(265c) Effect of Sulfur On CoMo/Al2O3 Catalyst for the Coal Pyrolysis

1. Introduction Pyrolysis is a common stage of many coal utilization processes, such as gasification, combustion, or liquefaction. Pyrolysis can also be considered as a possible pre-treating method to obtain clean feedstock and fuels from raw coal. However, during pyrolysis chemical forms and the state of trace elements may also change, which may affect the final fate of the elements and subsequently results in different environmental effect [1-5]. The presence of sulfur in coal is generally undesirable and presents problems in the utilization of coal in various processes, so the transformation of sulfur and desulfurization of coal during thermal processing have long been a topic of great importance. Sulfur transformation during pyrolysis directly influence and control the sulfur distribution in gas, tar and char for the whole process. Many studies have reported the effect of sulfur on the coal pyrolysis [5,6]. However, few studies were focused on the effect of sulfur on CoMo/Al2O3 catalyst for the coal pyrolysis. This paper will discuss the sulfur content on the catalytic performance of the CoMo/Al2O3 catalyst. Catalyst samples with different sulfur contents for the coal pyrolysis were characterized by BET and AAS. The results present in this study are aimed at providing a better understanding of the relationship between the catalyst modification with sulfur and the catalyst performances of CoMo/Al2O3 catalysts used in a PPFB during the coal pyrolysis process. 2. Experimental 2.1 Catalyst preparation. The CoMo catalyst precursors, the weight composition is CoO:MoO3: =4:13:83, used in this study were commercial product. The commercial CoMo/Al2O3 catalyst was labeled as CoMo/A. And the other four catalysts were prepared by the impregnant method. In brief, 2 g CoMo/A catalyst was impregnanted in different concentrate H2SO4 solutions by vigorous stirring for about 5 min at the room temperature. The catalyst samples were dried in air at 98 oC for 10 h. And then the dried catalysts were calcined in air at 600 oC for 2 h. The treated catalysts are CoMo/B with different sulfur, which were labeled as CoMo/B1, CoMo/B2, CoMo/B3, CoMo/B4 with the increasing sulfur content. The catalysts were ground and sieved to 250-500 µm before using. 2.2 Reactor system. The experiments were carried out in a power particle fluidized-bed (PPFB). A given weight of coal powder and the catalyst particle were packed continuously in a micro-feeder and the reactor, respectively. The cyclone wrapped by a ribbon heater for separating the char from gases. The gasbag is used to collect the gas product. The gas, liquid and solid products were collected to be analyzed quantitatively. H2, CO, CO2, CH4 (IOG: H2, CO, CO2)and C1-C3 hydrocarbons (HCG) in tail gas were collected in the gasbag and analyzed by Shimadzu gas chromatograph 2014 (GC-2014) with a MS-13X column ( N2 carrier) and a P-Q column (H2 carrier) equipped with a TCD and a FID. Light aromatic liquid (HCL), benzene, toluene, xylene and naphthalene (BTXN) were analyzed using the same GC with a 2 m activated carbon column (FID, N2 carrier). 2.3 Catalyst characterization. BET surface area of the catalysts was measured by N2 physisorption at -196 oC using a Micromeritics ASAP 2500 instruments. The samples were degassed under vacuum at 120 o C for 6 h prior to BET measurement. The composition of the catalyst and coal was determined by atomic absorption spectroscopy (AAS) on a EA1108-ELEMENTAL ANALYZER. 3. Results and Discussion 3.1 The sulfur loss To study the effect of sulfur content on the catalyst performances, a series of pyrolysis tests under the same reaction conditions are carried out. The sulfur loss during pyrolysis was shown in Fig. 1. It is found that sulfur loss in the catalysts during pyrolysis initial decreases notable with the extending pyrolysis duration and then gradually decreases to the stable values. Meanwhile, the sulfur loss increases with the increase of the sulfur content in the fresh catalysts. When the sulfur amount is high in the catalyst, the combination between the catalyst and sulfur is not fastness. And a part of sulfur lost with time on stream (TOS). From the beginning of TOS 25 min, the amount of sulfur of the catalysts reaches a stable value. Figure 1 should be placed here. 3.2 The catalyst activity and selectivity A series of pyrolysis tests under the same reaction conditions are carried out to systematically investigate the effect of sulfur content on CoMo/B catalyst activity and selectivity. The run time is typically for 40 min. The catalyst activity measured by the product yields is shown in Fig. 2. During the pyrolysis, the catalysts with different sulfur content exhibit significant differences in activity and selectivity. The catalyst activity decreases with the increase of sulfur content, from 35.02% (CoMo/B1) to 19.10% (CoMo/B4). The CoMo/A catalyst activity is 33.73%. The sulfur covered with the activitive site results in the catalyst activity decreasing. However, the activity of the CoMo/B1 and CoMo/A is very resemble. This result shows there is no obvious differences when the catalyst CoMo/A incorporated with a few of sulfur. The product distributions display considerable differences with the different catalyst sulfur content during pyrolysis. CoMo/B2 has the highest BTXN (benzene, toluene, xylene, and naphthalene) selectivity, however the lowest BTXN selectivity is obtained by CoMo/B1. CH4 is the main product during the coal pyrolysis using series of CoMo catalyst. Figure 2 should be placed here. 4. Conclusions The tests of the coal pyrolysis using CoMo catalyst were carried out in a power particle fluidized-bed (PPFB). It is found that the catalyst CoMo/A incorporated with a mount of sulfur could reform the catalyst activity. The catalyst activity decreases with the increase of sulfur content. There was no obvious differences in the total catalyst yields when the catalyst CoMo/A incorporated with a few of sulfur. CH4 was the main product during the coal pyrolysis using series of CoMo/B catalyst. The modified catalyst with sulfur, CH4 selectivity with time on stream was obviously changed. Acknowledgement We thank the National Natural Science Foundation of China for financial support under the contact numbers 20976132. References (1) Guo R. X.; Yang J. L.; Liu D. Y.; Liu Z. Y. Am. Chem. Soc., Div. Fuel Chem., 2002, 47(1), 201. (2) Attar A. Fuel, 1978, 57(4), 201. (3) Labiano F. G.; Hampartsoumian E.; Willians A. Fuel, 1995, 74(7), 1072-1079 (4) Ibarra J. V.; Bonet A. 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