Investigation into the Cleavage of Chemical Bonds Induced by CO2

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Investigation into the cleavage of chemical bonds induced by CO2 and its mechanism during the pressurized pyrolysis of coal Songping Gao, Lingrui Zhai, Yuhong Qin, Zhiqing Wang, Jiantao Zhao, and Yitian Fang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03950 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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Investigation into the cleavage of chemical bonds induced by CO2 and

2

its mechanism during the pressurized pyrolysis of coal

3 4

Songping Gao*,†,‡, Lingrui Zhai‡, Yuhong Qin‡, Zhiqing Wang§, Jiantao Zhao§, Yitian Fang*,§

5

‡State Key Laboratory Breeding Base of Coal Science and Technology Co-founded by Shanxi

6

Province and the Ministry of Science and Technology, Taiyuan University of Technology, Taiyuan

7

030024, P. R. China

8

§Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China

†Taiyuan Institute of technology, Taiyuan 030008, P. R. China

9 10

ABSTRACT

11

A Huo linhe coal sample and its N2-devolatilized char have been pyrolyzed in a

12

pressurized fixed-bed reactor under N2 and 50% CO2/50% N2 atmospheres, respectively.

13

With a view to avoiding any disturbance from the devolatilization process,

14

N2-devolatilized char was used to study the effect of CO2 on the cleavage of chemical

15

bonds. The results show that CO2 intensifies the consumption of H in the char before its

16

release in the form of H2. More H in the char is transferred into gaseous aliphatic

17

hydrocarbons and condensable volatiles, leading to a decrease in the yield of H2 under

18

50% CO2 atmosphere compared with that under N2 at elevated pressure in the range

19

550–900 °C. A CO2 atmosphere is the dominant factor in the release of H from char

20

below 700 °C, such that more H in char induced by CO2 is transferred into volatiles at

21

550 °C than that at 800 °C, above which temperature controls the release of H from char.

22

Thus, CO2 induces the decomposition of more H-containing organic groups to form CH4,

23

C2H6, C2H4, and H-rich low-molecular-weight tar at elevated pressure and 550 °C.

24

Dissociative adsorption of CO2 on coal/char can generate adsorbed active C(O) and

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active Cf at elevated pressure. The electronegativity of the O in active C(O) induces

2

cleavage of adjacent bonds, and the formation of active Cf may induce new structural

3

defects or active sites (Cf) capable of inducing bond cleavage, resulting in the cleavage

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of chemical bonds in the char at lower temperatures than under N2 atmosphere at

5

elevated pressure, such as those of aromatic rings, aliphatic chains of aromatic

6

hydrocarbons, and ether bonds.

7

Keywords: pressurized pyrolysis; CO2; chemical bond; cleavage; mechanism;

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1. INTRODUCTION

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Coal, acts as the main fuel and energy resource in China, will continue to be used,

10

due to its abundant reserves and low cost.1-3 However, the utilizations of coal always

11

associated with the huge CO2 emission, resulting in aggravating environmental

12

concerns.1,2,4 Hence, techniques involving CO2 capture and utilization have been

13

receiving more and more attention.5-9 In particular, oxy-fuel combustion with the

14

recirculation of flue gases (CO2-enriched)10,11 and a novel coal gasification process

15

involving the recycling of CO2 to the gasification system12,13 have been deemed to be

16

promising strategies for reducing CO2 emissions, which take advantage of the

17

characteristics of easy separation of CO2 at high concentration.14 Furthermore, CO2 can

18

be a valuable carbon resource for sustainable development.15,16 For example, catalytic

19

hydrogenation of CO2 leads to value-added products, such as methanol, syngas, and

20

dimethyl ether, which has been considered to be a promising reaction pathway for the

21

utilization of CO2. Therefore, research on fundamental aspects of coal behavior under a

22

CO2-enriched atmosphere has recently received fresh impetus,17,18 due to the distinct


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effects of introduced CO2 on thermochemical processes of coal.12,14,19 Indeed, CO2 is not

2

only a product of coal pyrolysis, but also a reactant (e.g., a gasifying agent) and the

3

pressurized gas for coal transport,12,14 which participates in chemical reactions in the gas

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phase and char gasification.20–23

5

Previous studies have revealed that CO2 affects the composition and nature of

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pyrolysis products and the thermochemical conversion of coal/char.12,14,18,23–29 Under

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CO2 atmosphere, the enhanced mass loss of char caused by its CO2 gasification reaction

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is the main reason for the increased yield of volatiles.12,23–25,27,28 CO2 is beneficial for the

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generation of CH4 and CO and the earlier release of H2,12,29 and accelerates the

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decomposition of sulfur compounds into the gas phase.14,24,30 Char under CO2 exhibits a

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lower reactivity compared to char under N2 during the fast pyrolysis of

12

high-volatile-content bituminous coal,12,18 in good agreement with the differences in the

13

distribution of C-O complexes on its char surface.18 In addition, the mode of action of

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CO2 has mostly been attributed to its gasification of nascent char at temperatures above

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700 °C.27,29,31,32 A CO2 atmosphere surrounding coal/char particles can greatly affect the

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formation of NH3 and HCN through its influence on the availability of H radicals during

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coal pyrolysis,27,31,32 and the chemisorption of CO2 on a nascent char surface could

18

consume H radicals or block access to N sites by H radicals for the formation of NH3

19

and HCN.32 In our previous work, we studied the effect of CO2 on the pyrolysis behavior

20

of lignite at atmosphere pressure.29 Our results showed that CO2 gasification of nascent

21

char destroyed its H-containing structure, and promoted the generation of H radicals,

22

leading to the cracking of chemical bonds, such as those in benzene rings and hydroxyl,


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methyl, and methylene groups. Based on our experimental results29 and those reported in

2

the literature,27,33 we conclude that the CO2 gasification reaction of char can occur at up

3

to 700 °C. Indeed, CO2 promotes the decrease in the yield of char 12,27–29 and affects the

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formation of pyrolysis products below 700 °C,12,29,30 which is unrelated to the CO2

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gasification reaction, and information on the mode of action of CO2 is still limited.

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Previous studies have nevertheless been carried out on the devolatilization process,

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which is apparently accompanied by the cleavage of chemical bonds. Moreover, there

8

are strong interactions between volatiles and char, based on the highly reactive nature of

9

these volatiles and the vulnerable structure of the nascent char,34,35 which affect the

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composition of the pyrolysis products. Thus, it is difficult to distinguish the effect

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between CO2 and volatiles. With a view to better understanding the mode of action of

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CO2, especially its induction of the cleavage of chemical bonds in char below 700 °C,

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N2-devolatilized char has also been studied in this work to avoid any disturbance arising

14

from the devolatilization process. The latter sample was obtained by coal pyrolysis to

15

constant weight under N2 atmosphere at a given temperature and 0.1 MPa. Thus, the

16

thermal cleavage of chemical bonds was completed at this temperature under N2

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atmosphere, indicating that no further volatiles could be generated under such conditions.

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Only those functional groups resistant to thermolysis remained in the N2-devolatilized

19

char. In this way, some less-active chemical bonds, which cannot be cleaved under N2

20

atmosphere and maybe can be cleaved under CO2, are left. These give great convenience

21

for analyzing the effect of CO2 on the cleavage of the chemical bonds. That is to say,

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certain active sites were made available for the analysis of the mode of action of CO2


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under 50% CO2 atmosphere during the pressurized pyrolysis of N2-devolatilized char.

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Therefore, the N2-devolatilized char is suitable to investigate the cleavage of chemical

3

bonds induced by CO2.

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In this study, pyrolysis experiments of Huo Linhe coal and its N2-devolatilized char

5

were carried out in a pressurized fixed-bed reactor under pure N2 and 50% CO2

6

atmospheres, respectively. The distribution of gaseous products and FTIR spectra,

7

elemental analyses, and surface structural properties of the solid products under both

8

atmospheres were comparatively analyzed. The mode of action of CO2 in influencing the

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pyrolysis behavior of coal has been investigated in detail.

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2. EXPERIMENTAL PROCEDURE

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2.1. Coal sample preparation

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Huo Linhe coal (HLH) is a typical lignite originating from Inner-Mongolia in China.

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The raw coal was crushed and sieved to yield particles with sizes in the range 154–180

14

μm. The powder was then dried in a vacuum oven at 383 K for 4 h and stored in an

15

air-tight container. The proximate and ultimate analyses of HLH are shown in Table 1.

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Table 1. Proximate and ultimate analyses of an HLH sample Proximate analysis, wt.ad (%)

Ultimate analysis, wt.daf (%)

M

A

V

FC

C

H

Oa

Stb

N

2.42

20.14

30.21

47.23

81.96

4.80

10.29

1.72

1.23

M: moisture; A: ash; V: volatile matter; ad: air-dry basis; daf: dry ash-free basis a determined by difference; 17

b determined total sulfur.

2.2. Apparatus and procedure of pressurized pyrolysis experiments


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Pyrolysis was carried out in an in-house-constructed pressurized fixed-bed reactor.

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Details of the experimental facility and operation have been described in our previous

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literature reports.36,37 As shown in Figure 1, the pyrolysis apparatus comprises a stainless

4

steel reactor (with quartz-lined tube, i.d. 26 mm), mass-flow controllers, a PID

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temperature controller, a product collection receptacle, and a sample transporter system.

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In addition, some features have been designed to facilitate fast pressurized pyrolysis. For

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example, the stainless steel tube reactor is divided into two chambers by ball valve 9. In

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each run, a sample in a quartz crucible (with a sintered quartz bottom) was suspended

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from a platinum wire and introduced into the upper chamber. The reactor was purged

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with N2 to displace the air, and then pressurized to the desired value with N2, heated to

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the pre-set temperature, and left to stabilize for 30 min. Thereafter, ball valve 9 was

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opened and the sample was rapidly lowered into the isothermal zone by the

13

magnetic-force-driven equipment (sample transporter 15), whereupon pyrolysis occurred.

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At the end of the experiment, the crucible was raised to the upper cold chamber, and the

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char was cooled to room temperature under N2 atmosphere. After depressurizing the

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reactor, the sample was removed, weighed, and stored for analysis. The gases derived

17

from the pyrolysis and gasification were collected in gas bag 7 placed in pressure tank

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20. After the experiment, the inlet and outlet valves (21, 22, and 25) of the pressure tank

19

were closed and valves (26 and 27) were opened to vent the higher pressure tank 20 to

20

the outside atmosphere. The elevated pressure then forced the gas collected in gas bag 7

21

to the exterior gas bag 28 at atmosphere pressure.


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Figure 1. Schematic diagram of pyrolysis apparatus.

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1: temperature controller; 2: furnace; 3: reactor; 4: thermocouples; 5: quartz hanging basket;

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6: quartz tube; 7, 28: air bags; 8: heater band; 9: spherical valve; 13: mass flow controller;

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10, 11, 12, 16, 21, 22, 23, 24, 25, 26 and 27: valves; 14, 15: sample transporter;

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17, 18: high-temperature cut-off valves; 19: flow meter; 20: pressure tank.

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2.3. Fast pyrolysis of coal at elevated pressures

8

In order to study the effect of CO2 on the distribution of H in the pyrolysis products,

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fast pyrolyses of coal at elevated pressures under 50% CO2/ 50% N2 and pure N2

10

atmospheres were carried out by the apparatus showed in Figure 1. The results reveal

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that CO2 can effectively affect the cleavage of chemical bonds in char during coal

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pyrolysis. The procedure was the same as that described in Section 2.2. The reactor was

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heated at a rate of 1000 °C·s-1 from ambient to a prescribed temperature in the range 550

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to 900 °C, with a holding period of 8 min, and pressures of 0.1–1.0 MPa under N2 and

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50% CO2/50% N2 atmospheres.

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As the accuracy of the reactor was very important, several pre-experiments with

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various alterations were performed to improve the performance of the reactor. The

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pyrolysis process conducted in the reactor could produce gas, char, and tar. The results


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showed that the amounts of tar, gas, and char could not be simultaneously determined

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with high accuracy in one apparatus in one run. Thus, the reactors used in this study

3

were specifically designed to produce and collect char and gases in a way to sacrifice the

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accuracy of tar determination, as the latter was not so well as the char or gas, where it

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could be controlled within 0.8%. Hence, the amounts of char and gas could be

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determined with acceptable accuracy, whereas the tar was not quantitatively analyzed in

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this study.

8

2.4. Preparation and pressurized pyrolysis of N2-devolatilized char

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In order to minimize and eliminate the disturbance originated from the

10

devolatilization process, N2-devolatilized char was pyrolyzed under CO2 to investigate

11

the cleavage of the chemical bonds induced by CO2 below 700 °C. While the

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N2-devolatilized char was obtained by coal pyrolysis to constant weight under N2

13

atmosphere at a given temperature and 0.1 MPa (Section 2.4.1.). Thus, the thermal

14

cleavage of chemical bonds under N2 atmosphere was completed at this temperature,

15

indicating that no further volatiles could be generated under such condition, and only

16

those chemical bonds resistant to thermolysis remained in the N2-devolatilized char. By

17

this way, some less-active chemical bonds, which cannot be cleaved under N2

18

atmosphere and maybe can be cleaved under CO2, are left, these give great convenience

19

for analyzing the effect of CO2 on the cleavage of the chemical bonds. That is to say,

20

certain active sites were made available for the analysis of the mode of action of CO2

21

under 50% CO2 atmosphere during the pressurized pyrolysis of N2-devolatilized char.

22

Pressurized pyrolysis experiments of N2-devolatilized char were conducted under 50%


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CO2 and pure N2 atmospheres, respectively (Section 2.4.2.).

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2.4.1. Preparation of N2-devolatilized char

3 4

The preparation procedure for N2-devolatilized char was the same as that described in Section 2.2.

5

Coal pyrolysis was performed at 0.1 MPa in the temperature range 550–900 °C.

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The carrier gas flow rate was set at 500 mL·min-1 (25 °C and 0.1 MPa). The pyrolysis

7

was carried out for 50 min, whereupon the char reached a constant weight.

8

2.4.2. Pressurized pyrolysis of N2-devolatilized char

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Pressurized pyrolyses of N2-devolatilized char under N2 and 50% CO2/50% N2

10

atmospheres were carried out by the apparatus showed in Figure 1, as described in

11

Section 2.2, at a flow rate of 1500 mL·min-1 and 1.0 MPa for 8 min. In particular, the

12

pressurized pyrolysis temperature of the N2-devolatilized char was kept the same as that

13

for its preparation. For example, when the N2-devolatilized char was prepared at 550 °C,

14

the pressurized pyrolysis was also carried out at 550 °C, and the solid residue was

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designated as Residue-550.

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2.5. Physicochemical analysis

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Mercury porosimetry was performed on a Micromeritics AutoPore IV9500 Series

18

instrument, to obtain the apparent surface areas, porosities, and pore volume

19

distributions of the samples. N2 adsorption isotherms were measured at 77.7 K using a

20

Micromeritics Tristar 3000 instrument, and BET surface areas were calculated using the

21

adsorption points at suitable relative p/p0. The elemental contents of char, including C, H,

22

and O, were determined by means of a Vario El Cube elemental analyzer.


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2.6. Infrared analysis of char

2

Changes in the chemical structures of samples were analyzed by means of FTIR

3

spectrometry on a Tensor 27 spectrometer. Samples were prepared at 1% in KBr pellets,

4

and spectra with a resolution of 4 cm−1 were obtained by accumulating 32 scans,

5

subtracting the background spectrum of pure KBr powder.

6

2.7. Analysis of gas composition

7

The contents of C1–C4 hydrocarbons in the gaseous products were analyzed using a

8

Shimadzu GC-14C equipped with an FID detector and an Rt-QPOT capillary column

9

(length 30 m, diameter 0.32 mm). N2, H2, CO, and CH4 were analyzed on another

10

Shimadzu GC-14C equipped with a TCD detector and a column packed with carbon

11

sieves. CH4 was used to calibrate the results obtained on the two instruments. The N2

12

volume percentage in the gas bag and its total volume detected by a mass flow meter

13

were used to compute the total amounts of N2, H2, CO, and CH4.

14

3. RESULTS AND DISCUSSION

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3.1. Effect of CO2 on the distribution of H in the products during coal pyrolysis.

16

3.1.1. Distribution of H in the pyrolysis products at 550 °C. N2 atmosphere 50% CO2 atmosphere

1.5 1.0 0.5 0.1 MPa a

0.0

17 18

550

600

650

700 750 O 800 Temperature ( C)

850

900

N2 atmosphere

1.5

The yield of H2 (wt.% , daf )

2.0

The yield of H2 (wt.% ,daf )

2.5 The yield of H2 (wt.% , 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

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50% CO2 atmosphere

1.2 0.9 0.6 0.3 0.0 550

600

650

700

750

O

0.5 MPa b 800 850 900

Temperature ( C )

1.4

N2 atmosphere

1.2

50% CO2 atmosphere

1.0 0.8 0.6 0.4 0.2 0.0 550

600

650

700

750

O

1.0 MPa c 800 850 900

Temperature ( C )

Figure 2. Yields of H2 at different temperatures and pressures of 0.1(a), 0.5(b), and 1.0(c) MPa.

19

Figure 2(a, b, and c) shows the yields of H2 under both atmospheres at different

20

temperatures and system pressures (e. g. 0.1, 0.5, and 1.0 MPa). From Figure 2, it can be

21

observed that the H2 yield was slightly lower under 50% CO2 atmosphere at 550 °C than


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that of char under N2. However, the H content of char under CO2 was lower than that of

2

char under N2 (Table 2). This implies that CO2 promotes the consumption of more H in

3

the char before it can be released in the form of H2, which is transferred to gaseous

4

aliphatic hydrocarbons and condensable volatiles at 550 °C. Indeed, the H content of

5

light gaseous aliphatic hydrocarbons and condensable volatiles was higher under 50%

6

CO2 atmosphere compared with that under N2 (Table 2). The higher yields of CH4, C2H6,

7

and C2H4 under 50% CO2 atmosphere than under N2, as shown in Figures 3(a) and 4,

8

indicate that CO2 contributes to the formation of gaseous aliphatic hydrocarbons, leading

9 10

Table 2. The H contents of the products from pyrolyses of 1g coal samples under 50% CO2/50% N2 and N2 atmospheres at 550 °C and different pressures. 550 °C-0.1MPa

550 °C-0.5MPa

550 °C-1.0MPa

atmosphere

atmosphere

Content of H in pyrolysis atmosphere products, wt.daf (%) N2

50% CO2

N2

50% CO2

N2

50% CO2

H2

0.11

0.06

0.105

0.046

0.101

0.041

CnHma

0.21

0.31

0.32

0.35

0.37

0.41

Char

2.71

2.26

2.35

2.16

2.31

2.02

Condensable volatilesb

1.77

2.17

2.025

2.244

2.01

2.33

a: CnHm represents gaseous aliphatic hydrocarbons, such as CH4, C2H6, and C2H4. b:Calculated from hydrogen balance, HCondensable volatiles =Hcoal－Hchar－H CnHm－HH2. 11

to higher H content in gaseous aliphatic hydrocarbons. The condensable volatiles mainly

12

comprised tar at 550 °C. Qu et al.38 carried out an experimental study on the pyrolysis of

13

Huo Linhe lignite with a solid heat carrier under N2 atmosphere, and found that the yield

14

of tar reached a maximum at 520 °C. Apparently, more tar should be generated at about


Energy & Fuels

N2 atmosphere

3.5

Yield of CH4 (wt.% , daf )

Yield of CH4 (wt.%, daf )

4.8

a

50% CO2 atmosphere

1.2

O

700 C

b

3.6

2.5

0.8

c

50% CO2 atmosphere

4.0

3.0

1.0

O

4.4

50% CO2 atmosphere

800 C

N2 atmosphere

Yield of CH4 wt.% ,daf )

O

550 C

N2 atmosphere

1.4

3.2 2.8

2.0

0.6

0.1

0.3

0.5

0.7

0.9

0.1

0.3

0.5 0.7 0.9 Pressure (MPa)

0.1

1.1

0.3

0.5

0.7

0.9

1.1

Pressure (MPa)

0.3 0.2

0.10 C2H6

C2H4

N2 atmosphere

0.1

50CO2 atmosphere

550

600

650

700

750

800

O

Temperature ( C)

850

900

0.05 0.00

0.18

0.6

0.12 C2H4

0.4

0.06

C2H6 N2 atmosphere

0.2

0.00

50 CO2 atmosphere

550

600

650 700 750 800 O Temperature ( C)

850

1.0

900

c

0.30

1.0 MPa

0.24

0.8

0.18

0.6 0.12 C2H4

0.4 C H 2 6 N2 atmosphere

0.2

0.00


550

600

650

700

750O 800

0.06

850

Yields of C2H4 (wt.%, daf)

0.15

0.4

0.24

b 0.5 MPa

0.8

Yields of C2H6 (wt.%, daf )

0.20

0.1 MPa

Yields of C2H4 (wt.% , daf )

a 0.5


Figure 3. Yields of CH4 at different pressures and temperatures of 550(a), 700(b), and 800(c) °C. 0.6

3

1.1

Pressure (MPa)


1 2


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900

Temperature ( C)

4 5

Figure 4. Yields of C2H6 and C2H4 at different temperatures and pressures of 0.1(a), 0.5(b), and 1.0(c) MPa.

6

550 °C and 0.1 MPa under 50% CO2 atmosphere because of the intensified coal

7

pyrolysis by CO2.29 Luo et al.19 studied the formation of tar during coal pyrolysis under

8

N2 and CO2 atmospheres at elevated pressures, and they found that tar yield under CO2

9

atmosphere at 0.1 MPa was higher than that under N2 atmosphere, while lower at 1.0

10

MPa. Furthermore, the decreasing tar yields with increasing pyrolysis pressure can be

11

attributed to increasingly intensive CO2–tar reforming and the secondary pyrolysis of

12

volatiles.19,39 Therefore, the decrease in the tar yield and the increase in the H content in

13

tar under CO2 atmosphere at elevated pressures indicate that CO2 promotes the

14

generation of enriched-H low-molecular-weight tar at elevated pressure and 550 °C. It

15

follows from the above results that more H·free radicals induced by CO2 are consumed

16

to stabilize free radical fragments from coal pyrolysis to form condensable products,

17

such as tar, along with gaseous aliphatic hydrocarbons, resulting in a lower yield of H2

18

under 50% CO2 atmosphere than under N2 at 550 °C. That is to say, CO2 promotes the

19

decomposition of H-containing groups in the char to form H-containing radicals, such


Page 13 of 51 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

as ·CH2-R-CH2·, R-CH2·, CH3·, CH2·, H·, and so on, and these further react to form

2

more CH4, C2H6, C2H4, and enriched-H low-molecular-weight tar at elevated pressure,

3

suggesting that more H·free radicals induced by CO2 were involved in coal pyrolysis

4

before H in the char can be released in the form of H2.

5

3.1.2. Distribution of H in the pyrolysis products at 800 °C.

6

Table 3. The H contents of the products from pyrolyses of 1g coal samples under

7

50% CO2/50% N2 and N2 atmospheres at 800 °C and different pressures. 800 °C-0.1MPa

800 °C-0.5MPa

800 °C-1.0MPa

atmosphere

atmosphere


50% CO2

N2

50% CO2

N2

50% CO2

H2

1.04

1.38

1.02

0.87

0.99

0.73

CnHma

0.76

0.81

1.08

1.14

1.13

1.27

Char

1.38

1.12

1.04

0.97

0.99

0.87


1.62

1.49

1.66

1.82

1.66

1.93

a: CnHm represents gaseous aliphatic hydrocarbons, such as CH4, C2H6, and C2H4. b: Calculated from hydrogen balance, HCondensable volatiles = Hcoal－Hchar－H CnHm－HH2 8

The yields of H2 under 50% CO2 atmosphere were higher than those under N2

9

atmosphere at 0.1 MPa, but lower at 1.0 MPa above 700 °C (Figure 2). It is known that

10

coal pyrolysis under inert atmosphere slowly produces H·free radicals,32 whereas CO2

11

gasification of char can destroy its H-containing structure, facilitating the generation of

12

H·free radicals to form volatiles.29,31 This accounts for the lower H content of char

13

under CO2 than that of char under N2 above 700 °C, as shown in Table 3.

14

Pyrolysis at 800 oC and 0.1 MPa led to a higher yield of H2 (Figure 2(a)), a higher

15

content of H in gaseous aliphatic hydrocarbons, and a lower content of H in condensable


Energy & Fuels

1

volatiles under 50% CO2 atmosphere compared with that under N2 atmosphere (Table 3).

2

The results indicated that CO2 promoted the generation of more H-rich gaseous products

Yields of volatiles (wt.%, 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

Page 14 of 51

O

30 800 C

N2 atmosphere 50% CO2 atmosphere

25 20 15 10

0.5

0.1

3

1.0

Pressure ( MPa)

4

Figure 5. Yields of volatiles under 50% CO2/50% N2 and N2 atmospheres at different pressures and

5

800 °C.

6

at 800 °C and 0.1 MPa. This can be attributed to the fact that the CO2 gasification

7

reaction of char intensifies coal pyrolysis to generate more H·free radicals and volatiles.

8

Furthermore, the pore structures of char play a crucial role in the gasification process

9

that mainly includes the gaseous internal diffusion and surface reaction process.40 The

10

pore networks of char can provide the reactive surfaces and transportation channels for

11

reactant gases and product gases. Figure 5 presents the increase in the yields of volatiles

12

under 50% CO2 atmosphere than that under N2 atmosphere, and the growth trend

13

becomes decreased with the pressure increasing from 0.1 to 1.0 MPa. These results are

14

consistent with the increase in the SBET and Vp of char under CO2 than that under N2,

15

which displays the decreased trend with the pressure increasing, as shown in Figures 6

16

and 7. Thus, it is deduced that CO2 can promote the porosity evolution in the char, and

17

make the carbon structure of the char become looser. Meanwhile, porosity can not only

18

motivate rapid evolution of gaseous products, but also be helpful for CO2 diffusing into

19

the channels, endowing more CO2 molecules accessible to the active sites in the char.


Page 15 of 51

2

generating more H2, CH4, C2H6, and C2H4 at high temperatures and 0.1 MPa, as shown

3

in Figures 2(a), 3(c), and 4(a). In addition, because the formation of simple gaseous

4

hydrocarbons is thermodynamically more favorable than that of H2,41 the higher yield of

5

H2 and the lower H content in char under CO2 indicate that the H·free radicals, induced

6

by CO2 gasification reaction of char, are generated in sufficient numbers to stabilize the

7

free radical fragments from coal pyrolysis. That is to say, CO2 promotes the generation

8

of more H·free radicals to participate in coal pyrolysis before H in the char can be

9

released in the form of H2.

10

200 150 Increase in SBET of char

100

under CO2 than that underr N2

50

o

800 C 0.2

0.4

0.6

0.8

1.0

0.20


3

50% CO2 atmosphere

under both atmospheres ( m /g )

2

N2 atmosphere

250

vpof char and Increases in vpof chars

This intensifies the decomposition of the H-containing organic groups in char,


1

SBETof char and Increase in SBET of chars

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

0.16

Increases in Vpof char under CO2than that under N2

0.12 0.08 0.04

o

800 C 0.2

0.4

0.6

0.8

1.0

Pressure (MPa)

Pressure (MPa)

11

Figure 6. SBET of chars and Increases in SBET

12

of char under CO2 than that under N2.

Figure 7. Vp of chars and Increases in Vp of char under CO2 than that under N2.

13

At 800 °C and 1.0 MPa, the distribution of H in the pyrolysis products shows the

14

same tendency as that at 550 °C under 50% CO2 atmosphere. The lower yields of H2

15

indicated in Figure 2(b, c), the higher yields of CH4, C2H6, and C2H4 indicated in Figures

16

3 and 4, and the higher H contents in gaseous aliphatic hydrocarbons and condensable

17

volatiles (Table 3), suggest that CO2 promotes the consumption of more H·free radicals

18

before H in the char can be released in the form of H2 at elevated pressure, especially at

19

higher temperature. Firstly, elevated systemic pressure can help the diffusion of CO2 into

20

the interior of the char, promoting sufficient contact for it to react at the active sites,


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

Page 16 of 51

1

resulting in the generation of more H·free radicals and volatiles than under N2 (Figure

2

5). Secondly, although the specific surface area and the pore volume of char under CO2

3

are higher than those of char under N2 at 800 °C and different pressures, the increased

4

magnitudes display the decreased trend with the pressure increasing, as shown in

5

Figures 6 and 7. This implies that the smaller diffusion channels improve the mass

6

transfer resistance for the evolution of volatiles at 1.0 MPa compared with that at

7

0.1MPa. Furthermore, elevated pressure inhibits the rapid release of volatiles from the

8

interior of particles, and the residence time of volatiles and H·free radicals in the char

9

particle pores increases, thus the reaction of auto-hydrogenation is enhanced and the

10

secondary

cracking

of

volatiles

is

further

promoted,

resulting

in

more

11

low-molecular-weight H-containing volatiles are formed. For example, the reforming

12

reaction between CO2 and primary tars is enhanced with increasing pressure, and

13

radicals, such as CH3·derived from the reforming reaction, can increase the number of

14

methyl substituents on aromatic rings,19 leading to an increase in the distribution of H in

15

tar. Therefore, the decreases in the yields of H2 and the H contents in char under CO2

16

with increasing pressure compared with those under N2 indicate that CO2 promotes the

17

consumption of H in char and its transfer into gaseous aliphatic hydrocarbons and

18

condensable volatiles at elevated pressure.

19

3.1.3. Effect of CO2 on the release of H from char at different temperatures.

20

Atmosphere and temperature are the main influences on the release of H from char

21

at the same pressure. However, the contribution of atmosphere to the release of H from

22

char is temperature-dependent. Figure 8 shows the contents of H in the char under the


Page 17 of 51

1

respective atmospheres at different temperatures and pressures during pyrolysis of coal.

2

It can be seen that the decrease in H content of char under CO2 compared to char under

3

N2 at 550 °C was greater than that at 800 °C at the same pressure. This implies that the Content of H in chars (wt. % , 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

3.0 O

O

550 C

2.4

5

50% CO2 atmosphere

1.8 O

800 C

1.2

O

800 C

0.6 0.1

4

N2 atmosphere

550 C

1.0 0.1 Pressure ( MPa)

1.0

Figure 8. Contents of H in chars under 50% CO2/50% N2 and N2 atmospheres at different

6

pressures and temperatures of 550 and 800°C.

7

CO2 atmosphere is the dominant factor for the release of H from char at 550 °C, whereas

8

temperature becomes the dominant factor at 800 °C, leading to a smaller difference in

9

the H contents of the chars under the respective atmospheres. Therefore, CO2 induces

10

more H in the char to be transferred into volatiles at 550 °C than at 800 °C. In other

11

words, CO2 induces the decomposition of more H-containing organic groups at 550 °C

12

to form CH4, C2H6, C2H4, and tar. Our previous studies showed that the CO2 gasification

13

reaction of Huo Linhe coal occurred at temperatures above 700 °C,29 which is consistent

14

with the results reported by Tromp et al., 33 who found that the endothermic gasification

15

of CO2 occurred above 1050 K (777 °C). Evidently, the CO2 gasification reaction of char

16

is not responsible for the effect of CO2 on the release of H from char at 550 °C.

17

In summary, CO2 intensifies the consumption of H in the char before it can be

18

released in the form of H2. More H in the char is transferred into gaseous aliphatic

19

hydrocarbons and condensable volatiles, leading to decreases in the yield of H2 under 50%


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

Page 18 of 51

1

CO2 atmosphere at elevated pressure and in the temperature range 550–900 °C. The CO2

2

atmosphere is the dominant factor in the release of H from char at 550 °C, such that

3

more H in the char is transferred into volatiles at 550 °C than at 800 °C, where

4

temperature becomes the dominant factor. Thus, CO2 induces the decomposition of more

5

H-containing

6

low-molecular-weight tar at 550 °C.

7

3.2 Cleavage of chemical bonds induced by CO2 during the pressurized pyrolysis of

8

N2-devolatilized char.

organic

groups

to

form

CH4,

C2H6,

C2H4,

and

enriched-H

9

In order to minimize and eliminate the disturbance originated from the

10

devolatilization process, N2-devolatilized char was pyrolyzed under 50% CO2

11

atmosphere to investigate the cleavage of the other chemical bonds induced by CO2

12

below 700 °C. N2-devolatilized char was obtained by coal pyrolysis to constant weight

13

under N2 atmosphere at a given temperature and 0.1 MPa. Thus, the thermal cleavage of

14

chemical bonds was completed at this temperature under N2 atmosphere, indicating that

15

no further volatiles could be generated under such condition. And only those functional

16

groups resistant to thermolysis remained in the N2-devolatilized char. In this way, some

17

less-active chemical bonds, which cannot be cleaved under N2 atmosphere and maybe

18

can be cleaved under CO2, are left, these give great convenience for analyzing the effect

19

of CO2 on the cleavage of the chemical bonds. In a word, certain active sites are made

20

available for analyzing the effect of CO2 on the cleavage of chemical bonds in

21

N2-devolatilized char at this given temperature under 50% CO2 atmosphere during the

22

pressurized pyrolysis.


Page 19 of 51

1

3.2.1. Distribution of pyrolysis gaseous products under the respective atmospheres.

2

Pressurized pyrolysis experiments on N2-devolatilized char were conducted under

3

50% CO2 and pure N2 atmospheres, respectively. The contents of H2, CH4, and CO in

4

the pyrolytic gaseous products evolved from pyrolyses of N2-devolatilized char under

5

the respective atmospheres at 1.0 MPa were analyzed by gas chromatography. Notably, H2, CH4, and CO became virtually undetectable under N2 atmosphere

7

during pyrolyses of N2-devolatilized chars at 1.0 MPa in the temperature range 550–

8

900 °C, whereas there was a discernible increase in the amounts of H2, CH4, and CO

9

under 50% CO2 atmosphere, as shown in Figures 9 and 10. This indicates that the

10

pyrolysis temperature has little influence on the cleavage of chemical bonds in

11

N2-devolatilized char under N2 atmosphere, because the temperature and atmosphere of

12

the pressurized pyrolysis were the same as those in the preparation of the

13

N2-devolatilized char, suggesting that the thermal cleavage of chemical bonds was

14

completed at this temperature under N2 atmosphere. However, when the N2-devolatilized

15

char was pyrolyzed at the same temperature and 1.0 MPa under 50% CO2 atmosphere, 0.30

50% CO /50%N atmosphere 2

0.25

H

2

2

at 1.0 MPa

CH

4

0.20 0.15

during the pyrolysis of the N2-devolatilized char

2

and CH

4

(wt. % , daf )

6

0.10

Yields of H

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

0.05 0.00 550

16

600

650

700

750

800

850

900

O

Temperature ( C)

17

Figure 9. Yields of H2 and CH4 at different temperatures under both atmospheres CO2+C

18

during pressurized pyrolysis of N2-devolatilized char.


2.5

Page 20 of 51

50% CO2/50% N2 atmosphere at 1.0 MPa

2.0

O

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 CO at 550 and 600 C (wt. %, daf )

Energy & Fuels

1.5

1.0

0.5

550

Temperature

(OC )

600

1 2 3

Figure 10. Yields of CO at 550 and 600 °C under both atmospheres during pressurized pyrolysis of N2-devolatilized char.

4

CH4, H2, and CO were detected (Figures 9 and 10), especially below 700 °C. Apparently,

5

CO2 induces the cleavage of chemical bonds below 700 °C, such as methyl, methylene,

6

and ether bonds, leading to the evolution of CH4, H2, and CO, which supplements our

7

previous results.29 As described above, CO2 gasification reaction of char above 700 °C

8

destroys its H-containing structure and promotes the cleavage of chemical bonds therein.

9

It had previously been established that CO2 gasification reaction of such samples can

10

occur at temperatures above 700 °C, but the information on the mode of action of CO2

11

below 700 °C was not revealed in our previous works.

12

3.2.2. Analysis of FTIR spectra of the solid residues from pressurized pyrolyses of

13

N2-devolatilized chars.

14

Figures 11–13 show FTIR spectra of the solid residues (residue-T) obtained from

15

pressurized pyrolyses of N2-devolatilized char at 1.0 MPa under the respective

16

atmospheres. Compared with those under N2 atmosphere, the C-H vibration absorption

17

intensities of aliphatic hydrocarbons of residue-550, residue-600, and residue-700 under

18

50% CO2 atmosphere were weaker at =2960, 2920, and 2850 cm-1, implying that

19

cleavage of methyl and methylene moieties occurred at a lower temperature under CO2

20

atmosphere than under N2. For example, the alkyl side chains of aromatic rings in the


Page 21 of 51

1

char were cracked at temperatures above 700 °C under N2 atmosphere.42 The presence of

2

absorption at =779 and 799 cm-1 reveals the substituted properties of aromatic rings,

3

the ortho tri-substituted (3H) and the ortho di-substituted (4H), respectively,43 at which

4

the weaker absorptions of the residue under 50% CO2 atmosphere compared to those

5

under N2 ( Figures 15–17) show that CO2 can indeed induce the cleavage of aliphatic

6

side chains on the aromatic rings in the range 550–700 °C. Therefore, CO2 may destroy 100.0

100.0

N2 CO2

99.9

2956

T (%)

Q

T(%)

E

CO2

99.7 99.6

2960 2850

99.5 99.4

2850

2940

2910

2880

2

1.0 MPa

2

99.6

2820

2970

50%CO atmosphere

2920

2940

2910

2

2880

2850

2820

-1

wavenumber (cm)

N500 (´‫؟‬N)

Figure 11. FTIR spectra of Residue-550 at 3000–2800 cm-1. 2970

N atmosphere

o

600 C

50%CO atmosphere

2920

2940 2910 2880 -1 wavenumber (cm )

7

2850

2

1.0 MPa 2970

99.8

99.7

N atmosphere

o

550 C

99.3

8 9

N2

99.9

99.8

2850

N700 (´‫؟‬N)

Figure 12. FTIR spectra of Residue-600 at 3000–2800 cm-1.

2820

100.0

700-1.0MPa

AC ©£2 ‫¨ص‬£‫ذق‬ 007÷×‫سأ‬

100.0

2 ‫ذق ص‬ S

99.9

T (%)

T (%)

99.9

K

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

99.8

2960

99.8 2960

2850

2850

O

550 C O

99.7

600 C O

700 C

N atmosphere

o

2

700 C

99.7

2970

10 11 12

2920

1.0MPa

50%CO atmosphere

O

800 C

(´‫؟‬N) 99.6 N600 1.0 MPa

O

2

2940

2910

2880

2850

2820

-1

2960

2920

Figure13. FTIR spectra of Residue-700. at 3000–2800 cm .

2880

-1

2840

2800

wavenumber(cm )

wavenumber (cm )

-1

900 C

2920

Figure 14. FTIR spectra of Residues under 50% CO2 atmosphere at 3000–2800 cm-1.

13

rich aliphatic chain structure of char to promote the generation of CH3·or CH2·and

14

H·free radicals, leading to the generation of more CH4, as shown in Figure 9. Moreover,

15

Figure 14 shows that the degree of cleavage of methyl and methylene groups was nearly

16

identical above 700 °C, implying that the pyrolysis temperature dictated the cleavage of

17

aliphatic chains. This corroborates the abovementioned finding that CO2 is the dominant


O-

Energy & Fuels

1

factor in inducing the decomposition of more H-containing organic groups at 550 °C,

2

whereas temperature becomes the dominant factor at 800 °C, and the effect of the

3

atmosphere diminishes.

4

The weaker absorptions of residue-550 and residue-600 at =1590 cm−1 under 50%

5

CO2 atmosphere (Figures 15 and 16), assigned to -C=C- stretching vibrations of the

6

aromatic rings,44 indicate the cleavage of more aromatic rings at 550 and 600 °C, which

7

occurs at lower temperature compared to that under N2 atmosphere. This may be

8

because the reaction of CO2 at active sites in char loosens the char structure and

9

destabilizes aromatic ring systems,45 facilitating their decomposition. The absorptions of the ether bonds in residue-T at =1100 and 1042 cm-1 under 50%

10 11

CO2 atmosphere are weaker than those under N2 between 550 and 700 °C, as shown in

12

Figures 15–17, which implies that CO2 promotes the cleavage of ether bonds at lower

13

temperature to generate CO. However, under N2 atmosphere, CO became undetectable at 100

100 1424

80

90

1590 779

1420

1590

799

80

799

60

779

70

T (%)

T (%)

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 22 of 51

40

60 50

20

40 0

N atmosphere 50%CO atmosphere

-20 1800

14 15 16

1100

1400

30

550 C

1042

1.0 MPa

2

1600

N atmosphere

o

2

1200

1000

800

600

20 1800

o

2

600 C

50% CO atmosphere 2

1600

1400

1100

1200

1042

1000

1.0 MPa

800

-1

-1

wavenumber (cm)

wavenumber (cm )

Figure 15. FTIR spectra of Residue-550 -1

at 1800–600 cm .



Page 23 of 51

100

100 1590

1420

1424

90

1590

T (%)

600 C O

1042

O

550 C

900 C

O

600 C

40

O

800 C

O

700 C

O

550 C

O

40

N atmosphere

30

50%CO atmosphere

o

2

3

O

60

60 50

1 2

779 799

799 779

70

1800

O

700 C

80

80

T (%)

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

2

1600

1100

1042

800 C

20

700 C

1100

O

900 C

1042

1.0MPa

1.0 MPa

1400 1200 1000 -1 wavenumber ( cm )

800

0 1800

1600

at 1800–600 cm .

1200

1000

800

600

-1

wavenumber (cm)

Figure 17. FTIR spectra of Residue-700. Figure 18. -1

1400

FTIR spectra of Residues under 50%

at CO2 atmosphere at 1800–600 cm-1.

4

550 and 600 °C during the pressurized pyrolysis of N2-devolatilized char, because the

5

ether, hydroxyl functionalities, and heterocyclic oxygen-containing structures are the

6

precursors for the high-temperature CO evolution during coal pyrolysis,40,46–48 and these

7

structures split off CO only at high temperatures above 700 °C.46–48 The energy provided

8

at 550 and 600 °C is insufficient for this process. On the contrary, under 50% CO2

9

atmosphere, CO is detected below 700 °C, as shown in Figure 10. This implies that CO2

10

lowers the stability of the structures of the precursors for the high-temperature CO

11

evolution, such as ether bonds (Figures 15–17), such that they can be cleaved with the

12

generation of CO at lower temperatures. Tromp et al.33 studied quantitative heat effects

13

under H2 and CO2 atmospheres, and found that the heat effects under a CO2 atmosphere

14

were quite similar to those under an inert atmosphere at up to 1050 K (777 °C),

15

indicating that the endothermic gasification reaction of char with CO2 does not

16

contribute to the generation of CO. That is to say, CO2 can induce the cleavage of

17

O-containing bonds, such as ether bonds, to form CO below 700 °C.

18

Above 700 °C, the characteristic bands of C-O-C in Ar-O-C and Ar-O-Ar at =1042

19

cm-1 largely vanish (Figure 18), further corroborating that temperature is the main

20

controlling factor for the cleavage of ether bonds. The absorption intensity at =1100


O550 (CO2£‫؛‬N2=1:)

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

Page 24 of 51

1

cm-1 becomes stronger above 700 °C, which may be attributed to the vibration

2

absorption of Si-O in ash.29

3

In summary, a CO2 atmosphere promotes the decomposition of more H-containing

4

and O-containing organic groups below 700 °C, whereas the effect of temperature is

5

dominant above 700 °C. CO2 can induce the cleavage of more chemical bonds at lower

6

temperatures than under N2 atmosphere, such as those of aromatic rings and aliphatic

7

chains of aromatic hydrocarbons, as well as ether bonds.

8

3.3 Mechanistic analysis of chemical bond cleavage induced by CO2

9 10

Chemical bond cleavage induced by CO2 includes the following two steps: (1) Activation of CO2 to form active C(O) and active Cf on char

11

Firstly, CO2 can be adsorbed on char. The O in the adsorbed CO2 can react with C

12

at active sites on char/coal under CO2 atmosphere, leading to dissociative adsorption of

13

CO2,49 as described by the following reactions:

14

C f  CO2  C (O)  CO

15

C (O)  C f  CO

16 17 18 19

(R.1) （R.2）

Cf represents an active site, at which oxygen-containing gases can potentially be adsorbed. C (O) represents a carbon–oxygen complex formed after the chemical adsorption of oxygen.

20

Secondly, Dissociative desorption of CO2 generates new active sites on the char

21

surfaces, e.g., -(Cf)solid and -(C(O))solid.40,50,51 Some of the adsorbed oxygen can form

22

carbon–oxygen complexes C(O) on the char,33,49 such as (CO)ad and other O-containing


Page 25 of 51 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

species.

2

(2) Cleavage of chemical bonds induced by the active C(O) and active Cf

3

Firstly, the inductive effect of active C(O)

4

CO2 can be adsorbed at the active sites of a coal/char surface, yielding CO,

5

adsorbed C(O), and adsorbed OH. While the adsorbed OH depends on the available Had

6

on the char surface.50 The strong electronegativity of O in active C(O) attracts electron,

7

also leads to increasing the electron density of the C in C(O). Thus, the chances of

8

cleavage of the bond linked to C(O) is increased. This is consistent with the results

9

reported by Kim et al.,52 who found that CO2 promoted the pyrolysis of functional

10

groups connected to C(O), resulting in enhanced weight loss from char during the

11

pyrolysis of phenol-formaldehyde under CO2 atmosphere. Intermediates such as (CO)ad

12

and (C(O))ad, generated from the dissociative desorption of CO2, would activate

13

aromatic ring systems, these also is helpful for further reactions.53 That is to say, the

14

oxygen derived from CO2 contributes to the oxygenation of aromatic ring systems. The

15

adsorbed active C(O) and OH can then induce the cleavage of a series of chemical bonds,

16

as shown in Figures 19–21. Besides, CO2 can prevent active sites on the char from

17

undergoing mutual polymerization.33,54,55 This allows the involvement of more H2 in the

18

hydrogenation of coal pyrolysis, further improving the degree of pyrolysis. CO CH2

19 20

CH3

CO2

CH2

CH3 (O)

HO CH2

H2C

CH2

CH2

Coal / char

Figure 19. Example of cleavage of aliphatic chains on aromatic hydrocarbons induced by CO2.


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

Page 26 of 51

CO

H2COCH2

CO2

OOH

OH

(O) H2COCH2

CHOCH2

OH CH2

[O] CHOCH2

OO

+

CH

Coal / char

1 2

Figure 20. Example of cleavage of ether bonds induced by CO2. H2O

CO

R

CO2

R

(O)

O

O

OH R

[O]

R

[O]

R

OH

COOH

+

OH COOH O

3 4 5

O

Coal / char

Figure 21. Example of cleavage of aromatic rings induced by CO2.

Secondly, the inductive effect of active Cf

6

Active Cf can also induce the cleavage of chemical bonds. Slaghuis et al.51 studied

7

the increased reactivity of char upon pyrolysis in a reactive atmosphere. They concluded

8

that during CO2 gasification, the removal of a carbon atom would not only create a new

9

active site (e.g., a free radical), but may also cause the remaining structure to be

10

rearranged in the vicinity of the original active site, which in turn may lead to new

11

structural defects or active sites(Cf), further accelerating the cleavage of chemical bonds.

12

In short, the dissociative adsorption of CO2 on coal/char can generate adsorbed

13

active C(O) and active Cf. The electronegativity of O in active C(O) induces cleavage of

14

adjacent bonds, and the formation of active Cf may lead to new structural defects or

15

active sites (Cf) capable of inducing bond cleavage. Therefore, active C(O) and active Cf

16

induce the cleavage of chemical bonds at lower temperatures than under an N2

17

atmosphere, such as those of aromatic rings, aliphatic chains of aromatic hydrocarbons,

18

ether moieties, and methyl and methylene groups.

19

4. CONCLUSIONS


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Energy & Fuels

1

A Huo Linhe coal sample and its N2-devolatilized char have been pyrolyzed in a

2

pressurized fixed-bed reactor under N2 and 50% CO2/50% N2 atmospheres, respectively.

3

With a view to minimizing the interference of the devolatilization process,

4

N2-devolatilized char has been used to study the effect of CO2 on the cleavage of

5

chemical bonds. The experimental results from this study not only provide further

6

evidence to support our proposed mechanisms for the effect of CO2 on coal pyrolysis,29

7

but also offer new insight.

8

(1) CO2 enhances the consumption of H in the char before it can be released in the

9

form of H2. More H in the char is transferred to gaseous aliphatic hydrocarbons and

10

condensable volatiles, leading to a decrease in the yield of H2 under 50% CO2

11

atmosphere at elevated pressure in the temperature range 550–900 °C.

12

(2) A CO2 atmosphere is the dominant factor favoring the release of H from char

13

below 700 °C, such that more H in char is transferred to volatiles at 550 °C than at

14

800 °C, above which temperature becomes the dominant factor. Thus, CO2 induces the

15

decomposition of more H-containing organic groups in char to form CH4, C2H6, C2H4,

16

and enriched-H low-molecular-weight tar at elevated pressure and 550 °C.

17

(3) The dissociative adsorption of CO2 on coal/char can generate adsorbed active

18

C(O) and active Cf. The electronegativity of O in active C(O) induces cleavage of

19

adjacent bonds, and the formation of active Cf may introduce new structural defects or

20

active sites(Cf) capable of inducing bond cleavage. As a result, chemical bonds in char

21

may be cleaved at lower temperatures than under N2 atmosphere, such as those of

22

aromatic rings, aliphatic chains of aromatic hydrocarbons, and ether moieties.


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

■ AUTHOR INFORMATION

2

Corresponding Authors

3

*Phone: +86 0351 4040492.

4

E-mail: [email protected] (Yitian. Fang),

5

*E-mail: [email protected] (Songping. Gao)

6

ORCID

7

Songping Gao: 0000-0003-2989-8267

8

Notes

9

The authors declare no competing financial interest.

10

■ ACKNOWLEDGMENTS

11

This work was financially supported by the National Natural Science Fund of

12

China (21676289), the State Key Laboratory Breeding Base of Coal Science and

13

Technology co-founded by Shanxi Province and the Ministry of Science and Technology,

14

the Taiyuan University of Technology (MKX201503), and Research Supported by the

15

CAS/SAFEA International Partnership Program for Creative Research Teams.

16

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Figure Captions

1 2

Figure 1. Schematic diagram of the pyrolysis apparatus.

3


4

Figure 3. Yields of CH4 at different pressures and temperatures of 550(a), 700(b), and 800(c) °C.

5

Figure 4. Yields of C2H6 and C2H4 at different temperatures and pressures of 0.1(a), 0.5(b), and 1.0(c)

6 7 8 9

MPa. Figure 5. Yields of volatiles under 50% CO2/50% N2 and N2 atmospheres at at different pressures and 800 oC. Figure 6 SBET of chars and Increases in SBET of char under CO2 than that under N2.

10

Figure 7 Vp of chars and Increases in Vp of char under CO2 than that under N2.

11

Figure 8. Contents of H in chars under 50% CO2/50% N2 and N2 atmospheres at different pressures

12

and temperatures of 550 and 800°C.

13 14 15 16

Figure 9. Yields of H2 and CH4 at different temperatures under both atmospheres during pressurized pyrolysis of N2-devolatilized char. Figure 10. Yields of CO at 550 and 600 °C under both atmospheres during pressurized pyrolysis of N2-devolatilized char.

17


18


19


20

Figure 14. FTIR spectra of Residues at 1.0MPa and different temperatures under 50% CO2

21

atmosphere at 3000–2800 cm-1.

22


23


24


25

Figure 18. FTIR spectra of Residues at 1.0MPa and different temperatures under 50% CO2

26

atmosphere at 1800–600 cm-1.

27

Figure 19. Example of cleavage of aliphatic chains on aromatic hydrocarbons induced by CO2.

28

Figure 20. Example of cleavage of ether bonds induced by CO2.

29

Figure 21. Example of cleavage of the aromatic rings induced by CO2.

30 31


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Energy & Fuels

1

2 3

Figure 1. Schematic diagram of pyrolysis apparatus.

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

1: temperature controller; 2: furnace; 3: reactor; 4: thermocouples; 5: quartz hanging basket; 6: quartz tube; 7, 28: air bags; 8: heater band; 9: spherical valve; 13: mass flow controller; 10, 11, 12, 16, 21, 22, 23, 24, 25, 26, 27: valves; 14, 15: sample transporter; 17, 18: high-temperature cut-off valves; 19: flow meter; 20: pressure tank.


Energy & Fuels

1 2.5 The yield of H2 (wt.% , daf)


2.0 1.5 1.0 0.5

0.1 MPa a

0.0 550

600

650

700 750 O 800 Temperature ( C)

The yield of H2 (wt.% ,daf )

2

0.9 0.6 0.3

600

650

700

750

O

0.5 MPa b 800 850 900

Temperature ( C )

3 1.4

N2 atmosphere

1.2

50% CO2 atmosphere

1.0 0.8 0.6 0.4 0.2 0.0 550

5 6 7 8

900

50% CO2 atmosphere

1.2

550

4

850

N2 atmosphere

1.5

0.0

The yield of H2 (wt.% , 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

Page 36 of 51

600

650

700

750

O

1.0 MPa c 800 850 900

Temperature ( C )



Page 37 of 51


N2 atmosphere

1.4

O

550 C a

50% CO2 atmosphere

1.2 1.0 0.8 0.6 0.1

0.3 0.5 0.7 Pressure (MPa)

1

N2 atmosphere

Yield of CH4 (wt.% , daf )

3.5

0.9

1.1

O

700 C

b

0.9

1.1

50% CO2 atmosphere

3.0 2.5 2.0 0.1

0.3

2

0.5 0.7 Pressure (MPa)

O

4.8

N2 atmosphere


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

800 C

c

50% CO2 atmosphere

4.4 4.0 3.6 3.2 2.8 0.1

3 4

0.3

0.5 0.7 Pressure (MPa)

0.9

1.1

Figure 3. Yields of CH4 at different pressures and temperatures of 550(a), 700(b), and 800(c) °C.


Energy & Fuels

0.6

0.20

0.1 MPa



a 0.5

0.15

0.4 0.3 0.2

0.10 C2H6

0.05

C2H4

N2 atmosphere

0.1

50CO2 atmosphere

550

600

650

700

750

800

O

850

0.00

900

0.24


b 0.5 MPa

0.8 0.18

0.6

0.12 C2H4

0.4

0.06

C2H6 N2 atmosphere

0.2

0.00


600

650 700 750 800 O Temperature ( C)

850

900

1.0

c

0.30

1.0 MPa

0.24

0.8

0.18

0.6 0.12 C2H4

0.4 C H 2 6 N2 atmosphere

0.2

0.00


550

600

650

700

750O 800

0.06

850

Yields of C2H4 (wt.%, daf)

550


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 38 of 51

900

Temperature ( C)

Figure 4. Yields of C2H6 and C2H4 at different temperatures and pressures of 0.1(a), 0.5(b), and 1.0(c) MPa.


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

Yields of volatiles (wt.%, daf )

Page 39 of 51

O

30 800 C


25 20 15 10

0.5

0.1

1.0

Pressure ( MPa)

Figure 5. Yields of volatiles under 50% CO2/50% N2 and N2 atmospheres at different pressures and 800 oC.


2


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

SBETof char and Increase in SBET of chars

Energy & Fuels

Page 40 of 51

N2 atmosphere

250

50% CO2 atmosphere

200 150 Increase in SBET of char

100

under CO2 than that underr N2

50

o

800 C 0.2

0.4

0.6

0.8

1.0

Pressure (MPa)

Figure 6. SBET of chars and Increases in SBET of char under CO2 than that under N2.


0.20


3


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

vpof char and Increases in vpof chars

Page 41 of 51

0.16

Increases in Vpof char under CO2than that under N2

0.12 0.08 0.04

o

800 C 0.2

0.4

0.6

0.8

1.0

Pressure (MPa)

Figure 7. Vp of chars and Increases in Vp of char under CO2 than that under N2.


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

Content of H in chars (wt. %, daf )

Energy & Fuels

Page 42 of 51

3.0 O

N2 atmosphere

550 C O

550 C

2.4

50% CO2 atmosphere

1.8 O

800 C

1.2

O

800 C

0.6 0.1

1.0 0.1 Pressure ( MPa)

1.0

Figure 8. Contents of H in chars under 50% CO2/50% N2 and N2 atmospheres at different pressures and temperatures of 550 and 800°C.


0.30

50% CO /50%N atmosphere 2

0.25

H

2

2

at 1.0 MPa

CH

4

0.20

4

and CH 2

Yields of H

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

(wt. % , daf )

Page 43 of 51

0.15

during the pyrolysis of the N2-devolatilized char 0.10 0.05 0.00 550

600

650

700

750

800

850

900

O

Temperature ( C)

Figure 9. Yields of H2 and CH4 at different temperatures under both atmospheres CO2+C during pressurized pyrolysis of N2-devolatilized char


2.5

Page 44 of 51

50% CO2/50% N2 atmosphere at 1.0 MPa

2.0

O

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 CO at 550 and 600 C (wt. %, daf )

Energy & Fuels

1.5

1.0

0.5

550

O

Temperature ( C )

600

Figure 10. Yields of CO at 550 and 600 °C under both atmospheres during pressurized pyrolysis of N2-devolatilized char.


Page 45 of 51

100.0

100.0

99.9 99.9

99.7

Q

2956

99.6

2960 2850

99.5 99.4 99.3

99.8

T (%)

T(%)

99.8

2850 99.7

N atmosphere

o

1.0 MPa

600 C

50%CO atmosphere

2920

2940 2910 2880 -1 wavenumber (cm )

2850

2820

2910

2

2880

2850

2820


at 3000–2800 cm . 2910

2940

-1

-1

2940

2970

50%CO atmosphere

2920

wavenumber (cm)

Figure 11. FTIR spectra of Residue-550 2970

2

1.0 MPa

2

99.6

2970

N atmosphere

o

2

550 C

2880

2850

2820

100.0 100.0

99.9

99.9

99.8

2960

T (%)

T (%)

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

2850

99.8 2960 2850

O

550 C O

99.7

600 C O

N atmosphere

o

1.0MPa 2970

O

2

700 C

99.7

700 C

2940

2920

50%CO atmosphere 2

2910

2880

2850

99.6

800 C

1.0 MPa

O

900 C

2920

2820

-1

2960

wavenumber (cm )

2920

2880

-1

2840

2800

wavenumber (cm )

Figure13. FTIR spectra of Residue-700 -1

at 3000–2800 cm .



Energy & Fuels

100

100 1424

80

779

799

80

60

779

T (%)

70

40

60 50

20

40 0

N atmosphere 50%CO atmosphere

-20 1800

1100

1400

1.0 MPa

1200

1000

800

600

o

2

30

550 C

1042

2

1600

N atmosphere

o

2

600 C

50% CO atmosphere

20 1800

1400

1200

1.0 MPa

1000

800

-1

-1

wavenumber (cm)

Figure 15. FTIR spectra of Residue-550

Figure 16. FTIR spectra of Residue-600

-1

at 1800–600 cm-1.

at 1800–600 cm . 100

100 1590

1420

1424

90

1590

O

80

779 799

700 C

80 799 779

70

60

60

O

O

550 C

40

50

600 C O

900 C

O

600 C O

O

800 C

700 C

O

550 C

O

40

N atmosphere

30

50%CO atmosphere

o

2

1800

1042

1100

2

1600

wavenumber (cm )

T (%)

T (%)

1420

1590

90

1590 799

T (%)

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 46 of 51

2

1600

1100

1042

20

700 C

800 C O

900 C

1100

1042

1.0MPa

1.0 MPa

1400 1200 1000 -1 wavenumber ( cm )

800

Figure 17. FTIR spectra of Residue-700. -1

at 1800–600 cm .

0 1800

1600

1400 1200 1000 -1 wavenumber (cm)

800

600



Page 47 of 51 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

CO CH2

CO2

CH2

CH3 (O)

CH2

CH2

CH2

Coal / char

1 2

CH3

H2C

HO

Figure 19. Example of cleavage of aliphatic chains on aromatic hydrocarbons induced by CO2. CO

H2COCH2

3

CO2

OOH

OH

(O) H2COCH2

CHOCH2

OH CH2

[O] CHOCH2

OO

+

CH

Coal / char

4

Figure 20. Example of cleavage of ether bonds induced by CO2. H2O

CO

R

CO2

R

(O) R

O

O

OH [O]

R

[O]

R

OH

COOH

+

OH COOH O

5 6

O

Coal / char

Figure 21. Example of cleavage of aromatic rings induced by CO2.

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22


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

Table 1 Proximate and ultimate analyses of an HLH sample.

3

Table 2 The H contents of the products from pyrolyses of 1g coal samples under

4 5 6

50% CO2/50% N2 and N2 atmospheres at 550 °C and different pressures. Table3 The H contents of the products from pyrolyses of 1g coal samples under 50% CO2/50% N2 and N2 atmospheres at 800 °C and different pressures.

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23


Page 48 of 51

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

1

Energy & Fuels

Table 1. Proximate and ultimate analyses of an HLH sample Proximate analysis, wt.ad (%)

Ultimate analysis, wt.daf (%)

M

A

V

FC

C

H

Oa

Stb

N

2.42

20.14

30.21

47.23

81.96

4.80

10.29

1.72

1.23

M: moisture; A: ash; V: volatile matter; ad: air-dry basis; daf: dry ash-free basis a determined by difference;

b determined total sulfur.

2


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

Page 50 of 51


550 °C-0.5MPa

550 °C-1.0MPa

atmosphere

atmosphere


50% CO2

N2

50% CO2

N2

50% CO2

H2

0.11

0.06

0.105

0.046

0.101

0.041

CnHma

0.21

0.31

0.32

0.35

0.37

0.41

Char

2.71

2.26

2.35

2.16

2.31

2.02


1.77

2.17

2.025

2.244

2.01

2.33

a: CnHm represents gaseous aliphatic hydrocarbons, such as CH4, C2H6, and C2H4. b:Calculated from hydrogen balance, HCondensable volatiles =Hcoal－Hchar－H CnHm－HH2. 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24


Page 51 of 51 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

Energy & Fuels


800 °C-0.5MPa

800 °C-1.0MPa

atmosphere

atmosphere


50% CO2

N2

50% CO2

N2

50% CO2

H2

1.04

1.38

1.02

0.87

0.99

0.73

CnHma

0.76

0.81

1.08

1.14

1.13

1.27

Char

1.38

1.12

1.04

0.97

0.99

0.87


1.62

1.49

1.66

1.82

1.66

1.93

a: CnHm represents gaseous aliphatic hydrocarbons, such as CH4, C2H6, and C2H4. b: Calculated from hydrogen balance, HCondensable volatiles = Hcoal－Hchar－H CnHm－HH2 3 4 5 6


Investigation into the Cleavage of Chemical Bonds Induced by CO2

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