Effects of demineralization on the structure and combustion properties of Shengli lignite

Fuel 183 (2016) 659–667

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Effects of demineralization on the structure and combustion properties of Shengli lignite Yinmin Song, Wei Feng, Na Li, Yang Li, Keduan Zhi, Yingyue Teng, Runxia He, Huacong Zhou, Quansheng Liu ⇑ College of Chemical Engineering, Inner Mongolia University of Technology, Inner Mongolia Key Laboratory of High-Value Functional Utilization of Low Rank Carbon Resources, Huhhot 010051, Inner Mongolia, China

h i g h l i g h t s
Inherent minerals could be demineralized from Shengli lignite by hydrochloric acid.
Demineralized treatment changed the structure characteristics of Shengli lignite.
HCl treatment decreased the combustion stability and combustion reactivity of lignite.

a r t i c l e

i n f o

Article history: Received 20 January 2016 Received in revised form 21 June 2016 Accepted 22 June 2016 Available online 6 July 2016 Keywords: Lignite Demineralization Structural characteristics Combustion

a b s t r a c t Investigation of the effects of demineralization of inherent minerals in Shengli lignite (SL) on its structure and combustion property in the field of coal conversion is an extremely important and interesting topic. In this study, inherent minerals in SL were demineralized by hydrochloric acid (HCl) treatment. Further, the metals cations Na+, Al3+, K+, Ca2+, Mn2+ and Fe3+, corresponding to the inherent metal ions in lignite were added into the demineralized samples by the impregnation method in order to compare their effects. The surface morphology and structure parameters of the raw and demineralized lignite samples were characterized by SEM, nitrogen adsorption–desorption, FTIR, XPS, XRD and Raman spectroscopy. The combustion properties of lignite samples were investigated by thermogravimetric analysis. Demineralization led to an improvement in the amount of side chains in SL and destroyed the pore structure and aromaticity; however, it did not have obvious effects on the surface morphology of SL and the main forms of carbon and oxygen. Moreover, demineralization increased the ignition temperature and apparent activation energy, and decreased the combustion stability index of SL. Added aluminum ions resulted in a decrease in the combustion stability and reactivity of the HCl treated sample; however, the added metals cations such as Na+, K+, Ca2+, Mn2+ and Fe3+, exhibited opposite effects. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction China is by far the world’s largest producer and consumer of coal. Coal accounts for about 70% of the top primary energy consumption in China. Most of the low-rank coals, including lignite and sub-bituminous coals have been large-scale exploited and used for combustion and chemical production [1]. As energy shortages continue to threaten industries worldwide, utilization of lowrank coal, in particular lignite has attracted significant interest. The Shengli coalfield is the largest lignite coalfield in China with the reserves of 24.2 billion tons of lignite. However, Shengli lignite

⇑ Corresponding author. E-mail address: [email protected] (Q. Liu). http://dx.doi.org/10.1016/j.fuel.2016.06.109 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

(SL) is not suitable for long distance transportation and longterm storage due to its high moisture content, high volatility and high oxygen content, which result in low ignition temperature and strong tendency toward spontaneous combustion. Moreover, SL exhibits a high chemical reactivity because of the presence of abundant oxygen-containing groups [2], of which can be used to rationally utilize SL. Coal is a composite material consisting of organic polymeric materials and inorganic minerals; not only their complex components, but also hierarchical micro-structures [3,4]. The inorganic minerals significantly influence the properties and reaction characteristics of coal [5–20]. Compared to high rank coals, lignite has lower temperature range of combustion and gasification reaction, and minerals present within lignite significantly affect the reaction characteristics. The content and composition of the minerals in

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lignite has a significant influence on its density. Moreover, lignite mineralogy greatly influences the combustion behavior. It is extremely important to know whether and how the minerals alter coal structure or reactivity [2]. Demineralization from lignite by hydrochloric acid (HCl) and/or hydrofluoric acid is often carried out in order to investigate the effects of minerals on the structure, properties, and reaction characteristics [1,21]. Many researchers have studied the effects of demineralization on the structures of coals by various approaches and characterized the structures of coals before and after demineralization by FTIR, XRD, XPS, Raman spectroscopy and 13C NMR [22–30]. Liang et al. [21] compared the surface species of the raw and demineralized Chinese Yimin lignite by FTIR spectroscopy and found that demineralization significantly affected the aromatic structures and oxygen-containing groups. Wei et al. [31] studied the carbon compositions of the raw Chinese Yongxing lignite and the demineralized samples by the solid state 13C-CP/MAS NMR analysis, and found that the double/multi-ring structure was decomposed into single-ring ones. Moreover, they also reported that the size of the aromatic clusters decreased after the demineralization treatment. Zhang et al. [32] reported that acid treatment removed not only minerals in lignite but also the volatiles, and thus resulted in a decrease in the combustion reactivity of low rank coals. Our previous studies showed that the HCl treatment did not have obvious effects on the surface morphology of SL [2]; however, it led to distinct changes in the steam gasification and pyrolysis characteristics [1,33,34]. The study of the effects of HCl treatment on the structure and the combustion performance of SL is still highly desirable. In this study, SL was demineralized by HCl. Further, the metal elements corresponding to the inherent metal elements in SL were added into the demineralized samples by the impregnation method in order to compare their effects. The surface morphology and structure parameters of the raw and the demineralized lignite samples were characterized by SEM, nitrogen adsorption–desorption, FTIR, XPS, XRD, and Raman spectroscopy. The combustion properties of raw and demineralized lignite, as well as the metaladded demineralized lignite samples were investigated by the thermogravimetric analysis (TGA). 2. Experimental section 2.1. Sample preparation SL was collected from Shengli coalfield in Inner Mongolia, China. The as-received lignite sample was crushed and ground to fine powder with size 38–75 lm, followed by drying at 105 °C for 4 h, and the corresponding sample was denoted as SL-D. Dried SL (30 g) was added to reagent-grade aqueous solution of HCl (180 mL) with stirring for 4 h at room temperature. The resulting slurry was filtered and rinsed continuously with deionized water. The solid so obtained was dried at temperature below 105 °C for 4 h and this sample was denoted as SL-HCl [1]. The proximate and ultimate analyses of the lignite samples were conducted and the corresponding results are listed in Table 1. The partial minerals were removed from SL by HCl treatment. The contents of the main

Table 2 Metal ions contents in SL-D and SL-HCl and the corresponding ash (wt%). Sample

SL-D SL-HCl

Coal based/Ash based Al3+

Na+

Ca2+

Si4+

Fen+

Mn2+

K+

2.40/ 35.50 0.90/ 32.94

0.58/ 8.59 0.00/ 0.16

0.41/ 6.08 0.01/ 0.19

3.11/ 46.09 2.89/ 64.58

0.11/ 0.69 0.03/ 0.92

0.08/ 1.18 0.03/ 1.18

0.06/ 0.88 0.00/ 0.03

metal elements in lignite samples and the corresponding ashes, detected by the inductively coupled plasma–optical emission spectroscopy (ICP–OES, Optima 7000, PerkinElmer), are listed in Table 2. Metal ions were loaded on the demineralized SL by the impregnation method according to the contents of the main metal ions in the raw lignite samples. Briefly, SL-HCl (2 g) was immersed in deionized water (40 mL, the Na and K concentrations were lower than 10 ppb) containing suitable amount of various metal chlorides including sodium chloride (NaCl), aluminum chloride (AlCl3), potassium chloride (KCl), calcium chloride (CaCl2), manganese chloride (MnCl2), or iron(III) chloride (FeCl3). The suspension was stirred for 24 h at room temperature and dried at 105 °C. The nominal content of each metal ion was 5.0% (mass) in dried SL-HCl, and the corresponding samples were labeled as SL-HCl-Na, SL-HCl-Al, SL-HCl-K, SL-HCl-Ca, SL-HCl-Mn, and SL-HCl-Fe [1], respectively. 2.2. Analysis of structural properties The surface morphology of lignite samples was observed by SEM (model S-3400N) equipped with an energy-dispersive spectrometry (EDS) detector. The specific surface areas of the lignite samples were measured using a 3H-2000PSZ surface area analyzer and determined by the Brunauer–Emmett–Teller (BET) method using the nitrogen adsorption–desorption process. The functional groups in lignite samples were characterized using a Nicolet 670 FTIR spectrometer by the KBr pellet technique. FTIR spectra recorded from 4000 to 400 cm1 at a resolution of 8 cm1 were compiled from 64 cumulative scans. The surface elemental compositions and body characteristics of lignite samples were detected using a PE PHI 5400 XPS analyzer equipped with a monochromatic Mg Ka X-ray source and operated at 150 W. The spectra were recorded in the fixed analyzer transmission mode. The XRD spectra of lignite samples were observed using a Bruker D8 powder diffractometer equipped with a Cu tube (Cu Ka radiation). The scan range was from 5 to 90° and the scan speed was 2° min1. Raman spectra were recorded using an American Thermo Fisher DXR laser Raman spectrometer with Nd-YAG laser source (532 nm exciting line was focused). The laser power at the sample surface was controlled at about 5 mW. The spectra were recorded in the range 500–3500 cm1.

Table 1 Proximate and ultimate analyses of SL-D and SL-HCl. Sample

SL-D SL-HCl

Proximate analysis (wt%)

Ultimate analysis (wt%, d)

Mad

Ad

Vd

FCd

C

H

N

S

Oa

1.52 2.14

13.92 7.53

33.37 39.77

52.71 52.70

57.59 61.42

3.58 3.34

0.89 0.86

1.81 1.74

22.21 25.10

ad, air-dried basis; d, dried basis. a By difference.

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Fig. 1. SEM micrographs of (a) SL-D and (b) SL-HCl.

2.3. Thermogravimetric analysis

Table 3 Changes of the porosity structure data. Samples

Surface area (m2 g1)

Pore volume (mL g1)

Pore diameter (nm)

SL-D SL-HCl

8.998 8.355

0.035 0.053

15.43 23.12

TGA was performed using a Diamond 6300 TG analyzer. The experiment was performed as follows: lignite sample (15–20 mg) was placed at the bottom of an alumina crucible. The temperature was increased from 20 to 900 °C at a heating rate of 10 °C min1, and the air flow rate during the experiment was maintained at 100 mL min1. During the thermogravimetric analysis, the mass loss and temperature were recorded simultaneously.

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The apparent activation energy E and pre-exponential factor A can be calculated from the linear fitting of ln[ln(1  x)/T2]  1/T. 3. Results and discussion 3.1. Morphology of lignite samples Fig. 1 shows the SEM image exhibiting the morphology of the lignite samples. Fig. 1 shows that demineralization has no obvious effects on the morphology of SL apart from the deprivation of some inorganic content, and this result was consistent with our previous reports [2]. The porosity characteristics and BET specific surface areas of lignite samples are listed in Table 3, which provides a good account of the physical changes that occur during the demineralization process. It is generally known that the diffusion of oxygen into the lignite particles influences the rate of lignite combustion. Therefore, in this study, it was found that demineralization resulted in the minimal reduction of the specific surface area which would not influence the diffusion of reactant gas and the combustion properties. The pore volume and pore size increased greatly attributed to demineralization process, which would facilitate the combustion reaction. So it should be minor considered the effects of morphological evolution in the demineralization process on the combustion properties of SL.

Fig. 2. FTIR spectra of SL-D and SL-HCl.

3.2. Fourier transform infrared spectra of samples

Fig. 3. XPS spectra of SL-D and SL-HCl.

2.4. Analysis of combustion reaction kinetics The combustion reaction kinetics of coal sample has been extensively reported [35–38]. Smith et al. [39] reported that coal combustion was the first-order reaction. The fundamental differential equation for non-isothermal reaction rate combustion can be described as Eqs. (1)–(3) [40,41] as follows:

dx=dt ¼ kð1  xÞ

ð1Þ

x ¼ ðw0  wt Þ  100%=ðw0  w1 Þ

ð2Þ

k ¼ A expðE=RTÞ

ð3Þ

where w0, w1, and wt represent the mass values corresponding to ignition temperature, terminate temperature (defined as 700 °C), and time during the reaction, respectively; A represents the pre-exponential factor (min1); T represents the absolute temperature (K); E represents the apparent activation energy (kJ mol1); R = 8.314 kJ mol1 K1 represents the gas constant; and b = 10 °C min1 represents the heating rate. The Coats–Redfern method was adopted by transforming Eq. (1) into Eq. (4) [42].

ln½ lnð1  xÞ=T 2  ¼ lnðAR=bEÞ  E=RT

ð4Þ

Fig. 2 shows the FTIR spectra of lignite samples. The three absorption bands from 3000 to 2800 cm1, from 900 to 700 cm1, and from 1800 to 1000 cm1 are attributed to aliphatic groups, aromatic groups, and oxygen-containing groups within lignite, respectively [43,44]. Specifically, the band at ca. 3400 cm1 is ascribed to the hydroxyl stretching vibration. The bands at ca. 2923 and 2855 cm1 are attributed to the aliphatic stretching vibration; and those at ca. 1695, 1593, 1436 cm1 and from 1300 to 1000 cm1 are attributed to the carboxyl absorption, stretching vibration of aromatic rings, aliphatic bending vibration, and stretching vibration of carbon and oxygen, respectively. Comparison of the FTIR spectra of SL-D and SL-HCl indicated that the spectrum of SL-D showed an approximate single peak band from 1800 to 1500 cm1; however, that of SL-HCl exhibited a two-peak band at 1695 and 1600 cm1, indicating that carboxyl groups in lignite were exposed after the demineralization treatment. The absorption bands at 1600–1500 cm1 and 2925–2850 cm1 are attributed to the aromatic rings and aliphatic hydrocarbons, respectively. The ratio of aromatic/aliphatic (Aar/Aal) was adopted to contrast the aromaticity of two samples. Higher value of Aar/ Aal corresponded to higher aromaticity [43]. The ratio of Aar/Aal for SL-D was found to be 9.83, and that for SL-HCl was 8.56, indicating that HCl treatment resulted in a decrease in the aromaticity of lignite sample. 3.3. X-ray photoelectron spectroscopic analysis of coal samples Fig. 3 shows the XPS spectra of lignite samples. Carbon and oxygen were found to be the main elements in lignite samples, which was consistent with the results obtained from ultimate analysis. In order to obtain better insight into the effects of demineralization on the coal sample, curve fitting was performed using a dedicated software XPSPEAK4.1. For XPS analysis, the binding energy of C 1s at 284.6 eV was used as reference to calibrate the position of other peaks. Fig. 4 shows the fitting results. The C 1s spectra of lignite samples demonstrates the presence of four carbon species on the surface of lignite samples, which are aromatic units with their alkyl substituent groups (CAC/CAH, 284.6 eV), phenolic or ether carbon

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Fig. 4. C 1s and O 1s fitting spectra of (a) SL-D and (b) SL-HCl.

Table 4 XPS analysis for the forms of carbon and oxygen in SL-D and SL-HCl. Elemental peaks

C 1s

O 1s

Functionality

CAC/CAH CAOA C@O COOA C@O CAOA COOA

Binding energy (eV)

284.6 286.3 287.5 289.0 531.3 532.5 534.0

Content (%) SL-D

SL-HCl

75.6 17.7 3.2 3.5 6.8 81.7 11.5

81.8 9.7 7.1 1.4 19.4 69.5 11.1

(CAOA, 286.1 eV), carbonyl (C@O, 287.6 eV), and carboxyl (COOA, 289.0 eV), respectively. The O 1s spectra were fitted according to the organic functional groups, including carbonyl (C@O, 531.3 eV), phenolic or ether (CAOA, 532.5 eV), and carboxyl (COOA, 534.0 eV) [23–26]. Table 4 lists the fitting parameters of lignite samples. The values listed in Table 4 indicated that the carbon element mainly existed as CAC/CAH, with marginal amounts of CAOA on the surface structure of lignite samples. The increase in CAC/CAH content and reduction in CAOA content indicated that the HCl treatment improved the amount of alkyl side chain of lignite. The increase in C@O content indicated that the HCl treatment had positive effects on C@O content, which was consistent with the results of FTIR.

Fig. 5. (a) Experimental and (b) fitting XRD analysis curves of SL-D and SL-HCl.

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Table 5 Crystalline structure parameters of SL-D and SL-HCl. Samples

fa

d002 (nm)

Lc (nm)

La (nm)

N

SL-D SL-HCl

0.54 0.53

0.37 0.39

0.83 0.70

0.69 0.46

2.26 1.17

disordered materials in the form of amorphous carbon. However, few crystals similar to graphite existed in the coal, as indicated by the peak at 25° (0 0 2) and the peak at 40° (1 0 0). Fig. 5(b) shows the fitting XRD spectra of lignite samples while neglecting the influence of the residual inorganic composition. Clearly, the broad hump in the 2h region 5–33° was fitted to two Gaussian peaks around 17° and 25°, denoted as c-band and Pband (d002), respectively. Theoretically, the areas under the c and (0 0 2)-peaks are considered to be equal to the number of aliphatic atoms (Cal) and aromatic atoms (Car), respectively [28]. Therefore, the aromaticity of coal, the ratio of carbon atoms in aliphatic chains vs. aromatic rings, can be defined as follows:

f a ¼ C ar =ðC ar þ C al Þ ¼ A002 =ðA002 þ Ac Þ

ð5Þ

where A is the integrated area under the corresponding peak. The interlayer space (d002), the layer size (La), and the stacking height (Lc) of coal are determined by using the conventional Scherer Eqs. (6)–(8) [27,28]:

Fig. 6. Raman spectra of SL-D and SL-HCl.

Table 6 Summary of peak/band assignment [15]. Band name

Band position (cm1)

Description

Bond type

GL G

1700 1590

sp2 sp2

GR

1540

VL

1465

VR

1380

D

1300

SL

1230

Carbonyl group C@O Graphite E22g; aromatic ring quadrant breathing; alkene C@C Aromatic with 3–5 rings; amorphous carbon structures Methylene or methyl; semi-circle breathing of aromatic rings; amorphous carbon structures Methyl group; semi-circle breathing of aromatic rings; amorphous carbon structures D band on highly ordered carbonaceous materials; CAC between aromatic rings and aromatics with not less than 6 rings Aryl–alkyl ether; para-aromatics

S

1185

SR

1060

R

960–800

CaromaticACalkyl; aromatic (aliphatic) ether; CAC on hydroaromatic rings; hexagonal diamond carbon sp3; CAH on aromatic rings CAH on aromatic rings; benzene (ortho-disubstituted) ring CAC on alkanes and cyclic alkanes; CAH on aromatic rings

sp2 sp2, sp3 sp2, sp3 sp2

sp2, sp3 sp2, sp3 sp2 sp2, sp3

Higher CAOA content was exhibited in the O 1s fitting spectra, which was in agreement with that reported by Shinn [45], indicating that phenolic hydroxyl was the most stable form of organic carbon–oxygen functional groups derived from the lone pair of electrons on the hydroxyl oxygen and aromatic ring [46]. The decrease in CAOA content indicated that HCl treatment had negative effects on the carbon–oxygen structure of lignite. 3.4. Analysis of X-ray diffraction patterns Fig. 5(a) shows the XRD profiles of SL-D and SL-HCl samples. Clearly, the profiles of two samples exhibit high background intensity, indicating that the coal contains a proportion of highly

d002 ¼ k=2 sinðh002 Þ

ð6Þ

Lc ¼ 0:89k=B002 cosðh002 Þ

ð7Þ

La ¼ 1:84k=B100 cosðh100 Þ

ð8Þ

where k is the wavelength of the incident X-ray, 0.154 nm for Cu Ka radiation, B100 and B002 are the full width and half maximum (FWHM) of the (1 0 0) and (0 0 2) peaks, respectively, and the h100 and h002 are the corresponding scattering angles or peaks positions. The results of above mentioned calculations are listed in Table 5. The aromaticity (fa) of SL-D and SL-HCl are 0.54 and 0.53, respectively, indicating insignificant effect of HCl treatment on the aromaticity of lignite sample. The interlayer spacing of SL-D and SL-HCl are higher than that of pure graphite (0.336– 0.337 nm) [28], indicating a low degree of crystallinity in the tested lignite samples. The interlayer spacing (d002) of SL-HCl is higher than that of SL-D; however, the stack thickness (Lc), aromatic layer size (La), and the number of aromatic layer (N) decreased due to HCl treatment. Thus, it was demonstrated that HCl treatment destroyed the degree of crystalline order of the coal. 3.5. Raman characterization of samples Raman spectroscopy analyses were performed in order to improve the understanding of the structure of lignite. Raman parameters were correlated with changes in the structure, which was used to characterize the structural ordering. Fig. 6 shows the Raman spectra of SL-D and SL-HCl. Both the curves exhibit two broad and overlapping peaks at 1350–1400 cm1 (D band) and 1580–1600 cm1 (G band), respectively [28–30,47]. The G and D bands of the two samples are relatively broad, indicating that large amounts of spectral residues were left behind if only the G and D bands were analyzed. Ten bands [30,48] were employed to fit the Raman spectra of two samples in the range 800–1800 cm1, which are briefly listed in Table 6. Fig. 7 shows the fitted Raman spectra of SL-D and SL-HCl. The G band in coal sample mainly represents the aromatic ring quadrant breathing with little contribution from graphitic structures. Similarly, the D band is ascribed to the aromatics having six or more fused benzene rings; however, less than that in graphite. The GR, VL, and VR bands, i.e. the ‘overlap’ between G and D bands are mainly ascribed to aromatic semi-quadrant ring breathing for aromatic ring systems having three to five fused benzene rings, which are, in general, found in amorphous carbon materials. The S band mainly represents the sp3-rich structures, such as alkyl–aryl CAC structures or methyl carbon dangling to aromatic ring, and can be considered as a brief measure of

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Fig. 7. The Raman fitted spectra of (a) SL-D and (b) SL-HCl.

Table 7 Spectroscopic parameter obtained from Raman spectra. Samples

ID/IG

La (nm)

IS/IG

ID/I(GR+VL+VR)

SL-D SL-HCl

1.46 1.59

3.39 3.12

0.32 0.54

2.35 1.81

cross-linking density and substitutional groups. One of the most important Raman parameters reported for the organizationdegree study of the carbon materials is the ID/IG ratio. Therefore, the ratio (the ID/IG ratio) between the D band and G band (peak areas) is related to the concentrations of aromatic rings having six or more fused benzene rings. The ratio of the band areas between the D and the combined GR, VL, and VR bands can be taken as a brief measure of the ratio between large and small aromatic ring systems. The ratio (the IS/IG ratio) between the S band and G

band (peak areas) indicates the amount of the cross-linking density or substitutional groups [30,48]. The average layer size La can be calculated by the integrated intensities of the D and G bands on the Raman spectra by using Eqs. (9) and (10) [29].

La ¼ CðkL Þ½ID =IG 1

ð9Þ

CðkL Þ ¼ C 0 þ kL C 1

ð10Þ

where C(kL) is the wavelength pre-factor, ID and IG are, respectively, the area of the D and G bands, C0 and C1 are 12.6 nm and 0.033, respectively. The value of kL is set as 532 nm; therefore, the C(kL) value was 4.956 nm. The related results are listed in Table 7. Clearly, the La value of each coal sample derived from the Raman spectra using Eq. (9) is higher than that obtained from XRD. This might be attributed to the overlap of the D and G bands due to the high

Fig. 8. Combustion reaction curves of lignite samples.

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Table 8 Combustion reaction parameters of lignite samples. Samples SL-D SL-HCl SL-HCl-Al SL-HCl-Mn SL-HCl-Na SL-HCl-Ca SL-HCl-K SL-HCl-Fe

Ti (°C)

Tp (°C)

Rw (S1 °C

345.54 386.37 400.98 343.84 341.80 327.38 325.29 293.67

384.04 436.14 452.84 373.23 389.81 423.84 370.34 310.31

1.506 0.868 0.749 1.702 1.737 1.232 1.813 2.002

2

)

E (kJ mol1)

A

R2

86.73 110.57 126.99 71.90 101.16 69.59 84.96 62.57

4.77  105 1.15  107 1.56  108 3.42  104 7.42  106 1.49  104 4.94  105 1.43  104

0.9825 0.9997 0.9999 0.9365 0.9980 0.9966 0.9963 0.9844

Fig. 9. Linear fitting of ln[ln(1  x)/T2]  1/T of lignite samples.

degree of disordered carbons in the two samples. The ID/IG ratio of demineralized lignite sample increased and the related average layer size La decreased, indicating that HCl treatment resulted in a decrease in the aromatic layer size of coal, which was consistent with the results of XRD. The improvement in IS/IG and the reduction in ID/I(GR+VL+VR) demonstrated the increase in side chain and the aromatic ring systems typically found in amorphous carbon. 3.6. Thermogravimetric analysis of samples TGA has been used to study the combustion and kinetic behavior of various coal samples. It is commonly performed by isothermal or non-isothermal procedure to understand the degradation behavior of coal and to estimate the kinetic parameters of char combustion or gasification. In this study, the burning characteristics of lignite were studied by non-isothermal combustion. Fig. 8 shows the comparison of non-isothermal combustion reactivity of lignite samples. The combustion stability index of lignite samples are calculated by using Eq. (11) [40], according to the measurements of pure carbon powder as follows:

Rw ¼

655 763 ðdx=dtÞmax ðdx=dtÞmax   ¼ 8:5875  107  Ti Tp 0:00582 TiTp

ð11Þ

where 655 represents the ignition temperature of carbon powder (°C); 763 represents the temperature corresponding to the highest burning rate of carbon powder (°C); 0.00582 represents the maximum weight loss rate of carbon powder (S1); Ti represents the ignition temperature of lignite samples, defined as the temperature corresponding to 5% weight loss on the TG curve (°C); Tp represents the temperature corresponding to the maximum weight loss rate of lignite samples (°C); and (dw/dt)max represents the maximum weight loss rate of lignite samples (S1). The higher value of Rw indicated more stability of combustion reaction. Table 8 lists the related

parameters of lignite samples. The Ti and Tp of SL-HCl increased, respectively, by 40.83 and 52.10 °C compared to those of SL-D. The Rw decreased from 1.506 to 0.868 S1 °C2, which indicated that demineralization adversely affected the combustion properties of SL. Various metal cations exhibited different effects on combustion reaction parameters of coal samples. Compared to SL-HCl, Ti and Tp of the metal loaded demineralized lignite samples other than SLHCl-Al decreased in different degree or became even lower than those of SL-D. Addition of Al led to a decrease in the value of Rw of SL-HCl; however, the metals cations, Na+, K+, Ca2+, Mn2+ and Fe3+, led to an increase in the Rw value of SL-HCl. The combustion reaction kinetics parameters are the essential basis data for evaluating the combustion properties of lignite. Therefore, herein, non-isothermal TG data was correlated with the kinetic parameters. Zhu et al. [49] reported that coal combustion reaction involve several steps as follows: mass transfer (by diffusion) of oxygen from the bulk gas phase to the sample surface and the gaseous products from the sample surface to the bulk gas phase; adsorption of oxygen on and desorption of the gaseous products from the sample surface; and rearrangements of the adsorbed surface species (surface reactions). Temperature has profound effects on the rate of chemical reaction and diffusion processes; therefore, different controlling mechanisms should be considered in different temperature region during the reaction of coal sample [41,49–51]. In low temperature region, chemical reaction is dominant in determining the reaction rate, thus the determined kinetic data represent the intrinsic kinetics. However, in the region with rather higher temperatures, the effect from heat and mass transfer including gas diffusion become dominant, thus leading to reduction in the estimated kinetic parameters [41]. In this study, the linear fitting of ln[ln(1  x)/T2]  1/T of the lignite samples corresponding to the conversion of 10–85% (obtained from TGA curves) was calculated by using Eq. (4) (Fig. 9). To enhance the clarity of the plot, the original lines were not included. The temperature range corresponding to the rectilinear region was approximately 308.63 and 505.87 °C (1/T = 1.28–1.72  103). The received activity energy was an apparent value including the considerable effect of transfer and diffusion. Table 8 lists the combustion kinetics parameters of lignite samples. Clearly, the fitting coefficient changes in the range 0.9396–0.9999. HCl treatment led to an increase in the apparent activation energy of SL by 27.49%, which indicated that the effect of demineralization on the combustion properties of SL was neglected. Addition of metal ions to SL-HCl exhibited different effects on its apparent activation energy, Al increasing the apparent activation energy and the other ions having opposite effects.

4. Conclusion The effects of hydrochloric acid treatment on the structure and combustion properties of Shengli lignite were comprehensively investigated. (1) The study showed that the carbon and oxygen mainly existed in the forms CAC/CAH and CAOA, respectively. The hydrochloric acid treatment could not change the surface morphology of Shengli lignite and the forms of carbon and oxygen on the surface structure. Further, demineralization improved the amount of side chains and aromatic ring systems typically found in amorphous carbon; however, destroyed the pore structure, crystalline order, and the aromatic layer size of Shengli lignite. (2) The combustion reaction results indicated that hydrochloric acid treatment destroyed the combustion stability and combustion reactivity of Shengli lignite. The metal ions

Y. Song et al. / Fuel 183 (2016) 659–667

demineralized from SL-D had different effects on the combustion reaction. Aluminum led to a decrease in the combustion stability and combustion reactivity of the hydrochloric acid treated sample; however, the metals cations, Na+, K+, Ca2+, Mn2+ and Fe3+, could improve these characteristics, in particular, Fe3+.

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