Energy & Fuels 2007, 21, 3063–3069
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Articles Catalytic Performance of Limonite in the Decomposition of Ammonia in the Coexistence of Typical Fuel Gas Components Produced in an Air-Blown Coal Gasification Process Naoto Tsubouchi,* Hiroyuki Hashimoto, and Yasuo Ohtsuka Research Center for Sustainable Materials Engineering, Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira, Aoba-ku, Sendai 980-8577, Japan ReceiVed February 19, 2007. ReVised Manuscript ReceiVed July 28, 2007
Catalytic decomposition of 2000 ppm NH3 in different atmospheres with an Australian R-FeOOH-rich limonite ore at 750–950 °C under a high space velocity of 45000 h-1 has been studied with a cylindrical quartz reactor to develop a novel hot gas cleanup method of removing NH3 from fuel gas produced in an air-blown coal gasification process for an integrated gasification combined cycle (IGCC) technology. The limonite shows very high catalytic activity for the decomposition of NH3 diluted with inert gas at 750 °C, regardless of whether the catalyst material is subjected to H2 reduction before the reaction or not. Conversion of NH3 to N2 over the reduced limonite reaches g99% at 750–950 °C, and the catalyst maintains the high performance for about 40 h at 750 °C. When the decomposition reaction is carried out in the presence of fuel gas components, the coexistence of syngas (20% CO/10% H2) causes not only the serious deactivation of the limonite catalyst but also the appreciable formation of deposited carbon and CO2. On the other hand, the addition of 10% CO2 or 3% H2O to the syngas improves the catalytic performance and concurrently suppresses the carbon deposition almost completely, and the NH3 conversion in the 3% H2O-containing syngas reaches about 90% and almost 100% at 750 and 850 °C, respectively. Influential factors controlling the catalytic activity of the limonite ore in the coexistence of fuel gas components are discussed on the basis of the results of the powder X-ray diffraction measurements, thermodynamic calculations, and some model experiments.
Introduction A hot gas cleanup method to remove a low concentration of ammonia (NH3), usually 1000–5000 ppm, in fuel gas produced in high-temperature coal gasification, in place of cold gas cleaning technologies with wet scrubbers, has attracted much attention as one of the most promising options to increase further the power generation efficiency of an integrated gasification combined cycle (IGCC) system under development.1 Several catalysts, such as Ni1–6,8-, Cu7,8-, Mo1,3-, and Ru1,3-based catalysts, have thus been developed to efficiently decompose * Corresponding author: phone +81-22-217-5654; Fax +81-22-2175655; e-mail
[email protected]. (1) Mitchell, S. C. Hot Gas Cleanup of Sulfur, Nitrogen, Minor and Trace Elements; IEA Coal Research: London, 1998 (CCC/12). (2) Leppälahti, J.; Simell, P.; Kurkela, E. Fuel Process. Technol. 1991, 29, 43–56. (3) Mojtahedi, W.; Abbasian, J. Fuel 1995, 74, 1698–1703. (4) Simell, P.; Kurkela, E.; Ståhlberg, P.; Hepola, J. Catal. Today 1996, 27, 55–62. (5) Jothimurugesan, K.; Gangwal, S. K. In High Temperature Gas Cleaning; Schmidt, E., Gäng, P., Pilz, T., Dittler, A., Eds.; G. Braun Printconsult GmbH: Karlsruhe, Germany, 1996; pp 383–392. (6) Wang, W.; Padban, N.; Ye, Z.; Olofsson, G.; Andersson, A.; Bjerle, I. Ind. Eng. Chem. Res. 2000, 39, 4075–4081. (7) Ismagilov, Z. R.; Shkrabina, R. A.; Yashnik, S. A.; Shikina, N. V.; Andrievskaya, I. P.; Khairulin, S. R.; Ushakov, V. A.; Moulijn, J. A.; Babich, I. V. Catal. Today 2001, 69, 351–356. (8) Dou, B.; Zhang, M.; Gao, J.; Shen, W.; Sha, X. Ind. Eng. Chem. Res. 2002, 41, 4195–4200.
NH3 into N2 and H2, and some inexpensive catalyst materials have been proposed for this purpose.2,9–12 The principal results and issues are summarized as follows: (1) Limestone2,9,10 and carbon-supported CaO11 can promote the decomposition of NH3 in inert gas, but these Ca-based catalysts lose the catalytic ability almost completely in a stream of fuel gas atmosphere. (2) Ferrous dolomite and sintered iron ore catalyze NH3 decomposition in the coexistence of fuel gas components and provide NH3 conversions of 75–85% at 900 °C.2 However, their catalytic effects are insufficient. (3) Fine particles of metallic iron (R-Fe), which are produced by pyrolysis of Fe3+ cations incorporated into brown coals, maintain conversion of NH3 to N2 at the high level of 95% at 750 °C in a simulated fuel gas, but part of the R-Fe is deactivated by the formation of iron carbides by the reaction with char substrate as the Fe support.11 These results clearly indicate that Fe-based catalysts are more active than Ca-based catalysts and that nanoscale R-Fe particles (9) Shimizu, T.; Karahashi, E.; Yamaguchi, T.; Inagaki, M. Energy Fuels 1995, 9, 962–965. (10) Chambers, A.; Yoshii, Y.; Inada, T.; Miyamoto, T. Can. J. Chem. Eng. 1996, 74, 929–934. (11) Ohtsuka, Y.; Xu, C.; Kong, D.; Tsubouchi, N. Fuel 2004, 83, 685– 692. (12) Xu, C.; Tsubouchi, N.; Hashimoto, H.; Ohtsuka, Y. Fuel 2005, 84, 1957–1967.
10.1021/ef070096j CCC: $37.00 2007 American Chemical Society Published on Web 09/19/2007
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prepared without carbonaceous materials exhibit high and stable catalytic performance in the decomposition of NH3 in fuel gas atmosphere. The present authors’ research group has recently found that an Australian low-valued iron ore (limonite) containing a large amount of goethite (R-FeOOH), which can readily be transformed into highly dispersed metallic Fe during H2 reduction, achieves the almost complete decomposition of 2000 ppm NH3 in inert gas to N2 at 500 °C and maintains the high catalytic activity in a long run for 50 h.13 We have also shown that the limonite catalyst has a high tolerance to several hundred ppm of H2S in the reaction at 750 °C and exhibits the high NH3 conversion of almost 100% for 50 h in the coexistence of 100 ppm H2S.14 Although these observations suggest that the limonite may be a promising catalyst material for NH3 decomposition, the influence of coexisting fuel gas on the catalytic activity should be investigated. In the present paper, therefore, we first examine the catalytic performance of the limonite in the decomposition of 2000 ppm NH3 not only in inert gas but also in the coexistence of fuel gas components, such as CO, H2, CO2, and H2O, produced in an air-blown coal gasifier for an IGCC system, and then make clear some factors that can affect the extent of the NH3 conversion. Experimental Section Catalyst Materials and Preparation. Limonite ore containing about 90 mass % of R-FeOOH, Yandi-Yellow limonite from Australia, was used as a catalyst precursor in the present study. The metal composition in the dried limonite is as follows: Fe, 55.6; Si, 2.3; Al, 1.4; Ca, 0.2; Mg, 0.1 mass %, the BET surface area and particle size being 20 m2/g and 99.99995%) was then passed at 200 cm3 (STP)/min until N2 concentration in the entire reaction system was decreased to less than 20 ppm. After such prudent precautions against leakage, the reactor was heated electrically in high-purity He up to 500 °C. At this temperature, the He was switched to pure H2, and the catalyst material was reduced with the H2 for 2 h. After the H2 reduction, the atmosphere was restored to the He, and the reactor was again heated to be held at a predetermined reaction temperature (750–950 °C). The standard reaction conditions are as follows: NH3 concentration, 2000 ppm; balance gas, high-purity He or simulated fuel gas; total flow rate, 300 cm3 (STP)/min; temperature, 750 °C; space velocity, 45000 h-1; apparent contact time between gas and catalyst, 0.080 s. Three kinds of the simulated gases, 20% CO/10% H2 (denoted as syngas), 20% CO/10% H2/ 10% CO2 and 20% CO/10% H2/3% H2O, were used, each gas being balanced with high-purity He. Nitrogen Analysis. The amount of N2 produced by NH3 decomposition was analyzed online at intervals of 2.5 min with a high-speed micro gas chromatograph (GC) (Hewlett-Packard) equipped with a thermal conductivity detector. NH3 unreacted and HCN formed were determined at intervals of 3 min with a multigas (13) Tsubouchi, N.; Hashimoto, H.; Ohtsuka, Y. Catal. Lett. 2005, 105, 203–208. (14) Tsubouchi, N.; Hashimoto, H.; Ohtsuka, Y. Powder Technol. 2007, in press.
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Figure 1. Effects of limonite (A), Fe3O4 (B), and R-Fe2O3 (C) as catalyst precursors on the decomposition of 2000 ppm NH3 diluted with He at 750 °C.
monitor (Innova) employing the photoacoustic infrared detection method. In the NH3 decomposition runs in the presence of H2O, the NH3 and HCN were analyzed independently by the Gastec standard detector tube (Gastec) because the analytical accuracy of the gas monitor may be deteriorated by the H2O. The nitrogen mass balance fell within the reasonable range of 100 ( 6% in all runs. Conversion of NH3 to N2 or HCN was estimated by using the amounts of both NH3 fed and N2 or HCN formed, respectively, and it was expressed in percent on a nitrogen basis. X-ray Diffraction Measurements. The powder X-ray diffraction (XRD) measurements of limonite samples after H2 reduction and after NH3 decomposition were made with an X-ray diffractometer (Shimadzu) using Mn-filtered Fe KR radiation. When the reduced limonite catalyst was exposed to laboratory air for recovery from the reactor, the largely exothermic reaction always occurred because of rapid oxidation of fine particles of metallic iron (R-Fe) formed and consequently caused the undesirable changes of the catalyst forms.13 To avoid this phenomenon, only the surface layer of the reduced limonite was first passivated with 1% O2/He at ambient temperature, and the resulting sample was then recovered for the XRD analysis. Such an O2-passivation pretreatment was not needed for the used limonite catalysts. The average crystalline size of R-Fe identified in the recovered samples was calculated by the Debye– Scherrer method.
Results Decomposition of Ammonia in Inert Gas. Figure 1 shows the effects of limonite, Fe3O4, and R-Fe2O3 as catalyst precursors on conversion of NH3 to N2 against time on stream at 750 °C. The conversion was as small as 99%) observed with the reduced limonite, but it increased with increasing time on stream, reached >99 % after 1 h, and did not change for further 3 h. The XRD measurement after the reaction showed the formation of metallic Fe, in other words, the reduction of R-FeOOH in the limonite by NH3 fed and/or H2 formed from NH3. It is thus evident that the H2-reduction pretreatment is not needed for realizing the high activity of the limonite catalyst. Figure 2 also shows the changes in the conversion over the reduced limonite in a long run at 750 °C and the concentration of NH3 in the reactor outlet. The limonite maintained the high conversion of more than 99% for a prolonged time of about 40 h. Such a stable catalytic performance of the limonite is interesting from a practical point of view, and it may thus indicate that the limonite is a promising catalyst material for NH3 decomposition in inert gas. As seen in Figure 2, the NH3 concentration was slightly larger at a longer time, and the value after 37 h was ∼25 ppm, which was higher than the concentration (850 °C are thus necessary for realizing the NH3 conversion of >90% under such conditions. The conversion of almost 100% observed at 850 °C in Figure 6 must be decreased in a pressurized N2-rich fuel gas. The limonite catalyst may rather be suitable for the decomposition of NH3 in a N2-free fuel gas produced from an O2-blown gasifier, though the catalytic activity should be examined in the future work. Conclusions Catalytic performance of R-FeOOH-rich limonite in the decomposition of 2000 ppm NH3 in fuel gas components, which can simulate product gas from an air-blown coal gasifier, has
Catalytic Decomposition of Ammonia
been studied with a fixed bed quartz reactor at a high space velocity of 45000 h-1. The principal conclusions are summarized as follows: (1) The limonite catalyst achieves the almost complete decomposition of NH3 in inert He at 750–950 °C and maintains conversion of NH3 to N2 at the high level of more than 99% for about 40 h. (2) Although the coexistence of 20% CO/10% H2 at 750–950 °C deactivates the limonite with appreciable formation of deposited carbon and CO2, 10% CO2 or 3% H2O added to the syngas improves the catalytic activity and restores the conversion at 750 °C to the high value of ∼90% without carbon deposition, the conversions at 850 and 950 °C in the H2O-containing syngas being almost 100%.
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(3) The XRD measurements reveal that metallic Fe formed from R-FeOOH upon H2 reduction is converted predominantly to Fe3C in the coexistence of syngas alone, whereas metallic Fe is the only Fe species after NH3 decomposition in the CO2- or H2O-containing syngas. These oxidizing gases work for suppressing of the carbon deposition and consequently keeping the limonite surface as catalytically active metallic Fe. Acknowledgment. This work was supported in part by the Japan Science and Technology Agency (JST). The authors acknowledge the supply of limonite from Kobe Steel Ltd. and Mitsubishi Chemical Corp. EF070096J
