Effect of Sodium Sulfate in Ash on Sulfur Trioxide Formation in


Jul 3, 2018 - To investigate the effects of different factors, including sodium sulfate (Na2SO4), etc., on the formation of sulfur trioxide (SO3) duri...
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Effect of sodium sulfate in ash on sulfur trioxide formation in post-flame region Haiping Xiao, Yu Ru, Qiyong Cheng, Gang Zhai, Chaozong Dou, Cong Qi, and Yuhui Chen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01856 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 4, 2018

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Effect of sodium sulfate in ash on sulfur trioxide formation in post-flame region Haiping Xiao, Yu Ru*, Qiyong Cheng, Gang Zhai, Chaozong Dou, Cong Qi, , Yuhui Chen School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, People’s Republic of China

Authors: Name

Email address

Haiping Xiao

[email protected]

Yu Ru*

[email protected]

Qiyong Cheng

[email protected]

Gang Zhai

[email protected]

Chaozong Dou

13905[email protected]

Cong Qi

[email protected]

Yuhui Chen

[email protected]

Full mailing address

Institution

Beinong Road 2#, Huilongguan

School of Energy, Power and Mechanical

Town, Changping District, Beijing

Engineering, North China Electric Power

102206, China

University

*Corresponding author. Yu Ru E-mail address: [email protected]; Tel: 86-18310939164; Fax: 86-10-61772811

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Abstract To investigate the effects of different factors including sodium sulfate (Na2SO4) etc. on the formation of sulfur trioxide (SO3) during oxy-fuel circulating combustion, the experiment was performed to simulate the SO3 formation condition in this work. In the experiments, the heterogeneous formations of SO3 were measured using the controlled condensation method. The results showed that the effect of sodium sulfate on SO3 formation was significant. For different ash, at high temperature, the SO3 concentration increased by 14.28 ~ 16.66 mg/m3 compared with that of raw ash after adding Na2SO4. The main reason that sodium sulfate enhanced the SO3 formation may be Na2SO4 can be combined with SiO2 and Al2O3 in ash to form NaAlSi3O8 (albite) and NaAlSiO4 (nepheline) with low fusion temperature and then release SO3. In addition, the results show that increasing SO2 concentration along with increasing temperature are favorable for enhancing SO3 formation over the range of tested parameters.

Keywords: Sulfur trioxide; Heterogeneous reaction; Reaction mechanism; Sulfates; High alkali coal; Ash

1 Introduction Zhundong is the largest integrated coalfield in China. The coal hold stored in this coal field is mainly high alkali coal. The sodium in high alkali coal has a strong ability to capture sulfur, and it may contain relatively abundant sodium sulfate (Na2SO4) in fly ash when high alkali coal is burnt. At present, the studies of high alkali coal mainly focus on the occurrence of forms of alkali metal Na in high alkali coal [1-2], the migration during combustion [3-5],

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and its influence on the ash fusibility[6-9]. Some studies have also paid attention to the effect of high sodium coal on the denitrification performance of SCR catalyst [10-13]. After the release of sodium in coal, many reactions will occur in the furnace. In general, the products of sodium in the furnace mainly include sodium chloride (NaCl), sodium sulfate (Na2SO4), sodium hydroxide (NaOH) and aluminosilicate after the reactions. NaCl and NaOH also will then generate Na2SO4. Glarborg and Marshall also thought that there would be Na2SO4 in the zone of the convective heating surface through software modeling [15]. In fact, Na2SO4 particles were found on the surface of fly ash by Dai et al. [8]. When the coal is burned, SO2 with a high concentration will be formed in the flue gas and some will be oxidized to SO3 in the gas. Besides, numerous laboratory and field tests have proved that the circulation of flue gas leads to high concentrations of SO2 and SO3 under oxy-fuel combustion [16-18]. SO3 has great harm to both production and living things. The SO3 would cause low temperature corrosion and serious ash deposits and blockage in the air preheater [19-23]. In fact, the fouling and blocking phenomena in the air preheater do indeed appear when mixedly burning Zhundong coal in the power plant [6, 9]. SO3 is also a health concern that plays an important role in the formation of photochemical smog and acid rain [24]. Although the pollution status of SO3 is very severe, only some countries have set relevant emission and governance standards [25]. There has been wide investigation of the oxidation mechanism of SO2 to SO3 under different combustion conditions. SO3 is formed mainly through the following reactions in the post-flame region [26-30]: SO + O(+M)SO (+M);

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SO + OH(+M)HOSO (+M), HOSO + O SO + HO . For heterogeneous formation, SO3 can be catalyzed by an SCR catalyst and some metal oxide, such as Fe2O3, CuO, etc. [31-33]. In addition, Marrier, Duan and Belo also found coal ash can also catalyze the SO3 formation because the ash contains Fe2O3 [34-36]. And the studies on the high temperature corrosion suggest the reactions of Na2SO4 to Na3Fe(SO4)3 and Na2S2O7 are the major reason for the high temperature corrosion in the furnace [37-39]. While Na3Fe(SO4)3 and Na2S2O7 would be decomposed easily and release SO3. Despite the importance of SO3, there are few studies reported on the SO3 formation during the combustion of Zhundong coal and the effect of Na2SO4 in ash on the SO3 formation. Therefore, it is very necessary to study the effect of Na2SO4 on the SO3 formation during the combustion of Zhundong coal. It is worth noting that the detection of formed SO3 is influenced by the analytical measurements. At present, all the methods used are closely related to the experimenter. Because the chemical property of SO3 is very active, and it is easy to combine with water vapor to form sulfuric acid, which will be condensed when the temperature is lower than the acid dew point. In previous studies, the measured values of SO3 varied greatly and the errors often reached dozens of ppm [35, 40-41]. As far as literature is concerned, the most practical and high precise method should be the control condensation method [42-44]. This paper is focused on the effects of sulfate on SO3 formation at high temperature and on revealing the formation mechanism of SO3 in the process of burning high alkali coal. In this study, the heterogeneous simulation experiments were carried out in a vertical tube flow

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reactor to simulate the SO3 formation in post-flame region (600~1000°C). In the experiments, a certain sodium sulfate was added into two kinds of ash. The reactant gas including SO2 and O2 etc. and carrier gas including CO2 and N2 were introduced into the tube reactor. The SO3 formed was captured and the amount was detected to analyze the SO3 formation. The samples of ash were characterized by XRD. Based on the above work, the effect of sulfate on the SO3 formation during the combustion of high alkali coal was studied.

2 Material and methods As shown in Fig. 1, the SO3 formation test was carried out on a self-made fixed bed reactor. The reactor is a vertical tube with 16 mm inner diameter. The reactor was placed in an electric heating furnace. The middle position of the quartz tube is designed with an inward three-jaw supporting structure, on which ash can be placed after putting quartz wool on it. The reaction region was in the center of the furnace.

1. CO2;2. N2;3. O2;4. SO2;5. Mass flow meter;6. Gas Mixer;7. Three-way valve;8. Quartz tube; 9. Catalyst;10. Tubular resistance furnace;11. Temperature controller;12. Water heater;13. Graham condenser;14.Flue gas analyzer

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Fig. 1 SO3 formation performance test device

The total flow of simulated flue gas was 3.22L/min, which made the residence time of gas was 1.5s. The main components of the flue gas were N2, SO2, CO2 and O2. Table. 1 showed the gas parameters of experiment. The N2, CO2 and O2 (99.99%) was measured by the gas mass flowmeter. The amount of reactant gases of the SO (volume fraction, 5%; carrier gas, N2) inlet was measured by a Testo350XL gas analyzer. Simulated flue gas passed through the tube furnace reactor, and through the 75~85 ℃ constant temperature water bath. The SO3 was collected by the graham condenser and water wasn’t condensed which can avoid the SO2 capture. The exhaust gas was absorbed by an impact gas collector equipped with hydrogen peroxide, sodium hydroxide solution and water, in succession. Deionized water was used to rinse the serpentine condenser and the pipeline repeatedly. Sulfuric acid formed by condensed SO3 was collected and detected by Method 8A control condensation method. In addition, to avoid SO3 condensation, the gas line was also heated. Table 1 Experimental flue gas conditions with the controlled condensation method. SO2

O2

CO2

N2

Temperature

ppm

%

%

%

2000

5

75

19.80

Ash1

Ash2

600

700

800

900

1000

2500

5

75

19.75

Ash1

Ash2

600

-

-

-

-

3000

5

75

19.70

Ash1

Ash2

600

-

-

-

-

Samples



3500

5

75

19.65

Ash1

Ash2

600

-

-

-

-

4000

5

75

19.60

Ash1

Ash2

600

-

-

-

-

2000

5

75

19.80

Ash1+Na2SO4

Ash2+Na2SO4

600

700

800

900

1000

2500

5

75

19.75

Ash1+Na2SO4

Ash2+Na2SO4

600

700

800

900

1000

3000

5

75

19.70

Ash1+Na2SO4

Ash2+Na2SO4

600

700

800

900

1000

3500

5

75

19.65

Ash1+Na2SO4

Ash2+Na2SO4

600

700

800

900

1000

Ash2+Na2SO4

600

700

800

900

1000

4000

5

75

19.60

Ash1+Na2SO4

0

5

75

20

Na SO

0

5

75

20

Na2SO4+SiO2+Al2O3

0

0

75

25

Na2SO4+SiO2+Al2O3

1000 600

1000 1000

Tests of each condition were performed in triplicate and the average was taken as the

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experimental data for that condition. SO3 formation ratio (η ) was calculated using the formula given by the equation,

η =

, c,

where , is SO3 concentration in the outlet flue gas, c, is SO2 concentration in the inlet gas. During the SO3 formation tests, the prepared sample (1g) with a particle size of 40–60 mesh and Na2SO4 (0.1g) with purity>99% were placed in the middle of a fixed bed quartz tube reactor. The ultimate and proximate analysis of two kinds of coals is shown in Table 2 and the ash composition is shown in Table 3. The coal1 was from Quanzhou in China and the coal2 was from Yulin in China. The ash was obtained by burningg coal at 900℃. Table 2 Ultimate and proximate analysis of coal (air-dried basis). Proximate analysis (wt.%)

Ultimate analysis (wt.%)

Aad

Vad

FCar

Mad

Car

Har

Sar

Nar

Oar

Coal1

35.74

11.12

52.28

0.86

69.28

4.28

3.66

1.16

0.90

Coal2

31.31

17.93

31.53

19.85

49.61

3.04

0.57

0.56

3.28

Table 3 Composition of ash (wt.%). Composition

SiO2

Ash1

44.84

18.47

15.64

5.67

0.72

3.52

2.60

1.22

0.52

0.11

n.d

93.31

Ash2

51.50

26.48

4.05

5.31

2.15

4.90

0.71

1.93

1.69

0.09

0.05

99.85

Al2O3

Fe2O3

CaO

MgO

SO3

TiO2

K2 O

Na2O

P2O5

MnO

total

X-ray diffraction (XRD) was used to determine the evolution of minerals in the ash. XRD was carried out on a German Bruker D8 X-ray diffractometer. The working voltage was 40 KV, and the operating current was 40 mA. The X-ray wavelength was 1.5406 Å. The scanning range was from10° to 80° (2Ө) and scanning rate was 0.2 °/s.

3 Results and discussion ACS Paragon Plus Environment

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3.1 Effect of temperature on SO3 formation 1.4

100

(a)

ash1 ash1+Na2SO4

(b)

ash1 ash1+Na2SO4

1.3

1.2

SO3 formation ratio (%)

3

SO3 concentration(mg/m )

90

80

70

60

1.1

1.0

0.9

0.8

50

0.7 600

700

800

900

1000

600

700

80

900

1000

900

1000

1.1

ash2 ash2+Na2SO4

ash2 ash2+Na2SO4

1.0

SO3 formation ratio(%)

75

800

Temperature (℃)

Temperature(℃)

70

3

SO3 concentration(mg/m )

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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65 60 55 50

0.9

0.8

0.7

45 0.6

40 600

700

800

900

1000

600

700

Temperature(℃)

800

Temperature(℃)

Fig. 2 Effect of temperature on SO3 formation

The SO3 formation of the ash and the ash with Na2SO4 added at different temperatures is shown in Fig. 2. For ash1 and ash2, both the SO3 concentration and formation ratio increased with increasing temperature. For ash1, at 600 °C, the SO3 concentration was 52.36 mg/m3 and the formation ratio was 0.73%. When the temperature reached 700 °C, the SO3 concentration and formation ratio significantly improved. At 1000 °C, they were the highest, and the SO3 concentration and formation ratio were 78.54 mg/m3 and 1.33%, respectively. Compared with SO3 formation at 600°C, the SO3 concentration was increased by 26.18 mg/m3 and the formation ratio was increased by 0.6%. For ash2, the SO3 concentration was relatively low, while the variation tendency of SO3 formation was consistent with ash1. The effect of temperature on the SO3 formation by ash is different from the study by

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Duan et al. [34]. They found that the catalytic activity of ash on SO3 first increases and then decreases as the temperature rises, and it reaches the maximum at 700 °C. The different results may be due to the different components in the ash. The content of Fe2O3 in the ash used in this paper was higher, while the content of CaO was lower. In Duan’s research, the Fe2O3 content in ash was 4.16~10.37%, while the CaO content was 8.66~19.99%. Fe2O3 has an obvious effect on the catalytic formation of SO3 [31, 36, 45], while CaO can inhibit the SO3 formation because of its adsorption capacity to SO3. The mechanism of Fe2O3 catalyzing the SO3 formation is as follows [31]: SO2 first combines with the lattice oxygen, O (latt), of Fe2O3, to form SO (ads) and a transferable electron is produced. Oxygen and oxygen vacancies are combined to produce adsorbed oxygen, O (ads). SO (ads) and O (ads) are combined to form SO3.

 () +    ( !")  ( !") +   2 ( !") $ () +  (% &&)$  ( !") +   $  ( !") +  ( !") → $ +  (% &&) While the reaction that CaO can inhibit the SO3 formation is as follows,

)  + $ → ) $ When the temperature increases, the two metal oxides will be sintered. The effects of the two substances on SO3 formation restrict each other. When the iron content is obviously higher, the promotion effect will be stronger than the inhibition effect, and the SO3 concentration will increase. The effect of ash1 promoting SO3 formation was stronger than that of ash2. Besides, the sulfur content in ash1 was significantly lower than that in ash2. Thus

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the experimental results show that the sulfur content in ash has a relatively weak influence on the catalytic formation of SO3, while Fe2O3 is the main composition affecting the ash catalyzing SO3. Compared with the raw ash, after adding Na2SO4, the SO3 concentration and SO3 formation ratio increased with the temperature. For ash1, when the sodium sulfate was added, the SO3 concentration was close to that of raw ash at 600 °C. With the increase of temperature, the effect of adding Na2SO4 on SO3 formation increased gradually. At 1000 °C, the SO3 concentration increased by 16.66 mg/m3 compared with raw ash. For ash2, the SO3 concentration also obviously increased compared with the raw ash. With the increase of temperature, after adding Na2SO4 the SO3 concentration increased gradually. The results show that the presence of Na2SO4 will obviously promote SO3 formation at high temperatures. The reason for the greater SO3 formation under high temperature conditions would be discussed in the later section numbered 3.3.

3.2 Effect of SO2 concentration on SO3 formation 160

1.4

(a)

(b)

140

3

SO3 formation ratio(%)

1.2

SO3 concentration(mg/m )

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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120

1.0

100

0.8

80

0.6

60 0.4

2000

2500

3000

3500

4000

2000

SO2 concentration(ppm)

2500

3000

3500

SO2 concentration(ppm)

Ash1+Na2SO4 at 600℃

Ash1+Na2SO4 at 700℃

Ash1+Na2SO4 at 900℃

Ash1+Na2SO4 at 1000℃

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Ash1+Na2SO4 at 800℃ Ash1 at 600℃

4000

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1.1

100

(d)

1.0

(c) 3

SO3 formation ratio(%)

90

SO3 concentration(mg/m )

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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80

70

60

0.9

0.8

0.7

0.6

0.5

50

0.4

40 2000

2500

3000

3500

4000

2000

SO2 concentration(ppm)

2500

3000

3500

4000

SO2 concentration(ppm)

Ash2+Na2SO4 at 600℃

Ash2+Na2SO4 at 700℃

Ash2+Na2SO4 at 800℃

Ash2+Na2SO4 at 900℃

Ash2+Na2SO4 at 1000℃

Ash2 at 600℃

Fig. 3 Effect of SO2 concentration on SO3 formation

As shown in Fig. 3, it can be predicted reasonably that SO3 concentrations will increase with the increase of input SO2 concentration. In order to simulate the condition of oxy-fuel combustion, this paper chose five SO2 concentrations, from 2000 ppm to 4000 ppm. For ash1 without Na2SO4, the SO3 concentration ranged from 52.36 mg/m3 for 2000 ppm of SO input to 69.02 mg/m3 for 4000 ppm of SO2 at 600 °C (Fig. 3a). After adding Na SO , the SO3 concentration ranged from 53.55 mg/m3 for 2000 ppm of SO input to 96.39 mg/m3 for 4000 ppm of SO2 at 600 °C, which increased by 76.7%; it ranged from 95.20 mg/m3 for 2000 ppm of SO2 input to 147.56 mg/m3 for 4000 ppm of SO2 at 1000 °C, which was an increase of 56.6%. The higher the concentration is, the more obvious the effect of Na2SO4 is. For ash2, the variation tendency of formed SO3 was consistent with ash1. These results are consistent with previous studies [24, 34-35, 45]. As for the reaction of SO3 formation, raising the SO2 concentration can shift the reaction to the right side and the reaction will be promoted to form more SO3. Meanwhile, the relatively high SO2 concentration can inhibit the decomposition of SO3. This also made the SO3 concentration increase. As shown in Fig.3, the formation ratio decreases with the increase of SO2 concentration.

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This is consistent with the previous studies [27, 35]. Taking the example of ash1, after adding Na2SO4, the SO3 formation ratio decreased from 0.76% for 2000 ppm of SO2 input to 0.63% for 4000 ppm of SO2 input at 600 °C; the formation ratio decreased from 1.33% to 1.06% at 1000 °C. This is because when the SO2 concentration increases, the temperature and the content of Fe2O3 etc. become the main factors that limit the SO3 formation. This is consistent with the expected result. At the same temperature, the SO3 concentration of ash1 was significantly higher than that of ash2. The content of Fe2O3 in the ash has a major impact on the SO3 formation, and the Fe2O3 content in ash1 was significantly higher than that in ash2. The results are consistent with previous studies.

3.3 Discussion on the reasons for Na2SO4 promoting SO3 formation. At present, studies on the high temperature corrosion suggest that the reactions of Na2SO4 to Na3Fe(SO4)3 and Na2S2O7 are the major reason for the high temperature corrosion by sulfate in the furnace [37-39]. In addition, the diffraction peak of Na3Fe(SO4)3 was also found in the slag when the high alkali coal burned [46]. This suggests that Na3Fe(SO4)3 and Na2S2O7 could be formed as intermediates to promote SO3 formation after adding Na2SO4. 1) Supposing Na2S2O7 is the intermediate, the possible reaction pathway is as follows:  In the presence of oxygen, SO2 and SO are combined to produce S O

* , then S O* is

decomposed into SO3 and SO

at high temperature. Namely the reaction is:

2$ () + 2+ +

 $ 

 $ * $ ()

+  ()2+

++

 $ *

 $

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(1) (2)

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2) Supposing Na3Fe(SO4)3 is the intermediate, the possible reaction pathway is as follows: In the presence of O2 and Fe2O3 , Na3Fe(SO4)3 is formed as the intermediate and then decomposed into SO3, Fe2O3 and Na2SO4. Namely the reaction is:

6$ () + 3 + 2.  + 6+ 2+

.($ )

 $

→ 4+

→ 3$ () + .  + 3+

.($ )

(3)

 $

(4)

It is undeniable that the SO3 formation increased significantly with the addition of Na2SO4. To determine the reasons for Na2SO4 promoting the SO3 formation, XRD was used to analyze the ash samples at different temperatures by taking the ash2 as example. The XRD spectra at different temperatures are shown in Fig. 4. This paper investigates the effect of Na2SO4 on the SO3 formation. Thus it is mainly concerned with the characteristic peaks of substances containing Na that may exist in the ash. Fe2O3 in the ash has a catalytic effect on the SO3 formation, so the peak of Fe2O3 was also marked.

1

Ash+Na2SO4-1000℃ Ash+Na2SO4-600℃

14

34 5 5 2 1

Ash 6

51 5

15 5

1

1

3 25251 6

1 5 5

155

51 5

15 5

1 SiO2 2 Na2SO4 3 NaAlSi3O8 4 NaAlSiO4 5 Fe2O3 6 MnO

1

1

10

20

3 25 51 6 30

40

50

60

θ/°

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70

80

90

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Fig. 4 XRD analysis of ash at different temperatures

The structure of the XRD spectra of the ash is not basically changed after adding Na2SO4 The characteristic peaks of the main substances in the samples are basically consistent and correspond well to each other. The diffraction peak of Na3Fe(SO4)3 and Na2S2O7 were not found in the all XRD samples. Thus, the assumption proposed above was not right. The main composition of ash2 is SiO2, and the iron content was relatively low. Thus the diffraction peak intensity of Fe2O3 was low. Due to their low iron content, the intensity of the diffraction peaks of Fe2O3 showed no obvious change. Besides, without adding Na2SO4, the diffraction peak intensities of Na2SO4 (31.9°, 34.1°) and NaAlSi3O8 (albite) were low, implying their contents were low. After adding Na2SO4, the intensity and amount of diffraction peak of Na2SO4 increased. While the intensity of diffraction peak of NaAlSi3O8 was basically unchanged. The diffraction peak strength of Na2SO4 decreased with increasing temperature, which means the content of Na2SO4 decreased. Meanwhile, the types of minerals containing sodium became gradually enriched with the increase of temperature. When the temperature reached 1000℃, both the intensity and the amount of diffraction peak of Na2SO4 decreased. The diffraction peak at 34.1°disappeared. And it can be found that the diffraction peak intensity of NaAlSi3O8 in the ash was obviously enhanced. The nepheline diffraction peak of NaAlSiO4 (nepheline) appeared at 29.7° and 23.1°. This is because Na2SO4 would be combined with SiO2 and Al2O3 in ash to form albite and nepheline with low fusion temperature. Then the two substances would further form low temperature co-melt with other minerals, which is the reason for the low fusion temperature of the ash with high sodium content [47-49]. And the effect of Na2SO4 on ash

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fusion was also confirmed by the experiment in this paper, as shown in Fig. 5. 1300

1300

Raw ash 1# Adding 10% Na2SO4

Raw ash 2# Adding 10% Na2SO4

1250

1250

Temperature/℃

Temperature/℃

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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1200

1150

1200

1150

1100

1100 DT

ST

HT

FT

DT

ST

HT

FT

Fig. 5 Effect of Na SO on the fusibility of the ash

After adding Na2SO4 in the ash, the minerals in ash may react as following equation at 1000℃.

+

 $

+ 0%  + 6$1 → 2+ 0%$1 2 ( %31&) + $ ↑

(5)

+

 $

+ 0%  + + 0%$1 2 → 3+ 0%$1 (56ℎ%15) + $ ↑

(6)

As temperature increasing, Na2SO4 had complex reactions with the minerals in the ash to form various kinds of minerals, which led to the gradual decrease of their content. Although the content of Na2SO4 decreased, the SO3 concentration was higher under high temperature (Fig.3). There is a complex evolution of Na2SO4 in ash with increasing temperature. With the respect to the SO3 formation, the decrease of the relative contents of Na2SO4 indicate that sulfur was released from Na2SO4 during the process of forming inorganic minerals, and some sulfur may be released in the form of SO3. It needs to be noted that if Na2SO4 exist alone, it wouldn’t be decomposed to release SO3. Only if Na2SO4 is in the ash, it would react with SiO2 and Al2O3 to for the compounds with low fusion temperature and release SO3 simultaneously. However, the mechanism is not clear and it requires further investigation. In order to confirm the inference and better understand the effect of Na2SO4 promoting

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SO3 formation, the experiments on Na2SO4 and Na2SO4 with SiO2/Al2O3 were carried out in this paper. In the test condition, SO2 was not included in the experimental gas, and only O2/N2/CO2 is introduced into the reactor. The experimental results and the XRD analysis of samples after experiment were shown in Fig. 6 and Fig. 7, respectively. It can be seen that Na2SO4 didn’t decompose to release SO3 when it existed alone. At low temperatures, Na2SO4 wouldn’t react with SiO2 and Al2O3 to produce the aluminosilicate with low fusion temperature. While at high temperature Na2SO4 would release SO3 through the above reactions. Besides, according to the condition without O2, the released gas containing sulfur is SO3 rather than SO2 and O2. The diffraction peaks of albite and nepheline can be clearly observed by XRD after experiment. The experiment results and XRD spectrum can be regarded as a support for the above inference. 20 18 16

SO3 concentration (ppm)

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

14

Na2SO4/O2-970 Na2SO4+Al2O3+SiO2/O2-600 Na2SO4+Al2O3+SiO2/O2-970 Na2SO4+Al2O3+SiO2-970

12 10 8 6 4 2 0

Fig. 6 SO3 formation by Na2SO4/SiO2/Al2O3

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1 NaAlSiO4 2 NaAlSi3O8

1 22

20

40



60

80

Fig. 7 XRD of Na2SO4/SiO2/Al2O3

4 Conclusions Experimental studies were conducted to determine the effects of SO2 concentration, temperature, coal and sulfates on the heterogeneous formations of SO3 in flue gas under oxy-fuel combustion. In the experiments, heterogeneous formations of SO3 were measured using the controlled condensation method. The results showed that the effect of sulfates on SO3 formation was significant. After adding Na2SO4, compared with the raw ash, the SO3 formation ratio increased greatly. Based on the experiment results, a mechanism of Na2SO4 promoting SO3 formation was built, which is Na2SO4 would be combined with SiO2 and Al2O3 in ash to form NaAlSi3O8 (albite) and NaAlSiO4 (nepheline) with low fusion temperature and then release SO3. In addition, the SO3 concentration was apparently affected by the SO2 concentrations and the temperature. With increase of the SO2 concentration, the SO3 concentration increased and the SO3 formation ratio decreased. With the increase of temperature, the SO3 formation ratio increased gradually after adding Na2SO4. The SO3 formation ratio changed with different

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chemical compositions in the ash. The higher Fe2O3 content and lower CaO content in the ash are more favorable to SO formation.

Declaration of interest The authors declare no competing financial interest.

Acknowledgements Financial support for this work by the National Natural Science Foundation of China (No. 51206047) is gratefully acknowledged.

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