Fuel 207 (2017) 365–372

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Fuel

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Full Length Article

Interactions between molten salts and ash components duringZhundong coal gasification in eutectic carbonates

http://dx.doi.org/10.1016/j.fuel.2017.06.0790016-2361/� 2017 Published by Elsevier Ltd.

⇑ Corresponding author.E-mail address: hongyunhu@hust.edu.cn (H. Hu).

Junhao Shen a, Hongyun Hu a,⇑, Mian Xu a, Huan Liu a, Kai Xu a, Xiuju Zhang a, Hong Yao a, Ichiro Naruse b

a State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, ChinabDepartment of Mechanical Science & Engineering, Nagoya University, Nagoya 464-8603, Japan

h i g h l i g h t s

� The distribution patterns of mineral matter in Zhundong coals were characterized.� The fate of Zhundong coal ashes was studied in the high-temperature molten salts.� Alkali/alkaline earth metals were predominantly dissolved in the molten salts.� Si/Al was separated from the molten salts including formed Li/K-aluminosilicates.� The thermal properties of the molten salts were inevitably affected by coal ashes.

a r t i c l e i n f o

Article history:Received 14 March 2017Received in revised form 17 June 2017Accepted 19 June 2017Available online xxxx

Keywords:Molten salt gasificationZhundong coalAsh formingMineral matterThermal property

a b s t r a c t

Molten salt gasification provides a promising way for the combined usage of solar energy and coal, duringwhich molten salts act as a heat storage and transfer medium. However, the interactions between coalash components and molten salts might have undesired effects on the molten salt gasification. The pre-sent study investigated the distribution of mineral matters in typical Zhundong coals and the fate of coalashes in the high-temperature Li2CO3-Na2CO3-K2CO3 eutectic system. The results showed that alkali andalkaline earth metals (AAEMs) were widely distributed in Zhundong coals in the forms of NaCl, CaCO3,CaSO4 and organic matter, which were predominantly dissolved in the molten salts. After the dissolutionof these AAEMs, the melting temperature and enthalpy of the molten salts were changed, while the vis-cosity of the mixtures was hardly affected in melting phase. On the other hand, Si/Al-compounds werealso of high content in some Zhundong coals, existing as quartz, kaolin and amorphous species. Someof these compounds tended to react with molten salts, by forming Li-silicate or Li/K-aluminosilicates,which were precipitated out of the system. Meanwhile, the suspension of the unreacted Si/Al-compounds increased the viscosity of molten salts. Additionally, interactions between coal ashes andmolten salts brought about the decrease in thermal diffusivity and thermal conductivity of the eutecticsystem as well as the increase in specific heat capacity.

� 2017 Published by Elsevier Ltd.

1. Introduction

In order to meet the large demand of energy and to solve thesevere environmental pollution problems, it is encouraged to userenewable energy and to take advantage of traditional energy ina clean way [1,2]. Based on this idea, molten salt gasification offersa promising way for the combined usage of solar energy and solidfuels [3]. In the process, molten salts act as heat storage mediumfor concentrated solar energy, because of their high thermal stabil-ity and high heat capacity [4]. On the other hand, molten salts were

used as catalytic medium and heat carrier for the gasification ofsolid fuels [5,6], meanwhile as in-situ capture agent of pollutinggases like HCl and H2S [7].

Located in northwest China, Xinjiang province is rich in bothcoal and solar energy resources [8,9]. Solar energy therein is cur-rently not exploited much due to its instability and intermittence[10]. And most of the coal in Xinjiang province (like Zhundongcoals) contains a high concentration of alkali and alkaline earthmetals (AAEMs), which causes serious fouling and slagging prob-lems in the traditional boiler [11,12]. Molten salt gasification tech-nology, of tremendous benefit to the storage of solar energy inconcentrated mode and the conversion of solar energy into chem-ical energy, could well solve the problems mentioned above [5,13].

366 J. Shen et al. / Fuel 207 (2017) 365–372

Simultaneously, AAEMs in coal could be stabilized in the moltensalts and partly substituted for raw molten salts. However, moltensalts could react not only with carbonaceous substances in thegasification process, but also with ash compounds [14]. The inter-actions between the molten salts and the ash components mightchange the properties of the molten salts, thus in turn affectingthe carbon conversion. Therefore, understanding the behavior ofash compounds in high-temperature molten salts is essential forusing Zhundong coals through molten salt gasification.

Apart from AAEMs, Zhundong coal ashes mainly consisted of Si,Al, and certain amount of Fe, S and Cl [15]. The characteristics ofthe ash varied among different mining areas [16]. And the distribu-tion patterns of mineral matter determined the ash forming behav-ior as well as the ash thermal behavior in high-temperature moltensalts [16,17]. Martín-Aranda et al. [14] investigated the fate of ashcomponents in the LiCl/KCl eutectic mixture, at 873 K with a heat-ing rate of 15 �C/min. It was found that [14] some minerals weredissolved into the molten salts while inert components were sep-arated from the molten salts and remained unreacted. The dissolv-ing of the ash compounds changed the composition of molten salts,affecting its melting temperature, viscosity and other properties[18,19]. Moreover, inert ash matrix in the fine scale could be sus-pended in the molten salts, which might play the similar role asnano particles in the nanofluid, by increasing the specific heatcapacity and thermal conductivity of the molten eutectic [20]. Usu-ally, alkali metals molten carbonates were widely used in moltengasification/pyrolysis of coal, biomass and waste solids at temper-atures ranging from 500 �C to 900 �C, which could be defined as thefollowing Eqs. (1)–(3) [3,5,6,21,22]. However, few researches havebeen reported to clarify the mechanisms regarding the interactionsbetween molten salts and fuel ashes.

M2CO3 þ 2C ¼ 2Mþ 3CO ð1Þ

2Mþ CO2 ¼ M2Oþ CO ð2Þ

M2Oþ CO2 ¼ M2CO3 M ¼ Li;Na and K ð3ÞThe present study aims to investigate the behavior of Zhundong

coal ashes in the ternary eutectic (Li2CO3-Na2CO3-K2CO3). More-over, the distribution patterns of the mineral matter in the coalsamples were observed through various ash forming processes.On this basis, several typical ash components were chosen to reactwith the molten salts to further understand the fate of Zhundongcoal ashes in the eutectic carbonates. Changes in the propertiesof the molten salts were also investigated after reacting with thecoal ashes or typical ash components.

2. Experimental

2.1. Materials

Six kinds of Zhundong coals were used in the present study,labelled as Coal 1# to 6#. The samples were ground and sievedto the desirable particle size (45–106 lm). Various ash sampleswere prepared in a muffle furnace by heating the coals, at 400 �Cwith a heating rate of 10 �C/min (denoted as LTA), at 815 �C witha slow heating rate of 15 �C/min (denoted as SHTA), or at 815 �Cwith a rapid heating rate (denoted as RHTA), respectively.

For the preparation of the molten salts, analytical grade reac-tants, including Li2CO3, Na2CO3 and K2CO3, were firstly physicallymixed at a mass ratio of 32.1:33.4:34.5. Then, the mixtures wereput in a corundum crucible and heated at 800 �C to form a singlephase liquid. After maintaining for 6 h, the molten salts werecooled to room temperature and finely ground in an agate mortar.

Other reactants used in the experiments, like CaCO3, CaSO4, NaCl,quartz and kaolin, are of analytical grade.

2.2. Experimental procedures

Fig. SM-1 shows the schematic of the interactions between mol-ten salts and coal ashes. Before heating process, the ash sampleswere thoroughly mixed with the molten salts at a mass ratio 1:9.Then the mixtures were heated in a muffle furnace from room tem-perature to 800 �C, with a heating rate of 10 �C/min. After reactionfor 2 h, the products were cooled to room temperature with a cool-ing rate of about 20 �C/min and collected. Typically, the productsmainly consisted of two different layers. Part of the products wereprecipitated and separated from the molten salts, forming thelower-layer. The rest was remained in the liquid phase and formedthe upper-layer product during the cooling process. Subsequently,products in various layers were separated and characterized.

In order to further illustrate the interactions between moltensalts and Zhundong coal ashes, the main components in the coalashes, like CaCO3, CaSO4, NaCl, quartz and kaolin, were chosen toreact with molten salts in the same way as coal ashes, and theproducts were collected and analyzed.

2.3. Analytical methods

To measure the distribution of elements in the products formedduring the interactions between coal ashes and molten salts, theupper layer was dissolved by diluted HNO3 and the lower layerwas washed by deionized water. Subsequently, the residues fromlower layer were centrifugally separated and dried in the oven.The concentration of the metals in the solution was determinedusing microwave plasma atomic emission spectrometer (MP-AES). And ion chromatography (IC) was applied to test the contentsof Cl� and SO4

2�. The characteristics of the residues were analyzedby X-ray powder diffraction (XRD) and X-ray fluorescence (XRF).

The molten salts, after reacting with typical ash components,were firstly cooled to room temperature with a cooling rate ofabout 20 �C/min. Then, the samples were ground into powderand dried at 105 �C for 6 h. The properties of the samples wereobserved by XRD and high-temperature Raman. The reacted mix-tures were divided into two parts (the upper layer and the lowerlayer), the elements contents in which were detected by XRF. Inaddition, differential scanning calorimetry (DSC) was used todetermine the melting temperature and melting enthalpy. The vis-cosity of molten salts was tested by high temperature rotationalviscometer. Moreover, thermal properties of molten salts, includ-ing thermal diffusivity, thermal conductivity and specific heatcapacity, were measured by the application of laser flash thermalanalyzer.

3. Results and discussion

3.1. The distribution patterns of mineral matter in Zhundong coalashes

Fig. 1 presents the ash yields of six kinds of Zhundong coals. Theresults indicated that the ash yields of Zhundong coals were below10 %, except for Coal 4#. Compared with the ash formation at hightemperatures, more ashes were obtained at 400 �C with a slow oxi-dation rate. Generally, the distribution patterns of mineral matterdetermined the ash forming process and low-temperature ashforming process had minimal alteration of the mineral species[17]. The differences of ash yields in three ash forming processesshowed various trends in different coal samples, suggesting thedifferent distribution patterns of mineral matter in each coal.

0

2

4

6

8

10

12

14

16 LTA SHTA RHTA

Coal 2#

Ash

yie

ld (

wt%

)

Coal 1# Coal 3# Coal 4# Coal 5# Coal 6#

Fig. 1. The ash yields of Zhundong coals formed in various ways (LTA, low temperature ash; SHTA, high temperature ash with slow heating rate; RHTA, high temperature ashwith rapid heating rate).

J. Shen et al. / Fuel 207 (2017) 365–372 367

The main compositions of each ash sample were shown inTable 1. Sodium was of high content in the low-temperature ashesof Coal 1#, 3# and 5#. According to the XRD patterns shown inFig. 2, sodium was mainly in the form of chloride in Coal 1# whileprobably in the organic forms in Coal 3# and 5#. And most ofsodium was released in the high-temperature ash forming process[16], resulting in the decrease of the ash yields. In the presence ofsome aluminosilicate compounds, sodium could be partly stabi-lized in the ash residues in the form of thermally stable Na-aluminosilicates [17,23].

CaCO3 was widely observed in the ash samples, either from thecalcite in the coal or the carbonation of organic-calcium [24]. Thedecomposition of CaCO3 (shown in Fig. 2 (b) and (e)) might slightlydecrease the ash yields. Besides, the ash yields were affected by thesulfation and/or silication of CaCO3. According to XRD patterns, fewpeaks of CaSO4 were found in ash samples except for coal 5#,demonstrating that gypsum was not the main initial mineral mat-

Table 1Main compositions of Zhundong coal ashes (wt%).

Coal Ashes SiO2 Al2O3 SO3 Fe2O3

Coal 1# LTAa 11.17 7.50 4.17 3.61SHTAb 17.60 12.47 4.47 5.82RHTAc 19.56 12.99 3.27 5.65

Coal 2# LTA 31.53 8.29 6.48 4.04SHTA 31.11 7.88 10.08 3.69RHTA 33.56 9.10 8.47 3.47

Coal 3# LTA 11.33 5.25 21.38 4.56SHTA 18.43 10.47 17.99 5.09RHTA 16.78 9.78 17.52 5.83

Coal 4# LTA 51.96 16.90 8.02 3.08SHTA 56.14 18.05 7.63 2.97RHTA 56.99 17.93 5.94 3.05

Coal 5# LTA 9.35 7.72 26.67 2.78SHTA 10.56 9.33 28.25 3.35RHTA 10.24 9.28 26.58 3.34

Coal 6# LTA 53.57 22.39 6.99 4.44SHTA 57.82 24.11 1.95 3.84RHTA 56.90 23.85 1.82 4.10

a LTA, low temperature ashb SHTA, high temperature ash with slow heating ratec RHTA, high temperature ash with rapid heating rated –, undetected.

ter in most of Zhundong coals. CaSO4 in the high-temperature ashwas probably formed through the interactions between CaCO3 andSO2. The formation of SO2 was attributed to the decomposition oforganic-sulfur compounds or the oxidation of pyrite [25]. On theother hand, calcium could react with quartz to form Ca2SiO4 or Ca3-Fe2(SiO4)3 (Fig. 2 (a) and (c)). Apart from quartz, the widely dis-tributed silicon was in the form of kaolin (Fig. 2 (f)) and otherundetected forms like amorphous compounds [11]. As summarizedin Table 2, other mineral matters, including Fe2O3 and MgO, werepossibly distributed in Zhundong coals [8,11,16,17,25–32].

3.2. Interactions between coal ashes and molten salts

For a better understand of the interactions between moltensalts and Zhundong coal ashes, low-temperature ashes of threetypical coals (Coal 1#, 5# and 6#) were selected to react with mol-ten salts. Fig. 3 shows the high temperature Raman spectra of the

CaO K2O Cl Na2O MgO P2O5

31.09 0.10 13.68 24.09 3.31 –d

45.83 – 5.18 – 6.45 –43.81 0.17 5.74 – 6.81 –

45.71 0.09 – – 2.06 –43.28 0.16 – – 2.04 –41.20 0.13 – – 2.49 –

26.65 0.06 – 21.10 8.79 –28.92 0.32 – – 17.77 –32.01 0.48 – – 16.31 –

7.91 1.12 – 4.86 3.94 0.897.30 1.16 – – 4.55 0.827.95 1.21 – – 4.08 1.43

32.90 0.16 – 10.63 8.62 –36.67 0.23 – – 10.19 –38.14 0.30 – – 10.72 –

3.80 0.42 – – – 5.493.21 0.43 – – – 5.873.68 0.68 – – – 5.98

10 20 30 40 50 60 70 80 90

(a)

21

2

311

213

2

1

1 1,2113

2θ

SHTA

RHTA

LTA

5

1-CaCO3 2-NaCl 3-SiO2

4-MgO 5-Ca3Fe2(SiO4)3 6-Ca2SiO4

5

6

5

5,6

54,54

456

5

56

5

5,6

54,5

445

6

10 20 30 40 50 60 70 80 90

(b)

Inte

nsity

( A

.U. )

2θ

111111

21

2 2

1

22

2

12

SHTA

RHTA

LTA

2

2 2

4

2

34

34

23

1-CaCO3 2-SiO2 3-CaO4-CaSO4

24

4

2

34

3

4

2 3

10 20 30 40 50 60 70 80 90

RHTA

SHTA

LTA

2θ

22 222

1

2

2

1 2

33 444 344 4

4

4

4

1

(c) 1-SiO2 2-CaCO3 3-MgO4-CaSO4 5-Ca2SiO4 6-Fe2O3

45

43

5

4

3

3

4

6

4

10 20 30 40 50 60 70 80 90

Inte

nsity

( A

.U. )

2θ

1112 1 1

1

1,21,21

111111,21,2

1

1

SHTA

RHTA

LTA

113

3

11111,331,3

13

(d)

1

1-SiO2 2-CaCO3 3-CaSO4

111 11 1

1

11

1,333

10 20 30 40 50 60 70 80 90

221

21

2111

2

21

1

2

SHTA

RHTA

LTA

2222 22223 22

23

222 3

2

2

3

(e) 1-CaCO3 2-CaSO4 3-CaO

2 232 222 23 2

2

2

2

10 20 30 40 50 60 70 80 90

11122

Inte

nsity

( A

.U. )

112

2

1

21

RHTA

SHTA

LTA

111 11111

1

(f) 1-SiO2 2-Al2Si2O5(OH)4

111111 11 1

1

1

Fig. 2. XRD patterns of ashes from (a) Coal 1#; (b) Coal 2#; (c) Coal 3#; (d) Coal 4#; (e) Coal 5#; (f) Coal 6#(LTA, low temperature ash; SHTA, high temperature ash with slowheating rate; RHTA, high temperature ash with rapid heating rate).

368 J. Shen et al. / Fuel 207 (2017) 365–372

rawmolten salts and the reacted mixtures with ashes from Coal 1#and 5#. Three signals were observed in the mixed carbonates at1062 cm�1, 1082 cm�1 and 1092 cm�1, which were assigned toK2CO3, Na2CO3 and Li2CO3 respectively [33]. When the salts were

heated at temperatures higher than the melting temperature, aeutectic mixture was formed, with an apparent signal in the regionof the CO3

2�. After reacting with the ash samples, the molten saltsshowed similar Raman spectra as the raw eutectic mixture, sug-

Table 2The distribution patterns of mineral matter in Zhungdong coals.

Authors Minerals References

Xinguo Zhuang, et al. Quartz, kaolinite, siderite, pyrite [8]Dongke Zhang, et al. Anhydrite, quartz, nepheline, hematite, lime, periclase, mullite [11]Chang’an Wang, et al. Halite, calcite, gehlenite, hauyne, gypsum, sodium aluminosilicate, Fe2O3, SiO2 [16]Houzhang Tan, et al. Anhydrite, quartz, Ca2SiO4, Na2SO4 [17,25]Hao Zhou, et al. NaCl, CaSO4, SiO2, CaCO3, FeS2 [26,28]Haixia Zhang, et al. CaSO4, SiO2, CaO, CaCO3, Fe2O3, MgO [27]Jing Jin, et al. Quartz, akemanite, gehlenite [29]Hai Zhang, et al. SiO2, CaCO3, NaCl, CaSO4, Ca3Si3O7 [30]Guoliang Song, et al. CaSO4, SiO2, CaCO3, NaCl, NaAlSi3O8 [31,32]Hongyun Hu, et al. CaCO3, CaSO4, NaCl, SiO2, kaolin This research

400 600 800 1000 1200 1400 1600

Molten salts

Molten salts with coal 1#

600 oC800 oC

400 oC

Wavenumber (cm-1)

25 oC

Inte

nsity

( A

.U. ) 800 oC

600 oC400 oC

25 oC

800 oC

600 oC

400 oC25 oC

Molten salts with coal 5#

Fig. 3. Raman spectra of raw molten salts and reacted products with different coal ashes.

0

1

2

3

4

5

6

7

1

2

3

4

5

6Lower layer

Upper layer Coal 1# Coal 5# Coal 6#

7

Con

tent

of m

ajor

ele

men

ts (

% )

Si Ca Al Cl- SO42-

Fig. 4. Content of the major elements in the extracted solution from upper-layerand lower-layer products after molten salts reacting with coal ashes.

J. Shen et al. / Fuel 207 (2017) 365–372 369

gesting that the main ash compounds (including CaCO3, CaSO4 andNaCl) were dissolved in the molten salts to form a single phaseliquid.

Fig. SM-2 compares the XRD patterns of physically mixture ofmolten salts with typical ash compounds, and the heated mixturesat 800 �C. The peaks for each reactant disappeared after theyreacted with molten salts, mainly due to the formation of eutecticmixture. The dissolution of these compounds in the molten saltswas further confirmed at high temperature. In a reaction systemof 6 g molten salts, the mass fraction of dissolved CaCO3 and CaSO4

could reach 1 g and 0.9 g. And more NaCl (above 6 g) was dissolvedto form a new eutectic system.

In addition, after molten salts reacted with ash samples, thecontents of the major elements in the extracted solutions fromthe upper- and lower-layer products were detected and the resultswere depicted in Fig. 4. Calcium was predominantly dissolved inthe molten salts regardless of its initial forms. Furthermore,sodium was mostly transformed into the molten salts with chlo-rine, as evident by the high content of Cl� in the extracted solutionfor Na-rich Coal 1# ash.

In contrast, silicon and aluminum were slightly dissolved in themolten salts even for the Si/Al-rich ash from Coal 6#. Table 3 dis-plays the main compositions of the residues in the lower-layerproducts. Most of silicon and aluminum were stably remained inthe ash residues. The content of calcium in the ash residues wasprobably over estimated. In the water washing process, the dis-

solved calcium in the molten salts was predominantly transformedinto the residues under the influence of CO3

2�. Similarly, Cl- andSO4

2� was found in the water extracted solution of the lower-

Table 3Main compositions of water washed lower-layer products after molten salts reacting with coal ashes (wt%).

Ash samples SiO2 Al2O3 SO3 Fe2O3 CaO K2O MgO P2O5

Coal 1# – LTA 20.94 16.84 0.5 11.42 36.39 3.91 6.99 –a

Coal 5# – LTA 16.44 23.9 1.07 10.24 31.74 0.76 13.41 –Coal 6# – LTA 57.88 31.26 1.35 4.88 0.30 0.80 – 2.42

a –, Undetected.

370 J. Shen et al. / Fuel 207 (2017) 365–372

layer product, which was from the unseparated molten saltsattached on the ash residues.

On the other hand, some Li-compounds were found in the resi-dues after molten salts reacted with Coal 5# and 6# ashes (as pre-sented in Fig. 5). It was probably caused by the reactions betweenLi+ and Si/Al-compounds. However, the reactions were determinedby the distribution patterns of Si/Al-compounds. Although, therewere some Si/Al-compounds in Coal 1# ash, few Li-compoundswere detected in the products. Moreover, the difference in the dis-tribution patterns of Si/Al-compounds resulted in the formation ofvarious products, like LiAlO2, Li2SiO3 and Li3AlSiO3. The reactionprocesses were demonstrated as Eqs. (4)–(6).

SiO2 þ Li2CO3 ! Li2SiO3 þ CO2 ð4Þ

1.0

Molten salts + 10% NaCl (wt) Exo

Al2O3 þ Li2CO3 ! 2LiAlO2 þ CO2 ð5Þ

1.6

0.0

0.5

1.0

0.0

0.5

o

oC

oC

Molten salts + 17% CaCO3 (wt)

C (m

W/m

g)

T= 356H= -218.5 J/g

Molten salts + 15% CaSO4 (wt)

T= 331H= -239.8 J/g

Al2Si2O5ðOHÞ4 þ 3Li2CO3 ! 2Li3AlSiO5 þ 3CO2 þ 2H2O ð6ÞThe mechanism of lithium’s thermal behavior was further clar-

ified by investigating the interactions between molten salts andanalytical quartz/kaolin. The XRD patterns of the residues weredisplayed in Fig. SM-3. The formation of Li2SiO3 and Li3AlSiO3

was found in the products. Sodium and potassium could also react

10 20 30 40 50 60 70 80 90

Inte

nsity

( A

.U. )

1-CaCO3 2-SiO2Coal 1#

1 112 2 11

1,21

111

1

1-CaCO3 2-LiAlO2 3-SiO2Coal 5#

21

231,21

2

111,3

12

111

1

2θ

222

1,22

1,21

1,2

12

11

1-Li3AlSiO5 2-Li2SiO3Coal 6#

Fig. 5. XRD patterns of water washed lower-layer products after molten saltsreacting with various coal ashes.

Table 4The compositions of upper-layer and lower-layer products after molten salts reactingwith kaolin and quartz (wt%).

Products K2O Na2O Al2O3 SiO2

Kaolin Upper layer 50.23 47.76 – a 1.28Lower layer 42.9 34.37 11.63 9.82

Quartz Upper layer 51.71 45.92 – 1.65Lower layer 48.2 38.27 – 12.68

a –, Undetected.

with the Si/Al-compounds, such as the formation of KAlSiO4 (Eqs.(7)). Moreover, the presence of Si in the upper-layer productswas observed in Table 4, which was mainly caused by the forma-tion of soluble (Na/K)SiO3. By comparison, aluminum was mostlystabilized in the residues, which was hardly introduced into themolten salts as demonstrated by Fig. 4 and Table 4.

Al2Si2O5ðOHÞ4 þ K2CO3 ! 2KAlSiO4 þ CO2 þ 2H2O ð7Þ

3.3. Effects of ash components on the characteristics of molten salts

The effects of the dissolution of typical AAEMs, including NaCl,CaCO3 and CaSO4, on the melting performance of the molten salts

200 300 400 500 6000

1

2

30.0

0.8

Molten salts

Temperature (oC)

oC

C

T= 383H= -238.3 J/g

DS T= 333

H= -233.7 J/g

Fig. 6. DSC curves of raw molten salts and the eutectic mixtures within typical ashcomponents at a heating rate of 10 �C/min(T, melting temperature; H, meltingenthalpy).

350 400 450 500 550 600

0

1000

2000

3000

4000

5000 Molten salts + 10% Quartz (wt) Molten salts + 10% Kaolin (wt) Molten salts + 15% CaSO4 (wt) Molten salts + 17% CaCO3 (wt) Molten salts + 10% NaCl (wt) Molten salts

Vis

cosi

ty (c

P)

Temperature (oC)

Fig. 7. Viscosity-temperature curves of raw molten salts and mixtures withintypical ash components.

J. Shen et al. / Fuel 207 (2017) 365–372 371

were firstly studied. Fig. 6 shows the DSC curves of raw moltensalts and the eutectic mixtures after reacting with different compo-nents. The raw molten salts showed a sharp endothermic peak,starting at 383 �C and reaching the peak around 406 �C. After thedissolution of NaCl, CaCO3 or CaSO4, several peaks were appeared

200 250 300 350 400 450 500 5500.00

0.05

0.10

0.15

0.20

0.25

0.30

Temperature ( oC )

Ther

mal

diff

usiv

ity (

mm

2 /s )

Molten salts Molten salts with coal 1# ash Molten salts with coal 5# ash Molten salts with calcium sulfate Molten salts with calcium carbonate

(a)

200 250 300 350 400 450 500 5500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Temperature ( oC )

Ther

mal

con

duct

ivity

( W

/(m·o C

) )

Molten salts Molten salts with coal 1# ash Molten salts with coal 5# ash Molten salts with calcium sulfate Molten salts with calcium carbonate

(b)

200 250 300 350 400 450 500 5500.0

0.5

1.0

1.5

2.0

2.5

3.0

Temperature ( oC )

Spec

ific

heat

cap

acity

( J/

(g·o C

) )

Molten salts Molten salts with coal 1# ash Molten salts with coal 5# ash Molten salts with calcium sulfate Molten salts with calcium carbonate

(c)

Fig. 8. The thermal properties of raw molten salts and products after reacting withcoal ashes or typical ash components, (a) Thermal diffusivity; (b) Thermalconductivity; (c) Specific heat capacity.

and the melting process lasted for a longer time at the same heat-ing rate (10 �C/min). Furthermore, the initial temperature for thesalts melting was slightly changed after the dissolving of CaCO3.In contrast, the melting of the eutectic mixture within CaSO4

tended to occur at lower temperatures ranging from 331 �C to498 �C, while the melting of the mixture with NaCl was carriedout at higher temperatures.

The melting enthalpy was an important index for the applica-tion of molten salts. According to Fig. 6, the melting enthalpy forthe raw molten salts was calculated (�238.3 J/g), which was con-sistent with the previous studies [3]. It was slightly affected afterthe dissolution of CaCO3 or CaSO4. However, the melting enthalpywas changed to �218.5 J/g when NaCl was dissolved in the moltensalts. As a result, less energy was needed for the melting of theeutectic mixture with NaCl.

Viscosity reflects the flow property and the structural relaxationtime of the melt [34], and the viscosity of the molten salts was alsostrongly affected after mixing/dissolving with the typical compo-nents in Zhungdong coal ashes. Fig. 7 shows the viscosity curvesof raw and reacted molten salts as a function of heating tempera-ture. The viscosity of raw molten salts was quite low in the liquidphase while increased rapidly as the temperature decreased belowa characterized temperature (critical viscosity temperature,labeled as Tcv). The viscosity curve of the molten salts was a typicalcrystalline slag (non-newtonian) [34]. And Tcv was lower than themelting point analyzed by DSC curves, which was caused by thesupercooling and superheating in the cooling and heating process.At temperatures above Tcv, the viscosity of raw molten salts wasaround 4cP and the low viscosity was conductive to mixing withsolid fuels. After the dissolution of CaCO3, CaSO4 and NaCl, the vis-cosity of the eutectic mixtures was rarely changed. Nevertheless,the Tcv of the samples was remarkably increased, which was ingood agreement of the DSC results shown in Fig. 6. The increasingof Tcv demonstrated that the transformation and the application ofthe eutectic mixtures should be at higher temperatures. The Tcvwas also obviously increased after the addition of quartz or kaolinpower in the molten salts. Even worse, the viscosity of the eutecticmixtures significantly increased. Unlike AAEMs, quartz and kaolinmostly existed in solid phase in the system which greatly affectedthe flow property.

Finally, the changes of the thermal properties, including thethermal diffusivity, thermal conductivity and specific heat capac-ity, of the molten salts were observed under the influence of coalashes and typical Ca-compounds. As shown in Fig. 8, the interac-tions in various cases had similar effects on the thermal propertiesof the molten salts. And the change of each thermal parametershowed the same trend as temperature increased. After reaction,the molten salts were of lower thermal diffusivity and thermalconductivity, and higher specific heat capacity. In addition, the dis-tribution patterns of mineral matter played a significant role in thethermal properties change of the molten eutectic. Compared withthe effect of CaCO3, the thermal diffusivity and thermal conductiv-ity of the molten salts had undergone more changes under theinfluence of CaSO4.

4. Conclusions

In this research, the distribution of mineral matter in typicalZhundong coals was investigated by observing the characteristicsof ashes formed by different processing methods. Then, the inter-actions between molten salts (Li2CO3-Na2CO3-K2CO3) and ash sam-ples were studied. According to the results, Zhundong coals mainlyconsisted of two parts, alkali and alkaline earth metals (in theforms of NaCl, CaCO3, CaSO4 and organic matter) and Si/Al-compounds (in the forms of quartz, kaolin and amorphous species).

372 J. Shen et al. / Fuel 207 (2017) 365–372

AAEMs tended to be dissolved in the molten salts, lowing the melt-ing temperature and extending the melting time. Si/Al-compoundswere predominantly separated from the molten salts and remainedin the ash residues, including some Li-silicate or Li/K-aluminosilicates formed. The addition of Si/Al-compoundsincreased the viscosity of the molten salts, whereas the dissolutionof AAEMs almost had few effects on it. Meanwhile, the critical vis-cosity temperature was increased under the influence of bothAAEMs and Si/Al-compounds. Finally, the interactions betweenmolten salts and Zhundong coal ashes increased the specific heatcapacity, while decreased the thermal diffusivity and thermal con-ductivity of the molten system.

Acknowledgments

The present study was supported by International Science &Technology Cooperation Program of China (2015DFA60410),National Natural Science Foundation of China (51606075,51506064) and General Financial Grant from the China Postdoc-toral Science Foundation (Grant, 2016M592330). The authors wishto thank the experimental measurements provided by the Analyt-ical and Testing Center of Huazhong University of Science andTechnology. We are so grateful to Prof. Jinglin You and Dr. XiaoyeGong in Shanghai University for the high-temperature Ramanoperation support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.fuel.2017.06.079.

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