Ind. Eng. Chem. Res. 1996, 35, 1251-1256
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Catalytic and Thermal Cracking of Coal-Derived Liquid in a Fixed-Bed Reactor Abolghasem Shamsi U.S. Department of Energy, Morgantown Energy Technology Center, P.O. Box 880, 3610 Collins Ferry Road, Morgantown, West Virginia 26507-0880
A coal-derived liquid, obtained from the Coal Technology Corp.’s mild gasification process, was cracked over char produced from Pittsburgh No. 8 coal mixed with Plum Run dolomite in the Foster Wheeler carbonizer. For the purpose of comparison, calcined Plum Run dolomite (PRD), char produced from Pittsburgh No. 8 coal, and silicon carbide (an inert material) were also studied. Coal liquid feed was analyzed by sulfur-selective gas chromatography (GC), liquid chromatography (LC), and proton nuclear magnetic resonance (NMR) and for elemental composition. The gaseous products of cracking were analyzed for hydrocarbons using GC. Most sulfur in the feed was present in molecules heavier than dibenzothiophene and was distributed in a variety of structures. The surviving coal liquid was analyzed by LC. The results indicated that deoxygenation of phenols, dealkylation of aromatic compounds (AR), and condensation of aromatic structures are some of the reactions occurring on the surface of bed materials. Energies of activation for homogeneous and for heterogeneous pyrolysis of the coal liquid were calculated after separating the rate of thermal cracking from the sum of rates of thermal and catalytic cracking. Introduction In a simplified, integrated gasification combined cycle (IGCC) system, using a fixed-bed gasifier, substantial amounts of sulfur are contained in the coal liquid which are formed in the gasifier and are carried through the hot gas desulfurization system. When the tars are burned in the gas turbine, sulfur dioxide is formed which limits the environmental performance of the IGCC system. In-bed cracking of tars removes the sulfur, which can then be captured by the desulfurization sorbent and can substantially improve the environmental performance of the IGCC system. Pyrolysis of coal at temperatures between 500 and 1200 °C produces, initially, subspecies that fall into three major groups: gases, liquid products, and char. These primary products further react, altering the nature and the composition of final products. The secondary reactions are usually cracking reactions such as dealkylation of AR and opening of hydroaromatic rings, resulting in lighter products, and condensation or polymerization reactions, giving heavier species. These reactions could take place on the walls of the reactor, on the surface of unconverted coal, or on the surface of char, therefore altering the composition and yields of the products of coal carbonization and changing the fuel value of gaseous products. Additional carbon may be deposited on chars, thus altering the reactivity of char. A detailed analysis of secondary reactions and an understanding of in-bed reduction of coal tar are useful in evaluating the performance of a gasifier. Coal liquids produced at about 500 °C, termed mild gasification coal liquids (MGCL), are not identical to the primary coal liquids. Nevertheless, catalytic pyrolysis of MGCL in the presence of char, and char produced in the presence of dolomite, provides an understanding of the secondary reactions, tar formation, and the composition of gaseous products in a gasifier. Adams et al. (1959) have observed that products of coal carbonization, if left in contact with hot char, undergo further reactions. In a similar work, Bond and co-workers (1959) have shown that, if coal is mixed with This article not subject to U.S. Copyright.
or overlaid by activated charcoal and heated in a slow stream of nitrogen to 600 °C, the coal liquid is cracked to light products. Griffiths and Mainhood (1967) have reported a more detailed work on the pyrolysis of coal tar and model AR on activated charcoal. Volatile products from low-temperature carbonization of a coal and model AR were passed through activated carbon, fused silica, and alumina at temperatures ranging from 350 to 900 °C. It was observed that activated carbon functions as a cracking catalyst for lower boiling components of the liquid but promotes coking of higher boiling components. Fused silica and alumina also function as cracking catalysts but are less reactive than activated carbon. The catalytic activity of activated carbon was attributed to its high surface area. Hesp and Waters (1970) have studied the thermal cracking of coal and volatile matter from coal carbonization on lumps of coke, obtained by carbonizing low-ash coal at 900 °C, and have identified several cracking reactions. These investigations suggest that tar cracking can occur on the hot surface of carbon including char. Tar cracking has also been investigated on several inorganic compounds. Longwell and co-workers (1985) and Ellig et al. (1985) have reported the pyrolysis of model organic compounds including n-heptane, AR, and m-cresol on quartz coated with calcium oxide (CCQ). Longwell et al. (1984) have also investigated the pyrolysis of freshly formed tars in the presence of CCQ. In this work, primary volatiles generated in an upstream reactor were rapidly swept to a downstream reactor to avoid any secondary reactions. In the second reactor the volatiles were passed over the catalyst, and products of pyrolysis were analyzed. Pyrolysis of a coal tar in a fixed-bed reactor has been investigated by Wen and Cain (1984) in the presence of quartz, clay minerals, and zeolites. Quartz exhibited no catalytic activity, and the product yields were similar to those obtained in a homogeneous cracking experiment. Linde LZ-Y82 and Zeolon 900-H were very active because of their large surface areas (600-900 m2/g) and high densities of acidic sites. The unreacted tar from the reaction at 550 °C on LZ-Y82 was 1 wt % of the injected tar, whereas
Published 1996 by the American Chemical Society
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Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996
56% of the tar was carbonized and the balance was cracked to light hydrocarbons. Kaolinite, on the other hand, with a surface area of 15-20 m2/g was much less active. Velegol (1993) has studied the cracking of coal tar in the presence of LZ-Y82, LZ-Y82/NiW, LZ-Y82/ ZnTi, and LZ-Y82/Zn in a fluid-bed reactor at different temperatures and residence times to estimate the effects of the latter two variables on the tar cracking process and to find an effective catalyst for cracking coal liquid. In the present investigation, thermal and catalytic cracking of a heavy fraction of a MGCL was investigated on a coal char generated in a laboratory-scale fixed-bed reactor and on the char generated in the Foster Wheeler carbonizer. The cracking of the coal liquid was also investigated in the presence of calcined dolomite to determine which component of the char was contributing to its catalytic activity and for the purpose of comparison on silicon carbide which is assumed to be a nonactive material. Experimental Section A laboratory-scale fixed-bed reactor was used in studying tar cracking. The carrier gas was nitrogen, preheated to 200 °C, at a flow rate of about 0.5 L/min. The reactor effluent stream contained the residual tar and gaseous products of pyrolysis. Tar was collected in cold traps, and the gaseous products were sampled for GC analysis. Foster Wheeler char was collected from the test runs TR2.2 and TR4.5, at 874-943 °C and 9-13 atm in the presence of Ohio PRD (Van Hook et al., 1993; Robertson and Bonk, 1993). Char was collected from the carbonizer drain (CD), the primary cyclone drain (PD), the secondary cyclone drain (SD), and the baghouse. The tar cracking activity of a mixture of CD, PD, and SD, mixed in a definite proportion, was tested. Because a significant portion of this mixture was -80 mesh (
