Article pubs.acs.org/EF
Reactivity of Al2O3- or SiO2‑Supported Cu‑, Mn‑, and Co-Based Oxygen Carriers for Chemical Looping Air Separation Hui Song,† Kalpit Shah,† Elham Doroodchi,‡ Terry Wall,† and Behdad Moghtaderi*,† †
Priority Research Centre for Energy, and ‡Priority Research Centre for Advanced Particle Processing and Transport, Chemical Engineering, School of Engineering, Faculty of Engineering and Built Environment, The University of Newcastle, Callaghan, New South Wales 2308, Australia ABSTRACT: The chemical looping air separation (CLAS) is a novel method for producing high-purity oxygen, which can be effectively integrated to oxy-fuel power plants. CuO/Cu2O, Mn2O3/Mn3O4, and Co3O4/CoO have been found to be the most thermodynamically suitable oxidation pairs for the CLAS process. In the current study, the reactivity and stability of these metal oxides were analyzed further. A total of six different oxygen carrier samples were prepared by the dry impregnation method on SiO2 and Al2O3 supports. Their redox behavior has been investigated in a thermogravimetric analyzer (TGA) at four different temperatures, i.e., 800, 850, 900, and 950 °C, where the temperature-programmed oxygen release and oxidation were applied for 5 continuous cycles using nitrogen and air, respectively. The results indicate that, although relatively all oxygen carriers exhibited good reactivity, CuO/Cu2O with SiO2 and Co3O4/CoO with Al2O3 were found to be most stable. Furthermore, oxygen transport capacity (OTC) (%) and rate of oxygen transport (ROT) (% min−1) were calculated. It was found that Cu oxide with SiO2 has the highest OTC of 4.77% as well as the highest ROT of 5.1 and 10.9% min−1 for oxygen release and oxidation, respectively, at 950 °C. The CuO/SiO2 oxygen carrier also exhibited better stability for the 41 continuous cycle test, with only 10.3% loss in OTC compared to 22.3% for Co3O4/Al2O3. The grain size growth was found to be the key cause in the loss of OTC. The oxygen concentration in the outlet stream for the CuO/SiO2 oxygen carrier was measured in packed-bed experiments at different temperatures. It was observed that the oxygen concentration at the outlet of the reactor was consistent with the equilibrium values at studied temperatures. proposed4,9,10 and studied in the current paper. The detailed description of these technologies can be found somewhere else.4,6,8−10 In the CLAS process (shown in Figure 1), continuous oxygen stream is separated from air through periodical oxidation and reduction reactions occurring in two reactors. The metal oxide oxygen carriers are circulated as oxygen- and heat-transferring media between the two reactors. A stream of air will be fed to oxidize (reaction 1) the reduced oxygen carriers, and CO2 is provided for facilitating the release of oxygen by the reduction (reaction 2) of oxidized oxygen carriers. To achieve the desired oxygen concentration required by oxy-fuel combustion, a steam is generally added with the CO2 sweeping gas. After condensation of the outlet gas from reduction, the desired oxygen concentration can be obtained depending upon the amount of steam used and the operating temperature of the CLAS. The operating costs for the integrated CLAS unit for oxy-fuel combustion have been explored elsewhere.10 A 20−40% less oxygen production cost for oxyfuel combustion was obtained with CLAS than the conventional systems. The CLAS process works in a way similar to the chemical looping oxygen uncoupling (CLOU) process particularly developed for burning solid fuel.12 In the CLOU process, the oxygen carriers produce oxygen during reduction in the fuel
1. INTRODUCTION The greenhouse gas (GHG) emissions from the power industry have increased global warming potential to a greater extent. According to the International Energy Agency (IEA) sources, electricity generation is responsible for approximately 37% of global emissions worldwide.1 The GHGs can be effectively tackled by different measures, such as energy conservation, improved levels of efficiency, incremental use of renewables, and carbon capture and storage (CCS). Because of the size and complexity involved in handling these methods, a mix approach is highly desirable. One of the efficient measures to control the GHGs is CCS.1−3 In this route, oxy-fuel combustion is developed for effective CO2 abatement. However, unlike conventional power plants requiring air for combustion, oxy-fuel combustion consumes high-purity oxygen, and thus, their feasibility will be largely determined by the economic oxygen production.3−5 As a mature technique, the cryogenic distillation has been widely adopted for providing a large amount of oxygen (>150 tons/day).4,6−8 Nevertheless, its high specific power requirement is still a major concern.5,9,10 For an oxy-fuel plant, the cryogenic air separation unit (CASU) consumes nearly 10−40% of the gross power output.10 Thus, development of more energy-efficient air separation technology for low-emission clean coal processes is highly desired. Such efforts have been made over the last several decades, and various methods have been introduced, targeting the reduction of energy footprints for oxygen production, e.g., ion transport membrane,6,8 Moltox,6 and ceramic autothermal recovery.8,11 As a step-change solution, chemical looping air separation (CLAS) has also been © 2014 American Chemical Society
Received: November 18, 2013 Revised: January 19, 2014 Published: January 20, 2014 1284
dx.doi.org/10.1021/ef402268t | Energy Fuels 2014, 28, 1284−1294
Energy & Fuels
Article
Figure 1. Schematic of the CLAS process.
MgO, to vary the thermodynamic properties of Mn- and Cobased oxides, few studies have successfully shown the positive results.24−26 Up to now, the published studies are still very limited for the CLAS process compared to the CLOU process. Only a few investigations are focusing on the two most abundantly available low-cost supports, i.e., Al2O3 and SiO2.16,17,21,27,28 More importantly, no experimental study has been reported on reactivity comparison of the three most potential metal oxide candidates, namely, CuO/Cu2O, Co3O4/CoO, and Mn2O3/ Mn3O4, using Al2O3 and SiO2 as supports for CLAS. On the other hand, the impregnation is considered to be a highly economic method for fabricating oxygen carriers.29,30 Moreover, de Diego et al. has elucidated that the impregnation method may also produce oxygen carriers with higher mechanical strength and lower attrition rate compared to other methods, e.g., mechanical mixing and co-precipitation.31,32 Therefore, the objective of the current study is to provide a detailed comparison of the reactivity and stability of the Al2O3- and SiO2-supported Cu-, Mn- and Co-based oxygen carriers prepared using a dry impregnation method for the CLAS process. Consequently, the most promising candidate for using an oxygen carrier in CLAS will be determined in terms of higher oxygen transport capacity (OTC) and rate of oxygen transport (ROT). The reactivity for both oxidation and reduction are evaluated for all prepared oxygen carriers in a thermogravimetric analyzer (TGA) at temperatures between 800 and 950 °C under conditions pertinent to CLAS. The longterm stability tests over 41 repeated redox cycles were also carried out for the selected oxygen carriers. Additionally, the packed-bed reactor test was conducted to analyze the oxygen content in the product stream for the most possible candidate.
reactor. However, the released oxygen will be subsequently consumed by the char and volatiles in the fuel reactor. There are a number of challenges associated with the solids handling (e.g., the separation between oxygen carriers and ash) in CLOU, and therefore, we propose the integration of CLAS with oxy-fuel combustion.10,13 One of the key issues for the CLAS process is the development of efficient metal oxide oxygen carriers. The oxygen carriers should have high reactivity, excellent stability, and low cost. Thermodynamically, the CuO/Cu2O, Co3O4/CoO, and Mn2O3/Mn3O4 metal oxide systems are the most favorable oxygen carriers for both CLOU and CLAS processes.12,14 The first experimental work regarding CLOU and CLAS was published in 2009 and 2010, respectively.4,12 In the past 4 years, there have been considerable papers published investigating different oxygen carriers for CLOU/CLAS. Table 1 provides a brief review of the experimental work carried out on the potential oxygen carrier materials. More detailed information about the experimental investigations on developing oxygen carrier materials can be found for CLOU from the recent review published by Mattisson.15 It can be observed that Cu-based oxygen carriers were investigated extensively for CLOU/CLAS processes because of their greater oxygen transport capacity, high reactivity, and low costs. However, the use of Cu-based oxygen carriers is also challenging, encountering many issues, e.g., low mechanical strength, deactivation, and serious agglomeration.16−22 To overcome these shortcomings, efforts have been made on identifying the suitable supports and better oxygen carrier preparation methods. The use of ZrO2 or MgAl2O4 as a support was found to improve the reactivity, mechanical strength, and resistance to agglomeration for Cu-based oxygen carriers.12,16−21 It can also be noted that various methods have been reported in the literature for oxygen carrier preparation, in particular, impregnation,16,17 mechanical mixing,16,17 freeze granulation,12 and spray drying18−20 methods. Because of the thermodynamic limitations associated with the use of Mn- and Co-based oxides, there is only very limited literature available on Mn- and Co-based oxygen carriers. It is well-known that the reduced forms of Mn- and Co-based oxides, i.e., CoO and Mn3O4, cannot be reoxidized in air at temperatures of around 890 and 900 °C, respectively.11,23−26 In addition, their reactivity and oxygen transport capacity are lower compared to Cu-based oxides. Although substantial efforts were made with using different supports, e.g., Fe2O3, NiO, and
2. EXPERIMENTAL SECTION 2.1. Preparation of Oxygen Carriers. A total six oxygen carrier samples of Mn, Co, and Cu oxides with Al2O3 and SiO2 were prepared in the laboratory by a dry impregnation method using commercial support materials and precursors of metal nitrates. Silica was purchased from Grace Davison, while the other binder alumina and precursory nitrates of copper trihydrate, cobalt hexahydrate, and manganese tetrahydrate were obtained from Sigma-Aldrich. Prior to use, the silica and alumina were dried overnight at 110 °C to remove the moisture entirely from the pore internals. A proportional amount of the metal nitrates were dissolved in deionized water, forming a solution at a volume the same as the pore volume of the binders. The prepared solution was then rapidly added to the dried binders. The resulted wet samples were again dried at 110 °C for 12 h and 1285
dx.doi.org/10.1021/ef402268t | Energy Fuels 2014, 28, 1284−1294
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50−80
60 (Co2O3/Mn3O4 at 1:1 weight ratio)
Mn3O4
mixture of Co2O3 and Mn3O4 MM
FG or SD
Fe2O3, NiO, MgO, TiO2, and SiO2 YSZ, Al2O3, and TiO2
IMP
CLAS
CLOU
CAR
CLOU
CLOU
PB
FB
TGA and PB
TGA and FB
FB
FB
TGA and DFB
FB
TGA and FB
facilityc
apparent density, 3860 kg/m3; crushing strength, 2.4 N real density, 4600 kg/m3; crushing strength, 2.4 N; BET surface area,
