Reduction Rate Enhancements for Coal Direct Chemical Looping

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Reduction Rate Enhancements for Coal Direct Chemical Looping Combustion with an Iron Oxide Oxygen Carrier Zhongliang Yu,†,‡ Chunyu Li,† Yitian Fang,*,† Jiejie Huang,† and Zhiqing Wang† †

Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China Graduate School of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China

‡

ABSTRACT: Chemical looping combustion (CLC) has been suggested as an energetically efficient approach for coal combustion with CO2 sequestration. An iron oxide oxygen carrier is an attractive option because of its low cost and environmental compatibility. However, the low reactivity between iron oxide and coal is a challenge for its application. In this paper, the effects of the C/Fe2O3 molar ratio and alkali carbonate addition on the reduction rate of coal char with an Fe2O3 oxygen carrier and the feasibility of coal char direct CLC with an alkali-carbonate-impregnated Fe2O3 oxygen carrier were investigated using thermogravimetric analysis coupled with a mass spectrometer (TGA−MS), an X-ray diffractometer (XRD), scanning electron microscopy (SEM), and carbon content tests. Results indicate that, on the premise of full consumption of C by an Fe2O3 oxygen carrier, higher C/Fe2O3 molar ratios are not only beneficial to the oxygen transport capacity but also to reduction kinetics. The kinetics enhancement by higher C/Fe2O3 molar ratios could be attributed to more iron oxide/carbon contacts, which improves the char gasification rate and, in turn, enhances the reduction rate. The reduction rate also increases with the increase of alkali carbonate addition, which could be ascribed to the synergistic effects of alkali carbonates and iron on char gasification. The catalytic activities of K2CO3, Na2CO3, and Li2CO3 decrease in the order of K2CO3 > Na2CO3 > Li2CO3. Overall, a high reduction rate of coal char with an Fe2O3 oxygen carrier can be achieved with the appropriate C/Fe2O3 molar ratio and alkali carbonate addition. Catalytic CLC by alkali carbonates appears to be an effective way to combust coal directly with an Fe2O3 oxygen carrier. by the oxygen carrier in the same reactor.23 Using CLOU, the slow gasification step can be avoided. However, limited materials are suitable for CLOU, because the oxygen carriers must have the property of releasing O2 and must be reoxidized to their original forms in the air reactor.26 Currently, the metal oxide pairs CuO/Cu2O, Mn2O3/Mn3O4, and Co3O4/CoO have been investigated as oxygen carriers for CLOU.23 Cu-, Ni-, and Fe-based oxides are the most promising candidates for CLC of solid fuels. Generally, Cu- and Ni-based oxygen carriers show higher reactivities than Fe-based carriers. However, they also possess some disadvantages over Fe-based carriers given their higher cost, lower melting temperatures, and some health issues. An iron oxide oxygen carrier has been considered as an attractive option for CLC of solid fuels because of its low cost, environmental compatibility, higher sintering temperature, and magnetic property, which is a benefit for separation. One of the most important problems of an iron oxide oxygen carrier for CLC of solid fuels is the low reactivity. Most of the existing studies for CLC of coal with an iron oxide oxygen carrier experienced comparatively low reduction rates.12,15,27−29 Studies on the reactivity enhancement of an iron oxide oxygen carrier are urgent for its application. It has been reported in the ironmaking field that the reduction rate of hematite by solid carbon fuels increases with the amount of carbon and alkali metal catalyst addition.30−32 However, these conclusions cannot be simply applied to CLC

1. INTRODUCTION Chemical looping combustion (CLC) is a novel, energetically efficient combustion technology for CO 2 capture and storage.1−3 Unlike traditional combustion, oxygen required for fuel combustion by CLC is transported from air by means of lattice oxygen, such as metal oxides, avoiding the direct contact between fuel and air. Therefore, CO2 from CLC can be inherently separated from atmospheric N2. In recent years, CLC of gaseous fuels, such as CH4, H2, CO, syngas, and coke oven gas, have been intensively reported.4−10 However, only a few studies are currently available on CLC of solid fuels, which have the properties of rich deposits, cheap cost, and wide distribution.11−21 There are four approaches for CLC with solid fuels: solidfuel direct CLC, CLC with syngas (syngas−CLC), in situ gasification CLC (iG−CLC), and chemical looping with oxygen uncoupling (CLOU).21−23 Because the solid−solid reactions are usually extremely slow compared to the gas−solid reactions, few researches have paid attention to solid-fuel direct CLC.24,25 Syngas−CLC is to gasify the solid fuel to produce a syngas without nitrogen first and then to supply the syngas to the CLC system.11 With this arrangement, an air separation unit and a separate gasifier are needed, which increase the investment cost of the CLC system. To avoid the need of the separate gasifier and air separation unit, iG−CLC was proposed, which allows for the gasification of solid fuel by H2O, CO2, or these mixtures directly in the fuel reactor of the CLC process.21 However, the gasification step is slow, which limits the overall rates of the reaction. CLOU allows for the combustion of solid fuel directly by the gaseous oxygen released © 2012 American Chemical Society

Received: November 29, 2011 Revised: February 24, 2012 Published: February 27, 2012 2505

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Table 1. Proximate and Ultimate Analyses of SM Coal/Char proximate analysis (wt %, air-dried basis)

a

ultimate analysis (wt %, air-dried basis)

sample

moisture

volatile matter

ash

fixed carbon

C

H

Oa

N

S

SM coal SM char

6.15 0.86

27.40 1.00

11.62 17.35

54.83 80.79

65.93 76.74

3.64 0.93

11.31 2.99

1.03 0.60

0.32 0.53

By difference. 1:6, 1:2, and 1.5:1, which correspond to the following stoichiometric transformations, respectively:

because of the process differences. For example, the C/Fe2O3 molar ratios investigated for ironmaking are usually higher than those required for CLC, because ironmaking focuses on the complete reduction of iron oxides, while CLC aims at complete conversion of the solid fuels. The catalyst recovery is an important issue for a catalyzed process. Because the oxygen carrier is recycled during CLC operation, the catalyst loaded on the oxygen carrier can alleviate the burden of catalyst recovery. No study has been conducted on CLC of solid fuels with an alkali-carbonate-impregnated Fe2O3 oxygen carrier. In this study, coal char was used as the solid fuel to eliminate the impacts of volatiles, which had been usually suggested as the gaseous intermediates to initiate the reaction with iron oxides.13 Influences of the C/Fe2O3 molar ratio and alkali carbonate addition on the reduction kinetics of Fe2O3 by coal char and CLC of coal char with a K2CO3-impregnated Fe2O3 oxygen carrier were studied through thermogravimetric analysis coupled with a mass spectrometer (TGA−MS). Lab-scale fixedbed reactor tests were also performed to prepare samples for characterization by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results suggest that CLC of coal directly with an Fe2O3 oxygen carrier can be feasibly conducted at a high combustion rate with the appropriate C/ Fe2O3 molar ratio and alkali carbonate addition.

C + 6Fe2O3 = 4Fe3O4 + CO2

(1)

C + 2Fe2O3 = 4FeO + CO2

(2)

1.5C + Fe2O3 = 2Fe + 1.5CO2

(3)

All mixtures were thoroughly mixed with agate mortar and pestle and were stored in a desiccator until used. 2.2. Experiments and Instrumentation. A Setaram Setsys TGA Instrument was used to investigate the effects of the C/Fe2O3 molar ratio and alkali carbonate addition on Fe2O3 reduction by SM char and CLC of SM char with an alkali-carbonate-impregnated Fe2O3 oxygen carrier. In each test, about 10−13 mg of Fe2O3−SM char mixture was heated in a platinum crucible from 30 to 870 °C with a ramp of 10 °C/ min under a high-purity N2 atmosphere (100 mL/min). Then, the sample was then kept isothermally at 870 °C for 60 min. Afterward, the purge gas (N2) was switched to air with 100 mL/min for about 10 min. When a Pfeiffer Omini mass spectrometer was used for characterization of the evolved gases from TGA, Ar was used instead of N2, owing to the same mass spectral peak of CO and N2 (m/z 28). Because of an inadequate amount of solid residue from TGA for XRD, SEM, and carbon content tests, a previously mentioned fixedbed with a 10 mm deep and 35 mm diameter alumina crucible was used to prepare samples for characterization. The fixed-bed tests were conducted with the same flow rate and temperature conditions as TGA tests. In each test, about 0.45 g of sample was used. Each sample and solid residue obtained from the fixed-bed was analyzed by a RIGAKU D/max-rB X-ray powder diffractometer, which operated with Cu Kα radiation (40 kV, 100 mA, Kα1 = 0.154 08 nm) and a step size of 0.01° at a scanning speed of 5° 2θ/min from 2θ = 10° to 90°. A FEI, NavaNano 430 SEM was used to study the morphology of the reaction products from fixed-bed tests. The carbon content of residues obtained from the fixed bed was determined with a LECO SC-444 carbon/sulfur analyzer. The element contents of K and Fe for the fresh mixtures and the residues after a redox cycle were determined by an inductively coupled plasma−atomic emission spectrometer (ICP− AES, Atomscan 16, TJA Corp.).

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The char used in this study was produced by pyrolysis of Shenmu (SM) bituminous coal from Shaanxi, China. Details of the fixed-bed reactor for pyrolysis have been reported by Ren et al.33 The char preparation procedures are briefly described as follows. First, SM coal was finely ground to less than 125 μm. Second, an alumina crucible (50 mm deep and 40 mm diameter) carrying 15 g of prepared coal particles was suspended in the cooler chamber of the reactor, before air in the reactor was replaced by N2 (100 mL min−1) and the temperature reached 900 °C. Then, the crucible was quickly lowered into the uniform temperature zone. After the pyrolysis was carried out under isothermal conditions for 30 min, the crucible was rapidly raised back to the cooler chamber and the sample was quenched under a stream of N2. The proximate and ultimate analyses of parent coal and the resulting char are listed in Table 1. The Fe2O3 powder (99.9%, Tianda Chemical Reagent Factory, China) sieved to a size range of 75−90 μm was used as the oxygen carrier. K2CO3 (99.0%, Tianda Chemical Reagent Factory, China), Na2CO3 (99.8%, Tianda Chemical Reagent Factory, China), and Li2CO3 (98.0%, Kermel Chemical Reagent Factory, China) were added separately to Fe2O3 by impregnation. The loading amount of the catalyst was 0−5% in terms of the mass ratio of received alkali carbonate/Fe2O3. The procedures are concluded as follows. First, 30 g of Fe2O3 particles were added to 20 mL of aqueous solution of alkali carbonate. Then, the slurry was stirred for 1 h at ambient conditions with a magnetic stirrer. After that, the slurry was dried in air at 115 °C for 24 h. Finally, the alkali-carbonate-impregnated Fe2O3 was moved into a desiccator for use. In this paper, SM char and graphite (99.0%, Tianda Chemical Reagent Factory, China) were used as the solid fuels. The C/Fe2O3 molar ratios of Fe2O3−SM char (or graphite) mixtures were fixed at

3. RESULTS AND DISCUSSION 3.1. Effect of the C/Fe2O3 Molar Ratio on the Reduction of Fe2O3 by SM Char. The reduction experiments of Fe2O3 by SM char with varied C/Fe2O3 molar ratios, namely, 1:6, 1:2, and 1.5:1, were conducted by TGA, as shown in Figure 1. It is evident that the reduction rate increases with an increasing C/Fe2O3 molar ratio. In other words, the reduction rate was improved by a higher carbon content. Because coal char mainly consists of carbon and mineral matter, the improved reduction could be caused by either the increasing carbon content or the accumulation of mineral matter. To further confirm the effect of the carbon content on the reduction, graphite was used to reduce Fe2O3, as shown in Figure 1. The reduction rate was also enhanced by the increasing graphite content. The data imply that the reduction of Fe2O3 by SM char proceeded faster in mixtures with a higher carbon content, which is consistent with the results by Rao and Srinivasan et al.,30,32 where all of the C/Fe2O3 molar ratios investigated were higher than 1.5:1. 2506

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particles under the investigated material ratios, higher C/Fe2O3 molar ratios provide more iron oxide/carbon contacts, which promote the rate of char gasification and, in turn, increase the reduction rate. The correspondingly higher reduction rate of the Fe2O3−SM char mixture than that of the Fe2O3−graphite mixture can be ascribed to the higher gasification rate of SM char than graphite. The direct reduction rate might also be enhanced by a higher carbon content. According to the “fuelinduced oxygen release” mechanism,35 more contacts could lead to a higher amount of reactive oxygen release. 3.2. Effect of Alkali Carbonate Addition on the Reduction of Fe2O3 by SM Char. Because the reduction of Fe2O3−SM char with the C/Fe2O3 molar ratio of 1.5:1 proceeds faster than those of 1:2 and 1:6, the C/Fe2O3 ratio was fixed at 1.5:1 to investigate the effect of alkali carbonate addition on the reduction rate. Figure 3 illustrates the effect of K2CO3 addition on the Fe2O3 reduction by SM char. It is apparent that the reduction rate increases with the addition of K2CO3, and the reduction can be completed with 4% K2CO3 addition under the experimental conditions. The maximum reduction rate of the 4% K2CO3-catalyzed sample is about 59.4 times faster than that of the uncatalyzed sample. Lab-scale fixed-bed experiments were also performed to verify the rate enhancement. Figure 4 compares XRD results of the residues of the uncatalyzed and 4% K2CO3-catalyzed samples. It can be observed that the residue of the uncatalyzed sample consisted of Fe3O4 and FeO, while the residue of the 4% K2CO3-added sample was composed of FeO and Fe; meanwhile, the carbon contents of them were 2.77 and 0.392%, respectively. It can be concluded that the reduction of Fe2O3 by SM char was significantly improved by K2CO3 addition. Because iron oxide and K2CO3 all contacted with SM char, the considerable enhancement of reduction kinetics could be attributed to the synergistic effects of K2CO3 and iron on the Boudouard reaction at the contact points, which dominates the overall reduction.31 Furthermore, the direct reduction rate of Fe2O3 by SM char might also be improved by K2CO3 addition, because K is an electron donor for the iron-based catalyst,36−38 which could weaken the strength of the Fe−O bond. To compare the effects of different alkali carbonates on the reduction kinetics, Li2CO3 and Na2CO3 were also used to impregnate Fe2O3. Figure 5 displays the catalytic reduction by

Figure 1. Effect of the C/Fe2O3 molar ratio on the reduction of Fe2O3 by SM char/graphite.

For the reduction of Fe2O3 by SM char, there is a mixed mechanism of indirect reduction and direct reduction.11 However, the indirect reduction has been believed to be the main mechanism, because the direct reduction does not occur at an appreciable rate.34 The indirect reduction proceeds through gaseous intermediates (CO and CO2) according to the following reactions 4 and 5: CO + FexOy = FexOy − 1 + CO2

(4)

CO2 + C = 2CO

(5)

where x = 1, 2, or 3 when y = 1, 3, or 4. Reaction 5 (Boudouard reaction) is the rate-limiting step of the overall reduction. Figure 2 shows the SEM images of the unreacted sample and the reduction residue with a C/Fe2O3 molar ratio of 1.5:1. From Figure 2a, it can be observed that the char particles were surrounded by Fe2O3 particles. During the reaction, char particles were still “held” by iron oxide particles, providing contacts between iron oxide and SM char (Figure 2b). These contacts can result in the occurrence of the catalyzed Boudouard reaction by iron. The C/Fe2O3 molar ratio of the mixture affects the number of iron oxide/carbon contacts.30,32 Because the Fe2O3 particles are much more than the char

Figure 2. SEM images of the uncatalyzed sample (C/Fe2O3 molar ratio of 1.5:1): (a) prior to reduction and (b) after reduction at 870 °C for 60 min (“C” represents coal char). 2507

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Figure 4. XRD patterns of 4% K2CO3-catalyzed and uncatalyzed samples after reduction.

Figure 5. Comparison of different alkali carbonates on the reduction kinetics of Fe2O3 by SM char.

between these two samples was observed (Figure 6b), which indicates that the K2CO3 loading of 4% is enough for the complete conversion of coal char at the given experimental conditions. It is interesting to note that the maximum reduction rates are even higher than the corresponding maximum oxidation rates (Figure 6b). After the reduction section, air was introduced to reoxidize the reduced iron oxide. Figure 7 displays XRD analysis of the 4% K2CO3-catalyzed sample after air oxidation for 10 min. Only the Fe2O3 phase was detected, which implies that the reduced iron oxides were fully oxidized to their original forms in 10 min. The calculated maximum weight loss of the 4% K2CO3-catalyzed mixture should be 34.52% if all C converted to CO2 and 21.97% if all C converted to CO. Ignoring the little impact of moisture, the actual total weight loss was 26.40%, which implies that some amount of C was generated to be CO instead of CO2. This speculation was also verified by MS tests. As shown in Figure 8, mass spectral peaks of CO were observed, which means partial combustion occurred. Under the conversion of Fe3O4−FeO and FeO−Fe, the generation of CO was inevitable because of the thermodynamic equilibrium.11

Figure 3. Effect of K2CO3 addition on the Fe2O3 reduction by SM char: (a) weight loss and (b) weight loss rate.

Li2CO3, Na2CO3, and K2CO3. As shown in Figure 5, the catalytic effects of the three carbonates decreased in the order of K2CO3, Na2CO3, and Li2CO3, which could be attributed to the different catalytic activities on the Boudouard reaction.39,40 3.3. K2CO3-Catalyzed CLC of Coal Char with an Fe2O3 Oxygen Carrier. Generally, it is difficult to achieve full conversion of solid fuels in CLC tests at relatively low temperatures because of the low reactivities.12,15 Nevertheless, from Figure 3, it can be inferred that SM char was completely converted by an Fe2O3 oxygen carrier with the K2CO3 addition of 4%. For the aim of full conversion of SM char, Fe2O3−SM char mixtures with K2CO3 loading of 4 and 5% were selected to perform CLC tests, as shown in Figure 6. The curves in Figure 6 indicate that the combustion initiated around 495 °C and experienced a fast reaction stage at 840 °C for about 10 min. No considerable difference of the weight loss rate profiles 2508

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Figure 8. Mass spectra of CO2 and CO evolved from the TGA test of the 4% K2CO3-catalyzed sample.

Figure 6. CLC of SM char with a K2CO3-impregnated Fe2O3 oxygen carrier: (a) weight loss and (b) weight loss rate.

Figure 9. Effect of the C/Fe2O3 molar ratio on the reduction of 4% K2CO3-impregnated Fe2O3 by SM char.

solid reaction. Therefore, it may be better to perform the coal direct CLC with higher C/Fe2O3 molar ratios to enhance the combustion rates and then take some subsequent measures to achieve maximum CO2 purity. For coal char direct CLC with an Fe2O3 oxygen carrier, the C/Fe2O3 molar ratio of 1.5:1 (calculated according to the transformation of Fe2O3−Fe) is more favorable than 1:2 (according to the transformation of Fe2O3−FeO) and 1:6 (according to the transformation of Fe2O3−Fe3O4) because of not only the maximum oxygen transport capacity but also the highest reduction rate. However, although higher reduction kinetics can be realized, the excessively high C/Fe2O3 molar ratios, especially greater than 3:1, are undesirable for a CLC process because of the inherent limitation for full conversion of carbon. Significant changes in surface morphology of an oxygen carrier because of deactivation should be prevented. Figure 10 shows SEM images of the 4% K2CO3-catalyzed sample prior to reduction, after reduction, and after oxidation. It is clear that fresh Fe2O3 particles were composed of small grains (Figure 10a). After reduction, the sizes of grains increased and the melt phase was found (Figure 10b). Because alkali carbonates can be used as fluxes (because of their lower melting points),41 this melting of iron oxides might also be promoted by K2CO3. When panels c and d of Figure 10 were compared to panel b of Figure 10, no obvious increase in the grain size was observed

Figure 7. XRD patterns of the 4% K2CO3-catalyzed sample after air oxidation for 10 min.

Theoretically, the undesirable CO can be removed by increasing the molar ratio of Fe2O3 to SM char.29 However, increasing the content of iron oxide will decrease the combustion rate, as shown in Figure 9. The reaction kinetics decreased markedly with the increase of the Fe2O3/C molar ratio. From a practical standpoint, performing a solid−solid reaction is usually much more difficult than conducting a gas− 2509

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Figure 10. SEM images of the 4% K2CO3-catalyzed sample: (a) prior to reduction, (b) after reduction at 870 °C for 60 min, (c) after oxidation at 870 °C for 10 min, and (d) after oxidation at 870 °C for 60 min.

after air oxidation for 10 min; however, significant surface sintering was found after oxidation for 60 min. Because the reduced oxygen carrier was fully oxidized to Fe2O3 in 10 min (as shown in Figure 7), it can be concluded that 10 min is enough for complete oxidation of the reduced iron oxide and excessive oxidation time should be avoided because of significant surface sintering. The catalyst loss is an important issue for catalytic processes by alkali carbonates. To study the catalyst loss for K2CO3catalyzed CLC of coal char, the atom contents of K and Fe were determined by ICP−AES. Because Fe would not evolve from the reaction, the possible K2CO3 loss can be indicated by K/Fe. The actual K/Fe weight ratios of the 4% K2CO3catalyzed sample prior to the reaction and after a redox cycle were 0.030 and 0.027, respectively. It can be concluded that a small amount of catalyst was lost. Measures to prevent the catalyst loss need further investigations.

molar ratio because of the improved char gasification rate (ratelimiting step) caused by more iron oxide/carbon contacts. On the premise of complete conversion of char by Fe2O3, higher C/Fe2O3 molar ratios represent higher oxygen transport capacity and conduce to increased reduction kinetics. (2) The reduction kinetics of Fe2O3 by SM char can be enhanced by alkali carbonate addition, which could be attributed to the synergistic effects of alkali carbonates and iron on the Boudouard reaction. The catalytic activities increase in the order of Li2CO3 < Na2CO3 < K2CO3. (3) A high combustion rate of coal char direct CLC with an Fe2O3 oxygen carrier can be realized with the appropriate C/Fe2O3 molar ratio and alkali carbonate addition. Catalytic CLC by alkali carbonates may be an effective approach to directly combust coal with an Fe2O3 oxygen carrier.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

4. CONCLUSION The effects of the C/Fe2O3 molar ratio and alkali carbonate addition on Fe2O3 reduction by coal char and CLC of coal char with a K2CO3-impregnated Fe2O3 oxygen carrier were investigated using TGA−MS. The main conclusions of this paper can be summarized as follows: (1) The reduction rate of Fe2O3 by SM char increases with the increase of the C/Fe2O3

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology of China under Grant 2011BWZ001. Thanks 2510

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are due to Profs. Wang Yang and Zhao Jiantao for helpful discussion and to Ma Li for her valuable help in writing the paper.

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dx.doi.org/10.1021/ef201884r | Energy Fuels 2012, 26, 2505−2511