Interpretation of Methane and Hydrogen Evolution in Coal Pyrolysis

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Interpretation of methane and hydrogen evolution in coal pyrolysis from the bond cleavage perspective Lei Shi, Qingya Liu, Bin Zhou, Xiaojin Guo, Zhengke Li, Xiaojie Cheng, Ru Yang, and Zhenyu Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02482 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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Interpretation of methane and hydrogen evolution in coal pyrolysis

2

from the bond cleavage perspective

3 4

Lei Shi a, Qingya Liu a, Bin Zhou a, Xiaojin Guo b, c, Zhengke Li a, Xiaojie Cheng a, Ru

5

Yang a and Zhenyu Liu a,*

6 7

a

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China

8 b

9

Thermophysics, Chinese Academy of Sciences, Beijing 100190, PR China

10 11 12

Key Laboratory of Advanced Energy and Power, Institute of Engineering

c

Research Center for Clean Energy and Power, Chinese Academy of Sciences, Lianyungang, Jiangsu 222069, PR China

13 14

ABSTRACT: Pyrolysis of 34 coals of different rank with carbon contents of

15

73.7-91.9% was studied in a TG-MS system to analyze CH4 and H2 evolution. Based on

16

the shape of evolution curves and dissociation energy of covalent bonds in coals, the

17

CH4 formation is attributed to the cleavage of Cal-Cal bond in a low temperature range

18

and that of Cal-Car bond in a high temperature range, while the H2 formation was

19

attributed to the cleavage of H-Cal bond in a low temperature range and that of Cal-Car

20

bond in a high temperature range. The yields of CH4 and H2 corresponding to cleavage

21

of each of these bonds are quantified by deconvolving each of the evolution curves into

22

two sub-curves. It is found that the cleavage of bonds containing only aliphatic carbon,

23

such as Cal-Cal and H-Cal peaking at 549 and 540 oC, respectively, contributes to small

24

fractions of CH4 and H2 generation. The cleavage of bonds containing an aromatic

25

carbon, such as Cal-Car and H-Car peaking at temperatures of 618 and 774 oC,

26

respectively, contributes to the major CH4 and H2 generation. The yields of the products

27

and the proportion of each bond in the coals that cleaved to generate these products are

28

found to be coal rank dependent. The changes of side-structure linked to these bonds are

29

also analyzed with respect to changes in coal rank. These results extend the 1

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1

understanding on the mechanism of CH4 and H2 formation in coal pyrolysis as well as

2

that on the bonding structure of coals.

3 4

KEY WORDS：：Coal pyrolysis; Methane; Hydrogen; Covalent bonds; Peak fit

5 6

1. INTRODUCTION

7

Methane (CH4) and hydrogen (H2) are important platform chemicals and clean

8

fuels that can be obtained directly from coal pyrolysis. Studies showed that about

9

10-50% H in coals transfers to CH4 and H2 during pyrolysis,1-6 and the CH4 generation

10

starts at 300-400 oC and peaks at around 500 oC,6-11 while the H2 generation starts at

11

about 500 oC and peaks at 700-800 oC.6,7 The yields of CH4 and H2 vary with pyrolysis

12

apparatus and operating conditions, as shown in Table 1.6-8,12-17

13

The mechanism of CH4 generation during coal pyrolysis has been studied

14

extensively, and categorized mainly into the radical mechanism due to the observation of

15

methyl intermediate.14 In detail CH4 is generated via four routes

16

desorption from the coal pores; (2) thermal cleavage of alkyl side chains and methylene

17

bridge bonds in coals; (3) cleavage of hydrogenated aromatic rings in coals; and (4)

18

cleavage of methyl side chains linked to aromatic rings during condensation of aromatic

19

nucleus into coal char. The latter three routes follow radical mechanism. Routes (2) and

20

(3) involve cleavage of bonds between aliphatic carbons (Cal-Cal) while route (4) is

21

attributed to the cleavage of aliphatic and aromatic carbons (CH3-Car), although the

22

cleavage of all these bonds does not necessarily yield only CH4 and the formation of

23

other paraffins is also possible. According to the bond dissociation energy of model

24

compounds in Table 2, the cleavage of Cal-Cal bond in compounds #1 and #2 may yield

25

CH4, while that of Cal-Cal bond in compounds #3 and #4 may yield C2 or C3

26

hydrocarbons, because the bond between the α-carbon and β-carbon in the alkyl chain is

27

the weakest among all the bonds. The cleavage of Cal-Cal bond in compound #5 may

28

generate radical fragments containing -CH2-Car bond which may crack further at

29

temperatures higher than 600 oC to generate CH4. In principle, the alkyl chain with two

30

carbons may be liable to generate CH4 at low temperatures while that with one carbon is 2


5,8,10,15,16,18

such as (1)

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liable to generate CH4 at high temperatures. The alkyl chain longer than C2 may not

2

generate CH4 directly. Although the actual mechanism of CH4 generation may be more

3

complex due to secondary reactions, the cleavage of bonds for CH4 formation may still

4

be attributed to cleavage of the bonds discussed.

5

It is generally recognized that H2 generation in coal pyrolysis results mainly from

6

condensation of macromolecules,7,20-22 including dehydrogenation of partially saturated

7

aromatic-ring structures in 450-600 oC and condensation of aromatic-ring structures at

8

temperatures higher than 600 oC.7,20 These reactions can be categorized into cleavage of

9

H-Cal bond and H-Car bond, respectively. It was reported that reactions of carbonaceous

10

matter with steam generate H2 at temperatures higher than 700 oC,23 but the quantities

11

should be low in coal pyrolysis because the amounts of water generated are low, which

12

is also evidenced by the trace amounts of CO2 and CO accompanied with H2

13

generation.20,24

14

Based on the above discussion it is clear that the complex parallel and sequential

15

reactions responsible for CH4 and H2 generation in coal pyrolysis may be categorized by

16

the cleavage of a few covalent bonds presented above, which occur sequentially with

17

increasing temperature according to their dissociation energies, from less than 150

18

kJ/mol to greater than 400 kJ/mol.25,26 This categorization may be clearly observed in a

19

system containing a thermal gravimetric analyzer (TGA) coupled online with a mass

20

spectrometer (MS), i.e. TG-MS, under fast purging to inhibit the reaction of volatiles by

21

fast cooling,26 and the MS curves of CH4 and H2 are deconvolved into sub-curves

22

corresponding to the bonds.7,27

23

Deconvolution of an experimentally obtained curve into a number of sub-curves

24

has been widely practiced using Gaussian distribution, including the curves of gaseous

25

products of coal pyrolysis.7,27,28 The Gaussian distribution function is shown in Equation

26

(1),29,30 where A is the amplitude, A =

27

one-half of the width of the two inflexion points; and µ is the mathematical expectation.

28

y=

1 ; σ is the standard deviation that equals to 2πσ

1 1 x−µ 2 1 x−µ 2 exp[− ( ) ] = A exp[− ( ) ] 2 σ 2 σ 2πσ 3


(1)

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This work studies the trends of CH4 and H2 evolution during pyrolysis of 34 coals,

2

from lignite to anthracite in a TGA-MS system, by deconvolving each of the evolution

3

curves into two sub-curves representing the linkage of -CH3 and -H with Cal and Car. The

4

trends are discussed with coal rank to promote understanding of cleavage of covalent

5

bonds in coal during pyrolysis.

6 7

2. EXPERIMENTAL SECTION

8

The 34 coals were produced in China with carbon contents (C%) of 73.7-91.9% as

9

shown in Table 3. All the coals were ground and sieved to sizes between 0.15 and 0.20

10

mm. The experiments were carried out in a TGA (SETSYS Evolution 24, SETARAM)

11

coupled online with a MS (OmniStar 200, Blazers) for analysis of CH4 and H2. The

12

pyrolysis conditions were 30 mg coal, ambient pressure with an Ar flow of 100 ml/min,

13

from room temperature to 110 oC at a rate of 10 oC/min and 30 min at 110 oC to remove

14

moisture, and from 110 oC to 900 oC at a rate of 10 oC/min and 20 min at 900 oC. The

15

tubing connecting the TGA and MS was kept at 180 oC to avoid condensation of

16

volatiles. Blank TGA-MS experiments (with empty crucible) were made to account for

17

buoyancy effect caused by the temperature raise. MS signals with mass/charge ratios

18

(m/z) of 16 for CH4, 2 for H2 and 40 for Ar were recorded. The detailed procedures have

19

been reported earlier.31

20

The mass of CH4 and H2 was determined by external calibration. The calibration

21

lines in Figure 1 were obtained by injecting CH4 and H2 of 99.99% and 99.999% purity,

22

respectively, into a flow of Ar (99.9997% pure) at 100 ml/min. To account for baseline

23

change in MS signal, the ratio of CH4 or H2 signal to Ar signal is used as the CH4 or H2

24

signal as reported earlier.11,31

25 26

3. RESULTS AND DISCUSSION

27

3.1. Methane and hydrogen release during coal pyrolysis. Figure 2 shows the MS

28

profiles of CH4 and H2 obtained during pyrolysis of the 34 coals. The curves are lined

29

from top to bottom according to the increasing carbon content of the coals as indicated

30

by the vertical arrows on the right side. Although the data are somewhat random and 4


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some curves overlap the major trends are obvious, as shown by the thin dash lines for

2

temperature range of the peaks and the thick dash lines for the peak temperature. The

3

temperature range and peak temperature of CH4 generation of lower rank coals do not

4

change much with changes in carbon content while those of higher rank coals increase

5

systematically with increasing carbon content. The temperature range of H2 generation is

6

broader than that of CH4 and the peak temperature of the main H2 peak does not vary

7

significantly with changes in carbon content. A small satellite H2 peak is notable but it

8

diminishes with increasing coal rank.

9

Figure 3 clearly shows that the peak temperatures of CH4 are approximately 525 oC

10

for coals with carbon contents of less than 88-89%, but increase with an increase in

11

carbon content, with a slope of approximately 42 °C/C%, for coals of higher carbon

12

content. The turning point, around 88-89% carbon, is similar to that reported for changes

13

in cleavage temperature of bonds determined by deconvolution of DTG data of the same

14

coals reported by us earlier.31 It is also the turning point in change of many physical

15

properties of coals, including the heat capacity,32 the conductivity,33 the heat of

16

wetting,34,35 the solubility parameter36 and many others.37-39 This turning point may be

17

attributed to evolution of coal structure in coalification, because after that the coal

18

entered the medium-volatile rank which is characterized by a significant decrease in

19

volatile matter content against increasing carbon content, due to decreases in aliphatic

20

bonds population and chain length, and release of methane as reported in literatures.40-42

21

The peak temperature of the major H2 peak changes little with an increase in carbon

22

content, except for coals of the lowest and the highest carbon contents. The peak

23

temperature of the small satellite H2 peak, however, increase linearly with an increase in

24

carbon content of the coals, from 450 oC at a carbon content of 73.7% (Coal #34) to 500

25

o

26

satellite peak disappears is also the turning point discussed above.

C at a carbon content of 88.5% (Coal #9). The carbon content at which the small

27

Figure 4 shows the mass of hydrogen in the coals and that transferred to CH4 and

28

H2 during the pyrolysis. The mass of hydrogen in the coals is determined from the

29

ultimate analysis in Table 3 while that in CH4 or H2 was determined from the MS data.

30

In particular, the mass of hydrogen in CH4 is 1/4 the mass of CH4. It can be seen that the 5


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mass of hydrogen in the coals increases with an increase in carbon content from less

2

than 40 mg/g of coal (daf) for a lignite with 73.7% carbon to around 46-48 mg/g of coal

3

(daf) for low rank bituminous coals with 79-82% carbon, and then decreases with a

4

further increase in carbon content, to approximately 25 mg/g of coal (daf) for an

5

anthracite with 91.9% carbon. The trends in mass of hydrogen in CH4 and in H2 are

6

similar but somewhat different from that in the coals. They increase with an increase in

7

carbon content of coals from 73.7 to 77%, fluctuate at around 9 mg/g of coal (daf) for

8

coals with carbon contents of 77 to 88-89%, and then decrease with a further increase in

9

carbon content of the coals.

10

The sum of hydrogen in CH4 and H2 is also shown in Figure 4. It increases from 5

11

to 15 mg/g of coal (daf) for coals with 73.7-77% carbon, corresponding to about 10-30%

12

hydrogen in the coals; maintains at around 15 mg/g of coal (daf) for coals with 77 to

13

88-89% carbon, corresponding to approximately 30-40% hydrogen in the coals; and then

14

decreases from 15 to 8 mg/g of coal (daf) for coals with a further increase in carbon

15

content, corresponding to about 35-55% hydrogen in the coals. These behaviors indicate

16

that the proportion of hydrogen in coals transferred to CH4 and H2 increases with

17

increasing coal rank, suggesting changes in coal structure in coalification as well as that

18

in pyrolysis. For example, low rank coals are rich in hydroxyl and carboxylic functional

19

groups18,43-45 which do not generated much CH4 and H2 in pyrolysis. The structures that

20

contain little oxygen, but -CH3 and -H linked to Cal and Car, increase with increasing

21

coal rank.

22

3.2. The covalent bonds relevant to CH4 and H2 formation. In principle, a

23

volatile product generated from cleavage of one type of covalent bond would yield only

24

one MS peak with a fixed peak temperature. If the product is generated from cleavage of

25

two covalent bonds that differ in bond dissociation energy it would show two MS peaks

26

unless the dissociation energies are close to each other. In the latter case the two peaks

27

merge to show a single peak, with a peak temperature close to that of the dominant peak.

28

The constant peak temperature of CH4 at around 525 oC for coals of less than 88-89%

29

carbon and the increasing peak temperature of CH4 with increasing carbon content for

30

coals of higher than 88-89% carbon in Figure 3 indicate that CH4 is generated from 6


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cleavage of at least two types of covalent bonds, and the bond of lower dissociation

2

energy dominates the lower rank coals while the bond of higher dissociation energy

3

become significant in the higher rank coals. This further indicates that the CH4 release

4

can be ascribed hypothetically to the cleavage of Cal-Cal at low temperatures and Cal-Car

5

bonds at high temperatures because the populations of other bonds in coals that may also

6

generate CH4, such as Cal-O and Cal-S bonds, are relatively low and their bond

7

dissociation energies are close to that of Cal-Cal bond.19,31,44,45

8

Similar to the above deduction on CH4 generation, the H2 generation may also be

9

ascribed to cleavage of two types of covalent bonds as indicated in Figure 3, with the

10

dominant peak at around 760 oC for the cleavage of H-Car bond and the small satellite

11

peak at 450-500 oC for the cleavage of H-Cal bond.

12

The bonds assignments made above for CH4 and H2 generation agree with the

13

relation between the bonds’ dissociation energy and the peak temperature of bonds

14

dissociation reported earlier as shown in Figure 5,31 where the bonds containing

15

aliphatic carbon such as Cal-Cal, Cal-O, Cal-S and H-Cal bonds dissociate at temperatures

16

lower than 550 oC while that containing aromatic carbon such as Cal-Car and H-Car bonds

17

dissociate at temperatures higher than 550 oC.

18

It should be noted that the relation in Figure 5 is only a general guide because the

19

bonds have a broad range of dissociation energies which correspond to a broad range of

20

dissociation peak temperatures. This is understandable because the dissociation energy

21

of each type of the bonds is affected by its side-structures, so does the bond dissociation

22

temperature. Since the CH4 and H2 generation in low rank coals results mainly from the

23

cleavage of Cal-Cal and H-Cal bonds, respectively, while that in high rank coals results

24

mainly from the cleavage of Cal-Car and H-Car bonds, the lowest and the highest peak

25

temperatures of CH4 and H2 in Figure 3 are used as the initial peak temperatures for

26

cleavage of the bonds with the lower and the higher bond dissociation energies,

27

respectively, in deconvolving each of the MS curves into two sub-curves. The low

28

temperature sub-curve for cleavage of Cal-Cal or H-Cal bond is denoted as P1 with the

29

standard deviation being equal to the range of temperature fluctuation, and the high

30

temperature sub-curve for cleavage of Cal-Car or H-Car bond is denoted as P2 with the 7


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1

standard deviation being equal to the range of temperature fluctuation. In such a way the

2

peak temperature ranges used in the deconvolution are 516±33 oC and 618±44 oC for P1

3

and P2 of CH4 respectively, and 500±40 oC and 774±21 oC for P1 and P2 of H2,

4

respectively.

5

Figure 6 shows CH4 and H2 curves of Coal #3 and Coal #15 and the corresponding

6

sub-curves determined from the deconvolution. It is clear that the sub-curves fit the

7

overall curves well although they differ slightly in a few regions, such as CH4 at

8

temperatures higher than 650 oC and H2 at temperatures of 400-600 oC and 700-800 oC.

9

The effectiveness of the fitting is quantified by the coefficient of determination (R2)

10

shown in Figure 7, where the R2 are generally higher than 0.9 except for a few coals,

11

mainly CH4 from high rank coals with more than 85% carbon.

12

It is generally accepted that the MS peak width of a volatile pyrolysis product tells

13

complexity of covalent bonds that underwent cleavage and yielded the product.7,20 The

14

wide the peak is, the complex of the side structure of the bond is. For example, the peak

15

width at half height of DTG curve for pyrolysis of ethylene-propylene rubber (EPR) or

16

polyvinylpyrrolidone (PVP) is less than 70 oC at a heating rate of 10 oC/min because

17

they involve cleavage of a single covalent bond.46,47 The DTG peak width for pyrolysis

18

of a more complex matter such as Coal #34 is much wider, approximately 220 °C,31

19

because it involves cleavage of many covalent bonds.

20

Figure 8 shows 2σ of CH4 and H2 sub-curves determined from the deconvolution,

21

which is a measure of peak width because σ is the half width of the two inflexion points

22

of a curve as stated earlier. It can be seen that the 2σ of P1 and P2 for both products

23

change systematically with increasing carbon content of coals, that of CH4 are narrower

24

than that of H2, that of P2 are higher than that of P1 and decrease slightly, and that of P1

25

decrease remarkably. These disclose the changes in coal structure with coal rank. For

26

example, the decrease in 2σ of P1 for CH4, from 130 to 33 oC for coals with carbon

27

contents of 73.7 to 91.9%, may be attributed to reduction of alkyl side chains in length

28

and in population with increasing coal rank. It has been reported that the low rank coals

29

are rich in alkyl chains with lengths longer than 3 carbon atoms,48 and the alkyl chains

30

are linked with many types of side-structure or functional groups.49 The length of chain 8


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become shorter and the population of side chains or functional groups become less with

2

an increase in coal rank.44,45,50 The slight decrease in 2σ of P2 for CH4, from 140 to 110

3

o

4

structures linked to Cal-Car bond (or CH3-Car bond) are relatively complex and stable,

5

such as the number of condensed aromatic rings as well as the number of side chains

6

attached to the condensed aromatic rings, so they do not change much with coal rank.

C with an increase in carbon content from 73.7 to 91.9%, may suggest that the side

7

Figure 8(b) shows that the 2σ of P1 and P2 for H2 are broader than that of CH4,

8

indicating that the H-Cal and H-Car bonds are influenced by more side structures. The

9

high and constant P2 may be attributed to H atoms linked to different positions on the

10

condensed aromatic rings. The high but fast decreasing P1 may be ascribed to a decrease

11

in aliphatic carbon with increasing coal rank, due to elimination of some aliphatic chains

12

or functional groups during coalification.

13

As mentioned in the introduction, the changes in the size of sub-peaks reflect

14

changes in coal structure. It can be seen in Figure 9(a) that the yield of P1 of CH4 may

15

be expressed roughly by 3 regions in coal rank: increasing from 3 to 11 mg/g-coal (daf)

16

for coals with 73.7 to 77% carbon, maintaining approximately at 11 mg/g-coal (daf) for

17

coals with 77 to around 88% carbon, and decreasing from 11 to 0 mg/g-coal (daf) for

18

coals with 88 to 91.9% carbon. The yields of P2 of CH4 may be expressed by a different

19

trend, it increases from 2 to 23 mg/g-coal (daf) for coals with 73.7 to 81% carbon, and

20

then decreases slowly to approximately 13 mg/g-coal (daf) for coals of higher carbon

21

content but the data are quite scattered. These data may indicate that CH4 are mainly

22

generated from the cleavage of Cal-Car bonds especially for coals with more than 77%

23

carbon, and the rich aliphatic content in lignite does not contribute to CH4 generation

24

probably due to the presence of large amounts of oxygen.

25

Figure 9(b) shows that the yield of H2 in P1 is negligible while that in P2 can be

26

expressed by 3 regions, increasing from 1 to 6 mg/g-coal (daf) for coals with 73.7 to

27

77% carbon, increasing further at a lower rate from 6 to 8 mg/g-coal (daf) for coals with

28

77 to 89% carbon, and then decreasing for coals with more than 89% carbon. This trend

29

is generally consistent with the increasing population of H-Car bond in coal with

30

increasing coal rank,44,45 and indicates that the decrease of aliphatic structures do not 9


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1

significantly alter the aromatic structure in coals until the carbon content is higher than

2

89%, and the condensation of aromatic structures become significant in coals with more

3

than 89% carbon.

4

In principle, the formation of one CH4 molecule requires the cleavage of one Cal-Cal

5

or Cal-Car bond as well as the cleavage of one H-Cal or H-Car bond, the formation of a H2

6

molecule requires the cleavage of two H-Cal or two H-Car bonds. Therefore the number

7

of bonds responsible for CH4 and H2 formation can be determined based on the

8

sub-curves of CH4 and H2 in Figure 9, if the formation of P1 products is ascribed only to

9

the dissociation of -Cal containing bonds while that of P2 is ascribed only to the

10

dissociation of -Car containing bonds. These assumptions are roughly correct because the

11

relative order of these bonds in dissociation energy. Based on the method proposed by

12

Zhou et al. for estimation of bonds population in coals,44,45 the proportion of each bond

13

in the coals that underwent cleavage for CH4 and H2 generation can be determined as

14

shown in Figure 10.

15

Figure 10(a) shows that the Cal-Cal bonds in coals contribute little to CH4 formation,

16

the percentage increases from 2 to 10% for coals with 73.7 to 77% carbon, maintains at

17

around 10% for coals with 77 to 88-89% carbon, and then decreases for coals with more

18

than 88-89% carbon. The Cal-Car bonds that generate CH4 are much large in proportion,

19

which increases from 5 to 70% for coals with 73.7 to 81% carbon, and scatters in a

20

range of 60-95% for coals with more than 81% carbon. These data indicate that the

21

cleavage of Cal-Cal bonds in coals is mainly for the generation of products larger than

22

CH4 in molecular weight, such as tar, and the cleavage of Cal-Car bonds are the main

23

source for CH4 generation due to shortened alkyl side chains connected to aromatic

24

structures with increasing coal rank.

25

Figure 10(b) shows that a few H-Cal bonds, less than 5%, in coals are responsible

26

for H2 generation. This may indicate that most of these bonds did not cleave in the

27

pyrolysis but stayed mainly in the volatile products generated from the cleavage of

28

Cal-Cal bonds which are lower than H-Cal bonds in dissociate energy. These volatile

29

products were purged out of the system before experiencing higher temperatures for

30

further cleavage. H2 is mainly derived from the cleavage of H-Car bond during 10


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Energy & Fuels

1

condensation of aromatic-ring structures in coals at temperatures higher than 650 oC.

2

The less than 50% H-Car bonds responsible for H2 generation, with a mean of 32%,

3

indicates that large portions of the bonds did not cleave, they stayed either in the volatile

4

products purged out off the system at low temperatures or in the char because the highest

5

pyrolysis temperature was 900 oC. It may also be possible, however, that some of H

6

generated from the cleavage of H-Car bonds contributed to CH4 formation, as well as to

7

other products, light or heavy. To account the amount of H for CH4 generation, Figure

8

10(c) shows overall H contributed to H2 and CH4 formation by H-Cal and H-Car bonds.

9

Apparently the contribution of H-Cal bonds to H2 and CH4 formation is low, with a mean

10

of around 10%. The contribution of H-Car bonds to H2 and CH4 formation is relatively

11

high; it increases from 10 to 38% for coals with 73.7 to 81% carbon, scatters in a range

12

of 30-55% for coals with 81 to 89% carbon, and then decreases to 20% for the coal with

13

91.9% carbon.

14 15

4. CONCLUSIONS

16

CH4 and H2 generated from pyrolysis of 34 coals in a TGA-MS system at a heating

17

rate of 10 °C/min are ascribed to cleavage of Cal-Cal and H-Cal bonds, respectively, at

18

low temperatures and Cal-Car and H-Car bonds, respectively, at high temperatures.

19

Contribution of these bonds to CH4 and H2 generation is quantified by deconvolving the

20

MS data. It is found that these products result mainly from the cleavage of bonds

21

involving an aromatic carbon, i.e. Cal-Car and H-Car bonds, with release peak

22

temperatures higher than 575 °C. The contribution of bonds involving only aliphatic

23

carbon, i.e. Cal-Cal and H-Cal bonds, is minor, with peak temperatures lower than 550 °C.

24

The amounts of these bonds cleaved to form CH4 and H2 are coal rank dependent.

25

The proportions of Cal-Car bonds that contribute to CH4 formation increase from 5 to 70

26

and then to 95% for coals with carbon contents from 73.7 to 81 and then to 91.9%. The

27

proportions of Cal-Cal bonds that contribute to CH4 formation are much lower, from 2%

28

for lignite with 73.7% carbon, to around 10% for coals with 77 to 88-89% carbon, and

29

then gradually to 0% for coals with 91.9% carbon. The proportion of H-Car bonds

30

contributing to H2 formation are small for lignite, around 9%, but increase to around 11


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

35% for coals with 77 to 88-89% carbon, and then gradually decrease to 20% for coals

2

with 91.9% carbon. The proportions of H-Cal bonds for H2 generation are less than 5%

3

for all the coals.

4

The side structures of bonds containing an aromatic carbon, i.e. Cal-Car and H-Car,

5

are more complex than that of bonds containing an aliphatic carbon, i.e. Cal-Cal and

6

H-Cal, and that of the latter become simpler with increasing coal rank.

7 8

AUTHOR INFORMATION

9

Corresponding Author

10

* E-mail: [email protected]. Tel.: +86-10-64421073. Fax: +86-10-64421077.

11

Notes

12

The authors declare no competing financial interest.

13 14 15 16

ACKNOWLEDGEMENTS The work is financially supported by the National Key Research and Development Program of China (2016YFB0600302).

17 18

REFERENCES

19

(1) Zhong, M.; Zhang, Z.; Zhou, Q.; Yue, J.; Gao, S.; Xu, G. Continuous high-temperature fluidized

20

bed pyrolysis of coal in complex atmospheres: Product distribution and pyrolysis gas. J. Anal.

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Appl. Pyrolysis 2012, 97, 123-129.

22 23 24 25 26 27

(2) Xu, Y.; Zhang, Y.; Wang, Y.; Zhang, G.; Chen, L. Gas evolution charcteristics of lignite during low-temperature pyrolysis. J. Anal. Appl. Pyrolysis 2013, 104, 625-631. (3) Safarova, M.; Kusy, J.; Andel, L. Pyrolysis of brown coal under different process conditions. Fuel 2005, 84, 2280-2285. (4) Zhao, Y.; Hu, H.; Jin, L.; Wu, B.; Zhu, S. Pyrolysis Behavior of Weakly Reductive Coals from Northwest China. Energy Fuels 2009, 23, 870-875.

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(5) Chen, L.; Zeng, C.; Guo, X.; Mao, Y.; Zhang, Y.; Zhang, X.; Li, W.; Long, Y.; Zhu, H.; Eiteneer,

29

B.; Zamansky, V. Gas evolution kinetics of two coal samples during rapid pyrolysis. Fuel Process.

30

Technol. 2010, 91, 848-852. 12


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(6) Porada, S. The influence of elevated pressure on the kinetics of evolution of selected gaseous products during coal pyrolysis. Fuel 2004, 83, 1071-1078. (7) Porada, S. The reactions of formation of selected gas products during coal pyrolysis. Fuel 2004, 83, 1191-1196. (8) Das, T. K. Evolution characteristics of gases during pyrolysis of maceral concentrates of Russian coking coals. Fuel 2001, 80, 489-500. (9) Shuai, Y.; Peng, P.; Zou, Y.; Zhang, S. Kinetic modeling of individual gaseous component formed from coal in a confined system. Org. Geochem. 2006, 37, 932-943. (10) Cramer, B. Methane generation from coal during open system pyrolysis investigated by isotope specific, Gaussian distributed reaction kinetics. Org. Geochem. 2004, 35, 379-392. (11) Arenillas, A.; Rubiera, F.; Pis, J. J. Simultaneous thermogravimetric-mass spectrometric study on the pyrolysis behaviour of different rank coals. J. Anal. Appl. Pyrolysis 1999, 50, 31-46. (12) Bermúdez, J. M.; Arenillas, A.; Luque, R.; Menéndez, J. A. An overview of novel technologies to valorise coke oven gas surplus. Fuel Process. Technol. 2013, 110, 150-159. (13) Elliott, M. A. Chemistry of coal utilization I; A wiley-interscience publication: New York, 1981; pp 702-703. (14) Poutsma, M. L. Free-radical thermolysis and hydrogenolysis of model hydrocarbons relevant to processing of coal. Energy Fuels 1990, 4, 113-131. (15) Charpenay, S.; Serio, M. A.; Bassilakis, R.; Solomon, P. R. Influence of maturation on the pyrolysis products from coals and kerogens. 1. Experiment. Energy Fuels 1996, 10, 19-25. (16) Van Heek, K. H.; Hodek, W. Structure and pyrolysis behaviour of different coals and relevant model substances. Fuel 1994, 73, 886-896. (17) Elliott, M. A. Chemistry of coal utilization I; A wiley-interscience publication: New York, 1981; pp 510. (18) Solomon, P. R.; Hamblen, D. G.; Serio, M. A.; Yu, Z.; Charpenay, S. A characterization method and model for predicting coal conversion behavior. Fuel 1993, 72, 469-488. (19) Luo, Y. Handbook of bond dissociation energies in organic compounds; Boca Raton: CRC Press, 2003, 105-221. (20) Jűntgen, H. Review of the kinetics of pyrolysis and hydropyrolysis in relation to the chemical constitution of coal. Fuel 1984, 63, 731-737. 13


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(21) Wang, M.; Li, Z,; Huang, W.; Yang, J.; Xue, H.; Coal pyrolysis characteristics by TG-MS and its late gas generation potential. Fuel 2015, 156, 243-253.

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(22) Li, X.; Krooss B. M.; Weniger, P.; Littke, R. Liberation of molecular hydrogen (H2) and methane

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(CH4) during non-isothermal pyrolysis of shales and coals: systematics and quantification. Int. J.

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Coal Geol. 2015, 137, 152-164.

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(23) Kříž, V.; Bičáková, O. Hydrogen from the two-stage pyrolysis of bituminous coal/waste plastics mixtures. Int. J. Hydrogen Energy 2011, 36, 9014-9022. (24) Ahmed, I. I.; Gupta A. K. Hydrogen production from polystyrene pyrolysis and gasification: characteristics and kinetics. Int. J. Hydrogen Energy 2009, 34, 6253-6264. (25) Liu, Z. Advancement in coal chemistry: structure and reactivity. Scientia Sinica Chimica 2014, 44, 1431–1435. (in Chinese) (26) Shi, L.; Liu Q.; Guo X.; He W.; Liu Z. Pyrolysis of coal in TGA: Extent of volatile condensation in crucible. Fuel Process. Technol. 2014, 121, 91-95. (27) Miura, K,; Mae K.; Shimada M.; Minami H. Analysis of formation rates of sulfur-containing gases during the pyrolysis of Various coals. Energy Fuels 2001, 15, 629-636.

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(28) Giroux, L.; Charland J. P.; MacPhee J. A. Application of thermogravimetric fourier transform

17

infrared spectroscopy (TG-FTIR) to the analysis of oxygen functional groups in coal. Energy

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Fuels 2006, 20, 1988-1996.

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(29) Perejón, A.; Sánchez-Jiménez, P. E.; Criado, J. M.; Pérez-Maqueda, L. A. Kinetic analysis of

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complex solid-state reactions. A new deconvolution procedure. J. Phys. Chem. B 2011, 115,

21

1780-1791.

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(30) Petrakis, L. Spectral line shapes: Gaussian and Lorentzian functions in magnetic resonance. J Chem. Educ. 1967, 44, 432-436. (31) Shi, L.; Liu, Q.; Guo, X.; Wu, W.; Liu, Z. Pyrolysis behavior and bonding information of coal – A TGA study. Fuel Process. Technol. 2013, 108, 125-132.

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(32) Guo, C. Chemistry of coal; Chemical Industry Press: Beijing, 1992; pp 49. (in Chinese)

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(33) Pope, M. J.; Gregg, S. J. The specific electrical conductivity of coals. Fuel 1961, 40, 123-129.

28

(34) Dryden, I. G. C. Action of solvents on coals at lower temperatures III – behaviour of a typical

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range of British coals towards specific solvents. Fuel 1951, 30, 217-239. (35) Iyengar, M. S.; Lahiri, A. The nature of reactive groups in coal. Fuel 1957, 36, 286-297. 14


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(36) van Krevelen, D. W. Chemical structure and properties of coal XXVIII-coal constitution and solvent extraction. Fuel 1965, 44, 229-242. (37) Dulhunty, J. A.; Penrose, R. E. Some relations between density and rank of coal. Fuel 1951, 30, 109-113 (38) Honda, H.; Ouchi K. Magnetochemistry of coal I- magnetic susceptibility of coal. Fuel 1957, 36, 159-175. (39) Marsh, H. The determination of surface areas of coals- some physicochemical considerations. Fuel 1965, 44, 253-268. (40) Kedzior, S. Methane contents and coal-rank variability in the upper Silesian coal basin, Poland. Int. J. Coal Geol. 2015, 139: 152-164. (41) Kopp, O. C.; Bennett III, M. E.; Clark, C. E. Volatiles lost during coalification. Int. J. Coal Geol. 2000, 44: 69-84. (42) Bustin, R. M.; Guo, Y. Abrupt changes (jumps) in reflectance values and chemical compositions of artificial charcoals and inertinite in coals. Int. J. Coal Geol. 1999, 38: 237-260. (43) Ibarra, J. V.; Moliner, R.; Gavilán, M. P. Functional group dependence of cross- linking reactions during pyrolysis of coal. Fuel 1991, 70, 408-413. (44) Zhou, B.; Shi, L.; Liu, Q.; Liu, Z. Examination of structural models and bonding characteristics of coals. Fuel 2016, 184, 799-807. (45) 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.

21

(46) Arenillas, A.; Pevida, C.; Rubiera, F.; García, R.; Pis J. J. Characterisation of model compounds

22

and a synthetic coal by TG/MS/FTIR to represent the pyrolysis behaviour of coal. J. Anal. Appl.

23

Pyrolysis 2004, 71, 747-763.

24

(47) Shi, L. Study on the cleavage of covalent bonds in coal pyrolysis from various temperature

25

stages. Dissertation for PhD degree, Beijing University of chemical technology, 2014; pp 68-84.

26

(in Chinese)

27 28 29 30

(48) Colin, E.; Snape, C. E.; Ladner, W. R.; Bartle, K. D. Fate of aliphatic groups in low-rank coals during extraction and pyrolysis processes. Fuel 1985, 64, 1394-1400. (49) Kidena, K.; Tani, Y.; Murata, S.; Nomura, M. Quantitative elucidation of bridge bonds and side chains in brown coals. Fuel 2004, 83, 1697-1702. 15


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(50) Obeng, M.; Stock, L. M. Distribution of pendant alkyl groups in the Argonne Premium coals. Energy Fuels 1996, 10, 988-995.

3

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Figure captions

2 3 4

Figure 1. Calibration of CH4 and H2 by MS. (R2: coefficient of determination; SE: standard error).

5

Figure 2. CH4 and H2 evolution detected by MS during pyrolysis of 34 coals.

6

Figure 3. Peak temperature distribution of CH4 and H2 formation in coal pyrolysis.

7

Figure 4. Hydrogen transfer to CH4 and H2 in coal pyrolysis.

8

Figure 5. Relation between bonds’ dissociation energy and pyrolysis peak temperature.

9

Figure 6. Deconvolution of CH4 and H2 curves into 2 sub-curves.

10

Figure 7. R2 for deconvolution of MS curves.

11

Figure 8. The 2σ of Gaussian distribution of sub-curves of CH4 and H2.

12

Figure 9. The yields of CH4 (a) and H2 (b) ascribed to cleavage of two types of bonds.

13

Figure 10. The proportions of chemical bonds in coals that cleaved to generate CH4 and

14

H2 .

15

17


Energy & Fuels

2.5

Peak area of MS signal (A/A)

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

Page 18 of 31

CH4 (m/z=16) Y = 0.4725 X 2 R = 0.9997 SE = 0.0012

2.0

1.5

1.0 H2 (m/e=2) 0.5

Y = 0.1846 X 2 R = 0.9995 SE = 0.0011

0.0 0

1

1

2

3

4

5

6

Injection volume (ml)

2 3

Figure 1. Calibration of CH4 and H2 by MS. (R2: coefficient of determination; SE: standard error).

4

18


High

200

400

600

800

H2 relative intensity ( A/A/g-coal, daf)

Low

High

0

1000

b

0.005

Carbon content (daf)

Low

0

1

a

0.01

Carbon content (daf)

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

CH4 relative intensity ( A/A/g-coal, daf)

Page 19 of 31

200

400

600

800 o

o

Temperature ( C)

Temperature ( C)

2 3

Figure 2. CH4 and H2 evolution detected by MS during pyrolysis of 34 coals.

4

19


1000

Energy & Fuels

1000

800

H2 - major peak

600

CH4

o

Peak temperature ( C)

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

Page 20 of 31

400

H2 - small satellite peak

200 75

1

80

85

90

95

Carbon content (%, daf)

2 3

Figure 3. Peak temperature distribution of CH4 and H2 formation in coal pyrolysis.

4

20


Page 21 of 31

70

Hydrogen content (mg/g-coal, daf)

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

Hydrogen in coal Hydrogen in H2 and CH4

60

Hydrogen in H2 Hydrogen in CH4

50 40 30 20 10 0 75

1 2 3

80

85

90

95


Figure 4. Hydrogen transfer to CH4 and H2 in coal pyrolysis.

4

21


Energy & Fuels

500

Bond dissociation energy (kJ/mol)

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

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H-Car 400 Cal-Car 300

Cal-Cal Cal-O Cal-S H-Cal

200

100 200

1

300

400

500

600

700

800

900

o

Peak temperature of bond cleavage ( C)

2 3

Figure 5. Relation between bonds’ dissociation energy and pyrolysis peak temperature.19,31

4

22


Page 23 of 31

CH4 - Coal #03 (90.8% C)

MS signal relative intensity (A/A/g-coal, daf)

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

H2 - Coal #03 (90.8% C)

P2

P2

P1

P1

CH4 - Coal #15 (85.8% C)

H2 - Coal #15 (85.8% C)

P2

P1 P2

P1

200

1 2 3

400

600

800

200

400

600

o

Temperature ( C)

Figure 6. Deconvolution of CH4 and H2 curves into 2 sub-curves.

4

23


800

Energy & Fuels

1.0

0.8

0.6 2

CH4

R

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

Page 24 of 31

H2

0.4

0.2 a

b

0.0 75

1

80

85

90

95

75

80


2 3

Figure 7. R2 for deconvolution of MS curves.

4

24


85

90

95

Page 25 of 31

400

a

CH4

300 200 P2

100 o

2σ ( C)

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

Energy & Fuels

0 400

P1

b

H2

300 200

P2

100 P1

0

1

75

80

85

90

95


2 3

Figure 8. The 2σ of Gaussian distribution of sub-curves of CH4 and H2.

4

25


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

Yields of CH4 and H2 (mg/g-coal (daf))

Energy & Fuels

60 a

b

CH4 - P2

H2 - P2

CH4 - P1

50

H2 - P1

40 30 20 10 0 75

1

Page 26 of 31

80

85

90

95

75

80

85

90


2 3

Figure 9. The yields of CH4 (a) and H2 (b) ascribed to cleavage of two types of bonds.

4

26


95

Page 27 of 31

100

Proportion of bonds that generate CH4 or H2 (%)

a

80

60 Percentage of Cal-Car bonds

40

in coal for CH4 in P2 Percentage of Cal-Cal bonds in coal for CH4 in P1

20

0 70

75

80

85

90

95


1 100

b


Percentage of H-Car bonds in coal for H2 in P2 Percentage of H-Cal bonds in coal for H2 in P1

80

60

40

20

0 70

75

80

85

90

95


2 100

c

Percentage of H-Car bonds in coals


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

for CH4 in P2 and H2 in P2

80

Percentage of H-Cal bonds in coals for CH4 in P1 and H2 in P1

60

40

20

0 70

3

75

80

85

90

95


4 5

Figure 10. The proportions of chemical bonds in coals that cleaved to generate CH4 and H2.

27


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

Table captions

2 3

Table 1. Yields of CH4 and H2 in coal pyrolysis (mg/g-dry coal).

4

Table 2. Bond dissociation energy of some of Cal-Cal and Cal-Car bonds.

5

Table 3. Proximate analyses and ultimate analyses of the coals.

6

28


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Energy & Fuels

Table 1. Yields of CH4 and H2 in coal pyrolysis (mg/g-dry coal).6-8,12-17

1 Gaseous

Flash pyrolysis

Fluidized-bed reactor

Fixed-bed reactor

Coke oven

products

(1050-1100 oC , 1500 oC/s)

(600-750 oC)

(650 oC)

(950 oC)

CH4

6-25

19-28

12-40

40-48

H2

6-11

1-6

4-16

12-15

2 3

29


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1

Page 30 of 31

Table 2. Bond dissociation energy of some of Cal-Cal and Cal-Car bonds.19 No.

Compounds

Length of alkyl side chain

1

2

2

2

3

3

4

4

5

-

2 3

30


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Energy & Fuels

1

Table 3. Proximate analyses and ultimate analyses of the coals. # 01 02 03 04 05 06 07 08 09 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

2 3 4

Name Taixi Jincheng Gaoping Jinshi Tongye Hebi Sanji Lingchuan Gujiao Pingdingshan Luxian Unknown-1 Unknown-2 Tangshan Wuhaixi Xuanzhong Kailuan Mengxi Zhaogezhuang Shenmu Unknown-3 Laoshidan Zhongliu Pingshuo Youyu Houan Yanzhou Yuling Bulianta Baorixile Huolinhe-1 Huolinhe-2 Chahaer Xiaolongtan

Proximate Analysis (wt.%)

Ultimate Analysis (wt.%, daf)

Mad

Ad

Vdaf

C

H

N

O*

S

0.4 1.7 0.9 1.5 1.1 0.6 0.6 0.7 0.6 0.6 0.6 0.8 4.9 0.9 1.0 7.8 1.0 1.2 0.7 3.2 7.0 0.8 1.3 3.6 3.1 3.4 2.7 1.8 3.9 13.2 17.0 2.2 7.6 16.4

1.8 10.1 18.5 14.4 20.7 7.3 10.5 12.4 9.4 19.8 18.1 15.2 21.4 11.7 10.9 3.0 12.6 11.1 12.2 8.5 2.3 11.8 7.2 19.7 30.2 32.8 2.8 18.9 5.5 6.0 8.5 24.7 12.4 14.5

7.2 4.9 8.7 6.3 11.6 13.6 16.1 7.8 19.3 16.3 20.1 22.6 14.9 32.1 27.8 32.3 32.3 30.5 31.7 41.9 30.8 28.8 28.4 37.1 37.6 39.8 44.7 42.6 36.5 33.4 49.0 48.9 44.5 50.7

91.9 91.8 90.8 90.2 90.1 90.0 89.1 88.9 88.5 88.4 88.1 87.3 86.5 86.0 85.8 85.6 85.5 85.4 85.3 85.1 85.1 85.0 83.9 83.1 82.8 82.7 81.5 81.4 80.3 79.1 77.5 76.6 75.7 73.7

2.1 2.9 3.0 3.0 3.5 3.7 3.8 2.8 3.8 4.1 4.1 4.0 3.8 4.2 4.3 4.2 4.3 4.2 4.3 4.7 4.3 4.3 4.2 4.7 4.8 4.9 5.9 4.8 4.9 3.9 3.7 4.6 4.9 3.9

0.9 1.0 1.2 1.0 1.2 1.2 1.2 0.8 1.3 1.2 1.3 1.2 1.2 1.3 1.2 1.0 1.2 1.2 1.2 1.1 1.0 1.3 1.3 1.3 1.4 1.3 1.3 1.3 0.9 1.1 1.2 1.5 1.9 1.3

5.0 3.9 4.5 3.7 4.8 4.7 4.7 3.9 4.9 5.9 6.2 5.9 7.6 7.8 8.6 9.0 8.0 8.1 7.9 8.8 9.3 8.0 8.6 9.6 9.1 9.8 8.6 9.9 13.7 15.7 17.3 16.6 17.1 20.1

0.1 0.4 0.5 2.1 0.4 0.4 1.2 3.6 1.5 0.4 0.3 1.6 0.9 0.7 0.1 0.2 1.0 1.1 1.3 0.3 0.3 1.4 2.0 1.3 1.9 1.3 2.7 2.6 0.2 0.2 0.3 0.7 0.4 1.0

ad: air-dry basis; d: dry basis; daf: dry-and-ash-free basis. M: Moisture; A: Ash; V: Volatile Matter Content. *: by difference

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Interpretation of Methane and Hydrogen Evolution in Coal Pyrolysis

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