Article pubs.acs.org/EF
Characterization and Refining Pathways of Straight-Run Heavy Naphtha and Distillate from the Solvent Extraction of Lignite Toluwanise Adesanwo, Moshfiqur Rahman, Rajender Gupta, and Arno de Klerk* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada S Supporting Information *
ABSTRACT: Coal liquids were produced by solvent extraction of Bienfait lignite at 415 °C and 4 MPa H2 for 1 h with a hydrotreated coal tar distillate in a 2:1 solvent to coal ratio. Detailed characterization was performed on four straight-run distillation fractions of the coal liquids in the 120−370 °C boiling range. It was found that the coal liquids contained very little aliphatic material. Most of the compounds were aromatics, with aromatic compounds having no alkyl substituents dominating the composition. The aromatic carbon content increased with boiling fraction from 80 wt % in the 120−250 °C fraction to 94 wt % in the 343−370 °C fraction. Major compounds identified in the coal liquids were acenaphthene, phenanthrene, fluoranthene, and pyrene, which constituted 62 wt % of the total product. The coal liquids also contained heteroatom species. Interestingly, the nitrogen content did not monotonically increase with an increase in boiling point. The maximum nitrogen content was found in the 300−343 °C boiling fraction as a result of a high concentration of carbazole. The refining pathways for transportation fuel production were evaluated. It was found that the naphtha fraction could be upgraded to a motor gasoline blending component just by hydrotreating. No subsequent catalytic reforming was necessary because of the low aliphatic content of the naphtha. The kerosene required severe hydrotreating in order to be acceptable as a jet fuel blending component, mainly because of the high dinuclear aromatic content of the straight-run kerosene. The distillate made a poor feed material for diesel fuel and required severe hydrotreating to achieve an acceptable cetane number. In general, the coal-derived distillate would benefit from ring opening to reduce its density. The prognosis for transportation fuel production from the coal liquids was not favorable. The production of aromatic chemicals was a better fit with the properties of the coal liquids.
1. INTRODUCTION Conventional crude oil can be substituted with other carbonbased materials as raw materials for the production of transportation fuels. The economic incentive to do so is derived from the difference in the raw material cost and the price of crude oil. Facilities to refine conventional crude oil already exist, and ideally one would like to make use of these refineries also to refine the synthetic crude oil produced from other carbon-based materials. One of the most abundant and cheap raw materials that can be employed for oil production is low-rank coal. Low-rank coal is mainly used as fuel for electric power generation, and the infrastructure to mine and transport coal is already in place. The two steps that are required for the conversion of low-rank coal into transportation fuels are the production of synthetic crude oil from the coal and the refining of the synthetic crude oil to produce on-specification fuels. This investigation deals only with the latter step, namely, the definition of a refining pathway for the synthetic crude oil. The scope of this work is further limited to focus only on synthetic crude oil produced by solvent extraction of coal. Solvent extraction of coal is a direct coal liquefaction (DCL) technology that involves the high-temperature and high-pressure dissolution of a coal in a solvent. The solvent is typically derived from the synthetic crude oil produced during DCL. The solvent is hydrotreated in a different reactor that is separate from the liquefaction reactor, and no hydrotreating catalyst is added to the liquefaction reactor employed for solvent extraction of coal. Examples of such processes are the Exxon Donor Solvent © 2014 American Chemical Society
(EDS) process and the Solvent-Refined Coal I (SRC-I) process.1 The approach that was followed in the investigation was to perform a detailed characterization of the straight-run heavy naphtha and distillate fractions obtained from the solvent extraction of coal. Some initial results have been reported before.2 The properties of the straight-run coal liquids were compared with the property requirements and fuel specifications of transportation fuels. Specific attention was paid to the compound classes present in the coal liquids and the refining pathways that would be required to meet fuel specifications. Needless to say, the work builds on previous literature dealing with this topic, mostly from the 1970s and 1980s.3−8 In the mid-1980s, when crude oil became too cheap for further development of DCL technology to be economically justified, transportation fuel specifications were quite different from current fuel specifications. It was therefore necessary to reinterpret the older literature. Although some research on coal liquid refining has appeared since the 1980s, such studies are not plentiful.9−13 There are far more studies concerned with increasing the oil yield from coal liquefaction than with refining of the oil to final products. In fact, in the overview of coal research by Mathews et al.,14 the number of studies dealing with refining of coal liquids was so small that this category was not worth mentioning. Received: April 12, 2014 Revised: May 22, 2014 Published: June 13, 2014 4486
dx.doi.org/10.1021/ef5008227 | Energy Fuels 2014, 28, 4486−4495
Energy & Fuels
Article
solvent was the industrial hydrotreated coal tar distillate, and solvent from the same batch was used for all of the digestions. The coal liquid was separated from the residual coal by filtration at 100 °C using a 1 μm Whatman glass microfiber filter. The residue was washed with tetrahydrofuran to recover residual coal liquids from the residue. The amount of undigested residue was around 30 wt % on a moisture- and ash-free (maf) basis. The coal liquid was subsequently separated into the following boiling fractions by distillation: 120−250, 250−300, 300−343, and 343−370 °C. The volumetric yield of each distillation fraction based on the total volume of coal liquid is given in Table 3.
2. EXPERIMENTAL SECTION 2.1. Materials. Canadian Bienfait lignite was employed as the coal feed for the solvent extraction process to produce coal liquids. The coal was crushed, and the
