Article pubs.acs.org/EF
Effect of H2S on Chemical Looping Combustion of Coal-Derived Synthesis Gas over Fe−Mn Oxides Supported on Sepiolite, ZrO2, and Al2O3 Ewelina Ksepko,† Ranjani V. Siriwardane,*,‡ Hanjing Tian,‡ Thomas Simonyi,‡ and Marek Sciazko† †
Institute for Chemical Processing of Coal, 1 Zamkowa, 41-803 Zabrze, Poland National Energy Technology Laboratory (NETL), United States Department of Energy (DOE), 3610 Collins Ferry Road, Post Office Box 10940, Morgantown, West Virginia 26507-0880, United States
‡
ABSTRACT: The performance of Fe−Mn oxide oxygen carriers supported on sepiolite, ZrO2, and Al2O3 with simulated synthesis gas/air in a novel combustion technology known as chemical looping combustion (CLC) was evaluated. Thermogravimetric analyses (TGAs) and bench-scale low-pressure (10 psi) flow reactor tests were conducted to evaluate the performance. Multicycle tests were conducted in atmospheric TGA with oxygen carriers using simulated synthesis gas with and without H2S. The effect of H2S impurities on both stability and oxygen transport capacity was also evaluated. Multicycle CLC tests were conducted in the bench-scale flow reactor at 800 °C with selected samples as well. Chemical-phase composition was investigated by the X-ray diffraction (XRD) technique. Five-cycle TGA tests at 800−900 °C indicated that all oxygen carriers exhibited stable performance. It was interesting to note that there was complete reduction−oxidation of the oxygen carrier during the five-cycle test. Fractional reduction, fractional oxidation, and global reaction rates were calculated from the data. It was found that the support-type had a significant effect on both fractional reduction−oxidation and reaction rate. The oxidation reaction was significantly faster than the reduction reaction for all oxygen carriers. The presence of H2S in the synthesis gas resulted in a positive effect on the reaction rate. Bench-scale low-pressure flow reactor data indicate stable reactivity, full consumption of oxygen from the oxygen carrier, and complete combustion of H2 and CO. XRD data of samples showed stable crystalline phases without the formation of sulfides or sulfites/sulfates and complete regeneration of the oxygen carrier.
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INTRODUCTION Carbon dioxide, the primary “greenhouse gas” for possible global climate change, is largely produced during fossil fuel combustion. Current commercial CO2 separation technologies carry a heavy energy penalty, meaning that they significantly increase the energy that a system requires for operation. Chemical looping combustion (CLC), which uses oxygen from a metal oxide instead of air to combust the fuel stream of a power system, is a novel 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 energy for separation or purification.1−5 As an added benefit, NOx production is greatly reduced in the process. Various transition-metal oxides, such as NiO, CuO, Mn2O3, Fe2O3, Co3O4, and WO3, have been investigated as oxygen carriers.6−27 However, there is limited work reported in the literature with mixed-metal oxides as oxygen carriers. Perovskite-type compounds (such as La0.8Sr0.2Co0.2Fe0.8O328) and bimetallic compounds (such as Co−Ni/Al2O329,30 and Fe−Mn oxides31,32) have been evaluated as oxygen carriers in the past. It was found that the bimetallic Co−Ni/Al2O3 sample displayed high reactivity and stable behavior over multiple reduction/ reoxidation experiments. Ryden et al.32 concluded that combined oxides of iron and manganese have very interesting thermodynamic properties and could potentially be suitable for CLC applications. However, they also found that the physical stability of the examined material was low. To address this © 2012 American Chemical Society
problem, they have suggested that improved stability could be achieved by the addition of inert material. The objective of the research reported in this paper was to prepare three mixed-metal oxide oxygen carriers, consisting of Fe2O3 and MnO2 supported on sepiolite [Mg4Si6(OH)2· 6H2O], ZrO2, and Al2O3 and to evaluate the performance of these carriers during the CLC process in the presence of synthesis gas/air. The advantages of using mixed-metal oxides and the effect of the support media on their performance will be discussed. Sulfur is the major impurity in coal synthesis gas, as well as in natural gas. Dependent upon the coal type and resource, coalderived synthesis gas may contain 200−8000 ppm H2S, which may interact with the metal oxide oxygen carrier during the combustion reaction, thus affecting the performance of the CLC system. There are only a few experimental papers that report studies on the effect of contaminants.33−35 Thermodynamic analyses indicated that there is an interaction between H2S and the metal oxide of an oxygen carrier.9 We have previously reported the interaction of H2S with metal oxide oxygen carriers supported on bentonite or sepiolite.26,35 It is important to systematically investigate the effect of H2S on the reaction performance of mixed-metal oxide oxygen carriers to successfully develop CLC systems based on coal synthesis gas or natural gas. Received: September 27, 2011 Revised: January 13, 2012 Published: February 15, 2012 2461
dx.doi.org/10.1021/ef201441k | Energy Fuels 2012, 26, 2461−2472
Energy & Fuels
Article
Figure 1. Five-cycle reduction−oxidation TGA data for (a) Fe−Mn oxides/ZrO2, (b) Fe oxide/ZrO2, and (c) Mn oxide/ZrO2, at 800 °C.
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In this study, the reduction−oxidation performance of Fe2O3−MnO2, supported on sepiolite, ZrO2, and Al2O3, was investigated by thermogravimetric analyses (TGAs)/mass spectrophotometry with simulated coal-derived synthesis gas in the presence of H2S during five-cycle tests. The reaction rate data obtained with synthesis gas containing H2S were compared to those obtained with synthesis gas in the absence of H2S. For comparison, monometallic oxides Fe2O3 and MnO2 supported on ZrO2 were also evaluated. The phase compositions of unreacted and reacted samples were analyzed by X-ray diffraction (XRD). Bench-scale flow reactor tests were also conducted to evaluate the performance of Fe2O3−MnO2 supported on ZrO2. There are many advantages of using Fe and Mn oxides as oxygen carriers. Both oxides are widely available at low cost, and they create minimal health and environmental concerns. They also contribute to high oxygen transport capacity and lend significant physical strength to the prepared carrier. Mixing a monometallic iron oxygen carrier with another metal oxide can improve physical stability over multiple reduction− oxidation cycles. Manganese oxides have displayed better reduction−oxidation kinetics; therefore, it is believed that they might improve Fe2O3 kinetics, which are usually slow.36 Additionally, thermodynamic calculations show that the formation of sulfides and sulfates with Fe2O3 is minimal within the temperature range of the CLC process; thus, Fe2O3 is likely to be more resistant to sulfur poisoning than oxygen carriers, such as copper oxide.35
EXPERIMENTAL SECTION
Preparation of Fe2O3−MnO2 Supported on ZrO2, Sepiolite, and Al2O3. The oxygen carriers with a composition of 60 wt % Fe2O3 and 20 wt % MnO2 supported on ZrO2, sepiolite, and Al2O3 were prepared using the solid-state mixing method. Fe2O3 and MnO2 were mixed thoroughly with sepiolite, ZrO2, or Al2O3, and then deionized water was added to obtain a paste. The paste was dried and calcined at 1050 °C in air for 20 h. After cooling, the sample was crushed and thoroughly mixed again. It was then calcined a second time at 1050 °C. The calcined sample was sieved, and 95% of the fraction consisted of grains below 200 μm. For comparison of results, additional samples were prepared with monometallic oxides on the ZrO2 support (60 wt % Fe2O3 and 60 wt % MnO2 on 40 wt % ZrO2). TGAs. TGA experiments were conducted in a thermogravimetric analyzer (TA Instruments model 2050), in which the weight change of the various metal oxide oxygen carriers was measured isothermally as a function of time during reduction−oxidation cycles. Five reduction− oxidation cycles were conducted at atmospheric pressure to determine the stability of the carriers. An approximately 20 mg sample was heated in nitrogen in a quartz bowl to the reaction temperature. A simulated coal-derived synthesis gas mixture of 4042 ppm H2S, 13% CO2, 38% CO, 17.8% He, and 30.8% H2 was used for the reduction segment, while zero air was used for the oxidation segment. All reaction gas flow rates were 45 standard cubic centimeters per minute (sccm). For all experiments, the reduction time was 120 min and the oxidation reaction time was 60−90 min. To avoid the mixing of reduction gases and air, the system was flushed with nitrogen for 10 min before and after each reduction reaction. For comparison purposes, tests were also conducted with a synthesis gas composed of 12% CO2, 36% CO, 25% He, and 27% H2 without H2S. 2462
dx.doi.org/10.1021/ef201441k | Energy Fuels 2012, 26, 2461−2472
Energy & Fuels
Article
To understand the effect of the temperature, TGA experiments were carried out at both 800 and 900 °C. The fractional conversions (fractional reduction and fractional oxidation) were calculated using the TGA data of the third cycle. The fractional conversion (X) is defined as follows: fractional reduction (X ) = (Moxd − M )/(Moxd − M red)
(1)
fractional oxidation (X ) = (M − M red)/(Moxd − M red)
(2)
monometallic/ZrO2 carriers is shown in Table 1. Both XRD (Table 2) and thermodynamic data (Figure 2) indicated the Table 1. Percentage of Oxygen Used from the Maximum Theoretical Capacity for the Reaction of Supported Fe−Mn Oxide Oxygen Carriers at 800 and 900 °C at the Third Cycle in Pure Syngas and Syngas/H2S syngas
where M is the instantaneous weight, Moxd is the weight of a completely oxidized sample in TGA (maximum weight after oxidation), and Mred is the weight of a completely reduced sample in TGA (minimum weight after reduction with synthesis gas). The percentage of oxygen consumption was obtained using the weight change data from TGA using the following equation: percent oxygen consumption
800 °C
900 °C
800 °C
Fe oxide/ZrO2 Mn oxide/ZrO2 Fe−Mn oxides/ZrO2 Fe−Mn oxides/sepiolite Fe−Mn oxides/Al2O3
98.14 32.80 76.10 79.50 65.81
98.69 27.44 77.89 74.92 72.66
83.81 78.10 53.74
Table 2. Phase Compositions of Fresh Samples
= (experimental oxygen consumption/theoretical capacity of oxygen present in the metal oxide) × 100
syngas/H2S
oxygen carrier
(3)
oxygen carrier
The fractional conversion data as a function of time was fitted to obtain the best polynomial regression equation. The global rates of reactions (dX/dt), fractional coverage change per minute at different fractional conversions (X), were calculated by differentiating the polynomial equation. Thermodynamic Analyses. The FactSage 6.0 thermochemical software and phase-diagram package, by GTT-Technologies and Thermfact/CRCT, was used to evaluate the possible phases formed during the interaction of mixed-metal oxides with an inert support at 1050 °C. X-ray Powder Diffraction. XRD patterns of a fresh Fe2O3−MnO2 oxygen carrier, after reactions with synthesis gas with H2S and without H2S, were performed using the X’Pert PRO diffractometer by PANalytical, with Cu Kα, λ = 1.540 56 Å radiation. The diffraction patterns were obtained in the 2Θ range of 15−90°. Data analyses were conducted with HighScore Plus, also supplied by PANalytical. An ICCSD database was used for identification of the phases. Brunauer−Emmett−Teller (BET) Surface Area Analyses. Specific surface area measurements were conducted with an ASAP 2020 model gas adsorption unit using nitrogen. Bench-Scale Flow Reactor Tests. The Fe−Mn oxides/ZrO2 particles at
