Energy Fuels 2010, 24, 4206–4214 Published on Web 07/16/2010
: DOI:10.1021/ef100490m
Comparative Investigation on Chemical Looping Combustion of Coal-Derived Synthesis Gas containing H2S over Supported NiO Oxygen Carriers Ewelina Ksepko,† Ranjani V. Siriwardane,*,‡ Hanjing Tian,‡,§ Thomas Simonyi,‡,§ and Marek Scia-z_ ko† †
Institute for Chemical Processing of Coal, 1 Zamkowa, 41-803 Zabrze, POLAND, ‡U.S. Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Road, P.O. Box 880, Morgantown, West Virginia 26507-0880, and §URS, 3610 Collins Ferry Road, Morgantown, West Virginia 26505 Received April 19, 2010. Revised Manuscript Received June 29, 2010
Chemical looping combustion (CLC) of simulated coal-derived synthesis gas was conducted with NiO oxygen carriers supported on SiO2, ZrO2, TiO2, and sepiolite. The effect of H2S on the performance of these samples for the CLC process was also evaluated. Five-cycle thermogravimetric analysis (TGA) tests at 800 °C indicated that all oxygen carriers had a stable performance at 800 °C, except NiO/SiO2. Full reduction/oxidation reactions of the oxygen carrier were obtained during the five-cycle test. It was found that support had a significant effect on reaction performance of NiO both in reduction and oxidation rates. The reduction reaction was significantly faster than the oxidation reaction for all oxygen carriers, while the oxidation reaction is fairly slow due to oxygen diffusion on NiO layers. The reaction profile was greatly affected by the presence of H2S, but there was no effect on the capacity due to the presence of H2S in synthesis gas. The presence of H2S decreased reduction reaction rates significantly, but oxidation rates of reduced samples increased. X-ray diffraction (XRD) data of the oxidized samples after a five-cycle test showed stable crystalline phases without any formation of sulfides or sulfites/sulfates. Increase in reaction temperature to 900 °C had a positive effect on the performance. Among various transition metal oxides (NiO, CuO, Mn2O3, Fe2O3, Co3O4, WO3),6-27 bulk or supported NiO oxygen carriers got a lot of attention because of their greater reactivity and better thermal stability.7-9,11-17 Our previous investigations on bentonite-supported NiO and CuO indicated that the supported materials possess better reactivity and stability than the corresponding bulk metal oxide.26,27 Adanez et al. investigated the support effect for Ni, Cu, and Fe oxides and concluded that Al2O3 and TiO2 were the best
Introduction Carbon dioxide (CO2), the primary “greenhouse gas” for possible global climate change, is largely produced during fossil fuel combustion. The current commercial CO2 separation technology has the disadvantage of requiring a large amount of energy. Chemical looping combustion (CLC), which utilizes the oxygen from metal oxide instead of air to combust fuels, is a novel combustion technology that may be an answer to the energy penalty problem.1 The significant advantage of a CLC system is that a concentrated CO2 stream can be obtained from the combustion gas stream after water condensation without requiring any energy for separation or purification.1-5 In addition, nitrogen oxide (NOx) production is also greatly reduced.
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*To whom correspondence should be addressed. Tel.: 304-285-4513. Fax: 304-285-0903. E-mail:
[email protected]. (1) Richter, H. J.; Knoche, K. ACS Symposium Series 235, ACS: Washington DC, 1983, p 71-85. (2) Anheden, M.; Svedberg, G. Energy Convers. Manage. 1998, 39 (16-18), 1967–1980. (3) Ishida, M.; Jin, H. Energy 1994, 19 (4), 415–422. (4) Brandvoll, Ø.; Bolland, O. J. Eng. Gas Turbines Power 2004, 126, 316–321. (5) Lyngfelt, A.; Krongber, B.; Adanez, J.; Morin, J.-X.; Hurst, P. Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies, GHGT-7, Vancouver, British Columbia, Canada, 2004. (6) Garcı´ a-Labiano, F.; Diego, L. F.; Adanez, J.; Abad, A. Ind. Eng. Chem. Res. 2004, 43, 8168–8177. (7) Garcı´ a-Labiano, F.; Adanez, J.; Diego, L. F.; Abad, A. Energy Fuels 2006, 20, 26–33. (8) Garcı´ a-Labiano, F.; Diego, L. F.; Adanez, J.; Abad, A.; Gayan, P. Chem. Eng. Sci. 2005, 60, 851–862. (9) Jerndal, E.; Mattisson, T.; Lyngfelt, A. Trans IchemE, Part A, Chem. Eng. Res. Des. 2006, 84 (A9), 795–806. (10) Diego, L. F.; Labiano, F. G.; Adanez, J.; Gayan, P.; Abad, B.; Corbella, M.; Palaciob, J. M. Fuel 2004, 83, 1749. (11) Cho, P.; Mattisson, T.; Lyngfelt, A. Fuel 2004, 83, 1215–1225. (12) Adanez, J.; Diego, L. F.; Labiano, F. G.; Gayan, P.; Abad., P. Energy Fuels 2004, 18, 371–377. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 4206–4214
: DOI:10.1021/ef100490m
Ksepko et al.
Figure 1. Five-cycle reduction (synthesis gas)/oxidation (air) TGA data for (a) NiO/sepiolite, (b) NiO/SiO2, (c) NiO/TiO2, and (d) NiO/ZrO2 at 800 °C.
supports for NiO.12 Cho et al. investigated the performance of Ni, Cu, and Fe supported on alumina. The results indicated that NiO/Al2O3 had limited strength.11 Therefore, most of the work was conducted to optimize the support selection and preparation methods to stabilize NiO. Zhao et al. prepared a NiO/NiAl2O4 oxygen carrier using a sol-gel method and reported stable reactivity from a multicycle TGA test with coal char and H2 above 850 °C.28 NiO/MgAl2O4 prepared by Ryden et al. using a freeze granulation method showed stable reactivity for 120 min with natural gas as fuels, while NiO/ Al2O3 prepared using a wet-impregnation method showed carbon formation during the similar tests.29 Gayan et al. showed that the agglomeration of Ni was very sensitive to the preparation technique of the NiO/Al2O3 oxygen carrier and the amount of Ni present in the oxygen carrier during the CLC reaction with natural gas.30 The spray-drying method was also used for the preparation of NiO oxygen carriers for large-scale testing of methane CLC.31 NiO/Al2O3 prepared by coprecipitation was reported for coal CLC, but its performance gradually declined due to sintering and sulfur poisoning.32 In these NiO/Al2O3 oxygen carriers, extra NiO must be added during the preparation since some of the NiO interacts with Al2O3 to form NiAl2O4 and this could increase
the cost of the oxygen carrier. Therefore, it is very important to evaluate low-cost supports which have minimal interaction with NiO for the preparation of NiO oxygen carriers. CLC reaction kinetics of NiO oxygen carriers with gaseous fuels have been investigated by many researchers. Ryu et al. have investigated the effect of particle size of NiO oxygen carriers by a macroscopic reaction model.33 They indicated that the reaction rate control resistances of oxidation and reduction are different due to a different extent of penetration and diffusion of reactant gas through the Ni or NiO layer. The oxidation reaction was found to be product layer diffusioncontrolled, and the reduction reaction was chemical-reaction controlled; the global oxidation rate was generally slower than reduction rates. Garcia-Labiano et al. developed a particle reaction model based on a shrinking-core model8 to determine the temperature variations along the radial direction of the oxygen carrier particles during both reduction and oxidation stages. The shrinking-core model is also applied for the NiO oxygen carrier system to explain reaction kinetics, and it is widely accepted as the best model for the oxidation reaction.34-36 Sepiolite37 is a naturally occurring, very inexpensive, and readily available mineral. It will be an excellent low-cost support for oxygen carrier preparation if sepiolite does not negatively influence the reaction performance of metal
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: DOI:10.1021/ef100490m
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Figure 2. Fractional reduction as a function of time for NiO/ sepiolite, NiO/SiO2, NiO/TiO2, and NiO/ZrO2 for the third cycle at 800 °C.
Figure 3. Reaction rate as a function of fractional reduction for NiO/sepiolite, NiO/SiO2, NiO/TiO2, and NiO/ZrO2 for the third cycle at 800 °C.
oxides. In this work, we report the CLC performance of NiO oxygen carriers prepared with sepiolite support which has a structural formula of Mg4Si6(OH)2 3 6H2O. The CLC performance was evaluated with simulated coal-derived synthesis gas. For comparison, the performances of NiO supported on SiO2, ZrO2, and TiO2 were also evaluated under similar conditions. Sulfur is the major impurity in coal synthesis gas, as well as in natural gas. Depending on the coal type and resource, coalderived synthesis gas may contain 200-8000 ppm H2S, which may interact with a metal-oxide oxygen carrier during the combustion reaction, thus affecting the performance of the CLC system. Interaction of H2S with metal oxides suitable for desulfurization sorbents have been reported at temperatures (500-700 °C)38-43 lower than those suitable for CLC. Our previous work with an NiO oxygen carrier supported on bentonite showed that polymeric NiSx species are formed during reduction and sulfur can be totally removed by reoxidation.44 NiO/bentonite showed stable performance during a multicycle test with synthesis gas in the presence of H2S, but both reduction and oxidation rates decreased in the presence of H2S. Bubbling fluidized bed tests conducted by other researchers confirmed that sulfur can negatively influence the reaction performance and suggested that the industrial CLC plant should decrease H2S concentration below 100 ppm.45 Therefore, it is important to systematically investigate the effect of H2S on the reaction performance of a metal-oxide oxygen carrier with various supports to successfully develop a chemical-looping combustion system based on coal synthesis gas or natural gas.
Figure 4. Fractional oxidation as a function of time for NiO/ sepiolite, NiO/SiO2, NiO/TiO2, and NiO/ZrO2 for the third cycle at 800 °C.
In this study, the reduction/oxidation performances of NiO supported on sepiolite, SiO2, ZrO2, and TiO2 were investigated by thermogravimetric analysis/mass spectrometry (TGA/MS) with simulated coal-derived synthesis gas in the presence of H2S during five-cycle reduction/oxidation tests. The effect of temperature was also investigated. The bulk compositions of reacted samples were analyzed by X-ray diffraction (XRD). Other physical characterizations such as BET surface areas, particle size analysis, attrition resistance, and melting behavior were also conducted.
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Experimental Section Preparation of NiO Supported on SiO2/ZrO2/TiO2/Sepiolite. Oxygen carriers containing 60 wt % NiO supported on SiO2, ZrO2, TiO2, and sepiolite were prepared by the mechanical mixing method. NiO (Sigma Aldrich,
