Article pubs.acs.org/EF
Laboratory Study on the Rising Temperature of Spontaneous Combustion in Coal Stockpiles and a Paste Foam Suppression Technique Yi Lu*,†,‡ †
Hunan University of Science and Technology, Work Safety Key Lab on Prevention and Control of Gas and Roof Disasters for Southern Coal Mines, Xiangtan, Hunan 411201, China ‡ Hunan University of Science and Technology, School of Resource, Environment and Safety Engineering, Xiangtan, Hunan 411201, China ABSTRACT: The spontaneous combustion of coal stockpiles is one of the major hazards during coal mining, storage, and transportation. To overcome the shortcomings of large-scale experiments and CFD modeling techniques, a mathematical model of rising temperature in a hemispherical coal stockpile with a constant heat source in the center was derived and simplified based on a previous study on the spatiotemporal temperature and heat transfer in simple bodies. From the rising temperature experiments in a coal stockpile, the actual temperature rise was greater than the predicted value from the theoretical mathematical model, and the difference in these values increased with an increase in test time. The distribution of the temperature field caused by the spontaneous combustion of coal was fit to revise the mathematical model for the temperature rise in a coal stockpile. In addition, a novel material, called paste foam, was prepared to prevent the spontaneous combustion of a coal stockpile, and the working principle of the device with a hollow spiral tube was studied. The prepared paste foam was uniform with an average pore size of 100 μm. The TG and DSC curves showed that the critical weight loss temperature was 250 °C, with a mass of 82.23%, while, at 400 °C, there was still a mass of approximately 54.67%. The fire extinguishing and cooling experiments indicated that the paste foam accumulated upward to block the fracture network along the radial direction. The temperature at different radial distances showed that the same law of variation can be divided into three stages, including an initial slowdown, followed by a rapid decrease, and a final slow and gentle reduction of the temperature. It maintained a better bubble shape with the temperature of the coal particles at approximately 670 K, and the paste foam had a favorable effect on the surface cooling of the coal stockpile because of oxygen isolation based on the infrared thermal imager analysis.
1. INTRODUCTION Spontaneous combustion of coal is one of the major hazards during coal mining, storage, and transportation, especially when a significant quantity of coal (a coal stockpile) is stored for extended periods.1−3 A coal stockpile is essentially a porous medium consisting of heterogeneous coal particles with fractures filled with fresh air.4 When the heat produced by the low-temperature reaction of coal with oxygen is not sufficiently dissipated to the surrounding environment, the temperature of the coal will continue to increase until it reaches the ignition temperature.5 This leads to the loss of precious coal resources and to the emission of greenhouse-relevant gases, such as carbon dioxide, methane, and other toxic gases, which threaten the health of the local inhabitants.6−8 Several academics and professionals have carried out a variety of experimental investigations to gain deeper insight into this combustion problem. For example, Stott et al.9,10 first built a vertical experimental stand with a length of 5 m and a diameter of 0.6 m, and then designed and constructed a 2 m long and 0.3 m diameter one-dimensional spontaneous combustion experimental apparatus. In China, Deng et al.11,12 designed and built large-scale, spontaneous combustion, coal experimental apparatuses with loading capacities of 0.5, 1.0, and 1.5 tons to simulate the actual conditions. These large-scale spontaneous combustion experiments are useful and can offer realistic results to determine when and where spontaneous ignition occurs. © XXXX American Chemical Society
However, these experiments are limited because a large-scale experiment is formidable and highly costly. It is more important that the change in the temperature field of the large-scale coal stockpile test is slow, and extensive time is needed to monitor the change in temperature of the coal stockpile. Often, numerous experiments are difficult to execute in practice. As an alternative to the experimental approach, computational fluid dynamics (CFD) modeling techniques have recently been used to study the self-heating process in a coal stockpile.13−20 Among the techniques applied, an important modeling approach is the theoretical formulation of the heat transfer, which includes the most significant governing equations. Therefore, it is necessary to analyze the change in the temperature field of a coal stockpile, which can be deduced from the mathematical models formulated from one-dimensional steady-state models into three-dimensional unsteadystate models. The generated heat is generally known to be transported into and out of the stockpile by conduction, convection, and radiation.21,22 Heat conduction occurs gas-togas, gas-to-coal, and coal-to-coal. The low thermal conductivity of the coal is the main reason why thermal energy can be wellcontained in a deep stockpile and cause a rise in temperature.23 Received: March 3, 2017 Revised: June 8, 2017 Published: June 12, 2017 A
DOI: 10.1021/acs.energyfuels.7b00649 Energy Fuels XXXX, XXX, XXX−XXX
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oxygen. Even if the foam cells rupture, the paste foam can still cover and compactly pack the coal, plug fractures, continuously and effectively hinder the coal from absorbing oxygen, and prevent the oxidation of the coal. In addition, the paste foam increases the humidity of the coal body, absorbs heat by evaporation and cooling, and seals air leakages. In this work, the preparation process, formation and characterization of paste foam, and its characteristics for extinguishing fire and cooling a coal stockpile were investigated in an experimental approach.
On the basis of the above analysis, a relatively long time is required to experimentally study the self-heating process of a coal stockpile. For this paper, a hemispherical coal stockpile with a constant output heat flow in its center was built. Among the approaches to model the temperature rise, since the real systems in the field are time-dependent (unsteady-state) with flow reaction behaviors in three dimensions, model tractability creates the need for simplification. Therefore, a one-dimensional mathematical model of the temperature field distribution along the radial direction of the hemispherical coal stockpile was established. The mathematical model was modified by arranging the measuring points in the stockpile, and the distribution of the temperature field caused by the spontaneous combustion of coal was obtained. Suppressing the spontaneous combustion of a coal stockpile is an important issue for the safe utilization of coal. Nevertheless, most studies have focused on modeling and predicting the self-heating or ignition of coal stockpiles, and only a few studies have explored the prevention of spontaneous ignition in coal stockpiles.14,24,25 Several well-known methods to suppress spontaneous combustion in coal stockpiles are periodic compaction,26 a low-angle stockpile,27 pile protection by wind barriers,28 installation of a dual barrier,29 water spray on the pile,30 and mantling loess over the surface of the coal stockpiles.31 The goal of these methods is to either maintain an extremely high temperature or greatly increase the rate of heat loss to create a “cold” coal stockpile. However, all the abovementioned methods have their own limitations. For example, water spray could increase the coal moisture content, which may also accelerate self-heating of the coal piles. Covering coal piles with loess could increase the coal ash content, which lowers the coal quality. Wind barriers are expensive, a low-angle stockpile occupies a large area, and periodic compaction is not effective to prevent spontaneous combustion of large coal stockpiles. Meanwhile, to prevent the spontaneous combustion of coal in the goaf, various traditional techniques have been developed and widely applied in mines, including grouting,32 spraying inhibitor,33 equalizing pressure,34 injecting inert gases,35 injecting gel,36 injecting three-phase foam,37 injecting sandsuspended colloids,38 injecting foamed gel,39 and injecting phase-transition aerosols.40 The most important considerations are that the coal stockpile has a certain scale with a big stacking height and the space is open to the environment. However, the grouting slurry and water only flow from high-lying areas to low-lying areas under natural conditions and cannot contact the high-temperature spots at high position in the stockpile. The inhibitor corrodes the equipment and harms the health of the workers, and pressure equalization by injecting noble gases can easily leak to the environment. With the injection of the gel, a small gel flow can barely extinguish the spontaneous combustion of coal in large areas. After rupture of the foam cells, the injection of three-phase foam allows extensive water slurry loss. Accordingly, new and fundamental methods are required to more effectively suppress the spontaneous ignition of coal stockpiles. A new technique, called paste foam, has been developed here. Paste foam not only has the properties of paste but also has the characteristics of foam. At the same time, paste foam can also overcome the disadvantages of both and, thus, significantly improve the efficiency of controlling the spontaneous combustion of a coal stockpile. Paste foam can cover the coal body to inhibit the contact between the coal and
2. EXPERIMENTAL SECTION 2.1. Physical Model of a Coal Stockpile. Coal samples for the experiment were selected from the No. 3-5 coal seam in the Tashan coal mine, Shanxi, China. The following physical parameters of the coal are tested: the density, ρ, is 958 kg/m3; the specific heat capacity, Cp, is 1420 J/(kg·K); and the thermal conductivity, λ, is 0.216 W/(m· K). As shown in Figure 1, the coal samples are stacked into a
Figure 1. Rising temperature test system for the coal stockpile. hemispherical pile with a radius (R) of 0.50 m. A heat source with constant heat flux is placed at the center of the coal stockpile with a heat flow rate (q0) of 5 W/m3. On the vertical section of the hemisphere, 5 groups of thermocouples are placed along the radial direction. There are 5 thermocouples in each group for a total of 25, which are connected with the temperature acquisition module through a wire and then connected to the computer for storage and display. The infrared thermal imager is placed on the top of the coal stockpile to monitor the surface temperature in real time. The ambient temperature is 27 °C. The convective heat transfer coefficient, h, is 0.1 W/(m2·K), and the thermal diffusivity, α, can be calculated by α = λ/ ρCp with the value of 1.588 × 10−7 (m2/s). In the process of fire suppression and the cooling tests, the paste foam outlet is located at the center of the hemisphere. The volume of paste foam is 0.15 m3, with a pressure injection flow rate of 0.75 m3/h. The foam injection time is 720 s, and the temperature monitoring time is 900 s. 2.2. Preparation of Paste Foam. 2.2.1. Materials. (1) Polyacrylamide (purity 98%, Sino German Chemical Co., Ltd.). (2) Portland cement (PC) with a compressive strength of 64.5 MPa at 28 days, conforming to the BS EN 197-1 type I cement.41 (3) Fly ash (FA) with a median particle size of 35 μm and loss on ignition (LOI) of 5.0%, conforming to the BS EN 450.42 (4) Silica fume (purity 96%, Chengdu Jin Technology Co. Ltd.). (5) Calcium chloride dihydrate (purity 99%, Chengdu Yu Trading Co. Ltd.). B
DOI: 10.1021/acs.energyfuels.7b00649 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels (6) Magnesium chloride hexahydrate (purity 99%, Shandong Hao Chemical Technology Co., Ltd.). (7) Sodium dodecyl sulfate (SDS) (purity 98%, Tianjin Beilian Fine Chemicals Development Co., Ltd.). (8) Acrylic acid (purity 98%, Normic Brand Agency). (9) Acrylamide (purity 98%, Dezhou Fukai Chemical Co. Ltd.). (10) Lauryl alcohol (purity 98%, Shanghai Ding Miao Chemical Co. Ltd.). 2.2.2. Preparation Process. The basic preparation process of the paste foam can be divided into three parts, namely, the mixing and stirring of the composite paste, the preparation of the aqueous foam, and the mixing of the composite paste and the aqueous foam. First, 80.00 wt % water, 0.80 wt % polyacrylamide, 0.80 wt % acrylic acid, 0.90 wt % acrylamide, 9.00 wt % fly ash, 3.00 wt % Portland cement, 3.00 wt % silica fume, 1.30 wt % magnesium chloride hexahydrate, and 1.30 wt % calcium chloride dihydrate are added to a blender and stirred to form a composite paste. Second, the compound surfactant, which is composed of 2.5 wt % sodium dodecyl sulfate and 2 wt % lauryl alcohol, is diluted 77 times with water. Then, the diluted surfactant is blown by pressurized air with a wind pressure of 0.3−0.4 MPa, and an air supply of 32−35 m3/h in the custom-designed porous foaming device forms a fine, uniform, aqueous foam. Third, the composite paste and the aqueous foam are mixed with a volume flow ratio of 10:1 in a helical channel to form the paste foam, as shown in Figure 2.
continuous medium. Then, the temperature distribution of the stockpile bed with combustible coal can be described by the body average energy equation. On the basis of the above analysis, the following assumptions can be made when calculating the heat transfer process in a coal stockpile bed: (1) The porous medium is isotropic, and the internal solid skeleton and air are in a state of thermal equilibrium. (2) The natural convection and radiation heat transfers caused by temperature differences are not considered. (3) The specific heat, density and thermal conductivity of the porous media and air in the voids are constants. (4) The influence of moisture in the porous media is ignored. The open-air coal stockpile is generally accumulated into a hemispherical shape. An analytical solution for the heat transfer process of a one-dimensional spherical coal stockpile bed in Cartesian coordinates is presented. We assume that the internal heat source does not change with temperature. The radius of the model is R, the contact surface with the ground is regarded as the adiabatic boundary, and the hemispherical surface of the coal stockpile is the convective heat transfer boundary. In spherical coordinates, the temperature field in a one-dimensional packed bed can be expressed by eq 1: q 1 ∂T 1 ∂ 2(rT ) = + 0 2 α ∂t r ∂r λ
(1)
The initial conditions are as follows: t=0
T0 = T∞
(2)
The boundary conditions are as follows: ∂T =0 ∂r
r=0
r = R −λ
∂T = h(T − T∞) ∂r
(3)
Assuming the equation is separable, a new variable can be defined: U (r ,t ) = r × T
(4)
Therefore, eq 1 can be changed into eq 5: q 1 ∂U ∂ 2U +r 0 = 2 λ α ∂t ∂r
Figure 2. Preparation system for the paste foam.
(5)
The corresponding initial conditions are as follows: 2.2.3. Microscopic Observation. The microstructure of the paste foam is observed by a Nomarski-type phase contrast interference microscope equipped with a digital camera, which can be used to photomicrograph the paste foam. A drop of the paste foam sample is mounted on a microscope slide, and the structure of the bubble is observed. 2.2.4. Thermal Stability Analysis. The thermal gravity is analyzed by a thermal analyzer (NETZSCH, STA409-PC) with a programmed temperature increase from 50 to 400 °C at a rate of 5 °C/min, with air as the testing environment. The sample with an initial mass of 9.2400 mg is scraped down directly from the filters and collected into the melting pot of the analyzer.
t=0
U = rT0
(6)
The corresponding boundary conditions are as follows: r = R , −λ
⎛h 1⎞ h ∂U = −⎜ − ⎟U + R T∞ ⎝λ ∂r λ⎠ λ
(7)
Then, the temperature distribution in the hemispherical coal stockpiles can be obtained as eq 8 q0
q0 R
2 (R − r ) + + T∞ + T= 6λ h R 2
2
βm2J0 (βmr )
3. RESULTS AND DISCUSSION 3.1. Rising Temperature Model for Coal Stockpiles. Due to the random stacking of the particles, the accumulation state and the spatial structure of local spots in the coalbed are very complex and difficult to accurately describe. In this paper, the particles in the bed and the gas in the gap are regarded as a
(
βm2 +
2
h λ2
)
∫0
·
∞ 2 m
∑ e−αβ t· m=1
R
rJ0 (βmr )·(T0 − Ts(r ))dr (8)
mπ
where βm = R ; h is the convective heat transfer coefficient, W/ (m2·K); R is the coal stockpile radius, m; r is the distance from C
DOI: 10.1021/acs.energyfuels.7b00649 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels the center, m; J0 is a geometric factor with a value of 2; λ is the thermal conductivity, W/(m·K); q0 is the constant internal heat source, J/m3; t is the time, s; and α is the thermal diffusivity, m2/s. From eq 8, we find that the analytical solution contains an infinite series, which is inconvenient for practical engineering calculations. The engineering sector has widely used the analytical solution of the first series of several curves (nomogram), which is used to determine the temperature distribution of the line called the Heisler graph. Although this calculation has the advantages of simplicity and convenience, its accuracy is affected by the limited line information and the precision of the graph. With the rapid development of modern computing technology, the method of approximate fitting has become increasingly important. Equation 8 is an infinite series, but when the Fourier coefficient is larger than 0.2 (Foiv > 0.2), the difference in the results calculated by the first term of the series and the complete series is less than 1%. Therefore, eq 8 can be simplified as eq 9: T=
q0 6λ
∫0
·
(R2 − r 2) +
q0 R h
+ T∞ +
composed of r = 0.1, 0.2, 0.3, 0.4, and 0.5 m are collected, and the average value of the 5 monitoring points in each group is calculated. The variation of the temperature with time at different radial distances is shown in Figure 3.
2 2 −αβ12t β1 J0 (β1r ) e · h2 R β12 + 2
(
λ
)
Figure 3. Variation of temperature with time at different radial distances.
R
rJ0 (β1r )·(T0 − Ts(r ))dr
Figure 3 shows that the temperature at different radial distances increases with the increase in the test time between 100 000 and 550 000 s. The temperature at the measurement point increases from 526.62 to 678.98 K at r = 0.1 m, from 525.99 to 676.00 K at r = 0.2 m, from 524.97 to 674.38 K at r = 0.3 m, from 523.52 to 672.14 K at r = 0.4 m, from 521.68 to 669.27 K at r = 0.5 m. With the increasing radial point of measurement, the temperature increases are 150.36, 150.01, 149.41, 148.62, and 147.59 K, respectively. The lack of variability in the radial temperature increase is not obvious, mainly because the internal heat source has a constant output. For the same radial distance, the temperature increases with time. However, the rate of increase from 10 000 to 30 000 s is less than from 30 000 to 550 000 s, mainly because the ambient temperature of the coal particles is relatively low in the early stages and the collision and contact between the coal particles and the oxygen molecules are very slow processes. At this point, the activation energy can be approximated as a constant, and the number of activated coal molecules is relatively small. The rate of the reaction between coal particles and oxygen is very slow. It is very difficult to break the original chemical bonds and to produce the new mass from the oxygen−coal reactions. Therefore, the rise in the temperature of the coal is not significant. Over time, the coal enters the self-heating period, due to the role of the latent process. When the coal temperature becomes relatively high, the activation energy increases, and the chemical reaction between the coal particles and the oxygen is accelerated. At this point, the energy required for the activation of a common reactant molecule into an active molecule is reduced, the chemical bonds between the molecules are broken, and the generation of the new molecules is increased. At the same time, with the increase in temperature, the energies of both the activated and reactant molecules increase. However, the increase in the average total energy of the reactant molecules is relatively much higher than for the activated molecules. Therefore, the overall activation energy decreases. Due to the reduction in the activation energy, the oxidation reaction rate is accelerated, and the reaction releases significant heat.
(9)
On the basis of a previous study of the rapid determination of the spatiotemporal temperatures and heat transfer in simple bodies cooled by convection,43 the fitting formula can be obtained as eq 10 [(1 − μ1)2 ·(λt / ρcR2)]
T = T∞ + T0RAe
B+
q0 R h
+
q0(R2 − r 2) 6λ (10)
where
μ1 = 1 − β1R A=2
B=
(11)
sin μ1 − μ1 cos μ1 μ1 − sin μ1 cos μ1
(12)
⎛ rμ ⎞ R sin⎜ 1 ⎟ rμ1 ⎝ R ⎠
(13)
Equation 10 is used to calculate the temperature field of the coal pile when the internal heat source (q0) is stable. However, when the constant heat flux from the internal heat source is moved from the center of the coal stockpile to the external environment, the coal particles in the stockpile are in a certain heat storage environment and are rich in oxygen in the fracture channel, and the stockpile reacts with oxygen and releases excess heat. Therefore, the calculation of the temperature field in the actual coal stockpile should consider the influence of spontaneous combustion. Therefore, a new interpolation function, F(t,r), is introduced to modify eq 10, and eq 14 is obtained: [(1 − μ1)2 ·(λt / ρcR2)]
T = T∞ + T0RAe + F(t ,r )
B+
q0 R h
+
q0(R2 − r 2) 6λ (14)
3.2. Temperature Rise Test of Coal Stockpiles. The entire test time is 550 000 s. Five sets of monitoring point data D
DOI: 10.1021/acs.energyfuels.7b00649 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels To further determine the heat production rules induced by the spontaneous combustion of the coal stockpile, the measured data of the coal stockpile temperature and the data calculated according to the mathematical model are drawn at the radial distance r = 0.3 m, as shown in Figure 4.
Table 1. Variation Trend of the Temperature Difference between the Measured and Theoretical Calculations at Different Radial Distances temperature difference between model and test data (K) time (s)
0.1
0.2
0.3
0.4
0.5
100 000 150 000 200 000 250 000 300 000 350 000 400 000 450 000 500 000 550 000
6.53 7.05 7.52 8.02 9.15 10.57 12.54 15.15 18.25 22.48
6.51 7.01 7.50 8.01 9.11 10.53 12.51 15.13 18.22 22.46
6.50 7.00 7.50 8.00 9.09 10.51 12.50 15.10 18.18 22.42
6.47 6.98 7.48 7.98 9.06 10.49 12.49 15.08 18.17 22.39
6.45 6.96 7.47 7.97 9.04 10.48 12.47 15.05 18.13 22.36
Figure 4. Theoretical values for the coal stockpile model of the rising temperature and actual measured value at a radial distance of r = 0.3 m.
As Figure 4 shows, when the radial distance is r = 0.3 m, the actual temperature rise is consistent with the predicted values of the mathematical model. This indicates that the mathematical model is suitable for a spherically piled coalbed with a constant internal heat source. In addition, the actual temperature rise is greater than the predicted value from the theoretical mathematical model, and the difference in values increases with an increase in the test time. Therefore, the difference in the values is likely not the same at the other radial distances because the heat production rate of the coal particles and oxygen is variable. Thus, we can determine the difference with respect to time at different radial distances and then modify the mathematical model of the coal stockpile temperature rise. For this reason, the actual, time-dependent measured temperature data at radial distances of r = 0.1 m, r = 0.2 m, r = 0.3 m, r = 0.4 m, and r = 0.5 m are determined, and the corresponding theoretical model data are calculated. The trend in the variation in the temperature difference between the measured and the theoretical calculation at different radial distances is shown in Table 1. The difference in the temperature rise with respect to the radial distance and time is fit by a nonlinear surface using the Origin software, as shown in Figure 5. From Figure 5, the fitting eq 15 for the temperature rise difference with varying radial distance and time is obtained
Figure 5. Fitting surface for the difference in the temperature rise as a function of radial distance and time.
⎡ qR kt ⎤ T = T∞ + T0RA exp⎢(1 − u1)2 × B+ 0 2⎥ h ⎣ ρcR ⎦ +
6k
⎛ ⎛ (t − tc) ⎞2 + TC + A × exp⎜⎜ −0.5 × ⎜ ⎟ ⎝ w1 ⎠ ⎝
⎛ (r − rc) ⎞2 ⎞ − 0.5 × ⎜ ⎟⎟ ⎝ w2 ⎠ ⎟⎠
(16)
3.3. Fire Suppression Test at High Temperature in the Coal Stockpile. 3.3.1. Formation and Characterization of Paste Foam. Due to the high viscosity (102.34 mPa·s) of the composite paste, it is difficult to process the aqueous foam into a paste. In addition, the traditional mixing process causes the aqueous foam to readily burst. Thus, a novel mixer is developed that includes a chamber and a built-in hollow screw rod. The hollow screw rod is provided with four rings and half-spiral blades. At the center of the screw pitch of each coil of the hollow rod, an outlet for the aqueous foam as a diversion port is opened, with adjacent diversion ports 72° apart. The guide mouth is composed of a drainage tube insertion hole formed in the radial hollow screw rod. The direction of the opening of the
⎛ ⎛ (t − tc) ⎞2 ⎜ δT = TC + A × exp⎜ −0.5 × ⎜ ⎟ ⎝ w1 ⎠ ⎝ ⎛ (r − rc) ⎞2 ⎞ − 0.5 × ⎜ ⎟⎟ ⎝ w2 ⎠ ⎟⎠
q0(R2 − r 2)
(15)
where Tc, tc, rc, w1, and w2 are all constants and the corresponding values are 5.7515, 2.4794 × 106, −17.9029, 567256.8639, and 23.2775, respectively. Therefore, the revised mathematical model for the temperature rise in a coal stockpile is shown in eq 16: E
DOI: 10.1021/acs.energyfuels.7b00649 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 6. Morphology of the paste foam.
guide hole does not pass through the central line of the hollow screw rod. The dynamic moment of the hollow screw rod from the reaction force of the foam jet improves its speed and stability. The high-speed composite paste is injected into the inlet and pushed forward along the helical blade passage. In this process, the vortex can be completely transformed into turbulent flow and generate a swirl at a given frequency. The loss of kinetic energy acts on the composite system of the paste and aqueous foam to form the paste foam. The rotation of the hollow screw rod causes the aqueous foam and the paste to move forward, improving the mixing quality, and the effect of the stirring paddle further enhances the mixing uniformity of the foam and the paste. The aqueous foam is added into the composite paste system by 5 diversion ports, which increases the contact area between the aqueous foam and the composite paste. As shown in Figure 6, the paste foam prepared by a hollow spiral mixer is uniform, with an average pore size of 100 μm. In addition, the bubble wall is uniformly dispersed with fly ash, cement, and silica particles, with these particles acting as a framework to jointly support the stability of the paste foam. However, as a type of fire extinguishing material for coal stockpiles, the thermal stability of the paste foam in a hightemperature environment is a key technical index. For this reason, a thermogravimetric analysis is used to test the weight loss and heat absorption performance of the paste foam in the temperature range from 50 to 400 °C. Figure 7 shows that, in the range of 50−400 °C, the paste foam gradually begins to lose weight with the increase in the test temperature. However, the weight loss rate shows two distinct stages. In the temperature range from 50 to 250 °C, the mass of the paste foam decreases from 100% to 82.23% with a TG curve (K1) slope of −0.08368. A small amount of heat loss in this temperature range is caused by the evaporation of water and the molecular weight of the unconverted monomer. Correspondingly, for the DSC curve, with an increase in temperature, the hydrogen bonds formed by the hydrophilic amide groups and water molecules are weakened, resulting in the intense contraction and entanglement of the molecular chains and enthalpy of heat changes. Therefore, an endothermic peak appears at 250 °C. However, the endothermic peak is wider than expected, which indicates that the reaction rate of the pyrolysis process is slower. This is because the molecular weight distribution of the various products in the paste foam is wide and the response temperature of different molecular weight chains is different.
Figure 7. TG and DSC curves for the paste foam.
The DSC peak temperature only for characterizing the molecular weight distribution is the most important decomposition temperature of the molecular chain concentration. The temperature response range of the paste foam due to the molecular weight distribution becomes wider and is gradually expanded. Therefore, the response rate decreases gradually. In the temperature range from 250 to 400 °C, the main mass loss occurs with a TG curve (K2) slope of −0.18711. However, even at 400 °C, the remaining mass is approximately at 54.67%, mainly because the global silicon powder, fly ash, and cement particles on the pore wall have a low thermal conductivity. At the same time, the paste foam is a porous structure material, and the heat transfer in the foam pores is primarily through the heat conduction of the gas, giving a relatively low thermal conductivity. To further analyze the influence of temperature on the morphology of the paste foam, the BT-1600 image particle analysis system is used to characterize its micromorphology at 150, 250, 350, and 450 °C, as shown in Figure 8. It can be seen from Figure 8 that, in the case of 150 °C, the bubbles in the paste foam are more uniform and the liquid is more stable. The bubbles become larger, and the particles in the liquid film appear to slide, indicating instability at the temperature of 250 °C. When the temperature reaches 350 °C, the liquid film has become thinner, and the bubbles have coalesced at the temperature of 450 °C. F
DOI: 10.1021/acs.energyfuels.7b00649 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 8. Microstructure of the paste foam under different temperature conditions (a: 150 °C; b: 250 °C; c: 350 °C; d: 450 °C).
3.3.2. Characteristics for Fire Extinguishing and Cooling of the Paste Foam. The variation in the temperature at different radial distances after injecting the paste foam is shown in Figure 9.
460 to 540 s, the temperature drops from 332.56 to 321.36 K, after which the temperature remains at approximately 320 K. At a radial distance of r = 0.4 m from 0 to 480 s, the temperature decreases from 669.44 to 581.14 K; from 480 to 580 s, the temperature drops from 581.14 to 336.48 K; from 580 to 660 s, the temperature drops from 336.48 to 327.37 K, after which the temperature remains at approximately 327 K. At a radial distance of r = 0.5 m from 0 to 700 s, the temperature decreases from 666.48 to 556.41 K; from 700 to 820 s, the temperature drops from 556.41 to 342.92 K; from 820 to 880 s, the temperature drops from 342.92 to 337.75 K, after which the temperature remains at approximately 337 K. The reasons for the above temperature changes are consistent with those for the radial distance r = 0.1 m. In addition, with the continuous injection and diffusion of the paste foam, because of the foam as the carrier with the ability to accumulate upward, the paste foam can gradually block the fracture network in the hemispherical coal pile and hinder the oxygen access into the stockpile body to slow the reaction rate of the coal spontaneous combustion. At 700 s, the paste foam flows out of the coal pile surface, as shown in Figure 10.
Figure 9. Variation of the temperature at different radial distances after injecting the paste foam.
With the test time, the paste foam is continuously injected into the cracks in the coal pile, and the temperature at different radial distances shows the same rule of variation. On the whole, the temperature decrease can be divided into three stages of cooling (shown as ①, ②, and ③ near the curve, Figure 9). The first is a slowdown, followed by a rapid decrease, and the final is the slow reduction of the temperature to a gentle decline. At a radial distance of r = 0.1 m from 0 to 100 s, the temperature decreases from 674.18 to 621.10 K; from 100 to 180 s, the temperature drops from 621.10 to 325.29 K; from 180 to 580 s, the temperature drops from 325.29 to 301.35 K, after which the temperature remains at approximately 301 K. The paste foam diffuses from the center and the oxygen supply channels in the stockpile body are gradually blocked. At 100 s, the paste foam diffuses into the cracks at r = 0.1 m, and the high-temperature coal particles are covered, wrapped, and cooled. Therefore, from 100 to 580 s, the paste foam directly affects the coal particles closer to the center. The cooling rate from the process is apparent, and the temperature at the measuring point decreases rapidly. At a radial distance of r = 0.2 m from 0 to 260 s, the temperature decreases from 673.43 to 593.37 K; from 260 to 340 s, the temperature drops from 593.37 to 324.97 K; from 340 to 540 s, the temperature drops from 324.97 to 308.54 K, after which the temperature remains at approximately 306 K. At a radial distance of r = 0.3 m from 0 to 380 s, the temperature decreases from 671.24 to 588.62 K; from 380 to 460 s, the temperature drops from 588.62 to 332.56 K; from
Figure 10. Effect of the paste foam covering the high-temperature coal stockpile.
Figure 10 shows that the paste foam has a high viscosity, which can produce wall hanging characteristics on the coal particle wall, which closes the cracks in the coal seam and maintains a better bubble shape in the high-temperature environment. In the experiment, the improved thermal stability of the foam can reduce the temperature of the hightemperature coal particles at approximately 670 K, which can be verified by the thermogravimetric analysis in Figure 7. To analyze the effect of covering and continuous cooling of the paste foam on the surface of the coal stockpile, an infrared thermal imager is used to monitor the real-time temperature field at the surface of the coal stockpile, as shown in Figure 11. Figure 11 shows that, at 660 s, there are six high-temperature spots in the surface temperature field of the coal stockpile, and the average temperature of the entire surface is 567 K. At 720 s, G
DOI: 10.1021/acs.energyfuels.7b00649 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 11. Temperature field on the surface of the coal stockpile from the infrared thermal imager.
the number of high-temperature spots is reduced to 3, and the average temperature of the surface is 523 K. This is due to the paste foam spreading to the surface of the coal stockpile after 700 s. At 780 s, the high-temperature area of the coal stockpile is further reduced, and the average temperature of the surface is reduced to 403 K. At 840 s, there is only one high-temperature point on the surface, and the average temperature is reduced to 340 K. The above temperature changes indicate that the paste foam has an acceptable effect on the surface cooling of the coal pile by blocking the high-temperature fracture channels and oxygen isolation.
temperature slowly reducing in a gentle decline. (2) The paste foam has a high viscosity, which can produce wall hanging characteristics on the coal particle wall to close the cracks in the coal stockpile and maintain a better bubble shape of the hightemperature coal particles at approximately 670 K. (3) The paste foam has an improved effect on the surface cooling of the coal stockpile by blocking the high-temperature fracture channels and oxygen isolation based on the infrared thermal imager analysis.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
4. CONCLUSIONS The mathematical model of the temperature rise of the hemispherical coal stockpile with a constant heat source in the center was derived and simplified according to a previous study on the rapid determination of the spatiotemporal temperature and heat transfer in simple bodies. With the increase in the radial distance, the increase in temperature was not obvious. For a given radial distance, the rate of increase from 10 000 to 30 000 s was less than the rate of increase from 30 000 to 550 000 s, due to the reduction of the activation energy. The actual temperature rise was clearly greater than the predicted value from the mathematical model, and the difference between these values increases at longer test times. The distribution of the temperature field caused by the spontaneous combustion of coal was fit by a nonlinear surface using the Origin software. A novel material, called paste foam, for preventing the spontaneous combustion of a coal stockpile was prepared, and the working principle of a device with a hollow spiral tube was studied. The paste foam was uniform with an average pore size of 100 μm, and the bubble wall was uniformly dispersed with fly ash, cement, and silica particles. The TG and DSC curves showed that the critical weight loss temperature was 250 °C, with a remaining mass of 82.23%. The fire extinguishing and cooling with the paste foam indicated the following: (1) The temperature change can be divided into three stages, including an initial slowdown, followed by a rapid decrease, and the final
ORCID
Yi Lu: 0000-0003-4285-5536 Notes
The author declares no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51604110, 51274100, U1361118, 51504093, and 51374003) and the Scientific Research Foundation for Doctor of Hunan University of Science and Technology (E51650).
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REFERENCES
(1) Pandey, J.; Mohalik, N. K.; Mishra, R. K.; Khalkho, A.; Kumar, D.; Singh, V. K. Investigation of the Role of Fire Retardants in Preventing Spontaneous Heating of Coal and Controlling Coal Mine Fires. Fire Technol. 2015, 51 (2), 227−245. (2) Deng, J.; Xiao, Y.; Li, Q.; Lu, J.; Wen, H. Experimental studies of spontaneous combustion and anaerobic cooling of coal. Fuel 2015, 157, 261−269. (3) Lu, Y.; Qin, B. Identification and control of spontaneous combustion of coal pillars: a case study in the Qianyingzi Mine, China. Natural Hazards 2015, 75 (3), 2683−2697. (4) Ejlali, A.; Aminossadati, S. M.; Hooman, K.; Beamish, B. B. A new criterion to design reactive coal stockpiles. Int. Commun. Heat Mass Transfer 2009, 36 (7), 669−673.
H
DOI: 10.1021/acs.energyfuels.7b00649 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels (5) Hooman, K.; Maas, U. Theoretical analysis of coal stockpile selfheating. Fire Saf. J. 2014, 67 (7), 107−112. (6) Kozinc, J.; Zupancickralj, L.; Zapusek, A. Evaluation of gas emissions from coal stockpile. Chemosphere 2004, 55 (8), 1121−1126. (7) Grossman, S. L.; Davidi, S.; Cohen, H. Emission of toxic and fire hazardous gases from atmospheric stockpiled coal. Fuel 1993, 72 (5), 708−708. (8) Miura, K. Adsorption of water vapor from ambient atmosphere onto coal fines leading to spontaneous heating of coal stockpile. Energy Fuels 2016, 30 (1), 219−229. (9) Stott, J. B.; Harris, B. J.; Hansen, P. J. A ‘full-scale’ laboratory test for the spontaneous heating of coal. Fuel 1987, 66 (7), 1012−1013. (10) Stott, J. B. Influence of moisture on the spontaneous heating of coal. Nature 1960, 188 (4744), 54−54. (11) Deng, J.; Xu, J. C.; Wang, H. Q. Numerical simulation study on the spontaneous combustion process of the column coal sample. J. Liaoning Tech. Univ. 2002, 90 (12), 505−509. (12) Wen, H.; Yu, Z.; Deng, J.; Zhai, X. Spontaneous ignition characteristics of coal in a large-scale furnace: An experimental and numerical investigation. Appl. Therm. Eng. 2017, 114, 583−592. (13) Taraba, B.; Michalec, Z. Effect of longwall face advance rate on spontaneous heating process in the gob area − CFD modelling. Fuel 2011, 90 (8), 2790−2797. (14) Kim, C. J.; Sohn, C. H. A novel method to suppress spontaneous ignition of coal stockpiles in a coal storage yard. Fuel Process. Technol. 2012, 100 (100), 73−83. (15) Zhu, H. Q.; Song, Z. Y.; Tan, B.; Hao, Y.-z. Numerical investigation and theoretical prediction of self-ignition characteristics of coarse coal stockpiles. J. Loss Prev. Process Ind. 2013, 26 (1), 236− 244. (16) Taraba, B.; Michalec, Z.; Blejchař, T.; Michalcová, V.; Bojko, M.; Kozubková, M. CFD simulations of the effect of wind on the spontaneous heating of coal stockpiles. Fuel 2014, 118 (1), 107−112. (17) Yang, Y.; Li, Z.; Tang, Y.; Liu, Z.; Ji, H. Fine coal covering for preventing spontaneous combustion of coal pile. Nat. Hazards 2014, 74 (2), 603−622. (18) Xia, T.; Wang, X.; Zhou, F.; Kang, J.; Liu, J.; Gao, F. Evolution of coal self-heating processes in longwall gob areas. Int. J. Heat Mass Transfer 2015, 86, 861−868. (19) Zhang, J.; Liang, Y.; Ren, T.; Wang, Z.; Wang, G. Transient CFD modelling of low-temperature spontaneous heating behaviour in multiple coal stockpiles with wind forced convection. Fuel Process. Technol. 2016, 149, 55−74. (20) Zhang, J.; Choi, W.; Ito, T.; Takahashi, K.; Fujita, M. Modelling and parametric investigations on spontaneous heating in coal pile. Fuel 2016, 176, 181−189. (21) Ejlali, A.; Hooman, K. Buoyancy Effects on Cooling a Heat Generating Porous Medium: Coal Stockpile. Transp. Porous Media 2011, 88 (2), 235−248. (22) Ejlali, A.; Aminossadati, S. M.; Ejlali, A. Numerical analysis of fluid flow and heat transfer through a reactive coal stockpile. Med. Oncol. 2009, 31 (9), 1−8. (23) Maloney, D. J.; Sampath, R.; Zondlo, J. W. Heat capacity and thermal conductivity considerations for coal particles during the early stages of rapid heating. Combust. Flame 1999, 116 (1−2), 94−104. (24) Kim, J.; Lee, Y.; Ryu, C.; Park, H. Y.; Lim, H. Low-temperature reactivity of coals for evaluation of spontaneous combustion propensity. Korean J. Chem. Eng. 2015, 32 (7), 1297−1304. (25) Fierro, V.; Miranda, J. L.; Romero, C.; Andrés, J. M.; Arriaga, A.; Schmal, D. Model predictions and experimental results on self-heating prevention of stockpiled coals. Fuel 2001, 80 (1), 125−134. (26) Fierro, V.; Miranda, J. L.; Romero, C.; Andrés, J. M.; Arriaga, A.; Schmal, D.; Visser, G. H. Prevention of spontaneous combustion in coal stockpiles: Experimental results in coal storage yard. Fuel Process. Technol. 1999, 59 (1), 23−34. (27) Arisoy, A.; Akgün, F. Effect of pile height on spontaneous heating of coal stockpiles. Combust. Sci. Technol. 2000, 153 (1), 157− 168.
(28) Moghtaderi, B.; Dlugogorski, B. Z.; Kennedy, E. M. Effects of wind flow on self-heating characteristics of coal stockpiles. Process Saf. Environ. Prot. 2000, 78 (6), 445−453. (29) Kim, C. J.; Sohn, C. H. Effects of Wind Barrier Design and Closed Coal Storage on Spontaneous Ignition of Coal Stockpiles. J. Loss Prev. Process Ind. 2016, 40, 529−536. (30) Shao, Z.; Wang, D.; Wang, Y.; Zhong, X.; Tang, X.; Hu, X. Controlling coal fires using the three-phase foam and water mist techniques in the Anjialing Open Pit Mine, China. Nat. Hazards 2015, 75 (2), 1833−1852. (31) Krishnaswamy, S.; Agarwal, P. K.; Gunn, R. D. Low-temperature oxidation of coal. 3. Modelling spontaneous combustion in coal stockpiles. Fuel 1996, 75 (3), 353−362. (32) Liu, X.; Xiao, Y.; Deng, J. Research and application on fire prevention and control performances of fly ash grouting. Coal Eng. 2011, 299 (2), 208−215. (33) Wang, D.; Dou, G.; Zhong, X.; Xin, H.; Qin, B. An experimental approach to selecting chemical inhibitors to retard the spontaneous combustion of coal. Fuel 2014, 117 (5), 218−223. (34) Yang, S. J.; Cheng, T.; Xu, Q. Theoretical analysis and numerical simulation of influence of the change of negative pressure and air volume of inner interlocked tail road on coal spontaneous combustion. J. China Coal Soc. 2011, 36 (2), 308−312. (35) Niu, H. Y.; Wang, H. Q. Influence analysis of flow rule in mine fire during injecting inert gases. J. Coal Sci. Eng. 2011, 17 (4), 422− 425. (36) Deng, J.; Xiao, Y.; Lu, J.; Wen, H.; Jin, Y. Application of composite fly ash gel to extinguish outcrop coal fires in China. Nat. Hazards 2015, 79 (2), 881−898. (37) Qin, B.; Lu, Y.; Li, Y.; Wang, D. Aqueous three-phase foam supported by fly ash for coal spontaneous combustion prevention and control. Adv. Powder Technol. 2014, 25 (5), 1527−1533. (38) Xu, Y.-l.; Wang, D.-m.; Wang, L.-y.; Zhong, X.-x.; Chu, T.-x. Experimental research on inhibition performances of the sandsuspended colloid for coal spontaneous combustion. Safety Sci. 2012, 50 (4), 822−827. (39) Ren, X. W.; Wang, F. Z.; Guo, Q.; Zuo, Z. B.; Fang, Q. S. Application of foam-gel technique to control CO exposure generated during spontaneous combustion of coal in coal mines. J. Occup. Environ. Hyg. 2015, 12 (11), D239−D245. (40) Tang, Y. Inhibition of low-temperature oxidation of bituminous coal using a novel phase-transition aerosol. Energy Fuels 2016, 30, 9303−9309. (41) Cement, Composition, Specifications and Conformity Criteria for Common Cements; Standard No. BS EN 197-1; British Standards Institution: London, U.K., 1995. (42) Fly Ash for Concrete: Definitions, Requirements and Quality Control; Standard No. BS EN 450; British Standards Institution: London, U.K., 1995. (43) Campo, A. Rapid determination of spatio-temporal temperatures and heat transfer in simple bodies cooled by convection: usage of calculators in lieu of heisler-gröber charts. Int. Commun. Heat Mass Transfer 1997, 24 (4), 553−564.
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DOI: 10.1021/acs.energyfuels.7b00649 Energy Fuels XXXX, XXX, XXX−XXX
