Insight into the chemical reaction process of coal self-heating after N2 drying

Fuel 255 (2019) 115780

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Full Length Article

Insight into the chemical reaction process of coal self-heating after N2 drying a,b

Jinhu Li a b

a,b,⁎

, Zenghua Li

, Yongliang Yang

a,b

, Junhao Niu

a,b

, Qingxia Meng

T

a,b

Key Laboratory of Gas and Fire Control for Coal Mines (China University of Mining and Technology), Ministry of Education, Xuzhou 221116, China School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China

G R A P H I C A L A B S T R A C T

There are original active sites on the surfaces of coal pores, and the thermal decomposition of oxygen-containing functional groups also produces secondary active sites, which are capable of producing an oxidative exotherm at room temperature and produce a large amount of CO and CO2 gaseous products. The generation and oxidation of active sites rather than changes in physical structures are the intrinsic reasons why brown coal is more likely to spontaneously combust after N2 drying.

A R T I C LE I N FO

A B S T R A C T

Keywords: Chemical structure Isothermal flow reactor Room temperature oxidation Active sites Oxygen-containing functional group Coal self-heating

It is generally considered that changes in physical rather than chemical structures are responsible for the increase in the spontaneous combustion tendency of coal after inert drying. In this paper, an isothermal flow reactor was combined with FTIR and ESR techniques to study the room temperature oxidation of coal samples under the effects of drying temperatures, numbers of drying-oxidation cycles and anaerobic heating temperatures. Macro-physical parameters such as gaseous generation, oxygen consumption, coal core temperature and micro-chemical structural parameters such as functional groups and free radicals were examined. After coal drying, the thermal decomposition of oxygen-containing functional groups generates free radical active sites, which can be oxidized and are exothermic at room temperature, accompanied by the generation of large amounts of CO and CO2 gaseous products. In addition, the active sites and oxygen-containing functional groups can be transformed into one another under certain conditions. The results indicate that the inherent reason responsible for self-heating and uncontrolled spontaneous combustion of N2-dried coal is the room temperature oxidation of the active sites, especially the room temperature oxidation of the secondary active sites generated by thermal decomposition of the oxygen-containing functional groups. The results presented in this research have applications to reveal the chemical reaction mechanism determining why lignite is more prone to spontaneous combustion after inert drying.

⁎

Corresponding author at: School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China. E-mail address: [email protected] (Z. Li).

https://doi.org/10.1016/j.fuel.2019.115780 Received 26 March 2019; Received in revised form 3 June 2019; Accepted 6 July 2019 Available online 09 July 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

Fuel 255 (2019) 115780

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1. Introduction

(approximately 70 °C) [25]. It has also been indicated that the carboxyl/carbonyl groups are thermally decomposed to generate CO2/CO, and active sites are formed along with the generation of gaseous products [26–29]. Unfortunately, the understanding of the role of these secondary active sites in the low temperature oxidation process of coal is still unclear. In the process of inert drying, whether the spontaneous combustion tendency of lignite increases only with the changes in the physical structure, the effect of the original active sites and secondary active sites generated by the thermal decomposition of oxygen-containing functional groups on the coal spontaneous combustion characteristics has not yet been discussed, which is needed for further study. In this study, the room temperature oxidation of coal samples after N2 drying under different conditions was carried out. Fourier infrared spectroscopy and electron paramagnetic resonance techniques were applied to analyse the chemical reaction involved during the dryingoxidation experiment. The purpose of this study is to explore the chemical mechanism of the increased spontaneous combustion tendency of lignite after N2 drying.

As an important part of coal resources, low-rank coal such as lignite accounts for more than half of the total coal resources worldwide [1]. In China, the storage of lignite is as high as 130 billion tons; in addition, lignite is cheap and low in sulfur, phosphorus and heavy metal contents and has broad application prospects in the electricity and chemical industries [2–6]. However, lignite contains high moisture content, usually between 20 and 60%, which significantly impacts its utilization [7]. To reduce the cost of long-distance transportation and the influence of moisture on the combustion, gasification and liquefaction of coal, industrial dehydration of lignite is required in advance [8,9]. Evaporative drying technologies have been widely applied as effective water removal methods. The corresponding evaporative technologies include drum drying, fluidized-bed drying, microwave drying, superheated steam drying and hot oil immersion drying [10,11]. These drying measures need to be performed under anoxic conditions to prevent energy loss and possible spontaneous combustion of coal. However, it was observed that coal after the drying process is prone to spontaneous combustion even if the coal temperature has been cooled to room temperature [12,13]. This effect not only reduces the calorific value but also poses a huge threat to the transportation and storage of dried coal. It is generally considered that the reason why lignite is more liable to spontaneous combustion after inert drying is mainly due to changes in physical structural characteristics such as pores and specific surface area [14]. Experiments such as mercury intrusion, nitrogen adsorption, and scanning electron microscope (SEM) have been applied to explain the reasons for this phenomenon. Tang et al. [15] compared the oxidation characteristics of lignite after heating treatments (< 200 °C) in N2, CO2 and 10% O2 environments and proposed that the spontaneous combustion propensity of coal samples increased after heating treatment in an inert environment. Using the CPT method, Zhang et al. [16] observed an increase in the spontaneous combustion tendency of Zhaotong lignite during nitrogen drying, which was attributed to the evolution of coal physical properties. Zhao et al. [17] also investigated the spontaneous combustion characteristics of Indonesian brown coal under nitrogen and vacuum conditions and considered that the moisture content and pore structure changes increased the susceptibility to spontaneous combustion. Generally, the emphasis of the above research was placed on the changes in physical structures during the process of coal drying. The increased specific surface area due to the collapse of macropores and mesopores during the drying process is considered to be an important factor increasing the spontaneous combustion tendency of lignite. However, little progress has been made regarding changes in chemical structures during the inert drying of coal. Recently, some investigators [18,19] believed that there are original active sites at the surface of coal pores, but the presence of a large amount of water prevents the interaction between active sites and molecular oxygen. When the moisture is removed, the oxidation of the original active sites leads to self-heating of coal. Currently, controversy exists in the following two aspects: First, although the original active sites are used to explain the initial oxidation process of low temperature oxidation, the above viewpoint is only a theoretical speculation, and the research on the room temperature oxidation of original active sites and its effect on coal spontaneous combustion have not yet been discussed. Second, the current proposed viewpoint mainly considers the role of the original active sites in coal. However, as coal is complex macromolecular organic matter, little attention has been devoted to the existence of secondary active sites formed by weak-bond cleavage. In fact, some weak bonds in coal are broken during the heating process. The weak chemical bonds in coal are mainly oxygen-containing functional groups, which contain carbonyl, carboxyl, hydroxyl and alkoxy structures [20–23]. Among them, the hydroxyl and alkoxy structures generally decompose above 250 °C [24]; however, the carbonyl groups and the carboxyl groups can be decomposed at a lower temperature

2. Experimental 2.1. Coal sample preparation The coal used in this study was a lignite coal obtained from Hulunbeier in Inner Mongolia. Due to the high moisture content, the industrial drying process of the coal should be carried out prior to transportation and utilization. First, the exposed fresh coal sample was wrapped in plastic bags and immersed in water for storage. Then, the coal sample was crushed and sieved into particles with a particle size of 0.125–0.075 mm in the laboratory. The proximate and ultimate analyses of the coal samples are shown in Table 1.

2.2. Room temperature oxidation experiments on coal samples after N2 drying A modified isothermal flow reactor (Fig. 1) was used for N2 drying and the subsequent room temperature oxidation process. The temperature-programmed device is a 2000-type programmed temperature oven, and the chromatographic analysis device is a FULI-9790 chromatographic analyser. The corresponding detailed parameters have been described elsewhere [28]. A mass of 40 g of the obtained coal was placed in the coal sample tank separately. After the nitrogen flow rate was controlled at 50 ml/min, the coal samples were kept for 10 h under different heating temperature conditions (60, 80, 120, 160, and 200 °C). Then, the coal temperature was cooled to room temperature (30 °C) and maintained for another 6 h. The room temperature oxidation experiment was carried out after switching the gas supply path to air with an oxygen content of 24.4%. In addition, continuous N2 drying and room temperature oxidation experiments on the coal were also carried out at 200 °C. A gas chromatograph was used to detect the gas components, and a K-type thermocouple with a precision of 0.1 °C and an error of less than 0.5% was inserted into the geometric centre of the sample tank to measure the coal core temperature. Table 1 Proximate and ultimate analyses of the coal samples. Industrial Analysis

2

Elemental Analysis Odaf

Cdaf

Hdaf

Ndaf

Sdaf

Vdaf %

Fixed carbon FCd %

%

%

%

%

%

30.46

36.38

20.29

74.01

4.36

0.86

0.48

Moisture

Ash content

Volatiles

Mad %

Ad %

19.32

13.84

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Fig. 1. Isothermal flow reactor for N2 drying and subsequent room temperature oxidation.

treated by the above steps were collected in a nitrogen glove box and prepared for infrared spectroscopy and the free radical test. Therefore, the relationships between the functional groups and the radical concentrations as a function of the heating temperature will be obtained.

2.3. FTIR spectroscopy analysis Infrared spectroscopy is a useful method for determining chemical changes during the N2 drying and oxidation process. The N2-dried coal samples under the effect of dried temperatures and drying-oxidation times were used for FTIR tests by using a Vertex 80 V FTIR spectrometer. The coal samples were ground and sieved to less than 0.075 mm, and then, KBr pellets were obtained by mixing and grinding 2 mg of coal sample with 2 g of dried KBr. The experimental wave number range was 400–4000 cm−1 with a resolution of 4 cm−1, and the sample was scanned 64 times. A blank background was collected before each experiment, and the obtained data were smoothed.

3. Results and discussion 3.1. Room temperature oxidation of N2-dried coal samples under different temperatures Room temperature oxidation experiments on Hulunbeier lignite treated with different temperatures under a nitrogen atmosphere were carried out. A mass of 40 g coal samples was heated to 60, 80, 120, 160, and 200 °C for 10 h, kept at 30 °C for 6 h and then subjected to a room temperature oxidation test for up to 3 h. The evolution of the coal core temperature and the gas concentrations at the gas outlet as a function of time are shown in Fig. 2. It is worth mentioning that only CO and CO2 gaseous products appeared during the room temperature oxidation of coal, and no other gases, such as alkanes and olefins, were detected. As shown in Fig. 2, under N2 drying at 60 °C, CO and CO2 gas products are generated during the room temperature oxidation of the coal sample, which is an impossible phenomenon for the raw coal samples. Although the generated amounts of CO and CO2 gases are small, a chemical oxidation reaction occurs at room temperature. It is generally believed that the decomposed temperature of oxygen-containing functional groups needed to produce the active sites is above 70 °C. Therefore, the occurrence of experimental phenomena indicates that there are original active sites in the coal sample, and with the removal of moisture, oxygen enters the pores of the coal sample and oxidizes these active sites. The reason for the small temperature change (0.3 °C) during oxidation is that there are fewer active sites associated with the drying process. With increasing N2 drying temperatures, the amount of gas generation in the subsequent room temperature

2.4. ESR spectroscopy analysis The above coal samples subjected to infrared spectroscopy were also used for the free radical test by a JES-FA200 type spin resonance analyser with a microwave frequency of 9.44 GHz and a modulation frequency of 100 kHz. 20 ± 0.5 mg of the prepared coal samples was loaded to continuously monitor free radical signals. The relevant parameters for ESR measurement were as follows: the microwave power was 10 mW, the centre magnetic field was 338.0 mT, the sweep width was 10 mT, the modulation amplitude was 0.2 mT, and the scanning time was 60 s. 2.5. Anaerobic heating experiment The room temperature oxidized coal that was treated at 200 °C in Section 2.2 was placed in the coal sample tank shown in Fig. 1. The nitrogen flow rate was controlled to 50 ml/min. The coal samples were heated to 40, 80, 120, 160, and 200 °C at a heating rate of 8 °C/min under nitrogen protection, kept at the set temperature for half an hour and then lowered to room temperature. After that, the coal samples 3

Fuel 255 (2019) 115780

J. Li, et al. o

o

60 C

33.5

o

80 C

T1

200 C

T1 (13 min, 33.3 C)

32.5

T3 (11 min, 31.4 oC)

32.0

o

T4 (9 min, 30.6 C) o

31.5

T5 (9 min, 30.3 C)

T3

31.0

T4

30.5 30.0

80 oC

120 oC

200 oC

ExpGro2 Fit of 60 oC ExpGro2 Fit of 80 oC o ExpGro2 Fit of 120 C ExpGro2 Fit of 160 oC o ExpGro2 Fit of 200 C

120 100 80 60 40 20

T5

0

29.5 -20

0

20

40

60

80

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0

200

20

40

60

80

180

60 oC

80 oC

160 140

120 oC

160 oC

24

200 oC

T5

20

T4

18

O2 Consumption (%)

T2

100 80

T1

60

0

20

40

60

80

100

120

140

160

140

160

180

200

60 oC

80 oC

120 oC

160 oC

200 oC

5 min 5

16

4

14

3

12

2

10

1

8 6

0

4

-1 6

8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38

2

T1 (60 min, 78.09 ppm) T3 (60 min, 144.35 ppm) T2 (60 min, 121.35 ppm) T4 (60 min, 151.12 ppm) T5 (60 min, 162.09ppm)

40

120

22

T3

120

100

Time (min)

Time (min)

CO2 Concentration (ppm)

160 oC

140

T2 (13 min, 32.3 oC)

T2

60 oC

160

CO Concentration (ppm)

o

o

160 C o

33.0

Coal Core Temperature ( C)

o

120 C

0 -2 180

200

-20

0

20

40

60

80

100

120

140

160

180

200

Time (min)

Time (min)

Fig. 2. Room temperature oxidation of coal samples after N2 drying at different temperatures.

curvature of the two descending segments, and B and C represent coefficient constants. This equation indicates that the concentration of CO gas emissions decreases rapidly with time, then decreases slowly, and finally tends to be a stable value. This finding is consistent with our previous study on the law of gas generation during high-temperature oxidation of active sites [28], indicating that the gas is produced by the room temperature oxidation of the active sites. The formation rule of CO2 with time is different from that of CO and shows a trend of increasing first, then stabilizing and then decreasing. This observation implies that the effect of heat treatment leads to the strong adsorption of CO2 by coal in the early stages of room temperature oxidation.

oxidation stage is also larger, and a rapid rise in the coal core temperature can be clearly seen. Among the samples, the core temperature of the coal sample after drying at 200 °C increased by 3.3 °C in the subsequent room temperature oxidation process, and the highest concentrations of CO and CO2 reached 140 ppm and 178 ppm, respectively. There are no similar experimental results in the literature for comparison because most gas emission experiments have been conducted at high temperatures [30–34]. This finding shows that the coal sample undergoes a significant chemical oxidation process at room temperature after drying in N2. Additionally, for coal samples after N2 drying at higher temperatures, it can also be found that the oxygen consumption is approximately 100% in the first 5 min. The strong physical adsorption of oxygen by pyrolyzed coal should be the main reason for the complete consumption of oxygen. In addition, with the same amount of oxygen consumed, the temperature increases and gas generation is not the same for different drying temperatures, indicating that different concentrations of active sites are involved in the room temperature oxidation reaction. Under nitrogen conditions, the higher the drying temperature is, the more active sites that are generated. The active sites are capable of undergoing a room temperature oxidation reaction to generate a large amount of heat while generating CO and CO2 gases, thereby rapidly increasing the coal core temperatures. This effect is also the intrinsic reason why coal after inert drying is more prone to spontaneous combustion. As shown in Table 2, the law of CO generation follows an exponential decay function with time and is in accordance with P = A + Be-t/K1 + Ce-t/K, where P stands for gas concentration, t stands for time, A stands for stable gas concentration, K1 and K2 represent the

3.2. Continuous N2 drying and room temperature oxidation experiments on coal samples The N2 drying experiments on coal samples under different temperatures indicate the existence of room temperature oxidation of active sites and prove that the reason responsible for the self-heating of N2-dried coal is the room temperature oxidation of active sites. However, determining whether the active sites participating in the oxidation process are only originally present in coal and whether there are secondary active sites due to thermal decomposition of oxygencontaining functional groups has not been solved and deserves further study. Therefore, continuous N2 drying and room temperature oxidation experiments on coal samples were designed. Under the condition of ensuring sufficient oxidation of the original active sites, the existence of secondary active sites and their influence on coal spontaneous combustion characteristics were analysed. The coal samples were heated to 4

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Table 2 Curve-fitting results of gas production during the room temperature oxidation of N2-dried coal at different temperatures. Drying conditions (°C)

Oxidation conditions (°C)

30 30 30 30 30

33.5

T1

Coal Core Temperature (oC)

33.0

1st

2st

3st

T3 T4

32.0

R2

PCO = 0.59 + 5.54exp(t/−11.01) + 2.66exp(t/−130.20) PCO = 4.80 + 14.67exp(t/−6.34) + 9.16exp(t/−29.20) PCO = 6.69 + 53.50exp(t/−10.54) + 16.08exp(t/−69.49) PCO = 8.61 + 80.12exp(t/−19.15) + 23.64exp(t/−240.91) PCO = 12.89 + 139.45exp(t/−11.20) + 39.38exp(t/−75.47)

94.64% 97.94% 99.97% 99.16% 99.94%

5st

T5

2st

31.0 30.5 30.0

4st

5st

140

ExpGro2 Fit of 1st

120

ExpGro2 Fit of 3st ExpGro2 Fit of 4st

100

ExpGro2 Fit of 5st

80 60 40 20

29.5

0 -20

0

20

40

60

80

100

120

140

160

180

200

0

20

40

60

80

180

1st

2st

160

100

120

140

160

180

200

Time (min)

Time (min)

3st

4st

24

5st

1st

22 20

T1

2st

3st

4st

5st

5 min

18

140

O2 Consumption (%)

CO2 Concentration (ppm)

3st

ExpGro2 Fit of 2st

T5 (13 min, 31.9 oC)

31.5

1st

160

o T1 (13 min, 33.3 C) T2 (12 min, 32.8 oC) T3 (12 min, 32.5 oC) o T4 (13min, 32.2 C)

T2

32.5

4st

Fitting formula

CO Concentration (ppm)

N2-60 N2-80 N2-120 N2-160 N2-200

CO

T2

120 T3

100

T4 T5

80

T1 (60 min, 162.09 ppm) T3

60

14

6

0

80

100

120

140

160

0

4

5

10

15

20

25

30

35

40

45

50

2

T4 (45min, .120.09 ppm) (45 min, 98.94 ppm)

60

2

8

-2

40

4

10

T5

20

6

12

T2 (45 min, 140.34 ppm)

40 0

(45 min, 130.19 ppm)

8

16

180

200

Time (min)

-20

0

20

40

60

80

100

120

140

160

180

200

Time (min)

Fig. 3. Continuous drying and room temperature oxidation of coal at 200 °C in nitrogen.

the formation of a large amount of gas products and the rapid increase in the coal core temperature during the continuous room temperature oxidation process are mainly caused by the oxidation of the secondary active sites generated by the thermal decomposition of the oxygencontaining functional groups. In addition, the dependence of CO generation during continuous drying and room temperature oxidation also follows an exponential decay function with time and is in accordance with P = A + Be-t/K1 + Ce-t/K, as shown in Table 3. After five consecutive experiments, it was still found that the coal sample showed obvious oxidation at room temperature, which proved that the oxidation of active sites can form oxygen-containing functional groups, and those functional groups can also produce active sites in the next decomposition process. That is, both the oxygen-containing functional groups and the active sites can be converted into each other. A similar result was reported in our previous work [28]. Therefore, in the N2 drying process of coal samples, a large number of active sites are

200 °C for 10 h and cooled to 30 °C, after which consecutive heating and room temperature oxidation experiments were carried out. The experimental results are shown in Fig. 3. As Fig. 3 demonstrates, the room temperature oxidation of coal after N2 drying can still produce a large number of CO and CO2 gaseous products and generate a large amount of heat, which causes the coal temperature to rise rapidly. The original active sites in the coal sample are consumed by oxidation at room temperature after the first drying process and have little effect on the subsequent room temperature oxidation process. Thus, this phenomenon indicates that the active sites involved in the subsequent oxidation reaction are substances produced from the decomposition of oxygen-containing functional groups rather than other original active structures such as methyl [35], methylene [36] or α-H [37]. According to a comparison of the first two dryingoxidation processes, it can be inferred that the secondary active sites play a major role in the room temperature oxidation process. That is, 5

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Table 3 Curve-fitting results of CO production during the continuous room temperature oxidation of N2-dried coal. Drying-oxidation times

Oxidation conditions (°C)

1st 2st 3st 4st 5st

CO

30 30 30 30 30

Fitting formula

R2

PCO = 12.89 + 139.45exp(t/−11.20) + 39.38exp(t/−75.47) PCO = 11.42 + 87.25exp(t/−10.33) + 29.26exp(t/−67.23) PCO = 10.37 + 64.99exp(t/−7.88) + 33.49exp(t/−47.18) PCO = 9.45 + 66.84exp(t/−11.95) + 17.84exp(t/−88.68) PCO = 8.77 + 64.17exp(t/−9.71) + 19.67exp(t/−69.11)

99.94% 99.82% 99.90% 99.87% 99.93%

generated along with the thermal decomposition of oxygen-containing functional groups. The reason for the decreases in the amount of gaseous products and the coal core temperatures after the continuous drying-oxidation process is presumed to be due to an increase in the activation energy of active site formation.

0.16

Original infrared absorption Fitting curve r2=99.999% Peak 1 Peak 2 Peak 3 Peak 4 Peak 5 Peak 6 Peak 7

0.14

Absorbance (a.u)

0.12

3.3. Changes in oxygen-containing functional groups of coal upon N2 drying 3.3.1. IR results after different N2 drying temperatures To study the changes in oxygen-containing functional group contents of coal samples after drying in N2, infrared spectrum analysis of coal samples after drying at 60, 80, 120, 160, and 200 °C was carried out, and the infrared spectra of the coal samples are shown in Fig. 4. Fig. 4 shows the IR spectra of N2-dried coal samples at different temperatures. In these spectra, 3200–3600 cm−1 (eOH or hydrogenbonded eOH) represents the characteristic absorption zone of hydroxyl groups, 2800–3000 cm−1 (aliphatic CeH stretching) is the absorption zone of alkyl groups, and 1500–1800 cm−1 (> C]O) corresponds to the absorption zone of oxygen-containing functional groups. To quantify changes in the oxygen-containing functional groups with the N2 drying temperature, curve fitting was performed, and seven bands were divided according to previous literature [38–40]. The fitting results are shown in Fig. 5, and the correlation coefficient of the fitting is over 99.999% As shown in Fig. 5, there are seven different characteristic bands in the 1500–1800 cm−1 zone, which are attributed to carboxylate characteristic stretching (1500, 1540 and 1576 cm−1); aromatic C]C group characteristic stretching (1610 cm−1); highly conjugated C]O characteristic stretching (1650 cm−1); aliphatic dicarboxylic acid characteristic stretching (1700 cm−1); and ester characteristic stretching (1725 cm−1). Semi-quantitative calculations of the chemical changes in carboxyl and carbonyl functional groups of N2-dried coal under

0.10 0.08 0.06 0.04 0.02 0.00 -0.02 1500

1550

1600

1650

1700

1750

1800

Wavenumber (cm-1) Fig. 5. Curve-fitted spectra of the 1500–1800 cm−1 zone for N2-dried coal at 200 °C.

different temperatures are presented in Table 4. Table 4 shows that the content of oxygen-containing functional groups significantly decreases with increasing drying temperature, indicating that the oxygen-containing functional groups undergo thermal decomposition at certain temperatures. As proposed by previous studies [18,28], the carboxyl and carbonyl groups are thermally decomposed to generate CO and CO2, accompanied by the formation of a large number of active sites. Therefore, as the N2 drying temperature increases, the oxygen-containing functional groups are thermally decomposed to generate active sites, and the higher the drying temperature is, the larger the number of active sites that are generated. This finding can also be obtained from a comparison of the amount of gaseous products

(B)

(A)

1500

1576

1610

1540

1650 1700

o

200 C

1725

200 oC

160 oC

160 oC

120 oC

120 oC

80 oC

80 oC

60 oC

500

60 oC 1000

1500

2000

2500

3000

3500

4000 1500

Wavenumber (cm-1)

1550

1600

Fig. 4. IR spectrum of N2-dried coal samples at different temperatures. 6

1650

1700

Wavenumber (cm-1)

1750

1800

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Table 4 FTIR semi-quantitative calculations for N2-dried coal at different temperatures (carboxyl-carbonyl functional groups). N2-drying Temeprature (°C)

60 80 120 160 200

Table 5 FTIR semi-quantitative calculations for continuous N2 drying and room temperature oxidation of coal (carboxyl-carbonyl functional groups).

Defined parameter and adsorption zone (cm−1)

Defined parameter and adsorption zone (cm−1)

Drying-oxidation times

Car A1610

COOH A1700

C=O A1650-1750

> C=O/Car A(1650-1750)/ A1615

COOH/Car A1700/A1615

6.08 1.82 3.08 5.02 4.18

2.11 0.55 0.78 1.2 0.98

6.26 1.83 2.89 4.57 3.84

1.03 1.01 0.94 0.91 0.92

0.35 0.30 0.25 0.24 0.23

200 °C-1st 200 °C-2st 200 °C-3st 200 °C-4st 200 °C-5st

Car A1610

COOH A1700

C=O A1650-1750

> C=O/Car A(1650-1750)/ A1615

COOH/Car A1700/A1615

6.08 1.82 3.08 5.02 4.18

0.98 0.88 1.46 2.08 0.77

3.84 3.82 5.14 6.91 2.50

0.92 0.93 0.93 0.94 0.95

0.23 0.21 0.26 0.28 0.29

containing functional groups, infrared spectroscopy experiments were carried out. The room temperature oxidation of the N2-dried coal samples were successively heated to 40, 80, 120, 160, and 200 °C in nitrogen. Then, the evolution of the contents of oxygen-containing functional groups with temperature was tested, as shown in Fig. 7. To carry out semi-quantitative analysis of the contents of carbonyl groups and carboxyl groups, the adsorption zone attributed to oxygencontaining functional groups was subjected to peak separation, and the evolution of the carboxyl and carbonyl groups as a function of heating temperature are shown in Table 6. It can be clearly seen from the table that the content of oxygencontaining functional groups in the coal increases significantly from room temperature to 40 °C under nitrogen conditions, which may be caused by the oxidation of the active sites in the coal. The active sites can react with oxygen in the air at room temperature to produce CO and CO2 gas and eventually form oxygen-containing functional groups. As the temperature increases, the remaining active sites in the coal react with the adsorbed oxygen, resulting in the increasing content of the oxygen-containing functional groups. This finding is consistent with previous studies conducted by Yuda [41], Murat [42] and Tahmasebi [43], which proposed that peroxides were first formed during the coal oxidation process and then converted to oxygen-containing functional groups such as hydroxyl, carbonyl and carboxyl groups at temperatures as low as 50 °C. However, the contents of carbonyl and carboxyl groups increased up to 80 °C and then significantly decreased at higher temperatures due to decomposition.

generated during the room temperature oxidation of the N2-dried coal samples. 3.3.2. IR analysis of continuous N2 drying and room temperature oxidation experiments To study the contents of oxygen-containing functional groups in coal after continuous drying and room temperature oxidation, infrared spectrum analysis of treated coal samples was carried out, as shown in Fig. 6. Similarly, the peak fitting of the absorption zone of oxygen-containing functional groups is obtained, and the semi-quantitative data of the carboxyl and carbonyl functional groups are shown in Table 5. It can be seen from Table 5 that the content of oxygen-containing functional groups increased with increasing N2 drying cycles. This result indicates that the active sites generated during the N2 drying process can also be oxidized to form new oxygen-containing functional groups under certain conditions, which is consistent with the conclusion obtained from gas generation during the continuous drying-room temperature oxidation process. However, according to the gas generation during the oxidation process, it was found that the number of active sites significantly decreased as the N2 drying time increased. The reason for this phenomenon may be that some of the active sites are oxidized to produce oxygen-containing functional groups that can be stably present and accumulate at high temperatures. 3.3.3. IR analysis of the generation of oxygen-containing functional groups It has been proven that oxygen-containing functional groups can be thermally decomposed to generate active sites, and the active sites can also be converted into oxygen-containing functional groups under certain conditions. To study the formation temperature of oxygen-

3.4. Analysis of free radicals of coal upon N2 drying The changes in free radical concentration under different drying

(A)

(B) 1500

o

200 C-5st

1540

1576

1610

1650

1700

200 oC-4st

200 oC-3st

200 oC-2st

200 oC-2st

200 oC-1st

1000

200 oC-5st

200 oC-4st

200 oC-3st

500

1725

200 oC-1st 1500

2000

2500

3000

3500

40001500

Wavenumber (cm-1)

1550

1600

1650

Fig. 6. IR spectrum of continuous room temperature oxidation of N2-dried coal at 200 °C. 7

1700

Wavenumber (cm-1)

1750

1800

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(B)

(A)

1500

200 oC

1540

1576

1610

1650

1700

1725

200 oC

o

160 C 160 oC

120 oC

120 oC

80 oC

80 oC

40 oC

40 oC

30 oC

30 oC

500

1000

1500

2000

2500

3000

3500

40001500

1550

1600

1650

1700

1750

1800

Fig. 7. IR spectrum of N2-dried and room temperature-oxidized coal treated at different anaerobic temperatures.

free radicals belonged to the same type of active free radical.

Table 6 FTIR semi-quantitative calculations for N2-dried and room temperature-oxidized coal treated at different anaerobic temperatures (carboxyl-carbonyl functional groups). Anaerobic Temperature (°C)

30 40 80 120 160 200

3.4.2. Analysis of free radicals for different drying-oxidation cycles To study the relationship between the number of drying cycles and the change in free radical concentration, a free radical test of coal samples under different numbers of drying-oxidation cycles was carried out. ESR images, free radical concentrations and the g factor of the coal samples under different numbers of drying-oxidation cycles are shown in Fig. 9. As shown in Fig. 9, as the number of drying-oxidation cycles increases, the concentration of free radicals increases slightly. Combined with the gas generation law of continuous drying and room temperature oxidation, it is speculated that some stable radicals that do not participate in the subsequent oxidation reaction process are generated. This effect leads to an increase in the concentration of free radicals after multiple drying processes, but few active free radicals are involved in the subsequent oxidation process, resulting in a reduction in the generation of gaseous products. The g factor of free radicals was also tested to confirm this viewpoint. It can be clearly shown that the g factor increased significantly with the number of drying-oxidation cycles. This phenomenon indicates that the type of active radicals in coal changes with increasing number of drying cycles, and the additional free radical species should belong to stable free radicals.

Defined parameter and adsorption zone (cm−1) Car A1610

COOH A1700

C=O A1650-1750

> C=O/Car A(1650-1750)/ A1615

COOH/Car A1700/A1615

4.18 3.86 2.55 2.65 3 3.38

0.77 1.36 0.79 0.79 0.81 0.85

2.50 3.805 2.33 2.39 2.48 2.87

0.95 0.99 0.91 0.90 0.83 0.85

0.29 0.35 0.31 0.30 0.27 0.25

temperatures, different drying-oxidation times and different anaerobic heating temperatures were investigated experimentally. In addition, as an important parameter of free radicals, the Lande factor g can infer changes in the type of free radicals [44,45]. The evolution of the corresponding g factor, which reflects the type of free radicals in the coal, was also obtained. 3.4.1. Analysis of free radicals at different N2 drying temperatures To study the changes in free radical concentration in coal samples during nitrogen drying, free radical tests of coal samples after nitrogen drying under different temperature conditions were carried out. ESR images, free radical concentrations and the g factor of coal samples under different treatment conditions are shown in Fig. 8. As shown in Fig. 8, during the N2 drying process of coal samples, the decomposition of oxygen-containing functional groups generates a large number of free radical active sites, and the higher the temperature is, the greater the amount of free radical active sites. When the temperature reaches 200 °C, the radical concentration increases to 1.7E17/ g. Additionally, as the temperature increases, the growth rate of the free radical concentration increases up to 160 °C and then decreases thereafter. The reason for this phenomenon may be that the increase in temperature accelerates the thermal decomposition of the oxygencontaining functional groups, but when the temperature is raised to a certain temperature, limited by the content of the oxygen-containing functional groups, the generation rate of active free radicals is lowered. In contrast to the increase in free radical concentration, the g factor first increased and then decreased with increasing N2 drying temperature, but the minor change in the g factor indicated that the additional

3.4.3. Analysis of free radicals of N2-dried and room temperature-oxidized coal treated at different anaerobic temperatures The active sites can be converted into oxygen-containing functional groups under certain conditions. To study the evolution of free radicals during the formation of oxygen-containing functional groups, N2-dried coal samples after room temperature oxidation were successively heated to 40, 80, 120, 160, 200 °C in nitrogen. ESR images, free radical concentrations and the g factor of the coal samples for different treatment conditions are shown in Fig. 10. As the temperature increased, the concentration of free radicals decreased when the coal was heated to 80 °C and increased thereafter, showing an inverse relationship with the change in oxygen-containing functional groups. This result indicates that as the temperature increases, the active sites are gradually consumed by adsorbed oxygen and converted into oxygen-containing functional groups. As the temperature continues to increase, the decomposition of the oxygen-containing functional groups begins to produce active sites, which leads to a rapid increase in the concentration of free radicals. Therefore, the decomposition of oxygen-containing functional groups in the N2 drying process generates free radical active sites that are capable of 8

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Fig. 8. Free radical tests of coal samples after N2 drying at different temperatures.

with molecular oxygen, oxidation reactions can occur at very low temperatures to produce CO and CO2 and release a large amount of heat. Therefore, even if the coal is cooled to room temperature, once the active sites become in contact with oxygen, oxidation reactions can occur quickly, and a large amount of heat is released, which leads to coal self-heating. As determined by means of infrared and electron paramagnetic resonance techniques, the active sites associated with the decomposition of oxygen-containing functional groups in coal are highly likely to be reactive free radicals. Therefore, the oxygen-containing functional groups can undergo thermal decomposition at certain temperatures accompanied by the generation of free radical active sites. In addition, the free radical active sites can also be oxidized with oxygen at room temperature to produce new oxygen-containing functional groups. The macroscopic experimental phenomena observed in the continuous N2 drying and room temperature oxidation of coal samples at 200 °C also support this speculation. The specific reaction diagram is shown in Fig. 11. During the N2 drying process, the volatilization of water leads to the exposure of the original active sites in the coal, and the thermal decomposition of oxygen-containing functional groups also produces secondary active sites. Once the dried coal sample is in contact with air, these free radical active sites will react rapidly with oxygen, which will lead to a rapid increase in the coal temperature accompanied by the formation of gaseous and solid products. When coal is in a good heat storage environment, once the coal temperature exceeds 70 °C, the conversion and heat release between free radical active sites and oxygen-containing functional groups will accelerate the oxidation process and even lead to the uncontrolled spontaneous combustion of

undergoing oxidation reactions at room temperature to produce oxygen-containing functional groups. From the above results, it can be concluded that the oxygen-containing functional groups and free radical active sites can be converted into one another under certain conditions. The evolution of the g factor as a function of heating temperature showed that the g factor increased with increasing temperature, and this increase was more significant at temperatures higher than 80 °C. During the slow increase phase, the oxidation reaction of active radicals is mainly carried out, and the number of new radicals generated during the whole process is small. Above 120 °C, the oxygen-containing functional groups begin to decompose in large amounts to produce new radicals, resulting in an increase in the types of free radicals, including inert and active free radicals, and the g-factor begins to grow rapidly.

3.5. Mechanism There are a small number of primary active sites and a large number of oxygen-containing functional groups on the pore surface of coal, and the presence of water isolates the interaction between original active sites and oxygen. During the drying process, with water evaporation, the original active sites on the coal pore surface are exposed. As the drying temperature of the coal increases, after reaching the decomposition temperature (approximately 70 °C), the original oxygen-containing functional groups in the coal begin to decompose in large quantities to produce secondary active sites. In a previous study [46], we reported that the active site has high activity and can stably exist under nitrogen conditions. In addition, when the active site is in contact

Fig. 9. Free radical tests for continuous N2 drying and room temperature oxidation of coal samples at 200 °C. 9

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Fig. 10. Free radical tests of N2-dried and room temperature-oxidized coal treated at different anaerobic temperatures.

after inert drying can be effectively suppressed, which has extremely important practical and commercial value.

coal. Therefore, the generation of free radical active sites and their oxidation at room temperature are the initial heat sources leading to self-heating of the N2-dried coal samples. Different from previous studies [47–49], although the changes in physical structures have a certain influence on the oxidation process of coal, we believe that the generation and oxidation of active sites are the intrinsic reasons why brown coal is more likely to spontaneously combust after N2 drying. In addition, according to a comparison of gas generation in continuous N2 drying and room temperature oxidation experiments, it can be inferred that the secondary rather than original free radical active sites play a major role in the room temperature oxidation process. In addition, the presence of carboxyl groups can increase the hydrophilicity of the coal surface, and these groups can also adsorb water molecules by means of chemical hydrogen bonds. Research has shown that the carboxyl functional groups of low rank coal have a significant linear correlation with moisture content [50,51]. Therefore, elimination of coal polar oxygen functional groups will cause a decrease in the oxygen content and moisture-holding capacity. However, during the drying process of lignite, with water evaporation, a large number of free radical active sites are also generated along with the decomposition of oxygen-containing functional groups. If the oxidation of the active sites cannot be well controlled, as the oxidation progresses, the content of oxygen-containing functional groups increases, and coal samples exposed to the natural environment not only lose a large amount of energy but also experience water re-absorption. Similarly, if the activity of the free radical active sites can be inhibited by the addition of blocking agents, self-heating and uncontrolled spontaneous combustion of coal

4. Conclusions In this paper, room temperature oxidation experiments on coal samples after nitrogen drying were carried out, and parameters such as gas generation, coal core temperature, functional groups and free radicals were investigated. The production and oxidation of active sites and their effects on coal spontaneous combustion are explored. The following conclusions can be drawn: (1) The heat release from the oxidation of active sites is the initial heat source responsible for the self-heating of N2-dried coal and is the intrinsic reason why lignite is more likely to spontaneously combust after inert drying. (2) There are original active sites on the surfaces of coal pores, and the thermal decomposition of oxygen-containing functional groups during the N2 drying process also produces secondary active sites. Room temperature oxidation of secondary active sites is the main reason for the self-heating of coal. (3) Thermal decomposition of oxygen-containing functional groups can generate free radical active sites, and the oxidation of those active sites results in the formation of new oxygen-containing functional groups. The free radical active sites and oxygen-containing functional groups can be converted into one another under certain conditions.

Fig. 11. Schematic diagram of the generation and oxidation of active sites during coal drying. 10

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Acknowledgment

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