Release and migration characteristics of sodium and potassium in high alkali coal under oxy-fuel fluidized bed combustion condition

Fuel 262 (2020) 116413

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Release and migration characteristics of sodium and potassium in high alkali coal under oxy-fuel fluidized bed combustion condition

T

Kaijun Yu, Xiaoping Chen , Tianyi Cai, Jiliang Ma, Daoyin Liu, Cai Liang ⁎

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy & Environment, Southeast University, Nanjing, Jiangsu 210096, People’s Republic of China

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Zhundong coal Oxy-fuel Ash formation Sodium and potassium Density functional theory

Zhundong coal is promising to replace inferior coal in fossil-fueled power plants, considering its high abundance, low contents of sulfur and nitrogen. However, the relative high contents of alkalis, especially sodium and potassium, damage the operational economy and safety badly since the release of alkali metals in flue gas can lead to fly ashrelated problems such as slagging, ash accumulation and corrosion. Although, oxy-fuel circulating fluidized bed (CFB) combustion is beneficial to reduce the release of sodium and potassium by lowering the operation temperature, the ash formation characteristics of Zhundong coal under the oxy-fuel CFB is still scarce. Our results showed that the residual amounts of sodium and potassium in the ash samples, both tend to decrease with increasing temperature no matter under air combustion or oxy-fuel combustion. However, the sodium content in the cinder obtained under the oxy-fuel combustion atmosphere was higher than that under the air combustion atmosphere, and the insoluble sodium, mainly existed in the form of aluminosilicates, was the main form. In addition, the increase of steam content in both atmospheres reduced the sodium content in the cinder while the residual amount of potassium increased. Density functional theory calculations verified the transformation behavior between sodium and potassium and the inhibiting effect of steam. This study elucidates the release and migration characteristics of sodium and potassium in Zhundong coal both experimentally and theoretically and will benefit the popularity of Zhundong coal.

⁎

Corresponding author. E-mail address: [email protected] (X. Chen).

https://doi.org/10.1016/j.fuel.2019.116413 Received 29 May 2019; Received in revised form 7 October 2019; Accepted 10 October 2019 0016-2361/ © 2019 Published by Elsevier Ltd.

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1. Introduction

Zhundong coal ash at different temperatures in oxy-fuel combustion atmosphere. They concluded that the oxy-fuel combustion atmosphere did not dramatically change the kinds of mineral phases, but did influence the relative amount of crystalline phases. They also found that the effect of temperature on the crystal phase composition of ash was greater than that of the atmosphere. During utilization of Zhundong coal, the release of sodium into gas phase has a significant effect on ash deposition and should be mainly responsible for the serious ash-related problems in Zhundong coal-fired power plants. Other countries have similar studies on high-sodium lignite. For example, Oleschko et al. carried out the experiment of sodium release when burning German lignite [30]. Laboratory combustion experiments with seven different German brown coals from the Rheinoland area were conducted at temperatures of 800 and 1200 °C. The results show that the release of Na- and K-species occurs mainly in the form of NaCl and KCl. Na2SO4 could also be observed in the gas phase at 1200 °C but not at 800 °C. Naruse et al [31] found that the residual fraction of sodium in ash obtained at 850 °C is between 60 and 90%. The difference in oxygen concentration in the atmosphere would affect sodium, calcium and iron migration process. However, the effects of steam on the migration of sodium and potassium of Zhundong coal under the oxy-fuel combustion conditions are still undefined by far. Moreover, the release and transformation of alkali metals in ash are mainly studied through correlation analysis of experimental phenomena, but not from an atomic level at present. The mechanism of causing these phenomena is still not revealed precisely. More importantly, the release and migration properties of sodium and potassium in Zhundong coal could be very different in the CFB combustion. In the dense phase of the CFB, the coal particles are actually in a local reducing atmosphere environment, meaning the surface is exposed to a low concentration of oxygen; the volatiles are rapidly precipitated; and the combustion is delayed. Therefore, motivated by the interesting possible future results, we quantified the release and migration characteristics of sodium and potassium in Zhundong coal under the atmosphere of Oxy-fuel CFB’s dense phase zone both experimentally and theoretically. The effect of atmosphere and temperature on the release and migration characteristics of sodium and potassium in zhundong coal were systematically discussed. Besides, the transformation behavior of sodium and potassium in coal was verified, using density functional theory calculations as well.

Currently, coal is still the main energy source in many countries, especially in China [1]. The Zhundong coal field, located in the east of the junggar basin, Xinjiang province, is the largest integrated coalfield with a coal reserve of 390 Gt [2]. The coal has the characteristics of extra low ash, extra low sulfur, high calorific value and low metamorphism [3]. It belongs to high-quality power coal, and has a broad prospect for power generation. However, the high content of alkali metals, especially sodium and potassium [4], seriously affects the safety and efficiency of equipment operation and limits its industrial applications because of the high coking property, serious slagging tendency, and corrosion in the heating surface [5]. Therefore, many researchers have focused on the release behavior of alkali metals in the combustion systems using Zhundong coals. As one of most promising carbon capture technologies, oxy-fuel combustion is believed to be both an effective and environment-friendly [6]. First, the concentration of CO2 in dry flue gas during oxy-combustion can reach more than 80%, resulting in a relatively low cost of the subsequent CO2 purification [7,8]. Second, oxy-fuel combustion has been widely proved secure in power generation [9]. Moreover, oxy-fuel combustion technology has relatively high CO2 capture efficiency as well as feasibility [7,10–13]. However, oxy-fuel combustion differs significantly in terms of atmosphere, thermodynamics, and chemical processes compared to conventional air combustion [14–16]. Although oxy-fuel combustion can reduce NOX emissions and unburned carbon in fly ash, it leads to the increase of SO2 content in the flue gas and the ash corrosion of the heating surface [17–21]. Another major feature of oxyfuel combustion is the high steam content caused by wet flue gas circulation. It can reach as much as 40%, (generally, between 15 and 30% in the conventional combustion), which seriously shortens the life of the equipment [22–24]. Fortunately, Oxy-CFB technology can solve the problems mentioned above since it has some certain advantages in lowcost furnace desulfurization, heating surface layout flexibility, as well as coal type adaptability, load regulation, and NOX emission control [23,25]. Hence, this paper focused on the release and migration characteristics of sodium and potassium under Oxy-CFB conditions. Alkali metals, especially sodium and potassium, show the strongest vaporization tendency among the main mineral matters in coal [26]. Hence, in the past decade, great efforts has been made to reveal the migration and release behavior of alkali metals of Zhundong coal. Because of the high content of water-soluble sodium and it is more likely to be discharged into gas phase [27], the utilization of Zhundong coal has given rise to some ash-related problems. Wang et al [28] studied the release and migration of sodium, calcium and iron of Zhundong coal under the oxy-fuel combustion conditions, they found out that it had a weak influence on the release of water-soluble sodium when the temperature between 800 and 1000 °C. Compared to the air case, oxy-fuel combustion with 21% oxygen led to more sodium and iron retained in residual ash. Zhou et al [29] investigated the sintering behavior of

2. Materials and methods 2.1. Coal properties This study selected the Wucaiwan bituminous in the Zhundong area. The coal sample was prepared following the ISO Standard (18283:2006(E)). The proximate and ultimate analyses, and the mineral contents of the coal are shown in Table 1, which was on dry basis. The ash content of Zhundong coal is only 5.41%, while the content of

Table 1 Typical properties of coal and ash used in this study. Coal

Proximate analysis (wt.%)

Ultimate analysis (wt.%daf)

M

V

FC

A

C

H

O

N

S

Zhundong coal

12.15

28.54

53.90

5.41

65.69

4.98

16.33

0.85

1.78

Coal

Ash characteristic temperature (°C)

Zhundong coal

Ash composition analysis(wt.%)

DT

ST

HT

FT

SiO2

Al2O3

CaO

Na2O

K2O

1040

1060

1070

1090

10.58

12.60

24.27

7.50

0.42

2

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The main experimental conditions of this paper are shown in the Table 3. A typical experimental run consisted of the following steps. First, the coal sample of 2 g was loaded in an alumina boat. Then, the alumina boat was placed into reaction zone of a tube furnace (The test system was shown in Fig. 1.). After that, the sample was heated at a preset temperature for 1 h. The carrier gas flow of 0.5 L/min was used to purge the reactor during both heating and cooling processes. At last, the resultant sample was collected and weighed. The sample boat was cleaned, washed, and dried before reuse in the next experimental run. Each experimental case was repeated 3 times and the results showed great repeatability. The change in the amount of ash obtained after combustion of coal in different atmospheres and temperatures is expressed by Eq. (1).

Table 2 Chemical fractionation analysis of Zhundong coal (ICP-OES results). Content(μg.g−1)

Ca Na K

W-Soluble

NH4Ac-Soluble

H-Soluble

Insoluble

Total content

56(1.0%) 2403(68%) 279(26%)

35(0.6%) 371(10%) 76(7%)

5308(95.9%) 185(5%) 50(5%)

137(2.5%) 616(17%) 657.5(62%)

5536 3575 1062.5

Na, Ca and Fe in ash is very high, and the content of Na2O is as high as 7.5%, Al2O3 and SiO2 is 12.60% and 10.58% respectively. The sodium content in Zhundong coal ash is much higher (more than 10 times) than that in common coal ashes.

mass of ash × 100% = Ashyield mass of coal

2.2. Experimental setup

(1)

where mass of ash is the quality of ash after coal combustion; mass of coal is the quality of coal before combustion. In order to visually compare the effects of different conditions on the release and migration of the sodium and potassium, this paper converted the sodium and potassium content of ash into the ratio of raw coal, defined as the residual ratio, and could be calculated by Eq. (2).

Chemical fractionation analysis was applied to measure the occurrence of sodium in the samples. The samples were sequentially extracted with deionized water, 1.0 mol.L−1 ammonium acetate (CH3COONH4), and 1.0 mol.L−1 hydrochloric acid (HCl) at a temperature of 60 °C for 24 h. The residual ash was digested with nitric acid (HNO3) and hydrofluoric acid. The solid to liquid ratio was solid sample of 1 g to solution of 50 mL. The leached and digested solutions were analyzed by inductive plasma equipped with atomic emission spectrometer (ICP-AES, Varian, America) to determine their sodium content. The sodium samples were named water-soluble sodium, CH3COONH4soluble sodium, HCl-soluble sodium and insoluble sodium, respectively.

m1 × 100% = Residual ratio m2

(2)

where m1 is the content of substance in ash; m2 is the content of substance in coal.

Table 3 Operation conditions of Different Cases. Experimental content

Atmosphere

Temperature(°C)

Particle size(mm)

Combustion temperature

CO2 + 15%H2O + 6%O2 + 1%SO2 N2 + 15%H2O + 6%O2 + 1%SO2 CO2+(0–45%)H2O + 6%O2 + 1%SO2 N2+(0–45%)H2O + 6%O2 + 1%SO2

750–950

< 0.25

850

< 0.25

Steam

peristaltic pump preheater

Gas mixer

CO2+SO2

N2+SO2

O2 Fig. 1. Fixed-bed reactor experimental system in the present study. 3

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70

5.7

CO2+SO2+O2+H2O

CO2+SO2+O2+H2O

5.6

N2+SO2+O2+H2O

N2+SO2+O2+H2O

5.5

65

Sodium content (%)

ash yield (%)

5.4 5.3 5.2 5.1 5.0 4.9

60

55

50

4.8 4.7 700

750

800

850

900

950

45 700

1000

750

800

°C

17

Ebulk

ENa

Potassium content (%)

15 14 13 12

10 700

750

800

850

900

950

1000

°C

(b) Effect of temperature on potassium content in ash Fig. 3. Effect of temperature on the release characteristics of sodium and potassium in two atmospheres.

where Em bulk , Em and Ebulk are the total energy of gas molecules in bulk KAlSi3O8 or its derivatives, the energy of gas molecules and the energy of bulk KAlSi3O8 or its derivatives, respectively. The definition of the substitution energy, Es , was the energy to substituting K atom of KAlSi3O8 with Na atom. It was calculated as below:

Es = ENa / K + EK

CO2+SO2+O2+H2O

11

(3)

Ebulk

1000

N2+SO2+O2+H2O

16

Calculations were performed using the Cambridge Sequential Total Energy Package (CASTEP) model in Materials Studio 2017 software (Accelrys, Inc.) [32,33]. Ultrasoft pseudopotentials (USP) (500 eV energy cutoff) [34] was used to describe the core and valence electron interaction and the Perdew-Burke-Ernzerhof functional (PBE) [35] of generalized gradient approximation (GGA) was employed for the exchange correlation energy. Monkhorst-Pack (3 × 2 × 4) k-point meshes [36] were used to sample for bulk KAlSi3O8 (CCSD-83531) The same convergence criteria for configuration optimization was applied to all calculations, including 1.0 × 10−6 eV/atom, 1.0 × 10−6 eV/atom, 0.01 eV/Å, 0.02 GPa and 5.0 × 10−4 Å for the SCF tolerance, energy, max force, max stress and max displacement, respectively. Gas-phase species were calculated as isolated molecules in a 15 × 15 × 15 Å3 periodic box. The formation energy, Ef , defined as the energy to filling gas molecules into bulk KAlSi3O8 or its derivatives, was calculated by:

Em

950

°C

2.3. DFT calculations

bulk

900

(a) Effect of temperature on sodium content in ash

Fig. 2. Effect of temperature on ash production rate in two atmospheres.

Ef = Em

850

and air combustion conditions is shown in Fig. 2. It can be seen that the ash yield under two atmospheres are all reduced obviously with increasing temperature. Under oxy-fuel combustion condition (the black line in Fig. 2), the overall ash yield is lower than that under the air combustion condition, When the temperature was higher than 850 °C, the ash yields of the two are almost the same and stable. Compared with the experimental results obtained by Wang et al. [28] in an atmosphere of 21% O2/79% CO2, it is found that the amount of ash obtained at the same temperature is lower. The effect of combustion temperature on residual contents of sodium and potassium in ash are shown in Fig. 3. The decrease of sodium and potassium content in ash by increasing temperature indicates more alkali metal emission in gas phase. Compared with air combustion shown in Fig. 3 (a), oxy-fuel combustion is more beneficial to inhibit the release of sodium in the range of temperature between 750 and 950 °C. The sodium content in ash is between 58.0% and 68.0%, at least 16.0% higher than that under air combustion condition. It can also be seen from Fig. 3 (b) the potassium content is much lower than the sodium content, decreasing from 16.6% (750 °C) to 12.0% (950 °C) under the oxy-fuel condition. When the temperature is higher than 900 °C, the potassium content under oxy-fuel condition sharply decreases and is lower than that at air condition, indicating the benefit of oxy-fuel condition no more than 900 °C. Song et al. [37] indicated that sodium in ash distinctly reduced as the temperature increased in both air

(4)

where ENa / K is the energy of KAlSi3O8 or its derivatives with one Na substitution of K atom. EK and ENa are the energies of K and Na atom, respectively. Corresponding author. 3. Results and discussion 3.1. Effect of temperature The occurrence modes of alkali and alkaline metals in zhundong coal was showed in Table 2. It can be seen that the content of watersoluble sodium is the predominant form, accounting for 68% of the total sodium. The fraction of insoluble sodium accounts for 17%. The present as NH4Ac-soluble and HCl-soluble sodium is quite low, with proportion of only 5–10%. The total content of potassium is only one third of sodium. The insoluble potassium is the manly chemical form in the coal, accounting for 62%. The effect of temperature on ash yield under oxy-fuel combustion 4

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Sodium content (%)

transformed proceeds through aluminosilicates as inter-media [38]. Compared with the experimental results obtained by Wang et al. [28] in an atmosphere of 21% O2/79% CO2, the tendency of sodium in different forms of existence is different, which may be due to the difference in coal type and atmosphere. Fig. 5 shows SEM micrographs of Zhundong coal ash at different temperatures under oxy-combustion condition. From these pictures, it can be seen that as the temperature increases, melting phenomenon gradually happens. Compared the five pictures, the ash at the combustion temperature of 750 °C is present as rough small particles without cohesiveness. When the temperature is higher than 850 °C, the ash exhibits clear agglomeration, and generates some large particles with irregular shapes. The particle surface are smoother, and the phenomenon of agglomeration is obvious. This phenomenon reflects that the melting point of Zhundong coal ash is relatively low. Meng et al [39] considered that during the higher temperature ashing process, the mineral compositions in ash compounded together to form low-meltingtemperature eutectic content. Therefore, the Zhundong coal-fired furnace is likely to be troubled by serious deposition and slagging. Kleinhans et al. [40] considered that the ash was not only affected by the surface viscosity of the heating surface, but also the chemical composition of the surface of the ash particles; Jensen et al. and Srinivasachar et al. [41,42] considered Al, Ca, Cl on the surface of ash particles, Fe, K, Mg, Na and other elements have a profound influence on the heated area ash. Hence, the sodium, potassium and calcium’s contents in the ash surface are detected by mapping and the result are shown in Table 4. The content of Na and K on the ash surface increases

Water-soluble NH4Ac-soluble HCl-soluble Insoluble

25 20 15 10 5 0 700

750

800

850

900

950

1000

°C Fig. 4. Effect of temperature on the forms of sodium in ash under Oxy-fuel combustion atmosphere.

combustion and gasification of Zhundong coal in a circulating fluidized bed. Sodium in Zhundong coal shows the similar behavior during combustion in this paper. Fig. 4 shows the evolution of sodium with combustion temperature under oxy-fuel atmosphere. It can be seen that when the temperature increases, the portion of insoluble sodium rises accompanying with the significantly reduction of both the water-soluble and NH4Ac-soluble sodium, especially at 800–850 °C; the HClSoluble is gradually rising at 750–850 °C, and then becomes stable between 850 and 950 °C. Contrary to the water-soluble sodium, the insoluble and HCl-soluble sodium exhibit positive correlation with an increasing temperature, hence, part of the reduced water-sodium is transformed into HCl-soluble and insoluble forms, while the rest part is released into gas phase. The possible conversion pathway is that the

(a) 750°C

Table 4 Surface content of Na, K and Ca.

Na(%) K(%) Ca(%)

750 °C

800 °C

850 °C

900 °C

950 °C

15.18 16.03 48.91

21.33 23.4 51.94

32.72 32.2 47.42

45.55 41.6 49.61

56.29 47.19 53.21

(b) 800°C

(d) 900°C

(c) 850°C

(e) 950°C

Fig. 5. Effect of temperature on the apparent morphology of ash under oxy-fuel combustion condition. 5

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CO2+SO2+O2+H2O

68

N2+SO2+O2+H2O

Sodium content (%)

66 64 62 60 58 56 54 52 50 0

5

10

15

20

25

30

35

40

45

50

H2O (%)

(a) Effect of steam on sodium content in ash

Fig. 6. Effect of temperature on crystal phase composition in coal ash under oxy-combustion conditions (1. CaSO4; 2. SiO2; 3. Fe2O3; 4. NaAlSi3O8; 5. Na2SO4; 6. Na6Si2O7; 7. Na(AlSi2O6); 8. Na2CO3; 9. Na2Si2O5; 10. Na(AlO2); 11. Na6Ca2Al6Si6O24(SO4)2).

13 12 11 10

2NaCl + 4SiO2 + Al2O3 + H2O = 2Na(AlSi2O6) + HCl

(1)

NaCl + KAlSi3O8 = NaAlSi3O8 + KCl

(2)

Na2O + 2SiO2 = Na2Si2O5

(3)

Potassium content (%)

rapidly with the increase of temperature, indicating that temperature can promote the outward diffusion of alkali metal from the interior of particles to the surfaces. On the contrary, the content of Ca is relatively uniform in the ash under the experimental temperature range. The increase of the Na and K contents on the ash particle surface forms a sticky surface layer, which can promote the occurrence of ash deposition. Fig. 6 shows the XRD patterns of the ash samples at different temperatures under oxy-fuel atmosphere. The main mineral present in all five ash samples are calcium sulfate (CaSO4) and Silicon dioxide (SiO2). It can be seen that crystalline compounds associated with sodium in the samples are strongly influenced by combustion temperature. Sodium chloride is undetectable in all samples. It can be concluded that part of NaCl releases in the gas phase, and the rest is transformed to some higher melting forms such as aluminosilicate. Li et al. [27] also got the similar conclusions mentioned above. The increase of insoluble sodium may be due to the reaction of soluble sodium with aluminosilicate, such as potassium aluminosilicate shown in reaction (1–3). When the temperature is 750–900 °C, the sodium aluminosilicate form of ash is mainly albite (NaAlSi3O8); when the temperature is higher than 900 °C, the main form of sodium is hauyne (Na6Ca2Al6Si6O24(SO4)2 and albite (NaAlSi3O8) in the ash. Li et al [27] also discovered this law. It can be concluded that at high temperature, CaSO4 may react with sodium aluminosilicate.

9 8

CO2+SO2+O2+H2O

7

N2+SO2+O2+H2O

6 5 4 3 2 1 0

0

5

10

15

20

25

30

35

40

45

50

H2O (%)

(a) Effect of steam on potassium content in ash Fig. 7. Effect of water vapor concentration on the content of Na and K in ash at 850 °C.

20%, the change gradually slows down. The rate of combustion increased because of the steam, leading to increased rates of alkali and alkaline earth metals release [43]. Therefore, it is reasonable to believe that Zhundong coal under oxy-fuel combustion atmosphere will promote the release of sodium, which may aggravate the ash-related problems. Fig. 8 shows the effect of steam on the forms of sodium in ash under two atmospheres. As the steam concentration increases, the insoluble, HCl-soluble, and water-soluble sodium contents in the ash decrease. When the steam concentration exceeds 20%, the contents of the four forms of sodium are basically stable. It can be concluded that the steam in the atmosphere impedes the conversion of water-soluble sodium to insoluble ones. Studies have shown that aluminosilicates removed hydrogen bonds and hydroxyl groups in the structure at high temperatures, forming crystal defects that promote the adsorption of sodium [44–46]. Therefore, the reason for the decrease of the sodium content in the ash may be that the steam suppresses the removal process. Fig. 9 shows SEM micrographs of steam concentrations of 0%, 10%, 20%, 30%, and 40% respectively, at 850 °C under oxy-fuel combustion condition. It can be seen that the apparent morphology of five ashes are significantly different with the increase of steam concentration. As the steam concentration increases, the surface of ash changes from rough and clear edges to smooth. Hence, steam can reduce the melting temperature of the ash.

3.2. Effect of steam The concentration of steam in the atmosphere during oxy-fuel combustion is higher than that in the air combustion. Fig. 7 shows the effect of steam concentration on sodium and potassium release under oxy-fuel and air combustion atmospheres at 850 °C. In both atmospheres, the sodium content in the ash decreases with increasing steam concentration, while the tendency of potassium is reversed. The sodium content in the ash under oxy-fuel atmosphere is higher than that under air condition, while for potassium is opposite. When the steam concentration is between 0 and 20%, the sodium content decreases significantly as the steam increases. When the concentration is higher than 6

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Sodium content (%)

Table 5 Effect of steam concentrations on the distribution of sodium on ash surface under oxy-fuel combustion condition.

Water-soluble NH4Ac-soluble HCl-soluble Insoluble

H2O (%) Na (%)

5 43.7

10 45.6

15 48.0

20 47.6

25 47.6

30 50.6

35 49.0

40 48.1

45 48.9

25 20

The chemical composition of the surface of ash particles at different steam concentrations under oxy-fuel combustion condition, using the Mapping method is shown in Table 5. The content of Na on ash surface increases with the increase of steam concentration (0–15%). As the steam concentration exceeds 15%, the trend is to be stable. Therefore, the increase of steam concentration can promote the internal migration of sodium to the surface, this may be the reason of fusion. The above results reveal that steam will change the physical form of ash and the content of sodium. The BET and XRD are chosen for further analysis. It can be seen from Table. 6 that when the steam concentration is between 0 and 15%, the surface area and pore diameter gradually increase with the increase of steam, and the pore volume’s change is not obvious. After that, the surface area and pore size tend to be stable. Therefore, it can be believed that the presence of steam in the atmosphere will influence the physical structure of ash. From the variation tendency of physical structure, it is known that the steam may accelerate the release of water soluble sodium, and reduce the probability of reacting with other substances in coal. Therefore, this leads to a decrease in both water-soluble and insoluble sodium. Fig. 10 shows the XRD patterns of the ash samples in the oxy-fuel combustion atmosphere with different proportions of steam. It can be observed that the main mineral phases for the five samples are the same: CaSO4, SiO2, Al2SiO5 and Fe2O3. In addition, the ash sample in low proportion of steam (0% and 10%) is rich in NaAlSi3O8, and poor in KAlSi3O8. As the steam concentration increases, the content of potassium aluminosilicate increases gradually, and the sodium aluminosilicate decreases accordingly. Therefore, the high concentration of steam in the atmosphere during oxy-fuel combustion plays a significant role in impeding the reaction between soluble sodium and potassium aluminosilicate.

15 10 5 0 0

5

10

15

20

25

30

35

40

45

50

H2O (%)

(a) The content of Na in the ash under oxy-combustion conditions 32

Water-soluble NH4Ac-soluble HCl-soluble Insoluble

28 24

Sodium content (%)

0 33.7

20 16 12 8 4 0 0

5

10

15

20

25

30

35

40

45

50

H2O (%)

(b) The content of Na in the ash under air-burning conditions Fig. 8. Effect of steam on the forms of sodium in ash under Oxy-fuel and air combustion atmospheres.

(a) 0%

(b) 10%

(d) 30%

(c) 20%

(e) 40%

Fig. 9. Effect of steam concentration on the apparent morphology of ash under oxy-fuel combustion condition. 7

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Table 6 Effect of steam on physical ash properties (BET test results). Water vapor(%) Surface area(m2/g) Pore volume(cm3/g) Pore diameter(nm)

0 0.52 0.004 43.17

5 0.49 0.005 45.91

10 0.54 0.004 46.48

Intensity

3 95 3 5 1 3 95 3 5 1

1

1

2 28

1 1

6

6

1 16 1

6

1 16 1

1 2 2 1 6 61 6

1 11 6 161 3 995 3 37 2 2 1 3 39 3

10

20

3 9

1

30

2 2 1 1 6

40

1

20 0.63 0.005 57.21

6 1

6 1

60

70

80

90

Two-Thta (deg) Fig. 10. Effect of water vapor on crystal phase composition in coal under oxyfuel combustion conditions (1. CaSO4; 2. Fe2O3; 3. NaAlSi3O8; 4. Na2Si3O8; 5. KAlSi3O8; 6. Al2SiO5; 7. K2SO4; 8. Na2SO4; 9. SiO2). Table 7 The experimental and optimized lattice parameters and percent errors for KAlSi3O8 and NaAlSi3O8.

KAlSi3O8 Exp.1 Opt. % NaAlSi3O8 Exp.2 Opt. % 1 2

30 0.64 0.006 56.68

35 0.66 0.005 55.10

40 0.65 0.004 55.29

45 0.64 0.004 53.06

Our experiments indicate that some Na melting from NaCl can break the crystal structure of KAlSi3O8 and occupy the position of some K atoms. Thus, the excessive K would escape from the mineral and go into gas phase, in form like KCl, consequently. However, this phenomenon seems to be obstructed by steam. In this section, we aim to reveal the feasible pathway of the sequential Na substitution in KAlSi3O8 and what the role water plays in the process at the atomic level. The KAlSi3O8 (CCSD-83531) and NaAlSi3O8 (CCSD-68913) bulk structures are both triclinic, belonging to C-1 space group. The experimental lattice parameters are listed in Table 7 as well as our calculation values. The percent errors, including lattice constants and angels, are all within 1.5%, indicating rationality of the calculation settings. As we can see from Table 7, the maximum percent error is 1.47% of lattice constant a in bulk NaAlSi3O8. Moreover, the bulk NaAlSi3O8 is a little smaller than KAlSi3O8 in all three dimensions, suggesting that Na substitution may compact the cells and thus is more stable and favorable to reside in the ash. The KAlSi3O8 and NaAlSi3O8 supercells are shown in Fig. 11. Each supercell contains four formula KAlSi3O8 or NaAlSi3O8. The oxygen, silica and alumina atoms construct almost the same bone structure by forming cavities of two different sizes, Cavity A and Cavity B (see Fig. 11a and 11b), and the potassium or sodium atoms fill the bone structure at the similar positions. Hence, we speculate that Na can easily replace K atoms to form the new mineral, NaAlSi3O8, leaving the original cell of KAlSi3O8 no change. To verify our hypothesis that Na can sequentially substitute K from bulk KAlSi3O8, we performed DFT simulations on KAlSi3O8 shown in Fig. 11a. The entire process can be divided into four steps by substituting K atoms once a time, following the order of K1, K2, K3 and K4 (see Fig. 10a). The substitution energies of each step are calculated by Eq. (4) and the results are listed in Table. 8, together with the optimized lattice constants. We can see that the lattice parameters, a, b and c, keep reducing along with the degree of Na substitution. The lattice parameters of SK4

1

50

25 0.66 0.005 55.43

3.3. DFT calculations

0% 10% 20% 30% 40%

1 1 11 6 3 9 5 3 5 82 28 1

15 0.61 0.004 48.07

a/Å

b/Å

c/Å

α/°

β/°

γ/°

8.57 8.67 1.17

12.94 13.06 0.98

7.21 7.30 1.23

90.53 90.95 0.47

115.97 115.57 −0.35

87.97 87.48 −0.56

8.14 8.26 1.47

12.79 12.90 0.87

7.16 7.23 1.01

94.26 94.25 −0.01

116.60 116.46 −0.12

87.71 87.49 −0.25

Experimental values from Ref. [47]. Experimental values from Ref. [48].

Fig. 11. Bulk structures of (a) KAlSi3O8 and (b) NaAlSi3O8 The red, yellow, pink, blue and purple spheres represent oxygen, silica, alumina, potassium and sodium, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 8

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K. Yu, et al.

(3) DFT calculations proved that H2O penetration into the mineral, KAlSi3O8, prevents Na substitution of K, resulting in more sodium release in gas phase.

Table 8 The optimized lattice parameters and substitution energies for the derivatives of KAlSi3O8.

SK1 SK2 SK3 SK4

a/Å

b/Å

c/Å

α/°

β/°

γ/°

Es /eV

8.58 8.48 8.38 8.25

13.05 13.02 12.97 12.90

7.29 7.27 7.25 7.23

91.69 92.90 93.74 94.35

115.48 115.42 115.72 116.25

87.38 87.39 87.46 87.51

0.58 0.46 0.46 0.26

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

*SK1, SK2, SK3 and SK4 are all derivatives of KAlSi3O8 with 1–4 substitution of Na atoms in the supercell.

Acknowledgment This work was supported by the National natural Science Foundation of China (51776040). References [1] Xin HH, Wang DM, Dou GL, Qi XY, Xu T, Qi GS. The infrared characterization and mechanism of oxygen adsorption in coal. Spectrosc Lett 2014;47:664–75. [2] Bloch H, Rafiq S, Salim R. Economic growth with coal, oil and renewable energy consumption in China: Prospects for fuel substitution. Econ Model 2015;44:104–15. [3] Yan XX, Lv S, et al. Great promotion of development of large scale integrative energy base in Xinjiang. Adv Technol Electric Eng Energy 2011;30:1–4. [4] Wei B, Tan HZ, Wang YB, Wang XB, Yang T, Ruan RH. Investigation of characteristics and formation mechanisms of deposits on different positions in full-scale boiler burning high alkali coal. Appl Therm Eng 2017;119:449–58. [5] Li GD, Li SQ, Huang Q, Yao Q. Fine particulate formation and ash deposition during pulverized coal combustion of high-sodium lignite in a down-fired furnace. Fuel 2015;143:430–7. [6] Sheng C, Lu Y, Gao X, Yao H. Fine ash formation during pulverized coal combustion. A comparison of O2/CO2 combustion versus air combustion. Energy Fuels 2007;21:435–40. [7] Tan Y, Croiset E, Douglas MA, Thambimuthu KV. Combustion characteristics of coal in a mixture of oxygen and recycled flue gas. Fuel 2006;85:507–12. [8] K. Jordal, M. Anheden, J. Yan, L. Stromberg, Oxyfuel combustion for coal-fired power generation with CO2 capture—Opportunities and challenges, I, 2005, 201–209. [9] Singh D, Croiset E, Douglas PL, Douglas MA. Techno-economic study of CO2 capture from an existing coal-fired power plant: MEA scrubbing vs. O2/CO2 recycle combustion. Energy Convers Manage 2003;44:3073–91. [10] Andersson K, Johnsson F. Process evaluation of an 865MWe lignite fired O2/CO2 power plant. Energy Convers Manage 2006;47:3487–98. [11] Buhre BJP, Elliott LK, Sheng CD, Gupta RP, Wall TF. Oxy-fuel combustion technology for coal-fired power generation. Prog Energy Combust Sci 2005;31:283–307. [12] Gibbins J, Chalmers H. Carbon capture and storage. Energy Policy 2008;36:4317–22. [13] Jordal K, Anheden M, Yan J, Stromberg L. Oxyfuel combustion for coal-fired power generation with CO2 capture—Opportunities and challenges. Greenhouse Gas Control Technol 2005;7 I:201–9. [14] Chen L, Yong SZ, Ghoniem AF. Oxy-fuel combustion of pulverized coal: Characterization, fundamentals, stabilization and CFD modeling. Prog Energy Combust Sci 2012;38:156–214. [15] Wilkinson GT, Chun BS. Interfacial tension in high-pressure carbon dioxide mixtures. Ind Eng Chem Res 1995;34:4371–7. [16] Fujimori T, Yamada T. Realization of oxyfuel combustion for near zero emission power generation. Proc Combust Inst 2013;34:2111–30. [17] Nsakalaya N, Gregory NL, David GT. Commercialization development of oxygen fired CFB for greenhouse gas control. Alstom Power, Incorporated 2007. [18] Scheffknecht G, Al-Makhadmeh L, Schnell U, Maier J. Oxy-fuel coal combustion—A review of the current state-of-the-art. Int J Greenhouse Gas Control 2011;5:S16–35. [19] Zeng Z, Natesan K, Cai Z, Rink DL. Effect of coal ash on the performance of alloys in simulated oxy-fuel environments. Fuel 2014;117:133–45. [20] H.A. Hirma T, Azuma N, Drastic reduction of NOX and N2O emission from BFBC of coal by means of CO2/O2 combustion: Effects of fuel gas recycle and coal type (C), Symp. of Engineering Foundation Fluidization IX, 1998. [21] Vuthaluru HB. Remediation of ash problems in pulverised coal-fired boilers. Fuel 1999;78:1789–803. [22] Anheden M, Burchhardt U, Ecke H, Faber R, Jidinger O, Giering R, et al. Overview of operational experience and results from test activities in Vattenfall’s 30 MWth oxyfuel pilot plant in Schwarze Pumpe. Energy Proc 2011;4:941–50. [23] Duan L, Jiang Z, Chen X, Zhao C. Investigation on water vapor effect on direct sulfation during wet-recycle oxy-coal combustion. Appl Energy 2013;108:121–7. [24] Hu Y, Yan J. Characterization of flue gas in oxy-coal combustion processes for CO2 capture. Appl Energy 2012;90:113–21. [25] Bläsing M, Melchior T, Müller M. Influence of the temperature on the release of inorganic species during high-temperature gasification of hard coal. Energy Fuels 2010;24:4153–60. [26] Sugawara K, Enda Y, Inoue H, Sugawara T, Shirai M. Dynamic behavior of trace elements during pyrolysis of coals. Fuel 2002;81:1439–43. [27] Li G, Wang CA, Yan Y, Jin X, Liu Y, Che D. Release and transformation of sodium

Fig. 12. Energy consumption of each Na substitution step in bulk KAlSi3O8 with (red line) or without (black line) H2O. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

are merely different from the optimized NaAlSi3O8 (see Table 6 and 7). Moreover, the first substitution step (Es = 0.58 eV) is the most thermodynamically unfavorable while the substitution energies also decrease following the extent of Na substitution. However, all four steps are facile to happen due to the experimental temperature, 850 °C. In practice, the flue gas from oxy-fuel combustion power plants is highly moist and our experiments confirmed less quantity of sodium resides in the ash because of steam effects. We suppose that water molecules can first occupy Cavity A or B in bulk KAlSi3O8 (see Fig. 11a) and thus impede Na substitution process. Our results show that Cavity A is easier to be occupied with the formation energy of 1.07 eV. Fig. 12 depicts the pathway of Na sequential substitution in KAlSi3O8 with/ without H2O molecule in the bulk. The substitution energies of each step in the case without water are almost stable and around 0.60 eV. Besides, the substitution energies are all higher than the corresponding step in the case of no water except the first Na substitution. The most distinguished difference is in the last step that 0.32 eV more energy is required when H2O exists in the bulk. Thus, the Na substitution is harder to perform when H2O exists in KAlSi3O8 structure. 4. Conclusions The release and migration of sodium and potassium in Zhundong coal were investigated under oxy-fuel combustion atmosphere. The mainly conclusions of this study are as follows: (1) The ash yield under two atmospheres are both reduced obviously with increasing temperature. Under oxy-fuel combustion condition, the overall ash yield is lower than that under the air combustion condition (2) In both atmospheres, the sodium content in the ash decreases with increasing steam concentration, while the tendency of potassium is reversed. The sodium content in the ash obtained under oxy-fuel atmosphere is higher than that under air condition, while for potassium is opposite. 9

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