overall coal-to-liquids process. If coal liquid refining becomes a significant cost component, then quality and not just quantity should be considered. Quality in the present context refers mainly to the effective hydrogen/ carbon ratio of the coal liquid. The effective hydrogen/carbon ratio is not just the molar H/C ratio but the molar ratio of H/C after subtracting the hydrogen needed to eliminate nitrogen, sulfur, and oxygen from the product.5 The question was therefore posed: what is the relationship between DCL operation, coal conversion, and coal liquid quality? It was anticipated that the hydrogen/carbon ratio as well as the heteroatom content of the coal liquids would be inversely © XXXX American Chemical Society
related to the coal liquid yield obtained by liquefaction. It was further anticipated that not all temperature−time combinations that resulted in the same coal liquid yield would produce coal liquids of the same quality. These aspects were experimentally investigated by performing non-catalytic liquefaction of a Canadian lignite coal using an industrial hydrotreated coal liquid as the solvent in a batch reactor in the temperature range of 350−450 °C and at different residence times.
2. EXPERIMENTAL SECTION 2.1. Materials. The liquefaction experiments were performed using Bienfait lignite and a hydrotreated industrial coal liquid as the solvent. The coal and solvent were previously characterized.4 Although the solubility parameter of the solvent was not experimentally determined, on the basis of analysis, it was estimated to be in the range of 18.5− 19.5 MPa1/2. The proximate and ultimate analyses of the coal feed and solvent are repeated for ease of reference (Table 1). Additional characterization data for the solvent is also included. The cylinder gases, H2 and N2, were obtained from Praxair. The cleaning solvent was tetrahydrofuran (Sigma-Aldrich, >99.9%). The solvents employed as mobile phases during chromatographic analysis were n-hexane [Fisher Scientific, high-performance liquid chromatography (HPLC) grade, 99.9%] and chloroform (Fisher Scientific, HPLC grade, 99.9%, stabilized with 0.75% ethanol). 2.2. Equipment and Procedure. The coal was crushed, and the
