Leaching of ashes and chars for examining transformations of trace

elements during coal combustion and pyrolysis

Jie Wang, Akira Takaya, Akira Tomita*

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan

Received 8 March 2003; revised 3 June 2003; accepted 4 June 2003; available online 6 December 2003

Abstract

One sub-bituminous coal and two bituminous coals were subjected to the combustion and pyrolysis by slow heating to a temperature

ranging 550–1150 8C. Leaching of raw coals, ashes and chars with dilute HCl and HNO3 was carried out, and leachate concentrations of

major and trace elements were determined. Such a comparative leaching method was validated for characterizing the modes of occurrence of

trace elements in coal and their transformations upon heating. Leaching results suggested that Be, V, Co, Cr and Ni were partially associated

with organic matter, and As was partially associated with pyrite. During the ashing at 550–750 8C, the organically associated trace elements

in coal formed some acid-soluble species. After the ashing at 1150 8C, Be, Co, Cr and Ni, together with Mn, Zn, and Pb, were immobilized in

ash against leaching, whereas As was not immobilized. After pyrolysis, the organically associated trace elements in chars remained insoluble

in both acids, and some HNO3-soluble As in coal turned to a HNO3-insoluble species.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: Mineral matter; Trace elements; Coal ash; Coal char; Acid leaching

1. Introduction

The physical and chemical transformations of trace

elements during coal combustion are of an important subject

of study, because of their predominant effect on the

distribution and speciation of trace elements in ash residues.

A large number of studies have reported the preferential

concentration of many toxic trace elements in submicron

ash particles during the combustion of pulverized coal due

to a volatilization-condensation mechanism [1–7]. This is

environmentally important because fine airborne aerosols

are readily inhaled and deposited in the human respiratory

system. Of importance, however, are not only the

concentration and distribution but also the speciation of

trace elements in ash residue, because the speciation of an

element determines its solubility and toxicity. It is thus

essential to clarify how the trace elements exist in coal and

how they are transformed upon various thermal treatments.

The modes of occurrence of trace elements in coal have

extensively been investigated [8 – 10]. The principal

methods for determining the modes of trace elements in

coal are the sequential leaching method and X-ray

absorption fine structure (XAFS) technique [10–14].

These two methods are also applied to determine the

speciation of trace elements in coal ash residues [15–17].

However, coal ash sample for analysis is usually limited to

the bottom ash and fly ash collected from coal-fired plants.

Little detailed information has been known with respect to

the transformations of trace elements under various thermal

conditions. Recently, we have proposed a comparative

leaching method using both coal and coal ashes [18]. This

method is suited for determining the organic and pyritic

associations of trace elements in coal. In this paper, a series

of coal ashes and chars are prepared under well-controlled

conditions using a laboratory reactor, and leaching results

are comprehensively compared between raw coals, coal

ashes and coal chars. We intend to further ascertain the

fundamental information on the modes of occurrence of

trace elements in coal and their chemical transformations

during coal combustion and pyrolysis. Moreover, acid

leaching is used for examining the mobility of elements in

ash residues to the acidic environment [16,19].

0016-2361/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2003.06.003

Fuel 83 (2004) 651–660

www.fuelfirst.com

* Corresponding author. Tel.: þ81-22-217-5625; fax: þ81-22-217-5626.

E-mail address: tomita@tagen.tohoku.ac.jp (A. Tomita).

2. Experimental

Three coals, Ermelo (EL) bituminous coal from Republic

of South Africa, Datong (DT) bituminous coal from China,

and Taiheiyo (THY) sub-bituminous coal from Japan, were

used in this study. The proximate and ultimate analyses of

these coals were given in Table 1. A reference coal (SARM)

from Republic of South Africa was also used, and the

concentrations of the major and trace elements in this coal

are certified. The particle size of all coal samples was

smaller than 0.15 mm.

Coal ash and char were prepared by combustion and by

pyrolysis, respectively, using a horizontal tubular reactor. In

coal combustion, some 1.5 g of coal sample loaded on an

alumina boat was heated in an air stream of 2 L/min at a rate

of 10 8C/min, and held at the final temperature for 1 h, and

then cooled down quickly. It was observed that coal

completely turned to ash below 550 8C. In the case of

pyrolysis, a nitrogen gas was employed in place of air, and

the other operating conditions were kept the same as used in

the combustion. The resultant ashes and chars were weighed

and subjected to the leaching experiment. In the case of

pyrolysis, no salient swelling was observed and all coal

chars were fragile. Before leaching, the coal char was gently

pulverized using a mortar pestle.

Quantification of the major (Al, Fe, Ca, Mg, and K) and

trace elements (Be, V, Cr, Mn, Co, Ni, Zn, As, and Pb) in

ash was performed with an inductively coupled plasma

atomic emission spectroscopy (ICP-AES) instrument

(Optima 3300XL, Perkin–Elmer), following the melting

of an ash sample with lithium tetraborate. The detailed

procedure was described elsewhere [18]. For quantification

of major and trace elements in coal and char, the coal and

char samples were first ashed at a rate of 10 8C/min up to

550 and 750 8C, respectively, and then the ash sample was

analyzed as stated above. We examined the volatilization

loss of the major and trace elements upon the ashing, and we

found that no vaporization occurred for the examined

metallic elements (see Section 3 in detail).

Leaching of raw coal (1.5 g) was conducted in a sealed

polyethylene container with 30 mL of 6% HCl or 6% HNO3

at 95 8C for 1 h. Leaching of coal ash and char was

conducted in a similar manner. The amount of coal ash or

char used in each leaching test resulted from the combustion

or pyrolysis of 1.5 g coal. After leaching, the slurry was

filtered with a piece of poly-tetrafluoroethylene membrane.

The concentrations of major and trace elements in the

filtrate were measured with ICP-AES.

3. Results

3.1. The amount of major and trace elements retained in ash

and char

First of all, it is necessary to determine the amounts of

major and trace elements in the raw coals. The certified and

reference concentrations [18] in raw coal are given for

SARM coal and EL coal, respectively. Table 2 shows the

amounts of major and trace elements measured from various

ashes and chars. Some important elements such as Na, Se

and Hg could not be determined with the ICP-AES

instrument. For easier comparison, the amount of an

element in ash or char is presented on the basis of initial

coal weight. In the repetitive ICP-AES measurements, the

variation ranged 5–15% for As, 1–5% for Pb and Zn, and

below 2% for the rest of the elements. The 550 8C ash was

prepared three times from SARM coal, and the repetitive

quantification of trace elements in three samples showed the

experimental error being less than 12% for all of the trace

elements.

Two important aspects of information are seen in Table 2.

First, a comparison of the certified and reference values with

measured ones indicates almost no volatilization loss

occurring during the ashing at 550 8C for all of the

examined elements in SARM and EL coals. This agreement

was also observed for four coals examined in the previous

study [18]. The data for the 550 8C ash are assumed to

represent the concentrations of these elements in the parent

coal. In the following section, the concentration in the

550 8C ash is accordingly used as that in the original coal for

the calculation of leaching percentage. Secondly, Zn and Pb

appreciably vaporized during the pyrolysis above 950 8C,

whereas the other elements showed little volatilization

during both the combustion and pyrolysis even at 1150 8C.

3.2. Leaching of major elements

Leaching results of major elements in the raw coals, coal

ashes, and coal chars with 6% HCl are shown in Fig. 1,

where the leaching percentage is referred to as the leached

amount of an element, divided by the amount of the element

Table 1

Proximate and ultimate analyses of the coal samples

Coal sample Proximate analysis (wt%, as-received) Ultimate analysis (wt%, daf)

Moisture Volatile matter Ash Fixed carbon C H N S O

EL 3.0 31.0 13.0 53.0 82.1 4.9 2.0 0.8 10.2

DT 3.2 28.7 9.7 58.4 82.7 4.7 1.1 0.7 10.8

THY 4.0 43.9 12.1 40.0 78.7 6.2 1.2 0.1 13.8

J. Wang et al. / Fuel 83 (2004) 651–660652

Table 2

Concentrations of major and trace elements in raw coal, ash, and char (unit: mg/g—coal for major elements; mg/g—coal for trace elements)

Element Raw coala 550 8C ash 750 8C ash 950 8C ash 1150 8C ash 550 8C char 750 8C char 950 8C char 1150 8C char

SARM coal

Al 39 39 39 39 40 39 37 40 40

Fe 11 12 12 12 12 11 10 11 11

Ca 9.9 9.5 9.7 9.5 9.8 9.1 8.6 9.2 9.3

Mg 1.2 1.2 1.2 1.2 1.2 1.1 1.0 1.1 1.1

K 2 2.3 2.3 2.3 2.3 2.7 2.7 2.8 2.8

Be 2.8 2.6 2.7 2.7 2.7 2.6 2.6 2.6 2.6

V 35 34 35 35 35 33 34 33 33

Cr 50 50 52 52 54 49 49 49 49

Mn 157 148 153 154 157 146 144 143 143

Co 5.6 8.3 8.4 8.5 8.5 8.2 8.3 8.3 8.2

Ni 16 16 16 17 17 15 15 15 15

Zn 12 14 15 15 15 15 13 2.7 1.7

As 7 7 8.3 6.4 7 7.2 7.3 7.4 6.9

Pb 20 21 21 21 20 22 22 5.7 3.5

EL coal

Al 23 21 21 21 21 21 21 22 22

Fe 4.9 5.6 5.4 5.5 5.3 5.6 5.5 5.7 5.7

Ca 7.0 6.0 6.0 5.3 4.8 7.4 7.3 7.6 7.5

Mg 2.2 2.1 2.2 2.2 2.1 2.2 2.1 2.2 2.2

K 1.1 1.3 1.3 1.3 1.3 1.6 1.5 1.5 1.6

Be na 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1

V 24 29 28 30 29 29 28 29 29

Cr 28 32 30 33 31 32 31 33 33

Mn 96 92 86 92 87 92 90 92 91

Co 11 12 12 12 12 12 12 12 12

Ni 19 24 24 25 24 25 25 26 25

Zn na 16 20 17 16 16 15 5.2 1.3

As 5.6 4.6 5.3 4.9 5.3 5.2 5.3 5.5 4.7

Pb na 12 11 12 12 13 13 4.0 2.2

DT coal

Al na 9.8 9.4 9.8 9.8 8.7 8.9 8.8 8.8

Fe na 11 11 11 11 10 11 11 11

Ca na 1.6 1.6 1.3 1.4 1.8 1.9 1.9 1.9

Mg na 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

K na 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3

Be na 2.3 2.2 2.3 2.4 2.3 2.4 2.3 2.3

V na 13 12 13 15 12 12 12 12

Cr na 20 20 21 20 20 21 22 22

Mn na 90 87 92 88 85 89 84 87

Co na 6.8 6.5 6.7 7.2 6.7 6.8 6.8 6.9

Ni na 25 24 24 25 20 21 21 22

Zn na 15 15 15 16 14 8.5 2.4 2.2

As na 4.2 3.6 4.4 3.6 3.9 4.1 4.1 3.9

Pb na 6.8 6.8 6.7 5.2 7.0 6.7 4.4 4.5

THY coal

Al na 22 22 21 21 22 23 23 23

Fe na 3.8 3.8 3.7 3.8 3.9 3.9 3.9 3.9

Ca na 6.4 6.3 6.2 6.1 7.4 6.9 7.2 7.2

Mg na 1.9 1.9 1.9 1.9 1.8 1.9 1.8 1.9

K na 1.1 1.1 1.1 1.1 1.2 1.1 1.2 1.3

Be na 1.4 1.4 1.4 1.4 1.3 1.3 1.3 1.3

V na 26 26 26 26 25 25 24 25

Cr na 26 26 26 25 24 25 24 25

Mn na 65 65 65 63 62 64 61 64

Co na 7.2 7.2 7.2 7.9 7.3 7.4 7.7 7.2

Ni na 15 15 15 16 15 15 16 17

Zn na 9.5 9.6 9.5 9.8 8.5 8.7 4.0 1.1

As na 3.2 3.8 2.9 3.5 3.2 3.4 3.0 3.5

Pb na 9.5 9.6 9.2 9.8 10.2 10.5 5.2 4.7

a The certified values for SARM coal, and the reference values for EL coal (cited from Ref. [18]).

J. Wang et al. / Fuel 83 (2004) 651–660 653

in the original coal. Interestingly, EL, THY, and DT coals

exhibited very similar leaching characteristics.

Ca and Mg in all raw coals were substantially dissolved

out, and the solubility of these two elements was not so

different between raw coal, 550–950 8C ashes, and chars.

Mg in the 1150 8C ashes and chars became less soluble.

Al and K were scarcely extracted from raw coal, whereas

these two elements in the 550–750 8C ashes and chars

became significantly more soluble. Above 950 8C, the

solubility of these elements from both ash and char was

dramatically reduced.

The solubility of Fe from EL coals was lower than that

from DT and THY coals. The solubility of Fe from the 550–

750 8C ashes was higher than from the parent coal, and it

sharply decreased above 750 8C. Contrary to the case of ash,

the leaching percentage of Fe in the chars was constant

between 550 and 1150 8C.

3.3. Leaching of nonvolatile trace elements

Leaching results of the nonvolatile trace elements (Be, V,

Cr, Mn, Co, Ni, and As) with 6% HCl are shown in Fig. 2.

Be, V, Co, Cr and Ni exhibited a similar leaching behavior;

the solubility of these elements from the 550–750 8C ashes

was significantly higher than from the parent coals. At

temperatures higher than 950 8C, the solubility of these

elements except for V sharply decreased. V in the 1150 8C

ashes of EL and THY coals behaved differently. In contrast

to a higher solubility of the five elements from the 550–

750 8C ashes, the solubility of Be, V, Co, Cr and Ni from the

chars remained as low as that from the parent coals.

Arsenic in all ashes was more soluble than that in the

parent coals, whereas As in the chars was slightly less

soluble than that in the parent coals. A large portion of Mn

was extracted from the raw coals, 550–750 8C ashes and

550–750 8C chars. Mn in the 1150 8C ashes and chars

became less soluble.

Fig. 3 shows the leaching results with 6% HNO3 for EL

and DT coals. The solubility of As from the 550 8C ashes

was lower than from the parent coals, and the solubility of

As increased from 550 8C ash to 1150 8C ash. These results

were quite dissimilar to those obtained with HCl leaching.

On the other hand, the solubility of As in HNO3 from the

chars was less soluble than that from the parent coals,

similar to the result of HCl leaching. The leaching behavior

of Be, V, Co, Cr and Ni was essentially similar to that shown

in the HCl leaching.

3.4. Leaching of volatile trace elements

Fig. 4 shows the volatilization loss of Pb and Zn upon

the combustion and pyrolysis, together with the HCl

leaching data for the resultant ashes and chars. Volatiliz-

ation loss is defined as a difference between the amount of

an element in the original coal (the initial amount) and that

in the residual ash or char, divided by the initial amount.

More detailed investigation on the volatilization of Pb and

Zn was conducted in the previous paper [20]. Three coals

exhibited very similar volatilization and leaching beha-

viors. The solubility of Pb and Zn from ash decreased with

increasing temperature. In the pyrolysis, a significant

portion of Pb and Zn vaporized above 950 8C so that

only a small portion of the two elements remained in the

chars. Thus the leaching percentage from the 950–1150 8C

chars was marginal.

4. Discussion

4.1. Mineralogy, transformation and solubility of major

elements

We examine the leaching behaviors of major metallic

elements with two principal intentions. First, we can

validate the present methodology for elucidating the thermal

transformations of major minerals from the leaching results

of major metallic elements, since the transformations of

major mineral are fairly known and one reason for this is

that the major minerals can be directly identified by XRD.

Second, trace elements in coal are mainly associated with

major minerals or organic matter. The information on the

leaching behaviors of major metallic elements is helpful to

Fig. 1. Leaching of the major metallic elements in the raw coals, coal ashes

and coal chars with 6% HCl.

J. Wang et al. / Fuel 83 (2004) 651–660654

postulate the modes of trace elements in coal and their

transformations upon heating according to similarities or

dissimilarities in the leaching behavior between major

metallic elements and trace elements.

Table 3 lists the major crystalline mineral species in the

coal samples, which were identified by XRD. The major

metallic elements in the coals are present in these minerals.

Calcite and dolomite are absent in DT coal and this accounts

Fig. 3. Leaching of the nonvolatile trace elements in the raw coals, coal ashes and coal chars with 6% HNO3.

Fig. 2. Leaching of the nonvolatile trace elements in the raw coals, coal ashes and coal chars with 6% HCl.

J. Wang et al. / Fuel 83 (2004) 651–660 655

for a low concentration of calcium. Pyrite and siderite are

relatively meager in THY coal and this corresponds to a low

concentration of iron.

Fig. 5 shows the XRD patterns of the DT coal ashes

prepared at different temperatures. Anhydrite (CaSO4)

occurred in the 550 8C ash. It is partly derived from by

the dehydration of gypsum [21]. At 1150 8C, anhydrite

disappeared and an anorthite phase (CaAl2Si2O8) appeared,

indicating the interaction of poorly crystalline calcium

oxide and/or anhydrite with kaolinite [22,23]. Hematite

(Fe2O3) occurred in the 550 8C ash due to the oxidation of

pyrite [23]. Siderite disappeared in the 550 8C ash, forming

hematite by decomposition and oxidation reactions. An

increase in the peak intensity of hematite with increasing

temperature indicates the developed crystallinity upon

heating. Kaolinite significantly present in the raw coal

disappeared in the 550 8C ash due to the formation of

amorphous meta-kaolinite (Al2Si2O8) [23]. Mullite (3Al2-

O3·2SiO2) and cristobalite (SiO2) in the 1150 8C ash are

formed from meta-kaolinite [22].

Similarly, the EL coal chars prepared at 550–1150 8C

were examined by XRD. Calcite and dolomite existed in the

550 8C char and they disappeared at 750 8C, forming

calcium sulfide (CaS). The crystallinity of calcium sulfide

increased with increasing temperature up to 1150 8C. No

anorthite was observed in the 1150 8C char. It is reported

[24] that in a reducing atmosphere, pyrite in coal is

dissociated to pyrrhotite (FeSx,1 , x , 2) above 400 8C.

However, the peaks of pyrrhotite in the chars were very

Fig. 4. The vaporization loss of zinc and lead upon the combustion and pyrolysis prior to the leaching test, together with the leaching fraction of remaining zinc

and lead in coal ashes and coal chars with 6% HCl.

Table 3

Major crystalline mineral species in the used coals

Coal sample Mineral speciesa

SARM Quartz (vs), kaolinite (s), calcite (m),

gypsum (m), pyrite (m), rutile (m)

EL Quartz (s), kaolinite (vs), calcite (m),

dolomite (m), gypsum (w), pyrite (m)

DT Quartz (vs), kaolinite (s), gypsum (m),

siderite (m), pyrite (m)

THY Quartz (vs), kaolinite (s), calcite (m),

gypsum (w), pyrite (w)

The relative XRD intensity is indicated in parentheses; vs: very strong,

s: strong, m: moderate, and w: weak.a Chemical formula; quartz: SiO2, kaolinite: Al2Si2O5(OH)4, gypsum:

CaSO4·2H2O, rutile: TiO2, calcite: CaCO3, dolomite: CaMg(CO3)2, pyrite:

FeS2, siderite: FeCO3.

J. Wang et al. / Fuel 83 (2004) 651–660656

weak. The change of kaolinite was similar to that occurring

in the combustion.

The leachability of the major metallic elements from the

raw coals, ashes and chars (Fig. 1) can be explained by the

coal mineralogy and mineral transformations. Ca was

substantially dissolved from all raw coals, ashes and chars

because the predominant calcium species in coal and the

products in ashes and chars, such as calcium carbonates,

calcium sulfate and calcium sulfide, were acid-soluble.

Anorthite formed at 1150 8C is also extracted with

hydrochloric acid [25]. Mg occurs in coal mainly as

carbonates; this allows a high solubility of Mg for EL and

DT coals. Nevertheless, about 40% of Mg in DT coal was

not leached, suggesting the presence of some acid-insoluble

magnesium-bearing minerals. For example, HCl-insoluble

serpentine (Mg3Si2O15(OH)4) is found in coal [26,27]. A

relatively low leachability of Mg from the 1150 8C ashes

and chars compared with Ca may be attributed to the

sintering of Mg with kaolinite above 1000 8C, forming acid-

insoluble cordierite ceramics [28]. In coal pyrolysis, such a

solid–solid interaction should be hindered by char matrix.

This may cause some differences in the degree of

leachability between 1150 8C ashes and 1150 8C chars.

Fe is present in coal primarily as HCl-soluble siderite and

iron-bearing dolomite as well as HCl-insoluble pyrite.

Analysis of the forms of sulfur showed that DT, EL, and

THY coals contain 0.5%, 0.4% and 0.1% pyritic sulfur (on

the dry basis), respectively. The ratios of pyritic Fe to total

Fe were calculated to be 0.6, 0.4 and 0.2, respectively. A

higher pyritic Fe ratio in EL coal accounts for a lower

solubility of Fe from this coal than from THY and DT coals.

Fe in the 550 8C ashes was more soluble than that in the

parent coals, but the solubility of Fe decreased with

increasing temperature. XRD showed that after HCl

leaching, the peaks of hematite in the 550– 750 8C

dramatically weakened, whereas those in the 950–1150 8C

ashes did not decline. Hematite produced at a low

temperature has a structure analogous to maghemite that

is readily susceptible to acid dissolution, but well-crystal-

lized hematite has no cation vacancies and thus is resistant

to acid leaching [29]. Fe was more dissolved from all chars

than from the parent coals as a consequence of the reduction

of pyrite to pyrrhotite that is soluble in dilute HCl acid

releasing H2S [30]. Although pyrrhotite was indistinct in the

XRD patterns of chars, the H2S gas was released upon

leaching.

Al was maximally leached from the 550 8C ashes and

550 8C chars. One major reason for this trend is a thermal

transformation of kaolinite in coal combustion and pyrol-

ysis. Phillips et al. [31] described that calcination of

kaolinite allowed a maximum extraction of kaolinite in

mineral acids when meta-kaolinite occurred. Above 900 8C,

meta-kaolinite gradually forms an acid-insoluble spinel

structure. The presence of carbon may accelerate the

formation of spinel phase, resulting in a lower leachability

of Al in the 950 8C chars. It is well known that calcination of

kaolinite is affected by many impurities [32]. Similar trend

in the leachabitity of K and Al is attributed to the portion of

K being associated with clay. In addition, K commonly

exists in coal as some other acid-insoluble mineral species

such as feldspar [26], resulting in a lower solubility of K

than Al.

4.2. Modes of occurrence, transformations and solubility of

trace elements

4.2.1. Beryllium, vanadium, chromium, manganese, cobalt

and nickel

Be, V, Co, Cr and Ni in the 550 8C ashes were

significantly more soluble in HCl than those in the parent

coals (Fig. 2). In a previous paper, we have described that

such a change is indicative of their association with the

organic matrix [18]. This association refers to two forms:

one is chemically bound to the organic matter and it is not

readily extracted with acid (except ion-exchanged

elements); the other is the matrix-encapsulated fine mineral

particles, to which a leaching solution is hardly accessible.

These organically associated elements are liberated upon

the combustion and they may be transformed to acid-soluble

Fig. 5. XRD patterns of the ashes derived from the combustion of DT coal

at different temperatures.

J. Wang et al. / Fuel 83 (2004) 651–660 657

species. In this study, leaching results showed the

persistently low solubility of Be, V, Co, Cr and Ni in acid

from the coal chars; this implied that although 25–35 wt%

of the coals was volatilized during the pyrolysis, these trace

elements probably were still associated with the carbon-

aceous structure and they thus remained insoluble.

How the trace elements are incorporated with the organic

matter is unknown. For example, although Huggins et al.

[33] identified that Cr is present partly as Crþ3 coordinated

with oxygen and hydroxyl group in bituminous coals, it is

not evident whether Crþ3 is physically disseminated as an

exceedingly fine CrO(OH) phase or chemically combined

with the organic matter. In an attempt to clarify this

ambiguity, we performed the following experiment. The

1150 8C chars were ashed at 750 8C, and then the ashes were

leached with 6% HCl. Leaching results of some major and

trace elements between the 1150 8C chars and its ashes are

shown in Fig. 6. It is interesting to note that Be, V, Co, Cr

and Ni more dissolved from the ashes than from the 1150 8C

chars. This observation still cannot clarify the mode of

occurrence of trace elements. The results can be explained

irrespective that these trace elements are chemically bound

to the organic matter or these are present as mineral phases.

We believe that these organically associated elements may

be at least partly chemically combined with the coal organic

matter, but in order to conclude more exclusively we need

more direct evidence.

Ashing at 1150 8C significantly lowered the solubility of

Be, Mn, Co, Cr and Ni in both acids (Figs. 2 and 3),

suggesting that high-temperature treatment stabilizes these

elements in ash. However, the interactions between trace

elements and minerals are too complex to clarify. We could

only postulate some possible reactions according to the

leaching results and the related literature. Sorensen et al.

[34] investigated thermal treatment for stabilization of

heavy metals in iron oxide. They found that heating at

900 8C reduces the leaching of Cr and Ni. The binding of Cr

increases probably due to the crystallization of Cr2O3 or the

interaction of chromium with hematite [35]. During the

thermal treatment, Ni and Co structurally integrates in iron

oxide and provide a stronger binding [36,37]. Mn occurs

mainly as carbonate [8] so that Mn in the raw coals was

highly acid-soluble. Upon heating, manganese carbonates

decomposes above 386 8C and interacts with hematite above

900 8C forming jacobsite (MnFe2O4) [38]; such an inter-

action possibly accounts for a marked decrease in the

leachability of Mn in the 1150 8C ashes. The effect of

thermal treatment on the leaching of V in ash was different

between coals; the degree of V leaching decreased with the

1150 8C ash for DT coal but unchanged for EL and THY

coals; this result was similar to the result of Ca (Fig. 1). A

low solubility of Ca and V from the DT 1150 8C ash is

probably due to the formation of glassy Ca and V.

4.2.2. Arsenic

It is known that As is a highly volatile element. However,

As was not vaporized in the slow heating combustion and

pyrolysis (Table 1), while it was vaporized during the rapid

heating combustion [20]. This has been explained in the

previous paper [20]. Little vaporization of As during

the slow heating combustion and pyrolysis may be due to

the formation of thermally stable As species, for example,

calcium arsenate (Ca3(AsO4)2) under an oxidizing atmos-

phere and iron arsenide (FeAs) under a reducing atmos-

phere. These postulations will be further verified by the

leaching results of As.

Part of As is identified to be associated with pyrite.

Pyritic arsenic in coal is insoluble in HCl but soluble in

HNO3, resulting in a lower solubility of As in 6% HCl

than in 6% HNO3 for pyrite-containing EL and DT coals

(Figs. 2 and 3). Pyritic arsenic is assumed to be

transformed to hematite-associated arsenic in the 550 8C

ash, and hematite in this ash is soluble in dilute HCl but

insoluble in dilute HNO3 (confirmed by XRD) [18]; this

well explains a higher leaching rate of As from the 550 8C

ash with 6% HCl than that from the raw coal but an

opposite result with 6% HNO3 (Figs. 2 and 3).

Nevertheless, ashing at a temperature higher than 550 8C

resulted in a constant solubility of As but a less solubility of

hematite in dilute HCl. Furthermore, such a heat treatment

increased the solubility of As in HNO3 from 550 to 1150 8C,

Fig. 6. Leaching of the 1150 8C chars and the ashes with 6% HCl. The ashes

were obtained from the combustion of the 1150 8C chars at 750 8C.

J. Wang et al. / Fuel 83 (2004) 651–660658

despite the insolubility of hematite in HNO3 (examined by

XRD). This inconsistent change in the solubility between As

and hematite could be explained by the liberation of As

from hematite at a higher temperature.

Arsenopyrite (FeAsS) is an example of pyritic arsenic,

and it is found in coal [39]. In an oxidizing atmosphere,

arsenopyrite forms ferric arsenate (FeAsO4) below 550 8C

and further dissociates to hematite and arsenic oxides

As2O5(g) at a higher temperature [40]. The released As2O5

is likely to be captured by lime, forming calcium arsenate

Ca3(AsO4)2 [41]. This oxyanion is soluble in acid. Turner

[42] observed that (AsO4)32 is a main species in the coal ash

leachate. Thus, pyritic arsenic in the 550 8C ash, like

FeAsO4, incorporates with hematite that is soluble in

hydrochloric acid but insoluble in nitric acid. At a higher

temperature, As is free from hematite so that the solubility

of As is not in line with that of hematite.

In a reducing atmosphere, As in coal is most likely

released as As ðgÞ: For example, arsenopyrite starts to

decompose to As at about 550 8C [43]. However, little

volatilization loss occurred for As during the pyrolysis,

implying the subsequent fixation of vaporous As. One

possibility is the capture of As by metallic iron, forming iron

arsenide (FeAs) [43]. As for the formation of metallic iron,

ferric or ferrous carbonate exists in coal in an amount much

greater than As, thermodynamic calculation shows ferric

carbonate can be reduced by carbon to Fe metal above

350 8C. Yamashita et al. [44] observed the presence of a-Fe

in the 650 8C char. Igeihon [43] reported that arsenic was

fixed by the reduction of arsenopyrite in the presence of

lime to form FeAs in the temperature range of 800–

1000 8C. Most portion of arsenic in the chars appeared not to

be soluble in both acids. However, ashing of the 1150 8C

chars significantly increased the leachability of As (Fig. 6),

most probably because of the oxidation of arsenide to acid-

soluble arsenate.

4.2.3. Zinc and lead

Pb and Zn behaved very similarly in the vaporization and

leaching. In the combustion, only slight vaporization of Pb

occurred at 1150 8C for EL and DT coals, whereas in the

pyrolysis, both Pb and Zn were substantially vaporized

above 950 8C. The leachability of Pb and Zn in the ashes

decreased with increasing temperature. These similarities

suggest their similar modes of occurrence in coal; both

elements are known to occurs mainly as sulfides, galena

(PbS) and sphalerite (ZnS) [8]. In an oxidizing atmosphere,

these sulfides are oxidized upon heating. A lowered

solubility of Pb with increasing the combustion temperature

implies the formation of less mobile species in the high-

temperature ashes. This may be attributed to the reaction of

Pb with clay [45] and/or with iron oxide [46]. In a reducing

atmosphere, the substantial vaporization of both elements is

explained by their carbothermic reaction to generate metal

vapors [20]. Because of most portion of Pb and Zn is

volatilized above 950 8C, leaching amount of Pb and Zn

decreased from the 950–1150 chars.

5. Conclusion

1. Leaching results of Be, V, Co, Cr and Ni in coal were

characteristic of their partial association with organic

matter, and likely the chemical binding to the organic

matter rather than the physically intimate association. Part

of As in coal was assigned to be associated with pyrite in the

coals.

2. After ashing at 550 8C, the organically associated Be,

V, Cr, Co and Ni were transformed to acid-soluble species,

and pyritic arsenic turned to hematite-associated arsenic.

Ashing at a temperature up to 1150 8C reduced the solubility

of Be, Cr, Mn, Co, Zn and As in both acids, suggesting their

stabilization reactions at higher temperatures. However, this

heat treatment did not reduce the solubility of As in both

acids, probably because of the formation of acid-extractable

arsenate.

3. Be, V, Co, Cr, and Ni in the chars remained insoluble

in both acids. In pyrolysis, a thermally stable and acid-

insoluble As phase was formed in the chars, and this phase

was assumed to be iron arsenide.

Acknowledgements

The study is financially supported partly by a Grand-in-

Aid for Scientific Research on Priority Areas (No.

11218201) from Ministry of Education, Culture, Sports,

Science and Technology, Japan.

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