Article pubs.acs.org/EF
Cu-Modified Manganese Ore as an Oxygen Carrier for Chemical Looping Combustion Lei Xu,† Rikard Edland,‡ Zhenshan Li,*,† Henrik Leion,‡ Dongmei Zhao,‡ and Ningsheng Cai† †
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, People’s Republic of China ‡ Department of Environmental Inorganic Chemistry, Chalmers University of Technology, S-412 96 Göteborg, Sweden ABSTRACT: A new type of Mn-based oxygen carrier was prepared by impregnating manganese ore with copper nitrate solution. Cyclic reduction and oxidation reactivity of the materials was investigated in a fluidized-bed reactor. The potential use of these oxygen carriers for chemical looping combustion (CLC) was examined. The reactivity of the manganese ore can be highly improved by impregnation of copper. The reactivity of the oxygen carrier reduction is higher with a larger amount of copper impregnated. However, the degree of the reactivity enhancement is not proportional to the amount of copper doped on the oxygen carriers. An important finding is that, even with low Cu loading, such as 0.5 wt % copper impregnated on manganese, the period with full CO conversion can be enhanced 6 times. A very interesting phenomenon is that the Cu-modified manganese ore can completely convert CO, even at a low temperature, such as 500 °C. This study proves that the reactivity of the manganese ore could be significantly improved by impregnating copper, even with a small amount. The copper impregnation method could be very promising to improve the reactivity of the manganese ore as oxygen carriers for CLC.
1. INTRODUCTION Chemical looping combustion (CLC) is a combustion technology with easy carbon dioxide separation at a low cost.1 CLC is usually accomplished by having a system consisting of two fluidized-bed reactors: one air reactor where the oxygen carriers are oxidized and one fuel reactor where the oxygen carriers are reduced by a fuel.2 The off-gases from both reactors never mix, and a stream of H2O and CO2 is obtained at the outlet of the fuel reactor. Pure CO2 can be obtained by condensing of water, resulting in the inherent separation of CO2 from N2. Two reactors are interconnected by a circulation of solid oxide particles, namely, oxygen carriers. A substantial part of research in the CLC community is finding suitable oxygen carriers. Aspects such as thermodynamic properties, durability, melting point, chemical stability, and cost of production must be considered. The most common oxygen carriers are oxides of copper, iron, manganese, and nickel.3 Copper-based oxygen carriers have good reactivity but have high tendency to agglomerate. Iron-based oxygen carriers are cheaper than copper-based oxygen carriers, but their reactivity is usually low.3 Manganese oxide as an oxygen carrier is also very cheap compared to copper and nickel oxides.3 There are several forms of manganese oxides, of which Mn2O3, Mn3O4, and MnO are the relevant manganese oxides for CLC applications. The redox system Mn2O3/Mn3O4 has been shown to have chemical looping with oxygen uncoupling (CLOU) properties,4 while the Mn3O4/MnO couple is most suitable for standard CLC.3 Synthetic manganese-based oxygen carriers3 have been investigated recently, although they are not investigated as much as copper-, nickel-, or iron-based materials. Research on improving the performance of Mnbased oxygen carriers was first about adding different inert materials. Mn-based particles with the addition of ZrO2 showed agglomeration problems.5 Mn-based oxygen carriers with ZrO2 © 2014 American Chemical Society
as an inert material stabilized by MgO, CaO, or CeO2 showed high reactivity and avoidance of agglomeration.6 Azad et al.7 examined an oxygen carrier made by mixing, extruding, and calcining a 1:1 molar mixture of CuO and Mn2O3 for CLOU, and the Mn−Cu−O spinel system shows good performance. However, the preparation procedure of synthetic oxygen carriers is usually complex, and use of the large amount of copper oxide in most cases increases the cost. Therefore, cheap materials, such as natural ores, have raised much interest as oxygen carriers. Furthermore, for solid fuels, losses of oxygen carriers because of a possible side reaction with ash as well as attrition may lead to large amounts of oxygen carrier makeup,8 which also increases the demand for low-cost oxygen carrier materials. Natural ores or industrial residues have been considered as oxygen carriers and are widely investigated,9−17 among which ilmenite is the most widely investigated material10,12,18 thus far. Most recently, Ajrmand et al.18 investigated several manganese ores as oxygen carriers for CLC, and these manganese ores all showed better reactivity than ilmenite. This suggests that manganese ore could be a better choice than ilmenite as an oxygen carrier. However, manganese oxide still has relatively low reactivity.19,20 In general, natural ores usually have lower reactivity than synthetic oxygen carriers. Therefore, improving the reactivity of natural ores with a simple method is needed for their practical use. For ilmenite, it has been found that the addition of potassium can enhance the reduction reactivity significantly.21 For manganese ore, simple methods for improving the reactivity can hardly be found in the literature in the CLC community. Received: August 6, 2014 Revised: October 4, 2014 Published: October 6, 2014 7085
dx.doi.org/10.1021/ef5017686 | Energy Fuels 2014, 28, 7085−7092
Energy & Fuels
Article
Table 1. Preparation of Three Cu-Modified Oxygen Carriers with Different Mass Ratios by Wet Impregnation sample name
mass of Cu(NO3)2·3H2O (g)
volume of water (mL)
mass of ore particles (g)
mass ratio of copper/manganese ore (%)
MnCu0.5 MnCu2 MnCu10
0.95 3.80 19.00
15 15 45
50 50 50
0.5 2.0 10.0
Table 2. Chemical Analysis of the Manganese Ore and Ilmenite wt %
MnO
SiO2
Fe2O3
Al2O3
BaO
K2O
P2O5
TiO2
MgO
Mn ore ilmenite
42.30 1.98
24.79 2.32
21.73 42.32
6.72 1.45
1.67
0.90
0.46
0.27 50.54
0.25
techniques. The composition of the manganese ore was analyzed by Xray fluorescence (XRF), and the result is summarized in Table 2. The Brunauer−Emmett−Teller (BET) surface area and Barrett−Joyner− Halenda (BJH) pore volume of the particles were measured with a Micromeritics micropore analyzer (Autosorb-iQ2-MP, NOVA4000). Some particles were fixed with epoxy and then ground with abrasive paper (Grit CW 2000) to obtain the cross-section. The cross-section morphology of the ground particles was observed under scanning electron microscopy (SEM, JSM-7001F), and the elemental distribution on the particle cross-section was mapped using the coupled energy-dispersive spectrometer (EDS). The crystalline chemical species were determined by powder X-ray diffraction (XRD, D/maxIIIB). The bulk densities of the oxygen carriers are obtained and listed in Table 3. 2.3. Reactivity Test in the Fluidized-Bed Reactor. The cyclic reduction and oxidation of the oxygen carriers were carried out in a single fluidized-bed quartz reactor with a total length of 800 mm and a porous plate of 30 mm in diameter. The temperature was measured by a K-type thermocouple enclosed in the quartz reactor 15 mm above the porous plate. All of the off-gases were introduced into the gas analyzer after passing through a filter. For each test, 30 g of oxygen carriers with a size of 125−300 μm were placed on the porous plate. The flow rate of fluidizing gases for both reduction and oxidation was 2 L/min [standard temperature and pressure (STP)]. The superficial velocity is 0.17 m/s at 800 °C, and the fluidization number U/Umf is 3.95, where Umf is calculated on the basis of relations by Kunii and Levenspiel.22 The reactor was first heated to 800 °C in 10 vol % O2 in N2. When the particles was adequately oxidized, they were reduced by the reducing agent (10 vol % CO in N2). Pure nitrogen was introduced as inert for 3 min after the CO concentration reached about 2%. After the inert period, the oxidizing agent (10 vol % O2 in N2) was introduced into the reactor. Both reduction and oxidation were conducted at 800 °C for the 10 redox cycles. After the 10 redox cycles were finished, the temperature during reduction was lowered to 700, 600, 500, 400, and 300 °C to test the reactivity of the oxygen carriers at lower temperatures. 2.4. Reactivity Test in the Fixed-Bed Reactor. Powders of manganese oxide, copper−manganese oxide, iron oxide, and copper− iron oxide were tested in the same quartz reactor with an inner diameter of 30 mm. In this test, a mixture of 1 g of oxides (
