Structural Characterization of Carbon in Blast Furnace Flue Dust and

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Structural Characterization of Carbon in Blast Furnace Flue Dust and Its Reactivity in Combustion Yiwei Zhong, Xinle Qiu, Jintao Gao, and Zhancheng Guo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01146 • Publication Date (Web): 25 Jun 2017 Downloaded from http://pubs.acs.org on June 26, 2017

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Structural Characterization of Carbon in Blast Furnace Flue Dust and Its Reactivity in Combustion Yiwei Zhong *, Xinle Qiu, Jintao Gao, Zhancheng Guo State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, P. R. China

Yiwei Zhong: Corresponding author, [email protected] Tel.: +86 010 82376018 Fax.: +86 010 82375042 Xinle Qiu: [email protected] Jintao Gao: [email protected] Zhancheng Guo: [email protected]

1

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ABSTRACT As an environmentally hazardous waste, blast furnace (BF) flue dust had a potential to reduce CO2 emission if recycled as fuels or reducing agents due to the high carbon content. The structure of carbon was a principal factor to the reactivity of carbon conversion and therefore was highly relevant to efficient utilization. In this work, the characteristics and chemical structures of carbonaceous materials in BF flue dust were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), and Raman analysis. The results showed that the aromatic structure of crystalline carbon was dominant in carbonaceous materials. Polymeric aromatic carbon and oxygen-containing groups (epoxide and esters carbon) existed on the surface. The stacking height (Lc), the in-plane crystallite sizes (La) and the interlayer spacing (d002) of the aromatic structure layer were 2.45, 3.31 and 0.347 nm, respectively. The mass ratios of chars and cokes to carbonaceous matter were estimated to be 90.56% and 9.44%, respectively, by Raman

spectroscopy.

Then

the

combustion

reactivity

was

studied

by

thermo-gravimetric analysis using the Kissinger-Akahira-Sunose kinetics method. The activation energy as a function of conversion degree was determined. The results thus provided fundamental information for the utilization of BF flue dust for thermochemical conversion.

Keywords: Blast furnace flue dust; Recycling; Carbon; Chemical structure; Combustion reactivity; Industrial solid waste 2


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1. Introduction During the production of steel, blast furnace (BF) dust or sludge collected from the gas-cleaning systems during the blast furnace processes was an important type of industrial solid waste [1-4]. The amount of blast furnace dust and sludge from dry and wet off-gas cleaning facilities with the mill scale was estimated to be about 70-110 kg per ton of steel production [4]. The blast furnace dust and sludge contained high contents of Fe (>30 %) and C (>15%) with major amounts of Si, Al, Ca, Mg, and K [1-4]. Besides, various trace heavy metals [1, 2, 5-7], including Zn, Pb, Cd, and Hg, was identified in the dust. These hazardous elements had negative effects not only on environment but also on steelmaking processes. Therefore, it was urgent to minimize the amount of metallurgical residues and to find an appropriate way to treat these wastes for recycling the valuable components. Currently, the prevalent treatment of these wastes was the cement production and landfilling, and the rest was recycled in the sintering unit. However, these solid wastes had many valuable components, such as carbon with a considerably high content. The carbonaceous materials in blast furnace dust had the potential to reduce CO2 emission and increase significant economic benefits if recycled economically as fuels or reducing agents for thermochemical conversion or metallurgy. A number of techniques have been developed to better utilize carbon from metallurgical dust wastes. Lanzerstorfer [4] used BF dust to replace coke breeze as alternative fuel for the sintering process. The carbon in the BF dust effectively replaced coke breeze, and 3


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the carbon replacement factor was very close to 1.0. Yehia [8] used kerosene flotation to recover carbon values in blast furnace flue dust. About 99% of carbon values were separated with a purity of 87%, which was suitable for reburning, waste-water treatment or production of carbon block. Briquetting was one of the possible methods applied in recycling and utilization of fine-sized materials and produced a suitable feed material for the metallurgical furnace [9-11]. El-Hussiny [9] and Polat [10] studied the self-reducing mixture of blast furnace flue dust and binders for utilization of iron oxides by briquetting. The carbon in the dust acted as a reductant, and thus the addition of coke was reduced. Hu [11] also investigated the feasibility and mechanisms of direct reduction of titanomagnetite using the blast furnace dust as a reductant. The recovery ratio of Fe was 89.19% with a purity of 91.25% when the dosage of blast furnace dust was 30%. Drobíková [12] used blast furnace sludge and starch binder to prepared briquettes, which were heated at 900-1000 °C in an inert atmosphere. The carbon materials in blast furnace sludge were gasified and transferred to waste gas. The main compounds of the waste gas were methane and hydrogen utilizable as fuels. Combustion and gasification were the most important approaches in the carbon conversion process. The reactivity of carbonaceous materials was therefore a critical aspect of understanding the behavior under combustion and gasification conditions. A key property affecting the reactivity of combustion and gasification was the physical and chemical structure of the carbonaceous materials, which strongly depended on devolatilization conditions and changed with carbon conversion. A good 4


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understanding of the structure and its effect on the reactivity behavior was beneficial to improve the reactivity. Therefore, it would be necessary to differentiate between the various carbonaceous materials in blast furnace flue dust for recycling as fuels or reducing agents. Gupta [13] used a chemical analysis method to study the characteristics of BF flue dust. The carbonaceous materials consisted of unconsumed coal, fine char and coke from the pulverized coal injection. Wu [14] studied the unconsumed fine coke and pulverized coal in BF flue dust by petrographical microanalysis. In that study, the unconsumed fine coal was found in styles of completely unconsumed coal, undeformed coal, deformed coal, and residue coal. For quantitative analysis, Machado [15] used the X-ray diffraction technique in combination with chemical analysis as a standard procedure to identify and differentiate the char and coke structures. Yu [16] and Wang [17] found that the parameters derived from Raman spectra can be used to calculate the percentage of char and coke and to determine the degree of graphitization for carbonaceous materials. However, these methods to analyze BF flue dust were mainly on the basis of petrographical microanalysis. Only the differentiation between mineralogical phases, such as coal, char and coke, was determined. The microstructure of carbon could not be resolved in detail, which was highly relevant to the combustion or gasification reactivity. Therefore, in this study the chemical structures, especially microcrystalline structure and C-C/C-O bonding, of the carbonaceous materials in BF flue dust was comprehensively characterized by XRD, XPS, FTIR, and Raman analysis. Then the 5


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combustion

reactivity

of

carbonaceous

materials

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was

examined

using

a

non-isothermal kinetics method, and the activation energy as a function of conversion degree was determined. 2. Experimental section 2.1. Materials The blast furnace (BF) flue dust samples used in this study were provided by Shougang Steel Group Corporation in Tangshan, China. The samples were collected from a bag-type dust separator installed in a blast furnace gas cleaning system. In order to eliminate the fluctuation of chemical composition, 63 samples collected during a period of 7 days were fully mixed and selected for the detailed investigations in this paper. The carbon content of the sample was 25.8% measured by an infrared absorption carbon/sulfur analyzer (EMIA-920V2, France). 2.2. Methods (1) Demineralization treatment In order to separate the carbonaceous substances from blast furnace flue dust, the sample was subject to demineralization treatment. A 10 g of sample was mixed with 150 ml of HCl (30%), and the slurry was stirred for 24 h at room temperature. After filtering, the HCl-washed sample was added into 150 ml of HF (40%) and stirred for 24 h at room temperature. Finally, the sample was filtered and washed by deionized water and then dried at 90 °C for 24 h. Most of the inorganic matter was removed by the demineralization process. Consequently, the reactivity was mainly dependent on the crystalline structure of carbon. 6


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(2) Combustion reactivity and kinetics The combustion kinetics of the carbonaceous materials in blast furnace flue dust was investigated by thermo-gravimetric analysis based on a non-isothermal method. Simultaneous thermo-gravimetric (TG) and differential thermal analysis (DTA) were performed using a thermo-gravimetric analyzer (STA-409C NETZSCH, Germany). For a typical test, about 15 mg of the sample after demineralization process was heated in a corundum crucible from ambient temperature to 1000 °C at the heating rates of 5, 10, and 15 °C/min under O2 atmosphere (>99.9%) at a flow rate of 100 ml/min. Standard α-Al2O3 was used as the reference material. 2.3. Characterization 2.3.1. X-ray powder diffraction X-ray diffraction (XRD, X’Pert PRO MPD, The Netherlands) was used to identify the phase and lattice structure of the carbonaceous materials. The X-ray diffraction data were collected in a board 2θ range using Cu-Kα1 radiation (45 kV, 40 mA) with a step size of 0.02 ° and a scanning speed of 6 °/min. The peak position and full width at half maximum (FWHM) in XRD patterns were analyzed by X'Pert HighScore Plus software. 2.3.2. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS, AXISULTRA-DLD, Japan) was performed to determine the states of carbon present in the carbonaceous materials. The Al Kα (1486.6 eV) line was used for the X-ray source. The peak positions for binding energy of samples were corrected by considering the charging effect. A 7


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binding energy of 284.7 eV was assumed for the C 1s peak maximum in correcting for surface charging. XPSPEAK 4.1 software was used for fitting and deconvolution of C 1s spectrum. 2.3.3. FTIR spectroscopy Fourier transform infrared spectrometer (FTIR, NICOIET-470, USA) was used to determine the chemical bonding of carbon. The FTIR spectra of the de-mineralized samples were recorded over the range of 4000-400 cm-1 using the KBr pellet technique. The total numbers of scans were 50 with spectral resolution of 4 cm-1. 2.3.4. Raman spectroscopy Raman analysis was employed to analyze the C-C and C-O structure. Raman scattering measurements were recorded using a high resolution Raman spectrometer (Lab RAM HR800, France). The 532.21 nm line of the Nd-YAG laser was used for excitation. The samples were scanned between 1000 and 2000 cm-1 with spectral resolution of 4 cm-1. Each measurement was taken at three different spots to eliminate the heterogeneity of the sample. ORIGIN 8.5 software was used for fitting and deconvolution of the spectrum. 3. Results and discussion 3.1. Characterization of Blast Furnace Flue Dust The mineral phase and chemical composition of blast furnace flue dust are listed in Figure 1 and Table 1, respectively. The principal components were KCl, NaCl, Fe2O3 and carbon. The major metal elements in the dust were Fe, K, Na and Zn with the mass fraction of 33.58%, 13.8%, 8.46% and 7.04%, respectively. The 8


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carbonaceous materials were the unconsumed coal, fine char and coke, which were entrained by the gas flow and carried up into the dust [16]. In blast furnace process, the peak temperature in the reaction zone reached >1800 °C. Generally, there was some potassium and sodium aluminosilicate (KAlSi3O8 and NaAlSi3O8) in the iron ore, and CaCl2 and CaO was added in sintering ore [18]. The aluminosilicate reacted with CaCl2 and CaO to generate KCl and NaCl in reducing atmosphere [19].

2KAlSi 3O 8 / NaAlSi 3O 8 + CaCl 2 + CaO = 2KCl / NaCl ↑ + CaAl 2Si 2O 8 + 2Ca 2SiO 4 + 2Ca 3SiO 5 KCl and NaCl were easy to volatilize duo to the high saturated vapor pressures (KCl: 18.4 kPa, NaCl: 10.8 kPa at 1200 °C [20]) in blast furnace. Then the KCl and NaCl vapor was condensed rapidly by the gas flow out of blast furnace and de-dusted by a gas-cleaning precipitator.

1

1 = KCl 2 = Fe2O3 3 = NaCl 4 = Carbon 5 = ZnS

Counts

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5 1 32 2

4 2

20

5

30

2 35 1 2 5 1 2

40

50

60

1 1

2

70

80

2θ (°)

Figure 1. XRD pattern of blast furnace flue dust sample in present study 9


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Table 1 Chemical analysis of blast furnace flue dust used in this paper (in oxide form) Component

SO3

Al2O3

SiO2

K2O Fe2O3

Na2O

CaO

ZnO

PbO

MgO

CuO

Cl

Content (wt %)

7.27

3.27

10.32

13.8

8.46

6.57

7.04

1.95

3.03

0.05

2.95

33.58

The granulometric distribution analysis of blast furnace flue dust was performed by using a laser particle size analyzer (Malvern Mastersizer 2000, UK). As shown in Figure 2, the blast furnace flue dust presented a heterogeneous distribution of particle sizes with two major size fractions: a fine grained portion (0.3-7.0 µm) and a coarser part (7.0-40.0 µm), where 90% of particles were below 23.3 µm. The mean particle size was 5.5 µm. Such an irregular granulo-metric distribution was probably due to agglomeration of the fine particles. Akiyama [21] had found that the amount of smaller sized particle was associated with the unconsumed coal and coke fines. As the injection rate increased, coal combustibility tended to decrease, and large amounts of the unburnt coal, char and coke with fine size was generated because of the rapid volatile evolution in blast furnace.

10


6

100

5

80

4

60

3 40

2 1

20

0

0

0.1

1

10

100

Cumulative volume (%)

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Frequency volume (%)

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1000

Particle size (µm)

Figure 2. Particle size distribution of blast furnace flue dust sample in present study

3.2. Structure of Carbonaceous Matter in Blast Furnace Flue Dust 3.2.1. XRD analysis As shown in Figure 3, there were apparent diffraction peaks at position with diffraction angles of 24°-27° and 42°-46°, referring to (0 0 2) and (1 0 0) planes of disordered carbon materials, respectively. (0 0 2) diffraction peaks were sharp with high intensity, whereas (1 0 0) diffraction peaks were weak. The carbonaceous materials contained two types of carbon structures: crystalline carbon and amorphous carbon (any non-aromatic carbon) [15, 22]. The microcrystalline carbon was usually aromatic structure. The aromatic structure was represented by structural parameters: the stacking height (Lc), the in-plane crystallite sizes (La), and the interlayer spacing (d002) of the aromatic structure layer [15, 23-25]. Various carbonaceous materials had different proportions of crystalline and amorphous carbon. As graphitization increased, 11


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more carbon layers were stacked. Thus, Lc and La were increased, while d002 was decreased. The Lc and La values of carbonaceous samples were estimated from XRD data using Scherrer equation [23-25]: Lc = 0.89 ⋅ λ β002 ⋅ cos θ 002

(1)

La = 1.84 ⋅ λ β 100 ⋅ cos θ 100

(2)

where λ is the wavelength of the X-ray source (Cu-Kα1 = 0.154060 nm); β002 and β100 are the FWHM of the (0 0 2) and (1 0 0) peaks expressed in radians, respectively; θ002 and θ100 are the diffraction angles at the centers of (0 0 2) and (1 0 0) peaks. The interlayer spacing of aromatic layers (d002) was calculated by Bragg equation [25]: d 002 =

λ 2 sin θ002

h2 + k 2 + l 2

(3)

where h, k, l are Miller indices. Then the average number of aromatic layers (Nc) was estimated from d002 and Lc [25]: Nc =

Lc d 002

(4)

The crystalline parameters of the carbonaceous materials calculated by Eqs. (1)-(4) are listed in Table 2. The value of d002 of the carbonaceous materials in BF flue dust was smaller than that of anthracite [26] and non-caking coal [25]. However, the values of Lc, La and Nc of the carbonaceous materials in BF flue dust were larger than those of anthracite [26] and non-caking coal [25]. The results showed that the microcrystalline structure of the carbonaceous materials was more ordered and the degree of graphitization was greater due to the growth and stacking of aromatic sheets. It was noted that the crystalline parameters of the carbonaceous materials were closer 12


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to those of coke from anthracite [26], which was heated at 2000 °C in N2 at atmospheric pressure. The BF flue dust originated from the pulverized coal injection in the blast furnace, where the temperature was 1800-2000 °C. Most of the organic functional groups in the pulverized coal were removed by pyrolysis, and the crystalline structure tended to graphitization. Therefore, structural ordering of the carbonaceous materials in BF flue dust was higher than that of anthracite and non-caking coal, but was similar to that of coke.

(0 0 2)

Counts

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(1 0 0)

20

25

30

35

40

45

50

55

60

2θ (°)

Figure 3. XRD pattern of the carbonaceous materials in BF flue dust

13


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Table 2 The values of Lc, La and d002 of the carbonaceous materials in BF flue dust

Sample

Lc (nm)

La (nm)

d002 (nm)

Nc (-)

Carbon in BF flue dust

2.45

3.31

0.347

7.1

Anthracite [26]

0.60

-

0.385

1.6

Non-caking coal [25]

0.71-1.89

1.65-2.57

0.348-0.368

2.0-5.4

Coke from anthracite*[26]

3.10

-

0.344

9.0

*: heated at 2000 °C in N2 at atmospheric pressure

3.2.2. XPS spectra

The XPS analysis was used to identify the different states of carbon on the surface. The C 1s spectrum ranging from 282 to 288 eV is shown in Figure 4. Asymmetry and broadening observed in C 1s spectra were due to the existence of different C environments, such as C-C, C=C, C-O, C=O, COOH, and so on. XPS curve was subjected to peak fitting using XPSPEAK 4.1. The C 1s spectrum was deconvoluted into three components at 284.65, 285.15 and 286.20 eV. The binding energies at 284.65, 285.15 and 286.20 eV were assigned to sp3 C-C, sp2 C-C, and C-O bonds, respectively [27, 28]. The sp3 C-C belonged to σ bonds of graphite-like stacking of carbon layers. The sp2 C-C was π bonds of hexagonal aromatic molecules of graphitic carbon. The C-O bonds can be attributed to the epoxy linkages (C-O-C) between the aromatic rings or alicyclic rings, indicating the polymerization of long-chain molecule structure. No evidence of C=O (287.5 eV) and COOH (288.8 eV) [27] structures were detected in the C 1s spectrum. A lower concentration of

14


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carbon-oxygen functional groups suggested a greater graphitization degree due to pyrolyzation [27]. The proportion of the carbon in different states was calculated by integrating the peak area. It was found that the contents of graphitic carbon, polymeric aromatic carbon, and epoxide carbon were 49.56%, 35.31% and 15.13%, respectively. Therefore, it was inferred that most of the carbon states were graphitic carbon. This result was in agreement with the XRD analysis.

Raw 3 sp C-C 2 sp C-C C-O

Intensity

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288

287

286

285

284

283

282

Binding energy (eV)

Figure 4. XPS spectra of C 1s for the carbonaceous materials in BF flue dust

15


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Table 3 Fitting parameters obtained from XPS spectra of carbonaceous materials in

BF flue dust Structure

Biding energy (eV)

Peak area (-)

Proportion (%)

sp3 C-C

284.65

56473.17

49.56

sp2 C-C

285.15

40244.67

35.31

C-O

286.20

17243.34

15.13

3.2.3. FTIR spectra

FTIR experiments were carried out to investigate the structures of C-C and C-O in the carbonaceous materials in BF flue dust. As shown in Figure 5, the bands at 1398-1642 cm-1 were assigned to the stretching vibration of C=C bonds in aromatic structure, probably as well as the skeletal C=C stretching vibrations in aromatic rings [24, 29-32]. Graphite did not show infrared bands in the spectra because there was no change in the dipole moment when the atoms vibrated in this symmetric crystal buildup of equal atoms [29]. The olefinic C=C vibrations were also not observed at 1645 cm-1. Therefore, the aromatic structure of crystalline carbon was dominant in carbonaceous materials of BF flue dust, which was consistent with the XRD analysis. The band at 1256 cm-1 was connected to the symmetric stretching of the ring in esters (-CO-O-) and epoxides (C-O-C), as well as acyclic C-O-C groups conjugated with carbon-carbon double bonds (C=C-O-C) in aromatic structures [29-32]. Comparing the standard coals [32], the absorption at 1398-1642 cm-1 shifted to higher wavenumbers, indicating the increase of aromatization due to the conjugation of C=O 16


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groups and C=C bonds. The weak band at 2815- 2907 cm-1 was assigned to the stretching vibration of -CH, -CH2 and -CH3 in aromatic and aliphatic rings [29-32]. As the graphitization degree increased the absorption (2850-2920 cm-1) shifted to lower wavenumbers [30]. The weak band at 1762-1785 cm-1 was assigned to the aromatic COOH or ester groups. These H-containing functional groups may be attributed to the HCl and HF treatment during demineralization.

Transmittance (a.u.)

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C=C in aromatic -CH in aromatic

COOH or ester in aromatic

-CO-O-, C-O-C C=C-O-C ring in aromatic

3000 2700 2400 2100 1800 1500 1200 900

600

-1

Wavenumbers (cm )

Figure 5. FTIR spectra of the carbonaceous materials in BF flue dust

3.2.4. Raman characterization

The Raman spectra of the carbonaceous materials in BF flue dust are presented in the 1000-3000 cm-1 region. As shown in Figure 6 (a), the curves exhibit three broad and overlapping peaks with intensity maxima at 1348 cm-1, 1586 cm-1 and 2700 cm-1. These bands observed in the studied samples were relatively sharp, indicating a high crystallite size of the carbon and graphitization. In order to improve the accuracy 17


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in the determination of spectroscopic parameters, Raman spectrum was subjected to peak fitting, which was deconvoluted into four bands (Lorentzian type) as shown in Figure 6 (b) and Table 4. Raman spectra of the carbonaceous materials were

commonly divided into first-order and second-order regions. In the first-order region, the band at about 1580 cm-1 (G band) was assigned to the stretching vibration mode with E2g symmetry in the aromatic layers of the graphite crystalline [17, 33-36]. For highly disordered carbons, additional bands induced by the defects in the microcrystalline lattices appeared at around 1350 cm-1 (D1), 1620 cm-1 (D2), 1500 cm-1 (D3), and 1200 cm-1 (D4) [17, 33-36]. The D1 and D2 bands were assigned to the stretching vibration of disordered graphitic lattice. D3 was associated with the amorphous sp2-bonded forms of carbon responsible for the reacting sites. D4 referred to sp3 or sp2-sp3 bonding and was O-containing functional groups maybe related with the active sites on the surface of carbon. Their intensities directly affected the reactivity of amorphous carbon materials, especially the D3 and D4 bands. The 1530 cm-1 band generally appeared as a very broad band around 1500-1550 cm-1, which originated from organic molecules and fragments of functional groups [33]. It may be responsible for the reacting sites and consequently the reactivity. The 1200 cm-1 band only appeared in very disordered carbonaceous materials, such as soot and coal chars [35]. In the second-order region, the band at 2700 cm-1 (2D band) was attributed to the ordered graphitic structure, which was more sensitive to the change of structural disorder [37]. The 2D band was inclined to broaden and shifted to lower wavenumbers or even disappear when the carbon structure became less ordered [16]. 18


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The intensity (band area) of the G, D1, D2, D3, D4, and 2D bands in Raman spectroscopy was highly associated with the degree of carbon structural order and the reactivity. As listed in Table 4, IG, ID1, ID2, ID3, ID4, and I2D were the area of the G, D1, D2, D3, D4 and 2D bands, respectively. A higher value of (ID3+ID4)/IG and a lower value of IG/Itotal referred to a higher combustion reactivity of carbonaceous materials [24, 36]. In this study, the values of (ID3+ID4)/IG and IG/Itotal of carbonaceous materials in BF flue dust were 1.71 and 0.22, respectively (Table 5). Sheng [36] had studied the char structure and its correlations with combustion reactivity by Raman spectroscopy. In the study of Sheng [36], the chars were prepared by pyrolysis in N2 at 1500 °C, and values of (ID3+ID4)/IG and IG/Itotal of char were 2.1 and 0.18, respectively. Compared with our experimental results, it was inferred that the combustion reactivity of carbonaceous materials in BF flue dust was lower than that of char prepared at 1500 °C. The BF flue dust originated from the unconsumed fine coal of the pulverized coal injection in the blast furnace, where the operation temperature (>1800 °C) was higher than 1500 °C. More organic functional groups in the pulverized coal were removed by pyrolysis. As a result, the degree of graphitization increased, and the crystalline structure became more ordered. Therefore, the reactivity of BF flue dust decreased. Yu [16] proposed a simplified quantification method to estimate carbon constituents of the BF flue dust by using the intensity ratios of ID1/IG and I2D/IG. ID1/IG was inversely proportional to the average crystal planar size for graphitic micro-crystallite size [38]. I2D/IG became weaker as the carbon structure was less 19


Energy & Fuels

ordered [38]. The decrease of ID1/IG and the increase of ID2/IG, therefore, implied the increase of the degree of graphitization. As listed in Table 5, the values of ID1/IG and ID2/IG were 1.69 and 0.51, respectively. According to the method as reported by Yu [16], the mass ratios of chars and cokes to carbonaceous matter in BF flue dusts were estimated to be 90.56% and 9.44%, respectively

140

(a) D

Intensity

120

G

100 80

2D

60 40 20 1200

1600

2000

2400

2800

-1

Raman shift (cm )

120

(b)

Raw D4 D1 D3 G D2

100

Intensity

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

Page 20 of 36

80 60 40 20 1000

1200

1400

1600

1800 -1

Raman shift (cm )

20


2000

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Energy & Fuels

Figure 6. Raman spectra of the carbonaceous materials in BF flue dust: (a) 1000-3000

cm-1 region; (b) Deconvolution of 1000-2000 cm-1 region

Table 4 Fitting parameters obtained from Raman spectra of carbonaceous materials in

BF flue dust Peak

Position (cm-1)

Structure

Intensity (area)

G

1582.09

Graphite crystalline

4776.57

D1

1346.06

Disordered graphite

6414.05

D2

1614.66

Disordered graphite

769.50

D3

1501.10

Amorphous carbon

3792.94

D4

1238.96

Disordered graphite or C-O groups

1672.72

2D

2700

Graphite crystalline

1913.23

Table 5 Intensity of the bands in Raman spectra and the calculated composition of

carbonaceous materials in BF flue dust Index

(ID3+ID4)/IG

IG/Itotal

ID1/IG

I2D/IG

Char (wt. %)

Coke (wt. %)

Value

1.71

0.22

1.34

0.40

90.56

8.44

3.3. Combustion Reactivity of Carbonaceous Matter in Blast Furnace Flue Dust 3.3.1. Kinetics Models

The thermo-gravimetric analysis was used to evaluate the combustion kinetics of the carbonaceous materials in BF flue dust. In order to determine the kinetics 21


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Page 22 of 36

parameters based on the TG data, the degree of conversion (X) of the thermal decomposition process is defined as:

X=

m0 − mt m0 − m f

(5)

where m0 is the initial mass of the sample (g), mt is the mass of the sample at a certain temperature (g), mf is the final mass of the sample in the reaction (g). Kinetics analysis of thermally stimulated reactions is traditionally expected to produce an adequate kinetics description of the process in terms of the reaction model and of the Arrhenius parameters using a solid- solid kinetics equation [39]: dX = k (T ) f ( X ) dt

(6)

where X is the degree of conversion, t is the reaction time (s), T is the reaction temperature in absolute units (K), f(X) is the reaction model, k(T) is the kinetics rate constant (s-1), which is given as: −E k (T ) = k 0 exp   RT 

(7)

where R is the gas constant (8.314 J·mol-1·K-1), k0 is the pre-exponential factor (s-1), E is the apparent activation energy (kJ/mol). Taking Eq. (7) into Eq. (6), its form is changed to: dX −E = k0 exp  f (X ) dt  RT 

(8)

In the TG process at a constant heating rate, the reaction temperature is a function of time, thus: β=

dT dt

(9) 22


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Energy & Fuels

where β is the heating rate in the TG experiment (°C/min). For non-isothermal conditions, Eq. (8) can be integrated to yield: G( X ) = ∫

X

0

dX k = 0 f(X) β

∫

T

0

 E  exp  −  dT  RT 

(10)

where G(X) is the integral form of the reaction model which depends on the reaction mechanism. Defining a variable: y=

E RT

(11)

The expression of G(X) is obtained by rearranging Eq. (10) [40]:

G( X ) = ∫

X

0

P( y) = ∫

∞

y

dX k E = 0 f(X) βR

∫

∞

y

exp( -y ) dy y2

(12)

exp( -y ) dy y2

(13)

where P(y) is the temperature integration. Many methods were carried out to derive the approximation of the temperature integral. To derive the approximation suited for an isoconversion method, the function of P(y) was expressed as [40]:

P( y) ≈

exp( -y ) y2

(14)

This approximation led to the Kissinger–Akahira–Sunose (KAS) method [41]. In this work, the KAS method was used to calculate the kinetics parameters. Substitute Eq. (14) into Eq. (12), G(X) was expressed as:

G( X ) =

k 0 E exp( -y ) k 0 E 2 −E = T exp( ) 2 βR y βR RT

(15)

At a constant degree of conversion, the relationship of E and T was obtained by taking 23


Energy & Fuels

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Page 24 of 36

the logarithm and rearranging Eq. (15):

E  k E   β  ln  2  = − + ln  0  RT T   G( X )R 

(16)

According to Eq. (16), the apparent activation energy was calculated from the slope of linear regression of ln(β/T2) versus the reciprocal of reaction temperature (1/T) at different degrees of conversion. 3.3.2. Model Evaluation

In the present study, four heating rates (5, 10, and 15 °C /min) were used to evaluate the relationship between the activation energy (E) and the degree of conversion (X) by the isoconversional method. The degrees of conversion obtained at different heating rates during combustion are shown in Figure 7(a). The degrees of conversion ranging from 0.2 to 0.9 were selected for the kinetic analysis. Substituting the data of TG into Eq. (5), the values of X can be calculated, and the corresponding temperature T was determined. Consequently, the values of ln(β/T2) at different degrees of conversion were determined as shown in Figure 7(b). According to Eq. (16), E can be estimated from the Arrhenius plot of ln(β/T2) with 1/T at the selected conversion degrees for different values of β and T. Different heating rates gave different Arrhenius plots. As a result, a series of E values were obtained from the slopes of linear regressions of ln(β/T2) with 1/T for different degrees of conversion. The data of ln(β/T2) and their linear regressions with 1/T calculated in Figure 7(b) were listed in Table S1. Regression analysis supported a good linear

relation between ln(β/T2) and 1/T. That is, the KAS method [Eq. (16)] fitted the kinetics data in this work. Therefore, it indicated that our assumption about the 24


Page 25 of 36

kinetics model was reasonable. The relationship of the apparent activation energy and the degree of conversion was shown in Figure 7(c). The activation energy significantly decreased from 478.4 to 168.3 kJ/mol when the degree of conversion increased from 0.2 to 0.9. López-Fonseca [42] reported that the activation energy calculated by the KAS method was 132-150 kJ/mol for the combustion of carbon blacks. The value was much lower than that in this work. According to XPS analysis (section 3.2.2), the polymerization of aromatic rings formed on the surface of carbon. The C-C bonds in the long-chain molecule structure were different to break during oxidization in O2. Thus, the activation energy was relatively high at the early stage of combustion. However, when the combustion of polymeric aromatic compounds completed, the aromatic layers of the graphite crystalline appeared on the reaction boundary. As a result, the kinetics characteristic and the activation energy at the later stage of combustion were similar to the results reported by López-Fonseca [42].

Degree of Conversion (-)

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

Energy & Fuels

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 360

5 K/min 10 K/min 15 K/min

380

400

(a)

420

440

460

Temperature (°C) 25


480

500

2

-10.3 -10.4 -10.5 -10.6 -10.7 X -10.8 X -10.9 X -11.0 X -11.1 X -11.2 X -11.3 X -11.4 X -11.5 1.40 1.41

Page 26 of 36

(b) = 0.2 = 0.3 = 0.4 = 0.5 = 0.6 = 0.7 = 0.8 = 0.9 1.42 1.43 1.44 1.45 1.46 1.47 1.48 -1

1000/T (K )

Activation Energy (kJ/mol)

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

ln(β /T )

Energy & Fuels

500

(c)

450 400 350 300 250 200 150 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Degree of Conversion (-)

Figure 7. Kinetics analysis of combustion: (a) The degree of conversion at different

heating rates; (b) The plot of ln(β/T2) versus 1/T; (c) Dependency of the activation energy on the degree of conversion

4. Conclusions

In this paper, the comprehensive characterization by several analytical techniques 26


Page 27 of 36

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Energy & Fuels

was carried out in order to analyze the chemical structure of carbonaceous materials in blast furnace flue dust for developing the recycling methods. The aromatic structure of crystalline carbon was dominant in carbonaceous materials with some epoxide and esters carbon on the surface. The stacking height (Lc), the in-plane crystallite sizes (La), and the interlayer spacing (d002) of the aromatic structure layer were 2.45, 3.31, and 0.347 nm, respectively. The mass ratios of chars and cokes to carbonaceous matter were estimated to be 90.56% and 9.44%, respectively, by Raman spectroscopy. For the combustion reactivity, the activation energy decreased from 478.4 to 168.3 kJ/mol when the degree of conversion increased from 0.2 to 0.9. The obtained data provided useful information for further studies of utilizing BF flue dust in thermochemical conversion process. Acknowledgements

This work was financially supported by National Key Research and Development Program of China (No. 2016YFB0601304), National Natural Science Foundation of China (No. 51604020 and 51234001) and Fundamental Research Funds for the Central Universities (No. FRF-TP-15-013A1). Supporting Information

Details about the data of combustion kinetics and linear regression analysis are given. This information is available free of charge via the Internet at http: //pubs.acs.org.

27


Energy & Fuels

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(13) Gupta, S.; Sahajwalla, V.; Chaubal, P.; Youmans, T. Carbon Structure of Coke at High Temperatures and Its Influence on Coke Fines in Blast Furnace Dust. Metall. Mater. Trans. B 2005, 36, 385-394. (14) Wu, K.; Ding, R.; Han, Q.; Yang, S.; Wei, S.; Ni, B. Research on Unconsumed Fine Coke and Pulverized Coal of BF Dust under Different PCI Rates in BF at Capital Steel Co. ISIJ Int. 2010, 50, 390-395. (15) Machado, A.S.; Mexias, A.S.; Vilela, A.C.F.; Osorio, E. Study of Coal, Char and Coke Fines Structures and Their Proportions in the Off-Gas Blast Furnace Samples by X-ray Diffraction. Fuel 2013, 114, 224-228. 29


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Figure captions Figure 1. XRD pattern of blast furnace flue dust sample in present study Figure 2. Particle size distribution of blast furnace flue dust sample in present study Figure 3. XRD pattern of the carbonaceous materials in BF flue dust Figure 4. XPS spectra of C 1s for the carbonaceous materials in BF flue dust Figure 5. FTIR spectra of the carbonaceous materials in BF flue dust Figure 6. Raman spectra of the carbonaceous materials in BF flue dust: (a) 1000-3000

cm-1 region; (b) Deconvolution of 1000-2000 cm-1 region Figure 7. Kinetics analysis of combustion: (a) The degree of conversion at different

heating rates; (b) The plot of ln(β/T2) versus 1/T; (c) Dependency of the activation energy on the degree of conversion

34


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Page 35 of 36

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Table captions Table 1. Chemical analysis of blast furnace flue dust used in this paper (in oxide

form) Table 2. The values of Lc, La and d002 of the carbonaceous materials in BF flue dust Table 3. Fitting parameters obtained from XPS spectra of carbonaceous materials in

BF flue dust Table 4. Fitting parameters obtained from Raman spectra of carbonaceous materials

in BF flue dust Table 5. Intensity of the bands in Raman spectra and the calculated composition of

carbonaceous materials in BF flue dust

35


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Table of Contents

(002)

XRD

Raw 3 sp C-C 2 sp C-C C-O

XPS

(100)

20 25 30 35 40 45 50 55 60288

2θ(°)

287

286

285

284

283

282

Bindingenergy(eV)

FTIR

Raw D4 D1 D3 G D2

Raman

C=C inaromatic -CH inaromatic

COOHorester inaromatic

-CO-O-, C-O-C C=C-O-Cring inaromatic

3000 2700 2400 2100 1800 1500 1200 900 6001000 -1

Wavenumbers (cm)

1200

1400

1600

1800

2000

-1

Ramanshift (cm )

36


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Structural Characterization of Carbon in Blast Furnace Flue Dust and

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