Inhibitive Effects of Antioxidants on Coal Spontaneous Combustion

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Inhibitive Effects of Antioxidants on Coal Spontaneous Combustion Jinhu Li, Zenghua Li, Yongliang Yang, Xiaoyan Zhang, Daocheng Yan, and Liwei Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02339 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 27, 2017

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Inhibitive Effects of Antioxidants on Coal Spontaneous Combustion Jinhu Li a, b, Zenghua Li a, b,* Yongliang Yang a, b，Xiaoyan Zhang a, b，Daocheng Yan a, b， Liwei Liu a, b a

Key Laboratory of Gas and Fire Control for Coal Mines (China University of Mining and

Technology), Ministry of Education,

Xuzhou，221116，China.;

b

School of Safety Engineering,

China University of Mining and Technology, Xuzhou 221116, China * Corresponding author. E-mail address: [email protected] Abstract: Based on the deep study of the mechanism of coal spontaneous combustion, this paper proposed new efficient inhibitors preventing coal spontaneous combustion from the perspective of inhibition of free radical chain reaction. Chifeng coal sample was used as the research object, six different types of antioxidants were selected as the inhibitor for low temperature oxidation experiment. A comparison study of the gas product concentration before and after inhibition was conducted; as well its inhibition mechanism was also studied and analyzed. On the basis of this, the specific indicators such as cross-point temperature (CPT), apparent activation energy, active functional groups and inhibitory rate at 100 °C were used to compare the inhibition effect of antioxidants. The experiments indicate that, except oxygen scavenger VC, another five antioxidants exhibit a good inhibition effect, and the order of inhibitory effect is: RTEMPO> RBHT> REDTA> RTPPI> RPA, indicating that TEMPO has the best inhibition effect with the activation energy of coal oxidation increased by 14.79 kJ•mol-1, and the inhibition rate reached 73.08%. The research results provide new ideas for prevention and control of coal spontaneous combustion. Keywords: inhibitor; coal spontaneous combustion; free radical; antioxidant; inhibition effect.

1 Introduction With the deepening mining depth and the increasing intensity, mine fire has become one of the main disasters in coal mine1-2. Coal spontaneous combustion not only results in substantial waste of resources, production of environment-polluted gases such as SO2, H2S; it may also lead to gas

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explosion, roof falls and other accidents, causing casualties3-5. For example, in 2013, in Babao coal mine, Tonghua City, Jilin Province, coal spontaneously combusted in a closed fire zone caused gas explosion resulting in 36 deaths; in 2012, a coal mine fire happened in Longshan Town, Shuangyashan City, Heilongjiang Province, resulted in the roof falls which took 12 lives. In order to reduce the occurrence of coal spontaneous combustion, scientific research institutions around the world have conducted a lot of theoretical researches, experimental researches and field applications on prevention and control the coal spontaneous combustion. Inhibitors have the following advantages: easy to use, a broader scope of application and low pollution, and become one of the commonly used fire prevention and extinguishment technology in domestic and foreign coal mine6-10. At present, the commonly used inhibitors are NaCl, MgCl2, CaCl2, however, these inhibitors doesn’t have ideal inhibition effect. The reason could be ascribed to that the researches are conducted from the perspective of physical properties such as water absorption rather than the inhibition mechanism11. The study of the new efficient inhibitors based on the mechanism of coal spontaneous combustion will certainly promote the prevention and control of coal spontaneous combustion. It is believed in the free radical theory of coal spontaneous combustion that coal generates a lot of alkyl free radicals after breaking, then alkyl radicals react with oxygen to form peroxides radicals; the heat generated could make the peroxide radicals further react to form hydroperoxide, which could decompose according to different structures, and ultimately CO, CO2 and other gases are generated12, 13

. During the oxidation of coal, the radical reactions produce activation centers, accompanied by the

generation of gases such as CO and CO2. Meanwhile, gas generation also reflects the number of activation centers, namely, the degree of free radical chain reaction14, 15. Therefore, based on the further research on coal spontaneous combustion mechanism, it is proposed to inhibit the coal spontaneous combustion by terminating the chain reaction with antioxidants and judging the effect of inhibition by gas generated. Antioxidants were mostly used in the petroleum, rubber industry, but were less used in the coal mines. Yang Yunliang et al.16 found that antiagers with different

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concentrations had a significant inhibitory effect on coal oxidation and spontaneous combustion, especially, showed a better inhibition effect than inorganic salts in the later period. On the basis of different concentration and dispersion, Yu Shuijun17 studied the amount of CO production in the process of coal low temperature oxidation with antiager A. It was concluded that after applying dispersant, dispersion degree of antiager A in water increased and the inhibition effect of resistance was improved. Deming Wang11 analyzed the effect of polyethylene glycol on coal spontaneous combustion with different metamorphism degrees using infrared spectroscopy, it was proved that this inhibitor could greatly inhibit the formation of surface functional groups in the coal oxidation process. Guolan Dou18 studied the inhibition effect of tea polyphenol on coal spontaneous combustion, and believed that the inhibition was ascribed to the inhibition of peroxide radicals. Liyang Ma19 studied the sustained inhibition effect of complex inhibitors of acrylic acid and ascorbic acid on five kinds of coal samples, it is believed that the complex inhibitor has the characteristics of eliminating heat accumulation and preventing free radical chain reaction. Zenghua Li20 selected the commonly used rubber antiager, diphenylamine, as the inhibitor to study its inhibitory effect on the spontaneous combustion of coal, it was proved that diphenylamine could inhibit the oxidation process of coal. These studies have demonstrated that the antioxidant has a good inhibitory effect on the spontaneous combustion of coal. However, the above studies simply analyzed the inhibitory effect of a certain kind of antioxidant and didn’t comprehensively and systematically study the various antioxidants and their mechanism. In view of this, it is necessary to conduct further research of the antioxidant type inhibitors and its mechanism of inhibition. Antioxidants can be classified into six categories according to the different antioxidant mechanisms21. The specific mechanism of the various antioxidants is shown in table 1:

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Table 1 Types of antioxidants and antioxidation mechanisms Types of Antioxidants Radicals absorber

Antioxidation mechanisms Inactivate the lipid radicals

Antioxidants Phenolic compounds Organic compounds of trivalent Phosphorus, organic compounds of Sulfur

The hydroperoxide converts to an inactive state

Hydroperoxide decomposer

Combined with free radicals to generate inactive substances Inhibit oxidation through its own redox reaction The metal ions are chelated into an inactive substance Enhance the activity of free radical absorbers

Free radical quencher Oxygen scavenger Chelating agents Synergist

Tetramethylpiperidine Vitamin C, SOD and so on Phosphate, Maillard reaction compounds, citric acid, EDTA Citric acid, phytic acid

In this study, six antioxidants with different mechanism were selected, the inhibition effect of each antioxidant on coal spontaneous combustion was analyzed by the Temperature-Programmed Reaction. Based on the change of gas components, crossing point temperature, apparent activation energy, and the inhibitory rate at 100 °C for various types of antioxidants before and after inhibition during the coal oxidation process, the inhibitory effect of various antioxidants were analyzed and compared.

2. Experiments and methods 2.1 Preparation of coal samples The used coal sample was Chifeng Diatomite with a low degree of metamorphism and was easy to spontaneous combustion. Table 2 Industrial analysis and elemental analysis Industrial Analysis Moisture Mad % 8.32

Ash content Ad % 13.89

Volatiles Vdaf % 30.46

Elemental Analysis Fixed carbon FCd % 47.33

Odaf

Cdaf

Hdaf

Ndaf

% 18.58

% 74.63

% 4.47

% 0.86

There are a lot of kinds of antioxidants, among which antioxidants were chosen according to the physical and chemical properties and the environment-friendly as well as the actual use conditions in coal mines. The selected antioxidants are shown in Table 3.

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Antioxidants type

Radicals absorber

Experimental chemicals

Butylated Hydroxytoluene (BHT)

Table 3 Choice of antioxidant experimental chemicals Hydroperoxide Free radical Oxygen Chelating decomposer quencher scavenger agents 2,2,6,6Triphenyl Tetramethyl-1Ascorbic Acid Edetic Acid Phosphite piperidine-N(VC) (EDTA) (TPPI) oxyl free radical (TEMPO)

Synergist

Phytic Acid (PA)

Molecular structure Purity Manufacturer

AR Sinopharm

CP Aladdin

CP Sinopharm

AR Sinopharm

AR Sinopharm

AR Sinopharm

Chifeng coal samples were coring crushed, and screened in accordance with the grades. Coal samples with a diameter of 0.180-0.380 micron meters were reserved. Weigh 0.5 g of every kind of antioxidants, 10 g of acetone, 10 g of water, which were stirred with a glass rod to make them evenly mixed; afterwards, the mixture was put into 40 g of prepared coal sample, after evenly mixing it was stored in the cool place for 24 hours. The inhibited coal samples were placed in a tray for drying and then placed in a coal sample tank to carry out a Temperature-Programmed Reaction. In order to ensure the objectivity and authenticity of experiments, the same volume of solvent was added into the comparison coal samples.

2.2 Study on low-temperature oxidation of coal gas composition Coal can produce a variety of gases (CO, CO2, C2H4, C2H2, etc.) in the process of oxidation, the relationship between the minimum temperature, the gas generation and coal temperatures varies with different coal samples22, 23. Therefore, the production of these gases can basically and accurately reflect the degree of coal spontaneous combustion, and then reflect the inhibition effect of antioxidants. The quantitative relationship between the coal temperature and the gas generation is systematically understood by the gas formation characteristics during the simulation experiment. The basic principle of the simulation experiment is that a certain amount of air flow was evenly mixed with the coal sample installed in a certain device, at the same time, the coal sample was heated at a certain temperature or was programmed heated to a certain temperature. The composition and 5


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content of produced gas and the oxygen consumption were analyzed to get the law of the change with the temperature, as well as build the quantitative relationship between gas content and the coal temperature. Each prepared coal sample was placed in the sample tank (see Fig.1). The air flow rate was 50 ml/min, the programmed heating experiment was initiated after a check of gas paths, with the heating rate of 0.5 °C/min. The gases were sent to the gas chromatograph for every 10 °C temperature rise within the range of 30-180 °C. The gas components and concentrations at the outlet were calculated.

sample tank 99.9¡ æ

Gas Chromatograph Programmed temperature oven Air

Hydrogen Nitrogen

PC

Gas generator

Figure 1. Temperature programmed and chromatographic analysis device

2.3 Cross point temperature experiment The metal basket method24 was used to test the crossing point temperature (CPT). The size of the metal basket is 5×5×5 cm, and the mesh aperture is smaller than 0.05mm. The coal samples which had been kept at 105 °C for 10 h under the protection of N2 were placed in the basket after it cooled down to room temperature, and the metal basket was suspended in the programmedtemperature device. Two thermocouples with a precision of 0.1 °C were arranged at the center and the edge of the coal sample respectively. With the heating rate controlled to 8 °C/min, the data acquisition system was turned on to automatically record the temperature data measured by the two sensors. 6


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At the initial stage, the temperature at the center of the coal sample is lower than that at the edge, then began to rise with the heat transfer of equipment temperature and the exothermic oxidation of coal, and the heating rate increased with the increase of oxidation degree. At a certain time point, coal temperature was bound to be higher than the furnace temperature, recorded the cross point temperature at this time. The cross point temperature reflects the level of difficulty for the reaction between the coal sample and the oxygen25. Therefore, the cross point temperature can be used as the auxiliary index to judge the inhibition ability of the antioxidant. The higher the cross point temperature is, the less coal self-heating and the spontaneous combustion tendency is, the better inhibition ability of antioxidants is.

2.4 Infrared spectrum experiment The surface functional groups of coal samples were tested before and after the inhibition of six kinds of antioxidants at 100 °C on the Vertex 80V Fourier transform infrared spectroscopy (Bruker Company, Germany). The inhibited samples were ground to smaller than 74μm, keep it for 30 min at 100 °C under air conditions, then cool naturally to room temperature. 1 mg of coal sample and 1g of dried KBr analytical reagent was weighed and put into a grinding machine together to be ground for about 10 min to enable them to mix well. The mixed sample was taken and evenly spread on a press mould that was 13 mm in diameter. Kept under 15 MP pressure for 1 min, the sample was pressed into an even flake for the spectroscopic measurement. The experimental wave range was 400-4000 cm-1, and the sample was scanned 64 times. The blank background was collected before each experiment; the data obtained were smoothed; and the baseline of the spectrum was corrected26.

3 Results and discussions 3.1 Analysis of gas composition and mechanism of antioxidant after inhibition 3.1.1 Analysis of Gas composition change and mechanism after BHT inhibition

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Concentration of every gas (CO, CO2, O2, C2H4) at the outlet of each reaction phase (30 °C180 °C) after BHT inhibition was measured and compared with the data of raw coal. The gas concentration curve at each temperature point is shown below: 7000

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It can be seen from the above figures; as for the oxygen consumption, the oxygen consumption and oxygen consumption rate of the raw coal are larger than those of the treated coal sample at the same temperature. Oxygen consumption of raw coal shows a steady acceleration trend lower than 140 °C, oxygen consumption rate shows a sudden increase after 140 °C, the oxygen concentration at the outlet drops to below 5% when the temperature is higher than 180 °C. The change of oxygen concentration after the BHT treatment is very small in the beginning 30-60 °C range, the oxygen consumption rate is much lower than that non-treated coal sample in the whole process, and the oxygen concentration at the outlet is 12% at 180 °C. To analyze by the oxidation products, when the coal temperature is lower than 60 °C, the degree of coal oxidation is relatively low, so the

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concentration of released CO and CO2 is relatively low; when the temperature is higher than 60 °C, with the increase of the temperature of coal samples, CO and CO2 concentrations of raw coal samples and the treated coal samples both show an increasing trend. Production rate of CO and CO2 are both accelerated and the oxygen consumption and oxygen consumption rate of raw coal samples are significantly higher than those of the treated coal sample. Compare the CO generation curve, the CO2 generation curve, and the oxygen consumption curve at each temperature point, it can be seen that the coal sample treated by BHT has an obvious inhibitory effect on coal spontaneous combustion. Antioxidant BHT is an excellent free radical absorber that can act as an electron or hydrogen donor to capture the oxygen-containing active free radicals produced in the oxidation reaction of the coal, and to convert the free radicals into relatively stable compounds, thereby to terminate the chain oxidation reaction of free radicals and lower the oxidation rate. The specific inhibition mechanism can be expressed as follows21, 27: R& + AH = RH + A& ROO& + AH = ROOH + A& RO& + AH = ROH + A&

Among them: AH is the antioxidant BHT

A& is free radicals formed by antioxidants BHT loses H atom. Since free radicals newly formed by BHT losing H atoms are more stable and are not easy to induce the generation of new free radicals, so the addition of BHT can inhibit the reaction chain to achieve the purpose of inhibiting coal oxidation. 3.1.2 Analysis of gas composition change and mechanism after TPPI inhibition Concentration of every gas (CO, CO2, O2, C2H4) at the outlet of each reaction phase (30 °C180 °C) after TPPI inhibition is measured and compared with the data of raw coal. The gas concentration curve at each temperature point is shown below:

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Figure 3. Curves of gas composition change after TPPI inhibition

As can be seen from the above figures, as for the oxygen consumption, the oxygen consumption of the TPPI treated coal was not obvious changes, the outlet concentration of oxygen drops from the initial 20.92% to 20%, a decrease of 0.95%. Under the same conditions, the oxygen concentration at the outlet of raw coal is rapidly reduced to 4.5%, a decrease of 16.41%. To analyze the oxidation product, after inhibition, the amount of CO, CO2 and C2H4 produced and the rate of production of coal samples was lower than that of raw coal. Compare the carbon monoxide generation curve, the carbon dioxide generation curve, the ethylene generation curve and the oxygen consumption curve at each temperature point, it can be seen that the coal sample treated by TPPI has an obvious inhibitory effect on coal spontaneous combustion. Hydroperoxides can be decomposed in two ways, homolytic process or heterolysis process: ROOH = RO& + O& H ROOH = ROO − + H +

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activation energy of the heterolysis process, so the hemolytic reaction can take place at room temperature. TPPI could effectively compete with the homolytic process of the hydroperoxide groups, in which hydroperoxides could be decomposed into the compounds without free radicals and no reactive products, thereby inhibiting the self-oxidation of branched chain free radicals to prevent the further oxidation reaction. In addition, TPPI and hydroperoxide radicals can also conduct the following reactions28: (PhO)3 P + ROO& → [ROOP(OPh )3 ]. → RO& + O(PhO)3 (PhO)3 P + RO& → [ROPH(OPh )3 ]. → PhO& + ROP(PhO)2

PhO& + ROO& → Inactive substance In addition to decompose hydroperoxides, TPPI is also a chain termination agent to achieve the purpose of inhibiting coal oxidation. 3.1.3 Analysis of gas composition change and mechanism for TEMPO inhibition Concentration of every gas (CO, CO2, O2, C2H4) at the outlet of each reaction phase (30 °C180 °C) after TEMPO inhibition was measured and compared with the data of raw coal. The gas concentration curve at each temperature point is shown below: 7000

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Figure 4. Curves of gas composition change after TEMPO inhibition

At the temperature range of 30 °C-70 °C, the oxygen consumption of TEMPO treated coal has slightly reduced as compared to the raw coal; in the temperature range of 80 °C-140 °C, the oxygen consumption of the inhibition treated coal sample decreased slowly from 19.68% to 17.95%, only a decrease of 1.73%, while oxygen consumption of the raw coal decreased by 4.3% in this stage. This conclusion can also be obtained from gas generation curves. Therefore, the effect of TEMPO addition on the low temperature oxidation of coal samples is not obvious in the early stage, but in the later stage, the inhibition effect begins to increase. Compare the carbon monoxide generation curve, the carbon dioxide generation curve, the ethylene generation curve and the oxygen consumption curve at each temperature point, it can be seen that the coal sample treated by TEMPO has an obvious inhibitory effect on coal spontaneous combustion. After adding TEMPO, it acts as an electron acceptor and reacts quickly with the active free radicals at the initial stage of the radical reaction with coal29. H3C H3C R

+

CH2

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HO

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lone paired electron of high activity in these free radicals to lose reactivity. In the early stage of low temperature oxidation of coal samples, the free radicals were less and the inhibition effect was not obvious, but the late inhibition effect was higher. This effect of TEMPO inhibits the chain reaction of free radicals, which greatly reduces the free radical concentration and free radical reactivity, so that the reaction rate was decreased. 3.1.4 Analysis of gas composition change and mechanism after VC inhibition Concentration of every gas (CO, CO2, O2, C2H4) at the outlet of each reaction phase (30 °C180 °C) after VC inhibition was measured and compared with the data of raw coal. The gas concentration curve at each temperature point is shown below: 10000

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As can be seen from the figure above, the addition of VC does not inhibit the oxidation. When the temperature is less than 140 °C, the oxygen variation curve of the coal sample with ascorbic acid is basically consistent with the variation curve of oxygen in the raw coal. The oxidation rate of coal

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added with ascorbic acid is even slightly larger than that of the raw coal after 140 °C. To analyze the oxidation product, when the temperature reaches 180 °C, the concentration of carbon monoxide in coal samples with VC is 6287 ppm, which is greater than that of 4863 ppm of raw coal. Carbon dioxide production law is similar to carbon monoxide, carbon dioxide concentration is 38452ppm, greater than 29837 ppm of raw coal. Thus, it can be seen that VC has no inhibitory effect on coal spontaneous combustion, but plays a certain role in promoting it. The main mechanism of oxygen scavenger is to provide directly electrons to reduce oxygen free radicals, and to remove oxygen and oxygen free radicals through their own oxidation reaction to delay the occurrence of oxidation30. VC itself cannot absorb and decompose free radicals, but it has strong reducibility, so that it can consume oxygen in the coal to reduce the oxygen concentration, at the same time it is oxidized to dehydrogenated ascorbic acid. It is relatively stable in dry air, iron and copper and other metal ions will accelerate the oxidation to form a stable metal salt, as well air and heating are likely to cause deterioration and failure. The stimulating effect of VC on the spontaneous combustion of coal may be related to this.

3.1.5 Analysis of gas composition change and mechanism after EDTA inhibition Concentration of every gas (CO, CO2, O2, C2H4) at the outlet of each reaction phase (30 °C180 °C) after EDTA inhibition was measured and compared with the data of raw coal. The gas concentration curve at each temperature point is shown below: 35000

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Figure 6. Curves of gas composition change after EDTA inhibition

As can be seen from the above figures, between 30 °C and 50 °C, the oxidation degree of coal is relatively low with no CO release; with the rise of coal sample temperature, the generation amount and rate of the raw coal are both significantly higher than those of the treated coal sample. Produced by the ethylene curve can be seen, after the treatment the initial temperature of ethylene is delayed from 110 °C to 130 °C. Compare the carbon monoxide generation curve, the carbon dioxide generation curve, the ethylene generation curve and the oxygen consumption curve at each temperature point, it can be seen that the coal sample treated with EDTA has an obvious inhibitory effect on coal spontaneous combustion. The transition metal ions in the coal can reduce the activation energy of the reaction to accelerate the Fenton reaction. The transition metal ions can react with the hydroperoxide to produce the alkoxy radicals and the hydroxyl radicals31, 32, thereby accelerate the free radical chain reaction :

Metal ion chelating agent EDTA is a good chemical compounding agent, its four carboxylic acids and two amine parts can be used as the teeth of ligand, it could bind with divalent metal ions except alkali metal ions by coordinating linkage and with coordination bonds 1: 1 to form a stable complex multiple pentacyclic complexes. The bound method of EDTA with metal ions is shown below33:

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Metal ion chelating agent EDTA reduces its catalytic activity by complexing metal ions to reduce the rate of Fenton reaction and reduce the formation of free radicals. At the same time, the complex structure enhances the stability of the active structure in coal and reduces the porosity of coal, so that the oxidation of coal is more difficult to happen. 3.1.6 Analysis of gas composition change and mechanism after PA inhibition Concentration of every gas (CO, CO2, O2, C2H4) at the outlet of each reaction phase (30 °C 180 °C) after PA inhibition was measured and compared with the data of raw coal. The gas concentration curve at each temperature point is shown below: 35000

7000

Raw coal coal—PA

6000

25000


5000


Raw coal coal—PA

30000

20000

4000

15000

3000

10000

2000 1000

5000

0

0 20

40

60

80

100

120


140

160

180

20

200

60

40

60

80

100

120


140

160

180

200

22

Raw coal coal—PA

50

Raw coal coal—PA

20 18 16


40


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

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14 12

30

10

20

8 6

10

4 2

0

0 20

40

60

80

100

120


140

160

180

200

20

40

60

80

100

120


140

160

180

200

Figure 7. Curves of gas composition change after PA inhibition

After adding PA, the oxygen consumption of coal sample dropped from 20.85% to 19.64% at 16


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Energy & Fuels

30-140°C. Meanwhile, the oxygen concentration of raw coal decreased from 20.86 % to 13.42 %, and decreased by 7.45%. The concentration of other gas products is lower than that of raw coal. Compare the carbon monoxide generation curve, the carbon dioxide generation curve, the ethylene generation curve and the oxygen consumption curve at each temperature point, it can be seen that the coal sample treated with PA has an inhibitory effect on coal spontaneous combustion. PA has a negatively charged phosphate in each carbon atom, plays a certain inhibitory effect on the coal spontaneous combustion through the chelating effect of metal ions, but it showed poor antioxidant activity. When it was used with other antioxidants, it can greatly improve the antioxidant effect, and it can be widely used in complex antioxidants34.

3.2 Comparison of inhibition effect of antioxidants In order to compare the antioxidants’ inhibition effect on spontaneous combustion of coal, the apparent activation energy of coal samples before and after the antioxidant treatment were compared, and the inhibition effect at 100 °C was calculated. 3.2.1 Change of cross point temperature Cross point temperatures of raw coal and the antioxidant added coal are shown in Table 4: Table 4 Cross point temperatures of raw coal and the antioxidant added coal Coal sample Size/mm CPT/°C

Raw coal 0.18-1.38 128.6

Coal-BHT 0.18-1.38 145.5

Coal-TPPI 0.18-1.38 135.3

Coal-TEMPO 0.18-1.38 148.4

Coal-VC 0.18-1.38 123.7

Coal-EDTA 0.18-1.38 138.4

Coal-PA 0.18-1.38 134.2

The cross temperature reflects the difficulty of coal oxidation, except the cross point temperature of VC treated coal showed a more forward moving trend than that of raw coal, other five antioxidants treated coal has a delayed cross point temperature, indicating that VC could promote the coal oxidation process to some extent; BHT, TPPI, TEMPO, EDTA, PA could inhibit coal from spontaneous combustion, among which cross point temperature of TEMPO inhibited coal sample reached 148.4 °C, delayed by 19.8 °C, showing the best inhibitory effect.

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Page 18 of 26

3.2.2 Apparent activation energy test From the Arrhenius equation, the rate of coal oxidation at any temperature is35： v(Ti ) = v(O2) = v(CO) / m = v(CO2) / g = Ac on2 exp(−E / RTi)

(1)

where ν is the reaction rate; Ti represents the thermodynamic temperature of coal, K; A is the pre-exponential factor; n standards for the reaction order which equals 1 the low-temperature oxygen adsorption stage; E is the activation energy, J/mol; and R is the molar gas constant, 8.314 J/(mol • K). In the heating experiment, the air flow was assumed to move along the axial direction of the coal sample. Then, the reaction rate of CO at a section dx taken from any axial is as follows:

Sv(CO)dx = kvgdc

(2)

By substituting Eq. (1) into Eq. (2), the following equation can be obtained: ASmc on2 exp(−E / RTi)dx = kvgdc

(3)

Eq. (4) can be acquired by integrating both ends of Eq. (3):

∫

L

0

ASmcon2 exp(− E / RTi ) dx = ∫

Cout

0

kvgdc

(4)

Eq. (5) can be acquired by logarithms on both sides of Eq. (4):

ln cout = −

 ASLmc on2 E + ln RTi  kvg

   

(5)

When the gas flow is constant, lnCout is linearly related to 1/T, and the apparent activation energy of the reaction can be obtained by calculating the slope of the straight line. As reported in the literature36, the apparent activation energy of coal samples at accelerated temperature oxidation stage (70 °C -120 °C) can better reflect the difficulty of oxidation, which is very important to understand and compare the inhibition effect of various antioxidants. According to the CO gas concentration curve of the coal samples in the low temperature oxidation experiment before and after the inhibition, the apparent activation energy change in the coal heating stage

18


Page 19 of 26

(70 °C-120 °C) was obtained by the relevant data and calculation model, which is shown in figure 8. 7.0

7.0

Raw coal Coal-BHT

6.5

6.0

Raw coal Coal-TPPI 6.5

y=19.22298+40.40431x 2 R =0.97642 lnCout

lnCout

6.0

y=19.22298+40.40431x 2 R =0.97642

5.5

5.0

y=20.91402+48.94927x 2 R =0.9688

4.5

5.5

5.0

y=20.16744+45.2264x 2 R =0.96236

4.0 4.5

3.5 -0.35

-0.34

-0.33

-0.32

-0.31

-0.30

-0.35

-0.34

7.0

7.0

Raw coal Coal-TEMPO

6.5

6.0

-0.33

-0.32

-0.31

-0.30

-1000/RT

-1000/RT Raw coal Coal-VC

6.8

y=19.22298+40.40431x 2 R =0.97642

6.6 6.4

y=18.48081+36.95884x 2 R =0.94925

lnCout

lnCout

6.2 5.5

5.0

4.5

6.0 5.8

y=19.22298+40.40431x 2 R =0.97642

5.6

y=23.0459+55.18747x 2 R =0.98176

5.4 5.2

4.0

5.0 -0.35

-0.34

-0.33

-0.32

-0.31

-0.30

-0.35

-0.34

-1000/RT

-0.33

-0.32

-0.31

-0.30

-1000/RT

7.0

7.0

Raw coal Coal-EDTA

Raw coal Coal-PA

6.5

6.5

6.0

y=19.22298+40.40431x 2 R =0.97642

6.0

y=19.22298+40.40431x 2 R =0.97642

5.5

lnCout

lnCout

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

Energy & Fuels

5.5

5.0

y=20.55644+46.54343x 2 R =0.96991

5.0

y=18.59477+42.56692x 2 R =0.9852

4.5

4.0

4.5 3.5

-0.35

-0.34

-0.33

-0.32

-0.31

-0.35

-0.30

-0.34

-1000/RT

-0.33

-0.32

-0.31

-0.30

-1000/RT

Figure 8. The apparent activation energy change in the coal heating stage (70 °C -120 °C)

CO production of all coal samples in the low temperature oxidation process and relevant data could be used to obtain the apparent activation energy. The apparent activation energy of raw coal and inhibition coal are shown in Table 5 below. Table 5 The apparent activation energy of raw coal and inhibition coal Coal sample

Size/mm

Raw coal Coal- BHT Coal-TPPI

0.180-1.380 0.180-1.380 0.180-1.380

Temperature range /°C 70-120 70-120 70-120 19


Apparent activation energy /kJ·mol-1 40.40 48.95 45.23

Energy & Fuels

Coal sample

Size/mm

Coal-TEMPO Coal-VC Coal-EDTA Coal-PA

0.180-1.380 0.180-1.380 0.180-1.380 0.180-1.380

Temperature range /°C 70-120 70-120 70-120 70-120

Apparent activation energy /kJ·mol-1 55.19 36.96 46.54 42.57

From the apparent activation energy data, it can be obtained that, except VC's apparent activation energy is 36.96 kJ·mol-1, slightly lower than that of the raw coal sample 40.40 kJ·mol-1; the apparent activation energy of other antioxidant is relatively higher than that of raw coal. Apparent activation energy could be ordered: ETEMPO>EBHT>EEDTA> ETPPI> EPA, among which apparent activation energy of TEMPO increased most, an increase of 14.79 kJ·mol-1. 3.2.3 Changes of surface functional groups Coal oxidation begins first from the surface of the coal and through the surface of the coal particles to deep inside, so the study of coal surface is of great significance7. After the oxidation of coal, the type and structure of the surface groups will change greatly. According to the change of the reactive functional groups, the oxidation degree of the coal can be clearly judged, and the resistance of the antioxidant effect can be further judged. The surface functional groups of coal samples before and after treatment with different antioxidants at 100 °C were shown in Figure 9. 0.35 0.30

Kubelka-munk（ a.u）

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

Page 20 of 26

0.25

Raw coal Coal-BHT Coal-TPPI Coal-TEMPO Coal-VC Coal-EDTA Coal-PA

0.20 0.15 0.10 0.05 0.00 4000

3500

3000

2500 2000 Wavenumber(cm-1)

1500

1000

500

Figure 9. Changes of surface functional groups of coal samples before and after treatment with different antioxidants

It can be seen from the above figure that in addition to the slight increase in the surface 20


Page 21 of 26

functional groups of the coal sample treated with VC, the functional groups of other antioxidants treated coal samples were reduced. The largest reduction of functional groups on coal surface is the coal samples treated by TEMPO and BHT. In order to quantitatively compare the change of active functional groups in coal before and after treatment, Peakfit software was used for deconvolution of the infrared spectrum images. The key issue before peak fitting was to deduct the baseline. The infrared spectrum of the coal sample is more complex and cannot be deducted simply within the range of 400-4000 cm-1. According to previous studies37, 38, the infrared spectrum of coal was divided into four regions: 3000-3750 cm-1 (hydroxyl and aromatic C-H stretching vibration), 2800-3000 cm-1 (aliphatic C-H stretching vibration, including methyl, methylene and methine), 1522-1800 cm-1 (C=O and aromatic C=C stretching vibration); 1000-1522 cm-1 (C-O stretching vibration and aliphatic functional groups deformation vibration). In each region, the lines made up of two endpoints were used as the baselines and the results were obtained using the mentioned Lorentz/Gauss combination. The changes of four common active functional groups before and after treatment with different antioxidants were shown in figure 10. 5.0 4.5

Raw coal Coal-VC

Coal-BHT Coal-EDTA

Coal-TPPI Coal-PA

Coal-TEMPO

4.0 3.5

Absorption rea

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Energy & Fuels

3.0 2.5 2.0 1.5 1.0 0.5 0.0

-COOH

-CH3

-OH

-CH2-

Figure 10. Changes of four active functional groups after treatment with different antioxidants

It can be seen from the figure that the four active functional groups have different degrees of

21


Energy & Fuels

change after treatment. The four active functional groups of the treated coal samples were decreased compared with those of the raw coal, in addition to the increase of the carboxyl group, methyl group and the methylene group of the VC-treated coal samples. The surface functional groups of coal samples decreased most obviously after TEMPO and BHT treatment, and the carboxyl groups and hydroxyl groups were reduced by more than half compared with the raw coal. 3.2.4 Inhibitory rate calculation The inhibition ability of the inhibitor is generally compared by the method of inhibitory rate method, the greater the inhibitory rate is, the stronger inhibition effect it is. It is possible to calculate the inhibitory rate by the production of CO of raw coal sample at 100 °C and of other antioxidant inhibited coal samples and to determine inhibitory rate and inhibition effect of antioxidants39. The mathematical formula is as follows: R= (A−B) ×100/A Where: R——coal sample resistance rate, %; A—— CO content produced by raw coal sample at 100 °C, ppm B—— CO content produced by inhibited coal samples at 100 °C, ppm According to the CO production of raw coal and five antioxidant inhibited coal samples at 100 °C the corresponding inhibitory rate can be calculated, as shown in Figure 11, inhibitory rate is the proportion of the white section to the entire section. 700

600

CO concentration at 100? ( ppm)

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

Page 22 of 26

A-B B R=61.25%

R=51.44%

R=73.08%

R=58.54%

R=45.73%

BHT

TPPI

TEMPO

EDTA

PA

500

400

300

200

100

0

Figure 11. CO production and corresponding inhibitory rate of antioxidant inhibited coal samples 22


Page 23 of 26

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Energy & Fuels

From the above figure, it could be concluded that five kinds of antioxidants have good inhibition effect on coal spontaneous combustion, and the order of inhibitory effect is RTEMPO> RBHT> REDTA> RTPPI> RPA, and TEMPO has the best inhibitory effect, reaching 73.08%.

4. Conclusions The existed halide salt inhibitor has a relatively low resistivity, considering the defects of these inhibitors and the free radical reaction theory in the process of coal oxidation, it is proposed to use the antioxidant which can inhibit the free radical chain reaction as the inhibitor of coal spontaneous combustion. Six different types of antioxidants were used for inhibition experiments. The oxidation products (CO, CO2 and C2H4) and O2 consumption before and after the inhibition of different antioxidants were compared by means of low temperature oxidation experiment. It is concluded that, except VC, other five kinds of antioxidants: free radical absorber BHT, free radical quencher TEMPO, metal ion chelator EDTA, hydroperoxide decomposer TPPI, synergist PA, have an inhibitory effect on coal spontaneous combustion. The mechanism of the inhibition of these antioxidants is to reduce the generation of free radicals directly or indirectly during the oxidation of coal, and inhibit the free radical chain reaction. The change of cross point temperatures, apparent activation energy at 70 -120 °C and the surface functional groups at 100 °C were compared before and after the antioxidant treatment. Finally, the resistivity of each antioxidant at 100 °C was calculated. The results showed that the inhibition effect could be ordered in the following sequence: TEMPO> BHT> EDTA> TPPI> PA, among which TEMPO was the best, the inhibitory rate reached 73.08%.

23


Energy & Fuels

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Inhibitive Effects of Antioxidants on Coal Spontaneous Combustion

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