Understanding Ash Deposition for the Combustion of Zhundong Coal

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Understanding ash deposition for the combustion of Zhundong coal: focusing on different additives effects Yongzhen Wang, Jing Jin, Dunyu Liu, Haoran Yang, and Shengjuan Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00384 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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Energy & Fuels

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Understanding ash deposition for the combustion of Zhundong coal: focusing on different additives effects

2

Yongzhen Wang 1，Jing Jin 1*，Dunyu Liu 1， Haoran Yang1, Shengjuan Li2

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1 School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China,

4

2 School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093,

5

China

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ABSTRACT: The industrial application burning Zhundong coal has been directly restricted due to severe ash deposition.

7

To attain the wide utilization for this coal, experiments about the effects of different additives on ash deposition were

8

conducted in a drop-tube furnace. The properties of ash deposits were characterized by ICP-OES and XRD, and mineral

9

properties were also calculated by molecular dynamics from the perspective of elemental reactions. The results indicate that

10

ash deposition can be divided into inner layer and outer layer. The deposited layer and the size of particles under vermiculite

11

additive is looser and bigger than that under raw coal and kaolin additive. Additionally, ash deposition for inner layer is

12

thinner, and ash ratio for outer layer is higher under vermiculite additive. The total deposited layer may be thinner under

13

kaolin additive, but the inner layer for ash deposition is denser than that under vermiculite additive. Ash deposition for raw

14

coal is mainly caused by the formation of hematite, magnesioferrite, anhydrite and a littlie sodium silicate. Vermiculite

15

additive mainly promotes the further reaction between calcium oxide with magnesium oxide, silicon dioxide, causing the

16

formation of high melting point merwinite. Additionally, vermiculite additive also reduce the formation of magnesioferrite

17

for the deposit layer. Kaolin additive can mainly reduce the formation of anhydrite, and accelerate the formation of anorthite.

18

Furthermore, kaolin additive does not change the types of iron - bearing minerals for the deposit layer. Based on molecular

19

dynamic simulations, α-Fe2O3 is more easily to combine with anhydrite, followed by merwinite, and finally anorthite based

20

on the binding energy and the radial distribution function. This shows the formation of anhydrite is a key mineral to cause

21

severe ash deposition for the combustion of Zhundong coal.

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KEY WORDS：Zhundong coal, ash deposition, vermiculite, kaolin, molecular dynamics

* Corresponding author. Tel: (021)55277768; E-mail address: [email protected]. 1

ACS Paragon Plus Environment

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1 Introduction

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Zhundong coal field with 390 billion tons reserve in Xinjiang province is currently the largest coalfield in China[1-2].

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Due to its low-to-medium ash yields, low phosphorus and sulfur contents, high volatile matters, and high calorific value,

4

Zhundong coal is considered as a high quality steam coal[3-5]. However, severe slagging and fouling occur during the

5

combustion of Zhundong coal, especially for these with high iron contents[6]. This greatly limits the application for this

6

type of coal. Therefore, understanding the relationship between iron content and slagging/fouling mechanism can be crucial.

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Slagging and fouling on heat transfer section in the boiler is also called ash deposition. Ash deposition on the heat

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transfer section of the boiler can be affected by the ash composition. Iron-bearing minerals (pyrrhotite, hematite, iron

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alumino-silicate et al.) in the ash can aggravate deposition phenomenon. Srinivasachar et al.[7] found that pyrrhotite formed

10

by the decomposition of pyrite will melt to form an iron oxysulfide droplet. Furthermore, hematite can be formed by

11

oxidation of the precipitated pyrrhotite with long residence time, and this may aggravate ash deposition[8]. Additionally,

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Mclennan[9] demonstrated that iron alumino-silicate ash particles can easily adhere to the heat transfer surfaces due to the

13

high viscosity property. In addition, alkali and alkali earth metal existing in coal ash may lead to severe ash deposition. Wu

14

et al.[10] studied the mechanism for ash deposition burning Xinjiang coal, and found wollastonite is formed on the radiant

15

heat transfer section, while sulfates formed by volatile alkali metal sulfation are formed on the convective heat transfer

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section. According to Li et al.[11], lime and anhydrite can be responsible for initiating the ash deposition, and sulphation of

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calcium species may occur in this initial layer. Previous research confirms the significance of iron, alkali, and alkaline earth

18

metal contents ash deposition.

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However, both iron and calcium in coal ash as a high content simultaneously exist, and the mechanism for ash

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deposition burning coal is not clear. Nigel et al.[12] investigated ash deposition for Spanish, American and South African

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coal. Ca and Fe in coal ash may lead to serious ash deposition. According to Song et al.[13], Ca, Na, Fe, Mg and Si in coal

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ash can easily form the low melting point sulfates and silicates, leading to severe ash deposition. Tao et al.[14] showed that

23

free CaO decomposed by CaSO4 can react with Fe and Si in ash to form the low temperature eutectic when the temperature

2


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is over 1000 ℃, and the low temperature eutectic reaches the molten state and further aggravate ash deposition. Although

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previous research reveals the formation of low melting point sulfates and silicates or low temperature eutectic when both

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calcium and iron based minerals exist, the relative importance of calcium based species and iron-bearing minerals on ash

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deposition has not been revealed. Thus, the formation mechanism on ash deposition when both of them exist in coal need to

5

be further studied.

6

To alleviate ash deposition, kaolin is widely applied as a coal additive due to low price, easy to attain and better

7

anti-deposition effect. However, kaolin additive is mainly used to inhibit ash deposition for coal with a high alkali or

8

alkaline earth metal content. The reliving mechanism may be explained by that kaolin may increase the formation of

9

feldspar minerals, and these minerals can relieve ash deposition due to the high melting point. Shen et al.[15] demonstrated

10

that anorthite can be formed to alleviate ash deposition. Zhang et al.[16] also found that kaolin can transform NaCl in flue

11

gas to albite, to alleviate sodium enrichment on heat transfer section of the boiler and further reduce ash deposition.

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Furthermore, kaolin may well curb the formation of small particles making up the initial layer for ash deposition, and cause

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a decrease of the sodium content in the initial layer. According to Takuwa et al.[17], sodium elements in fly ash are mainly

14

in the form of small particles. As kaolin is added, the number of small particles can be reduced and the number of big

15

particles can be increased. Therefore, kaolin additive can have a good inhibition effect on ash deposition for coal with a high

16

alkali or alkaline earth metal content.

17

Vermiculite may also be used as a potential anti-deposition additive due to rich resources, low price, large thermal

18

expansion and low sticky. Previous results[18] showed that some low-viscosity slices are formed to inhibit the

19

agglomeration of ash particles, as vermiculite can inflate after heat treatment under high temperature. In addition, high

20

melting point forsterite formed through combustion of vermiculite can cover the surface of sticky and spherical ash particles,

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and reduce the viscosity of spherical ash particles.

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Overall, kaolin and vermiculite are mainly used in high alkali coal, while few studies have been reported on the effects

23

of these two additives on ash deposition for the combustion of a high iron coal. The mechanism of kaolin and vermiculite on

3


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ash deposition for the combustion of Zhundong coal with high iron and calcium contents was studied in this article.

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Experiments were conducted in a drop-tube furnace and the properties of ash deposition caused by different additives were

3

characterized by ICP-OES and XRD. Furthermore, these properties for the forming minerals under different additives were

4

also calculated by molecular dynamics (MD) to reveal the mechanism for ash deposition from the perspective of elemental

5

reactions. This provides theoretical guidance for the utilization of additives.

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2 Experimental

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2.1 Experimental system

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Experiments were conducted in a 5000 W drop-tube furnace. This system was consisted of a main tube reactor (2400

9

mm of total height×100 mm of inner diameter), coal feeder, induced draft fan, a deposition tube and an ash collecting unit in

10

Fig.1. A constant temperature zone existed in the middle of the furnace. From the constant temperature zone to both ends of

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the exit for the furnace, the temperature was significantly reduced from 1200℃ to about 800℃. In addition, the ash

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deposition tube was made of a ring T91 steel pipe which had two openings. The outer diameter of the deposition probe was

13

45 mm. The distance for the ash deposition tube inserted into the furnace was 800 mm from the bottom of the main tube

14

reactor. And the length of deposition probe inserted into the furnace was 80 mm. Additionally, a slot was made in the top of

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the outer surface for the deposition tube, and the thermocouple was buried in the slot. The real-time monitor for the

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temperature of the tube was realized by the thermocouple located in the external case attached to the tube. In experiment,

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the ash deposition tube was horizontally inserted into the furnace and the surface temperature (600℃) for ash deposition

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was controlled through the adjustment of the flow rate of water inside the tube. This was to simulate ash deposition

19

characteristics in the actual boiler.

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Fig. 1 Schematic diagram of a 5000W drop-tube furnace system

2.2 Experimental sample properties

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A typical high iron content Jiangjun coal as the most representative coal with high iron and calcium contents in

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Zhundong coal was selected. The average diameter of coal particles and kaolin additive was 70 µm, while the average

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diameter of vermiculite additive was about 100 µm. Tab.1 gives the basic analysis for coal and ash. This coal was

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considered as a high quality steam coal due to high volatile and low ash content. However, basic oxides (Na2O, K2O, CaO,

8

Fe2O3 and MgO) in ash were high. Especially the contents of iron and calcium were high. To alleviate ash deposition

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burning coal, vermiculite and kaolin were selected as coal additives according to previous studies[15,18]. The amount for

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both

additives

was

5

wt

%

in

raw

coal.

The

chemical

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(Mg,Ca)0.7(Mg,Fe,Al)6.0[(Al,Si)8.0](OH4.8H2O), and its chemical analysis was also shown in Tab.2. Kaolin was purchased

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from China National Pharmaceutical Group, and the chemical formula of kaolin was Al2O3.2SiO2.2H2O. Additionally, the

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purity of kaolin was 98.5%.

14

5


formula

of

vermiculite

was

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1 2

Tab.1 The basic analysis for coal and ash Basic test method

Symbol

Value

Mad

12.22

Aad

6.71

Vad

26.19

FCad

54.89

Cd

71.12

Hd

3.30

Od

16.16

Nd

1.32

Sd

0.46

Proximate analysis (%)

Ultimate analysis (%)

Qnet.ad(kJ/kg)

24650 DT

1156

ST

1171

HT

1185

FT

1195

Na2O

4.63

K2O

2.19

CaO

17.63

Fe2O3

29.72

MgO

5.06

Al2O3

3.32

SiO2

7.68

SO3

26.44

Ash fusion temperatures (℃)

Ash composition(%)

3

M, Moisture;

4

temperatures; HT, hemispherical temperatures;

A, Ash; V, Volatile;

FC, Fixed Carbon;

ad, air dry basis;

d, dry basis; DT, deformation temperatures; ST, soft

FT, flow temperatures.

5 6 6


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Tab.2 Chemical analysis of vermiculite (wt %)

Na2O

K2O

CaO

MgO

Fe2O3

SiO2

Al2O3

H2O

1.35

2.56

1.82

17.60

6.57

40.28

16.24

6.28

3 4

2.3 Experimental and calculation method

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The original coal on air dry basis with different additives were fully mixed in a mixer. Then they was introduced into

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the furnace by a screw feeder. And the feeding rate was 16.67 g/min in combustion experiments. During the experiment, the

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ash deposition tube was laid in constant temperature area. The gas temperature tested by the thermocouple was 1200℃

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around the ash deposition tube. And the surface temperature for the ash deposition tube was 600℃. The excess air ratio was

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1.2 and the raw materials were continuously fed for 5 hours during each experiment. The gas flow rate in the furnace was 98

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L/min. The resulted burn-out rates were higher than 97%. This burned-out was measured by muffle furnace. And it was

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determined by the changes in quality before and after calcination under 1200℃. Ash deposit on the tube and bottom ash

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were taken out and quickly cooled to room temperature. Additionally, ash deposit for outer layer were attained by the fan

13

blowing under 0.3MPa. The whole blowing process was 2 minutes. The soot blower was vertically placed above the ash

14

deposition tube, and it was 200 mm from the ash deposition tube. Then ash deposits were collected by ash collecting unit in

15

the drop furnace. In addition, ash deposits at inner layer were difficult to remove. So they were gotten by weighing method

16

that the mass for the deposition tube with ash deposits at inner layer subtracted the mass for the smooth tube. The test was

17

conducted three times and averaged for the deposit mass and elemental content. Microstructure for ash deposits were also

18

analyzed by SEM from Phenom Prox. Last, elemental content for the samples from experiments were analyzed by

19

Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, Prodidy). XRD-6100 from Shimadzu was used to

20

analyze mineral composition of slag samples.

21

The binding energy between α-Fe2O3 and minerals based on molecular dynamics (MD) simulations was calculated. 7


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T91 steel pipe as a new austenitic stainless steel is widely used in a boiling water reactor component. Thus, the α-Fe2O3 (1 1

2

0) oxide film with the periodic boundary conditions was selected as the substrate[19]. The α-Fe2O3 (1 1 0) substrate is

3

prepared an area of 45.53×33.35 Å with a thickness of approximately 6.96 Å. And the simulation box is enlarged in the

4

z-direction with a vacuum of 40 Å. The MD simulations are performed in an NVT ensemble, and the temperature of the

5

substrate is chosen at 873 K, which is controlled by the Berendsen algorithm[20]. Additionally, the adsorbed clusters based

6

on conservation of calcium atoms were also calculated by molecular dynamics simulations.

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3 Results and Discussion

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To understand the effect of different additives on ash deposition burning Zhundong coal with a high iron, morphology, component and mineral properties are analyzed in sequence. 3.1 Morphological analysis

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Ash deposits can be classified in terms of inner layer and outer layer based on appearance. According to Zheng et al,

12

the inner layer is formed by fine particles while the outer layer is mainly composed of large particles[21]. Fig.2 shows

13

morphological images of ash deposits for raw coal combustion with vermiculite and kaolin additives respectively. The

14

colour of ash deposits for inner layer is generally lighter than that for outer layer. The inner layer is dense and hard to

15

remove by the the soot blower. Additionally, the particles for outer layer are relatively big and easily removed by the soot

16

blower [22].

17 18 19 20 21 22 23

8


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(a)

3 (b)

4

5 6 7 8

(c)

Fig. 2 Morphological images and SEM pictures (×5000) of ash deposition under different additives: (a) raw coal; (b) with vermiculite

additive; (c) with kaolin additive

9

The deposit mass for inner layer and outer layer under different additives is showed in Fig.3. The deposit mass at inner

10

layer under raw coal, vermiculite additive and Kaolin additive is 4.01g, 0.85g and 1.52g, respectively. And the deposit mass

11

at outer layer under raw coal, vermiculite additive and Kaolin additive is 2.11g, 2.18g and 0.60g, respectively. Combined

12

with SEM pictures of ash deposits for three cases in Fig.2, it is found that deposited layer for raw coal is the densest and the

9


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amount for the inner layer for raw coal is also the largest. These deposits are difficult to be removed. Moreover, the

2

deposition layer under vermiculite additive is relative loose and the size of particles is also big compared with that in raw

3

coal and under kaolin additive. Compared with deposit mass for raw coal and with kaolin additive, deposit mass at the inner

4

layer with vermiculite additive is the least, while deposit mass for outer layer is higher. Additionally, the total amount(2.12g)

5

of deposited layer under kaolin additive may be less than the total amount(6.12g) under raw coal and the total amount(3.03g)

6

under vermiculite additive and the colour of ash deposits under kaolin is relatively light. On the other hand, the inner layer

7

for ash deposition under kaolin additive consists of fine particles with sizes less than 10 µm, while that under vermiculite

8

additive is composed of much larger particles with sizes larger than 10µm in Fig.2. So, the inner layer for ash deposits under

9

kaolin additive is denser than that under vermiculite additive. Hence, both vermiculite additive and kaolin additive can

10

alleviate ash deposition for combustion of Zhundong coal. And the mechanism for ash deposition also need to be further

11

analyzed.

12 13 14

Fig.3 Deposit mass at different locations under different additives

3.2 Component analysis

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To further study the mechanism for ash deposition for combustion of Zhundong coal, the minerals for inner ash, outer

16

ash and bottom ash for raw coal are analyzed in Fig.4. Main minerals for deposited samples in inner layer include hematite, 10


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magnesioferrite, anhydrite, sodium silicate and a small amount of silicon dioxide. Hematite as oxidation products of

2

iron-bearing minerals (pyrite, siderite etc.) easily attaches to the tube and deposited layer, leading to severe ash deposition

3

[8]. In addition, the initial layer can be formed by the formation of calcium sulfate due to the low temperature tube

4

surface[23-24]. Calcium sulfate may be formed by the reaction between CaO produced by the decomposition of calcite

5

during coal combustion and SO3 or SO2 in the gas phase[25]. The sulfate particles with a high sticky may easily deposit on

6

the surface of the tube, causing the formation of a sticky trapping layer. The sticky trapping layer can effectively increase

7

the adhesion of fly ash. Then, a new capture layer may also be formed by continuously capturing ash particles by inertia

8

[26].

9

Hematite, magnesioferrite, anhydrite, sodium silicate, a small amount of silicon dioxide and forsterite are main

10

minerals in the outer layer for ash deposition. Additonally, the diffraction peaks for anhydrite obviously reduce. This may be

11

because part of anhydrite is decomposed under the high temperature of outer layer during ash deposition. Main minerals in

12

bottom ash include hematite, magnesioferrite, anhydrite, silicon dioxide, forsterite, sodium silicate and gehlenite. Gehlenite

13

is formed by the reaction among free CaO, SiO2 and Al2O3. Additionally, the diffracted intensity of deposition layer is lower

14

than the bottom ash due to cooling and crystallization for the molten ash adhering to the ash deposits. Especially, obvious

15

humps in around 25° in the XRD results suggest the presence of amorphous phases in the ash and deposition, causing the

16

formation of the low temperature eutectic between Ca, Fe, Al and S[27]. Overall, the diffraction peaks for hematite,

17

magnesioferrite and anhydrite at three locations are higher than other minerals. This indicates that iron - bearing minerals

18

(hematite and magnesioferrite) and anhydrite are preferably deposited.

11


Energy & Fuels 360

2

bottom ash 270 1

180 4

90

2

3 6 5 37 3

1

1

3

1

7

5

6

0 300

outer layer

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

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225

1

150

2 2 3

1

3 4 5 3 3 6

1

1

5

6

75 0 300

inner layer 2

225

3

150

4

5

3 3

1 2

3 1 3

75 0 10

1

20

30

40

1

1 3 44

3

50

5

60

2

70

80

2Θ

2

Fig.4 XRD patterns of ash deposits at different locations for raw coal combustion

3

1- Hematite(Fe2O3)；2- Magnesioferrite(MgFe2O4)；3-Anhydrite (CaSO4)；4- Sodium Silicate(Na2Si2O5)；

4

5-Silicon Dioxide (SiO2);

6-Forsterite (Mg2SiO4); 7-Gehlenite (Ca2Al2SiO7)

5

To further reveal the mechanism for different locations during the combustion of raw coal, the elemental analysis of

6

ash deposits at different locations is shown in Fig.5. The content of iron is the highest among all contents in three locations,

7

and the content of iron in inner layer is lower than that in outer layer and bottom ash. Combined with Fig.4, the diffraction

8

peaks for hematite and magnesioferrite in three locations are the highest, and the diffraction peaks for hematite and

9

magnesioferrite in inner layer are smaller than those in outer layer. Thus, hematite and magnesioferrite are main minerals in

10

ash deposition. Additionally, the weight percentages of sodium at different locations do not differ significantly, and the

11

content is not much. Furthermore, the contents of magnesium at three locations do not change greatly. Compared to outer

12

layer and bottom ash, the contents of calcium and sulphur are rich in the inner layer, and the amounts are relatively high.

13

This is because the deposition and condensation of anhydrite, combined with Fig.4. Additionally, the contents of silicon and

14

aluminum in deposit layers are smaller than those in bottom ash. Silicon and aluminum, as the network-forming elements 12


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can mitigate the ash deposited phenomenon. Based on the above result of the minerals and the elemental contents, it is

2

found that hematite, magnesioferrite and anhydrite are favored to deposit on the heat surfaces.

3 4

Fig.5 Elemental analysis of ash deposits at different locations for raw coal combustion

5

XRD patterns of ash deposits at different locations under vermiculite addition are shown in Fig.6. With vermiculite

6

addition, main minerals in the inner layer include hematite, anhydrite, merwinite, mullite, forsterite and potassium sodium

7

silicate while main minerals in outer layer include hematite, merwinite, forsterite, fayalite and quartz. Compared to the

8

composition in the outer layer, the types of minerals in bottom ash do not change, but the contents of minerals change.

9

Additionally, obvious humps in around 30° in the XRD results suggest the presence of amorphous phases in the ash and

10

deposition, causing the formation of the low temperature eutectic between Ca, Fe, Al and S.

11

Compared to inner layer for raw coal, magnesioferrite is not found with the addition of vermiculite. Instead, high

12

melting point merwinite(Ca3Mg(SiO4)2, ＞1300℃) is found[28]. This is that free CaO decomposed by anhydrite can react

13

with SiO2 to form wollastonite(CaSiO3). Wollastonite can further react with MgO from the combustion of vermiculite to

14

form akermanite (Ca2MgSi2O7). Furthermore, akermanite may further react with CaO and SiO2 to form merwinite. The

15

reaction process is as follows: (1) - (4). In addition, the high melting point mullite(1850℃) can be found in inner layer.

13


Energy & Fuels 1

CaSO4→CaO+SO3

(1)

2

CaO+SiO2→CaSiO3

(2)

2CaSiO3+MgO→Ca2MgSi2O7

3

(3)

CaO+SiO2+Ca2MgSi2O7→Ca3Mg(SiO4)2

4

(4)

5

Compared to the outer layer for raw coal, a large amount of magnesium-containing minerals (forsterite, merwinite)

6

appear after vermiculite addition. According to Yao et al.[18],the formations of these magnesium-containing minerals may

7

result in less sticky ash particles in outer layer, and make ash deposition in outer layer loose.

8

Therefore, vermiculite additive mainly inhibits the formation of anhydrite, and promotes the further reaction between

9

calcium oxide with magnesium oxide, silicon dioxide, causing the formation of high melting point merwinite. Meanwhile,

10

the high melting point mullite can also be formed to alleviate ash deposition. 360 1

bottom ash 3

270

5

5

3 3 7 7

180

1

5

90 3600

1 583

5 75

8

7 7

7

1

outer layer

270

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 14 of 26

5

180

3 7 7

3

5

31

1

5 8

90 3600

3

8

7 5

7 7 7

3 inner layer

270

1

2 1

180

3

2 3

90 0 10

20

30

5

6 2 25

61

40

1

54

50

60

70

80

2Θ

11 12

3

Fig.6 XRD patterns of ash deposits at different locations under vermiculite addition

13

1-Hematite (Fe2O3)；2- Anhydrite (CaSO4)；3-Merwinite (Ca3Mg(SiO4)2)；4-Mullite (Al2SiO5)；5-Forsterite ( Mg2SiO4 ); 6-Potassium sodium

14

silicate(Na1.3K0.7Si2O5)；7- Fayalite(Fe2SiO4); 8-Quartz (SiO2) 14


Page 15 of 26 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

XRD patterns of ash deposits at different locations under kaolin addition is shown in Fig.7. With kaolin additive,

2

hematite, anhydrite, magnesioferrite, anorthite, mullite and albite are main minerals in the deposited inner layer. In addition,

3

the types of mineral in the outer layer are the same with the inner layer, but their content of minerals is different from the

4

inner layer. Mineral compositions in tail ash mainly include hematite, magnesioferrite, anorthite and mullite. Additionally,

5

the diffracted intensity of inner layer is lower than outer layer and the bottom ash, and obvious humps in around 30° in the

6

XRD results suggest the presence of amorphous phases in inner layer. This is due to the formation of the low temperature

7

eutectic between Ca, Fe, Al and S.

8

Compared to inner layer for raw coal, sodium silicate disappears and anorthite and albite appear. It may be because

9

CaO or NaCl may react with silicon and aluminum in kaolin to form anorthite or albite, respectively[29-30]. The reaction

10

scheme is as follows: (5) ~ (8). In addition, sodium silicate and forsterite cannot be found in the bottom ash with kaolin

11

additive compared with raw coal. However, more anorthite and mullite exist in the bottom ash with kaolin additive than that

12

with raw coal. Furthermore, combined with Fig.4 and Fig.6, anorthite only exists in inner layer with kaolin additive

13

comparing with that for raw coal or vermiculite additive. Thus, kaolin additive can mainly reduce the formation of sulfates,

14

and accelerate the formation of anorthite.

15 16 17 18

Al2O3·2SiO2·2H2O(Kaolinite)→ Al2O3·2SiO2(Metakaolinite)+ 2H2O Al2O3·2SiO2→Al2O3+2SiO2 NaCl+Al2O3+3SiO2+H2O→Na2O·Al2O3·3SiO2+2HCl CaO+Al2O3·2SiO2(Metakaolin) →CaO·Al2O3·2SiO2(Anorthite)

15


(5) (6) (7) (8)

Energy & Fuels 550

6

fallout 440 330

1 6

220

1

1

1

3

110 0 320

6

160

3

1

1

4

4

6 6 1

140

1 3 3

4

3

inner layer

210

1 24

4

25

4

70

20

30

40

2

1

3

5

0 10

2

3 25

80 0 280

2 3 3 1

1 6

outer layer

240

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 16 of 26

50

3

3 1

60

70

80

2Θ

1 2

Fig.7 XRD patterns of ash deposits at different locations under kaolin addition

3

1- Hematite (Fe2O3)；2- Magnesioferrite(MgFe2O4)；3- Mullite (Al2SiO5)；4- Anhydrite (CaSO4)；5- Albite (Na(AlSi3O8))；

4

6-Anorthite(Ca(Al2Si2O8))

5

To further study the effects of different additives on ash deposition, elemental analysis of ash deposits under different

6

additives at different locations is shown in Fig.8. Fig.8(a) shows elemental analysis of ash deposits in inner layer. The

7

content of magnesium is highest under vermiculite additive, while the contents of iron is lowest under vermiculite additive.

8

A large amount of magnesium - bearing minerals appear due to the high content of magnesium in vermiculite. It is

9

speculated that this less sticky ash particles composed of magnesium - bearing minerals can reduce the adsorption of the

10

oxide film. This is verified in the article.

11

The content of sodium does not change with vermiculite addition compared with raw coal. Combined with Fig.6, it is

12

found that both the types and contents of sodium-bearing minerals do not change. Thus, vermiculite has little effect on

13

sodium-containing minerals. Although sodium contents with kaolin additive are lower than those with raw coal and

16


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Energy & Fuels

1

vermiculite additive, the total amount of sodium with raw coal and vermiculite additive is not high. Moreover, the content of

2

calcium reduces with vermiculite and kaolin additive, while the contents of silicon and aluminum increase. A certain

3

amount of silicon and aluminum as framework elements can increase the melting point of coal ash and reduces ash

4

deposition.

5

Fig.8(b) shows that elemental analysis of ash deposits in outer layer with different additives. The contents of silicon

6

and aluminum are higher in outer layer with vermiculite or kaolin additive than these under raw coal, while the iron and

7

calcium contents with vermiculite or kaolin additive are lower than these under raw coal. The content of magnesium in

8

outer layer with vermiculite additive is high, due to the high content of magnesium in vermiculite. Thus, the mineral

9

components have changed by vermiculite or kaolin addition. The contents of iron and calcium are reduced, while silicon

10

and aluminum are increased.

11

In addition, the molar ratio of Ca/S based on the elemental contents in inner layer under raw coal, vermiculite addition

12

and kaolin addition is 0.9, 1.5, or 2.7, respectively. While the molar ratio of Ca/S in outer layer under raw coal, vermiculite

13

addition and kaolin addition is 0.8, 3.7, or 4.3, respectively. This indicates that anhydrite is main mineral to aggravate ash

14

deposition. Based on the above results and discussion on the effects of different additives on ash deposition, both

15

vermiculite and kaolin can mitigate ash deposition. Vermiculite additive mainly inhibits the formation of anhydrite, and

16

promotes the further reaction between calcium oxide with magnesium oxide, silicon dioxide, causing the formation of high

17

melting point merwinite. Additionally, vermiculite additive also reduce the formation of magnesioferrite in inner and outer

18

layer for ash deposition. Kaolin additive can mainly reduce the formation of anhydrite, and accelerate the formation of

19

anorthite. Furthermore, kaolin additive does not change the types of iron - bearing minerals in inner or outer layer for ash

20

deposition. The translation of calcium-bearing minerals for different additives burning Zhundong coal is shown in Fig.9.

21

17


Energy & Fuels 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

1 (a)

2

3 (b)

4 5

Fig.8 Elemental analysis of ash deposits for different additives at different locations: (a) inner layer, (b) outer layer

18


Page 18 of 26

Page 19 of 26 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

1 2 3

Energy & Fuels

Fig.9 Translation of calcium-bearing minerals for different additives burning Zhundong coal

3.3 Analysis of mineral properties

4

The formation of inner layer for ash deposition is crucial for the following capturing of ash particles. Hence, it is

5

necessary to study inner mineral composition. Both vermiculite and kaolin can mitigate ash deposition by changing the

6

composition of the inner minerals. Anhydrite, merwinite and anorthite are three crucial components for ash deposition

7

phenomena. Thus, to study the binding energy between calcium-bearing minerals and α-Fe2O3 oxide film is important for

8

understanding the mechanism of ash deposition under different additives.

9

To evaluate adsorption intensity, the binding energy was calculated. The negative value for binding energy represents

10

the exothermic reaction, and the positive value indicates the endothermic reaction. When the bonding process is exothermic,

11

the larger the binding energy, more stable the system is[20, 31-32]. When the bonding process is endothermic, the system is

12

unstable. The formula for binding energy is as follow:

13

Einteration=Etotal-(Eα-Fe2O3 +EMinerals)

14

Here, Einteration is binding energy; Etotal is the total energy between calcium-bearing minerals and α-Fe2O3 oxide film;

15

Eα-Fe2O3 is total energy of α-Fe2O3 oxide film; EMinerals is the total energy of calcium-bearing minerals.

16

Fig.10 shows the schematic diagram of the different minerals adsorbed on the surface of α-Fe2O3. After 100,000 steps,

17

the system reaches equilibrium. It is found that an amount of SO42- in anhydrite can be easily adsorbed on the surface of

19


Energy & Fuels 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 26

1

α-Fe2O3. While anorthite and merwinite is not obvious to adsorb on the surface of α-Fe2O3. This indicates that α-Fe2O3 is

2

more easily to combine with anhydrite. Additionally, the binding energy between α-Fe2O3 and the different minerals is also

3

shown in Tab.3. The binding energy for α-Fe2O3 and anhydrite is negative, to be the exothermic process. While the binding

4

energy for α-Fe2O3 and anorthite, merwinite is positive, to be endothermic process. Furthermore, the binding energy

5

between α-Fe2O3 and anhydrite is -3743.38 kJ/mol. Binding energy between α-Fe2O3 and anorthite is 1857.24 kJ/mol, and

6

binding energy between α-Fe2O3 and merwinite is 644.25 kJ/mol. Therefore, α-Fe2O3 is more easily to combine with

7

anhydrite, followed by merwinite, and finally anorthite.

8 9

(a)

(b)

10 11

(c)

12 13

Fig.10 Schematic diagram of the different minerals adsorbed on the surface of α-Fe2O3: (a) anhydrite, (b) merwinite, (c) anorthite

14 20


Page 21 of 26 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

2

Tab.3 The binding energy between the different minerals and Fe2O3

Total energy(kcal/mol)

mineral(kcal/mol)

CaSO4-Fe2O3

-564303.91

-2550.69

Ca3Mg(SiO4)2-Fe2O3

-561183.04

-478.49

CaAl2(SiO4)2-Fe2O3

-560460.60

-45.96

Fe2O3(kcal/mol)

Binding energy(kJ/mol)

-3743.38

-560858.53

644.25

1857.24

1 kcal/mol =4.184 kJ/mol

3

Additional insights into the interaction modes of calcium-bearing minerals and α-Fe2O3(1 1 0) surface can be obtained

4

by performing a structural analysis of the MD simulations results. The radial distribution function (RDF) shown in Fig.11 is

5

used as a useful method to estimate the bond length. The peak occurs at 1 Å~3.5 Å, and this value is an indication of bond

6

length, which is correlated to chemisorption, while the physical interactions are associated with the peaks longer than 3.5

7

Å[33]. From the equilibrium configurations of the different minerals, the RDF shows that the bonding length for

8

CaSO4-Fe2O3(2.23 Å), Ca3Mg(Si2O8)-Fe2O3(2.61 Å), and CaAl(Si2O8)-Fe2O3(2.63 Å) are all ＜3.5 Å in Fig.11, suggesting

9

that the chemical interaction occurs with the different minerals and the oxide film. Furthermore, the bonding length of

10

CaSO4-Fe2O3 is less than those for Ca3Mg(Si2O8)-Fe2O3, and CaAl(Si2O8)-Fe2O3. While the bonding length for Fe2O3 and

11

anorthite, and the bonding length for Fe2O3 and merwinite differs little. Thus, the obtained results confirm the highest

12

reactivity of anhydrite with oxide film surface. This also shows that the formation of anhydrite is a key mineral that leads to

13

severe ash deposition for combustion of Zhundong coal.

21


Energy & Fuels

2.5

CaSO4-Fe2O3 Ca(Al2Si2O8)-Fe2O3 Ca3Mg(SiO4)2-Fe2O3

2.0

1.5

g(r)

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 22 of 26

1.0

0.5

0.0 0

4 5

4

6

8

10

12

x(Angstrom)

1 2 3

2

Fig.11. RDF analysis of the different minerals adsorbed on the surface of α-Fe2O3.

4 Conclusions (1) Ash deposition can be divided into inner layer and outer layer. The colour of ash deposits for inner layer is lighter than that for outer layer. The inner layer is dense and hard to remove by the the soot blower.

6

(2) The deposited layer and the size of particles under vermiculite additive is looser and bigger than that under raw coal

7

and kaolin additive. Additionally, ash deposition for inner layer is thinner, and ash ratio for outer layer is higher under

8

vermiculite additive. The total deposited layer may be thinner under kaolin additive, but the inner layer for ash deposition is

9

denser than that under vermiculite additive.

10

(3) Ash deposition for raw coal is serious due to the formation of hematite, magnesioferrite, anhydrite and a little

11

sodium silicate. Vermiculite additive mainly inhibits the formation of anhydrite, and causes the formation of high melting

12

point merwinite. Additionally, vermiculite additive also reduce the formation of magnesioferrite for the deposit layer. Kaolin

13

additive can mainly reduce the formation of anhydrite, and accelerate the formation of anorthite. Furthermore, kaolin

14

additive does not change the types of iron - bearing minerals for the deposit layer.

15

(4) Based on molecular dynamic simulations, α-Fe2O3 is more easily to combine with anhydrite, followed by

16

merwinite, and finally anorthite based on the binding energy and the radial distribution function. This shows the formation 22


Page 23 of 26 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

of anhydrite is a key mineral to cause severe ash deposition for the combustion of Zhundong coal.

2

Acknowledgment

3

The authors highly appreciate the financial support from National "Thirteenth Five-Year" Plan for Science &

4

Technology Support of China (No.2017YFF0209800) and the National Nature Science Foundation of China

5

(No.51402192).

6

5. References

7 8 9

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3

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Understanding Ash Deposition for the Combustion of Zhundong Coal

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