Potassium-Modified Iron Ore as Oxygen Carrier for Coal Chemical

Article pubs.acs.org/IECR

Potassium-Modified Iron Ore as Oxygen Carrier for Coal Chemical Looping Combustion: Continuous Test in 1 kW Reactor Haiming Gu,* Laihong Shen, Zhaoping Zhong, Xin Niu, Huijun Ge, Yufei Zhou, and Shen Xiao Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, 2 Sipailou, Nanjing 210096, China ABSTRACT: Chemical looping combustion (CLC) is a promising technology to capture CO2. However, the low conversion efficiency of fuel is the key challenge for the in situ gasification CLC with natural iron ore as an oxygen carrier. The reactivity of the selected 6% K2CO3-modified iron ore (6KMIO) was first determined by H2 and CO in TGA, and an enhanced reduction reactivity of 6KMIO was confirmed. Afterward, the feasibility of 6MKIO for coal CLC was tested in a 1 kW continuous reactor. The continuous reactor could be stably operated without any tendency of agglomeration or sintering among 6KMIO particles. Compared with use of the raw iron ore, the use of 6KMIO enhanced the coal conversion and the gasification-products conversion, promoting the CO2 capture efficiency. Furthermore, the mechanism of the catalyzed coal CLC process and the release of sulfur was explored.

1. INTRODUCTION In view of the intensive utilization of fossil fuel and the global warming due to the emission of greenhouse gas, the reduction of CO2 would be the main mission in the following decades. The technology of carbon capture and storage (CCS) will remain a critical component of low-carbon energy technologies. The traditional CCS technologies, that is, precombustion, oxyfuel, and postcombustion, present an intensive energy penalty, which is the major barrier to their applications.1 Therefore, the reduction of greenhouse gas emission is still one of the most important challenges in the following decades.2 The concept of chemical looping combustion (CLC) presents a novel combustion method with inherent separation of CO2 without energy penalty.3,4 The process of chemical looping combustion (CLC) changes the combustion mode from traditional incineration in which fuel and air are directly mixed. As is shown in Figure 1, the CLC process consists of oxygen carrier (mainly metal oxide) and two interconnected reactors, that is, a fuel reactor and an air reactor. In the air reactor the reduced oxygen carrier MexOy−1 is oxidized to MexOy, which is then transported to the fuel reactor where it is reduced back to MexOy−1. By this means,

oxygen is transported from air to fuel. Under a complete conversion, the flue gas from the fuel reactor, on a dry basis, is a CO2-rich product. Because of the combination of the air reactor and the fuel reactor, the oxygen carrier is the key factor to realize the CLC technology. The oxygen carrier should have a high reactivity, that is, high selective reactivity to fuel and oxygen and high redox reaction rate to obtain a high efficiency of fuel conversion and oxygen carrier conversion. A range of metal oxides as oxygen carrier, such as NiO, CuO, and Fe2O3, have been intensively tested as reviewed by Adánez5 and Lyngfelt.6 Natural iron ore has recently received intensive interest because of its low price,7−11 but generally this inexpensive material exhibits a poor performance when compared with either Nibased or Cu-based oxygen carriers, especially when solid fuel is employed.12 Additional development for the natural iron ore is necessary to improve CLC performance. For the solid fuel process, in situ gasification chemical looping combustion (IG-CLC)4 is the most acceptable approach to realize the CLC process. In this approach, the reaction patterns in the fuel reactor involve coal gasification and subsequent oxygen carrier reduction. The IG-CLC process allows these two-step reaction series to proceed in only one reactor, reducing the complexity and the cost of the CLC system. However, coal gasification in the fuel reactor is the ratelimiting step in the process, especially when a Fe-based oxygen carrier is used.12 As a result, a higher bed height of oxygen carrier is necessary to ensure adequate residual time of solid fuel or its gasification products in the bed material. Many efforts have been made to promote the char conversion rate by improving the Fe-based oxygen carrier with additions, including mechanically mixed and combined Received: Revised: Accepted: Published:

Figure 1. Schematic for chemical looping combustion. © 2014 American Chemical Society

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March 30, 2014 July 19, 2014 July 30, 2014 July 30, 2014 dx.doi.org/10.1021/ie501328h | Ind. Eng. Chem. Res. 2014, 53, 13006−13015

Industrial & Engineering Chemistry Research

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exposed to the reaction gas, that is, 15 vol % CO/N2 or 15 vol % H2/N2. The total flow of reaction gases was set to 500 mL/ min, regulated by mass flow controllers. The weight of oxygen carrier particles was recorded with a temporal resolution of 3 s. The reaction ended when the weight of oxygen carrier reached the balance. 2.3. Continuous Operation in a 1 kW Interconnected Fluidized Bed. The feasibility of 6KMIO as oxygen carrier for continuous coal CLC was evaluated in a 1 kW interconnected fluidized bed reactor, as is shown in Figure 2. The apparatus

oxygen carriers. The mechanically mixed oxygen carriers with additions, for example, lime and NiO, build on a synergistic effect on enhancing char conversion.13−16 An enhanced char conversion was also observed using the combined oxygen carrier with different materials, such as CaO, K2CO3, and Na2CO3.17−19 The concept of combined oxygen carrier involves the creation of new compounds with new properties. According to the open publications, Fe2O3 oxygen carrier impregnated with K2CO3 exhibits a relatively excellent performance.18 These investigations are based on TGA or single-bed reactor. The investigations of the catalyzed IG-CLC have not yet extended to continuous operation. The reactivity test of the K-modified oxygen carrier by gaseous fuel has not extended to H2, that is, another primary gasification product. Our previous work evaluated the catalyzed coal CLC in a single fluidized bed using a K2CO3 modified iron ore, and the optimized K2CO3 loading was 6 wt %.19 The present study aims at (1) investigating the reactivity of the novel 6 wt % K2CO3modified iron ore (6KMIO) using the main coal gasification products, H2 and CO, and (2) evaluating the feasibility of 6KMIO for the coal CLC process in a 1 kW continuous reactor.

2. EXPERIMENTAL SECTION 2.1. Oxygen Carrier and Fuel. South African iron ore modified with 6 wt % K2CO3 after calcinations was employed as an oxygen carrier, and the detailed information could be referred to our previous work.19 Simultaneously, the experiments using raw iron ore (RIO) were also conducted as reference conditions. Table 1 lists the chemical compositions of the RIO. The particles of the oxygen carrier were sieved to the size range of 100−300 μm. Table 1. Chemical Compositions of RIO components

content (wt %)

Fe2O3 Al2O3 SiO2 TiO2 P2O5 CaO K2O others

86.72 4.04 8.29 0.1 0.21 0.19 0.29 0.16

Figure 2. Configuration of the 1 kW coal CLC reactor.

was detailedly described in our previous work.20 Briefly, it consists of a fast fluidized bed as an air reactor, a cyclone, a spout-fluid bed as a fuel reactor, and an external loop-seal. Table 3 lists the main variables for the continuous operation. A 2 h continuous operation was first conducted at the fuel reactor temperature of 950 °C to evaluate the fluidization characteristic of 6KMIO during the coal CLC. The bed pressure of both reactors was monitored by the pressure sensors which were installed along the height of both reactors, as are marked in Figure 2. A stable time series of bed pressure drop represents that the oxygen carrier could be normally fluidized in the present reactor. Afterward, the effect of 6KMIO on the coal CLC process was evaluated by comparing with raw iron ore. In the present reactor, some gas from fuel reactor leaked into the downcomer, diluting the flue gas from air reactor, but not vice versa.21 However, it did not affect evaluating the performance of 6KMIO as oxygen carrier for the

Anthracite coal from Huaibei, China, was selected as solid fuel. The proximate analysis and ultimate analysis are listed in Table 2. The coal particles were crushed into small size and double-sieved into a size range of 200−450 μm. 2.2. Reactivity Test in TGA. The reactivity of the oxygen carrier was evaluated using the primary gaseous products from coal gasification, CO and H2, by thermogravimetric analysis (TGA, Thermax 500). For each case, a batch sample of 120 mg of oxygen carrier particles was initially placed in a quartz crucible in the furnace, which was then heated to 920 °C in N2 atmosphere. Afterward, the particles of oxygen carrier were Table 2. Proximate and Ultimate Analysis of Anthracite Coal ultimate analysis (ada, wt %)

a

proximate analysis (ada, wt %)

b

Nad

Sad

M

V

FC

ash

LHV (MJ/kg)

1.39

1.32

1.01

8.82

80.35

9.82

34.92

Cad

Had

Oad

80.85

4.62

2.31

Air-dried (ad) basis. bCalculated by difference. 13007

dx.doi.org/10.1021/ie501328h | Ind. Eng. Chem. Res. 2014, 53, 13006−13015


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Table 3. Variables for Continuous Operation in the 1 kW Reactor test

oxygen carrier

mcoal (g/h)

Fair,AR (m3/h)

FN2 (m3/h)

FH2O,LS (g/h)

FH2O,FR (g/h)

TFR (°C)

1 2 3

6KMIO 6KMIO RIO

100 100 100

0.78 0.78 0.78

0.42 0.42 0.42

198 198 198

180 180 180

950 900−975 900−975

Figure 3. Reactivity of RIO and 6KMIO.

As an electron donor, K in the oxygen carrier could weaken the strength of the Fe−O bond.22−24 As a result, 6KMIO could offer lattice oxygen to the gaseous fuel more easily in comparison to RIO. During the whole reaction period, the weight loss and the reduction rate of 6KMIO were always much larger in comparison to the case of RIO. The result was in accordance with that reported by Bao et al.25,26 The similar enhanced reaction performance of 6KMIO was also observed with 15% H2/N2 as fuel, as is shown in Figure 3. During the late reaction stage, RIO exhibited a higher reactivity than 6KMIO. This could be attributed to the relatively inert K−Fe−O series compounds in the oxygen carrier; that is,the lattice oxygen in some K−Fe−O compounds was unavailable for the gaseous fuel. Briefly, the 6KMIO exhibited an enhanced reaction performance with H2 and CO, the primary coal gasification products when not deeply reduced. The deep reduction of a Fe-based oxygen carrier should also be avoided due to the potential fluidization problem.27 3.2. Continuous Operation in the 1 kW Continuous Reactor. For the continuous CLC process in the 1 kW reactor, several key technical indexes were defined to evaluate the performance of the oxygen carrier, including coal conversion efficiency ηgasify, gas conversion efficiency f i, CO2 capture efficiency ηCO2, and carbon capture efficiency ηCC:

continuous coal CLC because the leakage proportion was only 2%. The gaseous products from both reactors, after water removal, were sampled by gas bags for offline analysis. For each condition, gas sampling began after 30 min of stable operation. The concentrations of CO2, CH4, CO, O2, and H2 were measured by an Emerson gas analyzer. To evaluate the wear characteristic of 6KMIO, the fine powder in the flue gas from both reactors was collected. The loss rate of oxygen carrier from both reactors could be obtained after the dry process of powder.

3. RESULTS 3.1. Reactivity Analysis in TGA. The reactivity of 6KMIO was determined using 15% CO/N 2 and 15% H 2 /N 2 , respectively. The weight change of oxygen carrier Xt and the reduction rate of oxygen carrier rt are defined to illustrate the reactivity of the oxygen carrier. m X t% = t 100 m0 (1) rt =

dX t dt

(2)

Figure 3 displays the reduction data of the oxygen carrier with 15% CO/N2 as gaseous fuel. On introducing the gaseous fuel, a fast weight loss of oxygen carrier occurred, corresponding to the reduction of Fe2O3 to Fe3O4. Afterward, the reaction rate slowed down due to the restraint of thermodynamics equilibrium. The weight of RIO and 6KMIO after 50 min was 90.3% and 79.6%, respectively. During the preparation process of 6KMIO, K in the oxygen carrier was transformed into a relatively stable potassium ferrite rather than vaporized.

ηgasify % =

FC,FR nC,coal × 22.4

100 (3)

FC,FR = Fout,FR (WCO,FR + WCO2,FR + WCH4,FR ) fi % = 13008

Wi,FR WCO,FR + WCO2,FR + WCH4,FR

(4)

100 (5)


Industrial & Engineering Chemistry Research ηCO % = 2

ηCC% =

FCO2,FR FC,FR + FC,AR

FC,FR FC,FR + FC,AR

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(6)

100 (7)

FCO2,FR = Fout,FR × WCO2,FR

(8)

FCO2,AR = Fout,AR × WCO2,AR

(9)

Fout,AR =

6 show the effect of the fuel reactor temperature on the gas concentrations during the experiments using RIO and 6KMIO, respectively. For the experiment using RIO, the concentrations of CO and CO2 in the fuel reactor increased as the temperature increased from 900 to 975 °C, whereas the CH4 concentration slightly decreased. The concentrations of CO2, CO, and CH4 were 6.19%, 1.3%, and 0.28% at 900 °C, and then, they became 8.24%, 1.68%, and 0.17% at 975 °C. In the air reactor, CO2 concentration decreased from 7.79% at 900 °C to 5.54% at 975 °C, and correspondingly, O2 concentration increased from 9.3% to 10.58%. The detailed explanation of the temperature effect on the gas compositions was discussed in previous work using an Australian iron ore as an oxygen carrier.15 In the present experiment, no or negligible H2 was observed in the flue gas from the fuel reactor. The situation was somewhat different from the investigation conducted in the Chalmers’ 10 kW reactor.28 It could be ascribed to the difference in reactor structure. For the experiment conducted in the Chalmers’ 10 kW reactor, coal particles were fed from the upper part of the fuel reactor, and only about 40% of the oxygen carrier was maintained in the fuel reactor. For the present 1 kW reactor, the coal-feeding position locates at the bottom of the fuel reactor and most of the oxygen carrier particles were maintained in the fuel reactor, as is indicated in Figure 4. It means that H2 from coal gasification has an adequate residence time in the bed material to be oxidized, and hence, no or negligible H2 was observed. For the experiment using 6KMIO as an oxygen carrier, the concentrations of CO, CO2, and CH4 versus the fuel reactor temperature exhibited the same trend with those in the case using RIO. However, the char conversion in the fuel reactor was improved, resulting in the concentration of (CO+CO2) almost 2.5 times of that in the case using RIO. The concentrations of CO2 and CO in the fuel reactor increased from 22.16% to 25.8% and from 0.19% to 0.57%, respectively, whereas CH4 concentration decreased from 0.39% to 0.31%. A larger circulation of oxygen carrier due to a larger amount of carbonaceous gases could promote the conversion of solid fuel in the fuel reactor.29 However, the experiments using these two oxygen carriers were conducted with the same variables. Therefore, the difference in gas concentrations in the case of those two oxygen carriers was primarily attributed to the reactivity of oxygen carrier rather than the circulation quantity. In the air reactor, CO2 concentration decreased from 6.19% at 900 °C to 1.5% at 975 °C, whereas the negligible O2 concentration remained at 0.15%−0.32%. The formation of CO2 depends on the circulation of residual char, after coal gasification, from the fuel reactor into the air reactor. As is shown in Figure 7, with temperature increasing from 900 to 975 °C, ηgasify in the cases using RIO and 6KMIO increased from 21.8% to 29% and from 72.8% to 91.1%, respectively. A higher temperature could result in a higher ηgasify for both oxygen carriers. However, the difference is that ηgasify in the case of 6KMIO was almost 3 times that in the case of RIO. Yu et al. reported that the addition of K2CO3 in Fe2O3 could efficiently enhance the direct reaction between coal char and oxygen carrier,18 contributing to the increase in ηgasify according to eq 3. Therefore, the concentration of CO2 in the air reactor in the case using 6KMIO was much lower than that in the case using RIO. Meanwhile, the consumption of O2 by the oxygen carrier and the amount of char was much larger in the case using 6KMIO. Furthermore, lower gasification efficiency would cause

100

Fair,AR × (1 − α) 1 − WO2,AR − WCO2,AR

Fout,FR = FN2,FR ×

1 − WCO,FR

(10)

1 − WCO2,FR − WCH4,FR (11)

3.2.1. Fluidization Characteristic. K-Containing compounds of low melting point may be formed in 6KMIO during the calcinations process at 950 °C. Agglomeration and sintering among the particles of oxygen carrier may occur thus causing the fluidization problem of the bed material at a fuel reactor temperature above 900 °C. Most of the established CLC reactors over the world are based on a fluidized bed, which is considered as the most suitable reactor to realize CLC technology.5 Therefore, the fluidization characteristic of 6KMIO was critical to the CLC process, and it was evaluated before the reactivity test. Bed pressure at different positions of the 1 kW reactor was monitored and recorded during the continuous operation using 6KMIO at the fuel reactor temperature 950 °C. The time series of P75 (P7−P5) and P14 (P1−P4) were employed to monitor the fluidization condition of bed material in the fuel reactor and air reactor, respectively. As is displayed in Figure 4, the typical time

Figure 4. Time series of P75, P14, and P34 during the continuous operation with 6KMIO as oxygen carrier.

series of both P75 and P14 are stable, and the average values were 4.4 and 13.5 kPa, respectively. The tendency of particle agglomeration or sintering among the oxygen carrier particles was never observed during the entire continuous operation. This indicates that 6KMIO as oxygen carrier exhibits a relatively excellent fluidization characteristic. Furthermore, P34 (P3−P4) was also stable during 2 h of continuous operation, and it provided evidence of circulation of the oxygen carrier. 3.2.2. Coal Conversion and Gas Conversion. The fuel reactor temperature is crucial to the gas compositions of both reactors during the continuous coal CLC process. Figures 5 and 13009



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Figure 5. Effect of the fuel reactor temperature on gas compositions with RIO as oxygen carrier.

Figure 6. Effect of the fuel reactor temperature on gas compositions with 6KMIO as oxygen carrier.

using RIO remained at about 16% and 80%, respectively, and those for 6KMIO remained at about 1.5% and 97%, respectively. Compared with the case of RIO, the use of 6KMIO gave a lower f CO but a larger f CO2. This effect could be ascribed to both the enhanced reduction reactivity of the oxygen carrier and the promoted water gas shift reaction R2. Table 4 also seems to indicate a positive effect of 6KMIO on converting CH4, and f CH4 decreased with temperature. 3.2.3. Carbon Capture. As is stated in the previous section, both the elevated temperature and the use of 6KMIO could promote the yield of carbonaceous gases (especially CO2) in the fuel reactor and also reduce the amount of char circulating with the oxygen carrier into the air reactor. Therefore, larger ηCC and ηCO2 was obtained at a higher temperature, as is indicated in Figure 8. When RIO was used, ηCC and ηCO2 increased from 35.7% to 52.1% and from 28.4% to 42.6%, respectively. When 6KMIO was used, ηCC and ηCO2 increased from 73.2% to 93.7% and from 71.2% to 90.4%, respectively. The difference Δη between ηCC and ηCO2, (ηCC − ηCO2), was due to the combustible carbonaceous gases in the fuel reactor. It reflects the conversion extent of carbonaceous gases in the fuel reactor, and Δη = 0 represents 100% conversion of carbonaceous gases. The Δη values for the cases using RIO and 6KMIO remained at 7.3%−9.6% and 2.0%−3.2%, respectively. This means that the conversion efficiency of carbonaceous gases in the case using 6KMIO was much higher than that in the case of RIO. Recirculation configuration of fly ash/char and combustible gases (CO) back to the fuel reactor could

Figure 7. Effect of the fuel reactor temperature on carbon gasification efficiency.

a larger carbon loss in fly ash. Thus, the use of 6KMIO could be a better choice. Table 4 shows the fractions of carbonaceous gases to evaluate their conversion into CO2. f CO and f CO2 during the experiment Table 4. Fractions of CO2, CO, and CH4 at Typical Temperature RIO

6KMIO

fraction

900 °C

950 °C

975 °C

900 °C

950 °C

975 °C

f CO f CO2 f CH4

16.7 79.7 3.6

15 82.9 2.1

16.7 81.7 1.7

0.8 97.4 1.7

1.8 97 1.3

2.1 96.7 1.2 13010



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3.3.1. Attrition Loss. A loss of oxygen carrier is inevitable due to attrition of the oxygen carrier during the continuous operation in the interconnected fluidized bed. Then, a supply of oxygen carrier is necessary to maintain the operation, and the capital cost of the CLC system would inevitably rise. In this sense, the antiwear characteristic is another crucial factor to evaluate the performance of oxygen carrier. Definitely, an oxygen carrier with poor mechanical strength would not seem to exhibit an excellent antiwear characteristic. Therefore, the crush strength, that is, the force needed to crush the oxygen carrier particles was first determined by a texture analyzer (TA XTplus) using the ASTM D4179-01 method. A batch sample of 60 particles was tested to obtain the average. The average crush strength of RIO and 6KMIO is 11.8 and 12.4 N, respectively. It indicates that the impregnation of K+ on iron ore would not weaken the mechanical strength of the oxygen carrier. During the continuous operation in the 1 kW interconnected fluidized-bed, the oxygen carrier particles underwent intensive thermal stress and attrition. Some particles turned into fine powder, too small to be captured by the cyclone, causing a loss of oxygen carrier. In this sense, the attrition loss of oxygen carrier was characterized by the fine powder of oxygen carrier that could not be captured by the cyclone. Therefore, the attrition loss was related to the gas−solid separation efficiency

Figure 8. Effect of the fuel reactor temperature on CO2 capture efficiency.

definitely promote the fuel conversion in the fuel reactor and the carbon capture efficiency. However, it would also increase the capital cost of the CLC system. At this point, the use of 6KMIO as oxygen carrier could be a better choice. 3.3. Characterization of Oxygen Carrier. The attrition characteristic, morphological features, and reductive extent of the oxygen carrier was characterized. It helps to understand the comprehensive effect of the decoration of K2CO3 on the iron ore oxygen carrier.

Figure 9. SEM images of both fresh and reacted oxygen carrier particles. 13011



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RI = IFe3O4 /IFe2O3

of the cyclone. The loss of 6KMIO was up to 0.013%/h at the fuel reactor temperature 950 °C. It was much lower than the loss of Australian iron ore when the experiment was conducted in the same reactor.15 It means that the lifetime of 6KMIO would be much longer than that of the Australia iron ore. Furthermore, owing to the structure feature of the present reactor, some fine powder of oxygen carrier together with coal ash was elutriated out from the fuel reactor, and this loss was about 0.4%/h. One more cyclone could be arranged after the fuel reactor to capture the fine particles of oxygen carrier for reuse. Since the flow rate of the fuel reactor was lower than that in the air reactor, the attrition loss of oxygen carrier from the future additional cyclone after the fuel reactor would be much lower than that from the cyclone after the air reactor. 3.3.2. Surface Morphology. The morphological features of both RIO and 6KMIO were characterized by field emission scanning electron microscope (SEM, FEI Quanta 200). Figure 9 displays the SEM images of oxygen carrier particles at a magnification of 20000 times. These SEM images clearly display the differences in structure between RIO and 6KMIO. For the fresh RIO, the particle surface is loosely covered with grains around 0−2 μm, and it has a relatively porous structure. With oxygen released, the reduced RIO is uniformly covered with smaller grains. Compared with that of RIO, the particle surface of 6KMIO was less porous, and the grains seem to have grown bigger with the size range of 0−0.5 μm. Unlike that of RIO, the grains on the surface of the reduced 6KMIO are much bigger than the fresh ones. Slight sintering on the particle surface could be attributed to the complex reactions between iron ore and the K-containing compounds. Although, the used 6KMIO particles still maintain the porous surface, the diffusion of gaseous products into the active center of the oxygen carrier was favored. Bao et al.25,26 ascribed the enhanced performance to the improvement of pore structure of iron ore, which, however, was not observed on the present 6KMIO. 3.3.3. Phase Characterization. The phase compositions of the reacted oxygen carrier were analyzed using an X-ray diffractometer (XRD, SHIMADZU), and the XRD spectra is indicated in Figure 10. There were only two phases of iron

(12)

The most intense reflections of the Fe2O3 phase (IFe2O3) and Fe3O4 phase (IFe3O4) are located at 2θ = 33.06° and 2θ = 35.54°, respectively. The RI values for the reduced RIO and 6KMIO are 0.74 and 1.45, respectively. This means that Fe2O3 in 6KMIO was more deeply reduced than that in RIO, that is, the oxygen transfer rate from 6KMIO in the fuel reactor was much larger. This result is in agreement with that discussed in the previous sections: the use of 6KMIO could enhance the conversion of carbonaceous gases into CO2.

4. DISCUSSION 4.1. Enhanced Performance Mechanism. Generally, solid fuel gasification in the presence of an oxygen carrier always shows an enhanced fuel conversion in comparison to the gasification using an inert bed material. To further enhance the solid fuel conversion in the CLC process, a series of oxygen carriers have been successfully developed including the mechanically mixed oxygen carriers13−16 and the combined oxygen carriers.17−19 The mixed oxygen carriers exhibit a synergy effect on enhancing the char conversion. For the combined oxygen carriers, new compounds are formed during the calcination process, and it could promote the char conversion. However, the catalytic mechanism exploration of coal conversion in the catalyzed chemical looping combustion is less clear. Song et al.30 and Bao et al.26 partially ascribed the enhanced performance of the oxygen carrier to the improved pore and surface structure. However, in the present study, the pore volume and the BET surface of the used 6KMIO was not efficiently improved. Therefore, the enhanced performance of the oxygen carrier should be attributed to its reactivity. The following several mechanisms may be at work simultaneously: (1) enhanced solid reaction between particles of coal and 6KMIO (2) enhanced consumption of CO and H2, that is, the primary gasification inhibitors (3) catalyzed char-steam gasification. In a CLC system, although the contact efficiency between the particles of oxygen carrier and char was low, direct reaction between them indeed occurred even in a fluidized bed reactor.31,32 In the present CLC system, the coal particles were surrounded by 6KMIO. Direct contacts among the particles of char, oxygen carriers, and K-containing catalysts were inevitable. The employed oxygen carrier may help to break the benzene rings to form some big molecules surrounding the char particles, and hence, promote the coal conversion. Yu et al. also reported the enhanced direct reaction between coal and a K2CO3-modified Fe2O3 oxygen carrier in a TGA analysis.18 However, this mechanism should be taken with caution because the contact efficiency between the catalyst and the char was even much lower than that between the particles of oxygen carrier and char. The second mechanism was the promoted consumption of gasification inhibitors, CO and H2, by 6KMIO. In the fuel reactor, coal was first gasified and the gaseous intermediates, mainly CO and H2, further reacted with the oxygen carrier. For 6KMIO, K2Fe22O34 and K2Fe10O16 were formed in the oxygen carrier during the calcinations process, depending on the K2CO3 loading.19 The K in the potassium ferrite was an electron donor, which could weaken the Fe−O bond strength.22−24 Therefore, the oxidation of gaseous fuel by 6KMIO was much easier than that by RIO, which was

Figure 10. XRD patterns of the reduced oxygen carriers.

oxides, that is, Fe2O3 and Fe3O4 in the reduced oxygen carrier. This indicates that the circulation of oxygen carrier particles could meet the demand of oxygen consumption by fuel. Thus, the oxygen carrier was not deeply reduced to FeO or even Fe. The relative content of Fe2O3 and Fe3O4 phases in the reduced oxygen carrier can be characterized by the intensity of their major peak. A reduction index, noted RI, was defined to evaluate the reduction extent of oxygen carrier: 13012



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Figure 11. Effect of the fuel reactor temperature on SO2 release.

reactor.20,34−36 The effect of temperature on the formation of SO2 was detailed in a previous work.21 Significant change in the formation of SO2 in both reactors occurred in the case of 6KMIO. With the fuel reactor temperature increasing from 900 to 975 °C, SO2 concentration in the fuel reactor increased from 15 to 125 ppm, approximately 250 ppm lower in comparison to the case of RIO at the same temperature. No SO2 was observed in the flue gas from the air reactor over the entire temperature range. The only difference between the case of 6KMIO and the case of RIO was the addition of K+. K-containing compounds in the oxygen carrier could act as sulfur absorbent to reduce the gaseous sulfur;37−39 that is, new S−K-containing species could be formed. However, this should be taken with caution because the intensity peak in XRD analysis was too low to be identified. Nonetheless, the interesting desulfurization capacity and stability of the 6KMIO for long-time operation deserve further investigation.

confirmed in section 3.1. The similar enhanced performance of K-promoted oxygen carrier was also reported by Bao et al.25 In the sense of reaction equilibrium, the reduction in CO and H2 could promote the coal gasification. Furthermore, the use of 6KMIO enhanced the water−gas shift reaction,19 promoting the conversion of CO into CO2 and the yield of H2. In the present continuous operation, H2 has adequate residence time in the fuel reactor to be oxidized, and no or negligible H2 concentration was observed in the exiting gas. According to the reaction equilibrium, the enhanced consumption of H2 and CO would efficiently promote the coal gasification. The third mechanism is the catalyzed steam-char gasification. Potassium groups were a well-known catalyst for the char gasification. However, the mechanism of traditional catalyzed coal gasification could not be simply applied to the present catalyzed CLC because of the process differences. For example, the catalyst was atomically dispersed in coal in the traditional process, whereas they were separated from each other in the CLC process. Although relative stable potassium ferrite was formed during the calcinations, potassium could be released in gaseous formation, such as KOH, in the presence of steam at high temperature. These gaseous K-containing species may adhere to the char particles and catalyze the char-steam gasification. A similar catalyzed char-steam gasification by the gaseous K released from the oxygen carrier was demonstrated with a manganese ore.33 Furthermore, undergoing intensive attrition during the CLC process, some particles of 6KMIO turned into powder. This K-containing powder could adhere to the char particles, enhancing the char gasification to a certain content. Nevertheless, the employed 6KMIO exhibited an obvious effect on promoting the coal conversion and gas conversion in the CLC process. It may help to develop an efficient oxygen carrier for industrial application; however, the catalytic mechanism of coal conversion using the 6KMIO is not clear and is worth a detailed study. 4.2. Release of Sulfur. In view of the existence of sulfur composition in coal, the emission of sulfur-containing gaseous products and the interactions between them and the oxygen carrier are critical to the CLC process. Figure 11 shows the emission of SO2 in the flue gas from both reactors with the fuel reactor temperature increasing from 900 to 975 °C. In the case of RIO, SO2 concentration in the fuel reactor increased from 288 to 346 ppm, and that in the air reactor decreased from 94 to 59 ppm. In the CLC process, a large proportion of sulfur in the fuel was oxidized by oxygen carrier to SO2 in the fuel

5. CONCLUSIONS In this study the 6% K2CO3-modified iron ore was further evaluated by both gaseous fuels (CO and H2) in a TGA and by anthracite in a 1 kW continuous reactor. During the continuous operation, the performance of the oxygen carrier was characterized by gas concentration, gasification efficiency, conversion of carbonaceous gases, and CO2 capture efficiency. The effect of the modified oxygen carrier on the release of sulfur was also discussed. The following conclusions may be drawn: • During the calcinations process of the K2CO3-improved iron ore at 950 °C, potassium ferrite was formed and coexistence with the active phase Fe2O3. As an electric donor, K in the oxygen carrier could weaken the Fe−O bond enhancing the reaction between the oxygen carrier and both H2 and CO, i.e., the main coal gasification products. • The K2CO3-improved iron ore as oxygen carrier could be stably operated in the 1 kW reactor during the coal CLC without any tendency of agglomeration or even sintering between particles. The attrition loss of oxygen carrier to fine powder from the cyclone was about 0.013%/h. • Compared with the case using raw iron ore, during the continuous coal catalyzed CLC process using the K2CO3improved iron ore, a higher CO2 concentration and a lower CO concentration in the fuel reactor were obtained. The efficiencies of coal conversion, gas 13013



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conversion into CO2, and carbon capture were all promoted. • The SEM results indicated that the grains on the particle surface of the K2CO3-improved iron ore have grown bigger with some sintering. However, the particle surface of the spent K2CO3-improved iron ore still maintained the porous structure. According to the XRD analysis, in comparison to the spent raw iron ore, the K2CO3improved iron ore was more deeply reduced; that is, it could more easily offer lattice oxygen, enhancing the reaction performance. • The use of K2CO3-modified iron ore as oxygen carrier reduced the SO2 emission before the sulfur equilibrium in the oxygen carrier in both air and fuel reactors. This topic is worth deeper exploration. Overall, this article presents an initial investigation on the evaluation of a K2CO3 modified iron for chemical-looping combustion of coal during continuous operation, and further investigations are necessary.

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Article

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-25-8379 5598. Fax: +86-25-5771 4489. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant No. 51276037), China Postdoctoral Science Foundation (2014M551489), Jiangsu Planned Projects for Postdoctoral Research Funds, Scientific Research Foundation of Graduate School of Southeast University (Grant No. YBJJ1206).

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NOTATION mt = instantaneous weight of oxygen carrier m0 = initial weight of oxygen Xt = weight change of oxygen carrier rt = reduction rate of oxygen carrier nC,coal = molar flow of carbon in the feeding coal Wi,FR = cocentration of i in the flue gases from fuel reactor (i = CO, CO2, CH4 and H2) Wi,AR = cocentration of i in the flue gases from air reactor (i = CO2 and O2) FCO2,FR = CO2 flow from fuel reactor FC,FR = flow of carbonaceous gas in the flue gases from fuel reactor FC,AR = flow of carbonaceous gas in the flue gases from air reactor Fout,FR = gas flow from the fuel reactor Fout,AR = gas flow from the air reactor FN2,FR = N2 flow fed into the fuel reactor Fair,AR = air flow fed into the air reactor ηCO2 = CO2 capture efficiency ηCC = carbon capture efficiency ηgasify = gasification efficiency of coal f i = fraction of carbonaceous gases (i = CO, CO2 and CH4) α = oxygen fraction in the air fed into the air reactor RI = reduction index of oxygen carrier Ia = most intense reflections of a phase (a = Fe2O3 and Fe3O4) 13014



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Potassium-Modified Iron Ore as Oxygen Carrier for Coal Chemical

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