Volatilization behavior of fluorine in coal during

fluidized-bed pyrolysis and CO2-gasification

Wen Li*, Hailiang Lu, Haokan Chen, Baoqing Li

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences,

Taiyuan South Road No. 27, Taiyuan 030001, People’s Republic of China

Received 9 July 2004; received in revised form 13 September 2004; accepted 13 September 2004

Available online 27 October 2004

Abstract

The volatilization behavior of fluorine in five Chinese coals was investigated during fluidized-bed pyrolysis and CO2-gasification at a

temperature range of 500–900 8C. The effect of co-existed and added calcium on fluorine volatility during pyrolysis was also determined.

With increasing pyrolysis temperature, the volatility of fluorine increases. However, the volatility is greatly dependent on the fluorine

chemical forms occurred in coal. Except for Datong and Zhungeer coal, more than 65% of fluorine in other three coals occurs as the steady

forms. Fluorapatite is not the major carrier of fluorine in the coals studied. Fluorine volatility is retarded by coexisting calcium during coal

pyrolysis, indicating that at least part of the stable forms of fluorine in coal might occur as calcium fluoride or calcium fluoride with complex

compounds which are stable even at high pyrolysis temperature. The addition of CaO and limestone can suppress the release of fluorine

during pyrolysis. The effect of CaO is better than that of limestone. The volatility of fluorine of coal during CO2-gasification depends on not

only the occurrence mode of fluorine, but also the gasification reactivity of the coal. Compared with N2 atmosphere, CO2 is more favorable to

the release of fluorine from coal.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Coal; Fluorine; Volatility; Calcium; Pyrolysis; CO2-gasification

1. Introduction

Coal is the most abundant fossil fuel in the world and it is

and will be the primary energy source in China in less than

50 years to come. In recent years, many toxic trace elements

including fluorine in coal attracted much attention [1–4].

Fluorine is one of the harmful trace elements in coal.

Though its content is very low, with a mean of 150 mg gK1

in the world coals [1] and 82 mg gK1 in Chinese coals [2], a

large amount of toxic compounds of fluorine such as HF,

SiF4, and CF4, were released into the atmosphere during

coal utilization, leading to harmful effects on the environ-

ment and human health. Thus, knowledge about transform-

ation of fluorine during coal conversion is needed. The

volatilization behavior of part of trace elements, such as Cl,

Hg, Cd, Pb, Mn, As, Be and Ni, during coal pyrolysis has

0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2004.09.008

* Corresponding author. Tel.: C86 351 4044335; fax: C86 351 4050320.

E-mail address: liwen@sxicc.ac.cn (W. Li).

been investigated [5–8]. And extensive studies have been

reported with respect to fluorine emission and control

technology during coal combustion [9–11]. However, there

is little information about the transformation of fluorine

under inert and reduction atmosphere. In this paper, the

volatilization of fluorine in five Chinese coals during

pyrolysis and CO2-gasification in fluidized-bed reactor

was investigated. The effect of calcium in coal and the

added calcium on fluorine volatility was also determined.

2. Experimental

2.1. Coal samples

Five Chinese lump coal samples were determined in this

study Huolinhe and Zhungeer coal are from Inner Mongolia.

Datong and Pingshuo coal are from Shanxi Province. Yima

coal is from Henan Province. Coal samples were ground

Fuel 84 (2005) 353–357

www.fuelfirst.com

Table 1

Proximate and ultimate analyses of coal sample

Sample Proximate analyses/wt% Ultimate analyses/wt%, daf

Mad Ad Vdaf C H S N

Datong 3.6 14.1 32.4 81.62 5.05 0.83 0.87

Pingshuo 4.8 19.9 39.2 78.76 5.39 0.49 1.46

Huolinhe 16.3 19.4 58.5 73.37 4.12 0.52 1.64

Yima 8.8 17.3 40.2 78.10 3.90 0.40 0.86

Zhungeer 4.9 23.4 37.8 74.33 5.65 0.27 1.10

W. Li et al. / Fuel 84 (2005) 353–357354

and sieved to 60–100 mesh before examination. Their

proximate and ultimate analyses are given in Table 1.

2.2. Pyrolysis and gasification tests

Pyrolysis tests were carried out in a quartz tube (with

inner diameter of 25 mm and length of 600 mm) fluidized-

bed reactor with nitrogen flow at temperature ranging from

400 to 900 8C. At a predetermined temperature about 5.0 g

coal samples was put into the quartz tube reactor quickly. A

mass flow meter controlled the nitrogen velocity ranging

from 500 to 900 ml/min for fluidization. Gasification runs

were performed at 800–950 8C for Yima and Zhungeer coal

with CO2 flow of 300 ml/min. After the desired residence

time, the quartz tube reactor was moved quickly to the

atmosphere and cooled down to the room temperature. Then

the chars were collected for analysis.

Table 2

Phosphorus and fluorine content in coal samples

Sample Ad/wt% P/mg gK1 F/mg gK1 Mass ratio (P/F)

Datong 14.1 68 75 0.9

Pingshuo 19.9 200 135 1.5

Huolinhe 19.3 135 105 1.3

Yima 17.3 234 127 1.8

Zhungeer 23.4 194 609 0.3

2.3. Determination and calculation method

Fluorine contents in coal and char were determined by

combustion-hydrolysis/fluoride-ion selective electrode

method based on Chinese standard method (GB/T 4633-

1997). 0.5 g sample was mixed with 0.5 g quartz sand in a

small earthen boat, and then a suitable amount of quartz

sand was spread on the mixture. The boat was gradually put

into the furnace tube preheated up to 1000 8C. 400 ml/min

of oxygen was flowing through the tube, during which the

steam was simultaneously introduced to control the volume

of condensate at 2.5–3 ml/min and the total volume within

85 ml. During this digestion process the fluorine in coal was

totally converted into HF and SiF4, and then dissolved into

water. Fluoride-ion selective electrode (pF-1 model, made

in Shanghai Leici Company, China) and saturated calomel

electrode were used as indicator and reference, respectively,

to determine the concentration of fluorine. The PH value of

digestion solution was adjusted to be 6, using C3H4-

OH(COONa)3–KNO3 as a buffer, to avoid the interference

of the background matters. The potential analyzer used in

this method was G301-A model, made in Institute of Coal

Chemistry, Chinese Academy of Sciences. Phosphorus

content of raw coal was determined by ICP-AES (induc-

tively coupled plasma-atomic emission spectroscopy, TJA

Company of America) using Atomscan16. Mineral matters

in coal were calculated from the data of ash analysis result.

Fluorine volatility during coal pyrolysis and CO2-

gasification was calculated by the following equation

V% Z 1 KC1;dm1;d

C0;dm0;d

� �100% Z 1 K

C1;dY

C0;d

� �100%

Where V%: fluorine volatility,%; C1,d: fluorine content of

char, mg gK1; C0,d: fluorine content of raw coal, mg gK1;

Y: yield of char, %; m1,d: mass of char, g; m0,d: mass of raw

coal, g.

3. Results and discussion

3.1. Mode of occurrence of fluorine in coal

The electronegativity of fluorine is 4.10 eV which is the

highest value in the known elements. Hence, fluorine is

the most active non-metallic element and cannot exist in the

simple substance in nature. Previous studies showed that the

main chemical forms of fluorine in coal were of inorganic

association [12,13]. Because of the similar radius of FK and

OHK, part offluorine in coal occurs as independent minerals,

such as CaF2 and MgF2; while others replace OHK in other

minerals, such as Ca10(PO4)6(OH)F. Due to the complexity

of coal composition and strong reaction capacity of fluorine,

additive or substitute reactions inevitably take place between

fluorine compounds and organic functional groups in coal.

Thus, trace amounts of organic fluorine must be existed in

coal. As geological environment during coalification and

coal ranks are different greatly, the modes of occurrence of

fluorine varies widely from coal to coal. Generally it is

believed that fluorapatite [Ca10(OH)2KxFx(PO4)6, 0%x%2]

is the most important carrier of fluorine [12–14]. Obviously,

the mass ratio of phosphorus to fluorine must be higher than

4.9, if fluoraptite is the major form of fluorine in coal. Table 2

gives the content of phosphorus and fluorine and the mass

Fig. 1. Effect of temperature on fluorine volatility during coal pyrolysis.

W. Li et al. / Fuel 84 (2005) 353–357 355

ratio of the corresponding samples as shown in Table 1. It is

seen that the mass ratios of phosphorus to fluorine in five coal

samples are all below 4.9, indicating that fluorapatite is not

the major form of fluorine in these five coal samples.

3.2. Effect of pyrolysis temperature on fluorine volatility

Relationship between pyrolysis temperature and fluorine

volatility is given in Fig. 1. It can be seen that fluorine

volatility increases with the increasing of pyrolysis

temperature. Contrary to expectation, only a small amount

of fluorine in coal volatilize during the temperature ranges

in this study. Except for Datong and Zhungeer coals,

fluorine volatilities are below 35%. It indicates that more

than 65% fluorine is thermal stable form in Pingshuo, Yima

and Huolinhe coals. The stable forms of fluorine include

the fluoride-bearing mineral matters and simple com-

pounds of fluorine. The decomposition temperature,

melting and boiling points of these minerals are higher

than pyrolysis temperature. The volatile fluorine contains

free state of FK adsorbed in coal seams and simple

compounds with boiling point below 900 8C. Some organic

fluorine might be present in volatile forms, which releases

as HF by cracking or condensation reactions during coal

pyrolysis.

The pyrolysis conditions, especially reaction tempera-

ture, greatly influence the volatilization behavior of fluorine

in coal. It is reported fluoraptite decomposes at 200 8C to

release fluorine which is volatilized up to 50% at 800 8C

[15]. The low volatility of fluorine of the five coals at 900 8C

indicates that fluoraptite is not the main carrier of fluorine in

Table 3

Analyses of ash compositions in coal (product of ash composition and ash conten

Sample SiO2 Al2O3 Fe2O3 CaO M

Datong 7.67 3.01 2.20 0.42 0.

Pingshuo 7.90 8.78 1.38 0.87 0.

Huolinhe 12.52 3.25 0.93 1.20 0.

Yima 7.65 3.27 2.56 1.81 0.

Zhungeer 6.10 13.46 0.93 1.30 0.

these coals, which is consistent with the result shown in

Table 2. In our previous work [8] the volatility of arsenic in

Yima coal was reported. It is found that nearly 70% of

arsenic in Yima coal was evaporated and increased slightly

when temperature is above 800 8C. But in the case of

fluorine in Yima coal less than 30% of fluorine was

evaporated even at 900 8C. This strongly suggests that the

thermal stability of fluorine-bearing minerals or fluorine-

containing compounds in Yima coal is much higher than

that of arsenic-bearing minerals which is mainly pyrite.

3.3. Effect of coexisting calcium on fluorine volatility

The mode of occurrence of fluorine in coal is an

important factor that influences its volatile behavior during

coal pyrolysis. The vaporization of fluorine might be

retarded by its host minerals or coexisting mineral

elements. Some fluoride-bearing minerals might be present

in coal, for example, fluorophlogopite (KMg3(AlSi3O10)F2,

DGFf ;1800 K ZK4171 kJ molK1), has a very high decompo-

sition temperature [15]. Previous studies showed that the

decomposition temperature of CaF2 is higher than 1000 8C,

and the decomposition temperature of calcium fluoride with

complex compounds such as CaF2 CaO, CaF2$CaO$Al2O3,

and CaF2$CaO$SiO2, etc, is even higher than 1300 8C [16].

The composition of ash composition in coal is shown in

Table 3. Obviously, there are more calcium, silicon and

aluminium than that required for formation of fluoride-

bearing mineral matters or complex compounds as men-

tioned above, which should be the stable forms of fluorine

in coal.

The relationship between the calcium content in raw coal

and the fluorine volatility during fluidized-bed pyrolysis is

shown in Fig. 2. For the coals studied, calcium content

increases in the following order: Datong, Pingshuo,

Huolinhe, Zhungeer and Yima. Except for Zhungeer coal

with a high content of fluorine, fluorine volatility decreases

as calcium content of original coal increases during coal

pyrolysis. This indicates a strong correlation between

fluorine and calcium in original coal at various pyrolysis

temperatures. The volatility of fluorine during coal pyrolysis

might be retarded by coexisting calcium in coal, indicating

that the stable forms of fluorine in coal might exist in a large

percentage as calcium fluoride or calcium fluoride based

complex compounds.

t)/wt%

gO TiO2 K2O Na2O P2O5

17 0.13 0.22 0.06 0.02

30 0.27 0.07 0.02 0.05

20 0.16 0.20 0.0 0.03

40 0.20 0.24 0.04 0.05

26 0.46 0.14 0.0 0.04

Fig. 2. Relationship between calcium content in raw coal and fluorine

volatility during fluidized-bed pyrolysis.

Fig. 4. Effect of quantity of added calcium on the restraining efficiency of

fluorine volatility during Zhungeer coal pyrolysis at 900 8C.

W. Li et al. / Fuel 84 (2005) 353–357356

3.4. The effect of added calcium on volatility of fluorine

To further understand the effect of calcium on the

fluorine volatility during pyrolysis, CaO and limestone were

added into coal with the ratio of 5 and 9%, respectively, to

keep the same Ca/S ratio of 5.7. Fig. 3 compares the fluorine

volatility of Zhungeer coal with and without calcium

additive. For the added CaO the volatility of fluorine greatly

decreases in the temperature range tested compared with

that of raw coal. For added limestone the fluorine volatility

changes little below 700 8C, while it decreases at high

temperatures. This suggests that the main fluorine fixation

reaction is: CaOC2HF/CaF2CH2O. Limestone is

decomposed little below 700 8C and the competitive

reaction of sulfur fixation reaction makes the added

limestone has little effect on the volatility of fluorine at

low temperature.

Fig. 4 shows the effect of calcium amount on the

volatility of fluorine at pyrolysis temperature of 900 8C.

The restraining efficiency is defined as the difference

between fluorine volatility of raw coal and that

of calcium added coal. With increasing amount of added

calcium the restraining efficiency increases, but more

Fig. 3. Effect of added calcium on fluorine volatility during fluidized-bed

pyrolysis of Zhungeer coal.

calcium is not favorable to improve fluorine fixation further.

The optimum ratio of Ca/S ratio for CaO and limestone to

restrain fluorine is about three and four, respectively.

3.5. Fluorine volatility of coal during CO2-gasification

The CO2-gasification of Yima and Zhungeer coal was

performed at 800–950 8C with residence time of 30 min.

Fig. 5 illustrates the yield of residue versus reaction

temperature. With increasing temperature the residue yield

of Yima coal decreases gradually, but that of Zhungeer coal

decreases linearly. The residue yield of Yima coal is much

lower than that of Zhungeer coal especially at low

temperature, suggesting the higher CO2-gasification reactiv-

ity of Yima coal. The content of fluorine in the gasification

residue is shown in Fig. 6 in which the value at 25 8C

represents the fluorine content in raw coal. The fluorine

content in Yima residue is higher than that in raw coal, but it

decreases remarkably with increasing temperature. For

Zhungeer coal the fluorine content in the residue is about

half of that in the raw coal. This implies that the volatility or

reactivity offluorine in Yima coal is lower than that of carbon

during CO2-gasification, but the case of Zhungeer coal is just

Fig. 5. Effect of gasification temperature on the yield of residue.

Fig. 6. Effect of gasification temperature on the fluorine contents in the

residue.

Fig. 7. Effect of gasification temperature on the fluorine volatility.

W. Li et al. / Fuel 84 (2005) 353–357 357

opposite. Fig. 7 shows the fluorine volatility of Zhungeer coal

gradually increases with increasing temperature and is higher

than that of Yima coal below 900 8C. The fluorine volatility

of Yima coal changes little at low temperature and increases

remarkably above 850 8C. The different volatility of fluorine

in Yima and Zhungeer coal suggests that the release of

fluorine depends on not only its occurrence mode, but also the

gasification reactivity of the raw coal.

Comparing the result in Fig. 1 with that in Fig. 7, one

can find that the fluorine volatility of the two coals

during CO2-gasification is obviously higher than that

during pyrolysis. Especially for Yima coal with high

gasification reactivity, the fluorine volatility in gasifica-

tion is three times higher than that in pyrolysis. It can be

concluded that fluorine is more easily released in the

reduction atmosphere than in the inert one.

4. Conclusions

The volatility of fluorine of five Chinese coals

during fluidized-bed pyrolysis and CO2-gasification was

investigated and the effect of co-existed and added calcium

was also examined. The following conclusions are drawn

from this work:

(1)

Fluorapatite is not the major form of fluorine in the coals

studied.

(2)

Fluorine volatility increases as reaction temperature

increases during coal pyrolysis. Except for Datong and

Zhungeer coal, more than 65% of fluorine in other three

coals occurs as the stable forms.

(3)

Fluorine volatility is retarded by co-existing calcium

during coal pyrolysis, indicating that at least part of the

steady forms of fluorine in coal might occur as calcium

fluoride or calcium fluoride with complex compounds.

(4)

The addition of CaO and limestone can suppress the

release of fluorine during pyrolysis. The effect of CaO is

better than that of limestone.

(5)

The volatility of fluorine of coal during CO2-gasification

depends on not only the occurrence mode of fluorine, but

also the gasification reactivity of the coal. Compared

with N2 atmosphere, CO2 is more favorable to the release

of fluorine from coal.

Acknowledgements

Financial support for this work by the Special Founds for

Major State Fundamental Research Project of China

(G1999022107) and by Natural Science Foundation of

China (29936090) is gratefully acknowledged.

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