1390
Ind. Eng. Chem. Res. 1987,26, 1390-1395
Flory, P. J.; Hocker, H. Trans. Faraday SOC.1971,67, 2258. Flory, P. J.; Orwoll, R. A.; Vrij, A. J.Am. Chem. SOC.1964,86, 3507. Fredenslund, Aa.; Gmehling, J.; Rasmussen, P. Vapor-Liquid Equilibria using UNIFAC; Elsevier Scientific: New York, 1977. Gmehling,J.; Onken, U. Vapor-Liquid Equilibrium Data Collection; DECHEMA Chemistry Data Series; DECHEMA: Frankfurt, ab 1977. Gmehling, J.; Rasmussen, P.; Fredenslund, Aa. Znd. Eng. Chem. Process Des. Deu. 1982, 21, 118. Guggenheim, E. A. Mixtures; Oxford: London, 1952. Hansen, C. M. J. Paint. Technol. 1967, 39(505), 104. Hellwege, K. H.; Knappe, N.; Lehmann, P. Kolloid-2. Z. Polym. 1962,183, 110. HGcker, H.; Blake, G. J.; Flory, P. J. Trans. Faraday SOC.1971,67, 2251. Hocker, H.; Flory, P. J. Trans. Faraday SOC.1971, 67, 2270. Huggins, M. L. Ann. N . Y . Acad. Sci. 1942, 43, 1. Landolt-Bornstein, T. Zahlenwerte und Funktionen, 6 ed.; Springer Verlag: New York, 1971; Vol. 2, Part 1.
Leung, Y.; Eichinger, B. E. Macromolecules 1974, 7, 685. McKinney, J. E.; Goldstein, M. J.Natl. Bureau St. A. Phys. Chem. 1974, 78A, 331. Mataumara, K.; Katayama, T. Kagaku Kogaku 1977, 38, 388. Nakajima, A.; Yamakawa, H.; Sakurada, I. J. Polym. Sci. 1959, 35, 189. Newmann, R. D.; Prausnitz, J. M. J . Paint Technol. 1973,45(585), 33. 1967,89, 26. Orwoll, R. A.; Flory, P. J. J. Am. Chem. SOC. Orwoll, R. A.; Flory, P. J. J. Am. Chem. SOC. 1967,89,6822. Prigogine, I. The Molecular Theory of Solutions; North Holland: Amsterdam, 1957. Ratzsch, M. T.; Glindemann, P.; Hamann, E. Acta Polym. 1980, 31(6), 377. Sanchez, I. C.; Lacombe, R. H. Macromolecules 1978, 11(6), 1145. Vera, J. H.; Prausnitz, J. M. Chem. Eng. J. 1972, 3, 1.
Received for review February 27, 1986 Accepted February 19, 1987
Catalytic Tar Conversion in Coal Gasification Systems Eddie G. Baker* and Lyle K. Mudge Battelle Pacific Northwest Laboratories, Richland, Washington 99352
Catalysts for coal tar conversion were tested in a fixed-bed catalytic reactor. Tar and water were mixed with a fuel gas to simulate coal gas from a fixed-bed gasifier. Tests were conducted at 450-690 O C , 1010-1810 kPa, and 1-8-s residence time. Acid cracking catalysts were found t o be the most effective of the catalysts tested. Coke was a primary product, and periodic regeneration was required t o maintain catalyst activity. Hydrocracking catalysts containing CoMo, NiW, and Pd reduced the coke yield from tar cracking but did not reduce the rate of catalyst deactivation.
Background The U.S. Department of Energy's Morgantown Energy Technology Center (METC) is developing technology to economically remove contaminants in hot gas streams produced by both coal gasification or combustion. Hot gas from coal gasifiers could be used in advanced power generation systems such as integrated gasification combined cycles using gas turbines and in fuel cell systems (Cicero and Jain, 1985). Fixed-bed gasification represents the most highly developed of all coal gasification technologies. However, one of its major drawbacks compared to other gasification systems is the production of tars and oils. If not removed from the gas, tars may deposit coke downstream on the high-temperature desulfurization sorbents being developed by METC, on fuel cell electrodes, or in a gas turbine. In addition, organic sulfur compounds in the tar are not removed by high-temperature desulfurization sorbents and may be detrimental to fuel cell systems or gas turbines. As a result, METC was interested in developing a means of removing tars from coal gas a t 500-750 "C and 10002000 kPa (10-20 atm) that could be integrated into advanced power plant systems utilizing fixed-bed gasifiers and hot gas cleanup. The objective of this study a t Battelle-Northwest (BNW) was to evaluate catalysts for removal or conversion of tar in hot coal gas streams. Tars account for only a small percentage of the gasification products, but they are responsible for many of the problems encountered in downstream processing. Therefore, the primary goal of this study was to remove the tar, particularly the organic sulfur compounds, from the gas. Sulfur in the tar should be converted to H2S or COS which could be removed by the high-temperature desulfurization sorbents. Ideally the tar would be converted to a gas product which would re0888-5885/87/2626-1390$01.50/0
cover the heating value of the tar in the gas stream and improve the process efficiency, but this was a secondary objective. I t was envisioned that a fixed bed of catalyst for tar conversion would be placed in front of a high-temperature desulfurization sorbent in the same vessel. The tar cracking catalyst would be operated and regenerated in conjunction with the desulfurization sorbent.
Catalysts for Tar Conversion Acidic cracking and hydrocracking catalysts have been studied extensively for conversion of coal liquids, including tars, to lighter more valuable products, and there are several excellent reviews of this technology (Crynes, 1981; Janardanarao, 1982). One recent study is particularly applicable to our effort. Wen (1984) cracked coal tar from the METC fixed-bed gasifier with a large number of catalysts including synthetic and natural zeolites and several natural clay minerals. A synthetic zeolite, LZ-Y82, was the most effective catalyst tested. The zeolites with wide pores (>0.7 nm) were generally more effective than the smaller pore materials. Coke and gas were the major products in the range of conditions studied by Wen, 400-800 "C, 1-5-s residence (contact) time, and 101 kPa (1 atm) of inert gas. Coal tar contains insufficient hydrogen and oxygen to convert all of the tar to gases by pyrolysis/cracking. Unless hydrogen or oxygen is added from an outside source, production of coke is inevitable. Hydrogen and oxygen (in the form of steam) are available in the raw coal gas. Raw coal gas from the METC gasifier contains about 30% steam on a wet gas basis. The steam, hydrogen, and carbon dioxide in the gas can gasify the coke off the catalyst surface after it forms. The rate of these reactions is too slow to be of significance at temperatures much 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 1391
1
A RD
-r
I
I
500 psig
1
n
cw and Motor
N2
Produc Gas
Condenser
Gas
e-
Separator
EPR CV
Back Pressure Regulator
CW
Meter
Cooling Water
Mass Flaw Indicator
F1 MV
Liquid Collector
Metering Valve
PI
Pressure Gage
TI
Thermocouple
RV
W e t Test
Check Valve
In-Line 02 Analyzer
Relief Valve ISPrlng Loaded)
Figure 1. Flow schematic of experimental system. Table I. Simulated Coal Gas Composition comcomcomponent vol % ponent vol % ponent vol % H2 16.3 CO 10.5 H20 31.0 30.1 tar 0.4 (2.3 w t %, C02 8.7 N2 0.4 125 mW) CHI 2.6 H2S
below 750 "C (Baker and Mudge, 1984). Raw gas from the METC fixed-bed gasifier contains 15-20% hydrogen on a dry gas basis (Pater, 1984), and at 1010-2020-kPa (10-20 atm) total pressure this gives 202-404 kPa (2-4 atm) of hydrogen pressure. To take advantage of the hydrogen in the gas will require a catalyst with a hydrogenation component. Hydrocracking catalysts consisting of acid sites for cracking and metal sites for hydrogenation are used in petroleum cracking. The metal sites keep the acid sites clean by hydrogenating the coke precursors formed (Choudhary and Saraf, 1975). At the low pressures and short residence times appropriate for coal gasification systems, hydrocracking catalysts will not likely eliminate coke production but they may reduce it and extend the lifetime of the catalyst before regeneration is required. Table 11. Tar Characteristics elemental anal. C H 0
N S
wt % (av of 5 samples)
84.3 f 0.3 6.7 f 0.1 4.4 f 0.4 0.9 f 0.1 3.7 f O.fi
Other catalysts that have been tested for conversion of tars or aromatic hydrocarbons include nickel and platinum steam reforming catalysts (Stern, 1982; Kertamus and Woolbert, 1976; Baker and Mudge, 1984), iron oxide (Tamhankar et al., 1985), and calcium oxide (Ellig et al., 1985). Deactivation by H,S was a problem with the steam reforming catalysts and with the iron oxide. With calcium oxide, coke was the major product and deactivation was rapid.
Experimental System The experimental system, shown schematically in Figure 1,was used to test catalysts a t conditions similar to those downstream of a fixed-bed coal gasifier. Coal-derived gases were simulated by combining a mixed fuel gas with a gas containing H,S in nitrogen. Both of the gases used were calibration gas mixtures supplied by Matheson Gas Products. Metering pumps were used to inject water and tar into the gas stream just upstream of the reactor. The composition of the simulated raw coal gas is shown in Table I. Coal tar from the fixed-bed gasification of Arkwright coal was provided by METC. The average analysis of five trace metal anal. Fe Ni Zn
ppm (av of 5 samples) 293 f 166 8 f 2 49 i 13
distillation-GC /MS analvsis fraction 1
2 3 4 5 a
normal bp, "C 110-210 210-270 270-370
amt, wt
370-450
17 28
>450
%
20 15 20
maior comuonents toluene, xylenes, phenol, cresols, indene naphthalene, methylnaphthalenes, dimethylnaphthalenes, thianaphthene acenaphthylene, dibenzofuran, fluorene, trimethylnaphthalene, xanthene, carbazols, methylnaphthol, anthracene, phenanthrene, methylanthracenes, methylphenanthrenes, dibenzothiophene fluoranthene, pyrene, methylpyrene, chrysene, benzfluoranthene, benzanthracenes a
Brittle, coke-like residue remaining in distillation flask.
1392 Ind. Eng. Chem. Res., Vol. 26, No. 7 , 1987
0, and N) analysis of the tars and an X-ray fluorescence
Table 111. Operating Conditions pressure, kPa (atm) temp, O C gas space velocity, GHSV residence (contact) time, s tar feed rate, g/h catalyst vol, cm3
1010-1818 (10-18) 450-700 1130-9070 1-8 4.2-4.7 25-100
different 1-L samples used in the tests is given in Table 11. Distillation of the coal tar showed that 72 wt % of the tar was volatile at 450 "C and atmospheric pressure. The remainder formed a coke-like solid in the distillation flask. Analysis of the distillation fractions with a gas chromatograph/mass spectrometer identified the major tar components as fused ring aromatics and heterocyclic compounds. The reactor in Figure 1 is a gradientless reactor made by Autoclave Engineers. A high-speed fan recirculates gases through the catalyst bed to maintain high gas velocities in the bed and minimize heat- and mass-transfer limitations. The reactor has a maximum allowable working pressure of 5000 kPa (50 atm) at 760 "C and has maximum bed volume of about 100 cm3. The simulated raw gas was preheated as it passed through the reactor head and was also heated by mixing with the recycled gas. The steam and tar were vaporized in the feed line as it passed through the reactor head or in a ceramic cup placed on top of the catalyst bed to catch any water or tar that entered the reactor as a liquid. A solid coke formed in the cup during each run. The amount of coke in the ceramic cup was about the same as the amount of residue in the distillation test (see Table 11). Gas from the reactor passes through a condenser and a separator to remove liquids. After pressure letdown through a back-pressure controller, the gas passed through a cold finger to remove additional condensate and was metered and sampled before venting. A side stream of gas was passed through scrubbers to remove light volatile oils (benzene, toluene, and xylene) for analysis. Table I11 shows the range of flow rates and operating conditions tested. The composition of the dry feed gas and the gas from the reactor was determined with a Carle Series 400 analytical gas chromatograph. Condensate was collected periodically, and the tars are separated, weighed, and analyzed. A Perkin-Elmer 240 was used for elemental (C, H,
spectrometer was used for sulfur analysis. A Perkin-Elmer 3920B gas chromatograph was used to analyze the light oil fraction. Compound identification in the tars and light oil fraction was made with a Hewlett-Packard 5992B gas chromatograph/mass spectrometer (GC/MS). Used catalysts were analyzed for carbon and sulfur with the Perkin-Elmer elemental analyzer and the X-ray fluorescence spectrometer. Catalysts. Table IV gives the properties of the catalysts tested for tar cracking. All were obtained commercially except for the CoMo/LZ-Y82 which was made by incipient wetness impregnation of ammonium molybdate and cobalt nitrate on LZ-Y82. All catalysts were calcined at 500 "C for 4 h prior to use.
Results and Discussion Between 20 and 30 wt % of the tar does not volatilize in the reactor system (and in distillation tests-see Table 11) and forms coke in the ceramic cup on top of the catalyst bed. The fact that this fraction of the tar does not revolatilize at conditions similar to the METC gasifier outlet conditions indicates these heavy polycyclic aromatics may have exited the gasifier as an aerosol mist. These compounds coke rapidly on the first hot surface they encounter, in our case the ceramic cup. In a larger system, a bed of inert material could be placed in front of the catalyst bed to remove these heavy compounds. The coke could be burned off of the inert material during the catalyst regeneration cycle. Tar conversion is based on the amount of tar fed and the amount of residual unconverted tar recovered from the product gas each hour during the test. By use of this definition, the nonvolatile tar converted to coke in the ceramic cup is counted as being converted. The coke in the ceramic cup is included in the coke yield for the process. Sulfur conversion was calculated from the tar conversion and the sulfur content of the feed and residual tar. In most all cases, some desulfurization of the residual tar does occur, so sulfur conversion is higher than tar conversion. In most all of the tests, tar conversion decreased with time on stream. Tar conversion results, except where noted, are the average for 4-h test runs. Catalyst Screening Tests. Table V shows the results obtained with nine different catalysts at the screening conditions of 550 "C, 1010 kPa, and 1-s residence time (GHSV = 9070). These results showed the acidic cracking
Table IV. Comoosition and Prooerties of Catalysts catalyst Y-zeolite (LZ-Y82)
composition, w t % 6670 SiO, 34% ALOq
