HCl treatment of 26 coals on their composition and

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Effect of a HF-HF/HCl treatment of 26 coals on their composition and pyrolysis behavior Xiaojie Cheng, Lei Shi, Qingya Liu, and Zhenyu Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04187 • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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Energy & Fuels

Effect of a HF-HF/HCl treatment of 26 coals on their

1 2

composition and pyrolysis behavior

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Xiaojie Cheng, Lei Shi, Qingya Liu, and Zhenyu Liu*

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State Key Laboratory of Chemical Resource Engineering, Beijing University of

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Chemical Technology, Beijing 100029, PR China

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Abstract:

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Coals contain many types of minerals. The minerals’ effect on coal pyrolysis has

9

been studied extensively in the literature by comparing only a few raw coals with the

10

corresponding acid treated coals. This paper studies an acid treatment of 26 coals from

11

lignite to anthracite and compares the elemental composition and pyrolysis behavior of

12

the acid-treated demineralized coals (D-Coals) with the raw coals (R-Coals). The acid

13

treatment involved leaching the coals with a HF solution and then a mixed HF/HCl

14

solution in air at 70 C and the pyrolysis was carried out in a temperature range of 120-

15

900 C in a thermal gravimetric analysis (TGA) coupled online with a mass

16

spectrometer (MS). The characteristic parameters of the differential mass-loss curves

17

(DTG) and the sub-peaks decoupled from the DTG curves are compared and discussed.

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It is found that the acid treatment not only removed most of the minerals from the coals

19

but also reduced the coals’ carbon content, increased their oxygen content, and altered

20

their organic structure, leading to reduced mass loss in pyrolysis, especially for mid-

21

rank coals. Some minerals removed by the acid treatment promote the coal pyrolysis

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in the temperature range of 400-600 C corresponding mainly to the cleavage of Cal-

23

Cal and Car-Cal bonds as well as to decomposition of crystal water. Some of the minerals

24

inhibited the condensation of aromatic structure at temperatures of higher than 700 C.

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The reduced carbon content and increased oxygen content of D-Coals are attributed to

26

the conversion of carboxylates to carboxylic acid which promoted the pyrolysis at

27

temperatures lower than 300 °C. The composition and pyrolysis behavior of anthracites

28

are not significantly affected by the acid treatment. 1 / 35

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Key words: coal; demineralization; pyrolysis; minerals; coal structure

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1. Introduction

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Coal is a heterogeneous mixture of complex organic macromolecular structures and

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inorganic minerals of different quantities. The pyrolysis of coals usually refers to the

7

reactions of organic structures of coal at high temperatures, which are inevitably

8

influenced by the types and contents of minerals in the coals.

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Many studies reported the effects of minerals on coal pyrolysis, and the methodology

10

used was mainly the comparison of pyrolysis behaviors of raw coals with the

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demineralized coals under the same conditions.1-3 The reported minerals’ effects on

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pyrolysis behaviors of coals, however, differ due to the complexity of coals’ organic

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structure, the minerals composition, as well as the pyrolysis conditions and the

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demineralization method employed. As indicated in Table 1 many studies used HCl

15

and HF solutions under ambient conditions to remove minerals from coal. For instances,

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Liu et al. studied pyrolysis reactivity of a subbituminous coal and a lignite, as well as

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their demineralized samples treated with HCl and HF solutions at room temperature,

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and found little minerals’ effect on the pyrolysis kinetics in a temperature range of 110-

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800 C (entry 1 in Table 1).3 Öztaş et al. reported that the pyrolysis conversion of a

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coal washed by HCl at 60 C was lower than that of the raw coal but a further HF wash

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increased the pyrolysis conversion in a temperature range of 300-500 C (entry 2 in

22

Table 1).4 Liu et al. showed that the pyrolysis rate of a coal demineralized by HCl and

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HF solutions in sequence at room temperature was slightly higher than that of the raw

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coal, suggesting little minerals’ effect on pyrolysis (entry 3 in Table 1).5 Yan et al.

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studied the effects of alkali/alkaline earth metals on pyrolysis yield of BTXN (benzene,

26

toluene, xylene and naphthalene) by washing 3 coals sequentially with HCl and HF

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solutions at room temperature and reported that the metals had little effect for a coal

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with 86.1% carbon but a significant promoting effect for coals with 69.0 and 75.9%

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carbon (entry 4 in Table 1).6 2 / 35


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It is certain that the minerals’ effect on pyrolysis of coals reported cannot be easily

2

generalized because the great variations in minerals composition and organic structure

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of different coals as well as in pyrolysis conditions. Furthermore, the different de-

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mineralization operations are not equally effective for all the minerals and alter the

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physicochemical structure of coals in different extents, leading to different pyrolysis

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behaviors of de-mineralized coals. For instances, HCl leaches carbonates from coals

7

but the aluminum and silicon oxides and pyritic compounds,7, 8 HF leaches aluminum

8

and silicon containing compounds but calcium fluoride and pyritic compounds,9,

9

while HNO3 leaches pyritic compounds but greatly increases the nitrogen and oxygen

10

10

contents of coals.11

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The effect of HF-HCl treatment on the organic structure of coals is long in dispute

12

as indicated in Table 2, for example. Larsen et al. treated 6 coals sequentially by HCl,

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HF and HCl at 60 °C in under N2, extracted the raw and acid treated coals with pyridine,

14

analyzed the amount and molecular weight distribution of the pyridine extracts, and

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reported little impact of the acid treatment on the coals’ organic structure (entry 1 in

16

Table 2).12 Liang et al. studied infrared (IR) spectra of a coal and the coal treated with

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HCl-HF-HCl in air at 60 °C and found significant increases of phenolic hydroxyl and

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carboxyl groups in the acid treated coal (entry 2 in Table 2).13 Strydom et al. found that

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a coal’s –COOH content increased slightly by the HCl-HF-HCl treatment in air at 20 °C

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(entry 3 in Table 2).14 Zhao et al. reported that a HCl-HF treatment of a coal in nitrogen

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increased the coal’s carboxyl group content and reduced the length of aliphatic side

22

chains and bridge bonds (entry 4 in Table 2).15 Kister et al. found that an HCl-HF-HCl

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treatment of a coal in N2 at 60 °C decreased the aliphatic structures and increased the

24

oxygenated species of the coal (entry 5 in Table 2).16 Although these studies yield

25

different results, they do show that the organic structures of coals are altered and the

26

changes are likely to be coal-rank dependent.

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Coals have been commonly characterized in many ways including X-ray diffraction

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(XRD), IR spectroscopy, nuclear magnetic resonance spectroscopy, and chemical

29

statistical calculation.17,

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These results, however, are average properties of coals, 3 / 35


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which do not clearly show the distribution of covalent bonds relevant to pyrolysis

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behaviors at different temperatures. Shi et al.19 reported that coals contain mainly 11

3

types of covalent bonds which can be lumpped to 5 groups according to their bond

4

dissociation energy. They fitted the differential thermal gravimetric (DTG) curve of

5

coals during pyrolysis in a thermal gravimetric analysis (TGA) into 5 sub-peaks (SPs)

6

corresponding to the 5 groups of covalent bonds and 1 SP representing the

7

decomposition of calcium carbonate. These organic SPs were ascribed in sequence to

8

the decomposition of carboxylic acid and carboxylates at temperatures < 300 C, the

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cleavage of bonds between aliphatic carbon and heteroatoms (Cal-S/O/N) at 300-400

10

C, between aliphatic carbons (Cal-Cal) at 400-500 C, between aliphatic carbon and

11

aromatic carbon (Cal-Car) at 500-600 C, and between aromatic carbon and hydrogen

12

(Car-H) at temperatures > 740 C. This approach was found correlating the coals’

13

pyrolysis behavior well with the coal rank (C%). In this regard, it is interesting to use

14

this method to study the effect of acid treatment of coals of different rank on their

15

organic structure and the pyrolysis behavior, and to better understand the role of

16

minerals in coal pyrolysis.

17

In this work, 26 coals (R-Coals), from lignite to anthracite, were subjected to an acid

18

treatment using HF and HCl/HF mixed solutions to yield corresponding demineralized

19

coals (D-Coals). The R-Coals and D-Coals were pyrolyzed in a TGA coupled with a

20

mass spectrometry (MS). The DTG curves were decoupled into sub-curves, including

21

1 for the decomposition of calcium carbonate and 5 for the cleavage of bonds in the

22

organic structure. The effects of acid treatment on the properties of coals and the bond

23

cleavage behavior in pyrolysis are discussed.

24

2. Experimental

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2.1. Coal samples and acid treatment method

26

The 26 coals used were from China with particle sizes within 80-100 mesh. All the

27

coal samples were divided into two parts. One part was pyrolyzed directly while the

28

other part was acid treated to remove most of the minerals before pyrolysis. The details

29

of the acid treatment include: placing 3 g R-Coal into a plastic container (100 mL) 4 / 35


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Energy & Fuels

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containing 30 mL HF solution (9 mol/L), agitating the mixture in air for 6 h at 70 C,

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filtering and washing the coal with deionized water to pH 7, mixing it with 50 mL HCl

3

solution (6 mol/L) and 50 mL HF solution (12 mol/L) for 12 h in air under agitation at

4

70 C, filtering and washing it with deionized water to pH 7, drying it in a vacuum for

5

5 h at 120 C and cooling it to the ambient temperature. The R-Coals and D-Coals were

6

subjected to the proximate and the ultimate analyses following the Chinese National

7

Standard GB/T12-2008 and GB/T476-2008, respectively. The oxygen content of the

8

coals was determined independently by Vario El Cube. The errors of this analysis were

9

within 0.4%. Table 3 shows the proximate and ultimate analyses of all the coals, from

10

the lowest C% to the highest C% of D-Coals.

11

2.2. Minerals analysis

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The minerals in R-Coals were determined with an S4-Explorer X-ray fluorescence

13

spectrometer (XRF, Bruker) under the sequential scanning mode and results are shown

14

in Table 4. The hardware parameters are 4 kW, maximum voltage of 60 kV, maximum

15

current of 170 mA, detection range of Be (4)-U (92), detection limit of PPM 100%.

16

The sample size was 5 g. XRD of the R-Coals was carried out on a D8 FOCUS (Bruker)

17

equipped with Cu Kα1 (λ = 0.15406 nm) radiation operated at 40 kV and 200 mA. The

18

samples were scanned over a 2θ range of 10−80° at a frequency of 10° min−1.

19

2.3. TG and pyrolysis

20

The pyrolysis experiments were carried out in a TGA (Setsys Evolution 1750,

21

Setaram) coupled online with an MS (Omnistar 200, Balzers) for the effluent analysis.

22

The alumina crucible used was 8 mm in height and 5 mm in diameter without a lid.

23

The purging gas was Argon with a purity of > 99.999% at a flow rate of 160 mL/min.

24

Each experiment used 30 mg coal that took 1/3 the crucible volume. The sample was

25

heated from the room temperature to 900 C at a rate of 10 C/min, with a 30 min stay

26

at 120 C to remove adsorbed water and 15 min stay at 900 C.

27

3. Results and discussion

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3.1. Proximate and ultimate analyses of R-Coals and D-coals

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Table 3 shows that the mineral contents of D-Coals are much lower than those of the

2

corresponding R-Coals even though they are on the dry-and-ash-free (daf) basis. In

3

principle, coal is heterogeneous, the sampling and the ultimate analysis involve errors.

4

The errors however should not as large as those between R-Coals and D-Coals,

5

suggesting that some changes in coal structure had taken place in the acid treatment.

6

To explore the structure change, C% and O% of R-Coals are compared with those of

7

D-Coals in Figure 1. It is seen that the effect of acid treatment is significant mainly for

8

coals with less than 80% carbon (Figure 1(a), the squares) and greater than 15% oxygen,

9

and perhaps also for coals with less than 5% oxygen (Figure 1(b), the squares).

10

It is known that carbonates contain carbon and oxygen, and there are many

11

carbonates in coals. Since the ultimate analysis cannot exclude all the carbonates in

12

coals the carbon and oxygen in carbonates would inevitably contribute to C% and O%

13

of R-Coals. Therefore, the removal of carbonates from R-Coals by the acid treatment

14

may result in D-Coals of different C% and O% on daf basis. It is seen in the XRD

15

spectra in Figure S1 in the supplementary material, the main carbonates are CaCO3 and

16

FeCO3 (siderite). The presence of siderite in coals is not common but has been reported

17

in the literature.20 If all the Ca and Fe in Table 4 are assumed to be in the carbonate

18

form, the maximum amounts of carbon and oxygen in the carbonates can be determined

19

and excluded from the elemental composition of R-Coals. The C% and O% of R-Coals

20

excluding the maximum contribution of carbonates are also plotted in Figure 1 (the

21

pentacles). Clearly, the carbon and oxygen in the carbonates do affect the C% and O%

22

of R-Coals, but the effect is relatively small and cannot explain the decreased C% and

23

increased O% of D-Coals from those of the R-Coals. These phenomena indicate that

24

the acid treatment not only removed many minerals from R-Coals but also changed the

25

organic structure of coals under the conditions used.

26

To evaluate the role of acid treatment on coals’ structure and reactivity, R-Coals and

27

D-Coals were pyrolyzed to 900 C in the TGA-MS. Figure 2 shows that the volatile

28

content of D-Coals are generally lower than that of R-Coals, especially for coals with

29

lower than 85% carbon. This phenomenon is similar to those reported by Zhao et al.21 6 / 35


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and Kister et al. .16 The former attributed the phenomenon to decreased methylene (-

2

CH2) and methyl (-CH3) groups by an acid treatment of a coal with 72.8% carbon based

3

on an IR study,21 while the latter attributed the phenomenon to the loss of aliphatic

4

groups attached to 3- and 4-ring polyaromatics and the increases of carbonyls and

5

carboxylic acids in a demineralized coal with 60.2% carbon.16 These findings were also

6

evidenced by Zhang et al.,22 they found that the filtrates from an HCl-HF-HCl treatment

7

of 5 coals of different rank (8.6-58.6% volatile matter, daf) at 60-70 C contained

8

aliphatic hydrocarbons from C12 to C33, and attributed the dissolved components to

9

the decreased C% and increased O% of the acid-treated coals. This information seems

10

to suggest that the decreased C% and increased O% of D-Coals in Figure 1 result from

11

the loss of hydrocarbons in the acid treatment. To explore this possibility the O% of D-

12

Coals is compared in Figure 3 with that of R-Coals corrected by excluding both the

13

maximum amount of carbonates and the decreased C%. It is apparent in Figure 3 that

14

the O% of most D-Coals are still higher than those of R-Coals, especially for coals

15

lower than anthracite in rank, indicating oxidation of coals in the acid treatment.

16

Figure 4 shows the increases in O% of D-Coals caused by oxidation. It is clear that

17

the extent of oxidation varies with coal rank, increasing with decreasing C% of coals

18

for coals with less than 80% carbon (mainly the lignite) but not vary significantly for

19

coals of higher rank. The increases in O% of coals are less than that reported for HNO3

20

treated coals, which are generally more 10% for bituminous coals 11, 23 and close to 15%

21

for a lignite,24 but are still significant.

22

3.2. Effect of acid treatment on pyrolysis of coal

23

The above study shows that the acid treatment used not only removed most of the

24

minerals and some organic components, but also oxidized the organic structure of coals.

25

Therefore the effect of acid treatment of coals on the total volatile yield in Figure 2

26

results from the accumulation or combination of many reactions in the broad pyrolysis

27

temperature range (120-900 C) and some of the reactions may offset each other and

28

do not explicitly show in the figure. For instance, the decomposition of some minerals

29

and crystal water as well as the catalytic effect of some minerals occurred in different 7 / 35


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1

temperature ranges increase the total volatile yield, while the inhibition of some

2

minerals on pyrolysis and the loss of organic components in the acid treatment decrease

3

the total volatile yield. It is interesting therefore to compare the DTG profiles of R-

4

Coals and D-Coals to study these effects on pyrolysis behavior in different temperature

5

ranges.

6

It is seen in Figure 5 that the DTG curves of R-Coals and D-Coals are similar at

7

temperatures lower than 600 C. With an increase in C% (from bottom to top), the

8

DTG peak temperature increases gradually. The peaks at around 700 C for some R-

9

Coals disappear in D-Coals due to the removal of calcium carbonate by the acid

10

treatment,19 as evidenced by the MS-CO2 peak in Figure S2 in the supporting material.

11

To more clearly identify the differences in DTG curves of R-Coals and D-Coals

12

Figure 6(a) shows the temperature of the main peak, Tp, of the coals. Obviously, the Tp

13

of R-Coals and D-Coals are basically the same, indicating that the minerals and the

14

organic components lost and the organic structures oxidized in the acid treatment do

15

not significantly alter the main pyrolysis behavior of the coals, which agrees with the

16

pyrolysis studies of Liu et al. on a lignite and a subbituminous coal3 and Zhao et al. on

17

a bituminous coal.21 It is worth noting that the Tp increases linearly with increasing C%

18

of coals, and the increases are relatively small in C% range of lower than 89%, about

19

4.3 C/C%, while relatively larger in higher C%.

20

Figure 6(b) compares the maximum mass loss, Rmax at TP, of R-Coals and D-Coals.

21

Clearly, Rmax of D-Coals generally decreases (approaches to zero) with increasing C%

22

although the data are somewhat scattered. The Rmax of R-Coals, however, is much more

23

scattered, with some of them being similar to that of D-Coals while some others being

24

greater (more negative) than that of D-Coals, especially the 4 coals with 70-85% carbon,

25

i.e., #4, #10, #11 and #14. Since Tp is lower than the decomposition temperature of

26

calcium carbonates, Rmax is likely affected by the decomposition of other minerals,

27

including some crystal water, and/or catalysis of some minerals in the pyrolysis. These

28

hypotheses, however, are difficult to be verified because the 4 R-Coals with the largest

29

Rmax, do not appear to contain extraordinary minerals in type and quantity compared 8 / 35


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with other coals. For instance, the total mineral contents of these 4 R-Coals (see Table

2

4) are not the highest, with #4 being the 4th highest (24.7%) while #10 is quite low

3

(8.5%). Although the Ca content of #4 is the highest and that of #11 is the 3rd (7.7 and

4

5.1%, respectively) those of #10 and #14 are low (1.3 and 0.6%, respectively). These

5

4 coals are not specifically rich in other minerals either, such as SiO2 and Al2O3, which

6

are likely to contain crystal water.

7

As discussed earlier, the difficulty in identifying the acid treatment effect on coal

8

pyrolysis may result from the multi-changes in coal structure and composition caused

9

by the acid treatment, which affect the pyrolysis at different temperatures in different

10

ways and some of these effects may offset each other when the overall pyrolysis

11

behavior is concerned. To decouple the possible offset effects Figure 7 shows the

12

difference in DTG between each pair of R-Coal and D-Coal at the same pyrolysis time,

13

ΔR (= RR - RD, where RR and RD are the DTG of a R-Coal and the corresponding D-

14

Coal, respectively). Since the RR and RD are negative values, ΔR > 0 indicates

15

increased de-volatilization by the acid treatment, i.e., the minerals removed by the acid

16

treatment inhibit the pyrolysis; while ΔR < 0 indicates decreased de-volatilization by

17

the acid treatment, i.e., the minerals removed by the acid treatment promote the

18

pyrolysis. For clearer presentation, the coals studied are shown in 3 sub-figures, Figure

19

7(a) for lignite (< 75.9% carbon), Figure 7(b) for bituminous coals, and Figure 7(c) for

20

anthracite (> 88.0% carbon).

21

It is seen that in the temperature range of 200-400 C, ΔR > 0 for almost all the coals,

22

although being relatively small for the anthracites, indicating increased de-

23

volatilization by the acid treatment. This behavior suggests that the minerals inhibiting

24

the pyrolysis of R-Coals include alkali and alkaline earth metals because they are

25

generally richer in low rank coals than in high rank coals and these metals are usually

26

in carboxylate form that decompose at temperatures lower than 300 C. This suggestion

27

agrees with the studies of Li et al.,25,

28

carboxylates in coals at relatively low temperatures (even less than 300 C) yielded

29

Ca2+, Mg2+, K+ and Na+ cations. These cations were then cross-linking the char matrix

26

who showed that the decomposition of

9 / 35


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(CM) to form CM-X (X is Ca, Mg, K and Na ) bonds that are more stable than CM and

2

pyrolyze at higher temperatures. Since these metals are absent in D-Coals, the pyrolysis

3

volatiles in D-Coals are easier to escape. This possibility is also evidenced by Zhang

4

et al., who showed increased 1-methynaphthalene extraction yield by acid treatment of

5

WY coal (68.4% carbon)27, and by Li et al., who showed increased N-methyl-2-

6

pyrrolidinone extraction yield by acid treatment of some low-rank coals.28

7

It is also seen in Figure 7 that for most of coals ΔR < 0 in the temperature range of

8

400-600 C, especially for the bituminous coals. This behavior indicates that some of

9

the minerals removed by the acid treatment promoted pyrolysis of certain organic

10

structure that is rich in bituminous coals. It is apparent that there is no clear trend

11

between the value of ΔR and C% of the coals, indicating that the true effects of the acid

12

treatment on coal pyrolysis are still not revealed. The ΔR in the temperature range of

13

greater than 600 C is not discussed here because most of them are close to zero and

14

the few exceptions are attributed to the decomposition of calcium carbonate as

15

evidenced earlier.

16

3.3. Effect of acid treatment on bond cleavage behavior during pyrolysis

17

The discussion presented so far shows that it is difficult to clearly identify the effects

18

of acid treatment on pyrolysis behavior of coals in terms of the DTG behavior. To

19

further explore the role of acid treatment on coal pyrolysis, the DTG curves were de-

20

coupled into six sub-curves (or sub-peaks, SP) by the generalized reduced gradient

21

method (GRG) as detailed by Shi et al.19 with the half peak width of less than 200 C

22

and the coefficients of determination (R2) generally greater than 0.99. This fitting

23

method yielded good results as evidenced by coal #9 in Figure 8 with a R2 of 0.997 for

24

R-Coal and 0.999 for D-Coal. The origin of these SPs has been briefly discussed earlier,

25

including SP1 for the release of bonded water and decomposition of carboxylic acid

26

and carboxylates,5, 19, 29 SP2 for the cleavage of Cal-O/S/N bonds in the side chains of

27

aromatic structures and S-S bond,19 SP3 for the cleavage of Cal-Cal

28

bonds,31 SP4 for the cleavage of Car-Cal and Car-O bonds,30, 31 SP5 for the decomposition

29

of carbonate that is absent in D-Coals,19 and SP6 for H2 release due to condensation of 10 / 35


19, 29, 30

and Cal-O

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Energy & Fuels

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aromatic structures19 and CO release for the decomposition of Phenolic hydroxyl.5

2

Clearly, the curve fitting yields two parameters, the SP’s peak temperature (TSP) and

3

peak area (ASP), the former is the average temperature for decomposition of specific

4

functional groups or cleavage of specific bonds while the latter is a measure of the

5

corresponding mass loss. The differences in TSP and ASP between an R-Coal and the

6

corresponding D-Coal are used to evaluate the influence of acid treatment of coal.

7

Figure 9 shows that with increasing C% the TSP trends of R-Coals and D-Coals

8

generally agree with that reported by Shi et al.,19 i.e. a slow linear increase for coals

9

with less than 89% carbon and a fast increase for coals with more than 89% carbon,

10

except that of SP2 of D-Coals with more than 89% carbon. In detail, it is apparent that

11

in the C% range of less than 89%, the TSP of SP2 (TSP2) of R-Coals differs randomly

12

from that of D-Coals but the trends of both coals can be represented by the same line.

13

This phenomenon is also oberved for TSP3 and TSP4, indicating that the minerals

14

removed by the acid treatment do not significantly alter the distribution of bonds that

15

cleaved in the temperature range of 300-600 C, or the organic structures oxidized by

16

the acid treatment are not the structures that decompose in the temperature range.

17

Figure 9 also shows that TSP1 and TSP6 of D-Coals differ from that of R-Coals and

18

show different trend. Since the content of each mineral does not vary progressively

19

with the changing C% of coals, the different behaviors of TSP1 and TSP6 of D-Coals

20

from those of R-Coals are attributed mainly to the changes in organic structure of coals

21

caused by the acid treatment. Accordingly, the decreasing difference between TSP1 of

22

D-Coals and that of R-Coals with increasing C% may be attributed to the loss of some

23

organic components in the acid treatment, and perhaps also to the oxidation of coal

24

structure. These effects perhaps overwhilmed the inhibitive effect of alkali and alkaline

25

earth metals on pyrolysis of R-Coals.

26

The consistently lower and the better trend of TSP6 of D-Coals than that of R-Coals

27

in Figure 9 suggest that the oxidation promoted the condensation of aromatic structures

28

in coals, resulting in the early releases of H2 and CO. This suggestion may be evidenced

29

by the conversion of carboxylate to carboxylic acid in the acid treatment converts,23, 28 11 / 35


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

which reduces the crosslinking between organic structures in D-Coals and promotes

2

the condensation of aromatic structure in D-Coals.32, 33 The behavior of TSP6 also shows

3

that the chars formed in pyrolysis at temperatures below 700 C, i.e., the reactants for

4

generation of SP6, vary in structure and reactivity with C% of coals, they are less

5

condensed and more reactive for a lower rank coal than for a higher rank coal.

6

The extraordinarily high TSP6 of some R-Coals than other R-Coals and D-Coals may

7

be attributed to the effect of some minerals that inhibited the condensation of aromatic

8

structure, resulting in delayed H2 and CO release, as reported by Xu et al. who showed

9

that the alkaline metals inhibited the graphitization progress of semi-coke.34

10

Figure 10 shows the ASP of each SP of R-Coals and D-Coals. The ASP of SP5 (ASP5)

11

is not shown because it resulted from the decomposition of calcium carbonate that is

12

absent in D-Coals. It is seen in Figure 10(a) that ASP1 is the smallest among those of all

13

SPs, less than 0.03 g/g-coal (daf) and decreases with increasing C% of coals. Since

14

ASP1 is mainly from the decomposition of carboxylic acid and carboxylates and is

15

affected by the organic components lost in the acid treatment, the higher ASP1 and TSP1

16

(Figure 9) of D-Coals than those of R-Coals indicate that the increased oxygen content

17

of D-Coals in the acid treatment is more influencial than the organics and crystal water

18

lost in the acid treatment. This is evidenced in Figure S3 in the supporting material,

19

where the mass differences of ASP1 between D-Coals and R-Coals in the C% range of

20

less than 80% are generally lower than the mass losses estimated from the increased O%

21

in D-Coals from R-Coals, assuming all the increased oxygen being in the form of

22

carboxylic acid. The higher ASP1 of D-Coals than that of R-Coals is consistent with that

23

reported by Geng et al., who showed increased carboxylic acid group contents in coals

24

treated by HCl and the effect decreased with increasing C% of coals, 5-8% for lignite

25

and sub-bituminous coals while 0-2% for bituminous coal.35 This behavior of ASP1 also

26

agrees with the findings of Liang et al.13 and Schafer et al.,36 the former showed a 24%

27

increase in C=O intensity in a coal treated by HCl-HF-HCl while the latter reported

28

easier decomposition of carboxylic acid than carboxylates.

12 / 35


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Energy & Fuels

1

Figure 10 (b) shows that ASP2 is less than 0.12 g/g-coal (daf), which is larger than

2

ASP1 and ASP6 but smaller than ASP3 and ASP4. Interestingly ASP2 of R-Coals are similar

3

to but more scattered than that of D-Coals, indicating that the acid treatment played

4

little role in altering the organic structure that generated the SP and the minerals’ effects

5

are small. Apparently, the similar ASP2 and TSP2 for R-Coals and D-Coals indicate that

6

the Cal-O, Cal-S and Cal-N bonds in the side chains of aromatic structures are not

7

significantly affected by the acid treatment. The decreasing trend of ASP2 with

8

increasing C% of coals agrees with the general understanding that the contents of Cal-

9

O, Cal-S, and Cal-N bonds in coals decrease with increasing coal rank.37, 38

10

It is clear in Figure 10(c) that ASP3 of D-Coals decreases linearly with increasing C%

11

for coals with less than 89% carbon. The ASP3 of many R-Coals is similar to that of the

12

corresponding D-Coals but ASP3 of some of the mid-rank R-Coals is greatly larger than

13

that of corresponding D-Coals and other R-Coals of similar C% as reported by Shi et

14

al.19 It seems that the linear relation of ASP3 with C% of coals is governed by the

15

pyrolysis of organic structure of coals while the higher ASP3 of some R-Coals may be

16

attributed to the catalytic effect of some minerals on the cleavage of Cal-Cal and Cal-O

17

bonds which are popular in mid-rank coals, or to the decomposition of some minerals

18

including crystal water. As indicated in Table S1, crystal water in some clay

19

components decomposes in the temperature range of 400-600 °C. These behaviors of

20

ASP3 of R-Coals explain the different minerals’ effects on pyrolysis, some promote coal

21

pyrolysis while some others do not; some contain crystal water while some others do

22

not. It is noted that the R-Coal that yielded a higher ASP3 than the corresponding D-

23

Coal shows similar TSP3 as the corresponding D-Coal, suggesting the minerals’

24

catalytic effect on cleavage of Cal-Cal and Cal-O/H bonds being small; the

25

decomposition of minerals is overwhelmed by the mass loss from the cleavage of Cal-

26

Cal and Cal-O/H bonds.

27

It is apparent that the trends of ASP4 in Figure 10(d) are similar to that of ASP3,

28

indicating similar acid treatment effects for both SPs. The smaller difference in ASP4

29

between R-Coals and D-Coals than that in ASP3 indicates that the promoting effect of 13 / 35


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

minerals on cleavage of Car-Cal bonds are not as large as that on Cal-Cal and Cal-O/H

2

bonds, and/or the decomposition of minerals is not extensive in 500-600 C compared

3

with that in 400-500 C.

4

Figure 10(e) shows that the differences in ASP6 between R-Coals and D-Coals are

5

minimal. This phenomenon and that of TSP6 indicate that the changes in aromatic

6

structures by the acid treatment affected only the temperature of aromatic condensation,

7

but the quantities of H2 and CO released.

8

It is clear so far that the above data and discussion indicate that the HF-HF/HCl

9

treatment of coals in air at 70 C not only removes most of the minerals in coals as

10

commonly recognized in the literature but also oxidizes the organic structure of coals,

11

which has not been well reported. To confirm the oxidation of coal structure, coals #3,

12

#6 and #14 were subjected to the same acid treatment under an Ar atmosphere (D-

13

Coals-in-Ar), and their O% are compared in Table 5 with those obtained under air listed

14

in Table 3 (D-Coals). It is seen in Table 5 that the O% of D-Coals-in-Ar is lower than

15

that of the corresponding D-Coals, indicating the participation of oxygen in air in the

16

commonly reported acid treatment processes.

17

4. Conclusions

18

(1) The commonly practiced sequential HF and HF/HCl treatment of coals in air at

19

70 C for demineralization of coals not only reduced the coals’ minerals content but

20

also reduced their C% and volatile content, and increased their O%, especially for coals

21

with less than 85% carbon.

22

(2) The acid treatment affects mainly devolatilization of coals in the temperature

23

range of 400-600 C, where the main DTG peak appears. This effect is attributed to the

24

decomposition of minerals, the loss of crystal water, and the catalytic effect of some

25

minerals, although they are not clearly distinguishable with the data available.

26

(3) Some minerals removed by the acid treatment promote the cleavage of Cal-O/S/N

27

and Cal-Cal bonds which are relatively more abundant in coals of mid-rank. Some

28

minerals removed inhibite the condensation of aromatic structure at temperatures of

29

greater than 700 C. 14 / 35


Page 14 of 35

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Energy & Fuels

1

(4) The oxidation of coals includes the conversion of carboxylates of alkali and/or

2

alkaline earth metals to carboxylic acid that pyrolyzes at temperatures less than 300 C

3

and promotes the condensation of aromatic structure to yield H2 and CO at

4

temperatures greater than 700 C.

5

Corresponding Author

6

* Telephone: +86 10 64421073. E-mail: [email protected]

7

Notes

8

The authors declare no competing financial interest.

9

Acknowledgments

10

The work was financially supported by the National Key Research and

11

Development Program of China through 2016YFB0600302-01 and Joint Funds of

12

Natural Science Foundation of China (U1610107).

13

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1

References

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(1) Zou, X.; Yao, J.; Yang, X.; Song, W.; Lin, W. Catalytic Effects of Metal Chlorides on the

3 4 5 6 7 8 9

Pyrolysis of Lignite. Energy & Fuels 2007, 2, 619-624. (2) Opaprakasit, P.; Scaroni, A. W.; Painter, P. C. Ionomer-Like Structures and π-Cation Interactions in Argonne Premium Coals. Energy & Fuels 2002, 16, 543-551. (3) Liu, Q.; Hu, H.; Zhou, Q.; Zhu, S.; Chen, G. Effect of inorganic matter on reactivity and kinetics of coal pyrolysis. Fuel 2004, 83, 713-718. (4) Öztaş, N.; Yürüm, Y. Pyrolysis of Turkish Zonguldak bituminous coal. Part 1. Effect of mineral matter. Fuel 2000, 79, 1221-1227.

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(5) Liu, L.; Kumar, S.; Wang, Z.; He, Y.; Liu, J.; Cen, K. Catalytic effect of metal chlorides on

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coal pyrolysis and gasification part I. Combined TG-FTIR study for coal pyrolysis.

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Thermochimica Acta 2017, 655, 331-336.

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(6) Yan, L. J.; Bai, Y. H.; Kong, X. J.; Li, F. Effects of alkali and alkaline earth metals on the

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formation of light aromatic hydrocarbons during coal pyrolysis. Journal of Analytical and

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Applied Pyrolysis 2016, 122, 169-174.

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(7) Bolat, E.; Saǧlam, S.; Pişkin, S. Chemical demineralization of a Turkish high ash bituminous coal. Fuel Processing Technology 1998, 57, 93-99.

18

(8) Steel, K. M.; Besida, J.; O'Donnell, T. A.; Wood, D. G. Production of ultra Clean coal : Part

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I-dissolution behaviour of mineral matter in black coal toward hydrochloric and hydrofluoric

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acids. Fuel Processing Technology 2001, 70, 171-192.

21 22 23 24 25 26 27 28

(9) Steel, K. M.; Patrick, J. W. The production of ultra clean coal by chemical demineralisation. Fuel 2001, 80, 2019-2023. (10) Meshram, P.; Purohit, B. K.; Sinha, M. K.; Sahu, S. K.; Pandey, B. D. Demineralization of low grade coal-A review. Renewable and Sustainable Energy Reviews 2015, 41, 745-761. (11) Steel, K. M.; Patrick, J. W. The production of ultra clean coal by sequential leaching with HF followed by HNO3. Fuel 2003, 82, 1917-1920. (12) Larsen, J. W.; Pan, C. S.; Shawver, S. Effect of demineralization on the macromolecular structure of coals. Energy & Fuels 1989, 3, 557-561.

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(13) Liang, H. Z.; Wang, C. G.; Zeng, F. G.; Li, M. F.; Xiang, J. H. Effect of demineralization on

2

lignite structure from Yinmin coalfield by FT-IR investigation (in Chinese). Journal of Fuel

3

Chemistry & Technology 2014, 42, 129-137.

4

(14) Strydom, C. A.; Bunt, J. R.; Schobert, H. H.; Raghoo, M. Changes to the organic functional

5

groups of an inertinite rich medium rank bituminous coal during acid treatment processes.

6

Fuel Processing Technology 2011, 92, 764-770.

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(15) Zhao, Y.; Liu, L.; Qiu, P. H.; Xie, X.; Chen, X. Y.; Lin, D.; Sun, S. Z. Impacts of chemical

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fractionation on Zhundong coal's chemical structure and pyrolysis reactivity. Fuel Processing

9

Technology 2017, 155, 144-152.

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(16) Kister J, G. M., Mille G. Changes in the chemical structure of low rank coal after low

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temperature oxidation or demineralization by acid treatment: analysis by FT-ir and uv

12

fluorescence. Fuel 1988, 67, 1076-1082.

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(17) Liu, Z. Y. Advancement in coal chemistry: Structure and reactivity (in Chinese) . Scientia Sinica Chimica 2014, 44, 1431-1438.

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(18) Okolo, G. N.; Neomagus, H. W. J. P.; Everson, R. C.; Roberts, M. J.; Bunt, J. R.; Sakurovs,

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R.; Mathews, J. P. Chemical-structural properties of South African bituminous coals: Insights

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from wide angle XRD-carbon fraction analysis, ATR-FTIR, solid state

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HRTEM techniques. Fuel 2015, 158, 779-792.

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13C

NMR, and

(19) Shi, L.; Liu, Q.; Guo, X.; Wu, W.; Liu, Z. Pyrolysis behavior and bonding information of coala TGA study. Fuel Processing Technology 2013, 108, 125-132. (20) Baruah, M. K.; Kotoky, P.; Borah, G. C. Distribution and nature of organic/mineral bound elements in Assam coals, India. Fuel 2003, 82, 1783-1791.

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(21) Zhao, H.; Wang, B.; Li, Y.; Song, Q.; Zhao, Y.; Zhang, R.; Hu, Y.; Liu, S.; Wang, X.; Shu, X.

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Effect of chemical fractionation treatment on structure and characteristics of pyrolysis

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products of Xinjiang long flame coal. Fuel 2018, 234, 1193-1204.

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(22) Zhang, H.; Pu, W. X.; Ha, S.; Li, Y.; Liu, D. Influence of acid treatment on the properties of

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pulverized coals with low ash content (in Chinese). Journal of Engineering Thermophysics

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2009, 30, 699-702.

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(23) Wijaya, N.; Zhang, L. A critical review of coal demineralization and its implication on

2

understanding the speciation of organically bound metals and submicrometer mineral grains

3

in coal. Energy & Fuels 2011, 25, 1-16.

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(24) Martyniuk, H.; Wieckowska, J. A study of sulphonated and oxidized coals by thermal analysis. Journal of Thermal Analysis 1993, 40, 217-224. (25) Li, C. Z. Importance of volatile–char interactions during the pyrolysis and gasification of lowrank fuels-A review. Fuel 2013, 112, 609-623. (26) Li, C. Z.; Sathe, C.; Kershaw, J. R.; Pang, Y. Fates and roles of alkali and alkaline earth metals during the pyrolysis of a Victorian brown coal. Fuel 2000, 79, 427-438.

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(27) Zhang, L.; Takanohashi, T.; Nakazato, T.; Saito, I.; Tao, H. Sequential leaching of coal to

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investigate the elution of inorganic elements into coal extract (HyperCoal). Energy & Fuels

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2008, 22, 2474-2481.

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(28) Li, C.; Takanohashi, T.; Saito, I. Elucidation of mechanisms involved in acid pretreatment and thermal extraction during ashless coal production. Energy & Fuels 2004, 18, 97-101. (29) He, Q.; Wan, K.; Hoadley, A.; Yeasmin, H.; Miao, Z. TG-GC-MS study of volatile products from Shengli lignite pyrolysis. Fuel 2015, 156, 121-128. (30) Mcmillen, D. F.; Malhotra, R.; Nigenda, S. E. The case for induced bond scission during coal pyrolysis. Fuel 1989, 68, 380-386. (31) Van Heek, K. H.; Hodek, W. Structure and pyrolysis behaviour of different coals and relevant model substances. Fuel 1994, 73, 886-896.

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(32) Eskay, T. P.; Britt, P. F.; Buchanan, A. C., III. Pyrolysis of aromatic carboxylic acids: potential

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involvement of anhydrides in retrograde reactions in low-rank coal. Energy & Fuels 1997, 11,

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1278-1287.

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(33) Eskay, T. P.; Britt, P. F.; Buchanan, A. C., III. Does decarboxylation lead to cross-linking in low-rank coals? Energy & Fuels 1996, 10, 1257-1261. (34) Xu, S.; Zhou, Z.; Xiong, J.; Yu, G.; Wang, F. Effects of alkaline metal on coal gasification at pyrolysis and gasification phases. Fuel 2011, 90, 1723-1730.

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(35) Geng, W.; Nakajima, T.; Takanashi, H.; Ohki, A. Analysis of carboxyl group in coal and coal

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aromaticity by Fourier transform infrared (FT-IR) spectrometry. Fuel 2009, 88, 139-144. 18 / 35


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(36) Schafer, H. N. Pyrolysis of brown coals. 2. Decomposition of acidic groups on heating in the range 100-900 °C. Fuel 1979, 58, 673-679. (37) Zhou, B.; Shi, L.; Liu, Q.; Liu, Z. Examination of structural models and bonding characteristics of coals. Fuel 2016, 184, 799-807. (38) Zhou, B.; Shi, L.; Liu, Q.; Liu, Z. Comigendum to "Examination of structural models and bonding characteristics of coals"[Fuel 184(2016)799-807]. Fuel 2016, 186, 864.

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1

Figure Captions

2

Figure 1. The C% (a) and O% (b) of R-Coals and D-coals

3

Figure 2. Volatile content of R-Coals and D-Coals

4

Figure 3. The O% of D-Coals compared with that of R-Coals corrected by excluding

5

both the maximum amount of carbonates and the decreased C% from R-

6

Coals

7

Figure 4. The increase of O% in D-Coals by oxidation compared with R-Coals

8

Figure 5. DTG curves of R-Coals (a) and D-Coals (b)

9

Figure 6. Tp (a) and Rmax (b) of R-Coals and D-Coals

10

Figure 7. Variation of ΔR with pyrolysis temperature for different coals

11

Figure 8. DTG curve of R-Coal and D-Coal fitted by six SPs

12

Figure 9. The TSP of SPs of R-Coals and D-Coals

13

Figure 10. Peak areas of the 5 SPs of R-Coals and D-Coals

14 15

Table Captions

16

Table 1. Effects of minerals on coal pyrolysis

17

Table 2. The effects of acid treatment on the organic structure of coals

18

Table 3. The proximate and ultimate analyses of D-Coals and R-Coals (in brackets)

19

Table 4. Mineral components of R-Coals

20

Table 5. O% of R-Coals and D-Coals acid treated in different atmospheres

21 22

20 / 35


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Energy & Fuels

1 2

Figure 1. The C% (a) and O% (b) of R-Coals and D-coals

21 / 35


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 2

Figure 2. Volatile content of R-Coals and D-Coals

22 / 35


Page 22 of 35

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1 2 3

Figure 3. The O% of D-Coals compared with that of R-Coals corrected by excluding both the maximum amount of carbonates and the decreased C% from R-Coals

4 5

23 / 35


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 2

Figure 4. The increase of O% in D-Coals by oxidation compared with R-Coals

24 / 35


Page 24 of 35

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Energy & Fuels

1 2

Figure 5. DTG curves of R-Coals (a) and D-Coals (b)

3

25 / 35


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1 2

Figure 6. Tp (a) and Rmax (b) of R-Coals and D-Coals

26 / 35


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Energy & Fuels

1 2

Figure 7. Variation of ΔR with pyrolysis temperature for different coals

27 / 35


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 2

Figure 8. DTG curve of R-Coal and D-Coal fitted by six SPs

28 / 35


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Energy & Fuels

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Figure 9. The TSP of SPs of R-Coals and D-Coals

29 / 35


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1 2

Figure 10. Peak areas of the 5 SPs of R-Coals and D-Coals

3 4

30 / 35


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Energy & Fuels

1

Table 1. Effects of minerals on coal pyrolysis Ref.

Coal type

C cont. wt.%

Dem. method

Dem. T (°C)

Dem. Atm.

Pyrolysis T (°C)

Results

[3]

2

76.2/54.4

HCl-HF

RM

Air

110-800

Minerals have little effect on pyrolysis

[4]

1

89.0

HCl-HF-HCl

60

N2

300-500

HCl lowers pyrolysis conversion; HCl-HF increases pyrolysis conversion

[5]

1

71.3

HCl-HF-HCl

RM

Air

105-1000

Minerals have little effect on pyrolysis

86.1

HCl-HF

RM

Air

RM-1000

Alkali metals have little effect on BTXN release

75.9/69.0

HCl-HF

RM

Air

RM-1000

Alkali/alkaline earth metals promote BTXN release

[6]

2 3

3

Note: C cont.: carbon content; Dem.: demineralization; T: temperature; RM: room temperature; Atm.:

atmosphere;

BTXN:

benzene,

toluene,

31 / 35


xylene,

and

naphthalene

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

2 3

Page 32 of 35

Table 2. The effects of acid treatment on the organic structure of coals Ref.

Coal type

C cont. wt.%

Dem. method

Dem. Atm.

Dem. T (°C)

Results

[12]

6

69.0-86.3

HCl-HF-HCl

N2

60

Little effect

[13]

1

75.7

HCl-HF-HCl

Air

60

Phenolic hydroxyl and carboxyl groups increase

[14]

1

78.7

HCl-HF-HCl

Air

20

–COOH content increase

[15]

1

73.4

HCl-HF

N2

RM

–COOH content increased; Aliphatic side chains and bridge bonds length reduced

[16]

1

60.2

HCl-HF-HCl

N2

60

Aliphatic structures decreased; Oxygenated species increase

Note: C cont.: carbon content; Dem.: demineralization; T: temperature; RM: room temperature; Atm.: atmosphere

32 / 35


Page 33 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

Table 3. The proximate and ultimate analyses of D-Coals and R-Coals (in brackets) Coal No.

2 3

Proximate analyses (wt.%) Mad

Ad

Vdaf

Ultimate analyses (wt.%, daf) C

H

N

O

S

01

6.8(16.4) 0.1(14.5) 47.3(50.7) 65.9(73.8) 4.2(3.9) 1.7(1.1) 27.6(19.5) 0.6(1.7)

02

4.7(0.7) 2.5(50.2) 43.8(48.8) 68.9(73.9) 4.7(5.1) 1.1(1.1) 25.0(19.4) 0.3(0.7)

03

4.8(12.5) 0.1(16.7) 41.5(43.4) 70.0(79.8) 4.7(2.8) 1.0(0.6) 23.8(16.2) 0.5(0.6)

04

5.6(2.2) 0.4(24.7) 45.7(48.9) 70.4(76.6) 5.1(4.6) 1.2(1.5) 22.8(16.6) 0.5(0.7)

05

3.3(7.6) 0.7(12.4) 43.2(44.5) 70.6(73.3) 4.6(4.5) 1.0(1.1) 23.7(21.0) 0.1(0.1)

06

4.3(16.3) 0.6(13.2) 39.4(40.9) 72.1(77.2) 5.2(3.9) 0.8(1.0) 21.8(17.8) 0.1(0.1)

07

6.3(8.8)

1.2(5.7)

35.4(34.9) 73.7(75.2) 3.8(3.2) 0.5(0.7) 21.8(20.3) 0.2(0.6)

08

5.9(13.2) 0.1(6.0)

37.4(33.4) 75.9(83.4) 4.4(4.0) 0.8(0.9) 18.8(11.6) 0.1(0.1)

09

3.4(3.9)

0.3(5.5)

35.4(36.5) 77.1(77.3) 4.7(4.8) 0.9(0.9) 17.2(16.9) 0.1(0.1)

10

1.9(3.2)

0.4(8.5)

40.0(41.9) 77.2(81.4) 4.8(4.7) 1.0(1.1) 15.8(11.5) 1.2(1.3)

11

3.1(1.8) 0.1(18.9) 39.2(42.6) 79.7(79.2) 5.4(5.3) 1.4(1.2) 11.6(12.1) 1.9(2.2)

12

3.0(3.4) 2.4(32.8) 29.7(39.8) 79.8(76.9) 4.9(4.8) 1.3(1.3) 13.4(16.3) 0.6(0.7)

13

2.2(3.1) 2.5(30.2) 30.8(37.6) 80.2(76.6) 4.6(4.5) 1.2(1.3) 13.1(15.8) 0.9(1.8)

14

1.4(1.0) 2.2(12.6) 23.4(32.3) 85.2(85.4) 4.7(5.0) 1.4(1.5)

8.0(7.3)

0.7(0.8)

15

1.5(0.9) 0.7(11.7) 25.7(32.1) 86.2(86.1) 4.9(5.3) 1.5(1.5)

7.0(6.5)

0.4(0.6)

16

1.8(1.3)

25.9(28.4) 86.6(85.6) 4.8(5.0) 1.4(1.1)

5.7(6.6)

1.5(1.7)

17

0.9(1.0) 0.4(10.9) 20.7(27.8) 86.7(87.2) 4.7(5.2) 1.4(1.4)

6.3(5.0)

0.9(1.2)

18

2.4(0.8) 0.1(11.8) 24.7(28.8) 87.6(86.6) 4.9(4.8) 1.4(1.3)

5.3(6.3)

0.8(1.0)

19

0.2(0.8) 0.1(15.2) 18.1(22.6) 88.0(87.6) 4.4(4.7) 1.3(1.3)

5.3(5.0)

1.0(1.4)

20

1.0(0.6) 0.5(10.5) 14.3(16.1) 90.1(89.6) 4.3(4.3) 1.2(1.3)

3.7(3.8)

0.7(1.0)

21

0.6(0.6) 0.5(19.8) 14.2(16.3) 90.2(89.7) 3.9(4.1) 1.2(1.4)

4.4(4.4)

0.3(0.4)

22

1.9(0.6)

13.9(13.6) 90.5(90.8) 3.9(3.9) 1.3(1.4)

4.0(3.4)

0.3(0.5)

23

1.9(1.1) 1.7(20.7)

7.8(11.6)

90.8(90.6) 3.6(3.7) 1.3(1.5)

4.0(3.8)

0.3(0.4)

24

2.1(0.9) 1.0(18.5)

9.2(8.7)

91.6(91.1) 3.5(3.0) 1.0(1.2)

3.6(4.3)

0.3(0.4)

25

0.9(1.7) 1.3(10.1)

7.8(4.9)

91.9(94.5) 2.7(3.1) 0.7(0.8)

4.4(1.3)

0.3(0.3)

26

1.7(0.4)

10.1(7.2)

92.3(93.4) 3.4(3.3) 0.6(0.7)

3.6(2.4)

0.1(0.2)

0.1(7.2)

0.1(7.3)

0.3(1.8)

ad: air-basis; d: dry basis; daf: dry-and-ash-free basis M: moisture; A: ash; V: volatile matter content.

33 / 35


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

Page 34 of 35

Table 4. Mineral components of R-Coals Content (wt.%, db)

Coal No.

Ash (wt.%, ad)

SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

SO3 K2O

Na2O

Other

01

14.5

6.38

4.18

1.31

0.85

0.09

0.35

1.20 0.10

0.00

0.04

02

50.2

30.13

9.89

2.44

2.22

1.18

0.62

1.94 1.29

0.33

0.16

03

16.7

9.22

1.63

2.20

1.94

0.28

0.18

1.00 0.15

0.10

0.01

04

24.7

8.05

2.71

2.56

7.67

0.34

0.21

2.81 0.24

0.05

0.06

05

12.4

4.89

1.60

0.93

2.28

0.24

0.14

0.63 1.57

0.10

0.03

06

13.2

1.67

0.88

2.15

5.85

0.37

0.10

2.10 0.05

0.00

0.03

07

5.7

1.28

0.46

0.38

1.63

0.37

0.03

1.22 0.03

0.29

0.00

08

6.0

2.11

0.76

0.16

2.02

0.23

0.12

0.53 0.03

0.03

0.00

09

5.5

2.59

0.97

0.39

0.67

0.06

0.06

0.63 0.07

0.05

0.01

10

8.5

2.48

1.22

2.00

1.29

0.22

0.00

0.68 0.06

0.00

0.55

11

18.9

4.34

2.53

2.93

5.10

0.55

0.25

2.55 0.06

0.38

0.23

12

32.8

11.59

7.42

3.05

1.49

0.11

0.39

8.43 0.30

0.00

0.02

13

30.2

13.60

8.20

1.34

1.62

0.13

0.84

4.15 0.24

0.00

0.06

14

12.6

4.39

2.61

0.78

0.60

0.10

0.29

3.62 0.08

0.00

0.13

15

11.7

3.30

2.64

1.99

1.08

0.14

0.54

1.85 0.08

0.00

0.10

16

7.2

1.95

1.08

0.51

0.13

0.04

0.11

3.24 0.07

0.03

0.04

17

10.9

3.69

2.27

0.61

0.32

0.12

0.20

3.54 0.07

0.05

0.03

18

11.8

3.77

2.46

0.54

0.38

0.12

0.43

3.92 0.07

0.00

0.10

19

15.2

7.93

4.15

1.27

0.95

0.11

0.54

0.00 0.19

0.00

0.05

20

10.5

4.09

2.15

0.54

0.32

0.04

0.23

2.97 0.10

0.04

0.02

21

19.8

9.98

4.96

0.78

1.22

0.14

0.41

1.80 0.43

0.04

0.05

22

7.3

1.59

1.12

0.77

1.82

0.10

0.15

1.66 0.02

0.07

0.01

23

20.7

10.47

5.00

1.06

1.12

0.15

0.53

1.80 0.37

0.14

0.08

24

18.5

9.09

4.70

1.01

1.25

0.13

0.28

1.59 0.27

0.14

0.03

25

10.1

3.77

2.45

0.91

0.84

0.09

0.11

1.38 0.07

0.08

0.40

26

1.8

0.57

0.35

0.23

0.19

0.03

0.03

0.33 0.01

0.05

0.01

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Page 35 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

2 3

Table 5. O% of R-Coals and D-Coals acid treated in different atmospheres O (wt.%, daf) Coal No.

R-Coals

D-Coals

D-Coals-in-Ar

03 06 14

16.2 17.8 7.3

23.8 21.8 8.0

21.6 21.4 7.6

daf: dry-and-ash-free basis

35 / 35


HCl treatment of 26 coals on their composition and

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