Article pubs.acs.org/EF
Kinetics of Iron Ore Reduction by Methane for Chemical Looping Combustion Somaye Nasr* and Kevin P. Plucknett Department of Process Engineering and Applied Science, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada ABSTRACT: Due to increasing atmospheric carbon dioxide (CO2) concentration, energy sources that release smaller amounts of CO2 to the atmosphere are of considerable interest. Attention is also now being paid to sequestering CO2 from the combustion process and eliminating discharge to the atmosphere from the major source points. Chemical-looping combustion (CLC) is a promising concept that can be used in power generation, which integrates power production and CO2 capture. In the present study, a commercially obtained iron ore was used as an oxygen carrier and the associated reduction reaction kinetics parameters have been estimated based on isothermal thermogravimetric analysis (TGA) in reducing environments. The iron oxide in the ore, which is initially Fe2O3, proceeds through a sequence of reaction steps and can ultimately end up as metallic iron. The reduction mechanism for the first stage reaction (i.e., Fe2O3 to Fe3O4) was evaluated using a number of different gas− solid reaction models. The results indicate that the Avrami−Erofe’ev model can be successfully applied to the experimental data. Through this approach, it was confirmed that the initial reaction stage is phase-boundary-controlled, which gradually transitions to diffusion control. The apparent activation energy was estimated and compared with values from the literature data.
1. INTRODUCTION In the process of trying to decrease CO2 emissions, one major target is to eliminate discharge to the atmosphere from major source points. Fossil fuel combustion, for the purpose of power generation, emits a significant amount of CO2 gas to the atmosphere. It has been established that emission of greenhouse gases is one of the main contributors to global warming, and CO2 is the most prevalent of these emissions. Published statistics revealed that the CO2 emissions resulting from human activity have led to an increase in the CO2 concentration, from a pre-industrial level of 280 to 380 ppm.1 However, there is still a significant reliance on fossil fuels, which currently account for approximately 70% of the world’s energy consumption. Deployment of low green house gas emission technologies is consequently required to lessen these emissions. With this in mind, chemical-looping combustion (CLC) is a promising concept that can be used in power generation, with the inherent capture of CO2 during the process cycle. The various possible technologies available for CO2 mitigation from combustion sources include (1) precombustion, in which the fuel is decarbonized prior to combustion, (2) oxy-fuel combustion, which utilizes pure oxygen obtained from cryogenic nitrogen separation from air, and (3) post-combustion, which separates CO2 from the flue gases using different approaches. All of these alternate options are energy-intensive in their own way. The energy penalty that relates to the respective CO2 capture processes is therefore a primary criterion that should be considered, as this will increase the electricity production costs.2 However, considering all these factors, the CLC process has been identified as a promising technology to carry out CO2 capture at a relatively low cost. Initially, CLC was intended to be an energy efficient combustion method,3 while its capability to be used as a ‘carbon capture’ technology was discovered subsequently. In this process, despite conventional combustion, the required oxygen is transferred from air to fuel by means of a solid oxygen © XXXX American Chemical Society
carrier. Direct contact between the air and fuel is consequently avoided. As a result of this, the combustion products are not diluted with N2, which is one of the main issues in conventional combustion processes. Due to this benefit, CLC technology has a low overall energy penalty. Almost pure CO2 is obtained in the exit stream after condensation of H2O, ready for sequestration without costly purification. The overall CLC process is accomplished with two reactors, an air and a fuel reactor, as shown schematically in Figure 1. This could be achieved with fluidized bed (e.g., Lyngfelt and co-workers4), fixed bed (e.g., Noorman and co-workers5), or moving bed (e.g., Gnanpragasam and co-workers6) reactors. In each case a solid oxygen carrier (metal oxide) is used to transfer
Figure 1. Schematic representation of the CLC air reactor (AR) and fuel reactor (FR). Received: October 29, 2013 Revised: January 17, 2014
A
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used was obtained in the form of small pellets. Various binders, such as Al2O3, SiO2, CaO, etc., were added by the manufacturer to the iron ore to make the pellets. In general, suitable metal oxides are combined with an inert support, which acts as a porous substrate, providing a more open structure and potentially higher surface area for reaction. These additives increase the mechanical strength, reactivity and ionic conductivity of the iron ore as an oxygen carrier agent; potentially, combining these elements could improve the oxygen carrier’s stability while maintaining a high reaction rate. The aim of the present work is therefore to study the reduction reaction kinetics of CLC using a low-cost, hematite ore. In order to generate a suitable CLC reactor design, it is important to have a good knowledge of the reaction kinetics for the material being utilized, which is the fundamental aim of the present work.
the oxygen from the air to the fuel. Regardless of which reactor arrangement is used, the oxygen carrier is of critical importance. The oxygen carrier circulates between the air reactor, where it is reconstituted to its original state by the air, and the fuel reactor, where it is reduced. For the case of the iron oxide hematite (Fe2O3), this reduction reaction is as follows when using methane (CH4): CH4(g) + 12Fe2O3(s) → CO2(g) + 2H 2O(g) + 8Fe3O4(s) (1)
while the related oxidation reaction for the magnetite product is 8Fe3O4(s) + 2O2(g) → 12Fe2O3(s)
(2)
As a consequence, the air is not mixed with the fuel, and the CO2 is not diluted by the N2 in the air. The outgoing gas from the oxidation step will contain N2 and unreacted O2, while the gas from the reduction step will be a mixture of CO2 and water vapor. The water vapor can be condensed, and a sequestrationready CO2 stream is then obtained. Some additional energy input is still needed to compress the CO2 into a liquid form, suitable for transportation and storage.4 The oxidation reaction is exothermic, while the reduction reaction can be either endothermic or exothermic, depending on the metal oxide used. The overall amount of energy released from each reaction consequently depends on the metal oxide and fuel that was used. Although significant research has been performed on the CLC process, it is still far from being a commercially available technology. Recently, variations of the CLC process have been developed, with one of the most promising being chemicallooping with oxygen uncoupling (CLOU).7−9 The CLOU process allows solid fuels to be burnt in gaseous oxygen without the need for an air separation unit, which significantly reduces energy consumption. A variety of oxygen carriers have been assessed using this approach.7−9 In terms of CLC, it is important that the metal oxide has a sufficient reduction and oxidation rate. It is also an advantage if the metal oxide is inexpensive and environmentally benign. The oxides of a number of different metals are possible candidates, including: Cu, Ni, Cd, Zn, Mn, Fe, and Co. A review of potential oxygen carrier candidate in the literature implies that a Cu-based oxygen carrier has a suitable reactivity, with an exothermic reduction reaction, but with a low melting point and tendency toward particle agglomeration (effectively ‘sintering’ of the particles) at temperatures exceeding 800 °C.10,11 Of the listed examples, Ni-based oxygen carriers are the most reactive and thermally tolerant but also the most expensive and toxic. Cd and Zn are not suitable, due to their low melting points. In comparison, Fe-based oxygen carriers may be a preferred choice because of their abundance, low price, reactivity, and their ability to endure physical stress and heat.12−16 Looking at Fe-based oxygen carriers more closely, the reduction of hematite (Fe2O3) to wüstite (FeO) is an important industry reaction, for both CLC and the methane reforming process, to produce pure hydrogen. Manganese oxide based materials have also shown significant promise for CLC processes in recent studies.17,18 In this respect, manganesebased ore, with a small iron content, also offers the potential for a low-cost precursor material while showing promising steam gasification rates.18 Low cost materials such as natural ores or minerals tend to be preferred for CLC.19,20 In the present study, the iron oxide ore, natural hematite, was selected as an oxygen carrier, as it exhibits several of the advantages outlined earlier. The iron ore
2. EXPERIMENTAL PROCEDURE The hematite iron ore used in the current study was supplied by the Iron Ore Company of Canada (Montreal, Quebec, Canada), in the form of small pellets, with an average diameter of approximately 15 mm. As noted previously, various binders, including Al2O3, SiO2, and CaO, are added to the iron ore to make the pellets. The ‘as-received’ ore was ground to reduce the particle size (e.g., 50 to 200 μm) using a ball mill. The resulting powder was then was sieved, to have a uniform powder size, through different size meshes. The mean size after sieving was assessed by particle size analysis (M3.1, Malvern Instruments, Malvern, Worcestershire, U.K.), while the specific surface area of the milled powder was determined using nitrogen BET analysis (Micromeritics Flowsorb II 2300, Norcross, GA, U.S.A.). The chemical composition of the ground iron ore was then measured using inductively coupled plasma optical emission spectroscopy (ICP-OES; Varian Vista Pro (Radial View), Varian, Inc., Mulgrave, Australia). The surface morphology of the ground ore particles was assessed using a scanning electron microscope (SEM; Model S-4700, Hitachi High Technologies, Tokyo, Japan), in order to characterize both the initial and reduced materials. In addition, the chemical compositions where determined using an associated energy dispersive X-ray spectroscopy (EDS; Model Inca X-MaxN 80 mm2, Oxford Instruments, Abingdon, U.K.) system. This characterization was performed in order to gain a further understanding of the chemical and physical transformations happening during the reduction reaction. To determine the fundamental kinetics of the iron ore reduction reactions, experiments were carried out in a thermogravimetric analyzer (TGA; Model SDT Q-600, TA Instruments, New Castle, DE, U.S.A.). The experiments were conducted in order to perform a reactivity analysis of the Fe-based oxygen carrier under a series of predetermined thermal and environmental conditions. Weight variation, as a function of time, was monitored throughout the reduction experiments, which were carried out isothermally. The ground Fe2O3-based powder was loaded into an alumina (Al2O3) crucible; the sample weight used was dependent upon the density of the carrier gas and the associated gas flow rate. The initial weights of the Fe2O3 samples were between 20 and 60 mg. The microbalance of the TGA apparatus has a nominal sensitivity of 0.1 μg. An initial equilibration hold was conducted at 50 °C, after which the ground ore samples were ramped to the test temperature at 20 °C/ min. The samples were preheated under an inert (Ar) atmosphere to the test temperature (selected within the range of 800−950 °C). After reaching the desired temperature, the reducing gas (CH4)/carrier gas (Ar) mixture was introduced during an isothermal hold (i.e., fixed temperature), with a flow rate of methane in the range of 15−20 mL/ min. The gas flow into the reactor was controlled using an electronic mass flow regulator. The inert Ar carrier gas was used for dilution of the methane. For the majority of the experiments the reducing gas composition was set as 33 vol % CH4, with the balance being the inert carrier gas. The reduction kinetics were then determined by B
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Table 1. ICP Metals Scan of the Iron Ore (All Elements Are Converted to Probable Oxide Stoichiometries) oxide wt %
Al2O3 0.27
BaO 1000 °C, gaseous diffusion chemical surface reaction, intraparticle diffusion through the reduced layer
phase boundary limited by gaseous diffusion
phase boundary
Fe2O3 → FeO
reduction mechanism/model random nucleation
Fe2O3 → Fe3O4
reduction step
115.62 179.52 89.13 70.41 162−246
experimental method
temperature programmed reduction, High surface area powders (74−108 m2/g)
temperature programmed reduction
isothermal TGA
isothermal TGA isothermal TGA isothermal TGA
isothermal TGA isothermal TGA isothermal TGA
23.81 14.98 19.84−42.15 271 105.7−220.2 34.4 39.3
isothermal TGA
isothermal TGA linear heating rate CRTA “rate-jump” isothermal TGA isothermal TGA isothermal TGA isothermal, 200 μm size isothermal, 100 μm size
isothermal TGA
linear heating rate
28.92
57−73 106 96 69−100 64.46−78.27 115.94 31.6−53.57 9.54−21.51
58.13
33.27−74
Ea (kJ/mol)
Table 4. Summary of the Activation Energy Values Reported in the Literature for Iron Oxide Reduction, Together with the Proposed Reduction Mechanism/Model
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that of Go et al.37 and Ghose et al.38 A variety of reasons may lead to such differences. These include the influence of temperature and the Fe2O3 source (i.e., size/surface area, secondary additions, etc.). In addition, differing gas combinations are occasionally used, such as a CH4/N2 mixture.39 It is likely that the addition of the previously mentioned elemental species, which was confirmed in the ICP-OES metal scan (Table 1), will therefore have an impact on the activation energy of the reduction reaction with the present materials. Some of these oxide components may act in a manner that also sees their partial reduction during the CH4 treatment (e.g., MnO, which is used in CLC processes).17,18 Alternatively, as noted above, there is potential for a low volume fraction of silicate glass formation, which could potentially partially choke the reaction. Either of these factors may affect the activation energy for the reduction process and emphasize the need for thorough precursor characterization when performing such mechanism analyses.
(6) Gnanapragasam, N. V.; Reddy, B. V.; Rosen, M. A. Int. J. Hydrogen Energy 2009, 6, 2606−2615. (7) Mattisson, T.; Lyngfelt, A.; Leion, H. Int. J. Greenhouse Gas Control 2009, 3, 11−19. (8) Leion, H.; Larring, Y.; Bakken, E.; Bredesen, R.; Mattisson, T.; Lyngfelt, A. Energy Fuels 2009, 23, 5276−5283. (9) Shulman, A.; Cleverstam, E.; Mattisson, T.; Lyngfelt, A. Energy Fuels 2009, 23, 5269−5275. (10) Forero, C. R.; Gayan, P.; Garcia-Labiano, F.; de Diego, L. F.; Abad, A.; Adanez, J. Int. J. Greenhouse Gas Control 2010, 5, 762−770. (11) Hoteit, A.; Chandel, M. K.; Delebarre, A. Chem. Eng. Technol. 2009, 3, 443−449. (12) Abad, A.; García-Labiano, F.; de Diego, L. F.; Gayán, P.; Adánez, J. Energy Fuels 2007, 21 (4), 1843−1853. (13) Zafar, Q.; Mattisson, T.; Gevert, B. Energy Fuels 2006, 20 (1), 34−44. (14) Mattisson, T.; Lyngfelt, A.; Cho, P. Fuel 2001, 80 (13), 1953− 1962. (15) Mattisson, T.; García-Labiano, F.; Kronberger, B.; Lyngfelt, A.; Adánez, J.; Hofbauer, H.; Int, J. Greenhouse Gas Control 2007, 1 (2), 158−169. (16) Roux, S.; Bensakhria, A.; Antonini, G. Int. J. Chem. React. Eng. 2006, 4, A38. (17) Fossdal, A.; Bakken, E.; Oye, B. A.; Schoning, C.; Kaus, I.; Mokkelbost, T.; Larring, Y. Int. J. Greenhouse Gas Control 2011, 5, 483−488. (18) Arjmand, M.; Leion, H.; Lyngfelt, A.; Mattisson, T. Int. J. Greenhouse Gas Control 2012, 8, 56−60. (19) Leion, H.; Lyngfelt, A.; Mattisson, T. Chem. Eng. Res. Des. 2009, 11A, 1543−1550. (20) Azis, M. M.; Jerndal, E.; Leion, H.; Mattisson, T.; Lyngfelt, A. Chem. Eng. Res. Design 2010, 11A, 1505−1514. (21) Zafar, Q.; Mattisson, T.; Gevert, B. Energy Fuels 2006, 1, 34−44. (22) Coetsee, T.; Pistorius, P. C.; de Villiers, E. E. Minerals Eng. 2002, 11, PII S0892−6875(02)00120−6. (23) Hancock, J. D.; Sharp, J. H. J. Am. Ceram. Soc. 1972, 2, 74−77. (24) Bamford, C.; H., Tipper; C. F., H.; Compton, R. G Kinetics and Chemical Technology; Elsevier: Amsterdam, 1980. (25) Liu, H. M.; Sullivan, R. M.; Hanson, J. C.; Grey, C. P.; Martin, J. D. J. Am. Chem. Soc. 2001, 31, 7564−7573. (26) Piotrowski, K.; Mondal, K.; Wiltowski, T.; Dydo, P.; Rizeg, G. Chem. Eng. J. 2007, 13, 73−82. (27) Piotrowski, K.; Mondal, K.; Lorethova, H.; Stonawski, L.; Szymanski, T.; Wiltowski, T. Int. J. Hydrogen Energy 2005, 15, 1543− 1554. (28) Go, K. S.; Son, S. R.; Kim, S. D. Int. J. Hydrogen Energy 2008, 21, 5986−5995. (29) Shimokawabe, M.; Furuichi, R.; Ishii, T. Thermochim. Acta 1979, 2, 287−305. (30) Sastri, M.; Viswanath, R.; Viswanathan, B. Int. J. Hydrogen Energy 1982, 12, 951−955. (31) Tiernan, M.; Barnes, P.; Parkes, G. J. Phys. Chem. B 2001, 1, 220−228. (32) Trushenski, S. P.; Li, K.; Philbroo, W. O. Metallurgical Trans. 1974, 5, 1149−1158. (33) El-Geassy, A.; Shehata, K.; Ezz, S. Iron Steel Int. 1977, 5, 329− 332. (34) El-Geassy, A.; Nasr, M.; Hessien, M. ISIJ Int. 1996, 6, 640−649. (35) Nasr, M. I.; Omar, A. A.; Hessien, M. M.; ElGeassy, A. A. ISIJ Int. 1996, 2, 164−171. (36) Moon, I.; Rhee, C.; Min, D. Steel Res. 1998, 8, 302−306. (37) Go, K. S.; Son, S. R.; Kim, S. D. Int. J. Hydrogen Energy 2008, 33, 5986−5995. (38) Ghosh, D.; Roy, A. K.; Ghosh, A. Trans. ISIJ 1986, 26, 186−193. (39) Monazam, E. R.; Breault, R. W.; Siriwardane, R.; Richards, G.; Carpenter, S. Chem. Eng. J. 2013, 232, 478−487. (40) Pena, J. A.; Lorente, E.; Romero, E.; Herguido, J. Catal. Today 2006, 116, 439−444.
4. CONCLUSIONS The reduction kinetics of a commercially obtained iron ore, by methane gas, were investigated using TGA experiments in the present work. Iron ore was used as an oxygen carrier/source due to its reduction−oxidation properties, combined with the fact that it is environmentally safe and low cost. In general, the oxygen carrier used in the CLC process is combined with a filler material, which acts as a porous support providing a variety of performance benefits (e.g., a higher surface area, increased mechanical strength and attrition resistance, etc.). In the present work, the effects of temperature and methane gas concentration were assessed in terms of the reduction response. A wide variety of gas−solid reaction kinetics models were evaluated to determine the best fit to the experimental data. By applying the Hancock and Sharp method to the data, an average value of n = 1.95 was obtained, which indicates the phase boundary control is likely to be dominant. It is demonstrated that the Avrami−Erofe’ev mechanism (eq 13) was the best suited for application to our experimental TGA data. In line with prior studies, it can be concluded that the iron oxide reduction process is an important industrial reaction, which could potentially be employed in the CLC and methane reforming processes.
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AUTHOR INFORMATION
Corresponding Author
* E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Halmann, M., Steinberg, M. Greenhouse Gas Carbon Dioxide Mitigation: Science and Technology; Lewis Publishers: Boca Raton, FL, 2000. (2) Hossain, M. M.; de lasa, H. I. Chem. Eng. Sci. 2008, 18, 4433− 4451. (3) Ritcher, H. J.; Knoche, K. F. Reversibility of Combustion Process. In Efficiency and Costing, Second Law Analysis of Process, ACS Symposium Series 235; Gaggioli, R. A., Ed.; American Chemical Society: Washington, DC, 1983; pp 71− 85. (4) Lyngfelt, A.; Leckner, B.; Mattisson, T. Chem. Eng. Sci. 2001, 10, 3101−3113. (5) Noorman, S.; Annaland, M. v. S.; Kuipers, J. A. M. Chem. Eng. Sci. 2010, 1, 92−97. H
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(41) Lin, H.-Y.; Chen, Y.-W.; Li, C. Thermochim. Acta 2003, 400, 61−67. (42) Munteanu, G.; Ilieva, L.; Andreeva, D. Thermochim. Acta 1999, 329, 157−162.
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