Carbon Deposition and Sintering Characteristics on Iron-Based

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Carbon Deposition and Sintering Characteristics on Iron-Based Oxygen Carriers in Catalytic Cracking Process of Coal Tar Yongpeng Li, Jian Gong, Fei Huang, Hongcun Bai, Fenyin Wang, and Cuiping Wang Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 6, 2017

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Carbon Deposition and Sintering Characteristics on Iron-Based Oxygen Carriers in Catalytic Cracking Process of Coal Tar Yongpeng Li a, JIan Gong a, Fei Huang b, Hongcun Bai c, Fengyin Wang a, Cuiping Wang a,* (a College of Electro-mechanical Engineering, Qingdao University, 266071, China ; b Sinopec Safety Engineering Institute, Qingdao 26607 , China ; c

College of Chemical Engineering, Ningxia University, Yinchuan,750021, China)

Abstract: Experiments on carbon black production from coal tar via chemical looping pyrolysis were performed in a fluidized bed reactor using natural hematite, hematite/γ-Al2O3 (Fe/Al), and hematite/NiO (Fe/Ni) composites as oxygen carriers (OCs). After three redox cycles, TGA-DSC results show that compared to natural hematite, the oxidation rate of Fe/Al OC became faster above 700 °C; however, the oxidation performance of Fe/Ni OC was always poor. Addition of γ-Al2O3 could effectively inhibit carbon deposition; nonetheless, NiO exhibited the opposite effect. Based on the burning temperature, the carbon deposition on reduced OCs was mainly hard coke. The XRD analysis showed that NiO was not only more effective than γ-Al2O3 to promote the reduction of Fe2O3 in compound OCs, but also increased the sintering trend. The SEM analysis indicated the melting of original grains at the surface of hematite OC under high temperature, and the surface sintering was serious. However, the Fe/Al OC always exhibited a relatively stable porous structure; thus the sintering was alleviated due to γ-Al2O3 addition. Owing to the weak mechanical strength, the Fe/Ni OC cracked after three-cycles, leading to deposition of large amount of coke, and its surface sintering was more serious compared to hematite OC. Key words: chemical looping pyrolysis; Fe-based oxygen carriers; TGA-DSC analysis; sintering; carbon deposition

1. INTRODUCTION

*Corresponding author at: Energy Engineering Research Institute, Qingdao University, Qingdao 266071, China E-mail address: [email protected](C.Wang).

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Carbon black (CB) is a type of widely employed carbon material that is primarily used in the rubber industry[1]. Moreover, it is also an important raw material in the fields of coatings, printing inks, sensors, conducting electrodes, and electronic industry[2,3]. With the rapid development of automobile industry, the demand for CB is continuously increasing. Currently, oil furnace method is the popular technology for CB production[4], and the CB produced via this method accounts for more than 90% of the total output. The basic principle involves the pyrolysis of atomizing coal tar in contact with high temperature flue gas (usually above 1800°C) to generate CB[5]. Therefore, problems related to high energy consumption and heavy pollution associated with this method require urgent solution[6].

The feasibility of CB production from coal tar via chemical looping pyrolysis (CLP) has also been investigated[7]. The principle of this method involves the enhancement of heat transfer by the fluidized oxygen carrier (OC) particles. Further the lattice oxygen of OCs catalyzes and oxidizes coal tar components, which promote the breaking of the hydrocarbon bonds to generate CBs. Therefore, it has obvious characteristics of energy conservation and thermal NOx emission reduction.

The OC particles are the key factor that affects the efficiency of the entire process of CLP[8], and their reactivity directly affects the yield of CBs. However, the OC particles remain in direct contact with coal tar in the reaction process; therefore, their surface is bound to have a relatively large amount of CB deposition. Theoretically, in the regeneration process, the OC particles with deposited carbon release more heat owing to the carbon combustion, which leads to more serious sintering of the OCs. After multiple cycles, the reactivity of OCs decreases severely due to the increase in carbon deposition and sintering, resulting in ineffectiveness and deactivation of the OCs[9,10]. Therefore, in


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view of the potential problems of carbon deposition and sintering, in this study two types of compound OCs, namely, hematitie/γ-Al2O3 (Fe/Al) and hematite/NiO (Fe/Ni) composites were prepared by mechanical mixing of a certain proportion of γ-Al2O3 and NiO in natural hematite, respectively[11,12]. The experiments of OC/coal tar co-pyrolysis and OC regeneration were conducted alternately in a fluidized bed reactor. The reduced hematite, Fe/Al, and Fe/Ni OCs after three-cycles were, respectively, investigated by thermogravimetric analysis combined with differential scanning calorimetry (TGA-DSC) and X-ray diffraction (XRD). The microstructures of OCs were characterized by scanning electron microscopy (SEM). The carbon deposition and sintering characteristics of OCs were investigated in this study, which lays a theoretical foundation for the development of CLP technology.

2. EXPERIMENTAI SECTION 2.1. Preparation of oxygen carriers. The natural OC used in the experiments was high-quality hematite obtained from Qingdao Iron and Steel Group Co., Ltd. The Fe/Al and Fe/Ni compound OCs were prepared by mechanical mixing. The specific steps are as follows. First, the hematite block was crushed into fine powder in the jaw crusher, and then the powder was evenly mixed with γ-Al2O3 or NiO powder according to the proportion listed in Table 1. Second, suitable amount of sesbania gum was added as binder, which was followed by the addition of water, and then the contents were stirred until a wet gel state was obtained. Finally, after extrusion molding and granulation, it was placed in a drying oven at 100 °C for 12 h, which resulted in the formation of the precursor[13]. Owing to the high price and poor environmental friendliness, NiO is not suitable for large scale applications. However,


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its reactivity is high; therefore, a small amount of NiO was added as a catalyst.

The composition analysis of hematite was carried out by X-ray diffractometer (Japan, Rigaku D/max-2500/PC, XRD), mainly the Fe2O3 and a little inertia SiO2 according to the diffraction pattern, and the lesser Al2O3, P2O5,TiO2, MgO, MnO and CaO by the element limit and peak searching. Under the mixing ratio shown as Table1, the compositions of fresh Fe/Al and Fe/Ni OCs could be calculated, shown as Table 2.

All the precursors were calcined for 3 h in a 1000 °C muffle furnace in order to improve their mechanical strength. After crushing and screening, the 0.5 mm particles were selected as OCs[7]. Table 1 Composition ratio of compound OCs Sample

Composition

Mass ratio

Fe/Al OC

hematite / γ-Al2O3

6:4

Fe/Ni OC

hematite / NiO

9:1

Table 2 Chemical composition of fresh Fe-based OCs Composition Composition content of hematite /% Composition content of Fe/Al OC /% Composition content of Fe/Ni OC /%

Fe2O3

SiO2

Al2O3

NiO

P2O5

TiO2

MgO

MnO

CaO

Other

86.30

8.28

4.45

/

0.30

0.20

0.12

0.10

0.10

0.15

51.78

4.97

42.67

/

0.18

0.12

0.07

0.06

0.06

0.09

77.67

7.45

4.00

10.00

0.27

0.18

0.11

0.09

0.09

0.14

2.2. Facility and procedure. The experimental apparatus and procedures of CLP have been described in detail in the literature report[7], thus not recalled herein. The reduced OC particles after three redox


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cycles were emphatically analyzed.

The carbon deposition characteristics of three types of reduced OCs were studied using the integrated thermogravimetric analyzer (Germany, Netzsch STA 409 PC). The specific steps are as follows.

1) Corundum crucible was calcined in the muffle furnace to constant mass in order to minimize the experimental bias;

2) Suitable amount of reduced OC was evenly spread in the crucible. After sealing the experimental system, appropriate amount of N2 was blown into the device in order to check the air tightness and exhaust the air.

3) During the heating process starting from room temperature, the heating rate was kept at 15 K min–1. At the same time, the air flow rate was controlled at 200 mL min–1, and the oxidation and calcination experiment was started.

4) During the testing process, the mass and heat flux changes of OC sample were automatically measured and recorded by TGA-DSC device.

5) When the temperature reached 900 °C, heating was stopped, the air flow was cut off, and the test was completed. After the device naturally cooled down to room temperature, the test of next batch of samples was carried out.

The micro-structure of OC was characterized by the SEM (Japan, JSM-6390LV). The composition types of reduced Fe/Al and Fe/Ni OC products were also determined by XRD, and the


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compositions variation after reaction with coal tar was compared and calculated.

3. RESULTS AND DISCUSSION

3.1. Thermogravimetric analysis combined with differential scanning calorimetry

The comparative analysis of TGA curves shown in Figures 1, 2, and 3 indicate the occurrence of two obvious mass-loss stages and three mass-gain stages in the entire oxidation and calcination processes of all types of reduced OCs. The analysis of DSC curves indicates that the all the three mass-gain stages are slight exothermic processes, thus implying that the three stages represent the oxidation processes of reduced OCs at different temperatures. The derivative TG patterns (DTG) show the reaction rates variation, the highest exothermic peak is corresponding to the highest reaction rate. So the following discussion only combined with the exothermic processes. 300

400

500

600

700

800

900 10

0.02

exo

0.00 8

98

DTG

96

TGA

94

DTG/%/K

-1

100

Heat flow/(mW mg )

200

102

TGA/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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DSC

-0.02

-0.06 4 -0.08

92

6

-0.04

2

-0.10 90 88 200

-0.12 0 300

400

500

600

700

800

900

Reaction temperature/℃

Figure 1. TGA-DSC analysis of reduced hematite OC


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200

300

400

500

600

700

102

800

98

DSC DTG

6 0.0

-1

Heat flow/(mW mg )

DTG/%/K

TGA

TGA/%

9008

exo

100

4

96 2 94 -0.1 92 200

300

400 500 600 700 Reaction temperature/℃

800

0

900

Figure 2. TGA-DSC analysis of reduced Fe/Al OC

exo

100

12 10

98

94 92

DTG/%/K

0.00 96

DTG TGA DSC

8 6

-0.05 4

90 -0.10

88 86 200

2

-1

0.05

Heat flow/(mW mg )

102

TGA/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 300

400 500 600 700 Reaction temperature/℃

800-0.15 900

Figure 3. TGA-DSC analysis of reduced Fe/Ni OC

In the two mass-loss stages, the maximum one corresponds to a strong exothermic process, thus indicating that this stage should be the burning process of deposited carbon. With the carbon burning out, the sample mass rapidly decreases and significant amount of heat is released. The DSC curve that corresponds to the minor mass-loss stage is relatively flat, and there is no obvious exothermic peak. The temperature corresponding to the maximum mass-loss rate of this stage is about 410 °C (as shown by the left second dotted line in Figures). These results indicate that this stage should be the


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heating volatilization process of the residual coal tar component attached on the OC particles. The residual coal tar is less; therefore, the heat absorption of volatilization process is slight. Moreover, at the same time, the oxidation of reduced OCs also leads to the release of heat. Therefore, the DSC curve becomes relatively flat when the heat balance is achieved.

There are some differences in the carbon deposition stages of three types of OCs. The specific analysis of these differences was carried out as follows:

In each mass-gain stage of reduced OCs, the temperature corresponding to the maximum mass-gain rate of the first stage is about 310 °C (as shown by the left first dotted line in Figures). At this time, the oxidation reaction of each OC becomes fast, and an obvious heat release peak is observed in the DSC curve. The oxidation exothermic peak is overlapped with the exothermic peak of carbon-burning stage.

The two points mentioned above are the common characteristics of three types of OCs, and the most significant difference lies in the third-stage. The oxidation rate of Fe/Al OC is accelerated after 680 °C and the mass-gain is obvious. The mass-gain rate reaches the maximum value at 700 °C and the DSC curve has an obvious exothermic peak. However, the DSC curves of hematite and Fe/Ni OC do not exhibit the appearance of exothermic peaks at this stage, thus indicating that their oxidation rates are low.

In particular, the oxidation rate of Fe/Ni OC is even less than that of hematite, and there is almost no significant mass-gain phenomenon during the entire process. Its oxidation rate just slightly increases above 850 °C.


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These phenomena demonstrated that the Fe/Al OC could still be rapidly oxidized after three-cycles, which indicated its excellent reactivity. Addition of γ-Al2O3 was found to be beneficial to improve and maintain the reactivity of Fe/Al OC. In contrast, due to the addition of NiO, the reactivity of Fe/Ni OC after three-cycles was seriously decreased; even the OC activity was almost lost. This may be attributed to the fact that the addition of NiO leads to more serious sintering of the Fe/Ni OC.

The comparative analysis of carbon-burning stages of all OCs shows that the highest exothermic peaks of hematite, Fe/Al, and Fe/Ni OC occur, respectively, at 640, 615, and 645 °C, and the peak widths are all about 150 °C. The carbon deposition contents of various types of OCs, respectively, account for about 12.5, 6.5, and 9.2% of the OC sample mass. These indicate that the addition of γ-Al2O3 could effectively inhibit carbon deposition; however, the NiO promoted it. Based on the research of Rossetti et al.[14], according to the carbon-burning temperature, the carbon deposition on reduced OC is mainly hard coke, which is a type of carbon material similar to graphite. Its formation may be attributed to the fact that CB generated on the surface of OC partly realized transition to graphite at high temperature.

3.2. X-ray diffraction analysis

The X-ray diffraction analysis of the reduced Fe/Al and Fe/Ni OC after three-cycles was carried out, and the results are shown in Figure 4. Furthermore, the phase identification was also carried out based on the diffraction pattern; and the corresponding results are listed in Table 3.

Table 3 Main composition analysis of reduced compound OCs


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Composition Composition content of Fe/Al OC /% Composition content of Fe/Ni OC /%

Fe3O4

FeO

Fe

Ni

Al2O3

C

Other

24.06

18.54

5.80

/

44.35

2.08

5.17

31.96

30.50

10.66

8.35

/

6.37

12.16

The comparison between the contents of Table 1 and 3 indicates the significant change in the main compositions of compound OCs after the reaction. The original Fe2O3 in two types of OCs gets reduced to Fe3O4 and FeO, and even the pure Fe is generated. The NiO in Fe/Ni OC also gets completely reduced to pure Ni. These results indicate that the pyrolysis reaction between coal tar and OC is very intense, and the OC is deeply reduced.

E

(a)

Intensity/(counts per second, CPS)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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E A

E B

E

D B E

B E

E E

A

A A C

A

10

20

40

B

D

C

30

B

50

60

2θ/(°)


70

80

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(b) B

F D

A

Intensity/(CPS)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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B

B F

B A

C

C

20

30

40

F

D

A

A

10

B

A

A

C

50

60

70

80

2θ/(°)

Figure 4. XRD characterization of reduced compound OCs (a) Fe/Al OC and (b) Fe/Ni OC A-Fe3O4; B-FeO; C-Carbon; D-Fe; E- Al2O3; and F-Ni

Figure 4 also shows the presence of a small amount of carbon in two types of reduced compound OCs. According to the position and intensity of the diffraction peak in curves, it should be the graphite crystal. Moreover, the carbon content of Fe/Ni OC is significantly higher than that of Fe/Al OC. These results are consistent with the above mentioned TGA-DSC analysis and provide mutual proof.

The Fe2O3 contents in two types of compound OCs are significantly different (as shown in Table 2). Therefore, in order to determine the reduction degree of Fe2O3, based on the Fe element conservation, the ratio of Fe2O3 that converted into Fe3O4, FeO, and Fe was, respectively, calculated. The results are shown in Figure 5, indicating that the Fe2O3 in Fe/Al OC gets mainly converted to Fe3O4; however, the Fe2O3 in Fe/Ni OC gets mostly converted into FeO. The ratio of Fe2O3 in Fe/Ni OC that gets converted to Fe is also higher than that in Fe/Al OC. These results showed that the


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addition of NiO was more effective in promoting the reduction of Fe2O3. However, the unit volume heat release of this OC also increased during the regeneration process, which promoted the trend of sintering. This perspective was also confirmed by the following SEM analysis.

Conversion proportion of Fe2O3/%

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Fe/Al OC Fe/Ni OC

40 30 20 10 0 Fe3O4

FeO

Fe

Reduced composition

Figure 5. The ratio of Fe2O3 in compound OC converted into the reduced components

3.3. Scanning electron microscopy characterization of oxygen carriers

Figure 6(a) exhibits the distribution of numerous grains on the surface of fresh hematite, and the porosity is more developed. However, Figure 6(b) shows the complete melting of the original grains on the surface of hematite OC after three cycles at high temperature. The liquid metal layers are in contact with each other, which lead to the closing of the surface pores. The surface sintering is rather serious. Furthermore, the OC surface also exhibits the distribution of a large number of fine carbon depositions. Overall, the anti-sintering/carbon deposition performance of hematite is poor.


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(a) Fresh hematite

(b) Reduced hematite

Figure 6. SEM images of the hematite OC

(a) Low magnification

(b) High magnification

Figure 7. SEM images of freshly-prepared Fe/Al OC

Figure 7(a) shows that compared to the natural hematite, the pore structure of freshly-prepared Fe/Al OC is more developed. Figure 7(b) represents the magnification of the area shown in the red box (so is in the following Figures). Figure 7(b) demonstrates that the small grains gather together sparsely to form grain-cluster whose surface distributes a large number of micro pores. Further these grain-clusters agglomerate with each other to form larger pores. According to the above mentioned phenomena, the addition of γ-Al2O3 leads to the more development of the space structure of Fe/Al OC. Thus, the specific surface area of Fe/Al OC is effectively improved, and the reactivity is also


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enhanced.



Figure 8. SEM images of reduced Fe/Al OC



Figure 9. SEM images of fresh Fe/Ni OC

Figure 8(a) shows that after the reaction the surface porosity of Fe/Al OC is decreased, in particular, the grain-cluster is obviously lost. However, Figure 8(b) still exhibits the appearance of a wide range of pores on the OC surface, and the porosity is also relatively high. The sintering and carbon deposition phenomena are not obvious. On the one hand, the γ-Al2O3 as an inert carrier could effectively prevent the surface grains of OC particles from melting at high temperature. On the other hand, the γ-Al2O3 could also effectively reduce the unit volume heat release of Fe/Al OC in the regeneration process. Therefore, the sintering was significantly relieved.


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Figure 9(a) shows that the porosity of fresh Fe/Ni OC is better than that of hematite shown in Figure 6(a); nonetheless, it is worse than that of Fe/Al OC. Figure 9(b) shows that different from the Fe/Al OC, the surface grains of Fe/Ni OC gather closely with each other. There are almost no pores at the surface of the grain-cluster.

(a) The overall appearance of Fe/Ni OC particle

(b) The partially enlarged view of particle crack

d

(c) The partially enlarged view of

(d) The partially enlarged view of

particle surface

carbon deposition

Figure 10. SEM images of reduced Fe/Ni OC

Figure 10(a) exhibits the distribution of different degrees of cracks on the surface of reduced Fe/Ni OC particle. Obviously, due to the low mechanical strength, the Fe/Ni OC gets cracked after three-cycles. Moreover, its surface is relatively "smooth", indicating extremely serious sintering of


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this OC. Figure 10(b) shows that the cracks are deep and attached with a large amount of carbon deposition. The long and narrow cracks provide a good environment for the formation and accumulation of carbon deposition. Figure 10c clearly displays that even the flat surface of Fe/Ni OC is attached with a lot of carbon deposition. There are almost no pores on the surface of OC particles, and the sintering is more serious than that of hematite. The reactivity of NiO is very high, and the above mentioned XRD analysis also shows that the NiO promotes the deep reduction of Fe2O3. Therefore, the unit volume heat release of Fe/Ni OC is huge during the regeneration process, which easily leads to sintering. Figure 10(d) shows that the significant carbon deposition is approximately spherical and the particle size range is about 30–100 nm. According to these two characteristics combined with the above mentioned TGA-DSC analysis, it is clear that the carbon deposition occurs by the graphitization conversion of CB at high temperature. This also provides a basis for the above mentioned relevant theoretical analysis.

4. CONCLUSIONS

Compared to natural hematite, addition of γ-Al2O3 is beneficial to inhibit carbon deposition, while the addition of NiO has the opposite effect. According to the carbon-burning temperature, the carbon deposition on reduced OC is mainly hard coke that is a type of carbon material similar to graphite. Its formation should be attributed to the graphitization conversion of CB at high temperature.

The SEM characterization indicates that γ-Al2O3 is helpful to improve and maintain the porosity of OC particles, and for significant alleviation of the sintering process. Owing to the weak


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mechanical strength, the Fe/Ni OC gets cracked after three redox cycles. Significant amount of coke gets deposited in the cracks, and the surface sintering of Fe/Ni OC is more serious than that of hematite.

The XRD analysis indicates that there are a large number of Fe3O4 and FeO in the reduced Fe/Al and Fe/Ni OCs. Compared to Fe/Al OC, the Fe2O3 in Fe/Ni OC mostly converts into FeO and Fe. NiO is more effective to promote the deep reduction of Fe2O3. Moreover, this also leads to an increase in the unit volume heat release of Fe/Ni OC during the regeneration process, which promotes the trend of sintering.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (51676102), Natural Science Foundation of Shandong Province (ZR2015EM004), and Thanks to Foundation of State Key Laboratory of Coal Clean Utilization and Ecological Chemical Engineering (2016-07) and Taishan Scholar Program of Shandong Province (201511029).

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Energy & Fuels

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Carbon Deposition and Sintering Characteristics on Iron-Based

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