Advertisement

Journal of Thermal Analysis and Calorimetry

, Volume 133, Issue 1, pp 453–463 | Cite as

Effects on the activities of coal microstructure and oxidation treated by imidazolium-based ionic liquids

  • Jun Deng
  • Zu-Jin Bai
  • Yang Xiao
  • Chi-Min Shu
Article

Abstract

To study the effect of ionic liquids (ILs) of the microstructure on the surface of the coal, four ILs ([Emim][BF4], [Bmim][BF4], [Bmim][NO3], and [Bmim][I]) were selected to treat the coal samples. Fourier transform infrared spectroscopy and synchronous thermal analyzer were employed to conduct the experimental tests. Active functional groups were analyzed when the ILs contained the same anion or cation. The results indicated that the quantity of the hydrocarbyls and the oxygen-containing functional groups for the coal sample treated by [Emim][BF4] was significantly less than the three coal samples by other ILs treated, in which the maximum area ratio of the hydrocarbons and the oxygen-containing functional groups was 0.553 and 1.159, respectively. However, ILs had lesser destructive effects on the aromatic hydrocarbons. The ILs containing [Emim]+ shared stronger destructibility to the coal’s micro-active structure than that containing [Bmim]+. The highest impact was the hydrocarbyl of the coal. While including the same [Bmim]+, the extent of destruction to the hydrocarbyls and the oxygen-containing functional groups of coal was varied in descending order as [NO3] > [I] > [BF4]. The aliphatic hydrocarbons were destroyed by the anion of ILs following the order: [I] > [BF4] > [NO3]. During the low-temperature oxidation stage, the apparent activation energy increased, whereas the reactivity of coal samples by ILs treated decreased in the order: [Bmim][NO3] < [Bmim][I] < [Bmim][BF4] < [Emim][BF4].

Keywords

Fourier transforms infrared spectroscopy Functional groups Destructive effects Apparent activation energy Reactivity 

Introduction

Coal spontaneous combustion is wide spread in the main coal-producing countries in the world [1, 2] and causes huge personal casualties and property losses as well as damaging the company’ social images [3, 4]. The existing technologies of mine fire prevention are mainly divided into inhibiting, inerting, and cooling. Among them, the inhibitor for the coal spontaneous combustion mainly achieves physical resistance to alter the physical properties, such as the temperatures of the coal itself and its adjacent surroundings. Nevertheless, the spontaneous combustion of coal cannot be fundamentally and effectively governed. With the passage of time, the effect of inhibition gradually fades out and is finally completely eliminated. While the chemical inhibitor can cause changes in the molecular structure of coal, it reduces reactive oxidation. Therefore, development of an inhibitor of chemical agent can break up microcosmic active groups of coal or inhibit the activity of coal structure to dictate its spontaneous combustion.

During the past century, the theory of coal spontaneous combustion substantially developed in a variety of directions. Based on the mechanism of coal and oxygen, coal spontaneous combustion is a complex physical and chemical process, which is a continuous composite reaction between reactive functional groups in coal molecules and oxygen. It emits heat and causes the coal to heat up and finally reaches the burning stage [5, 6]. Reactive groups are mainly distributed in the side chain and the bridge key position in the molecule of coal, so that the number and active strength of the coal in the process of low-temperature oxidation have vital effects [7, 8].

Ionic liquid (IL) [9, 10] is a kind of environmental-friendly green solvent, with had features such as low volatility, low distillation, tasteless, incombustibility, tractability, and high applicability. Therefore, we can use the high solubility properties of organic and inorganic in ILs to more readily dissolve oxidized functional groups in coal. This technology not only reduces the risk of spontaneous combustion of coal but also has prominent effects in safeguarding the environment. It provides a new direction and prevention for spontaneous combustion of coal. At present, extraction, dissolution liquefaction, depolymerization, and desulfurization of ILs to coal are being researched [11, 12].

Cao and Li et al. [13, 14] interpreted the effects of dissolution and inflation for coal samples pre-treated by ILs first. The molecular structure of the surface for coal sample pre-treated IL was changed based on the display of infrared spectrum. It is speculated that the physical and chemical forces are weakened in coal molecules that free energy is lessened and the oxidation performance is weakened in the coal structure. Furthermore, the performance of oxidation for coal sample structure is weakened. Painter [15, 16] and Pulati et al. [17] demonstrated that coal samples may be dissolved and destroyed in different degrees by [Bmim][Cl], [Bmim][CF3SO3], [Bmim][BF4], [Bmim][PF6], and [Pmim][I]. The ability to dissolve and destroy coal samples treated with [Bmim][Cl] is stronger than that of [Bmim][BF4]. Containing [Cl] counter ions of ILs can destroy and disperse coal samples to a greater extent than containing other larger anions. Kim et al. [18] implied that ILs were able to break and disperse coal samples. In addition, the solubility was enhanced with increased length of the alkyl ether linkage between the two imidazole rings. Bai et al. [19] found the interaction between the ILs and the hydrogen bond of the asphalt diluents, the π-cation, and the charge transfer complexes. As a result, a portion of the substance is dissolved.

Wang et al. [20, 21] and Zhang et al. [22, 23] reported that ILs had different effects on the aromatic structure, aliphatic chain hydrocarbons, and oxygen-containing functional groups, such as hydrocarbon groups and carbonyl groups. They indicated that the coal spontaneous combustion characteristics were alleviated by ILs.

Hu et al. [24] and Lei et al. [25, 26, 27] found that the ILs has an efficient depolymerization performance on the coal. Through products analysis, they found ILs destruction of coal molecules of various bonds and force. Moreover, it can loosen coal network structure and enhance the degree of swelling and depolymerization performance of coal. Similarly, Cummings et al. [28] determined that [Bpyd][Cl], [Bmim][Cl], [Bmim][TCM], and [Emim][DCA] could decompose the macromolecule structure of coal and increase the short chain of aliphatic hydrocarbons, in which [Emim][DCA] can dwindle the number of functional groups COOH and CO. However, these studies did not pay much attention to effects of cations and anions on specific functional groups in the microstructure of coal. Considering the sound dissolution and destruction properties, the effects of different anions and cations on the coal microstructure were explored, in which four kinds of imidazole ILs were selected as solvents. The impact of different anions and cations on the specific functional groups in the microscopic and the apparent activation energy (Ea) at the macroscale for coal oxidation with low-temperature oxidation stage were analyzed and discussed. The aim of the results was to provide a basis for inhibiting spontaneous coal combustion and preparing functionalized ILs for different coal types in coal fields and applications.

Experimental and methods

Specimens’ preparation

Preparation of raw coal samples: The experimental object was the fresh bituminous coal (weakly caking coal) from Wangwa II coalmine, Ningxia, China. The coal was pulverized, ground, and sieved to acquire a sample having a particle size of 200.0–280.0 μm and dried under vacuum at room temperature for 24.0 h and then placed in a shadow and sealed for preparation. The results of coal quality indicators are described in Table 1. All the ILs were received from Lanzhou Institute of Physical Chemistry in the Chinese Academy of Sciences. Their physical properties are given in Table 2 [29].
Table 1

Proximate analyses of four ionic liquids-treated coal samples and untreated samples

Sample

M ad

A ad

V ad

FC ad

[Emim][BF4]-tr

10.19

6.99

37.58

45.24

[Bmim][BF4]-tr

8.87

6.86

30.79

53.48

[Bmim][NO3]-tr

7.98

7.16

55.55

29.31

[Bmim][I]-tr

7.44

7.56

49.53

35.47

Un-tr

10.31

9.71

35.54

44.44

Table 2

Physical properties of four ILs [27]

Name

Schematic

Thermal decomposition temperature/°C

Soluble

Insoluble

Purity/%

Acidity

[Emim][BF4]

Open image in new window

412

Water, dichloromethane, acetonitrile, etc.

Ethyl acetate, ether, and alkane

> 99

Acid

[Bmim][I]

Open image in new window

265

Water, ethanol, acetonitrile, and dichloromethane, etc.

Ether and alkanes

> 99

Alkali

[Bmim][BF4]

Open image in new window

403

Water, ethanol, acetonitrile, dichloromethane, etc.

Ethyl acetate, ether, and alkanes

> 99

Acid

[Bmim][NO3]

Open image in new window

240

Water, methanol, acetone, and dichloromethane

Nonpolar solvent

> 99

Acid

Preparation of ILs-treated coal samples: The dried coal sample was divided into five and separately mixed in four shares with the ILs. The coal samples were thoroughly mixed with the four ILs at a ratio of 1:2 (g:mL) and agitated with a stirrer for 8.0 h. Figure 1 shows coal samples and ILs in initial contact at room temperature. To completely wash the samples, a centrifuge was applied in this experimental test, which was set at 2500.0 rpm for 20.0 min. The coal samples were rinsed several times with distilled water until presenting a neutral pH. Given coal samples were easily oxidized, coal samples were put vacuum dried in a 27.0 °C oven for 48.0 h until the mass was unchangeable. The ILs were replaced by distilled water, and the comparative coal sample was prepared by the same method. The distilled-washed coal sample which was not pre-treated by the ILs was named as the untreated coal (Un-tr), and the contrasts were the IL-treated coal (IL-tr) samples, where [Emim][BF4] treated, [Bmim][I] treated, [Bmim][BF4] treated, and [Bmim][NO3] treated were [Emim][BF4]-tr, [Bmim][I]-tr, [Bmim][BF4]-tr, and [Bmim][NO3]-tr, correspondingly.
Fig. 1

Appearance of coal samples mixed with ILs (from left to right): [Bmim][I]-tr, [Bmim][NO3]-tr, [Bmim][BF4]-tr, and [Emim][BF4]-tr

Experimental equipment

To investigate the change in type and quantity to compare with the functional groups before and after ILs-treated coal samples, Fourier transforms infrared spectrometer (VERTEX 70, Bruker, Germany) was employed in the experimental tests, which conducted the KBr compression method. (Coal sample and KBr were mixed at a ratio of 1:100.) Measurement conditions are: each coal sample can have 16 times ranging 4000–400 cm−1 with resolution 4 cm−1 scanning it at room temperature.

To acquire calories value, synchronous thermal analyzer TGA/SDC1 (Mettler Toledo, Switzerland) was applied to study variations in apparent activation energy. The initial mass of five coal samples each was 4.6 ± 0.4 mg, and the cell was open. Under room temperature, the temperature rose at the rates of 4.0, 6.0, 8.0, and 10.0 K min−1 and air was introduced at the rate of 100.0 mL min−1 until the coal was completely burned. Xiao and Deng et al. explored that a heating rate of 5.0 K min−1 is the closest to the actual situation for coal body oxidation [30, 31].

Method of identified functional groups

The FTIR experiments were employed to quantify the size of the functional group, which is based on the Lambert–Beer law, shown as follows:
$$A(v) = \lg \frac{1}{T(v)} = a(v)bc$$
(1)
where A(ν) is absorbance at the wave number (ν), T(ν) denotes the transmittance at the wave number (ν), a(ν) expresses the absorbance coefficient at the wave number (ν), which is the absorbance at the wave number (ν) of the measured sample at the unit concentration and unit thickness, b represents the optical path length (sample thickness), and c is the concentration of the sample. The product of b and c in the KBr compression represents the mass of the sample. Since the absorbance of the infrared spectrum is additive, the total absorbance at the wave number (ν) for the mixed sample of N components is:
$$A(v) = \sum\limits_{i = 1}^{n} {a_{\rm{i}} } (v)bc_{\rm{i}}$$
(2)
Moreover, Lambert–Beer law can also be converted to measure the area of the absorption peak. The peak area is also proportional to the thickness and concentration of sample which is more comparable than the absorption peak height [32]. To ensure the consistency of the spectral intensity of the same coal sample, we combined the above Lambert–Beer law and the solution of Schrodinger equation of the effect of quantum mechanics on the tunnel to calculate and compare the proportion behind [33]. To obtain the ratio of different content groups, quantum chemical software is used to quantify the different functional groups and frequency analysis inversion, which is described as follows:
$$C_{\text{x}} :C_{\text{y}} :C_{\text{z}} \ldots = \frac{{f_{\text{Ax}} }}{{f_{\text{x}} }}:\frac{{f_{\text{Ay}} }}{{f_{\text{y}} }}:\frac{{f_{\text{Az}} }}{{f_{\text{z}} }} \ldots$$
(3)
where fA is the area value of the infrared absorption peak of the functional group of coal sample and f is the unit absorption peak area of the functional group.

Method of apparent activation energy calculation

According to chemical reaction kinetics theory, the reaction rate of coal can be deduced as follows:
$$\frac{{{\text{d}}a}}{{{\text{d}}t}} = A{\text{e}}^{ - {\rm{RT}}} f(a)$$
(4)
where A is the pre-exponential factor and Ea is the apparent activation energy, kJ mol−1. R is the gas constant, 8.314 J mol−1 k−1. T is the differential function of the temperature, °C. f(a) is the differential function of the curve.
For the kinetic analysis of the DSC curve during the reaction of coal, the degree of reaction in DSC dynamics is proportional to the thermal effect of the reaction or absorption. Therefore, the conversion rate on the DSC curve can be defined as:
$$\frac{{{\text{d}}a}}{{{\text{d}}t}} = \frac{1}{{H_{\rm{T}} }}\frac{{{\text{d}}H}}{{{\text{d}}t}}$$
(5)
$$\frac{{{\text{d}}H}}{{{\text{d}}T}} = \nabla H$$
(6)
where dH/dT is the exothermic rate, mW mg−1. ΔHT is the total heat release after the reaction that is total enthalpy. Equation (6) is the heat flow rate of solid state. Equation (7) can be deduced by integrating Eq. (6), and it is shown as follows [34, 35, 36]:
$$\ln \frac{{{\text{d}}H}}{{{\text{d}}t}}\frac{1}{Ht} - n\ln \frac{{H_{\rm{t}} - H}}{{H_{\rm{t}} }} = - \frac{{E_{\rm{a}} }}{RT} + \ln A = \ln k$$
(7)

Furthermore, the relationship between the slope and Ea can be obtained as follows: k = −Ea/R, Ea = −kr, and the DSC of five kinds of coal samples can be linearly fitted. When n is 0, the reflected parameters and apparent activation energy of each coal sample can be obtained. (b is the intercept, k is the slope, and R is the correlation coefficient).

Results and discussion

Variations of microstructure

The FTIR peaks of the treated coal samples are shown in Fig. 2. It can be seen that the profiles at which the infrared peaks of the Un-tr and ILs-tr are basically similar. However, the strength of the peaks was different. The main functional groups were all present which shows that the ILs treatment does not change the main the structure of coal. However, the absorption intensity of some peaks had changed. According to the spectral peaks and the empirical analysis experience of the attribution position in main functional groups for the coal molecules, the absorption attribution table of the coal in the infrared spectrum was obtained [20, 37, 38]. We took the peak at 3600–3200 cm−1 range attributed to hydroxyl associative hydrogen bonds. The peak at 2983–2877 cm−1 was assigned to methyl, methylene asymmetric stretching vibration. The peak observed at 2871–2792 cm−1 was attributed to methyl and methylene symmetric stretching vibration. 1763–1505 cm−1 was ascribed to the carbonyl stretching vibration of aldehydes, ketones, and acids. 1482–1399 and 1396–1330 cm−1 were credited to the shear vibrations of methylene and methyl, respectively. 1324–1066 and 1065–956 cm−1 were assigned to phenol, alcohol, ether, ester oxygen bond, and minerals. Table 3 lists the absorption attribution of coal in the infrared spectra.
Fig. 2

FTIR spectra of coal samples treated by four ionic liquids and untreated

Table 3

Absorption peaks of FTIR for coal samples

Peak type

Peak number

Peak position

Functional group

Spectral attribution

Hydrocarbyls

1

3771–3715

–OH

Free hydroxyl

2

3708–3668

–OH

Intramolecular hydrogen bonds

3

3600–3200

–OH

Phenolic hydroxyl, alcoholic hydroxyl or hydrogen bonding between amino molecules

Aliphatic hydrocarbons

4

2983–2877

–CH3, –CH2

Methyl, methylene asymmetric stretching vibration

5

2871–2792

–CH3, –CH2

Methyl, methylene symmetric stretching vibration

6

1482–1399

–CH2–CH3

Methylene shear vibration

7

1396–1330

–CH3

Methyl shear vibration

8

787–711

–CH2

Methylene plane vibration

Aromatic hydrocarbons

9

3070–3005

–CH

Aromatics CH stretching vibration

10

1670–1505

C=C

Aromatic ring C=C stretching vibration

11

900–712

 

Outer bending vibration of various substituted aromatic hydrocarbons

Oxygen-containing functional groups

12

1765–1676

C=O

Ester carbonyl stretching vibration

   

Aldehyde, ketone, acid carbonyl stretching vibration

13

1324–1066

Ar–CO

Phenol, alcohol, ether, ester oxygen bond

Minerals

14

1065–956

Si–O

Si–O ether bond

Table 4 describes the calculation results of functional group f. The peak area values of the five main types of treated coal samples were obtained by integrating the infrared spectra. As given in Tables 4 and 5, and Eq. (3), the proportion of functional groups in different coal samples was obtained, as delineated in Figs. 36.
Table 4

Calculated results of functional group f [29]

 

Hydrocarbyls

Aliphatic hydrocarbons

Oxygen-containing functional groups

Benzene ring

f

632.00

164.14

331.62

7.18

Table 5

FTIR absorption peak area values of main functional groups in coal samples

Coal sample

Hydrocarbyls

Aliphatic hydrocarbons

Oxygen-containing functional groups

Aromatic hydrocarbons

[Emim][BF4]-tr

1.376

1.506

1.205

6.592

[Bmim][BF4]-tr

1.646

1.247

1.062

5.116

[Bmim][NO3]-tr

4.455

2.208

1.465

7.887

[Bmim][I]-tr

1.801

1.185

1.165

5.638

Un-tr

6.291

2.700

2.282

8.837

Fig. 3

Hydroxyl peak area ratio of coal samples

Hydrocarbyl groups

Figure 3 indicates the ratio of hydroxyl-bonded hydrogen bonds. From the FTIR spectra, it is inferred that there are free hydroxyl groups, intramolecular hydrogen bonds, and higher hydrogen bonds in the range of 3600–3200 cm−1. When the degree of association in the Un-tr was higher, the peak height of the hydroxyl-associated hydrogen bond for ILs-treated coal samples compared to Un-tr coal sample was significantly weakened. The order of the content ratio which was obtained from the histogram can be described as: [Emim][BF4]-tr > [Bmim][BF4]-tr > [Bmim][I]-tr > [Bmim][NO3]-tr > Un-tr. The difference in size indicates that the different ILs have different extents of damage to the hydrogen bonds. When containing the same [BF4] in the ILs, the damage of [Emim]+ was stronger than that of [Bmim]+. The results may be attributed to containing [Emim]+ ILs that were more likely to react with coal than the [Bmim]+ ILs and had a stronger effect on swelling for coal, thereby destroying the hydroxyl formed in the coal bonds. In addition, the hydrogen bond absorption peak intensity was crippled or even disappeared. The order of destruction intensity for the cations containing the same [Bmim]+ was [NO3] > [I] > [BF4]. This was caused by three anions weakened by the association of the hydrocarbyl groups.

Aliphatic hydrocarbons

Figure 4 describes the proportion of stretching vibration and symmetric stretching vibration for methyl and methylene asymmetric. According to the histogram, the damage strength of aliphatic hydrocarbon functional groups is in descending order as: [Bmim][I]-tr > [Emim][BF4]-tr > [Bmim][BF4]-tr > [Bmim][NO3]-tr > Un-tr. It can be concluded that the [Emim]+ is stronger than [Bmim]+ when containing the same anion [BF4] in the ILs.
Fig. 4

Aliphatic hydrocarbon area ratio of coal samples

The shorter the alkyl chain length is, the more the dissolution of the polar substance is. However, the cation [Emim]+ has an alkyl chain length less than [Bmim]+. Moreover, methyl and methylene are polar molecules. Therefore, the damage strength is that order [Emim]+ is greater than [Bmim]+. The damage of [I] > [BF4] > [NO3] is present when contained it in the same cation [Bmim]+. Although the order of the activity is [BF4] > [I] > [NO3], the main reason for this phenomenon is that the aliphatic hydrocarbons contain more short chain structures dominated by methylene functional groups and fewer side chains.

Oxygen-containing functional groups

Figure 5 illustrates that the ratio includes carbonyl stretching vibrations for the esters or the aldehydes, ketones, and acids for the carbonyl. It can be seen from the FTIR that the position of the peak is relatively unchanged in the range of the oxygen-containing functional groups, and it can be concluded that the intensity of the peak in the ILs-treated coal samples is not significant. Accordingly, we can determine the damage intensity in descending order as: [Emim][BF4]-tr > [Bmim][NO3]-tr > [Bmim][I]-tr > [Bmim][BF4]-tr > Un-tr. When containing the same anion [BF4], the destructive effect [Emim]+ was greater than [Bmim]+. When containing the same cation [Bmim]+, the destruction in order was [NO3] > [I] > [BF4]. However, the electronegativities of atoms N, I, and F in the anion gradually reduced; the adsorptions of these atoms with hydrogen ions were weakened. The three anions [NO3], [I], and [BF4] reacted with coal sample itself –OH to form hydrogen bonds. Hence, the hydrogen bond between C=O and –OH was destroyed, eventually destroying C=O.
Fig. 5

Oxygen-containing peak area ratio of coal samples

Aromatic hydrocarbons

Figure 6 describes the ratio of C=C stretching vibration content in the aromatic ring. From the FTIR, it can be inferred that the stretching vibration of the 3070–3005 cm−1 aromatics CH and the C=C stretching vibration of the 1670–1505 cm−1 aromatic ring and the outer bending vibration of 900–712 cm−1 kinds of substituted aromatic hydrocarbons can prove the presence of the aromatic ring in the coal. The effect of ILs on the aromatic ring was less, and it may be attributed to the ILs mainly destroying the active functional group at the edge of the coal molecular structure. However, the aromatic ring was distributed in the molecular structure of coal which is located in the middle position and hardly to destroy. In the process of ILs-treated coal samples, ILs will be a complex with the coal molecules, so that the coal molecules retain some of the remaining ILs, eventually resulting in an increase for C=C.
Fig. 6

Aromatic hydrocarbon peak area ratio of coal samples

Further discussion

The results revealed that the effect of ILs on the destruction of coal sample molecular structure is different. The aromatic ring functional group was not affected, as to readily compare the various ILs on the three functional groups of the size of the definition of ΔW. Figure 7 observes the effect of four kinds of ILs on ΔW. The value of ΔW was defined as the ratio of functional groups in the samples treated by between the ILs-tr and the Un-tr. The each functional group from Un-tr was chosen as a benchmark. The value of ΔW is the difference between the proportion of the molecular structure on the IL-tr and the Un-tr. The magnitude of the difference reflects the intensity of the ILs to the destruction of the coal molecular structure. It can be inferred from Fig. 7 that the four kinds of ILs have different damage degrees on each functional group in the coal sample, but the trend is roughly the same. Here, the difference of [Emim][BF4]-tr is the greatest. The maximum ΔW for the hydroxyl groups, the aliphatic hydrocarbons, and the oxygen-containing functional groups is 55.3, 42.5, and 15.9%, respectively. Therefore, [Emim][BF4]-tr was most obviously to damage the microstructure of coal surface at room temperature. In the destruction of aliphatic hydrocarbons, [Bmim][I]-tr reached a maximum of 39.8%. Four patterns of ILs on the destruction of hydroxyl were the most obvious. Meanwhile, the minimum proportion of the damage was 15.9%.
Fig. 7

Values of ΔW for coal samples

Enthalpy and apparent activation energy

Enthalpy

The DSC and DDSC curves of the coal samples in the whole burning stage at different heating rates (β = 4.0, 6.0, 8.0, and 10.0 K min−1) are shown in Fig. 8. To achieve the heat energy generated of a unit mass of the coal samples, the enthalpy expressed by the integration of the DSC curves in the oxidation process. The values of enthalpy for [Emim][BF4]-tr, [Bmim][BF4]-tr, [Bmim][NO3]-tr, [Bmim][I]-tr, and Un-tr were 1481.8, 1708.8, 1208.4, 1430.2, and 1301.4 J g−1, respectively. According to the DDSC curves, the exothermic rate of coal augmented with the increase in temperature, the rate of thermal released in descending order was [Bmim][NO3]-tr > Un-tr > [Bmim][I]-tr > [Bmim][BF4]-tr > [Emim][BF4]-tr. Therefore, we inferred that the enthalpy of other ILs-tr can augment during oxidation except [Bmim][NO3]-tr.
Fig. 8

DSC and DDSC curves of coal samples at heating rates of 4.0, 6.0, 8.0, and 10.0 K min−1. a [Emim][BF4]-tr, b [Bmim][BF4]-tr, c [Bmim][NO3]-tr, d [Bmim][I]-tr, e: Un-tr

Apparent activation energy

The ICTAC Kinetics Committee has developed recommendations for performing kinetic computations on thermal analysis data based on multiply heating rate experiments, and it can avoid to errors [39]. Therefore, the multi-heating rates can more accurately calculate the apparent activation energy in this paper. The apparent activation energy of the coal oxidation reflects the degree of the reactivity and rate of the coal in the oxidation process. The duration from the beginning at room temperature to 220.0 °C was considered the low-temperature oxidation stage. The greater the apparent activation energy is, the lower coal’s reaction rate is. The active molecular structure of coal plays a key role in the oxidative and exothermic properties of coal. Moreover, ILs can destroy the active groups of coal. Therefore, the study of the thermal properties of coal can reflect the influence of ILs on the microstructure of coal, which is essentially important in the low-temperature stage.

Figure 9 shows the linear fit of ln k against 1/T for [Bmim][BF4] coal samples at different heating rates (β = 4.0, 6.0, 8.0, and 10.0 K min−1), and the fitting curves of the other four coal samples can be obtained by the same method. Moreover, Table 6 presents the calculated values of apparent activation energy, which varied from 2041.93 to 8745.98 J mol−1 with a pre-exponential factor, approximately.
Fig. 9

Linear fit of ln k against 1/T for [Bmim][BF4]-tr coal samples at heating rates of 4.0, 6.0, 8.0, and 10.0 K min−1

Table 6

Fitting parameters and activation energy Ea of coal samples

β/K min−1

[Emim][BF4]-tr

[Bmim][BF4]-tr

[Bmim][NO3]-tr

[Bmim][I]-tr

Un-tr

4

T/°C

50.0–215.0

50.0–227.0

51.2–210.2

36.6–179.2

48.1–180.9

k

− 703.2

− 808.8

− 1201.3

− 1104.6

− 193.7

b

− 4.321

− 4.104

− 2.680

− 9.760

− 6.741

R

0.988

0.989

0.983

0.973

0.824

Ea/J mol−1

5846.40

6724.96

9988.29

9184.10

1610.45

lnA/s−1

− 4.321

− 4.104

− 2.680

− 9.760

− 6.741

6

T/°C

 

55.0–225.9

58.1–220.9

36.6–190.0

41.0–150.9

k

 

− 680.388

− 780.21

577.1

− 131.595

b

 

− 4.698

− 4.38076

− 8.46

− 6.37875

R

 

0.973

0.971

0.900

0.889

Ea/J mol−1

 

5656.74

6486.66

4798.05

1094.08

ln A/s−1

 

− 4.698

− 4.38076

− 8.46

− 6.37875

8

T/°C

40.0–181.0

55.0–215.0

55.2–215.6

 

45.9–180.9

k

− 432.6

818.9

− 789.2

 

− 295.4

b

− 5.549

− 4.168

− 4.337

 

− 5.849

R

0.988

0.985

0.958

 

0.970

Ea/J mol−1

3596.89

6809.14

6561.83

 

2456.39

ln A/s−1

− 5.549

− 4.168

− 4.337

 

− 5.849

10

T/°C

40.0–160.0

55.0–215.0

50.23–190.0

55.9–192.3

41.0–145.7

k

− 105.0

− 497.2

− 385.0

− 776.1

116.0

b

− 7.272

− 5.660

− 5.934

− 6.930

− 7.752

R

0.954

0.950

0.953

0.960

0.831

Ea/J mol−1

873.52

4133.71

3201.16

6453.31

964.88

ln A/s−1

− 7.272

− 5.660

− 5.934

− 6.930

− 7.752

\(\overline{{E_{\text{a}} }}\)/J mol−1

3438.93

5831.14

8745.98

6811.82

2041.93

As given in Table 6, it can be inferred that the Ea of most ILs-tr is elevated. It can be concluded that the reaction rate of [Emim][BF4]-tr, [Bmim][BF4]-tr, [Bmim][NO3]-tr, and [Bmim][I]-tr is relatively lower than that of Un-tr. Therefore, these four kinds of ILs can inhibit coal spontaneous combustion in the low-temperature oxidation stage, as consistent with the results of the damage degree for the microstructure of coal surface. The key functional group influencing the oxidation activity in the microstructure of coal surface is –OH. In addition, Ea is also increased when the damage degree is elevated.

Conclusions

The four kinds of ILs can swell and destroy coal samples in different degrees. The results in microscopic and macroscopic were consistent with each other. Detailed conclusions are summarized:
  1. 1.

    For the above four functional groups, the destruction of [Emim]+ is greater than that of [Bmim]+, which is consistent with the results of apparent activation energy Ea. Among the four kinds of ILs, [Emim][BF4] can most affect the various functional groups in coal sample.

     
  2. 2.

    With the same cation [BMIM]+, the effect of destruction lowers in the order of [NO3] > [I] > [BF4], for the hydrocarbyls and oxygen-containing functional groups. The order of the destruction of aliphatic hydrocarbons is as: [I] > [BF4] > [NO3]. The four ILs rarely destroy to aromatic hydrocarbon functional groups in microstructure of coal surface.

     
  3. 3.

    According to the destructive strength of the microstructure in coal surface, it is combined with the apparent activation energy Ea at macroscales. Therefore, the preferred inhibitor is the cation-containing [Emim]+ ILs for coal spontaneous combustion.

     

Notes

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 5120-4136), the China Postdoctoral Science Foundation (No. 2016-M-590963), and the Industrial Science and Technology Project of Shaanxi Province, China (No. 2016-GY-192).

References

  1. 1.
    Chen G, Ma X, Lin M, Lin Y, Yu Z. Study on thermochemical kinetic characteristics and interaction during low temperature oxidation of blended coals. J Energy Inst. 2015;88:221–8.CrossRefGoogle Scholar
  2. 2.
    Xu Y, Zhang Y, Zhang G, Guo Y, Zhang J, Li G. Pyrolysis characteristics and kinetics of two Chinese low-rank coals. J Therm Anal Calorim. 2015;122:975–84.CrossRefGoogle Scholar
  3. 3.
    Song Z, Kuenzer C. Coal fires in China over the last decade: a comprehensives review. Int J Coal Geol. 2014;133:72–99.CrossRefGoogle Scholar
  4. 4.
    Monnet A, Percebois J, Gabriel S. Assessing the potential production of uranium from coal–ash milling in the long term. Resour Policy. 2015;45:173–82.CrossRefGoogle Scholar
  5. 5.
    Tahmasebi A, Yu J, Han Y, Li X. A study of chemical structure changes of Chinese lignite during fluidized-bed drying in nitrogen and air. Fuel Process Technol. 2012;101:85–93.CrossRefGoogle Scholar
  6. 6.
    Deng J, Li QW, Xiao Y, Shu CM. Experimental study on the thermal properties of coal during pyrolysis, oxidation, and re-oxidation. Appl Therm Eng. 2017;110:1137–52.CrossRefGoogle Scholar
  7. 7.
    Tsai YT, Ho SC, Huang AC, Shu CM. Potential explosion hazard of polyester resin dust formed from a granulation process: Limiting oxygen concentration with different pressures. Appl Therm Eng. 2018;135:74–82.CrossRefGoogle Scholar
  8. 8.
    Wang DM, Xin HH, Qi XY, Dou GL, Ma LY. Reaction pathway of coal oxidation at low temperatures: a model of cyclic chain reactions and kinetic characteristics. Combust Flame. 2016;163:447–60.CrossRefGoogle Scholar
  9. 9.
    Rogers RD, Seddon KR. Chemistry. Ionic liquids–solvents of the future? Science. 2003;302:792–3.CrossRefPubMedGoogle Scholar
  10. 10.
    Wilkes JS. A short history of ionic liquids-from molten salts to neoteric solvents. Green Chem. 2002;4:73–80.CrossRefGoogle Scholar
  11. 11.
    Cummings J, Shah K, Atkin R, Moghtaderi B. Physicochemical interactions of ionic liquids with coal; the viability of ionic liquids for pre-treatments in coal liquefaction. Fuel. 2015;143:244–55.CrossRefGoogle Scholar
  12. 12.
    Qi Y, Verheyen TV, Vijayaraghavan R, Macfarlane DR, Chaffee AL. Ambient temperature solubilisation of brown coal in ammonium carbamate ionic liquids. Fuel. 2016;166:106–15.CrossRefGoogle Scholar
  13. 13.
    Cao M, Gu X, Zhang A, Ma M. Study on rheological properties about ionic liquid and coal slurry. Coal Convers. 2009;32:40–3.Google Scholar
  14. 14.
    Li Y, Zhang XP, Lai SY, Dong HF, Chen XL, Wang XL, Nie Y, Sheng Y, Zhang SJ. Ionic liquids to extract valuable components from direct coal liquefaction residues. Fuel. 2012;94:617–9.CrossRefGoogle Scholar
  15. 15.
    Painter P, Pulati N, Cetiner R, Sobkowiak M, Mitchell G, Mathews J. Dissolution and dispersion of coal in ionic liquids. Energy Fuels. 2010;24:1848–53.CrossRefGoogle Scholar
  16. 16.
    Painter P, Cetiner R, Pulati N, Sobkowiak M, Mathews J. Dispersion of liquefaction catalysts in coal using ionic liquids. Energy Fuels. 2010;24:3086–92.CrossRefGoogle Scholar
  17. 17.
    Pulati N, Sobkowiak M, Mathews JP, Painter P. Low-temperature treatment of Illinois No. 6 coal in ionic liquids. Energy Fuels. 2012;26:3548–52.CrossRefGoogle Scholar
  18. 18.
    Kim JW, Kim D, Ra CS, Han GB, Park NK, Lee TJ, Kang M. Synthesis of ionic liquids based on alkylimidazolium salts and their coal dissolution and dispersion properties. J Ind Eng Chem. 2014;20:372–8.CrossRefGoogle Scholar
  19. 19.
    Bai L, Nie Y, Li Y, Dong H, Zhang X. Protic ionic liquids extract asphaltenes from direct coal liquefaction residue at room temperature. Fuel Process Technol. 2013;108:94–100.CrossRefGoogle Scholar
  20. 20.
    Wang LY, Xu YL, Jiang SG, Yu MG, Chu TX, Zhang WQ, Wu ZY, Kou LW. Imidazolium based ionic liquids affecting functional groups and oxidation properties of bituminous coal. Saf Sci. 2012;50:1528–34.CrossRefGoogle Scholar
  21. 21.
    Wang L, Xu Y, Wang S, Song Z. Inhibiting effect of [HOEmim][BF4] and [Amim]Cl ionic liquids on the cross-linking reaction of bituminous coal. Int J Min Sci Technol. 2016;26:353–9.CrossRefGoogle Scholar
  22. 22.
    Zhang W, Jiang S, Wu Z, Wang K, Shao H. An experimental study of the effect of ionic liquids on the low temperature oxidation of coal. Int J Min Sci Technol. 2012;22:687–91.CrossRefGoogle Scholar
  23. 23.
    Zhang W, Jiang S, Wang K, Wang L, Wu Z, Kou L, Ju X. Study on coal spontaneous combustion characteristic structures affected by ionic liquids. Procedia Eng. 2011;26:480–5.CrossRefGoogle Scholar
  24. 24.
    Hu ZQ, Zhang SF, Lei ZP, Shui HF, Wang ZC, Ren SB. Study on the thermal extraction of Xianfeng lignite in ionic liquid 1-butyl-3-methyl-imidazolium trifluoromethanesulfonate. J Fuel Chem Technol. 2015;43:513–8.CrossRefGoogle Scholar
  25. 25.
    Lei ZP, Zhang SF, Zhang YQ, Shui HF, Wang ZC, Ren SB. Thermal extraction of lignite in ionic liquid and separation and characterization of its extracts. J Fuel Chem Technol. 2013;41:814–8.CrossRefGoogle Scholar
  26. 26.
    Lei Z, Wu L, Zhang Y, Shui H, Wang Z, Pan C, Li H, Ren S, Kang S. Microwave-assisted extraction of Xianfeng lignite in 1-butyl-3-methyl-imidazolium chloride. Fuel. 2012;95:630–3.CrossRefGoogle Scholar
  27. 27.
    Lei Z, Wu L, Zhang Y, Shui H, Wang Z, Ren S. Effect of noncovalent bonds on the successive sequential extraction of Xianfeng lignite. Fuel Process Technol. 2013;111:118–22.CrossRefGoogle Scholar
  28. 28.
    Cummings J, Tremain P, Shah K, Heldt E, Moghtaderi B, Atkin R, Kundu S, Vuthaluru H. Modification of lignites via low-temperature ionic liquid treatment. Fuel Process Technol. 2017;155:51–8.CrossRefGoogle Scholar
  29. 29.
    Hayes R, Warr GG, Atkin R. Structure and nanostructure in ionic liquids. Chem Rev. 2015;115:6357–426.CrossRefPubMedGoogle Scholar
  30. 30.
    Xiao Y, Li QW, Deng J, Shu CM, Wang W. Experimental study on the corresponding relationship between the index gases and critical temperature for coal spontaneous combustion. J Therm Anal Calorim. 2017;127:1009–17.CrossRefGoogle Scholar
  31. 31.
    Deng J, Xiao Y, Li QW, Lu JH, Wen H. Experimental studies of spontaneous combustion and anaerobic cooling of coal. Fuel. 2015;157:261–9.CrossRefGoogle Scholar
  32. 32.
    Weng SF, Xu YZ. Fourier transform infrared spectrometer. 3rd ed. Beijing: Chemical Industry Press; 2016 (in Chinese).Google Scholar
  33. 33.
    Wang DM. The coal oxidation dynamics: theory and application. Beijing: Science Press; 2012 (in Chinese).Google Scholar
  34. 34.
    Deng J, Zhao JY, Xiao Y, Zhang YN, Huang AC, Shu CM. Thermal analysis of the pyrolysis and oxidation behaviour of 1/3 coking coal. J Therm Anal Calorim. 2017.  https://doi.org/10.1007/s10973-017-6331-3.CrossRefGoogle Scholar
  35. 35.
    Hu RZ, Shi JZ. Thermal analysis kinetics. Beijing: Science Press; 2001 (in Chinese).Google Scholar
  36. 36.
    Furusaki A, Konno H, Furuichi R. Pyrolytic process of La (III)–Cr(VI) precursor for the perovskite type lanthanum chromium oxide. Thermochim Acta. 1995;253:253–64.CrossRefGoogle Scholar
  37. 37.
    Deng J, Zhao JY, Huang AC, Zhang YN, Wang CP, Shu CM. Thermal behaviour and microcharacterization analysis of second-oxidation coal. J Therm Anal Calorim. 2017;127:439–48.CrossRefGoogle Scholar
  38. 38.
    Sauer DN. Semi-quantitative FTIR analysis of a coal tar pitch and its extracts and residues in several organic solvents. Energy Fuels. 1992;6:518–25.CrossRefGoogle Scholar
  39. 39.
    Vyazovkin S, Burnham AK, Criado JM, Pérez-Maqueda LA, Popescu C, Sbirrazzuoli N. ICTAC Kinetics Committee recommendations for performing kinetic computations on thermal analysis data. Thermochim Acta. 2011;520:1–19.CrossRefGoogle Scholar

Copyright information

© Akadémiai Kiadó, Budapest, Hungary 2018

Authors and Affiliations

  1. 1.School of Safety Science and EngineeringXi’an University of Science and TechnologyXi’anPeople’s Republic of China
  2. 2.Shaanxi Key Laboratory of Prevention and Control of Coal FireXi’an University of Science and TechnologyXi’anPeople’s Republic of China
  3. 3.Graduate School of Engineering Science and TechnologyNational Yunlin University of Science and TechnologyYunlinTaiwan, ROC

Personalised recommendations