|Flow diagram of coal tar hydrogenation process. Credit: ACS, Kan et al. Click to enlarge.|
Researchers in China report the production of gasoline and diesel from coal tar via an optimized catalytic hydrogenation using two serial fixed beds, the first with a hydrofining catalyst of MoNi/γ-Al2O3 and the second with a hydrocracking catalyst of WNiP/γ-Al2O3-USY. Their paper was published in the ACS journal Energy & Fuels.
Coal tars—highly viscous liquids—are byproduct of the carbonization of coal to produce metallurgical coke and/or natural gas. In 2010, China’s coke output reached 387.571 million tons, accounting for 61.6% of the world’s total, according to a 2011 market research report; 9.94 million tons of coal tar was further processed. The current downstream markets of coal tar mainly include coal tar deep-processing (phenol, anthracene, industrial naphthalin, and coal tar pitch), carbon black, substitutes for heavy oil and exportation. Among these applications in China, deep processing ranked first with 66.4% of the total consumption volume in 2009, followed by carbon black raw oil sharing 28.1% of the total.
Concern on the petroleum depletion crisis and rising fuel prices is increasing. Hence, major efforts are being dedicated to the development of various usable energy sources. Abundant coal tar is produced every year by coal carbonization and gasification. Coal tar is a complex dark brown mixture consisting of aliphatic, alicyclic, aromatic, and heterocyclic compounds, especially some extremely complicated PAH (polycyclic aromatic hydrocarbon) compounds with molecular weight up to several thousands. Around 500 kinds of organic compounds have been identified in coal tar. There are some obvious defects in the physical and chemical properties of coal tar, including high viscosity, thermal instability, corrosiveness, etc., which limit its application as an energy fuel for combustion. Therefore, an upgrading process for coal tar is required before its application. By hydrogenation, coal tar can be used as an alternative source of transportation fuel such as gasoline and diesel.—Kan et al.
In an earlier study, the team from the Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, used MoCo/γ-Al2O3 and WNi/γ-Al2O3 catalysts in the dual reactor configuration to process coal tar. Although they achieved some promising results including high sulfur and nitrogen removals, some fuel indexes of products were still not satisfactory. For example, the cetane number of the diesel product obtained under the hydrogen pressure of 8 MPa was at a low level of 42.4—indicating that many PAH molecules still remained in the diesel product.
In the current study, they used the hydrofining and hydrocracking catalysts to enhance the hydrocracking of the feedstock and obtain high-quality fuel products.
After preliminary catalyst screening tests, the team determined the optimum combination of catalysts; as well, they investigated the effect of pressure on the hydrogenation performance was also investigated.
In the study, they obtained a gasoline fraction with low sulfur and nitrogen contents of 39 and 13 ppm, respectively and a diesel fraction with 43 and 9 ppm, respectively, as compared to the respective values of 0.96 and 1.69 wt % in the feedstock. GC−MS analyses revealed that the gasoline and diesel products mainly comprised substituted cyclohexanes, substituted and partially saturated naphthalenes, and saturated paraffins.
|Effect of pressure on properties of gasoline and diesel products. Credit: ACS, Kan et al. Click to enlarge.|
Compared to other previous studies, a relatively lower hydrogen pressure of 6 MPa was applied to obtain the good-quality products.
The catalysts showed good stability in activity in the test of catalyst life.
Tao Kan, Xiaoyan Sun, Hongyan Wang, Chunshan Li, and Usman Muhammad (2012) Production of Gasoline and Diesel from Coal Tar via Its Catalytic Hydrogenation in Serial Fixed Beds. Energy & Fuels doi: 10.1021/ef3004398