Chemical-looping Combustion of Coal-derived Synthesis Gas Over

Oct 25, 2008 - Hanjing Tian,†,‡ Karuna Chaudhari,†,‡ Thomas Simonyi,†,‡ James Poston,† Tengfei Liu,§. Tom Sanders,§ Götz Veser,§ and...
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Energy & Fuels 2008, 22, 3744–3755

Chemical-looping Combustion of Coal-derived Synthesis Gas Over Copper Oxide Oxygen Carriers Hanjing Tian,†,‡ Karuna Chaudhari,†,‡ Thomas Simonyi,†,‡ James Poston,† Tengfei Liu,§ Tom Sanders,§ Go¨tz Veser,§ and Ranjani Siriwardane*,† U.S. Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Road, P.O. Box 880, Morgantown, West Virginia 26507-0880, Parsons, P.O. Box 618, Pittsburgh, PennsylVania 15129, and Department of Chemical Engineering, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15261 ReceiVed June 6, 2008. ReVised Manuscript ReceiVed September 4, 2008

CuO/bentonite and CuO-BHA nanocomposites were studied as oxygen carriers in chemical-looping combustion (CLC) of simulated synthesis gas. Global reaction rates of reduction and oxidation, as the function of reaction conversion, were calculated from 10-cycle oxidation/reduction tests utilizing thermogravimetric analysis at atmospheric pressure between 700 and 900 °C. It was found that the reduction reactions are always faster than oxidation reactions; reaction temperature and particle size do not significantly affect the reaction performance of CuO/bentonite. Multicycle CLC tests conducted in a high-pressure flow reactor showed stable reactivity for production of CO2 from fuel gas at 800 and 900 °C and full consumption of hydrogen during the reaction. Results of the tapered element oscillating microbalance showed a negative effect of pressure on the global rates of reduction-oxidation reactions at higher fractional conversions. X-ray diffraction patterns confirmed the presence of CuO in the bulk phase of the oxidized sample. Electron microanalysis showed significant morphology changes of reacted CuO/bentonite samples after the 10 oxidation-reduction cycles above 700 °C in an atmospheric thermogravimetric analyzer. The nanostructured CuO-BHA carrier also showed excellent stability and, in comparison to the CuO/bentonite system, slightly accelerated redox kinetics albeit at the expense of significantly increased complexity of manufacturing. Overall, both types of CuO carriers exhibited excellent reaction performance and thermal stability for the CLC process at 700-900 °C.

Introduction The recently announced U.S. Global Climate Initiative (GCCI) to reduce the greenhouse gas (GHS) intensity by 2012 has accelerated CO2 management.1 Several methods were intensively investigated for CO2 capture and sequestration, including alkaline sorbents, scrubbing solutions, electrochemical pumps, and membrane separation.2-7 However, the main disadvantage of these techniques is that a large amount of energy is required for separation, thereby reducing the overall efficiency of a power plant. Chemical-looping combustion (CLC) is a new flameless combustion technology that involves the use of a regenerable metal oxide as an oxygen carrier that transports oxygen from * To whom correspondence should be sent. Phone: 304-285-4513; fax: 304-285-0903; e-mail: ranjani.siriwardane@ netl.doe.gov. † U.S. Department of Energy. ‡ Parsons. § University of Pittsburgh. (1) President Bush Announces Clear Skies and Global Climate Change Initiative” Feb 14, 2002; U.S. Global Climate Change Initiative, information available on the web at http://www.epa.gov. (2) Granite, E. J.; O’Brien, T. Fuel Process. Technol. 2005, 86, 1423– 1434. (3) Siriwardane, R. V. Solid sorbents for remoVal of carbon dioxide from gas Streams at low temperatures, U.S.Patent No. 6908497. (4) Siriwardane, R. V.; Shen, M.; Fisher, E. Energy Fuels 2003, 17 (3), 571–576. (5) Daeho, K.; Siriwardnae, R. V.; Biegler, L. T. Ind. Eng. Chem. Res. 2003, 42 (2), 339–348. (6) Akten, E. D.; Siriwardane, R. V.; Sholl, D. S. Energy Fuels 2003, 17 (4), 977–983. (7) Siriwardane, R. V.; Robison, C.; Shen, M.; Simonyi, T. Energy Fuels 2003, 21 (4), 2088–2097.

the air to the fuel.8-12 In this process, direct contact between fuel and air is avoided, and high-purity CO2 is produced from the combustion of fuel gas. The CLC system is composed of two reactors, air and fuel reactors. In the fuel reactor, the fuel in the gaseous form reacts with the metal oxide: fuel(CO, H2) + metal oxide f CO2 + H2O + Metal ⁄ reduced metal oxide (1) The metal or a reduced form of metal oxide is oxidized in the air reactor to form metal oxide: metal ⁄ reduced metal oxide + O2 f Metal oxide

(2)

The regenerated metal oxide is then ready to initiate a second cycle. The exit gas from the fuel reactor produces a pure CO2 stream for usage or sequestration after water condensation. The significant advantage of a CLC system compared to normal combustion is that the CO2 stream is so concentrated that potentially less energy is needed as compared to conventional CO2 separation processes. In addition, since the metal oxide serves as an oxygen carrier, NOx production is also greatly (8) Richter, H. J.; Knoche, K. ACS Symposium Series, 235; ACS: Washington DC, 1983; p 71-85. (9) Anheden, M.; Svedberg, G. Energy ConVers. Mgmt. 1998, 39 (1618), 1967–1980. (10) Ishida, M.; Jin, H. Energy 1994, 19 (4), 415–422. (11) Brandvoll, Ø.; Bolland, O. J. Eng. Gas Turbines Power 2004, 126, 316–321. (12) 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; International Energy Agency: Paris, France, 2004.

10.1021/ef800438x CCC: $40.75  2008 American Chemical Society Published on Web 10/25/2008

CLC of Coal-deriVed Synthesis Gas OVer CuOx

Energy & Fuels, Vol. 22, No. 6, 2008 3745

reduced. Different economic assessments performed in the framework of the GRACE project for a 200 MW CLC boiler for use at BP’s Grangemouth refinery and by the CO2 capture project indicated that CLC is among the best options for reducing the cost of CO2 capture using fuel gases.2,12 Development of efficient oxygen carriers is essential to successfully operate a CLC system. Various oxygen carriers composed of a metal oxide have been tested by researchers, mainly with methane as the fuel gas, including bulk or mixed CuO,13-21 NiO,14-16,18-26 Fe2O3,13-15,18-21,27-29 MnOx,17-19,21,30 CoO,15,19,31 CrOx,20 WO3,15 BaO,15 SrO,15 as well as those oxide materials supported on Al2O3, SiO2, ZrO2, TiO2, bentonite, sepiolite, and yttrium-stabilized zirconia (YSZ), etc. It is generally accepted that Ni, Cu, and Fe oxides possess higher activity and enough feasibility for a CLC system. Adanez et al. investigated the support effect for Ni, Cu, and Fe oxides and concluded that TiO2 and SiO2 were the best supports for a Cubased oxygen carrier, whereas Al2O3 and TiO2 were the best supports for Fe2O3 and NiO, respectively.19 Cho et al. investigated the feasibility of Ni, Cu, and Fe supported on alumina with free granulation preparation. In that study, copper oxide and iron oxide agglomerated during the reaction, whereas the NiO oxide displayed limited strength.18 De Diego et al. systemically investigated copper oxide supported on alumina, silica, sepiolite, titania, and zirconia prepared by different methods, such as mechanical mixing, coprecipitation, and impregnation. Those tests concluded that the support effect would greatly increase the reduction/oxidation rate of bulk CuO. The oxygen carrier prepared by the mechanical mixing and coprecipitation method underwent unacceptable degradation of mechanical properties after a multicycle test, whereas the CuO supported on titania and silica by the wet impregnation method exhibited high reactivities during multicycle tests (100 reduction/ oxidation cycles, 400 min, 800 °C, ambient pressure) in thermogravimetric analysis (TGA).17 In summary, these tests indicate that further work should be conducted to stabilize the

dispersed-oxide species on suitable supports in order to develop a better oxygen carrier with higher reactivity and stability. Few studies have been reported for the integration of coal gasification and CLC in power plants.17,18,25,26,32,33 Kronberger et al.33 reported a mathematical model of a CLC system with an integrated solution of the mass and energy balances for integrated gasification combined cycles (IGCC) systems. The results revealed that parameters such as reactor cooling and oxygen carrier flow have to be chosen properly to guarantee appropriate temperatures in the reactor and complete fuel gas combustion. Simulations performed by Jin and Ishida23 and Wolf et al.34 showed that IGCCCLC has a greater potential to increase efficiency than that of normal IGCC systems with conventional CO2 capture technology. However, few investigations had been reported on the oxygen carrier development for integrated IGCC-CLC systems. Our previous study on NiO/bentonite26 showed that the rate of reduction increased slightly with increased temperature, the rate of oxidation decreased at 900 °C, and higher fractional conversions could be obtained from the smaller particle size and higher pressure. Recently, Garcia-Labiano et al. prepared 10% alumina-supported copper oxide, iron oxide, and nickel oxide by the freeze granulation method. Negative pressure effects on the reaction rate were reported, and that was attributed to gas diffusion in oxide material particles.13 In this study, CuO/bentonite, prepared by a mechanical mixing method, and a CuO-hexaaluminate nanocomposite, prepared by a microemulsion-templated sol-gel method, were investigated as oxygen carriers in CLC with simulated coal synthesis gas for reduction at both low and high pressure. Initial screening tests and the effects of temperature and particle size on the performance were studied by TGA at atmospheric pressure. A tapered element oscillating microbalance (TEOM) was utilized to study the effect of pressure (up to 100 psi) on the CLC reactions. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electron microscopy (SEM and TEM) analyses were conducted to characterize the carriers before and after reaction.

(13) Garcia-Labiano, F.; Diego, L. F.; Ada´nez, J.; Abad, J. A.; Gaya´n, P. Ind. Eng. Chem. Res. 2004, 43, 8168–8177. (14) Garcia-Labiano, F.; Ada´nez, J.; Diego, L. F.; Abad, A. Energy Fuels 2006, 20, 26–33. (15) Garcia-Labiano, F.; Diego, L. F.; Ada´nez, J.; Abad, A.; Gaya´n, P. Chem. Eng. Sci. 2005, 60, 851–862. (16) Jerndal, E.; Mattisson, T.; Lyngfelt, A. Trans. IChemE, Part A 2006, 84 (A9), 795–806. (17) Diego, L. F.; Labiano, F. G.; Adanez, J.; Gayan, P.; Abad, B.; Corbella, M.; Palaciob, J. M. Fuel 2004, 83, 1749. (18) Cho, P.; Mattisson, T.; Lyngfelt, A. Fuel 2004, 83, 1215–1225. (19) Adanez, J.; Diego, L. F.; Labiano, F. G.; Gayan, P.; Abad, P. Energy Fuels 2004, 18, 371–377. (20) Mattisson, T.; Ja¨rdna¨s, A.; Lyngfelt, A. Energy Fuels 2003, 17, 643–651. (21) Takenaka, S, V.; DinhSon, T.; Otsuka, K. Energy Fuels 2004, 18, 820–829. (22) Anheden, M.; Svedberg, G. Energy ConVers. Magmt. 1998, 39, 16–18. (23) Jin, H.; Ishida, M. Proceedings of TAIES’97 International Conference, Newcastle, New South Wales, Australia, American Society of Mechanical Engineers: Washington, DC, 2001. (24) Jin, H.; Ishida, M. Ind. Eng. Chem. Res. 2002, 41, 4004–4007. (25) Jin, H.; Ishida, M. Fuel 2004, 83, 2411–2417. (26) Siriwardane, R. V.; Poston, J.; Chaudhari, K.; Zinn, A.; Simonyi, T.; Robinson, C. Energy Fuels 2007, 3, 1582–1591. (27) Mattisson, T.; Lyngfelt, A.; Cho, P. Fuels 2001, 80, 1953–1962. (28) Mattisson, T.; Johansson, M.; Lyngfelt, A. Energy Fuels 2004, 18, 628–637. (29) Johansson, M.; Mattisson, T.; Lyngfelt, A. Ind. Eng. Chem. Res. 2004, 43, 6978–6987. (30) Johansson, M.; Mattisson, T.; Lyngfelt, A. Trans IChemE, Part A 2006, 84 (A9), 807–818. (31) Los Rios, T. D.; Gutierrez, D. L.; Martinez, V. C.; Ortiz, A. L. Int. J. Chem. Reactor Eng. 2005, 3, 1–9.

Experimental Section 1. Preparation of CuO/Bentonite. A 60% CuO/bentonite oxygen carrier was prepared by the mechanical mixing method.26,35 Pure Cu2O (Aldrich, >99.95%) and bentonite (Fisher, laboratory grade) were mixed thoroughly with deionized water added to the powder mixture to obtain a paste. The paste was dried at 105 °C for 24 h. The dry material was then calcined at 900 °C in air for 6 h. The calcined sample was crushed into smaller particles of the desired mesh size. The mesh sizes used in the present study were (i) 100-200 mesh (150-74 µm, designated as 200 mesh), and g20 mesh (g840 µm, designated as 20 mesh). The sample was oxidized again prior to the test. 2. Preparation of CuO/Hexaaluminate. The synthesis of the nanocomposite carriers is based on a route previously developed for nanocomposite high-temperature catalysts.36,37 The synthesis is based on a reverse-microemulsion templated route in which the (32) Copeland, R. J.; Alptekin, G.; Cessario, M.; Gerhanovich, Y. Proceedings of the First National Conference on Carbon Sequestration, Washington DC, 2001; DOE/NETL, Pittsburgh, PA, 2001; LA-UR-00-1850. (33) Krongberger, B.; Lyngfelt, A.; Lo˜ffler, G.; Hofbauer, H. Ind. Eng. Chem. Res. 2005, 44, 546–556. (34) Wolf, J., Anheden, M.; Yan, I. Proceeding of 2001 International Pittsburgh Coal Conference, Newcastle, New South Wales, Australia, 2001; University of Pittsburgh: Pittsburgh, PA, 2001. (35) Ryu, H.-J.; Bae, D.-H.; Han, K.-H.; Lee, S.-Y.; Jin, G.-T.; Choi, J.-H. Kor. J. Chem. Eng. 2001, 18 (6), 831–837. (36) Kirchhoff, M.; Specht, U.; Veser, G. Nanotechnology 2005, 16, S401-S408. (37) Schicks, J.; Neumann, D.; Specht, U.; Veser, G. Catal. Today. 2003, 83, 287–296.

3746 Energy & Fuels, Vol. 22, No. 6, 2008 metal nanoparticles are simultaneously synthesized in a simple onepot synthesis with a high-temperature stable ceramic matrix (here: barium-hexaaluminate, “BHA”). For this purpose, a reverse microemulsion is prepared by mixing an aqueous solution of Cu(NO3)2 · 3H2O (99.999%, Aldrich) with isooctane (2,2,4-trimehtylpentane, 99.7% Aldrich) and a surfactant (poly(ethylene oxide)block- poly(propylene oxide)-block-poly(ethylene oxide) with 10 wt % ethylene oxide, Aldrich). 1-Pentanol (99+%, Aldrich) is added as a cosurfactant to obtain a Winsor type IV system. Aluminum isopropoxide and barium isopropoxide (both 99.9%, Aldrich) at a stoichiometric ratio of 1:12 are dissolved in dry isopropanol before addition to the reverse microemulsion. The isopropoxides diffuse through the oil phase and the surfactant shell into the aqueous phaseofthemicelles,wheretheyhydrolyzetoformabarium-aluminate structure. After aging for 48 h, the water phase and the organic phase are separated via temperature-induced phase separation (TIPS). The product phase is then washed several times with acetone and calcined for 5 h in air at 800 °C. This synthesis results in Cu and BHA nanoparticles with diameters of ∼10 nm for the BHA and ∼50 nm for the Cu phase. The nanoparticles form strongly bound agglomerates during the drying phase (most likely bound via oxygen bonds between the hexaaluminate particles) with a surface area of about 100 m2/g after initial redox treatment at 800 °C. Unless noted otherwise, the particles were used as prepared, that is, without further size selection. In these samples, we find typical size distribution of 35 wt % > 500 µm, 45 wt % between 180 and 500 µm, and 20 wt % < 180 µm. In the present study, 40 wt % Cu-BHA (corresponding to 45 wt % CuO-BHA) were prepared and studied. 3. Reactivity Tests. a. ThermograVimetric Analysis (TGA). TGA experiments were conducted in a thermogravimetric analyzer (TA Model 2050) in which the weight change of the CuO/bentonite was measured isothermally as a function of time. About 20 mg of the CuO/bentonite was heated in a quartz bowl to the reaction temperature with a nitrogen purge. A mixture of 12% CO2, 36% CO, 25% He, and 27% H2 was used for the reduction segment, and dry air was utilized for the oxidation segment. Reaction gas flow rates were set at 45 sccm, reduction reaction times were set at 10 min, and oxidation reaction times were 60 min for all experiments. To avoid the mixing of reduction gas mixture and air, the system was flushed with nitrogen for 5 min before and after each reaction phase. Testing was conducted between 700 and 900 °C for 10 cycles. The data analysis method has been reported in a previous report:26 The fractional conversions (fractional reduction and fractional oxidation) were calculated utilizing the TGA data. The fractional conversion (X) is defined as shown below,

Fractional Reduction (X) ) (Moxd-M) ⁄ (Moxd-Mred) Fractional Oxidation (X) ) (M-Mred) ⁄ (Moxd-Mred) where M is the instantaneous weight, Moxd is the weight of the completely oxidized sample, and Mred is the weight of the completely reduced sample. The fractional conversion data as a function of time was fitted to obtain the polynomial regression equation. The global rates of reactions (dX/dt) at different fractional conversions (X) were calculated by differentiating the polynomial equation. The number of moles of oxygen consumed during oxidation and released during reduction was calculated for each reductionoxidation cycle utilizing the weight change data obtained from the TGA at different temperatures and for different particles sizes using the following equation:

% oxygen consumption ) (experimentally determined gmoles of oxygen consumed ⁄ theoretical value of gmoles of oxygen present in CuO prior to reduction in the cycle) × 100 For evaluation of the reaction kinetics over the Cu-BHA nanocomposites and for direct and quantitative comparison with

Tian et al. Table 1. Dependence of Conversion on Time and Characteristic Time Constants Obtained from the Shrinking Core Model for the Three Possible Rate Limiting Steps rate-limiting step external mass transfer diffusion reaction

t/τ

τ

XB

FBmR/(3kAgbcAg)

[1 - 3(1 - XB)2/3 + 2(1 - XB)] [1 - (1 - XB)1/3 ]

[FBmR/(bcAg)](R/6De) [FBmR/(bcAg)](1/kAs)

the Cu-bentonite samples, the data (conversion vs time, as described above) was fitted against a shrinking core model (SCM).38 This model includes three major steps: external mass transfer of the reactant and product gases through the gas film surrounding the carrier particle, diffusion of the reactants and/or products through the product layer inside the carrier particles, and reaction between the gaseous reactants and the metal or metal oxide. The model yields the following dependence of conversion on reaction time (conveniently expressed as time as a function of conversion):

t)

{

FBmR XB R + [1 - 3(1 - XB)2⁄3 + 2(1 - XB)] + bcAg 3kAg 6De

}

1 [1 - (1 - XB)1⁄3] kAs

Where: FBm: molar density of solid B (Me during oxidation and MeO during reduction, mol/m3.); R: initial particle radius, m; b: stoichiometric ratio of O2 to metal in the oxidation step; cAg: reactant gas concentration in bulk phase, mol/m3; kAg: external mass transfer coefficient, m/s; De: internal diffusion coefficient, m2/s; kAs: surface reaction rate constant, m/s; In this final expression, the contributions of external mass transfer, diffusion, and reaction can be separated, as shown in Table 1. Therefore, a fit of the experimental data allows not only a determination of the rate-controlling step during oxidation and reduction, but also yields a characteristic time constant τ for a quantitative comparison between different carrier materials. b. High-pressure Flow Reactor Tests. CuO/bentonite particles of 20 mesh size were packed with quartz wool in an 11 in.long ceramic reactor (0.25 in. OD) inside a stainless steel shell. The reactant and inert gases were introduced to the reactor from gas cylinders through mass flow controllers. The product gas and reaction temperatures were continuously analyzed with an online mass spectrometer (Quadrupole Prisma, Pfeiffer). Multicycle CLC tests were conducted at different temperatures at 100 psi using the following mixed gases: 12% CO2, 36% CO, 25% He, and 27% H2 for reduction and air for oxidation. To avoid the mixing of reduction gas mixture and air, the system was flushed with argon for 5 min before and after each reaction phase. Labtech Notebook Pro was used to monitor the reactor pressure during the experiments. c. Tapered Element Oscillating Microbalance (TEOM). The TEOM Series 1500 Pulse Mass Analyzer (PMA 1500 from Rupprecht and Patashnick Co. Inc.) was used to study the effect of pressure on the uptake rate of CuO/bentonite.39,40 In TEOM, the steady flow of gas through the sample provides complete contact with the sample bed. The reaction gas was introduced to the CuO/ bentonite oxygen carrier bed at a flow rate of 45 sccm. The reaction gas utilized for reduction and oxidation reaction consisted of the same composition as TGA tests. Reaction times were 10 min for reduction and 60 min for oxidation for all experiments. Similar to TGA analysis, the system was flushed with argon for 5 min before and after each reaction phase. One-cycle tests were conducted at 800 °C and at pressures of 14.7, 50, and 100 psi. (38) Missen,R. W.; Mims,C. A.; Saville,B. A. Introduction to Chemical Reaction Engineering and Kinetics; John Wiley & Sons: New York, 1999. (39) Patachnick, H.; Rupprecht, G.; Wang, J. F. C. ACS Symposium Series 1980, 25, 188–193. (40) Jalani, N. H.; Datta, R. J. Membr. Sci. 2005, 264 (1-2), 167–175.

CLC of Coal-deriVed Synthesis Gas OVer CuOx

Energy & Fuels, Vol. 22, No. 6, 2008 3747

Table 2. Composition of the Simulated Synthesis Gas Used for Reduction component

mol %

CO H2 CH4 H2O CO2 Ar N2

38 30.8 0.0335 16.5 13.0 0.1218 1.5799

4. Characterization of Oxygen Carrier Particles. XRD, X-ray microanalysis, and XPS of CuO/bentonite oxygen carrier reacted in TGA tests were studied to determine phase transformations, chemical changes and changes in morphology that occurred during multiple-cycle CLC. XRD patterns of unreacted and used CuO/ bentonite were recorded using Cu KR radiation. The diffraction patterns were obtained between 2θ ) 10 and 80°. Data analysis was conducted with HighScore Plus analysis software supplied by Pan Analytical. XPS were recorded on a Physical Electronics (PHI) SAM-590 UHV system equipped with a PHI 50-096 X-ray photoelectron spectroscopy subsystem, a PHI spherical capacitortype analyzer with an Omni Focus III Lens, and a PHI 04-548 duel anode X-ray source. System calibration was in accordance with PHI procedures utilizing the photoemission lines EB (Cu2p3/2) ) 932.7 eV, EB (Au4f7/2) ) 84 eV, and EB (Ag3d5/2) ) 368.3.41 The system had a standing pressure of 10-9 torr (1.3 × 10-7 Pa) and was operated within a pressure range of 10-9 to 10-8 torr (1.3 × 10-7 to 1.3 × 10-6 Pa). The binding energies were corrected utilizing the reference binding energy for adventitious carbon, C (1s1/2), 284.8 eV. All reported intensities were experimentally determined as peak areas divided by the instrumental sensitivity factors. Data analysis was conducted with Multipak analysis software supplied by Physical Electronics. Surface morphology and X-ray microanalysis were determined utilizing a JEOLJSM6300 SEM (FESEM). The JEOL-6300 is equipped with an ETtype secondary electron detector and an Ultra-Dry Silicon Drift Detector interfaced to a Thermo-Noran System Six microanalysis system. The instrument was routinely operated within a pressure range of 10-7 to 10-6 torr (1.3 × 10-5 to 1.3 × 10-4 Pa). The microanalysis system, prior to analysis, was calibrated using a Cu standard, with the KR spectral peak being referenced to the energy value of 8.04 keV.

Figure 1. Ten-cycle thermo gravimetric analysis of CuO/bentonite (200 mesh) with coal-derived synthesis gas at 800 °C.

Both reactions are exothermic. If the fuel with the above gas composition is combusted in a conventional reactor, then the chemical equation could be represented by the following equation.

The heat evolved during CLC (eqs 3 and 4) is similar to that from the conventional reactor (eq 5). Thermogravimetric Analysis (TGA). To evaluate the reaction performance and stability of the CuO/bentonite oxygen carrier, 10 reduction and oxidation cycles were conducted in the TGA for 20 and 200 mesh samples within the temperature range 700-900 °C, at atmospheric pressure with simulated synthesis gas. The effect of impurities, such as H2S, will be reported in a separate paper. The original 10-cycle TGA analysis of CuO/bentonite (200 mesh, 800 °C) is shown in Figure 1. No mass loss was observed with either CuO/bentonite samples or pure CuO during the 10-cycle test under the reaction temperature range of 700-900 °C. Therefore, the bentonite-supported CuO species are very stable under high-temperature reduction/ oxidation conditions. In the previous investigations, it has been reported that alumina-supported CuO agglomerated and significantly deactivated in multicycle tests.18 De. Diego et al. successfully avoided the deactivation of CuO by utilizing the impregnation method, but the copper-oxide loading of the prepared oxygen carrier had to be limited to 10%.17 In this study, CuO species (as much as 60%) demonstrating stable activity and no deactivation were found during multiple cycles. Therefore, bentonite support was critical for modifying the reactivity of CuO during the 10-cycle test. In this study, the weight change data from cycle 5 were utilized to analyze the fractional reaction and to calculate the reaction rate. The plot of fractional reduction and fractional oxidation of CuO/bentonite (200 mesh) as a function of time at 800 °C is shown in Figure 2, panels a and b, respectively. These (Figures 2a and 2b) also include the fifth-cycle data of fractional reaction over pure CuO as a function of time. Pure bentonite was also tested in TGA, and no reactivity was observed. It clearly shows that the fractional conversions in reduction and oxidation of pure CuO were significantly lower than that of CuO/bentonite. Therefore, CuO provides the necessary oxygen for the reaction, whereas the bentonite support is necessary to disperse and stabilize CuO particles. At all temperatures, the reduction reaction was much faster than oxidation. For example, in the case of CuO/bentonite with 200 mesh size, the reaction time for 80% reduction was less than 1 min (∼0.50 min), whereas the reaction time of 90% fractional oxidation was 1.2-5.8 min.

0.552 CO + 0.447 H2 + 0.000487 CH4 + 0.5 O2 f 0.552 CO2 + 0.447 H2O ∆H ) -264.9905 MJ ⁄ kmol (5)

(41) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenburg, G. E. Handbook of X-Ray and Ultraviolet Photoelectron Spectroscopy; Muilenburg, G. E. Ed.; Perkin-Elmer: Physical Electronics Division, 1979.

Results and Discussion Thermodynamics. The composition of the simulated synthesis gas (without the impurities) used for reduction in a typical gasification system is given in Table 2. Chemical equations for oxidation and reduction reaction with the above gas composition can be presented as follows: Reduction: 0.552 CO + 0.447 H2 + 0.000487 CH4 + CuO f 0.552 CO2 + 0.447 H2O + Cu ∆H ) -107.52 MJ ⁄ kmol (3) Oxidation: Cu + 0.5 O2 f CuO H ) -157.3 MJ ⁄ kmol (4)

3748 Energy & Fuels, Vol. 22, No. 6, 2008

Figure 2. Fractional conversion as a function of time for 10 cycles of CLC on CuO/bentonite (200 mesh) at 800 °C and the fifth cycle of CuO; (a) fractional reduction and (b) fractional oxidation.

The rates of reduction and oxidation (dX/dt) of CuO/bentonite (200 mesh) at 800 °C at different fractional conversions (X) are shown in Figure 3, panels a and b, respectively. As shown in the figures, the reaction rates gradually decrease with increasing reduction/oxidation reaction because of slower reaction gas diffusion at higher coverage. The reaction rates at lower fractional conversion (1023 K).13 For NiO/bentonite and CuO/bentonite, our SCM analysis shows that the reduction reaction appears to be controlled by external mass transfer, explaining why the reduction rates are similar for both NiO and CuO. From a comparison of the solid-state diffusion rates of oxygen and the metal cations in the Cu/Cu2O and Ni/NiO systems (see Figure 11), it can be seen that the diffusion rates in the Cu/Cu2O system are an order of magnitude higher than in the Ni/NiO system. This explains the increased oxidation rates and the different ratecontrolling mechanisms for the Cu-based in comparison to the Ni-based system. It also explains why nanostructuring of the oxygen carrier has only comparatively minor effects on the performance of the Cu-based carrier, whereas the oxidation kinetics of the Ni-based carriers was drastically accelerated for similar NiO-BHA nanocomposite materials.48 The bulk structure of NiO and CuO also appeared to affect the reaction condition. Our previous XRD and SEM study on NiO/bentonite indicated that supported NiO did not undergo crystalline and morphological changes at high temperature (up to 900 °C). However, in the present study, morphological changes are clearly evident in CuO species on the bentonite as low as 700 °C. Garcia-Labiano et al. also suggested that Cu was well-dispersed on the entire porous structure of the inert support with a thin layer of about 0.4 nm from the BET surface area method.14 Thus, oxygen atoms may be more accessible and diffusion of oxygen in a CuO thin layer than in a crystalline NiO particle, and the easier diffusion of reactive gases and thin layer structure of CuO/bentonite structure may hence minimize the reaction performance dependence on temperature and particle size. High-pressure Flow-reactor Tests. The redox behavior of the oxygen carrier CuO/bentonite was tested for CLC in a high(47) Ishida, M.; Jin, H.; Okamoto, T. Energy Fuels 1996, 10, 958. (48) Karuna, C.; Liu, T.; Simonyi, T.; Sanders, T.; Siriwardane, R.; Veser, G. Proceeding of 2006 AICHE Annual Meeting, San Francisco, CA, November 14, 2006, The American Institute of Chemical Engineers: New York, NY, 2006.

3754 Energy & Fuels, Vol. 22, No. 6, 2008

Tian et al.

Table 7. Comparison of 60% CuO/Bentonite and NiO/Bentonite Reaction Performance in CLC CuO/bentonite

Ni/bentonite

oxygen uptake temperature effect

reduction rate > oxidation rate 100% fractional conversion dX/dt decrease during X ) 0.2-0.8 ∼90% minor

particle effect bulk structure surface structure oxygen diffusion rate

minor 2-D CuO thin layer Multioxidation state (metallic Cu, Cu2O and CuO) fast

reaction performance

pressure-packed bed reactor for three cycles. The outlet concentrations of gases produced during reduction reaction with CuO/bentonite particles at 800 °C and 100 psi with a gas space velocity of 1230 h-1 are shown in Figure 12. In the presence of oxidized CuO/bentonite oxygen-carrier particles, CO and H2 from the incoming reduction gas initially react to form CO2 and H2O. Thus, the concentration of CO and H2 decreased, whereas the concentration of CO2 increased with time and reached a steady state due to the reaction with oxygen as expected. The breakthrough times were similar for all three cycles. This indicated that the reactivity of CuO/bentonite was not changed during cyclic oxidation and reduction. The moles

Figure 11. Oxygen and metal diffusion rates in Cu, Cu2O, Ni, and NiO (adapted from ref48).

Figure 12. High-pressure (100 psig) flow-reactor data during CLC of simulated coal-derived synthesis gas with CuO/bentonite at 800 °C.

reduction rate > oxidation rate 100% fractional conversion dX/dt decrease during X ) 0.2-0.8 ∼60% slight at low conversion significant at high conversion small particle promotion reaction rate 3D NiO crystal NiO slow

Table 8. Effect of Pressure: TEOM Results of 20 Mesh CuO/ Bentonite in CLC at 800 °C and Various Pressures dX/dt pressure (psi)

X ) 0.2

X ) 0.3

X ) 0.5

X ) 0.8

14.7 50 100

0.916 0.504 0.318

Reduction 0.933 0.518 0.320

0.938 0.543 0.311

0.886 0.566 0.314

14.7 50 100

0.670 0.431 0.251

Oxidation 0.694 0.432 0.253

0.765 0.461 0.252

0.381 0.445 0.249

of CO2 produced as calculated from the data were similar for all three cycles, indicating that the performance was stable during the three cycles at high pressure. The oxygen available from the CuO was compared with oxygen utilized for CO2 formation and H2 conversion. It was observed that the oxygen from CuO was fully consumed during the reaction at high pressure. There was some unconverted CO but H2 was fully consumed. In the present reactor tests, it was not possible to lower the space velocity to achieve full consumption of CO. Stable reactivity during three-cycle flow reactor tests was also observed at 800 °C. The time required to achieve the maximum CO2 concentration was similar at 800 and 900 °C. Tapered Element Oscillating Microbalance (TEOM). A CuO/bentonite oxygen carrier (30 mesh) was tested for CLC in a pulse mass analyzer (PMA) at 800 °C under the pressure of 14.7, 50, and 100 psi. The global rates of reduction and oxidation reactions calculated at various fractional conversions (X ) 0.2, 0.4, 0.6, and 0.8) are shown in Table 8. TEOM data indicated that the reaction pressure has a negative effect on the reaction rates. With increasing reaction pressure, the reaction rate decreased for all fractional conversions. Similar phenomena were also observed by Garcia et al. on pressurized experiments of alumina-supported Fe, Cu, and Ni oxides.14 Very few studies are reported in the literature on the reaction performance of CLC systems under high-pressure conditions. Generally, the increase of the operation pressure would increase reacting gas partial pressure, and hence, increase reaction rate. However, in this work, negative pressure effects were found. Recently, Garcia-Labiano investigated the effect of pressure on kinetic parameters and gas dispersion.14 They concluded that a CGSM (changing grain size model) in the CLC reaction and the internal gas diffusion was not able to predict the experimental results obtained at pressurized condition when using kinetic parameters obtained at atmospheric pressure. In this study, we can tentatively explain the pressure effect in terms of bulk diffusion internal to the pores. Bulk diffusion is inversely proportional to the total pressure, as shown in eq 6;25

CLC of Coal-deriVed Synthesis Gas OVer CuOx

DB )

0.00100T1.75√(MW1 + MW2) ⁄ (MW1MW2)1⁄2 PTOT

[(∑ V)

1⁄3 1

+

(∑ V) ]

1⁄3 2

Energy & Fuels, Vol. 22, No. 6, 2008 3755

(6)

2

Where, DB ) bulk diffusivity T ) temperature (K) MW1, MW2 ) molecular weights of gas components PTOT ) pressure (atm) V1, V2 ) atomic diffusion volume of gas components The global reaction rate at higher conversions appears to be more favorable at lower pressures for a CuO/bentonite system, indicating that the bulk diffusion may be the rate-determining step at high pressure. Conclusions The CuO/bentonite prepared by solid-state mixing was found to be a good oxygen carrier for CLC of simulated synthesis gas without the impurities such as H2S for IGCC systems. Results of multicycle reduction/oxidation tests showed that CuO species on bentonite are stable in 10-cycle oxidation-reduction reactions. The particle size of a CuO/bentonite oxygen carrier and reaction temperature showed a minor effect on reaction rate, and this was attributed to the high diffusion rate of oxygen in CuO and morphological change in CuO at high temperature.

The reaction pressure had a negative effect on reaction performance due to slower bulk diffusion at higher pressure. XRD analysis revealed that the bulk phase of an oxygen carrier particle can be completely oxidized or reduced during multicycle CLC, whereas the XPS surface analysis suggests the presence of multioxidation states (metallic Cu, Cu(I), and Cu(II)) at the surface of the reacted sample. SEM analysis showed significant redistribution of CuO/bentonite after reactions above 700 °C in an atmospheric TGA for 10 oxidation-reduction cycles. This effect was found to be greater with increasing particle size of the oxygen carrier. Nanostructured CuO-BHA (barium-hexaaluminate) carrier also showed high activity and stable behavior over multiple redox cycles in the temperature range from 700 to 900 °C, where nanostructuring appeared to improve the morphological stability of the carrier and lead to a mild, but significant, acceleration of the redox kinetics. Acknowledgment. The authors gratefully acknowledge Anthony Zinn of REM, Clark Robinson of Parsons, and Rahual Solunke of the University of Pittsburgh for the experimental setup and result discussions. This work was funded in parts through grants from Department of Energy-National Energy Technology Laboratory and the Office of Energy Science. EF800438X