Catalytic effect of sodium components on the microstructure and steam gasification of demineralized Shengli lignite char

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Catalytic effect of sodium components on the microstructure and steam gasification of demineralized Shengli lignite char Rongrong Lu, Jie Wang, Quansheng Liu**, Yan Wang, Gusi Te, Yanpeng Ban, Na Li, Xiaorong Zhang, Runxia He, Huacong Zhou, Keduan Zhi* College of Chemical Engineering, Inner Mongolia University of Technology, Inner Mongolia Key Laboratory of HighValue Functional Utilization of Low Rank Carbon Resource, Hohhot 010051, Inner Mongolia, China

article info

abstract

Article history:

The catalytic effect of sodium on the demineralized Shengli (SLþ) lignite char micro-

Received 6 November 2016

structure and the performance of steam gasification were studied. Various sodium com-

Received in revised form

pounds including NaNO3, CH3COONa, Na2CO3 and NaOH were loaded on the demineralized

6 January 2017

coal samples, respectively, and the steam gasification was tested on the fix-bed reactor.

Accepted 11 January 2017

The char samples were characterized by X-ray diffraction (XRD), Raman, X-ray photo-

Available online 6 February 2017

electron spectroscopy (XPS) and FT-IR spectroscopy. Experimental results showed that sodium hydroxide loaded samples exhibited the highest gasification reactivity among the

Keywords:

coal samples prepared. With the increase of the alkalinity of sodium compounds, the

SLþ char

carbon crystallite structure tended to be disordered. In the process of pyrolysis, the

Sodium ion

introduction of sodium species promoted the ring-opening and polycondensation process

Steam gasification

of the chemicals in the coal samples. The possible reaction mechanism might be inferred

Carbon crystallite structure

that the sodium ions may replace the hydrogen ions in the oxygen-containing functional groups to form sodium phenolate intermediate, which may be critical for the catalytic effect of sodium species during gasification. It was speculated that the ring-opening of the condensation aromatic nucleus was the rate-limiting step in the whole process of gasification. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Low-rank coals, an abundant fossil resource on earth, become more and more important especially with the fast depletion of the high-rank coals. Unfortunately, low-rank coals suffer several disadvantages, such as high ash yield, high moisture

content, and low calorific value. Therefore, utilizing the lowrank coals in clean and high value approaches is important to improve the utilization efficiency of this carbon resource. Gasification of low-rank coals has been considered as a promising technology in terms of its efficiency and clean utilization [1]. Catalytic gasification is a potential way due to its

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Liu), [email protected] (K. Zhi). http://dx.doi.org/10.1016/j.ijhydene.2017.01.077 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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salient advantages of low operating temperature, low cost, high efficiency of energy conversion, and selective reaction pathways towards production of desired gases [2]. Up to now, extensive studies on the catalytic coal gasification with alkali and alkaline earth metals (AAEMs) [3e7] were conducted, because of their superior catalytic activity, low cost, and inherent presence in coals [8]. Although the inorganic matters take a very small proportion in the coal, they usually play a significant catalytic role in the gasification of the low-rank coals or their char. It has been proved that the AAEMs have obvious catalytic action in gasification [9e12,4,5]. Among various AAEMs, K, Ca, and Na are considered to be the most effective. Extensive studies have been reported in this field. Karimi and Gray [13] revealed that K2CO3 had a better catalytic activity than that of Na2CO3 for the steam gasification of bitumen coke. Zhang et al. [14] studied the steam gasification of tobacco stalk sample with potassium catalysts and found the addition of K2CO3 increased hydrogen yield and carbon conversion. Murakami et al. [15] investigated the catalytic effect of Ca catalysts prepared from CaCO3 on the steam gasification of Indonesian subbituminous coal. The results confirmed that CaCO3 is effective as a catalyst raw material in the steam gasification even at low catalyst loadings. Tang et al. [16] investigated the catalytic steam gasification of coal char with alkali carbonates and their synergic effects with calcium hydroxide. They found the addition of Ca(OH)2 to the coal substantially increases the rate of char gasification for alkali carbonates. Kopyscinski et al. [17] confirmed that K2CO3 could increase the steam gasification rate, and decrease the activation energy during the steam gasification of the ash-free coal. Kong YJ et al. [18] found K2CO3 with ash-free coals can significantly enhance the gasification rate and H2/CO ratio. Quyn et al. [19] studied the effect of different chemical forms of Na on catalytic gasification of char. Suzuki et al. [20] found that K2CO3, KCl, and Li2CO3 are the most effective additives for steam gasification of coal or char, and the presence of alkali additives allowed the catalytic gasification process to proceed at lower temperatures. What is well known is that some different calcium-bearing compounds, such as CaCO3, Ca(OH)2, Ca(COO)2, are catalytically active for lignite gasification [21e23]. The catalytic activity of minerals in char gasification was studied by adding catalysts in the preparation stage. The results demonstrated that the gasification rates were improved, but the mechanism of the catalytic behavior of the inherent inorganic matters was still unclear. Therefore, in this study, the catalytic effect of Nacontaining compounds on the microstructure and steam gasification of demineralized Shengli lignite char was investigated in detail. The possible mechanism of the catalytic effect of the sodium species was proposed.

sieved to the particle sizes between 0.038 mm and 0.075 mm. The SL-Raw samples (0.038e0.075 mm) was mixed with HCl (18.5%) with a coal 1 g:10 ml, the mixture was maintained for 24 h. Then the coal samples were washed with deionized water to neutrality to obtain demineralized coal samples (SLþ). NaOH, Na2CO3, CH3COONa and NaNO3 were added, respectively, in 5% proportion of the mass of SLþ samples by using the impregnation method. The suspension was stirred at 120 rpm using a motor stirrer at room temperature for 4 h. Then, the slurry evaporated in a vacuum oven at 105  C without filtration until the sample was thoroughly desiccated. These samples was abbreviated as SLþeNaOH, SLþeNa2CO3, SLþeCH3COONa, and SLþeNaNO3, respectively. The samples were ground into powder (200e400 mesh) and analyzed for moisture, ash and elemental composition. The ultimate analyzes of the samples are summarized in Table 1. The metal ions in and their contents in the lignite samples were determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Optima 7000, PerkinElmer) on the basis of coal (Table 2).

Char gasification The char gasification was performed in the fixed-bed gasification reactor. And the experimental process was as follows: a 0.15 g sample of coal was loaded into the reactor. Argon was used as a carrier gas, and the system pressure was maintained at 0.15 MPa. The reactor was heated from room temperature to 500  C at 15  C$min1, then up to 900  C at 2  C$min1. The deionized water was introduced into the vaporizer, and the vaporizer temperature was raised up 300  C previously. The tar and steam in the carrier gas was separated in a cold trap, and finally the gas entered into the gas chromatograph (SP3420). The major gaseous products H2, CO, CH4 and CO2, were quantitatively determined online using a rapid gas chromatograph (TCD), and sampling time interval for 4 min. The reaction was stopped when the content of resulting syngas (H2, CO, CH4 and CO2) at the largest relative rate was not more than 0.2%.

Analysis methods X-ray diffraction analysis The XRD profiles of the samples were measured using an Xray diffractometer (Bruker D8) with Cu-Ka radiation (40 kV, 40 mA) by a step-scanning method over the angular range of 5 e70 (2q). The broad hump in this region was fitted to two

Table 1 e Ultimate analyzes of the coals. Samples

Experimental Preparation of coal samples The used lignite was collected from Shengli coalfield in Inner Mongolia of China, which was abbreviated as SL-Raw. The coal sample was dried at 105  C for 4 h, and then ground and

þ

SL SLþeNaNO3 SLþeCH3COONa SLþeNa2CO3 SLþeNaOH

C

H

N

S

Oa

61.42 56.56 56.80 56.27 53.54

3.34 3.31 3.49 3.11 3.03

0.86 2.48 0.83 0.91 1.07

1.74 1.11 1.18 1.52 1.08

25.10 23.56 22.21 21.73 22.76

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

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FT,i,j represents the cumulative yield of the syngas at j C (j ¼ 900).

Table 2 e Metal ion percentage in coal samples (wt %). Element

Coal based

Coal sample

Al3þ

Naþ

Ca2þ

Si4þ

Fenþ

Kþ

Ni3þ

SL-Raw SL-HCl

4.53 1.70

0.78 0.00

0.57 0.01

3.17 2.94

0.15 0.04

0.10 0.04

0.08 0.00

Gaussian peaks around 15 and 35 , representing g-band and P-band respectively. Interplanar spacing (d002) of the crystallite was calculated from the peak position by means of Bragg's equation. The lateral size (La) and the stacking height (Lc) of the crystallite were determined using Scherrer's equations. The average number of stacking (N) estimated from d002 and Lc by means of the following equation N ¼ Lc =d002 [24].

Raman spectroscopy Raman spectroscopy with a 532 nm laser was used to investigate structural changes of chars. A laser power of 5 mW was selected. In this study the Raman spectra of chars over the range of 500e3500 cm1 were curve-fitted with 5 Gaussian bands representing major structures in the chars. The methodology of curve-fitted was given in detail elsewhere [24].

X-ray photoelectron spectroscopy analysis XPS spectra of PE in the U.S. company is done on PHI-5400 spectrometer, scanning area of 300  300 um, vacuum degree 3  107 Pa, with C1s do internal standard calibration (284.6 eV). In the XPS spectra, the abscissa represents the electron Binding energy (Binding energy, B.E), y coordinate for electronic counting. According to the wide range scanning figure, using area of sensitivity factor can be calculated quantitatively the surface atoms of the sample. Narrow sweep C1s and O1s spectra using XPSPEAK professional package peak fitting, for quantitative analysis of the existence of different atomic state.

 PT ¼ FT;H2 ;j FT;Co;j

Results and discussion X-ray diffraction analysis X-ray diffraction (XRD) study has played a significant role in the coal science. So XRD analysis becomes a widely and basically method to evaluate carbon-stacking structure. The XRD profiles of char samples were shown in Fig. 1. The (002) band between 22 and 35 reflected the aromatic ring stacking. The (g) band on the left of (002) band around 22 were mainly attributed to the presence of the aliphatic chains [26]. In addition, the (100) band belongs to the region of 40 e50 . The structures of SLþeNa2CO3 samples were analyzed in Fig. 2. The structural parameters such as interlayer spacing (d002), crystallite size (La, Lc), average number of aromatic layers (N) and aromaticity (fa) were listed in Table 3. The d002 values vary from 0.340 to 0.358 Å. This increased of char structural disorder simultaneously increased the d002 value. The crystallite sizes of char samples obtained by using the Scherrer equation. The aromaticity (fa) ratio for the samples ranged from 0.656 to 0.477 (Table 3). These changes indicated that different Na compounds added into coal samples limited the aromatic ring crystallite structure to grow during pyrolysis process in different extent. With the increase of the alkalinity of sodium compounds, the size of aromatic nucleus get smaller gradually, and the spacing of aromatic layers gets bigger. The results of XRD suggested that sodium compounds could effectively hinder the carbon crystallite structure ordering in the process of pyrolysis.

The FT-IR spectra of all samples were recorded over the range of 400e4000 cm1 on a FT-IR Spectrometer NEXUS670 using the KBr pellet technique. According to the proportion of 1:200 make the samples with KBr mixing and grinding, tabletting spectral scanning.

Gasification reaction rate The char gasification reaction rate is quantified by reactivity index (R0.5) [25], defined as R0:5 ¼ 0:5=t0:5 , where t0.5 is the gasification time (min) taken to reach a carbon conversion of 50%.

Date analysis In this study, carbon yield (X) was calculated using Eq. (1). Zj Fi;j dt 0

X¼

FT;Co2 ;j þ FT;CO;j þ FT;CH4 ;j  100% FT;Co2 ;900 þ FT;CO;900 þ FT;CH4 ;900

(1)

where i represent the syngas CO, CO2 and CH4, respectively;

(2)

where PT represent the cumulative H2/CO molar ratio.

FT-IR spectroscopy analysis

FT;i;j ¼

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Fig. 1 e XRD spectra of coal chars.

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Fig. 2 e Gauss fitting-curves of XRD spectra of SLþeNa2CO3.

Table 3 e Crystal structure parameters of the chars from the pyrolysis of acid washing coals added different sodium compounds at 500  C. Sample þ

SL SLþeNaNO3 SLþeCH3COONa SLþeNa2CO3 SLþeNaOH

d002 (Å)

La (Å)

Lc (Å)

N

fa

0.340 0.345 0.351 0.353 0.358

2.810 2.094 1.908 1.907 1.887

1.132 1.070 1.012 1.010 0.942

3 3 3 3 3

0.656 0.577 0.517 0.494 0.477

Raman characterization of the chars The Raman spectra of the coal chars in the range of 100e3500 cm1 were presented in Fig. 3. Two peaks with maximal intensity between 800 cm1 and 1800 cm1 were exhibited. In the earlier works [27e28] on carbon material, it was reported that Raman spectra of graphite and diamond

Fig. 3 e Raman spectra of coal chars.

have two typical Raman bands at 1580 cm1 and 1330 cm1, respectively. The work of curve-fitting mainly concentrated over the region of 800e1800 cm1. The curve-fitting analysis for SLþeNa2CO3 was shown as an example in Fig. 4. The two major bands, G and D1 bands, were presented, which is the same results with other works [29,30]. The D4 band at 1153e1174 cm1 mainly resulted from in the poorly organized materials. The D3 (1534e1540 cm1) band could mainly be attributed to 3e5 aromatic ring structures in coal char and sp2 heterocyclic amorphous carbon. The band at 1607e1620 cm1 was assigned to D2 band, resulted from a graphitic lattice mode E2g, but G band and D2 band, in some case, are difficult to be recognized [27]. Through the spectrum deconvolution with curve-fitting technique, the quantitative information about the evolution of different bands could be obtained. The band area ratio ID1/ IG, the ratio of the G band to the integrated area (IG/IAll) and microcrystalline plane size (La) [31] were calculated. The related parameters were shown in Table 4. As listed in Table 4, the ID1/IG of SLþ samples added different sodium compounds all increased in different extent, IG/IAll and La decreased during the pyrolysis process. The results indicated that the addition of sodium compounds make the defects increased, and the main structures of the char became disordered. Meanwhile, the proportion of aromatic ring structure in the coal chars reduced, especially for SLþeNaOH sample. These results were assistant with the analysis of XRD.

XPS measurements The form of C1s on the surface of coal chars were analyzed by XPS characterization. Fig. 5 shows typical C1s spectra for all the studied char samples. To get more detail information, the C1s spectra were deconvoluted into individual spectral lines, and fitted by using GausseLorentz. Table 5 summarized the XPS parameters of abovementioned species. And the SLþeNa2CO3-500J of deconvolution of the XPS spectra was given in Fig. 6 as an example. According to literature data [32,33], the peaks at 284.6 eV, 285.2 eV, 286.2 eV, 287.6 eV and 288.6 eV should be attributed to CeH or CeC, defects, CeO, C]

Fig. 4 e Curve-fitting analysis of SLþeNa2CO3-500J in the region of 1000e1800 cm¡1.

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Table 4 e Raman parameters of the chars from the pyrolysis of acid washing coals added different sodium compounds at 500  C. Samples

IG/IAll

ID1/IG

La (nm)

SLþ SLþeNaNO3 SLþeCH3COONa SLþeNa2CO3 SLþeNaOH

0.140 0.116 0.112 0.097 0.097

3.724 4.373 4.445 4.440 4.627

13.308 11.334 11.149 11.161 10.712

500J

+

SL + SL -NaNO3 +

SL -Na2CO3

Fig. 6 e The XPS C 1s spectra of SLþeNa2CO3-500J.

+

SL -CH3COONa +

SL -NaOH

292

290

288

286

284

Binding energy(BE)/ev

282

280

Fig. 5 e XPS C1s spectra of coal treatment 500J.

O and O]CeO, respectively. As shown in Table 5, the addition of different Na-containing compounds, the mass fraction of “CeC or CeH” reduced in different extent, especially SLþeNaOH-500J. The defects of samples of adding different Na-containing compounds significantly higher than SLþ, indicated the coal chars structure tend to be unstable. The proportion of “CeO” and “C]O” is relatively small, the difference is not great. The mass fraction of “COOe” increased, showed that the more alkaline warms up in the coal sample program is similar to the CoaleOeOeNa intermediates proportion increased [34].

FT-IR spectra of chars Fig. 7 illustrated the FT-IR spectra of the coal chars. The absorption bands in the spectra were assigned on the basis of a comparison with the standard patterns reported in the literature [35e38]. The broad absorption bands observed at

3429 cm1 in the coals were due to the eOH stretching vibration from residual water. The bands at 2920 cm1 and 2850 cm1 were due to aliphatic eCH, eCH2 and eCH3 stretching vibration, but the stretching vibration absorption bands almost disappeared after pyrolysis. Generally, most of the peaks below 1100 cm1 in FT-IR spectra of raw coal were assigned to clay minerals such as quartz, kaolinite, illite and montmorillonite groups [39e43]. The SLþ exhibited weak bands in the regions lower than 1000 cm1 due to the elution of the minerals during acid treatment. Adding different Nacontaining compounds to SLþ made the peaks below 1000 cm1 become stronger. The raw coals exhibit prominent bands at the 1038 cm1 and 770 cm1 region due to SieO bending vibration [38,39].

Reactivity of steam gasification Fig. 8 showed the carbon yields of syngas from the steam gasification of coal samples, which were calculated according to Eq. (1). The coal reactivity parameters and reactivity index were showed in Table 6 and Fig. 9, respectively. It was observed that the carbon yields of steam gasification of samples increased slowly with the increase of temperature before 650  C, and then increased noticeably after 650  C (as shown in Fig. 8), and increased gradually with a further increasing of temperatures. The carbon yields of SLþ samples were much lower than that of coal samples loaded different Nacontaining compounds. This suggested that loading sodium compounds dramatically increased the steam gasification efficiency of the SLþ.

Table 5 e The XPS C1s spectra data of the chars from the pyrolysis of SLþ added different sodium compounds at 500  C (at.%). Affiliation CeC/CeH Defects CeO C]O COOe

Position

SLþ

SLþeNaNO3

SLþeCH3COONa

SLþeNa2CO3

SLþeNaOH

284.6 285.2 286.2 287.6 288.6

79.14 4.78 8.20 3.23 4.66

69.68 9.70 9.55 5.28 5.79

75.12 9.91 6.46 0.49 8.02

77.22 10.44 2.31 1.82 8.22

52.98 10.99 3.56 3.09 29.38

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3429

2925

2850 2374

+

SL -NaOH

2068 1600 1348 1380

770

0.005 617

+

Reactivity index (min-1)

SL -Na2CO3

T/%

+

SL -CH3COONa +

SL -NaNO3 +

SL

1700

1107 1038

0.004 0.003 0.002 0.001 0.000

Fig. 7 e FT-IR spectra of SL coal adding different sodium compounds coal samples treatment at 500  C.

+

-N

aO

H

O 3 a 2C +

-N

SL

þ

SL

3

+

-C

H

C

+

Sample

SL

O

O

aN

N

O 3

500

-N

1000

SL

3000 2500 2000 1500 Wavenumber/cm-1

a

+

3500

SL

4000

Fig. 9 e Reactivity index from steam gasification of coal samples.

NaNO3 is relatively poor, which is consistent with the literature [44] findings.

Hydrogen yield and cumulative H2/CO molar ratio during steam gasification

Fig. 8 e Carbon yields of syngas from steam gasification of coal samples.

As shown in Table 6 and Fig. 9, loading different sodium compounds to coal samples could speed up the gasification reaction rate. The reactivity index of SLþ, SLþeNaNO3, SLþeCH3COONa, SLþeNa2CO3 and SLþeNaOH were 0.002659, 0.003571, 0.004273, 0.004504, and 0.004545 min1, respectively. Therefore, the catalytic activity of the steam gasification followed SLþeNaOH > SLþeNa2CO3 > SLþeNaAc > SLþeNaNO3 > SLþ. So NaOH had the best catalytic effect on SLþ. The dispersion of

Fig. 10 showed the hydrogen yield and cumulative H2/CO molar ratio in the syngas from the steam gasification of samples. The hydrogen yield from SLþ steam gasification was much lower than those from the steam gasification of coal samples were loaded Na-containing compounds. This clearly reflected that the acid treatment markedly reduced the hydrogen formation during the steam gasification of samples. After adding sodium compounds, the hydrogen yield and cumulative H2/CO molar ratio increased significantly, indicated that the sodium salts within the SLþ played critical roles in adjusting the hydrogen production in the steam gasification.

Gasification kinetics Coal char and steam gasification reaction belongs to the typical gasesolid heterogeneous reaction. Several models are often used: volumetric model (VM), shrinking core model (SCM), and random pore model (RPM). The SCM assumes that gaseous reactants diffuse through a gas film surrounding the particle, then diffuse through the ash layer and react on the unreacted core surface. Commonly used expression model are as follows: 2 3

Table 6 e Reactivity parameters of coal samples under steam gasification. SLþ t0.5/ 188 min t50/ C 874

SLþ eNaNO3

SLþ eCH3COONa

SLþ eNa2CO3

SLþ eNaOH

140

117

111

110

780

737

720

719

=

dx=dt ¼ kfðxÞ ¼ 3A expðE=RTÞð1  xÞ

(3)

and order GðxÞ ¼ ln kE=RT þ ln A

(4)

where x is carbon conversion, t is the time, k is the rate constant depending on the temperature, A and E are the preexponential factor and the activation energy, respectively, and T is the absolute temperature.

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Fig. 10 e The gas production from steam gasification of samples: (a) hydrogen yield and (b) cumulative H2/CO molar ratio. treatment and adding different sodium compounds, the activation energy of the samples was different. And activation energy has a close relation with the chemical reaction rate, the lower the activation energy, faster the reaction rate. This conclusion was consistent with the reactivity index results in Fig. 9.

Mechanisms of steam gasification

Fig. 11 e F(x)e1/T curves from TPSG of samples.

Table 7 e Kinetic parameters of gasification reaction of samples. Samples SLþeNaOH SLþeNa2CO3 SLþeCH3COONa SLþeNaNO3 SLþ

T/K

E/kJ mol1

851e881 847e877 851e881 888e930 995e1024

70.87 87.92 102.84 108.39 117.74

A/min1 1.21  1.68  9.98  1.29  6.74 

103 104 104 105 104

Fig. 11 showed the kinetic analysis of coal samples under the condition of heating steam gasification reaction shrinking core model of F(x)e1/T curve, and the kinetic parameters were shown in Table 7. As can be seen from Table 7, after acid

Based on the reported literatures and our results, possible mechanism on the catalytic effect of sodium was proposed. SLþ was loaded different sodium compounds, the carboxylic acid on the benzene ring was activated by sodium ions, and the hydrogen ions were replaced by sodium ions, and then formed the carboxyl salt. With release of a carbon monoxide molecular, the carboxyl salt structure translated into sodium phenolates. The process of sodium phenolates might open loop formed the quinones, which is considered to be the steam gasification ratecontrolling step. The macromolecules in the coal occurred condensation polymerization and depolymerization reaction, leading to rearrangement of the quinone compounds at 600e800  C. Finally quinone pyrolysis and open loop then formed into aliphatic hydrocarbons, of which took part in the seam gasification directly. Additionally, the carbon microcrystalline structures were formed by polycondensation, and then continue to join the steam gasification. According to the characterization of experimental analysis, and the SL-Raw main body structure and its possible mechanism of steam gasification reaction were inferred, as shown in Fig. 12.

Fig. 12 e The possible mechanism of steam gasification reaction.

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Conclusions The temperature-programmed steam gasification of the SLþ sample with different sodium compounds was conducted in a fixed-bed reactor. The main conclusions could be summarized as follows: (1) The steam gasification activity of sodium loaded coal samples is superior to SLþ. The active sequence is: SLþeNaOH > SLþeNa2CO3 > SLþeCH3COONa > SLþeNaNO3 > SLþ. The steam gasification activity depended on the interaction of sodium ions and alkaline environment, the stronger the reagent alkaline, the higher activity was. (2) The related parameters in the XRD showed that introduction of sodium compounds limited the aromatic ring crystallite structure to grow in different extent. With the alkaline increase of sodium compounds, the aromatic layer transverse diameter decreases, and the carbon crystallite structure towards disordered. The Raman spectroscopic data also indicated that the addition of sodium compounds make the defects increased, the main structure became disordered, the proportion of aromatic ring structure in the chars reduced, especially for SLþeNaOH. (3) After adding different alkaline sodium compounds to SLþ, the hydrogen yield and H2/CO molar ratio increased significantly, which indicated the addition of different alkaline sodium compounds prompted the hydrogen production during steam gasification process. (4) The kinetic analysis using shrinking core model indicated that the activation energy of sodium loaded samples was smaller than that of the SLþ coal.

Acknowledgments This study was financially supported by the National Natural Science Foundation of China (21676149, 21606134, 21566029, 21566028 and 21266017), the Major Basic Research Projects of Inner Mongolia (2014MS0220), the Major Basic Open Research Projects of Inner Mongolia (2015BS0206), the Natural Science Foundation of Inner Mongolia.

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