Enhanced performance of red mud-based oxygen carriers by CuO for chemical looping combustion of methane

Applied Energy 253 (2019) 113534

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Enhanced performance of red mud-based oxygen carriers by CuO for chemical looping combustion of methane

T

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Guixian Denga,b, Kongzhai Lia,b, , Guifang Zhanga, Zhenhua Gub, Xing Zhub, Yonggang Weib, Hua Wangb a b

Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, China

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

mud as oxygen carrier was mod• Red ified by CuO. addition of CuO enhanced activity • The and stability of red mud towards CLC of CH4.

addition of CuO effectively in• The creased the CO selectivity. presence of CuO promoted the • The reduction degree of Fe O in red mud.

In this paper, we used red mud as a cost-effective oxygen carrier for chemical looping combustion of methane and proposed a scheme of introduction CuO to improve the properties of native red mud. Both the reactivity and redox stability of the oxygen carrier were significantly enhanced due to the interaction between introduced CuO and the Fe2O3 inherent in red mud. This modification method is great significance for promoting the application of red mud in chemical looping combustion.

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A R T I C LE I N FO

A B S T R A C T

Keywords: Chemical looping combustion Red mud CuO Oxygen carriers CuFe2O4

The development of inexpensive oxygen carriers is of great significance for the large-scale application of chemical looping combustion technology. Red mud, a solid waste of alumina industry, is a very promising candidate as oxygen carrier for this technology due to its suitable content of Fe2O3 and low cost. In this study, the red mud oxygen carrier is modified by CuO, which is used as a cost-effective oxygen carrier for chemical looping combustion of methane. The results show that CuO as an additive strongly improve the activity and redox stability of the red mud oxygen carriers. The sample with 20 wt% content of CuO represents the highest CH4 conversion (80%), CO2 selectivity (100%) and oxidation efficiency (2.9 mmol‧g−1‧min−1) in the multiple redox testing, which are only 22%, 81% and 1.1 mmol‧g−1‧min−1 for the raw red mud. Comprehensive characterizations indicate that two kinds of Cu species (free CuO and CuFe2O4) are detected in the CuO-modified red mud after calcination at 900 ℃. Coper oxides in the both oxides can be firstly reduced to metallic Cu during the reaction with methane. The reduce Cu species may acts as active sites for methane activation and oxygen releasing, which would improve the reaction between iron oxides in red mud and methane. The element mapping indicates that the Cu species are well dispersed on the oxygen carrier after reduction by methane, which is beneficial for enhancing the catalytic function of Cu species. After the long-term redox experiment, the particle size of Cu oxides increases, while the interlacing distribution between Cu and Fe oxides are promoted. This may improve

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Corresponding author at: Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China. E-mail address: [email protected] (K. Li).

https://doi.org/10.1016/j.apenergy.2019.113534 Received 24 February 2019; Received in revised form 21 June 2019; Accepted 11 July 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

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the interaction between Cu and Fe oxides and help the CuO-modified red mud to maintain relatively high stability in the successive chemical looping combustion process.

1. Introduction

deterioration of the mechanical properties of the pellets. Red mud, a main polluting waste of the aluminum industry, also shows high potential as an low-cost oxygen carrier for CLC due to it specific physicochemical properties and composition. The main components of red mud are Fe2O3, Al2O3, SiO2, and TiO2 and the contents of Fe2O3 is in the range of 30–60% according to different bauxites [25]. This is similar with the composition of synthetic Fe2O3/Al2O3-based OCs [26,27]. In addition, since the red mud from the Bayer process is produced via a hydrometallurgy technology, it usually shows high specific surface area. On the other hand, red mud is very abundant and cheap. The annual output of red mud in the word is about 120 million tons and most of them are not effectively used, which has become a huge source of pollution [28,29]. Chen et al. [27] directly used a red mud with a Fe2O3 content of 51.14% as the OC for CLC of coal, and found that red mud can convert coal to CO2. However, its performance is much worse than that of the synthetic iron-based OC [30]. Especially, the presence of water vapor in the fuels can significantly reduce the oxygen carrying capacity and reactivity of red mud OC. Previously, we proposed a scheme of combining two types of red mud to modify the structure and the component distribution of red mud oxygen carriers. However, the redox experiments showed that the activity of the pure or combined red mud OCs declined sharply after several cycles [26]. As discussed above, the performance of raw and even modified red mud oxygen carriers, especially in the activity and redox stability, cannot meet the requirements of the chemical looping combustion technology. Some studies have shown that the presence of CuO in ironbased OCs can promote the reduction of Fe2O3 [31–33]. Yuzbasi et al. [31] revealed that the reduction of Fe2O3 to metallic Fe0 proceeded via Fe3O4 and FeO intermediate phases, where the transition FeO → Fe0 was the slowest reduction step. The presence of copper result in a six times faster reduction of FeO to metallic Fe0. In addition, Yang et al. [33] reported that the mixtures of iron ore and copper ore as OCs exhibit synergistic effects in improving reactivity of iron ore and resistance to agglomeration of copper ore. In this case, it is reasonable to expect a low-cost oxygen carrier with high performance for CLC technology by combining red mud with CuO. In the present work, red mud as an oxygen carrier for CLC of methane is modified by adding a small amount of CuO. The results show that the presence of CuO in red mud can strongly improve the CH4 conversion, CO2 selectivity and the redox stability due to the catalytic function of free Cu particles and the formation of CuFe2O4. This is of great significance for promoting the application of red mud in CLC technology and contributing to the development of inexpensive OCs.

CO2 capture and storage (CCS) has been considered as the most economical and feasible way to reduce greenhouse gas CO2 emissions on a large scale and slow down global warming in the future [1,2]. The types of fuel combustion process have a direct impact on the choice of CO2 capture systems. There are three main CO2 capture systems, namely, post-combustion capture, pre-combustion capture and oxycombustion capture [3,4]. For each method, reducing the energy consumption is the most important issue [5,6]. Chemical looping combustion (CLC) is a combustion technology for CO2 separation almost without energy penalty, and it has the potential to be more effective than other CO2 capture technologies [7,8]. The CLC process provides the oxygen needed for the combustion of fuels (both gaseous and solid fuels) by means of oxygen storage materials (i.e., oxygen carriers). In a typical reaction process, fuel firstly reacts with the oxidized oxygen carrier (OC) in the fuel reactor to produce CO2 and water vapor, and then the reduced OC can be recovered by O2 in the air reactor. The twostep process avoids the dilution of CO2 by N2 from air, and the separation and capture of CO2 therefore can be achieved by simple condensation with very low energy consumption [9]. Oxygen carrier is a crucial element in the CLC process [10]. A suitable OC should own high reactivity to oxidize the fuel, high stability to achieve multiple redox cycles at high temperatures, and sufficient mechanical strength. A large number of researchers have been working on the development of various OCs, and most of which focus on iron[11–13], copper- [14], nickel- [15] and manganese-based [16] oxides. Among the different OCs, iron-based oxides are considered as one of the most promising candidates due to their low cost and high agglomerate resistance [17]. Since the CLC technology requires large quantity of OCs to achieve relatively high fuel conversion, its running cost strongly relies on the price of OCs. The exploration of low-cost and environmental-friendly OCs is also a very important issue for the development of CLC technology. Natural ores such as ilmenite [18–20], manganese ore [21–23] and copper ore [24] are preferred candidates as low-cost OCs due to the relatively content of reducible oxides [18]. However, the ore oxygen carriers usually show poor performance due to the dense structure. For example, pure ilmenite showed very low acitivty for CLC of methane, and active oxides (CeO2, NiO and Mn2O3) were suggested to use as addtives [18]. Ortiz et al. [19] prepared a series of ilmenite pellets with different compositions and shapes for a packed-bed CLC process, and found that the thermal and chemical stresses result in a serious

Off gas CH4/N2

N2

O2/Ar

MFC 1

MFC 2

MFC 3

Valve 1

Valve 2

Valve 3

CH4/N2

N2

O2/Ar

Oxygen carrier & Quartz sand

CH4 H2 CO2 CO

Gas analyzer

T

Cold trap

Desiccant

Temperature control unit Fig. 1. Schematic diagram of the experimental setup for activity test. 2

Data acquisition

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(A)

(B)

Ca2Al2SiO7[35-0755] Na6(AlSiO4)6[42-0217] CuFe2O4[25-0283] CuO[48-1548] Fe2O3[33-0664]

TCD signal (a.u.)

Intensity (a.u.)

25wt%CuO-RM

Pristine-RM 5wt%CuO-RM 10wt%CuO-RM 15wt%CuO-RM 20wt%CuO-RM 25wt%CuO-RM

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Fig. 2. XRD patterns (A) and H2-TPR profiles (B) of the fresh pristine red mud and CuO-modified red mud with different contents of CuO. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

500 600 700 800 Temperature (oC)

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Fig. 3. Typical curves the main products and reactant during the temperature programmed reactions between CH4 (5% CH4/N2) and the different red mud OCs: (a) Pristine-RM, (b) 5 wt%CuO-RM, (c) 10 wt%CuO-RM, (d) 15 wt%CuO-RM, (e) 20 wt%CuO-RM and (f) 25 wt%CuO-RM. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2. Materials and methods

different samples was controlled at 0 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt% and 25 wt%, respectively. The corresponding samples are denoted as Pristine-RM, 5 wt%CuO-RM, 10 wt%CuO-RM, 15 wt%CuORM, 20 wt%CuO-RM and 25 wt%CuO-RM, respectively.

2.1. Oxygen carrier preparation The raw red mud from Wenshan Aluminum Co., Ltd. in Yunnan province of China, was washed by deionized water for several times until the pH of the filtering medium decreased to 7.0. The filter cake of red mud was dried at 110 ℃ for 24 h and then grinded to powers for 30 min. For the CuO modification, the required amounts of Cu (NO3)2·3H2O were dissolved in 200 mL of deionized water to obtain precursor solutions. Then, 2.0 g of dried red mud powders were immersed into the prepared precursor solution and stirred for 10 h at room temperature. After that, 1 mol/L of NaOH was added dropwise to the solid–liquid mixture under stirring until the pH value increased to 10.0. The resulting solution was further stirred for 10 h at room temperature. Thereafter, the mixtures were washed by deionized water for several times and filtrated. The obtained solid materials were then dried at 110 ℃ for 24 h and calcined in air at 900 ℃ for 2 h. The amount of CuO in

2.2. Oxygen carrier characterizations X-ray diffraction (XRD) measurements were performed at room temperature to analyze and identify the crystal phases of red mud OCs by an X-ray diffractometer (Rigaku, MiniFlex 600) using Cu Kα radiation (λ = 0.15406 nm). The XRD instrument was set to operate at 40 kV and 15 mA over a scanning range (2 Theta) of 10–90° at a scanning rate of 2°/min. Temperature-programmed reduction with hydrogen (H2-TPR) experiments were performed on a TPR Win instrument (produced by Quantanchrome Instruments Co.) under a flow rate of 10% H2/Ar mixture (25 mL/min) over 50 mg sample using a heating rate of 10 ℃/min from room temperature to 900 ℃. 3

Applied Energy 253 (2019) 113534

CH4 CO2 CO H2

4.8 4.2 3.6 3.0 2.4 1.8 1.2 0.6 0.0

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(d) CH4 CO2 CO H2

CH4 CO2 CO H2

4.8 4.2 3.6 3.0 2.4 1.8 1.2 0.6 0.0

0 2 4 6 8 10 12 14 16 18 20 Time (min)

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G. Deng, et al.

4.8 4.2 3.6 3.0 2.4 1.8 1.2 0.6 0.0

0 2 4 6 8 10 12 14 16 18 20 Time (min)

(f) CH4 CO2 CO H2

0 2 4 6 8 10 12 14 16 18 20 Time (min)

Fig. 4. Typical curves the main products and reactant during the isothermal reactions of CH4 (5% CH4/N2) with different the red mud OCs at 800 ℃: (a) Pristine-RM, (b) 5%wtCuO-RM, (c) 10 wt%CuO-RM, (d) 15 wt%CuO-RM, (e) 20 wt%CuO-RM and (f) 25 wt%CuO-RM. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Conversion or selectivity (%)

CO2 selectivity

Oxidation efficiency

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0 P

ine rist

-R M

RM -R M -R M -R M -R M OuO uO uO uO C C C C Cu % t% t% t% t% t 5w 20w 10w 25w 15w

For the temperature programmed reduction with methane, 2.0 g of OCs with a particle size range of 20–40 mesh were mixed with appropriate amounts of quartz sands (20–40 mesh) and placed in the middle of the quartz tube reactor. Before the reaction, N2 (99.99%) was firstly injected into the reactor at 300 ℃ for 20 min. Then, the reducing gas (5% CH4/N2) was introduced with a flow rate of 200 mL/min, and the temperature was increased to 900 ℃ at a heating rate of 10 ℃/min. The compositions and concentration of the effluent gas (e.g., CH4, CO2, CO, and H2) during the reactions was detected on line by a Nondispersive Infrared Radiation (NDIR) gas analyzer (GASBOARD-3100, Wuhan Cubic Optoelectronic Co., Ltd) and a gas chromatograph (Agilent 7890). The isothermal reaction of methane at 800 ℃ was performed over different OCs in the same equipment and under the same conditions. For the redox experiment, after the isothermal reaction with methane at 800 ℃ proceeded for 10 min, N2 was introduced for 30 min to purge reactor. Then, 10% O2/Ar with a flow rate of 200 mL/min was introduced to recover the reduced oxygen carriers. This redox cycle was repeated for 20 times to test the redox stability of the samples. The average CH4 conversion, CO2 selectivity and oxidation efficiency were used as indicators for quantitative evaluation of oxygen carrier activity, which are calculated by the Eqs. (1)–(3), respectively. The conversion and selectivity were measured at least 3 times and finally averaged. The oxidation efficiency is defined as the moles of CO2 that originates from complete oxidation of methane by per unit mass of oxygen carrier at per unit reaction time, and its value is positively correlated with the performance of oxygen carrier for CLC of methane.

Oxidation efficiency (mmol.g-1.min-1)

CH4 conversion

100

0.0

Fig. 5. Average CH4 conversion, CO2 selectivity and oxidation efficiency of different OCs in the isothermal reactions at 800 ℃.

The distribution of elements in the OCs was characterized using an EDAX energy dispersive spectrometer (EDS) that was attached to a NOVA NANOSEM 450 scanning electron microscopy (SEM). The XPS analysis test was carried out on K-Alpha+ X-ray photoelectron spectrometer (Thermo Fisher Scientific Co.) with monochromatic Al Kα X-ray source. The vacuum degree in the analysis room was ca. 2 × 10−7 mba, and the scanning mode was CAE. The binding energy was calibrated according to the standard of surface contaminated carbon C1s (284.8 eV).

CH4 conversion (%) =

2.3. Activity test

CO2 selectivity (%) =

The activity test for CLC of methane included temperature programmed reduction, isothermal reactions and multiple redox experiments. All of the above reactions were carried out in a quartz tube fixed-bed reactor with 600 mm long and 20 mm inner diameter at atmospheric pressure. HYPERLINK "SPS:refid::fig1" Fig . 1 shows the scheme of the experimental setup.

MCH4, in − MCH4, out × 100% MCH4, in

MCO2, out × 100% MCH4, in − MCH4, out

Oxidationefficiency (%) =

MCO2, out × 100% m (OC ) × tR

(1)

(2)

(3)

where MCH4, in and MCH4, out are the moles of introduced and discharged methane, respectively. MCO2, out denotes the moles of produced carbon 4

1.5 1.0 0.5 0.0 2

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Number of cycles

Fig. 6. Typical curves of CO2 and CO during the successive redox testing at 800 ℃ as well as the average CH4 conversion, CO2 selectivity and oxidation efficiency in each cycle over Pristine-RM (a, b), 20 wt%CuO-RM (c, d), 25 wt%CuO-RM (e, f). CH4 (5% CH4/N2) flow rate: 200 mL/min; time of methane reduction step: 10 min; time of air re-oxidation step: 20 min; dosage of OC: 2.0 g.

CuFe2O4. Fig. 2B shows the H2-TPR profiles of the red mud OCs with different contents of CuO. The reduction process of the Pristine-RM shows two peaks at 621.6 ℃ and 814.9 ℃, respectively. According to the reported literatures [36–40], the low-temperature peak (450–650 ℃) is attributed to the reduction process of Fe2O3 to Fe3O4, and the high-temperature peak (650–900 ℃) is related to the reduction process of Fe3O4 to FeO and Fe. It can be clearly seen from Fig. 2B that the addition of CuO results a new peak at very low temperatures (350–550 ℃), and the peak intensity is enhanced with the increase in the CuO content. This can be associated with the reduction of CuO reduction and the Cu oxidation in CuFe2O4 [41]. Among all the tested samples, the 20 wt %CuO-RM sample shows the highest H2 consumption. It is also noted that the reduction peaks (550–900 ℃) corresponding to the reduction iron oxides shift to lower temperatures due to the presence of CuO,

dioxide, m (OC ) is the mass of the OC, and tR is the reaction time.

3. Results and discussion 3.1. Characterization of oxygen carriers The XRD patterns of the prepared red mud-based oxygen carriers are shown in Fig. 2A. It can be seen that the main phase of the PristineRM is Fe2O3, while Ca2Al2SiO7 and Na6(AlSiO4)6 are also detected. The Fe2O3 is the oxygen storage component of red mud OC, and Ca2Al2SiO7 and Na6(AlSiO4)6 can be the carrier component because of their excellent thermal stability [34,35]. After the addition of CuO, CuFe2O4 and CuO phases are detected, and their intensity gradually increase with the increase of the amount of CuO. This indicates that parts of CuO reacts with the Fe2O3 in red mud during the calcination process to form 5

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O

Fe

Cu

Al

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Fe Cu

O Fe Cu Al Ca Na Si Ti

(B)

O

Fe

Cu

Al

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Fe Cu

O Fe Cu Al Ca Na Si Ti

(C)

O

Fe

Cu

Ca

Fe Cu

O Fe Cu Al Ca Na Si Ti

(A)

50 m

50 m

100 m Al

Fig. 7. EDS mapping images corresponding to O, Fe, Cu, Al, Ca, Na, Si elements for the fresh (A), reduced (B) and cycled (C) 20 wt%CuO-RM samples.

relatively low temperatures. This is probably ascribed to the relatively high reactivity of CuO and to the enhancement of Cu species to the reducibility of iron oxides in red mud, as observed in the H2-TPR measurements. The results for the isothermal reaction of OCs with methane at 800 ℃ are shown in Fig. 4. In the early stages of the reaction, the concentration of CO2 in the exhaust gas increased sharply. With the consumption of the active oxygen in the OCs, the concentration of methane continue to rise, while a decline in the CO2 concentration is observed. Further reaction results in presence of CO and H2, suggesting the occurrence of methane partial oxidation. As the addition of CuO increases, the time for the complete conversion of methane to CO2 increases and the formation of CO and H2 is delayed. It can be seen that the CO and H2 are detected after 6 min reaction for Pristine-RM, while it takes 16 min for the 25 wt%CuO-RM sample. This indicates that the addition of CuO in red mud can extend the reaction time for complete oxidation of methane to CO2, which is beneficial for improving the combustion efficiency.

which indicates that the addition of CuO can improve the reducibility of iron oxides in red mud. This is similar with the observation by Yuzbasi et al. [31] for the samples combining Fe2O3 with CuO.

3.2. Reactivity and stability of oxygen carriers Fig. 3 shows the CH4-TPR profiles of the red mud-based OCs. As shown in Fig. 3a, the pristine red mud starts to react with CH4 at ca. 500 ℃, and only CO2 is observed. When the reaction temperature increase to higher than 800 ℃, CO and H2 are detected, which is produced by the partial oxidation of methane. This indicates that 800 ℃ should be an optimum reaction temperature for chemical looping combustion of methane. For the conversion of CH4 to CO2, the CO2 concentration curve shows two peaks (α peak at low-temperature and β peak at hightemperature) with the reaction proceeding. It is very interesting that the α peak is enhanced, while the β peak is suppressed due to the addition of CuO. This indicates that the presence of CuO in the red mud can improve the reactivity of OCs for conversion of methane to CO2 at 6

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Na

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than the 20 wt%CuO-RM sample after the redox testing. This indicates that the excessive addition of CuO would reduce the stability of the OC probably because of the sintering of CuO. In this case, 20 wt% is the optimum content of CuO as an additive for red mud OC.

After the isothermal reactions last for 10 min, CO2 concentration has decreased to a very low level, indicating that most of the active oxygen is consumed. Therefore, 10 min is chosen as reduction time in the redox experiments. The average CH4 conversion, CO2 selectivity and oxidation efficiency are also calculated according to the data during the first 10 min reaction. As can be seen from Fig. 5, the CO2 selectivity in for the Pristine-RM is 89%, and it increase to 100% with the addition of only 5 wt% of CuO. As increasing the CuO content from 5 wt% to 25 wt %, the CH4 conversion and oxidation efficiency significantly increase from 37% and 1.69 mmol·g−1·min−1 to 75.6% and −1 −1 3.38 mmol·g ·min , respectively. Sequential CH4 reduction/O2 oxidation redox testing over the, Pristine-RM, 20 wt%CuO-RM and 25 wt%CuO-RM were carried out at 800 ℃ to identify the redox stability of the OCs in the chemical looping process. Fig. 6 presents the curves of CO2 and CO concentration as well as the average CH4 conversion, CO2 selectivity and oxidation efficiency during the sequential CH4 reduction/O2 oxidation cycling. It is clear from Fig. 6a and 6b that the reactivity of Pristine-RM sample suffers a serious decline during the testing. By contrast, the 20 wt%CuO-RM efficiently oxidizes methane to CO2 without the formation of CO during the whole process (Fig. 6c), and its reactivity is even enhanced in the later period the cycling (Fig. 6d). The deactivation of the raw red mud can be attribute to the serious sintering of Fe oxides during the long term redox reaction. The enhancement of the performance for 20 wt %CuO-RM is probably due to the promoted interaction between Cu and Fe oxides after cycling, which will be discussed in detail in the next section. It should be highlighted that the performance of 25 wt%CuORM also suffer a partial deactivation in the first 5 cycles (Fig. 6e and 6c). After that, it shows relatively lower CH4 conversion (ca. 70% vs. ca. 83%) and oxidation efficiency (ca. 2.4 vs. ca. 2.9 mmol·g−1·min−1)

3.3. Interaction between Cu and Fe species In order to observe the interaction of various elements (mainly Cu and Fe) during the service of OCs, EDS mapping were performed on the fresh, reduced and cycled 20 wt%CuO-RM samples. Fig. 7 shows the EDS mapping images corresponding to O, Fe, Cu, Al, Ca, Na, Si and the elements overlay. As can be seen, in the fresh OC (see Fig. 7A), Cu distributes uniformly in the whole region, while Fe concentrates in several regions. After the OC was reduced by methane (see Fig. 7B), Cu gathered into sample particles and well dispersed in the whole detection area. After the long-term redox cycling (see Fig. 7C), the distribution of Cu and Fe in the oxygen carrier has obviously changed. The agglomeration and growth of Cu oxides are detected, while the dispersion of Fe element is enhanced. Parts of Cu and Fe elements are bonded together, promoting the interlacing distribution between Cu and Fe oxides. This may improve the interaction between Cu and Fe oxides and help the CuO-modified red mud oxygen carriers to maintain relatively high stability in the successive CLC process. The distribution of inert and additive elements in OCs (e.g., Al) also varies at different stages (fresh, reduced, and cycled). Comparing the fresh and cycled OCs, it can be found that a very small part of Al and Fe are combined to form mixed oxide after redox cycles. The distribution of Ca is also improved after cycling. The enhanced dispersion of the active oxides (Cu and Fe oxides) and inert/additive oxides (Al and Ca oxides) may also improve the interaction between different 7

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530

528

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(d) O 1s Intensity (a.u.)

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Fig. 9. XPS spectra of the fresh (a, b, c), reduced (d, e, f), and cycled (g, h, (i) 20 wt%CuO-RM samples.

are CuO phases, and the phase region corresponding to Spot 4 is metal Cu phase. This indicates that separation of Cu and Fe oxides from the mixed Cu-Fe mixed oxides occurs, while they can react again to form the composite in the re-oxidation step. In addition, the phase distribution of Cu and Fe in the Cu-modified red mud OC is more uniform after redox cycle, which promotes the reaction performance of oxygen carrier. The fresh, reduced and cycled 20 wt%CuO-RM samples were also analyzed by XPS to explore the chemical states of O, Cu and Fe elements. As shown in Fig. 9(a, d and g) the spectra of O 1 s can be fitted into three peaks denoted as Ⅰ, II and III, respectively [32,42–44]. The peak Ⅰ located at ca. 529.5 eV is assigned to the lattice oxide oxygen of metal oxides (Fe-O and Cu-O), and the peak II at ca. 531 eV is attributed to the oxygen in hydroxyl groups on the surface. The peak III at ca. 532 eV is associated with surface adsorbed oxygen species [32,44,45]. As shown in Fig. 9b, e and h, the Cu 2p spectra of the OCs can be fitted into seven peaks [45,46]. The main peaks at ca. 932 eV and ca. 951.8 eV are assigned to Cu+, whereas the peaks located at ca. 933.6 eV and ca. 953.6 eV are attributed to Cu2+. Cu2+ also shows observable collection of satellite features at ca. 940.2 eV and ca. 961.2 eV, but there is only one satellite at ca. 942.8 eV in Cu+. For the Fe 2p spectra (Fig. 9c, f and i), five fitted peaks were observed at ca. 709.9, 712.8, 717.7, 722.7 and 725.1 eV, respectively. The peak at ca. 709.9 eV can be attributed to the binding energy of 2p3/2 for Fe3+ [42] and the peaks at ca. 712.8 and 722.7 eV are associated with the 2p3/2 and 2p1/2 of the Fe2+ [45], respectively. The peak at ca. 725.1 eV is assigned to the binding energies of 2p1/2 of Fe2+ and Fe3+ [47,48]. The peak at ca. 717.7 eV is a satellite for the above four peaks, which indicates that Fe2+ and Fe3+ coexist in the OCs. Compared with XRD patterns of the fresh, cycled and reduced 20 wt %CuO-RM samples (see Fig. 10), it can be found that the active

Intensity (a.u.)

Ca2Al2SiO7[35-0755] Na6(AlSiO4)6[42-0217] Fe2O3[33-0664] CuFe2O4[25-0283] CuO[48-1548] NaAlSiO4[33-1204] Cu[04-0836] FeO[06-0615]

reduced cycled fresh

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2 Theta (deg.) Fig. 10. XRD patterns of the fresh, cycled and reduced 20 wt%CuO-RM samples.

components, contributing to the relatively redox stability. Fig. 8 shows phase distribution of the fresh, reduced and cycled 20%CuO-RM samples on basis of the the elements analysis. The different phase regions are distinguished by different colors. The elemental distribution (see Fig. 7) and the XRD of the samples in different states (see Fig. 10) quantif the content of each element in the phase regions where Fe and Cu may be present to determine the phase evolution of the sample during the redox process. It is found that the Fe element exists in the form of Fe2O3 and CuFe2O4 in the fresh (Spot 1) and cycled (Spot 5) samples, and in the form of FeO in the reduced (Spot 3) sample. According to the XRD detection results(see Fig. 10), it can be confirmed that the phase regions corresponding to Spots 2 and 6 8

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components (i.e., Fe2O3, CuFe2O4 and CuO) were reduced to FeO and Cu after the reaction with CH4, and all the active components can be completely recovered after the re-oxidation step. The XPS measurement shows that the spectra of O 1s, Cu 2p and Fe 2p for the regenerated sample are similar with that for the fresh sample, which indicates that even the surface property keeps stable during successive redox cycling.

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4. Conclusions In this study, modification of red mud by CuO is investigated to improve the activity and redox stability of red mud oxygen carriers for chemical looping combustion of methane. Comprehensive characterizations (e.g., XRD, TPR, XPS and EDS mapping) were performed over the oxygen carriers with different components and different states, and the results are corelated to their chemical looping combustion performance. The conclusions are drawn as follows: (1) The addition of CuO strongly enhances the reducibility of red mud, and the oxygen carrier with 20 wt% of CuO (i.e., 20 wt% CuO-RM sample) shows the best performance for methane conversion. Due to the presence of Cu species, the reduction temperatures of iron oxides either by H2 or CH4 shift to lower temperatures. (2) The addition of CuO also increases the CO2 selectivity up to 100% during the reaction between red mud-based oxygen carriers and methane. The oxidation efficiency of methane increases from 1.68 to higher than 3.0 mmol‧g−1‧min−1 due to the addition of suitable amounts of CuO. In the long-term redox experiments, the 20 wt %CuO-RM sample shows very stable CH4 conversion (80%) and CO2 selectivity (100%). The oxidation efficiency slightly decreased in the first two cycles but keep stable in the further cycling. (3) Parts of CuO reacts with the Fe oxides in red mud to form CuFe2O4 after calcination, which possess higher reducibility than single iron oxides. In addition, both the coper oxide is easily reduced to metallic Cu, which can activate methane, promoting the reaction between iron oxide and methane. (4) The analysis on the element distribution and chemical states of the fresh and recycled samples indicates that the interlacing distribution between Cu and Fe oxides are promoted after long-term redox cycling, which can enhance the interaction between Cu and Fe oxides, contributing to high activity and stability for the successive chemical looping combustion process. Both the structure and surface property of the CuO-modified red mud oxygen carrier keep very stable during the redox cycling. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51604137 and 51774159), the National Key R&D Program of China (2018YFB0605401-02), and the Qinglan Project of Kunming University of Science and Technology. References [1] Bui M, Adjiman CS, Bardow A, Anthony EJ, Boston A, Brown S, et al. Carbon capture and storage (CCS): the way forward. Energy Environ Sci 2018;11:1062–176. [2] Adanez J, Abad A, Garcia-Labiano F, Gayan P, de Diego LF. Progress in chemicallooping combustion and reforming technologies. Prog Energy Combust Sci 2012;38:215–82. [3] Fan L-S, Zeng L, Wang W, Luo S. Chemical looping processes for CO2 capture and carbonaceous fuel conversion–prospect and opportunity. Energy Environ Sci 2012;5:7254–80. [4] Leung DYC, Caramanna G, Maroto-Valer MM. An overview of current status of carbon dioxide capture and storage technologies. Renew Sust Energ Rev 2014;39:426–43. [5] Rochelle GT. Amine scrubbing for CO2 capture. Science 2009;325:1652–4. [6] Haszeldine RS. Carbon capture and storage: how green can black be? Science 2009;325. 1647 1652. [7] Jing L, Zhang H, Gao Z, Jie F, Ao W, Dai J. CO2 capture with chemical looping combustion of gaseous fuels: An overview. Energy Fuels 2017;31:3475–524.

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