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Ind. Eng. Chem. Res. 2009, 48, 8418–8430
Effect of Hydrogen Sulfide on Chemical Looping Combustion of Coal-Derived Synthesis Gas over Bentonite-Supported Metal-Oxide Oxygen Carriers Hanjing Tian,†,‡ Thomas Simonyi,†,‡ James Poston,† 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, and Parsons, P.O. Box 618, Pittsburgh, PennsylVania 15129
The effect of hydrogen sulfide (H2S) on the chemical looping combustion of coal-derived synthesis gas with bentonite-supported metal oxidesssuch as iron oxide, nickel oxide, manganese oxide, and copper oxideswas investigated by thermogravimetric analysis, mass spectrometry, and X-ray photoelectron spectroscopy (XPS). During the reaction with synthesis gas containing H2S, metal-oxide oxygen carriers were first reduced by carbon monoxide and hydrogen, and then interacted with H2S to form metal sulfide, which resulted in a weight gain during the reduction/sulfidation step. The reduced/sulfurized compounds could be regenerated to form sulfur dioxide and oxides during the oxidation reaction with air. The reduction/oxidation capacities of iron oxide and nickel oxide were not affected by the presence of H2S, but both manganese oxide and copper oxide showed decreased reduction/oxidation capacities. However, the rates of reduction and oxidation decreased in the presence of H2S for all four metal oxides. 1. Introduction Carbon dioxide (CO2), the primary “greenhouse gas” for possible global climate change, is largely produced during fossil fuel combustion. The current technology that is available for CO2 separation or sequestration has the disadvantage of requiring a large amount of energy. To avoid the energy penalty problem associated with CO2 separation, Richter and Knoche introduced a novel combustion technologyschemical looping combustion (CLC)swhich utilizes the oxygen from metal oxide instead of air, to combust fuels, such as natural gas or synthesis gas.1 Since the gas stream during combustion reaction is not diluted with N2, the exiting gas stream gas contains only CO2 and H2O. The significant advantage of a CLC system is that a concentrated CO2 stream can be obtained after water condensation without requiring any energy for separation or purification.1-5 In addition, nitrogen oxide (NOx) production is also greatly reduced. Subsequently, chemical looping combustion has received a lot of attention and has been intensively investigated. Development of a high reactive oxygen carrier with stable performance is essential for success operation of the CLC process. Various transition metal oxides, such as NiO, CuO, Mn2O3, Fe2O3, Co3O4, WO3, have been investigated as oxygen carriers.6-27 Greater reactivity and better thermal stability have been observed when these metal oxides are supported on highsurface area supports, such as Al2O3 and TiO2. 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 and offer significant promise as materials for chemical looping combustion of coal synthesis gas.26,27 Only limited works in literature have been reported regarding the influence of sulfur species on the reaction performance of metal-oxide oxygen carriers. Sulfur is the major impurity in coal synthesis gas, as well as in natural gas. Depending on the coal type and resource, coal-derived synthesis gas may contain 200-8000 ppm H2S, which may * To whom correspondence should be addressed. Tel.: 304-285-4513. Fax: 304-285-0903. E-mail: ranjani.siriwardane@ netl.doe.gov. † U.S. Department of Energy, National Energy Technology Laboratory. ‡ Parsons.
interact with a metal-oxide oxygen carrier during combustion reaction, thus affecting the performance of the CLC system. Jerndal and his co-workers have conducted thermal analysis using HBC Chemistry 5.0 software to investigate the interaction of H2S and metal oxide, and concluded that the oxidation of most metal sulfide systems would be complete at the temperature range of 600-1200 °C. Exceptions are nickel, cobalt and tungsten, where the oxidation to form SO2 is slow at low temperatures and increased pressures, and metal sulfides formation takes place during H2S exposure.9 It should be noted that all these thermodynamic calculation results are applicable to equilibrium conditions, and reaction kinetics are not clearly known. Experimental studies on the interaction of H2S with oxygen carriers during CLC reaction have not been reported in literature. Therefore, it is important to systematically investigate the effect of H2S on the reaction performance of a metal-oxide oxygen carrier to successfully develop a chemical looping combustion system based on coal synthesis gas or natural gas. Metal oxides have been used as sorbents for removal of H2S from coal-derived synthesis gas.28-33 Although the reaction temperature for the H2S removal process is lower (500-800 °C) than the temperature of chemical looping combustion reaction (800-1200 °C), the basic chemistry on the interaction of H2S and metal oxides may be helpful to understand the influence of H2S on a CLC system. Mangnus et al. reported that it is easier to sulfide the surface and core of Ni(0) than that with Ni(II), and NiS could be readily formed at the temperature range of 370-850 K.28 Reshetenko et al. found that the H2S decomposition reaction on R-Fe2O3 leads to reduction of oxides and simultaneous formation of surface Fe(II)S.31 The interaction of H2S and CuO was more complex: CuS was the main product, but Cu2S and elemental Cu were formed at high temperature (>400 °C) and higher H2S concentration.29 As for manganese, MnCO3 is the stable solid below 400 °C, while MnO is stable above 400 °C. Importantly, manganese shows sulfidation potential at the higher temperature range of 600-700 °C.23,30 In this study, the reduction/oxidation performances of various metal oxides were investigated by TGA/MS with simulated coalderived synthesis gas in the presence of H2S. The effect of H2S
10.1021/ie900638p CCC: $40.75 2009 American Chemical Society Published on Web 08/24/2009
Ind. Eng. Chem. Res., Vol. 48, No. 18, 2009
on the reaction performance and stability of bentonite-supported Fe2O3, NiO, Mn2O3, and CuO was evaluated during 10-cycle reduction/oxidation tests. The reaction rate data obtained with syngas containing H2S were compared with those obtained with syngas in the absence of H2S. The total and surface sulfur concentrations of reacted samples were analyzed by LECO sulfur analysis and X-ray photoelectron spectroscopy (XPS), respectively. In addition, surface compositions and oxidation states of reduced sample were also investigated by XPS analysis. 2. Experimental Section 2.1. Preparation of Bentonite-Supported Metal Oxides. Metal-oxide oxygen carriers with 60 wt % bentonite support were prepared by the mechanical mixing method.26,27 Bentonite (Fisher) was mixed thoroughly with various metal oxides (Cu2O, Aldrich, >99.95%; NiO, Alfa Aesar, 99.98%; Mn2O3 Alfa Aesar, 98%,; Fe2O3 Fisher, 99.98%), and then deionized water was added to the powder mixture to obtain a paste. The paste was dried at 105 °C for 24 h, and then calcined at 900 °C in air for 6 h. The calcined sample was crushed into smaller particles of 170-200 mesh size. Fe2O3/bentonite sample with the particle size of 30-60 mesh was also prepared to investigate the particle size effect on CLC reaction. The samples were oxidized again prior to the test. 2.2. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) experiments were conducted in a Thermogravimetric Analyzer (model 2050) in which the weight change of the various metal-oxide oxygen carriers was measured isothermally as a function of time. Ten cycle reduction/oxidation cycles were conducted at atmospheric pressure to determine the stability of oxygen carriers. About 120 mg of samples were heated in a quartz bowl to the reaction temperature with nitrogen purge. A simulated coal-derived synthesis gas mixture of 4042 ppm H2S, 12% CO2, 36% CO, 25% He, and 27% H2 with nitrogen was used for the reduction segment, while zero air was utilized for the oxidation segment. All reaction gas flow rates were set at 45 sccm. Reduction and oxidation reaction times were set at 120 and 60 min, respectively. To avoid the mixing of reduction gases and air, the system was flushed with nitrogen for 5 min before and after each reaction segment. For comparison, tests were also conducted with a synthesis gas mixture with similar composition but without H2S. 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 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 completely oxidized sample, and Mred is the weight of completely reduced sample. The fractional conversion data as a function of time was fitted best 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. 2.3. Sulfur Analysis. Two and a half reduction/oxidation cycles were conducted in TGA, and samples were collected after the reduction reaction. Total sulfur concentrations in bentonite-
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supported oxygen carriers after 2.5 cycles were analyzed by a LECO SC-432DR sulfur analyzer in accordance with ASTM method D3176 at Galbreith Laboratories. 2.4. Surface Characterization of Oxygen Carrier Particles. Surface components and oxidation states of a bentonite-supported metal-oxide oxygen carrier were analyzed by an XPS spectra that were obtained with a Physical Electronics (PHI) model 32-096 X-ray source control and a 22-040 power supply interfaced to a model 04-548 X-ray source with an Omni Focus III spherical capacitance analyzer (SCA). The system is routinely operated within the pressure range of 10-8 to 10-9 Torr (1.3 × 10-6 to 1.3 × 10-7 Pa). The system was calibrated in accordance with the manufacturer’s procedures utilizing the photoemission lines Eb of Cu 2p3/2 ) 932.7 eV, Eb of Au 4f7/2 ) 84 eV and Eb of Ag 3d5/2 ) 368.3 for a magnesium anode.35 All reported intensities are experimentally determined peak areas divided by the instrumental sensitivity factors. Charge correction was obtained by referencing the adventitious C 1s peak to 284.8 eV.35 3. Results 3.1. Fe2O3/Bentonite Oxygen Carrier. To evaluate the effect of H2S on reaction performance and stability of the Fe2O3/ bentonite oxygen carrier, 10 reduction and oxidation cycles were conducted in the TGA with simulated synthesis gas, both in the absence and presence of 4042 ppm H2S at 800 and 900 °C. The 10-cycle TGA analysis of Fe2O3/bentonite in synthesis gas without H2S at 900 °C is shown in Figure 1a. Stable performance was observed with Fe2O3/bentonite samples during the 10-cycle test in synthesis gas without H2S at 900 °C. Similar data were also observed at 800 °C (not shown). Thus, TGA data indicated that bentonite-supported Fe2O3 species are very stable during high-temperature reduction/oxidation conditions. Pure bentonite was also tested in TGA and no reactivity was observed. It has been reported that alumina-supported Fe2O3 agglomerated and deactivated significantly during multicycle tests.23 In this study, bentonite-supported Fe2O3 species (as much as 60 wt %) demonstrated stable reactivity in multiple cycles. Thus, bentonite support was critical for obtaining stable reactivity of Fe2O3. Similarly, stable performance was observed with Fe2O3/bentonite oxygen carrier during 10 reduction/oxidation cycles in synthesis gas with H2S as shown in Figure 1b. Therefore, reduction/ oxidation capacity of bentonite-supported Fe2O3 appeared to be stable and resistant to H2S contaminant of coal-derived synthesis gas. In this study, the weight change data from the fifth cycle were utilized to determine the fractional reduction/oxidation and to calculate the reaction rate. The plot of fractional reduction and fractional oxidation of Fe2O3/bentonite in simulated synthesis gas with and without H2S as a function of time at 900 °C is shown in Figure 2a and 2b, respectively. The reaction profiles look similar both in the presence and absence of H2S. The rates of reduction and oxidation (dX/dt) of Fe2O3/bentonite at 800 and 900 °C and at various fractional conversions (X ) 0.2, 0.3, 0.5, and 0.8)sboth in the absence and presence of 4042 ppm hydrogen sulfide in simulated synthesis gassare listed in Table 1. The reaction rates at lower fractional conversion (
