Inhibition of Low-Temperature Oxidation of Bituminous Coal Using a

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Inhibition of Low-Temperature Oxidation of Bituminous Coal Using a Novel Phase-Transition Aerosol Yibo Tang* College of Mining Technology, Taiyuan University of Technology, Taiyuan, Shanxi 030024, People’s Republic of China ABSTRACT: At present, control of spontaneous combustion in goafs of coal mines mainly depends upon the injection of liquid or gas (inert) media. In this work, we attempted to use ultrasonic waves to produce a novel phase-transition aerosol. After treatment, gel layers were produced on the coal surface, whose formation is attributed to the reaction between aerosol particles of acrylamide and H3BO3. Additional experiments were performed to compare the effects of aerosol produced using MgCl2, CaCl2, and KH2PO4. Results show that the net weight of 10 g coal samples increased by 1.336 g after being treated with the phasetransition aerosol for 120 min. Thermogravimetry/differential scanning calorimetry tests indicated that the heat released from the treated coal samples was significantly inhibited and the activation energy of low-temperature oxidation at 50−150 °C also increased significantly. According to in situ infrared spectral studies, the phase-transition aerosol inhibited the activation of the −CH2− structure in coal. Importantly, the oxygen uptake of phase-transition-aerosol-treated coal declined by 34.1% following treatment. This result indicates that the phase-transition aerosol effectively prevented the contact between oxygen and coal and chemically inhibited the activation of groups during the low-temperature oxidation of coal.

1. INTRODUCTION Spontaneous combustion of coal is closely related to the oxygen supply and heat-storage conditions,1,2 and it generally occurs in the goafs of coal mines.3 The goaf is characterized by a large number of broken rocks and residual coal, which makes it increasingly difficult to accurately locate the fire. As a result, controlling self-ignition of coal in collieries is often restricted by many factors,4,5 such as ventilation conditions, mining techniques, coal properties, and susceptibility of the coal to spontaneous combustion. With the improvements in the degree of mine mechanization, fully mechanized top-coal caving technology has been widely applied in many coal mines. Consequently, the caving height in the goafs of coal mines, amount of residual coal extracted, and air leakage have significantly increased, thus increasing the frequency of occurrence of spontaneous combustion.6 The control methods used for extinguishing spontaneous combustion in goafs have constantly developed over the years.7 Previously, the ventilation in a coal mine was adjusted to minimize the air leakage in fire areas using pressure-equalizing technology;8,9 in recent years, researchers have attempted to prevent fire disasters by injecting new materials, such as inhibitors,10−12 inert gases,13 grout,14 foam, or gel.15,16 The inert gases (N2 and CO2) are nontoxic, and importantly, they are not involved in the combustion reaction. Injection of a large amount of inert gas decreases the oxygen concentration in a limited space.17 However, gaseous media cannot remain in a fixed position for a long time, and these gas molecules are weakly adsorbed by coal. Inert gases need to be constantly supplied to ensure the continuity of fireextinguishing effects. Besides, excessive injection of inert gases, including N2 or CO2, can easily cause suffocation. Other inert media, such as yellow slurry, gel, and foam, surround coal masses after being injected into goafs, which reduce the oxidation rate of coal and achieve the effects of preventing and controlling spontaneous combustion of coal. For example, Lu and Qin proposed that internally caused fires can be controlled © XXXX American Chemical Society

in various coal mines using an inorganic foam prepared by mixing base materials and accelerating agents in a certain proportion.18 Colaizzi has used a thermacell foam material (Thermocell) to replace traditional oxygen insulation materials for preventing coal fire.14 Zhou et al. developed foams with three phases, solid non-combustible materials (fly ash, yellow mud, etc.), inert gas (N2), and water, to control fire in mines and applied them in the Baijigou coal mine, Ningxia, China.19 In addition, through synthesis of P(NIPA-co-SA) temperaturesensitive polymeric hydrogels by phase separation, Deng et al. found that, when the temperature is higher than 90 °C, the P(NIPA-co-SA) gel exhibits better performance on fire extinguishing.20 With the combination of the advantages of foams and gels, Zhang et al. developed highly stable gel−foam materials through mechanical foaming using a surface-active agent, a cross-linking agent, and a macromolecule solution.21 Xu et al. developed novel colloids with suspended sediments that contained inorganic gel materials and organic polymers. The synthesized colloid was suitable for preventing and controlling spontaneous combustion in mines located in the northwestern China region, which has a severe water shortage.22 All of the aforementioned materials provide novel options for the prevention and control of spontaneous combustion of coal in coal mines. However, as a result of the gravity effect of the inert media, which is mainly liquid-based, it is difficult to maintain the slurry flow in a single area of the coal mine, because it usually flows into the deep layers along the gravity, which reduces its extinguishing effects. In particular, in mining areas with complicated conditions, such as those with a large steep angle, because the location of the fire remains uncertain, these injected inert media cannot effectively contact Received: August 13, 2016 Revised: October 15, 2016

A

DOI: 10.1021/acs.energyfuels.6b02040 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Technical Parameters of Experimental Coal Samples proximate analysis

elemental analysis

name

moisture (%)

ash (%)

volatile (%)

fixed carbon (%)

C (%)

H (%)

O (%)

N (%)

S (%)

calorific value (MJ kg −1)

coal rank

Bulianta coal

4.98

6.44

32.31

63.33

81.05

4.13

13.46

0.96

0.40

32.10

sub-bituminous

Figure 1. Experimental setup.

the fire source but, instead, directly drain into low strata of the mine. Therefore, a novel approach was proposed: multiphase gaseous materials are injected into goafs to improve the control effects of spontaneous combustion in coal mines with complex geological conditions. Moreover, owing to the diffusion of airflow, the aerosol particles are transformed into liquid or solid phase after interaction, agglomeration, and deposition, which are eventually deposited and adsorbed on the surface of residual coal to form dense protective layers with thin sheets. These protective layers seal off the microfractures on the surface of coal masses and prevent the oxidation and self-heating of coal masses, which can effectively control spontaneous combustion in goafs. In this work, we conducted experiments using ultrasonic atomization technology and simulated the injection of two aerosols into goafs of mining areas. Acrylamide and boric acid were injected into the sample reactor as aerosol particles. After their interaction and subsequent deposition on the coal mass surface, the injected aerosols effectively prevented spontaneous combustion of coal.

2. EXPERIMENTAL SECTION 2.1. Materials. Coal samples for the experiment were selected from No. 2 coal seam in the Bulianta coal mine, Inner Mongolia, China. The technical parameters and elemental composition of the coal samples are presented in Table 1. After the fresh coal blocks were cracked, coal samples (size of 0.180−0.250 mm) were screened using a sieve and dried prior to use in experiments. The chemical reagents used in all experiments were of chemical purity grade (i.e., purity of >99.5%) and were purchased from Sinopharm Chemical Reagent Co., Ltd., China. 2.2. Procedure. First, solutions of acrylamide and boric acid (5 wt %) were prepared. Approximately 100 mL of each solution was poured into a measuring cup, and the cup was then put into an ultrasonic atomizer to atomize the solution at an airflow rate of 1000 mL/min, as shown in Figure 1a. Two ultrasonic atomizers were then connected with the coal tank, which contained 30 g of coal samples, to continuously inject aerosols for 10−120 min. Afterward, the coal samples were taken out, weighed using an electronic balance (HC1100), dried for 24 h at 40 °C in a vacuum drying oven, and then weighed again. X-ray fluorescence (XRF) was conducted to study the changes of elements on the surface of coal samples before and after treatment. To investigate the heat-releasing characteristics and B

DOI: 10.1021/acs.energyfuels.6b02040 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 2. Distribution of Elements on the Surface of Coal before and after Treatment coal sample

Mg (%)

Ca (%)

Si (%)

Fe (%)

Al (%)

P (%)

S (%)

K (%)

Cl (%)

B (%)

raw coal coal−MgCl2 coal−CaCl2 coal−KH2PO4 coal−AAM−H3BO3

0.481 9.862 0.599 0.757 0.426

10.116 13.900 37.014 11.913 11.278

2.090 2.602 2.537 3.176 2.308

34.427 30.453 29.946 28.179 28.946

2.720 3.109 2.292 2.309 1.992

1.256 1.332 1.053 9.814 1.053

3.309 3.690 3.105 3.551 3.278

0.230 0.343 0.246 7.189 0.284

0.121 15.482 16.399 0.765 0.425

0.001 0.001 0.001 0.001 5.067

dynamic parameters of coal during low-temperature oxidation, thermogravimetry (TG)/differential scanning calorimetry (DSC) analysis using a simultaneous thermal analyzer (STA, NETZSCH STA 449 F5) was performed before and after the treatment of coal samples. The analysis was conducted in an air atmosphere from room temperature to 800 °C at a heating rate of 10 K/min. Then, 1 g of coal samples was selected, and their infrared spectra were studied using a Bruker VERTEX 70 infrared spectrometer, to analyze the changes in organic functional groups before and after treatment. The test range and scanning times were 500−4000 cm−1 and 32 times, respectively. Furthermore, the organic functional groups of the coal samples were evaluated using an in situ reactor, by increasing its temperature from 30 to 250 °C at a heating rate of 1 °C/min. To test the physical oxygen uptake of the coal samples subjected to the aforementioned treatment, the ZRJ-1 spontaneous combustion tendency analyzer was used. For this purpose, 1 g of coal samples (particle size of 0.10−0.15 mm) was added to the coal sample pipes of the analyzer. The coal samples were then pumped with nitrogen and heated to 105 °C. The samples were taken out and dried completely for more than 2 h. Once the samples dried, the analyzer was adjusted to a desired temperature (30−105 °C) and the samples were allowed to adsorb oxygen for 20 min to calculate the amount of oxygen uptake. Finally, coal samples (30 g) were placed in the reaction tank (5 × 10 cm) and subjected to the programmed temperature experiment (Figure 1b). The temperature for this experiment varied from room temperature to 200 °C, and the heating rate was 1 K/min. Dry air was allowed to flow through the reaction tank at a rate of 20 mL/min. From 60 °C, 5 mL of gases was extracted at every 20 °C rise in temperature, and the concentration of CO in gas was tested using a GC-950 gas chromatograph, to evaluate the effects of the inhibitor on the oxidation of coal. Additional experiments were conducted using MgCl2, CaCl2, and KH2PO4 to compare the inhibition effects of phase-transition aerosols to inorganic aerosols from these materials. All experiments were conducted twice to ensure the accuracy of experimental data.

degrees, indicating that these substances were adsorbed on the surface of the coal. In addition, it can be seen from Figure 2 that

3. RESULTS AND DISCUSSION 3.1. Surface Changes. Aerosols are stable suspension systems formed by liquid or solid particles uniformly dispersed in gases. When aerosols encounter obstacles, smaller particles pass through the obstacles and move with airflows, whereas larger particles do not change direction with airflows and, therefore, are intercepted by the obstacles. In the pretreatment step, aerosols entered the sample tank through the tubes and flowed through fine coal particles, during which some particles condensed and remained on the coal surface to form a protective layer, which impacted the oxidation of coal. To evaluate the distribution of elements on the coal surface, XRF analysis was performed both before and after coal treatment. Results of XRF analysis indicated that the raw coal surface contained high amounts of Fe, Ca, Al, Si, and S and few other elements in low amounts (Table 2). However, after treatment with aerosols containing inorganic salts, the elements on the coal surface changed significantly and the amounts of Cl and Mg on the surface of coal treated by MgCl2 reached up to 15.5 and 9.9%, respectively. Similarly, after treatment with aerosols containing CaCl2, KH2PO4, and AMM + H3BO3, the elemental content on the surface of the coal increased by different

Figure 2. Mass variation of coal samples before and after treatment.

the mass of the coal sample, weighed using an electronic balance, changed significantly following aerosol treatment. After treatment with MgCl2 aerosol for 10 min, the weight of coal samples increased slightly to 11.135 g at 120 min, corresponding to a 11.35% increase in weight. In contrast, the weight of the dried coal samples decreased significantly; for dried coal samples treated with MgCl2 aerosols, there was only a 2.52% increase (the maximum weight of samples was only 10.252 g). Similarly, when the coal samples were treated with C

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Energy & Fuels CaCl2 and KH2PO4 aerosols, the net increase in weight was no more than 6%. In contrast, the weight of the coal samples treated using AMM and H3BO3 showed a large increase (35.68%), and even after drying the samples, the weight increased by 13.36%. These results indicate that certain aerosol particles were adsorbed on the surface of treated coal samples and reached saturation. For a single kind of aerosol, once the water was evaporated, only limited amounts of solid residues remained on the coal surface. Obviously, the gels on the surface formed by the reaction between two kinds of aerosols greatly increased the adsorption capacity of coal. Furthermore, when a combination of AMM and H3BO3 was used, AMM was catalyzed by H3BO3 and polycondensed to polyacrylamide, which greatly improved the adsorption capacity of coal samples. This also provided an alternative method to overcome the shortcoming that gaseous media are difficult to remain on the coal surface. 3.2. Organic Functional Groups. For the main organic functional groups in BLT coal (raw samples), Figure 3

Figure 3. Fourier transform infrared (FTIR) spectra of coal samples before and after treatment.

demonstrates the out-of-plane stretching vibration of heteroatoms in the range of 508−872 cm−1, which replaced C−H and C−O vibration ranges from 1077 to 1374 cm−1. The CC vibration of aromatic rings was observed at 1591 and 1601 cm−1. Moreover, the −CH2− vibration at 2851 and 2921 cm−1 and OH stretching vibration in the range of 3006−3537 cm−1 were also observed for the organic functional groups. After the aerosol treatment, the organic functional groups of −OH and −CH2− demonstrated an obvious change. For the aerosoltreated coal samples, the −OH structure strengthened significantly, whereas the −CH2− structure weakened. In addition, the in situ infrared test results (Figure 4) showed that the area of the −OH absorption peak of the coal increased dramatically with the temperature from 30 °C, and then after reaching a peak at 70 °C, the absorption area reduced rapidly to a minimum value at about 150 °C. Afterward, it rose again slightly and stabilized at a low level. In contrast, the area of the −CH2− absorption peak remained stable at 100 °C, followed by a rapid increase, and then remained stable at about 200 °C. Apparently, the −CH2− structure of the treated coal samples was inhibited significantly when the temperature was increased, and the smallest peak area of −CH2− accounted for 61.6% of

Figure 4. Changes in the functional group of coal samples at different temperatures.

the raw coal surface. Furthermore, because −CH2− was easily oxidized at a low temperature,23 it is a critical functional group influencing the spontaneous combustion of coal; moreover, its content directly determines whether the coal is prone to spontaneous combustion. Therefore, aerosol treatment not only physically prevents the oxidation of coal but also chemically inhibits the activity of the active groups in coal. 3.3. Thermal Analysis. Figure 5 shows that, when the coal samples were treated by aerosols, the heat release from these samples was inhibited significantly and the exothermic peak temperature values of coal increased. The peaks of heat release for raw coal appeared at 516 °C, whereas after treatment with MgCl2, they appeared at 518 °C, showing a minor change and a slight decrease in the peak areas. After treatment by CaCl2, KH2PO4, and AMM + H3BO3, the peaks of heat release increased to 545, 532, and 593 °C, respectively, and the peak areas reduced dramatically, suggesting that the heat release of the coal samples was dramatically inhibited after the aerosol treatment. From Figure 6, it can be seen that the TG curves of D

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Energy & Fuels

and R indicate the reaction time, reaction temperature, preexponential factor, activation energy of oxidation and decomposition of coal, and universal gas constant, respectively. β is the temperature rising rate, and β = dT/dt. Therefore, eq 1 could change to the following equation: da /g (a) = (A /β)exp( − E /RT )dT

(2)

Performing a dynamic analysis on the weight loss and temperature change during coal combustion is complex. Assuming that the combustion of coal is a first-order reaction, using Gorbatchev integral formula, the following equation can be obtained:25 ⎡ ⎤ ⎡ g (a ) ⎤ AR E ln⎢ 2 ⎥ = ln⎢ ⎥− ⎣ T ⎦ ⎣ β(E + 2RT ) ⎦ RT

(3)

According to the dynamic mechanism of first-order chemical reaction, g(a) = −ln(1 − a), the experimental data obtained from 50 to 150 °C are processed and straight lines are obtained by plotting ln[−ln(1 − a)/T2] against 1/T. The values of dynamic parameter E and the pre-exponential factor are obtained as the slope and intercept of the line, and the magnitude of the activation energy directly reflects the possibility of occurrence of chemical reactions. As shown in Table 3, the activation energy of raw coal after low-temperature

Figure 5. DSC curves of coal before and after treatment.

Table 3. Activation Energy of Coal Samples

Figure 6. TG and derivative thermogravimetry (DTG) curves of coal before and after treatment.

activation energy (kJ mol−1)

raw coal coal−MgCl2 coal−CaCl2 coal−KH2PO4 coal−AAM−H3BO3

−8991.8 −9052.1 −9438.8 −9897.6 −10164.2

74.7 75.3 78.5 88.3 92.4

oxidation at 50−150 °C was 74.7 kJ mol−1, whereas that of coal samples after being treated increased significantly. The activation energy of coal samples treated with MgCl2 was 75.3 kJ mol−1 (least change), whereas that of coal samples simultaneously treated with AAM and H3BO3 reached 92.4 kJ mol−1. This indicated that it is difficult to oxidize the treated coal samples; that is, aerosol treatment inhibits the lowtemperature oxidization of coal samples. 3.4. Oxidation Products and Oxygen Uptake. As the main gaseous product released in the spontaneous combustion of coal, CO is considered as an important indicator gas. Figure 7 shows that, for the raw coal subjected to heat treatment at 60−200 °C, the CO concentration grew with the increase in oxidation at high temperatures. The highest concentration of CO was higher than 11 000 ppm. Although the coal samples treated by MgCl2 aerosol demonstrated significant inhibition of the CO concentration at the early stage, the inhibition effects weakened with the increase of the temperature and the maximum concentration of CO reached 8500 ppm at 200 °C. A similar tendency was demonstrated by coal samples treated using CaCl2 and KH2PO4. This can be explained as follows: single aerosols were mainly adsorbed in the form of tiny droplets on the surface of coal with a small adsorption capacity. Once water was evaporated, inorganic salts were unable to form oxygen barriers in a large area on the coal surface. By comparison, the combination of AMM and H3BO3 aerosol inhibited the release of CO for a long time, because when

coal samples also show a similar tendency. With an increasing temperature, water begins to evaporate and the weight of the coal samples slightly decreases at first, begins to increase at 105 °C, and reaches the maximum at about 245 °C, as they chemically adsorbed oxygen. Afterward, the weight of the coal samples declined rapidly with the progress of pyrolysis, and the combustion was terminated around 630 °C with the weight of coal samples remaining stable after that temperature. In addition, the TG curve of the coal samples treated by MgCl2 aerosol was similar to that of raw coal. In contrast, after treatment by other aerosols, the combustion rates of coal samples reduced significantly and the time needed to achieve a stable temperature showed a delay (i.e., increase in time). These phenomena indicated that, under the action of aerosols, the combustion process of the coal samples was blocked and the inhibition effects of AMM + H3BO3 were most favorable. The oxidation and decomposition of coal is a typical gas− solid reaction. According to chemical reaction dynamics, the reaction rate of coal can be expressed as follows:24 da = A exp( −E /RT )g (a) dt

coal sample

linear slope of ln[−ln(1 − a)/T2] against 1/T

(1)

where a represents the percent conversion during oxidation and decomposition of coal, g(a) represents the integral function of the function model for describing coal oxidation, and t, T, A, E, E

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Energy & Fuels

surface of coal and better fire-retardant effects were obtained. Furthermore, thermal analysis showed that the activation energy of the coal samples treated at a low temperature significantly increased, and therefore, it was difficult for the coal samples to react with oxygen. Infrared spectral analysis showed that the −CH2− structure of the treated coal samples weakened significantly, and the rate of change of CH2 showed an obvious decrease with an increase in the temperature compared to that of the raw coal. In general, the inhibition effects of aerosols (MgCl2, CaCl2, and KH2PO4) are weaker than the combination of AMM and H3BO3 aerosols, which showed better resistance in preventing coal from adsorbing oxygen. In conclusion, the method presented has the potential to control/limit spontaneous combustion of coal using aerosols, especially the phasetransition aerosols.

■

Figure 7. CO emission during low-temperature oxidation.

*E-mail: [email protected].

AMM and H3BO3 are used simultaneously, polyacrylamide was formed on the surface of coal and, eventually, polyacrylamide gels covered the coal surface. Although the gels were dehydrated, a layer of gel film remained on the coal surface that inhibited the reaction between coal and O2. Table 4 shows

Notes

The author declares no competing financial interest.

■

ACKNOWLEDGMENTS This study is funded by a project of the China National Natural Science Foundation (51604185), the Youth Foundation of Taiyuan University of Technology (2014QN002), and a research project supported by the Shanxi Scholarship Council of China (2015-037).

Table 4. Oxygen Absorption Quantity of Coal Samplesa oxygen absorption quantity (mL g−1)

a

process time (min)

coal−MgCl2

30 60 90 120

1.20 1.18 1.16 1.14

coal−CaCl2 coal−KH2PO4 coal−AAM 1.17 1.14 1.11 1.09

1.15 1.09 1.05 1.01

1.02 0.97 0.90 0.81

AUTHOR INFORMATION

Corresponding Author

■

raw coal

REFERENCES

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1.23

China National Standard GB/T 20104-2006.

the changes of oxygen uptake of the coal samples before and after treatment. It is obvious that the oxygen uptake of the coal samples remained unchanged after being treated by inorganic aerosol for 30 min. Even after treatment for 120 min, the oxygen uptake decreased only slightly. In contrast, the phasetransition aerosol reduced the oxygen uptake rapidly to the lowest value of 0.81 mL g−1, that is, a 34.2% decrease in oxygen uptake. In addition, once the oxygen uptake of coal was reduced, oxidation could not continue and, eventually, spontaneous combustion was interrupted.

4. CONCLUSION A novel approach involving the use of aerosols to decrease the spontaneous combustion of coal in underground collieries was investigated. This development not only solves the problem related to liquid slurry flow toward gravity in goafs, it also overcomes the shortcoming that gaseous media are difficult to be retained on the coal surface. Experimental results showed that, using the aerosol to treat coal samples, the aerosol particles were uniformly attached on the coal surface, which effectively inhibited the spontaneous combustion of coal. However, if a single inorganic aerosol was used, the aerosol particles were not favorably adsorbed on the surface of coal and only a small amount of materials remained on the coal surface. In contrast, if multiple aerosols are allowed to interact to form a gel, a higher concentration of liquids was adsorbed on the F

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DOI: 10.1021/acs.energyfuels.6b02040 Energy Fuels XXXX, XXX, XXX−XXX