Occurrence and transformation characteristics of recoverable soluble

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Occurrence and transformation characteristics of recoverable soluble sodium in high alkali, high carbon fly ash during Zhundong coal gasification in a circulating fluidized bed Guoliang Song, Shaobo Yang, Xiaobin Qi, and Zhao Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03410 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Energy & Fuels

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Occurrence and transformation characteristics of recoverable soluble sodium in high alkali,

2

high carbon fly ash during Zhundong coal gasification in a circulating fluidized bed

3

Guoliang Song1,2*, Shaobo Yang1,2, Xiaobin Qi1,2, Zhao Yang1,2

4

1

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China 2

5

6

University of Chinese Academy of Sciences, Beijing 100049, China

Abstract

7

In this reported study, the effects of bed temperature, air equivalence ratio (ER), heating surface

8

wall temperature, and coal type on the occurrence and transformation of recoverable soluble Na in

9

high alkali, high carbon fly ash during Zhundong (ZD) coal gasification were investigated in a 0.4 t/d

10

circulating fluidized bed test system. The experimental results illustrated that the quantity of soluble

11

Na in gasification fly ash of ZD coal was far greater than in the ZD coal. Na compounds in ZD coal

12

gasification fly ash were present mainly as NaCl, with other compounds such as Na2SO4 and sodium

13

aluminosilicates, including Na2SiO3 and NaAlSi3O8. The recoverable soluble Na accounted for 56 –

14

96.9% of the total Na in the ZD coal gasification fly ash. The proportion of soluble Na in ZD coal

15

gasification fly ash was greatly affected by the gasification conditions. As the bed temperature was

16

increased, the proportion of soluble Na decreased. As the ER was increased, the proportion of soluble

17

Na increased, with the maximum proportion present at an ER of 0.50. Compared to adiabatic

18

condition, cooling of the heating surface increased the proportion of soluble Na. The soluble Na

19

yield and the proportion of soluble Na were found to be high during gasification of Shaerhu (SEH)

20

and Tianchi (TC) coals, which made them more suitable for recovery of soluble Na from their

21

corresponding fly ashes: Shaerhu fly ash (SEHf) and Tianchi fly ash (TCf).

22

Key words: CFB, Zhundong coal, gasification fly ash, recoverable soluble Na, soluble Na 1

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proportion

2

1. Introduction

3

Coal has played a leading role in energy generation in China and will continue to be

4

predominant over the ensuing decades [1, 2]. Zhundong coalfield is the largest integrated coalfields

5

in China, where the coal reserves are estimated to be more than 390 billion tons [3-5]. Zhundong

6

(ZD) coal is a promising fuel, because of its low ash content and good reactivity. However, due to the

7

high sodium content in ZD coal, a series of ash-related problems including ash deposition and

8

slagging are generated during the combustion of ZD coal [6-8]. The catalytic effects of sodium help

9

the ZD coal to exhibit good gasification characteristics [9-11]. The circulating fluidized bed (CFB)

10

gasification process is operated at low temperatures (850 – 950 °C), and can be adapted to a variety

11

of fuels, so that CFB gasification is a promising choice for the clean utilization of ZD coal. Apart

12

from these benefits, one of the key factors limiting the CFB gasification of ZD coal is the quality of

13

its fly ash, particularly the high content of carbon and sodium in the fly ash. Application of CFB

14

gasification technology to ZD coal would produce large quantities of ZD coal gasification fly ash

15

which makes reuse of the gasification fly ash worthy of research attention.

16

During CFB gasification of ZD coal, the sodium in coal is volatilized so that sodium vapors are

17

condensed and enriched in the fly ash. Song et al. [12-15] conducted several CFB gasification tests

18

of ZD coals, and found that the sodium content in the gasification fly ash was as high as 1.0 – 4.0%,

19

which is much higher than the 0.2 – 0.9% found in ZD coal. Therefore, ZD coal gasification fly ash

20

can be considered to be a high alkali, high carbon residue. The sodium present in coal can be

21

classified as soluble sodium and insoluble sodium, where soluble sodium can be divided into water

22

soluble, NH4Ac soluble and HCl soluble sodium [16, 17]. The nature and relative content of these 2


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materials in coal are known to influence the slagging, fouling, and ash deposition, and soluble

2

sodium is considered to be the most harmful because of its volatility and low melting point [18, 19].

3

Upon combustion, most of the insoluble sodium in coal will be retained in the bottom ash in a stable

4

form of sodium aluminosilicate, while most of the soluble sodium will be released into the gaseous

5

phase and will be enriched in the fly ash. Water soluble sodium and HCl soluble sodium are the

6

predominant sodium occurrences in ZD coal gasification fly ash [12, 13]. Therefore, most of the

7

sodium in the ZD coal gasification fly ash is soluble sodium. On one hand, the soluble sodium in ZD

8

coal gasification fly ash can cause serious, ash-related problems during the process of fly ash

9

re-burning so that the application of this fly ash is limited. On the other hand, ZD coal gasification

10

fly ash is a rich source of sodium. Consequently, reuse of the ZD coal gasification fly ash and

11

utilization of the available sodium require the removal and recovery of the abundant soluble sodium

12

in ZD coal fly ash. Researchers have found that the soluble alkali metals in coal can be easily

13

recovered through physical and chemical methods [20-25]. For example, Rappas et al. [26, 27] found

14

that the soluble alkali metals in coal can be easily recovered from coal by water washing and ion

15

exchange with a weak electrolyte. Therefore, the study of the occurrence and transformation

16

characteristics of recoverable soluble sodium in high alkali, high carbon fly ash during ZD coal

17

gasification is vital for the re-burning of ZD coal gasification fly ash and the recovery of Na in ZD

18

coal gasification fly ash.

19

The amount of recoverable soluble sodium in ZD coal gasification fly ash is closely related to

20

the CFB gasification conditions including bed temperature, air equivalence ratio (ER), wall

21

temperature of heating surface, and coal type. These factors influence the coal feed rate and the

22

amount of introduced sodium. Also, the release and transformation of sodium, and the sodium

3


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content in fly ash are also directly affected by these factors. Gasification performance during ZD coal

2

gasification under different gasification parameters is important, and many researchers have studied

3

the gasification performance of ZD coal. For instance, Zhang et al. [9] found that the catalytic effect

4

of sodium improved the gasification performance of ZD coal, and that higher temperature favored

5

the formation of H2 and CO. Zhang et al. [28] studied the gasification characteristics of ZD coal in a

6

bench-scale BFB apparatus. The authors found that higher temperatures greatly favored the

7

gasification performance of ZD coal. ZD coal exhibited higher carbon conversion and cold gas

8

efficiency than Shigouyi coal at the same temperature, which is a good indication of the improved

9

adaptability of ZD coal for fluidized bed gasification. Because slagging and defluidization are prone

10

to occur during ZD coal gasification, the goal of this reported work is to study the effects of

11

gasification conditions on the occurrence and transformation characteristics of recoverable soluble

12

sodium in gasification fly ash, and gasification performance of ZD coal is not the focus of this study.

13

Zhang et al. [9] investigated the effect of temperature (850 – 1050 °C) on the sodium transformation

14

of ZD coal in a fluidized bed gasifier. These authors found that the sodium retention ratio initially

15

decreased, but then increased as the temperature was increased, so that more sodium was retained in

16

the residuals at higher temperature. Wang et al. [4] concluded that as the combustion temperature

17

was increased from 400 °C to 800 °C, 80% of the sodium was released from the coal. Song et al. [29]

18

studied the transformation of sodium in coal under various conditions in a 0.25 t/d CFB reactor.

19

These authors found that more sodium was released into the gaseous phase as the temperature was

20

increased, and this sodium existed primarily as Na2SO4 in the combustion fly ash and NaCl in the

21

gasification fly ash.

22

Song et al. [13] reported that less sodium was released into the gaseous phase with higher O2

4


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Energy & Fuels

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concentrations in the circulating fluidized bed. Na existed mainly as NaCl in fly ash and as

2

NaAlSiO4 in the bottom ash. The effect of the wall temperature of the heating surface on ash

3

deposition was investigated by Qi et al. [29] in a 0.4 t/d CFB reactor. Ash deposition was greatly

4

affected by the wall temperature, and gaseous NaCl was condensed and enriched on the cooled

5

surface. The presence and release of sodium varied with the type of coal used. Song et al. [30]

6

studied the nature and distribution of sodium during gasification of ZD coal in a 0.25 t/d CFB reactor.

7

These authors concluded that the bed temperature and type of coal exerted important effects on the

8

release of sodium during gasification. They found that H2O soluble sodium and NH4Ac soluble

9

sodium constituted a large proportion of the sodium in the fly ash. Therefore, an understanding of the

10

factors affecting the occurrence and transformation of the recoverable soluble sodium in the fly ash

11

during CFB gasification is essential in controlling the recovery of soluble sodium and re-burning of

12

the fly ash. Unfortunately, to date, there are few reports on the effects of gasification conditions on

13

the occurrence and transformation of recoverable soluble sodium in fly ash during the gasification of

14

ZD coal in a circulating fluidized bed reactor.

15

In this reported study, gasification tests of various ZD coals were conducted in a 0.4 t/d CFB

16

reactor. The effect of bed temperature, ER, wall temperature of the heating surface, and coal type on

17

the occurrence and transformation characteristics of recoverable soluble sodium in ZD coal

18

gasification fly ash were investigated. The occurrence and content of sodium were determined using

19

sequential chemical extraction and ICP-AES analysis. XRD analysis was employed to identify the

20

crystalline compounds of Na in the product. The proportion of soluble Na introduced into the bed

21

was varied, which was an indication of how the Na originating in the coal was converted to

22

recoverable soluble Na in gasification fly ash.

5


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2. Experimental

2

2.1. Experimental system

3

The gasification experiments of ZD coals were conducted in a CFB test system, where the coal

4

feed rate of the system was 0.4 t per day. A schematic diagram of the system is shown in Fig.1. The

5

0.4 t/d CFB test system was comprised of a reactor, a cyclone separator, a loop seal and an ash

6

collection system. The height of the reactor was 4200 mm and the inner diameter was 150 mm. The

7

bottom of the reactor was wrapped with a section of heating wire for ignition of the fuel in the initial

8

stage and providing extra heat for the test system to maintain stable operation during gasification

9

experiments. The air required for the experiment was provided by an air compressor. The flue gas

10

temperature and wall temperature (slagging probe surface temperature) were adjusted by slagging

11

probes (A-D) equipped with a cooling system. The temperatures of flue gas and surfaces of probes

12

were regulated by adjusting the cooling modes of the probes. The cooling of the actual boiler heating

13

surface was simulated by changing the cooling mode of the slagging probe. The schematic diagram

14

of the slagging probes (A-D) is shown in Fig. 2. The slagging probe was composed of Cr25Ni20

15

(GB/T20878-2007) and its length was 400 mm. Two thermocouples were set on the outside and

16

inside of the probe end to measure the wall temperature of the heating surface, the wall temperature

17

was calculated from the average temperature of the two thermocouples. The slagging probes were

18

cooled by different mediums (air or water) to adjust the flue gas temperature and wall temperature of

19

the heating surface, the flue gas temperature was measured using the thermocouples (T7 – T10). The

20

ash collecting can and bag filtering dust precipitator were used to collect the fly ash.

6


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Energy & Fuels

T6

T5

T7

A

T8

B

T9

C

T10

D

Bag-filtering Dust Precipitator

Chimney

Coal Hopper T4

Ash Collecting Can

T3

Loop seal Sample 2 T2 T1

Air Ash can Sample1

1 2

Fig. 1. Schematic diagram of 0.4 t/d CFB test system

Air out Air in

Thermocouple 1

400mm

Thermocouple 2

Ti To

3 4

Slagging Probe Fig. 2. Schematic diagram of the slagging probes (A-D)

5

The temperatures of the system were measured using K type thermocouples, the pressures were

6

measured using B300 type pressure transducers. All of the data, including pressures, temperatures

7

and air volume flow rates, was collected and displayed real-time by a Programmable Logic

8

Controller (PLC) data acquisition system.

9

2.2. Sampling and analysis methods

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Bottom ash (sample 1 in Fig.1) was collected in a can at the bottom of the reactor. Gasification 7


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1

fly ash (sample 2 in Fig.1) were collected in an ash collecting can during the gasification

2

experiments.

3

A sequential chemical extraction method was used to measure the presence of sodium in the

4

samples (coal, gasification fly ash and bottom ash) [12, 16]. The detailed chemical extraction

5

methods is shown in our previous works [13, 15].

6

The crystalline phases in the samples were determined by X-ray diffraction (XRD, PANalytical,

7

Netherlands). The diffractometer used Cu Kα radiation (λ = 1.5406 Ǻ). The ash compositions were

8

determined by X-ray fluorescence (XRF, PANalytical, Netherlands). In order to burn out the residue

9

carbon in samples and avoid the loss of sodium during ashing as far as possible, the ZD coal and its

10

gasification fly ash were ashed at 575°C in a muffle furnace before XRD and XRF tests [31].

11

2.3. Fuels and tests conditions

12

Three types of typical ZD coals were used for the gasification experiments, which were termed

13

Shenhua (SH), Tianchi (TC) and Shaerhu (SEH), and all ZD coals were produced from Xinjiang

14

Zhundong area of China. The coals were crushed and sieved to a size range of 0.1 – 1mm. The

15

proximate, ultimate analysis and chemical components of coals are listed in Table 1. As shown in

16

Table 1 the Na2O content in the ash of three ZD coals was 3.92%, 7.28% and 4.38%. Table 2 lists the

17

content of sodium in ZD coals, where water soluble Na appears to be the predominant type of Na

18

accounting for 56.7 – 88.4% of the total Na. Although the Na2O content in the ash of SEH is lower,

19

the Na content in SEH is higher than TC and SH, the higher ash content in SEH is the reason for this.

20

The Na content in TC coal is the lowest of the three types of ZD coal. The XRD analysis was used to

21

determine the crystal phases of the ash in ZD coal. The XRD results of ZD coals are shown in our

22

previous work, where the Na in ZD coal was mainly present as NaCl and Na2SO4 [30]. 8


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Energy & Fuels

Table 1 Basic properties of ZD coal

1

Proximate analysis (ad, wt %)

TC

SH

SEH

Water content

14.34

15.64

11.02

Ash

3.16

5.03

14.66

Volatile matter

27.02

34.06

30.46

Fixed carbon

55.48

45.27

43.86

Lower heating value (MJ/kg)

23.70

17.63

17.93

C

64.54

54.41

51.54

H

3.02

1.7

2.36

N

0.52

0.69

0.58

O

13.97

22.03

19.73

St

0.45

0.4

0.11

Cl

0.06

0.1

1.14

SiO2

3.73

17.24

41.98

Al2O3

6.16

11.9

17.59

Fe2O3

5.37

5.76

6.87

CaO

33.45

28.74

19.39

MgO

5.42

5.34

2.49

SO3

29.34

19.58

1.82

P2O5

0.03

0.05

0.18

K2O

0.45

0.38

0.66

Na2O

7.28

3.92

4.38

Ultimate analysis (ad, wt %)

Chemical components in ash (wt %)

2

Note: ad - as air dried basis

Table 2 Content of Na in ZD coal

3 Water soluble Na

NH4Ac soluble Na

HCl soluble Na

Insoluble Na

Total Na

(mg/g)

(mg/g)

(mg/g)

(mg/g)

(mg/g)

Fuels

9


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TC

2.37

1.51

0.11

0.19

4.18

SH

3.61

0.55

0.08

0.21

4.45

SEH

8.08

0.72

0.16

0.18

9.14

1

The experimental conditions for the gasification tests of ZD coal are shown in Table 3. The

2

effect of bed temperature (defined as the highest temperature in the reactor, test ID 1-3), ER (test ID

3

4-7), wall temperature of heating surface (test ID 8-10) and coal type (test ID 2, 5, 11) on the soluble

4

sodium in the gasification fly ash was determined. In the actual circulating fluidized bed (CFB)

5

gasification process, the temperature is usually about 950−980 °C. Higher bed temperatures produce

6

a higher gasification reaction rate and a better quality gas. However, the results of our previous work

7

have shown that when the bed temperature is above 950 °C, the tail heating surface is easily

8

corroded during gasification of SEH coal [13], and that slagging and defluidization of the bed easily

9

occurs during gasification of TC coal [14, 32]. While the sodium content in SH coal ash is lower than

10

the TC and SEH coal ash, and the gasification process is more stable when using SH coal at higher

11

bed temperatures [12, 30]. Therefore, in this reported work, SH coal was used as the fuel in the

12

experimental gasification tests at different bed temperatures (900−1000°C).

13

The ER of the actual CFB gasification process is generally about 0.35-0.45. The ER in this

14

reported work was established at about 0.40-0.50 during the gasification tests of the various types of

15

ZD coal. The sodium content of TC coal ash was higher than the SH and SEH coal ash, so the

16

quantities of soluble sodium in the TC coal fly ash and gaseous sodium compounds were greater

17

during gasification. Consequently, the ER and cooling modes of the heating surface during

18

gasification process had a greater impact on the occurrence and transformation of soluble Na in the

19

TC coal fly ash [29]. Therefore, we selected the TC coal as the best fuel candidate for studying the

20

effect of ER and cooling methods on the occurrence and transformation of soluble Na in fly ash. The 10


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Energy & Fuels

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bed temperature and the ER were modified by adjusting the coal feed rate. The feed rate of coal and

2

fly ash production at different operating conditions are also presented in Table 3. The wall

3

temperature of the heated surface was varied by adjusting the method used to cool (Adiabatic，Water

4

cooling，Air cooling) of the slagging probes. Table 3 Experimental conditions

5

ID

Fuel

1 2

SH

3

Bed temperature (°C)

ER

Coal feed

Fly ash production

rate (kg/h)

(kg/h)

Cooling method

900

0.48

13.43

1.54

950

0.46

14.08

0.93

1000

0.45

18.02

1.28

22.07

3.66

Air cooling

4

0.38

5

0.43

19.00

1.96

6

0.47

17.14

2.37

0.50

16.00

2.98

7

TC

935

8

0.42

Adiabatic

16.77

1.50

9

0.43

Water cooling

18.27

3.30

10

0.41

Air cooling

18.06

1.88

0.45

Air cooling

16.58

1.21

11

SEH

950

6

The wall temperatures attained using the various cooling methods are shown in Table 4. As

7

shown, the wall temperatures depended on the cooling method. The wall temperature was the lowest

8

with water cooling, while it was the highest under adiabatic conditions. The conventional operating

9

procedures employed in this study have been reported in our previous work [13, 14, 29]. Table 4 Wall temperatures with different cooling methods

10

Wall temperature (°C) Cooling method Adiabatic

A

B

C

D

745.0

750.5

677.0

621.5

11


Energy & Fuels

Water cooling

173.0

174.0

180.0

178.0

Air cooling

427.5

428.5

337.5

371.5

1

3. Results and discussion

2

3.1. Effect of bed temperature on soluble sodium occurrence transformation

3

Temperature is an important factor that affects the release and transformation of elemental Na in

4

the coal. The quantity of Na in the solid and gas phase vary with the bed temperatures of the CFB,

5

which also occurs in the gasification fly ash. SH coal was used as the fuel for the gasification

6

experiments conducted at different bed temperatures. The fly ash yield (g/kg, the mass of fly ash

7

produced per kilogram of coal) at different bed temperatures is shown Fig. 3. It can be seen from

8

these results that less fly ash was generated at higher bed temperatures, and the fly ash yield

9

decreased from 115 g/kg at 900 °C to 66 g/kg at 950 °C and 71 g/kg at 1000 °C, which meant that a

10

rise in the bed temperature favors carbon conversion. As the temperature increased, the effect of bed

11

temperature on the fly ash yield decreased. Zhang et al. [9] studied the effects of temperature on the

12

gasification performance of ZD coal, and obtained similar results where the rate of carbon

13

conversion increased as bed temperature was increased. 120 110 100

Fly ash yield (g/kg)

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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90 80 70 60 50 40 900

14 15

950

1000

Temperature (°C)

Fig. 3. Fly ash yield at different bed temperatures 12


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1

The distribution of Na in the solid and gaseous phases at different bed temperatures is shown in

2

Fig. 4. The distribution of Na is calculated based on the mass balance during the experiment, the

3

specific calculation method is shown in our previous work [32]. As is shown in Fig. 4, as the bed

4

temperature was increased from 900 °C to 1000 °C, fractions of Na in bottom ash and fly ash

5

initially decreased and then increased, whereas the opposite was true for Na fraction in gas phase.

6

Due to the strong volatility of Na species in ZD coal, their release would be enhanced as the bed

7

temperature increased. Meanwhile, at higher temperature, bed material would react with Na species

8

easier to form low-temperature eutectics. Hence, when temperature rose from 900 to 950 °C, more

9

Na species were released into gas phase, resulting in lower Na retention in bottom ash and fly ash.

10

The Na fraction in fly ash decreased from 37.7% to 9.7%. As the temperature further rose to

11

1000 °C, the enhanced reactions between bed material and Na species caused more Na species to

12

reside in bottom ash and fly ash [9, 30, 33]. The fraction of Na in fly ash was 13.7%. 100 Bottom ash

Fly ash

Gas phase

80

Sodium distribution (%)

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

60 40 20 0 -20 900

13 14

950

1000

Temperature (°C)

Fig. 4. Na distribution at different bed temperatures

15

Fig. 5 shows the content of Na in the gasification fly ash at different bed temperatures. In

16

contrast to the occurrences of Na in SH coal, the insoluble and HCl soluble Na predominated in the

17

gasification fly ash of SH coal, accounting for 50.9 – 95.0% of the total Na. As the bed temperature 13


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1

increased from 900 to 1000 °C, the amounts of insoluble and total Na in the gasification fly ash

2

initially increased and then decreased, while the soluble Na amount decreased a little. When the

3

temperature rose from 900 to 950 °C, the gasification reaction rate was enhanced and more carbon

4

was converted [34]. The specific surface area and porosity of the ash particles also increased, so that

5

more water soluble and NH4Ac soluble Na in the coal particles were released to gaseous phase.

6

However, insoluble and HCl soluble Na were combined with minerals in gasified fly ash and their

7

volatility was low, most of the Na was still retained in the fly ash, and the fly ash was further

8

concentrated, resulting in higher insoluble and total Na content in the gasified fly ash at 950 °C [19,

9

35]. The decrease in the content of water soluble Na also illustrated this point. However, as the bed

10

temperature rose to 1000 °C, the enhanced reactions between bed material and Na species caused

11

more Na species to reside in bottom ash, so that the insoluble and total Na content in fly ash was

12

lower [33].

13

As the bed temperature rose from 900 °C to 950 °C, the fraction of recoverable soluble Na

14

decreased from 42.4% to 31.6%. Transformation between different occurrences of Na occurred

15

during the process of thermal chemical transformation of ZD coal, where most of the water soluble

16

Na was released to the gaseous phase, and part of the remaining water soluble Na and HCl soluble

17

Na were converted to insoluble Na [19, 34], thus the fraction of recoverable soluble Na deceased.

18

However, as the bed temperature rose to 1000 °C, the fraction of recoverable soluble Na increased to

19

92.3%, which was caused by the decrease in the insoluble Na content. With the increase in bed

20

temperature, more insoluble Na was retained in the bottom ash due to the more intense interaction of

21

the bed material and the Na in the ash [14, 36, 37].

14


Page 14 of 35

Page 15 of 35

28 Water soluble

NH4Ac soluble

HCl soluble

Insoluble

24

Sodium content (mg/g)

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

20 16 12 8 4 0 900

Fig. 5. Na content in gasification fly ash at different bed temperatures

2

4

1000

Temperature (°C)

1

3

950

The Na yield (mg/kg, the mass of Na in the fly ash produced per kilogram of coal) is shown in Fig. 6. The Na yield (α) was calculated using the following formula:

α = ∗ β

5

(1)

6

Where, (mg/g) represents the Na content in gasification fly ash (shown in Fig. 5), and the

7

parameter β (g/kg) represents the fly ash yield (exhibited in Fig. 3). As is shown in Fig. 6, when the

8

temperature was 900 °C, the soluble Na yield was as high as 1014.7 mg/kg, while it was decreased to

9

510.6 mg/kg at 950 °C and then increased to 563.8 mg/kg at 950 °C at 1000 °C. Sodium yield is the

10

result of the combined effect of sodium content in fly ash and fly ash yield. At lower bed temperature,

11

the fly ash yield was higher (shown in Fig. 3), less soluble Na species was released to gas phase and

12

more soluble Na species were kept in fly ash (shown in Fig. 4), thus the soluble Na yield was much

13

higher at 900 °C. With the increase in bed temperature, the fly ash yield was lower, and more soluble

14

Na species were released to gas phase and retained in bottom ash, the soluble Na yield of fly ash was

15

lower.

15


Energy & Fuels

2800 Water soluble

NH4Ac soluble

HCl soluble

Insoluble

2400

Sodium yield (mg/kg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 35

2000 1600 1200 800 400 0 900

950

1000

Temperature (°C)

1

Fig. 6. Na yield at different bed temperatures

2 3

The proportion of soluble Na (η) was introduced to indicate how much Na introduced by coal

4

was converted to recoverable soluble Na in fly ash. The formula used for this calculation was as

5

follows:

6

η=

α ∗ 100%

(2)

7

Where, (mg/g) represents the total Na content in ZD coals (shown in Table 2). α (mg/kg)

8

represents the soluble Na yield (shown in Fig. 6). The proportion of soluble Na at different bed

9

temperatures is exhibited Fig. 7. As the bed temperature was increased from 900 °C to 1000 °C, the

10

proportion of soluble Na initially decreased from 22.8% to 11.5% and then increased to 12.6%. At

11

low operating temperatures (900 − 950 °C), bed temperature imposed important effects on the

12

proportion of soluble Na. While as the temperature increased further（>950 °C）, the effect of bed

13

temperature on the proportion of soluble Na was low. As bed temperature was increased, on one hand,

14

more water soluble Na and NH4Ac soluble Na were released from the coal to the gaseous phase, and

15

the content of gaseous Na increased. Also, correspondingly, the soluble Na content in gasification fly

16

ash. On the other hand, the gasification reaction rate was enhanced, so that more carbon reacted and

17

the yield of gasified fly ash decreased. Therefore, the proportion of soluble Na decreased with the 16


Page 17 of 35

1

increase of the bed temperature, and these analyses are consistent with the results shown in Fig. 3

2

and Fig. 4. 30 25

Soluble Na proportion (%)

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

20 15 10 5 0 900

3 4

5

950

1000

Temperature (°C)

Fig. 7. Proportion of soluble Na at different bed temperatures

3.2. Effect of ER on soluble sodium transformation

6

Release and transformation of Na are significantly influenced by the reaction atmosphere, and

7

the Na content in the ash is affected by the reaction atmosphere [12, 13, 38, 39]. Gasification

8

experiments were conducted at different ERs to determine the effect of ER on soluble Na

9

transformation in the gasified fly ash. The TC coal was chosen for these experiments, the ER of

10

gasification experiments was increased from 0.38 to 0.50, and the gasification temperature was

11

935°C. During the gasification experiments, the gasifier temperature was maintained constant

12

in-spite of variations in ER by adjusting the coal feed rate, and the heating power of the electric

13

furnace at the bottom of the riser. For instance, when the ER was decreased, the air flow rate was

14

basically the same, but more coal was fed into the test system, thus the heat absorption of the test

15

system increased and the test system needed more heat to maintain the gasifier temperature constant.

16

Therefore, the heating power of the electric furnace was increased to provide more heat to maintain 17


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

the gasifier temperature constant. The detailed experimental conditions are shown in Table 3 (test ID

2

4-7). The amount of soluble Na in the fly ash was found to be closely related to the fly ash yield. Fig.

3

8 shows the changes of fly ash yield and carbon conversion in the fly ash with variations in the ER.

4

When the ER was 0.38, the defluidization occurred after 8 hours of gasification, the detailed

5

description was shown in our previous work [32]. As the ER was increased from 0.43 to 0.50, the fly

6

ash yield increased from 103.2 g/kg to 186.2 g/kg. The carbon conversion increased from 70.3% to

7

81.4%. According to the experimental conditions shown in Table 3 (test ID 4-7), with the increase in

8

ER, the rate of coal feed decreased, while the volume of air was the same, so that more carbon was

9

consumed. It is noteworthy that the fly ash yield at the ER of 0.38 (defluidization) was higher than

10

the fly ash yield at the ER of 0.43, this phenomenon can be explained by the following reasons.

11

Compared to the ER of 0.43, the coal feed rate and fly ash production at the ER of 0.38 were higher

12

(shown in Table 3), and more sodium was introduced by coal to the test system. As a result, at the ER

13

of 0.38, the slagging and defluidization occurred [32], and more sodium reacted with minerals such

14

as SiO2 and Al2O3 to form sodium compounds with a low melting point, and the low melting point

15

sodium compounds covered the surface of the bottom ash and fly ash, and the sticky coating was

16

formed [14, 15, 29, 33]. The reactions between carbon and air were inhibited by the sticky coating,

17

and less carbon was consumed, and correspondingly more carbon was retained in bottom ash and fly

18

ash. Therefore, compared to the ER of 0.43, the fly ash yield was higher at the ER of 0.38

19

(defluidization).

18


Page 18 of 35

200

100

160

80

120

60

80

40

40

20

Carbon conversion (%)

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


Page 19 of 35

Defluidization 0

0 0.38

0.43

0.47

0.50

1

ER

2

Fig. 8. Fly ash yield at different ERs

3

The ER also affected the distribution of Na in the solid and gas phases. This is depicted in Fig. 9,

4

where a rise in the ER favored the release of Na from the solid phase to the gas phase. As the ER was

5

increased from 0.43 to 0.50 during gasification of TC coal, the fraction of Na in the gas phase

6

increased from 19.8% to 28.2%. The inhibition of carbon on the release of Na was probably

7

responsible for this phenomenon, because the carbon remaining in the ash was present as a carbon

8

matrix, which would inhibit the Na migration from the inside of the ash particles to the surface of the

9

ash particles. This clearly exhibited the inhibition of carbon on the release of Na. Takuwa et al. [12,

10

40] also reported an inhibitory effect of carbon on Na release. In addition, most of the Na was kept in

11

bottom ash during gasification of TC coal, which was caused by the high Na content in TC coal ash

12

(shown in Table 2). TC coal is a kind of coal which is easy to slagging, temperature and ER should

13

be strictly controlled during gasification [14, 32]. With the decrease in ER, the fraction of Na in

14

bottom ash was higher, resulting in defluidization at an ER of 0.38 [32].

19


Energy & Fuels

80 Gas phase

Fly ash

Bottom ash

64

Na distribution (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 35

48

32

16

0 0.43

0.47

ER

1 2

0.50

Fig. 9. Na distribution at different ERs

3

Fig. 10 shows the content of Na in the fly ash for different ERs. As these data show, water

4

soluble Na was the main type of Na compounds, and with an increase in ER, the contents of total and

5

insoluble Na increased, while the soluble Na content in fly ash decreased. As the ER was increased

6

from 0.43 to 0.50, the fraction of water soluble Na decreased from 72.1% to 37.8%, while HCl

7

soluble and insoluble Na fractions increased from 22.0% to 43.7%. These results can be explained by

8

the following rationale. The carbon conversion rate increased and the carbon content in the gasified

9

fly ash decreased as the ER increased. Water soluble Na was combined with carbon matrix in

10

gasified fly ash and its volatility was high, most of the water soluble Na in the fly ash was released to

11

gaseous phase with the increase in ER, while the fly ash was further concentrated, so that the total

12

and insoluble Na contents in the gasified fly ash were higher at higher ERs [19, 35]. The decrease in

13

the fraction of water soluble Na also illustrated this point. The content of soluble Na decreased from

14

5.72 mg/g to 5.07 mg/g as the ER was increased from 0.43 to 0.50, the decrease in water soluble Na

15

content was the cause of this phenomenon.

20


Page 21 of 35

12 Water soluble


10

NH4Ac soluble

HCl soluble

Insoluble

Defluidization

8 6 4 2 0 0.38

0.43

0.47

0.50

ER

1

Fig. 10. Na content in fly ash at different ERs

2 3

The Na yields at different ERs are exhibited in Fig. 11. As the ER was increased from 0.43 to

4

0.50, the total Na yield increased from 668.5 mg/g to 1270.2 mg/g, and the soluble Na yield

5

increased from 590.1 mg/g to 944.3 mg/g. The maximum total Na and recoverable soluble Na yield

6

appeared at an ER of 0.50, this might result from the higher fly ash yield (shown in Fig. 8) and

7

higher fraction of Na in fly ash (shown in Fig. 9) at an ER of 0.50. 2000 Water soluble

NH4Ac soluble

HCl soluble

Insoluble

1750 Defluidization


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

1500 1250 1000 750 500 250 0 0.38

0.43

0.50

ER

8 9

0.47

Fig. 11. Na yield at different ERs

10

Fig. 12 shows the proportion of soluble Na at different ERs. As shown, ER of gasification

11

imposed important effects on the soluble Na proportion in the fly ash. As the ER was increased from

21


Energy & Fuels

1

0.43 to 0.50, the proportion of soluble Na increased from 14.12% to 22.59%. When ER was 0.50, the

2

proportion of soluble Na attained a maximum and of about 22.59%, which meant that about 22.59%

3

of the total Na introduced by coal was transformed to the recoverable soluble Na in fly ash at an ER

4

of 0.50. The minimum proportion of soluble Na was 14.12% at an ER of 0.43. 40 Defluidization


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 35

30

20

10

0 0.38

0.43

0.47

ER

5 6

7

0.50

Fig. 12. Proportion of soluble Na at different ERs

3.3. Effect of heating surface wall temperature on soluble sodium transformation

8

Researchers have found that the temperature of the heating surface has significant an effect on

9

the deposition behavior of fly ash and the transformation of Na in fly ash [30, 41]. TC coal was used

10

as the fuel of the experiments employing different cooling methods, and the bed temperature was

11

maintained at 935 °C. Fig. 13 shows the wall temperatures of slagging probes (A – D) and the

12

temperatures of corresponding flue gas. It can be seen that under different cooling methods, there

13

was a big difference between the wall temperature and the corresponding flue gas temperature. The

14

flue gas temperature and wall temperature were the lowest when the water cooling method was used,

15

and the flue gas temperature and wall temperature were the highest in adiabatic condition. In addition,

16

the temperature difference between wall temperature and flue gas temperature was the largest when

22


Page 23 of 35

the water cooling method was used, from 296 °C to 490 °C, while the temperature difference was the

2

smallest in adiabatic conditions, only about 10 °C to 50 °C. Flue gas temperature (°C)

1

Wall temperature (°C)

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

900

Adiabatic

Air cooling

Water cooling

Adiabatic

Air cooling

Water cooling

750 600 450 300 900 600 300 0 A

3 4

B

C

D

Position

Fig. 13. Wall and flue gas temperature with different cooling methods

5

The yield of fly ash under different cooling methods is shown in Fig. 14. Compared with

6

adiabatic condition, the yield of fly ash was higher when the tail heated surface was cooled. This was

7

because that as the tail heated surface was cooled, the heat loss of the system increased, so that more

8

coal were needed to maintain the stable conditions. The bed temperature and ER varied little, so the

9

fly ash yield increased, which was consistent with the increased coal feed rate as shown in Table 3

10

(test ID 8-10). The feed rate of coal and fly ash yield were the highest in the water cooling method,

11

and were the lowest in adiabatic conditions. Based on the analysis of the data shown Fig. 13, the

12

temperature difference in the water cooling method was higher than the air cooling method, so more

13

fine particles were captured and deposited on the cooled surface as a result of the larger temperature

14

difference in the water cooling method. Therefore, more fly ash was collected under the water

15

cooling method. Li et al. [42, 43] found that when there was a large temperature difference on the

16

heated surface, the fine fly ash particles were easily deposited on the lower temperature surface due

17

to the influence of thermophoresis force and inertia. 23


Energy & Fuels

280


210

140

70

0 Adiabatic

2

Air cooling

Water cooling

Cooling method

1

Fig. 14. Fly ash yield with different cooling methods

3

The content of Na in gasification fly ash of TC coal is depicted in Fig. 15. These results

4

illustrate that water soluble Na was the major type of Na in gasification fly ash, accounting for 37.6 –

5

50.3% of the total Na in product. Under the water cooling method, the content of Na and soluble Na

6

in fly ash was higher, especially the content of water soluble Na and NH4Ac soluble Na. At lower gas

7

temperatures, more gaseous Na was condensed and enriched in the gasification fly ash, and gaseous

8

Na was present primarily as water soluble and NH4Ac soluble Na [30]. The amount of recoverable

9

soluble Na also increased as the temperature difference between flue gas and wall was increased. 8 Water soluble


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 35

NH4Ac soluble

HCl soluble

Insoluble

6

4

2

0 Adiabatic

10 11

Air cooling

Water cooling

Cooling method

Fig. 15. Na contents in fly ash with different cooling methods 24


Page 25 of 35

1

Fig. 16 shows the Na yields in different cooling methods. As shown, the yields of Na and

2

soluble Na were the highest in the air cooled method. This result may have been due to the increased

3

yield of fly ash in general in the air cooled method. As shown in Fig. 17, the proportion of soluble Na

4

in the water cooled method was 20.6%, and it was 11.6% in the air cooling method, while it was only

5

6.9% in the adiabatic condition. These results illustrated that the effective cooling of heating surface

6

improved the amount of soluble Na in gasification fly ash, and the proportion of soluble Na

7

increased with the increase in temperature difference of the flue gas and heating surface. 1200 Water soluble

NH4Ac soluble

HCl soluble

Insoluble


1000 800 600 400 200 0 Adiabatic

8 9

Air cooling

Water cooling

Cooling method

Fig. 16. Na yields with different cooling methods 32


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

24

16

8

0 Adiabatic

10 11

Air cooling

Water cooling

Cooling method

Fig. 17. Soluble Na proportion with different cooling methods

25


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

3.4. Effect of coal type on soluble sodium transformation

2

The quantity of Na in coal varies with the type of coal, so that the release and transformation of

3

Na is also influenced by the type of coal used in the gasification process [5, 30]. As shown in Table 3

4

(test ID 2, 5, 11), the gasification tests of SH, TC and SEH coals were conducted under the same

5

conditions, where the bed temperature was 950 °C, the ER was about 0.4, and the slagging probes

6

were cooled by air. Using these conditions, the dependence of the transformation of soluble Na was

7

studied based on the type of ZD coal used. Fig. 18 shows the content of Na in three types of ZD coal

8

and their resulting gasification fly ash. The gasification fly ashes of three kinds of ZD coals were

9

termed Shaerhu fly ash (SEHf), Tianchi fly ash (TCf) and Shenhua fly ash (SHf). For SEHf, the

10

content of Na was 25.0 mg/g, and the Na was present mainly as water soluble Na and HCl soluble

11

Na, and the recoverable soluble Na content was 96.9% of the total Na. For TCf, the content of Na

12

was 7.1 mg/g, water soluble Na and NH4Ac soluble Na were the predominant species of Na, and the

13

recoverable soluble Na content was 83.5% of the total Na content. However, for SHf, the Na content

14

was 24.3 mg/g, with insoluble Na as the main species of Na, and the fraction of recoverable soluble

15

Na was only 31.6%.

16

The content of soluble Na in ZD coal gasification fly ash was much greater than that in ZD raw

17

coal, which meant that the soluble sodium was enriched in fly ash during gasification. Our previous

18

work [12, 13, 30] also reached a similar conclusion. The gasification fly ash contains much

19

recoverable soluble Na, and provides a wealth of sodium resources, so that it is reasonable to try to

20

recover the soluble Na in gasification fly ash.

26


Page 26 of 35

Page 27 of 35

28

Water soluble

NH4Ac soluble

HCl soluble

Insoluble

24


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

20 16 12 8 4 0 SEH SEHf

TC

TCf

SH

SHf

Coal type

1

Fig. 18. Na content in different coal types

2 3

The fly ash yields of three types of ZD coal are shown in Fig. 19. As can be seen from this

4

figure, under the same reaction conditions, the fly ash yield of TC coal was the highest, and the fly

5

ash yield of SH coal was the lowest. The Na yields during gasification of SEH, TC and SH coals are

6

displayed in Fig. 20. It can be seen that the soluble Na yields of SEH and TC coal were much higher

7

than that of SH coal. Furthermore, as is shown in Fig. 21, the proportion of soluble Na during the

8

gasification of SEH and TC coals were higher than that of SH coal. The proportion of soluble Na

9

during gasification of SEH, TC and SH coals was 19.5%, 14.1% and 11.4%, respectively. However,

10

the water soluble and NH4Ac soluble Na contents in TCf and SEHf were much higher than those in

11

SHf. Compared with HCl soluble Na, water soluble and NH4Ac soluble Na were easier to be

12

removed and recovered [16, 17]. Therefore, compared with SHf, SEHf and TCf are more suitable for

13

the recovery of Na.

27


Energy & Fuels

120


90

60

30

0 SEH

TC

SH

Coal type

1

Fig. 19. Fly ash yield of different coal types

2

2000

Water soluble

NH4Ac soluble

HCl soluble

Insoluble


1600

1200

800

400

0 SEH

TC

SH

Coal type

3

Fig. 20. Na yield of different coal types

4 20


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 35

16

12

8

4

0 SEH

5 6

TC

SH

Coal type

Fig. 21. Soluble Na proportion of different coal types 28


Page 29 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

The XRD analyses of TCf, SHf and SEHf are displayed in Fig. 22. The bottom ash during ZD

2

coal gasification is present mainly as SiO2 because of the bed material, thus the XRD analysis of the

3

bottom ash is not shown in this reported study. For TCf, the Na was present mainly as Na2SO4 (water

4

soluble) and Na2SiO3 (water soluble), which is consistent with the analysis of Na content in TCf

5

shown in Fig. 18. For SEHf, NaCl (water soluble) was main form of Na, while, Na2SiO3 and Na2SO4

6

were also found. The insoluble NaAlSi3O8, was a high melting point (1100 °C) Na compound, and

7

was the primary form of Na in SHf, which was consistent with the high content of insoluble Na in

8

SHf. Researchers have found that the water soluble Na is weakly combined with the carbon on the

9

surface of coal, and can be easily recovered by water washing [20, 24]. The NH4Ac soluble Na is

10

organically combined with the coal matrix and existed in the form of carboxylate, which can be

11

recovered through ion exchange with a weak acid. The HCl soluble Na is associated with ionic clays

12

and can be recovered using acids, such as dilute hydrochloric acid [16]. Therefore, the existence

13

forms of Na in ZD coal gasification fly ash suggested that the soluble Na in ZD coal gasification fly

14

ash can be easily recovered. The combination of these analyses concerning the soluble Na yield and

15

the soluble Na proportion of three kinds of ZD coals, strongly suggests that it is more suitable to

16

remove and recover the soluble Na from TCf and SEHf before re-burning. SHf can be re-burned

17

directly, due to its low contents of total Na and soluble Na, and high fraction of insoluble Na. The

18

insoluble Na in SHf is stable and less harmful during combustion based on the reported results of

19

Song et al [12-15].

29


Energy & Fuels

10000

TCf

7500 5000 2500

Diffracted intensity (cps)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 35

a a o g h d qk pb a

gg

0

d

SHf

9000

s r

6000

d a r a d tdd d d d

3000 0

d

c

SEHf

7200 4800 2400 0 10

c h 20

u bh ck h u x 30

40

u 50

c

c

u c 60

70

c 80

90

2θ(°)

1 2

g

Fig. 22. XRD analysis of ZD coal gasification fly ash

3

a-CaSO4; b-Na2SO4; c-NaCl; d-SiO2; g- CaO; h-Ca12Al14O33; k-Na2SiO3; o-Ca2SiO4; p-Fe2O3;

4

q-Na2Ca(SO4)2;r-NaAlSi3O8; s- KAlSi3O8; t- K3Na(SO4)2; u-KCl; x-MgO

5

The proportions of recoverable soluble sodium in fly ash under different gasification conditions

6

are listed in Table 5. It can be seen that the bed temperature, ER, cooling method of the heating

7

surface, and coal type have great influence on the proportion of recoverable soluble sodium in fly ash.

8

The recoverable sodium proportion decreased with the increase in bed temperature. As the ER was

9

increased, the proportion of recoverable soluble Na increased. Compared with adiabatic condition,

10

cooling the heating surface improved the proportion of soluble Na in fly ash. Therefore, when the

11

bed temperature is 900 °C, the ER is 0.50 and the heating surface is cooled by air during the

12

gasification of ZD coal, the recoverable soluble sodium proportion is the highest. In addition,

13

compared with the gasification of SEH and SH coals, more Na from coal was transformed to

14

recoverable soluble sodium during the gasification process of TC coal. 30


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1

Energy & Fuels

Table 5 Proportion of recoverable soluble sodium in fly ash under different gasification conditions Recoverable soluble sodium ID

Fuel

Bed temperature (°C)

ER

Cooling method proportion in fly ash (%)

1 2

SH

3

0.48

22.80

950

0.46

11.47

1000

0.45

12.67 Air cooling

4

0.38

5

0.43

14.12

6

0.47

18.66

0.50

22.59

7

TC

935

36.50

8

0.42

Adiabatic

6.85

9

0.43

Water cooling

20.58

10

0.41

Air cooling

11.62

0.45

Air cooling

19.54

11

2

900

SEH

950

4. Conclusions

3

The occurrence and transformation characteristics of recoverable soluble sodium in high alkali,

4

high carbon fly ash during Zhundong coal gasification were investigated in a 0.4 t/d circulating

5

fluidized bed test system. The main conclusions are as follows.

6

(1) The quantity of soluble Na in ZD coal gasification fly ash is much greater than in the ZD

7

raw coal. The Na in the fly ash exists primarily as NaCl, Na2SO4 and sodium aluminosilicates, such

8

as Na2SiO3 and NaAlSi3O8. The soluble Na accounts for 56-96.9% of the total Na in the gasification

9

fly ash. Recovering Na from ZD coal gasification fly ash is feasible because of the high content of

10

soluble Na and the existence forms of Na in the fly ash.

11

(2) The proportion of soluble Na in ZD coal gasification fly ash is greatly affected by the

12

gasification conditions. With the increase in bed temperature, more Na was released from the ash to 31


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

the gas phase, and the proportion of the soluble Na decreased. As the ER was increased from 0.43 to

2

0.50, the proportion of soluble Na increased, and the maximum proportion of soluble Na (22.59%)

3

presented at an ER of 0.50. Compared to adiabatic condition, cooling of the heating surface improves

4

the proportion of soluble Na in fly ash.

5

(3) The yield of soluble Na and the proportion of soluble Na are found to be high during

6

gasification of Shaerhu (SEH) and Tianchi (TC) coals, and it was found that the gasification fly ash

7

of Shaerhu coal (SEHf) and the gasification fly ash of Tianchi coal (TCf) should be subjected to

8

sodium removal and recovery pretreatment before re-burning and more soluble Na can be recovered

9

from SEHf and TCf. The gasification fly ash of Shenhua coal (SHf) can be re-burned directly, due to

10

its low contents of total Na and recoverable soluble Na, and high fraction of insoluble Na.

11

Author Information

12

*Corresponding author. Guoliang Song, Tel.: +86-010-82543129;

13

E-mail address: [email protected].

14

Acknowledgements

15

This work was financially supported by the Strategic Priority Research Program of the Chinese

16

Academy of Sciences (No. XDA07030100) and the International Science & Technology Cooperation

17

Program of China (No. 2014DFG61680).

18

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Occurrence and transformation characteristics of recoverable soluble

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