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The effect of interaction between sodium and oxides of silicon and aluminum on the formation of fine particulates during synthetic char combustion Renhui Ruan, Jiafan Xiao, Yongle Du, Houzhang Tan, Xuebin Wang, and Fuxin Yang Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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Title page 1. Title: The effect of interaction between sodium and oxides of silicon and aluminum on the formation of fine particulates during synthetic char combustion 2. Author(s) and affiliation(s): Renhui Ruan Jiafan Xiao Yongle Du Houzhang Tan * Xuebin Wang Fuxin Yang MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China

3. Corresponding author: Houzhang Tan (1) Post address: School of Energy and Power Engineering, Boiler Laboratory Xi’an Jiaotong University 28 Xian Ning West Road, Xi’an, Shanxi, P.R.China.710049 (2)Telephone number: +086-029-82668051; +086-15829053541 (3)Fax number: +86-029-82668703 (4) E-mail: [email protected]

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The effect of interaction between sodium and oxides of silicon and aluminum on the formation of fine particulates during synthetic char combustion Renhui Ruan a, Jiafan Xiao a, Yongle Du a, Houzhang Tan a,*, Xuebin Wang a, Fuxin Yang a a

MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China

Abstract： Alkali and alkaline earth metals (AAEM) have great contribution to the formation of fine particulates during coal combustion, especially sodium. Extraction and loading through chemical method have been widely used for coal pretreatment to change the occurrence and content of sodium but could not load intrinsic minerals. In this study, synthetic char was used as mineral carrier to study the characteristic of fine particulate formation during coal combustion. The silica and alumina were added to synthetic char as intrinsic minerals to study the interaction between sodium and Si, Al compounds in coal. The results show that the inorganic water-soluble sodium is more likely to form stable fine particles, organic sodium is prefer to react with silica, alumina in the absence of chlorine. Chemical reactions and physical capture are two main ways for sodium capture by silica, alumina. The content of sodium captured through chemical reactions is 2.4 times of that by physical way. Key words：Sodium; Occurrence; Fine particles; Synthetic char

1. Introduction The Zhundong coalfield in Xinjiang province is expected to become the energy base of China in near future due to its huge reserves. Zhundong coal belongs to low rank coal with high content of AAEM1-3, especially sodium. Serious


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slagging and fouling problems occurred during low rank high-AAEM coal combustion will reduce the efficiency of power plant with different type of boilers4, 5. The high contents AAEM of Zhundong low rank coal have been proved to contribute to fine particulate formation greatly6, with sodium has been focused since 1980s in the world7. The smog and haze happened in recent China are endangering people’s health8, 9. Fine particulate matter generated from coal combustion contributes greatly to the atmospheric PM2.5 in eastern China10. It is worth studying the characteristic of PM formation from sodium rich coal combustion to realize clean coal technology. There are three main kinds of occurrences for sodium in coal11：Inorganic water-soluble sodium mainly exists as sodium chloride or hydrated ion, organic-bound sodium mainly combines with the coal matrix, acid insoluble sodium such as sodium-aluminosilicate. The different migration characteristics of these three kinds of sodium will affect the PM formation during coal combustion12. At present, the most widely used pretreatments for raw coal to remove AAEM include water wash, acetic acid wash and hydrochloric acid or nitric acid wash13, 14. By this way, the effect of sodium occurrence in raw coal on the formation of fine particulates can be revealed. The pretreatment process can remove most of the water soluble sodium, carboxylic sodium and acid soluble sodium gradually, but can also wash out some other minerals such as organically-bound magnesium, calcium, iron and silicon14. Gao14 analyzed the contribution of Na, Mg, Ca, Fe, Si, Al, Cl-, SO42- on PM10 formation of Victorian brown coal. The mineral components of water-washed coal and diluted-acid-washed coal are still too complex to illustrate the effect of interactions between different elements on PM formation. The coal structure will be changed during the pretreatment and it’s difficult to exclude these disturbing factors. For these reasons, some researchers9,13 control the target elements contents by adding specific compounds of target elements to acid-washed coal to study the influence of target elements on PM characteristics. It could explain the interactions between target elements and external minerals but is difficult to introduce the intrinsic minerals13. Wen13


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studied the influences of excluded quartz on PM formation characteristic from sodium-loaded coal. The quartz can only be mixed physically with pulverized coal. As the distribution of mineral matter in coal has great effect on the formation characteristic of fine particulate15, it is necessary to explore the influence of mineral distribution on PM formation. Pure substances mixture can also be used to study the interactions among mineral matters16. Si16 used pure kaolin, NaAc(sodium acetate) and NaCl to investigate the influence of interactions between kaolin and organic-bound sodium or water-soluble sodium. It could eliminate the interference of other mineral elements in coal, but is difficult to reproduce the local high temperature or strong reducing atmosphere17 inside or around pulverized coal particle during combustion. Synthetic carrier is another way to study the migration of target elements during coal combustion. Helble18 and Graham19 used synthetic char to study the contribution of mineral matter to fine particles during coal combustion, which was an early attempt that used synthetic char to study PM formation from solid fuel combustion. The char particle surface temperature was closed to that of real pulverized coal, which means the combustion characteristic of synthetic char is somewhat similar to coal and it can be used to model pulverized coal combustion. Synthetic carrier has been widely used to study char or coal combustion21 and it has great advantages on exploring the effect of mineral elements on fine particulate matter22. Wang21 compared the reactivities of synthetic model coal and real coal through TG (Thermal Gravimetric Analysis), the thermal reaction behaviors of synthetic fuel was similar with high volatile lignite. It provides a possibility to model lignite through synthetic carrier. Recently, Xu20 studied the gasification characteristic of silica and PM formation during coal combustion by adding silica to synthetic char. He developed an advanced silica vaporization model containing the influence of water steam which coincides well with experiment results. Although it is difficult to reproduce the devolatilization and volatile combustion process through synthetic char, the effect of mineral particle distribution and element occurrence on PM formation from coal combustion can be well


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studied by synthetic char due to its well performance on simulating char combustion. In order to eliminate the interference of other mineral elements on the characteristic of PM formation during coal combustion, we combine methods from previous researches18-22 and choose synthetic char as a carrier. Taking the experimental operability and the combustion similarity into account, carbon black and sucrose are chosen to prepare synthetic char. Sodium chloride and sodium acetate are used as additives for inorganic water-soluble sodium and organic-bound sodium respectively. Silica and alumina are chosen to simulate the common minerals of Si and Al in coal. We study the effect of sodium occurrence, interaction between sodium and silicon, aluminum compounds on the formation characteristic of fine particulate from coal combustion. To the best of our knowledge, there is no published research studying the effect of interaction between water-soluble, organic-bound sodium and intrinsic silicon, aluminum compounds on PM formation through synthetic char. It can reveal the sodium migration process of different occurrence in coal, and help deeply understanding the interaction between sodium and conventional Si, Al minerals in coal.

2 Experimental 2.1 Preparation of synthetic char In this paper, the preparation method of synthetic char is based on Graham’s19 research, the processes are shown in Figure 1 and Table 1. Each step in Figure 1 is explained as follows. A: Mixing the additives ① with deionized water and stirring for 1 hour. B: Heating the sample from step A at 353 K for 8 hours. C: Sample from step B undergoes pyrolysis from room temperature to 823 K in a horizontal furnace, the heating rate is 10 K/min and the holding time at 823 K is 30 minutes. D: The char sample from step C is sieved to below 97 um. E: Dissolving the


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additive ② into deionized water and immersing the D sample into the solution. F: Heating the sample from step E at 313 K until the moisture is evaporated. G: The char sample from step F is sieved to below 97 um. deionized water 1

80 furnace

mortar

sieve

N2

97µm

B

A

D

C

deionized water+ 2 D sample

40 sieve

mortar

97µm

E

F

G

Note: D sample represents the sample obtained from step D. Figure 1. Preparation methods of synthetic char

The manufacture methods and additives used in Figure 1 are shown in Table 1 as below. The SiO2 and Al2O3 used in our experiment are sieved to 13~20 um. Acetylene carbon black and analytical reagent sucrose are used in our study. The sodium containing compounds are added to ensure that the weight percentage of Na2O accounts for 1% of the total mass of the raw materials. Table 1. Production process of synthetic char Method Fuel

Sample preparation procedure A

B

C

D

C

√

√

√

√

NaCl+C

√

√

√

√

√

√

NaCl+SiO2+Al2O3+C

√

√

√

√

√

-COONa+C

√

√

√

√

-COONa+SiO2+Al2O3+C

√

√

√

√

a

E

F

Weight percentage a %

Additives G

①

②

Carbon black + Sucrose

None

100:0:0:0

√


NaCl

99:0:0:1

√

√

Carbon black + Sucrose + SiO2 + Al2O3

NaCl

90:6:3:1

√

√

√


CH3COONa

99:0:0:1

√

√

√

Carbon black + Sucrose + SiO2 + Al2O3

CH3COONa

90:6:3:1

(carbon black or carbon black + sucrose) : SiO2 : Al2O3 : Na2O


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In Table 1’s ‘Fuel’ list, ‘C’ represents pure synthetic char; ‘-COONa+C’ represents synthetic char loaded with organic-bound sodium; ‘NaCl+C’ represents synthetic char loaded with inorganic water-soluble sodium; ‘-COONa+SiO2+Al2O3+C’ represents synthetic char loaded with organic-bound sodium, silica and alumina; ‘NaCl+SiO2+Al2O3+C’ represents synthetic char loaded with inorganic water-soluble sodium, silica and alumina. For convenience, the symbols in Table 1’s ‘Fuel’ list are used below.

2.2 Analysis of synthetic char The typical properties of C and C+SiO2+Al2O3 are presented in Table 2. NaCl and CH3COONa were added to C or C+SiO2+Al2O3 through immersion method. Due to the low sodium loading level in our experiment, the proximate and ultimate properties of NaCl+C, -COONa+C are supposed to be similar with C while the NaCl+SiO2+Al2O3+C, -COONa+SiO2+Al2O3+C are supposed to be similar with C+SiO2+Al2O3. The ash was obtained in muffle furnace at 673 K to inhibit the release of AAEM (alkali and alkaline earth metal). The ash content of C is 0.35% which is close to that of acid-washed coal13. The purity of carbon black and sucrose used in our experiment is high enough to eliminate the effect of other mineral elements, and it is convenient to study the interaction between sodium and silicon, aluminum compounds during pulverized coal combustion. Table 2. Typical properties of synthetic char Fuel

Proximate Analysis (wt%)

Ultimate Analysis (wt%)

Mad

Aad

Vdaf

FCdaf

Cdaf

Hdaf

Odaf

Ndaf

St,daf

C

1.22

0.35

3.64

96.36

96.68

0.59

1.93

0.15

0.01

C+SiO2+Al2O3

0.95

8.26

3.14

96.86

93.64

0.55

5.39

0.40

0.02

Ash Compositions

(wt%)

Fe2O3

Al2O3

CaO

MgO

SiO2

SO3

Na2O

C

42.44

8.44

7.76

2.57

16.60

13.18

8.99

C+SiO2+Al2O3

3.82

32.12

0.35

0.92

61.21

1.02

0.56

ad: air dry basis, daf: dry ash free basis


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2.3 The drop tube furnace, sampling system and analysis methods The drop tube furnace used in this study is referred to our previous works23, 24. The combustion temperature during experiment is 1573 K. Using oxygen and nitrogen to simulate air condition at 1:4 ratio. A self-designed micro powder feeder is used to control the feeding rate at around 100 mg/min. The primary air is 0.5 L/min and the secondary air is 2.7 L/min. The flue gas temperature at the sampling position is 573 K for all conditions. Nitrogen quenching and PM10 cyclone are adopted to help sampling PM10 and the whole sampling system is heated around 423 K25, 26 to avoid acid gas condensation. DLPI (Dekati Low Pressure Impactor) is used to classify the particles into 13 fractions according to the aerodynamic diameter. The mass of particle on the aluminum foil are determined by a precision electronic balance (Sartorius M2P, 0.001 mg). The chemical composition of particulate matter on the aluminum foil are quantified by scanning electron microscopy equipped with an energy dispersive spectrometer (JSM-6390A)23, 25.

3. Results and discussion 3.1 Mass based particle size distributions of synthetic chars The mass based particle size distributions of PM10 are presented in Figure 2. The peaks of fine mode for C and -COONa+SiO2+Al2O3+C locate at around 0.1 um. The peaks of fine mode for -COONa+C and NaCl+SiO2+Al2O3+C locate at around 0.16 um. The fine mode peak for NaCl+C locates at around 0.2 um. The submicron particles are usually generated from mineral vaporization, nucleation, condensation and surface growth process27. The higher vapor concentration will result in larger size of submicron particles6. The peaks of coarse mode for C, -COONa+C and NaCl+C locate at around 1.6 um while that for -COONa+SiO2+Al2O3+C and NaCl+SiO2+Al2O3+C locate between 2.3 um and 4 um. The concentration of ultra-micron particles from C is low which means there are little coarse mineral


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particles in C. The ultra-micron peaks for synthetic chars without silicon-aluminum additives nearly coincide, indicating that the inorganic water-soluble sodium and organic-bound sodium loaded to synthetic char mainly contribute to submicron particulates.

dm/d log Dp mg PM/g fuel

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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15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0.01

NaCl+SiO2+Al2O3+C NaCl+C C

0.1

-COONa+SiO2+Al2O3+C -COONa+C

1

10

Dp (m)

Figure 2. Mass based particle size distributions of synthetic chars

We found the iron content of PM1 from C is almost 50% through the composition analysis. The iron in synthetic char is supposed to be introduced in the manufacturing process of carbon black. Compared with the amount of additives in our experiment, the ash content of C is low. For convenience of discussion, the iron in PM is removed.

3.2 PM components analysis of sodium-containing synthetic chars Figure 3 shows the element components of particulate matter from synthetic chars. The carbon content was around 4% according to SEM-eds results. It is convenient to comparing the Na, Si, Al contents in PM with carbon and oxygen removed. Figure 3(a) represents -COONa+C. The main elements of PM include sodium, sulfur and silicon. Sodium comes from sodium acetate while sulfur and silicon are the impurities in acetylene carbon black. Figure 3(b) represents -COONa+SiO2+Al2O3+C. The mass fractions of silicon and aluminum in PM increase obviously after adding silicon-aluminum compounds. Under the strong reducing atmosphere and high local temperature of char


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combustion, a part of SiO2 and Al2O3 additives will be reduced to sub-oxides or substances which are easier to gasify28. On the other hand, a small part of SiO2 and Al2O3 particles smaller than 10 um are fixed to the synthetic char because the sieving process cannot remove all SiO2 and Al2O3 particles with particle size less than 10 um. These small additive particles can transform to PM10 directly. The combined two factors above lead to the mass fraction increase of silicon and aluminum in PM from -COONa+SiO2+Al2O3+C. However, the reductive reaction for SiO2 is more easier than that for Al2O319, 29, 30. So the mass fraction of aluminum does not increase significantly. Figure 3(c) represents NaCl+C. Chlorine and sodium are the main elements in PM, and sodium chloride is supposed to be the main component. The sodium chloride is vaporized during char combustion and then nucleates to generate fine particles as the flue gas cooling down. Figure 3(d) represents NaCl+SiO2+Al2O3+C. The main elements are still sodium and chlorine, but the mass fractions decrease a little. The results indicate the influence of silicon, aluminum compounds on the composition of submicron particles. Similar to Figure 3(b), the silicon and aluminum content of particles larger than 0.6 um in Figure 3(d) increase obviously. In Figure 2, the mass based particle size distributions of -COONa+SiO2+Al2O3+C and NaCl+SiO2+Al2O3+C have a high coincidence degree in the ultra-micron region. It means the silicon, aluminum additives in -COONa+SiO2+Al2O3+C and NaCl+SiO2+Al2O3+C are the main sources for PM1-10. 100

100

90

90

Elements mass concentration %


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 50 40 30 20

70 60 50 40 30

Ca K Cl S P Si Al Mg Na


20 10

10 0

80

0

0.053

0.028 (a) -COONa+C

0.155 0.091

0.382 0.262

0.613

Dp (m)

0.946

1.592

2.378

3.972

6.642

0.382 6.642 0.946 0.155 2.378 3.972 0.613 1.592 0.262 0.091 (b) -COONa+SiO2+Al2O3+C D (m) 0.053

0.028

p


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Ca 100

100

90

90


K


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

80 70 60 50 40 30 20 10

80

Cl

70

S

60


P

50

Si

40 30

Al

20

Mg

10

Na 0

0

0.382 0.382 0.053 0.053 6.642 0.946 6.642 0.946 0.155 2.378 0.155 2.378 3.972 3.972 0.613 1.592 0.613 1.592 0.028 0.262 0.028 0.262 0.091 0.091 (c) NaCl+C Dp (m) Dp (m) (d) NaCl+SiO2+Al2O3+C

Fe

Ca

K

Cl

S

P

Si

Al

Mg

Na

Figure 3. Element components of particulate matter from synthetic chars

3.3 Sodium partitioning behavior in PM The results above show that the addition of silicon and aluminum additives has a great influence on the PM formation characteristics from sodium-loaded synthetic chars. Due to the 0.35% ash content of C, the interactions between sodium and silicon, aluminum compounds are considered to be main reason for the change of PM composition31. A main difference between –COONa+C and NaCl+C is that the sodium released from –COONa+C can only combine with oxygen if neglecting the little amount sulfur, chlorine and phosphorus in carbon black. The sodium released from synthetic char loaded with inorganic water-soluble sodium mainly combines with chlorine and forms sodium chloride. The reactivity of these two sodium compounds with silicon, aluminum compounds is different. Figure 4 depicts the Na/Cl molar ratio of synthetic chars loaded with sodium of different occurrence. The molar ratio of Na/Cl is higher when particle becomes larger. Sodium tends to combine with other components rather than chlorine or oxygen when particle size increases.


Energy & Fuels

100

Molar ratio of Na/Cl

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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10

1 -COONaC+C NaCl+C -COONa+Si+Al+C NaCl+Si+Al+C

0.1 0.01

0.1

1

10

Dp (m)

Figure 4. Distribution of Na/Cl molar ratio for synthetic chars

For particles dominated by nucleation and condensation mechanisms, sodium reacts with chlorine preferentially. Thus in submicron particle size region, the molar ratio of Na/Cl is lowest except for –COONa+C. When the amount of chlorine is sufficient, the Na/Cl molar ratio is closed to 1, for example NaCl+C and NaCl+SiO2+Al2O3+C in Figure 4. It is speculated the particulates in submicron range for NaCl+C and NaCl+SiO2+Al2O3+C are mainly sodium chloride which can be proved by the SEM image in Figure 5.

Figure 5. Crystal NaCl particles from NaCl+C combustion

With particle size increase, the reactions between sodium and silicon-aluminum compounds are shown below16：


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Al2O3(s)+Na2O(g,s)→2NaAlO2(s) Al2O3(s)+2NaCl(g,s)+H2O(g)→2NaAlO2(s)+2HCl(g)

(1) (2)

SiO2(s)+ Na2O(g,s)→Na2SiO3(s)

(3)

SiO2(s)+2NaCl(g,s)+H2O(g)→Na2SiO3(s)+2HCl(g)

(4)

Sodium is captured through chemical reactions above and chlorine is released to gas phase. This leads to the increase of Na/Cl molar ratio in lager particle size region and the four synthetic chars in Figure 4 conform to this trend. Beside the chemical reactions, physical process of surface deposition and adhesion also contribute to sodium capture. This part will be discussed in the following section.

3.4 Effects of interactions between sodium and silicon, aluminum compounds on PM Figure 6 compares the amount of PM generated from -COONa+C and NaCl+C before and after adding silica, alumina. The reduction percentage of PM1 generated from -COONa+C by adding silicon, alumina is highest as 83.3% while that for NaCl+C is 31.0%. The effect of silica, alumina on PM2.5 is similar for -COONa+C and NaCl+C. As for PM10, there is a 29.6% and 4.2% increase for -COONa+C and NaCl+C respectively due to the transform of silicon, aluminum additives into PM10 during synthetic char combustion. Combined with previous researches of other scholars32-34 and the results of our experiment, it may be interpreted that sodium is likely to form stable fine particulates in the presence of enough chlorine while the organic-bound sodium is easier to be captured by silica, alumina.


Energy & Fuels

100 -COONa+SiO2+Al2O3+C NaCl+SiO2+Al2O3+C

83.3 80

PM reduction ratio %

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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60 40

31.0

30.6 23.8

20 PM10 0 PM1

PM2.5

-4.2

-20 -29.6

-40

Figure 6. PM reduction ratio of synthetic chars

Figure 7 illustrates the effect mechanisms of silicon, aluminum compounds on the migration of sodium-containing fine particulates. Figure 7(a) and Figure 7(b) represent the evolution paths of fine particulates from synthetic chars. Figure 7(a): During the combustion of synthetic char without silica and alumina, the organic-bound sodium or inorganic water-soluble sodium release from char particles as Na(g) and NaCl(g). The sodium vapor will nucleate and condensate to generate Na2O(s) and NaCl(s) nano particles as flue gas cooling down. Then the nanoparticles will grow up through coagulation, collision and surface deposition process. These particles and gas phase sodium will flow downstream along the furnace without capture by silicon, aluminum compounds. The process above is named as (Ⅰ). Figure 7(b): Besides the path (Ⅰ) in Figure 7(a), there is another migration way due to the addition of silicon, aluminum compounds expressed as (Ⅱ) in Figure 7(b). The interactions occurred in (Ⅱ) are listed below: Na2O(g) and NaCl(g) will form low melting point compounds with silicon, aluminum additives through chemical reactions (see Equation (1)~(4)). The melting surface under high temperature will capture Na2O(s) and NaCl(s) particles through physical way. Chemical reactions and physical capture (melting surface adhesion, surface deposition) are two way of sodium capture by silicon, aluminum additives.


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Energy & Fuels

Ⅰ

Flue gas

Page 15 of 21

Ⅰ

Ⅱ

(a) Fuel: -COONa+C, NaCl+C

(b) Fuel: -COONa+SiO2+Al2O3+C, NaCl+SiO2+Al2O3+C

Ⅱ: Surface reaction and deposition

Nucleation Condensation

Collision and deposition

Fly ash with molten surface Flue gas compositions: Na2O(g) , NaCl(g) Na2O(s) , NaCl(s)

The main migration pathways:

Ⅰ Ⅱ

Remained in flue gas Captured by Si-Al additives

Al2O3(s) , SiO2(s)

Figure 7. Mechanisms of sodium capture by silicon, aluminum compounds

The contribution of chemical reactions and physical way in sodium capture process can be interpreted through the Na/Cl molar ratio in Figure 4. The sodium and chlorine are chosen as tracer element. There is no chlorine introduced in -COONa+C, we could not speculate the contribution of the two capture ways. In this paper, the data of NaCl+SiO2+Al2O3+C are used and the contribution ratio of chemical reaction way to physical way is calculated as below: 8  11 n   m ni , Na / Cl , PM1  mi  i   i , Na / Cl , PM110 i 9 i 1 =  11 8   m mi   i  i 9 i 1 

     

8

n i 1

i , Na / Cl , PM 1

 mi (5)

8

 m i 1

i

 represents the contribution ratio of chemical reaction way to physical way. ni , Na / Cl , PM

110

represents the mean

value of Na/Cl molar ratio on i stage of DLPI. mi represents the particle mass on i stage of DLPI. The first term in


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the molecule of right hand side in Equation (5) means the mass based average Na/Cl molar ratio for ultra-micron particles. The second term in the molecule of right hand side in Equation (5) represents the mass based average Na/Cl molar ratio of submicron particles. Sodium chloride is the main components of sodium in PM1 while both sodium chloride and sodium aluminum-silicate compounds exist in PM1-10. Thus the ratio of Na/Cl in PM1 should be 1 in theory, while the actual value in Figure 4 for different size particles in PM1 is range from 0.7 to 0.92 which we think the deviation is brought by the SEM-eds analysis. So the mass-based averaged ratio of Na/Cl in PM1 is taken as the characteristic value for NaCl. The ratio of Na/Cl in PM1-10 should be above 1 theoretically, and the actual value in Figure 4 is range from 1.2 to 2.9. When subtracting the Na/Cl ratio for PM1 from Na/Cl ratio for PM1-10, the ratio of sodium existing as Si-Al-Na compounds can be obtained. In our experiment,  is equal to 2.4 which indicates that the sodium content captured through chemical reaction way is 2.4 times of that through physical way. Chemical reaction way dominates the sodium capture process.

4. Conclusions 1. Fine particulate matter generated from synthetic char loading inorganic water-soluble sodium are mainly consist of sodium chloride while that from synthetic char loading organic-bound sodium are mainly consist of sodium oxide. Inorganic water-soluble sodium is much easier to translate into stable fine particulates than organic-bound sodium. 2. In the absence of chlorine, organic-bound sodium is more likely to react with silicon, aluminum compounds in fuel. This will reduce the amount of fine PM. In the presence of chlorine, sodium tends to convert to stable sodium chloride particles, which inhibits the reactions between sodium and silicon, aluminum compounds. 3. Silicon, aluminum compounds capture sodium through two ways. Chemical reaction way: Silicon, aluminum compounds react with sodium containing compounds (including sodium oxide and sodium chloride in this research).


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Physical way: Sodium vapor and sodium chloride vapor condensate on the surface of coarse silicon, aluminum particles; the melting surface of coarse particles capture the fine particles collide with the liquid phase. In our experiment, the sodium content captured through chemical reaction way is 2.4 times of that through physical way. Chemical reaction way plays a leading role in sodium fixation.

Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 91544108) and the National Key Research and Development Plan of China (No. 2016YFB0601504).

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