Mechanism of Gases Generation during Low-Temperature Oxidation

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Mechanism of Gases Generation during LowTemperature Oxidation of Coal and Model Compounds Jinliang Li, Wei Lu, Biao Kong, Yingjiazi Cao, Guansheng Qi, and Chuanrui Qin Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03571 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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Mechanism of Gases Generation during Low-Temperature Oxidation of Coal and Model Compounds Jin-liang Li1, Wei Lu*, 1, 2, 3, Biao Kong*, 1, Ying-jiazi Cao4, Guan-sheng Qi1, 2, Chuan-rui Qin1 (1. College of Mining and Safety engineering, Shandong University of Science and Technology, Shandong 266590, China 2. State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou 221116, China 3. Hebei State Key Laboratory of Mine Disaster Prevention, North China Institute of Science and Technology, Beijing 101601,China 4. Department of Civil Engineering, Monash University, Clayton, Victoria 3800, Australia)

ABSTRACT: This study focused on the mechanism of gas (such as CO and CO2) generation during low-temperature oxidation of coal to provide a better understanding of the progressing phases of coal spontaneous combustion. Considering the formation of coal body, internal structures of different coal bodies vary, so it is difficult to directly use coal samples to study the mechanism of gases generation. In this paper, we used model compounds, which include certain reactive functional groups discovered from the real coal body, to simulate the gas generation from oxidation of these functional groups. By comparing the gas generation during coal oxidation, we reached the conclusion of the gas generation mechanism during low-temperature oxidation of coal. It is found that the methylene and methyne groups attached to the aromatic rings are the main functional groups that generate peroxides. The peroxides are unstable and easy to decompose to produced alkoxy radicals and hydroxyl radicals. Then, the alkoxy radicals can generate aliphatic radicals and oxygen-containing groups (aldehyde, carboxyl) through beta-cleavage. And oxygen-containing groups then release CO and CO2, while oxidation/hydrogen adsorption/other reactions of 1

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aliphatic radicals happen, which also release CO, CO2 along with CH4, C2H4, C2H6 and C3H8.

Keywords: Coal spontaneous combustion, Index gas, Active groups, Peroxides

1. INTRODUCTION Coal spontaneous combustion is a complex process that involves the physical adsorption, chemical adsorption and chemical reaction between coal and oxygen. During which, heat release leads to the temperature rise of coal body and at the same time H2, CO, CO2 and some hydrocarbon gases with different concentration are released 1, 2. Because of the nature of these gases ( as they can be easily detected), they are commonly used as the index gas for coal spontaneous combustion

3-6.

Many

researchers have studied the gas generation during the low-temperature oxidation of coal, expecially on CO and CO2, 1, 2, 4, 7-10, which provided valuable experience for prevention and control of coal spontaneous combustion 11, 12. Commonly, researchers believe that the peroxides are generated firstly during the coal-oxygen reaction1, 8, and the generation of peroxides plays a key role in coal spontaneous combustion which connects the gas generation processes during coal’s oxidation. Clemens et al. 8, Wang et al.1 learned that peroxyl radicals (-O-O-) is produced by the coupling of oxygen molecules and free carbon atoms from the coal body’s aromatic or aliphatic structure during the chemisorption process, followed by abstraction of hydrogen atom by peroxide radicals from coal body’s aromatic or 2


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aliphatic structure. That leads to the formation of hydroperoxide (–O–O–H) and a new carbon center. Wang et al.7 believe that there are two ways that peroxides can be generated. One is the reaction between free carbon radical inside the coal body and oxygen. Another one is the reaction between methylene and oxygen. He believes that the reaction between methylene and oxygen takes the majority. However, in these experiments, no peroxide was detected, because of the variety of active functional groups existing in the coal body at the same time. While peroxide is generated by the oxidation of the functional groups, some other acidic groups or metal ions indeed promote the decomposition of peroxides13, 14, so it is difficult to detect the presence of peroxides in the process of coal oxidation. After the generation of peroxide, peroxide undergoes a series of transitional intermediate states and is ultimately transformed into CO, CO2, and hydrocarbon gases. An early study by Gethner 15 showed that the production of CO2 and CO is the consequence of the decomposition of both carboxyl and carbonyl groups. Conversely, some other observations7-9 concluded that the decomposition of the carboxyl leads to CO2 produced, while CO emission is the result of the decomposition of carbonyl groups. Wang et al.2 found that there are two parallel reaction sequences during coal spontaneous combustion–chemisorption and chemical reactions that lead to combustion. Both sequences can generate carbon dioxide and carbon monoxide. Zhang et al.16 later found that carbon oxides cannot only be generated by the reaction of oxygen and coal, but also from the decomposition of the oxygen-containing groups originally from the coal body. Wang et al.17 believed that the generation of 3


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hydrocarbons was due to the aliphatic hydrocarbons detached from the benzene. Dai18 suggested that the broken of side chains leads to CO and hydrocarbon gases generation during low-temperature oxidation of coal. Deng et al.19 found that ethylene appears when the coal temperature reaches 90 to 110℃, while Lu et al.20 found that the range of temperature at which ethylene could be detected was from 64.3 to 75.3℃. Although most researchers suggested that low temperature oxidation of coal can generate CO, CO2, the reactions in detail have not been fully elucidated and answers to that remain controversial, especially about the mechanism of hydrocarbon gases generation. In addition, the initial temperature at which hydrocarbon gases, especially ethylene produced is also unclear. Reason for the above stated phenomenon is due to the complexity of coal structure, which means when using different coal samples for testing, the active groups types and amount inside that sample are different vastly. However, the low-temperature oxidation of coal is closely related to the types and quantity of active groups in coal21, 22, therefore, the study of reactive groups will help to understand the mechanism of gas generation. As mentioned above, molecular structure of coal is complex, it is difficult to separate the specific group with others. To overcome this problem, model compounds can be used to simulate specific active groups inside the coal body. In this paper, we selected nine types of model compounds to be oxidized during the experiments, at the same time, studying the formation of peroxides, gaseous and liquid products during oxidation of each active group. The relationships between 4


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peroxides and gaseous (liquid) products were also analyzed. Based on these studies, the mechanism of different gases production by oxidation of specific active groups was concluded. Also, combined with the gases generation of coal, mechanism of gases generation during low-temperature oxidation of coal was concluded. 2. EXPERIMENTS AND METHODS 2.1. Coal Samples Three kinds of coal were analyzed in this paper. Lignite (PZ) is obtained from the Pingzhuang coal mine in the east of Haofeng, China; bituminous coal (TX) is collected from the Tongxi coal mine in the south of Datong, China; and anthracite coal (BJG) is obtained from the Bai jigou coal mine in the Ningxia, China. The proximate and ultimate analyses of the coal samples are shown in Table 1. Table 1. Coal rank

Proximate and ultimate analyses of coal samples

Proximate analysis, wt%

Ultimate analysis, wt%, daf

Mad

Aad

Vdaf

C

H

N

O

S

PZ

13.07

19.22

41.14

75.48

5.01

0.65

17.81

1.05

TX

1.92

18.45

34.80

83.35

5.51

1.47

8.87

0.81

BJG

0.86

3.60

7.99

90.16

3.35

0.64

5.74

0.10

Note: ad, air-dried basis; daf, dry and ash-free basis.

Coal samples were obtained directly from the work face of coal mine, and all samples were wrapped in a sealed plastic cling wrap, which was filled by nitrogen. After the samples were transported to the laboratory, the coal samples were crushed and sieved to a particle size range of 0.12–0.18 mm prior to the testing. 2.2. Selection of reagents Anisole reagent (99.0%) was purchased from Tianjin Damao Chemical Reagent Factory, China. Phenetole reagent (98.5%), ethylbenzene reagent (98.5%) were 5


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purchased from Sinopharm Group Chemical Reagent Co., Ltd., China. Benzaldehyde reagent (99.0%) and cumene reagent (99.0%) were purchased from Tianjin Guangfu Fine Chemical Co., Ltd., China. Phenylpropane reagent (98.0%), 1-phenylethanol reagent (98.0%), 1-phenylpropanol reagent (97.0%), diphenylmethane reagent （ 98.5% ） were purchased from Shanghai Macklin Biochemical Co. Ltd., China. Sodium thiosulfate pentahydrate (99.0%), acetic acid (99.5%), and potassium iodide (99.0%) were purchased from Chengdu Kelong Chemical Reagent Factory, China. All reagents were used directly, without further treatment. 2.3. Oxidation Experiments 2.3.1. Oxidation Procedures of Model Compounds The oxidation of model compound was performed in a flask reactor with three necks. The reactor (see Figure 1) consists of six parts: air system, oil bath, flask reactor, condensing tube, thermometer and sampler. During the test, 0.20 mol of the model compound was placed into the three-necked flask; air at a flow rate of 100.0 ml/min was passed into the model compound from the bottom of the reactor, and then magnetic stirring was started with 2000.0 r/min, which ensuring the absence of mass-transfer effects. Within the range of 30–150 °C, an increment of 10 °C was conducted and for every 10 °C temperature rise, the ending temperature was maintained unchanged for an hour. After that one-hour stabilization, the gas generated was taken out at the duct end via a 20.0 ml syringe and then was sent to the gas chromatograph (GC-4000). The peroxide production was analyzed by potassium iodide titration. Liquid chromatography (LC) was used to analyze the liquid products 6


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after the completion of oxidation reaction.

Figure 1.

The equipment for oxidation experiment

2.3.2. Oxidation of Coal Samples The oxidation experiment was conducted inside a low-temperature oxidation experimental device, which consists of an air system, sample container, programmed temperature controller, temperature control system and a data collection system (Figure 2-3). A 30-m copper pipe is located on left of the oven to ensure that the temperature of the inlet gas is the same as the oven when it reaches the sample. The oven and the door are double skinned and contain kaowool and high density breglass for insulation. Three sets of type K thermocouples are used for measuring and monitoring temperature. A resistive heater fitted at the back of the oven and a fan located at the rear of the oven, are used to ensure efficient temperature distribution inside the oven.

7


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Gas outlet Water drain valve

Gas Chromatograph

Control display screen

Copper pipe

Heating system

Gas system

Control and data acquisition system

Reactor

Mass flowmeter

Air

Flow regulation

Nitrogen

Figure 2. Temperature programmed oxidation experimental system

Figure 3. Inner view of temperature programming oxidation system A total of 25.0 g of coal was weighed and placed in the sample container, and the container was then placed into the furnace. 100.0 ml/min of high-purity nitrogen was let into the coal sample for 2 hours at 50 °C to reduce the influence of gas adsorption of the coal sample. Then air flow with 100.0 ml/min was passed into the coal sample from the bottom of the container to the top. The inner of furnace was heated by rise rate of 0.7℃ /min from 30℃. Within the range of 30–150 °C, the gas, which was sent to the gas chromatograph (GC-4000) every 10 °C temperature rise, was taken at the duct end via a 20.0 ml syringe. 8


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2.4. Peroxide Analysis by Iodimetry Peroxide concentration was measured by iodimetry23. In this test, 4.0ml mixture sample was put in a flask with plug initially with 20.0 mL of acetic acid and 5.0 mL of saturated KI solution coming after. Inside the container, peroxide reacts with KI solution to produce iodine (Eq. (1)): 2 KI+ ROOH+ 2 CH3COOH →I2 +ROH+2 CH3COOK+ H2O

(1)

After a fully stirring, titration with sodium thiosulfate, an equivalent amount of iodine was liberated, which was titrated with sodium thiosulfate solution (Eq. (2)); I2 +2Na2S2O3 → Na2S4O6+2NaI

(2)

The concentration of peroxide was calculated according to the equation: X

V1  V2  * c *0.008 *1000

(3)

m

Where X is peroxide concentration, mg/g, V1 is the volume of titrant used, mL, V2 is the volume of titrant is for the titration of the “blank” sample, mL; m is the sample weight, mg; c is the normality of the titrant in mol/L.

2.5. Liquid Oxidation Product Analysis by LC The liquid oxidation products were analyzed by liquid chromatography (LC) through an UltiMate3000 (Thermo Fisher Corp., USA.) equipped with a DB-WAX fused silica capillary column (30.0m*0.25 mm). Liquid analysis conditions are as following. Column temperature was held at 313 K; mobile phase: acetonitrile: H2O (60: 40); flow rate: 0.80 mL /min; detection wavelength: 220 nm; injection volume: 2.0μL. 2.6. Fourier Transform Infrared Spectroscopy (FT-IR) 9


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The present FT-IR study used the traditional KBr pellet method. FT-IR measurements were carried out using Thermo Scientific Nicolet Avatar360 spectrometer. Both the samples and KBr were dried in infrared lamp to minimize the effect of moisture on the spectrum. Subsequently, samples of 1.0 mg (±0.1 mg) were ground with 150.0 mg KBr for 2 min and pressed into pellets in an evacuated die under 10 MPa pressure for 2 min. Samples were analyzed at ambient temperature, from a collection of 32 scans per spectrum, with the scan range of 400–4000 cm-1. 3. RESULTS 3.1. Selection of Model Compounds 3.1.1. Test and Analysis of Main Active Groups of Coal Three coal samples with different degrees of metamorphosis were collected, including lignite (PZ), bituminous (TX) and anthracite (BJG). Infrared spectra can be used to describe the characteristics of functional groups in coal, and this technique was therefore used to analyze the active groups of the three coal samples. The infrared spectra of each sample are shown in Figure 4.

Figure 4.

FTIR spectra of the three raw coals. 10


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As shown in Figure 4, the 3200-3700 cm-1 range corresponds to the hydroxyl stretching modes, 2925 and 2854 cm-1 range corresponds to the methylene, 2960 cm-1 range corresponds to the methyl, 1660-1710cm-1 range corresponds to C=O, 1450-1620cm-1 range corresponds to the C=C stretching modes in the benzene, 1101-1228cm-1 range corresponds to the C-O. Spectroscopic curve of each band was peak fitted using peak software, to quantitatively describe the amount of the active functional groups. Hydroxyl groups are greatly affected by moisture and are not discussed here. According to the previous researches 24, 25, the peak areas of methyl, methylene, hydroxyl, carboxyl and carbonyl obtained from the regression curves are given in Table 2. Table 2. Structural parameters derived from curve-fitting analysis of IR Spectra Functional groups range（cm-1）

-CH22925

-CH3 2960

-COOH 1700

-C=O

PZ

1.20

0.86

2.30

2.42

TX

1.09

0.50

0.94

1.28

BJG

0.51

0.25

0.04

0.61

1660

It can be observed from Figure 4 and Table 2 that the characteristics of adsorption bands of different coal samples are similar, and it represents that these different coal samples have similar kind of functional groups which mainly contain -OH, -CH2-, -CH3, C=O, C-O. Thus, in this study, model compounds used all contain one or more of these kinds of functional groups. 3.1.2. Selection of Active Groups When using infrared to test the composition of active groups in coal body, researchers find it difficult to find specifically what active groups exist, because the 11


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experimental results only reveals the superposition of all similar active groups instead of a single active group. Therefore, it is necessary to use the molecular structure model of coal to select the representative active group. Based on the known model of coal molecular structure, main structural unit of coal is composed of condensed aromatic nucleus and other active structures. And these structural units are connected to each other by methylene bond, ether bond and aldehyde group bond. In general, the aromatic rings are considered stable and unreactive during coal oxidation, while the activity of the active groups are almost unaffected by the number of aromatic rings26. These factors determined that phenyl group can be used as a proxy for the main structure of the model compounds. Besides the main structure, coal also contains many aliphatic groups which are discovered as the main existence of active group in coal body

16, 27, 28,

so the model compounds should contain one or more methylene

(methyne) groups. There are also many oxygen-containing functional groups inside the coal body, which have reactivity during the coal oxidation process 29, and also act as intermediate products during low-temperature oxidation of coal. Therefore, the model compounds should include oxygen-containing groups. For the convenience of detection, the selected model compounds were all liquid. According to the above selection principles, model compounds consisting of aromatic ring and active groups chosen in this paper are shown in Table 3. Table 3.

Model compound of coal molecules

Active groups in coal molecular structure model

The source of active group

Molecular formula of model compound

12


Model compound

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O CH3

(1)lignite 30-32 (2)bituminous 33

O CH2 CH3

(1)bituminous 33

CH 2 CH 3

(1)lignite30, 34 (2)bituminous 33, 35

CH2 CH3

CH2 CH2 CH3

(1)lignite32, 34, 36, 37 (2)bituminous 33, 38

CH2 CH2 CH3

CH2

(1)lignite30, 31, 34 (2)bituminous 38, 39 (3) anthracite 32

CH3 HC CH3

(1)lignite 31 (2)bituminous 38, 40

OH CH CH3 OH HC CH2 CH3 O CH

O

CH3

（M1） O CH2 CH3

(M2) (M3)

(M4) CH2

(M5) CH3 CH CH3

Anisole Phenetole Ethylbenzene

Phenylpropane

Diphenylmethane

Cumene (M6)

OH

(1)lignite

31, 37

1-phenylethanol

CH CH3

(M7) OH

(1)lignite

31, 37

1-phenylpropanol

CH CH2 CH3

(M8)

(1)lignite

O CH

31, 37

Benzaldehyde (M9)

3.2. Peroxide Production of Model Compounds’ Oxidation 3.2.1. Peroxide Production of Model Compounds’ Oxidation - Methylene (Methyl) Compounds To examine the effect of temperature on the production of peroxide, the oxidation of methylene (methyl) compounds was performed at low temperature. The concentration of peroxide with temperature rise is shown in Figure 5. Since the boiling point of ethylbenzene is 136.2°C, data was collected before the temperature reaching 130°C. 13


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Figure 5.

The concentration of peroxide produced by methylene (methyl)

As shown in Figure 5, the concentrations of peroxide produced by methylene compounds increase with temperature rise. However, the oxidation of anisole that contains methyl did not show much production of peroxide, it testified that methyl is relatively stable and not easy to be oxidized. Phenetole oxidation did not generate peroxide below 140 °C, indicating that ether chain attached to the benzene is relatively stable and difficult to be activated and react with oxygen. This can be explained as p-π-conjugation of benzene and oxygen atom can generate electron-donating group to reduce the reactivity of methylene (methyl). The first temperature point where peroxide production can be observed during ethylbenzene and phenylpropane’s oxidation was 100°C, and the peroxide concentrations during these two model compounds’ oxidation were mostly the same. This suggests that both of them have the same active site, which is the methylene attached to the benzene. Benzene’s nature of electron-withdrawing led to the higher activity of methylene groups which are attached to the benzene. Because both ends of methylene in diphenylmethane are connected to benzene, the electron-withdrawing effect on the 14


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methylene

group

is

further

enhanced.

Therefore,

the

methylene

in

the

diphenylmethane was more active and could produce peroxide at 60°C. 3.2.2.

Peroxide

Production

of

Model

Compounds’

Oxidation

-

Methyne

(oxygen-containing group) compounds Figure 6 shows the peroxide concentration of model compounds containing methyne (oxygen-containing group) at different oxidation temperatures.

Figure 6. The concentration of peroxide produced by methyne oxidation (oxygen-containing group) Figure 6 demonstrates that all the model compounds containing methylene were oxidized to generated peroxide products. The concentration of peroxides reached the maximum amount at 130 ° C, indicating that the rate of peroxide production was the same as the rate of decomposition at 130 ° C. With the increase of temperature, the rate of peroxide decomposition was greater than that of its production, resulting in the decrease of peroxide concentration. The initial point where peroxide was produced by oxidation of 1-phenylethanol and 1-phenylpropanol was the same as cumene at 80°C, which means that these three model compounds have the same active site during the oxidation process, which is the 15


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methyne linked to benzene. Since the methyne is affected by the electron withdrawing of benzene, the hydrogen atoms of the methyne can be lost easily, and that lead to methylene’s strong reactivity. We can also observe from Figure 6 that during the oxidation of benzaldehyde, there is no peroxide detected. That is because, based on the results of previous studies 41, 42,

oxidation of benzaldehyde does produce peroxide, but that peroxide products are

easy to decompose, which means the production of peroxide is difficult to be detected during the oxidation process. What can be concluded from this chapter is that the main reaction of generating peroxide during coal’s low-temperature oxidation is the reaction of methylene or methyne structures that are linked to benzene. Also the strong electron-withdrawing effect of benzenes have a great influence on the activity of methylene and methyne groups. Based on the experimental results, initial reaction temperature of active groups can be ranked as, M9 > M5 > M6 = M7 = M8 > M3 = M4 > M2 > M1. 3.3. Gas Production Characteristics during Model Compounds Oxidation Process 3.3.1. CO2 and CO emission from methylene model compounds oxidation To study the oxidation of methylene model compounds and obtain its reaction temperatures, the generation of CO2 and CO from methylene model compounds oxidation under different temperatures was experimented. Experimental results are recorded in Figure 7.

16


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Figure 7.

CO and CO2 concentration of methylene (methyl)

It can be observed from Figure 7 that the generation of CO and CO2 during methylene model compounds oxidation increased with the temperature rise. But the initial points of CO and CO2 generation were different. The initial point of CO and CO2 production from oxidation of diphenylmethane was 60°C, and the concentration of CO and CO2 increased slowly with the temperature range of 60°C and 120°C. When the temperature is higher than 120 °C, CO and CO2 concentrations experienced a drastic increase with temperature rise. Ethylbenzene and propylbenzene’s oxidation produced CO and CO2 initially with temperature of 110°C, and the concentration changes were basically while temperature increased. From that, it can be concluded that ethylbenzene and propylbenzene have the same mechanism of gas release during oxidation process. Phenetole’s reaction did not generate CO and CO2 before 140 °C. During anisole’s reaction, no CO and CO2 were found within this temperature range. 3.3.2. CO2 and CO emission from methyne (oxygen-containing group) model compounds The curves of CO and CO2 production in the methyne (oxygen-containing group) samples oxidation over time are shown in Figure 8. 17


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Figure 8.

CO and CO2 concentration of methyne (-CH-)

As shown in Figure 8, the initial temperature of CO2 production by 1-phenylethanol and 1-phenylpropanol oxidation was 90°C and for CO that temperature was 100°C. The concentration of CO and CO2 all increased with temperature rise, while the increasing rate of CO2 was higher than CO, especially, after reaching 130°C. Combined with the previous experiments regarding the concentration of peroxide products during model compounds oxidation, that peroxide products concentration reached the maximum amount at 130°C (Figure 6), it can be concluded that the peroxides produced by these two compounds oxidation were mostly decomposed to generate CO2. The initial temperature of CO and CO2 production from cumene oxidation was 90°C, and production rate of CO and CO2 was similar, indicating that CO and CO2 were produced by the same intermediate products. Benzaldehyde could participate in the reaction and generated CO2 at 30°C, which reveals that the reactivity of aldehyde group is very high and benzaldehyde could directly react with oxygen to generate CO2 in normal temperature. However, the generation of CO happened after reaching 60°C, indicating that the energy required to generate CO is higher than that required from CO2 generation. 18


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3.3.3. Hydrocarbon gases emission from model compounds Based on the results of this experiment, there are five model compounds that can produce hydrocarbon gases during their oxidation process. The hydrocarbon gases concentration are shown in Figure 9.

(a)

(b)

(d)

(c)

Figure 9. Hydrocarbon gases concentration curves at each temperature As can be seen from the Figure 9(a), the initial point of methane production from cumene oxidation was 90°C, and the concentration of methane is very high. This is due to the structure of cumene. Cumene oxidation can only generate methyl radicals through β-cleavage, and methyl radicals then formed methane. Similarly, ethylbenzene and 1-phenylethanol experience this type of reactions to emit methane. It has been known from Figure 5 that ethylbenzene and propyl benzene have the same 19


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reaction site, however, the initial temperature points where methane is generated were different, which means they have different mechanisms of methane generation. Regarding propylbenzene oxidation, ethyl radicals were produced through β-cleavage before ethyl radicals were oxidized to yield methyl radical. Methane was produced at the final stage through the reaction of methyl radicals. 1-phenylpropanol had the similar reaction characteristics as propylbenzene regarding methane generation. Figure 9(b) shows that cumene, ethylbenzene and 1-phenylethanol which is only capable of producing methyl radicals during oxidation can also produce ethane, which indicates that ethane can be produced by the reaction of two methyl radicals. For propylbenzene and 1-phenylpropanol oxidation, ethyl radicals could be generated, and then the ethyl radicals reacted with each other to form ethane. It can be seen from Figure 9(c) that all the five model compounds can produce ethylene during their oxidation. Propylbenzene and 1-phenylpropanol oxidation produced ethylene initially at the temperature of 110°C, while the other three model compounds produced ethylene at 130 °C. Based on the results shown in Figure 9(b), these three model compounds oxidation produced ethane at 120°C. Then it can be inferred that ethylene was mainly produced by ethyl radicals. Figure 9(d) demonstrates the temperature at which propane was produced was much higher compared to other types of gases. This is because the model compounds used in this study could not simultaneously produce methyl and ethyl radicals at the initial temperature. Instead, methyl and ethyl radicals were produced separately with the increasing temperature, and then reacted with each other to produce propane. 20


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3.3.4. The relationship between active groups and indicator gases To study the relationship between active groups and indicator gases generation, initial reaction temperature of active groups and initial production temperature of different gases obtained from aforementioned experiments are summarized in Table 4. Table 4. The temperature of peroxides and indicator gases were initially detected Model compound

peroxide (℃)

CO (℃)

CO2 (℃)

CH4 (℃)

C2H4 (℃)

C2H6 (℃)

C3H8 (℃)

Ethylbenzene

100

110

110

110

130

120

-

Cumene

80

90

90

90

130

110

130

1-phenylethanol

80

100

90

100

140

120

140

1-phenylpropanol

80

100

90

130

110

100

140

Propylbenzene

100

110

110

130

110

110

140

Diphenylmethane Phenetole Benzaldehyde Anisole

60 140 -

60 140 60 -

60 140 30 -

-

-

-

-

Note Two linear carbon atoms linked to benzene Three linear carbon atoms linked to benzene One or zero linear carbon atoms linked to benzene

As shown in Table 4, the initial reaction temperature of different active groups can be roughly divided into three stages, 30-60℃, 60-80℃, and 80-100℃. The initial temperature of the oxidation has the characteristics as step continuity, which promotes the gradual oxidation of coal to produce indicator gases. It can also be seen from Table 4 that with the temperature increase, peroxides are produced before the generation of indicator gases. That means active groups can react to generate peroxides firstly during the oxidation process, after that, peroxides decomposed to produce indicator gases. When side chains attached to the benzene contain two straight-chain carbon atoms, peroxides from initial oxidation process experiences β-cleavage to form 21


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methyl radicals. Methyl radicals then undergo oxidation reaction and abstraction of hydrogen atom and this process emits CO, CO2 and methane. With the increase of temperature and rising of methyl radical’s concentration, the chance of collisions among methyl radicals increases, resulting in the formation of ethane. As temperature continues to rise and the concentration of ethane increases, the methyl radicals react with ethane to produce ethylene and propane. When the side chain attached to the benzene contains three straight-chain carbon atoms, peroxides from initial oxidation process experiences β-cleavage to form ethyl radicals. Ethyl radicals then undergo oxidation reaction, abstraction of hydrogen atom and disproportionation. This process emits CO, CO2, ethane and ethylene. Ethyl radicals can be oxidized to produce methyl radicals with the increase of temperature, and the chance of collision between methyl radicals and ethyl radicals’ increases, leading to the emission of propane. When the side chain attached to the benzene contains one or none straight-chain carbon atoms, model compounds did not produce alkane gases within the range of temperature in this experiment, only CO and CO2 were observed, which indicates the side chain connected to the benzene cannot generate methyl or ethyl radicals within the temperature range. Base on the above analysis, we can also find that alkanes can only be produced if the side chains contain at least two carbon atoms. The production of hydrocarbon gas is mainly due to oxygen attacks the side chain of coal to form peroxide, and then the

22


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peroxide decomposes to produce hydrocarbon radicals by β-cleavage. In the end, hydrocarbon radicals undergo various reactions to produce indicator gases. 3.4. Liquid Products from Oxidation of Model Compounds The analysis of liquid oxidation produced by each active group’s oxidation helps the understanding of the mechanism of oxidation reaction. Therefore, LC was used to analyze the liquid products after oxidation of each active group experiment. Combined with the peroxide data obtained from iodimetric method, the liquid products are shown in Table 5. Table 5. The main liquid oxidation products of model compounds. Active groups

Model Compound Diphenylmethane

CH2

Ethylbenzene Propylbenzene

O CH

Diphenylmethane peroxide, Benzophenone, Diphenylmethanol, Benzaldehyde, Benzoic acid Phenylethyl peroxide, Acetophenone, 1-Phenylethanol, Benzoic acid Phenylpropane peroxide, Phenylacetone, 1-Phenylpropanol, Benzaldehyde, Benzoic acid

1-Phenylethanol

Cumylhydroperoxide, Acetophenone, Dimethylbenzyl alcohol Phenylethanol peroxide, Acetophenone, Benzoic acid

1-Phenylpropanol

Phenylpropanol peroxide, Phenylacetone, Benzoic acid

Benzaldehyde

Benzoic acid

Cumene

HC

Liquid Products

As shown in Table 5, liquid products from methylene compounds oxidation mainly included peroxides, ketones, alcohols and a small number of aldehydes and acids, while liquid products from methyne compounds oxidation mainly included peroxides and ketones. Methyne containing hydroxyl group could produce benzoic acid, which indicates that hydroxyl group is an important pre-requisition of carboxylic acid formation. Aldehydes oxidation mainly generates carboxylic acids. In conclusion, the ketones can be detected from most of the model compounds 23


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after oxidation, indicating that the ketones are relatively stable and hard to be oxidized further. Methylene groups can generate alcohols, aldehydes, and acids, at the same time, alcohols and aldehydes can be further oxidized to acids. 4. DISCUSSION As concluded from this research, methyl groups barely participate in low-temperature oxidation, therefore, this chapter will focus on the gas production mechanism of the model compounds’ (containing methylene, methyne and oxygen-containing groups) oxidation. Combined with the study of gas production of coal oxidation, this chapter is going to propose the mechanism of gas production during the coal oxidation process. 4.1. The Mechanism of the Gases Generation by Oxidation of Methylene Compounds According to the peroxide data obtained in the previous chapters, methylene can react with oxygen to produce peroxides, and then peroxides undergone homogeneous cleavage to form hydroxyl radicals and alkoxy radicals43-45. Reactions are as following： O2 R CH2 1 R1

H R C OOH R1

2

..

H R C O + OH R1

(a)

Where, R represents the main structures, R1 represents an aliphatic hydrocarbon or aromatic hydrocarbon

Alkoxy radical is a key intermediate product which can further participate the oxidation. Based on the fact that methylene’s oxidation will generate methyl, ethyl radicals and aldehyde groups, it can be concluded that alkoxy radical can form 24


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Energy & Fuels

aldehydes and R1 radicals through reaction pathway 3a. At the same time, according to Table 5, the model compounds containing methylene could produce hydroxyls, which can be used to infer that the alkoxy radical also generates hydroxyl group and R radicals via pathway 3b. Reactions are as follows: O R C H + R1

.

. O

3a

R CH R1

3b RH

OH R CH R1 + R

.

(b)

Aldehyde generated by pathway 3a is more active and can continue to participate in the oxidation. According to the low-temperature oxidation process of benzaldehyde, it can directly react with oxygen to form CO2, it was concluded that the aldehyde group can produce CO2 by pathway 4. When there are free radicals, CO and CO2 can be generated by pathway 5. The reactions are as follows:

.OH

O R C H 4 O2

5

O2 +

O R COOH

O R C + H 2O

.

.

R + CO

RH

.

.

O R CO + OH

.

R + CO2

(c)

After formation of hydroxyl via pathway 3b, methylene was converted to methyne containing hydroxyl, which increases its reaction activity and encourage it to continue to react. As peroxides were generated by oxidation of model compounds containing hydroxyl groups, it is can be inferred that the hydroxyl continuous reaction follow pathway 6a and 6b. Reactions are as follows:

25


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OH R CH R1

.OH

O2 6a

O2

OH R C OO R1

.

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OH R C O + OH R1

. .

OH R C OOH R1

RH 6b

(d)

According to the reaction of model compounds containing hydroxyl groups and oxygen generating ketones, it can be concluded that alkoxy radicals containing hydroxyl groups can produce ketones through reaction pathway 7a. Oxidation of model compounds containing hydroxyl groups producing carboxylic acid and CO2 can be used to confirm the occurrence of pathway 7b, that is, the alkoxy radical generating carboxylic acid and R1 radicals through β-breaking. Reactions are as follows: OH R C O R1

.

RH 7a

O R C R1 + R + H 2O

.

7b

. R +R 1

.

O OH C OH

O R CO

.

CO2 + R

.

(e)

The radicals generated by reaction pathway from 3 to 7 are very active, and the radicals can continue to react with oxygen, other radicals or hydrogen donors. At the beginning of the reactions, the concentration of radical was relatively low but the concentration of oxygen and hydrogen donors were high, reactions among radicals were found difficult to occur. Oftentimes, at this stage, reactions are oxidation or hydrogen abstraction reactions as follows:

.

.

RH R or R1 8

.

.

R or R1

O2 9

RH or R1 H

(f)

.

ROO or R1OO

.

26


（g）

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Since R radicals are the main structure, it is difficult for its peroxides to directly produce CO and CO2. So, the main groups that react with oxygen and produce CO and CO2 are R1 peroxy radicals. When R1 is an aromatic hydrocarbon radical, because of its stability, it is less active. When R1 is an aliphatic hydrocarbon radical, according to previous research results46, the reactions of aliphatic hydrocarbon radicals can produce CO and CO2 as follows: 2 R1 OO

.+ O

2

10

.

O R'1 CH + HO2

CO + CO2

(h)

Where, R՛1 indicates a carbon atom less than R1

When R1 is an ethyl radical, ethane can be produced by reaction pathway 8. With the increase of ethyl radical concentration, ethylene can also be produced. At present, researchers believe that there are two main path ways to produce ethylene47-49:

.

C2H6 + C2H4

11a

2 C2 H5

C4H10

11b

. . CH + C H 3

2

5

12a 12b

（i）

CH4 + C2H4

C3 H8

（j）

From Table 4 we can see that model compounds which contain three straight-chain carbon atoms can directly generate ethyl radicals, and the initial reaction temperature of the ethyl radicals generated is 90 ℃ . Due to the lower concentration of ethyl radicals, and higher concentration of oxygen and hydrogen donors, ethyl radicals mainly undergo oxidation and hydrogen abstraction reactions to produce CO, CO2 and ethane at the initial stage of reaction. As the temperature and concentration of ethyl radicals increase, the reaction of pathway 11a occurs to 27


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generate ethylene, so the production temperature of ethylene is after 100°C.When methyl radicals and ethyl radicals are simultaneously present in the system, ethylene and propane can be generated by the reaction pathway 12. As what can be observed from the formation of ethylene products, ethyl radicals are prerequisites for the production of ethylene. When R1 is methyl radical, methane can be produced by reaction pathway 8. With the increase of methyl radical concentration, ethane and ethylene can also be produced. According to the results of this experiment, methyl radicals mainly react as follows:

.

.

CH3 + CH3

.

CH3 + C2H6

13

C2H6

（k）

.

14

C2H5 + CH4

（l）

The initial reaction temperature of the ethyl radical generated in reaction pathway 14 is 400K (127°C)

50,

and the ethylene and propane are generated by the

reaction pathway 11 and 12 after the generation of ethyl radical. This also explains the reason why model compounds which only generates methyl radicals could produce ethylene at higher temperature. 4.2. The Mechanism of the Gases Generation by Oxidation of Methyne Compounds It was observed during the experimental that the methyne compounds’ oxidation and gas release situations were in accordance with methylene. When R3 is a hydroxyl group and R2 is an aliphatic hydrocarbon, this process is carried out according to reaction pathway 6 and 7. When R3 and R2 are aliphatic hydrocarbons, according to 28


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Energy & Fuels

Table 5, the alkoxy radicals mainly have two reaction pathways. One is that hydrogen atoms are captured to generate tertiary alcohols and radicals by pathway 15; and another is that ketones and R2 radicals are generated by pathway 16, and the reactions are as follows:

.

R3 R C O R2

R-H 15 16

R3 R C OH + R R2

.

O R C R3(R2) + R2(R3)

.

(m)

The R2 (R3) radicals follow pathway 10 to generate CO and CO2, and follow pathway 9-12 to generate hydrocarbon gases. 4.3. Production of Gases during Low-Temperature Oxidation of Coal The oxidation of three coal samples was investigated to understand the gas release during coal oxidation. Results are shown in Table 6. Table 6. The initial temperature of gases produced by coal oxidation Coal

CO (℃)

CO2 (℃)

CH4 (℃)

C2H4 (℃)

C2H6 (℃)

C3H8 (℃)

PZ

30

30

100

100

100

100

TX

50

50

110

110

110

120

BJG

60

40

120

150

140

/

As shown in Table 6, the lower the metamorphic degree of coal, the lower the temperature is when gas was initially released from oxidation. For lower-rank coal such as PZ, gases were produced at lower temperatures, while hydrocarbon gases were produced at the same temperature. For TX coal, the temperature of gases production was slightly higher, the temperature when methane, ethane and ethylene were generated was basically consistent. Propane was produced at last. However, for BJG coal, the temperature at which gases were produced was much higher. Methane 29


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was first produced, followed by ethane and ethylene. But there is no propane generated during the oxidation process. The results of this experiment indicated that the metamorphism of coal has a great influence on the gas production of coal oxidation. That is because with the increase of metamorphic degree, the number of reactive groups in the coal body and the length of the side chains gradually decrease. 4.4. Mechanism of Gases Production during Low-Temperature Oxidation of Coal Comparative analysis of the initial temperatures of gases production in Table 4 and coal sample test results in Table 6, the possible active groups residing inside the coal body can be obtained. The active groups are shown in Table 7. Table 7. Active groups in coals Active groups

Coal PZ

O CH

TX

CH2

BJG

CH2

,

CH2

CH3 OH OH CH CH 3 , CH 2 CH 3 , HC CH3 , HC CH2 CH3 , CH2 CH2 CH3 ,

OH , CH CH3 , CH 2 CH 3 , CH2 CH2 CH3

, CH 2 CH 3

Based on the analysis of the similarity between the oxidation of active groups and coal samples from both macroscopic and microscopic aspects, it can be concluded that the mechanism of gas generation during low-temperature oxidation of coal is shown in Figure 10.

30


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Energy & Fuels

Note: red represents products detected; pink represents free radicals

Figure 10. Mechanism of gas production during low-temperature oxidation of coal The mechanism of gas generation during low-temperature oxidation of coal is as followed. First, bridge chains (or highly active oxygen-containing groups) directly linked to multiple aromatic rings or strong electron-withdrawing groups, react with oxygen to generate peroxides and release heat (Q1). Then, the peroxides decompose 31


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and produce CO, CO2, radicals and release heat (Q1). Second, the heat promotes the temperature rise of coal with certain heat storage conditions, and under the action of heat accumulation and free radicals, the active groups which require higher energy are activated to release more CO, CO2, radicals and heat (Q2). Third, when the temperature rises to a certain degree, the side chain alkanes which directly connected with the aromatic ring participate in the reaction and release more radicals and huge amount of heat (Q3). In the end, the alkoxy radicals continue to participate in the reaction to generate aliphatic radicals and oxygen-containing groups (aldehyde, carboxyl) through β-cleavage and the oxygen-containing groups produce CO and CO2, while aliphatic radicals react with oxygen, hydrogen donors, or other radicals to produce CO, CO2, CH4, C2H4, C2H6 and C3H8. 5. CONCLUSION The mechanism of the gradually generation of gases during low-temperature oxidation of coal has been understood by analyzing the reactions of main active groups through the method of model compounds. Peroxide plays an important role in gas production. The active groups in coal that can produce peroxides are mainly methylene and methylene groups linked to condensed aromatic nucleus. The strong electron-withdrawing groups such as aromatic rings have a great influence on the activity of methylene or methylene groups while peroxides are generated. The decomposition

of

peroxides

produces

intermediate

products

such

as

oxygen-containing groups and aliphatic hydrocarbon radicals, and these intermediate products are the main sources of indicator gases’ generation. 32


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This study also concludes that alkanes can only be produced if the side chains contain at least two carbon atoms. The production of hydrocarbon gas is mainly owing to oxygen attacking the side chain of coal. The hydrocarbon gases have different generation pathways, while methane is mainly produced by methyl radical capturing hydrogen from other active groups. Ethane is mainly produced by hydrogen abstraction and disproportionation reaction of ethyl radicals, and interaction among methyl radicals. Propane is mainly produced by the reactions among methyl radicals and ethyl radicals. Ethylene is mainly produced by disproportionation of ethyl radicals, and ethyl radical is a prerequisite for ethylene production.

AUTHOR INFORMATION Corresponding Authors *Telephone: +86 15902358877. E-mail: [email protected]. E-mail:[email protected]

NOTES The authors declare no competing financial interest.

ACKNOWLEDGMENTS We are grateful for joint funding by the National Key R&D Program of China (No. 2018YFC0807900 and 2018YFC0807906), National Natural Science Foundation of China (No. 51574279), Research Fund of The State Key Laboratory of Coal Resources and safe Mining, CUMT（SKLCRSM18KF013）and the Research Fund of Hebei State Key Laboratory of Mine Disaster Prevention, North China Institute of Science and Technology （ KJZH2017K07 ） . Natural Science Foundation of Shandong Province (ZR2018BEE003). 33


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Mechanism of Gases Generation during Low-Temperature Oxidation

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