Speciation and Distribution of Sodium during Zhundong Coal

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Speciation and distribution of sodium during Zhundong coal gasification in circulating fluidized bed Weijian Song, Guoliang Song, Xiaobin Qi, Shaobo Yang, Qinggang Lu, and Wojciech Nowak Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01610 • Publication Date (Web): 14 Jan 2017 Downloaded from http://pubs.acs.org on January 15, 2017

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Energy & Fuels

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Speciation and distribution of sodium during Zhundong coal gasification in circulating fluidized bed

4

Song Weijiana,b, Song Guoliang*, a ,b, Qi Xiaobina,b, Yang ShaobO a,b, Lu Qingganga,

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Wojciech Nowakc,d

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a

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

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b

University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China

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c

Czestochowa University of Technology, Dabrowskiego 73, 42-200 Czestochowa, Poland

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d

AGH University of Science and Technology, 30 Mickiewicza Av., Krakow, Poland

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ACS Paragon Plus Environment

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1

Abstract

2

The speciation and transformation of sodium during Zhundong coal circulating fluidized bed

3

gasification were investigated. Sodium speciation was investigated by chemical fractionation

4

analysis (CFA) using ICP-AES. XRD was employed to analyze crystalline compounds in the

5

ashes. H2O-soluble sodium is the predominant occurrence, accounting for 55-95% of the total

6

sodium. And insoluble potassium is the predominant occurrence, the total content of

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potassium is relatively low, approximately one-tenth of sodium in Zhundong coals.

8

Temperature and coal type are both important influencing factors on the volatilization of

9

sodium during Zhundong coal gasification. Sodium in SH ashes evidently decreases with the

10

increase of temperature, whereas increases in SEH and TC ashes. The occurrence of sodium

11

in the fly ash is different remarkably with that in the bottom ash. In the fly ash of gasification,

12

sodium exists mainly as H2O-soluble and NH4Ac-soluble, as HCl-soluble and insoluble

13

chemical form in the bottom ash. Sodium aluminum silicate and sodium silicate were formed

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during gasification process. The formation of sodium silicate leads to slagging and

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defluidization during the gasification of TC.

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Key words: CFB gasification; high-sodium Zhundong coal; slagging

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1 Introduction

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Coal plays an important role in the energy consumption of China and the consumption

3

will keep increasing in next decade1. Zhundong coalfield, is the biggest coal reserves newly

4

found in China. It has been estimated that the Zhundong coal reserves exceeds 390 billion

5

tons2, 3. Its volatility is ultra high and its sulfur content is ultra low. Zhundong coal is also

6

characterized by its high sodium content in the ash. Sodium in the coal would cause severe

7

slagging and ash deposition during combustion4, 5. At the same time, sodium can increase the

8

gasification reaction rate as catalyst6-10. High sodium Zhundong coal is suitable for

9

gasification. The circulating fluidized bed gasifier (CFBG) is operated at 850~950oC, much

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lower than pulverized coal boilers. CFB gasification is a potential choice for Zhundong coal

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utilization. Understanding the distribution and transformation behavior of sodium during CFB

12

gasification is essential to the practical use of Zhundong coal.

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Generally speaking, sodium in coals mainly exists in two main forms, inorganic sodium

14

and organic sodium. Inorganic sodium is dissolved within moisture and organic sodium is

15

organically bound to the coal matrix. In order to classify different forms of sodium, the

16

method of chemical fractionation analysis (CFA)

17

was classified as water (H2O) soluble sodium, ammonium acetate (NH4Ac) soluble sodium,

18

hydrochloric acid (HCl) soluble sodium and insoluble sodium. Their speciation and relative

19

content in the coals are known to influence fouling, slagging and ash deposition, and

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H2O-soluble sodium, ammonium acetate soluble sodium are considered to be the most

21

harmful14, 15. Wang et al.16 investigated the sodium transformation in CO2 and in N2, and

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found that in CO2, the volatilization of sodium is decreased and the transformation from

11-13

was introduced and sodium in coals

3


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1

water-soluble sodium to insoluble sodium was blocked. Yang et al.17,

18

2

influence of combustion temperature on the transformation of alkali metals during the

3

co-combustion of coal and rice straw, and pointed out that at the range 600°C and 800°C, the

4

content of H2O-soluble sodium in bottom ash changed little with the temperature. When the

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temperature exceeds 800°C, the fraction of water-soluble sodium decreased evidently as

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temperature increased. They also concluded that Na2SO4 was the dominant H2O-soluble

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sodium in the condensed phase. Kosminski et al.19-21 found that during the devolatilisation of

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brown coal, parts of both inorganic sodium and organic sodium release from the char.

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However, they did not show the exact proportion of sodium in different forms. Van Eyk et al.

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got a better understanding of sodium in different chemical forms. They concluded that 67%

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of the water-bound sodium retained in the char during the devolatilisation, and little of

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organically-bound sodium would release22,

13

investigate the distribution characteristics of sodium during combustion of Zhundong coals

14

and drew a conclusion that H2O-soluble sodium, ammonium acetate soluble sodium

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dominated the sodium release during combustion. Li et al.25 investigated the distribution and

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transformation characteristics of sodium during Zhundong coals combustion in a lab-scale

17

reactor, and the results show that sodium in Zhundong coals exist mainly as H2O-soluble

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form. The fraction of H2O-soluble sodium in residual ash decreases significantly, and more

19

sodium of HCl-soluble form is generated with the increase of combustion temperature.

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Manzoori et al.26 reported that organically bounded sodium would transform to insoluble

21

sodium under the pyrolysis condition. Song et al.27 reported that oxygen and steam could

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promote the generation of NaAlSiO4 during the gasification of Zhudong coal in circulating

investigated the

23

. He et al.24 adopted LIBS technique to

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fluidized bed gasifier. With the increase of oxygen and steam concentration, the fraction of

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NaAlSiO4 increased. These investigations are mainly focus on the combustion process, the

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occurrence of sodium differs with the coal types and ash types, the transformation routes also

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differ with the temperature for different coals during different thermal utilizations of

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Zhundong coals. The react apparatus has significant impact on the sodium transformation

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characteristics. There is lack of researches of the gasification process of Zhundong coal,

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especially on the transformation characteristics of different sodium forms. The transformation

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of sodium in Zhudnong coals during Gasification in circulating fluidized bed reactor operates

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at lower temperature and fluidized state compared with the pulverized coal furnace.

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Researches on the transformation of sodium in Zhudnong coals during CFB gasification is

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are still insufficient.

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The transformation characteristics of potassium during the thermal conversion of

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biomass have been investigated by many resaerchers7, 28-31. Content and chemical forms of

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sodium in coals are extremely different from potassium in biomass, leading to different

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transformation characteristics during thermal conversion. Investigation on transformation

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characteristics of sodium in different chemical forms is needed.

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The present work aims to reveal the transformation characteristics of sodium in different

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chemical forms during Zhundong coals CFB gasification at different temperatures. The

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experiments were carried out in a 0.25 t/d CFB experimental system. The speciation of

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sodium was investigated by chemical fractionation analysis (CFA) using ICP-AES. XRD and

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XRF were employed to analyze crystalline compounds and elemental compositions of

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Zhundong coal ash. And scanning electron microscopy with an energy dispersive X-ray

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spectrometer (SEM-EDX) were employed to obtain the microstructure of ash samples.

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2 Experimental section

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2.1 Experimental system

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Gasification experiments were carried out in a circulating fluidized bed gasifier. The

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coal feeding rate of the gasifier is 0.25t/d. Figure 1 shows the schematic diagram of the test

6

system. The reactor was equipped with electric wire. The electrical power heated to the

7

reactor is 9kW. More detailed description of the test system and the running procedure could

8

be found in our previous works32, 33.

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The superficial gas velocity of the circulating fluidized bed reactor is 0.11m/s at the

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starting period. During the experiments, gasification temperature was maintained at 850°C,

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900°C and 950°C respectively and kept for 4 hours. The equivalence ratio (the ratio of the

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amount of oxygen used for gasification to the amount of oxygen required for complete

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combustion of the coal, ER) was 0.4. The superficial gas velocity was maintained at 2.5m/s to

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3.0m/s.

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The carbon conversion ratios of Shenhua coal (SH for short) are 62.31%, 72.61% and

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84.53%, respectively at 850°C, 900°C and 950°C gasification. The carbon conversion ratios

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of Shaerhu coal (SEH for short) are 65.08%, 81.28% and 93.91%, respectively at 850°C,

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900°C and 950°C gasification. The carbon conversion ratio of Tianchi coal (TC for short) is

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67.69% at 850°C gasification.

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Energy & Fuels

Flue Gas B Chimney

Cyclone

Slagging probe

CFB reactor

Bag-filtering dust precipitator Coal Hopper A ah can Screw Feeder Loop seal Solid Recirculation Air

Bed Rem oval Air A

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Figure 1 0.25t/d CFB system for high alkali coal gasification

2.2 Methods of sample collecting and analyzing

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During the experiments, bottom ash was collected by a bottom ash can，shown as point

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A in Figure 1. Sampling cyclone was used to collect fly ash. Point B is the sampling cyclone

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for fly ash collecting.

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The chemical form of sodium in the samples, including fly ash, bottom ash and the raw

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coal, was determined by the chemical fractionation analysis. This method was first applied by

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Benson and Holm11 and improved by Yang et al.34 for determination of alkali metals and

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alkaline earth metals in Zhundong coals. The samples were leached by ultra-pure H2O,

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solution of CH3COONH4 (1 mol/L), solution of HCl (1 mol/L), sequentially. During each

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leaching step, the temperature was kept at 60°C and maintained at least 12 hours.

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H2O-soluble sodium, such as sodium chloride, sodium sulfate and sodium carbonate, was

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leached out by ultra-pure water (represented as H2O-soluble in figures). Subsequent

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extraction in ammonium acetate removed ammonium acetate soluble sodium which is bound 7


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organically in the sample matrix (represented as NH4Ac-soluble in figures). Hydrochloric

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acid dissolve the sodium associated with ionic clays (represented as HCl-soluble in figures).

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An accurately weighed amount of 0.1 g residual ash was mixed with 6 ml of 65% HNO3, 3

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ml of 40% H2O2 and 2 ml of 30% HF in a cleaned vessel. The samples were then

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microwaved by a setted program. Typically, the temperature rose from room temperature to

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120 °C in 5 min and held there for 5 min, next to 210 °C in 10 min and held there for 15 min.

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Following the evaporation of acid, the digestions were transferred to 100 ml volumetric flask

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and volume to 100 ml with ultra-pure water. The amount of sodium in the leached and

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digested solutions was analyzed by ICP-AES (Vista-MPX, Varian, America).

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The mineral compositions of ashes were analyzed by XRD (Empyrean, PANalytical,

11

Netherlands). The ash compositions were analyzed by XRF (Axios, PANalytical,

12

Netherlands). Samples collected from the gasification experiments, including fly ash and

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bottom ash, were ashed at 575oC in a muffle furnace before XRD and XRF test. The raw

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coals were ashed at the same condition. Ashing at 575oC could burn out all the carbon in the

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ash and avoid the loss of sodium during ashing35. SEM-EDX (S-4300, Hitachi, Japan) was

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employed to obtain the microstructure and elements distribution of ash samples.

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2.3 Fuel

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Three high sodium Zhundong coals (SH, SEH, TC), from Zhundong coalfield, were

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chosen as experiment fuel. The size range of the coals is 0.1~1 mm. Table 1 shows properties

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of Zhundong coals, including the proximate and ultimate analysis, ash fusion temperatures

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and the ash composition. The ash content is low (3.16%-14.66%, wt%, air dry basis). The

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content of sodium in Zhundong coal is high, reaching 3.92%, 4.38% and 7.28% in the coal 8


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ash, respectively. The content of CaO is higher than 20% in the coal ash, reaches 33.45% in

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TC. The content of total sulfur in Zhundong coal is less than 0.5%, whereas the content of

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chloride, which may cause severe corrosion, varies in different coal, reaches 1.138% in SEH

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coal. Table1 Properties of Zhundong coals

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Proximate analysis (ad,wt%) Moisture Ash Volitile matter Fixed carbon Lower heating value (MJ/kg) Ultimate analysis (ad, wt%) C H N O St Cl Ash fusion temperature (oC) Deformation temperature Softening temperature Hemispherical temperature Flowing temperature Ash compositions (wt%)

SH 15.64 5.03 34.06 45.27 17.63

SEH 11.02 14.66 30.46 43.86 17.93

TC 14.34 3.16 27.02 55.48 23.70

54.41 1.7 0.69 22.03 0.4 0.104

51.54 2.36 0.58 19.73 0.11 1.138

64.54 3.02 0.52 13.97 0.45 0.058

1320 1320 1330 1340

1120 1130 1140 1140

1360 1370 1370 1380

SiO2

17.24

41.89

3.73

Al2O3

11.9

17.59

6.16

Fe2O3 CaO MgO TiO2

5.76 28.74 5.34 0.6

6.87 19.39 2.49 1.08

5.37 33.45 5.42 0.41

SO3

19.58

1.82

29.34

P2O5

0.05

0.18

0.00

K2 O

0.38

0.66

0.45

Na2O

3.92

4.38

7.28

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Note: ad-as air dried basis.

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3 Result and discussion

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3.1 chemical forms of alkali metals in coals 9


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The content and chemical forms of alkali metals (sodium and potassium) in coals have a

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pronounced effect on their transformation behaviors during thermal conversions and vary

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largely in different coals. The content and occurrence of sodium and potassium are shown in

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figure 2(a) and figure 2(b), respectively. The mineral compositions in the coals are shown in

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Figure 3. Figure 2(a) shows that H2O-soluble sodium is the main occurrence in Zhundong

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coals, reaches to 55-95% of the total sodium in coals. The fraction of HCl-soluble and

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insoluble sodium is quite low, with a proportion of only 5−10%. Wang et al.16 and Li et al.4

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got the similar conclusions in their researches. The NH4Ac-soluble sodium varies in different

9

coal, ranging from 7% to 37%. Figure 3 shows that, sodium exits mainly as NaCl

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(H2O-soluble sodium) in Zhundong coals. Insoluble sodium was also detected in the coals,

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such as NaAlSi3O8 in SEH and Na2Si2O5 in TC. Specially, a small fraction of sodium exists

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as Na2SO4 bounding with CaSO4 in TC, which may lead to slagging and deposition during

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the thermal conversions of coals3.

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Figure 2(b) shows the content and occurrence of potassium in Zhundong coals. It can be

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found that the content and occurrence of potassium are remarkably different from that of

16

sodium. The total content of potassium is relatively low, approximately one-tenth of sodium.

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And insoluble potassium is the predominant occurrence in Zhundong coals. Considering the

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low content and the stable occurrence of potassium in Zhundong coals, the president work

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mainly focuses on the speciation and transformation of sodium.

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0.7

10

H2O-soluble

NH4Ac-soluble

HCl-soluble

H2O-soluble

insoluble

NH4Ac-soluble

HCl-soluble

insoluble

0.6

8

0.5

Content of K (mg/g)

Content of Na (mg/g)

6

4

0.4 0.3 0.2

2 0.1

0

0.0

SH

1 2 3

SEH

TC

SH

SEH

Coals

TC

Coals

(a) Sodium

(b) Potassium

Figure 2. Content and occurrence of alkali metals in Zhundong coals

2880

a

TC

2160 1440 Diffracted intensity (cps)

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Energy & Fuels

720

d b be a a d d f cea c f e a af a a aba

0 5700 3800 1900 0

f

d

i c ad i cg g

SEH d ic

d i i ii i c a i i

d

i

3600

a

i SH

a

2400 d i i c a i aiaa i d i a ai di a i

1200 0 10

20

30

40

50

60

70

80

90

2θ (°)

4 5

a-CaSO4; b-CaO; c-SiO2; d-NaCl; e-CaSO4•Na2SO4; f-Fe3O4; g-NaAlSi3O8;h-Na2Si2O5; i-CaCO3

6

Figure 3. XRD patterns of Zhundong coals

7

3.2 Sodium distribution

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The results of sodium distribution in the gasification ashes are shown in Figure 4. The

9

data of sodium content is based on silica free. This method of data analysis could eliminate

10

the interference of silica mixed with the gasification ashes. The data of SiO2 was taken from

11

the XRF test. Due to the high sodium content in TC coal ash, the defluidization occurred

12

when gasification temperature exceeded 900°C33. Sodium in TC coal ash reacted with bed

11


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1

material (mainly as silica) and formed sodium silicate with low melting point. Sodium

2

silicate would act as adhesive to accelerate the agglomeration of bed particles. The bed

3

particles with larger diameter were difficult to fluidize and defluidization occurred finally.

4

There lacks of TC gasification fly ash data above 900°C (Figure 4, Figure 5(c) and Figure

5

6(f)). It was shown that coal type affects the content of sodium in the gasification ash to a

6

large extent. Sodium in the SH gasification ash dropped quickly as the gasification

7

temperature increased. Namely, more sodium is released into gas phase. It is similar with

8

the conclusions in other published works

9

SEH and TC gasification ash increases with an increase of gasification temperature. The

10

contents of sodium in fly ashes increased little when the temperature increased from 850°C

11

to 900°C. The content of sodium in the 950°C fly ashes increased apparently. During

12

Zhundong coal gasification, temperature and coal type both are important factors on the

13

volatilization of sodium. The difference is due to the original occurrence of sodium in the

14

coals and the different transformation routes of sodium during gasification. More detailed

15

discussion was shown in section 3.3. The content of sodium in the bottom ash of TC is

16

about 2 to 3 times of that in the bottom ash of SH and SEH. The enrichment of sodium in

17

the bottom ash leads to defluidization easily during gasification of TC.

8, 25, 36-40

. Conversely, the content of sodium in the

12


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SH fly ash SH bottom ash SEH fly ash SEH bottom ash TC fly ash TC bottom ash

20 Content of Na (%)

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Energy & Fuels

15 10 5 0

850 900 950 Gasification temperature (°C)

1 2

3

Figure 4. Distribution of sodium in the ash after Zhundong coals gasification

3.3 Sodium transformation characteristics

4

The sodium occurrence in the CFB gasification ashes of SH, SEH and TC was shown in

5

figure 5(a), figure 5(b) and figure 5(c), respectively. It can be seen that the occurrence of

6

sodium in the fly ash is different remarkably with the existence of sodium in the bottom ash.

7

In the fly ash of gasification, sodium exists mainly as H2O-soluble, accounting for 60-90%.

8

The content of NH4Ac-soluble sodium and HCl-soluble sodium differ with the coal types.

9

The occurrence of sodium in fly ash was similar with that of raw Zhundong coal. Conversely,

10

the HCl-soluble and insoluble sodium are the predominant sodium in the gasification bottom

11

ash, account for 80-98%.

12

For SH coal, the fraction of NH4Ac-soluble sodium is higher in the ashes after

13

gasification than that in the raw coal. The occurrence of NH4Ac soluble sodium in the ash

14

demonstrates that after gasification, some of the sodium still remained in the char, bounding

15

with the organic matrix. Similar conclusions were drawn in the work of Jensen et al.41.

16

Andrea et al.13 concluded that 32% of sodium became bound to the char after gasification of

17

bagasse. The fraction of HCl-soluble sodium increases both in fly ash and bottom ash after

13


Energy & Fuels

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1

gasification in three kinds of Zhundong coals.

2

For SEH coal, the HCl-soluble sodium accounts for 30-35% of the total sodium in the

3

fly ash and 55-85% of the total sodium in the bottom ash. Accoding to the CFA result, as the

4

increase of temperature, the content of HCl-soluble sodium increases in the bottom ash, led to

5

the increase of sodium content in the bottom ash. The content of sodium in fly ash increases

6

little when the temperature increases from 850°C to 900°C. The content of sodium in the

7

950°C fly ash increases apparently. This can also be explained by the increase of HCl-soluble

8

sodium. Li et al.25 concluded that part of H2O-soluble sodium is transformed into HCl-soluble

9

and insoluble form and part is released into gas phase during the combustion of Zhundong

10

coals. Andrea et al.13 reported the transformation of potassium from a H2O-soluble form to a

11

HCl-soluble form, which is more stable, did occurred during the gasification of fuel cane

12

bagasse in a 50 kW air-blown down graft gasifier.

13

For TC coal, the fraction of insoluble sodium increases in the ashes, including the fly

14

ash and bottom ash after gasification. The fraction of sodium in insoluble chemical form in

15

the bottom ash reaches 80%, indicating the H2O-soluble sodium did transform into insoluble

16

sodium.

17

The gasification temperature is an important factor on the transformation of sodium

18

during gasification. With the increase of gasification temperature from 850°C to 950°C, the

19

fraction of H2O-soluble sodium and NH4Ac-soluble sodium increase from 83.72% to 89.94%

20

in the fly ash of SH and decrease from 24.49% to 9.45% in the bottom ash of SH. For SEH

21

fly ash, the fraction of H2O-soluble sodium and HCl-soluble sodium increase when the

22

gasification temperature increases from 850°C to 900°C. When gasification temperature

14


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reaches 950°C, the fraction of HCl-soluble sodium increases and accordingly the fraction of

2

sodium of H2O-soluble and NH4Ac-soluble chemical form decreases. In the SEH bottom ash,

3

the fraction of HCl-soluble sodium increases as the gasification temperature. High

4

temperature promotes the formation of HCl-soluble sodium. The sum of HCl soluble and

5

insoluble sodium in fly ash increased from 2.87% to 3.50% and 8.62% to 13.10% in bottom

6

ash when the gasification temperature increases from 850°C to 950°C. Li et al.25 reported that

7

the fraction of HCl-soluble sodium increased as the increasing of combustion temperature

8

during the combustion of ZJ coal and WCW coal. 120 H2O-soluble

NH4Ac-soluble

HCl-soluble

insoluble

Content of Na (%)

100 80 60 40 20 0 FA -850 FA -900 FA -950 BA -850 BA -900 BA -950

9 10

(a) SH 120 H2O-soluble

NH4Ac-soluble

HCl-soluble

insoluble

100

Content of Na (%)

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 60 40 20 0

11 12

FA -850 FA -900 FA -950 BA -850 BA -900 BA -950

(b) SEH

15


Energy & Fuels

120 H2O-soluble

NH4Ac-soluble

HCl-soluble

insoluble

100

Content of Na (%)

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 22

80 60 40 20 0 FA -850

BA -850

BA -900

1 2 3

(c) TC Note: FA-Fly ash; BA-Bottom ash; 850, 900, 950-gaisification temperature, °C

4

Figure 5. Occurrence of sodium in the Zhundong coals gasification ash

5

Mineralogical analyses of gasification ashes were conducted by X-ray powder

6

diffraction (XRD) analysis. Figure 6 shows the results of mineral forms analysis. In the

7

bottom ash of SH, SiO2 and CaSO4 is the main mineral phases. This can be explained as

8

follows: the bed material mainly is SiO2 and CaO accounts for about 30% of the coal ash

9

compositions. CaSO4 is one stable solid phase. The influence on CaSO4 on ash deposition42

10

could be ignored at the circulating fluidized bed gasifiers operating temperature range. Quartz

11

is the main phase of bottom ash of SEH coal and TC coal. Sodium in the SH and SEH bottom

12

ash of gasification exists mainly as sodium aluminosilicate (NaAlSi3O8, NaAlSiO4) whereas

13

as sodium silicates (Na2Si2O5) in the bottom ash of TC, namely the insoluble form, which is

14

consistent with the CFA results. Sodium aluminum silicate and sodium silicate were formed

15

during gasification process7-9, 19, 31. The chemical reaction equations are as follows:

16

Na2O+ Al2O3•2SiO2•2H2O=2NaAlSiO4+2H2O

(1)

17

Na2O+ Al2O3•2SiO2•2H2O+ 4SiO2=2NaAlSi3O8+2H2O

(2)

18

Na2O+ n•SiO2 = Na2O •nSiO2

(3)

19

The formation of sodium aluminum silicate and sodium silicate would retain sodium in the 16


Page 17 of 22

1

solid phase, accordingly gaseous phase sodium is dropped. The formation of low melting

2

compounds such as alkali silicates leads to the occurrence of slagging, whereas the sodium

3

aluminum silicate with relative high melting point could mitigate slagging. Piotrowska et al.43,

4

44

5

agglomeration or deposit during the combustion of rapeseed cake in CFB. Sodium in

6

different coal undergoes different transformation route, leading to different running

7

performance during gasification.

concluded that interactions between aluminum silicates and alkali metals could mitigate the

8

NaCl is the main chemical form of sodium in the fly ashes of all the three kind

9

Zhundong coals, which is in consistent with the CFA result. Gasification temperature has

10

little influence on the kinds of mineral phases, but the influence of gasification temperature

11

on the relative amount of mineral phases is obvious in the temperature range of 850°C and

12

950°C.

5000 0

c a c g ma

c c a

c

c

c

10000 c a

0

a

cb c

c nc

c c

c

c

15000

850°C bottom ash of SH

10000 5000

c a

a

0 10

13 14

b c c

1200


15000 5000

2400

Diffracted intensity (cps)

10000

20

30

a

3600


15000


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

c c c nc c 40 2θ (°)

c

50

c 60

a dn n a aa p d a a a a a

a

p

0 3600 2400 1200

900°C fly ash of SH a d n o a o an n an aaa pa d ao a a p a

0 a

3600 2400 1200

c

n

950°C fly ash of SH

850°C fly ash of SH

a n oa o n dn aoa aapa d ao a a

p

0 10

70

(a) Bottom ash of SH

20

30

40 2θ (°)

(b) Fly ash of SH

17


50

60

70

j jgg

c

c

c 900°C bottom ash of SEH c jj jg

c

c

c

c

850°C bottom ash of SEH

c c

cc c c c c c

jjg j g

k

0 1500 1000 500 0 1500 1000 500

950°C fly ash of SEH

a d g ac j kgk g d

d

d

d

900°C fly ash of SEH

j k d k j j j ac a jk g gg

dk g

d

d

c

850°C fly ash of SEH d j k j k j jga g a g k g d g c d

0 20

30

40

50

60

70

(c) Bottom ash of SEH c

c

20

30

40

50

60

70

80

90

(d) Fly ash of SEH

c

ce e c c

850°C bottom ash of TC

16000 c

cc c h g e c c

g 20

10

90

900°C bottom ash of TC h g e g c cc c c

c

80

2θ (°)

2

3

500

c

2θ (°)

4000 2000 0 10

d

1000 Diffracted intensity (cps)

cj

1

20000 16000 12000 8000 4000 0 18000

Page 18 of 22

1500 950°C bottom ash of SEH

c

30

40

50 2θ (°)

60

c 70

h ce e c c 80

90


8000 6000 4000 2000 0 8000 6000 4000 2000 0 8000 6000 4000 2000 0 10


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

a

6300

850°C fly ash of TC

4200 2100 0 10

e aa a a f hh e haeeb af df h hba h f 20

30

40

50 60 2θ (°)

da 70

f a 80

90

4 5 6

(e) Bottom ash of TC (f) Fly ash of TC a-CaSO4; b-CaO; c-SiO2; d-NaCl; e-CaSO4•Na2SO4; f-Fe3O4; g-NaAlSi3O8; h-Na2Si2O5; i-CaCO3; j-NaAlSiO4; k-CaO•Al2O3•SiO2; m-Ca2SiO5; n-CaO•Al2O3; o-KNaFeSi4O10; p-MgO

7

Figure 6. XRD patterns of Zhundong coals gasification ashes

8

3.4 Sodium transformation during Slagging

9

The running temperature curves of Zhundong coals gasification were shown in figure 7.

10

The gasification temperature was maintained at 900°C during the experiments. The running

11

temperature curves of SH and SEH are stable and smooth, whereas the running temperature

12

curve of TC fluctuates greatly. During the TC gasification experiment, the gasification

13

temperature increased sharply, reached 1029°C, slagging and defluidization occurred. The

18


Page 19 of 22

1

content of Na2O in the TC coal ash is relative high (7.28%) compared with the content of

2

Na2O in the SH coal and SEH coal (3.92% and 4.38%, respectively). Na2O in the coal ash

3

lowers the ash melting point and is one reason for the slagging and defluidization during TC

4

gasification. The micro photograph of bottom ash of TC at 900°C is shown in figure 8 and

5

the element contents analysis by EDX is presented in table 2. The enrichment of sodium and

6

silicate in the melted area (A in figure 7) is apparent and the fraction of Al was relative low,

7

about 1.41% by weight. Combining the results of chemical fraction analysis and XRD test,

8

the formation of sodium silicate leads to slagging and defluiidization during the gasification

9

of TC. 1000 800

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

SH

600 1000 800

SEH

600 1000 800

TC

600 00:00

10 11

01:00

02:00

03:00

Running time (h)

Figure 7. Gasification temperature curves of Zhundong coals gasification at 900°C

A

B

12 13

04:00

Figure 8. Micro photograph of TC bottom ash

19


Energy & Fuels

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

Table 2 Element distribution on TC bottom ash surface (wt%)

1

2

Page 20 of 22

Position

Na

Mg

Al

Si

K

Ca

Fe

C

O

A B

9.35 11.90

0.73 -

1.41 14.90

24.95 16.31

0.15 -

2.88 0.18

1.61 -

5.03 2.81

53.90 53.90

4 Conclusions

3

In this work, the speciation and transformation of sodium during Zhundong coal

4

gasification in a CFB gasifier were investigated. The results of which revealed the

5

following:

6

(1) H2O-soluble sodium is the predominant occurrence, accounting for 55-95% of the

7

total sodium. And insoluble potassium is the predominant occurrence. The total content of

8

potassium is relatively low, approximately one-tenth of sodium in Zhundong coals.

9

(2) Temperature and coal type are both important influencing factors on the

10

volatilization of sodium during Zhundong coal gasification. Sodium in SH ashes dropped

11

quickly with increasing gasification temperature for more gaseous Na is released, whereas

12

increases in SEH and TC ashes for more sodium was trapped by Al and Si in the ashes.

13

(3) The chemical form of sodium in the gasification fly ash and bottom ash are different

14

remarkably. H2O-soluble and NH4Ac-soluble sodium are the main chemical forms in the fly

15

ash, whereas HCl-soluble and insoluble sodium are the main chemical forms in the bottom

16

ash.

17

(4) During the gasification, Sodium aluminum silicate and sodium silicate were formed

18

during gasification process. The formation of sodium silicate leads to slagging and

19

defluidization during the gasification of TC.

20 21

Corresponding Author 20


Page 21 of 22

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

Energy & Fuels

*Phone number: +86-010-82543129, E-mail: [email protected].

2 3

Acknowledgment

4

The authors acknowledge the financial support provided by the Strategic Priority

5

Research Program of the Chinese Academy of Sciences (No.XDA07030100) and the

6

International Science & Technology Cooperation Program of China (No. 2014DFG61680).

7 8

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Speciation and Distribution of Sodium during Zhundong Coal

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