Article pubs.acs.org/EF
Investigation of a Canadian Ilmenite as an Oxygen Carrier for Chemical Looping Combustion Fang Liu,† Yi Zhang,† Liangyong Chen,† Dali Qian,† James K. Neathery,† Saito Kozo,‡ and Kunlei Liu*,† †
Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40511, United States Department of Mechanical Engineering, University of Kentucky, Lexington, Kentucky 40506, United States
‡
ABSTRACT: The use of chemical looping combustion (CLC) for power generation is an advanced energy technology that can capture CO2 inherently, which could prove to be the next electricity generation technology in a carbon-constrained future. For commercial-scale application of the CLC process, the availability of cost-effective oxygen carriers (OCs) with stable performance is imperative. Given the composition of ilmenite, there may be potential for its application as a cost-effective alternative OC for CLC. In this study, the performance of a Canadian ilmenite was investigated and showed promising results. Complete reduction of ilmenite takes a relatively long time; therefore, it is not practical for ilmenite to reach its maximum oxygen transport capacity by counter-balance of capital investment in the fuel reactor. Reduction in the first 30 min was selected as the effective reaction for the application of CLC, which gave an oxygen transport capacity of about 5.52%. To understand the phase and Fe valence state (Fe3+/Fe2+) transformations of calcined and reacted ilmenite, transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) were conducted. Using X-ray diffraction (XRD), the effective reaction was identified as Fe3+ reduced to Fe2+ at test conditions. The kinetics of this reaction were studied by doing experiments under different gas concentrations and temperatures based on a shrinking core model. The results showed that the activation energy was 169.6 × 103 J mol−1 with a reaction order of approximately 2.2. The chemical stability of the OC was studied by doing multiple redox cycles. To qualitatively show the difference between ilmenite and a synthetic OC, a freeze-granulated OC with a composition of 50 wt % Fe2O3 and 50 wt % Al2O3 was selected as a reference. The average reaction rate and crushing strength of each sample were compared. The results showed that ilmenite has favorable stability characteristics to be a viable, cost-effective OC for power generation.
1. INTRODUCTION According to the Intergovernmental Panel on Climate Change (IPCC) report, the global CO2 level was over 390 ppm or 39% above pre-industrial levels by the end of 2010.1 There is a general consensus on the need to reduce emissions of CO2 to limit or slow climate change.2 Chemical looping combustion (CLC) is a type of oxy-fuel combustion that has the advantage of in situ oxygen separation ability. Thus, CO2 can be separated from flue gas inherently without the use of an energy-intensive external air separation unit as employed for conventional oxyfuel pulverized coal combustion technology. Accordingly, CLC technology may be a promising method for fossil-based power generation in a carbon-constrained world. An ideal diagram of CLC is shown in Figure 1. CLC uses an oxygen carrier (OC), usually a metal oxide, to provide oxygen for combusting fuel without the presence of nitrogen from air. The reduced OC is then recycled to an air reactor to be reoxidized, where it is reused to provide oxygen in the subsequent fuel combustion cycle in the fuel reactor. Through the use of the OC, the flue gas is separated into two parts. The air reactor outlet is a high-temperature-depleted gas stream containing mostly N2 with a certain amount of excess O2. The fuel reactor outlet is primarily water vapor (H2O) and CO2. Water vapor is condensed, leaving an exhausted gas highly concentrated in CO2, which is ready to compress and store. Reactions in the fuel and air reactors can be expressed as follows: © 2013 American Chemical Society
Figure 1. CLC diagram.
fuel reactor
n⎞ 1 ⎛⎜ 2m + ⎟MexOy + CmHn z⎝ 2⎠ 1 ⎜⎛ n⎞ n 2m + ⎟MexOy − z + mCO2 + H 2O → ⎝ ⎠ z 2 2 Received: January 21, 2013 Revised: September 19, 2013 Published: September 23, 2013 5987
dx.doi.org/10.1021/ef401513p | Energy Fuels 2013, 27, 5987−5995
Energy & Fuels
Article
2. PERFORMANCE EVALUATION The performance of OCs was evaluated by the following parameters. 2.1. Oxygen Transport Capacity, Ro. Oxygen transport capacity, Ro, an important property of OCs, is defined as the mass fraction of usable oxygen in the OCs between the air reactor and fuel reactor
air reactor MexOy − z +
1 zO2 → MexOy 2
The overall reaction is ⎛ 1 ⎞ 1 CmHn + ⎜m + n⎟O2 → mCO2 + nH 2O ⎝ 4 ⎠ 2
R o = (mox − mred )/mox × 100%
(1)
where mox is the mass of OC in an oxidized state in grams and mred is the mass of OC in a reduced state in grams. 2.2. Conversion of Reduction, Xr, and Reaction Rate, w. Conversion of reduction is the degree of OC reduction and can be defined as the actual mass loss of OC divided by the mass of oxygen that the OC could provide.
Since the beginning of CLC research, OC development has always been a key focus of study because of the concern of slow reaction rates in the fuel reactor. Much of this development effort has been focused on developing synthetic OCs by means of freeze granulation, impregnation, spray-drying, etc., using various active metal compounds, such as Ni, Cu, Co, Mn, and Fe. Iron oxide is a relatively inexpensive and environmentally friendly compound, which is favored to make OCs. Typical Fe2O3-based OCs are pure Fe2O3 or hematite,3−12 Fe2O3 on a support of Al 2 O 3 , 1 3 − 2 6 Fe 2 O 3 on a support of MgAl2O4,8,21,27−30 and Fe2O3 mixed with a proportion of CuO or NiO on a support of Al2O3 and MgAl2O4.16,28,31−34 These synthetic OC particles perform well. However, they tend to require energy-intensive processing to enhance their integrity and reactivity and are therefore expensive to produce. For a commercial-scale CLC application, the large process OC inventory requirements and the inevitable OC loss because of attrition during circulation requires the use of cost-effective OCs with sufficient capacity. Leion et al.8,35 proposed the use of ilmenite as an OC for CLC, with experimental data indicating that Titania A/S (Norway) ilmenite was attractive and reacted with fuel as well as the synthetic particles of Fe2O3 supported on MgAl2O4 with an oxygen transport capacity of 5%. The study by Adánez et al.2 showed that reactivity of this ilmenite becomes stable after 4 cycles with an oxygen transport capacity of 4.8% (it should be pointed out that the number of cycles needed to activate the OC had a relation to the degree of reduction; for smaller conversion per cycle, more cycles will be needed to activate the ilmenite,2,51 i.e., 5−20 cycles). Azis et al.36 compared the same (Norway) ilmenite to South African ilmenite and found that Norwegian ilmenite was better than South African ilmenite. Berguerand and Lyngfelt37−40 also tested the Titania A/S ilmenite with a solid fuel of Mexican petroleum coke and South African bituminous coal in a 10 kW setup and concluded that Titania A/S ilmenite had good reactivity and was a suitable OC. Similar results were obtained by Kolbitsch et al.41 in a 120 kW dual-circulating fluidized-bed (CFB) unit. On the other hand, the study of Canadian ilmenite is limited. Schwebel et al.51 tested several kinds of ilmenite in a tubular quartz glass reactor and found that the activated Canadian rock ilmenite had a good performance close to the Norwegian ilmenite. The mechanism for oxygen transport in any ilmenite remains unclear. The purpose of this study was to elucidate the Fe phase and valence state transformations in the Canadian ilmenite and to better understand the reduction mechanism of ilmenite under CLC application conditions. Another purpose was to study the chemical stability, because a reusable OC with robust performance can greatly reduce the cost of OCs. Finally, this study also aimed to quantitatively show the performance difference between ilmenite and a synthetic OC.
X r = (mox − m)/(mox − mred ) × 100%
(2)
The reaction rate, w, is therefore defined by time rate of change of conversion. w = dX r /dt
(3)
2.3. Crushing Strength and Surface Area. Crushing strength reflects, to some degree, the particles resistant to attrition stresses. Particles with higher crushing strength possess a higher ability to retain their initial shape and, thus, exhibit good durability. It is worth noting that, for the OC crushing strength to be comparable in a study, a narrow particle size distribution must be used. Surface area must also be considered because it is an important factor in gas−solid reactions. 2.4. Shrinking Core Model (SCM). The shrinking core model was used to carry out a kinetics analysis; grain structure was observed in ilmenite particles. Typically, the shrinking core model is useful in describing the reaction kinetics with this type of particle morphology. The basic equation for the shrinking core model can be expressed as50 w = dX r /dt = 3(1 − X r)2/3 kC n
(4)
where the terms are defined as Xr, conversion, in %; t, time, in s; k, reaction rate constant, for reaction order of n, in Ln − 1 mol1 − n s−1; C, reacting gas concentration, in mol L−1; and n, reaction order. The reaction rate, k, according to Arrhenius law is expressed as k = A exp( −Ea /RT )
(5)
where the terms are defined as A, pre-exponential factor, for reaction order of n, in Ln − 1 mol1 − n s−1; Ea, activation energy, in J mol−1; R, universal gas constant of 8.314 J mol−1 K−1; and T, temperature, in K. Integration yields 1 − (1 − X r)1/3 = kC nt
(6)
3. EXPERIMENTAL SECTION Raw ilmenite ore was supplied by QIT Iron and Titanium Incorporation, Canada, from the Lac Tio mine, which is the largest hard-rock ilmenite deposit in the world. The ilmenite ore was crushed into small particles and sieved to obtain a range of 150−250 μm. Before using the crushed ilmenite particles as the OC, the particles were calcined at 1400 °C for 6 h in an oxidizing atmosphere to ensure complete oxidation of the ilmenite; XRD spectra measurements are shown in Figure 2. It can be seen that calcined ilmenite was in a completely oxidized state, with a main composition of Fe2O3 and 5988
dx.doi.org/10.1021/ef401513p | Energy Fuels 2013, 27, 5987−5995
Energy & Fuels
Article
Figure 3. SEM images of ilmenite samples: (a) overview of calcined particles in a completely oxidized state, (b) calcined particles in a completely oxidized state, (c) after a 30 min reduction, and (d) after 10 redox cycles in a completely oxidized state.
Figure 2. XRD spectra. Fe2TiO5. If we consider the ilmenite chemical composition in the form of metal oxides (Fe2O3, FeO, and TiO2), the resulting component of the calcined ilmenite particles in their completely oxidized state is shown in Table 1. The Fe2O3 weight concentration in the OC sample was 52.87%, with Fe3+ having a weight concentration of 37.01%. Scanning electron microscopy (SEM) images are shown in Figure 3a for an overview of the calcined ilmenite particles and Figure 3b for morphology of a calcined ilmenite particle. The ilmenite particles are irregular with a sharp-edged surface. The surface morphology of the ilmenite particle exhibits a more granular structure. The kinetics and stability experiments in this study were conducted in a thermogravimetric analysis (TGA) system (Netzsch STA449C), as shown in Figure 4. CO was used as the fuel and was diluted in argon to different concentrations before feeding into TGA. A set of mass flow controllers was used to control the appropriate flow rate for each gas composition and was controlled and monitored using LabView software. The program allowed the controller to automatically switch between an oxidizing and a reducing atmosphere, with a purge cycle of argon gas in between. About 500 mg of ilmenite was used for each run. To understand the Fe phase and valence state (Fe2+/Fe3+) transformations, transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) were operated at 200 kV in a JEM 2010 field-emission transmission electron microscope. To help analyze the reaction of ilmenite and to better characterize the ilmenite OC, the particles were examined with respect to composition, morphology, crushing strength, and Brunauer−Emmett−Teller (BET) surface area. The composition was identified by a Philips X-ray diffraction (XRD) instrument. The morphology of the particle was examined in a Hitachi S-4800 scanning electron microscope. The crushing strength was measured through the averaging of 30 measurements in a Shimpo FGE-10X. The surface area evaluation was based on the BET method and was determined in a Micromeritics Tristar 3000 gas adsorption analyzer. Reaction kinetics were investigated by varying the reaction temperature and the CO concentration. The chosen CO concentrations were 10, 20, and 30 vol %, with the balance being argon. The selected temperatures were 850, 950, and 1050 °C. The chemical stability of ilmenite was tested using TGA with 10 redox cycles, which was selected by considering achieving a stable performance state. In our experiment, the stable state was achieved
after 2 cycles. This result differs from the study by Adánez et al.2 because we are using a more extreme reducing gas condition, which promotes a faster activation. For each cycle, the sequence of gas purges was a 5 min purge of argon gas, a 20 min purge of 20 vol % O2 balanced with argon gas, a 5 min purge of argon gas, and a 30 min purge of 20 vol % CO balanced with argon gas. The intent of the argon purge was to flush the TGA furnace with an inert gas to avoid the possibility of CO and O2 ignition at high temperatures. Prior to the experiment, two precautions were taken to help reduce external diffusion effects on the OC during reduction/oxidation experiments. First, a TGA crucible with a larger open diameter (0.0162 m) was used to allow the gas to reach the sample easily. Second, a high gas flow rate of 0.20 L min−1 was used to provide sufficient excess flow around the sample crucible. After conducting several tests, it was determined that the gas flow above 0.18 L min−1 eliminated external diffusion. This flow rate was found by plotting the conversion−time curves at different gas flow rates (the CO concentration in the gas remained constant) and identifying the minimum flow rate at which the conversion−time relation was independent of the gas flow rate.42 In this study, a gas flow rate of 0.2 L min−1, well above the requisite 0.18 L min−1, was selected to minimize external mass-transfer resistance.
4. RESULTS AND DISCUSSION 4.1. OC TEM/EELS Analysis. To do TEM/EELS analysis, OC/resin film was microtomed using a diamond knife as reported by Ulan et al.43 Ideally, the sample film thickness needs to be thinner than 100 nm to avoid strong electron beam absorption during imaging and multiple scattering events within the sample during the EELS process. However, because of the large ilmenite particle size (over 150 μm in diameter, as shown in Figure 3a) and the brittle nature exhibited during cutting, 200 nm proved to be the thinnest film possible with available techniques. A TEM image of a typical 200 nm thick film microtomed from resin/ilmenite-calcined ore is shown in Figure 5a. The oval-shaped OC particle with a size of ∼100 μm,
Table 1. Composition of Completely Oxidized Ilmenite
a
composition
Fe2O3 (%)
TiO2 (%)
SiO2 (%)
Al2O3 (%)
CaO (%)
MgO (%)
othersa (%)
ilmenite
52.87
33.80
5.70
3.60
0.90
2.80
0.33
Others refer to MnO, S, Na2O, K2O, and P2O5. 5989
dx.doi.org/10.1021/ef401513p | Energy Fuels 2013, 27, 5987−5995
Energy & Fuels
Article
Figure 4. Diagram of the TGA test system.
powder is heterogeneous at the micrometer scale (see scale bar in panels b and c of Figure 5) along the debris edges, although their central regions are too thick (>100 nm) to perform meaningful EELS analysis. These Fe-rich domains could be the actual active OCs during the CLC process, considering that perfect FeTiO3 crystals contain only Fe2+ and Ti4+ and, therefore, not likely to take up or release oxygen. 4.2. Reduction Reaction of Ilmenite. The complete reduction curve of ilmenite is shown in Figure 6. The total
Figure 5. Characterizations of calcined ilmenite ore: (a) lowmagnification TEM image of a large OC particle (the dotted line traced its oval-like contour, and the dark bar is the Cu frame from the TEM grid) within a dark resin matrix, (b and c) scanning TEM image (zoom in the red-marked region in panel a), and representative EELS spectrum showing (d) Ti−Fe−O phase (from red spot regions), (e) Ti-deficient phase (from blue spots), and (f) Fe-deficient phase (from green spots) along the thinner edge (
