Experimental Investigation on a Sustained Release Type of Inhibitor

Oct 10, 2016 - reveals the synergism mechanism of PS-C as the caged-wrapping and sustained release of VC. With the optimum proportion of. PS to VC ...
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Experimental Investigation on a Sustained Release Type of Inhibitor for Retarding the Spontaneous Combustion of Coal Liyang Ma, Deming Wang,* Yang Wang, Haihui Xin, Guolan Dou, and Chaohang Xu Key Laboratory of Coal Methane and Fire Control, Ministry of Education, Faculty of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China ABSTRACT: A sustained release type of inhibitor (PS-C) prepared with poly(acrylic acid)/sodium alginate super absorbent (PS), ascorbic acid (VC), and water was developed, which exhibited a not only highly efficient but also sustained inhibition performance on the spontaneous combustion of coal. The impact of PS hydrogel on coal oxidation was examined thermodynamically by TGA−DSC tests, and the variations of functional groups on coal surface under the influence of VC were in situ monitored by an FT-IR technique. The inhibition performance was evaluated with a parameter defined as the emitted carbon monoxide concentration decay rate during coal oxidation. The results show that the PS hydrogel and VC perform distinctive inhibition characteristics by physically removing the accumulated heat and chemically interrupting the free radical chain reaction of coal oxidation, respectively. The composite PS-C is capable of combining the inhibition characteristics of PS hydrogel and VC as well as further realizing a dramatic synergism between PS hydrogel and VC, which is characterized by significant improvement of both the inhibitory efficiency and the temperature stability. Further observation of the SEM images reveals the synergism mechanism of PS-C as the caged-wrapping and sustained release of VC. With the optimum proportion of PS to VC identified as 1:3 and the optimum additive amount in coal found to be 8 wt %, PS-C exhibits an excellent applicability for inhibiting the spontaneous combustion of different ranks of coal efficiently and consistently.

1. INTRODUCTION Coal is the primary fossil fuel,1 but as time of exposure to air goes on, coal tends to absorb oxygen and trigger a chain of exothermic reactions, by which the generated heat accumulates, leading to self-heating and subsequent spontaneous combustion of coal.2 As coal demand in China stops growing gradually,3 the mining speed has slowed down, and the inventory level of coal has reached higher, resulting in frequent occurrence of coal spontaneous combustion disasters causing huge personal casualties, energy losses, and various environmental concerns. Accordingly, inhibiting spontaneous combustion of coal will be the focus of researchers in the coming years. Application of various inhibitors has been demonstrated to be effective for retarding the spontaneous combustion of coal.4−6 On this basis, there have been a wide range of inorganic and organic inhibitors developed and examined in recent years. In general, the role of inhibitors in retarding the spontaneous combustion of coal can be divided into two categories, one of which performs by insulating active sites of coal from oxygen as well as water release and heat absorption; for instance, inorganic matter (such as CaCl2, NaCl, and Mg(Ac)24) and three-phase foam,7 foamed hydrogels,8 and polymer emulsions9function on this principle. Li et al.9 reported that the CO emission of a polyethylene aqueous solution-treated coal sample was suppressed before 100 °C, but the efficacy faded as the temperature rose. The physically based inhibiting medium of oxygen barrier or water determines the low efficacy and short active lifetimes of this class of inhibitors. By comparison, the chemically based inhibitors are mainly aiming at modifying the structure activity of coal. Hao et al.10 examined the chemical inhibition of CaCl2 solution on coal oxidation using quantum chemistry calculation methods. The Ca2+ approached the active oxygen atoms of coal forming © 2016 American Chemical Society

coordination bonds under the recognition and guiding of hydrogen bonds of H2O, which improved the stability of oxygen groups on coal surface. Moreover, urea11 was also found to be an efficient chemical inhibitor which could cause up to 70% reduction of oxidation heat of coal when applied as a 10 wt % aqueous solution. Furthermore, some other chemical inhibitors based on either interrupting the free radical reactions or restraining the formation of reactive species have also been proposed. Ionic liquids such as imidazolium-based species12 and phosphonium-based species13 have been reported to be effective on dissolving the reductive groups and delaying the decomposition of aliphatic hydrocarbon groups and the formation of carbonyl groups during coal oxidation. Dou et al.14 investigated the effects of poly(ethylene glycol) on inhibiting coal oxidation by comparing the infrared spectra of additive-treated and untreated coal samples, with results suggesting that the active groups on coal surface were suppressed while the formation of stable ether linkages were accelerated. In addition, the inhibiting behaviors of Na3PO4 were also reported;15 it functioned by promoting the conversion from hydroxyl into ether linkages, which improved the thermal stability of coal. So far, approaches and criteria have been developed for selecting and evaluating the inhibiting properties of various inhibitors.16 However, due to the burst release pattern causing huge waste by excess dynamic oxygen at low temperature, current chemical inhibitors exhibit poor sustainability with temperature rising. The continuously highly Received: May 18, 2016 Revised: September 8, 2016 Published: October 10, 2016 8904

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Energy & Fuels Table 1. Basic Parameters of the Coal Samples wt %, air-dried basis

a

wt %, dry ash free basis

species

moisture

ash

volatiles

fixed carbon

carbon

hydrogen

Roa (%)

coal rank

BZ TLG CHS SJS XQ

9.86 8.54 10.57 4.99 0.95

20.65 15.22 4.02 6.81 14.47

30.01 35.56 28.82 32.21 12.04

39.48 40.68 56.59 55.99 72.54

73.28 75.05 83.26 82.51 93.72

4.84 5.41 4.14 5.26 2.08

0.37 0.49 0.82 1.05 2.16

lignite B sub-bituminous bituminous C bituminous B anthracite C

Ro, vitrinite reflectance index. were conducted with 10 mg of PS-H sample at a heating rate of 1 °C/ min in the temperature range 25−225 °C under an atmosphere of nitrogen (50 mL/min). Furthermore, differential scanning calorimetry (DSC) was performed using a Setaram C80 microcalorimeter to investigate the inhibition characteristic of PS-H on the spontaneous combustion of coal.23 About 1600 mg of the coal sample was loosely placed in the calorimeter cell and subsequently heated from 25 to 225 °C at 0.2 °C/min in dry air with an oxygen concentration of 20.96%. Similar experiments were carried out for 8 wt % PS-H-treated coals. In addition, as a contrasting sample, an equal amount of PS-H supported by inert α-Al2O3 instead of coal was measured. Besides, in order to investigate the thermal oxidative response behavior of PS-H, α-Al2O3 was also tested separately as a blank control. 2.3. In Situ FTIR Measurements and Quantum Chemistry Calculation. In situ tests were designed to investigate the impacts of VC on the active functional groups on the coal surface using a Nicolet 6700 Fourier transform infrared spectrometer. First, pure KBr powder was scanned as a baseline reference. Furthermore, the diffuse reflectance accessory was used, and raw coals with particle size of 0.038−0.075 mm were smoothly paved in the reaction camera. A Harrick automatic temperature controller was used to heat the samples from 25 to 225 °C at a rate of 1 °C/min under an atmosphere of dry air. Series spectra were recorded at a 30 s interval by adding 64 scans in the range of wavenumbers 400−4000 cm−1 at a resolution of 4 cm−1. A mixture with coal/VC ratio 50:1 was tested in the same way as mentioned above. All tested spectra were transformed on the basis of the Kubelka−Munk function to make absorption intensity proportional to the quantity of functional groups on the basis of the Beer− Lambert principle.24 Quantum chemistry methods based on density functional theory (DFT) were used to analyze the inhibition mechanism of VC. A hybrid meta-GGA with Becke’s three-parameter exchange function (B3LYP)25 was implemented to assist the calculation. The basis set 631 G was used to describe the atomic orbitals.26 Geometry optimization and molecular orbital calculation were performed for a VC molecule successively. 2.4. Examination of the Inhibition Performance. The concentration of emitted CO has been recognized as a critical parameter associated with the chemical kinetics of coal oxidation, which directly reflects the developing degree and trend of the spontaneous combustion of coal in real time.27,28 Therefore, the CO emission−temperature evolution was investigated using a self-designed experiment apparatus consisting of a coal oxidation simulation oven with programmed temperature enclosure which provides a surrounding environment at an accurate and uniform temperature by means of gas bath, a cylindrical copper reaction vessel with dimensions of 100 mm (H) × 45 mm (D), a gas chromatograph which can collect and analyze the emitted gases, and a data acquisition system to record the temperature.29 A 50 g (±0.01 g) coal sample was packed into the reaction vessel. First, the oven was set to the desired temperature of 30 °C, and as soon as the temperatures of the coal sample and the oven were equilibrated, 50 mL/min preheated dry air with an oxygen concentration of 20.96% was introduced to flow through the coal sample from bottom to top. After 15 min, gaseous products emitted from the outlet of the coal reaction vessel were measured, and CO concentration was analyzed. Meanwhile, the programmed temperature enclosure was set to run at a rate of 5 °C/min from 30 to 200 °C. Besides, due to the huge influence of temperature on CO formation

efficient inhibition of coal spontaneous combustion has not been achieved. Ascorbic acid (VC) is an attractive substance which has been widely applied in a variety of research fields including biomedicine,17 preservatives,18 and antiaging19due to its strong antioxidant activity.20 In addition, the sensitive eliminating capability toward activated free radicals also endows VC with a potential application value on inhibiting coal spontaneous combustion remaining to be investigated. Poly(acrylic acid)/ sodium alginate super absorbent (PS) hydrogel has been regarded as a promising material for application in biomaterial science and technology such as controlled drug delivery because of its excellent properties, e.g., controlled water-release ability, biodegradability, and nontoxicity;21,22 these characteristics also make PS hydrogel worth examining as an ideal loader as well as sustained release pump for chemical inhibitors. This paper is aimed at developing a class of continuously high-efficiency inhibitors (PS-C) of coal spontaneous combustion. Herein, VC is selected as the principal inhibitor while PS hydrogel plays the role of both loader and sustained release pump for VC. Specifically, the inhibition behaviors of PS hydrogel and VC were investigated using modern analytical instruments, respectively. Then, the inhibition performance of PS-C was compared with that of other additives using a selfdesigned experiment apparatus. Furthermore, dynamic changes of the micromorphology of PS-C with temperature rising were observed and analyzed to elucidate the continuously highefficiency inhibition performance.

2. EXPERIMENTAL AND METHODS SECTION 2.1. Coal Samples and Inhibitor Preparation. Five typical Chinese coal samples were collected from Beizao (BZ) Colliery in Shandong, Tuoluogai (TLG) Colliery in Xinjiang, Chahasu (CHS) Colliery in Inner Mongolia, Shanjiaoshu (SJS) Colliery in Guizhou, and Xinqiao (XQ) Colliery in Henan. The coal lumps were milled and sieved to the size ranging from 0.04 to 0.075 mm and then dried for 24 h at 40 °C. Proximate and ultimate analyses are summarized in Table 1. Poly(acrylic acid)/sodium alginate super absorbent (denoted as “PS”) and ascorbic acid (denoted as “VC”) were selected as raw materials to prepare the composite inhibitor (denoted as “PS-C”) for the experiments. Mixtures with different PS/VC ratios were added with deionized water and evenly stirred to the gel state; the hydrogels were then vacuum-dried at 40 °C and weighted in real time until the amount of holding water decreased to 9 times the weight of PS. The products were pulverized to particulates, and then PS-C samples with different PS/VC ratios were obtained, among which the product with a PS/VC ratio of 1:0 was PS hydrogel (denoted as “PS-H”). Prior to measurements, various inhibitors were blended with coal samples, mechanically homogenized, and then reserved in a thermostat at 25 °C for 48 h. 2.2. TGA−DSC Measurements. Thermogravimetric analysis (TGA, type TA SDT Q600) was employed to examine the waterrelease characteristic of PS-H with temperature rising. Measurements 8905

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Energy & Fuels rates, to avoid the errors caused by thermal lag, temperature was kept constant for 15 min at an interval of 10 °C during which the emitted CO concentrations were measured. The oxygen consumption rate at 70 °C (RO2,70) reflects the coal oxidation rate at low temperature (40−70 °C)29 while the crossing point temperature (CPT) signifies the onset of self-heating which is defined as the transient temperature at which the temperature of a coal sample exceeds the heating bath.30 These two indexes can be combined to characterize the whole course of coal spontaneous combustion. The same experimental system as mentioned above was employed to measure the RO2,70 and the CPT, and different approaches just present in the modification of the temperature program and gas flow. A slower heating rate of 0.8 °C/min was set after system equilibrium at 30 °C. In addition, because of the thermal sensitivity and low oxygen consumption at low temperature, a smaller flow rate of 8 mL/min preheated dry air was permitted to pass through the sample before 70 °C which would have little influence on the heating environment. Once the temperature reached 70 °C, the gas exiting the reactor outlet was sampled, the oxygen concentration was analyzed, and the RO2,70 was calculated by eq 1,31 after which the gas flow was adjusted to 50 mL/min to satisfy the oxygen requirement of coal oxidation in the rapid reaction stage. During this process, the oven temperature and the central point temperature of coal sample were monitored simultaneously and continuously by the data acquisition system until the CPT appeared.

R O2,70 =

Q inφin 22400SL

ln

observed from the curves, PS-H just undergoes a slight decline in its mass before 50 °C, which results from the drying temperature of 40 °C. As the temperature increases further, the TGA curve begins to decline rapidly and the mass loss rate peaks at 88.3 °C with a loss of 36.4 wt %; this phenomenon is mainly ascribed to the release of free moisture. At 146.2 °C, there appears to be another significant reduction of the TGA curve while the DTG reaches the second peak at 182.8 °C with 84.1 wt % loss. This process is attributed to the removal of bound moisture, which had been locked in the threedimensional cross-linked network structure of PS-H by the hydrogen-bonding with the strong hydrophilic groups in the polymer chain. Curves get flat at 208.3 °C with 9.8 wt % remnant, demonstrating that the remaining PS has good heat resistance. From the viewpoint of thermodynamics, coal spontaneous combustion is a self-accelerating process rooted in the heat storage of low-temperature oxidation.32 The accumulated heat raises the temperature and accelerates the oxidation rate, causing more heat to be released and the thermal reaction cycle to be formed. For this reason, we studied the effect of PS-H on displacing the accumulated heat during coal spontaneous combustion. Figure 2a shows the comparison of DSC curves of CHS coal samples with/without the addition of 8 wt % PS-H in the temperature range 25−225 °C. For the raw coal, a slight rise in heat flow can be seen at the beginning of heating (up to about 75 °C) due to the slow exothermic physisorption and early chemisorption of oxygen, followed by a rapid increase at temperatures of 75−225 °C, which is ascribed to the occurrence of some nonspontaneous exothermic reactions with higher activation energy (>40 kJ/mol) and larger heat release. As temperature rises, higher reaction barriers were overcome, and the cyclic chain reactions of coal oxidation were linked, which sped up the heat release. As we expected, the addition of 8 wt % PS-H results in a negative peak on the DSC curve characterizing an apparent endothermic stage at temperatures of 25−128.4 °C, indicating that the accumulated heat during coal oxidation at low temperatures was entirely removed; thereby, the self-heating process was interrupted, and the accelerated oxidation stage was delayed. This remarkable inhibition performance is mainly attributed to the property of slow-releasing water of PS-H. Figure 2b illustrates the thermal evolution process of 8 wt % PS-H supported by inert α-Al2O3, as well as that of separate αAl2O3, whose DSC curve advances almost horizontally around zero and shows little drift due to its high stability, indicating that α-Al2O3 could have little impact on the thermal oxidative response behavior of PS-H. Consistent with the TGA results above, the DSC curve presents a pattern of double endothermic peaks with peak points at 92.9 and 181.0 °C, respectively, which represents a slow-release process of water and a wide temperature range of heat absorption. The released water removed the accumulated heat from coal oxidation; simultaneously, one part penetrated into the micropores of coal forming liquid membrane on the internal surface, which isolated the active groups of coal from oxygen reducing heat production from coal oxidation.33 The reaction heat was calculated by an integral and summarized in Table 2. 3.2. Inhibition Performance of VC. 3.2.1. Variations in Active Functional Groups of Coal. It has been demonstrated that certain active alkyl and oxygen-containing groups on the coal surface tend to react exothermically with oxygen at low temperature and provide energy for subsequent reactions with

φin φout

(1)

Here, RO2,70 is the oxygen consumption rate at 70 °C, mol/(m3 s); Qin is the inlet flow rate, mL/s. S and L represent the bottom area and the height of the reaction vessel, respectively, m. φin is the inlet oxygen concentration while φout is the outlet oxygen concentration, %. Three repeated tests were performed for each set of measurements. The resulting data were averaged, and standard deviations were calculated to validate the data repeatability. 2.5. Examination of the Micromorphology. Quanta 250 scanning electron microscope (SEM) was employed to characterize the micromorphology of PS-H and PS-C with PS/VC ratios of 1:3, and the latter one was observed again after being heat-treated at 70 and 150 °C for 0.5 h, respectively. Before observation, samples were coated with Au using a sputtering coater which endowed the sample surfaces with electroconductivity.

3. RESULTS AND DISCUSSION 3.1. Inhibition Performance of PS-H. Figure 1 illustrates the TGA data obtained from the temperature-rising pyrolysis experiment for PS-H containing 10 wt % PS and 90 wt % water, as well as the differential thermal gravity (DTG) data. As

Figure 1. TGA−DTG curves of PS-H. 8906

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Figure 2. (a) DSC curves of CHS coal samples with/without the addition of 8 wt % PS-H; (b) DSC curve of 8 wt % PS-H supported by α-Al2O3.

phenols, carboxylic acids, and intermediates of chemisorption. The 1300−1100 cm−1 region characterizes the overlapping bands of CO bonds in various ether structures. A differential FT-IR spectrum at 70 °C is obtained for the CHS coal sample containing 2 wt % VC and the raw coal sample, as depicted in Figure 4. The appearance of obvious

Table 2. Reaction Heat Obtained from the DSC Curves sample

reaction heat (J/g)

CHS raw coal CHS coal containing 8 wt % PS-H α-Al2O3 α-Al2O3 containing 8 wt % PS-H

1161.61 816.69 3.23 −215.35

higher reaction barriers,34,35 which essentially reveals the initiation mechanism of self-heating and the subsequent spontaneous combustion of coal from microperspectives. Hence, effectively suppressing the crucial active groups is the key approach to stabilize coal. On this basis, in situ infrared spectrograms for coal samples with/without the addition of 2 wt % VC scanned from 25 to 225 °C at an interval of 0.5 °C under the atmosphere of dry air were obtained. A comparison of FT-IR spectra for 2 wt % VC-treated CHS coal scanned at 70 °C with that of untreated coal sample is exhibited in Figure 3. Figure 4. FT-IR difference spectrum for CHS coal with/without addition of 2 wt % VC scanned at 70 °C.

negative peaks at absorption bands for O−H species and aliphatic C−H species demonstrates that the concentrations of the crucial evocating groups (i.e., OH, •OH, CH3, CH2) for coal self-heating decreased substantially under the inhibition of 2 wt % VC. The negative peak located at carbonyl species absorption bands illustrates a slowing down in the formation of carbonyl intermediates. Besides, a significant positive absorption peak appears at the ether bands; this phenomenon is related to the etherification reaction between the hydroxyl groups bonded to exocyclic carbon atom of VC and the hydroxyl groups of coal.36 As a result, a large number of stable ether bonds were generated, which is indicative of the role of VC in improving the thermal stability of coal. Furthermore, Figure 5 reports the comparisons of in situ variations of methyl, methylene, hydroxyl, and carbonyl species between the raw CHS coal and the 2 wt % VC-treated coal sample. In general, the concentrations of methyl and methylene groups of raw coal both exhibit a trend of fluctuating growth. The similar evolution process is due to the analogous structure and reactivity of these two aliphatic groups; the irregular fluctuation of absorption signals results from the competition between the consumption rate of alkyl groups by oxidation and

Figure 3. FT-IR spectra for CHS raw coal (a) and 2 wt % VC-treated CHS coal (b) scanned at 70 °C.

The prominently changed absorption peaks as well as the corresponding functional groups are identified as follows. The diffuse absorption band of the 3600−3100 cm−1 region is assigned to the stretching vibration of hydroxyl species in the free state and the association state in water, alcohols, phenols, carboxylic acids, etc. The stretching vibration of CH3 groups is located at 2960 cm−1, while 2925 and 2845 cm−1 correspond to CH2 stretching vibrations. It can be seen that the amount of CH2 is much larger than that of CH3, indicating that CHS coal contains abundant alkyl side chains. The weak absorption peak at 1710 cm−1 is specific to the CO stretching vibration in 8907

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spondingly, the concentration evolution of hydroxyl species in VC-treated coal shows the same changing trend, where it shifts to a much lower level than that in raw coal, indicating that VC effectively eliminated hydroxyl radicals during coal oxidation. In the case of carbonyl species, it can be seen that the concentrations in both samples increase over time, which is attributed to the generation of secondary carbonyl (RCO) converted from unstable intermediates (R•CHO•) via the formation of a π bond between carbon and oxygen atom.37 In a comparison with properties of the raw coal, the addition of VC remarkably reduced the quantity of carbonyl species to a much lower level. 3.2.2. Inhibition Mechanism of VC. As acknowledged from the in situ FTIR analysis, VC exhibits an astonishing performance on suppressing the crucial active sites (including CH3, CH2, OH, •OH, CO) on the coal surface and improves the thermal stability of coal. Prior to reveal the mechanism of VC in slowing down coal oxidation, the reactive molecular bonds in VC molecular structure must be confirmed. Accordingly, on the basis of density functional theory (DFT),25,26 the molecular orbitals of VC were calculated, and the frontier orbitals were identified, as shown in Figure 6. According to molecular orbital theory, the bonds which contain the highest-energy level molecular orbitals especially the highest-occupied molecular orbital (HOMO) and the nexthighest-occupied molecular orbital (NHOMO) are the reactive molecular bonds.38 Figure 6 illustrates that the HOMO and NHOMO of the VC molecule are located in the bond between 10O and 11H of the hydroxyl in the dienol structure and the carbon−carbon double bond between 4C and 5C, respectively, which are recognized as the reactive molecular bonds. Furthermore, the molecular orbitals of the crucial radicals (i.e., ROO•, RO•, •OH) linking the chain reactions of coal oxidation2 were also calculated. Considering the view that active sites are almost unaffected by the number of benzene rings,39 we modeled the radicals with a single phenyl group instead of complicated aromatic structures, and the highestenergy level molecular orbitals were identified, as shown in Figure 7. The molecular orbital theory holds that, for free radical molecular models, the highest-energy level molecular orbital derives from the homolysis of molecular bonds, i.e., the singly occupied molecular orbital (SOMO) at free radical sites,2 which is consistent with the results illustrated in Figure 7. In essence, intermolecular reaction is the interaction between their respective highest-energy level molecular orbitals. Therefore, in terms of molecular orbital theory, the reactivity of crucial radicals with VC may be elucidated by the formation of HOMO/NHOMO−SOMO orbital interactions and the subsequent electron transfer. On this basis, a two-step reaction mechanism (R1−R6) between VC and the crucial radicals is illustrated in Figure 8a; simultaneously, the search for transition states (TS) and energy computation were performed by the QST2 method39 with the results presented in Figure 8b. It can be seen that all six reactions proceed with low-energy barriers (0.14−11.54 kJ/mol), indicating that R1−R6 can occur at ambient or higher temperature, which is indicative of the role of VC in eliminating the crucial linking radicals. During the oxidation process of VC-treated coal, with the inductive effect of oxygen and oxidative free radicals, reactive O−H bonds in the dienol structure of VC break successively and release hydrogen free radicals. As illustrated in Scheme 1, the hydrogen free radicals are able to react with the unstable peroxide radicals, which are generated from the oxygen

Figure 5. In situ variations over temperature in (a) methyl, (b) methylene, (c) hydroxyl, (d) carbonyl of CHS coal with/without addition of 2 wt % VC during oxidation.

the generation rate of secondary alkyl groups from the cleavage of unstable cycloalkanes and bridge bonds during oxidation. The concentrations of methyl and methylene groups in VCtreated coal, by comparison, demonstrate a minor amplitude rise with low fluctuation, and extend to a much lower extent than those in raw coal. As shown in Figure 5c, the quantity of hydroxyl species in raw coal initially undergoes a notable decline, followed by a progressive increase from around 105 °C due to the massive generation of hydroxyl radicals from the decomposition of oxidative intermediates (i.e., peroxide free radicals).37 Hydroxyl radicals play a crucial linking role in the reaction pathway for coal oxidation because of its extreme electron accepting ability, which can easily capture hydrogen from coal active groups and form various new free radicals (i.e., alkyl radicals, carbonyl radical, carboxyl free radical). Corre8908

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Figure 6. Frontier molecular orbitals of the VC molecule. (a) The highest-occupied molecular orbital (HOMO). (b) The next-highestoccupied molecular orbital (NHOMO).

Figure 7. Highest-energy level molecular orbitals of the crucial linking radicals: (a) peroxide radicals, (b) alkoxyl radicals, and (c) hydroxyl radicals.

chemisorption of C-centered radicals, and as a result, hydroperoxides are formed. Particularly, as temperature rises, the hydrogen free radicals tend to be captured by subsequent intermediates such as hydroxyl radicals and alkoxyl radicals which link the reaction pathway of coal oxidation, and convert these active radicals to water, alcohols, and further stable ether structure. The addition of VC blocks the further oxidation pathway of the active groups and interrupts the free radical chain reaction, which adequately explains the in situ variations in active groups of coal caused by VC. Therefore, it is convincing that VC acting as a chemical inhibitor can availably eliminate the reactive activity of crucial linking radicals (i.e., ROO•, RO•, •OH) and interrupt the free radical chain reaction of coal oxidation, and as a result, the spontaneous combustion of coal is inhibited. 3.3. Dramatic Synergism of PS-C. Coal spontaneous combustion is a kinetic process subjected to the coal oxidation rate. It has been demonstrated that the emitted CO concentration is a typical kinetic index reflecting the coal oxidation rate due to the fact that CO mainly generates from the decarboxylation reaction of chemisorption intermediates during coal oxidation.27,34,37 On this basis, we defined the

released CO concentration decay rate as an evaluation parameter to assess the inhibition performance of various additives on coal spontaneous combustion, which can be written as WTi = (Cco,Ti − Cco,Ti′)/Cco,Ti × 100%, where Ti is the test temperature, °C; Cco,Ti is the average value of emitted CO concentration at specific temperature of raw coal, while Cco,Ti′ represents that of the inhibited coal sample, ppm. Figure 9a illustrates the influence of various additives on the CO emission during CHS coal oxidation as a function of temperature. Accordingly, the corresponding WTi−temperature evolutions are exhibited in Figure 9b. As a physically based additive, 8 wt % PS-H (containing 0.8 wt % PS and 7.2 wt % water) just causes a slight decrease of the CO emission while the values of WTi fluctuate around a low extent of 3.22%− 14.88%, which can be explained as the temperature lag of CO emission caused by the endothermic effect of PS-H. Besides, the incorporation of 2 wt % VC in the coal sample reduces CO emission obviously. It can be seen that the values of WTi fluctuate above 80% before 80 °C, symbolizing an excellent 8909

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system came to an active radical-rich condition; the remaining VC could hardly provide sufficient hydrogen radicals to deactivation the large amount of active linking radicals generated with increasing temperature, which revived the rapid growth of CO emission. By comparison, amazing performance is present in the case of the coal sample with addition of 10 wt % PS-C (containing 0.8 wt % PS, 2 wt % VC, and 7.2 wt % water); the tested CO concentration curve of which shifts to a lower level substantially and gets flatter than that of a coal sample with the addition of a mixture of 8 wt % PS-H and 2 wt % VC, especially for the conditions above 80 °C. Simultaneously, the corresponding WTi values fluctuate steadily at the range 78.63−92.1%, avoiding the high-temperature failure of VC, indicating that the composite PS-C realized an astonishing synergism between PS hydrogel and VC, which shows an effect of “1 + 1 > 2” on retarding the spontaneous coal combustion. Furthermore, Figure 10 depicts the comparison of the RO2,70 and the CPT for CHS coal samples treated with various Figure 8. Two-step reaction mechanism of VC eliminating the reactive activity of crucial linking radicals of coal oxidation: (a) reaction pathways and (b) energy barrier analyses.

Scheme 1. Mechanism of VC in Retarding Coal Oxidation

inhibition of coal oxidation. Nevertheless, as temperature rises, the values of WTi continuously fall and remain low, until the lowest value of 55.05% at 170 °C, signifying that VC gradually lost efficacy at temperatures exceeding 80 °C. This phenomenon results from the fact that, during the coal oxidation at low temperature, the reactive system advanced in a VC-rich state. Relative to the active linking radicals in coal, the excess VC was depleted by the dynamic oxygen and lost the hydrogendonating capacity. As coal oxidation advanced, the reactive

Figure 10. RO2,70 and the CPT for CHS coal samples treated with various additives.

additives. The 10 wt % PS-C-treated coal sample displays the lowest but just slightly lower RO2,70 than the sample added with a mixture of 8 wt % PS-H and 2 wt % VC. Although both the samples reacted in a VC-rich state before 70 °C, the former provided a water environment for VC, which maintained and

Figure 9. (a) CO emission−temperature curves for CHS coal added with various additives, and (b) the corresponding WTi−temperature evolutions. 8910

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Figure 11. SEM images of (a) PS-H particle, (b) PS-C particle, (c) PS-C particle after heat treatment at 70 °C for 0.5 h, and (d) PS-C particle after heat treatment at 150 °C for 0.5 h.

surface of PS-C after 70 °C heat treatment, leading to the interaction pattern of PS hydrogel and VC transforming from enclosed-wrapping to caging-wrapping, so that the wrapped VC can gradually diffuse out through the pores and layered structures along with water release and enter the coal oxidation system. At a higher temperature of 150 °C, we can see that the PS-C particles thermally fracture and the internal microtopography is observed, as shown in Figure 11d; abundant pores and fold structures are distributed inside PS-C, which creates a huge specific surface area for VC adsorption, and therefore the densely adsorbed active VC is exposed to the coal oxidation system and performs an inhibitory effect continually. Consequently, a synergistic mechanism of PS-C between PS hydrogel and VC can be explained as follows. During the water absorbing and swelling process of the PS hydrogel, a large amount of three-dimensional cross-linked network structure is formed; therefore, VC tends to be captured and wrapped inside the water-rich environment or adsorbed on the surface forming PS-C. When acting on the coal oxidation system, PS-C constantly releases water and removes the accumulated heat; meanwhile, the three-dimensional cross-linked network structure is gradually destroyed, forming pores and layered structures through which the internal active VC diffuses out equally. Each PS-C particle is like a sustained release pump of VC, caged-wrapping and continually releasing active VC to the surrounding coal. With temperature rising, the PS-C particles

enhanced the reactivity of VC. In addition, dramatically, the addition of 10 wt % PS-C defers the CPT value to the highest point of 178.2 °C, signifying a more excellent ability for delaying the onset of coal self-heating and improving the thermal stability of coal. The data analyses fully confirm the synergism of PS-C between the PS hydrogel and VC by prominently improving the inhibitory efficiency and extending the inhibitory validity period of coal oxidation. In order to explain the mechanism of the synergism, SEM was employed to visually observe the interaction process of the PS hydrogel and VC in PS-C. Figure 11a,b shows the micromorphology of particles of PSH and PS-C, respectively. Typical sample particles with radial length of 80−100 μm were selected and compared with the uniform scale. As observed from the SEM images, the PS-H particles present as five or six angle geometries with a smooth surface; in contrast, in PS-C, we can observe that VC evenly spreads over the surface of PS hydrogel in flaky forms. Presumably, more VC is wrapped in the internal water-rich environment. Since there are relatively independent chemical properties, the PS hydrogel can effectively carry VC by surface absorption and enclosed-wrapping as a chemically stable cooperative system. Figure 11c exhibits the micromorphology of PS-C which had been heat-treated at 70 °C for 0.5 h. Observation reveals that a large number of pores and layered structures appear on the 8911

DOI: 10.1021/acs.energyfuels.6b01192 Energy Fuels 2016, 30, 8904−8914

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Energy & Fuels fracture along with the removal of bound water; immediately, the remaining VC is released and further retards coal oxidation. In general, convincingly, it is the sustained release characteristic of PS-C that makes the synergism between PS hydrogel and VC come true. On one hand, the depletion of excess VC by oxygen during the initial stage is averted, improving the effective utilization rate of VC significantly as well as extending the inhibitory validity period. On the other hand, the water-rich environment inside PS-C maintains the reactivity of VC during the whole inhibiting process, enhancing the inhibitory efficiency of the unit mass of VC. 3.4. Optimization. 3.4.1. Optimal Proportion of PS and VC. As discussed above, the excellent synergism between PS hydrogel and VC benefits from the sustained release characteristic of PS-C, which is closely related to the proportion of PS and VC. Figure 12 exhibits the WTi−temperature evolutions of

Figure 13. WTi−temperature evolutions for CHS coal with different additive quantities of PS-C.

gradually lower. With an additional 10 wt % PS-C, the values of WTi are almost unaffected with little improvement, which indicates that 8 wt % PS-C is the optimal additive quantity of PS-C in coal achieving the equilibrium between inhibiting performance and economic benefits. 3.4.3. Applicability for Different Coal Ranks. The RO2,70 and the CPT of five different ranks of coal with/without the addition of 8 wt % PS-C (PS:VC = 1:3) were tested; as a contrast, 8 wt % CaCl2 representing the traditional inhibitors4,6 was added to the five different ranks of coal, and these treated coal samples were also tested in the same way as mentioned above, with the results summarized in Figure 14. As illustrated, PS-C shows excellent performance for inhibiting the spontaneous combustion of all the five different ranks of coal, among which the lower rank coal samples are influenced more substantially, symbolizing an outstanding applicability of PS-C for various coal ranks. In addition, compared to the traditional inhibitor of CaCl2, as we expected, PS-C performs better with the lower oxygen consumption rate at 70 °C and the later CPT, which means greater improvement of thermal stability of coal as well as further reduction of the risk of coal spontaneous combustion.

Figure 12. WTi−temperature evolutions for CHS coal added with 8 wt % PS-C prepared with different PS/VC ratios.

CHS coal samples containing 8 wt % PS-C prepared with different PS/VC ratios, including 1:1, 1:2, 1:3, 1:4, and 1:5. As observed, in these cases of 1:1 and 1:2, the values of WTi maintain a rapid downward tendency as the temperature rises, indicating that VC is insufficient relative to PS so that PS-C gradually lost efficacy. In contrast, the other three samples all exhibit steady WTi−temperature evolutions at a high level above 85%. Before 70 °C, the curve of WTi slightly shifts to a higher position with an increasing amount of VC; however, in all three cases, the values of WTi at temperatures exceeding 70 °C evolve with little difference, demonstrating that VC is excessive relative to PS in the cases of 1:4 and 1:5. The excess VC cannot be adsorbed by PS hydrogel and drifts away from the synergism system, which tends to be depleted by oxygen prior to 70 °C. Consequently, the optimal proportion of PS and VC maximizing the synergism is identified as 1:3. 3.4.2. Optimal Additive Quantity of PS-C. It has been discussed that the role of PS-C in inhibiting coal spontaneous combustion is not physically based but chemically based, which means that the inhibitory effect may not increase linearly along with increasing the amount of PS-C. The WTi−temperature evolutions of CHS coal samples containing various weight percents of PS-C (PS:VC = 1:3) are displayed in Figure 13. Upon addition of 2−8 wt % PS-C to coal samples, the WTi values are successively improved along with increasing the additional ratio of PS-C, while the improvement rates get

4. CONCLUSIONS In summary, a sustained release type of composite inhibitor (i.e., PS-C) of coal spontaneous combustion was developed and characterized, which not only combined the inhibition characteristics of PS hydrogel and VC, but also realized a dramatic synergism between the two distinctive inhibitors. On the basis of the TGA−DSC results, PS-H exhibited a wide temperature range of water release and heat adsorption, which removed the accumulated heat as well as isolated active sites of coal from oxygen, retarding the self-heating process of coal effectively. Besides, the in situ FTIR spectra proved that VC could significantly suppress methyl, methylene, hydroxyl species, and carbonyl species during coal oxidation. The correlation between the variations of active groups and the inhibition of coal spontaneous combustion was elucidated by the free radical chain reaction of coal oxidation. Moreover, the inhibiting mechanism of VC was revealed via molecular orbital analysis; it was the hydrogen free radicals released from the reactive O−H bonds in the dienol structure of the VC molecule that eliminated the reactive activity of crucial linking radicals 8912

DOI: 10.1021/acs.energyfuels.6b01192 Energy Fuels 2016, 30, 8904−8914

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-15252001940. E-mail: dmwangcumt@outlook. com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Natural Science Foundation of China (51204172 and 51474210), the Innovation Project for Graduate Cultivation of Jiangsu Province (KYLX16_0573), as well as A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Figure 14. RO2,70 and the CPT for five different ranks of coal added with 8 wt % PS-C or 8 wt % CaCl2.

(i.e., ROO•, RO•, •OH), interrupting the free radical chain reaction as well as inhibiting the coal spontaneous combustion. The inhibition performance was assessed by the emitted CO−temperature evolution with a parameter WTi defined as the CO concentration decay rate. Comparison of the results for various additives showed that PS-C realized a dramatic synergism between the PS hydrogel and VC with significant improvement of both the inhibitory efficiency and the temperature stability. In addition, the RO2,70 and the CPT were measured and compared, verifying the synergism. Furthermore, mechanism of the synergism was revealed by observing and analyzing the interaction pattern of PS hydrogel and VC at different temperatures. By caged-wrapping VC in a water-rich environment and with sustained release of VC to the surrounding coal, PS-C remarkably improved the effective utilization rate as well as the inhibitory efficiency of unit mass of VC. With WTi−temperature evolution as the indicator, the optimal proportion of PS and VC was identified as 1:3 which maximized the synergism; in addition, the optimal addition quantity of PS-C in coal was found to be 8 wt %. An excellent applicability of the inhibition performance of PS-C for different coal ranks was proven, which was also better than that of the traditional inhibitor of CaCl2. 8913

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